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MBilARY

A TEXT-BOOK OF ZOOLOGY

MACMILLAN AND CO., Limited

LONDON . FOMBAY . 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, UUNEDIN, N.Z.

AND

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

TROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY, N.S.W.

IN TWO VOLUMES
VOL. I

WITH ILLUSTRATIONS

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

1910

QL
YX3

\q\0

v. I

KlCHAKD CLAV AND SONS, LIMITED,

BREAD STREET HILL, E.C., AND

BUNGAY, SUFFOLK.

First Edition, 1898.
Second Edition, 1910.

PREFACE TO THE FIRST 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 tbe 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 course 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.
We 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 tho 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 Ave 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 bis, 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 inave 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 studeuts, 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."

* 'Plie figures referred to are numbered 608, 609, 1080, 1081, 1082, 1085,
1128, 1132, 1 140, 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 parb 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.

PllEFACE 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
arc 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. Haswell.

CONTENTS

I'AOK

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 .

1. Amoeba ..

2. The Animal Cell

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

Germinal Layers

4. Tissues

5. Organs .....

6. The Reproduction of Animals

7. Symmetry .'....

8. The Primary Subdivisions or Phyla of the Animal Kingdom

10
10
14

19
23
31
40
41
43

SECTION II

Phylum Protozoa ...
Class I. Rhizopoda . . . .

1. Example of the Class — Amoeba proteus

2. Classification and General Organisation
Systematic Position of the Example
Appendix to the Rhizopoda

Class II. Mycetozoa

1. Example of the Class — Didymium difforme

2. General Remarks

Class III. Mastigophora ....

1. Example of the Class — Englena viridis

2. Classification and General Organisation
Systematic Position of the Example

Class IV. Sporozoa

1. Example of the Class — Monocystis agilis

2. Classification and General Organisation
Systematic Position of the Example

45
46
46

47
48
04

<;<;
oo
67
67
67

('»!•

70
so
80
81
82

xiv CONTENTS

PAGE

Phylum Protozoa — continued.

Class V. Infusoria 88

1. Example of the Class — Parametrium caadatum .... 88

2. Classification and General Organisation 91

Systematic Position of the Example 91

Further Remarks on the Protozoa . . . . - . . . 101

SECTION III

Phylum and Class Porifera [Parazoa] 105

1. Example Sf the Class — Sycon gelatinosum 105

2. Distinctive Characters and Classification Ill

Systematic Position of the Example 112

3. General Organisation 114

SECTION IV

Phylum Ccelenterata 128

Class I. Hydrozoa '. 128

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 Ccelenterata 226

Appendix to the Ccelenterata — The Mesozoa . . . . . 230

SECTION V

Phylum Platyhelmixthes 235

1. Examples of the Phylum . . 236

i. Planaria or Dendrocodum ■ . 236

ii. Fasciola hepaticci 240

iii. Tamia solium 245

CONTENTS xv

1'Al.K

Phylum Piati hii.mintuks 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 . 2'.»r>

SECTION VI

Phylum Nemathelminthes' 297

Class I. Nematoda 297

1. Example of the Class — Ascaris lumbricvides 297

2. Distinctive Characters and Classification 303

Systematic Position of the Example 304

3. General Organisation 305

Class II. Acanthocephala 312

Class III. Chsetognatha 316

Appendix to Nemathelminthes 319

Family Chatoso-micUe 319

,, Echinoderidce 311)

,, Desmoscolecidce 320

Affinities and Mutual Relationships of the Nemathelminthes . 320

SECTION VII

Phylum Tkochelminthes 322

(lass 1. Rotifera - 323

1. Example of the Class — Brachionu* 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 — Dinophilea and Histriobdellea . 336

SECTION VIII

Phylum Molluscoida 340

Class I. Polyzoa 340

1. Example of the Class — Bxigula 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 880

1. Example of the Class — Magellania lenticitlaris .... 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

PACJE

Phylum Echinodermata . 375

1. Example of the Asteroidea — Asterias rubens or Anthenea Jtdvetcens. 375

2. Example of the Echinoidea — Wrong i/locentrotus or Echinus . . 393

3. Example of the Holothuroidea — Cucnmaria or Colochirus . . 401

4. The Crinoidea — Antetkm rosacea . . 405

5. Distinctive Characters and Classification .410

Systematic Position of the Examples 414

6. General Organisation 415

SECTION X

Phylum Annulata 439

Class I. Chaetopoda 439

1. Examples of the Class 440

i. Nereis dumerilii 440

ii. Lwnribricus 454

2. Distinctive Characters and Classification 464

Systematic Position of the Examples . . . . . . 460

3. General Organisation 467

Appendix to the Chaetopoda — Class Myzostomida .... 489

( 'lass II. Gephyrea 491

1. Example of the Class — Sipuncidus 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 medicinalu and H. audraUs . 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 Arthropoda 526

Class I. Crustacea 526

1. Examples of the Class 526

i. Apus or Lepidurus 526

ii. Astacus fiuviatilis 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 til5

CONTENTS xvii

PAOK

Piivii m A i; i iikopoda — continued.

Class IV. Insecta 619

1. Example of the Class — Periplaneta oriental™ 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 — Euscorpio or Buthus 653

2. Distinctive Characters and Classification 660

3. General Organisation 662

Appendix to the Arachnida — the Pycnogonida, LinguatulicUi,

and Tardigrada 673

Relations of the Air-breathing Arthropoda 676

SECTION XII

Phylum Molixsca 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 ILLUSTBATIONS

VOL. I.

na page

1. Amoeba proteus 10

2. Amoeba polypodia, fission 13

.*!. Alveolar theory of protoplasm 15

4. Reticular theory of protoplasm 16

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 20

14. Reticular connective tissue 20

15. Fatty tissue 27

10. Hyaline cartilage 27

17. Fibro cartilage 27

18. Bone 28

l'». (nstriped muscle 29

20. Striped muscle 29

21. Nerve-cells 30

22. Nerve-fibres 30

23. Various forms of spermatozoa . . . . . . .30

24. Viscera of. Frog . . 33

25. Bones of human arm with biceps muscle 37

20. 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

6 2

tx LTST OF ILLUSTRATIONS

FIG. PAGE

34. Platoum stercoreum 51

35. Various forms of Foraminifera 52

36. Shells of Foraminifera 53

37. Ilastigerina murrayi 54

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

39. Actinophrys sol 57

40. Actinosphferium eichhornii 57

41. Various forms of Heliozoa 58

42. Actinophrys sol, conjugation .59

43. Lithocircus annularis 60

44. Tnalass°plancta 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

53. Trypanosome 72

54. Haematococcus 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. Gregarina . 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

Hi.. HAOB

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 Carpenteri 123

'.'.".. Larva of Clathrina blanca . 124

94. Development of Sycon raphanus 125

95. Obelia 130

96. ,, Vertical section of polype . 132

'.»7. 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 Leptolime 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

11 2. Two Trachymedusas 154

113. Two Narcomedusae 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

FIG. PACK

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. Astrsea 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. Alcyonium palmatum 203

154. Gorgonia verrucosa ..... 204

155. Structure of simple coral . . 206

156. Dendrophyllia and Madrepora 207

157. Adamsia palliata 209

158. Hormiphora 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 Ccelenterata . 229

175. Dicyema paradoxum with infusoriform embryos . 230

176. ,, ,, „ vermiform ,, .... 230

LIST OF ILLUSTRATIONS xxiii

no. PAM

177. Dicyema paradoxum, male 231

178. Rhopalura giardii, male . 232

171). ,, ,, 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

180. Distomum hepaticum 240

187. ,, ,, section of integument 241

188. ,, ,, internal organisation 242

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

190. ,, ,, development 244

191 . Taenia 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. Taenia echinococcus . . . . 261

205. Ligula . 262

206. Caryophyllfeus 263

207. Gyrocotyle • . 268,

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. Midler's larva 275

218. Embryos of Dendroccelum 276

219. Embryo of Temnocephala 278

220. „ „ 279

221. A Cysticercoid 280

222. ,, with head evaginated . 281

223. Cyst of Taenia echinococcus . . 282

224. Scolices „ 282

LIST OF ILLUSTRATIONS

FIG.

225.
226.

227.

228.
229.
230.
231.
232.
233.
234.
235.
236.
237.
238.
239.
240.
241.
242.
243.
244.
245.
246.
247.
248.
249.
250.
251.
252.
253.
254.
255.
256.
257.
258.
259.
260.
261.
262.
263.
264.
265.
266.
267.
268.
269.
270.
271.

Scolex of Taenia echinococcus . •

Process of budding in Microstomum

Diagram of the relationships of the Platyhelminthes
tinea

Diagram of Nemertine .

Proboscis of Nemertine

Tetrastemma ....

Anterior portion of Nemertine

Proboscis of Hoplonemertean, retracted
,, ,, everted

Transverse section of Nemertine .

Vascular and excretory systems of Nemertine

Pilidium

Ascaris lumbricoides

,, ,, transverse section

,, ,, muscle fibres .

,, ,, dissection of female .

Nervous system of Nematoda

Ascaris lumbricoides, dissection of male organs

Body- wall of platymyarian Nematode

Ankylostoma duodenale .

Transverse section of Gordius

Oxyuris

Gordius, anatomy ....

Development of Ascaris nigrovenosa

Trichinella spiralis ....

Two species of Echinorhynchus (Gigantorhynchus)

Echinorhynchus gigas, dissection of male .
,, ,, ,,- female

,, ,, ,, nephridia .

,, ,, female organs

Sagitta hexaptera

bipunctata, transverse
, , , , head

,, hexaptera, eye
Development of Sagitta
Cheetosoma
Echinoderes
Desmoscolex
A trochophore .
Brachionus rubens, female
,, ,, pharynx

,, ,, male and

Diagram of a Rotifer
Paiaseison asplanchnus .
Typical forms of Rotifera

mastax

sections

female, with attached

and Nemer-

LIST OF ILLUSTRATIONS xxv

27-. Cluetonotus maximus 896

l'7-"'. ,, „ anatomy . •!•">»>

274. Dinophilus tfeniatus 337

'_'7">. Stratiodrilus tasmanicus 338

_'7";. Mugula avicularia .... 34?

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

2.St>. ,, ,, 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 phylactolaematous 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 389

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

348b. ,, ,, setae 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. ,, setae ; 455

359. ,, transverse section 456

360. ,, sagittal section 457

361. ,, nervous system 459

362. ,, nephridium 460

363. ,, reproductive organs 462

364. ,, development . . .. . . . . i . 463

365. Polynoe setosissima 467

LIST OF ILLUSTRATIONS xxvii

no. paoe

366. Vermilia ccespitosa 468

367. Cluetopterus 469

368. Setae of various Polyclueta 470

369. Section of setigerous sac of an Oligochsete 470

370. Polynoe extenuata, anterior end 471

371. Polych«3ta, various, heads .* . . . 472

:*:•_'. Tubifex 473

373. Terebella 474

'•71. \phrodita, enteric canal 475

375. Saccocirrus, transverse section 477

376. Phyllodoce, nephridium . 479

."-77. Xephridia and coelomoducts 480

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

379. Spirorbis kevis 484

380. Eupomatus, development of trochophore 485

381. Autolytus cornutus, budding 487

382. Syllis ramosa 487

383. Serpulae 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 Rhynchobdellida 517

LIST OF ILLUSTRATIONS

FIG.

414.
415.
416.
417.
418.
419.

420.
421.
422.
423.
424.
425.
426.
427.
428.
429.
430a
430b
431.
432.
433.
434.
435.
436.
437.
438.
439.
440.
441.
442.
443.
444.
445.
446.
447.
448.
449.
450.
451.
452.
453.
454.
455.
456.
457.
458.
459.
460.

Proboscis of Clepsine

Nephridium of Herpobdella .

Pontobdella, nephridial system

Clepsine, development

Diagram of origin of metamerism .

Diagram illustrating the relationships of the Annulata

Trochelminthes ....
Apus cancriformis, dorsal aspect .
Lepidurus kirkii, side view .
Apus glacialis, ventral aspect

,, appendages . ...
Lepidurus kirkii, sagittal section .
Apus, transverse section
,, shell-gland ....
,, cancriformis, nervous system
,, structure of paired eye .
,, development ....
.Astacus fiuviatilis, male .

,, ,, transverse section of abdomen

,, ,, appendages ....

,, ,, articulations and muscles of leg

Section of skin and exoskeleton of Lobster
Articulations and muscles of abdomen of Crayfish
Astacus fiuviatilis, dissection from right side

,, „ gills

,, ,, kidney . . .

,, ,, transverse section of thorax .

,, ,, diagram of circulation

,, ,, nervous sj'stem

,, ,, reproductive organs

,, ,, formation of the blastoderm

,, ,, early embryo . •

,, ,, nauplius .

,, ,, section of embryo

,, ,, advanced embryo

Three Branchiopoda ....

,, Cladocera ....
Cypris ....
Cyclops and Calocalanus
Various forms of parasitic Eucopepoda
Argulus foliaceus
Lepas anatifera .
Balanus . . .
Sacculina carcini
Nebalia geoffroyi
Paranaspides
Mysis oculata . . .
Diastylis

LIST OF ILLUSTRATIONS xxix

no, PAGE

461. Gammarus 584

402. Asellus 685

403. Amphipoda 586

404. Isopoda 587

405. Shrimp and Prawn 588

4 till. Scyllarus arctus 589

407. Pagurus bernhardus 589

408. Cancer pagurus . . . . 590

409. Typical Brachyura 591

470. Squilla 592

471. Orchestia cavimana, anatomy 594

47•-,. Euphausia pellucida 595

473. Nervous system of Crab 590

474. Cypris-stage of Lepas 598

475. Larvse of Crabs 000

470. Diagram illustrating the mutual relationships of the orders of

Crustacea . . . 004

477. Dalmanites and Phacops 005

478. Triarthrus beckii 000

479. Peripatus capensis 007

480. ,, ,, ventral view of head . . . . . . 007

481. ,, anatomy 008

482. „ tracheal pit 009

483. ,, nephridium 010

484. ,, novse zealandiae, development . . . • . .011

485. ,, capensis 013

480. Scolopendrella immaculata 015

487. Scolopendra , 010

488. Lithobius forficatus 010

489. Pauropus huxleyi 017

490. Strongylostoma, development 018

491. Periplaneta orientalis 020

492. ,, mouth-parts 021

493- ,, americana, lateral view of head 021

494. ,, muscular system . . . 024

495. ,, anatomy 025

490. ,, salivary glands 635

497. Trachea of caterpillar 020

498. Periplaneta, tracheal system 027

499. ,, nervous system 027

500. ,, male reproductive organs 028

501. ,, female reproductive organs . . . . . 028

502. Segmentation of ovum of Insect 029

503. Ventral plate of embryo Cockroach 030

504. Embryo Cockroach 030

505. Lepisma 632

500. Podura 032

507. Locusta 033

508. Ephemera 633

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, tracheal, and digestive systems of the Honey-bee . 642

521. Trachea] gills of Ephemerid . . . 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

539. Embryo of Scorpion 659

540. Chelifer bravaisii 662

541. Phrynus 663

542. Galeodes dastuguei 663

543. Epeira diadema 664

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

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

546. Sarcoptes scabiaei 665

547. Trombidinm fuliginosum 665

548. Limulus 666

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

no. mom

558. Nymphon hispidum 674

559. Pentastomum trenioidcs 674

560. Macrobiotus hufelandi 675

Ml. I tiagmm to illustrate affinities of Arthropoda 678

MS. Aiiodonta cygnea 681

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

M4. ,, section of shell and mantle 683

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

MS. ,, ,, dissection from left side 686

.">t)7. ,, ,, structure of gills 687

MS. ,, ii transverse sections 688

569. ,, diagram of circulation 690

570. ,, statocyst 691

571. ,, early embryo 692

572. „ later- embryos 692

B7S. ,, advanced embryo 693

574. ,, metamorphosis 694

575. Anatomy of Pecten 697

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 man tie-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

MS. Eye of Pecten 708

593. Development of Ostrea 709

594. Veliger of Ostrea 709

595. Embryos of Cyclas 710

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

597. Chsetoderma nitidulum 713

598. Neoinenia carinata 714

599. Chiton, spinosus, dorsal view 714

600. „ ventral view 714

601. „ valves of shell 715

602. Chsetoderma nitidulum, 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 . . . . 728

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

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

618. ,, ,, section of eye 732

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

620. Solarium perspectivum 737

621. Terebra oculata . . . .738

622. Cyprsea moneta 739

623. Doris tuberculata 739

624. Carinaria mediterranea . . . . 739

625. Limax 739

626. Sigaretus leevigatus 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. ,, ,, ,, Limnseus 746

638. Eyes of Gastropoda 747

639. Osphradium of Murex 747

640. Reproductive organs 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. ,, larvoe .' 757

652. Rhodope . 758

653. Sepia, cultrata 760

654. Sepia ,, shell 762

655. ,, chromatophore 762

LIST OF [LLUSTRATION8 xxx.ii

m, PAOI

666. Sepia, cultrata, cranial cartilage 7<».'{

657. H >> nuchal cartilage 7<>3

658. ,, ,, mantle-cavity 7<»4

659. ,, officinalis, jaws 7'i-">

660. ,, section of buccal mass 7<><>

661. ,, officinalis, enteric canal 766

662. ,, cultrata, dissection of male from posterior aspect . . . 707

(it;.!. ,, ,, lateral dissection of male 768

664. ,, officinalis, longitudinal section of ink-sac .... 70'.t

666. ,, cultrata, vascular system 770

666. ,, ,, cephalic ganglia 770

667. ,, ii 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 pompilius, section of shell .... . 776

675. ,, ,, female in shell 778

676. Nautilus macromphalus, entire animal 77(»

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 780

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 7(»>

695. Shell of Belemnite 796

696. ,, Argonauta argo • • 796

697. Segmentation of Loligo 798

698. Blastoderm of Sepia 799

699. ,, i, sections 799

700. Development of Loligo 8(W

701 801

702- „ , ™

703. ,. • • °°f

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.

^1

?

ass I. BHIZOPODA.
Order 1. Lobosa.

,, 2. FORAMIXIFERA.
,, 3. HELtOZOA.

,, 4. Radiol aria.
MYCETOZOA.

MASTIGOPHORA.
Order 1. Flagellata.

„ 2. Choaxoflagellata.

,, 3. DlNOFLAUELLATA.

Class II
lass III

Order 4. Cystoflaoellata.
Class IV. SPOEOZOA.

Order 1. Greuarinida.
,, 2. Coccidiidka.
,, .3. h.emosporidia.
,, 4. Myxosporidea.
,, 5. Sarcocystidea.
Class V. INFUSOEIA.
Order 1. Ciliata.

,, 2. Textaculifera.

Phylum II. PORIFERA.

Class POEIFEEA.
Sub-class I. Calcarea.
Order 1. Homoccela.

Order 2. Heterocckla.
•Sub-class II. Hexactinellida.
,, III. Demospongia.

Phylum III. CCELENTERATA.

Class I. HYDEOZOA.

Order 1. Leptolin.e.

Sub-order a. Aiilhomedux^.
,, b. Lf-ptomedittii'.
Order 2. Trachylin.e.
Class II. SCYPHOZOA.

Order 1. Staitromedus.k.
,, 2. Coroxata.
,, 3. cubomedis.e.
,, 4. Djscomedus.e.
Sub-order a. SemoMomn'.
,, b. Rhizo8tom<i' .
Class III. ACTINOZOA.
VOL. I

Sub-class I. Zoantharia.
Order 1. Acplxiaria.
,, 2. Madreporaria.
,, 3. Axtjpatharia.
Sub-class II. Alcyonaria.

Sub-order a. Trachymedusce.
,, b. Xarcomedwuv.

Order 3. Hydrocorallina.

4. SlPHOXOPHORA.
."). (iRAPTOLITHIDA.

4. Alcyonacea.

5. Gorgonacea.

6. Pesnatulacea.

c *

xxvi CLASSIFICATION OF THE ANIMAL KINGDOM

Phylum III. CCELENTERATA— continued.

Class IV. CTENOPHORA.
Order 1. Cydippida.
,, 2. Lobata.
,, 3. Cestida.
4. Reroida.

Appendix to Ctenophora — Cttnoplana
and Gceloplana.

Order 1. Platyctexea.

Appendix (II) to Calenterata — Mesozoa.

Phylum IV. PLATYHELMINTHES.

Class I. TURBELLARIA. -
Order 1. Polycladida.
,, 2. Tricladida.

„ 3. RHABDOtXELIDA.

Class II. TREMATODA.^

Order 1. Monogexetica.
,, 2. Digenetica,

Order 3. Aspidocotylea.
,, 4. Temnocephalea.
U Class HI. CESTODA.

Order 1. Moxozoa.
,, 2. Polyzoa.

Appendix to Platyhelminthes — Class
NEMERTINEA.

Class I. NEMATODA. ^
Order 1. Nematoidea.
,, 2. Nematomorpha.

Class II. ACANTHOCEPHALA.

Phylum V. NEMATHELMINTHES. b

Class III. CHJETOGNATHA.

Appendix to Nemathelminthes — Chiv-
tosomidie, Echinoderidce, and Desmos-
colecidce.

Phylum VI. TROCHELMINTHES. w

Class I. ROTIFi-RA. u

Order 1. Rhizota.
,, 2. Bdelloiua. ;
,, 3. Ploima.
Sub-order a. Illorkata.
,, b. Loricata.

Order 4. Scirtopoda.

Order 5. Trochospha;rida.
„ 6. Seisoxida.

Class II. GASTROTRICHA.

Appendix to Trochelminthes— Dino-
philea and HistriobdeMea.

Phylum VII. MOLLUSCOIDA.

Class I. POLYZOA.

Sub-class I. Ectoprocta.
Order 1. Oymxol;emata.
Sub-order a. Cyclostomata. ■
,, b. Cheilostomata.
,, c. Ctenostomata.

Order 2. Phylactol;emata.
Sub-class II. Endoprocta.
Class II. PHORONIDA.
„ III. BRACHIOPODA.

Order 1. Inarticulata.
2. Articulata.

Phylum VIII. ECHINODERMATA.

Class I. ASTEROIDEA.

Order 1. Phaxerozoxia.
,, 2. Cryptozoxia.
Class II. OPHITJROIDEA.
Order 1. Lysophiurje.
,, 2. Streptophiurje.

Order 3. Cladophiukj;.

,, 4. Zygophiurte.
Class III. ECHINOIDEA.
Order 1. Regularia.

., 2. Clypeastridea.

,, 3. Spataxgoidea.

CLASSIFICATION OF THE ANIMAL KINGDOM xxxvii

1'iiYi.r.M VIII. ECHINODERMATA— continm-tl.

Class IV. HOLOTH UROIDEA.

Order 1. Elasipoda.

,, 2. Pedata.

,, 3. Aroi>A.

Class V. CRINOIDEA.

Sub-class I. Monocyclica.

Sub-class II. Dicyclica.
Class VI. CY8T0IDEA.
,, VII. BLASTOIDEA.
„ VIII. EDRIA8TEE0IDEA.
„ IX. CARPOIDEA.

Phylum IX. ANNULATA.

Class I. CHJETOPODA. y
Sub-class I. Polychaeta. ^
Order 1. Archi-Ch^topoda.
,, 2. Phaxerocephala,
,, 3. Cryptocephala.
Sub-class II. Oligochaeta. V
Order 1. Microdrili.
,, 2. Megadrili.
Appendix to the Chsetopoda-
MYZOSTOMIDA.

Class II. GEPHYEEA.

Order 1. Inermi a.
,, 2. Armata.

Class III. ARCHI ANNELIDA.

„ IV. HIRUDINEA. >

Order 1. Rhyxchobdellida.
,, 2. Arhyxchobdellida.
-Class Sub-order 1. Gnathobdellkbi.

,, 2. Herpobdellida.

Phylum X.

-+/

Class I. CRUSTACEA.

Sub-class I. Branchiopoda. ^
Order 1. Axostraca*

,, 2. NOTOSTRACAt

,, 3. coxchostraca.
,, 4. Cladogera.
Sub-class II. Ostracoda.
,, III. Copepoda.
Order 1. Eucopepolia.
,, 2. Branchiura.
Sub-class IV. Cirripedia.
Order 1. Eucirripedia.
,, 2. Rhizocephala.
Sub-class V. Halacostraca.
Order 1. Mysidacea.
,, 2. Cumacea.
,, 3. Taxaidacea.
,. 4. Isopoda.
,, o. Amphipoda.
Sub-order 1. Macrura.
,, 2. Anomura.
,, 3. Brachyura.

Appendix to Crustacea — Class TRI-
LOBITA.

Class II. ONYCHOPHORA* ^
„ III. MYRIAPODA. ^
Sub-class I. Progoneata.

APvTHROPODA.

Order 1. Pauropoda.
,, 2. Diplopoda.
,, 3. Symphyla.
Sub-class II. Opisthogoneata.
Order 1. Chilopoda.
Class IV. INSECTA.
Order 1. Aptera.

>>

.£. UKTHUlThliA.

3. Neuroptkua.

>>

4. Hemiptera.

tt

5. DlPTERA.

>>

6. Lkpidoptera.

))

7. Coleoptera.

JJ

8. Hymexoptera.

Class V.

ARACHNIDA. ^

Order 1. Scorpioxida.

? >

2. PsEUDOSCORPIOXIDA.

>>

3. Pedipalpida.

>>

4. SoLPUGIDA.

>>

5. Phalangioa.'

>>

6. Araxeida.

ii

7. Acarida.

)>

8. XlPHOSORA.

>>

9. ErRYPTERIDA.

Appendix to the Arachnida — The

Pycxogoxida, Lixguatclida, and Tar-

DIGRADA.

xxxviii CLASSIFICATION OF THE ANIMAL KINGDOM

Phylum XL MOLLUSCA.

Class I. PELECYPODA.

Order 1. Protobraxchia.

,, 2. FlLIBRANCHIA.

„ 3. PsEUDO-LAMELLIBRAXCHIA.

,, 4. EULAMELLIBRAXCHIA.

Sub-order a. Integripalliata.
,, b. Sinupal/iata.

Order 5. Septibraxchia.

Class II. AMPHINEUEA.
Order 1. Placophora.
,, 2. Aplacophora.

Class III. GASTEOPODA.

Sub-class I. Streptoneura.
Order 1. Aspidobraxchia.
Sub-order 1. Docoylossa.
,, 2. Rhipidogloasa.

Order 2. Pectixibraxchia.
Sub-order 1. Platypodia.
,, 2. Heteropoda.
Sub-class II. Euthyneura.
Order 1. Opisthobraxchia.
Sub-order 1. Tectihranchia.
,, 2. Nudibranchia.
Order 2. Pulmoxata.

Appendix to the Gastropoda-
SCAPHODA and RHODOPE.

Class V. CEPHALAPODA.

Sub-class I. Dibranchiata.
Order 1. Decapoda.
,, 2. Octopoda.
Sub-class II. Tetrabranchiata.

-Class

Phylum XII. CHORDATA.

Sub-phylum I. ADELOCHORDA.

Class ADELOCHOEDA.

Sub-phylum II. UROCHORDA.

Class UEOCHOEDA.

Order 1. Larvacea.
,, 2. Thaliacea.
Sub-order a. Cyclomyaria.
,, b. Hemimyaria.

,, <:. Pyrosomata.
Order 3. Ascidiacea.

Sub-order a. Ascidin' simplices.
,, b. Ascidiw compotitiv.

Sub-phylum III. EUCHORDA.
Section I. Acrania.

Section II. Craniata.

Class I. CYCLOSTOMATA.

Order 1. Petromyzoxtes.
,, 2. Myxinoidei.

Class II. PISCES.

Sub-class I. Elasmobranchii.

Order 1. Cladoselachil

,, 2. Pleuracaxthei.

,, 3. Acaxthodei.

4. Selachii.

Sub-order a. Protosdachii.
,, b. Euselachii.

Section o. Squalida.
,, 0. Rajida.

Sub-class II. Holocephali.

,, III. Teleostomi.

Order 1. Crossopterygil
,, 2. Chondrostei.
,, 3. Holostei.
,, 4. Teleostei.
Sub-order a. Physostomi.

,, b. Anacanthini.

,, c. Acanthopteri.

, , d. Pharynyoijnath i.

,, ft. Plectognathi.

,, f. Lophobranchii.

Sub-class IV. Dipnoi.

Order 1. Moxopxeumoxa.
,, 2. Dipxeumoxa.

Appendix to Pisces — The Ostracodermi.

Class III. AMPHIBIA.
Order 1. Urodela.
,, 2. Antra.
,, 3. Oymxophioxa.
4. Steoocephala.

CLASSIFICATION <>F TIIF ANIMAL KINGDOM

Phylum XII. CHORDATA— continued.

Order 16. Pterooletks.

17. Columba:.

18. Psittaci.

19. Striges.

20. Picarias.

21. Passeres.

Class VI. MAMMALIA.

Sub-class I. Prototheria.

„ II. Theria.
Section A. Metatheria (Marsupiau a).
Order 1. Polyprotodontia.

„ 2. DlPROTODONTIA.

Section B. Eu theria.
Order 1. Edentata.
,, 2. Cetacea.

Sub-order a. Mystacoceti.
,, b. Odontoceti ■
Order 3. Sirenia.
,, 4. Unoulata.
Section 1. Ungulata vera.
Sub-order a. Perissodactyla.
,, b. Artiodactyla.
Section 2. Subungulata.
Sub-order a. Hyracoidea.
,, b. Proboscidea.
Order 5. Carnivora.

Sub-order a. Carnivora vera.
,, b. Pinnepedia.
Order 6. Rodentia.
,, 7. Insectivora.
,, 8. Chiroptera.
Sub-order a. Megachiroptera.
,, b. Microchiroptera.
Order 9. Primates.
Sub-order a. Prosimii.

,, b. Anthropoidea

Class IV

. KEPTILIA.

Order 1. Sqcamata.

Sub-order a. Lacertilia.

., b. Ophidia.

„ c. Pythonomorpha

Order 2. Rhynchocephalia.

>>

3. Chelonia.

99

4. Theromorpha.

>t

5. Crocodilia.

99

6. Sauropterygia.

99

7. ICHTHYOSAURIA.

99

8. DlNOSAURIA.

99

9. Pterosauria.

Class V.

AVES.

Sub-class I. Archaeornithes.

Sub-cl

iss II. Neornithes.

Division A. Eatitas.

Order 1. Meoistakes.

,,

2. Apteryges.

99

3. DlNORMTHES.

,,

4. Rheas.

99

,"). Struthiones.

99

6. iEPYORNITHES.

,,

7. Gastornithes.

Division B. Carinatas.

Order 1. Stereornithes.

99

2. Odonto:lCjE.

99

3. ICHTHYORNITHES.

99

4. Pygopodes.

99

5. Impennes.

,,

6. TlTRBINARES.

,,

7. Steganopodes.

,,

8. Herodiones.

99

9. Anseres.

91

10. AOCIPITRES.

9>

11. Crypturi.

99

12. Gallin.k.

99

13. GRALLiB.

99

14. Gavia;.

99

15. L.IMICOLA5.

u

Zoology Vol. I.

u»

ERRATA.

17, description of Fig. 3, 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 1,/or "out" read "outer."

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

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

402, line 43, for " peripha>mal " read " perihaemal."

450, line 7, for "Fig. 346" read "Fig. 347."

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 Linnaeus.

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

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 Felis
domestica, the European Wild Cat F. catus, the Leopard F. pardus,
the Tiger F. tigris, the Lion F. leo. 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 Fclis. 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, Cynaelurus, 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 Cynaelurus.
The varying degree of difference is expressed in classification by
placing the Hyaenas in a separate family, the Hywnidai, while
Felis and Cynaelurus are placed together in the family Fclidce.
Similarly, the Civets and Mongooses form the family Viverridce ;
the Dogs, Wolves, Jackals, Foxes, &c, the family Canidoe ; 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 Carniwra,
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 Reptilia; 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 most 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-
tucfii 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
M(Mu90a, Polypes and Jelly-fishes the phylum Goelentcrata. 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 — Chordata,
Class — Mammalia.

Order — Carnivora.
Family — Felida:
Genus-r- 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
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
lorms. 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 duo to the action of the
internal heat of the globe, and which originate below the surface
and arc not arranged in layers or strata ; (2) Aqncons or sedimentary
rocks, which arise by the disintegration, at the surface of the earth,
of pre-existing rocks, the fragments or debris being carried off. by
shea ins 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
SKOA tsion 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 si/stems 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.

f 13. Quaternary and Recent.

III. Cainozoic or Tertiary. . <! ,.' «■

10. Eocene.
9, Cretaceous.
II. Mesozoic or Secondary . ( 8. Jurassic.

I. Palaeozoic or Primary

7. Triassic.
6. Permian.
5. Carboniferous.
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, 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 palaeontology we get a department of Zoology called Phyto-
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 " phylogenies " 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 bathy metrical 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 (abyssal species), while others (friagic
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 Holarctic Region includes the whole of Europe, Asia as
tar smith as the Himala}\is, Africa north of the Sahara, together
with the corresponding portion of Arabia, and North America as
tar si unli as Mexico. For convenience of reference it is often
customary to divide this region into two: its Eurasian portion is
thru called the Palcvarctie, its American portion the Ncarctic
region.

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

.'}. The Oriental Region includes India, Ceylon, South China,
the Malayan Peninsula, and what are known as the Indo-Malavan
islands, i.e. those islands of the Malayan Archipelago which lie to
the west of a line — called Wallaces 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

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 Amceba 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 {pscl.), 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. vac. contractile vacuole ; nu. nucleus ;
psd. pseudopods. (From Parker's Biology,
after Gruber.)

i STKI< II |;k AND PHYSIOLOGY OF ANIMALS II

watching shows that the Amoeba is also, with extreme slowness,
nhnngiTig 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, ~]5reservingr'a~~"siiarp contour* 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 watery 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 hulk of the Amoeba, which, if food par-
ticles are absent from the water, gradually diminishes.

Accompanying the degradation, or destructive metabolism ^,s 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

3&ary agent in the process of destructive metabolism, and

■■■'9 - •;, .-.' ~ V' &1:$ffc&.

:'A

\%

Fi<;. 2. -Amoeba polypodia in successive phases of division The light spot is the contractile
vacuole ; the dark the nucleus. (From Lang's Text-Book, after P. 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-
cesa 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 Anioeba 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 repro-
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 aricTto 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-
ments 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-

— iejise living beihgs~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
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-

Fio. 3. — Diagram to Illustrate the alveolar theory of
protoplasm. (After Dahlgren and Kepner.)

16

ZOOLOGY

Fi

4. — Diagram to illustrate the reticular theory of
protoplasm. (After Dahlgren and Kopuer.)

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 was 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,
similaTTcTthe nucleus of Amoeba, and usually of rounded shape, with
the appearance of being enclosed in a thin nuclear membrane
(A, nu.m), 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 nucleolus
(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 haryohinesis 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 chromatin 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 ; dir. chromatin ; rpl. cell-plate ; mi', nucleoli ; nu. m. nuclear membrane ; *. atrosphere ;
*p. spiudle. (From Parker's BMogy, after FIcmming, Habl, &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 cytoplakmic
structures — i.e. structures belougiug to the body of the cell — as well.

*..

18 m ZOOLOGY sect.

% termed the attraction-sphere or astrospltere (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 evils a narrow septum ; this is known as the cell-plate (I., c.pl.). *
But a cell-plate is not of general occurrsnce in the division of tlje
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 multicellular 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 ceTT(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-
nucleus (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

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

20

ZOOLOGY

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

microti
1*\

9 fir on

segrtad

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 ; B, formation
of segmentation-nucleus ; 9 cent, female centrosome ; 3 «»<• male centrosome ; mem. egg-
membrane ; microp. micropyle ; pol. polar bodies ; ? pron. female pronucleus ; <$ pron. 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
I 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 gamogencsis, 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 t« 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 tobe 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 minus its centrosome, and 'plus
the sperm with its nucleus and centrosome.

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

Fig. 8. — Various stages in the segmentation of the ovum. (From Gegeubaur's Comparative

Anatomy.)

segmentation) or only a part (mcroblastic 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

„ arcfv

ABC

Fig. 9.— Gastrulation.

arch, archenteron ; hi. blastopore ; ecto. ectoderm ; endo. endoderm.

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
singLe layer — the embryo (Fig. 9, A) assuming the form of a hollow

STRICTURE AND PHYSIOLOGY OF ANIMALS

23

sphere, the blastosphcre or blastula, with a wall composed of a
Bingle layer of cells enclosing a cavity — the segmentation cavity
or blastoccele.

( tee side of the hollow blastula next becomes pushed inwards or
invaginatcd (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
gastrulatian 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 a

archenteron or primitive digestive cavity ;
the opening is termed the blastopore, the
outer layer of the wall of the cup is the
ectoderm (or epiblast), the inner the endoderm
(or hypoblast). 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 mesoderm
(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.

Pio. 10

GaBtrula in longi-
tudinal section ; a,
blastopore ; 6, arch-
enteron ;c, endoderm;
d, ectoderm. (From
Gegenbaur'8 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

(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
sy £ 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 siikgle
layer of cells (a-h) ^k
may be many-layered S
(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, b) ; 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.

Fio. 11 — Various forms of epithelium, a, ciliated epi-
thelium ; b, columnar ; c^, surface view of the same ;
p, tesselated ; e, the same from the surface ; /, tlagel-
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.)

STIM'CTURE AND PHYSIOLOGY OF ANIMALS

2E

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 ((land, which is then termed a, unicellular gland
(Fig. 1 2, 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 (By. In other cases the gland is multicellular — 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. I ,

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 (ef), 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

!i av V present among the others, and may be so numerous as to

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
(duets) ; C, group of gland-cells ; D,
group of gland-cells lining a depression ;
B and F, simple multicellular gland ;
0, branched multicellular gland. (From
Lang.)

26

ZOOLOGY

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

Si J®

&

r.)

j>"~sA^'t^^

Fig. 13.— Gelatinous connective tissue of a Jelly-fish ; e, epithelium ; g, gelatinous matrix
b2, branching cells ; ef, 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

Via. 14. — Reticular connective tissue. (From Lang.)

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

STIMCTURE 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

Fio. 15.— Fatty tissue ; F, fat-cells ; B, connective-tissue fibrils. (From Lang, after Ranvier.)

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

In bone 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 bone-corpuscles, lie

Fio. 16.— Hyaline cartilage.

Fid. 17.— Fibro-cartilage.

in minute spaces — the lacunae — 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

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-siibstanee —
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 branched, 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

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

STRiriTRE AND I'll YSH )|,( ><; V OF UTCMALB

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

Fig. 19. — Xon-striated muscle-coll ; /, 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 nervc-jibres (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 oieuraxis (A, ax) — which is the

2 As

6

Fio. 20. — Striated muscle. A, part of a muscular fibre of a Frog; B, portion of striated muscle
teased out to show separation Into fibrillm. (From Uuxlcy's Lessons in Physiology.)
b, d, g, transverse bands and striaj ; ft, nuclei.

essential part of the fibre and is made up of numerous extremely
fine primitive fibrillar ; this is surrounded by a layer of a white
glistening material — the white substance of Schwann or medullary
sheath (mrd), 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.

irmed
<\x i near

Fig. 21.— Nerve-cells. A, multipolar ;
B, bipolar.

Fig. 22. — Nerve-fibres. A, mcdullated ;
B, non-medulated ; ax, neuraxis ;
ined, medullary sheath ; neur,
neurilemma.

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 (hcemoglobin) 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, en-
closes a large nucleus (germinal
vesicle) and in the interior of that
one or more nucleoli or. germinal
spots. The sperms (Fig. 23) are
extremely minute bodies, nearly
always motile, usually slender and whip-like, tapering towards
one extremity, and commonly with a rounded head at the other.

Fig. 23. — Various forms of spermatozoa.
a, of a Mammal ; 6, of a Turbellarian
worm ; c, and d, and e, of Nematode
worms ; /, of a Crustacean ; g, of a
Salamander ; h, the commonest form
with oval head and long flagellum.
(From Lang's Comparative Anatomy.)

The spc

nnrtoin

STKl "CTIKE 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 frequentty 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 exoskelcton {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 exoskelcton or
external skeleton ; in the latter an endoskeleton or internal skeleton.
In many groups of animals both systems are developed. An
exoskelcton 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 )evers 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 ^*-tain glands.

In the lowest groups in which a distinct alimentary ov 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 buccal cavity, a pharynx,
an oesophagus 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

SUM CTURE AND PHYSIOLOGY OF ANIMALS

33

tht 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 stomach, 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

Fk;. 24.— General view of the viscera of a male Frop, from the right side, a, stomach ; b, urinary
bladder; c, small intestine ; cl, cloacal aperture; d, large intestine; e, liver;/, bile-duct;
»/, gall-bladder ; A, spleen ; I, lung ; A-, larynx ; /, fat-body ; m, testis ; n, ureter ; o, kidney ;
p, pancreas ; *, cerebral hemisphere ; *p, spinal cord ; t, tongue ; u, auricle ; vr, urostyle ;
r, ventricle ; v», vesicnla seminalis ; ic, optic lobe ; x, cerebellum ; y, Eustachian recess ;
z, nasal sac. (From Marshall.)

mainly for the passage to the anal opening of the fmces 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, h), and of taking minute
particles of food into their interior to become digested and absorbed
(intracellular 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

V

SECT.

line special little pouch-like, simple or branched glands, opening
by a passage or duct 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
bile — 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 blood-vessels .

The essence of the process of respiration, as we have alreadyseen,
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 branchial ;
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 tracheal, 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 sj^stem 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 PHYSTOLOCiY 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 Amceba, 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

Fia. 25. — Bonos of the human arm and fore-arm with the biceps muscle, showing the shortening
and thickening of the muscle during contraction 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.

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 train. 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

Fig. 26. — Nervous system of the Frog.
Howes's At Ian.)

(From

i STRUCTURE AND PHYSIOLOGY OF ANIMALS M

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, iheiv 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, cars 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 car 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 moncecious 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
dioecious 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,
spermiducts, or vasa defer entia. 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 noticed 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, bd) 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 zooids. In some cases such a colony is produced
by a process which is more correctly termed incomplete fission
than budding.

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

STRUTUiE AND I'll YsrOLOfJY OF ANIMALS

41

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

Fir,. 27.— Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted,
.showing multiplication by budding. M.1 Id* Od.'S 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, AB) 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

— c

d-

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

Fig. 29. — Radial symmetry. Letters as
in Fig. 28. 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 antimcres,
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 4:]

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 hemiaomes by a median vertical plane passing through the
primary axis. This is the bilateral symmetry observable in all but
a few types 6f animals.

Sometimes the bilaterally symmetrical animal is unsegmented ;
xunetimes it is divided into a series of segments or metamercs.
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 or 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-
brata 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, t

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 Arthrcpoda. 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. Ccelenterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Nemathelminthes XI. 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.

o

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 plasmodvum.

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. Mycetozoa. — Terrestrial Protozoa in which the plas-
modial phase is specially characteristic, as also is the formation
of large and often complex cysts.

Class 3. Mastigophora. — Protozoa in which the flagellate form

46 ZOOLOGY sect.

is predominant, although the amceboid 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 amceboid.

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.

Amoeba 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 ^ 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 (mi), 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,f.,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.

Amoebae 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
chitinoid 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 binary fission ; direct or
amitotic division of the nucleus is followed by division into two of
the cell-body (i). Occasionally two Amcebse have been observed to

PHYLUM PROTOZOA

47

conj 11 n«t<- or undergo complete fusion, but nothing is known of the
result of this process or of its precise significance in this particular

cast'.

Fio. 30.— Amoeba. A, A. quarta ; B, the same killed and stained ; C, A. proteus ; D, encysted
specimen ; E, A. proteus ;"T\ nucleus of same, stained ; G, A. verrucosa ; H, nucleus of same,
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pscudopods ;
c. vac. contractile vacuole ; cy. cyst ; /. vac. food-vacuole ; nu. nucleus (numerous in
A. quarta) ; psd. pscudopod. (From Parker's Biology, after Leidy, Grubcr, 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 Amoeba,
belonging to the family Amoebidoz, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell, among the Amoebidse.. 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-

PHYLUM PROTOZOA

49

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

Fig. 81 .— Protamoeba primitiva. Showing changes of form and three stages in binary fission.
(After Haeckel, from Parker's Biology.)

Protamaiba (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.

#^^

Fig. 32.— A, Quadrula symmetrica; B, Hyalosphenia lata; C, Arcella vulgaris ;
I), Difflugia pyriformis. (From hang's OtmparatiK Anatomy.)

Skeleton. — We may understand the relation of the shelled to
the shell-less Lobosa by supposing an Amceba 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 a

50

ZOOLOGY

kECT.

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 Dijjlugia
(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 binary fission, with one of the daughter-cells creeping out of the shell ;
D, fiagellula ; 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

n PHYLUM PROTOZOA 51

fiflagrfhihi, is. a cell provided with one or more flagcl la, 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)

c- vac

sh

Fia. 3 1.— Platoum stercoreum. A, single zooid ; B, formation of colony ; e. vac. contractile
vacuole ; /. food particles ; nil. nucleus ; sli. shell. (From BUtschli'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 (sh.) 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. 35, #) 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 formirig"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
MKJrture. 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, 8), or alternately on opposite side3 of the original
chamber (5), or with each new chamber enclosing its predecessor
(.£), 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

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

''■III Wm\\\ \\\\\

Roralla

3.Squammulina

4.M i I i o I a

Fia. 35. -Various forms of Foraminifera. In U, Miliola, a, shows the living animal ;
6, the same killed and stained ; a. aperture of shell ; j. food particles ; nu. nucleus ; sh. 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

„ 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 .La gen a

4.Frondicularia 6.Globigerina

Q.PIanorbulina

ll.Nummulites

Fig. 3f>.— Shells of Foraminifera. In 3, /<, and 6, a shows the surface view, and b a section ;
8a is a diagram of a coiled cell without supplemental skeleton ; 8b of a similar form
with supplemental skeleton (n. nl\) ; and JO of a form with overlapping whorls ; in 11a half
the shell is shown in horizontal section ; b is a vertical section ; <t. aperture of shell ; 1 — 16,
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

64 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, Jf), or 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, £).

In many cases additional complexity is attained by the develop-
ment of what is called the supplemental skeleton (Fig. 36, 8b, s. sh).
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

Fig. 87. — Hastigerina murrayi. plsm. vacuolated protoplasm surrounding shell: psd.
pseudopods ; sh. shell ; sp. 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, Caving 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

THYLUM PROTOZOA

ft

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

***** J? I

^

Fio. 3R.— Dimorphism and alternation of generations in Poly stome 11a crispa. The arrows
indicate the direction of the life-cycle. A, young megaspheric individual ; B, full-grown
mega8pheric individual, decalcified ; C, megaspheric individual in the act of spore-formation,
the: protoplasm leaving the shell in the form of flagellulae ; D, fiagellula more highly
magnified ; B, microspheric individual developed from a fiagellula ; F, microspheric individual
in the act of producing amoeboid embryos. (From Lang, after Schaudinn.)

(sh.) 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 flagelluke — each provided with a
flagellum. In some cases the flagellulae 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 Globigerinse contained
in it.

From the palaeontological 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,

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,
dining 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-
sorbed, and their substance envelops
it, enclosing it in a vacuole. The
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

Fio. 3&— Actinophrys sol. o. axial
filaments of pseudopods ; n. nucleus ;
p. pseudopod. (From Lang's Com-
parative Anatomy, after Grenadier. )

Fio. 40.— Actinosphaerium eichhornli. A, the entire organism ; B, a small portion
highly magnified ; chr. chromatophore ; cort. cortex ; c. vac. contractile vacuole ; incd. medulla ;
n«. nuclei. (From BUtschli's Protozoa, after Hertwig and Lesser.)

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

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

58

ZOOLOGY

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

1.R aphid lofjhrys

2.Nuclearia

B.CIarhrulina

Fig. 41. — Various forms of Heliozoa. 3a, the entire animal; 3b, the flagellula ; e. vac.
contractile vacuole ; g. gelatinous investment ; nu. nucleus pad. pseudopods ; tie. siliceous
skeleton ; ap. spicules. (From Btitschli'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, 2,g.), 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 ;
f), 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 ; 3, nucleus ;
3, spindles concerned in maturation ; U, 5, outer and inner layers of cyst ; 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. Actinosphaerium, 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

60

ZOOLOGY

SECT.

throughout the winter, and in spring the protoplasm emerges
from each and assumes the form of the ordinary active Actino-
sphserium. In Clathrulina spore-formation takes place in the
active condition, and the spores (Fig. 41, 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
the 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).

Steel.

'Int. caps.jir
cent caps

. caps.pr.

Order 4. — Radiol aria.

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 (mi.), or sometimes
many nuclei : from the ex-
tracapsular 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 Aeantharia — 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

Fig. 43.— Lithocircus annularis, cent. caps.
central capsule ; ext. caps. pr. extra-capsular
protoplasm ; int. caps. pr. intra-capsular pro-
toplasm ; nu. nucleus ; psd. pseudopods ; si-el.
skeleton ; z. cells of Zoochlorella . (After
Btitschli, from Parker's Biology.)

THYW'.M I'KOTO/OA

61

r/i-

the perforated calcareous shell of Hastigerina with its investment
of vacuolated protoplasm. It is found in its simplest form in
Thulassoplancta (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 phccodium {ph.), prob-
ably partly of the nature of excreta. The
central capsule encloses, in addition to
the nucleus or nuclei, oil -drops, vacuoles,
protcid 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
Actinomma (Fig. 46), in which there are three concentric per-
forated spheres (A, sk. 1,8k. 2,sk. 8) 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 Collozoum 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 ; vt. oil drops ; en.
calymma ; rp. protoplasm
surrounding calymma ; «.
spicules. (From Lang's
Comjyarati cc A aatomy, after
Haeckel).

62

ZOOLOGY

ii ,""Vi

Si

Jl —

//A,— "-:

Pig. 4 j.— Aulactinium actinastrum. c. calymnia ; fan. central capsule ; n. nucleus ; op
operculum ; ph. phseodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

Fio. I. Actinomraa asteracanthiou. A, the shell with portions of the two outer
spheres broken away ; B, section showing the relations of the skeleton to the animal ;
cent. caps, central capsule ; ex. caps. pr. extra-eapsular protoplasm ; ntt. nucleus ; sk: 1, outer,
sh. 2, middle, sk. 3, inner sphere of skeleton. (From Butschli's Protozoa, after Haeckel and
Hertwig.)

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

C

Fie. 47. -Collozoum inerme. A— C, three forms of the entire colony, nat. size ; D, a small
colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with
vacuoles (vac.) ; E, spores containing crystals (c.) ; F, mega- and microspore. (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 (inegaspores). 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, z.) which multiply by fission
independently of the Radiolarian. It has been proved that these
are unicellular organisms, sometimes regarded as plants (Class
Alga?), sometimes as animals (Class Mastigophora of the Protozoa),
and named Zoochlorcllo3. This intimate association of two organisms
is called symbiosis : it is probably a mutually beneficial partner-
ship, the Radiolarian supplying the Zoochlorellae with carbon
dioxide and nitrogenous waste matters, while the Zoochlorellae

64

ZOOLOGY

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 Labyrinthula. .

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

Fig. 48.— Chlamydomyxa labyrinthuloides. A, active phase; c.w. cell-wall;/, frag-
ment of Alga ingested as food; sp. spindles in course of pseudopod,; B, resting-stage —
numerous individuals in the cells of a fragment of Sphagaum ; a, specimen completely enclosed
in cell ; 6 and c, specimens which have emerged through the ruptured cell-wall ; C, specimen
multiplying by budding; 1), by binary fission ; K, by internal fission. K may represent a
stage in spore-formation. (A after Archer, B— K after tieddes.)

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

PHYLUM PROTOZOA

65

numerous non-nucleated protoplasmic bodies or chromatophores, containing
chlorophyll and a green or brown colouring matter in varying proportions.
Thriv 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
finally 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 (D) ; 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 Alga3
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
unite to form larger masses of protoplasm at a considerable distance from the
original cell. At the same time the bluish spheres (ep.) 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

Fig. 40. — Xjabyrintbula 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 ; 1111, nucleus.
(From BUtschli's Protozoa, after Cienkowsky.)

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 contauiing two nuclei. These give rise to flagelluloe, the further history of
which has not been traced.

Labyriidhula (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-
domyxa. 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.

Fia. 50 — Didymium difforme. A, two sporangia (spg. 1 and 2) on a fragment of leaf(?.).

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

C, a flagellula with contractile vacuole (c. rac.) and nucleus (»«.)• D, the same after loss
of flagellum ; 6, an ingested Bacillus. E, an amcebula. F, conjugation of amcebulee to form
a small Plasmodium, G, a larger Plasmodium accompanied by numerous aincebulse; sp.
ingested spores, (4\fter Lister.)

CLASS II.— MYCET0Z0A.

1. Example of tke Class — Didymium 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-

PHYLUM PROTOZOA 67

ments 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, tpff, 1, spg. 2) is therefore not a mere resting capsule, like that of Amoeba,
but a sporangium or spore-case. Its wail 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,
winch 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 (tttt.), and a contractile vacuole
(c. vac.). The flagellulre move freely and ingest Bacilli (D, />.), and multiply by
fission : then, after a time, they become irregular in outline, draw in the
flagellum, and become amoeboid (E). The amoebube 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 amoebulae. Thus Didymium in its active condition is a
jAasmodium, i.e. a body formed by the concresence of amoebulae.

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 ai'e 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 flagellula?, but soon become amoeboid and fuse to form the plasmodium.

CLASS III.— MASTIG0PH0RA.

1. Example of the Class — Euglena 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, ces.), from the\ inner surface of which

F 2

68

ZOOLOGY

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 jin the outer
layer of the protoplasm may be due to the presence of contractile
fibrils. There is a nucleus {mi.) near the centre of the body, and
at the anterior end a contractile vacuole (H, c. vac), leading into

Fig. 51. — Euglena viridis. A — 1), 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 II, reservoir in E and F ; c//. cyst ; fl. 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 i2aramykjjit (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^inaLLprobabiJity, 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 flagellulae 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 1. — 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 Eit.glcnidce, 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
I 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 Mastigamceba (4-) has 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, Dimorpha (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 (b).

Nuclei of the ordinary character are universally present. In
addition there is present in the cytoplasm near the base of the
flagelluma muchmore minute, deeply-staining body, which is termed
the blepharoblast (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, IS), two (9, 10), three (6), or four (7).
Sometimes the flagella show a differentiation in function ; in

PHYLUM PROTOZOA

71

Hrfrromita, e.g. (Fig. 57) the anterior flagellum (Jl. 1) only is
use. I in progression, the second or ventral flagellum (fl. 2) is trailed

7.Tetram\ta
ff.Dallingeria

8.oiK<

9.Cryf>romooaS 10.Diblomirc

ft

il.Dinobryon 12. Sy ncry p t a 13. Anthobhysa H.Rhibidodendron

Flo. 52. — Various forms of Flagellata. — In 2, flagellate (a) and amoeboid (b) phases are
shown ; in 5, flagellate (a) and helio^oan (b) phases ; in 8 are shown two stages in the in-
gestion of a food-particle (/.); clr,\ chromatophores ; c. vac. contractile vacuole ; /. food par-
ticle g. gullet; n». nucleus ; I. lorica ; p. protoplasm ; per. peristome ; v.i. vacuole of ingestion.
(Mostly from BUtschli'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
Hwmoflagellata, 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

Fio. 53. — Trypanosomes of Fishes, c. blepharoblast ; /. flagellum ; fa, and fp. (In k) 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 (cm.),
and contains chromatophores (chr.) coloured either green by chloro-
phyll or red by haematochrome. 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 Algse 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.

PHYLUM PROTOZOA

73

hi Euglrna, as wc 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

Jto tn.m

Fin. 54.— Hsematococcus pluvialis. A, motile stage ; B, resting stage ; C, D, two modes
of fission ; E, Hcrmatococcv.s lacustrif, motile stage ; F, diagram of movements of fiagellum ;
chr. chromatophores ; c. vac. contractile vacuole ; c.ie. cell-wall; nw. nucleus; nu'. nucleolus;
?'."'"• pyrenoids. (From Parker's Biolnriii.)

marine Flagellates have siliceous skeletons similar to those of the
Radiolaria, 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 Rhipido-

74

ZOOLOGY

SECT.

dcndron (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.
Hoematococcus (Fig. 54) and Euglena (Fig. 51), for instance,
divide while in the encysted condition ; Heteromita (Fig. 57)

Fio. 55. — Pandorina morum. A, entire colony; H, asexual reproduction, each zooid
dividing into a daughter-colony ; C, liberation of garhetes ; D — P, three stages in conjugation
of gametes ; G, zygote ; H- -K, development of zygote into a new colony. (From Parker's
Biology, 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
Dinobryon (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

i:

PHYLUM PROTOZOA

75

which they are finally liberated. In Vokn.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
(Dl-D5), 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

Fio. 5(5.— Volvox globator. A, entire colony, enclosing several daughter-colonies;
B, the same during sexual maturity ; C, four zooids in optical section ; D*— D8, develop-
ment of parthenogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and
sperms ; H, oosperm ; a, parthenogonidia ; #. flagellum ; or. ovum ; on/, ovaries ; pff. pigment
spot ; ipij. spermaries. (From Parker's Biology, after Cohn and Kirchner.)

(Fig. 57, E1) and undergo complete fusion (E2 — E4) : the result of
this conjugation of the two gametes or conjugating cells is a thin-
walled sac, the zygote (E5), the protoplasm of which divides by
multiple fission into very minute spores. These, when first
liberated by the rupture of the zygote (E6), are mere granules,
but soon the ventral or trailing flagellum is developed, and after-
wards the anterior flagellum (F1 — F4). 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).

ZOOLOGY

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

nu c.vac

Fia. 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 ; U, fission of free-swimming form ; B, conjugation of free-swimming with anchored
form ; E5, zygote ; E<>, emission of spores from zygote ; F, development of spores ; fl.l, ante-
rior ; fl.S, ventral flagellum. (From Parker's Biology, after Dallinger.)

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

PHYLUM PROTOZOA

77

we find in the higher animals. Certain of the zooids enlarge and
form megagametea (B, '<//.), 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 agg. '

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., dcvcloirmcnt, the young organism being
far simpler in structure than the adult, and reaching its final form
by a gradual increase in complexity.

LMonosiga 2.Salpinao e ca.

3.Polyoeca. 4.Proterospongia.

Vic. f>S.— Various forms of Choanoflagellata. 2b illustrates longitudinal fission ; 2c, the pro-
duction of tiagellulw ; c. collar ; c. vac. contractile vacuole ; /t. flagelium ; I. lorica ; nu.
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 collar (Fig. 58, 7, c), surrounding
the base of the single flagelium (/I.). The collar is contractile, and,
although its precise functions are not 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 nagellum and assume an amoeboid form.

The nucleus (mt.) 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
Polyceca (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 Protcrospongia (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. gr. ), 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 nagellum (fl. 1),
which is directed forwards and serves as the chief organ of propulsion ; a second
nagellum (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 diatomin. 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 nematocysts (a, b) in the protoplasm, resembling
those of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes,
distinguished as meganuclei {nu. ), and micronuclei (nu'.).

PHYLUM PROTOZOA

79

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

Cleno dinium

2.Cerarium 3.Prorocentrum

4.Polykrikos

Fio. 59.;— Various forms of Dinoflapellata. 2 shows the shell only; Art is an undischarged,
and b a discharged stinging-capsule ; chr. chromatophores ; jt. 1, longitudinal flagellum ;
fl. ?, transverse flagellum ; I. <jr. longitudinal groove ; ntc. nematocyst ; nu. meganucleus ;
wit', micronucleus ; pg. pigment spot ; t. gr. transverse groove. (From BUtschli'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 gioup includes only two genera, Noctiluca and Leptodiscus. A descrip-
tion of Noctiluca miliaria, 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 (by.), 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 (/. ), 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.

Fie. 60. —Noctiluca miliaria, a. the adult
animal ; b, c. flagellula; ; !>;i. tentacle ; /. flagel-
hun ; m. mouth ; t». nucleus. (From Lang.)

80

ZOOLOGY

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

CLASS IV.— SP0R0Z0A

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 vesiculse 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
zygotes. E, Cyst enclosing ripe spores formcdirom the zygotes. F, single spore, showing
the (8) sporozoitcs 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 nucleolus 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
are 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 zygotes 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 sporozoitcs 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 flagellulae. 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. — H^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 amoeboid, 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 Monocystidat, 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
Oregarina (Figs. 62 and 63), the various species of which are
parasitic in the intestines of Crayfishes, Cockroaches, Centipedes,

Fig. 62. —Oregarina. A, two specimens of 0. blattarum partly embedded in enteric
epithelial cells of Cockroach ; B1, Ii2, two specimens of G. dvjardini ; in B~ the epimerite (</'•)
is cast off ; C, cyst of G. blattarum, from which most of the spores have been discharged ;
D, four stages in the development of G. gifjantea. cy. cyst ; deu. deutomerite ; cp. epimerite ;
(I. gelatinous investment of cyst ; nu. nucleus ; pr. protomerite ; psd. 1, short pseudopod ;
psd. 2, long pseudopod ; sp. mass of spores ; sjkI. sporoducta. (From BUtschli'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

PHYLUM PROTOZOA 83

to the protomerite there is sometimes found, especially in young
individuals, a third division, the cpimerite (ep.), which may be
provided with hooks (B1), 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 (B2), 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 (spd.) in the thickness of the wall,

■13

iio. 03.— Gregarina. Development from the sporozoite. 1, cells of the digestive epithelium
of the host ; 2, nuclei of the same ; 3, spore ; k, spore discharging sporozoites (6) leaving
residual mass (6); 7, sporozoites in the act of entering epithelial cells; 8, the same as
intracellular parasites ; 9-7,', different stages in the growth of the young Gregarines into the
lumen of the intestine ; 13, epimerite ; l/t. protomerite ; 15, dcutomerite. (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 amoebula (D1), with one long and one
short pseudopod (D2) ; 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 (D3, D4.) In
other species of Gregarina the sporozoites do not divide, but each
develops directly into the trophozoite (Fig. 63).

Order 2. — Coccidiidea.

Goccidium (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 blootl-corpuscles. A few are intra-nuclear parasites. Two
distinct modes of multiplication 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 — c) takes place. The nucleus divides to form a number of nuclei.
These migrate towards the surface, and each becomes surrounded by protoplasm,
with 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

lEi

rue n a

2.Coccidium

Fig. 64. — Coccidiidea. A, adult Eimeria (e) in enteric epithelial cell (ep.) of mouse ;
B, encysted form ; C, encysted form, the protoplasm contracting to form a spore ; D, formation
of falciform young (/.) in interior of spore (sp.) ; B, spore with falciform young ; F, adidt
encysted form of Coccidium from liver of rabbit ; G, division into spores ; JI, cyst containing
ripe spores (sp.), each with a single falciform young ; I, single spore with falciform young (/.).
(From Biitschli's Protozoa, after Leuckart and Bimer.>

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 (d), and become converted into either micro- or megagame-
tocytes. Each of the former (h, j) gives rise by division to a number of narrow
biflagellate microgametes or sperms. Each of the megagametocytes (e, f), 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 (zygote or oosperm) divides to form a varying number of cells
each enclosed in a resistant cyst (k). These give rise to spores with a firm, chitinous
spore-membrane, each containing two or more falciform young or sjjorozoiles (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.

PHYLUM PROTOZOA

*5

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

Flo. 65. — Life-History of Coccidium tchubergi. a. penetration of epithelium cell of host by
sporozoite; b-c, stuges of multiple fission (schizogony); d, gametocyte ; t, f, formation of
megagamete (ovum) : g, fertilisation ; h, j, formation of microgamctes (sperms) ; h, 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^mosporidea.

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

ZOOLOGY

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^-O) they live as amoeboid

-^ssmel

Fio. 66.— Life-History of Malaria Parasites. A-G, parasite of quartan fever, showing
development of trophozoite in a blood-corpuscle and the formation of inerozoites ; H,
gametocyte of the same ; I-M, parasite of tertian fever to the formation of the merozoites ;
N, gametocyte ; 0-T, crescentic gametocytes of Laverania ; PS, formation of micro-
gametes or sperms ; V- 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 ; k, 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 hlood of a malaria patient, all stages of the parasite
that oecur 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 — -If) 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 (/, 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 Hpemogregarines, 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 amoeboid
in the trophozoite phase, and which reproduce continuously by
spore-formation during that phase (Fig. 67, A). Many nuclei are present

Fio. 67.— A, Myxidium lieberkuhnii, amoeboid phase; B, Myxobolus mulleri,
spore with discharged nematoeysts {ate); C, spores (psorosperms) of a Myxosporidian ;
nte. nematoeysts. (From BUtschli'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 nematoeysts 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 Sarcocysiis (Fig. 68), which occurs in
the flesh of mammals, each parasite having the form of a long spindle embedded

Fig. 68. — Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From
Biitschli'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 — Paramcecium caudatum.

Structure. — Paramcecium, the "slipper-animalcule," is tolerably
common in stagnant ponds, organic infusions, &c. The body
(Fig. 69) is somewhat cylindrical, about \ mm. in length, rounded
at the anterior and bluntly pointed at the posterior end. On the
ventral face is a large oblique depression, the buccal groove (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 (port.) and
a semi-fluid medulla (med.), and is covered externally by a thin
pellicle or cuticle (pu.) 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. nit,), 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).. m

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

PHYLUM PROTOZOA

89

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

c.rac.

Ttuc. 97:

Fn;.»'"— Paramoecium caudatum A, the living animal from the ventral aspect; B, the

same in optical section : the arrow shows the course taken by food-particles ; C, a specimen

which has discharged its trichocysts ; D, diagram of binary fission ; buc. gr. buccal groove ;

cortex ; c». cuticle ; c. vac. contractile vacuole ; /. vac. food vacuole ; irul. gullet ; med.

medulla; Hi'.meganucleus; 3x1. nu. 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 (C).

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

flgSIU

Fio. 70. —Paramcecium caudatum, stages in conjugation, gul. gullet ; mg. nu. meganucleus ;
mi. nu. micronucleus ; Mg. nu. reconstructed meganucleus ; 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 meganocleus (mg, ///Oof 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 oilia in the young condition, tentacles in
the adult.

Systematic position of the Example.

Paramcecium aurelia is one of several species of the genus
Paramaxium, belonging the family Parmwcidm, 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-
moecidae 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 heterotrichous 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 adoral 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 Epistylis 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 (u. 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 Lophomonas (13), the ovoid body of
which bears a tuft of close-set cilia at its anterior end. Actino-
bolus (11/,) 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, nig. 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 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 umbcllaria, however,
there are found numerous minute capsules (Fig. 71, 9, ntc.)
arranged in pairs, each containing a coiled thread. They are

TtUh

isniHiini,.~, 16.Condvlosl-oma

l8.Trachelius IftOphryoglena

opsis

FlG. 71. — Various forms of Ciliata. 9a shows part of a colony, 6 a single zooid, and
c a couple of nematocysts ; a. anus ; c. (in 18) cuticle ; c. (in 19) excretory canals ; c. vac.
contractile vacuole; /. vac. food vacuole; <j. gullet; mff. nu. meganucleus ; mi. nu. micro-
nucleus ; mth. mouth ; nu. nucleus ; ntc. nematocyst ; p. (in 15) a Paramecium seized by
Didimium ; t. tentacle ; u. mb. undulating membrane ; vac. non-contractile vacuole ; vst.
vestibule. (From Btttschli's Protozoa, after various authors.)

II

1M1YU M 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. — Many 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.

I.Dictyocysl-a

3.Thuricola 4.0|>hryciium
2.Pyxicola

5. S t i chofricha

Fig. 72.— Various forms of Ciliata. In 1 the shell alone is shown ; »». 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, 2) and some other genera there is,
instead of the temporary anal spot described in Paramecium, a
distinct anal aperture (a.).

96

ZOOLOGY

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

Fig. 73. — Vorticella. A, B, living specimens in different positions , C, optical section ; Di, D-,
diagrams illustrating coiling of stalk; E', E'-*, two stages in binary fission; E3, free zooid ;
F1, F-, division into mega- and microzooids ; G1, G-, conjugation ; H1, multiple fission of
encysted form ; H-, H3, development of spores ; ax. f. axial fibre ; cort. cortex ; cu. cuticle ;
c. vac. contractile vacuole ; d. disc ; gull, gullet ; m. mierozooid ; mth. mouth ; nu. mega-
nucleus ; per. peristome. (From Parker's Biology.)

similar structure found in so many of the Mastigophora. Some
(Fig. 71, 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, #, 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 (-5) is

II

PHYLUM PROTOZOA

97

attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (m.), 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. 11, 9a;
Fig. 74). The stem of these may be a purely cuticular structure
and non-contractile (Fig. 71, 9, b), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 73, ax./.). 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

Fig. 74. — Zoothamnium arbuscula. A, entire colony; B, the same, natural size; C, the
same, retracted ; D, nutritive zooid ; E, reproductive zooid ; F1, F"2, development of reproduc-
tive zooid ; ax./, axial fibre ; c. vac. contractile vacuole ; nu. nucleus ; n.g. 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 Paramcecium, 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 (E3), 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. z.) 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 ; wit. 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-
larvai feed upon the water-plants, and in doing so frequently take
in the spores or encysted Opalinae 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 micron uclei, 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 (G1, G2).
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, ia), ovoid (lb), 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

3.Rhy

-1 v w

*•* 4.Dendrocomeres r -.. , \ 6. S pha«r obhrya

nchero _ 5. EbheMa r '

7.0(>hpyodenclron

a.Ef)heloha

9. Dendrosoma

Fig. 70. —Various forms of Tentaculifera. la and 6, two species of Podophrya; c, a
tentacle much enlarged ; Sa, Acineta jolyi ; 2b, A. tuberosa ; in 6 the animal has captured
several small Ciliata ; 8a, a specimen multiplying by budding ; 8b, a free ciliated bud ; 9a, the
entire colony ; 96, a portion of the stem ; 9c, 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 Btitschli 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 VortircUa. 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 (la), 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 (2n) 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.). Micronuclei of the ordinary character are present
as well.

Reproduction by Unary fission takes place in many species.
In Ephelota gerrtmipara (8) 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
tuberosa (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, bd.).

Further Remarks on tlve 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 8,000 to 10,000 feet
above 'sea-level, to a depth of from 2,000 to 3,000 fathoms. Some
forms, such as species of Amoeba and Gromia, 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, Ichthyophthirius, 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 Mesozod, 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-
mceba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 52. # 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 ]><>ssil>l\ be produced by the flowing protoplasm of a
ForaniininT or a Radiolarian. On the other hand, the nucleus
of the Radiolaria is a far more complex structure than that of
the Mastigophora : 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
Mastigamceba 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

Flagellata

Tentaculifera

Ciliata

-Sporozoa
Fio. 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, Multicilia and Lophomcnas (Fig. 71,12 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 secx 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]

i 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 Protogog, or uni-
cellular animals, and the JSlsiazoa or multicellular forms — the latter
comprising all the growfji^ffaT; 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 with —
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 — Syccn gelatinosum.

General External Appearance and Gross Structure. — :

Sycon gelatinosum,1 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 specie
which differs chiefly in the absence of the pore -membranes.

106

ZOOLOGY

( 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

Fio. 78. — Sycon gelatinosum.

— Entire sponge, consisting of a
group of branching cylinders
(natural size).

Fig. 5U Sycon gelatinosum.— A portion slightly
magnified ; one cylinder (that to the right) bisected
longitudinally 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,
dottedn'p. marks the position of three of the groups
of inhalant pores at the outer ends of the incurrent
canals ; o. osculum.

interior 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
fine 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
pass into its substance
through the minute in-
halant pores or ostia
already mentioned as
occurring in groups be-
tween the elevations on 7

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 Sycon is firmly squeezed, there will be
pressed out at first sea- water, and then, when greater pressure is

Fio. 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 ; IC, incurrent canals ;
ov. young ova ; Ji, radial canals ; tp. 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

J

ZOOLOGY

Fig. 81. — Sycon gre latino sum Transverse section
through the wall of a cylinder (parallel with the
course of the canals), showing one incurrent (/C),
and one radial (R) canal throughout their length ;
tp. triradiate spicules ; sp'. oxeote spicules of dermal
cortex (rfc.) ; sp". tetraradiate spicules of gastral
cortex (gc.) ; ec. ectoderm ; en. layer of flattened cells
lining the paragastric cavity ; pm. pore-membrane ;
pp. prosopyles ; ap. apopyle ; 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
Avail of the cylinders are a
large number of regularly-
arranged straight passages.
Of these there are two sets,
those of the one set — the
incurrent canals (Figs. 80j

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.

in PHYLUM AND CLASS PORIFERA 109

and 81 IC) — narrower, and lined by ectoderm similar to the ectoderm
of the surface ; those of the other set — the radial or flagellate canals
(II) — 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 f 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-mcmhranc (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 cxcurrcnt canal (Fig. 81 exc). Incurrent

\ J

■0^:. ,-

Fio. 82.— Sycon gelatinosum. Sur- Fin. 83.— Sycon gelatinosum.

face view of a pore-membrane highly An apopyle surrounded by its dia-

magnified ; p. ostium ; A. position of phrngm ; m. contractile cells,
the outer end of a radial canal.

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 choano-
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.

'i, teased out in sea-.ater, 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. 83), perforated by a
large circular central aperture — the apopyle (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 mcsoglcea — in which the spicules of the skeleton are
embedded. Each spicule is developed from cells termed sclero-
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 and 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

J

in PHYLUM AND CLASS PORIFERA 111

outer surface. The thick outer layer in which the bases of these
V oxeote spicules lie embedded, is termed the dermal cortex (dc). A
thick stratum at the inner ends of the canals and immediately
surrounding the paragastric cavity is termed the gastral cortex (gc).
It is supported by triradiate and also by tetraradiate spicules, one
ray of each of which (ftp") frequently projects freely into the para-
gastric cavity, covered over by a thin layer of flattened endoderm
cells.

The mesogloea 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 collencytes (Fig. fcO, 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, and in the same situation are to be
found developing embryos (em, em'), resembling in their various
stages those of Sycon raphanus, as described below, y

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 ectoderm1 — and the internal cavities, or a
part of them, are lined by a second single layer — the endoderm, —
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 mcsoglaa — 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.

i

\

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 sponginj in others of spongin-fibres supplemented
by microscopic siliceous spicules; in others of siliceous spicules
alone ; in others of spicules"or carbonate of lime. Reproduction
takes place both asexually by the formation of gemmules, 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 Syccttidm ; and the

hi PHYLUM AND CLASS PORIFERA 113

family Sycettidw is one of several families of the order Hetcrocoela
of the class Calcarea. Among the families of the Heteroccela,
that of the Sycettidcc 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 Sycettidcc 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 Sycettidcc 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- 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 lamellae ; 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, B). 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

TIT

PHYLUM AND CLASS PORIFERA

115

has resulted in the formation of a comparatively narrow basal
part or stalk, the Sponge expands distally, growing out into lobes
or 1 tranches 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

B.Psammoclema

D. Poherion

Fio.84 — External form of various Sponges. A, Oscaria, an encrusting form, with the
upper surfac.-.- raised up into a number of rounded prominences; B, Psarumoclema. a
ramifying subcylindrical Sponge ; C, 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. — Symn gelatinosnm 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 Ascctia or Olynthas
(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 far there is a
considerable resemblance to Sycon
gelatinosum ; but the structure of its
wall in Ascetta is extremely simple.
Regularly arranged over the surface
are a number of small rounded
apertures, the inhalant pores; but,
sinoe 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.

Fio. 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.)

Ill

PHYLUM AND CLASS PORIFERA

117

A somewhat more com
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 pares or ostia, and
lead directly into the
paragastric cavity by
means of ape/pyks —
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, U : 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-
rent centals, 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
prosopylcs (the equi-
valents of the inhalant
pores of the Oiynlhus),
with the"*radial canals.
Sponges similar to
Sycon gelatinosum,

plex type of structure than that of Ascetta

L

Li| M

-.&L , 0m / / ml

•i i ■'"' P« \fi

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
sndodena by short parallel strokes. A, cross-section
through ■ part of the wall of an Ascon ; B, cross-section
through a part <>f the wall of a Sycon ; C, cross-section
through a pirt of the wall of Livcilta ctmve&I / />, vertical
ii through OscarMa ; a, spaces of the incurrent canal
system ; b, spices of the excurrent canal system ; ot. oscu-
lum. (After Korseholt and Heider.)

118

ZOOLOGY

SECT.

but with flagellate canals arranged in groups, each group centred
round a main excurrent canal (Fig. 86, G) 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

0 P,G

Ex

Fio. 87. — Vertical section of a fresh-water sponge (Spongllla), showing the arrangement of the
canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; 60. openings
of the excurrent canals ; PG. paragastric cavity ; SD. subdermal cavities ; 0. osculum.
(Modified from Leuckart and Nitsche's diagrams.)

arranged flagellate chambers, are said to possess the Syt'on 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 Bhagon
type of canal-system. In the Ithagon proper the arrangement ox
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 01
zooids similar to the branches of Sycon gelatinosum ; and this effect

in 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, SB) — 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, gc.) 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 ^£ X^S^-'

the gelatinous substance of the **>;:.'■'■ ^^

mesoglcea are embedded connec- FlG 88>_Cells of the 'Jtoderm, very highly

tive-tissue Cells, amoeboid Wan- magnified. (After Von Lendcnfeld.)

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.

120

ZOOLOGY

SECT.

Fio. 89. — Development of a tri-radiate spicule of
Clathrina. scl, scleroblasts. (After Minchin.)

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 scleroblasts (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 Myxospmigia)
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.
These threads branch and
. 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 (Ewplectella), the Glass-Rope Sponge (Hyaloncma),
and Pher/mema (Fig. 92), the skeleton consists throughout of
siliceous spicules bound together by a siliceous cement.

Reproduction in the Sponges is effected either sexually or
asexuallv. The process by which, in all but the simplest forms of
Sponges, a colony of zooids is formed from the originally simple

Ill

rHYIJU AM) CLASS I'OlUFERA

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-

B.Pachychalina

Fir. skj — Microscopic structure of the skeleton in various sponges. A, Eusponeia network
of spongin fibres ; B, Pactaychalina, spongin strengthened by siliceous spicules ; C,
Spongelia, spongin itrangtbeBed 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 amphidiscs (Fig. 91, right side). These gemmules
are formed in the substance of the Sponge towards the end of the

122

ZOOLOGY

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 mesoglcea,
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, Cliona, 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 Ulynthus, the development
has been followed out in the case of Clalhrina 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 archceocytcs, some of which form the repro-

PHYLUM AND CLASS PORIFERA 123

ductive colls. Certain of the flagellate cells then withdraw their
flagella and pass into the internal cavity, becoming amoeboid. Soon

Fio. 92.— Pheronema carpenteri, one of the Huxactinellida.
(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 Ccelenterates (Sect. IV). This is the

124

ZOOLOGY

larval form known as the parenchy inula. 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 Glathrina hlanca, 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 archoeo-
cytes — at the posterior pole. The clear cells become elongated
and flagellate. The archoeocytes pass into the internal (segmenta-
tion) cavity and become completely enclosed by the flagellate
cells (stage of so-called pseudogastrula).

The cells at the posterior end then lose their flagella 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-

2><?c

Fig. 93. — Median longitudinal
section of the parenehyinula
larvaof Clathrinablanca.
p.g.c, posterior granular cells
which give rise to the archseo-
cytes. (From the Cambridge
Natural History, after Min-
chin.)

Ill

PHYLUM AND CLASS PORIFERA

125

I '/a*/ ula (e) — that the larval sponge becomes free. At a later
stage the flagellate cells become partly overgrown by the granular
cclis. 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

Fio. 94.— Development of Sycon raphanus. a, ovum ; b, c, ovum segmented— 6, as seen from
above, c, lateral view ; d, blastula ; e, amphiblastula ; /, commencement of invagination ;
;;, larva attached by its oral face<7i, I, young sponge— /(, lateral view ; ;', as seen from above.
(nam Sollas, after Suhulze,)

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 Silicispongise, 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 (Spoiir/illidce) 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 (Ceratosa) 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 Hcxactinellida and Ghoristida.

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, Mollusc's, and other Invertebrates.
None of the Sponges are true parasites. The little Boring
Sponge, Clioiia, 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 messmatcism, 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

m 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 (Hyalo7iema) is always found associated with a Zoophyte
(JPalythoa), and there are many other instances. Sponges often also
grow in very close association with certain low forms of plants
(Alga,).

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 Coelenterata, 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 Coelenterata.

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 Enlerozoa, 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
Coslenterata. 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.J

CLASS I.— HYDROZOA.

1. Example of the Class — Obclia.
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 line sewing-cotton: of
bhese, 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 (hydrorhizu) resembling a root_ar_
creeping stem, and 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, #) : 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 hydrothcca Qi.th), and produced distally into about two
dozen arms or tentacles (t) : these zooids are the polypes or hydrmith*.
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.lh), and bearing
numerous small lateral offshoots, varying greatly in form according
to their stage of develojfhient, and known as mcdusa>-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 trimorpkic, 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 manubrium or
hypostome (mnb), around the base of which are arranged the twenty-
four tentacles in a circle. Both body and manubrium are hollow,
containing a spacious canity, 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

Fxa 95. -Obelia .p. A, portion of a j£^U ^^ffi^^^SfSt ?5K \

B, medusa; C, the same with reversed umbrel a D, «« saj™, o™ i i , , ,

Us. blastos'tyle 5 <«• ccaiosarc ; «f. ■ ectodera ; J^^fQ' ^SubriuiZ; V
gonotheca; ».tt. hydrotheca ; J, iithocyst m. M. mea us a , (.tentacle;

mesogloea; m*A. mouth ; j>. pensarc; P. 1, *, 3, polypes, raa. c. reuuu «
W. velum.

MOT. iv PHYLUM CCELENTERATA 131

narrow or proximal end, it is produced inwards into a sort of
circular shelf (a&), perforated in t he cen^r^^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 (g.th), 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 (ect) and the endoderm (end) :
between them is a very delicate transparent membrane, the
mcsogloea or supporting lamella (insgl), 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.

ccenosarc, 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 ijo
cell-structure, but only a delicate lamination. It is, in fact, not a
"~ celfTHar 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. 90. — Obelia sp. Vertical section of a polype, highly magnified ; tct. ectoderm ; end. endo-
derm ; ent. enteric cavity ; h.th. hydrotheca ; msgl. mesogloea ; mth. mouth; ntc. nematocysts ;
»h. 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, Bdl,%).
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 nei've-

IV

PHYLUM CCELENTERATA

133

edU (Fig. 98, nv.c). The tentacles and the manubrium contain,
in addition, a layer of unstriped muscle-fibres between the ectoderm
and the mesoglcea : they are arranged longitudinally, and serve
for the rapid shortening of the tentacles (Fig. 98, inf). 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 ; otft,
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 nematocysts (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.

aWP^-^

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 cx-umbrclla, the

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 f. muscle-
fibres ; nic. nematocyst ; mi. nucleus ;
nv.c. nerve-cell. (After von Lendenfeld.)

iv PHYLUM C(ELENTERATA 135

concave inner surface as the sub-umbrella. From the centre of
the sub-umbrella proceeds the manubrium (mnb), 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
in mils {rod. 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

ffon
mnb

Kio. 99. — Obelia sp. A, mature medusa swimming with everted umbrella ; B, one quarter
of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; I. lithocyst ; mnb. manubrium ;
mth. mouth ; rod. 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 lithocysts : 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

precisely the same layers as that of the hydranth — ectoderm,
mesogloea, 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.fam

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
mesogloea black, tire. c. circular canal ; end. lam. endoderm lamella ; gon. gonad ; {. lithocyst ;
mnb. manubrium ; mth. mouth ; rod. c. radial canal ; vl. velum.

layer of mesogloea 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 lipex 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,

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 aJb) of form with tentacular
region extended into the form of a bell ; D, vertical, and D', transverse section (along the lineal)
of medusa. The ectoderm is dotted, the endodenn striated, and the mesoglcea black, cir. r.
circular canal; ect. ectoderm; end endoderm ; end. lam. endoderm lanialfcl ; ent. cav. enteric
cavity ; hyp. hypostomc or manubrium ; mnb. manubrium ; mst/l. mesoglcea ; villi, mouth ;
nv. nv', nerve-rings ; t. tentacle ; v. velum. (From Parker's Biology.)

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. cav') 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

rcvcUcce

radius

ad-rcuiciMf \

CrUtffOsdsiios \

SUb-radiu,j> » „ ^

a-d- radiws

sub radius
ptr- ra dius —

Fig 102.^Projections of polype (A) and medusa (B), showing the various orders of radii;
gon. gonad ; mnb. manubrium.

the endoderm-lamella (D', end. lam), and remaining only along
four meridional areas — the radial canals (rad. c), and a circular
area close to the edge of the bell — the circular canal (cir. 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

t.ikt-n 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 0/ 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 int» 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 noticedthat 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 hydrula 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

SECT.

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 medusoe — develop gonads, and from

Fig. 103 — Stages in the development of two Zoophytes (A— H, Laomedea. I— M, Endeu-
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 Allnian.)

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 go-niobium — 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

i\ 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 coenosarc 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 medusa?,
zoophyte colonies known not to produce free medusas, and medusae
known to have no zoophyte stage. Moreover, there are many
medusae 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 medusas, 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. — LEPTOLiNiE.

Hydrozoa in which there is a fixed zoophyte stage, and in which
the sense-organs are exclusively ectodermal.

Sub-Order a. — Anthomedusce.

Leptolinae in which the polypes are not protected by hydrothecae or the
reproductive zooids by gonothecae : the medusae bear the gonads on the manu-
brium and have no lithocysts.

Sub-Order b. — Leptomedusw.

Leptolinae in which hydro- and gonothecae are present : the medusae bear the
gonads in connection with the radial canals and usually have lithocysts.

142 ZOOLOGY sect.

Order 2. — Trachylinse.

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. — Trachymedusw.

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.

Trachylinse 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 which 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 hydrothecae, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusae. It is placed in the family Campanu-
lariidm, distinguished by having cup-shaped thecae 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 wTith 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

fcooids. The hydranths maybe colourless and quite invisible to
the naked eye, or, as in some Tubularia^ (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
JEquorea, 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 Anthomedusae are called Gymno-
blastea or naked-budded zoophytes (see also Fig. 105, 1, 4, 5). In
the Leptomedusae the cuticle is usually of a firmer consistency than
in the first sub-order, and furnishes hydrothecae for the hydranths
and gonothecae for the reproductive zooids : they are hence often
classified as Calyptoblastea 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 Anthomedusae the umbrella is usually strongly
arched, and may even be conical or mitre-shaped (Figs. 104 ; 105, 7 ;
1 09, 1 and 2) : its walls are thick, owing to a great development of
the gelatinous mesogloeaof 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(gon) 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
(oc) 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 Anthomedusae, 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 Leptolinae 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

curious genus Hydractinia (Fig. 105, 1) is remarkable for possessing
a massive ccenosarc, 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

Fio.104— Bougainviilea ramosa. A, entire cdlony, natural size; B, portion of the same
magnified ; C, immature medusa, cir. c. circular canal; cu. cuticle or perisarc ; ent. car.
enteric cavity; hyd. polype or hydranth ; hyp. hypostome or manubrium ; vied, medusa; mnb.
manubrium ; rod. c. radial canal ; t. tentacle ; v. velum. (Prom Parker's Biology, after
Allman.)

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 Anthomedusse : the Leptomedusan

L'.Myriofhela

3. Corymorpha
raef.c

4Syncoryne

rad.c
miib

o.Tubuloria

a

-runb
7. Sarsia

6. Clavarella

Fia. 10.0.— Various forms of Leptolinse. In 1, a shows the entire colony, 6 a portion highly
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 ; mnb. manubrium ; mth. mouth ;
oc. eye-spots ; rcul. c. radial canals ; *. sporosacs ; sp. spines ; /, (>, 1-, tentacles.

VOL. I L

146

ZOOLOGY

SECT.

Clathrozoon, an Australian genus, resembles it in having branched
and intertwined ccenosarcal 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). Ceratdla (Fig. 10G) has a similar fan-coral-like
appearance, with a branching axis composed of numerous inter-

Fio. 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 Myriothcla
(Fig. 105 2), 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 Corymorpha (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

nte

- SCALE FOR A

Fio. 107!— Hydra. A, vertical section of entire animal ; B, portion of transverse section, highly
magnified ; C, two large ectoderm cells ; D, endoderm cell of //. viridis ; E, large nematocyst ;
F, small nematocyst ; G, sperm, a, ingested diatom ; bd. 1, bd. 2, buds ; chr. chromatophores ;
cnbl. cnidoblast ; cnc. cnldocil ; ect. ectoderm ; end. endoderm ; ent. car. enteric cavity ; ent.
eav'. its prolongation into the tentacles ; fl. flagellum ; hyp. hypostome or manubrium ; int. c.
interstitial cells ; m. pr. muscle-processes ; mth. mouth : ttugl. mesogloea : nlc. large, and ntc\
small nematocysts ; nu. nucleus : ov. ovunj ; ovy. ovary ; psd. pseudopods ; tpy. 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, in. 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 Hydras, but these are always detached
sooner or later, so that a permanent colony is never formed. There

FiG.108..-Protoliydra leuckartii. (Prom 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
Protohydra (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 Siphoiiophora — an order yet to be dealt with
— than to the Leptolinse.

The polypes are usually cylindrical, as in Obelia, but in some
genera they are widened out into a vase-like form (Fig. 105, S), in
others elongated into a spindle-shape (J/). The tentacles may be
disposed in a single circlet, as in Obelia and Hydra, or there may
be an additional circlet rounft the hypostome (8, 5), or at the base of
the polype, or they may be scattered irregularly over the whole
surface (Jj). In Myriothela {$) they are short, and so numerous
as to have the appearance of close-set papillas. 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 dimotyhism of the hydranths obtains, some
of them being modified to form protective zooids. In Hydractinia
(1) 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 Leptomedusae, 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, 1), 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, 3).

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

8ECT. 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 be\ow the at-
tachment of the velum (Fig. 101, I), nv, nv). The medusae thus
furnish the first instance we have met with of a central nervous
t)/x(cm, 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

Fio. 110.— Diagram illustrating the formation of a sporosac by the degradation of a medusa. A,
medusa enclosed in ectodermal envelope (««) ; B, intermediate condition with vestiges of
umbrella (v.) and radial canals (ra) : C, sporosac. ec. ectoderm ; en, endoderm ; m, manubrium ;
or, ovary ; t, tentacle ; v, 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 asexual ly 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 sporosa&s (Fig. 105, lb,
5, s), each consisting of little more than a gonad, but shoeing an in-
dication of its true nature in a prolongation of the digestive^cavity

152

ZOOLOGY

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 medusa?, 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. 11 1.— Early development of Eucope. A, blastula<-stage ; B, planula with solid endoderm ;
C, planula with enteric cavity ; al. enteric cavity ; ep. ectoderm ; hy. endoderm. (From
Balfour's Embiyology, after Kowalevsky.)

sexual cells are not developed either in medusae or in sporosacs,
but are formed directly in the blastostyles.

In Obelia we found the medusa? 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 medusa? or sporosacs — may spring directly from the
coenosarc, as in Bougainvillea (Fig. 104), o"r 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 Leptolinae frequently, but not always,

iv PHYLUM CCELENTERATA 15.3

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 oosperm 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, and, after a period of rest, develops into the
adult form. \

Order 2. — Trachylin^e

General Structure. — The members of this order are all
medusas: 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 mesoglcea 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

2.Glossocodo'i

Fig. 112.^-Two Trachymedusse. cir. c. circular canal; ffon. gonad; mnb. manubrium; inth.
mouth ; rod. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst ; tg. tongue ;
vl. velum. (After Haeckel.)

rdd.c

l.Cunarcha

2.Polyco(pa

Fio. 113. — Two Narcomedusje. 2 in vertical section, gon. gonad ; mnb. manubrium ; mth.
mouth ; pr. peronium ; rad.c. 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

PHYLUM CCELENTERATA

155

distinguish them from the lithocysts of Leptomedusae, and to mark
the fact that they are modified tentacles, they are called tcntaculo-
cysls. 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 Trachymedusae, as in the
preceding order, they arise
near the edge of the um-
brella (Fig. 112), but in the
Narcomedusae 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 "
(Lr).

As to the position of the
reproductive organs, there
is the same difference be-
tween the two sub-orders
of Trachylinae as between
the two sub-orders of Lepto-

linae. In the Trachymedusae the gonads (Fig. 112, gon) are
developed in the course of fche radial canals : in the Narcomedusae
(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 Narcomedusae the manubrium is short ; in the
Trachymedusae 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 of the sub-umbrella (pseudo-
manubrium).

The simplest case of the development of Trachylinae is seen in
jEgirwpsis, one of the Narcomedusae. 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
iEginopsis is thus a hydrula, 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, erf. ectoderm ; end.
endoderm ; I. lithites ; nlc. nematocysts ;
nc.c. group of nerve-colls. (After Haeckel).

156 ZOOLOGY sect.

medusa. Thus the actual formation of the medusa from the
hydrula of yEginopsis 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, not quite unknown among the Trachy-
linse. In a parasitic Narcomedusa (Cttnina 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 medusae 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, M. 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, (j.p) ; the smaller are arranged more or less
irregularly round the gastropores, and are called dadylopores (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

PIIYLl M OKI.KNTKKATA

157

canals in immediate connection with the gastropores are traversed
by horizontal partitions, thf tabula} (tb).

In the living animal each pore is the place of origin of a zooid :
from the gastropores protrude polypes (Fig. 117, F) with hypostome
and four knobbed tentacles; from the dactylopores long, filamentous,
mouthless dactylozooids or feelers (D.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

« +

W

r^% d?>

I

Fio. 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. tabulw.
(After Nicholson and Lydekker.)

canals of the coral and represent a much-branched coenosarc,
recalling that of Hydractinia (p. 144).

The coenosarcal 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

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 zooids-— 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

end ect

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 ; mth. 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 medusoe
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

1\

PHI LITM CCKLENTRRATA

159

degraded reproductive zooida lodged in special chambers («) 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. lia— Stylaster sanguineus. A, portion of skeleton, natural size; B, small portion,
magnified ; a. ampulla? ; 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 far back as the Triassic
epoch.

Order 4.— Siphoxopkora.

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.

Halistemma (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 — the float or pncumatophore (pn). Next come a number
of closely set, transparent structures (net), having the general char-
acters of unsymmetrical medusae without manubria, each being a
deep, bell-like body, with a velum and radiating canals. During life
these swimming-bells or nectocalyces 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

hp/v

Fig. 11J>. — Halistemma tergestinum. A, the entire colony; B, a single group of zooids.
ecu. ccenosarc ; dz. dactylozooid ; hph. hydrophyllium or bract ; net. nectocalyx or swiniming-
bell; ntc. battery of nematocysts ; p. polype; pn. pneumatophore or float; s, s\ sporocysts ;
t. tentacle. (After Glaus.)

sect, iv PHYLUM CCKLENTERATA 1G1

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 sf hydroid polype as
Obelia, Halistemma is to be looked upon as a polymorphic floating
colony, the stem representing a coenosarc, 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, pri). 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 chai-acteristic
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

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 Siphonophore : the thick line represents endoderm ; the space ex-
ternal to it, ectoderm ; the internal space, the enteric cavity, cce. coenosarc ; dz. dactylozooid ;
hph. iiydrophyllium ; md. sporosac ; net, net', nectocalyces ; ntc. battery of nernatocysts ;
p. polype ; pn. pneumatophore ; t. tentacle. (After Glaus.)

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

1'IIVI.I M OKLKNTKUATA

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 in the 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-
• olonv as a medusa the manubrium of which has extended immensely and
produced lateral buds after the manner of some Anthomedusai (Fig. 105, 7 a).

Fig. 121.— Two stages in the development of Halistemma : the endoderm is shaded, the
ectoderm left whi£e. ep. pneumatocyst or air-chamber, of pneumatophore ; hy. endoderm
surrounding pneumatophore ; po. polype ; pp. pneumatophore; t. tentacle. (From Balfour,
after MetschnikofT.)

On this theory the entire coenosarc 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 («•) : 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
(goiwdendra) with groups of medusoids looking like bunches
of grapes of a deep blue colour, and long retractile tentacles (t),

M 2

164

ZOOLOGY

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

C7*

Fig. 122. — Physalia : the living animal floating on the surface of the sea. er. crest ; p. polype ;
pn. pneumatophore ; t. tentacle. (After Huxley.)

Halistemma, but the female apparently becomes detached as a
free medusa.

In Dlphycs the float is absent. Two swimming-bells (Fig. 123 A, m)
of proportionally immense size are situated at the proximal end
of the coensarc, and are followed by widely-separated groups of
zooids (B), each group containing a polype (n) with its tentacles (i),

IV

IMIVLCM OKLKNTKRATA

165

a meduzoid {<j), 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

Fia. 1*3. - Eiphyes campanulata A, the entire colony; B, single group of zooids. a,
ccenosarc ; c, cavity of swimming-bell ; e, groups of zooids ; ff, medusoid ; i, grappling line or
tentacle ; m, swimming-bell ; n, polype ; o, mouth of swimming-bell ; t, bract. (From
Parker's Biology, after Gcgenbaur.)

body, enclosing a chambered chitinoid shell (sh) containing air, and
obviously corresponding with the float of Physalia. The edge of
the disc is beset with long dactylozooids (t) and from its lower
surface depend numerous closely set blastostyles provided with
mouths and bearing medusas, while in the centre is a single large

166

ZOOLOGY

SECT.

gastrozooid (hy). The closely allied genus Vehlla 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, hy, large central gastro-
zooid ; hy'. blastostylcs ; sh, chambered shell ; t, dactylozooids. (From Parker's Biology, after
Duperry and Koellikcr.)

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 ccenosarc 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.th).
Tlic ccviiosarcal skeleton is strengthened by a slender axis, the viryida (v), the
proximal end of which is connected with a small dagger-shaped body, the
•tenia (•<), 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 gonothecse. But it must be added that
the evidence in favour of associating the Oraptolites
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 arc
marine, the only exceptions being Hydra,
found all over the world ; Microhydra, at
present known only in North America;
Cordylophom, one of the Anthomedusae,
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 ; Limnocodium, 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 . Zimnocnida, found
Lake Tanganyika, Africa.

The oldest known Hydrozoa are the Graptolites, found first in
the Cambrian rocks ; Hydractinia 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 Anthomedusoe, 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.

125. — Graptolites.

A, Monofiraptus colonics;

B, DimorphoaraptUB, both
magnified, hy. th. hydro-
theca ; s. sicula ; v. vir-
gula. (After Nicholson
and Lydekker.)

Ill

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 Amoeba 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).

t/rcn

Wn

Fig. i _■•■- Aurelia aurita. A, dorsal view, part of the ex-umbrella cut away t<i show part of
tho stomach and one of the four gastric pouches ; 15, ventral view — two of the oral arms, are
removed, a.r. c. adradial canal; </. /. gastric filaments; ijoa. gonads; «7. p. gastric pouch ;
i.r.c. inter-radial canal ; ma. lp. marginal lappet; uUk. mouth; or. a. oral arm; p.r.c. per-
radial canal ; s.g. p. sub-genital pit ; st. stomach ; t. tentacles.

170 ZOOLOGY sect.

A narrow region of the umbrella adjoining the edge is very thin

nd flexible : the structure thus constituted, with its marginal

notches and the fringe of marginal tentacles, is the velarium.

Unlike the true/ velum of the medusas 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 sub- genital pit (s.g.p).
which lies immediately beneath one of the conspicuously coloured
gonads (gon). 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 (p.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 mesoglcea, which, however,
is not structureless, but is traversed by branching fibres and
contains amoeboid cells derived from the endoderm. Both ex-
and sub-umbrellse are covered with ectoderm, and the stomach and
canal system are lined with endoderm, which is ciliated through-

IV

PHYLUM CCELENTERATA

171

I 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-

Fio. 127 Aurelia aurita. A, side view, one-fourth of the umbrella cut away; B, diagrammatic

vertical section, ectoderm dotted, endoderm striated, mesoglcea black, tire. c. circular canal ;
fj.f. gastric filaments ; (ion. gonad ; «7. p. gastric pouch ; trul. gullet ; h. hood ; i.r. c. inter-radial
canal ; mg. 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

f 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

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), /(, hood ; /, lithite ;
mg. Ip, marginal lappet ; oc, ocellus ; olf. 1, off. 2, 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 medusas. 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 inter-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 lit kite* (I) 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 (h) connecting them ; and in connection
with each are two depressions, one on the ex-umbrella (olf. 1), the
other immediately internal to the sense-club (olf. 2) : these
depressions are lined with sensory epithelium and are called
uJjudonj 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
w;ills 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, st), the bottom of which
becomes perforated so as to communicate - with the enteric cavity
(C, st): the depression is the stomodccnm, 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 adradia] tentacles appear. At the
same time~fche 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 tamioles (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 infandibula (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 Leptolinae, but distinguished by a pronounced differentia-
tion of structure, indicated by the sixteen tentacles developed in

174

ZOOLOGY

regular order, the stomodseum, 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 stomodseum ; D, transverse section of young scyphula ; E,
scyphula ; P, longitudinal section of same : the section passes through a per-radius on the
left of the dotted line, through an inter-radius on the light ; G, division of scyphula
into ephyrulse ; H, ephyrula from the side ; L, the same from beneath. In A — D and
P the ectoderm is unshaded, the endoderm striated, and the mesoglcea dotted, a. lobes
of umbrella ; mnb. manubrium ; mth. mouth ; *../". septal funnel ; st. storuodajum ; t. tentacle ;
in. taniioles. (Prom Korschelt and Heider'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,
tour 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 cpliyrula (H, I). The umbrella "of the ephyrula
is divided into eight long bifid arms (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 gamohinm V>pipg
represented by the adutTAlirelia, the agamobium by the scyphula.
BTft instead of the medusa being developed either as a bud on a
branched colony, as in Leptolina?, or by direct metamorphosis of a
polype, as in Trachylinae, 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 scyphulae which do not undergo
transverse division, the entire scyphula becoming metamorphosed
into a single adult.

176 ZOOLOGY sect.

2. General Structure and Classification.

The Scyjahozoa may be defined as medusoid Ccelenterata, 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 stomodaeum
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 (agamobium) 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 Luccmaria 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 stomodaeum.

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. — Cubomedusa.

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.

Discomedusaj in which the square mouth is produced into four long oral
arms.

Sub- Order b — Rhizostomcv.

DiscomedusoB 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 Semostomce,
and the order Discomedusai.

Its saucer-shaped umbrella and eight tentaculocysts place it at
once among the Disco-medusae : the presence of a distinct mouth
surrounded by four oral arms places it in the first sub-order
or Semostomae. This group contains six families, characterised
mainly by differences in the canal system : the Ulmaridae 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 (Lucernarida).

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-ratlial
{p.r.t.) and four inter-radial (i.r.t.), and movement is effected by a well-developed
system of circular and radial muscles.

Lucernaria (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 (g.f.) are very numerous — a distinct advance on Tessera — and
the gonads (gon. ) 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

(rudimentary eye), filenocyphits 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. 130.— Tessera princeps. A, external view; B, vertical section, g. f. gastric filament;
gon. gonad ; i.r. t. inter-radial tentacle ; rnnb. manubrium ; mth. mouth ; p.r. t. per-radial
tentacle ; it. stomach ; tn. tremolo. (After Haeckel.)

Fio 131.— Iiucernaria. A, oral aspect ; B, from the side, g. foot-gland ; g. f. gastric filaments
gon. gonad ; mth. mouth ; t. tentacles ; tn. tamioles. (After Clans.)

tentacles. The Depastridm, have an almost entire margin fringed with
tentacles.

IV

PHYLUM CCELENTERAJA

179

Order 2. — Coronata.

This group includes a number of rare and beautiful Medussi of curiously
complex structure, of which Pericolpa may be taken as aw^ 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. a

mnb

Fin. 132. Pericolpa quadrigata. A, external view; B, vertical section, circ. s. circular
sinus ; en. cone; jr. f. gastric filaments; ffom. gonads; tng. Ip. marginal lappets; mnb. manu-
brium ; mth. mouth ; ptl. I. pedal lobes ; st. stomach ; t. tentacles ; tc. tentaculocysts ; tn.
t.i-nioles. (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 (nig. Ip.), 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

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 (t.). 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 [tire. ».). As in Lucernaria, there are four wide inter-radial im-
fimdibula. The gastric filaments (g. y.) are very numerous, and the elongated
U-shaped gonads (gon.) 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

fO^

Fig. 13?.— Nausithoe. The entire animal from the oral aspect, ar. adradii ; g. gonads ; g.f.
gastric filaments ; ir. inter-radii ; m. circular muscle of sub-umbrella ; pr. per-radii ; rt. 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 (Peromedusce) resemble the
Lucemarida and the members of the order Cubomediisoi in the presence of
tamioles and inter-radial septa : Ej>hyropsis and its allies (Cannostomce)
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. — Cubomedus^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 marsupialis (Fig. 134), is
about 5 cm. in diameter and of very firm consistency.

IV

PHYLUM CCELENTERATi

181

As in the lower Coronata, the margin of the umbrella bears four' tentacles
(/.) and four tentaculocysts ((c) 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 hpm conspicuous
gelatinous lobes (/.), which probably answer to the pedal lobes of the preceding
order. These pedal lobes sometimes bear a number of supplementary tentacles.

'adjt

Fig. 134.— Chary bdaea 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 ; e.nil. lam. endoderm lamelln. ; end. lam', its pro-
longation into the velarium; g.f. gastric filaments; gon. gonad; yon', septum sepaiHtinpr
gonads ; I. lappet ; mnb. manubrium ; rati. p. radial pouch ; t. tentacle ; tc. tentaculocyst ;
vl. velarium. (After Claus, somewhat altered.)

The margin of the umbrella is produced, in most cases but not in all, into a
horizontal shell (vl.), resembling the velum of the hydroid Medusae, 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 (mnb.) 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 wide 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. /.).

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 Cubomedusaj 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
Rhizostomse 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 Semostomae
(Fig. 126), and altogether absent in the Rhizostomse (Fig. 135). In the
Semostomse 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 Rhizostoma;
each of the original four arms (Fig. 135, or. a.) becomes divided longitudinally in
the course of development, the adidt 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 tsenioles or inter-radial
septa (mesenteries). In the Semostomse (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 Rhizostomse (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.

1'HYU M OKLKNTKRATA

183

RhizofltomcR have been found wit li prey of consideralile size, such as fishes, em-
braced by the arms and partly drawn into these apertures, whiqh are therefore
called the mttoricd mouth*. 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
(at.). We thus get a, polystomn/oit* 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

rod.c

STTltil

Fie;. 135. — 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. 'i. oral arms ; nul. c. radial canal ; s. mth. suctorial mouths ; at. stomach ; tl, t2, tS, tentacles
on oral arms. (After Cuvier, Claus, 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 Rhizostomaj
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 Discomedusae 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, pi-oduces sexual medusae. In
Cassiopeia the scyphula arising from the fertilised ovum gives off buds which
become detached as free-swimming planulse, and these, coming to rest, develop
into scyphulse. But in other cases there is no alternation of generations, and
development is direct. For instance, in Pelagia (Fig. 136) — one of the
Semostom;e — a blastula is formed which becomes invaginated at one end,

Fig. 136.— Pelagia noctiluca : Three developmental stages, m. mouth; r. marginal lappet :
». tentaculocyst. (Prom 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, arc 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 Discomedusoe.

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,

i86

ZOOLOGY

SECT.

■mifo

OSt. t

9.TTV

rnesf

dL-'.nvee

mes. f

d.7nes

Fig. 137. — Tealia crassicornis. A, dissected specimen ; Si, transverse section, the half
above the line ab through the gullet, the lower half below the gullet.- d. mes. directive
mesenteries ; rjon. gonads ; gul. gullet ; I. m. longitudinal muscle ; I p. lappet ; mes. 1, primary,
mes. 2, secondary, mes. 3, tertiary mesenteries ; mes. f. mesenteric filaments ; mth. mouth ;
ost. 1, ost. 2, ostia ; p. m. parietal muscle ; $gx>h. siphonoglyphe ; s. i». 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 and 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. 2), 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 stomodaum, communicating with the exterior
by the mouth, and opening below into (2) a single main digestive
cavity, the stomach or mescnteron, 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 stomaSh 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. 2) 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
mesenteric 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 acontia — which may be
protruded through the mouth or through special apertures
(cinclides) 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 mesogloea,
which is extremely thick and tough. The gullet (yul.), 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 (mcs.)
consist of a supporting plate of mesogloea, covered on both sides by

mcs

■nl. ?nes. C

Fig. 138. — Diagrammatic vertical (A) and transverso (B) sections of a Sea^anemone. The
ectoderm is dotted, the endoderm striated, the mesogloea' black, ac. acontium ; en. cinclis ;
gul. gullet ; int. mes. c. inter-mesenteric chamber ; met. mesentery ; ines. f. mesenteric
filament ; mtk. mouth ; ost. ostium ; %y. 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 (XELENTERATA 189

is seen to be traversed by definite fibrous bands, the two most
obvious of which are the longitudinal or retractor muscle (Fig.
137, /.//>.), running as a narrow band from base to disc, and the
parietal muscle (/<.///. ), 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. mes.), 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 (mes. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary
mesenteries (mes. 8, mes. 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 sym-
metry, 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 (msgl.) 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 layer has, in fact, ceased to be a mere gelatinous
supporting lamella or mesoglcea, and has assumed, to a far greater

■msgl

Fio. 130.— Tealia crassicornis. .Trans-
verse section of tentacle, ect. ectoderm ;
end. endoderm; l.m. longitudinal muscles ;
m»gl. mesoglcea; nr.c. nerve -cells; nv.f.
nerve -fibres ; ntc. nematocysts ; t. m.
transverse muscles. (After Hertwig.)

Fio. 140. — Three nematocysts of
Sagartia. (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, gl.) are very abundant

of the gullet and in the mesenteric
trilobed in section, and the gland-
middle portion, the lateral divisions

in the ectodermal lining
filaments : the latter are
cells are confined to the

IV

PHYLUM CfELENTERATA

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

Fig. 141. — Transverse section of mesenteric filament of Sagartia. c. ciliated cells; fjl. gland-
cells ; ntc. 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 circyni-

i92 ZOOLOGY

SECT.

stance that, as we shall see, the muscles of all animals above
Ccelenterata 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,
nv.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 Medusa?.

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, gon.),
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 plamda, 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 stomodseum, 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

I'in I.IM OELENTKRATA

193

ami one of the sixtli pair ; a third couple is similarly formed by a mesentery of
the second and one of the liftli 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

SlcL

Flu. 142. — Traiisverse 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 parti}- 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 Mnbryo.

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 Ccelenterata 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 stomodoeum : 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 stomodaeum. 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 mesoglcea 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. — Aotiniaria.

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

,v PHYLUM CCELENTERATA 195

always possess an ectodermal calcareous skeleton. This order
includes the vast majority of Stony Corals (Figs. 146 and 156).

Order 3. — Axtipatharia.

Compound, tree-like Zoantharia in which the tentacles and
on -niteries 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
eight in number. The tentacles are pinnate, i.e. produced into
symmetrical branchlets. 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 fingers " (AUy.nium, Fig. 1-53) 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 colon}7, 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 (TuLipora, Fig. 148). In the " Blue Coral "
(Heliopom) 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.^— Gorgon acea.

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
siphonoglvphe. 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 Tealidce, which, with several
other families, make up the tribe Hexactinim, of the order
Actiniaria, of the sub-class Zoantharia. t

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 Hexactinige 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 stomodaeum has two siphono-
glyphes and two lappets.

The family Tealidse 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

sgph,

Fii.. 143. Zoanthus sociatus. A, entire colony ; xt. stolon. B, transverse section, xgph.
siphonoglyphes : d. <l. dorsal, and r. tl. ventral directive mesenteries. (After McMurrich and
Korsehelt and Heider.)

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 ccenosarc. 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 Astrcca (Fig.
146). In the Sea-pens (Penna-
tulacea) the proximal end of
the elongated colony (Fig.
147) is sunk in the mud, and
fio. 144. -Hartea eiegans. gui. gullet ; the distal end bears zooids
p^^f <-tentaCleS- springing either directly from

198

ZOOLOGY

SECT.

the crenosarc 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 (fubipora). 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 pint forms
(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 passu with the vertical

Fia 145.— Corallium rubrum, por-
tion of a branch. (From Claus,
after Laeaze-Duthiers.)

Fig. 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

IT

PHYLUM CCELENTERATA

199

Fig- 147. — Pexmatula sulcata. A, entire colony; B, portion of the same magnified.
/. lateral branch ; p. polype ; s. siphonozooid. (After Koelliker.)

po-

Fio. US.— Tubipora musica. A, skeleton oi' entire colony ; B, transverse sections of polype ;
('. sinple polype with tube and commencement of platform ; 1), growth of new polypes from
platform. /. m. longitudinal muscles ; pi. pi. polypes ; pi. platform ; gfjph. siphonoglyphe ; sp.
spicules ; std. stomodseum. (After Cuvier, Quoy and Gaimard, and Hickson.)

200

ZOOLOGY

K

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 Hexactinice) 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 Hexactiniae. 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 face inwards in the Alcyonaria,
outwards in Edwardsia.

Fig. 149 — Edwardsia claparedii.

A, the entire animal ; t. tube. K.
transverse section. (After Andres,
and Korschelt and Heider.)

iv PHYLUM CCELENTERATA L'Ol

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.

Fio. 150.— Antipathes ternatensis, portion of a branch, showing three zooids and the horny
axis >.osct 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
each edge (Fig. 144). Many Actiniaria have the tentacles
perforated at the tip (Fig. 138, A, p.) ; 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 Schizoimtlies (Fig. 151 , B)
there is thus recognisable an arrangement of the parts which might

FlG. 151.— Antipatharia. A, oral face of zooid of Parantipathes. Ii, oral face of zoojd of
Scltiznpathe*. (After Delage et Herouard.)

be interpreted as a dimorphism of the zooids, one set— the parts
containing the mouth and gullet — being regarded as gastrozooids,
and the others containing the gonads as gonuzooids.

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 (Fiff. 149) and Cervmthus,
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 Pcachia, 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
Minyas (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 homomorphic, 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. /. float.
(After Andre*.)"

PHYLUM CCELENTERATA

203

referred to, in which minute irregular deposits of calcium carbonate,
called spicules (sp.), are deposited in the mesogkea. A similar
spirillar skeleton occurs in the " Dead-men's finger " ( Alcyonium,
Fig. 153), where spicules of varying form are found distributed
throughout the mesoglcea 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

Fio. 1..3.— 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 mesoglcea of the coenosarc. In some

Fio. 154. —Gorgonia verrucosa A, entire colony; B, portion of the same magnified,
coenosarc ; />. polype. (After Koch and Cnvier.)

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 (XELENTERATA 205

which is still uncertain. Spicules occur in the mesoglcea, some of
them microscopic, others readil}' 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 polypa 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 Flabcllum (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 theca (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, b. pi.). From the inner surface of the theca a
number of radiating partitions, the septa (sep.), 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 epiiheca (C, e.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?, 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

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

Stp

eefi.ff

sep-1

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. 6. pi. basal plate ; col. colum-
ella ; e. tk. epitheca ; gul. gullet ; mes, mes. 1, mes. 2, mesenteries ; vies. 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-

PHYLUM CXELKNTERATA

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 be 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 Astrsea (Fig. 146) ; or tree-like forms, such

Vto. lOC.-Dendrophyllia nigrescans, B, Madrepora aspera. co. corallites;
cs. ensnosarc ; p. polypes. (After Dana )

as Dendrophyllia (Fig. 156, A), in which a common calcareous stem,
the caenenchyma,\s 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 (Madrepora, 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,
Astrasa, &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 ccenosarc, and placing the polypes of the
colony in communication.

The Blue Coral ( I 'feliup <or u), one of the Alcyonacea, has a massive
corallum 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 sipljhozoids.
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
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
Alcyonidae 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
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-
anemone, which, owing to its toughness, and to the pain caused
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

I • 1 1 V LUM CCELKNTKIUTA

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

,1 j/r

Fio. 157.— Adamsia palliata, four individuals attached to a Gasteropod shell inhabited by
a Ilcriuit-crab. nc. aci. acontia ; th. shell of Gasteropod. (After Andres.)

an intermediate size, in each of which is found a small choetopod
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 palceontological 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 Bugosa, the 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 fossula, where the septa are greatly
reduced.

1\

PHYLUM CCELENTERATA

211

CLASS IV.— CTENOPHORA.
1. EXAMPLE OP 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 H. plumosa is found in the Mediterranean ;
allied forms belonging either to the same genus (often called
('//</ ippc) or to the closely allied genus Pleurobrachia are common
pelagic forms all over the world.

From opposite sides of the broad end depend two long tentacles
(t.), provided with numerous little tag-like processes, and springing

mik

Fig. 158.— Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth ;
». pi. swimming plates ; t. and 6. tentacles. (After Chun.)

each from a deep cavity or sheath, into which it can be completely
retracted (Fig. 159, t.sJi.). 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

Fig. i . ■'. Horraiphora plumosa. A, dissected specimen having rather more than one
quarter of the body cut away. B, transverse section ; diagrammatic, adr. c. adradial
canal ; inf. infundibulum ; inf. c. infundibular canal ; int. c. inter-radial canal ; mrd^g.
meridional canal ; mth. mouth ; ovy. ovary ; per. e. per-radial canal ; s. o. sense-organ ; s, pi.
swimming-plate ; tpy. spermary ; std. stomodajum ; std. c. stomodseal canal ; std. r stomodreal
ridges ; t. tentacle ; t. b. base of tentacle ; t. r. tentacular canal ; t. ah. tentacular sheath.

bmt.it PHYLUM CCELENTERAT A 213

/

arc, 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 or
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
stomodmum. It reaches about two-thirds of the way towards the
aboral pole, and its 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 stomodgeum, 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 stomodreum gradually
narrows and opens into a cavity called the infundilmlum {inf.),
which probably answers to the stomach of an Actinozoon or a
medusa, and is flattened in a direction at right angles to the
stomodamm — i.e. in the transverse plane. From the infundibuluni
three tubes are given off: one, the inftindilndar 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 per-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 stomodaial canal
{std. c), which passes downwards, parallel to and in close contact

irU.o t>fc e^bso -^

/nrd^

adr.c

rrith

spl B
mrd.c

inr

pcrc

Fig. " Hormiphora plumcsa, 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. 159, except ex. p. excretory pore.

PHYLUM CQ5LENTKRATA

215

with the stomodreum, 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. b.),
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 stomodoeum
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-

a<i.c

PlO. 161, Hormiphora plumosa. A, transverse section of one of the branches of a
teataeb ; B, two adhesive cells (ad. c.) and a sensory cell (s. c.) highly magnified, cm. cuticle
nu. nucleus. (After Uertwig 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
Ccelenterata 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 papilhe, 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

Fig. I i-.-Hormiphora plumosa, Sense-organ : b. bell ; c. p. ciliated plate ; c. gr. ciliated
groove ; ex. p. excretory pore ; I. 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 tell (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
gonads are developed in the meridional canals (Fig. 159, B), each of
which has an ovary {ovy.) extending along the whole length of one
side, a spermary (spy.) along the whole length of the opposite side.

iv lMIYLUM CCELENTERATA 217

The oigans are so arranged that in adjacent canals those of the
Bame sex face one another. It will be seen that the reproductive
products have, as in Scyphozoa and Actinozoa, the position of
endodorm-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 (plsm.)
containing the nucleus (mo.), and of an internal mass of a frothy
or vacuolated nature (yk) : the bLsm

vacuoles contain a homo- ^*~ y^.

geneous substance which /^ / ^\^m'

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. 103._o-vum of^^ia MM. lllu.leIw.

a Considerable Space, filled With Plsm- protoplasm ; v. m. vitelline membrane ;

a clear jelly. ^yolk- <AfterCh»»>

After impregnation the oosperm segments, but the details of
the process are very different from those we are familiar with in
the other Coelenterata. 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 mcyamcrcs
(my.), .and eight small, dorsal, protoplasmic cells or mirro-
mercs (mi.).

The micromeres increase rapidly in number by division, and are
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

(Fig. 165, C), the final result being the production of an embryo
consisting of a central mass of large yolk-containing cells (ma.),

v ....

V

Fig. 164. — Segmentation of the oosperm ''in Ctenophora. mg. megameres ; mi. micromeros ;
plsm. protoplasm ; yk. yolk. (Modified from Korschelt 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-

Fio. 165. — Three stages in the development of Ctenophora. ma. megameres ; 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,

Fin. 106.— Three stages in the development of Callianira. d. infundibulum ; ec. ectoderm ;
en. endoderm ; me. mesoderm ; st. storaodneum. (From Lang's Comparative Anatomy.)

not by a process of invagination or tucking-in, but by one of cpiboly
or overgrowth.

The endoderm-cells increase in number, and become much
elongated and arranged obliquely, their long axes radiating,

PHYLUM CCELENTERATA

219

upwards and outwards, from the long axis of the entire embryo
( rig. 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 atomodaeum (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, g.\
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 (m«.j). The lithites are formed in
the ectoderm -cells of the apical pole, but
gradually make their way on to the free m
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 (sk.).

The most noteworthy points in this
somewhat complex process of develop-
ment 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 ccntrolecithal

Flo. 107. — Two later stages in the
development of Callianira.
d. infundibulum ; en. endoderm ;
r). niesogloea ; me. mesoderm ;
*k\ sense-organ ; st. stomodseum ;
t. tentacle. (From Lang's Com-
parative A natomy. )

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 tclolecithal 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 Ccelenterata. 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 Ctenophofa are pelagic Ccelenterata in which the formation
of colonies is entirely unknown. No indication of a polype-stage,
so characteristic of the remaining Ccelenterata, 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 stomodoeal 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 stomodaeal 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 Pleurobrachiidm 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 Plcuro-
brachiidoz 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 stomoda3al 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 body in the
transverse plane in some genera, e.g. Euchlora (Fig. 168, 2), the result being an

ICall

3.Lampeh'a
Pio. 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
Callianira (1), and in Lampetia (o) 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 Euchlora rubra minute nematocysts
have been found, and there is reason to believe that it was by the modification

IV

1'IIVHM ('(KLKXTKKATA

223

of these characteristic ccelenterate organs of offence that the adhesive cells of
Ctenophora were evolved.

