E3

I
I
I

I

Marine Biological Laboratory Library

Woods Hole, Mass.

Presented by

ESTATE OF HERBERT W. RAND
January 9, 1964

A.

I
I
I
I
I
I
I

CD

tr

i

CD-

a
a

a
m
a

A TEXT-BOOK OF ZOOLOGY

VOL. I

MACMILLAN AND CO., LIMITED

LONDON . I'OMBAY . CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD

TORONTO

A TEXT-BOOK
OF ZOOLOGY

BY

T. JEFFERY PARKER, D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, N.2.

AND

WILLIAM A. HASWELL, M.A., D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY, N.S.XV.

Lit / j

*
iss.

IN TWO VOLUMES
VOL, I

WITH ILLUSTRATIONS

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1910

RICHARD CLAY AND SONS, LIMITED,

BREAD STREET HILL, E.G., AND

BUNGAY, SUFFOLK.

First Edition, 1898.
Second Edition, 1010.

PKEFACE TO THE EIRST EDITION

IN spite of its bulk, the present work is strictly adapted to the
needs of the beginner. The mode of treatment of the subject is
such that no previous knowledge of Zoology is assumed, and
students of the first and second years should have no more
difficulty in following the accounts of the various groups than is
incidental to the first study of a complex and unfamiliar subject.

There can be little doubt that the study of Zoology is most
profitably as well as most pleasantly begun in the field and by the
sea-shore, in the Zoological Garden and the Aquarium. In a
very real sense it is true that the best zoologist is he who knows
the most animals, and there can certainly be no better foundation
for a strict and scientific study of the subject than a familiarity
with the general appearance and habits of the common members
of the principal animal classes. But Zoology as a branch of
academical study can hardly be pursued on the broad lines of
general natural history, and must be content to lose a little in
breadth of view at least in its earlier stages while insisting upon
accurate observation, comparison, and induction, within the limited
field of Laboratory and Museum work.

A not uncommon method of expounding the science of Zoology
is to begin the study of a given group by a definition, the very
terms of which it is impossible that the student should under-
stand ; then to take a general survey of the group, illustrated by
casual references to animals and to structures of which it is highly
unlikely he has ever heard ; and, finally, to descend to a survey of
the more important forms included in the group. It will probably
be generally agreed that, from the teacher's point of view, this
method begins at the wrong end, and is hardly more rational than

vi PREFACE TO THE FIRST EDITION

it would be to deliver a coarse on the general characteristics of
English Literature, suitably illustrated by " elegant extracts," to a
class of students who had never read a single English poet or
essayist.

There can be no question as to the vast improvement effected in
zoological teaching by the practice of preceding the study of a
given group as a whole by the accurate examination of a suitable
member of it. With the clear mental image of a particular animal,
in the totality of its organisation, the comparison of the parts and
organs of other animals of like build becomes a profitable study,
and the danger of the comparative method that the student may
learn a great deal of the systems of organs in a group without
getting a clear conception of a single animal belonging to it is
much diminished.

The method of " types " has, however, its own dangers. Students
are, in their way, great generalisers, and, unless carefully looked
after, are quite sure to take the type for the class, and to consider
all Arthropods but crayfishes and cockroaches, and all Molluscs but
mussels and snails, as non-typical. For this reason a course of
Zoology which confines itself entirely or largely to " types," or, as
we prefer to call them, 1 examples, is certain to be a singularly
narrow and barren affair, and to leave the student with the
vaguest and most erroneous ideas of the animal kingdom as a
whole. This is especially the case when the number of examples
is small, each of the Phyla being represented by only one or two
forms.

In our opinion every group which cannot readily and intel-
ligibly be described in terms of some other group should be
represented, in an elementary course of Zoology, by an example.
\Vu have, therefore, in the majority of cases, described, in some
detail, an example of every important class, and, in cases where
the diversity of organisation is very great as in Crustacea and
Fishes two or more examples are taken. The student is thus
furnished with a brief account of at least one member usually
readily accessible of all the principal groups of animals.

By the time the example has been studied, a definition of the
class and of its orders will convey some idea to the mind, and will

1 Following a suggestion for which we are indebted to Dr. Alexander Hill,
Master of Downing College, Cambridge.

PREFACE TO THE FIRST EDITION vii

serve to show which of the characters already met with are of
distinctive importance, and which special to the example itself
In order to bring out this point more clearly, to furnish a connec-
tion between the account of the example and that of the class as a
whole, and to give some idea of the meaning of specific, generic,
and family characters, we have introduced, after the classification,
a paragraph giving the systematic position of the example, some-
times in more, sometimes in less detail.

Following the table of classification with its brief definitions
comes the general account of the group. This is usually treated
according to the comparative method, the leading modifications of
the various parts and organs being described seriatim. In a few
cases this plan has been abandoned and the class described order
by order, but this is done only when the deviations from the type
are so considerable as to lead us to think the comparative method
unsuitable for beginners. On the other hand, when all the classes
of the phylum present a very uniform type of structure, the
phylum is studied comparatively as a whole. The description of
each group usually ends with some account of its ethology and
distribution, and with a discussion of its affinities and of the
mutual relationships of its various subdivisions.

We have done our best to make the space devoted to each group
proportional to its complexity and range of variation, and to
subdue the natural tendency to devote most attention to the more
recently investigated classes, or to those in which we ourselves
happen to be especially interested. A few lesser groups have been
put into small type, partly to economise space, partly because they
seem to us to be of minor importance to the beginner.

Following out the plan of deferring the discussion of general
questions until the facts with which they are connected have been
brought forward, we have placed the sections on Distribution, on
the Philosophy of Zoology, and on the History of Zoology at the
end of the book. We have, however, placed a general account
of the structure and physiology of animals immediately after the
Introduction, and one on the Craniate Vertebrata before the
description of the classes of that division, but it will be obvious
that these deviations from the strictly inductive method were
inevitable in order to avoid much needless repetition.

After a good deal of consideration we have decided to omit all

viii PREFACE TO THE FIRST EDITION

references to the literature of the subject in the body of the work.
Anything like consistent historical treatment would be out of place
in an elementary book ; and the introduction of casual references
to particular discoveries, while they might interest the more
advanced reader by giving a kind of personal colouring to the
subject, could hardly fail, from their necessarily limited character, to
be misleading to the beginner, and to increase rather than diminish
his difficulties. We have, therefore, postponed all reference to the
history of the science to the concluding Section, in which the main
lines of progress are set forth, and have given, as an Appendix, a
guide to the modern literature of Zoology. The latter is intended
merely to indicate the next step to be taken by the student who
wishes to acquire something more than a mere text-book
knowledge. 1

The various Sections have been written by the authors in fairly
equal proportions, but the work of each has been carefully read
and criticised by the other, and no disputed point has been allowed
to stand without thorough discussion. We are therefore jointly
and severally responsible for the whole work.

A very large proportion of the figures have been specially drawn
and engraved for the book. Those in which no source is named
are from our own drawings, with the exception of Figs. 571, 572,
1017, 1018, 1019, 1022, 1059, 1063, and 1071, for which we are
indebted to Mrs. W. A. Haswell. Figs. 1002 bis, 1005 fo's,are from
photographs kindly taken for us by Mr. A. Hamilton. 2 Many blocks
have been borrowed from well-known works, to the authors and
publishers of which we beg to return our sincere acknowledg-
ments. All the new figures have been drawn by Mr. M. P. Parker.

1 In this connection we cannot resist the pleasure of quoting two passages,
exactly expressing our own views, from the preface to Dr. Waller's Human
Physiology, which came under our notice after the above paragraph was in type '.
"I have given a Bibliography after some hesitation, feeling that references to
original papers are of no use to junior students, and must be too imperfect to
be satisfactory to more advanced students. . . . Attention has been paid to
recent work, but I have felt that the gradually-formed deposit of accepted know-
ledge must be of greater intrinsic value than the latest ' discovery ' or the
newest theory. An early mental diet in which these items are predominant is
an unwholesome diet ; their function in elementary instruction is that of condi-
ments, valuable only in conjunction with a foundation of solid food."

2 The figures referred to are numbered 608, 609, 1080, 1081, 1082, 1085,
1128, 1132, 1140, 1063, and 1067 in the new edition.

PREFACE TO THE FIRST EDITION ix

We have received generous assistance from Professors Arthur
Dendy, G. B. Howes, Baldwin Spencer, and J. T. Wilson, and from
Mr. J. P. Hill and Dr. Arthur Willey. Professor W. N. Parker
has very kindly read the whole of the proof-sheets and favoured us
with many valuable suggestions, besides acting as referee in
numerous minor difficulties which would otherwise have cost a
delay of many weeks.

It is a mere truism to say that a text-book can never really
reflect the existing state of the science of which it treats,
but must necessarily be to some extent out of date at the time
of publication. In the present instance, the revises of the earlier
pages, giving the last opportunity for any but minor alterations,
were corrected in the latter part of 1895, and the sheets passed
for press in the middle of 1896. We are, therefore, fully alive to
the fact that much of our work already needs a thorough revision ,
and can console ourselves only by reflecting that "to travel hope-
fully is a better thing than to arrive, and the true success is to
labour."

We may mention, in conclusion, that, whatever may be the merits
or demerits of the book, it enjoys the distinction of being unique
in one respect. The two authors have been separated from one
another, during the greater part of their collaboration, by a
distance of 1200 miles, and the manuscript, proofs, and drawings
have had to traverse half the circumference of the globe in their
journeys between the authors on the one hand, and the publishers,
printers, artist, and engravers on the other. It will, therefore, be
readily believed that all persons concerned have had every oppor-
tunity, during the progress of the work, of exercising the supreme
virtue of patience.

PREFACE TO THE SECOND EDITION

A NEW edition of this Text-Book has been called for on some-
what short notice, and, had it not been for the assistance generously
rendered by Professor W. Newton Parker, who has helped me
greatly in the revision of the proofs, and has made a large number
of useful suggestions, it would have been impossible for me to have
completed the work within the time prescribed. Fortunately, also,
materials for the most important of the alterations and additions
had been already, to a certain extent, prepared.

The original plan of the work has not been in any way altered,
and, though all parts have been subjected to careful revision, there
is a good deal, especially in the descriptions of many of the
examples, which has not been materially changed. On the other
hand, some parts have been to a great extent re-written, and a
good many illustrations have been added, a fair proportion of which
are new to text-books of this description.

I have the pleasure of acknowledging assistance on special points
received from Professor J. P. Hill, Mr. S. J. Johnston, B.A., B.Sc.,
Mr. E. J. Goddard, B.A, D.Sc, and Mr. H. L. Kesteven, B.Sc, all
of the University of Sydney. A good many of the new illustra-
tions were re-drawn by W. Birmingham, Laboratory Assistant,
Department of Biology.

W. A. HAS WELL.

[ Lie * j

MASS.

CONTENTS

PAGE

PREFACE .... . v

CONTENTS OF SECTIONS IN VOL. I xiii

LIST OF ILLUSTRATIONS IN VOL. I. ... .... xix

TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM . . . xxxv
INTRODUCTION 1

SECTION I

THE GENERAL STRUCTURE AND PHYSIOLOGY OF ANIMALS .... 10

1. Amoeba 10

2. The Animal Cell . . .... 14

3. The Ovum : Maturation, Impregnation, and Segmentation : the

Germinal Layers 19

4. Tissues . 23

5. Organs 31

6. The Reproduction of Animals 40

7. Symmetry .... 41

8. The Primary Subdivisions or Phyla of the Animal Kingdom . 43

SECTION II

PHYLUM PROTOZOA ... 45

Class I. Rhizopoda . .46

1. Example of the Class Amceba proteus 46

2. Classification and General Organisation 47

Systematic Position of the Example ... 48

Appendix to the Rhizopoda 64

Class II. Mycetozoa . ... 66

1. Example of the Class Didymium difforme <>6

2. General Remarks . 67

Class III. Mastigophora . 67

1. Example of the Class Englena viridis . . '>7

2. Classification and General Organisation . .... 69
Systematic Position of the Example .... .70

Class IV. Sporozoa . 80

1. Example of the Class Mouoci/stis ay His 80

2. Classification and General Organisation .... .81
Systematic Position of the Example . ... 82

8

ft

xiv CONTENTS

PAGE

PHYLUM PROTOZOA continued.
Class V. Infusoria

1. Example of the Class Paramceciv.m caudatuin ....

2. Classification and General Organisation ... . 91
Systematic Position of the Example

Further Remarks on the Protozoa . . . . . 101

SECTION III

PHYLUM AND CLASS PORIFERA [PARAZOA] .

1. Example of the Class Sycon yelatinosum . . 105

2. Distinctive Characters and Classification ... . Ill
Systematic Position of the Example .

3. General Organisation ... 114

SECTION IV

PHYLUM COZLENTERATA . 128

Class I. Hydrozoa

1. Example of the Class Obelia ... . . . 128

2. General Structure and Classification 140

Systematic Position of the Example .... . 142

Additional Remarks ... 167

Class II. Scyphozoa . . 168

1. Example of the Class Aurelia aurita 168

2. General Structure and Classification 176

Systematic Position of the Example 177

Additional Remarks 184

Class III. Actinozoa , . 185

1. Example of the Class Tealia crassicornis 185

2. Distinctive Characters and Classification 193

Systematic Position of the Example . . . 196

3. General Organisation . 196

Class IV. Ctenophora 211

1. Example of the Class Hormiphora plumosa 211

2. Distinctive Characters and Classification 220

Systematic Position of the Example ... . 221

3. General Organisation . 222

Appendix to Ctenophora Ctenoplana and Codoplana . . . 225

The Relationships of the Coelenterata 226

Appendix to the Coelenterata The Mesozoa 230

SECTION V

PHYLUM PLATYHELMINTHES . 235

1. Examples of the Phylum 236

i. Planaria or Dendrocoeliim 236

ii. Fasciola hepatica . 240

iii. Tania soliuni 245

CONTENTS xv

PAGE

PHYLUM PLATYHELMINTHES continued.

2. Distinctive Characters and Classification 251

Systematic Position of the Examples 253

.">. General Organisation 254

4. Distribution, Mode of Occurrence, and Mutual Relationships . 283

Appendix to Platyhelminthes Class Nemertinea . . . 288

Distinctive Characters and Classification 295

SECTION VI

PHYLUM NEMATHELMINTHES 297

Class I. Nematoda ... 297

1. Example of the Class Ascaris lumbricoides 297

2. Distinctive Characters and Classification ..... 303
Systematic Position of the Example 304

3. General Organisation 305

Class II. Acanthocephala . . 312

Class III. Chatognatha .... 316

Appendix to Nemathelminthes 319

Family Chcetosomidce 319

,, Echinoderidce 319

,, Desmoscolecidce 320

Affinities and Mutual Relationships of the Nemathelminthes . 320

SECTION VII

PHYLUM TKOCHELMINTHES 322

Class I. Rotifera . . . 323

1. Example of the Class Brachionus rubens 323

2. Distinctive Characters and Classification 327

Systematic Position of the Example 329

3. General Organisation 330

Class II. Gastrotricha , 335

Appendix to Trochelminthes Dinophttea and Histriobdellea . 336

SECTION VIII

PHYLUM MOLLUSCOIDA 340

Class I. Polyzoa . . ... .340

1. Example of the Class Bugida avicularia 341

2. Distinctive Characters and Classification . . . 347
Systematic Position of the Example 348

3. General Organisation 348

Class II. Phoronida . . 355

Class III. Brachiopoda 360

1. Example of the Class Magellania lenticultirix . . . 360

2. Distinctive Characters and Classification . . . 366
Systematic Position of the Example 367

3. General Organisation 367

Mutual Relationships of the Classes of the Molluscoida . . 372

xvi CONTENTS

SECTION IX

PAGE

Phylum Eehinodermata 375

1. Example of the Asteroidea Asterias rubens or Anthenea flavescens. 375

2. Example of the Echinoidea Strongylocentrotyg or .Echinus . . 393

3. Example of the Holothuroidea Cucumaria or Colochinix . . 401

4. The Crinoidea Antedon rosacea 405

5. Distinctive Characters and Classification .410

Systematic Position of the Examples 41 4

6. General Organisation 415

SECTION X

PHYLUM ANNULATA 439

Class I. Chsetopoda 439

1. Examples of the Class 440

i. Nereis dumerilii . . . 440

ii. Lumbricus 454

2. Distinctive Characters and Classification 464

Systematic Position of the Examples 466

3. General Organisation 467

Appendix to the Chsetopoda Class MyzostOHlida .... 489

Class II. Gephyrea 491

1. Example of the Class Sipuncvlus nudus 492

2. Distinctive Characters and Classification 495

3. General Organisation 496

Class III. Archi-annelida . 503

Class IV. Hirudinea 506

1. Example of the Class Hirudo medicinalis and H. aiistr<di* . 506

2. Distinctive Characters and Classification 515

Systematic Position of the Example 517

3. General Organisation 517

4. General Remarks on the Annulata 523

SECTION XI

PHYLUM ARTHROPOD A 526

Class I. Crustacea 526

1. Examples of the Class 526

i. Apus or Lepidurus 526

ii. Astacus fluviatilis 539

2. Distinctive Characters and Classification 561

Systematic Position of the Examples 569

3. General Organisation 570

Affinities and Mutual Relationships 602

Appendix to Crustacea Class Trilobita 604

Class II. Onychophora 607

Class III. Myriapoda 614

1. Distinctive Characters and Classification 614

2. General Organisation 615

CONTENTS xvii

PAGE

PHYLUM ARTHROPODA continued.

Class IV. Insecta . 619

1. Example of the Class Periplaneta orientalis or P. americana . 619

2. Distinctive Characters and Classification 631

Systematic Position of the Example . . 636

3. General Organisation . . ... . 636

Class V. Arachnida . . . 653

1. Example of the Class Enscorpio or Bit thus 653

2. Distinctive Characters and Classification . ... 660

3. General Organisation 662

Appendix to the Arachnida the Pijcnoyonida, Linguatulida,

and Tardigrade 673

Relations of the Air-breathing Arthropoda 676

SECTION XII

PHYLUM MOLLUSCA . 680

Class I. Pelecypoda . 680

1. Example of the Class Anodonta and Unio 680

2. Distinctive Characters and Classification 694

Systematic Position of the Examples 696

3. General Organisation . . . ... 696

Class II. Amphineura . 712

1. Distinctive Characters and Classification 712

2. General Organisation . .... 713
Class III. Gastropoda ... . 721

1. Example of the Class Triton nodiferus .... . 721

2. Distinctive Characters and Classification . ... 732
Systematic Position of the Example .... . 734

3. General Organisation . . . 735
Appendix to the Gastropoda . .... 756

A. Class IV. Scaphoda . 756

B. Rhodope . . 758
Class V. Cephalopoda . . . 759

1. Examples of the Class 759

i. Sepia, .... 759

ii. Nautilus pompilius . 776

2. Distinctive Characters and Classification 789

Systematic Position of the Examples .... . 790

3. General Organisation . 790

General Remarks on the Mollusca 804

VOL. I

LIST OF ILLUSTRATIONS

VOL. I.

FIG. PAGE

1 . Amoeba proteus 10

2. Amoeba polypodia, fission 13

3. Alveolar theory of protoplasm 15

4. Reticular theory of protoplasm 1G

5. Diagrams illustrating karyokinesis 17

6. Ovum of Sea-urchin 19

7. Maturation and fertilisation of ovum ... ... 20

8. Segmentation of ovum 22

9. Gastrulation 22

10. Gastrula .23

11. Various forms of epithelium 24

12. Diagram to illustrate structure of glands 25

13. Gelatinous connective tissue 26

14. Reticular connective tissue 26

15. Fatty tissue ... ... .... 27

16. Hyaline cartilage 27

17. Fibro cartilage ...... 27

18. Bone ...... .28

19. Unstriped muscle 29

20. Striped muscle 29

21. Nerve-cells 30 1

22. Nerve-fibres ... 30'

23. Various forms of spermatozoa .30'

24. Viscera of Frog .... 33

25. Bones of human arm with biceps muscle 37

26. Nervous system of Frog 38

27. Hydra . . . 41

28. Diagram of axes of body 42

29. Radial symmetry 42

30. Amoeba, various species . . . 47

31. Protamoeba primitiva . 49

32. Quadrula, Hyalosphenia, Arcella, and Difflugia .... 49

33. Microgromia socialis 50

b 2

xx LIST OF ILLUSTRATIONS

FIG. PAGE

34. Platoum stercoreum .51

35. Various forms of Foraminifera 52

36. Shells of Foraminifera . 53

37. Hastigerina murrayi .... .... 54

38. Dimorphism and alternation of generations in Polystomella . . 55

39. Actinophrys sol ... 57

40. Actinosphserium eichhornii 57

41. Various forms of Heliozoa 58

42. Actinophrys sol, conjugation 59

43. Lithocireus annularis 60

44. Thalassoplancta brevispicula 61

45. Aulactinium actinastrum ......... 62

46. Actinomma asteracanthion ... 62

47. Collozoum inerme . . ...... 63

48. Chlamydomyxa labyrinth uloides .... ... 64

49. Labyrinthula 65

50. Didymium difforme 66

51. Euglena viridis . 68

52. Various forms of Flagellata 71

ij

53. Trypanosome . . . .72

54. H^ematococcus pluvialis 73

55. Pandorina morum 74

56. Volvox globator .... 75

57. Heteromita rostra ta . . 76

58. Various forms of Choanoflagellata 77

59. Various forms of Dinoflagellata .79

60. Noctiluca miliaris 79

61. Monocystis . . . 80

62. Gregariiia . 82

63. ,, development 83

64. Eimeria and Coccidium ... 84

65. Coccidium, life-history .... ... .85

66. Malaria parasite 86

67. Myxidium and Myxobolus 87

68. Sarcocystis miescheri . 88

69. Paramoecium caudatum 89

70. ,, ,, conjugation . .... 90

71. Various forms of Ciliata 94

72. .95

73. Vorticella ...... 96

74. Zoothamnium arbuscula ....... .97

75. Opanna ranarum . .... . . 98

76. Various forms of Tentaculifera . 100

77. Diagram showing the mutual relationships of the Protozoa . 103

78. Sycon gelatinosum . . 106

79. ,, ,, magnified ... .... 106

80. ,, ,, transverse section .... . 107

81. ,, ,, vertical section 108

LIST OF ILLUSTRATIONS xxi

PIG.

82. Sycon gelatinosum, pore-membrane . . . 109

83. ,, ,, apopyle . 109

84. External form of various Sponges . . .115

85. Ascetta primordialis ...... . 116

86. Diagrams of canal-system of various Sponges . .117

87. Vertical Section of Spongilla .... . . 118

88. Cells of ectoderm of Sponge . . . . . . 119

89. Development of tri-radiate spicule ... . 120

90. Skeleton of various Sponges . . 121

91. Various forms of Sponge spicules . 122

92. Pheronema Carpenter! . . ... 123

93. Larva of Clathrina blanca . ... ... 124

94. Development of Sycoii raphanus . 125

95. Obelia . . .130

96. ,, Vertical section of polype .... ... 132

97. Nematocysts of Hydra . . . 133

98. Tentacle of Eucopella . 134

99. Obelia, medusa ........ . . 135

100. Diagram of medusa ... . 136

101. Derivation of medusa from polype . 137

102. Projections of polype and medusa. . . 138

103. Development of zoophyte . . . 140

104. Bougainvillea ramosa . . . . 144

105. Various forms of Leptolinre . . 145

106. Ceratella . . 146

107. Hydra . . .147

108. Protohydra leuckartii ... . . 148

109. Various forms of leptoline Medusas ... . 150

110. Diagram illustrating formation of sporosac by degeneration of

medusa ............. 151

111. Early development of Eucope .... . 152
1J2. Two Trachymedusje . . . 154

113. Two Narcomedusse ..... . 154

114. ^Eginura, tentaculocyst ... . 155

115. Larva of ^Eginopsis ... . . 156

116. Millepora alcicornis, skeleton . . .157

117. Millepora, diagram of structure . .158

118. Stylaster sanguineus, skeleton . . 159

119. Halistemma tergestinum .... . 160

120. Diagram of a Siphonophore . . . . . . 162

121. Development of Halistemma ...... . . 163

122. Physalia .... .164

123. Diphyes campanulata ..... . . 165

124. Porpita pacifica ......... . 166

125. Graptolites ..... . 167

126. Aurelia aurita, dorsal and ventral views . . . . 169

127. ,, ,, side view and vertical section ..... 171

128. ,, ,, portion of umbrella with tentaculocyst . .172

xxii LIST OF ILLUSTRATIONS

TIG. , PAGE

129. Aurelia aurita, development 174

130. Tessera princeps 178

131. Lucernaria 178

132. Pericolpa quadrigata 179

133. Nausithoe 180

134. Charybdsea marsupialis . 181

135. Pilema pulmo . 183

136. Pelagia noctiluca, development 184

137. Tealia crassicornis, dissection and transverse section . . .186

138. Diagrammatic sections of Sea-anemone 188

139. Tealia crassicornis, section of tentacle 190

140. Nematocysts of Sagartia 190

141. Section of mesenteric filament of Sagartia .... . 191

142. Transverse sections of embryos of Actinia 193

143. Zoanthus sociatus 197

144. Hartea elegans 197

145. Corallium rubrum 198

146. Astrrea pallida 198

147. Pennatula sulcata 199

148. -Tubipora musica 199

149. Edwardsia claparedii 200

150. Antipathes ternatensis 201

151. Parantipathes and Schizopathes 202

152. Minyas 202

153. Alcyonimn palmatum 203

154. Gorgonia verrucosa 204

155. Structure of simple coral 206

156. Dendrophyllia and Madrepora ... .... 207

157. Adamsia palliata 209

158. llormiphora plumosa 211

159. ,, ,, dissection and transverse section . . . 212

160. ,, ,, diagrammatic sections 214

161. ,, ,, section of branch of tentacle .... 215

162. ,, ,, sense-organ 216

163. Ovum of Lampetia 217

164. Segmentation of oosperm in Ctenophora 218

165. Development of Ctenophora 218

166. Development of Callianira 218

167. ,, ,, (later stages) 219

168. Three Cydippida . ... 222

169. Deiopea kaloknenota 223

170. Cestus veneris ... 223

171. Beroe forskalii . . 224

172. Ctenoplana kowalevskii 225

173. Sections of embryos of Actinia and Beroe 228

174. Diagram illustrating the mutual relationships of the Coalenterata . 229

175. Dicyema paradoxum with infusoriform embryos .... 230

176. ,, ,, ,, vermiform ,, . . . . 230

LIST OF ILLUSTRATIONS xxiii

FIG. PAGE

177. Dicyema paradoxum, male 231

178. Rhopalura giardii, male . 232

179. ,, ,, female 232

180. Salinella, longitudinal section 233

181. ,, transverse ,, . . 234

182. Planaria, digestive and excretory systems 237

183. ,, nervous system 237

184. ,, reproductive system 239

185. Transverse section of a Planarian 240

186. Distomum hepaticum 240

187. ,, ,, section of integument 241

188. ,, ,, internal organisation 242

189. ,, ,, terminal part of reproductive apparatus . 243

190. ,, ,, development 244

191. Tsenia solium . 246

192. ,, ,, head 247

193. ,, ,, transverse section 247

194. ,, ,, proglottis . ' 248

195. ,, ,, ripe proglottis 250

196. ,, ,, development 251

197. Various Planarians 255

198. Gunda segmentata 256

199. Digenetic Trematodes ... 257

200. Gyrodactylus and Polystomum 258

201. Temnocephala 259

202. Actinodactylella .... .260

203. Tetrarhynchus . 261

204. Trenia echinococcus 261

205. Ligula ... . 262

206. Caryophyllrcus . 263

207. Gyrocotyle . . .... .263

208. Archigetes 263

209. Section of body- wall of a Triclad . .... .264

210. Parenchyma of Flat-worm .... .... 265

211. Diagram of Rhabdocoele 266

212. ,, ,, Polyclad ... 266

213. ,, ,, Triclad . . 267

214. Flame-cell ... 269

215. Reproductive organs of Mesostomum ehrenbergii .... 272

216. Development of a Polyclad 274

217. Miiller's larva ..'... 275

218. Embryos of Denclroccelum 276

219. Embryo of Temnocephala . 278

220. ' 279

221. A Cysticercoid 280

222. ,, with head evaginated 281

223. Cyst of Trenia echinococcus 282

224. Scolices ,, ,, ... 282

xxiv LIST OF ILLUSTRATIONS

FIG. PAGE

225. Scolex of Trenia echinococcus . ' . 282

226. Process of buckling in Microstomum 283

227 Diagram of the relationships of the Platyhelminthes and Nemer-

tinea . .287

228. Diagram of Nemertine . 289

229. Proboscis of Nemertine . . . 290

230. Tetrastemrna . .291

231. Anterior portion of Nemertine . . . 292

232. Proboscis of Hoplonemertean, retracted . . . 292

233. ,, ,, everted . 292

234. Transverse section of Nemertine . . ... 293

235. Vascular and excretory systems of Nemertine ... . 293

236. Pilidium . .294

237. Ascaris lumbricoides . 298

238. ,, ,, transverse section ... . . 299

239. ,, ,, muscle fibres . 300

240. ,, ,, dissection of female .... . 301

241 . Nervous system of Nematoda ... . 302

242. Ascaris lumbricoides, dissection of male organs . 303

243. Body- wall of platymyariaii Nematode . . 305

244. Ankylostoma duodenale ........ . 306

245. Transverse section of Gordius . ... . 306

246. Oxyuris . . 307

247. Gordius, anatomy ... . . 308

248. Development of Ascaris nigrovenosa . . . 309

249. Trichinella spiralis . 311

250. Two species of Echinorhynchus (Gigantorhynehus) . . 313

251. Echinorhynchus gigas, dissection of male . . . 314

252. ,, ,, female . 314

253. ,, ,, ,, nephridia . . 315

254. ,, ,, female organs . . . 315

255. Sagitta hexaptera .... . . 316

256. ,, bipunctata, transverse sections . 317

257. ,, ,, head . 317

258. ,, hexaptera, eye . 318

259. Development of Sagitta . ... 318

260. Chtetosoma . . 319

261. Echinoderes . 320

262. Desmoscolex . 320

263. A trochophore . . 322

264. Brachionus rubens, female . . 324

265. ,, ,, pharynx 325

2(56. ,, ,, male and female, with attached eggs . 326

267. Diagram of a Rotifer . . 327

268. Pataseison asplanchnus . 329

269. Typical forms of Rotifera . . 331

270. ... .332

271. ,, ,, mastax 333

> i

LIST OF ILLUSTRATIONS xxv

FIG. PAGE

272. Chsetonotus maximus 336

273. ,, ,, anatomy .... ... 336

274. Dinophilus fcsBniatus . 337

275. Stratiodrilus tasmanicus . 338

276. Bugula avicularia .... - . . . 342

277. Development of Bugula . 345

278. ,, ....-. .... 346

279. Larva of Bugula . 346

280. Plumatella . . . .349

281. Cristatella . . . 350

282. Lophopus . 351

283. Pedicellina . . .355

284. Phoronis australis 356

285. ,, ,, free end .... .... 357

28(1 ,, ,, internal organisation . .... 357

287- ,, ,, section 358

288. ,, ,, development . . . . 359

289. Magellan ia flavescens, shell .... .... 361

290. ,, lenticularis, anatomy 363

291. ,, flavescens, lophophore . 364

292. ,, muscular system .... .... 364

293. Terebratula, nervous system, &c. . .... 365

294. Typical Brachiopods . . . .368

295. ,, ,, anatomy .... ... 369

296. Development of Cistella . . .370

297. Larva of Cistella . 370

298. Development of Cistella . .371

299. Lophophore of embryo Brachiopod . . . . 372

300. Diagrams of phylactoloematous Polyzoon and Phoronis . . . 373

301. Starfish, oral aspect 376

302. ,, vertical section of arm . 378

303. ,, ambulacral system 379

304. Starfish, portion of vertical section of arm 380

305. ,, diagrammatic sections 381

306. Asterias rubens, digestive system 382

307. Astropecten, section of stone-canal 383

308. Anthenea flavescens, dissection from dorsal aspect .... 384

309. Asterias rubens, structure 385

310. Anthenea flavescens, lateral dissection 386

311. ,, ,, aboral surface 387

312. oral surface 387

313. Asterina gibbosa, development

314. . 390

315. ,, ,, larva .... 390

316. . . . . . 391

317. ,, exigua, young after metamorphosis 391

318. Asterina gibbosa, development . 392

319. Apical system of young Starfish 393

xxvi LIST OF ILLUSTRATIONS

FIG. PAGE

320. Echinus esculentus, peristome 394

321. Strongylocentrotus 395

322. Corona of Sea-urchin 396

323. Apical disc of Sea-urchin 397

324. Echinus, lantern of Aristotle 397

325. Sea-urchin, anatomy, lateral view 398

326. Echinoid, transverse section of ambulacral zone .... 399

327. Sea-urchin, anatomy, oral view 400

328. Cucumaria planci 401

329. Anatomy of a Holothurian 403

330. Antedon . .405

331. Aboral view of Antedon 406

332. Antedon disc 406

333. ,, transverse section of pinnule 407

334. ,, sagittal section 408

335. Anthenea, ventral view 419

336. Ophioglypha lacertosa 420

337. Astrophyton arborescens 421

338. Diagram of spine of Sea-urchin 422

339. Pedicellaria of Arbacia punctulata 422

340. Hemipneustes radiatus 423

341. Clypeaster sub-depressus 423

342. Metacrinus interruptus 424

343. Development of Echinoderms 431

344. ,, ,, Antedon 432

345. Stalked larva of Antedon . 433

346. Diagram to illustrate the relationships of the classes of Echino-

dermata 437

347. Nereis dumerilii 440

348A. ,, ,, parapodium 441

348e. ,, ,, setse 441

349. Nereis diversicolor, proboscis 443

350. Nereis dumerilii, anatomy 444

351. ,, ,, transverse section 445

352. ,, ,, nervous system 446

353. ,, ,, eye 447

354. ,, ,, nephridium 448

355. ,, ,, development 451

356. ,, ,, ,, 453

357. Lumbricus herculeus 454

358. ,, setee 455

359. ,, transverse section 456

360. ,, sagittal section 457

361. ,, nervous system 459

362. ,, nephridium 460

363. ,, reproductive organs 462

364. ,, development . . . 463

365. Polynoe setosissima . 467

LIST OF ILLUSTRATIONS xxvii

FIG. PAGE

366. Vermilia ccespitosa 468

367. Chietopterus 469

368. Setee of various Polychreta 470

369. Section of setigerous sac of an Oligochtete 470

370. Polynoe extenuata, anterior end 471

371. Polychreta, various, heads 472

372. Tubifex .473

373. Terebella 474

374. Aphrodita, enteric canal 475

375. Saccocirrus, transverse section 477

376. Phyllodoce, nephridium 479

377. Nephridia and coelomoducts 480

378. Diagram illustrating development of gonad of Polychreta . . 482

379. Spirorbis lams . 484

380. Eupomatus, development of trochophore 485

381. Autolytus cornutus, budding 487

382. Syllis ramosa 487

383. Serpulre with their tubes 488

384. Myzostomum 490

385. ,, anatomy 491

386. Sipunculus nudus, anterior extremity 492

387. ,, ,, tentacular fold 493

388. ,, ,, anatomy 494

389. ,, ,, nervous system 494

390. Bonellia viridis, female 497

391. Echiurus 497

392. Priapulus 498

393. Bonellia, anatomy ... 499

394. Echiurus, ciliated funnel 499

395. ,, anatomy 500

396. ,, nervous system 500

397. Bonellia, male 501

398. Echiurus, trochophore 501

399. Polygordius neapolitanus 503

400. Protodrilus 504

401. Polygordius neapolitanus, transverse section 504

402. ,, ,, trochophore 505

403. ,, ,, ,, later stage .... 505

404. Hirudo medicinalis . . 507

405. ,, ,, transverse section 508

406. ,, jaw 509

407. ,, australis, dissection from dorsal aspect . . . 510

408. ,, australis, ,, ,, left side . . . . .511

409. ,, medicinalis, nephridium 512

410. ,, diagram of blood-channels 513

411. ,, section of eye 514

412. ,, cocoon 515

413. Three Rhynchobdellicla . . 517

xxviii LIST OF ILLUSTRATIONS

FIG.

414. Proboscis of Clepsine . .518

415. Nephridium of Herpobdella ... 519

416. Pontobdella, nephridial system 520

417. Clepsine, development . .... 521

418. Diagram of origin of metamerism . 524

419. Diagram illustrating the relationships of the Annulata and

Trochelminthes . . .... 525

420. A pus cancriformis, dorsal aspect 527

421. Lepidurus kirkii, side view . 528

422. Apus glacialis, ventral aspect 529

423. ,, appendages . . 530

424. Lepidurus kirkii, sagittal section . 532

425. Apus, transverse section 534

426. ,, shell-gland ... .535

427. ,, cancriformis, nervous system 536

428. ,, structure of paired eye 537

429. ,, development 538

430A. Astacus fiuviatilis, male ... ... . 540

430 B. ,, ,, transverse section of abdomen .... 540

431. ,, ,, appendages . . . 543

432. ,, ,, articulations and muscles of leg .... 545

433. Section of skin and exoskeleton of Lobster 546

434. Articulations and muscles of abdomen of Crayfish .... 547

435. Astacus fluviatilis, dissection from right side . . . 548

436. ,, gills . . . . 550

437. ,, kidney ... .552

438. ,, ,, transverse section of thorax 553

439. ,, ,, diagram of circulation 554

440. ,, ,, nervous system 555

44 1 . , , , , reproductive organs .... . 557

442. ,, ,. formation of the blastoderm 557

443. ,, ,, early embryo 558

444. ,, ,, nauplius 559

445. ,, ,, section of embryo 560

446. ... ... 560

447. ,, ,, advanced embryo 501

448. Three Branchiopoda 571

44 J. ,, Cladocera . 572

450. Cypris .... . .... 573.

451. Cyclops and Calocalanus 574

452. Various forms of parasitic Eucopepoda 576

453. Argulus foliaceus . . 577

454. Lepas anatifera . .... . . 578

455. Balanus . .... 579

456. Sacculina carcini . 580

457. Nebalia geoffroyi 581

458. Paranaspides 582

459. Mysis oculata . . 582

460. Diastylis 583

LIST OF ILLUSTRATIONS xxix

FIG. PAGE

461. Gammarus ... . 584

4(52. Asellus .... . 585

403. Amphipoda .... . 586

464. Isopoda . . . 587

465. Shrimp and Prawn . . . . . 588

466. Scyllarus arctus ... . . . . 589

467. Pagurus bernhardus ... ... . 589

468. Cancer pagurus . . . 590

469. Typical Brachyura . . 591

470. Squilla . . . 592

471. Orchestia cavimana, anatomy 594

472. Euphausia pellucida .... . . . 595

473. Nervous system of Crab . 596

474. Cypris-stage of Lepas 598

475. Larvre of Crabs . . . 600

476. Diagram illustrating the mutual relationships of the orders of

Crustacea . (504

477. Dalmanites and Phacops . . 605

478. Triarthrus beckii . . 606

479. Peripatus capensis 607

480. ,, ,, ventral view of head 607

481. ,, anatomy 608

482. ,, trachea! pit .......... 609

483. ,, nephridium .... 610

484. ,, riovse zealandite, development ... . 611

485. ,, capensis .... 613

486. Scolopendrella immaculata ... . 615

487. Scolopendra . 616

488. Lithobius forficatus 616

489. Pauropus huxleyi ... 617

490. Strongylostoma, development 618

491. Periplaneta orientalis .... 620

492. ,, mouth-parts .... ... . 621

493. ,, americana, lateral view of head ... . 621

494. ,, muscular system .... . . 624

495. ,, anatomy . . . . . . . . . 625

496. ,, salivary glands . . 625

497. Trachea of caterpillar . . . 626

498. Periplaneta, tracheal system ... 627

499. ,, nervous system 627

500. ,, male reproductive organs .... . 628

501. ,, female reproductive organs ... . 628

502. Segmentation of ovum of Insect . 029

503. Ventral plate of embryo Cockroach . . . . . 030

504. Embryo Cockroach . . . 630

505. Lepisma . 632

506. Podura . 632

507. Locusta . . ... 633

508. Ephemera ... . . 033

xxx LIST OF ILLUSTRATIONS

FIG. PAGE

509. Aphis rosre . . . 633

510. Cicada ... ... . . 634

511. Culex and larva ..... 634

512. Gastrophilus equi 634

513. Pieris ... . . .635

514. Crioceris . 635

515. Section of integument of Insect 636

516. Mouth-parts of Honey-bee 637

517. ,, ,, Diptera 638

518. ,, ,, Lepidoptera 639

519. Digestive organs of Beetle 641

520. Nervous, trachea!, and digestive systems of the Honey-bee . . 642

521. Trachea] gills of Epheinerid . ... .643

522. Heart of Cockchafer ... .643

523. Nervous system of Diptera 644

524. Ocellus of Dytiscus larva 645

525. Chordotonal organ of Isopteryx 645

526. Sexual apparatus- of Honey-bee 646

527. Segmentation of ovum of Insect 648

528. Germinal layers and amnion of Insect 649

529. Development of Hydrophilus 650

530. ,, 650

531. Apis mellifica, queen, worker, and drone 652

532. Formica rufa .652

533. Euscorpio . 654

534. Ventral surface of cephalothorax and pre-abdomen of Scorpion . 654

535. Endosternite of Scorpion 655

536. Scorpion, anatomy, lateral view . . 657

537. ,, ,, dorsal ,, . . . .658

538. ,, development 659

5.'!9. Embryo of Scorpion 659

540. Chelifer bravaisii 662

541. Phrynus .... . 663

542. Galeodes dastuguei 663

543. Epeira diadema 664

544. ,, ,, chelicerre and pedipalpi of female .... 664

545. ,, ,, ,, ,, male 664

546. Sarcoptes scabiaei 665

547. Trombidium fuliginosum 665

548. Limulus ... . . 66(5

549. ,, ventral view 667

550. Eurypterus fischeri ... 668

551. Anatomy of dipneumonous Spider ...... 669

552. Limulus, sagittal section . 670

553. Lung-book of spider .... 670

554. Tracheal system of Spider 670

555. Gill-books of Limulus ... 671

556. Lateral eye of Euscorpio . . . : . . 671

557. Central eye of Euscorpio .... ... 672

LIST OF ILLUSTRATIONS xxxi

FIG. PAGE

558. Nymphon hispidum ........... 674

559. Pentastomum trenioidcs ........ . . 674

560. Macrobiotus hufelandi ......... . 675

561. Diagram to illustrate affinities of Arthropoda ..... 678

562. Anodonta cygnea ....... ... 681

563. ,, ,, interior of valve and animal removed from shell . 682

564. ,, ,, section of shell and mantle ..... 683

565. ,. cygnea, animal after removal of mantle-lobe . . . 685

566. ,, ,, dissection from left side ...... 686

567. ,, ,, structure of gills ....... 687

568. ,, ,, transverse sections . ..... 688

569. ,, diagram of circulation ... .... 690

570. ,, statocyst ...... .... 691

571. ,, early embryo .......... 692

572. ,, later embryos .......... 692

573. ,, advanced embryo ......... 693

574. ,, metamorphosis ......... 694

575. Anatomy of Pecten .... .... . 61(7

576. Valves of Mya, Modiola, and Vulsella . . . 698

577. Cardium edule . .......... 698

578. Venus gnidia ..... ... . 699

579. Scrobicularia piperata ...... .... 699

580. Solecurtus strigillatus ......... 700

581. Diagram of concrescence of mantle-lobes ...... 700

582. Requienia and Hippurites .... ..... 701

583. Teredo navalis ............ 701

584. Aspergillum ............ 702

585. Mytilus edulis . . .... 702

586. Nucula delphinodonta .......... 703

587. Gills of Pelecypoda ... 704

588. Gill-filaments of Mytilus .... . 705

589. Dissection of Poromya .... ..... 705

590. Donax, enteric canal ...... . 706

591. Nervous system and auditory organs of Nucula ..... 707

592. Eye of Pecten .... . . .708

593. Development of Ostrea ..... ..... 709

594. Veliger of Ostrea . .... . 709

595. Embryos of Cyclas ........... 710

596. Diagram illustrating the mutual relationships of the Pelecypoda . 712

597. Chsetoderma iiitidulum ..... ..... 713

598. Neomenia carinata . .......... 714

599. Chiton, spinosus, dorsal view ........ 714

600. ,, ventral view . . . . ..... .714

601. ,, valves of shell .......... 715

602. Chsefcoderma iiitidulum, longitudinal section ..... 716

603. Chiton, longitudinal section ......... 717

604. Nervous system of Amphineura ........ 717

605. Neomenia carinata, reproductive organs .... .718

606. Chiton, nephridial and genital systems ...... 719

xxxii LIST OF ILLUSTRATIONS

FIG. PACK

607. Chiton, development ... . . . . 720

608. Triton nodiferus, shell . . . 722

609. Triton ,, shell, median section 723

610. ,, ,, operculum . 724

611. ,, ,, lateral view of body 724

612. ,, ,, diagram of introvert 725

613. ,, ,, dissection from dorsal side 727

614. ,, ,, buccal mass 728

615. ,, ,, vertical section of buccal cavity . . V . 728

616. ,, ,, nervous system from dorsal side .... 730

617. ,, ,, ,, ,, and related parts, lateral view . 731

618. ,, ,, section ofeye 732

619. Diagrams of displacement of mantle-cavity, &c. .... 736

620. Solarium perspectivum 737

621. Terebra oculata ..... 738

622. Cypraea moneta 739

623. Doris tuberculata 739

624. Carinaria mediterranea 739

625. Limax . ..... 739

626. Sigaretus Irevigatus 740

627. Aplysia . . . 740

628. Shell-bearing Pteropoda 741

629. Atlanta peronii ... 741

630. Pterotrachea scutata 742

631. Helix nemoralis . ... 742

632. Pleurophyllidia lineata . 743

633. Patella vulgata ... . 743

634. Pulmonary cavity and related parts in Limax . . 743

635. Nervous system of Patella . . . 745

636. Nervous system of Aplysia ... . . 746

637. ,, ,, ,, Limmeus . . 746

638. Eyes of Gastropoda ... . . 747

639. Osphradium of Murex . . . 747

640. Reproductive oigans of Helix ... . 748

641. Ovotestis of Gastropoda . 749

642. Forms of egg-cases in Gastropoda 749

643. Segmentation and formation of germinal layers in Gastropoda . 751

644. Early development of Patella .752

645. Trochophore of Patella ... . 753

646. Later trochophore of Patella ... . 754

647. Veliger of Vermetus 755

648. Diagram illustrating the relationships of the Gastropoda . . 756

649. Dentalium, section of shell ... 756

650. ,, anatomy . 757

651. ,, larvae .... . 757

652. Rhodope . . 758

653. Sepia, cultrata . . . 760

654. Sepia ,, shell . . 762

655. ,, chromatophore . . . 762

LIST OF ILLUSTRATIONS xxxiii

1 '' 1(; - PACK

656. Sepia, cultrata, cranial cartilage .... . 763

657. ,, ,, nuchal cartilage 763

658. ,, ,, mantle-cavity 764

659. ,, officinalis, jaws . 765

660. ,, section of buccal mass . 766

661. ,, officinalis, enteric canal , 766

662. ,, cultrata, dissection of male from posterior aspect . . . 767

663. ,, ,, lateral dissection of male .... . 768

664. ,, officinalis, longitudinal section of ink-sac . . . 769

665. ,, cultrata, vascular system 770

666. ,, ,, cephalic ganglia 770

667. ,, ,, pedal and pleuro-visceral ganglia .... 770

668. ,, section of eye . 771

669. ,, cultrata, statolith 772

670. ,, officinalis, renal organs . 773

671. ,, ,, diagrammatic sagittal section of female . . . 774

672. ,, male reproductive organs 775

673. ,, sperms and spermatophore 775

674. Nautilus pornpilius, section of shell .... . 776

675. ,, ,, female in shell 778

676. Nautilus macromphalus, entire animal 779

677. Nautilus pompilius, lobe of foot 780

678. ,, ,, spadix 781

679. ,, ,, cephalic cartilage 781

680. ,, ,, mantle-cavity of male 782

681. ,, ,, dissection of male from left side . . . 784

682. ,, ,, arteries 785

683. ,, ,, renal sacs, ctenidia, &c 786

684. , , , , male reproductive organs 788

685. ,, ,, female ,, ,, 788

686. ,, macromphalus, egg . 789

687. Octopus vulgaris 791

688. Loligo vulgaris 792

689. Argonauta argo 793

690. Octopus lentus, male 793

691. Amphitretus pelagicus . . 794

692. Shell of Spirula .794

693. Spirula peronii 795

694. Ammonite . 795

695. Shell of Belemnite .... .796

696. ,, Argonauta argo 796

697. Segmentation of Loligo .... 798

698. Blastoderm of Sepia .... . . 799

699. ,, ,, sections 799

700. Development of Loligo 800

701. . - 801

702. . . 801

703. . 802

704. Diagram to illustrate the relationships of the Cephalopoda . . 804

VOL. I C

CLASSIFICATION OF THE ANIMAL KINGDOM

IN THIS BOOK.

KINGDOM ANIMALIA.

PHYLUM I. PROTOZOA.

,,

,,

Class I. RHIZOPODA.
Order 1. LOBOSA.

2. FORAMIXIFERA.

3. HELIOZOA.

,, 4. RADIOLARIA.
Class II. MYCETOZOA.
Class III. MASTIGOPHORA.
Order 1. FLAGELLATA.

2. CHOAXOFLAGELLATA.

3. DlXOFLAGELLATA.

,,

Order 4. CYSTOFLAGELLATA.
Class IV. SPOROZOA.

Order 1. GREGARIXIDA.
,, 2. COCCIDIIDKA.
,, -3. H.^MOSPORIDIA.
,, 4. MYXOSPORIDEA.
5. SARCOCYSTIDEA.
INFUSORIA.

Class V.

Order 1. CILIATA.

2. TEXTACULIFERA.

PHYLUM II. PORIFERA.

Class PORIFERA.
Sub-class I. Calcarea.
Order 1. HOMOC<ELA.

Order 2. HETEROCIELA.
Sub-class II. Hexactinellida.
,, III. Demospongia.

PHYLUM III. CCELENTERATA.

Class I. HYDROZOA.

Order 1. LEPTOLIXJE.

Sub-order a. AntJiomeduscc

Order 2. TRACHYLIN.E.
Class II. SCYPHOZOA.

Order 1. STAUROMEDUS.*;.
,, 2. COROXATA.
,, 3. CTBOMEDUS^.
,, 4. DJSCOMEDUS.*:.
Sub-order a.
,, b.

Class III. ACTINOZOA.
VOL. I

Sub-class I. Zoantharia.
Order 1. ACTINIARIA.

,, 2. MADREPORARIA.

., 3. AXTIPATHARIA.
Sub-class II. Alcyonaria.

Sub-order <t. Tr<'hyme<ln$'

,_,

,,

Order 3. HYDROCORALLIXA.

4. SIPHOXOPHORA.

5. GRAPTOLITHIDA.
4. ALCYOXACEA.

o. (TORGOXACEA.
0. PENXATULACEA.

C *

xxvi CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM III. CCELENTERATA continued,

Class IV. CTENOPHORA. Appendix to Ctenophora Ctenoplann

Order 1. CYDIPPIDA. and Cceloplana.

,, 2. LOB ATA. Order 1. PLATYGTENEA.

,, 3. CESTIDA.

,, 4. REROIDA. Appendix (II) to Coelenterata Mesozoa.

PHYLUM IV. PLATYHELMINTHES.

lass I. TURBELLARIA. Order 3. ASPIDOCOTYLEA.

Order 1. POLYCLADIDA. ,, 4. TEMNOCEPHALEA.

2. TRICLADIDA. Class III. CESTODA.

,, 3. RHABDOCCELIDA. Order 1. MONOZOA.

,, 2. POLYZOA.
Class II. TREMATODA.

Order 1. MOXOGENETICA. Appendix to Platyhelminthes Class

2. DIGENETICA. NEMERTINEA.

PHYLUM V. NEMATHELMINTHES.

Class I. NEMATODA. Class III. CHJETOGNATHA.

Order 1. NEMATOIDEA.

,, 2. NEMATOMOBPHA. Appendix to Nemathelminthes Chit-

tosomidce, Echinoderidce, and Desmos-
ClassII. ACANTHOCEPHALA. colecid*.

PHYLUM VI. TROCHELMINTHES.

Class I. ROTIFARA. Order 5. TROCHOSPH^RIDA.

Order 1. RHIZOTA. ,, 6. SEISONIDA.
,, 2. BDELLOIDA.

3. PLOIMA. Class II. GASTROTRICHA.
Sub-order a. Illoricata,

,, b. Loricata. Appendix to Trochelininthes Dino-

Order 4. SCIRTOPODA. pliilta, and Histriobdellea.

PHYLUM VII. MOLLUSCOIDA.

Class I. POLYZOA. Order 2. PHYLACTOL^MATA.

Sub-class I. Ectoprocta. Sub-class II. Endoprocta.

Order 1. GYMXOLJEMATA. Class II. PHORONIDA.

Sub-order a. Cyclostomata, III. BRACHIOPODA.

,, b. Cheilostomata. Order 1. INARTICULATA.

,, c. Ctenobtomata. ,, 2. ARTICULATA.

PHYLUM VIII. ECHINODERMATA.

Class I. ASTEROIDEA. Order 3. CLADOPHIUK^:.

Order 1. PHANEROZONIA. ,, 4. ZYGOPHIURJS.

,, 2. CRYPTOZONIA. Class III. ECHINOIDEA.
Class II. OPHIUROIDEA. Order 1. REGULARIA.

Order 1. LYSOPHIUR.I:. ., 2. CLYPEASTRIDEA.

,, 2. STREPTOPHIUK^-:. ,, 3. SPATANGOIDEA.

CLASSIFICATION OF THE ANIMAL KINGDOM xxxvii

PHYLUM VIII. ECHINODERMATA continued.

Class IV. HOLOTHUROIDEA.

Order 1. ELASIPODA.
,, 2. PEDATA.
,, 3. APODA.
Class V. CRINOIDEA.
Sub-class I. Monocyclica.

Sub-class II. Dicyclica.
Class VI. CYSTOIDEA.
,, VII. BLASTOIDEA.
,, VIII. EDRIASTEROIDEA.
IX. CARPOIDEA.

PHYLUM IX. ANNULATA.

Class I. CHJETOPODA.

Sub-class I. Polychaeta.

Order 1. ARCH

,, 2. PHANEROCEPHALA.
,, 3. CRYPTOCEPHALA.
Sub-class II. Oligochaeta.
Order 1. MICRODRILI.

,, 2. JV1 EGADRILI.

Appendix to the Chsetopoda-
MYZOSTOMIDA.

Class II. GEPHYREA.
Order 1. IXERMIA.
,, 2. ARM ATA.

Class III. ARCHI ANNELIDA.

,, IV. HIRUDINEA.
Order 1. RHYXCHOBDELLIDA.

,, 2. ARHYXCHOBDELLIDA.
-Class Sub-order 1. Gnathobdettida.

,, 2. Herpobdellida.

PHYLUM X. ARTHROPOD A.

Class I. CRUSTACEA.

Sub-class I. Branchiopoda.
Order 1. ANOSTRACA.
,, 2. NOTOSTRACA.
, , 3. CONCHOSTRACA.
,, 4. CLADOGERA.
Sub-class II. Ostracoda.
,, III. Copepoda.
Order 1. EUCOPEPODA.
,, 2. BRANCHIURA.
Sub-class IV. Cirripedia.
Order 1. EUCIRRIPEDIA.
,, 2. RHIZOCEPHALA.
Sub-class V. Malacostraca.
Order 1. MYSIDACEA.
,, 2. CUM ACE A.
,, 3. TAXAIDACEA.
,. 4. ISOPODA.
,, 5. AMPHIPODA.
Sub- order 1. Macrura.
,, 2. A nomura.
,, 3. Brachyurcbi
Appendix to Crustacea Class
LOBITA.

Class II. ONYCHOPHORA.
,, III. MYRIAPODA.
Sub-class I. Progoneata.

Order 1. PAUROPOUA.
,, 2. DIPLOPODA.
,, 3. SYMPHYLA.
Sub-class II. Opisthogoneata.

Order 1. CHILOPODA.
Class IV. INSECTA.
Order 1. APTERA.

2. ORTHOPTERA.

3. NEUROPTERA.

4. HEMIPTERA.
DIPTERA.
LEPIDOPTERA.
COLEOPTERA.
HYMEXOPTERA.

Class V. ARACHNIDA.

Order 1. SCORPIOXIDA.

PSEUDOSCORPIOX IDA.
PEDIPALPIDA.
SOLPUGIDA.
PHALAXGIDA.
6. ARAXEIDA.
,, 7. ACARIDA.
TRI- ,, 8. XIPHOSURA.

EURYPTERIDA.

to the Arachnida The

55

55

55

} 5

55

5 J

55

55

55

55

5.
6.

8.

2.

3.
4.
5.

8.
9.
Appendix

55

PYCXOGONIDA, LIXGUATULIDA, and TAR-

DIGRADA.

ZOOLOGY

INTRODUCTION

of Natural History which deals with

ERRATA.

PAGE.

*7, description of Fig. 5, for " atrosphere " read " astrophere."
52, description of Fig. 35, for " Rotalla" read " Rotalia."
71, description of Fig. 52, for " Astasiopis " read " Astasiopsis. "
74, line 9, for " divison " read " division."

Ill, line I, for " out" read "outer."

208, line 10, for " siphnozoids " read " siphonozooids. "

272, line 2, for " prostrate" read " prostate."

402, line 43, for " periphwmal " read " perihaemal."

450, line 7, for "Fig. 846" read "Fig. 347."

in me position ami ciicuctcuuis ui uncn m uuj.-u.cn vj.^^^. ^..^ w^^^
can fail to see that these animals, in spite of differences of size,
colour, markings, &c., are all, in the broad sense of the word,
' Cats." This is expressed in the language of systematic Zoology
by saying that they are so many species of a single genus.

According to the system of binomial nomenclature introduced by
Linngeus, each kind of animal receives two names one the generic

ZOOLOGY

INTRODUCTION

Zoology, the branch of Natural History which deals with
animals, is one of the two subdivisions of the great science Biology,
which takes cognisance of all organisms, or things having life, as
distinguished from such lifeless natural objects as rocks and
minerals. The second of the two subdivisions of Biology is
Botany, which deals with plants.

The subject-matter of Zoology, then, is furnished by the animals
which inhabit the land-surface, the air, and the salt and fresh
waters of the globe : the aim of the science is to find out all that
can be known of these animals, their structure, their habits, their
mutual relationships, their origin.

The first step in the study of Zoology is the recognition of the
obvious fact that the innumerable individual animals known to
us may be grouped into what are called species, the members of
which resemble one another so closely that to know one is to know
all. The following example may serve to give the reader a fairly
accurate notion of what Zoologists understand by species, and of
the method of naming species which has been in use since the time
of the great Swedish naturalist Linnoeus.

The Domestic Cat, the European Wild Cat, the Ocelot, the Leopard,
the Tiger, and the Lion are animals which agree with one another in
the general features of their organisation in the number and form
of their bones and teeth, in the possession of retractile claws, and
in the position and characters of their internal organs. No one
can fail to see that these animals, in spite of differences of size,
colour, markings, &c., are all, in the broad sense of the word,
' Cats." This is expressed in the language of systematic Zoology
by saying that they are so many species of a single genus.

According to the system of binomial nomenclature introduced by
Linnaeus, each kind of animal receives two names one the generic

E B

2 ZOOLOGY

name, common to all species of the genus ; the other the specific
name, peculiar to the species in question. Both generic and specific
names are Latin in form, and are commonly Latin or Greek in
origin, although frequently modern names of persons or places, with
Latinised terminations, are employed. In giving the name of an
animal, the generic name is always placed first, and is written
with a capital letter, the specific name following it, and being
written, as a rule, with a small letter. For instance, to take the
examples already referred to, the Domestic Cat is called Fclis
domestica, the European Wild Cat F. catus, the Leopard F. pardus,
the Tiger F. tigris, the Lion F. ho. Thus the systematic name of an
animal is something more than a mere appellation, since it indicates
the affinity of the species with other members of the same genus :
to name an animal is, in fact, to classify it.

It is a matter of common observation that no two individuals of
a species are ever exactly alike : two tabby Cats, for instance,
however they may resemble one another in the general characters
of their colour and markings, invariably present differences in
detail by which they can be readily distinguished. Individual
variations of this kind are of universal occurrence. Moreover, it
often happens that the members of a species are divisible into
groups distinguishable by fairly constant characters : among
Domestic Cats, for instance, we find white, black, tabby, gray, and
tortoiseshell Cats, besides the large long-haired Persian breed, and
the tailless Manx Cat. All these are distinguished as varieties of
the single species Felis domestica.

It is often difficult to decide whether two kinds of animals should
be considered as distinct species or as varieties of a single species, and
no universal rule can be given for determining this point. Among
the higher animals mutual fertility is a fair practical test, the
varieties of a species usually breeding freely with one another and
producing fertile offspring, while distinct species either do not
breed together or produce infertile hybrids or mules. Compare,
for instance, the fertile mongrels produced by the union of the
various breeds of Domestic Dog with the infertile mule produced by
the union of the Horse and Ass. But this rule is not without
exception, and in the case of wild animals is, more often than not,
impossible of application : failing it, the only criterion of a " good
species " is usually the presence of constant differences from allied
species. Suppose, for instance, that a naturalist receives for
description a number of skins of wild Cats, and finds, after an
accurate examination, that in some specimens the tail is two-thirds
the length of the body and the skin of a uniform reddish tint with
a few markings on the head, while in the rest the tail is nearly half
as long as the body, and the skin tawny with black stripes. If
there are no intermediate gradations between these two sets of
individuals, they will be placed without hesitation in distinct

INTRODUCTION 3

species : if, on the other hand, there is a complete series of grada-
tions between them, they will be considered to form a single
variable species.

As, therefore, animals have to be distinguished from one another
largely by structural characters, it is evident that the foundations
of a scientific Zoology must be laid in Morphology, the branch of
science which deals with form and structure. Morphology may be
said to begin with an accurate examination of the external
characters ; the divisions of the body, the number and position of
the limbs, the characters of the skin, the position and relations of
the mouth, eyes, ears, and other important structures. Next the
internal structure has to be studied, the precise form, position,
&c., of the various organs, such as brain, heart, and stomach, being
made out : this branch of morphology is distinguished as Anatomy.
And, lastly, the various parts must be examined by the aid of the
microscope, and their minute structure, or Histology, accurately
determined. It is only when we have a fairly comprehensive
knowledge of these three aspects of a given animal its external
characters, its rough anatomy, and its histology- -that we can with
some degree of safety assign it to its proper position among its
fellows.

An accurate knowledge of the structure of an animal in its
adult condition is not, however, all-sufficient. Nothing has been
made more abundantly clear by the researches of the last half-
century than that the results of anatomy and histology must be
checked, and if necessary corrected, by Embryology i.e. by the
study of the changes undergone by animals in their develop-
ment from the egg to the adult condition, A striking instance is
afforded by the common Barnacles which grow in great numbers on
ships' bottoms, piers, &c. The older zoologists, such as Linnaeus,
grouped these creatures, along with Snails, Mussels, and the like,
in the group Mollusca, and even the great anatomical skill of
Cuvier failed to show their true position, which was made out only
when Vaughan Thompson, about sixty years ago, proved, from
a study of the newly hatched young, that their proper place
is among the Crustacea, in company with Crabs, Shrimps, and
Water-fleas.

Given a sound knowledge of the anatomy, histology, and em-
bryology of animals, their Classification may be attempted that
is, we may proceed to arrange them in groups and sub-groups,
each capable of accurate definition.

The general method of classification employed by zoologists is
that introduced by Linnaeus, and may be illustrated by reference
to the group of Cats which we have already used in the explanation
of the terms genus, species, and variety.

We have seen that the various kinds of true Cat Domestic Cat,
Lion, Tiger, &c. together constitute the genus Felis. Now there

B 2

4 ZOOLOGY

is one member of the cat-tribe, the Cheetah, or Hunting Leopard,
which differs from all its allies in having imperfectly retractile
claws and certain peculiarities in its teeth. It is therefore placed
in a distinct genus, Cyncelurus, to mark the fact that the differences
separating it from any species of Felis are of a more fundamental
character than those separating the species of Felis from one
another.

The nearest allies of the Cats are the Hyaenas, but the presence
of additional teeth and of non-retractile claws to mention only
two points makes the interval between Hyaenas and the two
genera of Cats far greater than that between Felis and Cynselurus.
The varying degree of difference is expressed in classification by
placing the Hyaenas in a separate family, the Hycenidce, while
Felis and Cynaelurus are placed together in the family Fdidce.
Similarly, the Civets and Mongooses form the family Vivcrridcc ;
the Dogs, Wolves, Jackals, Foxes, &c., the family Ganidoc ; Bears,
the family Ursidce ; and so on.

All the foregoing animals have sharp teeth adapted to a flesh
diet, and their toes are armed with claws. They therefore differ
fundamentally from such animals as Sheep, Deer, Pigs, and Horses,
which have flat teeth adapted for grinding vegetable food, and
hoofed feet. The differences here are obviously far greater than
those between any two of the families mentioned above, and are
emphasised by placing the flesh-eaters in the order Carnivora,
the hoofed animals in the order Ungulata. In the same way
gnawing animals, such as Rats, Mice, and Beavers, form the order
Bodentia ; pouched animals, such as Kangaroos and Opossums, the
order Marsupialia ; and so on,

Carnivora, Ungulata, Rodentia, Marsupialia, &c., although
differing from one another in many important respects, agree in
the possession of a hairy skin and in the fact that they all suckle
their young, They thus differ from Birds, which have a covering
of feathers and hatch their young from eggs. The differences here
are considerably more important than those between the orders
of quadrupeds referred to, and are expressed by placing the latter
in the class Mammalia, while Birds constitute the class Avcs. In
the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c.,
form the class Heptilia; the slimy-skinned, scaleless Frogs, Toads, and
Salamanders the class Amphibia ; and the finned, water-breathing
Fishes the class Pisces.

Mammals, Birds, Reptiles, Amphibians, and Fishes all agree with
one another in the possession of red blood and an internal skeleton-
an important part of which is an axial rod or vertebral column-
and in never having more than two pairs of limbs. They thus
differ in some of the mosfc fundamental features of their organisation
from such animals as Crabs, Insects, Scorpions, and Centipedes,
which have colourless blood, a jointed external skeleton, and

INTRODUCTION 5

numerous limbs. These differences far greater than those be-
tween classes are expressed by placing the backboned animals
in the phylum or sub-kingdom Chordata, the many-legged,
armoured forms in the phylum Arthropoda. Similarly, soft-bodied
animals with shells, such as Oysters and Snails, form the phylum
Mollusca, Polypes and Jelly-fishes the phylum Coelcntcmta. And
finally the various phyla recognised by zoologists together con-
stitute the kingdom Animalia.

Thus the animal kingdom is divided into phyla, the phyla into
classes, the classes into orders, the orders into families, the families
into genera, and the genera into species, while the species themselves
are assemblages of individual animals agreeing with one another
in certain constant characters. It will be seen that the individual
is the only term in the series which has a real existence : all the
others are mere groups formed, more or less arbitrarily, by man.

To return to the animal originally selected as an example, it will
be seen that the zoological position of the Domestic Cat is expressed
as follows :-

Kingdom ANIMALIA.
Phylum CHORD ATA.
Class MAMMALIA.

Order CARNIVORA.
Family Fc lidcv.
Genus Felis.

Species F. domestica.

The object of systematic zoologists has always been to find a
natural as opposed to an artificial classification of animals.
Good instances of artificial classification are the grouping of Bats
with Birds on the ground that they both possess wings, and of
Whales with Fishes on the ground that they both possess fins and
live in the water. An equally good example of a natural classi-
fication is the grouping of both Bats and Whales under the head of
Mammalia because of their agreement, in all essential points of
anatomy, histology, and embryology, with the hairy quadrupeds
which form the bulk of that class.

With the older zoologists the difficulty was to find some general
principle to guide them in their (arrangement of animals some
true criterion of classification. It was believed by all but a few
advanced thinkers that the individuals of each species of animal
were descended from a common ancestor, but that the original

o

progenitor of each species was totally unconnected with that of
every other, having, as Buffon puts it, " participated in the grace
of a distinct act of creation." To take an instance all Wolves
were allowed to be descended from a pair of ancestral Wolves, and
all Jackals from a pair of ancestral Jackals, but the original pair in
each case was supposed to have come into being by a supernatural

6 ZOOLOGY

process of which no explanation could or ought to be offered.
Nevertheless it was obvious that a Jackal was far more like a
Wolf than either of them was like a Tiger, and that in a natural
system of classification this fact should be expressed by placing the
Wolf and Jackal in one family, the Tiger in another.

All through the animal kingdom the same thing occurs : no
matter what group we take, we find the species composing it
resemble one another in varying degrees, or, as it is sometimes ex-
pressed, have varying degrees of relationship to one another. On
the view that each species was separately created the word relation-
ship was used in a purely metaphorical sense, as there could of
course be no real relationship between two groups of animals
having a totally independent origin. But it was assumed that
creation had taken place according to a certain scheme in the
Divine Mind, and that the various species had their places in this
scheme like the bits of glass in a mosaic. The problem of classifica-
tion was thus to discover the place of each species in the pattern of
the unknown design.

The point of view underwent a complete change when, after the
publication of Darwin's Origin of Species in 1859, the Doctrine
of Descent or of Organic Evolution came to be generally
accepted by biologists. A species is now looked upon, not as an
independent creation, but as having been derived by a natural
process of descent from some pre-existing species, just as the
various breeds of Domestic Fowl are descended from the little
Jungle-fowl of India. On this view the resemblances between
species referred to above are actually matters of relationship, and
species are truly allied to one another in varying degrees since
they are descended from a common ancestor. Thus a natural
classification becomes a genealogical tree, and the problem of
classification is the tracing of its branches.

This, however, is a matter of extreme difficulty. Representing
by a tree the whole of the animals which have ever lived on the
earth, those existing at the present day would be figured by the
topmost twigs, the trunk and main branches representing extinct
forms. Thus the task of arranging animals according to their
relationships would be an almost hopeless one but for two
circumstances : one, that remains of many extinct forms have been
preserved ; the other, that the series of changes undergone by an
animal in its development from the egg often forms an epitome of
the changes by which, in the course of ages, it has been evolved
from an ancestral type. Evidence furnished by the last-named
circumstance is, of course, furnished by embryology : the study of
extinct animals constitutes a special branch of morphology to
which the name Palaeontology is applied.

The solid crust of the earth is composed of various kinds of
rocks divisible into two groups : (1) Igneous rocks, such as granite

INTRODUCTION 7

and basalt, the structure of which is due to the action of the
internal heat of the globe, and which originate below the surface
and are not arranged in layers or strata ; (2) Aqueous or sedimentary
recks, which arise by the disintegration, at the surface of the earth,
of pre-existing rocks, the fragments or debris being carried off by
streams and rivers and deposited at the bottom of lakes or seas.
Being formed in this way by the deposition of successive layers or
strata, the sedimentary rocks have a stratified structure, the lowest
being in every case older than the more superficial layers. The
researches of geologists have shown that there is a general order of
succession of stratified rocks : that they may be divided into three
great groups, each representing an era of time of immense but
unknown duration, and that each group may be subdivided into
more or fewer systems of rocks, each representing a lesser period of
time. The following table shows the thirteen rock-systems usually
recognised, arranged under the three great groups in chronological
order, the oldest being at the bottom of the list.

13. Quaternary and Recent.

m n . m A- 12. Pliocene.

. Oamozoic or lertiary. . < -,-, ,.-.

11, Miocene.

[ 10. Eocene.

9: Cretaceous.
II. Mesozoic or Secondary . 8. Jurassic.

Hr m

7. Inassic.

6. Permian.

5. Carboniferous.

I. Palaeozoic or Primary

4. Devonian.
3. Silurian.

2. Cambrian.
1. Laurentian.

Imbedded in these rocks are found the remains of various extinct
animals in the form of what are called fossils, In the more recent
rocks the resemblance of these to the hard parts of existing
animals is perfectly clear : we find shells hardly differing from
those we pick up on the beach, bones easily recognisable as those
of Mammals, Birds, or Fishes, and so on. But in the older rocks the
fossils are in many cases so different in character from the animals
existing at the present day as to be referable to no existing order.
We find Birds with teeth, great aquatic Reptiles as large as Whales,
Fishes, Molluscs, Crustacea, &c., all of an entirely different type from
any now existing. We thus find that the former were in many
cases utterly unlike the present animal inhabitants of the globe,
and we arrive at the notion of a succession of life in time t and are
even able, in exceptionally favourable circumstances, to trace back
existing forms to their extinct ancestors.

By combining the results of comparative morphology, embryology,

8 ZOOLOGY

and paleontology we get a department of Zoology called Phylo-
geny, the object of which is to trace the pedigrees of the various
groups. There are, however, very few cases in which this can be
done with any approach to exactness : most " phytogenies ' are
purely hypothetical, and merely represent the views at which a
particular zoologist has arrived after a more or less exhaustive
study of the group under discussion.

Animals may also be studied from the point of view
of Distribution. One aspect of this study is inseparable from
Palaeontology, since it is obviously necessary to mention in con-
nection with a fossil the particular system or systems of rocks in
which it occurs : thus we distinguish geological distribution or
distribution in time.

The distribution of recent forms may be studied under two
aspects, their horizontal or geographical distribution, and their
vertical or bathymetrical distribution. To mention the latter
first, we find that some species exist only on plains, others hence
called alpine forms on the higher mountains ; that some marine
shells, fishes, &c., always keep near the shore (littoral species), others
live at great depths (ctbyssal species), while others (pelagic
species) swim on the surface of the ocean. Among aquatic
animals, moreover, whether marine or fresh-water, three principal
modes of life are to be distinguished. There are animals, such
as Jelly-fishes, which float on or near the surface of the water,
and are carried about passively by currents: such forms are
included under the term Plankton. Most Fishes, Whales, and
Cuttle-fishes, on the other hand, are strong swimmers, and are able
to traverse the water at will in any direction ; they together consti-
tute the Nekton. Finally, such animals as Crabs, Oysters, Sponges,
Zoophytes, &c., remain permanently fixed to or creep over the
surface of the bottom, and are grouped together as the Benthos.

Under the head of geographical distribution we have such facts
as the absence of all Land-mammals, except Bats, in New Zealand
and the Polynesian Islands, the presence of pouched Mammals,
such as Kangaroos and Opossums, only in some parts of America
and in Australia and the adjacent islands, the entire absence of
Finches in Australasia, and so on. We find, in fact, that the
fauna i.e. the total animal inhabitants of a country is to a
large extent independent of climate, and that the fauna? of
adjacent countries often differ widely. In fact, it is convenient
in studying the geographical distribution of animals largely to
ignore the ordinary division into continents, and to divide the
land-surface of the globe into what are called zoo- geographical
regions. The characteristics of these regions will be discussed in
a future section ; at present it is only necessary, for convenience of
reference, to give their names and boundaries.

INTRODUCTION 9

1. The Hokirctic Region includes the whole of Europe, Asia as
for south as the Himalayas, Africa north of the Sahara, together
with the corresponding portion of Arabia, and North America as
for south as Mexico. For convenience of reference it is often
customary to divide this region into two : its Eurasian portion is
then called the Palcearctic, its American portion the Nearctic
region.

2. The Ethiopian Region includes Africa south of the Sahara,
Southern Arabia, and Madagascar with the adjacent islands.

3. The Oriental Region includes India, Ceylon, South China,
the Malayan Peninsula, and what are known as the Indo-Malayan
islands, i.e. those islands of the Malayan Archipelago which lie to
the west of a line called Wallace's line passing to the east of
the Philippines, between Borneo and Celebes and between Bali
and Lombok.

4. The Australian Region includes Australia, Tasmania, and the
Austro-Malayan islands, i.e. the islands of the Malayan Archipelago
lying to the east of Wallace's line.

5. The New Zealand Region includes New Zealand and the
adjacent islands, such as the Chatham, Auckland, and Campbell
groups.

6. The numerous groups of islands lying between Australia
and Southern Asia to the west, and America to the east, are
conveniently grouped together as the Polynesian Region.

7. The Neotropical Region includes the whole of South and
Central America and part of Mexico.

There are still two departments of zoological science to be
mentioned. As it is impossible to have a right understanding of
a machine without knowing something of the purpose it is in-
tended to serve, so the morphological study of an animal is im-
perfect without some knowledge of its Physiology, i.e. of the
functions performed by its various parts, and the way in which
they work together for the welfare of the whole. It is hardly
possible to give more than occasional references to physiological
matters in a text-book of Zoology, but in order to pave the way
for such references a brief account of the general principles of
Physiology will be given in the next section.

Not only may we study the action of a given animal's organs,
but also the actions of the animal as a whole, its habits, its
relations to other animals whether as friends, as enemies, or as
prey, to the vegetable kingdom, and to its physical surroundings,
such as temperature, humidity, &c. In a word, the whole question
of the relation of the organism to its environment gives us a final
and most important branch of Natural History which has been
called Ethology or Bionomics.

SECTION I.

THE GENERAL STRUCTURE AND PHYSIOLOGY

OF ANIMALS

"psd

1. AMCEBA.

IF we examine under the microscope a drop of water containing
some of the slimy deposit which collects at the bottom of pools of
rain-water and in similar situations, we occasionally find it to
abound in microscopic life ; and among the minute moving creatures
in such a drop we frequently find examples of a remarkable or-
ganism the Ama-la or Proteus Animalcule (Fig. 1). This is

a little particle of irregular
shape, which we should be
likely, on a cursory examina-
tion, to put down as motion-
less ; it appears somewhat like
an irregular particle of some
colourless glass-like substance
with a more granular central
portion. If, however, we make
an exact drawing of the out-
line of the Amoeba, and, after
an interval, compare the draw-
ing with the original, we find
that the drawing appears no
longer to represent what we
see ; a change has taken place
in the shape of the Amoeba ;
and careful observation shows that this change is constantly going
on : the Amoeba is constantly varying in shape. This change is
effected by the pushing out of projections or processes, called
pseudopods (>srf.), which undergo various alterations of size
and shape, and may become withdrawn, other similar processes
being developed in their place. At the same time careful

FIG. 1. Amoeba proteus, a living specimen.
c. rac. contractile vacuole ; nu. nucleus ;
ps<t. pseudopods. (From Parker's Biology,
after Gruber.)

SECT, i STRUCTURE AND PHYSIOLOGY OF ANIMALS 11

watching shows that the Amoeba is also, with extreme slowness,
changing its position. This it effects by a kind of streaming
motion. A projection forms itself on one side, and the entire
substance of the Amoeba gradually streams into it ; a fresh
projection appears towards the same side, the streaming move-
ment is repeated, and, by a constant succession of such move-
ments, an extremely gradual locomotion, which it often takes very
close watching to detect, is brought about. In these movements,
it is to be noticed, the Amoeba is influenced to some extent by
contact with other minute objects; when the processes come in
contact with small grains of sand or other similar particles their
movements are modified in such a way that the Amoeba, in its
slow progress onwards, passes on one side of them, so that it
might be said to feel its way among the solid particles in the drop
of sediment.

Judging from the nature of these movements, we are obliged to
infer that the substance of which this remarkable object is com-
posed must be soft and semi-fluid, yet not miscible with the water,
and, therefore, preserving a sharp contoui\ These and other
characteristics to be mentioned subsequently enable us to conclude
that we have to do with the substance of complex chemical com-
position termed protoplasm, which constitutes the vital material of
all living organisms whether animals or plants. In Amoeba the
protoplasm is in many cases clearly distinguishable into two parts,
an outer homogeneous, glassy-looking layer completely enclosing
a more granular internal mass.

Examination of the Amoeba with a fairly high power of the
microscope reveals the presence in its interior of two objects which
with a low power we should be likely to overlook. One of these
is a small rounded body with well-defined contour, which
preserves its form during all the changes which the Amoeba as a
whole undergoes. This is termed the nucleus (Fig. 1, nu.)\ it is
enclosed in an extremely delicate membrane, and consists of a
protoplasmic material differing from that which forms the main
bulk of the Amoeba in containing a substance which refracts the
light more strongly and which has a stronger affinity for certain
colouring matters. The other minute object to be distinguished
in the interior appears as a clear rounded space (c. vac.) in the
protoplasm. When this is watched it will be observed to increase
gradually in size till it reaches a maximum of, let us say, a fifth of
the total diameter of the Amoeba, when, by a contraction of its walls,
it suddenly disappears, to reappear presently and gradually grow
again to its maximum size. This pulsating clear space is the
contractile vacuole. Other clear spaces which do not pulsate are
the non- contractile vacuoles.

By watching the Amoeba carefully for some time we may be
enabled to observe that the movements of the protoplasm of the
body not only effect locomotion, but are connected also with the

12 ZOOLOGY SECT.

reception of certain foreign particles of organic nature i.e. either
entire minute animals or plants, or minute fragments of larger
forms into the interior of the protoplasm. A process of the
protoplasm is pressed against such a particle, which becomes sunk
in the soft substance, and passes gradually into the interior. Here
it becomes enclosed in one of the non -contractile vacuoles, and by
degrees partially or wholly disappears ; the part, if any, which
remains subsequently passes outwards from the protoplasm into
the surrounding water. The matter which disappears evidently
mixes with the" protoplasm and adds to its bulk. All, in fact, of
the matter of the foreign body that is capable of doing so, becomes
digested and assimilated by the protoplasm. The fluid in the
vacuole enclosing the food-particle (for such is the true nature of the
foreign body) probably contains some ingredient of the nature of a
ferment, which is able to act on certain substances and render
them more soluble or capable of being more readily taken up by
the protoplasm. This we infer mainly from what we know of the
digestion and absorption of food in the higher animals ; but the
fact, which has been established by experiment, that the Amoeba
is able readily to digest certain classes of organic substances, while
others, when taken into the interior of the protoplasm, remain
unaltered, seems to indicate that some special property, similar to
those possessed by the digestive ferments of the higher animals,
is present in the w r atery fluid surrounding the food-particle.

The movements of the Amoeba, slow and gradual though they
are, must involve a certain expenditure of energy or working power ;
this can only be derived from the energy of chemical affinity
which the protoplasm possesses in virtue of its complex chemical
composition. The protoplasm loses some of this energy by its
conversion into energy of movement. This loss implies the break-
ing up of the complex chemical ingredients of which protoplasm
is composed into simpler ones; the protoplasm falls a grade in
the scale of chemical compounds, and by its fall generates the
force by means of which the Amoeba moves. The energy of
chemical affinity which the protoplasm possesses is thus analogous
to the potential energy which the weight of a clock has when it is
wound up. As the weight, by virtue of its position, is able as it
falls to deal out working power so as to cause the movement of the
machinery of the clock, so the protoplasm is able, by the degra-
dation or decomposition of its complex compounds, to deal out
working power enabling the Amoeba to move. In the case of the
clock-weight there comes a time when all the potential energy is
expended ; the weight reaches its lowest limit, and unless it is
wound up again the clock stops. The like holds good of the
Amoeba ; the protoplasm is continually being used up decomposed
into compounds of a lower order and, in course of time, the whole
potential energy would become exhausted, were it not that a new

STRUCTURE AND PHYSIOLOGY OF ANIMALS

13

supply is being constantly received. This new supply of energy is
derived from the substance of the food-particles ; and this at the
same time maintains the bulk of the Amoeba, which, if food par-
ticles are absent from the water, gradually diminishes.

Accompanying the degradation, or destructive metabolism as it
is termed, of the protoplasm, and intimately connected with it, is
the passage inwards of oxygen from the air dissolved in the water,
and the passage outwards of carbonic acid gas. Oxygen is a
necessary agent in the process of destructive metabolism, and

FIG. 2. Amoeba polypodiain successive phases of division- The light spot is the contractile-
vacuole ; the dark the nucleus. (From Lang's Text-Book, after F. E. Schulze.)

carbonic acid is a constant waste-product of such action. This
interchange of oxygen and carbonic acid is the essence of the pro-
cess of respiration observable in all living things. In addition to
the carbonic acid given off in this process, other waste-products are
formed and have to be got rid of. In all probability the contractile
vacuole already referred to has to do with this process the process
of excretion since uric acid, which in higher animals is the typical
form assumed by such waste-products, is said to have been detected
in the interior of the contractile vacuole in the case of certain near
relatives of Amoeba.

When food is abundant the Amoeba increases in bulk more

14 ZOOLOGY SECT.

food being ingested than is required for simply maintaining the
size unaltered and soon a remarkable change takes place. The
processes become withdrawn, and a fissure appears dividing the
Amoeba into two parts (Fig. 2). This fissure grows inwards, and the
two parts become more and more completely separated from one
another, till eventually the separation becomes complete, and we
have two distinct Amoebae resulting from the division of the one.
While the protoplasm has been undergoing this division into
halves the nucleus has also divided, and each of the two new
Amoebae possesses a nucleus similar to the original one, and
developed from it by division. It is mainly by this simple process
of division into two, or binary fission as it is called, that rcpro-
duction or multiplication takes place in the Amoeba.

In spite of the great simplicity of its structure, the Amoeba
thus carries on a number of different functions. The practically
structureless particle of protoplasm is able to act on matter
absorbed as food in such a way as to alter the chemical composition
of the latter and to assimilate it ; it is able to carry on movements
of locomotion, as well as movements those involved in the
taking in of food particles which may be looked upon as move-
-raents of prehension ; it exhibits a certain degree of sensitiveness
or irritability, as shown by the modifications of its movements
which result from contact with foreign bodies ; it is able to
respire ; it carries on processes of excretion ; and, finally, it is
capable of reproducing its kind. It is these functions that charac-
terise living beings as distinguished from non-living matter.
What is specially characteristic of the living organism in general
when compared with a non-living object is the capacity of the
former to respond by changes in itself to influences operating on
it from without. In the case of such an extremely simple
organism as Amoeba, these changes are also, necessarily, extremely
simple ; but they are of a quite definite character. In addition
to the effects produced on its actions by mechanical obstacles and
the presence of food-particles, it can be shown by experiment that
Amoeba responds by definite changes in itself to such external
influences as changes in the amount of oxygen supplied, in the
quantities of various salts present, in the temperature, and in the
electric conditions of the water in which it lives. The power of
locomotion, the capacity for assimilating organic substances, and
the absence of two special compounds chlorophyll and cellulose-
are specially characteristic of the animal as distinguished from
the plant.

2. THE ANIMAL CELL.

In all but the lowest animals the various functions just enume-
rated are carried on by means of a more or less complex machinery

STRUCTURE AND PHYSIOLOGY IN ANIMALS

15

of organs muscles, alimentary or enteric canal, glands, heart and
blood-vessels, gills or lungs, nervous system, organs of excretion, and
organs of reproduction. But in all animals, however complex, the
same substance, protoplasm, which in Amoeba constitutes the
bulk of the body, is the essential and active part. Wherever in
the body active functions are being discharged and active changes
are going on, there we find protoplasm present ; where there is
no protoplasm there is no vital activity. In the earliest stages of
their existence all animals are formed entirely of protoplasm.
Every animal consists at first of a single minute particle of proto-
plasm, not widely different from an Amoeba. Soon this particle
divides into a number of parts which, instead of separating
completely from one another, like the parts of a divided Amoeba,
remain associated together, forming a clump of minute particles
of protoplasm. Such minute protoplasmic particles are termed
cells ; every animal consists, at first, of a single cell, and afterwards,
in all higher animals, this single cell becomes converted by division
and subdivision into a little cluster or clump of cells.

It is time that we should inquire more particularly into the
meaning of these two terms cell and protoplasm evidently so
important in the study of both plants and animals. Protoplasm,
we have already seen, is a semi-fluid, gelatinous, clear or finely
granular substance of complex chemical composition. It is known
not to be a definite compound, but to be a somewhat varying
mixture of chemical compounds, the most essential of which are
bodies of the class of proteids highly complex substances, into the
composition of which the elements carbon, hydrogen, oxygen,
nitrogen, and sulphur all

T 1

enter. Living protoplasm
always contains a large
amount of water. It is
soluble in weak acids or
weak alkalies ; and is
capable of being coagu-
lated - - rendered firmer
and more opaque - - by
the action of heat and
of strong alcohol. Its re-
action is slightly alkaline.
As regards its minute
structure, it is generally
acknowledged that there
are two kinds of sub-
stance in the protoplasm, in some cases more, in others less, dis-
tinctly marked off from one another. One of these kinds of material
is apparently of less fluid consistency than the other. According
to one view (alveolar theory) the two kinds are intimately com-

FIG. 3. Diagram to illustrate the alveolar theory of
protoplasm. (After Dahlgren and Kepner.)

16

ZOOLOGY

SECT

FIG. 4. Diagram to illustrate the reticular theory of
protoplasm. (After Dahlgreu and Kepner.)

bined in the form of an emulsion or froth, the one forming the
minute vesicles or bubbles in the froth, the other the ground
substance in which the bubbles are embedded (Fig. 3). Accord-
ing to another view (reticular theory), one of these substances,

the less fluid, appears to
be arranged in the form
of a network of threads,
composed of numerous
minute rounded granules
enclosing the second,
more fluid substance in
its meshes (Fig. 4).

To a particle of pro-
toplasm, typically con-
taining a nucleus in its
interior, constituting the
entire body of such a
simple organism as
Amoeba, and forming one
of the constituent ele-
ments of which a higher plant or animal is made up, the term cell
is applied. The word \vas first employed in reference to the micro-
scopic structure of plants, in connection with which it is much more
appropriate than in connection with the microscopic structure of
animals ; for a plant-cell has, nearly always, a definite, firm, enclos-
ing envelope or cell-wall (Fig. 5, I, c.w) a structure which is only
exceptionally present in the case of animals. In the interior of
the cell-protoplasm, or cytoplasm, is a body termed the nucleus,
similar to the nucleus of Amoeba, and usually of rounded shape, with
the appearance of being enclosed in a thin nuclear membrane
(A, nu.ni), perforated by numerous minute apertures. In the
nucleus is a single coiled thread, or a network of threads, or one
or more rounded clumps, of a substance chromatin (chr.) which
differs from ordinary protoplasm in having a stronger affinity for
most staining agents. A rounded body termed the nuclcolus
(nu), which usually occurs in the interior of the nucleus, is
formed either of a solid mass of chromatin, or of a substance
differing somewhat from chromatin in its properties, and less
strongly affected by staining agents. When the nucleus divides
during the process of division of the cell, its contents, more
particularly the chromatin, in many cases go through a remarkable
series of changes, to which the term haryoltinesis or mitosis is
applied.

At the time when this mitotic division is about to be
initiated, either one or two minute bodies (Fig. 5, A, c) are to be
distinguished situated close together in the cytoplasm in the
immediate neighbourhood of the nucleus. When only one of

STRUCTURE AND PHYSIOLOGY OF ANIMALS

17

these bodies is present at the outset it subsequently becomes
divided into two. These are the centrosomes minute masses of a
specially modified protoplasmic substance, capable of being
rendered conspicuous by certain staining agents, -" surrounded
by a light zone. The centrosomes, at first close together,
gradually separate from one another, a spindle-shaped bundle
of very fine fibres of achromatic : material the nuclear spindle

FIG. 5. Diagrams illustrating karyokinesis. A, the resting cell ; B, C, D, successive phases in
the formation and arrangement of the chromatiii loops and of the nuclear spindle ; E, F ,G,
separation of the two sets of daughter-chromosomes and their passage towards the poles of
the spindle ; H, I, division of the cell-body and formation of the two new nuclei ; c. centro-
some ; chr. chromatin ; cpl. cell-plate ; me', nucleoli ; nu. in. nuclear membrane ; s. atrosphere ;
sp. spindle. (From Parker's Biology, after Flemming, Rabl, &c.)

-extending between them (Fig. 5, C). At the same time,
or at an earlier stage, each centrosome has become the centre
of a system of fine achromatin fibres (apparently made up,
like the fibres of the spindle, of rows of granules) which are
arranged round it in a radiating manner, forming a structure

1 The term achromatin is usually applied to all the matter of the nucleus
that has not the special characteristics of chromatin ; but it applies to cytoplasmic
structures i.e. structures belonging to the body of the cell as well.

VUL. I C

18 ZOOLOGY SECT.

termed the attraction-sphere or astrospkere (Fig. 5. A, s). Meantime
important changes have been in progress in the nucleus. The
chromatin first becomes arranged in a close tangle (spireme), and
then becomes divided up into a number of parts the chromatin
segments or chromosomes which frequently have the form of loop-
like threads (Fig. 5, B, C, chr), but often assume other forms. The
number of chromosomes varies, but is constant throughout the
cells of the same species of animal. The nuclear membrane
disappears. Each of the chromatin segments splits lengthwise
into two parts the daughter-segments of the chromatin or daughter-
chromosomes (Fig. 5, B D), and with these the filaments of the
spindle become connected.

At this point the segments of the chromatin form a single
group the equatorial plate extending across the axis of the
spindle. The latter has shifted its position, so that its fibres now
run across the original site of the nucleus. Each daughter-segment
of the chromatin now separates from its fellow, so that two groups
are formed, each containing a similar number of chromosomes.
The two groups then move apart from one another, each approach-
ing the corresponding end or pole of the spindle with its
centrosome (Fig. 5, E G). How this movement is effected
is not definitely known ; it has been supposed that it is due to the
contraction of spindle-fibres attached to the centrosomes ; but
since there is no appearance of the fibres shortening or thicken-
ing, it is unlikely that this can be the true explanation.

When the groups have approached the extremity of the spindle,
the segments of each unite, and eventually the entire chromatin of
each of the two groups assumes the arrangement which the
chromatin of the original nucleus exhibited before division began.
A new nuclear membrane becomes formed around each chromatin
group, and the whole assumes the character of a complete nucleus
the daughter -nucleus (Fig. 5, H, I). It is of importance to
note that, though in this mitotic division of the nucleus of
the animal cell the centrosomes are so conspicuous that it
would appear as if they had an important share in controlling
the process, yet mitosis takes place during cell-division of the
higher plants on the same general lines as in animals though
centrosomes have rarely, if ever, been observed in plants higher
than the Mosses.

A furrow which appears on the surface of the cell-protoplasm (Fig.
5,H,I), surrounding it in the form of a ring in a plane at right angles
to the long axis of the spindle, deepens gradually so as to give rise
to a cleft, eventually completely separating the substance of the
cell into two halves. Each of these halves encloses one of the
daughter-nuclei, and has assumed the character of a complete
daughter -cell. During this process there is sometimes distinguish-
able along the line corresponding to the division line between the

STRUCTURE AND PHYSIOLOGY OF ANIMALS

19

two cells a narrow septum ; this is known as the cellulate (I., c.pl.~).
But a cell-plate is not of general occurrence in the division of the
animal cell.

In some instances the division of the nucleus is direct
or amitotic, the nucleus simply becoming separated into two
equal parts, without disappearance of the nuclear membrane
and without any complicated re-arrangement of the chromatin.

3. THE OVUM : MATURATION, IMPREGNATION, AND SEGMENTATION :

THE GERMINAL LAYERS.

Amoeba is simply an independent animal cell ; or, to express
the same meaning in another way, is a unicellular animal, and as
such it is a member of the phylum of the Protozoa or unicellular
animals. All the rest of the animal kingdom, forming the
division Metazoa, are multicdlular in the fully developed condition ;
but each of these multicellular
animals or Metazoa originates from
a single cell the ovum. The
ovum is a typical cell (Fig. 6),
usually spherical in shape, with
one or more enclosing membranes,
with cell-protoplasm enclosing a
nucleus (germinal vesicle) in which
are contained one or more rounded
masses of chromatin (germinal spot
or spots). The ovum may contain
in addition to the protoplasm a
quantity of non-protoplasmic nu-
trient material or yolk.

Before the process of impregna-
tion or fertilisation which gives
the impulse to development, the
ovum undergoes a change which is
termed maturation (Fig. 7, A). This consists, in essence, of
the throwing out of portions of the nucleus.' The latter
approaches the surface and divides, mitotically, into two parts
one coming to project on the surface and finally the projection being
completely separated off from the ovum as a rounded particle-
the first polar body (pol.). A second division of the nucleus
results in the throwing off of a second polar body ; and, after this
has been formed, the portion which remains in the ovum resumes
its central position and forms what is termed the female pro-
nudeus (B, $ pron.). The essential ultimate result of maturation
is the reduction of the number of chromosomes in the ovum by
one-half.

In the process of impregnation a very minute body, the male

c 2

FIG. 6. Ovum of a Sea-Urchin, showing
the radially striated cell-membrane,
the protoplasm, containing yolk-
granules, the large nucleus (germinal
vesicle), with its network of chro-
matiii and a large nucleolus (ger-
minal spot). (From Balfour's Em-
bryology, after Hertwig.)

20

ZOOLOGY

SECT.

cell, sperm-cell, or sperm, penetrates into the interior of the female
cell or ovum, and the nucleus which it contains the male pro-
nucleus (C, $ pron.) coalesces with the female pronucleus to form a
single nucleus called the segmentation nucleus (E, seg. nucl.). The

pol

mem.

menv

seg.nucL

FIG. 7. Diagram illustrating the maturation and fertilisation of the ovum. A, formation of first
polar body ; B, beginning of fertilisation, sperms approaching the micropyle ; C, formation
of the male pronucleus ; D, approximation of the male and female pronuclei ; E, formation
of segmentation-nucleus ; $> cent, female centrosome ; <*, cent, male centrosome ; mem. egg-
membrane ; microp. micropyle; pol. polar bodies ; 9 pron. female pronucleus ; $ pt'on. male
pronucleus ; seg. nucl. segmentation nucleus.

principal part in the process of fertilisation is thus played by the
two nuclei. The female centrosome disappears: a male centrosome
enters with the sperm.

Apparently in this process of fertilisation some attraction is

STRUCTURE AND PHYSIOLOGY OF ANIMALS 21

operative between the male and female cells. In many instances
a prominence (the receptive prominence) is pushed out by the
ovum at the point where the sperm enters. The female
pronucleus, leaving its former central position, approaches the
male cell as it enters. In most cases a single sperm alone enters
the ovum in impregnation. According to the older observers,
as soon as a sperm enters the ovum, a membrane is formed
around the latter hindering the penetration of additional sperms.
But it has now been shown that such a membrane occurs
only in certain cases, and is quite exceptional. That,
as a general rule, only one sperm penetrates into the ovum
appears to be due to the circumstance that, as a result of the
entry of the one sperm, the peculiar attraction above referred
to becomes in some way destroyed or diminished. But, though
the entry of one sperm only is usual, cases of the entry
of several polyspermy, as it is termed are by no means
rare, and would appear to be quite normal in some groups of
animals.

In some animals the ovum develops parthenogenetically i.e.
without any process of fertilisation by means of a male cell.
This is a normal phenomenon in certain families of insects,
for example. In a considerable number of marine invertebrate
animals it has been shown that -though gamogenesis, i.e. develop-
ment as the result of fertilisation of ovum by male cell, is
the normal process, yet parthenogenesis can be produced by
various artificial means. By adding various salts to the water
in which the ova are contained, by changes of temperature,
or by subjection to the action of carbonic acid gas, the ova,
in the absence of sperms, may be caused to give rise to normal
embryos. Such experiments on artificial parthenogenesis, as it
is termed, show that the entry of a male cell into the ovum
is not necessary for the development of the embryo even in
cases in which gamogenesis is normal ; but that other exciting
influences may bring about the same result.

Though, as stated above, the female pronucleus, under normal
circumstances, plays so important a role in the development, it
has been shown that it can be dispensed with. When unfertilised
ova of a sea-urchin are broken up, and fragments devoid of
nuclei are placed in water along with sperms, the fragments may
be fertilised ; and, the nucleus of the sperm taking the place
of the segmentation-nucleus, normal young, differing from those
produced in the usual manner only in their smaller size, may
be developed. This phenomenon is known as merogony.

The result of fertilisation is the formation of the impregnated
ovum, or oosperm as it is called. The oosperm, it is to be noted,
before development begins, consists in general of the primary
ovum minus the portions of the substance of its nucleus removed

22

ZOOLOGY

SECT.

in the polar bodies and also minm its centrosome, and plus
the sperm with its nucleus and centrosome.

On impregnation follows shortly the process of division already
brieflj' referred to, which is known as segmentation (Fig. 8).
This either affects the entire substance (holoNastic or complete

FIG. S. Various stages in the segmentation of the ovum. (From Gegeubaur's Comparative

Anatomy.)

segmentation) or only a part (meroblastic or incomplete seg-
mentation) of the oosperm. In the former case the ovum usually
contains little or no food-yolk, consisting exclusively, or nearly
so, of protoplasmic matter. The first stage in the process of
segmentation is the mitotic division of the segmentation-nucleus,
accompanied by the division into two parts of the substance
of the protoplasm the result being the formation of two cells,
each with its nucleus (Fig. 8). Each of these two cells then divides
-four cells being thus formed ; the four divide to form eight ;
the eight divide to form sixteen, and so on ; until, by the process
of division and subdivision, the oosperm becomes segmented into
a large number of comparatively small cells which are termed the
blastomeres. This mass of cells is spherical in shape, and the

ardv

ABC

FIG. 9. Gastrulation.
a'.'di. archenteron ; II. blastopore ; ecto. ectoderm ; endo, endoderin.

rounded blastomeres of which it is composed project on its sur-
face so as to give it somewhat the appearance of the fruit of
the mulberry, whence it is termed the mulberry body or morula
stage. The blastomeres next become arranged regularly in a
singl/3 layer the embryo (Fig. 9, A) assuming the form of a hollow

STRUCTURE AND PHYSIOLOC4Y OF ANIMALS

23

sphere, the Uasfosphere or llastula, with a wall composed of a
single layer of cells enclosing a cavity the segmentation cavity
or Uastoccele.

One side of the hollow blastula next becomes pushed inwards or
invaginated (Fig. 9, B, C\ as one might push in one side of a hollow
india-rubber ball, the result of this process of invagination, or
gastrulation as it is termed, being the formation of a cup the
gastrula (Fig. 10) with a double wall. The
cavity of the cup-shaped gastrula is the
archenteron or primitive digestive cavity ;
the opening is termed the blastopore, the
outer layer of the wall of the cup is the
ectoderm (or cpiUast), the inner the cndoderm
(or liypoUast). The ectoderm and endoderm
are the primary germinal layers of the em-
bryo ; from one or both of them are developed
the cells of a third layer the mesodcrm
(mcsoblast) which is subsequently formed
between them.

This mode of formation of the primary
germinal layers in holoblastic oosperms by a
process of gastrulation prevails in a number
of different sections of the animal kingdom.
In many animals, however, it becomes modi-
fied or disguised in various ways ; and in many meroblastic
oosperms it is doubtful if there occurs anything of the nature of
true gastrulation.

The cells of the three germinal layers give rise to the various
organs of the body of the fully -formed animal each layer having
a special part to play in the history of the development. As the
various parts of the embryo become gradually moulded from the
cells of the germinal layers, it becomes evident on comparison
that their internal structure the form and arrangement of their
constituent cells is undergoing gradual modifications, the nature
of which is different in the case of different parts. A differentia-
tion of the cells is going on in the developing organs, resulting in
the formation of a variety of different kinds of tissues.

FIG. 10. Gastrula in longi-
tudinal section ; a,
blastopore ; b, arch-
enteron ; c, endoderm;
d, ectoderm. (From
Gegenbaur's Compara-
tive Anatomy.)

4. TISSUES.

The cells of the tissues of the animal body differ greatly in
form in different cases. Some are rounded, others cubical, others
polygonal ; some are shaped like a pyramid, others like a cone,
others like a column or cylinder ; others are flattened and tabular
or scale-like. Cells situated on free surfaces are in many cases
beset at their free ends with delicate, hair-like structures or cilia
which vibrate to and fro incessantly during the life of the cell

24

ZOOLOGY

SECT.

(Fig. 11, a); sometimes there is on each cell a single, relatively long,
whip-like cilium, which is then termed a flagellum (/, g\ Cells

provided with cilia are
termed ciliated, such as
bear flagella flagellate
cells.

Some tissues are com-
posed entirely of cells.
Others, though originat-
ing from cells or by the
agency of cells, consist in
greater or less measure of
non-protoplasmic matter
formed between the cells.
Tissues composed en-
tirely of cells take the
form, for the most part,
of membranes covering
various surfaces, external
and internal. Such mem-
branes are known under
the general name of
epithelia (Fig. 11); they
may consist of a single
layer of cells (afi) or
may be many-layered
(i) ; the former are
termed non-stratified, the
latter stratified, epithelia.
The cells of an epithe-
lium may be flattened
(c, e\ their edges being
cemented together so as
to form a continuous
membrane ; or they may
be cubical or cylindrical
or prismatic (a, 1}) ; in
the case of a stratified
epithelium the cells may
be of different forms in
different strata (i). The
epidermis, which covers the outer surface of the body of an animal,
is an example of an epithelium ; sometimes it is stratified, some-
times unstratified ; its cells sometimes possess cilia, sometimes are
devoid of them. Lining the internal cavities of the body are
layers of cells, or epithelia, sometimes in a single layer, sometimes
in several layers, sometimes ciliated, sometimes non-ciliated.

FIG. 11 Various forms of epithelium, a, ciliated epi-
thelium ; b, columnar ; d, surface view of the sauie ;
c, tesselated ; e, the same from the surface ; /, flagel-
late epithelium with collars ; g, flagellate epithelium
without collars ; h, epithelium of intestine with
pseudopods ; i, stratified epithelium ; k, deric epi-
thelium of a marine planarian with pigment cells,
rod-cells, and sub-epithelial glands. (From Lang's
Comparative Anatomy.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

Glands (Fig. 12) are formed for. the most part by the modifica-
tion of certain cells of epithelia. In many cases a single cell of the
epithelium forms a gland, which is then termed a unicellular gland
(Fig. 12, A). The secretion (or substance which it is the function of
the gland to form or collect) gathers in such a case in the interior
of the cell, and reaches the surface of the epithelium through a
narrow prolongation of the cell which serves as the duct of the
gland (). In other cases the gland is multiccllular formed of a
number of cells of the epithelium lining a depression or infolding,
simple or complex in form, of the
latter (D-G). In the central
cavity of such a gland the secre-
tion collects to reach the general
surface or cavity lined by the
epithelium through the passage
or duct.

A series of tissues in which the
cells are, in most instances, sub-
ordinate, as regards bulk, to sub-
stances formed between them, is
the group known as the con-
nective tissues, including gela-
tinous connective tissue, retiform
connective tissue, fibrous connective
tissue, cartilage, and bone. In the
majority of forms of connective
tissue the cells lie embedded in
an intermediate substance called
the matrix or ground-substance
of the connective tissue.

In the case of gelatinous con-
nective tissue (Fig. 13) the ground-
substance (g) is of a gelatinous
character, sometimes supported
by systems of fibres (<?/), and the
cells are usually stellate or star-
shaped with radiating processes. Retiform or reticulate connective
tissue (Fig 14) consists of stellate or branching cells with pro-
cesses which are prolonged into fibres the fibres from neigh-
bouring cells joining so as to form a network. In this form of
connective tissue there is no true ground-substance the inter-
spaces between the cells being filled with other tissue elements.

Fibrous connective tissue, which is a very common form, has a
ground-substance containing gelatin, consisting mainly of numerous
fibres, usually arranged in bundles. Thicker yellow elastic fibres
may be present among the others, and may be so numerous as to
give the entire tissue an elastic character. Associated with fibrous

FIG. 12. Diagram to illustrate the structure
of glands. A, unicellular glands in an
epithelium ; B, unicellular glands lying
below epithelium and communicating
with the surface by narrow processes
(ducts) ; C, group of gland-cells ; D,
group of gland-cells lining a depression ;
E and F, simple multicellular gland ;
G, branched multicellular gland. (From
Lang.)

26

ZOOLOGY

SECT.

tissue, and produced by modification of its cells, is adipose or fatty
tissue (Fig. 15), which consists of masses of large cells in which the
protoplasm has more or less completely become replaced by fat,

b,

J

FIG. 13. Gelatinous connective tissue of a Jelly-fish ; e, epithelium ; g, gelatinous matrix
62, branching cells ; ej\ elastic fibres. (From Lang's Comparative Anatomy.)

the cells being bound together into groups and masses or lobules
by means of fibrous connective tissue.

In the case of cartilage, the matrix is of a firm but elastic

\rni7

"Fio. 14. Reticular connective tissue. (From Lang.)

character, sometimes quite homogeneous in appearance (hyaline
cartilage, Fig. 16), sometimes permeated by systems of fibres (fibro-
cartilaf/e, Fig. 17), which may be of an elastic nature (yellow elastic

STRUCTURE AND PHYSIOLOGY OF ANIMALS

27

cartilage}. The cells are usually rounded, and as a rule several
occur together in spaces scattered through the matrix ; sometimes
condensation of the matrix round each of the spaces in which the
cells are contained forms a cell-capsule. The outer surface is

Fia. 15. Fatty tissue ; F, fat-cells ; B, connective-tissue fibrils. (From Lang, after Eanvier.)

covered over by a fibrous membrane \hQpcrichondrium. Carti-
lage is frequently hardened by the deposition in the matrix of salts
of lime and is then known as calcified cartilage.

In lone or osseous connective tissue (Fig. 18) the matrix is exceed-
ingly dense and hard owing to its being strongly impregnated with
carbonate and phosphate of lime. It consists typically of numer-
ous thin lamellae, which are arranged partly parallel with the sur-
face, partly concentrically around certain canals the Haversian
canals (c) in which blood-vessels lie. The cells, or lone-corpuscles, lie

FIG. 10. Hyaline cartilage.

Fia. 1 7. Fibro-cart ilage.

in minute spaces the lacunce between the lamellae, and a system
of exceedingly fine channels the canaliculi extend from lacuna
to lacuna, containing fine protoplasmic processes by means of which
neighbouring cells are placed in communication with one another
The outer surface of the bone is covered by a vascular fibrous

28

ZOOLOGY

SECT.

membrane the periosteum which takes an active part in its
growth and nutrition.

The connective tissues are all more or less passive in the
functions which they perform, serving mainly for support and for
binding together the various organs. Muscular tissue, on the

other hand, has an active part to
play this being the tissue by
means of which, in general, all
the movements of the body of
an animal are brought about.
Muscular tissue varies greatly in
minute structure in different
groups of animals, and even in
different parts of the same ani-
mal. It consists of microscopic
fibres aggregated together into
large bundles or layers. These
fibres are composed of a sub-
stance the muscle-substan ce
which when living has the special
property of contractility, contract-
ing or becoming shorter and
thicker on the application of a
stimulus. There are two princi-
pal varieties of muscular tissue
to be distinguished, termed re-
spectively non-striated and striated
muscle. Each fibre of non-striated
muscle (Fig. 19) is usually a
single, greatly elongated cell,
sometimes brai::hed, with a single
nucleus ; it may contain a core
of unaltered protoplasm, or all
except the nucleus may be altered
into muscle-substance ; cross-
striation is absent. A fibre of
striated muscular tissue (Fig. 20)
is formed by the close union
of several cells which are repre-
sented by their nuclei (n). Some-
times there is a core of proto-
plasm ; but more usually the entire fibre is composed of muscle-
substance, with perhaps a remnant of protoplasm in the neigh-
bourhood of each nucleus. The substance of the fibre is crossed
by numerous transverse bands and striae, the precise significance
of which is a matter of controversy. The fibre is usually en-
closed in a delicate sheath the sarcolemma. Striated muscular

pplPSaJS r*lPw7 : T&

Fid. 18. Transverse section of compact
bone, a, lamellse concentric with the
outer surface ; b, lamellae concentric
with the surface of the marrow cavity ;
c, section of Haversian canals ; c', sec-
tion of a Haversian canal just dividing
into two ; rf, interstitial lamellae. (From
Huxley's Lessons in Physiology.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

29

tissue is specially characteristic of parts in which rapid movement
is necessary.

The principal elements of nervous tissue are nerve-cells and
nerve-fibres.

Nerve-cells (Fig. 21) vary greatly in form ; they are relatively

-71

FIG. 19. Non-striated muscle-cell ; /, substance of fibre ; n, nucleus ; p, unaltered protoplasm in
the neighbourhood of the nucleus. (From Huxley's Lessons in Physiology.)

large cells with large nuclei and one or several processes, one of
which is always continuous with a nerve fibre.

The nerve-fibres (Fig. 22), which are to be looked upon as greatly
produced processes of nerve-cells, are arranged for the most part
in strands which are termed nerves. The fibres themselves vary
greatly in structure in different classes of animals. In the higher
animals the most characteristic form of nerve-fibre is that which is
termed the medullated nerve-fibre. In this there is a central
cylinder the axis-cylinder or neuraxis (A, ax) which is the

A

B

-rz

IB*/

' 6

*l /

Fio. 20. Striated muscle. A, part of a muscular fibre of a Frog; B, portion of striated muscle
teased out to show separation into fibrillfe. (From Huxley's Lessons in Physiology.)
b, d, g, transverse bands and striae ; n, nuclei.

essential part of the fibre and is made up of numerous extremely
fine primitive fibrillcc ; this is surrounded by a layer of a white
glistening material the white substance of Schivann or medullary
sheath (med), enclosed in turn in a very delicate membrane the
neurilemma (ncur^).

The blood, the lymph, and other similar fluids in the body of an
animal may be looked upon as liquid tissues, having certain cells

30

ZOOLOGY

SECT.

-the corpuscles disseminated through a liquid plasma, which
takes the place of the ground-substance of the connective tissues.

ax
\

med

, ,

/

tr

/ B

'

i.

e

FIG. 21. Nerve-cells. A, multipolur ;
B, bipolar.

FIG. 22. Nerve-fibres. A, mcdullated ;
B, iion-medulated ; ax, neuraxis ;
med, medullary sheath; ncur,
neurilemnia.

In a large proportion of cases such corpuscles are similar to
Amoebae in their form and movements (amoeboid corpuscles, leuco-
cytes). In the blood of Vertebrates leucocytes occur along with
coloured corpuscles of definite shape containing the red-colouring
matter (Ju&rnoglMn) of the blood. The leucocytes are able, like
Amoebae, to ingest solid particles, and under certain conditions a

number of them may unite to-
gether to form a single mass of
protoplasm with many nuclei,
termed a plasmodium.

The characteristic cells of the
reproductive tissues are the ova
and the spermatozoa or sperms. The
ova (Fig. 6), when fully formed, are
relatively large, usually spherical
cells, sometimes composed entirely
of protoplasm, but usually with an
addition of nutrient food-yolk. Each

OVUm > aS Already mentioned, 611-

closes a large nucleus (germinal
vesicle) and in the interior of that
one or more nucleoli or germinal

S p fa ^l ie S p erms (Fig. 23) ai'6

extremely -minute bodies, nearly

always motile, usually slender and whip-like, tapering towards
one extremity, and commonly with a rounded head at the other.

FIQ. 23. Various forms of spermatozoa,
a, of a Mammal ; b, of a Turbellariaii
worm ; c, and d, and e, of Nematudo
worms ; /, of a Crustacean ; g, of a
Salamander ; h, the commonest form
with oval head and lon# flagellum.
(From Lang's Comparative Anatomy.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 31

The sperms are developed by a succession of cell-divisions from
certain cells the primitive male cells similar in character to
immature ova.

