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A TEXT-BOOK OF ZOOLOGY 


VOL. I 


MACMILLAN AND CO., LimITED 
LONDON FOMBAY . CALCUTTA 
MELBOURNE 


THE MACMILLAN COMPANY 
NEW YORK . BOSTON . CHICAGO 
ATLANTA . SAN FRANCISCO 


THE MACMILLAN CO. OF CANADA, Ltp 
TORONTO 


A TEXT-BOOK 
OF ZOOLOGY 


BY 


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


PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, N.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 
IgIO 
T 


>t 
Aeses 7] 
Ricuarp CLay anp Sons, LimIrep, 


BREAD STREET HILL, E.C., AND 
BUNGAY, SUFFOLK. 


First Edition, 1898. 
Second Bdition, 1910. 


Zoology Vol. 1. 


ERRATA. 


PAGE. 
17, description of Fig. 5, for ‘‘ atrosphere ” read ‘* astrophere.” 
52, description of Fig. 35, for “‘ Rotalla” read ‘* Rotalia.” 
71, description of Fig. 52, for ‘‘ Astasiopis ” read  Astasiopsis.” 
74, line 9, for ‘‘divison” read “ division.” 

111, line 1, for ‘‘ out” read ‘ outer.” 

208, line 10, for ‘‘ siphnozoids” read ‘‘ siphonozooids. 

272, line 2, for “ prostrate” rend ‘ prostate.” 

402, line 43, for ‘‘ periphiemal” read ‘‘ perihaemal. 

450, line 7, for “ Fig. 346” read ‘* Fig. 347.7 ; 


Zoology. Vol. II. 


ERRATA. 


PAGE. 
9, description of Fig. 710, for ‘‘ Fig. 667” read ‘ Fig. 709.” 

17, line 8 from bottom, for ‘‘ end” read “(end)” 

19, line 1, for ‘‘ peribranchial” read ‘ peripharyngeal.” 
183, Fig. 840, for ‘‘ Pristurus ” read ‘ Pristiurus.”’ 
229, description of Fig. 885, for ‘‘ dorsa” read ‘‘ dorsal.’ 
322, line 20 from top, for ‘‘ innominiata” read ‘* innominala.” 
432, lines 3 and 20 from top, for ‘‘ Apertyx”’ read ‘* Apteryx.” 
500, line 1, for ‘‘ Arycteropus” read ‘“ Orycteropus.” 
606, line 9 from top, for ‘‘ Squolodontidoe” read ‘‘ Squalodontidiw.” 
620, line 3 from bottom, for ‘‘ import ” read ‘‘ important.” 


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

A not uncommon method of expounding the science of Zoology 
is to begin the study of a given group by a definition, the very 
terms of which it is impossible that the student should under- 
stand; then to take a general survey of the group, illustrated by 
casual references to animals and to structures of which it is highly 
unlikely he has ever heard; and, tinally, 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 Molluses 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,’ 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 the characters already met with are of 
distinctive importance, and which special to the example itself 
In order to bring out this point more clearly, to furnish a connec- 
tion between the account of the example and that of the class asa 
whole, and to give some idea of the meaning of specific, generic, 
and family characters, we have introduced, after the classification, 
a paragraph giving the systematic position of the example, some- 
times in more, sometimes in less detail. 

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

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

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

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


viii PREFACE TO THE FIRST EDITION 


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

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 bzs, are from 
photographs kindly taken for us by Mr. A. Hamilton.? Many blocks 
have been borrowed from well-known works, to the authors and 
publishers of which we beg to return our sincere acknowledg- 
ments. All the new figures have been drawn by Mr. M. P. Parker. 


1 Tn 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 !— 
‘‘T have given a Bibliography after some hesitation, feeling that references to 
original papers are of no use to junior students, and must be too imperfect to 
be satisfactory to more advanced students... Attention has been paid to 
recent work, but I have felt that the gradually-formed deposit of accepted know- 
ledge must be of greater intrinsic value than the latest ‘discovery’ or the 
newest theory. An early mental diet in which these items are predominant is 
an unwholesome diet ; their function in elementary instruction is that of condi- 
ments, valuable only in conjunction with a foundation of solid food.” 

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


PREFACE TO THE FIRST EDITION ix 


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

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

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


PREFACE TO THE SECOND EDITION 


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

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

T have the pleasure of acknowledging assistance on special points 
received from Professor J. P. Hill, Mr. 8. 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 


PREFACE 
List oF Lunumeeerions IN Won, I. 


TABLE OF THE CLASSIFICATION OF THE ANIMAL eens ‘ 


IntROpUCTION 


THE GENERAL STRUCTURE AND PHYSIOLOGY OF ANIMALS , 
. Ameeba. 

. The Animal Cell 
. The Ovum: Maturation, lnpregnation, sci Seotnentabion 


SECTION I 


Germinal Layers 


. Tissues 


Organs 


. The Reproduction of eels 
. Symmetry. . 
. The Primary Subaiisions or Phyla of the Antinal Kingdom 


SECTION TI 


Puytum Prorozoa 
Class I. Rhizopoda 


1. Example of the Class — Ameeben proteus 
Systematic Position of the Example 
Appendix to the Rhizopoda 


Class II. Mycetozoa 


1. Example of the Class— Didiwnines Rear me 
2. General Remarks . 


Class III. Mastigophora 


1. Example of the Class—Euglena ene 
2. Classification and General Organisation . 
Systematic Position of the Example 


Class IV. Sporozoa 


1, Example of the Clase onoeustis ag re 
2. Classification and General Organisation . 
Systematic Position of the Example 


the 


xiv CONTENTS 


Puytum Prorozoa—continued. 
Class V. Infusoria 
1. Example of the Class—Par comeectaume , cauddatrm 
2. Classification and General Organisation . 
Systematic Position of the Example 
Further Remarks on the Protozoa 


SECTION III 


Puytum anp Crass Porrrera [Parazoa] . 
1. Example of the Class—Sycon gelatinoswm 
2. Distinctive Characters and Classification 

Systematic Position of the Example 
3. General Organisation 


SECTION IV 


PuyLumM C&LENTERATA 
Class I. Hydrozoa.. 
1. Example of the Chacon 
2. General Structure and Classification 
Systematic Position of the Example 
Additional Remarks 
Class IL. Seyphozoa : 
1. Example of the Clings Anadtien aur he 
2. General Structure and Classification 
Systematic Position of the Example 
Additional Remarks 
Class III. Actinozoa 
: eels of the Class—Tealiv crussicor its 
! Distinctive Characters and Classification 
* askontatio Position of the Example 
3. General Organisation 
Class IV. Ctenophora 2 
: Example of the Class— Hor nabnioee: phenioen ‘ 
. Distinctive Characters and Classification 
~Gaatematic Position of the Example 
3. General Organisation . 


Appendix to Ctenophora— (tomipl nies and Cetopiaten. 


The Relationships of the Ccelenterata 
Appendix (II) to the Ceelenterata—The Mesozoc 


SECTION V 
Puytum PLATYHELMINTHES 
1. Examples of the Phylum 
i. Planaria or Dendrocelum 
ii. Fasciola hepatica 
ii. Tenia solium 


PAGE 


88 
88 
91 
91 
101 


105 
105 
111 
112 
114 


128 
128 
128 
140 
142 
167 
168 
168 
176 
177 
184 
185 
185 
193 
196 
196 
211 
211 
220 
221 
222 
225 
226 
230 


235 
236 
236 
240 
245 


CONTENTS 


Paytumw PLATYHELMINTHES-—condrivued. 
2. Distinctive Characters and Classification 
Systematic Position of the Examples 
3. General Organisation 
4. Distribution, Mode of Ceauhiense, iad Mutoal Relationships 
Appendix to Platyhelminthes—Class Nemertinea . 
Distinctive Characters and Classification 


SECTION VI 


Puyitum NEMATHELMINTHES 
Class I. Nematoda : 
1. Example of the Class—Ascamis toe oe 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation 
Class II. Acanthocephala . 
Class III. Chetognatha 
Appendix to Nemathelminthes 
Family Chetosomide 
>»,  Lichinoderidee 
3,  Desmoscolecide . 
Affinities and Mutual Relationships of the Neinathelminthes 


SECTION VII 


Puytum TROCHELMINTHES 
Class I. Rotifera . 
1. Example of the Class—Br wuchionus Geis 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation . 
Class II. Gastrotricha 
Appendix to Trochelminthes—Dinophilea and Histr iobdellen 


SECTION VIII 


Puytum Mo.uvuscoipa 
Class I. Polyzoa . 
1. Example of the Class—Bugula oe te 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation . : 
Class II. Phoronida 
Class III. Brachiopoda ; 
1. Example of the Class—Magellunia Tenbiouleg is 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation . 
Mutual Relationships of the Classes of the Molluscoida 


XV1 


H 


Shi ns 


CONTENTS 


SECTION IX 


Phylum Echinodermata 


. Example of the Asteroidea—Asterias rubens or Andie aien favrwons 
. Example of the Echinoidea—trongilocentrotus or Echinis 

. Example of the Holothuroidea—Cuenmaria or Colochiris 

. The Crinoidea—Antedon rosacea, 

. Distinctive Characters and Classification 


Systematic Position of the Examples . 


. General Organisation 


SECTION X 


Prytum ANNULATA 
Class I. Cheetopoda 


1. Examples of the Class 
1. Nereis dwmerilii 
LL. Lwmbricus . 
2. Distinctive Characters and Classification 
Systematic Position of the Examples 
. General Organisation 
poe to the Chetopoda—Class Myzostomida 


Class I. Gephyrea 


i Example of the Cis Shaan nadits 
. Distinctive Characters and Classification 
3 General Organisation 


Class III. Archi-annelida 
Class IV. Hirudinea . 


1. Example of the Class—Hirudo medicinulis aul Il. cuannilis 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation . 
4. General Remarks on the Annulata 


SECTION XI 


PHyLtum ARTHROPODA 
Class I. Crustacea 


1. Examples of the Class 
i. Apus or Lepidurus 
i. Astacus fluviatilis 
2. Distinctive Characters and Classification 
Systematic Position of the Examples 
3. General Organisation . 
Affinities and Mutual Relationships . 
Appendix to Crustacea—Class Trilobita 


Class II. Onychophora 
Class ITI. Myriapoda 


1. Distinctive Characters and Classification 
2. General Organisation , 


CONTENTS 


Payira ARTHROPODA —cond inwed. 
Class IV. Insecta ok ‘ 
- ea: uf the Class—Periplanetu or ‘untae, or P. comericone 
. Distinctive Characters and Classitication 
Systematic Position of the Example 
. General Organisation 
Class v Arachnida : 
1. Example of the Cassia isinepio ov Buthus 
2. Distinctive Characters and Classification 
3. General Oreanisation 


Appendix to the Aquelnile ihe Pre an Landa: 


and Tardiyrada , 
Relations of the Air- breathing ke siropuda 


SECTION XII 
Puyitum Mouuusca 
Class I. Pelecypoda 
1. Example of the Clad Audis wad Unio 
2. Distinctive Characters and Classification 
Systematic Position of the Examples 
3. General Organisation 
Class II. Amphineura 
1. Distinctive Characters and Chuadifention 
2. General Organisation ee ay i 
Class III. Gastropoda 
1. Example of the Class—T'r von wotlifer: us 
2. Distinctive Characters and Classification 
Systematic Position of the Example 
3. General Organisation , 
Appendix to the Gastropoda 
A. Class Scaphoda 
B. Rhodope 
Class V. Cephalopoda 
1. Examples of the Class 
i, Sepiu 
Neulilnus pompilins 
2. “DisGneive Characters and Classification 
Systematic Position of the Examples 
5. General Organisation . : 
General Remarks on the Mollascn : 


VoL. 1 b 


ne 
a 
5 


SO TED ot me ee 


LIST OF ILLUSTRATIONS 


VOL. I. 


Amceba proteus 


. Ameeba polypodia, fission 


Alveolar theory of Protoplasm 
Reticular theory of Protoplasm 


. Diagrams illustrating se him 
. Ovum of Sea-urchin . 


Maturation and fertilisation of ovum 
Segmentation of ovum 


. Gastrulation 

. Gastrula ‘ . 

. Various forms of apitheliun 

2. Diagram to illustrate structure of siands 
. Gelatinous connective tissue . 

. Reticular connective tissue 

. Fatty tissue 

. Hyaline cartilage 

. Fibro-cartilage . 

. Bone 


. Unstriped Musvle 


. Striped Muscle . 

. Nerve-cells 

. Nerve-fibres 

. Various forms of spermatozoa 

. Viscera of Frog 

. Bones of human arm with jiveng muscle 
. Nervous system of Frog . 

. Hydra. ‘ 

. Diagram of axes of oles 

. Radial symmetry 

. Amceba, various species . 

. Protamceba primitiva 

. Quadrula, Hyalosphenia, Rvcalle, wail Difflugia. 
3. Microgromia socialis d , : : 


LIST OF ILLUSTRATIONS 


. Platoum stercoreum 

. Various forms of Howmpinien 
5. Shells of Foraminifera 

. Hastigerina murrayi 

. Dimorphism and alternation at sncmuinne in Polystomella 
. Actinophrys sol. 

. Actinosphzrium eichhornii 

. Various forms of Heliozoa 

. Actinophrys sol, conjugation 

. Lithocircus annularis : 

. Thalassoplancta brevispicula . 

. Aulactinium actinastrum 

. Actinomma asteracanthion 

. Collozoum inerme 

. Chlamydomyxa inpyainbhalundes 
. Labyrinthula 


50. Didymium diffurme 


. Euglena viridis . 
. Various forms of Flagellata 
. Trypanosome 


” 
. Vorticella . : 
. Zoothamnium arbuscula . 
5. Opalina ranarum 
. Various forms of Paritiauliters 


. Diagram showing the mutual pie of the Protonod 
. Sycon gelatinosum 


Heematococcus lueialis 


. Pandorina morum 

. Volvox globator 

. Heteromita rostrata . 

. Various forms of Choanoflagellata 
. Various forms of Dinoflagellata 

. Noctiluca miliaris 

. Monocystis . 

. Gregarina 


re development 


. Eimeria and Coccidium 

. Coccidium, Life-history . 
. Malaria Parasite 

. Myxidium and Myzxobolus 
. Sarcocystis miescheri 

. Parameecium caudatum 


Br $5 conjugation 


71. Various forms of Ciliata . 


a 


o 5 nieniied: 
ite ‘is transverse section . 
» ‘9 vertical section 


LIST OF ILLUSTRATIONS xxi 


FIG. 


PAGE 
82. Sycon gelatinosum, pore-membrane —. i re é , . 109 
83. a apopyle .. ok . 109 
84. Extemal form of various arate ee . 1 
85. Ascetta primordialis. . . ; i -@ ( T6 
86. Diagrams of canal system of various Genes a: ‘ . 1a 
87. Vertical Section of Spongilla, .  . . 118 
88. Cells of ectoderm of Sponge . : .  . 11g 
89. Development of tri-radiate Spicule 120 
90. Skeleton of various Sponges... : . 11 
91. Various forms of Sponge Spicules ‘ F . 122 
92. Pheronema Carpenteri .  .. 123 
93. Larva of Clathrina blanca ; »  , 1d 
94. Development of Sycon raphanus : : : 125 
95. Obelia. , tz Hh ae Sg . 180 
96. 4, Vertical Seaton of polype P fh e 8 . 132 
97. Nematocysts of Hydra & @ Se of . . 183 
98. Tentacle of Eucopella .  . ; be 134 
99. Obelia, medusa . bo oR . 2. 1385 
100. Diagram of medusa . » oo. 136 
101. Derivation of medusa from oulpe ok . , 137 
102. Projections of polype and medusa. 2 »~  .  . 188 
103. Development of zoophyte DA at mow oa Ge a MO 
104, Bougainvillea ramosa é ae »-  .  . 144 
105. Various forms of Leptoline . i . 145 
106. Ceratella  . . . 146 
107. Hydra... =. oe : 147 
108. Protohydra Deckert . : . 148 
109. Various forms of leptoline Medusze 150 
110. Diagram illustrating formation of sporosac by degeneration ae 
medusa... ce ee ce oe 151 
111. Early development of Bucope a ee ee . 152 
112. Two Trachymeduse.  .  . .. » we 164 
113. Two Narcomedusze : : ae »  . 154 
114. Aiginura, tentaculocyst ‘ : ee 155 
115. Larva of Adginopsis . : , .  . 156 
116. Millepora alcicornis, skeleton co ah se . 157 
117. Millepora, diagram of structure .  .  .. : 158 
118. Stylaster sanguineus, skeleton . . . . . 159 
119. Halistemma tergestinum r ek bm te of . 160 
120. Diagram of a Siphonophore e OR ew Te oe ow og 162 
121. Development of Halistemma . RS . , 163 
122. Physalia .. : . 164 
123. Diphyes eanpanulas . 165 
124. Porpita Pacifica 3 P . 166 
125. Graptolites er . 167 
126. Aurelia aurita, dorsal ail ventral views : . 169 
127. 3 », side view and vertical section . . ii 


128, Re », portion of umbrella with tentaculocyst —, : . 172 


xxi 


FIG. 


120). 
130. 
131. 
132. 


133. 
134. 
135. 
136. 
137. 
138. 
139. 


140. 


141. 
142. 

143. 
144. 
145. 
146. 
147. 
148. 
149. 
150. 
151. 
152. 
153. 
154. 
155. 
156. 
157. 
158. 
159. 
160. 
161. 
162. 
163. 
164. 


165. 


166. 


167. 
168. 
169. 
170, 
171. 
172. 
173. 
174. 
175. 
176, 


LIST OF JLLUSTRATIONS 


Aurelia aurita, development , 
Tessera princeps 
Lucernaria . 
Pericolpa quadrigata 
Nausithée . 
Charybdzea marsupialis 
Pilema pulmo 
Pelagia noctiluca, dev epien 
Tealia crassicornis, dissection and transverse decead 
Diagrammatic sections of Sea-anemone 
Tealia crassicornis, section of tentacle , 
Nematocysts of Sagartia 
Section of mesenteric filament: of Savartia 
Transverse sections of embryos of Actinia . 
Zoanthus sociatus 
Hartea elegans . 
Corallium rubrum 
Astrea pallida 
Pennatula sulcata 
Tubipora musica 
Edwardsia claparédii 
Cirripathes anguina . 
Fenja mirabilis . 
Minyas : 
Alcyonium palmatum 
Gorgonia verrucosa 
Structure of simple coral 
Dendrophyllia and Madrepora 
Adamasia palliata 
Hormiphora plumosa ‘ : 
5 <9 dissection and transverse section 
a 3 diagrammatic sections 
5 aig section of branch of tentacle 
ee sense-organ . 
( Dvum af Lampetia 
Segmentation of vosperm in Cishophans 
Development of Ctenophora 
Development of Callianira : : 
” ” (later stages) 
Three Cydippida 
Deiopea kaloknenota 
Cestus veneris 
Berée forskalii 
Ctenoplana kowalevskii : 
Sections of embryos of Actinia sini Berve . 


Diagram illustrating the mutual relationships of the Cuslenterata 


Dicyema paradoxum with infusoriform embryos 
ys ” >» Vermiform 3 


PAaGk 
174 
178 
178 
179 
180 
181 
183 
184 
186 
188 
190 
190 
191 
195 
197 
197 
198 
198 
199 
199 
200 
201 
202 

202 
205 

204 

206 

207 
209 

211 

212 
214 

215 
216 
217 
218 
218 
218 
219 
222 
223 
223 
224 
225 
228 
229 
230 
230 


Fic. 

Wi 
178. 
179. 
180. 
181. 
182. 
183. 
184. 
185. 
186. 
187. 
188. 
189. 
190. 
191. 


192. 
193. 


194. 
195. 
196. 
197. 
198. 


199. 
200. 
201. 


202. 


203. 


204. 
205. 
206. 
207. 
208. 

209. 
210. 
211. 
212. 
213, 
214. 

215. 

216. 
217. 
218. 
219. 
220. 
221, 
222. 
223. 


224.. 


LIST OF ILLUSTRATIONS 


. Dicyema paradoxum, male 


Rhopalura giardii, male . 
» female 
Salinella, longitudinal section 


a5 transverse sig 

Planaria, digestive and excretory systems . 
sa nervous system 
3 reproductive system 


Transverse section of a Planarian . 
Distomum hepaticum ‘ 
iy 9 section of integument 


es i internal organisation 
= a terminal part of reproductive apaeatue 
3 - development 
Tenia solium 

~ », head 

ws ;, transverse section 

" 55  proglottis 

i » Tipe proglottis 

i » development 


Various Planarians 
Gunda segmentata 
Digenetic Trematodes 
Gyrodactylus and Polystomum 
Temnocephala 
Actinodactylella 
Tetrarhynchus 
Tenia echinococeus . 
Ligula . . 
Caryophylleus 
Gyrocotyle . 
Archigetes . . ; 
Section of body-wall ‘ol a Tr felad 
Parenchyma of Flat-worm 
Diagram of Rhabdoceele . 
. +, Polyclad . 
a », Triclad 
Flame-cell . 
Reproductive organs of Mesostomum ehrenbergii 
Development of a Polyclad 
Miiller’s larva 
Embryos of Dendroccelum 
Embryo of Temnocephala 
cE ” 
ACysticercoid . 2. 2... 
i with head evaginated 
Cyst of Tenia echinococcus 
Scolices ,, et 


he fo be iS be 


WS fH re tS 


re. te 


She ws 


LIST OF TLLUSTRATIONS 


225. Seolex of Tenia echinococcus 
3. Process of budding in Microstomum : 
. Diagram of the relationships of the Pliehelminthes ‘en Nemer- 


tinea 

. Diagram of N Serieviiie 

. Proboscis of Nemertine 

. Tetrastemima : 

. Anterior portion of Nemertine —. ‘ 
Proboscis of Hoplonemertean, retracted 

5 $3 5 everted 

. Transverse section of Nemertine 


5. Vascular and excretory systems of Beniwrkine 
36. Pilidium 


. Ascaris lumbricoides ; : 
+3 i transverse section 

55 muscle tibres 

hs es dissection of female 

. Nervous system of Nematoda : 
. Ascaris lumbricoides, dissection of male organs 


” 


3. Body-wall of platymyarian Nematode 


. Dochmius duodenalis r 
5. Transverse section of Gordius 
3. Oxyuris 


7. Gordius, paatany 


. Development of Ascaris nigrovenosa 

. Trichinella spiralis ; 

. Two species of Echinorhynchus (Gigantor lenichiues 
. Echinorhynchus gigas, dissection of male 


ii 7 re female 
as 33 Pe nephridia 
3 female organs 


dD. Sawin ‘helinten 


»,  bipunctata, transverse S Racitnaie 
+5 = head 

», hexaptera, eye 

. Development of Sagitta 


260. Cheetosoma 
31. Echinoderes 
32. Desmoscolex 
33. A trochophorc 


. Brachionus rubens, female 
33 33 pharynx . 
ay a male and female, witht acon re 


7. Diagram of a Rotifer 
iS, Pusasclanii asplanchnus 
9. Typical forms of Rotifera 


” ” ” 


” a mastax , fi . A ‘ 


PAQr 
282 
283 


287 
289 
290 
291 
292 
292 
292 
293 
293 
294 
298 
299 
300 
301 
302 
308 
305 
306 
306 
307 
308 
309 
311 
313 
314 
314 
315 
315 
316 
317 
317 
318 
318 
319 
320 
320 
322 
324 
325 
326 
327 
329 
331 
332 


333 


FIG. 
272. 
273. 
vv. 
275. 
270. 


ore 


mid. 
278. 


279, 


280. 
281. 
28y. 
283. 
284. 
285. 
286. 
287. 


288. 


289, 


290, 


291. 
292, 
293. 
294. 


295, 


296. 


297. 
298. 
299. 
300. 
301. 


302. 
303. 
304. 
305. 


306. 
307. 
308. 


309. 


310. 


311. 
312, 


313. 
314, 


315. 
316. 
317. 
318. 
319. 


LIST OF ILLUSTRATIONS 


Cheetonotus maximus , 
aa anatomy 
Dinephilus teeniatus . 
Stratiodrilus tasmanicus 
Bugula avicularia 
Development of Bugula 
” x 
Larva of Bugula 
Plumatella . 
Cristatella 
Lophopus 
Pedicellina . 
Phoronis australis 
se + free end 
oe 8 internal organisation 
a a section 
sip development 
Magellania flavescens, shell 
me lenticularis, anatomy . 
5 flavescens, lophophore 
sn muscular system . 
Terebratula, nervous system, &e. 
Typical Brachiopods 
a anatomy 
Tlevalopaient of Cistella . 
Larva of Cistella 
Development of Cistella . 
Lophophore of embryo Brachivpod 


Diagrams of oe Polyzoon and Phaiwnts 


Startish, oral aspect 


ss vertical section of arm 
#5 ambulacral system 

Starfish, portion of vertical section of arm 
"3 diagrammatic sections 


Asterias rubens, digestive system . 
Astropecten, section of stone-canal 
Anthenea flavescens, dissection from dorsal aapoct 
Asterias rubens, structure ‘ 
Anthenea flavescens, lateral disenation, 
dorsal surface 
$5 ventral surface 
Asterina gibbosa, development 


9 *Y 


M9 ” bed 
ah = larva 


ad ” ¥3 

35 exigua, young duties nammepliicis 
Asterina gibbosa, development 
Apical system of young Starfish 


“xvi LIST OF ILLUSTRATIONS 


FIG. PAGE 
320. Echinus esculentus, peristome  .  . B94 
321. Strongylocentrotus .. » 395 
3zz. Corona of Sea-urchin 396 
323. Apical dise of Sea-urchin 397 
324. Echinus, lantern of Aristotle . 897 
325. Sea-urchin, anatomy, lateral view ; . 398 
326. Echinoid, transverse section of ambulacral zone ; , 399 
327. Sea-urchin, anatomy, oral view. ‘ j 400 
328. Cucumaria planci —. fl ok is. ty . 401 
329, Anatomy of « Holothurian — , : . 403 
330. Antedon —. ; F . 405 
331. Aboral view of Antedon ‘ : . 406 
332. Antedon dise .  .  .. . + 406 
333. is transverse section of pinnule ' F . 407 
334, ss sagittal section . ‘ ; ; . 408 
335. Anthenea, ventral view : 419 
336. Ophioglypha lacertosa : , 420 
337. Astrophyton arborescens . 421 
338. Diagram of spine of Sea-urchin F : . 422 
339. Pedicellaria of Arbacia punctulata 5 : . 422 
340. Hemipneustes radiatus . . : : . 423 
341. Clypeaster sub-depressus se 428 
342. Metacrinus interruptus. . 424 
343. Development of Echinoderms . 431 
344. 6 » Antedon : & 4 .  . 432 
345, Diagram to illustrate the relationships of the classes of Echino- 
dermata . .  . : ae: , : ‘ . 433 
346. Nereis dumerilii 3 437 
B47. 4 “ parapodium 440 
348. 33 a sete. ‘ 441 
349, Nereis diversicolor, proboscis 443 
350. Nereis dumerilii, anatomy 444 
351. 5 % transverse section 445 
352. Pe ‘5 nervous system . 446 
353 ‘a ne eye S. 4 447 
Bod, i i dumerilii, nephridium . 448 
355. 5 = development —, ; 451 
356. is rr i“ .. +% 453 
357. Lumbricus herculeus : . 454 
358. a setze . ; j : : . 455 
359. a transverse section , . , ‘ . 456 
360. i herculeus, sagittal section . c ¢ w 457 
361. ee nervous system, . ; , . . 459 
362. 5 nephridium : : : . 460 
363. a reproductive organs . : 462 
364. 3 development . ; : . 463 
365. Polynde setosissima. 467 


366. Vermilia ccespitosa . 468 


aw ws 


wow te te 
TIT aT AT 


i OS 
fort 


eo) 
fo oie | 


ww oD 
wm ws 
Re Oe 


FC OR WI 


LIST OF ILLUSTRATIONS 


7. Chastopterus 
. Sete of various Polyelietn 
. Section of setigerous sac of an Oligochiete 


Polynée extenuata, anterior end 


. Polychzta, various, heads 

. Tubifex 

. Terebella 

. Aphrodita, enteric canal 

. Saccocirrus, transverse section 

. Phyllodoce, nephridium . 

. Nephridia and ccelomoducts 

. Diagram illustrating development. of gonad of Polychieta 
; Sotrorbis levis. 

. Eupomatus, development of Se ee 

. Autolytus cornutus, budding 

382. 
383. 
384. 
385. 
386. 
387. 
388. 
389, 
390. 
391. 
392. 
393. 
394. 
395. 
396. 
397. 
398. 
399. 
400. 
401. 
402. 
403. 
404, 
405. 
406. 
407. 
408. 
409. 
410. 
411. 
412. 
413. T 
414, 


Syllis ramosa 
Serpule with their tubes 
Myzostomum 

a anatomy . 
Sipunculus nudus, anterior extremity . 

ae “9 tentacular fold 

a5 aS anatomy 

4) Nervous system 
Pgielts viridis, female 
Echiurus 
Priapulus 
Bonellia, anatomy 
Echiurus, ciliated funnel 
3 anatomy 
sf nervous system 

Bonellia, male 
Echiurus, trochophore 
Polygordius neapolitanus 
Protodrilus 
Polygordius neapolitanus, transverse section 
trochophore 


” ” 


fs 5 later stage 


Gwuds medicinalis 


” 7 transverse section 


” ” jaw a 

», australis, dissection dean dorsal aspect 

» australis, 9 » left side 

»,  Medicinalis, nephridium . 3 : 


diagram of blood-channels 
section of eye 
cocoon . 
Three Rhynchobdellida 
Proboscis, of Clepsine 


exviii LIST OF ILLUSTRATIONS 


FIG. 


415. 
416. 
417. 
418. 
419. 


420, 
421. 


422 


Nephridium of Herpohdella 
Pontobdella, nephridial system 
Clepsine, development: : 
Diagram of origin of metamerism . 


Diagram illustrating the relationships of the Amnulata and 


Trochelminthes . 
Apus cancriformis, dorsal epee 
Lepidurus kirkii, side view 


2. Apus glacialis, ventral aspect 
423. 
424, 
425. 
426. 
427. 
428. 
429. 
430. 
431. 
432. 
433. 
434. 
435. 
436. 
437. 
438. 
459. 
440. 
441. 
442. 
443. 
444, 
445. 
446. 
447. 
448. 
449. 
450. 
451. 
452. 
453. 
454. 
455. 
456. 
457. 
458. 
459, 
460. 
461. 


+, appendages : 
Lepidurus kirkii, agiteal: section : 
Apus, transverse section 
», Shell-gland 
», cancriformis, nervous system 
», structure of paired eye 
», development 
Astacus fluviatilis, male . 
‘6 5% appendages : 
= a articulations and inuscles of es 
Section of skin and exoskeleton of Lobster 


Articulations and muscles of abdomen of Crayfish 
Astacus fluviatilis, dissection from right side 
56 44 gills 
is - kidney 
aa a transverse section of thorax . 
ste ie diagram of circulation 
a5 a nervous system 
3 Wns reproductive organs ' 
a ee formation of the blastoderm , 
is 4 early embryo 
” ie nauplius 
Aa Pe section of embryo 


advanced embryo 

Thre ee Bri shiclanpeda. 

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


539 


eo og 
wows: 
ole 


LIST OF ILLUSTRATIONS 


» Asellus 
. Amphipoda 
. Isopoda . 
. Shrimp and Prawn 
5. Scyllarus arctus 
. Pagurus bernhardus 
8. Cancer pagurus . 
. Typical Brachyura 
. Squilla : 
. Orchestia cavimana, anmbony 
72. Euphausia pellucida . 
3. Nervous system of Crab 
. Cypris-stage of Lepas 
5. Larvee of Crabs . 
. Diagram illustrating the mutual relationships “ot the orders at 


Crustacea . 


. Dalmanites and Phacops 
. Triarthrus beckii 
. Peripatus capensis 


35 a3 ventral view of head 
anatomy 

» tracheal pit 

»  nephridium . 

> hove zealandie, development 
» capensis 


. Scolopendrella immaculata 

. Scolopendra F 

. Lithobius forficatus . 

. Pauropus huxleyi 

. Strongylostoma, developrasnt. 
. Periplaneta americana 


oe mouth-parts . : 
= americana, lateral view of inaad 
<5 muscular system . 

ea anatomy 


salivary glands 


; Tisohes of caterpillar 
. Periplaneta, tracheal system . 


o nervous system : 
3 male reproductive organs . 
43 female reproductive organs 


. Segmentation of ovum of Insect 

. Ventral plate of embryo Cockroach 
. Embryo Cockroach 

. Lepisma 

. Podura 

. Locusta 

. Ephemera . 


LIST OF ILLUSTRATIONS 


. Aphis rose 


Cicada . 


. Culex and larva 

. Gastrophilus equi 

. Pieris 

. Crioceris 

. Section of integument of Taseet 
. Mouth-parts of Honey-bee 


ne », Diptera 
35 », Lepidoptera 


9. Digestive organs of Beetle 

. Nervous, tracheal, and digestive systems of the Honey-bee 
. Tracheal gills of Ephemerid 

. Heart of Cockchafer . 

. Nervous system of Diptera 

. Ocallus of Dytiscus larva 

. Chordotenal organ of Isopteryx 

3. Sexual apparatus of Honey-bee 

. Segmentation of ovum of Insect 

. Germinal layers and amnion of Insect . 
9. Development of Hydrophilus . 


” 


. Apis relies, queen, weukon. and annie 

. Formica rufa 

. Euscorpio 

. Ventral surface of cephalothorax and pre- sndonie of Scorpion 
. Endosternite of Scorpion 

3. Scorpion, anatomy, lateral view 


i i dorsal, 
development 


. Embryo of Scorpion 
. Chelifer bravaisii 

. Phrynus 

. Galeodes dastaguei 
3. Epeira diadenia d 


as », chelicere and pedipalpi of female 
” ” » male 


. Sarcoptes eatlinet 
. Trombidium fuliginosum . 
. Limulus 


a ventral view 


. Eurypterus fischeri . 

. Anatomy of dipneumonous Spider 
. Limulus, sayittal section 

3. Lung-book of spider 

. Tracheal system of Spider 

. Gill-books of Limulus 

. Lateral eye of Euscorpius 

. Central eye of Kuscorpius 


PAGE 


633 - 


634 
634 
634 
635 
635 
636 
637 
638 
639 
641 
642 
643 
643 
644 
645 
645 
646 
648 
649 
650 
650 
652 
652 
654 
654 
655 
657 
658 
659 
659 
662 
663 
663 
664 
664 
664 
665 
665 
666 
667 
668 
669 
670 
670 
670 
671 
671 
672 


bt oe he Ot Or se Ge Se 
Noe aS aes aS aS aS as S| 


Co ee 


LIST OF ILLUSTRATIONS 


. Nymphon hispidum . 

9. Pentastomum teenioides 

. Macrobiotus hufelandi , ‘4 

. Diagram to illustrate aftinities of Aiehnpods 


. Anodonta cygnea ; 1 oe A aa en eee: 
is »» interior of valve and avivial removed from shell . 
3 section of shell and mantle . ‘ 

- vygnea, animal after removal of mantle- lobe 
», dissection from left side 

a », structure of gills 

as », transverse sections 

A diayram of circulation 

a statocyst . 

si early embryu 

sy later embryos . 

~ advanced embryo 

ie metamorphosis 


. Anatomy of Pecten . 

. Valves of Mya, Modiola, and Velleclle 

. Cardium edule 

. Venus gnidia 

. Serobicularia piperata 

. Solecurtus strigillatus 

. Diagram of concrescence of mantle-lobes 
. Requienia and Hippurites 

. Teredo navalis 

. Aspergillum 

. Mytilus edulis 

3. Nucula delphinodonta 

. Gills of Pelecypoda 

. Gill-tilaments of Mytilus 

. Dissection of Poromya 

. Donax, enteric canal 5 

. Nervous system and auditory organs of N eeain:, 
. Eye of Pecten 

. Development of Ustrea 

. Veliger of Ostrea 

. Embryos of Cyclas . 

. Diagram illustrating the mutual ielshiuushipe of the Belkeypoda 
. Chetoderma nitidulum 

. Neomenia carinata . 

. Chiton, spinosus, dorsal view 


» ventral view 
5 valves of shell 


. Chetoderma nitidulum, longitudinal acetien 
. Chiton, longitudinal section 

. Nervous system of Amphineura 

. Neomenia carinata, reproductive organs 

3. Chiton, nephridijal and genital systems 


XXXI 


TAGE 
674 
674 
675 
678 
681 
682 
683 
685 
686 
687 
688 
690 
691 
692 
692 
693 
694 
697 
698 
698 
699 
699 
700 
700 
TOL 
701 
702 
702 
703 
704 
705 
705 
706 
707 
708 
709 
709 
710 
712 
713 
714 
714 
714 
71a 
716 
717 
717 
718 
719 


EXIM LIST OF ILLUSTRATIONS 


FIG, 


607. 
608. 
609, 
610. 
611. 
612. 
613. 
614. 
615. 
616. 
617. 
618. 
619. 
620. 
621. 
622. 
623. 
624. 
625. 
626. 
627. 
628. 
629. 
630. 
631. 
632. 
633. 
634. 
635. 
636. 
637. 
638. 
639. 
640. 
641. 
642. 
643. 
644. 
645. 
646, 
647. 
648. 
649. 
650. 
651. 
652. 
653. 
654. 
655. 


Chitun, development 
Triton nodiferus, shell 
Triton nodiferus, shell, median section 
operculum . 
lateral view of body 
diagram of introvert 
dissection from dorsal side 
buccal mass 
vertical section of yucca cavity 
nervous system from dorsal side 
” bel > ” 
a6 section of eye 
Diagrams ef digplacement of mantle-cavity, &c. 
Solarium perspectivum 
Terebra oculata 
Cypraea moneta . 
Doris tuberculata 
Carinaria mediterranea 
Limax . 
Sigaretus levigatus 
Aplysia 
Shell-bearing Peedi 
Atlanta peronii . 
Pterotrachea scutata 
Helix nemoralis 
Pleurophyllidia lineata 
Patella vulgata . : 
Pulmonary cavity and related parts in Limax 
Nervous system of Patella 
Nervous system of Aplysia 
de » Limneus . 
Eyes of Gastropoda 
Osphradium of Murex 
Reproductive organs of Helix 
Hermaphrodite gland of Gastropoda 
Forms of egg-cases in Gastropoda , 
Segmentation and formation of germinal Deen in acount 
Early development of Patella 
‘Trochophore of Patella 
Later trochophore of Patella 
Veliger of Vermetus 
Diagram illustrating the pelitionships of the Gastropoda 
Dentalium, section of shell 


- anatomy 
” larvee 
Rhodope 


Sepia cultrata 
Sepia, cultrata, shell 
agro chromatophore xt 


and related parts, lateral view . 


FIG. 


656. 
657. 
658. 
659. 
660. 
661. 
662. 
663. 
664, 
665. 
666. 
667. 
668. 
669. 
670. 
671. 
672. 
673. 
674. 
675. 


676. 


677. 
678. 
679. 
680. 
681. 
682. 
683. 
684. 
685. 
686. 
687. 
688. 
689. 
690. 
691. 
692. 
693. 
694. 
695. 
696. 
697. 
698. 
699. 
700. 
701. 
702. 
703. 


704. 


LIST OF ILLUSTRATIONS 


Sepia, cultrata, cranial cartilage 
56 i nuchal cartilage 
fe a mantle-cavity 
», officinalis, jaws 
»> section of buccal mass . 
», officinalis, enteric canal ‘ 
»,  cultrata, dissection of male from pnstedian aspect 
9 5 lateral dissection of male 
»,  Officinalis, longitudinal section of ink-sac 
»,  cultrata, vascular system 
34 ya cephalic ganglia ; 
“6 ia pedal and pleuro-visceral guiglia 
>, section of eye 
>,  cultrata, statolith . 
>,  Officinalis, renal organs 
an 55 diagrammatic sagittal aeobian ae femule.. 
»» male reproductive organs 
>> | Sperms and spermatophore 
Nautilus pompilius, section of shell 
5 female in shell 
Nautilus macromphalus, entire animal . 
Nautilus pompilius, lobe of foot 
5 3 spadix 
a 45 cephalic cartilage . 
mantle-cavity of male . : 
a dissection of male from left side 
i = arteries 
renal sacs, sentdin, Se. 
male reproductive organs 
female 9 i 
“3 minoramiphinluy; egs . 
Octopus vulgaris 
Loligo vulgaris 
Argonauta argo . 
Octopus lentus, male 
Amphitretus pelagicus 
Shell of Spirula . 
Spirula peronii . 
Ammonite . 
Shell of Belemnite 
a Argonauta argo . 
Segmentation of Loligo 
Blastoderm of Sepia 
», sections . 
Development of Loligo 


” ” 


ed 


”” ” 


” ” 


Dae to ‘liustrate the relationships of the Cephalopoda 


VOL, I 


XXNUi 


PAGE 
763 
763 
764 
765 
766 
766 
167 
768 
769 


CLASSIFICATION OF THE ANIMAL KINGDOM 
IN THIS BOOK. 


KINGDOM ANIMALIA. 


Paytrm I. PROTOZOA. 


Class I. RHIZOPODA. Order 4. CysToFLAGELLATA. 
Order 1. Loposa. Class IV. SPOROZOA. 
>, 2. FORAMINIFERA. Order 1. GREGARINIDA. 
>, 3. HELIOZOA. », 2. CoccrpiIpBA. 
sae aS RaDIOLARIA. 5, -3. HAMOSPORIDIA. 
Class II. MYCETOZOA. » 4. MyYXOSPORIDEA. 
Class III. MASTIGOPHORA. 5) 0. SARCOCYSTIDEA. 
Order 1. FLAGELLATA. Class V. INFUSORIA. 
3, 2. CHOANOFLAGELLATS. Order 1, CrLrava. 
>, 3 DINOFLAGELLATA. > 2. TENTACULIFERA. 


Puyittum Il. PORIFERA. 


Class PORIFERA. Order 2. Hrterocara. 
Sub-class I. Calcarea. Sub-class II. Hexactinellida. 
Order 1. Homocana. a IIT. Demospongia. 


Puytum TII. CAfALENTERATA. 


Class I. HYDROZOA. Sub-class I. Zoantharia. 
Order 1. LEPToLin x. Order 1. ACTINIARIA. 
Sub-order a. Anthomeduse, »> 2 MADREPORARIA, 
&n b, Leptomeduser, ., 3. ANTIPATHARIA, 
Order 2. TRACHYLIN-E. Sub-class IT. Aleyonaria. 
Class II. SCYPHOZOA. Sub-order «. Trachymedusu. 
Order 1. STAUROMEDUS.E. ie b. Narcomedusr, 
>, 2 Coronata, Order 3. HyDROCORALLINA. 
» 3 CUBOMEDUSE. >, + SIPHONOPHORA, 
» 4 DiscomEDUs£. >> 9 GRAPTOLITHIDA. 
Sub-order a. Semostome. » 4 ALCYONACEA. 
= b. Rhizostome, +) Oe GORGONACEA, 
Class III. ACTINOZOA. > 6. PENNATULACEA, 


VOL, I cm 


XXV1 


Puyium Il. 


Class IV. CTENOPHORA. 
Order 1. Cypippipa. 

>, 2. LosBata. 

> 3 CESTIDA, 

>, + Rerory. 


Puyiem IV. 


Class I. TURBELLARIA. 
Order 1. PoLtyenLabIpa. 
» 2 TRICLADIDA. 
» 38 REHABDOCELIDA, 


Class II]. TREMATODA. 
Order 1. MoxoukNetica. 
») 2 DIGENETICA. 


Puyirm V. 


Class I. NEMATODA. 
Order 1. NEMATOIDEA. 
>» 2. NEMATOMORPHA. 


Class II. ACANTHOCEPHALA. 


Puvirm VI. 


Class I. ROTIFLRA. 
Order 1. Ruizor., 
» 2 BdELLOIDA, 
152 Be EBON, 
Sub-order a. IMoricatau, 
ee bh. Lovicata, 


Order 4. ScIRTOPODA. 


Puyitm VII. 


Class I. POLYZOA. 
Sub-class I. Ectoprocta. 
Order 1. GYMNOL“MATA. 


Sub-order a. Cyclostomata. 


” b. 


” c. 


Pryium VIII. 


Class I. ASTEROIDEA. 
Order 1. PHANEROZONIA, 
> 2. CRYPTOZONIA. 
Class 1I. OPHIUROIDEA. 
Order 1. Lysopmiviz. 
» 2 STREPTOPHICE. 


Chetlostomutu, 
Clenostomata, 


CLASSIFICATION OF THE ANIMAL KINGDOM 


CURKLENTERATA—continued, 


Appendix to Ctenophora—Clenoplany 
and Cteloplana. 
Order 1. PLatycTENEa. 


Appendix (II) to Ciwlenterata—Jlesozoa. 


PLATYHELMINTHES. 


Order 3. ASPIDOCOTYLEA. 
» 4 TEMNOCEPHALEA. 
Class III. CESTODA. 
Order 1. Moxozoa. 
4 fe POINZOA. 


Appendix to Platyhelminthes—Class 
NEMERTINEA. 


NEMATHELMINTHES. 
Class III. CHAETOGNATHA. 


Appendix to Nemathelminthes—Chw- 
tosomidiv, Hehinodarvida, and Desmos- 
colecidir, 


TROCHELMINTHES. 


Order 5. TRoCHOSPH.ERIDA. 
> 6 SELSONIDA, 


Class II. GASTROTRICHA. 


Appendix to Trochelminthes—Dino- 
philew and Histriohdellea, 


MOLLUSCOIDA. 


Order 2. PHyLacroLaMAta, 
Sub-class II. Endoprocta. 
Class II]. PHORONIDA. 
» III. BRACHIOPODA. 
Order 1. INarticuLata. 
>> «2 ARTICULATA, 


ECHINODERMATA. 


Order 3. ChapoPHivin. 
»> 4&4 ZYGOPHIURE. 
Class III. ECHINOIDEA. 
Order 1. REecunartra. 
+ 2. CLYPEASTRIDEA. 
>, 38. SPATANGOIDEA. 


CLASSIFICATION OF THE ANIMAL KINGDOM 


Puytum VIII. 


Class IV. HOLOTHUROIDEA. 
Order 1, Enasreopa. 
» 2 PeDATA, 
» & APODA. 
Class V. CRINOIDEA. 
Sub-class I, Monocyclica. 


Pirytcm IX. 


Class I. CHETOPODA. 
Sub-class I. Polycheta. 
Order 1. ARcHI-CHATOPODA. 
> 2. PHANEROCEPHALA, 
5, 3. CRYPTOCEPHALA. 
Sub-class II. Oligocheta. 
Order 1, MicropRti. 
>> 2 MEGADRILI. 


Appendix to the Chetopoda—Class 


MYZOSTOMIDA. 


ECHINODERMATA—continued. 


Sub-class II. Dicyelica. 
Class VI. CYSTOIDEA,. 

», WII. BLASTOIDEA. 

» VIII. EDRIASTEROIDEA. 

» IX. CARPOIDEA, 


ANNULATA. / Mey Kapit © 


Class II. @EPHYREA. 
Order 1. Inermta. 
>» 2 ARMATA, 


Class III. ARCHI-ANNELIDA. 


», IV. HIRUDINEA. 

Order 1. RHYNCHOBDELLIDA. 
» 2. ARHYNCHOBDELLIDA. 
Sub-order 1. Gnathobdellida. 

oe 2. Herpohdellida. 


Paytum X. ARTHRORODA. 


Class I. CRUSTACEA. 
Sub-class I. Branchiopoda. 
Order 1. ANoSTRACA. 
» 2 Norostrac.. 
>, 938. CONCHOSTRACA. 
», + CLADOGERA, 
Sub-class II. Ostracoda. 
5, IIL. Copepoda. 
Order 1. Evcoprpopa. 
3») 2 BRANCHIURA. 
Sub-class IV. Cirripedia. 
Order 1. EuctrrIpEDIA. 
>> 2 RAIZOCEPHALA. 
Sub-class V. Malacostraca. 
Order 1. MysipacEa. 
2. CUMACEA. 
> 3. TANAIDACEA. 
4. Isopopa. 
3) oO. AMPHIPODA,. 
Sub-order 1. Macrura. 
me 2. Anomura, 
54 3. Brachyura. 
Appendix to Crustacea—Class 
LOBITA. ‘ 
Class II. ONYCHOPHORA. 
», II]. MYRIAPODA. 
Sub-class I. Progoneata, 


Order 1. Pavroropa. 
>» 2. Dreropopa. 
>», 93 SyYMPHYLA. 
Sub-class II. Opisthogoneata. 
Order 1. CHILOPoDA. 
Class IV. Insecta. (| ;+/. 
Order 1. ApTERa. 
. ORTHOPTERA. 
. NEUROPTERA. 
. HEMIPTERA. 
Diprrera, 
LepIpoprERa. 
. COLEOPTERA, 
- . HyMrENoprERA. 
Class V. ARACHNIDA. 
Order 1. Scorpronrpa. 
5, 2. PSEUDOSCORPIONIDA. 
. PEDIPALPIDA. 
. SOLPUGIDA. 
. PHALANGIDA. 
ARANEIDA, 
. ACARIDA, 
. NIPHOSURA. 
. EKURYPTERIDA. 
to the 


ee 8) 


TRI- 5a 


” 


Appendix 


OHOMTP HE we 


DIGRADA, 


XXxvii 


Arachnida—-The 
Pycxoconipa, Lincuatubipa, and Tar- 


XXXVili 


Puytum NI. 


Class I. PELECYPODA. 
Order 1, PRoTOBRANCHIA. 
2. FILIBRANCHTA. 


»” 


” 
> + EUChAMELLIBRANCHIA, 


Sub-order «a. Infeyripalliata. 
v5 db. Strupalliata. 
Order 5. SEPTIBRANCHIA,. 
Class Il. AMPHINEURA. 
Order 1. PLacopHora,. 
> 2. APLACOPHORA. 
Class III. GASTROPODA. 
Sub-class L. Streptoneura. 
Order 1. AsvIDOBRANCHTA. 
Sub-order 1. Docoglossa. 
16 2. Rhipidoylosse, 


Pavium XII. 
Sub-pHyLuM I. ADELOCHORDA. 
Class ADELOCHORDA. 
Sun-pHyLum II. UROCHORDA. 


Class UROCHORDA. 
Order 1. Larvaces. 
so 2s AABIACHS, 
Sub-order «. Cyclomyurii. 
se b, Hemimyaria. 
3 .. Pyrosomata, 
Order 3. ASCIDIACEA. 
Sub-order a. lscidi simplices, 
i hb. Aseidie composite, 


Sup-pHyLum IIT. EUCHORDA. 
Section I. Acrania. 


Section II. Craniata. 


Class I. CYCLOSTOMATA. 
Order 1. PETROMYZONTES. 
>» 2 Myninorpet. 


‘Class II. PISCES. 


Sub-class I. Elasmobranchii. 
Order 1, CLADOSELACHII. 
» 2 PLEURACANTIET. 
> 3 ACANTITODET, 
>> F SELACTIT, 


3. PskUDO-LAMELLIBRANCHIA, 


CLASSIFICATION OF THE ANIMAL KINGDOM 


MOLLUSCA. 


Order 2. PrcTINIBRANCHIA,. 
Sub-order 1. Platypoda. 
4 2. Heteropoda. 
Sub-class II. Euthyneura. 
Order 1. OristHOBRANCHIA. 
Sub-order 1. Teetthranchia, 
és 2. Nudibranchia. 
Order 2. PULMONATA. 


Appendix to the Gastropoda—Class 


SCAPHODA and RHODOPE. 


Class V. CEPHALAPODA. 
Sub-class I. Dibranchiata. 
Order 1. Decapopa. 
>> 2. Ocropopa. 
Sub-class II. Tetrabranchiata. 


CHORDATA. 


Sub-order «. Protoselachii. 
2a b. Buselachii. 
Section a. Squalide, 
os B. Rajida. 


Sub-class II. Holocephali. 
», III. Teleostomi. 


Order 1. ChossopreryGit. 
>» 2. CHONDROSTEL. 
>» 3. HoLostet. 
> + TELEOSTEI. 
Sub-order a. Physostomi. 


an b. Anacauthini. 

ae ev. Acanthopter?. 

en d. Pharyngoguathi. 
“a ce. Plectognathi. 

a J. Lophobranchii. 


Sub-class IV. Dipnoi. 
Order 1. MoNoPNEUMONA. 
» 2. Dipnecuona. 


Appendix to Pisces—The Ostracodermi. 


Class III]. AMPHIBIA. 
Order 1. UnopEna. 
jy. eo SUR AL 
>, 3. GYMNOPHIONA. 
.5 + STEGOCEPHALA, 


CLASSIFICATION OF THE ANIMAL KINGDOM 


Puytum XII. 


Class IV. REPTILIA. 


Order 1. Sguamatva. 
Sub-order a. Lacertilia. 


Order 2. 


” 


” 


b. Ophidia. 


c. Pythonomorpha. 
RHYNCHOCEPHALIA. 
. CHELONIA. 
. THEROMORPHA. 
. CROCODILIA. 
. SAUROPTERYGIA, 
. ICHTHYOSAURIA. 
. DINoOsAURIA. 
. PrEROSAURIA. 


Class V. AVES. 


Sub-class I. Archzornithes. 
Sub-class II. Neornithes. 
Division A. Ratite. 


Order 1. 
. APTERYGES. 
. DINORNITHES, 


” 


Division B. 
Order 1. 

. ODONTOLC.E. 

. ICHTHYORNITHES. 


MEGISTANES. 


RHE. 


. STRUTHIONES, 
. AE PYORNITHES, 
. GASTORNITHES. 


Carinate. 
STEREORNITHEs. 


PycGopopEs. 


. IMPENNES. 

. TURBINARES. 
. STEGANOPODES. 
. HERODIONES. 
. ANSERES. 

. ACCIPITRES. 
. CRYPTURI. 

. GALLIN®. 

. GRALLE. 

. GAVIA. 

. LimicoLa. 


XXXIX 


CHORDATA—continued. 


Order 16. PrEROULETES. 
> 17. CoLuMBA, 
» IS. Psrrract. 
», 19. STRICES. 
» 20. Prcarra. 
.» 2L. PAsSSERES. 


Class VI. MAMMALIA. 


Sub-class I. Prototheria. 
% II. Theria. 
Section A. Metatheria (MiRsupPIArta). 
Order 1. PoLypROTODONTIA. 
35 2. DIPROTODONTIA. 
Section B. Eutheria. 
Order 1. EprntTata. 
» 2. CETacka. 
Sub-order «. Mystucoceti. 
ah b. Odontoceti 
Order 3. SIRENIA. 
> 4 Uneunata. 
Section 1. Ungulata vera. 
Sub-order a. Perissodactyla. 
5 b. Artiodactyla. 
Section 2. Subungulata. 
Sub-order a. Hyracoideu. 
Pe b. Proboscidea. 
Order 5. CARNIVORA. 
Sub-order a. Carnivora vera. 
39 b. Pinnepedia. 
Order 6. RoDENTIA. 
»,» 7. INSECTIVORA. 
>, 8. CHIROPTERA. 
Sub-order a. Megachiroptera. 
or b. Microchiroptera. 
Order 9. PRIMATES. 
Sub-order a. Prosimit. 
3 b. Anthropoiden 


LOOLOGY 


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 Linnzeus. 

The DomesticCat, 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 
Linneus, each kind of animal receives two names—one the generic 


g B 


9 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 /. pardus, 
the Tiger F. tigris, the Lion /. 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. Jndividual 
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 varicties 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.c. by the 
study of the changes undergone by animals in their develop- 
ment from the egg to the adult condition. A striking instance 1s 
afforded by the common Barnacles which grow in great numbers on 
ships’ bottoms, piers, &c. The older zoologists, such as Linnzus, 
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 Linneus, 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 Fvlis. 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, Cynwlurus, 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 Hyznas, but the presence 
of additional teeth and of non-retractile claws—to inention only 
two points—makes the interval between Hyenas and the two 
genera of Cats far greater than that between Felis and Cynelurus. 
The varying degree of difference is expressed in classification by 
placing the Hyznas in a separate family, the Hywnide, while 
Felis and Cynzlurus are placed together in the family Felidae. 
Similarly, the Civets and Mongooses form the family Viverride ; 
the Dogs, Wolves, Jackals, Foxes, &c., the family Canide ; Bears, 
the family Urside ; and so on. 

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

Carnivora, Ungulata, Rodentia, Marsupialia, Wc, 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 Aves. 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 ; aud 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 differecnces—far greater than those be- 
tween classes—are expressed by placing the backboned animals 
in the phylum or sub-kingdom Chodi/a, the many-legged, 
armoured forms in the phylum Arthrapada, Similarly, soft-bodied 
animals with shells, such as Oysters and Snails, form the phylum 
Mollusea, Polypes and Jelly-fishes the phylum Coclenterala. 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—CuorData. 
Class—MAMMALIA. 
Order—CaRNIVORA. 
Family—Felidee. 
Genus—frlis. 
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 glassina 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 
iecepted 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 
sy a tree the whole of the animals which have ever lived on the 
2arth, those existing at the present day would be figured by the 
“opmost twigs, the trunk and main branches representing extinct 
orms. Thus the task of arranging animals according to their 
elationships would be an almost hopeless one but for two 
zircumstances : one, that remains of many extinct forms have been 
preserved ; the other, that the series of changes undergone by an 
wnimal in its development from the egg often forms an epitome of 
she changes by which, in the course of ages, it has been evolved 
rom an ancestral type. Evidence furnished by the last-named 
aircumstance is, of course, furnished by embryology : the study of 
»xtinct animals constitutes a special branch of morphology to 
vhich the name Paleontology is applied. 

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


INTRODUCTION 7 


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


13. Quaternary and Recent. 
12. Pliocene. 
11. Miocene. 
10. Eocene. 


Cretaceous. 
Jurassic. 
Triassic. 


Permian. 
Carboniferous. 
Devonian. 
Silurian. 

. Cambrian. 

. 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, 


III. Cainozoic or Tertiary. . 


II. Mesozoic or Secondary. 


J. Paleozoic or Primary .. 


fA po 00 He OLS ST 90 SO 


8 ZOOLOGY 


and paleontology we get a departinent of Zoology called Phylo- 
geny, the object of which is to trace the pedigrees of the various 
groups. There are, however, very few cases in which this can be 
done with any approach to exactness: most “phylogenies ” are 
*purcly 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 
Paleontology, since it is obviously necessary to mention in con- 
nection with a fossil the particular system or systems of rocks in 
which it occurs: thus we distinguish geological distribution or 
distribution in time. 

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

Under the head of geographical distribution we have such facts 
as the absence of all Land-mammals, except Bats, in New Zealand 
and the Polynesian Islands, the presence of pouched Mammals, 
such as Kangaroos and Opossums, only in some parts of America 
and in Australia and the adjacent islands, the entire absence of 
Finches in Australasia, and so on. We find, in fact, that the 
Jauna—ie. the total animal inhabitants—of a country is to a 
large extent independent of climate, and that the faune 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 z00-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 
far south as the Himalayas, Africa north of the Sahara, together 
with the corresponding portion of Arabia, and North America as 
far south as Mexico. For convenience of reference it 1s often 
customary to divide this region into two: its Eurasian portion is 
then called the Paleurctic, its American portion the Acaretie 
region, 

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

3. The Oriental Region includes India, Ceylon, South China, 
the Malayan Peninsula, and what are known as the Indo-Malayan 
islands, 7.e. those islands of the Malayan Archipelago which lhe to 
the west of a line—called Mudlace’s line—passing to the east of 
the Philippines, between Borneo and Celcbes and between Bali 
and Lombok. 

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

5. The New Zeeland 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, ic. 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, &e. 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. 


Ir 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 Amatba or Proteus Animaleule (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 Ameeba, and, after 
an interval, compare the draw- 
ing with the original, we find 
Fia. 1.—Ameoeba proteus, a living specimen. that the drawing appears no 

ce. vue. contractile vacuole; nv. nucleus; 

psd. pseudopods. (From Parker's Biology, longer to represent what we 

pene see ; a change has taken place 

in the shape of the Ameba; 
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 (psd.), which undergo various alterations of size 
and shape, and may become withdrawn, other similar processes 
bemg developed in their place. At the same time careful 


SECT. 1 STRUCTURE AND PHYSIOLOGY OF ANIMALS 1 


watching shows that the Amceba is also, with extreme slowness, 
changing its position. This it effects by a kind of streaming 
motion. A projection forms itself on one side, and the entire 
substance of the Ameeba 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 extremcly gradual locomotion, which it often takes very 
close watching to detect, is brought about. In these movements, 
it is to be noticed, the Ameeba 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 Amceba, in its 
slow progress onwards, passes on one side of them, so that it 
might be said to feel its way among the solid particles in the drop 
of sediment. 

Judging from the nature of these movements, we are obliged to 
infer that the substance of which this remarkable object is com- 
posed must be soft and semi-fluid, yet not miscible with the water, 
and, therefore, preserving a sharp 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 Ameeba 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 Amceba 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 Ameeba as a 
whole undergoes. This is termed the nucleus (Fig. 1, nw.); it is 
enclosed in an extremely delicate membrane, and consists of a 
protoplasmic material differing from that which forms the main 
bulk of the Amceba 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 (¢. 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 Amceba, 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 Ameeba 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—de. 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 ofa 
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 Amceba 
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 Amceba 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 1s 
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 Amceba 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 
Ameeba ; 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 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 13 


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

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


Fic, 2.—Amoeba polypodia in successive phases of division. The light spot is the contractile 
vacuole ; the dark the nucleus. (From Lang’s Zext-Book, after F. E. Schulze.) 


carbonic acid is a constant waste-product of such action. This 
interchange of oxygen and carbonic acid is the essence of the pro- 
cess of respiration observable in all living things. In addition to 
the carbonic acid given off in this process, other waste-products are 
formed and have to be got ridof. In all probability the contractile 
rucwole already referred to has to do with this process—the process 
of cacietion—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 Amceba. 

When food is abundant the Amceba 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 
Ameeba 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 Amcebe 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 
Amcebe 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 Amcba. 

In spite of the great simplicity of its structure, the Amoeba 
thus carries on a number of different functions. The practically 
structureless particle of protoplasm is able to act on matter 
absorbed as food in such a way as to alter the chemical composition 
of the latter and to assimilate it; 16 1s 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 
ov irritability, as shown by the modifications of its movements 
which result from contact with foreign bodies; it is able to 
respire ; it carries on processes of excretion ; and, finally, it is 
capable of reproducing its kind. It is these functions that charac- 
terise diving beings as distinguished from non-living matter. 
What is specially characteristic of the living organism in general 
when compared with a non-living object is the capacity of the 
former to respond by changes in itself to influences operating on 
it from without. In the case of such an extremely simple 
organism as Amoeba, these changes are also, necessarily, extremely 
simple ; but they are of a quite definite character. In addition 
to the effects produced on its actions by mechanical obstacles and 
the presence of food-particles, it can be shown by experiment that 
Amba 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 cnterie 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 Ameeba 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 Amceba. Soon this particle 
divides into a number of parts which, instead of separating 
completely from one another, like the parts of a divided Ameeba, 
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 coag:- 
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 


Fic. 3.—Diagram to illustrate the alveolar theory of 
acknowledged that there protoplasm. (After Dahlgren and Kepner.) 


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- 


16 ZOOLOGY SECT 


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 
Tio. 4 Diagn to ustats the reticular theory of 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 
anunals; for a plant-cell has, nearly always, a definite, firm, enclos- 
ing envelope or cell-wall (Fig. 5, I, ew)—a structure which is only 
exceptionally present in the case of animals. In the interior of 
the cell-protoplasm, or cyloplasm, is a body termed the nueleus, 
similar to the nucleus of Amceba, 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 
ditfers from ordinary protoplasm in having a stronger affinity for 
most staimiug agents. A rounded body termed the nuweleolus 
(ww), 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 karyokinesis or mvitosis is 
applicd. 

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


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 17 


ae bodies is present at the outset it subsequently becomes 
ivided 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 


Fic. 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; ¢. centro- 
some; chr. chromatin ; cpl. cell-plate ; nu’. nucleoli; nu. m. nuclear membrane ; s. atrosphere ; 
sp. spindle. (From Parker's Biology, after Flemming, Rabl, &c.) 

—extending between them (Fig. 5, C). At the same time, 

or at an earlier stage, each centrosome has become the centre 

of a system of fine achromatin fibres (apparently made up, 
like the fibres of the spindle, of rows of granules) which are 
arranged round it in a radiating manner, forming a structure 

_ 1 The term achromatin is usually applied to all the matter of the nucleus 

that has not the special characteristics of chromatin; but it applies to cytoplasmic 

structures—?.¢. structures belonging to the body of the cell—as well. 


VOL, I c 


18 ZOOLOGY SECT. 


termed the attraction-sphere or astrosphere (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, 1t 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 duughter-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 ina plane at right angles 
to the long axis of the spindle, deepens gradually so as to give rise 
to a cleft, eventually completely separating the substance of the 
cell into two halves. Each of these halves encloses one of the 
daughter-nuclei, and has assumed the character of a complete 
daughter-cell. During this process there is sometimes distinguish- 
able along the line corresponding to the division line between the 


STRUCTURE AND PHYSIOLOGY OF ANIMALS 19 


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

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


3. THE Ovum: Maturation, IMPREGNATION, AND SEGMENTATION : 
THE GERMINAL LAYERS. 


Ameeba is simply an independent animal cell; or, to express 
the same meaning in another way, is a unicellular aniinal, 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 J/ctazoa, 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 cell (Fig. 6), 
usually spherical in shape, with 
one or more enclosing membranes, 
with cell-protoplasm enclosing a 
nucleus (germinal vesicle) in which 
are contained one or more rounded 
masses of chromatin (germinal spot 
or spots). The ovum may contain 
in addition to the protoplasm a 
quantity of non-protoplasmic nu- Fro, 6—Ovum of a Sea-Urchin, showing 


] 7 NH. the radially striated cell-membrane. 
trient material or yolk. ‘ the protoplasm, containing yolk: 
Before the process of impregna- granules, the large nucleus (germinal 
. cane = 3 4 vesicle), with its network of chro- 
tion or fertilisation which gives matin and a large nucleolus (ger- 
= minal spot) (From Balfour's £m- 

the impulse to development, the bryology, after Hertwig.) 


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 /emale pro- 
nucleus (B, 2 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 

Co 2 


20 ZOOLOGY SECT. 


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


Fia. 7.—Diagram illustrating the maturation and fertilisation of the ovum. A, formation of first 
polar body; B, beginning of fertilisation, sperms approaching the micropyle; C, formation 
of the male pronucleus; D, approximation of the male and female pronuclei; E, formation 
of segmentation-nucleus ; 9 cert. female centrosome; g cent. 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 
a prominence (the receptive prominence) is pushed out by the 
ovum at the point where the sperm enters. The female 
pronucleus, leaving its former central position, approaches the 
male cell as it enters. In most cases a single sperm alone enters 
the ovum in impregnation. According to the older observers, 
as soon as a sperm enters the ovum, a membrane is formed 
around the latter hindering the penetration of additional sperms. 
But it has now been shown that such a membrane occurs 
only in certain cases, and is quite exceptional. That, 
as a general rule, only one sperm penetrates into the ovum 
appears to be due to the circumstance that, as a result of the 
entry of the one sperm, the peculiar attraction above referred 
to becomes in some way destroyed or diminished. But, though 
the entry of one sperm only is usual, cases of the entry 
of several—polyspermy, as it is termed—are by no means 
rare, and would appear to be quite normal in some groups of 
animals. . 

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

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

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


22 ZOOLOGY SECT. 


in the polar bodies and also 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 


Fic. $.—VYarious stages in the segmentation of the ovum. (From Gegenbaur’s Comparative 
Anatony.) 

segmentation) or only a part (meroblastic or ineomplete 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 


Fic. 9.—Gastrulation. 
avch, avchenteron ; Ul, 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, 4) assuming the form of a hollow 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 23 


sphere, the blustosphere or blastula, with a wall composed of a 
single layer of cells enclosing a cavity—the segmentation cavity 
or blastocele, 

_ One side of the hollow blastula next becomes pushed inwards or 
invaginated (Fig. 9,B, C), as one might push in one side of a hollow 
india-rubber ball, the result of this process of invagination, or 
gastrulation as it is termed, being the formation of a cup—the 
gastrula (Fig. 10)—with a double wall. The 
cavity of the cup-shaped gastrula is the 
archenteron or primitive digestive cavity ; 
the opening is termed the dlastopore, 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 
(mesoblast)—which is subsequently formed 
between them, 


This mode of formation of the primary  '%®-{0;;Gestruls in longi- 


germinal layers in holoblastic oosperms by a blastopore ;_ 8, arch- 

oe iS e enteron ;c, endoderm; 
process of gastrulation prevails in a number d, ectoderm, (From 
of different sections of the animal kingdom. eae 


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 ¢esswes. 


4. TISSUES. 


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


24 ZOOLOGY 


SECT. 


(Fig. 11, «); sometimes there is on each cell a single, relatively long, 
whip-like cilium, which is then termed a flagellum (f, 9). Cells 


Fia. 11—Various forms of epithelium. a, ciliated epi- 
thelium ; J, columnar ; «, surface view of the same ; 
ce, tesselated ; e, the same from the surface ; /, flagel- 
late epithelium with collars ; g, flagellate epithelium 
without collars; hk, epithelium of intestine with 
pseudopods; i, stratified epithelium ; &, deric epi- 
thelium of a marine planarian with pigment cells, 
rod-cells, and sub-epithelial glands. (From Lang’s 
Comparative Anatomy.) 