The Lobeta, for instance Deiojiea, are distinguished, as their name implies,
bj I Ih- presence of a pair of large lappets (Fig. 169 A, Ip.), into which the oral

mnLc

Fig. 160.— Deiopea kaloknenota A, adult ; B, young, aur. auricle ; Ip. lappet ; I. (. lateral
tentacles ; rard. c. meridional canal ; mth. 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 outrides (aur.), which bear swimming-plates. The meridional canals (mdr.c)
unite with one another, and, with the oesophageal 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 portions, 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 Ctxtida are represented by the remarkable "Venus's Girdle" (Cestus
veneris), a band-shaped Ctenophore (Fig. 170) which sometimes attains a length

sfo1 s.pl

B

Pio. 170.- Cestus veneris. A, adult ; B, young. I. t. lateral tentacles ; mth. mouth ; *. pj.i,
«. pi.'* swimming-plates ; t. tentacle. (After Chun.)

of 1 ^ 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.j*/.1) are very

224

ZOOLOGY

small ; the other four (s.pl.2) are continued all along the ahoral edge of the hody.

The bases of the two principal tentacles (t.) 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 which undergoes

gradual elongation in the median plane.

Beroe, the principal genus of theBero'ida, 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 (mth.).
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 stomodreal 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 arc usually perfectly
transparent, and quite colourless, save for
delicate tints of red, brown, or yellow
in the tentacles and stomodseal ridges.
Cestus has, however, a delicate violet hue,
and when irritated shows a beautiful
blue or bluish-green fluorescence. Beroe
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
nature, there is no trace of the group in
the fossil state.

A very remarkable fact has been made
V. swimming-piatcs. (After out ^jjj regard to Boliua hydcitina, 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 pedogenesis.

rrdh

Firs. 171.— Berce forskalii.
inf. mfundibulum ; mth. mouth ;

Chun.

PHYLUM CCELENTEB ATA

225

APPENDIX TO CTENOPHORA

CtENOIM.ANA AND CoCUH'LANA.

Before leaving the C'tenophora mention must be made of two remarkable
organisms nrhicb have been supposed to connect the present class with the
Torbellaria Polycladida, or Planariantt, a group of worms to be described in
the following section.

Ctenoplana (Fig. 17-) is a small marine animal, nearly circular in outline,
Battened dorso-ventrally, and about 6 mm. in diameter. It has hitherto been

Flo. 17:

-Ctenoplana kowalevskii. A, from above; D, from the side. cl. clefts; r.
radiating ridges ; s. o. sense-organ. (After Korotneff.)

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 on 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 tul>es of Flat- Worms. (See Section V.)

( 'aioplana 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.

Gtenoplana and Cceloplana 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 cost* (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 ccelenterate 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 stomodseum (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 medusas with no fixed zoophyte stage, as being
derived from a pelagic hydrula.

The Leptolinse 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 medusas,
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 Semostomae, Cubomedusae, and
RhizoetomSB 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 Hexactiniae 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 Anthomedusae (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 stomodaeum
of Hormiphora, the margin of the bell of Ctenaria with the mouth
of Hormiphora, and the mouth of Ctenaria with the aperture
between the stomodaeum and the infundibulum of Hormiphora.
But, as we have seen, the gullet of Ctenophora is a true stomo-
daeum 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 Hydrcctena, 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

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 medusse, 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

end

ect

end

Fig. 173. — Transverse section of embryos of Actinia (A) and Beroe (B), ect. ectoderm ;
end. endoderm ; 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,

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
■ •.institute an important connecting link. But 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

Cydippida
Platyctenea

Hydrocorallincte
Leptolinct

Cestida
Lobata Rhizostomeae Semostomcb

Hexactinia Madreporaria

Beroida

Coronatcfc

Edwardsia „

Alcyonaria

bomedusi*

ACTINULA

Lucernarjda

SCYPHULA

HYDRULA
Fii;. 174. — Diagram illustrating the mutual relationship! of the Coelentorata.

the larva, to correspond in no sense with the mouth of the
Coelenterate. 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 Ccelenterata, 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 endoderm either composed of a single
elongated cell or of several cells : a mesoglcea is not represented. The Mesozoa
comprise at least three families, the Dicyemidiv, the Heterocyemidce, and the
0>thoneetida>, all the members of which are internal parasites.

The Dici/emidcti are parasities in the kidneys of various Cuttle-fishes and
Octopods (Cephalopoda). Dicyema (Fig. 175), the length of which is between

:A...

Pig. 175.— Dicyema paradoxum,
with infusoriform embryos (males).
(From Broiin's Thierreich, after
KMliker.)

Fig. 170.— Dicyema paradoxum,
with vermiform embryos. (From
Bronn's Tltierreirh, after Kcilliker.)

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 pole nils. The cells of the head all
liear 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
< <>ntents, 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 non-
development 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 '. 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 in/woriform
embryo*. 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 '* 'iwevema M para-
cells, which at the posterior end take the form of a flat doxum. (From
ciliated epithelium. The complete development of the Bronn's Thierrtich,

i .. i i i Ii_ ii ^i after Kullikur.)

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

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 rhomboyene. 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 Heterocyemida', which are also parasites of the Cephalopoda, resemble
the DicyemidsB in most respects, but the head is wanting.

The family Orthonectidce comprises only two genera — Rhopalura and
Stmcharthrum — which live as parasites in a Polyclad (Ltptoplana), a Neinertine
(Linens), an Annelid, and a Brittle- Star (Amphiura). In the stage that
represents the asexual form of the Dicyemidaj 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 plasniodia the sexual forms are developed. A nucleus of the
Plasmodium surrounds itself with protoplasm and gives rise to a germ-cell,

Fio. 178.— Rhopalura Oiardii, male.
(From Brorm's Thierreicli, after Juliii.)

Fin. 170.— Rhopalura Giardii, female.
(From Broiin's Thierreick, 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. 179},

iv 1MIYLIM OELENTERATA 888

bear ■ considerable resemblance to the I)icyemida», 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 cire directed forwards : then
comes a shorter region devoid of cilia, and behind that is the longest region,
having cilia directed backward*. In shape the body is usually spindle like — the
males (Fig. 178) differing somewhat from the females. In about the middle of
the internal spare of the male is the compact oval testis containing small tailed
■perms. 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
tree 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 be mentioned in connection with the Dicyemida) and Orthonectida;, as
perhaps allied with them, are the remarkable parasites Amazbophrya and
Lohmanelfa — the former living in certain Radiolarians (Protozoa) the latter in
the lx)dy-cavity of a Frifillaria ( Urochorda). These both resemble the groups
described above, and differ from the other Metazoa, in the presence of only a
single lxxly-layer. This remarkable simplicity of body-structure occurs also in
SalineUa, 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

Pig. 180. Salinella, longitudinal section. (Aftw£Frem»l.j

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 ventrally
directed, is a circlet of from fifteen to twenty long whip-like cilia. The posterior
aperture (anus), 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 sette (non-
motile cilia). The internal cavity (enteron) is found to contain sand, plant

Fig. 181. — Salinella, transverse section. (After Frenzol.)

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 Tlatyhelminttyes, Nemathel-
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 Platyhehninthes 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 endoderm — as in
all higher groups of animals. An excretory vascular system of J

236 ZOOLOGY sect.

a peculiar kind — the water -vascular or protoncphridial system — is
present in nearly all members of the phylum. A body-cavity (see
following Sections) is not toresentAthe spaces between the various
organs and the wall of trie bodylbeing filled up with a peculiar
form of connective-tissue termed phe parenchyma. The egg is in
most instances composite, the egg-shell enclosing riot 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 vitelline 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. Examples of the Phylum.

i. A Fresh-water Triclad {Planaria or Dcndrocwlum)}

General Features. — Species of fresh-water Planarians of the
genera Planaria and Dendrocmlum 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 av te rior,. than at the other, the posterior,
which is more or less pointed. The animal (Figs. 182-184) is
very readily recognised to be bjilaterally symmetrical, with an upjJer
or dorsal and a lower or ventral surface, right and left borders,
and anterior and posterior e/ids. 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. 183, mo.) leads through

1 The account is sufficiently general to apply to species of either of these
genera.

PHYTATM PLATYHELMINTHES

237

B 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

l.n€>

Fig. 182.— Planaria. Digestive and excre-
tory systems, ex. openings of excretory
system ; int. intestine ; mo. mouth ;
o. ph. opening of pharyi IX, (After Jijinia
and Hatschek.)

Fkj. 183.— Planaria. Nervous system.
br. brain ; eye, eye ; /. ne. 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 blindly,
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
r>ocur in the course of some of the capillaries, though this is
doubi^';' This system of vessels is usually regarded as excretory;
but it may aL ^ave 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 (Ir),
from which proceed backwards two longitudinal nerve-cord's (1. nc.).
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 monoecious 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 deferentia,
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

r\

PHYLUM PLATYHELMTNTHES

239

of the testes and unite in the middle line posteriorly. The median

duct formed by the union of the two vasa deferentia traverses a

piotrusible muscular organ, the

picnis (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 (m.). 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
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.

tr.d

Fig. 184.— Flanaria. Reproductive system.
m. muscular sac ; ov. germarium ; ph.
pharynx ; p. penis ; pr. prostate ; tes.
testes ; ut. uterus ; v. d. vas deferens ;
vit. vitelline glands. (After Jijiroa and
Hatschek.)

240

ZOOLOGY

SECT.

par

eel

cere, mus

Itvt long.mus

dors vent, mus

o.d ™

I'ii;. IS"). — Transverse section of a Planarian. rirc. mm*, circular muscular fibres ; coec. in-
testinal ca;ca ; dors. vent. mus. dorso- ventral muscular fibres ; ect. ectoderm ; ext. long. mus.
external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. long,
mus. internal layer of longitudinal muscular fibres ; nc. nerve-cords ; o. <!. oviducts ; par.
parenchyma ; test, testes ; v. def. vas deferens ; vit. vitelline glands. (From Hatschek's
Lihrbuch.)

ii. The Liver-Fluke (Fasciola hepatica).

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 mouth (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 (rcpr.), 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

Fig. lSt>. — F asciola
hepatica, natural
size. excr. excretory
pore ; mo. mouth \repr.
reproductive aperture;
sckr. posterior sucker.

PHYLUM PLATYHELMINTHES

241

long.nuts *r

cut

^^m^ssm

i

ft

Fig. 187.— Fasciola hepatica. Section of the integu-
ment, circ. mut. layer of circular muscular fibres ;
cut. cuticle ; gl. unicellular glands ; long. mug. layer
of longitudinal muscular fibres ; sp. spinules. (After
Braun.)

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.
onus.) ; (3) a layer of longi-
tudinal muscular fibres
(long. mus.). A cellular
epidermis is wanting.
Beneath the muscles are
numerous unicellular
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 pharnyx (ph.), with thick muscular .
walls and a small cavity. From this a short passage, the oesophagus,
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 cosca, 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 /lame-
cell, similar to a flame-cell of the Planarian.

The Liver-Fluke has a well differentiated nervous system,
vol. i n

242 ZOOLOGY SECT-

which shares in the prevailing bilateral «^^^y£j
The central part of this system consists of a rag ner e^
which surrounds the oesophagus and V^t^Zzll 2 on n he
ings, or ganglia, containing nerve-cells, and y^^fj*r™
middle line below. From this are given off a number o nerves
S which the chief are a pair of lateral cords running back to the

Fig 188.-rasciolahepatica. . Internal orgu ^isatio • *»%£££ t/ie ventral aspect «.
of the body, showing the ™no«»%^™°{Z*Z anterior portion of the intestine (the
eiaculatory duct ; /. female »Product"«V dSTnvldJot* ow ovary (germariutn) ; p. cirrus; ph.
!£* <D ,„>f shownV. orf. commencement of oviduct , ov. ova jVs f do right vas

There are no

posterior end and giving off numerous branches.

The8 reSuctivr organs (Fig. 188) are constructed on the

tlTv^renlia, and «m„/ The M (*•) are two greatly

v PHYLUM PLATYHELMINTHES 243

.-.unified tulx-s which occupy the middle part of the body, one
situated behind the other. From each testis there runs forwards
a duct, the vas deferens, the two vasa deferentia (v.d.) opening
anteriorly into an elongated sac, the vesicula seminalis (v.s.) from
which a narrow tube— the ejaculatory duct (Fig. 189, eld.)— 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-
marium), an oviduct, a uterus, an
ootypc, 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

ZcfrT^jf TT^ uFr°m this a sinSIe media» ^elline
duct runs backwards for a short distance to join the oviduct

th at tefS F gr°Td a, maSS °f UnicelIular shelT^ands
(sh. gl.), each of which is produced into a narrow process or duct

opening into the end of the oviduct in the region of th flatter to
fluted teutrmf^ LS apfliGd- • The Ute™^ » a tide c^

vt£e&lTf Xth6 UDT °f thG °vidllCt and median
Whpn tL ' fuV* °penS close t0 the base of the cirrus

doaea i s fo3 ™ ™thdr»" "k* sma11 ^ity, the genital atrium*

and femat dnl T°n 'V^ eXternal aPertures of b°th male
and female ducts. A canal, termed the canal of Laurer leads
from the junction of the oviduct and median vitelline duct to
open externally on the dorsal surface vitelline duct to

rou^dld1^111611*'"^^-/^111 °n imPregnation becomes sur-

XtlanSs ItXnt Vltelhne T^ °r y°lk derived from ^e
on v2 «. then becomes enclosed, while passing through the

ootype m a chitinous shell, the substance of which is usuallv said
to be derived from the shell-glands The completed egg remaps

dealing with the general organisation? ^ ** "*"*"* t0 m the section

R 2

Fig. 189.— Fasciola hepatica. Ter-
minal part of the reproductive'
apparatus, ej. ejaculatory duct • /.
female aperture; g. unicellular
glands ; od. terminal part of oviduct ;
_ p. cirrus; ps. cirrus-sheath; «'
sucker; v. d. vasa deferentia; v i
.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

past

Fig. 190.— A—D. Development of Fasciola hepatica. A, ciliated larva ;
B, sporocyst, containing redise in various stages of development ; C,
redia, containing a daughter redia, and cercarise ; D, fully developed
cercaria. 6. o/>. birth opening ; ent. enteron of redia ; eye. eye-spots ;
gast. gastrula stage of redia ; germ, early stages in the formation of
cercarias ; int. intestine of cercaria ; mor. morula stage in the develop-
ment of cercarise ; oss. 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

{Lymncvus),aa 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 gastrulge, exactly as in
the case of the sporocyst; these gast'rulse either develop into
a fresh generation of rediae if the season should be winter, or, if it
should be summer, give rise to bodies termed cercariw. 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 (C, b. op), is formed in the wall
of the redia near the circular ridge, and through this the cercarise
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 ty

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 scohx ; 1 the rest of the animal is termed the body or
strobila ; the narrow part immediately behind the head is someJ

1 Though very probable, it is not certain that this end of the Tape- Worm
actually corresponds to the anterior end in the Liver-Fluke, as will be explained
later.

PHYLUM PLATYHELMINTHES

247

I 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-
ance, owing to its being made up of a string
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

Fig. 192

Head of Taenia
solium, magnified.
(After Leuckart.)

o.d.
Fig. 193. — Transverse section of Taenia solium, cm. circular layer of muscle ; ex. longitudinal
excretory vessel ; ne. longitudinal nerve ; o.d. oviduct : 01: ovary ; ut. uterus. (After
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 (jenitalj

248

ZOOLOGY

Ipapilla, 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

ne.ru. I

ou cjlvlt sited ou

Fig. 194. — A proglottis of Taenia solium with mature reproductive apparatus, can. txcret.
longitudinal excretory canals with transverse connecting vessels ; gl. tit. vitelline glands ;
nerv. I. longitudinal nerves ; ov, ov. ovaries (germaria) ; por. gen. genital pore ; schld. shell-
glands ; titer. uterusX; vag. vagina ; vas. def. 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 testes 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. J

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 receptacuhim
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
faeces of the host. For a time they exhibit
movements of contraction. The embryos con-
FlG'giottiT "of^Tsenia Gained within the eggs have meantime assumed
solium. (After the form of rounded bodies, each armed with
six chitinoid hooks — the six-hooked or hexacanth
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 hosNo^f 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
proscolex 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 Taenia solium ; this is the bladder- worm stagey

PHYLUM PLATYHELM1NTHES

251

/in- cysticercus. 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 Timid
gaginata ; C—B, 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 ; B, partly evaginated ; 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. serrata. (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.

"X

2. Distinctive Characters and Classification.

The Platyhelminthe#s 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

ay be absent, may be rudimentary, or may be highly developed ;

tfm

252 ZOOLOGY sE"ct.

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 byra metamorphosis.

CLASS I— TURBELLARIA.

Mostly non-parasitic Platyhelminthes with a ciliated cellular
epidermis; with a digestive cavity (except in the sub-division
Acada).

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. Acxela.

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 Temnocephala.

2 Except in the TemnocephcUea and Actinodactylella.

PHYLUM PIATYHELMINTHES 253

OUDEK 8. — Asi'lDOCOTYLEA.

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 Positive of the Examples.

Plarmria and Dendrocozlum are genera of the family Planaridw
or fresh-water Planarians, which is one of the two families of the
order Tricladida, differing from the other family, the Gcoplanidw
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 Distomidw 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 Tcenia,
of the family Tazniadce, 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 Ccelenterata
and primarily, as we have seen, associated with a fixed or stalked
condition, never being observable. A Flat- Worm has dorsal and

PHYLUM PLATYHELMTNTHES

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 Turbellaria (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-
coeles. 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,
r 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 Polycladida there is a small
ventral sucker, probably with a copulatory function ; and in
some Rhabdocoeles 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 segmentate/,, 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

f

Fig. 197.— Various Planarians. A, Con-
voluta ; B, Vortex ; C, Monotus ; D,
Thysanozoon ; B, Rhynchodemus ; F,
Bipalium ; (J, Polycelis. All natural size.
(After Von Graff.)

256

ZOOLOGY

int

znt

Fio. 19S. — Gunda segmentata. General view of the organisation, br. brain ; eye eye ;
gen. ap. genital aperture ; int. intestine with its cseca ; long. ne. longitudinal nerve-cord ;
ov. ovary ; ovd. oviduct : pe. penis ; ph. pharynx ; te. testes ; ut. uterus. (After hang.)

PHYLUM PLATYHELMINTHES

257

Turbellaria multiply by huiding, 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 Turbellarian, 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

Fio. 190. — Digenetic Trematodes. A, Amphistomum ; B, Homaloga»te>: g. p. genital
aperture ; in. mouth ; ». 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

eni

-v.d.

■vik

Fig. 200. — Monogenetic TrematOd.es. A, Gyrodactylus. til. 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 ; fcl. caudal disc of the first embryo ; h-. caudal disc of the
second embryo ; mo. mouth ; oogp. oosperm ; ov. ovary ; p. penis ; ph. pharynx ; te. testes.
B, Poh/stomum. 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

PHYU'M l'LATYHELMINTHES

259

digenetic forms — a single anterior and a single posterior sucker ; but
in some of the Digendica the posterior sucker is wanting. Adhesive
papilla 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 Gyrodadylidce

Fio. -Jul. Temnocephala minor, general view of the organisation, c. cirrus; e. s.
ejaculatory sac ; {/. c. genital atrium ; i. intestine ; o. germarium ; oo. ootype ; ph. pharynx ;
pr. prostate glands ; )•. d. strands of ducts of integumentary glands running forwards to the
tentacles ; r. g, groups of integumentary (rhabdite-forming) glands ; r.v. receptaculum ;
*. 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 Adinodactylella

s 2

260

ZOOLOGY

SECT.

(Fig. 202) a series of marginal tentacles are present in addition bo
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 (Temnocephala) vibratile cilia are

Fig. 202.— Actinodactylella. 6. c. bursa copulatrix ; br. brain ; c. penis ; i. intestine ; ov.
ovary; p}i. pharynx; r. v. receptaculum ; s. sucker; t., t. testes; ut. uterus; v. vitelline
glands ; v. 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 PLATYHELM 1 NTHES

201

by some fluid derived from their host. Pigment occurs in some
01 the ectoparasitic forms.

The relationship of the Gcstoda 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

Fig. Ji Li. Tetrarhynchus. n. nervous
system ; r. proboscides ; rs. sheaths,
with their muscles (»*■)• (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 scoleo:,
connected with the body by a narrow part or neck. The head is
usually rather radially than 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

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 Tetra-
rhynchus (Fig. 203) there are four very long and narrow rostella,
or " proboscides," covered with hooklets, 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 Caryophyllwus (Fig. 206),
Amphilina, Gyrocotyle {Amj)hiptyches — 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 Coelen-
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

lMIVIJM l'LATYHKL.MINTIIKK

263

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 Rhabdoccela, 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,

Fio. 206. — Caryophyllaug.
d. (7. vitelline duct ; d. tt. vi-
telline glands ; e. excretory
pore ; k. mobile organ ; od.
oviduct ; ov. germarium ; p.
cirrus ; r. *. receptaculum sc-
minis ; t. lobes of testes ; v. d.
tm deferens ; v. ». vesicula
seniinalis ; w.g.o. female aper-
ture. (After Leuckhart.)

Fig. 207.— Gyrocotyle < Amphiptyches
e. o. excretory opening ; ui. 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. «. receptaculum
seminis ; r. s. o. opening of vagina ; *.
sucker ; t. testes ; ut. uterus ; v. s. vesicula
seniinalis ; 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 Rhabdoccela 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

b.m

elm

i.l.m.

stinging capsules occur similar to those of the Ccelenterata, and
transition-forms between rhabdites and stinging capsules occur in

some cases. Adhesive cells with
rh 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
condition in the majority of
the Trematodes, and there is
only a homogeneous, non-
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

d.v.rn.

Fig. 209.— Section of the body-wall of a Triclad
6. m. basement membrane ; c. in. circular
muscles ; d. v. m. dorso-ventral muscles ;
e. 1. ni. external longitudinal muscles ; ep.
epidermis ; i. I. m. internal longitudinal
muscles ; p. parenchyma ; rh. rhabdites ;
rh. c. rhabdite-forming cells. (After Jijima.)

v PHYLUM PLATYHELMINTHES 265

been made in the descriptions of the examples — presenting many
varieties, filling up the interstices between tho organs and leaving
only, in some instances, very small spaces — sometimes regarded as
representing the body-cavity, or caslome, which we shall meet with
in other groups of worms: Sometimes the parenchyma appears to

Vrtv

Fig. 210. — Parenchyma of Distomum. a, b. 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-
docoele Turbellarians and a few Monogenetic Trematodes. In
some Turbellaria — species of Gonvoluta and Vortex — the paren-
chyma contains numerous cells enclosing chlorophyll or xantho-
phyll corpuscles ; these are symbiotic unicellular Algae, 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

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

Fig. 211. — General plan of the
structure of a Rhabdococle
Turbellarian. 6. c. bursa
copulatrix ; en. brain ; e. eye ;
g. germarium ; 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 ; <J ejacu-
latcry duct ; & $ common
genital aperture. (After Von
Graff.)

Fig. 212. — General plan of the structure of a Folyclad. en. brian ;
e. eye ; i., st. intestine ; In. longitudinal nerve cord ; m. mouth ;
ov. ovary ; ph. pharynx ; pM, sheath of pharynx ; t. testes ; v.
uterus vd. vas deferens ; vs. vesicula seminalis ; $ male
aperture ; 9 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

1MIYLI M l'l.ATVIIKI.MINTIIKS

207

exterior by an anal aperture. All the Turbellaria (except some
Acoela) and Trematoda have an alimentary apparatus consisting
of two well-defined parts — a muscular phaiynx 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
eversion and retraction. Actinodactylella (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 (Rhabdoccele 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-
wards from it, all three giving off
branches which again branch. In
some Polycladida 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 Rhabdocceles at the junction of
pharynx and intestine.

A bilateral nervous system is developed in all the Platy-

Fig. 213. — General plan of the structure of
a Triclad. en. brain ; e. oye ; g.
germarium : i. median limb of the in-
testine ; ii. right limb ; i3. left limb ;
In. longitudinal nerve-cord ; m. mouth ;
od' oviduct ; ph. pharynx ; t. testes ;
te. tentacles ; v. vitellaria ; v<l. vas
deferens ; u. uterus ; $ ejaculatory
duct ; 9 vagina ; £ ? common genital
aperture. (After Von Graff.)

268 ZOOLOGY sect.

helminthes. Its elements are nerve-fitoes 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-
coeles 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.
Byes 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 over 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 statocysts 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.

v PHYLUM PLATYHELMINTHES 269

The only vascular system present in the Platyhelminthes is the
system of water-ressels{p>rotonephridia) 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 _

'. ' & Fig. 214.— Flame-cell of a

posterior aperture ; frequently there are two. Turbeiiarian. /. processes ;
In the Tricladida there are two longitudinal «1 nucleus ; i?° vacuoles';
canals which open on the exterior through ££lg*jliary flame' (After
special branches by a series of pores.

In the Rhabdocoelida 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

270 ZOOLOGY sect.

separated off and does not in most cases become renewed, though
in at least one species of Tape-worm (Tamia cucumcrina), 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 hunch 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 of testes, with their ducts, thevasadeferentia, often
with a contractile terminal enlargement or vesicula seminalis, a
cirrus1 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 vaginse 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 bursa copulatrix, lined with
spines, acts as the female copulatory organ. A sac, the receptaculum,
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 Polyclad Turbellaria (Fig. 212) the testes 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-duct, with which the ducts of
the shell glands communicate (oolype), 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 copulatrix.

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 receptaculum 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 genito-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 ootype 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, arid 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 Rhabdocceles (Figs. 211 and 215) there are usually only

272

ZOOLOGY

SECT.

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
Trematodes. 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 Acwla 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 germarmm, 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

Fig. 215. — Reproductive organs of Meso-
stomum Ehrenbergii dg. duct of
vitelline glands ; do. vitelline glands ; go.
common reproductive aperture ; ov. ovary;
p. cirrus ; rs receptaculum seminis ; s.
pharynx ; t., t. testes ; ut. uterus ; vd.
vas deferens. (Prom Claus, after von
Graff and Schneider.)

H

v PHYLUM PLATYHELMINTIIKS 27.^

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 a oik -receptacle. There is nearly always a genital atrium
common to the ducts of both sexes.

In the Temnoeephalea (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-
ntlis, 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. Aetinodactylclla (Fig. 202) alone
has a htrsci copulatrix (b. c).

In the ordinary Cestodes each segment or proglottis contains a
set of reproductive organs similar. to those of a Trematode. 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. (/., Dibothriocephalas), 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 (Rhabdocoela,
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

separated off a small cell. The embryo at this stage consists of
eight cells, four large — the megameres, and four small — the
micromercs. The four micromeres increase rapidly by division, and
extend over the embryo, forming a la}^er, 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

Fio. 210.— 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 ; mes. mesoderm ; o. en. and u. en.
upper and lower endoderm. (From Korschelt and Heider, 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 PLATYUELMINTIIKS

275

become formed is, as in the Ctenophora (p. 218), a process of
cpibolic gastmlation. 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 Midlers larva (Fig. 217). It assumes

Pig. 217.— M tiller'* larva. A, longitudinal section; B, lateral view. ec. ectoderm; en.
endoderm ; g. brain ; hd. enteron ; 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 more
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

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') 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

Fia. 218.— Sections through embryos of Dendrccoclum lacteum (somewhat diagrammatic).
dz, yolk-cells ; ec, ectoderm ; en, endoderm ; ph', embryonal pharynx ; ph", permanent
pharynx : ioz, wandering cells. (From Korschelt and Heider, after Ilallez.)

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 Mcsostoma) 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 endodeim — 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
hepcUiea) which may be looked upon as typical of the Digenetic
Tiviiiatodi\s 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 Ccelenterata.
In the Trematoda, however, it is to be observed, it is an alterna-
tion of a sexual, not with an asexual, but with a parthenogenetic
generation (the sporocyst), the ova of which develop into a second
parthenogenetic generation (the rediae) ; and these finally produce
larvas (the cercariae) 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 cercariae 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 cercariae 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
cercariae of different Trematodes differ greatly, particularly with
regard to the nature of the tail. In some forms the cercaria is
tail-less : such cercariae 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 (A'), while still within the body of the parent worm,
develops another embryo (h2) 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 Polystomum passes through a larval stage with five rows of
cilia, and in Diplozoon paradoxitm, 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.

Temnocephala 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

ffim © Q o

or

Fig. 219.— Section through the blastoderm of Temnocephala, showing early stage of

endoccele (en).

endoccele 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 endoccele, 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 PLATYHELMINTHES

275)

en — vi--

si>pa rated 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

eSS-

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 Bothriocephalus ; 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
endoctele (en).

280

ZOOLOGY

CO

ccucct

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 embryo, 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 ci/stircrcus, 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 scolex 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 (Potyeercus) with
the head and rostellum enclosed by the
ciudal vesicle, a. aperture through which
evagination takes place ; bd. body ; c.
cavity of cyst ; caud. caudal vesicle ; ex.
aperture of excretory system ; ros. rostel-
lum ; s. sucker. (After Haswell and Hill.)

PHYLUM PLATYHELMINTHES

281

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 cysticercoid
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 cysticercus 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
fluid 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 in-
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 cysticcrcoid
with the three parts already described. One such form occurs

ccuui

FlO. 228. — A Cysticercoid with the rostel-
lum evaginated. ros. rostellum ; s. , x.
suckers ; raixl. caudal vesicle. (After
llnswell and Hill.)

282

ZOOLOGY

in certain Earthworms, another in a Myriapod (Glomcris
limbatus).

Taenia canurus 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 Taenia echinococcus with the developing daughter-cysts and scolices.

(After Lcuckart.)

and the same holds good of Taenia serialis from the Fox. But
the best known instance of multiple production of scolices in a
cysticercus is Tamia echinococcus — well known as cause of the
disease termed hydatids, 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

Fio. 224.— Scolices of T. echinococcus.

(After Cobbold.)

Fig. 225. — Separate scolex of
T. echinococcus. (After
Cobbold.)

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 PLATYHKLMINTHES

283

of the intestine and develop into the adult Tccnia echinocomis

— 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 feces of the Dog, may be taken into the digestive canal of

Man or of one of the domestic animals,

aiid 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.

%

Fio. 22(i. — Process of budding
in Microstomum <-., c'.
ciliated groove ; c. aye-spot ;
t. intestine ; m., »»', in..", >«.'"
mouth. (After Von Graff.)

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.

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 larva?, Water-mites.
Rotifers, small Worms, and the like ; or sometimes of Diatoms and
minute Algae 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 (Polystomnm), lives in the
urinary bladder of the Frog; another (Aspidogaster) lives in the
pericardial cavity of a Fresh- water Mussel. At least one family of
Trematodes (the Temnocephalea) 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

x PHY MM l'LATYHKLMINTHKS 285

ciliated larvae, they again enter into a state of parasitism as sporo-
cysts or rediae in a second host ; and, after a second free interval as
cercariae, may enter the body of a third host to become encysted.
The second host is, very generally, a Mollusc, and the cercaria
may beobme encysted in the same animal.

The Cestodesare, 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 Taenia cmssicollis of the intestine of
the domestic Cat, for example, the cysticercus-stage occurs in the
livers of Rats and Mice ; the cysticercus of Taenia 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
Jtepatica has occasionally been found in the human liver; Dis-
tomum rathousii is a common intestinal parasite in China ;
Opisthorchis sinensis occurs in the liver of Man in China and Japan ; •
Dicrcelium lanceolatum and various other species of the genus
occasionally occur in the human subject. Schistosomum haema-
tobium and S.japonicum, which differ from most other Trematodes
in being unisexual, are found in the human portal system of veins
ill various parts of Africa, in Arabia, the Philippines, and Japan.
Eggs with contained larvae are voided with the urine, and if they
reach water, the larvae 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
Timia 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. saginata 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 Tmnia solium
is the more prevalent ; where more beef, T. saginata. Bothrio-
cepltalus 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 Taenia eckinococeus, the presence of which produces what is termed
J hydatid disease (p. 282). The adult Taenia echinccoccus 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 cellulosa: of Tmnia 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. Cosloplana,
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 Ccelenterata 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-
i 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 may be a polyphyletic group—

v

PHYLUM PLATYHELMINTHES

287

/.< ., that different families may have become developed from
different families of Turbellaria altogether independently, some
of them appearing to be nearer the Rhabdoaeles, others nearer
the Pol \ cl.it Is, 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 pocdo-
genetic and parthenogenetic generation (hcterogeny). The sporocyst
and redia are to be regarded as intercalated stages — as cercaria)
which exhibit pedogenesis. 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. Caryophylla'us (Fig. 206) is a Cestode

Monogenetica

Nemertinea

Polycladida

iTemnocephalea
Tricladida

Rhabdocoelida

Diqenetica Polyzoa
Monozoa

Lower Coelenterata
Fio. 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), Amphilina, and Archigetes (Fig. 208).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 stuge, and have undergone
a precocious development of the sexual apparatus.

J

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., Moniezia)
tells in favour of the view that the scolex is anterior and
corresponds to a head.

APPENDIX TO PLATY HELMINTH ES.

Class Nemertjnea.

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.), the possession of which is one

PHYLUM PLATYHKLMINTHEK

289

ong.ne

of the most characteristic features of this class of Worms. The
proboscis is hollow ; when extended to its utmost, a part still
minims 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
it msists 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 DrejMnophoms 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 (rhynclwccele) 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

Fio. 228. — Diagram of the organs of a
Nemertine, from below, a, anus ; br.
brain ; div. cseca ; long. ne. longitudinal
nerve-cords ; m. mouth ; n. nephridia ;
ov. ovaries ; pr. proboscis. (After
Ilubrecht.)

290

ZOOLOGY

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 Bimyaria)
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 ccelome or body-cavity, except, perhaps, the

A

m.p

3P

Fig. 229.— Diagrammatic representation of proboscis : (A) in the retracted condition, (B) in the
everted condition, g. p. glandular portion of the proboscis ; m. muscle attaching the
proboscis to its sheath ; m. 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 (stomodecum)
— 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

{Metanemertini) 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 caeca 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 Tcmnocephala).

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 {dais.
res.) and two lateral {lot.
ves.). The blood is, in
most cases, colourless, and
contains rounded or ellipti-
cal, usually colourless, cor-
puscles.

proh a/b

lot. ne
lat.ves

dors.ves

op. neph

doTs.ves

retrmus

an

Fui. 230.— Tetrastemma. General view of the
internal organs, an. anus ; nc. St. accessory
stylet ; cer. g. brain ; cil. or. ciliated groove of
cerebral organ ; ilors. ves. dorsal vessel ; lat. ne.
lateral nerve ; lat. ves. lateral vessel ; neph.
nephridium ; op. neph. nephridial aperture ;
proM. eversible part of proboscis ; prob'*. non-
cversible part of proboscis ; prob. ap. aperture
for the protrusion of the proboscis ; retr. »»«*.
retractor muscle of the proboscis ; st. stylet.
(From Hatschek's Lehrbuch.)

u 2

292

ZOOLOGY

* The excretory system has a considerable resemblance to that
of the Platyhelminthes. It consists of a pair of longitudinal

vessels (Fig. 235, neph.) which give off
branches, by one or several of which each
communicates with the exterior. The
fine 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
Tnrbdlaria. The brain (Figs. 228 and
231, br., and Fig. 230 cer. g.) 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,
the proboscis and proboscis sheath, and
the mouth and oesophagus, „ From the brain
pair of thick longitudinal nerve-cords which

Flo. 231. — Anterior portion
the body of a Nemertine.
l>r. brain-lobes ; n. lateral
nerves ; p. o. external open-
ing through which the pro-
boscis is everted ; p. s. pro-
boscis-sheath ; pr. proboscis.
OEsophagus and mouth shown
by dotted lines. (After
Hubrecht.)

the anterior part
both being above
pass backwards a

of

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 PLATYHELMINTHES

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

lane;, m

longne

I'm;. 884.- Diagrammatic transverse section of a Nemertean (Carinclla). a, b, e. layers of
body-wall ; r. t. connective tissue between body-wall and enteron ; I. bv. lateral blood-vessels ;
lomj. ne. longitudinal nerves ; p. proboscis ; p. s. proboscis-slieath. (After Hubreebt.)

the Hetcronemertini is a nerve-plexus extending all over the body.
In the Mctancmertmi, 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,

ncph

teLblv

*— - medblv

Pig. SS6.— Anterior portion of a Nemertean (Drepanopfaorus), showing the bl l-Ta«ular

and excretory systems, lat.bl.v. lateral bl< Kid-vessels ; med. bl. v. median blood-vessels;
ntph. nephridial (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. 236.— A, Pilidium with advanced Nemertine 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 ; <x. gullet ; pr. proboscis ; st. stomach. (From Balfour, after Btitschli.)

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 dicecious. The ovaries
(Fig. 228), o?;.) and testes are situated in the intervals between the
intestinal ca>ca. 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 ccelome. 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 flames.

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.

296 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 caecum 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 dorso-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 arc extremely abundant in fresh and salt
water.

Class 2. Acanthocephala. — The " Hook-headed Worms," a
group of formidable internal parasites.

Class 3. Chjetognatha. — The " Arrow-worms," a small group of
pelagic organisms.

The affinities of the Acanthocephala and Chaetognatha with the
Nethatoda 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 lumh-icoides.)

r Ascaris lumbricoides 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. mcgalocephala) in the Horse. The following
description will apply to any of these. The female A. lumbricoides
is about 20-40 cm. (8-16 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 jS

298

ZOOLOGY

SECT.

fl

iving Worm, are respectively dorsal and ventral in position, and
are called the dorsal (Fig. 237, d.l.) and ventral (v. I.) 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. lp.), the other two ventro-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 sctai (pn. s.).
In the female the reproductive aperture or gonopore is separate
from the anus, and is situated on the ventral surface about one-
third of the length of the body from the anterior end (Fig. 240,
gnp.). The sexes are also distinguished externally by the form of

Fio. 237. — Ascaris lumbricoides. A, anterior end from above ; B. the same from below ;
C, posterior end of female, D, of male, side view. an. anus ; d. lp. dorsal lip ; d. I. dorsal line ;
ex. p. excretory pore ; p. papillae ; pn. s. penial setae ; v. 1. ventral line ; v. lp. 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 (G)
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}7ers, 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 a, 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 fibrc§ (m.),
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 t-uv (i.e. towards the body-cavity) into a large and almost
bladder-like mass of protoplasm (p) containing a nucleus (nu.).
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)i 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

der. eplhm, di

int

loll

acv

Flo. 23S. — Ascaris lumbricoides, transverse section, cm. cuticle ; d. I. dorsal line ; der. epthm.
deric epithelium or epidermis ; ex. v. excretory vessel ; int. intestine ; lat. I. lateral line ; m.
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-
lateral, 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. I.). 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 stomodamm (Fig. 240, /

..c ^

300

ZOOLOGY

SECT.

f 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

vksuaZ- \ -c^im to

Fig. 23!K— Ascaris lumbricoides. A, a single muscle fibre ; B, several fibres in transverse
section with portion of ectoderm (below), c, contractile substance ; /. fibrous processes ; ntc.
nucleus ; p. protoplasmic portion. (After Louck^rt.)

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 stomodreum. 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. The epithelium of the intestine, on the other hand, is
endodcrmal, this portion of the canal being derived from the
archenteron of the embryo.

y

PHYLUM NEMATHFXMINTIIKK

f

lift ween the enteric canal and
the body-wall is a distinct space,
the body-cavity, containing a

clear tin id 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 coelome, 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
(dln,vhi.). They are connected with
one another by transverse com-
missures (&), and the ventral nerve
swells into a ganglion just in
front of the anus. The pharyngeal
nerve-ring contains nerve-cells, and /

302

ZOOLOGY

SECT.

-din

its ventral portion (un.) is thickened and ganglion-like. The only
sense-organs are little elevations, the sensory papillw (Fig. 237, p.),
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 seminalis (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 setce (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. e.); 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 (vag.) which opens, as already seen, on
the ventral surface of the body (gnp.) at about one-third of the »
entire length from the head. —*

— haii

Fio. 241.— Diagram of nervous
system of Nematoda. c.
commissures ; din. dorsal
nerve ; km. posterior lateral
nerve ; on. upper and mm.
under portion of nerve-
ring ; s. g. lateral swellings ;
vln. ventral nerve. (From
Lang, after Butsehli.)

vi PHYLUM NEMATHELMINTHES 303

Development. — The eggs arc 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

tnt

der.e/jthm.

1

Fio. 242. — Asearis lumbricoides, posterior extremity of male, dissected, an, anus ;
cu. cuticle ; der. eptltm. 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. - S

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 stomodgeum.
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 pharyngeal 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 Gooxlius), 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 Ascaridw of the order Nematoidea.

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 Ascaridse are distinguished by
the possession of three lips furnished with papillae, and by the
body of the male being curved ventrally and being provided
with penial seta?. Ascaris is distinguished from the other

VI

PHYLUM NEMATHELM1NTHES

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 Gordms, 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 0*5 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) the 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 platymyarian
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

Fio. J 14— Ankylostoma duodenale. A, male and female in coitu. B, anterior eml,
showing — n\ gl. 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 m^er-cellular
but m£ra-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

lh*J

Fig. 245.— Transverse section of Gordius. bm. 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 oesophageal glands or intestinal
cseca. In Dochmius a pair of pear-shaped bodies of unknown
function, the cervical glands (Fig. 244, B, cv. gl.), lie one on each
side of the pharynx and probably open externally near the mouth.

VI

PHYLUM NEMATHELMINTHES

307

-.7*

int.

-bs

§■•'».!; J,

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

■<¥

,/brv.a

FI0. 24(>

Oxyuris, from the
right side. gz. gizzard ; int.
intestine ; ph. pharynx
pn. s. penial seta: ; ts. testis ;
v. df. vas deferens. (From
Shipley, after Galeb.)

308

ZOOLOGY

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, ovy.)
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, itt.) and are continued posteriorly by short
vaginae (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

sPtfl v.nv.cd v<*g

Pio. 247.— Gordius. A, horizontal section of female, showing ovaries (nvy) attached to mesen-
tery (me*.) ; 6. w. body-wall. B, posterior extremity of male, sagittal section, b. c. bursa
copulatrix ; cl. cloaca ; int. intestine ; t, tail ; v. nv.cd. ventral nerve-cord ; vs. sent, vesicula
seminalis. C, posterior extremity of female, sagittal section, gnp. gonopore ; spth. sperma-
theca ; ut. uterus ; vag. vagina ; v. nv. cd. ventral nerve-cord. (After Vejdowsky.)

early, after discharging their contents into large reservoirs or
vesicidce 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 Rhabdoncma nigrovcnosum (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

\ [

PHYLUM NEMATHKLM I NTHES

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 prododceu?n. 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

Fn;. '24S.— Development of Ascaris nigrovenosa. Up. blastopore ; ect. ectoderm ; end. endo-
derm ; ent. enterou ; g. genital cell ; mes. mesoderm ; n. nervous system ; stdm. stomodanim.
(From Korschelt and Heider, after Goette.)

ectoderm and the endoderm (E — H.) The body-cavity is not a
true coelome (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.

Rhdbdonema 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 Bhabditis- form, 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 dioecious form. This type of alternation of generations is
distinguished as heterogeny.
^ One of the most terrible parasites of man is Trichinella spiralis
/ (Fig. 249), a minute worm, the male (C) a little over 1 mm. (^ 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 Trichina; 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,

PHYLl M NEMATHELMINTHKS

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,

Pro. _'4.i.— TrichineUa spiralis. A, encysted form, in muscle of host ; B, female ; C, male.
bh. connective-tissue envelope ; cy. cyst; de. ejaculatory duct; e. embryos, /. lat-globules ;
ft. testis ; »i. /. muscle-fibre ; oe. pharynx ; ov. ovary ; wo. 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

1

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. (1— 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. bancrofti 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
Echinorhynchus 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. (Gigantorhynchus) gigas, 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 ehitinoid 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, n. )
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 (gnp.) : connected with this, in
the male, is a protrusible, bell-like structure, the bursa (b.), 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

PHYLUM NEMATHELMINTHES

313

t ! -ansverse, and then one of longtitudinal muscles. The body-wall thus constituted
encloses a spacious body-cavity or ccelome containing a clear fluid.

In oorreapondenoe 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-
w onus, 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

W

Fio. 250.— A, Echinorhynchus (Gigantorhynchus) gigas, female from the Pig (nat.
size ; B, B. lesinif oralis, 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 ; *. 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 (r.) 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 nepbridia (Fig. 253) have been
found to occur in Echinorhynchus gigas. These consist of a pair of ramified

314

ZOOLOGY

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.

■Lnv

s.lff

o"

rs.ovy

A

Fig. 2 •".!. Echinorhynchus gigas. Dissec-
tion of male. b. bursa ; c. gl. cement glands ;
ha. lemnisci ; nv. nerve-ganglion ; pr. pro-
boscis ; s. Ig. suspensory ligament ; fct. testis ;
v. df. vas deferens. (After Leuckart.)

Fig. 252.— Echinorhynchus

gigas. Dissection of female
(semi-diagrammatic), b. bell ;
Un, lemnisci ; pr. proboscis ;
a. ovy. swimming ovaries ; v.t.
uterus ; vg. vagina. .

In both sexes the gonads and their ducts are connected with a great suspt nsory
ligament (x.lg.), which extends backwards from the end of the proboscis-sheath.
In the male there are two pvoidal testes (Fig. 251, ts.) connected with the

VI

PHYLUM NEMATHKLM1NTHKS

315

suspensory ligament. From each a van deferens (v. df.), furnished with several
peawmta *< minales or sacs for the storage of spermatic fluid, passes backwards

Fi<;. 253.— A, longitudinal section through the terminal twigs of the nephridia of Echino-
rhynchuB gigas ; highly magnified, a, 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 ejaeulalory duct, with which are connected
alxmt half a dozen cement glands (c.gl.). The ejaculatory duct opens into the
bursa or bell-like copulatory organ (b), and has at its
opening a small papilla acting as a penis.

In the female the ovary is connected witli the sus-
pensory ligament (Figs. 252 and 254, *.ly.). 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 (b), "widely
open anteriorly (Fig. 254, x) into the coelomer and
having towards its posterior end a small aperture, also
communicating with the ccelome (y). The bell is con-
nected with a narrow double passage leading to a
uterus (tit.), 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 ccelome they are swallowed by the bell. The im-
mature eggs, which are globular, are passed back into I — -lit
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
cadome. Segmentation is regular, and a peculiar form
of gastrula is produced, having neither archenteron nor
blastoco3le--in other words the ectoderm and endoderm ^_Jr^-

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

Fio. 254. — Female organs of
Echinorhynchus .
b. uterine bell : ». l<i.
susjnmsory ligament ;
nl. uterus ; r<i. vagina ;
.'•. ?/. aperture of bell ;
z. apertures leading
from bell to uterus.
(After Hertwig.)

310

ZOOLOGY

-rft

%

a

in-

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 faeces. 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, Getonia aurata. The Echinorhynchi of
fresh-water Fish have for their intermediate host
<SC — A / certain small fresh-water Crustacea belonging to

the genera Gammarus and Asellus.

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 coelome. Part of the proboscis and
its sheath are also of endodermal origin. The
ectoderm gives rise to the protoplasmic layer of
—ova. tjjg 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

d. adult proportions and attains sexual maturity

only if the intermediate host is eaten by the
permanent host.

CLASS III.— CH.ET0GNATHA.

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 Jin («. fl.), and there are also
horizontal lateral fins (H.) — 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. eplhm) ,
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

•rfl—

Fig. 255.— Sagitta hexaptera,

from the ventral aspect, a.
anus ; 6,<7. ventral ganglion ;
d. intestine ; fl. lateral fins ;
ho. testis ; m. mouth; ov. ovary;
ovd. oviduct ; sc. oesophageal
connective ; sb. vesicula semin-
alis ; s. fl. tail fin ; $h, tail-
cavity ; si. spermiduct. (From
Lang's Qompctrativt Anatomy,
after Hertwig.)

preceding classes, is formed of
delicate basement membrane,

\ I

PHYLUM NEMATHELMINTHES

317

which are striated and disposed longitudinally in four hands — 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.eptfvm

Fio. 256.— Sagitta bipunctata. Transverse sections, A, of trunk ; B, of tail. coel. coelome ;
ctel. epthm. layer of nuclei of the muscle-cells formerly regarded as a coelomic epithelium ;
d. epthm. deric epithelium ; /. fin ; int. intestine ; m. muscles ; ovy. ovary ; U. testis. (After
Hertwig.)

chitinoid hooks (Fig. 257 yh.) 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 stomodaeum into a straight intes-
tine (d), which extends through the trunk
and opens by the anus (a) at the junction
of trunk and tail.

Coelome. — At the junction 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 ccelome 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, g), which
sends off on each side a long nerve-cord, the (esophageal connective («c). The two
connectives sweep round the enteric canal and unite on the ventral surface, not

Fio. 257 — Head of Sagitta bipunctata,
from above, an. optic nerve ; au. eye ;
g. brain : gh. hooks ; rn. olfactory nerve ;
ro: olfactory organ ; se. oesophageal
connective. (From Lang's Comparative
Anatomy, after Hertwig.;

318

ZOOLOGY

SKCT.

far from the middle of the trunk, in an elongated ventral ganglion (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 (ro.), 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 papillae
carrying stiff bristle-like processes, and probably serving as organs of touch.

There are two eyes (Fig. 258),

st

situated one on each side of the
dorsal surface of the head : each
is globular and contains three
biconvex lenses (I.), 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.

Reproduction. — The Clneto-
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, ts.) 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 ccelome. The spermi-
ducts or vasa'deferentia are delicate tubes (d.) opening at one end into the coelome
by a ciliated funnel-like extremity, and at the other end dilating into a reservoir
or vesicula aemrnaiis (.s-6. ), 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 (j/.)at the anterior end of the archenteron, i.e. the

Fig. 25S.— Section of eye of Sagitta hexaptera.
ep. epiderm ; I. lens ; p. pigment ; rz. visual
cells ; st. rods. (From Lang's Comparative
Anatomy, after O. Hertwig.)

Fig. 250.— Three stages in the development of Sagitta. hi. blastopore; cs. coelomic sacs; d.
mesenteron ; g. sexual cells ; pin. parietal layer of mesoderm ; st. stomodajum ; vm. 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, g.), 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
« a\ it \ into three parts a middle division or mutHteron ('/) the rudiment of the
intestine, and two lateral divisions- tin- /in/inhra, or cwlomic saes (cs.)- — which
give rise to the right and left compartments of the cudome 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
("I'lomic 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 invagination of ectoderm — the stomodamm (st. ) — takes place at the
anterior end, and finally communicates with the mesenteron.

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
remain* 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 Chcetosomidce.

This family includes three genera of small
worms, Chivtasoma (Fig. 260), Tristicocluvta, ami
Jihabdogcuiter, 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 (/),
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.

Pro, 260. — Mature female of
CU.-rtosoma claparedii.

x 57. a, oesophagus ; 6, in-
testine ; c, anus; d, ovary;
«. generative pore ; ./, loco-
motor rods. (From Shipley,
after Metschnikoff.)

2. Family Echinoderidce.

Echinoderei is a minute marine worm of cybndrical 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. 261).
The mouth is placed at the anterior, the anus at the posterior end of the body :

320

ZOOLOGY

the former opens into a sac, which can be everted so as to form a proboscis or

introvert, 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 are paired sacs

opening at the posterior end of the body.

3. Family Desmoscolecidaz.

Desmoscohx 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 setse, and a pair of

Fig 261 . Echinoderes, x about 210. 6, spine ;
c.s. caudal spine ; ph. pharynx ; s. and s'. spines
on the proboscis ; s.g. salivary glands ; at. stomach.
(After Hartog.)

Fio. 262.— Female Desmoscolex,
ventral view, x 260. a, ovary.
(From Shipley, after Panceri).

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, and the propriety of grouping them into a single plvylum
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 coelome ; there is a certain resemblance

m PHYLUM NEMATHELMINTHES 321

between Nematoda and Chaetognatha in the muscular system ;
and the leinnisci 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 Chaetognatha. 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
Nemathelminlhes. In connection with this question, the Chaeto-
somidae, briefly described in the Appendix (p. 319), seem to
require consideration. Other possible relationships suggested by
the mode of development of the ccelome 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 Chaetosomidae, and still
more in those of Desmoscolex. But the segmentation of the body
in both Desmoscolecidae and Echinoderidae 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 ^WSJHIk ,v.LA/
oe.LM .

d.LM

Neph
Msbr

Fio. 263. — Trochophore. A. anus ; d. IM. dorsal muscles ; ED. rectum ; J. stomach ; fl-. in-
testine ; Mstr. mesoderm-band ; n. nerves ; Neph. nephridia ; 0. mouth ; Oe. (esophagus ;
ceLM. oesophageal longitudinal muscle; SP. apical plate; v.LM. ventral muscle; v. IN.
lateral nerve ; Wkr, ickr., pre- and post-oral bands of cilia ; ]\'S. apical cilia ; wz. 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

vii PHYLUM TROCHELMTNTHES 323

behind the pre-onil 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, >vhich 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 Histriobdelka.

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 £ mm. (TVin.)in

Y 2

324

ZOOLOGY

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,

Fig. 264. — Brachionus rubens, female. A, from the dorsal aspect ; B, from the right side.
a. anus ; br. brain ; d. j. dorsal feeler ; c. gl. cement-gland ; cl. cloaca ; c. I. 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. d.
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 trochal 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

ui PHYLUM TROCHELMINTHES 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, mth.) 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 u.

of its cuticular lining, an y^ \f ^\

elaborate apparatus for tri- / jr^v^ Vp^\V-\\

turating the food. In the / /^/V]p|] \f^&//S\ \

middle line is a forked struc- / (/ ^vr^Yr^^^\K. \

ture, the incus, consisting of a ™\~~ff/7 <-' ^^A^\ \\ ) )
small base or fulcrum (j.) and \ \J (( ----- - 17 \" "Ay I

of two branches or rami (r.). ^i~£_-- -Jl\^-^~\~-^

On either side of the incus is y r

a hammer-like structure, the „ mmm _ . _

.. . . „ i li Fig. 265 —Pharynx of Brachionus rubens.

ma/leUS, Consisting OI a handle /. fulcrum ; m. manubrium ; u. uncus ; r.

or manubrium (m.) and of a ramu8- (After Hud8on and Gosse-)

toothed head or uncus (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 phaiynx 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 stomodaeum, the cloaca from the proctodeum. 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

The excretory system consists of paired nephridial tithes
(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, br.), of proportionally large

ov'

Fig. 2t>0. — Brachionus rubens. A, male ; B, female, with attached eggs ; c. gl. cement-
glands ; a), contractile vesicle ; nph. nephridial-tube ; ov. ovum in body ; ov^. 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 thetrochal 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,

\n PHYLUM TROCHELMINTHEX 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

Fk;. 'JOT. —Diagram of a Rotifer, a. anus ; br. brain ; c1. pre-oral, and c'2. post-oral circlet of cilia ;
c. gl. cement gland ; cl. cloaca ; eu. cuticle ; d. ep. deric epithelium ; d.f. dorsal feeler ; e. eye ;
fi. e. flame-cells ; int. intestine ; m. muscles ; mth. mouth ; nph. nephridial tube ; ov. ovum ;
ovd. oviduct ; 017/. 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
nephridiaKtubes^ 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-cesophageal 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. — Khizota.

Rotifera which are fixed in the adult state by the truncated end
of the non-retractile tail.

Including Floscularia, Stephanoceros, Melicerta, &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 distally.

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.

Ploima in which the trunk is not covered by a lorica.
Including HydatAna, Polyarthra Asplanchna, &c.

Sub-order b. — Loricata.

Ploima in which a lorica is present.
Including Brachionus, Euchlanis, &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 5. — Trochosph^erida.

Globular Rotifera having the trochal disc represented by an equa-
torial circlet of cilia ; tail absent.
Including Trochosphcera only.

PHYLUM TR0CHELM1NTHES

329

Order 6. — Seisonida.

Marine parasitic Rotifers (Fig. 268), with the trochal disc
reduced, the body long, narrow, and ringed, with a long slender

01/

Fio. 268.— Paraseison asplanchnus, female x 230. a. genital aperture ; br. ganglion ; /.
foot-glands ; to. mouth ; ma. mastax ; oe. oesophagus ; ov. ovary ; *(. stomach. (After Plate.)

neck-region, and an elongated foot provided at its extremity with
a perforated disc.

Systematic Position of the Example.

Brachionus rubens is one of the several species of the genus
Brachionus : it belongs to the family Brachionidm, and to the
sub-order Loricata of the order Ploima.

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 Brachionidte 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 Ploima 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 Ploima, e.g. Polyarthra (Fig. 269, S) possess simple
or fringed seta? moved by muscles attached to their bases.

In the Rhizota the adult is permanently fixed (Fig. 269, 1-Ji).
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 retract itself completely,
protruding the anterior end with the trochal disc when undis-
turbed. In most instances, as for example in Floscularia (1) and
Stephanocerus (#), the tube is formed of a delicate, transparent,
gelatinous secretion of the epidermis ; but in Melicerta (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 faeces.

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 (8), 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

l.Floscularia 2. 5ref)han oceros 3.Melicerra

4.Melicerra 5.Philodina 6. A s pla nchna

7.Hy;cIarina

10. N o rhoica

Fig. 269.— Typical forms of Kotifera :— 9 aiid 10 show the lorioa only. a. anus ; cK A ciliary
circlets ; int. intestine ; j». muscle ; ph. pharynx. (After Hudson and Gosse.)

332

ZOOLOGY

the first. The mouth in this case is always placed between the two
circlets on the ventral side (Fig. 2(57), so that the inner or anterior
circlet is pre-oral and corresponds with the chief ciliary band

E.Hex a rthro

3.Trocho s f>h a e r a

Fig. 270. — Typical forms of Rotifer a. In 1, a shows the outer form, 6 the muscular system.
a. anus ; br. brain ; A c2. ciliary circlets ; cl. cloaca ; d. gl. digestive gland ; d. I. dorsal limb ;
e. eye-spot; I. I., I. I', lateral limbs; in, muscles; mtk, mouth; nph. nephridial tube;
ov. ovary ; ph. pharynx ; ,«. 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 Trochosphwra (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

PHYLUM TR0CHELM1NTHES

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 (u). 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, ramate type.
/. fulcrum ; m. manubrium ; r. ramus ; w. 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 defalcation
(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 otT 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-cesophageal 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 oesophageal
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., l.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 Bdelloiida, 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 it, 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
stomodseum and proctodeum. The tail is formed as a prolongation

\n 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 Nebalia (Fig. 457). Others, again,
are internal parasites, such as Albertia in the coelome of Earthworms
and the intestines of fresh- water Oligochsetes (Nais), and Notommata
u-crneclcii 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 Trochosphcbra is, to all intents and purposes, a sexually
mature trochosphere with a mastax. The excretory organs recall
those of the Platyhelminthes, and 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 Gatfrotricha (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 partly sensory, partly
vibratile. The mouth, situated at the anterior end, leads by 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 set*. 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.

Fia. 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 TROCHELMLNTHES.

The Dinophilea and Histriobdellea.

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

PHYLUM TROCHELMINTHES

337

Annulata — a phylum to be treated of later. The Dinophilea are free-living animals,
mostly marine, one species living in brackish water. The Histriobdellea are
parasitic or commensal, ami live on the European lobster and the Australian
fresh-water crayfishes.

Dinophilus (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 on

Fi<;. 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 ; 6. rectum ; c. body-cavity ; d. vas
deferens ; m. pharynx ; n'. the first nephridium ; «. entrance to the oesophagus ; p., in left
fig., prostomium; p., in right fig., penis; st. stomach; $. x. vesiculse 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

tubes (n'). The inner ends of these do not open into the body-cavity, but are
provided with peculiarly modified flagellate cells known as solenocyte*, so that
these paired excretory tubes resemble closely the nephridia of some of the Poly-
chseta (phylum Annidata ; see Section X.). The sexes are separate. In the male
there is a conical ventral penis ; the last pair of nephridia act as vesiculas 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 Dinophilus 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 Histriob-
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
{t1, t2, t3) 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 Stratiodrilus, three pairs of two-
jointed non-retractile appendages or
cirri (c1, c2, c3) tipped with non-
motile cilia, and in the male a pair
of retractile appendages or claspers
(d). At the end of the tail is a pair
of large freely movable appendages
or legs (Ip), 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
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 oesophageal connectives, and a ventral
nerve cord (nc) with a series of ganglia which have a distinctly metameric

Fig. 275.— Stratiodrilus tasmanicus,

male. ac. accessory gland of male ap-
paratus ; br. c. brain ; gl, c2. c3. cirri ; cl.
claspers (appendages peculiar to the male) ;
ex. excretory tubes ; gr. gld. granule-gland ;
I. 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 ; &. i2. t:i. tentacles ;
vea. vesicula setninalis.

vn 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 Lb n<> metamorphosis.

There seems to be some reason for believing that Dinophilus and the
Histriobdellea may help to bridge over the interval between the Trochelminthes
and the higher segmented worms or Annulata. In this connection the
Kchinoderidie, which were noticed in an appendix to the last Section (p. 319),
have also to be kept in view.

z 2

SECTION VIII
PHYLUM MOLLUSCOIDA1

The phylum Molluscoida comprises three classes — the Polyzoa
(including, provisionally, the Endoprocta), the Brachiopoda,
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 (ccelome), lined
in most cases with a ccelomic 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 coelome, 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-cesophageal), 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,'' 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 coelome, 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 Chsetognatha. In some of the groups which are here
comprised in the ccelomate phyla, however, as will be seen, the coelome is
reduced, or entirely absent, or not typically developed.

340

m r. mm 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 Birds-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 zoaecia of the colony,
which are closely united together and arranged in four longitudinal
rows. The zocecia 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
" mouth " of the zoopcium — on either side of which is a short blunt
spine. A rounded structure — the ocecium — in many parts of the
colony lies in front of each zocecium (Fig. 276, ocec). On each-
zocecium, 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 zocecia 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 lophophore 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 ectocyst and eiidocyat are commonly applied respectively to the
hardened cuticle of the zooid and its soft body-wall.

342

ZOOLOGY

and with them food-particles, towards the mouth {mo) : they are
also capable of being bent in various directions. In the interior of

tent

OJJIC-

avw

Pig. 276. — Bugula avicularia. Two zooids, magnified, an. anus ; avk. avicularia ; emb.
embryo enclosed in the ooecium ; funic, funiculus ; gaxt. muscular bands passing from the
stomach to the body- wall ; int. intestine ; mo. mouth ; ok. ooeciunu; ces. oesophagus ; ov. ovary ;
ph. pharynx ; ret. parieto- vaginal muscles ; sp. spermatidia ; stom. stomach ; lent, 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

vni PHYLUM MOLLUSCOTDA 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 coelome 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 coelome 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 — the pharynx (ph) — just behind the bases of the tentacles ;
from this a somewhat narrower short tube, separated by a constric-
tion from the pliarynx, leads to the stomach (stom) from which it is
also separated by a constriction. The stomach gives off a long
conical prolongation or cozcum 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 zocecium ; 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 zocecium — 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 ccelome.
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 ooecium. 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

\ III

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.

cct

Fig. 277. — Early stages in the development of Bugula. cent, central mass of cells ; cor. corona
ect. ectoderm; end. endoderm ; «eg. 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 inaddition to
this sac, there is only a mass of undifferentiated tissue derived
from the original central tissue together with that derived

disc

cor

cent

suok

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. ectodermal groove ; p. pyri-
form organ ; pall, pallial groove ; such, 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 zocecium, 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 ; 8. basal' plate of everted
sucker ; v. vestibule. (From
Korschelt and Heider, after
Bari»is.)

\m PHYLUM MOLLUSCOIDA 347

described. In many of the zocecia 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 zocecium. 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 — jthe 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 calcified, 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 ccelome.

Order ]. — Gymnolaemata.

Almost exclusively marine Ectoprocta, with a circular lopho-
phore, and without an epistome.

Sub- order a. — Cyclostomata.

Gymnolaemata with tubular calcareous zooecia having circular
apertures devoid of closing apparatus.
Including Ci'isia, Idmonea, &c.

Sub-order b. — Cheilostomata.

Gymnolaemata with calcareous or chitinous zooecia usually pro-
vided with opercula.

Including Bugula, Flustra (" Sea-mat ") Membranipora, Gellepora,
Selenaria.

348 ZOOLOGY sect.

Sul-order c. — Ctenostomata.

Gymnoluemata with chitinous or gelatinous zocecia provided with
a series of tooth-like processes closing the aperture Avhen 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 Bicellariidse,
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 zocecia all facing in the same
direction. Bugula differs from the other genera of the family in
the arrangement of the zocecia 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 zocecia 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

vim PHYLUM MOLLUSCOIDA 349

colony varies in different families and genera in accordance with
differences in the shape of the constituent zocecia, 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 zocecia 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

stato

Fics. 280. — PlumateUa. Portion of a colony, magnified, funic, funiculus ; gang, ganglion ;
int. intestine ; mo. mouth ; ce. oesophagus ; repr. gonad ; retv. retractor muscle ; st. stomach ;
stato. stat "blasts. (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 Phylactolaemata
(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 zocecia open on the exterior by means of circular, semi-
circular, or crescentic apertures, which in the Phylactolaemata and
the Cyclostomata among the Gymnolaemata are devoid of any special
closing apparatus ; while in the Cheilostomata there is a movable

350

ZOOLOGY

lid or operculum closed by a pair of occlusor muscles when the
introvert is retracted ; and in the 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 zocecium. 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 ccelome. 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

\ III

PHYLUM MOLLUSCOIDA

351

which

used for prehension. In the Phylactolaemata (Fig. 280) the
lophophoro is horse-shoe-shaped, in the Gymnolapmata (Fig. 276)
circular : in the former, but not in the latter, there is a ciliated
lobe, the epistome (Fig. 282, ep) — which may have a sensory func-
tion— overhanging the mouth on the anal side. The retraction oi
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
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 (Phylactolaemata) by a
definite ccelomic 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. Crossipg 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
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, ga) 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

Fio. 282. — Anterior portion

body of
Lophopus. from the right side. an. anus ;
ep. epistome ; ga. ganglion ; o. mouth ; pr. in-
testine ; st. oesophagus ; t. tentacles, cut off
near the base. (From Lang's Comparative
Anatomy. After Allman.)

end of the zooecium ; in most

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 coelomic 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
arc 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 Gymnolsemata (Cheilostomata) these
ovicells or ocecia, 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 coelomic
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 coelome. 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 zooecium
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

\in PHYLUM MOLLCJSCOIDA 353

zooid has undergone degeneration, or through a special opening
formed for them in the wall of the zocecium. In some the fertilised
ova pass out through the intertentacular tube. In Crista and other
Cyclostomata each of the ripe ooecia 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 Bugula
(p. 344), a primary zocecium 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
Phylactolaemata 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 statoblasts
(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-
laemata, 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 Palaeozoic 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, Urnatclla, Myosoma, Gonopodaria 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 coelome 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-called foot-gland,
consisting of a small number of granular cells arranged around a
central space opening on the exterior. Buds are formed, but

vni 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

lent

yaruj

iKistf

Fig. 2S3.— Pedicellina. Showing successive stages (numbered 1 to 6) 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 ccelome, which is lined with a ccelomic
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 {inetacozlc),
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, m, m.), which extends from the oral
to the anal surface and supports both limbs of the alimentary

Fig. 284.— Phoronis
australis, natural
size.

\ 111

PHYLUM MOLLUSCOIDA

357

an nepkr nePh

Fig. 280.— Phoronis australis, free end,
magnified, an. anus ; ep. epistome ; mo. mouth ;
nephr. nephridial aperture ; neph. nephridium ;
(After Benham.)

canal ; and each of these is further subdivided by a longitudinal
mesentery extending from the body-wall to the oesophagus (os)
in the one compartment

(usually termed the right), ,«t>

and to the rectum (r) in
the other (left). The ali-
mentary canal is bent on
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 blood-
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
coelome — ), 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 is
formed with clearer ectoderm cells on one side ; invagination
takes place; and, as the embryo elongates, the blastopore is

In.. 886.— Phoronis australis, internal
organisation. a/. W. afferent blood vessel ;
an. anus ; ef.bl. efferent blood vessel ; ep.
epistome ; me*, mesentery ; mo. mouth ;
iitphr p. uepbridiopore ; nephr. d. duct
of nephridium ; nephrost. nepbrostome
(internal opening of nephridium ; ce». oeso-
phagus ; red. rectum ; red. met. recta.1
mesentery ; gjpt. septum ; tent, tentacles
(cut short). (After Benham.)

358

ZOOLOGY

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 metaccele
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

Fig. 287. — Phoronis, transverse section towards the anterior..end. of. v. afferent blood-vessel ;
c. mi. circular layer of muscular fibres ; ef. v. efferent blood-vessel ; ep. epidermis ; c. in. cir-
cular layer of muscle ; hi, m. mesenteries ; we. /. funnel-like opening of nephridium ; os.
oesophagus ; r. 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. — Fhoronls, 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 invagina-
tion partly everted ; E. invagination completely everted, m. mouth ; an. anus ; iv. involution
to form body. (From Balfour's Embryology.)

The ectoderm of the process on 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

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.— BRACHI0P0DA.

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. flavcscens.

Magellania lenticularis is found in great numbers, at moderate
depths, off the coast of New Zealand. An allied species, M. fiavescens,
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 fommen (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 (b), 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

VII!

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

=.- d. v,

Fio. 289.— Magellania 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 ; «. tooth-socket; *. I. shelly loop ;
sp. septum; t. hinge-tooth ; v. aj. m. adjustor 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 ccelome. The two
flaps thus formed are the dorsal (d. m) and ventral (v. m) mantle-
lobes. They are fringed with minute setse (s) lodged in muscular
sacs, like those of Chaetopods (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, Iph), 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 (Iph') 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 ccelome 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 (st), 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

PHYLUM MOLLUSCOIDA

:m

into it by a duct, is a large, branched digestive gland (d. gl). The
whole canal is lined with ciliated epithelium.

Fk;. 290.— A, body of Wlagellania lenticularis, removed from shell ; 13, 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. m. dorsal
mantle-lobe ; d. v. dorsal valve of shell ; gonX, gon'*. gonads ; ht. heart ; int. intestine ; Ip, Ip1.
lip; Iph. lophophore ; I phi. its coiled process; mth. mouth; nph. in B, nephridium, in A,
nephridial aperture ; pd. peduncle ; pi. «'. pallial sinuses ; *. setie ; 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.

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

she

wing

a thin membrane
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 divaricates (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

oui.nv

Fig. 291.— Magellania flavescens, the ventral
valve removed, c. p. cardinal process ; Iph. arm
of lophophore ; Iph1. its coiled process, with
the tentacles removed on the right side ; mth.
mouth. (After Davidson.)

■mth

<£.17V

Fig. 292. — Muscular system of Magellania. ad. m. adductors ; 6. beak ; d. aj. m. dorsal
adjusters ; d.m., d. »'. divaricators ; d. v. dorsal valve; int. intestine; vdh. mouth; pd.
peduncle ; pd. sh. sheath of peduncle ; p. m. protractor ; s. {. shelly loop ; v. aj. in. ventral
adjustors ; v. v. ventral valve. (After Hancock.)

inserted into the peduncle, are called adjustors (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

30.5

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 coelome is continued
into each of the mantle-lobes in the form of four canals or pallial
siniiscs (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

ujq

Fio. 203.— Anterior body-wall of Terebratula, to show nervous system, &c. <lm. dorsal mesen-
tery ; g. brain ; gf. genital folds ; n. nephridium ; nt. nepbrostome ; a>». gullet ; ov. ovary ; tic.
oesophageal connective; vsg. infra-nesophageal ganglion; vm. ventral mesentery; dmn, hn,
ian, tan. 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 nephridia
{nph) lying one on each side of the intestine. Each is funnel-
shaped, having a wide inner opening or nephroslome, 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-oesophageal 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, gori), 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 Lingula, Crania, Diseina, &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 Magellanic/,, Terebratula, Bhynchonella, Cistella
(Argiope), &c.

vmi PHYLUM MOLLDSCOIDA 367

Systematic piosition of the Example.

The genus Magellania, of which there are several species,
belongs to the family Terebratulidae, 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 Terebratulidae 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 type's : 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

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 Brachiopoda. 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 verj^ complex muscular
system by means of which the valves can be rubbed upon one
another, or moved laterally as well as opened and shut.

\ III

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 coelomic fluid, which is propelled by the
cilia lining that cavity, and circulate both in the ccelome itself and

Pig. 295.— Dissections of A, Cistella ; 15, Rhynchonella ; and C, Lingula. a. anus ; Iph,
lophophore ; mth. mouth. (After Schulgin aud 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-oeso-
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 . hrood-pouches,
A nf situated at the base of the lophophore.

Segmentation is regular and complete,
and results in the formation of a blas-
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 ccelomic 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 ccelome : their
outer walls thus become the somatic,
their inner walls the splanchnic layer
of mesoderm. The mesenteron remains
closed and surrounded by the ccelomic
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 oody-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.

I>v^

Fio. 2%. — Two stages in the
development >of Cistella
(Argiope). b. provisional set* ;
bl. blastopore ; me, mesen-
teron ; pv. ccelomic 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 (ra) become reflexed
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 stomodaeum 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 10G 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

11 b 2

Fin. 298.— Two later stages in the
development of Cistella. A,
free-swimming; B, after fixation.
hs. peduncular region ; m. mantle ;
ras. 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.

ruth

Mutual Relationships of the
Classes of the Molluscoida.

Fio. 299. — Lophophoro of embryo
of Terebratulina. ('. gl. di-
gestive gland ; int. intestine ;
Ip. lip ; Iph. lophophore ; mth.
mouth (From Korscheli 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 ho&y
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
coelome 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 (Ip) — 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 Polyzoa, since this represents the part by which the larval
Polyzoan becomes fixed, the everted " sucker " of the latter being

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 if 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

J'u.rtic

Fig. 300. — A, Diagrammatic median section of a phylactolawnatous Polyzoan. an. anus ;
ep. epistome ; ep. cav. epistome-cavity ; funic, funiculus ; gang, ganglion ; int. intestine ;
mo. mouth ; neph. nephridium ; ces. oesophagus ; st. stomach ; tent, tentacles. B, diagram-
matic median section of Fboronls. met. mesentery ; nr. nerve-riug. Other letters as in A.
(From Korschelt and Hcider, 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, viii

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 setas 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 {Aster video),
Sea-urchins (Echinoidea), 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
coelome 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 (Asterias rubens or Anthenea Jlavescens).

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

875

370

ZOOLOGY

their outer extremities. There are two surfaces — one, the dboral
or abactinal, 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 actinostomc, 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 papillm, 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
spines arranged in ir-
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 branchiae
or papulaz (Fig. 305, Resp. cm), which is capable of being entirely
retracted.

Very nearly, though not quite, in the centre of the aboral sur-
face is an aperture, the anys (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
madreporite (mad). The presence of this structure interferes to some

Fig. -301. — Starfish. (Asterias rubens). General view of the
oral or actinal surface, showing the tube-feet. (From
Leuckart and Nitsche's Diagrams.)

a 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 pedicellarim (Fig. 305, Peel).
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 pedicellariae 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 (t), 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 ccelome or body-cavity, arid 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. 305, A?nb. 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 of
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 pares, 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.F.): 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 ampulla;

Fig. 302.— Starfish. Vertical section through an arm.
amp. ampullae ; ep. epidermis ; rail. amb. radial vessel
of the ambulacral system ; rod. bl. v. points to the
septum dividing the perihsemal vessel into two parts ;
rod. ne. radial nerve of the epidermal system ; sp.
spaces in mesoderm of body-wall ; t. f. tube-feet.
(From Leuckart, after Hamann.)

PHYLUM ECHINODERMATA.

MT'-t.

(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, ?*') 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

Fkj. 803.— Ambulacral system of a Starfish.
a. ampulla) ; ap. Polian vesicles ; c. circular
canal ; m. madreporite ; »'. madreporic
canal ; t. tube-feet ; p. radial vessels ; r'.
branches to ampullae. (After Gegenbaur.)

380

ZOOLOGY

SECT.

(d. new.) 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 coelomic nervous
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 (rad. hi. v), extending, like the other parts that have been
mentioned, throughout the length of the arm, forms part of a
system of channels, the 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 perihernial sinus
into two runs a strand of a
kind of gelatinous connective
tissue containing many leuco-
cytes and perforated by ir-
regular channels or lacunar:
this is the radial strand of the lacunar or hcemal system. Like the
radial vessels of the perihsemal 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.

Fig. 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 perihsemal vessel ; rod. nerv.
radial nerve ; sept, septum of radial peri-
hsemal vessel ; sept', radial lacunar strand of the
hsemal system (here represented as solid).
(After Cuenot.)

I\

l'HYU M KCIIINODKKMATA

381

Body-wall and Coelome. — The entire outer surface is covered
with a layer of ciliated epithelium, the epidermis or cleric epi-
thelium (Fig. 305, Der. Epithm), which is continued over the
various appendages and processes— the tubercles aud spines, the
pedicellaria?, 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)

Fio. 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 (T. F.) 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. o*. ambulacral ossicles ; Amp. am-
pullae ; Ai>. aims ; ft Amb. V. circular ambulacral vessel : ft B. V. septum of ring perihajmal ;
vessel ; Cd. cce. cardiac caeca ; Ccel. coelome ; Ccel. Epithm. ccelomic epithelium ; Der. Epilhni.
deric epithelium ; Derm, mesoderm ; Ent. Epthm. enteric epithelium : Int. cce. intestinal caeca.
Mdpr. madreporite ; Mes. mesentery ; Mth. mouth ; Nv. R. nerve-ring ; oc. eye ; os. ossicles of
body-wall ; Ovd. oviduct ; Ped. pedicellaria; ; ph. perihaemal spaces ; Pyl. ccec. pyloric caeca ;
Rad. amb. v. radial ambulacral vessel ; Rod. B. V. points to septum in the radial perihaemal
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 coelome (Ccel.) or body-cavity is lined by a
ciliated epithelium, the ccelomic epithelium (Ccel. Epithm.), 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

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 ccelome is filled with a fluid, the coslomic fluid, consisting
mainly of sea-water, but containing a number of amoeboid cor-
puscles (amasbocytes) 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 ccelome.

Digestive System. — The mouth is found to open through a '
short passage, the wsophagm, 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
'^^^^M^^^M^^^^WfM^i^^ some object desired as

food, and then retracted
into the interior, the re-
traction being effected
by means of special
retractcrr 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
cmca (Figs. 305, 306, 308, 310, pyl. case). 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

Figs. 306
anus
ccec.

Asterias rubens. Digestive system.

; card. st. cardiac division of the stomach ;

intestinal cmca ; madr. madreporite ; pyl.

pyloric cseca ; pyl. st.
(From Leuckart.)

pyloric division of the stomach.

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 co3ca (Figs. 306 and 308, int. cose),
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, m. 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
madreporic canal. Below, the Fl°- so7.-TranBve™e section through the

~ . . ' madreporic canal of a Starfish (Astro-

latter Opens into a Wide, live- 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 ccelomic 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

pt/l.ccec

jbot.vei.

amp

Fig. 30S.— 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 niadre-
porite, has been completely removed. One of the five intestinal caeca 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 caeca have been cut away close to their bases. 1 — 5, the five rays
with their ambulacral ridges ; amp. ampullae ; an. anus ; int. caec. intestinal caeca ; i. p. cut
ends of the inter-radial partitions ; mad. madreporite with the madreporic canal ; ov. ovaries ;
pol. ves. Polian vesicles ; pyl. catc. pyloric caeca ; 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 ao:ial organ (Fig. 309,
g. stol) — the relationships and function of which have given rise

I\

PHYLUM ECHLNODERMATA

886

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 ccelome)
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 view of the axial sinus,
stone-canal and genital stolon cut across, audi. oss. ambulacral ossicle ; amp. ampulhe of the
tube-feet ; ax. ». axial sinus ; gon. gonad ; g. stol. genital stolon or axial organ ; marg. marginal
ossicle ; nerv. circ. nerve-ring ; oe. cut end of oesophagus ; pst. peristome ; ret. retractor muscle
of the stomach ; tept. inter-radial septum ; stone, c. stone-canal ; T. Tiedemann's vesicle ;
k. r. 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
perihsemal 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 ccelomic
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.st PyLaze . art

Jbi/Lcccc

isil.caec

7?tad.casv

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. ampulhe ; an. anus ; card. st. cardiac pouch of the stomach ; int. coec. intestinal
csecum ; ip. inter-radial partition ; mad. madreporite ; mad. can. madreporic canal ; ov. ovary :
pyl. cue. pyloric caeca ; r. cut ends of the ring-vessel of the ambulacral system ; ring v. posi-
tion of the ring-vessel ; relr. 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.

common

Anthenea flavescens (Figs. 308, 310, 311, 312), a
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

I\

PHYLUM ECHINODERMATA

387

Fio. U 1 . Anthenea, view of aboral surface.
(After Sladen.)

there among the tubercles, usually one in the middle of each

ossicle, are pcdicellarice, 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

pedicellariae 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
rifh 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-
cles, though fewer than
c c 2

Fig. 312.— Anthenea, view of oral surface.
(After Sladen.)

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 on^ of the
dermal branchiae. Numerous pedicellarise, 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 ampullar 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 cozca, which
are narrow tubes slightly enlarged and lobed at the extremities.

Development of a Starfish (Asterina gibbosa or A.
exigua l). — 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 blastula ; the cavity is the segmenta-
tion-cavity 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 invagination 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.

I\

PHYLUM ECHINODERMATA

380

a double-walled cup or gastrula (C) opening on the exterior by
an opening, the blastopore, which, at first very wide, gradually
beoomee 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

Fio. 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 ; D, section of
early gastrula ; E, section of later gastrula. arch, archenteron ; blastoc- blastocnele ; blp.
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 (coslome). 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-
coele 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

SKCT.

in such a way as to give rise to the coelome of the adult. The
anterior undivided part (anterior ccelome) forms the coelome of a
conspicuous larval structure, the pre-oral lobe, and it eventually

larv.org

'Om'.mo cnl.

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 ; hi. p. blastopore ; ect. ectoderm';
ent. enterocosle ; larv. mo. larval mouth ; larv. org. pre-oral lobe ; stom. stomodseum. (From
Ziegler's models.)

becomes cut off from the right and left pouches, giving off on
the left a five-lobed outgrowth, the hydroccele, which forms the
foundation of the entire ambulacral system of the adult : a right

B Car v. org

ccmJb

laruiTio

dorjjfr

ale

Fig. 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 an 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-
cral system or hydroccele ; dors. p. dorsal pore ; ent. enteric sacs and coelome ; larv. mo. larval
mouth ; larv. org. pre-oral lobe ; ces. oesophagus of adult ; r, r. lobes of hydroccele ; sept.
septum between the enterocoelic sacs. (A, after Ludwig ; B, from Ziegler's models.)

hydroccele is only represented by a small vesicle which in normal
embryos undergoes no further development. Before the hydro-
ccele is developed and before the right and left ccelomic
pouches have become cut off, two apertures make their appearance

a

PHYLUM ECHINODERMATA

391

Larv.org

Pro. 3it>

Larva of Asterina, view
of the left side, showing the fivc-
lobed prominence (mat.) formed by
the developing ambulacral system
on what is destined to become the
ventral surface of the body of the
Starfish ; larv. org. larval organ.

on the surface of the larva : one, on the ventral side, is the open-
ing of the stomodceum or larval mouth ; the other, on the dorsal
side, is the doi'sal 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.
fertl 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 hydroccele, at first a five-lobed outgrowth of the entero-
coele, grows into the form of a horse-shoe with five lobes, each of

r<xct.currf>

cis-camb

Fig. 317.— Asterina exigua. Young Starfish shortly
after the metamorphosis has baen completed, viewed
from the oral side. circ. amb. circular ambulacral
vessel ; dors. p. dorsal pore and madreporic canal ; rod.
amb, radial ambulacral vessel ; »t. stomach ; lent, tentacle ;
t. /. tube-feet.

392

ZOOLOGY

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 ccelome. This develops into a canal leading from the
hydrocoele to the anterior ccelome, 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

3 a

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.

I\

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 radials (rod) 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-basals (sec.
rad) is developed between the radials and the central. In the

bets

eecrud,

radL

te-rnc

Fio. 310. — Diagram showing the relations of the chief plates of the apical system in the young
Starfish, an. anus ; bos. basals ; dors, central ; raadr. 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 peristome 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

J^iG. 320. — Echinus esculentus; peristome. 1, tube-feet of the lower ends of the radii; 2,
branchia ; 3, teeth ; 4, buccal tube-foot ; 5, peristoraial membrane. (Prom MacBride, after
Kukenthal.)

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 pedicellariw
(see Fig. 340, p. 423), which are comparable to the stalked
pedicellariae 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 KCHINODERMATA

395

rounded bodies termed the spliccridia, which are perhaps, like the
pedicellariae, to be looked upon as modified spines : they contain
ga 11 l;-] ion-cells and are apparently organs of special sense, having
possibly the function of detecting changes in the composition of
tin' 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-

Fio. 321.- Strongylocentrotui, entire animal with the tube-feet extended. (From Brehm'e

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 inter-ambulacral 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 branchiae, are situated in

396

ZOOLOGY

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

i. 322.— Corona of Echinus esculentus, from the aboral surface, showing the arrangement
of the plates of the corona. 1, the anus ; 2, poriproct, with irregular plates ; 3, the madrc-
porite ; 4, one of the other genital plates ; 5, an ocular plate ; <;, an inter-ambulacral plate ;
7, an ambulacral plate ; 8, pores for the protrusion of the tube-feet ; 0, 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 inler-ambu-
lacral zones or areas, the plate's are not perforated. At its
anal end each area, ambulacra^ ^or inter-ambulacral, ends in a
single apical plate, so that the periproct is surrounded by a ring of

PHYLUM ECHINODERMATA

397

fiercer

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 bivium, 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,
our), 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

irU.ami.

Fig. 323. — Apical system of plates and
aboral extremities of zones of the shell
of a Sea-urchin, atnb. ambulacral
zones ; gen. genital plates ; int. amb.
inter-ambulacral zones ; vuulr. madre-
porite ; oc. ocular plates ; peripr. peri-
proct. (After Leuckart.)

Fit;. BM. — 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 ; 6'. suture with alveolus ; c. rotula ; d. radius ; e. tooth. (From
Huxley's Invei-tebrates, after MtUler.)

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.

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 rotulse
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 eyineural canals (Fig. 326, ep.), covered over by the
plates of the corona — is here more deeply situated ; the deep and
ccelomic systems are only feebly developed.

radne
rada,

Fig. 325.— Lateral view of the internal organs of a Sea-
urchin as seen on the removal of a half of the shell, ab. r.
ves. hsemal strand, aboral ring ; amb. r. ambulacral ring-
canal ; amp.&mpullx ; an. anus ; aur. auricle ; roil, ccelome ;
int. intestine ; int. ve«.iiitestinal 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 ; plex. ovoid gland ; pol. ves. Polian vesicle ; rad.
amb. radial ambulacral vessel ; rad. ne. radial nerve ; siph.
siphon ; sp. radial extension of the ccelome surrounding
the nerve ; t. f. tube-feet. (Prom Leuckart, after Hamann.)

I\

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 ampulla {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 Perih

Fie 3-20. — Diagrammatic transverse section of the ambulacral zone of an XSchlnoid. amb. ots.
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 ;
os*. ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perih. radial perihsemal canal ;
poi.l. 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 caeca
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 (siph) ; this,
together with the intestine itself, probably acts as an organ for the
respiration of the ccelomic fluid.

The cceiome contains a fluid in which, as in the Starfish,
there are numerous corpuscles. Of these there are two kinds
— amoeboid corpuscles {amabocytes) 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 ccelomic fluid.

The part of the ccelome 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 branchiae on the
peristome is evidently the oxygenation of the ccelomic fluid
enclosed in this compartment, which is known as the lantern-
ccelome.

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,

amp

ah. rves

Fig. 327. — Alimentary canal and other organs of Sea-urchin as seen when the oral half
of the corona has been removed, ah. r. ves. aboral ring of the hsemal system ; all. ali-
mentary canal; amp. ampullse ; int. ves. intestinal blood-vessels ; lant. lantern of Aristotle ;
ces. oesophagus ; or. r. v. oral ring-vessel of the hajmal system ; ov. ovary ; red. rectum ; siph.
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 phttevs, 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.

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 trivium
of the Starfish, the rest re-
presenting the bivhim. On
the dorsal surface, instead of
typical tube-feet, there are
papilke 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 papilla?. 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 spicides, 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

Fio. 328

Cucumaria planci. Entire animal
seen from the ventral surface. (From Hertwig's
Lehrbuch, after Ludwig.)

VOL. I

D D

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 ambulacral 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 codomic epithelium, lining the ccelome.
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, rod. amb) 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 ccelome.

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. bl. 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, simitar to those of the Starfish, and also a uuinber of

gen op

- raxji oss
inter oss

- qen gIm.
maa can
ini ves

^gen.gl

rcsft
— circmtcs

long.7KU8

b.w

Firs. 329. — Internal organs of a Holothurian as scon when the body-wall is divided along
the middle of the dorsal surface, b. w. body-wall ; circ. mus. circular layer of muscle
cl. cloaca ; el. op. cloacal opening ; Cur. org. Cuvieran organs ; gen. ap. genital aperture
gen. du. genital duct ; gen. gl. gonad ; int. intestine ; inter. oss. inter-ambulacral ossi
cles ; int. vet. intestinal haemal strands ; long. mus. longitudinal band of muscle ; unci. can.
madreporic canals ; mes. mesentery ; pol. ves. Polian vesicles ; rod. amb. radial ambulacral
vessel ; rod. ost. 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 buccal chamber, into which the mouth leads.
Surrounding the oesophagus, which lies immediately behind the
buccal chamber, is a circlet of ten circum-azsophageal ossicles, five
ambulacral (rad. oss) in position, and five inter-ambulacral (inter,
oss). Through each of the former pass the corresponding radial
ambulacral vessel, haemal strand, and nerve. The alimentary canal
itself is a simple cylindrical tube, only indistinctly marked out
into oesophagus, stomach (stom), and intestine. It forms several
coils within the ccelome, to the wall of which it is attached by a
thin membranous dorsal mesentery, and terminates behind in a
comparatively wide chamber, the cloaca (cl).

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 ccelomic 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 ccelome, 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. ap.).

The early stages of development are very similar to those of
the Starfish (p. 38.8). 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.

1\

PHYLUM ECHINODERMATA

405

4. The Crinoidea.

A Feather-Star. — Antedon rosacea.

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

ftO. 330.— Antedon. Side view of entire animal. (From Leuckart and Nitsehe'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),

si/z cir

Fig. 331.— Aboral view of Antedon. c. centro-dorsal ossicle ; dr. cirrus ; R,1 R* R3 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 (cirr)

Fig. 332.— Antedon, oral (upper) surface of the central disc. an. anus ; mo. mouth.
(From Vogt and Jung.)

consist each of a row of slender ossicles, covered, like all the rest
of the animal, with epidermis, and connected together by means

PHYLUM ECHINODERMATA

407

nui. amb
coel cart

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 (R1), also concealed by the centro-dorsal
ossicle : with each of these articulates a second radial (i22), which
is visible beyond the centro-dorsal. With each of the second
radials articulate two third radials (B3), each forming the base of
the corresponding arm-branch.

The ossicles of the arms — brachials (Br.1, Br.2) — 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 coelome 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-
bulacral grooves are three
canals which are prolongations

of the ccelome (Fig. 333, coel. 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 cceliac canal — runs between these and the supporting ossicles
(oss). The sub-tentacular canals and the cceliac 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 ; coel. can. sub-tentacular and
creliac canals ; mus. muscles ; neur. vet. radial
sinus of the perihaemal system ; rod. amb.
radial ambulacral vessel giving off branches
to the tentacles. Between the paired sub-
tentacular and unpaired cosliac canals is the
genital rachis. The small round bodies above
the line from rod. 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,rarf. amb.) 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

leftt

ceul.caps

• CITT

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 ;• Br .! Br* brachial ossicles ; CD. centro-dorsal
ossicle ; cent. caps, central capsule ; chainb. ovg. chambered organ ; cirr. cirri ; cct. ne.
ambulacral (epidermal) nerve-ring and radial nerve ; gen. st. genital stolon ; int. intestine ;
too. mouth; RA R.'-i iJ.s radials ; rot. 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, which are suspended within the ccelome, 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 —
superficial, deep, and axial or aboral. 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 ambulacral
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 ha?mal lacunae.

Like the rest of the Echinoderms, the Feather-star undergoes a
metamorphosis ( Fjgs. 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.— ELEUTHER0Z0A.

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 ccelome and of its contained organs. There
arc distinct oral and aboral surfaces, on the former of which the
anus and the madreporite are situated, and on the latter the

ix PHYLUM ECHINODERMATA 411

month and five narrow ambulacral grooves lodging the tube-feet.
The larva has the form either of a Hpinnaria or of a brachiolaria.
This class includes the Starfishes.

Order 1. — Phanerozonia.

Asteroidea with large marginal ossicles. The dermal branchia3
are present only on the aboral surface. The ambulacral ossicles
not closely crowded. Pedicellarise 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. Pedicellaria'
stalked or sessile.

CLASS II— OPHIUROLDEA.

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^:.

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^:.

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 phtteus.
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.— H0L0THUR0IDEA.

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.

» phylum echinodermata 413

Order 2. — Pedata.

Boldthuroidea 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 ambulacral, 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 on to a varying number of unbranched aims 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 (hydroxpires). 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 out 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 ruoens is a species of the genus Astcrias, which, with
several others, constitutes the family Asteriidm of the order
Gryptozonia. The family Asteriidm is characterised among the
families of the Gryptozonia by the following distinctive features : —
The ossicles of the aboral surface are small, unequal, reticulate
plates, bearing isolated or grouped spinelets {paxillm). The margin
of the actinostome is denned by the ambulacral plates. The
pedicellariae 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 wThich a short description has been given
are the genera Strongylocentrotus and Echinus, but the description
is sufficiently general to apply to any member of the family
Echinidce, to which these genera, with a number of others, belong.

i\ PHYLUM ECHINODERMATA 415

The family Echinidcc is one of about five families of the sub-order
Ectobranchiata, the members of which all differ from the other
sub-order — Entobranchiata — of the JRcgularia, or regular Sea-
urchins, in the possession of dermal branchiae, and in having the
auricles in the form of complete arches.

The Sea-cucumber {Gucumaria or Culochirus) is a member of
the Stichopoda — one of the families of the sub-order Dendrochirotai
of the Pedata, or foot-bearing Hoiothurians. The Dendrochirotce
differ from the Aspidochirotm — the other sub-order — mainly in
having arborescent instead of shield-shaped tentacles, and the
Stichopoda differ from the rest of the Dendrochirotce in having the
tube-feet arranged in five regular zones. The genus Gucumaria
is distinguished from the rest by having ten tentacles with the two
ventral smaller than the others. Golochirus is closely allied to
Gucumaria, the principal distinction being the presence in the
former of papillse taking the place of tube-feet in certain
situations, as already noted.

The Feather Star (Antedon rosacea) is a member of the family
Comatulidcc, which is distinguished from the four other living
families comprised in the class Grinoidea 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
ambulacra! and five inter-amlulacral 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 ambulacral system ; the remaining three
rays form the triviurn. 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

U 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
1 in >(li tied spines, termed pedicellariae, 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
perforin 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 pedicellariae 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. Pedicellariae 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.1 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
Brisingidai 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 ambulacral grooves, one of which is continued along

a

PHYLUM ECHINODERMATA

419

A

the oral Bur&oe of each arm to its extremity. Near to, but not

quite in the middle point of the aboral surface is the anal
aperture, absent is 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 madrepori$e;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 ambulacra!,
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
paxillm 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
ampullae, situated in the ccelome. 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

Fio. 33 '. Anthenea. View of or.il surface.
(After Sladen.)

420

ZOOLOGY

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 Ewrycdida (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 ambulacra] 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 Paleontology.)

Five pairs of slits on the oral surface (Fig. 336, C) lead into the
genital bursas, which receive the sperms and ova from the 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 ampullae, 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 ECHINODKRMATA

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

Fk;. 33".— 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

Fi(i. 33S. — Diagram of spine
of Sea-urchin showing
mode of articulation.
m. muscle ; 6. ligament.
(Prom Leuckart.)

arranged in ten zones, five ambulacral and five inter-ambulacral,
as described in the account of Echinus, with peristome, periproct,
ocular and genital plates, and madreporite. Spines (Fig. 338),
pcdiccllarice (Fig. 339), and sphceridia 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 branchial 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 clamdai, surround the anus in a ring and are
distributed elsewhere. A few pedicellarisB are present in the

IV:. 389.— Pedicellaria of
Arbacia punctulata.

(From Leuckart.)

PHYLUM ECHINODERMATA

423

neighbourhood of the mouth, and sphaeridia also occur. A series of

tree-like dermal branchiae 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

fenestras. 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. Sphaeridia are

present, but pedicellarige

and clavulae are absent.

An "Aristotle's lantern"

with teeth is present, as

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-

k... Hii.-ciypeaster »ub- tinguishable : it is most distinctly de-

depressus, view of aboral .© . . n- i x

surface showing the petaioid veloped in I solus and allien genera. In
SSo' (Fnnu HertWig'8 some Holothuroids the surface is enclosed

Fio. ; i • Hemipneustes radiatus. A. aboral,
and B, oral surface. C, apical plates. (From Bronn's
Tierreich.)

424

ZOOLOGY

Fig. : i WIetacrinus interruptxis,
(After P. H. 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 - like,
with a central disc and

U 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 ligamentous
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 centro-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 (Aclinomctm),
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
ambulacra I 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 branchial or papula, 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 branchiae in providing for the respiration of the compart-
ment of the ccelome enclosing Aristotle's lantern (lantern-ccelome)
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 bodv, 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 ampullar 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
Ticdemanns 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 ccelome 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 perihozmal system and
the hcemal 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

»■

-l

a 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
COBlomej from which they are developed. In Asteroidea and Ophi-
aroidea 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 perihaemal 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 gastrointestinal 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 lacunar is highly developed and
complicated in arrangement.

Whatever be its functions, this system is not a system of blood-

428 ZOOLOGY skct.

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 walls. 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 perihaemal 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 Ophiuroids the central mouth leads into a

i\ PHYLUM ECHINODERMATA 429

simple sac giving off short diverticula, and there is no anal
operi are. 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 caeca, and the
latter five pairs of very long branched diverticula — the pyloric
or hepatic caeca. 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 caeca 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 aboral. 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 Synaptidae of the Apoda there are also numerous examples
of hermaphroditism, the animal at first producing ova, later only
sperms. In Amphiura 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 bursa:., 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 brood-pouch, on the surface of the body of the parent. But in
most, development takes place externally, and the larvae 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 Coalenterata (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
enteroccele, 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 (hydrocosle) 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 ambtdacral vessels of the Echinoderm ; the
central part of the hydroccele 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

I\

PHYLUM ECHINODKRMATA

431

opens. A smaller adoral band ((tor) in the interior of the
mouth has the function of attracting nutrient particles. The

cuvr
fjur
rru>

Fro. 343.— Diagrams of the development of the larvae of Echinoderms. 1, Primitive form of
Ecl)inoderm larva ; 2 and 3, Development of an auricularia (Holothuroidea) ; 4, 5, and 6,
Development of a I'ipinnaria (Asteroidea) ; 7, 8, and 9, Development of a pluteiis (Echinoidea
and Ophiuroidea). aor. adoral band of cilia ; all. alimentary canal ; an. anus ; 6, b. processes
or arms ; mc. mouth ; por. peri-oral ciliated band and processes. (From Leuckart and
N'itsche'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

modified by the formation, except in the Crinoidea, of variously
arranged processes along the course of the peri-oral band. The
resulting larva, cchinopcedium or diphurula, 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 and
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 phiteus. 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 coelome, from which a
hydroccele 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

Fio. 344. — Free-swimming larva of
Antedon, from the left side.
I-V, ciliated bands ; ba-i to ba^,
the five basals ; ori to or '5, orals ;
1, vestibule; 2, intestinal vesicle;
3, right enterocoale ; 4, calcareous
joints of the stalk ; 5, pedal plate.
(From Lang, after Seeliger.)

!\

PHYLUM ECHINODERMATA

4M

d.

%

V.

with a tuft of sensory cilia. The vibratile cilia on the surface are
arranged in five transverse bauds (I-V). Between the second and
third of these is a wide shallow depression, the vestibule or
M"iiH>daum (I), which does not communicate with the mouth.
After remaining in the free condition ant

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, and
a solid rudiment of the adult
oesophagus arises in close apposition
with it. Round the oesophagus the
hydrocoele grows in the form of a
ring. The vestibule (5) with the
oesophagus and hydrocoele 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

Fio. 345.— Stalked larva of Antedon,
from the right side ; calcareous
plates not represented. 1, right
crelomic 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 larva? 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
(Peclata). 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 Comatuke, 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 Echinoids bury themselves in sand or mud; others creep
into narrow fissures in rock or Goral. 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
E&chinodermata than in the groups already dealt with.

A remarkable characteristic of the Echinoderms is the faculty
of si-//-))intilaHon 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 Comatulae 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 sj^stems of organs to the
conclusion that the Echinoderms are in no way closely or directly
related to the Ccelenterates. 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 Echinoderms
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 Ccelenterata 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

PHYLUM ECHJNODERMATA

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 dipleinida 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 ccelome, 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 °pW<""oidea

Cystoidea

Primitive Cystoid

Oipleurula

Fig. 340. — Diagram to ilhuitrate the relationships of the classes of the Eehinodcrmata.

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
Chcetopoda 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 nephridia or segmental 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 cazlomoducts, for the
passage outwards of the reproductive elements. )

CLASS I.— dLETOPODA.

The Chaetopoda, 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 setm (chcetce) 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

440

ZOOLOGY

the segments. There is a well-developed blood-vascular system
in the majority of the Cha^topoda, 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 ircchophore (cf. p. 322).

1. Examples of the Class.
a. Nereis dinner 'Mi.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 (chcelce). The head (Fig. 350)
consists of two parts, the pros- .

tomium (prccst) and the peristomium (jterist). The former bears )

1 Though Nerei* 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, Heteronerei?
ph ase. (After Claparedc.)

PHYLUM ANNULATA

til

noto

TU uro

vent.cirr

Fio. 34S, a.— Nereis dumerilii. A single para-
podimn, magnified : ac. aciculum ; dor*. ri,-r.
dorsal cirrus : neuro. neuropodium ; noto. noto-
podium ; vent. cirr. ventral cirrus. (After
Claparede.)

f on its dorsal surface four large rounded c//es, 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- dont.cirr

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
(pi rist. tent) : on its ventral
aspect is a transversely
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 setigcrous 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 blade 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 \

Fk;. 34s, i Nereis du-
merilii Set;«' highly mag-
nified. (Aftci- Claparede.)

442 ZOOLOGY sect.

f short and slightly hooked. On the dorsal side of the parapodium
l 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 amis ; 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 coelome, body-cavity, or peri-
visceral cavity — filled with a fluid, the cozlomic fluid, containing
amoeboid corpuscles. The walls of the ccelome (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 sepia passing
inwards from the body-wall to the wall of the alimentary canal
opposite the grooves between the segments, and thus dividing the
ccelome 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 {rns), which runs through about five
segments to open into the intestine. \

PHYLUM ANM I,\T\

443

L Running backwards and inwards from the wall of the peristomium
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
retract 'or sheet of muscle, which passes inwards and forwards to be

Fin. 349.— Nereis diversicolor, x 4. Head with buecal-'region everted. A, dorsal view ; .
B, ventral view, a, prostomium ; B, everted buccal region ; c, c', peristomial tentacles, 1, 2, 3, 4 ;
rf, denticles ; e, eyes ; h\ lower lip ; P, palp in A, entrance to pharynx in B : J, jaw ;
T, prostomial tentacle ; /, peristomium ; //, 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 co?ca (Fig. 350, gl), 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 ccelomic 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
(ent. 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 stomodmum, the rectum the proctodeum, the rest of the alimen-
tary canal the mesenteron.

The wall of the body consists of a cuticle, an epidermis or

444

ZOOLOGY

(

deric epithelium, muscular layers, and the parietal layer of the

coelomic epithelium {par,

firauts?; &$*

Pio. 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
whieh lie below, dors. vets, dorsal vessel ; gl. oeso-
phageal glands ; int. beginning of intestine ; ne. r.o.
nerve-cord ; neph. nephridia ; oti. oesophagus ; palp,
palp ; para, parapodia ; perist. peristome ; perist. tent.
pcristomial tentacles ; ph. pharnyx with its jaws ;
prmst. prostomium ; tent, prostomial tentacles ; rent,
vess. ventral vessel.

pert). 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
(ep) is very thin, except
on the ventral surface,
where \t 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

IS

)

PHYLUM ANNULATA

445

thr majority of the vessels, undergoes contractions which are of
*a peristaltic chamcter — 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

tiers Idfuf miu

cisv-.fnus

/ oiors. tress

coelr -

/ ctorsfxTng. mem

ov

■/uu*i#c4

vent. i/e&9 ne. co ci^c. mus Gu^ ep

Fio. 351.— Nereis dumerilii. Semi-diagrammatic transverse section of the middle region of
the body. eire. mus. (external), circular layer of muscle of body-wall ; circ. mus. (internal),
circular layer of muscle of wall of enteric canal ; ca-l. ccelpme ; cut. cuticle ; dors. long. mus.
dorsal longitudinal muscles of body-wall ; dors. ve*s. dorsal vessel ; ent. ep. enteric epithelium ;
ep. epidermis ; long. mus. 'longitudinal muscle of walPof enteric canal ; ne. co. nerve-cord ;
neph. nephridium ; neur. set. neurojxjdial setue and aciculum with their muscles ; not. set.
notopodial set« and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri, parietal layer
of coelomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent. vess. ventral vessel ;
vise. peri, visceral layer of coelomic epithelium. (The entire extent of the coelomic 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,
■vess), 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
I 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

Fio. 352. — Nereis. — Anterior portion of nervous system, comprising the brain, the oesophageal
connectives, and the anterior part of the ventral nerve-cord. (After Quatrefages.)

oi 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 oesophageal 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 peristomial tentacles.
Running backwards from the point <>f 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
neite-cord (/t). 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 stoma togastric or visceral
system, distributed to the anterior part of the alimentary canal.

Fia. 353. —Nereid— Section through one of the eyes. eo. cornea ; cu. cuticle ; I. lens ; r. layer
of rods ; re. retina. (After Andrews.)

f 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 peristomial 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 nuchal organs, all situated on the prostomiuny'
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

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.
f 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.
854) 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 {eoot. 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

Fig. 354.— Nereis dumerilii. One of the
nephridia. ext. op. external opening or
nephridiopore ; fun. internal funnel or
nephrostome opening into the ccelome ;
mes. transverse mesentery or septum.

x PHYLUM ANNULATA 440

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 testes, which
are developed towards the breeding season by a proliferation of
the cells of the membrane (coBlomic epithelium) lining the ccelome
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
coelomic fluid. 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 j

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. 346, 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 setae 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 setae. 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 phas^/

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 radiata) 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 what 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
Polyclad 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 somatoblast
(som. 1) ; a second somatoblast (som. 2) is soon given off from the
same megam ere 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 mesoderm of the Annelid. As the niicromeres multiply
by division, they form at first a can 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

som.l

micro

macro

som2

soml

Fio. 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, niicromeres; 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. cii). 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 stomodamm (st) or rudiment of the mouth and
oesophagus. The anus (<m)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 g 2

452 ZOOLOGY sect, x

behind the prototroch and mouth, 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 {new. 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.
S 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 stomodaeum, and behind with a
second smaller ectodermal invagination, the proctodeum, 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 seta'. 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

— Prvt

pigat

E \

fr.bod

£wa

Fig. 350. — Nereis. I.ater stages in the development. A, stage at which the prototrocli and the
apical tuft of cilia first become distinct. B, somewhat later stage, in which the stomodieal
invagination is being formed, and the rudiments of the mesoderm bands are distinct ; C, late
trochophore stage in which there are rudiments of the sctigerous sacs ; D, somewhat later
stage, in which the parapodia have begun to become prominent and the provisional seta;
project freely ; K, larva with three segments, an. anus ; ap. cil. apical cilia ; up. pi. apical
plate ; eye, eye;/)-, bod. frontal bodies; int. intestine ; I. mus. longitudinal muscle; Hie*,
mesoderm ; mo. mouth ; neur. pi. neural plate ; para, parapodia ; pig. ar. pigmented area ;
prot. prototroch ; ten*, h. sensory hairs ; set. sort, setigcrous sacs : torn, second somatoblast and
group of cells formed from it ; st. stomodreum ; tent, peristomial tentacles. (After £. 13.
Wilson.)

453

454

ZOOLOGY

SECT.

prostomium of the adult. The part immediately behind forms
the peristomium, which bears setse, 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.

b. The Earthworm (Lumbricus).

General External Features. — The Earthworm (Fig. 357)
has a long narrow body, which may be described as approximately

Fig. Lumbricus berculeus. A, entire specimen, lateral view ; B, ventral view of

anterior portion of the body, magnified. 1, 15, 33, first, fifteenth, and thirty-third segments.
Each of the black dots 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

PIIYW.M ANN I LATA

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 prostoniium,
immediately behind and below which is the opening of the mouth.
Next to the prostoniium 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
setae 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 mucri^Slenderer than the rest.
Along the middle line of the dorsal surface,
from about the eleventh segment backwards,
is a row of small aperTOres, 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 ccelome.
Through these coelomic 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"~withsomewhat
tumid lips, the male reproductive apertures ; and on the segment
immediately in front— the fmirteenth, are two smaller rounded
apertures, the female reproductive apertures. In the intervals
between the ninth and tenth, and tenthjmd eleventh segments
are two pairs of small pores, the openings of ihe~receptacula
seminis.

Fi<;. 358.— Lumbricus.
Sutai, 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 coelomic epithelium. The cuticle (mit.)
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, v ~

(>fiid. ^

circ.tuu.i

neph

loity.mus- *

let y

ext.nepTv

h rost

sub.rLvcss

Fig. 359. Lumbricus, transverse section of the middle region of the body. circ. mus. layer of
circular muscular fibres ; cwl. coslome ; cut. cuticle ; dors. v. dorsal vessel ; epid. epidermis ;
ext. neph. nephridiopore ; hep. layer of chloragen cells ; long. mus. longitudinal muscle ;
neph. ncphridium ; nephrost. nophrostome ; n. co. nerve-cord ; fct. setie ; sub. n. vess. sub-
neural vessel ; typh. typhldsole ; 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

1MIVIJM ANMLATA

457

a m nes 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 capsnlo-
genuus 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 coslome, lined
by a thin cellular mem-
brane, the peritoneum or
coelomic epithelium, and
filled with a fluid, the
ccelomic fluid, contain-
ing colourless .corpuscles.
The ccelome is divided
into a series of chambers
corresponding to the seg-
ments by a series \A
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 pluirynx {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 (oes), 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 oesophageal pouches — are hollow, and their cavities
are in communication with the lumen of the oesophagus {oes. 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 oesophageal 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 typhlosole (Fig. 359, typh), 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 chlora^en 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
• if these cells, more granular thai) 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 ace 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. hosmoglobin, 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 Ranches. 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 chining
through the former in the living worm. The ventral vessel {vent, v)

l'HYIJ.M ANMLATA

459

•prcsl

lies below the alimentary canal, the sub-neural below this again
under the nerve-cord ; the lateral neural lie on cither 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 oeso-
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 oesophageal connectives a
series of stoma togastric nerves pass to the pharynx and neighbouring
parts of the alimentary canal.

Fkj. M'.i. LnmbricuB. Anterior portion of
nervous system, cer. gang, oerebr&l ganglion
or brain ; com. oesophageal connectives ; ne. eo.
ventral nerve-cord ; frost, prostomium. (After
Leuckart.)

460

ZOOLOGY

SECT.

The Earthworm is devoid of organs of sights or hearing. It
exhibits sensitiveness to bright light, the sensitive elements being
large cells of the epidermis devoid of pigment. The sense of

tusl

Fig. 362. — Nephridium of XiUtnbricus (diagrammatic). — a. ampulla between ciliated and non-
ciliated parts of the intracellular canal ; ell. ciliated part of the intracellular canal ; eoe.
investment derived from the eoalomic epithelium ; ext. nephridioporc ; I.e. non-ciliated part
of the intracellular canal ; mes. septum ; nut. nephrostome ; t.v. intercellular canal of the
terminal vesicle. J. — ///. the three principal loops. (From Meisenheimer, after Maziarski. )

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 ANNULATA 461

narrow epidermal cells, most abundant on the prostomium and
l>.Hst milium, have probably to do with this faculty.

Tho organs of excretion — the segmental 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 ygphrosLome
(nst), opening into the cavity of the segment jn from, of that
in which 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 vesicular 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 vesiculae seminales (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 efferens, 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

cent, ves. sent ant. sp.res

mid. ves se.
fiost. sp. res

post, ves. sent-
vde/L.

ov.d

ov.cC

Fio. 303. — ZiUmbricus herculeus. Reproductive organs, ant. sp. res. anterior sperm reser-
voir ; ant. ves. sent, anterior left vesicula seminalis ; fun, funnel-like openings of vasa efferentia;
int. intermuscular partitions ; mid. ves. sem. middle vesicula seminalis ; n. co. nerve-cord ; ov.
ovaries ; ov. d. oviducts ; post. sp. res. posterior sperm -reservoir ; post ves. stm. posterior
vesicula seminalis ; rec. receptacula seminis ; te, anterior, and te', posterior testes ; v. e'ff.
anterior, and v. eff'. posterior vas efferens ; v. def. vasa deferentia. (After Vogt and Jung.)

The receptacula seminis
sacs which open on the exterior
ninth and tenth, and tenth and

the funnel-shaped ends of the oyiducts
(rec) are two pairs of rounTteasa
in the intervals between the
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

I'll M.I M \NM IATA

4G3

the eapBulogenous glands (p. 457). situated in the neighbourhood
of tlir reproductive organs. The sperms from the male apertures

of each pass along temporarily formed grooves to the teceptaeufa

semiuis 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

/TICS

blast op

ner-

Kk ;. WA.— Early stages in the development of Lumbricus. A, lateral view of flattened blastula ;
B. ventral view of gastrula with slit-like blastopore ; C; lateral view of later stage. Maxtor.
blastocoele ; blastop. blastopore ; ect. ectoderm ; en<l. endoderm ; m. primary mesoderm cell ;
mes. mesoderm bands ; ner. cell from which the primitive nerve-cord (ne. co.) takes origin ;
npli. cells taking part in the formation of the nephridia ; st. stomodteum. (After Wilson.)

(vide infra), after having being detained for a time in the
receptaculum ovorum.

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 wal^"of
which is formed of a viscid secretion from the glands of the
clitellum, hardened and toughened by exposure to the air. The

464 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 blastula
(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 teloblast. 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 oesophageal 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 Chaetopoda 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 ccelome 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 ccelome, and usually reach the exterior through ccelomoducts
or through modified or unmodified nephridia.

I PHYLUM ANNULATA 466

Sub-Class I.-POLYCHiETA.

Chstopoda 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
setee. There is usually a definite head with eyes and tentacles,
and often cirri and branchiae 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-Ch^topoda*

Aberrant or primitive Polychaeta1 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 branchiae, when present, are not confined
to the anterior end.

Order 3. — Cryptocephala*

Polychaeta 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 setae,
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.-OLIGOCKETA.

Chaetopoda 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 setae on each segment The
head is not distinct. A clitellum is usually present. There is no
metamorphosis. Mostly terrestrial or fresh-water forms.

1 The Arehi-Cluttopxxla are usually classed with the Polychceta, but their
alliances are perhaps quite as close with the Oliyochceta. In some respects
Saccocirrus resembles Polygordius and Protodrilus {Arhci- Annelida q-v.)
but is distinguished from them by the possession of seta-.

VOL. I H H

466 ZOOLOGY sect.

Order 1. — Microdrili.

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 Oligochaeta 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 Nercidw, 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
Nereidiformia 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 seta1, and dorsal and ventral cirri. The buccal

PHYLUM ANNULATA

407

region of the enteric canal is eversible as a proboscis, and there are
usually horny jaws.

There are several species of the genus Lumbricus, 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 setae 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 arc two pairs of receptacula seminis al-
ways situated in the ninth and tenth segments.

The family Lumbricidoe 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 seta; 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 vesicular
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, e.g.f
some members of the families Polynoidw
(Fig. 365) and Amphinomidw, 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 II 2

Pk». 3r»').— Polynbe seto-

sissima. Dorsal view
of entire animal, with
the pharynx protruded.
(After Quatrefagcs.)

468

ZOOLOGY

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 Oligochseta there is
a thickened zone, the cliteUum, comprising sometimes only one
segment, sometimes a number. Each segment,- with certain
exceptions to be noted presently, bears either a pair of parapodia
or merely a greater or smaller number of seta?. Parapodia are
lateral hollow processes of the body-wall bearing a number of

Fig. 3tS0. — A Serpulid (Vertnilia cuespitosa). Lateral view of animal removed from its tube.
abd. abdomen ; br. branchia; ; op. operculum ; th. 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 calcified. They are always
solid, except in Ewphrosyivi, entire, or divided into a number of
joints. In shape (Fig. 368) they vary greatly in different groups ;
often several very distinct forms of seta? 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, dois. 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 Polynoe (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 Stemaspis a ventral shield formed by a thickening of the
cuticle in the posterior region of the body bears a number of setae
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

Fio. 307.— Chsetopterus. Natural
size of a young specimen. A, an-
terior region of the body ; 13, middle
region ; C, hinder region, c, peri-
stomial cirri; </, "sucker" ; e, the
great "wings"; /, the first of the
three "fans"; m,< mouth. (From
Uenham, after Panceri.)

470

ZOOLOGY

peristomiwm, forms in many Polychseta a very distinct head ; the
prostomium in such a case bears eyes and tentacles and contains

Fig. 308.— Sette of various Polychasta. (From Claparede.)

the cerebral ganglion ; on the peristomium is the opening of the
mouth, and from it also arise the peristomial tentacles. A

Fig 360.— Section of the setigerous sac of an Oligochsete. bj, setigerous sac ; &._>, supplementary
follicle with seta ; e, cleric epithelium (epidermis) ; Im, longitudinal muscles of body-wall ;
tn, 'in. muscles of the setigerous sac ; ?-.»i, 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 peristomium 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
not lodge the cerebral ganglion. In Sternaspis spinosa the pro-
stomium is elongated and bifurcated like the proboscis of the
Gephyrea armata (vide infra).

■Jberistteni

dars.clrr-

dors. cirr

Fits. 370.— Polyaoe extenuata. Dorsal view of anterior extremity, dors. cirr. dorsal cirri ;
el. elytra ; perlst. tent, peristomial tentacles ; prcest. prostomium. (After Claparede.)

The la3t 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 Polychseta. 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

Tcrebellidm (Fig. 373) the branchiae are situated on the dorsal sur-
faces of some of the anterior segments. In the Scrpulidw (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

Fio. 371.— Heads of various Poly ch seta (diagrammatic). A,- Polynoid ; B, Syllid ; C, Nuphthys ;
D, Eunice ; E, Phyllodoce ; F, Trophonia. a, prostomium ; c, cirri of body segments ; &,
peristomial cirri (tentacles) ; &, cirrus of first body-segment ; c'\ cirrus of second body-seg-
ment; el', point of attachment of elytron; p, palp; s, nuchal organ; t, tentacle; /, peri-
stomium : II, 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 and 383) and its allies each
branchia consists of an elongated stem on which are borne two
rows of short filaments. The surface of the branchiae 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 ANNUL ATA

473

In the Oligochreta branchiffi
are rarely present ; but in certain
of the Naiidomorpha there are
metamerically 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 systems of fine
lines intersecting one another
at right angles : it is perforated
in many places by the ducts of
the unicellular glands of the
epidermis. The epidermis con-
sists of a single row of cells, in
some cases, with smaller cells
of replacement intercalated be-
tween their inner ends. In
shape the cells vary greatly in
different families and often in
different parts of the body of
the same worm, being some-
times flattened, sometimes cubi-
cal or polyhedral, but more
usually more or less vertically
elongated. Cilia occur on the
surface in certain parts in many
Chaetopoda. Among the ordin-
ary cells of the epidermis there
are usually numerous unicellu-
lar glands often containing rod-
like bodies. In the tubicolous
forms these unicellular glands
are active in secreting the ma-
terial for the construction of
the tube. In addition, the epi-
dermis frequently contains sen-
sory cells, which are in many
cases contained in certain special
elevations or sensory papilla?.

The muscular part of the
body-wall consists of two layers,
in the outer of which the fibres
are disposed circularly, while in

474

ZOOLOGY

the inner their arrangement is longitudinal. The circular layer is
continuous, or, more usually, is interrupted opposite the intervals
between the segments. The longitudinal layer is disposed in four
bands in the Polychseta, two dorso-lateral and two ventro-lateral.
In the Oligochaeta it is divided by the setigerous sacs which pass
through it.

The peritoneal or coslmnic epithelium consists of a single layer of
cells. These are usually non-ciliated, but are ciliated in the

-o

A

. WWV PI ll\ t ' .a V .

■M

Firs. 373.— Terebella. (After Quatrefages.)

Aphroditca, Glycera, and some others, the movement of the cilia
bringing about an active circulation of the ccelomic or perivisceral
fluid in the ccelome.

The body-cavity or coelome, a wide space intervening between
the wall of the body on the one hand and that of the enteric
canal on the other, is divided in many Chaetopoda by a series of
transverse septa into a series of chambers corresponding to the

PHYLUM ANNULATA

47 »

segments. The septa are not complete partitions, there being
always apertures of greater or less extent by which the
cavities of neighbouring segments communicate. The septa
consist of double folds of the peritoneum enclosing muscular
fibres.

The enteric canal is nn elongated, and nearly always straight
tube, running through the entire length of the body from mouth
to anus. A number of different parts are usually distinguishable :
but their disposition varies to a very great extent in the different
groups. The buccal cavity, into which the mouth leads, is followed
by a muscular pharynx ; these are both formed in the embryo by
invagination of the ectoderm, and therefore
correspond to a stomodreum. The muscular
pharynx is absent in some of the Cryptoce-
phala : when present it is frequently pro-
trusible to a greater or less extent (see
Figs. 349, 305) ; around its extremity, when
it is fully protruded, are to be seen a circlet
of papilhe in some forms ; and in many, one
or more horny teeth, situated in its interior,
are brought into play. A gizzard with
thick walls may follow upon this protrusible
pharynx, and is sometimes preceded by an
oesophagus, which may be dilated behind into
a crop. The intestine is nearly always
more or less deeply constricted in each seg-
ment, and in the A'phroditea, or " Sea-mice "
(Fig. 374), there are in each of the segments
(with the exception of one or two of the
most anterior and one or two of the most
posterior), a pair of cceea which are to a
greater or less extent branched at their
extremities. In the Hcsiontda and Syllida
a pair of caeca which open into the anterior
part of the intestine frequently contain gas,
and probably have a hydrostatic function.