5. ORGANS.

The chief systems of organs of an animal are the integumen-
tary, the skeletal, the muscular, the alimentary or digestive, the
vascular, the respiratory, the nervous, the excretory, and the repro-
ductive.

The skin or integument consists in the majority of animals
of a cellular membrane the epidermis to which reference has
already been made, with, superficial to it, in many animals, a non-
cellular layer the cuticle, and below it usually a fibrous layer which
is known as the dermis. The epidermis may consist of a single
layer or may be stratified ; it is frequently ciliated, and some of
its cells frequently assume the form of unicellular glands. Modi-
fication of its superficial layers of cells gives rise frequently to the
formation of hard structures contributing to the development of
an cxoskeleton (vide infra).

The cuticle, when present, varies greatly in thickness and con-
sistency. Sometimes it is very thin and delicate ; in many
animals it becomes greatly thickened and hardened so as to form
a strong protecting crust, sometimes of a material termed chitin,
somewhat akin to horn in consistency, sometimes solidified by the
deposition of calcareous salts. The cuticle is to be looked upon as
a secretion from the cells of the epidermis; but the term is
frequently applied in the case of the higher animals in which a
cuticle in the strict sense of the term is absent either to a super-
ficial part of the epidermis, in which the cells have become altered
and horny, or to the whole of that layer.

The layer or layers of the integument situated beneath the
epiderm consist of fibrous connective tissue and muscular fibres,
constituting, as mentioned above, the derm or dermis.

The term skeleton or skeletal system is applied to a system
of hard parts, external or internal, which serves for the protection
and support of softer organs and often for the attachment of muscles.
This system of hard parts may be external, enclosing the soft
parts, or it may lie deep within the latter, covered by integument
and muscles : in the former case it is termed an cxoskelcton or
external skeleton ; in the latter an endoskeleton or internal skeleton.
In many groups of animals both systems are developed. An
exoskcleton is formed by the thickening and hardening of a part
or the whole of one of the layers of the integument enumerated
above ; or more than one of these layers may take part in its
formation. In many invertebrate animals, such as Insects.
Crustaceans, and Molluscs, it is a greatly thickened and hardened

32 ZOOLOGY SECT.

cuticle which forms the exoskeleton. The horny scales of Reptiles,
the feathers of Birds, and the fur of Mammals are examples of
an exoskeleton derived from the epidermis, while the bony
shell of Turtles and the bony scales of Fishes are examples of a
dermal exoskeleton.

When an endoskeleton is present, it usually consists either of
cartilage or bone or of both ; but sometimes it is composed of
numerous minute bodies (spicules) of carbonate of lime or of a
siliceous material.

A skeleton, whether internal or external, is usually composed
of a number of pieces which are movably articulated together,
and which thus constitute a system of jointed Severs on which the
muscles act.

The alimentary or digestive system consists of a cavity or
system of cavities into which the food is received, in which it is
digested, and through the wall of which the nutrient matters are
absorbed ; together with certain glands.

In the lowest groups in which a distinct alimentary or enteric cavity
is present it is not distinct from the general cavity of the body ;
but in all higher forms there is an enteric canal which is sus-
pended within the cavity of the body, and the lumen of which is
completely shut off from the latter. It may have simply the form of
a sac or bag with a single opening which serves both as mouth and
anus ; in other cases the sac becomes branched and may take the
form of a system of branching canals. In most animals, however,
the alimentary canal has the form of a longer or shorter tube
beginning at the mouth and ending at the anal opening (Fig. 24).
In most cases there are organs in the neighbourhood of the mouth
serving for the seizure of food ; these may be simply tentacles or
soft finger-like appendages, or they may have the form of jaws, by
means of which the food is not only seized, but torn to pieces or
pounded up to small fragments in the process of mastication. The
alimentary canal itself is usually divided into a number of regions
which differ both in structure and in function.

In general there may be said to be three regions in the ali-
mentary canal the ingestive, the digestive and absorbent, and the
egestive or efferent. The ingestive region is the part following
behind the mouth, by which the food reaches the digestive and
absorbent region. But, besides serving as a passage, it may also
act as a region in which the food undergoes certain processes,
chiefly mechanical, which prepare it for digestion. This ingestive
region may comprise a mouth-cavity or l>uccal cavity, a pharynx,
an cesopkagus or gullet, with sometimes a muscular gizzard which
may be provided with a system of teeth for the further breaking
up of the food, and sometimes a crop or food -pouch.

The digestive and absorbent region is the part in which the
chemical processes of digestion go on, and from which takes place

STRUCTURE AND PHYSIOLOGY OF ANIMALS

33

the absorption of the digested food-substances. Into this part are
poured the secretions of the various digestive glands, which act on
the different ingredients of the food so as to render them more
soluble. Through the lining membrane of this part the digested
nutrient matter passes, to enter the blood-system. This region
may present a number of subdivisions; nearly always there are
at least two a wide sac, the stomachy and a narrow tube, the
intestine.

The egestive or efferent region of the alimentary canal is the
posterior part of the intestine, in which digestion and absorption
do not go on, or only go on to a limited extent, and which serves

FIG. 24. General view of the viscera of a male FrogTj from the right side, a, stomach ; I, urinary
bladder ; c, small intestine ; ct, cloacal aperture ; d. large intestine ; e, liver ; /, bile-duct ;
(j, gall-bladder ; //, spleen ; i, lung ; Ic, larynx ; f, fat-body ; in, testis ; n, ureter ; o, kidney ;
p, pancreas ; s, cerebral hemisphere ; sp, spinal cord ; t, tongue ; u, auricle ; ur, urostyle ;
r, ventricle ; vs, vesicula seminalis ; w, optic lobe ; x, cerebellum ; y, Eustachian recess ;
z, nasal sac. (From Marshall.)

mainly for the passage to the anal opening of the faeces or
unabsorbed effete matters of the food.

The whole of the interior of the alimentary canal is lined
by a layer of cells the alimentary or enteric epithelium. The
form and arrangement of the cells of this epithelium vary greatly
in different groups of animals. Usually, they are vertically
elongated, prismatic or columnar, or pyramidal in shape ;
frequently they are ciliated. In some lower forms, the cells lining
the alimentary cavity have the power, like Amoeba, of thrusting
forth processes of their protoplasm (Fig. 11, Ji), and of taking minute
particles of food into their interior to become digested and absorbed
(intracellidar digestion). Sometimes they are all more or less
active in secreting a fluid destined to act on the food and render
it more soluble ; sometimes this function is confined to certain of
the cells, which have a special form ; very often the secreting cells

VOL. I

D

34 ZOOLOGY

line special little pouch-like, simple or branched glands, opening
by a passage or dud into the main cavity of the alimentary
canal. Besides these glands formed from specially modified cells
of the enteric epithelium there are nearly always present certain
large special glands, separate from the alimentary canal itself, but
opening into it by means of ducts. Of these the most generally-
occurring are the glands termed salivary glands, liver, and pancreas.
The salivary glands have the function of secreting a fluid called
the saliva, which, in many cases at least, has a special action on
starchy matters, converting them into sugar. The ducts of these
glands open always, not into the digestive, but into some part of
the ingestive region of the alimentary system.

The most important function of the liver properly so called
is one distinct from the process of digestion ; its secretion the
l)ile has, however, at least a mechanical effect on this process,
and assists the secretion of the pancreas in its effects upon fat.
In lower forms the organ to which the term liver is commonly
applied appears in many cases to combine the functions of a true
liver with that of a pancreas, and is thus more appropriately
termed hepato-pancreas or liver -pancreas.

The pancreas secretes a fluid the pancreatic juice which has
a very important effect in digestion. It renders substances of the
nature of albumins soluble by converting them into modifications
termed peptones ; it converts starch into the soluble substance
sugar ; it acts on fatty matters in such a way as to convert them
into emulsions which are capable of being taken up and absorbed,
and it effects the splitting up of part of the fat into fatty acids
and glycerine.

When the food has been acted on by the various digestive
secretions, the soluble part of it is fitted to be taken up. and
absorbed through the wall of the alimentary canal into the blood
(in animals in which a blood-system exists), or into the fluid
which takes its place. In the higher animals a part of the
soluble matter of the food passes directly into the blood contained
in the blood-vessels ; while another part is taken up by a set of
special vessels, the lacteals which are a part of the lymphatic
system, and reaches the blood indirectly.

In some of the lower groups of animals there is no system of
blood-vessels, and the nutrient matter of the food, absorbed
through the alimentary canal, merely passes from cell to cell
throughout the body, or is received into a space or series of spaces
containing fluid intervening between the alimentary canal and the
wall of the body. But in the majority of animals there is a system
of branching tubes containing a special fluid the blood, and it is
into this that the nutrient matter absorbed from the food sooner
or later finds its way. The blood has for one of its principal
functions the conveyance .of the nutrient matters from the

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 35

alimentary canal throughout the body, so that the various organs
may select from it the material which they require for the carrying
on of their functions. To carry out this office the blood is con-
tained in a complicated system of branching tubes or Mcod-vessels .

The essence of the process of respiration, as we have already seen,
is an interchange of oxygen and carbonic acid which takes place
between the tissues of an organism and the surrounding medium,
whether air or water. During the vital changes which go on in
the bodies of all animals, as in Amoeba, oxygen is constantly
being used up and carbonic acid being formed. The necessary
supply of oxygen has to be got from the air, or, in the case of
aquatic animals, from the air dissolved in the surrounding water.
At the same time the carbonic acid has to be got rid of. In the
lowest animals as for instance Amoeba, and. many of higher
organisation the oxygen passes inwards and the carbonic acid
outwards through the general surface of the body. But in the
great majority of animals there is a special set of organs the
organs of respiration having this particular function. In some
animals these organs of respiration are processes, simple or
branched, lined by a very delicate membrane, and richly supplied
with blood-vessels. Such processes are called gills or branchice ;
they are specially adapted for the absorption of oxygen dissolved
in water.

In other animals the oxygen is obtained directly from the air ;
and in such air-breathing forms the organ of respiration is very
often a sac, either simple or compound, termed a lung. The
interior of this sac is lined with an epithelium of extreme delicacy,
immediately outside of which is a network of microscopic blood-
vessels or capillaries with thin walls ; and the oxygen readily passes
from the air in the cavity of the lung through its lining and
the thin wall of the blood-vessel into the blood. In other air-
breathing forms the organs of respiration are trachea:, which are
ramifying tubes, by means of which the air is conveyed to all parts
of the body. In such forms, of which the Insects are examples, the
air is conveyed, by means of these tubes, from openings on the
surface of the body to all parts, and respiration goes on in all the
organs.

In order that the air or water in contact with the surface of the
lungs or gills may be renewed, there are usually special mechanical
arrangements. In many gill-bearing animals the gills are attached
to the legs, and are thus moved about when the animal moves its
limbs. In others certain of the limbs are constantly moving in
such a way as to cause a current of water to flow over the gills.
In air-breathing forms there is usually a pumping apparatus, by
means of which the air is alternately drawn into and expelled
from the lungs.

In a great number of animals there is in the blood a substance

D2

36 ZOOLOGY SECT.

called haemoglobin, which has a strong affinity for oxygen ; and the
oxygen from the air, when it enters the blood, enters into a state
of loose chemical combination with it. In this state, or simply
dissolved in the fluid plasma of the blood, the oxygen is conveyed
throughout the body.

Thus the blood, besides receiving the solid and liquid food from
the alimentary canal and carrying it throughout the body for
distribution, receives also the oxygen or gaseous food, and supplies
it to the parts requiring it. In all parts of the body in which
vital action is taking place chemical changes are constantly going
on. These chemical changes in the tissues, having for their result
the production of heat, motion, secretion, and nerve-action, are
for the most part of the nature of oxidations, and involve a constant
consumption of oxygen; while a product which becomes formed
as a result of this action is carbonic acid gas.

To carry out all the functions which it has to perform as a
distributor of nourishment and oxygen and a remover of carbonic
acid, the blood has to be moved about through the vessels to
circulate throughout the various organs. In the lowest forms in
which a definite blood-system is to be recognised, this movement
is effected in great measure by the general movements of the
body of the animal. In others certain of the vessels contract and
drive the blood through the system ; such contractions are of a
peristaltic character, the contractions being of the nature of con-
strictions running in a definite direction along the course of the
vessel, with an effect similar to that produced by drawing the
hand along a compressible india-rubber tube.

In all higher forms the movement of the blood is effected by
means of a special organ the heart. The heart is a muscular
organ which by its contractions forces the blood through the
system of vessels. In its simplest form it usually consists of two
chambers, both with muscular walls, the one, called the auricle,
receiving the blood and driving it into the other, which is called
the ventricle. The latter, in turn, when it contracts, drives the blood
through the vessels to the various parts of the body the return
of the blood backwards to the auricle from the ventricle being
prevented by the presence of certain valves, which act like folding
doors opening from the auricle towards the ventricle, but closing
when pressure is exerted in the opposite direction. In the higher
animals the heart becomes a more complex organ than this, with a
larger number of chambers and a more elaborate system of valves.
Carbonic acid, as already mentioned, is a waste-product con-
stantly being produced in the tissues and being carried off by the
blood to pass out by the gills or lungs. Besides the carbonic
acid, there are constantly being formed waste-substances of another
class viz., substances containing nitrogen, of which uric acid and
urea are the principal ultimate forms. These are separated from

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 37

the blood and thrown out of the body by a distinct set of organs
called renal organs, or organs of urinary excretion. The

form of these organs varies greatly in the different groups;
in many cases they are more or less intimately connected with
the genital system.

In place of the simple contractions and extensions of the proto-
plasm which constitute the only movements of Amoeba, the higher
animals are capable of complex and definite movements. These
are brought about by the agency of a set of organs termed the
muscles. A muscle is a band or sheet of muscular fibres
endowed in the living state with the property of contractility, by
virtue of which, when stimulated in certain ways, it contracts in
the direction of its length, becoming shortened, and, at the same
time, thickened (Fig. 25). The extremities of the muscle are

FIG. 25 . Bones of the human arm and fore-arm with the biceps muscle, showing the shortening
and thickening of the muscle during extraction and the consequent change in the relative
position of the bones viz., flexion of the fore-arm on the upper arm. (From Huxley's Physiology.)

frequently composed, not of contractile muscular fibres, but of a
form of strong fibrous connective tissue the tendon of the muscle.
The ends of the muscle are usually firmly attached to two different
parts of the jointed framework or skeleton, external or internal;
and, when the muscle contracts and becomes shortened, these two
parts are drawn nearer to one another.

In all but the most lowly-organised animals there is a system
of organs the nervous system by means of which a communi-
cation is effected between the various parts of the body, enabling
them to work in harmony, and by means of which also a communi-
cation is established between the organism and the external
world. The two essential elements of the nervous system the
nerve-cells and nerve-fibres have a regular arrangement which
varies in the different animal types both as regards structural
details and the relations borne to the other systems of organs ;
but there are to be recognised two chief parts or sets of parts-
the central and the peripheral.

38

ZOOLOGY

SECT.

vo

c

The central parts of the nervous system consist (Fig. 26) of
certain aggregations of nerve-matter known as nerve-ganglia,
containing a large number of nerve -eel Is ; a relatively large mass

of this matter may be

/

collected together to
form a "brain. To or
from these central parts
pass all the systems of
nerve-fibres, constituting
the peripheral part of the
system ; the former have
the office both of re-
ceiving impressions con-
veyed by the nerve-fibres
from the surface, from
the organs of special
sense, and from the in-
ternal organs, and of
sending off messages
through similar channels
to the various parts of
the body to muscles, to
glands, to alimentary
canal, and to vascular
system. When a move-
ment is to be effected
a message passes from
the nerve-centre along a
nerve-fibre to a muscle
and causes it to contract ;
when an organ requires
the amount of blood sup-
plied to it to be in-
creased or diminished a
message is conveyed
along a nerve-fibre and
causes the dilatation or
contraction of the blood-
vessels of the part ; and
a similar initiatory or
controlling influence is
exerted over the activities
of all the organs.

In certain groups of animals all the impressions from the
external world are received through the integument of the general
surface, and this is the case in all animals with the general
impressions of touch and of heat and cold. The sensitiveness of

sc.

FIG. 26. Nervous system of the Frog.
Howes's At lax.)

(From

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 39

the integument to such general impressions may be increased by
the presence in it of a variety of tactile papillae or corpuscles
having nerve-fibres terminating in them. In most animals, how-
ever, there are certain organs, the organs of special sense,
adapted to receiving impressions of special kinds eyes for the
reception of the impressions produced by light, ears for the recep-
tion of those produced by the waves of sound, olfactory organs or
organs of smell, and gustatory organs or organs of taste. The most
rudimentary form of eye is little more than a dot of pigment
which absorbs some of the rays of bright light these producing
a nerve-disturbance in certain neighbouring nerve-cells. To this
may be added clear, highly-refracting bodies which intensify the
effect. In the higher types of eye there are the same character-
istic parts the clear, highly-refracting substance, the pigment, and
the nerve-cells ; but each has undergone a development resulting
in the construction of an organ adapted to the reception of light-
impressions of a very definite character. The highly-refracting body
assumes the form of a lens for the focussing of the light-rays ; the
nerve-cells are arranged within a regular layer, the retina, from
which nerve-fibres pass to the central part of the nervous system ;
the pigment is so arranged as to absorb the light-rays and prevent
their passage beyond the retina, and in certain cases also lines a
diaphragm, the iris, with a central aperture through which the
rays of light are admitted to the central parts of the eye. In
some animals (Insects, Crustacea) the eye consists of a very large
number of independent elements, each with its refracting apparatus,
its nervous element, and its absorbing pigment.

The ear in its simplest form is a membranous sac or otocyst with
internally projecting stiff cilia, and containing a liquid in which
there lie a number of particles of carbonate of lime. The sound-
waves evidently set in vibration the liquid and its contained cal-
careous particles, and by means of these vibrations acting on the
cilia, an impression of a definite character is produced in the cells
of a neighbouring nerve-ganglion. In higher forms the apparatus
for receiving the vibrations becomes extremely complex, and there
is elaborated a nervous mechanism by which sounds of different
pitch and intensity produce impressions of a distinct character.
The organ of hearing usually possesses the additional function
of an organ ministering to the sense of rotation, and thus has an
important part to play in the maintenance of the equilibrium of
the body.

The essential elements of the reproductive organs the ova
and spermatozoa have already been briefly alluded to (p. 30).
The ova are developed in an organ termed the ovary, and the
sperms in an organ called the spermary or testis. Sometimes
ovaries and testes are developed in the same individual, when the
arrangement is termed monoecious or hermaphrodite ; sometimes

40 ZOOLOGY SECT.

the ovaries occur in one set of individuals the females and the
testes in another set the males, when the term unisexual or
dicecious is employed. Very frequently the male differs from the
female in other respects besides the nature of the reproductive
elements in size, colour, and the like ; when such differences are
strongly marked the animal is said to be sexually dimorphic. The
ova and sperms are usually conveyed to the exterior by canals
or ducts the ovarian ducts or oviducts, and the testicular ducts,
spcrmiducts, or vasa defer enlia. In some instances the ova are
impregnated after being discharged from the oviducts, and the
development of the young takes place externally; in other cases
the impregnation takes place in the oviduct, and the young
become fully developed in the interior of a special enlargement
of the oviduct termed the uterus. In the former case the animal
is said to be oviparous, in the latter viviparous ; but there are
numerous intermediate gradations between these two extremes.

6. THE REPRODUCTION OF ANIMALS.

In a limited number of groups of animals reproduction takes
place by means of cells corresponding to ova developed in organs
similar to ovaries, but without impregnation by means of sperms.
This phenomenon is known as parthenogenesis (cf. p. 21).

Besides the sexual process of reproduction by means of ova and
spermatozoa, there are in many classes of animals various asexual
modes of multiplication. One of these the process of simple
fission has been already not iced in connection with the reproduction
of Amoeba. The formation of spores is an asexual mode of multi-
plication which occurs only in the Protozoa, and will be described
in the account of that group. Multiplication by budding takes
place in a number of different classes of animals. In this form of
reproduction a process or bud (Fig. 27, ltd) is given off from some
part of the parent animal ; this bud sooner or later assumes the
form of the complete animal, and may become detached from
the parent either before or after its development has been
completed or may remain in permanent vital connection with the
parent form.

When the buds, after becoming fully developed, remain in vital
continuity with the parent, a sort of compound animal, consisting
of a greater or smaller number of connected units, is the result.
Such a compound organism is termed a colony, and the component
units are termed zooiil*. In some cases such a colony is produced
by a process which is more correctly termed incomplete fission
than budding.

Alternation of generations ; heterogamy ; paedogenesis.-
In the life-history of a considerable number of animals, a stage in
which reproduction takes place by a process of budding or fission

STRUCTURE AND PHYSIOLOGY OF ANIMALS

41

alternates with a stage in which there occurs a true sexual mode
of reproduction. Such a phenomenon is termed alternation of
generations or metagenesis. The term heterogamy is applied to
cases in which two different sexual generations usually a true
sexual and a parthenogenetic alternate with one another.
Pcedogenesis, or the development of young by a sexual process from

FIG. 27. Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted,
showing multiplication by budding. W. 1 6t/.- W.3 buds in various stages of growth. (From
Parker's Biology.)

individuals that have not attained the adult condition, is a
phenomenon which is to be observed in some groups of animals.

7. SYMMETRY.

The general disposition or symmetry of the parts in an animal
presents two main modifications the radial and the bilateral.
The gastrula (p. 23) is the simplest and most generalised form among
multicellular animals or Metazoa ; but no adult animal retains
this simple shape. In the gastrula we may imagine a central
primary axis (Fig. 28, All) passing through the middle of the blas-
topore and of the archenteric cavity, and a series of secondary axes
(ab, cd,) running at right angles to this to the outer surface. In a
symmetrical gastrula the secondary axes would be all equal. Many

42

ZOOLOGY

SECT,

animals are in the adult condition similar in their symmetry to the
gastrula, except that there are special developments along a series
of regularly arranged radiating secondary axes ; these radial
developments may be in the form of tentacles or radially arranged
processes (Fig. 29), or may assume the character of a radial arrange-
ment of internal parts. Such an animal is said to be radially
symmetrical. The body of a radially symmetrical animal is capable

FIG. 28. Diagram of the axes of the body.
AB, primary axis ; ab, cd, secondary
axes. The lower figure is a transverse
section of the upper one showing its two
secondary axes. (From Gegenbaur.)

Fio. 20. Haciial symmetry. Letters as
in Fig. L'S. The processes at A are
the tentacles ; the lower figure repre-
sents the upper or oral surface. (From
Gegenbaur.)

of being divided into a series of equal radial parts or antimeres,
each of which is symmetrically disposed with regard to one of the
secondary or radial axes.

In animals which are not permanently fixed, locomotion usually
takes place in the direction of the primary axis of the body, and
one side, habitually directed downwards, becomes modified differ-
ently from the other which is habitually directed upwards : lower
or ventral surface becomes distinguishable from an upper or dorsal.
Thus the radial symmetry is now disturbed ; the secondary axes
have become unequal ; the dorso-ventral or vertical secondary axes

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 43

are, to a greater or less extent, different from the transverse or
horizontal secondary axes, and the body of an animal having such
a disposition of the parts is divisible into two equal lateral halves
or hemisomes by a median vertical plane passing through the
primary axis. This is the bilateral symmetry observable in all but
a few types of animals.

Sometimes the bilaterally symmetrical animal is unsegmented ;
sometimes it is divided into a series of segments or metamcrcs.
A distinct head may be present or absent. The head end or
anterior end is that which, save in exceptional cases, is directed
forwards in locomotion. It is towards this end that the organs of
special sense are situated, as well as the opening of the mouth and
the organs for the prehension and mastication of food. A head is
developed when the anterior part bearing these structures is
marked off externally from the rest. In segmented animals the
head consists of a number of segments amalgamated together, and
it contains the brain or the principal central ganglia of the nervous
system.

8. THE PRIMARY SUBDIVISIONS on PHYLA OF THE ANIMAL

KINGDOM.

The various systems of organ) digestive, circulatory, nervous,
excretory, etc. present under one form or another in all the higher
groups of animals, are variously arranged and occupy various
relative positions in different cases, producing a number of widely
different plans of animal structure. According as their structure
conforms to one or another of these great plans, animals are referred
to one or another of the corresponding great divisions or phyla of
the animal kingdom. That animals do present widely differing
plans of structure is a matter of common knowledge. We have
only to compare the true Fish, such as Cod, Haddock, etc., in a fish-
monger's shop with the Lobsters and the Oysters, to recognise the
general nature of such a distinction. The first-named are charac-
terised by the possession of a backbone and skull, with a brain and
spinal cord, and of two pairs of limbs (the paired fins) ; they belong
to the great vertebrate or backboned group the division Vertc-
Irata, of the. phylum Chordata. The Lobsters, on the other hand, in
which these special vertebrate structures are absent, possess a body
which is enclosed in a hard jointed case, and a number of pairs of
limbs also enclosed in hard jointed cases and adapted to different
purposes in different parts of the body some being feelers, others
jaws, others legs : their general type of structure is that which
characterises the phylum Arthropoda. The Oysters, again, with
their hard calcareous shell secreted by a pair of special folds
of the skin constituting what is termed the mantle, and with a
special arrangement of the nervous system and other organs which

44 ZOOLOGY SECT, i

need not be described here, are referable to the phylum Mollusca.
Other familiar animals are readily to be recognised as belonging to
one or other of these great phyla. A Prawn, a Crab, a Blue-bottle
Fly, a Spider, are all on the same general plan as the Lobster : they
are jointed animals with jointed limbs, and have the internal
organs occupying similar positions with relation to one another :
they are all members of the phylum Arthropoda. Again, a Mussel,
a Snail, and a Squid are all to be set side by side with the Oyster
as conforming to the same general type of structure : they are all
members of the phylum Mollusca. A Dog, a Lizard, and a Fowl,
again, are obviously nearer the Fish : they all have a skull and
backbone, brain and spinal cord, and two pairs of limbs, and are
members of the great group Chordata.

Altogether twelve phyla are to be recognised, viz. :-

I. Protozoa VII. Molluscoida

II. Porifera VIII. Echinodermata

III. Ccehnterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Nemathelminthes XL Mollusca

VI. Trochelminthes XII. Chordata

But these do not comprise all known animals. There are a
number of smaller groups which are only very doubtfully to be
associated with one or other of the phyla ; and it is in some cases
chiefly to avoid multiplication of the latter that such groups are
not treated as independent. Such forms, until their places are
more definitely fixed, are best dealt with as appendices to the
phyla to which they appear most nearly related.

SECTION II
PHYLUM PROTOZOA

IN the preceding section we learnt the essential structure of an
animal cell, and it was pointed out that in the lowest organisms
the entire individual consists of a single cell. All such unicellular
animals are placed in the lowest primary subdivision of the animal
kingdom the phylum Protozoa.

We have also learnt that cells vary considerably in character.
They may be amoeboid or capable of protruding temporary processes
of protoplasm called pseudopods ; flagellate, or produced into one
or more always a small number of threads having an intermit-
tent lashing movement ; ciliated, or produced into numerous
rhythmically moving threads of protoplasm ; or encysted, the proto-
plasm being enclosed in a cell- wall. Moreover, under certain
circumstances, amoeboid cells may fuse with one another to form
a plasmodium.

These well-marked phases in the life of the cell allow us to
divide the Protozoa into subdivisions called Classes. The same
organism may be amoeboid, flagellate, encysted, and plasmodial
at various stages of its existence, but nevertheless we find
certain forms in which the dominant phase in the life-history is
amoeboid, others which are characteristically flagellate or ciliated,
others again in which the tendency to form plasmodia is a
distinctive feature. In this way five well-marked groups of
unicellular organisms may be distinguished.

Class 1. RHIZOPODA. Protozoa in which the amoeboid form is
predominant, the animal always forming pseudopods. Flagella
are often present in the young, and occasionally in the adult.
Encystation frequently occurs.

Class 2. MYCETOZO A.- -Terrestrial Protozoa in which the plas-
modial phase is specially characteristic, as also is the formation
of large and often complex cysts.

Class 3. MASTIGOPHOUA. Protozoa in which the flagellate form

46 ZOOLOGY SECT.

is predominant, although the amoeboid and encysted conditions
frequently occur.

Class 4. SPOROZOA. Parasitic Protozoa without special loco-
motive parts in the adult. Encystation is almost universal, and
the young may be flagellate or amoeboid.

Class 5. INFUSORIA. Protozoa which are always ciliated, either
throughout life or in the young condition.

CLASS I. RHIZOPODA.

1. EXAMPLE OF THE CLASS Amoeba proteus.

Amoaba has been fully described in the preceding chapter ; it
will therefore be unnecessary to do more than recapitulate the
most essential features in its organisation.

It is an irregular mass of protoplasm (Fig. 30, E) about J mm.
in diameter, produced into irregular processes or pseudopods (psd)
of variable size and form and capable of being protruded and
retracted, often with considerable rapidity. The protoplasm is
divisible into a granular internal substance or endosarc and a clear
outer layer or ectosarc ; the difference between the two is hardly a
structural one, but depends simply on the accumulation of granules
in the central portion. The granules are, for the most part, various
products of metabolism proteinaceous or fatty.

Imbedded in the endosarc is a large nucleus (nu), of spherical form,
consisting of a clear achromatic substance, enclosed in a membrane,
and containing minute granules of chromatin. The contractile
vacuole (c. vac.),a, very characteristic structure of the Protozoa, lies
in the outer layer of the endosarc, and exhibits rhythmical move-
ments, contracting and expanding at more or less regular intervals.

Amoeba feeds by ingesting minute organisms (Fig. 30, c,/. vac.)
or fragments of organisms i.e., by enveloping them in its substance,
retaining them until the proteids they contain are dissolved and
assimilated, and then crawling away and leaving the undigested
remnants behind.

Amoeba are sometimes found to undergo encystation ; the
pseudopods are withdrawn and the protoplasm surrounds itself
with a cell- wall or cyst (D, cy\ from which, after a period of rest,
it emerges and resumes active life. The cyst is formed of a
ckitinoid material i.e., a nitrogenous substance allied in composi-
tion to horn and to the chitin of which the armour of Insects,
Crayfishes, etc., is composed.

Reproduction takes place by simple or Unary fission ; direct or
amitotic division of the nucleus is followed by division into two of
the cell-body (i). Occasionally two Amoebae have been observed to

II

PHYLUM PROTOZOA

conjugate or undergo complete fusion, but nothing is known of the
result of this process or of its precise significance in this particular

case.

A

FIG. 30. Amoeba. A, A. quarta; B, the same killed and stained ; C, A. protcus ; D, encysted
specimen ; E, A. proteus ; F, nucleus of same, stained ; G, A. verrucosa ; H, nucleus of same,
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pseudopods ;
c. vac. contractile vacaole ; cy. cyst ; /. vac. food-vacuole ; nu. nucleus (numerous in
A. quarta) ; psd. pseudopod. (From Parker's Biology, after Leidy, Gruber, and Howes.)

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Rhizopoda differ among themselves in the character of
their pseudopods, which may be short and blunt or long and

48 ZOOLOGY SECT.

delicate ; in the number of nuclei ; and in the presence or absence
of a hard shell within or around the protoplasm. The following
four orders may be distinguished :

ORDER 1. LOBOSA.
Rhizopoda with short, blunt pseudopods.

ORDER 2. FORAMIMFERA.

Shelled Rhizopoda with fine, branched, and anastomosing
pseudopods.

ORDER 3. HELIOZOA.
Rhizopoda with fine,; stiff, radiating pseudopods.

ORDER 4. RADIOLARIA.

Rhizopoda having a shell in the form of a perforated central
capsule, and usually, in addition, a siliceous skeleton : the pseudo-
pods are long and delicate.

Systematic Position of the Example.

Amoeba proteus is one of many species of the genus Amaibcb,
belonging to the family Amcebidce, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell, among the Amcebidse. The genus
Amoeba is distinguished by the presence of one or more nuclei,
and of a contractile vacuole. In A. proteus the pseudopods are
of considerable length and sometimes branched, and there is a
single nucleus, having its chromatin in the form of scattered
granules.

ORDER 1. LOBOSA.

General Structure.- -The members of this group all agree
with Amoeba in essential respects, their most characteristic feature
being the short, blunt pseudopods. The chief variations in struc-
ture upon which the genera and species are founded have to do
with the number and character of the nuclei, the form of the
pseudopods, and the presence or absence of a shell.

In Amoeba itself there may be one (Fig. 30, E) or several (B)
nuclei, the chromatin of the nucleus may be arranged in various
ways (F, H), and the pseudopods may be prolongations of con-

II

PHYLUM PROTOZOA

49

siderable relative size (c), or mere wave-like elevations of the
surface (G). Sometimes specimens are found in which neither
nucleus nor vacuole is present ; these are placed in the genus

FIG. 31. Protamceba primitiva. Showing changes of form and three stages in binary fission.

(After Haeckel, from Parker's Biology.)

Protamosba (Fig. 31). Very probably, however, future investigation
will show this and other non-nucleate forms to possess a potential
nucleus in the form of minute scattered granules of chromatin.

The largest of the naked or shell-less Lobosa is Pelomyxa, which
may be as much as 8 mm. in diameter ; it is multi-nucleate and is

,

further distinguished by the presence of numerous non-contractile
vacuoles in the endosarc.

D

FIG. 32. A, Quadrula symmetric a; 15, Hyalosphenia lata; 0, Arcella vulgaris :

L>, Difflugia pyriformis. (From Lang's Comparative Anatomy.)

Skeleton. --We may understand the relation of the shelled to
the shell-less Lobosa by supposing an Amoeba to draw in the
pseudopods from the greater part of its body, and to secrete, from
that part only, a cell-wall ; such a cell-wall or capsule would differ

VOL. i

E

50

ZOOLOGY

feECT.

from a cyst in having an aperture at one end to allow of the
protrusion of pseudopods from a small naked area. This is exactly
what we find in Arcclla and its allies (Fig. 32, A-c), in which the
shell is chitinoid. A different kind of shell is found in Difflugia
(D), which secretes a gelatinous coating to which minute sand-
grains and other foreign particles become attached.

ORDER 2. FORAMINIFERA

General Structure.- -The members of this order differ from
the Lobosa in the fact that their pseudopods are long and delicate
and unite to form networks ; moreover, with few exceptions, they
agree with Arcella and its allies in possessing a shell. In the
majority of cases this shell is formed of calcium carbonate.

One of the simplest members of the group is Microgromia (Fig.
33). It consists of a protoplasmic body (B), with a single nucleus

FIG. 33. Microgromia socialis. A, entire colony ; B, single zooid ; C, zooid which
has undergone biliary fission, with one of the daughter-cells creeping out of the shell ;
D, flagellula ; c. vac. contractile .vacuole ; nu. nucleus; sh. shell. (From Biitschli's Protozoa,
after Hertwig and Lesser.)

(nu.) and contractile vacuole (c. vac.), enclosed in a chitinoid cell-
wall or shell (sh.) with an aperture at one end through which the
protoplasm protrudes and is produced into delicate radiating
pseudopods. The animal multiplies by binary fission, and the
individuals or zooids thus produced remain united in larger or
smaller clusters, or cell-colonies (A). Sometimes the cell-body of a.
zooid divides and one of the daughter-cells creeps out of the cell-
wall (C), and, after moving about for a time like an Amoeba, draws
in its pseudopods, assumes an oval form, and sends out two
flagella by means of which it is propelled through the water (D).
We shall find other instances in which the young of a Rhizopod is

ii PHYLUM PROTOZOA 51

a, flagellula, i.e. a cell provided with one or more flagella, which,
if its history were not known, would be included among the
Mastigophora.

Platoum (Fig. 34, A) is a form resembling Microgromia, but
illustrating a very interesting type of colony. The protoplasm
flows out of the mouth of the shell in the form of a long plate (B)

O- vac

B

FIG. 34. Platoum stercoreum. A, single zooid ; B, formation of colony ; c. vac. contractile
vacuole ; /. food particles ; nu. nucleus ; 5/1. shell. (From Biitschli's Protozoa, after
Cienkowsky.)

which sends off rounded side branches, and each of these, acquiring
a cell-wall, becomes a zooid of the simple cell-colony.

Gromia (Fig. 35, 1) leads us to the more typical Foraminifera.
The protoplasm of this form protrudes from the mouth (a) of the
chitinoid shell (sA.) and flows around it so that the shell becomes
an internal structure. The pseudopods are very long and delicate
and unite to form a complicated network, exhibiting a streaming
movement of granules and serving, as usual, to capture prey.

Skeleton. Squammulina (Fig. 3 5, c?) differs from Gromia mainly
in having the shell formed of calcium carbonate and possessing the
character of a hollow, stony sphere, with an aperture at one end.
It appears that all the calcareous Foraminifera begin life in this
simple form ; but in the majority of cases the adult structure
attains a considerable degree of complexity. The protoplasm of
the original globular chamber overflows, as it were, through the
aperture ; but, instead of forming an elongated plate from which
side buds are given off, as in Platoum, the extended mass rounds
itself off, and secretes a calcareous shell in organic connection with
the original shell, and communicating with it by the original
aperture. In this way a two-chambered shell is produced, and a
repetition of the process gives us the many-chambered shell found
in most genera. New chambers may be added in a straight line
(Fig. 36, 3\ or alternately on opposite sides of the original
chamber (o), or with each new chamber enclosing its predecessor
(4\ or in a flat spiral, each new chamber being larger than its
predecessor (7, 8), or in a spire in which the newer chambers

E 2

52

ZOOLOGY

SECT.

overlap the older (9, 10), or in an irregular spiral of globular
chambers (6), or in an extremely compact spiral in which the new
chambers completely enclose their predecessors (11\ In all cases

\

\

\

I ,'', f f '

I If//// /

'

^MiL^^-<-v~

i\- ' j HI H \ \\\\

i i i V it) . \

3.Squammulina

4.M i I i o I a

FIG. 35. -Various forms of Foraminifera. In ft, Miliola, a, shows the living animal;
6. the same killed and stained ; a. aperture of shell ; /. food particles ; nu. nucleus ; &h. shell.
(From Biitschli's Protozoa and Claus's Zoology.)

adjacent chambers communicate with one another either by a
single large hole or by numerous small ones : the protoplasm
is thus perfectly continuous throughout the organism. With the

II

PHYLUM PROTOZOA

53

increase in the number of chambers there is a multiplication of
the nucleus (Fig. 35, 4, b, nu).

Not only does the shell increase in size by the formation of new

I.Saccommina

2.Lagena

7. Discorbina

5< Spiroloculma

3.Nodosaria

4.Frondicularia G.CIobigerina

s.sk.

9.Planorbulina

ll.Nummtrlires

FIG. 30. Shells of Foraminifera. In 3, It, and 5, a shows the surface view, and b a section ;
8a is a diagram of a coiled cell without supplemental skeleton ; Sb of a similar form
with supplemental skeleton (s. sk.); and 10 of a form with overlapping whorls ; in lla half
the shell is shown in horizontal section ; b is a vertical section ; a. aperture of shell ; 1 15,
successive chambers, 1 being always the oldest or initial chamber. (After Carpenter, Brady,
and Butschli.)

chambers : individual chambers become larger. In this process
Jayers of calcareous matter are added to the shell from without by
the agency of a thin layer of protoplasm that extends over the

54

ZOOLOGY

SECT.

surface, a corresponding thickness being, probably, removed by
solution from the inner side at the same time.

The shell presents two leading types of structure apart from
the form and arrangement of the chambers : either it is of a
porcelain-like texture and provided with a single terminal aperture,
(Fig. 35, 4), r the texture is glassy and the whole shell perforated
with very minute apertures, through which, as well as through the
terminal aperture, pseudopods are protruded (Fig. 35, 2).

In many cases additional complexity is attained by the develop-
ment of what is called the supplemental skeleton (Fig. 36, 8b, s. sk.).
This consists of a deposit of calcium carbonate outside the original
shell ; it is traversed by a complex system of canals containing pro-
toplasm, and is sometimes produced into large spines. Foraminifera

sfo

FIG. 37. Has tiger ina murrayi. filsm. vacuolated protoplasm surrounding shell : psd.
pseudopods ; sh. shell ; xj>. spines. (After Brady.)

in which this secondary skeleton occurs are sometimes of consider-
able size 2-3 cm. in diameter and of extraordinary complexity.

Many Foraminifera resemble Difflugia in having a skeleton
formed of sand-grains, sponge-spicules, and other foreign bodies
cemented together by a secretion from the protoplasm (Fig. 36, 1).
Some of these are formed on the imperforate type, having the
protoplasm protruded from a single terminal aperture ; others on
the perforate type, small pseudopods being protruded between the
particles forming the shell.

In many cases the pseudopods are the only portions of proto-
plasm outside the shell, whereas in Gromia, as we saw, the shell
is invested with a layer of protoplasm, and is thus in strictness
an internal structure. In one of the calcareous forms with

II

PHYLUM PROTOZOA

55

perforated spiral shell, called Hastigerina (Fig. 37), a very remark-
able modification of this condition of things obtains. The shell

F

i &4r^ 8 **^ ' * /

. . *J& A""" i^^3^ "' *" Q

~^*>i O*"* ' '' SL/

*:-fS

^\:^'- f

r? t

-.!,.'-* ;. ; ^

t-L.

FIG. 38. Dimorphism and alternation of generations in Polystomella crispa. The arrows
indicate the direction of the life-cycle. A, young megaspheric individual; B, full-grown
megaspheric individual, decalcified ; C, megaspheric individual in the act of spore-formation,
the protoplasm leaving the shell in the form of flagellulas ; D, flagellula more highly
magnified ; E, microspheric individual developed from a flagellula ; F, microspheric individual
in the act of producing amoeboid embryos. (From Lang, after Schaudinn.)

.) is surrounded with a mass of protoplasm (plsm.) many times
its own diameter, and so full of vacuoles as to present a bubbly or

56 ZOOLOGY SECT.

frothy appearance. The shell itself, moreover, in this and allied
forms is provided with numerous delicate, hollow, calcareous spines
(sp. ), which are only to be seen in perfect, freshly-caught specimens.

Many Foraminifera exhibit the phenomenon of dimorphism :
the individuals of a single species occur under two distinct forms
(megaspheric andmicrospheric) differing from one another in the size
of the central chamber, the shape and mode of growth of the suc-
ceeding chambers, and the number and size of the nuclei (Fig. 38).

The reproduction of Foraminifera is mainly by spore-formation,
with or without conjugation. The protoplasm has been observed
in some to divide into minute masses which may be amoeboid or
may be of the nature of flagellulae each provided with a
flagellum. In some cases the flagellula3 have been observed to
conjugate in pairs. The young may develop shells while still
within the shell of the parent or only after becoming free. In the
dimorphic Foraminifera there is evidence of the occurrence of an
alternation of generations (p. 41) the megaspheric form alternat-
ing with the microspheric, and the latter being developed as a
result of a process of conjugation, the former without it (alterna-
tion of sexual and asexual generations).

Distribution. Gromia, Microgromia, and a few other forms
are found in fresh-water : one species has been found in damp
earth, but the great majority of the Foraminifera are marine,
some being pelagic, i.e. occurring at or near the surface of the
ocean, others abyssal, i.e. living at great depths. In the Atlantic,
large areas of the sea-bottom are covered with a gray mud called
Globigerina-ooze from the vast number of Globigerinae contained
in it.

From the palasontological point of view, the Foraminifera are a
very important group. Remains of their shells occur in various
formations from the Silurian period to the present day, certain
rocks, such as the White -Chalk (Cretaceous period) and the
Nummulitic limestone (Eocene), being largely made up of them.

ORDER 3. HELIOZOA.

General Structure. The Heliozoa are at once distinguished
from the preceding groups by the character of their pseudopods,
which have the form of stiff filaments radiating outwards from
the more or less globular cell-body, presenting very little move-
ment beyond the characteristic streaming of granules, and not
uniting to form networks.

One of the simplest forms is the common " Sun-animalcule,"
Actinophrys sol (Fig. 39). The body is nearly spherical, and
contains a large nucleus and numerous vacuoles, some of which,
near the surface, are contractile. Each of the stiff radiating
pseudopods has a firm axis, apparently composed of protoplasm,

II

PHYLUM PROTOZOA

57

which is traceable through the general protoplasm as far as the

nucleus. Living organisms are de-

voured in much the same way as in

Amoeba: each is ingested along with

a droplet of water, and is thus seen,

during digestion, to lie in a de-

finite cavity of the protoplasm,

called a food-vacuole. If the or-

ganism be small, processes of the

protoplasm are developed, and sur-

round and engulf it. If it be larger,

several pseudopods are applied to

it, their axial fibres becoming ab-

11 j ,1 r i

sorbed, and their substance envelops

if Anplncinrr if in a var-nnlp Tho
It, enclosing It in a VaCUO

animal can fix itself by means of

its pseudopods, the ends of which become viscid, and it is able

to crawl slowly by their means. Sometimes it floats freely in the

FIG. 3<k Actinophrys sol. a. axial

filaments of pseudopods ; . nucleus ;

P- pseudopod. (From Lang's Com-
ive. Anatomy, after Grenadier.)

rrucl

nu

FIG. 40. Actinosphserium eichhornii. A, the entire organism ; B, a small portion
highly magnified ; chr. chromatophore ; cort. cortex ; c. vac. contractile vacuole ; med. medulla ;
n u. nuclei. (From Blitschli's Protozoa, after Hertwig and LesseT.)

water, and it possesses the power of rising or sinking by some
unknown means.

Actinosphcerium (Fig. 40, A), another fresh-water form, is more

58

ZOOLOGY

SECT.

complex. The protoplasm is distinctly divided into a central mass,
the medulla or endosarc (B, med.), in which the vacuoles are small,

2. Nude aria

3.ClaHiru.lma

FIG. 41. Various forms of Helicrzoa. 3a, the entire animal; 8b, the flagellula ; c. rac.
contractile vacuole ; g. gelatinous investment ; nu. nucleus psd. pseudopods ; si: siliceous
skeleton ; sp. spicules. (From Biitschli's Protozoa, after Schulze and Greeff.)

and an outer layer, the cortex or ectosarc (cort.), in which they are
very large. There are numerous nuclei (nu.) and chromatophores
(chr.), the latter coloured green by chlorophyll, the characteristic
pigment of green plants.

II

PHYLUM PROTOZOA

59

Many genera form colonies. Numerous zooids may be united
by bridges of protoplasm into an open network, or the connecting
bridges may be shorter and the zooids more numerous, giving the
colony a more compact appearance.

Transitional stages occur between the naked genera already re-
ferred to and forms with a distinct skeleton. Sometimes the body
simply surrounds itself with a temporary gelatinous investment
(Fig. 41, #,#.), in other cases it is surrounded by a capsule of loosely
woven fibres through which the pseudopods pass, thus reminding
us of the state of things characteristic of perforate Foraminifera.

Fio. 42. Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A. two indi-
viduals in the first phase of conjugation ; B, beginning of the encystation ; C, maturation ;
D, completion of maturation ; E, coalescence of nuclei ; F, completion of the first spindle of
the zygote resulting from the conjugation. 1, axial filaments of the pseudopods ; 2, nucleus ;
3, spindles concerned in maturation ; A, 5, outer and inner layers of <jyst ; 6, polar bodies ;
7, nucleus formed by the union of the two nuclei; S, first (mitotic) division. (From Lang,
after Schaudinn.)

One genus has a shell formed of agglutinated sand-grains ; in
another (Fig. 41, 1) the skeleton consists of loosely matted needles
of silica. Lastly, in the graceful Clathrulina (3) the body is
enclosed in a perforated sphere of silica, quite like the skeleton of
many of the Radiolaria (p. 61).

Reproduction ordinarily takes place by binary fission ; a
peculiar form of budding has been observed, and spore-formation
also occurs, with or without encystation. ActinosphaBrium, for
instance, encloses itself in a gelatinous cyst and undergoes
multiple fission, forming numerous spores each enclosed in a
siliceous cell-wall. These resting spores remain quiescent

CO

ZOOLOGY

SECT.

throughout the winter, and in spring the protoplasm emerges
from each and assumes the form of the ordinary active Actino-
sphaBrium. In Clathrulina spore-formation takes place in the
active condition, and the spores (Fig. 4*1, 3 b) are flagellulse, each
being an ovoid body provided with two flagella.

Conjugation has been observed in some instances between two
or more individuals, which may separate again without any nuclear
changes taking place; or the conjugation may be followed by
a sexual process, comprising the coalescence of the protoplasm of
fhe two individuals and the coalescence of the nuclei (Fig. 42)
after each has given off a part of its substance (6), as in the
maturation of an ovum in multicellular animals (p. 19).

ORDER 4. RADIOLARIA.

The Radiolaria are a large and well-defined group of Rhizopods,
noticeable, in most instances, by the presence of a siliceous skeleton
of great beauty and complexity. They are all marine.

General Structure. --The most important characteristic of
the group is the presence of a perforated membranous sac, called

the central capsule (Fig. 43,
cent. caps.\ which lies em-
bedded in the protoplasm,
dividing it into intra-capsular
(int. caps, pr.) and extra-
capsular (ext. caps.pr.) regions.
In the intra-capsular proto-
plasm is a large and complex
nucleus (nu.\ or sometimes
many nuclei: from the ex-
tra-capsular protoplasm the
pseudopods (psd.) are given
off in the form of delicate radi-
ating threads, which in some
cases remain free, in others,
e.g. Lithocircus, anastomose
freely, i.e. unite to form net-
works. In one large section the Acantharia the pseudopodia
contain firm axial rods similar to those in the pseudopods of the
Heliozoa. There is no contractile vacuole, but in many forms the
extra-capsular protoplasm contains numerous large non-contractile
vacuoles, which give it the frothy or bubbly appearance noticed
previously in Hastigerina. The vacuolated portion of the proto-
plasm has a gelatinous consistency, and is distinguished as the
calymma.

The central capsule may be looked upon as a chitinoid
internal skeleton, reminding us of the shell of Gromia and of

SKel.

FIG. 43. Lithocircus annularis. cent. caps.
central capsule ; ext. caps. pr. extra-capsular
protoplasm ; iiit. caps. pr. intra-capsular pro-
toplasm ; mi. nucleus ; psd. pseudopods ; skel.
skeleton ; z. cells of Zoochlorella (After
Biitschli, from Parker's Biology.)

II

PHYLUM PROTOZOA

61

the perforated calcareous shell of Hastigerina with its investment
of vacuolated protoplasm. It is found in its simplest form in
Thalassoplancta (Fig. 44), in which it is spherical and uniformly
perforated with minute holes. In other forms, such as Lithocircus
(Fig. 43), it is more or less conical in form, and the apertures are
restricted to the flat base of the cone. Lastly, in the most complex
forms (Fig. 45), the membrane of the capsule is double, and there
are three apertures a principal one having a central position and
provided with a lid or operculum (op), and
two subsidiary ones on the opposite side.
In relation with the principal or lidded
aperture there is found in the extra-
capsular protoplasm a heap of pigmented
matter called the phceodium (ph.), prob-
ably partly of the nature of excreta. The
central capsule encloses, in addition to
the nucleus or nuclei, oil -drops, vacuoles,
proteid crystals, and pigment.

In some genera the central capsule is
the only skeletal structure present, but in
most cases there is in addition a skeleton
mainly external formed, as a rule, of
silica, but in one subdivision of the class
of a substance called acanthin, composed
of strontium sulphate, so transparent that
it can only be distinguished from silica
by chemical tests. The siliceous skeleton
may consist of loosely woven spines
(Fig. 44), but usually (and the acanthin
skeleton always) has the form of a firm
frame-work of globular, conical, stellate,
or discoid shape, frequently produced into
simple or branched spines. In the forms
with an acanthin skeleton the spines fre-
quently have inserted into them a number
of contractile filaments arising from the
gelatinous extra-capsular layer. A very
beautiful form of skeleton is exhibited by

Adinomma (Fig. 46), in which there are three concentric per-
forated spheres (A, sk. 1, sk. 2, sk. 3) connected by radiating spicules.
The outer of these spheres occurs in the extra-capsular protoplasm
(B, ex. caps, pr.), the middle one in the intra-capsular protoplasm,
and the inner one in the nucleus (nu).

Colonial forms are comparatively rare in this order, but occur
in some genera by the central capsule undergoing repeated
divisions while the extra-capsular mass remains undivided. In
this way is produced in Coltozoum for instance (Fig. 47, A, B, C)

FIG. 44. Thalassoplancta
brevispicula, part of a
section, km. central cap-
sule ; ip. intra-capsular
protoplasm ; n. nucleus,
containing nl. numerous
nucleoli ; ot. oil drops ; ca.
calymma ; rp. protoplasm
surrounding calymma ; s.
spicules. (From Lang's
Compamti ce A natomy, after
Haeckel).

62

ZOOLOGY

SECT.

FIG. 45. Aulactinium actinastrum. c. calymiua ; km. central capsule ; n. nucleus ; op
operculum ; ph. phaeodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

FIG. 46. Actinomma asteracanthion. A, the shell with portions of the two outer
spheres broken away; B, section showing the relations of the skeleton to the animal;
<< nt. rn/>s. central capsule ; ex. <<!/>.*. ///. extra-capsular protoplasm ; nu. nucleus ; sk. 1, outer,
sk. 2, middle, sk. 3, inner sphere of skeleton. (From Biitschli's Protozoa, after Haeckel and
Hertwig.)

II

PHYLUM PROTOZOA

63

a firm gelatinous mass, the calymma or vacuolated extra-
capsular protoplasm (D, vac.) common to the entire colony,
having embedded in it numerous central capsules (c. caps.) each
indicating a zooid of the colony. Collozoum may attain a length
of 3 or 4 cm.

Reproduction by binary fission has been observed in some
cases, and is probably universal. The nucleus divides first, then
the central capsule, and finally the extra-capsular protoplasm.

Spore-formation has been observed in Collozoum and some other
genera : the intra-capsular protoplasm divides into small masses,
each of which becomes a flagellula (Fig. 47, E, F) provided with a
single flagellum. In some instances all the spores produced are

,.: ?; WS^JHfcVaPN fJ w ?

FIG. 47. Collozoum inermei A C, three forms of the entire colony, iiat. size ; D, a small
colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with
vacuoles(rac.) ; E, spores containing crystals (c.) ; F, mega- and inicrospore. (From Butschli's
Protozoa, after Hertwig and Brandt.)

alike (E), and each encloses a small crystal (c.): in other cases (F)
in the same species the spores are dimorphic, some being small
(microspores), others large (megaspores). Their development has not
been traced.

Symbiosis. One most characteristic and remarkable feature
of the group has yet to be mentioned. In most species there occur
in the extra-capsular protoplasm (in the intra-capsular in some
cases) minute yellow cells (Fig. 43, 2.) which multiply by fission
independently of the Radiolarian. It has been proved that these
are unicellular organisms, sometimes regarded as plants (Class
Algae), sometimes as animals (Class Mastigophora of the Protozoa),
and named Zoochlorellce. This intimate association of two organisms
is called symbiosis : it is probably a mutually beneficial partner-
ship, the Radiolarian supplying the Zoochlorellse with carbon
dioxide and nitrogenous waste matters, while the Zoochlorellae

64

ZOOLOGY

SECT.

give off oxygen and produce starch and other food- stuffs, some
of which must make their way by diffusion into the protoplasm
of the Radiolarian.

APPENDIX TO THE RHIZOPODA.

CHLAMYDOMYXA AND LABYBINTHULA.

Chlamydomyxa (Fig. 48), of which two species have been described, has been
found living on Bog-mosses (Sphagnum) in Ireland and in Germany and

' <' i ;/ ' , '
;.', i . ;////.,

!-}''::'/.

l-ff\/',U~-"-

FIG. 48. Chlamydomyxa labyrinthuloides. A, active phase ; c.w. cell-wall ; /. frag-
ment of Alga ingested as food ; s/>. spindles in course of pseudopods ; B, resting-stage
numerous individuals in the cells of a fragment of Sphaijiium. ; a, specimen completely enclosed
in cell ; b and c, specimens which have emerged through the ruptured cell-wall ; C, specimen
multiplying by budding; I), by binary fission ; E, by internal tission. 1] may represent a
stage in spore-formation. (A, after Areher, B .E after (Jeddes.)

Switzerland. It may occur either in the active or in the resting condition. In
the latter (B, a, b, c) it consists of a mass of protoplasm with a number of
nuclei surrounded by a laminated wall of cellulose (p. 14). In the protoplasm are

II

PHYLUM PROTOZOA

65

numerous non-nucleated protoplasmic bodies or chromatophores, containing
chlorophyll and a green or brown colouring matter in varying proportions.
There are also a number of minute rounded bodies of a bluish tint probably com-
posed of reserve food-materials. In the young condition (a) the resting cells are
globular and microscopic, lying enclosed within the cells of the Sphagnum, but
as they grow in this confined space they become elongated and irregular, and
tinally burst through the wall of the moss-cell, forming masses (b, c) quite visible
to the naked eye. These may bud (C) or undergo binary fission (I)) ; or the
protoplasm, retreating from the cell- wall, may divide into numerous small
uninucleated amoeboid masses, each of which subsequently surrounds itself with
a new cell-wall (E).

During the whole of the resting stage there is nothing to distinguish Chlamy-
domyxa from a plant, and it would certainly be placed among the lower Algas
if the active phase of its existence were unknown.

In the active stage (A) the protoplasm protrudes from the ruptured cell-wall
in the form of stiff pseudopods produced into a complex network of extremely
delicate filaments, which are much branched and perhaps anastomose, and may
imite to form larger masses of protoplasm at a considerable distance from the
original cell. At the same time the bluish spheres (-sp. ) found in the resting
stage take on a spindle shape and travel slowly along the filaments.

In one of the two known species the protoplasm entirely leaves the cyst wall
and becomes free in the water.

The filaments are used to capture living organisms (/. ) which are digested by
the protoplasm surrounding them, the products of nutrition being conveyed
along the network to all parts of the organism. Thus in the active condition the
nutrition of Chlamydomyxa is holozoic, i.e. strictly like that of an animal, the
food consisting of living protoplasm. In the resting stage, on the other hand,
nutrition is purely holophytic, i.e. like that of an ordinary green plant, the food

B

rtu.

Fio. -Hi. "Labyrinthula vitellina. A, specimen crawling on a fragment of Alga (a.) ; c. ceils
travelling in the filaments. B, part of specimen in resting condition with heap of cells (c.) ;
C, a single cell from an actively moving specimen with connecting threads ; nn. nucleus.
(From Biitschli's Protozoa, after Gienkowsky.)

consisting of the carbon dioxide and various mineral salts dissolved in the water.
Chlamydomyxa multiplies in the resting condition by the formation of spores
each containing two nuclei. These give rise to flagellulse, the further history of
which has not been traced.

Labyrinthula (Fig. 49) in the resting stage (B) consists of a heap of small

VOL. I F

66

ZOOLOGY

SECT.

nucleated cells (c. ) connected by a homogeneous substance. In the active condi-
tion (A) it is produced into long delicate stiff filaments of pseudopodial character,
along which the cells (c.) travel, in the same manner as the spindles of Chlamy-
clomyxa. Labyrinthula has, therefore, the character not of a single cell, but of
a cell-colony, formed of numerous cells connected together. Chlamydomyxa, on
the other hand, has the character of a single multinucleate cell. There is thus
no close connection between these two aberrant forms : but both may, perhaps,
best be regarded as Rhizopoda with nearer relationships to the Foraminifera
(Gromia in particular) than to any of the other orders.

cvac

FIG. -00 Didymium differ me. A, two sporangia (spg. 1 and 2) on a fragment of leaf(/.).

B, section of sporangium, with ruptured outer layer (a.); and threads of capillitiuni (cp.).

C, a flagellula with contractile vacuole (c. i-ac.) and nucleus (.). D, the same after loss
of flagellum ; l>, an ingested Bacillus. E, an amcebula, F, conjugation of amoebulae to form
a small plasmodium, G, a larger plasmodium accompanied by numerous anicebulae ; sp.
ingested spores, (*\iter Lister.)

CLASS II. MYCETOZOA.
1. EXAMPLE OF TKE CLkSSDidymium difforme.

Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 50, G),
often several centimetres across, which crawls, like a gigantic Amoeba, over
the surface of decaying leaves. It shows the characteristic streaming move-

ii PHYLUM PROTOZOA 67

merits of protoplasm, and feeds by ingesting various organic bodies, notably the
Bacilli which always occur in great numbers in decaying substances. Numerous
nuclei are present.

After leading an active existence for a longer or shorter time, the protoplasm
aggregates into a solid lump, surrounds itself with a cyst, and undergoes multiple
fission, dividing into an immense number of minute spores. The cyst (Fig. 50,
A, spy. 1, spg. 2) is therefore not a mere resting capsule, like that of Amoeba,
but a sporangium or spore-case. Its wall consists of two layers, an inner of a
dark purple colour and membranous texture, formed of cellulose, and an outer of
a pure white hue, formed of calcium carbonate. Thus the whole sporangium,
which may attain a diameter of 3 or 4 mm., resembles a minute egg. From the
inner surface of the wall of the sporangium spring a number of branched
filaments of cellulose, which extend into the cavity among the spores and together
constitute the capillitium (B, cp. ).

The spores consist of nucleated masses of protoplasm surrounded by a thick
cellulose wall of a dark reddish -brown colour. After a period of rest the proto-
plasm emerges in the form of an amoeboid mass which soon becomes a flagellula
(C), provided with a single flagellum, a nucleus (nu.), and a contractile vacuole
(c. vac.). The flagellulse move freely and ingest Bacilli (D, b. ), and multiply by
fission : then, after a time, they become irregular in outline, draw in the
flagellum, and become amoeboid (E). The amcebulas thus formed congregate in
considerable numbers and fuse with one another (F), the final result being the
production of the great amoeboid mass (G) with which we started. There is no
fusion of the nuclei of the amcebulne. Thus Didymium in its active condition is a
plasmodium, i.e. a body formed by the concresence of amoebulse.

2. GENERAL REMARKS ON THE MYCETOZOA.

Speaking generally, the Mycetozoa differ from all other Protozoa in their
terrestrial habit. They are neither aquatic, like most members of the phylum,
nor parasitic, like many other forms, but live habitually a sub-aerial life on
decaying organic matter. They are also remarkable for their close resemblance
in the structure of the sporangia and spores to certain Fungi, a group of parasitic
or saprophytic plants in which they are often included, most works on Botany
having a section on the Myxomycetes or " Slime-fungi," as these organisms are
then called. They are placed among animals on account of the structure and
physiology of the flagellate, amoeboid, and plasmodial phases, which exhibit
automatic movements and ingest solid food. The Mycetozoa are sometimes
included among the Rhizopoda, a course which their very peculiar reproductive
processes appears to render inadvisable.

An interesting organism, called Protomyxa, probably belongs to this group. In
its plasmodial phase it consists of orange-coloured masses of protoplasm, about
1 mm. in diameter, which crawl over sea-shells by means of their long, branched
pseudopods, and ingest living prey. No nuclei are known. The protoplasm
becomes encysted and breaks up into naked spores, which escape from the cyst
as flagellulte, but soon become amoeboid and fuse to form the plasmodium.

CLASS III. MASTIGOPHORA.

1. EXAMPLE OF THE CLASS Euglcna viridis.

Euglena (Fig. 51) is a flagellate organism commonly found in
the water of ponds and puddles, to which it imparts a green colour.
The' body (E, H) is spindle-shaped, and has at the blunt anterior
end a depression, the gullet (F, a j s.), from the inner surface of which

F 2

68

ZOOLOGY

SECT.

springs a single long flagellum (fl.). According to recent observa-
tions the flagellum is not a simple thread, but is beset with delicate
cilium-like processes. The organism is propelled through the
water by the lashing movements of the flagellum, which is always
directed forwards ; it can also perform slow worm-like movements
of contraction and expansion (A--D), but anything like the free
pseudopodial movements which characterise the Rhizopoda is
precluded by the presence of a very thin membrane or cuticle which
invests the body. Oblique and longitudinal lines ; in the outer
layer of the protoplasm may be due to the presence of contractile
fibrils. There is a nucleus (nu.) near the centre of the body, and
at the anterior end a contractile vacuole (H, c. me.), leading into

H ft

f.vac

FIG. 51. Eugleua viridis. A I), four views illustrating euglenoid movements; E and H,
enlarged views ; F, anterior end further enlarged ; G, resting form after binary fission ; c. vac.
contractile vacuole in H, reservoir in E and F ; cy. cyst ; ,rf. flagellum ; m. mouth ; nu. nucleus ;
ces, gullet ; p. paramylum bodies ; pg. pigment spot ; r. (in H), reservoir. (From Parker's
Biology, after Kent and Klebs.)

a large non-contractile space or reservoir (r.) which discharges into
the gullet.

The greater part of the body is coloured green by the charac-
teristic vegetable pigment, chlorophyll, and contains rod-shaped
grains of paramylum (H, p.), a carbohydrate allied to starch. In
contact with the reservoir is a bright red speck, the stigma (pg.),
formed of a pigment allied to chlorophyll and called hcematochrome.
It seems probable that the stigma is a light-perceiving organ or
rudimentary eye.

Euglena is nourished like a typical green plant : it decomposes
the carbon dioxide dissolved in the water, assimilating the carbon
and evolving the oxygen. Nitrogen and other elements it absorbs
in the form of mineral salts in solution in the water. But it has

ii PHYLUM PROTOZOA 69

also been shown that the movements of the flagellum create a
whirlpool by which minute fragments are propelled down the
gullet and into the soft internal protoplasm. There seems to be
no doubt that in this way minute organisms are taken in as food.
Euglena thus combines the characteristically animal (holozoic) with
the characteristically vegetable (holophytic) mode of nutrition.
But, in all probability, the Euglena is in large measure saprophytic,
the products of the decay of organic matter dissolved in the water
being absorbed through the general surface.

Sometimes the active movements cease, the animal comes to
rest and surrounds itself with a cyst or cell-wall of cellulose (G),
from which, after a quiescent period, it emerges to resume active
life. It is during the resting condition that reproduction takes
place by the division of the body in a median plane parallel to
the long axis (G). Under certain circumstances multiple fission
takes place, and flagellulse are produced, which, sometimes, after
passing through an amoeboid stage, develop into the adult form.

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Mastigophora form a very extensive group, the genera and
species of which show a wonderful diversity in structure and habit.
The only character common to them all is the presence of one or
more flagella. Some approach plants so closely as to be claimed
by many botanists ; others are hardly to be distinguished from
Rhizopods ; while the members of one order present an interesting
likeness to certain peculiar cells found in Sponges.

The class is divisible into four orders as follows :-

ORDER I.--FLAGELLATA.

Mastigophora having one or more flagella at the anterior end
of the body.

ORDER 2. CHOANOFLAGELLATA.

Mastigophora having a single flagellum surrounded at its base
by a contractile protoplasmic collar.

ORDER 3. DINOFLAGELLATA.

Mastigophora having two flagella, one anterior, the other
encircling the body like a girdle.

ORDER 4. CYSTOFLAGELLATA.

Mastigophora having two flagella, one of which is modified
into a long tentacle, while the other is small and contained within
the gullet.

70 ZOOLOGY SECT.

Systematic Position of the Example.

Euglena viridis is one of several species of the genus Euglena,
and belongs to the family Englenidce, sub-order .Euglenoidea, and
order Flagcllata.

The presence of an anterior flagellum and the absence of a
collar, transverse flagellum, or tentacle, indicate its position among
the Flagellata. It is placed among the Euglenoidea in virtue of
possessing a single flagellum and a small gullet into which
the reservoir opens. The genus Euglena is distinguished
by its centrally placed nucleus, green chromatophore, red stigma,
and euglenoid movements. E. viridis is separated from other
species of the genus by its spindle-shaped body with blunt ante-
rior and pointed posterior end, and by the flagellum being some-
what longer than the body.

ORDER 1. FLAGELLATA.

The cell-body is usually ovoid or flask-shaped (Fig. 52, 6, 7, 9,
&c.), but may be almost ^globular (1), or greatly elongated (3).
Anterior and posterior ends are always distinguishable, the flagella
being directed forwards in swimming, and, as a rule, dorsal and
ventral surfaces can be distinguished by the presence of a mouth
or by an additional flagellum on the ventral side. They are,
therefore, usually bilaterally symmetrical, or divisible into equal and
similar right and left halves by a vertical antero-posterior plane.

Some of the lower forms have no distinct cuticle, and are able,
under certain circumstances, to assume an amoeboid form (2).
The curious genus Mastigamccba (4) nas a permanently amoeboid
form, but possesses, in addition to pseudopods, a single, long
flagellum. It obviously connects the Mastigophora with the
Rhizopoda, and indeed there seems no reason why it should be
placed in the present group rather than with the Lobosa. Simi-
larly, Dimorpka (5) connects the Flagellata with the Heliozoa : in
its flagellate phase (a) it is ovoid and provided with two flagella,
but it may send out long stiff radiating pseudopods, while retaining
the flagella, or may draw in the latter and assume a purely helizoan
phase of existence provided with pseudopods only (&).

Nuclei of the ordinary character are universally present. In
addition there is present in the cytoplasm near the base of the
flagellum a much more minute, deeply-staining body, which is termed
the Uepliaroblast (Fig. 53). This has sometimes been taken for a
micronucleus such as is general in the Infusoria, but it is not of
nuclear origin, and does not take an active part in any reproductive
processes.

The number of flagella is subject to great variation. There
mav be one (Fig. 52, 1-S\ two \9, 10), three (6), or four (7).
Sometimes the flagella show a differentiation in function ; in

II

PHYLUM PROTOZOA

71

Hetcromita, e.g. (Fig. 57) the anterior flagellum (fl. 1) only is
used in progression, the second or ventral flagellum (fl. 2) is trailed

4.MasMg-

amoeba

e.Dallingeria

8.0ikomonas

H.DInobryon 12,Syncrybfa 13. An

H.Rhijjidodendron

FIG. 52. Various forms of Flagellata. In 2, flagellate () and amoeboid (b) phases are
shown ; in 5, flagellate () and helio^oan (/;) phases ; in 8 are shown two stages in the in-
gestion of a food-particle (/)! ^''- chromatophores ; c. rac. contractile vacuole ;/*. food par-
ticle g. gullet; nm. nucleus ; 1. lorica ; p. protoplasm ; per, peristome ; v.i. vacuole of ingestion.
(Mostly from Btitschli's Protozoa, after various authors.)

behind when the animal is swimming freely, or is used to anchor
it to various solid bodies. In some (Trypanosomes, Fig. 53) the

72

ZOOLOGY

SECT.

flagellum (or one of them, if two are present) is attacked through-
out its length, or in the greater part of its length, to the edge
of a wavy protoplasmic flange, or undulating membrane, running
along the body.

There are also important variations in structure correlated with
varied modes of nutrition. Many of the lower forms, such as
Heteromita, live in decomposing animal infusions : they have
neither mouth nor gullet and take no solid food, but live by
absorbing the nutrient matters in the solution ; their nutrition is,
in fact, saprophytic, like that of many fungi. A few live as para-
sites in various cavities of the body of the higher animals. The
HcBmoflagdlata, an extensive group, live as parasites in the
plasma of the blood of various vertebrates. Most of these appear
to be harmless, but some are the causes of serious diseases in Man

FIG. 53. Try pano somes of Fishes, c. blepharoblast ; /. flagellum ; /a. and fp. (in A) anterior
and posterior flagella ; m. undulating membrane ; n. nucleus. (After Laveran and Mesnil.)

and other higher animals. One Euglena-like form lives as an
intra-cellular parasite within the cells of one of the lower worms.

Hcematococcus (Fig. 54), Pandorina (Fig. 55), Volvox (Fig. 56),
and their allies present us with a totally different state of things.
The mouthless body is surrounded by a cellulose cell-wall (c.t0.),
and contains chromatophores (chr.) coloured either green by chloro-
phyll or red by hsematochrome. Nutrition is purely holophytic,
i.e. takes place by the absorption of a watery solution of mineral
salts and by the decomposition of carbon dioxide. It is, there-
fore, not surprising that these chlorophyll -containing Flagellata
are often included among the Algas or lower green plants.

Other genera live in a purely animal fashion by the ingestion of
solid proteinaceous food, usually in the form of minute living
organisms : in these cases there is always some contrivance for
capturing and swallowing the prey. In Oikomonas (Fig. 52, 8),
we have one of the simplest arrangements : near the base of the
flagellum is a slight projection containing a vacuole (v.i.) ; the
movements of the flagellum drive small particles (/.) against this
region, where the protoplasm is very thin and readily allows the
particles to penetrate into the vacuole, where they are digested.

II

PHYLUM PROTOZOA

73

In Euglena, as we have seen, there is a short, narrow gullet, and
in some genera (9, g} this tube becomes a large and well-marked
structure.

Skeleton. While a large proportion of genera are naked or
covered only by a thin cuticle, a few fabricate for themselves a
delicate chitinoid shell or lorica (10, /.), usually vase-shaped and
widely-open at one end so as to allow of the protrusion of the
contained animalcule. In the chlorophyll-containing forms there
is a closed cell-wall of cellulose (Fig. 54, c.w.). One group of

Fro. 54. HsematoCOCCUS pluvialis. A, motile stage ; B, resting stage ; C, D, two modes
of fission ; E, Hcematococcns lacustris, motile stage ; F, diagram of movements of flagellum ;
chr. chromatophores ; c. vac. contractile vacuole ; c.v. cell-wall ; mi. nucleus ; uu'. nucleolus ;
pyr. pyrenoids. (From Parker's Biology.)

marine Flagellates have siliceous skeletons similar to those of the
K/adiolaria, with which they were originally classed.

In many genera colonies of various forms are produced by
repeated budding. Some of these are singularly like a zoophyte
(see Sect. IV.) in general form (Fig. 52, 11), being branched colonies
composed of a number of connected monads, each enclosed in a
little glassy lorica ; or green (chlorophyll-containing) zooids are
enclosed in a common gelatinous sphere, through which their
flagella protrude (12) ; or tufts of zooids, reminding us of the
flower-heads of Acacia, are borne on a branched stem (13). In
Volvox (Fig. 56) the zooids of the colony are arranged in the form
of a hollow sphere, and in Pandorina (Fig. 55) in that of a solid
sphere enclosed in a delicate shell of cellulose. Lastly, in RJiipido-

74

ZOOLOGY

SECT.

dendron (Fig. 52, 14} a beautiful branched fan-shaped colony is
produced, the branches consisting of closely adpressed gelatin-
ous tubes each the dwelling of a single zooid.

Binary fission is the ordinary mode of asexual multiplication,
and may take place either in the active or in the resting condition.
Haematococcus (Fig. 54) and Euglena (Fig. 51), for instance,
divide while in the encysted condition ; Heteromita (Fig. 57)

FIG. r>5. Pandorina morum. A, entire colony; B, asexual reproduction, each zooid.
dividing into a daughter-colony ; C, liberation of gametes ; D F, three stages in conjugation
of gametes; G, zygote ; H- -K, development of zygote into a new colony. (From Parker's
mjti, after Goebel.)

and other saprophytic forms while actively swimming : in the
latter case the divison includes the almost infinitely fine flagellum.
In correspondence with their compound nature, the colonial
genera exhibit certain peculiarities in asexual multiplication. In
JDiTiobryon (Fig. 52, 11) a zooid divides within its cup, in which
one of the two products of division remains ; the other crawls out
of the lorica, fixes itself upon its edge, and then secretes a new
lorica for itself. In Pandorina (Fig. 55) each of the sixteen zooids
of the colony divides into sixteen (B), thus forming that number of
daughter-colonies within the original cell-wall, by the rupture of

II

PHYLUM PROTOZOA

75

which they are finally liberated. In Voh-o.r (Fig. 56), certain zooids,
called parthenogonidia (A, a), have specially assigned to them
the function of asexual reproduction: they divide by a process
resembling the segmentation of the egg in the higher animals
(D^D 5 ), and form daughter-colonies which become detached and
swim freely in the interior of the mother-colony.

A very interesting series of stages in sexual reproduction is
found in this group. In Heteromita two individuals come together

a

H

FIG. 5t>. Volvox globator. A, entire colony, enclosing several daughter-colonies;
B, the same during sexual maturity ; C, four zooids in optical section ; Di D5, develop-
ment of parthenogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and
sperms; H, oosperm ; a, parthenogonidia ; .r/. nagellmu ; or. ovum ; or)/, ovaries; jig. pigment
spot ; spit, spermaries. (From Parker's Biology, after Colin and Kirchner.)

(Fig. 57, E 1 ) and undergo complete fusion (E 2 - -E 4 ) : the result of
this conjugation of the two gametes or conjugating cells is a thin-
walled sac, the zygotc (E 5 ), the protoplasm of which divides by
multiple fission into very minute spores. These, when first
liberated by the rupture of the zygote (E), are mere granules,
but soon the ventral or trailing flagellum is developed, and after-
wards the anterior flagellum (F^-F 4 ). In Pandorina (Fig. 55)
the cells of the colony escape from the common gelatinous envelope
(C) and conjugate in pairs (D, E), forming a zygote (F, G), which,
after a period of rest (H), divides and forms a new colony (K).

76

ZOOLOGY

SECT.

In some cases the conjugating cells are of two sizes, union always
taking place between a large cell or megagamctc and a small cell

E

FIG. 57. Heteromita rostrata. A, the positions assumed in the springing movements
of the anchored form ; B, longitudinal fission of anchored form ; C, transverse fission of
the same ; D, fission of free-swimming form ; E, conjugation of free-swimming with anchored
form ; E=>, zygote ; E 6 , emission of spores from zygote ; F, development of spores ; jt.l, ante-
rior ; rt.2, ventral flagellum. (From Parker's Biolor/y, after Dallinger.)

or microgametc. In Volvox (Fig. 56) this dimorphism reaches its
extreme, producing a condition of things closely resembling what

II

PHYLUM PROTOZOA

77

we find in the higher animals. Certain of the zooids enlarge and
form megagametes (B, ovy.), others divide repeatedly and give rise
to groups of microgametes (B, spy. E, F), each in the form of an
elongated yellow body with a red pigment-spot and two flagella.
These are liberated, swim freely, and conjugate with the stationary
megagamete (G), producing a zygote (H), which, after a period of
rest, divides and reproduces the colony. It is obvious that the
megagamete corresponds with the ovum of the higher animals,
the microgamete with the sperm, and the zygote with the oosperm
or impregnated egg.

It should be noticed that in the more complex cases of repro-
duction just described we meet with a phenomenon not seen in
cases of binary fission, viz., development, the young organism being
far simpler in structure than the adult, and reaching its final form
by a gradual increase in complexity.

IMonosiga. 2.Salpinaoeca.

S.Polyoeca. 4.Proferospongia.

FIG. 58. Various forms of Choanofiagellata. 2b illustrates longitudinal fission ; 2c, the pro-
duction of flagellulaj ; c. collar ; c. vac. contractile vacuole ; ft. flagellum ; I. lorica ; ?'..
nucleus ; s. stalk. (After Saville Kent.)

ORDER 2. CHOANOFLAGELLATA.

General Structure. The members of this group are distin-
guished by the presence of a vase-like prolongation of the proto-
plasm, sometimes double, called the c0//a?'(Fig.58,/,c.), surrounding
the base of the single flagellum (fl.). The collar is contractile, and,
although its precise functions are nob yet certainly known, there is

78 ZOOLOGY SECT.

evidence to show that its movements cause vortices in the water which
draw in small bodies towards the outside of the collar to which they
adhere. By degrees such bodies are drawn towards the base, and
each is received into a vacuole which moves back into the interior
of the protoplasm, another vacuole taking its place. The animalcule
may draw in both collar and flagellum and assume an amoeboid form.

The nucleus (nu.) is spherical, and there are one or two con-
tractile vacuoles (c. vac.), but no trace of mouth or gullet. Some
forms are naked (1), others (2) enclosed in a chitinoid shell or
lorica of cup-like form. A stalk (s.) is usually present in the
loricate and sometimes also in the naked forms.

The genera mentioned in the preceding paragraph are all simple,
but in other cases colonies are produced by repeated fission. In
Polywca, (3) the colony has a tree-like form, which may reach
a high degree of complexity by repeated branching. A totally
different mode of aggregation is found in Proterospongia (4), in
which the zooids are enclosed in a common gelatinous matrix of
irregular form.

Reproduction.- -The " collared monads," as these organisms
are often called, multiply by longitudinal fission (2b). In some
cases multiple fission of encysted individuals has been observed
(2c), small simple flagellulse being produced which gradually
develop into the perfect form.

The order is especially interesting from the fact that, with the
exception of Sponges, it is the only group in the animal kingdom
in which the collar occurs.

ORDER 3. DINOFLAGELLATA.

The leading features of this group are the arrangement of the two flagella
which they always possess, and- the usual presence of a remarkable and often
very beautiful and complex shell.

The body (Fig. 59, 1) is usually bilaterally asymmetrical, i.e. it may be
divided into right and left halves, which are not precisely similar. On the
ventral surface is a longitudinal groove (I. yr. ), extending along the anterior half
only, and meeting a transverse groove (t. gr.), which is continued round the body
like a girdle. From the longitudinal groove springs a large flagellum (fl. 1),
which is directed forwards and serves as the chief organ of propulsion ; a second
flagellum (fl. 2) lies in the transverse groove, where its wave-like movements
formerly caused it to be mistaken for a ring of small cilia.

The body is covered with a shell (2} formed of cellulose, and often of very
complex form, being produced into long and ornamental process, and marked
with stripes, dots, &c. Besides a nucleus and a contractile vacuole, the proto-
plasm contains chromatophores (1, chr.) coloured with chlorophyll or an allied
pigment of a yellow colour, called diatom in. Nutrition is holophytic or holozoic.

The foregoing description applies to all the commoner genera. Prorocentrum
(3) is remarkable for the absence of the transverse groove, while Polykrikos (4)
has no fewer than eight transverse grooves and no shell. The latter genus
also has stinging-capsules or n< matocyxts (a, b) in the protoplasm, resembling
those of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes,
distinguished as mwjaiuidei (nu.), and micronuclti (nu'.).

II

PHYLUM PROTOZOA

79

Reproduction is, as usual, by binary fission, the process taking place some-
times in a free-swimming individual, sometimes in one which has lost its flagella
and come to rest.

vac

Glenodinium

S.CeraHum 3.Prorocentrum

4.Polykrikos

FIG. 59.; Various forms of Dinoflaerellata. 2 shows the shell only ; ka is an undischarged,
and 6 a discharged stinging-capsule; chr. chromatophores ; fl. 1, longitudinal flagellum ;
fi. 3, transverse flagellum; 1. ;//. longitudinal groove; ntc. nematocyst ; nu. meganucleus ;
ntf. micronucleus ; py. pigment spot ; t. gr. transverse groove^ (From Btitschli's Protozoa.)

The Dinoflagellata are mostly marine. Some are phosphorescent. Certain
kinds occasionally occur in such abundance in bays and estuaries as to cause a
deep brownish or red discoloration of the sea- water.

ORDER 4. CYSTOFLAGELLATA.

This group includes only two genera, Noctiluca and Leptodiscus. A descrip-
tion of Noctiluca miliaris, the organism to which the diffused phosphorescence

of the sea is largely due, will serve
to give a fair notion of the leading
characteristics of the order.

Noctiluca (Fig. 60) is a nearly
globular organism, about ^ mm. in
diameter. It is covered with a
delicate cuticle, and the medullary
protoplasm is greatly vacuolated.
On one side is a groove from
which springs a very large and
stout flagellum or tentacle (bg. ), no-
ticeable for its transverse striation.
Near the base of this flagellum is
the mouth (m.), leading into a short
gullet in which is a second flagel-
lum (f. ), very small in proportion
to the first. On the side opposite
to the mouth is a strongly marked
superficial ridge. The light-giving
region is the cortical protoplasm.
Reproduction takes place by binary fission, the nucleus dividing indirectly.
Spore-formation also occurs, sometimes preceded by conjugation, sometimes not.

Fin. 00. Noctiluca miliaris. n. the adult
animal ; I, c. flagellulai ; l<r. tentacle ; /. flagel-
lum ; nt. mouth ; n. nucleus. (From Lang.)

80

ZOOLOGY

SECT.

The spores (b, c), formed by the breaking up of the protoplasm of the parent,
escape as flagellulaf.

CLASS IV. SPOROZOA.

1. EXAMPLE OF THE CLASS Monocystis agilis.

One of the most readily procured Sporozoa is the microscopic
worm-like Monocystis agilis (Fig. 61, A), which is commonly found
leading a parasitic life in the vesicular seminales of the common
Earthworm. It is flattened, greatly elongated, pointed at both
ends, and performs slow movements of expansion and contraction,
reminding us of those of Euglena. In this, the trophozoite or adult

FIG. (51. Monocystis. A, Trophozoites in different stages of contraction. B, encysted
gametocytes. C, division of gametocytes into gametes. D, conjugation of gametes to form
zygotcs. E, Cyst enclosing ripe spores formed from the zygotes. F, single spore, showing
the (8) sporozoites in its interior. G, group of developing sperm-cells of the earthworm,
enclosing a sporozoite in the centre. H, young trophozites still surrounded with the tails of
the degenerated sperms, nu, nuclei. (From Parker's Practical Zoology.)

condition, the protoplasmic body is covered with a firm cuticle,
and is distinctly divided into a denser superficial portion, the cortex,
and a central semi-fluid mass, the medulla. There is a large clear
nucleus (nu.) with a distinct iiucleolus and nuclear membrane, but
the other organs of the protozoan cell-body are absent : there is
no trace of contractile vacuole, of flagella or pseudopods, of mouth
or gullet. Nutrition is effected entirely by absorption.

Reproduction takes place by a peculiar and characteristic
process of spore-formation. Two individuals come together,
and become rounded off and enclosed in a common cyst (B). The
nucleus of each divides repeatedly, until a large number of nuclei
arc formed (C). Each of the nuclei becomes surrounded by a thin
layer of protoplasm. The minute cells thus formed, after

ii PHYLUM PROTOZOA 81

moving to and fro actively for a time, unite in pairs after the
substance of the two individuals has become coalescent (D). From
each of the cells or zygotcs that are formed by the union of two
of the original small cells or gametes, a spore is formed, so that the
cyst now comes to contain numerous small spores (E). These are
spindle-shaped bodies, each enclosed in a strong chitinoid case (F),
and thus differing in a marked manner from the naked spores
of the Rhizopoda and Mastigophora. The protoplasm and nucleus
of each spore then undergo fission, becoming divided into a number
of somewhat sickle-shaped bodies which are arranged within the
spore-coat somewhat like a bundle of sausages. By the rupture
of the spore-coat these falciform young or sporozoites are liberated,
and at once begin active movements, the thin end of the body
moving to and fro like a clumsy flagellum. The falciform young
appear, in fact, to be greatly modified flagellulse. They make their
way to the clumps of developing sperms, bore their way in, and
are thus found surrounded by sperm-cells in various stages of
development (G). After thus living an intracellular life for
a time, they escape (H) into the cavity of the vesicula and grow
into the adult form.

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Sporozoa are exclusively parasitic, being the only group of
Protozoa of which this can be said. They have no organs of
locomotion and always multiply by spore-formation. The class is
divisible into the following five orders :

ORDER 1. GREGARINIDA.
Sporozoa in which the trophozoite is free and motile.

ORDER 2. COCCIDIIDEA.

Sporozoa in which the trophozoite is a minute intracellular
parasite.

ORDER 3. IL&MOSPORIDIA.

Sporozoa in which the trophozoite is amoeboid, and lives as
a parasite in the coloured blood-corpuscles of Vertebrates.

ORDER 4. MYXOSPORIDEA.

Sporozoa in which the trophozoite is amoaboid, but not intra-
cellular.

ORDER 5. SARCOCYSTIDEA.
Elongated Sporozoa, usually found in muscle.

VOL. I G

82

ZOOLOGY

SECT.

Systematic Position of the Example.

Monocystis agilis is a species of the genus Monocystis, belonging
to the family Monocystidce, of the order Gregarinida It is placed
in the Gregarinida on account of being free and motile in the tro-
phozoite state. The absence of partitions dividing the protoplasm
into segments indicates its position among the Monocystidse.
Monocystis is distinguished by its elongated form, by the absence
of any special apparatus in the cyst for the liberation and dispersal
of the spores, and by its spindle-shaped spores with thickened
ends, each producing 4 8 falciform young. The differences
between the species of Monocystis depend largely upon size.

ORDER 1. GREGARINIDA.

All the more typical members of the class belong to this
group. With the exception of Monocystis, already described, the
only genus to which it will be necessary to draw attention is
Gregarina (Figs. 62 and 63), the various species of which are
parasitic in the intestines of Crayfishes, Cockcoaches, Centipedes,

dcu,

spd

FIG. 6'2. -Gregarina. A, two specimens of G. blattannn partly embedded in enteric
epithelial cells of Cockroach ; B 1 , B-, two specimens of G. di(,m-ifiiu ; in B- the epimerite (.;>.)
is cast off ; C, cyst of <?. bln/tun'm, from which most of the spores have been discharged;
D, four stages in the development of G. {tirtant<:a. c>i. cyst ; den., deutomerite ; c/>. epimerite ;
ft. gelatinous investment of cyst; nu. nucleus ; pr. protomerite; -j>xd. 1, short pseudopod;
psd. ,?, long pseudopod ; sp. mass of spores ; spd. sporoducts. (From Biitschli's Protozoa.)

and other articulated animals. It differs from Monocystis in
having the medullary protoplasm of the adult divided into two
sections, an anterior, the protomerite (pr.), and a posterior, the
deutomerite (deu.\ in which the nucleus is situated. Anteriorly

II

PHYLUM PROTOZOA

83

to the protomerite there is sometimes found, especially in young
individuals, a third division, the cpimerite (cp.), which may be
provided with hooks (B 1 ), serving to attach the parasite to the
epithelium of the intestine of its host, by becoming embedded in
the substance of one of the cells. As maturity is reached the
epimerite is thrown off (B 2 ), and the parasite then lies freely in the
cavity of the intestine.

The cysts of Gregarina (C) are often very complex and
provided with delicate ducts (tyd.) in the thickness of the wall,

3

FIG. 03. Gregarina Development from the sporozoite. 1, cells of the digestive epithelium
of the host ; S, nuclei of the same ; 8, spore ; It, spore discharging sporozoites ('>) leaving
residual mass (6) ; 7, sporozoites in the act of entering epithelial cells ; S, the same as
intracellular parasites ; 9- 12, different stages in the growth of the young Gregarines into the
lumen of the intestine ; 13, epirnerite ; Ik. protomerite ; 15, deutomerite. (After Lang.)

through which the spores escape. In Gregarina gigantea of the
Lobster, the young (sporozoite) is liberated from the spore in the
form of a non-nucleated amcebula (D 1 ), with one long and one
short pseudopod (D 2 ) ; this divides by the long pseudopod (psd. 2)
becoming separated off, and each product of fission, developing a
nucleus, passes into the adult (trophozoite) form (D 3 , D 4 .) In
other species of Gregarina the sporozoites do not divide, but each
develops directly into the trophozoite (Fig. 63).

ORDER 2. COCCIDIIDEA.

Coccidium (Figs. 64, 65) and allied genera are parasites in the interior ot
cells, both in Vertebrates and Invertebrates. They live in the cells of various

G 2

84

ZOOLOGY

SECT.

organs, most frequently in those of the epithelium of the digestive canal. They
never inhabit blood-corpuscles. A few are intra-nuclear parasites. Two
distinct modes of irmltiplication occur by schizogony, a kind of multiple fission,
and by sporogony, a process of spore -formation preceded by conjugation between
male and female cells. The trophozoite, or adult phase, as we may term it, of
the parasite, grows to a certain size within the cell without destroying its
vitality the nucleus merely being pushed on one side. So far, in fact, from
impairing the nutrition of the cell, the presence of the parasite seems, in some
cases, for a time, rather to stimulate it At a certain stage of growth schizogony
(Fig. 65, b e) takes place. The nucleus divides to form a number of nuclei.
These migrate towards the surface, and each becomes surrounded by protoplasm,
w T ith the result that a number of small cells are formed. Each of these gives
rise to a club-shaped merozoite. The merozoites, when they become free, are
active bodies, which are able to penetrate into the interior of other epithelial
cells and develop into trophozoites like those from which they were derived.
This multiplication may take place on such an extensive scale that the

1 Ei

imena

S.CoccIdiutn

FIG. 04. Coccidiidea. A, adult Eimer'ta (E) in enteric epithelial cell (c?>.) of mouse ;
B, encysted form ; C, encysted form, the protoplasm contracting to form a spore ; D, formation
of falciform young(/.) in interior of spore (*/>.); E, spore with falciform young; F, adult
encysted form of CocciiHtim from liver of rabbit ; G, division into spores ; H, cyst containing
ripe spores (?/>.)> each with a single falciform young ; I, single spore with falciform young (/).
(From Biitschli's Protozoa, after Leuckart and Eimer."\

epithelium may be partially or completely destroyed. It is only, apparently,
when such extensive damage has been done, or is threatened, that multiplication
by sporogony takes place the invasion of a new host being by this process
rendered probable, and the continuance of the race being thus provided for in
the event of the death of the host in which the epithelium has become destroyed.
In this process certain of the merozoites, instead of developing into trophozoites,
grow more slowly ((/), and become converted into either micro- or megagame-
tocytes. Each of the former (k, j) gives rise by division to a number of narrow
biflagellate microgametes or sperms. Each of the megagametocytes (e, /), after a
process of the nature of maturation, forms a single rounded megagamete (ovum).
When this becomes fertilised by the penetration into it of a single microgamete, the
resulting body (zytjote or oosperni) divides to form a varying number of cells
each enclosed in a resistant cyst (/). These give rise to spores with a firm, chitinous
spore-membrane, each containing two or more falciform young or xporozoifes (I).
The cyst destroys the cell as it grows, and thus becomes free in the cavity by
which the epithelium is lined. The spores may thus pass out to the exterior,
and, if taken into the digestive canal of a new host, may liberate the now
active sporozoites, which may penetrate into epithelial cells (a) to become the
trophozoites with which the cycle began.

II

PHYLUM PROTOZOA

85

In some of the Coccidiidea this life cycle is modified in various ways, as, for
example, by the omission of schizogoiiy the trophozoites in such a case
developing directly into gametocytes.

Fio. 65. Life-History of Coccidium schubergi. a. penetration of epithelium cell of host by
sporozoite ; b-c, stages of multiple fission (schizogoiiy) ; d, gametocyte ; e, f, formation of
megagamete (ovum) ; g, fertilisation ; h, j, formation of microgarnetes (sperms) ; k, develop-
ment of fertilised ovum into four spores ; I, formation of two sporozoites (falciform young) in
each spore. (From Calkins, after Schaudinn.)

ORDER 3. H/EMOSPORIDEA.

These are Sporozoa which in the trophozoite condition live as parasites in
the interior of the coloured blood-corpuscles of all classes of Vertebrates, but
are occasionally found in other cells. In Man and in some other mammals and

86

ZOOLOGY

SECT.

in certain birds it has been found that their presence is the cause of various
feverish affections. The various forms of malaria in man have been proved to
be due to the presence in the blood-corpuscles of the patient of parasites
belonging to this order. The malaria-parasites, the history of which has been
carefully worked out, pass through a life-cycle comparable to that of Coccidium
described above. In the trophozoite stage (Fig. 66, A G) they live as amoeboid

D

r
f

Flo. 66. Life-History of Malaria Parasites. A-G, parasite of quartan fever, showing
development of trophozoite in a blood-corpuscle and the formation of merozoites ; //,
gametocyte of the same ; I-M, parasite of tertian fever to the formation of the merozoites ;
If, gametocyte ; 0-T, creseentio gametocytes of Laverania ; P-S, formation of micro-
gametes or sperms ; U- W, maturation of megagamete or ovum ; X, fertilisation ; Y, zygote.
a, zygote enlarging in stomach of mosquito ; b-e, passing into the body-cavity ; /, g, develop-
ment of the contents into a mass of sporozoites ; /t, sporozoites passing into the salivary
glands. (From Calkin's Protozoa, after Ross and Fielding Ould.)

intracellular parasites in the interior of the coloured corpuscles of their host.
Here they multiply by schizogony the products (merozoites) entering other
corpuscles. Some of the merozoites when they become established in the interior
of the corpuscles develop into rounded or crescentic bodies which become the
gametocytes (H, N, 0, T). In order that the life-cycle may be completed, it is
necessary that the parasite at this stage should be taken into the interior of a

II

PHYLUM PROTOZOA

87

second or intermediate host. In the case of the parasite of human malaria the
intermediate host is a mosquito of the genus Anopheles. On the mosquito
drawing up a drop of the blood of a malaria patient, all stages of the parasite
that occur in it are destroyed by the digestive juices of the insect with the
exception of the gametocytes ; these survive and form gametes in the
stomach of the mosquito. Each male gametocyte gives rise to a number of
slender filamentous microgametes (sperms, P, S) and each female gametocyte
forms a single megagamete (ovum). After maturation (U- -W) the megagamete
is fertilised (x) by one of the actively-moving microgamates, the result being the
formation of an active spindle-shaped ookinete. This perforates the stomach
wall and comes to rest in the subjacent tissues. It then becomese encysted
and increases greatly in size, bulging out into the body-cavity (b e). The
contents of the cyst eventually become divided up (f, g) into a large number of
long, narrow sporozoites. When the cyst becomes ruptured into the body-
cavity, these find their way to the salivary glands (h), and thence they may
readily be transferred to the blood-system of a human being when the mosquito
bites. Penetrating into the interior of coloured corpuscles they reach the
trophozoite condition.

The Hasmogregarines, which may most conveniently be referred to here, are
Sporozoa which live, like the malaria parasites, in the coloured blood-corpuscles
of all classes of Vertebrates ; but which in the mature or trophozoite condition
are not amoeboid, retaining the Gregarina-like form, and are therefore to be
regarded as belonging to the Gregarinida.

ORDER 4. MYXOSPORIDEA.

This group includes a small number of genera which are ainceboid
in the trophozoite phase, and which reproduce continuously by
spore-formation during that phase (Fig. 67, A). Many nuclei are present

FIG. 67. A, Myxidium lieberkiihnii, amoeboid phase; B, IWtyxobolus miilleri,

spore with discharged nernatocysts (ntc.); C, spores (psorosperms) of a Myxosporidian ;
ntc. nematocysts. (From Biitschli's Protozoa.)

in the amoeboid body, which may be of comparatively large size. The
spores (B) produced within the protoplasm of the trophozoite are provided
each with one or more bodies like the nematocysts of zoophytes and jelly-fish
[See Section IV]. Myxosporidea occur as parasites mainly of fishes
and amphibians, but very many occur in various groups of Invertebrates.
"Pebrine," the destructive silk- worm disease, is due to the presence of a
Sporozoan belonging to this order. A good example of the order is Myxidium,
found in the urinary bladder of the pike.

88 ZOOLOGY SECT.

ORDER 5. SARCOCYSTIDEA.

The best known form of this order is Sarcocystis (Fig. 68), which occurs in
the flesh of mammals, each parasite having the form of a long spindle embedded

FIG. CS. Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From
Butscmi's Protozoa, after Rainey.)

in a striped muscular fibre. They are often known as Rainey's or Mieschtr's
corpuscles. The protoplasm divides into spores from which falciform young are
liberated.

CLASS V. INFUSORIA.

1. EXAMPLE OF THE CLASS Paramcecinm caudatum.

Structure. Paramcecium, the "slipper-animalcule," is tolerably
common in stagnant ponds, organic infusions, &c. The body
(Fig. 69) is somewhat cylindrical, about J mm. in length, rounded
at the anterior and bluntly pointed at the posterior end. On the
ventral face is a large oblique depression, the luccal groom (hue. gr.\
leading into a short gullet (gul.\ which, as in Euglena, ends in the
soft internal protoplasm.

The body is covered with small cilia arranged in longitudinal
rows and continued down the gullet. The protoplasm is very
clearly differentiated into a comparatively dense cortex (cort.) and
a semi-fluid medulla (med.), and is covered externally by a thin
pellicle or cuticle, (cu.) which is continued down the gullet. The
cilia are continuous with the pellicle.

In the cortex are found two nuclei, the relations of which are
very characteristic. One, distinguished as the meganucleus (nu.),
is a large ovoid body staining evenly with aniline dyes, which,
when it divides, does so directly by a simple process of constriction.
The other, called the micronucleus (pa. nu.\ is a very small body
closely applied to the meganucleus; when it divides it goes
through the complex series of stages characteristic of mitosis
(p. 16).

The contractile vacuoles (c. vac.) are two in number, and are very
readily made out. Each is connected with a series of radiating
spindle-shaped cavities in the protoplasm which serve as feeders
to it. After the contraction of the vacuole these cavities are seen
gradually to fill, apparently receiving water from the surrounding

II

PHYLUM PROTOZOA

89

protoplasm : they then contract, discharging the water into the
vacuole, the latter rapidly enlarging while they disappear from

B

rac

C.TUC

me.

FIG. 69. Paramoecium caudatum. A, the living animal from the ventral aspect ; B, the
same in optical section : the arrow shows the covirse taken by food-particles ; C, a specimen
which has discharged its trichocysts ; D, diagram of binary fission ; buc. (jr. buccal groove ;
corf, cortex ; cu. cuticle ; c. vac. contractile vacuole ; /. vac. food vacuole j (iul. gullet ; med.
medulla; nu. meganucleus ; pa. mi. micronucleus ; trch. trichocysts. (From Parker's Biology. )

view ; finally the vacuole contracts and discharges its contents
externally.

The cortex contains minute radially arranged sacs called
trichocysts (trch.). When the animal is irritated, more or fewer of

90

ZOOLOGY

SECT.

these suddenly discharge a long delicate thread, which, in the
condition of rest, is very probably coiled up within the sac. In a
specimen killed with iodine or osmic acid the threads can fre-
quently be seen projecting in all directions from the surface (6').

Food, in the form of small living organisms, is taken in by
means of the current caused by the cilia of the buccal groove. The
food-particles, enclosed in a globule of water or " food-vacuole "
(/. vac.), circulate through the protoplasm, when the soluble parts
are gradually digested and assimilated. Starchy and fatty matters,
as well as proteids, are available as food, the digestive powers of
Paramcecium being thus considerably in advance of those of Amoeba.
Effete matters are egested at a definite anal spot posterior to the
mouth, where the cortex and cuticle are less resistent than else-
where. The whole feeding process can readily be observed in this
and other Infusoria by placing in the water some insoluble colour-
ing matter, such as carmine or indigo, in a fine state of division.

Reproduction. Multiplication takes place by transverse
fission (D), the division of the body being preceded by that of both
nuclei. As already mentioned, the meganucleus divides directly,
the micronucleus indirectly.

It has been proved, however, that multiplication by binary
fission cannot go on indefinitely ; but that after it has been repeated

Mg.nu

mi.nu

mJ.nu

FIG. 70. Paramoecium caudatum, stages in conjugation, gul. gullet ; mg. nu. meganucleus ;
mi. nu. micronucleus ; Mg. nu. reconstructed megauucleus ; Mi. nu. reconstructed micro-
nucleus. (From Parker's Biology, after Hertwig.)

a certain number of times it is interrupted by conjugation. In
this very remarkable and characteristic process two Paramcecia

ii PHYLUM PROTOZOA 91

become applied by their ventral faces (Fig. 70, A), but do not fuse.
The meganucleus (mg. nu.) of each breaks up into small masses,
which disappear, being apparently absorbed into the protoplasm.
At the same time the micronucleus (mi. nu.) of each divides,
each product of division immediately dividing again, so that each
gamete or conjugating body is provided with four micronuclei (B).
Two of these (mi. nu.' , mi. nu.") disappear; of the remaining two
one is distinguished as the stationary pronucleus, the other as the
active pronucleus. The active pronucleus of each Infusor now
passes into the body of the other and fuses with its stationary
pronucleus (D), each individual thus coming to possess a single
nuclear body derived in equal proportions from the two conjugat-
ing cells (E). The animalcules then separate from one another,
and the nucleus of each divides and gives rise to the permanent
mega- (G, Mg, nu.) and micronuclei (Mi. nu.).

2. CLASSIFICATION AND GENERAL ORGANISATION.

In the majority of the Infusoria the body is ciliated throughout
life, but in certain forms cilia are present only in the immature
condition, the adult being provided with peculiar organs of
prehension or tentacles. We thus get two orders, viz. :-

ORDER 1. CILIATA.
Infusoria provided with cilia throughout life.

ORDER 2.- -TENTACULIFERA.

Infusoria possessing cilia in the young condition, tentacles in
the adult.

Systematic position of the Example.

Paramoecium aurelia is one of several species of the genus
Paramcecium, belonging the family Parmcecidm, of the sub-order
Trichostomata, and order Ciliata. The presence of cilia in the
adult condition places it among the Ciliata : the presence of a
permanently open mouth into which food particles are swept by
the movement of the cilia, among the Trichostomata. The Para-
moecidse are free-swimming, asymmetrical, uniformly ciliated, with
a ventrally placed mouth. P. caudatum is about \--\ mm. in
length, its length about four times its breadth, rounded in front,
and bluntly pointed behind, and a single micronucleus is present.

ORDER 1. CILIATA.

This order presents a wider range of variations some of them
of a truly extraordinary character than any other group of
Protozoa.

92 ZOOLOGY SECT.

The form of the body is very varied : it may be ovoid (Fig.
71, 1), kidney-shaped (#), trumpet-shaped (#), vase or cup-shaped
(4, 9) ; produced into a long, flexible, neck-like process (5), or into
large paired lappets (6') ; flattened from above downwards, or
elongated and divided into segments reminding us of those of
a segmented worm (8}.

Most species are free-swimming, but some are attached to
weeds, stones, &c., by a stalk. This may be a purely cuticular
structure (9), or may contain a prolongation of the cortex in the
form of a delicate contractile axial fibre (Figs. 73 and 74, ax. /.),
which serves to retract the Infusor, its contraction causing the
stalk to coil up into a close spiral.

The arrangement of the cilia is also subject to great varia-
tion, and presents four chief types. In the holotrichous type, of
which Paramcecium is an example, the cilia are all small, equal-
sized or nearly so, and arranged in longitudinal rows (Fig. 69, Fig.
71, 1). The second or JieterotrwJwus type is seen in its simplest
form in Nyctotherus (Fig. 71, 2\ in which the left side of the
peristome is bordered by a row of specially large adored cilia, the
rest of the body being covered with small cilia. In Stentor (3)
the peristome is situated on the broad distal end of the trumpet-
shaped body, and the adoral band of cilia takes a spiral course.
This leads us to the peritrichous type of ciliation : in Vorticella
(Fig. 73) the vase-shaped body is, for the most part, quite bare of
cilia, but around the thickened edge of the peristome passes one
limb of a spiral band of large cilia united at their bases, the other
limb being continued round a raised lid-like structure, or disc, into
which the distal region is produced. This arrangement of cilia
reaches its greatest complexity in Episiylis plicatilis (Fig. 71, 9),
in which the ciliary spiral makes no fewer than four turns.

But it is in the hypotrichous type that the most extraordinary
modifications are found. The flattened body bears on its dorsal
surface mere vestiges of cilia in the form of very minute processes
of the cuticle, while on the ventral surface the cilia take the form
of large hooks, fans, bristles, and plates with fringed ends (Fig. 71,
7). The hooks and plates do not vibrate rhythmically like
ordinary cilia, but are moved as a whole at the will of the animal,
thus acting as legs. The hypotrichous Ciliata, in fact, in addition
to swimming freely in the water, creep over the surface of weeds,
&c., very much after the manner of Woodlice. One of the most
extraordinary forms in this group is Diophrys (7), the size and
arrangement of its polymorphic cilia giving it a very grotesque
appearance. In another genus (10) the distal end of the flask-
shaped body bears a circlet of large fringed cilia, giving the animal
the appearance of a Rotifer (vide Section VII.).

In addition to cilia, many genera possess delicate sheets of
protoplasm or undulating membranes in connection with the

n PHYLUM PROTOZOA 93

peristome. They contract so as to produce a wave-like movement
which aids in the ingestion of food. In some cases (Fig. 71, 11)
the undulating membrane (n, nib.) is a very large and obvious
structure.

Certain peculiar forms have yet to be mentioned. Multicilia (Fig.
71, 12) has an irregular body of varying form, and bears a small
number of very long flagellum-like cilia. Another genus in which
the cilia approach to flagella is LnpTiomonas (13), the ovoid body of
which bears a tuft of close-set cilia at its anterior end. Actino-
bolus (14) is remarkable for the possession, in addition to cilia, of
long retractile tentacles used for attachment. In Didinium ( 15)
the barrel-shaped body is encircled by two hoops of cilia.

As we have seen, the meganucleus in Paramcecium is ovoid : in
other genera it may be elongated and band-like (3, m-g. nu.), horse-
shoe-shaped (9), very long and constricted at intervals so as to
look like a string of beads (16), or much convoluted and branched
(17). In some genera the meganucleus undergoes repeated
divison, forming at last a very great number of small bodies only
discoverable by staining : this process of fragmentation of the nucleus
may proceed so far that the protoplasm of a stained specimen has
the appearance of being strewn with granules of chromatin. The
discovery of f this phenomenon has tended to throw doubt on the
reported total absence of a nucleus in some Rhizopods.

In nearly all species one or more micronuclei are present, the
number sometimes reaching nearly thirty. In Opalina (Fig. 75)
numerous nuclear bodies (nu.) are present, some of which on
account of their mitotic mode of division are to be regarded as
micronuclei, while the rest are meganuclei.

In Vorticella and other peritrichous genera there is a single
contractile vacuole (Fig. 73, c. vac), which, like that of Euglena,
opens through the intermediation of a reservoir into the vestibule.
In the remaining Ciliata there may be one, two, or many some-
times a hundred contractile vacuoles. They may be scattered
all over the cortex (Fig. 71, 18), or arranged in one or two rows
(8). The star-like arrangement of radiating canals, described in
Paramcecium, occurs in several genera : or there may be two long
canals, or the number of these channels in the protoplasm may
reach thirty (19, c). In some instances the protoplasm is hollowed
out by numerous non-contractile vacuoles (18, vac.) so as to
have a reticulate appearance, reminding us of the extra-capsular
protoplasm of Radiolaria.

Trichocysts, like those of Paramcecium, are found in many
holotrichous forms, but arc rarely present in the other subdivisions
of the order. In the peritrichous Epistylis umbellaria, however,
there are found numerous minute capsules (Fig. 71, 9, ntc.)
arranged in pairs, each containing a coiled thread. They are

?nth

mg.nu M,<

WS^$' ''-^ \ &

fey//. ..'- X

C-

vcw

A.-.*'. /-d V^i- ;.:),,-

*

2.Nycrorherus

S.Lacrymaria

10.TinMnnidium

3.E pisrylis

12.MulMcilia 13-Lophomonas

H.Cyclidium

H.Acfinobolus

. fore

,$sZSZ&~ law

'rmi.ntt \l\fvac

!8.Tracheliu$ mophryoglena
IdCondylos^ma r

17.0pa!inopsis

FIG. 71. Various forms of Ciliata. Pa shows part of a colony, 1> a single zooid, and
c a couple of nematocysts ; n. anus ; f. (in Ls) cuticle ; c. (in 1!) excretory canals ; c. rac.
contractile vacuole; ,/". vac. food vacuole; .'/. gullet; /////- nu, meganucleus ; -ini. nu. micro-
nucleus; intJt. mouth; nu. nucleus ; utc. nematocyst ; />. (in 15) a J'ftrama-civ.iii seized by
Didiiitiiuii', f. tentacle; i>. nth. undulating membrane; rac. non-contractile vacuole; rst.
vestibule. (From Biitschli's Protozoa, after various authors.)

II

PHYLUM PROTOZOA

95

obviously structures of the same character as trichocysts, and
their resemblance to the ncmatocysts so characteristic of Ccelenterata
(vide Section IV.) is singularly close.

Digestive Apparatus. --Man} parasitic forms (Fig. "71, 8, 17 ;
Fig. 75) have no mouth or gullet, and are nourished by absorption
of the digested food in the intestine of their host. The simplest
condition of the ingestive apparatus is found in Prorodon (Fig.
71, 1) and its allies, in which the mouth (mth.) is at one pole
of the ovoid body, and is closed except during the ingestion of
food, and the gullet (g.) is a short, straight tube. Such forms,
on account of the symmetrical disposition of their organs and the
want of differentiation of their cilia they are all holotrichous-
may be considered as the lowest or least specialised of the Ciliata.

*fi %

LDictyocysta

S.Thuricola
2. Pyxicola

5. Srichol-richa

FIG. 7:2. Various forms of Ciliata. In 1 the shell alone is shown ; m. contractile fibre ; op.
operculum. (From Butschli's Protozoa, after various authors.)

From them there is a fairly complete gradation to genera, like
Paramoecium, having the permanently open mouth on the left side
of the ventral surface, at the end of a well-marked buccal grove
or peristome. Vorticella (Fig. 73) and its allies are peculiar in
having the edge of the peristome (per.) thickened so as to form a
projecting rim, and in the development of an elevated disc (d.) from
the area thus enclosed : the mouth (mth.) lies between the peri-
stome and the disc, and between it and the gullet proper (gull.) is
interposed a section of the ingestive tube called the vestibule
into which the reservoir opens, and which contains the anal
spot. In Nyctotherus (Fig. 71, #) and some other genera there is,
instead of the temporary anal spot described in Paramcecium, a
distinct anal aperture (a.).

96

ZOOLOGY

SECT.

Most of the Ciliata are naked, having no shell or other form of
skeleton ; but in a few forms the body is provided with a shell or
lorica, formed of a chitinoid material, and reminding us of the

pen

FIG. 73. Vorticella. A, B, living specimens in different positions , C, optical section ; D 1 , D-,
diagrams illustrating coiling of stalk ; El, E-, two stages in binary fission; E3, free zooid ;
FI, F-, division into mega- and mierozooids ; G 1 , G 2 , conjugation ; H 1 , multiple fission of
encysted form ; H-, H : *, development of spores ; ax. f. axial fibre ; cort. cortex ; cu. cuticle ;
c. vac. contractile vacuole ; d. disc ; gull, gullet ; m. microzooid ; mth. mouth ; nil. mega-
nucleus ; per. peristome. (From Parker's Biology.)

similar structure found in so many of the Mastigophora. Some
(Fig. 7J, 4) have bell-like shells, variously ornamented, and in
others (Fig. 72, 1) the similarly shaped shell is perforated and
resembles the skeleton of some of the Radiolaria. A chitinoid
plate or operculum (Fig. 72, 2, op.) may be fixed to the edge of the
peristome, and, when the animal is retracted in its case accurately
closes the mouth of the latter, or a similar operculum (J) is

II

PHYLUM PROTOZOA

97

attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (w.), which acts as a retractor muscle.

Compound forms or colonies are common among the Peritricha,
rare in the other subdivisions. Many peritrichous forms occur as
branched, tree-like colonies, often of great complexity (Fig. 71, 9a;
Fig. 74). The stem of these may be a purely cuticular structure
and non-contractile (Fig. 71, 9, I), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 73, ax.f.}. In Ophridium
(Fig, 72, 4) the colony is an irregular mass, sometimes 3-4 cm. in
diameter, consisting of a gelatinous substance in which a delicate,
branching stem is embedded, each branch terminating in a zooid.
Some genera (Fig. 72, 5) secrete a hollow, brown, gelatinous tube,
branched dichotomously ; the end of each branch is the habitation
of one of the zooids.