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

Some tissues are com- 
posed entirely of cells. 
Others, though originat- 
ing from cells or by the 
agency of cells, consist in 
greater or less measure of 
non-protoplasmic matter 
formed between the cells. 
Tissues composed en- 
tirely of cells take the 
form, for the most part, 
of membranes covering 
various surfaces, external 
and internal. Such mem- 
branes are known under 
the general name of 
epithelia (Fig. 11); they 
may consist of a single 
layer -of cells (a-A) or 
may be many-layered 
(1); the former are 
termed non-stratzjied, 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, 6); in 
the case of a stratified 
epithelium the cells may 
be of different forms in 
different strata (7). 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. 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 25 


_ Glands (Fig. 12) are formed for the most part by the modifica- 
tion of certain cells of epithelia. In many cases a single cell of the 
epithelium forms a gland, which is then termed a wnicellular gland 
(Fig. 12,4). 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 (B). 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 le embedded in 
an intermediate substance called 


the matriz or ground-substance ; ary 

of the connective tissue. Berri acteenitre re ong 
In the case of gelatinous con- epithet, neva glands ting 

nective tissue (Fig. 13) the ground- with the surface by narrow processes 

substance (g) is of a gelatinous oe cette ee eae 

character, sometimes supported Urine sauna ee Gea Tee 


by systems of fibres (¢f), and the Lang.) 
cells are usually stellate or star- - 
shaped with radiating processes. Retiform or reticulate connective 
tissue (Fig 14) consists of stellate or branching cells with pro- 
cesses which are prolonged into fibres—the fibres from neigh- 
bouring cells joining so as to form a network. In this form of 
connective tissue there is no true ground-substance—the inter- 
spaces between the cells being filled with other tissue elements. 
Fibrous connective tissue, which is a very common form, has a 
ground-substance containing gelatin, consisting mainly of numerous 
fibres, usually arranged in bundles. Thicker yellow elastic fibres 
may be present among the others, and may be so numerous as to 
give the entire tissue an elastic character. Associated with fibrous 


26 ZOOLOGY SECT, 


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


— ie fee 2 iY aS Se 


Fig, 18.—Gelatinous connective tissue of a Jelly-fish; e, epithelium; g, gelatinous matrix 
bg, branching ells ; ef, elastic bres, (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 cartelage the matrix is of a firm but elastic 


h are 


fia. 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 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 27 


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


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


covered over by a fibrous membrane—the perichondriwm. 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 lamelle, which are arranged partly parallel with the sur- 
face, partly concentrically around certain canals—the Haversian 
canals (¢)—in which blood-vessels lie. The cells, or bone-corpuseles, lie 


Fic. 16.—Hyaline cartilage. Fia, 17.—Fibro-cartilage. 


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


28 ZOOLOGY SECT. 


membrane—the pertostewm—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-substance— 
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. Hach 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- 
Me a a anole Goleek Stee sttiation is absent. A fibre of 
outer surface; b, lamelle concentric gtrjiated muscular tissue (Fig. 20) 


with the surface of the marrow cavity ; : ; 
ec, section of Haversian canals; c’, svc- 18 formed by the close union 


tion of a Haversian canal just dividing : 
into two; d, interstitial lamelle, (From Of several cells which are repre- 
Huxley's Lessons in Physiology.) sented by their nuclei (7). 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 strie, 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 STRUCTURE AND PHYSIOLOGY OF ANIMALS 29 


tissue is specially characteristic of parts in which rapid movement 


1s necessary. 
The principal elements of nervous tissue are nerve-cells and 


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


a Se 


TL 


Fia. 19. —Non-striated muscle-cell ; f, substance of fibre > ”, nucleus ; p, unaltered protoplasm in 
the neighbourhood of the nucleus. (From Huxley’s Lessons in Physioloyy.) 
Ed 


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

The nerve-sibres (Fig. 22), which are to be looked upon as greatly 
produced processes of nerve-cells, are arranged for the most part 
im 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 asis-cylinder or newraxis (A, ax)—which is the 


B 
=! 
ska! 
J Aig, 
fer _— 
= d= 
a < i] 
‘= ee 
im oe! 
BELO SL ines | 
ws Ome 7 
bay ty 


A, part of a muscular fibre of a Frog; B, portion of striated muscle 


Fia. 20.—Striated muscle. 5 I 
(From Huxley's Lessons in Physiology.) 


teased out to show separation into fibrille. 
b, d, g, transverse bands and striw 3 2, nuclei. 
essential part of the fibre and is made up of numerous extremely 
fine primitive fibrille; this is surrounded by a layer of a white 
glistening material—the white substunce of Schwann or medullary 
sheath (med), enclosed in turn in a very delicate membrane—the 
neurilemma (newr). 
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. 


Fic. 21.—Nerve-cells. A, multipolar ; 


B, bipolar. 


Fic. 22.—Nerve-fibres. 4A, medullated ; 
B, non-medulated ; ax, neuraxis ; 
med, medullary sheath; neur, 
neurilemma. 


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


Fia, 23.—Various forms of spermatozoa. 
a, of a Mammal ; , 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.) 


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

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 Acad at the other. 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 31 


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


5. ORGANS. 


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

The skin or integument consists in the majority of animals 
of a cellular membrane—the epidermis—to which reference has 
already been made, with, superficial to it, in many animals, a non- 
cellular layer the cuticle, and below it usually a fibrous layer which 
is known as the dermis. The epidermis may consist of a single 
layer or may be stratified; it is frequently ciliated, and some of 
its cells frequently assume the form of unicellular glands. Modi- 
fication of its superficial layers of cells gives rise frequently to the 
formation of hard structures contributing to the development of 
an exoskeleton (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 soliditied 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 exoskeleton or 
external skeleton ; in the latter an endoskeleton or internal skeleton. 
In many groups of animals both systems are developed. An 
exoskeleton 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 Jevers on which the 
muscles act. 

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

In the lowest groups in which a distinct alimentary or enterte 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 ¢entacles or 
soft finger-like appendages, or they may have the form of saws, 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 1s 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 ov buccal cavity, a pharyna, 
an wsophagus 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 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 33 


the absorption of the digested food-substances. Into this part are 
poured the secretions of the various digestive glands, which act on 
the different ingredients of the food so as to render them more 
soluble. Through the ning 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 
antestine. 

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 toa limited extent, and which serves 


¢. 


Pic, 24.—General view of the viscera of a male Frog, from the right side. u, stomach ; b, urinary 
bladder ; ¢, small intestine; ¢/, cloacal aperture; d. large intestine; ¢, liver; 7, bile-duct: 
g, gall-bladder ; hk, spleen ; i, lung; k&, larynx ; /, fat-body ; m, testis; 7, ureter; 0, kidney ; 
Pp, pancreas ; s, cerebral hemisphere ; sp, spinal cord; t, tongue; w, auricle ; ur, urostyle ; 
v, ventricle ; vs, vesicula seminalis; w, optic lobe; x, cerebellum ; y, Eustachian recess ; 


z, nasal sac. (From Marshall.) 


mainly for the passage to the anal opening of the fwces 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, 2), and of taking minute 
particles of food into their interior to become digested and absorbed 
(intracellular digestion). Sometimes they are all more or legs 
active in secreting a fiuid destined to act on the food and render 
it more soluble; soinetimes this function is confined to certain of 
the cells, which have a special form; very often the secreting cells 


VOL. I D 


34 ZOOLOGY 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 /iver—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 jwice—which has 
a very important effect in digestion. It renders substances of the 
nature of albumins soluble by converting them into modifications 
termed peptones; 1t converts starch into the soluble substance 
sugar; 1t 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 dacteals—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 dlcod-vessels . 
; The essence of the process of respiration, as we have alreadyseen, 
1s 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 Amceba, 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 Amceba, and many of higher 
organisation—the oxygen passes inwards and the carbonic acid 
outwards through the general surface of the body. But in the 
great majority of animals there is a special set of organs—the 
organs of respiration—having this particular function. In some 
animals these organs of respiration are processes, simple or 
branched, lined by a very delicate membrane, and richly supplied 
with blood-vessels. Such processes are called gills or branchice ; 
they are specially adapted for the absorption of oxygen dissolved 
in water. 

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

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

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

D2 


6 ZOOLOGY SECT. 


alled hemoglobin, which has a strong affinity for oxygen ; and the 
xygen from the air, when it enters the blood, enters into a state 
f loose chemical combination with it. In this state, or simply 
issolved in the fluid plasma of the blood, the oxygen is conveyed 
hroughout the body. 

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

To carry out all the functions which it has to perform as a 
listributor of nourishment and oxygen and a remover of carbonic 
icid, the blood has to be moved about through the vessels—to 
irculate throughout the various organs. In the lowest forms in 
vhich a definite blood-system is to be recognised, this movement 
s effected in great measure by the general movements of the 
v0dy of the animal. In others certain of the vessels contract and 
lrive the blood through the system; such contractions are of a 
veristaltic character, the contractions being of the nature of con- 
‘trictions running in a definite direction along the course of the 
ressel, with an effect similar to that produced by drawing the 
iand along a compressible india-rubber tube. 

In all higher forms the movement of the blood is effected by 
neans of a special organ—the heart. The heart is a muscular 
wxrgan which by its contractions forces the blood through the 
system of vessels. In its simplest form it usually consists of two 
thambers, both with muscular walls,—the one, called the auricle, 
‘eceiving the blood and driving it into the other, which is called 
she ventricle. The latter, in turn, when it contracts, drives the blood 
shrough 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 
loors opening from the auricle towards the ventricle, but closing 
when pressure is exerted in the opposite direction. In the higher 
mimals the heart becomes a more complex organ than this, with a 
arger number of chambers and a more elaborate system of valves. 

Carbonic acid, as already mentioned, is a waste-product con- 
stantly being produced in the tissues and being carried off by the 
slood to pass out by the gills or lungs. Besides the carbonic 
acid, there are constantly being formed waste-substances of another 
slass—viz., substances containing nitrogen, of which wrie acid and 
wrea are the principal ultimate forms. These are separated from 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 37 


the blood and thrown out of the body by a distinct set of organs 
called renal organs, or organs of urinary excretion. The 
form of these organs varies greatly in the different groups; 
in many cases they are more or less intimately connected with 
the genital system. 

In place of the simple contractions and extensions of the proto- 
plasm which constitute the only movements of 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 


Fic. 25.—Bones 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 Physiol ogy.) 


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 
arts are drawn nearer to one another. 

Tn 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 tbe 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. 


ZOOLOGY SECT. 


The central parts of the nervous system consist (Fig. 26) of 
rtain aggregations of nerve-matter known as nerve-ganglia, 
ntaining a large number of nerve-cells; a relatively large mass 
of this matter may be 
collected together to 
form a brain. To or 
from these central parts 
pass all the systems of 
nerve-fibres, constituting 
the peripheral part of the 
system ; the former have 
the office both of re- 
ceiving impressions con- 
veyed by the nerve-fibres 
from the surface, from 
the organs of special 
sense, and from the in- 
ternal organs, and of 
sending off messages 
through similar channels 
to the various parts of 
the body—to muscles, to 
glands, to alimentary 
canal, and to vascular 
system. When a move- 
ment is to be effected 
a message passes from 
the nerve-centre along a 
nerve-fibre to a muscle 
and causes it to contract ; 
when an organ requires 
the amount of blood sup- 
plied to it to be in- 
creased or diminished a 
message is conveyed 
along a nerve-fibre and 
causes the dilatation or 
contraction of the blood- 
vessels of the part; and 
a similar initiatory or 
controlling influence is 


Fic. 26.—Nervous system of the Prog. (From eee 
Howes's Atlas.) exerted over the activities 


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


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 39 


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

The ear in its simplest form is a membranous sac or otocyst with 
internally projecting stiff cilia, and containing a liquid in which 
there he 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 ccoulial 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 monwcious or hermaphrodite; sometimes 


) ZOOLOGY SECT. 


ae ovaries occur in one set of individuals—the females—and the 
astes in another set—the males, when the term unisexual or 
iweinus is employed. Very frequently the male differs from the 
male in other respects besides the nature of the reproductive 
lements—in size, colour, and the like; when such differences are 
trongly marked the animal is said to be sexually dimorphic. The 
va and sperms are usually conveyed to the exterior by canals 
y ducts—the ovarian ducts or oviducts, and the testicular ducts, 
permiducts, or vasa deferentia. In some instances the ova are 
mpregnated after being discharged from the oviducts, and the 
levelopment of the young takes place externally; m other cases 
he impregnation takes place in the oviduct, and the young 
recome fully developed in the interior of a special enlargement 
f the oviduct termed the uterus. In the former case the animal 
s said to be oviparous, in the latter viviparous; but there are 
umerous intermediate gradations between these two extremes. 


6. THE REPRODUCTION OF ANIMALS. 


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

Besides the sexual process of reproduction by means of ova and 
spermatozoa, there are in many classes of animals various asexual 
nodes of multiplication. One of these—the process of simple 
fission—has been alreadynoticed in connection with the reproduction 
of Amceba. 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 
torm 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 


I STRUCTURE AND PHYSIOLOGY OF ANIMALS 41 


alternates with a stage in which there occurs a true sexual mode 
of reproduction. Such a phenomenon is termed alternation of 
generations or metuyenesix. The term heterogamy is applied to 
cases 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 


Fic. 27,—Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted, 
showing multiplication by budding. 6.1 bd.2 bd.3 buds in various stages of growth. (From 
Parker's Biology.) 


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


7. SYMMETRY. 


The general disposition or symmetry of the parts in an animal 
presents two main modifications—the radial and the bilateral. 
The gastrula (p. 23) is the simplest and most generalised form among 
multicellular animals or Metazoa; but no adult animal retains 
this simple shape. In the gastrula we may imagine a central 
primary avis (Fig. 28, 4/3) 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. Ina 
symmetrical gastrula the secondary axes would be all equal. Many 


2 ZOOLOGY SECT. 


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


A 


B 

LD 

(an 

< de 
73a. 28.—Diagram of the axes of the body. Fic. 29,—Radial symmetry. Letters as 
AB, primary axis; ab, cd, secondary in Fig. 28. The processes at A are 
axes. The lower figure is a transverse the tentacles; the lower figure repre- 
section of the upper one showing its two sents the upper or oral surface. (From 
secondary axes. (From Gegenbaur.) Gegenbaur.) 


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

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


a STRUCTURE AND PHYSIOLOGY OF ANIMALS 48 
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 Aemisomes by a median vertical plane passing through the 
primary axis. This is the bilateral syuumetry observable in all but 
a few types of animals. 

Sometimes the bilaterally symmetrical animal is unsegmented ; 
sometimes it is divided into a series of segments or metameres. 
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 
KInGpom. 


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, ete., 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 Verte- 
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 Mellusca. 
Other familiar animals are readily to be recognised as belonging to 
one or other of these great phyla. A Prawn, a Crab, a Blue-bottle 
Fly, a Spider, are all on the same general plan as the Lobster: they 
are jointed animals with jointed limbs, and have the internal 
organs occupying similar positions with relation to one another: 
they are all members of the phylum Arthropoda. Again, a Mussel, 
a Snail, and a Squid are all to be set side by side with the Oyster 
as conforming to the same general type of structure: they are all 
members of the phylum J/ollwsea. 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 VIL. Molluscoida 
IL Porifera VIII. Echinodermata 
TI. Celenterata 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. 


SECTION II 
FHYLUM PROTOZOA 


Iy the preceding section we learnt the essential structure of an 
animal cell, and it was pointed out that in the lowest organisnis 
the entire individual consists of a single cell. All such wnicedlular 
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 ameboid 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; ci/iated, or produced into numerous 
rhythmically moving threads of protoplasm ; or encysted, the proto- 
plasm being enclosed in a cell-wall. Moreover, under certain 
circumstances, amoeboid cells may fuse with one another to form 
a plasmodium. 

These well-marked phases in the life of the cell allow us to 
divide the Protozoa into subdivisions called Classes. The same 
organism may be ameeboid, 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 1s 
ameeboid, 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. Ruizopopa.—Protozoa in which the amceboid 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. Mycrrozoa.—Terrestrial Protozoa in which the plas- 
modial phase is specially characteristic, as also is the formation 
of large and often complex cysts. 

Class 3. MasricorHora.—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—Am«aba proteus. 


Ameeba 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, £) about 4 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 endosare and a clear 
outer layer or ectosare ; the difference between the two is hardly a 
structural one, but depends simply on the accumulation of granules 
in the central portion. The granules are, for the most part, various 
products of metabolism—proteinaceous or fatty. 

Imbedded in the endosarc is a large nucleus (nu), of spherical form, 
consisting ofa 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. 

Ameeba feeds by ingesting minute organisms (Fig. 30, c, f. vac.) 

or fragments of organisms—z.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. 
' Ameebse 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 
chatinoid waterial—i.c., a nitrogenous substance allied in composi- 
tion te horn and to the chitin of which the armour of Insects, 
Crayfishes, etc., is composed. 

Reproduction takes place by simple cr Linary fission ; direct or 
amitotic division of the nucleus is followed by division into two of 
the cell-body (1). Occasionaily two Amcebze have been observed to 


I PHYLUM PROTOZOA 47 


conjugate or undergo complete fusion, but nothing is known of the 


result of this process or of its precise significance im this particular 
case. 


Fic. 30.—Amoeba. A, A. quarta; B, the same killed and stained ; C, 4. proteus ; D, encysted 
specimen ; E, A. proteus ; F, nucleus of same, stained ; G, A, VETTUCOSA, H, nucleus of same, 
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pseudopods ; 
c. vac. contractile vacuole; cy. cyst; f. vac. food-vacuole; mv. nucleus (numerous in 
A, quarta); psd. pscudopod. (From Parker's Biology, after Leidy, Gruber, and Howes.) 


2. CLASSIFICATION AND GENERAL ORGANISATION. 


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


8 ZOOLOGY SECT. 


elicate ; in the number of nuclei; and in the presence or absence 
fa hard shell within or around the protoplasm. The following 
yur orders may be distinguished :— 


ORDER 1.—Loposa. 


Rhizopoda with short, blunt pseudopods. 


ORDER 2.—FoORAMINIFERA. 


Shelled Rhizopoda with fine, branched, and anastomosing 
iseudopods. 


ORDER 3.—HELIOZOA. 


Rhizopoda with fine, stiff, radiating pseudopods. 


ORDER 4.—RADIOLARIA. 


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


Systematic Position of the Lxample. 


Amoeba proteus is one of many species of the genus Amba, 
vclonging to the family Amebide, of the order Lobosa. The blunt 
»seudopods not uniting to forra networks place it among the 
sobosa: the absence of a shell,among the Amcebide. The genus 
Ameeba is distinguished by the presence of one or more nuclei, 
ind of a contractile vacuole. In A. proteus the pseudopods are 
f considerable length and sometimes branched, and there is a 
‘ingle nucleus, having its chromatin in the form of scattered 
sranules. 


ORDER 1.—LOBOSA. 


General Structure.—The members of this group all agree 
vith Amceba in essential respects, their most characteristic feature 
yeing the short, blunt pseudopods. The chief variations in struc- 
sure upon which the genera and species are founded have to do 
vith the number and character of the nuciei, the form of the 
sseudopods, and the presence or absence of a shell. 

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


I PHYLUM PROTOZOA 49 


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


Fic. 31.— Protameeba primitiva. Showing changes of form and three stages in binary fission. 
(After Haeckel, from Parker's Biology.) 


Protameba (Fig. 81). 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 8mm. in diameter ; it is multi-nucleate and is 
further distinguished by the presence of numerous non-contractile 
vacuoles in the endosare. 


Fig. 32.—A, Quadrula symmetrica; LB. Hyalosphenia lata; ©, Arcella vulgaris ; 
D, Difflugia pyriformis. (From Lang’s Comparutive Anatomy.) 


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


VOL. I ry 


50 ZOOLOGY SECT. 


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 Arcella and its allies (Fig. 32, A—c), in which the 
shell ischitinoid. A different kind of shell is found in Difflugia 
(D), which secretes a gelatinous coating to which minute sand- 
grains and other foreign particles become attached. 


ORDER 2.—FORAMINIFERA 


General Structure.—The members of this order differ from 


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

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


Fic. 83.—-Microgromia socialis. A, entire colony; B, single zooid; C, zooid which 
has undergone binary fission, with one of the daughter-cells creeping ont of the shell; 
D, flagellula; c. vac. contractile vacuole ; nu. nucleus; sk. shell. (From Biitschli’s Protozoa, 
after Hertwig and Lesser.) 


(nw.) and contractile vacuole (¢. vac.), enclosed in a chitinoid cell- 
wall or shell (sz.) 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-colomes (A). Sometimes the ccll-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 Ameeba, draws 
in its pseudopods, assumes an oval form, and sends out two 
flagella by means of which it is propelled through the water (D). 
We shall find other instances in which the young of a Rhizopod is 


II PHYLUM PROTOZOA 51 


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

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


Fic. 34.—Platoum stercoreum. A, single zooid; B, formation of colony; ¢. vae. contractile 
vacuole; /. food particles; au. nucleus; sk. shell. (From Biitschli's Protozoa, after 
Cienkowsky.) 


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

Gromia (Fig. 35, 1) leads us to the more typical Foraminifera. 
The protoplasm of this form protrudes from the mouth (a) of the 
chitinoid shell (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. 35,3) differs from Gromia mainly 
in having the shell formed of calcium carbonate and possessing the 
character of a hollow, stony sphere, with an aperture at one end. 
It appears that all the calcareous Foraminifera begin life in this 
simple form; but in the majority of cases the adult structure 
attains a considerable degree of complexity. The protoplasm of 
the original globular chamber overflows, as it were, through the 
aperture ; but, instead of forming an elongated plate from which 
side buds are given off, as in Platoum, the extended mass rounds 
itself off, and secretes a calcareous shell in organic connection with 
the original shell, and communicating with it by the original 
aperture. In this way a two-chambered shell is produced, and a 
repetition of the process gives us the many-chambered shell found 
in most genera. New chambers may be added in a straight line 
(Fig. 36, 3), or alternately on opposite sides of the original 
chamber (5), or with each new chamber enclosing its predecessor 
(4), or in a flat spiral, each new chamber being larger than its 
predecessor (7, 8), or in a spire in which the newer chambers 

E 2 


52 ZOOLOGY SECT. 


overlap the older (9, 10), or in an irregular spiral of globular 
chambers (6), or in an extremely compact spiral in which the new 
chambers completely enclose their predecessors (17). In all cases 


3.Squammulina 4.Miliola 
Fig. 3&-Various forms of Foraminifera. In 4, Miliola, a, shows the living animal; 


b, the same killed and stained ; a. aperture of shell; f. 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 


II PHYLUM PROTOZOA 53 


increase in the number of chambers there is a multiplication of 
the nucleus (Fig. 35, 4, b, nw). 
Not only does the shell increase in size by the formation of new 


6 oS 


1Saccammina 


v Z 


0 . 8. Spiroloculina 


2.Lagena 4.Frondicularia 6.Globigerina 


SSK. 


4, 
ih 


9.Planorbulina 10 11.Nummulites 


Fic. 36.—Shells of Foraminifera. In 3, 4, and 5, a shows the surface view, and } a section ; 
Sa is a diagram of a coiled cell without supplemental skeleton; Sb of a similar form 
with supplemental skeleton (s. sk.); and 10 of a form with overlapping whorls; in 1Ja half 
the shell is shown in horizontal section ; b is a vertical section ; a. aperture of shell; 1—15, 
successive chambers, 1 being always the oldest or initial chamber. (After Carpenter, Brady, 
and Biitschli.) 


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


ZOOLOGY SECT. 


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

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

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


37._Hastigerina murrayi. lism. vacuolated protoplasm surrounding shell: psd. 
pseudopods ; sh. shell; sp. spines. (After Brady.) 


which this secondary skeleton occurs are sometimes of consider- 
e size—-2-8 em. in diameter—and of extraordinary complexity. 
Many Foraminifera resemble Difflugia in having a skeleton 
med of sand-grains, sponge-spicules, and other foreign bodies 
nented together by a secretion from the protoplasm (Tig. 36, 2). 
me of these are formed on the imperforate type, having the 
toplasm protruded from a single terminal aperture; others on 
: perforate type, small pseudopods being protruded between the 
‘ticles forming the shell. 
In many cases the pseudopods are the only portions of proto- 
sm outside the shell, whereas in Gromia, as we saw, the shell 
invested with a layer of protoplasm, and is thus in strictness 
internal structure. In one of the calcareous forms with 


i PHYLUM PROTOZOA 55 


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


eee ee 


ao 


IY 


y fig! AZ eas 


Fic. 38.—Dimorphism and alternation of generations in Polystomella crispa. The arrows 
indicate the direction of the life-cycle. A, young megaspheric individual; B, full-grown 
megaspheric individual, decalcified ; C, megaspheric individual in the act of spore-formation, 
the protoplasm leaving the shell in the form of flagellule; D, flagellula more highly 
magnified ; #, microspheric individual developed from a flagellula ; ¥, microspheric individual 
in the act of producing ameeboid 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 and microspheric) ditfering 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 ameeboid or 
may be of the nature of flagellula—each provided with a 
flagellum. In some cases the flagellulee 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, 7c. occurring at or near the surface of the 
ocean, others abyssal, ze. 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 Globigerine contained 
in it. 

From the palzontological 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, 


Ir 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 
Ameeba: each is ingested along with 
a droplet of water, and is thus seen, 
during digestion, to lie in a de- 
finite cavity of the protoplasm, 
called a food-vacuole. If the or- 
ganism be small, processes of the 
protoplasm are developed, and sur- 
round and engulf it. If it be larger, 
several pseudopods are applied to 
it, their axial fibres becoming ab- : 
sorbed, and their substance envelops  ynhcactmoParys Son a axial 
it, enclosing it in a vacuole. The 2 meidopnl drum Langs One 
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 


Fic. 40.—Actinospherium eichhornii. A, the entire organism; B, a small portion 
highly magnified ; chr. chromatophore ; cort. cortex ; c. vue. contractile vacuole ; med. medulla ; 
nu. nuclei. (From Biitschli’s Protozoa, after Hertwig and Lesser.) 


water, and it possesses the power of rising or sinking by some 
unknown means. 
Actinospherium (Fig. 40, A), another fresh-water form, is more 


58 ZOOLOGY SECT. 


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


\ 


¢ a 


NER aphidiophrys 
\ 


\ 
: / \ 
pes \ 


2.Nuclearia 


3.Clathrulina 
Fic. 41.—Various forms of Heliozoa. 3a, the entire animal; 3b, the flagellula; c. vac. 
contractile vacuole; g. gelatinous investment; nu. nucleus psd. pseudopods ; sk. siliceous 
skeleton; sp. spicules. (From Biitschli’s Protozoa, after Schulze and Greeff.) 


and an outer layer, the cortex or ectosarc (cort.), in which they are 
very large. There are numerous nuclei (nu.) and chromatophores 


(chr.), the latter coloured green by chlorophyll, the characteristic 
pigment of green plants, 


Ir 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 witha distinct skeleton. Sometimes the body 
simply surrounds itself with a temporary gelatinous investment 
(Fig. 41, 2, 9.), 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. 


Sea 


Fic. 42.—Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A, two indi- 
viduals in the first phase of conjugation ; B, beginning of the encystation ; C, maturation ; 
D, completion of maturation ; EZ, coalescence of nuclei; /, completion of the first spindle of 
the zygote resulting from the conjugation. 1, axial filaments of the pseudopods ; 2, nucleus ; 
3, spindles concerned in maturation ; 4, 5, outer and inner layers of cyst ; 6, polar bodies ; 
7, nucleus formed by the union of the two nuclei; 8, first (mitotic) division, (From Lang, 
after Schaudinn.) 


One genus has a shell formed of agglutinated sand-grains; in 
another (Fig. 41, 7) 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. Actinospherium, 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- 
spherium. In Clathrulina spore-formation takes place in the 
active condition, and the spores (Fig. 41,30) are flagellule, 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). 


ORDER 4.—RADIOLARIA. 


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

General Structure.—The most important characteristic of 
the group is the presence of a perforated membranous sac, called 
the central capsule (Fig. 43, 
cent. caps.’, which lies em- 


Lu iz 
| 
Bi X 
CO) \ 


Shel. 


ie. 
ae 


Fia. 43.—Lithocircus annularis. cent. caps. 


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 (nw.), or sometimes 
many nuclei: from the ex- | 
tra-capsular protoplasm the 
pseudopods (psd.) are given 
off in the form of delicate radi- 


central capsule ; ert. caps. pr. extra-capsular 


ating threads, which in some 


protoplasm ; (vf. caps. pr. intra-capsular pro- 
toplasm ; rv. nucleus ; psd. pseudopods ; shel. } > 1 
skeleton; <. cells of Zoochlorelia. (After cases remain fr ee, others, 


Bittschli, from Parker's Biology.) e.g. Lrthocireus, anastomose 
freely, 7.c. unite to form net- 
works. In one large section—the Acantharia—the pseudopodia 
contain firm axial rods similar to those in the pseudopods of the 
Heliozoa. There is no contractile vacuole, but in many forms the 
extra-capsular protoplasm contains numerous large non-contractile 
vacuoles, which give it the frothy or bubbly appearance noticed 
previously in Hastigerina. The vacuolated portion of the proto- 
plasm has a gelatinous consistency, and is distinguished as the 
calymma. 

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


Bat PHYLUM PROTOZOA 61 


the perforated calcareous shell of Hastigerina with its investment 
of vacuolated protoplasm. It is found in its simplest form in 
Thalassoplancta (Fig. 44), in which it is spherical and uniformly 
perforated with minute holes. In other forms, such as Lithocireus 
(F 1g. 483), 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 phawodiwm (ph.), prob- 
ably partly of the nature of excreta. The 
central capsule encloses, in addition to 
the nucleus or nuclei, oi]-drops, vacuoles, 
proteid crystals, and pigment. 

In some genera the central capsule is 
the only skeletal structure present, but in 
most cases there is in addition a skeleton 
—mainly external—formed, as a rule, of 
silica, but in one subdivision of the class 
of a substance called acanthin, composed 
of strontium sulphate, so transparent that 
it can only be distinguished from silica 
by chemical tests. The siliceous skeleton 
may consist of loosely woven spines 
(Fig. 44), but usually (and the acanthin 
skeleton always) has the form of a firm 


Fic. 44.—Thalassoplancta 
brevispicula, part of a 


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 


section. km. central cap- 
sule; ip. intra-capsular 
protoplasm; 2. nucleus, 
containing 2. numerous 
nucleoli ; 6¢. oil drops ; ca. 
calymma ; 7p. protoplasm 
surrounding calymma,; s. 
spicules. (From Lang’s 
Comparative Anatomy, after 


gelatinous extra-capsular layer. A very a 


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. 3) connected by radiating spicules. 
The outer of these spheres occurs in the extra-capsular protoplasm 
(B, ex. caps. pr.), the middle one im the intra-capsular protoplasm, 
and the inner one in the nucleus (nw.). 

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 Collozowm for instance (Fig. 47, A, B, C) 


SECT. 


ZOOLOGY 


62 


Fic. 45,-Aulactinium actinastrum. c. calymma; km. central capsule; 7». nucleus; op 
operculum ; ph. pheeodium. (From Lang’s Comparative Anatomy, after Hacckel. 


Fic. 44.—Actinomma asteracanthion. A, the shell with portions of the two outer 
spheres broken away; B, section showing the relations of the skeleton to the animal; 
cent. caps, central capsule ; ex. caps. pr. extra-capsular protoplasm ; nu. nucleus ; sk. 1, outer, 
sk. 2, middle, sk. 3, inner sphere of skeleton. (From Biitschli’s Protozoa, after Haeckel and 


Hertwig.) 


I 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 (¢. 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 


Fic. 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 Biitschli’s 
Protozoa, after Hertwig and Brandt.) 

alike (E), and each encloses a small crystal (¢.): in other cases (F)— 

in the same species—the spores are dimorphic, some being small 

(microspores), others large (megaspores). Their development has not 

been traced. 

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


64 ZOOLOGY SECT. 


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


APPENDIX TO THE RHIZOPODA. 


CHLAMYDOMYXA AND 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 


Vic. 48.—Chlamydomyxa labyrinthuloides. 
ment of Alga ingested as food; sp. spindles in course uf pseudopods; B, resting-stage— 
numerous individuals in the cells ofa fragment of Sphagnum ; a, specimen completely enclosed 
in cell; 6 and c, specimens which have emerged through the ruptured cell-wall; C, specimen 


A, active phase; ¢.w. cell-wall; /. frag- 


multiplying by budding; D), by binary fission ; 4, by internal fission, J may represent a 
stage in spore-formation. (A after Archer, B—K after Geddes.) 


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 


Il PHYLUM PROTOZOA 65 


numerous non-nucleated protoplasmic bodies or chromatophores, containing 
chlorophyll and a green or brown colouring matter in varying proportions. 
There are also a number of minute rounded bodies of a bluish tint probably com- 
posed of reserve food-materials. In the young condition (a) the resting cells are 
globular and microscopic, lying enclosed within the cells of the Sphagnum, but 
as they grow in this confined space they become elongated and irregular, and 
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 amceboid masses, each of which subsequently surrounds itself with 
a new cell-wall (KE). 

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 Alge 
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 (sp.) found in the resting 
stage take on a spindle shape and travel slowly along the filaments. 

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

The filaments are used to capture living organisms (f.) 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, z.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 


Fie. 49. Labyrinthula vitellina. A, specimen crawling on a fragment of Alga (a.); ¢. 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; nu. nucleus, 

_ Crom Biitschli’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 containing two nuclei. These give rise to flagellule, the further history of 
which has not been traced. 

Labyrinthula (Fig. 49) in the resting stage (B) consists of » 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 Moraminifera 
(Gromia in particular) than to any of the other orders. 


Fic. 50 -Didymium difforme. A, two sporangia (spg. 1 and 2) on a fragment of leaf (/.). 
5, section of sporangium, with ruptured outer tayer (.); and threads of capillitium (cp.). 
C, a flagellula with contractile vacuole (c. rar.) and nucleus (nw.). D, the same after loss 
of flagellum ; b, an ingested Bacillus. EB, an amebula, F, conjugation of amcebule to form 
a small plasmodinm. G, a larger plasmodium accompanied by numerous amcebule; sp. 
ingested spores. (A\fter Lister.) 


CLASS II.—MYCETOZOA. 
J. EXAMPLE OF TAE CLAss—Didymium diffurme. 
Didymiun occurs as a whitish or yellow sheet of protoplasm (Fig. 50, G), 


often several centimetres across, which crawls, like a gigantic Amceba, over 
the surface of decaying leaves. It shows the charactezistic streaming move- 


II 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, spg. 1, spg. 2) is therefore not a mere resting capsule, like that of Amceba, 
but a sporangium or spore-case. Its wall consists of two layers, an inner of a 
dark purple colour and membranous texture, formed of cellulose, and an outer of 
a pure white hue, formed of calcium carbonate. Thus the whole sporangium, 
which may attain a diameter of 3 or 4 mm., resembles a minute egg. From the 
inner surface of the wall of the sporangiwn 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 amceboid mass which soon becomes a flagellula 
(C), provided with a single flagellum, a nucleus (nu.), and a contractile vacuole 
(c. vac.). The flagellule move freely and ingest Bacilli (D, 0.), and multiply by 
fission: then, after a time, they become irregular in outline, draw in the 
flagellum, and become amceboid (E). The amcebule thus formed congregate in 
considerable numbers and fuse with one another (F), the final result being the 
production of the great amceboid mass (G) with which we started. There is no 
fusion of the nuclei of the amcebule. Thus Didymium in its active condition isa 
plasmodium, i.e. w body formed by the concresence of amcebulee. 


2. GENERAL REMARKS ON THE MyYcETOZOA. 


Speaking generally, the Mycetozoa differ from all other Protozoa in their 
terrestrial habit. They are neither aquatic, like most members of the phylun, 
nor parasitic, like many other forms, but live habitually a sub-aérial life on 
decaying organic matter. They are also remarkable for their close resemblance 
in the structure of the sporangia and spores to certain Fungi, a group of parasitic 
or saprophytic plants in which they are often included, most works on Botany 
having a section on the J/yxomycetes or ‘‘ Slime-fungi,” as these organisms are 
then called. They are placed among animals on account of the structure and 
physiology of the flagellate, amceboid, 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 flagellule, but soon become amceboid and fuse to form the plasmodium. 


CLASS III—MASTIGOPHORA. 
1. EXAMPLE OF THE CLass—Luglena 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, ws.), from the inner surface of which 


F 2 


68 ZOOLOGY SECT. 


springs a single long flagellum (72.). According to recent observa- 
tions the flagellum is not a simple thread, but is beset with delicate 
cilium-like processes. The organism is propelled through the 
water by the lashing movements of the flagellum, which is always 
directed forwards ; it can also perform slow worm-like movements 
of contraction and expansion (A—D), but anything like the free 
pseudopodial movements which characterise the Rhizopoda is 
precluded by the presence of a very thin membrane or cuticle which 
invests the body. Oblique and longitudinal lines ;in the outer 
layer of the protoplasm may be due to the presence of contractile 
fibrils. There is a nucleus (xw.) near the centre of the body, and 
at the anterior end a contractile vacuole (H, ¢. vac.), leading into 


Fic. 51—BEuglena viridis. A—1), four views illustrating euglenoid movements; E and H, 
enlarged views ; F, anterior end further enlarged ; G, resting form after binary fission ; ¢. vac. 
contractile vacuole in H, reservoir in Eand F; cy. cyst; 1. flagellum ; #2. mouth ; nw. nucleus ; 
es, gullet; p. paramylum bodies; pg. pigment spot; 7. (in H), reservoir. (From Parker's 
Biology, after Kent and Klebs.) 


a large non-contractile space or reservoir (7.) which discharges into 
the gullet. 

The greater part of the body is coloured green by the charac- 
teristic vegetable pigment, chlorophyll, and contains rod-shaped 
grains of paramylum (H, p.), a carbohydrate allied to starch. In 
contact with the reservoir is a bright red speck, the stigma (pg.), 
formed of a pigment allied to chlorophyll and called hematochrome. 
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 


Ir PHYLUM PROTOZOA 69 


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

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


20 ZOOLOGY SECT. 


Systematic Position of the Hxample. 


Euglena viridis is one of several species of the genus Huglena, 
and belongs to the family Huglenide, sub-order Buglenoidea, and 
order Flagellata. 

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


ORDER 1,—FLAGELLATA. 


The cell-body is usually ovoid or flask-shaped (Fig. 52, 6, 7, 9, 
&c.), but may be almost globular (7), 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 belaterally 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 Mastigamaba (4) has a permanently amceboid 
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 (@) it is ovoid and provided with two flagella, 
butit 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 (8). 

Nuclei of the ordinary character are universally present. In 
addition there is present in the cytoplasm near the base of the 
flagelluma much more 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, 1-3), two (9, 10), three (6), or four (7). 
Sometimes the flagella show a differentiation in functiem ; in 


II PHYLUM PROTOZOA val 


Hetcromita, eg. (Fig. 57) the anterior flagellum (jl. Z) only is 
used in progression, the second or cies (f. 2) is trailed 


mS. 


6.Dalflingeria ee 9.Cryptomonas 
B.0ikomonas 


1. Dinobryon 12.Synerypta  13.Anthéphysa 14.Rhipidodendron 


Fia. 52.—Various forms of Flagellata.—In 2, flagellate (w) and amcbhboid (b) phases are 
shown ; in 6, flagellate (a) and heliozoan (b) phases; in 8 are shown two stages in the in- 
gestion of a food-particle (j.); chr. chromutophores ; c. vac. contractile vacuole ; f. food par- 
ticle g. gullet; nw. nucleus; /. lorica ; p. protoplasm ; per. peristome ; vi. vacuole of ingestion. 
(Mostly from Biitschli’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, Tig. 53) the 


72 ZOOLOGY SECT. 


flagellum (or one of them, if two are present) is attached through- 
out its length, or in the greater part of its length, to the edge 
of a wavy protoplasmic flange, or undulating membrane, ranning 
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 1s, 
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 
Hemoflagellata, 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 


fa 


"] 


f1a. 53.—Trypanosomes of Fishes. ¢. blepharoblast; /. flagellum ; fu, and fp. (in 4) 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. 

Heematococcus (Fig. 54), Pandorina (Fig. 55), Volvow (Fig. 56), 
and their allies present us with a totally different state of things. 
The mouthless body is surrounded by a cellulose cell-wall (¢.w.), 
and contains chromatophores (chr.) coloured either green by chloro- 
phyll or red by hematochrome. Nutrition is purely holophytic, 
4.¢. 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 Alge 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 Ockomonas (Fig. 52, 8), 
we have one of the simplest arrangements: near the base of the 
flagellum is a slight projection containing a vacuole (v.7.); the 
movements of the flagellum drive small particles (/) against this 
region, where the protoplasm is very thin and readily allows the 
particles to penetrate into the vacuole, where they are digested. 


II PHYLUM PROTOZOA 73 


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

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


Fic. 54.—Heematococcus pluvialis. A, motile stage; B, resting stage; C, D, two modes 
of fission; E, Hamatococcus lacustris, motile stage; F', diagram of movements of flagellum ; 
chr. chromatophores ; ¢. vac. contractile vacuole ; ¢,7. cell-wall; nw. nucleus ; nw’, nucleolus ; 
pyr. pyrenoids. (From Parker's Biology.) 


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, 71), 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. 


dendron (Fig. 52, 24) 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. 
Heematococcus (Fig. 54) and Euglena (Fig. 51), for instance, 
divide while in the encysted condition; Heteromita (Fig. 57) 


ris, 55,—Pandorina morum. A, entire colony; B, asexual reproduction, each zooid 
dividing into a daughter-colony ; (, liberation of gametes; D—F, three stages in conjugation 
of gametes; G, zygote; H--K, development of zygote into a new colony. (From Parker's 
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 Pandorinu (Fig. 55) each of the sixteen zooids 
of the colony divides into sixteen (B), thus forming that number of 
daughter-colonies within the original cell-wall, by the rupture of 


II PHYLUM PROTOZOA 75 


which they are finally liberated. In Volvo (Fig. 56), certain zooids, 
called purthenogonidia (A, a), have specially assigned to them 
the function of asexual reproduction: they divide by a process 
resembling the segmentation of the egg in the higher animals 
(D'=D*), 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 


Wan 
(Ui 


Fic. 53.—Volvox globator. A, entire colony, enclosing several daughter-colonies ; 
B, the same during sexual maturity; C, four zooids in optical section ; DI—D5, develop- 
ment of parthenogonidium ; E, ripe spermary; I°, sperm; G, ovary containing ovam and 
sperms; H, oosperm; a, parthenogonidia; #. flagellum; ov. ovum ; ory. ovaries 5 py. pigment 
spot; spy. spermayies. (From Parker's Biology, after Cohn and Kirchner.) 


(Fig. 57, E") and undergo complete fusion (E?—E*): the result of 
this conjugation of the two gametes or conjugating cells is a thin- 
walled sac, the zygote (E°), the protoplasm of which divides by 
multiple fission into very minute spores. These, when first 
liberated by the rupture of the zygote (E*), are mere granules, 
but soon the ventral or trailing flagellum is developed, and after- 
wards the anterior flagellum (F!—F"). 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 . SECT. 


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


Et 


Ic, 57,_Heteromita rostrata. A, the positions assumed in the springing movements 
of the anchored form; B, longitudinal fission of anchored form; C, transverse fission of 
the same; D, fission of free-swimming form ; E, conjugation of free-swimming with anchored 
form ; E5, zygote ; E6, emission of spores from zygote ; F, development of spores; 7.1, ante- 
rior; 7.2, ventral flagellum. (From Parker's Biology, after Dallinger.) 


r microgamete. In Volvox (Fig. 56) this dimorphism reaches its 
xtreme, producing a condition of things closely resembling what 


1 PHYLUM PROTOZOA 77 


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

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


1.Monosiga. 2.Salpingoeca. 3.Polyoeca. 4.Proterospongia. 


Vic, 58.—Various forms of Choanofiagellata. 20 illustrates longitudinal fission ; 2c, the pro- 
duction of flagellulz; ¢. collar; ¢c. vac. contractile vacuole; jl. flagelium ; J, lorica; nu. 
nucleus; 3. 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, /,¢.), surrounding 
the base of the single flagellum (7l.). The collar is contractile, and, 
although its precise functions are not yet certainly known, there is 


8 ZOOLOGY SECT. 


vidence to show that its movements cause vorticesin the water which 
raw in small bodies towards the outside of the collar to which they 
dhere. By degrees such bodies are drawn towards the base, and 
ach is received into a vacuole which moves back into the interior 
f the protoplasm, another vacuole taking its place. The animalcule 
aay draw in both collar and flagellum and assume an anceboid form. 

The nucleus (#w.) is spherical, and there are one or two con- 
ractile vacuoles (¢. vac.), but no trace of mouth or gullet. Some 
yrms are naked (2), others (2) enclosed in a chitinoid shell or 
mica of cup-like form. <A stalk (s.) is usually present in the 
wricate and sometimes also in the naked forms. 

The genera mentioned in the preceding paragraph are all simple, 
ut in other cases colonies are produced by repeated fission. In 
olyeca (3) the colony has a tree-like form, which may reach 

high degree of complexity by repeated branching. A totally 
ifferent mode of aggregation 1s found in Proterospongia (4), in 
‘hich the zooids are enclosed in a common gelatinous matrix of 
regular forma. 

Reproduction.—The “collared monads,” as these organisms 
re often called, multiply by longitudinal fission (20). In some 
ases multiple fission of encysted individuals has been observed 
Ze), small simple flagellule being produced which gradually 
evelop into the perfect form. 

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


ORDER 3.—DINOFLAGELLATA. 


The leading features of this group are the arrangement of the two flagella 

‘hich they always possess, and the usual presence of a remarkable and often 
ery beautiful and complex shell. 
The body (Fig. 59, 2) is usually bilaterally asymmetrical, i.e. it may be 
ivided into right and left halves, which are not precisely similar. On the 
antral surface is a longitudinal groove (I. gr.), extending along the anterior half 
ily, and meeting a transverse groove (t. gr.), which is continued round the body 
ke a girdle. From the longitudinal groove springs a large flagellum (fl. 1), 
hich is directed forwards and serves as the chief organ of propulsion ; a second 
igellum (jf. 2) lies in the transverse groove, where its wave-like movements 
rmerly 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 
mplex form, being produced into long and ornamental process, and marked 
ith stripes, dots, &c. Besides a nucleus and a contractile vacuole, the proto- 
asm contains chromatophores (1, chr.) coloured with chlorophyll or an allied 
gment of a yellow colour, called diatomin. Nutrition is holophytic or holozoic. 

The foregoing description applies to all the commoner genera. Prorocentrum 
) is remarkable for the absence of the transverse groove, while Polykrikos (4) 
sno fewer than eight transverse grooves and no shell. The latter genus 
30 has stinging-eapsules or nematocysts (a, b) in the protoplasm, resembling 
ose of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes, 
stinguished as meganuclei (nu.), and micronuclei (nw'.). 


Ir PHYLUM PROTOZOA 79 


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


2.Ceratium 3.Prorocentrum 


4.Polykrikos 


Fic. 59.—Various forms of Dinoflagellata. 2 shows the shell only; 4a is an undischarged, 
and b a discharged stinging-capsule; chr. chromatophores; fl. 1, longitudinal flagellum ; 
jl. 2, transverse flagellum; /. gr. longitudinal groove; ntc. nematocyst; nv. meganucleus ; 
nu’, micronucleus ; py. pigment spot; t. gr. transverse groove. (From Biitschli’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—CyYSTOFLAGELLATA. 


This group includes only two genera, Noctiluca and Leptodiscus. A descrip- 
tion of Noctiluca miliaris, the organism to which the diffused phosphorescence 
of the sea is largely due, will serve 
m f to give a fair notion of the leading 

- 4. ¥ characteristics of the order. 
i 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 (entacle (bg. ), no- 
ticeable for its transverse striation. 
Near the base of this flagellum is 
the mouth (m.), leading into a short 
es gullet in which is a second flagel- 
Lyi lum (f.), very small in proportion 
= to the first. On the side opposite 


Fic. 60. _—Noctiluca miliaris. «a. the adult : ; 
animal; 2, c. flagellule ; by. tentacle ; f. flagel- to the mouth is a strongly marked 


lum ; #2. mouth; 2. nucleus. (From Lang.) 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. 


80 ZOOLOGY SECT. 


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


CLASS IV.—SPOROZOA. 
1. EXAMPLE OF THE CLass—Monocystis agilis. 


One of the most readily procured Sporozoa is the microscopic 
worm-like Monocystis agilis (Fig. 61, A), which is commonly found 
leading a parasitic life in the vesicule 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 


B 


Fic. 61.—Monocystis. A, Trophozoites in different stages of contraction, B, encysted 
gametocytes, C,.division of gametocytes into gametes. D, conjugation of gamctes to form 
zygotes. E, Cyst cuclosing ripe spores formed from the zygotes. F, single spore, showing 
the (8) sporozoites in its interior. G, group of developing sperm-cells of the earthworm, 
enclosing a sporozoite in the centre. H, young trophozites still surrounded with the tails of 
the degenerated sperms. nu, nuclei. (From Parker's Practical Zoology.) 


condition, the protoplasmic body is covered with a firm cuticle, 
and is distinctly divided into a denser superficial portion, the cortex, 
and a central semi-fluid mass, the medulla. There is a large clear 
nucleus (xw.) 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 


Ir 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 (i). 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 hke a bundle of sausages. By the rupture 
of the spore-coat these falciform young or sporozoites are liberated, 
and at once begin active movements, the thin end of the body 
moving to and fro like a clumsy flagellum. The falciform young 
appear, in fact, to be greatly modified flagellule. 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 GENERAU 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.—H2MOSPORIDIA. 


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


OrpER 4.—MYxXOSPORIDEA. 


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


ORDER 5.—SARCOCYSTIDEA. 


Elongated Sporozoa, usually found in muscle. 
VOL, I G 


2 ZOOLOGY SECT. 


Systematic Position of the Example. 


Monocystis agilis is a species of the genus Monocystis, belonging 
0 the family Monocystida, of the order Gregarinida. It is placed 
1 the Gregarinida on account of being free and motile in the tro- 
hozoite state. The absence of partitions dividing the protoplasm 
ato segments indicates its position among the Monocystide. 
Tonocystis is distinguished by its elongated form, by the absence 
f any special apparatus in the cyst for the liberation and dispersal 
f the spores, and by its spindle-shaped spores with thickened 
nds, each producing 4—8 falciform young. The differences 
‘etween the species of Monocystis depend largely upon size. 


ORDER 1.—GREGARINIDA. 


All the more typical members of the class belong to this 
roup. With the exception of Monocystis, already described, the 
nly genus to which it will be necessary to draw attention is 
iregarina (Figs. 62 and 63), the various species of which are 
arasitic in the intestines of Crayfishes, Cockroaches, Centipedes, 


fic. 62.-Gregarina, <A, two specimens of (@, blattarum partly embedded in enteric 
epithelial cells of Cockroach ; Bl, B2, two specimens of G. dujardini; in B2 the epimerite («p.) 
is cast off ; OC, cyst of G@. blattarum, from which most of the spores have been discharged ; 
D, four stages in the development of @. gigantea. cy. cyst; deu. deutomerite ; cp. epimerite ; 
g. gelatinous investment of cyst; au. nucleus; pr. protomerite; ps7. 1, short pseudopod ; 
psd. 2, long pseudopod ; sp. mass of spores ; spd. sporoducts. (From Biitschli’s Protozoa.) 


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


II PHYLUM PROTOZOA 83 


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


Fic. 63.—Gregarina. Development from the sporozvite. 1, cells of the digestive epitheliu 
of the host; 2, nuclei of the same; 3, spore; 4, spore discharging sporcuolios ) leaving 
residual mass (6); '7, sporozoites in the act of entering epithelial cells; 8, the same as 
intracellular parasites ; 9-12, different stages in the growth of the young Gregarineés into the 
lumen of the intestine; 13,epimerite ; 14. protomerite; 15, deutomerite. (After Lang.) 


through which the spores escape. In Gregarina gigantea of the 
Lobster, fhe young (sporozoite) is liberated from the spore in the 
form of a non-nucleated amcebula (D1), with one long and one 
short pseudopod (D?) ; this divides by the long pseudopod (psd, 2) 
becoming separated off, and each product of fission, developing a 
nucleus, passes into the adult (trophozoite) form (D*, D‘.) In 
other species of Gregarina the sporozoites do not divide, but each 
develops directly into the trophozoite (Fig. 63). 


ORDER 2.—COCCIDIIDEA. 


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


G2 


84 ZOOLOGY SECT. 


organs, most frequently in those of the epithelium of the digestive canal. They 
never inhabit blood-corpuscles. A few are intra-nuclear parasites. Two 
distinct modes of multiplication occur—by schizogony, a kind of multiple fission, 
and by sporoyony, 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 Atacertain 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 


1 Eimeria 2.Coecidium 


Fic. 64.—Coceidiidea., A, adult Himeria (z) in enteric epithelial cell (ep.) of mouse; 
B, encysted form ; C, cncysted form, the protoplasm contracting to forma spore; D, formation 
of falciform young (/.) in interior of spore (sp.); E, spore with falciform young; F, adult 
encysted form of Cocer/iuin from liver of rabbit; G, division into spores; H, cyst containing 
ripe spores (sp.), each with a single falciform young ; I, single spore with falciform young (/,). 
(From Biitschli’s Protozon, after Leuckart and Eimer.) 


epithelium may be partially or completely destroyed. It is only, apparently, 
when such extensive damage has been done, or is threatened, that multiplication 
by sporogony takes place—the invasion of a new host being by this process 
rendered probable, and the continuance of the race being thus provided for in 
the event of the death of the host in which the epithelium has become destroyed. 
In this process certain of the merozoites, instead of developing into trophozoites, 
grow more slowly (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 ina resistant cyst (k). These give rise to spores with a firm, chitinous 
spore-membrane, each containing two or more falciform young or sporozoites (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 « new host, may liberate the now 
active sporozoites, which may penetrate into epithelial cells (a) to become the 
trophozoites with which the cycle began. 


II PHYLUM PROTOZOA 85 


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


Fic. 65.—Life-History of Coccidium schubergi. a, penetration of epithelium cell of host by 
sporozoite ; b-c, stuges of multiple fission (schizogony); d, gametocyte; e, f, formation of 
megagamete (ovum); g, fertilisation ; h, j, formation of microgametes (sperms); i, develop- 
ment of fertilised ovum into four spores ; 1, formation of two sporozoites (falciform young) in 
each spore. (From Calkins, after Schaudinn.) 


ORDER 3.—HA: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 


6 ZOOLOGY SECT. 


n certain birds it has been found that their presence is the cause of various 
everish affections. The various forms of malaria in man have been proved to 
»e due to the presence in the blood-corpuscles of the patient of parasites 
velonging to this order. The malaria-parasites, the history of which has heen 
arefully worked out, pass through a life-cycle comparable to that of Coecidium 
lescribed above. In the trophozoite stage (Fig. 66, A—G’) they live as amceboid 


B jc 2B 


(eeLoloT*lofe eel M101] 


Fia. 66.—Life-History of Malaria Parasites. 4-G, parasite of quartan fever, showing 
development of trophozoite in a blood-corpuscle and the formation of merozoites; H, 
gametucyte of the same ; J-M, parasite of tertian fever to the formation of the merozoites ; 
WN, gametocyte; 0-7, crescentic gametocytes of Laverania; P-S, formation of micro- 
gametes or sperms; U-JV, maturation of megagamete or ovum; X, fertilisation ; Y, zygote. 
a, zygote enlarging in stomach of mosquito ; b-e, passing into the body-cavity ; 7, g, develop- 
ment of the contents into a mass of sporozoites ; h, 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, NV, O, 7). In order that the life-cycle may be completed, it is 
necessary that the parasite at this stage should be taken into the interior of a 


II PHYLUM PROTOZOA 87 


second or intermediate host. In the case of the parasite of human malaria the 
intermediate host is a mosquito of the genus Anopheles. On the mosquito 
drawing up a drop of the blood of w malaria patient, all stages of the parasite 
that occur in it are destroyed by the digestive juices of the insect with the 
exception of the gametocytes; these survive and form gametes in the 
stomach of the mosquito. Each male gametocyte gives rise to a number of 
slender filamentous microgametes (sperms, P, S$) and each female gametocyte 
forms a single megagamete (ovum). After maturation (U— JW) the megagamete 
is fertilised (x) by one of the actively-moving microgamates, the result being the 
formation of an active spindle-shaped ookinete. This perforates the stomach 
wall and comes to rest in the subjacent tissues. It then becomese encysted 
and increases greatly in size, bulging out into the body-cavity (b—e), The 
contents of the cyst eventually become divided up (f, g) into a large number of 
long, narrow sporozoites. When the cyst becomes ruptured into the body- 
cavity, these find their way to the salivary glands (fh), 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 Hemogregarines, 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 ameeboid, 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 ameeboid 
in the trophozoite phase, and which reproduce continuously by 
spore-formation during that phase (Fig. 67, A). Many nuclei are present 


Fic. 67.—A, Myxidium lieberkthnii, amceboid phase; B, Myxobolus miilleri, 
spore with discharged nematocysts (xte.); C, spores (psorosperms) of a Myxosporidian ; 
ate. nematocysts. (From Biitschli’s Protozoa.) 


in the ameboid body, which may be of comparatively large size. The 
spores (B) produced within the protoplasm of the trophozoite are provided 
each with one or more bodies like the nematocysts of zoophytes and jelly-fish 
[See Section IV]. Myxosporidea occur as parasites mainly of fishes 
and amphibians, but very many occur in various groups of Invertebrates, 
“‘Pébrine,” 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. 


ZOOLOGY : SECT, 


ORDER 5.—SARCOCYSTIDEA. 


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


. 68 —Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From 
Biitschli's Protozoa, after Rainey.) : 


a striped muscular fibre. They are often known as Rainey’s or Miescher’s 
‘puscles. The protoplasm divides into spores from which falciform young are 
erated. 


CLASS V.—INFUSORIA. 
1. EXAMPLE OF THE CLAass—Paramecium caudatum. 


Structure.— Paramecium, the “slipper-animalcule,” is tolerably 
mmon in stagnant ponds, organic infusions, &c. The body 
‘ig. 69) is somewhat cylindrical, about + mm. in length, rounded 
the anterior and bluntly pointed at the posterior.end. On the 
ntral face is a large oblique depression, the buccal groove (buc. gr.), 
aiding into a short gullet (gul.), which, as in Euglena, ends in the 
ft internal protoplasm. 

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

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

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


II PHYLUM PROTOZOA 89 


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


Fic. 69—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 ; Ca specimen 
which has discharged its trichocysts ; D, diagram of binary fission ; buc. gi. “puccal groove ; 
cort. cortex ; ew. cuticle ; ¢. vac. contractile vacuole ; 7. vac. food vacuole ; gul. gullet ; med, 
medulla; av. meganucleus; pa. nv. micronucleus ; trck. trichocysts. (From Parker's Biology.) 


view ; finally the vacuole contracts and discharges its contents 


externally. 
The cortex contains minute radially arranged sacs called 


trichocysts (treh.). 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, cnclosed in a globule of water or “ food-vacuole” 
(/. vae.), 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 Amceba. 
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 


menu 


patty 


‘1c. 70,—Paramoecium caudatum, stages in conjugation. gul. gullet ; mg. nu. meganucleus; 
mt. nu. micronucleus; My. nu. reconstructed meganucleus; Mi. nu. reconstructed micro- 
nucleus, (From Parker's Biology, after Hertwig.) 


. certain number of times it is interrupted by conjugation. In 
his very remarkable and characteristic process two Paramecia 


II PHYLUM PROTOZOA 91 


become applied by their ventral faces (Fig. 70, A), but do not fuse. 
The meganucleus (ng. nv.) of cach breaks up into small masses, 
which chsappear, being apparently absorbed into the protoplasm. 
At the same time the micronucleus (m7. xu.) 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 (wi. nu.’, ma. 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 (EK). 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 (Jf, nw.). 


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.—CILiatTa. 
Infusoria provided with cilia throughout life. 


ORDER 2.—TENTACULIFERA. 


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


Systematic position of the Example. 


Paramcecium aurelia is one of several species of the genus 
Paramecium, belonging the family Parmecide, 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- 
moecide are free-swimming, asymmetrical, uniformly ciliated, with 
a ventrally placed mouth. P. caudatum is about 1—} 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. 


ZOOLOGY SECT, 


The form of the body is very varied: it may be ovoid (Fig. 
|, 1), kidney-shaped (2), trumpet-shaped (2), vase or cup-shaped 
, 9); produced into a long, flexible, neck-lhke process (4), or into 
rge paired lappets ((’); flattened from above downwards, or 
ongated and divided into segments reminding us of those of 
segmented worm (8). 

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

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

But it is in the hypotrichous type that the most extraordinary 
iodifications are found. The flattened body bears on its dorsal 
irface mere vestiges of cilia in the form of very minute processes 
f the cuticle, while on the ventral surface the cilia take the form 
“ large hooks, fans, bristles, and plates with fringed ends (Fig. 71, 
). The hooks and plates do not vibrate rhythmically like 
rdinary cilia, but are moved as a whole at the will of the animal, 
1us acting as legs. The hypotrichous Ciliata, in fact, in addition 
» swimming freely in the water, creep over the surface of weeds, 
¢., very much after the manner of Woodlice. One of the most 
xtraordinary forms in this group is Dtophrys (7), the size and 
rangement of its polymorphic cilia giving it a very grotesque 
ppearance. In another genus (10) the distal end of the flask- 
raped body bears a circlet of large fringed cilia, giving the animal 
1e appearance of a Rotifer (wide Section VIL.). 

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


Mi 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, 17) 
the undulating membrane (w. mb.) 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. Actono- 
bolus (14) 1s remarkable for the possession, in addition to cilia, of 
long retractile tentacles used for attachment. In Didiniwm (14) 
the barrel-shaped body is encircled by two hoops of cilia. 

As we have seen, the meganucleus in Paramecium is ovoid: in 
other genera it may be elongated and band-like (3, mg. nw.), 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 (nw.) 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, ¢. vuc.), 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, 78), or arranged in one or two rows 
(8). The star-like arrangement of radiating canals, described in 
Parameecium, 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 are rarely present in the other subdivisions 
of the order. In the peritrichous Epistylis wmbellaria, however, 
there are found numerous minute capsules (Fig. 71, 9, ante.) 
arranged in pairs, each containing a coiled thread. They are 


1.Prorodon 


(Page 
3. Stentor 


é 6.Folliculina 


1 / 


8.Anoplophrya c 
9.Epistylis ~ 


12.Multicilia  13.Lophomonas 


z CUAL ¢ 
15.Didinium 17.0 palinopsis 


Fic. 71.—Various forms of Ciliata. 9a shows part of a colony, b a single zooid, and 
ca couple of nematocysts; a. anus; ¢. (in 18) cuticle; ¢. (in 19) excretory canals; c. vac. 
contractile vacuole; jf. ruc. food vacuole; g. gullet; my. vu. meganucleus ; mi. nw. micro- 
nucleus; mth. mouth; nu, nucleus; nlc. nematocyst; p. (in 15) a Paramecium seized by 
Didimium; t. tentacle; uv. mb. undulating membrane; vuc. non-contractile vacuole; vst. 
vestibule. (From Biitschli's Protozoa, after various authors, ) 


I PHYLUM PROTOZOA 95 


obviously structures of the same character as trichocysts, and 
their resemblance to the nematocysts so characteristic of Coelenterata 
(vide Section 1V.) 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, 2) 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 Cilata. 


Wu 


| 

‘van 

Ni ; \; 
5. S 


3.Thuricola 4.0phrydium 
2.Pyxicola 


UDictyocysta K 2 
tichotricha 


Fic. 72.—Various forms of Ciliata. In 1 the shell alone is shown; m. contractile fibre; op. 
operculum. (From Biitschli’s Protozoa, after various authors.) 


From them there is a fairly complete gradation to genera, like 
Parameecium, having the permanently open mouth on the left side 
of the ventral surface, at the end of a well-marked buccal grove 
or peristome. Vortieella (Fig. 73) and its alles 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 (@.) from 
the area thus cnclosed: the mouth (a/h.) les 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 Paramcecium, a 
distinct anal aperture (a.). 


96 ZOOLOGY SECT. 


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


IG. 73. —Vorticella. A, B, living specimens in different positions , C, optical section ; D!, D2, 
diagrams illustrating coiling of stalk; E!, E?, two stages in binary fission; E3, free zooid ; 
Fl, F2, division into mega- and microzooids; G1, G2, conjugation ; H!, multiple fission of 
encysted form; H2, H3, development of spores; ax. f. axial fibre; cort. cortex; cu. cuticle; 
ce. vac. contractile vacuole; ¢. disc; gull. gullet; m. micruzooid; mth. mouth; nu. mega- 
nucleus ; pur. 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, 7) the similarly shaped shell is perforated and 
resembles the skeleton of some of the Radiolaria. A chitinoid 
alate or operculum (Fig. 72, 2, op.) may be fixed to the edge of the 
deristome, and, when the animal is retracted in its case accurately 
closes the mouth of the latter, or a similar operculum () 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. 71, 9a; 
Fig. 74). The stem of these may be a purely cuticular structure 
and non-contractile (Fig. 71, 9, 6), or may contain an axial 
fibre or muscle, like that of Vorticella (Fig. 73, av.f.). In Ophridium 
(Fig. 72, 4) the colony is an irregular mass, sometimes 3—4 ém. 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 


Fie. 74.-Zoothamnium arbuscula. A, entire colony; B, the same, natural size; C, the 
same, retracted ; D, nutritive zooid; E, reproductive zooid; F1, F2, development of reproduc- 
tive zooid; az,j. axial fibre; c. vac. contractile vacuole; nu. nucleus; 7.z. nutritive zooid ; 
1.2. 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 (E®), 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 inercased complexity of the colony, not in the development 
of anew one. The larger zooids (7. 2.) are globular and mouthless : 
they become detached, swim off, and, after a short free existence, 
settle down, develop a stalk (F), divide, and so form a new colony. 

In Vorticella multiplication by ludding also occurs: a small 
process is given off trom one side (Fig. 73, F), develops a basal 
circlet of cilia, and swims off as a méerozooid, the parent individual 


Fia. 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, yeung form liberated from cyst; K, the same after multiplication of the nucleus 
has begun; vw. 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 


It 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- 
larvee feed upon the water-plants, and in doing so frequently take 
in the spores or encysted Opaline along with their food. When 
this occurs the cyst 1s dissolved by the digestive juices of the host, 
and the protoplasm of the spore is sct free as a rounded body 
with a single nucleus (I), which rapidly grows into an adult 
Opalina (K). 

Conjugation, in the form of a temporary union accompanied by 
interchange of micronuclei, has been described in Paramcecium 
(p. 90), and takes place in many Ciliata. In others (2.9. Stylonychia 
histrio) there is a complete union of the two gametes. In 
Vorticella union is also permanent, and takes place, not between 
two ordinary forms, but between one of the ordinary stalked 
individuals, or megagametes, and a free-swimming, small form, or 
microgamete, produced, as described above, by budding (G!, G?). 
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, Za), ovoid (Zb), 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 (/c), 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 


i 


1.Podophrya 2.Acineta 


4.Dendrocometes 


BE phelora 6. aa ae 


148 


.Rhyncheta 


GLEE NE Cae 


Ophryodend = 
Pee ne Bie ph eleld 


9. Dendrosoma 


Fro. 76.—Various forms of Tentaculifera, a and b, two species of Podophrya; c, a 
tentacle much enlarged; 2a, Acineta jolyi; 2b, A. tuberosa; in 6 the animal has captured 
several small Ciliata ; 8a, a specimen multiplying by budding ; 8d, a free ciliated bud; Ya, the 
entire colony ; 9b, a portion of the stem ; 9c, a liberated bud; a, organism captured as food; 
bc. brood-cavity; bd. bud; ¢. vae contractile vacuole; J, lorica; mg. nu. meganucleus ; 
mi. nu. micronucleus ; t. tentacle. (After Blitschli and Saville Kent.) 


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


au PHYLUM PROTOZOA 101 


stalk of Vorticella. Infusors and other organisms are caught by 
the tentacles (4, 6), 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 (8), or the tentacle 
may be branched (4), the extremity of each branch being suc- 
torial. In some forms there are no terminal suckers (4), and the 
tentacles are waved about to catch the prey instead of standing 
out stiffly asin Acineta. In other cases there are one or more 
long, striated tentacles with tufted ends (7). 

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

Some genera are naked (Z): others form a stalked shell or 
lorica (2a) 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, na.). Micronucle: of the ordinary character are present 
as well. 

Reproduction by binary fission takes place in many species. 
In Ephelota gemmipara (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.): 1m this the buds take on 
the form of ciliated embryos, which finally escape from the parent. 
In Dendrcsoma the common stem of the colony produces internal 
buds (b, &d.). 


Further Remarks on the Protozoa. 


The majority of the Protozoa are aquatic, the phylum being 
equally well represented in fresh and salt water. They occur 
practically at all heights and depths, from 8,000"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 Amceba 
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, Zchthyophthirius, 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 AMesozoa, which will 
be discussed hereafter (Sect. LV.). Moreover, the Mycetozoa, the 
plasmodia of which are formed by the fusion of Ameebule, 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. IIT.). 


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- 
meeba, at the bottom of the list, and the Radiolaria at the top. 
Similarly, among the Mastigophora, such simple Flagellata as 
Oikomonas (Fig. 52, 2 and 8) and Heteromita are obviously the 
lowest forms, Noctiluca and the Dinoflagellata the highest. But 
whether the Rhizopoda, as a whole, are higher or lower than the 
Flagellata, is a question by no means easy to answer. A flagellum 
certainly seems to be a more specialised cell-organ than a 
pseudopod, and some of the Mastigophora rise above the highest 
of the Rhizopoda in the possession of a firm cortex and cuticle, 


II PHYLUM PROTOZOA 103 


and the consequent assumption of a more definite form of body 
than can possibly be produced by the flowing protoplasm of a 
Foraminifer or a Radiolarian. On the other hand, the nucleus 
of the Radiolaria is a far more complex structure than that of 
the 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 
Mastigameeba 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 


Tentaculifera 
Radiolaria Dinoflagellata 


Ciliata 
Cystoflagellata 


Foraminifera Heliozoa Choano- 


Flagellata 


Lobosa 
Flagellata 


Mycetozoa 


OS ae 


Fic. 77.—Diagram showing the mutual relationships of the chief groups of Protozoa. 


capable of assuming either the amceboid 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 amceboid 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 Lophomenas (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 dcvelop- 


104 ZOOLOGY SECx. II 


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 
amceboid-flagellate stock, since they are all predominantly ame- 
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 im accordance with their 
parasitic mode of life. 