In some of the terrestrial 01igoch;eta (Earthworms) a fold of
the intestinal wall, the typhlosole, projects into its lumen. The
intestine is straight in most, but is somewhat coiled in the
Chlorcemida\ Sternapsis, and others. The wall of the aliment-
ary canal consists (1) of the visceral layer of peritoneum ;
(2) of longitudinally arranged muscular fibres ; (3) of circularly
arranged muscular fibres ; (4) of enteric epithelium. The
peritoneum on the surface of the intestine has in many Chaetopoda
its cells enlarged and granular to form the so-called chhragen
nils, which probably have an excretory function. The enteric
epithelium is very generally ciliated ; it contains numerous gland -

I'm. 874. — Enteric canal of
Aphrodite. «, month ;
b, pharynx ; c, branching
caeca of intestine ;</, anus.
(From Gcgenbaur's Com-
parative Anatomy.)

476 ZOOLOGY sect.

cells. The stomodaeum and the proctodeum are lined internally
by a cuticular layer, which is continuous with the cuticle of the
general surface. The anus is usually terminal in position, some-
times directed towards the dorsal aspect. There is, in most
instances, a longitudinal mesentery running to the alimentary
canal from the dorsal body-wall ; sometimes a ventral mesentery
is also present bearing a corresponding relation to the ventral
surface.

Some Chaetopoda are entirely devoid of blood-vessels. In one
family in which this occurs (the Glyceridos among the Phaneroce-
phala), the perivisceral fluid, which assumes some of the functions
of the blood, contains numerous red corpuscles, the red colour of
which is due to the presence of haemoglobin (see p. 36). In the
majority of the Chaetopoda there is a highly developed vascular
system. Sometimes the blood is colourless : very commonly it
is bright red in colour, owing to the presence of haemoglobin,
which is not confined to the corpuscles, but is dissolved in the
plasma. In Serpula and its allies the blood is bright green, owing
to the presence of a green colouring matter, Avhich has an affinity
for oxygen similar to that possessed by haemoglobin.

The chief blood-vessels are usually dorsal and ventral longi-
tudinal trunks. These are connected together by metamerically
arranged transverse branches. In some of the Cryptocephala the
dorsal vessel is not present in the greater part of the length of the
body, its place being taken by a circumintestinal sinus or a
circumintestinal plexus of vessels lying in the wall of the ali-
mentary canal. This gives off in front a short thick- walled dorsal
vessel or " heart." The movement of the blood is effected in most
instances by peristaltic contractions of the dorsal vessel or of a
cirumintestinal sinus or plexus or of a short and wide dorsal
cardiac sac given off by the latter anteriorly, which have the effect of
driving the blood from behind forwards. In some instances, as in the
Earthworms and some Cryptocephala, specially dilated lateral
vessels are contractile, and by their pulsations bring about the
circulation of the blood through the system of vessels. Plexuses
of fine capillary vessels in the integument of various parts
frequently aid in respiration, and are particularly well developed
in certain forms in which definite organs of respiration are absent.

The nervous system consists of a cerebral ganglion or brain
and a double ventral chain of ganglia. The cerebral ganglion is
distinctly bilobed, and may be looked upon as composed of two
intimately united ganglia. It is almost invariably situated in the
prostomium, though placed a little further back in the Earth-
worms ; it gives off branches to the eyes and tentacles. ' From it
there run backwards and downwards the paired oesophageal con-
nectives, which embrace the anterior part of the alimentary canal
between them, and below join the anterior end of the ventral chain

PHYLUM ANNULATA

477

of ganglia. The hitter always exhibits indications of being made
up of two lateral halves in the double character of the connecting
commissures and frequently of the ganglia themselves. One of
these double ganglia occurs in each segment, and from it a number
of nerves pass out to the various parts of the segment. In certain
Cryptocephala (Serpitla and others) the two halves of the chain are
separated from one another by a wide space, across which trans-
verse commissures pass between the ganglia. Connected with the
cerebral ganglia, or with the ossophageal connectives, or with both,
there is a system of delicate stomato gastric nerves passing to the
walls of the anterior part of the alimentary canal. In the majority

cU^rs. vcss

set

Fio. ;,,'..- Saccocirrus, transverse section, to show the position of the nerve-cords, dors. vess.
dorsal vessel ; int. intestine ; tie. co. nerve-cord ; set. setse. (After Fraipout.)

of the Chaetopoda the cerebral ganglion and the ventral chain are
separated from the epidermis by muscular layers ; in some, how-
ever, the ventral chain is in contact with the epidermis, and in
certain primitive or aberrant forms, the Archi-Chsetopoda (Fig.
375) and Sternaspis, the cerebral ganglion is in close union with
the epidermis ; in these also the ventral cord is not segmented
into ganglia. Running longitudinally through the ventral cord in
many forms are certain giant fibres of very large size ; though these
may have rather a skeletal than a nervous function, they are simply
greatly enlarged and modified nerve-fibres. Nerve-cells may be
confined to the ganglia, or may be distributed over the entire sur-

478 ZOOLOGY sect.

face of the ventral cord. Giant nerve- cells occur in some forms in
certain regions. Small ganglia are found frequently in various
peripheral parts, more especially at the bases of cirri or of
sensory papillae.

The organs of special sense are eyes, tentacles and cirri, nuchal
organs, and otocysts. Eyes, absent in the Oligochoeta with a few
exceptions and in some of the tube-forming Polychseta as well as
in a few free forms of that sub-class, are very general in their
occurrence. Their structure is, as a rule, very simple, but in some
forms reaches quite a high grade of development. Usually they are
confined to the prostomium, but Polyophthalmiis,m addition to the
prostomial eyes, has pairs of eye-like organs on many of the seg-
ments of the body. Leptochone has a pair on each segment, and in
Fabricia there is a pair on the anal segment ; while in many species
of Isabella and all the species of Dasychonc there are eyes or eye-
like organs on the branchial filaments.

Most usually the eye is (as in Nereis, p. 447. Fig. 353) a
spherical capsule with a wall composed of a single layer of cells,
which are elongated on the inner side, i.e. the side turned towards
the brain, while on the outer side they are usually flattened. The
outer thin part of the wall of the capsule, or cornea, is some-
times united with the epidermis ; when the two layers remain
distinct, the outer one is the outer cornea, the inner the inner
cornea. In many cases a thickening of the surface cuticle over the
cornea forms a cuticular lens. The cells of the inner portion of
the wall of the capsule form the elements of the retina ; they are
long narrow cells, sometimes composed of three distinct segments
— (1) a clear rod, directed towards the central cavity ; (2)-a middle
segment which is densely pigmented ; and (3) a segment contain-
ing the nucleus of the cell and directed towards the brain or the
optic ganglion, with which it is connected by a nerve-fibre. Fre-
quently the second and third segments are not to be separately
recognised, the whole of that part of the cell which contains the
nucleus being densely pigmented. A refractive mass fills the
interior of the capsule, and is sometimes distinguishable into a
firmer outer part, the lens, and a more fluid inner part, the vitreous
body. This refractive mass is often continuous with the cuticle
externally, and internally may be in continuity with the rods. In
some cases the structure of the eye is very much simpler. The
eyes on the branchial filaments of many tube-forming Polychseta
consist each of a group of retinal cells having its own lens-like
body and is quite independent of the others ; the eye is thus
a compound one.

Nuchal organs (Fig. 371, B, s) are very general in the Polychseta.
They consist of a pair of special ciliated areas or pits on the
posterior part of the prostomium, eversible in certain cases.

Otocysts are only exceptionally present. They consist of capsules

PHYLUM ANNULATA

479

of ciliated cells, in the fluid contained in which there are one or
several calcareous otoliths.

Ciliated grooves occur on the prostomium of many forms; in
Aricia they are present on all the segments : they have a special
nerve-supply, but their function can only be conjectured. Tactile
cells of the epidermis, with or without a projecting tactile hair or
stiff cilium, are very common, especially on the prostomium in the
Oligoctueta and on the tentacles and cirri in the Polychreta.
Groups of these are often aggregated together in papilla' or
goblet-bodies, with special nerve-supply and often with a ganglion
or a single nerve-cell at the base.

The organs of excretion of the Cluetopoda are a series of
segmentally arranged tubes, the nephridia, of which a pair, as a
rule, occurs in each of the segments of the body with the exception
usually of a few at the anterior and a few at the posterior end.
In its simplest form the nephridium is a curved tube, ectodermal
in origin, ciliated internally, opening on the exterior by a laterally
placed pore at the one extremity, and at the other ending
in a ciliated funnel or nephrostomc, which opens into the coelome
either of the same segment as that on which the external aperture
is situated (most Polychseta) or of the segment in front (all
or most Oligochaeta, some Polychaeta). The nephridia thus in
such cases effect a communi-
cation between the coelome
and the exterior, and serve
to carry off waste-products
which have passed into the
coelomic fluid ; but in many
instances the cells lining the
tube are active in separating
out such waste-matters, and
are loaded with granules and
concretions.

In many Polychseta, how-
ever, there is no ciliated
coelomic aperture, the tube
ending blindly internally,
such a blindly ending ne-
phridium (Fig. 376) being
frequently branched. On the
inner extremities in such
cases, or on other parts of the tube, are situated a number of
peculiarly modified cells, the solenoeytes, sometimes separate, some-
times united together in groups. Each of these is a rounded
cell lying in the coelome, and connected with the nephridium by a
long, slender, tubular process : through the lumen of the process
extends a single, extremely long, vibratile flagellum, which may

-.solenoeytes

nephridial—
canal

Flo. 370. — Inner branched end of nephridium of
Phyllodoce paretti, showing the nephridial
canal and the solenoeytes. (After Goodrich.)

480

ZOOLOGY

8 EOT.

be prolonged for some distance in the interior of the nephridium
itself. The resemblance between those solenocytes and the
flame-cells of Platyhelminthes will at once be recognised.

In the Polychgeta another set of segmentally repeated structures
are frequently intimately connected with the nephridia. These
are a series of pairs of ciliated funnels, the ccelomoduds, opening
widely into the coelome, and, in a typical case, communicating with

Fig. 377. — Diagram to illustrate tlic various combinations of closed and open nephridia and
coelomoducts in the Polychceta.

la, Hypothetical stage with closed nephridia and separate coelomoducts ; b, condition in
which the coelomoducts have become united with the nephridia : this occurs in Pkyllo-
docidce and Goniadidcv ; c, condition in which the ccelomoduct becomes reduced to a ciliated
organ (Nephthyidte) ; Ila, combination of nephridia with nephrostomes and separate coelomo-
ducts (Dasybranchus) ; b, condition in which " segmental organs " are formed by the union
of nephridia with nephrostomes and coelomoducts (the most usual condition) ; c, condition in
which there are nephridia with nephrostomes, and the coelomoducts are reduced to ciliated
organs (Nereis, etc.). The nephridia are outlined with a thick line : the coelomoducts striated.
(After Goodrich.)

the exterior. In Nereis they are represented by the dorsal ciliated
organ, and are not known to open externally. When provided
with external apertures, as is usually the case, the coelomoducts
act as the efferent ducts for the sexual elements. In many of the
Polychseta they do not remain independent, but coalesce partially
or completely with the nephridia, and the functions of excretory
organs and reproductive ducts become combined in the one set of
" segmental organs " (Fig. 377). In some families of Polychasta

\ PHYLUM ANNILATA 481

(SerptUa and allies) there is a single pair of large nephridia in the
anterior region of the body, with smaller pairs in the posterior
segments, the former alone appearing to have an excretory function
while the latter act exclusively as genital ducts. In Sternaspis
only a single pair of nephridia are present, which, though they have
small ciliated funnels, are not known to communicate with the
exterior.

In the Oligochaeta the nephridia are usually simple, elongated
and coiled tubes, a pair or sometimes more than one pair in each
segment ; but in some, these are replaced or supplemented in
certain of the segments, or in all, by a branching system of tubes
with or without ciliated funnels. Sometimes the ordinary nephridia
are not developed in the segments lodging the reproductive organs,
their place being there taken by three pairs of tubes of the nature
of localised coelomoducts which become modified to give rise to the
reproductive ducts ; but ordinary nephridia may be present in these
segments as well. In some Oligochaeta the nephridia of the most
anterior segments open into the mouth or pharynx, and have
apparently taken on the function of digestive glands (peptoneph-
ridia), and all the nephridia of the posterior region of the body
in one species {Allolobophora antipce), instead of opening on the
exterior, communicate with a pair of longitudinal canals which
posteriorly open into a median vesicle communicating with the
rectum.

The permanent nephridia of the adult Chaetopod are preceded
in the larva by a series of provisional or embryonic nephridia of a
temporary character. These have been found to occur in the head
(prostomiuin) of many larval Oligochaeta and Polychneta. They are
ciliated intracellular tubes, sometimes branched, which do
not open into the cavity of the prostomium. Sometimes soleno-
cytes occur at the inner ends of the branches or of the undivided
tube. Embryonic nephridia have also been shown to occur in the
body in certain forms.

Phosphorescence, the production of light rendering the
animal brilliantly luminous in the dark, occurs in a few cases
(various Polynoids, Chwtopterus, &c).

In the arrangement of the reproductive organs in the
Chajtopoda there is an essential difference between the two sub-
classes, the Oligochaeta being hermaphrodite, and the Polycha;ta,
with only a very few exceptions, unisexual. In the latter the
gonads, ovaries or testes as the case may be, are masses of cells
which are developed as the result of a proliferation of the
coelomic epithelium in certain positions (Fig. 378). Usually these
organs, which are only conspicuous about the breeding season, occur
in the great majority of the segments of the body ; sometimes they
are confined to a certain region. The exact place which they
occupy in the interior of the segment varies in different cases :

vol. i i r

482

ZOOLOGY

SECT.

sometimes they surround one of* the principal blood-vessels,
sometimes they are situated laterally, in the bases of the para-
podia. The sperms frequently undergo the final stages of their
development after they have become detached from the testes,
while floating in the coelomic fluid, and the same sometimes
holds good of the ova. Both sperms and ova appear to reach the
exterior, in the majority of cases, through the " segmental organs,"
which may become modified and enlarged at the breeding season,
though in some forms it is stated that the reproductive cells escape

refir.gl

pcrib — ■

vent, cess

fir.yl

jbcrU:

Si

Vtopj,

XM3

D

-A.Jrl

berib

is v&rttvesa

Flo. 378.— Diagram to Illustrate the development of a gonad from the peritoneal (cielomie
epithelium in one of the Polycbwta. peril, peritoneal membrane; repr. ijl, gonad (repro-
ductive organ) ; vent, vests, ventral vessel. (After E. Meyer.)

through temporary or permanent openings in the body-wall. Im-
pregnation takes place externally in nearly all.

In the Oligochaeta the reproductive organs are confined to a
certain limited region of the body. There are either, as in the
Earthworms, two pairs of testes, or a single pair, as in the aquatic
forms. The testes are small, and frequently become reduced to
mere vestiges in the adult animal, having mainly become broken
up into sperm-mother-cells, which in some way reach the vesiculae
seminales to undergo development into mature sperms. The
vesicidce seminales are comparatively large sacs, which vary in
number and arrangement in the different genera. One or two
median sperm-sacs, formed by the coalescence of pairs of vesiculae,
may be present. In the same segments as the testes, and
opening into the sperm -sacs when the latter are developed, arc

y PHYLUM ANN l LATA 483

cither two or four ciliated funnels, according to the number of the
testes, Leading into efferent ducts. All the four ducts, when four
are present, may remain distinct, or the two ducts of each side
may open into a common atrium, or they may unite to form a
common elongated vas deferens, opening at the male genital
aperture. In connection with the terminal part of the vas
deferens in many Oligochtets is a gland known as the prostate or
spermiducal gland. Near the aperture of the vas deferens in
many Earthworms are special setae, the penial seta:

There are never more than two ovaries, which, like the testes,
are of very small size. The ova may become mature in the ovary,
or groups of cells may be detached from the latter and one
cell in each group ripen into an ovum. A reccptaeulum ovorwm
occasionally receives the ova after they leave the ovary. There
are two oviducts, which open by funnel-shaped apertures into the
coelome.

Development. — The Oligochaeta deposit the eggs in cocoons,
either buried in the earth or attached to water-plants. The
cocoon contains, in addition to a number of fertilised ova, a quan-
tity of an albuminous fluid which serves as nourishment to the
developing embryos. Segmentation is always unequal. In the forms
in which food-yolk is scanty there is a process of embolic in-,
vagination (Lumbricus rubcllus); in the others {Tubifer, &c.) the
process is of the epibolic type. In the former case a blastula and
an invaginate gastrula are formed in the way already described
in the case of the Earthworm. In Lumbricus trapezoides the
gastrula divides into two, each half subsequently giving rise to
an embryo. The micromeres spread over the megameres very
much as in the Polychseta. A pair of mesoderm cells early appear,
and by their division forms the mesoderm bands. No free larval
stage similar to the trochophore occurs in any of the Oligochseta,
but the stage intervening between the completion of the gastrula
and the commencement of the segmentation of the mesoderm
bands corresponds to the trochophore in essential respects ; and in
some forms there is recognisable a feebly developed circlet of
cilia comparable to the prototroch, and in some a pair of head-
nephridia.

Impregnation and the development of the embryo takes place
externally in all the Chsetopoda, with a very few exceptions in
which development occurs in the coelome or in the interior
of a dilated segmental organ. In the Polychseta, in the great
majority of cases, fertilisation takes place by the sperms coming
in contact with the ova when both have become discharged,
and the development of the embryos goes on while they are
floating freely in the sea. There are a few cases in which the
impregnated ova are received into a sort of brood-pouch and
there pass through at least the earlier stages of their development.

II 2

484

ZOOLOGY

SECT.

Such a brood-pouch is formed in certain Phanerocephala by the
raising up of the integument on the ventral surface. In some
species of Polynoe and allied genera, the fertilised ova and the

ov

Fig. 370.— Spirorbis laevis, a hermaphrodite tubieolous Polychset. Lateral view of entire
anim il. ant. neph. anterior nophridium ; br. branchiaj ; as. oesophagus ; o/>. operculum with
developing embryos in its interior ; ov. ova ; sp. sperms ; st. stomach. (After Claparede.)

resulting embryos adhere in masses to the dorsal surface under the
shelter of the elytra ; in some other Polycha^ta they are stuck by
means of some viscid secretion all over the dorsal surface, or
they may adhere singly to the ventral cirri. In certain Crypto-

PHYU'M ANNT LATA

48H

cephnla (Fig. 379) they develop in a cavity in the operculum ; in
others, in the interior of the tube, between the body of the worm
and the inner surface of the latter, or on its outer surface. In
some, again, though the ova do not remain in any way attached to
the parent worm, they may be deposited in clumps or packets
enclosed in gelatinous matter. Usually they have no other
covering but the egg-membrane.

The segmentation of the ovum in the Polychseta is unequal.
In the great majority the inequality between the megameres and
micromeres is very marked. In some Serpulids, however, the differ-
ence is very slight, and the two sets of cells are at first scarcely
distinguishable. In such cases the cells arrange themselves in

Fio. 380.— A, B, C, three stages in the development of the trocbopbore of Eupomatus, from
the side. an. anus ; j%. blastoccele ; m. i>olar cells of the mesoderm ; md. mid-gut ; n. larval
head-nephrijium ; ot. otolith ; .«,'• neural plate ; st. stomoduuim ; irl; preoral ciliated ring ;
nfc] post-oral ciliated ring. (From Lang's Comparative Anatomy, after Hatschek.)

such a way as to form the wall of a hollow sphere, the hlastula,
with an internal closed cavity, the scg mentation-cavity. The
megameres, which may or may not have been distinct from
the first, lie on one side of the blastula; and soon this side
becomes invaginated (Fig. 380 A), the result being the forma-
tion of an embolic gastmla. In the great majority of forms
however, an epibolic gastrula is formed after the manner already
described in the case of Nereis; but forms of the process
of gastrulation intermediate between these two extremes have
been observed. The blastopore of the gastrula, however formed,
does not give rise directly either to the mouth or to the anus.
It becomes elongated into a slit which becomes closed up, and
the anus and proctodeum are formed by a fresh invagination

486 ZOOLOGY ■ sect.

in the original position of its posterior end, while another in-
vagination of the ectoderm further forwards gives rise to the
mouth and stomodaeum. The embryo then passes into the
trochophorc stage.

The arrangement of the cilia on the surface of the trochophore
varies in different Polychaeta. Sometimes, though rarely, the pre-
oral circlet is absent and the surface is covered uniformly with
cilia : such larvae are said to be atrochal. Sometimes there are
two circlets close together, the one immediately in front of, and
the other immediately behind, the mouth. Sometimes in addition
to the pre-oral circlet, there is a peri-anal circlet round the anal
end {telotrochal larvae). In some cases, instead of a pre-oral circlet
there is one further back round the middle of the body (meso-
irochal), or there may be several between the mouth and the anal
end (polytrochal).

The post-oral portion of the larva elongates, and traces of
segmentation become visible ; sometimes a series of constrictions
are developed before there is any trace of parapodia, sometimes rudi-
ments of the latter with their seta3 are developed first. The number
of segments, at first very small, becomes added to from behind as the
body gradually elongates. The establishment of external segmenta-
tion is accompanied by the division of the mesoderm bands into
a series of segments, the history of which has been sketched in
describing the development of Nereis. The ectoderm of the ventral
plate develops a median thickening which gives rise to the ventral
nerve-cord. Anteriorly this becames connected by a pair of thick-
enings at the sides of the mouth — the rudiments of the oesophageal
connectives — with the developing cerebral ganglion.

The completion of metamorphosis is brought about by the
increase in length of the body and concomitant increase in the
number of segments, by the full development of the various
systems of internal organs, and by the formation of the tentacles
and other appendages. The parapodia, when first formed, very
usually bear relatively long provisional seta', which are subsequently
thrown off to make way for the those of the adult.

Asexual reproduction by simple fission followed by regenera-
tion of the lost segments, or by proliferation followed by fission,
occurs in certain groups of Chaetopoda both among the Oligochaeta
and the Polychaeta. Simple fission occurs in Salmacina, one of
the Serpulids : a constriction becomes formed at a certain point
towards the posterior end, rudiments of a new set of cephalic
branchiae bud out on one side at this point, and this posterior
part becomes a distinct zooid, which is eventually separated off
and develops the full number of segments characteristic of the
adult. This is not in any way a case of alternation of generations,
as both parent and offspring are similar and sexual (hermaphro-
dite). In Nais and Chwtogastcr (Oligochaeta) there is multiplica-

PHYLUM ANNULATA

4h:

tion by proliferation of the segments at the posterior end; then
the appearance of a constriction separating off five or six of the
most posterior segments followed by a fresh
proliferation in front of the constriction ; and
then a second constriction appears five or six
segments further forwards — the result being
the development of a chain of zooids which
remain for a time connected together. The
sexual cells become fully developed only after
the zooids have separated from one another.
/ In some of the Syllidae there is a distinct
alternation of generations. The asexual worm
developed from the ovum gives rise by a pro-
cess of posterior proliferation and constriction
(Fig. 381) to sexual zooids, a number of which
may remain for a time connected together
in a string before undergoing separation.
These sexual zooids become developed into
mature males or females, which may be re-
markably unlike the parent form in the shape
of the parapodia, the character of the seta4,
and other points ; and in some instances the
two sexes not only differ from the asexual
parent form but also from one another, so
that the three forms, before their relationship
was known, were set down as representing
three distinct "genera.

Splits ramosa (Fig 382) which occurs in the interior of certain
deep-sea sponges, is exceptional among the Chsetopoda in giving

to. 3S1. — Budding in one
of the Syllida; (Autolytv*
eorniitus) ; parent stock
with a male zooid nearly
ready to become de-
tached. (After Agassis.)

I'm. S8L— Portion of Syllis ramosa. (From the Cambridge Natural History, after Mcintosh.

rise by lateral branching to a colony from which sexual zooids
afterwards become separated off,

488

ZOOLOGY

Modes of Life, etc. — Very few Chsetopoda are true parasites ;
but a considerable number are to be set down as commensals,
habitually associating with another animal for the sake of food
and shelter. The Earthworms burrow in soil containing decaying
vegetable matter, passing the mould through their intestine and
subsequently throwing it out in the shape of castings on the
surface. They also feed on decaying leaves, and sometimes on
animal substances. Some of the fresh-wTater Oligochaeta {Tnoificidw)
manufacture tubes of mud held together by a tenacious secretion
from the epidermal unicellular glands. Some of the Phanerocephala
form temporary tubes of a gelatinous character, or more permanent

parchment-like tubes sometimes
strengthened by means of agglu-
tinated sand-grains. But the ma-
jority of the Phanerocephala, which
for the most part prey on other
small animals, are not confined to
tubes, but move about freely. Some
burrow in sand ; others even in
harder substances, such as the shells
of Mollusca, or in limestone, shale,
or sandstone. Many Cryptocephala
secrete tubes the substance of which
is derived from the epidermal
glands. These tubes are sometimes
membranous or parchment - like,
sometimes membranous but har-
dened by the deposition of grains
of sand or particles of broken shells
or bits of sea-weed ; sometimes
(Fig. 383) they are of a hard, shelly,
calcareous character, sometimes
composed entirely of foreign par-
ticles cemented together ; very fre-
quently they are permanently fixed
to foreign objects. Some, such as species of Poly dor a, and Sty-
larioides, near relatives of which construct tubes, excavate galleries
in rock or coral or in the shells of various Mollusca.

A few Polychseta, such as the Alciopidce and Tomopteris, as well
as, in a certain phase, the Nereicke and Syllida>, are pelagic ; but
the majority live on the sea-bottom. They occur in the greatest
abundance near the shore ; but are also found at all depths in the
ocean, the tube-dwelling forms being more abundant than the free
forms in the deeper zones.

Owing to the soft character of most of their parts, there are
comparatively few actual remains of Cheetopoda in the older
geological formations, though there are many burrows and tracks

Fio. 383.— Serpulse with their tubes.
(After Quatrefages.)

x PHYLUM ANNTLATA 4H«»

which have been ascribed to members of that class. Tubes of
tubiculous Polycha'ta have, however, been found in formations
dating from the Cambrian period onwards. Some tubes not
distinguishable from those of the existing genus Spirorhis, are
found as far back as the Silurian ; and others, apparently closely
related to the living Serpula, as far back as the Carboniferous.
In addition there are a number of tubes of extinct forms ascribed
to the tubicolous Polychoeta. The horny jaws of various Polychaeta
have been detected in strata from the Cambrian period onwards ;
and many tracks and burrows occurring in rocks of all ages are
ascribed, some with more, some with less certainty, to this group
of worms. No fossil remains of Oligochseta are known.

APPENDIX TO THE CH^TOPODA.
CLASS MYZOSTOMTDA.

The Myzostomida are a group of worms which appear to have
their nearest relatives in the Chaetopoda, though possessing certain
special features of their own. They are all external parasites of
various Crinoids — both of the stalked and the free varieties, or
internal parasites of certain Starfishes. They are disc-shaped
animals (Fig. 384) devoid of any trace of external segmenta-
tion. There are patches of cilia here and there on both dorsal
and ventral surfaces. At the sides there are five pairs of para-
podia (p), each with a chitinous hook and a supporting rod; in
the intervals between these there are in Myzostoma four pairs of
small " suckers " ; and round the margin are a series of ten or
more pairs of cirri provided terminally with motionless sensory
cilia, and with a ventral groove lined by adhesive cells. The
mouth, usually situated at the anterior extremity, leads into a
muscular pharynx (Fig. 3H5,ph.) capable of being protruded as a
proboscis ; from this a narrow oesophagus leads to the stomach,
which gives off a number of branched lateral diverticula (da.) A
short cloaca (klo.) leading from the stomach opens on the exterior,
in most cases at the posterior end of the body, sometimes on the
dorsal surface. There is no distinct ccelome, the space between
the alimentary canal and the body-wall being filled by connective
tissue (parenchyma), leaving only the cavities in which the sexual
elements are lodged. Bundles of dorso-ventral muscular fibres
form imperfect transverse septa, as in some Plat}melminthes.

There is no blood-vascular system, and specialised organs of
respiration are likewise wanting. There is a single pair of
nephridia with funnel-shaped internal apertures and with external
openings either into the cloaca or on the surface. The nervous

490

ZOOLOGY

system comprises a large stellate ganglion situated ventrally,
probably representing a number of fused ganglia, and giving off a
number of nerves ; and of two nerve-rings, one round the
oesophagus, the other round the pharynx, the two rings being
connected together by a series of longitudinal nerves. The

Fro. ::-i. Myzostoma. I-X, i-in-i ; m. month ; p. parapodift ; ». suckers. (After von Graff.)

oesophageal ring presents a very obscure dorsal thickening,
which is the only representative of a cerebral ganglion. '

Most of the Myzostomida are hermaphrodite. There is a pair of
ovaries formed by the proliferation of the layer of (coelomic)
epithelium covering the stomach ; and in the sexually mature
animal branching (coelomic) spaces in the parenchyma, between
the caeca, are found to be filled with ova (o). A posterior continua-
tion of these spaces (u) opens either into the cloaca or independently
of it. There are two elongated and usually branched testes (A),
each of which has two vasa deferentia leading to a vesicula
seminalis (sb) which opens near the lateral margins. The sexes

rilVLFM ANNFLATA

491

are united in most esses, separate in some. In the hermaphrodite
forms the testes are matured before the ovaries, and may have
ceased to be functional before the ova become ripe.

The development of the Myzoetomida closely resembles that of
the Polychjeta. A trochophore larva is first formed, and this

Myzoitoma. Diagrammatic view of the internal organs, <*, cirri ; da, branches of
the stomach ; *t, hind-gut : li, testes ; k-lo, aperture of cloaca ; m, stomach ; uto, male genital
aperture; o, ovaries; p, parapodia, with books and supporting rod; pli, pharynx; php,
pharyngeal tentacles ; pltt, pharyngeal pouch; xl>, vesicula seminalis ; v, uterus; wo, female
genital aperture. (rYom Lang's Comparative Anatomy, after von Graff.)

becomes metamorphosed into a larva with provisional setae bearing
a close resemblance to that of Nereis (p. 451).

V>

CLASS II.— GEPHYREA.

The Gejihyrca are marine Annulata devoid of any trace of segmen-
tation in the adult condition, without parapodia, and either without
setae or with only a limited number ; with either an invaginable
anterior body region or introvert, at the extremity of which is the
mouth surrounded by tentacles, or with a long, highly retractile
proboscis representing the pre-oral lobe of the larva, and having

492

ZOOLOGY

the mouth situated at the base. The anus is sometimes terminal
and posterior, sometimes anterior and dorsal. There is an exten-
sive ccelome filled with a corpusculated fluid, and not divided by
septa. The ventral nerve-cord is not made up of a series of
ganglia. There is, as a general rule, only a single pair of nephri-
dia. The sexes are separate ; the ovaries and teste's simple masses
of cells ; the nephridia act as reproductive ducts. The larva is
a trochophore.

1. Example of the Class — Sipun cuius nudus.

General External Features. — Bipunctdus occurs on sand at
moderate depths off the coast in most countries outside of the
tropics. It is an elongated worm of a cylindrical shape, somewhat
narrower towards one — the anterior — end. There is no trace of
division into segments. The anterior portion of the body, to the
extent of about a sixth of the total length, is capable of being
involuted within the part behind. The surface of this anterior
part, which is termed the introvert (Fig. 386), differs in appearance
from that of the rest of the body in being covered more or less
closely with chitinous papillae. The papillae of the posterior
portion of the introvert are shaped like the bowl of a spoon,

with the concavity turned to-
wards the body-wall and the
tip directed backwards ; they
are so closely arranged as to
overlap one another like the
shingles of the roof of a house :
further back they become longer
and narrower, mammilliform,

/|% and more scattered. When the

i*;.t*.'t;«~.'/,/ i.-J'th introvert is fully evaginated,

there appears at its extremity
a horseshoe-shaped fold of the
integument, the tentacular fold
{lent.), which is lobed and
plaited (Fig. 387) so as to as-
sume somewhat the appearance
of a circlet of tentacles. For
a little space immediately be-
hind the tentacular fold the
surface of the introvert is free
from papilla?. The posterior
portion 6f the body is devoid of papillae, but is marked out by a
series of narrow impressed lines into a number of elongated four-
sided areas.

post pap

Pig. 38(5. — Anterior extremity of Sipun-
culus nudus. ant. pap. anterior papil-
lary region ; post. pap. posterior papillary
region ; tent, tentacular fold. (After Ward.)

x PHYLUM ANNULATA 498

Body-wall. — The surface is covered by a chitinoid cuticle
having an iridescent lustre similar to that presented by the cuticle
of Nereis and Lumbricus, and due to the same cause — viz., the
presence of two systems of intercrossing lines. The papilla; on
the introvert are local thickenings
of this cuticular layer. Beneath
the cuticle is an epidermis con-
sisting of a single layer of cells,
usually sac-like, but capable of
being altered as a result of con-
traction or compression into a
spindle-like shape. Below the
epidermis is a layer of connective-
tissue, the dermis, in which, as
well as to some extent in the
epidermis itself, are a number
of dermal bodies. Of these there
are three kinds — bicellular glands, kig.sst.— Tentacular fold of sipuncuius
contained in papilla? ; multi- SBf "* "* ™M °Tg&a' (After
cellular glands, scattered through

the integument and not contained in papilla? ; and sense-papilla ',
small rounded thickenings of the epidermis in the anterior region
of the introvert, with their summits covered with cilia. There
are also numerous pigment-cells. A number of canals branch
through the dermis, beneath which are three layers of muscle —
(1) an outer circular layer, continuous in the introvert, but
divided into annular bands in the rest of the body ; (2) an oblique
layer, well developed only between the origins of the two retractor
muscles of the .introvert; (3) a longitudinal layer, which is
separated by spaces into a series of parallel bands. Between the
bundles of the longitudinal layer of muscle runs a series of canals
which communicate with the body-cavity by transverse branches.

There is a spacious coelome, but it is traversed in all directions
by filaments and strands of connective-tissue, with which are mixed
very fine muscular fibres ; these mostly run from the wall of the
body to the alimentary canal. Floating in the cuilomic fluid are
(1) colourless corpuscles ; (2) reproductive elements; (3) peculiar
ciliated bodies, the urns, which are developed by proliferation
from cells on the wall of the dorsal blood-vessel. These
are comparable in structure and function with the ciliated funnels
of the Hirudinea (q.v.).

The blood-vascular system is very feebly developed. It
consists of dorsal and ventral contractile vessels, the former known
as the " heart," communicating in front with a circular sinus
at the base of the tentacular fold.

The alimentary canal (Fig. 388) is a cylindrical tube of uniform
character throughout. It is twice the length of the body, running

494

ZOOLOGY

lent

f&

back from the mouth towards the posterior end, and then bending

sharply round to run forwards to the
anus, the two limbs being twisted
spirally round one another. Run-
ning along the inner surface of the
entire length of the alimentary canal,
with the exception of the terminal
part or rectum, is a narrow groove.

oes ^°nnected with the rectum is a nar-
row cwcum of variable length, which
opens into the beginning of the rec-
tum. Two tuft-like groups of rectal
glands occur close to the anal opening.
The nervous system (Fig. 389)

-red differs considerably from that of

dors retr -

neph-

dors, retr-

inl-

n.co-

tr'-

Fio. 388. — Dissection'of the internal organs
of Sipunculus nudus. dor*, retr.
dorsal retractor muscles of the intro-
vert ; int. intestine ; in. n. co. muscles
accompanying the nerve-cord ; n. co.
nerve-cord ; ne/th. nephridium ; eet.
oasophagus ; rtct. rectum ; tent, tenta-
cular fold. (After Vogt and J ung. )

PlO. 889. — Anterior part of the nervous system
<>f Sipunculus nudus. can. o. cab. cere-
bral organ ; corns, «, oesophagal connective ;
n.mu. ret. nerves to retractor muscles ; n. »j>l,
splanchnic nerves ; n. ta. 1-U, nerves to ten-
tacular fold ; /, //, nerves from ventral cord ;
24, main mass of brain. (After Ward.)

v PHYLUM ANNULATA 495

the rest of the Annulata. There is a relatively small bilobetl
cerebral ganglion situated on the dorsal aspect just behind the
tentacular circlet, to which it gives off on each side several pairs
of nerves. Arising from it anteriorly and dorsally are a number of
digitate processes lying in the coelome. The esophageal connectives
{corns, a) which it gives off behind are greatly elongated ; from each
arise muscular nerves (n. ma. ret), and also a visceral nerve (n. spl)
I Kissing to the alimentary canal. The two commissures unite behind
to form a ventral cord, which extends throughout the rest of the
length of the body. The ventral cord presents no appearance of
ganglia : it sends off laterally a large number of pairs of nerves
(I., II.); on section it appears distinctly double. Two delicate
muscular bands (Fig. 388, m. n. co.), which take origin anteriorly
from the body- wall, become attached to the nerve-cord, and follow
it throughout its length, giving off small branch-bands to accom-
pany the lateral nerves. A canal with folded and pigmented walls,
which opens in the middle line of the dorsal surface just behind
the tentacular fold (Fig. 387, cer. org.), extends backwards to the
anterior ventral surface of the cerebral ganglion, where it ends
blindly. It is possible that this, the cerebral organ, may be a
sensory organ of some kind. Eyes are entirely absent. The digi-
tate processes of the cerebral ganglion, which bear a number of
ciliated cups along their edges, may be sensory in character.

Sipunculus has only a single pair of nephridia. These
(Fig. 388, neph.) are situated tolerably far forwards, the external
openings being about 2 cm. in front of the anus. They are long,
nearly straight tubes, of a brown or yellowish colour, and very
mobile in the living condition. Near the external opening, which
is situated at the anterior end, is the internal opening into the
cielome. The sexes are separate. There are no definite gonads
except at a certain season of the year, when cellular elevations
developed in the connective tissue covering the ventral
retractor muscles of the introvert represent ovaries or testes
as the case may be. These give origin to cells which become
detached and develop into the fully-formed sexual elements
while Moating about in the ceelomic fluid. The nephridia act
as gonodacts.

V

2. Distinctive Characters and Classification.

The Gephyrea are Annulata with the body devoid of any ap-
pearance of segmentation in the adult condition. There is a large
ccelome, which is not divided into chambers by mesenteries or
septa. A blood- vascular system is sometimes present, sometimes
absent. The ventral nerve-cord is not composed of a chain of
ganglia. There is usually only one pair of nephridia. The sexes

496 ZOOLOGY sect.

are separate, the gonads simple, and the nephridia act as
gonoducts.

The larva is in most cases a trochophore, and may develop a
metameric segmentation which disappears as development pro-
ceeds.

Order 1. — Inermia (Sipunculoidea).

Gephyrea with an introvert and usually tentacles or a tentaclar
fold. The anus is dorsal. Seta? are absent. Nephridia a single
pair, or absent altogether.

Order 2. — Armata (Echiuroidea). .

Gephyrea with an elongated prostomial proboscis. The anus
is posterior. Two or more seta?. A single nephridium, or two
or three pairs of nephridia.

Systematic Position of the Example.

Sipunculus modus is one of several species of the genus Sipunculus.
Sipunculus differs from other genera of the family Sipunculidcc
of which it is a member, mainly in having a tentacular fold around
the mouth, instead of a series of distinct tentacles. The family
Sipunculidw is one of two families of the order Inermia ; and differs
from the other, the Priapulidw, in the presence of either tentacles
or a tentacular fold at the oral end, and the absence of filiform
appendages at the aboral end.

3. General Organisation.

The Gephyrea are a class of worms whose position among the
Annulata is determined more from a consideration of their develop-
ment than of their structure in the adult condition, though the
latter suggests a tolerably close affinity with the Chajtopoda. The
body of a Gephyrean is unsegmented, usually more or less com-
pletely cylindrical, broadest behind and narrowing towards the an-
terior end. The surface is covered with a chitinous cuticle developed
often into papilla, or tubercles, or hooks. In the Armata, seta: are
present, but they are always very few in number and not implanted
in parapodia ; in Bonellia there is only a single pair, situated about
the middle of the ventral surface ; in most species of Ecliiurus
(Fig. 391), in addition to this ventral pair, there are a number
arranged in one or two circlets around the posterior end. In the
Inermia the anterior part of the body is capable of being invagi-
nated within the part behind ; at the extreme anterior end of this
invaginable part or introvert, when it is evaginated, is the mouth,
surrounded by a circlet of sometimes pinnate, sometimes simple,

PHYLUM ANNULATA

40Y

t < ntacles, or by a lobod and plaited tentacular fold. The prostoniium
is in such tonus quite rudimentary. In the Armata there is no
introvert, bat an elongated, highly contractile, simple or bifurcated
proboscis, which is the greatly produced prostomium ; in Bonellia
(Fig. 390) the proboscis, when fully extended, is many times the
length of the body: in Echiurus (Fig. 391) it is much shorter:
at the base of the proboscis on the ventral side is the opening of
the mouth. In Priapulus (Fig. 392) there is at the posterior end
an elongated simple or bifurcated caudal appendage covered with

ant set

Fie. 390.— Bonellia viridis, entire
animal (female) with the proboscis
moderately extended. (After Grecf.)

pQ5t.Set

Fie. 301.— Echiurus, entire animal.
ant. set. anterior seta.' ; post. set. pos-
terior setaj ; prob. proboscis. (After
Greef.)

hollow papillae. The anus is situated at the posterior end of the
body in the Armata ; in the Inermia it lies far forwards on the dorsal
surface, except in the case of Priapulus, in which it is terminal.

Body-wall. — Beneath the cuticle is an epidermis, which is com-
posed of a single layer of cells. Among the cells are unicellular
(rarely multicellular) glands, and sensory cells. Various colouring
matters, such as the bright green characteristic of Bonellia, are
contained in the cells of the epidermis. The muscular wall of the
body consists of external circular and internal longitudinal layers,

VOL I K K

498

ZOOLOGY

sometimes with oblique and internal circular layers superadded.
There is an extensive undivided ccelome, lined, as in the case of
the Chaetopoda, with a coelomic epithelium, which is sometimes
ciliated.

The alimentary canal in the Incrmia consists of a muscular
pharynx, intestine, and rectum; in the Sipuncididce(Fig. 388) the
intestine is bent on itself, and spirally twisted as it runs forwards

to the anal opening, which, as already
noted, is situated far forwards on the
dorsal surface: at the junction of intestine
and rectum is a single simple coecum or a
pair of cseca ; and a number of small
branching glandular appendages are at-
tached to the rectum close to the anal
opening. Retractor muscles pass from
the body-wall to the pharynx. In the
Armata (Figs. 393 and 395) there is a
thin-walled buccal cavity, and an elong-
ated and coiled intestine, opening at
the posterior extremity of the body into
a dilated rectum : in most there is an
elongated caecum or siphon applied to the
ventral aspect of the intestine proper.
Into the rectum there open a pair of re-
markable caeca, the posterior nephridia
(Figs. 393 and 395, post, neph.), supposed
to be excretory in function ; these open
into the ccelome by means of a number
of ciliated funnels (Fig. 394).

There are no specialised organs of
respiration in the Gephyrea. A blood-
vascular system is sometimes present,
sometimes absent. When present, as it
is in most Gephyrea, it usually com-
prises a contractile dorsal vessel closely
applied to the intestine, and a peri-
pharyngeal ring or plexus. Cilia are
present in places in the interior of the
vessels.

The nervous system (Figs. 389 and 396) consists of a nerve-
ring, sometimes greatly elongated, surrounding the. anterior part of
the alimentary canal, with sometimes a dorsal and anterior thick-
ening representing a cerebral ganglion ; and of a nerve-cord, devoid
of ganglia, running backwards from this along the middle of the
ventral surface, and giving off pairs of branches at regular inter-
vals ; the branches of the same pair sometimes form complete
rings (Fig. 396, ne. ri.) by uniting dorsally. Eyes of a very

Fig. 302— Priapulus, entire
animal, resp. posterior papilte.
(After Bhlers.)

PHYLUM ANNUL AT A

4!)'.»

neph.fun

antneph

rvent.vcss

sinij)lo character, consisting of mere spots of pigment, are present
in some of the Inermia.

I'liapulus is devoid of nephridia. In the Armata a pair of
appendages of the
rectum are, as al-
ready mentioned,
to be regarded as
posterior nephridia.
In addition there
are present an-
U rut nephridia* In
Bonellia (Fig. 393,
ant. neph.), and
in some Inermia,
there is only a
single one : in the
majority of cases
there is one pair,
while in various
species of Thalas-
semia there are from
one to four pairs.
They are tubes
which open extern-
ally on the ventral
surface, and intern-
ally communicate
with the ccelome
by means of ciliated
apertures, the form
and position of which vary in different cases. They act as
efferent ducts for the reproductive elements (gonoducts) ; but
their function as excretory organs has not
been definitely established.

The sexes are usually distinct, and the
reproductive organs are of very simple
character, consisting merely of ridges or
clumps of cells {gonads), sometimes enclosed
in a membrane, developed at various points
on the body-wall or on the wall of one of
the main blood-vessels. The cells of these
ovaries or testes may develop in situ into
perfect ova or sperms ; more usually they
become detached, and undergo the later
stages of their development while float-
ing in the ccelomic fluid.
A remarkable instance of extreme sexual dimorphism occurs

K K 2

Fio. ::'.>:;. Bonellia, general view of the internal organs.
an. anus; ant. neph. anterior nephridium ; int. intestine;
neph. fun. nephrostome ; as. oesophagus; ov. ovary; ph.
pharynx ; 2'ost. neph. posterior nephridium ; prob. proboseis ;
vent. vess. ventral vessel. (After Grecf.)

Fio. 3!»4.— One of the ciliated
funnels of the posterior
nephridia of Echiurus.
(After Grcef.)

500

ZOOLOGY

in Bonellia. The ordinary large individuals (Fig. 390), to various
points in the structure of which reference has been already made,
are females. The single, greatly enlarged nephridium contains a
spacious cavity, which has been termed the uterus. In the
interior of this is found the very small male (Fig. 397), which is
not unlike a Planarian in appearance: it is compressed and covered
with cilia, with a pair of ventral hook-like setse. In the interior
of the body bundles of dorso- ventral muscular fibres placed at
regular intervals give an appearance of rudimentary segmenta-
tion. The alimentary canal is vestigial and completely closed,

ant.nep

r

post.neph

Fig. 395. — Echiurus, Internal organisation, an.
anus ; ant. neph. anterior ncphridia ; int. in-
testine ; int. vess. intestinal vessel ; at. oeso-
phagus ; post. neph. posterior ncphridia ; vent,
vess. ventral vessel. (After Greet.)

Fig. 3'Jti.— Echiurus, general out-
line of the animal, with the
nervous system (diagrammatic).
ne. co. nerve-cord ; ne. ri. ncrvc-
ring. (After Greef.)

both mouth and anus being absent. There is a pair of
nephridia placed posteriorly. The sperms, developed from
modified ccelomic cells, reach the exterior through a duct, dilated
externally into a vesicula seminalis, and opening internally into
the coalome by a funnel-shaped aperture. In Hamingia, also, there
are imperfectly developed males which are lodged in the nephridia
of the female.

Development. — The larva of Echiurus (Fig. 398) has a well-
developed pre-oral or prostomial lobe with pre-oral and post-
oral circlets of cilia, and in other respects closely resembles the
trochophore embryo of a Chsetopod. The posterior part of the

PHYLUM ANNULATA

501

body elongates, and the mesodorm-bands, developed as in the
Chaetopoda, become divided into as many as fifteen segments. A
circlet of setae is developed at the anal end, and subsequently
the two ventral setae are formed in the same manner as in
the Chietopoda. The pre-oral lobe becomes narrowed to form the
cylindrical proboscis of the adult ; and the rudimentary segmenta-
tion gradually disappears as development advances.

In Bonellia there is unequal segmentation, as in most Chaetopoda
resulting in the formation of four large megameres and eight small

micromeres : the latter multiply
,repr.ap rapidly, and grow over the mega-

meres so as eventually to enclose
the latter in a complete layer of
ectoderm, save at one point, where
there is a gap, the blastopore. Here
the ectoderm bends inwards to

coel

■ves.sem
ali

tlom

Fio. 307.— Male of Bonellia. ali.
alimentary-canal ; etel. groups of
eneloniic cells destined to give
rise to sperms ; repr. ap. repro-
ductive aperture ; vcs. sent, vesi-
cula seminalis. (After Greef.)

Fio. 308.— Trochophore of Echiurus.

in), anus ; ap. pi. apical plate ; int. in-
testine ; mo. month ; ne. ro. rudiment
of nerve-cord ; <e». wsophagrs ; **.
conn, oesophageal connective ; stom.
stomach. (After Hatschek.)

give rise to a continuous mesoderm layer superficial to the mega-
meres. The blastopore soon closes up. The megameres divide to
form the cells of the endoderm, among which a lumen only appears
comparatively late; mouth and oesophagus are developed as an
outgrowth, at first solid, from the endoderm. The anus becomes
formed still later by invagination at the hinder end of the body ;
and a pair of epidermal vesicles which appear at its sides, developed
as outgrowths from the terminal part of the intestine, form the
rudiments", of the posterior nephridia. A rudimentary pre-oral
lobe becomes established. The mesoderm remains unsegmented,

502 ZOOLOGY

SECT.

but splits into somatic and splanchnic layers going to form
the muscular system, blood-vessels, and other mesodermal organs.
Before the alimentary canal is formed the larva, which had
previously been spherical with two bands of cilia and a pair of
eye-spots, becomes elongated and dorso-ventrally compressed, and
becomes covered uniformly with cilia, so as to present the general
appearance of a Planarian. It becomes converted into the adult
female by a metamorphosis, including the elongation of the pre-
oral lobe to form the proboscis and the development of the pair of
setse of the adult. The male never goes through this metamor-
phosis, but remains in the Planarian stage: it at first adheres to the
proboscis of a female, then enters the oesophagus, and afterwards,
when sexually mature, passes into the cavity of the nephridium.

In the Inermia the early stages of the development closely
resemble those of the embryo of one of the Polychaeta, and a
stage corresponding to the trochophore of that class is developed,
but with the mouth situated further forward in front of the ring
of cilia and with the anus in front of the posterior extremity
on the dorsal side. But at no stage in the development has
any trace been observed of the temporary segmentation which
forms so marked a feature in the development of Echiurus.

Distribution Affinities, etc. — The Gephyrea are all marine.
They are only capable of very slow creeping locomotion, and live
for the most part either in natural rock-fissures, or in burrows
which they excavate for themselves either in sand or mud, coral or
rock. Their distribution is general ; and they occur at considerable
depths as well as in shallow water.

The differences between the Inermia and the Armata are so
considerable that there is some doubt whether they ought to be
united together in one class. The Inermia diverge most widely
from the Chaetopoda in the entire absence of setae and in the
want of segmentation at any stage. Priapulus differs in such
important points from the rest of the Sipunculoidea that
it is sometimes regarded as constituting a distinct order.

Affinities between Phoronis (p. 355) and the unarmed Gephyrea
have often been supposed to exist, and by some zoologists it has been
proposed to regard Phoronis as an outlying member of that class.
It seems probable, however, that the very manifest resemblances
which undoubtedly exist do not indicate a near relationship, but
are the result of converging modifications of originally widely
different stocks. The most striking of these points of resemblance
are two — (1) the approximation of the anus towards the oral
aperture, and (2) the presence of the tentacular circlet. But a
study of the development shows that these common features arise
in totally different ways in the two cases. The forward position of
the anus in the Sipunculida is brought about by a gradual displace-
ment resulting from the growth of the aboral region of the body ;
and the invagination and evagination by which the corresponding

PHYLUM ANNULATA

503

result is attained in Phoronis do not occur. Again, while in Phoronis
the tentacles of the adult may be looked upon as formed by the
development of processes along the line occupied by the post-oral
circlet of cilia, in the Sipunculida the tentacular lobes have
nothing to do with the post-oral circlet, but are formed by the
growth of a series of lobes from the margin of the mouth itself.
The larva of the Sipunculida again is, as already pointed out, very
nearly related to the larva of the Cha^topoda, and is a typical
trochophore ; while the Actinotrocha larva of Phoronis diverges
somewhat widely from that type.

CLASS III.— ARCHI-ANNELIDA.

More primitive in some respects than the other Annulata are the Archi-
Annelida, comprising only the family Polyyordiklai to which may perhaps be added

r,-.sb

Fio. 390.— Polygordius neapolitanus. A, the living animal, dorsal aspect, about five times
natural size ; B, anterior end, lateral view ; C, ventral view of the same ; D, portion of the
body showing the metameres ; E, ventral view of the posterior extremity; An. amis;
An. Deri, anal segment ; c. p. ciliated pit ; gr. grooves between metameres ; Mth. mouth ;
Mtwi: metameres ; />. papillae; per.xt. peristomium ; jn:*t. prostomium ; s. papilla; on
tentacles (<). (From Parker's Biology, after Fraipont.)

Ctenodrilus. They are marine worms with narrow, elongated, cylindrical hotly.
The prostomium (Fig. 399, pr. at) is small, the peristomium (per. at) large. The
segments (Mlmr) are only faintly marked off externally for the most part, though

504

ZOOLOGY

the internal division of the ccelome by means of septa is complete. Parapodia
and seta? are absent, but the prostomium bears a pair of tentacles (<). Several
pairs of simple nephridia are present. The position of the nervous system
(Fig. 401) is more primitive than in the Annulata in general ; it is continuous
with the epidermis, and not separated from it by mesodermal elements as in
most of the others. A pair of ciliated grooves (c. p. ) are probably to be looked
upon as organs of special sense.

The family Polygordiidce includes two genera — Polygordius and Protodrilus.
There are a pair of prostomial tentacles, long in Protodrilus, short in Polygor-
dius, and a pair of ciliated pits. The segmentation is only very indistinctly

marked externally in Protodrilus by circlets of
cilia ; in Polygordius it is indistinct in front, but
better marked behind. In Polygordius lacteus a
series of tooth-like processes occur round the anus,
and in front a circlet of adhesive papillae. In
Protodrilus there is a ventral ciliated groove.
_lnt There is a vascular system with dorsal and

D.V

Z>erE/bt7wi

KM'. Cd Cxl.Epthnf

Fig. 400.— Protodrilus, en-
tire animal, int. intestine ;
mus. w. muscular append-
age of oesophagus ; oes. oeso-
phagus. (After Hatschek.)

Fio. 401.— Polygordius neapolitanus, transverse
section of a male specimen. Coil. Epthm. parietal layer
of coelomic epithelium ; Ccel. Epthm.' visceral or
splanchnic layer of the same ; Cu. cuticle ; Jhr.
Epthm. deric epithelium ; D. V. dorsal vessel ; Ent.
Epthm. enteric epithelium ; M. PI. muscle-plates ;
0. M. oblique muscles ; Spy, immature gonads ;
V. Nv. Cd. ventral nerve cord continuous with deric
epithelium ; V. V. ventral vessel. (From Parker's
Biology, after Fraipont.)

ventral longitudinal vessels. In each segment is a pair of simple nephridia.
In Protodrilus there are two ventral nerve-cords, connected together by trans-
verse commissures: in Polygordius the cord (Fig. 401, V. Nv. Cd) is single;
in neither genus is there any trace of ganglia. The sexes are united in most
individuals of Protodrilus, ovaries occurring in all the first seven segments and
testes in some of those immediately following. In Polygordius the sexes are
separate; the ovaries or testes (Fig. 401, Spy) are developed in the posterior
segments. There are no special reproductive ducts.

The larva of Polygordius is a typical trochophore (Fig. 402), and its meta-
morphosis into the adult worm (Fig. 403) takes place as in the Polyclueta in all
essential respects.

Ctenodrilus resembles Polygordius in the ectodermal position of the nervous

PHYLUM ANN PLATA

505

system and in the presence of ciliated pits ; but it has a row of comb-
like seta.' on each segment, and it has only a single pair of nephridia, which are

Fio. 402.— Trochophore of Folygordius neapolitanus. A, lateral view of entire larva ;
B, diagrammatic vertical section ; C, transverse section through the plane ab in H ; An. anus ;
An. ci. anal cilia ; Bl. blastociele ; Br. apical plate ; Ent. enteron ; Msd. mesoderm ; Msd.
M. mesodermal bands ; Xph. head-kidney ; Oc. eye-spot ; Pr. or. ri. cilia of prototroch ; Pre.
dm. proctodeum ; 1't. or. ci. post-oral cilia : St. dm. stomodaeum ; V. Nv.C'd. ventral nerve-cord.
(From Parker's Biology, partly after Fraipont.)

tUBt

Msd .'/it )

Pre. dm.

Pr.an.ci.

An.ci

~An ci.

Fio. 403.— Later stage in the development of Folygordius neapolitanus, in which the
rostcrior part of the trochophore has become elongated and segmented ; A, entire larva ;
B, vertical section ; C, transverse section along the plane ab in B ; I)1 — D3, three stages in the
development of the somatic mesoderm : Co?l. ctelome : ail. Bpthm. ceelotaic epithelium ; her.
Ejithm. deric epithelium; M.pl. muscle-plate: Mid. (mh.), somatic mesoderm; Msd. (spl),
splanchnic mesoderm; 0. eye; t. tentacle. Other letters as in preceding figure. (From
Parker's Biology, partly after Fraipont.)

situated in the peristomium. There is only a 3ingle (longitudinal) layer of
muscles in the body-wall ; the reproductive apparatus is not known.

506 ZOOLOGY

CLASS IV.— HIRUDINEA.

1. Example of the Class — The Medicinal Leech {Hirudo medici-
nal is and H. (Limnobdclla) australis).

The medicinal Leech is found in ponds, swamps, and slowly
flowing streams in many parts of the world. H. medicinalis is the
common British species : H. australis is an allied Australian
form.

External Character. — The Leech is a vermiform animal, some
6-10 cm. (2-3 inches) in length, but is capable of contracting and
elongating itself so as to produce great alterations in form and
proportions. It moves by "looping" movementsi and is also a
good swimmer. The body (Fig. 404) is depressed or flattened
dorso-ventrally, the dorsal surface convex, the ventral flattened.
The anterior end presents a ventrally directed cup-like hollow, the
anterior sucker (a. s.), in the middle of which is a small aperture,
the mouth (mth.). The hinder end bears a disc-like posterior sucker
(p. s.), also directed downwards, and at its junction with the trunk,
on the dorsal surface, is the very small median amis (a.). The
animal is brightly coloured, the dorsal surface in H. medicinalis
being longitudinally banded with alternate stripes of greenish-
grey and rusty red, the ventral surface greenish-yellow, spotted
with black : in L. australis the whole under-surface is rust-
coloured.

The whole body is encircled by close-set transverse grooves,
dividing it into annuli. These, like the annuli of some Earth-
worms, are more numerous than the true segments or metameres,
the study of the internal organs showing that, except at the two
extremities, each segment contains five annuli. There are also
external characters by which the actual segmentation is plainly
indicated. The rust-coloured streaks on the back of H. medicina-
lis are spotted with black, and at every fifth annulus the spots are
larger than on the intervening rings : the annuli thus marked are
the second of their respective segments. Moreover, the same
rings bear on the ventral surface minute paired apertures, the
nephridiopores or excretory apertures {np. 1, np. 17) : of these there
are altogether seventeen pairs, marking the first ring of the
seventh and the second ring of the eighth to the twenty-third
segments.

In front of the first and behind the last pair of nephridiopores
one important external mark of segmentation fails, but a further
indication is furnished by the presence on the middle ring of each
undoubted metamere of a number of delicate transparent elevations,
the segmental papillcv (s. p .), which have probably a sensory func-
tion. These structures are found along the whole length of the

mlh

SjP <

Fio. in i.- Hirudo medicinaJig. A, dorsal ; B, vcntnil aspect ; a. anus ; a. s. anterior sucker ;
e. 1. first, and c. 0. fifth pair of eyes ; g. p. & . male gonopore ; ;i. p. ? . female gonopore ;
mth. mouth; np. 1. first, and np. 17. seventeenth pair of nephridioporcs ; p. s. posterior -
sucker ; a.p. segmental papillae ; I. — XXVI. segments.

508

ZOOLOGY

body, and as they mark the middle (third) ring of all those segments
the extent of which can be checked by the nephridiopores, it is
legitimate to assume their segmental value in the anterior and
posterior regions, where the controlling excretory apertures are
absent. By the clue thus furnished it is found that there are six
segments in front of that bearing the first pair of nephridiopores,
and three behind that bearing the last pair, making a total of
twenty-six metameres : of these the first seven and the last three
have less than the normal number of rings.

The anterior sucker bears on its dorsal surface five pairs of small
black spots, the eyes (e. 1, e. 5), the arrangement of which shows

nsl n.s vs

Fig. 405. — Hirudo medicinalis ; transverse section, b. t. botryoidal tissue ; c. to. circular
muscles ; cr. crop ; rr'. diverticula of crop ; cu. cuticle ; d. ep. epidermis ; d. s. dorsal sinus ;
d. v. /». dorso-ventral muscles ; I. m. longitudinal muscles ; I. v. lateral vessel ; n. c. nerve-
cord ; nph. 1 — h, nephridium ; n. s. nephrostomial sinus ; nst. nephrostome ; ts. testis ; v. d.
vas deferens ; vs. vesicle of nephridium ; v. s. ventral sinus. (After Marshall and Hurst.)

them to be special modifications of sensory papillae, since they
occupy in the first five segments the precise position occupied in
the sixth and following segments by segmental papillae.

The perfectly definite and comparatively small number of
metameres in the leech offers a striking point of contrast with
what we have met with in the Chsetopoda, and is to be looked
upon as a mark of higher differentiation.

Body-wall. — The body is covered externally by a thin cuticle
(Fig. 405, cu.), which is constantly being cast off in patches and
renewed. Beneath it is an epidermis (d. ep.) consisting of hammer-
shaped cells, separated at their inner ends by spaces in which

PHYLUM ANNULATA

509

blood-capillaries run. The blood is thus brought into close relation
with the surrounding water, and the skin becomes a highly effi-
cient respiratory organ. The space between the epidermis and
the enteric canal is filled by a peculiar form of connective-tissue,
consisting of a gelatinous matrix with interspersed cells and fibres,
many of the former large and branched. More immediately
surrounding the enteric canal is the peculiar and charac-
teristic botryoidal tissue (b. t.) consisting of branched canals, the
walls of which are formed of large cells loaded with black
pigment. This system of canals is in communication on the one
hand with the blood-vascular system and on the other with the
greatly reduced coelome.

Numerous unicellular glands are produced from the epidermis :
the gland-cells themselves lie in the connective-tissue, and are con-
tinued into long ducts which open on the surface. Special glands
in the ninth, tenth, and eleventh segments secrete the substance
from which the cocoon is formed (vide infra, p 515): the segments
in question therefore constitute the clitellum.

The muscular system is well developed, and consists of an
outer layer of circular (c. m.) and an inner of longitudinal (I. m.)
fibres. There are also dorso- ventral fibres (d. v. to.) passing vertically
between the pouches of the crop (vide
infra), and radial fibres extending from
the wall of the enteric canal to the in-
tegument: these take the place of the
septa of Chsetopods.

The alimentary organs are greatly
modified in accordance with the blood-
sucking habits of the animal. Surround-
ing the mouth are three jaws, one median
and dorsal (Fig. 408, d.j.), the other two
ventrolateral (v. I. /.). Each (Fig. 406)
has the form of a compressed muscular
cushion, with a sharp, evenly curved, free
edge covered with chitin, which is pro-
duced into numerous serrations or teeth :
by means of its muscles each jaw can be
moved backwards and forwards through
a certain arc, and the three, acting to-
gether, produce the characteristic triradiate bite in the skin of the
animal upon which the Leech preys.

The mouth leads into a muscular pharynx (Figs. 407 and 408,
ph.) situated in the fourth to the eighth segments. Radiating
muscles pass from its walls to the integument, and by their con-
traction dilate its cavity and suck in blood from the wounds made
by the jaws. Around the pharynx are numerous unicellular

Fk;. 406.— One of the jaws of
Hirudo medicinalis.

(After Leuckart.)

510

ZOOLOGY

-u.cL

•—\--l.u

cr.ll

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5s

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1 I

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crll'-t'

ts.10

gn.23

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3 fl*.

- - - :

8? .."3

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"H -.S u
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IB B •'

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1

salivary glands, which
open close to the
mouth : their secretion
has the effect of pre-
venting the coagula-
tion of the blood taken
as food.

The pharynx com-
municates by a very
small aperture with
the second and largest
division of the enteric
canal, the huge crop
(cr.), a thin-walled
tube extending from
the eighth to the
eighteenth segment,
and produced into
eleven pairs of lateral
pouches (cr. 1, cr, 11),
the first ten of which
are directed outwards
and correspond each
to a segment, while
the eleventh (cr. 11)
passes directly back-
wards as far as the
twenty - fourth seg-
ment. The crop is
capable of great dila-
tation, and its form
varies greatly accord-
ing to whether it is
empty or gorged with
blood. Posteriorly the
crop communicates by
a minute aperture with
the stomach (st.), a
tubular chamber with
a dilated anterior end,
and having its wall
produced internally
into a spiral fold : this
is the digestive portion
of the canal ; the blood
is passed into it from
the crop with extreme
slowness, and under-

PHYLUM ANNULATA

511

goes an immediate change, its
colour turning from red to
green. The digestion of a
whole cropful of blood takes
many months. The stomach
is continued into a narrow in-
/tsfi/ie (int.) : this passes into a
somewhat dilated rectum (ret.)
which turns slightly upwards
and opens by the anus (an.)
in the last annulus.

The excretory system
consists of seventeen pairs of
nephridia (nph. 1-17), situated
in segments 7-23. A typical
nephridium (Fig. 409) has the
general form of a loop passing
upwards from the ventral body-
wall, produced into an off-
shoot which extends inwards
(mesially) to the correspond-
ing testis, and connected pos-
teriorly with a small bladder
or vesicle (vs.). The principal
loop is divisible into two chief
parts, the main lobe (m. I.) and
the apical lobe (a. I.), connected
with one another by a short
recurrent lobe (r. I.): the off-
shoot to the testis is known as
the tcstis-lobc (t. I.) ; it is absent
in H. australis.

All these parts are formed
of a close-set mass of gland-
cells, traversed by a complex
system of minute intra-cellular
passages or ductules, which
finally unite into a compar-
atively wide inter-cellular tube
or duct : this winds through
the main and apical lobes,
and finally enters the vesicle,
which opens posteriorly in the
last annulus of the segment.
The free end of the testis-lobe
is swollen into a lobed mass
which lies in a sinus (Fig.
405 nst) in comiection with

512

ZOOLOGY

the testis. This lobed body is a modified ciliated funnel : it has a
great number of small ciliated openings into the sinus in which
it lies. The nephridia of the Leech differ from those of the
Earthworm, and also from those of Nereis, in the absence of
any internal openings, and in the absence of cilia in the interior
tof the canals. In most of the nephridia (all except the first
six pairs) ciliated funnels are present attached to the inner
ends of the nephridia, but these do not open into the canals of
the latter.

There is a complex vascular system, containing, like that of
the Earthworm, red blood, the plasma coloured with haemoglobin

Pig. 409.— Nephridium of Hirudo medicinalis. a. I. apical lobe; to. I. middle lobe";
np. nepliridiopore ; list, ciliated funnel ; r. 1. recurrent lobe ; t. 1. testis-lobe ; vs. vesicle ;
vs. d. vesicle-duct. The communication here represented as existing between the ciliated
funnel and the nephridial canals does not occur. (After Bourne.)

and containing sparsely distributed colourless corpuscles. But a
striking difference from the preceding annulate types is found in
the fact that the blood-containing spaces are of two kinds — blood-
vessels proper, having muscular walls ; and blood-sinuses, the walls
of which are devoid of muscle.

The two principal blood-vessels are lateral in position (Figs.
407 and 410, I. v.), running fore and aft at the level of the middle
of the nephridia and uniting with one another at the anterior and
posterior ends of the body. They send off branches both dorsally
and ventrally, some of which anastomose with one another. The
ultimate branches break up into capillaries in the integument,
nephridia, &c.

PHYLUM ANNULATA

513

The two principal sinuses are respectively dorsal (Figs. 405 and
410, d. s.) and ventral (v. s.), the former lying just above the
enteric canal in the middle dorsal line, the latter occupying a
similar position on the ventral side, and enclosing the ventral
nerve-cord. The two sinuses are in connection with one another
posteriorly, and are also in communication, by means of their
branches, with the capillaries of the skin. There is thus an
indirect connection, by means of capillaries, between the blood-
vessels and the sinuses, but no direct communication exists. The
sinuses in which the ciliated funnels are lodged open into the
ventral sinus. As we shall see more particularly in the general
account of the class, the sinuses represent a greatly reduced ccelome.

The nervous system is of the usual annulate type. There is
a small brain (Figs. 407 and 408, br.) situated above the anterior

^ \

Itr— i

Fit;. -110. — Diagram of principal blood-channels of Leech. <J. s. dorsal sinus ; I. v. lateral vessel ;
v. ». ventral sinus containing nerve-cord.

end of the pharynx immediately behind the median dorsal jaw.
It is connected by a very short pair of oesophageal connectives
with the ventral nerve-cord, which consists of twenty-three well-
marked rounded ganglia {gn. 1-23), situated in the third or middle
ring of each segment, united by delicate double connectives
and a slender median strand. The ganglion-cells are regularly
arranged in groups or packets. The first or sub-cesophageal
ganglion is larger than the others, and is shown by development
to be made up of five united pairs of embryonic ganglia: the last
ganglion is also of unusual size, and results from the fusion of six
pairs of ganglia distinct in the embryo. The whole ventral
nerve-cord is contained in the ventral sinus. Nerves are given off
from the ganglia, but not, as in the Earthworm, from the
connectives, in which also, nerve-cells are wholly absent.

VOL. I

L L

514

ZOOLOGY

The principal sense-organs are the eyes, of which there are five
pairs (Figs. 404 and 411) situated round the margin of the anterior
sucker, on the dorsal side, one pair in each of the first five segments.
They occupy positions taken in the succeeding segments by lateral

sense-organs, with which they are
obviously homologous. The structure
of the eyes is peculiar: they are
cylindrical in form (Fig. 411), the
long axis of the cylinder being at
right angles to the surface of the
body. The outer layer is formed of
black pigmented tissue (pi.), sur-
rounding a layer of large, clear,
refractive cells (p.), which occupy
the greater part of the organ. A
nerve (n.) enters at one side, and is
continued up the axis of the cylinder
by a row of sensory cells.

The margin of the anterior sucker
also bears a large number of goblet-
shaped organs, which are very pro-
bably organs of taste. The minute
structure both of these and of the
segmental sense-organs is very similar
to that of the eyes. The function of
the segmental sense-organs is un-
known.

Reproductive Organs. — The
Leech is monoecious. There are nine
or ten pairs of testes (Figs. 407 and
408, ts.), in the form of small spherical sacs, situated in segments
12 to 20 or 21. Each gives off from its outer surface a narrow
efferent duct, which opens into a common vas deferens (v. d.). In
the tenth segment the vas deferens increases in width and forms
a complex coil, the vesicula seminalis (v. sem.), from which is con-
tinued anteriorly a somewhat dilated muscular tube, the ductus
ejaculatorius (d. ej.). From each ejaculatory duct a narrow tube
passes to the base of the penis (p.), a curved eversible muscular
organ which opens on the ventral surface of the fourth annulus
of the eleventh segment, in the middle line. The base of the
penis is surrounded by a number of unicellular glands, which
constitute the prostate, and secrete a substance by which the
sperms are aggregated into masses called spermatophores.

The ovaries are coiled filamentous bodies, each enclosed in a
small globular ovarian sac (ov. s.), situated in the eleventh segment.
From each ovarian sac a short oviduct passes inwards and back-
wards, and unites with its fellow into a median duct, the walls of

Fig. 411. — Section of eye of Leech
c, cuticle ; d 8, gland-cells ; ep. epi
dermis ; g, nerve-cells ; n. nerve
p, refractive cells ; pi. pigment
(P'rom Lang's Comparative Anat
omy.)

x PHYLUM ANNULATA 515

which arc supplied with albumen-secreting gland-cells. The
o milium oviduct opens into a curved muscular tube, the vagina (va.),
which opens in the middle line on the ventral surface of the
fourth annulus of the twelfth segment, i.e. one segment behind
the male aperture.

It will be noticed that the ovaries of the Leech form a single
pair, while the testes are multiple and segmental : also that, while
the gonads and efferent ducts of both sexes are paired, the penis
and the vagina are median and unpaired. In the latter respect
the contrast between the Leech and the Annulata previously dis-
cussed is very striking. Further important peculiarities are the
enclosure of the ovary in a sac from which a duct leads directly to
the exterior, and the fact that the testes are hollow sacs discharg-
ing the sperms into a cavity from which they pass directly to the
efferent ducts. In Chuetopods, it will be remembered, the gonads
lie freely in the ccelome, their products — ova or sperms — are dis-
charged from their external surfaces and carried off either by
coelomoducts or by " segmental organs." It seems tolerably
certain that in the Leech the cavities both of the ovarian sacs
and of the testes represent shut-off portions of an almost
obsolete coelome, and that their ducts are coelomoducts.

Development. — When breeding two Leeches copulate, and one
impregnates the other by passing spermatophores through its
penis into the vagina. Simultaneous mutual impregnation
has also been described. The clitellar segments (ninth to
eleventh) secrete a cocoon (Fig. 412),
into which spermatophores, ova, and a
quantity of albumen, secreted by the
albumen-glands, are passed. The animal
then withdraws its head from the cocoon,
the two ends of which close up by their
own elasticity, producing a closed cap-
sule in which embryonic development fig. 412— The cocoon of Hirndo.
takes place. Segmentation is unequal, Lcuckart.) ' m "ec lon'
and results in the formation of a globular

embryo, which, after hatching, swims about in the cocoon, actively
devouring its albuminous contents, and finally escaping in a form
closely resembling the adult.

2. Distinctive Characters and Classification.

The Hirudinea are Annulata in which the body consists of a
limited and definite number of segments, and is marked externally
by secondary rings or annuli, a variable number of which go to
a segment. The anterior end of the body is suctorial, and several
of the hindmost segments are fused to form a powerful sucking-
disc, which is directed downwards and backwards. The mouth

l L 2

/

516 ZOOLOGY sect.

lies in the anterior sucker, the anus is usually dorsal and imme-
diately in front of the posterior sucker. The coelome is always
more or less obliterated by connective-tissue, and is represented
by sinuses of varying dimensions which contain blood. True
blood-vessels, with muscular walls, are also present. . The nervous
system consists of a brain united by short oesophageal connectives
to a ganglionated ventral nerve-cord. The excretory organs are
segmentally arranged nephridia. The sexes are united, the
testes numerous and usually segmentally arranged, the ovaries
a single pair. The testes have the form of sacs, and discharge
their products internally : the ovaries either have a similar struc-
ture or are band-like and enclosed in ovarian sacs, into which the
ova are set free. The penis and the vagina are unpaired, and open
by median apertures, the male anterior to the female, on the
ventral surface of the body. Development is usually direct, i.e.
unaccompanied by a metamorphosis. Leeches are either free-
living, or are permanently or intermittently parasitic : they inhabit
either the land, fresh-water, or the sea.

The class is divided into the following two orders : —

Order 1. — Rhynchobdellida.

Hirudinea in which the anterior part of the body can be pro-
truded and retracted so as to form a proboscis or introvert.

This order includes Glepsine (Glossiphonia), parasitic on Snails,
Frogs, &c. ; Piscicola, on fresh- water Fishes ; Pontobdella and
Branchellion, on marine Fishes (Fig. 413).

Order 2. — Arhynchobdellida.
Hirudinea in which there is no proboscis.

Sub-order 1. — Gnathobdcllida.

Arhynchobdellida in which the mouth is provided with two,
or more usually three, toothed jaws.

This sub-order includes Hirudo, the common Leech, parasitic
on Vertebrata; Aulostoma, the Horse-leech, free-living and
carnivorous ; JTcemadipsa, the Land-Leech.

Sub-order 2. — Herpobdellida.

Arhynchobdellida in. which the mouth is not armed with true
jaws.

This group includes Herpobdella (Nephelis) Troeheta, Orobdella,
etc. — all fresh-water or terrestrial forms.

PHYLUM ANNULATA

Systematic Position of the Example.

517

ffirudo belongs to the family Hiriulinidce, of the sub-order
Gnathobdcllida.

The presence of jaws places it in the sub-order Gnathobdcllida :
the possession of ten eyes, and the presence of five rings to all the
segments except a few at the anterior and posterior ends, dis-
tinguishes it as a member of the family Hirudinidse : the genus
Hirudo is distinguished by the constant presence of 26 segments
and of 102 annuli.

2. CI ep si ne

3.Branc hellion

I.Ponrobdella

Fio. 413.— Three Rhynchobdellida. br. gills ; pr. everted proboscis. (1, after Bourne ; 2 and 3

after Cuvier.)

3. General Organisation.

In the essential features of their organisation the Leeches are a
very uniform group : there are, however, a few interesting modifi-
cations of structure which must be referred to.

Form and Size. — Most kinds do not exceed a few centimetres
in length, but the American species Macrobdclla valdivania is said
to attain a length of 76 cm. (2£ feet). The number of annuli to
a segment varies from three to five, but the general form of the
body is remarkably uniform, the external differences between

518

ZOOLOGY

■nith

various species depending largely on colour and on the develop-
ment of papillae, which in some cases are large and prominent.
Setae are absent in all except one genus, AcarJhobdella, which has
two pairs on each side of the first five segments.

The proboscis (Fig. 414), the possession of which is distinctive
of the Rhynchobdellida, is simply the retractile anterior end of the
body, which, by the action of special muscles, can be drawn back
into a temporary sheath. The organ is thus an introvert, like that
of Gephyrea Inermia.

The chief differences in the structure of the enteric canal
depend upon the varying number, or, in some cases, the total

absence, of lateral
pouches to the crop :
for instance, the
horse-leech has only
a single pair, cor-
responding to the
eleventh pair in
Hirudo, while Ne-
phelis has none at
all. In the Rhyn-
chobdellida there is
a distinct slender
gullet (Fig. 414#m/.)
leading from the
pharynx to the crop
(cr.), and thrown
into a coil when
the proboscis is
retracted. Among
the Gnathobdellida
the median jaw is
absent in some
land-leeches, and in
other species all three jaws are rudimentary or absent.

The varying development of the blood- vessels and sinuses
presents many points of interest tending to explain the condition
of things described above in the medicinal leech. In the latter,
as we have seen, there are lateral vessels with contractile muscular
walls and dorsal and ventral sinuses with non-contractile walls.
In Pontobdelta, one of the Rhynchobdellida, there are dorsal
(Fig. 415 2, d.v.) and ventral (v.v.) as well as lateral vessels, and
lateral (l.s.) as well as dorsal and ventral sinuses, and in each case
the vessel is enclosed in the corresponding sinus. The ventral
sinus (v.s.) also contains the .nerve-cord (n.c.) and the ovaries (ov.),
and offshoots of it surround the testes (ts.) and the nephrostomes
(nst.). This arrangement clearly suggests the partial obliteration,

i~cr

^r"7

Fig. 414. — Proboscis of Clepsine. A, retracted; B, everted ;
cr. crop ; gul. gullet ; 'nith. mouth ; pr, introvert ; ». gl.
salivary glands. (After Bourne.)

1'IIYl.l M ANNI LATA

519

by growths of connective-tissue, of an originally continuous ccelome.
In the Rhynchobdellids in general the ccelomic spaces remain
fairly extensive, and are lined by a ccelomic epithelium. Another
interesting condition occurs in Nephelis (3), in which the middle
region of the body contains a series of paired, metamerically ar-
ranged spaces (c), surrounded by botryoidal tissue and containing
the nephrostomes. Development shows that these cavities are
derived from true ccelomic spaces in the embryo, formed, as in
Chaetopoda, by a splitting of
the mesoderm in each seg-
ment. Acantholdella, already
referred to as exceptional in
the possession of setae, is also
the only member of the class
which has a well-developed
and spacious ccelome, divided
by mesenteries into a number
of segments.

In most instances the skin,
with its abundant supply of
capillaries, constitutes the only
respiratory organ, but in
Branchellion (Fig. 413, 3) a
Rhynchobdellid parasitic on
the Electric Rays (Tooyedo and
Hypnos) and on .one of the Aus-
tralasian Skates {Raja nasuta),
differentiated respiratory or-
gans or gills (bi\) are present
in the form of delicate lateral
outgrowths of the segments.

In most members of the
class the nephridia are formed
on the same general type as
those of Hirudo, but differ in
the structure of the ciliated
funnels, which may be more
or less modified, as in Hirudo.

The funnels, where they occur, never open into the nephridial
canals. Each funnel leads by a narrow ciliated duct into a re-
ceptacle, in which leucocytes laden with waste matters are received
from the ccelomic spaces and sinuses, subsequently to undergo
degeneration and absorption (Fig. 415). By its outer side this
receptacle is in close relation to the inner end of the nephridium,
and the waste-matters from the disintegrated leucocytes are no
doubt received into the nephridial canals and thus passed out to
the exterior. In Hirudo and Herpobdella (Nephelis) the recep-

Fir.. 415.— Ciliated funnel of Herpobdella
(Clepsine). cr. crown-cells of funnel ; ex.
terminal cell of the nephridium ; Ic. leuco-
cytes ; st. duct leading to receptacle ; w. wall
of receptacle. (From Meiscnheimer, after L.
von Graff.)

520

ZOOLOGY

yrv.H

fltJi

Via. 416. — Nephridial system of Fon-
tobdella. gn. lh, gn. 17, ganglia
of nerve-cord ; np. nephridiopore ;
nph. nephridial network ; nst. cili-
ated funnel. (After Bourne.)

tacles appear to be the organs in which new blood -corpuscles are
manufactured. The ciliated funnels of the Hirudinea correspond
more closely with the coelomoducts or ciliated organs of the Poly-
cha3ta than with the nephrostomes ; they are to be compared
also with the " urns " of the Gephyrea. In the Rhynchobdellid

Pontobdella a very interesting modi-
fication of the nephridial system
occurs. Instead of distinct nephridia,
there is found on the ventral sur-
face of the body a very complex
network (Fig. 416, nph.), which
sends off on each side of each seg-
ment a short branch terminating in
a ciliated funnel, and a similar
branch which opens externally («jP.).
A similar modification occurs in
Branchellion.

The nervous system always
closely resembles that of Hirudo,
as also do the sense-organs. The
number of eyes is subject to con-
siderable variation : they may be
developed on the posterior sucker, or may be absent altogether.

Reproductive Organs. — The testes usually have the segmental
arrangement found in Hirudo, their number varying from five to
twelve pairs. But in Herpobdella (Nephelis) they are very
numerous, and are not arranged segmentary. In the Rhynchob-
dellida the muscular penis is absent, its place being taken by an
eversible sac or bursa copulatrix. The form of the ovary with its
containing sac in Hirudo is exceptional. As a rule, there is
an elongated hollow ovary, producing ova from its epithelial
lining, and thus agreeing very closely in structure with the
testis.

In Clepsine, a fresh-water Rhynchobdellid, copulation in the
ordinary sense of the word has never been observed, but one indi-
vidual has been seen to deposit one or more spermatophores on
any part of the body of another — often on the back. The spermato-
phore, which is nearly 3| mm. long, apparently exerts a solvent
action on the skin, since, after a short interval, the spermatic
substance streams through the skin into the coelomic spaces,
probably making its way at last to the ovaries. This extraordinary
process of hypodermic impregnation probably takes place in other
genera, but has been most closely followed in Clepsine.

It is in Clepsine that the early stages of development are
best known. Segmentation is unequal, the embryo consisting, in
the eight-celled stage (Fig. 417, A), of four large ventrally placed
megameres {mg.) and four dorsal micromeres (mi.). . One of the

PHYLUM ANNULATA

521

megamores, posterior in position, divides into two cells (B):
the so-called neuronephroblast and mesoblast, the latter of which at
once divides into two. As shewn by their subsequent history, the
neuronephroblast and the mesoblast correspond respectively to the
first and second somatoblasts of Nereis. The former divides and
sub-divides to form two symmetrical groups of four cells each,
situated at the posterior pole. The number of micromeres
increases, at first apparently by division of the megameres. The
latter subsequent!)' give off a number of small endoderm cells.

The embryo now consists of the three large megameres with a
number of endoderm cells, a cap of small micromeres forming an
ectodermal layer which is extending over the surface, with, at the
posterior pole, two symmetrical groups of neuronephroblast cells
(four in each), and, somewhat deeper, the two mesoblast cells.

-mill

Fio. 417. — Six stages in the development of Clepsine. a. 6. germinal bands ; mg. megameres
,„i. micromeres ; mth. mouth. (After Whitman.)

From each of the ten cells last mentioned new cells are given off
in front in such a way as to form ten rows of cells, five on
each side, four being derived from neuronephroblasts and one from
the mesoblast cell, These two sets of rows of cells constitute the
so-called germinal bands. From their subsequent fate it is clear
that they correspond to the mesoderm bands of Nereis plus the
neural plate. They grow forwards, the ectoderm extending with
them, over the endoderm and megameres. At first they diverge
widely, but their anterior ends subsequently meet towards the
anterior end of the embryo. Later the intermediate parts of the
bands, originally widely separated from one another owing to their
divergence during growth, approach one another and meet along
the middle line of the ventral surface. The germinal bands give
rise to the nerve-cord, the mesodermal segments, and the nephridia.
The layer of micromeres not only gives rise to the whole ectoderm

522 ZOOLOGY sect.

but also forms the head — the germinal bands not extending into
that region. The embryonic enteric cavity (mesenteron) becomes
formed by arrangement of the endoderm cells round the three
megameres, which break up to form nutrient material or yolk
destined to become absorbed in nourishing the embryo. The
pharynx is formed by an invagination of the ectoderm which
joins the mesenteron. At this stage the embryo leaves the egg,
and soon escapes from the cocoon to pass through its later stages
attached to the ventral surface of the parent.

In the Gnathobdellida the young are hatched at an early stage
of development, and their megameres contain but little yolk :
they are nourished up to the time of leaving the cocoon on the
albumen with which the latter is filled. One member of this
order, Herpobdella (Nephelis) is remarkable for undergoing a
metamorphosis : the anterior end of the embryo is ciliated, and it
possesses a provisional pharynx and several pairs of provisional
nephridia. Paired masses of cells, the head-germs, are developed
in the head, and from these and the germinal bands the whole
body of the adult is produced, the greater part of the larval body
being cast off. This process closely resembles the develop-
ment of the pilidium larva of certain Nemerteans (p. 295).

Habits, Distribution, &c. — The majority of the Hirudinea
are inhabitants of fresh-water, and live, like the Medicinal Leech,
by sucking the blood of higher animals — Vertebrates or Molluscs.
It is doubtless in correlation with this intermittent parasitism — the
chance of finding a vertebrate host being an infrequent one — that
the crop has attained such vast dimensions, holding, in the case of
the medicinal leech, as much blood as takes it a year to digest.
The allied species Hirudo sanguisuga has been found in the
nasal passages of man, producing serious results, and being, to all
intents and purposes, an internal parasite. The same is the case,
with the horse-leech, Hannopsis vorax, taken in, when young, by
horses and cattle while drinking. It attaches itself to the pharynx
and may even descend the trachea. Others are permanent ecto-
parasites : for instance, Branchellion occurs on the outer surface
of the Skate, Electric Ray, and other Fishes, entire families of this
leech, including individuals of all sizes, being sometimes found
crowded together on a small area of skin, which is distinctly
marked by their powerful posterior suckers. Other fish-parasites
are Pontobdella, on Rays, and Piscicola on fresh-water Fish.
Aulostoma, to which, as well as to Haemopsis, the name Horse-
leech is applied, is carnivorous, feeding on snails and other
Molluscs; so also are Glossiphonia (Glepsine), Herpobdella (Nephelis)
and the gigantic Macrobdella. The last-named genus and some
others are of subterranean habits, living in moist earth. The
Land- leeches (Hosmadipsa) live in the forests of many parts of

x PHYLUM ANNULATA 583

the world, and in spite of their small size, which does not exceed
30 mm. in length and 5 mm. in diameter, are much dreaded for
the persistent attacks they make on men and cattle.

Many genera are very widely distributed : for instance, the
Land-leeches (Hasmctdipsa) occur in India, Ceylon, the East
Indies, Japan, Australia, and South America, a distribution
which seems to indicate that the group is one of great antiquity.

GENERAL REMARKS ON THE ANNULATA.

A special feature of the Annulata, as distinguished from the
phyla previously dealt with, is metameric segmentation. In some
of the Platyhelminthes, as we have seen, there obtains a con-
dition to which the term pseudo-metamerism is applied. In such
cases there is a serial repetition of certain of the organs — gonads,
diverticula of the intestine, nerve-commissures, &c. — in such a
way as to produce a jointed appearance, though the body is not
divided into definite segments. An appearance resembling seg-
mentation is produced also in certain Rhabdocceles that multiply
by budding, chains of zooids remaining connected together
for a time. In the strobila of the Cestodes we recognise a con-
dition which might be described as combining pseudo-metamerism
with the formation of a chain of zooids. The condition of true
metamerism, as we observe it in the Annulata, is capable of being
deduced from a condition of pseudo-metamerism as it occurs in
Gunda (p. 255), the pseudo-metameres becoming converted into
true metameres by the development of inter-segmental constric-
tions and the completion of internal partitions. If we suppose that
during this process serially repeated outgrowths of the enteron
became separated off to form series of ccelomic sacs enclosing the
gonads, a condition would be reached not far removed from that
which characterises the Annulata. On the other hand, the meta-
meric condition is deducible from the condition of a linear colony
of zooids proliferating at the posterior end, the zooids, though
becoming each complete in itself, not, under ordinary circumstances,
becoming detached. The establishment of a closer connection
between the corresponding organs of the zooids in such a colony,
with the special differentiation of the anterior end, would result in a
condition closely resembling the metamerism of the Annulata. It
is conceivable that a condition of pseudo-metamerism was followed
by that of a linear series, not of zooids, but of comparatively
independent parts capable of readily reproducing the animal when
detached, and that a secondary closer connection established

524

ZOOLOGY

between the organs of all the series of parts resulted in the meta-
meric condition.

Metamerism is not universal in the phylum. In some (Archi-
Annelida) it may be said to be incipient or rudimentary ; in others
(Gephyrea) vanishing or vestigial. The Archi-Annelida are in this,
as in some other respects, the most primitive of the Annulata, and
through them it seems possible to connect the higher members of
the phylum with such lower forms as Dinophilus (p. 337) and the
Histriobdellea (p. 338). The general occurrence of the trocho-
phore larva may be taken as pointing to descent from an unseg-
mented ancestor having resemblances to the trochophore, and a

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Fig. 418. — Diagram to illustrate possible relations of the unsegmented to the metamerically
segmented worm. A, unsegmented worm with differentiated head end ; B, pseudo-meta-
merism ; C, linear series of zooids in which the first zooid differs in character from the others,
and in which the formation of new zooids takes place at the posterior end ; D, metamerically
segmented worm.

form like Dinophilus would afford us an intermediate link between
such a hypothetical ancestor and Polygordius or Protodrilus.

The position of the unarmed Gephyrea in the Annulata is, as
already noticed, a matter of doubt ; if we dissociate them from
the Armata there is little to connect them positively with the
other members of the phylum. But, on the whole perhaps, the
evidence in favour of regarding them as allied to the Armata,
and through them with the Cheetopoda, is sufficiently strong.

In adult structure, particularly in the absence of parapodia and
setae and the reduction of the coelome, the Hirudinea diverge some-
what widely from the Chsetopoda; but a study of their earlier
developmental stages shows unmistakably their close connection
with the latter group, more particularly with the Oligochseta ; and
the existence of an undoubted Leech (Acanthobdella) with setae
and with a well-developed ccelome traversed by mesenteries helps
still further to bridge over the gap between the two classes.

PHYLUM ANNULATA

525

The following diagram will serve to illustrate this view of the
relationships of the various groups referred to: —

Poly c haera

Myzostomida

Gephyrea

OligochaeTa

Hirudinea

Archi -Annelida

Dinophilea Roh'fera

-Gastrolricha

Tr ochojshore
Fn;. 419. — Diagram illustrating the relationships of the Annulata and the Trochelminthes.

It should be added, however, that it is not likely that the
trochophore actually represents the ancestral form, since, to some
extent at least, its special features, such as the special arrangement
of the cilia, may be adaptations to a pelagic mode of life.

SECTION XI
PHYLUM ARTHROPODA

In this large and important group of animals we meet with a
characteristic feature of the Cha^topoda, viz. metameric segmenta-
tion, and also with more or less perfect bilateral symmetry ; the
mouth and anus are at opposite ends_jQf-iIie elongated body, and
the central nervous system consists of a dorsal brain and a double
ventral chain of ganglia. There is, however, an important advance
on the segmented Worms in the circumstance that each typical
segment bears a pair of appendages, distinguished from the simple,
foot-stumps or parapodia of the Polychseta in being divisible into
distinct limb-segments or podomeres, separated from one another
by movable joints and acted upon by special muscles. Arthropods
are also characterised by the almost universal absence of cilia, by
their muscles being nearly always of the striped kind, and by the
body-cavity largely corresponding not to a true ccelome, but to
a hwmoccele, in free communication with the circulatory system and
developed from the latter.

The following are the most important subdivisions of the
phylum : —

Class 1. Crustacea, including Crayfishes, Crabs, Shrimps,
Wood-lice, Barnacles, Water-fleas, &c.

Class 2. Onychophora, including only the curious caterpillar-
like Peripatus, and a small number of closely related genera.

Class 3. Myriapoda, including the Centipedes and Millipedes.

Class 4. Insecta, including the true or six-legged Insects, such
as Cockroaches, Locusts, Flies, Beetles, Butterflies, and Bees.

Class 5. Arachnida, including Spiders, Scorpions, Mites, &c.

CLASS I.— CRUSTACEA.

1. Examples of the Class.
a. Apus or Lepidurus.

Apus and Lepidurus are two closely allied Crustaceans found
in the fresh-waters of most parts of the world, but curiously local

PlIYWM AKTIIKOPODA

527

in distribution and by no means common. They are so much
alike that, save in minor details, the same description will apply
to any sp.vi.-s of either genus.

External Characters. — The animal (Fig. 420) is from 20 to
SO mm. in length, and has the anterior two-thirds of the dorsal
surface covered by a thin chitinous shell or carapace, beyond the
posterior edge of which the hinder part of the body (abd.) projects
as a nearly cylindrical structure distinctly divided into segments.
The last or anal segment
bears a pair of long
processes, the caudal
styles (a. /.) between
which, in Lepidurus, is
a flat scale-like post-
anal plate (Fig. 421).
On the dorsal surface of
the carapace, near its
anterior border, are the
paired eyes (E), closely
approximated in front,
diverging posteriorly.
Immediately in front
of them is a small
black median eye (e.),
and between their di-
verging posterior ends
a semi-transparent oval
area, the dorscd organ
(d. o.). Passing trans-
versely across the cara-
pace, a short distance
behind the dorsal organ,
is a shallow furrow, the
cervical fold, immedi-
ately posterior to which
a pair of coiled tubes
(sh. gl.) are seen, one on
each side of the carapace
organs.

The carapace is attached only as far back as the cervical fold :
behind that level it is free, and, when lifted up or cut away
(Fig. 421), shows the greater- part of the body of the animal,
divided into segments like the posterior portion. From the
cervical groove backwards about twenty-eight or thirty segments
can be counted: the region in front of the cervical groove shows
no sign of segmentation, and is distinguished as the head. The
segments have the form of chitinous rings, often produced into

Via. 420.— Apui cancriformis, dorsal aspect, abd.
abdomen ; a. f. cavidal styles ; (/. o. dorsal organ ; B.
paired eye ; e. median eye; sh. gl. shell-gland ; tkf. 1,
endites of first thoracic foot. (From Uronn's Th ierrtich,}

these are the shell-glands or excretory

rT

528

ZOOLOGY

1'IIVI.l M ARTHROPODS

529

small spines: each ring slightly overlaps its successor, and is
connected with it by a narrow area, the articular membrane, the
chit inisat ion of which is less pronounced than that of the rings
t hriiiselves. By this arrangement the segments are freely movable
upon one another in all directions, the articular membranes acting
as joints. The last or anal segment is pierced by the terminal
anus (Fig. 424, an.).

The ventral surface of the head is formed by a flattened sub-
frontal plate (Fig. 422, s.f.pl.), con-
tinuous marginally with the cara-
pace. The posterior edge of this
plate is convex backwards, and is
produced in the middle line into a
shield-shaped process, the labrum
or upper lip (Ibr.), which over-
hangs the mmtth. From the sub-
frontal plate also arise, on each
side, two delicate processes, the
innermost called the antennule
(ant. 1), the outermost the an-
tenna (ant. 2) : these are the first
two pairs of appendages. The
third pair consists of two strong,
toothed bodies of a deep brown
colour, placed one on each side of
the mouth, and called the man-
dibles (md.). The remaining ap-
pendages form two rows of deli-
cate leaf-like processes, attached
to the segmented portions of the
body, and overlapping one another
from before backwards : their num-
ber varies from forty to nearly
seventy (th.f.,abd.f.).

Appendages. — The antennule
(Fig. 423, 1) consists of a bent
rod bearing delicate chitinous
bristles or sctai at its tip, and presenting, at the bend, a joint, due
to the presence of an articular membrane. The appendage is
thus made up of two podomercs or limb-segments, movably
articulated together. Its function is probably tactile.

The antenna (2) is absent in some species both of Apus and
Lepidurus : in A. cancriformis it is a very delicate hook-shaped
unjointed structure, probably functionless. As we shall see from
the study of development, it is a vestigial organ.

The mandible (J) is also an unjointed appendage. It has the
form of a deeply concavo-convex plate, strongly chitinised, and pro-

VOL. I U H

Fig. 422.— Apus glacialis, ventral
aspect, abd. f. abdominal feet ; ant1.
antennule ; ant2, antenna ; Ibr. labrum ;
md. mandible ; Hut. first maxilla ; ov.
aperture of oviduct ; *. /. pi. sub-frontal
plate ; sh. yl. shell-gland ; th.f. thoracic
feet ; th. /. 1, first thoracic foot. (After
Bernard.)

530

ZOOLOGY

duced along its inner edge into strong teeth. The mandibles lie
one on each side of the mouth, and are so articulated that, by
means of muscles, their toothed edges can be brought together in
the middle line, so as to rend the food.

The fourth and fifth appendages are very small, and are prob-
ably functionless or nearly so : they follow one another just behind
the mandible, and are called the first and second maxillai. The
first maxilla (4-) consists of two curved chitinous plates, the second
of a basal portion produced into two branches (5). Between the
first maxilla and the mandibles are a pair of delicate unjointed

LAnfennuls
2. Antenna

8. 7™ Thoracic Foot

lO.I^AbdominalFooh

Fia. 423. — Typical appendages of Apus. 1 — />, podomeres of axis ; br. bract ; en. 1, en. 7, endites ;
fl. flabellum ; ov. ova. (After Lankester.)

processes, the par agnatha (Fig. 421, pgn.): they form together a
sort of lower lip, and are not usually reckoned as appendages.

The foregoing appendages all spring from the unsegmented
anterior portion of the body or head. As we shall see, however,
the succeeding limbs arise each pair from its own segment, so
that the presence of five pairs of appendages on the head may
be taken provisionally as an indication that this region of the
body is composed of five fused segments.

The sixth appendage (0) springs from the ventro-lateral region
of the first clearly marked segment, and is the first of the long
row of appendages plainly visible in a ventral view. It consists
of an axis formed of four podomeres {1-4), and bearing a number
of offshoots : six of these, called endites {en. 1 — en. 6), arise from

\. PHYLUM ARTHROPODA, 531.

its inner or mesial border J two, called exiles (br.,Jl.), from its outer
or lateral border. The proximal enditc (en. 1) is small, and bears
Strong spines; in connection with its fellow of the opposite side
it is used to seize food-particles and pass them on to the mouth : it
is therefore conveniently distinguished as the gncdhobase. The
distal enditc is rudimentary (en. 6) : the remaining four (en. 8-6)
are long, jointed filaments. The proximal exite is nearly trian-
gular, and is called the fiabellum (Jl.); the distal exite is oval, and
is known as the bract (br.) ; both probably serve a respiratory
function.

The seventh appendage (7) has only two podomeres in the axis,
and the endites are comparatively short and flat. The next eight,
i.e. those borne on the third to the sixteenth free segments, closely
resemble one another : each (8) has an unjointed axis and short
leaf-like endites, the whole appendage having a distinctly foliaceous
character. The sixteenth appendage — that of the eleventh free
segment — resembles its predecessors in the male, but in the
female (9) is peculiarly modified. The distal portion of the axis
forms a hemispherical cup, over which the fiabellum (Jl.) fits
like a lid : in this way a capsule or brood-pouch is produced,
which serves for the reception of the eggs, and the appendage
is distinguished as the oostcgopod or brood-foot. The brood-feet
and the adjacent genital apertures allow of a very convenient
division of the body : all that region from the first free or post-
cephalic segment to that bearing the oostegopods, both inclusive,
is called the thorax, and its appendages the thoracic feet: it con-
sists of eleven metameres. The remaining segments, from the
twelfth to the last inclusive, constitute the aldomen, and their
appendages are called the abdominal feet.

The abdominal resemble the thoracic feet in general characters,
having the same foliaceous form (10), with unjointed axis, small
leaf-like endites, and large fiabellum and bract. They gradually
diminish in size from before backwards ; and, from the third abdo-
minal segment onwards, two or more pairs of appendages spring
from each segment, so that while the total number of abdominal
segments, in A. cancrif minis, is twenty-two, and the five hinder-
most of these are without appendages, there are altogether fifty-
two pairs of abdominal feet. It seemed probable that segments
bearing more than one pair of appendages represent two or more
fused — or, perhaps one should rather say, imperfectly differentiated
— metameres.

Body-wall. — The whole body is, as already mentioned, covered
by a layer of chitin of varying thickness, which constitutes an
exoskeleton or external supporting structure. Immediately under-
lying it is the deric epithelium or epidermis, from which the chitin
is secreted layer by layer. Thus the exoskeleton of Apus is a con-
tinous cuticular structure, exhibiting segmentation in virtue of the

M M 2

532

ZOOLOGY

fact that, while comparatively thick and strong in places where

no movement is required,
it is thin and flexible in
the intervening spaces, and
thus allows of the move-
ment of the harder parts
upon one another.

The setae, which occur
on many parts of the body,
and in particular fringe
the appendages, are hollow
offshoots of the chitinous
cuticle, containing a proto-
plasmic core continuous
with the epidermis (Fig.
433). They thus differ
fundamentally from the
setoe of Chsetopods, which
are solid rods sunk in
muscular sacs.

The muscular system
is well developed (Fig.
424). Underlying the epi-
dermis is a layer of con-
nective-tissue, and beneath
this is found, in the pos-
terior or limbless part of
the abdomen, a • layer of
longitudinal muscles encir-
cling the body and attached
by connective-tissue to each
segment. In this way the
muscular layer is itself seg-
mented, being divided by
the connective-tissue inser-
tions into muscle-segments
or myomeres. The action
of these muscles is to ap-
proximate adjacent seg-
ments : according as the
fibres on the dorsal, ventral,
or Hteral regions contract,
the abdomen will be raised,
lowered, or turned side-
ways. In the limb-bearing
portion of the abdomen and
in the thorax there is no longer a continuous muscular tube, but

\. PHYLUM ARTHROPOD A 688

paired dorsal (d.m.) and ventral bands, which pass respectively
above and below the origins of the limbs : the dorsal bands arise
in front from the head-region, the ventral from a strong fibrous
plate, the cephalic apodeme {cap.), lying just behind the gullet.

Each appendage is moved as a whole by muscles passing into it
from the trunk: its various parts are acted upon by delicate
muscular slips running to the various podomeres of the axis and
to the endites, thus rendering them separately movable. The only
example we have yet met with of appendages moved by definite
muscular bands is that of the curious Rotifer Pedalion (p. 330). The
muscles are all striped, a character which applies to the Arthropoda
generally, with the exception of the Onychophora.

Digestive Organs. — The mouth (Fig. 424, mth.) is situated on
the ventral surface of the head, and is bounded in front by the
labrum (Ibr.), on each side by the mandibles, and behind by the
paragnatha. The food appears to be pushed forwards towards
the mouth by the toothed bases of the thoracic feet, and is
broken up by the mandibles, which work laterally. The maxillae
are probably functionless, or nearly so.

The mouth leads into a narrow gullet (gul.), which passes
upwards and forwards into the head and enters a wide
stomach (st.), from which a straight intestine (int.) is continued
back to the terminal anus (an). From each side of the
stomach is given off a wide tube (d.gl.) which branches exten-
sively, its ramifications finally ending, in delicate caeca. The
larger branches. of these digestive glands contain food in process of
digestion : their ultimate caeca secrete a digestive juice: the walls
of the stomach itself are non-glandular. The walls of the enteric
canal consist of an inner layer of epithelium and an outer layer of
connective-tissue and muscle. In the gullet and in the posterior
end of the intestine the epithelium secretes a thin cuticle, which
thus comes to form the actual lining of the cavity. It is shown
by development that the portion of the canal devoid of a chitinous
lining is formed from the archenteron of the embryo : the gullet is
developed from the stomodeum, the posterior end of the intestine
from the proctodeum.

The body-cavity is divided into several parts by membranous
partitions (Fig. 425) : there is a large median cavity in which the
enteric canal (i) lies, called the intestinal sinus : on each side of this
are lateral sinuses containing the muscles : and in the dorsal region is
a median cavity, the pericardial sinus. All these spaces are
devoid of an epithelial liaing, and contain blood : as will become
evident later (cf. p. 593), they do not correspond with the coalome
of the higher worms.

The central organ of the circulatory system is the heart (Fig.
424, Jit, and Fig. 425, A), a narrow tube contained in the pericardial
sinus. It is pierced laterally by several pairs of apertures or ostia

534

ZOOLOGY

SECT.

provided with valves opening inwards, and is continued in front
into a narrow tube, the cephalic artery (c. art.), which extends into
the head and gives off near its origin a pair of arteries to the shell-
glands (Fig. 422). When the heart contracts, the blood is driven
through these arteries to the head and carapace : it then travels
backwards in the intestinal sinus, passes to the limbs, and is
returned to the pericardial sinus, finally re-entering the heart,
during its diastole, through the ostia. The plasma of the blood
is coloured red by haemoglobin, and contains amoeboid corpuscles.
As already mentioned, the function of respiration is discharged
by the flabella and bracts of the feet, which are abundantly sup-
plied with blood and the movements of which ensure a constant

-•m

Fig. 425. — Transverse section of Apus. em. muscles to feet ; dv. dorso-ventral muscles ; e. eggs ;
dm. dorsal muscles ; g. ovary ; dv. dorso-veutral muscles ; h. heart ; i. intestine ; m. partition
between intestinal and lateral sinus ; vm. ventral muscles. (From Bernard.)

renewal of the water in their neighbourhood. The renal organ

or shell-gland (Fig. 426) consists of a coiled urinary tube (nc.)
lying between the two layers of the carapace and lined by gland-
cells. At one end the tube is connected with an end-sac (ts.),
also lined with glandular epithelium ; at the other it dilates into
a small bladder (6.) which opens on the second maxilla (m.).

The nervous system (Fig. 427) is constructed on the annulate
type. There is a squarish brain {br.) situated in the dorsal region
of the head, beneath the eyes. From it a pair of aisophageal
connectives pass backwards and downwards to join the ventral nerve-
cord, which consists of a double chain of ganglia (gn. 1-4) united
by longitudinal connectives and transverse commissures so as to
have a ladder-like appearance. The first pair of ganglia lies
immediately behind the mouth, and sends off visceral nerves which
join to form a ring round the gullet, swollen in front into a small
visceral ganglion (v. gn.). Passing backwards the nerve-chain

PHYLUM ARTHROPODA

535

diminishes in size, and comes to an end at about the level of the
last pair of abdominal feet (Fig. 424).

The origin of the nerves given off from the central nervous
-\ -' i in presents many points of interest. From the fourth ganglion
of the ventral cord backwards each pair of appendages has its own
pair of ganglia, the metameric correspondence between the limbs
and the nervous system being complete. The mandibles and the
first maxilla?) also receive nerves, each from their own pair' of
ganglia, their serial homology with the more typical appendages
being thus confirmed. But the second maxillae receive their
nerves (mn. 2) from the connectives between the third and fourth
ganglia : the ganglion belonging to their segment may be assumed

Fio. 420.— Shell-gland of Apus, diagrammatic, ac. cephalic artery ; 6. bladder ; h. heart
vi. second maxilla ; ts. end-sac ; we. urinary tube. (From Bernard.)

to have atrophied. The antenna is supplied by a nerve (ant. £)
which springs from the oesophageal connective, but which can be
traced backwards to the first ganglion of the ventral chain : this
fact may be taken as an indication that the antennae are serially
homologous with the jaws and feet — that they are, in fact, meta-
meric or post-oral appendages which have shifted forwards, one
on each side of the mouth, thus becoming prc-oral. The nerve of
the antennule (ant. 1) also springs from the oesophageal connec-
tive, but is traceable forwards to the brain, where it is connected
with a special group of nerve-cells. This has been explained by
supposing that the antennule is a post-oral appendage the ganglion
of which has moved forwards along the oesophageal connective
and fused with the brain — a process which actually takes place

536

ZOOLOGY

ant J

ant.Z

-md

with the ganglia of the antennae in the higher Crustacea. But it
is also possible to consider the antennules as pre-oral appendages,
belonging, like the prostomial tentacles of Chsetopods, to the

prostomial region, and therefore
receiving their nerves from the
brain or prostomial ganglion. The
median and paired eyes are also
supplied by nerves from the brain.
Organs of Sense. — The setae
which occur on so many parts of
the body, and especially as fringes
to the limbs, are to be considered
as organs of touch : the only other
organs of special sense are the
eyes. The paired eyes are, as we
have seen, situated on the dorsal
surface of the head, just over the
brain : they are covered by trans-
parent cuticle forming the cornea,
beneath which is a narrow space
or water-sac, communicating with
the exterior by a pore, and there-
fore filled with water. The eye
itself is made up of a large num-
ber of radially arranged elements
called ommatidia (Fig. 428), each
of which consists of an outer and
an inner portion. The outer portion
is a group of clear glassy cells (cc.)
enclosing a transparent homogene-
ous vitreous body (cv.) : the whole
of this portion of the eye serves to
refract the rays of light — it is the
dioptric apparatus, like our own
The inner portion is a group of
retinula (re), and enclosing a re-

thj.l

Q?^-*

Fig. 427.- — Nervous system of Apus
cancriformis. ant.' nerve to nii-
tennule ; ant." to antenna ; br. brain ;
gn. 1—U, first four ganglia of ventral
nerve-cord ; md. mandibular nerve ;
mx. 1, nerve of first maxilla ; mx. 2, of
second maxilla ; as. con., oesophageal
connective; op., optic nerve ; th.f. 1,
of first thoracic foot ; v cjn. visceral
ganglion. (After Lankester and
Pelseneer.)

lens and vitreous humour,
sensory cells, constituting a

fractive rod, the rliabdome (rh.) : the retinula is the actual per-
cipient part of the ommatidium, its cells being comparable to our
own rods and cones. The retinula? of adjacent ommatidia are
separated from one another by cells full of black pigment (p.), so
that each ommatidium is in a state of optical isolation from its
fellows, and the whole eye is what is called a compound eye. The
optic nerve springing from the brain dilates into an optic ganglion,
from which fibres pass to the retinula?.

The median eye is an ovoid body, and consists of four groups
of large sensory cells enclosing a mass of pigmented tissue : it
is in immediate contact with the brain, and receives a narrow

\l

PHYLUM ARTIIROPODA

.-,:;:

canal from the water-sac beneath the cuticle of the paired

eyes.

Reproductive Organs. — The large majority of individuals
both of Apus and Lepidurus are females ; males are of com-
paratively rare occurrence. The ovary (Fig. 424, ovy.) is a branched
tube occupying a considerable portion of the body-cavity in
sexually mature individuals. The walls of the tube arc lined
with epithelium, and give rise to ova, which pass into the lumen
of the tube and thence to a duct (ovd.) opening on the eleventh or

Fio. 428. — Diagram of two ommatidia from the paired eyes of Apus. cc. vitreous cells ; rr. vit-
reous body ; H. connective-tissue fibre; Inj. epiderm cells; p. pigment cells; ;•, inner parts
of ommatidia ; re. rctiuula; ; rh. rbabdomc. (From Bernard.).

last thoracic segment. As in Leeches (p. 515), there is reason
for thinking that the cavity of the ovarian tube represents a
shut-off portion of the coelomc, and the oviduct a nephridium.
One species has been shown to be hermaphrodite : in others
males are occasionally found, but reproduction appears to be, as a
rule, parthenogenetic.

Development. — The eggs are centrolectthcU, i.e., have an
accumulation of yolk in the centre surrounded by a superficial
layer of protoplasm. The process of segmentation and the forma-
tion of the germ-layers have not been observed.

538

ZOOLOGY

The embryo is hatched in the form shown in Fig. 429, A. The
body is oval, and is divisible into three regions — a large anterior or
head-region ; an intermediate trunk-region, the hinder part of which
already shows signs of segmentation (I-V) and a posterior bilobed
anal region. The head-region bears a single median eye, and a
pair of small unjointed appendages (7), each with two large setae
at its extremity : these become the antennules of the adult. The
trunk region bears two pairs of appendages, the first of which (#)

Fig. 429. — Three stages in the development of Anus,
gland ; s. carapace ; 1 — U, cephalic appendagjs ; I-
(From Lang's Comparative Anatomy.)

ft. frontal sensory organ ; L, digestive
-XII I, body -tegmenta and appendages.

is very large and fringed with setae, but is chiefly remark-
able for being biramous or two-branched — being formed of a
proximal portion or stem, the protopodite ; a small inner branch, the
endopodite ; and a large outer branch, the exopoditc. This second
appendage becomes the antenna of the adult, and may be called
the antennary foot: it is the chief organ of locomotion of the
larva. The second-trunk appendage is the mandibular foot {3), so
called because it becomes converted into the mandible of the
adult : it is also biramous. The only internal structure to be

xi PHYLUM ARTHROPODA 539

noted is the straight enteric canal with its dilated anterior end
or stomach : the month opens between the bases of the antennary
and mandibular feet, and is bounded in front by a large labrum :
the anus is at the extremity of the anal region. This very
peculiar and characteristic larval form is called a nauplius.1

The nauplius swims freely, chiefly by vigorous strokes of
the great antennary feet, and after a time undergoes a series of
moults or ecdyses, the cuticle being cast off and the animal
emerging in the form shown in Fig. 429, B. The trunk-region has
elongated, new segments having been added, as in Choetopods,
between those previously present and the anal region. The
antennules have become shifted backwards, and rudiments of a
fourth pair of appendages, the first maxilla? {£), have appeared.
The carapace has grown out from the dorsal region of the head,
and a peculiar paired sense-organ (fs.) has appeared on the head.

After two more ecdyses the larva has assumed the form shown
in Fig. 429, C. Several new segments have been added, and the
anterior of these all bear leaf-like thoracic feet. The antennary
feet are still very large, and the bases of the mandibular feet have
become enlarged and toothed so as to form biting jaws. The
carapace (s) has increased greatly, and the caudal styles have
attained a considerable size. Further moults occur, new seg-
ments are added with their appendages, the antennules and
antenna? degenerate — the latter sometimes disappearing alto-
gether,— the mandibles become reduced to the enlarged basal
segment, and the larva passes by almost insensible gradations
into the adult form.

It will be seen that the development of Apus proves clearly
that the antenna? and mandibles arc ordinary trunk-appendages,
homologous with the thoracic and abdominal feet : a comparison
of the antennary and thoracic feet of the larva supports the view
that the endopodite of the former corresponds with the fifth endite
of the latter, and the exopodite with the sixth endite. The
antennules are from the first unbranched or uniramous, and are
originally situated quite at the anterior region of the body : they
do not, therefore, show a complete correspondence with the remain-
ing appendages, and, as was inferred from their nerve supply,
may perhaps be considered as prostomial and not metameric
appendages.

b. The Fresh-water Crayfish (Astacus Jluviatilis).

Astacus Jluviatilis is common in streams and rivers in England
and the continent of Europe ; allied species occur in Asia and
North America; and fresh-water Crayfishes belonging to other

1 More strictly Mekmauplius : the typical nauplius exhibits no segmentation
of the trunk region.

540

ZOOLOGY

genera, but agreeing with Astacus in all essential features, are
found in America, Australia, and New Zealand.

External Characters -The body of the Crayfish (Fig. 430,^4

and B) is divided into two regions — an anterior, the cephalothorax

Fio. 430^.— Astacus fluviatilus, side view of male, ~a\ antennule ; <fi, antenna ; ah. abdomen ;
etk. cephalothorax ; k-ti, gill-cover ; r. rostrum; 8, third maxillipede ; 0, first leg; 10 — 13,
remaining legs ; 1!», uropod ; XIV, first abdominal segment ; XIX, sixth abdominal segment.
(From Lang's Comparative Anatomy.)

Fio. 430/?. — Transverse section of abdomen of Crayfish. DA, dorsal abdominal artery; EM,
dorsal muscles of the abdomen ; El', space between the plcuron and the appendage; FM,
ventral muscles of the abdomen ; M, muscles of the appendage ; N, endopodite ; NG, nerve-
ganglion ; P, protopodite ; PL, pleuron ; Pit, hind-gut ; S, siernum ; T, tergum ; V, ventral
abdominal artery ; X, exopoditc. (From Parker's Practical Zoology, after Marshall and Hurat.

(cth.), which is unjointed, and is covered by a carapace resembling
that of Apus, but of smaller proportional size ; and a posterior, the
abdomen (ab), which is divided into distinct segments, movable upon

m PHYLUM ARTHROPODA -H

one another in a vertical plane. The cephalothorax is again divided
into two regions—' an anterior, t.hc head; and a posterior, the thorax
— by a transverse depression, the cervical groove. The divisions of
the body are thus the same as in Apus, but the abdomen alone is
movably segmented, owing to the fact that the carapace, instead
of being a purely cephalic structure continued backwards as a
loose fold over the thorax, is developed from the dorsal and
lateral regions of both head and thorax, and is free only at the
sid.s of the thorax, where it forms a flap or gill- cover (Jed) on each
side, separated from the actual body-wall by a narrow space in
which the gills are contained (Fig. 436). The carapace is made of
chitin, strongly impregnated with carbonate of lime so as to be
hard and but slightly elastic.

The abdomen is made up of six segments and a tail-piece or
telson: the six segments (XIV-XIX) have a ring-like form,
presenting a broad dorsal region or tcrgum, a narrow ventral region
or sternum, and downwardly directed lateral processes, the pleura
— the latter quite unrepresented in Apus. The telson is flattened
horizontally, and divided by a transverse groove into anterior and
posterior portions. All the segments and the telson are calcified,
and are united to one another by chitinous articular membranes :
the first segment is similarly joined to the thorax. Thus the exo-
skeleton of Astacus resembles that of Apus in being a continuous
cuticular structure, but differs from it in being discontinuously
calcified, so as to have the character of a hard jointed armour.

It has been stated that the abdominal _ segments are movable
u]i< hi one another in a vertical plane — i.e. the whole abdomen can be
extended or straightened, and flexed or bent under the cephalo-
thorax : the segments are incapable of movement from side to
side. This is due to the fact that, while adjacent segments are
connected dorsally and ventrally by flexible articular membranes,
they present at each side a hinge (Fig. 434, h), placed at the
junction of the tergum and pleuron, and formed by a little peg-
like process of one segment fitting into a depression or socket in
the other. A line drawn between the right and left hinges con-
stitutes the axis of articulation, and the only possible movement
is in a plane at right angles to this axis.

Owing to the presence of the carapace, the thoracic region is
immovable, and shows no distinction into segments either on its
dorsal (tergal) or lateral (pleural) aspect. But on the ventral
surface the sterna of the thoracic segments are clearly marked
off by transverse grooves, and the hindmost of them is slightly
movable. Altogether eight thoracic segments can be counted.

The ventral and lateral regions of the thoracic exoskeleton are
produced into the interior of the body in the form of a segmental
series of calcified plates, so arranged as to form a row of lateral
chambers in which the muscles of the limbs lie, and a median

542 ZOOLOGY sect.

tunnel-like passage or sternal canal, containing the thoracic portion
of the nervous system. The entire endophragmal system, as it is
called, constitutes a kind of internal skeleton : its anterior end is
formed by a plate, the cephalic apodeme, having the same anatomical
relations as the similarly named structure in Apus.

The head exhibits no segmentation : its sternal region is
formed largely by a shield-shaped plate, the epi stoma, nearly vertical
in position. The ventral surface of the head is, in fact, bent so as
to face forwards instead of downwards. The epistoma is bounded
laterally by the free edge of the carapace instead of passing
insensibly into it like the sub-frontal area of Apus, with
which, however, it agrees in having the labrum attached to the
middle of its posterior border. The cephalic region of the cara-
pace is produced in front into a large median spine, the rostrum
(Fig. 430, r) : immediately below it is a plate from which spring
two movably articulated cylindrical bodies, the eye-stalks, bearing
the eyes at their ends.

The appendages are seen at a glance to differ from those of
Apus in their vastly greater degree of differentiation : obvious at
a glance are the long feelers (Fig. 430, a. 1, a. 2) attached to the
head, the five pairs of legs A9-lo) springing from the thorax, and
the little fin-like bodies arising from the sterna of the abdomen.
It will be convenient to begin with the last-named region.

The third, fourth, and fifth segments of the abdomen bear
each a pair of small appendages, the swimming feet or phopods
(Fig. 431, 10), the resemblance of which to the biramous limbs
of the larval Apus is obvious. There is an axis or protc/podite
consisting of a very short proximal (pr. 1) and a long distal
(pr. 2) podomere, and bearing at its free end two jointed plates,
fringed with^etas, the endopoditc (en) and exopodite (ex). These
appendages act as fins, moving backwards and forwards with a
regular swing, and probably aiding in the animal's forward
movements.