Reproduction. Transverse fission is the universal method of
reproduction, the entire process taking from half an hour to two

F 2

--'"*

-

B

FIG. 74._ Zoothammum arbuscula. A, entire colony; B, the same, natural size ; C, the
same, retracted ; D, nutritive zooid ; E, reproductive zooid ; Fl, F 2 , development of reproduc-
tive zooid ; ax.f. axial fibre ; c. rac. contractile vacuole ; nu. nucleus ; n.z. nutritive zooid ;
r.z. reproductive zooid. (From Parker's Biology, after Saville Kent.)

hours in different species. In Vorticella (Fig. 73, E) and other
Peritricha the plane of division is parallel to the long axis of the
bell-shaped body, but as the distal surface probably corresponds
with the dorsal surface of such forms as Paramoecium, fission
is really transverse in this case also. In such simple Peritricha
as Vorticella division proceeds until two zooids are produced on
a single stalk ; one of the two then acquires a second circlet
of cilia near its proximal end, becomes detached (E 3 ), and, after
leading a free-swimming life for a time, settles down and develops
a stalk : in this way the dispersal of the non-locomotive species is
ensured. In many species of Zoothamnium (Fig. 74) the zooids
VOL. I H

98

ZOOLOGY

SECT.

are dimorphic : the ordinary bell-shaped forms (n.z.) divide in
the usual way, but as they remain attached, the process results only
in the increased complexity of the colony, not in the development
of a new one. The larger zooids (r. 2.) are globular and mouthless :
they become detached, swim off, and, after a short free existence,
settle down, develop a stalk (F), divide, and so form a new colony.
In Vorticella multiplication by ludding also occurs: a small
process is given off from one side (Fig. 73, F), develops a basal
circlet of cilia, and swims off as a microzooid, the parent individual

FIG. 75. Opalina ranarum. A, living specimen; B, stained specimen showing nuclei; C,
stages in nuclear division ; D F, stages in fission ; G, final product of fission ; H, encysted
form ; I, young form liberated from cyst ; K, the same after multiplication of the nucleus
has begun ; nu. nucleus. (From Parker's Biology, after Saville Kent and Zeller.)

or megazooid being left attached to the stalk. Obviously this
process is simply a modification of binary fission, the products of
division being of very different dimensions instead of equal-sized as
is the more usual case.

Spore-formation take place in Colpoda. The Infusor becomes
encysted, and divides into two, four, and finally eight masses, each
of which, becoming surrounded by a special investment, becomes
a spore. A somewhat similar process has been described in
Vorticella (Fig. 73, H} and others.

A peculiar kind of spore-formation, specially adapted to the
requirements of an internal parasite, takes place in Opalina

ii PHYLUM PROTOZOA 99

(Fig. 75), a parasite in the intestine of the Frog. Binary fission
takes place (D, E, F), and is repeated again and again so rapidly
that the daughter-cells are unable to grow to the adult size before
the next division. The final results of the process are small bodies
(G), each with only two or three nuclei instead of the large number
characteristic of the adult. These become encysted (H), and in
this passive condition are passed out of the Frog's intestine with
its faeces, frequently being deposited on water-weeds. All this
takes place during the Frog's breeding season : the tadpoles or Frog-
larvae feed upon the water-plants, and in doing so frequently take
in the spores or encysted Opalinoe along with their food. When
this occurs the cyst is dissolved by the digestive juices of the host,
and the protoplasm of the spore is set free as a rounded body
with a single nucleus (I), which rapidly grows into an adult
Opalina (K).

Conjugation, in the form of a temporary union accompanied by
interchange of micronuclei, has been described in Paramoecium
(p. 90), and takes place in many Ciliata. In others (e.g. Stylonychia
histrio) there is a complete union of the two gametes. In
Vorticella union is also permanent, and takes place, not between
two ordinary forms, but between one of the ordinary stalked
individuals, or megagametes, and a free-swimming, small form, or
microgamete, produced, as described above, by budding (G 1 , G' 2 ).
The essence of conjugation is the reception of nuclear material
derived from another individual : its effect appears to be a renewal
of vitality, usually manifesting itself in increased activity in
multiplication by fission.

ORDER 2. TENTACULIFERA.

Judged from the adult structure alone, the members of this
order would certainly be placed in a separate class of the Protozoa :
it is only in virtue of the facts of development that they are
united in a single class with the Ciliata.

The body may be globular (Fig. 76, la), ovoid (/b), or cup-
shaped (2a), but presents nothing like the variety of form met
with among the Ciliata. The distinguishing feature of the group
is furnished by the tentacles which are always present in greater
or less number, and which, in some cases at least, are the most
highly differentiated organs found in the whole group of Protozoa.
The characters of the tentacles vary strikingly in the different
genera.

In the common forms Acineta (2), and Podophrya (1\ the ten-
tacles spring either from the whole surface, or in groups from the
angles of the somewhat triangular body. Each tentacle is an elon-
gated cylindrical structure (7c), capable of protrusion and retrac-
tion, and having its distal end expanded into a sucker. It is, more-
over, practically tubular, the axial region consisting of a semi-fluid

H 2

100

ZOOLOGY

SECT.

protoplasm, while the outer portion is tolerably firm and resistant.
When partially retracted, a spiral ridge is sometimes observable

l.Podophrya

4.Dendrocometes c r- ,. 6. S f>haer obhrya

3.Rhynchero 5. Ephelota

! C VCU)

&<M/M.

, , \JBnfm\ H^W

wii-

!

T.Obhryodendron

c.wto

8.Ef>helora

9, Dendrosomo

FIG. 76. Various forms of Tentaculifera. la and b, two species of Fotlnphn/a ; c, a
tentacle much enlarged ; 2a, Acineta jolyi ; 2li, A. tuberosa ; in G the animal has captured
several small Ciliata ; 8a, a specimen multiplying by budding ; 6'fc, a free ciliated bud ; 9a, the
entire colony ; 9b, a portion of the stem ; He, a liberated bud ; a, organism captured as food ;
6.c. brood-cavity ; bd. bud ; c. vac contractile vacuole ; /. lorica ; mg. nu. meganucleus ;
mi. nu. micronucleus ; t. tentacle. (After Biitschli and Saville Kent.)

around the tentacle : this may indicate the presence of a band of
specially contractile protoplasm, resembling the axial fibre in the

II

PHYLUM PROTOZOA 101

stalk of Vorticdla. Infusors and other organisms are caught by
the tentacles (4, 0), the cuticle of the prey is pierced or dissolved
where the sucker touches it, and the semi-fluid protoplasm can
then be seen flowing down the tentacle into the body of the
captor, A single tentacle only may be present (3), or the tentacle
may be branched (4), the extremity of each branch being suc-
torial. In some forms there are no terminal suckers (5), and the
tentacles are waved about to catch the prey instead of standing
out stiffly as in Acineta. In other cases there are one or more
long, striated tentacles with tufted ends (7).

The nucleus may be ovoid (/a), horseshoe-shaped, or branched
(8, 9) : in many cases a micronucleus (1 a, mi. nu.) has been found
and it probably occurs in all. There are one or more contractile
vacuoles (c. vac.).

Some genera are naked (1) : others form a stalked shell or
lorica (a) like that met with in many of the Mastigophora.

The only colonial form is the wonderful Dendrosoma (9), in
which the entire colony attains a length of about 2 mm., and bears
an extraordinary resemblance to a zoophyte (vide Sect. IV.). It
consists of a creeping stem from which vertical branches spring,
and the various ramifications of these are terminated in Podo-
phrya-like zooids with suctorial tentacles. The nucleus is very
remarkable, extending as a branched axis throughout the colony
(b, nu.). Micron uclei of the ordinary character are present
as well.

Reproduction by Unary fission takes place in many species.
In Ephclota gemmipara (S) a peculiar process of budding occurs :
the distal end of the organism grows out into a number of
projections or buds, into which branches of the nucleus extend.
These become detached, acquire cilia on one surface, and swim
off (b). After a short active existence tentacles appear and the
cilia are lost. In this case budding is external, but in Acineta
tnberosa (2b) the buds become sunk in a depression, which is finally
converted into a closed brood-cavity (b.c.) : in this the buds take on
the form of ciliated embryos, which finally escape from the parent.
In Dendrosoma the common stem of the colony produces internal
buds (b, Id.).

Further Remarks on the Protozoa.

The majority of the Protozoa are aquatic, the phylum being
equally well represented in fresh and salt water. They occur
practically at all heights and depths, from 8,OOQ v to 10,000 feet
abovejsea-level, to a depth of from 2,000 to 3,000 fathoms. Some
forms, such as species of Amoeba and Groin ia, live in damp sand
and moss, and may therefore be almost considered as terrestrial
organisms. In accordance with their small size and the readiness
with which they are transported from place to place a large pro-

102 ZOOLOGY SECT.

portion of genera and even of species are universally distributed,
being found in all parts of the world where the microscopic fauna
has been investigated.

Numerous parasitic forms are known. Besides the entire class
of Sporozoa, species of Rhizopoda, Mastigophora, and of Infusoria
occur both as internal and external parasites. Species of Amoeba
are common in the intestines of the higher animals, and one
species has been found in connection with a cancerous disease in
Sheep. A ciliate Infusor, Icldhyophthirius, is found in the skin of
freshwater Fishes, where it gives rise to inflammation and death.

Many instances have been met with in our survey of the
Phylum of compound or colonial forms, the existence of which
seems at first sight to upset our definition of the Protozoa as
unicellular animals. But in all such cases the zooids or unicellular
individuals of the colony exhibit a quasi-independence, each, as a
rule, feeding, multiplying, and performing all other essential
animal functions independently of the rest, so that the only
division of labour is in such forms as Zoothamnium and Volvox,
in which certain zooids are incapable of feeding, and are set apart
for reproduction. In all animals above Protozoa, on the other
hand, the body is formed of an aggregate of cells, some of which
perform one function, some another, and none of which exhibit
the independent life of the zooid of a protozoan colony. It cannot,
however, be said that there is any absolute distinction between a
colony of unicellular zooids and a single multicellular individual :
Proterospongia and Volvox approach very near to the border-land
from the protozoan side, and a similar approach in the other
direction is made by certain animals known as Mesozoa, which will
be discussed hereafter (Sect. IV.). Moreover, the Mycetozoa, the
plasmodia of which are formed by the fusion of Amcebulae, the
nuclei of the latter remaining distinct and multiplying, are rather
non-cellular than unicellular. This point will also be referred to
at the conclusion of the section on Sponges (Sect. III.).

In each division of the Protozoa we have found comparatively
low or generalised forms side by side with comparatively high or
specialised genera. For instance, among the Rhizopoda, there
can be no hesitation in placing the Lobosa, and especially Prota-
moeba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 52. 2 and 8) and Heteromita are obviously the
lowest forms, Noctiluca and the Dinoflagellata the highest. But
whether the Rhizopoda, as a whole, are higher or lower than the
Flagellata, is a question by no means easy to answer. A flagellum
certainly seems to be a more specialised cell-organ than a
pseudopod, and some of the Mastigophora rise above the highest
of the Rhizopoda in the possession of a firm cortex and cuticle,

II

PHYLUM PROTOZOA

103

and the consequent assumption of a more definite form of body
than can possibly be produced by the flowing protoplasm of a
Foraminifer or a Radiolarian. On the other hand, the nucleus
of the Radiolaria is a far more complex structure than that of
the Mast rgophora:. and in Foraminifera, Radiolaria, and Heliozoa
the organism frequently begins life as a flagellula, a fact which,
on the hypothesis that the development of the individual recapitu-
lates that of the race, appears to indicate that these orders of
Rhizopoda are a more recently developed stock than at any rate
the lower Flagellata. These circumstances, and the fact that
Mastigamoeba might equally well be classed as a lobose Rhizopod
with a flagellum or as a Flagellate with pseudopods, seem to
indicate that the actual starting-point of the Protozoa was a form

Radiolaria
Foraminifera

Lobosa

Mycetozoa

Dinoflagellata

Cystoflagellata

Heliozoa Choano .
Flagellata

Tentaculifera

Ciliata

Flagellata

Sporozoa

FIG. 77. 'Diagram showing the mutual relationships of the chief groups of Protozoa.

capable of assuming either the amoeboid or the flagellate phase.
From such a starting-point the Lobosa, Foraminifera, Heliozoa,
Radiolaria, and Flagellata diverge in different directions, the first
four keeping mainly to the amoeboid form, but assuming the
flagellate form in the young condition, in the case of Foraminifera,
Heliozoa, and Radiolaria.

The Choanoflagellata, Dinoflagellata, and Cystoflagellata are
obviously special developments of the Flagellate type along
diverging lines.

As to the Ciliata, Midticilia and Lopliomcnas (Fig. 71,1% and 13)
appear to indicate the derivation of the order from the Flagellate
type, since their cilia are long and flageilum-like ; but the evidence
is not strong and no other is at hand. The derivation of the Tenta-
culifera from a ciliate type appears to be clear. The Tentaculifera
and the hypotrichous Ciliata are undoubtedly the highest develop-

104 ZOOLOGY sECi n

ment of the Protozoan series, since they show a degree of
differentiation attained nowhere else by a single cell.

The Mycetozoa appear to have been derived from the common
amoeboid-flagellate stock, since they are all predominantly amoe-
boid in the adult condition, flagellate when young. The Sporozoa
probably had a similar origin, but the characters of this class have
evidently been profoundly modified in accordance with their
parasitic mode of life.

The diagram on the previous page is an attempt to express
these relationships in a graphic form.

SECTION III
PHYLUM AND CLASS PORIFERA [PARAZOA]

The microscopic animals described in the preceding section
are, as already repeatedly pointed out, characterised by their
unicellular character, and in this respect stand in contrast to the
remainder of the animal kingdom. The animal kingdom is thus
capable of division into two great subdivisions, the Protozoa or uni-
cellular animals, and the Mctazoa or multicellular forms the latter
comprising all the groups that remain to be dealt with. In the
earliest stage of their existence all the multicellular animals or
Metazoa are, as already pointed out (p. 19), in a unicellular
condition, originating in a single cell, the fertilised ovum or
oosperm. By the process of segmentation or yolk-division the
unicellular oosperm becomes converted in all the Metazoa
into a mass of cells from which the body of the adult animal is
eventually built up. Of the Metazoa, the group which approxi-
mates most closely to the Protozoa is that now to be dealt vvith-
the Porifcra or Sponges. With all the other multicellular groups
the Sponges are so strongly in contrast that the Metazoa may
be regarded as falling into two main divisions the Porifera or
Parazoa, on the one hand, and all the rest of the Metazoa, grouped
together as Enterozoa, on the other.

1. EXAMPLE OF THE CLASS Si/con gdatinosum.

General External Appearance and Gross Structure.

Sycon gdatinosum^ one of the Calcareous Sponges, has the form of a
tuft, one to three inches long, of branching cylinders (Fig. 78),all con-
nected together at the base, where it is attached to the surface of a
rock or other solid body submerged in the sea. It is flexible, though
of tolerably firm consistency ; in colour it presents various shades
of gray or light brown. To the naked eye the surface appears
smooth, but when examined under the lens it is found to exhibit
a pattern of considerable regularity, formed by the presence of

1 This species is an inhabitant of southern seas. In all essential respects the
account of it given above will apply to S. ciliatum, a common European species
which differs chiefly in the absence of the pore-membranes,

J05

106

ZOOLOGY

SECT.

innumerable elevations of a polygonal shape, which cover the whole
surface and are separated off' from
one another by a system of depressed
lines. In these depressions between
the elevations are to be detected, under
the microscope, groups of minute
pores the ostia or inhalant pores.
At the free end of each of the cylin-
drical branches is a small but distinct
opening, surrounded by what appears
like a delicate fringe. When the
branches are bisected longitudinally
(Fig. 79), it is found that the terminal
openings (o) lead into narrow passages,
wide enough to admit a stout pin,
running through the axes of the
cylinders ; and the passages in the interior

FIG. 78. Sycon gelatinpsum.

Entire sponge, consisting of a
group of branching cylinders
(natural size).

FIG. 7tf. -Sycon gelatinosum.-A portion slightly

magnified; one cylinder (that to the right) bisected
longitudmally to show the central paragastric cavity
opening on the exterior by the osculum, and the
position of the incurrent and radial canals ; the
former indicated by the black bands, the latter,
dottediip. marks the position of three of the grot ips
of inhalant pores at the outer ends of the incurrent

canals ; o. oscuium.

of the various
branches join where the
branches join the pas-
sages thus forming a
communicating system.
On the wall of the
passages are numerous
tine apertures which re-
quire a strong lens for
their detection. The
larger apertures at the
ends of the branches
are the oscula of the
sponge, the passages the
paragastric cavities. If
a living Sycon is placed
in sea-water with which
has been mixed some
carmine powder, it will
be noticed that the
minute particles of the
carmine seem to be at-
tracted towards the sur-
face of the sponge, and
will often be seen to

P 8 " 88 intO its Substance

through the minute in-

, -,

halant pOrCS Or OStia

i J J

already mentioned as

nofnrrinfy in crrnnrm KP-
o 6 1<J1 4 X

tweeii the elevations on

Ill

PHYLUM AND CLASS PORIFERA

107

the outer surface. This would appear to be due to the passage of
a current of water into the interior of the sponge through these
minute openings dotted over the surface ; and the movement of
the floating particles shows that a current is at the same time
flowing out of each of the oscula. A constant circulation of
water would thus be seen to be carried on currents moved
by some invisible agency flowing through the walls of the sponge
to the central paragastric cavities, and passing out again by
the oscula.

If a portion of the S} 7 con is firmly squeezed, there will be
pressed out at first sea-water, and then, when greater pressure is

R

FIG. 80. Sycon gelatinosum. Section through the wall of a cylinder taken at right angles
to the long axes of the canals, highly magnified ; co, collencytes ; 1C, incurrent canals ;
ov. young ova ; R, radial canals ; sp. triradiate spicules.

exerted, a quantity of gelatinous-looking matter, which, on being
examined microscopically, proves to be partly composed of a
protoplasmic material consisting of innumerable, usually more or
less broken cells with their nuclei, and partly of a non-protoplasmic,
jelly-like substance. When this is all removed there remains
behind a toughish felt-like material, which maintains more or less
completely the original shape of the sponge. This is the skeleton
or supporting framework. A drop of acid causes it to dissolve
with effervescence, showing that it consists of carbonate of
lime. When some of it is teased out and examined under the
microscope, it proves to consist of innumerable, slender, mostly
three-rayed microscopic bodies (Figs. 80 and 81 , sp) of a clear
glassy appearance. These are the calcareous spicules which form

108

ZOOLOGY

FIG. 81. Sycon grelatinosum Transverse section
through the wall of a cylinder (parallel with the
course of the canals), showing one incurrent (1C),
and one radial (R) canal throughout their length ;
xp. triradiate spicules ; sp'. oxeote spicules of dermal
cortex (rfc.) ; sp". tetraradiate spicules of gastral
cortex ((]c.) ; ec. ectoderm ; en. layer of flattened cells
lining the paragastric cavity ; pm. pore-membrane ;
pp. prosopyles ; ap. apopylc ; di. diaphragm ; exc.
excurrent passage ; P.G. paragastric cavity ; em.
early embryo ; em', late embryo. The arrows in-
dicate the course of the water through the sponge.

SECT-

the skeleton of the
Sycon.

The arrangement of the
spicules, their relation to
the protoplasmic parts,
and the structure of the
latter, have to be studied
in thin sections of hard-
ened specimens (Figs. 80
and 81). An examination
of such sections leads to
the following results.

Microscopic struc-
ture. Covering the outer
surface of the sponge is
a single layer of cells the
dermal layer or ectoderm l
(Fig. 81, ec) through
which project regularly-
arranged groups of needle-
like and spear-like spicules
(sp'), forming the pattern
of polygonal elevations on
the outer surface. The
cells of the ectoderm are
in the form of thin scales,
which are closely cemented
together by their edges.
The paragastric cavities
are lined by a layer of
cells (en) which are, like
those of the ectoderm, thin
flattened scales. Running
radially through the thick
wall of the cylinders arc a
large number of regularly-
arranged straight passages.
Of these there are two sets,
those of the one set the
incurrent canals (Figs. 80

1 The terms ectoderm and
endoderm are here used as con-
venient terms for the outer and
inner layers of the Sponge,
though, as will appear later,
these layers differ completely in
their mode of formation from
the layers so named in the
higher phyla.

Ill

PHYLUM AND CLASS PORIFERA

109

and 81 1C) narrower, and lined by ectoderm similar to the ectoderm
of the surface ; those of the other set the radial or flagellate canals
(R) rather wider, octagonal in cross-section, and lined by endoderm
continuous with the lining of the paragastric cavity. The incurrent
canals end blindly at their inner extremities not reaching the
paragastric cavity ; externally each becomes somewhat dilated,
and the dilatations of neighbouring canals often communicate.
These dilated parts are closed externally by a thin membrane-
the pore-membrane (Fig. 81 pm, and Fig. 82), perforated by three
or four small openings (Fig. 82, p) the ostia already referred to.
The flagellate canals are blind at their outer ends, which lie at a
little distance below the surface opposite the polygonal projections
referred to above as forming a pattern on the outer surface ;
internally, each communicates with the paragastric cavity by a
short, wide passage the excurrent canal (Fig. 81 exc). Incurrent

'.

FIG. 82. Sycon gelatinosum. Sur-
face view of a pore-membrane highly
magnified ; p, ostium ; R. position of
the outer end of a radial canal.

FIG. 83. Sycon gelatine sum,

An apopyle surrounded by its dia-
phragm ; m. contractile cells.

and flagellate canals run side by side, separated by a thin layer of
sponge substance except at certain points, where there exist small
apertures of communication the prosopylcs (pp), uniting the
cavities of adjacent incurrent and flagellate canals. Each proso-
pyle is a perforation in a single cell termed a porocyte.

The ectoderm lining the incurrent canals is of the same char-
acter as that of the outer surface. The endoderm of the
flagellate canals, on the other hand, is totally different from that
which lines the paragastric cavity. It consists of cells of columnar
shape ranged closely together so as to form a continuous layer.
Each of these flagellate endoderm cells, or collared cells, or clwano-
cytes, as they are termed, is not unlike one of the Choanoflagellate
Protozoa (p. 77) ; it has a nucleus, one or more vacuoles, and, at
the inner end, a single, long, whip-like flagellum, surrounded at its
base by a delicate, transparent, collar-like upgrowth, similar to
that which has already been described as occurring in the
Choanoflagellata. If a portion of a living specimen of the sponge

110 ZOOLOGY SECT.

is teased out in sea-water, and the broken fragments examined
under a tolerably high power of the microscope, groups of these
collared cells will be detected here and there, and in many places
the movement of the flagella will be readily observed. The
flagellum is flexible but with a certain degree of stiffness,
especially towards the base, and its movements resemble those
which a very supple fishing-rod is made to undergo in the act of
casting a long line the movement being much swifter and
stronger in the one direction than in the other. The direction
of the stronger movement is seen, when some of the cells are
observed in their natural relations, to be from without inwards.
It is to these movements that the formation of the currents of
water passing along the canals is due. The collars of the cells in
specimens teased in this way become for the most part drawn back
into the protoplasm.

The short passage or excurrent canal, which leads inwards from
the flagellate canal to the paragastric cavity, differs from the
former in being lined by flattened cells similar to those of the
paragastric cavity ; it is partly separated from the flagellate canal
by a thin diaphragm (Fig. 81, di, and Fig. 88), perforated by a
large circular central aperture the apcrpyle (ap) which is capable
of being contracted or dilated : its opposite aperture of com-
munication with the paragastric cavity, which is very wide, is
termed the gastric ostium of the excurrent canal.

The effect of the movement of the flagella of the cells in the
flagellate canals is to produce currents of water running from
without inwards along the canals to the paragastric cavity. This
causes water to be drawn inwards through the prosopyles from
the incurrent canals, and, indirectly, from the exterior through the
perforated membranes at the outer ends of the latter.

Between the ectoderm of the outer surface and of the incurrent
canals, and the endoderm of the inner surface and of the flagellate
canals, are a number of spaces filled by an intermediate layer
the mesnglcea in which the spicules of the skeleton are
embedded. Each spicule is developed from cells termed sclcro-
blasts, which migrate inwards from the ectoderm. Each ray is
formed by the agency of a separate scleroblast, so that there are
three at least of the latter for each triradiate, and four for each
tetraradiate spicule. The spicules (Figs. 80 and 81, sp) are
regularly arranged, and connected together in such a way as to
protect arid support the soft parts of the sponge. Most are, as
already noticed, of triradiate form. Large numbers, however, are
of simple spear-like or club-like shape (sp') ; these, which are
termed the oxeote spicules, project on the outer surface beyond the
ectoderm, and are arranged in dense masses, one opposite the
outer end of each of the ciliated canals, this arrangement pro-
ducing the pattern already referred to as distinguishable on the

in PHYLUM AND CLASS PORIFERA 111

out surface. The thick outer layer in which the bases of these
oxeote spicules lie embedded, is termed the dermal cortex (do). A
thick stratum at the inner ends of the canals and immediately
surrounding the paragastric cavity is termed the (/astral cortex (gc).
It is supported by triradiate and also by tetraradiate spicules, one
ray of each of which (*p") frequently projects freely into the para-
gastric cavity, covered over by a thin layer of flattened endoderm
cells.

The mesoglcea itself, as distinguished from the spicules which
lie embedded in it, consists of a clear gelatinous substance
containing numerous nucleated cells of several different kinds.
Most of these are small cells of stellate shape, with radiating
processes the connective-tissue cells or collencytcs (Fig. 80, co) ;
others are fusiform ; a good many the amoeboid wandering cells
are Amoeba-like, and capable of moving about from one part of
the sponge to another.

Around the inhalant pores and the apopyles are elongated cells
(Figs. 82 and 83), sometimes prolonged into narrow fibres. These
are contractile effecting the closure of the apertures in question,
and are therefore to be looked upon as of the nature of muscular
fibres. In the case of the inhalant pores they are ectodermal ; in
that of the apopyles they are endodermal. A band of similar
fibres surrounds the osculum the oscular sphincter.

The sexual reproductive cells the ova (Figs. 80 and 81, ov) and
sperms are developed immediately below the flagellate endoderm
cells of the flagellate canals, arid in the same situation are to be
found developing embryos (cm, em'\ resembling in their various
stages those of Sycon raphanus, as described below.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

Sponges are plant-like, fixed, aquatic Metazoa, all, with the
exception of one family, inhabitants of the sea. The primary form
is that of a vase or cylinder, the sides of which are perforated by a
number of pores and in the interior of which is a single cavity;
but in the majority of Sponges a process of branching and folding
leads to the formation of a structure of a much more complex
character. The surface of the Sponge is covered by a single layer
of flattened cells the ectoderm l arid the internal cavities, or a
part of thorn, are lined by a second single layer the cndodcrm-
part or the whole of which consists of a single layer of choanocytcs,
i.e. columnar collared cells, each provided internally with a long
flagellum. Between these two layers is a quantity of tissue
usually of a gelatinous consistency the mcsogloba containing a
number of cells of various kinds. The wall of the Sponge is
pierced by a number of apertures. The skeleton or supporting

1 See footnote on p. 108.

112 ZOOLOGY SECT.

framework, developed in the mesogloea from cells derived from
the ectoderm, consists in some cases of fine, flexible fibres of a
material termed spongin ; in others of spongin-fibres supplemented
by microscopic siliceous spicules ; in others of siliceous spicules
alone ; in others of spicules of carbonate of lime. Reproduction
takes place both asexual ly by the formation of gcmmules, and
sexually by means of ova and sperms. The ovum develops into a
ciliated free-swimming larva, which afterwards becomes fixed and
develops into the plant-like adult Sponge.

The Sponges are sufficiently far removed in structure from the
rest of the Metazoa to justify us in looking upon them as con-
stituting one of the great divisions or phyla of the animal kingdom.
At the same time there is so much uniformity of structure within
the group that a division into classes is not demanded ; the phylum
Porifera contains a single class.

The class Porifera is classified as follows :

Sub-Class I. Calcarea.

Sponges with a skeleton of calcareous spicules, and with com-
paratively large collared cells.

ORDER 1. HOMOCCELA

Calcareous Sponges in which the internal lining membrane
consists throughout of flagellate collared cells.

ORDER 2. HETEROCCELA.

Calcareous Sponges in which the paragastric cavity is lined by
flattened cells, the collared cells being restricted to flagellate
canals or chambers.

Sub-Class II. Hexactinellida.

Sponges with six-rayed, tri-axon, siliceous spicules, and simple
canal system represented by unbranched or branched flagellate
chambers.

Sub-Class III. Demospongia.

Sponges either devoid of skeleton or with spongin fibres alone,
or a combination of spongin fibres and siliceous spicules, the
latter, when present, never six-rayed ; the canal system of the
Rhagon type (p. 118), usually complicated.

Systematic Position of the Example.

Sycon gelatinosum is one of many species of the genus Sycon.
Sycon is one of several genera of the family Syccttidw ; and the

in PHYLUM AND CLASS PORIFERA 113

family Sycettidce is one of several families of the order Heterocwla
of the class Calcarea. Among the families of the Heterocoela,
that of the Sycettidce is distinguished by the following features,
which characterise all its members :

" The flagellate chambers are elongated, arranged radially around
a central paragastric cavity, their distal ends projecting more or less
on the dermal surface, and not covered over by a continuous cortex.
The skeleton is radially symmetrical."

Of the genera into which the Sycettidce are divided, Sycon is
characterised as follows :-

" The flagellate chambers are not intercommunicating ; their
distal ends are provided each with a tuft of oxeote spicules."

The members of one of the other genera of the family Sycetta-
while possessing the general characteristics of the family, differ from
those of the genus Sycon in wanting the tufts of oxeote spicules ;
those of a third Sycantha have the flagellate chambers united
in groups ; the chambers of each group intercommunicating by
openings in their walls, and each group having a single common
opening into the gastric cavity. The members of this genus re-
semble Sycon, and differ from Sycetta, in the presence of tufts
of oxeote spicules at the distal ends of the flagellate chambers.

These distinctions between classes, orders, families, and genera
are of an entirely arbitrary character. No such divisions exist in
nature ; and they are merely established as a convenient way of
grouping the sponges and facilitating their classification. But a
classification of this kind, if carried out on sound principles, should
nevertheless have something corresponding to it in nature, inas-
much as the grouping of the various divisions and subdivisions
aims at expressing the relationships of their members to one
another. The members, for example, of the family Sycettidce are
all regarded, on account of the features which they possess in
common, as being more nearly related to one another than to the
members of the other families, and as 1 having been derived from a
common ancestor which also possessed those features the diver-
gences of structure which we observe in the different genera and
species being the result of a gradual process of change.

Within the limits of the genus Sycon, S. gelatinosum is distin-
guished from the rest as a group of individual Sponges all possess-
ing certain specific characters which it will be unnecessary to
detail here. But the individual Sponges referable to this species
frequently differ somewhat widely from one another : there are
numerous individual variations. If we compare a number of
specimens all possessing the species-characters of Sycon gelatino-
sum, we find that they differ in the number of branches, in the
shape of the cylinders some being relatively narrow, some re-
latively wide in the degree of development of the oscular crown

of spicules, in the ratio of the thickness of the wall to the width
VOL. I I

114 ZOOLOGY SECT.

of the contained paragastric cavities, and in many other more
minute points ; in fact, we find as a result of the comparison that no
two specimens are exactly alike. These differences are so great
that some very distinct races or varieties of S. gelatinosum have
been recognised, and some have received special names. Here
again, as in the case of the families and orders, the distinctions
are of an arbitrary character some writers on Sponges setting
down as several species what others regard merely as varieties of
one species. It is impossible, in fact, to draw a hard and fast line
of distinction between species and varieties. In the higher groups
of animals the attempt is made to establish a physiological dis-
tinction ; all the members of a species are regarded as being fertile
inter se, and capable of producing fertile offspring as a result of
their union ; but such a mode of distinguishing species is impos-
sible of application among lower forms such as the sponges. In
these lower groups, accordingly, a species can only be defined as
an assemblage of individuals which so closely resemble one an-
other that they might be supposed to be the offspring of a parent-
form similar to themselves in all the most essential features.
And, according to the view taken of the relative importance of
different points of colour, shape, and internal structure, the con-
ceptions of the species and their varieties and mutual relationships
formed by different observers must often differ widely from one
another.

3. GENERAL ORGANISATION.

General Form and Mode of Growth.- -The simplest Sponges
are vase-shaped or cylindrical in form, either branched or un-
branchedj and, if branched, with or without anastomosis or
coalescence between neighbouring branches. But the general
form of the less simple Sponges diverges widely from that of such
a branching cylinder as is presented by Sycon gelatinosum
(Fig. 78).

From the point to which the embryonic sponge becomes
attached it may spread out horizontally, following the irregulari-
ties of the surface on which it grows, and forming a more or less
closely adherent encrustation like that of an encrusting lichen
(Fig. 84, A). The surface of such an encrustation may be smooth ;
more commonly it is raised up into elevations rounded bosses,
cones, ridges or Iamella3 ; and the edges may be entire or lobed.
In other cases the sponge grows at first more actively in the
vertical than in the horizontal direction, and the result may be a
long, narrow structure, cylindrical or compressed, and more or less
branched (Fig. 84, />*). Sometimes vertical and horizontal growth
is almost equal, so that eventually there is formed a thick, solid
mass of a rounded or polyhedral shape (Fig. 84, C), with an even,
or lobed, or ridged surface. Very often, after active vertical growth

Ill

PHYLUM AND CLASS PORTFERA

115

has resulted in the formation of a comparatively narrow basal
part or stalk, the Sponge expands distally, growing out into lobes
or branches of a variety of different forms, and frequently anasto-
mosing. Sometimes, after the formation of the stalk with root-
like processes for attachment, the Sponge grows upwards in such
a way as to form a cup or tube with a terminal opening. Such a

A.Oscaria

C,E usbongia

B.Psammoclema

D. Poherion

Fin. 84. External form of various Sponges. A, Oscar ia. an encrusting form, with the
upper surface raised up into a number of rounded prominences ; J3, Psammoclema a
ramifying subcylindrical Sponge ; 0, Euspongria (toilet sponge), a massive form with
a broad base ; D, Poterion (Neptune's Cup), an example of a complex Sponge assuming
the form of a vase. (After Vosmaer.)

cup-shaped Sponge, exemplified in the gigantic Neptune's Cup
(Poterion, Fig. 84, D), is not to be confounded with the simple
vase or cup referred to above as the simplest type of Sponge,
being a much more complex structure with many oscula. Some-
times the Sponge grows from the narrow base of attachment into
a thin flat plate or lamella ; this may become divided up into a
number of parts or lobes, which may exhibit a divergent arrange-

i 2

116

ZOOLOGY

SECT.

ment like the ribs of an open fan. Often the lamella becomes
folded, and sometimes there is a coalescence between the folds,
resulting in the development of a honey-comb-like form of
sponge.

Sponges resemble plants, and differ from the higher groups of
animals, in the readiness with which, in many cases, their form

becomes modified during growth by
external conditions (environment).
Different individuals of the same
kind of Sponge, while still exhibiting
the same essential structure and the
same general mode of growth, may
present a variety of minor differences
of form, in accordance with differ-
ences in the form of the supporting
surface or in the action of waves and
currents.

Leading Modifications of
Structure. Sycon gelatinosum be-
longs to a type of Sponges interme-
diate between the very simplest forms
on the one hand, and the more com-
plex on the other. The simplest
type of Sponge-structure is that
of the so-called Ascctta or Olynthus
(Fig. 85). This is not a mature form
no adult Sponge retaining such
simplicity of structure. It is vase-
shaped, contracted at the base to
form a sort of stalk by the expanded
extremity of which it is attached ;
at the opposite or free end is the
circular osculum. So for there is a
considerable resemblance to Sycon
gelatinosum ; but the structure of its
wall in Ascetta is extremely simple.
Regularly arranged over the suri'ace
are a number of small rounded
apertures, the inhalant pores ; but,
sinje the wall of the Sponge is very thin, these apertures
lead directly into the central or paragastric cavity (Fig. 86 A),
the long passages or canals through which the communica-
tion is effected in Sycon being absent. The wall consists of the
same three layers as in Sycon, but the middle one, though it
contains a small number of spicules, is very thin. The ectoderm is
a thin layer of flat cells; the paragastric cavity is lined throughout
by choanocytes similar to those of the flagellate canals of Sycon.

FJO. 85. Olynthus stage of a simple
calcareous Sponge (Clalhrina). A
portion of the wall of the vase-like
sponge removed to show the para-
gastric cavity. (After Haeckel.)

Til

PHYLUM AND CLASS PORIFERA

117

A somewhat more complex type of structure than that of Ascetta
is exhibited by those
sponges in which the
wall becomes thick-
ened and perforated
by radially-arranged
canals, which open di-

rectly on the outer sur-
face by means of inhal-
ant pores or cstia, and
lead directly into the
paragastric cavity by
means of opapyles
the whole inner sur-
face as well as the
radial canals being
lined with flagellate
endoderm cells. In
forms which may be
regarded as represent-
ing the next stage
of development (Fig.
86, B : see also the
figures of Sycon gela-
tinosum), there are
formed by infolding
of the surface, in the
intervals between the
radial canals, canal-
like spaces, the incur-
nnt canals, lined by
ectoderm and com-
municating with the
exterior on the one
hand, either by a
wide opening or by
pores (ostia) perfor-
ating a pore-mem-
brane, and on the
other by means of
small openings, the
prosopyles (the equi-
valents of the inhalant
pores of the Olynthus),
with the radial canals.
Sponges similar to
Sycon gelatinosum,

/:v-: f *!/>#f$

'.: '. ::-',& /\V

&$&$&&

1 ''' '.-.;.! !':' .': ':.
\ '.::' r,^. .'.'' '.-:. -...
\:.-.;;'-.*-:?. \'v.>:?:.--.. :-::-.

FIG. 86. Diagram of the canal system of various sponges, the
ectoderm denoted by a continuous narrow line ; the flat-
tened endoderm by an interrupted line ; the flagellate
endoderm by short parallel strokes. A, cross-section
through a part of the wall of an Ascon ; B, cross-section
through a part of the wall of a Sycon ; C, cross-section
through a part of the wall of Leucillo con << xa ; V, vertical
section through Oscnrdln ; a, spaces of the incurrent canal
system ; b, spaces of the excurrent canal system ; os. oscu-
lum. (After Korschelt and Heider.)

118

ZOOLOGY

SECT.

but with flagellate canals arranged in groups, each group centred
round a main excurrent canal (Fig. 86, C) afford us the next
grade of advancing complexity. In these the incurrent canals may
form a branching system. In all the higher groups of Sponges
(Fig. 86, D and Fig. 87) the flagellate cells are confined to cer-
tain special enlargements of the canals the so-called *' ciliated
chambers " (C) and the rest of the canals are lined by flattened
cells.

Special names have been applied to the main types of canal-
system briefly sketched above. Forms in which the paragastric
cavity is lined by flagellate cells are said to belong to the Ascon
type, whether the paragastric cavity communicates directly or by
flagellate canals with the exterior. Forms in which there is a
paragastric cavity lined by flattened cells, and a system of radially

PG

CO

DP

In

FIG. 87. Vertical section of a fresh-water sponge (Spon^il la), showing the arrangement of the
canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; GO. openings
of the excurrent canals ; PG, paragastric cavity ; SD. subdermal cavities ; 0. osculuni.
(Modified from Leuckart and Nitsche's diagrams.)

arranged flagellate chambers, are said to possess the Sycon type of
structure. Such Sponges as have small rounded flagellate cham-
bers (" ciliated chambers "), communicating in most cases by
narrow branching incurrent canals with the exterior (directly or
indirectly) on the one hand, and by similar excurrent canals with
the paragastric cavity on the other the flagellate cells being
confined to the flagellate chambers are said to possess the Rhagon
type of canal-system. In the lilt agon proper the arrangement 01
parts is very simple. The Sponge has a paragastric cavity opening
on the exterior by an osculum. Opening into this central cavity
by wide apopyles are a number of rounded chambers each com-
municating with the exterior by an inhalant pore (prosopyle).

The development of branches from the originally simple Sponge,
and the coalescence of neighbouring branches with one another,
greatly obscure the essential nature of the Sponge as a colony or
zooids similar to the branches of Sycon gelatinosum ; and this effect

Ill

PHYLUM AND CLASS PORIFERA

119

is increased by the development of a variety of infoldings of the
ectoderm which appear in the higher forms. The oscula dis-
tributed over the surface of the mass may indicate the component
zooids, but these are not always recognisable, being carried inwards
by the infoldings or closed up altogether.

A thicker or thinner specialised outer layer the dermal cortex
situated immediately below the superficial ectoderm, is present
in many Sponges. This is a layer of mesoglcea with special
skeletal elements, usually containing spaces and canals lined by
ectoderm (subdermal cavities, Fig. 87, S2)) which communicate
directly with the exterior, and, internally, usually with more
deeply situated spaces (subcortical cavities), from which the in-
current canals lead to the ciliated chambers. This dermal cortex
is present, though not highly developed, in Sycon gelatinosum
(Fig. 81, dc\ and the enlarged outer ends of the incurrent canals
lying in the dermal cortex and closed externally by the pore-
bearing membrane, may be regarded as representing dermal
cavities. In most higher sponges a special inner layer is
developed ; this is the gastral cortex, represented in a rudi-
mentary form in Sycon gelatinosum (Fig. 81, yc.) as the internal
layer with special spicules, in which the excurrent canals are
situated.

Histology. In the protoplasmic elements or cells of the
various groups of Sponges there is little variation, except
in minor points. The cells
of the ectoderm (Fig. 88)
are flattened, and very rarely
assume other forms ; in some
cases each flattened ecto-
dermal cell is provided with a
flagellum. Lining the paraga-
stric cavities and canals is a
layer of flattened cells similar
to those of the ectoderm, or of
flagellate collared cells. In
the gelatinous substance of the
mesoglcea are embedded connec-
tive-tissue cells, amoeboid wan-
dering cells, and, in certain

positions (around orifices), muscle-cells. Unicellular glands (see
p. 25) are present in some sponges, both calcareous and siliceous ;
also cells containing the pigment to which the bright colour
of many sponges is due, though in most cases the pigment is not
confined to special cells, but occurs scattered through the con-
nective-tissue cells and flagellate cells. Fresh-water Sponges are
green, owing to the presence of chlorophyll, the colouring matter
to which the prevailing green colour of plants is due.

FIG. 88. Cells of the ectoderm, very highly
magnified. (After Von Lendenfeld.)

120

ZOOLOGY

SECT.

The elements of the skeleton differ in character in the different
classes. In the Calcarea they consist of calcareous spicules, usually
tri-radiate in form. Each of these spicules is developed from special

cells the sclcroblasts (Fig.
89). In the remaining groups
of Sponges the skeleton
either consists of spongin
fibres alone (Fig 90, A),
or of siliceous spicules
alone, or of a combination of
spongin fibres with siliceous
spicules (B) : in some Demo-
spongia (the Myxospongia)
skeletal parts are altogether
absent. Spongin is a sub-
stance allied to silk in chemi-
cal composition : the fibres
are exceedingly fine threads,
consisting of a soft granular
core and an outer tube of
concentric layers of spongin.

FIG. 89. Development of a tri-radiate spicule of rni_. j.v rl~, "U V, A

Clathrina. sd, scleroblasts. (After Minchin.)

anastomose, or are woven and

felted together in such a way as to form a firm, elastic,
supporting structure. They are secreted by the activity of
certain cells in the mesogloea which are called the spongin-
blasts, derived from the ectoderm. In certain exceptional cases
the spongin assumes the form of spicules. The siliceous spicules
(Fig. 91) are much more varied in shape than the spicules
of the Calcarea, and in a single kind of Sponge there may be
a number of widely differing forms of spicules, each form having
its special place in the skeleton of the various parts of the Sponge-
body. In most forms siliceous spicules and spongin fibres
combine to form the supporting framework, the relative develop-
ment of these two elements varying greatly in different cases.
But in certain groups, including the common Washing-sponges
(Fig. 90 A), spicules are completely absent, and the entire
skeleton consists of spongin. In some forms which are devoid
of spicules, the place of these is taken by foreign bodies -
shells of Radiolaria, grains of sand, or spicules from other
sponges (Fig. 90, C). In others, again, such as the Venus's
Flower-Basket (Euplectella), the Glass-Rope Sponge (Hyaloncma),
and Pheronema (Fig. 92), the skeleton consists throughout of
siliceous spicules bound together by a siliceous cement.

Reproduction in the Sponges is effected either sexually or
asexual ly. The process by which, in all but the simplest forms of
Sponges, a colony of zooids is formed from the originally simple

Ill

PHYLUM AND CLASS PORIFERA

121

cylinder or vase, may be looked upon as an asexual mode of repro-
duction by budding. In some cases asexual multiplication also takes
place by the production of external buds ; in others of internal buds
in the shape of groups of cells called gemmules, which eventually
become detached and develop into new individuals. In the Fresh-

C.Spongelia

A.EusJDongia

B. Pachychalina

FIG. 90 Microscopic structure of the skeleton in various sponges. A, Euspongia, network
of spongin fibres; B, Pachychalina, spongin strengthened by siliceous spicules; C,
Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules
of other sponges, &c. (After Vosmaer.)

water Sponges (Spongillidce) multiplication takes place very actively
by means of such gemmules, each of which is a spherical group of
cells enclosed in an envelope composed of peculiarly shaped siliceous
spicules, termed ainpliidiscs (Fig. 91, right side). These gemmules
are formed in the substance of the Sponge towards the end of the

122

ZOOLOGY

SECT.

year ; they are set free by the decay of the part of the parent
sponge in which they are developed, and fall to the bottom. In
spring the contained mass of protoplasmic matter reaches the
exterior through an aperture in the wall of the gemmule, and
develops into the adult form.

All Sponges multiply by a sexual process by means of male
cells, or sperms, and female cells, or ova. These are developed
from certain of the amoeboid wandering cells of the inesoglcea,
which take up a special position, usually immediately below the
collared cells of the endoderm. Ova and sperms are developed in
the same Sponge, but rarely at the same time. The amoeboid cell
destined to form sperms divides into a number of small cells, giving
rise to a rounded mass of sperms. The latter, when mature, have
oval or pear-shaped heads and a long tapering appendage or tail.
Each amoeboid cell destined to form an ovum enlarges, and

FIG. 91. Various forms of sponge spicules. (From Lang's Text-Book.)

eventually assumes a spherical form. After a sperm has penetrated
into its interior and effected impregnation, the ovum usually
becomes enclosed in a brood-capsule formed for it by certain
neighbouring cells, and in this situation, still enclosed in the parent
Sponge, it undergoes the earlier stages of its development. The
boring Sponge, Cliond, is the only one, so far as known, in which
the early stages of development are passed through externally.

In all known cases there is a free-swimming ciliated larval
stage ; but the form assumed by the larva differs profoundly in
different Sponges. Of the simpler types of calcareous sponges
with a structure resembling that of the Olynthus, the development
has been followed out in the case of Clathrina blanca. In this
sponge segmentation is followed by the formation of an oval
blastula, the wall of which consists of a single layer of cells all
alike in character elongated, columnar, and flagellate. At one
pole of the blastula is seen a pair of cells which are of a different
character, being large, rounded, and granular. These are destined
to give rise to the archwocytcs, some of which form the repro-

PHYLUM AND CLASS PORIFERA

123

d active cells. Certain of the flagellate cells then withdraw their
flagella and pass into the internal cavity, becoming amoeboid. Soon

FIG. 92. Pheronema carpenter!, one of the Hexactinellida.
(From Wyville Thomson.)

a large number of these amoeboid cells come to fill up a great part of
the cavity of the larva, which now passes into a stage corresponding
to the planula larva of the Coelenterates (Sect. IV). This is the

124

ZOOLOGY

SECT.

larval form known as the par enchy mulct. The parenchymula
(Fig. 93) consists of three kinds of cells : (1) an external layer
of flagellate cells ; (2) an inner mass of amoeboid cells ; (3) the
two posterior granular cells. In this condition it becomes fixed,

and develops into the form of a flat plate
with an irregular outline. Most of the
amoeboid cells now migrate to the outer
surface, passing between the flagellate
cells and then becoming arranged outside
them to form the ectoderm. The flagel-
late cells now form an irregular mass
together with a number of non-flagellate
cells derived from the ectoderm, which
are destined to give rise to the porocytes.
A cavity appears in the mass, and becomes
surrounded by a layer of porocytes. The
cavity increases in size, and is soon seen
to be bounded not by the porocytes alone,
but in part also by flagellate cells. Sub-
sequently the flagellate cells come to
form the entire boundary of the cavity,
the porocytes passing outwards to become
perforated by apertures the inhalant
apertures in the wall of the sponge.
Among the flagellate cells and porocytes
there are also amoeboid cells derived from the two original granular
cells ; some of these give rise to the reproductive cells. The
scleroblasts are formed of certain ectoderm cells which migrate
inwards, and at an early stage arrange themselves in threes to give
rise to the tri-radiate spicules. The development of the sponge
becomes completed by the enlargement of the internal cavity
(paragastric cavity) which is now lined by flagellate cells, and by
the development of the osculum.

In Sycon the early stages (Fig. 94, a-c) differ somewhat from
those in Clatkrina llanca, and the embryo leaves the parent sponge
in the peculiar stage to which the name of amphiblastula is
applied. When the blastula is formed the greater part of its wall
consists of clear cells, with a number of granular cells the archaeo-
cytes at the posterior pole. The clear cells become elongated
and flagellate. The archseocytes pass into the internal (segmenta-
tion) cavity and become completely enclosed by the flagellate
cells (stage of so-called pscudogastrula}.

The cells at the posterior end then lose their flagclla and
become large rounded granular cells, so that after a time the
wall of the embryo comes to be composed in one half of the
flagellate cells that have remained unaltered, and in the other half of
the large granular cells. It is in this stage termed the amphi-

FIG. 93. Median longitudinal
section of the parenchymula
larva of Clathriiia blanca.
p.g.c., posterior granular cells
which give rise to the archpeo-
cytes. (From the Cambridge
Natural History, after Min-
chin.)

Ill

PHYLUM AND CLASS PORIFERA

125

blastnia (e) that the larval sponge becomes free. At a later
stage the flagellate cells become partly overgrown by the granular
cells, the latter eventually giving rise to the ectoderm of the
adult, while the former become the flagellate collared cells. The
larva becomes fixed by one side, and soon assumes a cylindrical

FIG. 94. Development of Sycon raphanus. , ovum ; b, c, ovum segmented?*, as seen from
above, c, lateral view ; if, blastula ; e, amphiblastula ; /, commencement of invagination ;
ff, . larva attached by its oral face ; h, i, young sponge A, lateral view ; /, as seen from above.
(From Sollas, after Schulze.)

form (Fig. 94, h, i}. An aperture which is developed at the free
end becomes the osculum, and small perforations in the sides of
the cylinder form the inhalant apertures. As the wall of the
cylinder increases in thickness by the growth of the mesogloea,the
radial canals are formed, the endoderm extending into them and
its cells becoming flagellate.

126 ZOOLOGY SECT.

The amphiblastula type of larva is characteristic of the Calcarea,
and is probably universal in that sub- class except in such primi-
tive forms as Glathrina.

In the SilicispongiaB, on the other hand, the typical larva is a solid
body with a superficial layer of ciliated cells, and an internal mass
of granular cells. From the former, apparently, the collared cells
of the flagellate chambers are formed : from the latter the external
ectoderm and the other elements of the body of the Sponge. The
granular cells break through the ciliated cells at one end and grow
over the latter as an investing layer. This is a remarkable reversal
of what, as will be seen subsequently, is to be observed in the
Ccelenterata and in fact in the rest of the Metazoa, but is readily
reconcilable with what takes place in Sycon and the more complex
Calcarea.

Distribution and Mode of Occurrence of Sponges, and
their Position in the Animal Series. Fossil remains of Sponges
have been found in various formations from those of the Cambrian
period onwards, the greatest abundance being found in the Chalk.
No extinct class or order has been detected, the fossil forms all
being members of existing groups. Some of the orders of existing
Sponges such as the Myxospongiae are incapable of being
preserved as fossils, and the fossil forms belong, as we should
expect, to the more highly silicified groups and to the more
complex groups of the Calcarea.

Fresh-water Sponges (Spongillidce) occur in rivers, canals, and
lakes in all the great divisions of the earth's surface. Marine
Sponges occur in all seas, and at all depths, from the shore
between tide-marks to the deepest abysses of the ocean. The
Calcarea and the true horny sponges (Ccratosci) are most abundant
in shallow water, and have not been found below 450 fathoms.
The Sponges found at the greatest depths are members of the
groups Hcxadindlida and Choristida.

Sponges do not appear to be edible by Fishes or even the higher
Crustaceans or Molluscs. Countless lower animal forms, however,
burrow in their substance, if not for food, at least for shelter, and
the interior of a Sponge is frequently found to be teeming with
small Crustaceans, Annelids, Molluscs, and other Invertebrates.
None of the Sponges are true parasites. The little Boring
Sponge, Cliona, burrows in the shells of Oysters and other bivalves,
but for protection and not for food. But a Sponge frequently lives
in that close association with another animal or plant to which the
term mcssmateism, or commensalism, is applied, associations which
benefit one or both. Thus some species of Sponge are never found
growing except on the backs or legs of certain Crabs. In these
cases the Sponge protects the Crab and conceals it from its enemies,
while the Sponge benefits by being carried from place to place and
thus obtaining freer oxygenation. Certain Cirri pede Crustaceans

in PHYLUM AND CLASS PORIFERA 127

(members of the order to which the Barnacles and Acorn-shells
belong) are invariably found embedded in certain species of Sponge.
Frequently a Sponge and a Zoophyte grow in intimate association,
so that they seem almost to form one structure. Thus the Glass-
rope Sponge (Hyalonema) is always found associated with aZoophyte
(Palythocu), and there are many other instances. Sponges often also
grow in very close association with certain low forms of plants
(Algae).

The position of the Porifera in the animal series is unquestion-
ably among the Metazoa. The view that they are compound
Protozoa is now no longer maintained since the significance of
the facts of their development has been fully recognised. A
Sponge is to be regarded as a colony of Protozoa only in the sense
in which the same may be said of one of the higher animals. It
consists of a complex of cells, some of which have a consider-
able degree of independence, and some of which have a close
resemblance to certain Protozoa ; but the same is true of one of
the higher animals, the difference being one of degree and not
of kind. Like the rest of the Metazoa, the Sponge develops from
the oosperm by a process of yolk-division.

But the Porifera are perhaps somewhat nearer the Protozoa
than are any of the other types of Metazoa ; and among the
Protozoa they appear to approach nearest to certain colonial
Flagellata. The genus Proterospongia (Fig. 58), already referred
to (p. 78), appears to be the member of the latter group which of
all known forms most closely resembles a sponge. Proterospongia
consists of a colony of collared Flagellates (Choanoflagellata)
embedded in a mass of gelatinous substance, in which there are
also amoeboid zooids similar to the amoeboid wandering cells of
Sponges.

But, while the Porifera are clearly Metazoa, and not Protozoa,
there is some room for difference of opinion as regards their
relationships to the Coslenterata, with which great phylum they
have been sometimes amalgamated. The reasons for and against
such an arrangement will be discussed in considering the
general relationships of the Coalenterata.

SECTION IV
PHYLUM CCELENTERATA

THE possession of an interval cavity lined by a special internal
layer of cells the endoderm in which the digestive and absorp-
tive functions are centred, distinguishes all the remaining groups
of Metazoa from the Parazoa or Sponges. The former are grouped
together under the comprehensive title of Enterozoa, or animals
with enteric cavity. The simplest Enterozoa have an internal
cavity in which there is no separation between the enteric
or digestive cavity and the coelome or body-cavity one con-
tinuous space representing both and opening on the exterior by
the aperture of the mouth. These constitute the phylum
Ccelenterata. They are all animals of a low type of organisation
with a conspicuous radial symmetry, disguising, in some cases,
a more obscure bilateral arrangement, which may be more
primitive.

The most familiar examples of Ccelenterata are the horny,
seaweed-like " Zoophytes/' or, as they are sometimes called,
" Corallines," to be picked up on every sea-beach Jelly-fishes,
Sea-anemones, and Corals. The phylum is divided into four classes
as follows :

Class 1. HYDROZOA, including the Fresh-water Polypes, Zoo-
phytes, many Jelly-fishes mostly of small size, a few Stony
Corals, and the peculiar Palaeozoic fossils known as Graptolites.

Class 2. SCYPHOZOA, including most of the large Jelly-fishes.

Class 3. ACTINOZOA, including the Sea-anemones, and the vast
majority of Stony Corals.

Class 4. CTENOPHORA, including certain peculiar Jelly-fishes
known as " Comb-jellies."

CLASS I. HYDROZOA.

1. EXAMPLE OF THE CLASS Obelia.

General Structure. Obelia is a common zoophyte occurring
in the form of a delicate, whitish or light brown, almost fur-like

128

SECT, iv PHYLUM CCELENTERATA 129

growth on the wooden piles of piers and wharfs. It consists of
branched filaments about the thickness of fine sewing-cotton : of
these, some are closely adherent to the timber, and serve for
attachment, while others are given off at right angles, and present
at intervals short lateral branches, each terminating in a bud-like
enlargement.

The structure is better seen under a low power of the
microscope. The organism (Fig. 95) is a colony, consisting of a
common stem or axis, on which are borne numerous zooids. The axis
consists of a horizontal portion (hydrorkiza) resembling a root or
creeping stem, arid of vertical axes, which give off short lateral
branches in an alternate manner, bearing the zooids at their ends.
At the proximal ends of the vertical axes the branching often
becomes more complex : the offshoots of the main stem, instead of
ending at once in a zooid, send off branches of the third order on
which the zooids are borne. In many cases, also, branches are found
to end in simple club-like dilatations (Bd. 1, 2) : these are imma-
ture zooids.

The large majority of the zooids have the form of little conical
structures (P. 1 P. 4), each enclosed in a glassy, cup-like invest-
ment or hydrotheca (h.th), and produced clistally into about two
dozen arms or tentacles (t) : these zooids are the polypes or hydranths.
Less numerous, and found chiefly towards the proximal region of
the colony, are long cylindrical bodies or blastostyles (bis), each
enclosed in a transparent case, the gonotheca (g.th), and bearing
numerous small lateral offshoots, varying greatly in form according
to their stage of development, and known as medusa-buds (m.bd). By
studying the development of these structures, and by a comparison
with other forms, it is known that both blastostyles and medusa-
buds are zooids, so that the colony is trimorphic, having zooids of
three kinds.

To make out the structure in greater detail, living specimens
should be observed under a high power. A polype is then seen
to consist of a somewhat cylindrical, hollow body, of a yellowish
colour, joined to the common stem by its proximal end, and pro-
duced at its distal end into a conical elevation, the mamifirium or
hypostome (mrib\ around the base of which are arranged the twenty-
four tentacles in a circle. Both body and manubrium are hollow,
containing a spacious cavity, the enteron (cnt), which communicates
with the outer world by the mouth (mth), an aperture placed at
the summit of the manubrium. The mouth is capable of great
dilatation and contraction, and accordingly the manubrium appears
now conical, now trumpet-shaped. Under favourable circum-
stances small organisms may be seen to be caught by the polypes
and carried towards the mouth to be swallowed.

The hydrotheca (h.th) has the form of a vase or wine-glass, and

is perfectly transparent and colourless. A short distance from its
VOL. i K

^

FIG. 95. Obelia sp. A, portion of a colony with certain parts shown in longitudinal section;
B, medusa ; C, the same with reversed umbrella ; D, the same, oral aspect ; Bd. 1, 2, buds ;
bis. blastostyle ; c<x. coenosarc ; ect. ectoderm ; end. endoderm ; ent. enteric cavity ; g.th.
gonotheca; h.th. hydrotheca ; I, lithocyst; m.bd. medusa-bud; mnb. manubrium ; msgl.
mesogloea ; mth. mouth ; p. perisarc ; P. 1, 2, 2, polypes ; rod. c. radial cauui ; t. tentacle ;
vl. velum.

SECT, iv PHYLUM CCELENTERATA 131

narrow or proximal end, it is produced inwards into a sort of
circular shelf (sh), perforated in the centre : upon this the base of
the polype rests, and through the aperture it is continuous with the
common stem. When irritated by a touch or by the addition of
alcohol or other poison the polype undergoes a very marked con-
traction : it suddenly withdraws itself more or less completely into
the theca, and the tentacles become greatly shortened and curved
over the manubrium (P. 2).

The various branches of the common stem show a very obvious
distinction into two layers : a transparent, tough, outer membrane,
of a yellowish colour and horny consistency, the perisarc (p), and
an inner, delicate, granular layer, the ccenosarc (cce), continuous
by a sort of neck or constriction with the body of each hydranth.
The ccenosarc is hollow, its tubular cavity being continuous with
the cavities of the polypes, and containing a fluid in which a
flickering movement may be observed, due, as we shall see, to the
action of cilia. At the base of each zooid or branch the perisarc
presents several annular constrictions, giving it a ringed appear-
ance : for the most part it is separated by an interval from the
ccenosarc, but processes of the latter extend outwards to it at
irregular intervals, and in the undeveloped zooids (Bd. 2) the two
layers are in close apposition.

In the blastostyle both mouth and tentacles are absent, the
zooid ending distally in a flattened disc : the hydrotheca of a
polype is represented by the gonotheca (ff.tli), which is a cylindrical
capsule enclosing the whole structure, but ultimately becoming-
ruptured at its distal end to allow of the escape of the medusa-
buds. These latter are, in the young condition, mere hollow off-
shoots of the blastostyle : when fully developed they have the
appearance of saucers attached by the middle of the convex
surface to the blastostyle, produced at the edge into sixteen very
short tentacles, and having a blunt process, the manubrium,
projecting from the centre of the concave surface. They are ulti-
mately set free through the aperture in the gonotheca as little
medusae or jelly-fish (B D), which will be described hereafter.

The microscopical structure of a polype (Fig 96) reminds us,
in its general features, of that of such a simple sponge as Ascetta,
but with many characteristic differences. The body is composed
of two layers of cells, the ectoderm (ecf) and the endoderm (end) :
between them is a very delicate transparent membrane, the
mesoglcea or supporting lamella (msgl), which, unlike the inter-
mediate layer of sponges, contains no cells and is practically
structureless. The same three layers occur in the manubrium,
the ectoderm and endoderm being continuous with one another at
the margin of the mouth. The tentacles are formed of an outer
layer of ectoderm, then a layer of mesoglcea, and finally a solid
core of large endoderm cells arranged in a single series. The

K 2

132

ZOOLOGY

SECT.

coenosarc, blastostyles, and medusa-buds all consist of the same
layers, which are thus continuous through the entire colony.

The perisarc or transparent outer layer of the stem shows no
cell -structure, but only a delicate lamination. It is, in fact, not a
cellular membrane or epithelium, like the ectoderm and endoderm,
but a cuticle, formed, layer by layer, as a secretion from the ectoderm
cells (see p. 31). It is composed of a substance of chitinoid or horn-
like consistency, and, like the lorica of many Protozoa, serves as a
protective external skeleton. When first formed it is of course in
contact with the ectoderm, but when the full thickness is attained

FIG. 96. Obelia sp. Vertical section of a polype, highly magnified ; cct. ectoderm ; end. endo-
derm ; ent. enteric cavity ; h.th. hydrotheca ; msgl. mesoglcea ; mth. mouth ; ntc. nematocysts ;
sh. shelf -like prolongation of hydrotheca ; t. tentacle.

the latter retreats from it, the connection being maintained only
at irregular intervals. In the same way the hydro- and gonothecse
are cuticular products of the polypes and blastostyles respectively :
in the young condition both occur in the form of a closely fitting
investment of the knob-like rudiment of the zooid (Fig. 95, Bd 1, 2).
The ectoderm has the general character of a columnar epithelium
(see p. 24), but exhibits considerable differentiation of its component
cells. It is mainly composed of large conical cells with their bases
outwards, and having between their narrow inner ends clumps of
small rounded interstitial cells, and occasional large branched nerve-

IV

PHYLUM CCELENTERATA

133

cells (Fig 98, nv.c). The tentacles and the manubrium contain,
in addition, a layer of unstriped muscle-fibres between the ectoderm
and the mesogloea : they are arranged longitudinally, and serve
for the rapid shortening of the tentacles (Fig. 98, mf). This
muscular layer is a derivative of the ectoderm, and may be looked
upon as a rudimentary mesoderm.

FIG. 97. Nematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb,
cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nematocyst ; nv.c. nerve-cell. (From Parker's
Biology, after Schneider.)

Embedded in the ectoderm are numerous clear ovoid bodies, the
stinging -capsules or ncmatocysts (Figs. 96 98 ntc\ organs closely
resembling those of Epistylis umbellaria (p. 93), and like them,
serving as weapons of offence. Each consists (Fig. 97, A) of a tough
ovoid capsule, full of fluid, and invaginated at one end in the form
of a hollow process continued into a long, coiled, hollow thread.
The whole apparatus is developed into an interstitial cell called a
cnidoblast (cnb), which, as it approaches maturity, migrates towards

134

ZOOLOGY

SECT.

nit

end

nv.c

the surface and becomes embedded in one of the large ectoderm
cells. At one point of its surface the cnidoblast is produced into
a delicate protoplasmic process, the cnidocil or trigger-hair (cnc) :
when this is touched for instance by some small organism
brought into contact with the waving tentacle the cnidoblast
undergoes a sudden contraction, and the pressure upon the stinging
capsule causes an instantaneous eversion of the thread (B), at the
base of which are minute barbs. The threads are poisonous, and

exert a numbing effect on the
animals upon which Obelia
preys.

The endoderm also has the
general character of a columnar
epithelium. In the body of the
polype the cells are very large
and have the power of sending
out pseudopods at their free
ends (Fig. 96), which apparently
seize and ingest minute portions
of the partly-digested food. As
in many Protozoa, the pseudo-
pods may be drawn in and long
fiagella protruded, the contrac-
tion of which causes a constant
movement of the food particles
in the enteron. Amongst these
large cells are narrow cells with
very granular protoplasm : they
are gland-cells, and secrete a
digestive juice. In the manu-
brium a layer of endodermal
muscle-fibres has been described
taking a transverse direction,
and so serving to antagonise
the longitudinal muscles and
contract the cavity. In the
tentacles (Figs. 96 and 98) the
endoderm (end) consists of a

single row of short cylindrical cells, nearly cubical in longitudinal
section: their protoplasm is greatly vacuolated and their cell-walls
so thick that they may be considered as forming a sort of internal
skeleton to the tentacles.

The structure of the medusae formed, as we have seen, by the

development of medusa-buds liberated from a ruptured gonotheca

-yet remains to be considered. The convex outer surface of the

bell or umbrella (Fig. 95, B D) by which the zooid was originally

attached to the blastostyle is distinguished as the ex-umbrella, the

f

Fio. 98. Tentacle of Eucopella. The
lower part of the figure snows the ex-
ternal surface, in the middle part the
ectoderm is removed and the muscular
and nervous layer exposed, in the upper
part these latter are removed so as to
show the core of endoderm cells ; ect.
ectoderm ; end. endoderm ; m /. muscle-
fibres ; ntc. nematocyst ; nu. nucleus ;
nv.c. nerve-cell. (After von Lendenfeld.)

IV

PHYLUM CCELENTERATA

135

concave inner surface as the sub-umbrella. From the centre of
the sub-umbrella proceeds the manubrium (jnnb}, at the free end
of which is the four-sided mouth (mth). Very commonly, as the
medusa swims the umbrella becomes turned inside out, the sub-
umbrella then forming the convex surface and the manubrium
springing from its apex (Fig. 95, C, and Fig. 99. A).

The mouth (Figs. 95, 96, 99, and 100, mth) leads into an enteric
cavity which occupies the whole interior of the manubrium, and
from its dilated base sends off four delicate tubes, the radial
canals (rad. c), which pass at equal distances from each other
through the substance of the umbrella to its margin, where they all
open into a circular canal (circ. c), running parallel with and close
to the margin. By means of this system of canals the food, taken

yen,

FIG. 99. Obelia sp. A, mature medusa swimming with everted umbrella ; B, one quarter
of the same, oral aspect ; circ.c. circular canal ; gon. goiiad ; /. lithocyst ; mnb. manubrium ;
mth. mouth ; rad. c. radial canal ; t. tentacle. (After Haeckel.)

in at the mouth and digested in the manubrium, is distributed to
the entire medusa.

The edge of the umbrella is produced into a very narrow fold or
shelf, the velum (Fig. 100, vl), and gives off the tentacles (t), which
are sixteen in number in the newly-born medusa (Fig. 95), very
numerous in the adult (Fig. 99). At the bases of eight of the
tentacles two in each quadrant are minute globular sacs (/),
each containing a calcareous particle or lithitc. These are the
marginal sense-organs or litkocysis : they were formerly considered
to be organs of hearing, and are hence frequently called olocysts :
in all probability their function is to guide the medusa by
enabling it to- judge of the direction in which it is swimming.
The marginal organs, in this case, may therefore be looked upon
as organs of the sense of direction.

The manubrium (Fig. 100, mnb) of the medusa consists of

136

ZOOLOGY

SECT.

precisely the same layers as that of the hydranth ectoderm,
mesoglcea, and endoderm. The ectoderm is continued on to the
sub-umbrella, and then round the margin of the bell on to the
ex-umbrella, so that both surfaces of the bell are covered with
ectoderm. The endoderm is continued from the base of the enteric
cavity into the radial canals and thence to the circular canal,
so that the whole canal-system is lined by endoderm. In the
portions of the bell between the radial canals there is found,
between the outer and inner layers of ectoderm, a thin sheet of
endoderm, the endoderm-lamella (end. lam), which stretches
between adjacent radial canals and between the circular canal
and the enteric cavity. In the bell, as in the manubrium, a

end torn

FIG. 100. -Dissection of a medusa with rather more than one-quarter of the umbrella and manu-
brium cut away (diagrammatic). The ectoderm is dotted, the endoderm striated, and the
mesoglcea black, circ. c. circular canal ; end. lam. endoderm lamella ; gon. gonad ; I. lithocyst ;
mnb. manubrium ; mth. mouth ; rail. c. radial canal ; vl. velum.

layer of mesoglcea everywhere intervenes between ectoderm and
endoderm.

The velum (vl) consists of a double layer of ectoderm and a
middle one of mesogloea : there is no extension of endoderm into
it. The tentacles, like those of the hydranth, are formed of a
core of endoderm covered by ectoderm, the cells of the latter
being abundantly supplied with stinging-capsules.

Comparison of Polype and Medusa. Striking as is the
difference between a polype and a medusa, they are strictly
homologous structures, and the more complex medusa is readily
derivable from the simpler polype- form. It is obvious, in the
first instance, that the apex of the umbrella corresponds with the
base of a hydranth (Fig. 101, A and D), being the part by which
the zooid is attached in each case to the parent stem : the mouth
with the manubrium are also obviously homologous structures in

IV

PHYLUM CCELENTERATA

137

the two cases. Suppose the tentacular region of a polype to be
pulled out, as it were, into a disc-like form (B), and afterwards to
be bent into the form of a saucer (C) with the concavity distal,

B

eel

FIG. 101. Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal,
and A', transverse section (along the line ab) of polype-form ; B, polype-form with extended ten-
tacular region ; C, vertical, and C', transverse section (along the line ab) of form with tentacular
region extended into the form of a bell ; D, vertical, and D', transverse section (along the line ah)
of medusa. The ectoderm is dotted, the endoderm striated, and the mesogloea black, dr. r.
circular canal; net. ectoderm; end endoderm; on/, lam. endoderm lamella; cnt. cm\ enteric
cavity; lujp. hypostome or manubrium ; mnb. manubrium ; m#r2. mesogloea ; mth. mouth;
nv. >ii?', nerve-rings ; t. tentacle ; c. velum. (From Parker's Bioloyy.)

i.e. towards the manubrium. The result of this would be a medusa-
like body (C, C') with a double wall to the entire bell, the narrow
space between the two layers containing a prolongation of the

138

ZOOLOGY

SECT.

enteron (ent. cct-v') and being lined with endoderm. From such a
form the actual condition of things found in the medusa would be
produced by the continuous cavity in the bell being for the most
part obliterated by the growing together of its walls so as to form

\

\

a,d- radiws

sub radius ~_,

per-radius

FIG 102 '-Projections of polype (A) and medusa (B), showing the various orders of radii;
yon. gonad ; mnb. manubrium.

the endoderm-lamella (D', cwl. lam], and remaining only along
four meridional areas the radial canals (rail, r), and a circular
area close to the edge of the bell the circular canal (dr. c).

While both polype and medusa are radially symmetrical, the increase in
complexity of the medusa is accompanied by a differentiation of the structures
lying along certain radii. If a polype is projected on a plane surface (Fig. 102, A),

iv PHYLUM CCELENTERATA 139

taken at right angles to its long axis, a large number of radii about twenty-
four can be drawn from the centre outwards, all passing through similar parts,
i.e. along the axis of a tentacle and through similar portions of the body and
manubrium. But in the medusa (B) the case is different. The presence of the
four radial canals allows us to distinguish four principal radii or per-radii. Half
way between any two per-radii a radius of the second order, or inter-radius, may
be taken ; half way between any per-radius and the inter-radius on either side a
radius of the third order, or ad-radius, and half way between any ad-radius and
the adjacent per- or inter-radius, a radius of the fourth order, or sub-radius. Thus
there are four per-radii, four inter-radii, eight ad-radii, and sixteen sub-radii.
In Obelia the radial canals, the angles of the mouth, and four of the tentacles are
per-radial, four more tentacles are inter-radial, and the remaining eight tentacles,
bearing the lithocysts, are ad-radial. The sub-radii are of no importance in this
particular form.

Reproduction. In the description of the fixed Obelia-colony
no mention was made of cells set apart for reproduction, like
the ova and sperms of a sponge. As a matter of fact, such sexual
cells are found only in their fully developed condition at least
in the medusae. Hanging at equal distances from the sub-umbrella,
in immediate relation with the radial canal and therefore per-
radial in position, are four ovoid bodies (Figs. 99 and 100, gon),
each consisting of an outer layer of ectoderm continuous with
that of the sub-umbrella, an inner layer of endoderm continuous
with that of the radial canal and enclosing a prolongation of the
latter, and of an intermediate mass of cells which have become
differentiated into ova or sperms. As each medusa bears organs
of one sex only (testes or ovaries, as the case may be), the individual
medusae, are dioecious. It will be noticed that the gonad has the
same general structure as an immature zooid an outpushing
of the body-wall consisting of ectoderm and endoderm, and
containing a prolongation of the enteric cavity.

Development.- -When the gonads are ripe, the sperms of the
male medusae are shed into the water and carried by currents to
the females, impregnating the ova, which thus become oosperms
or unicellular embryos. The oosperm undergoes complete seg-
mentation (Fig. 103, A F), and is converted into an ovoidal body
called a planula (G, H), consisting of an outer layer of ciliated
ectoderm cells and an inner mass of endoderm cells in which a
space appears, the rudiment of the enteron. The planula swims
freely for a time (H), then settles down on a piece of timber, sea-
weed, &c., fixes itself by one end (K), and becomes converted into
a Jiydrulct or simple polype (L, M), having a disc of attachment at
its proximal end, and at its distal end a manubrium and circlet of
tentacles. Soon the hydrula sends out lateral buds, and, by a
frequent repetition of this process, becomes converted into the
complex Obelia-colony with which we started.

This remarkable life-history furnishes the first example we have
yet met with among the Metazoa of alternation of generations, or

140

ZOOLOGY

8ECT.

metagenesis (see p. 41). The Obelia-colony is sexless, having no
gonads, and developing only by the asexual process of budding ;
but certain of its buds the medusae develop gonads, and from

o

FIG. 103 Stages in the development of two Zoophytes (A H, Laomedea, I M, Ewden-
drium) allied to Obelia ; A F, stages in segmentation ; G, the planula enclosed in the
maternal tissues ; H, the free-swimming planula ; I M, fixation of the planula and develop-
ment of the hydrula. (From Parker's Biology, after Allman.)

their impregnated eggs new Obelia-colonies arise. We thus have
an alternation of an asexual generation, or agamobium the Obelia-
colony, with a sexual generation, or gamobium the medusa.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Hydrozoa may be defined as multicellular animals in which
the cells are arranged in two layers, ectoderm and endoderm,
separated by a gelatinous, non-cellular mesoglcea, and enclosing
a continuous digestive cavity which communicates directly with
the exterior by a single aperture the mouth and is lined through-
out by endoderm. The ectoderm consists of epithelial cells, inter-
stitial cells, muscle-fibres, and nerve-cells. Certain of the inter-
stitial cells give rise to characteristic organs of offence the
stinging-capsules. The endoderm consists of flagellate or amoeboid
cells, gland-cells, and sometimes muscle-fibres. There are two
main forms of zooids, polypes or nutritive zooids, which are
usually sexless, and medusae or reproductive zooids. In corre-
spondence with its locomotive habits, the medusa attains a higher

iv PHYLUM CCELENTERATA 141

degree of organisation than the polype, having more perfect
muscular and nervous systems, distinct sense-organs, and a diges-
tive cavity differentiated into central and peripheral portions, the
latter taking the form of radial and circular canals. The repro-
ductive products are discharged externally, and are very commonly,
though not always, of ectodermal origin.

Many Hydrozoa agree with Obelia in exhibiting alternation of
generations, the asexual generation being represented by a fixed,
more or less branched hydroid colony, the sexual generation by a
free-swimming medusa. In other forms there are no free medusae,
but the hydroid colony produces fixed reproductive zooids. In
others, again, there is no hydroid stage, the organism existing only
in the medusa-form. Then, while in most instances the only
skeleton or supporting structure is the horny perisarc, there are
some forms in which the ccenosarc secretes a skeleton of calcium
carbonate, forming a massive stony structure or coral. Lastly,
there are colonial forms which, instead of remaining fixed, swim
or float freely on the surface of the ocean, and such pelagic species
are always found to exhibit a remarkable degree of polymorphism,
the zooids being of very various forms and performing diverse
functions.

Thus we have zoophyte colonies known to produce free medusae,
zoophyte colonies known not to produce free medusae, and medusa
known to have no zoophyte stage. Moreover, there are many
medusas of which the life-history is unknown, so that it is un-
certain whether or not a zoophyte stage is present. It is also
found that in some cases closely allied zoophytes produce very
diverse medusae, while similar medusae, in other cases, may spring
from very different zoophytes. For these reasons a sort of double
classification of the Hydrozoa has come about, some zoologists
approaching the group from the point of view of the zoophyte,
others from that of the medusa. On the whole the following
scheme seems best adapted for bringing before the beginner the
leading modifications of the class.

ORDER 1. LEPTOLIN.E.

Hydrozoa in which there is a fixed zoophyte stage, and in which
the sense-organs are exclusively ectodermal.

Sul-Ordcr a. Anthomedit.sce.

Leptolinse in which the polypes are not protected by hydrothecre or the
reproductive zooids by gonotheca? : the medusa? bear the gonads on the manu-
brium and have no lithocysts.

I

Sub-Order 1). Leptomedusce.

Leptolinse in which hydro- and gonothecce are present : the medusa? bear the
gonacls in connection with the radial canals and usually have lithocysts.

142 ZOOLOGY SECT.

ORDER 2. TRACHYLIN.E.

Hydrozoa in which no fixed zoophyte stage is known to occur,
all members of the group being locomotive medusae, some of which
have been proved to develop directly from the egg. The sense-
organs are formed partly of endoderm.

Sub-Order a. Trachymeduscv.

Trachylinse in which the tentacles spring from the margin of the umbrella,
and the gonads are developed in connection with the radial canals.

Sub-Order b. Narcomedusce.

Trachylinee in which the tentacles spring from the ex-umbrella, some dis-
tance from the margin, and the gonads are developed in connection with the
manubrium.

ORDER 3. HYDROCORALLINA.

Hydrozoa in w r hich a massive skeleton of calcium carbonate is
secreted from the ccenosarc, the dried colony being a coral.

ORDER 4. SIPHONOPHORA.

^

Pelagic Hydrozoa in which the colony usually exhibits extreme
polymorphism of its zooids.

ORDER 5. GRAPTOLITHIDA.

An extinct group of Hydrozoa, found only in rocks of Palaeozoic
age, in the form of the fossilised perisarc of the branched colonies.

Systematic Position of the Example.

Obelia, in virtue of the possession of gono- and hydrotheca9, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusse. It is placed in the family Oampanu-
lariidcv, distinguished by having cup-shaped theca? borne at the
ends of distinct branchlets : the genus Obelia is distinguished
from other genera of the same family by the fact that the
reproductive zooids are free-swimming medusae.

ORDER 1. LEPTOLIN.E.

The more typical members of this group agree in all essential
respects with Obelia, consisting of branched colonies bearing two
principal forms of zooids, which serve for nutritive and reproductive
purposes respectively.

General Structure.- -The form and size of the colonies are
subject to great variation : they may be little insignificant tufts
growing on shells, sea-weeds, &c., or may take the form of com-
plex trees three feet in height, and containing many thousand

IV

PHYLUM CCELENTERATA 143

zooids. The hydranths may be colourless and quite invisible to
the naked eye, or, as in some Tubulariae (Fig. 105, 5) may be bril-
liantly coloured, flower-like structures, nearly an inch in diameter.
The medusae may be only just visible to the naked eye, or, as in
jffiquorca, may attain a diameter of 38 mm., or about 15 inches:
they are often seen with great difficulty owing to the bubble-like
transparency of the umbrella ; but frequently the manubrium is
brightly coloured, or brilliant dots of colour the ocelli or eye-spots
may occur around the margin of the umbrella. They are also
frequently phosphorescent, the phosphorescence of the ocean being
often due to whole fleets of medusae liberated in thousands from
the hydroid colonies beneath the surface.

The two sub-orders of Leptolinae are distinguished by the
arrangement of the perisarc. In the Anthomedusae, of which
Bouyainvillea (Fig. 104) is a good example, the cuticle stops short at
the bases of the hydranths, and the reproductive zooids are not
enclosed in gonothecse. It is for this reason that, in classifications
founded on the zoophyte stage, the Anthomedusse are called Gymno-
Uastea or naked-budded zoophytes (see also Fig. 105, 1, 4, ) In
the Leptomedusse the cuticle is usually of a firmer consistency than
in the first sub-order, and furnishes hydrothecae for the hydranths
and gonothecaa for the reproductive zooids : they are hence often
classified as Cafyptoblastea or covered-budded hydroids. To this
group belong the commonest species of hydroids found on the sea-
shore, and often mistaken for seaweeds the " Sea-firs " or Sertu-
larians.

The medusae also exhibit characteristic differences in the two
sub-orders. In the Anthomedusse the umbrella is usually strongly
arched, and may even be conical or mitre-shaped (Figs. 104 ; 105, 7 ;
109, 1 and 2) : its walls are thick, owing to a great development of
the gelatinous mesogkeaof the ex-umbrella, that of the sub-umbrella
remaining thin ; and the velum is considerably wider than in Obelia.
But the most important characteristics are the facts that the
gonads(//cw) are developed on the manubrium and that lithocysts are
absent. Sense-organs are, however, present in the form of specks
of red or black pigment at the bases of the tentacles. These ocelli
(pc) consist of groups of ectoderm cells containing pigment, and it
has been proved experimentally that they are sensitive to light :
they are, in fact, the simplest form of eyes. In the Leptomedusae
the umbrella is usually less convex, thinner, and of softer consist-
ency than in the Anthomedusaa, the gonads are developed as buds
formed in connection with the radial canals and projecting from
the sub-umbrella, the velum is feebly developed, and sense-organs
take the form sometimes of ocelli, but usually of lithocysts.

In the majority of LeptolinaB the coenosarc, as in Obelia, con-
sists of a more or less branched structure attached to stones, timber,
seaweeds, shells,&c., by a definite root-like portion (hydrorhiza). The

144

ZOOLOGY

SECT. IV

curious genus Hydractinia (Fig. 105, 1) is remarkable for possessing
a massive coenosarc, consisting of a complex arrangement of
branches which have undergone fusion, so as to form a firm
brownish crust on the surfaces of dead gastropod shells inhabited
by Hermit-crabs. The constant association of Hydractinia with

FIG. 104. Bougainviilea ramosa. A, entire colony, natural size; B, portion of the same
magnified; C, immature medusa, dr. c. circular canal; ci(. cuticle or perisarc ; tnt. car.
enteric cavity; hyd. polype or hydranth ; ky/>. hypostome or maiiubriuni ; med. medusa; mnb.
niaimbrium ; rod. c. radial canal ; t. tentacle ; c. velum. (From Parker's Biology, after
Alhaan.)

Hermit-crabs is a case of commensalism : the hydroid feeds upon
minute fragments of the Hermit-crab's food, and is thus its com-
mensal or messmate ; and the Hermit-crab is protected from its
enemies by the presence of the inedible, stinging hydroid.
Hydractinia belongs to the Anthomedusas : the Leptomedusan

1. HydracHnia

2.Myriofhela

3. Corymorpha

ntf.cs- A ^.Syncoryne

JL

7. Sarsia

6. Clavarella

FIG. 105. Various forms of Leptolinae. In 1, a shows the entire colony, b a portion higlily
magnified ; in 7, a is a species producing medusa-buds from the manubrium, b from the bases
of the tentacles ; dz. dactylozooids ; m. and M. medusae ; rnnb. manubrium ; inth. mouth ;
oc. eye-spots ; rod. c. radial canals ; s. sporosacs ; s^. spines ; t, t l , <-, tentacles.

VOL. I L

146

ZOOLOGY

SECT.

Clat7irozoon, an Australian genus, resembles it in having branched
and intertwined coenosarcal tubes, the perisarc of which under-
goes fusion ; but the complex mass thus produced, instead of
forming an incrustation on a shell, is a large, abundantly branched,
tree-like structure, resembling some of the fan-corals or Gorgonacea
(vide infra). Ceratella (Fig. 100) has a similar fan-coral-like
appearance, with a branching axis composed of numerous inter-

v<

4 $ 9 jf&iJt Hrfsr^^Wa '^ )/

U/J$ILJK* /^^ % b

>N> >\S\ ^ <

i

\ s -^*-O lr-iv>v._^ i/i.i MPi (i

VV "- L ^=-'^v""C '-L. ^v

K ^ix a vT^f^ >

f*3rj? "v\ -y? -rs /y

&i ^3C &

">^Tf? -f^i

FIG. 106. Ceratella fusca. About nat. size. (From Hickson, after Baldwin Spencer.)

twining and anastomosing tubes ; but while Clathrozoon possesses
thecse, in Ceratella they are absent.

A great simplification of the colony is produced in Myriothela
(Fig. 105 #), in which the short coenosarc bears a single large
terminal hydranth, and gives off numerous slender branches which
bear the reproductive zooids (s). Even greater simplicity is found
in Corymorplia (3), in which the entire organism consists of a
single stalked polype, from the tentacular region of which the
medusae (m) arise.

IV

PHYLUM CCELENTERATA

147

But the simplest members of the whole class, with the exception
of one or two imperfectly known forms which will be referred to

cnc

_! TMS _

100 mm

SCALE FOR A

FIG. 107! Hydra. A, vertical section of entire animal ; B, portion of transverse section, highly
magnified ; C, two large ectoderm cells ; D, endoderm cell of H. viridis ; E, large uematocyst ;
F, small nematocyst ; G, sperm, a, ingested diatom ; bd. 1, bd. 2, buds ; chr. chromatophores ;
cnbl. cnidoblast ; cnc. cnidocil ; ect. ectoderm ; end. endoderm ; ent. cav. enteric cavity ; ent.
cav'. its prolongation into the tentacles ; fl. flagellum ; liyp. hypostome or manubrium ; int. c.
interstitial cells ; m. pr. muscle-processes ; mth. mouth : msgl. mesogloaa ; ntc. large, and ntc'.
small nematocysts ; nu. nucleus ; ov. ovum ; ovy. ovary ; pad. pseudopods ; *py. spermary ;
vac. vacuole. (From Parker's Elementary Biology, after Lankester and Howes.)

below, are the Fresh-water Polypes of the genus Hydra. The
entire organism (Figs. 27 and 107) consists of a simple cylindrical

L 2

148 ZOOLOGY SECT.

body with a conical hypostome and a circlet of six or eight
tentacles. It is ordinarily attached, by virtue of a sticky secretion
from the proximal end, to weeds, &c., but is capable of detaching
itself and moving from place to place after the manner of a loop-
ing caterpillar. The tentacles are hollow, and communicate freely
with the enteron. Both the body and the tentacles are highly
contractile, the contractions being effected by means of a layer of
fibres which run longitudinally. These fibres are processes the
muscle processes (C, m. pr.) of the large ectoderm cells. Similar
shorter muscle-processes of some of the endoderm cells run
circularly and antagonise the longitudinal fibres. Nematocysts are
abundant in the ectoderm. The endoderm cells are mostly
amoeboid and vacuolated. Each usually bears one or more flagella,
but these may be retracted. Glandular cells occur here and there.
Nerve-cells (multipolar) occur in both layers, but present no regular
arrangement. There is no perisarc. Buds (bd. 1, bd. 2) are
produced which develop into Hydrse, but these are always detached
sooner or later, so that a permanent colony is never formed. There

FIG. lOS.r-Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the

disc of attachment to the right.

are no special reproductive zooids, but simple ovaries (ovy) and
testes (spy) are developed, the former at the proximal, the latter
at the distal end of the body. Even simpler than Hydra are
Protoliydra (Fig. 108) and Microhydra, in which the tentacles are
absent.

Pelagohydra is also solitary, but is pelagic. The part .corres-
ponding to the base in Hydra here takes the form of a float, and
there are tentacles distributed over the surface of the float as well as
in the neighbourhood of the mouth ; medusae are developed from
processes on the float. Pelagohydra, however, is perhaps more
nearly related to the Siphouopfiora an order yet to be dealt with
-than to the LeptolinaB.

The polypes are usually cylindrical, as in Obelia, but in some
genera they are widened out into a vase-like form (Fig. 105, 5), in
others elongated into a spindle-shape (4). The tentacles may be
disposed in a single circlet, as in Obelia and Hydra, or there may
be an additional circlet round the hypostome (3, 5), or at the base of
the polype, or they may be scattered irregularly over the whole
surface (4\ In Myriothela (#) they are short, and so numerous
as to have the appearance of close-set papillae. In some forms

iv PHYLUM CCELENTERATA 149

they are knobbed at the ends, the knobs being loaded with stinging-
capsules (4).

In some species a- dimorphism of the hydranths obtains, some
of them being modified to form protective zooids. In Hydractinia
(7) these are simply mouthless hydranths with very short tentacles
abundantly supplied with nematocysts, capable of very active
movements, and called dactylozooids (dz). In Plumularia there are
small structures called "guard-polypes," resembling tentacles in
structure, with very numerous nematocysts, and each enclosed
in a theca. In Hydractinia the coenosarc is also produced into
spines (sp), which may be much modified zooids.

But the most remarkable modifications occur in the repro-
ductive zooids. In a large proportion of genera, both of
Anthomedusae and Leptomedusse, these take the form of locomotive
medusae, agreeing in general structure with the descriptions
already given. Each appears at first as a hollow bud-like process
of the blastostyle, or of an ordinary polype, or, more exceptionally,
of the coenosarc. This becomes constricted at the junction and
rounded off. The ectoderm at its free extremity becomes
thickened, and this thickening, as it grows, pushes the endoderm
before it, producing a sort of involution. In the interior of the
mass of ectoderm a cavity appears : this is destined to form the
sub-umbrellar cavity. The ectodermal partition that at first
separates the cavity from the exterior, becomes perforated and
most of it is absorbed, what remains round the edge going
to form the velum. The endoderm is reduced to a thin layer
except along four radial lines where it gives rise to the four
radial canals, the thin parts between going to form the endoderm
lamella.

In different families and genera the medusae exhibit almost end-
less variety in detail. As to size they vary from about 1 mm. in
diameter up to 400 mm. (16 inches). The number of tentacles
may be very great (Fig. 109, 2) or these organs may be reduced to
two (Fig. 109, 7), or even to one (Fig. 105, 3) ; in the last-named
cases it will be noticed that the medusa is no longer radially, but
bilaterally symmetrical, i.e. it can be divided into two equal and
similar halves by a single plane only viz., the plane passing
through the one or two tentacles. With the increase in the
number of the tentacles a corresponding increase in that of the
radial canals often takes place (Fig. 109, J).

Some medusae creep over submarine surfaces, walking on the
tips of their peculiarly modified tentacles (Fig. 105, 6) but the
majority propel themselves through the water in a series of jerks
by alternately contracting and expanding the umbrella, and so,
by rhythmically driving out the contained water, moving with
the apex foremost. In correspondence with these energetic move-
ments there is a great development of both muscular and

SECT. IV

PHYLUM CCELENTERATA

151

nervous systems. The velum and the sub-umbrella possess
abundance of muscle-fibres, presenting a transverse striation,
and round the margin of the umbrella is a double ring of nerve-
cells and fibres, one ring being above, the other below the at-
tachment of the velum (Fig. 101, D, nv, nv). The medusae thus
furnish the first instance we have met with of a central nervous
system, i.e. a concentration of nervous tissue over a limited area
serving to control the movements of the whole organism. It has
been proved experimentally that the medusae is paralysed by
removal of the nerve-ring. Over the whole sub-umbrella is a
loose network of nerve-cells and fibres connected with the nerve-
ring, and forming a peripheral nervous system.

In some medusae the circular canal communicates with the
exterior by minute pores placed at the summits of papillae, the

FIG. 110 Diagram illustrating the formation of a sporosac by the degradation of a medusa. A,
medusa enclosed in ectoderrnal envelope (es) ; B, intermediate condition with vestiges of
umbrella () and radial canals (ra) ; C, sporosac. ec. ectoderm ; en, endoderiu ; m, nianubrium ;
ov, ovary ; t, tentacle ; r, velum. (From Lang's Comparative Anatomy.)

endoderm cells of which contain brown granules. There seems to
be little doubt that these are organs of excretion, the cells with-
drawing nitrogenous waste-matters from the tissues and passing
them out through the pores. If we except the contractile
vacuoles of Protozoa, this is the first appearance of specialised
excretory organs in the ascending series of animals.

Besides producing gonads, some medusae multiply asexually by
budding, the buds being developed either from the manubrium
(Fig. 105, 7a), or from the margin of the umbrella (76) or the base
of the tentacles. The buds always have the medusa form.

In many Leptolinae the reproductive zooids undergo a degrada-
tion of structure, various stages of the process being found in
different species. Almost every gradation is found, from perfect
medusae to ovoid pouch-like bodies called sporosacs (Fig. 105, Ib,
5, s), each consisting of little more than a gonad, but showing an in-
^dication of its true nature in a prolongation of the digestive cavity

152

ZOOLOGY

SECT.

of the colony, representing the stomach of the manubrium (Fig.
110). We thus have a reproductive zooid reduced to what is
practically a reproductive organ. It is obvious that a continua-
tion of the same process might result in the production of
a simple gonad like that of Hydra : there is, however, no evidence
to show that the Fresh- water Polype ever produced medusae, and
the probabilities are that its ovaries and testes are simply gonads,
and not degenerate zooids. The case is interesting as showing
how a simple structure may be imitated by the degradation of a
complex one. It is quite possible, on the other hand, that the
reproductive organs of the Leptomedusae (Fig. 100) are sporosacs,
i.e. reproductive zooids, not mere gonads. In some rare cases the

FIG. 111. Early development of Eucope. A, blastuhvstage ; B, planula with solid endoderm ;
C, planula with enteric cavity ; at. enteric cavity ; ep. ectoderm ; liy. endoderm. (From
Balfour's Embryology, after Kowalevsky.)

sexual cells are not developed either in medusae or in sporosacs,
but are formed directly in the blasfcostyles.

In Obelia we found the medusae to be budded off from pecu-
liarly modified mouthless zooids the blastostyles. This arrange-
ment, however, is by no means universal : the reproductive zooids
-whether medusae or sporosacs may spring directly from the
coenosarc, as in Bougainvillea (Fig. 104), oT from the ordinary
hydranths (Fig. 105, 4 and 5). The primitive sex-cells, from which
ova or sperms are ultimately developed, are sometimes formed
from the endoderm or (more usually) ectoderm cells of the repro-
ductive zooid; but in many cases originate in the coenosarc, and
slowly migrate to their destination in the ectoderm of the gonad,
where they metamorphose in the usual way into the definitive re-
productive products, which when mature pass into the space below
the ectoderm of the gonad.

The development of the Leptolinas frequently, but not always,

IV PHYLUM CCELENTERATA 153

begins within the maternal tissues, i.e. while the oosperm or im-
pregnated egg- cell is still contained in the gonad of the medusae or
in the sporosac. The oosperrn divides into two cells, then into
four, eight, sixteen, &c. Fluid accumulates in the interior of the
embryo, resulting in the formation of a blastula or hollow globe
formed of a single layer of cells (Fig. Ill, A). The blastula
elongates, and the cells at one pole undergo division, the daughter-
cells passing into the cavity, which they gradually fill (B). At
this stage the embryo is called a planula : it consists of an outer
layer of cylindrical cells the ectoderm which acquire cilia, and an
inner mass of polyhedral cells the endoderm. In some cases the
planula arises by a different process : a solid morula is formed, the
superficial cells of which become radially elongated and form
ectoderm, the central mass of cells becoming endoderm. By
means of its cilia the planula swims freely, and before long a
cavity appears in the middle of the solid mass of endoderm, the
cells of which then arrange themselves in a single layer around
the cavity or enteron (C, al). The planula then comes to rest, fixes
itself at one end to some suitable support, and becomes con-
verted into a simple polype or hydrula by the attached end
broadening into a disc and the opposite extremity forming a
manubrium and tentacles. The hydrula soon begins to send off
lateral buds, and so produces the branched colony.

In Tubularia the oosperm develops, while still enclosed in
the sporosac, into a short hydrula. which, after leading a free
existence for a short time, fixes itself by its proximal end, buds, and
produces the colony. In Hydra development begins in the ovary,
and is complicated by the fact that the ectoderm of the morula
gives rise to a sort of protective shell : in this condition the
embryo is set free, arid, after a period of rest, develops into the
adult form.

ORDER 2. TRACHYLIN.E

General Structure. The members of this order are all
medusae : no zoophyte stage is certainly known in any of them, and
several species have been proved to develop directly from the egg.
They thus differ from the members of the preceding order in the
fact that no alternation of generations ordinarily occurs in their
life-history.

Most species are of small or moderate size, the largest not
exceeding 100 mm. (4 inches) in diameter. The gelatinous tissue
or mesogloea of the ex-umbrella is usually well developed, giving
the medusa a more solid appearance than the delicate jelly-fish of
the preceding order: this is well shown in Fig. 112, in which the
apical region of the umbrella has a comparatively immense thick-
ness. The tentacles are also stiff and strong, and are always solid
in the young condition, although they may be replaced in the
adult by hollow tentacles.

154

ZOOLOGY

SECT.

But the most characteristic anatomical feature of the group is
the structure of the sense-organs, which are club-shaped bodies
(Figs. 112 and 113, tc) consisting of an outer layer of ectoderm

I.Pel'aeue

2. G I o sso c o dor>

V.

FIG. 112.^Two Trachy medusae, dr. c. circular canal; J/OH. gonad ; mub. maiiubrium ; mtk.
mouth ; rail. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentacul'ocyst ; tg. tongue ;
vl. velum. (After Haeckel.)

rad.c

mth
l.Cunarcha

2.Polycol|3a

Fio. 113. Two XVarcoxnedusJe, -1 in vertical section, rton. gonad; mnb. rnanubrium ; mtlt.
mouth ; pr. peronium ; rad.r. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ;
v.l. velum. (After Haeckel )

enclosing a central axis of endoderm cells (Fig. 114): they have,
therefore, the structure of tentacles. They contain one or more
lithites, which are always derived from the endoderm. To

IV

PHYLUM CCELENTERATA

155

cct

-csut.

distinguish them from the lithocysts of Leptomedusse, and to mark
the fact that they are modified tentacles, they are called tcntaculo-
cysts. They may either project freely from the margin of the
umbrella, or may become enclosed in a pouch-like growth of
ectoderm and more or less sunk in the tissue of the umbrella.
Eyes occur in some, and are always of simple structure.

The two sub-orders of Trachylinae are characterised by the mode
of origin of the tentacles.

In Trachymedusse. as in the

/ *

preceding order, they arise
near the edge of the um-
brella (Fig. 112), but in the
Narcomedusoe they spring
about half-way between the
edge and the vertex (Fig.
113), and are continued, at
their proximal ends, into the
ielly of the ex -umbrella in
the form of " tentacle-roots "
(t.r).

As to the position of the
reproductive organs, there
is the same difference be-
tween the two sub-orders
of Trachylinoe as between
the two sub-orders of Lepto-

linse. In the Trachyrnedusge the gonads (Fig. 112, goti) are
developed in the course of the radial canals: in the Narcomedusse
(Fig. 113) they lie on the manubrium, sometimes extending into
the pouch-like offshoots of its cavity.

There is always a well-developed velum, which, as in Fig. 113, 1,
may hang down vertically instead of taking the usual horizontal
position. In the NarcomedusaB the manubrium is short ; in the
Trachymedusye it is always well developed, and is sometimes (Fig.
112, 2) prolonged into a long, highly contractile peduncle, having
its inner surface produced into a tongue-like process (tg) which
protrudes through the mouth. In some the gastric cavity is
situated in the manubrium, which in such a case is looked upon as
partly of the nature of a process cf the sub-umbrella (pseudo-
manubrium).

The simplest case of the development of TrachylinaB is seen in
dSginopsis, one of the Narcomedusse. The oosperm gives rise to
a ciliated planula, which forms first two (Fig 115), then four
tentacles, and a mouth, hypostome, and stomach. The larva of
^ginopsis is thus a hyrfrula, closely resembling the corresponding
stage of Tubularia. After a time the tentacular region grows out,
carrying the tentacles with it, and becomes the umbrella of the

FIG. 114. .ffieinura myosura, a tentaculo-
cyst highly magnified, ect. ectoderm ; end.
endoderm ; /. lithites ; ntc. nematocysts ;
nv.c. group of nerve-cells. (After Haeckel).

156 ZOOLOGY SECT.

medusa. Thus the actual formation of the medusa from the
hydrula of ^Eginopsis corresponds precisely with the theoretical
derivation given above (p. 136). It will be seen that in the present
case there is no metagenesis or alternation of generations, but that
development is accompanied by a metamorphosis that is, the egg
gives rise to a larval form differing in a striking manner from the
adult, into which it becomes converted by a gradual series 01
changes.

Metagenesis is, however, ribt quite unknown among the Trachy-
linse. In a parasitic Narcomedusa (Cunina parasitica) the planula

FIG. 115. Larva of JEginopsis. m. mouth; t. tentacle. (From Balfour, after Metschnikoff.)

fixes itself to the manubrium of one of the Trachymedusse which
serves as its host, and develops into a hydrula. But the latter, in-
stead of itself becoming metamorphosed into a medusa, retains the
polype form and produces other hydrulae by budding, these last
becoming converted into medusse in the usual way.

ORDER 3. HYDROCORALLINA.

The best-known genus of Hydroid Corals is Millepora, one species
of which is the beautiful Elk-horn Coral, J\l. alcicornis. The dried
colony (Fig. 116 A) consists of an irregular lobed or branched mass
of carbonate of lime, the whole surface beset with the numerous
minute pores to which the genus owes its name. The pores are
of two sizes : the larger are about 1 or 2 mm. apart, and are called
gastropores (B, g.p) ; the smaller are arranged more or less
irregularly round the gastropores, and are called dactyloporcs (d.p).
The whole surface of the coral between the pores has a pitted
appearance. Sections (C) show that the entire stony mass is
traversed by a complex system of branched canals, which com-
municate with the exterior through the pores. The wide vertical

IV

PHYLUM CCELENTERATA

157

canals in immediate connection with the gastropores are traversed
by horizontal partitions, the tabular (tl)\

In the living animal each pore is the place of origin of a zooid :
from the gastropores protrude polypes (Fig. 117, P) with hypostome
and four knobbed tentacles ; from the dactylopores long, filamentous,
mouthless dactylozooids or feelers (J).Z\ with irregularly disposed
tentacles : the function of these latter is probably protective and
tactile, like that of the guard-polypes of Plumularia and the
dactylozooids of Hydractinia. The bases of the zooids are con-
nected with a system of delicate tubes, which ramify through the

FIG. 116. Millepora alcicornis. A, part of skeleton, natural size ; B, portion of surface,
magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; tb. tabulu-.
(After Nicholson and Lydekker.)

canals of the coral and represent a much-branched ccenosarc,
recalling that of Hydractinia (p. 144).

The ccenosarcal tubes have the usual structure, consisting of
ectoderm and endoderm with an intervening mesogloea. From
the relative position of the parts it will be obvious that the cal-
careous skeleton is in contact throughout with the ectoderm of the
colony : it is, in fact, like the horny perisarc of the Leptolinse, a
cuticular product of the ectoderm.

The only other genus to which we shall refer is Stylaster (Fig.
118), which forms a remarkably elegant tree-like colony, abund-
antly branched in one plane, and of a deep pink colour. On the
branches are little cup-like projections with radiating processes
passing from the wall of the cup towards the centre, and thus

158

ZOOLOGY

SECT.

closely resembling the true cup-corals belonging to the Actinozoa
(vide infra). But in the case of Stylaster each " cup ' is
the locus, not of one, but of several zooicls a polype projecting
from its centre, and a dactylozooid from each of the compartments
of its peripheral portion. A calcareous projection, the style, the
presence of which is the origin of the generic name, rises up from
the tabula at the bottom of each pore.

The gonophores in most species of Millepora are developed in
certain of the pores in dilatations or ampulla ; in one species at

enrf eef

FIG. 117. Millepora. Diagrammatic view of a portion of the living animal, partly from the
surface, partly in vertical section. In the sectional part the ectoderm is dotted, the endoderm
striated, and the skeleton black, ect. ectoderm ; end. endoderm ; d.p. dactylopore ; D.Z.
dactylozooid ; g.p. gastropore ; mtk. mouth ; P. polype ; t. tentacle. (Altered from Moseley.)

the apices of the dactylozooids. They are medusae, but never
have the complete medusa-form, being devoid of velum, mouth,
radial canals and tentacles. Both male and female medusae
become free, but the period of free existence is very
short.

In Stylaster the medusoid character is much more completely
lost ; and the gonophores are more of the nature of sporosacs or

IV

PHYLUM CGKLENTERATA

150

degraded reproductive zooids lodged in special chambers (a) of
the coral.

The Hydrocorallina occur only in the tropical portions of the
Pacific and Indian Oceans, where they are found on the ; ' coral-

Fio. 11&. Stylaster sanguincus. A, portion of skeleton, natural size; B, small portion,
magnified ; a. ampullaj ; d.p. dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.)

reefs " partly or entirely surrounding many of the islands in
those seas. Fossil forms arc found as for back as the Triassic
epoch.

ORDER 4. SIPHOXOPHORA.

The diversity of form exhibited by the members of this order is
so great that anything like a general account of it would only be
confusing to the beginner, and the most satisfactory method of
presentation will be by the study of a few typical genera.

Hnlistemma (Fig. 119 A) occurs in the Mediterranean and other
seas, and consists of a long, slender, floating stem, to which a
number of structures, differing greatly in form, are attached. At one
-the uppermost end of the stem is an ovoid, bubble-like body con-
taining air i\iQ float or pneumatopfiore (pn). Next come a number
of closely set, transparent structures (net), having the general char-
acters of unsymmetrical medusa without manubria, each being a
deep, bell-like body, with a velum and radiating canals. During life
these swimming-bells or nectocalyccs contract rhythmically i.e. at
regular intervals drawing water into their cavities, and immedi-
ately pumping it out, thus serving to propel the entire organism

B

FIG. 111K Halistemma tergestinum. A, the entire colony; B, a single group of zooids.
< . cttfimsarc ; dz. dactylozooid ; /</</<. hydrophyllium or )>ract; wt. nectocalyx or swinniiing-
bell; ntc. battery of neinatocysts ; t>. idlj T po ; pn. pneumatophore or float; s, a', sporoeysts ;
t. tentacle. (After Claus.)

SECT, iv PHYLUM CCELENTERATA 161

through the water. Below the last nectocalyx the character of the
structures borne by the stem changes completely : they are of
several kinds, and are arranged in groups which follow one
another at regular intervals, and thus divide the stem into seg-
ments, like the nodes and internodes of a plant.

Springing from certain of the " nodes " are unmistakable polypes
(p}, differing however from those we have hitherto met with in
having no circlet of tentacles round the mouth, but a single long
branched tentacle (t) arising from the proximal end, and bearing
numerous groups or " batteries " of stinging-capsules (ntc). In
the remaining nodes the place of the polypes is taken by dactylo-
zooids or feelers (dz) mouthless polypes, each with an unbranched
tentacle springing from its base. Near the bases of the polypes
and dactylozooids spring groups of sporosacs (B, s, s'), some male,
others female ; and finally delicate, leaf-like, transparent bodies
the bracts or hydrophyllia (hph) spring from the " internodes " and
partly cover the sporosacs.

It is obvious that on the analogy of such a hydroid polype as
Obelia, Halistemma is to be looked upon as a polymorphic floating
colony, the stem representing a ccenosarc, and the various struc-
tures attached to it zooids the polypes nutritive zooids, the
feelers tactile zooids, the sporosacs reproductive zooids, the bracts
protective zooids, and the swimming-bells locomotory zooids. The
float may be looked upon as the dilated end of the stem, which
has become invaginated or turned-in so as to form a bladder
filled with air, its outer and inner surfaces being furnished by
ectoderm, and the middle portion of its wall by two layers of
endoderm, between which the enteric cavity originally extended
(Fig. 120, pn). The upper or float-bearing end is proximal-
i.e. answers to the attached end of an Obelia-stem : it is the
opposite or distal end which grows and forms new zooids by
budding.

In some Siphonophora the bracts contain indications of radial
canals, so that these structures, as well as the swimming-bells
and sporosacs, are formed on the medusa-type, while the hydranths
and feelers are constructed on the polype-type.

It will be noticed that the radial symmetry, so characteristic
of most of the Hydrozoa previously studied, gives way, in the
case of Halistemma, to a bilateral symmetry. The swimming-bells
are placed obliquely, and the mouth of the bell is not at right
angles to the long axis, so that only one plane can be taken
dividing these structures into two equal halves : the same applies
to the polype and feelers with their single basal tentacle. When
first formed the various zooids are all on one side of the stem, but
the latter becomes spirally twisted during growth, and so causes
them to arise irregularly.

VOL. I M

162

ZOOLOGY

SECT.

The egg of Halistemma gives rise to a ciliated planula re-
sembling that of the other Hydrozoa. At one pole the ectoderm
becomes invaginated to form the float (Fig. 121, ep], the opposite
extremity is gradually converted into the first polype (po), and

net

FIG. 120. Diagram of a Siphpnophore : the thick line represents endoderm ; the space ex-
ternal to it, ectoderm ; the internal space, the enteric cavity. c<x. ccenosnrc ; Jz. dact3 T lozooid ;
kph. hydrophyllium ; m<l. sporosae ; net, net', nectocalyces ; ntc. battery of nernatocysts ;
p. polype ; p/i. pneuinatuphore ; /. tentacle. (After Clans.)

a bud appears on one side which becomes the first tentacle (t).
By gradual elongation, and the formation of new zooids as lateral
buds, the adult form is produced ; the various zooids are all
formed between the first polype and the float, so that the two

IV

PHYLUM CCELENTERATA

163

become further and further apart, being always situated at the
distal and proximal ends of the colony respectively.

In an allied form (Agalma) the first structure to appear iiTthe embryo is not
the float, but the first bract, which grows considerably and envelops the growing
embryo in much the same way as the umbrella of a medusa envelops the manu-
brium. On this and other grounds some zoologists look upon the Siphonophore-
colony as a medusa the manubriuin of which has extended immensely and
produced lateral buds after the manner of some Anthomedusa? (Fig. 105, 7 a).

FIG. 121. Two stages in the development of Halistemma : the endoderm is shaded, the
ectoderm left white. <:p. pneumatocyst or air-chamber of pneumatophore ; hi/, endoderm
surrounding pneumatophore ; po. polype ; pp. pneumatophore; t. tentacle. (From Balfour,
after Metschnikoff.)

On this theory the entire ccenosarc is an extended manubrium, and the first or
primary bract is the umbrella. But frequently as in Halistemma a primary
bract is not formed, and when present there appears to be no reason against
regarding it as a lateral bud of the axis, of quite the same nature as the remaining
zooids.

In the well-known " Portuguese man-of-war " (Physalia) there
is a great increase in proportional size of the float and a corre-
sponding reduction of the rest of the coenosarc. The float (Fig.
122, pn) has the form of an elongated bladder, from 3 to 12 cm.
long, pointed at both ends, and produced along its upper edge
into a crest or sail (cr) : as a rule it is of a brilliant peacock-blue
colour, but orange-coloured specimens are sometimes met with.
At one end is a minute aperture communicating with the exterior.
There are no swimming-bells, but from the underside of the float
hang gastrozooids (p), dactylozooids, branching blastostyles
(gonodendra) with groups of medusoids looking like bunches
of grapes of a deep blue colour, and long retractile tentacles (t),

M 2

164

ZOOLOGY

SECT.

sometimes several feet in length and containing batteries of
stinging-capsules powerful enough to sting the hand as severely
as a nettle. The male reproductive zooid remains attached, as in

**-.

FIG. 122. Physalia : the living animal floating on the surface of the sea. cr. crest ; p. polype ;
pn. pneumatophore ; t. tentacle. (After Huxley.)

Halistemma, but the female apparently becomes detached as a
free medusa.

In Diphycs the float is absent. Two swimming-bells (Fig. 123-4, m)
of proportionally immense size are situated at the proximal end
of the coansarc, and are followed by widely-separated groups of
/ooids (/?), each group containing a polype (n) with its tentacles (*'),

IV

PHYLUM CCELENTERATA

165

a meduzoid (y), and a large enveloping bract (t). The stem often
breaks at the internodes, and the detached groups of zooids then
swim about like independent organisms.

Porpita is formed on a different type, and has a close general
resemblance to a medusa. It consists (Fig. 124) of a discoid

FIG. 123. Diphyes campanulata A, the entire colony; B, single group of zooids. ,
cosnosarc ; c, cavity of swiniming-bell ; c, groups of zooids ; (/, medusoid ; i, grappling line or
tentacle ; m, swimming-bell ; n, polype ; o, mouth of swimming-bell ; t, bract. (From
Parker's Biology, after Gegenbaur.)

body, enclosing a chambered chitinoid shell (s/i) containing air, and
obviously corresponding with the float of Physalia. The edge of
the disc is beset with long dactylozooids (tf) and from its lower
surface depend numerous closely set blastostyles provided with
mouths and bearing medusae, while in the centre is a single large

166

ZOOLOGY

SECT.

gastrozooid (%). The closely allied genus Velclla is of rhomboidal
form, and bears on its upper surface an oblique sail.