The diagram on the previous page is an attempt to express 
these relationships in a graphic form, 


SECTION III 
PHYLUM AND CLASS PORIFERA [PARAZOA] 


The microscopic animals described in the preceding section 
are, as already repeatedly pointed out, characterised by their 
unicellular character, and in this respect stand in contrast to the 
remainder of the animal kingdom. ‘The animal kingdom is thus 
capable of division into two great subdivisions, the Protozoa or uni- 
cellular animals, and the JJetazoa or multicellular forms—the latter 
comprising all the groups that remain to be dealt with. In the 
earliest stage of their existence all the multicellular animals or 
Metazoa are, as already pointed out (p. 19), in a unicellular 
condition, originating in a single cell, the fertilised ovum or 
oosperm. By the process of segmentation or yolk-division the 
unicellular oosperm becomes converted in all the Metazoa 
into a mass of cells from which the body of the adult animal is 
eventually built up. Of the Metazoa, the group which approxi- 
mates most closely to the Protozoa is that now to be dealt with— 
the Porifera 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 Portfera 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—Sycen gelatinoswm. 


General External Appearance and Gross Structure.— 
Sycon gelatinosum, 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 ; im colour it presents various shades 
of gray or light brown. To the naked eye the surface appears 
smooth, but when examined under the lens it is found to exhibit 
a pattern of considerable regularity, formed by the presence of 


1 This species is an inhabitant of southern seas. In all essential respects the 
account of it given above will apply to S. ciliatum, a common European species 
which differs chiefly in the absence of the pore-membranes. 

105 


106 ZOOLOGY 


SECT. 


innumerable elevations of a polygonal shape, which cover the whole 


surface and are separated off from 
one another by a system of depressed 
lines. In these depressions between 
the elevations are to be detected, under 
the microscope, groups of minute 
pores—the ostza 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 (0) lead into narrow passages, 
wide enough to admit a stout pin, 
running through the axes of the 


lic.78-—Sycon gelatinosum. 
—Entire sponge, consisting of a 
group of branching cylinders 
(natural size). 


cylinders; and the passages in the interior of the various 


Fia. 79.—Sycon gelatinosum.—aA 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, 
dottedsip. marks the position of three of the groups 
of inhalant pores at the outer ends of the incurrent 
canals ; 0. osculum. 


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 


II PHYLUM AND CLASS PORIFERA 107 


the owter surface. This would appear to be duc 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 


Fic. 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; JC, incurreut canals ; 
ov. young ova; R, radial canals; sp. triradiate spicules. 


exerted, a quantity of gelatinous-looking matter, which, on being 
examined microscopically, proves to be partly composed of a 
protoplasmic material consisting of innumerable, usually more or 
less broken cells with their nuclei, and partly of a non-protoplasmic, 
jelly-like substance. When this is all removed there remains 
behind a toughish felt-like material, which maintains more or less 
completely the original shape of the sponge. This is the skeleton 
or supporting framework. A drop of acid causes it to dissolve 
with effervescence, showing that it consists of carbonate of 
lime. When some of it is teased out and examined under the 
microscope, it proves to consist of innumerable, slender, mostly 
three-rayed microscopic bodies (Figs. 80 and 81], sp) of a clear 
glassy appearance. These are the calcareous spicwles which form 


198 ZOOLOGY 


Ny 


mt zon \ i) 


wititi 


Fic. 81.—Sycon gelatinosum Transverse section 
through the wall of a cylinder (parallel with the 
course of the canals), showing one incurrent (/C), 
and one radial (2) canal throughout their length; 
sp. triradiate spicules ; sp’. oxcote spicules of dermal 
cortex (de.); 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 3; exc. 
excurrent passage; P.@. 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 
asingle layer of cells—the 
dermal layer or ectoderm 
(Fig. 81, ec)—through 
which project regularly- 
arranged groups of needle- 
like and spear-like spicules 
(sp), forming the pattern 
of polygonal elevations on 
the outer surface. The 
cells of the ectoderm are 
in the form of thin scales, 
which are closely cemented 
together by their edges. 
The paragastric cavities 
are lined by a layer of 
cells (en) which are, like 
those of the ectoderm, thin 
flattened scales. Running 
radially through the thick 
wall of the cylinders are a 
large number of regularly- 
arranged straight passages. 
Of these there are two sets, 
those of the one set—the 
incurrent canals (Figs. 80 


1 The terms ectoderm and 
endoderm are here used as con- 
venient terms for the outer and 
inner layers of the Sponge, 
though, as will appear later, 
these layers differ completely in 
their mode of formation from 
the layers so named in the 
higher phyla. 


III PHYLUM AND CLASS PORIFERA 109 


and 81 JC’)—narrower, and lined by ectoderm similar to the ectoderm 
of the surface; those of the other set—the radial or flagellate canals 
(L)—vather wider, octagonal in cross-section, and lined by endoderm 
continuous with the lining of the paragastric cavity. The incurrent 
canals end blindly at their inner extremities—not reaching the 
paragastric cavity; externally each becomes somewhat dilated, 
and the dilatations of neighbouring canals often communicate. 
These dilated parts are closed externally by a thin membrane— 
the pore-membrane (Fig. 81 pm, and Fig. 82), perforated by three 
or four small openings (Fig. 82, 7)—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 eacwrrent canal (Fig. 81 exc). Incurrent 


e 


Z 
| 


, 
i 
4 
4 
' 
‘ 
‘ 
4 
4 


ad Z 
tes? 
Fia. 82.—Sycon gelatinosum. Sur- Fia, 88.—Sycon gelatinosum, 
face view of a pore-membrane highly An apopyle surrounded by its dia- 
magnified ; p. ostium; &. position of phragm ; 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 prosopyles (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. 


is teased out in sea-water, and the broken fragments examined 
under a tolerably high power of the microscope, groups of these 
collared cells will be detected here and there, and in many places 
the movement of the flagella will be readily observed. The 
flagellum is flexible but with a certain degree of stiffness, 
especially towards the base, and its movements resemble those 
which a very supple fishing-rod is made to undergo in the act. of 
casting a long line—the movement being much swifter and 
stronger in the one direction than in the other. The direction 
of the stronger movement is seen, when some of the cells are 
observed in their natural relations, to be from without inwards. 
It is to these movements that the formation of the currents of 
water passing along the canals is due. The collars of the cells in 
specimens teased in this way become for the most part drawn back 
into the protoplasm. 

The short passage or excurrent canal, which leads inwards from 
the flagellate canal to the paragastric cavity, differs from the 
former in being lined by flattened cells similar to those of the 
paragastric cavity ; it is partly separated from the flagellate canal 
by a thin diaphragm (Fig. 81, di, and Fig. 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 ostiwm 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 mesoglva—in which the spicules of the skeleton are 
embedded. Each spicule is developed from cells termed selero- 
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 


IIL PHYLUM AND CLASS PORIFERA 111 


out surface. The thick outer layer in which the bases of these 
oxeote spicules lic 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 cortea: (ge). 
It is supported by triradiate and also by tetraradiate spicules, one 
ray of each of which (sp”) frequently projects freely into the para- 
gastric cavity, covered over by a thin layer of flattened endoderm 
cells. 

The mesoglcea itself, as distinguished from the spicules which 
he 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 eclls or collencytes (Fig. 80, co) ; 
others are fusiform ; a good many—the ameeboid 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 oscwlar 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. 


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 cctoderm4t—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 choanocytes, 
ae. 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 mesoglaa—containing a 
number of cells of various kinds. The wall of the Sponge is 
pierced by a number of apertures. The skeleton or supporting 


1 See footnote on p. 108, 


112 ZOOLOGY SECT. 


framework, developed in the mesoglca from cells derived from 
the ectoderm, consists in some cases of fine, flexible fibres of a 
material termed spongin ; in others of spongin-fibres supplemented 
by microscopic siliceous spicules; in others of siliceous spicules 
alone; in others of spicules of carbonate of lime. Reproduction 
takes place both asexually by the formation of gemmadles, 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. 


OrpDER 1.—HomoceLa 


Caleareous Sponges in which the internal lining membrane 
consists throughout of flagellate collared cells. 


ORDER 2.—HETEROCELA. 


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 Sycetttdw ; and the 


III PHYLUM AND CLASS PORIFERA 113 


family Sycettedw is one of several families of the order Heterocela 
of the class Calearea. Among the families of the Heterocwla, 
that of the Sycettide 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 Sycettidw 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 Syectta,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 Sycettide 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. gelatinoswm 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 othcr more 
minute points; in fact, we find asa result of the comparison that no 
two specimens are exactly alike. These differences are so great 
that some very distinct races or earictics of S. gelatinosum have 
been recognised, and some have received special names. Here 
again, asin 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 phystological 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 ottspring of a parent- 
form similar to themselves in all the most essential features. 
Arid, 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- 
branched; 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 lamelle ; 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 surtace. Very often, after active vertical growth 


Tit PHYLUM AND CLASS PORIFERA 116 


has resulted in the formation of a comparatively narrow basal 
part or stalk, the Sponge expands distally, growing out into lobes 
or branches of a variety of different forms, and frequently anasto- 
mosing. Sometimes, after the formation of the stalk with root- 
like processes for attachment, the Sponge grows upwards in such 
a way as to form a cup or tube with a terminal opening. Such a 


D.Poterion 


B.Psammoclema 


Fic, 84. External form of various Sponges. 4d, Oscaria, an cncrusting form, with the 
upper surface raised up into a number of rounded prominenccs; B, Psammoclema, a 
ramifying subcylindrical Sponge; C, Buspongia (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- 
12 


116 ZOOLOGY SECT. 


ment like the ribs of an open fan. Often the lamella becomes 
folded, and sometimes there is a coalescence between the folds, 
resulting in the development of a honey-comb-like form of 
sponge. 
Sponges resemble plants, and differ from the higher groups of 
animals, in the readiness with which, in many cases, their form 
becomes modified during growth by 
external conditions (environment). 
Different individuals of the same 
kind of Sponge, while still exhibiting 
the same essential structure and the 
same general mode of growth, may 
present a variety of minor differences 
of form, in accordance with differ- 
ences in the form of the supporting 
surface or in the action of waves and 
currents. 

Leading Modifications of 
Structure.—Sycon gelatinosum be- 
longs to a type of Sponges interme- 
diate between the very simplest forms 

- on the one hand, and the more com- 
plex on the other. The simplest 
type of Sponge-structure is that 
of the so-called Ascetta or Olynthus 
(Fig. 85). This isnot 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 Syeon 

= ! gelatinosum ; but the structure of its 

Mio a lasone Spores ee wall in Ascetta is extremely simple. 
portion of the wall of the vase-like Regularly arranged over the surface 
sponge removed to show the para- 
gastric cavity, (After Haeckel.) are a number of small rounded 

apertures, the inhalant pores; but, 
sinve the wall of the Sponge is very thin, these apertures 

Jead directly into the central or paragastric cavity (Fig. $6 .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. 


is 


inet PHYLUM AND CLASS PORIFERA 117 


A somewhat more complex type of structure than that of Ascetta 


is exhibited by those 
sponges in which the 
wall becomes thick- 
ened and_ perforated 
by radially-arranged 
canals, which open di- 
rectly onthe outersur- 
face by meansof tnhul- 
ant porcs or ostia, and 
lead directly into the 
paragastric cavity by 
means of apopyles— 
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, 2: 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 zeur- 
rent canals, lined by 
ectoderm and com- 
municating with the 
exterior on the one 
hand, either by a 
wide opening or by 
pores (ostia) perfor- 
ating a pore-mem- 
brane, and on the 
other by means of 
small openings, the 
prosopyles (the equi- 
valents of the inhalant 
pores of the Olynthus), 
with the radial canals. 
Sponges similar to 
Sycon  gelatinosum, 


Fic. 86.—Diagram of the canal system of various sponges, the 
ectoderm denoted by a continuous narrow line; the flat- 
tened endoderm by an interrupted line; the flagellate 
endoderm by short parallel strokes. A, cross-section 
through a part of the wall of an Ascon; B, cross-section 
through a part of the wall of a Sycon; C, cross-section 
through a part of the wall of Leucilla conveca ; D, vertical 
section through Oscarella , a, spaces of the incurrent canal 
system ; J, spaces of the excurrent canal system ; os. oscu- 
lum. (After Korschelt and Heider.) 


118 ZOOLOGY SECT. 


but with flagellate canals arranged in groups, each group centred 
round a main excurrent canal (Fig. 86, C) afford us the next 
grade of advancing complexity. In these the incurrent canals may 
form a branching system. In all the higher groups of Sponges 
(Fig. 86, D and Fig. 87) the flagellate cells are confined to cer- 
tain special enlargements of the canals—the so-called “ ciliated 
chambers” (C')—and the rest of the canals are lined by flattened 
cells. 

Special names have been applied to the main types of canal- 
system briefly sketched above. Forms in which the paragastric 
cavity is lined by flagellate cells are said to belong to the Ascon 
type, whether the paragastric cavity communicates directly or by 
flayellate canals with the exterior. Forms in which there is a 
paragastric cavity lined by flattened cells, and a system of radially 


o IR wy A 
COO 


(inns i 
Ex 
Fic. 87.—Vertical section of a fresh-water sponge (Sponzilla), showing the arrangement of the 
canal-system. C. ciliated chambers; DP. dermal pores; Ex. excurrent canals; GO. openings 
of the excurrent canals; PG. paragastric cavity; SD. subdermal cavities; 0. osculum. 
(Modified from Leuckart and Nitsche’s diagrams.) 
arranged flagellate chambers, are said to possess the Syron type of 
structure. Such Sponges as have small rounded flagellate chain- 
bers (“ciliated chambers”), communicating in most cases by 
narrow branching incurrent canals with the exterior (directly or 
indirectly) on the one hand, and by similar excurrent canals with 
the paragastric cavity on the other—the flagellate cells being 
confined to the flagellate chambers—are said to possess the Rhagon 
type of canal-system. In the L’haycn proper the arrangement o 
parts is very simple. The Sponge has a paragastric cavity opening 
on the exterior by an osculum. Opening into this central cavity 
by wide apopyles are a number of rounded chambers each com- 
municating with the exterior by an inhalant pore (prosopyle). 
The development of branches from the originally simple Sponge, 
and the coalescence of neighbouring branches with one another, 
greatly obscure the essential nature of the Sponge as a colony or 
zooids similar to the branches of Sycon gelatinosum ; and this effect 


s 


ty 


_. the gelatinous substance of the 


III 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. mesoglea with special 
skeletal elements, usually containing spaces and canals lined by 
ectoderm—(subdermal cavities, Fig. 87, SD)—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 


! mesogloea are embedded connec- Fic. 88,—Cells of the ectoderm, very highly 
tive-tissue cells, amceboid wan- magnified. (After Von Lendenfeld.) 
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. 


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 Jfyaxospongia) 
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 
vache concentric layers of spongin. 
Pg saa pennant ofa timate siete of 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 mesoglea 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 
aye (Fig. 90, C). In others, again, such as the Venus’s 
lower-Basket (Zuplectella), the Glass-Rope Sponge (Hyalonema), 
and Pheronema (Fig. 92), the skeleton consists throughout of 
siliceous spicules bound together by a siliceous cement. 

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


IIL PHYLUM AND CLASS PORIFERA 121 


cylinder or vase, may be looked upon as an asexual mode of repro- 
duction by budding. In some cases asexual multiplication also takes 
place by the production of external buds; in others of internal buds 
in the shape of groups of cells called gemmudes, which eventually 
become detached and develop into new individuals. In the Fresh- 


“B. Pachychalina 


Fic, 90 —Microscopic structure of the skeleton in various sponges. A, Euspongia, network 
of spongin fibres; B, Pachychalina, spongin strengthened by siliceous spicules; (, 
Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules 
of other sponges, &c. (After Vosmaer.) 


water Sponges (Spongillide) 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 SECT. 


year; they are set free by the decay of the part of the parent 
sponge in which they are developed, and fall to the bottom. In 
spring the contained mass of protoplasmic matter reaches the 
exterior through an aperture in the wall of the gemmule, and 
develops into the adult form. 

All Sponges multiply by a sexual process—by means of male 
cells, or sperms, and female cells, or ova. These are developed 
from certain of the amoeboid wandering cells of the mesoglea, 
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 amceboid 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 ameeboid cell destined to form an ovum enlarges, and 


Fic, 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 O/ynthus, the development 
has been followed out in the case of Cla/hrina 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 urchwocytes, some of which form the repro- 


PHYLUM AND CLASS PORIFERA 123 


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


Fic, 92.—Pheronema carpenteri, one of the Hexactinellida. 
(From Wyville Thomson.) 


a large number of these amceboid 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 Ceelenterates (Sect. IV). This is the 


24 ZOOLOGY SECT. 


wval form known as the parenchymula. The parenchymula 
Fig. 93) consists of three kinds of cells:—(1) an external layer 
f flagellate cells; (2) an inner mass of ameeboid cells; (3) the 
wo 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 
ameceboid 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- 
la. 93.—Median longitudinal sequently the flagellate cells come to 
sete Clatheina biowea’ form the entire boundary of the cavity, 


larvaof Clathrina blanca. 


p.g.¢c., posterior granular cells ‘ “7 
Che a ee the porocytes passing outwards to become 


cytes, (From the Cambridge perforated by apertures—the inhalant 
lai ce ic apertures—in the wall of the sponge. 

Among the flagellate cells and porocytes 
here are also amceboid cells derived from the two original granular 
ells; some of these give rise to the reproductive cells. The 
cleroblasts are formed of certain ectoderm cells which migrate 
nwards, and at an early stage arrange themselves in threes to give 
ise to the tri-radiate spicules. The development of the sponge 
recomes completed by the enlargement of the internal cavity 
paragastric cavity) which is now lined by flagellate cells, and by 
he development of the osculum. 

In Sycon the early stages (Fig. 94, a-c) differ somewhat from 
hose in Clathrina blanca, and the embryo leaves the parent sponge 
n the peculiar stage to which the name of amphablastula is 
ppled. When the blastula is formed the greater part of its wall 
onsists of clear cells, with a number of granular cells—-the archzeo- 
ytes—at the posterior pole. The clear cells become elongated 
nd flagellate. The archzeocytes pass into the internal (segmenta- 
ion) cavity and become completely enclosed by the flagellate 
ells (stage of so-called pseudogastrula). 

The cells at the posterior end then lose their flagella and 
recome large rounded granular cells, so that after a time the 
vall of the embryo comes to be composed in one half of the 
lagellate cells that have remained unaltered, and in the other half of 
he large granular cells. It is in this stage—terined the amphi- 


III PHYLUM AND CLASS PORIFERA 125 


blastula (c)—that the larval sponge becomes frec. At a later 
stage the flagellate cells become partly overgrown by the granular 
cells, the latter eventually giving rise to the ectoderm of the 
adult, while the former become the flagellate collared cells. The 
larva becomes fixed by one side, and soon assumes a cylindrical 


| HI i) 
\ Md Al 
TAI eo 
rey 
C) 


Uk 
GK Le 
4 en TERE [0) A} 


Fic. 94.—Development of Sycon raphanus. «, ovum; b, c, ovum segmented—2, as seen from 

- above, c, lateral view; d, blastula; e, amphiblastula; f, commencement of invagination ; 
g, larva attached by its oral face; h,/, young sponge—h, lateral view ; i, as seen from above. 
(From Sollas, after Schulze.) 


form (Fig. 94, 4,7). 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 mesoglca, 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, 
ind is probably universal in that sub-class except in such primi- 
sive forms as Cluthrina. 

In the Silicispongiz, on the other hand, the typical larva is a solid 
jody 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 
sxctoderm and the other elements of the body of the Sponge. The 
yranular cells break through the ciliated cells at one end and grow 
yer the latter as an investing layer. This is a remarkable reversal 
of what, as will be scen subsequently, is to be observed in the 
Scelenterata—and in fact in the rest of the Metazoa, but is readily 
-econcilable with what takes place in Sycon and the more complex 
Valcarea. 

Distribution and Mode of Occurrence of Sponges, and 
their Position in the Animal Series.—Fossil remains of Sponges 
aave been found in various formations from those of the Cambrian 
geriod onwards, the greatest abundance being found in the Chalk. 
No extinct class or order has been detected, the fossil forms all 
seing members of existing groups. Some of the orders of existing 
Sponges—such as the Myxospongia—are incapable of being 
oreserved as fossils, and the fossil forms belong, as we should 
xpect, to the more highly silicified groups and to the more 
somplex groups of the Calcarea, 

Fresh-water Sponges (Spongillid) occur in rivers, canals, and 
akes 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 
zroups Hexactinellida and Choristida. 

Sponges do not appear to be edible by Fishes or even the higher 
Crustaceans or Molluscs. Countless lower animal forms, however, 
burrow in their substance, if not for food, at least for shelter, and 
the interior of a Sponge is frequently found to be teeming with 
small Crustaceans, Annelids, Molluscs, and other Invertebrates. 
None of the Sponges are true parasites. The little Boring 
Sponge, Cliona, burrows in the shells of Oysters and other bivalves, 
but for protection and not for food. But a Sponge frequently lives 
in that close association with another animal or plant to which the 
serm messmatersm, or commensalism, is applied, associations which 
senefit one or both. Thus some species of Sponge are never found 
zrowing except on the backs or legs of certain Crabs. In these 
sases the Sponge protects the Crab and conceals it from its enemies, 
while the Sponge benefits by being carried from place to place and 
shus obtaining freer oxygenation. Certain Cirripede Crustaceans 


Ir 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 (Ayalonemc) is always found associated with a Zoophyte 
(Palythoa), and there are many other instances. Sponges often also 
grow in very close association with certain low forms of plants 
(Algve). 

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 amceboid zooids similar to the amceboid 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 Ceelenterata, 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 CQELENTERATA 


THE possession of an interfyal cavity lined by a special internal 
layer of cells—the endoderm—in which the digestive and absorp- 
tive functions are centred, distinguishes all the remaining groups 
of Metazoa from the Parazoa or Sponges. The former are grouped 
together under the comprehensive title of Enterozoa, or animals 
with enteric cavity. The simplest Enterozoa have an internal 
cavity in which there is no separation between the enteric 
or digestive cavity and the ceelome or body-cavity—one con- 
tinuous space representing both and opening on the exterior by 
the aperture of the mouth. These constitute the phylum 
Coelenterata, ‘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. Hyprozoa, including the Fresh-water Polypes, Zoo- 
phytes, many Jelly-fishes—mostly of small size,—a few Stony 
Corals, and the peculiar Paleozoic fossils known as Graptolites. 

Class 2. ScypHozoa, including most of the large Jelly-fishes. 

Class 3. AcTINOZOA, including the Sea-anemones, and the vast 
majority of Stony Corals. 

Class 4, CTENOPHORA, including certain peculiar Jelly-fishes 
known as “ Comb-jellies.” 


CLASS I—HYDROZOA. 


1. EXAMPLE OF THE CLASS—Obelia. 


General Structure.—Obelia is a common zoophyte occurring 


in the form of a delicate, whitish or light brown, almost fur-hke 
128 


SECT, IV PHYLUM C@LENTERATA 129 


growth on the wooden piles of piers and wharfs. It consists of 
branched filaments about the thickness of fine sewing-cotton : of 
these, some are closely adherent to the timber, and serve for 
attachment, while others are given off at right angles, and present 
at intervals short lateral branches, each terminating in a bud-like 
enlargement. 

The structure is better seen under a low power of the 
microscope. The organism (Fig. 95) is a colony, consisting of a 
common stem or axis, on which are borne numerous zooids. The axis 
consists of a horizontal portion (hydrorhiza) resembling a root or 
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. 7, 2): these are imma- 
ture zooids. 

The large majority of the zooids have the form of little conical 
structures (P. 1—P. 4), each enclosed in a glassy, cup-like invest- 
ment or hydrotheca (hth), and produced distally into about two 
dozen arms or tentacles (¢): these zooids are the polypes or hydranths, 
Less numerous, and found chiefly towards the proximal region of 
the colony, are long cylindrical bodies or blastostyles (bis), each 
enclosed in a transparent case, the gonotheca (y.th), and bearing 
numerous small lateral offshoots, varying greatly in form acccrding 
to their stage of development, and known as medusa-buds (m.bd). By 
studying the development of these structures, and by a comparison 
with other forms, it is known that both blastostyles and medusa- 
buds are zooids, so that the colony is trimorphic, having zooids of 
three kinds. 

To make out the structure in greater detail, living specimens 
should be observed under a high power. <A polype is then seen 
to consist of a somewhat cylindrical, hollow body, of a yellowish 
colour, joined to the common stem by its proximal end, and pro- 
duced at its distal end into a conical elevation, the manubriwm 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 cavity, the enteron (ent), 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 crcum- 
stances small organisms may be seen to be caught by the polypes 
and carried towards the mouth to be swallowed. 

The hydrotheca (2.th) has the form of a vase or wine-glass, and 
is perfectly transparent and colourless. A short distance from its 


VOL. I K 


Fic. 95.-Obelia sp. A, portion of a colony with certain parts shown in longitudinal section ; 

B, medusa ; C, the same with reversed umbrella; D, the same, oral aspect ; Ba. 1, 2, buds ; 
bls. blastostyle ; c@. coenosarc; ect. ectoderm; end. endoderm; ent. enteric cavity; g.th. 
gonotheca; h.th. hydrotheca; 1, lithocyst; m.bd. medusa-bud; mnb. manubrium ; magl. 


pe nee mth. mouth; p. perisarc; P. 1, 2, 3, polypes; rad, ¢, radial canal; t. tentacle; 
vl. velum. 


SECT. IV PHYLUM CQ@LENTERATA 131 


narrow or proximal end, it is produced inwards into a sort of 
circular shelf (sk), perforated in the centre: upon this the base of 
the polype rests, and through the aperture it is continuous with the 
common stem. When irritated—by a touch or by the addition of 
alcohol or other poison—the polype undergoes a very marked con- 
traction: it suddenly withdraws itself more or less completely into 
the theca, and the tentacles become greatly shortened and curved 
over the manubrium (P. 2). 

The various branches of the common stem show a very obvious 
distinction into two layers: a transparent, tough, outer membrane, 
of a yellowish colour and horny consistency, the perisarc (p), and 
an inner, delicate, granular layer, the cwnosare (ca), 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 perisare 
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 (Lu. 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 
medusz 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 
mesoglea or supporting lamella (msgl), which, unlike the inter- 
mediate layer of sponges, contains no cells and is practically 
structureless. The same three layers occur in the manubrium, 
the ectoderm and endoderm being continuous with one another at 
the margin of the mouth. The tentacles are formed of an outer 
layer of ectoderm, then a layer of mesogloea, and finally a solid 
core of large endoderm cells arranged in a single series. The 

K 2 


132 ZOOLOGY SECT. 


ceenosarc, blastostyles, and medusa-buds all consist of the same 
layers, which are thus continuous through the entire colony. 

The perisare or transparent outer layer of the stem shows no 
cell-structure, but only a delicate lamination. It is, in fact, not a 
cellular membrane or epithelium, like the ectoderm and endoderm, 
but a cuticle, formed, layer by layer, as a secretion from the ectoderm 
cells (see p. 31). It is composed of a substance of chitinoid or horn- 
like consistency, and, like the lorica of many Protozoa, serves as a 
protective external skeleton. When first formed it is of course in 
contact with the ectoderm, but when the full thickness is attained 


. Fic. 96.—Obelia sp. Vertical section of a polype, highly magnified ; ect. ectoderm ; end. endo- 
derm ; ent. enteric cavity ; h.th. hydrotheca; msyl. mesoglea; mth. mouth; ntc, nematocysts ; 
sh. shelf-like prolongation of hydrotheca ; ¢. tentacle. 


the latter retreats from it, the connection being maintained only 
at irregular intervals. In the same way the hydro- and gonothece 
are cuticular products of the polypes and blastostyles respectively : 
in the young condition both occur in the form of a closely fitting 
investment of the knob-like rudiment of the zooid (Fig. 95, Bd 1, 2). 
The ectoderm has the general character of a columnar epithelium 
(see p. 24), but exhibits considerable differentiation of its component 
cells. It is mainly composed of large conical cells with their bases 
outwards, and having between their narrow inner ends clumps of 
small rounded iéterstitial cel/s, and occasional large branched-nerve=— 


Iv PHYLUM CC@LENTERATA 133 


cells (Fig. 98, 2.0). The tentacles. and the manubrium contain, 


ening- of -the-tentaeles-(Fig--98;-mf). This 
muscular layer is a derivative of the ectoderm, and may be looked 
upon as a rudimentary mesoderm. 


Fic. 97.—Nematocysts of Hydra. A, undischarged ; B, discharged; C, nerye-supply ; enb, 
enidoblast ; enc. 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 nic), organs closely 
resembling those of Epistylis umbellaria (p. 93), and like them, 
serving as weapons of offence. Each consists (Fig. 97, A)ofa tough 
ovoid capsule, full of fluid, and invaginated at one end i 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 
enidoblast (enb), which, as it approaches maturity, migrates towards 


“nvaddition, a layerof anstriped mascle-fibees- between the-ectoderm — 
and _the_ me: : udinally,—and—serve 


134 ZOOLOGY SECT. 


the surface and becomes embedded in one of the large ectoderm 
cells. At one point of its surface the enidoblast is produced into 
a delicate protoplasmic process, the enidocil or trigger-hair (enc): 
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 
flagella ‘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- 

Fic. 98.—Tentacle of Eucopella. The brium a lay er of endodermal 
ns eee) ens Eee 
ectoderm is removed and the aiiseulAy taking a transverse direction, 
and nervous layer exposed, in the upper and so serving to antagonise 


part these latter are removed so as to 


show the core of endoderm cells; ect. the longitudinal muscles and 
ectoderm ; end. endoderm; mf. muscle- . 
fibres; fc. nematocyst; ‘nu nucleus; contract the cavity. In the 


nv.c.nerve-cell. (After von Lendenfeld.) 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 meduse— 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 ca-wmbrella, the 


ace i 


Iv PHYLUM CQiILENTERATA 135 


concave inner surface as the sub-wmbrella. From the centre of 
the sub-umbrella proceeds the manubrium (mb), at the free end 
of which is the four-sided mouth (mth). Very commonly, as the 
medusa swims the umbrella becomes turned inside out, the sub- 
umbrella then forming the convex surface and the manubrium 
springing from its apex (Fig. 95, C, and Fig. 99, A). 

The mouth (Figs. 95, 96, 99, and 100, mth) leads into an enteric 
cavity which occupies the whole interior of the manubrium, and 
from its dilated base sends off four delicate tubes, the radial 
canals (rad. ¢), 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 (cire. c), ranning parallel with and close 
to the margin. By means of this system of canals the food, taken 


Fic. 99.—Obelia sp. A, mature medusa swimming with everted umbrella; B, one quarter 
of the same, oral aspect; eirc.c. circular canal; gon. gonad; /. lithocyst ; mb. manubrium ; 
mth, mouth ; rad. c. radial canal; ¢. 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 velwm (Fig. 100, v), and gives off the tentacles (¢), 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 /ithite. These are the 
marginal sense-organs or lithocysts: they were formerly considered 
to be organs of hearing, and are hence frequently called otocysts : 
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 casc, may therefore be looked upon 
as organs of the sense of direction. 

The manubrium (Fig. 100, mnb) of the medusa consists of 


136 ZOOLOGY SECT. 


precisely the same layers as that of the hydranth—ectoderm, 
mesogloa, 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 tlie 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 cndoderm-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 lam 


Fic. 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. circ. c. circular canal; end. lam. endoderm lamella; gon. gonad ; lL. lithocyst ; 
mnb. Ianubrium ; mth. mouth ; rad. c. radial canal; vl. velum. 


layer of mesoglea everywhere intervenes between ectoderm and 
endoderm. 

The velum (v7) consists of a double layer of ectoderm and a 
middle one of mesoglea: there is no extension of endoderm into 
it. The tentacles, ike those of the hydranth, are formed of a 
core of endoderm covered by ectoderm, the cells of the latter 
being abundantly supplied with stinging-capsules. 

Comparison of Polype and Medusa.—Striking as is the 
difference between a polype and a medusa, they are strictly 
homologous structures, and the more complex medusa is readily 
derivable from the simpler polype-form. It is obvious, in the 
first instance, that the apex of the umbrella corresponds with the 
base of a hydranth (Fig. 101, A and D), being the part by which 
the zooid is attached in each case to the parent stem: the mouth 
with the manubrium are also obviously homologous structures in 


Iv 


the two cases. 


PHYLUM CQALENTERATA 137 
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, 


im 


OY 
UW, 


x 


\ a 
BS 


ssn 


eS 
3 


% 
% 
My 


uy 
0) 
ay 


sy f 
unig @Sss y 


Fic.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 a) of form with tentacular 
region extended into the form of a bell; D, vertical, and D’, transverse séction (along the line ab) 


of medusa. The ectoderm is dotted, the endoderm striated, and thi 
circular canal; ect. ectoderm; end endoderm; end. lam. endoderm 


mesoglea black. cir. c. 
mella; ent. cav. enteric 


cavity ; hyp. hypostome or manubrium; mn’. manubrium; msgl. mesoglea; mth. mouth ; 


nv, nv’, nerve-rings; t. tentacle; v. velum. (From Parker's Biology.) 


ae. 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 


radius 


radius 


tnter radius \ 


sub-radtus 


ad-radiws ~_ 
sUub-radiug ~~.___ 


per-radius 


LN f 
Sf NEON 
a, 


& 


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


the endoderm-lamella (D', en/. lam), and remaining only along 
four meridional areas—the radial canals (vad. ¢), and a circular 
area close to the cdye of the bell—the circular canal (rir. ¢). 

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 C@LENTERATA 139 


taken at right angles to its long axis, a large number of radii—about twenty- 
four—can be drawn from the centre outwards, all passing through similar parts, 
z.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-radit. Half 
way between any two per-radii a radius of the second order, or inter-radius, may 
be taken ; half way between any per-radius and the inter-radius on either side a 
radius of the third order, or ad-radius, and half way between any ad-radius and 
the adjacent per- or inter-radius, a radius of the fourth order, or sub-radius. Thus 
there are four per-radii, four inter-radii, eight ad-radii, and sixteen sub-radii. 
In Obelia the radial canals, the angles of the mouth, and four of the tentacles are 
per-radial, four more tentacles are inter-radial, and the remaining eight tentacles, 
bearing the lithocysts, are ad-radial. The sub-radii are of no importance in this 
particular form. 


Reproduction.—In the description of the fixed Obelia-colony 
no mention was made of cells set apart for reproduction, like 
the ova and sperms of a sponge. Asa matter of fact, such sexual 
cells are found only—in their fully developed condition at least— 
in the meduse. Hanging at equal distances from the sub-umbrella, 
in immediate relation with the radial canal and therefore per- 
radial in position, are four ovoid bodies (Figs. 99 and 100, gon), 
each consisting of an outer layer of ectoderm continuous with 
that of the sub-umbrella, an inner layer of endoderm continuous 
with that of the radial canal and enclosing a prolongation of the 
latter, and of an intermediate mass of cells which have become 
differentiated into ova or sperms. As each medusa bears organs 
of one sex only (testes or ovaries, as the case may be), the individual 
meduse are diecious.- It will be noticed that the gonad has the 
same general structure as an immature zooid—an_ outpushing 
of the body-wall consisting of ectoderm and endoderm, and 
containing a prolongation of the enteric cavity. 

Development.— When the gonads are ripe, the sperms of the 
male meduse 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 planwla (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. +1). The Obelia-colony is sexless, having no 
gonads, and developing only by the asexual process of budding ; 
but certain of its buds—the medusee—develop gonads, and from 


Fic, 103.—Stages in the development of two Zoophytes (A—H, Laomedea, I—M, Euden- 
drium) allied to Obelia; A—F, stages in segmentation; G, the planula enclosed in the 
maternal tissues ; H, the free-swimming planula ; I—M, fixation of the planula and develop- 
ment of the hydrula. (From Parker’s Biology, after Allman.) 


their impregnated eggs new Obelia-colonies arise. We thus have 
an alternation of an asexual generation, or agamobtwm—the Obelia- 
colony, with a sewual generation, or gamobium—the medusa. 


2. GENERAL STRUCTURE AND CLASSIFICATION. 


The Hydrozoa may be defined as multicellular animals in which 
the cells are arranged in two layers, ectoderm and endoderm, 
separated by a gelatinous, non-cellular mesoglea, 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 amceboid 
cells, gland-cells, and sometimes muscle-fibres. There are two 
main torms of zooids, polypes or nutritive zovids, which are 
usually sexless, and medusee or reproductive zooids. In corre- 
spondence with its locomotive habits, the medusa attains a higher 


IV PHYLUM CQILENTERATA 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 meduse, 
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 perisare, there are 
some forms in which the ccenosare 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 meduse, 
zoophyte colonies known not to produce free meduse, and medusa: 
known to have no zoophyte stage. “ Moreover, there are many 
meduse 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 medusze, while similar meduse, 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.—LEPTOLINA. 
Hydrozoa in which there is a fixed zoophyte stage, and in which 
the sense-organs are exclusively ectodermal. 
Sub-Order a.—Anthomedusee. 


Leptoline in which the polypes are not protected by hydrothece or the 
reproductive zooids by gonothece ; the meduse bear the gonads on the manu- 
brium and have no lithocysts. 


Sub-Order b.—Leptomeduse. 


Leptoline in which hydro- and gonothece are present: the meduse bear the 
gonads in connection with the radial canals and usually have lithocysts. 


142 ZOOLOGY SECT. 


ORDER 2.—TRACHYLINE. 


Hydrozoa in which no fixed zoophyte stage is known to occur, 
all members of the group being locomotive meduse, some of which 
have been proved to develop directly from the egg. The sense- 
organs are formed partly of endoderm. 


Sub-Order a—Trachymeduse. 


Trachyline in which the tentacles spring from the margin of the wnbrella, 
and the gonads are developed in connection with the radial canals. 


Sub-Order b.—Narcomeduse. 


Trachyline in which the tentacles spring from the ex-umbrella, some dis- 
tance from the margin, and the gonads are developed in connection with the 
manubriun, 


OrDER 3.—HyYDROCORALLINA. 


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 perisare of the branched colonies. 


Systematic Position of the Example. 


Obelia, in virtue of the possession of gono- and hydrothece, and 
of gonads formed in connection with the radial canals, belongs to 
the sub-order Leptomeduse. It is placed in the family Campanu- 
larvide, distinguished by having cup-shaped thece 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 meduse. 


ORDER 1.—LEPTOLINZ. 


The more typical members of this group agree in all essential 
respects with Obelia, consisting of branched colonies bearing two 
principal forms of zooids, which serve for nutritive and reproductive 
purposes respectively. 

General Structure.—The form and size of the colonies are 
subject to great variation: they may be little insignificant tufts 
growing on shells, sea-weeds, &c., or may take the form of com- 
plex trees three feet in height, and containing many thousand 


Iv PHYLUM CdiLENTERATA 143 


zooids. The hydranths may be colourless and quite invisible to 
the naked eye, or, as in some ‘Tubulariz (Fig. 105, 5) may be bril- 
hantly coloured, flower-like structures, nearly an inch in diameter. 
The meduse may be only just visible to the naked eye, or, as in 
Aiquorea, may attain a diameter of 88 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 medusze liberated in thousands from 
the hydroid colonies beneath the surface. 

The two sub-orders of Leptoline are distinguished by the 
arrangement of the perisarc. In the Anthomeduse, of which 
Bougainvillea (Fig. 104) isa good example, the cuticle stops short at 
the bases of the hydranths, and the reproductive zooids are not 
enclosed in gonothece. It is for this reason that, in classifications 
founded on the zoophyte stage, the Anthomedusz are called Gymno- 
blastea or naked-budded zoophytes (see also Fig. 105, 1, 4,5). In 
the Leptomeduse the cuticle is usually of a firmer consistency than 
in the first sub-order, and furnishes hydrothece for the hydranths 
and gonothece 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 medus@ also exhibit characteristic differences in the two 
sub-orders. In the Anthomedusz the umbrella is usually strongly 
arched, and may even be conical or mitre-shaped (Figs. 104; 105, 7; 
109, 7 and 2): its walls are thick, owing to a great development of 
the gelatinous mesoglcea of the ex-umbrella, that of thesub-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 ocella 
(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 Leptomeduse 
the umbrella is usually less convex, thinner, and of softer consist- 
ency than in the Anthomeduse, 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 Leptoline the ceenosarc, as in Obelia, con- 
sists of a more or less branched structure attached to stones, timber, 
seaweeds, shells, &., by a definite root-like portion (hydrorhiza). The 


144 ZOOLOGY SECT. IV 


curious genus Hydractinia (Fig. 105, 7) 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 


Fic.104.~Bougainviilea ramosa, A, entire colony, natural size; B, portion of the same 
magnified; C, immature medusa. cir. ¢, circular canal; ew. cuticle or perisare ; ent. cav. 
enteric cavity; hyd. polype or hydranth ; hyp. hypostome or manubrium ; med. medusa; mnb. 
manubrium; rad. ¢. radial canal; t. tentacle; v. velum. (From 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 Anthomeduse: the Leptomedusan 


3.Corymorpha 
4.Syncoryne 


6. Clavatella 


Fia. 105—Various forms of Leptolinze. In 1, a shows the entire colony, } a portion highly 
magnified ; in 7, « is a species producing medusa-buds from the manubrium, b from the bases 
of the tentacles; dz dactylozooids; m.and M. meduse ; mnt. manubrium; mth. mouth; 
oc. eye-spots ; rad. c. radial canals; s. sporosacs; sp. spines; ¢, ¢1, ¢2, tentacles. 


VOL. I L 


146 ZOOLOGY SECT. 


Clathrozoon, an Australian genus, resembles it in having branched 
and intertwined ccenosareal tubes, the perisare of which under- 
goes fusion; but the complex mass thus produced, instead of 
forming an incrustation on a shell, isa large, abundantly branched, 
tree-like structure, resembling some of the fan-corals or Gorgonacea 
(vide infra).  Ceratella (Fig. 106) has a similar fan-coral-like 
appearance, with a branching axis composed of numerous inter- 


Fia. 106.—Ceratella fusca. About nat. size. (from Hickson, after Baldwin Spencer.) 


twining and anastomosing tubes; but while Clathrozoon possesses 
thecx, in Ceratella they are absent. 

A great simplification of the colony is produced in Myriothela 
(Fig. 105 2), in which the short ccenosarc 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 
meduse (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 


ng 


EER 
- 


aT SCALE FoR A 
joo — 
Fia.107;—-Hydra, A, vertical section of entire animal; B, portion of transverse section, highly 

magnified ; C, two large ectoderm cells; D, endoderm cell of H. viridis; E, large nematocyst ; 
F, small nematocyst; G, sperm, a, ingested diatom ; bd. 1, bd. 2, buds ; chr. chromatophores ; 
enbl. cnidoblast; enc. cnidocil; ect. ectoderm ; end. endoderm ; ent. cav. enteric cavity ; ent. 
cav’. its prolongation into the tentacles; jl. flagellum ; hyp. hypostome or manubrium ; int. c. 
interstitial cells ; i. pr. muscle-processes ; mth. mouth ; msgl. mesoglea; ate. large, and nic’. 
small nematocysts; nu. nucleus; ov. ovum; ovy. ovary; psd. pseudopods; spy. 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 asticky secretion 
from the proximal end, to weeds, &c., but is capable of detaching 
itself and moving from place to place after the manner of a loop- 
ing caterpillar. The tentacles are hollow, and communicate freely 
with the enteron. Both the body and the tentacles are highly 
contractile, the contractions being effected by means of a layer of 
fibres which run longitudinally. These fibres are processes—the 
muscle processes—(C, m. pr.) of the large ectoderm cells. Similar 
shorter muscle-processes of some of the endoderm cells run 
circularly and antagonise the longitudinal fibres. Nematocysts are 
abundant in the ectoderm. The endoderm cells are mostly 
ameeboid and vacuolated. Hach 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 Hydra, but these are always detached 
sooner or later, so that a permanent colony is never formed. There 


Fia.108.-Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the 
disc of attachment to the right. 


are no special reproductive zooids, but simple ovaries (ovy) and 
testes (spy) are developed, the former at the proximal, the latter 
at the distal end of the body. Even simpler than Hydra are 
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 ; medusz are developed from 
processes on the float. Pelagohydra, however, is perhaps more 
nearly related to the Siphonophora—an order yet to be dealt with 
—than to the Leptoline. 

The polypes are usually cylindrical, as in Obelia, but in some 
genera they are widened out into a vase-like form (Fig. 105, 5), in 
others elongated into a spindle-shape (4). |The tentacles may be 
disposed in a single circlet, as in Obelia and Hydra, or there may 
be an additional circlet round the hypostome (3, 4), or at the base of 
the polype, or they may be scattered irregularly over the whole 
surface (4). In Myriothela (2) they are short, and so numerous 
as to have the appearance of close-set papilla. In some forms 


Iv PHYLUM CCQLENTERATA 149 


they are knobbed at the ends, the knobs being loaded with stinging- 
capsules (4). 

In some species a dimorphism of the hydranths obtains, some 
of them being modified to form protective zooids. In Hydractinia 
(J) 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 ccenosarc 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 
Anthomeduse and Leptomeduse, these take the form of locomotive 
meduse, 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 ceenosare. 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 meduse 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, ¢.c. 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 meduse 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 


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SECL. IV PHYLUM CCRLENTERATA 151 


nervous systems. The velum and the sub-umbrella possess 
abundance of muscle-fibres, presenting a transverse striation, 
and round the margin of the umbrella is a double ring of nerve- 
cells and fibres, one ring being above, the other below the at- 


tachment of the velum (Fig. 101, D, nv, nv’). The medusa—thus 
furnish the first. instance _we have met with of-a-central-nervous- 


system, @.¢. 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 medusz 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 meduse the circular canal communicates with the 
exterior by minute pores placed at the summits of papille, the 


Fic. 110—Diagram illustrating the formation of a sporosac by the degradation of a medusa. 4, 
medusa enclosed in ectodermal envelope (cx); B, intermediate condition with vestiges of 
umbrella (uw) and radial canals (a): (, sporosac. er, ectoderm ; en, endoderm ; i, manubrium ; 
ov, ovary ; t, tentacle; v, velum. (From Lang’s Comparative dnutomy.) 


endoderm cells of which contain brown granules. There seems to 
be little doubt that these are organs of eaeretion, the cells with- 
drawing nitrogenous waste-matters from the tissues and passing 
them out through the pores. If we excep’, the contractile 
vacuoles of Protozoa, this is the first appearance of specialised 
excretory organs in the ascending series of animals. 

Besides producing gonads, some meduse multiply asexually by 
budding, the buds being developed either from the manubrium 
(Fig. 105, 7a), or from the margin of the umbrella (72) or the base 
of the tentacles. The buds always have the medusa form. 

In many Leptoline 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 
meduse to ovoid pouch-like bodies called sporosacs (Fig. 105, 1b, 
5, s), each consisting of little more than a gonad, but showing an in- 
dication of its true nature in a prolongation of the digestive cavity 


152 ZOOLOGY SECT. 


of the colony, representing the stomach of the manubrium (Fig. 
110). We thus have a reproductive zooid reduced to what 1s 
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 meduse, 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 Leptomeduse (Fig. 100) are sporosacs, 
ie, reproductive zooids, not mere gonads. In some rare cases the 


Fic.111—Early development of Bucope. A, blastularstage; B, planula with solid endoderm ; 
C, planula with enteric cavity; al. enteric cavity; ep. ectoderm; hy. endoderm. (From 
Balfour’s Embryology, after Kowalevsky.) 


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

In Obelia we found the medusze to be budded off from pecu- 
liarly modified mouthless zooids—the blastostyles. This arrange- 
ment, however, is by no means universal: the reproductive zooids 
—whetler medusee or sporosacs—may spring directly from the 
ceenosare, as in Bougainvillea (Fig. 104), or from the ordinary 
hydranths (Fig. 105,4 and 45). 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 ccenosare, 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 Leptolinz frequently, but not always, 


IV PHYLUM CCELENTERATA 153 


begins within the maternal tissues, 7.c. while the oosperm or im- 
pregnated egg-cell is still contained in the gonad of the meduse 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. 111, 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,a/). The planula then comes to rest, fixes 
itself at one end to some suitable support, and becomes con- 
verted into a simple polype or Aydrula 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.—TRACHYLINE 


General Structure.—The members of this order are all 
meduse: 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 mesoglea 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, te) consisting of an outer layer of ectoderm 


Gh 1.Perasus Fee 2.Glossocodon xe 


Fic. 112.-Two Trachymedusze. civ. c. circular canal; gon. gonad; mnb. manubrium ; ath. 
mouth ; rad. c. radial canal; re. c. recurrent canal; ¢. tentacle ; tc. tentaculocyst ; tg. tongue ; 
vl. velum. (After Haeckel.) 


1.Cunarcha 2.Polycolpa 


Fia. 113.—Two Narcomeduse, 2 in vertical section. gon. gonad; mnb. manubrium ; mth. 
mouth; pr. peronium ; rad.c. radial canal; t. tentacle ; tc. tentaculocyst ; t.7. 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 


lv PHYLUM CQ@LENTERATA 155 


distinguish them from the lithocysts of Leptomeduse, and to mark 
the fact that they are modified tentacles, they are called tentaculo- 
cysts. They may either project freely from the margin of the 
umbrella, or may become enclosed in a pouch-like growth of 
ectoderm and more or less sunk in the tissue of the umbrella. 
Kyes occur in some, and are always of simple structure. 

The two sub-orders of Trachyline are characterised by the mode 
of origin of the tentacles. 
In Trachymeduse, as in the 
preceding order, they arise 
near the edge of the um- 
brella (Fig. 112), but in the 
Narcomeduse they spring 
about half-way between the 
edge and the vertex (Fig. 
113), and are continued, at 
their proximal ends, into the 
ielly of the ex-umbrella in 
the form of “ tentacle-roots ” 
(t.7). 

As to the position of the 
reproductive organs, there 
is the same difference be- 


Fic. 114.-#Hginura myosura, a tentaculo- 


tween the two sub-orders cyst highly magnified. ect. ectoderm ; end. 
: endoderm; J. lithites; afc. nematocysts ; 
of Trachyline as between nv.c. group of nerve-cells. (After Haeckel). 


the two sub-orders of Lepto- 

line. In the Trachymeduse the gonads (Fig. 112, gon) are 
developed in the course of the radial canals: in the Narcomedusee 
(Fig. 118) they le on the manubrium, sometimes extending into 
the pouch-like offshoots of its cavity. 

There is always a well-developed velum, which, as in Fig. 118, 1, 
may hang down vertically instead of taking the usual horizontal 
position. In the Narcomeduse the manubrium is short; in the 
Trachymedusz 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 (¢y) 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 Trachyline is seen in 
Aiginopsis, one of the Narcomeduse. 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 
Eginopsis 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 


156 ZOOLOGY SECT. 


medusa. Thus the actual formation of the medusa from the 
hydrula of Aiginopsis corresponds precisely with the theoretical 
derivation given above (p. 136), It will be seen that in the present 
case there 1s 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 or 
changes. 

Metagenesis is, however, nut quite unknown among the Trachy- 
line. Ina parasitic Narcomedusa (Cunina parasitica) the planula 


Fic. 115.—Larva of ABginopsis. m. mouth; ¢. tentacle. (From Balfour, after Metschnikoff.) 


fixes itself to the manubrium of one of the Trachymeduse 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 hydrule by budding, these last 
becoming converted into medusz in the usual way. 


ORDER 3.—-HYDROCORALLINA. 


The best-known genus of Hydroid Corals is Afillepora, one species 
of which is the beautiful Elk-horn Coral, J/. aleicornis. 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, gp); the smaller are arranged more or less 
irregularly round the gastropores, and are called dactylopores (d.p). 
The whole surface of the coral between the pores has a pitted 
appearance. Sections (C) show that the entire stony mass is 
traversed by a complex system of branched canals, which com- 
municate with the exterior through the pores. The wide vertical 


Iv PHYLUM CCHLENTERATA 157 


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

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


Fic, 116.—Mlillepora alcicornis. A, part of skeleton, natural size; B, portion of surface, 
magnified; C, vertical section, magnified ; ¢.p. dactylopores ; g.p. gastropores ; tb. tabule. 
(After Nicholson and Lydekker.) 


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

The ccenosarcal tubes have the usual structure, consisting of 
ectoderm and endoderm with an intervening mesoglea. 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 perisare of the Leptoline, 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-lke colony, abund- 
antly branched in one plane, and of a deep pink colour. On the 
branches are little cup-like projections with radiating processes 
passing from the wall of the cup towards the centre, and thus 


158 ZOOLOGY SECT. 


closely resembling the true cup-corals belonging to the Actinozoa 
(vide infra). But in the case of Stylaster each “cup” is 
the locus, not of one, but of several 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 ampulle ; in one species at 


Fia. 117.—Mfillepora. 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. 
dactylozovid ; g.p. gastropore ; mth. mouth; P. polype; t. tentacle. (Altered from Moseley.) 


the apices of the dactylozooids. They are meduse, but never 
have the complete medusa-form, being devoid of velum, mouth, 
radial canals and tentacles. Both male and female medusz 
become free, but the period of free existence is very 
short. 

In Stylaster the medusoid character is much more completely 
lost, and the gonophores are more of the nature of sporosacs or 


IV PHYLUM CCELENTERATA, 159 


degraded reproductive zooids 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- 


Fic. 118—Stylaster sanguineus. A, portion of skeleton, natural size; B, small portion, 
* magnified ; a. ampulla ; d.p. dactylopores ; g.p. gastrupores. (After Nicholson and Lydekker.) 


reefs” partly or entirely surrourding many of the islands in 
those seas. Fossil forms are found as far back as the Triassic 
epoch. 


ORDER 4.—SIPHONOPHORA, 


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 Jong, slender, floating stem, to which a 
number of structures, differing greatly in form, are attached. At one 
—the uppermost—end of the stem isan ovoid, bubble-like body con- 
taining air—the float or pneumatophore (pn). Next come a number 
of closely set, transparent structures (net), having the general char- 
acters of unsymmetrical medusz without manubria, each being a 
deep, bell-like body, with a velum and radiating canals. During life 
these swimming-bells or nectocalyces contract rhythmically—ze. at 
regular intervals—drawing water into their cavities, and immedi- 
ately pumping it out, thus serving to propel the entire organismr 


‘S, SUF 
ie B) 
N 


(Me 


1S} 


ISS 


Ly) 


Fic, 119.—Halistemma tergestinum, A, the entire colony; B, a single group of zooids. 
ce. cenosarc ; dz. dactylozooid ; khpk. hydrophyllium or bract; net. nectocalyx or swimming- 
bell; xtc. battery of nematocysts ; p. polypo; pr. pneumatophore or float; s, s’, sporocysts ; 
t, tentacle. (After Claus.) 


SECT. IV PHYLUM CQiLENTERATA 161 


through the water. Below the last nectocalyx the character of the 
structures borne by the stem changes completely: they are of 
several kinds, and are arranged in groups which follow one 
another at regular intervals, and thus divide the stem into seg- 
ments, like the nodes and internodes of a plant. 

Springing from certain of the “nodes” are unmistakable polypes 
(p), differing however from those we have hitherto met with in 
having no circlet of tentacles round the ‘mouth, but a single long 
branched tentacle (¢) arising from the proximal end, and bearing 
numerous groups or “batteries” of stinging-capsules (née). 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 hydrophy/lia (hph)—spring from the “ internodes ” and 
partly cover the sporosacs. 

It is obvious that on the analogy of such a hydroid polype as 
Obelia, Halistemma is to be looked upon as a polymorphic floating 
colony, the stem representing a ccenosarc, and the various struc- 
tures attached to it zooids—the polypes nutritive zooids, the 
feelers tactile zooids, the sporosacs reproductive zooids, the bracts 
protective zooids, and the swimming-bells locomotory zooids. The 
float may be looked upon as the dilated end of the stem, which 
has become invaginated or turned-in so as to form a bladder 
filled with air, its outer and inner surfaces being furnished by 
ectoderm, and the middle portion of its wall by two layers of 
endoderm, between which the enteric cavity originally extended 
(Fig. 120, pn). The upper or float-bearing end is proximal— 
ae. answers to the attached end of an Obelia-stem: it is the 
opposite or distal end which grows and forms new zooids by 
budding. 

In some Siphonophora the bracts contain indications of radial 
canals, so that these structures, as well as the swimming-bells 
and sporosacs, are formed on the medusa-type, while the hydranths 
and feelers are constructed on the polype-type. 

It will be noticed that the radial symmetry, so characteristic 
of most of the Hydrozoa previously studied, gives way, in the 
case of Halistemma, to a bilateral symmetry. The swimming-bells 
are placed obliquely, and the mouth of the bell is not at mght 
angles to the long axis, so that only one plane can be taken 
dividing these structures into two equal halves: the same applies 
to the polype and feelers with their single basal tentacle. When 
first formed the various zooids are all on one side of the stem, but 
the latter becomes spirally twisted during growth, and so causes 
them to arise irregularly. 


VOL. I M 


162 ZOOLOGY SECT. 


The egg of Halistemma gives rise to a ciliated planula re- 
sembling that of the other Hydrozoa. At one pole the ectoderm 
becomes invaginated to form the float (Fig. 121, ep), the opposite 
extremity is gradually converted into the first polype (wo), and 


rect’ 


Ap 


Fic. 120.—Diagram of a Siphonophore: the thick line represents endoderm ; the space ex- 
ternal to it, ectoderm ; the internal space, the enteric cavity. cw. comosare ; Jz, dactylozooid ; 
kph. hydrophyllium ; ind. sporosae ; nel, net’. nectocalyces ; ntc. battery of neraatocysts ; 
p. polype ; pr. pneuimatophore ; é. tentacle, (After Claus.) 


a bud appears on one side which becomes the first tentacle (é). 
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 


a 


Iv PHYLUM CQELENTERATA 163 


become further and further apart, being always situated at the 
distal and proximal ends of the colony respectively. 


In an allied form (4ga/ma) 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- 
colony as a medusa the manubrium of which has extended immensely and 
produced lateral buds after the manner of some Anthomeduse (Fig. 105, 7 a). 


Fig. 1:1.—Two stages in the development of Halistemma: the endoderm is shaded, the 
ectoderm left white. ep. pneumatocyst or air-chamber of pneumatophore ; hy. endoderm 
surrounding pneumatophore ; po. polype ; pp. pneumatophore ; t, tentacle. (From Balfvur, 
after Metschnikoff.) 


On this theory the entire cwnosare 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 asa 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 coenosare. 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 (¢7): as a rule it is of a brilliant peacock-blue 
colour, but orange-coloured specimens are sometimes met with. 
At one end is a minute aperture communicating with the exterior. 
There are no swimming-bells, but from the underside of the float 
hang gastrozooids (p), ‘dactylozooids, branching _ blastostyles 
(gonodendra) with groups of medusoids looking like bunches 
of grapes of a deep blue colour, and long retractile sri (¢), 

M 


164 ZOOLOGY SECT. 
sometimes several feet in length and containing batteries of 
stinging-capsules powerful enough to sting the hand as severely 
asa nettle. The male reproductive zooid remains attached, as in 


&. 


> 
‘ 
} 


> 


LN 


AN 


wt 


a TAGE Te ota ethernet 
ny 


sal 


“en, 


Fic. 122.—Physalia : the living animal floating on the surface of the sea. cv. crest ; ». polype ; 
pn, pneumatophore ; ¢, tentacle. (After Huxley.) 


Halistemma, but the female apparently becomes detached as a 


free medusa. 


In Diphyes the float is absent. Two swimming-bells (Fig. 123.4, m) 
of proportionally immense size are situated at the proximal end 
of the ccensare, and are followed by widely-separated groups of 
zooids (2), each group containing a polype (7) with its tentacles (2), 


Iv PHYLUM C@LENTERATA 165 


a meduzoid (y), and a large enveloping bract (¢). 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 (ig. 124) of a discoid 


Fic. 123.—LBiphyes campanulata_ A, the entire colony; B, single group of zooids. a, 
ccenosarc ; ¢, cavity of swimming-bell; ¢, groups of zooids; g, medusoid ; i, grappling line or 
tentacle; m, swimming-bell; », polype; 0, mouth of swimming-bell; ¢, bract. (From 
Parker’s Biology, after Gegenbaur.) 


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 (¢) and from its lower 
surface depend numerous closely sct blastostyles provided with 
mouths and bearing medusz, while in the centre is a single large 


166 ZOOLOGY SECT. 


gastrozooid (hy). The closely allied genus Velella is of rhomboidal 
form, and bears on its upper surface an oblique sail. 

The reproductive zooids are liberated as free meduse. The 
eggs give rise to young which have a close resemblance to flat 
medusx 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 meduse the 
sub-umbrella of which has given rise to buds forming the feelers 


Fic. 124.—Porpita pacifica. <A, from beneath ; B, vertical section. Ay, large central gastro- 
zooid ; hy’. blastostyles ; sk, chambered shell; ¢, dactylozooids. (From Parker's Biology, after 
Duperry and Koelliker.) 


and blastostyles. But, as their early development is not known, it 
is still quite legitimate to describe them in the same terms as the 
other Siphonophora—z.e, to consider them as hydroid colonies in 
which the ccenosare 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 dorble row, numerous small projections, the hydrothece (Fig. 125, h.th). 
The ceenosarcal skeleton is strengthened by a slender axis, the viryula (v), the 
proximal end of which is connected with a small dagger-shaped body, the 
sicula (s), supposed to be the skeleton of the primary 
zooid by the budding of which the colony was pro- 
duced. In connection with some species oval or 
cup-like capsules have been found: these may be of 
the nature of gonothece. But it must be added that 
the evidence in favour of associating the Graptolites 
with the Hydrozoa is by no means conclusive, and A B 
reasons have been adduced for regarding them as 
connected with groups much higher in the scale. 


hy the 
ADDITIONAL REMARKS ON THE Hyprozoa. 
“i 
The vast majority of Hydrozoa are 
marine, the only exceptions being Hydra, Tuyth 
found all over the world; Mtcrohydra, at y 
present known only in North America; 
Cordylophora, one of the Anthomeduse, 
found in Europe, America, Australia, and 
New Zealand ; Polypodium, also an Antho- ¥ g 
medusa, found in the Volga, where in 
one stage of its existence it 1s parasitic on "1% > Graptolites. 
the eggs of a Sturgeon; Limnocodium, a B, DN EROEE hoth 
doubtful Trachymedusa, hitherto found theca; s. siculas © vir- 
only in a tank in the Botanical Gardens, Le ot aan 


Regent’s Park, where it was probably in- 
troduced from the West Indies; and Limnocnida, found in 
Lake Tanganyika, Africa. 

The oldest known Hydrozoa are the Graptolites, found first in 
the Cambrian rocks; Hydractinia occurs in the Cretaceous epoch, 
and Hydrocoralline from the Cretaceous onwards. 

Parasitism, although rare, is not unknown in the class. Poly- 
podium, one of the Anthomeduse, is parasitic during part of its 
existence, in the ovary of the Sturgeon; and Cunina, one of the 
Narcomeduse, 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. 


168 ZOOLOGY SECT. IV 


This is still more notably the case in the lower Metazoa, such 
as Ascetta and Hydra, in which we have numerous cells combined 
to form a permanent two-layered sac with a terminal aperture, 
some of the cells having digestive, others tactile, others repro- 
ductive functions. Thus while an Amceba or a Parameecium 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 1s made over 
exclusively to the meduse, 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 af 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 winbrella, the convex surface of which, or ex- 
unbrella, is uppermost in the ordinary swimming position (Figs. 
126 and 127, A). The outline is approximately circular, but is 
broken by cight notches, in each of which lies a pair of delicate 
processes, the marginal lappets (mg. lp.): between the pairs of 
lappets the edge of the umbrella is fringed by numerous close-set 
marginal tentacles (t). 


en es met 5 acta 


arenes eng 


Fic, 126.—Aurelia aurita, A, dorsal view, part of the ex-umbrell 
the stomach and one of the four gastric pouches ; B, ventral view. 
removed, a.r.¢. adradial canal; g. /. gastric filaments ; gon. 
ir. c. inter-radial canal; my. /p. marginal lappet ; mth. mouth 
radial canal ; s.g. p. sub-genital pit; st. stomach ; ¢. tentacles, 


a cut away to show part of 
—two of the oral arms are 
gonads; 9. p. gustric pouch ; 
30rd oral arm; p.r. ¢. per- 


170 ZOOLOGY SECT. 


A narrow region of the umbrella adjoining the edge is very thin 
and flexible: the structure thus constituted, with its marginal 
notches and the fringe of marginal tentacles, is the velariwm. 
Unlike the true velum of the medus 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 manubriwm ; 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, zc. 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 suwb-genital pit (s.9.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.7.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 (y.7.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 mesoglea, which, however, 
is not structureless, but is traversed by branching fibres and 
contains ameceboid cells derived from the endoderm. Both ex- 
and sub-umbrell# are covered with ectoderm, and the stomach and 
canal system are lined with endoderm, which is ciliated through- 


Iv PHYLUM C@LENTERATA 171 


out. Some observations seem to show that the short tube 
described above as a gullet and a pair of the gastric pouches 
are lined, not by endoderm, but by an in-turned portion of the 
ectoderm, but this matter cannot be considered as definitely 
settled. 

It was mentioned above that in the free medusa the gonads 
appear through the transparent umbrella as coloured horseshoe- 


Fic. 127.—Aurelia aurita. A, side view, one-fouith of the umbrella cut away; B, diagrammatic 
vertical section, ectoderm dotted, endoderm striated, mesogloea black. cire. c. circular canal ; 
g.f. gastric filaments ; gon. gonad; g. p. gastric pouch; gul. gullet ; h. hood ; 7.7. c. inter-radial 
canal; mg. /p. 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, é.c. towards the mouth. Being developed from 
the floor of the enteric cavity, the gonad is obviously an:. 


172 ZOOLOGY SECT, 


endodermal structure: when mature, its products—ova or sperms 
—are discharged into the stomach and pass out by the mouth. 
Here, then, is an important difference from the Hydrozoa, in 
which the generative products are usually located in the ectoderm, 
and are always discharged directly on the exterior. The sexes 
are lodged in distinct individuals. 

Lying parallel with the inner or concave border of each gonad 
isa row of delicate filaments (Fig. 126, 127, g.f), formed of endoderm 
with a core of mesoglea and abundantly supplied with stinging- 
capsules. These are the gastric filaments or phacelle: 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 


Fic, 128. Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the 
tentaculocyst ; B, vertical section of the same region (diagrammatic), h, hood; /, lithite ; 
mg. tp, marginal lappet ; oc} ocellus ; olf. 1, olf. 2, olfactory pits. (Altercd 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 meduse. 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 CQiLENTERATA 173 


a prolongation of the circular canal, and thus representing a hollow 
iustead of a solid tentacle. At the extremity are calcareous con- 
cretions or /ithites (1) derived from the endoderm, and on the outer 
side is an ectodermal pigment-spot or ocellus (oc). The 
tentaculocysts are largely hidden by the marginal lappets (mg. lp) 
and by a hood-like process (/.) connecting them ; and in connection 
with each are two depressions, one on the ex-umbrella (0/7. 7), the 
other immediately internal to the sense-club (o/f. 2): these 
depressions are lined with sensory epithelium and are called 
olfactory pits. 

The development and life-history of Aurelia present several 
striking and characteristic features. The impregnated egg-cell 
or oosperm divides regularly and forms a morula, which, by accumu- 
lation of fluid in its interior, becomes a blastula—a closed sac with 
walls formed of a single layer of ceils. 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, sé), the bottom of which 
becomes perforated so as to communicate with the enteric cavity 
(C, st): the depression is the stomodwum, a structure of which 
there is no trace in the Hydrozoa. On two opposite sides of the 
mouth hollow processes grow out, forming the first two tentacles: 
soon two others appear at right angles to these, the organism 
thus being provided with four per-radial tentacles. Subsequently 
four inter-radial and eight adradial tentacles appear. At the 
same time the attached or proximal end is narrowed into a stalk- 
like organ of attachment (KE), and the endoderm of the enteric 
cavity is produced into four longitudinal ridges, inter-radial in 
position, and distinguished as the gastric ridges or tenioles (D tn.). 
The mouth (E, mth.) assumes a square outline, and its edges become 
raised so as to form a short manubrium (mzb.); and, finally, the 
ectoderm of the distal surface—ze. the region lying between the 
mouth and the circlet of tentacles—becomes invaginated in each 
inter-radius so as to produce four narrow funnel-like depressions— 
the septal funnels or infundibula (E and F,s. /.)—sunk in the four 
gastric ridges. 

The outcome of all these changes is the metamorphosis of the 
planula into a polype (E), not unlike a Hydra or the hydrula-stage 
of the Leptoline, but distinguished by a pronounced differentia- 
tion of structure, indicated by the sixteen tentacles developed in 


174 


ZOOLOGY SECT. 


regular order, the stomodwum, and the four gastric ridges with 
their septal funnels. The Scyphozoon-polype is called a scyphula 
or scyphistuma. 


Fic. 129.~Aurelia aurita, devclopment, A, planula, erroneously represented as completely 


closed; B, C, formation of stomodeum; D, transverse section of young seyphula; E, 
seyphula ; F, longitudinal section of same: the section passes through a per-radius on the 
left of the dotted line, through an inter-radius on the right; G, division of scyphula 
into ephyrule ; IH, ephyrula from the side; L, the same from beneath. In A—D_ and 
F the ectoderm is unshaded, the endoderm striated, and the mesuglea dotted. a, lobes 
of umbrella ; mab. manubrium ; mth. mouth ; 3,f. septal funnel ; s¢. stomodeeum ; t. tentacle ; 
tn, twnivles. (From Korschelt and Heider's Embryology.) 