In the female a similar appendage is borne on the second seg-
ment, while that of the first is more or less vestigial. In the
male the first and second pleopods (9) are modified into incom-
plete tubes which act as copulatory organs, serving to transfer
the spermatophores to the body of the female. The sixth pair of
abdominal limbs (11) are alike- in the two sexes : they are very
large, both endopodite and exopodite having the form of broad Hat
plates : in the natural position of the parts they lie one on each
side of the telson, forming with it a large five-lobed tail-fin
capable of being spread out after the manner of a fan ; they are
therefore conveniently called uropods or tail-feet. The telson itself
bears no appendages.

The thoracic appendages are very different. The four posterior
segments bear long slender, jointed legs (8), upon which the animal

\l

PHYLUM ARTHROPOD A

543

walks: in front of these is a pair of very large legs terminating
in h.nge claws or chelce, and hence called ckelipeds (Fig. 4.S0, 9).
The three anterior segments bear much smaller appendages
more or leas leg-like in form, but having their bases toothed to
serve as jaws: they are distinguished as ttujjjjlijwls or foot- jaws

(Fig. 4;n,£-7).

The structure of these appendages is best understood by a con-
sideration of the third maxiiliped (7). The main portion of the

5. 2^Maxilla 6. Pf Maxiiliped

7. 3r.d Maxiiliped

8. y." Leg

pr.t

O.Copula^ry Organs lO.SwimmingFoor

-bri
ll.Uropod

Fig. 431.— Typical appendages of Astacus. en. 1 — ~>, podonieres of endopodite ; ep. epipodite ;
ex. exopodite ; jt. flagella ; a. gill ; p>: 1, %n: t, podonieres of protopodite ; 1—3, podonieres of
axis of antennule. (After Huxley.)

limb is formed of seven podonieres arranged in a single series,
strongly calcined, and — with the exception of the second and third,
which are fused — movably articulated with one another. The second
podomere, counting from the proximal end, bears a many-jointed
feeler-like organ (ex), and from the first springs a thin folded
plate (ep) having a plume-like gill (<j) attached to it. Obviously
such an appendage is biramous, but with one of its branches

544 ZOOLOGY sect.

greatly in excess of the other : the first two segments of the axis
(pr. 1, pr. 2) form the protopodite, its remaining five segments
(en. 1-5) the endopodite, and the feeler, which is directed out-
wards, or away from the median plane, the exopodite (ex). The
folded plate (ep) is called the epipodite : in the natural position
of the parts it is directed upwards, and lies in the gill-cavity
between the proper wall of the thorax and the gill-cover (Fig. 438).
Its position is thus very similar to that of the flabellum of Apus,
while the gill attached to it is comparable to the bract.

The five legs (8) differ from the third maxilliped in their greater
size, and in having no exopodite : in the fifth or last the epipodite
also is absent. The first three of them have undergone a curious
modification, by which their ends are converted into pincers or
chela; : the fourth segment (en. 4-) of the endopodite (sixth of the
entire limb) is produced distally so as to form a claw-like projec-
tion (en. If, v), against which the terminal segment (en. 5) bites. The
first leg is much stouter than any of the others, and its chela is
of immense size and forms an important weapon of offence and
defence. The second maxilliped resembles the third, but is consid-
erably smaller : the first (6) has its endopodite greatly reduced,
the two segments of its protopodite large and leaf-like, and no gill
is connected with the epipodite.

As in Apus, the head bears a pair of mandibles and two pairs of
maxilla? in relation with the mouth, and in front of that aperture
a pair of antennules and one of antennae. The hindmost appen-
dage of the head is the second, maxilla (5), a markedly foliaceous
appendage : its protopodite (pr. 1, pr. 2) is cut up into lobes com-
parable with the four proximal endites in the thoracic feet of
Apus : its endopodite (en) corresponds with the fifth endite, while
the sixth endite is represented by the exopodite (ex), modified into
a boomerang-shaped plate, which, as we shall see, is an important
accessory organ of respiration. The first maxilla (4) is a very small
organ, having neither exopodite nor epipodite. The mandible (3)
is a large strongly calcified body, toothed along its inner edge,
and bearing on its anterior border a little three-jointed feeler-like
body, the palp, the two distal segments (en. 1, en. 2) of which
represents the endopodite, its proximal segment (pr. 2) together
with the mandible proper (pr. 1), the protopodite.

The antenna (2) is of great size, being nearly as long as the
whole body. It consists of an axis of five podomeres, the fifth or
last of which bears a long, flexible, many -jointed structure, or
flagellum (fl), while from the second segment springs a scale-like
body or squame (ex). It is fairly obvious that the two proximal
segments represent the protopodite, the remaining three, with the
flagellum, the endopodite, and the squame the exopodite.

The antennule (1) has an axis of three podomeres (1-3) ending
in two many -jointed flagella (fl. 1. and 2), which are sometimes

\l

I'll VLUM ARTHROPODA

r.-ir.

eru.S'

en,. 4-'.

Si-

en.-t--

exd-

- ccri. m.

ert.3-

Ctrt.;

considered as endopiM lite and exopodite. But in all the other limbs,
as we have seen, the exopodite springs from the second Began at
of the axis, and the probabilities are
t hat there is no exact correspondence
between the parts of the antennule and
those of the remaining appendages.

The eye-stalks, already noticed,
arise just above the antennules and
are formed each of a small proximal
and a large distal segment. They are
sometimes counted as appendages
serially homologous with the an-
tennae, legs, &c. But, as we have seen
in the case of Apus, the appendages
of Crustacea are always formed in
regular order from before backwards ;
the eye-stalks, on the other hand, .
always appear later, both in individual
development and in the Crustacean
series, than the normal anterior ap-
pendages. They are therefore more
properly to be looked upon as arti-
culated processes of the prostomium,
developed in connection with the need
for an increased range of vision. As-
suming this to be the case, it will be
seen that the body of the Crayfish
consists of a prostomium, nineteen
metameres, and a telson. The pro-
stomium bears eye-stalks: the first
five metameres are fused with the
prostomium to form the head, and
bear the antennules, antennae, man-
dibles, first maxillae, and second
maxillae : the next eight metameres
(5th-l 2th) constitute the thorax, and
bear the three pairs of maxillipeds and
the five pairs of legs : the remaining
six metameres (13th-18th), together
with the telson, constitute the ab-
domen, and bears five pairs of pleo-
pods and one of uropods.

The articulation of the various
podomeres of the appendages is on

the sama plan as that of the abdominal segments (p. 541). The
podomeres are, it must be remembered, rigid tubes : they are
connected with one another by flexible articular membranes

V( L. I N N

-ex£

?/*.£

Fin. 432.— Port ion of a leg of Astacus ,
with the exoskeleton partly re-
moved, showing articulations and
muscles, art. m. articular mem-
brane ; en. 2 — 5, podomeres of endo-
podite ; c.r(. extensor muscles ; I.
flexors ; li. hinge.

546

ZOOLOGY

(Fig. 432, art. m.), but at two points the adjacent ends of the
tubes come into contact with one another and are articulated by
peg-and-socket joints (li.), the two joints being at opposite ends of
a diameter which forms the axis of articulation. The two podo-
meres can, therefore, be moved upon one another in a plane at
right angles to the axis of articulation and in no other direction,
the joints being pure hinge-joints. As a rule, the range of move-
ment is from the perpendicular to a tolerably extensive flexion
on one side — the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of uni-
versal movement, owing to the fact that the axes of articula-
tion vary in direction in successive joints : the first joint of a limb
bending, for instance, up and down, the next backwards and for-
wards, the next obliquely, and so on. In some cases, e.g. in the
pleopods, peg-and-socket joints are absent, the articulation being
formed merely by an annular articular membrane and movement
being therefore possible in any plane.

Body- wall. — The exoskeleton is produced into spines of vary-
ing form and size, and many parts of it bear tufts or fringes of

setae, which also exhibit a wide varia-
tion in size and form. It is composed
of a thick laminated chitinous mem-
brane (Fig. 433, cu.), more or less im-
pregnated with lime-salts, and is shed
periodically — once a year during adult
life. Beneath it is the epidermis (ep.)
composed of a single layer of cells from
which the chitin is secreted, and under-
laid by a layer of connective-tissue (c. t.)
to which the muscles are attached.

The muscular system, like the
exoskeleton, shows a great advance in
complexity over that of Apus. In the
abdomen (Fig. 434) the muscles are of
great size, and are divisible into a
smaller dorsal and a larger ventral set.
The dorsal muscles (d. m.) are paired longitudinal bands, divided
into myomeres, and inserted by connective-tissue into the anterior
border of each segment: anteriorly they are traceable into the
thorax, where they arise from the side-walls of that region.
When these muscles contract, they draw the anterior edge of each
tergum under the posterior edge of its predecessor, and thus
extend or straighten the abdomen.

The ventral muscles are extraordinarily complex. Omitting de-
tails, there is on each side a wavy longitudinal band of muscle (c. m.),
nearly circular in section, which sends off a slip (ex.) to be inserted
into each segment above the hinge (h.) : the contraction of this

Pig. 433. —Vertical section of skin
and exoskeleton of Lobster.
c.t. connective-tissue ; cu. cuticle ;
ep. epidermis ; s. seta. (After
Gerstaecker.)

\I

PHYLUM ARTHROPODA

547

muscle must obviously tend to approximate the terga, and so aid
i lir dorsal muscles in extending the abdomen. Around this central
iiinxclc is wrapped, in each segment, a band of muscle {env. m.) in
the form of a loop, the outer limb of which turns forwards and is
inserted into a sternum, while the inner limb turns backwards and
is inserted into another and more posterior sternum. The con-
traction of this enveloping muscle produces an approximation of
the sterna, and thus flexes the abdomen, the central muscle always

art nv ia . _. cm, c£ m.

--T-

\

A ^^fe^^

****' 3t yi h, Cw.m yf

Fio. 434. — Fourrsegments of abdomen of Crayfish in sagittal section, with muscles (diagram-
matic). A, extension ; B, flexion; art, in., art. ?»'., articular membranes; r. m. central
muscles; d. in. dorsal muscle; ex. extensor slip of central muscle; env. in. enveloping
muscle ; fl..,fl.1, flexor slips ; h. hinge ; el. sternum ; tg. tergum.

keeping the middle of the loop in place. The ventral muscles
are, like the dorsal, traceable into the thorax, where they arise
from the endophragmal system (p. 542) : their various parts are
connected by a complex system of fibres extending between the
central and enveloping muscles, and connecting both with their
fellows of the opposite side. The flexor muscles are immensely
powerful, and produce, when acting together, a sudden and violent
bending of the abdomen upon the cephalothorax, causing the
Crayfish to dart backwards with great rapidity.

N N 2

548

ZOOLOGY

Fig. 435.— Astacus fluviatilis,

dissection from the right side.
Ha. antcnnary artery ; ab. abdo-
men ; an. anus ; b. d. aperture of
duct of right digestive gland ;
bf. It, cheliped ; bin, ventral nerve-
cord ; cs. anterior division of
gizzard ; ctk. cephalothorax ;
em, dorsal muscles; fin, ventral
muscles ; a, brain ; /*. heart ;
ltd, posterior part of intestine ;
Ir. left digestive gland ; md,
mid-gut ; o. ostium of heart ; oa.
right lateral artery ; oaa, dorsal
abdominal artery ; ve, ' gullet ;
pi, 1 — 5, pleopods ; pi. (i, uropod ;
pg, posterior division of gizzard ;
na. sternal artery ; t. testis and
tclson ; uaa^ ventral abdominal
artery; vd. vaa deferens; vde,
male genital aperture. (Prom
Lang, after Huxley.)

It will be seen that the body-
muscles of the Crayfish cannot be
said to form a layer of the body-wall,
as in Cha^topods, the abdomen of Apus,
&c., but constitute an immense fleshy
mass, filling up the greater part of the
body-cavity, and leaving a very small
space around the enteric canal.

In the limbs (Fig. 432) each podo-
mere is acted upon by two muscles
situated in the next proximal podo-
mere. These muscles are inserted, by
chitinous and often calcified tendons,
into the proximal edge of the segment
to be moved, the smaller on the ex-
tensor (ext.), the larger on the flexor
(fi.) side, in each case half-way be-
tween the two hinges, so that a line
joining the two muscular insertions is
at right angles to the axis of arti-
culation.

The digestive organs are con-
structed on the same general plan
as those of Apus, but present many
striking differences (Fig. 435). The
mouth lies in the middle ventral line
of the head, and is bounded in front
by the labrum, at the sides by the
mandibles, and behind by a pair of
delicate lobes, the paragnatha. It
leads by a short wide gullet (oe) into
a capacious gizzard (sometimes termed
stomach), which occupies a great part
of the interior of the head, and is
divided into a large anterior division
(c. s), and a small posterior division
(ps): the latter passes into a narrow
and very short portion of the intestine,
the mid-gut (md), from which the rest
of the intestine (hind-gut, hd) extends
to the anus (an), situated on the
ventral surface of the telson.

The outer layer of the enteric canal
consists of connective-tissue contain-
ing striped muscular fibres : within
this is a single layer of columnar epi-
thelial cells. In the gullet and gizzard,

\. PHYLUM ARTHROPODA 549

and in the hind-gut, the epithelium secretes a layer of chitin,
which thus constitutes the innermost lining of those cavities. It
is proved by development that the mid-gut, which has no
chitinous lining, is the only part of the enteric canal developed
from the mesenteron : the gullet and gizzard arise from the
stomoda3um, the hind-gut from the proctodeum. Thus a very
small portion of the enteric epithelium is endodermal.

In the anterior division of the gizzard the chitinous lining is
thickened and calcified in certain parts, so as to form a complex
articulated framework, the " gastric mill" on which are borne a
median and two lateral teeth, strongly calcified and projecting
into the cavity of the gizzard. Two pairs of strong muscles
arise from the carapace, and are inserted into the gizzard : when
they contract they move the mill in such a way that the three
teeth meet in the middle and complete the comminution of the
food begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles in
the walls of the gizzard. The posterior division of the gizzard
forms a strainer: its walls are thickened and produced into
numerous seta;, which extend quite across the narrow lumen and
prevent the passage of any but finely divided particles into the
intestine. Thus the gizzard has no digestive function, but is
merely a masticating and straining apparatus. On each side of
the anterior division is found at certain seasons of the year a
plano-convex mass of calcareous matter, the gastrolith.

The digestion of the food and to some extent the absorption of
the digested products are performed by a pair of large glands (Jr.),
lying one on each side of the gizzard and anterior end of the
intestine. They are formed, of finger-like sacs or caeca, which
discharge into wide ducts opening into the mid-gut, and
are lined with glandular epithelium derived from the endoderm
of the embryo. The glands are often called livers, but as the
yellow fluid they secrete digests proteids as well as fat, the name
hepato -pancreas is often applied to them, or they may be called
simply digestive glands. The Crayfish is carnivorous, its food con-
sisting largely of decaying animal matter. Microscopic glands
occur in the wall of the gullet.

The digestive organs and other viscera are surrounded by a
body-cavity, which is in free communication with the blood-
vessels and itself contains blood. As will be pointed out more
particularly hereafter, this cavity is to be looked upon as an
immense blood-sinus, and not as a true ca^lome.

There are well-developed respiratory organs, in the form of
gills, contained in a narrow branchial chamber, bounded internally
by the proper wall of the thorax (Fig. 438, ep), externally by the
gill-cover or pleural region of the carapace (led). Each gill con-
sists of a stem giving off numerous branchial filaments, so that

550

ZOOLOGY

SECT.

the whole organ is plume-like. The filaments are hollow, and
communicate with two parallel canals in the stem — an external,
the afferent branchial vein, and an internal, the efferent branchial
vein. The gill is to be considered as an out-pushing of the
body- wall, and contains the same layers — a thin layer of
chitin externally, then a single layer of epithelial cells, and

Pio. 436. — Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed and
the gills undisturbed ; in B the podobranchite are removed and the outer arthrobranehiaa
turned down, aj, antennule ; «•>, antenna; ab\, first; ab.i, second abdominal segment;
arb. 7 — 12, inner arthrobrarichiae ; arb'. 7 — 12, outer arthobranchias ; ep. 5, seaphognathito ;
plb. 11 — 13, pleurobranchiffi ; pdb. 7 — 13, podobranchs ; pi. 1, first pleopod ; 6 — 13, thoracic
appendages. (From Lang's Comparative Anatomy, after Huxley.)

beneath this connective-tissue, hollowed out for the blood
channels and containing gland-cells, which will be referred to
presently (p. 551).

According to their point of origin, the gills are divisible into
three sets — first, podobranchice or foot-gills, springing from the
epipodites of the thoracic appendages, from which they are only

\l

HIYUM AKTIIKOI'ODA

551

partially separable; secondly, arthrobranchicc or joint-gills, spring-
ing from the articular membranes connecting the thoracic
appendages with the trunk; and thirdly, plcurobranchim, or wall-
gills, springing from the lateral walls of the thorax, above the
attachment of the appendages. It is inferred from the study-of
other Crayfishes, that a typical thoracic segment bears four gills,
one podo-, two arthro-, and one pleurobranchia. But in Astacus
one or more of the gills in every segment are absent or vestigial,
and the following table, or " branchial formula," shows the actual
number and arrangement of these organs, ep standing for epipodite,
and r for the vestige of a gill.

Thoracic
Segments.

I.

II.

III.

IV.

V.

VI.

VII.

VIII.

Total.

Podobranchitc...
Artbrobranchiaj
Plcurobranchiaj

0 + cp
0
0

l+ep

1
0

\+ep
2
0

l + ep
2
0

l+ep
2
0

l+ep
2
r

l+ep
2
r

0
0
1

6 + 7ep

11

l+2r

Total ...

0 + cp

2 + cp

3 + cp

3 + cp

3 + ep

Z + r + ep

Z + r + ep

1

l8 + 2r + 7ep

By adding up the columns vertically we get the number of gills
in each segment ; by adding them horizontally, the number of each
kind of gill; and by adding together the results obtained by both
methods, the total number of gills, viz., eighteen complete gills
with two vestiges and seven epipodites.

The excretory organs differ both in position and in form
from those of Apus. There are no shell-glands, but at the base
of each antenna is an organ of a greenish colour, the antennary
or green gland, by which the function of renal excretion is per-
formed. The gland (Fig. 437) is cushion-shaped, and consists of
three parts — (1) a central saccule (s.) of a yellowish colour, occupy-
ing the mid-dorsal region, and consisting of a sac divided into
numerous compartments by partitions, and communicating with
(2) the outer or cortical portion {c. p.), of a green colour, consisting
of a glandular network formed of anastomosing canals, and com-
municating in its turn with (3) a white portion {w. p.), formed of a
single tube partly converted into a sponge- work by ingrowths
of its walls. The whole organ is lined by glandular epithelium,
and the white portion discharges into a thin-walled sac or urinary
bladder (bl.) which opens by a duct (d.) on the proximal segment of
the antenna. The glands already referred to as occurring in the
gills are also supposed to have an excretory function.

The circulatory organs are in a high state of development.
The heart (Figs. 435, 438, h.) is situated in the dorsal region of the

<\ '

552

ZOOLOGY

thorax, and is a roughly polygonal muscular organ pierced by
three pairs of apertures or ostia (o.), guarded by valves which open
inwards. It is enclosed in a spacious pericardial sinus (Fig. 438,
pc), which contains blood. From the heart spring a number of
narrow tubes, called arteries, which serve to convey the blood to
various parts of the body. At the origin of each artery from the
heart are valves which allow of the flow of blood in one direction

Fio. 437.— Diagram of kidney of Astacus fiuviatilis. I, unravelled ; II, the parts in their
natural relations. M. bladder ; c. p. cortical portion ; d. duct ; s. saccule ; ic. p. white
portion. (After Marchal.)

only, viz., from the heart to the artery. From the anterior end
of the heart arise five vessels — the median ophthalmic artery
(Fig. 435, oa.), which passes forwards to the eyes ; paired an-
tennary arteries (aa.), going to the antennules, antennae, green
glands, &c, and sending off branches to the gizzard ; and paired
hepatic arteries, going to the digestive glands. The posterior end
of the heart gives off two unpaired arteries practically united

XI

PHYLUM ARTHROPODA

553

at their origin, the dorsal abdominal artery (oaa.), which passes
backwards above the intestine, sending branches to it and to the
dorsal muscles ; and the large sternal artery (sa.), which extends
directly downwards, indifferently to right or left of the intestine,
passing between the connectives uniting the third and fourth
thoracic ganglia, and then turns forwards and runs in the sternal
canal, immediately beneath the nerve-cord, sending off branches
to the legs, jaws, &c. At the point where the sternal artery turns
forwards it gives off the \^

median ventral abdominal
artery (uaa.), which passes
backwards beneath the nerve-
cord, and supplies the ventral
muscles, pleopods, &c.

All these arteries branch
extensively in the various
organs they supply, becoming
divided into smaller and
smaller offshoots, which finally
end in microscopic vessels
called capillaries. These latter
end by open mouths which
communicate with the blood-
sinuses (Fig. 439, s.), spacious
cavities lying among the
muscles and viscera, and all
communicating, mediately or
immediately, with the sternal
sinus (st.s.), a great median
canal running longitudinally
along the thorax and ab-
domen, and containing the
ventral nerve-cord and the
sternal and ventral abdom-
inal arteries. In the thorax
the sternal sinus sends an
offshoot to each gill in the

form of a well-defined vessel, which passes up the outer side of
the gill and is called the afferent branchial vein (af.br.v. ; see also
Fig. 438). Spaces in the gill-filaments place the afferent in com-
munication with the efferent branchial vein (ef.br. v.), which occupies
the inner side of the gill-stem. The eighteen efferent branchial
veins open into six branchiocardiac veins (br.c.v.), which pass
dorsally in close contact with the lateral wall of the thorax
and open into the pericardial sinus (pcd.s.).

The whole of this system of cavities is full of blood, and the
heart is rhythmically contractile. When it contracts, the blood

vol. i N N 2*

Fio. 488. — Transverse section of thorax of Cray-
fish, diagrammatic, abm. ventral abdominal
muscles ; bj\ leg ; bm. ventral nerve cord ; </.
intestine ; dirm . dorsal muscles of abdomen ;
ep. wall of thorax ; h. heart ; k. gills ; *•</. gill-
cover ; I. liver ; ov. ovary ; pc. pericardial
sinus ;sa.sn., sternal artery ; vs. ventral sinus.
The arrows show the direction of the blood-
current. (From Lang's Comparative Anatomy.)

554 ZOOLOGY sect.

contained in it is prevented from entering the pericardial sinus by
the closure of the valves of the ostia, and therefore takes the only
other course open to it, viz., into the arteries. When the heart
relaxes, the blood in the arteries is prevented from regurgitating
by the valves at their origins, and the pressure of blood in the
pericardial sinus forces open the valves of the ostia and so fills
the heart. Thus in virtue of the successive contractions of the
heart and of the disposition of the valves, the blood is kept con-
stantly moving in one direction — viz., from the heart by the
arteries to the various organs of the body, where it receives car-
bonic acid and other waste matters ; thence by sinuses into the
great sternal sinus ; from the sternal sinus by afferent branchial
veins to the gills, where it exchanges carbonic acid for oxygen ;
from the gills by efferent branchial veins to the branchiocardiac

aJ. br v

st «

Fia. 439. — Diagram of the circulation in the Crayfish ; heart and arteries scarlet, veins and
sinuses containing non-aerated blood, blue ; those containing aerated blood, pink. «. artery ;
af.br.v. afferent branchial vein ; br.e.v. branchio-cardiac vein ; ef.br.v. efferent branchial
vein ; lit. heart ; pal.n. pericardial sinus ; s. sinus ; st.s. sternal sinus ; v1. ostium with
valves ; ifi. arterial valves. The arrows show the direction of the current.

veins, thence into the pericardial sinus, and so to the heart once
more.

It will be seen that -the circulatory system of the Crayfish con-
sists of three sections — (1) the heart or organ of propulsion ; (2) a
system of out-going channels, the arteries, which carry the blood
from the heart to the body generally ; and (3) a system of return-
ing channels, some of them, the sinuses, mere irregular cavities ;
others, the veins, with definite walls, which return it from the
various organs back to the heart. The respiratory organs, it
should be observed, are interposed in the returning current, so
that blood is taken both to and from the gills by veins.

Comparing the blood-vessels of Astacus with those of a
Chsetopod, it would seem that the ophthalmic artery, heart, and
dorsal abdominal artery together answer to a dorsal vessel, part
of which has become enlarged and muscular and discharges the

PHYLUM ARTHROPODA

:>:,:,

whole function of propelling the blood. The horizontal portion
of the sternal artery, together with the ventral abdominal,
represent a ventral vessel ; while the vertical portion of the
sternal artery is a commissure, developed sometimes on the right,
sometimes on the left side, its fellow being
suppressed. *

The blood when first drawn is colourless,
but after exposure to the air takes on a
bluish-grey tint. This is owing to the
presence of a colouring matter called
he mocyanin, which becomes blue when com-
bined with oxygen ; it is a respiratory
pigment, and serves, like haemoglobin, as a
carrier of oxygen from the external medium
to the tissues. The hsemocyanin is con-
tained in the plasma of the blood : the
corpuscles are all colourless leucocytes.

The nervous system (Fig. 440) con-
sists, like that of Apus, of a brain (g) and
a ventral nerve-cord, united by oesophageal
connectives (sc). But the right and left
halves of the ventral cord have undergone
partial fusion, so that the ganglia, and in
the abdomen the connectives also, appear
single instead of double. Moreover, the
brain supplies riot only the eyes and anten-
nules, but the antennae as well, and it is
found by development that the two pairs
of ganglia belonging to the antennulary
and antennary segments have fused with
the brain proper. Hence we have to dis-
tinguish between a primary brain or archi-
cerebmm, the ganglion of the prostomium,
and a secondary brain or syn-cerebrum
formed by the union of one or more pairs
of ganglia of the ventral cord with the
archi-cerebrum. A further case of con-
crescence of ganglia is seen in the ventral
nerve-cord, where the ganglia of the last
three cephalic and first three thoracic seg-
ments have united to form a large com-
pound mb-ceswphageal ganglion (bg). All the

remaining segments have their own ganglia, with the exception of
the telson, which is supplied from the ganglion of the preceding
segment. There is a visceral system of nerves (s) supplying the
gizzard, originating in part from the brain and in part from the
oesophageal connectives.

f£"

}Y

Fig. 440. — Nervous system of
Astacus fluviatilis.
bg. sub-<esophageal gang-
lion ; eg. commissural
ganglion ; g, brain ; *, vis-
ceral nerve ; sc, oesopha-
geal connective ; y, post-
oesophageal commissure ;
IV — VIII, thoracic gang-
lia ; 1 — 6, abdominal gang-
lia. (From Lang's Com-
parative Anatomy, after
Vogt and Yung.)

556 ZOOLOGY

SECT.

Sensory Organs. — The eyes have the same essential structure
as the compound eye of Apus. The chitinous cuticle covering the
distal end of the eye-stalk is transparent, is divided by delicate
lines into square areas or facets, and constitutes the cornea.
Beneath each facet of the cornea is an ommatidium, optically
separated from its neighbours by black pigment, and consisting
of an outer segment or vitreous body, and an inner segment or
retinula, formed of sensory cells enclosing a rhabdome.

The antennules contain two sensory organs, to which are assigned
the functions of smell and hearing respectively. The olfactory
organ is constituted by a number of extremely delicate olfactory
setce, borne on the external flagellum and supplied by branches of
the antennulary nerve. The so-called auditory organ is a sac
formed by invagination of the dorsal surface of the proximal
segment, and is in free communication with the surrounding water
by a small aperture. The chitinous lining of the sac is produced
into delicate feathered auditory setm, supplied by branches of the
antennulary nerve ; and in the water which fills the sac are minute
sand-grains, which take the place of otoliths, but, instead of being
formed by the animal itself, are taken in after each ecdysis, when
the lining of the sac is shed. Probably the main function of these
organs is connected with the equilibration of the body rather than
with the sense of hearing. Many of the setae on the body
generally have a definite nerve-supply, and are probably tactile
organs.

Reproduction. — The Crayfish is dioecious, and presents a very
obvious sexual dimorphism. The abdomen of the female- is much
broader than that of the male : the first and second pleopods of
the male are modified into tubular or rather spout-like copulatory
organs (Fig. 431, 9) ; and the reproductive aperture is situated in
the male on the proximal podomere of the fifth leg, in the female
on that of the third.

The testis (Fig. 441, B, t, u) lies in the thorax, just beneath the
floor of the pericardial sinus, and consists of paired anterior lobes
(t) and an upaired posterior lobe (w). From each side goes off a
convoluted vas deferens (vd), which opens on the proximal segment
of the last leg. The sperms are curious amoeboid bodies produced
into a number of stiff processes or pseudopodia (Fig. 23, /) : they
are aggregated into vermicelli-like spermatophores by a secretion
of the vas deferens.

The ovary (A, ov, u) is also a three-lobed body, and is similarly
situated to the testis : from each side proceeds a thin-walled
oviduct (od), which passes downwards, without convolutions, to
open on the proximal segment of the third or antepenultimate
leg. The eggs are of considerable size and are centrolecithal.

As in Apus, both ovary and testis are hollow organs, discharging
their products internally. The ova, when laid, are fastened to the

\l

PHYLUM ARTHROPODA

;,;,:

seta on t In* pleopods of the female by the sticky secretion of glands
occurring both on those appendages and on the segments them-
selves : they are fertilised immediately after laying, the male

H

Fi<;. Ml. —Reproductive organs of Astacus fluviatilis. A, female ; B, male. oil. oviduct ;
ot, its external opening ; ov. ovary ; t. testis ; it, unpaired posterior portion of gonad ; vd. vas
deferens. (From Lang's Comparative Anatomy, after Huxley.)

depositing spermatophores on the ventraL surface of the female's
body just before ovi position.

Development. — The process of segmentation of the oosperm pre-
sents certain striking peculiarities. The nucleus (Fig. 442, A, nu)
divides repeatedly, but no corresponding division of the protoplasm
takes place, with the result that the morula-stage, instead of being
a heap of cells, is multinucleate but non-cellular. Soon the nuclei

Fig. 442.— Three stages in tho formation of the blastoderm of Astacus fluviatilis. tiu.
nuclei ; yp. yolk-pyramids. (From Korschclt and Heider, after Morin and Ueichenbach.)

thus formed retreat from the centre of the embryo, and arrange
themselves in a single layer close to the surface (B) : around each
of these protoplasm accumulates, the central part of the embryo
consisting entirely of yolk-material. We thus get a superficial

7

558

ZOOLOGY

segmentation, characterised by a central mass of yolk and a super-
ficial layer of cells collectively known as the blastoderm (C). Sub-
sequently the yolk itself undergoes a process of segmentation,
becoming divided into radiating yolk pyramids (y.p.), each with its
base in contact with one of the cells of the blastoderm and its
apex pointing to the centre of the egg : before long, however,
these pyramids fuse into an undivided mass of yolk.

The first indications of the future Crayfish take the form of
thickenings on what will become the ventral surface. There are
at first five of these thickenings — two anterior, the head-lobes
(Fig. 443, K), on which the eyes subsequently appear ; two some-
what further back, the thoracico-abdominal rudiments (T A) \ and
one, posterior and unpaired, the endoderm-disc {E8). On the latter

Fig. 443. — Early embryo of Astacus. BM, mesoderm ; ES, endoderm disc ; K' , head-lobes ;
TA, thoracico-abdominal rudiments. (From Lang's Comparative Anatomy, after Reichenbach.)

an invagination of the blastoderm takes place, giving rise to a
small sac, the archenteron, which communicates with the exterior
by an aperture, the blastopore. By this process the embryo passes
into the gastrula-slage, which, however, differs from the corre-
sponding stage in the types previously studied in the immense
quantity of food-yolk filling up the space (blastocoele) between
ectoderm and endoderm. Very soon the embryo become tri-
ploblastic, or three-layered, by the budding off of cells from the
endoderm in the neighbourhood of the blastopore : these accumu-
late between the ectoderm and endoderm, and constitute the
mesoderm.

Before long the blastopore closes, converting the archenteron
into a shut sac (Fig. 445, A) : the thoracico-abdominal rudiments
unite with one another, forming a well-marked oval elevation

PHYLUM ARTHROPODA

:,.->«)

(Fig. 444, TA), and three pairs of elevations appear between it
and the head-lobes. These are the rudiments of the first three
] tail's of appendages, the antennules (av), antennae (n.2.), and man-
dibles (m.) : by their appearance the embryo passes into the
nauplius-stage, which in this case is passed through in the egg,
instead of being active and free-swimming as in Apus.

Between the bases of the antennules and antennae a pit appears,
which soon deepens and widens: it is the stomodecum (Fig. 455,
stdm.), and its aperture the mouth. A similar but narrower and
more cylindrical pit appears on the thoracico-abdominal rudiment :
it is the proctodeum (pedm.), and its aperture the anus. For a

Fn;. 444. KaapHuaitage I Astacus. A, (above) eye; A, (below) anus; at. autennule ;
a... antenna ; (V, cerebral ganglion ; f/<i... antennary ganglion ; am, mandibular ganglion ;
/. labrum ; hi. mandible; TA, thoracico-abdominal rudiment. (From Lang's Conqtarative
Anatomy, after Reichenl»acli.)

considerable time both stomodamm and proctodeum remain in
the condition of blind sacs, but after a time they open into the
archenteron, a complete enteric canal being thus constituted. In
the meantime the endoderm cells lining the archenteron grow
outwards in a radial direction, ingesting the yolk as they do so,
until they take the form of long columns, in contact by their outer
ends with the ectoderm (Fig. 446, B).

The thoracico-abdominal rudiment soon begins to increase
rapidly in length, but, being enclosed in the egg-membranes, it
grows not baekwards but forwards, being in fact folded upon the
anterior part of the body in much the same way as the abdomen
of the adult during extreme flexion. Thus in Fig. 447 the ventral
surface of the head and anterior thoracic region faces the observer,

560

ZOOLOGY

but the dorsal surface of the posterior thoracic and abdominal
regions : in order to bring the parts into their adult position, the
abdomen must be supposed to be lifted up and turned backwards.
In the meantime the post-mandibular appendages are formed
in regular order from before backwards : the eye-stalks appear

7 O CCfl

abet

Figs. 445 .and 446. — Sections of embryos of Astacus. A, Nauplius-stagc (rj. Fig. 444 ; B, after
development of thoracic appendages (ef. Fig. 447). abd. abdomen ; an. anus ; br. brain ;
ect. ectoderm; end. endodcrm ; ent. enteron ; lit. heart; mes. mesoderm; mes.' splanchnic
layer of mesoderm ; mth. mouth ; pedm. proctodwum ; stdm stomod«um ; th. abd. thoracico-
abdominal rudiment ; v. nv. cd. ventral nerve-cord. (From Korschelt and Hcider, after
Rcichenbach.)

(Fig. 447, A), as well as the labrum (/.), and a fold on each side
of the thorax, which is the rudiment of the carapace (7s), "and this
gradually extends dorsal ly until it meets with its fellow of the
opposite side and covers in the cephalothorax. The embyro now
consists of a nearly globular cephalothorax with a small abdomen

PHYLUM ARTHROPODA

561

and a nearly complete set of appendages, all tucked in under the
cephalothorax and closely packed together within the egg-mem-
branea In this condition the embiyo is hatched, and for some
time clings to the pleopods of the mother by means of the peculiarly
hooked chelie of its first pair of legs.

The development of the principal internal organs must be
referred to very briefly. From the ectoderm arise, not only the
epidermis of the adult, but the epithelium of the gullet and
gizzard and of the hind-gut, the epithelium of the gills, the
nervous system, the vitreous cells and retinulrc of the eyes, and

Fjti. 447. — Embryo of Astacus after development of thoracic appendages. A, eyes; oj. an-
tennulo ; 03. antenna ; ab. abdomen ; g, archicerebrum and ganglion of antennule ; <io, optic
ganglion ; I. labruru ; m. mandible ; mx\. )i«2- maxillae ; t. 1—8, thoracic appendages ;
t. telson ; ts, carapace. (From Lang's Comparative Anatomy.)

the epithelium of the auditory sac. From the endoderm arises
the epithelium of the mid-gut and of the digestive glands,
the latter being formed as tubular branching outgrowths of the
archenteron. The connective-tissues, the muscles, the vascular
system, the gonads, and perhaps the kidneys, are all of mesodermal
origin.

2. Distinctive Characters and Classification.

The Crustacea are Arthropods in which the five anterior seg-
ments are fused with the prostomium to form the head, while the
rest are usually divisible into two regions, the thorax and the
abdomen. More or fewer of the thoracic segments may be fused
with the head to form a cephalothorax. The head may bear a

562 ZOOLOGY sect.

median eye, which frequently disappears in the adult, and a pair
of compound eyes, both belonging to the prostomial region : the
latter frequently become elevated on jointed eye stalks. The
appendages of the head are (1) the antennules, which are usually
considered as belonging to the first metamere ; (2) the antennae,
which are certainly post-oral or metameric appendages shifted for-
wards to a pre-oral position ; (3) the mandibles or crushing jaws ; (4)
the first maxillae ; and (5) the second maxillae. The thoracic and
abdominal appendages are variously modified as jaws, legs, fins, or
accessory reproductive organs. With the exception of the anten-
nules, the appendages are typically biramous, consisting of a stem
or protopodite bearing two branches, the endopodite and exopodite.

The body is covered externally by a chitinous cuticle, which
becomes thickened and sometimes calcified in regions where no
movement is required, forming a series of hard parts or sclcritcs,
separated by flexible chitin : the whole chitinous cuticle thus
constitutes an exoskeleton. Typically there is one sclerite to each
metamere behind the head, and to each podomere in the append-
ages, but concrescence of sclerites frequently takes place. The
exoskeleton is produced into seta?, which are hollow processes of
the cuticle containing prolongations of the underlying epidermis.

Respiration takes place either by the general surface of the
body or by gills, which are hollow offshoots of the thoracic wall or
of the thoracic or abdominal limbs. The stomodaeum and proc-
todaeum form a considerable portion of the enteric canal, and are
lined with chitin : the mesenteron gives rise to digestive glands.
The body-cavity is divided into comparments, most of which
contain blood and are portions of the vascular system : the true
coelome may be represented by compartments of the body-cavity
not containing blood and by the cavities of the reproductive organs.
There is a vascular system which nearly always includes a con-
tractile heart, formed as a muscular dilatation of a dorsal vessel,
and communicating by valvular ostia with an enclosing pericardial
sinus. The blood is taken from the heart to the various organs
by arteries, and is returned to the pericardial sinus by sinuses
and veins : the respiratory organs are interposed in the returning
current. The renal organs are peculiarly modified nephridia, .
which may take the form either of shell-glands opening on the;
second maxillae, or of antennary (green) glands opening on the*
antennae.

The nervous system consists of a brain united by oesophageal
connectives with a ventral nerve-cord, formed of a double chain
of ganglia joined together by commissures and connectives. The
first three pairs of embryonic ganglia commonly unite to form
the brain, which is therefore a syn-cerebrum. The sexes are
separate or united : sexual dimorphism is common : partheno-
genesis frequently occurs. The sperms are either amoeboid with

n PHYLUM ARTHROPODA 563

radiating pseadopodia, <>r flagellate: the eggs are usually centro-
lecithal but may be telolecithal, or almost alecithal. The muscles
are striped, and there are no cilia.

Segmentation of the oosperm is usually superficial, but may
be complete or discoid. The embryo very usually has a distinct
nauplius-stage, which may be a free-swimming larva or may
be passed through before hatching, and is characterised by the
presence of three pairs of appendages which become the
antennules, antennae, and mandibles of the adult.

The Crustacea are classified as follows : —

Sub-class I.— Branchiopoda.

Crustacea with a varying number of body-segments, provided
with appendages of a uniform character, usually foliaceous, rarely
leg-like, the posterior region (abdomen) devoid of appendages and
provided with a pair of many-jointed or unjointed caudal styles.
A cephalic carapace is sometimes absent : when present it may be
either shield-like or bivalve. Paired eyes are usually present.
The antennules and the maxillae are reduced or absent : the
mandibles devoid of, or with a vestigial, palp. The larva is a
nauplius or metanauplius.

Order. 1. — Anostraca.

Branchiopoda in which a carapace is not developed. The eyes
are stalked : the antennae are prehensile in the male, reduced
in the female. The appendages of the body-segments number
11 or 19 pairs. The caudal styles are not jointed.

This order includes firanchijnis and Artemia.

Order 2. — Notostraca.

Branchiopoda in which there is a large dorsal shield-shaped
carapace. The eyes are sessile. The antenna; are reduced. There
are 40 to 63 pairs of trunk appendages. The caudal styles are
many-jointed.

Including Apus and Lcpidurus.

Order 3. — Conchostraca.

Branchiopoda with a carapace divided into two lateral portions
or valves like the shell of a bivalve mollusc, and enclosing the
entire animal. The antennte are biramous and are used as
swimming appendages. The eyes are sessile, coalescent. The

564 ZOOLOGY sect.

appendages of the body-segments number 10 to 27 pairs. The
caudal styles are in the form of unjointed, curved claws.

In this order are included Estheria, Limnctis (Fig. 448) and one
or two other genera.

Order 4. — Cladocera.

Branchiopoda of small size with a bivalved carapace which
encloses the trunk but not the head. The eyes are sessile;, and
united together. The antenme are biramous and used as
swimming appendages. Only 4 to 6 trunk appendages ; caudal
styles unjointed, claw-like.

To this order belong Daphnia, Polyphemus, Lcptodora (Fig.
449), etc.

Sub-class II. — Ostracoda.

Crustacea with unsegmented, or indistinctly segmented, body,
bearing not more than four pairs of appendages on the trunk, the
limbless posterior part provided with a pair of caudal styles.
There is a well-developed bivalved carapace. Paired eyes may be
present or absent. Both antennules and antenna; are used in
swimming ; the latter are generally biramous. The mandibles
have a palp. The young escapes from the egg as a nauplius.

In this sub-class are comprised Cypris, Cy there, etc. (Fig. 450).

Sub-class III. — Copepoda.

Crustacea with elongated, distinctly segmented body, bearing
usually five pairs of limbs, the last four having the character of
biramous swimming appendages, sometimes with a sixth pair
which may be vestigial : the posterior region (abdomen) without
appendages, provided with a pair of caudal styles. The cephalic
dorsal shield is not extended backwards, but usually coalesces with
the exoskeleton of the first (and sometimes also the second) body-
segment. Paired eyes are absent except in the Branchiura. Both
antennules and antennae are usually well developed, and the latter
are sometimes biramous : they may both be used as swimming
organs or for prehension. The mandibles may be provided with a
palp. The young is a nauplius. In the parasitic forms more or
fewer of these general characteristics may become lost in the
adult.

Order 1. — Eucopepoda.

Free or parasitic Copepoda without paired compound eyes.
The appendages of the body-segments are devoid of a flagellum.
The genital apertures are situated on the seventh body-segment.
In this group are included (a) free-swimming forms, such as
Cyclops (Water-flea) (Fig. 451) and (b) parasitic forms or Fish-
lice — e.g, Ergasilus, Ghondr acanthus. Lemaia (Fig. 452).

xi PHYLUM ARTHROPODA <- .

Order 2.- Brawn uka.

Parasitic Copepoda with compound eyes and a suctorial mouth.
Some of the appendages of the body-segments are usually provided
with peculiar appendages — the flagclla. The genital apertures are
situated on the fifth body -segment. This order includes the
( \u ji-lice, Argulus (Fig. 453) and two other genera.

Sub-class IV. -Cirripedia.

Imperfectly segmented Crustacea, which are always fixed in
the adult condition, and may be parasitic. There are usually six
pairs of biramous cirri form appendages of the body region. The
limbless posterior region (abdomen) is rudimentary, and is usually
provided with a pair of caudal styles. The carapace forms a pair
of folds, the mantle, completely enclosing the animal, and usually
supported by a system of calcareous plates giving rise to a hard
shell. Paired eyes are absent in the adult. The antennules of
the larva give rise to organs of attachment and become vestigial
in the adult : the antenna? usually disappear. The mandibles have
no palp. The sexes are united in the great majority. The
young animal is hatched in the nauplius form and passes later
through a stage — the cypvis stage — in which it is provided with
a bivalved shell.

Order 1. — Eucirripedia.

Cirripedia some of which are parasitic, -while the rest are non-
parasitic but are permanently fixed in the adult condition.
There are usually six pairs of biramous trunk appendages.

In this order are included (a) fixed forms such as Lepas
(Barnacle) (Fig. 454) and Balamis (Acorn-shell) (Fig. 455) and (b)
parasites — e.g., Pctrarca, Alcippe, Proteokpas.

Order 2. — Rhizocephala.

Parasitic Cirripedia in which the body has undergone extreme
degeneration, and has lost all trace of appendages and of ali-
mentary canal in the adult condition.

Including Sacculina (Fig. 456) and Peltogastcr.

Sub-class V.— Malacostraca.

Crustacea in which the body is always distinctly segmented and
is made up in all cases except the Leptostraca of an anterior region
(thorax) of eight segments, and a posterior (abdomen) of six. with a
terminal tail-piece or telson — the total number of segments,
leaving the prostomium and the telson out of account, being always
nineteen. The appendages of the thorax and abdomen are sharply

vol. I o o

566 ZOOLOGY sect.

marked off from one another. The abdomen is devoid of caudal
styles. The exoskeleton of the head unites with that of more or
fewer of the thoracic segments to form a cephalothoracic carapace.
Paired eyes are usually present and may be sessile or stalked.
The antennules are biramous in most cases. The mandibles are
provided with a palp. There is usually a metamorphosis, but a
nauplius-stage rarely occurs.

Series I. — Leptostraca (Phyllocarida).

Malacostraca in which the abdomen contains seven segments
and a telson — the last segment devoid of appendages, the telson
bearing a pair of caudal styles. There is a large bivalved carapace
with an adductor muscle, enclosing the greater part of the body.
The thoracic appendages are foliaceous, the abdominal biramous.

Includes only one order, the Nebaliacea, with Nebalia (Fig. 457)
and three allied genera.

Series II. — Eumalacostraca.

Malacostraca with six segments and a telson in the abdomen,
the latter never provided with caudal styles. Carapace never
bivalve. Thoracic appendages nearly always leg-like, but seldom
all uniform : their protopodite always made up of two podomeres
except in the Stomatopoda.

Division 1. — Syncarida.

Eumalacostraca devoid of carapace, with the first • thoracic
segment united with the head or marked off from it by a groove.
Heart elongated, tubular.

Order— Anaspidacea.

Syncarida in which the thoracic appendages are' provided (except
the last or the last two) with exopodites, and (except the last)
with a double series of lamellar epipodites (gills). The abdominal
appendages, except the first two in the male and the last in both
sexes, have the endopodite reduced or absent. The last pair of
abdominal appendages (uropods) expanded, and forming with
the telson a fan-like tail-fin. This, the only order of the Syncarida,
comprises the genera Anaspides, Koonunga, and Paranaspides
(Fig. 458).

Division 2. — Peracarida.

Eumalacostraca in which the carapace, when present, leaves at
least four of the thoracic segments free. Heart elongated,
tubular.

\. PHYLUM ARTHROPOD A 507

Okdek 1. — Mysidacea.

Peracarida in which, though the carapace extends over the
it. i pari of the thorax, it does not coalesce dorsally with more
than the first three segments. The eyes, when present, are
supported on movable stalks ; the antennules are biramous, and
the antennae have a scale-like exopodite or squamc. The first
pair of thoracic appendages are specialised as maxillipedes. The
thoracic appendages (except sometimes the first and second pairs)
arc biramous. The uropods with the telson form a broad fan-like
tail-fin.

This order includes My sis (Fig. 459), Lophogastcr, and other
genera.

Order 2. — Cumacea.

Peracarida in which the carapace coalesces with the first three
or four segments of the thorax, is produced on each side to enclose
a branchial cavity, and in front is drawn out into a rostrum. The
eyes usually coalesce into one, which is not borne on a movable
stalk. The antennules are sometimes biramous ; the antenna?
have no exopodites. Some of the thoracic appendages are
biramous. The telson may coalesce with the last segment of
the abdomen. The uropods are styliform. and there is no fan-
like tail-fin.

Includes Cuma (Bodotria), Diastylis, etc. (Fig. 460).

Order 3. — Tanaidacea. f

Paracarida in which the carapace coalesces with the first two
thoracic segments and is produced on each side to enclose a
branchial cavity. The eyes, when present, are usually supported
on short stalks which are not movable. The antennules are some-
times biramous : the antenna? may possess small exopodites.
The first pair of thoracic limbs are modified as maxillipedes. The
second and third thoracic limbs sometimes have vestigial exopodites.
The uropods are usually narrow.

This order includes Apseudes, Tanais, Leptochclia, etc.

Order 4. — Isopoda.

Peracarida in which the dorsal exoskeleton of the head is not
produced into a carapace, but in which the first and sometimes
also the second segment of the thorax coalesce with the head.
The eyes are sessile or borne on short processes which are not
movable. The antennules are nearly always uniramous : the
antennae sometimes bear a minute exopodite. The thoracic limbs
have no exopodites : the first pair are modified as maxillipedes ;

o o 2

568 ZOOLOGY skct.

the rest are usually alike in character. The abdominal appendages
are usually biramous ; the rami function as branchiae The body
is nearly always dorso-ventrally compressed. This is a large order
including many families, e.g. — Asellus, Phrcatoicus, Anthura
Sphaeroma, Idotea, Oniscus, Bopyrus (Figs. 462, 464).

Order 5. — Amphipoda.

Peracarida with the characters of the preceding order, except
that (1) the body is nearly always laterally compressed; (2) the
second and third pairs of thoracic appendages are nearly always
modified as prehensile organs (gnathopods) ; (3) there are vesicular
or lamellar branchiae attached to the bases of more or fewer of the
thoracic limbs ; (4) the abdominal appendages are distinguishable
into two sets, the three anterior pairs with many -jointed rami, the
three posterior (including the uropods) with unjointed styliform
rami.

Includes Orchestia, Gammarus (Fig. 461), Hypcria, Gaprclla,
Gyamus (Fig. 463), and many other genera.

Division 3. — Eucarida.

Malacostraca in which the carapace coalesces with all the
thoracic segments, forming a cephalothorax. The eyes are borne
on movable stalks. The heart is short, sac-like, and situated in
the thorax.

Order 1. — Euphausiacea.

Eucarida in which none of the thoracic limbs take the form of
maxillipedes ; with a single series of branchiae (podobranchs)
attached to the bases of the thoracic limbs. The larva is a
nauplius.

This is a comparatively small order of pelagic Malacostraca,
including Ewphausia (Fig. 472), Thysanopoda, Nydiphanes, and a
few other genera.

Order 2.— Decapoda.

Eucarida in which the first three pairs of thoracic appendages
are modified as maxillipedes, with the branchiae usually in several
scries — podobranchs, arthrobranchs, and pleurobranchs.

Sub -order 1 . — Macrura.

Decapoda with well-developed, elongated abdomen, which is
usually held in the extended position, and terminates in an
expanded fan-like tail-fin composed of the telson and the uropods.
The eyes are not enclosed in orbits. The antennules and antennae

xr PHYLUM ARTHROPODA W9

are both largo; the former arc not sunk in pits, and the antennae
usually have a scale-like extipodite (squame).

Included among the Macrura are (a) swimming forms — Pencms
and Palwmon (Prawns), Crangon (Shrimps), Lucifer, etc. ; and (b)
creeping forma — Homarus (Lobster), Astacus, Astacopsis, Para-
nephrops, Oamiarm (Fresh-water Crayfishes), Palinnrus (Rock-
lobsters), Scyllarus, etc. (Figs. 465, 466).

Sub-order 2. — Anamura.

Decapoda with the abdomen more or less reduced, usually held
in a flexed position, and not provided with such a well-developed
tail-fin as in the Macrura.

In most respects the Anomura are intermediate between the
Macrura and the Brachyura. Examples are the Hermit-crabs —
Pag a rus (Fig. 467), and other genera, the Cocoa-nut crab — Birgus
— Galathca, Hippa, Porccllana, etc.

Sub-order 3. — Brachyura.

Decapoda in which the abdomen is greatly reduced, shorter
than the cephalothorax, and permanently flexed beneath it. The
antennules and the eyes are both capable of being retracted,
into cavities. There is a metamorphosis comprising zona and
megalopa stages.

Including the true Crabs such as Cancer, Maia, Grapms, etc.
(Figs. 468, 469).

Division 4. — Hoplocarida.

Malacostraca in which the carapace does not coalesce with
at least the last four thoracic segments, so that the cephalothorax
is relatively short. In front of the head proper are two movable
segments, one bearing the stalked eyes, the other the antennules.
The branchiae are borne on the abdominal appendages. The
heart is elongated. There is a metamorphosis, but a nauplius
stage is not known to occur.

Order Stomatopoda.

This, the only order of Hoplocarida, includes Squilla (Fig. 470),
Gouodactylvs, and other genera.

Syteinatic Position of the Examples.

The genera Apus and Lepidurus belong to the family Apodidw
of the order Notostraca of the sub -class Branchiopoda.

The foliaceous character of the swimming-feet is alone sufficient
to assign them to the Branchiopoda, and the large number of seg-

5?° ZOOLOGY

SECT.

ments (considerably more than ten) and swimming-feet, together
with the presence of the large shield-like carapace, decides their
position among the Notostraca.

K Jhey-?i!"e,fcd ln th° fami1^ AP°did* in virtue of the elongated
body with 40-60 pairs of swimming-feet, diminishing in size from
before backwards and showing considerable differentiation ; and of
the elongated heart reaching to the twelfth post-cephalic segment
Apus is distinguished by the absence of a post-anal plate, and
by elongated flagella (endites) to the first pair of thoracic feet ■ in
Lepidurus the post-anal plate is present, and the flagella of the
nrst thoracic feet are short.

Astacusfluviatilis is one of several species of the genus Astacus
belonging to the family Potamobiidw, tribe Astacoidca, sub-order
Macrura, order Decapoda, and sub-class Malacostraca

Ihe possession of a thorax made up of eight segments and an
abdomen of six, places it among the Malacostraca : the presence of
a cephalothorax formed by coalescence of head and thorax, toother
with that of movable eye-stalks, determines its position in the
division Eucanda: the modification of the first three pairs of
thoracic appendages as maxillipedes, and the arrangement of the
brancniffi in three sets, place it in the Decapoda.

The possession of a squame to the antenna, and of legs having
all seven podomeres distinct-the first three pairs chelate, and the
first pair greatly enlarged— determine its position in the tribe
Astacoidea, which includes all the fresh-water Crayfishes and the
true Lobsters The family Potamobiid* is distinguished by
having the podobranchue partly united to the epipodites, and bv
possessing appendages on the first abdominal segment of the
male and usually on that of the female.

3. General Organisation.

There is no class in the animal kingdom which presents so wide
a range of organisation as the Crustacea, or in which the devia-
tions in structure from the "type-form" are so striking and so
interesting from their obvious adaptation to the mode of life

Ihe most interesting modifications are those connected with
the external characters and the structure of the append-
ages As we have seen, the body consists of a prostomium a
variable number of metameres, and an anal segment. The first
five metameres fuse with the prostomium to form a head, which
as well as the anal segment, is homologous throughout the class'
Un the other hand, there is no strict homology between the
various post-cephalic metameres in different forms until we come
to the Malacostraca, in which their number is constant

ihere is considerable diversity of form among the Branchiopoda.
Apus has already been described. Branchipus (Fig. 448 1) and

PHYLUM ARTIIROPODA

r»:i

Artemia (the Brine-shrimp) (Jnostraca) arc small shrimp-like
forms, tli«' former living in fresh-water lakes, the latter in brine-
pools; they have no carapace, and the eyes are raised on unjointed
stalks. In Linnxtis (2), on the other hand, and in Estheria (3)
the carapace takes the form of a shell, formed of two parts
or valves, united in Estheria by a hinge, and resembling the
shell of a cockle or other bivalved mollusc. The limbs have the
same general structure as those of Apus, but the antennae are

sh.gl

ant.2

ant.2^vT=-!— !
ant.1

2.LlmneHs

l.Branchipus

ant.2

3.E st.heri a

Kic. 44S.— Tlirec Branchiopoda. In 3, n is the shell ; h the animal with one valve of the shell
'removed. untK antcimule ; <t,tl~. antenna; ht. heart; m. adductor muscle ; /.<>'. mandiblo;
or. ovary ; a. unpaired process from head ; j>. copulatory appendages ; rft.gl. shell-gland ;
t. testis. (After Gerstaeeker.)

often of considerable size, and are sometimes modified into
prehensile organs.

In the Cladocera, of which the common fresh-water Daphnia
(Fig. 449, 1) is a good example, there is a great reduction in size
(1-2 mm.), and a corresponding shortening of the body by a
reduction in the number of metameres. Segmentation is very
imperfect, and the whole body, but not the head, is covered
by a large folded carapace. The abdomen is turned downwards
and is in constant movement, sweeping out any foreign particles
which may have made their way among the feet. Between the
abdomen of the female and the posterior part of the carapace
is a large brood-pouch (Ir.p), in which the eggs are stored. The

572

ZOOLOGY

liVcx

SECT-

paired eyes {E) have fused into a single organ, whiM exhibits
a contstant trembling movement. The antennules {ant. 1)
are small, the antennas {ant. 2) very large, biramous, and con-
stitute the chief organs of locomotion. The mandibles are large,
the second maxillae absent in the adult, and there are usually
five pairs of leaf-like swimming-feet (J) on the thorax. The
abdomen is devoid of appendages. Many of the Cladocera have
an extraordinarily grotesque form (2, 3), owing to the peculiar

1 . D /q p h nia

o. 449. — Three Cladocera. ant. 1, anteunule ; ant. 2, antenna ; br. brain ; hr.p. brood-pouch ;
E. eye ; d.gl. digestive gland ; /. swimming-feet ; ht. heart ; md. mandible ; »k.ffl, shell-
gland. (1 after Clans, 2 and 3 after Gerstaecker.)

shape of the head, the immense antennae, and the great hump-like
brood-pouch.

The Odracoda are usually not more than 1-2 mm. in length,
and are found both in fresh and sea-water. One of the commonest
genera is Gypris, which occurs in immense numbers in stagnant
pools. Cythere is a common marine form.

The body (Fig. 450) is unsegmented, and is completely enclosed
in a carapace (A), the right and left halves of which are articu-
lated together along the dorsal edge so as to form a bivalved shell
(C), which may be variously ornamented or sculptured. The
valves are opened by the elasticity of a ligament, which passes

\1

PHYLUM AKTHIIOPODA

573

from one^Q another at the hinge, and are closed by a large
adductor muscle (m.), which extends transversely from valve to valve,
its insertions giving rise to markings on the shell (A, m.), often
of systematic value.

At the anterior end is a median eye (c), and in some forms
compound eyes are present as well. There are only seven pairs
of appendages. The antennules (ant.l) and antennae (ant.2) are
large and uniramous. The mandible (md.) has a large leg-like palp
and a flabellum-like offshoot. The first maxilla (mx.l) also bears
a large plate resembling a flabellum of Apus. The last cephalic

Fio. 450.— A, external view of Cypris ; B, the same with the appendages exposed by the removal
of the left valve of the shell ; C, transverse section ; D, a single sperm, abd. abdomen ;
ant.l, antennulc ; ant.S, antenna; d.gl. digestive gland; e. median eye; f.l,f.Z, thoracic
feet; int. intestine ; m. adductor muscle; mil. mandible ; mx.l, m.r.2, maxilla-; or. ovary;
xlt. shell ; t. testis. (After Gerstaecker.)

appendage (second maxilla, rnxJH) is jaw-like in some forms
(Cypris), leg-like in others (Cythcrc). The only thoracic appendages
are two pairs of slender legs (f.l, f.<2). The abdomen (abd.) is
devoid of appendages, and is terminated by a pair of small
caudal styles.

The diversity of form among the Copepoda is so great that it
will be advisable to consider separately the free-swimming
Eucopepoda, the parasitic Eucopepoda, and the Branch iura.

The free-sunmming Eucopepoda are well represented by the
common water-flea (Cyclops), found everywhere in fresh and
brackish water, and easily recognisable, in spite of its minute
size, by its elongated form, its rapid, jerky movements, and by the
egg-sacs of the female.

574

ZOOLOGY

Cyclops (Fig. 451, 1) has been compared in form to a split pear,
the broad end being anterior, and the convex surface dorsal.
The first thoracic segment is fused with the head, and the

anlf.

2. Ca I o c a lanus

Pig. 451. — Free-swimming Eucopepoda. la, female Cyclops, from the right side ; b, dorsal
view ; C, antenna of male ; 1), swimming-foot, abd.l, first abdominal segment ; ant.l,
antennule ; ant.S, antenna ; c th. cephalothorax ; e. median eye ; en. endopodite ; e.s. egg-
sac ; ex. exopodite ; ov. ovary ; pr.l, pr.2, protopodite ; r. rostrum ; .«..;• swimming-feet ;
th.%, th.6, thoracic segments. (After Huxley, Gerstaecker, Hartog, and Giesbrecht.)

cephalothorax (c. th.) thus formed is covered with a carapace pro-
duced in front into a short spine or rostrum (r), near the base
of which, on the dorsal surface, is the median eye (e). There are
five free thoracic segments: the last (th. 6) bears the genital

\i PHYLUM ARTHROPODA 575

aperture, and is fused in the female with the first abdominal
(abd. 1). There are four abdominal segments : the last bears the
dorsal anus (an), and a pair of caudal styles produced into plumed
setes.

The antennules (ant. 1) are very large, and are the principal
organs of locomotion. In the male they are modified (C.) — by a
peculiar form of joint and long setae — as clasping organs, used
for holding the female during copulation. The antenna? (ant. 2)
are comparatively short and uniramous. Mandibles and maxilla?
are present, and the first four thoracic segments bear biramous
swimming-feet (la s.f.), those of the right and left sides being
connected by transverse plates or couplers. The fifth thoracic
segment bears a pair of vestigial limbs : the abdominal segments
are limbless.

Some of the pelagic marine Eucopepoda (Fig. 451, 2) are re-
markable for their brilliant colours and for the extraordinary
development of their setae, especially those of the caudal
styles.

The parasitic Eucopepoda, or Fish-lice, present a very interesting
series of modifications, illustrating the degeneration of structure
which so often accompanies parasitism, Ergasilus (Fig. 452, 1) is
found on the gills of the Bass (Morone labrav) ; it is readily recog-
nisable as a Copepod, but the appendages are greatly reduced, the
antenna? modified into hooks for holding on to the host, and the eyes
absent. Anthosoma (2), found in the mouth of the Porbeagle Shark
(Lamna cormibica), has recognisable appendages, but the form of
the body is much modified by the development of curious overlap-
ping lobes. Nicothbe ($), found on the gills -of the Lobster, has
antenna? and mouth-parts modified for suction : the abdomen is
normal, but the thorax is produced imSo. huge lobes, which give
it a curiously deformed appearance. In Clwndracanthus (4), the
various species of which are parasites on the gills of Bony Fishes,
there is, at the first glance, nothing to suggest that the animal
is a Crustacean, except the characteristic copepod egg-sacs : the
body is depressed, unsegmented, and produced into crinkled lobes,
and it requires careful examination to discover that antennules,
hooked antenna? (ant.%) — used for attachment — mandibles, maxilla?,
and two pairs of legs (J.l, f.2) are present. The male (b) is of
higher organisation than the female, but of minute size — about ^
the length of its mate — and is permanently attached to her body,
close to the genital aperture (a, M). In Lemma (7) and its allies
the body is vermiform with a curiously lobed anterior end : the
maxilla? are adapted for piercing the skin of the host and sucking
its juices, and there are minute vestiges of feet. In Lesteira (J)
the degradation is even more marked : the female reaches a large
size — 70 mm. in length, excluding the egg-sacs — and is found
with the swollen head between the skin and flesh of a fish

576

ZOOLOGY

(Gcnypterus blacodes), and the rest of the body hanging freely
into the water. Lastly, in Tracheliastes (6) the second maxilla?

ani.Z

7. Lernaea

5. Les r e ira

6.T racheliasres

Fio. 452. — Various forms of parasitic Eucopepoda. 4a, female ; 4b, male, ant.l, antcnnule ;
ant.S, antenna; e. median eye ; e.#. egg-sac ; f.l, f.S, thoracic feet; M. male; mx.S, second
maxillae. (After Gerstaccker, Claus, Cuvier, and G. M. Thompson.)

(mx.2) are greatly enlarged, and form a characteristic organ of
attachment.

Argtdus (Fig. 453) is the most familiar example of the Branchiura, or Carp-
lice. It is an external parasite on fresh-water Fishes (Carp, Stickleback, &c), not

XI

PHYLUM ARTHUOPODA

577

permanently attached like the degenerate forms just described, but crawling
tivi'lv over the surface of the host. The body consists of an oval flattened
oephalo-thorax, and a small bilobed abdomen (ab.). The mandibles and maxilla-
arc pierdng organs enclosed in a sucking-tube or proboscis (r.), in front of which
is a median tube ending in a spine (xt.). The second maxilla; are divided into
two portions, the anterior of which (//. /) are modified into sucking-discs by

Flo. 453. — Argulus foliaceus, young male, a*, untennulc ; «2> antenna; ab. abdomen ;
b\ — 64, thoracic feet ; d. digestive glands connected with intestine ; ifl, anterior or suctorial
feet ; kji, posterior or leg-like portion of second maxillst ; pa. paired eye ; /•. rostrum : ad. shell-
gland ; at. stylet ; It. testis ; tia, median eye. (From Lang's Comparative Anatomy.)

which the parasite clings to the surface of its host, and there are four pairs of
swimming-feet (6/ —hlf). Alone among the Copepoda the Branchiura have no
egg-sacs, and they are exceptional also in the possession of compound eyes {pa. ).

The most familiar examples of the Eucirripedia are the Barnacles
found on ships' bottoms, piles, &c, and the Acorn-shells or Sessile
Barnacles, which occurin immense numbers on rocks between tide-
marks in all parts of the world.

The common Barnacle {Lepas anati/era) is attached by a long
stalk or peduncle (Fig. 454, A,£>), covered with a wrinkled skin, and
bearing at its distal end the body proper enclosed in a sort of

578

ZOOLOGY

SECT.

bivalved carapace, formed by a fold of the skin, and strengthened
by five calcareous plates. Of these one is median and dorsal, and
is called the carina (c); two are lateral and proximal, the scuta (s):
and two lateral and distal, the terga (t). During life the carapace
is partly open, and from the ventrally placed aperture delicate
setose filaments are protruded and keep up a constant grasping
movement : these are the endo- and exopodites of the biramous
thoracic feet, of which there are six pairs. Removal of the carapace
shows the feet to arise from a vermiform unsegmented body
(B), attached on the ventral aspect to the stalk and carapace by

a'

Fig. i Lcpas anatifera. A, the entire animal; B, anatomy. «j, antennule ; c. carina
cd. cement-gland ; /. digestive gland ; m. adductor muscle ; od. oviduct ; ov. ovary ; p. (in B)
penis and (in A) peduncle ; s. scutum ; t. tergum and testis ; vd. vas deferens. (From Lang's
Comparative Anatomy, after Darwin and Claus.)

its anterior end, while its posterior end is free and terminates in
a long filament, the penis (p), immediately dorsal to which is the
anus. The mouth is ventral and anterior, and is provided with
a pair of mandibles and two pairs of maxillae. There are no
antennae : at first sight 'the antennules appear to be absent, but
a careful examination shows the presence of a pair of minute
structures (a) on the proximal or attached surface of the stalk,
and embedded in the cement by which the animal is fixed to its
support ; these are the antennules, and their position relatively to
the mandibles shows that the stalk is formed by an elongation of
the anterior region of the head.

\l

PHYLUM ARTHROPOD A

579

The Sessile Barnacles or Acorn-shells (Balanus) have no stalk
( [Pig. 455), the head-region being short and broad. The scuta (s)
and terga (t) support a valvular carapace, through the opening of
which the feet are protruded, and the whole animal is surrounded
by a sort of parapet (sk) formed of six calcareous pieces. One
of these, dorsal in position, is the carina, the others appear to be
represented by small ossicles developed on the peduncle of certain
stalked forms such as Pollicipcs.

Many of the Eucirripedia are parasitic. Some of these (Pctrarca,
&c.), parasitic in Actinozoa, resemble the attached forms in
essential respects; others {Alcippc), parasitic in the shells of
Molluscs and Cirripedes, have abdominal but no thoracic feet.

FjO. 466. Balanus. A. external view ; B, anatomy. a\. antennules ; ad. adductor muscle;
m. muscles of scuta and terga ; o, edge of parapet ; ov. ovary ; ovi. oviduct ; s. scutum ;
at. parapet; t. tergum ; too. female aperture, (from Lang's Comparative Anatomy, after
Darwin.)

Proteolepas, also parasitic on other Cirripedes, has a maggot-like,
segmented, limbless body, and a suctorial mouth.

The Rhizoccphala are represented by Sacculiiia (Fig. 456), parasitic
on Crabs, and Pdtogastcr on Hermit-Crabs. Both genera have the
appearance of an immense tumour (ks) on the abdomen of the host,
snowing no sign of segmentation, no appendages, no mouth or
From the attached end go off a number of delicate root

anus.

like filaments, which extend through the body of the host and
absorb nutriment. Obviously degeneration is here as complete as
it can well be, and nothing but the developmental history of the
parasite (p. 599) would justify its inclusion among the Crustacea.

The most striking general character in the external features of
the Malacostraca is the limitation in the number of segments. The

580 ZOOLOGY

head has the same composition as in the Entomostraca, but the
thorax is invariably formed of eight segments, and, except in
the Phyllocarida, the abdomen of six segments and a telson.

Fig. 450. -Sacculina carcini, on abdomen of crab. br. branchial region of crab ; I, hepatic
region ; d, intestinal region ; ks, body of parasite ; p. peduncle ; mb, basilar membrane,
giving off root-like processes which are seen extending through the body of the host. (From
Lang's Comparative Anatomy, after Delage.)

The limbs are strikingly modified for the performance of various
functions.

The Phyllocarida are interesting from the fact that they are
annectent or linking forms between the Branchiopoda and the
Copepoda on the one hand, and the higher Crustacea, particularly
the Schizopoda and Decapoda, on the other. The order contains
only three genera, the commonest of which, Nebalia (Fig. 457),
is a little shrimp-like marine Crustacean about 6-8 mm. in
length. The body is divisible into head, thorax, and abdomen,
all having the normal malacostracan number of segments ex-
cept the abdomen, which is formed of eight segments, the last
bearing caudal styles — structures not found elsewhere in the
sub-class. There is a bivalved cephalic carapace (s), closed by
an adductor muscle (sm) and extending backwards to the fourth
abdominal segment : it is terminated in front by a movable
rostrum (r).

PHYLUM ARTHROPODA

581

The eyes («) are large, compound, and raised on movably articu-
lated stalk-. Thf antennules («,) and antenna? («2) are large, the
mandibles (md.) have palps
(mt), and the exopodite of
the second maxilla {mxt) has
the form of a slender filament
which acts as a "cleaning-
foot " to keep the cavity of
the carapace free from foreign
bodies. There are eight thor-
acic appendages {brf), all of
them leaf-like, and recalling
those of Apus. The first
four abdominal appendages
(pi — f>£) are large biramous
swimming-feet, like those of
Gopepods; the fifth and sixth
(pS, 2*6) are small and uni-
ramous.

The Syncarida (Anaspid-
acea) (Fig. 458) are small,
shrimp-like, fresh -water Crus-
taceans, which, though re-
sembling the rest of the
Malacostraca (Eumalacos-
traca) in the presence of only
six segments in the abdomen
ami the absence of caudal
styles, differ from them in
the possession of a combina-
tion of features which con-
nect them more closely with
certain fossil forms of Car-
boniferous age. Thus there
is no carapace, the thoracic
appendages are provided Avith
slender respiratory exopo-
dites, and bear a double
series of epipodites or bran-
chiae ; there are stalked eyes
and a fan-like tail-fin formed
of the telson and the ex-
panded uropods.

The Mysidacca (Fig. 459)
are small, transparent, shrimp-like forms, mostly from 2 — 6 mm.
in length. They agree with the Crayfish in the general form
of the body, in the union of the head and thorax, in the

Fio. 457.— lfebalia geoffroyi, male, a. eye ;
rtj, antennule ; «._>, antenna ; c, head ; brf. thoracic
feet; (/, intestine; h. heart; km, gizzard; md.
mandible; mt, mandibular palp; mnt. exo-
podite of second maxilla; p\ — p^, pleopods ;
r. rostrum ; s, carapace ; nm, adductor muscle ;
t. testis ; /— Vlll, thoracic segments. (From
Lang's Comparative Anatomy, after Claus.)

1

VOL. I

P P

582

ZOOLOGY

presence of a carapace — which leaves some of the posterior
thoracic segments free — and in the number both of segments
and appendages, but present several interesting characters

Fig. 458. — Faranaspides lacustris, x4. a1, antennules ; a?, antenna; ; Ab.l, first
abdominal segment : ep, epipodites or gills on the thoracic legs ; md, mandible ; Pl.l, first
abdominal appendage ; T, telson ; Th.S, eighth free thoracic segment ; U, uropod. (After
Geoffrey Smith.)

indicating a lower grade of organisation. One of the most
notable of these is the absence of differentiation in the thoracic
appendages, which, though they have a leg-like and not a leaf-like
form, are all alike, none of them being modified into maxillipedes,

Fio;. 459. — Mysis oculata. end. endopodite; ex. exopodite ; ot. otocyst. (After Gerstaeeker.)

except to a very slight degree in some forms. Moreover, the legs
all possess exopodites {ex), thus retaining the primitive biramous or
" split-footed " form which is lost in the Decapoda. The first five
pleopods are large in the male, small in the female : the sixth

XI

PHYLUM ARTHROPODA

583

18 a uropod, i.e., assists the telson in the formation of the
characteristic malaeostracan tail-
fin : there is no trace of the
entomostracan caudal styles

The Cumacea are also a very
small group : Diastylis (Fig. 460)
is a good example. They are
little shrimp-like animals, differ-
ing from all the Malacostraca pre-
viously considered in having poorly
developed sessile eyes, sometimes
{used together, and in some genera
altogether absent. The carapace
(cth) is so small as to leave the
five posterior segments (thlV —
VIII) uncovered. The first two
pairs of thoracic limbs are maxiili-
pedes, the last six, legs : of these
two or three pairs have exopo-
dites (ex).

The Tanaidacea, the Isopoda and
the Amphipoda are often grouped
together under the heading of
Arthrostraca. These orders, par-
ticularly the two last, comprise a
great number of genera and
species, many of them strangely
modified in correspondence with
special habits of life. The best
known examples of the Amphipoda
are the little Fresh-water Shrimp
(Gammarus, Fig. 461) and the
Sandhoppers (Talilnis, Orchcstia)
so common on the sea-shore. Of
the Isopoda very convenient ex-
amples are Asellus (Fig. 462),
common in fresh-water, and the
well-known Wood-lice or Slaters
(Oniscus, Fig. 464, /), found
under almost any piece of wood,
stone, &c, which has lain undis-
turbed on the ground for a few
weeks.

The body is usually compressed ^^T^SS^SSUeS X

Or flattened from Side tO Side in dominal segments J cth. cephalothorax ;

. , . , /T-,. tn-,\ i i «'l> cndopodito ; ex, cxopuditc ; /).l,p.O,

AmphipodS (rig. 461), depressed pleopods; IV-VII, tii VUI, free thoracic

or flattened from above down- 'fi^JStotf*** o-*~*»

p p 2

534

ZOOLOGY

SECT.

wards in Isopods (Fig. 462). The normal malacostracan number
of segments is present, but the first thoracic segment is always
united with the head, so that the apparent head is really an incom-
plete or partial cephalothorax (c.th). In the Tanaidacea (Tanais,
&c.) the second segment of the thorax also unites with the head,
and such forms — sometimes included under a distinct sub-order,
Anisopoda — form a transition to the other Malacostraca, and
especially the Cumacea. In the Amphipoda and Isopoda, the pos-
terior seven thoracic segments (th.2 — th8) are free, and those of the
short abdomen are usually free in Amphipods (Fig. 461, ahd,

th.Z

th.S

Fig. 461. — Gammarus neglectus. abd.l — abd.U, abdominal segments; ant.1, antennule ;
ant. 2, antenna ; eth. cephalothorax ; E. eye ; ,/'. /. 1, first jumping foot ; I. 1 — I. 7, .legs ;
mtp. maxillipede ; o.i. oostegite ; ov. ova; s.f.l, first swimming foot ; tli.S—th..!, free thoracic
segments. (After Gcrstaeckcr.)

1-6), often more or less fused in Isopods (Fig. 462, aid). In some
Isopoda the thoracic segments are produced laterally into large
and prominent pleura.

The eyes (E) are compound and usually sessile : they are, how-
ever, stalked in some of the less specialised members of the order,
a circumstance which lends support to the view that the sessile
eyes have, in this particular group, arisen by the atrophy of eye-
stalks. The antennas {ant. 2) as well as the antennules (ant.l) are
uniramous, or the former bear a minute exopodite. The first pair
of thoracic appendages (mxp) are modified to form maxillipedes,
which are sometimes united together in the middle line so as to
form a sort of lower lip. The remaining seven thoracic append-
ages take the form of legs (1.1-1.7) which are usually arranged in

PHYLUM ARTHROPODA

:,*:,

two groups, four of them directed forwards and three backwards, or
tend. The legs end either in simple claws or in large sub-
chelae : vestigial exopodites are present in some Tanaidacea.
In the female, certain of the legs bear flat plates, the oostegites
(Fig. 461, on), probably modified epipodites, which enclose a brood-
pouch for the reception of the eggs. In Amphipods the gills are
also borne on the legs.

The abdominal appendages are very different in the two orders.
In Amphipoda the first three are biramous swimming-feet (Fig.

Pro. 4t>2.— Aiellus aquaticus. A, dorsal ; 15, ventral view, abd, abdomen ; ant.l, antennule ;
ant.i, antenna; bp. brt>od-pouch ; c.th, cephalothorax : £, eye; 1.1 — 1.7, legs; pl.l — pl.7,
pleopods ; th.2—lh.8, free thoracic segments. (After Gerstaecker.)

461, sf.), the last three peculiar stiff processes used for jumping
(//). In Isopods more or fewer of the pleopods have broad plate-
like endo- and exopodites (Fig. 462, pl.3). the former thin and
vascular and acting as gills: the sixth pair (pl.O) are either leg-
like or aid in the formation of a tail-fin.

Interesting modifications occur in both sub-orders. Among the
Amphipoda, Phronima (Fig. 463, /) is a marine form of glassy
transparency, the female of which inhabits a transparent barrel-
like structure — the test of a pelagic Tunicate — in which she

586

ZOOLOGY

brings up her young. Gaprella (3) is a singular creature in Avhich
the abdomen is quite vestigial, and the rest of the body, as well as
the appendages, extremely slender. It creeps about on colonies
of Hydrozoa and Polyzoa, to the branches of which its own form
and colour are so closely assimilated as to render it difficult of
detection. The allied Gyamus (Whale-louse, #) is parasitic on the
skin of whales : it also has a vestigial abdomen, but the body
— exceptionally among Amphipods — is broad and depressed, and
the legs are curiously swollen.

3. C a p r e 1 1 a
u.Cyamus
Fig. 463. — Amphipoda. 3, o, male ; 6, female. (After Gerstaecker, and Bate and Westwood.

Among the Isopoda, one of the most interesting forms is the
common Wood-louse (Fig. 464, 1), which is almost unique among
Crustacea for its perfect adaptation to terrestrial life. The allied
" Pill-bugs " (Armadillidium, 2) have the habit of rolling them-
selves up into a ball when disturbed. Gymothoa and its allies are
large species (6-8 cm. in length) parasitic in the mouths of Fishes,
where they hold on to the mucous membrane with their short, clawed
legs: their mouth-parts are often modified for sucking. In the Bopy-
rini, found in the gill-cavities of various Crustacea, parasitism is
accompanied by great degeneration and asymmetry (3) as well as
by a notable degree of sexual dimorphism, the males (3, b, m) being
very small and permanently attached to the bodies of the females.
Lastly, in Cryptoniscus, parasitic on Crabs, the adult female (4-> b) nas

PHYLUM ARTHROPODA

587

no trace of crustacean organisation, and it is only by the study of
development that its true systematic position can be guessed.

In the division Eucarida, the Euphaimaeea (Fig. 472) are
pelagic forms in which none of the thoracic appendages are modi-
fied so as to take the form of maxillipedes, and in which there
is only a single series of branchiae (podobranchs).

Amongst the Dccapoda are included nearly all the largest and
most familiar Crustacea — the Prawns and Shrimps, Lobsters, Cray-
fishes, and Crabs. The cephalothorax is always completely covered
by the carapace. The three anterior pairs of thoracic appendages
are modified into maxillipedes, which retain the original biramous
character, but the five posterior pairs are enlarged, and form legs,

2. Armadillidium.

3. Gyge.

4. Cryptoniscus.

Fio. 464. — Isopoda. 3,a. entire animal ; b, posterior end with attached male (m) ; U,a, larva
b, adult female. (After Cuvier, Claus, and Gerstaecker.)

which are always — except as an individual variation — devoid of
exopodites in the adult.

In the Shrimps and Prawns (Fig. 465) the body is compressed,
and the exoskeleton is not calcified. The abdomen is very large
in proportion to the cephalothorax, and has a peculiar bend close
to its junction with the thorax. The legs are very slender, are
used for swimming, not walking, and sometimes one pair, sometimes
another, is enlarged to form the chelipeds. The rostrum is large
— sometimes longer than the rest of the carapace — and the eye-
stalks, antennae, and legs may attain extraordinary dimensions.

The Lobsters and fresh-water Crayfishes agree with Astacus in
all essentia1 details, but the sea-Crayfishes (Palinurus) present some
striking modifications. There are no chelae, the legs all ending in
simple claws : the antennae are of immense size, and their proximal
segments are fused with one another and with the carapace, quite
crowding out the epistoma: the rostrum is reduced, or even
vestigial, and the pleopods are very broad and fin-like. In Scyllarus
(Fig. 466) and its allies the body is broad and depressed, the bases

588

ZOOLOGY

SECT.

of the legs widely senarated from one another by the broad
sterna, the antennae {ant. 2s) short and plate-like, and the eye-stalks
(E) enclosed in socket-like grooves of the carapace. Most of

2.Palaemon.

Via. 465.— Shrimp (dorsal view) and Prawn (side view). (After Cuvier.)

these characters show an approximation to what is found in
the Crabs.

Of the Anomura, the Hermit-crabs (Pagumos, &c, Fig. 467)
are very strangely modified in relation with their peculiar mode

\I

IMIYW'.M ARTHROPODA

589

of life. They are always found inhabiting the empty shells of
(iastropods (Whelks, Periwinkles, &c.), the abdomen, which has
become spirally twisted, completely enclosed within the shell and
only the cephalothorax protruding. In correspondence with this
mode of protection, the abdomen is soft, having only vestiges
of terga (t) on the dorsal side, and its appendages are more or less
atrophied except the sixth pair (up), which take the form
of a pair of hooks, and are used to hold on to the columella of
the shell. The fifth pair of legs (1.5) are much reduced, and in

Fin. 4(56. ScyUarus arctus.
unt.l, antennule ;ant.2, antenna ;
E, eye. (After Cuvier.)

Fi<;. 487.— Fagurus bernhardus. ch. chela of
first right leg; Lit, 1.6, fourth and fifth legs;
t, abdominal terga ; up. uropods. (After Bell.)

some species one of the chelipeds is greatly enlarged and its
chela (ch) acts as an operculum, completely closing the mouth
of the shell when the animal is retracted. As the Hermit- Crab
grows it takes up its abode in larger and larger shells, sometimes
killing and removing piecemeal the original inhabitant.

Other Anomura, such as the Cocoa-nut Crab (Biryiis), Hippa, &c,
approach the Brachyura in the short, more or less permanently
flexed abdomen, but are clearly separated from them by the
structure of the head and its appendages.

In the Brachyura, or true Crabs, we reach the highest degree of

590

ZOOLOGY

specialisation known among the Crustacea. The cephalothorax
(Fig. 468) is always of great proportional breadth, and is frequently
much broader than long. The abdomen, on the other, hand is greatly
reduced, its sternal region is uncalcified, and it lies permanently

mccp

Fig. 468. — Cancer pagurus. A, dorsal, B, ventral aspect, ant.l, antennule ; ant.3, antenna ;
abd.l, abd.3, abd. 7, abdominal segments ; E, eye-stalk; l.l, 1.5, legs; mxp.S, third maxilli-
pedes. (A, after Bell.)

flexed in a groove on the very broad thoracic sterna, so as to be
often quite hidden in a view from above. In correspondence with
this the pleopods are much reduced, the male retaining only two
pairs as copulatory organs, the female four pairs for the attachment
of the eggs. The uropods are absent, so that there is no tail-fin.

XI

PHYLUM ARTHROPODA

591

The eye-stalks {E) are contained in orbits or sockets of the carapace,
which are so prolonged that the eyes appear to arise behind the
antennules and antenna?. Both pairs of feelers are small, and the

Fi<i. 409.— Typical Bracliyura (After Bell and de Ilaan.)

bases of the antennules are contained in sockets or fossdtes. The
third maxillipedes (mxp.) are broad, flat, and valve-like, not leg-
like as in the Macrura. The first legs (1.1) form chelipeds often
of great size : the remaining legs generally end in simple claws,

592

ZOOLOGY

but in the Swimming-crabs (Fig. 469, 1) the distal segment
in the fifth pair is flattened and forms a fin. The range of
variation in form, proportions, colour, markings, &c, among Crabs
is very great (Fig. 469).

Unlike the Decapoda, the Stomatcywda form a very small order,
comprising a few genera varying from the size of a Shrimp to that
of a Lobster. Squilla (Fig. 470) is the best known genus.

The abdomen (al — a7) is very large in proportion to the
cephalothorax, and the carapace (cth), which is thin and uncalci-
fied, leaves the last three thoracic segments (VI — VIII) un-
covered. The rostrum is movably articulated, and covers the
anterior head-region, which is divided into two distinct segments,
the first bearing the large stalked eyes, the second the antennules.

Fio. 470. — Squilla. al, antennule ; a%, antenna ; al — a7, abdominal segments ; br, gills ; cth,
cephalothorax ; p, copulatory organ ; pi — p.5, pleopods ; p6, uropods ; VI — VIII, free thoracic
segments ; 1 — 8, thoracic appendages. (From Lang's Comparative Anatomy.)

This arrangement appears to support the view that the anten-
nulary region is a metamere distinct from the prostornium ; but
the division in question is absent in the larva, and does not
appear till the proper segmentation of the body is established :
probably it has a physiological meaning, and is connected with
the necessity of extreme mobility of the eyes and olfactory organs
in an animal which lives in a burrow with only the anterior end of
the head exposed.

The antennule (al) has three flagolla ; the antenna (a8) a single
flagellum and a very large exopodite. The first five pairs of
thoracic limbs (1 — 5) are turned forwards towards the mouth, and
act as maxillipedes ; the second of these — corresponding with the
second maxillipede of Astacus — is very large (2), and its distal
segment is turned back and articulated to the penultimate seg-
ment like the blade of a pocket-knife to the handle. In this way
a very efficient weapon called a sub-chela is produced, both of the
segments of which are produced into strong spines. The re-

a PHYLUM ARTHROPODA 593

maining three thoracic appendages {6 — 6') are slender legs pro-
vided with exopodites : the last of them has a styliform copu-
latory organ (p) developed from its proximal segment. The
pleopods are large and biramous : the first five (pi, po) have gill-
filamenta (br) attached to their plate-like exopodites: the sixth
(pO) form large uropods or lateral tail-lobes, as in Astacus.

With regard to the texture of the exoskeleton, there is
every graduation from the delicate polished cuticle of most
Branchiopoda, Ostracoda, Copepoda, &c, through the calcified but
still flexible cuticle of Astacus, to the thick, tuberculated, stony
armour of many Crabs (Fig. 4G9, 3), or the shelly pieces of Oirri-
pedes. The exoskeleton is sjlreted from a single-layered ectoderm,
and undergoes periodical fnoults or ecdyses. There is no trans-
verse layer of muscle, and the longitudinal layer is broken up
into paired dorsal and ventral bands. As a rule, each limb-
segment is acted upon by two muscles : the joints are nearly
always hinge-joints.

The body-cavity consists of several chambers separated from
one another by partitions. In Palwmonetcs, one of the Prawns,
there is a median dorsal chamber enclosing the ophthalmic artery,
and not containing blood : it is probably a portion of the ccelome
in the strict sense of the word. The cavities of the gonads are
also coelomic, and the ducts by which they communicate with the
exterior are probably modified ccelomoducts. In addition to these
cavities there is a large central space, in which the enteric canal,
digestive glands, gonads, &c, lie ; paired lateral spaces containing
portions of the shell-gland ; spaces in the limbs ; and the peri-
cardial sinus, in which the heart lies. All these cavities contain
blood, and constitute a kind of secondary body-cavity, formed by
the enlargement of blood-vessels, which have largely replaced the
true coelome. Such a secondary or blood-containing body-cavity
is called a Jummocaelc. »

The enteric canal consists of a vertical gullet, an expanded
" stomach," and a nearly straight horizontal intestine. In some of
the Cladocera the intestine is coiled, but this is quite exceptional.
In the lower Crustacea, part or the whole of the " stomach " is
formed from the mesenteron, but in Malacostraca both gullet and
" stomach " (gizzard) are developed from the stomodaeum. A
" gastric mill " is present in Malacostraca, and a rudiment of
such an apparatus occurs in Ostracoda. The digestive glands
are usually branched caeca formed as offshoots of the mesenteron :
in the Isopoda and Amphipoda (Fig. 471, Z) they are unbranched
caeca extending into the abdomen : in Stomatopoda they consist
of ten metamerically arutnged organs opening into the intestine.
In Amphipods there are intestinal caeca (ud) which may have an
excretory function. ►So-called salivary glands, opening on the
labrum, have been found in several genera.

594

ZOOLOGY

SECT.

In most of the the Branchiopoda, Ostracoda, Copepoda, and
Cirripedia, respiration takes place by the general surface of the body,
and the only respiratory organs are specially modified parts of the
appendages. In the stalked Barnacles, however, there are delicate

Fio. 47].— Orchestia cavimana, male.

eye; a\, antennule ; a%, antenna; aoa, anterior

aorta; aop, posterior aorta ; bm, ventral nerve-cord ; br, gills; C+T, cephalothorax ; de, vas
deferens ; ed, rectum ; g, brain ; h, heart ; ltd, intestine ; kf, maxillipede ; I, digestive glands ;
<e, gullet ; p 1 — j> 7, abdominal segments ; am, " 8tomacli " ; ud, intestinal caecum ; vs. vesicula
seminalis ; t, testis ; [I— VIII, free thoracic segments. (From Lang's Comparative Anatomy,
after Nebesky.)

processes attached to the feet, which are supposed to be rudiment-
ary gills. Amongst the Malacostraca also, the Phyllocarida, many
Mysidacea, and the Cumacea have no specialised respiratory organs,
but the Euphausiacea possess tufted podobranchiae (Fig. 472) quite

\[

PHYLUM ARTHROPODA

;-,«»:,

uncovered by the carapace. In the Decapoda the gills may be
either plume-like, as in Astacus and its allies, or the delicate
cylindrical gill-filaments may be replaced by flat plates, as in
Crabs and many Prawns. It is in this order only that. we find the
three types of gill described in Astacus, and the examination

3 *

J6i

Fig. 472.— Anterior portion of Euphausia pellucida. «j, antennule ; ant.2, antenna; ab.l,
first abdominal segment ; mi, eye ; br. 1—8, podobranchia ; cth. cephalothorax ; en.l, en J,
cndopoditcs of first two thoracic limbs; ex.1 — ex.6, exopodites of first six thoracic limbs;
h. heart ; /, digestive gland ; m, "stomach " ; ov. ovary ; ovd. oviduct ; I—V1I1, protopodites of
thoracic limbs. (From Lang's Comparative Anatomy^)

of numerous for*ms leads to the conclusion that the typical
theoretical branchial formula for the group is as follows : —

or

Thoracic

Ski.mknts.

I.

II. III.

•

IV.

V.

VI.

VII.

VIII.

\
Total.

Podobranchia;

Arthrobranchise

Pleurobranchia?

1 + ep
2

1

l+ep
2

1

4 + ep

\-\-ep
2

1

1+ep
2
1

1 + ep
2
1

1 + ep
2

J

1+ep
2

1

1 + ep
2

1

j S + 8ep
■16
8

Total ...

4 + ep

4 + ep

4 + ep

4 + ep

4 + ep

4 + ep

4 + ep

!

32 + Sep

1

Actually, however, this formula never occurs, as there is always
more or less reduction in the number of gills. Palinurus has the
highest number known, viz., twenty-one, and in the Common Crab
the total number is only nine.

Many Crabs live on land, and their gills are enabled to discharge
their function in virtue of the moisture retained in the nearly
closed gill-chamber. In the Cocoa-nut Crab (Birgits) the upper
part of the gill-chamber is separated from the rest and forms an
almost closed cavity into which vascular tufts project : it thus

596

ZOOLOGY

functions as a true lung. Probably the inner surface of the gill-
cover or branchiostegite performs a respiratory function in the
Crayfishes.

In Amphipoda, also, the gills (Fig. 471, br) are outgrowths of
the thoracic limbs : in Isopods they are the modified endopodites
of the second to the fifth pleopods ; in some of the terrestrial
forms, in adaptation to aerial respiration, a system of air-tubes are
developed in the exopodites ; in Stomatopoda, gill-filaments (Fig.
470, br) spring from the exopodites of the first to the fifth pleopods.
Moreover many Crustacea perform rhythmical contractions of the
intestine, taking in and expelling water : such anal respiration
is common among the lower groups, and is especially noticeable in
Cyclops.

The heart is absent in many Copepods (including Cyclops), in
some Ostracoda (including Cypris), and in Cirripedia: it is an
elongated tube with several pairs of ostia in Euphyllopoda,
Leptostraca, Stomatopoda, Anaspidacea, Tanaidacea, Isopoda, and
Amphipoda (Fig. 471, h); in Cladocera and Decapoda'it is
shortened to an ovoid sac with one or more pairs of ostia.

Excretory Organs. — In many larval Crustacea two pairs of
modified mesonephridia are present — the antennary glands opening
on the bases of the antenna?, and the maxil-
p lary or shell-glands opening on the bases of

the second maxillae. But as development
proceeds one pair nearly always atrophies,
the maxillary gland alone being usually
retained in the Branchiopoda, Ostracoda,
Copepoda and Cirripedia, the antennary
gland in the Malacostraca. In the Stoma-
topoda, however, there is no antennary
gland, and the function of renal excretion
may be discharged by a pair of glandular
tubes opening into the rectum ; and in
Amphipoda a similar function is assigned to
caeca opening into the posterior end of the
mesenteron. In some of the Cirripedia the
maxillary gland is described as opening into
one of the compartments of the body-cavity
like a typical nephridium.

The nervous system is always formed
on the ordinary arthropod type, as de-
scribed in Apus and Astacus, and the chief
variations it presents are connected with
the greater or less amount of concrescence
of ganglia. In the sessile Barnacles and in the Crabs (Fig. 473)
this process reaches its limit, the whole ventral nerve-cord being
represented by a single immense thoracic ganglion (bg).

Fig. 473. — Nervous system of a
Crab (OTaja squinado).
bg, thoracic ganglion ; eg.
commissural ganglion ; g,
brain ; m, " stomach " ; sc,
oesophageal connective ; sg,
visceral nerves ; y, post -oeso-
phageal connective. (From
Lang's Comparative Anat-
omy, after Milne-Edwards.)

xi PHYLUM ARTHROPOD A 597

The sense-organs are mostly of the same character as those of
the two examples. The median or nauplius-eye always occurs in
the larva, and can frequently be shown to exist in the adult of
even the higher groups (Decapoda). The Cirripedia and many
parasitic Copepods are eyeless in the adult, as also are certain
subterranean Malacostraca. Olfactory setse occur, as a rule, on the
antennules, and the auditory organs (or statocysts) of Decapoda
are open sacs in the basal segment of the same appendages, but
in Mysidacea they occur as closed cysts (Fig. 459, ot) in the
endopodites of the uropods.

Reproduction. — In most Crustacea the sexes are separate, but
hermaphroditism occurs in some Branchiopods, in nearly all Cirri-
pedes, and in certain parasitic Isopods (Cymothoa). In the latter
case the animals are protandrous, male organs being developed first,
and female organs at a later stage. In many Cirripedia minute
complemental males are found attached, like parasites, to the body
of the ordinary or hermaphrodite individual, the male organs of
which appear to be inadequate for the full discharge of the ferti-
lising function. Sexual dimorphism is almost universal, and
reaches its maximum in the parasitic Copepods and Isopods
already referred to.

The gonads are always a single pair of hollow organs discharg-
ing their products into a central cavity .or lumen, whence they
pass directly into the gonoducts and so_to the exterior. The
gonads may be single or branched, and frequently there is more or
less concrescence between those of the right and left sides, as in
Astacus and Cyclops. The sperms vary greatly in form, and are
usually motionless: in Cirripedia, however, they are motile, and
in Ostracoda they perform movements after reaching the female
ducts. In some Ostracoda they are about three times as long as
the animal itself (Fig. 450, D). In many Branchiopoda and
Ostracoda reproduction is parthenogenetic. In Daphnia, for
instance, the animal reproduces throughout the summer by
parthenogenetic summer eggs, which develop rapidly in the brood-
pouch (Fig. 449, 1, br. p.). In the autumn winter eggs are produced,
which are fertilised by the males : they pass into the brood-pouch,
a portion of which becomes specially modified and forms the
ephippium or saddle. At the next moult the ephippium is
detached and forms a sort of bivalved capsule in which the eggs
remain in an inactive state during the winter, developing in the
following spring.

Development. — In some Crustacea segmentation is complete,
and a hollow blastula is formed : in others segmentation is
followed by an accumulation of yolk in the interior, resulting
in the formation of a superficial blastoderm, as in Astacus :
in others, again, the egg is telolecithal, and the protoplasm,
accumulated at one pole, divides so as to form a disc of cells

VOL. i q y

598

ZOOLOGY

which afterwards spreads over the whole yolk. But in most
cases the egg is ccntrolecithal and segmentation superficial, as
in Astacus.

Development is always accompanied by more or less metamor-
phosis. In most Branchiopoda the young is hatched in the form of
a nauplius (Fig. 429, A), and further changes are of the same char-
acter as in Apus. In Cladocera development is direct, the nauplius-
stage being passed through in the egg, and the young hatched in
a form closely resembling the adult. In one of the Cladocera,
however, Zeptodora (Fig. 449, 3), while development of the summer
eggs is indirect, the winter eggs give rise to free nauplii. In the
Ostracoda the nauplius is peculiar in having a bivalved shell and
all three pairs of appendages uniramous. In all the Copepoda
there is a free nauplius, which, in the parasitic forms, leads a
free existence for a time, and then attaches itself to its particular
host and undergoes retrograde metamorphosis.

In the Cirri pedia, also, there is a free nauplius, the body of which
is often produced into long spines. After several moults, the

Fig. 474.— Cypris-stage of Lepas fascicularis. ah. abdomen ; pa. paired eye ; rj, thoracic
feet ; ua, unpaired eye ; 1, antcnnule. (From Lang's Comparative Anatomy, after Claus.)

nauplius passes into a form called the Cypris-stage (Fig. 474),
characterised by the presence of a bivalved shell, like that of an
Ostracod: the antennules (1) also have become modified into organs
of adhesion by the development of the penultimate segment into a
disc, the antennae have disappeared, and six pairs of swimming-feet
like those of a Copepod have made their appearance : there are
paired compound eyes, and the shell is closed by an adductor
muscle. After leading a free existence for a time, the Cypris-
larva attaches itself by its antennules, aided by the secretion
of cement-glands, and becomes a pupa : the carina, terga, and
scuta appear beneath the shell, and within the skin of the mouth-
parts and legs of the pupa appear the corresponding appendages

xi PHYLUM ARTHROPOD A 599

of the adult. In Lopas the anterior region of the head grows out
into a peduncle. The pupal integument is then thrown off, the
paired eyes disappear, and the adult form is assumed.

In Sacculina a still more extraordinary metamorphosis takes
place. The young is hatched as a nauplius, and passes into a
Cypris-stage. In this condition, after a brief free existence, it
attaches itself to the body of a young Crab, near the base of a seta.
The thorax with its appendages is thrown off, and the rest of the
body is converted into a rounded mass, from the anterior end of
which an arrow-like process is developed. This perforates the
cuticle of the host, and, through the communication thus formed,
the whole body of the parasite passes into the interior of the Crab,
and becomes surrounded by a new cuticle, the old cuticle being
left empty on the outside of the Crab's body. The Sacculina now
sends out root-like processes, grows immensely, and, pressing upon
the body- wall of the Crab, causes atrophy of the tissues: this
allows the now greatly-swollen parasite to project on the exterior
as the tumour-like adult described above (p. 579).

The embryo of Euphausia leaves the egg as a typical
free-swimming nauplius; this passes into what is called the
protozocva-slaye, distinguished by the possession of an elongated,
unsegmented abdomen without appendages. After successive
moults, the rest of the appendages appear-, and the adult form is
assumed. In Mysis (Fig. 459) the nauplius is maggot-like,
and undergoes development in the brood-pouch, emerging in a
condition closely resembling the adult.

The development of the Decapoda presents a very interesting
series of modifications. In two genera of prawns {Petucus and
Lucifer) the embryo leaves the egg as a nauplius, and passes by
successive moults through a protozoaea stage, a zocva-staye, with
segmented but limbless abdomen, and a mysis or schizopcd-slaye
in which it resembles an adult Mysis, having exopodites to all
the thoracic limbs.

In the Crabs the nauplius stage is passed through in the egg,
and the young is hatched in the form of a peculiarly modified
zoaea (Fig. 475, A), with an immense cephalothorax produced into
spines, large stalked eyes, and a slender abdomen. This passes
by successive moults into the meyalopa-staye, (B), which resembles
an adult Macruran, having an extended abdomen with well-
developed pleopods. The megalopa passes by successive moults
into the adult form.

In the Lobster (LTomarus) both nauplius and zosea-stages are
passed through in the egg, and the embryo is hatched in the
mysis-stage with exopodites to all the thoracic limbs. In the
Rock-lobster {Palinurus) and its allies, the newly hatched young
is a strangely modified Mysis-form called a Glass-Crab or Phyllo-

600

ZOOLOGY

soma: it has broad, depressed cephalic and thoracic shields of
glassy transparency : the abdomen is very small and the legs
extremely long and biramous. Lastly, in the Fresh-water Cray-
fish the young resemble the adult in all but proportions and
certain unimportant details of structure. Thus in the series of
Decapoda we get a gradual abbreviation in development, stages
which are free larval forms in the lower types being hurried
through before hatching in the higher.

The larvas of Stomatopoda are grotesque little creatures with a
very large spiny carapace. In Amphipoda there is no free larval

Fig. 475.— Larva) of Crabs. A, Zowa-stago of Maja ; B, Megalopa-stago of Portunus.
It, heart ; a% — a6, aVjdominal segments ; 1, antennule ; tt antenna ; J — VI11, thoracic append-
ages. (From Lang's Comparative Anatomy, after Claus.)

form, but in Isopoda the young leave the egg in the form of a
curious maggot-like modification of the nauplius, which remains
in the brood-pouch until it has attained the adult form.

Ethology.— The Crustacea are remarkable for their very perfect
adaptation to the most various conditions of life : they occur in
fresh-water, in the sea, in brine-pools, in subterranean caves, and
on land : of the marine forms some are littoral, some pelagic, some
abyssal, descending to over 3,000 fathoms. One species of Copepod,
Puntcllina mediterranea , may almost be considered as aerial : it
is described as taking long flying leaps out of the water, after the
manner ef a Flying-fish. Some, like Lobsters, Crayfishes, &c, are

\i PHYLUM ARTHROPOD A G01

solitary ; others, like Shrimps, are gregarious, occurring in immense
shoals. Most of them either prey on living animals or devour
carrion, but, as we have seen, the barnacles are fixed, and feed on
minute particles after the fashion of many of the lower animals,
and the members of more than one order are parasites remark-
able for their deviation from the typical structure of the class
and their adaptation to their peculiar mode of life. In size
they present almost every gradation from microscopic Water-fleas
to Crabs two feet across the carapace, or four feet from tip to
tip of legs.

As to geographical distribution, all the chief groups are cosmo-
politan, and it is only among the families, genera, and species that
matters of interest from this point of view are met with. Fossil
remains are known from very ancient periods. The oldest forms
are usually referred to the Phyllocarida, and occur from the Cam-
brian to the Trias. The shells of Ostracoda are also known from
the Cambrian upwards, and those of Cirripedia from the Silurian.
Peracarida are known from Palaeozoic times, but are rare as
fossils: the earliest Macruran is a shrimp-like form from the
Devonian, while the highly differentiated Brachyura are not
known with absolute certainty until the Cretaceous period.

It was in the Crustacea that the recapitulation theory so often
alluded to was first worked out in detail. Embryology shows
that all Crustacea may be traced back in individual development
to the nauplius, upon which follows some kind of zoaea-stage, many
of the lower forms progressing no further. But in Malacostraca
the zosea is followed by the mysis-stage, which is permanent in
Schizopods, transient in Decapods. It was certainly a tempting
hypothesis that this series of forms represented as many ancestral
stages in the evolution of the class. But we have to remember
that all such free larva; are subject to the action of the struggle
for existence, and have no doubt been modified in accordance with
their own special needs and without exclusive reference to their
ancestors or to the adult species into which they finally
change.

Many Crustacea present instances of 'protective and aggres-
sive characters, i.e., modifications in form, colour, &c, which serve
to conceal them from their enemies or from their prey. Probably
the most striking example is that of certain crabs {Paramithrax),
which deliberately plant Sea-weeds, Sponges, Alcyonarians, Zoo-
phytes, &c, all over the carapace, and are thus perfectly concealed
except when in motion. Another Crab, a species of Dromia, carries
a relatively immense Ascidian or Sea-squirt on its back, and in
another member of the same family the hinder legs are used to
hold umbrella-wise over the back a single valve of a bivalve
shell.

602 ZOOLOGY sect.

Several instances of commensalism occur in the class. The
association of Hermit-crabs with sea-anemones, has already been
referred to (p. 208) : another interesting example is the occurrence
of the little Pea-crab {Pinnotheres) in the mantle-cavity of Mussels.
Other Decapods are found in the intestines of Sea-urchins and
Holothurians, and one genus of Crab lives in a cavity in a Coral,
the aperture being only just sufficient to allow of a due supply
of food and water.

It is in Crustacea that we find the first indication of characters
the purpose of which appears to be their attractiveness to the
opposite sex. The immensely enlarged and highly coloured chelae
of some male crabs {Gelasimus, Fig. 469, S) are said to be used for
attracting the female as well as for fighting. The sound-producing
organs of some Decapoda have probably also a sexual significance.
The Rock-lobster (Palinnrus vulgaris) has a soft chitinous pad on
the antenna, which it rubs against a projecting keel on the sternal
region of the head, producing a peculiar creaking sound ; and
Alpheus, another Macruran, makes noises by clapping together the
fixed and movable fingers of its large chelae. The fact that these
sounds can be produced at the will of the animals seems to show
that the latter undoubtedly possess a sense of hearing, and
that the auditory sac is not merely an organ of the sense of
direction.

Affinities and Mutual Relationships. — That the Crustacea
belong to the same general type of organisation as the articu-
lated worms is clear enough. The advance in structure is
shown in the reduction in number and in the differentiation
of the segments, and in the concrescence of those at the
anterior end to form a head ; in the hardening of the cuticle
into sclerites so as to form a jointed armour; in the jointing
and mobility of the limbs ; and in the differentiation of the
dorsal vessel into a heart by which the propulsion of the
blood is alone performed. The resemblance of the foliaceous
limbs of Phyllopods to the parapodia of the higher worms
is so striking that one can hardly believe it to be without
significance. On the other hand, the absence of transverse
muscles and of cilia, and the replacement of the ccelome by
blood-spaces, are fundamental points of difference from any known
Chaetopod.

As to the mutual relations of the various orders, the Branchio-
poda, with their very generalised structure and parapod-like
limbs, may be taken as the base of the series. The Ostracoda,
Copepoda, and Cirripedia are best conceived as derivatives, along

a PHYLUM ARTHROPODA 003

separate lines, of an ancestral form common to them and the
Branchiopoda. By a differentiation of the post-cephalic limbs,
ami a reduction in the number of segments, the branchiopod-
type easily passes into that of the Phyllocarida, which, though
they nearly conform to the malacostracan type of segmen-
tation, have still marked traces of relationship with lower
groups in the presence of caudal styles and in their bivalved
carapace and foliaceous thoracic appendages. Next to these in
ascending order would come the Cumacea with their cephalic
carapace coalescent with the first three or four thoracic
segments and bounding branchial cavities at the sides of the
thorax, but with — as more primitive features — a biramous character
in some of the thoracic appendages and the absence of the fan-
like tail-fin. Then a little higher, the Arthrostraca (Tanaidacea,
Isopoda and Amphipoda) and the Anaspidacea may be supposed
to have branched off from the main trunk at about the same level,
and may be regarded, on account of a number of resemblances, as
having had a common origin from it. Probably the Anaspidacea
are to be looked upon as more primitive than the other two groups
in view of their less advanced coalescence of the first thoracic
segment with the head, the absence of specialised maxillipedes,
and the biramous character of the thoracic limbs ; but, on the
other hand, they show a higher development in the possession of
the fan-like tail-ifin and the stalked movable eyes such as charac-
terise the Decapoda.

A stage nearer the latter group are the Mysidacea, with their
single pair of maxillipedes, their stalked eyes, their rudimentary
podobranchia? and their fan-like tail-fin ; but these still show some
primitive features, more especially in their incomplete cephalo-
thorax and their biramous thoracic appendages. But without
doubt it is in the Euphausiacea that we find the nearest connections
with the Decapoda. This is shown, in spite of the absence of
maxillipedes, in their completed cephalothorax, their series of
podobranchiaB, and sac-like heart, in addition to their stalked eyes
and fan-like tail-fin.

From the Euphausiacea the Macrura are derivable by the
differentiation of three pairs of foot-jaws and the disappearance of
the exopodites of the legs. In the series of the Macrura we find,
on passing from the Prawns through such forms as Astacus,
Palinurus, and Scyllarus, a gradual shortening of the abdomen,
accompanied by a broadening and flattening of the whole body.
In Birgus, Hippa, &c, this process goes a step further, and the
abdomen becomes permanently flexed under the cephalothorax,
thus leading to the high degree of specialisation found in the
Crabs.

604 ZOOLOGY sm

These relationships are expressed in the following diagam : —

Brachyura

Anomur*a
Macrura

Ar^hroslraca

Anaspid

Tri I obi ra

OsYra

Euphausiacea

Shomahopoda

Phyllocarida

Copepoda

Cirripedfa

Annulaha

Fig. 476. — Diagram illustrating the mutual relationships of the orders of Crustacea.

APPENDIX TO CRUSTACEA.

Class Trilobita.

The Trilobita are extinct Arthropods peculiar to, and characteristic of, the
Palaeozoic rocks : they are specially abundant from the upper Cambrian to the
Carboniferous. They are often found in a wonderfully good state of preservation,
owing to the hard exoskeleton covering the dorsal surface : the greater part of
the ventral region and the appendages were, however, very delicate, and are
preserved only in exceptionally favourable cases.

The body is depressed, more or less oval in outline, and divided into three
regions, the head (Fig. 477, c.xh), the thorax (th), and the abdomen (p), all of
which usually present an elevated median ridge and depressed lateral portions,
whence the trilobation generally characteristic of the group. The head is

PHYLUM ARTHROPODA

605

covered by a carapace or cephalic shield (c.sh), the elevated median region of which,
known as the glabella («//), usually presents three or four transverse grooves,
probably indicating the presence of four or five segments. The lateral regions
of tin- carapace are divided by an oblique line of separation, the frontal or facial
suture (./".••<), into an inner or mesial portion, thefxed cheek (f.c), continues with
the glabella, and an outer free portion, the movable cheek (m.c) ; the latter
bears the large paired compound eye (e). In some cases there is an indication
of a dona) organ, like that of Apus, on the last cephalic segment. Ventrally
the carapace is continued, as in Apus, into a sub-frontal plate (B, s.f.p), to the
posterior edge of which is attached a large labrum or hypostome (Ibv). In many
Trilobites the hypostome bears a pair of small compound eyes. The posterior
angles of the carapace are often produced into spines.

Fm. 477.— Dalmanites socialis, dorsal aspect; B, the same rolled up; C, under-side of
head of Fbacops fecundus. c.sh. cephalic shield ; e. eye ; f.c. fixed cheek ; /.«. frontal
suture ; gl. glabella ; Ihr. labrum ; m.r. movable cheek ; p. pygidium ; pi. pleura ; *.f.j>.
sub-frontal plate ; th. thorax. (After Gerstaecker.)

The thorax (th) is composed of a variable number (2-29) of movably articulated
segments, which are commonly trilobed, consisting of a median region or axis,
and of lateral pleura (pi) often produced backwards and downwards into spines.
The abdomen is covered by a caudal shield or pyaidium (p), formed of a variable
number of fused segments. Owing to the mobility of the thorax, the Trilobites
were able in many cases to roll themselves up like Wood-lice (B). Each of the
segments, with the sole exception of the last or anal, bore a pair of appendages.

The appendages are known only in a few cases. Quite recently a single pair of
antenna? (Fig. 478) has been shown to exist in one species, probably attached to
the sub-frontal plate. Four pairs of biramous leg-like cephalic appendages
have been demonstrated, and the thorax bears slender biramous legs with
endo- and exo-podites, and bearing spiral gills. Similar limbs are present on the
abdomen.

606

ZOOLOGY

The larvae of several species of Trilobites have been found in the fossil state.
In some of these the body consists only of carapace and pygidium in the youngest
stages, and the thoracic segments are subsequently intercalated in regular order.
In other species the earliest stage has the form of a rounded plate, the posterior
portion of which elongates and segments to form the thorax and abdomen.

Fio. 478.— Triarthrus beckl, x2J.

Hy, hypostome. B, second thoracic appendage
the Cambridge Natural History, after Beecher.)

A, ventral surface with appendages; Ep, metastome ;
ippendage. en. endopodite ; ex. exopodite,xl2. (From

Nothing is known of the larval appendages, and none of the stages hitherto dis-
covered can be considered as nauplii.

The precise systematic position of the Trilobites is uncertain, but their
nearest affinities seem to be, on the whole, with such Branchiopoda as Apus :
but the relationship is by no means a close one.

\I

PHYLUM ARTHROPODA

m

CLASS II.-ONYCHOPHORA.

The class Onychophora comprises only the aberrant arthropod
genus Peripatus, with several sub-genera, which differs very widely
in certain important features of its organisation from all the rest
of the Arthropoda, and in some respects enables us to bridge over

Fig. 479.— Peripatus capensis, lateral view. (From Balfour.)

the interval between the latter and some of the lower phyla,
more particularly the Annulata.

General external features. — Peripatus (Fig. 479) is a cater-
pillar-like animal of approximately cylindrical form, and not divided
into segments : it has a fairly well-marked head, and a series (14 —
42, according to the species) of pairs of short stumpy appendages.

Fio. 4S0.— Ventral view of head of Peripatus capensis, with antennas jaws, oral papilla;,
and first pair of legs. (After Balfour.)

The integument is thrown into a number of fine, transverse
wrinkles, and is beset with numerous conical papilla1, each
capped with a little chitinous spine. The head (Fig. 480) bears
a pair of antennae, a pair of eyes, a pair of jaws, and a pair of
short processes known as the oral papilla. The antennae are made

608

ZOOLOGY

up of a number of short rings bearing minute spines. The eyes
are constructed somewhat after the model of the chaetopod eye as
described on p. 478. On the surface of the oral papillae are
situated the apertures of a pair of glands — the slime glands.

Each jaw is composed of two
curved, falciform, pointed,
chitinous plates, the inner
toothed on their posterior con-
cave edge ; they lie at the sides
of the mouth enclosed by a cir-
cular lip. The jaws, as well
as the oral papilla?, are de-
veloped as modified limbs.

The legs are not jointed, but
rows of papillae give them a
ringed appearance ; each con-
sists of a conical proximal part
and a small distal part or foot,
the latter terminating in a
pair of horny claws.

The ventral surface is reddish
in colour, the dorsal darker :
the latter presents an elaborate
pattern — which varies greatly
in different individuals — pro-
duced by minute mottlings of
various colours and tints —
green, red, and brown, and the
arrangement of these in stripes
and bands.

Body - wall and body -
cavity. — The wall of the body
consists of a cuticle, a layer of
deric epithelium with an un-
derlying layer of fine fibres, a
layer of circularly arranged
muscular fibres, and a layer of
longitudinal fibres divided into
a series of bundles. A layer of
epithelium lines the wall of the
body-cavity and invests the
contained organs. Incomplete muscular partitions divide the cavity
into a median and two lateral compartments, in addition to the
pericardium, or space in which the heart is lodged ; the lateral com-
partments send prolongations into the legs. As in the Arthropoda
in general, the body- cavity is a haemoccele, and is filled with blood.
The enteric canal (Fig. 481) begins with a small buccal cavity,

Cr.r y/r/

Fio. 4S1. — Dorsal view of the internal organs of
Peripatus, cm. anus ; ant. antennas ; brn.
brain ; rox. gld. coxal gland of the seven-
teenth leg ; $ gen. male genital aperture ;
ne. co. nerve-cord ; neph. nephridia ; or. pap.
oral papillse ; p/iar. pharynx ; sal. gld. salivary
gland ; si. gld. slime-gland ; stoni. stomach.
(Combined from Balfour.)

PHYLUM ARTHROPOD A

(509

tr.o.

enclosed by the circular lip raised up into a number of papilla-
bearing a few spines, and having on its roof a slight prominence,
the tongue, with a row of small spines or teeth. This is followed by
a thick-walled 'pharynx (pilar.) leading to a narrow oesophagus.
The part which follows, the mescnteron or stomach-intestine, a wide
somewhat thin-walled tube, extends nearly to the posterior end of
the body. The narrower rectum leads to an anal aperture situated
on the last segment of the body. A diverticulum leading back-
wards from the buccal cavity receives the secretion of two long
narrow tubular salivary glands (sal. gld.).

Circulatory system. — The heart is an elongated tube run-
ning through nearly the entire length of the body. It presents a
number of pairs of ostia
arranged segmentally —
i.e., one opposite each pair
of legs. It is enclosed in
a pericardial sinus imper-
fectly cut off from the
general body-cavity by
a longitudinal partition.
The only other vessel is
a median ventral vessel.

The organs of respir-
ation {Fig. 482) are de-
licate, unbranched or
rarely branched tracheal
tubes, lined with a thin
chitinous layer exhibiting
fine transverse striations.
Groups of these open in
little depressions of the integument, the tracheal pits (tr.p.), the
external openings of which are known as the stigmata (tr.o.). The
stigmata in some of the species are distributed irregularly over
the surface ; in others are arranged in longitudinal rows. By
means of these tubes air is conveyed to all parts of the body.

A series of pairs of glands, the coxal glands (Fig. 481,
co.r. gld.), lie in the lateral compartments of the body-cavity, and
their ducts open on the lower surfaces of the legs just outside the
nephridial apertures. Their distribution varies in the two sexes
and in the different species : in one species — P. edwardsii — they
are only developed in the male. A pair of larger glands — the
slime glands (si. gld.) — opening at the extremities of the oral
papillae, may be modified coxal glands : the secretion of these
is discharged in the form of a number of fine viscid threads when
the animal is irritated, and appears to serve a defensive purpose.

The nervous system consists of a brain (brn.) situated in the
head, and of two longitudinal nerve cords (ne. co.) which run parallel

Flo. iS'2. — Section through a tracheal pit and diverg-
ing bundles of tracheal tubes of Peripatus. tr.
trachea; ; tr. c. cells in walls of trachea; ; tr. o.
tracheal stigma ; tr. p. tracheal pit. (From Camb.
Nat. Hist., after Balfour.)

610

ZOOLOGY

SECT.

with one another throughout the body to the posterior end, where
they join together behind the anal aperture. A number of very
fine transverse commissures, more numerous than the segments,
(i.e., than the pairs of limbs) connect the two cords together to
form a ladder-like nervous system comparable to that of some
of the Flat Worms. The cords are very slightly swollen opposite
each pair of limbs : nerve-cells cover them uniformly throughout
their entire length. The brain gives off nerves to the antennae.
The nerves to the jaws are just where the brain passes into the
longitudinal nerve cords.

The excretory organs are nephridia (Fig. 483) of the type of
those of the Annulata, situated in pairs in the lateral compartments
of the body-cavity, and opening on the lower surfaces of the legs at
their bases. Each nephridium consists of a thin-walled closed

internal vesicle, a looped
tube (s.c), and a dilated ter-
minal vesicle (s.), situated
close to the external open-
ing. The salivary glands
and the reproductive ducts
are, as shown by the study
of their development, speci-
ally modified nephridia, as
apparently also are a pair
of glands — the anal glands
— opening close to the anus.
Reproductive organs.
— Peripatus has the sexes
distinct. In the female
there are two tubular
ovaries and two uteri, the
latter in the form of long
curved tubes which unite behind in a median vagina opening on
the exterior on the ventral surface just behind the anus, between
the legs of the last pair or behind them. In the oviparous forms
the opening is situated at the end of a long cylindrical process —
the ovipositor. In some species, connected with each uterus where
it leaves the ovary, are two diverticula — the reeeptaculnm seminis
and receptacuhtm ovorum. In certain species one or other of
these may be absent.

In the male there are two tubular testes, each with a narrow vas
efferens opening by a funnel-like aperture into a vcsicula seminalis ;
this is followed by a long, narrow, coiled vas deferens. The two
vasa deferentia unite together to form a median tube — the ductus
ejaculatorius — opening on the exterior, in the same position as the
vagina of the female. The wall of the proximal part of the
ejaculatory duct is glandular, and secretes a. substance forming

Fio. 483.— PeripatUS capensis, nephridium fruin
the ninth pair of legs, o.s, external opening ;
p.f', internal opening into the lateral compartment
of the body-cavity ; s, vesicle of nephridium ;
s.e. 1, s.e. 2, s.c. 3, s.c. 4, successive regions of
coiled portion ; s.o.t., third portion of nephridium
broken off at p.f from the internal vesicle, which
is not shown. (From the Camb. Nat. Hist., after
Balfour.)

\l

PHYLUM ARTHROPODA

611

complicated cases which enclose masses of sperms to form
spermatophoree.