The reproductive zooids are liberated as free medusae. The
eggs give rise to young which have a close resemblance to flat
medusae with manubrium, marginal tentacles, and an air-chamber
or float developed in the ex-umbrella. Thus it is quite possible
that the Siphonophora of the Porpita-type may be medusae the
sub-umbrella of which has given rise to buds forming the feelers

FIG. 124. Porpita pacifica. A, from beneath ; B, vertical section, hij, large central gastro-
zooid ; hy'. blastostyles ; sh, chambered shell ; t, dactylozooids. (From Parker's Biology, after
Duperry and Koelliker.)

and blastostyles. But, as their early development is not known, it
is still quite legitimate to describe them in the same terms as the
other Siphonophora i.e. to consider them as hydroid colonies in
which the cosnosarc is represented by the discoid or rhomboid
body with its contained air-chamber.

ORDER 5. GRAPTOLITHIDA.

The " Graptolites " are fossil Hydrozoa found in the Upper Cambrian and
Silurian rocks. They are known only by their fossilised chitinoid skeleton, all
trace of the soft parts having, as in the majority of fossils, disappeared.

With one doubtful exception they are compound, consisting of an elongated
tube the perisarc of the common stem, having attached to it, either in a single

IV

PHYLUM CCELENTERATA

167

or a double row, numerous small projections, the hydrothecae (Fig. 125, h.lh).
The coenosarcal skeleton is strengthened by a slender axis, the virgula (v), the
proximal end of which is connected with a small dagger-shaped body, the
siculn (s\ supposed to be the skeleton of the primary
zooid by the budding of which the colony was pro-
duced. In connection with some species oval or
cup-like capsules have been found : these may be of
the nature of goiiothecre. But it must be added that
the evidence in favour of associating the Graptolites -^

with the Hydrozoa is by no means conclusive, and
reasons have been adduced for regarding them as
connected with groups much higher in the scale.

ADDITIONAL REMARKS ON THE HYDROZOA.

The vast majority of Hydrozoa are
marine, the only exceptions being Hydra,
found all over the world ; MicroJiydra, at
present known only in North America ;
Cordylophora, one of the Anthomedusse,
found in Europe, America, Australia, and
New Zealand ; Polypodium, also an Antho-
medusa, found in the Volga, where in
one stage of its existence it is parasitic on
the eggs of a Sturgeon; Limnocodiwn, a
doubtful Trachymedusa, hitherto found
only in a tank in the Botanical Gardens,
Regent's Park, where it was probably in-
troduced from the West Indies ; and Limnocnida, found in
Lake Tanganyika, Africa.

The oldest known Hydrozoa are the Graptolites, found first in
the Cambrian rocks ; Hydr actinia occurs in the Cretaceous epoch,
and Hydrocorallinse from the Cretaceous onwards.

Parasitism, although rare, is not unknown in the class. Poly-
podium, one of the Anthomedusse, is parasitic during part of its
existence, in the ovary of the Sturgeon ; and Cunina, one of the
Narcomedusse, is parasitic on a Trachymedusa.

In the section on the Protozoa we saw that while the majority
of species are independent cells, each performing alone all the
essential functions of an animal, others, such as Pandorina,
Volvox, and Proterospongia, consist of numerous unicellular
zooids associated to form a colony in which a certain division of
labour obtains, the function of reproduction, for instance, being
assigned to certain definite cells and not performed by all alike.
Thus the colonial Protozoa furnish an example of individuation,
numerous cells combining to form a colony in which the several
parts are dependent one upon another, and which may therefore
be said to constitute, from the physiological point of view, an
individual of a higher order than the cell.

FIG. 125. Graptolites.

A, Monograptus coionus;

B, DimorpJiograptus, both
magnified, hy. th. hydro-
theca ; s. sicula ; -c. vir-
gula. (After Nicholson
and Lydekker.)

168 ZOOLOGY SECT, iv

This is still more notably the case in the lower Metazoa, such
as Ascetta and Hydra, in which we have numerous cells combined
to form a permanent two-layered sac with a terminal aperture,
some of the cells having digestive, others tactile, others repro-
ductive functions. Thus while an Amceba or a Paramcecium is
an individual of the first order, Hydra and Ascetta are individuals
of the second order, each the equivalent of an indefinite number of
individuals of the first order.

In the Hydrozoa we see this process carried a step further.
Budding takes place and colonies are produced, the various zooids
of which each the equivalent of a Hydra instead of remaining
all alike, become differentiated both morphologically and physio-
logically, so as to differ immensely from one another both in form
and function. In Obelia, for instance, reproduction is made over
exclusively to the medusae, while in Halistemma we have zooids
specially set apart, not only for reproductive, but for tactile and
protective purposes. Thus in Halistemma and the other Siphono-
phora there is a very complete subordination of the individual
zooids to the purposes of the colony as a whole, the colony thus
assuming, from the physiological point of view, the characteristics
of a single individual, and its zooids the character of organs. In
this way we get an individual of the third order, consisting of an
aggregate of polymorphic zooids, just as the zooid or individual
of the second order is an aggregate of polymorphic cells or
individuals of the first order.

CLASS II. SCYPHOZOA.

1. EXAMPLE OF THE CLASS- -THE COMMON JELLY-FISH

(Aurelia aurita).

Aurelia is the commonest of the larger jelly-fishes, and is often
found cast up on the sea-shore, when it is readily recognisable by
its gelatinous, saucer-shaped umbrella, three or four inches in
diameter, and by having near the centre four red or purple horse-
shoe-shaped bodies the gonads lying embedded in the jelly.

External Characteristics. The general arrangement of the
parts of the body is very similar to what we are already familiar
with in the hydrozoan jelly-fishes. Most conspicuous is the
concavo-convex umbrella, the convex surface of which, or ex-
umbrella, is uppermost in the ordinary swimming position (Figs.
126 and 127, A). The outline is approximately circular, but is
broken by eight notches, in each of which lies a pair of delicate
processes, the marginal lappets (mg. Ip.) : between the pairs of
lappets the edge of the umbrella is fringed by numerous close-set
marginal tentacles (t).

S

'///;"*>,

/ ///fr<'^*-^

^m^^ f

"'a am

FIG. r2ti. Aurelia aurita. A, dorsal view, part of the ex-umbrella cut away to show part of
tht! stomach and one of the four gastric pouches ; B, ventral view two of the oral arms are
removed, a.r. '. adradial canal; g. f. gastric filaments; gon. gonads ; ft. j>. gastric pouch;
i.r. c. inter-radial canal ; m>i. I p. marginal lappet; ntih. mouth; or. a. oral arm; p.r. c. per-
radial canal ; s.y. p. sub-genital pit ; st. stomach ; t. tentacles.

170 ZOOLOGY

A narrow region of the umbrella adjoining the edge is very thin
and flexible: the structure thus constituted, with its marginal
notches and the fringe of marginal tentacles, is the velarium.
Unlike the true velum of the medusae of the Hydrozoa the
velarium contains endodermal canals.

In the centre of the lower or sub-umbrellar surface is a four-
sided aperture, the mouth (mth), borne at the end of an extremely
short and inconspicuous manubrium : surrounding it are four long
delicate processes, the oral arms (or. a*), lying one at each angle
of the mouth and uniting around it. Each arm consists of a
folded membrane, tapering to a point at its distal end, beset
along its edges with minute lobules, and abundantly provided
with stinging-capsules. The angles of the mouth and the arms
lie in the four per-radii, i.e. at the end of the two principal axes
of the radially symmetrical body : of the marginal notches with
their lappets, four are per-radial and four inter-radial.

At a short distance from each of the straight sides of the
mouth, and therefore inter-radial in position, is a nearly circular
aperture leading into a shallow pouch, the svJb-genital pit (s.g.p).
which lies immediately beneath one of the conspicuously coloured
gonads (gori). The sub-genital pits have no connection with the
reproductive system, and are probably respiratory in function.

Digestive Cavity and Canal-System. The mouth leads by
a short tube or gullet (gul), contained in the manubrium, into a
spacious stomach (st), which occupies the whole middle region of
the umbrella, and is produced into four wide inter-radial gastric
pouches (g.p), which extend about half way from the centre to
the circumference, and are separated from one another by thick
pillar-like portions of the umbrella-jelly. In the outer or peri-
pheral wall of each gastric pouch are three small apertures,
leading into as many radial canals, which pass to the edge of
the umbrella and there unite in a very narrow circular canal
(circ. c). The canal which opens by the middle of the three
holes, is of course inter-radial (i.r.c) : it divides immediately
into three, and each division branches again : the canals from the
other two holes are ad-radial (a.r.c), and pass to the circular canal
without branching. There is also an aperture in the re-entering
angle between each two gastric pouches : this leads into a per-
radial canal (r>.r.c), which, like the inter-radial, branches
extensively on its way to the edge of the umbrella.

The general arrangement of the cell-layers in Aurelia is the
same as in a hydroid medusa (Fig. 127, B). The main mass of
the umbrella is formed of gelatinous mesogloea, which, however,
is not structureless, but is traversed by branching fibres and
contains amoeboid cells derived from the endoderm. Both ex-
and sub-umbrellae are covered with ectoderm, and the stomach and
canal system are lined with endoderm, which is ciliated through-

IV

PHYLUM CCELENTERATA

171

out. Some observations seem to show that the short tube
described above as a gullet and a pair of the gastric pouches
are lined, not by endoderm, but by an in-turned portion of the
ectoderm, but this matter cannot be considered as definitely
settled.

It was mentioned above that in the free medusa the gonads
appear through the transparent umbrella as coloured horseshoe-

B

I. PC

FIG. 127. Aurelia aurita. A, side view, oiie-fomth of the umbrella cut away ; B, diagrammatic
vertical section, ectoderm dotted, endoderm strmted, mesogloea black, circ. c. circular canal ;
g. f. gastric filaments ; cion. gonad ; ft. p. gastric pouch ; gul. gullet ; h. hood ; i.r. c. inter-radial
canal ; tug. lp. marginal lappet ; mth. mouth ; or. a. oral arm ; s.g. p. sub-genital pit ; st.
stomach.

shaped patches. Their precise position is seen by cutting away a
portion of the ex-umbrella so as to expose one of the gastric
pouches from above (Fig. 126, A). It is then seen that the
gonad (gon) is a frill-like structure lying on the floor of the
pouch and bent in the form of a horse-shoe with its concavity
looking inwards, i.e. towards the mouth. Being developed from
the floor of the enteric cavity, the gonad is obviously an

172 ZOOLOGY SECT.

endodermal structure : when mature, its products ova or sperms
-are discharged into the stomach and pass out by the mouth.
Here, then, is an important difference from the Hydrozoa, in
which the generative products are usually located in the ectoderm,
and are always discharged directly on the exterior. The sexes
are lodged in distinct individuals.

Lying parallel with the inner or concave border of each gonad
is a row of delicate filaments (Fig. 126, 127, g.f), formed of endoderm
with a core of mesogloea and abundantly supplied with stinging-
capsules. These are the gastric filaments or phacellae : their
function is to kill or paralyse the prey taken alive into the
stomach. No such endodermal tentacles are known in the
Hydrozoa.

Muscular and Nervous Systems. The contractions of the
bell by which the animal is propelled through the water are

B

FIG. 128 Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the
tentaculocyst ; B, vertical section of the same region (diagrammatic), ft, hood ; /, lithite ;
mrj. lp, marginal lappet ; oc, ocellus ; o/f. 1, o/f. 3, olfactory pits. (Altered from Lankester.)

effected by means of a muscular zone round the edge of the sub-
umbrella. The nervous system is formed on a different plan
from that of the hydroid medusae. Extending over the sub-
umbrellar surface between the superficial epithelial layer of
ectoderm and the muscular layer is a plexus of simple nerve-fibres.
This presents radial thickenings, most strongly developed
externally in the per-radii and iriter-radii, corresponding to the
position of the marginal notches and sense-organs. About the
base of each of the latter are special groups of nerve-cells. A
slight ring-like thickening of the plexus extends round the margin
in the neighbourhood of the marginal canal.

The sense organs (Fig. 128) are lodged in the marginal
notches in close relation with the nerve-patches : like the latter,
therefore, four of them are per-radial and four inter-radial. Each
consists of a peculiar form of sense-club or tentaculocyst, containing

iv PHYLUM CCELENTERATA 173

a prolongation of the circular canal, and thus representing a hollow
instead of a solid tentacle. At the extremity are calcareous con-
cretions or lithites (1) derived from the endoderm, and on the outer
side is an ectodermal pigment-spot or ocellus (cc). The
tentaculocysts are largely hidden by the marginal lappets (mg. Ip)
and by a hood-like process (A) connecting them ; and in connection
with each are two depressions, one on the ex-umbrella (plf. 1), the
other immediately internal to the sense-club (off. 2) : these
depressions are lined with sensory epithelium and are called
oljactory pits.

The development and life-history of Aurelia present several
striking and characteristic features. The impregnated egg-cell
or oosperm divides regularly and forms a morula, which, by accumu-
lation of fluid in its interior, becomes a blastula a closed sac with
walls formed of a single layer of cells. One end of this sac becomes
invaginated to form the gastrula. The blastopore or gastrula-
mouth does not completely close, the resulting two layered planula
(Fig. 129) differing in this respect, as well as in its mode of
formation, from the corresponding stage of a Hydrozoan.

The planula swims about by means of the cilia with which its
ectodermal cells are provided, and, after a brief free existence,
settles down, loses its cilia, and becomes attached by one pole.
At the opposite pole a mouth is formed, the process taking place
by a sinking-in or invagination of the surface so as to produce a
depression lined with ectoderm (B, s), the bottom of which
becomes perforated so as to communicate with the enteric cavity
(C, st) : the depression is the stomodceum, a structure of which
there is no trace in the Hydrozoa. On two opposite sides of the
mouth hollow processes grow out, forming the first two tentacles :
soon two others appear at right angles to these, the organism
thus being provided with four per-radial tentacles. Subsequently
four inter-radial and eight adradial tentacles appear. At the
same time the attached or proximal end is narrowed into a stalk-
like organ of attachment (E), and the endoderm of the enteric
cavity is produced into four longitudinal ridges, inter-radial in
position, and distinguished as the gastric ridges or tcenioles (D tn.).
The mouth (E, mth.') assumes a square outline, and its edges become
raised so as to form a short manubrium (mnb.} ; and, finally, the
ectoderm of the distal surface i.e. the region lying between the
mouth and the circlet of tentacles becomes invaginated in each
inter-radius so as to produce four narrow funnel-like depressions-
the septal funnels or infundibula (E and F, s. /.) sunk in the four
gastric ridges.

The outcome of all these changes is the metamorphosis of the
planula into a polype (E), not unlike a Hydra or the hydrula-stage
of the Leptolinse, but distinguished by a pronounced differentia-
tion of structure, indicated by the sixteen tentacles developed in

174

ZOOLOGY

SECT.

regular order, the stomodoeum, and the four gastric ridges with
their septal funnels. The Scyphozoon-polype is called a scyphula
or scyphistoma.

FIG. 129. Aurelia aurita, development. A. planula, erroneously represented as completely
closed ; B, C, formation of stomodteum ; D, transverse section of young scyphula ; E,
scyphula ; F, longitudinal section of same : the section passes through a per-radius 011 the
left of the dotted line, through an inter-radius on the right ; G, division of scyphula
into ephyrulae ; H, ephyrula from the side ; L, the same from beneath. In A D and
F the ectoderm is unshaded, the endoderm striated, and the mesoglcea dotted, a. lobes
of umbrella ; mnb. manubrium ; mth. mouth ; s.f. septal funnel ; st. stoniodseum ; t. tentacle ;
in. ttenioles. (From Korschelt and Holder's Embryology.)

The scyphula may grow to a height of half an inch, and some-
times multiplies by budding. After a time it undergoes a process

iv PHYLUM CCELENTERATA 175

of transverse fission (G), becoming divided by a series of constric-
tions which deepen until the polype assumes the appearance of a
pile of saucers, each with its edge produced into eight bifid lobes,
four per- and four inter-radial. Soon the process of constriction
is completed, the saucer-like bodies separate from one another,
and each, turning upside down, begins to swim about as a small
jelly-fish called an ephyrula (H, I). The umbrella of the ephyrula
is divided into eight long bifid a.rms (a) with deep (per-radial or
inter-radial) notches : it has of course carried away with it a
segment of the stomach with the gastric ridges of the scyphula :
during the process of constriction this becomes closed in on the
proximal or ex-umbrellar side, while on the sub-umbrellar side it
remains open, and its edges grow out to form a manubrium.
Round the margin there are the bases of eight per-radial and
inter-radial tentacles, each in the notch of one of the arms, and
eight ad-radial tentacles in the intervals between the lobes : the
latter disappear completely ; the former may persist as the
tentaculocysts. On each gastric ridge appears a single gastric
filament, soon to be followed by others, and in the notches at the
extremities of the eight arms tentaculocysts are now recognisable.
In the meantime the spacious enteric cavity is continued into the
eight arms in the form of wide radiating canals.

As the ephyrula grows the adradial regions at first deeply
notched grow more rapidly than the rest, the result being that
the notches become gradually filled up, and the umbrella, from an
eight-rayed star, becomes a nearly circular disc. Four oral arms are
developed and numerous marginal tentacles, and the ephyrula
gradually assumes the form of the adult Aurelia. It seems
probable that the sub-genital pits of the medusa are formed
from sections of the septal funnels of the scyphula.

Thus the life-history of Aurelia differs in several marked
respects from that of any of the Hydrozoa. There is, in a sense,
an alternation of generations as in Obelia, the gamobium being
represented by the adult Aurelia, the agamobium by the scyphula.
But instead of the medusa being developed either as a bud on a
branched colony, as in Leptolinas, or by direct metamorphosis of a
polype, as in Trachylinaa, it is formed by the metamorphosis of an
ephyrula developed as one of several transverse segments of a
polype ; so that the life -history might be described as a metamor-
phosis complicated by multiplication in the larval (scyphula)
condition, rather than a true alternation of generations.

It has been shown that, under exceptional circumstances, the
egg of Aurelia develops into scyphulse which do not undergo
transverse division, the entire scyphula becoming metamorphosed
into a single adult.

176 ZOOLOGY SECT.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Scyphozoa may be defined as medusoid Coelenterata, having
the same general structure and arrangement of the layers as the
medusoid Hydrozoa, but differing from them in the possession of
endodermal gastric tentacles ; in having gonads the sexual cells
of which are lodged in the endoderm and which discharge their
products into the digestive cavity ; in the absence of a true velum,
and in nearly all cases, in the presence of sense-organs in the form
of hollow sense-clubs or tentaculocysts. Whether a stomodseum
or ectodermal gullet occurs is uncertain. As in the Hydrozoa, the
medusa develops directly from the egg in some Scyphozoa, while
in others there is a sort of alternation of generations, a polype-
form (again obium) giving rise to the medusa-form (gamobium) by
a process of transverse fission. In the majority, however, nothing
is known of the life-history, the process of development having
been worked out only in a few cases.

As far as is known, the segmenting embryo gives rise to a gastrula
by invagination in all with the exception of Lucernaria. and its
allies : by the partial or complete closure of the blastopore a
planula is produced, at one end of which a second invagination
takes place, forming the stomodseum.

The Scyphozoa are divisible into four orders, as follows :

ORDER 1. STAUROMEDUS.E (LUCERNARIDA).

Scyphozoa having a conical or vase-shaped umbrella, sometimes
attached to external objects by an ex-umbrellar peduncle : no
tentaculocysts.

ORDER 2. CORONATA.

Scyphozoa having the umbrella divided by a horizontal coronary
groove : four to sixteen tentaculocysts.

ORDER 3. CUBOMEDUS.E.

Scyphozoa with a four-sided cup-shaped umbrella : four per-
radial tentaculocysts.

ORDER 4 DISCO-MEDUSA.

Scyphozoa with a flattened saucer- or disc-shaped umbrella:
not fewer than eight tentaculocysts four per- and four 'inter-
radial.

iv PHYLUM CCELENTERATA 177

Sub-Order a Semostomce.

Discomedusfc in which the square mouth is produced into four long oral
arms.

Sub- Order b Rhizostomcv.

Discomedusse having the mouth obliterated by the growth across it of the
oral arms : the stomach is continued into canals which open by funnel-shaped
apertures on the edges of the arms.

Systematic Position of the Example.

Aurelia aurita is one of several species of the genus Aurelia,
and is placed in the family Ulmaridce, the sub-order Semostontce,
and the order Discomedusce.

Its saucer-shaped umbrella and eight tentaculocysts place it at
once among the Discomedusse : the presence of a distinct mouth
surrounded by four oral arms places it in the first sub-order
or Semostomse. This group contains six families, characterised
mainly by differences in the canal system : the Ulmaridse are
distinguished by narrow branched radial canals opening into a
circular canal. Of the eight genera in this family, Aurelia stands
alone in having its tentacles attached on the dorsal or ex-umbrellar
side of the margin, and in the oral arms showing no trace of bi-
furcation. Eight species of Aurelia are recognised, A. aurita,
being distinguished by having the oral arms slightly shorter
than the radius of the umbrella, and by possessing a trichotomous
inter-radial canal and two unbranched adradial canals springing
from each gastric pouch.

ORDER 1. STAUROMEDUS.E (LUCERNARTDA).

Tessera (Fig. 130), formerly regarded as the simplest member of this group,
is now looked upon as probably not a mature form. It is described as a small
medusa about 4 mm. in diameter having the same general characters as the
scyphula-stage of Aurelia, except that the bell-shaped body is free-swimming.
The edge of the umbrella is surrounded by eight tentacles, four per-radial
(p.r.t.) and four inter-radial (i.r.t.), and movement is effected by a well-developed
system of circular and radial muscles.

Lucemaria (Fig. 131), a genus not uncommon on the British coasts, is in one
respect even more like a scyphula, since it is attached by a peduncle developed
from the centre of the ex-umbrella. The margin of the umbrella is prolonged
into eight short hollow adradial arms, bearing at their ends groups of short
adhesive tentacles (/.). As in the scyphula, each gastric ridge contains an
infundibulum, lined with ectoderm and opening on the sub-umbrella. The
gastric filaments (#./.) are very numerous a distinct advance on Tessera and
the gonads ((jon.) are band-like. There are no sense-organs in Lucernaria, but
in an allied genus, Halicystus, there are eight per-radial and inter-radial
marginal bodies (anchors) of the nature of reduced and modified tentacles, each
surrounded at its base by a cushion-like thickening containing many adhesive
cells. Internal to each anchor on the sub-umbrellar side is a pigment spot

VOL. I N

178

ZOOLOGY

SECT.

(rudimentary eye). Stenocyphus is an allied form which probably is able to
move by creeping (looping) movements like those of a leech. Capria has no

i.r

FIG. iso. Tessera princeps, A, external view; B, vertical section, r/./. gastric filament;
ijmi. gonad ; i.r. t. inter-radial tentacle; mub. manubriuni ; ruth, mouth; p.r. t. per-radial
tentacle ; st. stomach ; tn. tamiole. (After Haeckel.)

Fio. 131. Xiucernaria. A, oral aspect ; B, from the side, p. foot-gland ; 57. /. gastric filaments
gon. gonad ; mlh. month ; t. tentacles ; tn. tsenioles. (After Claus.)

tentacles. The Depaxtruhe have an almost entire margin fringed with
tentacles.

IV

PHYLUM CCELENTERATA

179

ORDER 2. CORONATA.

This group includes a number of rare and beautiful Medussft of curiously
complex structure, of which Pericolpa may be taken as an example. The
umbrella (Fig. 132) is usually conical, and is divided by a horizontal furrow
(coronary groove) into an apical region or cone, (en.) and a marginal region or

circ. 8

TfinJb

FIG. 132. Pericolpa quadriarata. A, external view; B, vertical section, circ. s. circular
sinus ; en. cone ; </. f. gastric filaments ; yon. gonads ; mg. lp. marginal lappets ; mnb. manu-
Lrium ; mtk. mouth ; pi I. I. pedal lobes ; st. stomach ; t. tentacles ; tc. tentaculocysts ; tn.
tjenioles. (After Haeckel.)

crown; the crown is again divided by a second, rather irregular horizontal
furrow into a series of pedal lobes (pel. I.), adjacent to the cone, and a series of
marginal lappets (my. lp.), forming the free edge of the bell. In some of the
Coronata, such as Pericolpa, the pedal lobes and marginal lappets correspond
(i.e. are in the same radii) ; in others (Periphylla, Ephyropsis) they alternate.

N 2

180

ZOOLOGY

SECT.

In Pericolpa four of the pedal lobes, inter-radial in position, bear
tentaculocysts (tc. ) ; four others, per-radially situated, give origin to long,
hollow tentacles ((,.). In the more complex genera there are eight additional
adradial tentacles.

The mouth (mth.) is very large, and leads by a wide manubrium (mnb.) into
a spacious stomach (st.}, which is continued quite to the apex of the cone. In
the wall of the stomach are four wide per-radial slits, leading into an immense
circular sinus (circ. .<?.). As in Lucernaria, there are four wide inter-radial im-
fundibula. The gastric filaments (g. y.) are very numerous, and the elongated
U-shaped gonads (/o.) are eight in number and adradial.

The coronary groove is characteristic of the group : but in other points -
such as the number of pedal and marginal lobes, tentaculocysts, and tentacles

f]

FIG. 133. Nausithoe. The entire animal from the oral aspect, ar. adradii ; g. gonads; g.f.
gastric filaments ; ir. inter-radii ; TO. circular muscle of sub-umbrella ; pr. per-radii ; rl. tenta-
culocysts ; sr. sub-radii ; t. tentacles. The black cross in the centre represents the mouth.
(From Lang's Comparative Anatomy.)

there is great variation. Pericolpa and its allies (Peromedusca} resemble the
Lucernarida and the members of the order Culo/nednw in the presence of
ttenioles and inter-radial septa : Ephyropsis and its allies (Cannostonur]
resemble the order Discophora in the absence of these structures. The scyphula
larva of Nausithoe (Fig. 133) lives as a parasite in the interior of a horny
sponge.

ORDER 3. CUBOMEMJS.E.

The Jelly-fishes forming this order are, as the name implies, of a more or less
cubical form, resembling a deep bell with somewhat flattened top and square
transverse section. They resemble the hydrozoan Medusa? more than any of the
other Scyphozoa. The best known species, Charybdcea marsupialix (Fig. 134), is
about 5 cm. in diameter and of very firm consistency.

IV

PHYLUM CCELENTERAT^

181

As in the lower Coronata, the margin of the umbrella bears four tentacles
(t.) and four tentaculocysts (tc.} but the position of these organs is reversed, the
tentaculocysts being per-radial, the tentacles inter-radial. The tentaculocysts
are set in deep marginal notches, and the tentacles spring from conspicuous
gelatinous lobes (I.}, which probably answer to the pedal lobes of the preceding
order. These pedal lobes sometimes bear a number of supplementary tentacles.

gon.

B

endMtm

circ.

FIG. 134. Charybdsea marsupialis . A, side view of the entire animal ; B, vertical section
passing on the left side through an inter-radius, on the right through a per-radius ;
C, transverse section, circ. c. circular canal; <in<(. lam. endoderm lamella; cn<L lam', its pro-
longation into the velarium ; g. f. gastric filaments ; gon. goiiad ; gon'. septum separating
gonads ; f. lappet ; m>ib. manubrium ; rod. p. radial pouch ; t. tentacle ; tc. tentaculocy.st ;
vl. velarium. (After Claus, somewhat altered.)

The margin of the umbrella is produced, in most cases but not in all, into a
horizontal shelf (vl.}, resembling the velum of the hydroid Medusa, but differing
from it in containing a series of branched vessels (end. lam'. ) continuous with the
canal-system and of course lined with endoderm. In the Hydrozoa, it will be
remembered, the velum is formed simply of a double layer of ectoderm with a

182 ZOOLOGY SECT.

supporting layer of mesoglcea. Such a false velum, like the produced thin edge
of the umbrella in Aurelia, is known as a velarium.

The mouth is situated at the end of a short manubrium (inn 6.) leading into a
wide stomach, from which go off four very broad shallow per-radial pouches
(rad. p.}, occupying the whole of the four flat sides of the umbrella, and
separated from one another by narrow inter-radial septa or partitions (mesenteries]
placed at the four corners. These pouches are equivalent to \vide radial canals,
and the partitions between them to a poorly developed endoderm lamella (end.
lam.}. At the margin of the umbrella the pouches communicate with one
another by apertures in the septa, so that a kind of circular canal is produced
(circ. c. ), which is divided into chambers by the mesenteries. Near the junction
of the gastric pouches with the stomach are the usual four groups of gastric
filaments (g. f. ).

The gonads (yon.) are four pairs of narrow plate-like organs, attached one
along each side of each inter-radial septum. The nervous system takes the form
of a sinuous nerve-ring round *the margin of the bell, bearing a distinct group of
nerve-cells at the base of each tentaculocyst and tentacle. The Cubomedusae are
the only Scyphozoa which, like the Hydrozoa, have a complete nerve-ring. The
tentaculocysts are very complex, each bearing a lithocyst and several eye-spots.

ORDER 4. DISCOMEDUS.E.

The preceding orders are all small ones, i.e., include a small number of genera
and species. The vast majority of Scyphozoa belong to the present order the
" Disc- jellies " or " Sea-blubbers " as ordinarily understood.

The umbrella is always comparatively flat, having the form of an inverted
saucer. The edge is produced primitively into eight pairs of marginal lappets,
but in some of the more highly differentiated forms the number both of lappets
and of tentaculocysts becomes greatly increased. Most of the Semostomae and
Rhizostomae are large, and one of the former group Cyanea arctica may attain
a diameter of 2 metres and upwards, while its marginal tentacles reach the
astonishing length of 40 metres or about 130 feet. But in spite of their size and
apparent solidity, the amount of solid matter in these great Jelly-fishes is extra-
ordinarily small ; some of them have been proved to contain more than 99 per
cent, of sea- water.

The marginal tentacles are hollow and often of great length in the Semostomaj
(Fig. 126), and altogether absent in the Rhizostomae (Fig. 135). In the
Semostomae there are four oral arms (Fig. 126, o r. a.), each resembling a leaf
folded along its midrib, and having more or less frilled edges : in the Rhizostomae
each of the original four arms (Fig. 135, or. a.) becomes divided longitudinally in
the course of development, the adult members of the group being characterised
by the presence of eight arms, often of great length, and variously lobed and
folded so as to present a more or less root-like appearance.

The arrangement of the enteric cavity and its offshoots presents an interest-
ing series of modifications. In no case are there any twnioles or inter-radial
septa (mesenteries). In the Semostomae (Fig. 126) the stomach-lobes give off
well-defined radial canals, which are frequently more or less branched, often
unite into complex networks, and sometimes open into a circular canal round the
margin of the umbrella.

In the Rhizostornae (Fig. 135, B) a similar network of canals is found in the
umbrella, but an extraordinary change has befallen the oral or ingestive portion
of the enteric system. Looking at the oral or lower surface of one of these Jelly-
fishes, such as Pilema, no mouth is to be seen, but a careful examination of the
oral arms shows the presence of large numbers hundreds, or even thousands in
some cases of small funnel-like apertures (B, C, s.mth.) with frilled margins.

IV

PHYLUM CCELENTERATA

183

Rhizostomes have been found with prey of considerable size, such as fishes, em-
braced by the arms and partly drawn into these apertures, which are therefore
called the suctorial mouths. They lead into canals in the thickness of the arms
(B, c. ), the lesser canals unite into larger, and then finally open into the stomach
(st.). We thus get a polystomatous or many-mouthed condition which is practi-
cally unique in the animal kingdom, the only parallel to it being furnished
by the Sponges, in which the inhalant pores are roughly comparable with the
suctorial mouths of a Rhizostome.

It has been found that this characteristic arrangement is brought about by
certain changes taking place during growth. The young Rhizostome has a single
mouth in the usual position, and more or less leaf-like arms, folded along the
midrib so as to enclose a deep groove, from which secondary grooves pass, like

St

s.mfft,

FIG, 13'). Pilema pulmo. A, side view of the entire animal ; B, vertical section, diagrammatic ;
C, one of the suctorial mouths, magnified, c. arm canal ; </. /. gastric filaments ; yon. gonads ;
or. a. oral arms ; rail. r. radial canal ; .?. mth. suctorial mouths ; st. stomach ; tl, t2, t3, tentacles
on oral arms. (After Cuvier, Glaus, and Huxley.)

the veins of a leaf, towards the edge of the arm. As development proceeds, these
grooves become converted into canals by the union of their edges, thus forming
a system of branching tubes opening proximally into the angles of the mouth and
distally by small apertures the suctorial mouths on the edges of the arms.
At the same time the proximal ends of the arms grow towards one another and
finally unite across the mouth, closing it completely, and forming a strong
horizontal brachial disc, which in the adult occupies the centre of the sub-
umbrellar surface.

The gastric filaments are usually very numerous. In the higher Rhizostom?e
a remarkable modification is produced in connection with the sub-genital pouches ;
the four pouches approach the centre and fuse with one another, forming a single
spacious chamber, the sub-genital portico, which lies immediately below the
floor of the stomach and above the brachial disc.

184

ZOOLOGY

SECT.

In many of the Discomedusa? development takes place in the same general
way as in Aurelia, i.e. the impregnated egg gives rise to a scyphula or asexual
polype stage, which, by transverse division, produces sexual medusae. In
Cassiopeia the scyphula arising from the fertilised ovum gives oft' buds which
become detached as free-swimming planulse, and these, coming to rest, develop
into scyphula?. But in other cases there is no alternation of generations, and
development is direct. For instance, in Pelagia (Fig. 136) one of the
Semostonue a blastula is formed which becomes invaginated at one end,

B

FIG. 136. Pelagia noctiluca : Three developmental stages, m. month ; r. marginal lappet ;
s. tentaculocyst. (From Korschelt and Heider, after Krohn.)

forming a gastrula. The blastopore or gastrula-mouth remains open, and a
considerable space is left between the invaginated endoderm and the ectoderm.
Next the mouth region becomes elevated, forming a manubrium, and around
this a circular depression appears the rudiment of the sub-umbrellar cavity
surrounded by a raised ridge, the umbrella margin, which soon becomes divided
into lobes, the marginal lappets. Up to this time the embryo is ciliated
externally, but soon the cilia disappear, and the little creatures assume somewhat
the form of an ephyrula, which gradually develops into the adult Pelagia.

ADDITIONAL REMARKS ON THE SCYPHOZOA.

The Scyphozoa are all marine, and the majority are pelagic, i.e.
swim freely on the surface of the ocean. A few inhabit the deep
sea, and have been dredged from as great a depth as 2,000 fathoms.
Nearly all are free-swimming in the adult state : some, however,
live on coral-reefs or mud-banks, and are found resting, in an
inverted position, on the ex-umbrella ; and a few, such as Lucern-
aria, are able to attach themselves at will by a definite ex-
umbrellar peduncle.

Many of the Scyphozoa are semi-transparent and glassy, but
often with brilliantly coloured gonads, tentacles, or radial canals.
In many cases the umbrella, oral arms, &c., are highly coloured,
and some species, e.g. Pelagia noctiluca, are phosphorescent. They
are all carnivorous, and although mostly living upon small

iv PHYLUM CCELENTERATA 185

organisms, are able, in the case of the larger species, to capture
and digest Crustaceans and Fishes of considerable size. In many
cases small fishes accompany the larger forms and take shelter
under the umbrella.

Considering the extremely perishable nature of these organisms,
and the fact that many of them contain not more than 1 per cent.
of solid matter, it is not to be expected that many of them should
have left traces of their existence in the fossil state. Nevertheless,
in the finely grained limestone of Solenhofen, in Bavaria, belong-
ing to the Upper Jurassic period, remarkably perfect impressions
of Jelly-fishes have been found, some of them readily recognisable
as Discomedusse.

CLASS III. ACTINOZOA.

1. EXAMPLE OF THE CLASS. A SEA- ANEMONE

(Tealia crassicornis).

Sea-anemones are amongst the most abundant and best known
of shore-animals. They are found attached to rocks, sea-weeds,
shells, &c., either in rock-pools or on rocks left high and dry by the
ebbing tide. Usually their flower-like form and brilliant colour
make them very conspicuous objects, but many kinds cover them-
selves more or less completely with sand and stones, and contract
so much when left uncovered by water, that they appear like soft
shapeless lumps stuck over with stones, and thus easily escape
observation. Any of the numerous species will serve as an
example of the group : the form specially selected is the " Dahlia
Wartlet" (Tealia crassicornis), one of the commonest British
species.

External characters.- -Tealia (Fig. 137, A) has the form of a
cylinder, the diameter of which slightly exceeds its height. It is
often as much as 3 inches (8 cm.) across, is of a green or red colour,
and habitually covers itself with bits of shell, small stones, &c. It
is attached to a rock or other support by a broad sole-like base,
sharply separated from an upright cylindrical wall or column, the
surface of which is beset with rows of adhesive warts or tubercles :
at its upper or distal end the column passes into a horizontal plate,
the disc or peristome. In the middle of the disc, and slightly
elevated above its surface, is an elongated slit-like aperture, the
mouth (mth.\ from which streaks of colour radiate outwards.
Springing from the disc and encircling the mouth are numerous
short conical tentacles (t.\ which appear at first sight to be
arranged irregularly, but are actually disposed in five circlets, of
which the innermost contains five, the next five, the third ten,

136

ZOOLOGY

SECT.

FIG. 137. Tealia crassicornis. A, dissected specimen ; B, transverse section, the half
above the line ab thnm^li tlie y""'- 1 ^ the lower half below the gullet.- <i. mes. directive
mesenteries; r/on. gnnads ; <n<i. gullet ; /. //;. longitudinal muscle; Ip. lappet; mcs. 1, primary,
me.?. 2, secondary, mcs. 3, tertiary mesenteries; mix. /. inesenteric filaments; milt, mouth ;
ost, 1, ost. 2, ostia ; p. m. parietal muscle ; syph. siphonoglyphe ; s. n>~ sphincter muscle ; t. m.
transverse muscle.

iv PHYLUM CCELENTERATA 187

the fourth twenty, and the fifth or outermost forty, making a total
of eighty.

Obviously the Sea-anemone is a polype, formed on the same
general lines as a Hydra or a scyphula, but differing from them in
having numerous tentacles arranged in multiples of five, and in
the absence of a hypostome, the mouth being nearly flush with the
surface of the disc. Its great size and bulk, and the comparative
firmness of its substance, are also striking points of difference
between Tealia and the polypes belonging to the classes Hydrozoa
and Scyphozoa.

Enteric System. Still more fundamental differences are found
when we come to consider the internal structure. The mouth does
not lead at once into a spacious undivided enteric cavity, but into
a short tube (gul.), having the form of a flattened cylinder, which
hangs downwards into the interior of the body, and terminates in
a free edge, produced at each end of the long diameter into a
descending lobe or lappet (lp.}. This tube is the gullet or stomodceum,
a structure we have already met with in the Scyphozoa, but which
here attains a far greater size and importance. Its inner surface is
marked with two longitudinal grooves (A arid B, sgph.\ placed one
at each end of the long diameter, and therefore corresponding with
the lappets : they are known as the gullet-grooves or siphonoglyphes.

The gullet does not simply hang freely in the enteric cavity,
but is connected with the body-wall by a number of radiating
partitions, the complete or primary mesenteries (mes. 1) : between
these are incomplete secondary mesenteries (mes. #), which extend
only part of the way from the body-wall to the gullet, and
tertiary mesenteries (mes. 3), which are hardly more than
ridges on the inner surface of the body-wall. Thus the entire
internal cavity of a Sea-anemone is divisible into three regions :
(1) the gullet or stomodceum, communicating with the exterior
by the mouth, and opening below into (2) a single main digestive
cavity, the stomach or mesenteron, which gives off (3) a number of
radially arranged cavities, the inter-mesenteric chambers or metentera.
It is obvious that we may compare the gullet and stomach with
the similarly named structures in the scyphu la-stage of Aurelia,
and the mesenteries with the gastric ridges ; indeed, there seems to
be little doubt that these structures are severally homologous. A
further correspondence is furnished by the presence of an aperture
or ostium (ost. 1) in each mesentery, placing the adjacent inter-
mesenteric chambers in direct communication with one another:
in Tealia a second ostium (ost. ) is present near the outer edge
of the mesentery. Moreover, the free edge of the mesentery
below the gullet is produced into a curious twisted cord, the
mcscntcric filament (mes. /.), answering to a gastric filament of
the Scyphozoa. In many Sea-Anemones the mesenteric filaments

188

ZOOLOGY

SECT.

are produced into slender threads the a ronl ia which may be
protruded through the mouth or through special apertures
(cinolidcs) of the body-wall (Fig. 138, A.)

The general arrangement of the cell-layers is the same as in
the two preceding classes. The body-wall (Fig 138) base, column,
and disc consists of a layer of ectoderm outside, one of endoderm
within, and between them an intermediate layer or mesoglcea,
which is extremely thick and tough. The gullet (f/nl.), which, like
that of the scyphula, is an in-turned portion of the body-wall, is
lined with ectoderm, and its outer surface i.e. that facing the
inter-mesenteric chambers is endodermal. The mesenteries (mes.)
consist of a supporting plate of mesogloea, covered on both sides by

B

mcs

nZ.

. c

FIG. 138. Diagrammatic vertical (A) and transverse (B) sections of a Sesu-anemone. The
ectoderm is dotted, the endoderm striated, the mesogloea' black, ac. acontium ; en. cinclis ;
gul. gullet ; -int. -nits. c. inter-mesenteric chamber ; mes. mesentery ; mcs. /. in-esenteric
filament ; ruth, mouth ; ost. ostiuin ; p. pore ; t. tentacle.

endoderm. The tentacles (t) are hollow out-pushings of the disc,
and contain the same layers.

Muscular System. Sea-anemones perform various charac-
teristic movements : the column may be extended or retracted, the
tentacles extended to a considerable length, or drawn back and
completely hidden by the upper end of the column being folded
over them like the mouth of a bag; the gullet, and even the
mesenteries, may be partially everted through the mouth ; and
lastly, the whole animal is able, very slowly, to change its position
by creeping movements of its base.

These movements are performed by means of a very well-
developed set of muscles. A mesentery examined from the surface

iv PHYLUM CCELENTERATA 189

is seen to be traversed by definite fibrous bands, the two most
obvious of which are the longitudinal or retractor muscle (Fig.
137, l.m.), running as a narrow band from base to disc, and the
parietal muscle (p.m.), passing obliquely across the lower and outer
angle of the mesentery. Both these muscles are very thick, and
cause a projection or bulging on one side of the mesentery,
specially obvious in a transverse section (B. l.m.) : a third set of
fibres, forming the transverse muscle (t.m), crosses the longitudinal
set at right angles, but is not specially prominent. The longi-
tudinal muscles shorten the mesentery, and draw the disc
downwards or towards the base, thus retracting the tentacles ; the
parietal muscles approximate the column to the base, and the
transverse fibres produce a narrowing of the mesentery and thus,
opposing the action of the longitudinal muscles, act as extensors of
the whole body. The withdrawal of disc and tentacles, during
complete retraction, has been compared to the closure of a bag by
tightening the string, and is performed in much the same way, the
string being represented by a very strong band of fibres, the
circular or sphincter muscle (s.m.), which encircles the body at the
junction of the column and disc.

The foregoing muscles can all be seen by the naked eye, or
under a low magnifying power. They are supplemented by fibres,
only to be made out by microscopic examination, occurring both in
the body-wall and in the tentacles. The latter organs, for instance,
are able to perform independent movements of extension and re-
traction by means of delicate transverse and longitudinal fibres.

It was mentioned above that the thickness of the longitudinal
and parietal muscles produces a bulging on one surface of the
mesenteries. A transverse section shows that the arrangement of
the mesenteries and of their muscles is very definite and charac-
teristic (Fig. 137, B). At each end of the gullet, opposite the
siphonoglyphe, are two mesenteries (d. mcs.), having their longi-
tudinal muscles turned away from one another : they are distin-
guished as the directive mesenteries, and, in the case of Tealia,
there are two couples of directive mesenteries, one at each end of
the long axis of the gullet. Of the remaining complete or
primary mesenteries there are four couples on each side (mcs. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary
mesenteries (mes. 2, mcs. S) are also arranged in couples, and in all
of them the longitudinal muscles of each couple face one another.

Symmetry .- -It will be noticed that Tealia, unlike the typical
hydrozoan and scyphozoan polypes, presents a distinct bilateral x//v/?-
mctry, underlying, as it were, its superficial radial symmetry. It
is divisible into equal and similar halves by two planes only, viz. a
vertical plane taken through the long diameter of the gullet, and a
transverse plane taken through its short diameter.

190

ZOOLOGY

SECT.

The general microscopic structure of a Sea-anemone is well
shown by a section through a tentacle (Fig. 139). Both ectoderm
(ect.) and endoderm (end.) consist mainly of very long columnar,
ciliated, epithelial cells, and the mesoglcea (m-sgl.) is not only ex-
tremely thick, but has the general characters of connective tissue,
being traversed by a network of delicate fibres with interspersed
cells. The middle la} T er has, in fact, ceased to be a mere gelatinous
supporting lamella or mesoglcea, and has assumed, to a far greater

ntc

FIG. 140. Three iiematocysts of
Sagartia. (After Hertwig.)

FIG. 139. Tealia crassicornis. Trans-
verse section of tentacle, ect. ectoderm ;
end. endoderm; l.m. longitudinal muscles ;
msgl. mesoglcea; nr.c. nerve -cells; nr.f.
nerve -fibres ; ntc. iiematocysts ; t. m.
transverse muscles. (After Hertwig.)

extent than in any of the lower groups, the characters of an inter-
mediate cell-layer or mesoderm.

Stinging-capsules occur in the ectoderm, and are also very
abundant in the mesenteric filaments. They (Fig. 140) resemble
in general characters the nematocysts of Hydrozoa, but are of
a more elongated form, and the thread is usually provided at
the base with very numerous slender barbs (B). Very fre-
quently the coiled thread is readily seen in the undischarged
capsule (A). Gland-cells (Fig. 141, <//.) are very abundant
in the ectodermal lining of the gullet and in the mesenteric
filaments : the latter are trilobed in section, and the gland-
cells are confined to the middle portion, the lateral divisions

IV

PHYLUM CCELENTERATA

191

being invested with ordinary ciliated cells (c.). In virtue of
possessing both stinging-capsules and gland-cells, the mesenteric
filaments perform a double function. The animal is very voracious,
and is able to capture and swallow small Fishes, Molluscs, Sea-
urchins, &c. The prey is partly paralysed, before ingestion, by the
nematocysts of the tentacles, but the process is completed, after
swallowing, by those of the mesenteric filaments. Then as the
captured animal lies in the stomach, the edges of the filaments
come into close contact with one another and practically surround

rite

FIG. 141. Transverse section of mesenteric filament of Sagartia. c. ciliated cells; gl. gland-
cells ; -iitc. nematocysts. (After Hertwig.)

it, pouring out, at the same time, a digestive juice secreted by their
gland-cells.

The muscles described above consist partly of spindle-shaped
nucleated fibres, and partly of muscle-processes, like those of
Hydra : the latter occur chiefly in the transverse muscular layer of
the tentacles and are endodermal, the longitudinal layer is formed
of distinct fibres of ectodermal origin : the great muscles of the
mesenteries are of course endodermal. Although always derived
either from the ectoderm or endoderm, many of the muscle-fibres
of Tealia undergo a remarkable change of position by becoming
sunk in the mesoglcea, and thus appearing to belong to that
layer (Fig. 139 /. m.). This fact is significant from the circum-

ZOOLOGY SECT.

stance that, as we shall see, the muscles of all animals above
Coelenterata are mesodermal structures.

The nervous system is very simple. It consists of a layer of
delicate fibres lying between the epithelial and muscular layers of
the ectoderm. Among the fibres are found nerve-cells (Fig. 139,
nc.c.\ often of large size, and occurring chiefly in the disc and
tentacles. Thus, as in the polype-forms previously described, the
nervous system is in a generalised condition, and shows no con-
centration into a definite central nervous system such as occurs
in Medusae.

Reproductive organs. Sea-anemones are dioecious, the sexes
being lodged in distinct individuals. The gonads ovaries or testes
-are developed in the substance of the mesenteries (Fig. 137,ffon.),
a short distance from the edge, and, when mature, often form very
noticeable structures. The reproductive products are obviously, as
in the Scyphozoa, lodged in the endoderm. The sperms, when
ripe, are discharged into the stomach and escape by the mouth :
they are then carried, partly by their own movements, partly by
ciliary action, down the gullet of a female, where they find their
way to the ovaries and impregnate the eggs.

The development of Sea-anemones resembles, in its main features,
that of Scyphozoa. The oosperm undergoes more or less regular
division, the details differing considerably in individual cases, and
becomes converted into a planula, an elongated ovoidal body with
an outer layer of ciliated ectoderm, and an inner layer of large
endoderm cells, surrounding a closed enteric cavity, usually filled
with a mass of yolk, which serves as a store of nutriment.

In this condition the embryo escapes from the parent, through
the mouth, swims about for a time, and then settles down, becom-
ing attached by its broader or anterior end. At the opposite or
narrow end a pit appears, the rudiment of the stomodseum ; this
deepens, and its lower or blind end becoming perforated, effects a
communication with the enteron.

The mesenteries are developed in regular order, but in a way which would
certainly not be suspected from their arrangement in the adult. First of all, a
single pair of mesenteries (Fig. 142; A, 1} grow from the body-wall to the gullet,
being situated one on each side of the vertical plane, at right angles to the long
diameter of the stomodaeum, and near one end of that tube. The enteron thus
becomes divided into two chambers, a larger or dorsal and a smaller or ventral,
and the embryo acquires a distinct bilateral symmetry. Next a pair of mesen-
teries (2) appear in the dorsal chamber, dividing it into a median and two lateral
compartments ; then a third pair (3) in the ventral chamber, producing a similar
division ; then a fourth pair (4) in the middle compartment of the dorsal
chamber ; then a fifth pair (B, 5) in the lateral compartments of the dorsal
chamber ; and a sixth (6) in the lateral compartments of the ventral chamber.
Soon the longitudinal muscles are developed, and the fate of these primitive
pairs of mesenteries can be seen. The third and fourth pairs become the two
directive couples of the adult ; another couple of primary mesenteries is consti-
tuted, on each side of the vertical plane, by one of the mesenteries of the first

IV

PHYLUM CCELENTERATA

193

and one of the sixth pair ; a third couple is similarly formed by a mesentery of
the second and one of the fifth pair. Thus it is only in the case of the directive
mesenteries that an adult couple coincides with an embryonic pair : in other
instances the two mesenteries of a couple are of different orders, belonging
to distinct embryonic pairs. The mesenteric filaments of the first cycle of

Std,

A

B

FIG. 142. Transverse sections of early (A) and later (B) stages of an embryo Sea-anemone
(Actinia.) The mesenteries are numbered in the order of their development ; std. stomo-
dseum. (After Korschelt and Heider.)

mesenteries are partly ectodermal, partly endodermal in origin, those of the
remainder entirely endodermal.

The tentacles are developed in a somewhat similar order to that of the
development of the mesenteries. The first to make its appearance is connected
with the larger or dorsal enteric chamber mentioned above : for some time it
remains much longer than any of its successors, and thus accentuates in a marked
degree the bilateral 'symmetry of the embryo.

It will be noticed that the development of the Sea-anemone is
accompanied by a well-marked metamorphosis, but that there is no
alternation of generations. In this respect its life-history offers a
marked contrast with that of Obelia.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Actinozoa are Coelenterata which exist only in the polype-
form, no medusa-stage being known in any member of the class.
The actinozoan differs from the hydrozoan polype mainly in
possessing a stomod^aum : it differs from the hydrozoan and many
scyphozoan polypes in the possession of mesenteries or vertical
radiating partitions, which extend inwards from the body- wall
and some of which join the stomodum. The free margins of the
mesenteries bear coiled mesenteric filaments, which appear to
answer to the gastric filaments of Scyphozoa, but may be partly
ectodermal in origin. The mesenteries are developed in pairs,

VOL. I O

194 ZOOLOGY SECT.

symmetrically on each side of a vertical plane : their final radial
arrangement is secondary.

The body-wall consists of ectoderm and endoderm separated by
a stout mesogloea containing fibres and cells. The stomodseum
consists of the same layers reversed i.e. its lining membrane is
ectodermal. The mesenteries are formed of a double layer of
endoderm with a supporting plate of mesogloea. Nematocysts,
frequently of a more complex form than those of Hydrozoa and
Scyphozoa, are present in the tentacles, body-wall, stomodseum,
and mesenteric filaments. The muscular system is well developed,
and contains both ectodermal and endodermal fibres and endo-
dermal muscle-processes. The nervous system consists of irregu-
larly disposed cells and fibres ; there is no concentration of these
elements to form a central nervous system.

/

The gonads are developed in the mesenteries, the sex-cells are
lodged in the endoderm, and the ripe sexual products are dis-
charged into the enteron. The impregnated egg develops into a
planula, which, after a short free existence, settles down and
undergoes metamorphosis into the adult form. Except in one
doubtful instance there is no alternation of generations.

In some Actinozoa the animal remains simple throughout life,
but in most members of the class an extensive process of budding
takes place, the result being the formation of colonies of very various
form and often of great size. Some kinds, again, resemble Tealia
in having no hard parts or skeletal structures of any kind : but the
majority possess a skeleton, formed either of carbonate of lime or
of a horn-like or chitinoid material, and developed, in most cases
though not in all, from the ectoderm.

The Actinozoa are classified as follows :

Sub-Class I. Zoantharia.

Actinozoa in which the tentacles and mesenteries are usually
very numerous and are frequently arranged in multiples of five or
six. The tentacles are usually simple, unbranched, hollow cones.
There are commonly two siphonoglyphes and two pairs of directive
mesenteries : the remaining mesenteries are usually arranged
in couples with the longitudinal muscles of each couple facing one
another.

ORDER 1. ACTIXIARIA.

Zoantharia which usually remain simple, but in a few instances
form small colonies. The tentacles and mesenteries are numerous,
and there is no skeleton. This order includes the Sea-anemones.

ORDER 2. MADREPORARIA.

Zoantharia which resemble the Actiniaria in the general
structure of the soft parts, but which usually form colonies, and

IV PHYLUM CCELENTERATA 195

always possess an ectodermal calcareous skeleton. This order
includes the vast majority of Stony Corals (Figs. 146 and 156).

ORDER 3. ANTIPATHARIA.

Compound, tree-like Zoantharia in which the tentacles and
mesenteries are comparatively few (6 24) in number. A skeleton
is present in the form of a branched chitinoid axis, developed from
the ectoderm, which extends throughout the colony. This order
includes the Black Corals" (Fig. 150).

Sub-Class II.- Alcyonaria.

Actinozoa in which the tentacles and mesenteries are always

u

eight in number. The tentacles are pinnate, -i.e. produced into
symmetrical brahchlets. There is never more than one siphono-
glyphe, which is ventral in position, i.e. faces the proximal end of
the colony. The mesenteries are not arranged in couples, and
their longitudinal muscles are all directed ventrally, i.e. towards the
same side as the siphonoglyphe.

ORDER 4. ALCYONACEA.

Alcyonaria in which the skeleton usually consists of calcareous
spicules or small irregular bodies occurring in the mesogloea, but
probably originating from wandering ectoderm cells. The common
" Dead men's lingers " (Alcyonium, Fig. 153) has a skeleton of this
type. In some cases the spicules become aggregated so as to pro-
duce a coherent skeleton, which may form a branched axis to the
whole colony, as in the precious Red Coral (Corallium, Fig. 145),
or a series of connected tubes for the individual polypes, as in
the Organ-pipe Coral (Tubipora, Fig. 148). In the " Blue Coral '
(Heliopora) the skeleton is a massive structure resembling that of
the Madreporaria. Most genera are compound ; a few, such as
Hartea which, however, is probably a larval form (Fig. 144)
are simple.

ORDER 5. GORGOXACEA.

Compound tree-like Alcyonaria, with a calcareous or horny
skeleton of ectodermal origin forming a branched axis throughout
the colony. Spicules are present in the mesogloea. There is no
siphonoglyphe. The beautiful *' Sea-fans " belong to this group
(Fig. J 54).

ORDER 6. PENNATULACEA.

Alcyonaria in which the colony is usually elongated, and has
one end embedded in the mud at the sea-bottom, while the
opposite or distal end bears the polypes, usually on lateral

196 ZOOLOGY SECT.

branches. The stem is supported by a calcareous or horny
skeleton. The polypes are dimorphic. The " Sea-pens" (Pennatula)
are the commonest members of this group (Fig. 147).

Systematic Position of the Example.

Tealia crassicornis is one of several species of the genus
Tealia: it belongs to the family Tcalidce, which, with several
other families, make up the tribe Hexactinicc, of the order
Actiniaria, of the sub-class Zoantharia.

The presence of numerous tentacles, arranged in multiples of
five, places it at once among the Zoantharia. The fact that it is
simple and devoid of a skeleton causes it to be assigned to the
Actiniaria. This order is divided into tribes characterised by
differences in the arrangement of the mesenteries, especially by
the presence of one or two couples of directive mesenteries,
and by the direction in which the longitudinal muscles face.
In the Hexactinise the mesenteries are all arranged in couples
with the longitudinal muscles of each couple facing one another,
except in the case of the two directive couples. The mesenteries
are in multiples of five, and the stomodseutn has two siphono-
glyphes and two lappets.

The family Tealidas is characterised by the possession of
numerous mesenteries, of tentacles of moderate length which are
completely covered by the closed-in disc during retraction, and
by the presence of a large endodermal sphincter muscle. The
genus Tealia is distinguished from other members of the same
family by being broader than high, by having numerous retractile,
equal-sized tentacles, and by the presence of longitudinal series
of warts on the column. The species crassicornis is distinguished
from other species of the genus by the warts being of approxi-
mately equal size.

3 GENERAL ORGANISATION.

The chief variations in the external form of the Actinozoa are
due to the diverse modes of budding : as we shall see, the structure
of the individual polypes or zooids is remarkably uniform at
least as regards all the essentials of their organisation.

Nearly all the Actiniaria or Sea-anemones are simple, and, in the
few instances where colonies are formed, these are usually small,
and contain a very limited number of zooids. In Zoanthus
(Fig. 143), for instance, the original polype sends out a horizontal
branch or stolon (st.}, from which new polypes arise. Besides the
Sea-anemones the only simple forms are certain Madreporarian
corals, such as Flabellum (Fig. 155, A, B), and three genera of
Alcyonacea, of which Hartca (Fig. 144) may be taken as an
example.

IV

PHYLUM CCELENTERATA

197

The simplest mode of budding is that just described in Zoan-
thus, in which new zooids are developed from a narrow band-like

B

FJC. 143 Zoanthus SOCiatus. A, entire colony ; st. stolon. B, transverse section, sgph.
siphonoglyphes ; d. <1. dorsal, and r. (I. ventral directive mesenteries. (After McMurrich and
Kurschelt and H eider.)

or tubular stolon (Fig. 143, st). A more usual method resembles that
with which we are already familiar in Hydrozoa, new buds being

formed as lateral outgrowths,
and a tree-like colony arising
with numerous zooids spring-
ing from a common stem or
coenosarc. Corallium and Gor-
gonia (Figs. 145 and 154) are
good examples of this type of
growth. In other cases the
buds grow more or less paral-
lel with one another, producing
massive colonies either of close-
set zooids or of zooids separ-
ated by a solid coenosarc. As
examples of this type we may
take Palythoa, the most com-
plex of the Actiniaria, and
many of the common Madre-
poraria, such as Astroea (Fig.
146). In the Sea-pens (Penna-
tulacea) the proximal end of
the elongated colony (Fig.
147) is sunk in the mud, and
FIG. 144. -Hartea eiegans. <ii. gullet ; the distal end bears zooids

><*. mesentery ; ;>. spicules ; t. tentacles. orrincrmcr PifViPv rlirprtlv frnm
(After Perceval Wright.) Springing tiy

198

ZOOLOGY

SECT.

the coenosarc or, as in Pcnnatula itself, from flattened lateral
branches. The stem itself is the equivalent of a polype.

A very peculiar mode of budding occurs in the Organ-pipe
Coral (Tubipom). The base of the original polype (Fig. 148)

grows out into a flattened expansion
from which new polypes arise, diverg-
ing slightly from one another as
they grow, and separated by toler-
ably wide intervals. The distal ends
of the polypes then grow out into
horizontal expansions or pliiforms
(pi.}, formed at first of ectoderm and
mesogloea only, but finally receiving
prolongations of the endoderm. The
platforms extend, come in contact
with one another, and fuse. In this
way platfoims of considerable extent
are formed (A, pi.), uniting the
polypes with one another. From
the upper surfaces of the platforms,
between the older polypes, new
buds arise, and in this way the colony tends to assume the form
of an inverted pyramid, the number of zooids, and consequently
the diameter of the colony, increasing pari iwssu with the vertical

Fio. 145. Corallium rubrum, por-
tion of a branch. (From Glaus,
after Lacaze-Duthiers.)

Kir;. 146, Astrsea pallida, the living colony. (After Dana.)

growth of the latter. The skeleton of this remarkable coral will
be referred to hereafter.

Although the general structure of the individual polypes

of the Actinozoa is, as mentioned above, very uniform, the varia-
tions in detail are numerous and interesting, especially among
the Actiniaria. One of the most important points to consider

IV

PHYLUM CCELENTERATA

199

FIG- 147. Peimatula sulcata. A, entire colony; B, portion of the same magnified.
/. lateral branch ; p. polype ; s. siphonozooid. (After Koelliker.)

t.-m>

FIG. 14S. Tubipora musica. A, skeleton of entire colony ; B, transverse sections of polype ;
C. single polype with tube and commencement of platform ; I), growth of new polypes from
platform. /. /. longitudinal muscles ; pi. p'*. polypes ; pi. platform ; MI ph. siphonoglyphe ; sp.
spicules ; std. stomocUeum. (After Cuvier, Quoy and Gaimard, and Hickson.)

200

ZOOLOGY

SECT.

B

is the arrangement of the mesenteries. In Edwardsia (Fig. 149),
a genus which burrows in sand instead of attaching itself to
rocks, &c., there are only eight mesenteries (B) the usual two
couples of directives, and two others on each side of the vertical
plane, having their longitudinal muscles directed ventrally, and
therefore not arranged in couples. The adult Edwardsia thus
corresponds with a temporary stage in the development of one of
the more typical sea-anemones, viz., the stage with eight mesen-
teries shown in Fig. 142, A. ; it is probably to be looked upon as
the most primitive or generalised member of the order. In

Zoanthus (Fig. 143, B) the dorsal
directives (d.d.) do not reach the
gullet, and each lateral couple con-
sists of one perfect and one small
and imperfect mesentery. In Ceri-
anthus, another burrowing form,
there is a couple of very small
ventral directives, and the remain-
ing mesenteries are very numerous,
not arranged in couples, and all
directed ventrally at their outer
ends, so as to have a very obviously
bilateral arrangement : in this genus,
as growth proceeds, new mesen-
teries are added on the dorsal side,
and not, as is usual, between already
formed couples. On the other hand,
the newly discovered Gyractis ex-
hibits a perfectly radial arrange-
ment : the mesenteries are all

arranged in couples with the longitudinal muscles facing one
another. Lastly, in all the more typical Sea-anemones (forming
the tribe Hexactinioe) there are either six, eight or ten pairs of
perfect mesenteries, which, as well as the secondary and tertiary
cycles, are all arranged in couples, the longitudinal muscles of
all but the one or two directive couples facing one another.

In the Madreporaria the mesenteries are arranged, so far as is
known, in the way just described for the HexactiniaB. In the
Antipatharia there are six primary, and sometimes either four or
six secondary mesenteries. In the whole of the Alcyonaria the
mesenteries are eight in number : they are not arranged in
couples, and their longitudinal muscles all face the same
way, viz., towards the ventral aspect (Fig. 148, B). In this
whole sub-class, therefore, the resemblance to Edwardsia is very
close, the main difference being that the longitudinal muscles
of the ventral directives lace inwards in the Alcyonaria,
outwards in Edwardsia.

FIG. 149 Edwardsia claparedii.

A, the entire animal ; t. tube. B.
transverse section. (After Andres,
and Korschelt and Heider.)

iv PHYLUM CCELENTERATA 201

The tentacles in Zoantharia are usually very numerous, and in
nearly all cases have the form of simple glove-finger-like out-
pushings of the disc. In Edwardsia, however, they may be
reduced to sixteen, and in some genera of Sea-anemones they are
branched. In the Antipatharia (Fig. 150) they vary in number
from six to twenty-four. When more than six are present, six
of them are larger than the others.

FIG. 150. Antipathes ternatensis, portion of a branch, showing three zooids and the horny
axis beset with spines. (From the Cambridge Natural History, after Schultze.)

In the Alcyonaria, on the other hand, the tentacles, like the
mesenteries, are eight in number and are always pinnate, i.e.
slightly flattened and with a row of small branchlets along
ach edge (Fig. 144). Many Actiniaria have the tentacles
perforated at the tip (Fig. 138, A, j>j>.) ; and in some species
these organs undergo degeneration, being reduced to apertures
on the disc, which represent the terminal pores of the vanished
tentacles and are called stomidia.

Many Sea-anemones possess curious organs of offence called
acontia (Fig. 138, A, and Fig. 157, ac.). These are long
delicate threads springing from the edges of the mesen-
teries : they are loaded with nematocysts, and can be protruded
through minute apertures in the column, called " port-holes " or
cinclides (en.).

Enteric System.- The gullet in the Actiniaria presents some
remarkable modifications. It is usually a compressed tube with two
siphonoglyphes, but in Zoanthus and some other genera the ventral
gullet-groove alone is present (Fig. 143, B), and in Gyractis both
grooves are absent, and the tube itself is cylindrical with a circular
mouth. The ordinary compressed form of gullet often assumes, in
the position of rest, an oo-shaped transverse section, owing to
its walls coming together in the middle and leaving the two ends
wide open. In most of the Antipatharia the zooid is drawn out
in the direction of the long axis of the branch (Fig. 151), and in
some it becomes constricted into three parts (B) which may have
the appearance of separate zooids, the central part containing the
gullet with the mouth, while the lateral parts each contains a gonad;
each of these apparent zooids bears two of the six tentacles ; the
median one has all six mesenteries attached internally to the gullet;
in each lateral part there is only the outer portion of one of the

202

ZOOLOGY

SECT.

transverse mesenteries. In such a form as Schizopathes (Fig. 151, B)
there is thus recognisable an arrangement of the parts which might

FIG. 151. Antipatharia. A, oral face of zooid of Parantipathes. B, oral face of zooid of

Schiznpathe*. (After Delage et Herouard.)

be interpreted as a dimorphism of the zooids, one set the parts
containing the mouth and gullet being regarded as yastrozooids,
and the others containing the gonads as gonozouids.

Fixed and Free Forms. A large proportion of Actinozoa are
permanently fixed, such, for instance, as most of the Stony Corals,
the Sea fans, Black Corals, &c. Most Sea-anemones are tempo-
rarily attached by the base, but are able slowly to change their
position : some forms, such as Edwardsia (Fig. 149) and Ocrianthus,
usually live partly buried in sand enclosed in a tube formed of
discharged stinging-capsules, the oral end with its crown of
tentacles alone being exposed : others, such as Peachia, live an
actually free life, habitually lying on the sea-bottom with the
longitudinal axis horizontal like that of a worm : a few, such as
Mini/as (Fig. 152), have the aboral end dilated into a sac containing
air and serving as a float ; by it? means
these animals can swim at the surface of
the sea, and are thus, alone among the
Actinozoa, pelagic.

Dimorphism. --With the exception of
one genus of Stony Corals, the Zoantharia
are all homornorphic, i.e. there is no dif-
ferentiation of the zooids of a colony. But
in the Alcyonaria dimorphism is common :
the ordinary zooids or polypes are ac-
companied by smaller individuals, called

siphonozooids (Fig. 147, s.), having no tentacles, longitudinal
muscles, or gonads.

None of the Actiniaria have a true skeleton : in some, how-
ever, there is a thick cuticle, and several kinds enclose themselves
in a more or less complete tube (Fig. 149). which may be largely
formed of discharged nematocysts. The simplest form of skeleton
is found in the solitary Alcyonarian genus Hartea (Fig. 144), already

FIG. 152. Minyas. f. float.
(After Andres.)

IV

PHYLUM CCELENTERATA

203

referred to, in which minute irregular deposits of calcium carbonate,
called spicules (sp.), are deposited in the mesogloea. A similar
spicular skeleton occurs in the " Dead-men's finger ' ( Alcyonium,
Fig. 153), where spicules of varying form are found distributed
throughout the mesogloea of the coenosarc. In Tubipora (Fig. 148),
the " Organ-pipe Coral," the mesogloeal spicules become closely
fitted together, and form a continuous tube for each polype, the
tubes being united by horizontal calcareous platforms (pi.) formed
by deposits of spicules in the expansions of the same name already
referred to. The skeleton of Tubipora is, therefore, an internal

FIG. 153. Alcyonium palmatum, A, entire colony ; B, spicules (After Cuvier.)

skeleton, and in the living state is covered by ectoderm. In the
Red Coral of commerce (Corallium, Fig. 145) the originally separate
spicules are embedded in a cement-like deposit of carbonate of
lime, the result being the production of an extremely hard and
dense branched rod, which extends as an axis through the coenosarc.
In the Blue Coral (Heliopora\ on the other hand, the stony
calcareous skeleton is not made up of fused spicules, but is solid
from the first.

Another type of skeleton is found in the Antipatharia (Fig. 150)
and in the Gorgonacea (Fig. 154). It also consists of an axial rod,
extending all through the colony and branching with it, but is

204

ZOOLOGY

SECT.

formed of a flexible horn-like material. Moreover it is not meso-
gloeal, but ectodermal in origin : in close contact with it is an
epithelium, from the cells of which it is produced as a cuticular
secretion, and this epithelium is formed as an invagination of the
base of the colony. In addition to its axis, Gorgonia contains
numerous spicules in the mesogloea of the coenosarc. In some

FIG. 154. Gorgonia verrucosa A, entire colony; B, portion of the same magnified, c.

coenosarc ; /. polype. (After Koch and Cuvier.)

of the Gorgonacea the axial skeleton is partly horny, partly
calcareous.

In the Sea-pen (Pennatula, Fig. 147) and its allies the stem of
the colony is supported by a horny axis which is unbranched, not
extending into the lateral branches. In this case the axis is
contained in a closed cavity lined by an epithelium, the origin of

iv PHYLUM CGELENTERATA 205

which is still uncertain. Spicules occur in the mesoglcea, some of
them microscopic, others readily visible to the naked eye.

In the Madreporaria we have a skeleton of an entirely different
type, consisting, in fact, of a more or less cup-like calcareous
structure, secreted from the ectoderm of the base and column of
the polype. When formed by a solitary polype, such a " cup-
coral " is known as a corallite : in the majority of species a large
number sometimes many thousands of corallites combine to-
form a corallum, the skeleton of an entire coral-colony.

The structure of a corallite is conveniently illustrated by that
of the solitary genus Flabellum (Fig. 155, A, B). It has the form
of a short conical cup, much compressed so as to be oval in section.
Its wall or tlicca (th.) is formed of dense stony calcium carbonate,
white and smooth inside, rough and of a brownish colour outside,
except towards the margin, where it is white. Its proximal or
aboral end is produced into a short stalk or peduncle, by which the
Coral is attached in the young state, becoming free when adult :
in many other simple Corals there is no stalk, but attachment to-
the support is effected by means of a flattened proximal surface
or basal plate (C, I. pi.). From the inner surface of the theca a
number of radiating partitions, the septa (scp.), proceed inwards or
towards the axis of the cup, and, like the mesenteries of a polype,
are of several orders, those extending furthest towards the
centre being called primary septa, the others secondary, tertiary,
and so on. Towards the bottom of the cup the primary septa
meet in the middle to form an irregular central mass, the columella
(col.). In some Corals the columella is an independent pillar-like
structure arising from the basal plate (D, col.).

In many Corals there is a distinct calcareous layer investing the
proximal portion of the theca, and called the epitheca (C, c.th.}. Some
species have the inner portions of the septa detached so as to form
a circlet of narrow upright columns, the pali. In others there are
horizontal partitions or dissepiments passing from septum to septum,
and in others, again, complete partitions or tabula 1 , like those of
Millepora (p. 157), extending across the whole corallite. In the
Mushroom-coral (Fungia), the corallite is discoid, the theca is con-
fined to the lower surface, and small calcareous rods, the synapticula'^
connect the septa with one another.

In the living condition the polype fills the whole interior of the
corallite and projects beyond its edge to a greater or less degree
according to its state of expansion (C). The proximal part of the
body- wall is thus in contact with the theca, which has the relation
of a cuticle, and is, in fact, a product of the ectoderm. The free
portion of the body-wall is frequently, in the extended state, folded
down over the edge of the theca so as to cover its distal portion.
The septa alternate with the mesenteries, each lying in the space
between the two mesenteries of one couple, and each being in-

206

ZOOLOGY

SECT.

vested by an in-turned portion of the body-wall (E, F). Thus the
septa, which appear at first sight to be internal structures, are
really external : they lie altogether outside the enteric cavity, and
are in contact throughout with ectoderm.

The ectodermal nature of the entire corallite is further proved by
its development. The first part to appear is a ring-shaped

GCfi.ff

Fio. 155. A, B, two views of Flabellum curvatum. C, semi-diagrammatic view of a simple
coral ; D, portion of a corallite ; E, F, diagram of a simple coral in longitudinal and transverse
section ; ectoderm dotted, endoderm striated, skeleton black, b. pi. basal plate ; col. colurn-
ella ; e. th. epitheca ; yul. gullet ; mes, mes. 1, mes. 2, mesenteries ; mes. f. mesenteric filaments ;
sep. septa ; t. tentacle ; th. theca. (A and B after Moseley ; C and D after Gilbert Bourne.)

deposit of carbonate of lime between the base of the polype and the
body to which it adheres : sections show this ring to be formed by
the ectoderm cells of the base. The ring is soon converted into a
disc, the basal plate, from the upper surfaces of which a number of
ridges arise, arrayed in a star-like fashion : these are the rudiments
of the septa Here, again, sections show that each septum corre-

IV

PHYLUM CCELENTERATA

207

spends with a radial in-pushing of the base, and is formed as a
secretion of the invaginated ectoderm. As the septa grow they
unite with one another at their outer ends, and thus form the theca.
In some cases, however, the theca appears to he an independent
structure.

The almost infinite variety in form of the compound corals is
due, in the main, to the various methods of budding, a subject
which has already been referred to in treating of the actinozoan
colony as a whole. According to the mode of budding, massive
Corals are produced in which the corallites are in close contact
with one another, as in Astraea (Fig. 146) ; or tree-like forms, such

FIG. 156. Dendrophyllia nigrescans, B, Madrepora aspera. co. corallites;

cs. coenosarc ; p. polypes. (After Dana )

as Dendrophyllia (Fig. 156, A), in which a common-calcareous stem,
the ccenenckyma,i$ formed by calcification of the ccenosarc (cs.), and
gives origin to the individual corallites. It is by this last-named
method, the ccenosarc attaining great dimensions and the indivi-
dual corallites being small and very numerous, that the most
complex of all Corals, the Madrepores (Madrepom, Fig. 156, B)
are produced.

The microscopic structure of corals presents two main varieties.
In what are called the aporose or poreless corals, such as Flabellum,
Astra^a, &c., the various parts of the corallite are solid and stony,
while in the perforate forms, such as Madrepora, all parts both of

208 ZOOLOGY SECT.

the corallites and of the connecting coenenchyma, have the charac-
ters of a mesh-work, consisting of delicate strands of carbonate of
lime united with one another in such a way as to leave interstices,
which in the living state are traversed by a network of interlacing
tubes, representing the coenosarc, and placing the polypes of the
colony in communication.

The Blue Coral (/fdiupora),one of the Alcyonacea, has a massive
coral lum the same general appearance as a Madreporarian. The
lobed surface bears apertures of two sizes, the larger being for the
exit of the ordinary polypes, the smaller for the siphnozoids.
Tabulae are present, and septum-like ridges, which, however, have
no definite relations to the mesenteries and are inconstant in
number.

Colour. The Actinozoa are remarkable for the variety and

t/

brilliancy of their colour during life. Every one must have noticed
the vivid and varied tints of Sea-anemones ; but most dwellers in
temperate regions get into the habit of thinking of Corals as white,
and have no conception of their marvellously varied and gorgeous
colouring during life. The Madrepores, for instance, may be pink,
yellow, green, brown, or purple : Tubipora has green polypes, con-
trasting strongly with its crimson skeleton ; and the effect of the
bright red axis of Corallium is greatly heightened by its pure white
polypes. In Heliopora the whole coral is bright blue ; the tropical
Alcyonidse are remarkable for their elaborate patterns and gor-
geous coloration ; and Pennatula, in addition to its vivid colours,
is phosphorescent.

In most cases the significance of these colours is quite unknown.
In some species, however, " yellow-cells " or symbiotic Algae have
been found in the endoderm, where they probably serve the same
purpose as the similar structures which we have already studied
in Radiolaria (p. 63).

Many Actinozoa, like many sponges (p. 126), furnish examples of
commensalism, a term used for a mutually beneficial association

ij

of two organisms of a less intimate nature than occurs in symbiosis.
An interesting example is furnished by the Sea-anemone Adam sin
palliata (Fig. 157). This species is always found on a univalve shell
-such as that of a Whelk inhabited by a Hermit-crab. The
Sea-anemone is carried from place to place by the Hermit-crab, and
in this way secures a more varied and abundant food-supply than
would fall to its lot if it remained in one place. On the other
hand, the Hermit-crab is protected from the attack of predaceous
Fishes by retreating into its shell and leaving exposed the Sea-
anernone, which, owing to its toughness, and to the pain erased
by its poisonous stinging-capsules, is usually avoided as an article
of food.

Other Sea-anemones such as the gigantic Discosoma of the
great Barrier-Reef are found associated with Small Fishes or

IV

PHYLUM CCELENTKRATA

209

Crustacea, which have their abode in the enteric cavity. In this
case the Fish secures shelter in a place where it is very unlikely to
be disturbed, and the two animals are strictly commensals or " mess-
mates " since they share a common table. A somewhat similar
instance is furnished by the Blue Coral (Heliopora), already referred
to more than once. The corallum contains, not only the apertures
for the polypes and siphonozooids, but also tubular cavities of

Fin. 157. Adamsia palliata, four individuals attached to a Gasteropod shell inhabited by
a Hermit-crab, ric. ac 1 . acontia ; s/t. shell of Gasteropod. (After Andres.)

an intermediate size, in each of which is found a small chsetopod
Worm, belonging to the genus Leucodorc. As the polypes are
frequently found retracted at a time when the Worms are protruded
from their holes in search of food, it is not surprising that the
latter should have been credited with the fabrication of the coral.
Trapezia, a genus of Crabs, always lives in interstices of a par-
ticular species of Madrepore.

The distribution of the Actiniaria is world-wide, and in
many cases the same genera are found in widely separated parts

VOL. i P

210 ZOOLOGY SECT.

of the world. They are, however, larger, and of more varied form
and colour in tropical regions, for instance on coral-reefs. The
largest reef-anemone, Discosoma, found also in the Mediterranean,
attains a diameter of 2 feet. Most members of the order are
littoral, living either between tide-marks or at slight depths, but a
few are pelagic, and several species have been dredged from depths
of from 10 to 2,900 fathoms.

The Madreporaria, taken as a whole, have also a wide distribu-
tion ; but the number of forms in temperate regions is small, and
the majority including the whole of what are called reef- building
Corals are confined to the tropical parts of the Atlantic, Indian,
and Pacific Oceans, flourishing only where the lowest winter tem-
perature does not sink below 68 F. (20 C.). Thus their northern-
most limits are the Bermudas in the Atlantic, and Southern Japan
in the Pacific ; their southernmost limits, Rio and St. Helena in
the Atlantic, Queensland and Easter Island in the Pacific : in other
words, they extend to about 30 on each side of the equator.
Moreover, they have a curiously limited bathymetrical distribu-
tion, flourishing only from high-water mark down to a depth of
about 20 fathoms, but not lower.

Many of the Pacific Islands are formed entirely of coral rock,
others are fringed with reefs of the same, and the whole east coast
of Northern Queensland is bounded, for a distance of 1,250 miles,
by the Great Barrier Reef, a line of coral rock more or less parallel
to and at a distance of from 10 to 90 miles from the land.
Such reefs consist of gigantic masses of coral rock fringed by living
coral, the latter growing upon a basis of dead coral, the interstices
of which have been filled up with debris of various kinds, so as to
convert the whole into a dense limestone.

The Antipatharia, and many of the Alcyonaria, such as the Gor-
gonacea and Pennatulacea, have also a world-wide distribution,
and, even in temperate regions, Black Corals and Sea-fans may
attain a great size : the members of both these groups, as well as
the Sea-pens, are found at moderate depths. The Red Coral is
found only in the Mediterranean, at a depth of 10 to 30 fathoms.
Tubipora and Heliopora have the same distribution as the reef-
building Corals.

From the palaeontological point of view, corals are of great im-
portance : they are known in the fossil condition from the Silurian
epoch upwards, and in many formations occur in vast quantities,
forming what are called coral limestones. The majority of fossil
forms are referable to existing families, but in the Palaeozoic era
the dominant group was the 2ingosa,ihe affinities of which are still
very obscure. In these the corallites are usually bilaterally sym-
metrical, the septa are arranged in multiples of four, and the cup
presents on one side a pit, the fossnla, where the septa are greatly
reduced.

IV

PHYLUM CCELENTERATA

211

CLASS IV. CTENOPHORA.

1. EXAMPLE OF THE CLASS Hormiphora plumosa.

External Characters. Hormiphora is a pear-shaped organism
about 5-20 mm. in diameter, and of glassy transparency (Figs. 158
and 159). The species //. plumosa is found in the Mediterranean ;
allied forms belonging either to the same genus (often called
Cydippe) or to the closely allied genus Plcurolrachia are common
pelagic forms all over the world.

From opposite sides of the broad end depend two long tentacles
.), provided with numerous little tag-like processes, and springing

FIG. 158. Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth ;
s. pi. swimming plates ; t. and b. tentacles. (After Chun.)

each from a deep cavity or sheath, into which it can be completely
retracted (Fig. 159, t.sh.). At the narrow end where the stalk
of a pear would be inserted is a slit-like aperture, the mouth
(mth.) : this end is therefore oral. At the opposite or aboral pole
is a slight depression, in which lies a prominent sense-organ (s.o.),
to be described hereafter.

But the most striking and characteristic feature in the external
structure of Hormiphora is the presence of eight equidistant meri-
dional bands (s.pl.), starting from near the aboral pole, and extend-
ing about two-thirds of the distance towards the oral pole. Each
band is constituted by a row of transversely arranged comb-like
structures, consisting of narrow plates frayed at their outer ends.
During life the frayed ends are in constant movement, lashing to
and fro, and so propelling the animal through the water. The combs

p 2

S.O

Fig. 159. Hormiphcra plumosa. A, dissected specimen having rather more than one
quarter of the body cut away. 13, transverse section ; diagrammatic, adr. c. adradial
canal ; inf. infundibulum ; inf. c. infundibular canal ; int. c. inter-radial canal ; rurd. i:
meridional canal ; iiilh. mouth ; orii. ovary ; per. c. pcr-radial canal ; s. o. sense-organ ; .s. pi.
swimming-plate ; tpy. spermary ; xtd. stomodamm ; std. c. stomodseal canal ; .s^</. / stunn>d;ral
ridges; t. tentacle ; t. li. }>ase <>f tentacle; t. r. tentacular canal ; /. .<(//. tentacular sheath.

SECT, iv PHYLUM CCELENTERATA 213

are, in fact, rows of immense cilia, fused at their proximal ends :
their presence and mode of occurrence arranged in meridional
comb-ribs or swimming-plates are strictly characteristic of the
class, and indeed give it its name.

It will be seen at once that apart from all considerations of
internal structure Hormiphora presents a similar combination of:
radial with bilateral symmetry as in some Hydrozoa, such as
Ctenaria (Fig. 109, 1), and as in the majority of Actinozoa. The
swimming-plates are radially arranged, and mark the eight adradii,
but the slit-like mouth and the two tentacles indicate a very
marked and characteristic bilateral symmetry. A plane passing
through the longitudinal axis of the body, parallel with the long
axis of the mouth, is called, as in Actinozoa (see p. 189), the vertical
plane : it includes two per-radii, which are respectively dorsal and
ventral. A plane at right angles to this, passing through both
tentacles, and including right and left per-radii, is called the
transverse plane.

Enteric System.- -The mouth leads into a flattened tube (Fig.
159, std.), often called the stomach, but more correctly the gullet or
stomodaium. It reaches about two-thirds of the way towards the
aboral pole, audits walls are produced internally into ridges (std.r.),
which increase the area for the absorption of digested food.
Living prey is seized by the tentacles, ingested by the aid of the
mobile edges of the mouth, and digested in the stomodseum, which
is thus physiologically, though not morphologically, a stomach.
The products of digestion make their way into the various parts
of the canal-system, presently to be described, and indigestible
matters are passed out at the mouth.

Towards its upper or aboral end the stomodseum gradually
narrows and opens into a cavity called the infundibulum (inf.),
which probably answers to the stomach of an Actinozoon or a
medusa, and is flattened in a direction at right angles to the
stomodaBum i.e. in the transverse plane. From the infundibulum
three tubes are given off: one, the infundibular canal (inf. c.), passes
directly upwards, and immediately beneath the aboral pole divides
into four short branches, two of which open on the exterior by
minute apertures, the excretory pores (Fig. 160, A, ex. p.). The two
other canals given off from the infundibulum are the pcr-radial
canals (per. c.) : they pass directly outwards, in the transverse plane,
and each divides into two inter-radial canals (int. c.), which in their
turn divide each into two adradial canals (adr. c.). These succes-
sive bifurcations of the canal-system all take place in a horizontal
plane (Fig. 160, B), and each of the ultimate branches or adradial
canals opens into a meridional canal (mrd. c.), which extends up-
wards and downwards beneath the corresponding swimming-plate.
Furthermore, each per-radial canal gives off a stomodceal canal
(std. c.), which passes downwards, parallel to and in close contact

fierc ex* so

adxc

S.pl

std.c

std

rritk

t.c

inf

FIG. 1(50. Ilorniiplicra plume sa, diagrammatic longitudinal (A) and transverse (B)
sections. The ectoderm is dotted, the endoderm striated, the mesogloea black, and the
muscular axis of the tentacles gray. Lettering as in Fig. 15'J, except ex. p. excretory pore.

SECT. IV

PHYLUM CCELENTERATA

215

with the stomodseum, and a tentacular canal (t. c.) which ex-
tends outwards and downwards into the base of the correspond-
ing tentacle. Each tentacle presents a thickened base (t. &.),
closely attached to the wall of the sheath, and giving off a long
flexible filament, beset with processes of two kinds one simple
and colourless, the other leaf-like, beset with branchlets, and of a
yellow colour.

Cell-layers.- -The body is covered externally by a delicate
ectodermal epithelium (Fig. 160), the cells from which the combs
arise being particularly large. The epithelium of the stomoda3um
is found by development to be ectodermal, that of the infundibulum
and its canals endodermal : both are ciliated. The interval between
the external ectoderm and the canal-system is filled by a soft jelly-
like mesogloea. The tentacle-sheath is an invagination of the ecto-

ad,.c

Vfesi

FIG. Kil. Hormiphora plumosa. A, transverse section of one of the branches of a
tentacle ; B, two adhesive cells (ad. c.) and a sensory ceil (s. c.) highly magnified, cu. cuticle
nu. nucleus. (After Hertwig and Chun.)

derm, and the tentacle itself is covered by a layer of ectoderm,
within which is a core or axis formed by a strong bundle of longi-
tudinal muscular fibres, which, as we shall see, are of mesodermal
origin, and which serve to retract the tentacle into its sheath.

Delicate muscle-fibres lie beneath the external epithelium and
beneath the epithelium of the canal-system, and also traverse
the mesogloea in various directions. The feeble development
of the muscular system is, of course, correlated with the fact
that the swimming-plates are the main organs of progression,
the Ctenophora differing from all other Ccelenterata in retaining
cilia as locomotory organs throughout life.

A further striking difference between our present type and the
Coelenterata previously studied is the absence, in Hormiphora, of
stinging-capsules. The place of these structures is taken by the
peculiar adhesive-cells with which the branches of the tentacles

216 ZOOLOGY SECT.

are covered. An adhesive-cell (Fig. 161, ad. c.) has a convex surface,
produced into small papilla, which readily adheres to any object
with which it comes in contact and is with difficulty separated.
In the interior of the cell is a spirally coiled filament, the
delicate inner end of which can be traced to the muscular axis of
the tentacular branch. These spiral threads act as springs, and
tend to prevent the adhesive-cells being torn away by the
struggles of the captured prey.

Both the central nervous system and the principal sense-
organ are represented by a peculiar apparatus situated, as already
mentioned, at the aboral pole. In this region is a shallow depres-
sion (Fig. 162, c. p.) lined by ciliated epithelium and produced in
the transverse plane into two narrow ciliated areas, the polar
plates (p. pi-}. From the depression arise four equidistant groups
of very large S-shaped cilia (sp), united to form as many springs (*p.),
which support a mass of calcareous particles (/.), like the lithites of

Fie. lt)2. Hormiphora plumosa, Sense-organ: b. bell; c. p. ciliated plate ; c. gr. ciliated
groove ; ex. p. excretory pore ; 1. lithites ; p. pi. polar plate ; sp. spring. (Modified from Chun.)

Hydrozoa and Scyphozoa. From each spring a ciliated groove (c. gr).
proceeds outwards, bifurcates, and passes to the two swimming-
plates of the corresponding quadrant. The lithitic mass, with
its springs, is enclosed in a transparent case or bell (b.), formed of
coalesced cilia. It appears that the whole apparatus acts as a
kind of steering-gear, or apparatus for the maintenance of equili-
brium. Any inclination of the long axis must cause the calcareous
mass to bear more heavily upon one or other of the springs : the
stimulus appears to be transmitted by the corresponding ciliated
groove to a swimming-plate, and results in a vigorous movement
of the combs. Thus the sensory pit acts as a central nervous
system, and the ciliated grooves as nerves. A sub-epithelial
plexus of nerve-fibres with nerve-cells extends all over the surface
of the body.

Reproductive Organs. --The animal is hermaphrodite, the
organs of both sexes being found in the same individual. The
yonads are developed in the meridional canals (Fig. 159, B), each of
which has an ovary (pvy.) extending along the whole length of one
side, a spermary (spy.) along the whole length of the opposite side.

IV

PHYLUM CCELENTERATA

217

The oigans are so arranged that in adjacent canals those of the
same sex face one another. It will be seen that the reproductive
products have, as in Scyphozoa and Actinozoa, the position of
endoderm-cells : whether they are developed, in the first instance,
from that layer is uncertain. When ripe, the ova and sperms are
discharged into the canals, make their way to the infundibulum,
thence to the stomodseum, and finally escape by the mouth. Im-
pregnation takes place in the water.

Development.- -The process of development has been traced
in several genera closely allied to Hormiphora, so that there is
every reason to believe that, in all essential particulars, the
following description will apply to that genus.

The egg (Fig. 163) consists of an outer layer of protoplasm (plstn.)
containing the nucleus (nu.), and of an internal mass of a frothy
or vacuolated nature (yk) : the hism

vacuoles contain a homo- -yL.

i x. i T- / "\ v.m-

geneous substance which

serves as a store of nutri-
ment to the growing embryo,
and apparently corresponds
with the yolk which we shall
find to occur in a large pro-
portion of animal eggs. En-
closing the egg is a thin
vitelline membrane (v.m.), sepa-
rated from the protoplasm by FlG 1G3 ._ 0vum of Lam p etia MM . nucleus;

a Considerable Space, filled With P ?s) - protoplasm ; r. m. vitullino membrane ;

. ,, yk. yolk. (After Chun.)

a clear jelly.

After impregnation the oosperm segments, but the details of
the process are very different from those we are familiar with in
the other Ccelenterata. The protoplasmic layer accumulates on
the side which will become dorsal, and the oosperm divides along
a vertical plane, forming two cells each with a sort of protoplasmic
cap (Fig. 164, A, plsm.). A second division takes place at right
angles to the first, producing a four-celled stage (B), and each of
the four cells divides again into daughter-cells of unequal size, the
result being an eight-celled embryo, each cell with a protoplasmic
cap at its dorsal end (C, D). Next a horizontal division takes
place, dividing off the protoplasmic caps as distinct cells, and so
producing a sixteen-celled stage (E, F) in which we can dis-
tinguish eight large, ventral, yolk-containing cells or megameres
(ing.), and eight small, dorsal, protoplasmic cells or micro-
meres (mi.).

The micromeres increase rapidly in number by division, and arc
further added to by new, small cells being budded off from the
megameres (Fig. 164, G, H, and Fig. 165, A). The result of this
increase is that the micromeres gradually overspread the megameres

218

ZOOLOGY

SECT.

(Fig. 165, C), the final result being the production of an embryo
consisting of a central mass of large yolk-containing cells (ma.),

FIG. 1(34. Segmentation of the oospsrm -.in Ctenophora. mg. meganieres ; mi. micromeres ;

plsm. protoplasm ; yk. yolk. (Modified from Korsehelt and Heider.)

partly surrounded by an epithelium-like layer, incomplete below,
of small cells (mi.). This stage corresponds with the gastrula of
preceding types, the micromeres forming the ectoderm, the mega-

FIG. 165. Three stages in the development of Ctenophora. ma. meganieres ; mi. micromeres.

(From Lang's Comparative Anatomy.)

meres the endoderm, and the ventral edge of the ectodermal
investment representing the blastopore. There is, however, no
archenteron or gastrula-cavity, and the stage has been produced,

B C

tne

me

Kic. hi*;. Three stages in the duvulopment of Callianira. <l. infundibiihim ; cc. ectoderm ;
en. endoderm ; me. mesoderm ; st. stomodajum. (From Lang's Comparative Anatomy.)

not by a process of invagination or tucking-in, but by one of epiboly

or overgrowth.

The endoderm-cells increase in number, and become much
elongated and arranged obliquely, their long axes radiating,

IV

PHYLUM CCELENTERATA

219

upwards and outwards, from the long axis of the entire embryo
(Fig. 166, A). Their lower (ventral) ends then become divided off,
forming a number of small cells, which constitute the rudiment
of a true middle cell-layer or mesoderm (A, me.). A kind of in-
vagination of the megameres with their mesoderm cells then takes
place, resulting in the formation of a cavity the infundibulum
(B, d.) bounded below by the megameres, now placed horizontally,
and above by the mesoderm. The mesoderm gradually retreats to
the dorsal surface (C), finally spreading out between the dorsal
ectoderm and the infundibulum. At the same time the ectoderm
cells bounding the aperture of the infundibulum grow into it so
as to line its ventral portion : in this way the ytomodseum (st.) is
produced. The remainder of the cavity widens out and becomes
the definite infundibulum (d.), and before
long sends off four adradial pouches, the
rudiments of the canal -system. At the
same time a gelatinous layer (Fig. 167, //.),
the mesoglcea, makes its appearance be-
tween the ectoderm and endoderm.

The later processes of development
may be described very briefly. The
canal-system gradually assumes its adult
complexity and the swimming - plates
appear. A thickening of the ectoderm
on each side of the body gives rise to
the epithelium of the tentacle and of its
pouch. The muscle-fibres forming the
axis of the tentacle (B, me.) are derived
from the mesoderm, which also gives rise
to the contractile fibres of the meso-
glcea (MC.J). The lithites are formed in
the ectoderm -cells of the apical pole, but
gradually make their way on to the free cn -
surface of the cells, and become supported
on four groups of fused cilia. Four outer
groups of cilia unite with one another to
form the bell (sJc.).

The most noteworthy points in this n
somewhat complex process of develop-
mcnt are the following :-

1. The distinction between a purely
protoplasmic part of the egg and a yolk-
containing portion. In the Hydrozoa
and Actinozoa the yolk-material is small

in amount and evenly distributed, the egg being described as
alecithal or yolkless. In the present instance the yolk is at first
accumulated in the centre of the egg, which is thus centrolecithal

li>7. Two later stages in the
development of Callianira.
d. infundibulum ; en. endoderm ;
a. mesogkea ; me. mesoderm ;
.s7,-. sense-organ ; .s/. stomodamm ;
L tentacle. (From Lang's Coin-
jxirative Anatomy.)

220 ZOOLOGY SECT.

or mid-yolked, but soon the protoplasm accumulates at one end
and the yolk at the opposite end of the developing embryo, pro-
ducing a tclokcitlial or end-yolked condition.

2. The fact that segmentation is unequal, there being a distinc-
tion into large cells or megameres, containing yolk, and purely
protoplasmic small cells or micromeres.

3. The formation of a peculiar type of gastrula by epiboly or
overgrowth, the ectoderm cells (micromeres) growing over and
partly enclosing the endoderm cells (megameres).

4. "The presence, for the first time in the ascending animal
series, of a true middle embryonic layer or mesoderm. In the
other Coelenterata, as well as in the Sponges, two embryonic layers
only are formed, and the intermediate layer of the adult is formed
by the comparatively late separation of muscle-cells and connec-
tive-tissue fibres either from ectoderm or endoderm. In the
present case a definite layer of mesoderm cells becomes separated
from the endoderm during the gastrula stage.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Ctenophora are pelagic Coelenterata in which the formation
of colonies is entirely unknown. No indication of a polype-stage,
so characteristic of the remaining Coelenterata, can be detected
either in the adult or in the embryonic condition. Ciliary move-
ment, instead of being a merely embryonic form of locomotion as
in the preceding classes, is retained throughout life, the cilia being
fused to form comb-like structures, which are arranged in eight
meridional rows or swimming-plates. Tentacles, when present,
are usually two in number, situated in opposite (right and left)
per-radii, and retractile into pouches. The enteron communicates
with the exterior by a large stomodseum which functions as the
chief digestive cavity. From the enteron is given off" a system of
canals, the ultimate branches of which are adradial and have a
meridional position, lying beneath the swimming-plates ; a single
axial canal is continued to the aboral pole, where it commonly
opens by two excretory pores. There are no gastric filaments.
The central nervous system is represented by a ciliated area on
the aboral pole, and is connected with a single sensory organ,
having the character of a peculiarly modified lithocyst. The
gonads of both sexes are lodged in the same individual, the ovaries
and testes being formed on opposite sides of the meridional canals.
The oosperm undergoes unequal segmentation, the gastrula is
formed by epiboly or overgrowth, and a definite mesoderm is
established during the gastrula stage. There is no alternation of

iv PHYLUM CCELENTERATA 221

generations ; but in some cases development is accompanied by a
well-marked metamorphosis.

The Ctenophora are divisible into four orders as follows :-

ORDER 1. CYDIPPIDA.

Ctenophora having two tentacles, retractile into sheaths, and
unbranched meridional and stomodreal vessels. The body is
either circular in section or is slightly compressed in the trans-
verse plane (Figs. 158 and 168).

ORDER 2. LOBATA.

Ctenophora having numerous non-retractile lateral tentacles
contained in a groove : the bases of the two principal tentacles are
also present, but have no sheaths. The stomodaeal and meri-
dional vessels unite with one another. The body is compressed in
the transverse plane, and is produced into two large oral lobes or
lappets and into four pointed processes or auricles (Fig. 169).

ORDER 3. CESTIDA.

Ctenophora having a band-like form, owing to the extreme
compression of the body in the vertical plane. The bases of the
two principal tentacles are present, enclosed in sheaths, and there
are also numerous lateral tentacles contained in a groove. Union
or anastomosis of the meridional and stomodoeal vessels takes
place (Fig. 170).

ORDER 4. BEROIDA.

Ctenophora having no tentacles. The mouth is very wide, and
the gullet occupies the greater part of the interior of the body.
The meridional vessels are produced into a complex system of
anastomosing branches (Fig. 171).

Systematic Position of the Example.

Hormiphora plumosa is a species of the genus Hormiphora, be-
longing to the family Pkurobrachiidcv and to the order Cydippida.

The presence of two tentacles, retractile into sheaths, and of
unbranched meridional canals places it in the order Cydippida.
In this order there are three families, amongst which the Pleuro-
Irachiidce are distinguished by the absence of any compression of
the body, the transverse section being circular. The genus
Hormiphora is distinguished by having a rounded body somewhat
produced at the oral pole, and by the aperture of the tentacle-

222

ZOOLOGY

SECT.

sheath being on a higher level than the funnel. In the species
plumosa the stomodreal ridges are of a brown colour, and the leaf-
like branchlets of the tentacles yellow.

3. GENERAL ORGANISATION.

Compared with the two former classes of Ccelenterates, the Hydrozoa and
Actinozoa, the organisation of the Ctenophora is remarkably uniform. This is
due to the fact that all the species are pelagic, none are colonial, and none form
skeletons. Nevertheless a very great diversity of form is produced in virtue of
differences in proportion and modifications of the tentacular and canal systems.

The Cydippida agree in all essential respects with Hormiphora, the most
important deviation from the type-form being the compression of the bocty in the
transverse plane in some genera, e.g. Euchlora (Fig. 168, 2], the result being an

mlti
l.Callianira

2.Euchlora

S.LampeMa

FIG. 168. Three Cydippida. ab. p. aboral process ; mth. mouth. (After Chun.)

oval instead of a circular transverse section, with the tentacles at the end of the
long axis. The aboral pole may be produced into wing-like appendages, as in
CaUianira (1), and in Lampetia (3) the mouth is so dilatable as to form, when
expanded, a sole-like plate by which the animal retains itself on the surface of the
water or creeps over submarine objects. In EnchJora rubra minute nematocysts
have been found, and there is reason to believe that it was by the modification

IV

PHYLUM CCELENTERATA

223

of these characteristic ccelenterate organs of offence that the adhesive cells of
Ctenophora were evolved.

The Lobata, for instance Deiopea, are distinguished, as their name implies,
by the presence of a pair of large lappets (Fig. 169 A, fy>.), into which the oral

mrd.c

Fir;. 109. Deiopea kaloknenota. A, adult ; B, young, aur. auricle ; lp. lappet ; I. t. lateral
tentacles ; nird. t. meridional canal ; mtli. mouth. (After Chun.)

surface is produced at either end of the vertical plane. Four of the swimming
plates are shorter than the others, and at their bases arise elongated processes
called auricle* (aur.), which bear swimming-plates. The meridional canals (mdr.c)
unite with one another, and, with the cesophageal canals, are continued into the
lappets, where they become curiously coiled. The principal tentacles are
usually absent in the adult, but are represented by their basal portion^, which
are small, situated at the oral end, and devoid of sheaths. From each tentacle-
base grooves are continued along the oral surface to the auricles, and from the
grooves depend numerous small lateral tentacles (l.t.}. In the young condition
the Lobata resemble such compressed Cydippida as Euchlora, having a pair of
long principal tentacles, no lappets, and unbranched vessels (B).

The Cextida are represented by the remarkable " Venus's Girdle" (Cestux
renerix), a band-shaped Ctenophore (Fig. 170) which sometimes attains a length

B

FIG. 170.- Cestus veneris. A, adult ; B, young. /. t. latci-al tentacles ; mth. mouth ; . p/J,

s. pi.'- swimming-plates ; t. tentacle. (After Chun.)

of 1 J metre, or nearly five feet. The body is greatly elongated horizontally in the
vertical, and compressed in the transverse plane, so as to have the form of a
ribbon, which progresses by undulations of the whole body as well as by the
action of its swimming-plates. Four of the swimming -plates (x.pl. 1 ) are very

224

ZOOLOGY

SECT.

small ; the other four (s.pl. 2 ) are continued all along the aboral edge of the body.

The bases of the two principal tentacles (V.) are large and are enclosed in sheaths,

and, as in Lobata, numerous small lateral tentacles (l.t.) spring from grooves

which, in the present case, are continued the whole length of the oral edge.

The young of Cestus (B) resembles a compressed Cydippid svhich undergoes

gradual elongation in the median plane.

Bei'o< ; , the principal genus of the Beroida, has the form of a cylinder (Fig. 171),

one end of which is rounded and bears the sense-organ, the other truncated

and occupied entirely by the immense mouth (m(h.}.
The greater part of the body is taken up by the
huge gullet; the infundibulum (inf.'), per-radial
and infundibular canals, &c. , all being crowded
into a small space at the aboral pole. The
meridional canals send off branches which unite
with one another, forming a complex network of
tubes, and at their -oral ends the four meridional
canals of each (right and left) side and the corre-
sponding stomodseal canal unite into a horizontal
tube, which runs parallel with the margin of the
mouth. There is no trace of tentacles either in
the adult or in the embryonic condition.

The Ctenophora .are usually perfectly
transparent, and quite colourless, save for
delicate tints of red, brown, or yellow
in the tentacles and stomodaeal ridges.
Cestus has, however, a delicate violet hue,
and when irritated shows a beautiful
blue or bluish-green fluorescence. Beroe
if \\ is coloured rose-pink.

Ctenophora are found in all seas from
the Arctic regions to the tropics. As is
to be expected from their perishable
?nlh na ^ ure > there is no trace of the group in
the fossil state.

A very remarkable fact has been made
c'hmi ) swimmin s- plates - (Aftcr out with regard to Bolina hydatina, one of

the Lobata, a Ctenophore which attains a

diameter of 25-40 mm. While still in the larval or cydippid con-
dition and not more than 0'5-2 mm. in diameter, it becomes
sexually mature, the gonads producing ripe ova and sperms ; and
the eggs are impregnated and develop in the usual manner. Soon
the gonads degenerate, the larva metamorphoses into the adult
form, and a second period of sexual maturity supervenes. This
precocious ripening of sex-cells, occurs as we shall see in other
animal groups, and is called pcedogenesis.

IV

PHYLUM CCELENTERATA

225

APPENDIX TO CTENOPHORA

CTENOPLANA AND COSLOPLANA.

Before leaving the Ctenophora mention must be made of two remarkable
organisms which have been supposed to connect the present class with the
Turbellaria Polycladida, or Planarians, a group of worms to be described in
the following section.

Ctenoplana (Fig. 172) is a small marine animal, nearly circular in outline,
flattened dorso-ventrally, and about 6 mm. in diameter. It has hitherto been

'^v^cs'-''^-'--' v : *\*\ ^/^' " ---.'."..;>' ".-

c "i i-} "-, .---. -'4V ~\-,/*//-- .~.-^.C--

%mm ;^r

-r.r

'

'" , - A A

- ' - ; \ ,-, - * * < *

\s\s-

JL/~

Fio. 172. Ctenoplana kowalevskii. A, from above ; B, from the side. r?. clefts ; r. r.
radiating ridges ; .. o. sense-organ. (After Korotnetf.)

found only twice once in the Indian Ocean and once in New Britain. Instead
of swimming freely, like a Ctenophoran, it creeps on its ventral surface, like
a worm. In the centre of the dorsal surface is a vesicle (.s-.o.) containing a mass
of lithites surrounded by eight radiating ridges (r.r.}, alternating with which
are as many clefts (cl.}, each containing a protrusible row of stiff processes,
resembling the swimming-plates of Ctenophora. The mouth is in the centre
of the ventral surface, and leads into a stomach, from which are given off
numerous anastomosing canals, as well as a vertical canal which passes upwards
and ends beneath the sense-organ. In diverticula of this system are found the
testes, which have independent ducts opening 011 the exterior. There are two
solid tentacles contained in sacs, and a nerve-centre lies beneath the sense-organ
(.s.o.). Beneath the ectoderm is a basement-membrane, which acts as an organ
of support, and the muscular system is complex. Near each tentacle is an
aperture leading into a branched canal which is probably excretory, like the
nephridial tubes of Flat-Worms. (See Section V.)

Cwloplana is found in the Red Sea. It is also flattened dorso-ventrally, but
is oval instead of circular in outline, its dimensions being about 6 by 4 mm. It
resembles Ctenoplana in its ventral mouth, dorsal sense-organ, paired retractile
tentacles, and complex system of anastomosing canals from the stomach. There
are, however, no swimming-plates, and the ectoderm is ciliated.

Nothing is known of the development of either genus.

Q

226 ZOOLOGY SECT.

Ctenoydana and C&loplana are perhaps best looked upon as forming an
additional, somewhat aberrant, order of the Ctenophora, viz.-

ORDER 5. PLATYCTENEA.

Flattened Ctenophora of creeping habit, with a pair of retractile lateral
tentacles. The costs? (swimming-plates), when present, are retractile. There are
no meridional canals, but a system of anastomosing peripheral vessels.

THE RELATIONSHIPS OF THE CCELENTERATA.

There can be little doubt that the lowest coelenterate form
known to us is the simple hydrozoan polype, represented by
Hydra and by the hydrula stage of many Hydrozoa. Somewhat
more complex, in virtue of its stomodaeum (if a true stomodseum
be indeed represented) and its gastric ridges and filaments, is
the scyphozoan polype, represented by the scyphula of Aurelia.
Still more complex is the actinozoan polype, or actinula, as
it may be called, with its large stomodseum, mesenteries and
mesenteric filaments, and elaborate muscular system. Speaking
generally, one may say that these three polype-forms represent as
many grades of organisation along a single line of descent.

The medusa-form in the Hydrozoa is, as we have seen, readily
derived from the hydrula by the widening out of the tentacular
region into an umbrella. We may thus conceive of the Trachy-
linse, or hydroid medusae with no fixed zoophyte stage, as being
derived from a pelagic hydrula.

The Leptolinas may be considered to have arisen in consequence
of the adoption of asexual multiplication, by budding, during the
larval or hydrula stage. Instead of the hydrula giving rise
directly to a medusa, we may suppose it to have formed a temporary
colony by budding, after the manner of the Hydra, the individual
zooids being ultimately set free as medusae. The next stage
would be the establisment of a division of labour, in virtue
of which a certain proportion only of the zooids became medusa3,
the rest retaining the polype-form, remaining permanently
attached, and serving for the nourishment of the asexual colony.

The Hydrocorallina appear to be a special development of the
leptoline stock, the nearest affinities of the order being with such
forms as Hydractinia.

The Siphonophora may be conceived as having originated from
a hydrula specially modified for pelagic life by the conversion of
the basic disc into a float something after the fashion of Minyas
(Fig. 152). In such a form extensive budding, accompanied by
division of labour, would give rise to the complex siphonophoran
colony.

The lowest Scyphozoa are the Lucernarida, some of which,

iv PHYLUM CCELENTERATA 227

however, show evidence of degeneration, so that it is quite possible
to conceive them as having been derived from more highly
organised forms, instead of springing directly from simple polypes
of the scyphula type. The Semostomse, Cubomedusse, and
Rhizostomae clearly represent three grades of increasing com-
plexity along the same general line of descent, the Coronata
diverging somewhat. It is to be noted, however, that such a
supposed line does not lead towards the simpler Actinozoa, but
towards a type which diverges from the latter as well as from the
Lucernarida, Cubomedusae and Peromedusae in the absence of
septa or mesenteries in the adult condition.

The close similarity of Edwardsia and the Alcyonaria in the
number and arrangement of the mesenteries seems to indicate the
derivation of both Zoantharia and Alcyonaria from a common
ancestor in the form of a simple actinozoan polype or actinula,
Edwardsia clearly leads us to the Hexactinise or typical Sea-
anemones, and the Madreporaria are undoubtedly to be looked
upon as skeleton-forming Hexactinia?.

The relationships of the Ctenophora to the other Ccelenterata are
very doubtful. Ctenaria, one of the Anthomedusaj (Fig. 109, 1\
presents some remarkable resemblances to a Cydippid, such as
Hormiphora. It has two tentacles, situated in opposite per-radii,
and each having at its base a deep pouch in the umbrella resem-
bling the sheath of Hormiphora. There are eight radial canals
formed by the bifurcation of four inter-radial offshoots of the
stomach, and corresponding with them are eight bands of nema-
tocysts diverging from the apex of the ex-umbrella. If these
striking resemblances indicate true homologies, we must compare
the whole sub-umbrellar cavity of Ctenaria with the stomodseum
of Hormiphora, the margin of the bell of Ctenaria with the mouth
of Hormiphora, and the mouth of Ctenaria with the aperture
between the stomodseum and the infundibulum of Hormiphora.
But, as we have seen, the gullet of Ctenophora is a true stomo-
doeum developed as an in-pushing of the oral ectoderm, and has
therefore a totally different origin from the sub-umbrella of a
medusa. Moreover, the tentacles of Ctenaria have no muscular
base contained in the sheath, but spring from the margin of
the umbrella as in other Hydrozoa : its gonads are developed in
the manubrium, not in the radial canals, and there is no trace of
an aboral sense-organ.

Of Hydroctena, which has also been supposed to afford us a
connecting link between the Hydrozoa and the Ctenophora, almost
the same may be said. Hydroctena is bell-like, and provided with
a velum. At its apex is an ampulla bearing two lithites supported
on spring-like processes of the epithelium. From the apex of the
gastric cavity a canal is given off which extends to the sense-
organ, where it terminates blindly, and from the sides a pair of

Q2

228

ZOOLOGY

SECT.

short canals, each of which terminates blindly at the base of the
corresponding tentacular sheath. Only two tentacles are present,
with sheaths at their bases : these are situated, not on the margin
of the bell, as in a medusae, but between it and the apex. There
are no traces of swimming plates, and, so far as the evidence at
present forthcoming goes, there is not sufficient evidence to
establish Ctenophoran affinities.

On the other hand, the resemblance between transverse sections
of an embryo Ctenophore (Fig. 173, B) and of an embryo Actinian

A

end

end

FIG. 173. Transverse section of embryos of Actinia (A) and Beroe (B), crt. ectoderm ;
end. ciidoderm ; inf. infundibulum. (After Chun.)

(A) is very striking, and the presence of a well- developed stomo-
dseum, and of gonads developed in connection with the endoderm
and discharging their products through the mouth, may be taken
as further evidences of affinity between the Ctenophora and the
Actinozoa.

The special characteristics of the Ctenophora are, however, so
numerous and so striking, and their development so utterly unlike
that of any of the other Ccelenterata, that in our present state of
knowledge it is impossible to determine their affinity with the
other classes with any degree of certainty.

As to the orders of Ctenophora, it seems tolerably clear that
both Lobata and Cestida are derived from cydippid forms, since
they both pass through, in the course of development, a stage
closely resembling the lower Cydippida. The Beroi'da are more
highly organised in certain respects, e.g. in the details of their
histology, than the other Ctenophora, and it seems quite possible
that they may be derived from tentaculate forms. Whether the
Platyctenea are primitive or specially modified, remains doubtful,
especially in the absence of data regarding their development ;
but the latter appears the more probable conclusion.

These relationships are expressed in the diagram on the
opposite page.