The scyphula may grow to a height of half an inch, and some- 


times nultiplies by budding. After a time it undergoes a process 


Iv PHYLUM COBLENTERATA 175 


of transverse fission (G), becoming divided by a series of constric- 
tions which deepen until the polype assumes the appearance of a 
pile of saucers, each with its edge produced into eight bifid lobes, 
four per- and four inter-radial. Soon the process of constriction 
is completed, the saucer-like bodies separate from one another, 
and each, turning upside down, begins to swim about as a small 
jelly-fish called an ephyrula(H, I). The umbrella of the ephyrula 
1s divided into eight long bifid arms («) with deep (per-radial or 
inter-radial) notches: it has of course carried away with it a 
segment of the stomach with the gastric ridges of the scyphula: 
during the process of constriction this becomes closed in on the 
proximal or ex-umbrellar side, while on the sub-umbrellar side it 
remains open, and its edges grow out to form a manubrium. 
Round the margin there are the bases of eight per-radial and 
inter-radial tentacles, each in the notch of one of the arms, and 
eight ad-radial tentacles in the intervals between the lobes: the 
latter disappear completely; the former may persist as the 
tentaculocysts. On each gastric ridge appears a single gastric 
filament, soon to be followed by others, and in the notches at the 
extremities of the eight arms tentaculocysts are now recognisable. 
In the meantime the spacious enteric cavity is continued into the 
eight arms in the form of wide radiating canals. 

As the ephyrula grows the adradial regions—at first deeply 
notched—grow more rapidly than the rest, the result being that 
the notches become gradually filled up, and the umbrella, from an 
eight-rayed star, becomes a nearly circular disc. Four oral arms are 
developed and numerous marginal tentacles, and the ephyrula 
gradually assumes the form of the adult Aurelia. It seems 
probable that the sub-genital pits of the medusa are formed 
from sections of the septal funnels of the scyphula. 

Thus the life-history of Aurelia differs in several marked 
respects from that of any of the Hydrozoa. There is, in a sense, 
an alternation of generations as in Obelia, the gamobium being 
represented by the adult Aurelia, the agamobium by the scyphula. 
But instead of the medusa being developed either as a bud on a 
branched colony, as in Leptoline, or by direct metamorphosis of a 
polype, as in Trachyline, 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 scyphule which do not undergo 
transverse division, the entire scyphula becoming metamorphosed 
into a single adult. 


| 


1 


176 ZOOLOGY SECT. 


2. GENERAL STRUCTURE AND CLASSIFICATION. 


The Scyphozoa may be defined as medusoid Coelenterata, having 
the same general structure and arrangement of the layers as the 
medusoid Hydrozoa, but differing from them in the possession of 
endodermal gastric tentacles; in having gonads the sexual cells 
of which are lodged in the endoderm and which discharge their 
products into the digestive cavity; in the absence of a true velum, 
and in nearly all cases, in the presence of sense-organs in the form 
of hollow sense-clubs or tentaculocysts. Whether a stomodeum 
or ectodermal gullet occurs is uncertain. Asin 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 Zucernaria 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 stomodeeum. 

The Scyphozoa are divisible into four orders, as follows :— 


ORDER 1.—STAUROMEDUS& (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.—CUBOMEDUS2. 
Scyphozoa with a four-sided cup-shaped umbrella: four per- 
radial tentaculocysts. 
ORDER 4.—DISCOMEDUS&. 


Scyphozoa with a flattened saucer- or disc-shaped umbrella: 
not fewer than eight tentaculocysts—four per- and four inter- 
radial. 


Iv PHYLUM CQ@LENTERATA WW 


Sub-Order a—Semostome. 


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


Sub- Order b—Rhizostome. 


Discomeduse 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 Bvample. 


Aurelia aurita is one of several species of the genus Aurelia, 
and is placed in the family Ulmaridw, the sub-order Semostome, 
and the order Discomedusw. 

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


Tessera (Fig. 130), formerly regarded as the simplest member of this group, 
is now looked upon as probably not 4 mature form. It is described as a small 
medusa about 4 mm. in diameter having the same general characters as the 
scyphula-stage of Aurelia, except that the bell-shaped body is free-swimming. 
The edge of the umbrella is surrounded by eight tentacles, four per-radial 
(p.r.t.) and four inter-radial (2.7.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 (t.). As in the scyphula, each gastric ridge contains an 
infundibulum, lined with ectoderm and opening on the sub-umbrella. The 
gastric filaments (9.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 SECT. 


(rudimentary eye). Slenoeyphus is an allied form which probably is able to 
move by creeping (looping) movements like those of u leech. Capria has no 


Fic. 130.—Tessera princeps. A, external view; B, vertical section, g.,f. gastric filament, 
gon. gonad; a7. t. inter-radial tentacle; mnub. manubriuin ; mth. mouth; p.r. ¢. per-radial 
tentacle ; st. stomach; tn. teeniole. (After Haeckel.) 


Bae 2 a : 


Fic, 131.—Lucernaria. A, oral aspect ; B, from the side, g. foot-gland ; y. /. gastric filaments 
gon. gonad ; wik, mouth ; ¢. tentacles; ta. tenioles, (After Claus.) 


tentacles. he Depastride have an almost entire margin fringed with 
tentacles. 


Iv PHYLUM CQ@LENTERATA 179 


ORDER 2.—CoRONATA. 


This group includes a number of rare and beautiful Medus» of curiously 
complex structure, of which Pericolja may be taken as an example. The 
umbrella (Fig. 132) is usually conical, and is divided by a horizontal furrow 
(coronary groove) into an apical region or cone (cn.) and a marginal region or 


Fic. 132,—Pericolpa quadrigata. A, external view; B, vertical section. circ. s. circular 
sinus ; cn. cone; g. f. gastric filaments; gon. gonads; mg. lp. marginal lappets; mnb. manu- 
brium ; mth. mouth; pd. 2. pedal lobes; st. stomach; ¢. tentacles; tc. tentaculocysts ; tz. 
tenioles. (After Haeckel.) 


crown; the crown is again divided by 2 second, rather irregular horizontal 
furrow into a series of pedal lobes (pd. l.), adjacent to the cone, and a series of 
marginal lappets (mg. lp.), forming the free edge of the bell. In some of the 
Coronata, such as Pericolpa, the pedal lobes and marginal lappets correspond 
(i.e. are in the same radii) ; in others (Periphylla, Hphyropsis) they alternate. 


N 2 


180 ZOOLOGY SECT. 


In Pericolpa four of the pedal lobes, inter-radial in position, bear 
tentaculocysts (¢c.); four others, per-radially situated, give origin to long, 
hollow tentacles (¢.). In the more complex genera there are eight additional 
adradial tentacles. 

The mouth (mth.) is very large, and leads by a wide manubrium (mnb.) into 
a spacious stomach (sf.), 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 (cire. s.), As in Lucernaria, there are four wide inter-radial im- 
fundibula, The gastric filaments (g. /) 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 


Fic. 133.—MNausithoe. The entire animal from the oral aspect. ar. adradii; g. gonads; g. f. 
gastric filaments ; i. inter-radii; m. circular muscle of sub-umbrella ; pr. per-radii; rl. tenta- 
culocysts; sr. sub-radii; ¢. tentacles. The black cross in the centre represents the mouth. 
(From Lang's Comparative Anutomy.) 


—there is great variation. Pericolpa and its allies (Peromeduse) resemble the 
Lucernarida and the members of the order Cubomedusew in the presence of 
twnioles and inter-radial septa: Ephyropsis and its allies (Cannostome) 
resemble the order Discophora in the absence of these structures, The scyphula 
larva of Nausithoé (Fig. 133) lives as a parasite in the interior of a horny 
sponge. 


OrvDER 3.—CUBOMEDUS. 


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 Meduse more than any of the 
other Scyphozoa. The best known species, Charybdea marsupialis (Fig. 134), is 
about 5 cm. in diameter and of very firm consistency. 


Iv’ PHYLUM CQiLENTERATS 181 


As in the lower Coronata, the margin of the umbrella bears four tentacles 
(t.) and four tentaculocysts (¢v.) but the position of these organs is reversed, the 
tentaculocysts being per-radial, the tentacles inter-radial. The tentaculocysts 
are set in deep marginal notches, and the tentacles spring from conspicuous 
gelatinous lobes (/.), which probably answer to the pedal lobes of the preceding 
order, These pedal lobes sometimes bear a number of supplementary tentacles. 


Fic. 134.—Charybdza 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; end. lam. endoderm lamells; vn. dam’. its pro- 
longation into the velarium; g. f. gastric filaments; gon. gonad; gon’. septum separating 
gonads; J. lappet; mnb. manubrium ; rad. p. radial pouch; ¢, 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 shelf (v/.), resembling the velum of the hydroid Meduse, 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 mesoglea. 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 cquivalent 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 
(cire. ¢.), which is divided into chambers by the mesenteries. Near the junction 
of the gastric pouches with the stomach are the usual four groups of gastric 
filaments (g. f-). 

The gonads (yon.) are four pairs of narrow plate-like organs, attached one 
along each side of each inter-radial septum. The nervous system takes the form 
of a sinuous nerve-ring round the margin of the bell, bearing a distinct group of 
nerve-cells at the base of each tentaculocyst and tentacle. The Cubomeduse 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&. 


The preceding orders are all small ones, z.¢., 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 Semostome and 
Rhizostome are large, and one of the former group— Cyanca. 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 Semostome 
(Fig. 126), and altogether absent in the Rhizostome (Fig. 135). In the 
Semostome there are four oral arms (Fig. 126, 0 r. a.), each resembling a leaf 
folded along its midrib, and having more or less frilled edges : in the Rhizostome 
each of the original four arms (Fig. 135, or. a.) becomes divided longitudinally in 
the course of development, the adult members of the group being characterised 
by the presence of eight arms, often of great length, and variously lobed and 
folded so as to present a more or less root-like appearance. 

The arrangement of the enteric cavity and its offshoots presents an interest- 
ing series of modifications. In no case are there any texnioles or inter-radial 
septa (mesenteries). In the Semostome (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 intoa circular canal round the 
margin of the umbrella. 

In the Rhizostome (Fig. 135, B) a similar network of canals is found in the 
umbrella, but an extraordinary change has befallen the oral or ingestive portion 
of the enteric system. Looking at the oral or lower surface of one of these Jelly- 
fishes, such as Pilema, no mouth is to be seen, but a careful examination of the 
oral arms shows the presence of large numbers—hundreds, or even thousands in 
some cases—of small funnel-like apertures (B, C, s.mth.) with frilled margins. 


Iv PHYLUM CQ&LENTERATA 183 


Rhizostomes have been found with prey of considerable size, such as fishes, em- 
braced by the arms and partly drawn into these apertures, which are therefore 
called the suctorial mouths. They lead into canals in the thickness of the arms 
(B, c.), the lesser canals unite into larger, and then finally open into the stomach 
(st.). We thus get a polystomatous or many-mouthed condition which is practi- 
cally unique in the animal kingdom, the only parallel to it being furnished 
by the Sponges, in which the inhalant pores are roughly comparable with the 
suctorial mouths of a Rhizostome. - 
It has been found that this characteristic arrangement is brought about by 
certain changes taking place during growth. The young Rhizostome hasa 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 


“< 


Sarah, 


M 


LLL 


Fic.135._Pilema pulmo. A, side view of the entire animal; B, vertical section, diagrammatic ; 
C, one of the suctorial mouths, magnified. ¢. arm canal; ¢.,/: gastric filamments; gon. gonads 5 
or. a. oral arms 3; rad. ¢. radial canal ; s. mth. suctorial mouths ; s¢. stomach ; ¢/, (2, ¢3, 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 Rhizostomz 
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 suh-genital portico, which lies immediately below the 
floor of the stomach and above the brachial disc. 


184 ZOOLOGY SECT. 


In many of the Discomedusx development takes place in the same general 
way as in Aurelia, 7c. the impregnated egg gives rise to a scyphula or asexual 
polype stage, which, hy transverse division, produces sexual meduse. In. 
Cassiopeia the scyphula arising from the fertilised ovum gives off buds which 
become detached as free-swimming planule, and these, coming to rest, develop 
into scyphule. But in other cases there is no alternation of generations, and 
development is direct. For instance, in Pelagia (Fig. 136)—one of the 
Semostome—a blastula is formed which becomes invaginated at one end, 


Fic. 136.—Pelagia noctiluca : Three developmental stages. m. mouth; +. marginal lappet ; 
s. tentaculocyst. (From Korschelt and Heider, after Krohn.) 


forming a gastrula. The blastopore or gastrula-mouth remains open, and a 
considerable space is left between the invaginated endoderm and the ectoderm. 
Next the mouth region becomes elevated, forming a manubrium, and around 
this a circular depression appears—-the rudiment of the sub-umbrellar cavity— 
surrounded by a raised ridge, the umbrella margin, which soon becomes divided 
into lobes, the marginal lappets. Up to this time the embryo is ciliated 
externally, but soon the cilia disappear, and the little creatures assume somewhat 
the form of an ephyrula, which gradually develops into the adult Pelagia. 


ADDITIONAL REMARKS ON THE SCYPHOZOA. 


The Scyphozoa are all marine, and the majority are pelagic, ac. 
swim freely on the surface of the ocean. A few inhabit the deep 
sea, and have been dredged from as great adepth 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, ¢.g. Pelagia noctiluca, are phosphorescent. They 
are all carnivorous, and although mostly living upon small 


Iv PHYLUM CCLENTERATA 185 


organisms, are able, in the case of the larger species, to capture 
and digest Crustaceans and Fishes of considerable size. In many 
cases small fishes accompany the larger forms and take shelter 
under the umbrella. 

Considering the extremely perishable nature of these organisms, 
and the fact that many of them contain not more than 1 per cent. 
of solid matter, it is not to be expected that many of them should 
have left traces of their existence in the fossil state. Nevertheless, 
in the finely grained limestone of Solenhofen, in Bavaria, belong- 
ing to the Upper Jurassic period, remarkably perfect impressions 
of Jelly-fishes have been found, some of them readily recognisable 
as Discomedusee. 


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” (Lealta crassicornis), one of the commonest British 
species. 

External characters.—Tealia (Fig. 187, 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, &. 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.j, from which streaks of colour radiate outwards. 
Springing from the disc and encircling the mouth are numerous 
short conical tentacles (¢.), 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, 


ZOOLOGY SECTS 


Fic. 137.—Tealia crassicornis. A, dissected specimen; 8, transverse section, the half 
above the line ab through the gullct, the lower half below the gullet. d. mes, directive 
mesenteries ; gon. gonads ; qul. gullet ; 1. m. longitudinal muscle ; /p.lappet ; mes. 1, primary» 
mes. 2, secondary, mes. 3, tertiary mesentcries ; mes. jf. mesenteric filantents; mfh. mouth ; 
ost. 1, ost. 2, ostia; p. m. parietal muscle ; sgph. siphonoglyphe ; s.m- sphincter muscle ; ¢. 7: 
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 (gl.), 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 (/p.), This tube is the gullet or stomodcum, 
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 mesentertes (mes. 1): between 
these are incomplete secondary mesenterics (mes. 2), which extend 
only part of the way from the body-wall tothe gullet, and 
tertiary mesenteries (mes. 3), which are hardly more than 
ridges on the inner surface of the body-wal]. Thus the entire 
internal cavity of a Sea-anemone is divisible into three regions: 
(1) the gullet or stomodeuwm, communicating with the exterior 
by the mouth, and opening below into (2) a single main digestive 
cavity, the stomach or mescnteron, which gives otf (8) a number of 
radially arranged cavities, the inter-mesenteric chambers or metentera. 
It is obvious that we may compare the gullet and stomach with 
the similarly named structures in the scyphula-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 ostiwm (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. f.), answering to a gastric filament of 
the Scyphozoa. In many Sea~-Anemones the mesenteric filaments 


188 ZOOLOGY SECT. 


are produced into slender threads—the aconlia—which may be 
protruded through the mouth or through special apertures 
(cinelides) of the body-wall (Fig. 138, A.) 
The general arrangement of the cell-layers is the same as in 
the two preceding classes. The body-wall (Fig 138)—base, column, 
and disc—consists of a layer of ectoderm outside, one of endoderm 
within, and between them an intermediate layer or mesoglcea, 
which is extremely thick and tough. The gullet (gul.), which, like 
that of the scyphula, is an in-turned portion of the body-wall, is 
lined with ectoderm, and its outer surface—ie. that facing the 
inter-mesenteric chambers—is endodermal. The mesenteries (7s.) 
consist of a supporting plate of mesogliwa, covered on both sides by 


Fig. 138.—Diagrammatic vertical (A) and transverse (B) sections of a Searanemone. The 
ectoderm is dotted, the endoderm striated, the mesoglea black. «ar. acontium ; en. cinclis ; 
gul. gullet; tnt. wes. ce. inter-mesenteriec chamber; sacs. mesentery; mes. 7. mesenteric 
filament; mth. mouth ; ost. ostium; p. pore; ¢. tentacle. 


endoderm. The tentacles (¢) 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 CQLENTERATA 189 


is scen to be traversed by definite fibrous bands, the two most 

obvious of which are the longitudinal or retractor muscle (Fig. 

187, /.w.), running as a narrow band from base to disc, and the 

parietal avuscle (pan.), passing obliquely across the lower and outer 

angle of the mesentery. Both these muscles are very thick, and ” 
cause a projection or bulgmg on one side of the mesentery, 

specially obvious in a transverse section (B. lm.): 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 dise 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 

etrcular 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. 187, B). At each end of the gullet, opposite the 
siphonoglyphe, are two mesenteries (//. 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. ?, mes. 3) 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 
verticr! plane taken through the long diameter of the gullet, and a 
transverse lune 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 
(vet) and endoderm (cz/.) consist mainly of very long columnar, 
ciliated, epithelial cells, and the mesogloea (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 mesoylu, and has assumed, to a far greater 


A 
Fic. 139.—Tealia crassicornis, Trans- Fic. 140.—Three nematocysts of 
verse section of tentacle. ect. ectoderm ; Sagartia. (After Hertwig.) 


end. endoderm; l.m. longitudinal muscles ; 
msgl. mesoglea ; nv.c. nerve-cells; ne. f. 
nerve -fibres; atc. nemutocysts; ¢t. im. 
transverse muscles. (After Hertwig.) 


extent than in any of the lower groups, the characters of an inter- 
mediate cell-layer or mesoderm. 

Stinging-capsules occur in the ectoderm, and are also very 
abundant in the mesenteric filaments. They (Fig. 140) resemble 
in general characters the nematocysts of Hydrozoa, but are of 
a more elongated form, and the thread is usually provided at 
the base with very numerous slender barbs (B). Very fre- 
quently the coiled thread is readily seen in the undischarged 
capsule (A). Gland-cells (Fig. 141, gl.) are very abundant 
in the ectodermal lining of the gullet and in the mesenteric 
filaments: the latter are trilobed in section, and the gland- 
cells are confined to the middle portion, the lateral divisions 


‘. 


IV PHYLUM CORLENTERATA 191 


being invested with ordinary ciliated cells (¢). In virtue of 
possessing both stinging-capsules and gland-cells, the mesenteric 
filaments perform adouble function. The animal is very voracious, 
and is able to capture and swallow small Fishes, Molluscs, Sea- 
urchins, We. 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 


Fic. 141,—Transverse section of mesenteric filament of Sagartia. c. ciliated cells; gl. gland- 
cells; ate. nemnatocysts. (After Hertwig.) 


it, pouring out, at the same time, adigestive 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 mesoglea, and thus appearing to belong to that 
layer (Fig. 189 7. m.). This fact is significant from the cireum- 


4 


192 ZOOLOGY SECT, 


stance that, as we shall see, the muscles of all animals above 
Cuwlenterata 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. 189, 
neve.), 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 dicecious, 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. 

Thedevelopment of Sea-anemones resembles,in its main features, 
that of Scyphozoa. The oosperm undergoes more or less regular 
division, the details differing considerably in individual cases, and 
becomes converted into a planula, an elongated ovoidal body with 
an outer layer of ciliated ectoderm, and an inner layer of large 
endoderm cells, surrounding a closed enteric cavity, usually filled 
with a mass of yolk, which serves as a store of nutriment. 

In this condition the embryo escapes from the parent, through 
the mouth, swims about for a time, and then settles down, becom- 
ing attached by its broader or anterior end. At the opposite or 
narrow end a pit appears, the rudiment of the stomodeum ; 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 stomodieum, and near one end of that tube. The enteron thus 
becomes divided into two chambers, a larger or dorsal and a smaller or ventral, 
and the embryo acquires a distinct bilateral symmetry. Next a pair of mesen- 
teries (2) appear in the dorsal chamber, dividing it into a median and two lateral 
compartments ; then a third pair (3) in the ventral chamber, producing a similar 
division; then a fourth pair (4) in the middle compartment of the dorsal 
chamber ; then a fifth pair (B, 5) in the lateral compartments of the dorsal 
chamber ; and a sixth (6) in the lateral compartments of the ventral chamber. 
Soon the longitudinal muscles are developed, and the fate of these primitive 
pairs of mesenteries can be seen. The third and fourth pairs become the two 
directive couples of the adult ; another couple of primary mesenteries is consti- 
tuted, on each side of the vertical plane, by one of the mesenteries of the first 


Iv PHYLUM CCELENTERATA 193 


and one of the sixth pair ; a third couple is similarly formed by a mesentery of 
the second and one of the fifth pair. ‘Thus it is only in the case of the directive 
mesenteries that an adult couple coincides with an embryonic pair: in other 
instances the two mesenteries of w couple are of different orders, belonging 
to distinct embryonic pairs. The mesenteric filaments of the first cycle of 


Wy 
NIT 


YN: 
A 


Fic. 142,—Transverse sections of early (A) and later (B) stages of an embryo Sea-anemoune 
(Actinia.) The mesenteries are numbered in the order of their development; std. stomo- 
deum. (After Korschelt and Heider.) : 


mesenteries are partly ectodermal, partly endodermal in origin, those of the 
remainder entirely endodermal. 

The tentacles are developed in a somewhat similar order to that of the 
development of the mesenteries. The first to make its appearance is connected 
with the larger or dorsal enteric chamber mentioned above: for some time it 
remains much longer than any of its successors, and thus accentuates in a marked 


degree the bilateral symmetry of the embryo. 
of 


It will be noticed that the development of the Sea-anemone is 
accompanied by a well-marked metamorphosis, but that there is no 
alternation of generations. In this respect its life-history offers a 
marked contrast with that of Obelia. 


2, DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Actinozoa are Coelenterata which exist only in the polype- 
form, no medusa-stage being known in any member of the. class. 
The actinozoan. differs from the hydrozoan polype mainly in 
possessing a stomodzeum : it differs from the hydrozaan and many 
scyphozoan polypes in the possession of mesenteries or vertical 
radiating partitions, which extend inwards from the body-wall 
and some of which juin the stomod#um. The free margins of the 
mesenteries bear coiled mesenteric filaments, which appcar to 
answer to the gastric filaments of Seyphozoa, but may be partly 
ectodermal in origin. The mesenteries are developed in pairs, 


VOL. I oO 


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 mesoglea containing fibres and cells. The stomodeum 
consists of the same layers reversed—z.. its lining membrane is 
ectodermal. The mesenteries are formed of a double layer of 
endoderm with a supporting plate of mesoglea. Nematocysts, 
frequently of a more complex form than those of Hydrozoa and 
Sevphozoa, are present in the tentacles, body-wall, stomodeum, 
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. 

‘he 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 
plannla, which, after a short free existence, ‘settles down and 
undergues 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 
mesenterics: the remaining mesenteries are usually arranged 
in couples with the longitudinal muscles of each couple facing one 
another. 

ORDER 1.—ACTINIARIA. 


Zoantharia which usually remain simple, but in a few instances 
form small colonies. The tentacles and mescnteries are numerous, 
and there is no skeleton. This order includes the Sea-anemones. 


ORDER 2.—MADREPORARLA. 


Zoantharia which resemble the Actiniaria in the general 
structure of the soft parts, but which usually form colonies, and 


Iv PHYLUM CCELENTERATA 195 


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


ORDER 3.—ANTIPATHARIA. 


Compound, tree-hke Zoantharia in which the tentacles and 
mesenteries are comparatively few (6—24) in number. A skeleton 
is present in the form of a branched chitinoid axis, developed from 
the ectoderm, which extends throughout the colony. This order 
includes the “ Black Corals” (Fig. 150). 


Sub-Class II.—Alcyonaria. 


Actinozoa in which the tentacles and mesenteries are always 
eight in number. The tentacles are pinnate, ic. produced into 
symmetrical branchlets. There is never more than one siphono- 
glyphe, which is ventral in position, zc. faces the proximal end of 
the colony. The mesenteries are not arranged in couples, and 
their longitudinal muscles are all directed ventrally, ze. 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 ” (A/ry miwm, Fig. 153) has a skeleton of this 
type. In some cases the spicules become aggregated so as to pro- 
duce a coherent skeleton, which may form a branched axis to the 
whole colony, as in the precious Red Coral (Coralliwm, Fig. 145), 
or a series of connected tubes for the individual polypes, as in 
the Organ-pipe Coral (Zuhipora, Fig. 148). In the “ Blue Coral” 
(Heliopora) the skeleton is a massive structure resembling that of 
the Madreporaria. Most genera are compound; a few, such as 
Hartea—which, however, is probably a larval form (Fig. 144)— 
are simple. 


ORDER 5.—GORGONACEA. 


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 mesoglea. There is no 
siphonoglyphe. The beautiful “Sea-fans” belong to this group 
(Fig. 154). 

ORDER 6.—PENNATULACEA. 


Aleyonaria in which the colony is usually elongated, and ha» 
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” (Peanatulu) 
are the commonest members of this group (Fig. 147). 


Systematee Position of the Haanvple. 


Tealia crassicornis is one of several species of the genus 
Teulia. it belongs to the family Zvewdidw, which, with several 
other families, make up the tribe Heractiniw, of the order 
cletiniariu, of the sub-class Zoanthariu. 

The presence of numerous tentacles, arranged in multiples of 
five, places it at once among the Zoantharia. The fact that it is 
siaiple 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 Hexactiniee 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 stomodeum has two siphono- 
glyphes and two lappets. 

The family Tealidee 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 
Alevonacea, of which Hartea (Fig. 144) may be taken as an 
example, 


Iv PHYLUM CQLENTERATA 197 


The simplest mode of budding is that just described in Zoan- 
thus, in which new zooids are developed from a narrow band-like 


Fic. 143.—Zoanthus sociatus. 


A, entire colony ; st. stolon. 


whee 


B, transverse section. syph. 


siphonoglyphes ; d. ¢. dorsal, and +. d. ventral directive mesenteries. (After McMurrich and 


Korschelt 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 


Fic. 144.—Hartea elegans. 


gul. gullet ; 
mes. mesentery ; sp. spicules; ¢. tentacles. 
(After Perceval Wright.) 


formed as lateral outgrowths, 
and a tree-lke colony arising 
with numerous zooids spring- 
ing from a common stem or 
cenosare. Corallium and Gor- 
gonia (Figs. 145 and 154) are 
good examples of this type of 
growth. In other cases the 
buds grow more or less paral- 
lel with one another, producing 
massive colonies either of close- 
set zooids or of zooids separ- 
ated by a solid coenosarc. As 
examples of this type we may 
take Palythoa, the most com- 
plex of the Actiniaria, and 
many of the common Madre- 
poraria, such as Astrea (Fig. 
146). In the Sea-pens (Penna- 
tulavea) the proximal end of 
the elongated colony (Fig. 
147) is sunk in the mud, and 
the distal end bears zooids 
springing either directly from 


198 ZOOLOGY SECT. 


the ccenosare or, as in Prnnatula 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 (Tulipora). The base of the original polype (Fig. 148) 
grows out into a flattened expansion 
from which uew polypes arise, diverg- 
ing slightly from one another as 
they grow, and separated by tolcr- 
ably wide intervals. The distal ends 
of the polypes then grow out into 
horizontal expansions or platforms 
(pl.), formed at first of ectoderm and 
mesoglcea 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, pil.), uniting the 
Tio. 145.-Corallium rubrum, por- polypes with one another. From 

tion of a branch, (From Claus, 

after Lacaze-Duthiers. ) the upper surfaces of the platforins, 

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 part passwu with the vertical 


Fic. 146.—Astreea 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 1s, as mentioned above, very uniform, the varia- 
tions in detail are numerous and interesting, especially among 
the Actiniaria. One of the most important points to consider 


Iv PHYLUM CCELENTERATA 199 


Fic. 147.—Pennatula suleata. A, entire colony; B, portion of the same magnified. 
l. lateral branch ; p. polype; s. siphonozooid. (After Koelliker.) 


CUTTING 


Fic. 148.—Tubipora musica. A, skeleton of entire colony ; B, transverse sections of polype 3 
C. single polype with tube and commencement of platform; D, growth of new polypes from 
platform, /. m. longitudinal muscles ; p1. p2. polypes; pl. platform ; s7ph. siphonoglyphe ; sp. 
spicules ; std. stomodzeum. (After Cuvier, Quoy and Gaimard, and Hickson.) 


200 ZOOLOGY SECT. 


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./.) 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 


Fio, 149.—Bdwardsia elaparédii. formed couples. On the other hand, 
, the entire animal; ¢. tube. B. ] anys , 

transverse section. (After Andres, the. newly discovered Gy) achis eXx- 

and Korschelt and Heider.) hibits a perfectly radial arrange- 


ment: the mesenteries are all 
arranged in couples with the longitudinal muscles facing one 
another. Jastly, in all the more typical Sea-anemones (forming 
the tribe Heaactiniw) 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 Hexactinie. 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. 


Iv PHYLUM CCQHLENTERATA 201 


The tentacles in Zoantharia are usually very numerous, and in 
nearly all cases have the form of simple glove-finger-like out- 
pushings of the dise. 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. 


Fic. 150.—Antipathes ternatensis, portion of a branch, showing three zvoids and the horny 
axis besct with spines. (From the Ciunbridge Natural History, after Schultze.) 


In the Alcyonaria, on the other hand, the tentacles, like the 
mesenteries, are eight in number and are always pinnate, ‘r. 
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 (ci.). 

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 %-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 (6) 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 Schizepathes (Fig. 151, B) 
there is thus recognisable an arrangement of the parts which night 


Fic. 151.—Antipatharia. A, oral face of zovid of Parantipathes. B, oral face of zooid of 
Schizopathes, (After Delage et Hérouard.) 


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 gonozouids. 

Fixed and Free Forms.—<A large proportion of Actinozoa are 
permanently fixed, such, for instance, as most of the Stony Corals, 
the Sea fans, Black Corals, &. Most Sea-anemones are tempo- 
rarily attached by the base, but are able slowly to change their 
position: some forms, such as Bdwardsia (Fig. 149) and Cerianthus, 
usually live partly buried in sand enclosed in a tube formed of 
discharged stinging-capsules, the oral en] with its crown of 
tentacles alone being exposed: others, such as Peachia, live an 
actually free life, habitually lying on the sea-bottom with the 
longitudinal axis horizontal like that of a worm: a few, such as 
Minyas (Fig. 152), have the aboral end dilated into a sac containing 
air and serving as a float; by its 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, 7.2. there is no dif- 
ferentiation of the zooids of a colony. But ni 
in the Alcyonaria dimorphism is common: 4, 459 pes 5 goal 

. : » Lon. yas. jf. float. 
the ordinary zooids or polypes are ac- (After Andres.) 
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 Aleyonarian genus Hartea (Fig. 144), already 


IV PHYLUM CCELENTERATA 203 


referred to, in which minute irregular deposits of calcium carbonate, 
called spicules (sp.), are deposited in the mesoglea, A similar 
sprewlar skeleton occurs in the “ Dead-men’s finger” (Alcyonium, 
Fig. 153), where spicules of varying form are found distributed 
throughout the mesogloea of the ccenosare. In Z'ubipora (Fig. 148), 
the “ Organ-pipe Coral,” the mesoglceal spicules become closely 
fitted together, and form a continuous tube for each polype, the 
tubes being united by horizontal calcareous platforms (j/ ) formed 
by deposits of spicules in the expansions of the same name already 
referred to. The skeleton of Tubipora is, therefore, an internal 


Fic. 153,—Aleyonium 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 coenosare. 
In the Blue Coral (Heltopora), 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 


ZOOLOGY SECT, 


204 


formed of a flexible horn-like material. 
gleal, 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 


In addition to its axis, Gorgonia contains 
In some 


Moreover it is not meso- 


base of the colony. 
numerous spicules in the mesogloea of the ccenosare. 


4 
A 
i 
fy 
is 
4 i 
4 
are), 
H POWs 
ae oR ioe 
Bes 
3 4 Bi 
a iS 
A i 
Hk 
a Of 
gS fee 
ny eA 


3; 


RS 
EEL, 


IES Se OS 


EST EIS 
See 


Fic. 154.-Gorgonia verrucosa A, entire colony; B, portion of the same magnified, c. 
ceenosare ; p. polype. (After Koch and Cuvier.) 


of the Gorgonacea the axial skeleton is partly horny, partly 


calcareous. 

In the Sea-pen (Pennatula, Fig. 147) and its allies the stem of 
the colony is supported by a horny axis which is unbranched, not 
extending into the lateral branches. In this case the axis 1s 
contained in a closed cavity lined by an epithelium, the origin of 


Iv PHYLUM CC@LENTERATA 205 


which is still uncertain, Spicules occur in the mesoglea, some of 
them microscopic, others readily visible to the naked cye. 

In the Madreporaria we have a skeleton of an entirely different 
type, consisting, in fact, of a more or less cup-like calcareous 
structure, secreted from the ectoderm of the base and column of 
the polype. When formed by a solitary polype, such a “ cup- 
coral” is known as a corallite: in the majority of species a large 
number—sometimes many thousands—of corallites combine to 
form a cora/llun, the skeleton of an entire coral-colony. 

The structure of a corallite is conveniently illustrated by that 
of the solitary genus Flabellwm (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 thee (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, 0. pl.). 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, co/.). 

In many Corals there is a distinct calcareous layer investing the 
proximal portion of the theca, and called the epitheca (C, ¢.th.). Some 
species have the inner portions of the septa detached so as to form 
a circlet of narrow upright columns, the pali. In others there are 
horizontal partitions or dissepiments passing from septum to septum, 
and in others, again, complete partitions or tabule, like those of 
Millepora (p. 157), extending across the whole corallite. In the 
Mushroom-coral (Fungia), the corallite is discoid, the theca is con- 
fined to the lower surface, and small calcareous rods, the synapticula, 
connect the septa with one another. 

In the living condition the polype fills the whole interior of the 
corallite and projects beyond its edge to a greater or less degree 
according to its state of expansion (C). The proximal part of the 
body-wall is thus in contact with the theca, which has the relation 
of a cuticle, and is, in fact, a product of the ectoderm. The free 
portion of the body-wall is frequently, in the extended state, folded 
down over the edge of the theca so as to cover its distal portion. 
The septa alternate with the mesenteries, each lying in the space 
between the two mesenteries of one couple, and each being in- 


206 ZOOLOGY SECT. 


vested by an in-turned portion of the body-wall (E, F). Thus the 
septa, which appear at first sight to be internal structures, are 
really external: they lie altogether outside the enteric cavity, and 
are in contact throughout with ectoderm. 

The ectodermal nature of the entire corallite is further proved by 
its development. he first part to appear is a ring-shaped 


spac sep.7 


Fic. 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. 0. pl. basal plate ; col. colum- 
ella; e.th epitheca ; gul. gullet ; mes, mes. 1, mes. 2, mesenteries ; mes. /. 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 
dise, the basal plute, from the upper surfaces of which a number of 
ridges arise, arrayed in a star-like fashion: these are the rudiments 
of the septa Here, again, sections show that each septum corre- 


Iv PHYLUM CdGiLENTERATA 207 


sponds 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 ave produced in which the corallites are in close contact 
with one another, as in Astrea (Fig. 146); or tree-like forms, such 


Fic. 156.—Dendrophyllia nigrescans, B, Madrepora aspera. co. corallites; 
cs. ceenosare ; p. polypes. (After Dana ) 


as Dendrophyllia (Fig. 156, A), in which a common calcareous stem, 
the cenenchyma,is formed by calcification of the ccenosare (es.), and 
gives origin to the individual corallites. It is by this last-named 
method, the coenosare 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. 
Tn what are called the aporose or poreless corals, such as Flabellum, 
Astrea, &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 ccenenchyma, 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 bya network of interlacing 
tubes, representing the ccenosare, and placing the polypes of the 
colony In communication. 

The Blue Coral (/Zedivpora), one of the Aleyonacea, 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 siphnozoids. 
Tabulz are present, and septum-like ridges, which, however, have 
no definite relations to the mesenterics 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 
Alcyonide are remarkable for their claborate 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 symbivtic Algwe have 
been fount 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 Adamsia 
pulliata (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 Disccsoma of the 
great Barrier-Reef—are found associated with Small Fishes or 


IV PHYLUM CCELENTERATA 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 onee. ‘The corallum contains, not only the apertures 
for the polypes and siphonozooids, but also tubular cavities of 


Fia. 157.—Adamsia palliata, four individuals attached to a Gasteropnd shell inhabited by 
a Hermit-crab. «ae. acl. acontia ; sh. shell of Gasteropod. (After Andres.) 


an intermediate size, in each of which is found a small cheetopod 
Worm, belonging to the genus ZLeucodore. 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 débris 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 paleontological 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 Rugosa, 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 fosswla, where the septa are greatly 
reduced, 


IV PHYLUM C@LENTERATA 211 


CLASS 1V.—CTENOPHORA. 
1. EXAMPLE OF THE CLAass—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 ; 
alhed forms belonging either to the same genus (often called 
Cydippe) or to the closely allied genus Plewrobrachia are common 
pelagic forms all over the world. 

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


Fic, 158,—Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth ; 
s. pl. swimming plates; t. and b. tentacles. (After Chun.) 


each from a deep cavity or sheath, into which it can be completely 
retracted (Fig. 159, ¢.sh.). At the narrow end—where the stalk 
of a pear would be inserted—is a slit-like aperture, the mouth 
(mth.) : this end is therefore oral. At the opposite or aboral pole 
is a slight depression, in which lies a prominent sense-organ (s.0.), 
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 


Pp 2 


zntc Perc 


Fig. 159.—Hormiphora plumosa. A, dissected specimen having rather more than one 
quarter of the body cut away. B, transverse section; diagrammatic. adr. ¢. adradial 
canal; inf. infundibulum; inf. ¢. infundibular canal; ind. c. inter-radial canal; mid. ¢. 
meridional canal ; mth. mouth; ovy. ovary ; per. e. per-radial canal ; s, 0. sense-organ 5 &. pl. 
swimming-plate ; spy. spermary ; std. stomodzeum ; std. c. stomodzeal canal ; std. 7 stomodval 
ridges ; t. tentacle; ¢ b. base of tentacle; ¢. ¢. tentacular canal ; t. sh. tentacular sheath. 


SECT. IV PHYLUM CQ@LENTERATA 213 


are, in fact, rows of immense cilia, fused at their proximal ends : 
their presence and mode of occurrence—arranged in meridional 
comb-ribs or swimming-plates—are strictly characteristic of the 
class, and indeed give it its name. 

It will be seen at once that—apart from all considerations of 
internal structure—Hormiphora presents a similar combination of 
radial with bilateral symmetry as in some Hydrozoa, such as 
Ctenaria (Fig. 109, 7), and as in the majority of Actinozoa. The 
swimming-plates are radially arranged, and mark the eight adradii, 
but the slit-lke 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 mght 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 
stomodewm. It reaches about two-thirds of the way towards the 
aboral pole, and its walls are produced internally into ridges (std. 7.), 
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 stomodzum, 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 stomodeum gradually 
narrows and opens into a cavity called the infundibulum (inf), 
which probably answers to the stomach of an Actinozoon or a 
medusa, and is flattened in a direction at right angles to the 
stomodeum—z.e. in the transverse plane. From the infundibulum 
three tubes are given off: one, the infundibular canal (inf. ¢.), 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 exeretory pores (Fig. 160, A, cx. 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. ¢.), which in their 
turn divide each into two adradial canals (adr. ¢.). 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. ¢.), which extends up- 
wards and downwards beneath the corresponding swimming-plate. 
Furthermore, each per-radial canal gives off a stomodeal canal 
(std. ¢.), which passes downwards, parallel to and in close contact 


Fic, 160.-Hormiphora plumesa, diagrammatic longitudinal (A) and transverse (B) 
sections. The ectoderm is dotted, the endoderm striated, the mesoglea black, and the 
muscular axis of the tentacles gray. Lettering as in Fig. 159, except ex. p. excretory pore. 


SECT, IV PHYLUM C@LENTERATA 215 


with the stomodeum, 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 (¢. 0.), 
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 stomodeum 
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 mesoglea. The tentacle-sheath is an invagination of the ecto- 


WG ell 
ee f A ‘ 


yo 7Lee 


Seat a NN 


Fic. 1o1.—Hormiphora plumosa. A, transverse scction of one of the branches of a 
tentacle ; B, two adhesive cells (ad. ¢.) and a sensory cell (s. c.) highly magnified. cu. cuticle 
nu. nucleus. (After Hertwig and Chun.) 


derm, and the tentacle itself is covered by a layer of ectoderm, 
within which is a core or axis formed by a strong bundle of longi- 
tudinal muscular fibres, which, as we shall see, are of mesodermal 
origin, and which serve to retract the tentacle into its sheath. 

Delicate muscle-fibres lie beneath the external epithelium and 
beneath the epithelium of the canal-system, and also traverse 
the mesoglea 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 
Celenterata previously studied is the absence, in Hormiphora, of 
stinging-vapsules. 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 papillae, 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, ¢. 9.) lined by ciliated epithelium and produced in 
the transverse plane into two narrow ciliated areas, the polar 
plates (p. pl.). From the depression arise four equidistant groups 
of very large S-shaped cilia (sp.), united to form as many springs (sp.), 
which support a mass of calcareous particles (/.), like the lithites of 


Fic. 142.—Hormiphora plumosa, Sense-organ: b. bell; ¢. p. ciliated plate; ¢. gv. ciliated 
groove; ex. p. excretory pore; l. lithites ; p. pl. 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 bed/ (0.), 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 (svy.) extending along the whole length of one 
side, a spermary (spy.) along the whole length of the opposite side. 


IV PHYLUM CCELENTERATA 217 


The organs are so arranged that in adjacent canals those of the 
same sex face one another. It will be seen that the reproductive 
products have, as in Scyphozoa and Actinozoa, the position of 
endoderm-cells: whether they are developed, in the first instance, 
from that layer is uncertain. When ripe, the ova and sperms are 
discharged intvu the canals, make their way to the infundibulum, 
thence to the stomodzeum, and finally escape by the mouth. Imn- 
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 (plsi.) 
containing the nucleus (2w.), and of an internal mass of a frothy 
or vacuolated nature (yk): the 
vacuoles contain a homo- 
geneous substance which 
serves as a store of nutri- 
ment to the growing embryo, 
and apparently corresponds 
with the yolk which we shall 
find to occur in a large pro- 
portion of animal eggs. En- 
closing the egg is a thin 
vitelline membrane (v.m.), sepa- 
rated from the protoplasm by Fic, 163.-Ovum of Lampetia. 2x. nucleus; 
a considerable space, tilled with plsm. protoplasm ; 7. i. vitelline membrane ; 

é yk, yolk, (After Chun ) 
a clear jelly. 

After impregnation the oosperm segments, but the details of 
the process are very different from those we are familiar with in 
the other Ceelenterata. 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, p/sm.). 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 (EH, F) in which we can dlis- 
tinguish eight large, ventral, yolk-containing cells or megameres 
(mg.), and eight small, dorsal, protoplasmic cells or miero- 
meres (mt). 

The micromeres increase rapidly in number by division, and arc 
further added to by new, small cells being budded off from the 
megameres (Fig. 164, G, H, and Fig. 165, A). The result of this 
increase is that the micromeres gradually overspread the megameres 


218 ZOOLOGY SECT. 


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


Fic. 164,—Segmentation of the oosperm»in Ctenophora. mg. megameres ; mi. micromeres ; 
plsm. protoplasm ; yk. yolk. (Modified from Korschelt and Heider.) 


partly surrounded by an epithelium-like layer, incomplete below, 
of small cells (mz.). This stage corresponds with the gastrula of 
preceding types, the micromeres forming the ectoderm, the mega- 


Fic. 165,—Three stages in the development of Ctenophora. ma. megameres ; mi. micromcres. 
(From Lang’s Comparative Anutomy.) 


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, 


Fic. 166.—Three stages in the development of Callianira. «. infundibulum ; ec. ectoderm ; 
en. endoderin ; me. mesoderm ; st. stomudwum., (From Lang’s Comparative Anatomy.) 
not by a process of invagination or tucking-in, but by one of eptboly 

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


Iv PHYLUM CCQLENTERATA 219 


upwards and outwards, from the long axis of the entire embryo 
(Fig. 166, A). Their lower (ventral) ends then become divided off, 
forming a number of small cells, which constitute the rudiment 
of a true middle cell-layer or mesoderm (A, me.). A kind of in- 
vagination of the megameres with their mesoderm cells then takes 
place, resulting in the formation of a cavity—the infundibulum 
(B, d.)\—bounded below by the megameres, now placed horizontally, 
and above by the mesoderm. The mesoderm gradually retreats to 
the dorsal surface (C), finally spreading out between the dorsal 
ectoderm and the infundibulum. At the same time the ectoderm 
cells bounding the aperture of the infundibulum grow into it so 
as to line its ventral portion: in this way the stomodzeum (sé¢.) 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, 4.), 
the mesoglea, 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- 
gleea (me.,). The lithites are formed in 
the ectoderm-cells of the apical pole, but 
gradually make their way on to the free 
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 (ss). 

The most noteworthy points in this 
somewhat complex process of develop- 1. jie wo lator stares in the 
ment are the following :— ‘lovelopment of Gallianira. 

1. The distinction between a purely — (-Mfandipmum som modern: 
protoplasmic part of the egg anda yolk- ee ee 
containing portion. In the Hydrozoa — parative Anatomy.) 
and Actinozoa the yolk-material 1s small ; 
in amount and evenly distributed, the egg being described as 
alecithal or yolkless. In the present instance the yolk is at first 
accumulated in the centre of the egg, which is thus centrolecithal 


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 telolecithal or end-yolked condition. 

2. The fact that segmentation is wnequal, there being a distine- 
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 Ceelenterata, 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 
trom the endoderm during the gastrula stage. 


2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Ctenophora are pelagic Celenterata in which the formation 
of colonies is entirely unknown. No indication of a polype-stage, 
so characteristic of the remaining Ceelenterata, 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 throughont 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-radu, and retractile into pouches. The enteron communicates 
with the exterior by a large stomodeeum 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 oospermi 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 C@LENTERATA 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 stomodeal 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 stomodeal 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 ina groove. Union 
or anastomosis of the meridional and stomodeeal 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 Plewrobrachiidw 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 Plewre- 
brachiide 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 stomodzeal 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 Ceelenterates, 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. Huchlora (Fig. 168, 2), the result being an 


1LCallianira 


2.Euchlora 


3.Lampetia 


Fic. 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 (3) the mouth is so dilatable as to form, when 
expanded, a sole-like plate by which the animal retains itself on the surface of the 
water or creeps over submarine objects. In Huchlora rubra minute nematocysts 
have been found, and there is reason to believe that it was by the modification 


Iv PHYLUM CQiLENTERATA 223 


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

The Lobata, for instance Deiopea, are distinguished, as their name implies, 
hy the presence of a pair of large lappets (Fig. 169 A, /p.), into which the oral 


Fic. 169,—Deiopea kaloknenota. A, adult; B, young. aus. auricle; lp. lappet ; U. ¢. lateral 
tentacles ; mrd. 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 auricles (aur.), which bear swimming-plates. The meridional canals (mdr.c) 
unite with one another, and, with the esophageal 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 (/.t.). In the young condition 
the Lobata resemble such compressed Cydippida as Kuchlora, having a pair of 
long principal tentacles, no lappets, and unbranched vessels (B). 

The Cestida are represented by the remarkable ‘‘ Venus’s Girdle” (Cestus 
veneris), &@ band-shaped Ctenophore (Fig. 170) which sometimes attains a length 


Fia. 170.- Cestus veneris. A, adult; B, young. J, ¢. lateral tentacles ; mth. mouth; s. pl.}, 
s. pl.2 swimminug-plates ; ¢. tentacle. (After Chun.) 


of 14 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 (s.7/.!) are very 


224 ZOOLOGY SECT. 


small ; the other four (s.p/.”) are continued all along the aboral edge of the body. 
The bases of the two principal tentacles (t.) are large and are enclosed in sheaths, 
and, as in Lobata, numerous small lateral tentacles (/./.) 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 clongation in the median plane. 

Bevo’, the principal genus of the Beroida, has the form of a cylinder (Fig. 171), 
one end of which is rounded and bears the sense-organ, the other truncated 
and occupied entirely by the immense mouth (mth. ). 
The greater part of the body is taken up by the 
huge gullet; the infundibulum (/u/.), 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 stomodeal canal unite into a horizontal 
tube, which runs parallel with the margin of the 
mouth. There is no trace of tentacles either in 
the adult or in the embryonic condition. 


The Ctenophora are usually perfectly 
transparent, and quite colourless, save for 
delicate tints of red, brown, or yellow 
in the tentacles and stomodzeal ridges. 
Cestus has, however, a delicate violet hue, 
and when irritated shows a_ beautiful 
blue or bluish-green fluorescence. Beroé 
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. 
sg a es hth ated A Laces : A very remarkable fact has been made 

G2 Wane plates. After out with regard to Bolina hydatina, one of 
the Lobata, a Ctenophore which attains a 
diameter of 25-40 mm. While still in the larval or cydippid con- 
dition and not more than 0-5-2 mm. in diameter, it becomes 
sexually mature, the gonads producing ripe ova and sperms ; and 
the eggs are impregnated and develop in the usual manner. Soon 
the gonads degenerate, the larva metamorphoses into the adult 
form, and a second period of sexual maturity supervenes. This 
precocious ripening of sex-ceils, occurs as we shall see in other 
animal groups, and is called pedogenesis. 


Iv PHYLUM CCELENTERATA 225 


APPENDIX TO CTENOPHORA 
OTENOPLANA AND Ca@LOPLANA. 


Before leaving the Ctenophora mention must be made of two remarkable 
organisms which have been supposed to connect the present class with the 
Turbellaria Polycladida, ov Planarians, a group of worms to be described in 
the following section. 

Cienoplane (Fig. 172) is a small marine animal, nearly circular in outline, 
flattened dorso-ventrally, and about 6 mm. in diameter. It has hitherto been 


Fic. 172.-Ctenoplana kowalevskii. <A, from above; B, from the side. cl. clefts 3. 1. 
radiating ridges ; s. 0, 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.0.) containing a mass 
of lithites surrounded by eight radiating ridges (7.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.0.). Beneath the ectoderm is a basement-membrane, which acts as an organ 
of support, and the muscular system is complex. Near each tentacle is an 
aperture leading into a branched canal which is probably excretory, like the 
nephridial tubes of Flat-Worms. (See Section V.) 

Celoplana 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 + mm. Mt 
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. 


226 ZOOLOGY SECT. 


Ctenoplana and Celoplana 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 CQELENTERATA. 


There can be little doubt that the lowest coelenterate form 
known to us is the simple hydrozoan polype, represented by 
Hydra and by the hydrula stage of many Hydrozoa. Somewhat 
more complex, in virtue of its stomodeum (if a true stomodeum 
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 stomodzeum, 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- 
line, or hydroid medusze with no fixed zoophyte stage, as being 
derived from a pelagic hydrula. 

The Leptolinze 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 toa 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 meduse. The next stage 
would be the establisment of a division of labour, in virtue 
of which a certain proportion only of the zooids became medusa, 
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 dise 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 CQiLENTERATA 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 Semostome, Cubomeduse, and 
Rhizostome 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, Cubomeduse and Peromedusse—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 Aleyonaria from a common 
ancestor in the form of a simple actinozoan polype or actinula. 
Edwardsia clearly leads us to the Hexactiniw or typical Sea- 
anemones, and the Madreporaria are undoubtedly to be looked 
upon as skeleton-forming Hexactinie. 

The relationships of the Ctenophora to the other Ceelenterata are 
very doubtful. Ctenaria, one of the Anthomedusz (Fig. 109, 2), 
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 stomodeum 
of Hormiphora, the margin of the bell of Ctenaria with the mouth 
of Hormiphora, and the mouth of Ctenaria with the aperture 
between the stomodeum and the infundibulum of Hormiphora. 
But, as we have seen, the gullet of Ctenophora is a true stomo- 
dzum developed as an in-pushing of the oral ectoderm, and has 
therefore a totally different origin from the sub-umbrella of a 
medusa. Moreover, the tentacles of Ctenaria have no muscular 
base contained in the sheath, but spring from the margin of 
the umbrella as in other Hydrozoa: its gonads are developed in 
the manubrium, not in the radial canals, and there is no trace of 
an aboral sense-organ. 

Of Hydroctena, which has also been supposed to afford us a 
connecting link between the Hydrozoa and the Ctenophora, almost 
the same may be said. Hydroctena is bell-like, and provided with 
a velum. At its apex is an ampulla bearing two hthites supported 
on spring-like processes of the epithelium. From the apex of the 
gastric cavity a canal is given off which extends to the sense- 
organ, where it terminates blindly, and from the sides a pair of 

Q2 


228 ZOOLOGY SECT. 


short canals, each of which terminates blindly at the base of the 
corresponding tentacular sheath. Only two tentacles are present, 
with sheaths at their bases: these are situated, not on the margin 
of the bell, as in a meduse, 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 


ead. endoderm ; inj. infundibulum. (After Chun.) 


(A) is very striking, and the presence of a well-developed stomo- 
dum, 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 Celenterata, 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 Beroida are more 
highly organised in certain respects, ¢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 Coelenterata that they may be regarded 
as members of the same great phylum. The points of resemblance 
are readily to be recognised: the simple structure, with the large cen- 
tral cavity into which a wide opening—the mouth or the osculum, 


Iv PHYLUM CCELENTERATA 229 


as the case may be—leads ; the absence of a well-developed meso- 
derm, the fixed mode of life, and associated with it, the tendency 
to form compound structures or colonies by a process of budding. 
In addition, the occurrence of larval stages which have at least 
a superficial correspondence in the two phyla would appear to 
constitute an important connecting link. 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 Ccelenterates too important to 
allow us to suppose that a close relationship exists. One of these 
diterences stands out beyond the others as the most radical. The 
osculum of a sponge is found, when we trace the development of 


Hexactinia Madreporaria 
Cestida 
Lobata Rhizostomeae  Semostomee 
Cydippida Beroida 
Platyctenea Coronate: Edwardsia eee 
Cubomedusce 
ACTINULA 


Lucernarida 


Hinge 
se a SCYPHULA 


Siponophora 


HYDRULA 


Fic, 174.—Diagram illustrating the mutual relationships of the Cazslenterata. 


the larva, to correspond in no sense with the mouth of the 
Celenterate. This alone, apart from important differences in 
the adult structure, such as the presence in the wall of the Sponge 
of the system of inhalant apertures, the presence of the peculiar 
collared endoderm cells, and the absence of stinging capsules, 
would suffice to remove the Sponges from the Coelenterata, and 
place them in a phylum apart. But not only is the grouping 
of Sponges and Ccelenterates 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 (IL) TO THE CCQLENTERATA. 
THE MEsozoa. 


Under the designation Mxsozoa 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 Moruoidea, 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 
clongated cell or of several cells : a mesoglcea is not represented. The Mesozoa 
comprise at least three families, the Dicyemide, the Heterocyemide, and the 
Orthonectide, all the members of which are internal parasites. 

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


Fic. 175.—Diceyema paradoxum, Fic, 176.—Dieyema paradoxum, 
with infusvriform cmbryos (males). with vermiform embryos. (From 
(Pron Bronn's) Thicrreich, after Bronn’s Thierreich, after Kolliker.) 


Kolliker.) 


0°75 and 6 or 7 millimetres, consists of a head-part or calotte, and an elongated 
body. The form of the calotte varies a good deal, according to age; in young 
specimens it is isotropic (7.e. symmetrical around the long axis) ; in the adult 


IV PHYLUM CGiILENTERATA 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 cells. The cells of the head all 
bear cilia, which are shorter and thicker than those of the body-cells. 

The body consists of a single large axial cell, and of a single layer of outer 
cells which completely invest the axial cell. The outer cells which follow 
immediately on the head are distinguishable from the rest by their granular 
contents, and by their being dilated internally in such a way that the apex of 
the axial cell is constricted. 

The axial cell is either almost completely cylindrical or spindle-shaped, and 
is covered in its entire extent by the outer cells. It presents a differentiated 
cortical layer, beneath which the finely granular gelatinous contents are at first 
homogeneous, but afterwards become vacuolated. In the middle of the cell is a 
large oval or ellipsoidal nucleus. 

The life-history is a true alternation of generations. The primitive nucleus of 
the axial cell divides mitotically to form a smaller asexual germ-nucleus and 
a larger nucleus—the definitive nucleus (somatic nucleus) of the axial cell. 
Further germ-nuclei result from subsequent divisions. The germ-cells undergo 
a process similar to segmentation. Of the cells thus formed one gives rise to 
the axial cell of the embryo: the others, increasing in numbers and becoming 
smaller, gradually grow over the axial cell until they at length completely 
enclose it. The embryo increases greatly in length, and escapes from the 
interior of the parent, which it completely resembles, by perforating the body- 
wall. This phase of the parent animal (Fig. 176) is the phase to which the term 
nematogene is applied : the asexually developed young are the so-called vermiform 
embryos. The latter swim about for a time in the fluid of the kidney of the 
host ; afterwards they attach themselves by means of the head to certain 
appendages—the venous appendages—of the walls of the cavity. A number of 
generations of these asexually developed forms succeed one another until, when 
the venous appendages of the kidney have become thickly infested with the 
parasites, a change takes place, and a sexual process of multiplication becomes 
initiated. In the interior of the nematogene a change is observable, and female 
sexual individuals are formed instead of vermiform embryos. Unlike the latter 
the former do not leave the body of the parent ; they also differ in the 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 infusoriform 
embryos. In its mature form the male is approximately 
pear-shaped, the narrower end being posterior. Several 
axial cells are present: these form the testes, in which the = 
sperms are developed. They are surrounded by the outer yee ae 
cells, which at the posterior end take the form of a flat doxum. (irom 
ciliated epithelium. The complete development of the  Bronn’s  Phierreich, 

after Kélliker.) 
sperms only takes place when the young male leaves the 
host in which it was formed and seeks a new one; thus it is only by the 
sperms of a male from another host that the ova can be fertilised. The males 


232 ZOOLOGY SECT. 


are developed from the fertilised ova and subsequently escape, their develop- 
ment being similar to that of the vermiform embryos: the phase of the parent 
form to which they are developed is that known as the rhombogene. After a 
number of generations of males have been formed in this way, the rhombogene 
undergoes modification, and the last generation of fertilised ova gives rise, not to 
males, but to vermiform embryos—7.¢., to an asexual generation—and with these 
the cycle begins anew. 

The Heterocycmidie, which are also parasites of the Cephalopoda, resemble 
the Dicyemidz in most respects, but the head is wanting. 

The family Orthonectide comprises only two genera — Rhopalura and 
Stecharthrum—which live as parasites in a Polyclad (Leptoplana), a Nemertine 
(Lineus), an Annelid, and a Brittle-Star (Amphiura). In the stage that 
represents the asexual form of the Dicyemide the Orthonectid assumes the 
character of «a plasmodium, or mass of finely granular protoplasm contain- 
ing many nuclei, and is capable of active amceboid movements. In the 
interior of the plasmodia the sexual forms are developed. A nucleus of the 
plasmodium surrounds itself with protoplasm and gives rise to a germ-cell, 


nil 
mt 
Fic. 17s —Rhopalura Giardii, male. Fic. 179,—Rhopalura Giardii, female. 
(From Bronn’s Thierreich, after Julim:) (From Bronn's Vhierreich, after Julin. 


which hy 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 PHYLUM CQiLENTERATA 233 


bear a considerable resemblance to the Dicyemida, but instead of the axial cell 
there are a number of cells, the ova or sperm-cells. The outer cells are 
arranged in segments or rings. In front is usually a region composed of a few 
rings in which the outer cells bear cilia which are directed forwards: then 
comes a shorter region devoid of cilia, and behind that is the longest region, 
having cilia directed backwards. In shape the body is usually spindle-like—the 
males (Fig. 178) differing somewhat from the females. In about the middle of 
the internal space of the male is the compact oval testis containing small tailed 
sperms. Beneath the outer layer in the male, but not in the female, is a 
layer of fibres sometimes regarded as muscular. The plasmodia multiply by 
fragmentation, The development of the embryos either goes on in the intact 
plasmodium, or the latter breaks up and the embryos are to be found at various 
stages free in the host. 

In the development of a male from the germ-cell the first segmentation is 
unequal. The further segmentation results in the formation of a solid morula, 
The outer cells become differentiated into two distinct groups, the one giving 
rise to the external layer of the anterior region, the other to that of the 
posterior region of the body. The inner cells multiply and give rise to the 
numerous small spermatocytes of the testis. The formation of the layer of 
fibres only takes place later. 

In the case of the female the segmentation appears to be equal from the first, 
and results in the formation of a blastula-like stage, which becomes converted 
into a solid morula-like body by the passing inwards of a number of cells. As 
in the male, the central cells multiply to form the sexual cells, and the outer 
cells form the external layer with its segments. In all probability, though this 
has not been actually proved, the mature sexual animals become free from the 
plasmodia, and the females, after fertilisation, find their way to another host 
where they become transformed into plasmodia, the germ-cells of which are the 
fertilised ova. 

To be mentioned in connection with the Dicyemide and Orthonectide, as 
perhaps allied with them, are the remarkable parasites Amebophrya and 
Lohmane//a—the former living in certain Radiolarians (Protozoa) the latter in 
the body-cavity of a fritilaria (Urochorda). These both resemble the groups 
described above, and differ from the other Metazoa, in the presence of only a 
single body-layer. This remarkable simplicity of body-structure occurs also in 
Nalinella, 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 heen dissolved, is a 


BARRAUITLACRIDSFUERIFTETERuEry ee 


asad 
roy ees 
YL 


\ dot G2) i@) @ ae 
Se ei? f ne So ih i a) a 
W @: i g: . 


; 1 wesesaaies ase Sees = 
“ i He nt 


Fic. 180.—Salinella, longitudinal section. (After Frenzel.) 


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, isa circlet of from fifteen to twenty long whip-like cilia. The posterior 
aperture (anus), which is usually closed, is surrounded by a few stiff set. 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 sete (non- 
motile cilia). The internal cavity (enteron) is found to contain sand, plant 


Fie. is1.—Salinella, transverse section. (After Frenzcl.) 


fragments, and Bacteria ; its surface is beset with long cilia. Multiplication is 
said to take place ly transverse fission ; and a process of conjugation followed 
by encystation has also been observed. 

Trichoplux and Treptoplax, which have been supposed to be Mesozoa, appear 
to be merely special moditications 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 Celenterata, 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 Platyhelminthes, Nemathel- 
minthes, Trochelminthes, Mollwscoida, and Annulata, with their 
appendices, all consist of forms which are or have been comprised 
in the Vermes. They differ from the Coelenterata in the presence 
of three well-developed body-layers—of which the middle one, or 
mesoderm, is of relatively predominant importance; and for the 
most part, in the much higher stage of complexity attained by the 
various systems of organs. The first four phyla present no meta- 
meric segmentation (p. 43): in the Annulata, metamerism is more 
or less strongly pronounced. 


The Platyhelminthes or Flat-Worms are a group of soft-bodied, 
bilateral, usually flattened animals, which are devoid of true 
metameric segmentation. With a sufficient degree of uni- 
formity of structure to render the phylum a fairly compact and 
well-defined one, there is yet a considerable range in complexity, 
from the simplest forms—certain of which have been supposed to be 
nearly connected with the Ctenophora among the Coelenterata—to 
the highest, which have all the various systems of organs very 
much more highly developed. The body is built up from 
three embryonic layers—ectoderm, mesoderm, and endoderm—as in 
all higher groups of animals. An excretory vascular system of 


236 ZOOLOGY SECT. 


a peculiar kind—the water-vascular or protonephridial system—is 
present in nearly all members of the phylum. A body-cavity (see 
following Sections) is not present, the spaces between the various 
organs and the wall of the body being filled up with a peculiar 
form of connective-tissue termed the parenchyma. The egg is in 
inost instances composite, the egg-shell enclosing not only the 
oosperm or impregnated ovum, but a quantity of nutrient material 
or food-yolk, derived, in most instances, from a special set of glands, 
the yolk or 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. 


J. EXAMPLES OF THE PHYLUM. 


1. A Fresh-water Triclad (Planaria or Dendrocwelum) 


General Features.—Species of fresh-water Planarians of the 
genera Planaria and Dendrocelum are common in the mud at 
the bottom of ponds of fresh-water in all quarters of the globe. 
They are small, thin, flattened worms a few millimetres in length, 
broader at one end, the anterior, than at the other, the postertor, 
which is more or less pointed. The animal (Figs. 182-184) is 
very readily recognised to he bilaterally symmetrical, with an upper 
or dorsal and a lower or ventral surface, right and left borders, 
and anterior and posterior ends. The colour varies in different 
species and in different individuals; but is usually gray, red, 
brown, or black. Movements of locomotion in the direction of the 
long axis of the body are recognisable in the living animal. Some- 
times this is a steady gliding movement, which is brought about 
by the action of a coating of vibratile cilia on the surface ; some- 
times the worm moves along somewhat after the fashion of a 
Leech, the ventral surface of the anterior end of the body being 
of a sticky adhesive character, and performing the part of the 
anterior sucker of the Leech. 

Close to the anterior extremity on the dorsal surface are two 
rounded black spots, the eyes (Fig. 183). On the ventral surface, 
a considerable distance behind the middle of the body, is the 
opening of the mouth (Fig. 182, mo.), and further back still, near 
the posterior pointed end, is a smaller median opening, the genital 
aperture (Fig. 184). 

Digestive System.—The mouth (Fig. 183, mo.) leads through 


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


v PHYLUM PLATYHELMINTHES . 237 


a short mouth-cavity into a cylindrical thick-walled chamber, the 
pharyna (ph.), which is highly mobile, and is capable of being 
thrust out as a proboscis through the mouth, beyond which it 


Fic, 182.—Planaria. Digestive and excre- Fic. 183.—Planaria. Nervous system, 
tory systems. ex. openings of excretory br. brain; eye, eye; t. ne. longitu- 
system; inf. intestine; mo. mouth ; dinal nerve; ph. pharynx. (After 
o. ph opening of pharynx. (After Jijima Jijima and Hatschek.) 


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 
runping 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 toa 
number of branches, which in turn give off a system of extremely 
fine capillary vessels, many of which terminate in flame-cells (Fig. 
214, p. 269). A flame-cell is a nucleated cell having in its proto- 
plasm a small space into which one of the capillaries leads; in this 
space lies a bundle of vibratile cilia, or a single thick cilium, which 
performs regular undulating movements, giving it somewhat the 
appearance of a flickering candle-flame. Cilia are said also to 
occur in the course of some of the capillaries, though this is 
doubtful. This system of vessels is usually regarded as excretory ; 
but it may also have a respiratory function. 

A well-developed nervous system (Fig. 183) is present. At 
the anterior end is a central knot of nerve-matter, the brain (Lr), 


~~. from which proceed backwards two longitudinal nerve-cords (1. ne.). 


The brain consists partly of transverse fibres connecting  to- 
gether the two longitudinal nerve-cords, partly of groups of nerve- 
cells situated at the ends, or in the course of, the nerve-fibres. 
The nerve-cords give off both internally and externally numerous 
transverse branches, which divide into finer twigs; the internal 
branches of the two cords frequently anastomose, thus forming 
commissures or connecting nerve-strands between the two. A 
number of nerves extend forwards to the anterior margin, which 
is highly sensitive. 

Reproductive System.—The reproductive organs (Fig. 184) 
are hermaphrodite or monecious in their arrangement, both male 
and female organs occurring in the same individual. The genital 
aperture leads into a small chamber, the genital atriwm 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 


v PHYLUM PLATYHELMINTHES 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 
protrusible muscular organ, the 
penis (p), to open into the genital 
cloaca. At the base of the penis, 
where the vasa deferentia meet, 
the median canal is slightly 
enlarged to form a rounded 
dilatation, the vesicula seminalis, 
Into the median canal open the 
narrow ducts of a number of 
unicellular glands, the prostate 
glands (pr). 

The female part of the repro- 
ductive apparatus consists of 
ovaries (germaria), oviducts, vitel- 
line glands, uterus, and muscular 
sac, The germaria (ov.) are two 
in number—small rounded bodies 
situated near the anterior end, 
each connected with an elongated 
duct, the oviduct. The two ovi- 
ducts (od.) unite posteriorly to 
form a short median common ovi- 
duct opening into the genital 
atrium. With this cavity are 
connected also the uterus (at.), a 
median rounded chamber, and a 
thick-walled muscular body, the 
muscular sac (m.). Numerous Boe 
branching tubes — the vitelline Sey: a a 
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 ™q°“tamcwar me; eo germanium ph 
and gullet (see Fig. 218), and Plata; n penie:!'p:" prostate, fo. 
swallows the food-yolk, by the — st, vitelline glands. (After Jijima and 
aid of which it grows rapidly. oo 
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. 


les 


240 


od vd int ¥v dof 


ZOOLOGY SECT. 


TC. tus 
had a ext longmus 


ent long russ 


l 
iS eee. 


‘Ses 
ee EE 
AAA 


ee ae 
(ml gg RAUCH l sa ie 


ccc. 
vee 
od TH 


Fia. 185.—Transverse section of a Planarian. circ. ius. circular muscular fibres; cece. in- 
testinal ceca; dors. vent. mus. dorso-ventral muscular fibres ; ect. ae ext, long. mus. 
external layer of longitudinal muscular fibres ; int. central lumen of the intestine; int. long. 
mus, internal layer of longitudinal muscular fibres; xe. nerve-cords ; 0. d. oviducts ; par. 
parenchyma ; fesf. testes; v. def vas deferens; vit. vitelline glands. (Irom Hatschek’s 


Lehrbuch.) 


i. The Liver-Fluke (Fasciola hepatiea). 