Development. — The differences between the species of Peripatus
;is regards the segmentation of the egg and the formation of the
germinal layers as described by various observers are very con-
siderable. Nearly all the species are viviparous, but in some the
egg, before the completion of embryonic development, is enclosed
in a well-formed shell, and in certain species the eggs pass out to
the exterior before the emergence of the embryo. In some species
the i'gg encloses a considerable amount of food-yolk, in others the
quantity of food-yolk is small, and nutriment is obtained from the
parent.

In P. iiovcc-zealanduc there is a superficial segmentation. The
first segmentation-nucleus is itself superficial, and segmentation

I ••(, ■. i g i.-iVo early stages in the development of Peripatus novae -Zealand! ae. A, transverse
section of an ovum in which the yolk is nearly covered by the blastoderm (W) ; B, transverse
section of an ovum in which the blastopore (W./>.) is formed. (After Sheldon.)

results in the development of a number of nuclei, each with its
island of protoplasm, which arrange themselves on what is destined
to become the dorsal side (Fig. 484 A), opposite the site of the
future blastopore, while some pass inwards to the central part of
the ovum. The peripheral nuclei multiply rapidly and grow round
the yolk so as completely to enclose it except on a small space
(blastopore) in the middle of the ventral side (B). There a
thickening takes place, and an involution of the lips of the
blastopore results in a sort of invagination, the floor of the
invagination-cavity being formed of yolk with scattered nuclei.

In another species — 1\ capensis — the segmentation has the
appearance of being total ; but the cells, though separated by
fissures externally, are fused internally. A peripheral ectodermal
layer becomes formed, enclosing a central, nucleated, vacuolated

612 ZOOLOGY sect.

mass, except at one point where a small area, the blastopore,
is uncovered. The central mass is the endoderm; the lumen
of the enteron is formed by coalescence of the vacuoles.

In accordance with the smaller size of the ova and the rela-
tionship of the embryo with the wall of the uterus, the American
species show a totally different mode of development. The eggs,
which are almost entirely devoid of yolk, undergo a total and
tolerably equal process of segmentation. Even at this stage the
embryo, which increases considerably in size, appears to receive
nutrient lymph from the uterine wall. When it has reached the
32-cell stage the embryo, according to one observer, consists of a
solid mass closely invested by the epithelium of the wall of the
uterus. It then becomes reduced in size, and owing to exosmosis,
assumes the form of a disk placed in close apposition to one side
of the wall of the uterus. The embryo subsequently loses its
flattened form and becomes somewhat vesicular, the cavity of the
vesicle opening into the cavity of the uterus. From its surface
are given off isolated cells which become applied in part to the
wall of the uterus, and finally unite to form a complete envelope
(amnion) enclosing the embryo. The vesicle then becomes closed
and the embryo raised from the surface of the uterine wall,
the part applied to the latter narrowing so as to form a sort
of stalk, at the base of which is a growth of cells termed the
placenta. Into close relation with this placenta comes a ring-
shaped thickening of the uterine wall, the uterine placenta.

In P. capensis (Fig. 485) proliferation of cells gives rise
to an oval thickening behind the elongated blastopore. The
mesoderm takes its origin at this point and extends forwards
in the form of two germinal lands, one on the right of the
blastopore and the other on the left. These bands undergo a
division into rudiments of segments — the division beginning in
front. The lips of the blastopore meanwhile become approximated,
and fuse throughout the greater part of their length, leaving only
an anterior and a posterior opening ; these go to form the mouth
and the anus respectively. The division into segments soon
becomes well marked. The cavities of the segments give rise
only to the nephridia and the generative ducts, which thus alone
represent the coelome. At the anterior end the head lobes
become distinguishable. The body elongates, and the head and
trunk become differentiated. The limbs now arise as ventro-
lateral outgrowths which are developed from before backwards.

Distribution. — The various species of Peripatus are all terres-
trial, and are found in damp localities, under bark, or dead timber,
or stones. Some twenty-nine species occur in the Neotropical
region ; one in South America ; eight in Africa ; four in Malaya ;
one in New Britain, and eight in Australasia.

Relationships. — Peripatus is the most primitive of existing

PHYLUM ARTHROPODA

613

Arthropods, and presents some striking points of resemblance to
the Ch;etopoda. The development is in the main arthropodan,
especially as regards the mode of segmentation (at least in the
forms with much food-yolk, which are probably the more

Fig. 485. — Three somewhat later stages in the development of Peripatus capensis, showing
the mode of closure of the blastopore and the appearance of the primitive segments. A, stage
in which the blastopore (W.) has the form of an elongated slit ; B, stage in which the blastopore
is closing up in its middle part ; C, stage in which the blastopore has become closed up except
the anterior part which has gone to form the mouth (mo.), and the posterior part which has
formed the anus (an.); the whole embryo has now become strongly curved towards the dorsal
side. (After lialfour.)

primitive), the mode of closure of the blastopore, and of the
development of the mesodermal strands. Arthropodan also are
the relatively large size of the brain and the presence of trachea^,
the character of the heart with its pairs of ostia, together with
the clawed appendages, and the jaws in the form of modified
vol. I it R

C14 ZOOLOGY sect.

limbs. The nephridia on the other hand, and their modification
in certain segments to form the gonoducts, which are ciliated
internally, are annulate in character ; and in all probability the
slime-glands and coxal glands correspond to the setigerous glands
of the Chastopoda. The nervous system is peculiar, and is most
nearly paralleled among the Platyhelminthes and the Mollusca.
Also peculiar, and serving to distinguish Peripatus from the rest
of the Arthropoda, are the large number of stigmata and their
irregular arrangement, the presence of only a single pair of jaws,
and the nature of the cuticle.

CLASS III.-MYRIAPODA.1

The class Myriapoda, including the Centipedes and the Millipedes,
consists of tracheate Arthropoda, which present many features
of resemblance to the Insects. There is a distinct head, bearing
many-jointed antennas, a pair of eyes, and two or three pairs of
jaws; the body is not distinguishable into regions, but consists
of a number of similar segments, each bearing either one pair of
legs or two pairs. A system of air-tubes or tracheae, similar to
those of Peripatus and the Insects, open by a series of stigmata,
usually in considerable numbers, on the sides or lower surfaces of
the segments.

A. — Distinctive Characters and Classification.

The Myriapoda are tracheate Arthropoda in which there is a
head, bearing antennas and jaws, and a trunk made up of a number
of similar segments, provided with leg-like appendages. Groups
of ocelli are present on the head.

Sub-Class I.-PROGONEATA.

Myriapoda in which the genital apertures are situated far
forwards towards the anterior end of the body.

Order 1. — Pauropoda.

Progoneata with ten trunk-segments and nine pairs of legs, one
pair to each segment except the first. Antennas with several
nagella. Tracheae not known. The order includes only the single
genus Pauropus (Fig. 489).

1 As will appear subsequently, the class Myriapoda, as formerly understood,
comprises two groups which are separated from one another by such important
differences that they might very well be looked upon as constituting two distinct
and independent classes. The old class Myriapoda is retained here as a matter
of convenience, and the two constituent groups are ranked as sub-classes.

\I

PHYLUM ARTHROPODA

615

Order 2. — Diplopoda (Chilognatha).

Progoneata with a body composed of a considerable number of
apparent segments, each of which, with the exception of the first
three, bears two pairs of legs. There
are no maxillipedes.

This order includes the Millipedes.

Order 3.— Symphyla.

Progoneata in which there are not
more than twelve leg-bearing segments,
and in which there is only a single
pair of branching trachea?, the external
apertures of which are situated in the
head. Not more than three pairs of
jaws. Feet with two claws.

This order includes only the two
genera Scolopendrella (Fig. 486), and
Scutigerella.

Sub-Class II.-OPISTHOGONEATA.

Myriapoda in which the genital aper-
tures are situated at the posterior ex- Vm. 486. — scoiopendreua

i--^™^.. „f it„ l j„ immaculata. (From IjoucIc-

tremity Ot the body. art, after Latzel.)

Order 1. — Chilopoda (Syngnatha).

Opisthogoneata with numerous (15 — 173) trunk-segments,
each bearing a single pair of legs. Numerous tracheae opening in
pairs of stigmata on the sides of a number of the segments.
Four pairs of jaws, including a pair of poison-jaws. Feet with
a single claw.

This order includes the Centipedes (Fig. 487) and Scutigera.

General Organisation.

External features. — The head in the Myriapoda is as well
marked oft' as in an Insect ; it appears to be composed of about
four fused segments. The antenna' consist sometimes of many,
sometimes of comparatively few segments ; in Pauropus they are
branched. A pair of eyes, situated on the dorsal surface of the
head, consist of aggregations of ocelli except in Scutigera, in which
there are compound eyes, differing, however, in their structure
from those of Insects. There is a movable labium, a pair of
mandibles, and two pairs of maxilla?. The mandibles have no

r r 2

610

ZOOLOGY

SECT.

palps ; one or both pairs of maxilla? usually possess palps ; the
second pair of maxillae are in some groups more or less united
together. In the Chilopoda the first pair of legs of the trunk
are specially modified to act as poison-jaws (maxillipedes), by
means of which the Centipede inflicts its poisonous bite.

-brn

tnal

slom.

Fir. 48V.— Scolopendra. (From
Cuvier's Animal Kingdom.)

Fio. 48S.— LithobiUB forficatUS scon from the
ventral side. ant. antennae ; brn. brain : cox. up.
coxie of ai>pendages ; ft. 15, fifteenth pair of legs ;
int. intestine ; mal. Malpighi in tubes ; inxp.
maxillipedes ; ne. co. nerve cord ; ft's. oesophagus ;
atom, stomach. (From Lcuckart.)

The number of segments in the body varies from 10 to 173. In
the Millipedes the dorsal walls of the segments are very strongly
arched ; in the Centipedes the segments are all dorso-ventrally
compressed, with distinct tergal and sternal shields separated
laterally by intervals of comparatively soft skin on which the

PHYLUM ARTHROPODA

017

stigmata open. In the Chilopoda each segment bears a pair of
jointed legs ; of these the most anterior pair is extended forwards,
as already stated, to form a pair of poison-jaws (maxillipedes), at
the extremity of the pointed terminal joint of which opens the
duct of a poison-gland. In the Diplopoda each segment behind
the fourth or fifth bears two pairs of legs, the four or five most
anterior having only one pair each. In most of the Diplopoda
the appendages of the seventh segment are modified in the male
to form copulatory organs.

The integument and body-wall do not differ widely from
those of Insects (see p. 036). The exoskeleton is a thickened
chitinous cuticle which is calcified in Diplopoda. Odoriferous
glands are present in most Diplopoda on some of the body-
segments, and open on the dorsal surface. Scolopendrella possesses
spinning glands.

The alimentary canal is straight, and is much simpler in
character than that of the Insecta. There are a pair of salivary
glands ; and one or two pairs of Malpighian tubes, having a renal
function, open into the beginning of the hind-gut.

The heart is a greatly elongated tube, divided into a number
of chambers.

The respiratory system resembles that of Insects, which will
be fully dealt with later (p. 626, Fig. 497), consisting of air-tubes
or tracheae. There is one pair of stigmata in
each segment in the Diplopoda, and the branch-
ing tracheal do not anastomose. In the Chilo-
poda the number of stigmata is in most cases
less than the number of segments, and the
tracheal anastomose, often forming longitudinal
trunks which may extend throughout the body.
In Scutigera, the stigmata are unpaired and
dorsal, and lead, not into tracheal, but into air-
sacs or lurujs. In the Symphyla there are only
two stigmata, and these are situated on the
head.

The nervous system is, in accordance with
the form of the body, much less concentrated
than in the Insecta (see below, p. 643). There
is a brain, a pair of oesophageal connectives,
and a ventral nerve-cord consisting of a series
of double nerve-ganglia, one in each segment,
with double connectives between them. The
double character of the ventral cord is much
more distinctly marked in the Chilopoda than in the Diplopoda,
the ganglia are more distinct, and the first three are intimately
united together into an infra-cesophageal mass. A sympathetic
or visceral nervous system is present, at least in the Diplopoda.

Via. 480.-Fauropus
huxleyi. (From

LoiK-kart, after L-.it-
zel.)

618

ZOOLOGY

The sexes are always separate. There is usually an unpaired
gonad with paired ducts. In the Chilopoda the single genital
aperture is situated at the posterior end of the body : in the
Diplopoda and Pauropoda the two apertures are placed far forwards
towards the anterior end.

The ovum, as in most Arthropods, contains a large quantity of
food-yolk. The centrally-placed segmentation-nucleus divides so
as to give rise to a number of nuclei, this division being accom-
panied by a division of the yolk into a number of masses, which,
however, are more numerous than the nuclei. The nuclei then, for
the most part, migrate to the surface, some being left behind in
the yolk. Those that reach the surface, surrounded each by its
little clump of protoplasm, become arranged into a continuous
superficial layer of cells — the blastoderm. On the surface of this

b32^~

Fig. 41H).— Two stages in the development of Strongylostoma, one of the Diplopoda.

A, early stage in the formation of the larva, which already exhibits distinct segments.

B, larva immediately after hatching. (From Balfour, after Metschnikoff.)

appears a thickening, and along the thickening is formed a groove
which may perhaps represent the blastopore, though the endoderm
is formed by direct modification of the cells in the interior of the
yolk. Stomodseum and proctoda3um are developed as invagina-
tions of the surface layer. The thickening of the blastoderm gives
rise to a germinal band in Avhich rudiments of the segments soon
become recognisable. Larval membranes do not occur.

In some of the Diplopoda there is a metamorphosis, such as
will shortly be described in the embryo Insect, and the larva
(Fig. 490, B) has a singular superficial resemblance to an Insect,
owing to the presence at first of only three pairs of appendages on
the anterior trunk region.

Fossil remains of Myriapoda have been found in strata as far
back as the Devonian. The more ancient fossil forms are not

a PHYLUM ARTHROPODA 619

capable of being grouped in the same orders as the living repre-
sentatives of the class, and arc looked upon as constituting at
least two orders, the members of which are all extinct. While the
Progoneata, and, more especially, the Symphyla, show marked
resemblances to the Insccta — more particularly to some of the
members of the order Aptera, the Opisthogoneata have features
connecting them through the Onychophora with the Annulata.

CLASS IV.-INSECTA.

The class of Insects (comprising the Cockroaches, Grass-hoppers,
Dragon-flies, House-flies, Butterflies, Beetles and Bees, with their
many allies) though it is a very extensive one — including as it does
a larger number of species than any of the other classes of the
Arthropoda — is yet characterised by a remarkable degree of uni-
formity, no such extremes of modification occurring as are
observable within the class Crustacea.

Characteristic of all the members of the class is the presence of
three clearly-defined regions — the head, thorax, and abdomen.
There are present on the head, antennae, mandibles, and two pairs
of maxillae, the jaws being variously modified in the different
orders. All Insects have three pairs of thoracic legs, and most
have either one or two pairs of wings likewise borne on the thorax ;
the abdomen is not provided with paired appendages.

The organs of respiration are tracheae similar to those of the
Myriapoda.

The various systems of internal organs attain a ^very high
grade of structure in all the higher groups of Insects. In most
the development is complicated by the occurrence of a strongly-
marked metamorphosis. Insects are terrestrial or aerial, only a
few groups living on the surface of fresh or salt water ; but many
are aquatic throughout their larval condition.

Many groups of Insects are remarkable for the high grade of
their intelligence as compared with the members of other classes
of the animal kingdom. This manifests itself mainly in a number
of instincts, often of a remarkable character, having to do with
the protection and rearing of the young; and in some cases leading to
the formation of communities consisting of individuals of various
different kinds (workers, soldiers, sexual individuals) for mutual
support and protection.

1. Example of the Class — The Cockroach (Periplaneta
orientalis or P. americana).

The Cockroach, familiarly known by the misleading title of
" Black Beetle," is a common pest of kitchens, bakeries, and store-
rooms. It is nocturnal in its habits, rarely coming out of its

620

ZOOLOGY

lurking-places in the day-time, and is almost omnivorous in its
diet. It is a good example of the Insecta, not only on account of its
large size, which renders it convenient for dissection, but also
because of its generalised structure, which makes it a fairly
central member of the class, devoid of any extreme modifications.
Three regions are very distinctly recognisable in the body of
the Cockroach (Fig. 491). In front is the head, elongated verti-
cally, bearing the very long slender feelers or antenna) and the

Fio. 491. — Feriplaneta orientalis, male. A, dors.il view. B, ventral view x2J. ab\ aft*
ab9. ah™, first, second, ninth, and tenth segments of abdomen; ant. antenna;; c. cerci ; cl.
clypeus ; ex. coxa of third leg ; E. eye ; cl. elytra ; ep. epicranium ; /. fenestra ; ft, femur of
third leg ; lid. head ; Ig1. h/2. Ifp. legs ; l.p. labial palp ; ir. labrum ; inn. mandible ; m.p.
maxillary palp ; p.p. style on ninth abdominal segment, internally to which a podical plate
is seen ; flW. (th. in B) tlfl. tltf. segments ;of thorax ; ti. tibia ; tor. trochanter ; ts. tarsus ;
w. posterior wing.

large eyes, and contracted behind to form a narrow neck. In the
middle is the thorax, consisting of three segments, bearing the
three pairs of legs and the two pairs of wings. Behind is the
abdomen, consisting of ten segments covered over above by the
wings in the male. The entire surface is invested by a chitinous
cuticle, which is especially thickened on the head, on certain
parts of the thorax, and on the anterior pair of wings.

The head consists of four parts — the epicranium behind, com-
prising the region between and behind the eyes ; the clypeus,
or portion extending vertically downwards ; and two lateral parts,

PHYLUM ARTHROPODA

(521

the gentr, in front,. The eyes are a pair of reniform black patches

on the sides of the .,

head; each is seen

when examined with a

lens to be divided into

a number of minute

hexagonal areas or
facets, like those in the

eye of the Crayfish.

Borne in sockets just

below the eyes are the

long, slender, highly

mobile feelers or an-

tcniuc, each made up

of a large number of

small segments, the

first three being larger

than the others. In-
ternal to the base of

each antenna is a

rounded white space

— the fenestra — the

nature of which is not

known, but which- may

be an abortive repre-
sentative of the simple

eyes or ocelli found in

most Insects.

Movably articulated with the lower or ventral end of the

clypeus is a broad plate, the labrum or upper lip (Fig. 492, Ibr.)

overhanging the aperture of the
mouth. Below the genae and arti-
culating with the sides both of the
epicranium and of the clypeus are a
pair of stout mandibles (Fig. 492,
md., and 493, man.) which work
rerv horizontally like those of the Cray-
fish; their inner edges are divided
into a number of teeth. Behind
the mandibles are a more flexible
pair of jaws — the first pair of maxilla
(mx.1, max.1). Each maxilla exhibits
a structure comparable to the funda-

Fl°^ral^Po^et.X^naaP: mental type of the appendages of
pendages. etn. one of the cervical the Crayfish : — a basal part or proto-

sclerites ; ey. eye ; gen. gena ; man. ■ •• • . • t> J~

mandible ; niaj., first pair of max- p/Mlte, Consisting 01 tWO Segments

fj&S?" sec°nd ■* °f maxllla; (podomeres), supporting an internal

mx,

Fio. 402.— Mouth parts of "the Cockroach. Ibr. labrum ;
m. mentum ; md. mandible ; mxA. anterior pair of maxilla' :,
rue. and mi. outer and inner divisions of the first and
second pair of maxilla; ; nueg. second maxilla;; pi. labial
palp ; pui. maxillary palp ; xt. stipes ; urn. submentuin.
(From Lang's Comparative Anatomy.)

622 ZOOLOGY

SECT.

ramus or endopodite, and an external ramus or exopodite. The
former consists of two parts : an inner, pointed, hard blade — the
lacinia (mi.), and an outer, softer, more elongated— the galea (me.).
The exopodite forms a palp, the maxillary palp (mn.), consisting
of five podomeres. Behind these are the second maxillae, which
are reducible to the same type, but which have their two basal
segments (those of the protopodites) united together in the
middle line to form two median sclerites, known respectively as
mentum (??i.) and momentum (sm), so that the two appendages
form a sort of lower lip called the labium. The endopodites taken
together constitute what is termed the ligula ; each is divided into
two parts like the endopodite of the first maxillae. The exopo-
dites form three-jointed palps, the labial palps (pi.).

The neck, or narrow region between the head proper and the
thorax, is covered for the most part by a thin flexible cuticle, but
supporting it are eight thickened and hardened patches — the
cervical sclerites (cerv.).

Each of the three segments of the thorax — known respectively
as prothorax, mesothorax, and metathorax — is covered over dorsal ly
by a chitinous plate — the tergum, and ventrally by another — the
sternum. The tergum and sternum of each segment are distinct
from one another, not united into a continuous sclerite as in the
Crayfish. The tergum of the prothorax is larger than that of the
other two segments, and overlaps the neck above. Attached to
the anterior border of the tergum of the mesothorax in the male
are the anterior wings or elytra — a pair of thick opaque plates,
which, in their ordinary position, extend backwards over the
abdomen to some little distance beyond its extremity. Articulating
with the tergum of the metathorax are the posterior wings — a pair
of extremely delicate membranous expansions, which, when at rest,
are folded up longitudinally, like a fan, under the elytra. In the
female of P. orientalis the wings are only represented -by small
vestiges. Attached to the sternum of each segment of the thorax
is a pair of legs. Each leg consists of a stout flattened proximal
podomere or coxa ; a small second, or trochanter ; a third, the femur,
similar to the coxa but narrower ; a fourth slender and spinose, the
tibia ; and finally the tarsus or foot, composed of six very short
segments provided ventrally with patches of setae to give adhesive
power; the last segment (pulvillus)' is armed in addition with a
pair of claws (Fig. 491).

Of the segments of the abdomen the most posterior are over-
lapped by those just in front. Each is enclosed in a dorsal tergum
and a ventral sternum, both of which are thinnish and flexible —
the terga and sterna of succesive segments overlapping one another
from before backwards. The eighth and ninth terga are hidden
from view by being overlapped by the seventh. The tenth is
produced backwards into a thin flexible plate, the posterior border

m PHYLUM ARTHROPODA 623

of which presents a deep notch ; below this is the opening of the
anus, at the sides of which are a pair of small hard plates — the
podical plates ; at the sides of the tergum are a pair of many-
jointed palp-like bodies — the cerci. The sternum of the first
abdominal segment is rudimentary. In the male that of the ninth
bears a pair of short styles. In the female the sternum of the
seventh is very much more prominent than in the male. The
genital aperture is situated on the ventral aspect of the posterior
extremity of the abdomen beneath the anal opening.

When compared with the Crayfish, as regards the external
anatomy, the Cockroach is found to differ (1) in the arrangement
of the segments into regions ; (2) in the form and position of the
appendages. The head and thorax together correspond to the
cephalothorax of the Crayfish, but comprise fewer segments ; the
abdomen contains a larger number of segments. The single pair
of antennae probably correspond to the antennules of the Crayfish
— the antennae of the latter not being represented. On this view
the homologies of the anterior appendages in the two animals may
•be expressed in the following table : —

Crayfish. Cockroach.

Antennules. Antenna?.

Antenna?.. Absent.

Mandibles. Mandibles.

First maxilla?. First maxilla?.

Second maxilla?.
First maxillipedes.
Second maxillipedes.
Third maxillipedes.

Second maxilla? (labium).
First legs.
Second legs.
Third legs.

Representatives of the five pairs of thoracic legs of the Crayfish
would thus appear to be absent in the Cockroach, and evanescent
rudiments, no traces of which remain in the adult, alone represent
in the latter the well-developed abdominal appendages of the
former.

In the living Cockroach respiratory movements are to be
observed, in which the abdomen becomes alternately expanded
and contracted ; these movements bring about the alternate
inhalation and exhalation of air through certain apertures — the
stigmata — at the sides of the body. Two of these are situated on
each side of the thorax, one between the prothorax and meso-
thorax, and the other between the mesothorax and the meta-
thorax. Eight occur on each side in the abdomen between the
terga and sterna of the segments. Just internal to each spiracle
the main air- tube or trachea into which it leads presents an
elastic ring or spiral, acting as a valve for closing the passage.

624

ZOOLOGY

SECT.

aM.cc j

abd.ccoc

cxt.fem.
latere/, stern

long, stern

The principal sets of muscles of the trunk of the Cockroach
are (1) the longitudinal sternal muscles (Fig. 494, long, stern.),
which form a transversely segmented sheet, extending between
adjoining sterna of the thorax and abdomen : (2) oblique sternal
muscles {obi. stem.), confined to the abdomen ; and (3) longitudinal
tergal muscles, best developed in the abdomen. The various
segments of the limbs are capable of being flexed or extended on
one another, as in the Crayfish, by the contractions of special
muscles. The wings are little used, the female Cockroach being
incapable of flight, and the male not a strong flier : accordingly

the wing muscles are not very
strongly developed.

Between the body-wall and
the alimentary canal is a cavity
taking the place of the ccelome,
but in reality forming a speci-
ally developed part of the
blood-vascular system {hcemo-
caile). This is- bounded extern-
ally by an irregular wall, formed
of a mass of polygonal cells
constituting the fat-body.

Digestive system. — The
mouth opens into a buccal
cavity, which receives the -ducts
of the salivary glands (Fig. 495,
sal. gld.). Each gland is divided
into two lobes, each made up
of numerous ramifications. In
close relation to each gland is
an elongated thin-walled sac —
the salivary receptacle (scd. rcc).
The duct given off from the
salivary receptacle joins that
of' the opposite side, and the
median duct thus formed is
joined by a single duct (sal. du.), formed by the union of the two
ducts of the salivary glands ; the common duct thus formed
opens into the buccal cavity (Fig 496). A chitinous fold of
the floor of the mouth forms the lingua or tongue.

From the buccal cavity there proceeds backwards a narrow
cesophagus (oss.), which leads to an elongated saccular dilatable sac
— the crop (cr.). On this there follows the provcntricidus or
gizzard (gizz.) — a pear-shaped chamber with the broad end directed
forwards, its chitinous internal lining raised up into a number of
teeth. A narrow passage leads from this to the stomach —
a wide tube with glandular walls ; from its anterior end are

obi. stem
ier-tji. stern,

Fio. 404. — Ventral portion of the muscular
system of the Cockroach, add. cox. ad-
ductor of coxa ; abd. cox. abductor of coxa ;
ext. fem. extensor of femur ; 1st toy. stern.
first tergo-sternal ; long, stern, longitudinal
sternal ; obi. stern, oblique sternal. (After
Miall and Denny.)

\!

PHYLUM ARTHROPODA

626

given off eight tubular hepatic cmca {hep. cm.) — blind tubes
soiih what narrower than the stomach. The point of junction
of the stomach with the intestine is marked by the presence of

zl.fl Ofl Pulnce Ik * a.id' ■ gbi

oil

Flo. 495. — Semi-diagrammatic view of the internal organs of female Cockroach, dissected
from the left side. The heart is not represented, abdj, abd.*>, first and fifth abdominal segments;
obi. Jin a ;/.^ sixth abdominal ganglion ; an. anus; ant. antennary nerve; bm. brain; cer.
ccrctis ; car. ataca ; coll. colleterial glands ; cr. crop ; gizz. gizzard ; <jon. gonapophyses ;
in/, ijainj. sub-o3sophageal ganglion ; int. intestine ; lb. pip. labial palp ; l.ov. left ovary ;
malp. Malpighian tubes ; tax. pip. maxillary palp. ; od, points to the external opening of the
median oviduct (vagina) ; as. oesophagus ; opt. optic nerve ; we. rectum ; r.ov. right ovary ;
ml. (ild. salivary glands ; *al. rer. salivary receptacle (left) ; sal. da. salivary ducts, indicating
the point at which the median duct of the salivary glands unites with the ipedian duct of
the salivary receptacles ; *p'u: stigmata ; $t. 7, sternum of the seventh segment ; te. 10,
tergum of the tenth segment ; tM, tt&, th^, first, second, and third segments of the thorax ;
thor. gangl, first thoracic ganglion.

very numerous thread-like yellow appendages — the Malpighian
tubes {malp.) — which are the renal organs of the animal. The
intestine {int.) terminates
in a dilated portion — the
rectum {ret.) — the walls of '
which are longitudinally
folded. Of the entire ali-
mentary canal only a small
part — the stomach — with
the appended hepatic caeca,
is of the nature of a mesen-
teron, the region in front
being a stomodamm, and
that behind a proctodeum.

The heart is an elongated tube, closed behind, open in front,
running along the middle line of the abdomen and thorax, imme-
diately beneath the terga. Internally the tube is divided into a

I'i<;. 490. Right salivary glands and salivary recep-
tacle of Cockroach. (After Wall and Denny.)

G26

ZOOLOGY

number of chambers ; its walls are perforated by a series of pairs
of valvular apertures or ostia. Running from the wall of the heart
to the terga are a series of segmentally-arranged fan-shaped
bundles of muscles — the alary muscles (Fig. 522, m.).

Respiration takes place through the instrumentality of. a
system of air-tubes or trachea; (Figs. 497 and 498), opening on the
surface at the stigmata, to which reference has already been made.
These tracheae form a richly ramifying system extending to all
parts of the body. They possess a chitinous internal lining,

supported by means of a spirally-
wound, fibre-like thickening. By
means of this system of air-tubes
air is conveyed throughout the
body to all parts, and there is
thus ensured the rapid and com-
plete oxygenation which the
functional activity of the Insect
requires.

The nervous system consists
of a brain (Fig. 495, brn., and
499, br.), a sub-wsophageal pair
of ganglia (infr. gang.), three
thoracic (Fig. 499, thor. 1, 2, and 3),
and six abdominal pairs of ganglia
(the members of each pair being
united), a system of connectives
uniting the ganglia together, and
a series of nerves given off to the
various parts of the body. The
brain consists of a bilobed mass
of nerve-matter situated in the
head, and divisible into two parts,
anterior and posterior. From the
anterior part is given off on each
side the optic nerve passing to
the eye to become expanded into an optic ganglion, and from
the posterior part the nerves to the antenna?. It is supported by
a chitinous framework — the tentorium. From the brain there run
backwards a pair of oesophageal connectives {conn.), passing, one on
each side of the oesophagus, downwards and backwards to the sub-
cesophageal ganglia. The latter, which are situated between the
submentum and oesophagus, give off a pair of connectives, passing
backwards to the first thoracic ganglia. From the sub-oesophageal
ganglia are given off the nerves to the labrum, the mandibles
and both pairs of maxilla?. The three pairs of thoracic and six of
abdominal ganglia are connected together into a chain by a series
of double connectives ; the last pair of abdominal ganglia, situated

Fig. 497.— Portion of a trachea of a Cater-
pillar. B, C, J), branches ; a, cellular
layer ; 6, nuclei. (Prom Gegenbaur.)

PHYLUM ARTHROPODA

627

in the sixth segment of the abdomen (abd°), are larger than the
others, and supply the segments behind. A visceral nervous system,
ramifying on the anterior part of the alimentary canal, is con-
nected with the two oesophageal connectives by two nerves, which
join above the oesophagus to form a median frontal ganglion.

The organs of special sense are the eyes, the antennae, and
the palpi. The eyes are compound — each being made up of a
large number of simple elements similar to those that go to
make up the eye of Apus (see p. 537). The antennae and palpi,

\ Q, -f — rccnn

tiu> i

ltwr.2

llwr3

Fio. 4!>s. Cockroach. View of Fig. 499.— Cockroach. General view of the

the arrangement of the principal
trunks of the tracheal system.
(After Miall and Denny.)

nervous system, aba"*, sixth abdominal gang-
lion ; ant. antennary nerve ; br. brain ; conn.
oesophageal connective ; inf. sub-oesophageal
ganglion ; opt. optic nerve ; thor* thorp thorp
first, second, and third thoracic ganglia.
(After Miall and Denny. f~.

together with the anal cerci, act as organs of touch. In addition,
certain setae on the antennae appear to have an olfactory function.

Reproductive organs. — In the male the testes (Fig. 500, test.),
are a pair of small bodies which lie in the fourth and fifth seg-
ments of the abdomen immediately below the terga. From these
a pair of delicate tubes, the vasa deferentia, lead to the vesicuku
seminales, two tufts of whitish caeca, which together constitute
what is known as the " mushroom-shaped gland " ; these open
into the anterior end of the ejaculatory duct {duct, ej.), an un-

628

ZOOLOGY

paired tube with muscular walls opening on the exterior imme-
diately below the anus. Around the genital aperture are a series
of chitinous processes, the gonapophyses, which subserve copula-
tion.

In the female there are two groups of ovarian tubes or ovariolcs,
each group or ovary (Fig 501, ov.) consisting of eight. The
ovarioles of each group are united together anteriorly, where they
are connected by a ligament to the dorsal body-wall. Posteriorly
each group is connected with a lateral oviduct (cd.). Each ovarian
tube has a beaded appearance, owing to its containing a row of
ova, which increase in size posteriorly. The two oviducts unite to
open by a median aperture on the sternal surface of the eighth seg-
ment of the abdomen, A pair of unsymmetrical sacs opening
together in the middle of the sternum of the ninth segment

Fio. 500.— Cockroach. Male

reproductive organs, lateral
view. duct. ej. ductus ejacula-
torius with mushroom-shaped
gland ; stem. 7, sternum of
seventh segment of abdomen ;
terg. 7, tergum of the same
segment ; test, testis. (After
Miall and Denny.)

ccH.c/id-

Fig. 501.— Cockroach. Female re-
productive organs, coll. (fid. colleterial
glands ; od. oviducts ; ov. ovaries.
(After Miall and Denny.)

constitute the spermatheca or receptaculum seminis. A pair of
ramifying glandular tubules, the colleterial glandsj^coll. gld.), open
behind the spermatheca. A series of chitinous gonapophyses, which
aid in carrying the eggs, are situated between the female genital
aperture and the anus.

Development. — The eggs are enclosed about sixteen together
in chitinous capsules, the substance of which is secreted by the
colleterial glands. They are laterally compressed, concave on one
side (the future ventral side), convex on the other (the future
dorsal side).

The mature egg in the lower part of the ovary is enclosed in a
follicle composed of a single layer of cells, within which is the thin
chitinoid egg-shell or chorion perforated by a number of micropylar
apertures. After the processes of maturation and fertilisation, the
segmentation-nucleus undergoes division, the result being the
formation of a number of irregular amoeboid cells, which are

XI

PHYLUM ARTHROPODA

629

pen

serf

blast

distributed through a considerable portion of the yolk. Thete
(Fig. 502) all migrate to the surface, where they multiply rapidly
and form a layer, the blastoderm, which becomes thickened along
the ventral surface by the cells being elongated in a vertical
direction. From the blastoderm a number of cells pass inwards
into the substance of the
yolk, where their function is
to convert the yolk-material
into various soluble substances
for the nourishment of the
blastoderm. The ventral thick-
ening of the latter is the ven-
tral plate : its cells prolifer-
ate, and the plate comes to
be several cells thick: in front
it becomes broader — an indi-
cation of the position of the
future head-lobes. At the op-
posite end there is a specially
thickened area of the ventral
plate with a slight depression
on its surface ; the depression
perhaps represents the blas-
topore, since it is from this
point forwards that the for-
mation of the mesoderm pro-
ceeds. The latter is formed
as a longitudinal band which
bifurcates in front in the
position of the head-lobes.

The mode of origin of the
endoderm in the Cockroach
is not known with certainty.
It appears beneath the meso-
derm, in two separate portions,
as a thin layer of cells — one
portion, the anterior, coming
into relation with the begin-
nings of the stomodaeum,
which arises as an invagina-
tion from the surface in the

region of the head-lobes — and the other, the posterior, uniting
with the proctodeum, a similar ectodermal invagination at
the posterior end of the ventral plate. These two rudiments
of the endoderm grow towards one another, and eventually
meet to form a continuous layer destined to form the wall of
the mesenteron. The ventral, plate early becomes divided by a

blast

yk.c

Flu

502. — A— D, successive stages in the seg-
mentation of the ovum of an Insect ; blast.
blastoderm ; peri, peripheral protoplasm ; xty.
segmentation cells ; yk. yolk ; yk\ c. yolk-cell*.
(From KorscheltandHeider, after Blochmann)

VOL. I

s s

630

ZOOLOGY

number of narrow transverse lines which indicate the boundaries
of the future segments.

Rudiments of appendages (Figs. 503, 504) appear on the head
and thorax, and a series also appears on the abdomen ; all of the
latter, however, subsequently disappear with the exception of the
last pair, which give rise to the cerci. The segment on which the
rudiments of the antennas appear is at first post-oral in position,

Fig. 503 —Ventral plate of embryo Cockroach
(Blatta germanica),--isolated from the
yolk. as. amnion and serosa ; at. antennary
lobe ; r.gl. brain ; r.p[. caudal plate ; lb.
labrum ; md. mandible l~mxl, mx", first and
second maxillpe ; p1, %&, jfi, legs. (After
Wheeler.)

Fig. 504.— Embryo Cockroach just after the
rupture of the amnion and serosa, lateral
view of entire egg. Letters as in preceding
figure. In addition, at, fatty body ; ast.
caudal styles ; b. cephalic end of yolk ; oc.
eye. (After Wheeler.)

but subsequently becomes fused with a pre-oral segment {pros-
tomium), so that the antennas acquire their permanent pre-oral
position only secondarily. The prostomial segment, the antennary
segment, a segment devoid of appendages, the segment bearing
the rudimentary mandibles, and those bearing the two pairs of
maxillae, all unite to form the head of the adult.

Then follows the appearance of the larval membranes. On
either side arises a fold of the blastoderm ; and the two folds
grow inwards and eventually unite- over the body of the embryo,

n PHYLUM ARTHROPOD A fi31

forming a complete two-layered covering for it The outer layer
is termed serosa, the inner amnion.1

Each of the two mesoderm bands undergoes transverse division
into a series of segments, which become hollow and are then
closely applied to one another, eventually coalescing, so that the
cavities of all of them unite to form the coelome, the outer walls
becoming applied to the ectoderm to form a somatopleure, or lamina
consisting of somatic layer of mesoderm and of ectoderm ; the
inner being applied to the endoderm to form a splanchnopleure, or
lamina consisting of splanchnic layer of mesoderm and endoderm.
The body-cavity of the adult (hamiocaile) is not derived from the
coelome of the embryo.

The ventral plate gradually grows upwards at the sides, and
eventually its borders meet and unite along the dorsal middle line,
the entire yolk thus becoming enclosed by it.

The ventral nerve-chain is developed from a groove of the
ectoderm, bounded by thickenings which become detached from
the surface-ectoderm and form the chain of ganglia. The brain
is developed from a pair of ectodermal thickenings. That part
which is developed in the prostomial region — the archicerebrum —
becomes united with that developed in the following two segments
to form the completed brain or syncerebrum.

It can hardly be said that the Cockroach undergoes a metamor-
phosis, . the young Insect when it escapes from the egg differing
from the adult only in its smaller size and in the absence of
wings, which grow out subsequently from the terga of the meso-
and metathorax. Between its hatching and its complete develop-
ment the young Cockroach undergoes no fewer than seven
" moults " or ecdyses, in which all the chitinous parts become
thrown off and renewed.

2. Distinctive Characters and Classification.

The Insecta are air-breathing Arthropoda in which the
body consists of three well-marked regions — head, thorax, and
abdomen ; the head devoid of external segmentation, nearly
always bearing compound eyes, a pair of antennae situated on
the prostomium, mandibles, and two pairs of maxillae; the
thorax of three segments each bearing a pair of legs, and the
second and third usually bearing wings ; the abdomen composed
of a varying number of segments (7 — 11), which are devoid of
appendages in the adult condition. A liver is absent, but salivary
glands are always present. There is an elongated tubular heart,

1 This term is derived from one used in the Vertebrata, in which there is
an analogous membrane, occupying, however, a dorsal instead of a ventral
position as regards the body of the embryo.

S S 2

632

ZOOLOGY

divided into eight chambers, situated in the abdomen ; the vessels
themselves are not highly developed. The Insecta are, almost
without exception, air-breathers, and the organs of respiration

take the form of branching
tubes, the tracheae, by means
of which air is conveyed
to all parts of the body.
The nervous system and
sense-organs are highly de-
veloped. The excretory organs
are a number of blind tubes,
the Malpighian tubes, ap-
pended to the intestine. The
sexes are separate ; develop-
ment is sometimes direct,
more usually complicated by a
metamorphosis.

Order 1. — Aptera (Collem-
bola and Thysanura).

Insecta in which the wings
are absent, and the surface is
covered either with scales or
hairs. Eyes are sometimes
absent ; sometimes there are groups of ocelli ; sometimes com-
pound eyes. The segments of the thorax are not fused. Some
progress by running, others by springing movements effected by
a special springing apparatus on the ab-
domen. Some have elongated, many -jointed
filaments or cerci at the extremity of the
abdomen. Development is direct.

This order includes the Spring- tails
(Podura, Fig. 506), and "Silver-fish"
(Lepisma, Fig. 505).

Order 2. — Orthoptera.

Insects in which there are two pairs of
wings, of which, in most cases, the anterior
pair are hard and tough, and the posterior
pair delicate and transparent. The parts of
the mouth are masticatory. The prothorax
is not united with the other segments of

the thorax. Development is direct, or there is a gradual and
incomplete metamorphosis.

Fit;, .in... Lepisma. (After Guerin and
Pcrcheron.)

Fia. 500. —Podura. (After
Guerin and Percheron.)

XI

1MIYLUM ARTHROPOD A

633

This order includes Earwigs, Cockroaches, Stick- and Leaf-
insects, Grasshoppers, and Locusts (Fig. 507).

Fir; ".i 17. I.ocusta. (From Cuvicr's
Animal Kiviidom.)

Fio.508.— Ephemera (May-fly)and
larva. (After Guerin and Pcrcheron.)

Order 3. — Nelu&qptera.

Insects with two pairs of netted membranous wings. The parts
of the mouth are adapted for biting. The prothorax is free from
the other segments of the thorax. The metamorphosis is some-
times complete, sometimes incomplete.

This order includes Termites (" White Ants "), May-flies (Fig.
508), Dragon-flies, Ant-lions, and Caddis-nTes. '

Order 4. — Hemiptera.

Insects in which wings are usually present, sometimes similar,
sometimes dissimilar, and in which there is a jointed suctorial

Fio. 509.— Aphis rogae and larva. (From Cuvier's Animal Kingdom.)

634

ZOOLOGY

rostrum formed from the labium, enclosing the jaws in the form of
piercing organs. The prothorax is free from the other segments
of the thorax. The metamorphosis is incomplete.

Fio. 510.— Cicada. (After Guerin and Percheron.)

This order includes Bugs, Water-bugs, Lice, Scale-insects, Plant-
lice (Fig. 509), and Cicadas (Fig. 510).

Order 5. — Diptera.

Insects provided (except in the Fleas) with a single pair of
transparent membranous wings,
representing the anterior pair of
other orders. The mouth parts are

Fig. ."il I . Culex (mosquito) and larva.
(After Guerin and Percheron).

Fio. 512.— Bot-fly ofj the^horse (Gastro-
philus equi). a, -mature insect ; b,
egg attached to a hair ; c, d, and e, stages
in the larval development. (After Brehm.)

adapted for piercing and sucking. The prothorax is fused with the
other segmentsof the thorax. There is a complete metamorphosis.

PHYLUM ARTHROPODA

635

This order includes Fleas, Gnats, and Mosquitoes (Fig. 511),
Bouse-flies and Blow-Hies, Bot-flies (Fig. 512), Crane-flies, and
' Daddy-long-legs."

Via'. 513. — Butterfly (Pieris), with caterpillar and chrysalis stages. (After Querinand Percheron.)

' • ; ■

Order 6. — Lefidoptera. .

' Insects with both pairs of- wings' well developed and covered
with scales (modified hairs). The maxillae are modified to form
an elongated sucking tube, which can be
rolled up spirally ; the other parts of the
mouth are rudimentary, with the' ex-
ception of the labial palpi. The pror
thorax is fused with the meso^horax.
The metamorphosis is complete.

This order includes Butterflies (Fig.
513) and Moths.

Order 7.— Coleoptera.

Insects in which the anterior pair of
wings take the form of hard horny
wing-cases, or elytra, which, when at rest,
are folded up along the back and cover
over the fplded-up membranous pos-
terior wings. The prothorax is movable
on the other segments. The jaws are fully
developed, and adapted for biting and chewing. The metamor-
phosis is complete.

This order includes the true Beetles (Fig. 514), ^

Fio. 514.- Beetle (Crioccns)
with larva. (After Guerin
and Percheron.)

636 ZOOLOGY sect.

Order 8. — Hymenoptera.

Insects in which both pairs of wings are present and membran-
ous. The mouth parts are adapted both for biting and licking.
The prothorax is united with the other segments of the thorax.
There is a complete metamorphosis.

Included in this order are Bees (Fig. 531) and Wasps, Ants
(Fig. 532), Gall-flies, and Ichneumons.

Systematic Position of the Example.

The Cockroach is a member of the order Orthoptera and of the
sub-order Orthoptera gcmiina, which comprises all the members of
the order with the exception of the aberrant group of the Earwigs
(sub-order Dcrmaptera). Of the Orthoptera genuina there are
three divisions, the Gursoria, to which the Cockroaches belong;
the Gressoria, comprising the Mantidce and Phasmida\ or Stick- and
Leaf-insects and their allies ; and the Saltatoria, including the
Grasshoppers, Locusts, and Crickets. The division Gursoria com-
prises the single family of the Cockroaches (Blattida'), characterised
by the deflexed head, the flat oval body, the large prothoracic
tergum, the long antennae, the three pairs of legs similar, with
large coxae entirely covering the sternal surface of the thorax, the
five-jointed tarsi, and the presence of anal cerci. Pcriplaneta
belongs to a section of the family distinguished from the rest by
the femora being spiny underneath, and by the valvular character
of the last sternum in the female.

3. General Organisation.

The exoskeleton of the Insecta (Fig. 515) consists of a chitinous
cuticle (cut.), which varies in hardness and thickness in different

Insects and in different parts of the
body of the same Insect, but is very rarely
calcified. Frequently it presents hexa-
gonal markings ; sometimes it is perfor-
ated by numerous pores; sometimes it
is covered with thin scales; in many
cases it is developed into tactile hairs or
setos, which may be scattered over the
no. sis-section through the D°dy> or maybe located only on certain
integument of an insect. 0f the appendages — the antennse, the

brme. basement membrane; ... i 1 i • 1 i • iji

cut. layers of the cuticle ; maxillary and labial palpi, and the tarsi
(After M^iiTnd5 Denny. >**" of the legs. In some, glands are present
in the integument — odoriferous, honey-
secreting, or wax-forming glands ; poison glands are present in
connection with an abdominal sting in certain Insects ; spinning
glands, forming a silky material, are confined to the larvae.

XI

PHYLUM ARTHROPODA

637

—Jilt

The head presents no trace of segmentation, but the history of
its development indicates that it may be looked upon as composed
of a prostomium and about five segments, intimately united
together. It varies a good deal in shape, but always presents the
regions that have already been described in the case of the Cock-
roach. Of these the cpicranium is the most extensive ; the clypcus,
situated in front of it, supports the labrum ; the genm are situated
laterally, and a median piece, the gula, occupies the middle of the
ventral surface. Some-
times the head is sunk
within the anterior part
of the thorax ; sometimes
it is free from the latter ;
and there may be. as in
the Cockroach, a short
narrow region or neck,
covered with soft skin,
supported only by iso-
lated cervical scleritcs, on
the ventral aspect.

The three segments of
the thorax — pro-, mcso-,
and meta - thorax — are
usually firmly united to-
gether ; but in some
Insects the prothorax is
movable upon the other
segments : it is usually
the smallest of the three
segments. In each the
exoskeleton consists of
doi'sal or tergal and ven-
tral or sternal elements,
sometimes separate from
one another laterally,
sometimes united to-
gether in such a way as
to form complete rings
round the segments.
Laterally projecting processes or pleura are sometimes developed.

The abdomen contains from seven to eleven segments, enclosed
in tergal and sternal shields. In some Insects the first abdominal
segment is united with the thorax so as to appear to belong to
the latter region.

The appendages of the head are four pairs, as in the Cock-
roach ; but a considerable variation is observable in the different
orders, especially as regards the jaws. In certain of the Aptera

Pio. :.1C— A, mouth parts of the Honey-bee (Apis
mellificai ; ]$. the two pairs of maxilla;, aw. eye;
a. antenna; c. cardo ; ep. epipharynx ; Ibr. labrum ;
li. ligula ; m. men turn ; mm, BMC1, first pair of maxill;c ;
mil. mandible ; pi. labial palpi ; pm. palp of the first
pair of maxilla; ; pry. paraglossa ; tui. submentum ;
*tin. stipes of the first maxillae. (From hang.)

638

ZOOLOGY

SECT.

an additional pair — the so-called maxillula; — occur between the
mandibles and first maxillae. In a few eyes are absent. Most
have large compound or faceted eyes, and many have simple eyes
or ocelli as well ; in a few groups the latter are alone present.
The antenna? vary in shape in different groups and sometimes
even in the sexes of the same species. They may be tapering,
moniliform, club-shaped, pectinate, or plume-like. In addition to
functioning as tactile appendages they bear the olfactory setae, and
there seems reason to believe that they act also as organs con-
cerned in the maintenance of the equilibrium of the body. The

A &

uuc,

Fig. 517.— Mouth parts of the Diptera. 'A, of Tahanus ; B, of Culex. Lettering as in pre-
ceding figure : py. hypopharynx ; oc. ocellus. (From Lang.)

mandibles are always one-jointed, and differ from those of the
Crustacea in never being provided with a palp. An arrangement
of the mouth-parts adapted for biting or chewing has already been
described in the case of the Cockroach : this type is characteristic of
the order Orthoptera, to which the Cockroach belongs, and a very
similar type characterises the Coleoptera. In theHymenoptera(Fig.
516) the mouth-parts are adapted both for biting and for licking
and sucking ; the mandibles (ind.) and maxillae (mx1.) are sharp and
lancet-like, the middle part of the labium is produced into a long
median tongue {ligula, li.) at the sides of which are a pair of
accessory tongues or paraglossce (prg.). In the Hemiptera there
is a proboscis formed from the labium and enclosing the stylet-like

PHYLUM ARTHROPOD \

039

mandibles and maxilla'. I In the Diptera (Fig. 517) the mandibles
(md-.), usually not developed in the males, are biting or piercing
organs, while the basal parts of the labium form a proboscis (mx2.)
enclosing ;l spine or seta (hp.) — which is a process from the hypo-
pharynx, a chitinous process on the floor of the mouth — and
sometimes stylet-like maxillae (mx1.). In the Lepidoptera (Fig.
518) the mandibles are aborted in the adult, and the maxillae are
developed into elongated half-tubes, which when applied together
form a complete tube (sr.) capable of being coiled up in a spiral
manner under the head,
the extremity provided
with hooks or spines for
rupturing the nectaries of
flowers.

Appendages of the
thorax. — Each of the seg-
ments of the thorax bears
a pair of five-jointed legs ;
the terminal section or
tarsus being made up of a
number of short segments
and ending in a pair of
claws, often with an ad-
hesive pad or sucking disc
between them. In accord-
ance with variations in
the uses to which .they are
put, considerable differ-
ences are observable in the
form of the legs in different
groups of Insects. In most
they are adapted for walk-
ing, and are long and
slender; in some they
are expanded to enable
them to act as swimming
paddles ; in some the first

pair are prehensile, and develop a sub-chelate extremity ; in
others, again, the legs, or the first pair of them, are stout and
adapted for burrowing. In addition to the legs the meso- and
meta-thorax may each bear a pair of wings. The wings are thin
transparent expansions of the integument of the body, supported
by a system of branching ribs or nervures consisting of chitinous
material with branches of the tracheae, nerves, and tubular diver-
ticula of the body-cavity. In most Lepidoptera the wings are
opaque, owing to their being covered with numerous overlapping
microscopic scales, to which the various colours of the wing' are

Fig. 518.— Mouth parts of the Lepidoptera. B, the
second uiaxillie. Lettering as in preceding figures :
'pi. labial palp ; pm. palp of the anterior maxilla; ;
m: sucking tube. (From Lang.)

640 ZOOLOGY sect.

due. In some insects — e.g., Beetles and Orthoptera — the posterior
wings alone are delicate and membranous, the anterior pair being
converted into hard or tough cases — the elytra — which when
folded up cover over and protect the delicate posterior wings.
In some Beetles the elytra are permanently united together along
the back of the Insect. In some Insects (Bugs) the anterior wings
are chitinous at the bases only. In the Diptera the anterior
wings alone are developed, the posterior being represented by
vestiges — the halteres or balancers. In the Strepsiptera, or Bee-
parasites, an aberrant group of Neuroptera, on the other hand,
it is the anterior pair that are vestigial. In some Insects
(Spring-tails, Lice, Fleas) wings are entirely absent in all stages.
In others again they are present in one sex — usually the male —
and absent or vestigial in the other. In the Aptera there is
no vestige whatever of wings at any stage, and this, taken in
connection with the simplicity of the structure in other respects,
seems to indicate that in these Insects we have to do with the
descendants of a primitive group in which wings had not yet
become developed.

The segments of the abdomen are entirely devoid of paired
appendages in the adult condition (except in the Thysanura),
though vestiges of them may be present in the young at an
early stage. Each segment is enclosed in dorsal tergal and
ventral sternal plates, which usually remain separate laterally, but
may be united. At the extremity of the abdomen there are
frequently appendages which are perhaps of the nature of limbs,
having the function of stings, ovipositors, and genital processes.

Haemocaele. — The cavity intervening in an Insect between the
body- wall and the various internal organs does not correspond,
as already explained (p. 631), to the coelome of other groups ; but is
found, when we study its mode of development, to be a hcemoccele
— an extended part of the blood-vascular system. The coelome
is apparently represented only by the lumen of the reproductive
organs.

A fat-body is always present, either in the larval condition or
throughout life. It consists of a mass of polygonal cells bounding
the haemoccele externally. When young the cells are nucleated
and possess a protoplasmic body. At a later stage a fluid loaded
with minute granules takes the place of the protoplasm, and
crystals containing uric acid are formed. These crystals afterwards
become absorbed ; their appearance and subsequent absorption
would seem to point to the probability that the fat-body
is concerned in separating out nitrogenous waste matters, which
subsequently reach the exterior through the Malpighian tubes.
Its chief function is to serve as a reserve-store of nutrient material.

Digestive system. — Some Insects do not feed in the adult
condition, and when this is the case the mouth may be absent,

XI

PHYLUM ARTHROPODA

641

as, for example, is the case in the Day-flies (Ephemeridcc). When
a mouth is developed, as it is in the vast majority of Insects,
it is situated on the lower aspect of the head, bounded in front
by the labrum, and behind by the labium. It leads into the
buccal cavity, into which open the ducts of a pair of salivary
glands, each of which often has associated with it a thin-walled
sac or salivary receptacle. Also in the neighbourhood of the mouth
in such larval Insects as spin a cocoon, the ducts of a pair of
spinning glands open. A projection
of the roof of the mouth-cavity (epi-
pharynx) is present in some Insects ;
in others it is replaced by a projec-
tion from the floor, the hypopharynx or
lingua.

The alimentary canal is nearly al-
ways considerably longer than the
body ; it is longer in vegetable-feeding
than in carnivorous forms. The mouth
leads into a long, narrow passage — the
oesophagus (Figs. 519 and 520 ce.) —
which dilates behind into a crop {in)
for the storage of food. The place of
this in sucking Insects is taken by a
stalked sac, usually termed the sucking
stomach. The essential processes of
digestion are carried on in an elongated
chamber with glandular walls — the
stomach (cd) — which may be divided
into several parts. Sometimes between
the crop and stomach is intercalated
a muscular-walled chamber, frequently
containing chitinous teeth, the pro-
vent7-iculus or gizzard (pv). Appended
to the stomach at its anterior end are,
in many Insects, a varying number of
tubular blind pouches, the hepatic cmca.
At its junction with the small intes-
tine, or further back, there open a
number (from 2 to over 100) of narrow

tubular appendages, the Malpighian tubes (vm), which are the organs
of renal excretion. In the cases in which the development of the
alimentary canal has been traced, it has been found that the
Malpighian tubes mark the point where the mesenteron passes
into the proctodeum, and it is assumed that this holds good
generally. The lumen of the tubes is sometimes filled up with
cells. In some insects, the Malpighian tubes open into a paired
or unpaired sac — the urinary bladder. The intestine is usually

Fiii. 519. — Digestive apparatus of a
Beetle (Carabus auratus).
ad, anal glands : alt, their muscu-
lar api>endages ; cd, stomach ;
td, hind gut ; in, crop ; k, head
with mouth parts ; ce. oesopha-
gus ; pv. proventriculus ; vm.
Malpighian tubes. (From Lang,
after Dufour.)

642

ZOOLOGY

SECT.

elongated, and its posterior portion (ed.) is dilated to form a wide
rectum (r.), which opens on the exterior by an anal aperture
situated on the ventral side of the last segment of the abdomen.
Anal glands (ad.), producing an odoriferous secretion, often open
into the rectum.

The tracheal system (Fig. 520) communicates with the ex-
terior through a number of apertures— the stigmata (st)— which

vary in the details of
their arrangement in
the different orders.
They are always pro-
tected against the
entry of foreign par-
ticles by some means
—either by being
surrounded by spe-
cial bundles of hairs,
or by being provided
with a special sieve-
like membrane. In
most cases they are
capable of being
closed by muscular
action. In some In-
sects, mainly those
adapted for active
flight, such as the
Hymenoptera, the
tracheal system is
dilated in certain
parts of the body to
form comparatively
large aio'-sacs or air-
reservoirs (tb). In the
aquatic larvae of some
Insects there is a
series of soft external,
simple or divided,
processes — the tracheal gills (Fig. 521) — attached to the abdominal
segments and richly supplied with tracheae, which have no com-
munication with the exterior.

The blood-vascular system is, in comparison with the other
systems of organs, not very highly developed, the need of an
elaborate system of vessels being greatly diminished by the way in
which all the tissues and organs are supplied with oxygen through
the system of trachea?. The blood is colourless or faintly yellowish
or greenish, and contains colourless corpuscles. A- contractile

Fic<. 520. — Nervous, tracheal, and digestive systems of the
Honey-bee. a. antenna; au, compound eye; &], 62, 63,
the three pairs of legs ; an, stomach ; td, hind-gut ; hm,
honey stomach (crop) ; rd, rectal glands ; st, stigmata ;
tb, vesicle of tracheal system ; vm, Malpighian vessels.
(From Lang's Comparative Anatomy.)

XI

1 'I IV I.I M ARTHROPOD A

643

dorsal vcssrl or heart (Fig. 522) extends through the abdomen — and
sometimes thorax — immediately below the tcrga. Its cavity is
divided internally into a series usually of eight chambers by
a system of valves. In its walls are a series of slits or ostia, by
which a communication is effected between the internal cavity and
a surrounding pericardial sinus. In front the heart gives origin to
a main vessel, or aorta (a), by means of which the blood is
conveyed throughout the body to enter a system of sinuses in free
communication with the general body-cavity, from the various
parts of which it finds its way back to the pericardial sinus.

The nervous system (Figs. 520 and 523) is on the same general
plan as in the Crustacea. There is a double supra-cesophageal

Fig. 521. — Thorax and anterior abdominal segments
of a larval Ephemerid with tracheal gills.
HF, hind wings ; tri, tr-, to*, tracheal gills ; tl,
longitudinal tracheal trunks; VF, fore wings.
(From Lang's Comparative Anatomy.)

FlU. bfi. — Heart of Cockchafer
(Melolontha). a aorta;

//(, ;;(, alary muscles. (From
Gegenbaur.)

ganglion or brain, a sub- oesophageal ganglion, also double, and a series
of thoracic and abdominal pairs of ganglia, which are closely united
together in the middle line. The brain is relatively large in the
higher Insects, and is divided into several lobes. It gives off nerves
to the antennae, the ocelli and the labrum, and on each side arises
a large lobe — the optic ganglion — on which the compound eye
rests. A pair of oesophageal connectives pass backwards on either
side of the mouth from the brain to the sub-oesophageal ganglia*
These connectives are very short, and, as a consequence, the brain
and sub-oesophageal ganglia are closely approximated. From the
latter there originate nerves to the appendages of the mouth — the
mandibles and the two pairs of maxilla:. There are sometimes three
pairs of thoracic, and as many as eight of abdominal ganglia in
the adult insect; but in many* cases there is a greater or. less

644

ZOOLOGY

SECT.

degree of concentration of the ventral ganglionic chain (Fig. 523),
and in some of the Diptera this reaches such an extreme that all
the ventral ganglia, with the exception of the sub-oesophageal, are
united into one continuous elongated mass. The Insects, like the
higher Crustacea, possess a visceral or sympathetic nervous system,
connected with the oesophageal connectives, and passing backwards
on the oesophagus and crop.

The most highly-developed organs of special sense are the
large compound eyes. The surface of the compound eye is
marked out, as in the case of the Crayfish, into a great number of
minute hexagonal facets, each of which represents one of the

Flu. 523 — Nervous systems of four species of Diptera to illustrate various degrees of concentration.
A, non -concentrated nervous systems of Chironomus plumosus with three thoracic, and
six abdominal ganglia ; B, nervous system of Empis stercorea with two thoracic and five
abdominal ganglia ; C, nervous system of Tabanus bovinus, with one thoracic ganglion
and with the abdominal ganglia closely approximated ; O, nervous system of Sarcopliaga
carnaria, with all the ganglia of the ventral chain united together with the exception
of the sub-oesophageal. (From Lang's Comparative Anatomy.)

elements (ommatidia) of the eye. Of these there may be as
many as 28,000 (Dragon-fly). When the eye is examined in
section, each ommatidium is found to consist of a cornea-lens — the
outer surface of which forms the facet, a crystalline cone, and a
rhabdome. The crystalline cone is not always developed, its place
being taken in the eyes of some Insects by four crystal cells. The
rhabdome is an elongated rod. Beneath the rhabdomes is a
fenestrated membrane, beneath which, again, is a dense plexus of
nerve-fibres. Nerve-fibres pass through the fenestrated membrane
and terminate in a delicate sheath which incloses each rhabdome,
the sheath, together with the nerves that end in it, constituting
the retinula. Pigment surrounds the crystalline cones and retinulse.

PHYLUM ARTHROPODA

ci;,

Ficj. 524.— Section through tho ocellus of a young
Dytiscus larva, ct. cuticle : <jk, cells of the vitreous
body ; hy, epidermis ; I, cuticular lens ; no, optic
nerve ; re, retinal cells ; it, rods. (From Lang, after
Grenacher.)

The ocelli, or simple eyes (Fig. 524), consist of a bi-convex
transparent thickening of the cuticle— the lens — and beneath it
of a group of specially modified epidermal cells. Some of these,
situated beneath the
lens, form a transparent
mass, the vitreoits body,
another set of elongated
cells being arranged to
form the retina.

The antennae and palpi
are the organs of touch,
and these appendages
seem to be also the seat
of the olfactory sense. A
number of minute pro-
cesses sometimes sunk
in pits, and each having a
special nerve-plate con-
nected with it, are regarded as being specially concerned with this
sense ; and similar processes or pits on the maxillae and the
epipharynx are perhaps connected with the sense of taste. The
results of experiments on the action of the antennae seem to
lead to the conclusion that one of their -main functions is to

act as- organs for regulat-
ing the equilibrium of the
body.

, Peculiar nerve-endings,
supposed to be auditory,
have been found in various
parts of the body. Each
consists of a ganglion-cell
(Fig. 525, gz.) giving off a
process which is enclosed
in an elongated tube, and
which ends externally in a
slender rod (sc). Groups
of these are associated to-
gether to form the auditory
organ.

In certain Insects — the
Fireflies and Glowworms,
belonging to the order Col-
eoptera, occur luminous organs for the production of light.

Sounds are emitted by many Insects, and are produced by a

variety of different means. Often the sound is the result of the

rubbing together of opposed rough surfaces of the integument.

The chirp of the Grasshopper, for example, is produced by the

VOL. I T T

-tt

Fk:. bio.— Chordotonal (auditory) organ in the tibia
of Xsopteryx apicalla. bk; blood-corpuscles ;
e. integument ; es. terminal fibrous strands at-
tached to the integument ; gz, nerve-cells : *c ter-
minal rods ; tr. trachea. (From Lang, after v.
Graber.)

646

ZOOLOGY

rubbing of the femur of the last pair of legs over a series of ridges
on the anterior wing, and that of the Locust by the rubbing against
one another of the roughened basal parts of the first pair of wings.
In other cases the sound results from the rapid vibratory move-
ment of the wings ; this is the case with the buzzing of many
Diptera and Hymenoptera. Again, the humming sounds charac-
teristic of many of the last named order are produced partly by
the vibrations of the wings in flight, partly by the vibration of
leaf-like appendages in the tracheae set in motion by strong
expiratory currents of air. The loud shrill note of the Cicada is
produced by the rapidly recurring contractions of the fibres of a
muscle inserted into a stiff chitinous membrane,' the result being

Fio. 526.— A, female and B, male sexual apparatus of the Honey-bee ; ad, accessory glands ;
de, c >mmon ejaculatory duct ; r/d, poison-glands ; gb, poison-vesicle ; ks, bulb of the stinging
appa'/atus ; md, rectum, twisted back and cut off ; nva, accessory sac of the vagina (bursa
copulatrix) ; od, oviduct ; ov, ovary ; p, penis ; rs, receptaculum seminis ; ad, colleterial gland ;
t, testes ; va. vagina ; rd, sperm-ducts. (From Lang's Comparative Anatomy.)

a series of crackling sounds, which follow one another so rapidly
as to give rise to a continuous note.

Reproductive organs. — The sexes are always separate in
Insects, as in Arthropoda in general; and the males and females
are very commonly distinguishable from one another by various
modifications of form and of coloration. There are two ovaries
each of which consists of a greater or smaller number of narrow
tubes or ovariolcs ; in each of these the ova are arranged in a single
row- — the early stages in their formation being situated at the
anterior end, the more mature ova towards the posterior extremity.
Each group of ovarian tubes opens into a lateral oviduct, and the
two lateral oviducts, right and left (Fig. 526, A, od.), in most cases
unite behind to form a median oviduct or vagina (va.), which
opens towards the posterior end of the abdomen. Connected with

xi PHYLUM ARTHROPODA 647

this median oviduct, or opening close to it, are receptacula seminis
( rx.) and colleterial or cement -glands (sd.). Sometimes there is a
copulatory sac, or bursa copulatrix (nva.). In the male the paired
testes (B, t.) vary greatly in form : sometimes each is a long narrow-
tube ; sometimes several such tubes combine to form the testis ; or
it may be of more compact rounded form and entire or lobed.
Each testis has a slender duct or vas deferens (B, vd), the two
vasa deferentia uniting to form a median ejacnlatory duct. A
vesicula seminalis is appended to each vas deferens or to the
ejaculatory duct. Accessory glands, opening into the vas deferens
or the ejaculatory duct, secrete cementing material for uniting the
sperms into masses, the spermatophores. In most instances the
eggs are laid shortly after their fertilisation, only a comparatively
few forms, such as the Aphides or Plant-lice, many Diptera, and
some Coleoptera, being viviparous. Some Insects, such as the
Aphides and the Bees and Wasps, as well as some Lepidoptera
and Neuroptera, present us with the unusual phenomenon of
parthenogenesis ; i.e., ova are formed, as in ordinary female insects,
in organs corresponding to the ovaries of the latter, and are
developed without fertilisation. In the case of the Aphides, an
autumn generation of completely-developed males and females is
followed by a spring generation consisting entirely of females ;
these are both parthenogenetic and viviparous. In the Bees,
the workers (imperfectly developed femalesy occasionally produce
ova which, without fertilisation, develop into drones (males). In
one or two groups, including the Scale-Insects (Corcida;) and
Gall-Insects {Cynipidw), males are never developed, so that repro-
duction is exclusively parthenogenetic. Pedogenesis accompanies
parthenogenesis in certain Diptera; ie., the larva; produce ova and
embryos without impregnation.

The eggs when laid are protected from injury by a number of
methods ; they may be firmly fixed to the substratum, buried in
the earth, or laid in the interior of certain plants or even of
animals. The deposition of the eggs, by means of ovipositors, in
the leaves or other parts of plants gives rise to swellings — the
so-called galls, in the interior of which the young Insects live.
In the case of many Insects the eggs are enclosed in a cocoon ; in
others they are surrounded by gelatinous or waxy material. The
eggs are, for the most part, of relatively considerable size. In form
they vary, but the long oval prevails in most instances. The ripe
egg is enclosed in two egg-membranes — an inner, the vitelline
membrane, produced by the egg itself, and an outer, the chorion,
formed from the follicle-cells. The chorion, which usually exhibits
a more or less elaborate pattern, has one or more apertures or
micropyles for the entry of the sperm. The contents are dis-
tinguishable into two layers — a superficial, consisting of proto-
plasm, and a central, of nutrient yolk.

T T 2

648

ZOOLOGY

SECT.

seg

blast

Development. — The segmentation is usually of a type already
referred to (p. 597) as very common among the Crustacea, viz.,
superficial segmentation. The actual segmentation (Fig. 527)
has chiefly been observed in the case of certain Insects with very
little yolk ; but there can be very little doubt that in ordinary

forms with abundant yolk the
process is in essence the same.
The segmentation-nucleus, ori-
ginally situated near the middle
of the ovum, divides into a
number of nuclei, and most or
all of these migrate towards
the surface, and arrange them-
selves in the form of a sphere
almost parallel with the latter ;
eventually they reach the sur-
face and coalesce with the peri-
pheral protoplasm, which then
becomes divided into cell-areas
corresponding with the nuclei.
The layer of cells thus formed
constitutes the blastoderm.
This thickens along one side
to form the ventral plate, as
already described in the case of
the Cockroach (p. 629), and the
changes which this structure
undergoes, together with the
mede of formation of the ap-
pendages, are similar in most
members of the class, except
that in most Insects the forma-
tion of the lower layers is
associated with a more or less
distinct invagination (Fig. 528).
The same holds good of the
formation of the amnion and
the development of the meso-
derm and endoderm. In some
cases there is developed be-
tween the serosa and the true
amnion a space filled with yolk, and the ventral plate appears
sunk within the yolk. The nervous system is developed from
the ectoderm in the manner indicated in the account of the
Cockroach (p. 631). The tracheal system is derived from a
series of pairs of segmental ly arranged ectodermal involutions
(Fig. 530, st).

Hast

yk.u

Fib. 527. — A — D, successive stages in the seg-
mentation of the ovum of an Insect blast.
blastoderm ; peri, peripheral protoplasm ;
seft. segmentation-cells ; yk. yolk ; ykc. yolk-
cells. (From Korschelt and Heider, after
Blochmann.)

PHYLUM ARTHROPODA

649

blast

ser

amn

amn.f

Metamorphosis. — In some instances the young Insect, when
it escapes from the egg-mem-
branes, has exactly the form of (rast __^^R^^^r-'vent-P^
the parent, except that, as a
rule, the wings have not yet
grown. But in most cases
there is a metamorphosis. In
some this is comparatively
slight and gradual, the adult
Insect differing from the larva
only in comparatively unim-
portant points, and the seg-
ments and appendages of the
latter becoming directly con-
verted into those of the former.
Such a metamorphosis, in
which there is no quiescent
stage, is said to be incomplete.
The term complete is applied
to the metamorphosis of the
majority of Insects, in which
the larva differs so completely
from the imago, or perfect In-
sect, in external form, the
nature of the appendages, and
the internal organisation, that
there is need of a quiescent
or pupa stage, during which
the whole animal, or a con-
siderable part of it, undergoes
an entire transformation. The
metamorphosis is complete in
the Diptera, Lepidoptera, Cole-
optera, and Hymenoptera, ab-
sent or incomplete in the
other orders. In the most
lowly organised larvae (many
Diptera) the body of the larva
or "maggot" is completely
worm-like, without any ap-
pendages, and without any dis-
tinct head. In other cases
(Lepidoptera, &c), there is a
distinct head ; the three thor-
acic segments have three pairs
of jointed legs, and the abdominal segments short unjointed pro-
legs (Fig. 513). In most instances the larva? differ widely from the

blast

ect

amn ser

Fig. 528. — A — C, transverse sections through the
developing ovum of an Insect at successive
stages to show the mode of development of
the germinal layers and of the amnion.
amn. amnion ; amn. f. fold of the amnion ;
amn. cav. cavity of the amnion ; blunt, blasto-
derm covering the yolk ; ect. ectoderm ; end.
endoderm ; yast. invagination of ventral
plate ; *er. serosa ; vent. pi. ventral plate ;
yk. yolk. (After Korechelt and Heider.)

650

ZOOLOGY

SECT.

adults in their food and mode of life ; very generally the jaws are
adapted for biting, even when the mouth of the adult is suctorial.

B _ C _ D ^ E

Fin. 529.— A— E, ventral view of five stages in the development of Hydrophilus. a and b,
points at which the blastopore first closes ; af. edge of the amnion fold ; af, caudal fold ; af",
paired head-fold ; an. antenna ; es, terminal segment ; g, pit-like invagination to form the
rudiment of the amnion cavity ; k, procephalic lobes ; r, groove-like medio-ventral in-
vagination ; x, germinal bands covered by the amnion. (From Lang, after Heider.)

B

Fig. 530.-^4 and B, later stages of the embryo of Hydrophilus with the rudiments of the
extremities ; in B the abdominal appendages are visible, a. anus ; an. antenna ; g, rudiment
of the ventral nerve-chain ; m. mouth ; md. mandible ; mx\ first maxilla ; nufi, second
maxilla; jt\, j>2> Pit thoracic legs; p±, p$, pj, py, rudiments of the appendages of the first,
second, fourth, and sixth abdominal appendages ; st. stigmata ; xk, prostomium, (From
Lang, after Heider.)

After a longer or shorter period passed in this larval condition, in
which it is usually active and very voracious, the young Insect

xi PHYLUM ARTHROPODA 651

passes into a quiescent or pupa stage, during which it remains
passive, enclosed in a tough integument, while a more or less
complete reconstruction of the organs goes on, resulting in the
development of all the parts of the perfect Insect. The develop-
ment of the new parts takes place from certain patches of cells,
the imagined discs, present in the larva.

In the Diptera the larva or maggot is sometimes completely
devoid of jaws. In some Diptera, however, the jaws are well
developed, and there is a distinct head. After frequent moultings
the maggot passes either into a quiescent or pupa stage enclosed
in a hard skin, or into the stage of an active aquatic. pupa, which
swims about actively in water and may possess tracheal gills.

In the Lepidoptera the larvae (" caterpillars ") are worm-like,
but with well-developed jaws, three pairs of jointed thoracic legs,
and a number of unjointed stumpy abdominal legs (pro-legs).
Lepidopterous larvae are often brilliantly coloured, are very
active, and feed with voracity, chiefly on leaves and other succulent
parts of plants. Eventually they spin a cocoon of a silky substance,
enclosed within which, and covered with a tough skin, they pass
through a quiescent or pupa condition — the condition of the '
chrysalis (Fig. 513). From the interior of this the imago subse-
quently emerges with all the parts of the adult Insect fully formed.

In mode of life there is a very considerable difference between
different orders and families of Insects. Some are parasites in the
strict sense throughout life. This is the case, for instance, in the
Strepsiptera (Bee-parasites), the females of which live permanently
lodged between the joints of the abdomen of their hosts. The
Lice and Bird-lice are external parasites throughout life ; Bugs and
Fleas, though not adhering to their hosts, are parasites as regards
their diet. Many Insects are parasites in the larval condition,
though free in the adult state. This holds good, for example, of
the larvae of the Ichneumons, which develop in the interior of the
bodies of other insect-larvae ; also of the larvae of the Bot-flies
(Fig. 512), which inhabit the alimentary canal of mammalian hosts
(Horses, Oxen, Sheep, Rhinoceroses, Tapirs). The blood-sucking
Insects act in certain cases as the carriers or intermediate hosts of
the protozoan or bacterial parasites that are the causes of various
diseases in man. Thus, as was stated in the account of the
malaria-parasite (Section II, p 86), mosquitoes are the means of
conveying that disease from one person to another.

In accordance with the high grade of the structure of their
various system of organs, Insects exhibit a correspondingly high
degree of functional activity. The quantity of food consumed
and assimilated is great in comparison with the bulk of the body,
and the energy expended in muscular contractions is of very con-
siderable amount. It is estimated that while the muscular force
exerted by a Horse bears a ratio of about 0'7 to its own weight

652 ZOOLOGY SECT

(reckoned as 1) the muscular force of an Insect bears a ratio to
its weight of from about 14 to about 23. Insects ar" also dis

eS TheTng ^ ^bl»te V the keennSs of th r"
senses Ihe sense of sight is, as we should expect from the
e aborate character of the optic' organs, the most highly deveToped
many Insects having been shown by experiment to have a keen
sense of colour; but a sense of smell, the seat of which Ts in the
antennae and palpi, can be shown to exist in a high degree and

FI°" 531-H01--bee (^l?S»n«T«^Se); '

the parts about the mouth bear nerve-endings concerned in a well
developed sense of taste. A sense of hearing does nTapraa? to
be universally present, but is well marked in such forms aTproduce
sounds At the same time Insects are remarkable fo ^he instincts
often leading to results of an elaborate character, wW^S
them in the pursuit of food and the protection and reariS of
their young Among the insects which are the 1st Shlv
endowed m this respect are some-the Ants Bees Wasns ,nH
Termites-which live together in organised a^ti^Tc^m
mimities, the various individuals composing which ared^tW.T"
able into smtal individuals, neuter KfJd Sf^^I

Pig. 532.-Bed Ant (Pormica rufa);

male, worker, and female. (After Brehm.)

and 532), each specially organised for the part which it has to play
in the economy of the community V J

Distribution in time.-The earliest known fossil remains of
Insects have been found in rocks of Silurian age. A good many
fossil Insects have been found in the Devonian; bu/they on"v
become abundant in the Carboniferous. All the PateozoicTnsec I
belong to a group which has been regarded as a distinct order
and has been named the Palceodictyoptera. The members of thTs'

cSteaSti:trteris^,rather b^he absence ^ thT ts

characteristics of any of the existing orders than by any positive

xi PHYLUM ARTHROPODA 653

features of their own ; but different families of the order approxi-
mate to a certain extent towards the groups of living Insects.
Amongst them, for example, are forms representing the Cock-
roaches and the Phasmidoe among the Orthoptera ; others repre-
senting the modern Day-flies among the^europtera ; others the
Coleoptera.

Of the existing orders, the Neuroptera, Orthoptera, and Coleop-
tera are first found in the Trias ; the Hemiptera, Diptera, Hymen-
optera, and Lepidoptera in the Jurassic.

CLASS V.-ARACHNIDA.

The class Arachnida, comprising the Scorpions and Spiders, the
Mites and Ticks, the King-crabs, and a number of other families,
is a much less homogeneous group than the Insecta, approaching
the Crustacea in the variety which it presents in the arrangement
of the segments and their appendages. In most members of the
class, however, there is an anterior region of the body — the cephalo-
thorax — representing both head and thorax, and a posterior part,
or abdomen, which is typically composed of a number of distinct
segments ; in some cases cephalothorax and abdomen are amalga-
mated. There are no antennae in the adult Arachnid, though
rudiments of them have been found in the larvae of some species.
The first pair of appendages of the cephalothorax (probably repre-
senting the antennae of the Crayfish) are the chelicerce ; the second
are the pedipalpi, the representative of the Crayfish's and Cock-
roach's mandibles. Behind these are four pairs of legs. The
organs of respiration are sometimes tracheae, similar to those of
the Insects, sometimes book-lungs, or sacs containing numerous
book-leaf-like plates : sometimes leaf-like external appendages
or gills.

1. Example of the Class. — The Scorpion (Euscoiyio or

Buthus).

Scorpions are inhabitants of warm countries — the largest kinds
being found in tropical Africa and America. They are nocturnal
animals, remaining in holes and crevices during the day, and
issuing forth at night to hunt for their prey, which consists of
Spiders and Insects. These they seize with their pincer-claws
and sting to death with their caudal spine, afterwards sucking
their juices.

There are a number of different species of Scorpions, divided
into several genera, which differ from one another in compara-
tively unimportant points, so that the following general descrip-
tion will apply almost equally well to any of them.

654

ZOOLOGY

SECT.

External features. — A Scorpion (Fig. 533) has a long narrow
body, in superficial appearance not unlike that of a Crayfish.
There is a small cephalothoracic shield or carapace, covering over
dorsally a short anterior region, cephalothorax or prosoma. This
is followed by a long posterior region or abdomen, the terminal
part of which in the living animal is habitually carried over the
back (536), constituting the " tail," at the end of which the sting
is placed. The carapace bears a pair of large eyes about its
middle, and several pairs of smaller eyes on the antero-lateral

peei

Fig. :. . Euscorpio. (From
Cuvier's Animal Kingdom.)

Fig. 534.— Scorpion. Ventral surface of the
cephalothorax and pre abdomen, chel. cheli-
ceraj ; op. operculum ; pect. pectines ; ped.
pedipalpi ; stiff, stigmata. (From Leuckart
and Nitsche's Diagrams.)

margin. The anterior, broader part of the abdomen, which is
termed the pre-abdomen or mesosoma, consists of seven segments,
each of which is enclosed in firm, chitinous, dorsal and ventral
plates, or terga and sterna. The tergum and sternum of each
segment are separated from one another laterally by intervals
of soft skin, except in the seventh, where they are united laterally
for a longer or shorter distance. The posterior, narrower part of
the abdomen, known as the post-abdomen or metasoma, consists of
five segments, each enclosed in a complete investing ring of hard
chitinous matter. Articulating with the last segment of the

xi PHYLUM ARTHROPODA 655

post-abdomen is a terminal appendage, the caudal spine or sting,
swollen at the base and acutely pointed at the apex, where open
the ducts of two poison-glands. The anal opening is situated on
the ventral surface of the last segment of the post-abdomen,
immediately in front of the sting.

The aperture of the mouth, which is very small, is at the anterior
end of the cephalothorax on its ventral aspect ; a lobe which over-
hangs it in front is the labruin. On each side of the mouth is a
three-jointed appendage — the chelicera (Fig. 534, chel.) — which is
terminated by a chela. Behind these are the very large pincer-
claws or pedipalpi (ped)., each composed of six podomeres and
terminating in a powerful chela. The basal joint of each pedipalp
has a process which bites against the corresponding process of the
other pedipalp, these processes thus performing the function of
jaws. Following upon the pedipalpi are four pairs of walking
legs, each composed of seven podomeres, the last of which is
provided with curved and pointed horny claws. The basal segments
of the first two pairs of walking legs are modified so as to perform
to some extent the function of jaws.

• All the six pairs of appendages hitherto described — the cheli-
cerae, the pedipalpi, and the four pairs of walking legs — belong to
the cephalothorax. The first segment of the pre-abdomen (Fig.
534) has a narrow sternum, on which there is a soft rounded
median lobe divided by a cleft ; this is termed the genital
operculum (op.) ; at its base is the opening of the genital duct. To
the sternum of the second segment of the pre-abdomen are attached
a pair of remarkable appendages of a comb-like shape — the
pectines (pert.) — each consisting of a stem, along the posterior
margin of which is a row of narrow processes, somewhat like the
teeth of a comb ; the function of these appendages is doubtful,
but is probably sensory. The remainder of the segments of the
pre-abdomen, and all those of the post-abdomen, are devoid of
appendages. The sterna of the third, fourth, fifth, and sixth
segments of the pre-abdomen, which are very
broad, bear each a pair of oblique slits — the
stigmata (stig.) — leading into the pulmonary
sacs.

In the interior of the cephalothorax, over
the nervous system, is a cartilaginous plate —
the endosternite (Fig. 535) — which serves to
give attachment to muscles, and is comparable
to the cephalic apodeme of Apus (p. 533). Fl(). 535.-Endo8temite

All the appendages of the Scorpion are post- °f SjJHpP** (After
oral in position, and the most anterior — the
chelicerae — are probably best regarded as cor-
responding to the antennae of the Crayfish, the equivalent of
the Crayfish's antennules and of the antennae of the Cockroach

656

ZOOLOGY

not being present. The pedipalpi would then be the homologues
of the mandibles of the Insect and the Crustacean.

Crayfish.

Antennules.
Antennae.
Mandibles.
First maxillae.
Second maxillae.
First maxillipedes.
Second maxillipedes.
Third maxillipedes.

Cockroach.

Antennae.
Absent.
Mandibles.
First maxillae.
Second maxillae.
First legs.
Second legs.
Third legs.

Scorpion.

Absent.
Chelicerae.
Pedipalpi.
First legs.
Second legs.
Third legs.
Fourth legs.

Digestive system. — The narrow mouth leads into a large
chamber with elastic walls, the pharynx ; this is capable of being
greatly dilated by the action of a number of radiating bundles of
muscular fibres which run outwards from it to the walls of the
cephalothorax, the result of this being to cause suction through
the mouth, by which means the juices of the Scorpion's prey are
drawn in. A second dilatation, to which a narrow oesophagus
leads, receives the ducts of a pair of salivary glands (Fig. 537,
sal. gld.). Upon this follows the mesenteron (mesent.), which is
an elongated, wide, straight tube, with glandular walls corre-
sponding to the stomach of the Insect. Opening into the
mesenteron are five pairs of narrow tubes (Figs. 536 and 537.
hep. du.) leading into the substance of a large glandular body,
usually termed the liver {hep.), though its hepatic functions are
doubtful. Into the long narrow intestine, in the first segment of
the post-abdomen, open one or two pairs of delicate tubes —
the Malpighian tubes (mat.) — which act as the organs of renal
excretion.

Circulatory organs. — An elongated tubular heart (Fig. 536,
hrt.) lies in the pre-abdomen enclosed in a pericardial sinus ; it is
divided internally into a series of eight chambers by transverse
partitions ; into each of these chambers the blood passes by a pair
of valvular apertures or ostia. The heart ends both in front and
behind in main arteries, the anterior and 'posterior aortas (ant.
art., post, art.); and a series of pairs of lateral arteries are
given off from the various chambers. The anterior aorta soon
bifurcates to form a pair of vessels which embrace between them
the (esophagus, and meet below in a median ventral trunk which
runs backwards above the nerve-cord. The blood carried to the
various parts of the body by the arteries is gathered up into a
large ventral sinus from which it passes to the book-lungs. From
these it is carried by a series of veins to the pericardial sinus to
enter the heart through the ostia.

XI

PHYLUM ARTHROPODA

657

The organs of respiration in the Scorpions are in the form of
pulmonary sacs or book-lungs (pul.), the stigmata or external openings
of which have already been referred to. Each pulmonary sac is

a compressed chamber lined with a thin cuticle. The lining
membrane is raised up into numerous delicate laminae lying
parallel with one another like the leaves of a book. Into the
numerous narrow spaces between the lamina; the air penetrates,

658

ZOOLOGY

SECT.

and oxygenates the blood which enters the interior of the laminae
from the ventral sinus.

A pair of coxal glands, situated near the base of the fifth pair
of appendages, are, in the embryo Scorpion, represented by tubes
which, like nephridia, effect a communication between the body-
cavity and the exterior ; in the adult Scorpion the tube assumes
the form of a closed gland, and its function is quite uncertain.

The nervous system is constructed on a plan which bears a
considerable resemblance to that of the Crayfish and that of the
Cockroach. There is a bilobed cerebral ganglion or brain (Fig. 536,

Irn.) from which nerves are

chel

stoma

salqlct

he/} die

given off to the eyes ; a nerve-
collar formed of a pair of
esophageal connectives unites
ventrally in a sub-wsophageal
ganglion, forming the anterior
part of a ventral nerve-cord
{ne. co.). The connectives and
sub-oesophageal ganglion give
rise to the nerves of the first six
pairs of appendages and of the
operculum, the pectines, and the
two following segments. The
first ganglion behind the sub-
cesophageal ganglion appears in
the eleventh segment (reckoning
the cephalothorax as made up
of six) ; behind which a ganglion
occurs regularly in each seg-
ment as far back as the fourth
of the post- abdomen.

The organs of special sense

are the eyes and pectines. The

lateral eyes (Fig. 556) are

similar in character to the

simple eyes or ocelli of Insects.

The two larger central eyes (Fig. 557) differ from them in having

the retinal cells arranged in groups as in the compound eye, but

resemble them in the presence of a single cuticular lens.

Reproductive organs. — In the male the testes consist of two
pairs of longitudinal tubules united by cross branches. These
are connected with a median vas deferens, the terminal portion of
which, provided with accessory glands, is modified to form a double
penis ; its external opening is just behind the operculum, as
already noticed. There is an unpaired ovary, which is made up of
three longitudinal tubules with transverse connecting branches ;
the oviducts open on the operculum.

Fig. 537. — Dorsal view of the internal organs
of Scorpion, chel. chelicera: ; hep. liver ;
hep. du. hepatic ducts ; inal. Malpighian
tubes ; mesent. mesenteron ; proct. intestine ;
sal . f/ld. salivary glands ; stomo. stomodajum.
(From Leuckart, after Blanchard.)

\l

PHYLUM ARTHROPODA

659

Scorpions are viviparous. The eggs, which are spherical or oval,
and in most species contain a large amount of food-yolk, lie in a
follicle formed of a diverticulum of the oviduct. Fertilisation
either occurs in the follicle or after the egg has escaped into
the oviduct. The further development takes place in the oviducts ;
and, when born, the young Scorpion differs from the parent very
little save in size.

Development. — The segmentation is of the type to which the
term discoidal is applied. On one side are formed a number of cells
in the form of a one-layered disc or cap, which gradually spreads over
the yolk. On this appears a thickening — the ventral plate (Fig. 538)
corresponding to that of the Insect. A longitudinal groove which

Fi<;. M8. — Three surface views of the ventral
plate of a developing Scorpion. A,
before the appearance of segments ; B, after
five segments have become formed ; C, after
the appendages have begun to be formed.
(From Balfour, after Mctschnikoff.)

Fia. 530.— Embryo of Scorpion lEuscor-
pius italicus), later stage. ap.
II. — VI., abdominal appendages; cli.
chclicera ; p. 1—4, legs ; m. mouth ;
ped. pedipalpi; pab. po3t-abdomen,
(From Korschelt and Heider, after
Mctschnikoff.)

appears on the surface of this may be regarded as representing
an elongated blastopore (Fig. 538, A). The cells of the blastoderm
of the ventral plate become divisible into three layers — ectoderm,
endoderm, and mesoderm. The mesoderm undergoes division into
a series of masses which are hollowed out to form the primitive
segments (B) and their cavities. Embryonic membranes — serosa
and amnion — are formed as in the Insects. When about ten seg-
ments have become distinguishable, the rudiments of appendages
(Fig. 538 C, and Fig. 539) appear in the form of hollow processes
of the segments on either side of the middle line. Behind the
rudiments of the thoracic limbs appear a series of six pairs of
abdominal appendages («/?, I. — VI.); the place of the first of these

660 ZOOLOGY sect.

is afterwards taken by the operculum ; the second develops into
the pectines. The four posterior pairs become aborted, though
they apparently have some relation to the development of the
book-lungs.

2. Distinctive Characters and Classification.

The Arachnida are air-breathing Arthropoda in which the body
is usually distinguishable into two regions — cephalothorax and
abdomen. The cephalothorax bears sessile, usually simple, eyes,
two pairs of jointed appendages — the chelicerae and pedipalpi — and
four pairs of legs. There are no antennae. The organs of respira-
tion, when present, are usually either tracheae or book-lungs, but in
the Xiphosura take the form of book -gills. Heart and vascular
system are usually present ; the heart is tubular, like that of the
Insects. The sexes are nearly always separate, and there is usually
no metamorphosis.

The class is divided into the following orders : —

Order 1. — Scorpionida.

Arachnida in which the body consists of a continuous cephalo-
thorax and an abdomen, the latter consisting of an anterior
broader pre-abdomen of seven segments, and a posterior, narrower
post-abdomen of five segments, with a caudal spine in the form
of a sting. There are small chelate chelicerse and large chelate
pedipalpi. A pair of comb-like pectines occur on the second
segment of the pre-abdomen. The organs of respiration are four
pairs of book-lungs in the third, fourth, fifth and sixth segments
of the pre-abdomen.

This order includes the Scorpions.

Order 2. — Pseudoscorpionida.

Arachnida in which there is a continuous cephalothorax, some-
times marked dorsally with two transverse grooves, and a broad
abdomen, not divided into pre- and post-abdomen, and not pro-
vided with a sting. The chelicerse are very small, the pedipalpi
similar to those of the Scorpions. The organs of respiration are
a system of tracheae. A pair of spinning-glands are present.

This order includes the Book-scorpions (Fig. 540).

Order 3. — Pedipalpida.

Arachnida in which the body consists of unsegmented cephalo-
thorax and flattened abdomen of eleven to twelve segments.
The chelicerse are simple, the pedipalpi simple or chelate, and the

\i PHYLUM ARTHROPODA 061

first pair of legs terminate in a many-jointed flagellum. The
organs of respiration are two pairs of book-lungs on the second
and third segments of the abdomen.

This order includes the Scorpion-spiders (Fig. 541).

Order 4. — Solpugida.

Arachnida with three regions — head, thorax (of three segments)
and abdomen (of ten segments). The chelicerae are chelate ; the
pedipalpi elongated and leg-like. The organs of respiration are
tracheae.

This order includes Galeodes (Fig. 542).

Order 5. — Phalangida.

Arachnida with an unsegmented cephalothorax, and an abdomen
of six segments. The chelicerae are chelate, the pedipalpi leg-like.
The organs of respiration are trachea?. No spinning glands are
developed.

This order includes the Harvest-men.

Order 6. — Araneida.

Arachnida in • which the body is composed of an undivided
cephalothorax and an unsegmented abdomen, which is usually soft
and rounded, and attached to the cephalothorax by a narrow neck.
The chelicerse are sub-chelate, with poison-glands ; the pedipalpi
simple. The organs of respiration are book-lungs alone, or book-
lungs combined with tracheae.

This order comprises all the true SpidersjCEig. 543).

Order 7. — Acarida.

Arachnida in which the body exhibits no division into regions.
The mouth-parts are adapted either for biting or piercing and
sucking. The organs of respiration, when present, are in the
form of tracheae.

This order includes the Mites and Ticks (Figs. 546 and 547).

Order 8. — Xiphosura.

Arachnida in which the body consists of a cephalothorax
covered over by a broad carapace, and an abdomen of seven
firmly united segments, with a long narrow tail-piece or telson.
The cephalothorax bears a pair of short chelate appendages
and five pairs of legs. The abdomen bears in front a pair of
. united plate-like appendages, forming the operculum, followed
by five pairs of flat appendages overlapped by the operculum.
VOL. I U U

662

ZOOLOGY

SECT.

The organs of respiration are lamelliform gills attached to the
abdominal appendages.

This order includes the King-crabs (Limulus, Figs. 548 and
549).

Order 9. — Eurypterida.

Arachnida with a relatively small cephalothorax, followed by
twelve free segments and a terminal, elongated, narrow telson.
There are a pair of pre-oral leg-like or chelate appendages and
four more leg-like appendages on the cephalothorax, the last
expanded to form swimming paddles. A broad operculum is
situated immediately behind the cephalothorax. There are pairs
of lamellate appendages on certain of the anterior free segments.
The exoskeleton is characteristically sculptured.

This order includes only a number of extinct (Palaeozoic) forms
of large size (Fig. 550).

3. General Organisation.

The external form in the Scorpionida has already been suffi-
ciently described. Most nearly related to that order in this
respect are the Pseudoscorpionida or Book-scorpions and their
allies. In these (Fig. 540) there is an unsegmented cephalo-
thorax, or the carapace is crossed by
two transverse grooves which may indi-
cate segmental divisions. There is a
broad abdomen consisting of eleven, or
more rarely ten, segments ; the post-
abdomen is not represented, nor the
caudal sting. The chelicerse are small ;
the pedipalpi are large, and resemble
those of the Scorpions in their chelate
form. Spinning glands are present.

The Pedipalpi, or Scorpion-spiders
(Fig. 541), are intermediate in some of
their external features between the
Scorpions and the Spiders. The abdomen
is broad and marked out into a series
of eleven or twelve distinct segments ;
in one of the genera of the order there
is a short post-abdomen formed of the last three segments, with
an elongated, many -jointed anal filament. The chelicerse end
in simple claws ; they are probably provided with poison-glands ;
the pedipalps are very long, either claw-like or chelate; the
first pair of legs are very long and slender, their terminal part
made up like antennae of numerous short joints. There are eight
eyes on the carapace, two larger central, and six smaller marginal.
The Solpugida (Fig. 542) have, at least superficially, the

Fig. 540.— Chelifer bravaisii.

2—0, second to sixth pairs of
appendages. (From Lang's
Comparative Anatomy.)

XI

PHYLUM ARTHROPODA

663

Fio. 541. — PhrynuB. (From Cuvier's Aiiimal Kiwjdom.)

Fig. . 1-. — Oaleodes dastuguei 9 , natural size. 1—6, the six pairs of appendages ;
1, chelicerse ; 2, pedipalpi ; c, head ; th. thorax ; ab. abdomen. (From Lang, after Dufour.)

u u 2

664

ZOOLOGY

appearance of being intermediate between the Insecta and the
other groups of Arachnida. The cephalothoracic region is divided

by a constriction into two parts,
head and thorax, the latter made up
of three segments. The chelicerae
are chelate ; the pedipalpi resemble
the legs, and are used in locomo-
tion. The first pair of legs are at-
tached to the head. The abdomen
is distinctly segmented, and there
is no caudal appendage. A pair of
poison-glands open at the bases of
the chelicerse. There are two simple
eyes on the head.

In the true Spiders (Fig. 543) the
abdomen is rounded, unsegmented,
and separated off from the cephalo-
thorax by'a constriction. The chelicerae (Fig. 544, A) are sub-
chelate, and the duct of a large poison-gland opens at the extremity.

Pig. 543.— Spider (Epeira diadema).

B

Fig. 544.-^4, Ckelicera;, and B, pedipalpi of female of Epeira diadema. (After Leuckart.)

The pedipalpi (Fig. 544, B) are elongated, and end in simple
extremities ; in the male (Fig. 545) the terminal joint is modified
to serve for the reception and trans-
ference of the sperms. At the ex-
tremity of the abdomen is the spinning
apparatus or arachnidiutn (Fig. 551 ,
arach.). This consists of four or six
elevations, the spinnerets, sometimes
jointed, probably derived from em-
bryonic rudiments of abdominal ap-
, pendages. On the surfaces of these
open the numerous fine ducts of the
spinning glands (sp. glds.), secreting
the material of which the spider's web

is composed. The fine threads of viscid secretion issuing from
the ducts harden on exposure to the air, and are worked up

Fig. 545. — Pedipalpi of male of
Epeira diadema. //. ///.

IV. V. podomeres : lib, sac ; sph.
spiral tube. (After Leuckart.)

XI

PHYLUM ARTHROPODA

665

into the web by means of the
posterior legs. There are six
or eight eyes on the carapace.

In the spider-like Phalangida,
or " Harvest-men," the cephalo-
thorax is not constricted off
from the abdomen. The cheli-
cerae are chelate, the pedipalpi
short and leg-like, the legs long
and slender.

In the Acarida, or Mites and
Ticks (Figs. 546 and 547), the
distinction into regions is no
longer recognisable. The form
of the mouth parts varies some-
what in the different families.
Sometimes the basal portions of
the pedipalpi form a sucking
proboscis enclosing the stylet-like chelicerne, modified
piercing • organs ; sometimes these appendages are claw

Fig. 546.

Itch mite (Sarcoptes scabiau).

(After Leuckart.)-

to form
-like or

Fig ".47.— Water mite (Trombidium fuliginosum), female ehtt. chelicene ; i»dK pedipalpi .

(After Leuekait.)

chelate. The legs vary somewhat in shape in the different groups,
according as they* are used for prehension, for creeping, for running,

666

ZOOLOGY

SECT.

or for swimming ; they end usually in two olaws, between which
there may be discs or stalked suckers.

In the Xiphosura or King-crabs (Fig. 548), the body consists of
two well-marked regions — cephalothorax and abdomen. The former
is covered over by a wide, dorsally convex, sub-crescentic shield or
carapace, bearing two large compound eyes, and two smaller simple
eyes. The segments of the abdomen (seven in number) are united

together, being covered dor-
sally by a continuous ab-
dominal carapace. At the
posterior end is attached a
very long, narrow, caudal
spine or telson. The anterior
appendages (Fig. 549) re-
semble those of the Scor-
pion. In front of the mouth
is a pair of short, three-
jointed, chelate appendages,
the chelicercv (i), at the
sides of a labrum {rostrum)
or upper lip. Behind these
follow a series of five pairs
of legs, the bases of all of
which, with the exception
of the last, are covered
with spines, and have the
action of jaws, while the
extremities are for the most
part chelate. The first pair
of appendages of the ab-
domen are flat plates, which
are united together in the
middle line and together
form the broad operculum
(operc), overlapping all the
posterior appendages ; on its
posterior face are the two
genital apertures. The pos-
terior appendages, of which
there are five pairs, are thin flat plates to which the gills are
attached ; each of them is divided by sutures into a small inner
ramus or endopodite, and a larger external ramus or exopodite.
Between the sixth pair of appendages is a pair of processes, the
chilarla.

In the Eurypterida (Fig. 550) there is a small cephalothorax
bearing a pair of large eyes and a pair of ocelli, and an elongated
segmented region containing twelve segments, followed by a

Fin. ,'iIn Limulus

(After Leuckart.)

Dorsal aspect.

PHYLUM ARTHROPOD A

607

o/berc

narrow pointed telson. There are usually five pair of limbs sur-
rounding the mouth and, with the exception of the first, toothed
at the bases in order to perform the functions of jaws ; the last
pair are stouter than the others and are expanded so as, ap-
parently, to assume the character of swimming paddles. Certain
of the more anterior of the free segments bear paired lamel-
liform appendages
which probably car-
ried the branchiae, as
in the Xiphosura.
The exoskeleton is in
many cases elabor-
ately sculptured.

A cartilaginous in-
ternal endosternite
of the same nature
as that which has
been described as oc-
curring in the Scor-
pions is found in
Limulus and in cer-
tain Spiders, but
not in the other
groups.

Coxal glands,
similar to those that
have been described
in the Scorpion, oc-
cur also in most
Spiders, in the Sol-
pugida and Phalan-
gida, in some Acarida,
and in the Xiphosura.
In the Solpugida and
Phalangida they oc-
cur on the bases of
the last pair of legs :
in the Araneida arid
Xiphosura, as in the
Scorpion, they are
appendages.

Alimentary system. — The oesophagus (Fig. 551, ces.) of the
Spiders is expanded behind into a special sucking stomach (suck. st.).
The mesenteron (mesenl.) gives off in the cephalothorax a pair
of large diverticula from each of which arise five narrow
diverticula (ccec.) which enter the bases of the pedipalps and legs ;
in the abdomen it is surrounded by a mass of cells commonly

Fio. 540.— Ventral view of Limulus. 1—6, appendages of
cephalothorax ; abd. abdomen ; ceph. cephalothorax ; operr.
operculum, behind which are seen the series of abdominal
appendages : tds. caudal spine or telson. (After Leuckart.)

found on the bases of the fifth pair of

668

ZOOLOGY

termed " liver" {hep.), the ducts of which open into it. The rectum
or proctodeum (rect.) is dilated ; the dilated portion (red. ccec.)
gives off two pairs of Malpighian tuoes (mat.).

In the Pseudoscorpionida the mesenteron, which is bent into a
loop, gives off three diverticula ; the proctodeum has also a
diverticulum. In the Solpugida the mesenteron also gives off
diverticula ; the occurrence of Malpighian tubes is doubtful. In
the Acarida there are always diverticula, the number and arrange-
ment of which vary,
connected with the
mesenteron. There
are usually two long
coiled Malpighian
tubes.

In the Xiphosura,
the mouth (Fig. 552,
mo.), which is situ-
ated some distance
behind the anterior
extremity of . the
body, leads into a
suctorial pharynx, fol-
lowed by a stomach,
which opens into
the elongated mesen-
teron ; the procto-
deum, a short tube
with folded walls,
opens on the ex-
terior at the posterior
extremity of the ab-
domen. Into the
mesenteron, as in the.
Scorpion, open the
ducts of a large gland,
usually termed the
" liver " (I. liv.).

A heart is absent

in all the Mites with

the exception of one family. In the other Arachnida a heart is

present and has the same general form as in the Scorpions,

though always more concentrated. e *

In the various orders the organs of respiration differ a good
deal in their character. In the Pseudoscorpionida they take the
form of branching tracheae similar to those of Insects. In the
Pedipalpi there are two pulmonary sacs or oooh-lungs similar to
those of the Scorpions. In the Solpugida there is a system of

Fig,

550.— Eurypterus fischeri (Silurian).
(From Nicholson and Lydekker.)

\[

PHYLUM ARTHROPODA

669

tracheae. In the Spiders there are either four pulmonary sacs
(Fig. 553), or two pulmonary sacs and a system of tracheae (Fig. 554).

Tracheae are present in the Phalangida and also in the majority
of the Acarida. In the Xiphosura the organs of respiration are

670

ZOOLOGY

external appendages or gills (book-gills), in the shape of delicate
laminae attached to the abdominal appendages (Fig. 555).

The nervous system is, in most instances, more concentrated
than in the Scorpions. There may be one or two separate

Fig. 552. — Diagrammatic view of a median longitudinal section of Limulus. abd.app. abdominal
appendages ; an. anus ; bm. brain : chit, ehilaria ; hep. du. opening of one of the hepatic
ducts ; lit. heart ; int. intestine ; I. liv. " liver " ; mo. mouth ; ne. co. nerve-cord ; its. oesophagus ;
operc. operculum ; tels. telson ; ren. sinus, venous sinus ; 1— S, legs. (From Leuckart, partly
after Packard.)

abdominal ganglia behind the mass formed by the united cephalo-
thoracic and anterior abdominal (Pseudoscorpionida, Pedipalpida,
some Araneida, Solpugida, Phalangida). In most of the .Araneida
and in the Acarida all the abdominal are united with all the

Flo. 553. — Book-lung of a Spider
(Zilla callophylla). a,

axis ; b, lamina; ; st. stigma.
(From Hertwig.)

Fio. 554.— Main branches of the tracheal
system of a Spider, st. stigma.
(From Hertwig, after Bertkau.)

cephalothoracic ganglia to form a single mass perforated by the
oesophagus, the part lying behind, which is much the larger,
representing the ventral nerve-cord.

Sense-organs. — Eyes are present in all except in some of the
Acarida. Their number and arrangement have been given with

PHYLUM ARTHROPODA

671

Fio. 555— One of the book-gills of Limulus, with
the appendage to which it is attached. (After
Lankester.)

the external characters of the groups. They are all (Fig. 556) of
the type of the ocelli or simple eyes of Insects, except the central
eyes of the Scorpions
(Fig. 557) and the com-
pound eyes of Limulus.
The former are intermedi-
ate in character between
ocelli and faceted eyes,
possessing the single cuti-
cular lens {lens) of the
ocellus, and resembling the
faceted eye in having the
retinal cells arranged in
groups corresponding to
ommatidia. Each retinula,
composed of five cells, con-
tains a thick axial rod or
•rhabdome (rhabd.).

In Limulus the com-
pound eye has a continu-
ous chitinous cornea-lens of the nature of a thickening of the
cuticle. This, though non-faceted, differs from the corresponding
part in the compound eye of the Scorpion in being produced
internally into a number of conical papilla?, each of which lies
over one of the ommatidia and may be looked upon as its lens.

A considerable variety is observable in the exact arrangement
of the parts of the reproductive apparatus in different groups
te s of the Arachnida. In

general, testes or ovaries
are either paired or
(more rarely) unpaired
tubes, with paired vasa
deferentia or oviducts,
which unite in a median
duct opening on the ex-
terior by an unpaired
genital opening. Vivi-
parity is exceptional. In
the Spiders the ovaries
(Fig. 551, ov.) are two
wide tubes, on the sur-
face of which follicles
project prominently ;
sometimes they unite
into a single circular
There are two short oviducts even when the ovary is

Fio. 556. — Section of the lateral eye of Euscorpius
italicus. int. intermediate cells ; lent, cuticular
lens ; nerr. c. terminal nerve-cells ; nerv. /. nerve-
fibres of optic nerve ; rhubd. rhabdomes. (After
Lankester and Bourne.)

ovary.

single ; these unite in a median vagina, which opens on the

672

ZOOLOGY

SECT.

lens

vilr-

exterior by a median genital aperture at the base of the abdomen.
One, two, or three receptacula seminis (rec. sem.) are present, and
either open into the vagina or independently on the surface. In
the male there are two elongated tubular testes with two narrow
and often greatly coiled efferent ducts, which unite in a short
median vas deferens, the aperture of which is on the base of the
abdomen between the stigmata of the first pair. The pedipalpi
of the male (Fig. 545) are modified to act as intromittent organs :
the terminal segment is swollen, and contains a twisted tube (sph.)
into which the sperms from the reproductive aperture are re-
ceived in order to be transferred in the act of copulation to the
reproductive aperture of the female. The eggs of spiders are

laid in nests or cocoons,
and are usually guarded
by the mother, some-
times carried about
by her.

In their mode of
life the Arachnida pre-
sent almost as great
a diversity as the In-
secta. Some Acarida
are parasites through-
out life. Most of the
other groups of Arach-
nida are predaceous
— preying for the most
part on Insects or
other Arachnids. To
capture the Insects
which constitute their
food, the majority of
Spiders construct a web formed of the threads secreted by the
arachnidium. The primary function of the threads formed from
the secretion of the spinning organ is to constitute the material
for the manufacture of a cocoon enclosing the eggs, and in some
Arachnids this is the sole purpose to which they are devoted. In
others there is added a nest for the protection of the eggs and of
the parent itself; this in many cases becomes a permanent lurking-
place, which the Spider inhabits at all seasons, and from which it
darts out to capture its prey ; in the Trap-door Spider the nest has
a closely fitting hinged lid. In very many Spiders the secretion
is used mainly to form the web by means of which the prey is
snared, and frequently also a nest in which the Spider lies in wait.
A subsidiary function of the threads is to aid in locomotion, the
Spider being enabled by means of them to let itself down safely
from considerable heights, and even to float in the air.

rhabd

Fio. .557. - Section of the central eye of Euscorpius.
Letters as in preceding figure, 'pigm. cells containing
pigment ; vtr. vitreous body (a specialised part of the
ectoderm). (After Lankester and Bourne.)

xi PHYLUM ARTHROPODA 673

Some of the Mites, as already mentioned, are parasitic ; others
feed on various kinds of fresh or decaying animal or vegetable
substances. Most free Acarida are terrestrial ; some are aquatic.

The Xiphosura are marine, living at a depth of a few fathoms
in warm seas, burrowing in sand ; their food consists of various
kinds of marine Annelids.

Geological History. — The most ancient of the living groups of
the Arachnida are the Scorpions, which are represented in Silurian
rocks by various fossil forms not differing very widely from those
existing at the present day. The earliest known fossil Spiders
have been found in deposits of Carboniferous age ; and remains of
Pedipalpida occur in the same formation. In Tertiary deposits
there have been found representatives of all the principal groups
of living Arachnida.

The earliest fossil-remains of Xiphosura that have been found
occur in strata of the Triassic period. Other fossil species occur
in later formations. These are all nearly related to the living
species of Limulus. The Eurypterida, as already noted, are
entirely Palaeozoic, ranging from the Lower Silurian rocks to the
Devonian.

APPENDIX TO THE ARACHNIDA.

THE PYCSOGONIDA, LINGUATULIDA, AND TARDIGRADA.

These three groups, though not in any way related to one another, and of
doubtful relationships to the Arachnida, are, as a matter of convenience, men-
tioned together here.

The Pyonogonida.

These are marine Spider-like Arthropods (Fig. 558) in which the body consists
of a cepthalothorax composed of an anterior proboscis (s), three head-segments,
and one thoracic segment, followed by three free thoracic segments and a rudi-
mentary abdomen (ab.). The cephalothorax bears usually four simple eyes and
four pairs of appendages, one or both of the first two of which may be chelate.
To these succeed a pair of usually ten-jointed ovigerous legs (3), and the first
pair of thoracic legs (4). The free thoracic segments bear lateral processes for
the articulation of the remaining three pairs of legs. The rudimentary abdomen
(ab.) is devoid of appendages.

Diverticula from the mesenteron penetrate for a considerable distance into
the limbs. Malpighian vessels are absent. There is a tubular heart with two or
three pairs of ostia. Organs of respiration are absent. The nervous system
consists of brain, sub-cesophageal ganglia and three other ganglia in the cephalo-
thorax, and one or two small pairs in the abdomen. The testes in the male
are partly, and the ovaries in the female either partly or completely contained
in the bases of the thoracic appendages on which they open. In the male 4-7
cement-glands are situated in the fourth joints of certain of the appendages ;
their secretion cements the eggs together into masses which are carried on the
ovigerous legs of the male, and in one species on those of the female also.

A metamorphosis occurs in most cases. The larva usually has three pairs of
appendages, so that it bears a superficial resemblance to a nauplius ; but the

674

ZOOLOGY

appendages are simple, and in other respects the larva has no essential likeness
to the nauplius form. Additional segments with their appendages are formed

Fig. 558. — Nymphon hispidum. 1—7, appendages ; ah. abdomen
(From Lang, after Hoek.)

proboscis.

behind the original three until the form of the adult is completed. Different
kinds of Pycnogonids occur at various depths from between tidal limits to
considerable depths in the ocean. The larvse of the species of one genus are

internal parasites in certain hydroid

Zoophytes.

The Linguatulida ok Pentastomida.

The Linguatulida (Fig. 559) are para-
sitic animals, which, whe.n superficially
examined, present little appearance of
affinity with the Arthropoda. The body
is completely worm-like, not divided
into regions, and presenting only a super-
ficial annulation, which in no way cor-
responds with division of the body into
segments. The sole representatives of
limbs are four hooks (hk.) at the sides of
the mouth. The muscular fibres are
striated. The alimentary canal is simple
and straight, and Malpighian tubes are
absent. Heart and organs of respiration
are wanting. The nervous system is
greatly reduced. A narrow nerve-collar
surrounds the oesophagus, presenting no
brain-enlargement, and connected behind
with a single ventral nerve-mass. Organs
of special sense are absent.

Some species of Pentastomum are in
the adult condition parasites in the lungs
of snakes. One species {Pentastomum
tcenioide*) inhabits certain cavities — the
frontal sinuses and maxillary antra con-
nected with the nasal chambers — in the

aex.cfi

Fig. v.i. Pentastomum tsenioides,

young female, an. anus ; gang, ganglion ;
hk. hooks; mo. mouth; ces. oesophagus ;
ov. ovary ; ovd. oviduct ; rec. se>n. re-
ceptaculum seminis ; sex. ap. sexual
aperture ; stout, stomach ; ut. uterus.
(After Leuckart.)

PHYLUM ARTHROPOD A

675

Dog tul Wolf. Its embryos, escaping and falling on grass and other herbage,
which form the food of Hares and Rabbits, are taken up by these animals, and
perforating the wall of the alimentary canal, by means of a boring apparatus
composed of several chitinous pieces, lodge themselves in the liver, where they
become encysted and undergo a metamorphosis. Afterwards they leave the
cysts and move about. If the young Pentastomum should be received into the
mouth of a Dog (still contained probably in most cases in the tissues of the Hare
or Rabbit) it may find its way to the frontal sinuses or maxillary antra, there
to undergo its final transformation into the adult form. The larva possesses two
paii-s of short legs.

The Tardigrada.

The Tardigrada (" Bear-animalcules") are soft-skinned animals (Fig. 560) of
minute size, not exceeding a millimetre in length. The body is unsegmented and
not distinguishable into regions, except
that in some a slight constriction separates
off an anterior part or head from the rest.
The mouth is provided with a sucking
proboscis. There are four pairs of short
unjointed legs (I. — IV.), the last of which
is terminal, and each is provided with
two or four claws. The mouth is sur-
rounded by papillae ; the buccal cavity
contains a pair of horny, sometimes partly
calcified, teeth (styl.). The ducts of a pair
of salivary (?) glands (ncUi) open into the
cavity of the mouth ;■ there is a muscular
pharynx (ph.), a narrow oesophagus, and
a wide mesenteron (stom.) ; the anus is
sub-terminal, situated in front of the last
pair of limbs. A pair of tubes (mal.)
which open into the terminal part of the
intestine are perhaps representatives of
Malpighian tubes. The muscles are all
non-striated. There are no organs of re-
spiration, and heart and blood-vessels are
likewise absent. There is a brain and a
ventral nerve-cord with four ganglia.
Two eye- spots situated at the anterior
end are the only representatives of
organs of special sense. The gonads in
both sexes are saccular, and open into
the terminal part of the intestine. Seg-
mentation is complete and regular. The
young animal at one stage has only two
pairs of rudimentary legs, but develops the

full number before being hatched. The larva possesses a head and four distinct
segments.

Some of the Tardigrada live among damp moss ; others in fresh or in salt
water.

Fig. 560.— Macrobiotus hufelandi.

I — IV, appendages ; bucc. buccal cavity;
(//«/. accessory gland ; mal. Malpighian
tube ; or. ovary ; red. rectum ; soli.
salivary glands ; stout, stomach ; styl.
teeth. (From Hertwig's Lehrbuch,
after Greef and Plate.)

676 ZOOLOGY sect.

Relationships of the Air-breathing Arthropoda.1

Notwithstanding the existence of some striking superficial
resemblances between the Arachnida and the Insecta, the evidence
afforded by anatomy and embryology points to the conclusion that
there is no direct genetic relationship between the two groups.
The occurrence in both of a peculiar form of respiratory organs,
the tracheae, seems at first sight to indicate such a relationship ;
but the evidence of an independent origin is so strong that it
must be supposed that the tracheae have been independently
developed in the two classes. The most important points of
difference are — the separation of head and thorax in the Insecta,
the mode of development of the eyes, the presence in the Arachnida
of an extensive " liver " and (perhaps) the endodermal origin of
the Malpighian tubes in the latter class.

Resemblances between Limulus and the Scorpions are readily
apparent. In both there is a cephalothorax bearing six pairs
of appendages, together with two median and several lateral
eyes. The appendages in both are all originally post-oral, the
first pair becoming pre-oral in course of growth, and the ganglia
belonging to it coalescing with the brain. The upper lip be-
tween the bases of these appendages is similarly developed in
both. The pair of processes situated behind the sixth pair of
appendages, which in Limulus form the chilaria, are represented
in the Scorpions by a small pentagonal plate in front of the oper-
culum. The abdomen of Limulus corresponds to the pre- and
post-abdomen of the Scorpion ; it contains only eight segments,
inclusive of the telson ; but there is evidence, from a comparison
with certain fossil forms, that the telson represents several united
metameres. A certain amount of correspondence is also traceable
in the appendages of the abdomen. In both the first pair form
the operculum ; in the Scorpion the second pair form the pectines,
while the rest disappear; in Limulus all persist as the lamelliform
appendages to which the book -gills are attached. In structure
there is considerable similarity between the book-gills of Limulus
and the book-lungs of the Scorpion, but how far they are equiva-
lent to one another remains doubtful in view of the difference
in their position, the book-gills being attached to the dorsal surface
of the abdominal appendages and the book-lungs sunk within the
segments.

The presence in both of the large " liver," of a circum-cesophageal
artery, of a cartilaginous endosternite, and of a pair of coxal

1 The Xiphosura, and also the Pentastomida, though not air-breathing, are
discussed here.

xi PHYLUM ARTHROPODA 677

glands on the basal joints of the fifth pair of appendages, are some
of the points of correspondence in the internal anatomy.

While Limulus is thus closely related to the/ Scorpions on the
one hand, it exhibits, on the other, indications of affinities with the
Trilobites, a group of extinct Arthropods probably finding their
nearest existing allies in the Branchiopod Crustacea (p. 563). This
resemblance to the Trilobites is most marked in the stage — the
trilobite-stage — in which the young King-crab escapes from the
egg. Certain fossil representatives of the Xiphosura come still
nearer to the Trilobites than the adult Limulus, and thus increase
the probability that there is a genetic connection between the two
groups.

It seems probable that the air-breathing Arachnida were
derived through Limulus-like ancestors from primitive Crustacea,
and that the tracheae were developed without genetic relationship
Avith those of the other air-breathing groups — perhaps as modifica-
tions of the pulmonary sacs, the latter having been originally
derived from gills like those of Limulus. That air-tubes can
be developed in air-breathing members of what are, fundamentally,
aquatic groups, is shown by the case of certain terrestrial Isopoda
among the Crustacea (p. 596).

There is a very evident close relationship between the Myriapoda
i.e., the Progoneata, and the Insecta. The Insects are more highly
specialised, and have their structure modified in adaptation
to a special mode of locomotion, but the resemblances in many
respects are very strong. One of the most striking points
of difference is the indefiniteness in the number of the segments
in the Myriapoda, and their constant and definite arrangement
in the Insecta. The well-defined thorax of the Insects is wanting
in the Myriapods in general, but certain of the segments following
the head differ from the rest in various respects, and might
be looked upon as constituting a thoracic region. The presence
in both groups of a sharply marked-off head bearing antennas
and jaws is an important point of resemblance ; so is the absence
in both of the voluminous " liver " of the Crustacea and Arachnida.
The gap between the two classes is narrowed by two converging
groups — the Symphyla among the Myriapoda on the one hand,
and the wingless — and in other respects primitive — Aptera among
the Insecta on the other.

While the Insecta thus appear to be nearly related to the
Progoneata, there are indications of relationship between the
Opisthogoneata and the Onychophora, and, through these, the
Chaetopoda. The elongated, homonomously segmented body, the
well-defined head with its antennae, the occurrence of similar
appendages on all the body-segments, all point in this direction.
Accordingly, instead of placing the branchiate Arthropoda
in one group and all the air-breathing forms in another, and
VOL. I xx

678

ZOOLOGY

SECT.

deriving the latter from the former, we should probably express
more correctly the affinities of the various groups of Arthropods
by some such scheme as that expressed in the diagram (Fig. 561).
Here an intermediate link between Annelida and the existing
Arthropoda is supposed to have been constituted by hypothetical
primitive forms from which Peripatus, the Insecta, and the
Myriapoda are supposed to have been evolved in the one direction,

Air- breathing
Arac holds

Insecta

Myriof>oda

Onych

Pycnogonida
Crustacea

Tardtgrada
Pentasfomida

Xijahosura
Euryfjherida

Primitive Arthropods

Annelida

Fig. 5(31

and the Crustacea, Eurypterida, Xiphosura, and air-breathing
Arachnida in the other.