By many authors the Sponges have been looked upon as so
closely related to the Ccelenterata that they may be regarded
as members of the same great phylum. The points of resemblance
are readily to be recognised : the simple structure, with the large cen-
tral cavity into which a wide opening the mouth or the osculum,

IV

PHYLUM CCELENTERATA

229

as the case may be leads ; the absence of a well-developed meso-
derm, the fixed mode of life, and associated with it, the tendency
to form compound structures or colonies by a process of budding.
In addition, the occurrence of larval stages which have at least
a superficial correspondence in the two phyla would appear to
constitute an important connecting link. Bat a closer examina-
tion of the subject shows that some of these apparent points of
resemblance are superficial only, and establishes a number of
differences between Sponges and Coelenterates too important to
allow us to suppose that a close relationship exists. One of these
differences stands out beyond the others as the most radical. The
osculum of a sponge is found, when we trace the development of

Hexactinia^ Madreporaria
Cestida

Lobata / Rhizostomeue Semostomcfe

Cydippida. / / Beroida

V / / \/ Edwardsia^

Platyctenea y / Cororiatcfc V -^ Alcvonaria

-^ \ / ^**^^ \

Cubomeduscfe

ACTINULA

v

Lucernarida

Hydrocorallinoe
Leptolinofc

SCYPHULA

Siponophora
Trachylincfc

HYDRULA

Fni. 174. Diagram illustrating the mutual relationships of the Coelenterata.

the larva, to correspond in no sense with the mouth of the
Ccelenterate. This alone, apart from important differences in
the adult structure, such as the presence in the wall of the Sponge
of the system of inhalant apertures, the presence of the peculiar
collared endoderm cells, and the absence of stinging capsules,
would suffice to remove the Sponges from the Coelenterata, and
place them in a phylum apart. But not only is the grouping
of Sponges and Coelenterates in one phylum thus rendered
impossible by important differences in their structure and develop-
ment ; a comparison of the mode of formation of the embryonic
layers in the two groups shows such radical dissimilarity that it is
scarcely possible to find sufficient evidence for regarding them as
having been derived from the same metazoan ancestors, and there
is much to be said in favour of the view that they have originated
separately from the Protozoa.

230

ZOOLOGY

SECT.

APPENDIX (II.) TO THE CCELENTERATA.

THE MESOZOA.

Under the designation MESOZOA have been comprised certain lowly organised
animal forms, formerly supposed to afford us something of the nature of a
connecting link between the Protozoa and the Metazoa, but now more generally
looked upon as degenerate members of the latter subdivision. It has been
proposed to term them the 'Moruloidea, from the resemblance which they bear
to the morula stage in embryonic development.

They are all multicellular, with an ectoderm composed of a single layer of
cells ciliated in whole or in part, and an eiidoderm either composed of a single
elongated cell or of several cells : a mesogkea is not represented. The Mesozoa
comprise at least three families, the Dicy&midcR, the Heterocyemidce, and the
0) fhoneciidw, all the members of which are internal parasites.

The Dicyemidai are parasities in the kidneys of various Cuttle-fishes and
Octopods (Cephalopoda}. Dicyema (Fig. 175), the length of which is between

FIG. 175. Dicyema paradoxum,

with infusoriform embryos (males).

(From J5romi's T/</< /v< /VA, after
Kolliker.)

FK;. 170. Dicyema paradoxxim,

with vermiform embryos. (From
JJruim's T/t tern ifh, after Kolliker.)

0'75 and 6 or 7 millimetres, consists of a head-part or calotte, and an elongated
body. The form of the calotte varies a good deal, according to age ; in young
specimens it is isotropic (i.e. symmetrical around the long axis) ; in the adult

iv PHYLUM CCELENTERATA 231

condition ventral and dorsal sides are distinguishable. It consists of a swollen
disc of four cells and a ring of four or five yole cells. The cells of the head all
bear cilia, which are shorter and thicker than those of the body-cells.

The body consists of a single large axial cell, and of a single layer of outer
cells which completely invest the axial cell. The outer cells which follow
immediately on the head are distinguishable from the rest by their granular
contents, and by their being dilated internally in such a way that the apex of
the axial cell is constricted.

The axial cell is either almost completely cylindrical or spindle-shaped, and
is covered in its entire extent by the outer cells. It presents a differentiated
cortical layer, beneath which the finely granular gelatinous contents are at first
homogeneous, but afterwards become vacuolated. In the middle of the cell is a
large oval or ellipsoidal nucleus.

The life-history is a true alternation of generations. The primitive nucleus of
the axial cell divides, mitotically to form a smaller asexual germ-nucleus and
a larger nucleus the definitive nucleus (somatic nucleus) of the axial cell.
Further germ- nuclei result from subsequent divisions. The germ-cells undergo
a process similar to segmentation. Of the cells thus formed one gives rise to
the axial cell of the embryo : the others, increasing in numbers and becoming
smaller, gradually grow over the axial cell until they at length completely
enclose it. The embryo increases greatly in length, and escapes from the
interior of the parent, which it completely resembles, by perforating the body-
wall. This phase of the parent animal (Fig. 176) is the phase to which the term
nematogene is applied : the asexually developed young are the so-called vermiform
embryos. The latter swim about for a time in the fluid of the kidney of the
host ; afterwards they attach themselves by means of the head to certain
appendages the venous appendages of the walls of the cavity. A number of
generations of these asexually developed forms succeed one another until, when
the venous appendages of the kidney have become thickly infested with the
parasites, a change takes place, and a sexual process of multiplication becomes
initiated. In the interior of the nematogene a change is observable, and female
sexual individuals are formed instead of vermiform embryos. Unlike the latter
the former do not leave the body of the parent ; they also differ in the 11011-
developmeiit of an enclosing layer. In their development, as in that of the
vermiform embryos, the first nucleus of the axial cell of the parent divides into
two. One of these becomes the permanent somatic nucleus of the axial cell : the
other becomes the nucleus of the primitive ovum, and surrounds itself with
protoplasm. This divides to give rise to a number of ova, which become more
numerous till they come to fill the axial cell. Then the first generation of these
ova are discharged into the protoplasm of the parent axial cell I this first
generation of ova are derived from the cells representing the outer layer.
Later further generations of ova, which are the descendants of the primitive ova,
make their escape. These all eventually wander away into
the protoplasm of the axial cell of the parent, increase in
size, undergo a process of maturation, and become fertilised.
Fertilisation is effected by means of typical tailed sperms
developed in a second set of sexual individuals, the males
(Fig. 177), which were formerly known as the infusoriform
embryos. In its mature form the male is approximately
pear-shaped, the narrower end being posterior. Several
axial cells are present : these form the testes, in which the

sperms are developed. They are surrounded by the outer Fl ;. ] ' ' ~

n 1-1 . , re Dicyema ^..

cells, which at the posterior end take the form of a flat doxum (From

ciliated epithelium. The complete development of the Broim's_ Thien;
sperms only takes place when the young male leaves the

host in which it was formed and seeks a new one ; thus it is only by the
sperms of a male from another host that the ova can be fertilised. The males

232

ZOOLOGY

SECT.

are developed from the fertilised ova and subsequently escape, their develop-
ment being similar to that of the vermiform embryos : the phase of the parent
form to which they are developed is that known as the rhombogene. After a
number of generations of males have been formed in this way, the rhombogene
undergoes modification, and the last generation of fertilised ova gives rise, not to
males, but to vermiform embryos i.e. , to an asexual generation and with these
the cycle begins anew.

The Heterocyemidce, which are also parasites of the Cephalopoda, resemble
the Uicyemidse in most respects, but the head is w r anting.

The family Orthonactidfe comprises only two genera - - Rhopahira and
Stcscharfhrum which live as parasites in a Poly clad (Leptoplaua), a Nemertine
(Linen*), an Annelid, and a Brittle-Star (Ampliinra). In the stage that
represents the asexual form of the Dicyemida? the Orthonectid assumes the
character of a plasmodium, or mass of finely granular protoplasm contain-
ing many nuclei, and is capable of active amoeboid movements. In the
interior of the plasmodia the sexual forms are developed. A nucleus of the
plasmodium surrounds itself with protoplasm and gives rise to a germ-cell,

In;. 1 7s. Bhopalura Giardii, male.
(From Bronn's T/iivrreich, after Julin.)

Fie;. 170. Rhopalura Giardii, female.
(From Bromi's Tltici'i'i ir/t, after Julin.

which by a process of segmentation develops into the sexual stage. In some
cases only males are developed in one plasmodium and females in another : in
others both sexes are formed together in the same plasmodium. In some forms
the sexes are united. The sexual animals, especially the females (Fig. 179J,

IV

PHYLUM CCELENTERATA

233

bear a considerable resemblance to the Dicyemidse, but instead of the axial cell
there are a number of cells, the ova or sperm-cells. The outer cells are
arranged in segments or rings. In front is usually a region composed of a few
rings in which the outer cells bear cilia which are directed forwards : then
comes a shorter region devoid of cilia, and behind that is the longest region,
having cilia directed backwards. In shape the body is usually spindle-like the
males (Fig. 178) differing somewhat from the females. In about the middle of
the internal space of the male is the compact oval testis containing small tailed
sperms. Beneath the outer layer in the male, but not in the female, is a
layer of fibres sometimes regarded as muscular. The plasmodia multiply by
fragmentation. The development of the embryos either goes on in the intact
plasmodium, or the latter breaks up and the embryos are to be found at various
stages free in the host.

In the development of a male from the germ-cell the first segmentation is
unequal. The further segmentation results in the formation of a solid morula.
The outer cells become differentiated into two distinct groups, the one giving
rise to the external layer of the anterior region, the other to that of the
posterior region of the body. The inner cells multiply and give rise to the
numerous small spermatocytes of the testis. The formation of the layer of
fibres only takes place later.

In the case of the female the segmentation appears to be equal from the first,
and results in the formation of a blastula-like stage, which becomes converted
into a solid morula-like body by the passing inwards of a number of cells. As
in the male, the central cells multiply to form the sexual cells, and the outer
cells form the external layer with its segments. In all probability, though this
has not been actually proved, the mature sexual animals become free from the
plasmodia, and the females, after fertilisation, find their way to another host
where they become transformed into plasmodia, the germ-cells of which are the
fertilised ova.

To l)e mentioned in connection with the Dicyemidae and Orthoiiectido 1 , as
perhaps allied with them, are the remarkable parasites Amfebophrya and
Lolimctnella the former living in certain Radiolariaiis (Protozoa) the latter in
the body-cavity of a Fritillaria ( Urochorda), These both resemble the groups
described above, and differ from the other Metazoa, in the presence of only a
single body-layer. This remarkable simplicity of body-structure occurs also in
tiafineUa, though too little is known with regard to this animal to provide
adequate data for determining its affinities with certainty.

Salinella (Figs. 180 and 181), which has only been found on one occasion in
water in which some salts from the Argentine Republic had been dissolved, is a

FIG. ISO. Salinella, longitudinal section. (After Frenzul.)

minute animal in the form of a somewhat depressed cylinder, open at both ends,
and with a wall composed of a single layer of cells. The anterior end is some-
what pointed ; around the anterior opening or mouth, which is veiitrally
directed, is a circlet of from fifteen to twenty long whip-like cilia. The posterior
aperture (ami*), which is usually closed, is surrounded by a few stiff seta?. The

234

ZOOLOGY

SECT. IV

ventral surface is flattened, and is covered with fine vibratile cilia, while on the
dorsal surface and the sides are regularly arranged rows of straight setae (non-
motile cilia). The internal cavity (enteroii) is found to contain sand, plant

Flu. 181. Salinella, transverse section. (After Frenzel.)

fragments, and Bacteria ; its surface is beset with long cilia. Multiplication is
said to take place by transverse fission ; and a process of conjugation followed
by encystation has also been observed.

Trichoplax and Treptoplax, which have been supposed to be Mesozoa, appear
to be merely special modifications of developmental phases of Hydrozoa.

SECTION V
PHYLUM PLATYHELMINTHES

A NUMBER of classes of Metazoa, some a little, others very de-
cidedly, higher in organisation than the Coelenterata, were formerly
regarded as constituting one great sub-kingdom or phylum the
Vermes or Worms. The groups ordinarily referred to the Vermes
differ, however, very widely from one another : points of agree-
ment, except such as are merely negative, are, in fact, frequently
hardly recognisable : and rather than group together under one
common designation such a heterogeneous assemblage of forms, it
is usually considered to be more expedient to avoid the term
Vermes altogether, and to endeavour to divide the " Worms " into
phyla the members of which shall have points of positive resem-
blance to one another. The four phyla Platylulminthes, Ncnmthel-
minthes, Trochelminthes, Molluscoida, and Annulata, with their
appendices, all consist of forms which are or have been comprised
in the Vermes. They differ from the Coelenterata in the presence
of three well- developed body-layers of which the middle one, or
mesoderm, is of relatively predominant importance ; and for the
most part, in the much higher stage of complexity attained by the
various systems of organs. The first four phyla present no meta-
meric segmentation (p. 43) : in the Annulata, metamerism is more
or less strongly pronounced.

The Platyhelminthes or Flat-Worms are a group of soft-bodied,
bilateral, usually flattened animals, which are devoid of true
metameric segmentation. With a sufficient degree of uni-
formity of structure to render the phylum a fairly compact and
well-defined one, there is yet a considerable range in complexity,
from the simplest forms certain of which have been supposed to be
nearly connected with the Ctenophora among the Coelenterata to
the highest, which have all the various systems of organs very
much more highly developed. The body is built up from
three embryonic layers ectoderm, mesoderm, and cndoderm as in
all higher groups of animals. An excretory vascular system of

23(5 ZOOLOGY SECT.

a peculiar kind the water -vascular or protonephridial system is
present in nearly all members of the phylum. A body-cavity (see
following Sections) is not present, the spaces between the various
organs and the wall of the body being filled up with a peculiar
form of connective-tissue termed the parenchyma. The egg is in
most instances composite, the egg-shell enclosing not only the
oosperm or impregnated ovum, but a quantity of nutrient material
or food-yolk, derived, in most instances, from a special set of glands,
the yolk or vitclline glands.

The main features which distinguish the Platyhelminthes from
the Coelenterata are the pronounced bilateral symmetry with
the many secondary features which it involves, the presence
of a middle embryonic layer or mesoderm, and the non-occurrence
of fixed colonies formed by budding.

1. EXAA1PLES OF THE PHYLUM.

i. A Fresh-water Triclad (Planaria or Dendroccelum). 1

General Features. Species of fresh-water Planarians of the
genera Planaria and Dendroccelum are common in the mud at
the bottom of ponds of fresh-water in all quarters of the globe.
They are small, thin, flattened worms a few millimetres in length,
broader at one end, the anterior, than at the other, the posterior,
which is more or less pointed. The animal (Figs. 182-184) is
very readily recognised to be bilaterally symmetrical, with an upper
or dorsal and a lower or ventral surface, right and left borders,
and anterior and posterior ends. The colour varies in different
species and in different individuals ; but is usually gray, red,
brown, or black. Movements of locomotion in the direction of the
long axis of the body are recognisable in the living animal. Some-
times this is a steady gliding movement, which is brought about
by the action of a coating of vibratile cilia on the surface ; some-
times the worm moves along somewhat after the fashion of a
Leech, the ventral surface of the anterior end of the body being
of a sticky adhesive character, and performing the part of the
anterior sucker of the Leech.

Close to the anterior extremity on the dorsal surface are two
rounded black spots, the eyes (Fig. 183). On the ventral surface,
a considerable distance behind the middle of the body, is the
opening of the mouth (Fig. 182, mo.), and further back still, near
the posterior pointed end, is a smaller median opening, the genital
aperture (Fig. 184).

Digestive System. The mouth (Fig. 18'3, mo.) leads through

1 The account is sufficiently general to apply to species of either of these
genera.

PHYLUM PLATYHELMINTHES

237

a short mouth-cavity into a cylindrical thick-walled chamber, the
pharynx (ph.}, which is highly mobile, and is capable of being
thrust out as a proboscis through the mouth, beyond which it

FIG. 182. Planaria. Digestive and excre-
tory systems, ex. openings of excretory
system ; int. intestine ; mo, mouth ;
o. ph. opening of pharynx. (After Jijinia
and Hatschek.)

FKI. 183. Planaria. Nervous system.
br. brain ; ri/e, eye ; I. nc. longitu-
dinal nerve ; ph.. pharynx. (After
Jijima and Hatschek.)

may then be extended to a relatively considerable distance.
When retracted it lies within an enclosing muscular sheath. The

238 ZOOLOGY SECT.

cavity of the pharnyx opens in front into the intestine (int.), which
almost immediately divides into three narrow main branches, one
running forward in the middle line, the other two running back-
wards. Each of these three main branches gives off numerous
smaller branches, which in turn become branched, so that the
whole intestine forms a ramifying system, extending throughout
the greater part of the body ; all the branches terminate blindty,
an anal aperture being absent.

A system of vessels the "water-vessels or vessels of the excre-
tory system (ex.) sends ramifications through all parts of the body.
There are two main, considerably coiled, pairs of longitudinal
trunks, right and left, which open externally on the dorsal surface
by means of several pairs of minute pores ; in front they are
connected together by a transverse vessel. The vessels of each pair
often join and separate again. Each main trunk gives origin to a
number of branches, which in turn give off a system of extremely
fine capillary vessels, many of which terminate in flame-cells (Fig.
214, p. 269). A flame-cell is a nucleated cell having in its proto-
plasm a small space into which one of the capillaries leads ; in this
space lies a bundle of vibratile cilia, or a single thick cilium, which
performs regular undulating movements, giving it somewhat the
appearance of a flickering candle-flame. Cilia are said also to
occur in the course of some of the capillaries, though this is
doubtful. This system of vessels is usually regarded as excretory ;
but it may also have a respiratory function.

A well-developed nervous system (Fig. 183) is present. At
the anterior end is a central knot of nerve-matter, the brain (Lr),
from which proceed backwards two longitudinal nerve-cords (I. ne.).
The brain consists partly of transverse fibres connecting to-
gether the two longitudinal nerve-cords, partly of groups of nerve-
cells situated at the ends, or in the course of, the nerve-fibres.
The nerve-cords give off both internally and externally numerous
transverse branches, which divide into finer twigs ; the internal
branches of the two cords frequently anastomose, thus forming
commissures or connecting nerve-strands between the two. A
number of nerves extend forwards to the anterior margin, which
is highly sensitive.

Reproductive System. The reproductive organs (Fig. 184)
are hermaphrodite or moncecious in their arrangement, both male
and female organs occurring in the same individual. The genital
aperture leads into a small chamber, the genital atrium or cloaca,
which is common to both the male and the female reproductive
systems.

The male part of the apparatus consists of testes, vasa defercntia,
and penis. The testes (tes.) are numerous rounded glands,
situated near the right and left borders. Two ducts, the right
and left vasa deferentia (v.d.), run backwards from the neighbourhood

V

PHYLUM PLATYHELMINTHES

239

ov

uit

oci

of the testes and unite in the middle line posteriorly. The median

duct formed by the union of the two vasa deferentia traverses a

protrusible muscular organ, the

penis (p) } to open into the genital

cloaca. At the base of the penis,

where the vasa deferentia meet,

the median canal is slightly

enlarged to form a rounded

dilatation, the vesicula seminalis.

Into the median canal open the

narrow ducts of a number of

unicellular glands, the prostate

glands (pr).

The female part of the repro-
ductive apparatus consists of
ovaries (germaria), oviducts, vitel-
line glands, uterus, and muscular
sac. The germaria (ov.) are two
in number small rounded bodies
situated near the anterior end,
each connected with an elongated
duct, the oviduct. The two ovi-
ducts (od.) unite posteriorly to
form a short median common ovi-
duct opening into the genital
atrium. With this cavity are
connected also the uterus (ut), a
median rounded chamber, and a
thick-walled muscular body, the
muscular sac (in.). Numerous
branching tubes - the vitelline
glands (vit.) open into the ovi-
ducts.

Reproduction is entirely sexual.
The oosperm is enclosed within a
protecting case or shell, which
contains also a quantity of food-
yolk derived from the vitelline
glands. When the larva has
reached a certain stage it de-
velops a temporary larval mouth [G \1 S4 \
and gullet (see Fig. 218), and
swallows the food-yolk, by the
aid of which it grows rapidly.
The -larval mouth disappears, and a new one the permanent
mouth is developed in its place. When the embryo leaves the
shell, it has assumed the characteristic shape of the parent.

Planaria. Reproductive system.

muscular sac ; ov. germariuni ; j>lt.
pharynx ; p. penis ; pr. prostate ; tes.
testes ; ut. uterus ; v. d. vas deferens ;
vit. vitelline glands. (After Jijima and
Hatschek.)

240

ZOOLOGY

SECT.

ect

a.<yrs vent, mus

^ fl

od

ccec

o.d

FIG. 185. Transverse section of a Planarian. circ. mus. circular muscular fibres ; car. in-
testinal cseca ; dors. vent. mus. dorse-ventral muscular fibres ; ect. ectoderm ; ext. long. mus.
external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. long.
'at vs. internal layer of longitudinal muscular fibres; ne. nerve-cords; o. d. oviducts; par.
parenchyma ; text, testes ; v. def. vas deferens ; vit. vitelline glands. (From Hatschek's
Lchrbuch.)

ii. The Liver-Fluke (Fasciola licpatica).

General Features.- -The Liver-Fluke of the Sheep, which is to
be found in the interior of the larger bile-ducts of the infested
animal, is a soft-bodied worm of flattened leaf-like shape (Fig.
186), with a triangular process, the head-lobe, projecting from the

broader end. The symmetry of the parts is
distinctly bilateral, as in the Planarian. Ex-
ternally the body is quite equilateral, the right
and left portions exactly balancing one another,
but, as will appear subsequently, this complete
symmetry does not extend to all the internal
organs.

The surface is devoid of vibratile cilia, but
is covered with innumerable minute spinulcs
or papillae-, which are prolongations of the
homogeneous external layer or cuticle investing
the whole animal. At the extreme anterior
end of the triangular head-lobe is the small
opening of the month (mo.) surrounded by a
muscular oral sucker. A short distance back
on the ventral surface, just behind the head-
lobe, is a second much larger posterior sucker (sckr.). Between
the two suckers, but rather nearer the posterior one, is a
median aperture, the genital opening (repr.), through which a
curved muscular process, the cirrus or penis may be protruded. In
the middle of the posterior end of the body is a minute opening,
the excretory pore (excr.).

excr

Fin. 18U. Fasciola
liepatica natural

size, c.i'cr. excretory
pore ; mo. mouth ; ?*< /.
reproductive aperture;
s<:kr. posterior sucker.

PHYLUM PLATYHELMINTHES

241

Body-wall.- -The body- wall (Fig. 187) is found on section to
comprise three layers : (1) a homogenous cuticle (cut.) of which
the spinules (sp.) are
special developments ; (2)
a layer of circularly dis-
posed muscular fibres (circ.
mus.) ; (3) a layer of longi-
tudinal muscular fibres
(lung. mus.). A cellular
epidermis is wanting.
Beneath the muscles are
unicellular

arc

numerous

FIG. 1ST. Fasciola hepatica. Section of the integu-
ment, circ. mus. layer of circular muscular fibres ;
cut. cuticle ; gl. unicellular glands ; long. mus. layer
of longitudinal muscular fibres ; sp. spinules. (After
Braun.)

glands (gl.), the ducts of
which, in the form of pro-
cesses of the cells, open on
the outer surface. Inter-
nally, the interspaces
between the organs are
filled by a peculiar form of connective - tissue, the paren-
chyma.

Digestive System. The mouth (Fig. 188) leads to a small
rounded bulb-like body, the pliarnyx (ph.), with thick muscular
walls and a small cavity. From this a short passage, the (esophagus,
opens into the intestine. The latter (int.) is frequently a very
conspicuous structure, owing to its being filled with the dark
biliary matter mixed with blood on which the Fluke feeds. It
divides almost immediately into two main limbs, right and left,
and from each of these are given off, both internally and externally, a
number of blind branches or caeca, those on the inner side being short
and simple, while those on the outer side are longer and branched.
The two limbs of the intestine with their branches thus form, as
in the Planarian, a complicated system, the ramifications of which
extend throughout the whole of the body. There is no aperture
of communication between the intestine and the exterior, the only
external opening of the alimentary system being through the
mouth.

A branching system of vessels the water- vessels or vessels of
the excretory system ramify throughout the body. A longi-
tudinal main trunk opens behind by means of the excretory pore
already mentioned as occurring at the posterior end. In front it
gives off four large trunks, each of which branches repeatedly, the
branches giving off smaller vessels, and these again still smaller
twigs, until we reach a system of extremely fine microscopic vessels
or capillaries. Each of these ends internally in a slight enlarge-
ment situated in the interior of a large cell, an excretory cell or flame-
cell, similar to a flame-cell of the Planarian.

The Liver-Fluke has a well differentiated nervous system,

VOL. I

R

242

ZOOLOGY

SECT.

which shares in the prevailing bilateral arrangement of the parts.
The central part of this system consists of a ring of nerve-matter
which surrounds the oesophagus and presents two lateral thicken-
ings, or ganglia, containing nerve-cells, and a single ganglion in the
middle line below. From this are given off a number of nerves,
of which the chief are a pair of lateral cords running back to the

vit

ov-

FIG. 188. Fasciola hepatica. Internal organisation. General view of the anterior portion
of the body, showing the various systems of organs as seen from the ventral aspect, ej.
ejaculatory duct ; /. female reproductive aperture ; int. anterior portion of the intestine (the
rest is not shown); od. commencement of oviduct; ov. ovary (germariuro) ; p. cirrus; ph.
pharynx ; sh. shell-gland ; te. testes ; ut. uterus ; vtft. left vas deferens ; txf-. right vas
deferens ; vit. lobes of vitelline glands ; vs. vesicula seminalis. (After Sommer.)

posterior end and giving off numerous branches. There are no
organs of special sense.

The reproductive organs (Fig. 188) are constructed on the
hermaphrodite plan, i.e. both male and female organs occur in
the same individual. The male part of the apparatus consists of
testes, vasa defer entia, and cirrus. The testes (te.) are two greatly

PHYLUM PLATYHELMINTHES

243

V. S

ramified tubes, which occupy the middle part of the body, one
situated behind the other. From each testis there runs forwards
a duct, the vas dcfcrens, the two vasa deferentia (v.d.) opening
anteriorly into an elongated sac, the vesicula seminalis (v.s.), from
which a narrow tube the cjaculatory duct (Fig. 189, ej.d.) leads
to the male aperture at the ex-
tremity of the cirrus. The female
part of the reproductive apparatus
consists of a single ovary (ger-
mctrium}, an oviduct, a uterus, an
ootype, vitelline glands, vitelline ducts,
and shell- glands. The germarium
(Fig. 188, ov.) is a branched tube
situated on the right-hand side in
front of the testes ; the branches
open into a common narrow tube,
the oviduct (od.\ The vitelline
glands (vit.~) consist of very nu-
merous, minute, rounded follicles,
which occupy a considerable zone in
the lateral regions of the body. On
each side are two large ducts,
anterior and posterior, uniting to
form a single main lateral duct,
right or left ; and these run nearly
transversely inwards to open into a
small sac, the yolk reservoir. From this a single median vitelline
duct runs backwards for a short distance to join the oviduct.
Around the junction are grouped a mass of unicellular shell-glands
(sh. gL), each of which is produced into a narrow process or duct
opening into the end of the oviduct in the region of the latter to
which the term ootype is applied. The uterus (ut.) is a wide con-
voluted tube, formed by the union of the oviduct and median
vitelline duct ; in front it opens close to the base of the cirrus.
When the cirrus is withdrawn, a small cavity, the genital atrium or
cloaca, is formed common to the external apertures of both male
and female ducts. A canal, termed the canal of Laurcr, leads
from the junction of the oviduct and median vitelline duct to
open externally on the dorsal surface.

Development. Each ovum on impregnation becomes sur-
rounded by a mass of vitelline matter or yolk derived from the
yolk-glands. It then becomes enclosed, while passing through the
ootype, in a chitinous shell, the substance of which is usually said
to be derived from the shell-glands. 1 The completed egg remains

1 The agency of the shell-glands in producing the material of the egg-shell is
not universally acknowledged. This point will be referred to in the section
dealing with the general organisation.

R 2

v.d

FIG. 189. Fasciola hepatica. Ter-
minal part of the reproductive
apparatus, ej. cjaculatory duct ; /.
female aperture ; g. unicellular
glands ; od. terminal part of oviduct ;
p. cirrus ; ps. cirrus-sheath ; s.
sucker ; v. d. vasa deferentia ; v. s.
vesicula seminalis. (After Sommer.)

244

ZOOLOGY

SECT.

for a little time in the uterus ; eventually it is discharged, and,
passing down the bile-ducts of the Sheep into the intestine,
reaches the exterior with the faeces. Active development only
begins at this stage, and, three to six weeks later, a portion of
the egg-shell at one end becomes separated off as a sort of
lid or operculum, and gives exit to the contained embryo. This,
the ciliated embryo or miracidium (Fig. 190, A), is a somewhat
conical body covered all over with vibratile cilia, and with two

B

eye

t?S|

\m

: - ?-'*
' , '--, t

#A

' <i 3"

Pi

perm- 1

FIG. 190. AD. Development of Fasciola hepatica. A, ciliated larva ;
B, sporocyst, containing redife in various stages of development ; C,
redia, containing a daughter redia, and cercariae ; D, fully developed
cercaria. b. op. birth opening ; ent. enteron of redia ; eye. eye-spots ;
gast. gastrula stage of redia ; germ, early stages in the formation of
cercarise ; int. intestine of cercaria ; mor. morula stage in the develop-
ment of cercarise ; ces. oesophagus ; or. su. oral sucker ; pap. head-papilla
of ciliated embryo ; ph. pharynx ; proc. processes of redia ; vent. su.
ventral sucker. (After Thomas.)

spots of pigment, the eye-spots (eye), near the broader or anterior
end, which is provided with a triangular head-lobe (pap.). There
is an imperfectly developed intestine and a pair of flame-cells,
each with a fine canal opening on the surface. The rest of
the interior is filled with a mass of germ-cells. The ciliated
larva swims about in water, or moves over damp herbage
for a time, and perishes unless it happens to reach a Pond-snail,

v PHYLUM PLATYHELMINTHES 245

(Lymnceus),as a parasite of which it is alone able to enter upon the
next phase in its life-history. When it meets with the Snail,
the embryo bores into it by means of the head-lobe, coming to
rest in the pulmonary sac or some other organ of the mollusc.
Established in the interior of the Snail, it loses its ectoderm and
grows rapidly into the form of an elongated sac, the sporocyst (Fig.
190, B), with an internal cavity containing germ-cells and lined
by a layer of cells, with remnants of the eye-spots, and with
flame-cells. The sporocyst may divide into two similar bodies by
a process of transverse fission, but this is exceptional. Eventually
cells are budded off from the layer that lines the internal
cavity of the sporocyst or from the germ-cells, and these
undergo a process of segmentation similar to the holoblastic
segmentation of the impregnated ovum, resulting in the
formation of a morula, which becomes converted into a stage
resembling a gastrula. The gastrula elongates and gives rise
to a body called a redia (C), which begins to move about, and,
eventually forcing its way out of the interior of the sporocyst,
finds its way to some other part of the Snail, usually the liver.
When fully formed, the redia is a cylindrical body with a pair
of short processes (proc.) near the posterior end, and with a
circular ridge near the anterior end. It possesses a mouth
leading to a pharynx and simple sac-like intestine, and there
is a system of excretory vessels. In the interior of the redia
cells are budded off and develop into gastrulae, exactly as in
the case of the sporocyst; these gastrulse either develop into
a fresh generation of redise if the season should be winter, or, if it
should be summer, give rise to bodies termed cercarice. The latter
(D) are provided with long tails : they have anterior and posterior
suckers, and a mouth and pharynx, followed by a bifid intestine.
An opening, the birth-opening (G t b. op.), is formed in the wall
of the redia near the circular ridge, and through this the cercariae
escape ; they move actively by means of their tails, and force their
way out of the body of the Snail. They then, losing the tail,
become encysted, attached to blades of grass or leaves of other
herbage. The transference of the larval Fluke in this stage to its
final host, the Sheep, is effected if the latter swallow the grass on
which the cercaria has become encysted. The young Fluke then
escapes from the cyst and forces its way up the bile-ducts to the
liver, in which it rapidly grows, and, developing reproductive
organs, attains the adult condition.

iii. The Common Tape- Worm of Man (Tcenia solium).

General Features. Tania solium occurs as a parasite in the
intestine of man. It has the form of a narrow ribbon (Fig. 191),
which may attain a length of several yards, attached at one end to

246

ZOOLOGY

SECT.

the wall of the intestine, the remainder hanging freely in the
interior. Towards the attached end the ribbon becomes very much
narrower than it is towards the opposite end ; and at this narrower

FIG. 191. Taenia SOlium. Entire specimen, "reduced ; cap. head. (After Leuckart.)

extremity is a small, rounded, terminal knob, which is known as the
head or scolex ; l the rest of the animal is termed the body or
strobila; the narrow part immediately behind the head is some-

Tliough very probable, it is not certain that this end of the Tape- Worm
jtually corresponds to the anterior end in the Liver-Fluke, as will be explained

ac:
later.

PHYLUM PLATYHELMINTHES

247

times called the neck. The attachment of the Tape-worm to the
wall of the intestine is slight and temporary ; it is effected by
certain organs of adhesion, the hooks and suckers on the head.

The head (Fig. 192) may be roughly described as pear-shaped,
but becomes four-sided at the broader end. In the middle of this
broader, anterior end is a rounded prominence,
the rostellum, round the base of which there is
a double row of usually about twenty-eight
curved and pointed chitinous hooks. The
rostellum is capable of being protruded and
retracted to a slight extent, and the position
of the hooks varies accordingly : when the
rostellum is fully retracted the points of the
hooks are directed forwards, and may even
meet in the centre ; as the rostellum is pro-
truded the hooks become rotated until their
apices come to be directed backwards. Four
cup-shaped suckers project slightly from the
surface behind the circlet of hooks.

The body or strobila has a jointed appear- FIG. 19-2. -Head of Tsenia
ance, owing to its being made up of a string ^Se?TeucSirf 1 ) ified '
of segments, or proglottides about 850 alto-
gether. These are narrower and shorter in front, gradually
increasing in size towards the posterior free extremity. The neck
or part immediately following the head is devoid of any trace
of segmentation. The two surfaces of the proglottides are not to
be distinguished by any differences visible to the unassisted eye ;
but that side towards which the female reproductive organs are

o.d.

Fig. 193. Transverse section of Teenia solium. c.m, circular layer of muscle ; ex. longitudinal
excretory vessel; ne. longitudinal nerve; o.d. oviduct; ov. ovary; ut. uterus. (Aftei-
Shipley.)

more nearly approximated is regarded as the ventral, the opposite
as the dorsal surface. On one border, alternately on the right
and left, of each proglottis, is a little prominence, the genital

248

ZOOLOGY

SECT.

papilla, on which is the opening of a chamber, the genital cloaca,
into which both the male and female reproductive ducts open.

An examination of entire living and of preserved and stained
Tape- Worms under the microscope shows (1) that an alimentary
system is not present ; (2) that nervous and excretory systems are
represented ; (3) that there is a complete set of reproductive
organs, constructed on the same general plan as those of the
Liver-Fluke, present in each of the proglottides.

The nervous system consists of two not very well-defined
ganglia united by a broad transverse commissure in the head ;
of slender nerves passing from these to the suckers, and of two
longitudinal nerves which run backwards through all the proglot-
tides to the posterior end of the body. The ganglia seem to
correspond to the ganglia on the nerve-ring of the Liver-Fluke.

The excretory organs consist of a richly branched system
of excretory vessels. There are four main longitudinal trunks
(Fig. 194, can. excret.\ two near each lateral margin in the more
anterior part of the strobila ; in the more posterior region one of
these becomes lost on each side. The two pairs of longitudinal
vessels are connected together in the head by a ring-like vessel

can-.excret

can. ejccret

ne.ru. I

FIG. 194. A proglottis of Tacnia SOlium with mature reproductive apparatus, can. excret.
longitudinal excretory canals with transverse connecting vessels ; gl. tit. vitelline glands ;
nerv. 1. longitudinal nerves ; ov, ov. ovaries (germaria) ; por. gen. genital pore ; schld. shell-
glands ; vt?.r. uterus; rar/. vagina; vas. <lef. vas deferens. The numerous small round
bodies are the lobes of the testes. (After Leuckart.)

and in each proglottis near its posterior margin by a straight,
transverse, connecting branch. Posteriorly the longitudinal trunks
open into a pulsatile caudal vesicle, communicating with the
exterior in the last proglottis. When the latter becomes thrown
off, the vesicle is lost with it, and, subsequently, the longitudinal
vessels have their separate openings on the exterior. These
main trunks of the excretory system give origin to a number of
branches, and these in turn give off numerous fine canalicules, or
capillaries, terminating in flame-cells similar to those of the
Fluke.

v PHYLUM PLATYHELM1NTHES 249

The reproductive organs (Fig. 194), repeated in each fully
formed proglottis, are in essential respects very similar to those of
the Liver-Fluke. In the most anterior proglottides they are not
developed ; it is only at about the 200th proglottis that they first
appear : at first the male parts of the system are alone differen-
tiated ; then in the succeeding proglottides, till we approach near
the posterior extremity of the body, the female organs are like-
wise developed. In the most posterior segments modifications
and reductions of some of the parts take place, owing to the great
increase in size of the uterus. The male portion of the apparatus
consists of the testcs with their efferent ducts, the vas deferens
(vas. def.), and the cirrus, with its sac. The testes consists of
numerous rounded lobes situated nearer the dorsal than the
ventral surface, and extending throughout the greater part of the
length and breadth of the proglottis. With each lobe is con-
nected a fine efferent duct ; the ducts of neighbouring lobes unite
together to form somewhat larger ducts; and the larger ducts,
receiving numerous tributaries, eventually open into the inner
extremity of the vas deferens, or main duct of the testis. The
vas deferens is a convoluted tube which extends outwards towards
the lateral margin (right or left as the case may be) of the
proglottis.

The terminal part of the vas deferens, which is somewhat nar-
rower than the rest, traverses a narrow protrusible process, the
cirrus, and opens at its extremity by the male genital aperture in
the genital atrium or cloaca. The cirrus is enclosed at the base
by a muscular sac, the cirrus-sac.

The ovary (germarium) (ov.) differs from that of the Liver-
Fluke in being a paired organ, consisting of two approximately
equal, right and left, halves. It is situated towards the posterior
border of the proglottis. Like that of the Liver-Fluke, it consists
of a number of branching tubes, in the interior of which the ova
are developed. From opposite sides these tubes converge towards
the median line, where they open into the oviduct. A yolk-gland
(gl. vit.), of less relative extent than in the Liver-Fluke, consists
of a number of minute lobules ; a duct, the yolk-duct, which runs
forward from it, opens into the oviduct. The numerous lobules of
a rounded shell-gland (schld.) surround the yolk-duct where it passes
forward to join the oviduct ; and the many shell-gland ducts open
into the oviduct near its junction with the yolk-duct: this part of
the oviduct is the ootype the part in which the egg becomes
completed. In front this passes into the uterus. The female
genital pore, situated in the genital atrium, leads into a narrow
passage which runs inwards and backwards towards the middle
line of the proglottis, where it ends in a dilatation usually filled
with sperms the receptaculum seminis. From this a narrow duct
-the fertilising duct or spermatic duct runs to join the oviduct.

250

ZOOLOGY

SECT.

The uterus, in the segments in which it first makes its appearance,
is a simple cylindrical diverticulum of the oviduct; it retains
its simple form as far back as about the 600th proglottis, where it
begins to branch, the ramifications increasing in extent and volume
in the posterior segments. It has no opening on the exterior.

Masses of sperms (probably from the same proglottis) pass
in the act of copulation along the vagina to the receptaculum
seminis; through the fertilising duct they pass to the oviduct
to fertilise the ova. As in the case of the Liver-Fluke, the oosperm
proper becomes surrounded by a quantity of food-yolk developed
in the yolk-glands, and is then enclosed in a firm chitinous shell
formed for it by the secretion of the shell-gland. It then passes
into the uterus. The first completed eggs are found in the uterus
in some proglottis between the 400th and the 500th. From this
point backwards they rapidly accumulate, until the cavity of the
uterus, which now becomes branched, is filled and distended with

them. Eventually in the most posterior, so-
called "ripe," proglottides (Fig. 195), the
uterus, with its contained accumulation of eggs,
becomes so large as to fill the greater part of
the interior of the proglottis, the remainder of
the reproductive apparatus meanwhile having
become absorbed.

Development. --When the ripe proglottides
are detached they pass to the exterior with the
fa3ces of the host. For a time they exhibit
movements of contraction. The embryos con-
tained within the eggs have meantime assumed
the form of rounded bodies, each armed with
six chitinoid hooks the six-hooked OY licxacantli
embryo (Fig. 196, A) enclosed within two membranes. If the pro-
glottides, or the eggs which have escaped from them, should now
be taken into the alimentary canal of the Pig, which forms the
ordinary second host of the parasite, the hooked embryos, becoming
freed from their coverings, bore their way with the aid of their
hooks through the wall of the alimentary canal, and reach the
voluntary muscles. Here they increase greatly in size, and develop
into rounded cysts with a large cavity filled with watery fluid the
proscolcx stage (B). On the wall of the proscolex, at one side, is
formed a hollow ingrowth, or invagination (C) ; and on the inner
surface of this are developed the hooks and suckers characteristic
of the head or scolex of the adult (D). When these are fully formed
the hollow ingrowth becomes everted (E), the suckers and hooks thus
coming to be situated on the outer surface (F). The whole embryo
has now the form of a bladder or vesicle, with which is connected
at one point a process having all the characters of the head and
neck of the mature Tasnia solium ; this is the bladder- worm stage,

FIG. 105. "Ripe" pro-
glottis of Tacnia
solium. (After
Leuckart.)

PHYLUM PLATYHELMINTHES

251

or cysticercm. If a portion of Pig's muscle containing cysticerci
which have not been killed by cooking is taken into the stomach

FIG. 196. Development of Tapeworm. A, hexacanth embryo; B, proscolex of Tc/uVt
saginata ; C , stages in the formation of the scolex of the same ; C, the invagination before
the hooks and suckers have become developed ; D, after the appearance of the hooks and
suckers ; E, partly evaginatcd ; F, fully evaginated scolex of T. solium with caudal vesicle ;
G, scolex of T. serrata with remains of the vesicle ; H, young tapeworm of T. scrrata. (After
Leuckart.)

of Man, the bladder is thrown off, the scolex attaches itself to the
wall of the intestine by its hooks and suckers, and develops the
series of proglottides of the adult Tape-Worm.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Platyhelminthes are bilaterally symmetrical, usually dorso-
ventrally compressed animals, devoid of hard supporting skeleton-
either external or internal, and also of metameric segmentation ;
with three embryonic layers ectoderm, mesoderm, and endoderm
-entering into the formation of the body. A body-cavity is not
present. There is a system of excretory vessels, communicating in
the majority of cases with the exterior, and furnished with ciliary
flames. There is no blood-vascular system. An enteric cavity
may be absent, may be rudimentary, or may be highly developed ;

252 ZOOLOGY SECT.

it is never provided with an anal aperture. The completed egg
contains, in addition to the oosperm, a quantity of yolk-matter,
usually in the form of definite yolk-cells, and usually produced by
a special set of "yolk-glands. Development is sometimes direct,
sometimes accompanied by a metamorphosis.

CLASS L TURBELLARIA,

Mostly non -parasitic Platyhelminthes with a ciliated cellular
epidermis; with a digestive cavity (except in the sub-division
Accela).

ORDER 1. POLYCLADIDA.

Flattened leaf-shaped Turbellaria, without separate yolk-glands ;
testes and ovaries numerous ; male and female genital apertures
usually separate ; intestine complexly branched.

ORDER 2.- -TRICLADIDA.

Turbellaria with elongate depressed body ; with numerous yolk-
glands, two ovaries, numerous testes ; a single genital aperture ;
intestine consisting of a median anterior division and two lateral
posterior limbs which are provided with side branches.

ORDER 3. RHABDOCCELIDA, incl. ACCELA.

Comparatively small Turbellaria, with the body usually elongate
and cylindrical or compressed : with simple, or nearly simple,
sac-like intestine ; with or without yolk-glands ; with one or two
ovaries and two or many testes.

CLASS II. -TREMATODA.

Ecto- or endo-parasitic Platyhelminthes devoid of cilia, 1 or of a
cellular epidermis ; 2 with a well-developed digestive apparatus.

ORDER 1. MONOGENETICA (HETEROCOTYLEA).
Mostly ectoparasitic Trematodes ; with direct development.

ORDER 2. DIGENETICA (MALACOCOTYLEA).
Endoparasitic Trematodes with complicated life-history.

1 Except in certain species of T<-mn<><-<'j>liala.

2 Except in the Temnocejrfialea and Actinodactylella.

PHYLUM PLATYHELMINTHES 253

ORDER 3. ASPIDOCOTYLEA.

Endoparasitic Trematodes with direct development; adhesive
apparatus in the form of a large sucker, which is divided by
septa into compartments, and occupies nearly the entire ventral
surface.

ORDER 4. TEMNOCEPHALEA.

Trematodes with direct development, which live on the outer
surface or in the respiratory cavities of various animals e.g., Crusta-
ceans ; most non-parasitic as regards their nutrition, with organs
of adhesion in the form of a simple posterior sucker and a system
of anterior or marginal tentacle-like appendages.

CLASS III. CESTODA.

Endoparasitic Platyhelminthes without cilia and without di-
gestive cavity, the animal consisting in most cases of a rounded
head bearing organs of adhesion in the form of suckers and hooks,
and an elongated compressed body consisting of a string of similar
proglottides, each containing a complete set of hermaphrodite
reproductive organs.

ORDER 1. MONOZOA.
The body not divided into proglottides.

ORDER 2. POLYZOA (MEROZOA).
The body consisting of head or scolex, and string of proglottides.

Systematic Position of the Examples.

Planaria and Dendrocoelum are genera of the family Planaridcc
or fresh-water Planarians, which is one of the two families of the
order Tricladida, differing from the other family, the Gcoplanidce
or Land Planarians, mainly in having the body less elongated and
more dorso-ventrally compressed.

The genus Fasciola, to which the Liver-Fluke belongs, is a
member of the family Distomidce of the Monogenetic Trematodes.
The Distomidce are characterised by the following features : They
have a cylindrical or more or less flattened body, always provided
with two suckers the anterior terminal or nearly so, the posterior
ventral and either terminal, or in a varying position on the ventral
surface. A pharynx may be present or absent. The intestine is
always forked, the limbs simple or branched. The genital pore is

254 ZOOLOGY SECT.

ventral, either median or lateral, sometimes at the posterior end.
There are two testes, sometimes fused into one, sometimes broken
up into more or less numerous follicles, but always provided with
only two vasa deferentia. There is a single germarium, not un-
commonly lobed or divided up into a number of separate parts.
A receptaculum seminis, or a Laurer's canal, or both, are present.
The vitelline glands are, in most instances, paired, more or less
richly branched, extending towards the lateral borders of the
body.

The genus Fasciola is a member of the sub-family Fasciolince of
the Distomidce, and this is distinguished from the other sub-families
by the following characteristics. The Fasciolince are broad, leaf-
like Distomidce, with the integument spinose or scaly. They have
a well-developed pharynx. The intestinal limbs are simple or
branched. The genital aperture is median, and situated in front
of the posterior sucker. The testes are situated one behind the
other, directly or obliquely : they are either simple, divided into
lobes, or branched. The ovary is immediately in front of the
testes, the uterus in front of the ovary. A Laurer's canal is
present. The receptaculum seminis is absent or small. Among
the many genera into which this sub-family is now divided the
genus Fasciola presents the following distinctive features :- -The
anterior end is distinctly differentiated into a head-lobe; the
intestinal limbs have long branched diverticula on the outer side,
short on the inner ; the gonads are all richly branched ; there is
no receptaculum seminis.

Tcenia solium is one of the many species of the genus Tcunia,
of the family Tceniadce, which is distinguished from the other
families of Cestodes by the possession of four suckers, with or
without a circlet of hooks, and by the development of well-defined
proglottides which become separated off when mature.

3. GENERAL ORGANISATION.

General External Features. As the name of the phylum
denotes, the body in the Platyhelminthes is, in the great majority
of cases, much compressed in the dorso-ventral direction ; very
thin, so that when very short it may be described as leaf-
like, or, when more elongated, as ribbon-like ; or thickish in
the middle and becoming thinner towards the margin. Some,
however, have the body comparatively thick, usually with a certain
amount of dorso-ventral compression ; a few are approximately
cylindrical or fusiform. The symmetry is always bilateral (p. 43),
the radial arrangement of parts so prevalent in the Coelenterata
and primarily, as we have seen, associated with a fixed or stalked
condition, never being observable. A Flat- Worm has dorsal and

PHYLUM PLATYHELMINTHES

255

ventral surfaces, right and left sides or borders, and anterior
and posterior ends. The anterior end is that which is directed
forwards in ordinary locomotion : it usually has some of the
features which distinguish a head-end ; but a distinct head is
rarely developed, and the mouth, when present, is usually placed
some distance back on the- ventral surface.

In the Turlellaria (Fig. 197) the leaf-form is the prevailing one,
a shape resembling that described for Planaria being very common.
In many, however, the body is
greatly elongated, and it may
assume the shape of a thin
ribbon with puckered edges, as
in some marine forms ; or may
be thickened and band-like, as
in the Land Planarians ; or it
may approach the shape of a
cylinder, as in some Rhabdo-
cceles. A head -region is not
usually distinct; but there is
always something to mark off
the anterior from the posterior
end a difference in shape, the
presence of eyes, and, sometimes,
of a pair of short tentacles ; in
some a slight constriction sepa-
rates off" an anterior lobe, on
which the eyes are borne, from
the rest of the body. In others
the anterior end is retractile,
and may be everted as a pro-
boscis. The mouth is never at
the extreme anterior end, but
always ventrally placed, some-
times behind the middle. In
some Poly clad id a there is a small

ventral sucker, probably with a copulatory function ; and
some Rhabdocceles both the anterior and posterior ends, though
not provided with suckers, are adhesive, so that the animal
can loop along like a Hydra or a Caterpillar. There is never
any external appearance of segmentation, though in at least
one exceptional instance (Gunda segmentata, Fig. 198) the internal
parts may be so disposed as to approximate to the metameric
arrangement (pseudo-metamerism). In such a case a number
of transverse muscular septa are present, imperfectly dividing the
body internally into a series of segments ; and various internal
organs intestinal caeca, gonads, transverse commissures of the
nervous system are arranged in pairs following this division. A few

Fir.. 197. Various Planarians A, Con-
voluta ; J>, Vortex ; C, Monotus ; D,
Thysanozoon ; E, Rhyuchodemus ; F,
Bipalium ; Cf, Polycelis. All natural size.
(After Von Graff.)

Ill

256

ZOOLOGY

SECT.

Fio. 198. Gunda segmentata. Geiieral view of the organisation, br. brain ; eye eye ;
gen. ap. genital aperture ; int. intestine with its caeca ; long. ne. longitudinal nerve-cord ;
ov. ovary ; ovd. oviduct : pe. penis ; ph. pharynx ; te. testes ; ut. uterus. (After Lang.)

PHYLUM PLATYHELMINTHES

257

Turbellaria multiply by budding, and these form long chains, having
something in common with the string of proglottides of a Cestode,
but differing radically, as will be shown later, in the mode of
development. Colour is very general in the Tnrbellarian, though
some are transparent and colourless. The most vivid coloration
characterises some of the marine Polyclads, the Rhabdocceles being
comparatively obscure. The surface is covered with a coating of
fine vibratile cilia, the vibration of which subserves respiration as
well as (in the smaller forms) locomotion. Among the ordinary
cilia are frequently disposed longer whip-like cilia or flagella, like-
wise motile ; and sometimes non-motile (sensory) cilia may occur
here and there.

The Trematodes (Figs. 186, 199, 200, 201), nearly related to the
Turbellarians in internal organisation, resemble them also in

B

ftk&5*9*a!$a < *
?5o2<ii?a*

FIG. 199. Digenetic Trematodes. A, Amphistomum ; B, Homalogaster. g. p. genital
aperture ; m. mouth ; s. posterior sucker ; te. testes ; vit. vitelline glands. (After M. Braun.)

external form, with certain modifications connected with a parasitic
mode of life. As in the latter class, the leaf-shape prevails ; an
elongated form also occurs, though more rarely. The body is
usually thicker and more solid than in most Turbellaria. The
anterior end is distinguished from the posterior by its shape, by
the arrangement of the suckers, and, in many of those Trematodes
that are external parasites, by the presence of eyes. Suckers,
present in the Turbellaria only in some of the Polycladida and a
few Tricladida, are universal in their occurrence. They are always
ventrally placed, their chief function being to fix the parasite to
the surface of its host in such a way as to facilitate the taking in
by the mouth of animal juices and epithelial debris ; their
number and arrangement vary considerably. There are nearly

VOL. I

s

258

ZOOLOGY

SECT.

always present an anterior set of suckers (or a single anterior
sucker surrounding the mouth) and a posterior set, or a single

B

jbh

en-

ff-P

vit.

i/it-

FIG. 200. Monogenetic Trematodes. A, Gyrodactiilus. cil. disc bearing hooks and pro-
cesses at the posterior end ; ent. intestine ; gl. unicellular glands whose ducts open on the
surface about the anterior end ; A 1 , caudal disc of the first embryo ; k~. caudal disc of the
second embryo ; uio. mouth ; oosp. oosperm ; ov. ovary ; p. penis ; ph. pharynx ; te. testes.
B, Polystomuin. en. intestine ; g. p. genital pore; mo. mouth ; ph. pharynx; ov. ovary ; te.
testes; u. uterus; v., v. d. vas deferens ; vit. vitelline glands ; vit. d. vitelline ducts; x.
marks the position of the genito-intestinal canal connecting the oviduct with the intestine.
(From M. Braun.)

large posterior sucker. The arrangement already described as
characterising the Liver-Fluke is that which is typical in the

PHYLUM PLATYHELMINTHES

259

digenetic forms a single anterior and a single posterior sucker ; but
in some of the Digenetica the posterior sucker is wanting. Adhesive
papillae on the dorsal or ventral surface may supplement the
adhesive action of the suckers (Fig. 199, B.). In the Monogenetica
the suckers are often more numerous ; in the family Gyrodactylidce

FIG. 201. Temnocephala minor, general view of the organisation, c. cirrus ; e. s.
ejaculatory sac ; ft. c. genital atrium ; i. intestine ; o. gerinarium ; oo. ootype ; ph. pharynx ;
pr. prostate glands ; r. d. strands of ducts of integumentary glands running forwards to the
tentacles ; r. g, groups of integumentary (rhabdite-forming) glands ; r.v. receptaculum ;
s. sucker ; t. testes ; te. tentacles ; t. s. terminal sacs of excretory system ; v. s. vesicula
seminalis.

(Fig. 200, A) there is no anterior sucker, but at the posterior end
one or two discs armed with hooks; in the Polystomece (Fig. 200,
B) there is also a posterior disc on which are six suckers with
several hooks ; in the Temnocephalea (Fig. 201) there is no
anterior sucker, but the anterior end develops a row (two only in
Scutariella) of adhesive tentacles, while in Actinodactylella

s 2

260

ZOOLOGY

SECT.

(Fig. 202) a series of marginal tentacles are present in addition to
both anterior and posterior suckers. In the Aspidocotylea there is
only a single sucker ; but it extends over nearly the whole of the
ventral surface, and is complicated in structure owing to its cavity
being divided into a number of compartments by a system of
partitions.

Save in two exceptional cases (Tcmnocepkala) vibratile cilia are

s

FIG. 202. Actinodactylella. 1. c. bursa copulatrix ; br. brain ; c. penis ; i. intestine ; or.
ovary; ph. pharynx; r. v. receptaculum ; s. sucker; t., t. testes ; ut. uterus; v. vitelline
glands ; r. s. vesicula seminalis.

not known to occur on the surface in the adult condition ; in some
there are groups of non-motile cilia, situated on little conical
elevations the tactile cones. Pigment is rare in the endoparasitic
Digenetica, save in a few that live in the interior of transparent
animals ; though many appear coloured variously by the internal
organs shining through the translucent body-wall, or are stained

PHYLUM PLATYHELM1NTHES

261

by some fluid derived from their host. Pigment occurs in some
of the ectoparasitic forms.

The relationship of the Cestoda to the Trematoda is, as will be
subsequently shown, fairly close ; but though there are connecting
forms between the two classes, the shape of the average Cestode
is very different from that of such an average Trematode as the
Liver-Fluke. The body of an ordinary Cestode is of great length
sometimes extending even to a good many feet and relatively
narrow, being compressed into the form of a ribbon. One end,
which it will be convenient to designate anterior (though it may

rb

FIG. 203. Tetrarhynchus. n. nervous
system ; r. proboscides ; rs. sheaths,
with their muscles (rb.). (From
Leuckart, after Pintner.)

FIG. 204. Taenia echinococcus

(After Cobbold.)

not, perhaps, correspond to the anterior end in a Trematode or a
Turbellarian), is, in most cases, attached to the host by means of
suckers and hooks placed on a rounded lobe, the head or scale?,
connected with the body by a narrow part or neck. The head is
usually rather radially hhan bilaterally symmetrical, with four
suckers and a circlet of hooks. The hooks, when present, are borne
on a longer or shorter retractile process, the rostellum, the long
axis of which is in line with the long axis of the body. In Bothrio-
cephalus and allied forms a pair of longitudinal grooves take the
place of suckers, and there are no hooks. In many Cestodes para-

262

ZOOLOGY

SECT.

sitic in Fishes the head bears four prominent thin folded flaps
the bothridia, which are exceedingly mobile, and are used more as
creeping organs than as organs of fixation. In relation to each of
these bothridia, which, by coalescence, may appear to be reduced
to two, may be a small sucker of the ordinary kind. In Tctra-
rkynchus (Fig. 203) there are four very long and narrow rostella,
or " proboscides," covered with booklets, and capable of being
retracted into sheaths.

The Cestoda are devoid of mouth, and in most of them the
genital apertures are marginally placed, so that, externally, there
is except in the case of a few in which the genital apertures are
not marginal nothing to distinguish the dorsal surface from the
ventral. The body, or strobila, which is narrower in front than it
becomes further back, is made up throughout its length of a series
of segments, or proglottides, which become larger and more dis-
tinctly marked off from one another as we pass backwards. Tccnia
echinococcus (Fig. 204) is exceptional in possessing only three or
four proglottides. In a few (Ligula and its allies Fig. 205),

FIG. 205. Ligula. (After Leuckhart.)

though the body has the normal elongated ribbon-like form, the
segments are not distinct, and in Caryopliyllwns (Fig. 206),
Amphilina, Gyrocotyle (Amphiptyches Fig. 207), and Archigetes
(Fig. 208) (Monozoa), segmentation is entirely absent, the whole
body in these genera consisting of a single proglottis. The surface
in the Cestodes is devoid of cilia, and there is no pigment.

Integument and Muscular Layers. In the Platyhelminthes
in general there are integumentary layers and underlying layers of
muscle, which are more highly differentiated than in the Ccelen-
terates. But considerable differences exist in this respect between
the members of the three classes. In the Turbellaria (Fig. 209)
there is, as already noticed in the account given of the Planarian,
a distinct epidermis (ep.) in the form of a layer of cells, most of
which are ciliated. A delicate cuticle is usually, though not always,
distinguishable, investing the epidermis externally. In one family
the cuticle is developed, along the margin of the body, into a series

PHYLUM PLATYHELMINTHES

203

of chitinous bristles. Among the ordinary epidermal cells there
are in the Polycladida numerous cells containing short rod-like
bodies the rhabdites (rh.) ; in the other orders of the Turbellaria
these rhabdite-forming cells are sunk deeply within the paren-
chyma, and, in the Rhabdocoela, have very long ducts, formed of
processes of the cells, by means of which the rods, together with a
viscid matter, reach the exterior at certain points of the surface,

r.s

FIG. 206. Caryophyllaeus.

d. e/. vitelline duct ; d. st. vi-
telline glands ; e. excretory
pore ; k. mobile organ ; od.
oviduct ; ov. germarium ; p.
cirrus ; r. s. receptaculum se-
minis ; t . lobes of testes ; v. d.
vas deferens ; v. s. vesicula
seminalis ; w.g.o. female aper-
ture. (After Leuckhart.)

eo

vs

rso

7710

FIG. 207. Gyrocotyle (Amphiptyches).

e. o. excretory opening ; m. o. male opening ;
n. longitudinal nerve ; n'. anterior nerve-
ring ; n. r. posterior nerve-ring ; o. opening
of uterus ; o. ovary ; o'. receptaculum
ovorum ;p. base of cirrus ; r. s. receptaculum
seminis ; r. s. o. opening of vagina ; s.
sucker ; t. testes ; ut. uterus ; v. s. vesicula
seminalis ; yk. vitelline glands. ( After
Spencer.) The end here directed downwards
represents the scolex-end of other Cestodes.

FIG. 208.-
Archigetes

(After Leuckhart )

chiefly around the anterior extremity. The function of these
rhabdites is not in all cases certain ; they have been supposed to
add to the sensitiveness of the parts in which they are situated
after the fashion of hairs or nails, or to have a skeletal function.
In the Rhabdocoela and Tricladida they undoubtedly aid in
adhesion, and probably have the function of assisting in the
entanglement and capture of food. In certain of the Turbellaria

264

ZOOLOGY

SECT.

li.rn.
e.l.m

c.m

stinging capsules occur similar to those of the Ccelenterata, and
transition-forms between rhabdites and stinging capsules occur in

some cases. Adhesive celts with
processes also frequently occur
in the epidermis. Beneath the
epidermis is a basement mem-
brane (b. m.), which in the
Polycladida is of a thick re-
sistent character, and contains
stellate cells.

In a small number of the
Trematoda three layers are
distinguishable in the integu-
ment - - a homogeneous, or
nearly homogeneous, outer
cuticle ; a cellular, or at least,
nucleated, epidermis, and a
basement membrane; but the
cellular epidermal layer is
absent as such in the adult

nr . rif | 1 >i nn i n fVi P mainritv nf
COnOLtlOD in tne majority 01

Trematodes, and there is

i.l.ni

rh.c

d.v.rrL

FIG. 209. Section of the body-wall of a Triclad.
b. m. basement membrane; c. m. circular

muscles; d. v . m dorso-ventrai muscles;

e. 1. m. external longitudinal muscles ; ep.

epidermis; i. 1. m. internal longitudinal Only a honiOgeneOUS,

muscles ; p. parenchyma ; rh. rhabdites ; i i i i i

rh. c. rhabdite-forming ceils. (After jijima.) nucleated outer layer, which

may be the modified epidermis,

or may be the cuticle, with or without a basement-membrane.
Rhabdite-forming, and other unicellular glands derived from the
epidermis, are frequently present beneath the integument.

In the Cestodes, as in the majority of the Trematodes, no
definite epidermis is present. The external layer, sometimes
divided into two or more strata, is of a homogeneous non-cellular
character, and is usually termed cuticle. Beneath this is a thin
layer of parenchyma, the basal membrane. Beneath this again
is a layer of fusiform cells, narrow prolongations of which pass
to the cuticle, into the inner part of which they penetrate and
spread out into a thin layer. These cells are by some authors
regarded as the cells that secrete the cuticle ; but they may be
concerned in the absorption of nutrient matter, and some of them
are undoubtedly of the nature of nerve-cells and have nerve-fibres
connected with them.

The muscular layers of the body-wall vary somewhat in their
arrangement in the different groups of Platyhelminthes. Most
commonly there is an external layer of circularly arranged, and
an internal layer of longitudinally arranged fibres ; frequently
layers of fibres running in a diagonal direction are present also.

Characteristic of the Flat-worms is a peculiar form of connective-
tissue, the parenchyma (Fig. 210) mention of which has already

PHYLUM PLATYHELMINTHES

265

been made in the descriptions of the examples presenting many
varieties, filling up the interstices between the organs and leaving
only, in some instances, very small spaces sometimes regarded as
representing the body-cavity, or cwlome, which we shall meet with
in other groups of worms. Sometimes the parenchyma appears to

FIG. 210. Parenchyma of Distomum. , &. intercellular spaces ; bm. basement membrane ;

c. nuclei ; d. nuclei ; ep. epidermis. (After Braun.)

consist of distinct large cells with greatly vacuolated protoplasm,
with interspaces here and there in which groups of rounded cells
are enclosed. Sometimes the constituent cells run together, and
the parenchyma then appears as a nucleated, finely fibrillated,
vacuolated mass in which the boundaries of the cells are not
recognisable. Pigment occurs in the parenchyma in some Rhab-
doccele Turbellarians and a few Monogenetic Trematodes. In
some Turbellaria species of Convoluta and Vortex the paren-
chyma contains numerous cells enclosing chlorophyll or xantho-
phyll corpuscles ; these are symbiotic unicellular Alga3, similar
in their mode of occurrence to the yellow cells which have been
referred to as found in the Radiolaria. Running through the
body, for the most part in a dorso-ventral direction, are numerous
slender muscular fibres, the fibres of the parenchyma muscle;
many of these become inserted externally into the basement
membrane.

Great differences exist between the various groups of Platy-
helminthes as regards the development of the alimentary
system, differences which are, broadly, to be correlated with
differences in the mode of nutrition. Some of the Flat-worms
-the Turbellaria and some of the Monogenetic Trematodes
procure their food, in the shape of small living animal or vege-

266

ZOOLOGY

SECT.

table organisms, or floating organic debris, by their own active
efforts. Others the Digenetic Trematodes and the Cestodes-
having reached a favourable situation in the interior of their host,
remain relatively or completely passive. An alimentary canal is

ov-

771,

FIG. 211. General plan of the
structure of a Rhabdococle
Turbellarian. b. c. bursa
copulatrix ; en. brain ; e. eye ;
g. gerraarium ; i. intestine ;
In. longitudinal nerve ; m.
mouth ; ph. -pharynx ; r.s.
receptaculum seminis ; s. uni-
cellular glands ; t. testis ; u.
uterus ; v. vitellarium ; vs.
vesicula seminalis ; <5 ejacu-
latcry duct ; (J 9 common
genital aperture. (After Von
Graff.)

- 0V

FIG. 212. General plan of the structure of a Polyclad en. brian ;
e. eye ; i., st. intestine ; In. longitudinal nerve cord ; m. mouth ;
ov. ovary; ph. pharynx; p/ii. sheath of pharynx; t. testes ; K.
uterus vd. vas deferens ; vs. vesicula seminalis ; $ male
aperture ; $ female aperture. (After Von Graff.)

completely absent in the last-named group, nutrition being effected
by the absorption of digested matter from the interior of the
animal in which the Cestode lives. In all the rest of the Platy-
helminthes there is an alimentary canal, which never opens on the

PHYLUM PLATYHELMINTHES

207

In

exterior by an anal aperture. All the Turbellaria (except some
Accela) and Trematoda have an alimentary apparatus consisting
of two well-defined parts a muscular pharynx and an intestine.
The pharynx is usually a rounded muscular bulb, but is sometimes
(some Turbellaria) of a cylindrical shape ; it is usually capable of
aversion and retraction. Actinodadylella (Fig. 202) is exceptional
in having in addition to a large muscular pharynx, an extensile
proboscis with a pin-shaped style, which becomes retracted within
the opening of the mouth. Uni-
cellular glands open into the pharynx
in most cases.

The mouth is always ventral, but
varies greatly in its position on the
ventral surface, being sometimes
central, sometimes situated behind,
sometimes in front of, the middle of
the length of the body. In the
most lowly organised group of Tur-
bellaria (the Accela} the intestine is
represented merely by a vacuolated,
nucleated mass of protoplasm with-
out, or with only an irregular, lumen.
In the others it is sometimes a simple
sac (Rhabdocoele Turbellaria Fig.
211, a few Trematoda), with or
without short lateral diverticula. In
the majority of the Trematodes it
consists of a pair of simple canals ;
but in some, as in the Liver-Fluke,
there is a pair of canals which give
off numerous branches. In the Poly-
cladida (Fig. 212) there is a central
cavity from which numerous branch-
ing canals are given off. In the
Tricladida (Fig. 213) one median
canal passes forwards from the
pharynx, and a pair of canals back- FIG 913
wards from it, all three giving off
branches which again branch. In
some Polydadida there are minute
pores, by means of which certain of
the canals are placed in communica-
tion with the exterior. A number
of unicellular glands, which probably produce a digestive secretion,
open in many Trematodes and Rh'abdocoeles at the junction of
pharynx and intestine.

A bilateral nervous system is developed in all the Platy-

u

od

General plan of the structure of
a Triclad. en. brain ; e. eye ; <j.
germarium : i. median limb of the in-
testine ; \i. right limb ; ig. left limb ;
In. longitudinal nerve-cord ; m. mouth ;
od' oviduct ; ph. pharynx ; t. testes ;
te. tentacles ; v. vitellaria ; vd. vas
deferens ; u. uterus ; <$ ejaculatory
duct ; 9 vagina ; ? common genital
aperture. (After Von Graff.)

268 ZOOLOGY SECT.

helminthes. Its elements are nerve-fibres and nerve-cells. The
nerve-cells, which are usually bipolar, more rarely uni- or multi-
polar, lie in the course of these fibres, with which the substance
of the cells is in continuity. The degree of development of a
central part of the nervous system, or brain, varies in the different
groups ; it is best developed in some Polycladida and some Mono-
genetic Trematodes. It consists of numerous nerve-fibres which
here converge from the various parts of the body and pass across
from one side to the other, together with a central mass of fine
fibrils, and a number of nerve-cells. It is situated in the anterior
portion of the body, almost invariably in front of the mouth.
When the peripheral part of the nervous system is best developed,
as it is in the Polycladida, the Tricladida, and some Trematodes,
there are three pairs of longitudinal nerve-cords running backwards
from the brain throughout the body, connected together by
frequent transverse connecting nerves, or commissures. To these
there are sometimes superadded fine net- works or plexuses of
nerves, situated superficially under the dorsal integument, or
on both dorsal and ventral surfaces. Sometimes nerves run
forwards from the brain as well as backwards. In the Rhabdo-
cosles and some of the Trematodes the whole system is simpler,
and the number of longitudinal cords fewer. In the Cestodes
there are two principal longitudinal trunks which run throughout
the length of the body, and are connected together in the head by
commissures, variously thickened to form ganglia representing the
brain of other Platyhelminthes.

In addition to the tactile cones of some Trematodes and the
sensory cilia of the Turbellaria, already referred to, the sensory
organs of the Platyhelminthes are the eyes and the statocysts.
Eyes occur in the Turbellaria and some Monogenetic Trematodes,
but are wanting in the Digenetic Trematodes and in the Cestodes.
In some of the Polycladida they are extremely numerous, collected
into groups ovor the brain, and frequently arranged also round the
margin of the body. In the Rhabdocoeles and Monogenetic
Trematodes they are much less numerous usually two to four.
In some cases each eye simply consists of a pigment spot ; to
this may be added a refractive body. When most highly de-
veloped the eye is still of very simple structure, consisting of
a cup formed of one or more pigment-cells enclosing refractive
bodies (rods), and having nerve-cells in close relation to it with
processes (nerve-fibres) passing to the brain. The statoci/sts are
sacs containing statoliths of carbonate of lime. The function of
these bodies, which occur only in a small number of the Turbellaria,
is unknown ; there is no sufficient evidence that they are organs of
hearing; it is more likely that they are organs connected with
the maintenance of the equilibrium. Ciliated pits which appear to
be sensory are developed in some Rhabdocoeles in the head region.

PHYLUM PLATYHELMTNTHES

269

The only vascular system present in the Platyhelminthes is the
system of 'water-vessels (protonephridia) which are commonly regarded
as performing an excretory function. The arrangement of these,
the mode of ending internally of the finest branches, and the way in
which the system communicates with the exterior, vary greatly in
the different groups. A series of main longitudinal trunks give
off branches which subdivide to form a system of minute inter-
lacing branches or capillaries. In little spaces at the ends of the
capillaries are a number of highly characteristic structures the
ciliary flames. Each ciliary flame consists of a bundle of vibratile
cilia ; typically each is situated in the interior of a cell the
flame- cell (Fig. 214) terminating one of the capillary branches.
But there are some cases in which there are
several flames in each flame-cell. The finer
branches, and in some cases the larger trunks
also, are intra-cellular, and are to be looked
upon as perforations in linear rows of
elongated cells. In the Cestoda, at least
the larger trunks are inter-cellular, being
lined by an epithelium of small cells. This
system of water-vessels opens on the exterior
in a variety of different ways : sometimes it
opens by a number of minute pores ; some-
times, as in the Liver-Fluke, there is a single
posterior aperture ; frequently there are two.
In the Tricladida there are two longitudinal
canals which open on the exterior through
special branches by a series of pores.
In the Rhabdoccelida there are either two longitudinal main
vessels or a single median one ; the communication with the
exterior in the former case may be by a pair of ventral apertures,
or indirectly through the pharynx ; or there may be a common
short passage in which the two trunks unite, opening by a
posterior median aperture. When a single main trunk is present
it opens at the posterior end of the body. In the Trematodes
there are usually two principal longitudinal trunks, which either
unite behind to open at the posterior end of the body, or
(Monogenetica) remain separate and open independently on the
dorsal surface, each having, where it opens, a contractile excretory
sac. In Temnocephala each dorsally opening excretory sac has
ramifying through its wall which consists mainly of a single large
cell a system of capillary vessels containing ciliary flames.

In the Cestodes there are usually four longitudinal trunks, which
open through a contractile excretory sac at the posterior end of
the body. In many cases it has been shown that the main trunks
communicate with the exterior at intervals by means of fine canals.
The excretory sac is thrown off when the last proglottis becomes

;j. 214. Flame-cell of a
Turbellarian. /. processes ;
k. termination of capillary ;
n. nucleus ; v. vacuoles ;
wf. ciliary flame. (After
Lang.)

270 ZOOLOGY SECT.

separated off and does not in most cases become renewed, though
in at least one species of Tape-worm (Tcenia cucumerina), a new
vesicle is developed again and again at the end of the body as a
fresh segment is thrown off. The main trunks are connected
together by a ring-vessel in the head and in some cases by a
transverse branch in each proglottis, and where the latter originate
from the main trunks are valves formed by folds of the wall of the
vessel. In the posterior region only two of the longitudinal
trunks (one on each side) may be retained.

The sexes are united in all the Platyhelminthes with only
one or two exceptions, and the reproductive organs are
sometimes somewhat complicated presenting a remarkable ad-
vance on those of the Ccelenterata. The male part of the
apparatus consists oftestes, with their ducts, ihevasadeferentia, often
with a contractile terminal enlargement or vesicula seminalis, a
cirrus 1 or a penis, and often prostate or granule- glands. The female
part comprises ovary or ovaries, receptaculum seminis, oviduct, uterus,
an ootype, often a bursa copulatrix, shell- glands, vitelline or yolk-
glands, and cement- glands. In most, though not in all, there is a
single or paired germarium (ovary), in which the ova are formed,
and a set of vitellaria or vitelline glands, producing material which
surrounds each of the mature impregnated ova before it becomes
enclosed in its shell. In some, on the other hand, ova and vitelline
matter are formed in the same organ the germ-vitellarium. The
shell-glands are so named because they are usually supposed to
secrete the chitinoid substance of the egg-shells; but the
share which they take in this process is uncertain. The
cement- glands secrete a viscid material for causing the eggs to
adhere together, enclosing them in a cocoon or fastening them to
some foreign body. The oviduct is the passage by which the ova
reach the exterior from the ovary ; but an enlarged part of this
passage, into which ducts of the shell glands open, is distinguish-
able as the ootype, while a terminal part, leading to the female
aperture may be modified as a vagina. In some cases (Heterocoty-
lean Trematodes) there is a vagina or a pair of vaginoe in the shape
of a passage, or a pair of passages, distinct from the oviduct and
opening independently on the exterior. A uterus in the form of an
enlarged part of the oviduct or of an outgrowth from the latter or
from the atrium is very usually developed for the reception of
the completed eggs. A special sac or Itursa copulatrix, lined with
spines, acts as the female copulatory organ. A sac, the reccptaculum;
opening into the oviduct or into the atrium (Figs. 201, 202, r.v.),
may serve as a reservoir for the semen received in copulation
or for the vitelline matter or yolk, or for surplus reproductive

1 The term cirrus is here restricted to cases in which the terminal part of the
male duct, often provided with spines and other chitinous structures, is
involuted within a sheath when at rest.

v PHYLUM PLATYHELMINTHES 271

material. Male and female ducts sometimes have separate and
independent openings ; but very commonly there is a common
chamber or genital atrium into which both lead, opening on the
exterior by a single aperture.

In the Poly clad Turbellaria (Fig. 212) the testcs are numerous,
and there are a corresponding number of fine tubes which
combine to form the two vasa deferentia, leading to the male
aperture with its penis. The latter is sometimes multiple. The
ovaries consist of numerous small rounded masses of cells, and
there are no separate yolk-glands. Numerous narrow oviducts
lead from the ovaries, and unite to form larger ducts ; these, in
turn, open into elongated uteri, in which numerous eggs collect.
The uteri open into a median egg-dud, with which the ducts of
the shell glands communicate (ootype), and in which the eggs
receive their chitinoid investment. This leads to the female
aperture, a part of it being, in some cases, surrounded by a
muscular sheath to form a bursa ccpulatrix.

In many cases the egg-duct gives off posteriorly a narrow duct
which usually terminates behind in a vesicle known as the accessory
sac or reccptaculum seminis, which may be double. In a few Polyclads
this duct opens on the exterior on the ventral surface some
distance behind the main female aperture, in one instance on the
dorsal surface. A gcnito -intestinal canal connecting this duct with
one of the intestinal caeca has been found in one Polyclad. In
most cases male and female apertures are distinct from one
another, the former being situated in front of the latter. But
sometimes, though rarely, both lead into a common chamber or
atrium with a single opening on the exterior.

In the Tricladida (Fig. 213) there are also numerous testes, but
the fine tubes connecting them with the two vasa deferentia are
absent. There are two germaria, situated far forwards, and
numerous yolk-glands. Two oviducts, into which the yolk is dis-
charged from the yolk-glands by a series of lateral apertures, lead
from the ovaries to unite in a median ootypc or vagina, receiving
the ducts of glands which may secrete the substance of the cocoon.
The condition is thus intermediate between that observable in
most of the Rhabdocceles and that which characterises the Poly-
clads. Though germaria and vitellaria are separate, they have a
common duct, and might be regarded as distinct lobes of one
germo-vitellarium. A uterus is present, formed as an outgrowth
of the vagina or of the atrium, or as an independent sac or pair of
sacs opening independently on the exterior. There may be a
receptaculum seminis, and in some there is a duct of communica-
tion between this and the intestine (genito-intestinal canal). A
common genital atrium with a single external aperture receives
the ducts of both sexes.

In the Rhabdocoeles (Figs. 211 and 215) there are usually only

272

ZOOLOGY

SECT.

Ut

two compact testes and two vasa deferentia leading to the unpaired
male aperture at the extremity of the cirrus. The prostrate or
granule glands a set of unicellular glands, which secrete round,

bright granules destined to mix
with the sperms are specially
well developed in the Rhabdo-
cceles, and are present in some
other Turbellaria and in certain
T re mat odes. Ovaries (germ-
vitellaria) alone occur in some,
separate germaria and vitellaria
in others ; there are either
two germaria or one only. A
receptaculum seminis may be
present as a swelling or diver-
ticulum of the main female duct,
or of the atrium. The terminal
part of this duct may form a
muscular vagina, or there may be
a muscular bursa copulatrix de-
veloped from the wall of the
atrium. A uterus is present in
most cases as an outgrowth from
the wall of the atrium. Male
and female ducts have a common
chamber or genital atrium with a
single external opening.

In the Acoela there are in
nearly all cases separate male
and female apertures. The two
testes are divided into numerous small lobes. There are no
vitellaria in most cases the two ovaries producing large ova
containing abundant food-yolk. Oviducts are absent in most
cases. Into the main female genital passage opens a peculiar
single or double sac or bursa, usually provided with chitinous
structures.

The Trematodes nearly all have two testes, usually compact,
sometimes branched ; in a few instances there are four. The vasa
deferentia unite into a median duct, which is dilated at the base
of the cirrus to form a vesicula seminalis. There is a single oval
or branched germarium, and two sets of vitelline glands. A canal
termed Laurers canal in some Malacocoty leans, such as some
species of Distomum, leads from the exterior to the oviduct or
vitelline duct. This may be replaced by a receptaculum
seminis, or both structures may co-exist. The distal part of
the oviduct is enlarged to act as a uterus. In the Hetero-
cotylea there is a vagina, which is sometimes paired, opening

nt

FIG. 215. Reproductive organs of IVEeso-
stomum Ehrenbergii d<j. duct of
vitelline glands ; do. vitelline glands ; go.
common reproductive aperture ; ov. ovary;
p. ciiTus ; rs receptaculum seminis ; s.
pharynx ; t., t. testes ; ut. uterus ; rd.
vas deferens. (From Glaus, after von
Graff and Schneider.)

v PHYLUM PLATYHELMINTHES 273

on the surface independently of the uterus : internally it com-
municates with the oviduct through the main vitelline duct.
In some of the Heterocotylea there is a genito-intestinal canal
occupying a corresponding position to Laurer's canal, but opening
into the intestine. In the Aspidocotylca this is replaced by a
stalked yolk-receptacle. There is nearly always a genital atrium
common to the ducts of both sexes.

In the Temnocephalea (Figs. 201, 202) there is a genital atrium
and a single genital aperture. Thare are two pairs of compact
testes ; the right and left vasa deferentia unite in a vesicula semi-
nalis, and granule or prostate glands are well developed. The
cirrus has a chitinous tube and a variety of eversible spines. There
is a single compact ovary ; the oviduct has connected with it a
large receptaculum, and dilates posteriorly to form an ootype into
which the shell-glands open. Actinodactylclla (Fig. 202) alone
has a bursa copulatrix (b. c.}.

In the ordinary Cestodes each segment or proglottis contains a
set of reproductive organs similar to those of a Trernatode. There
may be a single genital aperture leading into a genital cloaca, into
which both male and female ducts open ; or the male and female
apertures may be distinct. The testis is divided into numerous
minute lobes, from which proceed a number of fine canals joining
together to form the vas deferens, at the extremity of which is the
chitinous cirrus. There are two germaria, and either a single
vitelline gland, or two. The oviduct has its origin in a sort of
isthmus connecting the two germaria. It receives a narrow
fertilising duct from the receptaculum seminis and the vitelline
ducts and becomes surrounded by a rounded mass of shell-glands
to form the ootype, which is not definitely enlarged. Further
forward it gives off the uterus. The latter is at first a simple
cylindrical outgrowth from the oviduct, but it usually becomes
large and may be extensively ramified. It has no external
opening in most instances, so that the eggs only escape from it by
the breaking down of the proglottis or by dehiscence. But in
some (c. ()., Dibnthriocephal'iis), it has an independent exter-
nal opening. The female aperture leads into a narrow canal
-the vagina which ends in a receptaculum seminis from which
the narrow fertilising duct conveys the sperms to the oviduct.

The development of some of the Platyhelminthes (Rhabdoccela,
Monogenetic Trematodes) is direct i.e., not complicated by the
occurrence of a metamorphosis ; in the Digenetic Trematodes, the
Cestodes, and some of the Planarians a metamorphosis occurs.

The eggs of the Polyclads, each of which consists merely of the
fertilised ovum (oosperm) usually enclosed in an egg-shell, are,
in most instances, laid in large numbers embedded in a plate of
slimy secretion. The ovum (Fig. 216) divides first into two
equal parts, then into four. From each of these four cells is then

VOL. i T

274

ZOOLOGY

SECT.

separated off a small cell. The embryo at this stage consists of
eight cells, four large the megameres, and four small the
micromeres. The four micromeres increase rapidly by division, and
extend over the embryo, forming a layer, the ectoderm, completely
covering it in all parts except for a median fissure, the blastopore,
which runs along what is destined to become the middle ventral
line : this soon closes up. The ectoderm cells develop a
coating of cilia. The four megameres have previously given off

mes

mes

en

en

u.en

Fi<;. 215. Early stages in the development of a Polyclad. A, stage of four cells, of which
those lettered v and h correspond to the anterior and posterior portions of the body ; B to D,
later stages ; B and C, seen from above ; D, from the side ; E, earlier, and F, later stage of
epibolic gastrula, lateral view. ec. ectoderm ; en. endoderm ; rues, mesoderm ; o. en. and u. en.
upper and lower endoderm. (From Korschelt and Holder, after Lang.)

four more small cells or micromeres, which increase in number by
division, and eventually form the middle layer or mesoderm of the
embryo. These extend over the surface below the ectoderm as
four mesodermal bands which subsequently fuse together to form
a continuous layer. The megameres give off a number of additional
micromeres which form the endoderm layer, giving rise to the
epithelium of the intestine ; finally the megameres become disinte-
grated, and their substance goes to nourish the cells of the develop-
ing embryo. The process by which the germinal layers have

PHYLUM PLATYHELMINTHES

275

become formed is, as in the Ctenophora (p. 218), a process of
cpibolic gastrulation. The brain is developed from a pair of
thickenings of the ectoderm : these unite into a common mass from
which the longitudinal nerves are formed as backward outgrowths.
The mouth is developed as an ingrowth from the ectoderm in the
position of the former blastopore, the involuted epidermal cells
giving rise to the epithelium of the pharnyx and pharyngeal sac,
while the muscular tissues of the wall of the pharnyx are formed
from surrounding mesodermal elements. The intestine is at
first simple in form ; the caeca are developed as a result of
the formation of vertical mesodermal septa which, growing inwards,
constrict the enteric wall and the enclosed mass of nutrient
material. The embryo, which has assumed an ellipsoidal shape,
becomes flattened in the dorso-ventral direction, and, having
absorbed the greater part of the nutrient matter, escapes by
rupture of the egg-shell.

In many cases the embryo develops into a characteristic larval
form, such as that known as Mullers larva (Fig. 217). It assumes

FIG. 217. Miiller's larva. A, longitudinal section ; B, lateral view. cc. ectoderai ; en.
endoderm ; </. brain; hd. enterou ; o. mouth; ph. pharynx; pt. pharyngeal pouch; sn
sucker. (From Lang.)

an oval shape, with a series of eight elongated processes, covered
with long cilia, and connected together by a ciliated band. There
are eye-spots at the anterior end and a mouth in the middle of
the ventral surface. The form of the body alters after a time,
becoming gradually longer and flatter, and the arms are
gradually reduced in length, till, eventually, they become completely
absorbed.

The development of the Triclads is very different from that of
the Polyclads. Each egg-capsule or cocoon encloses a number of
oosperms and a quantity of yolk-cells. After segmentation a
blastoderm is formed composed of a rounded mass of cells
surrounded by the yolk-material, the cells of which become moi*e
or less fused. Around the periphery of the blastoderm a layer
of cells form a thin membrane the ectoderm. About the
middle appears a rounded group of cells which passes to the
periphery and becomes connected with the ectoderm ; a cavity
is formed in its interior and it becomes converted into the

T 2

276

ZOOLOGY

SECT.

embryonal pharynx. Meanwhile a group of four cells enclosing a
cavity are modified to form the foundation of the endoderm,
and. increasing in number give rise to the embryonal intestine,
into which the embryonal pharynx soon opens, the latter opening
on the exterior by a mouth aperture.

The embryonal pharynx (Fig. 218, ph f ) has the function of
swallowing the yolk-matter with which the embryonal intestine
becomes greatly distended. At a subsequent stage the embryonal
pharynx and intestine are aborted, and the former comes to be
represented merely by a mass of cells. In this a cavity arises
-the cavity of the permanent pharynx. The permanent intestine

FIG. 218. Sections through embryos of Dendrcccelum la cteum (somewhat diagrammatic).
dz, yolk-cells ; cc, ectoderm ; en, endoderm ; pit', embryonal pharynx ; ph", permanent
pharynx : wz, wandering cells. (From Korschelt and Heider, after Hallez.)

becomes formed and the cavity of the pharynx opens into its lumen.
Subsequently the permanent mouth makes its appearance. The
brain is formed in the thickness of the blastoderm, and thus
appears to be of mesodermal origin, not of ectodermal, as in
the Polyclads.

In some Rhabdocceles (certain species of Mesostoma) two distinct
kinds of eggs are formed summer and winter eggs. The oosperms
are deposited singly, and each, together with a mass of yolk-cells, is
enclosed in a chitinous, usually stalked, shell. Segmentation
takes place very much as in the Polyclads. No embryonal
pharynx is formed, the permanent pharynx performing the function
of swallowing the yolk-material : it appears to be of endodermal
derivation. The intestine arises from a group of cells the

v PHYLUM PLATYHELMINTHES 277

primitive endoderm among which a cavity appears. But in
some Rhabdocceles no definite intestinal epithelium is developed,
and the syncytial mass which represents it is only to be dis-
tinguished from the surrounding mesodermal syncytium by its
enclosing the remains of the yolk. No metamorphosis is known
to occur.

An account has already been given (p. 244, Fig. 190) of the
development and metamorphosis of the Liver-Fluke (Fasciola
lupatica) which may be looked upon as typical of the Digenetic
Trematodes in general. There is thus to be recognised in the
Digenetic Trematodes an alternation of generations comparable to
that which has been described as so general in the Coelenterata.
In the Trematoda, however, it is to be observed, it is an alterna-
tion of a sexual, not with an asexual, but with a parfchenogenetic
generation (the sporocyst), the ova of which develop into a second
parthenogenetic generation (the redire) ; and these finally produce
larvaa (the cercarias) capable of developing into the sexually
mature form. The term heterogeny is applied to a life-history
of this kind, in which several distinct generations succeed one
another in a regular series.

In some of the Distomidae the eggs, instead of becoming free as
in the case of the Liver-Fluke, are taken directly into the digestive
canal of the intermediate host, and there hatched out. The
sporocyst stage may take the form of a branching tube in the
interior of which cercarire are developed the redia stage being
omitted. Sometimes the sporocyst becomes directly developed
into a redia instead of giving rise to a generation of the latter by
such a process of internal development as that described in the
case of the Liver-Fluke. The cercariaB in most Digenetic Trema-
todes only develop further if they succeed in establishing themselves
in a second intermediate host instead of merely becoming encysted
on the surface of herbage, as in the case of the Liver-Fluke. The
cercariaa of different Trematodes differ greatly, particularly with
regard to the nature of the tail. In some forms the cercaria is
tail-less : such cercaria3 do not become free, but are taken directly

-with the intermediate host in which they have been developed-
into the digestive canal of the final host.

Among the Heterocotylea, Gyrodactylus (Fig. 200, A) is vivi-
parous, and the remarkable phenomenon is observed that the
embryo (ti), while still within the body of the parent worm,
develops another embryo (A 2 ) in its interior, and this again
develops a third. The rest of the Heterocotylea deposit eggs
each of which, within a chitinous shell, contains an oosperm and
a number of yolk-cells. Usually there is a stalk and often an
operculum. In general the development appears to be direct ;
but Polystomnm passes through a larval stage with five rows of
cilia, and in Dipluzoon paradoxum, a parasite on the gills of certain

278

ZOOLOGY

SECT.

fresh-water fishes, in which there is also a ciliated larval stage, the
young animals do not become sexually mature until two of them
have permanently united with one another.

Temnoccphala produces relatively large eggs, stalked or sessile,
with a thick chitinous shell, enclosing a single oosperm and a mass
of yolk-cells, which later become fused into a continuous mass.
Segmentation results in the formation of an irregular, massive
blastoderm composed of cells of several sizes. In this collects
a rounded group of larger cells, in the middle of which a space
appears (Figs. 219, 220). The space (endoccele) increases greatly
in size, the boundary cells becoming spread out to form a thin
layer, and approaches the periphery of the egg. A part of the

.0 ..'-'

FIG. 219. Section through the blastoderm of Temnocephala, showing early stage of

cndoccele (en).

endocoale becomes subsequently rounded off to form the internal
lining membrane of the pharynx, the cavity of which remains cut
off from the exterior by a partition, which is the outer part of the
wall of the endocoele, until the young worm is ready to leave the
egg. A short prolongation backwards from this cavity ends blindly
in a mass composed mainly of yolk, still containing degenerate
nuclei and cells which have wandered into it from the blastoderm.
Only at a late stage, when the yolk has become taken up, is an
arrangement of cells recognisable in the form of an intestinal
epithelium (endoderm) enclosing a lumen (intestinal lumen). An
ectoderm likewise does not appear as an embryonic layer, the
epidermis only appearing late, and extending over the surface
evidently by modification in situ of cells that have become

PHYLUM PLATYHELM1NTHES

270

en-

separated from the main body of the blastoderm. The rudiments
of the brain are formed in the blastoderm near the wall of the
endocoele, and thus have no connection with an ectoderm. The
excretory sacs and main vessels are formed from a small number
of large cells connected with the wall of the endocoele : subse-
quently these rudiments shift their position to the dorsal surface
on which the sacs form their
permanent apertures pierc-
ing the epidermis. There
is no metamorphosis of any
kind all the organs, includ-
ing even the male part of
the reproductive apparatus,
being well advanced towards
full development before the
young animal leaves the
egg.

The egg of a Cestode is
similar in essential respects
to that of a Trematode :
there is a tough, chitinoid
membrane or egg-shell,
which encloses not only the
ovum but a number of yolk-
cells. The result of seg-
mentation is the formation
of a superficial layer of cells
(ectoderm) and a central
mass, all enclosed in a mem-
brane composed of a single
layer of cells thrown off
when the embryo escapes
from the egg. The ecto-
dermal cells become ciliated,
so far as is known, only
in Botlirioccplialus ; in the
others they are thrown off
or ultimately absorbed with-
out developing cilia. The central mass of cells alone forms the
embryo. The embryo, while still consisting of a small number of
cells, develops a series of six chitinous hooks. These early changes
all take place in the majority of Cestodes while the egg is still in
the uterus of one of the most posterior of the proglottides of the
parent worm. When the proglottis in question becomes separated
off, and has passed out from the body of the final host, the eggs
are discharged.

In order that development may proceed further, the embryo

FIG. 220. Longitudinal section through the entire
egg of Temnocephala with the shell re-
moved, showing blastoderm with developing
endocoale {en).

280

ZOOLOGY

SECT.

must in most cases reach the interior of a second or intermediate
host. This is a passive migration, since the embryo of the Cestode is
still confined within the egg-shell, and the transference has to
take place in the water or food. The digestive fluids of this inter-
mediate host dissolve the egg-shell and set free the contained six-
hooked or hexacanth cmlri/o, which bores its way by means of its
hooks to some part of the body in which it is destined to pass
through the next phase in its life- history, and there becomes
encysted.

The phase which follows presents two main varieties. In
cases in which the intermediate host is an invertebrate animal
the hooked embryo develops into a form to which the name of

cysticercoid is given ; when, on
the other hand, the intermediate
host is a vertebrate, the form
assumed is nearly always that
termed ei/sticcrcus, or bladder-
worm. The cysticercoid form
(Figs. 221 and 222) is to be re-
garded as the more primitive
and less modified. Cysticercoids
of various tape- worms occur in
a great variety of different in-
vertebrates e.g., Insects of all

kinds, Water-fleas, Centipedes,
Earthworms. The hooked em-
bryo loses its hooks and de-
velops into the cysticercoid in
some part of the invertebrate
intermediate host. The cysti-
cercoid consists of three parts-
a tape- worm head or scolcx with
the hooks and suckers of the
mature worm, a so-called body,
and a caudal vesicle. Some-
times there is a tail recalling to some extent the tail of a cercaria.
Sometimes the caudal vesicle is absent : when present, either from
the first, or as a result of later changes, it encloses the head as
well as the body after the manner of a cyst. While undergoing
these changes the cysticercoid is usually enclosed in an adventitious
cyst formed for it by the tissues of its host, but it often lies free
in the body-cavity. The transference to the final host is effected
by the intermediate host, or the part of it containing the cysti-
cercoid, being taken into the alimentary canal of the final host.
Sometimes, if the intermediate host is a relatively small animal,
such as a water-flea, this may take place " accidentally " ; in other
cases the invertebrate intermediate host actually forms the food

eac

FIG. 221. A Cysticercoid (Polycercus) with
the head and rostellum enclosed l>y the
cnudal vesicle, a. aperture through which
evagination takes place ; M. body ; r.
cavity of cyst ; caud. caudal vesicle ; ex.
aperture of excretory system ; ros. rostel-
lum ; s. sucker. (After Haswell and Hill.)

PHYLUM PLATYHELMINTHES

281

ros

of the final host. Thus a cysticercoid having as an intermediate
host an Earthworm is taken with the latter into the alimentary
canal of a Sea-Gull its final host. In this way the cj^sticercoid
is set free in the alimentary canal of the final host, the head
becomes pushed out from the enclosing caudal vesicle and body
(probably owing to the stimulus of the higher temperature), so
that the suckers and hooks come into play and attach the young
tape-worm to the wall of the alimentary canal.

The cysticcrc'us or bladder-worm differs from the cysticercoid
mainly in its much greater size and in the development of a
relatively large caudal vesicle or
caudal bladder. When the hooked
embryo has reached that part of
the vertebrate host in which it
is destined to develop into the
cysticercus it undergoes a remark-
able change ; it becomes greatly
enlarged, and a cavity, filled with
fiu id or with a very loose form
of connective-tissue, appears in
its interior, so that it assumes
the appearance of a relatively
large bladder. On one side of
this bladder appears a small iii-
vagination with a cavity opening
freely on the exterior. On the
bottom of this is formed an
elevation projecting into its in-
terior; this is the rudiment of
the rostellum on which the hooks
are borne ; at its base, on the
inner surface of the side walls
of the invagination, appear the
suckers. When inverted this in-
vagination corresponds closely
with the head and body of the cysticercoid ; the bladder corre-
sponds to the caudal vesicle. Thus the chief difference between
a cysticercus and a cysticercoid is that in the former the caudal
vesicle is relatively very large and that the order of development
of the parts is somewhat modified.

A very small number both of cysticercoids and cysticerci
multiply by proliferation by the formation of more than one
tape-worm head from one embryo. In the few instances in which
this occurs among the cysticercoids the hooked embryo gives rise,
not directly to a cysticercoid, but to a mass of cells from which
are given off a number of buds, each developing into a cysticercoid
with the three parts already described. One such form occurs

ccuui

Fin. -2-2-2. A Cysticercoid with the rustd-
lum yvaginated. ro.s". rcstellum ; s. , s.
suckers ; r<nn(. caudal vesicle. (After
Ilaswell and Hill.)

282

ZOOLOGY

SECT.

in certain Earthworms, another in a Myriapod (Glomcris
limbatus).

Tccnia ccenurus of the Dog has a bladder-worm stage in the
Sheep and Rabbit which gives rise to several tape-worm heads,

FIG. 223. Cyst of Tccnia ecliinococcus with the developing daughter-cysts and scolices.

(After Leuckart.)

and the same holds good of Tccnia serialis from the Fox. But
the best known instance of multiple production of scolices in a
cysticercus is Tccnia ecliinococcus well known as cause of the
disease termed hi/datids, common in Man and in various domestic
animals. In this case the hooked embryo develops into a large

mother- cyst, from the interior
of which daughter- cysts are
budded off (Fig. 223). Event-
ually from the walls of these
daughter-cysts there are
formed numerous tape- worm
heads, or scolices (Figs. 224

FIG. 224. Scolices of T. echinococcus.

(After Cobbold.)

FIG. 225. Separate scolex of
T. echinococcus. (After

and 225), which, when fully formed, assume the appearance of
cysticercoids without the caudal vesicle. These are readily de-
tached, and, should the organ in which the cyst has been
developed be devoured by a Dog which is the final host of the
parasite some of these scolices become attached to the wall

PHYLUM PLATYHELMINTHES

283

m

1

77I/

TTL

of the intestine and develop into the adult Tccnia ccTiinococcus
-which are very small as compared with the size of the cyst and
as compared with other tape-worms. The eggs, passing out with
the faaces of the Dog, may be taken into the digestive canal of
Man or of one of the domestic animals,
and the minute embryos escaping, reach
some organ, such as the liver or lung, in
which they are capable of developing into
a comparatively enormous cyst.

Asexual reproduction also occurs in
some Platyhelminthes. In some Rhabdo-
ccele Turbellaria (Microstomum) a process
of budding (Fig. 226) results in the forma-
tion of strings of sexual individuals which
may eventually separate ; the new bud is
always formed from the posterior end of
the last individual of the string.

The sporocyst stage in the Trematodes
may, as already mentioned, multiply by
budding or fission. The formation of new
proglottides in the Tape-worm may be
looked upon either simply as growth ac-
companied by segmentation, or as asexual
multiplication, according as we regard the
proglottides as segments of a simple animal
or as zooids of a colony, There is this
essential difference between the formation
of proglottides and the asexual multipli-
cation by budding in Microstomum, that in
the former case the proglottides, when they
have been formed by segmentation of the
undivided part behind the head, do not
in turn give rise by budding to new pro-
glottides. Spontaneous transverse fission
has been observed in certain Tricladida, and is often followed by
the regeneration of the lost portion.

6. DISTRIBUTION, MODE OF OCCURRENCE, AND MUTUAL

RELATIONSHIPS.

Of all the great groups of the animal kingdom above the
Protozoa the Platyhelminthes are the widest in their distribution.
Members of the phylum occur on land, in fresh-water down to the
bottom of some of the deepest lakes, on the sea-shore, in the deep
sea, and on the surface of the ocean ; and parasitic Flat- worms live,
in one phase or another, in animals of nearly every class of the
Metazoa.

i

Fir:. 226. Process of budding
in Microstomum. <:, c'.
ciliated groove ; c. cye-spi >t ;
j. intestine; m.,m',m.",m.'."
mouth. (After Von Graff.)

284 ZOOLOGY SECT.

As regards their mode of life, they present almost every possible
gradation between free-living forms which procure their food con-
sisting of minute animals and plants by their own exertions, and
forms that are only capable of living in a special part of the
interior of a certain other animal, and are quite incapable of pro-
curing food for themselves, living by the passive absorption of the
juices of their host or of its digested food. The Turbellaria are
for the most part free living, and their food consists of small
Crustacea or the larvae of larger forms, Insect larvae, Water-mites.
Rotifers, small Worms, and the like ; or sometimes of Diatoms and
minute Alga? of various kinds. Some, however, live a life of true
parasitism. Such are certain Rhabdocceles which are parasitic in
the alimentary canal of various Holothurians and Gephyreans (vide
Sections IX. and X.). In these there is correlated with the in-
active mode of life a tendency to degradation of structure, a degrada-
tion which is characteristic of parasites in general : the pharnyx
is reduced in size as compared with that of non-parasitic allied
forms, not being required for the capture and swallowing of living
prey; and the eyes, useless to an animal living in complete dark-
ness, are absent. Some of the Turbellaria, though not parasitic
in the strict sense, live in a state of commensalism with another,
larger animal : that is to say, are more or less constantly associated
with it, living on its surface or in one of its cavities that open
freely on the exterior, and often sharing its food. An example of
this mode of life is the Triclad Bdelloura, which lives on the surface
of the King-Crab (Limulus).

While a free existence is the rule in the Turbellaria, true
parasitism is the rule in the Trematodes, and is universal in the
Cestodes. The majority of the Monogenetic Trematodes are ex-
ternal parasites, living on a part of the outer surface of a
larger animal : and feeding on mucus and other secretions of the
integument. Many are parasites on the gills of Fishes. A few,
however, inhabit the interior of various organs, and are true
internal parasites : one, for example (Polystomum*), lives in the
urinary bladder of the Frog ; another (Aspidogastcr) lives in the
pericardial cavity of a Fresh- water Mussel. At least one family of
Trematodes (the Temnoccphalea) are not parasites at all in the
strict sense of the term, living on the surface of the " host "
animal, depositing their eggs there, and being carried about by it,
but subsisting on minute living animals captured in the water.

The Digenetic Trematodes are all internal parasites, and in the
adult condition inhabit, in nearly all cases, the alimentary canal,
liver, or lungs of some vertebrate animal, swallowing the
digested food or various secretions of their host. But, as mentioned
before in the account given of their development, they are internal
parasites, not only in the adult condition, but throughout the
greater part of their life. After a short period of freedom as

v PHYLUM PLATYHELMINTHES 285

ciliated larvre, they again enter into a state of parasitism as sporo-
cysts or redise in a second host ; arid, after a second free interval as
cercarise, may enter the body of a third host to become encysted.
The second host is, very generally, a Mollusc, and the cercaria
may become encysted in the same animal.

/ i/

The Cestodes are, of all the Platyhelminthes., those that are most
modified in accordance with the condition of internal parasitism
in which they remain throughout life. The adult Cestode is
almost always an inhabitant of the alimentary canal of a verte-
brate. The intermediate host is frequently also a vertebrate-
commonly of a kind which is liable to become the prey of the
final host. In the case of Tcvnict cmssicollis of the intestine of
the domestic Cat, for example, the cysticercus-stage occurs in the
livers of Rats and Mice ; the cysticercus of Tccnia serrata of the
Dog is found in Hares and Rabbits. But in many cases the inter-
mediate host is an invertebrate. In either case the passage from
one host to another is a passive translation, not an active
migration as in the Trematodes.

A few human parasites belong to the Trematoda, but none
that are of very common occurrence among Europeans. Fasciola
hepatica has occasionally been found in the human liver; Dis-
tomvjin rathousii is a common intestinal parasite in China ;
Opisthorchis sincnsis occurs in the liver of Man in China and Japan ;
Dicrcelium lanceolatnm and various other species of the genus
occasionally occur in the human subject. Schistosomum kccma-
tobium and S.japonicum, which differ from most other Trematodes
in being unisexual, are found in the human portal system of veins
in various parts of Africa, in Arabia, the Philippines, and Japan.
Eggs with contained Iarva3 are voided with the urine, and if they
reach water, the larvaa may gain access to the human host by
being swallowed in drinking water or by perforating the skin.

The commonest human Cestode parasites among Europeans are
Tconia solium and T. saginata (otherwise called T. mediocanellata).
The cysticercus stage of T, solium (Cysticercus cellulose) occurs,
as already stated, chiefly in the muscles of the Pig, that of
T. sayinata in the muscles of the Ox ; and the relative prevalence
in different countries of these two Tape- Worms varies with the
habits of the people with regard to flesh-eating : where more
swine's flesh is eaten in an imperfectly cooked state Tcvnia solium
is the more prevalent ; where more beef, T. saginata. Botlirio-
cepludus latus, a very large tape-worm without hooks, and with a
pair of longitudinal sucking-grooves on the head instead of ordinary
suckers, is a common human parasite in eastern countries. Its
cysticercus, which is elongated and solid, occurs in the Pike and
certain other fresh-water Fishes.

Of all the Cestode parasites of man, however, the most formid-
able is one which occurs in the human body, not in the sexually

286 ZOOLOGY SECT.

mature or strobila condition, but in that of the cysticercus. This
is Tcenia echinococcus, the presence of which produces what is termed
hydatid disease (p. 282). The adult Tccnia ecliino coccus is a very
small tape-worm with only three or four proglottides, occurring in
the intestine of the domestic Dog. The eggs passing out with a
liberated proglottis in the faeces, may reach the alimentary canal of
Man uninjured in drinking-water, on the surface of salad vegetables,
and the like ; and, the egg-shells becoming dissolved, the contained
hooked embryos bore their way to the liver or the lungs or some
other organ. Arrived at its final destination, the embryo develops
into a cyst, which may become of enormous size. In the interior
of the primary or mother-cyst are developed a number of secondary
or daughter-cysts, and from the walls of these, both internally and
externally, are formed very numerous scolices in the way already
described (p. 282). Hydatid cysts are very common in some
domestic animals (Oxen. Sheep), as well as in Man. Various other
Cestodes occur in the bladder-worm stage occasionally in Man
e.g., the Cysticercus celluloses of Tcenia, solium.

The most primitive of the Platyhelminthes are, without doubt,
some of the simplest Turbellaria, and it is among these that we
must look for the nearest existing relatives to the Ccelenterata.
In none, however, is the relationship very close. Cceloplana
and Ctenoplana (p. 225) are probably rather to be looked upon
as Ctenophores specially modified in accordance with a creeping
mode of progression than as intermediate forms between Cteno-
phores and Turbellaria. The relationship with the Coelenterata is
shown, perhaps, most strikingly when we take into account the
development of the Turbellaria, in the earlier stages of which there
is to be recognised a marked tendency towards a radial symmetry.
In their development the Turbellaria, that is to say the Planarians,
show some special points of resemblance to the Ctenophora ; the
ectoderm cells are formed and spread over the rest in a similar
way, and the bands of cilia have a disposition and mode of move-
ment that strongly bring to mind the ciliary swimming plates of
the Ctenophora. But though there is much to be said in favour
of the view that the Turbellaria and the Ctenophora were derived
from a common, not very distant stock, the latter are too specially
modified to be looked upon as the direct ancestors of the former.

The connection between the Turbellaria and the Monogenetic
Trematodes is very close so much so that it is difficult to give
any characters of universal occurrence distinguishing all the
members of the two classes. The Trematodes are, in fact, to be
looked upon as Turbellaria some of whose external characteristics
-and, in the case of the Digenetica, whose life-history have been
specially modified in accordance with a parasitic mode of life. It
is not unlikely that the Trematodes maybe a polypliyletic group-

PHYLUM PLATYHELMINTHES

287

i.e., that different families may have become developed from
different families of Turbellaria altogether independently, some
of them appearing to be nearer the Rhabdocoeles, others nearer
the Polyclads, and others, again, nearer the Triclads, in the majority
of their characters.

The remarkable life-history of the Digenetic Trematodes is, as
already pointed out, to be looked upon as a special form of alter-
nation of generations the alternation of a sexual with a pcedo-
gcnctic and partheno genetic generation (heterogeny). The sporocyst
and redia are to be regarded as intercalated stages as cercariie
which exhibit psedogenesis. The cercaria is the characteristic
larval stage of the Trematodes, and corresponds to the cysticercus
or cysticercoid of the Cestode. The most important difference
between these is in the presence in the former of an enteric cavity,
and its absence in the latter. There seems to be something to be
said in favour of the view that the enteric cavity of the cercaria is
represented by the frontal sucker of some scolices, and by the
rostellum of the majority.

Between the adult Cestodes and the Trematodes an intimate
relationship is traceable. Caryophyllceus (Fig. 206) is a Cestode

Monogenetica

Nemertinea

Polycladida

'.Temnocephalea
Tricladida

Digenetica Polyzoa
Monozoa

Clenophora

Rhabdocoelida

Lower Coelenterata
FIG. 227. - Diagram of the relationships of the Platyhelminthes (together with the Nemertinea).

which, but for the absence of an enteric cavity and the want
of organs of adhesion at the posterior end, is not far distant
from the Trematodes ; and the same might be said of Gyrocotylc
(Fig. 207), Ainphilina, and Archigetcs (Fig. 20S). 1 The most

1 It is possible, however, that in the last two forms we have to do with larval
Cestodes which have failed to reach the mature stage, and have undergone
a precocious development of the sexual apparatus.

288 ZOOLOGY SECT.

important differences between a Cestode and a Trematode, in
addition to the absence of an enteric cavity in the former and
its presence in the latter, is the occurrence in the Cestodes of
strobilation. Ligula in a certain sense forms a connecting link in
this respect between the Trematode and the ordinary Cestode, the
body being elongated and the reproductive organs repeated as in
the normal Tape- Worm, but there being no corresponding division
of the body into a string of definitely separated proglottides.

Of importance in connection with the subject of the relationship
of Trematodes and Cestodes is the question whether the scolex of
the latter is at the end corresponding to the anterior end of the
former, or whether it is the free end of the strobilia that is in
reality anterior. In favour of the latter conclusion is the fact
that the hooks of the hexacanth larva, developed at its anterior
end, are found in the cysticercoid to lie in the tail region, i.e., the
region most remote from that which develops the scolex, and thus
at the end which should represent the free extremity of the
strobila. On the other hand, the specialisation of the nervous
system to form quite definite and comparatively elaborate nerve-
centres (brain) in the scolex of some Cestodes (e.g., Moniczia)
tells in favour of the view that the scolex is anterior and
corresponds to a head.

APPENDIX TO PLATYHELMINTHES.
CLASS NEMERTINEA.

General Features.- -The Nemerteans are non-parasitic, unseg-
mented worms, most of which are marine, only a Few forms living
on land or in fresh-water. They are commonly looked upon as nearly
related to the Turbellaria and were formerly included in that
class ; but in some respects they are higher in organisation than
the Turbellaria, and they exhibit certain special features distin-
guishing them from the rest of the lower Worms.

The body (Fig. 228) is nearly always narrow and elongated,
cylindrical or depressed, unsegmented and devoid of appendages.
In length it varies from a few millimetres to as much as ten
metres. In some cases there is a short narrower posterior region
or " tail " ; a head is rarely marked off from the body proper.
The entire surface is covered with vibratile cilia, and frequently the
integument is vividly coloured. Gland-cells of the epidermis
secrete a mucous matter, which may serve as a sheath or tube for
the animal. The mouth (m.) is at or near the anterior extremity
on the ventral aspect. Near it in front (rarely united with it)
there is an opening through which can be protruded a very long
muscular organ, the proboscis (pr.) y the possession of which is one

PHYLUM PLATYHELMINTHES

Ir

7TL

of the most characteristic features of this class of Worms. The
proboscis is hollow ; when extended to its utmost, a part still
remains which is not capable of being everted. This hollow tube
(Fig. 229) is open in front, where its edges are continuous with
the body-wall, and closed behind. Its wall in the eversible part
consists of an epithelium (internal when at rest) continuous with
the epidermis and similar to the latter, a basement-membrane,
and either two or three layers of
muscle, circular and longitudinal,
with an external thin epithelium
of flat cells. The circular muscular
fibres are not continued back on
the non-eversible part, but the
longitudinal fibres pass backwards
to form the retractor muscle, by
means of which the proboscis is
attached to the sheath in which
it is enclosed, and by means of
which also it is retracted. The
internal epithelium of the pro-
boscis develops variously formed
and arranged papillae, and in
most cases its cells form rods of
a similar character to that of the
rods or rhabdites of Turbellaria.
Exceptionally the cells contain
nematocysts similar to those of
Coelenterates. In the part be-
tween the eversible and non-
eversible regions, a part which
may itself become elongated and
complicated in structure, is de-
veloped in many Nemerteans
(Metanemertini) a median cal-
careous stylet (Figs. 232, 233)
with groups of smaller accessory
stylets at the sides. In the
everted proboscis these are borne
at the free anterior extremity,
and are thus capable of being
used as weapons. In Drepanophorus there are a number of small
stylets supported on a narrow curved plate, together with accessory
stylets. In the rest of the Nemerteans stylets are not developed.
It is by contraction of the muscular walls of the sheath, the
cavity of which (rliy nchoccele) contains a corpusculated fluid, that
the proboscis becomes everted. The abundant nerve-supply of
the proboscis points to its being used partly as a tactile organ.

VOL. I U

dii

FIG. 228. Diagram of the organs of a
Nemertine, from below, a. anus ; br.
brain ; div. cpeca ; lonci. ne. longitudinal '
nerve-cords ; m. mouth ; n. nephridia ;
o-v. ovaries ; pr. proboscis. (After
Hubrecht.)