General Features.—The Liver-Fluke of the Sheep, which is to 


be found inthe in 


terior 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 


70 
reer 
Schr 


excr 


Fic, 186.—Pasciola 
hepatica, natural 
size. exer. excretory 
pore 3 mo.mouth ; repr. 
reproductive aperture; 
scky. posterior sucker, 


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 spinules 
or papitle, 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 (repr.), through which a 
curved muscular process, the cirrus or penis may be protruded. In 
the middle of the posterior end of the body is a minute opening, 
the excretory pore (excr.). 


ors vert. mus 


v PHYLUM PLATYHELMINTHES 241 


Body-wall.—The body-wall (Fig. 187) is found on section to 
comprise three layers:—(1) a homogenous cuticle (cut.) of which 
the spinules (sp.) are 
special developments; (2) 
a layer of circularly dis- 
posed muscular fibres (cir. 
mus.) ; (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 Fic. 187.—Fasciola hepatica. Section of the integu- 


th fs Int ment. circ. mus. layer of circular muscular fibres ; 
e outer suriace. nter- cut. cuticle ; gl. unicellular glands; long. mus. layer 


nally, the interspaces arc aaa muscular fibres ; sp. spinules. (After 
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 esophagus, 
opens into the intestine. The latter (ant.) 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 cea, those on the inner side being short 
and simple, while those on the outer side are longer and branched. 
The two limbs of the intestine with their branches thus form, as 
in the Planarian, a complicated system, the ramifications of which 
extend throughout the whole of the body. There is no aperture 
of communication between the intestine and the exterior, the only 
external opening of the alimentary system being through the 
mouth. 

A branching system of vessels—the water-vessels or vessels of 
the excretory system—ramify throughout the body. A longi- 
tudinal main trunk opens behind by means of the excretory pore 
already mentioned as occurring at the posterior end. In front it 
gives off four large trunks, each of which branches repeatedly, the 
branches giving off smaller vessels, and these again still smaller 
twigs, until we reach a system of extremely fine microscopic vessels 
or capillaries, Each of these ends internally in a slight enlarge- 
ment situated in the interior of a large cell, an excretory cell or flame- 
cell, similar to a flame-cell of the Planarian. 

The Liver-Fluke has a well differentiated nervous system, 


VOL. I R 


242 ZOOLOGY SECT. 


which shares in the prevailing bilateral arrangement of the parts. 
The central part of this system consists of a ring of nerve-matter 
which surrounds the cesophagus and presents two lateral thicken- 
ings, or ganglia, containing nerve-cells, and a single ganglion in the 
middle line below. From this are given off a number of nerves, 
of which the chief are a pair of lateral cords running back to the 


Fic, 188.—Fasciola hepatica. Internal organisation. General view of the anterior portion 
of the body, showing the various systems of organs as seen from the ventral aspect. @. 
ejaculatory duct ; /. female reproductive aperture ; int. anterior portion of the intestine (the 
rest is not shown); od. commencement of oviduct ; ov. ovary (germarium); p. cirrus; ph. 
pharynx; sh. shell-gland:; te. testes; ut. uterus; vdl. left vas deferens; vd?. right vas 
deferens ; vit. lobes of vitelline glands ; vs. vesicula seminalis. (After Sommer.) 


posterior end and giving off numerous branches. There are no 
organs of special sense, 

The reproductive organs (Fig. 188) are constructed on the 
hermaphrodite plan, ic. both male and female organs occur in 
the same individual. The male part of the apparatus consists of 
testes, vasa deferentia, and cirrus. The testes (te.) are two greatly 


v PHYLUM PLATYHELMINTHES 243 


ramified tubes, which occupy the middle part of the body, one 
situated behind the other. From each testis there runs forwards 
a duct, the vas deferens, the two vasa deferentia (v.d.) opening 
anteriorly into an elongated sac, the vesteu/a seminalis (v.s.), from 
which a narrow tube—the ¢juweulatory duct (Fig. 189, e.d.)—leads 
to the male aperture at the ex- 
tremity of the cirrus. The female 
part of the reproductive apparatus 
consists of a single ovary (ger- 
marium), an oviduct, a uterus, an 
ootype, vitelline glands, vitelline ducts, 
and shell-glands. The germariwm 
(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 


: Fic, 189.—Fasciola hepatica. Ter- 
each side are two large ducts, minal part of the reproductive 
anterior and posterior, uniting to appease et f 
form a single main lateral duct, eine ae eel uae eT oeiiacts 
: p. cirrus; ps. =! hy s. 
right or left; and these run nearly sucker; v.’d; vasa deferentia v. & 

. . j 7 j AL - 2 
transversely inwards to open into a Yesouloigerinaliss: (etverisommer:) 


small sac, the yolk reservoir. From this a single median vitelline 
duct runs backwards for a short distance to join the oviduct, 
Around the junction are grouped a mass of unicellular shell-glands 
(sh. gl.), each of which is produced into a narrow process or duct 
opening into the end of the oviduct in the region of the latter to 
which the term votype is applied. The uterus (ut.) is a wide con- 
voluted tube, formed by the union of the oviduct and median 
vitelline duct; in front it opens close to the base of the cirrus. 
When the cirrus is withdrawn, a small cavity, the genital atrium or 
cloaca, is formed common to the external apertures of both male 
and female ducts. A canal, termed’ the canal of Laurer, leads 
from the junction of the oviduct and median vitelline duct to 
open externally on the dorsal surface. 

Development.—Each ovum on impregnation becomes sur- 
rounded by a mass of vitelline matter or yolk derived from the 
yolk-glands. It then becomes enclosed, while passing through the 
ootype, in a chitinous shell, the substance of which is usually said 
to be derived fiom the shell-glands.1. The completed egg remains 


1 The agency of the shell-glands in producing the material of the egg-shell is 
not universally acknowledged. This point will be referred to in the section 
dealing with the general organisation. 


R 2 


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 feces. 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 


Fra. 190.—A—D. Development of Fasciola hepatica. 4, ciliated larva ; 
B, sporocyst, containing redize in various stages of development ; C, 
redia, containing a daughter redia, and cercarie; D, fully deveioped 
cercaria. 0. op. birth opening ; ent. enteron of redia; eye. eye-spots ; 
gast. gastrula stage of redia; germ. early stages in the formation of 
cercariz ; int. intestine of cercaria; mor. morula stage in the develop- 
ment of cercariz ; ws. cesophagus; ov. sv. 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 


(Lymncus), as a parasite of which it is alone able to enter upon the 
next phase in its life-history. When it meets with the Snail, 
the embryo bores into it by means of the head-lobe, coming to 
rest in the pulmonary sac or some other organ of the mollusc. 
Established in the interior of the Snail, it loses its ectoderm and 
grows rapidly into the form of an elongated sac, the sporocyst (Fig. 
190, B), with an internal cavity containing germ-cells and lined 
by a layer of cells, with remnants of the eye-spots, and with 
flame-cells. The sporocyst may divide into two similar bodies by 
a process of transverse fission, but this is exceptional. Eventually 
cells are budded off from the layer that lines the internal 
cavity of the sporocyst or from the germ-cells, and these 
undergo a process of segmentation similar to the holoblastic 
segmentation of the impregnated ovum, resulting in the 
formation of a morula, which becomes converted into a stage 
resembling a gastrula. The gastrula elongates and gives rise 
to a body called a redia (C), which begins to move about, and, 
eventually forcing its way out of the interior of the sporocyst, 
finds its way to some other part of the Snail, usually the liver. 
When fully formed, the redia is a cylindrical body with a pair 
of short processes (proc.) near the posterior end, and with a 
circular ridge near the anterior end. It possesses a mouth 
leading to a pharynx and simple sac-like intestine, and there 
is a system of excretory vessels. In the interior of the redia 
cells are budded off and develop into gastrule, exactly as in 
the case of the sporocyst; these gastrule either develop into 
a fresh generation of rediz if the season should be winter, or, if it 
should be summer, give rise to bodies termed cercaria. 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 cercarize 
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. 


ii. The Common Tape-Worm of Man (Tenia solium). 


General Features.— 7ania soliwm occurs as a parasite in the 
intestine of man. It has the form of a narrow ribbon (Fig. 191), 
which may attain a length of several yards, attached at one end to 


246 ZOOLOGY SECT. 


the wall of the intestine, the remainder hanging freely in the 
interior. Towards the attached end the mbbon becomes very much 
narrower than it is towards the opposite end; and at this narrower 


Fic. 191.—Teenia solium. Entire specimen, reduced ; cap. head. (After Leuckart.) 


extremity 1s a small, rounded, terminal knob, which is known as the 
head or scoler;1 the rest of the animal is termed the body or 
strobila ; the narrow part immediately behind the head is some- 


? 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, 


Vv PHYLUM PLATYHELMINTHES 247 


times called the neck. The attachment of the Tape-worm to the 
wall of the intestine is slight and temporary; it is effected by 
certain organs of adhesion, the hooks and suckers on the head. 

The head (Fig. 192) may be roughly described as pear-shaped, 
but becomes four-sided at the broader end. In the middle of this 
broader, anterior end is a rounded prominence, 
the vostel/wm, 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 1s 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 joimted appear- pre. 192.—MHead of Tenia 
ance, owing to its being made up of a string ERNE, magnified. 
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 


Coit. ul, ov. te. en. ne. 


I 


At 4 i 

< i Vial | 
roe 
Was So) 


Fe RMT 
eal 


EP TV =0\7 7 alto Toe oF (8 : 
Cull a) se 
bibiadday iyte {Aba 


| 


od. 
Fig. 193.—Transverse section of Teenia solium. c.. circular layer of muscle ; ex. longitudinal 
excretory vessel; ne. longitudinal nerve; 0.d. oviduct: ov. ovary; uf, 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 genital 


248 ZOOLOGY SECT. 


papilla, on which is the opening of a chamber, the genital cloaca, 
into which both the male and female reproductive ducts open. 

An examination of entire living and of preserved and stained 
Tape-Worms under the microscope shows (1) that an alimentary 
system is not present; (2) that nervous and excretory systems are 
represented; (3) that there is a complete set of reproductive 
organs, constructed on the same general plan as those of the 
Liver-Fluke, present in each of the proglottides. 

The nervous system consists of two not very well-defined 
ganglia—united by a broad transverse commissure—in the head ; 
of slender nerves passing from these to the suckers, and of two 
longitudinal nerves which run backwards through all the proglot- 
tides to the posterior end of the body. The ganglia seem to 
correspond to the ganglia on the nerve-ring of the Liver-Fluke. 

The excretory organs consist of a richly branched system 
of excretory vessels. There are four main longitudinal trunks 
(Fig. 194, can. excret.), two near each lateral margin in the more 
anterior part of the strobila; in the more posterior region one of 
these becomes lost on each side. The two pairs of longitudinal 
vessels are connected together in the head by a ring-like vessel 


can. excret can excret 


‘= pees | 


meryu 1 


—= —— 
ie XY 
glut aie ou ot 
Fic. 194.—A proglottis of Teenia solium with mature reproductive apparatus. can, eccvel. 
longitudinal excretory canals with transverse connecting vessels ; gl. vit. vitelline glands ; 
nerv. l. longitudinal nerves ; ov, ov. ovaries (germaria); por. gen. genital pore; schld. shell- 


glands ; uter, uterus; 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. 


Vv PHYLUM PLATYHELMINTHES 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 ¢estes 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 termina] 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 atriwm 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 
arounded shell-gland (schid.) 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 wterus. The female 
genital pore, situated in the genital atrium, leads into a narrow 
passage which runs inwards and backwards towards the middle 
line of the proglottis, where it ends in a dilatation usually filled 
with sperms—the receptaculum seminis. From this a narrow duct 
—the fertilising duct or spermatic duct—runs to join the oviduct. 


250 ZOOLOGY SECT. 


The uterus, in the segments in which it first makes its appearance, 
is a simple cylindrical diverticulum of the oviduct; it retains 
its simple form as far back as about the 600th proglottis, where it 
begins to branch, the ramifications increasing in extent and volume 
in the posterior segments. It has no opening on the exterior. 
Masses of sperms (probably from the same proglottis) pass 
in the act of copulation along the vagina to the receptaculwm 
seminis; through the fertilising duct they pass to the oviduct 
to fertilise the ova. Asin 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. =x 
Development.— When the ripe proglottides 
are detached they pass to the exterior with the 
feeces of the host. For a time they exhibit 
movements of contraction. The embryos con- 
MO. iota of tenia tained within the eggs have meantime assumed 
sou. (After the form of rounded bodies, each armed with 
; six chitinoid hooks—the stx-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 host of the parasite, the hooked embryos, becoming 
freed from their coverings, bore their way with the aid of their 
hooks through the wall of the alimentary canal, and reach the 
voluntary muscles. Here they increase greatly in size, and develop 
into rounded cysts with a large cavity filled with watery fluid—the 
proscolea 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 Tenia solium ; this is the bladder-worm stage, 


Vv PHYLUM PLATYHELMINTHES 251 


or cysticercus. If a portion of Pig’s muscle containing cysticerci 
which have not been killed by cooking is taken into the stomach 


Fic. 196.—Development of Tapeworm. 4A, hexacanth embryo; B, proscolex of Twria 
saginata ; C—E£, stages in the formation of the scolex of the same; C, the invagination before 
the hvoks and suckers have become developed ; PD, after the appearance of the hooks and 
suckers ; £, partly evaginated ; F, fully evaginated scolex of 7’. solium with caudal vesicle ; 
G, scolex of 7’. serrata with remains of the vesicle ; H, young tapeworm of 7’. 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. 


2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Platyhelminthes are bilaterally symmetrical, usually dorso- 
ventrally compressed animals, devoid of hard supporting skeleton— 
either external or internal, and also of metameric segmentation ; 
with three embryonic layers—ectoderm, mesoderm, and endoderm 
—entering into the formation of the body. A body-cavity is not 
present. There is a system of excretory vessels, communicating in 
the majority of cases with the exterior, and furnished with ciliary 
flames. There is no blood-vascular system. An enteric cavity 
may be absent, may be rudimentary, or may be highly developed ; 


252 ZOOLOGY SECT. 


it is never provided with an anal aperture. The completed egg 
contains, in addition to the oosperm, a quantity of yolk-matter, 
usually in the form of definite yolk-cells, and usually produced by 
a special set of yolk-glands. Development is sometimes direct, 
sometimes accompanied by a metamorphosis. 


CLASS I.—TURBELLARIA. 


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


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.—RHABDOCELIDA, tncl. ACHELA. 


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, or of a 
cellular epidermis ;* 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 Zemnocephalea and Actinodactylella. 


v PHYLUM PLATYHELMINTHES 253 


ORDER 3.—ASPIDOCOTYLEA. 


Endoparasitic Trematodes with direct development; adhesive 
apparatus in the form of a large sucker, which is divided by 
septa into compartments, and occupies nearly the entire ventral 
surface. 


ORDER 4.—TEMNOCEPHALEA. 


Trematodes with direct development, which live on the outer 
surface or in the respiratory cavities of various animals—e.g., Crusta- 
ceans; most non-parasitic as regards their nutrition, with organs 
of adhesion in the form of a simple posterior sucker and a system 
of anterior or marginal tentacle-like appendages. 


CLASS III.—CESTODA. 


Endoparasitic Platyhelminthes without cilia and without dli- 
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.—PoOLYZOA (MEROZOA). 
The body consisting of head or scolex, and string of proglottides. 


Systematic Position of the Hxamoples. 


Planaria and Dendrocelum are genera of the family Planaride 
or fresh-water Planarians, which is one of the two families of the 
order Tricladida, dittering from the other family, the @coplanide: 
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 Distomide of the Monogenetic Trematodes. 
The Distomide 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, cither 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 Fascioline of 
the Distomede, and this is distinguished from the other sub-families 
by the following characteristics. The Fascioline are broad, leaf- 
like Distomide, 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. 

Tenia soliwm is one of the many species of the genus Tenia, 
of the family Teniade, 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 


v PHYLUM PLATYHELMINTHES 255 


ventral surfaces, right and left sides or borders, and anterior 
and posterior ends. The anterior end is that which is directed 
forwards in ordinary locomotion: it usually has some of the 
features which distinguish a head-end; but a distinct head is 
rarely developed, and the mouth, when present, is usually placed 
some distance back on the ventral surface. 

In the 7urbellaria (Fig. 197) the Jeaf-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- 
celes. A head-region is not 
usually distinct; but there is 
always something to mark off 
the anterior from the posterior 
end—a difference in shape, the 
presence of eyes, and, sometimes, 
of a pair of short tentacles; in 
some a slight constriction sepa- 
rates off an anterior lobe, on 
which the eyes are borne, from 
the rest of the body. In others 
the anterior end is retractile, 
and may be everted as a pro- 
boscis. The mouth is never at 
the extreme anterior end, but Fic. 197.—Various Planarians. 4, Con- 


voluta; B, Vortex; C, Monotus; D, 
always ventrally placed, some- Thysanozoon ; BE, Rhynchodemus; F, 
: : : Bipalium ; G, Polycelis. All natural size. 
times behind the middle. In (After Von Graff.) 


some Polycladida there is a small 

ventral sucker, probably with a copulatory function; and in 
some Rhabdocceles both the anterior and posterior ends, though 
not provided with suckers, are adhesive, so that the animal 
can loop along like a Hydra or a Caterpillar. There is never 
any external appearance of segmentation, though in at least 
one exceptional instance (Gunda segmentata, Fig. 198) the internal 
parts may be so disposed as to approximate to the metameric 
arrangement (psewdo-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 ceca, gonads, transverse commissures of the 
nervous system—are arranged in pairs following this division. A few 


256 ZOOLOGY SECT. 


Fic, 198.—Gunda segmentata. General view of the organisation. br. brain; eye eye; 
gen. ap. genital aperture ; int. intestine with its ceca; long. ne. longitudinal nerve-cord ; 
ov. ovary ; ovd. oviduct: pe. penis ; ph. pharynx ; te. testes ; wt. uterus. (After Lang.) 


v PHYLUM PLATYHELMINTHES 257 


Turbellaria multiply by budding, and these form long chains, having 
something in common with the string of proglottides of a Cestode, 
but differmg 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 Zrematodes (Figs. 186, 199, 200, 201), nearly related to the 
Turbellarians in internal organisation, resemble them also in 


Fic. 199.—Digenetic Trematodes. A, Amphistomum, B, Homalogaster. g. p. genital 
aperture; m. mouth ; s. posterior sucker ; te. testes; vit. vitelline glands. (After M. Braun.) 


external form, with certain modifications connected with a parasitic 
mode of life. As in the latter class, the leaf-shape prevails; an 
elongated form also occurs, though more rarely. The body is 
usually thicker and more solid than in most Turbellaria. The 
anterior end is distinguished from the posterior by its shape, by 
the arrangement of the suckers, and, in many of those Trematodes 
that are external parasites, by the presence of eyes. Suckers, 
present in the Turbellaria only in some of the Polycladida and a 
few Tricladida, are universal in their occurrence. They are always 
ventrally placed, their chief function being to fix the parasite to 
the surface of its host in such a way as to facilitate the taking in 
by the mouth of animal juices and epithelial débris; their 
number and arrangement vary considerably. There are nearly 


VOL, I 8 


258 ZOOLOGY SECT. 


always present an anterior set of suckers (ora single anterior 
sucker surrounding the mouth) and a posterior set, or a single 


A \y) 


IN A. B 
oe A sana 
gl ae ° 
y fe tac ph 
| (il p 
Se A elk 
Es at \ } 
A 2 lie G 
Hit i Aes ; 
Bij 
© MU tise 
oN C)), E O0sp 
: le 
i | 
ov 


<= cil 


Fia. 200.—IMfonogenetic Trematodes. A, Gyrodactylus. cil. disc bearing hooks and pro- 
cesses at the posterior end ; ent. intestine ; gl. unicellular glands whose ducts open on the 
surface about the anterior end ; kJ. caudal dise of the first embryo; h2. caudal disc of the 
second embryo ; mo. mouth ; oosp. oosperm ; ov. ovary; p. penis; ph. pharynx; te. testes. 
B, Polystomum. 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 


Vv PHYLUM PLATYHELMINTHES 259 


digenetic forms—a single anterior and a single posterior sucker ; but 
in some of the Digenctica the posterior sucker is wanting. Adhesive 
papillon 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 Gyrodactylide. 


te 
r.a r.d 
tes: an} 
p.h 
rg lf] 
i 
t t 
z 
Pe , 
Aes S 7 
U.S 00 
p.r g.c 
pr eB : ae 
S 


Fic. 201.—Temnocephala minor, general view of the organisation. ¢. cirrus; ¢. s. 
ejaculatory sac; g. c. genital atrium ; 7. intestjne; 0. germarium ; oo. ootype; ph. pharynx; 
pr. prostate glands ; 7. d. strands of ducts of integumentary glands running forwards to the 
tentacles ; 7. g, groups of integumentary (rhabdite-forming) glands; +.v. receptaculum ; 
s. sucker; ¢. 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 Polystomee (Fig. 200, 

B) there is also a posterior disc on which are six suckers with 

several hooks; in the TZemmnocephalea (Fig. 201) there is no 

anterior sucker, but the anterior end develops a row (two only in 

Scutariella). of adhesive tentacles, while in <Actinodactylella 
$2 


260 ZOOLOGY SECT. 


(Fig. 202) a series of marginal tentacles are present in addition to 
both anterior and posterior suckers. In the Aspidocotylea there is 
only a single sucker; but it extends over nearly the whole of the 
ventral surface, and is complicated in structure owing to its cavity 
being divided into a number of compartments by a system of 
partitions. 

Save in two exceptional cases (Zemnocephala) vibratile cilia are 


Ss 


Fic. 202.—Actinodactylella. b.c. bursa copulatrix; br. brain; c. penis; i. intestine; ov. 
ovary; ph. pharynx; 7, v. receptaculum ; s. sucker; ¢., ¢. testes; wt. 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 
anunals; though many appear coloured variously by the internal 
organs shining through the translucent body-wall, or are stained 


Vv PHYLUM PLATYHELMINTHES 261 


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

The relationship of the Cestoda to the Trematoda is, as will be 
subsequently shown, fairly close ; but though there are connecting 
forms between the two classes, the shape of the average Cestode 
is very different from that of such an average Trematode as the 
Liver-Fluke. The body of an ordinary Cestode is of great length-— 
sometimes extending even to a good many feet—and relatively 
narrow, being compressed into the form of a ribbon. One end, 
which it will be convenient to designate anterior (though it may 


Fic. 203.—Tetrarhynehus. i. nervous Fic. 204.—Teenia echinococcus. 

system ; 7. proboscides; 7s. sheaths, (After Cobbold.) 

with their muscles (rb.). (From 

Leuckart, after Pintner.) 
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 scolex, 
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 rostellwm, the long 
axis of which is in line with the long axis of the body. In Bothrio- 
cephalus and allied forms a pair of longitudinal grooves take the 
place of suckers, and there are no hooks. In many Cestodes para- 


262 ZOOLOGY SECT. 


sitic in Fishes the head bears four prominent thin folded flaps— 
the bothridia, which are exceedingly mobile, and are used more as 
creeping organs than as organs of fixation. In relation to each of 
these bothridia, which, by coalescence, may appear to be reduced 
to two, may be a small sucker of the ordinary kind. In Tetra- 
rhynchus (Fig. 203) there are four very long and narrow rostella, 

r “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 
1s—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. Tenia 
echinococcus (Fig. 204) is exceptional in possessing only three or 
four proglottides. In a few (Ligula and its allies—Fig. 205), 


Fic. 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 (Amphiptyches—Fig. 207), and Archigetes 
(Fig. 208)—(Monozoa), segmentation is entirely absent, the whole 
body in these genera consisting of a single proglottis. The surface 
in the Cestodes is devoid of cilia, and there is no pigment. 
Integument and Muscular Layers.—In the Platyhelminthes 
in general there are integumentary layers and underlying layers of 
muscle, which are more highly differentiated than in the Ceelen- 
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 


Vv PHYLUM PLATYHELMINTHES 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, 


Fic. 206. — Caryophylleus. Fic. 207.—Gyrocotyle (Amphiptyches). Fic. 208.— 
dg, vitelline duct ; d. st. vi- e. 0. excretory opening ; m. 0. male opening ; Archigetes. 
telline glands; e. excretory n. longitudinal nerve ; 2’. anterior nerve- (After Leuckhart ) 
pore; k. mobile organ; od. ring ; 2. r. posterior nerve-ring ; 0. opening 
oviduct; ov. yermarium ; p. of uterus; 0. ovary; o’. receptaculum 
cirrus; 7. 8. receptaculum sc- ovorum ; p. base of cirrus ; 7. 8, receptaculum 
minis ; t. lobes of testes ; v. i. seminis; 7. s. 0. opening of vagina; s. 
vas deferens; v. s. vesicula sucker ; ¢. testes ; ut. uterus; v. s. vesicula 
seminalis ; w.g.0. female aper- seminalis; yk. vitelline glands. (After 
ture, (After Leuckhart.) Spencer.) The end here directed downwards 


represents the scolex-end of other Cestodes. 


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 Rhabdocela and Tricladida they undoubtedly aid in 
adhesion, and probably have the function of assisting in the 
entanglement and capture of food. In certain of the Turbellaria 


264 ZOOLOGY SECT. 


stinging capsules occur similar to those of the Celenterata, and 


transition-forms between rhabdites and stinging capsules occur in 
some cases. Adhesive cells with 


processes also frequently occur 
in the epidermis. Beneath the 
epidermis is a basement mem- 
brane (b. m.), which in the 
Polycladida is of a thick re- 
sistent character, and contains 
stellate cells. 

In a small number of the 
Trematoda three layers are 
distinguishable in the integu- 
ment —a homogeneous, or 
nearly homogeneous, outer 
cuticle ; a cellular, or at least, 
nucleated, epidermis, and a 
basement membrane; but the 
: cellular epidermal layer is 

ania : absent as such in the adult 
Fic. 209.—Section of the hody-wall of .a Triclad. condition in the majority of 


b. m. basement membrane; c. m. circular 


muscles; d. v. m. dorso-ventral muscles; 1 
el. m. external longitudinal muscles; ep. the Trematodes, and there - 


epidermis ; Ge Ue hs internal longitudinal only a homogeneous, non- 

Hee tate tearing cae a eee 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 tegument. 

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 


ony 


v PHYLUM PLATYHELMINTHES 265 


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


Fic. 210.—Parenchyma of Distomum. 4g, b. intercellular spaces ; bm. basement membrane ; 
ec. nuclei; d. nuclei; ep. epidermis. (After Braun.) 


consist of distinct large cells with greatly vacuolated protoplasm, 
with interspaces here and there in which groups of rounded cells 
are enclosed. Sometimes the constituent cells run together, and 
the parenchyma then appears as a nucleated, finely fibrillated, 
vacuolated mass in which the boundaries of the cells are not 
recognisable. Pigment occurs in the parenchyma in some Rhab- 
doccele Turbellarians and a few Monogenetic Trematodes. In 
some Turbellaria—species of Convoluta and Vortez—the paren- 
chyma contains numerous cells enclosing chlorophyll or xantho- 
phyll corpuscles; these are symbiotic unicellular Alge, similar 
in their mode of occurrence to the yellow cells which have been 
referred to as found in the Radiolaria. Running through the 
body, for the most part in a dorso-ventral direction, are numerous 
slender muscular fibres, the fibres of the parenchyma muscle ; 
many of these become inserted externally into the basement 
membrane. 

Great differences exist between the various groups of Platy- 
helminthes as regards the development of the alimentary 
system, differences which are, broadly, to be correlated with 
differences in the mode of nutrition. Some of the Flat-worms 
—the Turbellaria and some of the Monogenetic Trematodes— 
procure their food, in the shape of small living animal or vege- 


266 ZOOLOGY SECT. 


table organisms, or floating organic débris, 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 


id 


Tn 
23 


IZ 


sam 


Be 


Lay 


ph, 


7m 


Fic. 211.—General plan of the 
structure of a Rhabdoceele 
Turbellarian. 0. c. bursa 
copulatrix ; cn. brain ; e. eye ; 
g. germarium ; i. intestine; 
tv. longitudinal nerve; m 
mouth; ph. pharynx; 7s. 
receptaculum seminis ; s. uni- 
cellular glands ; /. testis; u. 
uterus; v. vitellarium; vs. 
vesicula seminalis ; ¢ ejacu- 
latery duct; 82 common 
genital aperture. (After Von aperture; ? female aperture. 
Graff.) 

completely absent in the last-named group, nutrition being effected 

G : 3 . 4 
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 


Fic. 212.—General plan of the structure of a Polyelad. cn. brian ; 
c. eye ; i., st. intestine ; 2a. longitudinal nerve cord ; im. mouth ; 
ov. ovary; ph. pharynx; phl. sheath of pharynx; t. testes; v. 
uterus vd. vas deferens; vs. vesicula seminalis; 4 male 
(After Von Graff.) 


Vv PHYLUM PLATYHELMINTHES 207 


exterior by an anal aperture. All the Turbellaria (except some 
Acela) and Trematoda have an alimentary apparatus consisting 
of two well-defined parts—a muscular pharyna 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. RUS cn 
The mouth is always ventral, but ; € 
varies greatly in its position on the ef 
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 Acewla) 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 (Rhabdocele Turbellaria—Fig. ra : 
211, a few Trematoda), with or | Geaeinre 
without short lateral diverticula. In % i 
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 é Sperone 
off numerous branches. In the Poly- \ ee 
cladida (Fig. 212) there isa central 9 V@ Y yes 
cavity from which numerous branch- DR seek od 
ing canals are given off. In the 4 A ¢ 
Tricladida (Fig. 213) one median “A by 
canal passes forwards from the 
pharynx, and a pair of canals back- Fig. 213.—General plau of the structure of 
wards from it, all three giving off 2 Triclad. cn. brain; « oye; 9. 
: : germarium : i. median limb of the in- 
branches which again branch. Intestine; io. right limb; is. left lim) ; 
some Polycladida there are minute ln. longitudinal nerve-cord ; m. mouth ; 
pores, by means of which certain of te. tentacles; v. vitellaria; vd. vas 


we 


od’ oviduct; ph. pharynx; ¢. testes ; 

B z deferens; uv. uterus; g ejaculatory 

the canals are placedin communica- — duet; @ vagina; g ¢ commun genital 
tion with the exterior. A number “"™'™* “¥er¥= ene) 

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- 


268 ZOOLOGY SECT. 


helminthes. Its elements are nerve-fibres and nerve-cells. The 
nerve-cells, which are usually bipolar, more rarely uni- or multi- 
polar, lie in the course of these fibres, with which the substance 
of the cells is in continuity. The degree of development of a 
central part of the nervous system, or brain, varies in the different 
groups; it is best developed in some Polycladida and some Mono- 
genetic Trematodes. It consists of numerous nerve-fibres which 
here converge from the various parts of the body and pass across 
from one side to the other, together with a central mass of fine 
fibrils, and a number of nerve-cells. It is situated in the anterior 
portion of the body, almost invariably in front of the mouth. 
When the peripheral part of the nervous system is best developed, 
as it 1s 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 commuissures. To these 
there are sometimes superadded fine net-works or pleawses 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 
commiussures, variously thickened to form ganglia representing the 
brain of other Platyhelminthes. 

In addition to the tactile cones of some Trematodes and the 
sensory cilia of the Turbellaria, already referred to, the sensory 
organs of the Platyhelminthes are the eyes and the statocysts. 
Eyes occur in the Turbellaria and some Monogenetic Trematodes, 
but are wanting in the Digenetic Trematodes and in the Cestodes. 
In some of the Polycladida they are extremely numerous, collected 
into groups over the brain, and frequently arranged also round the 
margin of the body. In the Rhabdoceles 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 Rhabdocceles in the head region. 


v PHYLUM PLATYHELMINTHES 269 


The only vascular system present in the Platyhelminthes is the 

system of water-vessels (protonephridia) which are commonly regarded 
as performing an excretory function. The arrangement of these, 
the mode of ending internally of the finest branches, and the way in 
which the system communicates with the exterior, vary greatly in 
the different groups. A series of main longitudinal trunks give 
off branches which subdivide to form a system of minute inter- 
lacing branches or capillaries. In little spaces at the ends of the 
capillaries are a number of highly characteristic structures—the 
ciliary flames. Each ciliary flame consists of a bundle of vibratile 
cilia; typically each is situated in the interior of a cell—the 
flame-cell (Fig. 214)—terminating one of the capillary branches. 
But there are some cases in which there are 
several flames in each flame-cell. The finer 
branches, and in some cases the larger trunks 
also, are intra-cellular, and are to be looked 
upon as perforations in linear rows of 
elongated cells. In the Cestoda, at least 
the larger trunks are inter-cellular, being 
lined by an epithelium of small cells. This 
system of water-vessels opens on the exterior 
in a variety of different ways: sometimes it 
opens by a number of minute pores; some- 
times, as in the Liver-Fluke, there isasingle .., 914 mamoccll of a 
posterior aperture ; frequently there are two. — Turvellarian. F. processes 
In the Tricladida there are two longitudinal 7 seas ee aettes | 
canals which open on the exterior through = j,“jliwy fame. (After 
special branches by a series of pores. 
In the Rhabdoccelida there are either two longitudinal main 
vessels or a single median one; the communication with the 
exterior in the former case may be by a pair of ventral apertures, 
or indirectly through the pharynx; or there may be a common 
short passage in which the two trunks unite, opening by a 
posterior median aperture. When a single main trunk is present 
it opens at the posterior end of the body. In the Trematodes 
there are usually two principal longitudinal trunks, which either 
unite behind to open at the posterior end of the body, or 
(Monogenetica) remain separate and open independently on the 
dorsal surface, each having, where it opens, a contractile eacretory 
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 (Zania cucumerina), a new 
vesicle is developed again and again at the end of the body as a 
fresh segment is thrown off. The main trunks are connected 
together by a ring-vessel in the head and in some cases by a 
transverse branch in each proglottis, and where the latter originate 
from the main trunks are valves formed by folds of the wall of the 
vessel. In the posterior region only two of the longitudinal 
trunks (one on each side) may be retained. 

The sexes are united in all the Platyhelminthes with only 
one or two exceptions, and the reproductive organs are 
sometimes somewhat complicated—presenting a remarkable ad- 
vance on those of the Coelenterata. The male part of the 
apparatus consists of ¢estes, with their ducts, the vasa deferentia, often 
with a contractile terminal enlargement or vesicwla seminalis, a 
cirrus! or a penis, and often prostate or granwle-glands. The female 
part comprises ovary or ovaries, receptaculum seminis, oviduct, uterus, 
an ootype, often a dursa copulatria, shell-glands, vitelline or yolk- 
glands, and cement-glands. In most, though not in all, there is a 
single or paired germariwm (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 octype, 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 vagine in the shape 
of a passage, or a pair of passages, distinct from the oviduct and 
opening independently on the exterior. A wterws 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 atriwm 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 wteri, in which numerous eggs collect. 
The uteri open into a median egg-duct, with which the ducts of 
the shell glands communicate (ootype), and in which the eggs 
receive their chitinoid investment. This leads to the female 
aperture, a part of it being, in some cases, surrounded by a 
muscular sheath to form a bursa copwlatria. 

In many cases the egg-duct gives off posteriorly a narrow duct 
which usually terminates behind ina vesicle known as the accessory 
sac or receptaculum seminis, which may be double. Ina 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 ceca 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, and in some there is a duct of communica- 
tion between this and the intestine (genito-intestinal canal). A 
common genital atrium with a single external aperture receives 
the ducts of both sexes. 

In the Rhabdoceeles (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- 
coeles, 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 copulatria de- 
veloped from the wall of the 
atrium. A wterus is present in 
most cases as an outgrowth from 
ut the wall of the atrium. Male 
Fie, 215.—Reproduetive organs of Meso- and female ducts have a common 

stomum Ehrenbergli. dg. duct of é A + 

vitelline glands ; do. vitelline glands; go. Chamber or genital atrium with a 


common reproductive aperture ; ov. ovary; - £ 
p. cirrus; vs receptaculum seminis; s. single external opening. 


pharynx; ¢., t. testes; ut. uterus; vd. In the Acela there are in 
vas deferens. (From Claus, after von 
Graff and Schneider.) 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 dwrsa, usually provided with chitinous 
structures. 

The Trematodes nearly all have two testes, usually compact, 
sometimes branched; in a few instances there are four. The vasa 
deferentia unite into a median duct, which is dilated at the base 
of the cirrus to form a vesicula seminalis. There is a single oval 
or branched germarium, and two sets of vitelline glands. A canal 
termed Lawrer’s canal in some Malacocotyleans, 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 


v PHYLUM PLATYHELMINTHES 273 


on the surface independently of the uterus: internally it com- 
municates with the oviduct through the main vitelline duct. 
In some of the Hetcrocotylea there is a genito-intestinal canal 
occupying a corresponding position to Laurer’s canal, but opening 
into the intestine. In the Aspidocotylea this is replaced by a 
stalked yolk-receptacle. There is nearly always a genital atrium 
common to the ducts of both sexes. 

In the Lemnieephalea (Figs. 201, 202) there is a genital atrium 
and a single genital aperture. There are two pairs of compact 
testes; the right and left vasa deferentia unite in a vesicula semi- 
nalis, and granule—or prostate glands are well developed. The 
cirrus has a chitinous tube and a variety of eversible spines. There 
is a single compact ovary; the oviduct has connected with it a 
large receptaculum, and dilates posteriorly to form an ootype into 
which the shell-glands open. <Actinodactylella (Fig. 202) alone 
has a bursa copulatrix (b. ¢.). 

In the ordinary Cestodes each segment or proglottis contains a 
set of reproductive organs similar to those of a Trematode. There 
may be asingle 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 (v.¢., Dibothriocephalus), 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 (Rhabdocesla, 
Monogenetic Trematodes) is direct—i.ec., 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 ecgg-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. 1 T 


ord ZOOLOGY SECT. 


separated off a small cell. The embryo at this stage consists of 
eight cells, four large—the megameres, and four small—the 
miecromercs. The four micromeres increase rapidly by division, and 
extend over the embryo, forming a layer, the ectoderm, completely 
covering it in all parts except for a median fissure, the Dlastopurr, 
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 


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 


v PHYLUM PLATYHELMINTHES 275 


become formed is, as in the Ctenophora (p. 218), a process of 
cpibolie gastrulation. The brain is developed from a pair of 
thickenings of the ectoderm : these unite into acommon 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 ceca 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 Miiller’s larva (Fig. 217). It assumes 


a) 


aa 


Fic. 217.—Miiiller’s larva. A, longitudinal section; B, lateral view. ce. ectoderm; er. 
endoderm; g. Jnain; Ad. enteron; 0. mouth; ph. pharynx; pt. pharyngeal pouch; sn 
sucker. (From Lang.) 


an oval shape, with a series of cight elongated processes, covered 
with long cilia, and connected together by a ciliated band. ‘There 
are eye-spots at the anterior end anda 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 SECT. 


embryonal pharynx. Meanwhile a group of four cells enclosing a 
cavity are modified to form the foundation of the endoderm, 
and increasing in number give rise to the embryonal intestine, 
into which the embryonal pharynx soon opens, the latter opening 
on the exterior by a mouth aperture. 

The embryonal pharynx (Fig. 218, ph’) 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 


Dh, 


Fic, 218.—Sections through embryos of Dendreceelum lacteum (somewhat diagrammatic). 
dz, yolk-cells; ec, ectoderm; en, endoderm; ph’, embryonal pharynx ; ph”, permanent 
pharynx : wz, wandering cells. (From Korschelt and Heider, after Hallez.) 


becomes formed and the cavity of the pharynx opens into its lumen. 
Subsequently the permanent mouth makes its appearance. The 
brain is formed in the thickness of the blastoderm, and thus 
appears to be of mesodermal origin, not of ectodermal, as in 
the Polyclads. 

In some Rhabdocceles (certain species of Mcesostoma) two distinct 
kinds of eggs are formed—summer and winter eggs. The oosperms 
are deposited singly, and each, together with a mass of yolk-cells, is 
enclosed in a chitinous, usually stalked, shell. Segmentation 
takes place very much as in the Polyclads. No embryonal 
pharynx is formed, the permanent pharynx performing the function 
of swallowing the yolk-material: it appears to be of endodermal 
derivation. The intestine arises from a group of cells—the 


v PHYLUM PLATYHELMINTHES 277 


primitive endoderm—among which a cavity appears. But in 
some Rhabdocceles no definite intestinal epithelium is developed, 
and the syncytial mass which represents it is only to be dis- 
tinguished from the surrounding mesodermal syncytium by its 
enclosing the remains of the yolk. No metamorphosis is known 
to occur. 

An account has already been given (p. 244, Fig. 190) of the 
development and metamorphosis of the Liver-Fluke (Fasciola 
hepatica) which may be looked upon as typical of the Digenetic 
Trematodes in general. There is thus to be recognised in the 
Digenetic Trematodes an alternation of generations comparable to 
that which has been described as so general in the Coelenterata, 
In the Trematoda, however, it is to be observed, it is an alterna- 
tion of a sexual, not with an asexual, but with a parthenogenetic 
generation (the sporocyst), the ova of which develop into a second 
parthenogenetic generation (the rediz) ; and these finally produce 
larvee (the cercarie) 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 Distomide 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 cercarize 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 cercariz in most Digenetic Trema- 
todes only develop further if they succeed in establishing themselves 
in asecond intermediate host instead of merely becoming encysted 
on the surface of herbage, as in the case of the Liver-Fluke. The 
cercarle of different Trematodes differ greatly, particularly with 
regard to the nature of the tail. In some forms the cercaria is 
tail-less: such cercariee do not become free, but arc 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 (h’), while still within the body of the parent worm, 
develops another embryo (7) 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 paradoxwm, 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 (endocele) 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 


Fic. 219.—Section through the blastoderm of Temmnocephala, showing carly stage of 
endoccele (en). 


endoceele 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 docs not appear as an embryonic layer, the 
epidermis only appearing late, and extending over the surface 
evidently by modification im sttw of cells that have become 


au PHYLUM PLATYHELMINTHES 279 


separated from the main body of the blastoderm. The rudiments 
of the brain are formed in the blastoderm near the wall of the 
endoccele, 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 endoccele: subse- 
quently these rudiments shift their position to the dorsal surface 
on which the sacs form their 
permanent apertures pierc- 
ing the epidermis. There 
is no metamorphosis of any 
kind—all the organs, includ- 
ing even the male part of 
the reproductive apparatus, 
being well advanced towards 
full development before the 
young animal leaves the 
egg. ; 
The egg of a Cestode is 
similar in essential respects 
to that of a Trematode: 
there is a tough, chitinoid 
membrane or egg-shell, 
which encloses not only the 
ovum but a number of yolk- 
cells. The result of seg- 
mentation is the formation 
of a superficial layer of cells 
(ectoderm) and a central 
mass, all enclosed in a mem- 
brane composed of a single 
layer of cells thrown off 
when the embryo escapes 
from the egg. The ecto- 
dermal cells become ciliated, 


So far as 1S known, only Fic. 220.—Longitudinal section through the entire 
in Bothriocephalus ; in the egg of Temnocephala with the shell re- 


others they are thrown off a eae Meg CRM Sty nete eae 
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 jinal host, the eggs 
are discharged. 

In order that development may proceed further, the embryo 


28a ZOOLOGY SECT. 


must in most cases reach the interior of a second or intermediate 
host. This is a passive migration, since the embryo of the Cestode is 
still confined within the egg-shell, and the transference has to 
take place in the water or food. The digestive fluids of this inter- 
mediate host dissolve the egg-shell and set free the contained six- 
hooked or hexracanth 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 
cystwercoid 1s given; when, on 
the other hand, the intermediate 
host is a vertebrate, the form 
assumed is nearly always tliat 
termed cysticercus, 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—c.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 
Fic, 221.—A Cysticercoid (Polycercus) with intermediate host. The eysti- 

the head and rostellum enclosed hy the  cercoid consists of three parts— 

ciudal vesicle. a. aperture through which af 
evagination takes place; bd. hudy; «. & tape-worm head or scolea: with 
cavity of vst cd, caudal vesicle: ce-  the hooks and suckers of the 
lum ; s. sucker, (After Haswell and Hill.) 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 


eases the invertebrate intermediate host actually forms the food 
¢ 


\ 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 cysticcrcus 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 lum cyaginated. ros. rostellum ; 8.5 8. 
suckers. When inverted this in- oa 
vagination corresponds closely 
with the head and body of the cysticercoid; the bladder corre- 
sponds to the caudal vesicle. Thus the chief difference between 
a cysticercus and a cysticercoid is that in the former the caudal 
vesicle is relatively very large and that the order of development 
of the parts is somewhat modified. 

A very small number both of cysticercoids and cysticerci 
multiply by proliferation—by the formation of more than one 
tape-worm—head from one embryo. In the few instances in which 
this occurs among the cysticercoids the hooked embryo gives rise, 
not directly to a cysticercoid, but to a mass of cells from which 
are given off a number of buds, each developing into a cysticercoid 
with the three parts already described. One such form occurs 


282 ZOOLOGY SECT. 


in certain Earthworms, another in a Myriapod (Glomeris 
linbatas). 

Tania canurus of the Dog has a bladder-worm stage in the 
Sheep and Rabbit which gives rise to several tape-worm heads, 


Fic, 223,-Cyst of Teenia echinococcus with the developing daughter-cysts and scolices. 
(After Leuckart.) 


and the same holds good of Tenia serialis from the Fox. But 
the best known instance of multiple production of scolices in a 
cysticercus is Zeenia 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 


Fu:, 224.—Sceovlices of I, echinococcus,. Fic, 225.—Separate scolex of 
(After Cobbold.) T. echinococcus. (After 
Cobbold.) 


and 225), which, when fully formed, assume the appearance of 
eysticercoids 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 


v PHYLUM PLATYHELMINTHES 283 


of the intestine and develop into the adult Tania cchinococcus 
—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, 
and the minute embryos escaping, reach 
some organ, such as the liver or lung, in 
which they are capable of developing into 
a comparatively enormous cyst. 

Asexual reproduction also occurs in 
some Platyhelminthes. In some Rhabdo- 
ecele Turbellaria (Jfirrostomum) 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 xooids 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 — Fi. 226.—Process of budding 
in Microstomum. c., ¢’. 


undivided part behind the head, do not ciliated groove; ¢. eye;-pot 
in turn give rise by budding to new pro- mouth, CA Hea vin Guar) 


glottides. Spontaneous transverse fission 
has been observed in certain Tricladida, and is often followed by 
the regeneration of the lost portion. 


6. DisrripuTion, 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 distrabutzon. 
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-woris live, 
in one phase or another, in animals of nearly every class of the 
Metazoa, 


284 ZOOLOGY SECT. 


As regards their mode r 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 larvee of larger fornis, Insect larvee, Water-mites, 
Rotifers, small Worms, and the like; or sometimes of Diatoms and 
minute Algve of various kinds. Some, however, live a life of true 
parasitism. Such are certain Rhabdoccles 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 ina 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 Bdellowra, 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 (Polystomwm), lives in the 
urinary bladder of the Frog; another (Aspzdogaster) 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 


Vv PHYLUM PLATYHELMINTHES 285 


ciliated larvee, they again enter into a state of parasitism as sporo- 
eysts or rediz in a second host; and, after a second free interval as 
cercarie, may enter the body of a third host to become encysted. 
The second host is, very gencrally, a Mollusc, and the cercaria 
may become encysted in the same animal. 

The Cestodes are, of all the Platyhelminthes, those that are most 
modified in accordance with the condition of internal parasitism 
in which they remain throughout life. The adult Cestode is 
almost always an inhabitant of the alimentary canal of a verte- 
brate. The intermediate host is frequently also a vertebrate— 
commonly of a kind which is liable to become the prey of the 
final host. In the case of Zwnia crassicollis of the intestine of 
the domestic Cat, for example, the cysticercus-stage occurs in the 
livers of Rats and Mice ; the cysticercus of Z'wnia 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 dranslation, 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 Kuropeans. asciola 
hepatica has occasionally been found in the human liver; Dis- 
tonum rathousii is a common intestinal parasite in China ; 
Opisthorchis sinensis occurs in the liver of Man in China and Japan ; 
Dicrelium lanceolatum and various other species of the genus 
occasionally occur in the human subject. Schistosomum hama- 
tobium and S. japonicum, which differ from most other Trematodes 
in being unisexual, are found in the human portal system of veins 
in various parts of Africa, in Arabia, the Philippines, and Japan. 
Eggs with contained larvee are voided with the urine, and if they 
reach water, the larvee 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 
Tenia solium and 7’. saginata (otherwise called 7. mediocanellata). 
The cysticercus stage of 7. solium (Cysticercus cellulose) occurs, 
as already stated, chiefly in the muscles of the Pig, that of 
1. sagina/a 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 Tania soliwm 
is the more prevalent; where more beef, 7. saginata. Bothrio- 
cephalus 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 Tenia erhinococeus, the presence of which produces what is termed 
hydatid disease (p. 282). The adult Zenia echinocorrus 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 they 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 danghter-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— 
eg., the Cystecercus cellulose: of Tania soliwm. 


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. Celoplana 
and Ctenoplana (p. 225) are probably rather to be looked upon 
as Ctenophores specially modified in accordance with a creeping 
mode of progression than as intermediate forms between Cteno- 
phores and Turbellaria. The relationship with the Coelenterata is 
shown, perhaps, most strikingly when we take into account the 
development of the Turbellaria, in the earlier stages of which there 
is to be recognised a marked tendency towards a radial symmetry. 
In their development the Turbellaria, that is to say the Planarians, 
show some special points of resemblance to the Ctenophora; the 
ectoderm cells are formed and spread over the rest in a similar 
way, and the bands of cilia have a disposition and mode of move- 
ment that strongly bring to mind the ciliary swimming plates of 
the Ctenophora. But though there is much to be said in favour 
of the view that the Turbellaria and the Ctenophora were derived 
from a common, not very distant stock, the latter are too specially 
modified to be looked upon as the direct ancestors of the former. 

The connection between the Turbellaria and the Monogenetic 
Trematodes is very close—so niuch 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— 


Vv PHYLUM PLATYHELMINTHES : 


to 


ove 


ie, that different families may have become developed from 
different families of Turbellaria altogether independently, some 
of them appearing to be nearer the Rhabdocwles, others nearer 
the Polyclads, and others, again, nearer the Triclads, in the majority 
of their characters. 

The remarkable life-history of the Digenetic Trematodes is, as 
already pointed out, to be looked upon as a speeial form of alter- 
nation of generations—the alternation of a seaval with a pado- 
genetic and parthenogenetic generation (heterogeny). The sporocyst 
and redia are to be regarded as intercalated stages—as cercarie 
which exhibit peedogenesis. 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 Trcematodes an intimate 
relationship is traceable. Caryophyllaus (Fig. 206) is a Cestode 


Monogenetica 


Polycladida 


Digenetica Pol 
Temnocephalea 3 ees 


Nemertinea Tricladida 
Monozoa 


id 
Clenophora Rhabdocoelida 


Lower Coelenterata 


Fic, 227.— Diagram of the relationships of the Platyhelminthes (together with the Nemertinca). 


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 Gyrocotyle 
(Fig. 207), Amphilina, and Archigetes (Fig. 208). The most 

1 It is possible, however, that in the last two forms we have to do with larval 


Cestodes which have failed to reach the mature stage, and have undergone 
4 precocious development of the sexual apparatus. 


288 ZOOLOGY SECT. 


important differences between a Cestode and a Trematode, in 
addition to the absence of an enteric cavity in the former and 
its presence in the latter, is the occurrence in the Cestodes of 
strobilation. Ligwla in a certain sense forms a connecting link in 
this respect between the Trematode and the ordinary Cestode, the 
bedy 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, 7. 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 (¢9., Aonivzia) 
tells in favour of the view that the scolex is anterior and 
corresponds to a head. 


APPENDIX TO PLATYHELMINTHES. 
Ciass NEMERTINEA. 


General Features.— The Nemerteans are non-parasitic, unseg- 
mented worms, most of which are marine, only a tew 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 (p7.), the possession of which is one 


v PHYLUM PLATYHELMINTHES 289 


of the most characteristic features of this class of Worms. The 
proboscis is hollow; when extended to its utmost, a part. still 
remains which is not capable of being everted. This hollow tube 
(Fig. 229) is open in front, where its edges are continuous with 
the body-wall, and closed behind. Its wall in the eversible part 
consists of an epithelium (internal when at rest) continuous with 
the epidermis and similar to the latter, a basement-membrane, 
and either two or three layers of 
muscle, circular and longitudinal, 
with an external thin epithelium 
of flat cells. The circular muscular 
fibres are not continued back on 
the non-eversible part, but the 
longitudinal fibres pass backwards 
to form the retractor muscle, by 
means of which the proboscis is 
attached to the sheath in which 
it is enclosed, and by means of 
which also it is retracted. The 
internal epithelium of the pro- 
boscis develops variously formed 
and arranged papille, 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 
Celenterates. 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 
(Metanemertint)—a median cal- 
careous stylet (Figs. 232, 233) 


long.ne 


: a 
with groups of smaller accessory Fic. 228.—Diagram of the organs of a 


atyleis. al the ides “Inc the) Mavetie fon tor gues 
everted proboscis these are borne nerve-cords ; m. mouth; ». sreplinaia 

‘ . : ov. ovarics; pr. proboscis. er 
at the free anterior extremity, Hubrecht.) : 


and are thus capable of being 

used as weapons. In Drepanophorus there are a number of small 
stylets supported on a narrow curved plate, together with accessory 
stylets. In the rest of the Nemerteans stylets are not developed. 
It is by contraction of the muscular walls of the sheath, the 
cavity of which (rhynchocele) 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 


290 ZOOLOGY SECT. 


The outermost layer of the body-wall is an epidermis of 
columnar cells many of which are ciliated, while others are 
unicellular glands, some of which are arranged in groups; these 
secrete the mucus with which the surface is usually covered, and 
which may form a gelatinous tube. Beneath the epidermis is a 
basement membrane, very thin in most cases, followed by the 
muscular layers. In some Nemerteans (whence called Dimyaria) 
there are only two layers of muscular fibres, an outer circular and 
an inner longitudinal ; in the rest (Zrimyaria) a third (longitudinal) 
layer is super-added. Another circular layer of muscular fibres 
closely encompasses the digestive canal. The interspace enclosed 
by the outer inuscular layers does not comprise any cavity corres- 
ponding to a true ceelome or body-cavity, except, perhaps, the 


Fic, 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 ; i. p. muscular portion of the proboscis; p. p. in A, proboscis pore ; 
p- p. in B represents the position of the provoscis-pore in the retracted condition of the 
proboscis ; p. 4. 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.'| 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—wsophagus (stomodeum) 
—but may be more complicated, and divided into various 
regions. Posteriorly it opens into the intestine. The latter may 


7 When a fui’ is present the intestine may, or may not, be continued 
through it. 


v PHYLUM PLATYHELMINTHES 


(Metanemertini) project for- 
wards below the esophagus 
as a ventral cecum, 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 ceca, 
which may be branched. The 
cxeca are separated from one 
another by incomplete trans- 
verse septa of dorso-ventral 
muscular fibres—the  ar- 
rangement of the ceca 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 Temnocephala). 

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 (dors. 
ves.) and two lateral (lat. 
ves.). The blood is, in 
most cases, colourless, and 
contains rounded or ellipti- 
cal, usually colourless, cor- 
puscles, 


291 


dors.ves 


retr mis By 


Fic. 230.—Tetrastemma. General view of the 
internal organs. an. anus; uc. st. accessory 
stylet; cer. g. brain ; cil. qv. ciliated groove of 
cerebral organ ; dors. ves. dorsal vessel ; lat. ne. 
lateral nerve; lat. ves. lateral vessel; neph. 
nephridium ; op. neph. nephridial aperture ; 
provl, eversible part of proboscis ; prob. non- 
eversible part of proboscis ; prob. ap. aperture 
for the protrusion of the pruboscis 3 retr. mus. 
retractor muscle of the proboscis; sé. stylet. 
(From Hatschek’s Lehrbuch.) 


u 2 


292 ZOOLOGY SECT. 


The excretory system has a considerable resemblance to that 
of the Platyhelminthes. It consists of a pair of longitudinal 
vessels (Fig. 235, 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 cesophagus. 

The nervous system is in some re- 

er carr Pootine  Spects more highly developed than in the 

eae Turbellaria. The brain (Figs. 228 and 

ing through which the pro. 231, br., and Fig. 230 ce. y.) 1s composed 
pboscis is everted; p. s. pro- : : : 

hoscis-sheath ; pr. proboscis. of two pairs of ganglia, dorsal and ventral, 

Se eae noe wane «the ganglia of each pair being con- 

Hubrecht.) nected together by commissures, the 

dorsal situated above, the ventral below, 
the anterior part of the proboscis and proboscis sheath, and 
both being above the mouth and cesophagus. From the brain 
pass backwards a pair of thick longitudinal nerve-cords which 


Fics. 232 and 233.—Proboscis of a Hoplonemertean, with stylet rescrve-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 


Vv 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 


Fic. 234.—Diagrammatic transverse section of a NWemertean (Carinella). a,b, ¢. layers of 
body-wall ; ¢. t. connective tissue between body-wall and enteron ; 0. be. lateral blood-vessuls ; 
long. ne. longitudinal nerves ; p. proboscis ; p. s. proboscis-sheath, (After Hubrecht.) 

the Heteronemertini is a nerve-plexus extending all over the body. 

In the Aletanemertini, 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, 


lat. blv 


med bly 


Fic, 235.—Anterior portion of a Nemertean (Drepanvphorus), showing the blood-vascular 
and excretory systems. Jat. bl. v. lateral bluod-vesscls ; med. bl. v. median bloud-vessels ; 
neph. 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 


Fic. 236.—A, Pilidium with advanced Nemertine worm ; B, ripe embryo of Nemertes from 
interivr of pilidium. an. amnion, or investment of the embryo; i. intestine ; lp. lateral pit ; 
a. hervous system ; @. gullet; pr. proboscis; st. stomach. (From Balfour, after Biitschli.) 


the region of the brain or of a pair of lobes separate from the 
latter. This apparatus may have a respiratory function, more 
especially for the oxygenation of the substance of the brain, but 
perhaps 1t has also a sensory function. It has some resemblance 
to the ciliated pits developed in certain Z'urbellaria. 

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 diwcious. The ovaries 
(Fig. 228), ov.) and testes are situated in the intervals between the 
intestinal cuca. 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 pilidiwm (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 hody-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 segmentata 
(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. 


OrpDER 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. 


OrvDER 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 itis 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 clongated and cylindrical, 
whence their general name of Round- or Thread-worms. The 
phylum includes the following classes :-— 

Class 1. NEMATODA.—The Round-worms in the strict sense of 
the term. The best known forms are internal parasites, but many 
genera and species are extremely abundant in fresh and salt 
water. 

Class 2. ACANTHOCEPHALA.—The “Hook-headed Worms,’ a 
group of formidable internal parasites. 

Class 8. CHETOGNATHA.—The “ Arrow-worms,” a small group of 
pelagic organisms. 

The affinities of the Acanthocephala and Chetognatha with the 
Nematoda are somewhat doubtful, and the association of the three 
classes is largely a matter of convenience. 


CLASS I.—NEMATODA. 


1. EXAMPLE OF THE CLASS—THE COMMON ROUND-WORM OF 
Man. (Ascaris lumbricoides.) 


Ascaris lumbricoides is a common parasite in the human intes- 
tine: a closely allied if not identical form (A. sui//a) occurs in the 
Pig, and another (A. mcegalocephala) 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. (4 inch) 
in diameter; the male is considerably smaller. 

External Characters.— When fresh the animal is of a light 
yellowish-brown colour: it is marked with four longitudinal 
streaks, two of which, very narrow and pure white in the 

297 


298 ZOOLOGY SECT. 


living Worm, are respectively dorsal and ventral in position, and 
are called the dorsal (Fig. 237, d./.) and ventral (v.l.) lines: the other 
two are lateral in position, thicker than the former, and brown in 
colour, and ave distinguished as the lateral lines. The mouth is 
anterior and terminal in position, and is bounded by three lobes, 
or /aps, one median and dorsal (d. /p.), 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 ereretory pore (ex. p.), At about 
the saine 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 sete (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 


A ip B : a 


Fic, 237.—Ascaris lumbricoides, A, anterior end from above; B. the same from below; 

i C, posterior end of female, D, of male, side view. un. anus ; d. lp. dorsal lip ; d. l. dorsal line ; 
ex. p. excretory pore; p. papillae; pn. s. penial sete; v. Z. ventral line; v. lp. ventral lip. 
(After Leuckart.) 


the short ¢azl, or post-anal portion of the body, which in the male 
is sharply curved downwards (Fig. 237, D), while in the female (C) 
its ventral contour is nearly straight. 

Body-wall.—The outer surface of the body is furnished by a 
delicate, transparent, elastic membrane, of a firm material of 
albuminoid composition, the cuticle (Fig. 238, cu.). It is divisible 
into several layers, 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—t.e., an ectoderm 
is which the cell-boundaries are not differentiated, and whose 
cellular nature is recognisable only by the nuclei. The cuticle is, 
as usual, a secretion of the ectoderm. 

Beneath the ectoderm is a single layer of muscular fibres (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 face (i.c. towards the body-cavity) into a large and almost 
bladder-like mass of protoplasm (p) containing a nucleus (nw.). 
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. ¢) has the form of a plate bent upon itself 
so as to be, as it were, wrapped round the protoplasmic portion 
(p). The protoplasmic processes project to a greater or less extent 
into the body-cavity, sometimes practically obliterating it, and are 
produced into delicate filaments (7) which take a transverse 


Ce 
aes 


ay 


i 
<< 


Fic. 238.—Ascaris lumbricoides, transverse section. cu. cuticle ; d.7. dorsal line ; der. epthm, 
deric epithelium or epidermis ; ec. v. excretory vessel ; int. intestine ; Jat. J. lateral line 5 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.1.). 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 pharyna or stomodewm (Fig. 240, 


300 ZOOLOGY SECT. 


ph.): its walls are very muscular, its cavity is three-rayed in cross- 
section, and it is lined by a cuticle secreted from the epithelial layer 
and continuous, at the mouth, with that of the body-wall. 
Posteniorly the pharynx opens into the tntestine (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, indt.). 
Posteriorly the intestine narrows considerably to form the short 
rectuin, Which has a tew muscular fibres in its walls and opens 
externally by the anus (Fig. 240, az.). The food, consisting of the 
semi-Huid contents of the intestine of the host, is sucked in by 
movements of the pharynx, and is then absorbed into the system 


Fic. 239.—Ascaris lumbricoides. A, a single muscle fibre; B, several fibres in transverse 
section with portion of ectoderm (below). e¢, contractile substance ; J. fibrous processes ; nu. 
nucleus ; p. protoplasmic portion. (After Leuckart.) 


through the walls of the intestine. The food being already 
digested by the host, there is no need of digestive gland-cells, 
such as occur in animals which prepare their own food for 
absorption. 

It will be noticed that in the above description the pharynx is 
also called stomodeum. 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 epithclitm of the intestine, on the other hand, is 
endodermal, this portion of the canal being derived from the 
archenteron of the embryo. 


VI PHYLUM NEMATHELMINTHES 301 


Between the enteric canal and 
the body-wall is a distinct space, 
the body-cavity, containing a 
clear fluid and more or less en- 
croached upon by the protoplasmic 
processes of the muscle-cells. The 
cavity is bounded externally by 
these processes, internally by the 
outer cuticle of the intestine : there 
is no trace of epithelial lining 
such as occurs in most of the higher 
animals The body-cavity of the 
Nematode, in fact, does not exactly 
correspond to the ccelome 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 (ez. v.), one 
in each lateral line. Anteriorly 
these pass to the ventral surface, 
unite with one another, and open 
by the minute excretory pore (ew. 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, éach 
cell having a single nucleus. 

The nervous system consists 
of a ring (nv. 7.) 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 
(din, vln.). They are connected with 
one another by transverse com- 
missures (c.), and the ventral nerve 
swells into a ganglion just in 
front of the anus. The pharyngeal 
nerve-ring contains nerve-cells, and 


3 mth. mouth; nv. 7. nerve-ring ; 


Taine 


deric epithelium 3; cx. p. excretory 


=) 


eo 


| 


an. anus; eu. cuticl 


utestine, partly cut away ; lat. l. lateral line ; m.1 
ead out; gk. pharynx, partly cut away ; wt. uterus ; 


atic dissection of the female. 


der. epthon 


Semi-diagr: 


pore ; ec. v7. excretory vessel; gap. gonopore ; i 
ovy. ovary, that of the right side in situ, the left 


240._Ascaris lumbricoides. 


Fic. 


302 


ZOOLOGY SECT. 


its ventral portion (wz.) is thickened and ganglion-like. The only 
sense-organs are little clevations, the sensory papille (Fig. 237, p.), 


on the lips. 


The reproductive organs are formed on a peculiar and very 


characteristic pattern. 


vuln 


din: 


}— han 


Fic. 241,—Diagram of nervous 
systein of Nematoda. e. 
commissures; din. dorsal 
nerve ; isn. posterior lateral 
nerve; on. upper and un. 
under portion of nerve- 
ring ; 8. g. lateral swellings ; 
vin. ventral nerve. (From 
Lang, after Btitschli.) 


The ¢estis (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 1s 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 ejacwlatorius, into the rectum. 
Behind the rectum, and opening into its 
dorsal wall, are paired muscular sacs (s.), 
containing the pental sete (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, 
ce. d.¢.); when transferred into the body 
of the female they exhibit amcboid 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 (wé.). 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 (gzp.) at about one-third of the 
entire length from the head. 


VI PHYLUM NEMATHELMINTHES 303 


Development.—The eggs are produced in immense numbers— 
at the rate, it has been reckoned, of about 15,000 a day. They 
are fertilised in the upper part of the uterus, each becoming 
enclosed in a chitinoid egg-shell, and are passed out of the body of 
the host with its feces. 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 


Fic. 242—Ascearis lumbricoides, postcrior extremity of male, dissected. an. anus; 
cu, cuticle ; der. epthm. epidermis; m. muscular layer; p. s. penial seta; s. sac containing 
penial seta ; ts. testis; vs. sem. vesicula scminalis, 


water, or in soil accidentally swallowed, into the intestine of a new 
human host, in which the embryos, escaping from the eggs, become 
mature. 


2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Nematoda are Nemathelminthes having a cylindrical body 
of great length in proportion to its diameter, and pointed at both 
ends The body-wall consists of a tough external cuticle, an 
ectoderm in the form of a syncytium or protoplasmic layer con- 
taining nuclei and rarely exhibiting cell-structure, and a single 
layer of longitudinal muscular fibres which are interrupted along 
one or more (dorsal, ventral, and lateral) lines. The body-wall 
encloses a body-cavity containing a clear fluid, and more or less 
encroached upon by processes of the muscle-cells or other meso- 
dermal tissues. The enteric canal is straight, and consists of 
pharynx, intestine, and rectum: the pharynx is a stomodeum. 
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 dicecious: 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- 
hum. The pharyngeal nerve-ring sends off a single large ventral 
nerve-cord well supplied with nerve-cells. The gonads, or at least 
the ovaries, are arranged metamerically, and the reproductive pro- 
ducts are discharged into the body-cavity and pass thence into the 
gonoducts. This order includes a small number of greatly 
elongated, thread-like worms (species of the genus Gordius), which 
are parasitic in the asexual, free-living in the sexual stage; and 
also the genus Neetwrus, 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 Ascarida 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 Ascarid are distinguished by 
the possession of three lips furnished with papille, and by the 
body of the male being curved ventrally and being provided 
with penial sete. Ascaris is distinguished from the other 


VI PHYLUM NEMATHELMINTHES 305 


genera of the family by the absence of a bulb-like enlargement at 
the posterior end of the pharynx, by thé posterior extremity of the 
body having the form of a short blunt cone, and by the presence 
of two penial sete in the male. 


3. GENERAL ORGANISATION. 


External Characters.—The Nematoda vary much in size: the 
little Anguzllula, 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 Gordius, 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 cap.c 
Ascaris, 7. consists of a single 1) 
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 woe Fic. 243.—The body-wall of a platymyarian 
tains only two rows, the total "Nematode, spread out. Jat. 1. lateral lines. 

: : (After Leuckart.) 
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. | x 


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 


Wy Ra 


Fia. 244.—Ankylostoma duodenale. A, male and female in coitu.  B, anterior end, 
showing—cv. 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 T’richinella, 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 dter-cellular 
but intra-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 


Vic, 245.—Transverse section of Gordius, lm. ventral nerve-cord; c. cuticle; et. epithelium 
lining body-cavity ; hy. epiderm ; lh. budy-cavity ; 7m. muscular layer; md. intestine ; mes. 
mesentery ; ov. ovary ; wu. uterus. (From Lang, after Vejdovsky.) 


hollow appendages in the form of cesophageal glands or intestinal 
ceca. In Dochmius a pair of pear-shaped bodies of unknown 
function, the cervical glands (Fig. 244, B, cv. gi.), lie one on each 
side of the pharynx and probably open externally near the mouth. 


VI PHYLUM NEMATHELMINTHES 307 


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 1s, 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 pro, 245.-Oxyuris, from the 


‘ | z right side. gz. gizzard ; int. 
the fact that in the smaller forms the eee ph pune 
entire genital tube (gonad plus gonoduct) pn. s. penial setee ; ts, testis ; 
é § a v. df. vas deferens, (From 
is short and not coiled (Fig. 246, ts. and Shipley, after Galeb.) 