On account mainly of general resemblances to the Spiders, the
Pycnogonida have frequently been grouped with the Arachnida,
and attempts have been made to homologise their appendages
with those of the Spiders and Scorpions. There is one pair more
in the Pycnogonida ; and either the last pair would have to be set
down as corresponding to the vestigial first abdominal pair of the
ordinary Arachnida, or the ovigerous legs would have to be
reckoned, not as independent appendages, but as parts of the
second pair, a view for which there is some ground. A close
relationship with the Arachnida, however, cannot be traced, and
their affinities are perhaps best expressed, as in the diagram, by
connecting them with the Arachnid branch of the Arthropod tree
at a point below that at which the air-breathing forms had become
developed from forms allied to the Xiphosura.

m PHYLUM ARTHROPODA 670

The position of the Pentastomida is a matter of uncertainty.
In the absence of organs of respiration and excretion, the only
feature in the adult which distinctly points to arthropod affinities
is the striated character of the muscular tissue. The presence of
two pairs of legs in the larva, however, is sufficient to confirm the
view that they are aberrant and probably degenerate Arthro-
pods, while leaving it uncertain in what class they find their near-
est allies. The Tardigrada are still more aberrant in some respects.
They differ from Arthropods in general in the absence of external
segmentation in the adult state, in the simple unjointed character
of the appendages, in the absence of striation in the muscular
fibres, and in the absence of organs of respiration and circulation.
It is impossible to place them in any of the great classes, and they
are perhaps best looked upon as a special offset of the Arthropod
tree given off near the base.

x x 2

SECTION XII
PHYLUM MOLLUSCA

The Mollusca, like the Arthropoda, form one of the chief
divisions of the animal kingdom, both as regards diversity of
organisation and number of genera and species. They are sharply
distinguished from Arthropods by the absence of segmentation,
and by having, as a rule, an exoskeleton in the form of a shell,
usually external, sometimes internal. An enumeration of the
Classes of the Phylum will serve to give some notion of its
extent.

Glass 1. — Pelecypoda, including the bivalved shell-fish, such
as Mussels, Cockles, Oysters, &c.

Glass 2. — Amphineura, including the Chitons and their allies.

Class 3. — Gastropoda, including the univalved Shell-fish, such
as Periwinkles, Whelks, Snails, Slugs, &c.

Glass 4. — Scaphopoda, including the Tooth-shells.

Glass 5. — Cephalopoda, including the Cuttle-fishes, Squids,
Octopods, and Nautili.

CLASS I.-PELECYPODA.

1. Example of the Class— The Fresh-water Mussels

{Anodonta and Unio).

Fresh-water Mussels are found in rivers and lakes in most parts
of the world. Anodonta cygnea, the Swan-mussel, is the commonest
species in England ; but the Pearl-mussel, Unio margaritifcr, is
found in mountain streams, and other species of the same genus
are universally distributed.

The Mussel (Fig. 562) is enclosed in a brown shell formed of
two separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand, with the
valves slightly gaping, and in the narrow cleft thus formed a

080

\

skit, xii PHYLUM MOLLUSCA 681

delicate, semi-transparent substance (m.) is seen — the edge of the
mantle or pallium. The mantle really consists of separate halves
or lobes corresponding with the valves of the shell, but in the
position of rest the two lobes are so closely approximated as to
appear simply like a membrane uniting the valves. At one end,
however, the mantle projects between the valves in the form of
two short tubes, one {ex. sph.) smooth-walled, the other (in. sph.)
beset with delicate processes or fimbria}. By diffusing particles of
carmine or indigo in the water it can be seen that a current is
always passing in at the fimbriated tube, hence called the inhalant
siphon, and out at the smooth or exhalant siphon. Frequently a
semi-transparent, tongue-like body (ft.) is protruded between the
valves at the opposite side from the hinge and at the end furthest
from the siphons : this is the foot, by its means the animal is able
slowly to plough its way through the sand or mud. When the

ft ' '

Fio. 562. — Anodonta cygnea. The entire animal. A, from the left side ; 'B, from the
posterior end ; d. p. a. dorsal pallial aperture ; ex. xph. exhalant siphon ; ft. foot ; in. uph.
inhalant siphon ; Iff. ligament ; m. mantle ; um. umbo. (After Howes.)

Mussel is irritated the foot and siphons are withdrawn and the
valves tightly closed. In a dead animal, on the other hand, the
shell always gapes, and it can then be seen that each valve is
lined by the corresponding lobe of the mantle, and that the
exhalant siphon is formed by the union of the lobes above and below
it and is thus an actual tube ; but that the boundary of the
inhalant siphon facing the gape of the shell is simply formed by the
approximation of the mantle-lobes, so that this tube is a tem-
porary one.

The hinge of the shell is dorsal, the gape ventral, the end
bearing the siphons posterior, the end from which the foot is
protruded anterior : hence the valves and mantle-lobes are re-
spectively right and left.

In a dead and gaping Mussel the general disposition of the
parts of the animal is readily seen. The main part of the body

682

ZOOLOGY

lies between the dorsal ends of the valves : it is produced in the
middle ventral line into the keel-like foot, and on each side,
between the foot and the corresponding mantle-lobe, are two deli-
cate, striated plates, the gills (Figs. 566-568). Thus the whole
animal has been compared to a book, the back being represented
by the hinge, the covers by the valves, the fly-leaves by the mantle-
lobes, the two first and the two last pages by the gills, and the
remainder of the leaves by the foot.

The Shell. — When the body of the mussel is removed from
the shell the two valves are seen to be united, along a straight

cad.

jbrc/

Fio. 503.— Anodonta cygnea. A, interior of right valve ; B, the animal removed from th
shell, a. ad. anterior adductor or its impression ; a. r. anterior retractor or its impression ;
d.gl. digestive gland, seen through mantle ; ex. sph. exhalant siphon ; /(. foot ; gl. gills, seen
through mantle ; It. I. hinge-line ; in. sph. inhalant siphon ; kd. kidney, seen through mantle ;
k. o. Keber's organ, seen through mantle; m. mantle; p. ad. posterior adductor or its
impression ; pe. pericardium, seen through mantle ; pi. I. pallial line ; pi. m. pallial muscles ;
i>. r. posterior retractor or its impression ; pre. protractor or its impression.

hinge-line (Fig. 563, A, h.l.), by a tough, elastic substance, the
hinge-ligament (Figs. 562 and 568,/^.) passing transversely from valve
to valve. It is by the elasticity of this ligament that the shell is
opened : it is closed, as we shall see, by muscular action : hence
the mere relaxation of the muscles opens the shell. In Anodonta
the only junction between the two valves is afforded by the liga-
ment, but in Unio each is produced into strong projections and

XII

PHYLUM MOLLUSC A

683

pre

prs

ridges, the hinge-teeth, separated by grooves or sockets, and so
arranged that the teeth of one valve fit into the sockets of the
other.

The valves are marked externally by a series of concentric lines
(Fig. 562) parallel with the free edge or gape, and starting from a
swollen knob or elevation, the umbo {urn.), situated towards the
anterior edge of the hinge-line. These lines are lines of growth.
The shell is thickest at the umbo, which represents the part
first formed in the young animal, and new layers are deposited
under this original portion, as secretions from the mantle. As the
animal grows each layer projects beyond its predecessor, and in
this way successive outcrops are produced giving rise to the
markings in question. In
the region of the umbo
the shell is usually more
or less eroded by the action
of the carbonic acid in the
water.

The inner surface of the
shell also presents charac-
teristic markings (Fig.
563, A). Parallel with
the gape and at a short
distance from it is a deli-
cate streak (pi. I.) caused
by the insertion into the
shell of muscular fibres
from the edge of the
mantle : the streak is
hence called the pallial
line. Beneath the anterior
end of the hinge the
pallia] line ends in an
oval mark, the anterior

adductor impression (a. ad.), into which is inserted one of the
muscles which close the shell. A similar but larger posterior
adductor impression (p. ad.) lies beneath the posterior end of the
hinge. Two smaller markings in close relation with the anterior
adductor impression mark the origin of the anterior retractor (a. r.)
and of the protractor (pre.) of the foot : one connected with the
posterior adductor impression is that of the posterior retractor (p. r.)
of the foot. From all these impressions faint converging lines can
be traced to the umbo: they mark the gradual shifting of the
muscles during the growth of the animal.

The shell consists of three layers. Outside is a brown horn-like
layer, the periostracwm (Fig. 564, pre), composed of conchiolin, a
substance allied in composition to chitin. Beneath this is a

VOL. i x x 2*

e.p.1

ep.s.

Fig. 564. — Vertical section of shell and mantle of
^*1?fl9B* a — c- '• connective-tissue layer of
fiTantle ; ep. 1, its outer epithelium ; ep. 2, its
inner epithelium ; n. nacreous layer of shell ;
pre. pcriostracum ; prs. prismatic layer. (After
Clans.)

684 ZOOLOGY

SECT.

prismatic layer (prs.) formed of minute prisms of calcium car-
bonate separated by thin layers of conchiolin ; and, lastly, forming
the internal part of the shell is the nacre (n.),or "mother-of-pearl,"
formed of alternate layers of carbonate of lime and conchiolin
arranged parallel to the surface. The periostracum and the pris-
matic layer are secreted from the edge of the mantle only, the
pearly layer from the whole of its outer surface. The hinge-
ligament is continuous with the periostracum, and is to be looked
upon simply as a median uncalcified portion of the shell, which is
therefore, in strictness, a single continuous structure.

By the removal of the shell the body of the animal (Fig. 563, B)
is seen to be elongated from before backwards, narrow from side
to side, produced on each side into a mantle-lobe (m.) and con-
tinued ventrally into a keel-like visceral mass (Fig. 565, v.m.), which
passes below and in front into the foot (ft.). Thus each valve of
the shell is in contact with the dorso-lateral region of the body
of its own side together with the corresponding mantle-lobe, and
it is from the epithelium (Fig. 564, ep.1) covering these parts that
the shell is formed as a cuticular secretion. The whole space
between the two mantle-lobes, containing the gills, visceral mass,
and foot, is called the mantle-cavity.

A single layer of epithelial cells covers the whole external sur-
face, i.e. the body proper, both surfaces of the mantle, the gills,
and foot ; that of the gills and of the inner surface of the mantle
(Fig. 564, ep.2) is ciliated. Beneath the epidermis come connective
and muscular tissue, which occupy nearly the whole of the interior
of the body not taken up by the viscera, the ccelome being, as we
shall see, much reduced. The muscles are all unstriped, and are
arranged in distinct bands or sheets, many of them very large and
conspicuous. The largest are the anterior and posterior adductors
(Figs. 563 and 565, a. ad., p. ad.), great cylindrical muscles, pass-
ing transversely across the body and inserted at either end into
the valves of the shell, which are approximated by their con-
traction. Two muscles of much smaller size pass from the shell
to the foot, which they serve to draw back ; they are the anterior
(a. r.) and posterior (p. r.) retractors of the foot. A third muscle
(pre.) arises from the shell, close to the anterior adductor, and has
its fibres spread fan-wise over the visceral mass, which it serves to
compress, thus forcing out the foot and acting as a protractor of
that organ. The substance of the foot itself consists of a complex
mass of fibres, the intrinsic muscles of the foot, many of which also
act as protractors. Lastly, all along the border of the mantle is a
row of delicate pallial muscles (Fig. 563, pi. m.), which, by their
insertion into the shell, give rise to the pallial line already seen.

The coelome is reduced to a single ovoidal chamber, the peri-
cardium (Fig. 565. pc), lying in the dorsal region of the body and
containing the heart and part of the intestine ; it is lined by

XII

PHYLUM MOLLUSCA

685

ccelomic epithelium. In the remainder of the body the space
between the ectoderm and the viscera is filled by the muscles
and connective-tissue.

Digestive organs. — The mouth (Fig. 565, mth) lies in the
middle line, just below the anterior adductor. On each side of
it are two triangular flaps, the internal and external labial palps
(I. int. pi})-, l.ext.plp.) ; the external palps unite with one another
in front of the mouth, forming an upper lip ; the internal palps are
similarly united behind the mouth, forming a lower lip ; both are
ciliated externally. The mouth leads by a short gullet (Fig. 566,
gill.) into a large stomach (st.), which receives the ducts (d.d.) of a
pair of irregular, dark-brown digestive glands (d.gl.).

The intestine

<zvt, tc <£ u^

ex-- sp/v

I ext.plp

v.m.

Fig. 565.— Anodonta cygnea. The animal with most of the left mantle-lobe removed,
or. anus ; a. ad. anterior adductor ; a. r. anterior retractor ; au. auricle ; d. p. a. dorsal
pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; in. sph. inhalant siphon ; led. kidney ;
I . txt. >il. left external gill-lamina ; /. ext. pip. left external labial palp ; I. int. gl. left internal
gill-lamina; /. int. pip. left internal labial palp ; I. m. cut edge of left mantle-lobe; mth.
mouth ; p. ad. posterior adductor ; pc. pericardium ; p. r. posterior retractor ; pre. protractor ;
ret, rectum ; r. m. right mantle-lobe ; v. ventricle ; v. m. visceral mass.

(int.) is given off from the posterior end of the stomach, descends
into the visceral mass, where it is coiled upon itself, then ascends
parallel to its first portion, turns sharply backwards, and proceeds,
as the rectum (ret.), through the pericardium — where it traverses the
ventricle of the heart — and above the posterior adductor, finally
discharging by the anus (a.) into the exhalant siphon, or cloaca.
The wall of the rectum is produced into a longitudinal ridge, or
typhlosole (ty.), like that of the Earthworm, and two similar ridges
begin in the stomach and are continued into the first portion of
the intestine. The stomach contains, under certain conditions,
a gelatinous rod, the crystalline style.

On each side is a single gill or ctenidium composed of two
plates or lamina, an inner and an outer. We have thus right outer

686

ZOOLOGY

and inner gill-laminae, and left outer and inner gill-laminae (Fig.
565, l.ext. gl., Lint. gl.). Seen from the surface, each lamina
presents a delicate double striation. being marked by faint lines
running parallel with, and by more pronounced lines running at
right angles to, the long axis of the organ. Moreover, each
lamina is double, being formed of two similar plates, the inner and
outer lamellrc, united with one another along the anterior, ventral,
and posterior edges of the lamina, but free dorsally. The lamina
has thus the form of a long and extremely narrow bag open above
(Figs. 566, 567 and 568) : its cavity is subdivided by vertical bars of
tissue, the inter-lamellar junctions (i.*l.j.), which extend between the

Fig. 566. — Anodonta cygnea. Dissection from the left side. a. anus; a. ad. anterior
adductor ; a. ao. anterior aorta ; a. v. ap. auriculo-ventricular aperture ; bl. urinary bladder ;

c. pi. gn. cerebro-pleural ganglion ; d.d. duet of digestive gland; d. gl. digestive gland ;

d. p. a. dorsal pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; g. ap. genital aperture ;
gon. gonad ; gul. gullet ; i. 1. j. inter-lamellar junction ; in. sph. inhalant siphon ; int. intes-
tine ; led. kidney ; m. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ;
pc. pericardium ; pd. gn. pedal ganglion ; r. ap. renal aperture ; r. au. right auricle ;
ret. rectum ; r. p. a. reno-pericardial aperture ; st. stomach ; ty. typhlosole ; v. ventricle ;
v. gn. visceral ganglion ; 10. t. water tubes.

two lamellae, and divide the intervening space into distinct compart-
ments or water-tubes (w.t.), closed ventrally, but freely open along
the dorsal edge of the gill. The vertical striation of the laminae is
due to the fact that each lamella is made up of a number of close-
set gill-filaments (/.) : the longitudinal striation to the circumstance
that these filaments are connected by horizontal bars, the inter -
filamentar junctions i. f. j.). At the thin free or ventral edge of the
lamina the filaments of the two lamellae are continuous with one
another, so that each lamina has actually a single set of V-shaped
filaments, the outer limbs of which go to form the outer lamella,
their inner limbs the inner lamella. Between the filaments, and
bounded above and below by the inter-filamentar junctions, are

XII

PHYLIM MOLUCCA

087

minute apertures, or ostia (os.), which lead from the mantle-cavity
through a more or less irregular series of cavities into the interior
of the water-tubes. The filaments themselves are supported by
chitinous rods (?•.), and are covered with ciliated epithelium, the
large cilia (Fig. 567, D) of which produce a current running from
the exterior through the ostia into the water-tubes, and finally

ep

Fig. 067. — Anodonta cygnea. A, transverse section of outer, and B, of inner gill-lamina ;
C, diagram of gill-structure; D, transverse section of gill filament, b. c. blood-corpuscle;
)>. r. blood-vessels ; eh. cbitin ; /. branchial filaments ; ep. epithelium ; i. f. j. inter-
filamentar junction ; i. I. inner lamella ; i. I. j. inter-lamellar junction ; o. I. outer lamella ;
os. external ostium ; oh internal ostium ; r, chitinous rods ; w. t. water tubes. (A, B, and D,
after Peck.)

escaping by the wide dorsal apertures of the latter. The whole
organ is traversed by blood-vessels (b. v.).

The mode of attachment of the gills presents certain features of
importance. The outer lamella of the outer lamina is attached
along its whole length to the mantle (Fig. 568) : the inner lamella
of the outer and the outer lamella of the inner lamina are attached
together to the sides of the visceral mass a little below the origin

G88

ZOOLOGY

of the mantle : the inner lamella of the inner lamina is also attached
to the visceral mass in front (A), but is free further back (B). The
gills are longer than the visceral mass, and project behind it, below
the posterior adductor (C), as far as the posterior edge of the

Flo. 568.— Anodonta cygnea. Three transverse sections, A. B. C. au. auricle ; bl. urinary
bladder ; ext. gl. external gill-lamina ; ft. foot ; i. l.j. inter-lamellar junction ; int. intestine ;
int. gl. internal gill-lamina ; led. kidney ; k. o. Keber's organ ; Ig. ligament ; m. mantle ; p. ad.
posterior adductor ; pc. pericardium^; ret. rectum ; s. brc. supra-branchial chamber ; sh. shell ;
ty. typhlosole ; v. ventricle ; vc. vena cava ; v. gn. visceral ganglion. (After Howes, slightly
altered.

mantle : in this region the inner lamella? of the right and left
inner laminae are united with one another, and the dorsal edges
of all four laminae constitute a horizontal partition between the
paliial cavity below and the exhalant chamber or cloaca above.
Owing to this arrangement it will be seen that the water-tubes all

xii PHYLUM MOLLUSCA 689

open dorsally into a supra-branchial chamber (s. br. c.) continuous
posteriorly with the cloaca and thus opening on the exterior by
the exhalant siphon.

The physiological importance of the gills will now be obvious.
By the action of their cilia a current is produced which sets in
through the inhalant siphon into the pallial cavity, through the
ostia into the water tubes, into the supra-branchial chamber, and
out at the exhalant siphon. The in-going current carries with it
not only oxygen for the aeration of the blood, but also Diatoms,
Infusoria and other microscopic organisms, which are swept into
the mouth by the cilia covering the labial palps. The out-going
current carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in producing
the food-current is of more importance than their respiratory
function, which they share with the mantle.

The excretory organs are a single pair of curiously-modified
nephridia (portions of the true ccelome), situated one on each side
of the body just below the pericardium. Each nephridium consists
of two parts, a brown spongy glandular portion or kidney (Fig. 566,
kd.), and a thin- walled non-glandular part or bladder (bl.), which
communicates with its fellow anteriorly by a large oval .aperture.
The two parts lie parallel to one another, the bladder being placed
dorsally and immediately below the floor of the pericardium :
they communicate with one another posteriorly, while in front each
glandular part opens into the pericardium (r. p. ap.), and the
bladder on to the exterior by a minute aperture (r. ap.), situated
between the inner lamina of the gill and the visceral mass. Thus
the whole organ, often called after its discoverer, the organ of
Bojanus, is simply a tube bent upon itself, opening at one end into
the ccelome, and at the other on the external surface of the body :
it has thus the normal relations of a nephridium. The epithelium
of the bladder is ciliated, and produces an outward current.

An excretory function is also discharged by a large glandular
mass of reddish-brown colour, called the pericardial gland or Kebers
organ (Fig. 568, B, k.o.). It lies in the anterior region of the body
just in front of the pericardium, into which it discharges.

The circulatory system is well developed. The heart lies in
the pericardium and consists of a single ventricle (Figs. 566, 568,
and 569, v.) and of right and left auricles (aw.). The ventricle is
a muscular chamber which has the peculiarity of surrounding the
rectum (Figs. 566 and 568, B) : the auricles are thin-walled
chambers communicating with the ventricle by valvular apertures
opening towards the latter. From each end of the ventricle an
artery is given off, the anterior aorta (Fig. 566, a. ao.) passing
above, the posterior aorta (p. ao.) below the rectum. From the
aortse the blood passes into arteries (Fig. 569, art.1 art.2) which
ramify all over the body, finally forming an extensive network of

690

ZOOLOGY

SECT.

vessels, many of which are devoid of proper walls and have there-
fore the nature of sinuses. The returning blood passes into a
large longitudinal vein, the vena cava (v. c.) placed between the
nephridia, whence it is taken to the kidneys themselves (nph. v.),
thence by afferent branchial veins (of. br. v.) to the gills, and is finally
returned by efferent branchial veins (ef. br. v.) to the auricles. The
mantle has a very extensive blood supply, and, as mentioned above,
probably acts as the chief respiratory organ : its blood {art.1) is
returned directly to the auricles without passing through either

Fig. 569. — Diagram of the circulatory system of Anodonta. Vessels containing aerated blood
red, non-ae'rated blue. af. br. v. afferent branchial veins ; ao. aorta ; art. 1, artery to mantle ;
art. 2, artery to body generally ; au. auricle ; ef.br.v. efferent branchial veins ; nph.v. nephridial
veins ; pc. pericardium ; v. ventricle ; v. c. vena cava. The arrows show the direction of the
current.

the kidneys or the gills. The blood is colourless and contains
leucocytes.

The nervous system is formed on a type quite different from
anything we have yet met with. On each side of the gullet is a
small cerebro-pleural ganglion (Fig. 566, c. pi. gn.) united with its
fellow of the opposite side by a nerve-cord, the cerebral commissure,
passing above the gullet. Each cerebro-pleural ganglion also gives
off a cord, the cerebro-pedal connective, which passes downwards
and backwards to a pedal ganglion (pd. gn.) situated at the junction
of the visceral mass with the foot : the two pedal ganglia are so
closely united as to form a single bilobed mass. From each
cerebro-pleural ganglion there further proceeds a long cerebro-
visccral connective which passes directly backwards, through the
kidney, and ends in a visceral ganglion (v. gn.) placed on the ventral

PHYLUM MOLLUSCA

691

side of the posterior adductor muscle. The visceral, like the
pedal ganglia, are fused together. The cerebro-pleural ganglia
supply the labial palps and the anterior part of the mantle ; the
pedal the foot and its muscles ; the visceral the enteric canal, heart,
gills, and posterior portion of the mantle.

It will be seen that the cerebral commissures and cerebro-pedal
connectives together with the cerebro-pleural and pedal ganglia,
form a nerve-ring which surrounds the gullet : the cerebro-pleural
ganglia may be looked upon as a supra-oesophageal nerve mass
corresponding in part with the brain of Annelids and Arthropods,
and the pedal ganglia as an infra-oesophageal mass representing the
ventral nerve-cord.

Sensory organs are poorly developed, as might be expected in
an animal of such sedentary
habits. In connection with each
visceral ganglion is a patch of
sensory epithelium forming
the so-called olfactory organ
or, better, osphradium, the

I function of which is apparently
to test the purity of the water
entering by the respiratory
current. Close* to the pedal
ganglion a minute statocyst
(" otocyst ") (Fig. 570) is some-
times found, the nerve of which
is said to spring from the
cerebro-pedal connective, being
probably derived from the cere-
bral ganglion. Sensory cells
— probably tactile — also occur
round the edge of the mantle, and especially on the fimbriae of
the inhalant siphon.

Reproductive organs. — The sexes are separate. The gonads
(Fig. 566, gon.) are large, paired, racemose glands, occupying a
considerable portion of the visceral mass amongst the coils of
the intestine : the testis is white, the ovary reddish. The gonad
of each side has a short duct which opens (g. ap.) on the surface
of the visceral mass just in front of the renal aperture.

In the breeding season the eggs, extruded from the genital
aperture, pass into the suprabranchial chamber and so to the
cloaca. There, in all probability, they are impregnated by sperms
introduced with the respiratory current. The oosperms are then
passed into the cavities of the outer gill-laminae, which they
distend enormously. Thus the outer gill-laminae act as brood-
pouches, and in them the embryo develops into the peculiar larval
form presently to be described.

Fig. 570.— Statocyst of Anodonta. a, b,c, c',
cellular layers surrounding the statocyst ; ot.
statolith. (From the Cambridge Natural
History.)

692

ZOOLOGY

sdl-~

Development. — Segmentation of the oosperm is complete, but
unequal. A gastrula is formed by the invagination of the mega-
meres into the micro-
rk meres, but the archen-

>«» teron (Fig. 571, ent.)

~Nk thus formed is quite

tJf £j<J~& .'' \ small and insignifi-

cant, and has no
physiological import-
ance until a late period
of larval life. Certain
of the cells of the
gastrula are budded
off into the blasto-
coele, where they ac-
cumulate and form the
mesoderm (mes.). At
about the same time a
deep invagination (ad.)
is formed, which might
easily be mistaken for
the archen teron, but is
really a very characteristic molluscan organ, the shell-gland : it
marks the dorsal surface of the embryo. The posterior end is
distinguished by a tuft of long cilia.

The shell-gland becomes converted into a plate of long, cylin-
drical cells (Fig. 572, sd.), from which an unpaired shell (s.) is

mes

Fio. 571.— Early embryo of Anodonta. eh, vitelline
membrane ; ent. archen teron ; ». micropyle ; mes. meso-
derm ; rk, polar cells ; sd, shell-gland ; si, lateral cells ;
?r, cilia. (From Korschelt and Heider's Embryology.)

R

B-

mes.

TO:

Fio. 572.— Two later stages in the development of Anodonta. ent. archen teron ; mes. meso-
derm ; s. shell ; sd, shell-gland ; so. sense-organs ; to, cilia. (From Korschelt and Heider's
Embryology.)

secreted. This is replaced before long by a bivalved shell of
triangular form, its ventral angles produced into incurved hooks

XII

PHYLUM MOLLUSCA

693

beset with spines (Fig. 573, sh). At the same time the body of
the larva, which has hitherto been an undivided mass projecting
between the two valves of the shell, becomes cleft from below up-
wards, and thus divided into a single dorsally-placed body proper,
and paired — right and left — mantle-lobes. Upon the latter
peculiar brush-like sense-organs make their appearance, and on
the ventral surface of the body is formed a glandular pouch, which
secretes a long thread, the provisional byssus (/). The mesoderm
cells give rise to a single immense adductor muscle (sm), the fibres
of which extend from valve to valve.

The larva is now called a glochidium : it remains in. the brood-
pouch, nourished by a secretion from the walls of the latter, and
entangled with its fellows by means of the byssus. At this stage
the outer gill-lamina appears as if stuffed full of closely
aggregated sand-grains. Before long the larvae are ejected

B.

ID*''

Fig. 573.— A, advanced embryo of Anodonta. 11, free glochidium ; /, provisional byssus
*. shell ; sli, hooks ; sm, adductor muscle ; so. sense-organs ; ic, cilia. (From Korschelt and
Heider's Embi-yology.)

through the exhalant siphon, and if they happen to come in
contact with a passing Stickleback or other fresh-water Fish,
fix themselves on some part of its body by means of the hooked
valves. The glochidia of Unio usually become attached to the
gills, those of Anodonta to the skin or the fins. In this position
they become encysted by an overgrowth of the skin or mucous
membrane of the host, and are nourished by its juices absorbed
through processes of the mantle. They thus lead a truly ecto-
parasitic existence for about ten weeks.

While in this condition a metamorphosis takes place. The pro-
visional byssus and sense-organs disappear (Fig. 574), and imme-
diately posterior to the former an invagination, the stomodceum (m),
is formed, and soon communicates with the archenteron. The
posterior end of this cavity is in close contact with the ectoderm,
so that the anus is formed by a simple process of rupture, and
without the development of a proctodeum. The foot (fu) arises
as a median ventral elevation behind the mouth, and on each side of

694

ZOOLOGY

it two papillae (k) appear, the rudiments of the gills. The larva is
now fitted for free existence ; it drops from its host, and gradually
assumes the adult form and mode of* life.

so.

2. Distinctive Characters and Classification.

The Pelecypoda are bilaterally symmetrical, compressed Molluscs,
in which the mantle consists of paired right and left lobes, secret-
ing a bivalved cal-
f careous shell. There

is no distinct head.
The ventral region of
the body is differenti-
ated into a muscular
foot, which is usu-
ally ploughshare- or
tongue - shaped : in
some cases there is a
hyssus-gland posterior
to the foot, which se-
cretes a mass of horny
fibres, the byssus, by
which the animal may
be permanently at-
tached. There are two
gills or denidia, one
on each side : the chief
function of the gills is
the production of a
respiratory and food-
carrying current of
water. The body is
covered by a one-
layered epidermis,
which is ciliated on
the gills and on the
inner surface of the
mantle. The muscular
system is well-de-
veloped, the largest
muscles being either
one or two adductors,
which close the shell,
and several bands connected with the foot and byssus ; the
muscles are usually unstriped. The ccelome is reduced to a
dorsally-placed pericardium. The mouth is bounded by two pairs
of flat, triangular tentacles or labial palps, the cilia of which

Fig. 574. — Three stages in the metamorphosis of Ano-
donta. d, enteric canal ; /. provisional byssus ; fu,
foot ; .</, lateral pits ; k, rudiments of gills ; m. mouth ;
sh. shell ; sm, adductor muscle ; so. sense-organs ; ie,
cilia. (Prom Korschelt and Heider's Embryology.)

xn PHYLUM MOLLUSCA 095

scnr to carry food-particles to the mouth : the enteric canal is
coiled, and is formed mainly from the mesenteron : there are large
paired digestive glands : the rectum passes through the pericardium,
usually perforates the ventricle, and ends above the posterior
adductor. The heart is contained within the pericardium, and
consists of a median ventricle and of right and left auricles : the
blood, which is usually colourless, is taken from the ventricle to
the body by one or two aortse, and is returned partly directly,
partly by way of the renal organs and gills, to the auricles. The
renal organs are a single pair of ccelomic nephridia, which
usually open at one end into the pericardium, at the other on the
exterior. The nervous system consists typically of four pairs of
ganglia called respectively cerebral, pleural, pedal, and visceral :
the cerebral and pleural of each side are usually fused into a single
cerebro-pleural ganglion. The chief sense-organs are statocysts
and osphradia or water-testing organs. The sexes are separate or
united : there are no accessoiy organs of reproduction. Develop-
ment is accompanied by a metamorphosis, which usually includes
a trochophore stage.

The classification of the Pelecypoda is as follows : —

Order 1. — Protobran'chia.

Pelecypoda in which the gills take the form of a single pair of
plume-like organs or ctenidia, each with two rows of flattened
gill-filaments. The foot is not compressed, but has a flattened
ventral surface or sole upon which the animal creeps. There are
two adductor muscles.

This group includes only four genera — Nucula (Fig. 586),
Yoldia, Leda, and Solenomya.

Order 2. — Filibranchia.

Pelecypoda in which there is a pair of plate-like gills formed
of distinct V-shaped filaments: interfilamentar junctions are either
absent or formed by groups of interlocking cilia : interlamellar
junctions are either absent or non- vascular. As a rule there are
two adductor muscles, but the anterior may be greatly reduced or
absent.

Including the Noah's ark shell {Area), Sea-mussel (My tikes,
Fig. 585), Anomia, Trigonia, &c.

Order 3. — Pseudo-lamellibranchia.

Pelecypoda in which the gills are plaited so as to present
vertical folds : the interfilamentar junctions may be ciliary or
vascular : the interlamellar junctions vascular or non-vascular.

696 ZOOLOGY sect.

There is a single large (posterior) adductor muscle. The shell is
frequently inequivalve.

Including the Scallop (Pecten, Fig. 575), Oyster (Ostrea), Pearl
Oyster (Meleagrina), Lima, Pinna. &c.

Order 4. — Eulamellibranchia.

Pelecypoda in which the gill-filaments are united by vascular
interfilamentar and interlamellar junctions, firm, basket-like gills
being the result : the gills may be smooth or plaited. There are
two equal-sized adductor muscles.

Sub-order a. — Integripalliata.

Eulamellibranchia in which the siphons are small or absent,
and the pallial line on the shell is entire.

Including the Fresh- water mussels (Anodonta and Unio).

Sub-order b. — Sinupalliata.

Eulamellibranchia in which the siphons are of considerable
size, and the pallial line is inflected to form a sinus.

Including the Cockle {Cardium), Mya, Pholas, Teredo (Ship-
worm), Aspergillum, &c. (Figs. 577 — 580).

Order 5. — Septibranchia.

Pelecypoda in which the gills are reduced to a horizontal
muscular partition. There are two adductor muscles.
Including Poromt/a, Cuspidaria, &c.

Systematic Position of the Examples.

Anodonta and Unio are two genera belonging to the family
Unionidoz, sub-order Integripalliata, order Eulamellibranchia.

Their complex basket-like gills are alone sufficient to place
them among the Eulamellibranchia. The incomplete ventral
siphon and the correlated entire pallial line (see p. 683) indicate
their position among the Integripalliata. The regular shell, with
thick brown periostracum and large external ligament, the elon-
gated branchial or inhalant aperture, the long, compressed foot,
and the absence of a byssus, place them among the Unionidse.
Anodonta is distinguished from Unio by the absence of hinge-
teeth.

3. General Organisation.

The most important variations in structure in the present class
are connected with modifications of the gills, the foot, the muscular
system, and the siphons. With the structure of the muscles and
of the siphons are correlated important variations in the shell

XII

PHYLUM MOLLUSC A

697

which arc of great systematic value, especially in cases where, as
with fossils, the shell is the only part available for examination.

In all the Protobranchia, some of the Filibranchia, such as
Arm, and all the Eulamellibranchia and Septibranchia, there
are two almost equal-sized adductor muscles, as in Anodonta.
In many Filibranchs, such as the common sea-mussel (Mytilits
ci/iilis), the anterior adductor becomes greatly reduced and the
posterior correspondingly enlarged ; and in another species of the
same genus (M. lotus) the anterior adductor has completely
atrophied, the function of closing the shell being performed by the

XIV

Fig.. 575. — Anatomy of Fecten. I, palpi ; II, foot; III, aperture of gonad into kidney; IV,
external renal aperture; V, male, and VI, female portion of gonad; VII, pallia! eye;
VIII, visceral ganglion ; VIII', gill ; IX, anus ; X, striated portion of adductor ; XI, smooth
portion; XII, retractor of foot; XIII, heart; XIV, liver; XV, stomach. (From Pelseneer's
Mollusques.)

great posterior adductor alone. In* Anomia and in the Pseudo-
lamellibranchs there is a single immense adductor (Fig. 575, X, XI)
placed nearly in the middle of the greatly shortened body, and
known to represent the posterior adductor — both from the fact that
the rectum passes over it, and from the circumstance that, in the
embryo Oyster, two adductors are present, the anterior of which
atrophies, while the posterior enlarges to form the single muscle
of the adult.

These peculiarities in the muscular system bear their mark
upon the shell, in which impressions corresponding to the

VOL. I Y Y

698

200L00Y

SKOT.

adductors are clearly marked on the inner surface (Fig. 576). The
whole class is, in fact, frequently classified on this basis, species
with equal-sized adductors (Protobranchs, some Filibranchs, and
all Eulamellibranchs and Septibranchs) being called Isomyaria (A),

Fig. 576.— Left valves of A, Mya ; B, Madiola ; C, Vulsella. The upper dotted line passes
through the hinge-lines, the lower connects the anterior and posterior adductor muscles.
(From the Cambridge Natural History.)

those with a large posterior and a reduced anterior adductor (most
Filibranchs) Hekromyaria (B), and those with large centrally
placed posterior and no anterior adductor (Pseudolamellibranchs
and Anomia among Filibranchs) Monomyaria (C).

In many forms, such as Nucula (Fig. 586), Ostrea, &c, the right
and left mantle-lobes are quite free from each other, so that there
are no siphons. In Anodonta and Unio,as we have seen, the two
lobes unite along the line of attachment of the gills so as to
enclose a dorsal or exhalant siphon, a ventral or inhalant siphon
being formed simply by apposition of the lobes ventrally. In such
cases the pallial muscles in their neighbourhood act as retractors
of the short and imperfect tubes thus formed. In other species
a second concrescence of the mantle-lobes takes place so as to con-
vert the inhalant siphon into an actual circumscribed aperture or

Fia. 577.— Cardium edule.

A, exhalant siphon; B, inhalant siphon; F, foot. (From Ihe
Cambridge Natural History.)

short tube. In the Sinupalliata the two siphons are prolonged
into distinct muscular tubes (Fig. 577, A, B) which, in the posi-
tion of extension, project beyond the posterior margin of the
shell and may even be considerably longer than the body. Under

XII

PHYLUM MOLLUSCA

these circumstances the posterior pallial muscles become en-
larged to form retractors of the siphons, and the portion of the
pallial line from which they arise becomes, as it were, pushed
forwards so as to form a bay qy pallial sinus (Fig. 578, p.s). Thus
the shells of species with well-
il.veloped siphons are sinupal-
liate, or have an indented
pallial line, while those with
.small or no siphons are in-
tegripalliate, or have an entire
pallial line. The larger the
siphons the stronger are their
muscles and the deeper is the
pallial sinus : when very large
they cannot be completely
retracted, and the posterior
border of the shell then gapes
permanently. The siphons may
be separate (Fig. 579) or
united (Fig. 580). They are
specially adapted for species
of burrowing habits, which are
able to remain buried in the
mud or sand, only the ends of

the siphons being exposed for the supply of aerated water and
food, and even these can be instantly withdrawn in the event of
danger.

In addition to their union posteriorly to form the siphons, the
mantle-lobes may concresce to a greater or less extent along their
ventral border (Fig. 581), forming a more or less tubular invest-
ment for the body, and leaving an anterior pedal aperture for the

Pig. 578.— Venus gnidia, inner surface of
left valve, al, anterior lateral tooth ; am,
anterior adductor impression ; c, cardinal
teeth ; I. ligament ; lie. lunule ; p, pallial
line ; p. I, posterior lateral tooth ; p. m, pos-
terior adductor impression ; p. x. pallial
sinus ; u. umbo. (From the CaiuOrhlye
Natural -History.)

Fn;. 679.— Scrobiculariapiperata, in its natural position, partly buried in sand. A, exhalant
siphon ; IJ, inhalant siphon. (From the Cambridge Natural History.)

protrusion of the foot. Their anterior portions may also be united
to form a sort of hood.

To return to the shell, the muscular impressions and the pallial
line on which have already been referred to. As a general rule
the right and left valves are alike, or nearly so, the shell being
therefore equivalve. Each valve is inequilateral, being divided into

Y Y 2

700

ZOOLOGY

SECT.

unequal portions by a line drawn from the
umbo to the gape. It will be remembered
that in the Brachiopoda, the only other class
of bivalved animals, the precise opposite is
the case, the shell being equilateral and in-
equivalved. Some Pseudolamellibranchs are,
however, nearly equilateral and markedly in-
equivalved, such as the scallop (Pcctcn), and
the inequi valve character is still more marked
in the oyster, in which the right valve is
deeply concavo-convex and permanently at-
tached to a rock, while the left is flat and
forms a sort of lid. This condition of
things reaches its maximum in the extinct
Hippurites (Fig. 582, B), in which the left
valve has the form of a long tube closed at
one end by the flat lid-like right valve. In
the extinct Requienia (A) the left valve is
spirally coiled, so that it resembles a snail-
shell, and its aperture is closed by the flat
lid-like right valve : in Diceras, also extinct,
both valves are coiled.

The hinge-teeth (Fig. 578) vary greatly
in form and size or may be absent altogether :
the hinge-ligament is usually band-like, but
in Pecten takes the form of a cylindrical cord.
The variations in form, ornamentation, colour,
&c, among the many thousand known species of shell are too
numerous to mention ; but reference must be made to peculiar
modifications found in certain burrowing forms. In Pholas, a

Fig. 580.— Solecurtus
strigillatus. s. af,
inhalant siphon, s. ef,
exhalant siphon, the
two united at SS.
(From the Cambridge
Natural History.)

Fia. 581. — Diagram illustrating the various degrees of union of the mantle-lobes, b.o, byssal
aperture ; /. foot ; s. a, exhalant siphon ; s. b, inhalant siphon ; 1, first point of union between
siphons ; f, second, between inhalant siphon and foot ; S, third, between byssal aperture and
foot. (From the Cambridge Natural History.)

siphonate genus which burrows in stone, the shell is weak and
brittle, and additional calcareous pieces are developed between

Ml

PHYLUM MOLLUSCA

701

the two valves. In Teredo (Fig. 583), the so-called Ship-worm,
which causes great destruction by boring into piles, ships'-timbers,
&c, the valves (v.) remain very small and weak but movable, and
the general surface of the mantle secretes a continuous shelly
tube which lines the burrow. In
Aspcrgillnm (Fig. 584), which lives
buried in sand, there is a similar, but
\vi( lor calcareous tube, with which the
valves are completely fused, and the
anterior end of the tube which ap-
pears above the surface of the sand
is closed by a plate perforated with
numerous holes like the rose of a
watering-pot. The larval shell

IS

Fir,. 582.— Requienia ammonea ; B, Hip-
purites cornu-vaccinum. a, right
valve ; /, point of fixation. (Prom the Cam-
bridge Natural History.)

sometimes, though not always, dis-
tinguishable at the apex of each
valve in the Pelecypoda in general.

In Nucula, Area, &c, the foot
(Fig. 586, ft) presents what may be
considered as its most primitive form,
having a flat ventral surface or sole
upon which the animal creeps. Far
more common is the ploughshare-like
form we are already familiar with in
Anodonta and Unio, adapted for slowly
making its way through sand or mud.
In a few forms, e.g. Trigonia and

Cardium (Fig. 577), it is bent upon itself and is capable of being
suddenly straightened so as to act as a leaping organ : in Mytilus
it is cylindrical (Fig. 585, F): in the Oyster it is absent. In addition
to the anterior and posterior retractors and a pair of protractors,

Fio. 583.— Teredo navalis, in
a piece of timber. P. pallets
(small calcareous plates support-
ing the siphons) ; xs. siphons ;
T. tube ; V. valve of shell.
(From the Cambridgt Natural

llixtoi ■■//.)

702

ZOOLOGY

684 . — Aspergillum.

(After Sowerby.)

the foot is sometimes provided with a levator muscle, particularly
well developed in Nucula and its allies.

Immediately posterior to the foot a byssus-gland is fre-
quently found : it secretes a silky substance in the form of
threads which serve to anchor the animal
permanently or temporarily. It is by means
of the byssus that the Sea-mussel (Mytilus)
is attached to the rocks (Fig. 585, By) : in
Pinna the threads are fine enough to be
woven in a fabric. In Lima the threads of
the byssus are spun into a kind of nest in
which the animals lie protected, and in
species of Modiola similar modifications of
the byssus occur. In such forms as Mytilus
the muscles which ordinarily serve to retract
the foot are inserted mainly into the byssus :
the latter being fixed, they serve to rotate
the animal in various directions or, in other
words, act as adjustors and also as retractors
of the byssus. It must be borne in mind
that the definite byssus just described is not
homologous with the provisional byssus of
Anodonta (p. 693) which lies in front of the mouth.

The gills or ctenida are two in number, right and left. Each
consists of a horizontal axis bearing two rows of filaments, outer
and inner, which are outgrowths from it. In the Protobranchia
(e.g. Nucula) the filaments are short, compressed, and free from one
another (Figs. 586, g,
and 587, A). In Amu-
sium (B) the gill-fila-
ments are much elon-
gated and thread-like
instead of triangular.

In the common Ark-
shell (Area, C) a great
change is seen. The
gill-filaments are deli-
cate and somewhat flat-
tened threads, each bent
upon itself into the
form of an elongated
U, and therefore con-
sisting of a proximal

or fixed limb and a distal or free limb. The flexure takes place
in such a way that the free limb is external in the outer row
of filaments, internal in the inner row. Adjacent filaments are
loosely united by groups of large interlocking cilia (see Fig. 588),

Fio. 586. — Mytilus edulis, attached by byssus (£y.
to a piece of wood. F. foot ; S, exhalant siphon.
(From the Cambridge Natural History.)

lMIYLUM MOLLUSCA

m

placed at regular intervals, and in this way the outer and
inner limbs of the filaments are respectively joined together
so as to convert each longitudinal row of U-shaped filaments
into a double plate, fairly coherent unless the ciliary junctions
are forcibly separated. In this way the single ctenidium of
Nucula has given place to two plate-like simple laminae, each
formed of an outer and an inner lamella : the inner lamella of the
outer and the outer lamella of the inner laminae are united along
their dorsal edges, the line of junction representing the axis of the

Fio. 586.— Adult specimen of Nucula delphinodonta, represented as seen from the right
side. Reconstructed to ibow Internal organs. Folly grown specimens are 4 mm. long.
an. anterior adductor muscle; hg. byssal gland; eg. cerebral ganglion; /. foot; g. gill:
h, heart ; int. intestine ; lp. labial palp ; en. (esophagus ; ot. statocyst ; pa. posterior adductoi
muscle ; pap. j alp-appendage ; pg. pedal ganglion; «<o. stomach; vy. visceral ganglion.
(After Drew.)

ctenidium: the outer lamella of the outer and the inner lamella
of the inner laminae are free dorsally.

In Myiilus (Fig. 587, D) the gill is strengthened by the develop-
ment of delicate non-vascular bars or interlamellar junctions
between the two limbs of each filament. In Lucina these
junctions are large and provided with blood-vessels; and vascular
bars of tissue, the interhlamentar junctions, replace the ciliary
junctions of the lower forms. Thus by a regular series of grada-
tions the ctenidium is replaced by the complex double gill we are

ro4

ZOOLOGY

SKC'T.

already familiar with in Anodonta. In all the higher forms the
outer lamella of the outer lamina unites with the mantle and
the inner lamella of the inner lamina with the visceral mass,
while, posterior to the latter, the inner lamellae of the right and
left inner laminae unite with one another. The blood-vessels,
which are confined to the filaments in the simpler types, occur
also in the interfilamentar and interlamellar junctions in the

Fig. 587. — Half transverse suctions of various Pelecypoda to show the chief kinds of gill.
A, Nucula ; B, Amusinm ; C, Area ; D, Mytilus ; K. Anodonta ; F, Poromya.
a. aperture in branchial septum ; b. v. blood-vessel ; ft. foot ; i. f. inner row of filaments ;
i. g. inner lamina ; i. I. inner lamella ; i, l.j. interlamellar junctions ; m. mantle ; o. f. outer
row of filaments ; o. g. outer lamina ; o. I. outer lamella ; sep. branchial septum. (Modified
from Korschelt and Heider, and Lang.)

more complex forms of gills. In the Septibranchia the gills are
degenerate, being represented by a horizontal muscular partition
or septum (Fig. 587, F, and Fig. 589 IX), which divides the
inhalant and exhalant chambers from one another. Respiration
in this case is performed entirely by the internal face of the
mantle.

Digestive Organs. — The mouth is anterior ; in forms with two
adductor muscles it is always placed immediately behind the

MI

PHYLUM MOLLUSCA

705

anterior adductor. It is usually bounded by two pairs of labial
jnljts which sometimes attain an immense size (Fig. 586); there is
never any trace of jaws or other masticatory apparatus. The
convolutions of the intestine are some-
times very complex. The crystalline style
either lies freely in the stomach and
anterior part of the intestine, or is con-
tained in a ca?cal pouch of the stomach
(Fig. 590), which may be prolonged into
one of the lobes of the mantle. The
anterior end of the style, which projects
into the stomach, appears to be slowly
dissolved by the digestive juice, forming
a sort of cement to enclose the hard
particles of the food and prevent any
harmful effect on the mucous membrane.
It is possible also that the dissolved
substance of the style may play the part
of a digestive secretion, since it appears

to contain a substance of the nature of a digestive ferment
capable of acting upon starchy matters.

The excretory organs occur in their, simplest form in the
Protobranchia, in which they have the form of cylindrical curved

Fio. 588.— Four gill-filaments of
Mytilus. e.j, ciliary junc-
tion ; /. filaments. (From the
Cambridge Natural History.)

VIII

VII VI

Fio. 580.— Dissection of Poromya. I, anterior palp ; II, foot ; III, lamella on branchial septum ;
IV, valve of branchial aperture ; IV, anal siphon ; V, posterior adductor ; VI, posterior re-
tractor of foot ; VII, heart ; VIII, ovary ; IX, branchial septum ; X, anterior adductor. (From
Pelseueer.)

tubes or nephridia, opening at one end into the pericardium
and at the other on to the exterior ; the whole nephridium
is lined with glandular epithelium, and has no communication with
its fellow of the opposite side. In the higher forms the organ

706

ZOOLOGY

becomes differentiated into a secreting portion or kidney — which
is very spongy in texture, and opens into the pericardium,
and a non-secretory portion or bladder, which opens externally.
Frequently there is a communication between the right and left
nephridia, and in some genera, such as the Oyster, the organs
become extensively branched. Also taking a share in the process
of excretion are the pericardial glands, or Kebers organs, glandular
developments of the wall of the pericardium.

Circulatory Organs. — The heart is usually perforated by
the rectum, but lies altogether above it in Nucula (Fig. 586, h)
and some other genera ; the ordinary arrangement seems to have

Pro. 500. — Sagittal section of part of enteric canal of Dcnax. I, lower lip; II, intestine ; III,
pyloric crecum ; IV, crystalline style; V, cuticle; VI, stomach; VII, gullet; VIII, upper
lip ; IX, mouth. (From Pelseneer.)

been brought about by the heart becoming folded over the intes-
tine and united below it. In the Oyster and some other forms the
heart is below the rectum. In Area the ventricle is divided into
two by a constriction. Pores are often found on the surface
of the foot, and it# has been asserted that through them the
external water mixes with the blood ; this, however, is certainly
not the case : the blood-system is everywhere closed. The blood
is red in some forms (e. g., Area) owing to haBmoglobin in the
corpuscles ; in some cases it is of a bluish tint owing to the
presence of haemocyanin.

The nervous system is found in its most primitive condition
in Nucula (Fig. 591). Instead of a single cerebro-pleural ganglion
there are, on each side, distinct cerebral (XVI) and pleural (I)
ganglia, each united by a connective with the pedal,

xtr

PTTYLFM MOLLUSC A

707

The most characteristic sense-organs are the statocysts
( • < >t ocysts ") and the osphradia. The statocyst — "auditory" or
directive organ — is always placed in the foot, in close relation to
the pedal ganglion, sometimes embedded in the latter. The
statocysts are developed as involutions of
the ectoderm and retain their connection
with the exterior in Nucula (Fig. 586 <>t)
.iiid some others. In most cases they
bei ome closed sacs. The cavity is usually
ciliated, but the cilia may be wanting.
Each statocyst may contain a number of
minute statocones or, more usually, a
single, larger statolith. The nerves sup-
plying the statocysts are given off not from
the pedal ganglia, but from the cerebro-
pleural connectives, and their fibres are
derived from the cerebral ganglia.

The osphradia — " olfactory " or water-
testing organs — are patches of sensory
epithelium with an accessory or osphra-
dial ganglion situated in the immediate
relation with the visceral ganglia (Fig.
591, viii), but connected by nerve-fibres
with the cerebral ganglion. Patches of
sensory epithelium, very similar to the
osphradia, and called the abdominal sense-
organs, occur one on each side of the
anus in Area and other forms devoid of
siphons, and a similar organ has been
described beside the retractor muscles of
the siphons in several Sinupalliata.

In a few instances eyes are present,
but never in what we are accustomed to
consider as the normal position for such
organs, at the anterior or head -end of the
body. They occur, in fact, in the only
situation where they can be of any use,
namely, along the edge of the mantle.
The best-known form in which they occur
is the common Scallop (Pecten), which has
a single row (Fig. 575, VII) all round the
mantle-border. Each has a cornea (Fig. 592 1), a cellular (not
cuticular) lens (£), a retina (5) — formed of cells, the inner ends
of which are modified into visual rods, and an optic nerve (7),
one branch of which spreads over the front of the retina and
sends branches backwards to the visual rods. In this peculiarity,
as well as in the cellular lens, the eye of Pecten is singularly

Flo. 591. — Nervous system and
"auditory" organs of Nu-
cula. I, pleural ganglion ;

II, pleuropedal-conneetive ;

III, common connective from
cerebral and pleural to pedal
ganglia ; IV, " auditory "
nerve ; V, pedal ganglion ;
VI, visceral ganglion ; VII,
posterior pallial nerve ; VIII,
osphradial ganglion ; IX,
visceral connective ; X, stato-
cyst ; XI, canal of statocyst ;

XII, its external aperture ;

XIII, cerebro-pedal connec-
tive ; XIV, anterior pallial
nerve ; XV, nerve to palps ;
XVI, cerebral ganglion . ( Fron i
Pelseneer.)

708

ZOOLOGY

SECT.

like that of Vertebrates. The palliah eyes of Pelecypoda are
probably to be looked upon as modified tentacles. The only
cephalic eyes that occur, in this class are a pair of small but well-
developed organs which occur in the bases of the most anterior

Fig. 592.— Vertical section of eye of Pecten. 1, cornea; S, lens; 8, external epithelium;
U, blood-sinus ; 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and Heidcr.)

filaments of the inner lamina of the gill in Mytilus and some other
genera.

Reproduction and Development. — Most Pelecypoda are
dioecious, but several hermaphrodite forms are known: Some of
these, such as the Oyster, are protandrous, the gonad producing
first sperms and afterwards ova : in others part of the gonad serves
as an ovary, part as a testis, the two opening into a common duct :
in others again there is a distinct ovary and testis on each side
opening by separate ducts. There are never any accessory organs
of reproduction, such as spermatheca, penis, &c. Fertilisation

XII

PHYLUM MOLLUSCA

709

frequently takes place i»'vthc water after the eggs are laid. Seg-
mentation is total but unequal, and the gastrula is formed either

mes-

E.

dm ../.J*

TTltS- ^-^ /

'.—Five stages in the development of Ostrea. a. anus ; M. blastopore ; m. mouth ;
ma, stomach ; vies, mesoderm : rk; polar bodies ; s. shell ; sd, shell-gland ; sm. anterior adductor ;
tt>, prc-oral circlet of cilia. (From Korschelt and Heider.)

by invagination or by epiboly. A shell-gland (Fig. 593, sd.) is
formed as an invagination of the dorsal surface, a stomodaeum (m)
as an invagination of the
ventral surface, and the
larva of most forms, un-
like that of Anodonta or
Unio, passes into a stage
in which it closely re-
sembles the trochophore
of Chsetopods (Fig. 593),
having a pre-oral and a
post-oral circlet of cilia, a
tuft of cilia round the
anus, and an apical tuft
in the middle of the pros-
tomium. There is also an
ectodermal thickening on
the prostomium which
becomes the cerebral gang-
lion, and a similar ventral
thickening which gives
rise to the pedal ganglion and corresponds with the rudi-
ment of the ventral nerve-cord in Polychaeta. The pelecypod

Fii;. 0!'4. — Veliger larva of Ostrea. «. anus; 4ml
dorsal longitudinal muscle ; I. " liver " ; n. mouth ;
«<", stomach ; ». shell ; sm. adductor muscle ; st.
hinge of shell ; Vel. velum ; vm. ventral longitu-
dinal muscle. (From Korschelt aud Heider.)

710

ZOOLOGY

SfiUT.

trochophore is, however, distinguished from the corresponding stage
in Worms by the presence of the shell-gland, which soon secretes
a delicate unpaired shell. The prostomial region grows out into a
thickened retractile rim bearing the pre-oral circlet of cilia, and
called the velum (Fig. 594 ml): the larva at this stage is
distinguished as a veliger — a very characteristic molluscan phase
of development. The shell soon becomes bivalved and extends
ventrally on each side, paired processes of the dorsal region of the
body accompanying it and forming the mantle-lobes. A projection
grows out from the ventral surface, between mouth and anus, and
forms the foot (Fig. 595,/), and on the sides of the body the gill-
filaments (k) arise as a row of delicate processes, at first simple

a

n.--

"T /j

£F-

. - "\"f

mr.-

i,-

Pia. 696.— Two embryos of Cyclas. a. anus; by. byssus-gland ; /. foot; g. gonad; k, gill;
m. mouth ; m+l. stomach and " liver " ; mr. edge of mantle ; n. kidney ; p. pericardium ; s'.
unpaired shell ; «". rudiment of paired shell ; sd. shell-gland ; vd. gullet ; vel. velar area.
(From Korschelt and Heider.)

but afterwards becoming bent upon themselves so as to assume a
V-shape. Eyes are often present in the larva at the base of the
velum.

General Remarks. — Although none of the Pelecypoda are
microscopic, they present a considerable range in size, from the
minute Nucula, about 4 mm. long, to the Giant Clam ( Tridacna
gigas) of the Indian and Pacific islands, which is sometimes 60 cm.
(two feet) in length and 500 pounds in weight.

Many pelecypod shells are white or dull brown in colour, but
in several genera brilliant tints are the rule, the various species of
Scallop (Pecten) being specially remarkable in this respect. The
inner surface of the shell often exhibits beautiful iridescent tints,
noticeably in the so-called Pearl-oyster (Meleagrina) and the
Australian Trigonia. As far as is known, the colours are all what
are called " non-significant," i.e. are of no physiological or ethological
importance. In this connection the formation of pearls by some

Kit PHYLUM MOLLUSCA 711

species must be mentioned : they are deposits of nacre formed
usually round encysted parasitic worms, either between the
mantle and shell or in the soft parts. They are produced, amongst
other species, by the "Pearl-oyster" (Meleagrma margaritifera)
and by the Pearl-mussel (Unio margaritifera). Some species,
such as the common boring Pholas, are phosphorescent.

Most Pelecypoda are sluggish in habit, progressing only by slow
contractions of the foot, and some are permanently fixed during
adult life by the byssus, or are only able to change their position
after throwing off the byssus, which becomes replaced by a new
one. The Scallops, however, swim freely by clapping the valves
together. The Cockles {Gardium), Trigonia, &c, jump by sudden
movements of the foot, and the Razor-fish (Solen) jerks itself
forward by suddenly withdrawing its foot and thus ejecting water
through the siphons. The only parasitic genus is Entovalva,
found in the gullet of a Holothurian.

Pelecypoda are abundant both in fresh water and the sea ; the
marine forms are mainly littoral. None are pelagic or terrestrial.
They are very abundant in the fossil condition, occurring in all
formations from the Upper Cambrian upwards, and, owing to
their gregarious habits, frequently forming extensive deposits or
shell-beds. The oldest forms are all iso- or hetero-myarian ; the
monomyarian types (Pseudolamellibranchia) appear first in the
Carboniferous, and "the Siphoniata not until the Triassic period.
The modern genus Area dates from the Upper Cambrian, and thus
furnishes as striking an example of a " persistent type " as some
of the Brachiopods.

There seems to be little doubt that the Protobranchia, and
especially Nucula, exhibit the most primitive type of pelecypod
organisation, as indicated by the plume-like gills with separate
filaments, the simple nephridia, and the distinct cerebral and
pleural ganglia ; absence of concrescence is always a mark of low
or generalised organisation. The Filibranchia with imperfectly
united gill-filaments come next, and are divisible into two groups
— isomyarian with equal-sized adductors, and heteromyarian with
more or less atrophied anterior and proportionally enlarged posterior
adductor; the latter group is to be looked upon as the more
specialised, and leads to the Pseudolamellibranchia (monomyarian
type) in which the anterior adductor disappears completely in the
adult, while the posterior is immensely enlarged and assumes a
central position. Similarly, the isomyarian Filibranchia lead to the
Eulamellibranchia, which are equal-muscled, but have the gill-
filaments united into a complete basket-work. In the Eulamelli-
branchia, lastly, there is a gradual series of stages from compara-
tively generalised forms with free mantle-lobes up to the highly
specialised species with large siphons. That the Pseudolamelli-
branchia and the siphoniate Eulamellibranchia are to be looked

712 ZOOLOGY

SECT.

upon as the highest members of the class is indicated, not only by
morphological evidence but by their comparatively late appearance
in time.

SINUPALLIATE
EULAMELLIBRANCHIA

INTEGRIPALLIATE
EULAMELLIBRANCHIA

PSEUDO-LAMELLIBRANCHIA

HETEROMYARIAN
FILIBRANCHIA

ISOMYARIAN
FILIBRANCHIA

PROTOBRANCHIA

Pig. 596. — Diagram illustrating the mutual relationships of the Pelceypoda.

CLASS II.-AMPHINEURA.

The Amphineura are a class of marine Mollusca formerly
grouped with the Gastropoda, but now recognised as sufficiently
far removed from the latter to require separation as a distinct
class. The commonest, as well as the most highly organised, of
the Amphineura are the Chitons, a group of remarkably sluggish
Limpet-like Molluscs with a shell composed of eight pieces. The
other members of the class are lowly organised, comprising the
most primitive forms of the entire phylum, all of which are devoid
of a shell.

1. Distinctive Characters and Classification.

The Amphineura may be defined as bilaterally symmetrical,
more or less elongated Mollusca, with terminal mouth and anus,
either devoid of a shell, or possessing one which consists of eight
median valves. The mantle contains numerous spicules of carbon-
ate of lime, and is not divided into paired lateral lobes. The
ctenidia are either absent, or there is a single pair, or they
occur as a circlet round the anus, or as two lateral rows situated
between the edge of the mantle and the side of the foot. A
radula (vide infra) is sometimes present, sometimes absent.
The nervous system consists of two pairs of nerve-cords, pedal and
pallial, connected in front with a nerve-ring.

The class is divisible into two orders :

\II

PHYLUM MOLLUSCA

713

Order 1. — Placophora.

Amphineura with a broad foot, and with a shell which consists
of eight transverse valves. There is a row of ctenidia on either side.
This order includes the Chitons.

Order 2. — Aplacophora [Solenogastres].

Amphineura with an elongated body covered completely by the
mantle, without shell, but with calcareous spicules. There is no
foot, but generally a ventral longitudinal groove along which
usually runs a low ciliated ridge. In some there is a posterior
cavity (cloaca or mantle-cavity), containing a pair or a circlet
of ctenidia.

This order includes Neomcnia, Proneomenia, Chcctoderma, and
a number of other genera.

2. General Organisation.

External Features. — The Aplacophora are distinguished
by their worm-like body, sometimes elongated and narrow and
capable of being coiled into a spiral, some-
times comparatively short and thick. In
most instances there is little difference in
external appearance between the anterior
and posterior ends. In Chcctoderma (Fig. 597)
alone is there a distinct head, separated off
from the body by a constriction, as well as
a posterior cloacal region which is similarly
marked off. A shell is completely absent.
The mantle covering the surface possesses
a cuticle, in the substance or on the surface
of which are spicules of calcified material.
Along the middle of the ventral surface runs,
in most instances, a groove, in some cases
merely represented by a narrow strip from
which the cuticle and spicules are absent.
The ventral groove, when present, usually
contains a slight longitudinal ridge, and this
is all that in these simple forms represents the foot, an organ
so highly developed in other Molluscs. In Cha3toderma it is
entirely absent. With the ventral groove is connected in front
an anterior ciliated groove, while behind it is in direct communica-
tion with the cavity of the cloaca.

In Proneomenia ctenidia are absent. In the remaining genera
there is either a pair or a circlet of gills situated in the cloaca

Vir,. Mir. — Choetoderma
nitidulum. a. anus ;
in. mouth. (From the
('a mbridye Natural His-
tory.)

VOL. I

/. Z

714

ZOOLOGY

SECT.

— a cavity at the posterior end of the body into which the anus
opens (Fig. 602).

In Chiton (Figs. 599 and 600) the body is dorso- ventral ly com-
pressed, convex above, and presents below a broad flat foot
(narrow in Chitonellus) which acts not only
as an organ for effecting creeping movements,
but also as a sucker for enabling the animal
when at rest to adhere firmly, like a Limpet,
to the surface of a rock. The head region is
not distinctly separated off, and is not pro-
vided with eyes or tentacles. The most re-
markable external feature of Chiton is the
presence on the dorsal surface of a calcareous
shell (Figs. 599 and 601) made up of no fewer
than eight transversely elongated pieces or
valves, arranged in a longitudinal row, articulating together and
partly overlapping one another. They are sometimes partly, some-
times completely, covered over by the mantle. Each valve consists
of two very distinct layers, a more superficial and a deeper, the latter
formed of compact calcareous substance, the former perforated by

Fio. 598.— Neomenia
carinata. a. anus ;
gr. ventral groove ;
m. mouth. (Prom the
Cambridge Natural
History.)

Fig. 599.— Chiton spinosus, dorsal view.
(From the Cambridge Natural History.)

pip

jnutil

alert.

Fio. 600.— Chiton, ventral view. an. anus ;
cten. ctenidia ; ft. foot ; mant. mantle edge ;
mo. mouth ; pip. palp. (After Pelseneer.)

numerous vertical canals for the lodgment of the sense-organs to
be presently referred to ; the former alone represents the shell of
other Molluscs. External to the valves the dorsal integument
(mantle) of Chiton and its allies is usually beset with a number of
horny or calcified tubercles and. spicules. The mantle develops
only very slight lateral flaps, and under cover of these are a series
of small ctenidia (Figs. 600 and 606, cten.) to the number of

XII

PHYLUM MOLLUSCA

715

From fourteen to eighty. The mouth and anus are both median,
situated at the anterior and posterior extremities respectively.

Alimentary System. — In the Aplacophora the mouth is usually
a longitudinal, rarely (Ghaitoderma) a transverse, slit, situated
veutrally near the anterior extremity. There is a buccal cavity,
with a radula x in some cases (Fig. 602. rad), and in others a single
chitinous tooth supporting smaller denticles : sometimes teeth are
entirely absent. There are both salivary and buccal glands.
-Very characteristic of the group as compared with other Molluscs
is the presence of a straight intestine devoid of coils, and having
connected with it either a single caecum or
a double row of caeca. In the Placophora
the buccal cavity always contains a well-
developed odontophore and radula. The
intestine is elongated and coiled. There
are salivary glands and a large paired
liver (Fig. 003, liv.).

Body-Cavity. — In the Aplacophora the
interstices between the organs and the
body-wall are filled with a form of con-
nective-tissue with muscular fibres ; a
vertical diaphragm (Fig. 602, dia.) separates .
the posterior part of the body, contain-
ing the pericardium (peri), from the
rest. In the Placophora the coelome
(Fig. 603) is an extensive cavity, lined
with a coelomic epithelium, and divided
into three completely separated parts —
the pericardium, the genital cavity, and
the general body-cavity.

Vascular System. — The vascular sys-
tem of the Aplacophora is very rudi-
mentary. There is a heart enclosed in a
pericardium, and composed, when best
developed, of an auricle and a ventricle (Figs. 602 and 605, peri).

In Chiton there is a well-developed heart (Fig. 603, hi.) consist-
ing of a median ventricle and two lateral auricles. The pericardial
cavity in which it lies is a space of considerable extent in the
posterior region of the body, below the two last valves of the
shell.

The Nervous System consists in the Aplacophora (Fig. 604,
A,B,C) of four longitudinal nerve-cords — two pedal and tviopallial.
These are connected together by an oesophageal ring, thickened
dorsally into a single or double cerebral ganglion ; and in front of
this is a second, more slender stomato gastric nerve-ring with small

1 For a description of the structure of this characteristic organ, see the
account of Triton (p. 126).

z z 2

Fig. t>01.— Chiton, valvos of
shell. (tYoin the Cambridge
Natural History.)

716

ZOOLOGY

SECT.

ganglia. The pedal cords (v.vy may present" in front a pair of
ganglionic thickenings connected by a commissure, and further
back there maybe a series of enlargements united by commissures.
The pallial cords (/, /) are connected behind, above the rectum,
by a commissure (p, c) which usually bears a median enlargement.
Sometimes a union takes place posteriorly between the cords of
the two pairs. There are no eyes, or statocysts, or tentacles.
Some have a sensory frontal lobe and a sensory pit or elevation in
the middle line of the dorsal surface near the posterior end.

br/t

perv

Fig. C02.— ChsetOderma nitidulum, longitudinal section, an. anus ; brn. brain ; ccec. gland-
ular cascaof mosenteron ; cten. ctenidium ; ilia, diaphragm separating off the posterior portion
of the body ; mo. mouth ; 'perl, pericardium and heart ; rod. radula ; net. rectum. (After
Simroth.)

In the Placophora (Fig. 604, D) there is an oesophageal nerve-
ring consisting of a thicker dorsal cerebral portion not differentiated
into ganglia, and a thinner ventral buccal commissure. The cerebral
part sends off nerves to the labial palps, the lips, and the buccal
apparatus. Two pairs of longitudinal nerve-cords, pedal'and pallial,
are given off posteriorly : the former, from which arise nerves to
the foot, are joined by numerous commissures passing beneath
the enteric canal ; the latter, which send off nerves chiefly
to the mantle and the ctcnidia, are united together by a supra-
rectal commissure at the posterior end of the body. Just behind
its origin each pallial cord gives off a slender visceral commissure,

Ml

thvum molu'sca

717

which unites with its fellow of the opposite side : two small ganglia
lie in this visceral commissure near the middle line. The large
cords contain nerve-cells throughout their length.

The conspicuous organs of special sense, present on the head of
Gastropods (vide infra), are absent in the Placophora, as in the

hd.

Fio. 60S.— Diagrammatic longitudinal section of Chiton, specially intended to show the
relations of the parts of the crelome, which, except the genital |>arts, arc bordered with a
thick line. an. anus ; ent. enteric cavity ; ft. foot ; gon. gonad ; hd. head-lobes ; lit. heart ;
lit: liver; ,no. mouth; ntpk. nophridium ; peri, pericardial cavity. (From Siniroth, after
Haller.)

Aplacophora. A pair of processes situated in front, at the sides of
the mouth, have the character of labial palps. In the buccal
cavity there are cup-shaped gustatory _ organs supplied with
nerves from the cerebral commissures, and in front of the
odontophore is a thickening of the epithelium — the suhradular
organ — containing nerve-endings. Remarkable sensory organs,
the micrmsthetes and the megalwsthetes, lie in the canals already

"~* — v^-

I T

5

Fi(i. UOi.— Nervous system of Amphineura. A, Pnmmmenia: B, Ntomenia: C, Chatoderma :
D, Chiton, r, cerebral ganglia ; I, I, pallial cords ; pc. posterior commissure ; s, stomatogastric
commissure or ring, with ganglia ; v, v, pedal cords. (From the Ctnabridg* Natural History,
after Hubrecht.)

mentioned as occurring in the superficial layer of the shell- valves.
The megala>sthetes may take the form of eyes, with cornea,
lens, pigment-layer, iris, and retina ; in some cases the lens is
absent.

718

ZOOLOGY

Reproductive and Renal Organs — In the Placophora
the sexes are distinct : in the Aplacophora, with the exception
of Chgetoderma, they are united. In the Aplacophora (Fig. 605),
with the exception of Chsetoderma, the gonads are paired.
The sexual products pass into the pericardial cavity and thence
are carried to the exterior by a pair of ducts (coelomoducts)
opening into the cloaca. Nephridia are unknown in the
Aplacophora.

In the Placophora (Fig. 606) there are two symmetrical
nephridia, each opening internally into the pericardium by a

ciliated funnel-like opening
(n. peri, ap), and externally
{neph. ap) between two of the
posterior ctenidia some little
distance in front of the anus.
Each consists of a looped
main tube, into which open
numerous minute tubules
which ramify among the vis-
cera. The testis and ovary (gon)
are similar in appearance, dif-
fering only in colour when the
products are mature. Each is
an unpaired sac marked by a
series of slight lateral constric-
tions. There are two gono-
ducts, each opening immedi-
ately in front of the corre-
sponding nephridial duct.

Little is known of the
development of the Aplaco-
phora. The eggs undergo
complete segmentation, and
give rise to a gastrula by in-
vagination. This develops into
a form of trochophore with a
ciliated ring, the prototroch.
The larva is provided for
a time with a row of seven calcareous plates on the dorsal
surface.

The eggs of Chiton are fertilised in the mantle-cavity, where in
one species they are retained until the embryos are fully developed.
At first the segmentation is tolerably equal — the ovum becoming
divided into four approximately equal blastomeres ; but at the
stage of eight cells, four on one side are to be distinguished
as larger than the other four. These two sets undergo further
divisions and arrange themselves in such a way as to form

red

aonooi

■perv

Fio. iiiii. Neomenia carinata, reproduc-
tive organs, cop. copulatory organs ; (/on.
gonads enclosed in extensions of the peri-
cardial cavity ; gonod. gonoducts ; peri, peri-
cardium ; red. receptaculum seniinis. (From
Simroth, after Wiren.)

PIIYLUM MOLLUSCA

71!>

a somewhat flattened blastula, one side of which (vegetal pole)
is composed of a comparatively small number of large cells.
Then follows the invagination of the cells of the vegetal side
and the resulting formation of a gastrula : this soon becomes
elongated in the direction of the future long axis. Two
endoderm cells of specially large size in the neighbourhood
of the blastopore, with several others in their proximity, constitute
the rudiments of the mesoderm (Fig. 607, B, mes.); these pass

tfonocl

COfi. — Chiton, ncphridial and genital systems, an. anus ; cten. ctcnidia ; gen.ap genital
aperture ; ;ton. gonad ; gonad, gonoduct ; mo. mouth ; ntph.ap. ncphridial aperture ; n. per op.
aperture from nephridia to pericardium. (From Simroth, after Haller and hang.

into the segmentation-cavity and speedily assume a bilateral
arrangement.

Two rings of cells surrounding the embryo develop cilia (cil.), and
owing to the double circlet thus formed an anterior and a posterior
region are distinguishable in the larva. The blastopore becomes
shifted from its original posterior position forwards on the ventral
surface until it comes to be situated just behind the circlet of
cilia ; it undergoes elongation, and an invagination of ectoderm
round its anterior end forms the mouth (mo.) and stomodreum. A

720

ZOOLOGY

ventral diverticulum of this forms the rudiment of the radular sac
(rd.). By greater relative growth of the post-oral part the embryo
assumes the form of a pear ; and in this trochophore stage, with a
pre-oral circlet and a bunch of cilia in the middle of the apical area,
it becomes free in the case of certain of the species, while in others
it remains enclosed in the egg up to a later stage of development.
As yet there is no anus, that aperture, with the proctodeum,
being formed later by invagination. An apical plate is not
present in the early larva; but the rudiments of the cerebral
ganglia (C, cer. g.), which subsequently appear at the apical pole,
probably represent it. Primitive nephridia, such as occur in
Annulate and many Molluscan trochophores, are not present.

The post-oral region now becomes greatly elongated ; the
mesoderm increases greatly in extent, and forms two well-defined

cil.l

esmt

Fio. C07. — Chiton, development. A, general view of larva ; B, section of early, and C, of later
trochophore. calc. calcifications (rudiments of shell) ; cer. g. cerebral ganglion ; cil. ciliary ring ;
cil. t. ciliary tuft at apical pole ; eye, eye ;ft.gl. foot-gland ; mes. mesoderm ;mesent. mesenteron ;
mo. mouth; rd, radular sac; sp. spines; vise. g. visceral ganglion. (From Korschelt and
Heider, after Kowalewsky.)

streaks, which are afterwards divided into parietal and visceral
layers with a coelomic space between them. The post-oral part
of the embryo now presents an appearance resembling rudimentary
segmentation. This is due to the development of a series of
rudiments of the eight pieces of the shell (B, calc), each of which
is formed independently after the fashion of the entire shell of
other Mollusca.

Ethology, Distribution, &c. — All the Amphineura are ma-
rine. The Placophora occur at all depths, though most abundant
on the shore between tidal limits. The Aplacophora, on the
other hand, are rare in very shallow water, and absent alto-
gether from the littoral zone: some have been found at con-
siderable depths (down to 1,250 fathoms). Some of them burrow

xii PHYLUM MOLLUSCA 721

in mud, others live in association with various colonial
C< i 'It 'literates. The Placophora are all vegetable feeders, their food
consisting of minute alga? and diatoms. The Placophora when at
rest adhere firmly to the surface of a rock or a block of coral by
means of the sucker-like foot. When forcibly detached the animal
curls itself up into a ball, and will only after a considerable time
slowly extend itself again: all its movements are extremely
sluggish.

The Aplacophora have no hard parts that would be recognis-
able in the fossil condition ; but numerous fossil Placophora are
known from the Silurian onwards. The valves of the Silurian
genera differ from those of recent forms in the absence of the
articulations.

CLASS III.— GASTROPODA.

The Gastropoda, including the Snails and Slugs, Limpets,
Whelks, Periwinkles, Sea-hares, and the like, are Mollusca in
which there is, as a rule, a shell composed of a single piece,
and in which the mantle is not divided into two lateral folds as
in the Pelecypoda. The body is inequilateral, owing to the
one-sided development of the visceral mass. There is a well-
developed ventraL foot, usually with a broad, flat surface on which
the animal creeps. A head-region bearing eyes and tentacles is
distinguishable in front of the foot. The alimentary canal is
characterised by the presence in the buccal region of a peculiar
organ, the odontophore, present also in some of the Amphineura,
bearing rowS~oi minute chitinous teeth. Plume like ctenidia are
usually present. A metamorphosis occurs in the development,
during which the young Gastropod passes successively through
trochophore and veliger stages. The majority of the families
of Gastropoda are marine, a few of these being pelagic ; but some
inhabit fresh water, and others are terrestrial.

1. Example of the Class. — The Triton (Triton nodiferus).

Triton is a marine Gastropod living in shallow water, usually
close inshore. The species to which the following description
specially applies has a very wide range, from the English Channel
to the South Pacific, and occurs as a fossil as far back as the
Miocene. In most respects the English Whelk (Buccinum tin-
datum) will be found to conform to the description.

The shell (Fig. 608) is a very hard and dense calcareous
structure, presenting no trace of division into valves such as
compose the shell of the fresh-water Mussel, and lacking also
its bilateral symmetry. It is in the form of an elongated hollow
cone closely wound round a central axis. The apex of the cone

722

ZOOLOGY

SECT.

is the organic apex of the shell, corresponding to the umbo of
the fresh-water Mussel, and is the point from which the growth
of the shell has proceeded: the base is represented by the wide
oblique opening — the mouth or peristome of the shell. Starting
from the apex along the internal cavity of the spirally-wound
cone, in order to reach the mouth in an adult shell, we pass
completely round the central axis five times — i.e. the spiral consists
of five turns. In following the turns, the direction taken is to

the right, that is to say, the
spiral of the shell is a right-
handed or dextral one. The
axis (Fig. 609) is in the
shape of a twisted shelly
rod — the columella — con-
taining a narrow lumen ; it
is formed by the close union
of the axial portions of the
wall of the spiral. The
windings of the spiral are
marked on the outer surface
of the shell by a narrow im-
pressed spiral line or suture,
parallel with which are
numerous fine ridges and
depressions — the lines of
growth ; the increase in size
of the shell takes place in
the direction of these lines,
not at right angles to them
as in the shell of the fresh-
water Mussel, and the lines
that more strictly correspond
to the lines of growth of the
latter are excessively fine
striae which run transversely
to the stronger lines. At
certain points, usually three
in a full-grown shell, the
spiral is interrupted by a transversely directed edge which appears
to overlap the succeeding portion ; this edge marks the position
which the mouth of the shell has occupied during regularly
recurring periods of arrest of growth, probably annual.

The mouth of the shell is bordered on the side turned away
from the columella by a prominent rim or outer lip of the
peristome ; this is produced at the extremity farthest from the apex
of the shell into a spout-like process — the siphonal process. The
prominent edge of the peristome is in relation to the dorsal

Firs. 60S.— Shell of Triton nodiferus.

Natural size.

Ml

PHYLUM MOLLUSCA

723

surface of the body of the animal ; the opposite side has no
prominent edge, but is rounded off to form a smooth inner lip ;
a couple of ridges on this inner lip towards the apical end aid the
animal in drawing itself out after it has become retracted into the
interior of the shell. The outer lip is in relation to the dorsal
surface of the body of the animal, the inner lip in relation to the
ventral surface; the si phonal
process is for the lodgment
of a spout-like process of
the edge of the mantle —
the svpjwn*.

When removed from the
water, or disturbed in any
other way, the animal be-
comes completely with-
drawn into the interior of
the shell, when the latter
is observed to become closed
by a plate — the operculum
(Fig. 610)— which" fi ts ac-
curately across the passage
some distance internal to
the peristome. The oper-
culum is an oval plate of
chitinoid material hardened
by calcareous deposits ; like
the shell itself, it exhibits
lines of growth marking
what has been its edge at
successive stages in the
development of the shell.

The minute structure of
the shell is in the main
similar to that of the fresh-
water Mussel (p. 683). Its
outer surface is covered
with a thin layer of uncal-
cified chitinoid material, the

periodracum, beneath which is a thick prismatic layer, and, lining
the inner surface, a layer of nacre.

External Features of Soft Parts. — The Triton is able to
extend itself to a considerable degree beyond the mouth of the
shell ; but a portion of the body always remains concealed in the
interior, even when the animal has extended itself to its utmost,
the body being, like that of the fresh-water Mussel (and of nearly
all the Mollusca) organically connected with the shell. In Triton
the connection is by means of a strong muscle — the columellar

Fin. 609. — Longitudinal median section of the shell
of Triton nodiferus.

'24

ZOOLOGY

Fio. 610. — Operculum of
Triton nodiferus.

muscle (Fig. 611, col. m.) — which extends from the concave right
side of the animal to the columella, into which it is inserted; it
is by means of this muscle that the anterior portion of the body,
capable of being thrust out through the
mouth of the shell, may again be with-
drawn.

If the Triton be examined in the ex-
tended condition (Fig. 611) it will be
found to present a distinct head, which
bears dorsally a pair of appendages — the
tentacles (tent) — of a sub-cylindrical shape,
slightly compressed towards their bases, and narrowing some-
what towards their free extremities ; these are capable of being
extended -and contracted, but not of being completely retracted.
Each bears on its outer side, some little distance from the base
a prominent eye. At the anterior end of the head on its ventral
aspect is the opening of the mouth. When the animal is
feeding, an elongated cylindrical introvert (Fig. 612), comparable
to that of Sipunculus (p. 492), is extended forwards, bearing
the mouth at its anterior end; at other times the introvert is
completely involuted within the head and anterior portion of
the body. In the male, on the right-hand side of the body some
little distance behind the head, is a long, narrow fleshy process,
broader at the base than at the free end, and deeply grooved
longitudinally ; this is the penis. Running back from its base

manlcav cou

tent

frve.ta.fid rnesopct

Fio. Oil.— Lateral view of the body of a female specimen of Triton nodiferus, removed from
the shell, moderately extended, col. m. columellar muscle ; coll. collar of mantle ; eye, eye ;
liv. liver ; mant. car. mantle-cavity ; meso. mesopodium ; neph. nephridium ; op. operculum ;
M>. ovary ; prop, propodium ; atom, stomach ; tent, tentacles

is a narrow grove with prominent lips — the sperm-groove, con-
tinuous with that of the penis; in the female these parts are
not represented.

Foot. — On the side of the body (ventral) which the animal
applies to the surface of the ground when it extends from the
shell is a flat surface elongated in the antero-posterior direction.

mi PHYLUM MOLLUSCA 725

The wall of the body in this region is composed of a dense mass
of muscular fibres : this is the principal part of the foot {pro-
podium and mesopodium combined) ; the posterior portion (meta-
podium) is a thick process projecting behind this and bearing
the operculum on its surface. The foot is highly contractile, and
it is by means of contractions passing over it in a succession of
undulations that the animal creeps along, dragging after it the
rest of the body enclosed in the shell. In the middle line of its
flat surface, nearer the anterior than the posterior end, is a slit-
like aperture leading into a cavity lined with unicellular glands —
the pedal gland.

When the remainder of the body has been removed from the
shell, it is found to be twisted up into a coil — the visceral spiral,
corresponding to the spire of the shell within which it was lodged.
This is unsymmetrical, the axis of
the spiral being directed not straight
backwards, but backwards, upwards,
and to the right. The external asym-

metry of the body is not strongly intro

marked in the part which is capable

of being protruded from the Shell, but fteJIM.— Diagram of the introvert of

. .,, ° L • i i i • Triton, in longitudinal section,

is still recognisable ; and an examina- as it appears when almost com-

tion of the internal organs shows a 'JgSSStt^Jtu^S.
marked excess of development on the fiwotaw canal ; the cross-hatched

,. i • i • i -i i • i lKlrt th<-' wall of the introvert ;

lett-hand Side, I.e. the Side Which COr- the dotted line marks the position

i •.% .% i • j of the opening through which the

responds with the longer outer side introvert passes. t*£o. introvert;

of the spiral of the shell. The sur- p^gu8°utb: "*" hmm °f ^°"

face of the part of the animal which
is capable of being pushed out from

the shell is covered with a thick integument, which is darkly
pigmented except on the lower surface of the foot. Over the
visceral spiral the mantle forms a thin, delicate, colourless
layer. Anteriorly the mantle becomes thickened and pigmented,
and at the posterior limit of the protrusible part gives rise to a
thickened ridge, the collar (Fig. 611, coll.), forming a semicircle
over the dorsal and lateral regions. In the middle the collar
is not in close contact with the body, but leaves a large cleft
leading into a very wide space extending backwards for a consider-
able distance. This space, which is formed by an infolding of the
mantle, is termed the mantle- or vallial cavity. In it are to be
found the ctenidium, the ospnradium, ana tne anal, excretory, and
reproductive apertures. The wall of the cavity is much folded
and plaited, and contains a quantity of glandular tissue, the
plaits being most numerous on the right-hand side in front of
the anus.

The ctenidium or gill (Fig. 613, ctcn.) is closely applied to the
wall of the mantle-cavity to the left of the middle. It consists of

726 ZOOLOGY sect, xii

a main stem, with which are connected a row of delicate flexible
laminae set at right angles to it : these are broadest in the middle,
becoming smaller towards the ends.

The osphradium (osph.) lies close to the ctenidium on its right-
hand side, i.e. nearer the middle of the body. It presents a central
axis, connected at right angles with which are two rows
of close-set delicate lamellae. Like the ctenidium, it is closely
applied to the wall of the mantle-cavity throughout its length.
The laminae of the osphradium are supplied with nerves which
ramify over them, and its function seems to be to ascertain the
condition of the water that enters the mantle-cavity.

Digestive System. — The mouth, situated at the anterior end
of the introvert, leads into a large chamber with muscular walls,
the buccal cavity (hue). At the sides of the entrance to this cavity
the investing cuticle is thickened to form two distinct horny plates
— the jaws (Figs. 613, 614, and 615, jaw). The jaws are flexible,
and on examination under the microscope are found to be com-
posed of numerous rows of minute bodies, the denticles; the
anterior edge is minutely denticulated. From the floor of the
cavity rises an elevation, the odontophore /(Fig. 613, od., Fig. 614,
odont.), which is somewhat elongated in the direction of the long
axis of the body and compressed laterally. Over the summit of
the odontophore runs longitudinally a narrow strap-like body, the
radula or lingual ribbon (Figs. 614, and 615, rad.), beset with
numerous minute horny teeth arranged in transverse rows. Pos-
teriorly this toothed ribbon passes into a narrow curved pouch
— the radular sac (Fig. 613, rad. s, Fig. 615, rad sac.) — extending
backwards from the posterior and lower aspect of the buccal
cavity. Anteriorly it does not extend beyond the odontophore-
prominence. The latter contains cartilages (Fig. 615, cart.)
serving for the support of the whole apparatus, and is capable
of being extended, with the radula which it bears, through the
opening of the mouth, by the contraction of sets of protractor
muscular fibres. Muscles inserted into the odontophore also
effect the movements of the radula by means of which it
produces a rasping effect on the food, and probably the radula
itself is capable of a certain degree of to-and-fro movement on the
membrane (sub-radular membrane) which lies beneath it. The
entire buccal cavity is capable of being drawn forwards towards
the mouth opening, or backwards into the introvert, by the con-
traction of strands of muscular fibres passing from its wall to the
wall of the body.

From the buccal cavity runs backwards a long narrow tube
with sacculated walls — the oesophagus (Figs. 613 and 615, oe&.).
Posteriorly this opens into a large ovoid sac — the crop
(Fig. 613, crop). The outer surface of the crop appears marked
with numerous close-set fine lines, transverse or oblique in

jaus

Fig 613. — Triton nodiferus. Dissection of the Internal organs of a female, viewed from the
dorsal side. The roof of the mantle-cavity has been divided by a longitudinal incision and the
naps laid out, that on the left bearing the ctenidium and osphradiuui,and thaton the right the
rectum and terminal part of the oviduct. The muscular dorsal wall of the l»ody and the
introvert have been divided so as to bring into view the anterior part of the alimentary canal
and a portion of the nervous system. The buccal cavity has been tilted up and opeued so as
to show the odoutophore, and the oesophagus has been cut through near the anterior end.
A portion of the ventral wall of the crop has been removed so as to bring the internal
folds into view, and the interior of the nephridium with the contained portion of the
intestine has been exposed. Note. The complete course of the intestine through the
nephridium is not shown ; the stomach is not seen, being hidden by the nephridium, and
the ovary is not represented, an. anus ; ant. aort. anterior aorta ; aur. auricle ; hue. buccal
cavity; cev. Iiuc. con. cerebro-buccal connective; ccr. g. cerebral ganglia; crop, crop : cten.
ctenidium ; int. intestine ; jaic, jaw ; I. hue. q. left boon] ganglia ; I. nil. <jl. left salivary
gland; neph. nephridium ; neph. ap. nephridial aperture ; od. odoutophore; »>.«. < esophagus ;
'/,»■'. anterior end of same, cut and turned aside; otpli. osphradium ; odd. oviduct; ovid'.
terminal thick-walled portion of oviduct ; p'evr. n. pleural ganglion ; post, aort. posterior
aorta ; post, or. posterkr oesophagus ; rati. s. radula sac ; red. rectum ; r sal. pi. right salivary
gland ; sal. du. salivary duct ; siph. siphon ; su/>ra. a, supra-oesophageal visceral ganglion ;
tent, tentacle ; tent. n. tentacular nerve ; vent, ventricle.

728

ZOOLOGY

SECT.

vyaw

rad.

Fig. 614.— Triton nodiferus. Interior of the
buccal mass, from above, magnified. I. jaw, left
jaw : odont. lateral surface of odontophorc ; rad.
radula ; r. jaw, right jaw.

direction ; and, when the cavity of the organ is opened, it is found

that these correspond to numerous delicate folds which extend

far inwards, and almost
completely block up the
lumen. On either side of
the crop is a large gland
— the salivary gland (Fig.
613, I sal. gl, Fig. 617,
r. sal. gl.) — partly composed
of a compact glandular
substance, partly of spongy
tissue in which the secre-
tion collects. The two
salivary glands are unlike
in size and shape, that on
the left-hand side being
much longer than that on
the right. Each has a nar-
row duct (sal. du.) which
runs forwards and inwards
to the dorsal aspect of the

oesophagus, where the two come into close apposition, becoming

embedded in the wall of the oesophagus, along which they run

forwards to open into the buccal cavity.

From the crop leads backwards and to the left a narrow

cylindrical tube — the 'posterior oesophagus. On this follows

a stomach (Fig.

611,stom.) which

is in the form of

a U-shaped tube

partly embedded

in the substance

of the digestive

gland or " liver,"

the hepatic ducts

from which open

into it. The tu-
bular stomach is

followed by a

somewhat nar-
rower tube — the

intestine (Fig.

613, int.). This

enters the cavity

of the nephridium, round the interior of which it bends; and,

leaving it at its right-hand side, runs forwards in a straight

course as the rectum (red.), embedded in the glandular wall of the

rad hocL.cccv

jcvw

"bocL.ccuv

CU.7'1

ra.ciL.sac

Fio. 615.— Triton nodiferus. Diagrammatic longitudinal ver-
tical section of buccal cavity, bod. car. body-cavity ; cart, car-
tilage of odontophore ; jaw, right jaw ; of*, oesophagus ; rad.
radula ; rad. sac. radula sac.

xii PHYLUM MOLLUSCA 729

mantle-cavity, to near the anterior end, where it terminates in
a short, freely projecting, spout-like portion, with the anus (an.)
at its extremity.

The digestive gland or " liver " forms a mass of reddish-brown
glandular follicules which compose the greater part of the bulk of
the visceral coil.

Vascular System. — Close to the base of the ctenidium, behind
it and a little to the right, is the heart, lodged, like that of the
fresh- water Mussel, in a cavity, the pericardium, lined by a trans-
parent membrane — the pericardial membrane. The heart consists
of two chambers, an auricle (Fig. 613, aur.) and a ventricle. The
auricle, which is the smaller of the two, is situated somewhat in
front of the ventricle, close to the ctenidium, from the main
central vessel of which it receives the blood. The ventricle (vent.)
is of somewhat pyramidal shape, but with the edges rounded
off. Its wall is extremely thick and muscular. Passing out from
the ventricle towards the right is a thick artery, which soon
divides into two, one running forwards, the other backwards — the
anterior (ant. aort.) and posterior (post, aort.) aortw. The former
is a very large trunk which runs forwards below the posterior
(esophagus, crop, and anterior oesophagus, giving off branches to
the region of the head as it goes. The posterior aorta, narrower
than the anterior, passes into the visceral spiral, where it breaks
up into branches for the supply of the various parts. The blood-
system consists in large measure of sinuses, as in the fresh-water
Mussel, and the general course of the circulation is similar to
what has already been described in that Mollusc (p. 690).

Excretory System. — There is only one nephridium (neph.),
a large organ situated dorsally, behind the pericardium. It is
a sac with thick, glandular, aud highly vascular walls, the
inner surface of which is thrown into numerous complex folds.
In front it communicates directly by a large aperture (neph. ap.)
with the mantle-cavity, and by a narrower passage with the
pericardium.

The nervous system (Figs. 616 and 617) is more highly
elaborated than in the fresh- water Mussel. Two pairs of nerve-
ganglia — the cerebral (cer. g.) and the pleural (pi. g.) — lie close
together over the posterior part of the (esophagus, just where it
passes into the crop. The right and left cerebral ganglia are
fused together in the middle line, though separated by a con-
striction, and the ganglia of the two pairs are placed very close
together, though quite distinct. From each cerebral ganglion
there passes forwards a stout cercbro-buccal connective (cer. hue. con.)
to a buccal ganglion (r. buc. g.) situated on the posterior surface of
the buccal chamber. Also given off anteriorly from the cerebral
ganglia are optic nerves (opt. n.) to the eye and tentacular nerves
(tent, n.) to the tentacles. From each cerebral ganglion passes

VOL I 3 A

730

ZOOLOGY

—cerrhuccorv

tsisc.n

r.ahd.

Fig. C16. — Triton nodiferus. Nervous system, from the dorsal side. cer. buc. con. cerebro-
buccal connective ; cer. g. cerebral ganglion ; col. n. nerves to the columellar muscle ; infra, g.
infra-oasophagcal visceral ganglion ; I. abd. g. left abdominal ganglion ; I. Or. n. left branchial
nerve ; I. br. n'. nerves to branchia and osphradium ; I. mant. n. left mantle-nerve ; opt. n.
optic nerve ; ped. con. cerebro-pedal and pleuro-pedal connectives ; ped,. g. pedal ganglia ; pi. >/.
pleural ganglion ; r. abd. g. right abdominal ganglion ; r. br. n. right branchial nerve ; r. buc.
gang, right buccal ganglion ; r. mant. n. right mantle-nerve ; supra, g. supra-intestinal visceral
ganglion ; tent. n. nerve to tentacle ; vise. n. visceral nerve-branches

\n PHYLUM MOLLUSCA 731

downwards and forwards a stout cerebro-pedal connective, and from
each pleural ganglion a plcuro-pcdal connective, to a large pair of
closely-united pedal ganglia (Figs. 616 and 617, ped. g.) embedded
in the upper layers of the muscles of the foot, to which they give
off numerous nerves. The right pleural ganglion gives off behind a
supra-intestinal visceral connective, which bends across to the left,
over the oesophagus, and, some distance to the left of the ali-
mentary canal, expands into a triangular supra-intestinal visceral
ganglim {supra, g.), situated below the superficial layer of muscular
fibres. The left pleural ganglion gives off an infra-intestinal
visceral connective, which passes obliquely backwards and to the
right, below the alimentary canal, to a ganglion situated a little to
the right of the middle line — the infra-intestinal visceral ganglion
{infra, g.). The supra-intestinal ganglion gives off a nerve which

r.sal.gl

.cLil

xrvfra^ ^^fe cer.buc.con.

pedgr

Pkj. til".— Triton nodiferus. Lateral viow of nerve-ganglia and related parts. Letter! as in
Fig. Olti ; in addition — ant. aort. anterior aorta ; cr. crop ; <e*. oesophagus ; sal. du. salivary
duet ; r. sal. <jl. right salivary gland.

runs towards the osphradium and ctenidium, which it supplies
with branch nerves, and unites with a stout mantle-nerve {I. mant. n.),
which is given off from the left pleural ganglion. The right
pleural also gives off a stout connecting nerve to the infra-intes-
tinal ganglion. From the supra- and infra-intestinal ganglia the
left and right visceral connectives are continued backwards and
unite behind in the neighbourhood of the stomach ; each ends
in a triangular abdominal ganglion {I. abd. g. ; r. abd. g.), and these
are joined by a transverse commissure, from which a number of
visceral nerves (vise, n.) are given off. A remarkable torsion of
the nerve connectives is here to be observed, the two visceral
connectives becoming twisted into the form of the figure 8.

The organs of special sense of Triton, in addition to the
tentacles and the osphradiion, which have been already referred to,
are the eyes and the statoeysts. The eye (Fig. 618) is a rounded
invagination of the epidermis with an inner wall or retina {ret.)

3 A 2

732

ZOOLOGY

CO

rets

composed of pigmented and sensory cells. The latter (retinophores)
are elongated cells narrowed at their central free ends, and pro-
duced at the opposite extremity to become continuous with nerve-
fibres of the optic nerve. The former (retinulce) have their free

extremities much enlarged, and
surround the slender ends of the
retinophores. A layer of short
rods (rds.) lies within the retina
proper. The outer wall is thin,
and, with the overlying epidermis,
forms a transparent cornea. In
the interior of the eye is a clear
rounded lens (I.) of dense cuti-
cular matter secreted by the cells
of the retina ; this is surrounded
by a less dense vitreous body.

The sexes are distinct. There
is a single gonad — ovary or testis
as the case may be — lodged in
the visceral spiral. The sperm-
duct is a white tube, thickish and
much convoluted where it leaves
the testis, narrower and straight
distally ; it opens in front in the mantle-cavity into the proximal
end of the sperm-groove, which, as already mentioned, runs
forwards along the right side and becomes continous with the
groove traversing the penis. The oviduct (Fig. 613, ovid.) is
proximally a very delicate tube with colourless, transparent walls.
This runs forwards to the right side of the mantle-cavity, where
it assumes the character of a stout tube (ovid.') with thickened
glandular walls, which passes forwards close to and parallel with
the rectum, and opens on the exterior near the anus.

opl.rv

Fig. (318.— Triton. Section of the eye-
co. cornea ; ep. epidermis ; I. lens ; opt.
optic nerve ; rds. layer of rods (the line
is not continued far enough inwards) ;
ret. retina.

2. Distinctive Characters and Classification.

The Gastropoda are unsymmetrical Mollusca, with a mantle
which is not divided into two lateral portions and usually a shell,
which does not consist of two lateral valves, but of a single, un-
symmetrical, usually spirally coiled valve, enclosing a visceral mass
of corresponding form. There are, typically, two plume-like
ctenidia enclosed in a mantle-cavity, but there may be only one ;
and in air-breathing forms ctenidia are not developed, respiration
taking place through the wall of the mantle-cavity itself. A dis-
dinct head bearing eyes and tentacles is present in the majority.
The foot is situated behind the head, and usually has an extensive
flattened ventral surface. The buccal cavity contains an odonto-
phore. The nephridium is usually single. The nervous system

mi PHYLUM MOLLUSCA. 733

contains distinct cerebral and pleural, besides pedal, visceral, abdo-
minal, and buccal ganglia. The sexes are sometimes separate,
sometimes united. The larva passes through trochophore and

\ t'liger stages.

Sub-Class I.— Streptoneura.

Gastropoda in which the visceral connectives are in most cases
twisted into a figure of 8, and in which the sexes are distinct.

Order 1. — Aspidobranchia.

Streptoneura with the nervous system but little concentrated :
the pedal ganglia are produced into long cords with the anterior
ends of which the pleural ganglia are fused ; the cerebral ganglia
wide apart; the osphradium little developed. There is nearly
always a single ctenidium or a pair, plume-like and free distally.
The auricles and the nephridia are usually paired.

Sub-Order 1. — Docoglossa.

Aspidobranchia in which the pleural ganglia are not connected
with the opposite visceral connective. The eye is in the form of
an open pit, without lens. There are two osphradia, a single jaw,
and no operculum: The visceral mass is conical.

This section includes the Limpets (Patellidce).

Sub-Order 2. — Jihipidoglossa.

Aspidobranchia in which each pleural ganglion is connected
with the opposite visceral connective. The eye is a closed sac
and contains a lens. There are nearly always a single osphradium,
a pair of jaws, and two auricles.

This sub-order includes the Ear-shells (Haliotidce), Trochus,
Turbo, and others.

Order 2. — Pectinibranchia.

Streptoneura with a somewhat concentrated nervous system.
There is a single osphradium which is often pectinate. The
primarily right ctenidium is alone developed. The heart has a
single auricle. The ctenidium consists of a. stem with a single row
of lamellae, attached throughout its length to the wall of the
mantle-cavity.

Sub-Order 1. — Platypcda.

Pectinibranchia with the foot flattened ventrally, at least in
front. Jaws are nearly always present.

This sub-order includes the Cowries, the Vermetes, the Tritons,
the Whelks, the Cones, and a number of other groups.

734 ZOOLOGY . sect.

Sub-Order 2. — Heteropoda.

Pelagic Pectinibranchia with the foot laterally compressed and
bearing, at least in the male, a ventral sucker. The visceral sac
and mantle form only a small part of the mass of the body. Jaws
are absent.

Sub-Class II.— Euthyneura.

Gastropoda in which the visceral connectives are not twisted
into a figure of 8, and in which the sexes are united.

Order 1. — Opisthobranchia.

Marine Euthyneura with aquatic respiration, the auricle of the
heart usually posterior to the ventricle. The mantle-cavity, when
present, opens by a wide aperture.

Sub- Order 1 . — Tectibranchia.

Opisthobranchs provided in nearly all cases with a mantle and
a shell, nearly always with a true ctenidium, and an osphradium.

This section comprises the Aflysiidw, or Sea-hares, and several
other families, including certain pelagic Gastropoda, some shell-
bearing, some shell-less, formerly regarded as constituting a
distinct class — the Pteropoda.

Sub-Order 2. — Nudibranchia.

Opisthobranchs which are devoid of a shell in the adult condition,
and have no true ctenidia or osphradia, respiration being carried
on by means of secondary branchiae usually arranged in a circlet
around the anus, or in rows on the dorsal surface, or laterally tinder
the edge of the mantle.

This sub-order includes Doris, Eolis, Tetliys, and other shell-less
forms.

Order 2. — Pulmonata.

Euthyneura devoid of ctenidia, respiration being carried on
through the walls of the mantle-cavity, which has a narrow
contractile aperture.

This sub-order includes the Land-Snails and Slugs.

Systematic Position of the Example.

Triton nodiferus is one of several species of the genus Triton,
which is the only member of the family Tritonidw, belonging to
the sub-order Platypoda. The family Tritonidae differs from the

xii PHYLUM MOLLUSCA 735

other families of the sub-order in the possession of a proboscis, of
a well-developed, but not greatly elongated, siphon, and of a short
foot.

3. General Organisation.

External Features, Symmetry, &c. — Few Gastropods make
an approach towards even superficial symmetry, and in cases in
which there is a near approximation towards such a state of
things, it seems clear from the results of the study of develop-
ment and of a comparison with allied forms that the symmetry
presented is not primitive, but has been secondarily acquired —
such symmetrical forms having been derived from unsymmetrical
ancestors.

The departure from symmetry is most marked in the majority
of the Streptoneura. It may be said to be due to the develop-
ment of a protective shell composed of one piece and extensive
enough to be capable of enclosing all the soft parts; and to
the extension of the foot on the ventral side as an elongated
muscular creeping organ. The development of the shell rendered
necessary an arrangement of the parts whereby the mantle-cavity
with the anus, the ctenidia and the excretory apertures, should
come to be situated in the neighbourhood of the opening of
the shell, i.e., towards the heacl-end of the animal. The mantle-
cavity and associated parts {yallidl complex, as the whole is termed)
had, therefore, to be shifted forward from its primitive posterior
position, and this was probably effected by arrest or retardation of
growth on one side and active extension on the other. In the
majority of cases it is the right side the growth of which becomes
retarded, and, in consequence, it is on the right side that the
pallial complex comes to travel forwards. The effect is as if, the
head retaining its symmetry, the parts between it and the anus
had undergone a process of rotation or torsion through about 180°
around a vertical axis passing in a dorso-ventral direction —
the direction of torsion being opposite that of the movement
of the hands of a watch (Fig. 619).

With regard to the spiral form assumed by the shell in all
highly developed Gastropods, it can only be pointed out here that,
given the necessity for complete protection, compactness, and
a provision for continuous growth, the spirally-coiled cone is
the form of shell best adapted to all the conditions. A straight
cone, however directed, would be a great impediment to active
progression, and the coiling in a compact spiral would seem to be
the line of development best adapted to secure concentration and
strength.

The rotation around a dorso-ventral axis is not the only form of
torsion leading to the markedly unsymmetrical disposition of parts
observable in most Gastropoda. There is also a process of torsion

vol. i 3 A 2*

736

ZOOLOGY

SECT.

around a horizontal axis which, the head and foot being regarded
as fixed, results in the visceral mass enclosed in the spiral shell
coming to occupy a more or less dorsal position with the 'apex
directed backwards and to the right.

■vtsc.com.

Fig. 619. — Diagrammatic representation of the displacement of the mantle-cavity and associated
parts in the Gastropoda. Enteric canal blue, blood-system red. A, represents a nearly
symmetrical arrangement ; in B, C, D, are represented successive stages of displacement of
the mantle-cavity to the right and forwards ; in E, the anus and (primarily) right ctenidium
have passed the middle line. an. anus: aort. aorta; cer. g. cerebral ganglion; I. cten. left
ctenidium ; I. vise. com. left visceral connective ; mant. mantle ; mo. mouth ; neph. ap. nephri-
dial apertures ; ped. g. pedal ganglion ; pi. g. pleural ganglion ; r. cten. right ctenidium ; r.
vise. com. right visceral connective ; vise. com. visceral connectives. (After Korschelt and
Heider.)

The shifting of the pallial complex in many Streptoneura
proceeds so far that the complex completely or partially passes
across the middle line, and the anus comes to be situated to
the left of the mouth. The displacement of the pallial complex
involves two important series of changes in the internal organs, in
addition to the suppression of certain structures to be referred to

m, PHYLUM MOLLUSCA 737

presently. In the first placed there is necessarily involved a
throwing of the enteric canal into a loop (Fig. 619), and in
the second place the long pleuro-visceral connectives (viae, com.)
become twisted in such a way as to assume the form of the figure
8 — the right connective becoming supra-intestinal and the left
infra -in testina I.

Universally accompanying the process of forward displacement
of the pallial complex, except in Haliotis and Fissurella and allied
Rhipidoglossa, occurs the reduction of its paired parts. Thus in
all the more highly developed Streptoneura the primitively right
(topographically left) ctenidium alone persists, and one of the two
nephridia is alone fully developed and functional.

In the Euthyneura there are distinguishable various stages in a
process of detorsion by which the torsion tends to be reversed and
the pallial complex carried back towards the posterior end along
the right side. The pleuro-visceral connectives lose their twisted
arrangement in nearly all such cases ; but there is the same
reduction of the paired parts of the pallial complex.

The shell in the adult limpets (Patella and allied genera) is in
the form of a short cone. In most of the Gastropoda it is in
the shape of a spiral .with the turns
usually in close contact with one
another, the inner walls of the turns
coalescing to form an axial, hollow or
solid column. — the columella. The por-
tion of the shell projecting inwards
between the turns of the spiral some-
times becomes absorbed. In certain
cases, on the other hand, the cavity of
the apical portion of the spiral may
be cut off from the cavity of the
rest of the shell by the formation of a
transverse partition, the animal then fio. 62o.-sheii of solarium per-

* . , j ,i 'Ui Bpectivnm, from the under

becoming restricted. tO tne Dasai side- (Kroin the Cambridge Ratvral

portion ; or several such partitions «*wr)
may be formed. By far the greater

number of such spiral shells are dextral, i.e. if we begin at the
apex of the spiral, to reach the opening of the shell we have
to pass from left to right, with the columella always on our right-
hand side : in a few cases, however, the spiral is sinistral, taking
the opposite direction from that of the ordinary dextral shell.
The form of the shell varies with the degree of obliquity with
which the whorls are set on the axis. When the obliquity is very
slight (Fig. 620) the spiral is nearly flat ; when the obliquity is
great, an elongated tapering shell such as that represented in
Fig. 621 is the result. Sometimes the later whorls completely
cover over the earlier ones, so that the spiral for,m of the shell

738

ZOOLOGY

is concealed. Sometimes only the apical portion of the shell
is spiral, the remainder being a straight or sinuous cylinder. The
spiral form of the shell and the parts enclosed in it, as well as the
direction of the spiral — whether dextral or sinistral — are, it may be
here pointed out, very fundamental features of the organisation of
the Gastropod, and are foreshadowed at an early stage in the
segmentation of the ovum. The mouth of the shell has usually
a prominent margin or peristome, which is
sometimes entire and continuous, sometimes
broken by a deep notch or a spout-like process
or canal, formed in connection with the de-
velopment of a spout-like prolongation of the
mantle, the siphon, which lies in it. The
mouth of the shell in many Gastropoda is
capable of being closed by means of an oper-
culum borne on the foot. In some terrestrial
forms in which an operculum is absent, the
opening may be closed up during winter by a
layer of hardened mucous matter to which
the name of epiphragm is applied. The margin
of the mantle in some cases bears a series of
tentacles. Lateral folds of the mantle are in
some of the Gastropoda (Fig. 622) reflected
over the shell and may completely cover it.
In some cases these folds unite by their edge,
so that the shell comes to be enclosed in a
complete sac of the mantle ; such enclosed
shells are always imperfectly developed and
incapable of covering the body. Thus in
Aplysia and some other Opisthobranchs the
shell is greatly reduced, thin and horn-like,
and concealed within the mantle, while in the
nudibranch members of the same sub-order it
is entirely absent (Fig. 623). The shell is also
completely absent in some of the pelagic forms
( Ileteropoda and Pteropoda) ; in others, though
present and external, it is too small to enclose
the animal (Fig. 624). In the slugs, among
the Pulmonata, the shell is vestigial and in
most cases is concealed by the mantle (Fig. 625).

The foot varies in the extent of its development in the different
families of the class. It usually presents an elongated fiat ventral
surface on which the animal creeps by wave-like contractions
of the muscular tissue. An exceptional case is that of Caecum, in
which the creeping movement is entirely due to the action
of cilia covering the ventral surface. In the typical Gastropods
the foot is usually distinguishable into three parts, a middle part

Fir.. 621.— Shell of
Terebra oculata.

MI

PHYLUM MOLLUSCA

F39

or mesopodium, which is the most important, with a smaller
anterior propodium
and posterior mcta-
podium. In many bur-
rowing forms (Fig.
626) the propodium
is well developed and
sharply marked off to
act as a burrowing
organ. In a few cases
a pair of tentacles —
the pedal tentacles —
are situated at the anterior end of the foot ; still rarer is a pair
of similar appendages at the posterior end. The whole foot

becomes reduced in the few Gastropods
i*. ^a^tete^ ,-fL that remain fixed. The metapodium

very usually in the Streptoneura bears

Fio. 022.— Cypraea moneta (Cowrie). Showing the mantle,
provided with marginal tentacles, partly enveloping the
shell. Br. siphon ; M.M. mantle ; F. foot ; T. tentacles at the
edge of the mantle. (Prom Cooke, after Quoy and Gaimard.)

Pi ... B8.— Porta (Archidoris
tuberculata. a. anus ; br.
braachte; <«, jxjnis ; rh, rh,
tentacles. (From the t'ainbriilge
Sut, nil History.)

Fin. Carinaria mediterranea. a. anus;

br. branchia ; /. foot ; i. intestine ; m. mouth ; p.
penis ; ». sucker ; ah. shell ; t. tentacles. (From the
( i inil n- idy t Natural History.)

a disc or stopper — the operculum already referred to — usually
horn-like, rarely completely calcified, more commonly horn-like with

a thin calcareous invest-
ment— by means of which
the aperture of the shell is
closed when the animal is
retracted.

In some forms, such as
the Sea-hares (Aplysia, Fig.
627), the foot develops a
pair of lateral lobes — the
parapodia — which act as
fins ; and in the Ptero-
pods (Fig. 628) which are

P». 825.— A Slug (Umax). PO, pulmonary QT,APir,llv moHifipd for a
aperture. (From the Cambridge Natural Hittory.) Specially mOUineU IOl d

740

ZOOLOGY

SECT.

pelagic existence, these constitute the largest part of the foot.
In the Heteropoda (Figs. 629, 630) which are also pelagic,
the foot is also modified to act as a swimming organ. In one
family of this sub- order (Fig. 629) all three parts of the foot

js.aji

Fir. 626.— Sigaretus laevigatas, exemplifying great development of propodium (pr.) and
metapodium (met.), in a burrowing Gastropod. The shell has been removed, f. mesopodium ;
I. "liver" ; s. ap. aperture of siphon ; t. t. tentacles. (From the Cambridge Natural History,
after Quoy and Gaimard.)

are well-developed, the mesopodium bears a sucker, and the
metapodium an operculum ; in the rest the mesopodium is alone
well developed and forms a laterally-compressed, vertically-
elongated fin. The term epipodium is applied to a ridge or
fold, which, when best developed, runs around the entire edge
of the creeping sole of the foot, and is beset
with papillae or tentacle-like processes.

A pedal gland is present in the majority :
it is a simple or branched invagination of
the integument, lined by mucus-secreting
cells. Very commonly, as in Triton, it opens
on the exterior in the middle line of the
ventral surface of the foot.

The Gastropoda have a well-marked
head, separated from the body by a con-
striction or neck. The mouth, situated at
the anterior end of the head on its ventral
aspect, is in many instances provided with
a protrusible proboscis or introvert, some-
times of considerable length. On the dorsal
surface of the head are a pair of tentacles
which vary a good deal in shape, but are
usually cylindrical or club-shaped. In most
cases the eyes are situated on tubercles at the
bases of the tentacles, or elevated towards
the middle ; but in the snails and slugs (Pulmonata, Fig. 631) the
eyes are elevated on the extremities of a second, longer, pair of
tentacles (oc. tent) placed behind the first.

The mantle is usually developed into a fold — the mantle flap —

Fig. 627.— Aplysia, dorsal
view, r, parapodia. (After
Keferstein.)

PHYLUM MOLLUSCA

741

originally posterior, but subsequently becoming shifted round, in
the course of the displacement already referred to, to the right-
hand side. This covers over a cavity— the mantle-cavity—
situated anteriorly, in which are situated the anal and nephridial

Fig. 628.— Shell-bearing Pteropoda. /. /. fins; I. liver; o. ovary; sh. shell. (From Cooke

after Souleyet.)

apertures and the ctenidia. The edges of the mantle-flap may
become united together in such a way as to form a chamber
opening on the exterior by a comparatively narrow opening.
In many of the Prosobranchia the edges- of this aperture are
drawn out into a- spout-like prolongation - open ventrally — the

Fig. 629.— Atlanta peronii. a, cerebral ganglia ; b, pedal ganglia ; e. eye ; g, ctenidia ;
h. heart ; k, nephridinm ; I. " liver" ; in. mouth ; o. ovary ; p, operculum ; t. testis

siphon — which liss in the corresponding prolongation of the peri-
stome of the shell and serves as a channel for the ingress and egress
of water. In some Gastropods, however, there is no definite
mantle-cavity, the anus, nephridial apertures, and ctenidia
merely lying under cover of a comparatively slightly-developed

742

ZOOLOGY

lateral mantle-flap. Usually there is on the inner surface of the
mantle a glandular area — the pallial mucus-gland.

Respiratory Organs. — There are typically two ctenidia, one on
the right side and the other on the left, contained in the mantle-

pr*ob

Fig. 630. — Pterotracliea scutata. ali. alimentary canal; cten. gills; eye, eye; fi. float;
mo. mouth ; pvob. proboscis ; repr. gonad ; sh. shield covering a portion of the dorsalsurface ;
«!. sucker.

cavity ; but in the great majority of the Streptoneura and branchiate
Euthyneura the primitively right (actually left) ctenidium alone is
retained. In those Gastropoda that possess two ctenidia, and in
many forms with only one, the axis of the ctenidium bears two
rows of compressed filaments, and is attached only towards its
base. But in the majority of those with one ctenidium there
is, as in Triton, only a single row of filaments retained, and the
organ is attached throughout its length.

fe. In the Nudibranchs true ctenidia are absent, but their place
as breathing organs is taken by a number of secondary branchial,

oc.lbenc

lent

ci-rv

pulm.

Fig. 631. — Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye-bearing
tentacles ; pulm. opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.)

sometimes simple, sometimes branched or pinnate processes, which
are distributed over the dorsal surface, as in Eolis\ or, as in
Doris (Fig. 623), form a circlet surrounding the anus; or, as in
Pleurophyllidia (Fig. 632), a row on each side beneath the
mantle-flap.

\II

PHYLUM MOLLUSCA

74*

In the limpets (Patella and its allies, Fig. 633) the true
ctenidia are represented only by a pair of vestiges, and respiration

L*«r

|TO. 632. Pleurophyllidia lineata,
(ran the ventral surface, a. anno;
In: secondary branchiae: iii. month;
*. o. sexual opening. (Prom the Cam-
bridge Natural History. y

PlO.fSSS. — Patella vulgata, seen from theven
tral side. /. foot ; </. I. circlet of gill-lamelhe ;
rii. e. edge of the mantle; mu. attachment-muscle;
si. slits in the attachment-muscle ; sh. shell ;
r. efferent branchial vessel ; ?•'. aorta ; »-e. smaller
vessels. (From the Cambridge Natural History.)

is carried on by a number of secondary branchiae (g. I.) in the form
of lamellae situated between the short lateral fold of the mantle and
the foot. In the Pulmonata, and in some members of other groups,
ctenidia are absent, vuj v

and the mantle-
cavity, completely
enclosed except for
a small rounded
opening, has the
function of a pul-
monary sac or lung
(Fig. 634), its roof
being richly supplied
with blood-vessels :
in the aquatic forms
its function is ap-
parently as much
hydrostatic as re-
spiratory. In one
family of Pulmonata,
the pulmonary chamber gives off a number of branching air-tubes
or traduce. In some of the Pulmonata there is a return to

perve

I'ent

Fio

634. — Pulmonary cavity and related parts in a slug
Limaxi. aort. aorta; aur. auricle; neph. nephridium ;
peric. pericardium, laid open ; put. ap. pulmonary aperture ;
pul. r. pulmonary vein with its ramifications ; rert. rectum ;
ur. ureter ; rent, ventricle. (After Pelseneer.)

744 ZOOLOGY sect.

a completely aquatic mode of respiration accompanied by the
development of secondary gills — vascular processes of the Avail
of the mantle-cavity.

Near the base of each ctenidium is an elevation — the
osphradium — corresponding to the body of that name in other
Mollusca and having a similar function.

Digestive Organs. — In many Streptoneura there is a long
introvert, capable of being everted and retracted, at the
extremity of which the mouth is placed. A single curved horny
jaw lies on the roof of the buccal cavity in the Pulmonata; in
most Streptoneura (as in Triton) the place of this is taken by
two lateral pieces.

A characteristic feature of the alimentary canal of the Gastro-
poda, which, however, they share with some Amphineura and with
the Cephalopoda, is the possession of an odontophore and radula,
a typical example of which has been described in that of Triton.
In the different groups differences are observable in the odonto-
phore as regards the proportions of the parts, and the size, form,
and arrangement of the teeth. The structure and relations of the
alimentary canal are similar to what has already been described in
Triton, and salivary glands and " liver " (hepato-pancreas) are
always present. The former may be tubular, but are usually
botryoidal : the latter varies in relative extent and in the arrange-
ment of its lobes in different forms.

In some Opisthobranchia the stomach contains a series of teeth
which are sometimes sharp and chitinous, sometimes plate-like and
calcified. Frequently a special development of the cuticular lining
of the stomach forms a hard rod — the crystalline style, lodged in
a caecum and comparable to the body of the same name in the
Pelecypoda (p. 705). A pyloric caecum is frequently appended to
the stomach. The intestine is long and thrown into folds in the
vegetable-feeding forms, short and straight in the carnivorous.
In some cases, e.g. Haliotis, it traverses the ventricle, in others
the pericardium ; in others it passes through the nephridium. In
Eolis (Nudibranchia) the stomach gives off a number of glandular
caeca which penetrate into the interior of the secondary branchiae
or cerata on the dorsal surface ; these caeca take the place of
the " liver " of other Gastropoda. In some of the Pectinibranchia
there is a peculiar ad-rectal gland, situated at the side of the rectum
and secreting a colourless fluid, which in Murex and Purpura turns
purple on exposure to the air, and was anciently used as a dye —
the " Tyrian purple."

The heart is, as in other Molluscs, enclosed in a special cavity —
the pericardium — a specialised part of the ccelome, communicating
with the cavity of the nephridia. It consists usually, as in
Triton, of two chambers — auricle and ventricle ; but in some,
e.g., Haliotis, there are two auricles and a ventricle. In the

MI

PHYLUM MOLLUSCA

745

ce7'.#

osphrcj

7Tta.Tit.Tl

Opisthobranchia, as already mentioned, it lies in front of the
ctenidia ; in the Streptoneura at the side or behind. Given off'
from the apex of the ventricle is a large vessel which soon bifur-
cates to form anterior and posterior aortse. These are the main
trunks of the arterial system, which is more highly developed than
in the Pelecypoda ; the finest branches terminate in sinuses, as in
the latter class.