290 ZOOLOGY SECT.

The outermost layer of the body-wall is an epidermis of
columnar cells many of which are ciliated, while others are
unicellular glands, some of which are arranged in groups ; these
secrete the mucus with which the surface is usually covered, and
which may form a gelatinous tube. Beneath the epidermis is a
basement membrane, very thin in most cases, followed by the
muscular layers. In some Nemerteans (whence called Dimyaria)
there are only two layers of muscular fibres, an outer circular and
an inner longitudinal ; in the rest (Trimyaria) a third (longitudinal)
layer is super-added. Another circular layer of muscular fibres
closely encompasses the digestive canal. The interspace enclosed
by the outer muscular layers does not comprise any cavity corres-
ponding to a true coelome or body-cavity, except, perhaps, the

B

Fio. 229. Diagrammatic representation of proboscis : (A) in the retracted condition, (B) in the
everted condition, ti. p. glandular portion of the proboscis ; m. muscle attaching the
proboscis to its sheath ; r i>i. p. muscular portion of the proboscis ; p. p. in A, proboscis pore ;
p. p. in B represents the position of the proboscis-pore in the retracted condition of the
proboscis ; p. s. proboscis sheath. (After Sheldon.)

cavities of the gonads, the interspaces between the organs being
filled with parenchyma (Fig. 234).

The digestive canal consists of a tube which extends throughout
the length of the body from the mouth situated near the anterior
extremity on the ventral side, to the anus at the posterior
extremity. 1 The mouth is usually placed some distance behind
the proboscis pore, but may be shifted forwards so as to lie close
to the latter, or to be incorporated with it. The first part of the
digestive canal is usually a simple tube oesophagus (stomodceum)
-but may be more complicated, and divided into various
regions. Posteriorly it opens into the intestine. The latter may

1 When a tail is present the intestine may, or may not, be continued
through it.

PHYLUM PLATYHELMINTHES

291

(Afetanemertini) project for-
wards below the oesophagus
as a ventral caecum, which
may give off paired lateral
diverticula. The intestine,
constituting by far the
greater part of the length of
the canal, may be a simple
unconstricted tube, or may
be only slightly constricted
at intervals by the paired
gonads. In most cases the
constrictions corresponding
to the gonads are very deep,
so that the intestine comes
to be provided with two rows
of lateral diverticula or caeca,
which may be branched. The
caeca are separated from one
another by incomplete trans-
verse septa of dorso-ventral
muscular fibres the ar-
rangement of the ca?ca and
septa with the alternately
arranged gonads bringing
about an appearance of im-
perfect metamerism such as"
is observable in some of
the Platyhelminthes (Gunda,
species of Temnoeephala).

The Nemerteans possess
a system of vessels usually
regarded as representing a
blood - vascular system
(Figs. 230 and 235), with
well-defined walls consisting
of a layer of epithelium
surrounded by a thin layer
of muscular fibres arranged
circularly. There are three
principal longitudinal trunks
a median dorsal (dens.
ves.) and two lateral (lat.
ves.). The blood is, in
most cases, colourless, and
contains rounded or ellipti-
cal, usually colourless, cor-
puscles.

cil gr
cer.g

lat. ne
I at. res

dors.ves

=- doTS.VBS

retr.tnus

FK

an

. 230. Tetrastemma. General view of the
internal organs, an. anus ; ac. st. accessory
stylet ; cer. g. brain ; cit. <jr. ciliated groove of
cerebral organ ; dors. ves. dorsal vessel ; lat. ne.
lateral nerve; lat. ves. lateral vessel; neph.
nephridium ; op. neph. nephridial aperture ;
prob 1 . eversible part of proboscis ; prob-. non-
eversible part of proboscis ; prob. ap. aperture
for the protrusion of the proboscis ; retr. mus.
retractor muscle of the proboscis; st. stylet.
(From Hatschek's Lehrbuch.)

u 2

292

ZOOLOGY

SECT.

The excretory system has a considerable resemblance to that
of the Platyhelminthes. It consists of a pair of longitudinal

vessels (Fig. 235, ncpli.) which give off
branches, by one or several of which each
communicates with the exterior. The
line terminal branches of the system are
provided with ciliary flames, each situated
in the midst of a group of cells, not in
the interior of a single flame-cell as in
the Flat-worms.

There are no special organs of re-
spiration in any of the group. But
there is evidence that this function is
carried out, in part at least, by the taking
in and giving out of water through the
mouth by the oesophagus.

The nervous system is in some re-
spects more highly developed than in the
Turbellaria. The brain (Figs. 228 and
231, lr., and Fig. 230 cer. #.) is composed
of two pairs of ganglia, dorsal and ventral,
the ganglia of each pair being con-
nected together by commissures, the
dorsal situated above, the ventral below r ,

the anterior part of the proboscis and proboscis sheath, and
both being above the mouth and oesophagus. From the brain
pass backwards a pair of thick longitudinal nerve-cords which

rt

FIG. 231. Anterior portion of
the body of a Nemertine.
br. brain-lobes ; n. lateral
nerves ; p. o. external open-
ing through which the pro-
boscis is everted ; p. s. pro-
boscis-sheath ; pr. proboscis.
OZsophagus and mouth shown
by dotted lines. (After
Hubrecht.)

FIGS. 232 and 233. Proboscis of a Hoplonemertean. with stylet reserve-sacs and muscular
bulb. Fig. 232 retracted, Fig. 233 everted. (After Hubrecht.)

run throughout the length of the body. Usually these are
lateral in position, sometimes approximated dorsally, sometimes
ventrally. Usually the lateral nerve-cords meet posteriorly in a
commissure usually situated above, but in one genus below, the

PHYLUM PLATYHELM1NTHES

293

anus. A third median dorsal nerve of smaller size than the
lateral cords extends backwards from the dorsal commissure of the
brain. Associated with the nerve-cords in the Protonemertini and

c.t

c.t

lon^.nc

long.ne

Uv

x 234. Diagrammatic transverse section of a Werner lean. (Carinella). n,l>,<\ layers of
body-wall ; c. t. connective tissue between body-wall and enturoii ; I. l>v. lateral blood-vessels ;
long. ne. longitudinal nerves ; j>. proboscis ; p. s. pi-oboscis-sheath. (After Hubrecht.)

the Hetcronemertini is a nerve-plexus extending all over the body.
In the Mctancmcrtini, instead of a nerve-plexus there is a series
of slender transverse connectives running across at short intervals
between the lateral nerve-cords, and from each cord are given off
numerous branches arranged with some regularity.

The position of the brain and lateral nerve-cords and the
nerve-plexus, or the system of commissures and nerve-branches,

mph

latblv

FIG. 235. Anterior portion of a Wemerteaii (Drepanophorus), showing the blood-vascular
and excretory systems, hit. li. v. lateral blood-vessels ; med. bl. v. median blood-vessels ;
ntph. iiephridial (excretory) tubes. (After Oudemans.)

varies in the different groups. In the Protonemertini (Fig. 234)
they occupy the most primitive position, being quite superficially
situated at the bases of the epidermal cells. In the rest they are
deeper : in the Metanemertini they lie in the parenchyma within

294

ZOOLOGY

SECT.

the muscular layers. The median cord is always, except in the
Heteronemertines, superficially placed.

A remarkable apparatus connected with the nervous system
is that composed of a pair of peculiar structures known as the
cerebral organs. When most highly developed these consist
of a pair of ciliated tubes (Fig. 230, cil. gr.\ opening externally in

FIG. 23<j. A, Piliclium with advanced Nernertine worm ; B, ripe embryo of Nemertes from
interior of pilidium. .an. amnion, or investment of the embryo ; i. intestine ; Ip. lateral pit ;
n. nervous system ; o>. gullet ; pr. proboscis ; st. stomach. (From Balfour, after Biitschli.)

the region of the brain or of a pair of lobes separate from the
latter. This apparatus may have a respiratory function, more
especially for the oxygenation of the substance of the brain, but
perhaps it has also a sensory function. It has some resemblance
to the ciliated pits developed in certain Turbellaria.

Eyes are present in the majority of Nemerteans, and in the
more highly organised species occur in considerable numbers.
Sometimes they are of extremely simple structure ; in other cases

v PHYLUM PLATYHELMINTHES 295

they are more highly developed, having a spherical refractive
body with a cellular " vitreous body," and a ' retina ' : consisting
of a layer of rods enclosed in a sheath of dark pigment, each rod
having a separate nerve-branch connected with it. Statocysts
containing statoliths have been found in only a few of the
Nemerteans.

Reproductive System. Most species are dioecious. The ovaries
(Fig. 228), ov.) and tcstcs are situated in the intervals between the
intestinal caeca. The ovary or testis is a sac the cells lining which
give rise to ova or spermatozoa ; when these are mature each sac
opens by means of a narrow duct leading to the dorsal, rarely
to the ventral surface, on which it opens by a pore. In all
probability the cavities of these hollow gonads are all that
represent the ccelome of higher forms.

Development. Some of the Nemerteans go through a meta-
morphosis ; in the others the development is direct. The charac-
teristic larval form is the pilidium (Fig. 236). This is a helmet-
shaped body with side lobes like ear-lappets, and a bunch
of cilia representing a spike. In the metamorphosis two pairs of
ectodermal invaginations, growing inwards around the intestine,
fuse together and form the integument and body-wall of the future
worm, which subsequently frees itself from its investment and
develops into the adult form. In others there is a ciliated
creeping larva called the " larva of Desor" in the interior of which
the larval worm is developed much as in the case of the pilidium.

Though none of the Nemerteans exhibit metameric segmenta-
tion, yet in some of them there is, as in Gunda segmenlata
(p. 255) among the Turbellaria, a serial repetition of the internal
parts (pseudo-metamerism, associated with the presence of
regularly arranged transverse partitions of dorso-ventral muscular
fibres). Transverse fission is of frequent occurrence.

DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nemertinea are ciliated, unsegmented worms with
elongated body, without distinct coelome. There is an eversible
proboscis enclosed in a sheath and capable of being protruded to
a great length through an aperture situated usually in front of
and above the mouth. The intestine usually has distinct lateral
diverticula, and there is a posteriorly situated anus. There is a
blood-vascular system and also a system of excretory vessels with
ciliary fiames.

ORDER 1. PROTONEMERTINI.

Dimyarian Nemertines with the lateral nerve-cords situated
outside the muscular layers. The mouth is situated behind the
brain. The proboscis is devoid of stylet.

290 ZOOLOGY SECT, v

ORDER 2. MESONEMERTINI.

Dimyarian Nemertines having the lateral nerve-cords withdrawn
within the musculature of the body-wall. The mouth is situated
behind the brain. There are no stylets.

ORDER 3. METANEMERTINI.

Dimyarian Nemertines, in which the lateral nerve -cords lie
inside the muscular layers in the parenchyma. The mouth is
situated in front of the brain. The proboscis is provided with
stylets. A ventral crecum is present.

ORDER 4. HETERONEMERTINI.

Trimyarian Nemertines, in which the lateral nerve-cords are in
the muscle-layers, between the outer longitudinal and the circular
layers. The mouth is situated behind the brain. The proboscis
has no stylet.

The Nemerteans are almost exclusively marine ; and the greater
number live between tide-marks or at moderate depths ; a few
have been obtained from considerable depths. The comparatively
small number of terrestrial and fresh -water forms are all
Metanemertini. The Nemerteans progress for the most part by
slow crawling movements, leaving a track of slime behind them.
Some burrow freely in mud or sand, the proboscis being made use
of to help in the process. Some are able to swim by means of
undulating movements of the body. Nearly all are carnivorous,
and either capture living prey in the shape of small invertebrates
of various kinds, or feed on dead fragments. The chief function
of the proboscis is the capture of living prey, around which it
becomes coiled and then draws the prey towards the mouth. One
Nemertean lives in the interior of a Crustacean, and is probably a
true parasite. Others, live, apparently as commensals or mess-
mates, in the pharynx or atrial cavity of Ascidians, or within the
mantle cavity of bivalve Mollusca.

A striking feature of the Nemerteans is the readiness with
which, on being irritated by handling or by the action of some
chemical agent, they break up transversely into fragments. This
takes place most freely when the body is highly charged with
sexual products, but is by no means confined to that condition.
The process probably takes place spontaneously under certain
circumstances. The broken-off fragments may remain alive for a
considerable time, and under suitable conditions regeneration of
the lost parts is readily effected, so that it is possible to look upon
the entire process as a form of asexual reproduction.

SECTION VI
PHYLUM NEMATHELMINTHES.

THE members of the preceding phylum are characterised, as a
whole, by a marked dorse-ventral flattening. In the Worms in-
cluded in the present group the body is elongated and cylindrical,
whence their general name of Round- or Thread-worms. The
phylum includes the following classes :-

Class 1. NEMATODA.- -The Round-worms in the strict sense of
the term. The best known forms are internal parasites, but many
genera and species are extremely abundant in fresh and salt
water.

Class 2. AcANTHOCEPHALA.--The "Hook-headed Worms," a
group of formidable internal parasites.

Class 3. CH^TOGNATHA. The " Arrow- worms," a small group of
pelagic organisms.

The affinities of the Acanthocephala and Chsetognatha with the
Nematoda are somewhat doubtful, and the association of the three
classes is largely a matter of convenience.

CLASS I. NEMATODA.

1. EXAMPLE OF THE CLASS THE COMMON ROUND- WORM OF

MAN. (Ascaris lumbricoidcs.)

Asr.aris lumbricoidcs is a common parasite in the human intes-
tine : a closely allied if not identical form (A. suilla) occurs in the
Pig, and another (A. megaloccphala) in the Horse. The following
description will apply to any of these. The female A. lumbricoides
is about 20-40 cm. (8-1(3 inches) long, and about 6-8 mm. (j inch)
in diameter: the male is considerably smaller.

External Characters. --When fresh the animal is of a light
yellowish-brown colour: it is marked with four longitudinal
streaks, two of which, very narrow and pure white in the

297

298

ZOOLOGY

SECT.

living Worm, are respectively dorsal and ventral in position, and
are called the dwsal (Fig. 237, d.L) and ventral (v.l.) lines : the other
two are lateral in position, thicker than the former, and brown in
colour, and are distinguished as the lateral lines. The mouth is
anterior and terminal in position, and is bounded by three lobes,
or lips, one median and dorsal (d. Ip), the other two venire-lateral
(v. lp). A very minute aperture on the ventral side, and about 2
mm. from the anterior end, is the excretory pore (ex. p.). At about
the same distance from the pointed and down-turned posterior end
is a transverse aperture with thickened lips, the anus (an.), which
in the male serves also as a reproductive aperture and gives exit
to a pair of needle-like chitinoid bodies, the penial setce (pn. s.).
In the female the reproductive aperture or gonopore is separate
from the anus, and is situated on the ven-tral surface about one-
third of the length of the body from the anterior end (Fig. 240,
ynp.). The sexes are also distinguished externally by the form of

FIG. 237. Ascaris lumbricoides. A, anterior end from above ; B. the same from below ;
C, posterior end of female, I), of male, side view. an. anus ; d. Ip. dorsal lip ; d. I. dorsal line ;
ex. p. excretory pore ; p. papilla? ; pn. s. penial setre ; v. 1. ventral line ; v. Ip. ventral lip.
(After Leuckart.)

the short tail, or post-anal portion of the body, which in the male
is sharply curved downwards (Fig. 237, D), while in the female (C)
its ventral contour is nearly straight.

Body-wall. The outer surface of the body is furnished by a
delicate, transparent, elastic membrane, of a firm material of
albuminoid composition, the cuticle (Fig. 238, cu.). It is divisible
into several la} T ers, and is wrinkled transversely, so as to give the
animal a segmented appearance. Beneath the cuticle is a proto-
plasmic layer (der. epthm.) containing scattered nuclei and longitu-
dinal fibres, and representing &syncytial ectoderm 'i.e., an ectoderm
is which the cell-boundaries are not differentiated, and whose
cellular nature is recognisable only by the nuclei. The cuticle is,
as usual, a secretion of the ectoderm.

Beneath the ectoderm is a single layer of muscular fibres (in),
arranged longitudinally, and bounding the body-cavity. The
structure of the muscles is very peculiar : each (Fig. 239, A) has

VI

PHYLUM NEMATHELMINTHES

299

the form of a spindle, striated longitudinally, and produced on its
inner face (i.e. towards the body-cavity) into a large and almost
bladder-like mass of protoplasm (p) containing a nucleus (mi.).
Apparently the whole of this structure is derived from a single cell,
part of which has become differentiated into contractile substance
(c), the rest remaining protoplasmic. In transverse section the
contractile portion (B. c) has the form of a plate bent upon itself
so as to be, as it were, wrapped round the protoplasmic portion
(p). The protoplasmic processes project to a greater or less extent
into the body-cavity, sometimes practically obliterating it, and are
produced into delicate filaments (/.) which take a transverse

i?tt

cu-

/m /i i// vr SVSL MlW'-/ k.v -\- -$ri aj

ex

Ot'U

*'

FIG. 238. Ascaris lumbricoides, transverse section, cu. cuticle ; cZ. I. dorsal line ; der. cptlim.
deric epithelium or epidermis ; ex. v. excretory vessel ; int. intestine ; fat. I. lateral line ; in.
muscular layer ; ovy. ovary ; ut. uterus ; v. v. ventral line. (After Vogt and Yung.)

direction, and are mostly inserted into the dorsal and ventral
lines.

The muscular layer is not continuous, but is divided into four
longitudinal bands or quadrants, two dorso-lateral and two ventro-
iateral, owing to the fact that at the dorsal, ventral, and lateral
lines the ectoderm undergoes a great thickening and projects
inwards, between the muscles, in the form of four longitudinal
ridges (Fig. 238, d.L, v.v., lat. L). The ridges are composed of fibres
continuous with the fibres of the ectoderm. It is this arrange-
ment that gives rise to the lines seen externally. The ridges
forming the lateral lines are much more prominent than the other
two.

Digestive Organs.- -The mouth leads into the anterior
division of the enteric canal, the pharynx or stomodceum (Fig. 240,

300

ZOOLOGY

SECT.

ph.): its walls are very muscular, its cavity is three-rayed in cross-
section, and it is lined by a cuticle secreted from the epithelial layer
and continuous, at the mouth, with that of the body-wall.
Posteriorly the pharynx opens into the intestine (int.), a thin-
walled tube, flattened from above downwards, and formed of a
layer of epithelial cells bounded both internally and externally
by a delicate cuticle : it has no muscular layer (Fig. 238, int.).
Posteriorly the intestine narrows considerably to form the short
rectum, which has a few muscular fibres in its walls and opens
externally by the anus (Fig. 240, an.). The food, consisting of the
semi-fluid contents of the intestine of the host, is sucked in by
movements of the pharynx, and is then absorbed into the system

FIG. 230. Ascaris lumbricoides. A, a single muscle fibre ; B, several fibres in transverse
section with portion of ectoderm (below), c, contractile substance ; /. fibrous processes ; uu.
nucleus; p. protoplasmic portion. (After Leuckart.)

through the walls of the intestine. The food being already
digested by the host, there is no need of digestive gland-cells,
such as occur in animals which prepare their own food for
absorption.

It will be noticed that in the above description the pharynx is
also called stomodseum. This must not be taken to indicate that
the two terms are synonymous, but that, in the present instance, the
epithelial lining of the pharynx is derived from the ectoderm,
being formed as an in-turned portion of the outer layer of the
body-wall. Tho epithelium of the intestine, on the other hand, is

</ 1.

endodermal, this portion of the canal being derived from the
archenteron of the embryo.

VI

PHYLUM NEMATHELMINTHES

301

Between the enteric canal and
the body-wall is a distinct space,
the body-cavity, containing a
clear fluid and more or less en-
croached upon by the protoplasmic
processes of the muscle-cells. The
cavity is bounded externally by
these processes, internally by the
outer cuticle of the intestine : there
is no trace of epithelial lining
such as occurs in most of the higher
animals The body-cavity of the
Nematode, in fact, does not exactly
correspond to the coelome to be
met with in most higher phyla.
It is not to be derived, directly or
indirectly, from the archenteron of
the embryo, and it does not lie,
like a true crelome, between layers
of the mesoderm.

The excretory system presents
a certain resemblance to that of
Platyhelminthes. It consists of
two longitudinal canals (ex. v.), one
in each lateral line. Anteriorly
these pass to the ventral surface,
unite with one another, and open
by the minute excretory pore (ex. p.)
already noticed. The system is not
ciliated, and contains no flame-
cells. Both canals are excavations
in a small number of enormously
elongated and branched cells, each
cell having a single nucleus.

The nervous system consists
of a ring (nv. r.) surrounding the
pharynx and giving off six nerves
forwards and six backwards (Fig.
241). Of the latter two are of
considerable size, and run in the
dorsal and ventral lines respectively
(clln,vln.). They are connected with
one another by transverse com-
missures (c.), and the ventral nerve
swells into a ganglion just in
front of the anus. The pharyngeal
nerve-ring contains nerve-cells, and

0) 3
J 3

'CO

=3

-X s

. ^

M & 'J

3 H >>

S <U cS

h

as r- -i- 3
-'23
O'-C o

,, tc

cj

c o

S '- | 'i

. a

G to 33

S -

rt

I - o

o 9

~f JH jSi

302

ZOOLOGY

SECT.

its ventral portion (un.) is thickened and ganglion-like. The only
sense-organs are little elevations, the sensory papillae (Fig. 237,>.),
on the lips.

The reproductive organs are formed on a peculiar and very
characteristic pattern. The testis (Fig. 242, ts.) is a long, coiled

thread, about the thickness of fine sewing-
cotton, and occupying a considerable por-
tion of the body-cavity. At its posterior
end it is continuous with the vas deferens,
the two passing insensibly into one
another so that the junction is not visible
externally. The vas deferens, in its turn,
becomes continuous with a wide canal,
the vesicula scminaiis (vs. sem.), which
opens by a short, narrow muscular tube,
the ductus ejaculatorius, into the rectum.
Behind the rectum, and opening into its
dorsal wall, are paired muscular sacs (s.),
containing the penial setcc (pn. s.) already
noticed. The anterior end of the testis con-
sists of a solid mass of sex-cells ; passing
backwards there is found a cord or
rachis occupying the axis of the tube and
having the sperm-cells attached to it ;
still further back the sperms become
gradually differentiated, and are finally
set free in the vas deferens. The sperms
are peculiar rounded cells (Fig. 23, p. 30,
c. d. c.) ; when transferred into the body
of the female they exhibit amoeboid move-
ments, but as long as they remain in the
male ducts they are non-motile : they
have no trace at any stage of the char-
acteristic tail of the typical sperm. In
this connection it may be mentioned that
the tissues of Ascaris are remarkable for
the total absence of cilia.

The organs of the female (Fig. 240) re-
semble those of the male, but are double
instead of single. There are two coiled,
thread-like ovaries (ovy.), each passing in-
sensibly into a uterus (ut.\ In the ovary,
as in the testis, the eggs are developed
in connection with an axial cord or rachis. The two uteri unite
in a short muscular vagina (vcig.) which opens, as already seen, on
the ventral surface of the body (gnp.) at about one- third of the
entire length from the head.

Fin. 241. Diagram of nervous
system of Xematoda. c.
commissures ; din. dorsal
nerve ; Itsn. posterior lateral
nerve ; on. upper and un.
under portion of nerve-
ring ; s. (/.lateral swellings ;
vln. ventral nerve. (From
Lang, after Butschli.)

VI

PHYLUM NEMATHELMINTHES

303

Development. The eggs are produced in immense numbers
at the rate, it has been reckoned, of about 15,000 a day. They
are fertilised in the upper part of the uterus, each becoming
enclosed in a chitinoid egg-shell, and are passed out of the body of
the host with its faeces. Segmentation is complete, but the
details of development are not known in this species. The results
of experiments render it probable that infection is direct, without
intermediate host, the embryo-containing eggs being taken, in

FIG. 242. Ascaris lumbricoid.es, posterior extremity of male, dissected. an. anus ;
cu. euticle ; der. eptltui. epidermis; m. muscular layer; p. s. penial seta ; s. sac containing
penial seta ; ts. testis ; vs. sem. vesicula seminalis.

water, or in soil accidentally swallowed, into the intestine of a new
human host, in which the embryos, escaping from the eggs, become
mature.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nematoda are Nemathelminthes having a cylindrical body
of great length in proportion to its diameter, and pointed at both
ends. The body-wall consists of a tough external cuticle, an
ectoderm in the form of a syncytium or protoplasmic layer con-
taining nuclei and rarely exhibiting cell-structure, and a single
layer of longitudinal muscular fibres which are interrupted along
one or more (dorsal, ventral, and lateral) lines. The body-wall
encloses a body-cavity containing a clear fluid, and more or less
encroached upon by processes of the muscle-cells or other meso-
dermal tissues. The enteric canal is straight, and consists of
pharynx, intestine, and rectum : the pharynx is a stomodaeum.
The mouth is anterior and terminal, the anus ventral and situated
a short distance from the posterior end. Excretory canals, running
in the lateral lines, are usually present. The nervous system con-
sists of a pharyrigeal ring containing nerve-cells and giving off
nerves forwards and backwards : of these there is either a single
ventral-cord, or there are two cords, respectively dorsal and ventral,

304 ZOOLOGY SECT.

of considerable size and extending to the posterior end of the body.
The Nematoda are in nearly all cases dioecious : eggs are pro-
duced in immense numbers, and are impregnated within the body
of the female. The sperms are non-motile, or perform amoeboid
movements only after entering the female organs. Cilia are
wholly absent.

A large proportion of Nematoda are free-living, spending their
whole life in fresh or salt water, damp earth, decaying matter,
&c. ; the remainder are parasitic during the whole or a part of
life.

The class is divided into two orders.

ORDER 1. NEMATOIDEA.

Nematoda in which the body-cavity is not lined by epithelium,
but is bounded directly by the body-muscles. Two chief nerve-
cords are given off backwards from the pharyngeal ring and
lie in the dorsal and ventral lines. There are two excretory
canals lying in the lateral lines and opening anteriorly and
ventrally. The gonads are continuous with their ducts, and con-
sist of long, more or less convoluted cords. This order includes
the whole of the free-living Nematodes as well as the large
majority of parasitic forms.

ORDER 2. NEMATOMORPHA.

Nematoda in which the body-cavity is lined by a distinct epithe-
lium. The pharyngeal nerve-ring sends off a single large ventral
nerve-cord well supplied with nerve-cells. The gonads, or at least
the ovaries, are arranged metamerically, and the reproductive pro-
ducts are discharged into the body-cavity and pass thence into the
gonoducts. This order includes a small number of greatly
elongated, thread-like worms (species of the genus Gordius), which
are parasitic in the asexual, free-living in the sexual stage ; and
also the genus Necturus, which has only been found swimming
in the sea.

Systematic Position of the Example.

Ascaris lumbricoides is one of many species of the genus Ascaris,
and belongs to the family Ascaridcc of the order Ncmatoidea.

The absence of an epithelial lining to the body-cavity, and the
presence of elongated gonads continuous with their ducts, indicate
its position as one of the Nematoidea. Among the numerous
families constituting this order, the Ascarida? are distinguished by
the possession of three lips furnished with papilla?, and by the
body of the male being curved ventrally and being provided
with penial seta?. Ascaris is distinguished from the other

VI

PHYLUM NEMATHELMINTHES

305

genera of the family by the absence of a bulb-like enlargement at
the posterior end of the pharynx, by the posterior extremity of the
body having the form of a short blunt cone, and by the presence
of two penial setae in the male.

3. GENERAL ORGANISATION.

External Characters. The Nematoda vary much in size : the
little Anguillula, one of the commonest of aquatic animals, does
not exceed 1 mm. in length, while the dreaded parasite known as
the Guinea-worm (Filaria medinensis) is sometimes as much as
2 metres (6 feet) long. The length is always great in proportion
to the diameter, and the body is always bluntly pointed at the
anterior end and either pointed or forked posteriorly. One of the
most striking cases of disproportion between length and breadth is
exhibited by the free, sexual form of Grordius, one of the Nemato-
morpha ; it is found in earth or water and resembles a tangle of
brown string, the length being frequently as much as 15 or 16 cm.
while the diameter does not exceed O5 mm.

Body-wall. --The body is always covered by a cuticle secreted
by the deric epithelium or external ectoderm : the latter usually
takes the form of a protoplasmic layer with scattered nuclei, but
in the Nematomorpha it consists in part of a true epithelium a
single layer of distinct cells. Beneath the ectoderm is a muscular
layer, which in many genera
has the same structure as in
Ascaris, i.e. consists of a single
layer of longitudinal fibres, in-
terrupted at the dorsal, ventral,
and lateral lines, each fibre
being spindle-shaped and pro-
duced into a protoplasmic pro-
cess which projects into the
body-cavity. But in many forms
(e.g., Strongylus)thQ muscle-cells
are flat rhomboidal plates (Fig.
243), and each quadrant con-
tains only two rows, the total
number in a transverse section
being therefore eight. In the

Nematomorpha the muscles are interrupted along the ventral line
only, the dorsal and lateral lines being absent (Fig. 245). More-
over the muscular layer in this order is lined by a layer of
epithelial cells which bounds the body-cavity.

Enteric Canal. The mouth is frequently armed with spines
(Fig. 244, C), by means of which the worms draw blood from the

VOL. i X

FIG. 243. The body-wall of a platymyariaii
Nematode, spread out. lat. I. lateral lines.
(After Leuckart.)

306

ZOOLOGY

SECT.

intestinal blood-vessels of their host. Many free-living forms have
a sharp stylet for piercing the tissues -of the plants on which they
feed, and a suctorial apparatus for absorbing the juices. The

FIG. 244. Ankylostoma duodenale. A, male and female in coitv.. B, anterior end,
showing cr. (il. cervical glands; ph. pharynx. C, mouth with spines; D, posterior end of
male, with bursa. (After Leuckart.)

posterior end of the pharynx is often dilated to form a globular
chamber with muscular walls, the gizzard (Fig. 246, gz.). The only
specially interesting variation in the structure of the intestine
is that occurring in Trichinella, one of the Nematodes parasitic
in Man, in which this part of the enteric canal consists of a single
row of perforated cells : the lumen is therefore not inter-cellular
but intm-cellular, like the gullet of an Infusor. In the sexual stage
of Gordius the enteric canal undergoes more or less complete
degeneration. The alimentary canal in some rare cases has

J7H?

FIG. 245. Transverse section of Gordius. Im. ventral nerve-cord; c. cuticle; et. epithelium
lining body-cavity ; hy. epiderm ; Ih. body-cavity ; Im. muscular layer ; md. intestine ; mes.
mesentery ; ov. ovary ; u. uterus. (From Lang, after Vejdovsky.)

hollow appendages in the form of cesophageal glands or intestinal
caeca. In Dochmius a pair of pear-shaped bodies of unknown
function, the cervical glands (Fig. 244, B t cv. gL), lie one on each
side of the pharynx and probably open externally near the mouth.

VI

PHYLUM NEMATHELMINTHES

307

-ph,

ts

In Nematoidea the body-cavity is always a single continuous
chamber crossed in various directions by delicate fibres, but in
Gordius certain partitions or mesenteries (Fig. 245, mes.) extend
longitudinally through it, dividing it into several compart-
ments. The most important of these are
a median ventral compartment containing
the intestine and the nerve-cord, a pair
of large lateral compartments containing
the ovaries, and a pair of small dorso-
median canals which act as oviducts. It
is stated that the median ventral com-
partment acts as an excretory canal
and opens posteriorly along with the ovi-
ducts : in the Nematomorpha there are
no lateral excretory canals like those of
Ascaris and the other typical Nematodes.
In the Nematoidea, when definite
excretory organs are developed, they
take the form of longitudinal canals
similar to those described as occurring in
Ascaris. Sometimes only one canal is
present. In some cases it is stated that
the canal or canals open into the body-
cavity.

In the Nematoidea the nervous
system has the structure already de-
scribed in Ascaris ; it is, however,
apparently absent in some free-living
forms. But in Gordius it is much more
highly developed : the pharyngeal ring
is of great thickness and is. continued
into a single ventral cord (Fig. 245, bm.)
containing nerve-cells. Eye-spots have
been described in the sexual form of
Gordius.

The reproductive organs in all the
Nematoidea resemble those of Ascaris, the
only important variation depending upon
the fact that in the smaller forms the
entire genital tube (gonad plus gonoduct)
is short and not coiled (Fig. 246, ts. and
v. df.). A few forms are hermaphrodite,

but, instead of having a double set of reproductive organs,
as in Platyhelminthes, organs of the ordinary female nematode-
type are present, and the gonads produce first sperms and
afterwards ova. Such animals are said to be protandrous (male
products ripe first), and self-impregnation is as effectually

x 2

,~.

.f * I3

* e tui

* '

FIG. 246. Oxyuris, from the
right side. gz. gizzard ; int.
intestine ; ph. pharynx
pn. s. penial setfe ; ts. testis ;
r. df. vas deferens. (From
Shipley, after Galeb.)

308

ZOOLOGY

SECT.

prevented as if the organs of the two sexes were distinct. A
totally different arrangement is met with in the Nematomorpha,
the female having numerous pairs of ovaries (Fig 247, A, ovij.)
arranged segmentally and attached to one of the partitions (mes.)
of the body-cavity. The ripe eggs are discharged into large
egg-sacs, formed by the lateral compartments of the body-cavity,
and finally make their way into the medio-dorsal compartments
which act as uteri (C, ut.) and are continued posteriorly by short
vagina (vag.) into a median chamber. The latter opens externally,
and also receives the duct of a large spermotheca (spth.) or chamber
for storing the sperms received in copulation. In the male
Gordius the testes are not known : they seem to disappear very

int

v.nv.cd "<*&

FIG. 247. Gordius. A, horizontal section of female, showing ovaries (nvy) attached to mesen-
tery (ie.<i.) ; b. w. body-wall. B, posterior extremity of male, sagittal section, b. c. bursa
copulatrix ; cl. cloaca ; int. intestine ; t. tail ; r. nv.cd. ventral nerve-cord ; rs. sem. vesicula
seminalis. C, posterior extremity of female, sagittal section, gnp. gonopore ; spth. sperma-
theca ; ut. uterus ; vag. vagina ; v. nr. cd. ventral nerve-cord. (After Vejdowsky.)

early, after discharging their contents into large reservoirs or
vesiculce seminales (B, vs. sem.) : from these, vasa deferentia are
continued into the cloaca (cl.) or dilated extremity of the intestine,
part of which can be everted as a bursa copulatrix (b.c.).

In the development of Nematodes segmentation may be un-
equal from the outset, or equal at first, becoming unequal after the
first two or three divisions. There may be an invagination (embolic
gastrulation), or, as in Rluibdonema nigrovenosum (Fig. 248) a kind
of epiboly, or a process of an intermediate character. The blastopore
always disappears, taking no part in the formation of the apertures
of the adult. The archenteron also becomes obliterated, and the
lumen of the intestine has no connection with it, but is formed
anew by the development of a fissure between the endoderm cells

VI

PHYLUM NEMATHELM1NTHES

309

which have become arranged in two rows. The epithelium of the
pharynx is formed by an involution of the ectoderm, so that this
division of the enteric canal forms a stomodceum (Fig. 248, 1, stdm.\
and the rectum appears also to be lined by ectodermal cells and
thus to be of the nature of a proctodceum. The middle layer is
formed from a pair of cells (D, mes.) of the inner layer which
enlarge and multiply to form a layer of cells between the

FIG. '248. Development of Ascaris nigrovenosa. Up. blastopore ; cct. ectoderm ; end. endo-
derm ; ent. enteron ; g. genital cell ; mes. mesodei'm ; n. nervous system ; stdm. stomodpeum.
(From Korschelt and Heider, after Goette.)

ectoderm and the endoderm (E H.) The body-cavity is not a
true ccelome (i.e., a cavity formed in the interior of a mass of
mesoderm or arising as an outgrowth from the archenteron), but is
derived from the primitive blastula- or segmentation-cavity. The
nervous sytem is developed from certain cells (G--I, n.) which
bud off from the ectoderm at the anterior end of the body. The
reproductive cells originate from a pair of the inner cells (H, I, g.}
which become differentiated from the rest at an early stage.

310 ZOOLOGY SECT.

Many of the Nematoda have a curious and complex life-
history : a few examples will be selected for description.

Rhabdonema nigrovenosum lives, in the sexual condition, in the
lungs of Frogs and Toads : it is remarkable among members of
the class in being hermaphrodite. The eggs are laid and the
embryos pass from the lungs into the enteric canal of the host, are
expelled with its faeces, and develop in water into a sexual
Nematode, called the RhaMitis-foTm, in which the sexes are
separate : in this the fertilised eggs develop in the body of the
female, and, when fully formed, make their way through the wall
of the uterus and proceed to devour the whole of the maternal
tissues, leaving nothing but the cuticle. Being set free, they live
in mud until they succeed in gaining access to a frog's mouth,
when they pass into the lung, develop hermaphrodite reproductive
organs, and so re-commence the cycle. It will be seen that we
have here a peculiar form of alternation of generations, distinguished
not by the alternation of a sexual with an asexual form (meta-
genesis) as in Hydrozoa, but by the alternation of a hermaphrodite
with a dicBcious form. This type of alternation of generations is
distinguished as heterogeny.

One of the most terrible parasites of man is Tricliinella spiralis
(Fig. 249), a minute worm, the male (C) a little over 1 mm. ( T V in.)
in length, the female (B) about 3 mm. ( in.). In the adult or
sexual condition it lives in the intestine of Man, the Pig, and
other Mammals.

The adult females, which are viviparous, leave the cavity of the
intestine and bore into its wall, usually reaching the interior of
one of the lacteal vessels of the lymphatic system. Here
they deposit their young (B, e) to the number of as many as a
thousand or more at a time. These are carried passively in the
stream of lymph, perhaps ultimately in the blood-stream, and thus
distributed throughout the body. Eventually they travel into the
system of voluntary muscles, such as those of the limbs, back,
tongue, etc. Each worm then penetrates the sarcolemma of a
muscle-fibre and coils itself up in the muscle substance (A) ; a
spindle-shaped cyst (cy.) is formed round it, and the muscle
undergoes more or less degeneration. This process gives rise to
various morbid symptoms in the host, but, after some months the
cysts become calcified, and the danger to the infected individual is
over. The flesh of a " trichinised ' human subject has been
estimated to contain 100,000,000 encysted worms, and that of an
infected pig 85,000 to the ounce. In order that further develop-
ment of the encysted and sexless Trichinae should take place, it is
necessary for the infected flesh of the host to be eaten by another
animal in which the worm is capable of living, e.g. that of Man
by a Pig or Rat, or that of a Pig by Man. When this is done the
cysts are dissolved by the digestive juices, the worms escape,

VI

PHYLUM NEMATHELMINTHES

311

develop reproductive organs, and copulate, the young migrating
into the muscles and producing the disease as before. The
result of eating an ounce of " trichinised ' or " measly ' pork,

B

Fir;. 241*. Trichinella spiralis. A, encysted form, in muscle of host ; B, female ; C, male.
bh. connective-tissue envelope ; cy. cyst; </e. ejaculatory duct; e. embryos, /. lat-globules ;
h. testis ; m. f. muscle-fibre ; oe. pharynx ; or. ovary ; v;o. gonopore ; zh. cell-masses in
intestine. (From Lang's Comparative Anatomy, after Claus.)

improperly cooked, might be the liberation in the human
intestine of perhaps 80,000 worms ; and, if half of these were
females, each producing 1,000 embryos, some 40,000,000 worms

312 ZOOLOGY SECT.

would shortly begin to migrate into the muscles, and produce the
various symptoms of " trichiniasis."

It will be noted that in this case the parasite is able to exist in
various hosts, and that both sexual and asexual stages are passed
through in the same host, dispersal of the species taking place by
the flesh of an infected animal being eaten by another, either of
the same or of a different species.

The female Guinea-worm (Filaria medinensis) attains a length
of 30-200 cm. (l-6ft.), and lives in the subcutaneous connective-
tissue of Man. The eggs develop in the uterus, and the new-born
young pass out of the body of the host through abscesses caused
by the presence of the parasite. If, as must often be the case,
they escape into water, they make their way into the body of a
Water-flea (Cyclops), which is the intermediate host, and in this
condition probably reach their human host once more in his
unfiltered drinking-water. Filaria "bancrofti and other species are,
in the larval condition, parasites in the blood of man. The adult
females of F. bancrqfti live normally in the lymphatic vessels.
They are viviparous, and the young when they escape reach the
blood and are thus distributed. Normally they are to be found in
the peripheral vessels only at night-time, when the superficial
vessels are more dilated and thus permit of their passage. They
are transmitted from one human host to another by the agency
of mosquitoes, which act as intermediate hosts. F. bancrofti is
very widely distributed in tropical countries, and is the cause of a
disease called filariasis, with a variety of symptoms such as
anaemia, lymphatic tumours, elephantiasis.

CLASS II. ACANTHOCEPHALA.

This class contains a number of genera of parasitic worms, of which
EcMnorJiynchus is the chief. The present section will be devoted to this genus,
a not uncommon parasite in the intestine of Mammals, Birds, Reptiles,
Amphibians, and Fishes. The largest species, E. (Gigantorhynchux) giga*, is
found in the Pig (Fig. 250, A), and has once been recorded in the human
subject : it may attain, in the female, a length of 50 cm. , or more than half a
yard. Most species are small, not exceeding 1 cm. in length,

External Characters. The body is cylindrical, and ends in front in a
protrusible portion, the -proboscis (A, p. , B, pr. ), which is cylindrical and is
covered with many rows of recurved chitinoid hooks. The worm lies with the
proboscis sunk in the wall of the intestine of its host, which is sometimes riddled
with holes formed in this way. In some species there is a distinct neck (B, ?t.)
between the proboscis and the trunk, and there may be a globular dilatation at
the anterior end of the neck. At the hinder end of the body is a single
aperture, the gonopore or reproductive aperture (f/np.~) : connected with this, in
the male, is a protrusible, bell-like structure, the bursa (6.), which acts as a
copulatory organ, like the somewhat similar organ in certain Nematoda. There
is no trace of mouth, anus, or excretory pore.

The body-wall is covered with a stout cuticle, beneath which is a striated
protoplasmic layer, probably representing the ectoderm. Then comes a layer of

VI

PHYLUM NEMATHELMINTHES

313

transverse, and then one of longtitudinal muscles. The body- wall thus constituted
encloses a spacious body-cavity or coelome containing a clear fluid.

In correspondence with the absence of mouth and anus there is no trace of
enteric canal, the Acanthocephala resembling in this respect the Cestoda, the only
other class of Metazoa which is entirely anenterous. Food is thus, as in tape-
worms, taken entirely by absorption by the general surface of the body.

The proximal end of the proboscis is contained in a muscular sheath sunk
in the anterior end of the trunk, and is provided with four retractor muscles
(Fig. 250, r.m.). The muscles of the sheath are circular and act as protractors,
At the sides of the base of the proboscis two club-shaped organs, the lemnisci

B

i-rrt

FIG. 250. A, Echinorhynchus (Gigantorhynchus) gigas, female from the Pig (nat.
size ; B, E. lesiniformis, male from the edible Frog (magnified). 6. bursa ; c.gl. cement
glands ; gnp. gonopore ; Im. lemnisci ; n. neck ; p. or pr. proboscis ; r. m. retractor muscle
of proboscis ; .s. Ig. suspensory ligament ; t. testis ; v. vessel.

(Im.}, hang down into the body-cavity. Their function is quite unknown, but
they have been compared with the cervical glands of Nematodes (p. 306).

In the body-wall run two longitudinal vessels (v.) containing a granular
fluid, and connected with a network of fine canals in the proboscis, bursa, &c.
The function of these vessels is not known with certainty : they may have to do
with the absorption and circulation of nourishment.

The central nervous system (Fig, 251, nv.) is represented by a single large
ganglion placed at the base of the proboscis, and sending off nerves in various
directions. In the male there are also two ganglia supplying the reproductive
organs. Organs of sense are wholly absent.

A pair of remarkable excretory organs or nephridia (Fig. 253) have been
found to occur in Echinorhynchus gigas. These consist of a pair of ramified

314

ZOOLOGY

SECT.

ru>

tn-i

s.lg

-Jbr

protoplasmic masses situated in the body-cavity at the posterior end, near the
genital aperture. In the interior is a system of branching canals, the terminal
branches of which, each contained in one of the
terminal lobes of the tree-like nephridium, are
provided with ciliary flames ; at the end of each
lobe are a number of fine perforations placing the
contained canal in communication with the body-
cavity. The stalk of each nephridium contains a
single main canal ; these unite to form a wide
median dorsal channel which opens behind in the
female into the unpaired portion of the oviduct
and in the male into the ejaculatory duct.

The greater part of the body-cavity is occupied
by the reproductive organs. The sexes are
separate, and the female is larger than the male.

;0 ,0!

FIG. 251. Echinorhynchus gigas. Dissec-
tion of male. b. bursa ; c. yl. cement glands ;
I in. lemnisci ; nv. nerve-ganglion ; ?>/. pro-
boscis ; 8. 1(1. suspensory ligament ; tx. testis ;
v. df. vas deferens. (After Leuckart.)

Fin. 2VJ. Echinprhynchus

gigas. Dissection of female
(semi-diagrammatic). !>. bell ;
Im, lemnisci ; pr. proboscis ;
s. or//, swimming ovaries ; i>i.
uterus ; ///. vagina.

In both sexes the gonads and their ducts are connected with a great
li<l<ini< at (*./'/.), which extends backwards from the end of the proboscis- sheath.
In the male there are two ovoidal /f-.s/e-s (Fig. 251, fs.) connected with the

VI

PHYLUM NEMATHELMINTHES

315

suspensory ligament. From each a vas deferent (v. r//.), furnished with several
vesiculce seminal ex or sacs for the storage of spermatic fluid, passes backwards

FIG. 253. A, longitudinal section through the terminal twigs of the nephridia of Echino-
rhynchus gigas ; highly magnified. , nucleus. B, a terminal twig more highly magni-
fied. 6, the porous membrane. (From Shipley, after Kaiser.)

and unites with its fellow to form an ejacitlatory duct, with
about half a dozen cement glands (c.gL). The ejaculatory
bursa or bell-like copulatory organ (6), and has at its
opening a small papilla acting as a penis.

In the female the ovary is connected with the sus-
pensory ligament (Figs. 252 and 254, s.lg.). When
ripe, groups of ova known as the " swimming ovaries "
(x.ovy.) become detached and swim freely in the body-
cavity, where they are impregnated. The ducts are
very peculiar. Connected with the end of the sus-
pensory ligament is a muscular uterine bell (6), widely
open anteriorly (Fig. 254, cc) into the ccelome, and
having towards its posterior end a small aperture, also
communicating with the coelome (y). The bell is con-
nected with a narrow double passage leading to a
uterus (nt.}, which itself opens by the genital aperture
at the posterior end of the body. The uterine bell
performs rhythmical swallowing movements, and as the
eggs containing partly developed embryos float in
the ccfilome they are swallowed by the bell. The im-
mature eggs, which are globular, are passed back into
the coelome through the posterior aperture (y] of the
bell ; but the mature eggs, which are spindle-shaped
and covered with a chitinous investment, make their
way from the bell to the uterus through the narrow
passages, and so to the vagina.

The early stages of development take place in the
ccelome. Segmentation is regular, and a peculiar form
of gastrula is produced, having neither archenteron nor
blastocrele--in other words the ectoderm and endoderm
are in close contact with one another, and no central
cavity is enclosed by the latter. The ectoderm layer,
which is devoid of cell-limits, secretes a cuticular
membrane investing the embryo, then a second mem-
brane is formed within the first, and a third within the
second ; the embryo thus comes to be enclosed in a
triple case, which differs from an egg-shell in being

which are connected
duct opens into the

-sly

FIG. 254. Female organs of
Echinorhy nchus .

b. uterine bell ; .. Iff.
suspensory ligament ;
n t. uterus ; v<i. vagina ;
.'. ?/. aperture of bell ;
z. apertures leading
from bell to uterus.
(After Hertwig.)

310

ZOOLOGY

SECT.

formed by the developing ectoderm. At what will become the anterior end

chitinoid hooks appear.

At about this period the embryo is born, and reaching the intestine of the

host, is extruded with its foeces. Its further
development depends upon its being swallowed
by an intermediate host, which, in the case of
E. gigas of the Pig is a maggot, the larva of a
Beetle, Cetonia aurata. The Echinorhynchi of
fresh-water Fish have for their intermediate host
certain small fresh-water Crustacea belonging to
the genera Gammarus and Axellut.

Having reached the intestine of the inter-
mediate host, the chitinoid embryonic membranes
are dissolved by its digestive juices, and the
embryo either fixes itself to the wall of the in-
testine or makes its way into the coelome ; in
either case it soon begins to undergo further de-
velopment. The endoderm, hitherto a solid mass
of cells, undergoes a process of splitting, be-
coming divided into an outer layer in contact
with the ectoderm and a solid central axis. The
latter gives rise to the reproductive organs and
the suspensory ligament, the outer layer to an
epithelium, from which the body-muscles arise ;
the cavity formed by the splitting of the endo-
derm is the ccelome. Part of the proboscis and
its sheath are also of endodermal origin. The
ectoderm gives rise to the protoplasmic layer of
the body-wall, to the whole system of vessels,
and to the lemnisci. The larval cuticle is thrown
off and a new one formed. The larva reaches
adult proportions and attains sexual maturity
only if the intermediate host is eaten by the
permanent host.

-QV

-ovci

a-

-ho

FIG. 255. Sagitta hexaptera,
from the ventral aspect, a.
anus ; bg. ventral ganglion ;
d. intestine ; jl. lateral fins ;
ho. testis ; in. mouth; ov. ovary;
ovd. oviduct ; sc. cesophageal
connective ; sb. vesicula semin-
alis ; s. Jl. tail fin; *//, tail-
cavity;^, spermiduct. (From
Lang's Coiii/Hirti.tit-e Anatomy,
after Hertwig.)

preceding classes, is formed of
delicate basement membrane,

CLASS III, CH^ETOGNATHA.

The present group, like that just discussed,
is a very small one, containing only three genera
(Sagitta, Spadella and Krohnia) of curious arrow-
shaped worms, all but one species of which are
pelagic.

External characters. The body (Fig. 255)
is elongated and nearly cylindrical, and is divided
into head, trunk, and tail, the head being marked
off by its somewhat rounded form, while the junc-
tion of trunk and tail is indicated by the ventrally
placed anus (a). The tail bears a horizontal ex-
pansion, or caudal fin (.s. Jl.}, and there are also
horizontal lateral fin* (#. ) a single pair in
Spadella, two pairs in Sagitta.

Body-wall. There is no cuticle, but the
outer layer of the body- wall is formed by an
epidermis or deric epithelium (Fig. 256, d. eptlun],
which, instead of being syncytial as in the two
several layers of epithelial cells. Next comes a
and then a layer of muscles (m.), the fibres of

VI

PHYLUM NEMATHELMINTHES

317

which are striated and disposed longitudinally in four Lands two dorso-lateral
and two-ventro-lateral an arrangement which recalls that of the corresponding
layer in Nematoda.

Enteric Canal. --The mouth (Fig. 255, m.) is a longitudinal slit-like aperture
on the ventral surface of the head ; on either side of it are several sickle-shaped

d.ejatfvnt

m^ JL -^

Fir;. 256. Sagitta bipunctata. Transverse sections, A, of trunk ; B, of tail. coel. coelome ;
cvel. ept/im. layer of nuclei of the muscle-cells formerly regarded as a coelomic epithelium ;
(1. epthm. deric epithelium ; /. fin ; int. intestine ; m. muscles ; ovij. ovary ; s. testis. (After
Hertwig. )

chitinoid hooks (Fig. 257 gli-} which are moved by muscles in a horizontal plane
and serve as jaws. The anterior region of the head also bears spines, and is
strengthened by chitinoid plates and partly covered by a hood-like fold of the
integument.

The mouth leads by a muscular pharynx or stomodseum into a straight intes-
tine (rf), which extends through the trunk
and opens by the anus (a) at the junction
of trunk and tail.

Coelome. At the j auction of the head
with the trunk, and of the trunk with
the tail, are transverse partitions or
septa, dividing the coelome into compart-
ments. The trunk region of that cavity
is further sub-divided by two longitudinal
partitions, the dorsal and ventral mesen-
teries, which respectively connect the
dorsal and ventral surfaces of the intes-
tine with the body-wall : the tail-region
of the coelome is similarly divided into
right and left chambers by a longitudinal
vertical partition (Fig. 256, A and B).

There is no trace of vascular system
or of excretory canals. The nervous
system, on the other hand, is much
better developed than in either of the

preceding classes, in accordance with a free life and active movements. On
the dorsal side of the pharynx is a comparatively large brain (Fig. 257, </), which
sends off on each side a long nerve-cord, the cesophageal connective (sc.). The two
connectives sweep round the enteric canal and unite on the ventral surface, not

/tf

FIG. 257 Head of Sagitta bipunctata,

from above, an. optic nerve ; au. eye ;
g. brain : gh. hooks ; rn. olfactory nerve ;
ro. olfactory organ ; sc. oesophageal
connective. (From Lang's Comparative
Anatomy, after Hertwig.)

318

ZOOLOGY

SECT.

far from the middle of the trunk, in an elongated ventral yanylion (Fig. 255,
bg.), from which numerous nerves are given off. The brain sends nerves to the
eyes (Fig. 257, an.} and to the olfactory organs (?'O. ), and is also connected with
two pairs of ganglia in the head, which lie deeply sunk in the mesoderm : all the
rest of the nervous system retains its primitive connection with the ectoderm.

Sensory Organs. On the surface of the body are numerous little papilla?
carrying stiff bristle-like processes, and probably serving as organs of touch.

There are two eyes (Fig. 258),
situated one on each side of the
dorsal surface of the head : each
is globular and contains three
biconvex lenses (.), separated by
pigment (p. ) and surrounded by
rod-like sensory cells (rz. ). Behind
the head is a ring-like structure,
of the nature of an annular ridge
of peculiarly modified and in part
ciliated cells (Fig. 257, ro.) : to
this an olfactory function has been
assigned.

FIG. 208. Section of eye of Sagitta hexaptera.
e/i. epiderm ; L lens ; p. pigment ; rz. visual
cells ; st. rods. (From Lang's Comparative
Anatomy, after O. Hertwig.)

Reproduction. - - The Chseto-

gnatha are monoecious. The ovaries
(Fig. 255, ov., Fig. 256, ovy.} are
elongated organs situated one on
each side of the trunk-region of

the coelome, and opening by a narrow oviduct just in front of the posterior
septum. The testes (Fig. 255, ho. , Fig. 256, t*. ) are similarly situated in the tail-
region of the ccelome, and have the form of narrow ridges from which immature
seminal cells are given off and develop into sperms in the coelome. The spermi-
ducts or vasa deferentia are delicate tubes (*L ) opening at one end into the ccelome
by a ciliated funnel-like extremity, and at the other end dilating into a reservoir
or vesicula xeininalis (*&. ), which opens externally in the posterior region of the tail.
Development. Internal impregnation takes place, and the oosperm, seg-
menting completely and regularly, forms a typical gastrula by invagination (Fig.
259, A). Twoendoderm cells (#.)at the anterior end of the archenteron, i.e. the

FIG. 259. Three stages i?i the development of Sagitta. bl. blastopore ; at. ccelomic sacs; <L
mesenteron ; g. sexual cells ; pm. parietal layer of mesoderrn ; st. stomodamm ; vtn. visceral
layer of mesoderm. (From Lang's Comparative Anatomy, after O. Hertwig.)

end opposite to the blastopore, soon increase greatly in size, and are the rudiments
of the gonads. This precocious differentiation of the sex-cells is a point of con-
siderable importance, as will be seen hereafter. Before long these cells migrate
into the archenteron and divide, forming a group of four cells (B, <j. ), two of which
subsequently become the ovar and two the testes. At the same time two folds

VI

PHYLUM NEMATHELMINTHES

319

of endoderm grow into the archenteron from its anterior end, partly dividing the
cavity into three parts a middle division or rnwitfvron (d) the rudiment of the
intestine, and two lateral divisions the meteittera, or coelomic sacs (c..s. ) which
give rise to the right and left compartments of the ccelome of the trunk. From
the latter are given off in front a pair of small head-cavities. Owing to the
rapid elongation of the embryo in the stages following, all the cavities become
for a time obliterated : subsequently the cavities of the enteric canal and
ccelomic sacs re-appear ; the tail-region of the body-cavity is formed from the
posterior, undivided portion of the archenteron. The blastopore (bl. ) now closes
and an imagination of ectoderm the stomodajum ($t.) takes place at the
anterior end, and finally communicates with the mesenteroii.

From this it will be seen that the ectoderm of the gastrula gives rise to the
deric epithelium of the adult and to the epithelium of the pharynx, which is
therefore a stomodseum ; from the same layer the nervous system arises at a later
stage. The epithelium of the intestine arises from the mesial (inwardly -turned)
layers of the two endodermal ridges. The mus-
cular layer of the body-wall arises from the rest
of the endoderm, i.e. that portion of it which
remains in immediate contact with the ectoderm.
Thus in Sagitta the mesoderm is entirely derived
from the endoderm of the gastrula.

APPENDIX TO NEMATHEL-
MINTHES.

1. Family Chatosomidce.

This family includes three genera of small
worms, Chcvtosoma (Fig. 260), Tristicochceta, and
Rhabdogaster, which are sometimes included
among the Nematoda.

The body is elongated, its anterior region
being sometimes dilated to form a head. Either
the whole body, or the dorsal surface only, is
beset with fine setae, and there may be a double
row of movable chitinoid hooks round the head,
reminding us of the "jaws" of Sagitta. The
ventral surface bears curious locomotor rods (f),
either hooked or with knobbed ends : by these
the animals crawl. The mouth is anterior and
terminal, the anus posterior and ventral, and there
is a muscular pharynx. The sexes are separate.
The male has a single testis : the vas deferens
opens along with the anus : there are two penial

seta?. The female has paired ovaries and a single vagina opening near the
middle of the body on the ventral side.

FIG. 260. Mature female of
Chaetosoma claparedii.

X 57. a, oesophagus ; 6, in-
testine ; c, anus ; d, ovaiy ;
e, generative pore ; /, loco-
motor rods. (From Shipley,
after Metschmkoff.)

2. Family Echinoderidce.

Echinoderes is a minute marine worm of cylindrical form with a flattened
ventral surface. The body is segmented, or divided into rings, eleven or twelve
in number, all strongly cuticularised, and most of them bearing spines (Fig. 26 1 ).
The mouth is placed at the anterior, the anus at the posterior end of the body :

320

ZOOLOGY

SECT.

the former opens into a sac, which can be everted so as to form a proboscis or

in fro vert, and is armed with spines. The enteric canal consists of a muscular

pharynx and a straight intestine. A pair of

sacs opening by ciliated ducts on the tenth

segment appear to be excretory organs. The

sexes are separate : the gonads ai'e paired sacs

opening at the posterior end of the body.

3. Family Desmoscolecidce.

Desmoscolex is also a minute marine worm,
having a globular head and a variable number
of segments (Fig. 262). The head bears four
movable chitinoid rods or seta?, and a pair of

FIG 261. Echinoderes, x about 210. b, spine ;
c.s. caudal spine ; ph. pharynx ; s. and s'. spines
on the proboscis ; s.g. salivary glands ; st. stomach.
(After Hartog.)

FIG. 2(32. Female Desmoscolex,
venti'al view, x 260. a, ovary.
(From Shipley, after Pancei - i).

similar structures occurs on many of the other segments. The terminal mouth
leads by a muscular pharynx into a straight intestine : the anus is dorsal in posi-
tion. The animal is dioecious ; the gonads have the form of simple sacs, the testis
opening along with the anus, the ovary on the ventral surface anterior to the
anus. The male has a pair of penial seta?.

AFFINITIES AND MUTUAL RELATIONSHIPS OF THE

NEMATHELMINTHES.

The affinities of all the classes of Nemathelminthes are very
obscure, arid the propriety of grouping them into a single phylum
is extremely doubtful. They all agree in being elongated, cylin-
drical worms with a body-cavity, which is sometimes of the
nature of a true ccelome ; there is a certain resemblance

vi PHYLUM NEMATHELMINTHES 321

between Nematoda and Chsetognatha in the muscular system ;
and the lemnisci of Acanthocephala have been compared with the
cervical glands of Nematoda. Beyond these points there is little
to unite the three classes ; and, on the other hand, the proboscis
of Acanthocephala recalls the rostellum of Cestoda.

Very various views have been put forward as to the affinities ot
the Chsetognatha. But, in the absence of adequate evidence of
any near relationship with higher phyla, they may be regarded as
having their nearest known relatives, even if very remote, in the
Nemathelmintkes. In connection with this question, the Chaeto-
somida3, briefly described in the Appendix (p. 319), seem to
require consideration. Other possible relationships suggested by
the mode of development of the coelome from hollow diverticula of
the archenteron and by other features will be referred to in later
sections.

The three families placed as an Appendix to the phylum present
some undoubted resemblance to the Nematoidea : this is especially
the case in the reproductive organs of the Chaetosomidse, and still
more in those of Desmoscolex. But the segmentation of the body
in both Desmoscolecidse and Echinoderida3 and the presence of
setae show a certain resemblance to higher worms or Annulata,
which will be more fully appreciated when that phylum has been
studied.

VOL. I

SECTION VII

PHYLUM TROCHELMINTHES

THE typical larval form of a number of the groups which have
yet to be studied is a form which is known as the trochosphere
or, more usually, trochophore. It is necessary that a clear idea
should be formed at this stage of this important larva, reference
to which will very frequently be made in the sections that follow.
The general shape of a typical trochophore is oval or pear-like
(Fig. 263) with a broader and a narrower end and distinct bilateral

v.LM
oe.LM

SP
v.LN

O
Oe

-wz

wkr

d.LM

n

Neph
Msbr

J

J.

FIG. 263. Trochophore. A. amis ; d. IM. dorsal muscles ; ED. rectum ; /. stomach ; J l . in-
testine ; Mstr. mesoderm-band ; n. nerves ; A'tph. nephridia ; 0. mouth ; Oe. oesophagus ;
ceLM. cesophageal longitudinal muscle; SP. apical plate; v.LM. ventral muscle; v.LN.
lateral nerve ; W'Av, ickr., pr'e- and post-oral bands of cilia ; WS. apical cilia ; icz. adoral cilia.
(From Hertwig's Zoology, after Hatschek.)

symmetry. Encircling the body about the middle, or rather
nearer the broad than the narrow end, is a double circlet of strong
cilia, the pre-oral circlet ( Wkr.) or prototroch, situated on a corre-
sponding ring-like thickening of the ectoderm ; behind the mouth
is often a second circlet of cilia, the post-oral circlet (wkr.) and a
ciliated groove or ciliated streak usually runs backwards from it
along the middle of the ventral surface. The mouth, situated just

322

SECT, vii PHYLUM TROCHELMINTHES 323

behind the pre-oral circlet, leads into an alimentary canal, which at
first runs nearly transversely, and then bends round so as to extend
back towards the narrow end, near which it opens on the exterior
by an anal aperture. About the middle of the broader (anterior)
end of the trochophore is a thickening, the apical plate (SP.), pro-
jecting from which are usually a number of sensory cilia ( WS.) ; and
in many trochophores eye-spots and a pair of short tentacles occur
in close relation with the apical plate, which is the nerve-centre
of the larva. A pair of ciliated tubes the excretory organs or
nephridia (Neph.) may be present.

In the higher groups in which this form of larva occurs, the
adult condition is attained by modifications and new developments
of so radical a nature that the transition from larva to adult is of
the nature of a metamorphosis. Sometimes the narrow part of the
larva elongates and becomes divided into a series of sections fore-
shadowing the metameres of the adult animal ; in other cases, in
which no metamerism occurs, radical changes of other kinds lead
to the adult form. But in all these higher groups, whatever the
nature of the changes involved, there is a metamorphosis, and the
adult animal is totally unlike the larva. In a small number of
forms now to be dealt with, however, there is no such radical
change, and the adult may be looked upon as a somewhat modified
trochophore. The groups thus associated together may not be
be genetically related : they may have become independently
developed from trochophore-like ancestors, but the possession of
the general characters which have been referred to above renders
it convenient to group them together and regard them as con-
stituting a small but well-marked phylum. The groups referred
to are the Rotifera or Wheel-animalcules, together with the
Gastrotricha. Associated with these, though scarcely to be in-
cluded in the same phylum, are the Dinophilea and HistrioMeUea.

CLASS I. ROTIFERA.

The Rotifers or ' Wheel-animalcules "are microscopic creatures,
very abundant in pools, gutters, &c., and formerly classed with the
Infusoria, to which several of them bear a superficial resemblance.
But in spite of their minute size they are multicellular animals,
having an enteric canal, a spacious body- cavity, nephridial tubes,
gonads, a nervous system, and sense-organs, and have therefore no
real relationship with the Protozoa.

1. EXAMPLE OF THE CLASS Brachionus rubens.

External Characters. --Brachionus (Fig. 264) is one of the
commonest members of the class, being frequently found in abun-
dance in ponds, ditches, &c. The female is about J mm. ( T Vin.)in

Y 2

324

ZOOLOGY

SECT.

length, and is divisible into two distinct parts a broad anterior
region, the trunk, and a slender movable tail (t.). The trunk is
enclosed in a glassy cuirass or lorica (lr.), formed by a thickening
of the cuticle and produced into several spines : the tail is
wrinkled superficially and ends in two slender processes, together
forming a kind of forceps. One surface of the trunk is flattened,
and owing to the position of the mouth, is considered as ventral,

rn-

Lr

FIG. 264. Brachionus rubens, female. A, from the dorsal aspect ; B, from the right side,
a. anus ; br. brain ; d. /. dorsal feeler ; c. gl. cement-gland ; cl. cloaca ; c. 1. ciliary lobes ; c. v.
contractile vesicle ; e. eye-spot ; int. intestine ; lr. lorica ; I. f. lateral feeler ; m. muscular
bands ; nph. nephridial tubes ; ov. germarium ; ph. pharynx ; st. stomach ; t. tail ; tr. <L
trochal disc ; vt. vitellarium. (After Hudson and Gosse.)

the opposite or dorsal surface is convex both from before back-
wards, and from side to side.

The anterior portion of the body projects from the lorica in the
form of a transverse disc (tr.d.) with a prominent edge fringed with
cilia : this is the Irochal disc, and is one of the most characteristic
organs of the class. By the action of the cilia the animal is
propelled through the water, and, as in Vorticella, their successive
flexion gives an appearance of rotation to the disc or " wheel-
organ " whence the name of the class is derived. Within the circlet
of cilia arise three prominences (c.l.) covered with cilia of large

vii PHYLUM TROCHELMESTHES 325

size. The trochal disc is not perfectly symmetrical, but has at one
part of its circumference a depression in which the mouth lies :
this marks the ventral surface. The anus (a.) is dorsal in position,
and is placed at the junction of the tail with the trunk.

The body-wall consists of an epidermal layer, without cell-
limits, covered by a chitinoid cuticle : it is by a thickening of the
latter in the region of the trunk that the lorica is produced.
There is no continuous muscular layer, but several bands of
unstriped muscle (m.) pass from the lorica to the trochal disc
in front and to the tail behind, and act as retractors of those
organs.

Digestive Organs. --The mouth (Fig. 267, wA.) lies, as already
mentioned, in the ventral region of the trochal disc, anterior to the
ciliary circlet but posterior to the three ciliated lobes ; it leads by
a short buccal cavity into a pharynx (ph.) of peculiar structure
known as the mastax, and constituting one of the most character-
istic organs of the class. The mastax is a muscular chamber
(Fig. 265) of rounded form,
and contains, as a thickening
of its cuticular lining, an
elaborate apparatus for tri-
turating the food. In the
middle line is a forked struc-
ture, the incus, consisting of a
small base or fulcrum (/.) and
of two branches or rami (r.).
On either side of the incus is
a hammer-like structure, the

j, ... PI FIG. 265 Pharynx of Brachionus rubens.

ma/leUS, Consisting OI a handle /. fulcrum; m. manubrium; . uncus; r.

or manubrium (m.) and of a

toothed head or iincus (u.).

By means of the muscular walls of the chamber the heads of the

mallei are worked backwards and forwards upon the forked uncus,

and thus reduce the organisms taken as food to a fine state of

division.

The pharynx leads by a short gullet into a spacious stomach (st.),
having a wall composed of very large epithelial cells, ciliated
internally : with it are connected paired digestive glands. The
stomach opens into a rounded intestine (int.), also ciliated internally,
which communicates, by means of a short cloaca (cl.), with the ex-
terior. The stomach and intestine are formed from the archenteron
of the embryo and are therefore lined by endoderm : the rest of the
enteric epithelium is ectodermal, the pharynx being derived from
the stomodseum, the cloaca from the proctodasum. Between the
body- wall and the enteric canal is a spacious body-cavity contain-
ing a fluid which serves the purpose of blood and contains minute
granules.

326

ZOOLOGY

SECT.

The excretory system consists of paired nepliridial tubes
(Figs. 264 and 267, nph.) resembling those of the Platyhelminthes.
Their general direction is longitudinal, but they are a good deal
coiled and give off little tag-like processes ending in flame-cells.
With the end of each tag, projecting into the body-cavity, is a long
flagellum. The lumen of the tubes is intra- cellular : it is uncertain
whether or not the cavities of the flame-cells communicate with the
body-cavity by apertures in their walls. Posteriorly the nephridial
tubes open into a bladder or contractile vesicle (c. v.\ the contents
of which are discharged, by periodical contractions, into the
cloaca.

Nervous System and Sense Organs.- -There is a single
ganglion or brain (Figs. 264 and 267, lr.\ of proportionally large

ov

FIG. 266. Brachionus rubens. A, male ; B, female, with attached eggs ; c. gl. cement-
glands ; rv, contractile vesicle ; nph. nephridial-tube ; or. ovum in body ; ov 1 . ova attached
to base of tail ; p. penis ; t. tail ; ts. testis. (After Hudson and Gosse.)

size, situated at the anterior end of the body, above (dorsal to) the
mouth and pharynx. On the dorsal surface of the brain, where it
comes into contact with the body-wall, is a small red eye-spot (e.).
The only other organs which can be considered as sensory are
three structures known as tactile rods or feelers ; one of these (d.f.)
is a small cylindrical process tipped with stiff hair-like bodies,
which projects from the dorsal surface just behind the trochal disc :
the other two (/./.) are paired, situated on the dorsal surface of
the lorica and not prominent.

The tail contains a pair of cement glands (c. gl.), by the secretion
of which the animal is able temporarily to attach itself.

Reproduction and Development.- -The sexes are lodged
in distinct individuals, which present a striking degree of sexual
dimorphism. The preceding description applies to the female,

VII

PHYLUM TROCHELM1NTHES

327

which is the form most commonly met with. In addition to the
organs already mentioned, it has a germarium (ov,, ovy.}, connected
with a large vitellarium (vt.) and opening by an oviduct into the
cloaca.

The male (Fig. 266, A) is a very minute creature, not more than
one-fourth the size of the female, and is strangely degenerate in
structure. The enteric canal is absent, the trochal disc simple in
structure, the nervous system and nephridial tubes greatly reduced,
and the greater part of the body occupied by a large testis (ts.)
which opens by a duct at the extremity of a protrusible, dorsally
placed penis (p.).

After extrusion the eggs are attached to the base of the tail
of the female (B, ov'.), where they undergo development: they are

c.yl

FIG. 267. Diagram of a Rotifer, o. anus ; br. brain ; cl. pro-oral, and A post-oral circlet of cilia ;
c. gl. cement gland ; cl. cloaca ; cu. cuticle ; (/. ep. deric epithelium ; d.f. dorsal feeler ; e. eye ;
fl. c. flame-cells ; int. intestine ; TO. muscles ; mth. mouth ; nph. nephridial tube ; ov. ovum ;
ovd. oviduct ; ovy. germarium. ; ph. pharynx ; st. stomach ; vt. vitellarium.

of two sizes, the larger giving rise to females, the smaller to
males. Probably both kinds develop parthenogenetically, but in
the autumn thick-shelled winter eggs are produced which appear
to require fertilisation. These remain quiescent during the winter,
and in the spring develop into females.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Rotifera are Trochelminthes of microscopic size. The ante-
rior end is modified into a retractile trochal disc, with variously
arranged cilia ; the posterior end usually forms a mobile and
often telescopically jointed tail. The mouth is anterior and more
or less ventral in position, the pharynx contains a chitinous
masticatory apparatus, and the anus is placed dorsally at the
junction of the trunk with the tail. There is a spacious body-cavity
devoid of epithelial lining. The excretory organs are a pair of
nephridial tubes provided with flame-cells. The central nervous

328 ZOOLOGY SECT.

system consists of a single dorsal ganglion, with, in a few cases, a
smaller ventral or sub-oesophageal ganglion. The sexes are
separate, and the males are, in nearly all cases, smaller than the
females and degenerate in structure.

The class is divided into five orders as follows :

ORDER 1. RHIZOTA.

Rotifera which are fixed in the adult state by the truncated end
of the non-retractile tail.

Including Floscularia, Stephanoceros, Mdiccrta, &c.

ORDER 2. BDELLOIDA.

Rotifera which both swim freely by means of the cilia of the
disc and creep after the manner of a Leech. The tail is telescopic
and forked distal ly.

Including Rotifer, Philodina, &c.

ORDER 3. PLOIMA.

Rotifera in which locomotion is performed by the ciliated disc
only. The tail is usually forked and more or less retractile.

Sub- order a. Illoricata.

Plo'ima in which the trunk is not covered by a lorica.
Including Hydatina, Polyarthra Asplanchna, &c.

Sub-order b. Loricata.

Ploima in which a lorica is present.
Including Brackionus, JBucklanis, &c.

ORDER 4. SCIRTOPODA.

Rotifera provided with setose appendages moved by striped
muscles : skipping movements are performed by the aid of these,
as well as swimming movements by the trochal disc. The tail is
either absent or is represented by a pair of ciliated processes.

Including Pedalion and Hexarthra.

ORDER S.TROCHOSPPLERIDA.

Globular Rotifera having the trochal disc represented by an equa-
torial circlet of cilia ; tail absent.
Including Trochosphcera only.

VII

PHYLUM TROCHELMINTHES

329

ORDER 6. SEISONIDA.

Marine parasitic Rotifers (Fig. 268), with the trochal disc
reduced, the body long, narrow, and ringed, with a long slender

ce

br

ov

FIG. 268. Paraseison asplanchnus, female x 230. a. genital aperture ; br. ganglion ; /.
foot-glands ; m. mouth ; ma. mastax ; oe. oesophagus ; ov. ovary ; st. stomach. (After Plate.)

neck-region, and an elongated foot provided at its extremity with
a perforated disc.

Systematic Position of the Example.

BracJiionus rubens is one of the several species of the genus
BracJiionus : it belongs to the family Brachionidce, and to the
sub-order Loricata of the order Plowia.

It is placed in the order Ploima in virtue of its active swimming
habits and the absence of looping or skipping movements. The
presence of a distinct lorica places it in the sub-order Loricata.
The family Brachionidse is distinguished by having a box-like
lorica open at both ends, and a long, flexible, retractile tail with
wrinkled surface and forceps-like termination. In the genus
Brachionus the lorica is not marked with ridges, and the tail is

330 ZOOLOGY SECT, vii

very long and perfectly retractile. In B. rubens the anterior edge
of the lorica is produced dorsally into six spines and is sinuous
ventrally.

3. GENERAL ORGANISATION.

External Characters. The majority of the Rotifera are free-
swimming, being propelled rapidly through the water by the action
of the trochal disc. But in the Bdelloida (Fig. 269, 5), in addition
to this mode of progression, the animal performs looping move-
ments like those of a leech : the tail in this order is freely jointed,
the various segments fitting into one another like the tubes of a
telescope, and the body is fixed alternately by it and by the anterior
end, the trochal disc being kept retracted while the animal moves
in this way. Many of the Pioima also have a telescopic tail, but
in some, e.g., Asplanchna (Fig. 269, 6), this organ is absent. In
Pedalion (Fig. 270, 1) curious skipping movements are performed
by the aid of six hollow limbs or appendages, one dorsal, one
ventral, and two on each side. These curious organs are ter-
minated by feathered seta?, and closely resemble the limbs of
some of the lower Crustacea : each is moved by two opposing
muscles which extend into its cavity (1, B, m). Three pairs of
similar appendages are present in the other genus of Scirtopoda,
Hexarthra (Fig. 270, #), the resemblance of which to the nauplius
larva of Crustacea is very striking (see Fig. 429), and four genera
of unarmoured Pioima, e.g. Polyarthra (Fig. 269, 8} possess simple
or fringed setse moved by muscles attached to their bases.

In the Rhizota the adult is permanently fixed (Fig. 269, 1-4).
The end of the tail is devoid of the characteristic fork, and is
attached to plants or other supports. Moreover the animal is
surrounded by a tube into which it can retrace itself completely,
protruding the anterior end with the trochal disc when undis-
turbed. In most instances, as for example in Floscularia (1) and
Stephanoceros (2), the tube is formed of a delicate, transparent,
gelatinous secretion of the epidermis ; but in Mdicerta (3) it is
built up of rounded pellets, which the animal moulds in a cup-like
depression on the dorsal surface and places in position one by one.
The pellets are usually formed of foreign particles, but in some
species are made of the animal's own fasces.

The ciliation of the trochal disc is subject to considerable
variation. In its simplest form the disc is surrounded by a single
circlet of cilia, within which lies the mouth. A modification of
this type may be produced by the prolongation of the ciliary
crown into long arm-like processes fringed with cilia, as in
Stephanoceros (.2), or, as in Floscularia (1\ into blunt elevations
bearing long stiff cilia like those of the Heliozoa. The single
circlet may be folded upon itself, or a second type may be pro-
duced by the addition of a second circlet within and parallel to

c~

I.Flos cular ia

2.Srephanoceros 3. MelicerTa

a

4.Melicerra

5. Phi lod ina

6. A s p I a nchna

8. Po ly arl-hra

9 . N o r e u s

7. H y.d a N n a

10.N orhoica

FIG. 269. Typical forms uf Rotifera : 1 and 10 shuw the lorica only. a. anus ; c j . <:-. ciliary
circlets ; inf. intestine ; HI. muscle ; ph. pharynx. (After Hudson and Gosse.)

332

ZOOLOGY

SECT.

the first. The mouth in this case is always placed between the two
circlets on the ventral side (Fig. 207), so that the inner or anterior
circlet is pre-oral and corresponds with the chief ciliary band

m.

2. H e x a r I- h r a

nph-

3-Trocho sfjhaera

FIG. 270. Typical forms of Rotifera. In 1, a shows the outer form, b the muscular system.
a. anus ; br. brain ; ci. c 2 . ciliary circlets ; cl. cloaca ; d. gl. digestive gland ; d. !. dorsal limb ;
e. eye-spot; /. L, 1. I', lateral limbs; >/i, muscles; mtlt, mouth; nph. nephridial tube;
ov. ovary ; ph. pharynx ; s. sense-organ ; v. I. ventral limb. (After Hudson and Gosse
(1 and 2) and Korschelt and Heider (3).)

of a trochosphere larva., while the outer or posterior circlet corre-
sponds with the post-oral band found in many worm-larvae. In the
curious globular Trochosphvera (Fig. 270, 3) there is a single
equatorial circlet, which is pre-oral, and a few post-oral cilia : here
the correspondence with the typical worm-larva is singularly

VII

PHYLUM TROCHELM]NTHES

333

close. Lastly, both the pre- and post-oral circlets may be pro-
duced into more or less complex lobes, as in Melicerta (Fig. 269,4),
or may be interrupted as in Brachionus, in which the pre-oral
circlet is represented by three distinct lobes, or as in Pedalion,
in which both circlets are divided into right and left moieties.
In one genus the trochal disc is absent.

Digestive Organs. --The typical form of mastax or pharyngeal
mill is that described in Brachionus (Fig. 265). There is an un-
paired incus consisting of a short stem or fulcrum (/) and of two
broad branches or rami (r), and a pair of mallei, each consisting
of a stout handle or manubrium (m) and a broad, toothed head or
uncus (11). In some forms all the parts of the apparatus become
very slender, the incus assuming the form of forceps (Fig. 271, A).
Or the mallei may be absent and the two rami movable upon
one another so as to convert the incus into a pair of forceps (B)

FIG. 271. Typical forms of mastax. A, forcipate type ; B, incudate type ; C, rarnate type.
/. fulcrum ; TO. manubrium ; r. ramus ; u. uncus. (After Hudson and Gosse.)

used to seize prey, the mastax being in this case protrusible.
Lastly, the fulcrum and manubrium may be absent, and the unci
and rami very strong and massive (C). Glands, supposed to be
salivary, open into the mastax or oesophagus.

The stomach is always large, and usually has a pair of digestive
glands opening into it : it may pass insensibly into the intestine,
or the latter may be a distinct chamber of more or less globular
form. In the Rhizota the intestine turns forwards so as to allow of
the anus being brought over the edge of the tube in defecation
(Fig. 269, 4, a )- In Asplanchna (6) the stomach ends blindly, the
intestine, cloaca, and anus being absent.

The excretory system is very uniform in structure. It con-
sists of a pair of more or less coiled nephridial tubes, placed
longitudinally and giving off lateral branchlets which end in
flame-cells. The outer surface of each flame-cell usually bears
one or sometimes two flagella, which lie free in the body-cavity.

334 ZOOLOGY SECT.

Frequently, but not always, the two tubes open posteriorly into
a contractile vesicle or bladder which discharges into the
cloaca.

Nervous System and Sense Organs.- -The nervous system
usually consists of a single ganglion (Fig. 267, br) towards the
dorsal aspect of the anterior part of the body, and representing the
brain or supra-oesophageal ganglion of the higher Worms : it sends
nerves to the muscles, trochal disc, and tactile organs. In some
cases a smaller ventral or infra-oesophageal ganglion is present
as well, connected with the first by a pair of slender cesophageal
connectives. Connected with the dorsal ganglion are a pair
of lateral longitudinal nerves which run backwards to the tail,
giving off branches in their course. One or more eyes (e) are
usually present in close relation with the brain, and are sometimes
mere spots of pigment, but may be provided with a refractive
body or lens. The only other organs of sense are the tactile rods
(d.f., //.), of which there is usually one on the dorsal surface near
the anterior end of the body, and frequently two others,
one on each side of the trunk. They are more or less rod-like
structures, tipped with delicate sensory hairs and receiving nerves
from the brain.

Reproduction and Development. In most cases the female
reproductive organs have the same general character as in Brachi-
onus, i.e. the gonad is unpaired (Fig. 264), consists of germarium and
vitellarium, and is provided with an oviduct (Fig. 267). But in
some of the Bdelloida, such as Philodina, there are two ovaries, not
divisible into germ-gland and yolk-gland, and the oviduct is absent.
The males are smaller than the females and degenerate in structure,
the enteric canal being atrophied (Fig. 266, A). There is a large
testis (t) with a duct opening at the end of a protrusible penis (_p),
which is dorsal in all but Asplanchna, in which ifc, as well as the
cloacal opening of the female, appear to be ventral. Apparently
hypodermic impregnation sometimes takes place, i.e. the body- wall
of the female may be perforated at any place for the entrance
of the sperms.

Three kinds of eggs are produced : large and small summer eggs,
which always develop parthenogenetically, the larger giving rise to
females, the smaller to males ; and thick-shelled winter eggs, which
probably require impregnation, and remain in an inert condition all
through the winter, finally developing in the spring. Most Rotifers
are oviparous, but some (Philodina, &c.) bring forth living young,
which are born by breaking through the body-wall or through the
cloaca, thus causing the death of the parent.

Segmentation is total and irregular, the oosperm dividing into
megameres and micromeres. An epibolic gastrula is formed, the
blastopore closes, and invaginations of ectoderm give rise to the
stomodgeum and proctodseum. The tail is formed as a prolongation

vii PHYLUM TROCHELMINTHES 335

of the postero-ventral region of the embryo, and contains at first
an extension of the endoderm. No metamorphosis is known to
take place in any member of the class.

Ethology. A few Rotifers live in the sea, but the majority
are fresh-water forms, occurring in lakes, streams, ponds, and even
in puddles the water of which is rendered foul and opaque by mud
and sewage. Frequently the water in which they live is dried up,
and the thick-shelled winter eggs may then be widely dispersed
by wind. It is even stated that the adult animals may survive
prolonged desiccation and resume active life when again placed in
water. They are able to survive prolonged exposure to tem-
peratures far below the freezing point of water. Many forms
cling to the bodies of higher animals in order to obtain a share
of their food, thus leading a kind of commensal existence.
Others go a step further and become true external parasites,
like Drilophaga on a fresh-water Oligochaete (vide Section X),
or Seison on the little Crustacean Nebalid (Fig. 457). Others, again,
are internal parasites, such as Albcrtia in the ccelome of Earthworms
and the intestines of fresh- water Oligochaates (Nais), and Notommata
u'erneckii in the cells of the fresh-water Alga Vaucheria.

Affinities. The affinities of the Rotifera are very obscure.
Their general resemblance to the free-swimming larvae of Annelids
(phylum Annulata) is extremely close, and, in particular, the
curious TrocJiospTicara is, to all intents and purposes, a sexually
mature trochosphere with a mastax. The excretory organs recall
those of the Platyhelminthes. arid also resemble the provisional
nephridia or head-kidneys of Annulate larvae. Lastly, the hollow
muscular appendages of Pedalion and Hexarthra give those genera
a certain resemblance which is probably, however, merely adaptive
to the nauplius or free-swimming larva of Crustacea.

Class II. GASTROTRICHA.

The GttHtrofricJut (Figs. 272 and 273) are a small group of minute fresh-water
animals, which are apparently allied, though certainly not very closely, to the
Rotifera, and are on that account placed in the present phylum. The body is
spindle-shaped with flattened ventral surface. The ventral surface bears two
longitudinal bands of cilia ; the dorsal is non-ciliated, but in some forms bears a
number of longitudinal rows of slender, pointed, cuticular processes. The aboral
end is narrow and usually bifurcated.

On the head are four tufts of flagella, which are parti} 7 sensory, partly
vibratile. The mouth, situated at the anterior end, leads bv a narrow tube into

/

the thick-walled oesophagus. At the beginning of the latter are a number of
small chitinous denticles, and in front of them a circlet of sets. The oesophagus
leads to a wide elongated stomach followed by a short intestine which terminates
in an anal aperture at the posterior extremity. The nephridia are a pair of
unbranched coiled tubes each opening on the ventral surface and terminating

336

ZOOLOGY

SECT.

internally in a flame-cell. The nervous system consists of a large dorsally and
anteriorly situated cerebral ganglion or brain giving off a pair of ventro-lateral
longitudinal nerves. The sexes are united, and there is no metamorphosis.

W- M

rn.

O&S

FIG. 272. Chaetonotus maximus.

Highly magnified. (After Zelinka.)

FIG. 273. Chaetonotus maximus (or-
ganisation), brn. brain ; gld. adhesive
gland ; mes. mesenteron ; mo. mouth ;
ces. oesophagus ; ov. ovum ; ovar. ovary ;
retr. retractor muscles ; vent. mus. ventral
muscle. (After Zelinka.)

APPENDIX TO THE TROCHELMINTHES.

The Dinophilea and Hislriobdell&a.

These are two isolated groups of minute animals which may most conveniently
be dealt with in association with the Trochelminthes, since they bear certain
striking resemblances, now to one, now to another, member of that phylum ;
but they differ from all of them in the assumption of a simple kind of meta-
merism (p. 43), by virtue of which they have claims to association with the

VII

PHYLUM TROCHELMINTHES

337

Annulata a phylum to be treated of later. The Diiiophilea are free-living animals,
mostly marine, one species living in brackish water. The Histriobdellea are
parasitic or commensal, and live on the European lobster and the Australian
fresh-water crayfishes.

Diiiophilus (Fig. 274) is a minute worm-like animal with a head or pro-
stomium, a body composed of from five to eight segments separated from one
another by constrictions, and a short ventral tail. The prostomium bears two
eye-spots and some sensory hairs : it is either covered uniformly with cilia, or
bears two or three annular ciliated bands apparently representing the prototroch
of the trochophore. The body is in some of the species uniformly ciliated ; in
others the cilia are disposed in rings corresponding to the segments, except 011

_ - n

3.X

Fin. 274. Dinophilus taeniatus. The left figure represents the dorsal surface of a young
individual, x 76 ; the mouth and alimentary tract are seen by transparency. The right
figure shows the anatomy of the male, x 38. an. anus ; b. rectum ; c. body-cavity ; d. vas
deferens ; m. pharynx; n'. the first nephridium ; ce. entrance to the oesophagus ; p., in left
fig., prostomium; p., in right fig., penis; st. stomach; s. x. vesiculte seminalis. (From
Sheldon, after Harmer.)

the ventral surface, where the ciliation is always uniform. The mouth, which is
situated on the ventral aspect of the prostomium, leads into an alimentary canal
consisting of oesophagus, stomach, and intestine, all of which are ciliated ; the
anus (an) is placed dorsally over the tail. A protrusible muscular proboscis lies,
when retracted, in a recess opening close to the mouth. There is an imperfectly
developed ccelome which is crossed by strands of connective tissue. A nervous
system is present, and consists of a large dorsal ganglion in the prostomium,
giving off two anterior, and two posterior nerves or ventral cords (sometimes
segmented into a series of ganglia connected in each segment by commissures),
all situated in the epidermis.

The excretory system consists of a series of metamerically arranged pairs of

VOL. 1

Z

338

ZOOLOGY

SECT.

br.

tubes (??/). The inner ends of these do not open into the body-cavity, but are
provided with peculiarly modified flagellate cells known as solenocytes, so that
these paired excretory tubes resemble closely the nephridia of some of the Poly-
chseta (phylum Aiimtlata ; see Section X.). The sexes are separate. In the male
there is a conical ventral penis ; the last pair of nephridia act as vesiculse seminales.
In the ovary two sets of ova are developed, the larger destined to give rise to
females, and the smaller destined to form males. They pass into the body-cavity
and reach the exterior by an aperture on the ventral surface in front of the anus.

A process of unequal segmentation is
followed by the formation of an
epibolic gastrula. What is known
of the development is in favour of
the view that Dinopliilus is to be
looked upon as a trochophore-like form
that has made some progress in the
evolution of metamerism.

The Histriobdellea comprise only
the two nearly-allied genera Hixtriob-
della and Stratiodrilus (Fig. 275) the
former found on the eggs of the Euro-
pean lobster, the latter in the gill-
cavities of Australian and Tasmanian
fresh -water crayfishes. The animal is
narrow, almost cylindrical, with a
well-marked head, a body of six seg-
ments, and a narrower tail-region in
which segmentation is not clearly
marked. The head bears five tentacles
(t l , t 2 , 3 ) tipped with non-motile sen-
sory cilia, and a pair of retractile
appendages or limbs (I, a), with basal
glands the ducts of which open at
their extremities. The head has the
mouth at its anterior extremity on
the ventral aspect. The body bears,
in Stratiodrilu.*;, three pairs of two-
jointed non-retractile appendages or
cirri (c 1 , c 2 , c 3 ) tipped with non-
motile cilia, and in the male a pair
of retractile appendages or claspers
(eZ). At the end of the tail is a pair
of large freely movable appendages
or legs (lp), which are the organs of
locomotion : at the end of each of
these open the ducts of a mass of
unicellular glands. The anus is situ-
ated posteriorly between the bases
of the legs. Opening from the mouth -
cavity on its ventral aspect is a
muscular sac in which are enclosed,
when retracted, a system of chitinous
jaws reducible to the same general

FIG. 275. Stratiodrilus tasmanicus,
male. ac. accessory gland of male ap-
paratus ; br. c brain; c j . c' 2 . c3. cirri; d.
claspers (appendages peculiar to the male) ;
ex. excretory tubes ; gr. gld. granule-gland ;
1. a. anterior limb ; I. gl. gland at base of
anterior limb ; I. gld. gland at base of pos-
terior limb; I. p. posterior limb; n. c.
nerve-cord ; p. penis ; fl. t~. t'-\ tentacles ;
ves. vesicula seminalis.

type as the mastax of the Rotifera,

but with the relative position of malleus and incus inverted. There is a
highly developed nervous system consisting of a large brain (br. c.) situated
dorsally in the prostomium, a pair of cesophageal connectives, and a ventral
nerve cord (we) with a series of ganglia which have a distinctly metameric

vii PHYLUM TROCHELMINTHES 339

arrangement. The excretory system takes the form of ciliated tubes (ex}, closed
internally, and showing a tendency to metamerism : these extend into the head.
The sexes are distinct : the male has a protrusible penis, directed ventrally.
There is no metamorphosis.

There seems to be some reason for believing that Dinophilns and the
Histriobdellea may help to bridge over the interval between the Trochelminthes
and the higher segmented worms or Ammlata. In this connection the
Echiuoderidse, which were noticed in an appendix to the last Section (p. 319),
have also to be kept in view.

z 2

SECTION VIII
PHYLUM MOLLUSCOIDA 1

THE phylum Molluscoida comprises three classes the Polyzoa
(including, provisionally, the Endoprocta), the Bracliiopoda,
and the Phoronida. The members of these three classes are
tolerably widely divergent, so that it is somewhat difficult to
frame a general account of the entire phylum ; but the following
are the most important common features :

There is, except in the Endoprocta, a body-cavity (coelome), lined
in most cases with a coelomic epithelium, within which the ali-
mentary canal is suspended by means of mesenteries or by means
of funicular strands taking their place. The dorsal region of the
body is abbreviated, being represented only by a short space
between the mouth and anus, which are closely approximated.
There is a lophophore or tentacle-bearing ridge, usually of a horse-
shoe shape, containing a special compartment of the ccelome, and
overhanging the mouth on its anal side there is in most cases a
sensitive process the epistome also containing a special com-
partment of the body-cavity. The central part of the nervous
system consists of a single ganglion (supra-cesophageal), or of two
ganglia (supra-cesophageal and infra-oesophageal), or of a nerve-
ring. The nephridia when present are in nearly all cases a single
pair of ciliated tubes, which act also as gonoducts.

CLASS I. POLYZOA.

The Polyzoa form colonies known as " Sea-mats, 3 ' or " Coral-
lines," which in many cases bear a close general resemblance to

1 This and all the remaining phyla of the animal kingdom are characterised
by the possession of a true <:(?lome, i.e. of a cavity interposed between the
wall of the body and that of the enteron, and developed either directly by
outgrowth from the archenteron, or formed from clefts that appear in solid
masses of mesoderm cells. The only group hitherto dealt with in which a definite
coelome is present is the Cheetognatha. In some of the groups which are here
comprised in the coelomate phyla, however, as will be seen, the coelome is
reduced, or entirely absent, or not typically developed.

340

SECT, viii PHYLUM MOLLUSCOIDA 341

Hydroid Zoophytes, and only on a more minute inspection are
found to differ totally from the latter and to exhibit a very much
higher type of structure.

1. EXAMPLE OF THE CLASS. BUGULA AVICULARIA.

Bugula avicularia, the common Bird's-Head Coralline (Fig. 276),
occurs in brown or purple bushy tufts, two or three inches long, on
rocks, piles of jetties, and similar situations on the sea-shore in all
parts of the world. On a naked-eye examination it presents a
considerable resemblance to a Hydroid Zoophyte, and might readily
be taken for a member of that group. It consists of dichotomously
branching narrow stems, which are rooted by a number of slender
root-filaments. Each stem is found, when examined with a lens, to
be made up of a number of elements, the zocecia of the colony,
which are closely united together and arranged in four longitudinal
rows. The zooecia are approximately cylindrical in shape, but
broader distally than proximally, four or five times as long as broad,
and have, near the distal end, a wide crescentic aperture the
" moutli " of the zoopcium on either side of which is a short blunt
spine. A rounded structure the ooscium in many parts of the
colony lies in front of each zooecium (Fig. 276, ocec.). On each
zooecium, except a few at the extremities of the branches, is a
remarkable appendage, the avicularium (avic), having very much
the appearance of a bird's head supported on a very short stalk :
if the Bugula is examined under the microscope in the living
condition, the avicularia will be found to be in almost constant
movement, turning from side to side ; and a movable part, com-
parable to the lower jaw of the bird's head, will often be seen to
be moved in such a way that the mouth of the avicularium is
opened very widely and then becomes closed up with a quick
" snap." All the parts hitherto mentioned can be shown, by using
appropriate tests, to be composed of some material akin to chitin
in composition. The chitinous wall of the zooecia is the hardened
and thickened cuticle of the zooids, having beneath it the soft body-
wall. 1 The anterior region of the body of the zooid forms an
introvert, i.e. is capable of being involuted like the finger of a
glove within the more posterior part : the cuticle covering this, con-
tinuous behind with the thick ectocyst, is quite thin and flexible.
When the introvert is everted it is seen to bear at its anterior end a
circlet of usually fourteen long, slender filiform tentacles (tent) on a
circular ridge or loplwplwre surrounding the mouth of the zooid. The
tentacles are densely ciliated except along their outer surfaces : the
cilia vibrate actively in such a way as to drive currents of water,

1 The terms ectocyxt and mdocyst are commonly applied respectively to the
hardened cuticle of the zooid and its soft body-wall.

342

ZOOLOGY

SECT.

and with them food-particles, towards the mouth (mo) : they are
also capable of being bent in various directions. In the interior of

auic

avw

FIG. 276. Bugula ayicularia. Two zooids, magnified, an. anus ; tv>. avicularia ; emb.
embryo enclosed in the o<ecium ; 'funic. funiculus ; gaxt. muscular bands passing from the
stomach to the body-wall ; int. intestine ; j/io. mouth ; on . miM-ium-; ten. (esophagus ; ov. ovary ;
ph. pharynx; ret. parieto- vaginal muscles; sp. spermatidia ; stoiit. stomach; tenf. tentacles.
The ganglion, which is not indicated, lies just below the middle of the stroke from mo.

each is a narrow prolongation of the coelome. In all probability,
besides bringing minute particles of food to the mouth of the zooid
by the action of their cilia, the tentacles are prehensile as well as

vin PHYLUM MOLLUSCOIDA 343

tactile, and also act as organs of respiration. When retracted they
become enclosed by the walls of the introvert as by a sheath-
the tentacle-sheath. A pair of bands of muscular fibres the parieto-
vaginal muscles (ret.) passing to the introvert from the body- wall,
serve to retract the introvert and tentacles.

The body- wall consists, in addition to the cuticle, of an epidermis
composed of a single layer of large flattened cells, two muscular
layers, the outer circular and the inner longitudinal, and a layer of
an irregular cellular tissue, or parenchyma.

The ccelome is extensive ; it is lined externally by the parietal
layer of parenchyma forming the innermost layer of the body-wall,
and internally by a visceral layer of the same tissue, ensheathing
the alimentary canal. Across the cavity between the parietal and
visceral layers of the parenchyma pass numerous strands of spindle-
shaped cells. A large double strand (funic) passes from the
proximal or aboral end of the alimentary canal to the aboral
wall of the zooecium ; this is the funiculus. A transverse partition
cuts off (though not completely) a small anterior compartment of
the ccelome from the rest. The former surrounds the basis of
the tentacles, the narrow internal cavities of which are in com-
munication with it : this is known as the circular canal. The
coelomic fluid contains a number of colourless corpuscles or
leucocytes.

Alimentary Canal.- -The mouth (mo) leads into a wide
chamber ihe pharynx (ph) just behind the bases of the tentacles ;
from this a somewhat narrower short tube, separated by a constric-
tion from the pharynx, leads to the stomach (storn) from which it is
also separated by a constriction. The stomach gives off a long
conical prolongation or ccecum passing towards the aboral end of
the zooecium, to which it is attached by the funiculus. The
intestine (int) comes off from the oral aspect of the stomach, not
far from the oesophagus, with which it lies nearly parallel : it ter-
minates in a rounded anal aperture (an) capable of being dis-
tended to a considerable size, situated not far from the mouth,
but outside the lophophore. The entire alimentary canal is lined
by an epithelium, which is ciliated throughout except in a portion
of the stomach : the cells of the epithelium, which are arranged
in a single layer, vary in length in different regions, being longest
in the pharynx, which is comparatively thick- walled. A pair of
slender muscles (gast) passing from the body-wall to the stomach
act as .retractors of the alimentary canal when the introvert is
drawn back.

There are no blood-vessels.

The nervous system consists of a small round ganglion situated
between the mouth and the anus, giving off nerves to the
various parts ; organs of special sense are absent. Definite ex-
cretory organs do not occur in Bugula, the function of excretion

344 ZOOLOGY SECT.

(i.e. the collection of the nitrogenous waste-matters) being appar-
ently carried on by the leucocytes and the cells of the funicular
tissue.

Reproductive Organs. Ovary and testis are found to occur
together in the same zooid. They are both formed from specially
modified cells of the parenchyma, either of the funiculus or of the
body-wall. The testis, developed from the cells of the funicular
tissue, gives origin to spherical masses of cells the spermatidia
(sp) which develop into sperms with very long motile tails. These
become free from one another and move about in the body-cavity
or in its prolongations into the tentacles. There is no spermiduct,
and it is doubtful if the sperms pass to the exterior. The ovary
(ov) is a small rounded body formed from the parietal layer of the
parenchyma about the middle of the zooecium ; it consists of only
a small number of cells of which only one at a time becomes a
mature ovum, certain smaller cells forming an enclosing follicle.
The mature ovum is perhaps fertilised in the ccelome ; it passes
into the interior of a rounded outgrowth of the zoceciurn the
ocecium (ocec) lined with parenchyma, and forming a sort of brood-
pouch in which it undergoes development.

Development. Segmentation (Fig. 277) is complete and
nearly regular. A blastula is formed having the shape of a
bi-convex lens. In the interior of the blastocoele or cavity
of the blastula, four cells (end) the primitive endoderm
cells- -become distinguishable: these increase in number by
division, and form a mass of free cells which almost completely
fill the blastocoele ; this mass apparently represents both
endoderm and mesoderm. Small cavities which appear in it
subsequently unite together to form the primitive coelome.
A very broad ring-shaped thickening the corona (G, cor.)
is formed round the equator of the embryo and becomes
provided with cilia. A circular pallial groove arises on the
oral side of the corona. A sac-like, afterwards beaker-shaped
invagination of the ectoderm on what is destined to become
the oral side of the ciliated ridge, forms a larval structure, termed
the sucker (Fig. 278, suck), which afterwards serves to fix the larva.
A second depression of the ectoderm in the region of the corona
on the oral side forms the ectodermal groove. At the aboral pole is
developed, also from the ectoderm, a second larval structure the
calotte or retractile disc (disc), on which motionless sensory cilia
appear. In close relation to the ectodermal groove is formed a
mass of cells, the pyriform organ (p).

An alimentary canal is absent in the larva of Bugula when it
escapes from the orecium. After an interval of free existence as a
ciliated larva, certain changes appear which lead to a very
complete metamorphosis. The sucker becomes everted by a
strong contraction of the body, and fixes the larva to some foreign

VIII

PHYLUM MOLLUSCOIDA

345

object. The aboral side of the larva becomes greatly extended, so
that almost the entire integument of the primary zooid is devel-
oped from this part (i.e. from the region occupied by the retractile
disc and pallial groove). Accompanying the extension of the
aboral surface are the obliteration of the pallial groove and the
bending down of the corona towards the oral side. Thus the stage
of the larva termed the umbrella-shaped stage is reached. The
sucker is everted, and by means of it the larva becomes attached.
The edge of the " umbrella " becomes bent downwards, and

end.

eel:

cor

FIG. 277. Early stages in the development of Bugula. cent, central mass of cells ; cor. corona
ect. ectoderm ; end. endoderrn ; se<j, segmentation-cavity. (After Vigelius.)

fused with the broad plate into which the sucker has ex-
panded, thus enclosing a circular cavity, the so-called vestibule
(Fig. 279, v). The walls of this, consisting of the coronal cells
and a portion of the original sucker, become broken up and the
cavity is merged in the general cavity in the interior of the
larva. All the larval structures have now disappeared with the
exception of the basal plate of the sucker and the retractile disc.
The former gives rise to the basal part of the wall of the primary
zocecium. From the latter, which becomes invaginated, or from
a sac which is developed to replace it, are developed both
the ectodermal and endodermal structures of the primary zooid.

346

ZOOLOGY

SECT

Occupying the interior of the larva at this stage in addition to
this sac, there is only a mass of undifferentiated tissue derived
from the original central tissue together with that derived

disc

B

cor

cent

cor

suclt

FIG. 278. A, Larva of Bugula plumosa ; B, Sagittal section of larva of Bugula (diagram-
matic), cent, central tissue ; cor. corona ; disc, retractile disc ; e. ectodernial groove ; p. pyri-
form organ ; pall, pallial groove ; suck, sucker. (From Korschelt and Heider, after Barrois.)

from the disintegrated corona, pyriform organ, and part of the
sucker. The outer wall forms the wall of the primary zooecium, the
surface of which becomes covered with a chitinous cuticle or
ectocyst. Most of the internal mass goes to form a brown body,
which now becomes developed, but a part of it seems to form the

mesoderm of the zooid. A diverticulum
of the sac constitutes the first rudiment
of stomach and intestine ; a second
diverticulum forms the rudiment of the
oesophagus ; these become applied to
one another and fuse to form the con-
tinuous alimentary canal. The ganglion
arises as an invagination of the ecto-
derm in the space between mouth and
anus. The upper part of the cavity
of the primitive sac, after the rudi-
ment of the alimentary canal has been
separated off, forms a space termed the
atrium ; the walls of this become con-
verted into the tentacle sheath, while
on its base appear the rudiments of the
tentacles and lophophore. During the

development of the organs of the adult zooid the brown body
becomes closely applied to the stomach and gradually absorbed.

The primary zooid thus formed gives rise asexually by a process
of repeated budding to the branching structure which has been

FIG. 279. Longitudinal section of
attached larva of Bugula.
c. cells of corona; r. rudiment
of the zooid in the form of a
sac ; s. basal plate of everted
sucker ; v. vestibule. (From
Korschelt and Heider, after
Barrois.)

vm PHYLUM MOLLUSCOIDA 347

described. In many of the zooecia of a fully-developed colony no
zooid is found to be present, but, instead, there is a dark brown
body similar to that which occurs in the primary zooecium. This
is a zooid that has undergone degeneration the lophophore,
tentacles, and alimentary canal having become absorbed. Such
degenerated zooids are capable of regeneration, the organs becoming
re-developed and the brown body re-absorbed.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Polyzoa are Molluscoida which, with one exception, form
colonies of zooids connected together by a common organic sub-
stance. There is a lophophore bearing a series of slender, cilated,
post-oral tentacles. The anterior part of the body forms, in the
majority, a short introvert, within which the lophophore and the
tentacles are capable of being withdrawn. In some the pro-
stomium is represented by a small lobe the epistome. The
alimentary canal is U-shaped, and the anus is anterior, within, or
just outside, the tentacular circlet. In most the nervous system
is represented only by a small ganglion between the mouth and
the anus. A cuticle, sometimes gelatinous, sometimes horny,
sometimes calcined, forms a firm exoskeletal layer for the support
of the colony. Nephridia (corresponding to the head-nephridia of
the trochophore) occur only in the Endoproda. There is no
vascular system. The sexes are usually united. The majority of
Polyzoa occur in the sea ; a limited number are inhabitants of
fresh water.

Sub-Class I. Ectoprocta.

Colonial Polyzoa with the anus outside the lophophore, with a
well-developed introvert and a spacious coelome.

ORDER 1 . GYMNOL^MATA.

Almost exclusively marine Ectoprocta, with a circular lopho-
phore, and without an epistome.

Sub- order a. Cyclostomata.

Gymnolsemata with tubular calcareous zooecia having circular
apertures devoid of closing apparatus.
Including Grisia, Idmonea, &c.

Sub-order b. Cheilostomata.

Gymnola3mata with calcareous or chitinous zooecia usually pro-
vided with opercula.

Including Bugula, Flustra (" Sea-mat ") Membranipora, Cellepora,
Selenaria.

348 ZOOLOGY SECT.

Sub-order c. Ctenostomata.

Gymnolyemata with chitinous or gelatinous zooecia provided with
a series of tooth-like processes closing the aperture when the
tentacles are retracted.

Including Alcyonidium, Serialaria, Paludicella.

ORDER 2. PHYLACTOL^EMATA.

Fresh-water Ectoprocta with horse-shoe-shaped lophophore and
with an epistome.

Including Cristatella, Plumatella, Fredericella.

Sub-Class II. Endoprocta.

Colonial or solitary Polyzoa multiplying by the formation of
buds, which in Loxosoma soon become separated off, while in
Pedicellina they remain connected together by a creeping stolon.
The anus, as well as the mouth, is internal to the lophophore.
The introvert is slightly or not at all developed. A pair of ciliated
nephridial tubes are present.

Systematic position of the Example.

Bugula avicularia is an example of the sub -order Cheilostomata
of the Gymnolsemata. It is a member of the family Bicellariidas,
which is characterised by the erect plant-like colony, with narrow
compressed branches, and attached by root-like fibres; by the
avicularia, when present, being stalked and bird's-head shaped ; and
by the wide oblique apertures of the zooecia all facing in the same
direction. Bugula differs from the other genera of the family in
the arrangement of the zooecia in double or multiple rows, in their
close union, and in the avicularia, when present, being on the side
on which the mouth is situated. The various species differ in the
exact shape of the zooecia and of the avicularia.

3. GENERAL ORGANISATION.
Sub-Class I. Ectoprocta.

The Ectoprocta and the Endoprocta differ so considerably from
one another that it is advantageous to deal with them separately.
The Ectoprocta are all colonial the colonies being capable, in
most cases, like the colonies of hydroid zoophytes, of increasing in
size to an apparently indefinite extent by continuous budding.
The thickened cuticle which forms the support of the colony is
sometimes gelatinous, sometimes chitinous, sometimes chitinous
with sand-grains affixed, sometimes calcareous. The form of the

viii PHYLUM MOLLUSCOIDA 349

colony varies in different families and genera in accordance with
differences in the shape of the constituent zooecia, and differences
in their mode of budding and consequent arrangement. The
zocecia are sometimes tubular, sometimes ovoid, sometimes poly-
hedral. In some cases the buds are so developed that the colony
assumes the form of a thin, flat expansion, which may be encrusting,
and consist of a single layer of zocecia in close contact with one
another or connected together by tubular processes ; or may be
erect, and with the zooecia either in one or two layers : sometimes
the lamellar colony thus formed may be fenestrated or divided into
lobes ; sometimes it is twisted into a spiral. In other cases the
colony, instead of being lamellar, has the form of an erect, shrub-
like structure, consisting of numerous cylindrical, many-sided, or

I /" tf^'Xar^F' * L*risl4-;

state

y

FIG. 280. Plumatella. Portion of a colony, magnified, funic. funiculus ; gang, ganglion;
int. intestine ; mo. mouth ; a 1 , oesophagus ; repr. gonad ; retr. retractor muscle ; st. stomach ;
stato. statoblasts. (After Allman.)

strap-shaped branches arising from a common root. Sometimes
there is a creeping cylindrical stolon, simple or branched, having
the zooids arranged along it in a single or double row. The colony
is free only in Cristatella (Fig. 281) in which it performs creeping
movements, in some other (American) forms of Phylactola3inata
(in the younger stages of the colony), in one family of the
Cheilostomata the Selenariidce, (in which it moves along with
the aid of certain peculiar appendages the vibracula to be
described subsequently), and in one or two other cases.

The zooecia open on the exterior by means of circular, semi-
circular, or crescentic apertures, which in the Phylactolsemata and
the Cyclostomata among the Gymnolsemata are devoid of any special
closing apparatus ; while in the Cheilostomata there is a movable

350

ZOOLOGY

SECT.

lid or operculum closed by a pair of occlusor muscles when the
introvert is retracted ; and in i/he Ctenostomata there is a series of
lobes or teeth which close in together over the opening. The
cavities of the neighbouring zocecia are in some forms completely
cut off from one another by a continuation of the chitinous or
calcareous exoskeleton ; in others there is free communication ; in
others, again, there is communication through a number of minute
perforations.

The oral (anterior) part of the body of each zooid is, as already
described in the case of Bugula, covered only with a thin and

[FiG. 281. Cristatella mucedo. Entire colony. (After Allman.)

flexible cuticle, and forms an introvert capable of being retracted
into the interior of the zooecium. At the free end of the introvert
is the mouth surrounded by a lophophore bearing tentacles. The
tentacles are always simple, filiform, and hollow, each containing a
narrow diverticulum of the circular canal or anterior compartment
of the coelome. They are beset with vibratile cilia by means
of which currents are created subserving alimentation and
respiration. They are also highly sensitive, and are capable
of being bent about in various directions by the contraction
of muscular fibres in their walls, so that they can be

VIII

PHYLUM MOLLUSCOIDA

351

used for prehension. In the Phylactolsemata (Fig. 280) the
lophophore is horse-shoe-shaped, in the Gymnola^mata (Fig. 276)
circular : in the former, but not in the latter, there is a ciliated
lobe, the cpistomc (Fig. 282, cp) which may have a sensory func-
tion overhanging the mouth on the anal side. The retraction 01
the introvert is effected by a pair of bands of muscular fibres, the
parieto-vaginal muscles, passing to it from the body-wall, and
by a pair of retractor muscles passing from the latter to the ali-
mentary canal.

Structure of body- wall. Beneath the cuticle is an epi-
dermis, consisting of a single layer of flattened polygonal cells,
firmly united together by their edges. Beneath this there
is usually, but not always, a layer of muscle, which is
arranged in two strata an
external composed of circular,
and an internal of longitu-
dinal fibres. There is an ex-
tensive ccelome lined in some
forms (Phylactolsemata) by a
definite coelomic epithelium,
in part ciliated ; while in
others there is no such de-
finite epithelium, but its place
is taken by thin parietal and
visceral layers of an irregular
cellular tissue the paren-
chyma. Crossing the coelome
are strands, in some instances
very numerous, of spindle-
shaped cells. In some cases
two mesenteric bands sus-
pend the alimentary canal -
an anterior attached near the
mouth and a posterior passing
from the caecum to the aboral end of the zocecium ; in most
cases the latter, to which the special name of funiculus is given, is
alone present.

The alimentary canal has in all species the parts that have
been already described in the case of Bugula. In some of the
Cheilostomata it is stated that the cells of the oesophagus bear
numerous striated muscle-fibre processes. In some Ctenosto-
mata there is in addition a thick-walled chamber the gizzard
with chitinous teeth, between the oesophagus and stomach.

The nervous system consists of a single, sometimes bilobed,
ganglion (Fig. 280, gang, and Fig. 282, go) placed between the
mouth and the anal aperture, and of nerves passing from it
to the various parts. There are never any organs of special

FIG. 282. Anterior portion of the body of
Lophopus, from the right side. an. anus ;
ep. epistoma ; ga. ganglion ; o. rnouth ; pr. in-
testine ; st. oesophagus ; t. tentacles, cut off
near the base. (From Lang's Comparative
Anatomy. After Allman.)

352 ZOOLOGY SECT.

sense, unless the epistome of the Phylactolsemata be of that
nature.