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 


308 ZOOLOGY SECT. 


prevented as if the organs of the two sexes were distinct. A 
totally different arrangement is met with in the Nematomorpha, 
the female having numerous pairs of ovaries (Fig 247, A, 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, vt.) and are continued posteriorly by short 
vagine (vag.) into a median chamber. The latter opens externally, 
and also receives the duct of a large spermotheca (spth.) or chamber 
for stormg the sperms received in copulation. In the male 
Gordius the testes are not known: they seem to disappear very 


pe 


Fic. 247.—Gordius, A, hvurizontal section of female, showing ovaries (ovy) attached to mesen- 
tery (mes.); b. w. body-wall. B, posterior extremity of male, sagittal section, b. c. bursa 
copulatrix ; cl. cloaca ; int. intestine ; ¢t. tail; v. nv.rd. ventral nerve-cord ; vs. sem. vesicula 
serminalis. C, posterior extremity of female, sagittal section. gnp. gonopore ; spth. sperma- 
theca ; ut. uterus ; vag. vagina ; v. nv. cl. ventral nerve-cord. (After Vejdowsky.) 


early, after discharging their contents into large reservoirs or 
vesiculee seminales (B, vs. sem.): from these, vasa deferentia are 
continued into the cloaca (el.) or dilated extremity of the intestine, 
part of which can be everted as a bursa copulatria (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 Rhabdonema nigrovenosum (Fig. 248) a kind 
of epiboly, or a process of an intermediate character. The blastopore 
always disappears, taking no part in the formation of the apertures 
of the adult. The archenteron also becomes obliterated, and the 
lumen of the intestine has no connection with it, but is formed 
anew by the development of a fissure between the endoderm cells 


VI PHYLUM NEMATHELMINTHES 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 stomodwum (Fig. 248, I, stdm.), 
and the rectum appears also to be lined by ectodermal cells and 
thus to be of the nature of a proctodwwm. 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 


Fic, 248.—Development of Ascaris nigrovenosa. Jip. blastorore; ect. ectoderm send. endo- 
derm ; ent. enterou; g. genital cell; mes. mesoderm ; 2. nervous system ; stdm. stomodzum. 
(Frormo Korschelt and Heider, after Goette.) 


ectoderm and the endoderm (E—H.) The body-cavity is not a 
true coelome (iv., a cavity formed in the interior of a mass of 
mesoderm) or arising as an outgrowth from the archenteron), but is 
derived from the primitive blastula- or segmentation-cavity. The 
nervous sytem is developed from certain cells (G—I, n.) which 
bud off from the ectoderm at the anterior end of the body. The 
reproductive cells originate from a pair of the inner cells (H, I, g.) 
which become differentiated from the rest at an early stage. 


310 ZOOLOGY SECT. 


Many of the Nematoda have a curious and complex life- 
history : a few examples will be selected for description. 

Rhabdonema nigrovenosum lives, in the sexual condition, in the 
lungs of Frogs and Toads: it is remarkable among members of 
the class in being hermaphrodite. The eggs are laid and the 
embryos pass from the lungs into the enteric canal of the host, are 
expelled with its feces, and develop in water into a sexual 
Nematode, called the Rhabditis-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 dicecious form. This type of alternation of generations is 
distinguished as heterogeny. 

One of the most terrible parasites of man is Z'richinella spiralis 
(Fig. 249), a minute worm, the male (C) a little over 1 mm. (4 in.) 
in length, the female (B) about 3 mm. (4 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, ¢) 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 Trichine 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, eg. that of Man 
by a Pig or Rat, or that of a Pig by Man. When this is done the 
cysts are dissolved by the digestive juices, the worms escape, 


vI PHYLUM NEMATHELMINTHES 311 


develop reproductive organs, and copulate, the young migrating 
into the muscles and producing the disease as before. The 
result of eating an ounce of “trichinised” or “measly” pork, 


Fic. 249.—Trichinella spiralis. A, encysted form, in muscle of host ; B, female; C, male. 
bh. connevtive-tissue envelope ; cy. cyst; Je. ejaculatory duct; e. embryos, /. lat-globules ; 
h. testis; m. f. muscle-fibre; oc. 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 


312 7 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 bancrofld 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. & banerofti 1s 
very widely distributed in tropical countries, and is the cause of a 
disease called filariasis, with a variety of symptoms—such as 
anemia, 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, £. (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 ] cm. in length, 

External Characters.—The body is cylindrical, and ends in front in a 
protrusible portion, the proboscis (A, p., B, pr.), which is cylindrical and is 
covered with many rows of recurved chitinoid hooks. The worm lies with the 
proboscis sunk in the wall of the intestine of its host, which is sometimes riddled 
with holes formed in this way. In some species there is a distinct neck (B, 2.) 
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 


VI PHYLUM NEMATHELMINTHES 313 


transverse, and then one of longtitudinal muscles, The body-wall thus constituted 
encloses a spacious body-cavity or coelome containing a clear fluid. 

In correspondence with the absence of mouth and anus there is no trace of 
enteric canal, the Acanthocephala resembling in this respect the Cestoda, the only 
other class of Metazoa which is entirely anenterous. Food is thus, as in tape- 
worms, taken entirely by absorption by the general surface of the body. 

The proximal end of the proboscis is contained in a muscular sheath sunk 
in the anterior end of the trunk, and is provided with four retractor muscles 
(Fig. 250, r.m.). The muscles of the sheath are circular and act as protractors, 
At the sides of the base of the proboscis two club-shaped organs, the /emnisci 


Fic. 250.—A, Echinorhynchus (Gigantorhynchus) gigas, female from the Pig (nat. 
size; B, BE. lesiniformis, male from the edible Frog (magnified). 0b. bursa; c.gl. cement 
glands; gup. gonopore ; lm, lemnisci ; 2. neck ; p. or pr. proboscis ; 7. m. retractor muscle 
of proboscis; s./g. suspensory ligament ; ¢. testis ; v. vessel. 


(/m.), hang down into the body-cavity. Their function is quite unknown, but 
they have been compared with the cervical glands of Nematodes (p. 306). 

In the body-wall run two longitudinal vessels (v.) containing a granular 
fluid, and connected with a network of fine canals in the proboscis, bursa, &c. 
The function of these vessels is not known with certainty : they may have to do 
with the absorption and circulation of nourishment. 

The central nervous system (Fig, 251, nv.) is represented by asingle large 
ganglion placed at the base of the proboscis, and sending off nerves in various 
directions. In the male there are also two ganglia supplying the reproductive 
organs. Organs of sense are wholly absent. 

A pair of remarkable excretory organs or nephridia (Fig. 253) have been 
found to occur in Echinorhynchus gigas. These consist of a pair of ramified 


314 ZOOLOGY SECT. 


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 hody-cavity is occupied 
by the reproductive organs. The sexes are 
separate, and the female is larger than the male. 


Fic. 251.—Behinorhynchus gigas, Disscc- Fic. 252.—Eehinorhynchus 
tion of male. b. bursa; c. gl. cement glands ; gigas. Dissection of female 
im. lemnisci; nv. nerve-ganglion; pr. pro- (semi-diagrammatic). b. bell ; 
boscis ; s. /g. suspensory ligament ; ts. testis ; im, lemnisci; pr. proboscis ; 
v. df. vas deferens. (After Leuckart.) 8. ovy. swimming ovaries ; ut. 


uterus ; ¢y. vagina. 


In both sexes the gonads and their ducts are connected with a great suspensory 
hiyameut (s.dg.), which extends backwards from the end of the proboscis-sheath. 
In the male there are two ovoidal festes (Fig. 251, ts.) connected with the 


VI PHYLUM NEMATHELMINTHES 


315 


suspensory ligament. From each a vas deferens (v. df.), furnished with several 
vesicule seminales ov sacs for the storage of spermatic fluid, passes backwards 


Fic, 253.—A, longitudinal section through the terminal twigs of the nephridia of Eehino- 
rhynchus gigas; highly magnified. a, nucleus. B, a te:minal twig more highly magni- 


fied. b, the porous membrane. (From Shipley, after Kaiser.) 


and unites with its fellow to form an ejaculatory duct, with which are connected 
about half a dozen cement glands (c.gl.). The ejaculatory duct opens into the 


bursa or hell-like copulatory organ (6), and has at its 
opening a small papilla acting as a penis. 

In the female the ovary is connected with the sus- 
pensory ligament (Figs. 252 and 254, s./g.). When 
ripe, groups of ova—known as the ‘‘ swimming ovaries ” 
(s.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 werine bell (b), widely 
open anteriorly (Fig. 254, x) into the ccelome, 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 (vf.), which itself opens by the genital aperture 
at the posterior end of the body. The uterine bell 
perforins 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 
the ccelome 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 
celome. Segmentation is regular, and a peculiar form 
of gastrula is produced, having neither archenteron nor 
blastoccele--in other words the ectoderm and endoderm 
are in close contact with one another, and no central 
cavity is enclosed by the latter. The ectoderm layer, 
which is devoid of cell-limits, secretes a cuticular 
membrane investing the embryo, then a second mem- 
brane is formed within the first, and a third within the 
second; the embryo thus comes to be enclosed in a 
triple case, which differs from an egg-shell in being 


Vic, 254.—Female organs of 


Echinorhynchus. 
b. uterine bell; s. /g. 
suspensory ligament ; 
ut. uterus ; vg. vagina 5 
x.y aperture of bell; 
z. apertures leading 
from bell to uterus. 
‘After Hertwig.) 


316 ZOOLOGY SECT. 


formed by the developing ectoderm. At what will become the anterior end 


chitinoid hooks appear. 


At about this period the embryo is born, and reaching the intestine of the 


Fic. 255Sagitta hexaptera, 
from the ventral aspect. a. 
anus; bg. ventral ganglion ; 
d, intestine; j#. lateral fins; 
ho. testis ; m. mouth; ov. ovary; 
ovd, oviduct ; sc. cesophageal 
connective; sb. vesicula semin- 
alis; s. fl. tail fin; sh, tail- 
cavity ; sf. spermiduct. (From 
Lang’s Comparative Anatomy, 
after Hertwig.) 


preceding classes, is formed of several layers of epithelial cells. 
delicate basement membrane, and then a layer of muscles (m.), the fibres of 


host, is extruded with its feces. Its further 
development depends upon its being swallowed 
by an intermediate host, which, in the case of 
E. gigas of the Pig is a maggot, the larva of a 
Beetle, Cetonia aurata. The Echinorhynchi of 
fresh-water Fish have for their intermediate host 
certain small fresh-water Crustacea belonging to 
the genera Gammarus and 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 ccelome ; 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 celome. Part of the proboscis and 
its sheath are also of endodermal origin. The 
ectoderm gives rise to the protoplasmic layer of 
the body-wall, to the whole system of vessels, 
and tothe lemnisci. The larval cuticle is thrown 
off and a new one formed. The larva reaches 
adult proportions and attains sexual maturity 
only if the intermediate host is eaten by the 
permanent host. 


CLASS III.—_CHATOGNATHA. 


The present group, like that just discussed, 
is a very small one, containing only three genera 
(Sagitta, Spadelia and Krohnia) of curious arrow- 
shaped worms, all but one species of which are 
pelagic. 

External characters. —The body (Fig. 255) 
is elongated and nearly cylindrical, and is divided 
into head, trunk, and tail, the head being marked 
off by its somewhat rounded form, while the junc- 
tion of trunk and tail is indicated by the ventrally 
placed anus (a). The tail bears a horizontal ex- 
pansion, or caudal fin (s. fl.), and there are also 
horizontal lateral fins (f.)—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. epthm), 
which, instead of being syncytial as in the two 
Next comes a 


vI PHYLUM NEMATHELMINTHES 317 


which are striated and disposed longitudinally in four bands—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 


Fic. 256._Sagitta bipunctata. Transverse sections, A, of trunk; B, of tail. coel. ccelome ; 
cel. epthm. layer of nuclei of the muscle-cells formerly regarded as a ccelomic epithelium ; 
d. epthm. deric epithelium ; /. fin; int. intestine ; m. muscles; ovy. ovary; ts, testis. (After 
Hertwig.) 


chitinoid hooks (Fig. 257 gh.) 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 stomodeum 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 ceelome 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 ceelome is similarly divided into - 
right and left chambers byalongitudinal Frc, 257-—Head of Sagitta bipunctata, 


vertical partition (Fig. 256, A and B). from above. an. optic nerve; aw. eye ; 
There is tiotrace-of vascular system g. brain : gh, hooks ; rn. olfactory nerve ; 

y ro. olfactory organ; sc. cesophageal 

or of excretory canals. The nervous connective. (From Lang’s Comparative 


system, on the other hand, is much Anatomy, after Hertwig.) 


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, 9), which 
sends off on each side a long nerve-cord, the wsophageal connective (sc.). The two 
connectives sweep round the enteric canal and unite on the ventral surface, not 


318 


ZOOLOGY 


SECT. 


far from the middle of the trunk, in an elongated rentral ganglion (Fig. 255, 
by.), from which numerous nerves are given off. The brain sends nerves to the 
eyes (Fig. 257, av.) 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 papille 
carrying stiff bristle-like processes, and probably serving as organs of touch, 


cells; st. rods. (From 
Anatomy, after O. Hertwig.) 


There are two eyes (Fig. 258), 
situated one on each side of the 
dorsal surface of the head: each 
is globular and contains three 
biconvex lenses (/.), separated by 
pigment (p.) and surrounded by 
rod-like sensory cells (rz.). Behind 
the head is a ring-like structure, 
of the nature of an annular ridge 
of peculiarly modified and in part 
ciliated cells (Fig. 257, 70.): to 
this an olfactory function has been 
assigned. 

Reproduction. — The Chto- 
gnatha are monecious. The ovaries 
(Fig. 255, ov., Fig. 256, ovy.) are 
elongated organs situated one on 
each side of the trunk-region of 


the ccelome, 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 ceelome, 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 (s/.) opening at one end into the celome 
by a ciliated funnel-like extremity, and at the other end dilating into a reservoir 
or vesicula seminalis (sb.), 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). Two endoderm cells (y.) at the anterior end of the archenteron, #.¢, the 


Fra. 259.—Three stuges in the development of Sagitta. UI. blastopore ; cs. cozlomic sacs ; «/. 
mesenteron ; g. sexual cells ; pm. parietal layer of mesoderm ; st. stomodeum ; vm. visceral 
layer of mesederm. (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, y.), two of which 


subsequently become the ovar 


and two the testes. At the same time two folds 


v1 PHYLUM NEMATHELMINTHES 319 


of endoderm grow into the archenteron from its anterior end, partly dividing the 
cavity into three parts—a middle division or mesenferou (d)—the rudiment of the 
intestine, and two lateral dlivisions—-the meftentera, or cwlomic sacs (c.s.)—which 
give rise to the right and left compartments of the cwlome 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 
cceelomic sacs re-appear; the tail-region of the body-cavity is formed from the 
posterior, undivided portion of the archenteron. The blastopore (b/.) now closes 
and an invagination of ectoderm—the stomodeum (s/.)—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 stomodeum ; 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, 7.¢. that portion of it which 
remains in immediate contact with the ectoderm. 
Thus in Sagitta the mesoderm is entirely derived 
from the endoderm of the gastrula. 


APPENDIX TO NEMATHEL- 
MINTHES. 


1. Family Chetosomide. 


This family includes three genera of small 
worms, Chetosoma (Fig. 260), T'risticochwta, and 
Rhabdogaster, which are sometimes included 
among the Nematoda. 

The body is elongated, its anterior region 
being sometimes dilated to form ahead. Either 
the whole body, or the dorsal surface only, is 
beset with fine sete, and there may be a double 
row of movable chitinoid hooks round the head, 
reminding us of the ‘‘jaws” of Sagitta. The 


ventral surface bears curious locomotor rods (f),  ¥14,_ 260.—Mature | female _ of 
Cheetosoma claparedii. 


either hooked or with knobbed ends: by these x 57. Gd, oesophagus; b, in- 
the animals crawl. The mouth is anterior and testine ; cs anus; d, Se he 

i a e. generative pore; jf, loco- 
terminal, the anus posterior and ventral, and there thoter rods.. (Rrony Shipley, 


is a muscular pharynx. The sexes are separate. after Metschnikoff.) 

The male has a single testis: the vas deferens 

opens along with the anus: there are two penial 

sete. ‘The female has paired ovaries and a single vagina opening near the 
middle of the body on the ventral side. 


2. Family Echinoderide. 


Echinoderes is a minute marine worm of cylindrical form with a flattened 
ventral surface. The body is segmented, or divided into rings, eleven or twelve 
in number, all strongly cuticularised, and most of them bearing spines (Fig. 261). 
The mouth is placed at the anterior, the anus at the posterior end of the body : 


320 ZOOLOGY SECT. 


the former opens into w sac, which can be everted so as to form a proboscis or 
introvert, and is armed with spines. The enteric canal consists of w 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 Desmoscolecide. 


Desmoscolex is also a minute marine worm, 
having a globular head and a variable number 
of segments (Fig. 262). The head bears four 
movable chitinoid rods or sete, and a pair of 


aaegl 


Fic 261.—Eehinoderes, x about 210. 0, spine; Fic. 262.—Female Desmoscolex, 
e.s. caudal spine; ph. pharynx; s. and s’. spines ventral view, x 260. a, ovary. 
on the proboscis ; s.g. salivary glands; st. stomach. (From Shipley, after Panceri). 


(After Hartog.) 


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 dicecious ; 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 sete. 


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 phylum 
is extremely doubtful. They all agree in being elongated, cylin- 
drical worms with a body-cavity, which is sometimes of the 
nature of a true ccelome; there is a certain resemblance 


VI PHYLUM NEMATHELMINTHES 321 


between Nematoda and Chetognatha in the muscular system ; 
and the lemnisci of Acanthocephala have been compared with the 
cervical glands of Nematoda. Beyond these points there is little 
to unite the three classes ; and, on the other hand, the proboscis 
of Acanthocephala recalls the rostellum of Cestoda. 

Very various views have been put forward as to the affinities of 
the Chetognatha. 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 
Nemathelminthes. In connection with this question, the Cheeto- 
somid, 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 phylura present 
some undoubted resemblance to the Nematoidea: this is especially 
the case in the reproductive organs of the Chetosomide, and still 
more in those of Desmoscolex. But the segmentation of the body 
in both Desmoscclecide and Echinoderide and the presence of 
sete show a certain resemblance to higher worms or Annulata, 
which will be more fully appreciated when that phylum has been 
studied. 


VOL. I Y 


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, trochophure. 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 


Fic. 263 —Trochophore. 4. anus; d.lM. dorsal muscles ; ED. rectum; J. stomach ; J}. in- 
testine ; Mstr. mesoderm-band ; 2. nerves; Neph. nephridia; 0. mouth; Oe, cesophagus ; 
@wLM, vesophageal longitudinal muscle; SP. apical plate; v.LM. ventral muscle; v.LN. 
lateral nerve; Whr, wkr., pre- aud post-oral bands of cilia; WS. 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 cirelet (Wkr.) or prototroch, situated on a corre- 
sponding ring-like thickening of the ectoderm; behind the mouth 
ig often a second circlet of cilia, the post-oral circlet (wkr.) and a 
cihated groove or ciliated streak usually runs backwards from it 


long the middle of the ventral surface. The mouth, situated just 
322 


SECT. VII PHYLUM TROCHELMINTHES 323 


behind the pre-oral circlet, leads into an alimentary canal, which at 
first runs nearly transversely, and then bends round so as to extend 
back towards the narrow end, near which it opens on the exterior 
by an anal aperture. About the middle of the broader (anterior) 
end of the trochophore is a thickening, the apical plate (SP.), pro- 
jecting from which are usually a number of sensory cilia ( WS.) ; and 
in many trochophores eye-spots and a pair of short tentacles occur 
in close relation with the apical plate, which is the nerve-centre 
of the larva. A pair of ciliated tubes—the excretory organs or 
nephridia (Neph.)—may be present. 

In the higher groups in which this form of larva occurs, the 
adult condition is attained by modifications and new developments 
of so radical a nature that the transition from larva to adult is of 
the nature of a metamorphosis. Sometimes the narrow part of the 
larva elongates and becomes divided into a series of sections fore- 
shadowing the metameres of the adult animal; in other cases, in 
which no metamerism occurs, radical changes of other kinds lead 
to the adult form. But in all these higher groups, whatever the 
nature of the changes involved, there is a metamorphosis, and the 
adult animal is totally unlike the larva. In a small number of 
forms now to be dealt with, however, there is no such radical 
change, and the adult may be looked upon as a somewhat modified 
trochophore. The groups thus associated together may not be 
be genetically related: they may have become independently 
developed from trochophore-like ancestors, but the possession of 
the general characters which have been referred to above renders 
it convenient to group them together and regard them as con- 
stituting a small but well-marked phylum. The groups referred 
to are the Rotifera or Wheel-animalcules, together with the 
Gastrotricha, Associated with these, though scarcely to be in- 
cluded in the same phylum, are the Dinophilea and Histriobdellea. 


CLASS I.—ROTIFERA. 


The Rotifers or “ Wheel-animalcules ” are microscopic creatures, 
very abundant in pools, gutters, &., 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-zavity, 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 3 mm. (,); in.) in 

Y 2 


324 ZOOLOGY SECT. 


length, and is divisible into two distinct parts—a broad anterior 
region, the ¢rwnk, and a slender movable tai (¢.). The trunk is 
enclosed in a glassy cuirass or lorica (Ir.), 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, 


ee \ 
= OY \e Ss 
i 


tne 


Fic, 264.—Brachionus rubens, female. A, from the dorsal aspect; B, from the right side. 
a.anus ; br. brain ; d. #. dorsal feeler ; c. gl. cement-gland ; cl. cloaca ; ¢. 1. ciliary lobes ; ¢. v. 
contractile vesicle ; e. eye-spot; iat. intestine; lr. lorica ; 1. f. lateral feeler; m. muscular 
bands ; nph. nephridial tubes4 ov. germarium; ph. pharynx; st. stomach; ¢. tail; tr. d. 
trochal disc ; vf. 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 (¢r.d.) with a prominent edge fringed with 
cilia: this is the ¢rochal 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 classis derived. Within the circlet 
of cilia arise three prominences (¢l.) covered with cilia of large 


VII 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 (@.) 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 
of its cuticular lining, an 
elaborate apparatus for tri- 
turating the food. In the 
middle line is a forked struc- 
ture, the incus, consisting of a 
small base or fulerwm (7.) and 
of two branches or rami (7.). 
On either side of the incus is 
* hammer-like structure, the Fic. 265 —Pharynx of Brachionus rubens. 
malleus, consisting of a handle f. fulcrum; a. manubrium; uw. uncus; 7. 
or manubrium (m.) and of a ramus. (After Hudson and Gosse.) 
toothed head or uneus (w.). 
By means of the muscular walls of the chamber the heads of the 
mallei are worked backwards and forwards upon the forked uncus, 
and thus reduce the organisms taken as food to a fine state of 
division. 

The pharynx leads by a short gullet into a spacious stomach (st.), 
having a wall composed of very large epithelial cells, ciliated 
internally: with it are connected paired digestive glands. The 
stomach opens into a rounded ¢ntestine (int.), also ciliated internally, 
which communicates, by means of a short c/oaca (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 stomodzeum, 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 SECT. 


The excretory system consists of paired nephridial tubes 
(Figs. 264 and 267, nph.) resembling those of the Platyhelminthes. 
Their general direction is longitudinal, but they are a good deal 
coiled and give off little tag-like processes ending in flame-cells. 
With the end of each tag, projecting into the body-cavity, 1s a long 
flagellum. The lumen of the tubes 1s 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 


Fic. 266.—Brachionus rubens. A, male; B, female, with attached eggs; c. gi. cement- 
glands ; cv, contractile vesicle ; nph. nephridial-tube ; ov. ovum in body ; ovl, ova attached 
to base of tail ; p. penis ; ¢. tail; ts. testis. (After Hudson and Gosse.) 


size, situated at the anterior end of the body, above (dorsal to) the 
mouth and pharynx. On the dorsal surface of the brain, where it 
comes into contact with the body-wall, is a small red eye-spot (e.). 
The only other organs which can be considered as sensory are 
three structures known as tactile rods or feelers ; one of these (d. f.) 
is a small cylindrical process tipped with stiff hair-like bodies, 
which projects from the dorsal surface just behind the trochal disc : 
the other two (/. /.) are paired, situated on the dorsal surface of 
the lorica and not prominent. 

The tail contains a pair of cement glands (c. gl.), by the secretion 
of which the animal is able temporarily to attach itself. 

Reproduction and Development.—The sexes are lodged 
in distinct individuals, which present a striking degree of sexual 
dimorphism. The preceding description applies to the female, 


vi PHYLUM TROCHELMINTHES 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 vitedlariwm (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 (¢s.) 
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 


Fic, 267.—Diagram ofa Rotifer. a. anus; Ur. brain ; cl. pre-oral, and c?. post-oral circlet of cilia ; 
ec. gl. cement gland ; cl. cloaca ; cu. cuticle ; «/. ep. deric epithelium ; d. f. dorsal feeler ; ¢. eye ; 
fl. c. flame-cells ; int. intestine ; m. muscles ; mth. mouth ; nph. nephridial tube ; ov. ovum ; 
ovd. oviduct ; ovy. germarium. ; ph. pharynx ; st. stomach ; vt. vitellarium. 

of two sizes, the larger giving rise to females, the smaller to 

males. Probably both kinds develop parthenogenetically, but in 

the autumn thick-shelled winter eggs are produced which appear 
to require fertilisation. These remain quiescent during the winter, 
and in the spring develop into females. 


2.—DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Rotifera are Trochelminthes of microscopic size. The ante- 
rior end is modified into a retractile trochal disc, with variously 
arranged cilia; the posterior end usually forms a mobile and 
often telescopically jointed tail. The mouth is anterior and more 
or less ventral in position, the pharynx contains a. chitinous 
masticatory apparatus, and the anus 1s placed dorsally at the 
junction of the trunk with the tail. There is a spacious body-cavity 
devoid of epithelial liming. The excretory organs are a pair of 
nephridial tubes provided with flame-cells. The central nervous 


328 ZOOLOGY SECT. 


system consists of a single dorsal ganglion, with, in a few cases, a 
smaller ventral or sub-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.—RHIZOTA. 


Rotifera which are fixed in the adult state by the truncated end 
of the non-retractile tail. 
Including Moscularia, Stephanoceros, Melicerta, &e. 


ORDER 2.—BDELLOIDA. 


Rotifera which both swim freely by means of the cilia of the 
dise and creep after the manner of a Leech. The tail is telescopic 
and forked distally. 

Including Rotifer, Philodina, &e. 


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.—IJlloricata. 


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


Sub-order b.—Loricata. 


Ploima in which a lorica is present. 
Including Brachionus, Huchlanis, &e. 


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 Heaarthra, 


ORDER 5.—TROCHOSPHARIDA. 


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


vir PHYLUM TROCHELMINTHES 329 


ORDER 6.—SEISONIDA. 


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


Fi. 268.—Paraseison asplanchnus, female x 230. w. genital aperture; br. ganglion; jf. 
foot-glands ; m. mouth ; ma. mastax ; oe. esophagus ; ov. ovary ; st. stomach. (After Plate.) 


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


Systematic Position of the Example. 


Brachionus rubens is one of the several species of the genus 
Brachionus: it belongs to the family Brachionide, 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 Brachionidw is distinguished by having a box-hke 
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, e9., Asplanchna (Fig. 269, 6), this organ is absent. In 
Pedation (Fig. 270, 7) 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 sete, and closely resemble the lhmbs of 
some of the lower Crustacea: each is moved by two opposing 
muscles which extend into its cavity (J, B, m). Three pairs of 
similar appendages are present in the other genus of Scirtopoda, 
Henxarthra (Fig. 270, 2), 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, 8) possess simple 
or fringed setae moved by muscles attached to their bases. 

In the Rhizota the adult is permanently fixed (Fig. 269, 1-4). 
The end of the tail is devoid of the characteristic fork, and is 
attached to plants or other supports. Moreover the animal is 
surrounded by a tube into which it can retract itself completely, 
protruding the anterior end with the trochal disc’ when undis- 
turbed. In most instances, as for example in Mosewlaria (Z) and 
Stephanocervs (2), the tube is formed of a delicate, transparent, 
gelatinous secretion of the epidermis; but in MMelicerta (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 fices. 

The ciliation of the trochal dise is subject to considerable 
variation. In its simplest form the disc is surrounded by a single 
eirclet of cilia, within which lies the mouth. A modification of 
this type may be produced by the prolongation of the ciliary 
crown into long arm-like processes fringed with cilia, as in 
Stephanoceros (:2), or, as in Floscularia (2), 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 


8.Polyarthra 


Foes 
10.Notholca 


7.Hydatina 


Fic. 269.—Typical forms uf Rotifera :—9 and 10 show the lorica only, a. anus ; el}. c%, ciliary 
<circlets ; int, intestine ; m. muscle ; ph. pharynx. (After Hudson and Gosse.) 


3382 ZOOLOGY SECT. 


the first. The mouth in this case is always placed between the two 
circlets on the ventral side (Fig. 267), so that the inner or anterior 
circlet is pre-oral and corresponds with the chief ciliary band 


SUP edad ten 


\\ 


2.Hexarthra 


a 
3.Trochosphaera 


Fic. 270.—Typical forms of Rotifera. In i, a shows the outer form, b the muscular system. 
a, anus ; br. brain ; cl. c2. ciliary circlets ; cl. cloaca; d. gl. digestive gland ; d. /. dorsal limb ; 
e. eye-spot; l. 1, 1. U. lateral limbs; im, muscles; mth, mouth; ph. nephridial tube ; 
ov, ovary; ph. pharynx; s. sense-organ ; v. /, ventral hmb. (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-ora] band found in many worm-larvee. In the 
curious globular Tyrochosphera (Fig. 270, 3) there is a single 
equatorial circlet, which is pre-oral,and a few post-oral cilia: here 
the correspondence with the typical worm-larva is singularly 


VII PHYLUM TROCHELMINTHES 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 fulerwm (7) and of two 
broad branches or rwmt (r), and a pair of mallei, each consisting 
of a stout handle or manubriwim (av) and a broad, toothed head or 
uncus (wv). In some forms all the parts of the apparatus become 
very slender, the incus assuming the form of forceps (Fig. 271, A). 
Or the mallei1 may be absent and the two rami movable upon 
one another so as to convert the incus into a pair of forceps (B) 


— 


l 


ae 
(\ mil Li] 
= ci 


> 


(at 
i 


" 


B\\\¥ Cc 


Fic, 271.—Typical forms of mastax. A, forcipate type; B, incudate type; C, ramate type. 
Jf. fulerum ; m. manubrium; 7. ramus; uw. 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 cesophagus. 

The stomach is always large, and usually has a pair of digestive 
glands opening into it: it may pass insensibly into the intestine, 
or the latter may be a distinct chamber of more or less globular 
form. In the Rhizota the intestine turns forwards so as to allow of 
the anus being brought over the edge of the tube in defecation 
(Fig. 269, 4,a). In Asplanchna (6) the stomach ends blindly, the 
intestine, cloaca, and anus being absent. 

The excretory system is very uniform in structure. It con- 
sists of a pair of more or less coiled nephridial tubes, placed 
longitudinally and giving off lateral branchlets which end in 
fiame-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-cesophageal ganglion is present 
as well, connected with the first by a pair of slender cesophageal 
connectives. Connected with the dorsal ganglion are a pair 
of lateral longitudinal nerves which run backwards to the tail, 
giving off branches in their course. One or more eyes (¢) 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 
(ay, Lf.), 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, te. the gonad is unpaired (Fig. 264), consists of germarium and 
vitellarium, and is provided with an oviduct (Fig. 267). But in 
some of the Bdelloida, such as Philodina, there are two ovaries, not 
divisible into germ-gland and yolk-gland, and the oviduct is absent. 
The males are smaller than the females and degenerate in structure, 
the enteric canal being atrophied (Fig. 266, A). There is a large 
testis (¢) 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, t.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 swmmer 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 
stomodzum and proctodeum. The tail is formed asa prolongation 


VII PHYLUM TROCHELMINTHES 335 


of the postero-ventral region of the embryo, and contains at first 
an extension of the endoderm. No metamorphosis is known to 
take place in any member of the class. 

Ethology.—A few Rotifers live in the sea, but the majority 
are fresh-water forms, occurring in lakes, streams, ponds, and even 
in puddles the water of which is rendered foul and opaque by mud 
and sewage. Frequently the water in which they live is dried up, 
and the thick-shelled winter eggs may then be widely dispersed 
by wind. It is even stated that the adult animals may survive 
prolonged desiccation and resume active life when again placed in 
water. They are able to survive prolonged exposure to tem- 
peratures far below the freezing point of water. Many forms 
cling to the bodies of higher animals in order to obtain a share 
of their food, thus leading a kind of commensal existence. 
Others go a step further and become true external parasites, 
like Drilophaga on a fresh-water Oligochete (vide Section X), 
or Seison on the little Crustacean Webalia (Fig. 457). Others, again, 
are internal parasites, such as Albertia in the celome of Earthworms 
and the intestines of fresh-water Oligochzetes (Mais), and Nolommata 
werneckit 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 larve of Annelids 
(phylum Annulata) is extremely close, and, in particular, the 
curlous Trochosphera 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 larve. 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.—GaAsTROTRICHA. 


The Gastrotricha (Figs. 272 and 273) are a small group of minute fresh-water 
animals, which ave 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 hears a 
nwnber 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 cesophagus. At the beginning of the latter are a number of 
small chitinous denticles, and in front of them a circlet of seta. The cwsophagus 
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 


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. 


Fic. 272.—Chaetonotus maximus. Fic. 273.—-Chaetonotus maximus (or- 
Highly magnified. (After Zelinka.) ganisation). brn. brain; gld. adhesive 
gland; mes. mesenteron; mo. mouth; 
es, cesophagus; ov. ovum; ovar. ovary ; 
retr, retractor muscles ; vent. mus, ventral 

muscle. (After Zelinka.) 


APPENDIX TO THE TROCHELMINTHES. 


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 


vir 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, and live on the European lobster and the Australian 
fresh-water cray fishes. 

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 


NUL nan LL a 
"| aera 


Fic. 274.—Dinophilus taeniatus. The left figure represents the dorsal surface of a young 
individual, x 76; the mouth and alimentary tract are seen by transparency. The right 
figure shows the anatomy of the male, x 38, an. anus; b, rectum; c. body-cavity ; d. vas 
deferens ; m. pharynx ; 7’. the first nephridium ; @. entrance to the wsophagus ; p., in left 
fig., prostomium; p., in right fig., penis; st. stomach; s. x. vesicule 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 cesophagus, 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 Zz 


338 ZOOLOGY SECT, 
tubes (v’). The inner ends of these do not open into the hody-cavity, but are 
provided with peculiarly modified flagellate cells known as so/cnocytes, so that 
these paired excretory tubes resemble closely the nephridia of some of the Poly- 
chieta (phylum Annulata ; see Section X.). The sexes are separate. In the male 
there is a conical ventral penis ; the last pair of nephridia act as vesicule 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 
(t!, 7, ) tipped with non-motile sen- 


A) 


—— 
\" 
8 


vas { : oy ES 
ZN 


Dp 2 ol sory cilia, and a pair of retractile 
ee 
70 _p appendages or limbs (J, a), with basal 


ac 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 (cl, ¢?, ¢3) tipped with non- 
motile cilia, and in the male a pair 
of retractile appendages or claspers 
(cl). At the end of the tail is a pair 
of large freely movable appendages 
or legs (lp), which are the organs of 
locomotion: at the end of each of 


q 
=! 
tf 


Fic. 275.—Stratiodrilus tasmanicus, 
male. ae. accessory gland of male ap- 
paratus; br. ¢ brain; ¢!. ¢2. ¢3. cirri; el. 
claspers (appendages peculiar to the male) ; 
ex, excretory tubes; gr. gld. granule-gland 3; 
1. a, anterior limb; J. gl. gland at base of 
anterior limb; l. gld. gland at base of pos- 
terior limb; Jl. p. posterior limb; 7. ce. 
nerve-cord; p. penis; ?¢1. ¢2. ¢3. tentacles ; 
ves, vesicula seminalis. 


but with the relative position of malleus and incus inverted. 


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, 
There is a 


highly developed nervous system consisting of a large brain (br. c.) situated 
dorsally in the prostomium, a pair of cesophageal connectives, and a ventral 
nerve cord (ac) with a series of ganglia which have a distinctly metameric 


vil PHYLUM TROCHELMINTHES 339 


arangement. 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 distinet : the male has a protrusible penis, directed ventrally. 
There is no 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 
Echinoderide, which were noticed in an appendix to the last Section (p. 319), 
have also to be kept in view. 


SECTION VIII 
° PHYLUM MOLLUSCOIDA ? 


THE phylum Molluscoida comprises three classes—the Polyzoa 
Gncluding, provisionally, the #ndoproctu), 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 ccelome, and 
overhanging the mouth on its anal side there is in most cases a 
sensitive process—the epistome—also -containing a special com- 
partment of the body-cavity. The central part of the nervous 
system consists of a single ganglion (supra-cesophageal), or of two 
ganglia (supra-cesophageal and infra-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- 
ines,” which in many cases bear a close general resemblance to 
lines, y 


1 This and all the remaining phyla of the animal kingdom are characterised 
by the possession of a true celome, 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 
celome is present is the Chetognatha. In some of the groups which are here 
comprised in the ccelomate phyla, however, as will be seen, the cclome is 
reduced, or entirely absent, or not typically developed. 

340 


SECT. VIII PHYLUM MOLLUSCOIDA 341 


Hydroid Zoophytes, and only on a more minute inspection are 
found to differ totally from the latter and to exhibit a very much 
higher type of structure. 


1. EXAMPLE OF THE CLASS.—BUGULA AVICULARIA. 


Bugula avicwlaria, the common Bird’s-Head Coralline (Fig. 276), 
occurs in brown or purple bushy tufts, two or three inches long, on 
rocks, piles of jetties, and similar situations on the sea-shore in all 
parts of the world. On a naked-eye examination it presents a 
considerable resemblance to a Hydroid Zoophyte, and might readily 
be taken for a member of that group. It consists of dichotomously 
branching narrow stems, which are rooted by a number of slender 
root-filaments. Each stem is found, when examined with a lens, to 
be made up of a number of elements, the zowcia 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 zoucium—on either side of which is a short blunt 
spine. A rounded structure—the owciwm—in many parts of the 
colony lies in front of each zocecium (Fig. 276, owe.). On each 
zocecium, except a few at the extremities of the branches, is a 
remarkable appendage, the aviculariwm (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! The anterior region of the body of the zooid forms an 
introvert, te. is capable of being involuted lke 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 (¢ent) 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 endocyst are commonly applied respectively to the 
hardened cuticle of the zooid and its soft body-wall, 


342 ZOOLOGY SECT. 


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


Fic, 276.—Bugula avicularia. Two zvoids, magnified. an. anus; avie. avicularia; emb. 
embryo enclosed in the ocecium ; ‘funic. funiculus ; gast, muscular bands passing from the 
stomach to the body-wall ; ‘at. intestine ; 20. mouth ; or. oceciumy; ws. cesophagus ; ov. ovary 5 
ph. pharynx ; ret. parieto-vaginal muscles; sp. spermatidia ; stom. stomach ; tent. tentacles. 
The ganglion, which is not indicated, lies just below the middle of the stroke from mo. 


each.is a narrow prolongation of the ceelome. In all probability, 
besides bringing minute particles of food to the mouth of the zooid 
by the action of their cila, the tentacles are prehensile as well as 


VIL PHYLUM MOLLUSCOIDA 34 


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 ceelome is extensive ; it is lined extervally 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 (/uwnic) passes from the 
proximal or aboral end of the alimentary canal to the aboral 
wall of the zocecium ; this is the funiculus. A transverse partition 
cuts off (though not completely) a smal] anterior compartment of 
the ccelome from the rest. ‘he 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 
ccelomic 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 asomewhat narrower short tube, separated by a constric- 
tion from the pharynx, leads to the stomach (stom) from which it is 
also separated by a constriction. The stomach gives off a long 
conical prolongation or cwcwm passing towards the aboral end of 
the zocecium, to which it is attached by the funiculus. The 
intestine (int) comes off from the oral aspect of the stomach, not 
far from the cesophagus, 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 clo 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 celome; it passes 
into the interior of a rounded outgrowth of the zocecium—the 
oecium (oec)—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 blastoccele 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 blastoccele; this mass apparently represents both 
endoderm and mesoderm. Small cavities which appear in it 
subsequently unite together to form the primitive cclome. 
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 dise (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 ocecium. After an interval of free existence as a 
ciated 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 


VUI 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 (te. 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 uwmbrella-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 


Fic. 277.—Early stages in the development of Bugula. cent. central mass of cells ; cor. corona 
ect. ectoderm ; end. endoderm ; sey. 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 
zoecium. From the latter, which becomes invaginated, or from 
a sac which is developed to replace it, are developed both 
the ectodermal and endodermal structures of the primary zooid. 


346 ZOOLOGY SECT 
Occupying the interior of the larva at this stage in addition to 


this sac, there is only a mass of undifferentiated tissue derived 
from the original central tissue together with that derived 


cor 


Fic. 278,—A, Larva of Bugula plumosa ; B, Sagittal section of larva of Bugula (diagram- 
matic). cent. central tissue; cor. corona; disc. retvactile disc ; e. ectodermal groove ; p. pyri- 
form organ ; pall. pallial groove; suck. sucker. (From Korschelt and Heider, after Barrois.) 


from the disintegrated corona, pyriform organ, and part of the 
sucker. The outer wall forms the wall of the primary zocecium, the 
surface of which becomes covered with a chitinous cuticle or 
ectocyst. Most of the mternal 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 
cesophagus; 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- 
MIG Pehed, Lote or Baeaia, ment of the alimentary canal has been 
c. cells of corona; r. rudiment separated off, forms a space termed the 
of the zooid in the form of a “ : 
sac; s, basal plate of everted atrium; the walls of this become con- 
Ronchit’ and’ Holder, “ates Vetted into the tentacle sheath, while 
Barrois.) 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 


Vu PHYLUM MOLLUSCOIDA B47 


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—the epistome. The 
alimentary canal is U-shaped, and the anus is anterior, within, or 
just outside, the tentacular circlet. Jn 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 #ndoprocta. 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 J].—GYMNOLAEMATA. 


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


Sub-order a.—Cyclostomata. 


Gymnolemata with tubular calcareous zocecia having circular 
apertures devoid of closing apparatus. 
Including Crista, Idmonea, ce. 


Sub-order b.—Cheilostomata. 


Gymnolemata with calcareous or chitinous zocecia usually pro- 
vided with opercula. 

Including Bugula, Flustra (“Sea-mat”) Membranipora, Cellepora, 
Selenaria. 


348 ZOOLOGY SECT. 


Sub-order e.—Ctenostomata. 


Gymnolemata with chitinous or gelatinous zocecia provided with 
a series of tooth-like processes closing the aperture when the 
tentacles are retracted. 

Including Aleyonidium, Serialaria, Paludicella, 


ORDER 2.—PHYLACTOLEMATA. 


Fresh-water Ectoprocta with horse-shoe-shaped lophophore and 


with an epistome. 
Including Cristatella, Plumatella, Predericella. 


Sub-Class II.—Endoprocta. 


Colonial or solitary Polyzoa multiplying by the formation of 
buds, which in Zoxosoma 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 Ecameple. 


Bugula avicularia is an example of the sub-order Cheilostomata 
of the Gymnolemata. It is a member of the family Bicellariide, 
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 


VIII 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 


Fic. 280.—Plumatella, Portion of a colony, magnified. funic. funiculus; gang. ganglion ; 
int. intestine ; mo. mouth; @. esophagus; repr. gonad ; retr, retractor muscle ; st. stomach ; 
stato. statoblasts. (After Allman.) 


strap-shaped branches arising from a common root. Sometimes 
there is a creeping cylindrical stolon, simple or branched, having 
the zooids arranged along it in a single or double row. The colony 
is free only in Cristatella (Fig. 281)—in which it performs creeping 
movements, in some other (American) forms of Phylactolemata 
(in the younger stages of the colony), in one family of the 
Cheilostomata—the Selenartide, (in which it moves along with 
the aid of certain peculiar appendages—the vbracula—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 Phylactolemata and 
the Cyclostomata among the Gymnolemata are devoid of anyspecial 
closing apparatus; while in the Cheilostomata there is a movable 


350 ZOOLOGY SECT. 


lid or operculum closed by a pair of oerlusor muscles when the 
introvert 1s retracted ; and in the Clenostomata there is a series of 
lobes or teeth which close in together over the opening. The 
cavities of the neighbouring zoecia are in some forms completely 
cut off from one another by a continuation of the chitinous -or 
calcareous exoskeleton ; in others there 1s 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 


Fic, 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 mtrovert 
is the mouth surrounded by a lophophore bearing tentacles. The 
tentacles are always simple, filiform, and hollow, each containing a 
narrow diverticulum of the civewlar canal or anterior compartment 
of the celome. 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 


Vil PHYLUM MOLLUSCOIDA 351 


used for prehension. In the Phylactolemata (Fig. 280) the 
lophophore is horse-shoe-shaped, in the Gymnolamata (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 or 
the introvert is effected by a pair of bands of muscular fibres, the 
parieto-vaginal muscles, passing to it from the body-wall, and 
by a pair of retractor muscles passing from the latter to the ali- 
mentary canal. 

Structure of body-wall.—Beneath the cuticle is an epi- 
dermis, consisting of a single layer of flattened polygonal cells, 
firmly united together by their edges. Beneath this there 
is usually, but not always, a layer of muscle, which is 
arranged in two strata—an t 
external composed of circular, a 
and an internal of longitu- 
dinal fibres. There is an ex- 
tensive coelome lined in some 
forms (Phylactolemata) by a 
definite coelomic epithelium, 
in part ciliated; while in 
others there is no such de- 
finite epithelium, but its place 
is taken by thin parietal and 
visceral layers of an irregular eA 
cellular tissue—the paren- 
chyma. Crossing the ccelome 
are strands, in some instances 
very numerous, of spindle- 
shaped cells. In some cases 


Fig. 282.—Anterior portion of the body of 


two mesenteric bands sus- Lophopus, from the right side. an. anus; 
= ep. epistome ; ga. ganglion ; 0. mouth; pr. in- 
pend the alimentary canal— testine; sf. cesophagus; ¢. tentacles, cut off 
a h d 7 h near the base. (From Lang's Comparative 

an anterior attached near the ‘Anatomy. After Allman.) 


mouth and a posterior passing 

from the cecum to the aboral end of the zocecium; in most 
cases the latter, to which the special name of fwniculus 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 cesophagus 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 cesophagus 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 


352 ZOOLOGY SECT. 


sense, unless the epistome of the Phylactolemata be of that 
nature. 

Nephridia are not known with certainty to exist in any of 
the Ectoprocta. In some there is a pore through which water 
enters the body-cavity, or a ciliated intertentacular tube opening 
at the base of the tentacles. Excretion appears to be performed 
by certain cells of the funicular tissue and of the parenchyma 
or ccelomic epithelium. These become loaded with the products 
of excretion, and are set free as leucocytes in the ccelome, whence 
they may pass out through the intertentacular tube or may 
accumulate in the cells of the brown body. 

In many Ectoprocta the colony bears a series of remarkable 
appendages—the avicularia—which are of the nature of modified 
zooids. In typical cases the avicularium has the bird’s-head-like 
form that has been already described in the case of Bugula; 
sometimes it is completely sessile. A second set of movable 
appendages found in some forms are the vibracula; these 
are long tapering whip-like appendages which execute to-and-fro 
movements. The avicularia are frequently found to have seized 
in their jaws minute Worms or Crustaceans, and it is probable 
that their function, as well as that of the vibracula, is defensive ; 
in the case of the Selenartide, 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, 
o«c.), and in many of the Gymnolemata (Cheilostomata) these 
ovicells or owcia, as they are termed, take on a very definite 
shape. 

Reproduction and Development.—As a general rule the 
Ectoprocta are hermaphrodite. Both ovary and testis are derived 
from the layer lining the ccelome (parenchyma or ccelomic 
epithelium as the case may be), or from the funicular tissue. The 
testis may be single or double. The spermatidia,as in Bugula, or 
the mature sperms, become free in the ccelome. The ovary is very 
generally situated towards the oral end or about the middle, the 
testis towards the base. The mature ova escape into the celome, 
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 ccelome or in a special diverticulum of 
it; in the Cheilostomata the fertilised ova pass into the ovicells ; 
in some cases, both among the Phylactolemata and the 
Gymnolemata, they are received into a sheath formed by the 
tentacles of an imperfectly-developed zooid formed in a zocecium 
in which the original zooid had undergone degeneration. _ 

In those cases in which the early stages of development are 
passed through in the body-cavity of the parent, the ciliated 
embryos may either escape through the zocecial aperture after the 


VIII PHYLUM MOLLUSCOIDA 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 occia 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 dose 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. 844), 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 
Phylactolemata 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 Phylactolemata. 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- 
lermata, 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 AA 


354 ZOOLOGY SECT. 


represented in the Cambrian and later Palsozoic 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, Urnatella, 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 epistomve 
overhangs the mouth. The ccelome is almost or quite obliterated; 
the space between the alimentary canal and the wall of the body 
being filled, more or less completely, with a gelatinous hyaline 
matrix. A pair of nephridia are present. In Loxosoma they lie 
one on each side of the cesophagus 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 
ovarves 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 


vit PHYLUM MOLLUSCOIDA 355 


become detached before reaching maturity. Segmentation of the 
ovum is complete, and a gastrula is formed by invagination. 
Certain ditferences in the larval history have sometimes been 
regarded as separating very widely the Endoprocta from the 
Ketoprocta. 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 


Fic. 283.—Pedicellina. Showing successive stages (numbered 1 to 6) in the development of 
zovids 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, 


AA2 


356 ZOOLOGY SECT. 


to the Phylactolamata, 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 Moiluscoida. 

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 xe- 
phridial tubes (neph) of mesodermal origin, 
which open internally each by two apertures 
into the posterior chamber of the ccelome. 

The celome, which is lined with a coelomic 
epithelium, consists of three main parts of 
very unequal extent. The first (prosocwle) is 
a narrow cavity in the epistome. The second 
(mesocele), which is in communication with the 
Me tealin sae first, lies in front of a transverse septum or 

size, mesentery extending between the mouth and 

anus, and perforated by the cesophagus 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 (metacele), 
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 alunentary 


VIII PHYLUM MOLLUSCOIDA 357 
canal; and each of these is further subdivided by a longitudinal 
mesentery extending from the body-wall to the cesophagus (@) 
in the one compartment 
(usually termed the right), 
and to the rectum (7) in 
the other (left). The alc- 
mentary canal is bent on 
itself to form a loop, as in 
the Polyzoa: it is distinguish- 
able into esophageal, gastric 
and intestinal regions. There 
is a closed system of blood- 
vessels with contractile walls 
containing red blood-cor- 
puscles. The nerrous 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- 


Fic. 


285.—Phoronis australis, 
magnified. 
nephr, nephridial aperture ; neph, nephridium ; 
(After Benham.) 


free end, 
an, anus 3 ep. epistome ; mo. mouth ; 


tacles. There are no organs of 
special sense. 
Phoronis is hermaphrodite. 


Ova and sperms are developed 
in the ccelome 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 
ceelome—), and they go through 
the early stages of development 
fixed to the tentacles. The 


Fic. 286.—Phoronis australis, internal 
organisation. ay. bl. afferent blood vessel ; 


an. anus ; ef. bl. efferent blood vessel ; ep. 
epistome ; mes. mesentery; mo. mouth ; 
wyphe p. nepbridiopore; nephr. d. duct 
of nephridium; aephrost. nephrostome 
(internal opening of nephridium ; es. ceso- 
phagus ; rect. rectum; rect. mes. rectal 
mesentery ; sept. septum; tent. tentacles 
(cut short). (After Benham.) 


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 


358 ZOOLOGY SECT. 


drawn out into a slit which eventually becomes closed up behind, 
the anterior portion alone remaining open to form the mouth. 
The anus is developed later as an invagination in the position 
of the posterior part of the former blastopore. The mesoderm 
arises from cells budded off from the endoderm. The prosoccele 
and mesoceele 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 
ET 


ss ms 1 RA Bek 


ff 
/ 
ong 


Fic. 287.—Phoronis, transverse section towards the anterior end. af. v. afferent blood-vessel ; 
ce. m. circular layer of muscular fibres; ef. v. efferent blood-vessel; ep. epidermis; c. m. cir- 
cular layer of muscle; m,m. mesenteries; ve. f. funnel-like opening of nephridium; @. 
cesophagus ; 7. 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 


Vu 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 cesophagus 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. 


Fic. 288.—Phoronis, 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, ww), 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 (£), the 
projection thus formed, which grows out at right angles with the 


BOO ZOOLOGY SECT. 


long axis of the larva, becoming the body of the future animal ; 
the larval tentacles and pre-oral lobe become thrown off, and the 
lophophore is developed. 


CLASS III BRACHIOPODA. 


The Brachiopoda are the fabricators of the well-known “ Lamp- 
shells” found in most parts of the world. They occur in the sea 
at various depths, and were formerly classed under the Mollusca, 
their characteristic bivalved shell being compared with that of 
oysters, mussels, &. 


1. EXAMPLE OF THE CLASS—-Iagellania (Waldheimia) lenticularis 
or M. flavescens. 


Magellania lenticularis is found in great numbers, at moderate 
depths, off the coast of New Zealand. An allied species, If, flavescens, 
is equally common in the Australian seas, and several other species 
are known in various parts of the world. 

The body is entirely covered by a shell (Fig. 289) of oval form 
and pink colour, composed of two pieces or valves, one of which, dis- 
tinguished as the ventral valve (v. v), projects beyond the other 
or dorsal valve (d. v), in the form of a short conical beak (6) perfor- 
ated at the end by an aperture, the foramen (b), through which 
passes a dark brown stalk or peduncle (Fig. 290, B, pd) of horny 
consistency. In the natural state the peduncle is attached to a 
rock or other support, and the animal hes 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 (0), 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 deltidiwm (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-tecth (¢). On the inner surface of the 
valve, towards its posterior end, are certain shallow depressions 
inarking 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 (¢. p) with a curiously folded surface: when the two valves 
are in position this process fits between the hinge-teeth of the 


VI 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. 1), which projects 


Fic. 289.—Magellania flavescens, A, the entire shell 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; 6. beak ; ¢. p. cardinal process; ¢. deltidium; d. m. divaricator impressions; d. v. 
dorsal valve; 7. foramen; p. m. protractor impressions; s. tooth-socket; s. 1. 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 celome. The two 
flaps thus formed are the dorsal (d. m) and ventral (v. m) mantle- 
lobes. They are fringed with minute setze (s) lodged in muscular 
sacs, like those of Cheetopods (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, lph), 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 (lph’) 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 frmged throughout its whole extent with 
long ciliated tentacles which form the outer boundary of a ciliated 
food-grovve, bounded on the inner side by a wavy ridge or lip 
(p, 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 gudleé 
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 


Vu PHYLUM MOLLUSCOIDA 363 


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


Fic, 200.—A, body of Magellania lenticularis, removed from shell; B, sagittal section of 
the entire animal. Both semi-diagrammatic, the lophophore being represented as of smaller 
proportional size than in the actual animal (ef. Fig. 291). d. gl. digestive gland ; d. im. dorsal 
mantle-lobe ; d. v. dorsal valve of shell; gon!, gon. gonads ; ht. heart ; int. intestine ; lp, lpl. 
lip; lphk. lophophore; lphl. its coiled process; mth. mouth; npk. in B, nephridium, in A, 
nephridial aperture ; pd. peduncle ; pl. si. pallial sinuses ; s. setae; 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 
ccelomic epithelium lining the body-cavity. On the outer surfaces 
of the mantle-lobes, where they are in contact with the shell, 
the epidermis is replaced by 
a thin membrane showing 
no cell-structure. 

The muscular system 
(Fig. 292) is well developed. 
Two large adductor muscles 
(ad. m) arise on each side 
from the dorsal valve, and 
passing downwards, unite 
with one another so as to 
have a single insertion on 
the ventral valve: their 
action is to approximate the 
valves and so to close the 
Fic. 291.—Magellania flavescens, the ventral shell. A large and a small 

valve removed. c. p. cardinal process ; lph. arm A : 5 

of lophophore; Iphl, its coiled process, with pair of divaricators (d. m, dm’) 

aoe Ee eitter Daviiion) no Tent side; mil. 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 


Fic. 292.—Muscular system of Magellania. ad. m. adductors; b. beak; d. aj. m. dorsal 
adjustors; d.m., d. m’. divaricators; d. v. dorsal valve; int. intestine ; mth. mouth; pd. 
peduncle; pd. sh, sheath of peduncle; ». m. protractor ; s.7. shelly loop; v. aj. m. ventral 
adjustors ; v. v, ventral valve. (After Hancock.) 


inserted into the peduncle, are called adjustors (a.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. 


vul PHYLUM MOLLUSCOIDA 365 

The ceelome 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 ceelome is continued 
into each of the mantle-lobes in the form of four canals or pallial 
sinuses (Fig, 290, pl. sz), the two outer of which are extensively 
branched. 

Blood-System.—Attached to the posterior region of the 
stomach is a small, almost globular sac (1), 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 


Fic. 293,-—-Anterior body-wall of Terebratula, to show nervous system, &c. dim. dorsal mesen- 
tery ; g. brain; gf. genital folds; 2. nephridinm ; nt. nephrostome ; es. gullet ; ov,ovary ; se. 
cesophageal connective; vsg. infra-cesophageal ganglion ; vm. ventral mesentery; dmn, hn, 
ian, san, nerves. (From Lang’s Comparative Anatomy, after van Bemmelen.) 


of the whole circulatory system and the course of the circulation 
are very imperfectly known. 

The excretory organs consist of a pair of very large nephridia 
(nph) lying one on each side of the intestine. Each is funnel- 
shaped, having a wide inner opening or nephrostome, with plaited 
walls, opening into the ceelome, and a narrow, curved, outer portion 
which opens into the mantle-cavity not far from the mouth. As 
in many cases which have already come under our notice, the 
nephridia act also as gonoducts. 

The nervous system (Fig. 293) is a ring round the gullet pre- 
senting supra- (g) and infra- (usg) oesophageal swellings or ganglia, 
of which the infra-cesophageal is the larger. Nerves are given off 


366 ZOOLOGY SECT. 


to the mantle, lophophore, &c. No special sense-organs are 
known. 

Reproductive Organs.—'The sexes are separate. There are 
two pairs of gonads (Fig. 290, gon), one dorsal and one ventral, in 
the form of irregular organs sending off branches into the pallial 
sinuses, 


2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Brachiopoda are Molluscoida in which the body is enclosed 
in a shell formed of two parts or valves which are respectively 
dorsal and ventral in position. The body occupies only a small 
portion of the space enclosed by the shell, and is usually attached 
to foreign objects by a posteriorly placed stalk or peduncle: it 
gives off dorsal and ventral reduplications, the mantle-lobes, which 
line the valves of the shell and enclose a large mantle-cavity. 
From the anterior surface of the body is given off a lophophore 
which surrounds the mouth, and is beset with ciliated tentacles. 
There is a ridge-like pre-oral lip which is continued on to the 
lophophore. The enteric canal is usually V-shaped, and is 
divisible into gullet, stomach, and intestine: there is a pair 
of digestive glands. The ccelome is spacious, and is continued 
into the mantle-lobes. A heart is usually present, attached to 
the stomach. The excretory organs are one or two pairs of 
nephridia which act also as gonoducts. The nervous system is a 
ganglionated circum-cesophageal ring: sense-organs are usually 
absent in the adult. The sexes are separate or united. Develop- 
ment is accompanied by a metamorphosis. 

The class is divided into two orders :— 


OrvDER 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, Discina, &e. 


ORDER 2.—ARTICULATA. 


Brachiopoda in which the shell is formed of oblique prisms or 
spicules of calcium carbonate: the two valves unite by a definite 
hinge, and there is usually a shelly loop, for the support of the 
lophophore, developed in connection with the dorsal valve. The 
intestine ends blindly. 

Including Magellania, Terebratula, Rhynchonella,  Cistella 
(Argiope), &e. 


Vil PHYLUM MOLLUSCOIDA 367 


Systematic position of the Evrample. 


The genus Magellania, of which there are several species, 
belongs to the family Terebratulide, 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 Terebratulide 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 hasa foramen partly bounded bya 
deltidium of one or two pieces: there is a shelly loop springing 
from the hinge-line of the dorsal valve. The genus Magellania is 
characterised by having the shelly loop fully half as long as the 
shell itself, and by the presence of a median septum on the inner 
face of the dorsal valve. 

The specific differences between M. lenticularis and M. flavescens 
are largely matters of detail, depending upon the precise form of 
the shell and loop. More obvious differences are seen in the shell, 
which is pink, evenly-rounded, and short-beaked in M. lenticularis, 
while in M. flavescens it is horn-coloured, almost pentagonal, and 
has a prominent beak. 


3. GENERAL ORGANISATION. 


The shell presents two distinct types: in the Articulata, the 
order to which Magellania belongs, the dorsal and ventral valves 
are dissimilar, the dorsal valve having a cardinal process and usually 
a shelly loop, the ventral a spout-like beak for the peduncle ; while 
in the Inarticulata, of which Lingula is a good example (Fig. 294,A), 
the two valves are nearly alike, and there is no shelly loop and no 
beak. These differences are accompanied by differences in micro- 
scopic structure; in the Articulata the shell is dense and stony, 
and is formed of obliquely placed calcareous prisms, while in 
the Inarticulata it has no prismatic structure, but usually con- 
sists of a chitinoid material more or less strengthened by calcareous 
spicules. Among the Articulata the loop may be absent; when 
present, it varies greatly in form and size, being sometimes very 
small and simple (Fig. 294, C, D), sometimes bent upon itself, as 
in Magellania, sometimes attached to the septum or to the interior 
of the dorsal valve (E), sometimes, as in the extinct Spirifera, 
represented by a complex double spiral (F), sometimes reduced to 
short, paired rods springing from the septum (G). 

The majority of both orders are attached by a longer or shorter 
peduncle which passes between the proximal ends of the valves in 
Lingula (Fig. 294, A), through a perforation in the ventral valve in 
Discina (C), and through a foramen in the spout-like posterior end 


368 ZOOLOGY SECT. 


of the ventral valve in the Articulata. Crania (B) has the ventral 
valve fixed directly to foreign objects, the peduncle being absent. 

The lophophore is found in its simplest form in Cistella 
(Fig. 295, A), in which it is a horse-shoe-shaped disc with very 
short arms, attached to the dorsal mantle-lobe and surrounded 
with flexible tentacles which project between the valves. From 
this the lophophore of Magellania, which may be considered as 
typical for the Articulata, is easily derived by an increase in size, 
and by the prolongation of the middle region of the concave edge 
into a coiled offshoot. In the Inarticulata (C), and in Rhyn- 
chonella (B) among the Articulata, each arm of the horse-shoe is 


Fic, 294.—Typical Brachivpoda, A, Lingula; B, Crania; C, Discina ; D, Terebratula ; 
E, Cistella ; , 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 very complex muscular 
system by means of which the valves can be rubbed upon one 
another, or moved laterally as well as opened and shut. 


Vil PHYLUM MOLLUSCOIDA 369 


In the Articulata the enteric canal is V-shaped, as in Magel- 
lania, the intestine being straight or nearly so, and ending blindly. 
In the Inarticulata, on the other hand, the intestine is usually 
coiled, and always ends in an anus (Fig. 295, C, a), which generally 
opens into the mantle-cavity, but in one genus (Crania) into a 
pouch or sinus at the posterior end of the body between the 
valves. 

A heart is usually present, but the function of blood is per- 
formed mainly by the ccelomic fluid, which is propelled by the 
cilia lining that cavity, and circulate both in the ccelome itself and 


AOE 
eS 
TTI 


SO 
sipre’s 
Ue 
Mdina 


lph 


Fic. 295,—Dissections of A, Cistella ; B, Rhynchonella; and C, Lingula. u«. anus; /ph. 
lophophore ; mth. mouth. (After Schulgin and Hancock,) 


in the pallial sinuses, each sinus presenting—in Lingula at least 
—both an outgoing and an ingoing current. 

A single pair of nephridia, resembling those of Magellania, 
occurs in all known genera except Rhynchonella, in which there are 
two pairs, one dorsal and one ventral. Besides discharging an 
excretory function they act as gonoducts. 

The nervous system always takes the form of a circum-ceso- 
phageal ring with ganglionic enlargements, the largest of which 
is ventral or sub-cesophageal in position. Otocysts have been 
described in Lingula, rudimentary eyes in Megerlia, and patches 
of sensory epithelium in Cistella: with these exceptions sensory 
organs are unknown. 

There are usually four gonads, two dorsal and two ventral, 


VOL. I BB 


370 


ZOOLOGY SECT. 


sending prolongations into the pallial sinuses. Some genera are 
dicecious, others hermaphrodite, the epithelium of the gonads 
producing, in the latter case, both ova and sperms. ate 

The development of the Brachiopoda is best known in Cistella, 
in which the first stages of development are passed through 


Fic. 


296.—Two stages in the 
development ‘of Cistella 
(Argiope). b. provisional setze ; 
bl. Pplastopore ; me. mesen- 
teron; pv. coelomic pouches. 
(From Balfour’s Embryology, 
after Kowalevsky.) 


in a pair of cavities, the brood-pouches, 
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 celomic pouches (p.v), grow out 
from the archenteron, and the blastopore 
closes. The coelomic sacs separate from 
the mesenteron (B, me) or middle portion 
of the archenteron, and extend between 
it and the ectoderm, forming the right 
and left divisions of the coelome: their 
outer walls thus become the somatic, 
their inner walls the splanchnic layer 
of mesoderm. The mesenteron remains 
closed and surrounded by the 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 body-region, and 
the peduncular region (Fig. 297). 

Next the head-region grows out into an 
umbrella-like disc surrounded with cilia and 
bearing four eye-spots (Fig. 298, A), and on 
the body-region a backwardly-directed an- 
nular fold (m) appears, bearing four groups 
of provisional sete. In Cistella, which has 
no sete in the adult condition, the pro- 
visional setze are subsequently lost, and are 


Fic. 297.—Young larva of 
Cistella, with the 
three segments, two 
eye-spots, and two 
bundles of setee (From 
the Cambridge Natural 
History, after Kowal- 
evsky.) 


not replaced. In forms which possess sete in the adult condition 
the provisional sete are likewise lost, but are replaced by the per- 
manent set. Soon this mantle-fold divides into dorsal and ventral 
lobes, which, being directed backwards, cover the peduncular region. 


VIII PHYLUM MOLLUSCOIDA 371 


In this condition the larva swims freely like a trochophore. 
After a time it comes to rest and fixes itself by its peduncular seg- 
ment (B). The two lobes of the mantle-fold (m) become refexed 
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 stomodeum 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, lph). 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 


: 7] 3 Fic. 298.—Two later stages in the 
the present day the class includes eee gre ees 
only about 20 genera and 100 free-swimming ; B, after fixation. 

7 b . . ‘ h hs. peduncular region ; m. mantle ; 

species, ut in past times the case ese Gk tar are lag 

: 5 = wk. ciliated ring ; vs. head-region. 

was very different. Brachiopods ap (From Lang’s Comparative Ana- 
pear first in the lower Cambrian tomy, after Kowalevsky.) 


rocks, where the existing genera 
Lingula and Discina are found. No more striking examples can 
be adduced of persistent types—organisms which have existed 
almost unchanged for the vast period during which the whole of 
the fossiliferous rocks have been in process of formation. Alto- 
gether 106 genera are known from the Paleozoic 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 


BB 2 


72 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. 


MutTuaL RELATIONSHIPS OF THE 
CLASSES OF THE MOLLUSCOIDA. 


In adult structure Phoronis ex- 
hibits marked resemblances to the 
Ectoprocta, more especially to the 
Fro, 299.-Lophophore of embryo Phylactolemata—resemblances which 

of Terebratulina. . y!. di- will be rendered clear by a comparison 

gestive gland; int. intestine; s é : 

lp. Vip ; Tpke. lophophore ; mth. of the diagrams A and B in Fig. 300. 

Heller, alter Morse) “"" «In. both, the ventral side of the body 

Heider, after Morse.) ? 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 
ccelome 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 Phylactolemata, and a lobe—the arm- 
fold or lip (4v)—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 


VIE PHYLUM MOLLUSCOIDA 373 


evidently homologous with the foot-segment of the larval Brachio- 
pod. The end of the body of the Brachiopod from which the 
peduncle proceeds is thus the ventral portion. From the position 
of the epistome and lophophore, it follows that the dorsal valve 
of the Brachiopod, being on the same side of the mouth as the 
epistome, lies on the side of the body corresponding with the anal 
side of the Polyzoan, though the intestine is bent round in the 


Fic. 300.—A, Diagrammatic median section uf a phylactolematous Polyzoan. «mn. anus; 
ep. epistome ; ep. cav. epistome-cavity ; funic. funiculus; gang. ganglion ; int, intestine ; 
mo. mouth ; reph. nephridium ; os. esophagus ; sf. stomach ; tent. tentacles. B, diagrani- 
matic median section of iPhoronis. mcs. mesentery; nr. nerve-ring. Other letters asin A. 
(From Korschelt and Heider, after Cori.) 


opposite direction and directed towards the ventral valve. The 
supra-cesophageal ganglion of the Brachiopod represents the single 
ganglion of the Polyzoa, though it is subordinate in importance to 
the infra-cesophageal 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 sete of Brachiopods, sunk in muscular sacs, are marks of 
annulate affinities, since such organs are found elsewhere only 
among Cheetopoda and Gephyrea (Sect. X.). The form of the 
larva tells in the same direction, the eye-bearing head region 
or prostomium and the provisional sete being very striking charac- 
ters. But the segmentation of the Brachiopod is quite different 
from that of the annulate larva, in which new segments are always 
added behind those previously formed, and in which metamerism 
always affects the mesoderm. 


SECTION IX 
PHYLUM ECHINODERMATA 


THE phylum Echinodermata comprises the Starfishes (Asteroidea), 
Sea-urchins (Hchinoidea), Brittle-stars (Ophiuroidea), Feather-stars 
(Crinoidea), and Sea-cucumbers (Holothuroidea). All exhibit a 
radial arrangement of parts, which is recognisable as well in the 
globular Sea-urchins and elongated Sea-cucumbers as in the star- 
shaped Starfishes, Brittle-stars and Feather-stars. Another uni- 
versal feature is the presence of a calcareous exoskeleton, sometimes 
in the form of definitely shaped plates, which may fit together by 
their edges so as to form a continuous shell; sometimes merely in 
the form of scattered particles or spicules. In very many the 
surface is beset with tubercles or spines, from which feature the 
name of the phylum is derived. The various systems of organs 
attain a comparatively high degree of complexity. An extensive 
ccelome is present, developed in the embryo from hollow outgrowths 
from the archenteron. The Echinoderms are rarely capable of 
rapid locomotion, and are sometimes permanently fixed by means 
of a stalk; they never give rise to colonies by budding. Without 
a single exception, all the members of this phylum are inhabitants 
of the sea. 