The nervous system varies considerably in the different
groups in regard to the arrangement of the ganglia and their
commissures and connectives.

In the majority the arrangement is nearly that which has been
described as occurring in Triton. There is a pair of cerebral
ganglia usually closely united, but in Patella (Fig. 635) widely
separated, situated over the
gullet, and giving off behind
a pair of visceral nerve-cords,
in the course of which there
is placed laterally a pair
of 'pleural ganglia, which
are united together behind
in a median abdominal gang-
lion (or a paired ganglion, as
in Triton). In the course
of these visceral cords there
is a pair of visceral ganglia.
A pair of pedal ganglia
united together by a trans-
verse commissure and joined
to the cerebral ganglia by
connectives, give off behind
one or two pairs of pedal
nerves, as already mentioned.
A pair of buccal ganglia are

connected by slender nerves with the cerebral. At the base of
each osphradium is usually a small osphradial ganglion connected
by a slender nerve with the visceral. In most Streptoneura
(Fig. 619), in accordance with the displacement of the anus, the
visceral cords are twisted, as already described in the case of
Triton, into a figure of 8.

In Patella (Fig. 635) the pedal ganglia (ped. g.) give origin to
a pair of elongated pedal nerve-cords. In Haliotis and Fissurella
there is a similar pair of pedal cords which are connected together
by transverse commissures, and, in the latter genus, join one
another posteriorly.

In the Euthyneura (Fig. 636) except in Actazon and Chilina,
the visceral cords are not caught up in the twist of the visceral
mass, and do not cross one another.

Fit;. 035. — Nervous system of Patella, cer. g.
cerebral ganglia; want. «., mantle-nerves;
osj>h. <j. osphradial ganglia ; ped. >j. pedal
ganglia and pedal nerve-cords ; pl.y. pleural
ganglion. (After Spengel.)

746

ZOOLOGY

In the Snails and other Puhnonata (Fig. 637) the ganglia of the
nervous system are more closely aggregated together. A pair
of cerebral ganglia overlie the oesophagus,
and below it is a mass of ganglia in which
are to be made out a pair of pedal ganglia
and at least two pairs of ganglia representing
the visceral and pleural. A pair of small
buccal ganglia are connected with the cere-
bral by means of slender connectives.

The organs of special sense are the
eyes, the statocysts, and the osphradia. In
nearly all cases there are two cephalic eyes
(Fig. 638), the position of which has already
been referred to in the account given of the
external characters. In structure they are
simplest in Patella (A), where each con-
sists of a pit-like depression, lined by pig-
mented cells connected with nerve-fibres.
In the majority they have the structure
described in the case of Triton. In certain
species of Oncidium, a littoral Pulmonate,
there are numerous eyes of a simple type
scattered over the dorsal surface. In this
case the optic nerve pierces the retina and
the cells of the latter have their free ends
directed away from the centre of the eye, as
in Pecten (see p. 707) and in the Vertebrata,
instead of towards it, as in other Mollusca. The internal cavity
of the eye is occupied by a refractive body composed of a few
large transparent cells. The
statocysts are usually placed in
close relation to the pedal
ganglia, but are always in-
nervated from the cerebral.
An olfactory, organ is present
in the shape of groups of cells
on the tentacles, in which the
fibres of an olfactory nerve
terminate. *

The osphradia are promin-
ences, usually of simple form,
situated close to the base of
the ctenidium. In many of
the branchiate Streptoneura
(Fig. 639), as already men-
tioned in the case of Triton
(see p. 726, Fig. 613), the

Flo. 630. — Nervous system
of Aplysia (Opistho
branchia). abd. abdo
minal ganglion ; cer. g
cerebral ganglion ; ospkr
g. osphradial ganglion
ped. g. pedal ganglion
pi. g. pleural ganglion
(After Spengel.)

osplwjr

Fiu. 037.— Nervous system of Limnacus (Pul-
monata). abd. g. abdominal ganglion ; cer. g.
cerebral ganglion ; ospkr. g. osphradhd gang-
lion ; ped. g. pedal ganglion ; pi. g. pleural
ganglion. (After Spengel.)

\I1

PHYLUM MOLLUSCA

747

primitively right osphradium, which is alone developed, assumes
the form of a pectinate body with a central ridge, on either side

lean

Ki... 6S8.— Eyes of Gastropoda. A, Patella; li, Trochus ; C, Turbo; I), Blurex.
</(. epidermis; '. lens; op. n. optic nerve; ;: retina; c. U. vitreous humour. (From the
Caiitbihlgv A'alural History, after Helger.)

of which is a row of close-set lateral lamina;, and is commonly
t en ned the pardbranchia from its resemblance in appearance to
a gill. In some cases it is of even more complicated shape than in
Triton, owing to the branch-
ing of the lateral ridges.

The nephridia of the
Gastropoda are dorsally
placed glandular tubes or
chambers, which communi-
cate internally with the peri-
cardium, and open on the
exterior, either directly or
through a duct — the ureter.
Both right and left neph-
ridia may be present, though unequal in size, the one situated
to the right of the anus being larger than that situated to
the left ; or the former may alone be developed (Euthyneura).

br.rt.

Fie. 03i>. — Transverse section of osphradium of
Murex. hr. ft. branch nerve passing to lamina ;
/«,/*. hnihm; o*phr. n. main osphradial nerve.
(After Spengcl.)

748

ZOOLOGY

herrn.Cj/l

7term.cL

recsern

sp.d

In a very limited number of Gastropoda the gonad opens into
the nephridium.

The sexes are separate in nearly all the Streptoneura, united
in the Euthyneura. Special gonoducts are present, except in one
or two forms in which the nephridia perform that function. In
the unisexual forms the reproductive apparatus is of a compara-
tively simple character, consisting merely of a racemose repro-
ductive organ, ovary
or testis as the case
may be, situated dor-
sally in the visceral
spiral, with the gono-
duct opening far for-
wards on the right-
hand side, and, in the
male, a penis, which
is grooved longitudin-
ally and non-retractile.
In the hermaphrodite
forms, such as the
Pulmonata (Fig. 640),
on the other hand, a
considerable degree of
complexity is observ-
able. There is an
ovotestis or "herma-
phrodite gland " (herm.
gl., Fig. 641, A) — some
of the follicles of which
produce ova, while
others produce sperms,
a convoluted " herma-
phrodite dud " (herm.
d.), an albumen-gland,
in which the albumen
of the relatively large
eggs is formed, and
sometimes a separate
oviduct and sperm duct
leading to a common
genital opening; sometimes there is a single duct undivided through-
out. A receptaculum seminis (rec.sem.) is connected with the oviduct,
and also a number of narrow accessory oviducal glands (muc. gl.) ;
frequently a gland termed prostate is connected with the sperm duct,
and there is an eversible sac — the sac of the dart (d. s.) containing a
crystalline stylet, and a penis (pen.), which is perforated by a canal
and is capable of being retracted by a special muscle. The duct

muc.tjt
muc.cjl.ap

rec.sern.ap \pen.

Fig. (340. — Reproductive organs of Helix, alb. gl. albumen-
gland ; d. s. dart-sac ; flag, flagellum of the penis ;
herm. gl. hermaphrodite gland or ovotestis ; herm. d. duct of
ovotestis ; muc. gl. mucous gland ; muc. gl. ap. apertures of
mucous glands into vestibule ; oeid. oviducal part of the
common duct ; oeid. ap. aperture of oviduct into vesti-
bule ; pen. penis ; rec. sem. receptaculum seminis ; rec.
tiem. ap. aperture of receptaculum seminis ; up. d. sperm
duct ; sp. a", spermiducal part of common duct. (After
Pelseneer.)

PHYLUM MOLLUSCA

749

Fig. 641.— Follicles of the ovotestis of the Gastropoda. A
of Helix hortensis (Pulmonata) : if, of the Eolidic.
a. a, ova ; h, masses of sperms ; c. common efferent duct.
(From Gegenbaur.)

may be simple or may be incompletely divided longitudinally into
two parts. In the Pulmonata the first part (" hermaphrodite duct "
proper) is simple, and
serves for the pas- A

sage both of ova and
sperms : the middle
part is incompletely
divided internally into
two passages, one serv-
ing as oviduct, the
other as sperm-duct.
In the distal part
oviduct and sperm-
duct are completely
separate. Where the
sperm-duct enters the
penis, there is given
off a long, slender, tapering diverticulum, the flagelhnn (flag.),
in which the sperms are made up into elongated masses or

spermatophores.

Development. — The
limpets (Patella) are ex-
ceptional in laying the
eggs one by one and un-
f e r t i 1 i sed — impregnation
taking place in the water
after they have been dis-
charged. In almost all
the Gastropoda fertilisa-
tion is internal, and the
eggs are laid in great
masses, embedded in jelly,
each egg having its own
hyaline envelope. Very
often the mass of spawn
consisting of the jelly-like
substance, with the eggs
embedded in it, attains
a relatively considerable
size. In form it varies
greatly : frequently it is in
the shape of long strings
which are cylindrical or
band-like: sometimes
several such strings are
twisted together into a cord. Sometimes the spawn is fixed to
sea-weed or other objects ; sometimes it is unattached, and may
VOL. I 3 B

Fi<;. 643. — Forms of egg-easus in Oartropodn. A and
D, Pyrula or Busycon ; B, Conns ; C,
Voluta musica ; E, AmpuUaria. (From
tlie Oeembridfft Natural History.)

750 ZOOLOGY sect.

float about freely. In the Streptoneura (Fig. 642), instead of a
jelly-like mass, the eggs are enclosed in a firm, parchment-like
capsule, in which is contained, in addition to the eggs, a quantity
of an albuminous fluid, serving to nourish the developing embryos.
The shape of the capsule varies greatly in the different genera :
sometimes it is stalked, sometimes sessile ; in some cases there
is a lid or operculum, the opening of which permits the embryos
to escape. Very commonly large numbers of these capsules are
aggregated together, and usually they are attached to a rock or a
sea- weed or similar object. In many cases only a limited number
— sometimes only one — of the embryos contained in the capsule
become developed, the rest serving as nutriment for the survivors.

In the land Pulmonata each ovum may be embedded in
gelatinous matter enclosed in a firmer envelope, and a number of
them are arranged in a string ; sometimes a large number are em-
bedded in a rounded gelatinous mass. Usually, as in some species
of Helix and other genera, the outer layers of the albumen-like
substance enclosing the egg become toughened and impregnated
with salts of lime, so as to assume the character of a calcareous
shell ; a number of such eggs, which are of relatively considerable
size, are laid in holes excavated in the earth.

In a few marine and fresh-water Gastropoda the ova undergo
their development in the body of the parent, enclosed in an
enlargement of the oviduct which serves as a uterus.

The egg contains a considerable quantity of food-yolk, which
may be evenly distributed, or a clear protoplasmic and an opaque
yolk-laden segment may be distinguishable. There is a fairly
close agreement throughout the class in the nature of the segmen-
tation (Fig. 643). In all cases it is total, sometimes equal at first,
but soon afterwards becoming unequal. The first four blastomeres
are usually equal or nearly so ; they are so arranged that two of
them are in contact in the middle, and thus separate the other
two : the line of contact of the former pair becomes the transverse
axis of the embryo.

From the four first-formed cells four small cells or micromeres
become constricted off, the larger cells being the mc gamer es ;
then four more micromeres are divided off, and again the
same process is repeated. The embryo now consists of the four
megameres and twelve micromeres. The latter then increase by
division and form a cap of small cells (ectoderm) on the surface of
the megameres. The whole process, as will be noticed, has a
remarkably close resemblance to the process of segmentation of
the ovum of a Polyclad as described on p. 273.

The megameres then give off internally four small endoderm
cells, and from one of these (endo-mesoderm cell) are formed two
primitive mesoderm cells, from which the cells of the mesoderm
are developed. In some cases (Paludina) the mesoderm is formed

\ll

PHYLUM MOLLUSCA

751

entirely from cells that migrate inwards from the ectoderm and
come to fill the segmentation-cavity. The megameres themselves
eventually become converted into endoderm cells. A segmentation-
cavity is developed between the micromeres and the megameres,
and the result is the formation of the blastula, one side of which
{vegetal pole) is greatly thickened owing to its consisting of the
large megameres, the opposite side {animal pole) being made up of
micromeres. This may become a gastrula by epiboly or over-growth

Tries

Fi<;. »U3 — Diagram of the segmentation and formation of the germinal layers of the Gastropoda.
A and B, lateral view; C—F, viewed from the-animal (upper) pole ; H, from the vegetal (lower)
pole ; 0, in optical section ; ett. ectoderm ; did. endoderm ; me. micromeres ; me<j. megameres ;
met. mesoderm ; pM. polar bodies. (After Korschelt and Heider.)

of the ectoderm over the megameres; or, if the segmentation-
cavity is of considerable size, an invagination takes place.

The two larval stages, the trochophore and the veliger, are
characteristic of the development of the Gastropoda. The former
is most typically developed in Patella; in other Gastropods it
undergoes more or less modification. In Patella (Fig. 644) there
is a ciliated blastula {A) which has on one side the large megameres
The latter become enclosed by the micromeres, and the foundation

3 u 2

752

ZOOLOGY

SECT.

of the mesoderm is laid in the manner already described. The
blastopore is situated at the vegetal pole, destined to become
the hinder end of the larva, but it soon changes its position
and extends forwards on the ventral side, and a ciliated ring
— the prototroch or future velum — is formed. Subsequently

Fig. 644. — Earlier stages hi the development of Fatella. A, blastula ; B, beginning of endo-
dermal invagination ; C, completion of gastrula ; D, frontal section of somewhat later stage.
ap. pi. apical plate ; W. blastopore ; endm. endo-mesoderm cell ; end. endoderm ; men. meso-
derm ; mesent. mescnteron ; proto. prototroch ; sh. gl. shell-gland. (From Korschelt and
Heider, after Patten.)

the position of the blastopore becomes still further shifted and
its form U-shaped and then slit-like. It undergoes elongation
(Fig. 645, A) and eventually becomes partly closed up, the
closure taking place from behind forwards ; the most anterior
part remains open to form the mouth — or, perhaps more correctly,
there is in the position of the anterior part a sinking-in of the

PHYLUM MOLLUSCA

753

ectoderm which pushes the blastopore inwards and forms the
rudiment ot* the stomodseum. The originally solid mass of
endoderm develops a lumen, and its cells become arranged to
form the enteric epithelium. From the posterior end, where
the mesoderm cells are situated, proceed two very regularly
formed mesoderm-bands (Fig. 645, B). On the dorsal surface
the shell-gland has already appeared as a pit lined by elongated
ectoderm cells ; on the surface of this appears the embryonic
shell. The rudiment of the foot (Fig. 645, A) arises at a re-
markably early stage as two protuberances lying on the ventral
side of the posterior end of the larva at the sides of the blastopore ;
these coalesce to form the median foot.

too, r,4.">. — a and B, Trochophores of Patella at different stages. In A are to be seen the
circular blastopore and the two foot-elevations ; in B the blastopore is drawn out, at the sides
of it are the two mesoderm bands. (From Korschelt and Ucider, after Patten.)

The larva (Fig. 646) has now assumed the trochophore form. The
pre-oral part is large and convex, with an apical plate on which is
borne a bunch of long cilia, and near it two small ciliated elevations,
each consisting of a single cell. The pre-oral part of the larva
then becomes much flattened, and the apical plate (ap. pi) increases
in size and importance. At the posterior end is a bunch of cilia
which are borne on two special large cells, the anal cells (an. e). The
embryonic shell becomes saucer-shaped. A slight ridge in the
neighbourhood of the shell represents the border of the mantle.
The mid-gut (mesent) has become considerably widened : a
diverticulum from it is recognisable, and this afterwards opens
on the exterior to form the anus. A diverticulum of the fore-
gut (rati) at the same time forms the rudiment of the radular
sac. The statolith-sacs appear as depressions of the ectoderm at

754

ZOOLOGY

SFX'T.

the sides of the mouth : these grow inwards and become sac-like,
subsequently lying at the sides of- the foot, which, has meantime
attained a considerable size.

The trochophore-stage, which is so well marked in the case
of Patella, occurs in other Gastropods, though, as a rule, present-
ing modifications perhaps traceable to the enclosure of the embryo
in an egg-shell and to the presence of much food-yolk. The
history of the blastopore is not the same in all cases ; in Paludina
it -becomes converted into the anus ; in some the mouth is
developed from its anterior portion ; in others the stomodseal in-
vagination arises after
its complete closure, or
may, with the mantle-
cavity, only become
developed after the
symmetry has been
disturbed by torsion.
In most of the Gas-
tropoda the pre-oral
circlet or velum (Fig.
647, ml.) undergoes a
development not ob-
servable in the Pele-
cypod embryo, and
becomes greatly ex-
tended as a bilobed
flap, the strong cilia
with which it is bor-
dered rendering it a
very efficient organ
of locomotion for the
larva. With the full
development of the
velum the larva passes
into the veliger stage (Fig. 647). In this stage the shell (sh.)
increases in size, loses its simple form, and begins to develop a
spiral. A cleft-like depression in the border of the mantle on the
right-hand side forms the rudiment of the mantle-cavity in which,
later, the gills are developed. The anus when it first appears may
be symmetrically placed, but later becomes shifted to the right
side and forwards as well as dorsal ly. A pair of larval nephridia
are developed, having a remarkable resemblance to the excretory
canals of the Flat- worms. Each consists of a longer or shorter tube
sometimes intra-cellular, sometimes inter-cellular, opening on the
exterior at one end, and at the other terminating in one or several
flame-cells. The foot (/.) may attain a considerable development
during the veliger stage, and on its posterior and dorsal part appears

ctn.c

Pig. 646. — Later trochophore of larva of Patella in longi-
tudinal section, an. c. anal cells with cilia ; ap. pi. apical
plate ; /. foot ; vies, mesoderm cells ; mesent. mesenteron ;
too. month ; rail, rudiment of radula-sac ; xh. shell. (From
Korschelt and Heider, after Patten.)

XII

PHYLUM MOLLUSC A

765

the operculum. Two little processes on the velar area develop into
the tentacles (tent.), and the eyes (ey.) appear at their bases. As
the foot and other organs advance in development the velum
decreases in size and gradually aborts, but in some cases a portion
of it persists as the subtentacular lobes or labial tentacles in the
neighbourhood of the mouth.

In the Pulmonata the velum is not well developed, except in
Oncidium, though the trochophore stage is well marked.

The young Gastropod is at first bilaterally symmetrical ; the
prevailing asymmetry is the result of unequal growth of the two
sides of the body. In the majority of cases it is the left side that
grows more actively than the right, a result of which is that the

Fio. 647.— Veliger stage of Vermetus. cer. g. cerebral ganglia ; ty. eye ; ). foot ; mo. mouth ;
ot. statocyst ; $h. shell ; tent, tentacle ; vel. velum. (After Lacaze-Duthiers.)

posterior parts — the anus and the region surrounding it — are dis-
placed forwards towards the right, the space between the anus and
the mouth on that side undergoing little or no increase in length.
In the Opisthobranchia and the Pulmonata the anus with the
mantle-cavity and its contents become displaced forwards towards
the anterior end ; in most of the Streptoneura the anus, &c, in
their displacement forward pass beyond the middle line, one of the
most striking effects of which is the crossing of the pleuro-visceral
connectives already referred to (p. 737).

Ethology and Distribution. — Only a few aberrant families of
Gastropoda are parasites. Most are aquatic, all the most primitive
forms being inhabitants of the sea. Of the marine families the
majority move by creeping over the sea-bottom, some burrowing
in mud or sand, some in solid rock; some are able to. float in

756

ZOOLOGY

a reversed position, adhering to frothy mucus secreted by the
glands of the foot ; certain exceptional forms such as Vermetus
are fixed in the adult condition by the substance of the shell.
A few families — the Heteropoda and the Pteropoda — are specially
modified for a pelagic mode of existence, and swim through the
water by flapping movements of the lobes of the foot, which act
as fins. Gastropods are found in the ocean at considerable
depths — up to nearly 3,000 fathoms. Many forms, however, are
inhabitants of fresh water, while many Pulmonata are terrestrial,
and occur even towards the summits of the highest mountains.

Fossil Gastropoda are known from almost the earliest fossil-
bearing rocks, and all the major divisions of the class are repre-
sented in formations of Palaeozoic age.

The mutual relationships of the various groups of Gastropoda
are shown in the following diagram (Fig. 648): —

Platypoda Heteroboda

Rhibidoglossa

Docoglossa

Pulmonata

Tecfibranchia

Nudibranchia

Scabho^oda

Fig. (J48. — Diagram to illustrate the relationships of the Gastropoda.

APPENDIX TO THE GASTROPODA.
A. CLASS IV. -SCAPHODA.

The Scaphopoda or Elephant's tusk-shells are aberrant marine Molluscs
comprising only three genera — Dentalium, Siphonodentalium, and Pulsellum. The
body is elongated so as to be
almost worm-like, with complete
bilateral symmetry. The mantle-
folds are almost completely united
to form a cylindrical tube en-
closed by the shell (Fig. 649),
which is in the form of a delicate,
curved tube, open at both ends
and wider at the anterior or

oral end than at the other. The foot (Fig. 650,/) is narrow, trilobed at the
extremity or provided with a terminal disc, capable of being protruded through
the oral opening of the. shell, and used for burrowing in sand. The mouth is

Fio. 649. — Dentalium, longitudinal section of
shell. (After Keferstein.)

XII

PHYLUM MOLLUSCA

757

situated mi a short oral proboscis, and is sometimes surrounded by Iobed pro-
of pinnate palpi. Further back are a pair of tentaculiferous lobes, each
bearing a large number of filiform tentacles, which are
probably respiratory in function. The mouth leads into
a buccal cavity containing an odontophore. Connected
wit h the meeenteron is a large bilobed digestive gland (/. ).
The anus is situated ventrally behind the lw.se of the
foot. The vascular system is extremely simple, con-
sisting of sinuses without definite walls, and there is
no distinct heart, though in the neighbourhood of the
rectum there is a specially contractile part of the prin-
cipal sinus. Two nephridia open near the anus, the
right one acting as a gonoduct, the left (k) entirely
renal in function. The sexes are distinct. There is an
elongated unpaired gonad (g.), divided by lateral in-
cisions into a number of lobes, occupying all the posterior
and dorsal parts of the body. Anteriorly it narrows
to form a duct opening into the right nephridium.

The nervous system consists of paired cerebral, pleural,
pedal, and visceral ganglia ; the cerebral ganglia are
situated close together. There are no eyes or statocysts.
• In the gastrula stage the embryo, which is provided
with cilia, becomes free. The ciliated cells are arranged
in a characteristic manner in three rows which, at first
situated close together about the middle of the body,
become shifted at a later stage near the apical pole, and
amalgamated into a broad band representing thepre-
oral circlet of other molluscan larvae ; at the same time
a bunch of cilia previously developed at the apical
pole becomes more conspicuous and a considerable part
of the general surface covered with more delicate cilia.
The blastopore, at first terminal, is shifted forwards on
the ventral surface until it comes to be immediately
behind the ciliated circlet. At its anterior end an in-
vagination gives rise to the mouth and stomodamm.

The larva (Fig. 651) has now attained the stage of a trochophore, in which,
however, both apical plate and primitive nephridia are wanting. A shell-gland

Fn;. 660.— Dentalium,

anatomy, a. anterior
aperture of mantle ; ./'.
foot; (/. gonad ; i-, neph-
ridium ; I. digestive
gland. (From the Cam-
bridae Natural History,
after Lacaze-Uuthiers.)

Fio. 051.— Veliger of Dentalium. A, longitudinal section of a larva 14 hours old; B, larva
of 37 hours ; C, longitudinal section of larva of 34 hours. M. mouth ; c, c. velum. (From
Cooke, after Kowalewsky.)

is developed, and soon the rudiment of the shell. The post-oral region, at first
inconsiderable in size, soon undergoes an increase, until it forms eventually by

758

ZOOLOGY

far the longest part of the body, while the pre-oral region almost completely
aborts. When the post-oral region has attained a certain size, there are
developed on it two lateral folds, the rudiments of the mantle (B), which grow
inwards towards the middle ventral line, and later on unite by their free
margins. The pre-oral circlet or velum changes its
form — at first it is conical, later it becomes plate-like,
rmUi and is then gradually reduced, the larva sinking

to the bottom ; and though still occasionally swim-
aai.at ming with the aid of the velum, coming to use the

foot as a creeping organ. The shell now increases
in size step by step with the growth of the mantle,
and bends round the body of the larva until its edges
meet and coalesce in the ventral median line. Later
it assumes the elongated conical form, curved towards
the dorsal side, characteristic of the adult. The foot
at the same time elongates and takes on the charac-
teristic three-lobed shape.

HaP

B. RHODOPE.

i

t

Rhodope (Fig. 652) is a minute, elongated, fusi-
form animal, ciliated externally, with complete
(external) bilateral symmetry. There is no shell, but
within the body-wall, in the parenchyma between it
and the enteric canal, are numerous irregularly shaped
calcareous spicules. There are neither jaws nor odonto-
phore. The enteric canal — which is a narrow tube,
consisting of buccal cavity, with salivary glands,
oesophagus, mid-gut with a caecum, and rectum — opens
in an anal aperture situated to the right of the
posterior extremity of the body. A digestive gland
is absent. The central part of the nervous system
consists of a supra-cesophageal mass made of three
pairs of ganglia— cerebro-visceral, pedal and buccal —
and a single ventral ganglion. An eye and a stato-
cyst are situated on each side in close relation to the
cerebro-visceral ganglion. The nephridial system
opens on the right side in front of the anus : it
consists of a narrow ciliated canal, running out from
which are two longitudinal excretory canals with a
number of flame-cells similar to those of the Flat-
worms, but of a multicellular structure.

There are no blood-vessels, and specialised organs
of respiration are also absent.

The sexes are united. The gonads consist of about
twenty ventrally situated masses of cells, the an-
terior being ovaries and the posterior testes. There
is a common duct receiving the products of all the
gonads ; and a single hermaphrodite aperture, with
a muscular penis, a receptaculum seminis, and an
accessory gland.
There is no metamorphosis, and the larva is not provided at any stage with

any representatives of either shell-gland or foot.

Though the occurrence of flame-cells is unique, there can be little doubt that

Rhodope is best regarded as a degenerate member of the Mollusca, and it probably

finds its nearest relatives among the Gastropoda.

Fio. 652.— Rhodope ver-

anii. General view. The
scattered curved bodies
are the spicules. $ ap.
male aperture ; 9 «?>•
female aperture ; hue.
buccal cavity ; brn. cen-
tral nervous system ; ccec.
cfecum ; int. intestine ;
with, mouth ; ov. ovary ;
pgm, layer of pigment ;
sal. gld. salivary gland ; te.
testes. (After von Graff.)

\n PHYLT'M MOLLUSCA 759

Class V.-CEPHALOPODA.

The Cephalopoda, including the Cuttle-fishes, Squids, Oetopods,
and Nautili, are marine Mollusca of a high grade of organisation.
There is a very definitely-formed head, bearing a pair of highly-
developed eyes, and surrounded by the anterior portion of the foot,
modified into arms or tentacles. The body is bilaterally symmetrical.
The posterior part of the foot is modified to form a funnel leading
out from the large mantle-cavity. A shell is sometimes present,
sometimes absent. When present it is usually internal, but
sometimes external, and in the Nautili is capableof containing
the body of the animal.

1. Examples of the Class.
i. The Cuttle-Fish (Sepia1).

Cuttle-fishes are marine Molluscs, which live usually at a depth
of a few fathoms, but often come into shallower water, and are
frequently caught in the trawl or the seine. The animal arrests
attention when compared with Unio or Triton by the strength,
and more particularly the rapidity, of its movements; by the
possession of a pair of eyes resembling in size and complexity
those of a Fish ; and by various other features, all pointing to
a higher grade of organisation than is attained by the members
of the classes of Mollusca dealt with in the preceding pages.

External Features.— The Cuttle-fish (Fig. 653) has a distinct
head, bearing ten long arms, and a pair of large, highly-developed
eyes. The head is connected with the body by a constricted
region or neck. The trunk is elongated and shield-shaped, the
base of the shield being towards the head. The long axes of
head and trunk are in line with one another. Not only the head,
but also the trunk, are completely equilateral, in which respect
there is a marked contrast to Triton ; and this symmetry extends
to most of the systems of internal organs. The free extremity of
the head bears the mouth, and is accordingly termed the oral
extremity, the opposite extremity, the apex of the shield-shaped
body, is the aboral end. The surfaces of the shield are anterior
or antero-dorsal and posterior1 or. postero-ventral, its borders right
and left. The anterior surface is to be distinguished by its darker
colour, and by the firmness of the body-wall, due to the presence
in this position of a hard internal shell.

1 Most of the figures have reference to a common Australian species — S. cnltrata
— bat the differences between the various species of the genus are slight and
unimportant, and the description given will apply fairly well to any other
species, ... . . .

760

ZOOLOGY

The aperture of the mouth is surrounded by the bases of the
ten arms. These are in pairs, situated to the right and left of the
median plane. All of them, with the exception of the fourth pair
(the most anteriorly situated pair being reckoned as the first), are
stout at the base and taper towards the extremity. When extended

they are about two-thirds of the
length of the body. The outer
surface of each {i.e. that turned
away from the mouth) is strongly
convex, the inner flat, and beset
throughout its length with a
number of suckers, which are
arranged in four longitudinal
rows. Each sucker is in the
form of a shallow cup, supported
on a short, thick stalk (st.) ; the
lip of the cup is membranous,
and immediately within it is a
narrow, horny rim (dent.). Into
the floor and walls of the cup
are inserted numerous muscular
fibres. When the sucker is being
brought into use it is firmly
applied to the surface of the
object ; by the contraction of the
muscular fibres the cavity of the
cup is then enlarged, and a partial
vacuum is formed, the result
being firm adhesion, owing to
the pressure of the surrounding
water. The fourth pair of arms,
usually known as the tentacles,
are comparatively long and nar-
row, and provided with suckers
only towards their free ends,
which are somewhat thickened
and club-like. In the male the
fifth arm on the left side presents
a slight modification, some of
the suckers being absent. This
is an indication of a change termed hectocotylisalion, which, as
will be pointed out in the general account of the class, assumes
in some cases a very remarkable character. As the nerves which
supply them are derived from the pedal ganglia, there is no
doubt that the arms of Sepia represent a portion of the foot
of other Molluscs ; but there is some doubt as to whether they
correspond to the fore-foot or to the epipodia of the Gastropoda.

Fig. 653.— Sepia cultrata. Entire animal
viewed from the antero-dorsal aspect.

mi PHYLUM MOLLUSCA 761

The head-region, comprising as it does the arms (which are
the chief part of the foot) and the head proper, is termed the
a •'jthahypodium.

The trunk is covered over by the thick integument of the mantle,
which terminates toward the oral end in a ridge round the neck.
Anteriorly this ridge projects as a prominent rounded lobe under
cover of which the head can be partially retracted. Posteriorly it
forms the posterior lip of the opening of a large cavity bounded by
the mantle — the mantle-cavity — which extends along the entire
posterior face of the body almost to the apex. The wide cleft
between the oral edge of the mantle and the posterior surface of
the body is not the only aperture leading into the mantle-cavity.
On the oral side of this cleft is a large tube — the funnel (Fig. 658,
inf.) — opening on the exterior behind the neck, and internally
communicating by a wide aperture with the mantle-cavity. The
cleft is capable of being almost completely closed by the
apposition of a pair of oval projections (mant. cart.) of the
inner surface of the posterior mantle-wall near its oral border,
and a pair of concave depressions (inf. cart.) on the opposite
(posterior) face of the funnel. The funnel is thus, under ordi-
nary circumstances, the main outlet of the mantle-cavity.
As such it not only carries to the exterior the effete water
of respiration, the faecal matters from the intestine, and the
products of the excretory and reproductive organs, but also
takes an important part in locomotion, the most important
movements of the Cuttle-fish — by which it darts rapidly through
the water in the direction of the aboral pointed end of the body —
being effected by rhythmical contractions of the muscular walls of
the mantle-cavity causing jets of water to be forced in the oral
direction through the funnel. The free passage of water inwards
through the funnel is prevented by the presence in its interior of
a flap-like valve opening outwards. The water required for re-
spiration and in locomotion is thus drawn in, not through the
funnel, but through the partially-closed slit-like pallial aperture
previously referred to. The funnel seems, from the source of the
nerves which supply it, to be, like the arms, a specially modified
part of the foot.

Fringing each lateral margin of the body is a thin muscular
fold — the Jin — which is used as a swimming organ.

The anterior wall of the body exhibits,' as already mentioned, a
hard and resistant character owing to the presence of the internal
shell (Fig. 654). This is completely enclosed in a sac of the
mantle. Like the body itself, it is bilaterally symmetrical. In
shape it may be described as leaf-like, with a rounded and
comparatively broad oral end, and a narrower aboral end, provided
with a sharp, anteriorly-projecting spine. The posterior surface is
convex; the anterior convex towards its oral end, but deeply

762

ZOOLOGY

concave aborally, and bounded laterally by thin prominent wing-
like ridges which converge to meet at the aboral extremity. The
main mass of the shell consists of numerous,
closely-arranged, thin laminae of calcareous com-
position, between which are interspaces contain-
ing gas. On the surface is a thin layer of
chitinoid material, and slightly thicker strips of
similar composition run along the margins.

The living Cuttle-fish will be observed to
undergo frequent changes of colour, and blushes
of different hues are to be observed passing over
the surface. These are due to the presence of
numerous contractile pigment-containing cells or
chromatophores (Fig. 655) situated in the
deeper layers of the integument over the entire
surface. The chromatophores are flattened sacs
with elastic walls, the contracting tendency of
which is capable of being counteracted by the
action of bundles of muscular fibres radiating
outwards from the edge of the sac into the sur-
rounding tissues. When these radiating fibres
are in action the edge of the chromatophore is
drawn outwards in different directions, and as a
result the flattened sac becomes more expanded and thinner,
the pigment being spread out into a thinner layer. ' When the
fibres are relaxed the elasticity of the wall comes into play, and
the chromatophore contracts,

Fl(i. (554.— Shell of
Sepia cultrata,
posterior view. Re-
duced.

the contained pigment re-
suming its former arrange-
ment. A peculiar iridescence
which, in addition to the
play of colours, is recognisable
in the integument of Sepia,
is due to the presence of a
number of cells, the iridocysts.
When the mantle-cavity
is laid open (Fig. 658) there
is seen on each side of it one
of the two . plume-shaped
ctenidia {den.). In the middle
line of the posterior surface,
close to the internal opening
of the funnel, is the anal
aperture (an.) situated at the
oral extremity of a longi-
tudinal tube — the rectum.

retd.mus

Fig. 655.— Chromatophore of Sepia, magnified.
nuc. nuclei in wall of sac ; pigm. pigment ;
rod. mus. radiating strands of muscle. (After
Vogt and Jung.)

On either side of the rectum is a

much narrower projecting tube with a terminal opening — the

PHYLUM MOLLUSCA

763

pa.llji

-Vvsc.tv

I'ii.. li.-.ii. — Sepia cultrata, cranial car-
tilage seen from the posterior aspect,
with the cavities of the statocysts ex-
posed, eye, position of eye indicated by
dotted line ; ot. statocyst ; pall. n.
pallial nerve ; vise. n. visceral nerves.

nephridial aperture (neph.). On the left-hand side is the opening
of the sperm-duct or oviduct (ovid.) as the case may be.

In addition to the shell, which is an important protective
structure, and gives support to the muscles of the fins, Sepia also
has a remarkably well developed internal skeleton composed of
cartilage. An important part of this — the cranial cartilage (Fig.
656) — protects the principal nerve-
centres, encloses the statocysts,
and gives support to the eyes.
Other cartilages support the bases
of the arms. A thin shield-
shaped plate — the nuclial cartilage
(Fig. 657) — lies on the posterior
surface of the neck. The pair of
elevations on the posterior wall
of the funnel and the correspond-
ing depressions on the anterior
surface of the body are borne
each on a thin plate of cartilage,
and other thin cartilages support
the bases of the fins.

Alimentary System. — The
mouth is surrounded by a thin

perwtomial membrane, within which is a circular lip beset with
numerous minute elevations. Lodged within the circular lip is a
pair of powerful horny jaws (Fig. 659, Fig. 660, jaw1, jaw'1; Fig.
661, y. ; Fig. 663, jaw). These have somewhat the appearance of
the beak of a parrot, the posterior jaw being larger and more
strongly bent than the other, which it partly encloses. The mouth
leads into a thick-walled buccal cavity, which
contains an odontophore bearing numerous minute
horny teeth. The wsophagus (Figs. 660 and 661, ce ;
Fig. 663, as), following on the buccal cavity, is a
narrow straight tube, which runs between the
halves of the " liver " towards the aboral end of the
body. It opens into a rounded thick-walled
stomach (st.), and, close to the pyloric aperture
leading from the latter into the intestine, opens
a wide caecum (c). The alimentary canal at this
point bends sharply round upon itself, and the
intestine runs nearly parallel with the oesophagus to open into
the mantle-cavity as already described.

A pair of glands (Fig. 661, s.g. ; Fig. 663, sal.), which are
commonly termed salivary, though their functional correspondence
with salivary glands has not been proved, are situated in the
head behind the cranial cai'tilage. The ducts of these two glands
run inwards and unite to form a median duct, which opens into

I'm.. 867. — Sepia
cultrata, nnchal

cartilage.

764

ZOOLOGY

SECT.

the buccal cavity. The name of" liver " (Fig. 660, 11 ; Fig. 662, liv.)
or digestive gland is given to a large brown glandular mass which
extends from the neighbourhood of the salivary glands nearly to
the aboral end of the body. It consists of two partly united right

mtxnl.cart-lj^
Lit/ —

dC.Tvtd.

Via. 658. — Sepia cultrata, female seen from the posterio- ventral aspect, the wall of the mantle
cavity divided along the middle line and the two flaps thus formed spread out so as to expose
the contents, ac. nid. accessory nidamental glands ; an. anal aperture with its lateral append-
ages ; /. membranous fold attaching the ctenidium to the wall of the mantle-cavity ; inf.
external opening of funnel ; inf. cart, infundibular cartilage ; ink. d. ink-duct ; ink. s. ink-sac ;
Kg. ligamentous band which extends from the anterior wall of the mantle-cavity to the ovary,
cut across ; lie. "liver" ; I. cten. left ctenidium ; /. neph. left nephridial aperture ; /. nid, left
nidamental gland ; I. st. g. left stellate ganglion ; mant. cart, mantle-cartilage ; mo. mouth ;
mus. neck-muscles ; oe. ovary ; ovid. oviduct ; red. rectum.

and left portions, each of which has a duct opening into the
cavity of the alimentary canal opposite the point where stomach,
caocum, and intestine meet. Surrounding the ducts and opening
into them are masses of minute vesicles (Fig. 661, b, d.) ; the
secretion of these has the property of converting starchy matters

\II

l'HYU M MoLLl'SCA

7.;:,

into sugar; they sometimes, though without sufficient reason,
receive the name of 'pancreas.

Immediately below the thin integument of the anterior wall of
the mantle-cavity lies a characteristic organ — the ink-sac (Fig.
658, ink. s. ; Fig. 661, i). This is a pear-shaped body, a portion of
the interior of which is glandular and secretes a black substance —
the ink or sepia — which collects in the main cavity of the sac and
is discharged by a cylindrical duct
opening into the rectum close to
the anal aperture. When the
Cuttle-fish is startled it discharges
the ink, which, mixing with the
water in the mantle-cavity, is
ejected through the funnel as a
black cloud, under cover of which
the animal may escape from a
threatened attack.

Vascular System. — The heart
(Figs. 662, 663, and 665) of the
Cuttle-fish consists of a ventricle
and two auricles. The ventricle
{vent.) which is divided into two
lobes by a constriction, is some-
what obliquely placed, but. the
rest of the vascular system is
almost completely equilateral. At
its oral end the ventricle gives
off a large vessel — the oral aorta
(aort.); aborally it gives origin
to be a much smaller aboral
aorta (aort'), which bends over
the ink-sac and supplies the
aboral portions of the body. The
arteries which lead off from the
aorta communicate by their
ultimate branches with a system
of capillaries, and these with a

system of veins. A large median vein, the vena cava (c. cav.), runs
from the head to the neighbourhood of the rectum, in front of
which it bifurcates to form the left and right afferent branchial
veins (/. aff. br. v., r. off. br. v.), each running through the cavity of
the corresponding renal organ to the base of the gill, where it is
joined by veins from the aboral region. At the base of the
gill the afferent branchial vein becomes dilated to form a con-
tractile sac — the branchial heart (r. br. ht.) — appended to which
is a rounded body of a glandular character — the appendage
of the branchial heart, representing the pericardial glands of the

vol. i 3 c

In.. 669. — Sepia officinalis, jaws. A,
in situ ; B, romoved iind slightly en-
larged. (From the Cambridge Natural
History.)

766

ZOOLOGY

Pelecypoda. The afferent branchial vein runs through the axis of
the branchia, giving off branches as it goes. The blood is carried
back to the ventricle on either side by a dilated contractile vessel,
the auricle or efferent branchial vein (I. aur., r. aur.).

The coelome (Fig. 671) is a pouch of considerable size, divided
by a constriction into oral and aboral parts. The former is the
-pericardium, or cavity in which the heart is lodged ; it gives off a
pair of diverticula, right and left, each lodging the corresponding
branchial heart, and communicates by a pair of apertures with

pcrist

Fig. 660. — Sepia, median section through
the buccal mass. g. hue. buccal ganglia ;
(j. stom. stomatogastric ganglia ; gust, sup-
posed gustatory organ ; jaw1, posterior
jaw ; jawV, anterior jaw ; ce. oesophagus ;
purist, peristomial membrane ; rail, radula.
(After Keferstein.)

Fig. 661.— Sepia officinalis, enteric
canal, a. anus ; b. d. duct of one of
the portions of the digestive gland ;
b. m. buccal mass ; c. cajcum ; i. ink-
sac ; i. d. ink-duct ; j. jaws ; 1. 1.
digestive gland ; eg. oesophagus ; j).
pancreatic appendages ; r. rectum ;
s. g. salivary glands ; st. stomach.
(From the Cambridge Natural History.)

the cavities of the nephridia or renal sacs. The aboral part of
the coelome forms the capsule {gonocoele) which encloses the ovary
or testis.

The paired, plume-shaped ctenidium lies parallel with the
long axis of the body. It is attached throughout the greater part
of its length to the wall of the mantle-cavity by a thin muscular
fold, and consists of numerous pairs of delicate lamellae, the surface
of which is increased by the presence of a complex system of fold-
ings. Internally the lamellae are not completely in contact, an

MI

PTIYLUM MOLLUSCA

767

axial canal being left through which the water penetrates freely to
all parts of the gill. The blood carried to the gill by the afferent
branchial vessel passes in a system of minute branches through
the lamellae, and is gathered up again into vessels which open into
the main efferent vessel leading to the auricle.

iiifca.rt

Fn.. i'ii;-2.— Sepia cultrata, mala specimen seen from the postero-ventral aspect, the mantle-
cavity opened as in Fig. 058, the posterior body-wall partly dissected off, so as to expose tho
organs in the visceral sac, the ink-sac and duct removed, aort. main aorta ; aort.' aboral
aorta ; app. appendage of left branchial heart ; cier. cajcum ; inf. cart, funnel cartilages ; tiv.
digestive gland ; I. alul. r. left abdominal vein ; /. njt'. br. left afferent branchial vessel ; I. ottn
left auricle ; /. br. ht. left branchial heart ; I. ct>n. left ctenidium ; t. st. </. left stellate
ganglion ; mant. cart, mantle-cartilage ; mo. mouth ; pen. penis ; pro**, prostate ; r. aljd. r.
right abdominal vein ; r. rten. right ctenidium ; rect. rectum ; r. ven. a/*;), appendages of
right afferent branchial vessel ; te. testis ; tc. v. vein to testis ; ra. valve of funnel ; rent.
ventricle.

Nervous System. — Though parts homologous with those
of Triton are recognisable in the nervous system of Sepia, their pro-
portions and arrangement indicate a higher grade of organisation.
The cerebral, pedal, and pleuro-visceral ganglia (Fig. 666), all of
relatively large size, are closely aggregated together around the

3 c 2

768

ZOOLOGY

SECT.

oesophagus, supported and protected by the cranial cartilage. The
cerebral ganglia (cer. g.) are fused together into a rounded mass,
lodged in a hollow of the cranial cartilage, and covered over
anteriorly by a strong fibrous membrane. Laterally are given off

Fig. t>o3.— Sepia cultrata, lateral dissection of male. The left-hand half of the head has
been removed by an approximately median longitudinal section, the buccal mass, however,
being left intact ; the funnel and the anterior and posterior walls of the mantle-cavity are
likewise bisected longitudinally. The left ctenidium with the left nephridial sac and
left branchial heart have been removed from their natural position and displaced backwards
so as to expose the other organs. The digestive gland with its ducts and the pancreatic
appendages have been removed, but the position of the former is indicated by a dotted line.
app. appendage of left branchial heart ; aort. aorta ; aort'. aboral aorta ; hue. buccal mass ; Or.
cart, section of cartilage supporting the arms ; cer. g. cerebral ganglia ; ffiz. gizzard ; ink. s.
ink-sac ; in/, funnel ; jaio, jaw ; I. aur. left auricle ; I. br. ht. left branchial heart ; I. cten.
left ctenidium ; lie. position of digestive gland ; /. neph. left nephridial sac ; n. cart, nuchal
cartilage ; est. oesophagus ; ot. cavity of statocyst laid open ; ped. g. section of pedal gang-
lion ; perist. peristomial membrane ; post. v. abdominal vein ; r. aur. right auricle ; r. cten.
right ctenidium ; rect. rectum ; sal. salivary gland ; sh. shell ; st. stomach ; tc. testis ; va.
valve of funnel ; *;. cav. vena cava ; vent, ventricle.

a pair of short thick processes — the optic nerves or optic stalks
(opt. st.) — which expand almost immediately into large masses —
the optic ganglia (opt. g.) — in immediate contact with the eyes.
At the sides and posteriorly a pair of very thick commissural
bands of nerve-matter pass round the oesophagus to unite with the

PHYLUM MOLLUSCA

7(i«.i

pedal and pleuro-visceral ganglia, which lie behind. The pedal
ganglia (Fig. 067) are, like the cerebral, nnited into a single
mass; orally this is prolonged and expanded into a broad mass
from which the ten brachial nerves (br. n.) are given off to the
The pleuro-visceral ganglia, also nnited into one, are in

arms.

T

1 &

immediate contact with the pedal behind
the oesophagus.

Besides the optic nerves the cerebral
ganglia also give off a pair of slender
nerves which join a smaller pair of closely
united buccal ganglia (Fig. 666, hue"),
situated close to the buccal mass on the
anterior aspect of the oesophagus. The
buccal ganglia again (which are some-
times looked upon as separated portions
of the cerebral) are connected by slender
connectives with a pair of stomatogastric
ganglia (Fig. 660, g. stow.), also closely
united, situated on the posterior aspect
of the oesophagus. Besides the ten
brachial nerves, each of which, expanding
at the base of the arm into a brachial
ganglion, runs along the axis of the arm
to its extremity, the pedal ganglia also
give off nerves to the funnel, and also a
pair to the statocysts ; but the latter are
found, when their fibres are traced to
their origin, to be derived from the cere-
bral ganglia. The pleuro-visceral ganglia
give off two visceral nerves (Fig. 667,
vise, n.) supplying the various internal
organs, one pair of branches, the branchials,
having each a branchial ganglion at the
base of the ctenidium, and running along
its axis to its extremity. Two other
ganglia of considerable size — the visceral
and the gastric — occur in the course of
this system. The pleuro-visceral ganglia
also give off two very stout pallial nerves
(pall, n.) which run through the neck to
the inner surface of the mantle-cavity,
where each expands into a large, flat, pallial or stellate ganglion
(Fig. 658, /. st. g.), which is visible in front of the ctenidium when
the mantle-cavity is opened. From the outer edge of this arise a
number of nerves supplying the various parts of the mantle.

The organs of special sense of the Cuttle-fish are much more
highly developed than those of Triton. The eyes (Fig. 668) are

1.7*

Fk;. 064.— Sepia officinalis,

longitudinal section of ink-
sac, a. anus ; </. ink-duct ;
i. ft. ink-gland ; i. r. cavity
of ink-sac ; o. orifice of ink-
gland ; r. rectum ; up. sphinc-
ter muscles. (Prom the Cum-
h.tihi,- Natural History, after
Oirod.)

r7o

ZOOLOGY

supported by curved plates of cartillage forming a sort of orbit,
connected with the cranial cartilage. The significance of the

™&*-

aorl

oi/.i;

Fin. 66.r). — Sepia cultrata, heart and main blood-vessels from the posterior aspect, ant. no,
aorl. aorta ; aort'. aboral aorta ; app. appendage of right branchial heart ; eff. br. v. right
efferent branchial vessel ; ink. a. artery to ink-sac ; ink. v. vein from ink-sac ; I. aff'. br. r. left
afferent branchial vessel; I. aur. left auricle; or. r. deep ovarian vein; ov. v'. superficial
ovarian vein ; pall. v. pallial vein ; r. abd. v. right abdominal vein ; r. aff. br. v. right
afferent branchial vein ; r. cten. right ctenidium ; r. br. lit. right branchial heart ; r. car.
vena cava : ven. app. venous appendages ; rent, ventricle.

bice
cer.bzccjcon

a.LL n.

vlsc.jl

Fio. 600.— Sepia cultrata, cephalic gang-
lia from the anterior aspect, ao. aorta ;
buc. buccal ganglion ; cer. buc. con. cere-
bro-buccal connective ; cer. g. cerebral
ganglion ; opt. ci. optic ganglion (removed
on the left side) ; opt. st. optic stalk ;
pall. n. pallial nerve ; pi. p. pleural gang-
liop ; rite. n. visceral nerves.

Fio. GOV.— Sepia cultrata, anterior view
of pedal and pleuro- visceral ganglia after
removal of the cerebral and optic, br. n.
brachial nerves ; conn, connectives be-
tween the cerebral and the pedal and
pleuro- visceral ganglia (cut across); inf. n.
nerve to funnel ; pall. n. pallial nerves ;
rise. n. visceral nerves.

various parts of the eye will not be fully understood till the struc-
ture of that of the Vertebrata has been studied. A transparent

\II

PHYLUM MOLLUSCA

771

portion of the integument covering the exposed face of the eye is
termed the false cornea {corn). The eye-ball has a firm wall, or
sclerotic, strengthened by plates of cartillage (scl. carl). Externally,
i.e., on the side turned towards the surface of the head, this presents a
large opening — the pupil. The part of the sclerotic which imme-
diately bounds the pupil is termed the iris (ir) ; it contains muscu-
lar fibres by whose action the size of the pupil can, to a limited
extent, be increased or diminished. Just internal to the iris and
projecting slightly through the pupil is the lens — a dense glassy-
looking body of a spherical shape. The lens consists of two plano-
convex lenses in close apposition ; it is supported by an annular

scLcarl

corn.

ir

orb.ca.ri

Flo. 668. — Sepia, section of eye. cil. proc. ciliary processes ; corn, false cornea; ir. iris; lens,
lens ; opt. g. optic ganglion ; orb. cart, orbital cartilage ; rds. rods ; ret. retina ; scl. curt, sclerotic
cartilage. (From Vogt and Jung, after Hensen.)

process — the ciliary process (cil. proc.) — projecting inwards from the
sclerotic. Between the two parts of the lens lies a thin layer of
cells — the cornea. The lens with the ciliary process divides the
cavity of the eye into two portions, a smaller outer — the cavity of
the aqueous humour, — containing water, and a larger inner, contain-
ing a gelatinous substance — the vitreous humour. Over the wall
of this inner chamber extends the retina (ret), the sensitive part of
the eye, in which the optic nerve- fibres derived from the optic
ganglion terminate. The retina is of somewhat complicated
structure, consisting of a number of layers: of these that which

772 ZOOLOGY sect.

immediately bounds the internal cavity of the eye is a layer of
short narrow prismatic bodies — the layer of rods (rd), while the
outermost is a layer of optic nerve-fibres connected with the nerve-
cells of the optic ganglion on the one hand, and with the other
elements of the retina on the other.

In immediate contact with the eye, in addition to the optic
ganglion, is a large soft body of unknown function, the so-called
optic gland or white body, Bundles of muscular fibres bring about
limited movements of the eyeball in various directions. A pair of
integumentary folds of the character of eyelids are capable of
being drawn over the cornea.

The statocyst (" otocyst ") (Fig. 656), though not of such compli-
cated structure as the eye, is very much more highly developed
than that of the Pelecypoda or Gastropoda. The two statocysts are
embedded in the cartilage of the posterior portion of the cranium
close to the pleuro-visceral ganglion. The cavities of the two
oi'gans, which are about 3 mm. in diameter, are separated by a
median cartilaginous septum. The inner surface presents a
number of rounded and pear-shaped elevations, and is lined
with a flattened epithelium raised up on the posterior surface into
a ridge or crista acustica and a macula acustica composed of
large cylindrical cells provided at their free extremities with short
cilia, and produced at their bases, into processes continuous with
nerve-fibres derived from the statocyst-nerve. Enclosed in the
cavity of the statocyst and attached to the macula is a large
statolith (Fig. 669) of dense composition and
complicated form. The function of the stato-
cysts as organs of hearing is quite unproved ; it
has been shown by experiment that their re-
moval leads to a loss of the power of co-
ordinating the movements in such a way as to
maintain the equilibrium.

Supposed to be olfactory in function is a
pair of ciliated pits, which open by slits on the
Fm. ceo .-sepia cui- surface behind each eye ; amongf the ciliated

trata. .statolith, . . . J ' o

highly magnified. cells lining the pit are numerous narrow sensory
cells connected at their bases with the fibres of
a nerve derived from a small ganglion situated close to the optic
ganglion. A small elevation (Fig. 660, gust), covered with papilla?,
on the floor of the buccal cavity just in front of the odontophore,
is perhaps an organ of taste.

The excretory organs or nephridia of Sepia (Figs. 670 and
671) are a pair of thin-walled sacs, which open into the mantle-
cavity by the conspicuous excretory apertures already described.
On either side is an aperture {ap}) placing the cavity of the sac in
communication with the pericardium, and the right and left sacs
communicate with one another anteriorly and posteriorly. From

XII

PHYLUM MOLLUSC A

their posterior junction is given off a median diverticulum
(Fig. 671, med. s), into which the pancreatic follicles (pane.) project.
Through each excretory sac runs the corresponding afferent
branchial vein, formed by the bifurcation of the vena cava,
and surrounding it are masses of glandular tissue (Fig. 670,
ven. app), by whose agency the process of renal excretion (the
products of which, in the shape of a nitrogenous excretory

jned.s

abd.i>

inh.s.f

Fir,, r.70.— Sepia officinalis, excretory organs. aJ>d. v. abdominal vein ; ap\ funnel-like opening
from the pericardium ; a>>2, aperture of communication between the left and the median
nephridial sac ; ink: *. v. ink-sac vein ; mal. s. median sac ; pall, v. pallial vein ; ur. ureter ;
t, Mm vena cava ; ren. app. venous appendages of the afferent branchial veins. (From Vogt
and Jung, after Grobben.)

substance called guanin, are to be detected in the internal cavity)
is carried on.

Reproductive system. — In the male the testis (Fig. 672, te.)
forms a compact mass of minute tubules situated in the aboral
region of the body and enclosed in a capsule. The single spermi-
duct (v. def) is a greatly convoluted tube which leads from the
cavity of the capsule towards the right ; it opens into an elon-
gated veaicula seminalis (ves.), to which is appended a glandular
body, the prostate (pr.). In the interior of the vesicula seminalis

774

ZOOLOGY

SECT,

the sperms are rolled up by the action of a system of grooves and
ridges into long narrow bundles of about 2 cm. in length, each
of which becomes enclosed by. a chitinoid capsule of a narrow
cylindrical shape, forming a spcrmatophorc (Fig. 673, i>) ; at one
end of the spermatophore is a complicated apparatus of the nature
of a spring for causing the rupture of the wall and the discharge
of the sperms. The vesicula seminalis expands into a wide sac —

Fig. 671.— Sepia Officinalis, diagram of a median vertical section of a female specimen, to
show the relations of the cavities, ap. aperture between the secondary body-cavity (peri-
cardium) and the lateral nephridial sac ; l>r. Id. bronchia] heart ; inf. funnel ; ink. s. ink-sac ;
int. intestine; lat. s. lateral nephridial sac; lir. liver; med. «. median nephridial sac; or.
ovary ; or. ap. aperture leading from oviduct to secondary body-cavity ; pane, pancreatic
appendages ; sh. shell ; st. stomach ; ur. ureter ; rent, ventricle. (From Vogt and Jung, after
Grobben.)

the spermatophoral sac or Ncedhams sac (Fig. 672, sp. s.) — in the
interior of which the spermatophores are stored. This opens into
the mantle-cavity by the aperture already described at the
extremity of the penis to the left of the middle line.

In the female the ovary (Fig. 658, ov.) occupies a position exactly
corresponding to that of the testis in the male, and is enclosed in
a similar capsule, with the cavity of which the lumen of the
oviduct is continuous. An axial swelling bears numerous follicles,

PHYLUM MOLLUSCA

775

each containing a single ovum at various stages of development,
and supported on a long slender stalk. At the breeding season
the ovary becomes a compact mass of ova, which assume a
polygonal shape owing to mutual pressure. The oviduct (ovid.)
is a wide tube, opening, as already described, into the mantle-
cavity to the left of the rectum. Occupying a conspicuous
position on the anterior wall of the mantle-cavity of the female
is a pair of large flattened glands of somewhat oval outline, the

spr

Fin. G72.— Sepia, reproductive organs of male. pn. penis;
pr. prostate ; »p. x. sperm-sac ; it. testis ; v. def. vas
deferens ; ve*. vesicula seminalis. (After Kefcrstein.)

Flo. 673.— Sepia. A, sperms,
highly magnified ; B, sperma-
tophoro. up. mass of sperms ;
.«/</•. .spring apparatus hy which
the wall of the spermatophore
is ruptured. (From Yogt and
Jung.)

nidamental glands (nid), situated to the
right and left of the ink-duct. In the
long axis of each is a median canal, on
either side of which is a range of closely-set delicate lamellae ; the
median canal opens into the mantle-cavity by a slit bounded by a
number of plaits situated at the narrower oral end. The nida-
mental glands secrete the viscid material by means of which the
eggs when deposited adhere together in masses. A glandular
mass of unknown function, known as the accessory nidamental
glands (ac. nid.), lies at the sides and around the oral ends of the
nidamental glands proper.

776

ZOOLOGY

ii. The Pearly Nautilus (Nautilus pompilius).

The three living species of Nautilus, of which N. pompilius is
the best known, are inhabitants of moderately shallow water about
the shores and coral-reefs of the South Pacific, usually swimming
near the bottom, and probably rarely, if ever, coming voluntarily
to the surface. The body is enclosed in a calcareous, spirally-
coiled shell (Fig. 674), into which the entire animal can be with-
drawn for protection. The cavity of the shell is divided by a
system of septa into a series of chambers, the last and largest of
which, opening widely on the exterior, alone lodges the body of the

Fio. fi74. — Section of the shell of Nautilus pompilius, showing the septa (.?, .<*), the septal
necks.(s. n.,s. n.), the siphuncle, si. (represented by clotted lines), and the large body-chamber
(ch). (From the Cambridge Natural History.)

animal. Between the animal and its shell there is a direct organic
connection through the intermediation of a narrow, tubular, vascular
prolongation of the visceral region, which perforates the entire
series of the septa to the apex of the spiral. This tube, which is
termed the siphuncle (si.), has its wall supported by scattered
spicules of carbonate of lime ; but, in addition, as it passes through
each septum, there is produced over it for some distance a shelly
tube — the septal 'neck (s. n.) — continuous with the substance of the
septum. The apical or initial chamber presents a small scar, the
cicatrix, which may indicate the original presence of the larval
shell, or protoeonch, which has fallen off in the course of develop-
ment.

mi PHYLUM MOLLUSCA 777

When the shell of the Nautilus is compared with that of Triton
some points of resemblance, together with important points of
difference, will be at once recognised. In both the growth of the
shell has taken place in such a way as to produce a gradual and
regular increase in the width of the internal cavity, from the apex
to the mouth, the result being a form of shell which, if it were
straightened out, would be a long cone. In both the growth has
not taken place in a straight line, but in a spiral, and a spiral of
so close a character that successive turns are in immediate contact
and their walls fused together. But in Nautilus all the turns of
the spiral are in the same plane ; the spiral in other words, is a
flat one, as has already been -found to be the case in certain of the
Gastropoda (p. 737), whereas in Triton the spiral is an elongated
helix : in other words, the spiral of Nautilus is that of a watch-
spring, that of Triton that of a corkscrew. The possession by
Nautilus of the series of septa marking the position which the
animal has occupied at successive stages in its growth is another
. striking difference. Moreover the relations of the soft parts of
the shell are radically different in the two cases. In Triton the
body is attached to the shell by the columellar muscle ; in
Nautilus the main organic connection is by means of the siphuncle ;
for, though it is chiefly through the pressure exerted by two great
lateral masses of.muscle (Fig. 675, onus.) that the Nautilus retains
its hold of the shell, the muscular fibres are not attached to the
latter in the same intimate way as those of the columellar muscle
of Triton, but are inserted into a horny cuticular membrane inter-
vening between the muscle and the shell. Again, while the curva-
ture of the body of Triton with the enclosing shell is towards the
ventral side (endogasti'ic), in Nautilus it is towards the dorsal side
(exoyastric).

When the animal is removed from the shell it is found to possess
two regions, a distinct and relatively large, obtusely conical head
bearing eyes and a system of tentacles, and a rounded sac-like
trunk. Both head (or cephalopodium) and trunk are very slightly
compressed, the direction of the compression being, as in Sepia,
from the antero-dorsal towards the postero-ventral side, and
they are almost complete bilaterally symmetrical, only a very slight
disturbance of the symmetry being discernible. The mouth,
situated at the free extremity., is provided with a pair of relatively
enormous, partly calcified jaws (Fig 675). Surrounding the mouth
is a series of bilaterally arranged lobes which represent the fore-
foot or the epipodia of other Molluscs. These are beset with
numerous slender, three-sided tentacles, each provided with an
elongated tubular sheath, in the interior of which the greater part
of the tentacle in the retracted condition lies enclosed, only a
small portion protruding. Minute ring-like markings on the
tentacle are due to the presence of a number of annular constric-

778

ZOOLOGY

tions, which give the tentacle a transversely ridged character.
There are no suckers : but the ridged surfaces enable the tentacle
to adhere firmly to -rough objects. The tentacles are arranged in
two series, an outer and an inner. The outer, which are borne on
an annular muscular ridge of the foot, are nineteen on each
side in both sexes. Anteriorly this muscular ridge is thickened to
form a massive lobe — the hood (Figs. 675, 676, hd.) — in which
there is a concavity for the reception of the coil of the shell. The

Pig. 675. — Nautilus pompilius, diagrammatic lateral view of a female specimen enclosed
in its shell, cart, cartilage ; cten. ctenidia ; hd. hood ; inf. funnel ; jaws, jaws ; maiit. mantle ;
mant'. dorsal mantle fold overlapping the coil of the shell ; mus. position of lateral mass of
muscle : nid. nidamental glands ; sept, first septum ; sij>k. siphuncle. (After Keferstein.)

hood bears two tentacles, and has the appearance of being com-
posed of the immensely developed sheat,hs of these, completely
fused together in the middle line : on each side the enlarged
sheaths of a second pair of tentacles are closely applied to, though
not completely coalescent with, the hood, being separated from the
latter by a narrow groove. The hood, with these two enlarged
sheaths, is covered with a thickened tuberculated skin, and acts
after the manner of an operculum for protecting the tentacles

XII

PHYLUM MOLLUSCA

779

and other soft parts about the head. Altogether there are forty-
two tentacles of the outer series, including four ophthalmic tentacles,
one situated on the oral and another on the aboral side of each
eye. The latter (ophthalmic) differ from the rest in being highly

Fig. 676. — Nautilus macromphalui, adhering to tfae substratum in a vertical position by
means of its tentacles, e. eye ; h. hood ; n. )<t. nuchal membrane detached from coil of shell ;
o. t. ophthalmic tentacles ; *A. shell ; ic. /. wing of funnel. (After Willey.)

sensitive, ciliated, and with the ridges on the inner side produced
into lamella-. The tentacles of the inner series differ strikingly in
number and arrangement in the two sexes. In the female there
are two inner lateral lobes, right and left, quite symmetrically
developed, each bearing twelve tentacles, and an inner posterior

780

ZOOLOGY

lobe (Fig. 677) divided by a deep median notch into two,
each half bearing twelve to fourteen tentacles. On the middle of
the oral surface of the latter, close to the median notch, is an oval
patch raised up into numerous closely set ridges {organ of Owen).
In the male the inner posterior lobe with its ridged organ is only
represented by a median posterior body consisting of two oval
elevations, each divided into a number of folds {organ of Van der
Hoeven). The internal lateral lobes are greatly modified, four of
the tentacles on either the right side or the left, usually the latter,
being modified to form a structure termed the spadix (Fig. 678),

ucxl

Fig. 677. — Inner posterior lobe of foot of female of Nautilus pompilius, with neighbouring
parts of cephalopodium. oio. organ of Owen ; t. one of the tentacles of the outer wing ;
val. organ of Valenciennes. (After Willey.)

which is supposed to represent the hectocotylised arm of the male
Sepia. It has the form of a large compressed cone formed by the
union of the enlarged sheaths of three of the tentacles. The
corresponding tentacles themselves are in the adult male enor-
mously thickened, and the outer surface of the most posterior (3)
is covered with regularly arranged rows of minute pits. A fourth
tentacle, much smaller than the others, is closely applied to the
outer surface of the organ. In the internal lateral lobe, right or
left as the case may be, opposite that bearing the spadix, the
latter is represented by a group of four tentacles forming what is
termed the anti-spadix.

PHYLUM MOLLUSCA

781

Fie. 678.— Nautilus pompilius,
spadix of full-grown male, seen
from the outer side. 1, 2, 3, 4,
modified tentacles ; 1, withdrawn
into its sheath, its position and
shape indicated by the dotted
line ; 3, the flattened tentacle
with the rows of minute cavities ;
x, patch of modified integument.
Two-thirds of the natural size.
(After Haswell.)

A further difference between the male and the female with

regard to the foot is the presence in

the latter, but not in the former, on

the inner surface of the outer ring,

close to the inner posterior lobe on

either side, of an area thickly beset

with delicate membranous ridges

{organ of Valenciennes, Fig. 677, val.).
On the posterior side of the head is

a funnel corresponding with that of

Sepia, but extending further forwards ;

this, however, does not form a com-
pletely closed tube, the edges of its

right and left moieties being simply

in apposition posteriorly without being

united together. Near the oral end

is a large, somewhat triangular valve

arranged like that of Sepia.

There is an internal skeleton of

cartilage (Fig. 679), as in Sepia, but its

relationships with the nerve-ganglia

are much less intimate in the case of

Nautilus than in that of Sepia.

Mantle and Mantle-Cavity. — The mantle is produced around

the head into a free flap, longer and looser than the mantle-flap of
Sepia. Dorsally this splits into two layers,
reflected over the convexity of the shell,
which fits into a hollow behind the hood.
Ventral ly and posteriorly the mantle en-
closes a large mantle-cavity (Fig. 680), cor-
responding to that of Sepia. In this are
lodged two pairs of ctenidia (cten.), having
the same general structure as the single
pair present in Sepia. Between the bases
of the ctenidia of each side is a small knob-
like elevation, the oral osphradium (ant. os.),1
and behind the bases of the more aborally
situated pair are two compressed, bilobed
projections, more or less completely united
in the middle so as to form a transverse
ridge ; these are the aboral osphradia (p. os.).

In the middle line of the mantle-cavity is the anus (an.), a large

1 As in Sepia, it is convenient to use the term oral for parts towards the
mouth end, and aboral for those situated towards the opposite extremity, the
same terms being also used to indicate relafivt position of different parts. The
relative position of the parts is, however, for the sake of simplicity given here
as they lie when the mantle-cavity is opened by turning back its thin postero-
ventral wall.

VOL. I 3 D

Fu;. 07'.).— Nautilus pom
pilius, cartilaginous in

ternal skeleton.
Keferstein.)

(After

782

ZOOLOGY

aperture with minutely lobed margin, situated on a slight eleva-
tion, but by no means so prominent as in Sepia. On each side
are two apertures, the oral and aboral nephridial apertures (Fig.
680, a. I. neph., pt. neph.), corresponding to the single pair of
Sepia, but not elevated on papillae. Close to each posterior

1.6 ap

, (it. neph

r. anl.os

cterv

pi. neph
l.visc.ap

Fig. (580. — Nautilus pompilius, interior of mantle-cavity of a male specimen with the
postero-ventral wall reflected, a. I. neph. oral left nephridial aperture; an. anus; cten.
ctenidia ; ey. eye ; /. funnel ; I. <$ ap. left reproductive aperture indicated by a bristle passed
through it ; 1. rise. ap. left viscero-pericardial aperture; n. s. Needham's sac; pen. penis;
pi. neph. aboral left nephridial aperture ; p. os. aboral osphradia ; r. ant. os. right oral osphra-
dium ; v. n. visceral nerves. (After Willey.)

nephridial aperture is an opening — the viscero-pericardial (l.visc.ap.,
r. vise, ap.) — leading into the pericardial section of the ccelome ;
these are not represented in Sepia. In both sexes there are two
reproductive ducts, right and left ; but in both the right alone
appears to be functional, and the left is much the smaller. The

mi PHYLUM MOLLUSCA 783

opening of the right sperm-duct of the male is situated on a
cylindrical prominence — the penis (pen.) — placed close to the
middle line. In the female the nidamental glands are, as in Sepia,
conspicuous objects when the mantle-cavity is exposed ; but they
are mainly situated on its posterior instead of its anterior wall.

Enteric Canal. — The mouth is surrounded by a peristomial
membrane beset with numerous papilla1. There is a pair of
jaws (Fig. 681, yaw) of similar shape to those of Sepia, but much
more powerful, and calcified towards the tips. The buccal mass
is a large rounded body with thick muscular walls. On the floor
of the contained cavity is a large and prominent odontophore
(odotit.), with long and pointed, curved teeth. In front of the
odontophore is a large bilobed soft prominence, the tongue (tong.).
Behind the odontophore, between it and the opening of the
oesophagus, are one large median and two lateral tongue-like pro-
minences beset with papillae ; on the inner surface of the latter
are the apertures of a pair of salivary glands.

The oesophagus (as.) becomes dilated aborally into a very
spacious crop (cr.) for the storage of the food, which consists of
small prawn-like Crustaceans and small Fishes broken up by the
jawsandradula. This opens into a rounded stomach (stom) having
very much the appearance of the gizzard-like caecum of Sepia.
The intestine (int.), shortly after it leaves the stomach, develops
a rounded caecum (coec.) with complexly folded walls, into which
the ducts of the digestive gland or " liver" open. The intestine
does not pass straight to the anus as in Sepia, but first bends
round in a short coil. The ink-sac and duct of Sepia are
not represented. There is a very large digestive gland divided
into four main portions or lobes, each of which is made up of
a number of lobules. The ducts ("bile-ducts," b. du.), opening
as above mentioned into the caecum, have a series of small
diverticula which may represent the pancreatic appendages of
Sepia.

The coelome consists of the pericardium and the gonocosle — the
cavity in which the gonad is enclosed : these communicate with
one another by three apertures. The pericardium contains the
ventricle, the four auricles, and parts of the renal glandular
appendages. It communicates with the exterior by the viscero-
pericardial apertures.

Heart and Vascular System. — The vascular system consists of
the heart, the arteries and veins, and certain large spaces constituting
the haemoccele. The latter consists of three chief parts — the peri-
stomial, peri-oesophageal and peri-hepatic haemocoeles, the first sur-
rounding the buccal mass, the second the oesophagus, and the third
the liver.

The ventricle (Figs. 681 and 683, vent.) is a bilobed, transversely
placed, muscular sac, very similar to that of Sepia. On either side

3 D 2

Fig. 681. — Nautilus pompilius, dissection of the Internal organs of a male, from the left side. The
funnel and the hood have been divided by a longitudinal median section. A portion of the wall of the
buccal cavity has been removed to show the odontophore and the tongue, (tec. yl. vesicula seminalis ;
an. anus ; aort. oral aorta ; aort'. posterior pallia] artery ; (>. du. bile-ducts ; hue. n. buccal nerves ; hur.
pap. papillae of peristomial membrane ; cer. g. cerebral ganglion ; exec, cascum ; er. crop ; hd. hood ; i,tf.
funnel ; inf. n. infundibular nerve ; int.* part of intestine between stomach and c.ecum ; int.'* part of
intestine following caecum ; jaw, larger (posterior) jaw ; /. eff. !»: c. left efferent branchial vessels ; I. tent,
int. left internal tentacular lobe ; need. s. Needham's sac ; odont. odontophore ; ie'. style passed from buccal
cavity into the opening of the oesophagus ; at. oesophagus ; olf. n. olfactory nerve ; opt. n. optic nerve ;
oto. statocyst ; pall. n. pallial nerves ; ped. g, pedal ganglion ; pi. <j. pleural ganglion ; r. eff. br. v. right
efferent branchial vessel ; retr. retractor muscle of the buccal mass ; /'. liv. right lobe of " liver" ; stoui.
stomach ; test, testis ; tony, tongue-shaped elevation of the floor of the mouth ; *a. valve of funnel ;
ven, c. vena cava ; vent, ventricle.

PHYLUM MOLLUSCA

785

there open into it two auricles or efferent branchial vessels (a.or.),
one from each of the four ctenidia. The ventricle gives off a large
main amta (aort.), which passes to the head after giving off
arteries to the stomach, the crop,the digestive gland, and the mantle.
From the aboral surface of the ventricle arises a smaller artery, the
Utter aorta, which immediately bifurcates. One of its branches —
the posterior pallia I artery (Fig. 682, -post. pall, a.) — passes to the
ana of the mantle applied to the septum, bifurcates to supply this
area, and gives off a branch to the siphuncle. The other — anterior

effbr.v

gfe.rvA.3

—Nautilus pompilius (male), origin of pullial and genital arteries, ant. pal. a.
anterior pallial artery ; eff. In: c, efferent branchial veins ; gen. a. 1, artery to vesicula
seniinalis (>: sem.) ; gen. a. 2, testicular artery and its branches ; gen. a. 3, artery to pyrifonn
sac ; n. s. spermatophore-sac ; post. pall. a. posterior pallial artery ; pyr. pyrifonn sac ; rtct.
rectum ; test, testis. (After Willey.)

pallial (ant. pall, a.) — after giving off arteries to the intestine
and rectum, and to the branchiae and osphradia, passes to the
muscular edge of the mantle, bifurcating anteriorly. Three genital
arteries (gen. a. 1, 2, 3), supplying the various parts of the re-
productive apparatus, are likewise given off directly from the
ventricle.

A large vena cava (Figs. 681 and 683, ven. c.) occupies a position
corresponding closely with that which it occupies in Sepia. It
presents the remarkable peculiarity of being in free communication
by numerous (valvular) apertures with the general cavity of the

(86

ZOOLOGY

heemoccele. At its aboral end it presents a dilatation from which
four afferent branchial veins (Fig. 683 a.l.aff; p.l.aff, p.r.aff,
r.ant. aff.) — two right and two left — proceed to the corresponding
ctenidia, at the bases of which veins from the aboral region join
them. There are no branchial hearts.

The renal organs (Fig. 683) are, like the ctenidia and the afferent
and efferent vessels, four in number, instead of two as in Sepia.
Each renal sac (I. neph. s.; r. neph. s., I. post. neph. s., r. post. neph. s.)
opens into the mantle-cavity, as already stated, by an orifice
which is not drawn out into a tube. There is no communication
between the cavities of the different sacs, and thus no median

l,ne-ph..i

post.nep7t*. cup

T.pcst. nef>h. aja

rposlnephs

r.post.aur \ ve,n£
ren.ct.pp

'" Ipost.neph.s

iScpens-

Fig. 688. — Nautilus pompilius, renal sacs, with ctenidia and other related parts, as seen from
the posterior aspect ; the boundaries of the four renal sacs represented by dotted lines.
a. I. aff. left oral afferent vessel ; cten. right ctenidia ; I. neph. s. left nephridial sac ;
I. neph. ap. left oral nephridial aperture ; I. post. neph. ap. left aboral nephridial
aperture ; I. post. neph. s. left aboral nephridial sac ; l.v.ap, left viscero-pericardial aperture ;
p. I. aff. left aboral afferent vessel ; p. r. aff. right aboral afferent vessel ; r. ant. aff. right
oral afferent vessel ; r. ant. aur. right oral auricle ; ren. app. renal appendages ; r. neph. ap.
right nephridial aperture ; r, post. aur. right aboral auricle ; r. post. neph. s. right aboral
nephridial sac ; r. v. ap. right viscero-pericardial aperture ; ven. c. vena cava ; vent, ventricle ;
vise. per. s. viscero-pericardial sac.

chamber as in Sepia, and there is also no communication with the
pericardium. The cavities are found to contain phosphate of
lime. Into each projects, from the corresponding afferent
branchial vein, a compact rounded group of venous appendages
(ren. app.), consisting of two symmetrical portions. Internal to
these, each afferent vein has connected with it a second group of
glandular appendages, which are cylindrical or club-like in form ;
they project, not into the nephridial sac, but into the peri-
cardial compartment of the ccelome. They have been compared
with the appendages of the branchial heart of Sepia, but differ in
their relations to the renal appendages.

mi IMIYLT'M MOLLUSC A 7-S7

Nervous System. — Nautilus differs strikingly from Sepia, and
somewhat resembles Chiton (p. 710, Fig. 604) in the form assumed
by the central parts of the nervous system (Fig. 681, cer. g.),
distinct ganglia being absent. A very thick nerve-collar,
the posterior portion of which is double, surrounds the oesophagus
just behind the buccal mass. The anterior part of the collar
(Fig. 681, cer. g.) represents the cerebral ganglia, the oral
portion of the posterior part the pedal (ped. g.), the aboral
portion the pleuro-visceral (pi. g.) ; while the lateral parts, not
distinctly marked off from the rest, represent the cerebro-pedal and
cerebro-pleural connectives. From the cerebral ganglia pass
nerves to the buccal mass, to the olfactory organs (olf. n.) and
the statocysts, and a pair of very thick optic nerves supply the
eyes (opt. n.). The pedal ganglion gives off numerous nerves
to the tentacles and the funnel, and from the pleuro-visceral arise
pallial and visceral nerves.

Sense Organs. — The statocysts (otocysts) are a pair of sacs
embedded in recesses close to the cerebral ganglia, but not enclosed
in the cartilage of the endoskeleton ; each contains a number of
microscropic statoconcs. An olfactory function is ascribed to a
process (the rhinophore) with a ciliated pit at its base, situated on
the aboral side of the eye. The ophthalmic tentacles (Fig. 676, o.t.)
are supposed to act as accessory olfactory organs. The osphradia
(p. 781) contain ganglion-cells, are beset with sensory cilia, and
are undoubtedly organs of special sense.

The eyes, situated at the sides of the head, are very large but
extremely simple in structure, presenting a marked contrast to
those of Sepia, and scarcely comparable to those of any other
animal with the exception perhaps of Patella (p. 746). Each is
of the shape of a saucer, attached to the head by its convex side by
means of a short thick stalk, the mouth being closed in by a slightly
convex disc, with a circular aperture at about its centre. A
slightly raised rim runs round close to the margin on the posterior
half, and a narrow groove extends inwards from this to the central

O ...

aperture. In the interior of the cup is neither lens, vitreous
humour, nor iris. The sea-water, passing in through the central
aperture, directly bathes the retina, which is spread over the
interior in a thick layer.

Reproductive Organs. — The gonad (testis, Fig. 684, test., or
ovary, Fig. 685, ov.), like that of Sepia, is single and median, enclosed
in a special sac towards the aboral end of the body. The duct
is paired in both sexes, but in both the right alone appears to
be functional. In the male a large glandular vesicula seminalis,
in which the spermatophores are formed (ace.) is connected with
the right duct, and this appears to be represented on the left-hand
side by a vestige — the so-called pyriform sac (pyr.), situated close
to the ventricle. The distal part of the right duct dilates to form

ZOOLOGY

a receptacle, the spermatophoral sac or Needham's sac (sp. s.), and
opens, nearly in the middle line at the end of a prominence — the
penis (Fig. 680, pen.). In the female the right oviduct has a

rcjertop

I gen op

P$r

post, aort

FlG. C84. — Nautilus pompilius, male reproductive organs, ace. vesieula seminalis ; eff. vest.
efferent branchial vessels ; I. gen. op. left genital opening ; post. ao. posterior aorta ; pyr.
pyriform appendage ; r, (ten. op. right genital opening ; sp. s. spermatophore-sac ; test.
testis ; vent, ventricle.

Tuje-n.op

l.gen.op

Fig. 685. — Nautilus pompilius, female reproductive organs, alb. albumen-gland ; I. gen. op.
left genital opening ; op. ovary ; pyr. pyriform appendage ; r. gen. op. right genital opening ;
vent, ventricle. (After Lankester and Bourne.)

glandular dilatation, which is supposed to be an albumen gland.
The ova are of large size, greatly exceeding those of Sepia in
dimensions, containing a large proportion of food-yolk. Nidamental

\n PHYLUM MOLLUSCA 7M

glands are present, but are mainly situated, as already pointed
out, on the posterior instead of the anterior wall of the mantle-
cavity. Each egg becomes enclosed
in an elaborate capsule (Fig. 686),
probably moulded by the agency of
the organ of Owen, on the inner
posterior lobe of the foot of the female
(Fig. 677). The development is not
known.

2 Distinctive Characters and
Classification.

The Cephalopoda are bilaterally
symmetrical Mollusca, which have the
main part of the foot displaced for-
wards to the neighbourhood of the
mouth and divided into a series of
arms bearing suckers, or of lobes
bearing tentacles, while the remainder F,,Ucromfhaiu?aenS
of the foot forms a funnel for the in its capsule. (After wuiey.)

egress of water from the mantle-cavity.

The visceral mass is symmetrical and not coiled. The mantle
encloses posteriorly and ventrally a large mantle-cavity, in which
are situated the ctenidia and the nephridial, reproductive, and
anal apertures. The shell may be absent or rudimentary ; when
present and well developed, it may be internal or external, undivided,
or divided internally by septa into a series of chambers. There is
an internal cartilaginous skeleton, supporting and protecting the
nerve-centres and giving attachment to muscles. The mouth is
provided with a pair of horny jaws, and an odontophore is present.
In the majority there is an ink-gland with a duct opening into
the rectum. The ctenidia and nephridia are either two or four in
number. The nervous system is highly developed ; and the
principal nerve-ganglia are aggregated together around the
oesophagus. The sexes are separate ; the segmentation of the
ovum is meroblastic, and there is no metamorphosis.

Sub-Class I.— Dibranchiata.

Cephalopoda in which the main part of the foot assumes the
character of a circlet of either eight or ten arms, bearing suckers,
and surrounding the mouth. The funnel forms a complete tube. The
shell is usually internal ; when external its cavity is not divided by
septa. There are two ctenidia, two nephridia, and two branchio-
cardiac vessels or auricles. An ink-gland and duct are present.

*90 ZOOLOGY

Order 1. — Decapoda.

Dibranchiata possessing ten arms, with stalked suckers provided
with horny rims, and with a well-developed internal shell.

This order includes the Cuttle-fishes, Squids, Spirula, and others,
as well as the extinct Belemnites.

Order 2. — Octopoda.

Dibranchiata provided with eight arms, the suckers on which are
sessile and devoid of horny rims : with or without slight vestiges
of an internal shell. An external shell, secreted by a specially-
modified pair of arms, is present in the female Argonaut only.

This order includes the Octopods and the Argonauts.

Sub-Class II.— Tetrabranchiata.

Cephalopoda in which the main part of the foot has the
character of lobes bearing numerous tentacles. The funnel does
not form a complete tube. There is an external, spiral, chambered
shell. There are four ctenidia, four nephridia, and four auricles.
An ink-gland is absent.

This sub-class includes only one living genus, Nautilus, but the
Ammonites and other extinct forms are usually referred to it.

Systematic Position of the Examples.

The genus Sepia is a member of the family Sepiidos of the order
Decapoda, which is distinguished from the seven other families of
the order by the combination of the following features : — The body is
compressed and comparatively broad ; the fins are narrow and
elongated ; the internal shell consists almost entirely of calcareous
material.

Nautilus is the sole living representative of the sub-class Tetra-
branchiata.

3. General Organisation.

The uniformity of structure among the Dibranchiate Cephal-
opoda is very great, and, as already stated, Nautilus is the only
living member of the Tetrabranchiata, so that comparatively little
has to be said to supplement the descriptions of the two
examples.

External Features. — The general external shape differs very
little in the different members of the Dibranchiata : the body in
some is more elongated, in others, less ; the degree of compression
likewise varies. Fins may be absent, and the animal may pro-

xn

PHYLUM MOLLUSC A

791

gress entirely by creeping with the aid of the long arms, or
by swimming by the movements of the arms, or under the
propulsion of a current of water forcibly ejected through the
ninnel by the contraction of the muscular mantle (Fig. 687).
When fins are present they may take the form of a continuous
lateral flap as in Sepia, but, more usually, are of the nature of
flattened lobes situated towards the aboral extremity of the body
(Fig. 688) ; in Ctenopteryx they have the character of fringes of
filaments. The arms vary in length and proportions and in the
form and arrangement of the suckers. Eight arms are present in
the Octopoda and ten in the Decapoda. In the former group the

Flo. 687.— Octopus vulgaris. A, at rest; B, in motion;/, funnel, the arrow showing the
direction of the propelling current through the water. (From Cooke, after Merculiano.)

Argonauts (Fig. 689) have, in the female, one pair of arms
(wa.) flattened and expanded at the extremities for the secretion
and support of the shell (sh.). In the Decapoda one pair of
arms, the fourth, is always specially modified, as in Sepia, to
act as prehensile appendages or tentacles capable of being partly
or entirely retracted within certain sacs situated at their bases.
In nearly all one of the arms is specially modified (or heclocotylised)
to act as an intromittent organ. This modification is only very
slight in Sepia and confined to the base, and is most marked in
certain of the Octopoda (Fig. 690), including the Argonauts. In
the latter, before the breeding season, the third arm in the male
is found to be represented by a rounded sac, which subsequently

r92

ZOOLOGY

bursts and sets free the elongated hectocotylised arm. Spermato-
phores are taken by the arm from the genital opening, and in the
act of copulation the entire arm is detached and left in the
mantle-cavity of the female. In other cases the arm is not
detached. The suckers are sometimes stalked, sometimes sessile,
sometimes armed with hooks, sometimes replaced by hooks. In
many cases the arms are united by a web-like fold, the inter-

na. 688.— LoUgo vulgaris.

A, entire animal, dorsal view ; B, horny internal shell or pen.
(From Keferstein.)

brachial membrane (Fig. 691), which may reach nearly to their
extremities.

In the Tetrabranchiata the series of groups of slender, ringed,
sheathed tentacles, situated on lobes of the foot surrounding the
mouth, take the place of the arms, and suckers are not present.
In the males the spadix probably represents, functionally at least,
the hectocotylised arm of the Dibranchiata.

In all the Dibranchiata the funnel is a complete tube. In the
Nautilus, on the other hand, as we have seen, the folds which form
the funnel have their edges merely in apposition, and not united.

\ll

PHYLUM MOLLUSCA

793

A valve, such as has been described in Sepia, occurs in most
Decapoda and in Nautilus, but is absent in the Octopoda.

In.. 089. Argonauta argo, female, showing the relations of the animal to the shell in the living
state, the arrow showing the direction of movement. /. funnel ; m. mouth, with jaws project-
ing ; sh. shell, with arms as seen through it ; wa, webbed arm clasping the shell. (From
Cooke, after Lacaze-Duthiers.)

Chromatophorcs, similar to those of Sepia^ are universal in the
Dibranchiata but absent in Nautilus.

Shell. — The shell of Nautilus is the most complete and yet in
a certain sense the most primitive. As already stated, it is an
external shell of a spiral character, divided internally by septa
into a series of chambers. The last of the chambers is occupied

Fn;. 090. Octopus lentils, male specimen, showing the structure of the hectocotylised arm
(h. a). (From Cooke, after Verrill.)

by the body of the animal ; the rest are filled with gas. Perforat-
ing the middle of all the septa in succession is a spiral tube — the
siphundc — continuous with the centro-dorsal region of the visceral

794

ZOOLOGY

SECT.

prominence. In the course of its growth the body of the Nautilus
shifts forwards at intervals into a newly formed chamber, and a

new septum is formed closing the
latter off from the cavity last oc-
cupied. It is only after the last
septum has been formed that the
animal attains sexual maturity.

Of existing Dibranchiata, Spirilla
alone has a shell (Fig. 692) com-
parable to that of Nautilus. The
shell of Spirula is of spiral form,
the turns of the spiral, however,
not being in close contact. In-
ternally it is divided into chambers
by a series of septa, and these are
perforated by a siphuncle. But
the initial chamber (protoconch)
instead of being, like the initial
chamber in Nautilus, similar to the
others though smaller, is dilated
into a spherical shape, constricted
off from the succeeding chamber, and has passing through it a
tube — the prosiphon — not continuous with the siphuncle. Again,
as will be seen by comparing Figs. 675 and 693, the relation of

Fig. IS h— Amphitretus pelagicus,
an Octopod with the arms united by a
web. e. eyes ; /. funnel ; p. pouch in
the mantle. (From Cooke, after Hoyle.)

Fig. 692.— Shell of Spirula. A, outside view ; B, showing last chamber and position of
siphuncle ; C, in section, showing the septa and the course of the siphuncle ; D, shell broken
to show the convexity of the inner side of the septa ; ZS, portion of a septal neck. (After
Cooke.)

the soft parts to the shell is the reverse of what obtains in
Nautilus, the shell of Spirula curving backwards (endogastric

PHYLUM MOLLUSCA

795

curvature), that of Nautilus forwards (exogastric curvature). More-
over the shell of Spirilla is an internal structure, being almost
completely covered by the mantle.

The shell of the extinct Ammonites (Fig. 694), which are usually
referred to the Tetrabranchiata, resembles
that of the Nautilus in many respects,
being a chambered spiral shell with a
large terminal chamber, and with a
siphuncle. The chief external difference
is in the form of the sutures, or lines of
union of the edges of the septa with the
side wall of the shell ; these are more or
less complexly lobed, instead of being
entire as in Nautilus. But in one im-
portant respect the shell of an Ammonite
differs from that of Nautilus and ap-
proaches that of the dibranchiate Spirula.
At the apex of the spiral is an initial
chamber or protoconch, which is dilated
and separated from the first of the or-
dinary chambers by a constriction, and
has passing into it a prosiphon not con-
tinuous with the siphuncle. The Am-
monite was also characterised by the
possession of a paired or unpaired struc-
ture, sometimes horny, sometimes cal-
careous, called theaptychus,riot represented
in any existing form. The aptychus,
which was composed of two parts, may have been of the nature
of an operculum for closing the mouth of the shell, but was
more probably endoskeletal. Young Ammonites, each with its

aptychus, have been found within the
shell of the parent, in which they
must have remained protected during
their development.

In the ordinary decapod Dibranch-
iata the shell may consist of three
parts — a horny pen or pro-ostracum, a
calcareous guard, and a part termed
the phragmacone. The last, which
alone represents the shell of Spirula,
has the form of a cone divided intern-
ally by a series of septa perforated by
a siphuncle. These parts* are most
completely developed in the extinct genus Belemnites, in which
the shell (Fig. 695) consists of a straight, conical, chambered
phragmacone (phr.), with a siphuncle, enclosed in a calcareous

1 ic . 60S.— Spirula peronii,
lateral view, d, terminal
sucker ; /. funnel ; », **, *-.
projecting portions of the
shell, the internal part of
which is indicated by dotted
lines. (Krom Cooke.)

Fit;. 004.— An Ammonite (Ceratites
nodosus).

796

ZOOLOGY

SECT.

pen

phr

sheath, the guard, produced into a horny or calcareous plate, the
pro-ostracum (pen.). In Sepia the spine-like projecting point
represents the guard, and the main sub-
stance of the shell is to be looked upon as
the pro-ostracum and phragmacone, the
septa of the latter being represented by
the calcareous lamellae. In the Squids
(e.g.,Loligo) the shell (Fig. 688, B) is long,
narrow, and completely horny ; it corre-
sponds to the pro-ostracum, the phrag-
macone being entirely absent.

In Octopus the shell is represented only
by a pair of vestiges with which muscles
are connected. In Argonauta there is no
shell in the male, but the female has an
external shell (Fig. 696) of a remarkable
character. This is a delicate spiral struc-
ture the internal cavity of which is not
divided into chambers. It is not secreted
by the mantle like the shells of other
Mollusca, but by the surfaces of a pair of
the arms ending in expanded disc-like
extremities, which become applied to its
outer surface (Fig. 689) ; its chief function is to carry the eggs.
An internal cartilaginous skeleton is present not only in
Sepia and Nautilus, as already described, but in all the Cephalo-
poda. Such an internal skeleton occurs in other ; groups — some

Fig. (505.— Shell of ;i Belem-
nite. gd. guard ; pen,
pro-ostracum ; phr. phrag-
niocone. (From Nicholson
and Lydekker's Palaonto-

Fig. 606.— Shell of Argonauto argo.

Chaetopoda (p. 472), Crustacea, and Arachnida (p. 667) — but
attains a much more elaborate character in the present group than
in any other Invertebrates.

\n PHYLUM MOLLUSCA 7'»7

The plume-shaped gills, lodged in the mantle-cavity, are two
in Dumber in all the Dibranchiata, as in Sepia. In the Tetra-
branchiata there are four gills, similar in general character to
those of the Dibranchiata.

The ccelome in the Dibranchiata has the the extent already
indicated (p, 766) in the case of Sepia, except that in the Octopoda
the oral part does not exist. In Nautilus it encloses, besides the
heart and gonad, a part of the glandular appendages of the
afferent branchial vessels. In the Dibranchiata the pericardial
portion communicates with the nephridia ; in Nautilus this com-
munication is absent, but the ccelome opens on the exterior by
two symmetrical viscero-pericardial orifices placed at the side of
the openings of the aboral nephridia.

Alimentary Organs. — Jaws similar to those of Sepia are
present in all the members of the class; in Nautilus, instead of
being completely horny, they are partly calcified. Buccal mass,
oesophagus, stomach, intestine, salivary glands, and digestive gland
are all of the same general character throughout all the members
of the class. In some of the Dibranchiata, such as Octopus, there
are two pairs of salivary glands. In Nautilus the salivary glands are
absent, so far as known, the oesophagus is dilated to form a sort
of crop, and the stomach is gizzard-like. In that genus also the
ink-gland, general in the Dibranchiata, is absent, and there is a
< a eal appendage to the intestine; the digestive gland is four-lobed,
each lobe having its duct. The so-called pancreas, described in
Sepia, is similarly developed in all the Dibranchiata, and is present
also, though only feebly developed, in the Tetrabranchiata.

Heart and vascular system are well developed in the
Cephalopoda, and their structure and arrangement closely corre-
spond with what has been described in Sepia, except that in
Tetrabranchiata there are, as already stated, in accordance with
the double number of gills, four auricles instead of two, and
branchial hearts are absent.

Nervous system and sense-organs. — The ganglia of the
central nervous system are in all closely aggregated together round
the oesophagus, as already stated to be the case in Sepia ; and the
general disposition is the same as that described. In Octopus the
ganglia are much less sharply marked off. In Nautilus, as already
mentioned, there is less concentration, and distinct ganglia are not
recognisable. All the Dibranchiata possess highly developed eyes
similar to those of Sepia; but in Nautilus the eyes are of a much
simpler character, each consisting of a sac opening on the exterior
by a small rounded aperture, lined internally by a two-layered
retina similar to that of Sepia, but without lens, vitreous humour,
or cornea. In the embryo of the Dibranchiata, the eye passes
through a stage in which it is in the condition of an open cup
similar to the adult eye of Nautilus. Osphradia are present, as

VOL. I 3 E

798

ZOOLOGY

already mentioned, only in the Tetrabranchiata ; but in both the
Dibranchiata and the Tetrabranchiata certain sensory processess
or depressions conjectured to possess an olfactory function are
developed on the head. Statoct/sts are universally present.

All the Dibranchiata have two nephridia similar in character
to those of Sepia, and communicating with one another ; in Octopus
they are completely united. In the Tetrabranchiata there are
four nephridia, each opening on the exterior.

The sexes are distinct in all the Cephalopoda, and in addition
to the hectocotylised arm, there are frequently other external
differences between male and female. In all the Dibranchiata
the arrangement of the gonads and gonoducts is, as regards
general features, similar to what we find in Sepia. In Octopus,
however, there are two oviducts instead of one, and in one other
member of the Octopoda (Eledone moschata) the same holds good
of the spermiducts.

Development. — The development of the Dibranchiata alone is
known. The eggs are very large, containing a relatively large
amount of food-yolk. They are usually laid in masses or strings
embedded in a soft gelatinous, or a tougher, more leathery
substance, usually attached to some foreign body ; in some cases
each egg, enclosed in a gelatinous sheath, has a longer or shorter
stalk. A chorion or delicate transparent egg-membrane, in which
there is an aperture — the micropyle — immediately invests the
egg itself. In shape the egg is oval or spherical. The greater
part of the comparatively small quantity of protoplasm lies as

a disc-like elevation on the sur-
dors face °f the yolk on the side of

the egg at which the micropyle
is situated. Continuous with
this germinal disc is a thin
layer of peripheral protoplasm
investing the entire ovum.

Segmentation (Figs. 697 and
698) is incomplete, being con-
fined to the germinal disc. At
an early stage in the process of
division, the blastoderm exhibits
a distinct bilateral symmetry.
This meroblastic segmentation
results in the formation of a
nearly circular blastoderm, the
outer cells of which tend to
separate off. At first the blas-
toderm consists of only a single layer of cells — the ectoderm,
which gradually extends. At a later stage a second layer
(Fig. 699, B, G) appears below the margin of the blastoderm, and

a-nl

post

vent

Fig. 097.— Segmenting ovum of Loligo
(From Korschelt and Heider, after Watase.)

PHYLUM MOLLlSt A

:<.»<)

extends inwards until it comes t<» underlie the whole of the
embryonic part of the blastoderm : separating this from the yolk

hi

Fiu. til's.— Sepia, blastoderm at a late stage of segmentation, hf. blastoderm ; yk. yolk. (From
Korschelt and Heider, after Vialleton.)

is a thin layer of uncertain derivation — the yolk-cpithclivm (Fig.
699, yk. itp..). There is some doubt as to the nature of the second

Fio. 099.— Sections through the edge of the blastoderm of Septa at three successive stages ;
W. blastoderm ; yk. yolk ; yk. ep. yolk-epithelium. (From Korschelt and Heider, after
Vialleton.)

layer ; it certainly gives rise to the mesodermal structures, and
by some observers it is also said to form the epithelium of the

3 E 2

800

ZOOLOGY

mesenteron. From whatever source it may be derived, the latter
becomes distinguishable as a cell-plate which is converted
into a vesicle opening below against the yolk-epithelium, there
never being any direct communication with the yolk. An exten-
sive stomodseum eventually opens into this ; a proctodeum is
merely represented by the ectodermal pit forming the anus.

Fig. 700. — Early stages in the development of Loligo. A, stage at which the rudiments of the eyes
and of the shell-gland are first distinguishable ; B, later embryo from the oral side ; 0 and D,
from the anal side. ant. f. f. anterior funnel-fold ; ar. rudiments of arms ; cten. ctenidia ;
eye, eye ; mo. mouth ; want, rudiment of mantle ; ot. statocyst ; post. f. f. posterior funnel-fold ;
sh. gl. shell-gland ; yk. s. yolk-sac. (After Korschelt and Heider.)

About the middle of the blastoderm appears a thickening
of a cap-like shape, the edges of which become raised above the
general level of the blastoderm ; this is the rudiment of the
mantle. On the surface of this is developed a depression which
subsequently forms a closed sac — the shell- gland (Fig. 700, sh.gl.).
Below the mantle — i.e. nearer the vegetal pole — appear two eleva-
tions each with a pit-like depression, which are the rudiments of

MI

IMIY1J M MOLIJ S( A

801

the eyes; and still nearer the vegetal pole a series of paired
elevations, the rudiments of the arms.

cterv

ar.t

1/k.S

yJz.s

Via. 701.— Two later stages in the development of Loligo. A, from the funnel side. B, obliquely
from above. Letters as in preceding figures ; ne. cart, nuchal cartilage. (After Korsehelt and
Heider.)

B

-<fin

poster

cten.

fk.S

yks

Fin. 702.— Two stages in the development of LoligO, later tlian tin >se represented in Fig. 701.
From the anal or funnel side. Letters as in preceding figure ; in addition, rin, fins. (After
Korsehelt and Heider.)

After the complete enclosure of the yolk by the blastoderm, the
mouth (mo.) is developed as an oval depression between the nidi-

802

ZOOLOGY

SECT.

merits of the eyes. Immediately in front of the edge of the mantle
appear two short ridges, the beginnings of the gills (den.), and a pair
of folds — the posterior funnel-folds (post.ff.) — which are formed
between these and the eyes, are the first rudiments of the funnel,
the greater part of which, however, is formed from a second pair
of folds — the anterior funnel-folds (ant. f. /.) — developed further
forwards. Behind the anterior funnel-folds appear two pit-like
depressions, which subsequently develop into the statocysts.

The elevations on which the eyes {eye) are situated become
more and more prominent. The eyes themselves are formed

dcTV

Fig. 703.— Two late stages in the development of Loligo, seen from the funnel side. Letters as
in preceding figures. (After Korschelt and Heider.)

from a part only of these elevations ; each is a pit which sub-
sequently becomes closed to form a vesicle — the optic vesicle :
later an ingrowth of the ectoderm over this gives rise to
the lens.

The embryo covers only a part of the egg, and as it develops, it
withdraws itself more towards the animal pole, at which the
germinal disc was originally situated — a constriction, which soon
becomes very deep, separating it off from the rest of the egg ; the
latter, consisting of the greater part of the yolk enclosed in a thin
layer of blastoderm, forms a rounded appendage of the embryo — the

xii PHYLUM MOLLUSCA 803

j/<>lL-sac {yk. s.). The yolk-sac undergoes contractions, which are
due to the action of contractile cells in the thin mesoderm
lining it, and by this means the yolk is forced into the interior of
the body of the embryo.

The anus appears as an aperture situated on a little papilla — the
anal papilla. A row of cilia, which are developed in the neighbour-
hood of the mouth in some forms, perhaps represent the velum
or pre-oral circlet of other molluscan embryos. The mantle now
increases in extent, and its margins become more prominent.
The anterior funnel-folds grow out and unite in the middle
line ; and these, with the posterior folds, go to form the
completed funnel together with the " neck-muscles." For a time
the edges of the two folds which form the funnel remain free ;
eventually they coalesce into a complete tube.

The edges of the mantle grow out into prominent folds to form
the mantle-cavity, into which the gills are drawn. Lateral out-
growths have already given rise to the rudiments of the fins. The
arms grow out into more and more prominent processes on which
the suckers are developed, the second pair — the prehensile arms
(ar. 2) — soon becoming distinguishable from the rest by their
greater length.

As the embryo increases in size, the yolk is gradually
absorbed, and the yolk-sac decreases in bulk, until, when the
embryo leaves the egg, it has almost completely disappeared.

Distribution. — The Cephalopoda are all marine, and range from
tidal limits to a considerable depth. A large number (Loligo, etc.)
are pelagic and move together in great shoals. Sepia lives chiefly
between stones and in rock-fissures in the littoral zone, and often
burrows in sand. Octopus constructs a den or shelter of stones to
which it always returns after excursions in search of food. Cephalo-
pods are, nearly without exception, carnivorous. In length they
range from an inch or two to as much as fifty feet — the gigantic
members of the group, such as Architeuthis, being by a long way
the largest of invertebrate animals. Like the other classes of
Mollusca they are most abundant in tropical and warm-temperate
seas.

If the Ammonites are to be included among the Tetrabran-
chiata, that sub-class was most abundantly represented during
the Mesozoic period. The nautiloid Tetrabranchiata were
most abundant in the Palaeozoic epoch, during which there
lived a great variety of forms of this group, the shell
being straight (Orthoeeras), or curved (Phragmoceras), or in a flat
spiral with the turns not in contact, or in a helix, or a flat close
spiral (Nautilus and others). The earliest representatives of the
Nautiloids are found in rocks of Cambrian age ; they are com-
paratively scarce in the Mesozoic epoch and in the Tertiary,
and are represented at the present day only by the genus

804 ZOOLOGY sect.

Nautilus itself. The Ammonites are mainly Mesozoic, the repre-
sentatives found in the earlier rocks (from the Upper Silurian
onwards) being few in number and simpler in structure than
the more typical later forms. The oldest fossil representatives
of undoubted Dibranchiata belong to the extinct order of the
Belemnites; which flourished in the Mesozoic period from the
Trias to the Cretaceous, and survived in scanty number into the
Tertiary. Unlike the Tetrabranchiata, the Dibranchiata would
appear to have reached their maximum at the present day.

The mutual relationship of the various groups of Cephalopoda
are indicated, as nearly as the information at our disposal will
allow, in the following diagram (Fig. 704).

Decapoda

Belemnites
Naurlloids \ / ^Ocropoda

Ammonites

Fig. 704. — Diagram to illustrate the relationships of the groups of Cephalopoda.

General Remarks on the Mollusca.

The Mollusca, like the Arthropoda, form an extremely well-
defined phylum, none of the adult members of which approach
the lower groups of animals in any marked degree. There are,
however, clear indications of affinity with " Worms," especially
in the frequent occurrence of a trochophore stage in develop-
ment, in the presence of nephridia, and in the occurrence, in
Amphineura and some of the lower Gastropoda, of a ladder-like
nervous system resembling that of some Turbellaria and of the
most worm-like of Arthropods — Peripatus. The head-kidneys or
primitive nephridia of the molluscan and annelid trochophore
are practically identical, and are probably homologous with the
various types of nephridial tubes found in " Worms " from Platy-
helminthes to Chsetopoda.

If the occurrence of the trochophore be taken as a guide towards
the ancestry of the Mollusca, it need not necessarily be regarded
as leading back to the Annulata. In fact the presence of not
more than a single pair of nephridia (and of ctendia) in all with

mi PHYLUM MOLLUSCA 805

the exception of Nautilus, would seem to indicate the derivation
of the phylum from a group in which metamerism had not arisen.
It will be readily recognised that the gap between the typical
trochophore and certain forms of Turbellarian larvae (Muller's larva)
is not a very wide one, and might be covered by adaptation of the
larval Flat-worm to a freer pelagic life. If we were to suppose that
the most primitive Mollusca were derived from Turbellarian-like
ancestors, the conversion of a larva of the type of Muller's larva
into a larval form like the molluscan trochophore would also have
to be postulated. This might involve a common platyhelminth origin
for Annulata and Mollusca, with subsequent extreme divergence —
a divergence in which the respective trochophores would take part,
though in a limited degree. The chief changes which the adult
animal would have to undergo in order to assume the character
of a primitive Mollusc on this supposition, would be — (1) The
development of some kind of protective layer of hard material,
perhaps composed at first of spicules in a thickend integument, on
the dorsal surface — the rudiment of the shell ; (2) The greater
development of the muscular layers of the body-wall on the ventral
side to give rise to a more efficient and specialised creeping organ
than was possessed by the Turbellarian ancestor ; (3) The develop-
ment of specialised respiratory organs in the form of ctenidia — a
change rendered necessary by the great reduction in the available
respiratory area brought about by the development of the shell ;
(4) The formation of an anus and proctodeum ; and (5) the develop-
ment of a ccelome.

With regard to the relationships of the various classes of
Mollusca, the following points are some of the most important to
be borne in mind.

The lowest members of the phylum are undoubtedly the Proto-
branchia among Pelecypoda, and the Aplacophora among Amphi-
neura, The latter take the lowest rank in virtue of the absence of
both foot and shell, but the possession by some of a radula indi-
cates a comparatively high degree of specialisation. On the other
hand, while there is no indication of an odontophore, even in a
rudimentary condition, in the Pelecypoda, the foot and shell are
well developed even in Nucula and its allies. There is no actual
evidence to show that the foot and shell have been lost by degenera-
tion in the Aplacophora or the odontophore in Pelecypoda ; and it
would appear, therefore, that the two groups are to be derived
independently from some primitive form.

The facts that the pelecypod shell, at its first appearance, is
univalve, and that the foot of the Protobranchia is of the creeping
type and their ctenidia plume-like, suggest the derivation of the
class from a form resembling a simple type of Gastropod with no
odontophore and with undisturbed bilateral symmetry. The Amphi-
neura are also bilaterally symmetrical, with paired ctenidia, kidneys,

806 ZOOLOGY sect, xii

and auricles, and the fact that these organs are also paired in the
lower Gastropoda, seems to point to a common ancestor for Pele-
cypoda, Amphineura, and Gastropoda, which was bilaterally sym-
metrical, had a creeping foot, a simple shell, paired auricles,
kidneys, and gills, and no odontophore.

While the leading feature in the evolution of the Pelecypoda
has been the splitting of the mantle into two halves and the
resulting bivalve shell, the most noticeable fact in that of Gastro-
poda, apart from the appearance of the odontophore, has been the
torsion of the visceral mass, producing a characteristic asymmetry.
In the Cephalopoda, on the other hand, the primitive bilateral
symmetry is retained, and the most characteristic special feature
of the group is the extraordinary modification of the foot into arms
or tentacles, and funnel. The class is raised far above the remain-
ing Mollusca by its wonderfully high organisation, especially of
the nervous system and the eye, and there is nothing to indicate
close relationship with any of the lower classes beyond the general
conformity to the molluscan plan of organisation and the presence
of an odontophore. The Cephalopods form, in fact, a singularly
isolated group. Palaeontology has not hitherto given any indica-
tion of their origin, and embryology is equally silent ; the absence
of a free larva, and the profound modification in development
produced by the enormous mass of food-yolk, sharply separating
them from all other members of the phylum.

<r°7

INDEX

INDEX

All numbers refer to pages : words in italics are names of families, genera and
species: words in thick type are names of higher divisions: words in small
capitals are names of examples. Numbers in thick type are numbers of pages on
which there are figures : an asterisk after a number indicates a definition of the
term or of the group.

A,

lBactinal, 376*, 418

Abdomen, of Aptu, 531 i Attacue, f>41 :
Periplaneta. 820, 622

Aboral. 376*, 418

Absorption, 34

Abyssal species, 8*

Aiiiiitharin, 60

Ai anthin, 61*

Acanthocephala, 297* : External charac-
ters. 31$ 313 : Body-wall, 312 : Body-
cavity, 313 : Proboscis, 313 : Vessels,
313 : Nervous-system, 313 : Excretory
organs, 313, 315 : Reproductive
organs, 314, 315 : Development, 315

Acatdhobdella, 518, 519

Acarida, 661*, 665, 667, 668, 669, 672,
673

Achromatin, 17*

Aciculum, 441*

Adaeta, 99, 100, 101

Acaia, 262, 267, 272

Acontia, 188*, 201

Acorn-shells, 565

Actceon, 745

Actinal, 376*, 418

Actinia, 193, 228

Actiniaria, 194*, 196, 197

Acttnobohu, 83, 94

ArimodnrtyleUa, 259, 260, 267, 273

Actinometra, 42.~>

A'/iitomni't astterarniithion, 61, 62

Actinojihrys sol, 56, 57, 59

Acfinosphmrium, 57

Actinostome, 376*

Aclinotrocha, 358, 359, 503

Actinozoa, 128 : Example, 185 : Distinc-
tive characters and classification, 193 :
Systematic position of example, 196 :

General organisation, 196 : Budding,
197 : Structure of polypes, 19S : Enteric
system, 201 : Fixed and free forms,
202 : Dimorphism, 202 : Skeleton,
202 : Colour, 208 : Commensalism.
208 : Distribution, 209

Actinula, 226*, 229

Adamsia palliata, 208, 209

Adductor impressions, 682, 683, 694,
697, 698, 699

Adductor muscles, 683, 697, 698

Adhesive cells, of Hormiphora, 215*:
Turbellaria, 264

Adipose tissue, 26*, 27

Adjusters, 364*

Adradius, 139

Adrectal gland, 744

JSgkioptia, 155, 156

Alyinura, 155

dlq norea, 143

Affinities — See Relationships

A yalma, 163

Agamobium, 140*, 176

Aggressive characters, in Crustacea, 601

Air-sacs, of Insects, 642

Albert ia. 335

Atciopidce, 488

Aleippe, 565, 579

Alcyonacea, 195, 197, 198, 199, 203

Alcyonaria, 1!)5*, 196, 197, 199, 200, 201,
202, 203, 227

Alcyonidir, 208

Alcyonidium, 348

Alcyonium, 195, 203

Alecithal, 219*

Alimentary canal — See Digestive system

Ali mentary system — See Digesti ve system

Allolobophnra antipir, 481

Alpfieti*, 602

810

INDEX

Alpine forms, 8*

Alternation of generations — See Meta-
genesis.

Alveolus, of Sea-urchin, 397

Ambulacral area, 395

Ambulacral grooves, 376*

Ambulacral ossicles, 378, 381, 419

Ambulacral pores, 378, 396

Ambulacral ridges, 380

Ambulacral spines, 376, 387

Ambulacral system, of Asterias, 383 :
Echinus, 396, 399 : Sea-cucumber, 402,
403 : Antedon, 408 : Echinodermata,
415

Amitotic division, 19*

Ammonites, 790, 795, 803, 804

Amnion, of Peripatus, 612 : Periplaneta,
631 : Scorpion, 659

Amoeba, 10, 11, 12, 13, 14: Pseudopods,
47 : Endosarc, 46 : Ectosarc, 46 : Con-
tractile vacuole, 46 : Encystation,
46 : Fission, 46 : Systematic position,
48

Amcebidiv, 48

Amcebocytes, 382, 399

Amcebophyra, 233

Anuebula, of Didymium, 66, 67 : of
Gregarina, 82, 83

Amphiblastula, 124*, 125

Amphidiscs, 121*, 122

Amphilina, 262, 287

Amphineura, 680, 712* : Distinctive char-
acters and classification, 712 : General
organisation, 713 : External features,
713, 714 : Ctenidia, 713 : Alimentary
system, 715: Body cavity, 715: Vas-
cular system, 715 : Nervous system,
715, 717 : Reproductive and renal
organs, 718, 719 : Development, 718,
720: Ethology, distribution, &c,
720

Amphinomidve, 467

Amphipoda, 568*, 583, 584, 585, 586, 593,
596, 603

Amphiptyches, 262, 263

Amphislomum, 257

Amphitretus pelagicus, 794

Amphiura, 232

Amphiura squamata, 429

Ampulla-, 158*, 378*, 399

Ampullaria, 749

Amusium, 702, 704

Anal filament, 663

Anal glands, of Peripatus, 610

Anal respiration, 596

Anal spot, of Parametrium, 90

Anaspidacea, 566*, 581, 582, 596, 603

Anaspides, 566

Anatomy, 3*

Anchors, 177

Anguilhda, 305

Anisopoda, 584*

Ankylostoma duodenale, 306

Annulata, 439* : General remarks on,
523 : Relationships, 525

Annuli of Leech, 506

Anodonta, 680 : Shell, 682 : Body, 684 :
Muscles, 684 : Cu-lome, 684 : Digestive
organs, 684, 685 : Gills, 685 ; Excretory
organs, 689 : Circulatory system, 689,
690 : Nervous system, 690 : Sensory
organs, 691 : Reproductive organs, 691 :
Development, 692, 693, 694: Syste-
matic position, 696

Anodonta, 696, 697, 698, 704

Anomia, 695, 697, 698

Anomura, 569*, 588, 589

Anopheles, 87

Anoplophyra, 94

Anostraca, 563*, 571

Antedon rosacea, 405 : General external
features, 405, 406 : Ossicles, 406
Cudome, 407 : Enteric canal, 407 :
Amhulacx-al system, 408 : Nervou:
system, 408 : Perihsemal and luemas
system, 409 : Sacculi, 409 : Reproducl
tive organs, 409 : Metamorphosis, 409-
432, 433 : Systematic position, 414 :
Development, 432

Antenna, of Astacus, 544 — See also Ap-
pendages

Antennary gland, of Astacus, 551, 552

Antennary glands, 551, 596

Antennule, of Astacus, 544 — See also
Appendages

Anthenea flavescens, 384, 386, 387,
388

Anthomedusee, 141*, 143, 144, 149

Anthophysa, 576

Anthosoma, 575, 576

Anthura, 568

Antimeres, 42* 415

Antipatharia, 195*, 201, 202, 203, 204,
210

Antipathes, 201

Antispadix, 780

Ant-lions, 633

Ants, 636, 652

Aorta — See Vascular system

Aphides, 647

Aphis ros<e, 633

Aphrodite, 475

Aphroditea, 474, 475

Apical plate, of Trochosphere, 323

Apical system of plates, 393, 397

Apis mellifica, 637, 652

Aplacophora, 713*, 715, 717, 718, 720,
721, 805

Aplysia, 738, 739, 740, 746

Aplysiidic, 734

Apoda (Holothuroidea), 413*, 426, 428

Apodiida', 569, 570*

Apopyle, 108, 109, 110*

Appendages, of Rotifera, 330 : Apus,
529, 530 : Astacus, 542, 543, 544: Crus-
tacea, 570 : Peripatus, 607, 608 : Peri-

INDEX

811

pfaneta, 621 : Insecta, 037 : Scorpion,

<;.v>

Apm n<!> t, ~><>7

Aptera, 632*, 640, 677

Aptyohus, 795*

Aim E8, 526 : External characters, 527
528 : Appendages, 529, 530 : Body
wall, 531 : Muscular system, 532
Digestive organs, 532, 533: Body
cavity. 'V.V.I -. Circulatory system, 533
Respiration. 534 : Renal organ, 534
535 : Nervous system, 534, 536
< )rgans of sense, 536, 537 : Reproduc
tive organs, 537 : Development, 537
538 : Systematic position, 569

A pu<, 534. 535, 537. 538

A /ins cancriformia, 528, 536

A /his glackdi*, 529

Aquatic pupa, 651

Arachnida, 526, 653*, 677 : Example,
<>."">,"{ : Distinctive characters and classi-
fication, 660 : General organisation,
(i&2 -. External form. 662-667 : Endo-
stcniite, 667 : Coxal glands, 667 :
Alimentary system, 667 : Heart, 668 :
Organs of respiration, 668 : Nervous
system, 670 : Sense-organs, 670 : Re-
productive apparatus, 671 : Mode of
life, 672: Geological history, 073:
Appendix, 673

Arachnidinm, 664

Araneida, 661*, 664, 667, 669, 670

A rhmia punctidata, pedicellaria, 422

Arm. 695, 897, 701, 702, 704, 706, 711

Arcella, 4B, BQ

Archwocytes, 122

Arehenteron 23*

Archi-Annelida, 439, 503*, 504, 505. 524

Archi-cerebrum. 665

Archi-cerebrum, of Periplaneta, 631

Archi Chaetopoda, 465*, 477

ArckigeUs, 262, 263, 287

ArvJuteuthit, 803

A rgiope, 366

A njonauta argo, 793, 796

Argonauts, 790, 791, 796

Argu/us, 565, 576, 577

Arhynchobdellida, 516*

Aricia, 479

Aristotle's lantern, 397

Ark-shell, 702

Arnmdillidium, 586, 587 •

Armadillo, 524. 543

Armata, 496*, 497, 498, 499, 502

Arrow worms, 297

Artemia,5Q3, 571

Arthrobranchiaj, 551

Arthropoda. 526* : Affinities of air-
breathing, 677

Arthrostraca. 583*

Articulata, 366*, 367, 368, 369: Shell,
398

Axcaridn', 304*

Am akis lumisuh <ui>ks, 297 : External
characters, 297 : Body-wall, 298 :
Digestive organs, 299, 300 : Ccelome,
301 : Excretory system, 301 : Nervous
system, 301, 302 : Reproductive
organs, 302, 303 : Development, 303 :
Systematic position, 304

A scar is megaloeepha/a, 297

Ascaris nigrovenow, 308, 309, 310

Ascaris suit la, 297

Ascttta, 116

Ascon, 117, 118*

Ascopodaria, 354

Asellus, 568, 583, 585

Asexual reproduction, 40 : in Amoeba,
46 : Heliozoa, 59 : Radiolaria, 63
Euglena, 69 : Flagellata, 74 : Choano
flagellata, 78 : Dinoflagellata, 79
Cystoflagellata, 79 : Sporozoa, 80
Coccidiidea, 84 : Haemosporidea, 86
Myxosporidea, 87 : Paramcecium, 90
Ciliata, 98 : Tentaculifera, 101
Sponges, 121 : Actinozoa, 197 : Platy
helminthes, 283: Bugula, 346
Chaetopoda, 486

Aspergillus, 096, 701, 702

Aspidobranchia, 733*

Aspidochirotae, 415

Aspidocotylea, 253*, 260, 273

Aspidogasterj 284

Asplanchna, 328, 330, 331

Astacopsis, 569

Aetactu, 569

Astacus fluviatilis, 539, External
characters, 540 : Abdomen, 541,
Thoracic region, 541 : Head, 542 :
Appendages, 542, 543 : Articulations,
545 : Body-wall, 546 : Muscular
system, 546, 547 : Digestive organs,
548 : Respiratory organs, 54it. 550 :
Excretory organs, 551, 552 : Circula-
tory organs, 551, 552, 553, 554:
Nervous .system, 555 : Sensory organs,
556 : Reproduction, 556, 557 : Develop-
ment, 557, 558, 559, 560, 561 : System-
atic position, 57' I

Astasioptis, 71

Astkkias RUBBN8, 375 : General external
features, 375, 376, 377 : Transverse
section of an arm. 378 : Vascular and
nervous systems, 379 : Structure of the
disc, 380 : Body-wall andcudome, 381 :
Digestive system, 382 : Ambulacra!
system, 383 : Reproductive system,
386 : Systematic position, 414

Affteriidtv, 414*

Asterina, development, 388, 389, 390,
391. 392, 393

.!>'< rina yibbosa, 429, 432

Asteroidea, Example, 375 : Development,
388 : Distinctive characters and classi-
fication, 410 : Apical system, 417 :
Modifications of form, 418, 419 :

S12

INDEX

Ccelome, 425 : Ambulacral system,
425 : Blood- vascular system, 426 :
Haemal system, 427 : Axial organ,
428 : Enteric canal, 429 : Nervous
system, 429 : Reproductive organs,
429 : Development, 430 : Ethology, 434

Asthenosoma, 421

Astracoidea, 570*

Astrcea, 197, 198, 207

Astropecten, 419, 420

Astrophyton, 421

Astrosphere, 17, 18*

A tlanta peronii, 741

Atrium, 249, 346

Atrochal, 486*

Attraction-sphere, 18*

Auditory organs, 39, 645

Aulacliniwm actinastrum, 62

Aulostoma, 516, 522

Aurelia aurita, External character-
istics, 168, 169 : Digestive-cavity and
canal system, 170, 171 : Cell-layers,
170 : Gonads, 171 : Gastric filaments,
172 : Muscular and nervous systems,
172 : Sense-organs, 172 : Development
and life-history, 173, 174, 175:
Systematic position, 177

Auricle, of heart, 36*

Auricles, of Sea-urchin, 397 : of Cteno-
phora, 223*

Auriadaria, 404, 413, 431, 432

Australian region, 9*

Autolytns cornutus, 487

Avicularium, 341, 342, 352*

Axes, 42*

Axial fibre, 92, 96, 97

Axial nerve, 409

Axial organ, 384, 400, 428

Axial sinus, 380

Axis-cylinder, 29*, 30

B.