Nephridia are not known with certainty to exist in any of
the Ectoprocta. In some there is a pore through which water
enters the body-cavity, or a ciliated intertentacular tube opening
at the base of the tentacles. Excretion appears to be performed
by certain cells of the funicular tissue and of the parenchyma
or ccelomic epithelium. These become loaded with the products
of excretion, and are set free as leucocytes in the ccelome, whence
they may pass out through the intertentacular tube or may
accumulate in the cells of the brown body.

In many Ectoprocta the colony bears a series of remarkable
appendages the avicularia which are of the nature of modified
zooids. In typical cases the avicularium has the bird's-head-like
form that has been already described in the case of Bugula ;
sometimes it is completely sessile. A second set of movable
appendages found in some forms are the vibracula ; these
are long tapering whip-like appendages which execute to-and-fro
movements. The avicularia are frequently found to have seized
in their jaws minute Worms or Crustaceans, and it is probable
that their function, as well as that of the vibracula, is defensive ;
in the case of the Selenariidce, which form unattached colonies, it
is said that the movements of the vibracula subserve locomotion.

The impregnated ova in many cases undergo the early stages of
their development in certain dilatations of the colony (Fig. 276,
oac.\ and in many of the Gymnolaemata (Cheilostomata) these
ovicells or occcia, as they are termed, take on a very definite
shape.

Reproduction and Development. As a general rule the
Ectoprocta are hermaphrodite. Both ovary and testis are derived
from the layer lining the ccelome (parenchyma or ccelomic
epithelium as the case may be), or from the funicular tissue. The
testis may be single or double. The spermatidia, as in Bugula, or
the mature sperms, become free in the ccelome. The ovary is very
generally situated towards the oral end or about the middle, the
testis towards the base. The mature ova escape into the ccelome,
and in some forms there become impregnated apparently by the
spermatozoa of the same individual. The development of the
larva may take place in the coelome or in a special diverticulum of
it ; in the Cheilostomata the fertilised ova pass into the ovicells ;
in some cases, both among the Phylactolsemata and the
Gymnolsemata, they are received into a sheath formed by the
tentacles of an imperfectly-developed zooid formed in a zocecium
in which the original zooid had undergone degeneration.

In those cases in which the early stages of development are
passed through in the body-cavity of the parent, the ciliated
embryos may either escape through the zocecial aperture after the

vin PHYLUM MOLLUSCOIDA 353

zooid has undergone degeneration, or through a special opening
formed for them in the wall of the zooacium. In some the fertilised
ova pass out through the intertentacular tube. In Crisia&nd other
Cyclostomata each of the ripe ocecia is found to contain a large
number of embryos, developed from one ovum. The ovum in this
genus segments to form a mass of cells from which finger-like pro-
cesses arise, the end of each of these becoming constricted off to
form an embryo.

Segmentation is total and approximately equal. The form of
the free-swimming larva varies considerably, but in most there is a
circular band with very long cilia, the corona, which may represent
the tentacular crown of the adult ; this divides the surface into
two regions oral and aboral. The larva may or may not be
provided with a digestive canal. The aboral portion of the body
presents a ciliated retractile disc or calotte ; on the oral side is the
sucker by which the larva afterwards becomes fixed. By a metamor-
phosis similar to that which has been described in the case of Eugula
(p. 344), a primary zooecium with a primary zooid is developed from
the previously free ciliated larva. In the Cyclostomata the larva
is barrel-shaped, with the mouth at one end, and at the other a
prominence corresponding to the retractile disc. In the
Phylactolsemata the larva is in the form of a ciliated hollow cyst
from which the colony is formed by gemmation. A special form
of asexual multiplication by means of bodies termed statob/asts
(Fig. 280, stato) is observable in the Phylactolaemata. The
statoblasts are internal buds formed from the funiculus and
enclosed in a chitinous shell ; they are set free eventually by the
death and decay of the parent colony, and in spring each gives
rise to a small zooid which fixes itself and develops into a
colony.

Ethology and Distribution. None of the Ectoprocta are
parasites in the strict sense of the term, but very many of them
live in intimate association with other organisms, often growing
over and through them so as to form with them one complex
structure. Certain genera are able by some means to excavate
minute burrows in the shells of bivalves.

The majority of Ectoprocta are marine ; but all the Phylacto-
kernata, together with Paludicella of the Ctenostomata, are in-
habitants of fresh water. The fresh-water forms inhabit both
running and stagnant waters ; they occur at all elevations and
are represented in all the great regions of the earth's surface.
The marine forms are most abundant at moderate depths ;
but representatives of the group have been dredged from as
great a depth as over 3,000 fathoms. In certain localities the
larger kinds grow in great luxuriance, so as to form miniature
forests.

Geologically the Ectoprocta are a very ancient group, being

VOL. I A A

354 ZOOLOGY SECT.

represented in the Cambrian and later Palasozoic formations by
forms which appear to have belonged mainly, if not exclusively,
to the Cyclostomata. In the later formations of the Mesozoic period
the Cheilostomata are also abundantly represented, and in the
Tertiary the latter sub-order greatly outnumbers the Cyclostomata.
The Tertiary Polyzoa flourished in certain localities in such
luxuriance that their remains form calcareous deposits of very
great extent.

Sub-Class II. Endoprocta.

While the sub-class of the Ectoprocta comprises a large number
of genera, that of the Endoprocta includes only Pedicellina (Fig. 283),
Loxosoma, Urnatdla, Myosoma, G-onopodaria and Ascopodaria, with
one or two other less completely known forms. They are all
marine except Urnatella an American fresh-water genus. The
feature indicated by the name of the sub-class viz. the position
of the anus within the circlet of the tentacles, is an important
point of difference from the rest of the class ; but there are others
of as great or greater importance.

In none of the Endoprocta is there is a distinct introvert. The
body is cup-shaped, with a rim which is capable of being inverted
over a cavity the vestibule within which the tentacles can be
withdrawn, and which contains both mouth and anus. An epistome
overhangs the mouth. The ccelome is almost or quite obliterated,
the space between the alimentary canal and the wall of the body
being filled, more or less completely, with a gelatinous hyaline
matrix. A pair of nephridia are present. In Loxosoma they lie
one on each side of the oesophagus and open separately on the
exterior ; they are ciliated intra- cellular tubes, each of which
probably begins in a flame cell. In Urnatella the two nephridial
tubes unite to open into the cloaca a diverticulum of the
vestibule. The ganglion (Fig. 283, gang), situated between mouth
and anus as in the Ectoprocta, is bilobed in Loxosoma. Testes and
ovaries occur in the same individual in some, but appear to mature
at different times : they are provided with special ducts ; in others
the sexes are separate.

Pedicellina and Urnatella are colonial, Loxosoma solitary. In
Pedicellina (Fig. 283) there is a creeping stolon with which a
number of zooids are connected ; a diaphragm separates the body
of each zooid from the stalk. Gonopodaria ramosa has a branching
stalk. Urnatella has a disc of attachment with one to six, jointed,
branching stems. In Loxosoma, which is found attached to
various Annulata, two parts are distinguishable the calyx or body
and the stalk. In the base of the latter is the so-c&lled foot-gland,
consisting of a small number of granular cells arranged around a
central space opening on the exterior. Buds are formed, but

VIII

PHYLUM MOLLUSCOIDA

355

become detached before reaching maturity. Segmentation of the
ovum is complete, and a gastrula is formed by invagination.

Certain differences in the larval history have sometimes been
regarded as separating very widely the Endoprocta from the
Ectoprocta. The former, like the latter, have a free-swimming
ciliated larva, provided with a corona and a ciliated disc. This
develops directly into the primary zooid after becoming attached
by means of the oral surface. The ectoproct larva also, as stated
previously (p. 344), becomes attached by the oral surface ; but any
rudiments of a zooid such as an alimentary canal which may

terrf

tent

rrw

FIG. 283. Pedicellina. Showing successive stages (numbered 1 to (5) in the development of
zooids by budding, an. anus ; gang, ganglion ; mo. mouth ; tent, tentacles (retracted). (After
Hatschek.)

have been developed, become absorbed, and the primary zooid is
developed at the free or aboral end of the larva, with its oral
surface directed upwards, away from the base of attachment.
The difference, however, is not so important as it may at
first appear, for the parts of the larval Endoproct do not remain in
the reversed position in which they are situated when attachment
first takes place, with the vestibule, mouth, and anus directed
downwards. Very soon a rotation is observed to take place, by
virtue of which the vestibule and developing tentacles, with the
mouth and anus, become carried to their permanent position on
the free-surface of the animal.

CLASS II.-PHORONIDA.

The position of Phoronis, a worm-like marine animal, is a
matter on which widely divergent views are held. On account of
certain strong resemblances to the Polyzoa, and, more particularly,

A A 2

350

ZOOLOGY

SECT.

to the Phylactolsemata, it is most commonly looked upon as
related to that class and to the Brachiopoda, and the Phoronida
may thus conveniently be dealt with as a class of the Molluscoida.
Phoronis (Fig. 284) lives in associations consisting of a number
of individuals, all of which are developed from ova, there being no
process of asexual formation of buds. Each worm is enclosed in a

membranous or leathery tube, within which it
is capable of being completely retracted. The
body is cylindrical, elongated, and unsegmented.
At one end there is a crown of numerous
slender, ciliated tentacles borne on a horse-shoe-
shaped lophophore, the lateral cornua of which
are spirally coiled in the larger species ; these
are supported by a mesodermal skeleton and
are non-retractile.

Both mouth and anus (Fig. 285, mo, an) are
situated at this tentacular extremity of the
body, separated from one another by only a
short space. This short space between mouth
and anus represents, as in the Polyzoa, the
greatly abbreviated dorsal surface ; but it will
be convenient to term this end of the animal
the anterior, and the opposite the posterior
end : the side of the elongated body towards
which the mouth is approximated may be dis-
tinguished as the oral, the opposite as the
anal. A small lobe the epistome (ep) over-
hangs the mouth and lies between it and the
anus. Near the anus open two ciliated ne-
phridial tubes (neph) of mesodermal origin,
which open internally each by two apertures
into the posterior chamber of the ccelome.

The coelome, which is lined with a coelomic
epithelium, consists of three main parts of
very unequal extent. The first (prosoccele) is
a narrow cavity in the epistome. The second
(mesoccele), which is in communication with the
first, lies in front of a transverse septum or
mesentery extending between the mouth and
anus, and perforated by the oesophagus but
not by the rectum ; it is prolonged round the lophophore and
gives off narrow diverticula to the hollow tentacles. The third,
and by far the most extensive part of the ccelome (metaccde),
occupies the whole of the length of the body behind the trans-
verse septum. It is subdivided into two by a median longi-
tudinal mesentery (Fig. 287, ra, m.), which extends from the oral
to the anal surface and supports both limbs of the alimentary

FIG. 284. Phoronis
australis, natural
size.

VIII

PHYLUM MOLLUSCOIDA

357

FIG. 285. Phoronis australis, free end,
magnified, an. anus ; ep. epistome ; mo. mouth ;
nephr. nephridial aperture ; neph. nephridium ;
(After Benham.)

tent

canal ; and each of these is further subdivided by a longitudinal

mesentery extending from the body-wall to the oesophagus (a?)

in the one compartment

(usually termed the right),

and to the rectum (?) in

the other (left). The ali-

mentani canal is bent on

V ,

itself to form a loop, as in
the Polyzoa: it is distinguish-
able into oesophageal, gastric
and intestinal regions. There
is a closed system of Hood-
vessels with contractile walls
containing red blood-cor-
puscles. The nervous system
lies immediately below the
cells of the epidermis. Nerve-
elements are generally distributed over the surface, but are
specially concentrated in the form of a ring surrounding the
body just behind the mouth, but not enclosing the anus,

thickened into a ganglion be-
tween mouth and anus, and
giving off nerves to the ten-
tacles. There are no organs of
special sense.

Phoronis is hermaphrodite.
Ova and sperms are developed
in the coelome towards the pos-
terior end from cells on the wall
of one of the large blood-vessels.
When mature these pass out
through the nephridia to the
spaces enclosed by the tentacles,
where the ova are impregnated
( according to another account,
fertilisation takes place in the
ccelome ), and they go through
the early stages of development
fixed to the tentacles. The
segmentation is complete and
slightly unequal : when four blas-
tomeres are formed two larger,
darker endoderm and two smaller,
clearer ectoderm cells are to be
distinguished. A blastula

FIG. 280. Phoronis australis, internal
organisation, af. bl. afferent blood vessel ;
an. anus ; ef.M.' efferent blood vessel ; ep.
epistome ; mes. mesentery ; mo. mouth ;
iici>ltr p. nepbridiopore ; nephr. d. duct
of nephridium ; nephrost. nephrostome
(internal opening of nephridium ; ces. oeso-
phagus ; reel, rectum ; rect. mes. rectal
mesentery ; sept, septum ; tent, tentacles
(cut short). (After Benham.)

IS

formed with clearer ectoderm cells on one side ; imagination
takes place; and, as the embryo elongates, the blastopore is

358

ZOOLOGY

SECT.

drawn out into a slit which eventually becomes closed up behind,
the anterior portion alone remaining open to form the mouth.
The anus is developed later as an invagination in the position
of the posterior part of the former blastopore. The mesoderm
arises from cells budded off from the endoderm. The prosocoele
and mesocoele arise by the formation of fissures ; the metacoele
by a process of folding off from the archenteron. A large pre-
oral lobe is formed, and the anus becomes surrounded by a circlet
of cilia (Fig. 288, A). The part of the body on which the anus

ef. v

m

ne

777

FIG. 287. Phoronis, transverse section towards the anterior-end, nf. v. afferent blood-vessel ;
c. m. circular layer of muscular fibres ; ef. v. efferent blood-vessel ; ep. epidermis ; c. in. cir-
cular layer of muscle ; in, m. mesenteries ; ne. f. funnel-like opening of nephridium ; ce.
oesophagus ; '/. rectum. (After Benham.)

is situated becomes elevated into a conspicuous process. Behind
the mouth there is a circlet of cilia, and from this region grow
out a circlet of processes the rudiments of the larval tentacles
(B). The larva has now reached the stage to which the term
actinotrocha is applied. It has a large hood-like lobe overhang-
ing the mouth and a circlet of ciliated larval tentacles ; the
anus is situated on a prominent process.

There is a pair of larval excretory organs corresponding to those
of the trochophore larva (p. 322) : these apparently do not
become converted into the nephridia of the adult. A thickening

VIII

PHYLUM MOLLUSCOIDA

359

of the ectoderm of the pre-oral lobe, sometimes bearing eyespots,
appears to represent the apical plate of the trochophore. At the
point where the oesophagus opens into it, the gastric region of the
alimentary canal gives off forwards in one species a pair of hollow
diverticula, the cells of which contain vacuoles like those of the
neighbouring parts of the stomach itself.

FIG. 288. Fhoronis, development. A, young larva ; B, larva after the formation of the post-
oral circlet of tentacles ; C. larva with commencing pit-like involution ; D, larva with imagina-
tion partly everted ; E. invagination completely everted, in. mouth ; an. anus ; iv. involution
to form body. (From Balfour's Embryology.)

The ectoderm of the process 011 which the anus is situated
subsequently becomes involuted to form a deep pit (C, iv), and
rudiments of the adult tentacles are formed as a ring of processes
at the bases of the larval tentacles. The metamorphosis from this
point is completed with great rapidity. The larva sinks to the
bottom ; the pit at the side of the anal elevation becomes everted
(D), and the alimentary canal of the larva is drawn into it (E\ the
projection thus formed, which grows out at right angles with the

360 ZOOLOGY SECT.

long axis of the larva, becoming the body of the future animal ;
the larval tentacles and pre-oral lobe become thrown off, and the
lophophore is developed.

CLASS III. BRACHIOPODA,

The Brachiopoda are the fabricators of the well-known "Lamp-
shells " found in most parts of the world. They occur in the sea
at various depths, and were formerly classed under the Mollusca,
their characteristic bivalved shell being compared with that of
oysters, mussels, &c.

1. EXAMPLE OF THE CLASS Magellania (Waldheimia) lenticularis

or M. flavescens.

Magellania lenticularis is found in great numbers, at moderate
depths, off the coast of New Zealand. An allied species, M. flavescens,
is equally common in the Australian seas, and several other species
are known in various parts of the world.

The body is entirely covered by a shell (Fig. 289) of oval form
and pink colour, composed of two pieces or valves, one of which, dis-
tinguished as the ventral valve (v. v\ projects beyond the other
or dorsal valve (d. v\ in the form of a short conical beak (b) perfor-
ated at the end by an aperture, the foramen (b), through which
passes a dark brown stalk or peduncle (Fig. 290, B, pd) of horny
consistency. In the natural state the peduncle is attached to a
rock or other support, and the animal lies with the ventral valve
uppermost and with the valves gaping slightly. The pointed or
peduncular end of the shell is considered to be posterior in posi-
tion, the opposite end or gape anterior.

It will be convenient to consider the shell first. Both valves are
deeply concavo-convex, of a pinkish colour outside, white within.
The ventral valve (Fig. 289), as already stated, is produced poste-
riorly into a beak (5), terminating in a foramen (/) for the peduncle.
The distal margin of the foramen is left incomplete by the shell
proper, but is closed by a small double plate, the deltidium (d).
Immediately anterior to the beak is the curved hinge-line along
which the valve articulates with its fellow, and just anterior to
the hinge-line the inner surface of the shell is produced into a pair
of massive, irregular hinge-teeth (t). On the inner surface of the
valve, towards its posterior end, are certain shallow depressions
marking the attachments of muscles (ad. m, d. m).

The dorsal valve (D) has no beak, but its posterior edge forms
a hinge-line which is produced in the middle into a strong cardinal
process (c. p) with a curiously folded surface : when the two valves
are in position this process fits between the hinge-teeth of the

VIII

PHYLUM MOLLUSCOIDA

361

ventral valve, the hinge-teeth in their turn being received into de-
pressions (s) placed on each side of the cardinal process. The inner
surface of the dorsal valve is produced into a median ridge or
septum (sp\ continuous posteriorly with the cardinal process, and
attached on either side of the base of the latter are the two ends
of a delicate calcareous ribbon, the shelly loop (s. I), which projects

G

t/.v

d.-m'

cut, m.

FIG. 289. IVIagellania flavescens. A, the entire sliell from the dorsal aspect, and B, from
the left side ; C, interior of ventral valve, and D, of dorsal valve, ad. m. adductor impres-
sions ; b. beak ; c. p. cardinal process ; d. deltidium ; d. m. divaricator impressions ; d. v.
dorsal valve ; /. foramen ; p. m. protractor impressions ; s. tooth-socket; s. I. shelly loop ;
sp. septum; t. hinge-tooth; v. aj. m. adjuster impressions; v. v. ventral valve. (After
Davidson.)

freely into the cavity enclosed between the two valves, and has the
form of a simple loop bent upon itself. The inside of the dorsal
valve also has muscular impressions.

Externally both valves present a series of concentric markings
parallel with the edge or gape : these are lines of growth, the
shell being built up by new layers being deposited within those
previously formed, and projecting beyond them so as to form a
series of outcrops.

362 ZOOLOGY SECT.

Microscopically the shell consists of prismatic rods or spicules
of carbonate of lime, placed obliquely to the surface and separated
from one another by a thin layer of membrane. It is also tra-
versed, perpendicularly to the surface, by delicate tubules which
begin on the inner surface in microscopic apertures and extend
to within a short distance of the outer surface.

The actual body of the animal (Fig. 290, B) lies at the posterior
end of the shell, occupying not more than a third of the space
enclosed between the two valves : it is consequently more or less
wedge-shaped in form, and presents dorsal and ventral surfaces in
contact with the two valves, and an anterior surface looking
towards the gape. The dorsal is of greater extent than the
ventral surface, so that the anterior surface is placed obliquely.

The dorsal and ventral regions are continued each into a flat
reduplication of the body-wall, closely applied to the correspond-
ing valve and containing a prolongation of the coelome. The two
flaps thus formed are the dorsal (d. m) and ventral (v. tii) mantle-
lobes. They are fringed with minute setae (s) lodged in muscular
sacs, like those of Cha?topods (vide Sect. X.), and give off from their
outer surfaces hollow processes which extend, into the tubules of
the shell mentioned above.

The large wedge-shaped space or mantle-cavity, bounded by the
mantle-lobes above and below, and behind by the anterior surface
of the body, is occupied by a huge and complex lophophore (Figs.
290 and 291, Ipti), which springs from the anterior surface of the
body, and, like that of the fresh-water Polyzoa and of Phoronis,
has the general form of a horse-shoe. It is, however, peculiarly
modified : the two limbs of the horse-shoe curve towards one
another so as to adapt themselves to the mantle-cavity ; and the
middle of the concave edge, which is dorsal in position, is pro-
duced into a spirally coiled offshoot (IpJi) which lies between the
two arms and is coiled towards the dorsal side. The lophophore
is hollow, containing a spacious cavity or sinus: its two main arms
also receive prolongations of the coelome into which the digestive
glands project : it is fringed throughout its whole extent with
long ciliated tentacles which form the outer boundary of a ciliated
food-groove, bounded on the inner side by a wavy ridge or lip
(Ip, lp'}. By the action of the cilia microscopic particles are swept
along the food-groove to the mouth.

Digestive Organs. The mouth (mth) is a narrow crescentic
aperture situated in the middle of the lophophore, towards its
convex or ventral edge, and is bounded dorsally by the lip. It
leads into a V-shaped enteric canal which consists of a gullet
passing upwards from the mouth, an expanded stomach (sty, and a
straight intestine (int.) which extends from the stomach downwards
and backwards towards the ventral surface and ends blindly,
there being no anus. On each side of the stomach, and opening

VIII

PHYLUM MOLLUSCOIDA

363

into it by a duct, is a large, branched digestive gland (d. gl}.
whole canal is lined with ciliated epithelium.

fil.si,

The

fil.si

FIG. 290. A, body of IVXagellania lenticularis, removed from shell ; B, sagittal section of
the entire animal. Both semi-diagrammatic, the lophophore being represented as of smaller
proportional size than in the actual animal (cf. Fig. 291). d.gl. digestive gland ; d. HI. dorsal
mantle-lobe ; d. v. dorsal valve of shell ; gon 1 , gon' 2 . gonads ; ht. heart ; int. intestine ; ip, lp l .
lip; Ipli. lophophore ; IplJ-. its coiled process; inth. mouth; nph. in B, nephridium, in A,
nephridial aperture ; pd. peduncle ; pi. si. pallial sinuses ; s. seta? ; st. stomach ; v. m. ventral
lobe of mantle ; v. v. ventral valve of shell.

The body-wall consists externally of an epidermis formed of a
single layer of cells, then of a layer of connective tissue, of a

364

ZOOLOGY

SECT.

.th.

cartilaginous consistency in many parts, and finally of a ciliated
coelomic epithelium lining the body-cavity. On the outer surfaces
of the mantle-lobes, where they are in contact with the shell,

the epidermis is replaced by
a thin membrane showing
no cell-structure.

The muscular system
(Fig. 292) is well developed.
Two large adductor muscles
(ad. m) arise on each side
from the dorsal valve, and
passing downwards, unite
with one another so as to
have a single insertion on
the ventral valve : their
action is to approximate the
valves and so to close the
shell. A large and a small
pair of divaricators (d. m, dm')
arise from the ventral valves,
and are inserted into the
cardinal process, which they depress : as this process is situated
posteriorly to the hinge-line, its depression raises the rest of
the dorsal valve and so opens the shell. Two pairs of muscles
arising, one from the ventral, the other from the dorsal valve, and

tyh.

FIG. 291. IVIagellania flavescens, the ventral
valve removed, c. p. cardinal process ; Iph. arm
of lophophore ; Iphi. its coiled process, with
the tentacles removed on the right side ; mth.
mouth. (After Davidson.)

cL.m,

FIG. 292. Muscular system of IVIagellania. ad. m. adductors ; b. beak ; d. aj. m. dorsal
adjusters; d. m., d. m'. divaricators; d. v. dorsal valve; int. intestine; ruth, mouth; pd.
peduncle ; pd. sh. sheath of peduncle ; p. m. protractor ; s. 1. shelly loop ; v. aj. m. ventral
adjusters ; v v. ventral valve. (After Hancock.)

inserted into the peduncle, are called adjusters (aj. m) : the
peduncle being fixed, they serve to alter or adjust the position
of the animal as a whole by turning it in various directions.

VIII

PHYLUM MOLLUSCOIDA

3C.5

The coelome is a spacious cavity more or less encroached upon
by the muscles and other organs, and traversed by sheets and
bands of membrane which connect the enteric canal with the
body-wall, and thus act as mesenteries. The ccelome is continued
into each of the mantle-lobes in the form of four canals or pallial
sinuses (Fig. 290, pi. si), the two outer of which are extensively
branched.

Blood-System. Attached to the posterior region of the
stomach is a small, almost globular sac (h), which has been proved
to be contractile and is to be considered as a heart. Vessels have
been traced from it to various parts of the body, but the relations

Ol/

FIG. 293. Anterior body-wall of Terebratula, to show nervous system, &c. dm. dorsal mesen-
tery ; g. brain ; gf. genital folds ; n. iiephridium ; nt. nephrostome ; <xs. gullet ;.ov. ovary ; sc.
cesophageal connective ; usg. infra-resophageal ganglion ; vm. ventral mesentery ; dmn, hn,
ian, san. nerves. (From Lang's Comparative Anatomy, after van Bemmelen.)

of the whole circulatory system and the course of the circulation
are very imperfectly known.

The excretory organs consist of a pair of very large ncpliridia
(nph) lying one on each side of the intestine. Each is funnel-
shaped, having a wide inner opening or nephrostome, with plaited
walls, opening into the coelome, and a narrow, curved, outer portion
which opens into the mantle-cavity not far from the mouth. As
in many cases which have already come under our notice, the
nephridia act also as gonoducts.

The nervous system (Fig. 293) is a ring round the gullet pre-
senting supra- (g) and infra- (usg) oesophageal swellings or ganglia,
of which the infra-cesophageal is the larger. Nerves are given off

366 ZOOLOGY SECT.

to the mantle, lophophore, &c. No special sense-organs are
known.

Reproductive Organs. The sexes are separate. There are
two pairs of gonads (Fig. 290, gon), one dorsal and one ventral, in
the form of irregular organs sending off branches into the pallial
sinuses.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Brachiopoda are Molluscoida in which the body is enclosed
in a shell formed of two parts or valves which are respectively
dorsal and ventral in position. The body occupies only a small
portion of the space enclosed by the shell, and is usually attached
to foreign objects by a posteriorly placed stalk or peduncle : it
gives off dorsal and ventral reduplications, the mantle-lobes, which
line the valves of the shell and enclose a large mantle-cavity.
From the anterior surface of the body is given off a lophophore
which surrounds the mouth, and is beset with ciliated tentacles.
There is a ridge-like pre-oral lip which is continued on to the
lophophore. The enteric canal is usually V-shaped, and is
divisible into gullet, stomach, and intestine : there is a pair
of digestive glands. The ccelome is spacious, and is continued
into the mantle-lobes. A heart is usually present, attached to
the stomach. The excretory organs are one or two pairs of
nephridia which act also as gonoducts. The nervous system is a
ganglionated circum-cesophageal ring : sense-organs are usually
absent in the adult. The sexes are separate or united. Develop-
ment is accompanied by a metamorphosis.

The class is divided into two orders :

ORDER 1. INARTICULATA.

Brachiopoda in which the shell is not composed of oblique
prisms : the valves are not united by a hinge, and there is no
shelly loop for the support of the lophophore. An anus is
present.

Including Lingnla, Crania, Discina, &c.

ORDER 2. ARTICULATA.

Brachiopoda in which the shell is formed of oblique prisms or
spicules of calcium carbonate : the two valves unite by a definite
hinge, and there is usually a shelly loop, for the support of the
lophophore, developed in connection with the dorsal valve. The
intestine ends blindly.

Including Magellania, Terebratula, Rhynchonella, Cistella
(Argiope), &c.

viii PHYLUM MOLLUSCOIDA 367

Systematic position of the Example.

The genus Magellania, of which there are several species,
belongs to the family Terebratulidse, and to the order Articulata.

The dissimilar valves of the shell articulated by teeth and
sockets, and the absence of an anus, place it among the Articulata.
The Terebratulidas are distinguished by an oval or rounded shell,
the structure of which is punctate, the dots corresponding with
blind tubes receiving processes of the mantle ; the beak of the
ventral valve is prominent, and has a foramen partly bounded by a
deltidium of one or two pieces : there is a shelly loop springing
from the hinge-line of the dorsal valve. The genus Magellania is
characterised by having the shelly loop fully half as long as the
shell itself, and by the presence of a median septum on the inner
face of the dorsal valve.

The specific differences between M. lenticularis and M. flavescens
are largely matters of detail, depending upon the precise form of
the shell and loop. More obvious differences are seen in the shell,
which is pink, evenly-rounded, and short-beaked in M. lenticularis,
while in M. flavescens it is horn-coloured, almost pentagonal, and
has a prominent beak.

3. GENERAL ORGANISATION.

The shell presents two distinct types : in the Articulata. the
order to which Magellania belongs, the dorsal and ventral valves
are dissimilar, the dorsal valve having a cardinal process and usually
a shelly loop, the ventral a spout-like beak for the peduncle ; while
in the Inarticulata, of which Lingula is a good example (Fig. 294, A),
the two valves are nearly alike, and there is no shelly loop and no
beak. These differences are accompanied by differences in micro-
scopic structure ; in the Articulata the shell "is dense and stony,
and is formed of obliquely placed calcareous prisms, while in
the Inarticulata it has no prismatic structure, but usually con-
sists of a chitinoid material more or less strengthened by calcareous
spicules. Among the Articulata the loop may be absent ; when
present, it varies greatly in form and size, being sometimes very
small and simple (Fig. 294, C, D), sometimes bent upon itself, as
in Magellania, sometimes attached to the septum or to the interior
of the dorsal valve (E), sometimes, as in the extinct Spirifera,
represented by a complex double spiral (F), sometimes reduced to
short, paired rods springing from the septum (G).

The majority of both orders are attached by a longer or shorter
peduncle which passes between the proximal ends of the valves in
Lingula (Fig. 294, A), through a perforation in the ventral valve in
Discina (C), and through a foramen in the spout-like posterior end

368

ZOOLOGY

SECT.

of the ventral valve in the Articulata. Crania (B) has the ventral
valve fixed directly to foreign objects, the peduncle being absent.

The lophophore is found in its simplest form in Cistella
(Fig. 295, A), in which it is a horse-shoe-shaped disc with very
short arms, attached to the dorsal mantle-lobe and surrounded
with flexible tentacles which project between the valves. From
this the lophophore of Magellania, which may be considered as
typical for the Articulata, is easily derived by an increase in size,
and by the prolongation of the middle region of the concave edge
into a coiled offshoot. In the Inarticulata (C), and in Rhyn-
chonella (B) among the Articulata, each arm of the horse-shoe is

FIG. 294. Typical Brachiupoda. A, Lingula ; B, Crania ; C, Discina ; D, Terebratula ;
E, Cistella ; F, Spirifera ; G, Kraussina. (After Bronn.)

coiled into a conical spiral, which in some cases can be protruded
between the valves.

The most noteworthy point about the muscular system is the
fact that the shell is both opened and closed by muscular action.
The dorsal valve may be taken to represent a lever of which the
hinge-line is the fulcrum, the cardinal process the short arm, and
the main portion of the valve the long arm. The muscles all arise
from the ventral valve, the adductors being inserted into the inner
face of the dorsal valve, which they depress, the divaricators into
the cardinal process, their action depressing it and thus elevating
the valve itself. In Lingula there is a ver}^ complex muscular
system by means of which the valves can be rubbed upon one
another, or moved laterally as well as opened and shut.

VIII

PHYLUM MOLLUSCOIDA

369

In the Articulata the enteric canal is V-shaped, as in Magel-
lania, the intestine being straight or nearly so, and ending blindly.
In the Inarticulata, on the other hand, the intestine is usually
coiled, and always ends in an anus (Fig. 295, C, a), which generally
opens into the mantle-cavity, but in one genus (Crania) into a
pouch or sinus at the posterior end of the body between the
valves.

A heart is usually present, but the function of blood is per-
formed mainly by the ccelomic fluid, which is propelled by the
cilia lining that cavity, and circulate both in the ccelome itself and

FIG. 295. Dissections of A, Cistella ; B, Rhynchonella ; and C, Lingula. . anus ; Ipii.
lophophore ; mth. mouth. (After Schulgin and Hancock,)

in the pallial sinuses, each sinus presenting in Lingula at least
both an outgoing and an ingoing current.

A single pair of nephridia, resembling those of Magellania,
occurs in all known genera except Rhynchonella, in which there are
two pairs, one dorsal and one ventral. Besides discharging an
excretory function they act as gonoducts.

The nervous system always takes the form of a circum-ceso-
phageal ring with ganglionic enlargements, the largest of which
is ventral or sub-cesophageal in position. Otocysts have been
described in Lingula, rudimentary eyes in Megerlia, and patches
of sensory epithelium in Cistella : with these exceptions sensory
organs are unknown.

There are usually four gonads, two dorsal and two ventral,

VOL. I

B B

370

ZOOLOGY

SECT.

sending prolongations into the pallial sinuses. Some genera are
dioecious, others hermaphrodite, the epithelium of the gonads
producing, in the latter case, both ova and sperms.

The development of the Brachiopoda is best known in Cistella,
in which the first stages of development are passed through

in a pair of cavities, the Iwood-pouches,
situated at the base of the lophophore.
Segmentation is regular and complete,
and results in the formation of a bias-
tula, which is converted into a gastrula
by invagination (Fig. 296, A). Paired
sacs, the ccelomic pouches (p.v), grow out
from the archenteron, and the blastopore
closes. The coelomic sacs separate from
the mesenteron (B, me) or middle portion
of the archenteron, and extend between
it and the ectoderm, forming the right
and left divisions of the coelome : their
outer walls thus become the somatic,
their inner walls the splanchnic layer
of mesoderm. The mesenteron remains
closed and surrounded by the coelomic
sacs during the whole of larval life.

The embryo now elongates and be-
comes divided by an annular groove
into two divisions, an anterior and a posterior : a second groove
soon appears in the anterior division, the embryo then consisting
of three regions (B), which, from a superficial
point of view, might be looked upon as meta-
meres. But as the segmentation affects only
the body-wall and not the internal parts, the
process is not one of metamerism, and the
three apparent segments are called respect-
ively the head-region, the body-region, and
the peduncular region (Fig. 297).

Next the head-region grows out into an
umbrella-like disc surrounded with cilia and
bearing four eye-spots (Fig. 298, A), and on
the body-region a backwardly-directed an-
nular fold (m) appears, bearing four groups
of provisional setae. In Cistella, which has
no setae in the adult condition, the pro-
visional setae are subsequently lost, and are
not replaced. In forms which possess setae in the adult condition
the provisional setae are likewise lost, but are replaced by the per-
manent setae. Soon this mantle-fold divides into dorsal and ventral
lobes, which, being directed backwards, cover the peduncular region.

FIG. 296. Two stages in the
development iof Cistella

(Argiope). b. provisional setse ;
bl. blastopore ; me. mesen-
teron ; pv. coalomic pouches.
(From Balfour's Embryology,
after Kowalevsky.)

FIG. 297. Young larva of
Cistella, with the
three segments, two
eye-spots, and two
bundles of seta? (From
the Cambridge Natural
History, after Kowal-
evsky.)

VIII

PHYLUM MOLLUSCOIDA

371

In this condition the larva swims freely like a trochophore.
After a time it comes to rest and fixes itself by its peduncular seg-
ment (B). The two lobes of the mantle-fold (m) become refiexed
so as to point forwards instead of backwards, thus leaving the
peduncular region exposed and covering the head-region : by this
process the outer surface of the larval mantle becomes internal,
and vice versa. A stomodaDum is
formed on the head-region, and,
communicating with the mesenteron,
establishes the enteric canal. The
umbrella-like head-region decreases
in size, and perhaps forms the lip,
which is at first confined to the
part immediately dorsal to the
mouth. The lophophore appears at
first on the inner surface of the
dorsal mantle-lobe, but gradually
extends and surrounds the mouth ;
in its earlier stages it is circular, but
afterwards assumes the horse-shoe
form by sending out paired exten-
sions. In genera with a complex
lophophore, like Magellania, this
organ has at first a simple horse-
shoe form (Fig. 299, Iph}. A shell
is secreted by the mantle-lobes, and
the peduncular region becomes the
peduncle of the adult.

Distribution. The Brachiopoda
are all marine. They are widely
distributed geographically, and live
at various depths from between
tide-marks to 2,900 fathoms. At
the present day the class includes
only about 20 genera and 100
species, but in past times the case
was very different. Brachiopods ap-
pear first in the lower Cambrian
rocks, where the existing genera
Lingula and Discina are found. No more striking examples can
be adduced of persistent types organisms which have existed
almost unchanged for the vast period during which the whole of
the fossiliferous rocks have been in process of formation. Alto-
gether 106 genera are known from the Palaeozoic rocks, 34
from the Mesozoic, and 21 in the Cainozoic and Recent periods.
Obviously the group is tending, though slowly, towards extinction.

Researches on fossil and recent forms have shown the

B B 2

B

Fin. 298. Two later stages in the
development of Cistella. A,
free-swimming ; B, after fixation.
hs. peduncular region ; m. mantle ;
ms. body-region ; md. mesenteron ;
wk. ciliated ring ; vs. head-region .
(From Lang's Comparative Ana-
tomy, after Kowalevsky.)

372

ZOOLOGY

SECT.

Brachiopoda to illustrate, in a remarkable manner, the recapitu-
lation theory already referred to : the theory, that is, that
ontogeny or individual development is a more or less modified

recapitulation of phylogeny or ancestral
development. It has been shown that
there is a striking and almost com-
plete parallelism between the stages in
the development of the shelly loop in
such highly organised forms as Magel-
lania, and the entire series of articu-
lated Brachiopods from those with the
simplest to those with the most complex
loop.

Iph

mtli

d.gl

MUTUAL RELATIONSHIPS OF THE
CLASSES OF THE MOLLUSCOIDA.

FIG. '299. Lophophore of embryo
of Terebratulina. ('. gl. di-
gestive gland ; int. intestine ;
lp. lip; Ipli. lophophore ; mtlt.
month (From Korschelt and
Heider, after Morse.)

In adult structure Phoronis ex-
hibits marked resemblances to the
Ectoprocta, more especially to the
Phylactolsemata resemblances which
will be rendered clear by a comparison
of the diagrams A and B in Fig. 300.
In both, the ventral side of the body
is greatly produced and elongated, and,
by the approximation of the mouth and anus, the dorsal surface
is reduced to a very short space between those two apertures.
The form of the lophophore, the presence of an epistome having
similar relationships in the two groups, and the fact that the
coelorne is similarly developed in both, point in the same direc-
tion. Some points which are supposed to indicate relationships
with the Annulata and with the Chordata are referred to at a
later stage.

The resemblances between the Brachiopoda and the other two
classes of the phylum are somewhat disguised by the development
of the shell, but are very obvious more particularly when we take
into account certain features of the development. One of the
most striking points of resemblance between the three classes
is the presence of the lophophore with its tentacles ; in the earlier
stages of its development in the Brachiopod, as we have seen, this
structure (Fig. 299) has the horse-shoe shape which it retains in
the adult Phoronida and Phylactolsemata, and a lobe the arm-
fold or lip (lp) comparable to the epistome, is present overhanging
the mouth. The end of the body of the Brachiopod with which
the peduncle is connected must correspond to the aboral extremity
in the Polyzo-a, since this represents the part by which the larval
Polyzoan becomes fixed, the everted " sucker " of the latter being

VIII

PHYLUM MOLLUSCOIDA

373

evidently homologous with the foot-segment of the larval Brachio-
pod. The end of the body of the Brachiopod from which the
peduncle proceeds is thus the ventral portion. From the position
of the epistome and lophophore, it follows that the dorsal valve
of the Brachiopod, being on the same side of the mouth as the
epistome, lies on the side of the body corresponding with the anal
side of the Polyzoan, though the intestine is bent round in the

B

-tent

FK;. 300. A, Diagrammatic median section of a phylactolsematous Polyzoan. an. anus ;
ep. epistome ; tp. cav. epistome-cavity ; funic. funiculus ; gang, ganglion ; int. intestine ;
mo. mouth ; neph. nephridium ; o's. oesophagus ; st. stomach ; tent, tentacles. B, diagram-
matic median section of Phoronis. mes. mesentery ; nr. nerve-ring. Other letters as in A.
(From Korschelt and Heider, after Cori.)

opposite direction and directed towards the ventral valve. The
supra-oesophageal ganglion of the Brachiopod represents the single
ganglion of the Polyzoa, though it is subordinate in importance to
the infra-oesophageal ganglion not represented in the latter group.
Other important points of resemblance between the Brachiopoda
and the Phoronida are seen in the character of the nephridia and
the presence in both of larval forms which may very well be looked
upon as modified trochophores.

374 ZOOLOGY SECT, vm

The setae of Brachiopods, sunk in muscular sacs, are marks of
annulate affinities, since such organs are found elsewhere only
among Chsetopoda and Gephyrea (Sect. X.). The form of the
larva tells in the same direction, the eye-bearing head region
or prostomium and the provisional seta? being very striking charac-
ters. But the segmentation of the Brachiopod is quite different
from that of the annulate larva, in which new segments are always
added behind those previously formed, and in which metamerism
always affects the mesoderm.

SECTION IX
PHYLUM ECHINODERMATA

THE phylum Echinodermata comprises the Starfishes (Asteroidea),
Sea-urchins (IJJchinoidea), Brittle-stars (Ophiuroidea), Feather-stars
(Crinoidea), and Sea-cucumbers (Holothuroidea). All exhibit a
radial arrangement of parts, which is recognisable as well in the
globular Sea-urchins and elongated Sea-cucumbers as in the star-
shaped Starfishes, Brittle-stars and Feather-stars. Another uni-
versal feature is the presence of a calcareous exoskeleton, sometimes
in the form of definitely shaped plates, which may fit together by
their edges so as to form a continuous shell ; sometimes merely in
the form of scattered particles or spicules. In very many the
surface is beset with tubercles or spines, from which feature the
name of the phylum is derived. The various systems of organs
attain a comparatively high degree of complexity. An extensive
ccelome is present, developed in the embryo from hollow outgrowths
from the archenteron. The Echinoderms are rarely capable of
rapid locomotion, and are sometimes permanently fixed by means
of a stalk ; they never give rise to colonies by budding. Without
a single exception, all the members of this phylum are inhabitants
of the sea.

1. EXAMPLE OF THE ASTEROIDEA.
A Starfish (Astcrias rubens or Anthenea flavesccns).

General External Features of Asterias rubens. --The

body of the Starfish is enclosed in a tough, hard integument,
containing numerous plates, or ossicles as they are termed, of
calcareous material. This exoskeleton is not completely rigid in
the fresh condition, but presents a certain limited degree of flexi-
bility. The body (Fig. 301) is star-shaped, consisting of a central
part, the central disc, and five symmetrically arranged processes,
the arms or rays, which, broad at the base, taper slightly towards

375

376

ZOOLOGY

SECT.

their outer extremities. There are two surfaces one, the dboral
or dbactinal, directed upwards in the natural position of the living
animal ; the other, the oral or actinal, directed downwards. The
aboral surface is convex, the oral flat ; the colour of the former is
much darker than that of the latter.

In the centre of the oral surface (Fig. 301) is a five-rayed
aperture, the actinostome, and running out from this in a radiating
manner are five narrow grooves, the ambulacral grooves, each extend-
ing along the middle of the oral surface of one of the arms to its
extremity. Bordering each of the ambulacral grooves there are
either two or three rows of movable calcareous spines, the

ambulacral spines. At
the central ends of the
grooves the ambulacral
spines of contiguous
sides of adjacent grooves
form five groups, the
mouth papillce, one at
each angle of the mouth.,
External to the am-
bulacral spines are three
rows of stout spines
which are not movable ;
and a third series runs
along the border separ-
ating the oral from the
aboral surface.

On the convex aboral
surface there are a
number of short stout

WiG.--BOl.Sta.V&tib.(A8tffria8rubens). General view of the Spines arranged ill IF-

, .l*-i 1 t'\V il /*T"1T*1 1 O 11 *TO r>l C"Vl /^TXTl 11 <~f 4" Vl i* 4" 1 1 Vtl_TlOT / EVvtYyi ^ '

oral or actinal surface, showing the tube-feet.
Leuckart and Nitsche's Diagrams.)

(From

regular rows parallel
with the long axes of the
rays. These are supported on irregularly-shaped ossicles buried in
the integument. In the soft interspaces between the ossicles are
a number of minute pores, the dermal pores, scarcely visible with-
out the aid of a lens. Through each of these pores projects
a very small, soft, filiform process, one of the dermal branchial
or papuloe (Fig. 305, Rcsp. cce), which is capable of being entirely
retracted.

Very nearly, though not quite, in the centre of the aboral sur-
face is an aperture, the anus (Fig 310), wide enough to admit
of the passage of a moderately stout pin. On the same surface,
midway between the bases of two of the rays, is a flat, nearly
circular plate, the surface of which is marked by a number of
radiating, narrow, straight, or slightly wavy grooves ; this is the
madreporitc (mad). The presence of this structure interferes to some

ix PHYLUM ECHINODERMATA 377

extent with the radial symmetry of the Starfish, two of the anti-
meres (p. 42), viz. those between which the madreporite is placed,
being different from the rest. There thus arises a bilateral sym-
metry, there being one vertical plane, and only one that passing
through the middle of the madreporite and through the middle of
the opposite arm along which it is possible to divide the Starfish
into two equal right and left portions. 1 The two rays between
which the madreporite lies are termed the bivium, the three
remaining the trivium.

Attached to the spines of the oral surface, in the intervals
between them, and in the intervals between the spines of the
dorsal surface, are a number of very small, almost microscopic
bodies, which are termed the pedicellarice (Fig. 305, Ped).
Each of these is supported on a longer or shorter flexible stalk,
and consists of three calcareous pieces a basilar piece at the
extremity of the stalk, and two jaws, which are movably articu-
lated with the basilar piece, and are capable of being moved by
certain sets of muscular fibres so as to open and close on one
another like the jaws of a bird. In some of the pedicellarise the
jaws, when closed, meet throughout their entire length, while in
the case of others, mostly arranged in circles round the spines on
the aboral surface, one jaw crosses the other at the end like the
mandibles of a Crossbill.

In a well-preserved specimen there will be seen in each of the
ambulacral grooves two double rows of soft tubular bodies ending
in sucker-like extremities; these are the tube-feet (Fig. 301). In
a living specimen they are found to act as the locomotive organs
of the animal. They are capable of being greatly extended, and
when the Starfish is moving along, it will be observed to do so by
the tube-feet being extended outwards and forwards (i.e. in the
direction in which the animal is moving), their extremities be-
coming fixed by the suckers, and then the whole tube-foot con-
tracting so as to draw the body forwards ; the hold of the sucker
then becomes relaxed, the tube-foot is stretched forwards again,
and so on. The action of all the tube-feet, extending and con-
tracting in this way, results in the steady progress of the Starfish
over the surface. With the aid of the tube-feet the Starfish is
also able to right itself if it is turned over on its back.

At the extremity of each of the ambulacral grooves is to be
distinguished a small bright red speck, the eye (Fig. 305, A, oc),
over which is a median process, the tentacle (), similar to the tube-
feet, but smaller and without the terminal sucker. The tentacles
have been ascertained by experiment to be olfactory organs, the
Starfish being guided to its food much more by this means than
by the sense of sight.

1 The slightly eccentric position of the anal aperture introduces a correspond-
ingly slight inequality between the right and left portions.

378

ZOOLOGY

SECT.

Transverse Section of an Arm. If one of the arms be cut
across transversely (Fig. 302 and Fig. 305, B) and the cut surface
examined, the aboral part of the thick, hard wall of the arm will
present the appearance of an arch (with its convexity upwards),
and the oral part the form of an inverted V, the ends of the
limbs of which are connected with the oral ends of the aboral
arch by a very short, flat, horizontal portion. Enclosed by these
parts is a space, a part of the coslome or body-cavity, and below,
between the two limbs of the V, is the ambulacral groove. The
aboral arch is supported by a number of irregular ossicles and is
perforated by the numerous small dermal pores, through which the

dermal branchiae project.
The V-shaped oral part
of the body-wall i.e. the
walls of the ambulacral
groove is supported by
two rows of elongated
ossicles, the ambulacral
ossicles (Fig. 3Q5,Amb. os),
which meet together at
the apex or summit of
the groove like the
rafters supporting the
roof of a house, but
with a movable articu-
lation allowing of separa-
tion or approximation ot
the two rows so as to
open or close the groove.
At the end of the ray
the ambulacral ossicles
end in a median ter-
minal ossicle. At the
edges of the groove a

row of ossicles support the ambulacral spines and prominent
tubercles. Between the ambulacral ossicles of each row are
a series of oval openings, the ambulacral pores, one between
each contiguous pair of ossicles, and so arranged that they form
two rows on each side, one row higher than the other, the
pores of the higher row alternating with those of the lower. In
the ventral groove lie the contracted tube-feet (T. J<\) : each tube-
foot is found to correspond to one of the ambulacral pores, so
that the former, like the latter, are arranged in a double alter-
nating row on each side of the groove. When the tube-foot is
drawn upon, it is seen to be continuous with one of a series of
little bladder-like bodies, which lie on the other side of the ambu-
lacral ossicles, i.e. in the cavity of the arm. These the ampullce

FIG. 302. Starfish. Vertical section through an arm.
ump. ampullae ; ep. epidermis ; ra<L amb. radial vessel
of the ambulacral system ; rad. bl. v. points to the
septum dividing the perihsemal vessel into two parts ;
rad. ne. radial nerve of the epidermal system ; sp.
spaces in mesoderm of body-wall ; t. f. tube-feet.
(From Leuckart, after Harnann.)

IX

PHYLUM ECHINODERMATA

379

(Figs. 302 and 305, amp. ; Fig. 303, ap) are arranged like the tube-
feet, in a double row on each side, a higher row and a lower,
there being one opposite each ambulacral pore. When one of
them is squeezed, the corresponding tube-foot is distended and
protruded, the cavities of the tube- foot and ampulla being in
communication by means of a narrow canal running through the
ambulacral pore and provided with a valve. It is in this way
that the foot is protruded in the living animal : the corresponding
ampulla being contracted by the contraction of the muscular
fibres in its walls, the contained fluid is injected into the tube-
foot and causes its protrusion,
the return of the water back-
wards through the canal being
prevented by the closing of
the valve.

Vascular and Nervous
System. Running along the
ambulacral groove, immedi-
ately below where the ambu-
lacral ossicles of opposite sides
articulate, is a fine tube, the
radial ambulacral vessel (Fig.
302, rod. amb, Fig. 303, r),
which appears in the trans-
verse section as a small rounded
aperture. From this short side-
branches (Fig. 303, r') pass out
on either side to open into the
bases of the tube-feet. Below
the radial ambulacral vessel
is a median thickening of the
integument covering the am-
bulacral groove : this marks
the position of the radial nerve
(Fig. 302, rad. ne) of the

epidermal nervous system, and is traceable as a narrow thickened
band running throughout the length of the groove, and ter-
minating in the eye at its extremity, while internally it be-
comes continuous with one of the angles of a pentagonal
thickening of a similar character, the nerve-pentagon, which
surrounds the mouth. In thin sections (Fig. 304) the ventral
median thickening, or radial nerve {rad. nerv.), as well as the nerve-
pentagon, are seen to be thickenings of the epidermis, consisting
of numerous vertically-placed, fibre-like cells, with their nuclei at
their outer (lower) ends, intermixed with longitudinal nerve-fibres
and with nerve-cells. Above this, on each side of the epidermal
nerve-thickening constituting the radial nerve, is a band of cells

FIG. 303. Ambulacral system of a Starfish.
a. ampulla? ; ap. Polian vesicles ; c. circular
canal ; m. madreporite ; m'. madreporic
canal ; t. tube-feet ; p. radial vessels ; ?'.
branches to ampulla?. (After Gegenbaur.)

380

ZOOLOGY

SECT.

(d. nerv.} also of a nervous character. These more deeply placed
nerve-bands are the radial parts of the deep nervous system : like
the epidermal, the deep nervous system has a central part in the
form of a pentagon, which in this case is double, surrounding the
mouth. A third set of nerve elements (the ccelomic nervoi^s
system} extends along the roof of the arm superficial to the
muscles.

The two radial nerve-bands of the deep nervous system are
thickenings of the lining membrane of a space overlying the
radial nerve and underlying the radial ambulacral system. This
space (rod. II. v), extending, like the other parts that have been
mentioned, throughout the length of the arm, forms part of a
system of channels, ihe perihcemal system, which have been regarded
as constituting a blood-vascular system. This radial perihcemal

vessel or sinus, as it is termed,
is divided longitudinally by
a vertical septum (sept.) into
two lateral halves. Internally
it communicates with an oral
ring-vessel surrounding the
mouth and likewise divided
into two by a septum. The
inner division of the ring-
vessel is connected with the
axial sinus referred to on
p. 384.

In the septum dividing
the radial perihsemal sinus
into two runs a strand of a
kind of gelatinous connective
tissue containing many leuco-
cytes and perforated by ir-
regular channels or lacuna? :

this is the radial strand of the lacunar or licemal system. Like the
radial vessels of the perihoemal system, the radial strands of the
lacunar system are connected internally with an oral ring.

Structure of the Disc.- -When the aboral wall of the central
disc is dissected away, the remainder of the organs come into view
(see Fig. 308). The rows of ambulacral ossicles appear in this
view as ridges, the ambulacral ridges, one running along the
middle of the oral surface of each arm to its extremity, and
extending inwards to the corresponding angle of the mouth. At
the sides of each of these ridges appear the rows of ampullae.
Within the pentagonal actinostome is a space, the peristome,
covered with a soft integument, and in the centre of this is a
circular opening, the true mouth, the size of which is capable of
being greatly increased or diminished.

Fio. 304. Starfish. Lower part of a vertical
section through the arm, to show the structure
of the radial nerve and the position of the
deep nervous system and radial perihsemal
vessels, d. nerv. strand of deep nervous system ;
rad. bl. v. radial perihfemal vessel ; roe/, nerv.
radial nerve ; sept, septum of radial peri-
hfemal vessel ; sept', radial lacunar strand of the
htemal system (here represented as solid).
(After Cuenot.)

IX

PHYLUM ECHINODERMATA

381

Body-wall and Ccelome.- -The entire outer surface is covered
with a layer of ciliated epithelium, the epidermis or deric epi-
thelium (Fig. 305, Der. Epithm), which is continued over the
various appendages and processes the tubercles aud spines, the
pedicellariae, the dermal branchiae, and the tube-feet. Beneath
it is a network of nerve-fibrils with occasional nerve-cells. The
mesoderm (Derm) of the wall of the body beneath this consists of
two layers, between which are a number of spaces : the ossicles (os)

m

FIG. 305. Diagrammatic sections of a Starfish. A, vertical section passing on the right through
a radius, on the left through an inter-radius. The off-side of the ambulacral groove with the
tube-feet (2'. /'.) and ampullae (Amp.) are shown in perspective. B, transverse section through
an arm. The ectoderm is coarsely dotted, the nervous system finely dotted, the ectoderm
radially striated, the mesoderm evenly shaded, the ossicles of the skeleton black, and the
ccelomic epithelium represented by a beaded line. Amb. os. ambulacral ossicles ; Amp. am-
pullae ; An. anus ; C. Amb. V. circular ambulacral vessel : C. B. V. septum of ring perihsemal ;
vessel ; Cd. at. cardiac caeca ; Cvel. coalome ; del. Epithm. ccelornic epithelium ; Der. Epitliiu.
deric epithelium ; Derm, mesoderm ; Eiit. Ept/tm. enteric epithelium : Int. tee. intestinal caeca.
Mdpr. madreporite ; Mes. mesentery ; Mtk. mouth ; Nv. R. nerve-ring ; oc. eye ; os. ossicles of
body-wall ; Ovd. oviduct ; Fed. pedicellarise ; %)h perihaumal spaces ; Pyl. ccec. pyloric caeca ;
Had. amb. v. radial ambulacral vessel ; Hud. B. V. points to septum in the radial perihsemal
vessel ; Rad. Nv. radial nerve ; Resp. cce. dermal branchiae ; St. stomach ; St. c. stone-canal ;
t . tentacle ; T. F. tube-feet. (From Parker's Biology.')

are all, except the ambulacral ossicles and the inter-radial par-
titions, developed in the outer of these two layers. Each ossicle
consists of a close network of calcareous rods. Between contiguous
ossicles extend bands of muscular fibres.

The interior of the caelome (Ccel.) or body-cavity is lined by a
ciliated epithelium, the ccelomic epithelium (Gael. Mpithm.}, which
not only covers the inner surface of the body-wall as the parietal
layer, but also forms an investment for the contained organs
the various parts of the alimentary canal and its appendages,
the gonads, the madreporic canal, ampullae, etc. In addition

382

ZOOLOGY

SECT.

to this visceral layer of the peritoneum, the wall of the ali-
mentary canal and its caeca consists of a muscular layer and an
internal lining, the enteric epithelium or endoderm (Ent. Epthm).
The coelome is filled with a fluid, the ccelomic fluid, consisting
mainly of sea-water, but containing a number of amoeboid cor-
puscles (amceljocytes) containing a brown pigment. The dermal
branchiae consist of a muscular layer, an external epidermal layer,
and an internal peritoneal layer, the internal cavities of the hollow
branchiae being in free communication with the coelome.

Digestive System.- -The mouth is found to open through a
short passage, the cesophagus, into a wide sac, the cardiac division
of the stomach (Fig. 305, St, Figs. 308, 310, card. st). This is a

five-lobed sac, each of
the lobes of which is
opposite one of the five
arms. The walls of the
sac are greatly folded,
and the whole is cap-
able of being everted
through the opening of
the mouth, wrapped over
some object desired as

FIG. 306. Asterias rubens. Digestive system, an.
anus ; card. st. cardiac division of the stomach ; int.
co:c. intestinal cfeca ; madr. madreporite ; pyl. coec.
pyloric creca ; pyl. st. pyloric division of the stomach.
(From Leuckart. )

food, and then retracted
into the interior, the re-
traction being effected
by means of special
retractor muscles (Fig.
308, retr) which arise
from the sides of the
ambulacral ridges. This
cardiac division of the
stomach communicates
aborally with a much
smaller chamber, the
pyloric division of the

stomach, and this in turn opens into a very short conical in-
testine, which leads directly upwards to open at the anal aperture.
The pyloric division of the stomach is pentagonal, each angle
being drawn out to form a pair of large appendages, the pyloric
cceca (Figs. 305, 306, 308, 310, pyl. coec). Each pair of pyloric
caeca commences, as a cylindrical canal or duct, the lumen of
which is continuous with the cavity of the pyloric chamber.
This soon bifurcates to form two hollow stems, extending to near
the extremity of the cavity of the arm, and giving off laterally two
series of short branches, each having connected with it a number
of small bladder-like pouches. The walls of the pyloric caeca are
glandular : they secrete a digestive fluid, and are therefore to be

ix PHYLUM ECHINODERMATA 383

looked upon as digestive glands. It is found by experimenting
with this digestive fluid that it has an action on food-matters
similar to that exerted by the secretion of the pancreas in the
Vertebrata, converting starch into sugar, proteids into peptones,
and bringing about the emulsification of fats. While the pouches
of the cardiac division of the stomach are attached to the oral
wall of the body, the pyloric caeca are connected with the aboral
wall. From the short intestine are given off inter-radially two
hollow appendages, the intestinal cceca (Figs. 306 and 308, int. ccec),
each with several short branches of irregular shape.

Ambulacral System. Running downwards from the madre-
porite to near the border of the mouth is an S-shaped cylinder,
the madreporic or stone-canal (Figs. 303, iri. 310, mad. can). The
walls of this canal are supported by a series of calcareous rings,
and projecting into it is a ridge which bifurcates to form two
spirally rolled lamellae occupying a considerable part of the lumen of
the canal. In some Starfishes, such as Astropecten (Fig. 307), the
internal structure is more com-
plicated owing to the branching
of the lamellae. The interior of
the madreporic canal communi-
cates above with the exterior
through the grooves of the madre-
porite. At the bottom of each
of the grooves is a row of pores
leading into a sac, the ampulla,
which in turn leads into the

Canal. Below, the FlG - 307. Transverse section through the
. , f, madreporic canal of a Starfish (Astro-

latter Opens into a Wide, Five- pecten). (From Gegenbaur, after Teuscher.)

sided, ring-like canal, the ring-

vessel of the ambulacral system. From this are given off the five
radial ambulacral vessels, passing to the extremities of the arms.
From the pentagonal canal are given off also in most Starfishes,
but not in Asterias, a series of five pairs of appendages, the Polian
vesicles (Fig. 303, ap ; Fig. 308, pol. ves) pear-shaped, thin-walled
bladders with long narrow necks which are placed inter-radially.
At the sides of the neck of each Polian vesicle (except in the
inter-radius containing the madreporic canal, where there is one
on one side only) project inwards a pair of little rounded glandular
bodies, the racemose or Tiedemann's vesicles (Fig. 309, T), the cavity
in the interior of each of which, opening into the ring-vessel, is
divided into a number of chambers.

The various parts of the ambulacral system of vessels have a
muscular wall and an internal lining epithelium in addition to the
coverings which they may derive, according to their situation,
either from the external epidermis or the internal coelomic epi-
thelium. The muscular layer is most strongly developed on the

384

ZOOLOGY

SECT.

tube-feet, where it consists of two strata, and is also well developed
on the ampullae and Polian vesicles.

The stone-canal is enfolded in the wall of a wider canal, the
axial sinus (Fig. 309, ax. s), which forms a part of the perihaemal
system already referred to. The axial sinus runs nearly vertically.
At its oral end it opens into the internal division of the oral ring

fit/I. CGC

cccc

3

t*eo

FIG. 308. Anthenea flavescens. Upper view of a dissection of the internal organs. The
aboral wall of the body, with the exception of a small portion round the anus and the madre-
porite, has been completely removed. One of the five intestinal cteca has been removed with
the exception of its proximal part. All the ovaries have been removed except one pair, and
four of the pairs of pyloric cteca have been cut away close to their bases. 1 5, the five rays
with their ambulacral ridges ; amp. ampulla? ; an. anus ; int. c<xc. intestinal cpeca ; /. p. cut
ends of the inter-radial partitions ; mad. madreporite with the madreporic canal ; ov. ovaries ;
pol. ves. Polian vesicles ; pyt. CCP.C, pyloric cajca ; retr. retractor muscles inserted into the
cardiac division of the stomach.

sinus ; aborally it approaches close to, if it does not actually open
into, an aboral ring sinus : it also communicates aborally with the
stone-canal, and perhaps opens on the exterior through certain of
the pores in the madreporite.

Accompanying the madreporic canal and also enfolded in the wall
of the axial sinus there is an organ the axial organ (Fig. 309,
g. stol) the relationships and function of which have given rise

IX

PHYLUM ECHINODERMATA

,,85

to a considerable amount of difference of opinion. It is a fusiform
body, the interior of which assumes an appearance of com-
plexity -largely due to both its inner surface (i.e., that turned
towards the axial sinus) and its outer (that facing the coelome)
being folded in a complicated manner. The axial organ contains
strands of lacunar tissue, i.e. of the same tissue that composes the

FIG. 309. A, view of the "under part of a specimen of Asterias rubens, which has been
horizontally divided into two nearly equal portions. B, enlarged rview of the axial sinus,
stone-canal and genital stolon cut across, amb. oss. arnbulacral ossicle ; aiiip. ampulla} of the
tube-feet ; ax. s. axial sinus ; gon. gonad ; g. slot, genital stolon or axial organ ; marg. marginal
ossicle ; new. circ. nerve-ring ; oe. cut end of oesophagus ; pst. peristome ; ret. retractor muscle
of the stomach ; sept, inter-radial septum ; stone, c. stone-canal ; T. Tiedemann's vesicle ;
v}. v. r. water- vascular ring-canal. (After MacBride.)

so-called haemal system, and is intimately related with the
latter. Its essential morphological character, however, appears to
be that of a genital stolon. At its aboral end it is continuous with
a genital rachis, which, in the form of a ring, runs in the aboral
perihaemal sinus, and gives off branches to the gonads. There is
evidence that the sexual cells originate in the aboral end of the
axial organ, and travel through the genital rachis and its branches

VOL. I

c c

386

ZOOLOGY

SECT.

to the gonads, which are to be looked upon as the greatly expanded
extremities of the latter. Strands of the lacunar tissue accompany
the genital rachis and its branches to the gonads.

Reproductive System. The Starfish is unisexual, each in-
dividual possessing either ovaries (Figs. 308, 309, and 310, ov) or
testes, which appear very similar until they are examined micro-
scopically. They consist of masses of rounded follicles, like
bunches of minute grapes a pair in each inter-radial interval.
Ova and sperms are alike developed from cells of the same
character as those which become the amcebocytes of the coelomic
and other cavities of the body. The ducts, by means of which
the ova or sperms reach the exterior, open on the aboral surface

card.*

Cf/

cunfi

ifl ?nad .

FIG. 310. Anthenea flavescens. Lateral view of a dissection in which one of the rays and
a portion of a second have been removed, and in which the alimentary canal has been laid
open. amp. ampulla; ; an. anus ; card. st. cardiac pouch of the stomach ; int. cctc. intestinal
caecum ; ip. inter-radial partition ; mad. madreporite ; mad. can. madreporic canal ; ov. ovary :
pyl. cctc. pyloric caeca ; r. cut ends of the ring-vessel of the ambulacral system ; ring v. posi-
tion of the ring-vessel ; retr. retractor muscle of cardiac pouch of stomach ; s. cavity of the
stomach.

through a number of perforations on a pair of sieve-like plates,
situated inter-radially close to the bases of the arms.

Anthenea flavescens (Figs. 308, 310, 311, 312), a common
Australian Starfish, which may be taken as an example instead of
Asterias rubens, differs from the latter in the following main
points.

The animal consists of a relatively large central disc and five
relatively short arms, which taper rapidly towards their extremities.
On the oral surface the comparatively broad, flat surfaces be-
tween the ambulacral grooves are roughish, owing to the plate-like
ossicles being beset with a number of minute rounded tubercles,
which, in the immediate neighbourhood of the ambulacral
grooves, assume the character of short, blunt spines. Here and

IX

PHYLUM ECHINODERMATA

387

there among the tubercles, usually one in the middle of each

ossicle, are pedicellarics, which differ widely from those of Asterias.

Each pedicellaria in An-

thenea is a small, narrow,

oblong, calcareous body,

consisting of two parallel

narrow valves or jaws :

these, instead of being

supported on a flexible

stalk, are articulated with

the edges of a slit-like

depression on the surface

of the flat ossicle, and are

thus on a level with the

general surface. The term

valvulate is applied to

pedicellarire of this de-

scription. In a living

Anthenea many of the

pedicellariae will be found

to have their valves widely

open ; when they are

touched the valves close

together, gradually open-

ing again after a little time. The ambulacral spines bounding

the ambulacral grooves are flattened and blunt, and arranged

in fan-like fasciculi. Round
the border separating the
aboral and oral surfaces
the plates are arranged
in two somewhat irregular
rows.

The aboral surface is
strongly convex, but not
uniformly so, there being a
more or less distinct de-
pression in the form of a
shallow open groove, the
inter- radial depression, op-
posite each of the intervals
between the arms. The
surface is dotted over with
numerous small rounded
tubercles, arranged in some-
what irregular radiating
lines. These aboral tuber-

FIG. 311. Anthenea, view of aboral surface.

(After Sladen.)

F,c, 3,2-

, w )1)f oral Sl ,rfce.

c c 2

388 ZOOLOGY SECT.

those on the oral surface, are for the most part more prominent,
so that they assume the character of short spines. The ossicles
on which they are borne are star-shaped with six rays, a
spine being borne in the centre of each ossicle, and one on
each of the rays. Between the ossicles the surface is covered
with a soft, slimy skin, perforated by a large number of minute
dermal pores, each of which is enclosed by a minute irregular ring of
calcareous matter ; each pore serves for the lodgment of one of the
dermal branchia?. Numerous pedicellariae, similar to those on
the ventral surface, but smaller, are borne on the ossicles, usually
taking the place normally occupied by the central spine. The
tube-feet are arranged in a single row on each side of each ambu-
lacral groove ; but the an^ullce are in two rows, an upper and a
lower, and each tube-foot has two ampullae connected with it,
one of the upper row and one of the lower row.

Anthenea has vertical calcareous inter-radial partitions not de-
veloped in Asterias. There are five bifid intestinal cceco, which
are narrow tubes slightly enlarged and lobed at the extremities.

Development of a Starfish (Asterina gibbosa or A.
exigua 1 ). In these Starfishes the reproductive apertures are
placed on the ventral surface. When the ova have been dis-
charged and impregnated, they adhere by means of a viscid
investment to the surface (rock or stone) on which they are laid,
and go through all the stages of their development in this position,
never passing through a free pelagic stage. The eggs are about
half a millimetre in diameter, and of a spherical shape. Each con-
sists of a perfectly opaque central mass of yellow or orange yolk,
and of a glassy layer enclosing this. After fertilisation the process
of segmentation begins by the division of the ovum into two blasto-
meres almost equal in size, but one, which may be termed cell
I., slightly smaller than the other (cell II.). Both I. and II. soon
afterwards divide, I. somewhat earlier than II. The resulting
four cells again divide, leading to the formation of an eight-celled
stage (Fig. 313, A), in which the four cells derived from I. form
an incomplete ring not closed below, and the four derived from
II. form an incomplete ring open above.

The eight cells then divide by meridional fissures into
sixteen, and a further division results in the formation of thirty-
two. The thirty-two cells become arranged in such a way as to
enclose a central cavity which had been present in the four-celled
stage : this stage (B) is the Nastula ; the cavity is the segments -
tion-eavity or blastoccele. The number of cells in the wall of this
cavity increases by further divisions, and the whole surface becomes
covered with vibratile cilia. A process of imagination then
follows, one side of the blastula being pushed inwards to form

1 The development of these has been described in preference to that of the
examples, as it is more completely known.

IX

PHYLUM ECHINODERMATA

389

a double-walled cup or gastrula (C) opening on the exterior by
an opening, the blastopore, which, at first very wide, gradually
becomes narrowed. At the same time the shape of the larva
alters, so as to be somewhat elongated, the blastopore, lying at first
midway between the two poles, afterwards gradually drawing
nearer to what becomes the posterior end.

Of the two layers of the gastrula (D and E), the outer is the
ectoderm, the inner the endoderm ; between them is a space, at first
filled with gelatinous matter, in which cells soon appear, giving

FIG. 313. Early stages in the development of a Starfish (Asterina gibbosa). A, eight-celled
stage ; B, stage of about thirty -two cells seen in section ; C, gastrula stage ; Z>, section of
early gastrula ; E, section of later gastrula. arch, archenteron ; blastoc, blastoccele ; blj).
blastopore ; ect. ectoderm ; end. endoderm. (Modified after Ludwig.)

rise subsequently to an intermediate mass of tissue, the
mesenchyme.

The cavity in the gastrula is early distinguishable into two
parts (Fig. 314, B) that part into which the blastopore leads
(arch), and a wider terminal part (ent) ; the former becomes the
stomach and intestine of the larva, the blastopore giving rise to the
larval anus ; the latter is termed the enteroccele (ccelome). The wall
of the enteroccele becomes thinner, and it gives off two lateral
swellings, the right and left enteroccelic pouches (C, ent), which
are closely applied to the sides of the larval alimentary canal : the
left pouch is soon seen to be larger than the right. The entero-
ccele is subsequently completely closed off from the enteric
canal. It now consists of three parts, an anterior undivided part,
and the two pouches, right and left. Of the latter the left grows
more rapidly than the right : both extend posteriorly in the space
between the enteric canal and the body-wall to coalesce posteriorly

390

ZOOLOGY

SECT.

in such a way as to give rise to the coelome of the adult. The
anterior undivided part (anterior coelome} forms the coelome of a
conspicuous larval structure, the pre-oral lobe, and it eventually

'larv.org

iarv.oig

e&,

arch,

FIG. 314. Later stages in the development of the larva of Asterina gibbosa A, newly
hatched larva, ventral surface with the beginning of the larval organ at the anterior end and
with the larval mouth. B, dorsal half of an embryo of the same age as A. C, somewhat older
larva with larger larval organ, the ectoderm of the left side removed to expose the alimentary
canal and the walls of the body-cavity, arch, archenteron ; bl. p. blastopore ; ect. ectoderm ;
ent. enteroccele ; larv. mo. larval mouth ; Ian: org. pre-oral lobe ; stom. stomodajum. (From
Ziegler's models.)

becomes cut off from the right and left pouches, giving off on
the left a five-lobed outgrowth, the hydroaale, which forms the
foundation of the entire ambulacral system of the adult : a right

B ktrv.

ec

Fit;. 315. Larva of Asterina gibbosa. A, diagrammatic lateral view ; the alimentary
canal dotted, the ambulacral system -striated, the ectoderm shaded. B, Larva seen from the
left as ail opaque object, the body-wall of the left side removed ; hydroccele separated off
from left enteric sac and partly surrounding oesophagus, all. alimentary canal ; amb. ambula-
cra! system or hydroccele ; dors. p. dorsal pore ; <>ht. enteric sacs and coelome ; larv. mo. larval
mouth ; larv. org. pre-oral lobe ; oe.<?. oesophagus of adult ; r, r. lobes of hydrociele ; sept.
septum between the enterocielic sacs. (A, after Ludwig ; B, from Ziegler's models.)

hydrocoele is only represented by a small vesicle which in normal
embryos undergoes no further development. Before the hydro-
coele is developed and before the right and left coelomic
pouches have become cut off, two apertures make their appearance

IX

PHYLUM ECHINODERMATA

391

tew.

FIG. 316. Larva of Asterina, view
of the left side, showing the five-
lobed prominence (am&.) formed by

the developing ambulacral system
on what is destined to become the
ventral surface of the body of the
Starfish ; tarv. org. larval organ.

tertt

on the surface of the larva : one, on the ventral side, is the open-
ing of the stomodeewm or larval mouth ; the other, on the dorsal
side, is the dorsal pore. The mouth
subsequently opens into the larval
stomach, and for a time the enteric
canal of the larva opens on the ex-
terior both by mouth and anus :
soon, however, the larval anus be-
comes closed up. The dorsal pore
is developed as an outgrowth of the
anterior part of the enterocoele, a
little to the left of the middle line,
meeting a thickening of the ecto-
derm about the middle of the dorsal
surface, where an aperture is formed.
The pre-oral lobe appears at an
early stage as a dilatation at the
anterior end of the larva. This

takes an antero-posterior direction, and assumes the character
of an elongated, almost cylindrical, hollow appendage at the
anterior end of the larva, consisting of a shorter, anterior, and a

longer, posterior, part.
On the anterior sur-
face of the pre-oral
lobe a flattened area
appears surrounded by
a raised rim, which is
beset with specially
large cilia : this is the
larval organ. In the
middle of the larval
organ appears an ele-
vation, the rudiment
of a sucker by means
of which the larva be-
comes attached when
the metamorphosis is
about to begin. At
this stage the larva
(Fig. 316) is able to
creep by contractions
of the pre-oral lobe,
and also to swim by
the action of the cilia,
more especially the cilia of the larval organ.

The hydrocoele, at first a five-lobed outgrowth of the entero-
coele, grows into the form of a horse-shoe with five lobes, each of

FIG. 317. Asterina exigua. Young Starfish shortly
after the metamorphosis has b^en completed, viewed
from the oral side. circ. amb. circular ambulacral
vessel ; dors. p. dorsal pore and madreporic canal ; rad.
amb, radial ambulacral vessel ; st. stomach ; tent, tentacle ;
t. f. tube-feet.

392

ZOOLOGY

SECT.

which represents one of the radial parts of the ambulacral system,
the horse-shoe itself representing the ring-vessel. The rudiment
of the madreporic canal arises as a groove on the posterior wall of
the anterior coelome. This develops into a canal leading from the
hydrocoele to the anterior coelome, and eventually entering into
connection with the dorsal pore, forms a tube, the madreporic
canal, leading from the ring canal to the madreporite, of which the
dorsal pore represents the first-formed aperture.

As the hydrocoele develops, its form influences the external
shape of the larva ; on the left-hand side there grows out a five-
lobed elevation (Fig. 316, ami)), each of the lobes corresponding
to one of the five lobes of the hydrocoele. Each of the latter then
becomes divided, first into three rounded processes (Fig. 315, B,
amb), and then into five, and these project freely on the

ABO

3 31

FIG. 318. Views of the larva of Asterina gibbosa in the course of metamorphosis. A, larva
of eight days, from the right ; B, left, and C, right view of the larva of nine days ; 1-5, lobes
of hydrocoele ; I-V, rudiments of arms. (From MacBride, after Ludwig.)

surface ; the middle one is the rudiment of the tentacle, the
lateral processes are the first two pairs of tube-feet. At the same
time five elevations of the opposite wall become evident, and give
rise to the beginnings of the dorsal regions of the arms (Fig. 318).
The transition from the larval stage to the condition of the five-
rayed Starfish (Fig. 317) is effected by the abortion of the pre-
oral lobe (which, on the larva becoming fixed by means of the
sucker, degenerates into a temporary stalk and eventually becomes
completely absorbed) by the further development of the arms and
tube-feet, and by certain changes which take place in the internal
organs. Of these, one of the most important is the formation of
a new mouth and oesophagus (Fig. 315, B, ces), the larval mouth and
oesophagus becoming abolished during the metamorphosis. Round
this new mouth grows the ring- vessel of the ambulacral system.
From the stomach, diverticula grow out radially into the developing
arms to give rise to the caeca : and later the permanent anal
opening is formed on the dorsal surface.

IX

PHYLUM ECHINODERMATA

393

When the first ossicles are definitely formed they present the
following arrangement (Fig. 319). In the middle of the abactinal
surface is a single central plate (dors). Around this are five basals
(bas) one of which becomes merged into the madreporite. External
to these, five radial s (rad) appear somewhat later. At the end of
each developing arm is a single terminal or ocular plate (term),
which is carried outwards as the ambulacral and adambulacral
ossicles of the arm are developed, supporting the corresponding
eye and tentacle. A ring of secondary radials or infra-nasals (sec.
rad) is developed between the radials and the central. In the

seo.ra.ct

term.

FIG. 310. Diagram showing the relations of the chief plates of the apical system in the young
Starfish, an. anus ; bas. basals ; dors, central ; madr. madreporite ; rad. radials ; sec. rad.
secondary radials (infra-basals).

adult, by the intercalary development of numerous additional
ossicles, these primary plates of the apical system, as it is termed,
lose their original arrangement, and become no longer recognisable.

2. EXAMPLE OF THE ECHINOIDEA.
A Sea-Urchin. (Strongyloccntrotus or Echinus.)

General External Features. The Sea-Urchin (Figs. 321 and
322) is globular in shape, but somewhat compressed in one direc-
tion, so that two poles are distinctly recognisable. At one of these
the degree of flattening is greater than at the other ; this is the
oral pole, the opposite pole being termed the anal or aboral. At
the oral pole is a rounded aperture, the mouth, through which may
be seen projecting five hard white points, the extremities of the

394

ZOOLOGY

SECT.

teeth, surrounding the mouth is a thin, soft membrane known as
the pcristome or peristomial membrane (Fig. 320). At the anal pole
is a much smaller aperture, the anus, the space immediately
surrounding which is termed the periproct (Fig. 322).

The entire surface, with the exception of the peristome and
periproct, is bristling with spines cylindrical, pointed, solid ap-
pendages, the surface of which is longitudinally fluted. These are
movably articulated with the body so that they may be turned
about in all directions. When one of them is removed (see Fig.
338, p. 422), it is found that the joint is of the character of a ball

FIG. 320. Echinus esculentus, peristome. 1, tube-feet of the lower ends of the radii; 2,
branchia ; 3, teeth ; 4, buccal tube-foot ; 5, peristomial membrane. (From Mac-Bride, after
Kiikenthal.)

and socket, a concavity on the base of the spine fitting over a
hemispherical elevation on the surface of the Sea-urchin, and the
spine being retained in place and caused to move by means of a
capsule of muscular fibres enclosing the joint. Around the bases
of the large spines are a number of very small spinules. Here and
there among the spines are to be observed minute pcdice/larice
(see Fig. 340, p. 423), which are comparable to the stalked
pedicellarise of Asterias ; but each has three jaws instead
of two, and a relatively long stalk, which is supported by a
slender calcareous rod. Here and there are to be found also small

IX

PHYLUM ECHINODERMATA

395

rounded bodies termed the sphceridia, which are perhaps, like the
pedicellariae, to be looked upon as modified spines : they contain
ganglion-cells and are apparently organs of special sense, having
possibly the function of detecting changes in the composition of
the water.

Projecting from the surface among the spines all the way from
the peristome to the periproct will be observed five double rows of
tube-feet (Fig. 321), which in a living specimen will be found to
be capable of great extension. These are similar to the tube-

FIG. 321. Strongylocentrotus, entire animal with the tube-feet extended. (From Brehm's

Tierleben.)

feet of the Starfish, and have similar functions : the sucker-like
extremity of each is supported by a perforated sieve-like plate of
calcareous matter. Each double row of tube-feet occupies a
meridional zone of the surface, termed the ambulacral area,
corresponding to the ambulacral groove of the Starfish-: the inter-
mediate zones are termed the intcr-ambidacral areas. At the oral
end of each ambulacral area on the peristome (Fig. 320) is a pair
of appendages similar to tube-feet, but shorter, and termed tentacles.
Ten shrub-like appendages, the dermal branchice, are situated in

396

ZOOLOGY

SECT.

the peripheral part of the peristome, a pair opposite each inter-
ambulacral area.

When the spines are removed, the body is found to be enclosed
in a rigid globular shell, or corona (Fig. 322) as it is termed,
formed of a system of plate-like ossicles, the edges of which fit
accurately and firmly together, and the surfaces of which are
ornamented with the rounded elevations or tubercles for the articu-
lation of the spines. These plates are arranged in ten zones, each
consisting of two rows, running in a meridional direction from the

---4

L-. 5

7 u,

8

FIG. 322. Corona of Echinus esculentus, from the aboral surface, showing the arrangement
of the plates of the corona. 1, the anus; 2, periproct, with irregular plates; 3, the madre-
porite ; 4, one of the other genital plates ; 5, an ocular plate ; 6, an inter-ambulacral plate ;
7, an ambulacral plate ; S, pores for the protrusion of the tube-feet ; 9, tubercles. (After
MacBride.)

edge of the peristome to the neighbourhood of the periproct. Of
the zones of plates there are two sets, each consisting of five, the
members of which alternate with one another. In the case of one
of these sets of zones the ambulacral zones or ambulacral areas
already referred to each of the plates is perforated towards
its outer end by two minute pores, the ambulacral pores, for the pro-
trusion of the tube feet. In the other five zones, the inter-ambu-
lacral zones or areas, the plates are not perforated. At its
anal end each area, ambulacral or inter-ambulacral, ends in a
single apical plate, so that the periproct is surrounded by a ring of

PHYLUM ECHINODERMATA

397

a nib

am-b

ten. plates, the apical system of plates (Fig. 323). Of these, the
five that are situated at the ends of the ambulacral areas are
termed the ocular plates (oc), owing
to the fact that each of them bears
a structure once supposed to be a
rudimentary eye, but now known
to be a tentacle; while the five
opposite the inter-ambulacral areas
are termed the genital plates (gen),
each of them being perforated by
an opening which is the aperture
of one of the five genital ducts
the ducts of the ovaries or testes
as the case may be. One of these
genital plates (madr*) has a swollen
and spongy appearance, which dis-
tinguishes it from the others : this
is the madreporite, through which,
as in the case of the structure of the same name in the Star-
fishes, the madreporic canal communicates with the exterior. The
two ambulacral areas between which the madreporite lies con-
stitute the bimum, the remaining three the trivium.

On the inner surface of the shell, close to the edge of the peri-
stome, there project inwards five processes, the auricles (Fig. 325,
aur\ one opposite each ambulacral area. Within the ring of auricles
lies a complex structure termed Aristotle's lantern (Fig. 324).
This consists of the five teeth (e), the apices of which are to be

inZ.amb.

Fir;. 323. Apical system of plates and
aboral extremities of zones of the shell
of a Sea-urchin, amb. ambulacral
zones ; gen. genital plates ; int. aidb.
inter-ambulacral zones ; madr. madre-
porite ; oc. ocular plates ; peripr. peri-
proct. (After Leuckart.)

B

Fir;. 324. Lantern of Aristotle of Echinus. A, two of the five chief component parts apposed
and viewed laterally. B, lateral, and C internal view of a single part. a. alveolus ; a', suture
with its fellow ; 6. epiphysis ; b'. suture with alveolus ; c. rotula ; <?. radius ; e. tooth. (From
Huxley's Invertebrates, after M tiller.)

seen projecting through the mouth, together with a system of
ossicles. The teeth are long, curved, and pointed : proximally each
is supported by and partly embedded in a pyramidal ossicle, the
alveolus (a}, consisting of two halves united by a longitudinal suture.

398

ZOOLOGY

SECT.

racLne
radam.

Firmly united to the base of the alveolus is a stout bar, the
epiphysis (b). Adjacent epiphyses are in close contact with one
another, and running inwards from their points of union are five
radially-directed, stout bars, the rotulce (c), the inner ends of which
unite to bound a circular aperture through which the oesophagus
passes. With the inner end of each rotula is movably articulated
a more slender bar, the radius (d), which runs outwards, parallel
with, and closely applied to, the rotula, to end in a free, bifurcated
extremity. Aristotle's lantern as a whole is in the shape of a five-
sided pyramid, at the apex of which project the five teeth ; the
pyramid is hollow, containing a passage which is the beginning of

the oesophagus. The
base has the appear-
ance of a wheel, the
tyre of which is re-
presented by the five
epiphyses, the spokes
by the five rotula3
with the five radii in
close contact with
them, and the hub by
the rounded central
aperture. Passing be-
tween the various os-
sicles of the lantern,
and from them to the
auricles, are systems
of muscles by means
of the contractions of
some of which the
lantern as a whole
can be protruded or
retracted, while the

action of others is to cause the movements of the alveoli by
which the teeth are brought to bear on the food.

Nervous System. Passing outwards through each auricle,
and running along the inner surface of the corona opposite the
middle of each ambulacral area, is a radial nerve (Fig. 325, rad.
ne). Within the ring of auricles the five radial nerves are con-
nected with a nerve-ring (nerv. r) surrounding the mouth. At its
distal end each radial nerve is connected with the so-called eye (oc),
borne by the corresponding ocular plate. These parts correspond
to the epidermal nervous system of the Starfish, which, owing to
the ambulacral grooves having become closed in to form narrow
canals the epineural canals (Fig. 326, ep.) t covered over by the
plates of the corona is here more deeply situated ; the deep and
ccelomic systems are only feebly developed.

nerv.r

FIG. 325. Lateral view of the internal organs of a Sea-
urchin as seen on the removal of a half of the shell, aft. r.
ves. haemal strand, aboral ring ; amh. r. ambulacral ring-
canal ; trap, ampullae ; an. anus ; aur. auricle ; rid. ccelome ;
int. intestine ; int.ves.mtestinal haemal strands ; mad. mad-
reporite ; mad. can. madreporic canal ; mo. mouth ; mus.
muscles passing from the auricles to Aristotle's lantern ;
nerv. r. nerve-ring ; oc. ocular plate ; or.r.ves. haemal strand,
oral ring ; pi ex. ovoid gland ; pol. ves. Polian vesicle ; rod.
amb. radial ambulacral vessel ; rad. ne. radial nerve ; sipli.
siphon ; sp. radial extension of the coelome sin-rounding
the nerve ; t. f. tube-feet. (From Leuckart, after Hamann.)

IX

PHYLUM ECHINODERMATA

399

Ambulacral System. Internal to each radial nerve, and pur-
suing a corresponding course, runs a radial ambulacral vessel (Figs.
325 and 326). From this are given off on each side a series of short
branches to the tube-feet, with each of which is connected one of
a series of compressed sacs, the ampullae (amp), by two canals, one
passing through each of the two pores. At their oral extremities
the five radial ambulacral vessels unite with a ring-vessel
surrounding the oesophagus. Appended to the ring-vessel are
five Polian vesicles (pol. ves.) in the form of small mammillated
bodies. A madreporic canal (mad. can.), corresponding to that of
the Starfish, but with soft membranous walls devoid of ossicles,

wv.r

---amp.

FIG. 32(3. Diagrammatic transverse section of the ambulacral zone of an Echinoid. amb. oss.
ambulacral ossicle ; amp. ampulla of a tube-foot ; ep. epineural canal ; muse, muscles
attaching spine to its tubercle ; nerv. nervous ring in base of spine ; n. r. radial nerve-cord ;
oss. ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perili. radial perihfemal canal ;
pod. tube-foot ; wv. r. radial ambulacral vessel. (After MacBride.)

runs from the madreporite at the side of the periproct to the
ring-canal.

The enteric canal (Fig. 327, ali) is devoid of the radial cteca
which it presents in the Starfish : it is a wide, soft-walled tube,
which winds round the interior of the corona in its passage from
the mouth to the anus, held in place by a band of threads, the
mesentery, passing out from it to the inner surface of the shell.
It gives off a short blind diverticulum, the siphon (sipli) ; this,
together with the intestine itself, probably acts as an organ for the
respiration of the ccelomic fluid.

The coelome contains a fluid in which, as in the Starfish,

there are numerous corpuscles. Of these there are two kinds

-amoeboid corpuscles (amcebocytcs) with long pseudopodia, and

vibratile corpuscles, which closely resemble sperms, having a rounded

400 ZOOLOGY SECT.

head and a slender vibratile tail : the latter aid in bringing about
a constant circulation of the coelomic fluid.

The part of the coelome containing Aristotle's lantern is com-
pletely cut off from the rest by the arrangement of the membrane
enclosing the lantern, and the function of the branchia3 on the
peristome is evidently the oxygenation of the coelomic fluid
enclosed in this compartment, which is known as the lantern-

The perihaemal and haemal or lacunar systems, as well as
the axial organ, will be referred to in the account of the general
structure of the phylum.

The reproductive organs consist of five masses of minute
rounded follicles (Fig. 327, ov) situated in the anal portion of the
shell, and each communicating with the exterior by its duct,

ves

reef

.orrves
la.nl

aU

FIG. 327. Alimentary canal and other organs of Sea-urchin as seen when the ora half
of the corona has been removed, ab. r. ves. aboral ring of the ha?mal system ; all. ali-
mentary canal ; amp. ampullae ; int. ves. intestinal blood-vessels ; lant. lantern of Aristotle ;
oes, oesophagus ; or. r. v. oral ring-vessel of the haemal system ; ov. ovary ; rect. rectum ; sip/i.
siphon ; z. teeth. (From Leuckart, partly after Cuvier.)

which perforates the corresponding genital plate. The sexes are
distinct ; as in the Starfish, there is little difference to be
observed between the ovaries of the female and the testes of the
male until we come to examine their microscopic structure. The
genital rachides which in the Starfish connect the gonads
with the genital stolon (p. 385) are aborted in the adult Sea-
urchin.

The early stages in the development of the Sea-urchin are
very similar to the corresponding stages in the development
of the Starfish described on page 388. The bilateral larva
of the Sea-urchin, which is termed a plutevs, is provided with
a number of elongated arms or processes supported by delicate
calcareous rods. A metamorphosis, in which the bilateral larva
becomes converted into the radial adult, takes place as in the
Starfish.

IX

PHYLUM ECHINODERMATA

401

3. EXAMPLE OF THE HOLOTHUROIDEA.
A Sea-cucumber. Cucumaria or Colochirus.

General External Features.- -The body (Fig. 328) is elon-
gated, in shape not unlike a miniature cucumber, somewhat
irregularly five-sided, with an opening at each end. One end is
somewhat thicker than the other, and the opening at this thicker
(oral or anterior) end is the mouth, that at the opposite (aboral or
posterior) end is the anus. The body is five-sided, and along each
side there extends a double
row of tube-feet. In Colo-
chirus there is a very distinct
ventral surface, into which
three of the five sides enter,
distinguished by the absence
of the rows of tubercles that
occur on the dorsal portion
of the surface, and by the
presence of three distinct
bands of tube-feet. This
ventral part of the body with
its three ambulacral areas is
the equivalent of the trimum
of the Starfish, the rest re-
presenting the 5wmw. On
the dorsal surface, instead of
typical tube-feet, there are
papillse devoid of sucking ex-
tremities, and similar appen-
dages take the place of tube-
feet at the ends of the three
ventral bands. In Cucumaria
the ventral surface is less
distinctly defined, but its
position is to be deter-
mined by reference to the
tentacles (vide p. 402) ; there
are no papillse. The ventral surface is. it is to be noticed, parallel
with the axis joining mouth and anus, and the body, when
compared with that of the Starfish or Sea-urchin, is greatly drawn
out in the direction of the line joining mouth and anus.

There are no definite calcareous plates ; but the integument is
tolerably hard, owing to the presence in its substance of innumer-
able microscopic calcareous spiculcs, very variable in shape in
different species of Cucumaria, and in Colochirus having the form
of sieve-like or lattice-like plates, some of which are to be found

VOL. I D D

FIG. 328. Cucumaria planci. Entire animal
seen from the ventral surface. (From Hert wig's
Lehrbuch, after Ludwig.)

402 ZOOLOGY SECT.

even in the walls of the tube-feet. The tube-feet are, like those
of the Starfish, used in locomotion, progression being effected by
creeping with the ventral surface applied to the ground. In a
Sea-cucumber living undisturbed under natural conditions there
will be found protruding through the mouth a circlet of ten
tentacles, which are to be looked upon as greatly developed and
specially modified tube-feet. These are tree-like in shape a
central stem giving off a number of short branches, which may in
turn be branched and they are highly sensitive and contractile.
Two of these tentacles will be seen to correspond to each of
the ambulacra! areas. The pair situated opposite the middle
ambulacral area of the ventral surface are very much smaller than
the others, and will be observed to perform the special function of
pushing the food-particles into the mouth. All the tentacles are
drawn completely back within the mouth when the animal is
disturbed.

Structure of Body- wall.- -When the wall of the body is
divided, it is found to consist, in addition to the hardened integu-
mentary layer, of two layers of muscle in addition to a thin layer
of cells, the peritoneum or ccelomic epithelium, lining the coelome.
The outer layer of muscle is a complete, continuous layer of
muscular fibres which have a circular arrangement, i.e. are
arranged in a ring-like manner around the long axis of the body ;
while the inner layer is not continuous, consisting, in fact, merely
of five flattened bands which run longitudinally from the oral to
the anal extremities, each underlying one of the ambulacral areas.
In close contact with each of these bands, on its inner surface,
runs a radial ambulacral vessel (Fig. 329, rad. ami}) together with a
radial nerve.

Ambulacral System. Just behind the bases of the tentacles,
and surrounding the beginning of the oesophagus, is a circular
ambulacral vessel (ring, ves) which gives off the five radial vessels',
these first run forwards and give off branches to the tentacles,
and then backwards, passing along the ambulacral areas and
giving off branches to the tube-feet, each of which is provided
with its ampulla. From the ring- vessel also arises a large pear-
shaped Polian vesicle (pol. ves), and a short sinuous canal, the
madreporic canal (mad. can), which ends in a perforated extremity-
not situated, like the madreporite of the Starfish or Sea-urchin,
on the outer surface of the body, but in the interior of the coelome.

A nerve-ring surrounds the mouth and gives off the five radial
nerves.

Both periphaemal and haemal systems are well developed.
The latter comprises a ring-like strand (ri. U. ves) situated close
to the nerve-ring and sending off five radial strands, as well as
dorsal and ventral strands (int. ves) accompanying the enteric
canal, and a plexus surrounding the left respiratory tree (p. 404).

IX

PHYLUM ECHINODERMATA

403

The ccelome contains a fluid in which float numerous amcebo-
cytes, similar to those of the Starfish, and also a number of

radainh

ri.bl.vcs

pol. ves

s torn
intv&s

ci.rc

res

cciv org

cl

Cl.oj}

Fin. 329. Internal organs of a Holothurian as scon when the body-wall is divided along
the middle of the dorsal surface. 6. w. body-wall ; circ. mus. circular layer of muscle ;
cl. cloaca ; cl. op. cloacal opening ; Cuv. org. Cuvieran organs ; gen. ap. genital aperture ;
gen. du. genital duct ; gen. gl. gonad ; int. intestine ; inter, oss. inter-ambulacra! ossi-
cles ; int. ves. intestinal haemal strands ; long. mus. longitudinal band of muscle ; mad. can.
madreporic canals ; mes. mesentery ; pol. ves. Poliau vesicles ; rad. ami), radial ambulacral
vessel ; rad. oss. ambulacral ossicles ; ri. bl. ves. ring strand of haemal system ; resp. respira-
tory trees ; ring-ves. ring-vessel of the ambulacral system ; stom. stomach. (After Leuckart.)

flattened nucleated corpuscles containing a red colouring matter
-haemoglobin almost identical with that which gives the red
colour to the blood of the higher animals.

D D 2

404 ZOOLOGY SECT.

The enteric canal is, as already mentioned, surrounded at its
oral extremity by the circlet of tentacles, and within these, when
they are fully exserted, is a narrow peristome with the mouth in
the centre. When the tentacles are retracted the peristome be-
comes inverted, so that peristome and tentacles are enclosed
within a chamber, the bnccal chamber, into which the mouth leads.
Surrounding the oesophagus, which lies immediately behind the
buccal chamber, is a circlet of ten circum-azsophagcal ossicles, five
ambulacra! (rad. oss) in position, and five inter-ambulacral (inter,
oss). Through each of the former pass the corresponding radial
ambulacral vessel, hsemal strand, and nerve. The alimentary canal
itself is a simple cylindrical tube, only indistinctly marked out
into ossophagus, stomach (stom), and intestine. It forms several
coils within the coelome, to the wall of which it is attached by a
thin membranous dorsal mesentery, and terminates behind in a
comparatively wide chamber, the cloaca (el).

Opening into the cloaca is a pair of remarkable organs of
doubtful function, the so-called respiratory trees (resp). Each of
these, beginning behind in a single tubular stem, becomes elabo-
rately branched in front, some of the branches reaching nearly to
the anterior end of the body-cavity. Each of the terminal branches
ends in a small enlargement or ampulla. Besides having to do,
most probably, with the respiration of the coelomic fluid and with
the excretion of waste-matters, these organs have a hydrostatic func-
tion ; it is through them also that, when the tentacles are with-
drawn, the overplus of fluid which would impede their retraction is
got rid of, and by their means, in like manner, that the quantity is
again ' increased when the tentacles are protruded again. In all
probability it is through the permeable walls of these organs that
additional supplies of sea-water are received into the coelome, and
thus reach the ambulacral system through the perforated end of
the madreporic canal.

Reproductive Organs.- -The Sea-cucumber, like the Starfish
and Sea-urchin, has the sexes separate. Ovaries and testes (gen. gl)
are very like one another, and consist of bunches of tubular
follicles, which communicate with the exterior by means of a duct
opening on the dorsal surface some little distance behind the oral
end (gen. >.).

The early stages of development are very similar to those of
the Starfish (p. 388). The bilateral, however, assumes a shape
somewhat different from that of the Asteroidea, and is
termed the auricularia (Fig. 343) : it has a number of short
processes developed in the course of the ciliated bands. The
larval mouth and oesophagus, instead of being abolished as in the
case of the Starfish, persist to the adult condition.

IX

PHYLUM ECHINODERMATA

405

4. THE CRINOIDEA.
A Feat her- Star. Antedon rosacca.

General External Features. In the Feather-Star (Fig. 330),
as in the Starfish, there are to be recognised a central disc and a
series of five radiating arms. In the natural position of the animal
the side of the disc which corresponds to the oral or actinal
surface of the Starfish is directed upwards, and the aboral or
abactinal surface downwards. The five arms are bifurcated at
their bases ; they are feather-like and highly flexible, acting as
the locomotive organs of the animal, their alternate flexions and
extensions resulting in a slow movement through the water. On

FIG. 330. Antedon. Side view of entire animal. (From Leuckart and Nitschc's Diagrams.)

the aboral side of the disc are whorls of slender, curved, cylindrical
appendages, the cirri (Fig. 331), by means of which the Feather-star
is enabled to anchor itself temporarily to a rock or a sea-weed.

On the oral side of the disc the body- wall is soft and flexible,
containing only scattered irregular spicules of calcareous matter ;
and nearly, but not quite, in the centre of this surface is an opening,
the mouth (Fig. 332, mo). From the mouth five very narrow
grooves, the ambulacral grooves, radiate outwards towards the bases
of the arms, near which they bifurcate, so that ten grooves are
formed, one passing along the oral surface of each of the ten
arm-branches to its extremity. The anal opening (an) is likewise
on the oral surface, being situated on a papilliform elevation in
the interspace between two of the radiating canals.

406

ZOOLOGY

SECT.

The aboral side of the disc is occupied by a large, flat, pentagonal
ossicle, the centro-dorsal ossicle (Fig. 331, c ; and Fig. 334, CD),

syz cir

FIG. 331. Aboral view of Antedon. c. ccntro-dorsal ossicle; cir. cirrus; /J,i R,~ R,3 the three
radial plates of one column ; syz. syzygy or articulation. (After MacBride.)

bearing on its outer surface a number of little cup-like depressions,
with which the bases of the cirri are connected. The cirri (tirr)

Fie. 332. Antedon, mal (upper) surface of the central disc. an. anus ; mo. mouth.

(Krnni \ n-t and .lung.)

consist each of a row of slender ossicles, covered, like all the rest
of the animal, with epidermis, and connected together by means

IX

PHYLUM ECHINODERMATA

407

rod,, a nib
cod curt

OSS

of muscular fibres. Concealed from view by the centro-dorsal
ossicle is a thin plate termed the " rosette ''' (ros), formed by the
coalescence of the basals of the larva. At the sides are five
first radial ossicles (R l ), also concealed by the centro-dorsal
ossicle : with each of these articulates a second radial (H 2 ), which
is visible beyond the centro-dorsal. With each of the second
radials articulate two third radials (R 3 ), each forming the base of
the corresponding arm-branch.

The ossicles of the arms brachials (Er. 1 , Br?} are arranged
in a single row in each arm. They are somewhat elongated
in the direction of the long
axis of the arm, strongly con-
vex on their aboral surfaces,
longitudinally grooved on the
oral surface, and connected to-
gether by the investing epi-
dermis and by bundles of
muscular fibres, by the con-
tractions of which the move-
ments of the arms are brought
about. Fringing the sides of
each arm are two rows of side-
branches, or pinnules, each sup-
ported by its row of connected
ossicles, and each grooved along
its oral surface.

The ccelome contains num-
erous strands of connective-
tissue which serve to suspend
the various organs.

Extending through the arms
and pinnules between the sup-
porting ossicles and the am-
bulacra! grooves are three
canals which are prolongations
of the coelome (Fig. 333, cod. can}. Two of these the sub-
tentacular canals form a pair separated from one another by a
median septum underlying the ambulacral groove. The other
the ccdiac canal runs between these and the supporting ossicles
(oss). The sub-tentacular canals and the coeliac canal communicate
with one another at the extremity of each arm.

The enteric canal begins with a wide, funnel-shaped oeso-
phagus leading to a spacious stomach which gives off a number of
short, blunt diverticula and a pair of longer, narrower, " hepatic '
caeca, which are slightly branched at the ends. Distally the stomach
becomes contracted and opens into a wide intestine, which winds
round the ccelome, becoming narrower where it passes upwards to

FIG. 333. Antedon, transverse section of a
pinnule, amb. ne. radial nerve of the super-
ficial (ambulacral) nervous system ; ax. ne.
axial nerve ; crel. can. sub-tentacular and
coeliac canals ; mus. muscles ; neur. ves. radial
sinus of the perihfemal system ; rad. amb.
radial ambulacral vessel giving off branches
to the tentacles. Between the paired sub-
tentacular and unpaired coeliac canals is the
genital rachis. The small round bodies above
the line from rad. amb. are the sacculi. (After
Teuscher.)

408

ZOOLOGY

SECT.

open on the exterior, the terminal part, or rectum, projecting as a
tubular papilla on the surface. In the living animal the rectal tube
is observed to undergo frequent movements of contraction and
dilatation, by means of which water is drawn into and expelled
from the intestine ; so that here, as in the Sea-urchin, there
would appear to be a process of intestinal respiration.

The ambulacral system consists of a ring-vessel surrounding
the mouth, and a series of radial vessels (Fig. 333, rad. ami).) which
run in the ambulacral grooves, giving off branches to the pinnules.
Connected with the radial vessels and their branches are a series
of minute tubular appendages, the so-called tentacles (Fig. 334,
tent.), which are homologous with the tube-feet of the Starfishes

amb

ival.p

co

cJmmb.org

ceTil.caps

cirr

FIG. 334. Antedon. Diagrammatic view of a median vertical section through the disc, passing-
through one radius and one inter-radius, amb. ambulacral vessels ; ax. co. axial nerve-cord
passing through the ossicles of the arm ; Brl Br2 brachial ossicles ; CD. centro-dorsal
ossicle ; cent. caps, central capsule ; chamb. org. chambered organ ; cirr. cirri ; cct. nc.
ambulacral (epidermal) nerve-ring and radial nerve ; gen. st. genital stolon ; int. intestine ;
mo. mouth ; 7Z.1 R2 R.3 radials ; ros. rosette; tent, tentacles; wat.p. water-pores. (After
Milnes Marshall.)

and Sea-urchins, but are devoid of terminal suckers. These are
not organs of locomotion : they bear numerous sensory papillae, and
are therefore to be looked upon as tactile organs, but they probably
also have a respiratory function. Connected with the ring-vessel
are a number of ciliated, branched, tubular diverticula, the water-
tubes t which are suspended within the coelome, and may open freely
into it at their extremities. A large number of vessels with
minute ciliated openings the water-pores (wat. p) lead through
the actinal wall of the disc : these and the ciliated tubes are to be
considered as together representing the madreporic canal and its
openings in the Star-fish and Sea-urchin.

The nervous system consists of three perfectly distinct parts-
superfaial, deep, and axial or ciboral. A superficial radial nerve-

ix PHYLUM ECHINODERMATA 409

ring (ect. ne) surrounds the mouth, and from it are given off
a series of nerves thickenings of the epidermis of the ambulacra!
grooves and their offsets which extend throughout the length
of the arms and pinnules. The deep nervous system follows the
same general arrangement as the superficial. In the axis of the
supporting ossicles of the arm is an axial nerve (ax. co), which gives
off branches (Fig. 333, ax. ne) running through the axes of the
ossicles of the pinnules. The axial nerves are connected internally,
not with the circum-oral nerve-ring, but with a central body
situated below the rosette, in the interior of the centro-dorsal ossicle.
This, the central capsule (Fig. 334, cent, caps), forms the investment
of a body termed the five-chambered organ (chamb. org), divided
into five parts by radial septa, and continuous with the aboral end
of the genital stolon. Processes from the five angles of the central
capsule combine to form a pentagonal ring from which pass out-
wards the axial nerves of the arms. Aborally the central capsule
gives off nerves to the cirri.

A system corresponding to the perihaemal system of the
Starfish is present, though reduced, and there is a highly developed
and complicated lacunar or haemal system.

Numerous bodies termed the sacculi, the character of which
has given rise to much discussion, occur regularly arranged along
the ambulacral grooves and also in other parts. They are small,
spherical bodies which become vividly coloured when treated with
staining agents. They are sometimes supposed to be parasitic
Algae ; but the regularity of their arrangement is opposed to
such a view. It has been suggested with more appearance of
probability that they may be masses of reserve materials, stored
up for the nutrition of the animal, or may consist of excretory
matters.

The reproductive organs ovaries or testes, as the case may
be are lodged in the dilated bases of the pinnules, which become
considerably enlarged as the ova or sperms mature, those next to
the bases of the arms alone remaining sterile. When mature, the
sexual elements escape by means of short ducts. Each gonad
is one of the terminal parts of a system of tubes lined by an
epithelium, and extending from a central part or genital stolon
(gen. st) lodged in the vascular plexus that surrounds the oesopha-
gus and connected dorsally with the chambered organ outwards
through the arms ; the terminal portions, lying in the pinnules,
aie dilated to form the reproductive organs, and the cells
of their epithelium become developed into ova or sperms, while
the rest constitute a non-fertile connecting rachis. This system
is enclosed throughout by a plexus of haemal lacunae.

Like the rest of the Echinoderms, the Feather-star undergoes a
metamorphosis (Figs. 344 and 345). It passes through a free-
swimming ciliated larval stage, which is followed by a fixed

410 ZOOLOGY SECT.

stalked stage known as the " pentacrinoid " larva on account of the
resemblance which it bears to the adult Pentacrinus, one
of the permanently fixed members of the same class. This fixed
pentacrinoid larva passes into the adult free-swimming Feather-
star by the development of the dorsal cirri, the elongation of the
arms, and the absorption of the stalk.

5. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Echinodermata are radially symmetrical animals, the radial
arrangement of whose parts imperfectly conceals a more obscure
bilateral symmetry. The surface is covered with an exoskeleton
of calcareous plates or ossicles, which usually support a system of
movable or immovable calcareous spines. There is a large body-
cavity or ccelome, and well-developed alimentary, nervous, and
vascular systems. A characteristic system of vessels, the ambu-
lacral system, is connected with the locomotion of the animal, as
well as with other functions : the organs of locomotion are in most
cases elastic and contractile tubular bodies, the tube-feet, which are
appendages of the ambulacral system. Nearly all the systems of
organs of the animal partake to a greater or less extent of the
general radial form of the body. Reproduction is entirely sexual.
In the course of its development from the egg the Echinoderm
passes through a peculiar larval stage, in which the symmetry of
parts is bilateral, instead of radial as in the adult animal. All the
Echinodermata are marine.

The Echinodermata are classified as follows :-

SUB-PHYLUM I. ELEUTHEROZOA.

Echinodermata devoid of a stalk, and always freely locomotive in
the adult condition : with a system of radial ambulacra in the form
of grooves or areas radiating out from the mouth, and containing
a double series of tubular appendages of the ambulacral system,
the tube-feet, usually employed in locomotion, and in the majority
of cases provided with terminal suckers : the anus usually aboral ;
the mouth on the surface that is habitually directed downwards, or
at the end habitually directed forwards in locomotion.

CLASS I. ASTEROIDEA.

Free Echinoderms with star-shaped or pentagonal body, in
which a central disc and usually five arms are more or less readily
distinguishable, the arms being hollow, and each containing a
prolongation of the co3lome and of its contained organs. There
are distinct oral and aboral surfaces, on the former of which the
anus and the madrcporite are situated, and on the latter the

ix PHYLUM ECHINODERMATA 411

mouth and five narrow ambulacral grooves lodging the tube-feet.
The larva has the form either of a bipinnaria or of a brachiolaria.
This class includes the Starfishes.

ORDER 1. PHANEROZONIA.

Asteroidea with large marginal ossicles. The dermal branchise
are present only on the aboral surface. The ambulacral ossicles
not closely crowded. Pedicellarias sessile.

ORDER 2. CRYPTOZONIA.

Asteroidea with the marginal ossicles inconspicuous. Dermal
branchiae not restricted to the aboral, but often present on the
oral surface. Ambulacral ossicles crowded together. Pedicellarise
stalked or sessile.

CLASS II. OPHIUROIDEA.

Star-shaped free Echinoderms, with a central disc and five arms,
which are more sharply marked off from the disc than in the
Asteroidea and which contain no spacious prolongations of the
coelome. There are distinct oral and aboral surfaces. The anus
is absent ; the mouth, as well as the madreporite, on the oral
surface. Except in one fossil order there are no ambulacral
grooves. The larva is a pluteus. This class includes the Sand-
stars and Brittle-stars (Figs. 336 and 337).

ORDER 1. LYSOPHIUR.E.

Extinct Ophiuroids with ambulacral grooves.
Silurian and Devonian.

ORDER 2. STREPTOPHIUR/E.

Ophiuroids in which the ambulacral ossicles articulate with one
another by simple ball-and-socket joints.

ORDER 3. CLADOPHIUR^E.

Ophiuroids in which the ambulacral ossicles articulate with one
another by means of hour-glass-shaped surfaces. The arms may
be branched.

ORDER 4. ZYGOPHIUR.K.

Ophiuroids in which the movement of the ambulacral ossicles on
one another is restricted by the presence of lateral processes and
pits.

412 ZOOLOGY SECT.

CLASS III. ECHINOIDEA.

Free Echinoderms with globular, heart-shaped, or disc-shaped
body enclosed in a shell or corona of close-fitting, firmly united
calcareous plates. The mouth is nearly always polar ; the anus
usually at the opposite (aboral) pole ; the madreporite is close to
the latter. There are no ambulacral grooves ; but the surface is
divided into alternating ambulacral and inter-ambulacral zones or
areas, which usually run from pole to pole. The larva is a pluteus.
This class includes the Sea-urchins, with the Heart- urchins and
Cake-urchins.

ORDER 1. REGULARIA.

Echinoidea with globular corona containing, in most cases,
twenty meridional rows of plates. Mouth and anus polar. A
lantern of Aristotle is present. This order includes the Sea-
urchins.

ORDER 2. CLYPEASTRIDEA.

Echinoidea with more or less flattened corona, with the mouth
central, the anus excentric. A lantern of Aristotle is present.
This order includes the Cake-urchins (Fig. 341).

ORDER 3. SPATANGOIDEA.

Heart-shaped Echinoidea with the mouth and anus excentric.
No lantern of Aristotle. This order includes the Heart-urchins
(Fig. 340).

CLASS IV. HOLOTHUROIDEA.

Free Echinoderms with elongated, cylindrical or five-sided body,
having the mouth and anus at opposite extremities. The body-
wall is usually only supported by scattered ossicles or spicules.
There is no external opening to the madreporic canal (except in
some Elasipoda). The surface usually exhibits five ambulacral
areas ; but these may be absent. There is a circlet of large oral
tentacles. The larva is an auricularia. This class includes the
Sea-cucumbers and " Beche-de-mer."

ORDER 1. ELASIPODA.

Holothuroidea with well-marked bilateral symmetry, with tube-
feet on the ventral surface (which is flattened) and papillae on the
dorsal. Confined to the deep sea.

ix PHYLUM ECHINODERMATA 413

ORDER 2. PEDATA.

Holothuroidea with tube-feet either in longitudinal rows or
scattered irregularly over the surface.

ORDER 3. APODA.

Holothuroidea devoid of tube-feet and of radial ambulacral
vessels.

SUB-PHYLUM II. PELMATOZOA.

Echinodermata which are usually fixed at the base, and usually
supported on a stalk composed of a row or rows of ossicles
(Fig. 342) : the mouth on the free surface, near or in the centre, and
having extending out from it on the oral surface a "radially arranged
system of narrow, ciliated ambulacral grooves, having the function
of food-grooves, which may run between the plates of the theca,
on the surface of the theca, or along the oral surfaces of a system
of radial processes or arms given off from it. The tube-feet of
other Echinoderms, when represented, take the form of small,
tubular, strongly ciliated appendages (tentacles) without suckers :
the anus usually on the oral surface.