1. EXAMPLE OF THE ASTEROIDEA. 
A Starfish (Astertas rubens or Anthenea flavescens). 


General External Features of Asterias rubens.—The 
body of the Starfish is enclosed in a tough, hard integument, 
containing numerous plates, or ossicles as they are termed, of 
calcareous material. This exoskeleton is not completely rigid in 
the fresh condition, but presents a certain limited degree of flexi- 
bility. The body (Fig. 301) is star-shaped, consisting of a central 
part, the central disc, and five symmetrically arranged processes, 


the arms or rays, which, broad at the base, taper slightly towards 
375 


376 ZOOLOGY SECT. 


their outer extremitics. There are two surfaces—one, the aboral 
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 actinostome, and running out from this in a radiating 
manner are five narrow grooves, the ambulacral grooves, each extend- 
ing along the middle of the oral surface of one of the arms to its 
extremity. Bordering each of the ambulacral grooves there are 
either two or three rows of movable calcareous spines, the 
ambulacral spines. At 
the central ends of the 
grooves the ambulacral 
spines of contiguous 
sides of adjacent grooves 
form five groups, the 
mouth palle, 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 
vboral surface. 

On the convex aboral 
surface there are a 
by number of short stout 
Pre 301.— Starfish (Asterias rubens), General view of the spines arranged in ir- 

oral or actinal surface, showing the tube-feet. (From 

Leuckart and Nitsche’s Diagrams.) regular TOWS 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 branchia 
or papule (Fig. 305, Resp. cw), which is capable of being entirely 
retracted. 

Very nearly, though not quite, in the centre of the aboral sur- 
face is an aperture, the anus (Fig 310), wide enough to admit 
of the passage of a moderately stout pin. On the same surface, 
midway between the bases of two of the rays, is a flat, nearly 
circular plate, the surface of which is marked by a number of 
radiating, narrow, straight, or slightly wavy grooves; this is the 
madreporite (mad.). The presence of this structure interferes to some 


Ix PHYLUM ECHINODERMATA 377 


extent with the radial symmetry of the Starfish, two of the anti- 
meres (p. 42), viz. those between which the madreporite is placed, 
being different from the rest. There thus arises a bilateral sym- 
metry, there being one vertical plane, and only one—that passing 
through the middle of the madreporite and through the middle of 
the opposite arm—along which it is possible to divide the Starfish 
into two equal—right and left—portions.! The two rays between 
which the madreporite lies are termed the biviwm, the three 
remaining the triviwm. 

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 jpedicellarie (Fig. 305, Ped). 
Each of these is supported on a longer or shorter flexible stalk, 
and consists of three calcareous pieces—a basilar piece at the 
extremity of the stalk, and two jaws, which are movably articu- 
lated with the basilar piece, and are capable of being moved by 
certain sets of muscular fibres so as to open and close on one 
another like the jaws of a bird. In some of the pedicellariz 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 (7.¢. in the 
direction in which the animal is moving), their extremities be- 
coming fixed by the suckers, and then the whole tube-foot con- 
tracting so as to draw the body forwards; the hold of the sucker 
then becomes relaxed, the tube-foot is stretched forwards again, 
and so on. The action of all the tube-feet, extending and con- 
tracting in this way, results in the steady progress of the Starfish 
over the surface. With the aid of the tube-feet the Starfish is 
also able to right itself if it is turned over on its back. 

At the extremity of each of the ambulacral grooves is to be 
distinguished a small bright red speck, the eye (Fig. 305, A, oc), 
over which is a median process, the tentacle (¢), similar to the tube- 
feet, but smaller and without the terminal sucker. The tentacles 
have been ascertained by experiment to be olfactory organs, the 
Starfish being guided to its food much more by this means than 
by the sense of sight. 


1 The slightly eccentric position of the anal aperture introduces a correspond- 
ingly slight inequality between the right and left portions. 


378 ZOOLOGY SECT. 


Transverse Section of an Arm.—TIf 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 calome or body-cavity, and below, 
between the two limbs of the V, is the ambulacral groove. The 
aboral arch is supported by a number of irregular ossicles and is 
perforated by the numerous small dermal pores, through which the 
dermal branchiz 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, Amb. os), 
which meet together at 
the apex or summit of 
the groove like the 
rafters supporting the 
roof of a house, but 
with a movable articu- 
lation allowing of separa- 
tion or approximation ot 
the two rows so as to 


Fic. 302.—Starfish. Vertical section through an arm. open or close the groove. 


amp. ampulle ; ep. epidermis; rad. amb. radial vessel 
of the ambulacral system; rad.bl.v. points to the At the end of the Tay’ 


septum dividing the perihemal vessel into two parts ; the ambulacral ossicles 
rad. ne. radial nerve of the epidermal system; sp. : * 
spaces in mesoderm of body-wall; t. 7 tube-feet. end in a inedian ter- 


(From Leuckart, after Hamann.) 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 pures, 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 (7. #.): 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 le on the other side of the ambu- 
lacral ossicles, i.e. in the cavity of the arm. These—the ampulle 


IX PHYLUM ECHINODERMATA 379 
(Figs. 302 and 305, amp.; Fig. 303, ap)—are arranged like the tube- 
feet, in a double row on each side, a higher row and a lower, 
there being one opposite each ambulacral pore. When one of 
them is squeezed, the corresponding tube-foot is distended and 
protruded, the cavities of the tube-foot and ampulla being in 
communication by means of a narrow canal running through the 
ambulacral pore and provided with a valve. It is in this way 
that the foot is protruded in the living animal: the corresponding 
ampulla being contracted by the contraction of the muscular 
fibres in its walls, the contained fluid is injected into the tube- 
foot and causes its protrusion, 
the return of the water back- SY 
wards through the canal being 
prevented by the closing of 
the valve. fe hn 
Vascular and Nervous 
System.—Running along the 
ambulacral groove, immedi- 
ately below where the ambu- 
lacral ossicles of opposite sides ap 
articulate, is a fine tube, the 
radial ambulacral vessel (Fig. 
302, rad. amb, Fig. 303, 7), 
which appears in the trans- 
verse section as a small rounded 
aperture. From this short side- 
branches (Fig. 303, 7’) pass out 
on either side to open into the 
bases of the tube-feet. Below 
the radial ambulacral vessel 


eit 


is a median thickening of the 
integument covering the am- 
bulacral groove: this marks 


Fic. 303.—Ambulacral system of a Starfish. 
a. ampulle; ap. Polian vesicles; c. circular 
canal; m. madreporite ;  m’. madreporic 


canal; ¢. tube-feet; p. radial vessels; 7’, 


the position of the radial nerve branches to ampulle. (After Gegenbaur.) 


(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 


380 ZOOLOGY SECT. 


(d. nerv.) also of a nervous character. These more deeply placed 
nerve-bands are the radial parts of the deep nervous system: like 
the epidermal, the deep nervous system has a central part in the 
form of a pentagon, which in this case is double, surrounding the 
mouth. A third set of nerve elements (the calomic 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. bi, v), extending, like the other parts that have been 
mentioned, throughout the length of the arm, forms part of a 
system of channels, the perihemal system, which have been regarded 
as constituting a blood-vascular system. This radial perthemal 
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 


i rod nerv cS 
, oe p- 384. 
Fic. 304.—Starfish. Lower part of a vertical een bs 
section through the arm, to show the structure In the septum dividing 


of the radial nerve and the position of the the radial perihemal sinus 
deep nervous system ‘and radial perihemal 


vessels. d. nerv. strazid of deep nervous system ; into two runs a strand of a 
radial nerve; sept. septim of radial pert. Kind of gelatinous connective 
peal vane pf radial nowneestrandeltNe tissue containing many leuco- 
(After Cuénot.) cytes and perforated by ir- 
regular channels or lacune: 
this is the radial strand of the /acwnar or hemal system. Like the 
radial vessels of the perihemal 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 ampulle. 
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. 


IX PHYLUM ECHINODERMATA 381 


Body-wall and Celome.—The entire outer surface is covered 
with a layer of ciliated epithelium, the epidermis or deric epi- 
thelium (Fig. 305, Der. Epithm), which is continued over the 
various appendages and processes—the tubercles and spines, the 
pedicellarie, the dermal branchiz, 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 (08) 


Fic. 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 (7. /.) and ampulle (4.p.) are shown in perspective. B, trausverse section through 
anarm. 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 
ecelomic epithelium represented by a beaded line. Amb. 0s. ambulacral ossicles ; Amp. am- 
pulle ; An.anus; C. Amb. V. circular ambulacral vessel: C. B. V. septum of ring perihzemal ; 
vessel ; Cd. ewe. cardiac ceca; (il. celome; Cel. Epithm. ccelomic epithelium ; Der. Epithm. 
deric epithelium ; Dern.mesoderm ; Ent. Epthi. enteric epithelium : Jnt. ce. intestinal czeca. 
Mdpr. madreporite ; Mes. mesentery; Mth. mouth; Nv. R. nerve-ring ; oc. eye ; 0s. ossicles of 
body-wall; Ovd. oviduct ; Ped. pedicellarie ; ph. periheemal spaces; Pyl. cee. pyloric czeca ; 
Rad, amb, v. radial amlulacral vessel; Rad. B. V. points to septum in the radial periheemal 
vessel; Rad. Nv. radial nerve; Resp. ce. dermal branchie ; St. stomach ; S¢. c. stone-canal ; 
t. tentacle; 7. 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 osstcle 
consists of a close network of calcareous rods. Between contiguous 
ossicles extend bands of muscular fibres. 

The interior of the cwlome (Cel.) or body-cavity is lined by a 
ciliated epithelium, the celomic epithelium (Cal. Hpithm.), 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, ampulle, etc. In addition 


382 ZOOLOGY SECT, 


to this visceral layer of the peritoneum, the wall of the ali- 
mentary canal and its ceeca consists of a muscular layer and an 
internal lining, the enteric epithelium or endoderm (Knt. Epthm). 
The coelome is filled with a fluid, the celomie fluid, consisting 
mainly of sea-water, but containing a number of amceboid cor- 
puscles (amabocytes) containing a brown pigment. The dermal 
branchiz cousist of a muscular layer, an eaternal epidermal layer, 
and an internal peritoneal layer, the internal cavities of the hollow 
branchiz being in free communication with the ccelome. 
Digestive System.—The mouth is found to open through a 
short passage, the wsophagus, into a wide sac, the cardiac division 
of the stomach (Fig. 305, St, Figs. 308, 310, card. st). This is a 
five-lobed sac, each of 
the lobes of which is 
opposite one of the five 
arms. The walls of the 
sac are greatly folded, 
and the whole is cap- 
able of being everted 
through the opening of 
the mouth, wrapped over 
some object desired as 
food, and then retracted 
into the interior, the re- 
traction being effected 
by means of special 
retractor muscles (Fig. 
308, retr) which arise 
from the sides of the 
Set ambulacral ridges. This 
Bln Goee cardiac division of the 


Fic, 306,—Asterias rubens. Digestive system. ua. ; 
anus; card. st. cardiac division of the stomach ; int. stomach communicates 


eae, intestinal ceca; madr, madreporite; pyl. ewe. aborally with a much 
pyloric ceca; pyl. st. pyloric division of the stomach. 
(From Leuckart. ) smaller chamber, the 
pyloric division of the 
stomach, and this in turn opens into a very short conical i- 
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 pylori 
ceca (Figs. 305, 306, 308, 310, pyl. cee). Each pair of pyloric 
ceeca commences as a cylindrical canal or duct, the lumen of 
which is continuous with the cavity of the pyloric chamber. 
This soon bifurcates to form two hollow stems, extending to near 
the extremity of the cavity of the arm, and giving off laterally two 
series of short branches, each having connected with it a number 
of small bladder-like pouches. The walls of the pyloric caeca are 
glandular: they secrete a digestive fluid, and are therefore to be 


Ix PHYLUM ECHINODERMATA 383 


looked upon as digestive glands. It is found by experimenting 
with this digestive fluid that it has an action on food-matters 
similar to that exerted by the secretion of the pancreas in the 
Vertebrata, converting starch into sugar, proteids into peptones, 
and bringing about the emulsification of fats. While the pouches 
of the cardiac division of the stomach are attached to the oral 
wall of the body, the pyloric ceca are connected with the aboral 
wall, From the short intestine are given off inter-radially two 
hollow appendages, the intestinal ceca (Figs. 306 and 308, int. cc), 
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 madreporie or stone-canal (Figs. 308, 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 lamellz 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 lamelle. 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 = 
inadreponie canal, Below, Che TN Wewusamia s surdsn Cole 
latter opens into a wide, five- pecten). (From Gegenbaur, after Teuscher.) 
sided, ring-like canal, the ring- 
vessel of the ambulacral system. From this are given off the five 
radial ambulacral vessels, passing to the extremities of the arms. 
From the pentagonal canal are given off also in most Starfishes, 
but not in Asterias, a series of five pairs of appendages, the Polian 
vesicles (Fig. 303, ap; Fig. 308, pol. ves)—pear-shaped, thin-walled 
bladders with long narrow necks—which are placed inter-radially. 
At the sides of the neck of each Polian vesicle (except in the 
inter-radius containing the madreporic canal, where there is one 
on one side only) project inwards a pair of little rounded glandular 
bodies, the racemose or Tiedemann’s vesicles (Fig. 309, 7’), the cavity 
in the interior of each of which, opening into the ring-vessel, is 
divided into a number of chambers. 

The various parts of the ambulacral system of vessels have a 
muscular wall and an internal lining epithelium in addition to the 
coverings which they may derive, according to their situation, 
either from the external epidermis or the internal coelomic epi- 
thelium. The muscular layer is most strongly developed on the 


384 ZOOLOGY SECT, 


tube-feet, where it consists of two strata, and is also well developed 
on the ampulle 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 perihzemal 
system already referred to. The axial sinus runs nearly vertically. 
At its oral end it opens into the internal division of the oral ring 


Lyl.cec ¢ pyl.cec 


fel. ves 


Fic, 308.—Anthenea flavescens. Upper view of a dissection of the internal organs. The 
aboral wall of the body, with the exception of a small portion round the anus and the madre- 
porite, has been completely removed. One of the five intestinal ceca 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 czeca have been cut away close to their bases. 1—5, the five rays 
with their ambulacral ridges ; amp. ampulle; an. anus; int. cwc. intestinal ceca; 7. p. cut 
ends of the inter-radial partitions ; mad. madreporite with the madreporic canal ; ov. ovaries ; 
pol. ves. Polian vesicles; pyl. ca@c. pyloric ceca; retr. retractor muscles inserted into the 
cardiac division of the stomach. 


sinus ; aborally it approaches close to, if it does not actually open 
into, an aboral ring sinus: it also communicates aborally with the 
stone-canal, and perhaps opens on the exterior through certain of 
the pores in the madreporite. 

Accompanying the madreporic canal and also enfolded in the wall 
of the axial sinus there is an organ—the axial organ (Fig. 309, 
g. stol)—the relationships and function of which have given rise 


IX PHYLUM ECHINODERMATA 385 


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 (ie. 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, 7.¢. of the same tissue that composes the 


awe 


“\ 


ys 


net 


i 


gus 
NY 

TITY 
3 


gu 


a 
ih 
\s 


MM 


Fic. 309.—4, 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. azab. oss. arabulacral ossicle ; aap. ampuille of the 
tube-feet ; az. s. axial sinus ; gon. gonad ; g. stol. genital stolon or axial organ ; marg. marginal 
ossicle ; nerv. circ. nerve-ring ; o¢. cut end of esophagus ; pst. peristome ; ret. retractor muscle 
of the stomach ; sept. inter-radial septum; stone. c. stone-canal; 7. Tiedemann’s vesicle ; 
w.v. 7. Water-vascular ring-canal. (After MacBride.) 


so-called hemal 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 
perihemal 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 cc 


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 wnisexual, 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 ameebocytes 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 


Fic, 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.ampulle; an. anus ; card. st. cardiac pouch of the stomach ; int. cece, intestinal 
execu ; ip. inter-radial partition ; mad. madreporite ; mad. can. madreporic canal ; ov. ovary : 
pyl. cee, pyloric ceca; 7. cut ends of the ring-vessel of the ambulacral system ; ring v. posi- 
tion a ring-vessel ; retv. retractor muscle of cardiac pouch uf stomach ; s. cavity of the 
stomach. 


through a number of perforations on a pair of sieve-like plates, 
situated inter-radially close to the bases of the arms. 


Anthenea flavescens (Figs. 308, 310, 311, 312), a common 
Australian Starfish, which may be taken as an example instead of 
Asterias rubens, differs from the latter in the following main 
points. 

The animal consists of a relatively large central disc and five 
relatively short arms, which taper rapidly towards their extremities. 
On the oral surface the comparatively broad, flat surfaces be- 
tween the ambulacral grooves are roughish, owing to the plate-like 
ossicles being beset with a number of minute rounded tubercles, 
which, in the immediate neighbourhood of the ambulacral 
grooves, assume the character of short, blunt spines. Here and 


Ix PHYLUM ECHINODERMATA 387 


there among the tubercles, usually one in the middle of each 
ossicle, are pedicellarie, 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-lke 
depression on the surface 
of the flat ossicle, and are 
thus on a level with the 
general surface. The term 
valvulate is applied to 
pedicellarize of this de- 
scription. In a living 
Anthenea many of the 
pedicellarize 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. 


Fic. 311.—Anthenea, view of aboral surface. 


(After Sladen.) 


The ambulacral spines bounding 


the ambulacral grooves are flattened and blunt, and arranged 


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


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 


co 2 


388 ZOOLOGY SECT. 


those on the oral surface, are for the most part more promineht, 
so that they assume the character of short spines. The ossicles 
on which they are borne are star-shaped with six rays, a 
spine being borne in the centre of each ossicle, and one on 
each of the rays. Between the ossicles the surface is covered 
with a soft, slimy skin, perforated by a large number of minute 
dermal pores, each of which is enclosed by a minute irregular ring of 
calcareous matter; each pore serves for the lodgment of one of the 
dermal branchiw. Numerous pedicellariz, 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-fect are arranged in a single row on each side of each ambu- 
lacral groove ; but the ampulle are in two rows, an upper and a 
lower, and each tube-foot has two ampulle connected with it, 
one of the upper row and one of the lower row. 

Anthenea has vertical calcareous tnter-radial partitions not de- 
veloped in Asterias. There are five bifid intestinal cceca, which 
are narrow tubes slightly enlarged and lobed at the extremities. 

Development of a Starfish (Asterina gibbosa or A. 
exigua ').—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 alinost equal in size, but one, which may be termed cell 
I, shghtly 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. 318, A), in which the four cells derived from I. form 
an incomplete ring not closed below, and the four derived from 
IT. 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) 1s the blastula; the cavity is the segmenta- 
tion-cuvity or blastocele. 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. 


IX PHYLUM ECHINODERMATA 389 


a double-walled cup or gastrwla (C) opening on the exterior by 
an opening, the blastopore, which, at first very wide, gradually 
becomes narrowed. At the same time the shape of the larva 
alters, so as to be somewhat elongated, the blastopore, lying at first 
midway between the two poles, afterwards gradually drawing 
nearer to what becomes the posterior end. 

Of the two layers of the gastrula (D and #), 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 


lic. 313,—Early stages in the development of a Starfish (Asterina gibbosa). 4, cight-celled 
stage; B, stage of about thirty-two cells scen in section; C, gastrula stage; D, section of 
early gastrula; £, section of later gastrula. arch. archenteron; blastoc, blastoccele ; 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 enterocele (celome). The wall 
of the enterocceele becomes thinner, and it gives off two lateral 
swellings, the right and left enterocwlic 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 SECT. 


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

lary. org B 


lary, or 
< 
Le. 


Fig. 314.—Later stages in the development of the larva of Asterina gibbosa. 4, newly 
hatched larva, ventral surface with the beginning of the larval organ at the anterior end and 
with the larval mouth. 8B, dorsal half of an embryo of the same age as 4. C, somewhat older 
larva with larger larval organ, the ectoderm of the left side removed to expose the alimentary 
canal-and the walls of the body-cavity. arch. archenteron ; bl. p. blastopore ; ect. ectoderm ; 
ent. enteroceele ; larv. mo. larval mouth ; larv. org. pre-oral lobe ; stom. stomodzeum. (From 
Ziegler’s models.) 


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


larv. OG, 
\ ere 


oept : 


Fic. 315.—Larva of Asterina gibbosa. 4, 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 cesophagus. ali. alimentary canal ; amb. ambula- 
cral system or hydroceele ; dors. p. dorsal pore ; eat. enteric sacs and ccelome ; larv. mo. larval 
mouth; larv. org. pre-oral lobe; @s. cesophagus of adult ; 7,7. lobes of hydroceele ;« sept. 
septum between the enteroccelic sacs. (4, 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 nght and left coelomic 
pouches have become cut off, two apertures make their appearance 


Ix PHYLUM ECHINODERMATA 391 


on the surface of the larva: one, on the ventral side, is the open- 
ing of the stemedeuwm or larval mouth; the other, on the dorsal 


side, is the dorsal pore. The mouth 
subsequently opens into the larval 
stomach, and for a time the enteric 
canal of the larva opens on the ex- 
terior both by mouth and anus: 
soon, however, the larval anus be- 
comes closed up. The dorsal pore 
is developed as an outgrowth of the 
anterior part of the enterocele, 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 


Fic. 316,—Larva of Asterina, view 


of the left side, showing the five- 
lobed prominence (anb.) 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. 


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 


Fic. 317.—Asterina exigua. Young Starfish shortly 
after the metamorphosis has bzen completed, viewed 
from the oral side. circ. amb. circular ambulacral 
vessel ; dors. p. dorsal pore and madreporic canal ; rad. 
amb, radial ambulacral vessel ; s¢. stomach ; tent. tentacle ; 


t. f. tube-feet. 


longer, posterior, part. 
On the anterior sur- 
face of the pre-oral 
lobe a flattened area 
appears surrounded by 
a raised rim, which is 
beset with specially 
large cilia: this is the 
larval organ. In the 
middle of the larval 
organ appears an ele- 
vation, the rudiment 
of a sucker by means 
of which the larva be- 
comes attached when 
the metamorphosis is 
about to begin. At 
this stage the larva 
(Fig. 316) is able to 
creep by contractions 
of the pre-oral lobe, 
and also to swim by 
the action of the cilia, 


more especially the cilia of the larval organ. 
The hydroceele, at first a five-lobed outgrowth of the entero- 
cele, grows into the form of a horse-shoe with five lobes, each of 


BND ZOOLOGY SECT. 


which represents one of the radial parts of the ambulacral system, 
the horse-shoe itself representing the ring-vessel. The rudiment 
of the madreporic canal arises as a groove on the posterior wall of 
the anterior ccelome. This develops into a canal leading from the 
hydroccele 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 hydroceele develops, its form influences the external 
shape of the larva; on the left-hand side there grows out a five- 
lobed elevation (Fig. 316, amb), each of the lobes corresponding 
to one of the five lobes of the hydroccele. 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 


Fic. 318,—Views of the larva of Asterina gibbosa in the course of metamorphosis, 4, larva 
of eight days, from the right; B, left, and C, right view of the larva of nine days ; 1-5, lobes 
of hydroccele ; 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 cesophagus (Fig. 315, B, es), the larval mouth and 
cesophagus becoming abolished during the metamorphosis. Round 
this new mouth grows the ring-vessel of the ambulacral system. 
From the stomach, diverticula grow out radially into the developing 
arms to give rise to the caeca; and later the permanent anal 
opening is formed on the dorsal surface. 


Ix PHYLUM ECHINODERMATA 393 


When the first ossicles are definitely formed they present the 
following arrangement (Fig. 319). In the middle of the abactinal 
surface is a single central plate (dors). Around this are five basals 
(bas) one of which becomes merged into the madreporite. External 
to these, five radials (rad) appear somewhat later. At the end of 
each developing arm is a single terminal or ocular plate (erm), 
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 


Fic. 319.—Diagrata showing the relations of the chief plates of the apical system in the young 
Starfish. az. anus; bas. basals; dovs. central ; madi, 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.—(Strongylocentrotus 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 men.brane (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. 
388, p. 422), itis found that the joint is of the character of a ball 


Fic. 320.—Eehinus esculentus, peristome, 1, tube-feet of the lower ends of the radii; 2, 
branchia; 3, teeth ; 4, buccal tube-foot; 5, peristomial membrane. (From MacBride, after 
Kiikenthal.) 


and socket, a concavity on the base of the spine fitting over a 
hemispherical elevation on the surface of the Sea-urchin, and the 
spine being retained in place and caused to move by means of a 
capsule of muscular fibres enclosing the joint. Around the bases 
of the large spines are a number of very small spinules. Here and 
there among the spines are to be observed minute pedicellariw 
(see Fig. 340, p. 423), which are comparable to the stalked 
pedicellarie of Asterias; but each has three jaws instead 
of two, and a relatively long stalk, which is supported by a 
slender calcareous rod. Here and there are to be found also small 


IX PHYLUM ECHINODERMATA 395 


rounded bodies termed the spheridia, which are perhaps, like the 
pedicellariz, to be looked upon as modified spines: they contain 
ganglion-cells and are apparently organs of special sense, having 
possibly the function of detecting changes in the composition of 
the water. 

Projecting from the surface among the spines all the way from 
the peristome to the periproct will be observed five double rows of 
tube-feet (Fig. 321), which in a living specimen will be found to 
be capable of great extension. These are similar to the tube- 


Fic, 321.—_Strongylocentrotus, entire animal with the tube-feet extended. (From Brehm’s 
Tierleben.) 


feet of the Starfish, and have similar functions: the sucker-like 
extremity of each is supported by a perforated sieve-like plate of 
calcareous matter. Each double row of tube-feet occupies a 
meridional zone of the surface, termed the ambulacral areu, 
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 branchie, are situated in. 


396 ZOOLOGY SECT. 


the peripheral part of the peristome, a pair opposite each inter- 
ambulacral area. 

When the spines are removed, the body is found to be enclosed 
in a rigid globular shell, or corona (Fig. 322) as it is termed, 
formed of a system of plate-like ossicles, the edges of which fit 
accurately and firmly together, and the surfaces of which are 
ornamented with the rounded elevations or tubercles for the articu- 
lation of the spines. These plates are arranged in ten zones, each 
consisting of two rows, running in a meridional direction from the 


Fic, 322,—Corona of Echinus esculentus, from the aboral surface, showing the arrangement 
of the plates of the corona. 1, the anus; 2, periproct, with irregular plates ; 3, the madre- 
porite; 4, one of the other genital plates; 5, an ocular plate ; 6, an inter-ambulacral plate ; 
7, an ambulacral plate; 8, pores for the protrusion of the tube-feet ; 9, tubercles. (After 
MacBride.) 


edge of the peristome to the neighbourhood of the periproct. Of 
the zones of plates there are two sets, each consisting of five, the 
members of which alternate with one another. In the case of one 
of these sets of zones—the ambulacral zones or ambulacral areas 
already referred to—each of the plates is perforated towards 
its outer end by two minute pores, the ambulacral pores, for the pro- 
trusion of the tube-feet. In the other five zones, the zler-ambu- 
lacral zones or areas, the plates are not perforated. At its 
anal end each area, ambulacral or inter-ambulacral, ends in a 
single apical plate, so that the periproct is surrounded by a ring of 


1X PHYLUM ECHINODERMATA 397 


ten plates, the wpieal 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 


Fic. 323.—Apical system of plates aud 

as the case may be. One of these epee extremiiips of zones of Laer 

= ofa Sea-urchin. amb. ambulacra 

genital plates (mad?) has a swollen zones ; gen. genital plates; int. ainb. 

| S inter-ambulacral zones ; madi. madre- 

and spongy, appearance, which dis- porite ; oc. ocular plates ; peripr. peri- 
tinguishes it from the others: this proct. (After Leuckart.) 


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 biviwm, 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. 825, 
aur), one opposite each ambulacral area. Within the ring of auricles 
lies a complex structure termed Aristotle’s lantern (Fig. 324). 
This consists of the five teeth (c), the apices of which are to be 


Fic. 324.—Lantern of Aristotle of Echinus. 4, two of the five chief component parts apposed 
and viewed laterally. 3B, lateral, and C internal view of a single part. a. alveolus; a’. suture 
with its fellow; b. epiphysis; b’. suture with alveolus ; ¢. rotula; d. radius; e. tooth. (From 
Huxley’s /avertebrates, after Miller.) 


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 rotule (c), the inner ends of which 
unite to bound a circular aperture through which the cesophagus 
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 endin 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 cesophagus. 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 rotule 
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, 


Fic. 325.—Lateral view of the internal organs of a Sea- 


urchin as seen on the removal of ahalf of the shell. ab.r. 
ves. heemal strand, aboral ring ; amb. 7, ambulacral ring- 
canal ;amp.ampulle ; an. anus ; aur. auricle ; cel. celome ; 
int. intestine ; int.ves.intestinal hemal strands ; mad. mad- 
reporite ; mad. can. madreporic canal; mo. mouth; mus. 
muscles passing from the auricles to Aristotle's laitern ; 
neve. 7. nerve-ring ; oc. ocular plate ; 07.7. ves. heemal strand, 
oral ring ; plerc. ovoid gland ; pol. ves. Polian vesicle ; rad. 
amb, radial ambulacral vessel ; rad. ne. radial nerve ; siph. 
siphon ; sp. radial extension of the coelome surrounding 
the nerve ; ¢. f. tube-feet. (From Leuckart, after Hamann.) 


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 ihe 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. 7) surrounding the mouth. At its 
distal end each radial nerve is connected with the so-called eye (0c), 
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 epinewral canals (Fig. 326, ep.), covered over by the 
plates of the corona—is here more deeply situated ; the deep and 
coelomic systems are only feebly developed. 


IX PHYLUM ECHINODERMATA 399 


Ambulacral System.—Internal to each radial nerve, and pur- 
suing a corresponding course, runs a radial ambulacral vessel (Figs. 
325 and 326). From this are given off on each side a series of short 
branches to the tube-feet, with each of which is connected one of 
a series of compressed sacs, the ampulle (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 cesophagus. 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, 


perih 


Fic, 326.—Diagrammatic transverse section of the ambulacral zone of an Echinoid. amb. oss. 
ambulacral ossicle; amp. ampulla of a tube-foot; ep. epineural canal; muse. muscles 
attaching spine to its tubercle ; nerv. nervous ring in base of spine ; n. 7. radial nerve-cord ; 
oss, ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perih. radial periheemal canal ; 
pod. tube-foot ; wv. r. radial ambulacral vessel. (After MacBride.) 


runs from the madreporite at the side of the periproct to the 
ring-canal. 

The enteric canal (Fig. 327, ali) is devoid of the radial czca 
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 coelomic fluid. 

The celome contains a fluid in which, as in the Starfish, 
there are numerous corpuscles. Of these there are two kinds 
—ameboid corpuscles (amebocytes) with long pseudopodia, and 
vibratile corpuscles, which closely resemble sperms, having a rounded 


400 ZOOLOGY SECT. 


head and a slender vibratile tail: the latter aid in bringing about 
a constant circulation of the coelomic fluid. 

The part of the 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 branchize on the 
peristome is evidently the oxygenation of the ccelomic fluid 
enclosed in this compartment, which is known as the lantern- 
celome. 

The perihemal and hemal 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, 


Fic. 327.—Alimentary canal and other organs of Sea-urchin as seen when the ora half 
of the corona has been removed, ab. r. ves. aboral ring of the hemal system; ali. ali- 
mentary canal; amp. ampulle ; int. ves. intestinal blood-vessels ; ant. lantern of Aristotle ; 
es, cesophagus ; or. r. v. oral ring-vessel of the hemal system ; ov. ovary ; rect. 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 plutews, is provided with 
a number of elongated arms or processes supported by delicate 
calcareous rods. A metamorphosis, in which the bilateral larva 
becomes converted into the radial adult, takes place as in the 
Starfish. 


IX PHYLUM ECHINODERMATA 401 


3. EXAMPLE OF THE HOoLoTHUROIDEA, 
A Sea-cucumber.—Cucuwmaria 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 NY 
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 triviwm 
of the Starfish, the rest re- 
presenting the biviwm. On 
the dorsal surface, instead of 
typical tube-feet, there are 
papillze 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 Fiq, 328,—Cucumaria planci. Entire animal 
position is to be deter- seen from the ventral surface, (From Hertwig's 
mined by reference to the Lehrbuch, after Ludwig.) 
tentacles (vide p. 402); there 
are no papillz. 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 spicules, very variable in shape in 
different species of Cucumaria, and in Colochirus having the form 
of sieve-like or lattice-like plates, some of which are to be found 


VOL. I DD 


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 surtace 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 peritonewm or calomic epitheliwm, liming the ccelome. 
The outer layer of muscle is a complete, continuous layer of 
muscular fibres which have a circular arrangement, we. are 
arranged in a ring-like manner around the long axis of the body ; 
while the inner layer is not continuous, consisting, in fact, merely 
of five flattened bands which run longitudinally from the oral to 
the anal extremities, each underlying one of the ambulacral areas. 
In close contact with each of these bands, on its inner surface, 
runs a radial ambulacral vessel (Fig. 329, rad. amb) together with a 
radial nerve. 

Ambulacral System.—Just behind the bases of the tentacles, 
and surrounding the beginning of the esophagus, is a cireular 
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 sinuons canal, the 
madreporie 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 periphemal and hemal systems are well developed. 
The latter comprises a ring-like strand (7. 62. ves) situated close 
to the nerve-ring and sending off five radial strands, as well as 
dorsal and ventral strands (cnt. ves) accompanying the enteric 
canal, and a plexus surrounding the left respiratory tree (p. 404). 


IX PHYLUM ECHINODERMATA 403 
The ceelome contains a fluid in which float numerous amebo- 


cytes, similar to those of the Starfish, and also a number of 


radams UT a wea 
; rad oss 
CGE, < eneer oss 
7c. bl.ves Vie 2 gen. du 
ol Ue jn <= es 
Pol ves Lf 2 i 77Lad Can 
; stom - a es tnt.ves 
ind ves , ; ee 
! if j 
/ : . We 
| 2 
wrt aE aN pa 
j 3 a ee 
1 Bld, be ; AS Z 
long mus 4 Ne r a Jer G 
long mus [ = ison 
y ent ves 
Y. ce . 
i ent Ves 
resp } el, 
= Long. mus 
=a 
tnu.ves ~ } od 
Auf, z 
ata teres 
CLre mua ANN’ Cf SHE CLC. 1s, 
C4Lu org } ’ UTE cong. ices 
ZF bw 
a long. mus 


ot cL.op 


Fic. 329,—Internal organs of a Holothurian as scen 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; Cuv. org. Cuvieran organs; gen, ap. genital aperture ; 
gen. du. genital duct; gen. gl. gonad; int, intestine; inter. oss. inter-ambulacral ossi- 
cles ; iat. ves. intestinal hemal strands; long. mus. longitudinal band of muscle; mad. can. 
madreporic canals; mes. mesentery ; pol. ves. Polian vesicles ; rad. amb. radial ambulacral 
vessel ; rad. oss. ambulacral ossicles ; 7%. b/. ves. ring strand of heemal system ; resp. respira- 

tory trees ; ring-ves. ring-vessel of the ambulacral system ; stom. stumach. (After Leuckart.) 


flattened nucleated corpuscles containing a red colouring matter 
—hzmoglobin—almost identical with that which gives the red 


colour to the blood of the higher animals. 
DD 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 esophagus, which lies immediately behind the 
buccal chamber, is a circlet of ten circwm-asophageal ossicles, five 
ambulacral (rad. oss) in position, and five inter-ambulacral (dnter. 
oss). Through each of the former pass the corresponding radial 
ambulacral vessel, heemal strand, and nerve. The alimentary canal 
itself is a simple cylindrical tube, only indistinctly marked out 
into cesophagus, stomach (stom), and intestine. It forms several 
coils within the coelome, to the wall of which it is attached by a 
thin membranous dorsal mesentery, and terminates behind in a 
comparatively wide chamber, the cloaca (cl). 

Opening into the cloaca is a pair of remarkable organs of 
doubtful function, the so-called respiratory trecs (resp). Hach 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. 388). The bilateral, however, assumes a shape 
somewhat different from that of the Asteroidea, and is 
termed the auricularia (Fig. 343): it has a number of short 
processes developed in the course of the ciliated bands. The 
larval mouth and ceesophagus, instead of being abolished as in the 
case of the Starfish, persist to the adult condition. 


-IX 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 


Fic. 330.-Antedon. Side view of entire animal. (From Leuckart and Nitsche’s Diagrams.) 


the aboral side of the disc are whorls of slender, curved, cylindrical 
appendages, the cirri (Fig. 3381), 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 cach 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 dise is occupied by a large, flat, pentagonal 
ossicle, the centro-dorsal ossicle (Fig. 331, ¢; and Fig. 334, CD), 


oe je 
C2 


Fic. 331.—Aboral view of Antedon. c. centro-doursa] ossicle ; ¢ir. cirrus; R,! #2 R,3 the three 
radial plates of one column ; syz. syzygy or articulation. (After MacBride.) 


bearing on its outer surface a number of little cup-like depressions, 
with which the bases of the cirri are connected. The cirri (cir) 


Fic, 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 cpidermis, and connected together by means 


IX PHYLUM ECHINODERMATA 407 


of muscular fibres. Concealed from view by the centro-dorsal 
ossicle is a thin plate termed the “ rosette” (vos), formed by the 
coalescence of the basals of the larva. At the sides are five 
first radial ossicles (BR), also concealed by the centro-dorsal 
ossicle: with each of these articulates a second radial (R?), which 
is visible beyond the centro-dorsal. With each of the second 
radials articulate two third radials (R*), 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 celome contains num- ; 

: Fic. 333.—Antedon, transverse section of a 
erous strands of connective- pinnule. amb. ne. radial nerve of the super- 


S 4 ficial (ambulacral) nervous system; az. ne. 
tissue which serve to suspend axial nerve; cel. can. sub-tentacular and 


the various organs. celiac canals ; mus, muscles ; neur. ves. radial 

a sinus of the perihzeemal system; rad. amb. 

Extending through the arms radial ambulacral vessel giving off branches 

° to the tentacles. Between the paired sub- 

and pinnules between the sup- tentacular and eS ee ne 

i | : enital rachis. The small round bodies above 

porting ossicles and the eine the line from rad. amb. are the sacculi. (After 
bulacral grooves are three Teuscher.) 


canals which are prolongations 

of the celome (Fig. 333, cal. 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 caliac canai—runs between these and the supporting ossicles 
(oss). The sub-tentacular canals and the coeliac canal communicate 
with one another at the extremity of each arm. 

The enteric canal begins with a wide, funnel-shaped e@so- 
phagus leading to a spacious stomach which gives off a number of 
short, blunt diverticula and a pair of longer, narrower, “hepatic ” 
ceca, which are slightly branched at the ends. Distally the stomach 
becomes contracted and opens into a wide intestine, which winds 
round the celome, becoming narrower where it passes upwards to 


408 ZOOLOGY SECT. 


open on the exterior, the terminal part, or rectum, projecting as a 
tubular papilla on the surface. In the living animal the rectal tube 
is observed to undergo frequent movements of contraction and 
dilatation, by means of which water is drawn into and expelled 
from the intestine; so that here, as in the Sea-urchin, there 
would appear to be a process of intestinal respiration. 

The ambulacral system consists of a ring-vessel surrounding 
the mouth, and a series of radial vessels (Fig. 333, rad. 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 


Lent 


A 


al 

cirr 

Fic. 334.—Antedon, Diagrammatic view of a median vertical section through the dise, passing 
through one radius and one inter-radius. amb. ambulacral vessels ; az. co. axial nerve-cord 
passing through the ossicles of the arm; Br.l Br.2 brachial ossicles; CD, centro-dorsal 
ossicle ; cent. eaps. central capsule; chamb. org. chambered organ; cirr. cirri; ect. ne. 
ambulacral (epidermal) nerve-ring and radial nerve ; gen. st. genital stolon ; iat. intestine ; 
mo. mouth ; Rl R.2 R.3 radials; ros. rosette; tent. tentacles; wat. p. water-pores. (Aftcr 
Milnes Marshall.) 


cent.caps 


and Sea-urchins, but are devoid of terminal suckers. These are 
not organs of locomotion : they bear numerous sensory papille, 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- 


1X 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 aaial nerve (ax. co), which gives 
off branches (Fig. 333, av. 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-chambercd 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 perihemal system of the 
Starfish is present, though reduced, and there is a highly developed 
and complicated lacunar or hemal 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 
Alge; 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 cesopha- 
gus and connected dorsally with the chambered organ—outwards 
through the arms; the terminal portions, lying in the pinnules, 
are 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 hemal lacunee. 

Like the rest of the Echinoderms, the Feather-star undergoes a 
metamorphosis (Figs. 344 and 345). It passes through a frce- 
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 coelome, and well-developed alimentary, nervous, and 
vascular systems. A characteristic system of vessels, the ambu- 
lacral system, is connected with the locomotion of the animal, as 
well as with other functions: the organs of locomotion are in most 
cases elastic and contractile tubular bodies, the tube-feet, which are 
appendages of the ambulacral system. Nearly all the systems of 
organs of the animal partake to a greater or less extent of the 
general radial form of the body. Reproduction is entirely sexual. 
In the course of its development from the egg the Echinoderm 
passes through a peculiar larval stage, in which the symmetry of 
parts is bilateral, instead of radial as in the adult animal. All the 
Echinodermata are marine. 

The Echinodermata are classified as follows :— 


SUB-PHYLUM I.—_ELEUTHEROZOA. 


Echinodermata devoid of a stalk, and always freely locomotive in 
the adult condition: with a system of radial ambulacra in the form 
of grooves or areas radiating out from the mouth, and containing 
a double series of tubular appendages of the ambulacral system, 
the tube-feet, usually employed in locomotion, and in the majority 
of cases provided with terminal suckers: the anus usually aboral ; 
the mouth on the surface that is habitually directed downwards, or 
at the end habitually directed forwards in locomotion. 


CLASS I.—ASTEROIDEA. 


Free Echinoderms with star-shaped or pentagonal body, in 
which a central disc and usually five arms are more or less readily 
distinguishable, the arms being hollow, and each containing a 
prolongation of the coelome and of its contained organs. There 
are 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 


mouth and five narrow ambulacral grooves lodging the tube-feet. 
The larva has the form either of a dipinnarta or of a brachiolaria. 
This class includes the Starfishes. 


ORDER 1.—PHANEROZONTA. 


Asteroidea with large marginal ossicles. The dermal branchiz 
are present only on the aboral surface. The ambulacral ossicles 
not closely crowded. Pedicellariz sessile. 


ORDER 2.—CRYPTOZONIA. 


Asteroidea with the marginal ossicles inconspicuous. Dermal 
branchiz not restricted to the aboral, but often present on the 
oral surface. Ambulacral ossicles crowded together. Pedicellari 
stalked or sessile. 


CLASS II.—OPHIUROIDEA. 


Star-shaped free Echinoderms, with a central disc and five arms, 
which are more sharply marked off from the disc than in the 
Asteroidea and which contain no spacious prolongations of the 
celome. There are distinct ora] and abora] 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.. 
Extinct Ophiuroids with ambulacral grooves. 
Silurian and Devonian. 
ORDER 2.—STREPTOPHIURE. 
Ophiuroids in which the ambulacral ossicles articulate with one 
another by simple ball-and-socket joints. 
ORDER 3.—CLADOPHIUR. 


Ophiuroids in which the ambulacral ossicles articulate with one 
another by means of hour-glass-shaped surfaces. The arms may 
be branched. 


ORDER 4.—ZyYGOPHIUIE. 


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 IIL—ECHINOIDEA. 


Free Echinoderms with globular, heart-shaped, or disc-shaped 
body enclosed in a shell or corona of close-fitting, firmly united 
calcareous plates. The mouth is nearly always polar; the anus 
usually at the opposite (aboral) pole; the madreporite is close to 
the latter. There are no ambulacral grooves; but the surface is 
divided into alternating ambulacral and inter-ambulacral zones or 
areas, which usually run from pole to pole. The larva is a pluteus. 
This class includes the Sea-urchins, with the Heart-urchins and 
Cake-urchins. 


ORDER 1.—REGULARIA. 


Echinoidea with globular corona containing, in most cases, 
twenty meridional rows of plates. Mouth and anus polar. A 
lantern of Aristotle is present. This order includes the Sea- 
urchins. 


ORDER 2.—CLYPEASTRIDEA. 


Echinoidea with more or less flattened corona, with the mouth 
central, the anus excentric. A lantern of Aristotle is present. 
This order includes the Cake-urchins (Fig. 341). 


ORDER 3.—SPATANGOIDEA. 


Heart-shaped Echinoidea with the mouth and anus excentric. 
No lantern of Aristotle. This order includes the Heart-urchins 
(Fig. 340). 


CLASS IV.—HOLOTHUROIDEA. 


Free Echinoderms with elongated, cylindrical or five-sided body, 
having the mouth and anus at opposite extremities. The body- 
wall is usually only supported by scattered ossicles or spicules. 
There is no external opening to the madreporic canal (except in 
some Llasipoda). The surface usually exhibits five ambulacral 
areas; but these may be absent. There isa circlet of large oral 
tentacles. The larva is an auricularia. This class includes the 
Sea-cucumbers and “ Béche-de-mer.” 


ORDER 1.—ELASIPODA. 


Holothureidea with well-marked bilateral symmetry, with tube- 
feet on the ventral surface (which is flattened) and papilla on the 
dorsal. Contined to the deep sea. 


IX PHYLUM ECHINODERMATA 413 


ORDER 2.—PEDATA. 


Holothuroidea: with tube-feet either in longitudinal rows or 
scattered irregularly over the surface. 


ORDER 3.—APODA. 


Holothuroidea devoid of tube-feet and of radial ambulacral 
vessels. 


SUB-PHYLUM II.—PELMATOZOA. 


Echinodermata which are usually fixed at the base, and usually 
supported on a stalk composed of a row or rows of ossicles 
(Fig. 342): the mouth on the free surface, near or in the centre, and 
having extending out from it on the oral surface a radially arranged 
system of narrow, ciliated ambulacral grooves, having the function 
of food-grooves, which may run between the plates of the theca, 
on the surface of the theca, or along the oral surfaces of a system 
of radial processes or arms given off from it. The tube-feet of 
other Echinoderms, when represented, take the form of small, 
tubular, strongly ciliated appendages (tentacles) without suckers: 
the anus usually on the oral surface. 


CLASS I—CRINOIDEA. 


Mostly fixed, stalked Pelmatozoa in which there is a theca 
comprising five regularly arranged radial and five basal plates, 
giving off five, usually branched, jointed processes or arms; with 
food-grooves radiating out from ‘the mouth along the oral surfaces 
of the arms, and extending along their branches: the central parts 
of the ambulacral, nervous, and reproductive systems, and of the 
ceelome lodged in the theca, send extensions through the arms. 

This class comprises, together with many extinct forms, the 
only living Pelmatozoa, 


Sus-Criass J.—Monocyctica. 


Crinoidea in which the base of the theca comprises basals 
only. 


Sup-Cxiass II.—Dicycrica. 


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 arms or “ fingers”; the theca 
perforated completely or partially by numerous pores which are supposed to 
have lodged respiratory processes. 

Lower Silurian to Carboniferous. 


CLASS III.—BLASTOIDEA. 


Fixed Pelmatozoa with well-developed stalk, and theca with a regular 
system of plates; with five, rarely four, food-grooves radiating out from the 
central mouth, and each borne on a special ‘lancet plate,” the inter-radial 
intervals between which are occupied by a corresponding number of oral or 
“deltoid” plates. The grooves are bordered by a series of side plates bearing 
small branches or ‘ fingers” to which side branches of the grooves extend. In 
the intervals between the grooves on the aboral sides of the deltoids are a whorl 
of plates perforated by the apertures of groups of internally situated respiratory 
folds (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 saec-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 rubens 1s a species of the genus Asterias, which, with 
several others, constitutes the family <Asteriada of the order 
Cryptozonia. The family Asteriidew is characterised among the 
families of the Cryplozonia by the following distinctive features :— 
The ossicles of the aboral surface are small, unequal, reticulate 
plates, bearing isolated or grouped spinelets (paxillw), The margin 
of the actinostome is defined by the ambulacral plates. The 
pedicellariz are of two forms, forceps-like and scissors-like. The 
tube-feet are in four rows. Asterias differs from the other genera 
of the family in having well-developed reticulate dorsal ossicles 
bearing definite spines. 

The Sea-urchins of which a short description has been given 
are the genera Stronyylocentrotus and Echinus, but the description 
is sufficiently general to apply to any member of the family 
Echinide, to which these genera, with a number of others, belong. 


IX PHYLUM ECHINODERMATA 415 


The family Achinid@ 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 Regulariv, or regular Sea- 
urchins, in the possession of dermal branchie, and in having the 
auricles in the form of complete arches, 

The Sea-cucumber (Cucwmaria or Colochirus) is a member of 
the Stzchopoda—one of the families of the sub-order Dendrochirote. 
of the Pedata, or foot-bearing Holothurians. The Dendrochirote 
differ from the <Aspidochirote—the other sub-order—mainly in 
having arborescent instead of shield-shaped tentacles, and the 
Stichopoda differ from the rest of the Dendrochirote in having the 
tube-feet arranged in five regular zones. The genus Cucumaria 
is distinguished from the rest by having ten tentacles with the two 
ventral smaller than the others. Colochirus is closely allied to 
Cucumaria, the principal distinction beimg the presence in the 
former of papille taking the place of tube-feet in certain 
situations, as already noted. 

The Feather Star (Antedon rosacea) is a member of the family 
Comatulide, which is distinguished from the four other living 
families comprised in the class Crinoider 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 sinaller or larger central disc. Butin 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 
ambulacral and five inter-cmbulacral areas. In the Ophiuroidea 
and Asteroidea two of the rays—constituting the diviwm—have 
between them the madreporite, marking the position of the 
madreporie canal of the ambulacral system; the remaining three 
rays form the ¢rivium. 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 twbe-fect—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 along way from the 
surface of the body. In the majority of cases the tube-foot has at 


1X PHYLUM ECHINODERMATA 417 


its extremity a suching-disc, by means of which it can be attached ; 
in a few, however, this sucking-dise is absent. 

The epiermis is ciliated in all but Holothuroidea. In the 
subjacent dermal layers there are always present calcareous bodies 
or ossicles, varying very greatly in form and arrangement in the 
different groups. Movable or immovable calcareous spines or 
tubercles projecting on the surface are very general. Peculiarly 
modified spines, termed pedicellariw, are commonly, though not 
universally, present in certain parts in the Echinoidea and 
Asteroidea. A pedicellaria consists in essence of two or three 
calcareous jaw-like pieces or valves, movably articulated together, 
and capable of being separated or approximated by the con- 
traction of bundles of muscular fibres; sometimes there is a long 
stalk ; sometimes (as in the case of Anthenea, p. 387) a stalk is 
absent ; during life the jaws or valves keep opening and closing. 
That such specialised structures have some important function to 
perform there can be no doubt, but there is some uncertainty as 
to what their special purpose is. According to some observers. 
the pedicellarizee of the Sea-urchin have been seen passing from 
one to another the particles of fecal 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 pedicellarize 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. Pedicellariz 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 sphwridia, 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 clavule— 
slender spines covered with strong cilia, which occur in bands on 
the surface of the Spatangoids. Larger spines, resembling the 
clavule 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 branchiz. 

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 Lasals which are inter-radial in position ; (3) of five radials 


VOL. I EE 


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 éerminal 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. Tn 
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 1s 
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 isa 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, elght, 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 
Brisingide differ from all the rest of the class in having the arms 
almost as sharply separated off from the central dise 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 


IX PHYLUM ECHINODERMATA 419 


the oral surface of cach arm to its extremity. Near to, but not 
quite in the middle point of the aboral surface is the anal 
uperture, absent in a few instances; and on the same surface, 
nearer the margin, between the two rays of the bivium in the 
five-rayed Starfishes, is the madreporite, a finely grooved calearcous 
plate perforated by a number of minute apertures. In some fossil 
Starfishes it is situated on the oral surface. Sometimes instead of 
one madreporite there are several. 

The wall of the body in the Starfishes contains a number 
of calcareous ossicles, movably articulated together and connected 
by bands of muscle, so that, though the body is firm, and in the 
dried condition often quite 
rigid, the arms are capable 
during life of slow move- 
ments of flexion and exten- 
sion, enabling the animal 
to creep through compar- 
atively small fissures and 
crannies. A special system 
of ossicles—the ambulacral 
ossicles—are arranged in a 
double row along each am- 
bulacral groove, the ossicles 
of the two rows articulating 
movably with onc 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 Fic. 335.—Anthenea. View of oral surface. 
are sometimes confined to (After Sladen.) 
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 
paxille 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 
ampullz, 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 


EE 2 


420 ZOOLOGY SECT. 


function. The tube-feet are provided (except in Astropecten) with 
terminal suckers. 

In the Ophiuroidea (Fig. 336) the central disc is much more 
sharply marked off from the arms than in the Asteroidea. The 
arms, which are five in number, are comparatively slender, and 
cylindrical, tapering towards the free extremities; in one group, 
the Luryalida (Fig. 337), they are branched. The mouth is in the 
middle of the oral surface of the disc, as in the Asteroidea, but 
there are no ambulacral grooves, and there is no anal aperture. 


Fic. 336.—Ophioglypha lacertosa, 4, 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 Palwontology.) 


Five pairs of slits on the oral surface (Fig. 336, C) lead into the 
genital bursee, 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. ach 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 ampulle, and 
locomotion is effected in the majority of the Ophiuroids by active 
flexions and extensions of the arms. In one genus there is a pair 


IX PHYLUM ECHINODERMATA 421 


of fin-like appendages, supported by slender spines, on each joint 
of the arms. The madreporite is situated inter-radially on the 
oral, and not on the aboral surface as in the Asteroidea. In 
the Euryalida there are five madreporites and five madreporic 
canals. 

In the Echinoidea the body is either globular, or heart-shaped, 
or flattened and disc-like. The exoskeleton is in the form of a 


Fic. 337,—Astrophyton arborescens, aboral surface. (After Ludwig.)- 


rigidly articulated system of calcareous plates, fitting closely 
together by sutures, so as to form a continuous shell or corona. 
Asthenosoma and allies, deep-sea forms, differ from all the rest in 
having a corona possessing a certain degree of flexibility and 
performing movements which are brought about by the contrac- 
tions of five longitudinal bands of muscle running along the 
ambulacral areas on the inner surface. 

In the globular forms, or regular Sea-urchins, the mouth is situ- 
ated at the oral pole of the globe, the anus at the aboral, and 
the plates of the corona are in twenty regular meridional rows, 


429 ZOOLOGY SECT. 


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), 
pedicellarie (Fig. 339), and spheridia 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 sack- 
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 
Fr ae bine likewise of the nature of tentacles, and are 
ee es Wee sometimes devoid of sucking-membranes. 
(From Leuckart.) ’ Corresponding to the dermal branchice of 
the Asteroidea are, in the majority, five 
pairs of branched, hollow appendages surrounding the peristome. 
Surrounding the mouth are five teeth, supported by an elaborate 
system of ossicles (Avistotle’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. = 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 Ba 
pores, The genital and ocular plates are — Fic. 339.—Podicellaria of 
in the middle of the aboral surface, where Tee eee amma 
the ambulacra converge, and are thus widely 
separated from the anus; there are usually only four genital plates, 
and the genital apertur es may be reduced to two. Slender spines 
beset the entire surface and are the chief organs of locomotion. 
Modified spines, the elavule, surround the anus in a ring and are 
distributed elsewhere. A few pedicellarize are present in the 


IX PHYLUM ECHINODERMATA 423 


neighbourhood of the mouth, and spheeridia also occur. A series of 
tree-like dermal branchiw 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 
fenestre. The mouth is 
in the middle of the flat 
or concave eral 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. Spheeridia are 
present, but pedicellarice 
and clavule are absent. 
An “Aristotle's lantern” fic, $0—“Hembpneustes radiatns. 4, or 
with teeth is present, as Ticrveich.) 
in the globular forms. 

In the Holothuroidea the body is more or Jess 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- 
tinguishable: it is most distinctly de- 


Fic, 341.—Clypeaster sub- 
depressus, vicw of aboral i é : 
surface showing the petaloid veloped in Psolus and allied genera. In 


iehrbueny Cem Bates some Holothuroids the surface is enclosed 


424 ZOOLOGY 


WWE 
ee 


Ty 


anu 


eg 
wa 


Pi) 


aw 


IK 


< 


“SER 


is 
ee is 
7 
CANS 
Oh BED 
SEA g 


Fic, 342.—Metacrinus interruptus, 
(After P, H, Carpenter,) 


SECT. 


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 papille. 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 


IX PHYLUM ECHINODERMATA 425 


a series of radiating arms, which usually branch dichotomously. 
In the stalked forms (Fig. 342) a stalk, consisting of a row of 
elongated ossicles connected together by bundles of 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 cirrv. 

The mouth in all the Crinoidea, with one exception (Actinometra), 
is situated in the centre of the oral (upper) surface, and the anus 
in all, with the same exception, is eccentric and inter-radial. 
Running outwards from the mouth are a series of very narrow 
ambulacral grooves, one of which extends along the oral sur- 
face of each arm, giving off branches to the arm-branches and 
to the pinnules. Bordering the ambulacral grooves and their 
branches are a pair of rows of short tubular tentacles, which 
correspond morphologically with the tube-feet of the other classes, 
but are devoid of the terminal suckers, and are not locomotor, 
but probably sensory and respiratory in function. 

The celome in the Echinoderms is a wide cavity lined by 
a ciliated cceelomic 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 branchie or papule, 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 branchi in providing for the respiration of the compart- 
ment of the celome 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 body, to the ambulacral system of vessels. A 


426 ZOOLOGY SECT. 


ring-like cirewm-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 ampulle 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 ampulla’ 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-lke appendages—the Polian vesicles—are in 
most cases connected with the ring-vessel. The racemose vesicles, or 
Ticdemann’s 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 (Alasipoda) 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 cceelome by means of a system of ciliated 
water-tubes, while the coelome 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 hemoglobin, 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 perthemal system and 
the hemeal system. Neither of these systems comprises vessels 
with contractile walls, and there is no definite circulation of 
the contained fluid. The perihemal, or, as it is sometimes 
termed, pseudohamal 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 


1X 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 
celome, from which they are developed. In Asteroidea and Ophi- 
uroidea the radial and ring-vessels, which lie between the corres- 
ponding parts of the ambulacral and epidermal nervous systems, 
are divided into two parts by a longitudinal septum, vertical in 
the radial, oblique in the ring-vessel. The axial sinus is nearly 
vertical in direction and partly encloses the axial organ in the 
way already described (p. 384). At its oral end it opens into the 
inner division of the circum-oral vessel : at its aboral end it opens 
into, or becomes closely applied to, the aboral vessel, which is in 
the form of a ring giving off radial branches towards the gonads : 
it also may communicate aborally with several of the pore-canals 
of the madreporite, and opens into the madreporic canal itself. In 
the Echinoidea the arrangement of the parts 1s modified in certain 
important respects. An oral ring-sinus is absent unless it be 
represented by the lantern-ccelome. The radial vessels of the 
system do not open orally into the lantern-ccelome: 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 perihemal 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 hamal 
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 
theircourse occupy the typical position between the superficial radial 
nerve and the radial ambulacral vessel; aborally they terminate 
blindly. A gastro-intestinal system given off from the oral ring 
is highly developed, and there 1s 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 lacune is highly developed and 
complicated in arrangement. 

Whatever be its functions, this system is not a system of blood- 


428 ZOOLOGY SECT. 


vessels. It is made up of strands of a kind of gelatinous connec- 
tive tissue, with many leucocytes, permeated in a very irregular 
way by minute lacune without definite walls. The great 
development of the gastric and intestinal branches of this system 
in some (Kchinoids, 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 perihemal and hemal 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 Kchinoidea 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 cecum, 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 


1X PHYLUM ECHINODERMATA 429 


simple sac giving off short diverticula, and there is no anal 
aperture. In the Asteroidea the alimentary canal is more complex 
than in the other classes. The stomach is divided, as already 
described in the account of the examples, into two portions, the 
cardiac and the pyloric, the former giving off five large rounded 
radial diverticula—the cardiac pouches or cardiac ceca, and the 
latter five pairs of very long branched diverticula—the pyloric 
or hepatic ceca. 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 ceca ; 
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 ceeca connected with the cesophagus. 

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 Synaptide 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 Asteroicea 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. 


Ophinreidea there are five pairs of gonads, a pair in the walls 
of cach 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 larvee 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 Ceelenterata (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 
enterocale, 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 coelomic 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 inio 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 enterocele (hydrocele) destined to give rise to 
the ambulacral system, at first rounded, becomes compressed, and 
subsequently divided round the border into five lobes. Each of 
these lobes grows outwards to become developed subsequently into 
one of the five radial ambulacral vessels of the Echinoderm ; the 
central part of the hydroceele gives rise to the ring-vessel sur- 
rounding the cesophagus. 

The cilia, which at first (in the gastrula stage) covered the sur- 
face of the larva uniformly, become restricted to a peri-oral band 
(Fig. 343, por) surrounding a concave area on which the mouth 


IX PHYLUM ECHINODERMATA 431 


opens. A smaller adoral band (wor) in the interior of the 
mouth has the function of attracting nutrient particles. The 


Fic. 343.—Diagrams of the development of the larve of Echinoderms. 1, Primitive form of 
Echinoderm larva; 2 and 3, Development of an auricularia (Holothurovideca); 4, 5, and 6, 
Development of a bipinnaria (Asteroidea) ; 7, 8, and 9, Development of a plutews (Echinoidea 
and Ophiuroidea). aor. adoral band of cilia; ali. alimentary canal; an. anus; 6, b. processes 
or arms; mo. mouth; por. peri-oral ciliated band and processes, (From Leuckart and 
Nitsche’s Diagrams.) 


peri-oral band undergoes characteristic changes in the different 
classes, and the form of the larva at the same time becomes 


432 ZOOLOGY SECT. 


modified by the formation, except in the Crinoidea, of variously 
arranged processes along the course of the peri-oral band. The 
resulting larva, cchinopedium or diplewrula, 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. 891), 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- 
x naria, developed in a brood-pouch, 

Ot, adheres to the parent by means of the 
Fie, 344.—Free-swimming larva of pre-oral lobe which takes the form of 
Pree ender ba to ke, short stalk. In both the Ophiu- 


ba, to bas, 

gaan eae 54071 to oF) ovals ; yroidea and the Echinoidea (Fig. 3438, 
3, right enteraceele ; 4, calcareous 7 to 9) the larva has the form 
fre, vate stor Sete) Pte = which is known as the pluteus. The 
pluteus has a series of slender arms 

directed forwards and supported by a skeleton of delicate calcareous 
rods. The larva of the Holothuroidea, the auricularta (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 ceelome, 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 


IX PHYLUM ECHINODERMATA 433 


with a tuft of sensory cilia. The vibratile cilia on the surface are 
arranged in five transverse bands (I-V). Between the second and 
third of these is a wide shallow depression, the vestibule or 
stomodeum (1), which does not communicate with the mouth. 
After remaining in the free condition ant 

for a short time, the larva (Fig. 345) eS 
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 
cesophagus arises in close apposition 
with it. Round the cesophagus the 
hydroccele grows in the form of a 
ring. The vestibule (5) with the 
cesophagus and hydroceele 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 cesophagus (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 


post 


Fic. 345.—Stalked larva of Antedon, 


six months this pentacrinoid larva 
becomes free by the absorption of 
the stalk and develops into the adult 
Antedon. 


from the right side; calcareous 
plates not represented. 1, right 
cozlomic sac; 2, enteric cavity ; 3, 
left ccelomic sac; 4, sacculi; 5, 
vestibule, still closed; 6, primary 
tentacles ; 7, secondary tentacles ; 


8, esophagus ; 9, rectum ; 10, axial 
organ; 11, fibrous strands in the 


In the transition from the bi- stalk, (From Lang, after Seeliger.) 


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 Jarval arms or processes become absorbed. In the 
Holothuroidea and Ophiuroidea all the organs of the larva are 


VOL, I FF 


434 ZOOLOGY SECT. 


carried on into the adult; in the Asteroidea and Echinoidea the 
larval mouth and cesophagus are abolished and a new permanent. 
mouth and esophagus 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,! 
inhabitants of the sea. In the adult condition the majority 
creep on the sea-shore or on the sea-bottom, the stalked Crimoids 
being exceptional in their permanently attached condition ; but 
the larvee of the great majority are pelagic—ie. 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 
(Pedata). 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 Comatule, 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 coral. Movements of manducation 
are performed by the tentacles in the Holothurians: in the Star- 
fishes the mouth papille are separated from one another and the 
cardiac part of the stomach everted in order to enfold the prey, 
often of relatively large size. In those Echinoidea that possess a 
lantern of Aristotle there are very powerful and efficient move- 
ments of mastication. On the whole, as might be expected from 
the comparatively highly developed muscular and nervous systems, 

1 One species of Synapta is said to inhabit brackish water. 


Ix PHYLUM ECHINODERMATA 435 


the co-ordination of movement is very much more complete in the 
Echinodermata than in the groups already dealt with. 

A remarkable characteristic of the Echinoderms is the faculty 
of self-mutilation which many of them possess, together with 
the capacity for replacing parts lost in this way or by acei- 
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 reraoved. 

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 cesophagus, 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 (Paleozoic) 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 Comatule 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 


FF 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 Ccelentcrata, 
which were grouped with the Echinodermata under the comprehen - 
sive class-designation of Radiata (see section on the History of 
Zoology). But, leaving out of account the presence of a bilateral 
symmetry underlying and partly concealed by the radial, we are led 
by a study of the anatomy of the various systems of organs to the 
conclusion that the 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 celome 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 Ccelenterates, 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, 2.2. non-colonial, animals, and each of them is developed, 
save in certain very exceptional cases, as a result of a sexual 
process from an impregnated ovum. In spite, then, of the radial 
symmetry, we are forced to the conclusion that the Echinodermata 
are not more nearly related to the Coelenterata than to some of 
the groups of Worms. They are, in fact, a singularly isolated 
group, and we Jook 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 ot zooids 
connected together centrally, each ray being a zooid equivalent 
to an entire simple worm-like animal. But the history of the 
development is entirely at variance with such a view. 

Whatever may have been the group of animals from which the 
Echinodermata were developed, there is every probability that it 
was a group with bilateral and not radial symmetry. The radial 
symmetry is evidently, as has already been pointed out, of a 
secondary character ; it is only assumed at a comparatively late 
period of development, and even in the adult condition it does 
not completely disguise a more primitive bilateral arrangement of 
the parts. Accordingly, within the phylum itself, it is reasonable 


IX PHYLUM ECHINODERMATA 437 


to regard those classes as the more ancient which have the radial 
symmetry less completely developed. Again, the free condition 
which characterises all existing Kchinoderms 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 diplewrula larva (p. 432); 2.¢., 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). 


Echinoidea 


Holothuroidea Asteroidea OPhiuroidea 


Crinoidea 


Blastoidea : 
Cystoidea 


Primitive Cystoid 


Dipleurula 


Fic. 346,.—Diagram to illustrate the relationships of the classes of the Echinodermata, 


438 ZOOLOGY SECT. IX 


According to another view, however, the most primitive of 
existing Echinoderms are Synapta and its allies (Holothuroidea 
apoda). The other Holothuroids are supposed, according to this 
conception of the relationships of the various classes, to have been 
derived from a Synapta-like ancestor. From the primitive stock 
of the Holothuroids is supposed to have been derived a form 
which gave origin to all the stalked classes. From this ancestral 
stalked Echinoderm, again, the remainder of the free classes—the 
Echinoidea, Asteroidea and Ophiuroidea—are regarded as having 
been descended. 

Possible relationships between the Echinodermata and the 
Chordata will be referred to in the discussion of the affinities of 
the latter phylum. 


SECTION X 
PHYLUM ANNULATA 


THE phylum Annulata comprises four classes of Worms—the 
Chetopoda or Earthwormsand marine Annelids, the Archi-Annelida, 
the Gephyrea, and the Hirudinea 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, cesophageal 
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 cwlomoducts, for the 
passage outwards of the reproductive elements. 


CLASS I—CHATOPODA. 


The Chetopoda, 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 parapodi«, 
which do duty as limbs, bearing bundles of se/a (chet) 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 1.c. opposite the middle of the 
segments, or inter-segmental 2c. opposite the intervals between 

439 


440 ZOOLOGY SECT. 


the segments. There is a well-developed blood-vascular system 
in the majority of the Cheetopoda, and organs of respiration in 
the shape of gills or branchiz 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 trcchophore (cf. p. 322). 


]. EXAMPLES OF THE CLASS. 
a. Nereis dumerilti 


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 elemeuts. 
In N. dumerilit 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 serles of segments or metameres, 
which are about eighty in number 
altogether ; and each of these bears 
laterally a pair of movable mus- 
cular processes called the parapodia, 
ar ee provided with bundles of bristles or 

phase, (After Claparéde.) sete (chele). The head (Fig. 350) 

consists of two parts, the pros- 
tomium (prast) and the peristomiwin (perist). The former bears 


renee 


sone 


rannnees 


1 Though Nereis 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. 


x PHYLUM 


ANNULATA 441 


on its dorsal surface four large rounded eyes, in front a pair of 
short cylindrical tentacles (tent), and further back a pair of some- 


what longer stout appendages 
or palpr (palp). The peri- 
stomium, Which has some 
resemblance to the seg- 
ments of the body, though 
wanting the parapodia, bears 
laterally four pairs of long, 
slender, cylindrical tentacles 
(porist. 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 


dors.cirr 


ac / sebienen noto 


vent.cirr 


Fic. 348, a.—Nereis dumerilii. A single para- 
podium, magnified: ae. aciculum; dors. ciz7. 
dorsal cirrus: neuro. neuropodium 3 noto. noto- 
podium ; vert. cirr. ventral cirrus, (After 
Claparéde.) 


throughout the length of the worm. Each bears laterally a pair 
of parapodia, which in the living animal are usually in active 


Vic, 348, B.—Nereis du- 


merilii. Sete highly mag- 


nified. 