B

►alanus, 565, 577, 579
Barnacles, 3, 526, 565, 577, 579, 594,

596
Barrier reef, 210
Basal plate, of coral, 205*, 206
Batteries, 161*
Bdelloida, 328*, 330, 334
Bdelloura, 284
Bear-animalcules, 673
Bee-parasites, 651
Bees, 526, 619, 636, 647, 652
Beetles, 525, 619, 635, 640
Belemnites, 790, 795, 796, 805
Benthos, 8*
Berenice, 150
Beroe, 224, 228
Beroida, 221*, 224
Bicellariidw, 348*
Bicellular glands, 493*

Bilateral symmetry, 41, 43*

Bile, 34*

Binomial nomenclature, 1*

Biology, 1*

Bionomics, 9*

Bipalium, 255

Bipinnaria, 411, 432

Bird-lice, 651

Bird's-Head Coralline, 341

Birgus, 569, 589, 595, 603

Birth-opening, 245*

Bivium, 377, 416 : of Sea-cucumber, 4<»1

Black coral, 195*, 202, 210

Blastoccele, 23*

Blastoidea, 414*, 435, 437

Blastomeres, 22*

Blastopore, 22, 23*

Blastosphere, 23*

Blastostyle, of Obefia, 129*, 130 : Lepto-
linte, 151 : Porpita, 165, 166

Blastula, 23*

Blatta — See Periplaneta

Blattidas, 636, 653

Blepharoblast, 70*, 72

Blood, 29*, 34*

Blood-corpuscles. 30

Blood- vascular system — See Vascular
system

Blood-vessels, 34

Blow -flies, 635

Blue coral, 195

Bodotria, 567

Body-cavity — See Coelome

Body-wall, of Sea-anemone, 185: Hormi-
phora, 215 : Liver-fluke, 241 : Platy-
helminthes, 262 : Nemertinea, 290 :
Ascnris, 298: Nematoda, 305: Chaato-
gnatha, 316 : Brachionux rubens, 325 :
Bugula, 343 : Ectoprocta, 351 : Mayel-
lania, 362 : Asterias, 381 : Sea-cucum-
ber, 402 : Nereis, 443 : Earthworm,
456 : Chsetopoda, 473 : Sipuncnbus, 493 :
Gephyrea, 497 : Hirudo, 508 : Apus,
531 : Astacvs, 546 : Crustacea, 594 :
Peripatus, 608 : Myriapoda, 617 : In-
secta, 636

Bojanus, organs of, 689

Bolina hydatina, 224

Bone, 25, 27*, 28

Bone-corpuscles, 27*

BoneUia, 496, 497, 499, 500, 501

Book-gills, 671 •

Book-lungs, 657, 668

Book-scorpions, 662

Bopyrini, 586

Bopyrus, 568

Botany, 1*

Bot-fly, 635, 651

Bothridia, 262*

Bothriocephalus, 261, 279, 285

Bothriorephalus latus, 285

Botryoidal tissue, of Leech, 509

Bougainvillea, 143, 144, 152

INDEX

SIS

Brachial disc, of Discomedusie, 18.3

Brachial ossicles, 407

Braehiolaria, 411, 432

/Irnr/iioiiitf'i , 329*

Brackionut, 324, 325. 326, 888, 888

BbaCHIOXOS RUBENS: External charac-
ters, 888, 324 : Body-wall, 325 : Diges-
tive organs, 886: (Vlome. 886: Ex-
cretory system, 326 : Nervous system,
and sense organs, 3*26 : Reproduction
and development, 326, 327 : Systematic
position, 888

Brachiopoda, 340, 360* : Example, 360
Distinctive characters and classifica
tion, 366: Systematic position of ex
ample, 367 : General organisation, 367
Shell, 367, 368 : Peduncle, 367 : Lo
phophore, 368 : Muscular system, 368
Enteric canal, 369 : Heart, 369
Nephridia, 369 : Nervous system, 369
Gonads, 369 : Development, 370, 371
Distribution, 371

Brachyura, 569*, 589, 590, 591, 601

Bract, 160, 161*, 531

Brain, 38

liranchellion, 516, 517, 519, 522

Branchia.', 35* : of Asterias, 376 ." Sea-
urchin, 395: Polyeha-ta, 471 : Oligo-
( hata, 473 : Branchellion, 519 :
Astacus, 549; Crustacea, 593: Ano-
donta, 686, 687, 688 : Pelecypoda, 702 :
Triton, 725 : Gastropoda, 742

Branchial formula, of A star us, &c , 551

Branchiopoda, 563*, 569, 570, 571, 593,
594, 596. 697, 598, 602, 603, 677

Braivhipu*, 668, 670, 571

Branchiura, 565*, 573, 577

Brine-shrimp, 571

Brisingidte, 418

Brood-pouch, 370, 531

Brood-cavity, 101

Brown body, 346

Buccal cavity, 32

Buecinum mutatum, 722

Budding, 40*, 41— See Asexual repro-
duction

Budding, in Turb.'Uaria, 257

Buflbu, 5

Bugs, 634, 640, 651

Bugula, 347

Bugula avicularia, 341, 342: Body-
wall, 343 : Coelome, 343 : Alimentary
canal, 343 : Nervous system, 343 :
Excretory organs, 343 : Reproductive
organs, 344 : Development, 344, 345,
346 : Systematic position, 348
Bugula plumose/., 346
Bursa copulatrix, 270, 271, 272, 273, 647
Busycon, 749

Buthus, 653 : External features, 654,
655 : Digestive system, 65(5, 657, 658 :
Circulatory organs, 656, 657 : Organs of
respiration, 657 : Nervous system, 657,

VOL. I

(>.uS : Organs of special sense, 658 : Re
productive organs, 658 : Development,
659

Butterflies, 526, 619, 635

Byssus, 694, 702

Byssus-gland, 694, 702

Byssus, provisional, 693

c

'addis-fi.iks, 633
Cake-urchins, 412*, 417, 423
Calcarea, 112*, 120, 122, 123
Calcareous spicules, of Sponges, 107, 108,

120. 122
Calciferous glands, 458
Cattianira, 218, 219, 222
Callitiara, 150
Oalocalanus, 574
Calotte 230*, 344
Calymma, of Radiolaria, 80
Calyptoblastea 143*
Cambarue, 569
Cambrian, 7
Campaunlarii<la>, 142*
Canalicular (bone), 27*
Canals, Haversian, 27*, 28: incurrent,
radial of flagellate, excurrent of
Sponge, 107, 108, 109*, 117 : of Medusa.
135, 136
Canal system of Sponges, 117, 118
Cancer, 569, 590
Cannostomse, 180

Capillaries, 35*

Capillary vessels, 238

Capillitium, of Mycetozoa, 66, 67

Caprella, 568, 586

Capria, 178

Capsulogenous glands, 457

Carabus auratus, 641

Carapace, Apus, 527: Astacus, 540:
of Scorpion, 654

Carboniferous, 7

Cardiac sac, of Polychreta, 476

Cardinal process, 360*

Cardium, 696, 698, 701, 711

Jarina, of Cirripedia, 578

larinaria mediterranea, 739

Jarp-lice, 565

Carpoidea, 414*, 488

Cartilage, 25, 26*: Hyaline, 26, 27:
Fibrous, 26, 27 : Yellow elastic, 26 :
Calcified, 27

Carpophyllasux, 262, 263, 287

Cassiopeia, 184

Caudal spine, 666

Caudal styles, of Apus, 527

Caudal vesicle, 248*, 280

Cell, animal, 14, 16*

Cell, 14, 16*, 17: Forms, 23: Ciliated,
23, 45 : Flagellate, 24, 45 : Anueboid,
45 : Encysted, 45

3 F

814

INDEX

Cell-colony, 50*, 66

Cell-division, 17, 18

Cell-plate, 17, 19*

Cell-wall, 16*

Cellepora, 347

Cellulose, 14, 64, 67, 69, 72, 73, 78

Cement glands, 270, 326

Centipedes, 526, 614, 615

Central capsule (Antedon), 409

Central capsule of Radiolaria, 60

Central nervous system of Medusa*, 151

Centro-dorsal ossicle, of Antedon, 406

Centrolecithal, 537, 219*

Centrosome, 17*

Cephalic apodeme, of Apus, 533 : Aba-
cus, 542

Cephalopoda, 680, 759*. Examples, 759,
776 : Distinctive characters <i.nl classi-
fication, 789 : Systematic position of
the examples, 790 : General organisa-
tion, 790 : External features, 790 :
Shell, 793 : Internal skeleton, 796 :
Gills, 797 : Osphradia, 797 : Ccelome,
797 : Alimentary organs, 797 : Heart
and vascular system, 797 : Nervous
system and sense-organs, 797 : Neph-
ridia, 798 : Sexes, 798 : Development,
798, 799, 800, 801, 802 : Distribution,
&c, 803 : Relationships, 804

Cephalopodium, 761

Cephalothorax, of Antaeus, 541

Cerata, 744

Ceratella, 146

Ceratella Jusca, 145

Geratites nodosw, 795

Geratium, 79

Ceratosa, 126

Cercaria, 244, 245*

Cerci, 623

Cerebral organ, 294*, 495

Cerianthus, 200, 202

Cervical fold, of Apus, 527

Cervical glands, 306

Cervical groove, of Astacus, 541

Cervical sclerites, 622

Cestida, 221*, 223

Cestoda, 253*, 261, 262, 263, 264, 266,
268, 269, 270, 273, 279, 280, 281, 282,
285, 286, 287, 288 : Example, 245

Cestus veneris, 223

Cetonia aurata, 316

Chaeta, 440 — See Seta

Chmtoderma, 713, 715, 716, 717, 718

Chcetogasta, 486

Chsetognatha, 297*, 316 : External char-
acters, 316: Body-wall, 316: Enteric
canal, 317 : Ccelome, 317 : Nervous
system, 317 : Sensory organs, 318 :
Reproduction, 318 : Development,
318

Chcetonotus, 336

Ghwtopterus, 469, 481

Chcetosoma, 319

Chd'tosomidre, 319*

Chatopoda, 439 : Examples, 440, 454 !
Distinctive characters and classifica-
tion, 464 : Systematic position of ex-
amples, 466 : General organisation,
467 : General form, 467, 468 : Para-
•podia and setae, 468, 469, 470 : Bran-
chiae, 471: Body-wall, 473: Calomc,
474 : Enteric canal, 475 : Blood-
vessels, 476 : Nervous system, 476 :
Organs of special sense, 478 : Organs
of excretion, 479, 480, 481 : Phosphor-
escence, 481 : Reproductive organs,
481 : Development, 483, 485: Asexual
reproduction, 486, 487 : Mode of life,
&c, 488: Appendix, 489

Ck(Hosomid(tj, 319

Chalk, 56

Charybdiva marsupialis, 181

Cheilostomata, 347*, 348, 349, 351, 352,
354

Chela;, 543*

Chelicera, 653*, 662, 664, 665, 666

Chelifer bravaisii, 662

Chelipeds, 543*

Chilaria, 666

Ch'dina, 745

Chilognatha, 615*, 617, 618

Chilopoda, 615*, 616, 617, 018

Chironomus, 644

Chitin, 31, 46*

Chiton, 680, 713, 714, 715, 717, 718, 719,
720

Chitonellus, 714

Ghlamydomyxa, 64, 65

Chloratmidw, 475

Chloragen cells, 458

Chlorophyll, 14, 58, 65, 68, 72, 78, 119

Choanocytes, 107, 108, 109*, 111

Choanoflagellata, 69* : General structure,
77 : Collar, 77 : Colonies, 77, 78 : Re-
production, 78

Chcetopterus, 469

Chondracanthiis, 564, 57-r>, 576

Chordotonal organ, 645

Chorion, of Cephalopoda, 798 : of Insects,
628

Choristida, 126

Chromatin, 16*, 17

Chromatophores, ActinospJurrium, 57, 58 :
Ghlamydomyxa, 65 : Flagellata, 71. 72:
Dinoflagellata, 78, 79: Sepia, 762:
Cephalopoda, 793

Chromosome, 17, 18*

Chrj'salis, 651

Cicada, 634, 646

Cicatrix, 776

Gidaris, 426

Cilia, 23*

Ciliary flames, 269

Ciliary process, 771

Ciliata, 91* : Form of body, 92, 93, 94,
95, 96, 97, 98: Stalk, 92, 96, 97:

INDEX

815

Arrangement of cilia, 92, 94, 96 I Un-
dulating membranes, 92, 94 : Mega-
nucleus, 93, 94: Micro-nuclei, 93, 94
Contractile vacuole, 98, 94, 96, 97
Non-contractile vacuoles, 93, 94
Trichocysts, 93, 94 : Digestive ap
paratus, 94, 95 : Skeleton, lorica,* 94,
95, % : Operculum, 95, 96 : Colonies,
94, 95, 97 : Reproduction, 96, 97, 98 :
( on j ligation, 99

Ciliated chambers, 118

Cinclides, 188, 901

( 'iiTidation — See Vascular system

Circulatory svstein —See Vascular system

Cirri, 405*, 439

Cirripedia, 565*, 593, 594, 596, 597, 698,
601, 602

Cirrus, 240

Cirrus sac, 249

Ciste/fa, 366, 368, 369, 370, 371

Cladocera, 564*, 572, 573, 596, 598

CladophiursB, 411*

Class, 4*

Classification, 3*, 5 : of Rhizopoda, 47
Mastigophora, 69* Sporozoa, 81 : In
fusoria, 91 : Porifera, 112: Hydrozoa
140 : Scyphozoa, 176 : Actinozoa, 193
Ctenophora, 220 : Platyhelminthes
•25 1 : Nemertinea, 295 : Nematoda
303 : Rotifera, 327 : Polyzoa, 347
Brachiopoda, 366 : Echinodermata
410 : Chaetopoda, 464: Gephyrea, 495
Hirudinea, 515 : Crustacea, 561 : In
secta, 631 : Arachnida, 660 : Pelecy
poda, 694 : Amphineura, 712 : Gastro
poda, 732 : Cephalopoda, 789

Clathrina, 116, 120, 122, 124, 126

('lathrina blanca, 124,

( 'Inthrozoon, 146

Clathrulina, 58

Clavatilla, 145

Clavula'. 417*

Cleaning foot, 581

depame, 516, 517, 518, 519, 521, 522

Cliona, 122, 126

Clitellum, 455, 468, 509

Cloaca, 249

Clypeaster snbdepretsiui, 423

Clypeastridea, 412*, 417, 423

Clvpeus, 620*, 037

Cnidoblast, 133*

Cnidocil, 133, 134*

i ktecidtB, 647

Coccidiidea, 81* : Characteristic features,
83, 84, 85

Coccidium, 83, 84, 85

Coccoon, 463, 483

Cockchafer, 643

Cockles, 680, 696, 698

Cockroach— See Periplaneta

Cockroaches, 526, 619, 633, 636, 638,
648, 653, 656, 658

Cocoa-nut crab, 569

Codonella, 94

Coelenterata, Classes, 128 : Examples,
128, 10S, 185,211 : Relationships, 226-
.229 : Appendix, 230 : Relationships to
Sponges, 228

Coeliac canal, 407

Cteloine and body-cavity, of Aacaritf,
301 : Nematoda, 307 : Acanthocephala,
313 : Clnetognatha, 317 : Buyula, 343 :
Eudoprocta, 354 : Phoroiu'*, 308 :
Magellania, 365 : Axterias, 381 : Sea
urchin, 399 : Sea-cucumber, 403 :
Antedon, 407 : Echinodermata, 425 :
Nereis, 442 : Cha;topoda, 474: 6ij>mt-
culu8, 493 : Gephyrea, 498 : Apua,
533 : Crustacea, 593 : Peripatux, 608 :
Insecta, 640 : Anodonta, 684 : Am-
phineura, 715: Sepia, 766: Cephalo-
poda, 797

Ccelomoducts, 439, 480

Cceloplaiui, 225, 286

Coenenchyma, 207*

Coenosarc, 131*

Coleoptera, 035*, 638, 640, 645, 647, 649,
653

Collar of choanoflagellata, 77

Collared cells, 107,' 108, 109*

Collared monads, 77, 78

Collencytes, 107, 111*

Colleterial glands, 628

Collozoum, 61, 63

Colochirus, 401 : General external fea-
tures, 401 : Structure of body-wall,
402 : Ambulacral system, 402 : Nerve-
ring, 402 : Perihsemal and hamial
systems, 402 : Ccelome, 403 : Enteric
canal, 403, 404 : Reproductive organs,
404 : Development, 404 : Systematic-
position, 414

Colony, 40* : of Foraminifera, 51 :
Heliozoa, 59 : Radiolaria, 61 : Flagel-
lata, 73 : Choanoflagellata, 78 : Ciliata,
97: Tentaculifera, 101: Obelia, 129:
Leptolinat, 143 : Actinozoa, 197 : Poly-
zoa, 340 : Bugula, 341 : Ectoprocta,
348 : Endoprocta, 355

Colpoda, 98

Columella, of Coral, 205*: of Triton,
722

Column, 185*

Comatulct, 434, 435

Comntulid" \ 415

Comb-jellies, 128

Combs, of Hormiphont, 211*, 212

Comb-ribs, 213*

Commensalism, in Sponges, 126: ////-
dractinia, 144 : Actinozoa, 208 :
Platyhelminthes, 284 : Chietopoda, 488 :
Crustacea, 602

Complemental males, 597

Conchiolin, 683*

Conchostraca, 563*, 569

Condyloatoma, 94

3 f 2

816

INDEX

Cone, of Coronata, 179
Cones, 733

Conjugation of Amseba, 47: Foraminifera,

56 : Heliozoa, 60 : Cystoflagellata, 79 :

Ciliata, 99 : Flagellata, 75 : Para-

m(ecium, 90

Connective tissue, 25* : Gelatinous, 25,

26 : Fibrous, 25 : Retiform, 25, 26
Connective tissue cells of Sponges, 1 1 1
Contractile vacuole, 11*, 13, 47, 66, 68,

77, 88, 89
Contractility of muscles, 28, 37*
Conns, 749
Convoluta, 255, 265
Copepoda, 564*, 573, 593, 594, 596, 597,

598
Coral, Aporose, 207* : Black, 195 : Blue,
195 : Fossil, 210 : Organ pipe, 195 :
Perforate, 207*: Red, 195, 210: Reef-
building, 210 : Stony, 195
Coral limestones, 210
Coral reefs, 159, 210
Corallines, 340
Corallite, 128, 205*

Corallium, 195, 197, 198, 203, 208, 210
Corallum, 205*
Gordylophora, 167
Cornea, 771 : false, 771
Corona, of Folyzoa, 344 : Sea-urchin,

396
Coronary groove, 176, 179*
Coronata, 176*, 179, 180
Corpuscles, 30 : amoeboid, 30 : Miescher's

or Rainey's, 88
Cortex, of Aclinosphverium, 57, 58 :
Monocystis, 80 : Paramecium, 88,
89: Sponges, 111, 119
Corymorpha, 145, 146
Couplers, 573

Covered-budded Hydroids, 143
Cowries, 733
Coxa, 622

Coxal glands, Peripatus, 609 : Scor-
pion, 658 : Arachnida, 667
"NCrabs, 526, 569, 587, 590, 593, 595, 596,
599, 603
Crane-flies, 635
Grangon, 569, 588
Crania, 366, 368
Crayfish, 539 — See Astacus
Crayfishes, 526, 569,~587, 596, 600
Cretaceous, 7
Crickets, 636

Crinoidea, example, 405 : Distinctive
characters and classification, 413 :
Apical system, 417 : Modifications of
form, 424 : Coelome, 425 : Ambnlacral
system, 426 : Blood-vascular system,
427 : Haemal system, 427 : Axial organ,
428 : Enteric canal, 428 : Nervous
system, 429 : Reproductive organs,
429 : Development, 430 : Ethology,
434

Crioceris, 635
Crisia, 347
Crista acustica, 772*
Cristatella, 348, 350
Crop, 32

Crown, of Coronata, 179
Crustacea, 526, 678 : Example a, 526 :
Example ft, 539 : Distinctive characters
and classification, 561 : Systematic
position of the examples, 569 : General
organisation, 570 ; External characters'
and structure of appendages, 570 :
Texture of the exoskeleton, 593 : Body-
cavity, 593 : Enteric canal, 593 : Re-
spiratory organs, 594 : Heart, 596 :
Excretory organs, 596 : Nervous
system, 596 : Sense-organs, 597 : Re-
production, 597 : Development, 597 :
Ethology, 600 : Affinities and mutual
relationships, 602 : Appendix, 604
Cryptocephala, 465*, 468, 475, 476, 484,

488
Cryptomonas, 71
Cryptoniscus, 586, 587
Cryptozonia, 411*,H14
Crystalline style, 685 : of Gastropoda,

744
Ctenaria, 150, 227
Ctenidium, 685, 687, 688, 702 : of Triton.

725 : of Gastropoda, 742
Ctenodrilus, 503, 504
Ctenophora, 128, 211: Example, 211:
Distinctive characters and classifica-
tion, 220: Systematic position of the
example, 221 : General organisation,
222, 223, 224 : Appendix, 225 : Re-
lationships, 226
Ctenoplana, 225, 286
Ctenopteryx, 791

Ctenostomata, 348*, 350, 351, 353
Cabomedusse, 176*, 180, 181
Cucumaria planci, 401
Cucumaria — See Colochirit*
Culex, 586, 634, 638
Cama, 567

Cumacea, 567*, 583, 594, 603
Cunarcha, 154
Cunina, 167
Cunina parasitica, 156
Cup-coral, 205
Cursoria, 636
Cursoria, 636
Cuspidaria, 696
Cuticle, 31*

Cuttle-fish, 680, 759, 790
Cuvier, 3

Cuvieran organs, 428
Cyamus, 568, 586
Cyanea arctica, 182
Cyclas, 710
Cyclidium, 94

Cyclops, 564, 573, 574, 596, 597
Cyclostomata. 347*, 349, 352, 354

INDEX

817

Cydippida, 221 *, 222

( ymotkoa, 688, .v.tT

( 'i/>iipi</<t\ 647

Cyprea, 739

( ')///>•/>, 664, 572, 573, 596

( 'yjirix stage, of Cirripedes, 565, 598

Cyst, Anuelxt, 46*, 47: Didymium, (iT :

JBWgfaw, 68, 69 : Monocyx/'is, 80
Cysticercoid, 280*
Cyslicercus, 251, 280, 281*
( 'ysticr, -in* aUulox<v, 285, 2.S6
Cystoflagellata, 69* : Characteristic

features, 79
Cystoidea. 413*, 430, 437
Cytfu n , .">64. 672
( ytn)tlasm, 16*

I)

D,

"actyloporbs, of Millepora, 156* :

Stylaaler, 159
Dactylozooids, 149*, 163: of Millepora,

158 : Stylaxter, 158 : Halixtemma, 160,

161
Daddy long-legs, 635
Dahlia wartlet, 185] '
Dallingeria, 71
Dalmanites xocialix, 605
Daphnia, 564, 572
Darwin, 6
Daxychone, 478
Daughter-cell, 18*
Daughter-chromosomes, 18*
Daughter-cysts, 282
Daughter-nucleus, 18*
Daughter-segments, 16
Day-flies, 641, 653
Dead men's fingers, 195, 203
Decapoda, 568*, 570, 687, 584, .~>!iii. ~>«»T.

599, 6<»2. 803
Decapoda (Cephalopoda). 7!H>*. 7!H. Sii4
Degeneration, in Copepoda, 575 : in Cir-

ripedia, 578 : in Isopoda, 586
Deiopea, 223
Dcltidium, 360*
Deltoid plates, 414
Demospongia. 1 12*
Dendrochirotee, 415*
Dendrocadum — See Pla na ria
Dendrocometex, 100
l>< iiilrojihi/l/ia, 207
Dendroxoma, 100, 101
Devtalium, 756, 757
Denticles, 442
Depastridm, 178
Derm (dermis), 31*
Dermal branchise of Echinoidea, 395
Dermal branchiae, 376, 395, 425

Dermal cortex, 108, 111*, 119
Dermal pores, 376
Dermaptera, 636
Dermis, 31*

Dexmoxcolecido', 320, 321
Dtxmoscolex, 320 _^ — >
Destructive metabolism, 13*
Deutomerite of (Treyarina, 82*, 83
Development, of Sycon, 124, 125 : Sponges,
122: Obelia, 139: Leptoliwv, 152:
Trachylinw. 155: Aurelia, 173, 174,
175: Sea-anemone, 192: Hormiphora,
217 : Dkytmido', 231 : Bhopalura,
233: Planaria, 239 : Liver-Fluke, 243:
Ta nia, 250 : Platyhelminth.es, 273-282:
Nemertinea, 294, 295 : Axcarix, 303 :
Nematoda, 308, 309 : Chajtognatha,
318 : BrachiontLsrubens, 326 : Rotifera,
334 : Buyida, 344 : Ectoprocta, 352 :
Endoprocta, 355 : Phoronis, 358, 359 :
Brachiopoda, 370: Axterina, 388, 389:
Sea urchin, 400: Sea-cucumber, 404:
Antedou, 432 : Echinodermata, 430 :
X< rci«, 460,481, 453 : Lumbricux, 463,
464: Chaetopoda, 483 : Gephyrea, 500 :
Hirudo, 515 : Hirudinea, 520, 521 :
Apitx, 538: Astacus, 557: Crustacea,
597 : Peripal <m, 611, 612, 613 : Myria-
poda, 618 : Periplaucta, 628 : Scor-
pion, 659: Anodovta, 692 : Pelecypoda,
708 : Amphineura, 718 : Gastropoda,
749 : Scaphopoda, 757 : Cephalopoda,
798
Devonian, 7
Dinttylit,, 567, 583
Diatomin, 78*
Dibothriocephalux, 273
Dibranchiata, 789*, 792, 793, 794, 786,

797, 798, 804
Dieertu, 700
Dirliyocyxta, 95
Dicyclica, 413*
Dicpama, 230, 231
Dicyemida-, 230, 231 ;
Ditli nium, 93, 94
Didymium dipfokmk, 66, 67
Differentiation, 23*
/'ijflii'/in, 49, 50
Digenetica, 252*. 257, 25!), 266. 268, 272,

273, 277, 284, 286, 287
Digestion, intracellular, 33
Digestive glands, 33
Digestive system, 32*
Digestive system, of Paramecium, 90 :
Aurelia, 170, 171; Sea-anemone, 187:
Hormiphora, 213 : Planaria, 288, 237:
Liver-Fluke, 241 : Platyhelminthes,
265, 266, 267 : Neinertinea, 290 : Ax-
caris, 299, 300: Nematoda, 305, 306 :
Cha?tognatha, 317 : Brachionux ruin ■*,
325 : Rotifera, 333 : Buyula, 343 :
Ectoprocta, 351 : Endoprocta, 354 ;

P

818

INDEX

Phoronis, 357 : Magellania, 362 : .4s-
terias, 382 : Sea-urchin, 399, 400 : Sea-
cucumber, 403, 404: Antedon, 407
Echinodermata, 428 : Nereis, 442
Lumbricus, 457 : Chuetopoda, 475
Sipunculus, 493, 494 : Gephyrea, 498
Hirudo, 409, 410: Hirudi'nea, 518
Apus, 532, 533 : Astacus, 548 : Crus-
tacea, 593 : Peripafus, 608 : Myria-
poda, 617: Periplaneta, 624, 625 ; In-
secta, 640, 641 : Scorpion, 656, 657,
658 : Arachnida, 667 : Anodonta, 684,
685 : Pelecypoda, 704 : Aniphineura,
715 : Triton, 726, 727, 728 : Gastro-
poda, 744 : Scaphopoda, 757 ; Sepia,
763 : Nautilus, 786 ; Cephalopoda, 797

Dimorpha, 70, 71

Dimorphism, in Foraminifera, 56 : in
Radiolaria, 63 : in Flagellata, 76 : in
Ciliata, 97, 98

Dimorphism, sexual, 40*

Dimorphograptus, 167

Dimyaria, 290

Dinobryon, 71, 74

Dinoflagellata, 69* : Characteristic fea-
tures, 78, 79

Dinophilea, 336, 337

Dinophilus, 524, 337

Dioecious, 40*, 139*

Diophrys, 92, 94

Diphyes, 164, 165

Dipleurula, 432*

Diplomita, 71

Diplopoda, 615*, 617, 618

Diplozoon, 277

Diptera, 634*, 638, 639, 640, 644, 646,
647, 649, 651, 653

Directive mesenteries, of Sea-anemone,
189

Discina, 189, 366, 368

Disc, 185*

Disc- jellies, 182

Discoidal segmentation, 659

Discomedusae, 176*, 182, 183, 189

Discorbina, 53

Discosoma, 208, 210

Discrotlhim lanceolatum, 285

Dissepiments, 205*

Distinctive characters — See Classification

Distomidm, 253*, 254

Distomurn, 265

Distomcm hepaticum, 240 — See
Fasciola hepatica

Distomurn, rathousii, 285

Distribution, 8 : of Sponges, 126: Hydro-
corollime, 159 : Scyphozoa, 184 : Ac-
tinozoa, 209 : Ctenophora, 224 : Platy-
helminthes, 283 : Ectoprocta, 353 :
Brachiopoda, 371: Gephyrea, 502;
Hirudinea, 524: Onychophora, 612:
Pelecypoda, 710: Aniphineura, 720:
Gastropoda, 755 : Cephalopoda, 803

Distribution, geographical, 8

Distribution, geological, 8* : of Forami-
nifera, 56 : Sponges, 126 : Corals, 210 :
Brachiopoda, 371 : Cha'topoda, 488 :
Insecta, 652 : Arachnida, 673 : Cephalo-
poda, 803

Distribution, vertical, 8*

Divaricators, 364

Dochmius duodenalis, 306

Docoglossa. 733*, 756

Doctrine of descent, 6

Donax, 706

Doris, 734, 742

Doris (Archidoris) tuberctdata, 739

Dorsal, 42* •

Dorsal cirri, of Antedon, 405

Dorsal organ, of Apus, 527

Dorsal pores, of Earthworm, 455

Dragon flies, 633, 644

Drepanophorus, 289

Drilophaga, 335

Dromia, 601

Ductus ejaculatorius, 302

Dytiscus, 645

E

E

JAR, 39*

Far-shells, 733

Earthworm — See Lumbricus

Earthworms, 439

Earwigs, 636

Ecdyses, of Apus, 539

Echiniidce, 414*

Echinoderes, 319, 320

Krhinoderid<e, 319, 320

Echinodermata, 375* : Examples, 375,
393, 401, 405 : Distinctive characters
and classification, 410 : Systematic
position of examples, 414 : General or-
ganisation, 415 : General form and
symmetry, 415: Systems of plates, 417 :
Modifications of form, 418 : Ccelome,
425 : Ambulacral system, 425 : Blood-
vascular system, 426 : Hremal system,

427 ; Axial organ, 428 : Enteric canal,

428 : Nervous system, 429 : Sexes, 429 :
Development and metamorphosis, 430 :
Echinopaidium, 432: Ethology, &c. ,
434 : Self-mutilation and regeneration,
435: Affinities, 436

Echinoidea, example, 393 : Distinctive
characters and classification, 412 :
Apical system, 417 : Modifications of
form, 421 : Dermal branchiae, 425 :
Stewart's organs, 425 : Ambulacral
system, 426 : Blood-vascular system,

427 : Hamial syBtem, 427 : Axial organ,

428 : Enteric canal, 428 : Nervous
system, 429 : Reproductive organs, 429 :
Development, 430 : Ethology, 434

Echinopa>divm, 432*
Echinorhynchus, 312, 313, 314, 315

TNDEX

819

Echini s, 393: General external features,
393, 394. 395: Corona, 396: Aristotle's
lantern, 397 : Nervous system, 398 ■
Ambulacra! system, 899: En terie canal,
999, 400 : ( '■! •iomc, 399 : Blood-vascular
system, 400 : Reproductive organs,
400 : Development , 400 : Systematic
position, 414

Echiuroidea, 496*, 407. 498, 499, 502

KcJtiwm*, 496, 497. 500, 501

Ectobranchiata, 415*

Bctocyst, 341*

Ectoderm, 23*— See Body-wall

Ectoprocta, 347*, 348 : Structure of body-
wall, 351 : Alimentary canal, 361 :
Nervous system, 351 : Nephridia, 302 :
Avicularia, 352; Vibraoala, 392: Re-
production and development, 352 :
Ethology and distribution, 353

Ectosarc, 46

Edriasteroidea. 414*. 435

gdwardna, 200. 292, 227, •_,-_".i

Eggs — See Development

Eimeria, 84

Ejaculatory duct, '243

Elasipoda, 412*, 428, 434

SXedone moackata, 7i*<s

Elephant's tusk shells, 7~>fi

Eleutherozoa. 410*

Elk-born coral, 156, 157

Elytra, of Cockroach. 622: Polyclueta,
469: Coleoptera, 6*5, 640

Embryology, 3*— See Development

Km pis, 644

Emulsions, 34

Encystation, 45*, 46, 59

Endites, 530

Endocyst, 341*

Endoderm, 23*

Endoderm-disc, of A slant*, 558

Endoderm lamella, 136* Z

Kndogastric, 777*

Endophragmal system, of Aetactu, 542

Endopodite, 538*

Endoprocta, 340, 348*, 354: Vestibule,
354 : Nephridia, 354 : Cloaca. 354 :
Oanglion, 354: Testes and ovaries,
3."»4 : Foot-gland , 354 : Development,
355

Endosarc, 46

Kndoskeieton. 31*

Endosternite, 655, 667

End-sac, 534

Enteric canal— See Digestive system

Enterocosle, 389

Enterozoa, 105

Entobranchiata, 415*

Bntovahn, 711

Environment, 9*

Eocene, 7

Eolis, 734, 742, 71 1

Epeira diadema, 664

Ephdota, 100, 101

Ephemera, 633

EphemeridAv, 641, 653

Ephippium, 597

Ephyropsis, 179, lsn

Ephyrula, of Ann/in, 175*

Kpiblast, 2a*

Kpibolic gastrulation, 273

Epiboly, 218*

Epicranium, 620*, 637

Epidermis, 24, 31 — See Body-wall

Bpimerite of Gregarinida, 82, 83*

Epineural canals, 398*

Epipharynx, 641

Epiphragm, 738

Epiphysis, of Bsa-urohin,*398

Epipodite, 544*

Epipodium, 740

Epistoma, of Antaeus, 542

Epistome, 340, 351, 356

Ejiixtylis plicati/is, 92, 94

Epistylis iimhdlaria, 93

Epitheca, 205*

Epithelia, 24*: Non-stratified, 24*:
Stratified, 24*

Epithelium, enteric, 33*

Equatorial plate, 18*

Ecpii valve, 699*

Brgeutttu, 564, 575, 576

Eetheria, 564, 571

Ethiopian, 9*

Ethology, 9* : of Platyhelminthes, 283 :
Rotifera, 335 : Ectoprocta, 353 : Echi-
nodermata, 434 : Chajtopoda, 488 :
Hirudinea, 522 : Crustacea, 600 : In-
secta, 651 : Arachnida, 672 : Pelecy-
poda, 710: Amphineura, 720: Gastro-
poda, 755

Eurhlatiis, 328

Bvchlora, 222

Eucirripedia, 56.")*, 577, 579

Eucope, development, 152
Eucopepoda, 5f**> 573, 574, 575, 602

Emh inlrium, 140

Euglena viridis, 67, 68 : Systematic

position, 70
Eulamellibranchiata, 696*, 697, 698, 711,

712
Eucarida, 568*, 687
Europe/ fa, 134
Euglenida>, 70
En < ili uoidea, 70
Eumalacostraca. 568*, 581
Euphcmsia, 568, 595, 599
Euphausiacea, 568*, 587, 594, 596, 599,

603
E>i/i/ii-o*i//ie., 468
Euphyllopoda, 596
Euptfctella, 120
Enpomatus, 485
Euryalida, 417, 420, 421
Eurypterida, 662*, 666, 668, (573, 678
Eurypterus fischeri, 668
Euscorpjo — See Buthf/s

820

INDEX

Euscorpius italkus, 659. 671, 672
Euspongia, 115, 121

Euthyneura, 734*, 737, 742, 745, 747,
748

Evolution, 6

Examples, of Rhizopoda, 46 : Mycetozoa
66 : Mastigophora, 67 : Sporozoa, 80 :
Infusoria, 88 : Porifera, 105 : Hydro-
zoa, 128 : Scyphozoa, 168 : Actinozoa,
185: Ctenophora, 211: Platyhel-
minthes, 236, 240, 245 : Nematoda,
297 : Rotifera, 323 : Polyzoa, 342 :
Branchiopoda, 360 : Asteroidea, 375 :
Echinoidea, 393 : Crinoidea, 405 :
Chaetopoda, 440, 454 : Oephyrea, 492 :
Crustacea, 526, 539 : Insecta, 619 :
Arachnida, 653 : Pelecypoda, 680 :
Gastropoda, 721 : Cephalopoda, 759-
776 : Holothuroidea, 401

Excretion, 14*, 37

Excretory pores, 213, 290, 298

Excretory system, of Leptolin-e, 151 :
Planaria, 238 : Liver-Flnke, 241 :
T<p>iia, 248: Platyhelminthes, 269:
Nemertinea, 292: A tear is, 301 : Echino-
rhynchus, 313 : Brachionus rubtns, 326 :
Rotifera, 333 : Ectoprocta, 352 : Endo-
procta, 354 : Phoronis, 356 : Magel-
lania, 365 : Brachiopoda, 369 : Nereis,
448 : Lnmbricus, 460, 4(51 : Chtetopoda,
479, 480 : Sijmnculus, 495 : Oephyrea,
499 : Hirwlo, 510, 512 : Hirudinea,
519, 520: Apus, 534, 535: Astacus,
551, 552 : Crustacea, 596 : Peripatus,
610 : Periplaneta, 625 : Insecta, 641 :
Scorpion, 656 : Arachnida, 667 : Ano-
donta, 689 : Pelecypoda, 705 : Amphi-
neura, 718, 719 : Triton, 729 : Gastro-
poda, 747 : Scaphopoda, 757 : Sepia,
772, 773 : Nautilus. 786 : Cephalopoda,
798

Excurrent canals, of Sponges, 108, 109*

Exhalant siphon, 681

Exites, 531

Exogastric, 777*

tfxopodite, 538*

Exoskeleton, 31* — See Body-wall

External features, of Sycon, 105, 106 :
Porifera, 114, 115, 116 : Obelia, 130:
Aurelia, 168, 169 : Sea-anemone, 185,
186 : Actinozoa, 196 : Hormiphora,
211, 212 : Planaria and Dendrocalum,
236: Liver-Fluke, 240, 242: Taenia
solium, 245 : Platyhelminthes, 254 :
Nemertinea, 288: Ascaris lumbricoides.
297 : Nematoda, 305 : Echinorhynchus,
312 : Ohaetognatha, 316 : Brachionus
rubens, 323, 324 : Rotifera, 330 : Bugula
avicidaria, 341, 342 : Ectoprocta, 349,
350 : Endoprocta, 354 : Phoronis, 356 :
Magellania, 362 : Brachiopoda, 366 :
A sterias rubens, 375, 376,377: Anthenea
fiavescens, 386 : Sea-urchin, 393, 394,

395: Sea-cucumber, 401: Antedon
rosacea, 405, 406 : Echinodermata, 415 :
Asteroidea, 418: Ophiuroidea, 415:
Echinoidea, 421 : Holothuroidea, 416 :
Crinoidea, 424 : Nereis, 440, 441 :
Lumbricus, 454 : Cluetopoda. 467, 468 :
Sipunculu* nudus, 492,493 : Gephyrea,
496: Arclii-annelida, 503: Hirvdo,
506, 507, Hirudinea, 517 : Apus, 527 :
Astacns, 540 : Crustacea, 570 : Peri-
pat us, 607, 608 : Myriapoda, 615 :
Periplaneta, 619, 620: Insecta, 636:
Scorpion, 654,655: Arachnida, 662:
Anodonta, 684: Pelecypoda, 69(5 :
Amphineura, 713, 714 : Triton, 723 :
Gastropoda, 735 : Scaphopoda, 75(5 :
Sepia, 759, 760 : Nautilus, 777 :
Cephalopoda, 790

Ex-umbrella, 134*

Eyes, 39* : of Euglena, 68 : Medusae,
155 : Planaria, 236 : Platyhelminthes,
268 : Nemertinea, 294 : Nematoda,
307 : Chrctognatha, 318 : Brachionus,
326: Rotifera, 334: Dinophilus, 337:
Brachiopoda, 369 : Asterias, 377 :
Nereis, 447 : Chtetopoda, 478 : Hirudo,
514: Apus, 536, 537: Astacus, 556:
Crustacea, 597 : Periplaneta, 627 :
Insecta, 644, 645 : Arachnida, 670 :
Pelecypoda, 707 : Chiton, 111: Triton,
731 : Gastropoda, 746 : Sepia, 771 :
Nautilus, 787

V

F*

ABK1CIA, 478

Facets, 556*

Facial suture, 605*

Faeces, 33*

Falciform young — See Sporozoites

Family, 4*

Fasciola hepatica, 240 : General tea
tures, 240, 242 : Body wall, 241
Digestive system, 241 : Water-vessels
241 : Nervous system, 241 : Repro
ducti ve organs, 242, 243 : Development
243, 244 : Systematic position, 254

Fasciolinoe, 254

Fat, 26*, 27

Fat body, of Periplaneta, 624

Fauna, 8*

Feather-star — See Antedon rosacea

Feeding, method of, Anueba, 12, 46
Actinophrys, 57: Chlamydomyxa, 65
Euglena, 68, 69 : Flagellata, 72
Choanoflagellata, 78 : Monocyslis, 80
Tentaculifera, 101

Femur, of Cockroach, 622

Fenestra, of Periplaneta, 621*

Ferment, 12*

Fertilisation— See Impregnation

INDEX

821

l'-\ ar, parasite of, quartan, 86 : Tertian.
86

Fibres, nerve, 29*, 30

Fibro-cartilage. 26*, 27

Filaria bancrofli. 312

Filaria medinensu>, 305, 312

Filariasis, 812

Filibranchia. ()!).">*, 697, 698, 711, 712

Fimbria, of Fresh -water Mussels, 6S1

Fire-flies, 645

Fission, 14, 40*, 46, 50, .")<», (».->. 80, 73, 74,
78, 79, 81, 90, 101

Fissurella, 737. 74.">

Five chambered organ, 409*

Fixed cheek of Trilobites, 605*

FUMhm, 208, 206, 207

Flagella of Copepods, 565*

Flagellata, 69* : Cell body, 70, 71 :
Flagella, 70, 71 : Modes of nutrition,
72 : Skeleton, 73 : Colonies, 73 :
Asexual multiplication, 74 : Sexual
reproduction, 75

Flagellate canals, of Sponges, 107. 108,
109*, 118

Flagellate cells, 108. 109

Flagellula, 50, .">!*, ."><), 58, 60, 63, (>."), 66,
69, 78, 79

Flagelluin, 24*, 68. 08

Flagellum, of Adactts, 544

Flame-cells; 238, 269

Flat-worms - See Platvhelminthes

Fleas, 635, 640, 651

Flies. 526, 619

Float, 159*, 160

Floscularia, 328, 330, 331

Flmtra, 347

Fol He til inn, 94

Food-yolk, 30

Food vacuole, of A'tinophn/*, .">7 : Fara-
ni'icitim, 90*

Foot, of Anodonta, 681 : Pelecypoda,
701: Amphineura, 713 : Triton, 724:
• ia.-tropods, 738, 739: Sepia, 760:
Nan/ Hit*, 777 : Cephalopoda, 791

Foot-gland, 354

Foraminifera, 4S* : General structure. 50,
51 : Skeleton. 51, 52, 53 : Protoplasm,
54 : Dimorphism, 55, 56* : Repro-
duction, 56 : Distribution, 88

Formica ruja, 652

Fossettes, 591*

Fossils, 7*

Fosmla, 210

Fredericella, 348

Fresh-water Crayfish, 539

Fresh- water Mussel — See AnwloiUa

Fresh-water Sponges, 126

Fresh- water Worms, 439

Fritillaria, 233

Frondicidaria, 53

FYontal suture, 605*

Fulcrum (of Rotifera), 325

Fungia, 205

Funic, 343*
Funiculus, 343, 861
Funnel, 792
F'unnel folds, 792

a

G

AI.KA, 622

tliilitthea, 569

Oaleodes, 663, 661

(Jails, 647

Gall-flies, 636, 647

(Jail-insects, 647

Gametes, of Flagellata,74, 75* : Sporozoa,
80, 81

Gametocytes, 80, 86

Uammarus, 584, 568, 583

Gamobium, 140*, 176

Gamogenesis, 21*

Ganglia, 38, 242

Gastral cortex, 108, 111*, 119

Gastric filaments, of Aurelia, 172*

Gastric mill, 593 : of Astacux, 549

Gastric ostium, 1 10*

( Jastric ridges, of A urelia, 173* : pouches,
170*

Gastrolith, 549*

Oantrophiljis eqiti, 634

Gastropoda. 680, 721* : Example, 721 :
Distinctive characters and classifica-
tion, 732: Systematic position of ex-
ample, 734 : General organisation, 735 :
External features, symmetry, &c. , 735 :
Shell, 737 : Foot, 738, 739 : Head,
740: Mantle, 740: Respiratory organs,
742 : Osphradium, 744 : Digestive
organs, 744 : Heart, 744 : Nervous
system, 745 : Organs of special sense,
746 : Nephridia, 747 : Reproduction,
748 : Development, 749, 751, 752, 753,
754, 755 : Ethology and distribution,
7">.") : Relationships, 756 : Appendix,
756

Gastropores, of Millepora, 156*

Gastrotricha, 323, 335*, 336

Gastrozooids, 163, 202

Gastrula, 23*

Gastrulation, 22, 23*

Qetatimua, 591, 602

Gemmules, 112, 121*

Genae, 621*

Generic, 1*

Genital cloaca, 248* : radii*. 38.-), 430 :
stolon, 385, 430 : plates, 396, 397 :
bursa?, 430 : operculum, 655*

Genital system — SeeReproductivesystem

Gi-nus, 1*

Geological distribution — See Distribu-
tion, geological

Geoplanida;, 253

Gephyrea, 439, 491* : Example, 492 : Dis-
tinctive characters and classification,

822

INDEX

495 : Systematic position of the ex-
amples, 496 : General organisation,
496: Body-wall, 496: Ccelome, 498:
Alimentary canal, 498 : Vascular
system, 498 : Nervous system, 498 :
Eyes, 498 : Nephridia, 499 : Repro-
ductive organs, 499 : Sexual dimor-
phism, 499 : Development, 500, 501 :
Distribution, affinities, &c, 502, 524

Germarium, 270, 272, 239*

Germinal bands, of Clepsine, 521 : of
Peripatus, 612

Germinal disc, 798 %

Germinal layers, 23*

Germinal spot, 19*, 30

Germinal vesicle, 19*, 30

Germ-vitellarium, 270

Giant Clam, 710

Giant fibres, 477

Giant nerve-cells, 478

Oigantorhynchus, 312, 313, 314, 315

Gill-cover, of Astacus, 541

Gills, 35* — See Respiration

Gizzard, 32

Glabella, 605*

Glands, 25*

Glands, Multicellular, 25* : Unicellular,
25* : Ducts. 25 : Salivary, 34*

Glass -crab, 599

Glass-rope sponge, 120, 127

Glenodinium, 79 J

Globigerina, 53, 56

Glockidium, 693

Glomeris, 282

Glossiphonia, 516

Glossocodon, 154

Glow-worms, 645

Glycera, 472, 474

Glyceridie, 476

Gnathobase, 531

Gnathobdellida, 516*, 517, 518, 522

Gnats, 635

Goblet-shaped bodies, 460|

Goblet-shaped organs, of Leech, 514

Gonads — See Reproductive system

Gonapophyses, 628*

Gonocoele, 783*

Gonodactylus, 569

Gonodendra, 163*

Gonopodaria, 354

Gonopore, 298

Gonotheca, of Obelia, 129*, 130

Gonozooids, 202*

Gordius, 304, 306, 308

Gorgonacea, 195*, 203, 204

Gorgdnia, 197, 204

Granule glands, 270, 271

Grapsus, 569

Graptolifluda, 142*, 166, 167

Grasshoppers, 619, 633, 636, 645

Green gland, of Astacm, 551

Gregarina, JQ^^Jr

Gregarina blatlarmn, 82

Gregarina dujardini, 82
Gregarina gigantea, 82, 83
Gregarinida, 81* : Characteristic fea-
tures, 82, 83
Gressoria, 636
Gromia, 51, 52, 56
Guard, 795*
Guard-polypes, 149*
Guinea-worm, 305, 312
Gula, 637*

Gullet, 32 — See Digestive organs
Gunda segmentata, 255, 256, 291, 295
Gustatory organ, 39*
Gyge, 587

Gymnoblastea, 143*
Gymnoleemata, 347*, 348, 351, 352
Gyractis, 200, 201
Gyrocotyle, 262, 263, 287
Gyrodactylidai, 259
Gyrodactylus, 277, 258

II

H,

.abits— See Ethology

Hmnadipsa, 516, 522

Haemal system, 380

Hsematochrome, 68, 72

Hiematococcus, 72, 73

Hcemoccfile, of Crustacea, 52S>__i>93* :
Peripatus, 624 : Insecta, 640

Haemocyanin, 555

Haemoflagellata, 72

Haemoglobin, 30, 36

Ificmopsis vorax, &22

Haemosporidea, 81* : Characteristic fea-
tures, 85, 86

Halicystm, 177

Haliotidas, 733

Haliotis, 737, 744, 745

Halidemma, 159, 161, 162, 168

Halintemma tergestiuum, 160, 163

Halteres, 640

Hamingia, 500

Hartea, 195, 202, 197

Harvest-men, 661, 665

Hastigerina, 54, 55

Haversian canals, 27*, 28

Head, 43*

Head-germs, 522*

Head-kidneys, 452

Head-lobes, of Astacu-s, 558: of Fasciola,
240*

Heart, 36*

Heart — See Vascular system

Heart-urchins, 412*, 417, 422

Hectocotylisation, 760, 792

Heliozoa, 48* : General structure, 56,
57, 58 : Colonies, 59 : Skeleton, 58,
59 : Reproduction, 59 : Conjugation,
59, 60

Heliopora, 195, 203, 208, 209, 210

INDEX

Belie, 748, 749

//« lix >i< month*, 742
HemipwButtet radiiatu*, 423

Hemiptera, 033*. 638, (>.">3
Ht'iiiisoincs. 43*
H<l>atic cieca, 407
Hepato-pancreas, 34, .">IH
Hermaphrodite, 39*
Hermit-crabs, 569, 688, 689
Hoi init-crabs and Hvdractinia, 144
HerpolxUUa, 516, 519, 620, 522
Herpobdellida, 516*

Hi MOmtfa, 47.")

Heterocoela, 112*

Heterocotylea— Nee Monogenetica, 262

//. U >•<><• t/i mi'/" , 230, 232

Heterogamy, 41*

Heterogeny, 277*. 287, 310*

Heteromita, 71, 72, 75, 76

Heteromyaria, 698*

Heteronemertini, 296*

//. •■ rout r< /,*, 440, 4.~>0

Heteropoda, 733*, 738. 740, 756

Heterotrichous, 92*. 94

Hexacanth embryo, of Tun in, 250:

Cestodes, 280
Hexactinellida, 112*, 120,123, 126

Ihjnrtiui.f, 196, 200

Hexarthra, 328, 33(1, 332, 335

Hinge ligament, 682 .

Hinge-line, 361, 682

Hinge-teeth, 361, 683

Hippa, 569, 589, 603

Hippurites, 700, 701

Hirudinea. 439, 506: Example, 50ft:
Distinctive characters and classifica-
tion, 515: (Jeneral organisation, 517:
Form and size, 517 : Seta*, 518 : Pro-
boscis, 518: Enteric canal, 518 : Blood-
vessels, 518 : Respiratory organs, 519 :
Nephridia, 519, 520 : Nervous system,
520 : Sense-organs, 520 : Reproductive
organs, 520 : Development, 520, 521 :
Habits, distribution, &c, 522, 524

Hiriidinidir, 517*

Hirudo, 520, 522

Hirudo muetmlit, 506, 510, 511

Hirudo medicinai.is, .">< Ki : External
characters, 506, 507: Body-wall, 508.
Muscular system, 508, 509 : Alimentary
organs, 509. 510 : Excretory system,
510,511, 512: Vascular system, 512,
513 : Nervous system, 513 : Sense-
organs, 514 : Reproductive organs,
514: Development, 515: Systematic
position, 517

Hi KIT DO QCINQUESTRIATA— See H. MKIH
(IN AI.IS

Hirudo mMmguisuga, 522

Histology, 3*

Hisiriolxldlea, 336, 338, 339, 524

Holarctic. 9*

Holoblastic, 22*

Holophytic, 65*, 69, 72, 78

/ln/i>/hnrin — tiee Sea-cucumber

Holothurian — See Colochirus

Holothuroidea : Example, 401 : Distinc-
tive characters and classification, 412 :
( Uneral form, 416 : Modifications of
form, 423 : Coelome, 425 : Ambulacral
system of vessels, 426 : Blood- vascular
system, 426 : Hremal system, 427 :
Axial organ, 428 : Enteric canal, 428 :
Respiratory trees, 428 : Cuvierian
organs, 428 : Nervous system, 429 :
Reproductive organs. 429 : Develop-
ment, 430 : Ethology, 434

Holotrichous, 89, 92*, 94

Holozoic, 65*, 69

Homalogaster, 257

Honiara*, 569, 599

Homocoela. 112*

Hood, 779*

Hook-headed worms, 297

Hoplocarida, 569*

Hormiphora plumosa, 211 : External
characters, 211, 212 : Enteric system,
213, 214 : Cell-layers, 215 : Nervous
system, 216 : Sense-organs, 216 : Re-
productive organs, 216 : Development,
217, 218, 219, 220 : Systematic posi-
tion, 221

Host, of parasite, 84: intermediate, 87

Houseiflies, 619, 635

Hyaline cartilage, 26*, 27

H yalonema, 120, 127

Hyafoxphenia, 49

Hybrid 2*

Hydatids, 282*

Hydatina, 328, 331

Hydra, 133, 147, 148, 153, 167, 168

Jli/dractiiiia, 144, 145, 149, 167

Hydranths, of Obdia, 129*, 148, 187

Hydrocele, 390

Hydrocorallina, 142*, 156, 157, 158, 159,
167

Hydrocorallina, distribution, 159

Hydrocteiia, 227

Hvdroids, naked-budded, 143* : covered-
budded, 143*

lfi/i/ro/,fii/ns, 650

Hydrophyllia, of Halistemma, 1 GO*, 160

Hydrorhiza, 128*

Hydrospires, 414*

Hydrotheca, of Obdia, 129*, 130, 167

Hydrozoa, 128: Example, 128: (Jenoral
structure and classification, 140: Alter-
nation of generations, 141 : System-
atic position of example, 142 : General
remarks on, 167

1 1, id ntlu. 139*. 140, 153, 229

Hymenoptera, 630*, G3S. 642, 646, 649,
653

Hyperia, 568

Hypoblast, 23*

Hypodermic impregnation, 334, 520

824

|1NDEX

Hypopharynx, 641
Hypostome, of Obtfiri, 129*
Hypotrichous, 92*, 94

I

1

CHNEUMONS, 636, 651 j

Ichthyophthirius, 102

Idmonea, 347

Idotea, 568

Illoricata, 328*

Imaginal discs, 651

Imago, 649

Impregnation, 19*, 20

Impregnation, hypodermic, 309

Inarticulata, 366*, 367, 368, 369

Incurrent canals, of Sponges, 107, 108*,
109, 117

Incus (of Rotifera), 325

Individualdifferences, in Nereis dumerilii,
449

Individual of the first order, 168*

Individual of the second order, 168*

Individual of the third order, 168*

Individual variations, 2*, 113

Individuation, in Hydrozoa, 167

Inequilateral, 699*

Inermia, 496*, 497, 498, 499, 502

Infundibula, of Aurelia, 173*

Infundibular canal, 213*

Infusoria, 46* : Example, 88 : Classifica-
tion, 91 : Systematic position of
example, 91

Infusoriform embrvos, of Dicyemida.',
230, 231*

Inhalant pores, 106, 117

Inhalant siphon, 681

Ink -sac, of Sepia, 765

Insecta, 526, 619*, 677, 678: Example,
619 : Distinctive characters and classi-
fication, 631 : Systematic position of
the example, 636 : General organisa-
tion, 636 : Exoskeleton, 636 : Head,
637 : Thorax, 637 : Abdomen, 637 :
Appendages of head, 637, 638, 639:
Appendages of thorax, 639 : Abdomen,
640 : Heemoccele, 640 : Fat body, 640 :
Digestive system, 640, 641 : Malpighian
tubes, 641 : Tracheal system, 642 :
Blood-vascular system, 642 : Nervous
•system, 643, 644 ; Organs of special
sense, 644, 645 : Luminous organs,
645 ; Production of sounds, 645 : Re-
productive organs, 646 : Eggs, 647 :
Development, 648, 649, 650 : Meta-
morphosis, 649 : Mode of life, 651 :
Distribution in time, 652

Integripalliata, 696*

Integripalliate, 699*

Integument— See Body-wall

Inter-filamentar junctions, 686

Inter- lamellar junctions, 686

Inter-mesenteric chambers, 187*

Internodes, of Siphonophora, 161

Inter-radius, 138, 139*

Interstitial cells, of Obelia, 132

Intertentacular tube, 352

Intestinal cajca, 383, 388

Introvert, of Polyzoa, 341 : Sipuncu/u*,

493 : Gephyrea, 496 : Triton, 724 :

Gastropoda, 744
Invagination, 23*
Iridocytes, 762*
Iris, 39, 771*
Isomyaria, 698*
Isopoda, 567*, 583, 584, 585, 586, 587,

593, 596, 597, 603, 677
Isopteryx, 645*
Isopteryx apicalu, 645

fj aws, 32

Jelly-fish, Common (Aurelia aurita), 168

Jelly-fishes, 128

Jurassic, 7

IL

K

..ARYOGAMY, 59

Karyokinesis, 16, 17
Keber's organ, 689, 706
Kidneys — See Excretory system
King-Crabs, 653, 662, 666 '
Kingdom, 5*
Koonunga, 566
Kramsina, 368
Krohnia, 316

L,

Jabial palpi, of Pelecypoda, 685
Labial palps, 622
Labium, 622
Labrum, 620
Labium, of Apus, 529 : of Astacus, 542 :

of Scorpion, 655 : of Periplaueta, 620
Labyrinthula, 65, 66
Lacinia, 622
Lacrymaria, 94
Lacteals, 34
Lacuna? (bone), 27*
Lacunar system, 380
Lagena, 53

Lamellae (of bone), 27, 28
Lampetia, 217, 222
Lamp- shells, 360
Lancet plate, 414
Land-snails, 734
Lantern ccelome, 400

INDEX

825

Laomedva, 140

Lappets, of Sea-anemone, 1ST!

Larva, of Desor, 295

Larval membranes, of Perijrfaneta, 630*

Larval organ, 391

/.utri i/fia, 591

Laurentian, 7

Laurer's canal, 243

l.in nuiin, 86

Leaf-insects, 633, 636

Leda, 695

Leech — See Hirudo

Leeches — See Hirudinea

Lemnisci, 313*

Lemnobdella — See Hirudo

Lens, 39— See Eve

Lqpat, 565, 577, 578

Lepas anati/era, 578

Lepa* fasricn/arix, 598

Lepidoptera, 635*, 639, 647, 649, 651,
653

Ltpidunis — See Apus

Ltpidnrus kirlii, 528, 532

Lepixma, 632

Leptoehelia, 567

Lcptochone, 478

Leptodiscus, 79

Leptodora, 564, 572, 598

Leptolinae, 141* : General structure, 142,
143,144,145: Perisarc, 143: Medusa-,
143, 144, 145 : Coenosarc, 143 : Repro-
ductive zooids, 149: Development, 152

Leptomedusae. 141*, 143, 144, 149

Leptostraca, r>(Mi*, 580, 581, 595, 596, 601,
603

Lernaa, 564, 575, 576

Lesteira, 576

Leucilla eonvexa, 117

Leucocytes, 30

Lencodore, 209

Lice, 634, 640, 651

Ligula, 262, 288

Ligula, 622*

Lima, 696, 702

Limax, 739, 743

Limnmus, 746

Limnetis, 564, 571

Limnobdella — see Hirudo

Limnocnida, 167

Limnocodium, 167

Limpets. 721, 733, 743— See Patella

Limulus, 666, 667, 670, 671

Linem, 232

Lingua, 641 •

Lingual ribbon, 726

Linguatulida, 673, 674*, 675, 678
Lingida, 366, 367. 368, 369, 371

Linna?us, 1, 3

Liquid tissues, 29

Lithite, 136*

Lithites, of AnreJia, 173
Lithobius forficatm, 615

Lithocirais annularis, 60

Lithocysts, 135*

Littoral forms, 8*

Liver, 34*

Liver-Fluke — See Faxciota hepaticd

Liver-pancreas, 34

Lobata, 221*, 223, 224

Lobosa, 48*, 49: General structure, 48,
49 : Skeleton, 49

Lobsters, 569, 587, 599

Locomotion, of Am<eba, 1 1, 46 : Heliozoa,
57 : Eitgltna, 68 : Flagellata, 72 :
Choanoflagellata, 78 : Sporozoa, 80 :
Paramecium, 90: Tentaculifera, 101

Locomotor rods, 319

Locusta, 633

Locusts, 526, 633, 636, 646

Lohmanella, 233

LoJigo, 796, 798, 800, 801, 802, 803

Loligo mUgarii, 792

LophogaMzr, 567

Lophomonas, 93, 94

Lophophore, 340, 341, 362, 368

Lopkopus, 351

Lorica, of Ciliata, 96: Flagellata, 71,
73: Choanoflagellata, 77, 78: Tenta-
culifera, 101 : Rotifera, 324

Loricata, 328*, 329

Loxosoma, 348, 354

Lucernarida — See Staurome<lus;e, 176

Lucernaria, 176, 177, 178

Lucifer, 569, 599

Lucina, 703

Lumbricidw, 467*

LiTMBRicus, 454 : General external fea-
tures, 454 : Body-wall, 456 : Seta-,
454, 457 ; Setigerous sacs, 457 : Enteric
canal, 457 : Vrascular system, 458 :
Nervous system, 459 : Organs of ex-
cretion, 460, 461 : Reproductive organs,
461, 462: Development, 463, 464;
Systematic position, 466

Lumbricu8 rubellus, 483

Lumbricus trapezcndes, 483

Luminous organs, 645

Lung, 35* : of Scorpion, 657 : of Pulmo-
nata, 743

Lymph, 29*

Lysophiurae, 411*

M

M

ACROBDELLA VALDIVIAHA, f>17
Macrobiotua hu/elandi, 675
Macula acustica, 772
Macrura, 568*, 601, 602, 603, 670
Afadrepora, 207
Madreporaria, 194*, 197, 198, 200, SOS,

207, 210, 227
Madrepores, 207, 210
Madreporic canal, 383, 399, 402
Madreporite, of Starfish, 376, 419 : of

Sea-urchin, 397

826

INDEX

Magellania, Shell, 360,361: Body, 362:
Mantle-lobes, 362 : Mantle-cavity,
362: Lophophore, 362, 363; Food-
groove, 362 : Digestive organs, 362 :
Body- wall, 363 : Muscular system, 364 :
Cudome, 365 : Blood system, 365 : Ex-
cretory organs, 365 : Nervous system,
365 : Reproductive organs, 366 : Posi-
tion of example, 367
Magellania Jlavencens, 360
Maggot, 649*
Maia, 569, 596, 600
Malacocotylea, 252 — See Digenetica
Malacostraca, 565*, 570, 579, 593, 594,

596, 597, 601
Malaria parasites, 86
Malleus (of Rotifera), 325
Malpighian tubes, 617, 625
Malpighian tubes, of Scorpion, 656 : of

Arachnida, 668 : of Tardigrada, 675
Mandibles— See Appendages
Manducation, 434*
Mantidm, 636
Mantle lobes, 362

Mantle, of Anodonta, 684 : Pelecypoda,

698 : Amphineura, 713 : Triton, 725 :

Gastropoda, 740 : Scaphopoda, 757 :

Sepia, 762 : Nautilus, 781

Manubrium, of Medusa, 129 : of Obelia,

130
Marginal lappets, of Aurelia, 168*, 169,

179
Marginal sense-organs, 135*
Marginal tentacles, of Aurelia, 168*,

169
Marine Annelids, 439
Mastax of Rotifera, 325
Mastigamceba, 70, 71
Mastigophora, 45* : Example, 67 : Clas-
sification, 69 : General organisation,
39: Systematic position of the example,
70
Matrix of connective tissue, 25*
Maturation, 19*, 20
Maxilla — See Appendages
Maxillary palp — See Appendages
Maxillipeds — See Appendages
Maxilluta?, 638*
May-flies, 633
Measly pork, 310
Medulla, of Actinospherinm, 57, 58 :

Monocystis, 80 : Paramecium, 88, 89
Medullary sheath, of Nerve-fibre, 29*, 30
Medusa- buds, of Obelia, 129
Medusae, of Obelia, 134, 135
Megadrili, 466*, 467
Megagametes, of Flagellata, 76* : Spo-

rozoa, 84, 85, 86 : Ciliata, 99
M egagametocyte, 84, 85
Megalaesthetes, 717
Megalopa stage, 599

Megameres, of Ctenophora, 217, 218 :
of Polyclad, 274 j

Meganucleus, of Ciliata, 93 : Pant*

meecium, 88, 89 :' DinoHagellata, 78
Megaspheric forms, 55, 56
Megaspores, of Radiolaria, 63
Megazooid, of Vorticella. 96, 98
Meleagrina, 696, 710
Meleagrina margaritifera, 711
Melic'erta, 328, 330, 331
Melolontha, 643
Membranes, 24
Membranipora, 347
Mentum, 622
Meridional canal, 213
Meroblastic, 22*
Merogony, 21*
Merozoa, 253* — See Polyzoa
Merozoite, 84*, 85, 86
Mesenchyme, 389
Mesenteric filaments, of F/abel/mn, 2C6 :

of Sea-anemone, 187*
Mesenteries, of Sea-anemone, 187, 182*
Mesenteries, development of, in Sea-
anemones, 192, 193 : Arrangement in
Actinozoa, 200
Mesenteron, 187
Mesoblast, 23*
Mesoccele, 356*
Mesoderm, 23*, 219*
Mesoderm bands, of Periplaneta, S3]
Mesogbea, 110*, 111: of Obelia, 131*
Mesonemertini, 296*
Mesopodium, 725, 739
Mesosoma, 654*
Mesostomum, 272
Mesothorax, 622
Mesotrochal, 486*
Mesozoa, 230

Messmateism — See Commensalism
Metabolism, 13*
Metacoele, 356*
Metacrinus interruptu-s, 424
Metagenesis, 40* : of Foraminifera, 66 :
Obelia, 139 : Leptolinse, 152 : Tra-
chylinfle, 156 : Aurelia, 173 : Livei-
Fluke, 243: Tmnia, 250: Platyhel-
minthes, 273 : Nematoda, 310
Metameres, 43*, 523
Metamerism, 523

Metamorphosis, of Trachylinse, 155: Sea-
anemone, 193 : Platyhelminthes, 273 :
Nemertinea, 295 : Phoronis, 359 : A 8-
terina, 388 : Antedon, 432: Echino-
dermata, 430 : Chaetopoda, 487 : Apn»,
537 : Crustacea, 597 : Insecta, 649
Metamorphosis, retrogressive — See De-
generation
Metanauplius, 539
Metanemertini, 296*
Metapodium, 725, 739
Metathorax, 622
Metazoa, 19*, 105
Metentera, 187*
Micrsesthetes, 717

TNDEX

827

Microdrili, 466*

Microgametes, of Flagellata, 76*: Spo-

rozoa, 84, 85. 86 : Ciliata, 99
Microgametocyte, 84, 85
Microgromia, 50, 56
Microkpdra, L48, 167
Micromeres, of Ctenophora, 217, 218
Micromeres, of Polyclad, 274
Mil ronucleus, of Ciliata, 93 : of Dino-

flagellata, 78
Micronucleus, of Paramwciiun, 88, 89
Micropyle, 20

Micropyle, of Cephalopoda, 798
Microspheric forms, 55, 56
Microspores, of Railiolaria, 63
Microstomnm, '282, 283
Microzooid of Vortitelta, 96, 98
Mieseher's corpuscles, 88*
Migration, 285*
Mil tola, 52
Millipora, 156

Milh pora alcieornia, 157, 158
.Millipedes, 526, 614, 616
Miiu/'t*, 202, 226
Miocene, 7
Miracidiuni, 244*
Mites, 526, 653, 661, 665, 668, 673
Mitosis, 16*, 17
Mitotic division — See Mitosis
Modiola, 698, 702 .
Mollusca. 680* : General remarks,

804
Molluscoida, 340*
Molluscoida, mutual relationships of the

classes, 372
Mongrels, 2*
Mnniezin, 288
Monocyclica, 413*
Monocvsiis AOIUS, 80, 81 : Systematic

position, 82
Momecious, 39*
Monogenetica, 252*, 258, 259. 966, 268,

969, -,72. 273. 277. 2S4. 286, 287
Monomyaria. 698*
Monosiga, 77
Mount us, 255
Monozoa, 253*
Morphology, 3*
Morula, 22*
Moruloidea, 230*
Mosquitoes, 635, 661
Mother-cyst, 282
Mother-of-pearl, 684
Moths, 635
Mouth papilla;, 37<»*
Movable cheek, 605*
Mulberry body, 22*
Multicellular, 19*
Multicellular gland, 25*
Miiller's larva, 275
Mullicilia, 93, 94
Multiple fission, of Euglena, 69
Mit rex, 744, 747

Muscle, striated, 28*, 29 : non-striated,
28*, 29

Muscle processes, Hydra, 148

Muscles, 37*

Muscular fibres of Sponges, 111

Muscular system, of Aure/ia, 172 : St a
anemone, 186, 188: Magellan in, 364:
Brachiopoda, 368: Hirndo, 508, 509:
Apt**, 532: Astaaix, 546, 547: Crus-
tacea, 593: Periplaneta, 624: Ano-
donta, 684 : Pelecypoda, 697

Muscular tissue, 28*, 29

Mushroom coral, 205

Mussels, 680

Mya, 696

Mya arenaria, 698

Mycetozoa, 45* : Example, 66, 67 : Spo-
rangium, 66, 67 : Capillitium, 66, 67 :
Spores, 66, 67 : Flagellula, 66, 67 :
Plasmodium, 66, 67: General remarks,
67 : Protomyxa, 67

Myomeres, of Apus, 532

Myosoma, 354

Myriapoda, 526, 614*, 677, 678 : Distinc-
tive characters and classification, (ill:
General organisation, 614 : External
features, 615 : Integument and Body
wall, 617 : Alimentary canal, 617
Heart, 617: Respiratory system, 617:
Nervous system 617 : Reproduction.
618 : Ovum, 618: Fossil remains, 618

Myriothela, 145, 146, 148

Mysidacea, 567', 581, 582, 594, 597, 599,
603

Mysis, 567, 582

Mytilus, 695, 697, 702, 703, 7<i8

Mytiliis eduiis, 697, 702

Mytiht8 lotus, 697

Myxidium lieberkuhnii, 87

Myxoholm midleri, 87

Myxospongiae, 120, 126

Myxosporidea. 87* : Characteristic
features, 87

Myzortoma, 489, 490, 491

Myzostomida, 489*, 490, 491

N.

x

ACM, 684*

Naiidomorpha. 473

Xais, 486

Narcomedusae, 142*, 154, L56

Natural History, 1*

Nauplius, 538. 539

NammtkBt, 18C

Nautiloids, 680, 790

Nautilus mocromphaJii", 779, 789

Nautilus pompilius, 776 : Shell, 776 :
External characters of soft parts, 777 :
Mantle and mantle-cavit3r, 781 : Kn-
teric canal, 783 : Ccelome, 783 : Heart
and circulation, 783 : Renal organs,
786 : Nervous system, 787 : Sense-

828

TNDEX

organs, 787 : Reproductive organs,
787, 788 : Systematic position, 790

Neartic region, 9*

Ndwlia, 566, 579, 581

Neck, of cockroach, 620

Nectocalyx, 159*

Necturus, 304

Needham's sac, 774*, 788

Nekton, 8*

Nematomorpha, 304*, 305, 307

Nemathelmintb.es. 297* : Appendix, 319 :
Affinities and relationships, 320 *~^

Nematocyst, 78, 79, 87, 95, 133*

Nematoda, 297* : Example, 297 : Ex-

. , ternal characters, 305 : Body-wall, 305 :
Enteric canal, 305, 306 : Ccelome, 307 :
Excretory canal, 307 : Nervous system,
307 : Eye-spots, 307 : Reproductive
organs, 307 : Development, 308, 309 :
Life-history, 310

Nematogene, 231*

Nematoidea, 304*, 307

Nematomorpha, 304*, 305, 307

Nemertinea, 288,295* : General features,
288, 289, 291, 293 : Body-wall, 290 :
Alimentary canal, 290 : Blood-vessels,
291, 293 : Excretory vessels, 292, 293 :
Nervous system, 291,292, 293: Cerebral
organs, 291, 294 : Eyes, 294 : Stato-
cysts, 295 : Reproductive system, 295 :
Development, 294, 295 : Distinctive
characters and classification, 295

Neomenia, 713, 714, 717, 718

Neotropical region, 9*

Nephelis, 516

Nephridiopore, 448, 455, 506

Nephridium, 439, 448*, 460, 461, 479,
495, 511, 689

Nephridium, provisional, 481

Nephrostome, 365, 448, 461

Nereidce, 466*, 488

Nereidiformia, 466*

Nereis, External features, 440, 441 :
Enteric canal, 442, Ccelome, 442 : Body-
wall, 443 : Vascular system, 444 :
Nervous system, 446 : Sense-organs,
447 : Excretory organs, 448 : Repro-
ductive organs, 449 : Individual varia-
tion, 449 : Development, 450, 451, 452,
453 : Systematic position, 466

Nerve-cell, 29*, 30

Nerve-fibres, 29*, 30

Nerve ganglia, 38

Nerve pentagon, 379

Nervous system, 14, 29, 37*, 38 : Obelia,
132 : Leptolina;, 151 : Aurelia, 172 :
Tealia,\92: Hormiphora,2\6: Planaria,
238 : Fasciola, 241 : Tamia, 248 : Platy-
helminthes, 266, 267 : Nemertinea,
291, 292, 293: Ascaris, 301, 302:
Nematoda, 307 : Acanthocephala, 313 :
Chaetognatha, 317 : Brachionus, 326 :
Rotifera, 334 : Dinophilea, 337 : Gastro-

tricha, 336 : Polyzoa, 351 : Endoprocta,
354 : Phoronis, 357 : Mage/lania, 365
Brachiopoda, 369 : Starfish, 379 : Sea
urchin, 398 : Holothurian, 402 : Ante
don, 408 : Echinodermata, 429'
Nereis, 446 : Lumhricus, 459 : Chseto
poda, 476 : Myzostomida, 490 : Sipun
cuius, 494 : Gephyrea, 498 : Archi
annelida, 504 : Rirudo, 513 ; Hiru
dinea, 520 : Apiis, 534, 536 : Astacus
555 : Crustacea, 596 : Peripatus, 609
Myriapoda, 617, 626, 627 : Periplaneta
626, 627 : Insects, 643, 644 : Scorpio
657, 658 : Arachnida, 670 : Mussels
690 : Pelecypoda, 706 : Amphineura
715, 717 : Triton, 729 : Gastropoda
745 : Scaphopoda, 757 : Rhodope, 758
Sepia, 767 : Nautilus, 787 : Cephalo-
poda, 797

Nervous tissue, 29

Nervures, 639*

Neuraxis, 29*, 30

Neurilemma, 29*, 30

Neuronephroblast, 521*

Neuropodium, 441*, 468

Neuroptera, 633*, 640, 647, 653

New Zealand region, 9*

Nicothoe, 576

Nida mental gland, 775*

Noah's Ark shell, 695

Noctiluca, 79

Nodes, of Siphonophora, 161

Nodosaria, 53

Nomenclature, binomial, 1*

Non-contractile vacuoles, 11*, 60

Noteus, 331

Notholca, 331

Notommata wernecMi, 335

Notopodium, 441, 468

Notostraca, 563*

Nuchal cartilage, 763

Nuchal organs, 447*, 448

Nuclear membrane, 16*, 17, 80

Nuclear spindle, 17*

Nuclearia, 58

Nucleolus, 16*, 19, 80

Nucleus, 11*, 16, 19

Nucula, 695, 698, 701, 702, 703, 706, 707,
710

Nudibranchia, 734*, 738, 742, 744, 756

Nummulites, 53

Nyctiphanes, 568

Nyctothertis, 92, 94, 95

Nymphon hispidium, 674

0

o

beija, General structure, 128, 130 :
Microscopic structure, 131, 132 :
Medusae, 134, 135, 136 : Comparison
of polype with medusa, 136, 137, 138 :

INDEX

821>

Reproduction, 139: Development, 188 :

Systematic position, 142

Ooolusor muscle, •'{.mi
Ocelli, 148. 173

Octopoda. 680, 790*, 791, 7!I8, 7!)7, 804
Octoptu, 791. 793, 798
Octorchandra, 150
Ocular plaits. 396. :>'.»7
Odentophora, 670*, 678
Odontophore, 721, 726, 727
(Esophagus See Digestive system
OihomonaSi 71. 72
Olfactory organs, 39 i pits, 173*
Oligochata. 465*, 168, 409, 471, 478, 474,
175, 478, 479, 4Sl, 482, 4S8, 486, 488,
489
OhiuthiK, 116
Ommatidium, 536*, 537
Oii'/titliinii, 74U
Ontsctw, 568, 583, 587
Onychophora, 526, 607*, 677, 678— See

Peripatus
( )ociuin. 841
< lokinete, 87*
Oosperm, 21*
Oostegites. 585*
Oostegopod, ">31*
Ootype, 243, 270, 271
Opattm, 93, 98, 99
Opalinqpeu, 94
Operculum Radiolaria, "61, 62: Ciliata,

95. 98 : Gastropoda, 723*, 724. 780
Opkiogljfpha, 420

Ophiuroidea. Distinctive characters and
classification, 411*: General form and
symmetry, 41.~> : System of plates,
418 : Modifications of form. 420. 421 :
Coelome, 42."» : Amlmlacral system,
426; Blood-vascular s3'Stem, 42(i :
Sauna] system, 427: Axial organ,
428 : Enteric canal, 428 : Nervous
system. 429 : Reproductive organs,
429: Development, 480: Ethology,
&c, 434
Ophrydhtm, 95, 97
Ophryod* ndron, 100
Ophryoi/1) na, 94
Opistho'brancnia, 734*, 738, 744, 745'

7").")
Opisthogoneata, 615*, 619, 677
OpiethtrckU mnensM, 288
Opossum-Shrimp, 523
Optic gland, 772
Optic vesicle, 802
Oral, 376*: Arms, 170*
Orekeatia, 588, 594
Order, 4*

Organ of Bojanus, 689*: cf. Owen, 780*
Organic evolution — See Evolution
Organism, 1*
Organ-pipe Coral. 1!).">
Organs, 81*
Oriental region, 9*

VOL. I.

Orobdeila, 516

Orthocaxu, 803

Orthonecticke, 230, 232, 233, 234

Orthoptera, 682*, 638, 640, 653

Othoptera genuina, 636

Oeeamfa, 117

Oncaria, 115

Oscular sphincter, 111*

Osculum, 106*

Osphradium, 691, 707, 731, 707

Ossicles, 375

Ostia, 106*, 109, 533*

Ostium, 187*

Ostracoda, 564*, 572, 573, 593, 594, 597,

598, 601, 602
Oslrea, 696, 698, 701, 706, 708, 709 4

Otocyst, 39*, 478
Ovariole, 628*, 646
Ovary, 39*
Ovicells, 352
Oviduct, 40*
Oviparous, 40*
Ovipositor, 610
Ovum, 19*, 30
Owen, organ of, 780
Oxygen, oxidation, 86
Oxeole spicules, 108, 110*
Oxyiiri*. 307
Oysters, 680, 696

-* ACHYCHALINA, 121
Pedogenesis, 41*, 224*, 647
Pagurus, 569, 589
PaUemon, 569, 588

P'lln monfites, 593

Pahearctic region, 9*

Palaeodictyoptera. 652

Paheontologv, 6*

Pnllnuriix, 589, 587, 595, 599, 602

Pallial line, 683*

Pallial groove, .844

Pallial sinus, 365

Pallial muscles, 684

Pallial complex, 735

Pallium— See Mantle

Palpus, 470

Patudicella, 348, 353

Pallidum, 754

Pains, 205*
Paiytkoa, 127, 197
Pancreas, 34*
Pancreatic juice, 34*
PnnihifiiKi. 72. 78, 74, 7-">
Papula, 424*

Parapodium, 439, 440, 441, 468, 739
Paragastric cavitv, 106*, 108
P.iragnatha, 530*, 548
Paramaeidas, 91

Pak.\M(Kcm-m, 88, 89, 90 : Systematic-
position, 91

3 a

830

INDEX

Paramithrax, 601

Paramitome, 15*

Paramylum, 68*

Paranaxpides, 566, 582

Paranepkrops, 569

Parasitism, Protozoa, 72, 81, 82, 85, 87,
95 : Hydrozoa, ] 67 : Mesozoa, 230 :
Rotifera, 335

Parenchyma, 236*, 264, 351

Parenchyma muscle, 265

Parenchymula, 124*

Parthenogenesis, 21*, 40*, 287, 647

Parthenogonidium, 75*

Parthenope, 591

Patella, 737, 743, 745, 746, 747, 749, 751,
752, 753, 754

Paraseison axplanchitii*, 329

Farazoa, 105

Patellidw., 733

Pauropoda, 614*, 618

Pauropus, 614, 615, 617

Paxilla, 414

Peachia, 202

Pea-crab, 602

Pearl-mussel, 710

Pearl-oyster, 696

Pearls, 711

Pebrine, 87*

Pecten, 696, 697, 700, 708, 710

Pectines, 655*

Pectinibranchia, 733,* 744

Pedal gland, 725* : Lobes, 179

Pedalion, 328, 330, 332, 335

Pedata, 413*, 434

Pedicellaria, 377, 381, 387, 394, 417

Pedicellina, 348, 354, 355

Pedipalpi, 653

Pedipalpida, 660*, 662, 663, 668, 670,
673

Peduncle, 205*
Pelagia, 184

Pellicle, 88

Ptlomyxa, 49

Pelagic, 8*

Pelayohydra, 148

Pelecypoda, 680 : Example, 680 : Dis-
tinctive Characters and classification,
694* : General organisation, 696 : Ad-
ductor muscles, 697 : Shell, 697 :
Siphons, 698 : Foot, 701 : Byssus
gland, 702 : Gills, 702, 704 : Digestive
organs, 704 : Excretory organs, 705 :
Circulatory organs, 706 : Nervous
system, 706 : Sense organs, 707 : Re-
production and development, 708, 709,
710 : General remarks, 710 : Mutual
relationship, 712
Pelmatozoa, 413*, 434
Pe/toyaster, 565, 579
Pen, 795*
Peruvus, 569, 599
Penial seta?, 298, 304
Pennahda, 196, 197, 198, 199, 204, 208

Pennatulacea, 195*, 197, 199, 204
Pentacrinoid larva, 410, 433
Pentacrinus, 410

Pentastomida — See Linguatulida
Pentastomum ttmioide*, 674
Peptones, 34
Peptonephridia, 481*
Peracarida, 566*, 601
Perforate corals, 207*
Pericardial sinus, 533*
Perichondrium, 27*
Pericolpa, 179, 180
Perihaemal system, 380*
Periosteum, 28*
Periostracum, 683*

Peripa i us, 526, 607 : External features,
607, 608 : Body-wall and body-cavity,
608 : Enteric canal, 608 : Circulatory
system, 609 : Organs of respiration,
609 : Coxal and slime glands, 609 :
Nervous system, 609 : Nephridia, 610 :
Reproductive organs, 610 : Develop
ment, 611,612,613: Distribution, 612:
Relationship, 612
Peripatus capensis, 607, 608, 609, 610,

613
Peripa'us noiue-zealandin*, 611
Periphylla, 179

Periplaneta AMERICANA, 619, 620 :
Head, 620, 621 : Neck, 622 : Thorax,
622 : Abdomen, 622 : Respiratory
movements, 623 : Muscles, 624 : H«emo-
coele, 624 : Digestive - system, 624,
625 : Renal organs, 625 : Heart, 625 :
Respiration, 626, 627 : Nervous system,
626, 627 ; Organs of special sense, 627 :
Reproductive organs, 627, 628 : De-
velopment, 628, 629, 630 ; Systematic
position, 636
Periplaneta orientalis, 620, 621
Periproct, 394*
Perisarc, 130, 131*
Perisaltic movements, 36*, 445
Peristome, 92, 95, 96. 185*, 380*, 394*
Peristomium, 441*, 455, 469
Peritoneum, 442*
Peritrichous, 92*, 96
Perivisceral cavity, 442*
Periwinkles, 680
Peromedusae, 180
Per-radius, 139*
Petaloid ambulacra, 422
Petatms, 154
Petrarca, 565, 579
Phacelbe, 172*
Phacops fecundus, 605
Phaeodium, 61, 62

Phalangida, 661*, 665, 667, 669, 670
Phanerocephala, 465*, 466, 476, 484,

488
Phanerozonia. 411*
Pharynx — See Digestive system
Phasmidiv, 636, 653

INDKX

831

Phenmema carpenter*, ISO, 123

PkUodina, 828, 331. 334

Pfcofoa, 898, 7ix», 711

Phoronida. S40, S65*

Phoronis, 356, 356, 357, 358, 359

Phraymoi ■ M, sn ■'!

I'hragmocone, ?!».")*, 796

Phreatoiai*, 568

Phroninnt, 585, 586

Pkrymu, 663

Phyiactolaemata. 348*, 849, SSI, 352, 353

Phyllocarida, ."WO*, 580, 581. :>!I4, 596,

601, BOS
PhyUoflwt. pan ft i. 479
Phyllosoma, 600
Phvlogeny, 8*
Phylum, 5*, 48*
Physalia, 163
Physiology, 9*
Pima, 635
Pigment, 68, 119
Pi/, ma, 183
Pilidium, "29.")

Pill-bug, 586
Pm«j, 696, 702
Pinnothere*, 602

Pinnules, 407*

Pi*cicola, 516, 522

Placophora, 713*, 715, 716, 717, 718,
720, 721

Planaria, 236: General features, 236,
237, 240: Digestive system, '236, 237 i
Water vessels, 237, 238 : Nervous
system, 237, 238 : Reproductive sys-
tem. 238, 239 : Systematic i><>sition,
268

PlanaridtB. 253

Plankton, 8*

P/iiiiorhidiim, 58

Plant lice, 634, 647

Pianola, 139*, 140, 153, 173, 192

Plasma, of blood, SO

Plasmodium, 30*, 66. <>7

P/atoum stereo eum, 51

Platyctenea. 226*

Platyhelminthe8, 235, 251*: Examples,
236. MO, 846: Systematic position,
253 : General external features, 2.">4 :
Integument and muscular layers, 262,
264: Parenchyma, 264, 265 : Alimen-
tary systems, 265, 266, 267 : Nervous
system, 266, 267 : Water vascular
system, 269 : Reproductive organs,
270, 271, 272: Development, 273, 274,
275, 276, 277, 278, 280, 281, 282, 379 :
Asexual reproduction, 283 : Distribu-
tion occurrence and relationships, 28
Appendix, 288
Platypoda, 733*. 734
Pleopod, 542*
Ph mohrarhia, 211
Phiirohrachiid'i', 221
Pleurobranchia, 550, 551*

Pleoron, .vil*
Plevrophyilidia, 742, 743
Ploima, 328*. 329, 880
PlumctieUa, 348, 349

Phimitldiin. I I '.»

Pluteus, 400, 411, 412, 431. 432

Pneumatophore, 1">9*

Podical plates, 623*

Podobranchia, 550*

Podonierc, 526*

Podopkrya, 99, 100

Podura, 632, 633

Polar body, l!l*, 20

Polar plates, 216*

Polian vesicle, 383*, 399, 402, 426

Pol lie i/,, ' s, 579

Polyarthra, 328, 330, 331

Polyrtli.^ 255

Polychaeta, 465*, 470, 471, 474, 478, 479
480, 481, 483, 484, 485, 486, 488, 489

Polycladida, 282*, 255, 256, 257, 263,
264, 266, 267, 268, 271, 273, 274, 287

Poly col pa, 154

Polydora, 488

PotygordUdtB, 503

Pofygordiut laeteus, 504

Poljffxtrdku nevpolitaniu, 503. 504, 505 '
Polykrlko*, 78, 79

Polymorphism, 141*
Polynesian region, 9*
PotynSe, 469, 484
Polynbe extenua/a, 471
PolyiKi' tetosuaima, 467
Poly noii In <■. 467, 481
Polyaca, 77, 78
Po/i/op/itlm/i/nix, 479
Polype, 129*, 148
Po/yph,i,i>i«, .1()4, 572
Polyphyletic, 286*
Polypoili inn. 167
Poly sperm}', 21*
Polystomatons, 183*
Polysomia, 257, 258
Po/i/x/onu lla, 55
Polystounnn, 258, 277, 284
Polytrochal, 486*

Polyzoa. 340* : Kxample. 341 : Distinc-
tive characters and classification, 347 :

See Ectoprocta and Em/oprortn
Polyzoa (Cestoda), 253*
Pontellina medilcrranea, 600
Pontohdi'lii, 616, 517, 518, 520
Porctllnnn, ."><>! I
Pore-membrane, 108, 109*
Porifera, Kxample, 106 : Distinctive

characters ar.d classification, 111 :

General form and mode of growth,

114: Leading modifications of struc-
ture, 116: Histology, 119: Skeleton.

120 : Reproduction, 120 : Develop-
ment, 122 : Distribution, affinities,

&c, 126, 228
Porocyte, 109*, 124

3 G 2

832

INDEX

Poromya, 696, 785

Porpita, 165, 166

Port-hole — See Cinclis

Portuguese Man-of-War, 163

Portunus, 591

Post-abdomen of Scorpion, 654

Potamobiida?, 570*

Poterion, 115

Prae-abdomen of Scorpion, 654

Prawns, 587, 588

Priapulidce, 496

Priaptdus, 497, 498, 499

Primary axis, 41*, 42

Prismatic layer, 684

Proboscis, 237

Prodandrous, 307*

Proglottides, 247*

Progoneata, 614*, 619, 677

Pro-legs, 649

Proneomenia, 713, 717

Pronucleus, active and stationary, 91

Pronucleus, male and female, 19*, 20

Pro-ostracum, 795*

Propodium, 725, 739

Prorocentmm, 78, 79

Prorodon, 94, 95

Proscolex, 250*

Prosiphon, 741*

Prosobranchia — See Streptoneura

Prosocoele, 356*

Prosoma, 654
Prosopyle, 108, 109*
Prostate, 270, 271, 483

Prostomium, 440*, 455, 469

Profamaba, 49

Protandrous, 307*

Protective characters, 601

Proteid, 15*

Proteolepas, 565, 579

Proterospongia, 77, 78, 127

Proteus Animalcule, 10

Prothorax, 622

Protobranchia, 695*, 697, 698, 702, 705,
711, 712, 805

Protoconch, 776*, 794

Protodrilus, 504, 524

Protohydra, 148

Protomerite, 82*, 83

Protomyxa, 67

Protonemertini, 295*

Protonephridial system, 236*, 269

Protoplasm, 11*, 14, 15*, 16

Protopodite, 538*, 542

Prototroch, 322, 451

Protozoa, 45

Protozoal, 599

Protractor muscle, 683

Proventriculus, 624

Pmmmoclema, 115

Pseudo-gastrula, 124*

Pseudo-lamellibranchia, 695*, 697, 698,
700, 711, 712

Pseudo-manubrium, 155*

Pseudo-metamerism, 255*

Pseudopod, 10*, 45* : of Amaba, 46

Pseudo-scorpionida, 660*, 662, 668, 670

Padua, 423

Psorosperms, 87

Pteropoda, 734*, 741, 756

Pterolmchea, 742

Pulmonary sac, 743*

Pulmonata, 734*, 738, 740, 743, 744, 746,

748, 749, 750, 755, 756
Pulsellum, 756
Pupa (of Cirripedia), 598
Pupa (of Insects), 649
Purpura, 744

Pycnogonida, 673*, 674, 678
Pygidium, 605*
Pyloric caeca, 382
Pyrenoids, 73
Pyriform organ, 344
Pyriform sac, 787
Pyrula, 749
Pyxicola, 95

Q

UADRULA, 49

II

Ha

Uachis, 302*

Radial Canals, 107, 108, 109*, 118

Radial Symmetry, 42*

Radiata, 436

Radii, orders of, 139

Radiolaria, 48* : General structure, 60
Central capsule, 60 : Skeleton, 61
Colonial forms, 61 : Reproduction, 63
Symbiosis, 63

Radula, 726*, 728

Radidar, sac, 726*, 782

Rainey's corpuscles, 88*

Raphidiophrys, 58

Razor-fish, 711

Receptaeula ovorum, 462

Red coral, 195

Redia, 245*

Regularia, 412*, 415

Relationship, 6*

Relationships, of Protozoa, 102 : Sponges,
127, 228 : Coelenterata, 226 : Platyhel-
minthes, 283 : Nemathelminthes, 320 :
Rotifera, 335 : Dinophilea, 336 : Mol-
luscoida, 372 : Echinodermata, 436,
502 : Annulata, 524, 602 : Air-breath-
ing Arthropoda, 676 : Pelecypoda,
712: Cephalopoda, 804: Mollusca, 804

Relationships, diagrams of : of Protozoa,
103 : Coelenterata, 229 : Platyhel-
minthes, 287 : Echinodermata, 437 :
Annulata and Trochelminthes, 525 :

INDEX

H33

i riiM.irni, 604 : Arthropod*, <\~* :
Peleoypoda, 712: Gastropoda, 75ii :
Cephalopoda, 804

Renal organs— See Excretory system
Reproduction, Reproductive System, 14,
30, 39: Aui'ilxi, 14, 40 : Foraminifcra,
."><i : Ht'liu/.oa, 50: Radiolaria. 63 :
Mveetozoa, 67: Eni/h nn, 69: Flagel-
lata, 74 : Choanottagellata, 78 : Dino-
flagellata, 79 : Cystoflagellata, 79 :
Mo)io<u*ti.*, 80 : Gregarinida, 83 :
Coccidiidea, 84 : H:emosporidea, 86 :
ParumCBdum, 90: Ciliata, 97: Ten-
taculifera, 101 : Porifera, 120 : Obelia,
ISO : Leptolin;v, 149 : Trachvunse, 155 :
B vdroeorallina, 158: Siphonophora,
162, 104, 166: Aiirdia, 171: Tea/ in.
192: Aetinozoa, 197: Hormiphom,
216: Mesozoa, 231, 234: I'innnriii .
238, 239: Fcuciola, 242, 243: Taenia,
248. 249, 250: Platyhelminthes, 270:
Nemertinea, 295 : AscarU, 302, 303 :
Nematoda, 307 : Acanthocephala, 314,
315: Chretognatha, 318: Brachionns,
326 : Rotifera, 334 : Dinophilea, 337 :
HiKjtila, 344 : Ectoprocta, 352 : Eudo-
procta, 355 : Phoronifi, 357 : Magt I-
Innin, 366: Brachiopoda, 369: Star-
fish, 386: Sea-urchin, 400: Ilo/othnrinn.
104: An/'i/on, 409: Echinodermata,
429: Xerei*. 449: Lumbricus, 461,
462, 481 : Chaetopoda, 481 : Myzosto-
inida, 490, 495 : Sipuncidut, 495 •
(iephyrea, 499: Archi-annelida, 504:
Struck, 514 : Hirudinea, 520 : Apttx,
537: Astaeut, 666: Crustacea, Peri-
petttu, 610 : Myriapoda, 618 : Pi ri-
planeta, 627, 628 : Insects, 646 : 8oor-
jiloii, 658: Araclinida, 671: Mussel,
691: Pelecypoda, 708: Amphineura,
718, 719 : Triimi, 732 : Gastropoda,
748 : Scaphopoda. 757 : Rhwlojn . 768 :
Sepia, 773. 775: Nautilus, 787, 788:
Cephalopoda, 798

/,'- 'J nil ii in, 7*x», 701

Reservoir (Euglena), 68

Respiration, 13*, 35

Respirator}' organs, 35* : Starfish, 370 ;
Sea-urchin, 399: Holothurian, 404:
Cluetopoda, 471: Hirudinea, 602:
Apus, 534: Attacus, 54!». 550: Crus-
tacea, 594: PeripOtUS, 609: Mvria-
poda, 617: Periptaw/a, 626, 627:
Insects, 641, 642: Scorpion, 657:
Arachnida, 668 : Mussel, 685 : Pelecy-
poda, 702, 704: Triton. 725: Gas-
tropoda, 742: Sepia, 766: IfautSus,
781 : Cephalopoda, 797

Respiratory trees, 404, 428

Retina, 39

Retinophore, 732*

Retinula, 536*, 537, 556

Retractor muscles, 683

Rhabditis, 263

Rhabditisform, 310

Rhabdoccelida, 252*, 255, 257, 263, 265,
2%, 207, 20S, 269, 271, 272, 273, 276,
283, 2S4

Rhabdogatter, 319

Rbabdome, 563*, 537

Rhabdonema nigrorcnmnm, 308, 310

Rhagon-type of Sponge, 118*

Rhinophore, 787*

Rhipictodendron, 71, 73, 74

Rhipidoglossa, 733*, 750

Rhizocephala, 565*, 579, 580

Rhizopoda. 45* : Example, 46 : Clas-
sification and general organisation,
47

Rhizostomee, 177*, 182, 183, 227

Rhizota, 328*, 330, 333

Rhodope, 758

Rhombogene, 232

Rhopalura, 232

Rhynchobdellida, 516*, 517, 518, 519,
520

Rhynchodemus, 255*

Rhynchonella, 366, 369

Rhynchota, 100

Rhyncoccele, 289*

Ring- vessel, 270*, 283

Rock-lobster, 569

Rock systems, 7

Rocks, igneous and aqueous, 7*

Rosette, 407*

Rosette (Earthworm) 461

Rosette plate, 418

Rostellum, 246*

Rostrum, 542*

Rotalia, 52

Rotation, sense of, 39

Rotifer, 328

Rotifera, 323* : Distinctive characters
and classification, 327* : External
characters, 330 : Digestive organs,
333 : Excretory system, 333 : Nervous
system and Sense-organs, 334 : Re-
production and Development, 344 :
Ethology, 335 : Affinities, 335

Rotate, 398*

Round-worms, 297 — See Nemathelmin-
thes

Rugosa, 210*

s.

lobelia, 478
Saccammina, 53
Saccocirrus, 465, 477
Sacculi, 409*

Sacculina, 565, 579, 580, 599
Sagartia, 190. 191
Sagitta, 316, 317. 318
Sail, of Siphonophora, 163, 166
Salinella, 233, 234

834

INDEX

Saliva, 34*

Salivary glands, 34*

Salivary receptacle, 624

Salmacina, 486

Salpingoeca, 77

Saltatoria, 636

Sand-hopper — See Orchestia, 568

Saprophytic, 69*, 72

Sarcocystidea, 81*, 88

Sarcocystis, 88

Sarcolemma, 28*

Sarcophaga, 644

Sarcoptes scabicci, 665

Sarsia, 145

Scale-insects, 634, 647

Scallop, 696

Scaphoda, 756", 757

Scaphopoda, 680, 756*, 757

Schist osomum haematobium, 285

Schistosomum japonicum, 285

Schizogony, 84*, 85, 86

Schizopathes, 202

Schizopod-stage, 599

Schizopoda, 601

Scirtopoda, 328*, 330

Sclerite, 562*

Scleroblast, 110*, 120, 124

Sclerotic, 771

Scolex, 246*, 280

Scolopendra, 615

Scolopcndrella, 615, 617

Scorpion — See Buthus.

Scorpions, 526, 653

Scorpionida, 660*, 673, 676, 677

Scorpion-spiders, 632

Scrobicularia piperata, 699

Scuta, 578

Scutariella, 259

Scutigera, 615, 617

Scutigerella, 615

Scyllarus, 587, 589

Scyllis ramosa, 487

Scyphistoma — See Scyphula

Scyphozoa, 128 : Example, 168 : Struc-
ture and classification, 176 : System-
atic position of example, 177: Ad-
ditional remarks on, 184

Scyphula, 174*, 175, 229

Sea-anemones, 128, 185, 194, 196, 200—
See Tealia

Sea-blubbers, 182

Sea-cucumber, External features, 401 :
Structure of body-wall, 402: Ambu-
lacral system, 402 : Nervous- vascular
systems, 402 : Ccelome, 403 : Enteric
canal, 403, 404 : Reproductive organs,
404 : Development, 404 : Systematic
position, 414

Sea-cucumbers, 412 — See Holothuroidea

Sea-fans, 195

Sea-firs (Sertularians), 143

Sea-hares, 721

Sea-mats, 340 — See Polyzoa

Sea-mice, 475

Sea-mussel, 695

Sea-pens, 196

Sea-urchin, External features, 393, 394,
395 : Corona, 396 : Aristotle's lantern,
397 : Nervous system, 398 : Ambu-
lacral system, 399 : Enteric canal, 399,
400 : Coelome, 399 : Blood-vascular
system, 400 : Reproductive organs,
400 : Development, 400 : Systematic
position, 414

Sea-urchins, 412* — See Echinoidea

Secondary axis, 41*, 42

Secretion, 25*

Segment, 43*

Segmental organ — See Nepredium

Segmentation of oosperm, 22"

Segmentation-cavity — See Blastoc* ele

Segmentation-nucleus, 20*

Seison, 335

Seisonida, 329

Sehnaria, 347

Selenariidw, 349, 352

Self-mutilation, 435

Semostomae, 177*, 182, 227

Sense-organs, 39* : Obelia, 135 : Trachy-
lime, 154: Aurelia, 172: Cubomedusre,
182 : Hormiphora, 216 : Ctenoplana,
225 : Platyhelminthes, 268 : Nemer-
tinea, 294 : Chsetognatha, 318 : Bra-
chiomis, 326 : Rotifera, 334': Phoronis,
357 : Starfish, 377 : Sea-urchin, 395 :
Nereis, 447 : Chretopoda, 438 : Sipun-
culus, 454 • Hirudo, 514 : Hirudinea,
520 : Apus, 536 : Astacus, 556 : Crus-
tacea, 597 : Peripatus, 609 : Peri-
planeta, 627 : Insects, 644, 645 :
Scorpio, 658 : Arachnida, 670 : Mussel,
691 : Pelecypoda, 707 : Amphineura,
715 : Triton. 731 : Gastropoda, 746 :
Rhodope, 758 : Sepia, 769, 771 :
Nautilus, 787 : Cephalopoda, 797— See
also Eyes, Auditory organs, Olfactory
organs, Gustatory organs, Tactile or-
gans, Osphradia

Sense papilla;, 302, 493

Sepia cultrata, 759

Sepia, External features, 759, 760 :
Shell, 761, 762: Chromatophores, 762 =
Mantle-cavity, 762: Internal skeleton,
763: Alimentary system, 763: Ink-sac,

765 : Vascular system, 765 ; Ccelome,

766 : Ctenidia, 766, 767 : Nervous
system, 767 : Sensory organs, 769, 771:
Excretory organs, 772, 773 : Repro-
ductive organs, 773, 775 : Systematic
position, 790

Sepiido3, 790*
Septal funnel, 173*
Septal neck, 776*
Septibranchia, 696*, 697, 698

INDEX

835

Septum, 206

.s'» ri'i/'iri'i. :;js

Serosa, 631, 669

Serpula, I7J. lT«i, 477, 488

Serptdidce, 468. 172, 185, 186

S< rtulatians, 143
*3!>. 440, 441

Seta, provisional, 4.V2

Setigerooa sac, 441*, 4.V2, 4.~>3, 4.">7

Sexual dimorphism, 40*

Sexual generation, 140

Shell-gland. SMS, 529, 682, 800

Shell-gland -(Apus), 534, 635* : (Crus-
tacea), 596

Shell, Magellania, 360. 361 : Braehiofpda,
3(17, 368 : M*S$el, 682 : Pelecypoda.
697: Chiton, 714: Triton, 721, 722,
723 : Gastropoda, 737 : Scaphopoda,
7 ."">»> : Sepia, 781, 762: Nemtuiu, 776,
Cephalopoda, 793

Shelly loop, 380

Ship-worm, 696

Shrimp. 526, 588, 587

Sicula, 167*

SigarkiHS, 740

Silicispongiae. 126*

Silver fish, 632

Sinupalliata, 696*, 698, 707

Sinupalliate, 699*

Sinus, 554

Siphon, 399, 428, 498," 723

Siphonal process, 722*

Siphoniata. 711

Siphonodentalium, 756

Siphonoglyphe, 187*

Siphonophora, 142*, 188, 160, 162, 163,
164, 165. 166, 226

Siphonozooid, 202*

Siphons, inhalant andexhalant, 681*, 698

Siphuncle, 776*

Sipunculidce, 496, 498, 502, 503

Sipunculoidea, 496*, 497, 498, 499, 502

Siphni tu's ncdus, General external
features, 492. 493: Body-wall, 493:
Coelome, 493 : Blood-vascular system,
493 : Alimentary canal, 493, 494 :
Nervous system, 494: Nephridiaand
gonads, 495 : Systematic position, 496

Skeleton, 31* : Lobosa, 49 : Foraminifera,
51, 53 : Heliozoa, 59 : Radiolaria, • 61 :
Mastigophora, 73 : Ciliata, 96 : Pori-
fera, 120 : Actinozoa, 202 : Stjria, 763 :
Cephalopoda, 796 — See also Shell and
Body-wall

Skin, 31* — See Body-wall

Slime glands, 608

Slugs, (.so. 734

Snails, 680, 734

Solariwn, 7 7

Solecurtiut strigillatii*, 700

Solen, 711

Solenocytes, 338, 479

Solenogastres, 713*, 715, 717, 7ls, 720,
721, so.")

Solenomya , 685

Solpugida. 881*. 882, 663, 667, 668, 670

Somatolilast. 450

Somatopleure, 631*

Spadella, 316

Spadix, 780*, 781

Spatangoidea, 412*, 417, 422

Species, l*, | [?* -

Specific name, 2*

Sperm, 20, 30*

Spermary, 39*

Spermatidia, 344

Spermatozoon — Se.e Sperm

Spermiduct, 40*

Spermotheca, 308*

Sphairidia, 395*, 417

Sphaeroma, 568

Spharophrya, 100

Spicules, 32*, 203

Spiders, 526, 863

Spinnerets, 664

Spinning-glands, 664

Spinules, 240*

Spireme, 18*

Spirifera, 664,

Spirolocu/iiui, 53

Spirorhi*^ 489

Spirorbi* lams, 484

Spirula, 790, 794, 795

Splanchnopleure, 631*

Spongdia, 121

SpongiUa, 118

SpongUlida>, 121, 126

Spongin, 112, 120*, 121

Spongin-blattt, 120*

Sporangium (Mycetozoa), 66, 67

Spore, 40, 67, 75, 80, 81, 88

Spore formation, 40, 56, 59, 60, 63, 65,
66, 67, 75, 79, 80, 87, 89, 98

Sporocyst 245*

Sporogony, 84*, 85

Sporosac, 151*

Sporozoa, 46* : Example, 80 : Classifica-
tion and general organization, 81

Sporozoites, 80, 81, 83, 84, 85, 86, 88

Springtails, 632, 640

Squame, 544*

Squammulhia, 51, 52

Squids, 790

Squilla, 592

S/'irfui rth ru »t , 232

Statoblasts, 353

Statocones, 707*, 7S7

StatocvBts, 268, 285, 691, 707, 788

Statolith, 268, 707*

Starfish, Kxternal characters, 875, 376,
377: Transverse action of arm, 378:
Vascular and nervous systems, 379 :
Structure of disc, 380 : Body-wall and
ccelome, 381 : Digestive system, 382 :

836

INDEX

Ambulacral system, 383 : Reproduc-
tive system 386 : Development, 388,
389, 390, 391, 392, 393 : Systematic
position, 414

Starfishes, 374 — See Asteroidea

Stauromedusae, 176*, 177, 118, 227

Stenocyphus, 178

Stentor, 92, 94

Stephanoceros, 328, 330, 331

gterneupis, 469, 471, 475, 477, 481

Sternaspis spinosa, 471

Sternum, 541*

Stewart's organs, 425*

Stichopoda, 415

Stichotricha, 95

Stick-insects, 633, 636

Stigma (Euglena), 68

Stigmata, 609, 623, 642, 655

Stinging capsule — See Nematocyst

Stolon, 196*

Stomach — See Digestive system

Stomatogastric nerves, 447

Stomatopoda, 569*, 592, 593, 596

Stomidium, 201*

Stomoda;al canal, 213

Stomodaeum, 173*

Stone-canal, 383*, 399

Stony-corals, 128, 195 202

Stratiodrilus tasmanicus, 338

Strepsiptera, 640, 651,

Streptophiurae, 411*

Streptoneura, 733*, 735, 736, 739, 742,
744, 745, 746, 748, 750, 755

Strichotrkha, 95

Strobila, 246*, 247, 262

Strongylocentrotus, 393, 395, 397, 414-
See Sea-urchin

Strongyloxtoma, 615

Strongyhis, 305

Stylarioides, 488

Stylaster, 157, 158, 159

Style, 158*

Stylonychia, 99

Subcortical cavity of sponges, 119*

Subdermal cavity of sponges, 118, 1 19

Sub-genital pit (Aurelia), 170*

Sub-genital portico, 183*

Sub-kingdom — See Phylum

Sub-mentum, 622

Sub-radius, 139*

Sub-tentacular canal, 407

Sub-umbrella, 135*

Succession of Life in time, 7

Sucker (Sepia), 760*

Sucking-disc, 417

Suctorial mouths (Rhizostomie), 183 :
Discomedusaj, 183

Summer eggs, 597

Supplemental skeleton, 53, 54*

Supporting lamella — See Mesogkea

Swimming-bell — See Nectocalyx

Swimming ovaries, 315

Swimming-plate, 213*

Sy cant ha, 113

Sycetta, 113

Sycettide, 113*, 172

Sycon : External characters, 105, 106,
107 : Microscopic structure, 108, 109 :
Systematic position, 112: Develop-
ment, 124, 125

Sycon raphanus, 125

Sycon-type of sponge, 117, 118*

Syliida, 475, 487, 488

Syllii ramosa, 487

Symbiosis, 63*

Symmetry, 41*, 42: Polype and Medusa,
138 : Tealia, 189, 192

Symphyla, 615*, 617, 619, 677

Syuapta, 424, 434, 438

Synaptide, 429

Synapticula, 205*

Syncarida, 566*, 581

Syn-cerebrum, 555*, 631

Syncoryne, 145

Syncrypta, 71

Syncytium, 298*

Syngnatha, 615*, 616, 617, 618

T

L A ban us, 638, 644

Tabula, 157*, 205, 208

Tactile cones, 260*, 268

Tactile organs, 39

Tenia ccenurus, 252

Tenia crasskollis, 285

Tenia cncumerina, 270

Teniade, 254

Tenia echinococcm, 261, 262, 282, 286

T<niia mediocanellafa, 285

Tenia saginata, 285

Tenia serialis, 282

T(vnia serrata, 285

T^knia solium, General features, 245,
246, 247 : Nervous system, 248 :
Excretory organs, 248 : Reproductive
organs, 248, 249, 250 : Development,
250, 251 : Systematic position, 254

Tamiole, 173*

Talitrm, 583

Tanaidacea, 567*, 583, 584, 585, 603

Tanais, 567, 583

Tape-worm— See Taenia and Cestoda

Tardigrada, 673, 675*, 678

Tealia : External characters, 185, 186 :
Enteric system, 187, 188, Cell layers,
188: Muscular system, 186, 188: Sym-
metry, 189 : Microscopic structure,
190, 191 : Nervous system, 192 : Re-
productive organs, 193 : Development,
192, 193 : Systematic position, 196

INDEX

837

?/.«, 196*
ectibranchia. 734 \ 7~><»
Teloblast, 4(il
Telolecithai; --'lit*
Telotrochal, 486*
Telson, 541*
Temnocephala, 259. 280, 269, 277,278,

279
Temnocephalea, BBS*, 259, 260. 260, 273,

278. 279. 284, 287
Tendon, 37*

Tentacle sheath, 210*: Knots, 155*

Tentacle*, 32*, 91

Tentacular canal, 213

Tentaculifera, 01* : Body and tentacles,

99, 100: Nucleus, contractile vacuoles.

shell, colonies, reproduction, 100, I'd
Tentaoulocyst, 166 , 172
Tentorium,' 626*
TerebeUa, 474
Ttrebdtidn, 472, 474
Terebra, 738
T< rebratula, 3<>i>, 368
Terebratiiliilu, ,H67*
TerebratvUna, 372
Teredo, (i'tfi, 701
Terga, 578*
Tergum, 541*
Termite*, 833
Tem rn, 177, 178
Testis -See Spermary
Tethya, 734
Tetrabranchiata, 790*. 792, 795, 797, 798,

803, sot
Tetremita, 71
Tetrarhynckua, 261, 282
Tetrastemma, 291
Thaiastoplancta, 61
Thalkuema, 499
Theca, Jo.",

Thomson, J. Vaughan. 3
Thorax, of Apn», 631 : Attaeiu, 541

/'. riptam in , 622: Solpugida, 664
Thread-worms, 297 — See Neinathel-

mintlns
Thymnopoda, 568
Thymnozoon, 265
Thysanura, 640
Thurirnl'i. 95

Ticks, 663, 661 Sue Aearida
Tiedemami's vesicle, 383*, 428
Tmtinniditim, 94
Tissues, 23*
TomopU rie, 488

Tooth-shells, 680 — See Scaphopoda
Trachea?, 36*, 600, 743
Tracheal gills, 642, 643
Tvachelkutes, 516
Trnr/i, /;>,,<, 94
Tmekefomomu, 71

Trachylinae, 142* : General structure,
153 : Sense organs, 154 : Tentacles,

166: Reproductive organs, 154. 166:
Development, 166, 156

Trachymedusae. 142*, 154, 166, 168
Translation, 286
Trap-door Spider, 672
Trapezia, 209

Trematoda, 252*, 257, 258, 259, 260, 264.
265, 286, 267, 268, 269, 270, 272, 273,
277. 278, 279, 284, 285, 286, 287, 288 :
Example, 240
Treptoplax, 233
1'rin, -thru* beckU, 606
Trichina, 310, 311
TrickineOa, 306, 311
Triehiniasis, 312
Tricliocyst, 89, 93
Trichoplax, 233
Trichostomata, 91
Tricladida, 252*, 253, 255, 257, 263, 264,

267, 268, 269, 271, 275, 276, 283, 284
Tridacna gigas, 710
Trigger-hair— See Cnidocil
Tniimiia, 695, 701, 710, 711
Trilobita, 604, 605, 606, 677
Trimorphism, 129*
Trimyaria, 290
TrittlcothoHa, 319

Triton noiukkkis, 721 : Shell, 721, 722.
723 : External features of soft parts,
723 : Foot, 724 ; Visceral spiral, 725 :
Manthv 725 ; Ctenidium, 725 ; Os-
phradinm, 726 : Digestive system, 726,
727, 728 : Vascular system, 729 : Ex-
cretory system, 729: Nervous system,
72!). 730, 731 : Sensory organs, 731 :
Reproductive organs, 732 : Systematic
position, 734
Tritonidai, 733, 734*, 745
Trivium, 377*, 416
Troche! disc, 324

Trochelminthes, 322* : Appendix, 336
Trorheta, 516

Trocliophore — See Trochosphere
Trochoephara, 328, 332. 336
Trochosphaerida, 328*
Troohosphere, 322, 440
troehuf, 733, 747
Trombidium fuliginoettm, 665
Trophozoite, 80
TrypoMODOmet, 71. 72
Tube-feet, 377*. 376, 395, 401
Tiilnfix, 473, 483
TubUcida, 188

Tub'ipora, 195, 108, 199, 203, 208, 210
Tvbviaria, 145, 161
Tubularioj, 143

Turbellaria, 252*, 866, 256. 257. 282,
283, 264, 286, 266. 267. 268, 269, 271,
272, 273, 274, 275, 276, 284, 288 : Ex-
ample, 236
Turbo, 733, 747
Typhlosole, 458*, 475, 685

838

INDEX

Ul

V

LMARIDJE, 177*
Umbo, 683
Umbrella, 168*
Uncus, 325*

Undulating membrane, 72*, 92, 94
Unicellular, 19*
Unicellular gland, 25*
Unio, 680, 696— See Anodonta
Unio margaritifer, 680, 711
Unionida, 696*
Unisexual, 40*
Urea, 36
Uric acid, 13, 36
Urinary organs, 37*
Urnatella, 354
Urns, 493*
Uropod, 542*
Uterine bell, 313
Uterus, 40*

Vermetes, 733

Vermetus, 755, 756

Vermiform embryos, 231

Vermilia coespitosa, 468

Vertebral column, 4

Vestibule, 95

Vibracula, 349, 352*

Vibratile corpuscles, 399

Virgula, 167*

Visceral spiral, of Triton, 725

Vitellaria, 270

Vitelline glands— See Yolk -glands

Vitelline membrane, 217

Vitreous body, 536*

Viviparous, 40*

Voluta, 749

Volvox, 72, 73, 75, 76, 77

Vortex, 265

Vorticella, 92, 93, 95, 96, 97, 98

Vulsella, 698

W

V

V,

acuole, contractile, 11*, 13, 47, ,66
68, 86, 88, 93, 96, 101 : non contractile,
11*, 60, 93, 94
Valenciennes, organ of, 781*
Valvulate, 387*

Van der Hoefen, organ of, 780*
Variation, individual, 2*, 113
Variety, 2*, 114*

Vascular system, 34 : Nemertinea, 291
Acanthocepbala, 313 : Phortmis, 357
Magellania, 365 : Brachioppda, 369
Starfish, 379: Holothurian, 426
A ntedon ,* 409 : Echinodermata, 426
Nereis, 444: Lmnhricus, 458: Sipun
cuius, 493 : Gephyrea, 498 : Archi-
annelida, 504: Hirudo, 512: Hirudinea,
518 : Apus, 533 : Astacus, 551 : Crus-
tacea, 596: Peripatus, 609: Peri-
planeta, 625 : Insects, 642 : Scorpio ,
656: Aracbnida, 668: Mussel, 689:
Pelecypoda, 706: Amphineura, 715:
Triton, 729 : Gastropoda, 744 : Scapho-
poda, 757 : Sepia, 765 : Nautilus, 783 ;
Cephalopoda, 797

Vas deferens — See Spermiduct

Vegetal pole, 751

Velarium, 170*, 182

Velella, 166

Veliger, 709, 710*, 751

Velum, 135*, 136, 170, 710, 752 ■

Ventral, 42*

Ventricle, 36*

Venus gnidia, 699

Venus's Flower-basket, 120

Venus's Girdle, 223

Vermes, 235

W

ALDHEIMIA—See Magellania
Wallace's line, 9*
Wandering cells, 111
Wasps, 636, 647, 652
Water-bugs, 634
Water-flea, 526, 564
Water-pores, 408*, 426
Water-sac, 536
Water-tubes, 408*, 426, 686
Water-vascular system, 236*
Whale-louse, 586
Wheel-animalcules — See Rotifera
Wheel-organ, 324
Whelks, 680, 733
White body, 772

White substance of Schwann, 29*
Winter eggs, 327, 597
Wood-louse, 526, 568— See Oniscus

X

iphosura, 661*, 666, 667, 668, 669,
673, 677, 678

Y,

. ellow Cells — See Zoochlorella.
Yellow elastic cartilage, 26*
Yoldia, 695
Yolk, 19*
Yolk -glands, 236*

Yolk, epithelium, 799 : reservoir, 243
Yolk-sac, 803

INDEX 890

Zoochlorella, 60, 68, 808, 841

Z Zoo-geographical Regions, 8

ZZooi<l, 40* 50, 7-"), 283
II.LA CALLOPHYILA, 670 Zoology, 1*

Zona, BW1 Zoophyte, 1-28

Zoantharia. 194" 227 Zoothamninm, 97

Zoanthu*, 196, 197. 200, 201 Zygophiurse, 411*

Zoucium, 341 Zygote, 74, 7">* 80, 81

R. CLAY 4MB SOS'S, LTD., BRKAD ST. HILL, E.C., ANI> BUNGAY, SUFFOLK.

0 '

QL Parker, Thomas Jeffrey

| 47 A text-book of zoology.

: P23 2d ed.

1910

v.l

BioMed

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