CLASS I CRINOIDEA.

Mostly fixed, stalked Pelmatozoa in which there is a theca
comprising five regularly arranged radial and five basal plates,
giving off five, usually branched, jointed processes or arms ; with
food-grooves radiating out from the mouth along the oral surfaces
of the arms, and extending along their branches : the central parts
of the ambulacra!, nervous, and reproductive systems, and of the
ccelome lodged in the theca, send extensions through the arms.

This class comprises, together with many extinct forms, the
only living Pelmatozoa.

SUB-CLASS I. MONOCYCLICA.

Crinoidea in which the base of the theca comprises basals
only.

SUB-CLASS II. DICYCLICA.
Crinoidea in which the base comprises basals and infra-basals.

CLASS II. CYSTOIDEA.

Fixed, stalked, or sessile Pelmatoza, with the plates of the theca sometimes
irregular, sometimes arranged in a regular radial system, with food-grooves
extending for a longer or shorter distance over the surface of the theca, some-
times on special plates lying above those of the latter, their terminal parts

414 ZOOLOGY SECT.

extending 011 to a varying number of unbranched arms or " fingers" ; the theca
perforated completely or partially by numerous pores which are supposed to
have lodged respiratory processes.
Lower Silurian to Carboniferous.

CLASS III. BLASTOIDEA.

Fixed Pelmatozoa with well-developed stalk, and theca with a regular
system of plates ; with five, rarely four, food-grooves radiating out from the
central mouth, and each borne on a special "lancet plate," the inter-radial
intervals between which are occupied by a corresponding number of oral or
" deltoid " plates. The grooves are bordered by a series of side plates bearing
small branches or "fingers" to which side branches of the grooves extend. In
the intervals between the grooves on the aboral sides of the deltoids are a whorl
of plates perforated by the apertures of groups of internally situated respiratory
folds (hydr aspires). The anus is eccentrically situated on the oral surface.
Upper Silurian to Carboniferous.

CLASS IV. EDRIASTEROIDEA.

Fixed (or sometimes free ?) Pelmatozoa, usually sessile, rarely with a short
stalk ; with sac-like, cushion-shaped or disc-shaped theca made up of numerous
plates devoid of any regular arrangement and without any appendages ; with
central mouth and five straight or curved radiating food-grooves bordered by
covering plates : anus and madreporite on oral side.

Cambrian to Carboniferous.

CLASS V. CARPOIDEA.

Pelmatozoa with a well-developed stalk, with the body laterally compressed,
with only two food-grooves running oat from the mouth. Theca composed of
numerous small irregular plates with larger lateral plates forming a framework
along the margins.

Cambrian and Silurian.

Systematic Position of the Examples.

Asterias rubens is a species of the genus Asterias, which, with
several others, constitutes the family Asteriidce of the order
Cryptozonia. The family Asteriidce is characterised among the
families of the Cryptozonia by the following distinctive features :
The ossicles of the aboral surface are small, unequal, reticulate
plates, bearing isolated or grouped spinelets (paxillce). The margin
of the actinostome is denned by the ambulacral plates. The
pedicellarise are of two forms, forceps-like and scissors-like. The
tube-feet are in four rows. Asterias differs from the other genera
of the family in having well-developed reticulate dorsal ossicles
bearing definite spines.

The Sea-urchins of which a short description has been given
are the genera Stronyylocentrotus and Echinus, but the description
is sufficiently general to apply to any member of the family
Ecliinidw, to which these genera, with a number of others, belong.

ix PHYLUM ECHINODERMATA 415

The family Ecliinidce is one of about five families of the sab-order
Ectobranchiata, the members of which all differ from the other
sub-order Entobranchiata of the Rcgularia, or regular Sea-
urchins, in the possession of dermal branchiae, and in having the
auricles in the form of complete arches.

The Sea-cucumber (Cucumaria or Colochirus) is a member of
the Stichopoda one of the families of the sub-order Dendrochiwtce
of the Pedata, or foot-bearing Holothurians. The Dcndrochirotce
differ from the Aspidochirotce the other sub-order mainly in
having arborescent instead of shield-shaped tentacles, and the
Stichopoda differ from the rest of the DendrocJiirotce in having the
tube-feet arranged in five regular zones. The genus Cucumaria
is distinguished from the rest by having ten tentacles with the two
ventral smaller than the others. Colochirus is closely allied to
Cucumaria, the principal distinction being the presence in the
former of papillae taking the place of tube-feet in certain
situations, as already noted.

The Feather Star (Antedon rosacea) is a member of the family
Comatulidce, which is distinguished from the four other living
families comprised in the class Crinoidea of the Pelmatozoa, by
the absence of a stalk in the adult condition.

6. GENERAL ORGANISATION.

General Form and Symmetry. Like the Ccelenterata, the
Echinodermata are radially symmetrical, the body being capable
of division into a series of sub-equal antimeres along a series of
radiating planes at right angles to the principal axis. In the
majority of existing forms (Asteroidea, Ophiuroidea, and Crinoidea)
the radial symmetry is expressed in the external form of the body,
which is produced into a number of radially disposed parts, the arms
or rays, arranged around a smaller or larger central disc. But in the
Echinoidea the body is sub-spherical, and in the Holothuroidea
sub-cylindrical, the radiate arrangement being in these classes
indicated externally only by the distribution of the tube-feet, and
internally by that of certain of the systems of organs.

Although, however, the general external form and the arrange-
ment of some of the internal organs in the Echinodermata indicates
a radial symmetry, it is invariably found that this radial arrange-
ment serves to hide a more primitive and more fundamental
bilateral symmetry. This is best marked in the larva, which has
pronounced bilateral instead of radial symmetry, but is quite
recognisable in the adult. In all Echinoderms there is, passing
through the primary axis, a plane the median plane along
which, and along which alone, the body is capable of being divided
into two equal or, to speak more correctly, approximately equal-
right and left halves. The existence of such a single median

416 ZOOLOGY SECT.

plane is, as already explained (p. 377), indicative of the bilateral
form of symmetry.

The body is most usually five-rayed (Ophiuroidea, most Aste-
roidea, Crinoidea), cylindrical (most Holothuroidea) or globular
(most Echinoidea), the surface in the two last cases being marked
by five bands or zones of tube-feet, which divide it into five
anibulacral and five inter-ambulacral areas. In the Ophiuroidea
and Asteroidea two of the rays constituting the bivium have
between them the madreporite, marking the position of the
madreporic canal of the anibulacral system ; the remaining three
rays form the trimum. The median plane passes through the
madreporite, and thus midway between the two rays of the
bivium, and bisects longitudinally the middle ray of the trivium.
A corresponding disposition of the parts is traceable also, as
will be subsequently shown, in the cylindrical and globular
Echinoderms.

In all the Echinodermata aboral or abactinal, and oral or
actinal surfaces are more or less distinctly recognisable. In the
Asteroidea, Ophiuroidea, and Echinoidea, the actinal surface is
that in the middle of which the mouth is situated, and which is,
in the natural position of the animal, directed downwards or
towards the surface to which it is clinging. The opposite abactinal
surface is, in the majority of the Asteroidea and Echinoidea,
marked by the presence of the anal aperture : in the Ophiuroidea
and some Asteroidea the anus is absent ; in some Echinoidea it is
situated on the border between the two surfaces, or even on the
oral surface. In the Crinoidea the oral surface, which is habitually
directed upwards in the natural position of the animal, bears both
mouth and anus, the former central, the latter eccentric and inter-
radial. In the fixed Crinoids the abactinal or aboral surface has
attached to its centre the distal end of the stalk ; in the free forms it
has connected with it whorls of slender curved appendages, the dor-
sal cirri, by means of which temporary attachment is effected. In the
Holothurians, owing to the elongation of the body in the direction of
the line joining mouth and anus, oral and aboral surfaces correspond-
ing to those of the other classes are not distinguishable ; but in
many, as for example in Colochirus, there is a marked difference be-
tween one surface the dorsal, which is habitually directed upwards,
and another the ventral, which is habitually directed downwards.

In considering the general external form in the various classes
of Echinoderms, we have to take into account the arrangement of
the tube-feet the organs of locomotion as these have important
relations to the other parts and to the whole plan of organisation
of the animal. These organs, as previously explained, are tubular
appendages with highly elastic and contractile muscular walls,
capable of being stretched out so as to extend a long way from the
surface of the body. In the majority of cases the tube-foot has at

ix PHYLUM ECHINODERMATA 417

its extremity a sucking-disc, by means of which it can be attached ;
in a few, however, this sucking-disc is absent.

The epidermis is ciliated in all but Holothuroidea. In the
subjacent dermal layers there are always present calcareous bodies
or ossicles, varying very greatly in form and arrangement in the
different groups. Movable or immovable calcareous spines or
tubercles projecting on the surface are very general. Peculiarly
modified spines, termed pediccllariw, are commonly, though not
universally, present in certain parts in the Echinoidea and
Asteroidea. A pedicellaria consists in essence of two or three
calcareous jaw-like pieces or valves, movably articulated together,
and capable of being separated or approximated by the con-
traction of bundles of muscular fibres ; sometimes there is a long
stalk ; sometimes (as in the case of Anthenea, p. 387) a stalk is
absent ; during life the jaws or valves keep opening and closing.
That such specialised structures have some important function to
perform there can be no doubt, but there is some uncertainty as
to what their special purpose is. According to some observers,
the pedicellariae of the Sea-urchin have been seen passing from
one to another the particles of fsecal matter discharged from the
anus, and their function would thus appear to be a cleansing one.
On the other hand, it is stated that when a Sea-urchin is attacked,
the spines may be bent aside from the assailed portion of the
surface so as to allow of the pedicellame being brought to bear as
defensive weapons on the assailant, and from these and other
observations that have been recorded, both on Asteroids and on
Echinoids, it is concluded that the main function of these appen-
dages is to act as defensive organs. Pedicellaria? are absent in the
Ophiuroids, but in the Euryalida there are peculiar hook-like
organs of adhesion, most abundant on the oral surface and
towards the extremities of the arms. The sphceridia, which have
already been referred to as occurring in the Sea-urchin, are only
doubtfully to be regarded as modified spines ; they are confined
to the Echinoidea. Also confined to that class are the clavulce-
slender spines covered with strong cilia, which occur in bands on
the surface of the Spatangoids. Larger spines, resembling the
clavulse in being covered with strong cilia, occur also in the
Clypeastroids and some Asteroids. The currents produced by the
action of their cilia serve to keep constantly renewed the water
in the neighbourhood of the anus and of the branchiae.

There are two principal systems of plates to be recognised,
an oral and an apical ; the former corresponding with the oral or
actinal, and the latter with the aboral or abactinal surface. The
former vary considerably in the different classes : the constant
elements are five orals, which may or may not be recognisable in
the adult. The apical system consists (1) of a central plate ; (2)
of five basals which are inter-radial in position ; (3) of five radials

VOL. I E E

418 ZOOLOGY SECT.

which are radial in position. In the Asteroidea (Fig. 319) the
radials are late in making their appearance ; before they are
developed five terminal plates have become distinct, one at the end
of each rudimentary arm', these are carried outwards by the
extension of the arm, and each supports the corresponding tentacle.
As a rule these plates of the apical system are only distinct in the
young condition. In the Ophiuroidea the arrangement resembles
that observable in the Asteroidea. In the Echinoidea (Fig. 323)
the basals (genitals) are perforated by the ducts of the repro-
ductive organs ; the radials (oculars) are perforated for the tentacle :
the central (anal) rarely persists as a single plate in the adult,
usually becoming broken up into a series of irregular plates. In
the stalked Crinoidea the term central has been applied to a plate
which is transformed into the disc of attachment at the base of
the stalk, but the correspondence between this and the similarly
named plate in the other classes is very doubtful ; the ossicles of
the stalk intervene between it and the basals. In the free forms
the uppermost segment of the larval stalk, uniting with the central
and the infra-basals, is transformed into a centro-dorsal plate, and
the basals nearly always unite into a rossette-plate, which is
concealed from view by the centro-dorsal and the radials. The
apical system of plates is apparently not represented in the
Holothuroidea.

Modifications of Form in the Five Classes.- -The general
shape in the Asteroidea is, as already pointed out, that of a
star. There is a central part, or central disc, from which proceed
a series of radially disposed arms or rays. The central disc and
the rays are usually compressed in the vertical direction, as in
Anthenea and Asterina, but in some Starfishes the rays are
approximately cylindrical ; they nearly always taper distally. In
the majority of Starfishes, as in the examples described, the arms
are five in number, except in malformed individuals ; but in
some they are six, in others seven, eight, or more. The propor-
tions borne by the arms to the central disc are subject to consider-
able variation. In some, as in Asterias, the arms are long, and
the central disc appears as little more than their point of union ;
in others, again, owing to coalescence of the arms, the whole
Starfish has the form of a five-sided disc, in which the arms are
represented only by the five angles ; while between these two
extremes there are numerous intermediate gradations. The
Brisingidce differ from all the rest of the class in having the arms
almost as sharply separated off from the central disc as in the
Ophiuroids.

The abactinal or aboral, and the actinal or oral surfaces are always
distinctly marked off from one another. In the middle of the
latter (Fig. 335) is the mouth, running out from which are five or
more narrow amoulacral grooves, one of which is continued along

IX

PHYLUM ECHINODERMATA

419

the oral surface of each arm to its extremity. Near to, but not
quite in the middle point of the aboral surface is the anal
((pcrturc, absent in a few instances ; and on the same surface,
nearer the margin, between the two rays of the bivium in the
five-rayed Starfishes, is the madreporite, a finely grooved calcareous
plate perforated by a number of minute apertures. In some fossil
Starfishes it is situated on the oral surface. Sometimes instead of
one madreporite there are several.

The wall of the body in the Starfishes contains a number
of calcareous ossicles, movably articulated together and connected
by bands of muscle, so that, though the body is firm, and in the
dried condition often quite
rigid, the arms are capable
during life of slow move-
ments of flexion and exten-
sion, enabling the animal
to creep through compar-
atively small fissures and
crannies. A special system
of ossicles the ambulacral
ossicles are arranged in a
double row along each am-
bulacral groove, the ossicles
of the two rows articulating
movably with one another
at the apex of the groove.
At the end of the arm the
two rows of ambulacral
ossicles end in a terminal
ossicle which supports the
unpaired tentacle. Spines
are invariably present, but
are sometimes confined to
the margins of the ambula-
cral grooves, in which position they are movably articulated with
the underlying ossicles. Tubercles take the place of spines over
most of the surface in many forms. In Astropecten the ossicles
of the aboral surface take the special form to which the term
paxillce is applied. Each paxilla is a plate which is produced into
a short rod, divided at its extremity into a number of radiating
processes.

The tube-feet are arranged in a double row along each of the
ambulacral grooves, each connected through an aperture between
the ambulacral ossicles with an ampulla, or, exceptionally, with two
ampulla, situated in the coelome. Each double row of tube-feet
terminates at the extremity of the arm in an unpaired appendage,
the tentacle, which is tactile and olfactory, and not locomotive in

E E 2

FIG. 335. Anthenea. View of oral surface.
(After Sladen.)

420

ZOOLOGY

SECT.

function. The tube-feet are provided (except in Astropecten) with
terminal suckers.

In the Ophiuroidea (Fig. 336) the central disc is much more
sharply marked off from the arms than in the Asteroidea. The
arms, which are five in number, are comparatively slender, and
cylindrical, tapering towards the free extremities ; in one group,
the Eurycdida (Fig. 337), they are branched. The mouth is in the
middle of the oral surface of the disc, as in the Asteroidea, but
there are no ambulacral grooves, and there is no anal aperture.

FIG. 336. Ophioglypha lacertosa. A, outline, of the natural size. B, central disc, aboral
surface. C, the disc, oral surface showing the mouth and genital fissures. (From Nicholson
and Lydekker's Paheontology.)

Five pairs of slits on the oral surface (Fig. 336, C) lead into the
genital bursas, Avhich receive the sperms and ova from he gonads,
and which appear also to act as organs of respiration and perhaps
also of excretion. The surface is covered with thin plate-like
ossicles, usually beset along their edges with longer or shorter
spines ; sometimes irregular calcareous granules take the place
of plates. Hook-like organs of adhesion are present only in the
Euryalida. Each of the arms is supported by a row of internally
situated ambulacral ossicles. Tube-feet are present and are pro-
truded at the sides of the arms between the lateral plate- like
ossicles; but they have no sucking-discs and no ampulla?, and
locomotion is effected in the majority of the Ophiuroids by active
flexions and extensions of the arms. In one genus there is a pair

IX

PHYLUM ECHINODERMATA

421

of fin- like appendages, supported by slender spines, on each joint
of the arms. The madreporite is situated inter-radially on the
oral, and not on the aboral surface as in the Asteroidea. In
the Euryalida there are five madreporites and five madreporic
canals.

In the Echinoidea the body is either globular, or heart-shaped,
or flattened and disc-like. The exoskeleton is in the form of a

FIG. 337. Astrophyton arborescens, aboral surface. (After Ludwig.)-

rigidly articulated system of calcareous plates, fitting closely
together by sutures, so as to form a continuous shell or corona.
Asthenosoma and allies, deep-sea forms, differ from all the rest in
having a corona possessing a certain degree of flexibility and
performing movements which are brought about by the contrac-
tions of five longitudinal bands of muscle running along the
ambulacral areas on the inner surface.

In the globular forms, or regular Sea-urchins, the mouth is situ-
ated at the oral pole of the globe, the anus at the aboral, and
the plates of the corona are in twenty regular meridional rows,

422

ZOOLOGY

SECT.

Fn;. 338. Diagram of spine
of Sea-urchin showing
mode of articulation.
TO. muscle ; b. ligament.
(From Leuckart.)

arranged in ten zones, five ambulacral and five mter-ambulacral,
as described in the account of Echinus, with peristome, periproct,
ocular and genital plates, and madreporite. Spines (Fig. 338),
pedicellarice (Fig. 339), and splmridia are present, as already

described (p. 394), the last-named appen-
dages, however, being absent in one group.
The spines are -usually defensive organs
simply, but in some Sea-urchins they act
also as the locomotive organs, the animal
moving by their agency along the sea-bottom.
The tube-feet, which are arranged in a
double row in each ambulacral zone, are
extremely extensible, and terminate in suck-
ing-membranes strengthened by a calcareous
rosette. An unpaired tentacle, corresponding
to that of the Asteroidea, is supported on
each of the ocular plates at the ends of the
ambulacral zones. Two tube-feet in each
double row, situated on the peristome, are
likewise of the nature of tentacles, and are
sometimes devoid of sucking-membranes.
Corresponding to the dermal branchice of
the Asteroidea are, in the majority, five
pairs of branched, hollow appendages surrounding the peristome.
Surrounding the mouth are five teeth, supported by an elaborate
system of ossicles (Aristotle's lantern, see p. 397), and a ring of
processes, the auricles, from the interior of the corona, surrounds
this and gives attachment to some of the muscles by which the
ossicles are moved.

In the heart-shaped forms or Heart- urchins (Fig. 340) the
corona is heart-shaped, the mouth is usually
more or less eccentrically placed on the oral
surface, and the peristome is usually trans-
versely elongated ; the anus is on or near
the border beween the two surfaces. The
ambulacral areas do not run continuously,
but stop short at the margin (petaloid am-
bulacra*) ; one of them, the anterior, is usually
unlike the others and frequently devoid of
pores. The genital and ocular plates are
in the middle of the aboral surface, where
the ambulacra converge, and are thus widely
separated from the anus ; there are usually only four genital plates,
and the genital apertures may be reduced to two. Slender spines
beset the entire surface and are the chief organs of locomotion.
Modified spines, the davulce, surround the anus in a ring and are
distributed elsewhere. A few pedicel laiitu are present in the

'lo. 339. Pedicellaria of
Arbacia punctulata.

(From Leuckart.)

IX

PHYLUM ECHINODERMATA

423

B

neighbourhood of the mouth, and sphseridia also occur. A series of
tree-like dermal branchia? surround the peristome. The " lantern
of Aristotle," with its teeth,
is not represented.

In the Clypeastridea or
Cake- urchins the whole
corona (Fig. 341) is usually
greatly compressed so as
to assume the form of a
disc, sometimes notched at
the edges or pierced by
fenestra?. The mouth is
in the middle of the flat
or concave oral surface, the
anus eccentrically situated
near the margin. The am-
bulacra are petaloid. The
genital and ocular plates
are usually more or less
fused together at their
edges, and the genital
apertures are often not in
the genital plates, but in
the corresponding ambu-
lacral zones. The spines
are exceedingly fine and
hair-like. Sphseridia are
present, but pedicellariaB
and clavulaB are absent.

An " AvitntlA' lanfprn " FIG. 340. Hemipneustes radiatus. A, aboral,
An A11ST and ^ oral surfaco _ c> apical plates. (From Bromi's

with teeth is present, as Tien-etch.)

in the globular forms.

In the Holothuroidea the body is more or less elongated in

the direction of the axis joining mouth with anus, which are placed

at opposite (anterior or oral, and posterior
aboral or anal) extremities of the body.
The shape is sometimes completely cylin-
drical, sometimes five-sided ; in many
there is more or less dorso-ventral com-
pression, and the dorsal and ventral sur-
faces may differ greatly from one another.
A flattened sole-like ventral surface bear-
ing the three rows of tube-feet of the
trivium is, as already stated, often dis-

FIG. 34i.-ciypeasterj:sub- tiiiguishable : it is most distinctly de-

depressus, view of aboral i T- ? i IV l T

surface showing the petaloid velopecl in 1 solus and allied genera.
SSg' < FromHertwi e' E some Holothuroids the surface is enclosed

424

ZOOLOGY

SECT.

Fie. 342. Bfletacrinus interruptus,

(After P. II. Carpenter,)

in an armour of close-
fitting plates ; but in
the vast majority the
body- wall is comparat-
ively soft, being strength-
ened merely by a great
number of minute os-
sicles of a variety
of shapes. In Synapta
(Apoda) numerous mi-
nute anchor-like spicules,
each connected with a
latticed plate, project
from the surface, and
cause the animal to ad-
here to soft bodies with
which it comes in con-
tact. Around the mouth
is a whorl of tentacles
pinnate, shield-shaped, or
arborescent. The tube-
feet are sometimes en-
tirely absent. When pre-
sent they are usually
uniform in character
throughout, and may be
arranged in five regular
longitudinal rows, or
scattered over the entire
surface. Sometimes, as
has already been stated
in the account of Colo-
chirus, the tube-feet of
the dorsal and even some
of those of the ventral
surface may assume the
form of papillae. In the
Elasipoda the tube-feet
of the dorsal surface
are remarkably modified,
taking the form of greatly
elongated processes.

In the Crinoidea the
general shape is that
which has been described
in the case' of the
Feather- star star - li kc,
with a central disc and

ix PHYLUM ECHINODERMATA 425

a series of radiating arms, which usually branch dichotomously.
In the stalked forms (Fig. 342) a stalk, consisting of a row of
elongated ossicles connected together by bundles of ligarnentous
fibres, attaches the animal to the sea-bottom. Along some of
the joints of the stalk are usually arranged a number of slender,
many-jointed appendages the cirri. At its base the stalk
usually breaks up into a number of root-like processes ; distally
it becomes continuous with the central disc. The ossicles forming
the skeleton of the central disc are the basals and the radials : with
the latter articulate externally the brachials, a single row of which
gives support to each of the arms and its branches, while similar rows
of smaller ossicles support the pinnules the lateral appendages
which fringe the arms in a double row. In the free forms
the stalk is absent in the adult condition, though present
in the larva, and from its terminal ossicle and other neighbouring
plates is formed by coalescence a plate the ccntro-dorsal ossicle
of the disc. To the centro-dorsal ossicle are attached whorls
of many -jointed, slender, curved cirri.

The mouth in all the Crinoidea, with one exception (Adinomctra),
is situated in the centre of the oral (upper) surface, and the anus
in all, with the same exception, is eccentric and inter-radial.
Running outwards from the mouth are a series of very narrow
ambulacral grooves, one of which extends along the oral sur-
face of each arm, giving off branches to the arm-branches and
to the pinnules. Bordering the ambulacral grooves and their
branches are a pair of rows of short tubular tentacles, which
correspond morphologically with the tube-feet of the other classes,
but are devoid of the terminal suckers, and are not locomotor,
but probably sensory and respiratory in function.

The ccelome in the Echinoderms is a wide cavity lined by
a ciliated ccelomic epithelium and containing a corpusculated
fluid. Prolongations of it pass out into the rays, and, in the
Ophiuroidea and Asteroidea, between the layers of the body-wall.
In the Crinoidea it contains numerous strands of connective tissue.
Special organs providing for the respiration of this fluid are the
dermal branchice or papidce, the Stewart's organs, and the respiratory
trees. The first of these, which are confined to the Asteroidea
and Echinoidea, have been described in the accounts of the
Starfish and Sea-urchin. In most Asteroidea they occur only on
the dorsal surface, but in some forms they are present on the
ventral surface as well. In some of the Echinoids the place of
dermal branchiaB in providing for the respiration of the compart-
ment of the coelome enclosing Aristotle's lantern (lantern-coelome)
is taken by Stewart's organs, arborescent bodies which project
inwards from the peristome. The respiratory trees are referred
to below in connection with the enteric canal.

Some reference has already been made, in describing the general
form of the body, to the ambulacral system of vessels, A

426 ZOOLOGY SECT.

ring-like circum-oral vessel (ring-vessel) in nearly all cases sends off
a series of radial branches, one passing along each of the rays or
ambulacral areas and giving off branches to the ampullae of the
tube-feet or to the tentacles. In most of the Holothuroidea
branches pass forwards to the circlet of shield-shaped or branched
oral tentacles, and in some cases there are vesicles or ampulloe at
their bases. In the Apoda, in which tube-feet are wanting, radial
vessels are also absent, and the vessels to the tentacles come off
directly from the ring-vessel. In all the classes, except Crinoidea,
one or more bladder-like appendages the Polian vesicles are in
most cases connected with the ring-vessel. The racemose vesicles, or
Tiedemanris vesicles (p. 383), are characteristic of the Asteroidea. In
all, except the Crinoidea and the majority of the Holothuroidea,
there is a communication between the ring- vessel and the surround-
ing water through the madreporic canal. In the Asteroidea, and in
Cidaris among the Echinoidea, the wall of this tube is strengthened
by numerous calcareous ossicles. In the Asteroidea, Ophiuroidea,
and Echinoidea, the communication with the exterior is through
the madreporite. The fine pores perforating the madreporite
and placing the madreporic canal in communication with the
exterior, and the madreporic canal itself, are lined with strong cilia
which move so as to drive a strong current inwards the effect
being to keep all parts of the ambulacral system in a condition of
turgidity. In the few Holothuroids in which such a communication
exists (Elasipoda) there is usually a simple opening, but sometimes
a number of pores crowded together. In the remainder of the
Holothuroidea the distal end of the madreporic canal, or canals,
lies free in the interior of the body-cavity, with which it is placed
in communication by a number of perforations. In the Crinoidea
there is no madreporic canal ; but the ring-vessel is placed in
communication with the ccelome by means of a system of ciliated
water-tubes, while the crelome communicates with the exterior
through a number of minute water-pores, which perforate the
oral body- wall. The fluid contained in the ambulacral system
is similar to that in the ccelome, and contains similar corpuscles.
In one Ophiuroid, however, the ambulacral system contains
corpuscles coloured red with haemoglobin. Tiedemann's vesicles
appear to have the function of manufacturing the corpuscles.

It cannot be definitely stated that a blood-vascular system
exists in the Echinoderms. But two systems have been regarded
as playing the part of blood-vessels the perihwmal system and
the hcemctl system. Neither of these systems comprises vessels
with contractile walls, and there is no definite circulation of
the contained fluid. The perihaemal, or, as it is sometimes
termed, pseudohaemal system, is present in all the classes of the
phylum. When typically developed (Asteroidea, Ophiuroidea) it
consists of a. ring-like circum-oral vessel or sinus and five radial

ix PHYLUM ECHINODERMATA 427

vessels given off from it, together with an axial sinus and aboral
ring-vessels. These ' vessels " are channels with a definite epi-
thelial lining, and are of the nature of specialised parts of the
coelome, from which they are developed. In Asteroidea and Ophi-
uroidea the radial and ring-vessels, which lie between the corres-
ponding parts of the ambulacral and epidermal nervous systems,
are divided into two parts by a longitudinal septum, vertical in
the radial, oblique in the ring- vessel. The axial sinus is nearly
vertical in direction and partly encloses the axial organ in the
way already described (p. 384). At its oral end it opens into the
inner division of the circum-oral vessel : at its aboral end it opens
into, or becomes closely applied to, the aboral vessel, which is in
the form of a ring giving off radial branches towards the gonads :
it also may communicate aborally with several of the pore-canals
of the madreporite, and opens into the madreporic canal itself. In
the Echinoidea the arrangement of the parts is modified in certain
important respects. An oral ring-sinus is absent unless it be
represented by the lantern-coelome. The radial vessels of the
system do not open orally into the lantern-coelome : aborally they
also terminate blindly, not opening into the aboral ring-sinus.
The axial sinus is largely filled by the axial organ : it terminates
blindly at the oral end ; aborally it communicates with the
madreporic canal and is not connected with the aboral sinus. In
the Holothuroidea there are five radial sinuses extending through
the ambulacral areas between the superficial radial nerve and the
radial ambulacral vessel, ending blindly aborally and opening
orally into an oral ring-sinus. There is no axial sinus. In the
Crinoidea the perihsemal system is greatly reduced, though
representatives of the radial sinuses are present in the same
situation as in the other classes.

The general disposition of the lacunar or so-called haemal
system in the Asteroidea has been described in the account given
of the structure of the Starfish (p. 380). Save for certain minor
alterations which are involved in the change in the position of the
madreporite, the system is arranged in the Ophiuroidea on the
same plan as in the Asteroidea. In the Echinoidea there is an
oral ring giving off five radial strands which in the greater part of
their course occupy the typical position between the superficial radial
nerve and the radial ambulacral vessel; aborally they terminate
blindly. A gastro-intestinal system given off from the oral ring
is highly developed, and there is an axial plexus in the axial organ
and an aboral ring, with strands passing to the gonads, as in the
Asteroidea. In the Holothuroidea there is an oral ring with
radial strands, and a well-developed gastro-intestinal system. In
the Crinoidea this system of lacuna 1 is highly developed and
complicated in arrangement.

Whatever be its functions, this system is not a system of blood-

428 ZOOLOGY SECT.

vessels. It is made up of strands of a kind of gelatinous connec-
tive tissue, with many leucocytes, permeated in a very irregular
way by minute lacunae without definite Avails. The great
development of the gastric and intestinal branches of this system
in some (Echinoids, Holothuroids), lends support to the view that
its main functions are connected with the absorption and
distribution of nourishment.

The axial organ (genital stolon) of the Echinodermata is
closely connected both with the perihsemal and haemal systems.
Its general structure and relations in the Asteroidea have already
been described (p. 384). In the Ophiuroidea there is a close
correspondence with the Asteroidea, the chief differences being
such as are involved in the change in the position of the
madreporite from the aboral to the oral surface, and the
resulting change in the direction of the madreporic canal and
associated axial sinus and axial organ. In the Echinoidea the
essentials are the same ; but the axial organ has grown round the
axial sinus so as to enclose it completely.

The enteric canal varies in the five classes more than any
of the other systems of organs. It is a simple tube in the Holo-
thurians and Echinoids, passing spirally through the body from the
mouth at the oral pole to the anus at the opposite pole. In most
of the latter group a complex masticatory apparatus with five
teeth the so-called "lantern of Aristotle" is situated at its
anterior extremity ; the corresponding region in the Holothurians
is surrounded by a circlet of ossicles, which protect the nervous
and vascular rings and into which the longitudinal muscles of the
body-wall are inserted.

In the Echinoidea there is a tubular caecum, the siphon, con-
nected with the intestine. In the Holothurians the so-called
" respiratory trees " (absent in the Elasipoda and the Apoda) are
branched appendages usually two in number, sometimes single
-of the cloaca or posterior wider portion of the intestine, and
the " Cuvierian organs ' are simple filiform glandular tubes, also
connected with the cloaca.

The functions of the siphon and of the respiratory trees have
already been referred to in the accounts of Echinus and Cucu-
maria. The Cuvierian organs, which occur only in a limited
number of Holothurians, correspond to undivided basal branches
of the respiratory trees : they are defensive organs, the animal
when attacked throwing out numbers of these long filaments,
which are very viscid and have the effect of entangling and
hampering the assailant ; but they may also have an excretory
function.

In the Crinoidea the alimentary canal is simply a coiled tube
with both mouth and anal opening on the same (actinal) surface
of the body, In the Ophiurokls the central mouth leads into a

ix PHYLUM ECHINODERMATA 429

simple sac giving off short diverfcicula, and there is no anal
aperture. In the Asteroidea the alimentary canal is more complex
than in the other classes. The stomach is divided, as already
described in the account of the examples, into two portions, the
cardiac and the pyloric, the former giving off five large rounded
radial diverticula the cardiac pouches or cardiac ca?ca, and the
latter five pairs of very long branched diverticula the pyloric
or hepatic ca?ca. The intestine is short and conical, and opens, in
all but a few, by an anal aperture. In some Asteroidea (as in
Anthenea, Figs. 308 and 310) the intestine has connected with it
a system of five elongated bifurcated inter-radial intestinal caeca ;
in others (as in Asterias, Fig. 306) these are represented only by
two or three lobed diverticula. In one member of the class there
are also ten ca^ca connected with the oesophagus.

In the nervous system of the Echinodermata three distinct
parts, the relative development of which differs in the different
classes, are to be recognised. These are the epidermal or super-
ficial, the deep, and the dboral. The epidermal system is well
developed in all the classes : its principal parts are a circum-
oral nerve-ring and radial branches, but a plexus of nerve-fibres
with occasional nerve-cells extends from it through the epi-
dermis. In the Ophiuroids the radial nerves and the ring
nerve are similar in their arrangement to what is to be observed
in the Asteroids, but are more deeply placed, being covered over
by the investing calcareous plates. The deep-lying nervous system
is absent in the Crinoidea, very feebly developed in the Echinoidea,
but well developed in the Asteroidea, Ophiuroidea, and Holo-
thuroidea. Its general arrangement has already been described
in the account of the Starfish. The aboral system is best
developed in the Crinoidea and is absent altogether in the
Holothuroidea.

The sexes are distinct in all the Echinoderms, with one or two
exceptions ; but there is very rarely any trace of sexual dimorphism.
Asterina gibbosa, the Starfish the development of which has been
described (p. 388), is one of the exceptional hermaphrodite forms ;
the young animals of this species are male, producing sperms, but
at a later stage they become female and produce only ova. In the
family SynaptidaB of the Apoda there are also numerous examples
of hermaphroditism, the animal at first producing ova, later only
sperms. In Ampliiura squamata, an Ophiuroid, both ovaries and
testes are present at once. The gonads, ovaries or testes as the
case may be, are branching bodies, inter-radial in position, and
usually in pairs. In the Asteroidea there are five pairs, the ducts
from which open usually on a special plate on the aboral sur-
face, but in one or two species on the oral surface. In the
Echinoidea there are five ovaries or testes, the five ducts of
which open on the genital plates of the apical system. In the

430 ZOOLOGY SECT.

Ophiuroidea there are five pairs of gonads, a pair in the walls
of each of five genital Imrscv, which open on the exterior by slits
on the oral surface close to the mouth. In the Holothuroidea
there is only a single branched gonad, sometimes imperfectly
divided into two, with a duct opening on the dorsal surface not
far from the mouth. In the Crinoidea the ovaries and testes occupy
a remarkable position, being situated in the dilated bases of the
pinnules ; but, as in the other classes, they are connected by means
of a genital rachis running through the arm with a centrally
situated genital stolon.

Development and Metamorphosis. A few of the members
of each class of Echinoderms are viviparous, in the sense that the
development of the young takes place in some sheltering cavity,
or Irood-pouch, on the surface of the body of the parent. But in
most, development takes place externally, and the larvas are free-
swimming. The ovum in all undergoes regular and nearly equal
segmentation, resulting in the formation of a ciliated blastula,
which becomes invaginated so as to form a typical gastrula, like
that of some Ccelenterata (p. 173). The invaginated cells form the
lining membrane (the endoderm layer) of an internal cavity the
primitive alimentary cavity or archenteron ; the enclosing cells
form the ectoderm ; between the endoderm and ectoderm, and
derived from the former, appear the cells of the mesoderm or middle
layer. From the archenteron is given off a hollow outgrowth, the
cnterocwle, from which are derived the body-cavity with its
enclosing peritoneal membrane, and the vessels of the ambulacral
system with their various appendages. In the Crinoidea the
vesicle destined to form the ambulacral system is developed
independently of the ccelomic vesicles destined to form the body-
cavity. A canal opening on the exterior by a dorsally situated
opening, the dorsal pore (sometimes double), is formed by
invagination from the surface ectoderm, and comes into relation
with a canal arising as an outgrowth from the rudimentary am-
bulacral system to form the foundation of the madreporic canal of
the adult. In the Crinoidea five dorsal pores and five canals are
developed, but the two sets of structures do not enter into direct
communication (see p. 408).

The part of the enterocoele (liydroccele) destined to give rise to
the ambulacral system, at first rounded, becomes compressed, and
subsequently divided round the border into five lobes. Each of
these lobes grows outwards to become developed subsequently into
one of the five radial ambulacral vessels of the Echinoderm ; the
central part of the hydrocoele gives rise to the ring-vessel sur-
rounding the oesophagus.

The cilia, which at first (in the gastrula stage) covered the sur-
face of the larva uniformly, become restricted to a peri-oral band
(Fig. 343, por) surrounding a concave area on which the mouth

IX

PHYLUM ECHINODERMATA

431

opens. A smaller adored band (aor) in the interior of the
mouth has the function of attracting nutrient particles. The

FIG. 343. Diagrams of the development of the larvre of Echinoderms. 1, Primitive form of
Echinoderm larva ; 2 and 3, Development of an auricularia (Holothuroidea) ; 4, 5, and <>,
Development of a Inpinnaria (Asteroidea) ; 7, 8, and 9, Development of a pluteus (Echinoidea
and Ophiuroidea). aor. adoral band of cilia ; all. alimentary canal ; an. anus ; b, b. processes
or arms; mo. mouth; por. peri-oral ciliated band and processes. (From Leuckart and
Nitsche's Diagrams.)

peri-oral band undergoes characteristic changes in the different
classes, and the form of the larva at the same time becomes

432

ZOOLOGY

SECT.

modified by the formation, except in the Crinoidea, of variously
arranged processes along the course of the peri-oral band. The
resulting larva, eckinopcedium or dipleurula, always exhibits
marked bilateral symmetry. It has a pre-oral lobe on which an
apical plate comparable to that of the trochophore (p. 322) may
be developed.

In the Asteroidea the larva is either a bipinnaria (Fig. 343,
4 to 6) or a brachiolaria. The former has a series of bilaterally
arranged processes or arms ; the latter has, in addition, three
processes not developed in the course of the ciliated band arid
used for fixation. The larva of Asterina, the development of which

has been described and illustrated
on pp. 388-393, is a greatly modified
bipinnaria with the pre-oral lobe large
and eventually serving as a stalk, and
the pre-oral band of cilia confined to
the edge of the larval organ and de-
void of the bilateral processes of
the normal bipinnaria. The bipin-
naria is usually free-swimming, but
sometimes, as in the case of Asterina
(p. 391), creeps on the surface of a rock
by means of the pre-oral lobe, and
subsequently becomes fixed by means
of the latter modified to act as a
stalk. In at least one form the bipin-
naria, developed in a brood-pouch,
adheres to the parent by means of the
pre-oral lobe which takes the form of
a short stalk. In both the Ophiu-
roidea and the Echinoidea (Fig. 343,
7 to 9) the larva has the form
which is known as the pluteus. The
pluteus has a series of slender arms
directed forwards and supported by a skeleton of delicate calcareous
rods. The larva of the Holothuroidea, the auricularia (2 and 3),
has a number of short processes developed in the course of the
ciliated bands ; subsequently, in the pupa stage, the ciliated bands
become broken up into a series of ciliated hoops encircling the
body. Of the Crinoidea the development of Antedon alone is
known. Blastula and gastrula stages occur as in the Starfish,
but the history of the archenteron and its diverticula is widely
different, though the outcome is the same viz., the differentiation
of a primitive enteric canal, an anterior ccelome, from which a
hydrocoele becomes separated off, and a pair of coelomic sacs.
The larva (Fig. 344) becomes barrel-shaped, and the pre-oral lobe,
which is not very conspicuous, develops an ectodermal thickening

FIG. 344. Free-swimming larva of
Antedon, from the left side.
I-V, ciliated bands ; ba^ to 6a 5 ,
the five basals ; or\ to or?,, orals ;
1, vestibule ; 2, intestinal vesicle ;
3, right enteroccele ; 4, calcareous
joints of the stalk ; 5, pedal plate.
(From Lang, after Seeliger.)

IX

PHYLUM ECHINODERMATA

433

ant

with a tuft of sensory cilia. The vibratile cilia on the surface are
arranged in five transverse bands (I-V). Between the second and
third of these is a wide shallow depression, the vestibule or
stomodseum (1), which does not communicate with the mouth.
After remaining in the free condition
for a short time, the larva (Fig. 345)
fixes itself by means of the pre-oral
lobe, which elongates into a stalk
(11), the cilia meanwhile being
lost, and the apical plate absorbed.
The vestibule becomes closed, arid
a solid rudiment of the adult
oesophagus arises in close apposition
with it. Round the oesophagus the
hydroccele grows in the form of a
ring. The vestibule (5) with the
oesophagus and hydroccele are rotated
so as to come to lie at the free
extremity. The radial canals first
appear as five tentacles which at
first project into the cavity of the
vestibule, and subsequently when
the latter opens out, as it soon does-
on the exterior. The oesophagus (8),
meanwhile, has become completed,
and the mouth pierces the bottom of
the now open vestibular cavity. The
arms appear as five processes which
soon bifurcate : the five radial canals
become applied to them and un-
dergo a corresponding division. The
first plates are formed while the
larva is still in the free condition ; in
the fixed condition they undergo
further development, and extend into
the arms as they grow. After about
six months this pentacrinoid larva
becomes free by the absorption of
the stalk and develops into the adult
Antedon.

In the transition from the bi-
lateral larva pluteus, bipinnaria,
brachiolaria, or auricularia to the radial adult there is a marked
metamorphosis. As the adult form is developed on one side
of the larva, with its principal axis at right angles to that of
the latter, the larval arms or processes become absorbed. In the
Holothuroidea and Ophiuroidea all the organs of the larva are

VOL. I F F

post

FIG. 345. Stalked larva of Antedon,
from the right side ; calcareous
plates not represented. 1, right
ciBlomic sac ; 2, enteric cavity ; 3,
left coelomic sac ; 4, sacculi ; 5,
vestibule, still closed ; 6, primary
tentacles ; 7, secondary tentacles ;
8, oesophagus ; 9, rectum ; 10, axial
organ; 11, fibrous strands in the
stalk. (From Lang, after Seeliger.)

434 ZOOLOGY SECT.

carried on into the adult ; in the Asteroidea and Echinoidea the
larval mouth and oesophagus are abolished and a new permanent
mouth and oesophagus formed as a fresh invagination from the
surface. In the very limited number of Echinoderms which are
viviparous there is no such marked metamorphosis ; but even in
these the larva is at first distinctly bilateral in its symmetry.

Ethology, etc.- -The Echinodermata are without exception, 1
inhabitants of the sea. In the adult condition the majority
creep on the sea-shore or on the sea-bottom, the stalked Crinoids
being exceptional in their permanently attached condition ; but
the larvae of the great majority are pelagic i.e. live swimming in
the upper strata of the ocean.

Echinoderms inhabit all depths of the sea, ranging from the
shore between low and high-water limits to the greatest depths.
Members of all the classes are found at all depths ; but the stalked
Crinoids, and the Elasipoda among the Holothuroidea are virtually
confined to the deepest waters of the ocean, only one genus of the
former and one species of the latter occurring in comparatively
shallow water. Echinoderms are found in the seas of all parts of
the globe. Like the majority of marine invertebrate groups, the
phylum is more abundantly represented, as regards the number of
genera and species as well as of individuals, in the warmer regions ;
the Crinoidea, the Holothuroidea and the Echinoidea are all much
more abundant in tropical and warm temperate seas than in colder
latitudes.

Echinoderms are of gregarious habits, large numbers of the
same species frequently being found closely associated together in
a comparatively narrow area. The movement of locomotion in
the Starfishes is, as previously described (p. 377), a slow creeping
one, through the agency of the tube-feet : the same holds good of
the Echinoidea and those of the Holothuroidea that possess tube-feet
(Pedaia). The footless Holothurians (Apoda, such as Synapta)
creep along with the help of the tentacles. Most of the Ophiuroids
move by lateral flexions, sometimes sluggish, sometimes remark-
ably rapid, of the arms. The Comatulse, on the other hand, swim
along by the flexion and extension of the pinnate arms pro-
pelling them through the water. Many Asteroids, Ophiuroids,
and Echirioids bury themselves in sand or mud ; others creep
into narrow fissures in rock or coral. Movements of manducation
are performed by the tentacles in the Holothurians : in the Star-
fishes the mouth papilla? are separated from one another and the
cardiac part of the stomach everted in order to enfold the prey,
often of relatively large size. In those Echinoidea that possess a
lantern of Aristotle there are very powerful and efficient move-
ments of mastication. On the whole, as might be expected from
the comparatively highly developed muscular and nervous systems,
1 One species of Synapta is said to inhabit brackish water.

ix PHYLUM ECHINODERMATA 435

the co-ordination of movement is very much more complete in the
Echinodermata than in the groups already dealt with.

A remarkable characteristic of the Echinoderms is the faculty
of self -mutilation which many of them possess, together with
the capacity for replacing parts lost in this way or by acci-
dental injury. This is most marked in many Ophiuroids, some
Asteroids, and some Holothurians, and does not occur at all among
the Echinoids. Many Brittle-stars and some Starfishes, when
removed from the water, or when molested in any way, break off
portions of their arms piece by piece until, it may be, the whole
of them are thrown off to the very bases, leaving the central disc
entirely bereft of arms. A central disc thus partly or completely
deprived of its arms is capable in many cases of developing a new
set ; and a separated arm is capable in some instances of develop-
ing a new disc and a completed series of arms. In some Star-
fishes (Ophiuroids and Asteroids) a process of separation of the
arms and their development into complete individuals frequently
occurs altogether independently of injury, and seems to be a
regular mode of reproduction in these exceptional cases. Many
Crinoids, also, readily part with their arms when touched and are
able to renew them again ; and some, at least, are capable
of renewing the visceral sac of the central disc when it has become
accidentally removed.

In the case of many Holothurians it is the internal organs, or
rather portions of them, that are capable of being thrown off and
replaced the oesophagus, or the cloaca with the Cuvierian organs,
or the entire alimentary canal, being ejected from the body by
strong contractions of the muscular fibres of the body-wall,
and in some instances, at least, afterwards becoming completely
renewed.

Four out of the nine classes of the phylum Echinodermata-
the Cystoidea, Blastoidea, Edriasteroidea, and Carpoidea are
represented only by fossil forms ; and these are found only
in rocks of the older (Palaeozoic) formations, no representatives
having survived to more recent times. Of the five classes that
have living members, one, the Crinoidea, was very much more
abundantly represented in the older geological periods than
it is at the present day, the remains of stalked Crinoids
forming great beds of limestone of Silurian to Carboniferous
age : the free Comatulte only appeared at a much later period.
The other classes, or at least the Echinoidea, Asteroidea,
and Ophiuroidea, were represented at a very early period by
forms not very widely different from those now living ; but the
earliest Echinoids were peculiar in having the number of rows
of plates variable, and in the plates overlapping one another.
The Holothuroidea, owing to their comparatively soft integument,
were less fitted to leave any remains in the form of fossils, and it

F F 2

436 ZOOLOGY SECT.

is not till we come to the Mesozoic Period that undoubted traces
of their existence are found.

Affinities. --The presence of radial symmetry was formerly
regarded as involving a near relationship with the Ccelenterata,
which were grouped with the Echinodermata under the comprehen-
sive class-designation of Radiata (see section on the History of
Zoology). But, leaving out of account the presence of a bilateral
symmetry underlying and partly concealed by the radial, we are led
by a study of the anatomy of the various systems of organs to the
conclusion that the Echinoderrns are in no way closely or directly
related to the Coelenterates. One very great and very important
difference between the two phyla consists in the presence in
the Echinodermata of an extensive ccelome or body-cavity
lined by mesodermal epithelium between the alimentary canal
and the body-wall. In addition to this the Echinoderrns
are characterised by the possession of highly elaborated
systems of organs alimentary, vascular, and nervous such as
occur in none of the Coelenterates, all of which exhibit ex-
treme simplicity in their internal structure. A further point of
difference, not perhaps of so much importance, is the absence in
the Echinoderms of any tendency to form colonies of zooids by
asexual multiplication by means of buds : all Echinoderms are
simple, i.e. non-colonial, animals, and each of them is developed,
save in certain very exceptional cases, as a result of a sexual
process from an impregnated ovum. In spite, then, of the radial
symmetry, we are forced to the conclusion that the Echinodermata
are not more nearly related to the Coelenterata than to some of
the groups of Worms. They are, in fact, a singularly isolated
group, and we look in vain among the known members, living and
fossil, of other phyla, for any really close allies. The intermediate
forms whatever they may have been like between the Echino-
derms and other groups have become extinct, and have left no
remains in the form of fossils, or such remains have not yet been
discovered. So difficult has it been found to connect the Echino-
derms with other animal types that it has even been proposed to
regard an Echinoderm as a radially arranged colony of zooids
connected together centrally, each ray being a zooid equivalent
to an entire simple worm-like animal. But the history of the
development is entirely at variance with such a view.

Whatever may have been the group of animals from which the
Echinodermata were developed, there is every probability that it
was a group with bilateral and not radial symmetry. The radial
symmetry is evidently, as has already been pointed out, of a
secondary character ; it is only assumed at a comparatively late
period of development, and even in the adult condition it does
not completely disguise a more primitive bilateral arrangement of
the parts. Accordingly, within the phylum itself, it is reasonable

IX

PHYLUM ECHINODERMATA

437

to regard those classes as the more ancient which have the radial
symmetry less completely developed. Again, the free condition
which characterises all existing Echinoderms with the exception
of a few Crinoids, is probably less primitive than the attached,
since in other phyla the radial symmetry is co-ordinated with,
and seems to be developed on account of, a fixed, usually stalked
condition. Probably, then, stalked Echinoderms were the pro-
genitors of the existing free forms, and these were preceded by
primitive free forms with pronounced bilateral symmetry. It
appears to be most probable that this ancestral form possessed the
most essential features of the diplcnrula larva (p. 432); i.e., that
it was a bilaterally symmetrical form with a pre-oral lobe, simple
alimentary canal with mouth on ventral surface and anus at
posterior end ; that it had a coelome, originally developed from the
archenteron of the gastrula ; and that it had a band of strong
cilia running around the concave ventral surface. Such a
dipleurula-like form became converted, it is supposed, into a fixed
form, such as that represented by some of the extinct class of the
Cystoidea. The fixation must be supposed to have become
effected through the medium of the pre-oral lobe, and further
changes must have involved the shifting of the mouth to about
the middle of the free surface. From this primitive Cystoid, thus
regarded as the most primitive of all known Echinoderms, the
remaining classes, both fixed and free, might have been derived by
some such order of succession as that indicated in the diagram
below (Fig. 346).

Holothuroidea

Echinoidea Asteroidea Ophiuroidea

Crinoidea

Cystoidea

Primitive Cystoid

Dipleurula
FIG. 346. Diagram to illustrate the relationships of the classes of the Echinodermata.

438 ZOOLOGY SECT, ix

According to another view, however, the most primitive of
existing Echinoderms are Synapta and its allies (Holothuroidea
apoda). The other Holothuroids are supposed, according to this
conception of the relationships of the various classes, to have been
derived from a Synapta-like ancestor. From the primitive stock
of the Holothuroids is supposed to have been derived a form
which gave origin to all the stalked classes. From this ancestral
stalked Echinoderm, again, the remainder of the free classes the
Echinoidea, Asteroidea and Ophiuroidea are regarded as having
been descended.

Possible relationships between the Echinodermata and the
Chordata will be referred to in the discussion of the affinities of
the latter phylum.

SECTION X
PHYLUM ANNULATA

THE phylum Annulata comprises four classes of Worms the
Chcvtopoda or Earthworms and marine Annelids, the Archi- Annelida,
the Gephyrea, and the Hirudinca or Leeches. All of these, except
the Gephyrea, have the elongated body divided externally into a
number of rings, which represent a division of the internal parts
into a series of segments or metameres. There is usually an
extensive ccelome, and there is in most a system of blood-vessels.
The nervous system consists of a cerebral ganglion, oesophageal
connectives, and a double ventral nerve-cord, which in all but the
Gephyrea is segmented into a series of ganglia. The organs of
excretion are in the form of metamerically arranged pairs of tubes,
the ncphridia or segmented organs, closed internally or leading from
the ccelome to the exterior ; and united with these, or distinct
from them, are a series of paired ducts, the ccelomoducts, for the
passage outwards of the reproductive elements.

CLASS I, CHJETOPODA.

The Chretopoda, comprising the Earthworms, Fresh-Water
Worms, and Marine Annelids, are Worms the body of which, un-
like that of a Flat-worm or a Round-worm, is made up of a series
of more or less completely similar segments or metameres, each
containing a chamber or compartment of the body-cavity and a
section of the alimentary canal and other organs. At the sides
of each are typically a pair of muscular processes, the parapodia,
which do duty as limbs, bearing bundles of setce (chcetcc) or bristles
and usually also certain tactile appendages, the cirri. There
is an extensive ccelome, incompletely divided into a series of
chambers corresponding to the segments by a series of muscular
partitions which act also as mesenteres, being attached internally
to the alimentary canal. The latter extends throughout the
length of the body ; the intestine is usually constricted, the con-
strictions being either segmental i.e. opposite the middle of the
segments, or inter-segmental i.e. opposite the intervals between

439

440

ZOOLOGY

SECT.

B

the segments. There is a well-developed blood-vascular system
in the majority of the Chsetopoda, and organs of respiration in
the shape of gills or branchiae are usually developed. The
excretory organs are in the form of segmentally arranged pairs of
tubes, the nephridia. The nervous system consists of a bilateral
principal ganglion or brain situated in the prostomium, and a
double chain of ganglia extending throughout the body. The
sexes are in some distinct, in others united. When a definite
larval form occurs it is a trcchopliore (cf. p. 322).

1. EXAMPLES OF THE CLASS.

a. Nereis dumerilii. 1

General External Features.- -Various species of Nereis occur
abundantly between tide-marks on the sea-shore, under stones, and

among sea-weed, in all parts of the
world. The worm varies consider-
ably in colour even in the same
species, the differences being partly
due to differences in the stage of
development of the sexual elements.
In N. dumerilii the prevailing colour
is some shade of violet, with a blush
of red in the more vascular parts
due to the bright red colour of the
blood. In shape (Fig. 347) the
body, which may be about 7 or 8
centimetres in length, is long and
narrow, approximately cylindrical,
somewhat narrower towards the
posterior end. A very distinct head,
bearing eyes and tentacles, is recog-
nisable at the anterior end ; the rest
is divided by a series of ring-like
narrow grooves into a correspond-
ing series of segments or mctameres,
which are about eighty in number
altogether ; and each of these bears
laterally a pair of movable mus-
cular processes called the parapodia,
provided with bundles of bristles or
setce (chcelcB) The head (Fig. 350)
consists of two parts, the pros-
tomium (prccst) and the pcristomium (perist). The former bears

1 Though Nerd* dumerilii, is here named as the example, and the majority of
the figures refer specially to that species, the description given would apply
almost equally well to a considerable number of species of the genus.

FIG. 347. Nereis dumerilii, natural
size. A, Nereis phase ; B, Heteronereis
ph ase. (After Claparede.)

PHYLUM ANNULATA

441

dors, cirr

noto

rieuro

FIG. 348, A. Nereis dumerilii. A single para-
podium, magnified : ac. aciculum ; dors. cirr.
dorsal cirrus : neuro. neuropodium ; noto. noto-
podium ; vent. cirr. ventral cirrus. (After
Claparede.)

on its dorsal surface four large rounded ci/rx, in front a pair of
short cylindrical tentacles (tent), and further back a pair of some-
what longer stout appendages
or palpi (palp). The peri-
stomium, which has some
resemblance to the seg-
ments of the body, though
wanting the parapodia, bears
laterally four pairs of long,
slender, cylindrical tentacles
(pcrist. tent): on its ventral
aspect is a transversely vent.cirr

elongated aperture, the
mouth. The segments of the
body differ little in external
characters from one another
throughout the length of the worm. Each bears laterally a pair
of parapodia, which in the living animal are usually in active

movement, aiding in creeping, or acting as
a series of oars for propelling it through
the water. When one of the parapodia
(Fig. 348, A) is examined more attentively
it is found to be biramous, or to consist of
two distinct divisions a dorsal, which is
termed the notopodium (noto), and a ventral,
which is called the neuropodium (neuro).
Each of these is further subdivided into
several lobes, and each bears a bundle of
setae. Each of the bundles of setae is lodged
in a sac formed by invagination of the epi-
dermis the sctifjerous sac and is capable
of being protruded or retracted and turned
in various directions by bundles of muscular
fibres in the interior of the parapodium. In
each bundle there is, in addition to the
ordinary setae, a stouter, straight dark-
coloured seta (ac.), the pointed apex of
which projects only a short distance on the
surface ; this is termed the aciculum. The
ordinary setae (Fig. 348, B) are exceedingly
fine, but stiffish, chitinous rods, of which
two principal kinds are recognisable : both
have a terminal Uadc articulating with
the main shaft of the seta by a distinct
joint ; but in the one variety the shaft of

the seta is finer than in the other, and the terminal blade long,
slender, and nearly straight, whereas in the other variety it is

1

jo. 348, B. Nereis du-

merilii. Seta; highly mag-
nified. (After Claparede.)

442 ZOOLOGY SECT.

short and slightly hooked. On the dorsal side of the parapodium
is a short cylindrical, tentacle-like appendage, the dorsal cirrus
(Fig. 348, A, dors, cirr), and a similar, somewhat shorter appendage,
the ventral cirrus (vent. cirr), is situated on its ventral side. The
last segment of the body, the anal segment, bears posteriorly a small
rounded aperture, the anus ; this segment is devoid of parapodia,
but bears a pair of appendages, the anal cirri, similar in character
to the cirri of the ordinary segments, but considerably longer.

On the ventral surface, near the bases of the parapodia, there is
in each segment a pair of very fine apertures, the openings of the
nephridia.

The enteric canal is a straight tube running throughout the
length of the body from the mouth to the anus. Between the
outer surface of this tube and the inner surface of the wall of the
body is a considerable space the ccelome, body-cavity, or pcri-
visccral cavity filled with a fluid, the codomic fluid, containing
amoeboid corpuscles. The walls of the coelome (Fig. 351) are
lined with a thin membrane, the peritoneum or coelomic epithelium,
of which the outer layer that lining the body-wall is the
parietal layer (par. peri), that covering the outer surface of the
alimentary canal the splanchnic or visceral layer (vise. peri}. The
space is divided by a series of transverse partitions or septa passing
inwards from the body-wall to the wall of the alimentary canal
opposite the grooves between the segments, and thus dividing, the
coelome into a series of chambers, each of which corresponds to one
of the segments. These partitions are not complete, spaces being
left around the alimentary canal and elsewhere through which
neighbouring chambers communicate.

The mouth leads into a wide cavity, the buccal cavity, con-
tinued back into a pharynx (Fig. 350, ph). These two chambers
extend through the peristomium and the first to the fourth seg-
ments of the body. They are lined with a tolerably thick cuticle,
continuous with a similar layer lining the outer surface of the body,
and in the buccal cavity are a number of very small dark brown
chitinous denticles, which are very regularly arranged. The
posterior part of the pharynx (dentary region) has very thick
walls composed of bundles of muscular fibres, which are concerned
in the movements of a pair of laterally placed chitinous jaws.
Each jaw is elongated in the direction of the long axis of the body,
rounded at the posterior end or base where it is embedded in
muscle, pointed at the apex, which is strongly incurved ; the inner
edge is divided into a number of strong serrations or teeth : the
whole jaw might be compared to a pruning-hook with its cutting
edge deeply serrated (Fig. 349, B).

Behind the pharynx the alimentary canal narrows considerably
to form a tube, the oesophagus (ass), which runs through about five
segments to open into the intestine.

PHYLUM ANNULATA

44;}

Running backwards and inwards from the wall of the peristomium
to the wall of the buccal cavity and pharynx are a number of
bands or sheets of muscle, the protractor muscles, by the contraction
of which, and the pressure of the ccelomic fluid, this anterior part
of the alimentary canal can be everted so as to form a proboscis
(Fig. 349), and thus the jaws are thrust forth and rendered
capable of being brought to bear on some small living animal or
fragment of animal matter, to be seized and swallowed as food.
The eversion is arrested at a certain point by means of a muscular
diaphragm passing from the wall of the buccal cavity to that of
the first body-segment. The proboscis is withdrawn again by a
retractor sheet of muscle, which passes inwards and forwards to be

FIQ. 349. Nereis diver Sic olor, x 4. Head with buccal-'region everted. A, dorsal view ;
B, ventral view, a, prostonrium ; B, everted buccal region ; c, c', peristomial tentacles, 1, 2, 3, 4 ;
(7, denticles ; e, eyes ; E, lower lip ; P, palp in A, entrance to pharynx in B ; J, jaw ;
2', prostomial tentacle ; I, peristomium ; 11, parapodium of first body-segment. (From the
Cambridge Natural History.)

inserted into the wall of the alimentary canal at the junction of
the pharynx and oesophagus.

Into the oesophagus open a pair of large unbranched glandular
pouches, or cceca (Fig. 350, #/), which probably are of the nature of
digestive glands. The intestine (int) is a straight tube of nearly
uniform character throughout, regularly constricted in each segment
-the constrictions becoming much deeper towards the posterior
end of the body. The part of the intestine which lies in the last
segment is termed the rectum.

The wall of the alimentary canal (Fig. 351) consists (1) of the
visceral layer of the coelomic epithelium (vise, peri) ; (2) of a layer of
longitudinal muscular fibres (long, mus) ; (3) of a layer of circular
muscular fibres (circ. mus) ; and (4) of the enteric epithelium
(cnt. ep), consisting of close-set, long, narrow cells. To these
layers is superadded in the buccal cavity and the pharynx an
internal chitinous cuticle, continuous with that of the general
outer surface.

Developmentally the buccal cavity and the pharynx constitute
the stomodceum, the rectum the proctodceum, the rest of the alimen-
tary canal the mesentcron.

The wall of the body consists of a cuticle, an epidermis or

444

ZOOLOGY

SECT.

deric epithelium, muscular layers, and the parietal layer of the

ccelomic epithelium (pi'.

fonaust t^ AZ#>^ peri). The cuticle (cut)

is a thin chitinous layer
which exhibits an iri-
descent lustre due to the
presence of two intersect-
ing systems of fine lines ;
it is perforated by numer-
ous minute openings, the
openings of the epidermal
glands. The epidermis
(cp) is very thin, except
on the ventral surface,
where it becomes consider-
ably thickened. It consists
of a layer of cells con-
taining numerous twisted
unicellular glands, which
are most abundant on the
ventral surface, particu-
larly near the bases of the
parapodia ; on the dorsal
surface the epidermis
contains plexuses of fine
blood-vessels. The mus-
cular layers are two in
number an external, in
which the fibres run cir-
cularly (circ. mus), and an
internal, in which they
run longitudinally. The
latter is not a continuous
layer, but consists of four
bundles of fibres, two
dorso- lateral (dors. long,
mus) and two ventro-
lateral (vent. long. mus).

Nereis has a well-de-
veloped system of vessels
filled with blood of a
bright red colour. A
main dorsal vessel (Figs.
350 and 351, dors, vess)
runs from one end of the

body to the other above the alimentary canal, and is visible in
places through the body-wall in the living animal. It, as well as

FIG. 350. Nereis dumerilii. Semi-diagrammatic
view of the anterior portion of the body with the
dorsal body-wall removed, so as to show the ali-
mentary canal, the septa, the blood-vessels, and the
nephridia ; a portion of the intestine removed so as
to show the ventral blood-vessel and nerve-cord
which lie below, dors. vess. dorsal vessel ; gl. osso-
phageal glands ; int. beginning of intestine ; ne. ro.
nerve-cord ; neph. nephridia ; a *. (esophagus ; palp,
palp ; para, parapodia iperixf. peristome ; / rist. tent.
peristomial tentacles ; ph. pharnyx with its jaws ;
!>!<! *t. prostomium ; tent, prostomial tentacles ; vent,
vesit. ventral vessel.

PHYLUM ANNULATA

445

the majority of the vessels, undergoes contractions which are of
a peristaltic character waves of contraction passing along the
wall of the vessel so as to cause the movement of the contained
blood. These peristaltic contractions are more powerful in the
case of the dorsal vessel than in that of any of the others, and
run with great regularity from behind forwards, so as to drive a
current of blood in that direction. The contractions are brought
about partly by a series of muscular fibres which are arranged in

CIST-, fnus

dcrs

ru>t. set

ev

FIG. 351. Nereis dumerilii. Semi-diagrammatic transverse section of the middle region of
the body. clrc. mus. (external), circular layer of muscle of body-wall ; circ. i,ms. (internal),
circular layer of muscle of wall of enteric canal ; caul, coelome ; cut. cuticle ; dors. long. mus.
dorsal longitudinal muscles of body-wall ; dors. vess. dorsal vessel ; ent. ep. enteric epithelium ;
ep. epidermis ; long. mus. 'longitudinal muscle of wall of enteric canal ; tie. co. nerve-cord ;
nepli. nephridium ; neur. set. neuropodial setae and aciculum with their muscles ; not. set.
notopodial sette and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri, parietal layer
of coelomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent.vesx. ventral vessel ;
vise., peri, visceral layer of ccelomic epithelium. (The entire extent of the ccelomic epithelium is
not represented.)

rings round the wall of the vessel at short intervals ; but the wall
of the vessel is itself contractile,

, Along the middle of the ventral surface below the alimentary
canal runs another large longitudinal vessel, the ventral vessel (vent,
wss), in which the current of blood takes a direction from before
backwards. Connecting the dorsal and ventral vessels, there are in
each segment two pairs of loop-like transverse vessels which give
off branches to the parapodia, the alimentary canal, and neighbour-
ing parts. Some of these branches communicate with plexuses
of fine vessels in the interior of the lobes of the parapodia and in
the integument of the dorsal surface, and with dilatations or sinuses

446 ZOOLOGY SECT.

situated in the bases of the parapodia. A delicate longitudinal
neural vessel accompanies the nerve-cord.

Nereis is devoid of any branchiae ; but there can be little doubt
that the lobes of the parapods with their rich blood-supply, and the
areas of integument occupied by plexuses of blood-vessels, subserve
the function of respiration.

There is a well-developed nervous system (Fig. 352), which is
bilateral and metameric in its arrangement, like the other systems

n-

FIG. 352. Nereis. Anterior portion of nervous system, comprising the brain, the oesophageal
connectives, and the anterior part of the ventral nerve-cord. (After Quatrefages.)

01 organs. Situated in the prostomium is a large bilobed mass
of nerve-matter containing numerous nerve-cells, the cerelral
ganglion or brain (c). This gives off tentacular nerves to the tentacles
and palpi, and two pairs of short thick optic nerves to the eyes.
Behind, two thick nerve-strands, the assophageal connectives (d), curve
round the mouth in the peristomium to meet on the ventral
aspect behind the mouth and below the pharynx. The oesopha-
geal connectives with the cerebral ganglion thus form a ring
around the anterior part of the enteric canal. From them are

PHYLUM ANNULATA

447

given off nerves to the two anterior pairs of perisfcomial tentacles.
Running backwards from the point of union of the oesophageal
connectives along the entire length of the body of the worm, on
the ventral aspect, is a thick cord of nerve-matter, the ventral
nerve-cord (/i). In each segment this cord presents a little dilata-
tion from which nerves are given off to the various parts of the
segment : and each of these enlargements is really double, consist-
ing of a pair of closely-united ganglia. The intermediate parts of the
cord, between successive pairs of ganglia, are also double, consisting
of a pair of longitudinal connectives enclosed in a common sheath.
Given off behind from the cerebral ganglion is a system of fine
nerves with occasional small ganglia, the stomatogastric or visceral
system, distributed to the anterior part of the al im en taiTv canal.

FIG. 353. Nereis. Section through one of the eyes. co. cornea ; cu. cuticle ; I. lens ; r. layer

of rods ; re. retina. (After Andrews.)

The first ganglion of the ventral cord, which is situated in the
third segment, represents at least two double ganglia which have
coalesced, as is shown by the fact that it gives off nerves to the
two posterior pairs of peristornial tentacles and to the first pair of
parapodia.

The tentacles and palpi, as well as the cirri, are probably organs
of the sense of touch. The only other sense-organs are the four
eyes and the two nuclvd organs, all situated on the prostornium.
The eye (Fig. 353) consists of a darkly pigmented cup, the retina
(re.), with a small rounded aperture, the pupil, and enclosing a
mass of gelatinous matter, the lens (I.) The wall of the cup is
composed of numerous long and narrow cells lying parallel with
one another in a radial direction. The outer end of each cell
narrows into a nerve-fibre forming part of the optic nerve ; near
this end is a nucleus ; the main body of the cell is densely

448

ZOOLOGY

SECT.

pigmented ; the inner part projects towards the lens as a clear
hyaline rod (r). The cuticle of the general surface passes over
the eye, and a continuation of the epidermis with its cells some-
what flattened, constitutes the cornea (co). The nuchal organs
consist of a pair of pits lined by a special ciliated epithelium with
gland-cells, situated in close contact with the posterior part of the
brain near the posterior part of the prostomium on the dorsal
side. They are regarded as olfactory in function.

The organs which are supposed to perform the function of
excretion are a series of metamerically arranged pairs of tubes,

the segmental organs or nephridia
(Figs. 350 and 351, neph, Fig.
354) occurring in all segments
of the body with the exception
of several at the anterior and
posterior ends. The nephridium
consists of two parts a body
and a narrow anterior prolonga-
tion. The body is of an irregular
oval shape directed nearly trans-
versely, but slanting somewhat ;
the outer end, situated in the
base of the parapodium near its
middle, is much the narrower ;
the inner end is continuous with
a narrow prolongation about
equal in length to the body,
which runs forwards and in-
wards to become attached to the
mesentery. The external open-
ing or nephridiopore (ext. op) is
a fine circular pore capable of
being widened or contracted,
situated on the ventral surface
not far from the base of the
ventral cirrus. This leads into
a canal, ciliated except in its
most external part, which runs
through the anterior prolonga-
tion to its extremity, where it bends sharply back again and runs
to the body ; through which it pursues an extremely tortuous
course to the outer end, and then bends back again and runs in
the anterior prolongation to the extremity of the latter, where it
opens into the coelome through a ciliated bell or funnel (fun), the
nephrostome, projecting through the septum into the cavity of
the segment next in front of that in which the body of the organ
lies. The edge of the nephrostome is produced into a number

Fie. 354. Nereis dumerilii. One of the
nephridia. ext. op. external opening or
nephridiopore ; fun. internal funnel or
nephrostome opening into the coslome ;
nii-s. transverse mesentery or septum.

x PHYLUM ANNULATA 449

of narrow ciliated processes not represented in the figure.
Throughout its course the canal is excavated in a mass of nucleated
material of a granular character not distinguishable into cells.

On the dorsal side of each segment, in close relation to the longi-
tudinal muscular bundle, is a specially developed ciliated tract of
the coelomic epithelium of the nature of a short funnel without
external aperture, the dorsal ciliated organ. It is possible that at
the time of sexual maturity an aperture is formed through the
body-wall opposite this funnel, and that thus a genital duct of a
temporary character becomes formed : but no such opening has
ever been observed.

Nereis is unisexual. The sexual elements, ova or sperms,
are formed from temporary masses of cells, ovaries or testcs, which
are developed towards the breeding season by a proliferation of
the cells of the membrane (coelomic epithelium) lining the coelome
and the structures it contains. In Nereis dumerilii there is in the
male only a single pair of these proliferating masses of cells (testes),
situated in one of the segments between the nineteenth and the
twenty-fifth. But in other species of Nereis they are much more
numerous. These, during the season of their active development,
give off groups of cells which become disseminated throughout the
ccelomic fiuid. The original cells (mother-cells) undergo division
into smaller cells, each of which develops into a sperm with a
minute rod-shaped head and a long vibratile flagellum or tail. In
the female the ovaries (Fig. 351, ov), formed by a similar process
of proliferation, take the form of rounded masses of cells, meta-
merically arranged, surrounding the principal vessels throughout
the length of the body. The young ova become detached from
the ovaries, and attain their full development while floating
about in the coelomic fluid. Both ovaries and testes dwindle after
they have given off the sexual cells, and at the non-breeding season
of the year are not to be detected.

Ova and sperms, when fully ripe, are discharged, reaching the
exterior probably through apertures temporarily formed by
rupture of the body-wall (cf. above), and impregnation takes place
by contact between the two sets of elements while floating freely
in the sea-water.

Nereis dumerilii is an extremely variable species. If we
compare a number of specimens, we find numerous individual
differences between them. The most striking of these are
differences of colour and of the number of segments in the body ;
but a careful examination reveals many other points in which
individuals differ. Thus the precise form of the lobes of the
parapodia, together with the number of setae- in the two bundles,
vary ; so also do the relative length of the tentacles, the
number of teeth on the jaws, and the number and arrangement
of the denticles in the buccal cavity. Not only are such individual

VOL. I G G

450 ZOOLOGY SECT.

differences common, but the species occurs in two distinct forms
or phases, which differ from one another so widely that they have
been referred to distinct genera. One of these is the Nereis phase,
which is that described in the preceding paragraphs. A Nereis
dumerilii may become sexually mature in this form, or may first
undergo a series of changes by which it becomes converted into
the second or Heteronereis phase (Fig. 34G, B\ The principal
changes which take place during this metamorphosis are a great
increase in the size of the eyes, and a marked modification of the
parapodia in the posterior portion of the body, the lobes becoming
larger and more leaf-like, and the setse of the Nereis being
superseded by others which are considerably longer, more nume-
rous, and somewhat oar-shaped. The Heteronereis, instead of
creeping about on the bottom, swims about actively through the
water by wriggling movements of the body combined with active
paddling movements of the parapodia with their long setoe. After
a time the Heteronereis, like the Nereis, becomes sexually mature,
developing ova and sperms, the latter of which differ remarkably
in shape from those of the Nereis phase.

Development. --The egg of Nereis when first discharged is
enclosed in a transparent thick gelatinous envelope, within which
are two membranes an outer very thin and delicate, and an inner
(zona radiate) thicker and very distinctly striated in a radial
direction. The protoplasm of the ovum contains a number of
oil-drops and yolk-spherules. When fertilisation takes place
the yolk-spherules move away Irom Avhat is destined to become
the upper pole of the egg, leaving a polar area composed of
granular protoplasm. The zona radiata disappears, and the
contents of the ovum undergo for a time amoeboid changes of
form. Then the spherical form is reassumed, two small bodies-
the polar bodies (p. 19) are thrown off at the upper pole, and the
process of segmentation begins (Fig. 355). Up to a fairly advanced
stage this corresponds very closely with the segmentation of the
Poly clad oosperm as described on page 273. The oosperm divides
first into two parts, then into four. From these four cells the
megameres there are separated off in succession three sets of
micromeres, making twelve in all. One of these, belonging to the
second set, somewhat larger than the others and differing from
them in its subsequent history, is termed the first somatcUast
(som. 1) ; a second somatoblast (som. 2) is soon given off from the
same megamere that gave origin to the first.

The germinal layers are now all established. The micromeres
constitute the ectoderm, destined to give rise to the epidermis and
all its derivatives, to the cerebral ganglion and nerve-cord, to the
oesophagus and rectum. The megameres eventually give origin
to the cells of the endoderm, forming the internal epithelium of
the alimentary canal. The second somatoblast gives rise to

PHYLUM ANNULATA

451

the entire mcsodcrm of the Annelid. As the micromeres multiply
by division, they form at first a cap of small cells over the upper
pole of the embryo ; eventually the cap extends so as completely
to cover the four megameres and the descendants of the somato-
blasts except at one point, the blastopore, at the lower pole,
where the investment remains for a time incomplete. When
the blastopore closes, the process of epibolic gastrulation is
completed. A thickening of the layer of ectoderm cells, the apical
plate, in the middle of what is destined to form the head-end of the
embryo, is the rudiment of the cerebral ganglion : in close relation
to it are formed a pair of pigment-spots, the larval eyes. From

micro

micro

B

macro

macro
macro

micro

D

som.l

micro

macro

som2

som.l

FIG. 355. Nereis. Early stages in the development. A, lateral view of eight-celled stage ;
B, the same from above ; C, stage of the formation of the first somatoblast ; D, stage at which
both somatoblasts are present; macro, megameres; micro, micromeres; som. 1, som. 2. first
and second somatoblasts. (After Westinghausen.)

the middle of the head-end projects a tuft of cilia (Fig. 356, A,
ap. oil.). Encircling the body of the larva behind this is a thick-
ened ridge, the prototroch (prot), the cells of which develop strong
cilia. Just behind the prototroch the cells of the ectoderm
become pushed inwards in the middle of what will eventually
become the ventral surface, so as to line a sort of depression or
pouch ; this is the stomodcuum (sf) or rudiment of the mouth and
oesophagus. The anus (an) does not appear until later; the position
which it will subsequently occupy is indicated at this stage by a
pigmented area (pig. ar) marking the point at which the blasto-
pore becomes closed. The first and second somatoblasts divide
to form a mass of small cells which extend on the ventral surface

G

2

452 ZOOLOGY SECT, x

behind the prototroch and month, constituting what is termed
the ventral plate ; of this plate the more superficial cells are
descendants of the first somatoblast one of the twelve original
micromeres ; and those situated more deeply are derived from
the second somatoblast or mesomere. A superficial thickening
of the ectoderm along the middle of the ventral plate is the
rudiment of the ventral nerve-cord (neur. pi) ; the deeper cells
divide and extend to form a pair of mesoderm bands or muscle-
plates, from which the muscles of the body-wall are developed ; the
muscular layers of the wall of the alimentary canal are derived
from certain of the same set of cells which migrate inwards
from the lower end.

A pair of micromeres separated from the rest at an early stage
are destined to form the larval excretory organs, the head-kidneys or
larval nephridia : at first situated at the upper end, they
sink below the surface and migrate downwards till they come
to lie below the prototroch ; each then elongates, and a number
of vacuoles which have become formed in the interior coalesce
in such a way as to form a long, narrow canal. The embryo has
now reached the completed trochophore stage.

The endoderm cells become arranged so as to bound a canal-
like space, the beginning of the lumen of the middle part of the
alimentary canal (oesophagus and intestine, int.), the cells subse-
quently giving rise to the enteric epithelium. This canal becomes
continuous in front with the stomodseum, and behind with a
second smaller ectodermal invagination, the proctodwum, which
arises in the position of the former pigment-area. The part of
the larva behind the prototroch now elongates, and two pairs of
invaginations, the setigerous sacs (set. sacs), appear at its sides : in
the interior of these, to which a third pair is soon added, are
developed setae which grow out to a great relative length as the
larval or provisional setce. Constrictions soon appear marking off
the first three segments, and at the same time the mesoderm bands
undergo a corresponding division into three pairs of mesoderm
segments. The mesoderm segments of each pair grow inwards
towards one another and surround the alimentary canal : in the
interior of each appears a cavity which is the beginning of a
segment or chamber of the coelome. As the two mesoderm
segments become closely applied to one another and unite around
the alimentary canal, their two cavities also come into close
relation, and eventually are separated from one another only by
thin vertical septa, forming dorsal and ventral mesenteries which
subsequently disappear. Successive mesoderm segments also
come into close relationship with one another, their cavities
eventually only remaining separated by thin transverse partitions,
which form the intersegmental septa.

The region in front of the prototroch becomes modified to form the

prot(

l.rnus

$et.sncs

prof.

prvt

pig.ar

an

. h

^L -fr.loJ.

FIG. 356. Nereis. Later stages in the development. A, stage at which the prototroch and the
apical tuft of cilia first become distinct. B, somewhat later stage, in which the stomodajal
invagination is being formed, and the rudiments of the mesoderm bands are distinct ; C, late
trochophore stage in which there are rudiments of the setigerous sacs ; Z>, somewhat later
stage, in which the parapodia have begun to become prominent and the provisional setsv.
project freely ; E, larva with three segments, an. anus ; up. fit. apical cilia ; ap. pi. apical
plate ; eye, eye ; j'r. bod. frontal bodies ; int. intestine ; 1. mug. longitudinal muscle ; mes.
mesoderm ; mo. mouth ; neur. pi. neural plate ; para, parapodia ; pig. ar. pigmented area ;
prof, prototroch ; sens. h. sensory hairs ; set. sacs, setigerous sacs : som. second somatoblast and
group of cells formed from it ; st. stomodfeum ; tent, peristomial tentacles. (After E. B.
Wilson.)

453

454

ZOOLOGY

SECT.

prostomium of the adult. The part immediately behind forms
the peristomium, which bears setae, and is to be looked upon as
the specially modified first segment. The body increases in length,
and additional segments with their setigerous sacs become dis-
tinguishable (E) until, on the development of the tentacles, the
outgrowth of the parapodia (para] with their cirri and the
permanent setae (which replace those first formed), the formation
of the full number of segments, and the completion of the internal
organs, the adult condition of the worm is attained.

1. THE EARTHWORM (Lumbricus).

General External Features.- -The Earthworm (Fig. 357)
has a long narrow body, which may be described as approximately

FIG. 357. Lumbricus herculeus. A, entire specimen, lateral view ; B, ventral view of
anterior portion of the body, magnified. 1, 15, 33, first, fifteenth, and thirty -third segments.
K u-h of the black clots represents a pair of seta-. (After Vogt and Jung.)

cylindrical, but slightly depressed towards the posterior end.
Dorsal and ventral surfaces are readily recognisable, the latter
being much paler in colour than the former, and exhibiting a

PHYLUM ANNULATA

455

slight flattening ; the anterior end is distinguishable in the living
animal as that which is directed forwards in the ordinary creeping
movements of the worm. The surface, as in the case of Nereis,
is very distinctly marked out into segments or metameres by a
series of ring-like constrictions ; the segments, which are very
numerous amounting to about 150, are somewhat longer towards
the anterior end than they are further back.

At the extreme anterior end is a rounded lobe, the prostomium,
immediately behind and below which is the opening of the mouth.
Next to the prostomium is the most anterior segment, the peri-
stomium, which bounds the mouth behind. The eyes and tentacles
present in Nereis are not represented. On the most posterior
segment, the anal segment, is a small median opening, the anal
aperture. A limited region of the body in front of the middle,
comprising segments from the thirty-second to the thirty-seventh,
has a swollen appearance ; this is termed the
clitellum. There are no parapodia like those
of Nereis, but running along the lower sur-
face of the worm are to be recognised with the
aid of a lens four double rows of short bristles
or setae (Fig. 358), a pair of each row occur-
ring in each segment, which thus possesses
eight altogether. The extremities of all these
setce are directed backwards, and they act as
fulcra for the forward movements of the worm
on the surface of the ground or in the interior
of its burrow. The setae in the clitellum, and
those in the neighbourhood of the genital
apertures, are much slenderer than the rest.

,, J i 11 T c ,1 i i f

Along the middle line ot the dorsal surface,
from about the eleventh segment backwards,
is a row of small aperture's, one at the line of division between each
contiguous pair of segments : these, which are termed the dorsal
pores, perforate the body- wall and open internally into the coelome.
Through these ccelomic fluid is capable of being discharged,
covering the surface with a thin layer which may protect the
worm from desiccation or from contact with irritating sub-
stances. On the ventral surface are two rows of minute
apertures a pair on each segment the excretory apertures or
nephridiopores. On the ventral surface of the fifteenth segment
(Fig. 357, 15), is a pair of slit-like apertures with somewhat
tumid lips, the male reproductive apertures; and on the segment
immediately in front the fourteenth, are two smaller rounded
apertures, the female reproductive apertures. In the intervals
between the ninth and tenth, and tenth and eleventh segments
are two pairs of small pores, the openings of the receptacula
seminis.

G. 358. Lumbricus.
Seta*, highly magnified.

456

ZOOLOGY

SECT.

The body-wall (Fig. 359) consists of a cuticle, an epidermis
or deric epithelium, a dermis, muscular layers with associated con-
nective-tissue, and, lining the inner surface, a thin cellular
membrane, the peritoneum or ccelomic epithelium. The cuticle (cut.)
is similar to that of Nereis, and has a similar iridescent lustre ; it
is perforated by numerous minute apertures. The epidermis
consists, except on the clitellum, of a single layer of cells
elongated in the vertical direction : many of these cells have the
character of unicellular glands ; many others are sensory cells,

dors. i r

typh

cut

e/iid

Cll~C.tr MX

It in a. tti.ua

neph

set

FIG. 359. Immbricus, transverse section of the middle region of the body. circ. IHH.S. layer of
circular muscular fibres ; cccl. ccelome ; cut. cuticle ; (tors. v. dorsal vessel ; epi-i.l. epidermis ;
ext. nepk. nephridiopore ; hep. layer of chloragen cells ; Imui. mus. longitudinal muscle ;
neplt. nephridium ; nepkrost. nephrostome ; n. co. nerve-cord; fet. seta;; sub. n. vess. sub-
neural vessel ; typh. typhlosole ; vent. v. ventral vessel. (After Marshall and Hurst.)

and are connected by fine nerve-fibres with the nerve-cord. On
the clitellum the epidermis is thickened, and blood-vessels extend
between the cells. Below the epidermis is a layer of connective-
tissue, the dermis. The muscular fibres which make up the
greater- part of the thickness of the body- wall are arranged in two
principal sets a layer of circularly arranged fibres (circ. mus]
situated externally, immediately below the dermis and a layer of
longitudinally arranged fibres (long, mus) situated internally.
The circular layer is interrupted at all the intervals between
the segments ; the longitudinal layer is interrupted along

PHYLUM ANNUL ATA

457

a series of longitudinal lines, so as to be divided into seven
bundles.

The setae (Fig. 358) are lodged in sacs, the setigerous sacs (see
Fig. 369), lined by a
continuation of the epi-
dermis. In the region
of the body in which the
reproductive organs are
lodged some of these sacs
are enlarged and glan-
dular, and receive the
special name of capsulo-
genous glands.

The enteric canal
(Fig. 360) is, as in
Nereis, a tube which
runs through the entire
length of the body from
the mouth at the an-
terior to the anus at the
posterior end. As in the
case of Nereis, it lies in a
cavity, the ccelomc, lined
by a thin cellular mem-
brane, the peritoneum or
ccelomic epithelium, and
filled with a fluid, the
ccelomic fluid, contain-
ing colourless corpuscles.
The coelome is divided
into a series of chambers
corresponding to the seg-
ments by a series of
delicate transverse parti-
tions, the septa or mesen-
teries, consisting of folds
of the peritoneal mem-
brane enclosing muscular
fibres.

The mouth leads into
a small buccal cavity.
This is followed by
a much larger, thick-
walled, rounded chamber, the pharynx (ph.}. From the wall
of the pharynx there run outwards to the body-wall a number
of radially arranged bundles of muscular fibres which, when they
contract, draw the pharynx backwards, and at the same time

458 ZOOLOGY SECT.

dilate it. Behind the pharynx follows a comparatively narrow
tube, the oesophagus (a?s\ which extends through about seven
segments. At the sides of the oesophagus, in each of the segments
ten, eleven, and twelve, is a pair of rounded projections. The
first pair the cesophagccd pouches are hollow, and their cavities
are in communication with the lumen of the oesophagus (ces. gl).
The other two pairs the calciferous glands are thickenings of the
wall of the oesophagus, the fluid in the interior of which is milky,
owing to its containing numerous particles of carbonate of lime ;
the numerous small cavities which they contain are in communi-
cation with the cesophageal pouches. Posteriorly the oesophagus
is continuous with a rounded thin-walled chamber, the
crop (cr) and this is followed by a very thick-walled chamber, also
of rounded form, the gizzard (giz). From this the intestine (int)
extends throughout the rest of the length of the body to the anal
aperture. It is wide, with thick but soft walls, constricted
opposite the septa, i.e. in the intervals between the segments.
Running along the middle of its dorsal surface is a longitudinal
fold, the typlilosolc (Fig. 359, typli), projecting downwards into the
lumen. On the wall of the intestine outside the muscular layers
and surrounding the intestinal blood-vessels are a number of
granular, yellow cells the chloragcn cells (hep) : these are specially
abundant in the typhlosole. The terminal part, situated in the
last segment, is termed the rectum.

The whole alimentary canal is lined internally by a cuticle which
is thicker in the gizzard than elsewhere, and by a single layer
of ciliated columnar epithelial cells, the enteric epithelium. Some
of these cells, more granular than the others, grouped in certain
regions more particularly along the typhlosole, are of the nature
of unicellular digestive glands, secreting a digestive fluid. Others
seem to be specially concerned in the absorption of the digested
food. External to this is a layer of connective-tissue, between
which and the external covering of yellow cells are muscular
fibres, of which there are two layers, an external longitudinal and
an internal circular. These layers are greatly thickened in the
walls of the pharynx and of the gizzard.

The Earthworm, like Nereis, has a well-developed vascular
system, consisting of blood-vessels with well-defined walls. The
blood is bright red, the colour being due to the same colouring-
matter, viz. haemoglobin, as in the case of the blood of the higher
animals, occurring, however, not in corpuscles, but in the liquid
part or plasma ; corpuscles are present, but they are colourless.
The main trunks are the dorsal, the ventral, the sub-neural, the
two lateral neural, and a series of transverse branches. The dorsal
vessel (Fig. 359, dors, v) runs along the middle of the dorsal surface
between the body-wall and the intestine ; it is readily visible shining
through the former in the living worm. The ventral vessel (vent, v)

PHYLUM ANNULATA

459

foresl

ctr.

lies below the alimentary canal, the sub-neural below this again
under the nerve-cord ; the lateral neural lie on either side of the
nerve-cord. The transverse branches correspond in number to the
segments ; they run round from the dorsal vessel to the ventral,
giving off branches in their course. Five of them, viz. those in
the seventh to the eleventh segments inclusively, are dilated and
pulsate rhythmically ; these have the function of driving the
blood through the system of vessels, and are hence frequently
termed the " hearts." The walls of the principal vessels are
contractile, and assist in bringing about the movement of the
blood, which is propelled in such a way as to run forwards in the
dorsal vessel and backwards in the ventral, its direction of move-
ment being regulated by a number of valves in the " hearts," the
dorsal vessel, and the chief vessels connected with it.

The nervous system (Fig. 361) consists of a dorsal bilobed
brain or cerebral ganglion and a double ventral nerve-cord
together with a pair of ceso-
phageal connectives, by which
the former is connected with
the anterior end of the latter.
The brain, which is of small
size, is situated in the third
segment, above the beginning
of the alimentary canal ; it is
divided by a median constric-
tion into two lateral parts of
pyriform shape with their
broad ends in contact. The
connectives pass from this
round the sides of the ali-
mentary canal to unite in the
middle below with the anterior
end of the ventral nerve-cord.
In this way a complete nerve-
ring or nerve-collar surrounds
the anterior part of the enteric
canal in the third segment.
From this the ventral nerve-
cord extends backwards to the posterior end of the body, and in
each segment it presents a slight enlargement or ganglion, as it is
usually termed, most conspicuous in the more posterior segments.
The whole cord is double, consisting of two intimately united
right and left parts. From the brain, nerves are given off to
the prostomium ; and from the ventral cord three pairs of nerves
arise in each segment. From the cesophageal connectives a
series of stomatogastric nerves pass to the pharynx and neighbouring
parts of the alimentary canal.

-com.

-n-e.co

FKI. 3i.il. Iumbricus. Anterior portion of
nervous system, cer. yang. cerebral ganglion
or brain ; cont. cesophageal connectives ; ne. co.
ventral nerve-cord ; yrost. prostomium. (After
Leuckart.)

460

ZOOLOGY

SECT.

The Earthworm is devoid of organs of sight or hearing. It
exhibits sensitiveness to bright light, the sensitive elements being
large cells of the epidermis devoid of pigment. The sense of

coe

ext

JL

Fi<;. 362. Nephridium of Lumbricus (diagrammatic). a. ampulla between ciliated and non-
ciliated parts of the intracellular canal ; cil. ciliated part of the iutracellular canal ; <<><.
investment derived from the crelomic epithelium ; eft. nephridiopore ; (.<: non-ciliated part
of the intracellular canal ; //^'.s\ .septum ; ;.s<. nephrostome ; t.v. intercellular canal of the
terminal vesicle. I. III. the three principal loops. (From Meisenheimer, after Ma/Jarski. )

hearing appears to be absent ; but a faculty analogous to taste
or smell, enabling the animal to distinguish between different
kinds of food, is well developed. The goblet-shaped bodies, groups of

x PHYLUM ANNUL AT A 461

narrow epidermal cells, most abundant on the prostomium and
peristomium, have probably to do with this faculty.

Tho organs of excretion the segmented organs or nephridia
-(Fig. 362) are similar to those of Nereis, but somewhat more
complicated. They are slender tubes which occur in pairs in all
the segments of the body except the first three and the last.
Externally each nephridium opens by one of the small nephridio-
pores which have already been mentioned as occurring on the
ventral surface ; internally it ends in a funnel-shaped ciliated
extremity with a crescentic slit-like aperture, the nephrostome
(nst), opening into the cavity of the segment in front of that
in Avhich the external aperture occurs. The tube is thrown into
three loops attached to the posterior surface of the corresponding
septum by a fold of membrane. Two parts are clearly recognis-
able an inner narrow and an outer wide part : in the former the
narrow central lumen is a perforation through the axis of a string
of cells, and is thus intracellular : it is lined in parts with cilia
arranged in two rows ; in the latter (the terminal vesicle} the
passage is lined by cells, and is thus intercellular, and there is a
thick muscular investment. The nephridia are abundantly
supplied with blood by means of nephridial branches of the
ventral vessel.

Reproductive Organs. The Earthworm is hermaphrodite.
There are two pairs of very small flattened testes (Figs. 360, 363, le,
te), partly divided into a number of digitate lobes, situated in the
tenth and eleventh segments. A pair of comparatively large sacs,
the anterior vesiculce seminalcs (ant. ves. sem) lie partly in the
cavity of the ninth segment, but extend into the tenth, where
they coalesce in the middle to form a large median sac of some-
what irregular form, the anterior sperm-reservoir (ant. sp. res).
The anterior pair of testes project into this, and the cells destined
to form the sperms, developed in the former, pass by dehiscence
into the large median cavity. On either side is a large ciliated
funnel, or rosette (fun), leading outwards from the interior of the
reservoir. A second pair of vesiculse seminal es (mid. ves. sem),
situated in the eleventh segment, also open into the anterior
sperm-reservoir. A third pair (post. ves. sem), situated in the
twelfth segment, unite in front to form the posterior sperm-reservoir
(post. sp. res), which lies in the middle of the cavity of the
eleventh segment. The posterior pairs of testes have the same
relation to this as the anterior pair have to the anterior reservoir ;
and a posterior pair of ciliated funnels (fun) lead outwards from
its cavity. Each ciliated funnel passes into a narrow, somewhat
convoluted duct, the vas cfferens, and the two vasa efferentia of each
side unite to form a vas deferens or spermiduct (v. def), right or
left as the case may be, which passes almost straight backwards to
open by the corresponding male aperture on the fifteenth segment.

462

ZOOLOGY

SECT.

The female reproductive organs consist of a pair of ovaries, a
pair of oviducts with a pair of receptacula ovorum, and two pairs of
receptacula seminis. The ovaries (ov) are minute pear-shaped bodies,
which are situated in the thirteenth segment, attached to the
septum between the twelfth and thirteenth. The oviducts (ov. d)
are a pair of short tubes, each with a comparatively wide funnel-
shaped opening into the cavity of the thirteenth segment, and
extending backwards and outwards in the fourteenth segment to
open at the female aperture on the ventral surface of the latter.
The receptacula ovorum are a pair of reniform sacs which open into

ctnt. ves.

i?tt

ant. sp.res
n.co int I int

rec

rec

post

intd, v&s se
fiost.sp.res

ov.d

ov.d-

FIG. 363. Lumbricus herculeus. Reproductive organs, ant. sp. res. anterior sperm reser-
voir ; ant. ves. sem. anterior left vesicula seminalis ; fun. funnel-like openings of vasa efferentia;
i tit. intermuscular partitions ; mid. ves. sem. middle vesicula seminalis ; n. co. nerve-cord ; ov.
ovaries ; or. d. oviducts ; post. sp. res. posterior sperm-reservoir ; post v(s. sim. posterior
vesicula seminalis ; rec. receptacula seminis ; te, anterior, and te', posterior testes ; r. <-ff.
anterior, and v. eff' . posterior vas efferens ; v. <lef. vasa deferentia. (After Vogt and Jung.)

the funnel-shaped ends of the oviducts. The receptacula seminis
(rec) are two pairs of rounded sacs which open on the exterior
in the intervals between the ninth and tenth, and tenth and
eleventh segments.

Though hermaphrodite, the Earthworm is not self-impregnating,
but two individuals provide for mutual fertilisation by an act of
copulation. The copulating individuals apply themselves together
by their ventral surfaces, the heads pointing in opposite directions,
and become attached in this position by the setae of the genital
region and by a viscid secretion from the clitellum and of

PHYLUM ANNULATA

463

the capsulogenous glands (p. 457), situated in the neighbourhood
of the reproductive organs. The sperms from the male apertures
of each pass along temporarily formed grooves to the receptacu/a
seminis of the other.

When the ova are mature they are discharged from the ovary
into the cavity of the thirteenth segment, whence they pass out
to the exterior through the oviducts, to be enclosed in the cocoon

eel

%o s v

;

mes-

FIG. 3(34. Early stages in the development of Liimbricus. A,. lateral view of flattened blastula ;
B. ventral view of gastrula with slit-like blastopore ; L', lateral view of later stage, blastoc.
blastoccele ; blastop. blastopore ; vet. ectoderm ; en<l. endoderm ; in. primary mesoderm cell ;
mes. mesoderm bands ; ,ier. cell from which the primitive nerve-cord (ne. co.) takes origin ;
npk. cells taking part in the formation of the nephridia ; .s. stomodseum. (After Wilson.)

(vide infra), after having being detained for a time in the
receptaculum ovoruin.

Development. The oosperms or fertilised ova of the Earth-
worm are enclosed, together with a quantity of an albuminous fluid
derived from the capsulogenous glands, in a cocoon, the wall of
which is formed of a viscid secretion from the glands of the
clitellum, hardened and toughened by exposure to the air. The

4f54 ZOOLOGY SECT.

cocoon is deposited in the earth and the embryos develop into
complete, though minute, worms before they make their escape.
The segmentation is somewhat unequal. A flattened blastnla
(Fig. 364, A) is formed, with a large but flattened segmentation-
cavity. This becomes invaginated to form a cylindrical gastrula
(B) ; the blastopore narrows and subsequently gives rise to the
mouth of the adult. A pair of large mesoderm cells (m) are early
marked off from the other cells of the gastrula ; these undergo
division to form a pair of mesoderm bands composed of several
rows of small cells which grow forwards towards the mouth.
By swallowing movements the embryo at this stage, having
burst through the enclosing vitelline membrane, takes in the
albuminous fluid in the interior of the cocoon, and increases rapidly
in size. As the embryo elongates, the mesoderm bands become
divided into segments, and the subsequent history of these is
essentially similiar to what has been already described in the case
of Nereis. The ectoderm is thickened on each side along
the line of the mesoderm bands, and the mass of ectoderm cells so
formed becomes arranged in a number of rows each originating be-
hind in a larger rounded cell or teloUast. The innermost of these
rows (Fig. 364, C, ner, ne. co) give rise to the ventral nerve-cord.
The next two rows (nph) are said by some observers to give rise
to the nephridia all but the funnels : but according to others the
nephridia, or at least all their inner glandular portions, are of
mesodermal derivation. The brain and cesophageal connectives are
formed in continuity with the rudiments of the ventral nerve-cord.
On the whole the development resembles that, of Nereis, the
chief differences being such as may be traced to the non-occurrence
in the Earthworm of any free-swimming trochophore stage, and
the absence of such larval structures as the large pre-oral lobe,
the apical plate, the prototroch, and the larval nephridia or head-
kidneys.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Chsetopoda are Annulata with the body made up of distinct
metameres, which are usually numerous and similar throughout.
The metameres are provided with chitinous setae developed in sacs
(setigerous sacs) of the epidermis, and usually elevated on muscular
appendages, the parapodia. There is a large coelome divided
internally into chambers by transverse septa, and not in com-
munication with the blood- vascular system, which is nearly
always highly developed. The ventral nerve-cord consists
of a chain of ganglia. The reproductive cells are formed by a
proliferation of certain parts of the peritoneum or membrane lining
the coelome, and usually reach the exterior through coelomoducts
or through modified or unmodified nephridia.

x PHYLUM ANNULATA 465

Sub-Class I.-POLYCILETA.

Chsetopoda with the sexes distinct, and the ovaries and testes of
simple character and metamerically repeated. Highly developed
parapodia are present, in most instances, bearing numerous long
setae. There is usually a definite head with eyes and tentacles,
and often cirri and branchiaB on the segments of the body. A
clitellum is never developed. A metamorphosis takes place: the
larva is a trochophore. Nearly all the Polychaeta are marine.

ORDER 1. ARCHI-CFLETOPODA.

Aberrant or primitive PolychaBta 1 in which the nervous system
is not separated from the epidermis, and the ventral cord is not
segmented into ganglia. Only one genus (Saccocirrus).

ORDER 2. PHANEROCEPHALA.

Polychaeta with protrusible pharynx usually armed with chitinous
jaws. There is a well-developed head. The segments are
completely or nearly similar throughout the length of the body,
and the parapodia are usually equally developed throughout and
provided with cirri. The branchia?, when present, are not confined
to the anterior end.

ORDER 3. CRYPTOCEPHALA.

Polychseta devoid of protrusible pharynx and of jaws or teeth.
The head is frequently very small, and sometimes is devoid of
eyes or of tentacles, the prostomium sometimes much reduced and
covered over by the peristomium. The body is distinguishable, by
differences in the form of the segments, parapodia, and setaa,
into two or even three regions. The parapodia are little prominent
in the posterior parts, and usually without cirri. The branchiae,
when present, are usually confined to the anterior end, and are
sometimes represented by modified cephalic palpi.

Sub-Class II.-OLIGOCILETA.

Cha^topoda with the sexes united, the reproductive system com-
plicated, the ovaries and testes compact and never more than two
pairs of each. No definite paropodia are developed and no cirri,
and only a small number of simple setas on each segment The
head is not distinct. A clitellum is usually present. There is no
metamorphosis. Mostly terrestrial or fresh-w r ater forms.

1 The Archi-CluKtopoda are usually classed with the Polychfzta, but their
alliances are perhaps quite as close with the Oliyochceta. In some respects
Saccocirrus resembles Polygordius and Protodrilus (Arhci- Annelida q.r.)
but is distinguished from them by the possession of seta-.

VOL. I H H

466 ZOOLOGY SECT.

ORDER 1. MICRODBILI.

Small Oligochaeta with relatively few segments, often multiply-
ing asexually. The male genital pores are on, or in front of, the
seventh segment. The vasa deferentia are short, opening on the
segment immediately behind that in which the internal apertures
are situated. The anterior part of the body is often distinguished
from the rest by a difference in the form and arrangement of the
setae. The clitellum, which is composed of only one layer of cells
is situated comparatively far forward. Eye-spots are frequently
present.

ORDER 2 MEGADRILI.

Mostly large Oligochseta with many segments, never multiply-
ing asexually. The male genital pores are behind the seventh
segment. The vasa deferentia are elongated, passing through two
or more segments. The anterior part of the body is never special-
ised as regards its setae. The clitellum, which consists of two
layers of cells, never begins in front of the twelfth segment.
Eye-spots are not developed.

Systematic Position of the Examples.

Nereis damerilii is one of many species of Nereis differing from
one another in certain minor details of their structure such as
the relative length of the palpi and tentacles, the size and form of
the eyes, the shape of the parapodia, the form of the setae, and the
like. The genus Nereis differs from the other genera of the
family Nereidce, to which it belongs, in having the parapodia
biramous and the cirri simple, and in the presence of a series of
denticles in the buccal cavity in addition to the pair of jaws. The
family Nereidae differs from all the other families of the sub-order
Ncreidiformia of the Phanerocephala in the union of the following
characters :- -The body is always elongated and made up of a con-
siderable number of segments. The prostomium is well developed,
and bears a pair of tentacles, a pair of palpi, and four eyes. The
peristomium is devoid of parapodia, and has four pairs of tentacles.
The parapodia are either uniramous or biramous ; both dorsal and
ventral cirri are present ; the setae are compound (articulated).
There is a pair of anal cirri. In the pharynx there is always a
pair of horny jaws, and usually a number of denticles in the
buccal cavity.

The members of the sub-order Nereidiformia are all character-
ised by the possession of well-developed tentacles and palpi, and
usually peristomial cirri. There are highly developed parapodia
with acicula, jointed seta-, and dorsal and ventral cirri. The buccal

PHYLUM ANNUL AT A

407

region of the enteric canal is eversible as a proboscis, and there are
usually horny jaws.

There are several species of the genus Lunibricus, differing from
one another in the general form of the body, the number of the
segments, the shape of the prostomial lobe, and other minor
points. All of them agree in the presence of the following features,
which characterise the genus and distinguish it from the many
other genera of the family Lumbricidce :-

The prostomium is dovetailed completely into the peristomium.
The setce are always in couples. There are longer and straighter
setae on the clitellum. The male apertures are always on the
fifteenth segment. There are three pairs of vesicuhe seminales,
in the ninth, eleventh, and twelfth segments, connected across the
middle line in the tenth and eleventh by sacs enclosing the
ciliated funnels. There are two pairs of receptacula seminis al-
ways situated in the ninth and tenth segments.

The family Lumbricidou is distinguished from the other families
of the sub-order Megadrili, which comprises all the Earthworms,
by the combination of the following features :

The clitellum usually begins behind the twentieth segment and
occupies from six to nine segments ; it is incomplete ventrally.
Dorsal pores are present. The seta3 on the clitellum differ from
the others. The male apertures are not situated further back than
the fifteenth segment. There are three or four pairs of vesicuhe
seminales, in the ninth to the twelfth seg-
ments. The testes and ciliated funnels are
usually in the tenth and eleventh segments :
the female apertures on the fourteenth.

3. GENERAL ORGANISATION.

The general form of the body in the
Chaetopoda is cylindrical, but in many, tyy.,
some members of the families Polynoidce
(Fig. 365) and Amphinomidce, there is a
very considerable degree of dorso- ventral
compression. In most the body is very long
in comparison with its breadth ; but this is
not a universal rule, the length being in
some cases not more than five or six times
the breadth. The surface is marked out
by a number of more or less distinct
annular constrictions or impressed lines
into a corresponding series of segments or
metameres, which are usually very numerous,

often some hundreds in number, though in some cases there are
not more than from twenty to thirty. These segments are

H 11 2

FKJ. 3G5. Folynde seto-
sissima. Dorsal view
of entire animal, with
the pharynx protruded.
(After Quatrefages.)

468

ZOOLOGY

SECT.

usually very similar throughout the length of the body ; but in
the Cryptocephala (Figs. 366, 367, 373) there may be two or even
more regions distinguishable from one another by the form of
the segments and of their appendages. In the Oligochaeta there is
a thickened zone, the clitdium, comprising sometimes only one
segment, sometimes a number. Each segment, with certain
exceptions to be noted presently, bears either a pair of parayodia
or merely a greater or smaller number of setae. Parapodia are
lateral hollow processes of the body-wall bearing a number of

l.mv

FK;. 3<3G. A Serpulid (Yurmilia cn.'.s]>itosa). Lateral view of animal removed from its tube.
abd. abdomen ; b>: branehi; ; o/. operculum ; l/t. thorax.

bristles or setae. Frequently the parapodium is divided horizon-
tally into two distinct lobes or branches a dorsal which is termed
the notopodium, and a ventral which is termed the neuropodium.
Even when this is not the case there may be two bundles of setae
representing the the two parts. The setae are nearly always
chitinous ; in Euphrosyne they are calcined. They are always
solid, except in EiipJirosym, entire, or divided into a number of
joints. In shape (Fig. 368) they vary greatly in different groups ;
often several very distinct forms of setie are present in different

PHYLUM ANNULATA

469

parts of each parapodium of a single worm, or in parapodia of
different regions of the body. Some are exceedingly delicate and
hair-like, others needle-shaped, others compressed and sabre-like,
others bayonet-like. Very often there is a long, straight, narrow
part or handle with which is articulated a terminal blade, or
bayonet, or hook. Sometimes the setae are quite short, projecting
little beyond the parapodia, and are hook-like or comb-like.
Usually each bundle contains, in addition to the ordinary setae,
a stouter, straight, simple seta, which scarcely projects on the sur-
face ; this is termed the aciculum. Each seta, or each bundle of
setae, is lodged in a sac, the setigerous
sac (Fig. 369), formed by an invagina-
tion of the integument, and lined by
cells continuous with the epidermis.
Each seta is derived from one of these
cells, and is to be looked upon as a
specially developed part of the cuticle
of the general outer suface. The
setigerous sacs are usually provided
with protractor and retractor muscles,
by the action of which the setae may
be thrust out or retracted.

In addition to the setae the para-
podium bears very commonly certain
soft appendages of a sensory character,
the cirri (Fig. 347, dors, cirr., vent.
cirr.). There are usually both dorsal
and ventral cirri, the latter nearly
always much smaller than the former.
The cirri are usually filamentous,
sometimes jointed ; sometimes they
are laterally compressed and leaf-like.
In Polyndc (Figs. 365 and 370) and
its allies certain of the parapodia
bear, instead of dorsal cirri, flattened
scales, the elytra (el.), richly supplied

with nerves : these are sometimes looked upon as modified dorsal
cirri, but in some members of the group cirri and elytra occur
together on the same segment.

In Sternaspis a ventral shield formed by a thickening of the
cuticle in the posterior region of the body bears a number of seta
round its edge.

In the Oligochaeta (Fig. 372) the parapodia are absent as pro-
cesses of the body-wall, and are merely represented by a small
number of short setae each lodged in its sac ; cirri are not
developed. In certain Oligochaeta setae are absent.

The first segment or prostomium, together with the second or

FK;. 3(i7. Chaetopterus. Natural
size of a young specimen. A, an-
terior region of the bocty ; B, middle
region ; C, hinder region, c, peri-
stomial cirri; <l, "sucker" ; e, the
great " wings " ; /, the first of the
three "fans"; //;, mouth. (From
JJenham, after Paneeri.)

470

ZOOLOGY

SECT.

pcristomium, forms in many Polychseta a very distinct head ; the
prostomium in such a case bears eyes and tentacles and contains

Fie. Mils. Seta? of various Polyrlwta. (From Chiparede.)

the cerebral ganglion ; on the peristomium is the opening of the
mouth, and from it also arise the perislomial tentacles. A

rrn

Fii; 3i>;i. Section of tlic setigerous sac of an Oligochsete. bj, setigerous sac ; Z>g, supplementary
follicle with seta ; e, deric epithelium (epidermis); ?m, longitudinal muscles of body-wall ;
~m, ,K. muscles of the setigcrous sac; r.m, circular muscular layer of body-wall. (From
Hatschek, after Vejdovsky.)

ventral pair of prostomial tentacles, somewhat thicker than the
rest, are sometimes to be distinguished, and are termed the palpi.

PHYLUM ANNULATA

471

Neither prostomium nor peristoinium bears parapodia, though an
aciculum is sometimes developed in the latter ; the prostomium
in fact, is not quite correctly termed a segment, being different
from the true segments both in structure and in mode of develop-
ment. In the Oligochaeta there is no definite head, tentacles are
entirely absent, and in the terrestrial forms the prostomium does
riot lodge the cerebral ganglion. In Sternaspis spinosa the pro-
stomium is elongated and bifurcated like the proboscis of the
Gephyrea armata (vide infra).

cirr-

dors. cirr

Fm. 370. Polyaoe extenuata. Dorsal view of anterior extremity, dors. cirr. dorsal cirri ;
el. elytra; purist, tfnt. peristomial tentacles ; prmt. prostomium. (After Claparede.)

The last segment is termed the anal segment, owing to its
bearing the anal opening ; it usually also differs from the preceding
segments in wanting the parapodia and in having a pair of special
cirri, the anal cirri.

Branchiae are borne on the dorsal surfaces of more or fewer
of the segments in many of the Polychoeta. Sometimes they
occur on all, or nearly all, the segments ; sometimes they are
confined to the middle region of the body ; sometimes they are
present only at the anterior end, as in the majority of the Poly-
chseta living habitually in tubes (Figs. 366 and 373). In the

472

ZOOLOGY

SECT.

Terebellidci' (Fig. 373) the branchise are situated on the dorsal sur-
faces of some of the anterior segments. In the Serpulidce (Fig.
366) they form two incomplete lateral circlets of elongated
appendages situated at the anterior end of the body, apparently
representing modified palpi, and sometimes supported by a carti-
laginous skeleton ; one of them is enlarged to form a stopper or
operculum (op.}, often armed with calcareous plates and spines, for
the closure of the mouth of the tube in which the annelid lives. In

B

Fir;. 371. Heads of various Polychacta (diagrammatic). A, Polynoid ; B, Syllid ; C, Xt-

I), Eunice; E, Phyllodoce ; F, Tropltonia. a, prostoraium ; c, cirri of body segments; c 1 ,
peristomial cirri (tentacles) ; r-, cirrus of first body-segment ; <'*, cirrus of second body-seg-
ment ; e/', point of attachment of elytron ; p, palp ; x, nuchal organ ; t, tentacle ; /, peri-
stomium : JJ, III, IV, segments. (From the Cambridge Natural History.)

shape the branchiae are sometimes filiform, sometimes compressed
and leaf-like, sometimes branched in a tree-like manner, some-
times pinnate. In Serpula (Figs. 366 arid 383) and its allies each
branchia consists of an elongated stem on which are borne two
rows of short filaments. The surface of the branchiaB is usually
ciliated. They are richly supplied with blood-vessels when a
blood-vascular system is developed ; in Glycera, in which there
are no blood-vessels, each branchia contains a diverticulum of the
coelome.

PHYLUM ANNULATA

473

In the Oligochreta branchiae
are rarely present ; but in certain
of the Naiidomorpha there are
metainerically arranged simple
or branched branchiae, sometimes
retractile, on the segments of
the posterior region.

The body-wall consists of a
cuticle, an epidermis, muscular
layers, and a layer of peri-
toneum. The cuticle, composed
of a chitinoid material, usually
presents two sj 7 stems of fine
lines i