(After Claparede.) 


movement, aiding in creeping, or acting as 
a series of oars for propelling it through 
the water. When one of the parapodia 
(Fig. 848, A) is examined more attentively 
it is found to be biramous, or to consist of 
two distinct divisions—a dorsal, which is 
termed the notopodiwm (noto), and a ventral, 
which is called the newropodium (neuro). 
Each of these is further subdivided into 
several lobes, and each bears a bundle of 
sete. Each of the bundles of setze is lodged 
in a sac formed by invagination of the epi- 
dermis—the settgerous saec—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 sete, a stouter, straight dark- 
coloured seta (ac.), the pointed apex of 
which projects only a short distance on the 
surface; this is termed the actculam. The 
ordinary sete (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 1s 


442 ZOOLOGY SECT. 


short and slightly hooked. On the dorsal side of the parapodium 
is a short cylindrical, tentacle-like appendage, the dorsal cirrus 
(Fig. 344, a, dors, cirr), and a similar, somewhat shorter appendage, 
the ventral cirrus (vent. cirr), is situated on its ventral side. The 
last segment of the body, the anal segment, bears posteriorly a small 
rounded aperture, the anus ; this segment is devoid of parapodia, 
but bears a pair of appendages, the anal cari, 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 celome, body-cavity, or peri- 
visceral cavity—filled with a fluid, the celomice fluid, containing 
ameeboid corpuscles. The walls of the ccelome (Fig. 351) are 
lined with a thin membrane, the peritonewm or calumac epithelium, 
of which the outer Jayer—that lining the body-wall—is the 
parietal layer (par. pert), that covering the outer surface of the 
alimentary canal the splanchnic or visceral layer (vise. pert). The 
space is divided bya series of transverse partitions or sepla passing 
inwards fron: the body-wall to the wall of the alimentary canal 
opposite the grooves between the segments, and thns dividing the 
cwelome into a series of chambers, each of which corresponds to one 
of the segments. ‘Phese 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 
cdge decply serrated (Fig. 849, £). 

Behind the pharynx the alimentary canal narrows considerably 
to form a tube, the esophagus (es), which runs through about five 
segments tu open into the intestine. 


x PHYLUM ANNULATA 443 


Running backwards and inwards from the wall of the peristomium 
to the wall of the buccal cavity and pharynx are a number of 
bands or sheets of muscle, the protractor muscles, by the contraction 
of which, and the pressure of the ccelomic fluid, this anterior part 
of the alimentary canal can be everted so as to form a proboscis 
(Fig. 349), and thus the jaws are thrust forth and rendered 
capable of being brought to bear on some small living animal or 
fragment of animal matter, to be seized and swallowed as food. 
The eversion is arrested at a certain point by means of a muscular 
diaphragm passing from the wall of the buccal cavity to that of 
the first body-segment. The proboscis is withdrawn again by a 
retractor sheet of muscle, which passes inwards and forwards to be 


Fic, 349.—Nereis diversicolor, x 4. Head wit buceal“region everted. A, dorsal view ; 
B, ventral view. a, prostomium ; B, everted buccyl region ; ¢, ec’, peristomial tentacles, 1, 2, 3, 4 3 


d, denticles; e, eyes; 4, lower lip; P, palp in A, entrance to pharynx in B; 7, jaw : 
7, prostomial tentacle ; /, peristomiun ; //, 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 cesophagus. : 

Into the esophagus open a pair of large unbranched glandular 
pouches, or eceea (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 coelomic epithelium (vise. pert) ; (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 stomodewm, the rectum the pructodaum, the rest of the alimen- 
tary canal the mesenteron. 

The wall of the body consists of a cuticle, an epidermis or 


444 ZOOLOGY 


SECT. 


deric epithelium, muscular layers, and the parictal layer of the 


frat 8 Bgip -7>* 


7 
@ 2 


3 yy past led 

==> = ¥ Vey, aoa 
a0) ie 
le a 


VENE. VES S — 


TL, CO— 


Fic. 350.—Nereis dumerilii. Semi-diagrammatic 
view of the anterior portion of the body with the 
dorsal body-wall removed, su as to show the ali- 
mentary canal, the septa, the blood-vessels, and the 
nephridia ; a portion of the intestine removed so as 
to show the ventral blood-vessel and nerve-cord 
which lie below. dors. vess, dorsal vessel ; gl. ceso- 
phageal glands ; int. beginning of intestine ; ne. co. 
uerve-cord ; reph. nephridia ; a's, esophagus ; palp, 
palp ; para, parapodia ; perist, peristome ; perist. tent. 
peristomial tentacles ; ph. pharnyx with its jaws ; 
prest, prostomium ; tert, prostomial tentacles ; vent. 
vers, ventral vessel. 


ccelomic epithelium (par. 
pert. The cuticle (cut) 
is a thin chitinous layer 
which exhibits an im- 
descent lustre due to the 
presence of two intersect- 
ing systems of fine lines; 
it 1s perforated by numer- 
ous minute openings, the 
openings of the epidermal 
glands. The epidermis 
(ep) 1s very thin, except 
on the ventral surface, 
where it becomes consider- 
ably thickened. It consists 
of a layer of cells con- 
taining numerous twisted. 
unicellular glands, which 
are most abundant on the 
ventral surface, particu- 
larly near the bases of the 
parapodia; on the dorsal 
surface the epidermis 
contains plexuses of fine 
blood-vessels. The mus- 
cular layers are two in 
number—an external, in 
which the fibres run cir- 
cularly (etre. mus), and an 
internal, in which they 
run longitudinally. The 
latter is not a continuous 
layer, but consists of four 
bundles of fibres, two 
dorso-lateral (dors. long. 
mus) and two ventro- 
lateral (vent. long. mus). 
Nereis has a_ well-de- 
veloped system of vessels 
filled with blood of a 
bright red colour. A 
main dorsal vessel (Figs. 
350 and 351, dors. vess) 
runs from one end of the 


body to the other above the alimentary canal, and is visible in 
places through the body-wall in the living animal. It, as well as 


ec 


x PHYLUM ANNULATA 445 


the majority of the vessels, undergoes contractions which are of 
a peristaltic character—waves of contraction passing along the 
wall of the vessel so as to cause the movement of the contained 
blood. These peristaltic contractions are more powerful in the 
case of the dorsal vessel than in that of any of the others, and 
run with great regularity from behind forwards, so as to drive a 
current of blood m that direction. The contractions are brought 
about partly by a series of muscular fibres which are arranged in 


ders long mus cul § / dors.vess © 
ge FF dlors.long. mits 


eh =4 
BAT WE Nia iiss 
1 A nies Ke 
ie 
ra Sa aye) 
ibas 


ov i 
iA TA 
be\ 4, " briff v ip " 7] Tl 
WM SS TA ee 

Loss =Y LS itt BS 


vent. vess 70.CO  CUC-TMUS 4 \, GB 


Fic. 351.—-Nereis dumerilii. Semi-diagrammatic transverse section of the middle region of 
the body. circ. mus. (external), circular layer of muscle of body-wall 3 ear. ius. (internal), 
circnlar layer of muscle of wall of enteric canal ; co’. cceelome ; ews, cuticle ; dors. long. mus. 
dorsal longitudinal muscles of body-wall ; dors. vess. dorsal vessel ; ent. ep. enteric epithelium ; 
ep. epidermis ; long. mus. longitudinal muscle of wall of enteric canul; ne. co. nerve-cord ; 
neph. nephridium ; neur. set. neuropodial setz and aciculum with their muscles ; not. set. 
notopodial setz and aciculum ; obl, aus. oblique muscle ; ov. ovary ; par. peri. parietal layer 
of ceelomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent. vess. ventral vessel ; 
vise. peri. visceral layer of coelomic epithelium. (The entire extent of the celomic 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 
neural vessel accompanies the nerve-cord. 

Nereis 1s devoid of any branchie ; 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 (lig. 352), which is 
bilateral and metameric in its arrangement, like the other systems 


, 
e\\, ¢ 
eC 


Fia, 352.—Nereis.—Anterior portion of nervous system, comprising the brain, the cesophageal 
connectives, and the anterior part of the ventral nerve-cord. (After Quatrefages.) 


ot organs. Situated in the prostomium is a large bilobed mass 
of nerve-matter containing numerous nerve-cells, the cerebral 
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 esophageal connectives (d), curve 
round the mouth in the peristomium to meet on the ventral 
aspect behind the mouth and below the pharynx. The cesopha- 
geal connectives with the cerebral ganglion thus form a ring 
around the anterior part of the enteric canal. From them are 


x PHYLUM ANNULATA 447 


given off nerves to the two anterior pairs of peristomial tentacles. 

Running backwards from the point of union of the cesophageal 
connectives along the entire length of the body of the worm, on 
the ventral aspect, is a thick cord of nerve-matter, the sieniea 
nerve-cord (kt). 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 stometogastric or visceral 
system, distributed to the anterior part of the alimentary canal. 


Fira. 353.—Nereis.—Section through one of the eyes. co, cornea; cu. cuticle ; 1. lens; 7. layer 
of rods; +e. retina. (After Andrews.) 


The first ganglion of the ventral cord, which is situated in the 
third segment, represents at least two double ganglia which have 
coalesced, as is shown By the fact that it gives off nerves to the 


two posterior pairs of 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 orguns, all situated on the prostomium. 
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 dens (/.) 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 


7 


448 ZOOLOGY SECT. 


pigmented; the inner part projects towards the lens as a clear 
hyaline rod (v). 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 ftnction. 

The organs which are supposed to perform the function of 
excretion are a series of metamerically arranged pairs of tubes, 
the segmental organs or nephridia 
(Figs. 350 and 351, neph, Fig. 
354) occurring in all segments 
of the body with the exception 
of several at the anterior and 
posterior ends. The nephridium 
consists of two parts—a_ body 
and a narrow anterior prolonga- 
tion. The body is of an irregular 
oval shape directed nearly trans- 
versely, but slanting somewhat ; 
the outer end, situated in the 
base of the parapodium near its 
middle, is much the narrower ; 
the inner end is continuous with 
a narrow prolongation about 
equal in length to the body, 
which runs forwards and in- 
wards to become attached to the 
mesentery. The external open- 
ing or nephridiopore (eat. op) is 
a fine circular pore capable of 
being widened or contracted, 
situated on the ventral surface 
Fic. 354.—Wereis dumerilii. One of the not far from the b ase of the 

nephridia. ert. op. external opening or ventral cirrus. This leads into 

nophrostome opening inte the eaiome; & canal, ciliated except in its 
mes. transverse mesentery or septum. 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 ccelome 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 


x PHYLUM ANNULATA 449 


of narrow ciliated processes not represented in the figure. 
Throughout its course the canal is excavated in a mass of nucleated 
material of a granular character not distinguishable into cells. 

On the dorsal side of each segment, in close relation to the longi- 
tudinal muscular bundle, is a specially developed ciliated tract of 
the ceelomic 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, cvaries or testes, which 
are developed towards the breeding season by a proliferation of 
the cells of the membrane (coelomic epithelium) lining the ecelome ~ 
and the structures it contains. In Nereis dwmerilii there is in the ° 
male only a single pair of these proliferating masses of cells (testes), | 
situated in one of the segments between the nineteenth and the 
twenty-fifth. But in other species of Nereis they are much more 
numerous. These, during the season of their active development, 
give off groups of cells which become disseminated throughout the 
ccelomic 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 ¢az/. 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 cceelomic 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 sete in the two bundles, 
vary; so also do the relative length of the tentacles, the 
number of teeth on the jaws, and the number and arrangement 
of the denticles in the buccal cavity. Not only are such individual 


VOL. I GG 


450 ZOOLOGY SECT. 


differences common, but the species occurs in tivo distinct forms 
or phases, which differ from one another so widely that they have 
been referred to distinct genera. One of these is the Nerets phase, 
which is that described in the preceding paragraphs. A Nereis 
dumerilii may become sexually mature in this form, or may first 
undergo a serics of changes by which it becomes converted into 
the second or Heteronereis phase (Fig. 346, B). The principal 
changes which take place during this inetamorphosis 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 sete 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 sete. After 
a time the Heteronereis, like the Nereis, becomes sexually mature, 
developing ova and sperms, the latter of which differ remarkably 
in shape from those of the Nereis phase. 

Development.—The egg of Nereis when first discharged is 
enclosed in a transparent thick gelatinous envelope, within which 
are two membranes—an outer very thin and delicate, and an inner 
(zona 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 from 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 amceboid 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 toa 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 megamere that gave origin to the first. 

The germinal layers are now all established. The micromeres 
constitute the ectoderm, destined to give rise to the epidermis and 
all its derivatives, to the cerebral ganglion and nerve-cord, to the 
cesophagus 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 


Xx PHYLUM ANNULATA 451 


the entire mesoderm of the Annelid. As the micromeres multiply 
by division, they form at first a cap of small cells over the upper 
pole of the embryo; eventually the cap extends so as completely 
to cover the four megameres and the descendants of the somato- 
blasts except at one point, the b/astopore, 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 


Fia. 355.—Nereis. Early stages in the development. 4, lateral view of eight-celled stage ; 
B, the same from alove ; C, stage of the formation of the first somatoblast ; D, stage at which 
both somatoblasts are present; macro. megameres ; micro. micromeres; som. 1, som. 2. first 
and second somatoblasts. (After Westinghausen.) 


the middle of the head-end projects a tuft of cilia (Fig. 356, A, 
ap. cil.). Encireling 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 stomodewm (st) or rudiment of the mouth and 
cesophagus. The anus (az) does not appear until later; the position 
which it will subsequently occupy is indicated at this stage by a 
pigmented area (pig. ur) 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 
Ga 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 (xcwr. pl); the deeper cells 
divide and extend to forma pair of mesoderm bands or musele- 
plates, from which the muscles of the body-wall are developed ; the 
muscular layers of the wall of the alimentary canal are derived 
from certain of the same set of cells which migrate inwards 
from the lower end. 

A pair of micromeres separated from the rest at an early stage 
are destined to form the larval excretory organs, the head-kidneys or 
larval nephridia: at first situated at the upper end, they 
sink below the surface and migrate downwards till they come 
to lie below the prototroch ; each then elongates, and a number 
of vacuoles which have become formed in the interior coalesce 
in such a way as to form a long, narrow canal. The embryo has 
now reached the completed trochophore stage. 

The endoderm cells become arranged so as to bound a canal- 
like space, the beginning of the lumen of the middle part of the 
alimentary canal (cesophagus and intestine, aé.), the cells subse- 
quently giving rise to the enteric epithelium. This canal becomes 
continuous in front with the stomodseum, and behind with a 
second smaller ectodermal invagination, the proctodwum, which 
arises in the position of the former pigment-area. The part of 
the larva behind the prototroch now elongates, and two pairs of 
invaginations, the setigerous sacs (set. sacs), appear at its sides: in 
the interior of these, to which a third pair is soon added, are 
developed sete which grow out toa great relative length as the 
larval or provisional sete. 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 celome. 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 


Jebod 


sens. h 


Fic. 356.—Nereis. Later stagesin thedevelopment. A, stage at which the prototroch and the 
apical tuft of cilia first become distinct. B, somewhat later stage, in which the stomodzal 
invagination is being formed, and the rudiments of the mesoderm bands are distinct ; C, late 
trochophore stage in which there are rudiments of the setigerous sacs; D, somewhat later 
stage, in which the parapodia have begun to become prominent and the provisional sete 
project freely ; £, larva with three segments. an. anus; ap. cil. apical cilia; ap. pl. apical 
plate ; eve, eye; fr. bod. frontal bodies; ivt. intestine; /. mus. longitudinal muscle ; mes. 
mesoderm ; mo. mouth ; newr. pl. neural plate ; para. parapodia ; pig. ar. pigmented area ; 
prot. prototroch ; sens, i. sensory hairs ; set. sacs. setigerous sacs: som. second somatoblast and 
group of cells formed from it; s¢. stomodxum ; tent, peristomial tentacles. (After E. B. 
Wilson.) 

453 


454 ZOOLOGY i SECT. 


prostomium of the adult. The part immediately behind forms 
the peristomium, which bears setz, 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 (#) until, on the development of the tentacles, the 
outgrowth of the parapodia (para) with their cirri and the 
permanent sete (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. 


bd. THe Earraworm (Lumbricus). 


General External Features.—The Earthworm (Fig. 357) 
has a long narrow body, which may be described as approximately 


Fic, 357.-Lumbricus herculeus. A, entire specimen, latcral 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 u pair of sctw. (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 


x PHYLUM ANNULATA 455 


slight flattening ; the anterior end is distinguishable in the living 
anmnal as that which is directed forwards in the ordinary creeping 
movements of the worm. The surface, as in the case of Nereis, 
is very distinctly marked out into segments or metameres by a 
series of ring-like constrictions ; the segments, which are very 
numerous—amounting to about 150, are somewhat longer towards 
the anterior end than they are further back. 

At the extreme anterior end is a rounded lobe, the prostomium., 
immediately behind and below which is the opening of the mouth. 
Next to the prostomium is the most anterior segment, the pert- 
stomiwm, 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 sete (Fig. 358), a pair of each row occur- 
ring in each segment, which thus possesses 
eight altogether. The extremities of all these 
sete are directed backwards, and they act as 
fulera for the forward movements of the worm 
on the surface of the ground or in the interior 
of its burrow. The sete in the clitellum, and 
those in the neighbourhood of the genital 
apertures, are much slenderer than the rest. |...” pumbricus 
Along the middle line of the dorsal surface, ‘Sob, bighly amenihed, 
from about the eleventh segment backwards, 
is a row of small apertures, one at the line of division between each 
contiguous pair of segments: these, which are termed the dorsal 
pores, perforate the body-wall and open internally into the coelome. 
Through these ccelomic fluid is capable of being discharged, 
covering the surface with a thin layer which may protect the 
worm from desiccation or from contact with irritating sub- 
stances. On the ventral surface are two rows of minute 
apertures—a pair on each segment—the excretory apertures or 
nephridiopores. On the ventral surface of the fifteenth segment 
(Fig. 357, 15), is a pair of slit-lke apertures with somewhat 
tumid lips, the male reproductive apertures ; and on the segment 
immediately in front—the fourteenth, are two smaller rounded 
apertures, the female reproductive apertures. In the intervals 
between the ninth and tenth, and tenth and eleventh segments 
are two pairs of small pores, the openings of the receplacula 
sements. 


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 eeloniic epithelium. The cuticle (cut.) 
is similar to that of Nereis, and has a similar iridescent lustre ; 16 
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.7 


CAATTERA ALANS 4 
eel f ie 


CA RACAAL 
bie 


SUBILVCES 


Fic. 359,—Lumbricus, transverse section of the middle region of the body. ere. mus. layer of 
circular muscular fibres ; cut. ewlome ; cut. cuticle; dors. v. dorsal vessel ; epid. epidermis ; 
ext. neph. nephridiopore ; hep. layer of chloragen cells; /ong. mus. longitudinal muscle ; 
neph. nephridium ; nephrost. nephrostome; 2. co. nerve-cord 3 svt. setw; sub. n. vess. sub- 
neural vessel ; typh. typhlosole ; vent. v. ventral vessel. (After Marshall and Hurst.) 


and are connected by fine nerve-fibres with the nerve-cord. On 
the clitellum the epidermis is thickened, and blood-vessels extend 
between the cells. Below the epidermis is a layer of connective- 
tissue, the dermis. The muscular fibres which make up the 
greater part of the thickness of the body-wall are arranged in two 
principal sets—a layer of circularly arranged fibres (cire. mus) 
situated externally, immediately below the dermis and a layer of 
longitudinally ~ arranged fibres (dony. mus) situated internally. 
The circular layer is interrupted at all the intervals between 
the segments; the longitudinal layer is interrupted along 


x PHYLUM ANNULATA 457 


a series of longitudinal lines, so as to be divided into seven 
bundles. 

The sete (Fig. 358) are lodged in sacs, the setiyerous saes (sce 
Fig. 369), lined by a 
continuation of the epi- 
dermis. In the region 
of the body in which the 
reproductive organs are 
lodged some of these sacs 
are enlarged and glan- 
dular, and receive the 
special name of capsulo- 
genous glands, 

The enteric canal 
(Fig. 360) is, as in 
Nereis, a tube which 
runs through the entire 
length of the body from 
the mouth at the an- 
terior to the anus at the 
posterior end. As in the 
case of Nereis, it lies in a 
cavity, the cwlome, lined 
by a thin cellular mem- 
brane, the peritoneum or 
celomic epithelium, and 
filled with a fluid, the 
celomic fluid, contain- 
ing colourless corpuscles. 
The ccelome is divided 
into a series of chambers 
corresponding to the seg- 
ments by a series of 
delicate transverse parti- 
tions, the septa or mesen- 
teries, consisting of folds 
of the peritoneal mem- 
brane enclosing muscular 
fibres. 

The mouth leads into 
a small buccal cavity. 
This is followed by 
a much larger, thick- 
walled, rounded chamber, the pharynx (ph.). From the wall 
of the pharynx there run outwards to the body-wall a number 
of radially arranged bundles of muscular fibres which, when they 
contract, draw the pharynx backwards, and at the same, time 


3 m.c. nerve-cord ; neph. neph: 


e anterior halfof the animal. br.brain ; cr. crop + 
(After Marshall and Hurst.) 


middle seminal vesicle 
pouch ; ov. ovary; pk. pharynx; 7.0. receptaculum ovorum ; 


n through th 
lateral vesiculz seminales, 


‘ 
% 
8 
5 
S 


Longitudinal vertical sectio 


es. gl. aperture of a 


d: 
deferens ; ves. sem. posterior 


. gizzar 


seminal funnel; gi 
od, oviduct ; &s. cesophagus ; 


te, anterior testis; 7. d. vas 


re 


Fic. 360.—Lumbricus hereuleus. 


458 ZOOLOGY SECT. 


dilate it. Behind the pharynx follows a comparatively narrow 
tube, the esophagus (ws), which extends through about seven 
segments. At the sides of the wsophagus, in each of the segments 
ten, eleven, and twelve, is a pair of rounded projections. The 
first pair—the esophageal ponches—are hollow, and their cavities 
are in communication with the lumen of the cesophagus (ws. gl). 
The other two pairs—the caleiferows glands—are thickenings of the 
wall of the cesophagus, the fluid in the interior of which is milky, 
owing to its containing numerous particles of carbonate of lime ; 
the numerous small cavities which they contain are in communi- 
cation with the cesophageal pouches. Posteriorly the cesophagus 
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 g7zzard (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, ze. 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 chloragen cells (hep): these are specially 
abundant in the typhlosole. The terminal part, situated in the 
last segment, is termed the rectum. 

The whole alimentary canal is lined internally by a cuticle—which 
is thicker in the gizzard than elsewhere, and by a single layer 
of ciliated columnar epithelial cells, the enteric epithelium. Some 
of these cells, more granular than the others, grouped in certain 
regions—more particularly along the typhlosole, are of the nature 
of unicellular digestive glands, secreting a digestive fluid. Others 
seem to be specially concerned in the absorption of the digested 
food. External to this is a layer of connective-tissue, between 
which and the external covering of yellow cells are muscular 
fibres, of which there are two layers, an external longitudinal and 
an internal circular. These layers are greatly thickened in the 
walls of the pharynx and of the gizzard. 

The Earthworm, like Nereis, has a well-developed vascular 
system, consisting of blood-vessels with well-defined walls. The 
blood is bright red, the colour being due to the same colouring 
matter, viz. haemoglobin, as in the case of the blood of the higher 
animals, occurring, however, not in corpuscles, but in the liquid 
part or plasma; corpuscles are present, but they are colourless. 
The main trunks are the dorscl, the ventral, the sub-neurel, the 
two lateral nenral, and a series of transverse branches. The dorsal 
vessrl (Fig. 359, dors. v) runs along the middle of the dorsal surface 
between the body-wall and the intestine ; it is readily visible shining 
through the former in the living worm. The ventral vessel (vent. v) 


x PHYLUM ANNULATA 459 


hes below the alimentary canal, the swb-newral below this again 
under the nerve-cord; the lateral neural lie on cither side of the 
nerve-cord. The transverse brunches 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 ventrul nerve-cord 
together with a pair of @so- 
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. ne co 

In this way a complete me7ve- Fic. 361.—Lumbricus. Anterior portion of 

ring or nerve-collar surrounds nervous system. cer. gung. cerebral ganglion 
é ‘ or brain ; com. cesophageal connectives ; ne. co. 

the anterior part of the enteric ventral nerve-cord ; prost. prostomium. (After 


canal in the third segment. Beat) 


From this the ventral nerve- 

cord extends backwards to the posterior end of the body, and in 
each segment it presents a slight enlargement or ganglion, as it is 
usually termed, most conspicuous in the more posterior segments. 
The whole cord is double, consisting of two intimately united 
right and left parts. From the brain, nerves are given off to 
the prostomium; and from the ventral cord three pairs of nerves 
arise in each segment. From the cesophageal connectives a 
series of stomatogastric nerves pass to the pharynx and neighbouring 
parts of the alimentary canal. 


460 ZOOLOGY SECT. 

The Earthworm is devoid of organs of sight or hearing. It 
exhibits sensitiveness to bright light, the sensitive elements being 
large cells of the epidermis devoid of pigment. The sense of 


coe 


ext 


erro 


ee eget ede CNbaE 


payee aed 


eee 


sarc h 


cut 


WES 


Fic. 362.—Nephridium of Kumbricus (diagrammatic).—¢. ampulla between ciliated and non- 
ciliated parts of the intracellular canal; c:/. ciliated part of the intracellular canal; coe. 
investnient derived from the ceelomic epithelium ; es. nephridiopore ; /.c. non-ciliated part 
of the intracellular canal ; mes. septum; vst. nephrostome ; t.v. intercellular canal of the 
terminal vesicle. /.—W//. 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 


N PHYLUM ANNULATA 461 


narrow epidermal cells, most abundant on the prostomium and 
peristomium, 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 nephrostome 
(nst), opening into the cavity of the segment in front 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 lied 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, fe, 
te’), partly divided into a number of digitate lobes, situated in the 
tenth and eleventh segments. A pair of comparatively large sacs, 
the anterior vesiculee seminales (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 vesicule seminales (mid. ves. sem), 
situated in the eleventh segment, also open into the anterior 
sperm-reservoir. A third pair (post. ves, senv), situated in the 
twelfth segment, unite in front to form the posterior sperm-reservoir 
(post. sp. ves), 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 (/wn) 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 reeeptacule 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. ¢) 
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 receptacle ovorum are a pair of reniform sacs which open into 


Ail ves. SET ane. Le 
ri taut 
na mn al | TA 
if th il i i 
i ' ut i (i ul 
TEC iit mT 
i dit 
z lini 
A i 
MEAT 
ae 
mud. ves ser} Hh WAY 
post.sp.res| a ni Ly 


Post ves Sem 


Fic, 363.—Lumbricus herculeus. Reproductive organs. ane. sp. res. anterior sperm reser- 
voir ; ant. ves. sem. anterior left vesicula seminalis ; fun. funnel-like openings of vasa efferentia; 
dat. intermuscular partitions ; mid. ves. sem. middle vesicula seininalis ; 2. co. nerve-cord ; 0 
ovaries; ov. d. oviducts; post. sp. res. posterior sperm-reservoir ; post. ves. sem. posterior 
vesicula seminalis ; rec. receptacula seminis; f, anterior, and te’, posterior testes; v. eff. 
anterior, and 7. ef’. posterior vas efferens ; v. vef. vasa deferentia. (After Vogt aud Jung.) 


the funnel-shaped ends of the oviducts. The reecptacula seminis 
(ver) are two pairs of rounded sacs which open on the exterior 
in the intervals between the ninth and tenth, and tenth and 
eleventh segments. 

Though hermaphrodite, the Earthworm is not self-impregnating, 
but two individuals provide for mutual fertilisation by an act of 
copulation. The copulating individuals apply themselves together 
by their ventral surfaces, the heads pointing in opposite directions, 
and become attached in this position by the sete of the genital 
region and by a viscid secretion from the clitellum and _ of 


x PHYLUM ANNULATA 463 


the eapsulogenous glands (p. 457), situated in the neighbourhood 
of the reproductive organs. The sperms from the male apertures 
of each pass along temporarily formed grooves to the receptaeula 
seminis of the other. 

When the ova are mature they are discharged from the ovary 
into the cavity of the thirteenth segment, whence they pass out 
to the exterior through the oviducts, to be enclosed in the cocoon 


EX 


OMS 


LG} 
So. 


4 
o) 


Tteé7r° 
Tic. 364.—Early stages in the development of Lumbricus. 4, lateral view of flattened blastula ; 
B. ventral view of gastrula with slit-like blastopore ; C, lateral view of latcr stage. blastoc. 
blastoceele ; blastop. blastopore ; ert. ectoderm ; end. endoderm ; m. primary mesoderm cell ; 
mes. mesoderm bands ; ner. cell from which the primitive nerve-cord (ne. co.) takes origin ; 
nph. cells taking part in the forination of the nephridia ; st. stomodeeum. (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 wall 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 SHOT, 


eocoon 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 scgmentation- 
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 mrsoderm 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 vitelle 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 similar 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 anumber of rows each originating be- 
hind in a larger rounded cell or ¢eloblast. The innermost of these 
rows (Fig. 364, C, ner, ne. co) give rise to the ventral nerve-cord. 
The next two rows (ph) are said by some observers to give rise 
to the nephridia all but the funnels: but according to others the 
nephridia, or at least all their inner glandular portions, are of 
mesodermal derivation. The brain and cesophageal connectives are 
formed in continuity with the rudiments of the ventral nerve-cord. 

On the whole the development resembles that of Nereis, the 
chief differences being such as may be traced to the non-occurrence 
in the Earthworm of any free-swimming trochophore stage, and 
the absence of such larval structures as the large pre-oral lobe, 
the apical plate, the prototroch, and the larval nephridia or head- 
kidneys. 


2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 


The Cheetopoda 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 ecelomoducts 
or through modified or unmodified nephridia. 


x PHYLUM ANNULATA 465 


Sub-Class I.—POLYCH ATA. 


Chetopoda 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 
sete. There is usually a definite head with eyes and tentacles, 
and often cirri and branchize on the segments of the body. A 
clitellum is never developed. A metamorphosis takes place: the 
larva is a trochophore. Nearly all the Polycheta are marine. 


ORDER 1—ARCHI-CHEHTOPODA. 


Aberrant or primitive Polychzeta! in which the nervous system 
is not separated from the epidermis, and the ventral cord is not 
segmented into ganglia. Only one genus (Succocirrus). 


ORDER 2.—PHANEROCEPHALA. 


Polycheta with protrusible pharynx usually armed with chitinous 
Jaws. There is a well-developed head. The segments are 
completeiy or nearly similar throughout the length of the body, 
and the parapodia are usually equally devcloped throughout and 
provided with cir. The branchiz, when present, are not confined 
to the anterior end. 


ORDER 3.—CRYPTOCEPHALA. 


Polycheta 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 sete, 
into two or even three regions. The parapodia are liftle prominent 
in the posterior parts, and usually without cirri. The branchiz, 
when present, are usually confined to the anterior end, and are 
sometimes represented by modified cephalic palpi. 


Sub-Class II.—_ OLIGOCH ATA. 


Cheetopoda 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 setze on cach segment The 
head is not distinct. A clitellum is usually present. ‘here is no 
metamorphosis. Mostly terrestrial or fresh-water forms. 

I The Arehi-Chietopoda ave usually classed with the Polychreta, but their 
alliances are perhaps quite as close with the Oligocheta. In some respects 


Naccocirrus resembles Polygordius and Protodrilus (4rhei-Annelidu gv.) 
but is distinguished from them by the possession of setiw. 


VOL, [ HEL 


466 ZOOLOGY SECT. 


OrpdeER 1.-—MICRODRILI. 


Small Oligocheta 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 
sete. 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 Oligocheta 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 setw. 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 Hxamples. 


Nereis dumerilit 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 sete, and the 
like. The genus Nereis differs from the other genera of the 
family Nereidew, 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 Nereide 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 sete 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 sete, and dorsal and ventral cirri. The buccal 


x PHYLUM ANNULATA 467 


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 Lwmbricide :— 

The prostomium is dovetailed completely into the peristomium. 
The seta are always in couples. There are longer and straighter 
sete on the clitellum. The male apertures are always on the 
fifteenth segment. There are three pairs of vesicule seminales, 
in the ninth, eleventh, and twelfth segments, connected across the 
middle line in the tenth and eleventh by sacs enclosing the 
ciliated funnels. There are two pairs of receptacula seminis al- 
ways situated in the ninth and tenth segments. 

The family Lumbricid is distinguished from the other families 
of the sub-order Megadrilt, 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 1s incomplete ventrally. 
Dorsal pores are present. ‘The sete 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 vesicule 
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 
Cheetopoda is cylindrical, but in many, e7., 
some members of the families Polynoide 
(Fig. 365) and Amphinomida, 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 Fic. 365.—Polynde seto- 
sissima. Dorsal vicw 


by a number of more or less distinct of entire animal, -with 
is tot] : 7 j the pharynx protruded. 
annular constrictions or impressed lines (ith Ouetnoees) 


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 


HH 2 


468 ZOOLOGY SECT. 


usually very similar throughout the length of the body ; but in 
the Cryptocephala (Figs. 366, 367, 3873) there may be two or even 
more regions distinguishable from one another by the form of 
the segments and of their appendages. In the Oligochieta there is 
a thickened zone, the elctel/um, comprising sometimes only one 
segment, sometimes a number. Each segment, with certain 
exceptions to be noted presently, bears either a pair of parapodia 
vr merely a greater or smaller number of seta. Parapodia are 
lateral hollow processes of the body-wall bearing a number of 


Fic, 366.--A Serpulid (Vermilia coespitosi. Lateral view of animal removed from its tube. 
ubd. abdomen ; by, branchie ; op. operculum ; th. thorax. 


bristles or set. 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 newropodium. 
Even when this is not the case there may be two bundles of sete 
representing the the two parts. The seta: are nearly always 
chitinous; in Buphrosyne they are calcified. They are always 
solid, except in Huphrosyn, entire, or divided into a number of 
joints. In shape (Fig. 368) they vary greatly in different groups ; 
often several very distinct forms uf setw are present in different 


x 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 set are quite short, projecting 
little beyond the parapodia, and are hook-like or comb-like. 
Usually each bundle contains, in addition to the ordinary set, 
a stouter, straight, simple seta, which scarcely projects on the sur- 
face ; this is termed the aciewlwm. Each seta, or each bundle of 
setae, is lodged in a sac, the setigerous 
sae (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 
setigcrous sacs are usually provided 
with protractor and retractor muscles, 
by the action of which the sete may 
be thrust out or retracted. 

In addition to the sete the para- 
podium bears very commonly certain 
soft appendages of a sensory character, 
the cirri (Fig. 347, dors. cirr., vent. 
eirv.). 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 


Fic. 367.-Chetopterus. Natural 
size of a young specimen. A, an- 


are laterally compressed and leaf-like. 
In Polynée (Figs. 365 and 870) and 
its allies certain of the parapodia 
bear, instead of dorsal cirri, flattened 


terior region of the body ; B, middle 
region; C, hinder region. c, peri- 
stomial cirri; d, “sucker” ; e, the 
great ‘‘ wings”; f, the first of the 
three ‘‘fans” ; m,- mouth, (From 
Benham, after Pancevi.) 


scales, the elytra (el.), richly supplied 

with nerves: these are sometimes looked upon as modified dorsal 
cirri, but in some members of the group cirri and elytra occur 
together on the same segment. 

In Sternaspis a ventral shield formed by a thickening of the 
cuticle in the posterior region of the body bears a number of setze 
round its edge. 

In the Oligocheta (Fig. 372) the parapodia are absent as pro- 
cesses of the body-wall, and are merely represented by a small 
number of short sete each lodged in its sac; cirrl are not 
developed. In certain Oligocheta set are absent. 

The first segment or prostomiwm, together with the second or 


470 ZOOLOGY SECT. 


peristomium, forms in many Polycheta a very distinct head; the 
prostomitwn in such a case bears eyes and tentacles and contains 


Fic. 308.—Setve of various Polycheeta. (From Claparéde.) 


the cerebral ganglion ; on the peristomium is the opening of the 
mouth, and from it also arise the peristomial tentacles. A 


Fic 369.—Section of the setigerous sac of an Oligochwete. 6), setigerous sac ; bo, supplementary 
follicle with seta; e, deric epithelium (epidermis); 7m, longitudinal muscles of body-wall ; 
m, m, muscles of the setigerous sac; 7.m, circular muscular layer of body-wall. (From 
Hatschek, after Vejdovsky.) 


ventral pair of prostomial tentacles, somewhat thicker than the 
rest, are sometimes to be distinguished, and are termed the palpi. 


x 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 Oligocheta 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). 


Fic. 370.—Polynoe extenuata. Dorsal view of anterior extremity. dors, cirr, dorsal cirri ; 
el. elytra; perist. tent, peristomial tentacles ; prest, prostomium. (After Claparéde.) 


The last segment is termed the anal segment, owing to its 
bearing the anal opening ; it usually also differs from the preceding 
segments in wanting the parapodia and in having a pair of special 
cirri, the anal cirri. 

Branchiz are borne on the dorsal surfaces of more or fewer 
of the segments in many of the Polycheta. 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- 
cheeta living habitually in tubes (Figs. 366 and 373). In the 


472 ZOOLOGY SECT. 


Terebellidw (Fig. 873) the branchix are situated on the dorsal sur- 
faces of some of the anterior segments. In the Serpulida (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 


Fic. 371.—Heads of various Polycheta (diagrammatic). A, Polynoid ; B, Syllid; C, Nephthys : 
D, Eunice; BE, Phyllodoce; F, Trophonia, a, prostomium ; c, cirri of body segments; c!, 
peristomial cirri (tentacles) ; ¢?, cirrus of first body-segment ; ¢?, cirrus of sccond body-seg- 
ment; ¢’, point of attachment of elytron; p, palp; s, nuchal organ; t, tentacle; /, peri- 
stomium : //, //, JV, segments. (From the Cambridge Natural History.) 


shape the branchiz 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 branchie 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 
ceelome. 


x PHYLUM ANNULATA 


In the Oligochzta branchia 
are rarely present ; but in certain 
of the Naudomorpha there are 
metamerically arranged simple 
or branched branchiz, 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 
Chetopoda. 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 papille. 

The muscular part of the 
body-wall consists of two layers, 
in the outer of which the fibres 
are disposed circularly, while in 


<j 
Uw 


(After D’Udekem.) 


Fig. 372.—Tubifex. General view of entire animal. 


474 ZOOLOGY SECT. 


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 Polycheta, two dorso-lateral and two ventro-lateral. 
In the Oligoch#ta it is divided by the setigerous sacs which pass 
through it. 

The peritoneal or ecelomie epithelium consists of a single layer of 
cells. These are usually non-ciliated, but are ciated in the 


Fic. 373.—Terebella. (After Quatrefages.) 


Aphroditea, Glycera, and some others, the movement of the cilia 
bringing about an active circulation of the calomie or perivisceral 
fluid in the ecelome. 

The body-cavity or celome, a wide space intervening between 
the wall of the body on the one hand and that of the enteric 
canal on the other, 1s divided in many Cheetopoda by a series of 
transverse septa into a series of chambers corresponding to the 


x PHYLUM ANNULATA 475 


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 

tak of double folds of the peritoneum enclosing muscular 
res. 

The enteric canal is an 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 stomodewm. 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, 365); around its extremity, when 
it is fully protruded, are to be seen a circlet 
of papillee 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 
esophagus, 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 Aphroditea, 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 cca which are to a 
greater or less extent branched at their PA iio Ruteric canal of 


oe ‘ : Aphrodite. a, mouth; 
extremities. In the Hesionida and Syliida b, pharynx ;_ ¢, branching 


5 " s A czeca of intestine ;d, anus. 
a pair of ceca which open into the anterior — (From Gegenbaur’s Com- 
part of the intestine frequently contain gas, 9 ”"("* Antony) 
and probably have a hydrostatic function. 
In some of the terrestrial Oligocheta (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 
Chloremide, 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 Chetopoda 
its cells enlarged and granular to form the so-called chloragen 
cells, which probably have an excretory function. The enteric 
epithelium is very generally ciliated ; 1t contains numerous gland- 


476 ZOOLOGY SECT. 


cells, The stomodaum and the proctodum are lined internally 
by a cuticular Jayer, 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, 2 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 Cheetopoda are entirely devoid of blood-vessels. In one 
family in which this occurs (the Glyceridir 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 hemoglobin (see p. 86). In the 
majority of the Chietopoda 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 hemoglobin, 
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 prescnec of a green colouring matter, which 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 vesselis 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 ettected in most 
instances by peristaltic contractions of the dorsal vessel or of a 
clrumintestinal 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 tio 
intimately united ganglia. It is almost invariably situated in the 
prostomium, though placed a little further back in the Earth- 
worms ; 1t gives off branches to the eyes and tentacles. From it 
there run backwards and downwards the paired wsophageal con- 
nectives, which embrace the anterior part of the alimentary canal 
between them, and below join the anterior end of the ventral chain 


x PHYLUM ANNULATA 477 


of gangha. The latter 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 (Serpi/ 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 cesophageal connectives, or with both, 
there is a system of delicate stomatogastric nerves passing to the 
walls of the anterior part of the alimentary canal. In the majority 


dors. vEess 


Le Tee,» 


ea Bre! 


We.CO 


Fic. 375.—Saccocirrus, transverse section, to show the position of the nerve-cords. dors. cess. 
dorsal vessel ; iat. intestine ; ne. co. nerve-cord ; set. setee. (After Fraipout.) 


of the Cheetopoda 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-Chutopoda (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 papille. 

The organs of special sense are eyes, tentacles and cirri, nuchal 
organs, and otocysts. Eyes, absent in the Oligocheta with a few 
exceptions and in some of the tube-forming Polycheta 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 Polyophthalmus, in 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 Sabella and all the species of Dasychone 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, z.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 (8) 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 ens,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 Polychaeta 
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 Polycheta. 
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 


Xe PHYLUM ANNULATA 479 


of ciliated cells, in the Huid contained in which there are one or 
several calcareous otoliths. 

Ciliated grooves occur on the prostomium of many forms; in 
Avicia 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 cium, are very common, especially on the prostomium in the 
Oligocheta and on the tentacles and cirri in the Polycheta. 
Groups of these are often aggregated together in papille or 
goblet-bodies, with special nerve-supply and often with a ganglion 
ur a single nerve-cell at the base. 

The organs of excretion of the Chetopoda are a series of 
segmentally arranged tubes, the wcephridia, 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 nephrostome, which opens into the ccelome 
either of the same segment as that on which the external aperture 
is situated (most Polycheta) or of the segment in front (all 
or most Oligocheta, some Polycheta). The nephridia thus in 
such cases effect a communi- 
cation between the ccelome 
and the exterior, and serve 
to carry off waste-products 
which have passed into the 
celomic fluid; but in many 
instances the cells lining the 
tube are active in separating 
out such waste-matters, and 
are loaded with granules and 
coneretions. ies ool aaa 

In many Polycheta, how- 
ever, there is no. ciliated 
ceelomic aperture, the tube 
ending blindly internally, 
such a blindly ending ne- 
phridium (Fig. 376) being Fic, 376.—Iuner branched end of nephridium ot 
frequently branched. Onthe  — FAYMOPTREERNStytee “Catter Goodiuch,) 
inner extremities in such 
cases, or on other parts of the tube, are situated a number of 
peculiarly modified cells, the solenocytes, sometimes separate, some- 
times united together in groups. Each of these 1s a rounded 
cell lying in the ccelome, 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 


480 ZOOLOGY: SECT. 


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 Polycheta another set of segmentally repeated structures 
are frequently intimately connected with the nephridia. These 
are a series of pairs of ciliated funnels, the calomoducts, opening 
widely into the cwlome, and, in a typical case, communicating with 


we) Ag 
sd WIV 


Fic, 877.—Diagram to illustrate the various combinations of closed wid open nephridia and 
celomoducts in the Polychwta. 
Iv, Hypothetical stage with closed nephridia and scparate cozlomoducts ; b, condition in 
which the cvwlomoducts have becume united with the nephridia: this occurs in Phyl/o- 
docidee and Goniadide ; c, condition in which the ca@lomoduct becomes reduced to a ciliated 
organ (Nephthyidw) ; Iu, combination of nephridia with nephrostomes and separate coelomo- 
ducts (Dasybraachus) ; 6, condition in which ‘segmental organs” are formed by the union 
of nephridia with nephrostoines and ceelomoducts (the most usual condition) ; ¢, condition in 
which there are nephridia with nephrostemes, and the eelomoducts are reduced to ciliated 
organs (Nerecs, etc.). The nephridia are outlined with a thick lime: the ccelomoducts 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 ecelomoducts 
act as the efferent ducts for the sexual clements. In many of the 
Polycheeta 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 Polychwta 


x PHYLUM ANNULATA 481 


(Serpula 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 Oligocheta 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 ccelomoducts which become modified to give rise to the 
reproductive ducts ; but ordinary nephridia may be present in these 
segments as well. In some Oligocheta 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 antipe), 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 Chatopod 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 
(prostomium) of many larval Oligochzta and Polychrta. They are 
ciliated intracellular tubes, sometimes branched, which do 
not open into the cavity of the prostomium. Sometimes solenu- 
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, Chatopterus, &c.). 

In the arrangement of the reproductive organs in the 
Chetopoda there is an essential difference between the two sub- 
classes, the Oligocheata being hermaphrodite, and the Polychwta, 
with only a very few exceptions, unisexual. In the latter the 
gonads, ovaries or testesas 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 
vecupy in the interior of the segment varies in different cases: 


VOL, I fT 


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 ccelomic 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 


verl.vess 


B repr gl 
pera 


VENL.VESS 


repr. gl 


peril YT ofa\e. aim _— 


eal vess wenl.vess 


Fic. 378.—Diagram to illustrate the development of a gonad from the peritoneal (ccelomic) 
epithelium in one of the Polychaeta. peri. peritoneal membrane ; repr. gl. gonad (repro- 
ductive organ) ; vent. vess. ventral vessel, (After E. Meyer.) 


through temporary or permanent openings in the body-wall. Im- 
pregnation takes place externally in nearly all. 

In the Oligocheta 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 vesicule 
seminales to undergo development into mature sperms. The 
vesicule 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 vesicule, 
may be present. In the same segments as the testes, and 
opening into the sperm-sacs when the latter are developed, are 


x PHYLUM ANNULATA 483 


either two or four cilzated 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 Oligochets is a gland known as the prostate or 
spermiducal gland. Near the aperture of the vas deferens in 
many Earthworms are special sete, the penial sete. 

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 im each group ripen into an ovum. A receptaculwm ovorum 
occasionally receives the ova after they leave the ovary. There 
are two oviducts, which open by funnel-shaped apertures into the 
ccelome. 

Development.—The Oligocheta deposit the eggs in cocvons, 
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 (Zwbifex, &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 asin the Polycheta. 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 Oligocheta, 
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 Chetopoda, with a very few exceptions in 
which development occurs in the ccelome or in the interior 
of a dilated segmental organ. In the Polycheta, 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 ther development. 


112 


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 Pulynée and allied genera, the fertilised ova and the 


¥iu, 379.—Spirorbis levis, a hermaphrodite tubicolous Polychat. Lateral view of entire 
anim. «t. neph. anterior nephridium ; br. branchi ; es. @sophagus ; op. operculum with 
developing cibryos in its interior ; ov. ova; sy. sperms ; st. stomach. (After Claparéde.) 


resulting embryos adhere in masscs to the dorsal surface under the 
shelter of the elytra; in some other Polycheta they are stuck by 
means of some viscid secretion all over the dorsal surface, or 
they may adhere singly to the ventral cirm. In certain Crypto- 


x PHYLUM ANNUTLATA 485 


cephala (Fig. 879) they develop in a cavity in the operculum; in 
others, in the interior of the tube, between the bedy 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 ege-membrane. 

The segmentation of the ovum in the Polycheta 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 themsclves in 


Fic. 380.—A, B, C, three stages in the development of the trochophore of BEupomatus, from 
the side. an. anus ; sh. blastoccele ; i. polar cells of the mesoderm 3 wi, mid-gut; 2. larval 
head-nephridium ; of. otolith ; sp. neural plate ; sf. stomodeeum ; wk, preoral ciliated ring ; 
why, post-oral ciliated ring. (From Lang’s Comparative Anatomy, after Hatschek.) 


such a way as to form the wall of a hollow sphere, the dlastala, 
with an internal closed cavity, the segmentation-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 embolie gastrula. In the great majority of forms 
however, an epibolic gastrula is tormed 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 proctodseum 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 stomodwum. The embryo then passes into the 
trochophore stage. 

The arrangement of the cilia on the surface of the trochophore 
varies in different Polycheta. Sometimes, though rarely, the pre- 
oral circlet is absent and the surface is covered uniformly with 
cilia: such larvee 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 (éelotrochal \arvee). In some cases, instead of a pre-oral circlet 
there is one further back round the middle of the body (meso- 
trochal), 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 sete 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 cesophageal 
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 sete, 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 Chetopoda both among the Ohgocheta 
and the Polychta. Simple fission occurs in Salmacina, one of 
the Serpulids: a constriction becomes formed ata certain point 
towards the posterior end, rudiments of a new set of cephalic 
branchiz 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 ofalternation of generations, 
as both parent and offspring are similar and sexual (hermaphro- 
dite). In Nais and Chetogaster (Oligochzeta) there is multiplica- 


x PHYLUM ANNULATA 487 


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 Syllide 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 sete, 
and other points; and in some instances the 
two sexes not only differ from the asexual — yyq, 391,—Buading in one 
parent form but also from one another, so at therSy Miche (ailorg tus 


that the three forms, before their relationship with atm oid neeny 

was known, were set down as representing ea STAs: Mooasts 

three distinct genera. ; 
Syllis ramosa (Fig 382) which occurs in the interior of certain 


deep-sea sponges, is exceptional among the Chetopoda in giving 


Fic. 382.—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 SECT, 


Modes of Life, etc.—Very few Chetopoda 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 
veyctable 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-water Oligocheeta (Zubificide) 
manutacture 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 - lke, 
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 
Pia, 383,—Serpule with their tubes. composed entirely of foreign par- 

(After Quatrefages. ) 0 
ticles cemented together; very fre- 
quently they are permanently fixed 
to foreign objects. Some, such as species of Polydora and Sty- 
lartoides, near relatives of which construct tubes, excavate galleries 
in rock or coral or in the shells of various Mollusca. 

A few Polycheta, such as the Aletopide and Tomopteris, as well 
as, in a certain phase, the Nereida and Syllide, ave 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 
geolocical formations, though there are many burrows and_ tracks 


x PHYLUM ANNULATA 489 


which have been ascribed to members of that class. Tubes of 
tubiculous Polychaeta have, however, been found in formations 
dating from the Cambrian period onwards. Some tubes not 
distinguishable from those of the existing genus Syirurbis, 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 Polycheta. The horny jaws of various Polycheta 
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 Oligochieta are known. 


APPENDIX TO THE CHATOPOD A. 
CLASS MYZOSTOMIDA. 


The Myzostomida are a group of worms which appear to have 
their nearest relatives in the Chietopoda, 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 MMyzostoma 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. 385, ph.) capable of being protruded as a 
proboscis; from this a narrow cesophagus leads to the stomach, 
which gives off a number of branched lateral diverticula (da.) A 
short cloaca (J:lo.) 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 Platyhelminthes. 

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 SECT. 


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 
cesophagus, the other round the pharynx, the two rings being 
connected together by a series of longitudinal nerves. The 


Fic, 884.-Myzostoma. I-X, cirri; m. mouth; p. parapodia ; s. suckers. (After von Graff.) 


esophageal ring presents a very obscure dorsal thickening, 
which is the only representative of a cerebral ganglion. 

Most of the Myzostomida are hermaphrodite. There isa pair of 
ovaries formed by the proliferation of the layer of (ccelomic) 
epithelium covering the stomach; and in the sexually mature 
animal branching (ccelomic) spaces in the parenchyma, between 
the ceeca, are found to be filled with ova (0). A posterior continua- 
tion of these spaces (w) opens either into the cloaca or independently 
of it. There are two elongated and usually branched testes (1), 
each of which has two vasa deferentia leading to a vesicula 
seminalis (sb) which opens near the lateral margins, The sexes 


x PHYLUM ANNULATA 491 


are united in most cases, 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 Myzostomida closely resembles that. of 
the Polychwta. A trochophore larva is first formed, and this 


Fic. 385.-Myzostoma. Diagrammatic view of the internal organs. c, cirri ; da, branches of 
the stomach ; ed, hind-gut: h, testes; k/o, aperture of cloaca; im, stomach ; mo, male genital 
aperture; 0, ovaries; , parapodia, with hooks and supporting rod; ph, pharynx ; php, 
pharyngeal tentacles ; pkt, pharyngeal pouch ; sb, vesicula seminalis ; v, uterus ; wo, female 
genital aperture. (From Lang's Comparatire Anatomy, after von Graff.) 


becomes metamorphosed into a larva with provisional scti» bearing 
a close resemblance to that of Nereis (p. 451). 


CLASS II._GEPHYREA. 


The Gephyrea are marine Annulata devoid of any trace of segmen- 
tation in the adult condition, without parapodia, and either without 
setze 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 SECT. 


the mouth situated at the base. The anus is sometimes terminal 
and posterior, sometimes anterior and dorsal. There is an exten- 
sive celome 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 testes simple masses 
of cells; the nephridia act as reproductive ducts. The larva is 
a trochophore. 


1. EXAMPLE OF THE CLass—Sipunculus nudus. 


General External Features.—Sipunciulus 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 papilla. The papille 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 
introvert is fully evaginated, 
there appears at its extremity 
a horseshoe-shaped fold of the 
integument, the tentacular fold 
(/ent.), which is lobed and 
plaited (Fig. 387) so as to as- 
sume somewhat the appearance 
of a cirelet of tentacles. For 
Fic. 386,—Anterior extremity of Sipun- a little space immediately be- 

culus nudus. if, uy, anterior papil- A 

lary region ; post, pap, posterior papillary hind the tentacular fold the 

region ; fenf. teutacular fold, (After Ward.) surface of the introvert is free 

from papille. The posterior 
portion of the body is devoid of papillz, but is marked out by a 
series of narrow impressed lines into a number of elongated four- 
sided areas. 


X PHYLUM ANNULATA £93 


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 papillae 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 
ot dermal bodies. Of these there 
are three kinds—bicellular giunds, — ¥ic. 387.—Tentacular fold of Sipunculus 
contained in papillee ; multi- neces cer, org. cerebral organ, (After 
cellular glands, scattered through 
the integument and not contained in papille; and sense-papille, 
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 circulur 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 ceelome, 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 celomic fluid are 
(1) colourless corpuscles; (2) reproductive elements ; (3) peculiar 
ciliated bodies, the wrns, 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 e/reuler sinus 
at the base of the tentacular fold. 

The alimentary canal (Fig. 388) 1s a cylindrical tube of uniform 
character throughout. It is twice the length of the body, running 


494, ZOOLOGY SECT. 


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. 
Connected with the rectum is a nar- 
row cecwm 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) 
differs considerably from that of 


rq 

Fic. 888.—Dissection of the internal organs Fie, 38,—Anterior part of the nervous system 
of Sipunculus nudus. dors. vetr. of Sipunculus nudus. can. o. ceb. cere- 
dorsal retractor muscles of the intvro- bral organ ; coms. a, cesophagal connective ; 
vert; int. intestine ; i. 2. co. muscles nn, vet. nerves to retractor muscles ; n. spl, 
accompanying the nerve-cord; 7. ¢o. splanchnic nerves ; n. ta. 1-4, nerves to ten- 
nerve-cord; mneph. nephridium; «s. tacular fold ; J, I/, nerves from ventral cord ; 
cesophagus ; rect. rectum; tent. tenta- 24, main mass of brain. (After Ward.) 


cular fold. (After Vogt and Jung.) 


x PHYLUM ANNULATA 495 


the rest of the Annulata. There is a relatively small bilobed 
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 celome. The wsophageal connectives 
(coms. @) which it gives off behind are greatly elongated ; from each 
arise muscular nerves (n. mu. ret), and also a visceral nerve (n. spl) 
passing 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 
(1., II); on section it appears distinctly double. Two delicate 
muscular bands (Fig. 388, m. . 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. yes 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 
coelome. 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 floating about in the ccelomic fluid. The nephridia act 
as gonoducts. 


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. Sete are absent. Nephridia a single 
pair, or absent altogether. 


ORDER 2.—ARMATA (ECHIUROIDEA). 


Gephyrea with an elongated prostomial proboscis. The anus 
1s posterior. Two or more sete. A single nephridium, or two 
vr three pairs of nephridia. 


Systematic Position of the Example. 


Sipunculus nudus is one of several species of the genus Sipunculus, 
Sipunculus differs from other genera of the family Sipunculida: 
ot which it is a member, mainly in having a tentacular fold around 
the mouth, instead of a series of distinct tentacles. The family 
Sipunculide is one of two families of the order Jnermia ; and differs 
from the other, the Priapulide, in the presence of cither tentacles 
ora 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 1s 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 Chatopoda, 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 papille, or tubercles, or hooks. In the Armata, set 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 Eechiurus 
(Fig. 391), in addition to this ventral pair, there are a number 
arranged in one or two cirelets 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, 


x PHYLUM ANNULATA 497 


tentacles, or by a lobed and plaited tentacular fold. The prostomium 
is in such forms quite rudimentary. In the Armata there is no 
introvert, but an elongated, highly contractile, simple or bifurcated 
proboscis, which is the greatly produced prostomiuin; 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 Privpulus (Fig. 392) there is at the posterior end 
an elongated simple or bifurcated caudal appendage covered with 


% a 4 < 
. POSSE 
Fic. 390,.-Bonellia viridis, cntire Fic, 301.—Eechiurus, cntire animil. 
animal (female) with the proboscis ant, set. anterior scte 5 post. set. pos- 
moderately extended. (After Greef.) terior setie ; prob. proboscis. (After 


Greef.) 


hollow papille. 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 SECT. 


sometimes with oblique and internal circular layers superadded. 
There is an extensive undivided ccelome, lined, as in the case of 
the Cheetopoda, with a ccelomic epithelium, which is sometimes 
ciliated, 

The alimentary canal in the Jnwrmia consists of a muscular 
pharynx, intestine, and rectum; in the Sipunculide (Fig. 888) the 
intestine 1s 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 rectun is a single sumple caecum or a 
pair of cwea; 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 buecal 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 cecum or siphon applied to the 
ventral aspect of the intestine proper. 
Into the rectum there open a pair of re- 
markable cieca, the postertor nephridia 
(Figs. 393 and 395, post. neph.), supposed 
to be exeretory 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 suinetimes present, 
sometimes absent. When present, as it 
is in most Gephyrea, it usually com- 
prises a contractile dorsal vessel closely 
o applied to the intestine, and a peri- 
Mu. 30° —Priapulus, entire pharyngeal ring or plexus. Cilia are 

(After Ela) "PP resent in places in the interior of the 

vessels. 

The nervous system (Figs. 389 and 396) consists of a nerve- 
ving, 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 gangha, 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. 77.) by uniting dorsally. Byes of a very 


Xx 


PHYLUM ANNULATA 499 


simple character, consisting of mere spots of pigment, are present 
in some of the Inermia. 


Priapulus is devoid of nephridia. 


appendages of the 
rectum are, as al- 
ready mentioned, 
to be regarded as 
posterior nephaidia. 
In addition there 
are present = ai- 
terior nephredia. 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 Zhalas- 
sema 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. 


In the Armata a pair of 


Fic. 393,—Bonellia, general view of the internal organs. 
an. anus; wal. neph. anterior nephridium ; int. intestine ; 
ph. Jun. nephrostome ; os. esophagus; ov. ovary; ph. 
pharynx ; post. nepk. posterior nephridium ; prob. proboscis ; 
vent, vess. ventral vessel. (After Greef.) 


They act as 


efferent ducts for the reproductive elements (goneducts); but 


Fic. 394.—One of the ciliated 
funnels of the posterior 
nephridia of Echiurus. 
(After Greef.) 


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 im sitw 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 


kK Kk 2 


500 ZOOLOGY SECT. 


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. 897), which is 
not unlike a Planarian in appearance: it is compressed and covered 
with cilia, with a pair of ventral hook-like sete. In the interior 
of the body bundles of dorso-veutral muscular fibres placed at 
regular intervals give an appearance of rudimentary segmenta- 
tion. The alimentary canal is vestigial and completely closed, 


Fic, 395,—Echiurus, inteinal organisation. an. Fic, 34,—Behiurus, general out- 
anus; ant. neph. anterior nephridia; int. in- line of the animal, with the 
testine; def. cess. intestinal vessel; os. weso- nervous system (diagrammatic). 
phagus ; post. neph. posterior nephridia ; vent. ne. co. uerve-cord ; ne. vi, nerve- 
vess. ventral vessel. (After Greef.) ring. (After Greef.) 


both mouth and anus being absent. There is a pair of 
nephridia placed posteriorly. The sperms, developed from 
modified ccelomie cells, reach the exterior through a duct, dilated 
externally into a vesicula seminalis, and opening internally into 
the cwluine by a funnel-shaped aperture. In Haméingia, 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- 
develuped 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 Chatopod. The posterior part of the 


x PHYLUM ANNULATA 5OL 


body elongates, and the mesoderm-bands, developed as in the 
Cheetopoda, become divided into as many as fifteen segments. A 
circlet of sete is developed at the anal end, and subsequently 
the two ventral setw are formed in the same manner as in 
the Cheetopoda. 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 


Fia, 397.—Male of Bonellia. «li. Fic. 398.—Trochophore of Eehiurus. 
alimentary-canal ; cel. groups of wa. anus; ap. pl. apical plate 5 int. in- 
ceelomic cells destined to give testine ; mo. mouth ; ve. co. rudiment 
rise to sperms ; repr. ap. repro- of nerve-cord; @s. cesophagt s 5 ws. 
ductive aperture ; ves. xe. vesi- conn, cesophageal connective ; stoi. 
cula seminalis. (After Greef.) stomach. (After Hatschck.) 


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 cesophagus are developed as an 
outgrowth, at first solid, from the endoderm. The anus becomcs 
formed still later by invagination at the hinder end of the body ; 
and a pair of epidermal vesicles which appear at 1ts 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 
sete 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 cesophagus, 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 Polychwta, 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 Chzetopoda in the entire absence of sete and in the 
want of segmentation at any stage. Priapulus differs in such 
important points from the rest of the Sipunculoidea that 
it 1s sometimes regarded as constituting a distinct order. 

Affinities between Phoronis (p. 855) 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 


x 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 Cheetopoda, 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 Polyyordiide to which may perhaps be added 


B Pst 


(iT 
AAA HUTA 
Y {| i i) 


\\ 


Fic. 399.—Polygordius neapolitanus. 4, the living animal, dorsal aspect, aout five times 
natural size; B, anterior end, lateral view; ¢, ventral view of the same; D, portion of the 
hody showing the metameres; #, ventral view of the posterior extremity ; dx. anus ; 
An. seg. anal segment; c¢. p. ciliated pit ; gr. grooves between metameres ; Mth. mouth ; 
Mtr. metameres; p. papillae; per.st. peristomium 5 pr.st. prostomium ; s. papille on 
tentacles (t), (From Parker's Biology, after Fraipont.) 


Ctenodrilus. They are marine worms with narrow, elongated, cylindrical body. 
The prostomium (Fig. 399, pr. st) is small, the peristomium (per. st) large. The 
segments (M¢mr) are only faintly marked off externally for the most part, though 


504 ZOOLOGY SECT, 


the internal division of the celome by means of septa is complete. Parapodia 
and sete are absent, but the prostomium bears a pair of tentacles (t). 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 hy mesodermal elements as in 
most of the others. A pair of ciliated grooves (¢. p.) are probably to be looked 
upon as organs of special sense. 

The family Polyyordiae includes two genera—Polyyordius 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 luctens a 
series of tooth-like processes occur round the anus, 
and in front a circlet of adhesive papille. In 
Protodrilus there is a ventral ciliated groove. 
There is a vascular system with dorsal and 


Fic. 401.—Polygordius neapolitanus, transverse 
section of a male specimen. Col, Epthm. parietal layer 
of ceelomic epithelium; Cul. Epthin.’ visceral or 
splanchnic layer of the same; Cu. cuticle; Der. 
Epthm, Aeric epithelium; D. V. dorsal vessel; Lut. 


Fic, 400.—Protodrilus, cn- Hyak. cuterice epithelium; MM. P/ muscle-plates ; 
tire animal. if, intestine ; O. M. oblique muscles; Spv, immature gonads ; 
mus, jw. musenlar append- V, Ne. Cd. veutral nerve cord continuous with deric 
age of wwsophagus ; cs. ceso- epithelium; V. V. ventral vessel. (From Parker's 
phagus, (After Hatschek.) 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, TV. Nr. 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 Polyyordius 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 Polychita in all 
essential respects. 

Clenodrilus resembles Polygordius in the ectodermal position of the nervous 


x PHYLUM ANNULATA a05 


system and in the presence of ciliated pits; but it has a row of comb- 
like setw on each segment, and it has only a single pair of nephridia, which are 


Fie, 402.—Trochophore of Polygordius neapolitanus. 4, lateral view of entire larva ; 
B, diagrammatic vertical section ; C, transverse section through the plane ab in By An. auus ; 
An. ct. anal cilia; B/. blastoceele ; Br. apical plate; Fat. enteron; Msd. mesoderur: AMsil. 
bd. mesodermal bands ; Nph. head-kidney ; Oc. eye-spot ; Pr. ov. ci. cilia of protetroch ; Pre. 
dia, proctodwun 3 Pb. or. cf. post-oral cilia : St. die, stomodeeum 3 . Nv.Cd. ventral nerve-cord, 


(From Parker's Biology, partly after Fraipont.) 


= 
Sy, 
‘Derlpthar 


a 


Der Epthin 


a (Spl. 
eo Spr) 2 


Fic, 403.—Later stage in the development of Polygordius neapolitanus, in which the 
yosterior part of the trochophore has become elongated and segmented ; 4, entire larva ; 
B, vertical section ; ¢, transverse section along the plane a) in B ; DI—D%, three stages in the 
development of the somatic mesoderm: Col. clone : ral. Hpthm. coelomic epithelium ; Der. 
Epthim. deric epithelium ; V.p!. muscle-plate : Msd. (som.), somatic mesoderm; Msd. (spl), 
splanchnic mesoderm; 0. eye; t. tentacle. Othvr letters as in preceding figure. (From 


Parker's Biology, partly after Fraipont.) 


situated in the peristomium, There is only a single (longitudinal) layer of 
muscles in the body-wall; the reproductive apparatus is not known. 


DOG ZOOLOGY SECT. X 


CLASS IV.—HIRUDINEA. 


J. EXAMPLE OF THE CLASS—The Medicinal Leech (Hirudo medici- 
nalis and H. (Limnobdella) australis). 


The medicinal Leech is found in ponds, swamps, and slowly 
flowing streams in many parts of the world. 2H. medicinalis is the 
common British species: A. 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” movements, 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 sucher 
(p. s.), also directed downwards, and at its junction with the trunk, 
on the dorsal surface, is the very small median anus (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 Z. wustralis 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 papille (s.p.), which have probably a sensory func- 
tion. These structures are found along the whole length of the 


& 
XQ 


a 


Xill 


XIV 


SSSSSSSSSSSS 


Beenenanenne 


Fic. 404.—Hirudo medicinalis. A, dorsal ; B, ventral aspect ; a. anus; a@.s anterior sucker ; 
e. 1. first, and e. 5. fifth pair of eyes; g. p. g. male gonopore; g. p. 9. female gonopore ; 
mth. mouth; np. 1. first, and np. 17. seventeenth pair of nephridiopores; p. s. posterior 
sucker ; s.p, segmental papille ; I.—X XVI. segments. 


508 ZOOLOGY SECT. 


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 antenor and 
posterior regions, where the controlling excretory apertures are 
absent. By the clue thus furnished it 1s 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 mctameres: 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 (r.1, e.5), the arrangement of which shows 


Fia. 405.—Hirudo medicinalis ; transverse section. b. t. botryoidal tissue 3 ¢. m. circular 
muscles ; er. erop; ¢r!. diverticula of crop; cv, cuticle; d. ep. epidermis; d. s. dorsal sinus; 
d.v. iv. dorso-ventral muscles; /. mw. longitudinal muscles ; /. v. lateral vessel; ». ¢. nerve- 


cord; nph. 1—4, nephridium ; 2. s. nephrostomial sinus ; 2st. nephrostome ; ts. testis; v. d. 
vas deferens; vs, vesicle of nephridium ; ~. s. ventral sinus. (After Marshall and Hurst.) 
them to be special modifications of sensory papille, since they 
occupy in the first five segments the precise position occupied in 

the sixth and following segments by segmental papille. 

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 Cheetopoda, 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, ew.), 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 


x 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 cceelome. 

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 clited/wim. 

The muscular system is well developed, and consists of an 
outer layer of circular (¢. m.) and an inner of longitudinal (/. m.) 
fibres. There are also dorso-ventral fibres (d. v. m.) passing vertically 
between the pouches of the crop (vide 
infra), and radial fibres extending trom 
the wall of the enteric canal to the in- 
tegument: these take the place of the 
septa of Cheetopods. 

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. 7.), the other two 
ventro-lateral (v. i. 7.). 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 tecth : 


. : Fic. 406.—O: f the jaws of 
by means of its muscles each jaw can be ““Mixvudo medieinalis. 
moved backwards and forwards through CALEY Levick ants) 


a certain arc, and the three, acting to- 
gether, produce the characteristic triradiate bite in the skin of the 
animal upon which the Lecch preys. 

The mouth leads into a muscular pharyna (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 


510 


ZOOLOGY 


3 Ov. S. OV: 


i? ii’, the same 
vesicula semin 


uspect ; a. anus ; 


U. See. 


a of the crop, contracted ; 
v. d. vas deferens ; 


vessel ; 


38 
a 
n 
o 
» 
Q 
oO 
~ 
7 
nN 
6 
a 


SECT. 


salivary glands, which 
vpen 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 
(er.), a thin-walled 
tube extending from 
the eighth to the 
eighteenth segment, 
and produced into 
eleven pairs of lateral 
pouches (cr. 1, cv, 11), 
the first ten of which 
are directed outwards 
and correspond each 
to a segment, while 
the eleventh (er. 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- 


x PHYLUM ANNULATA d11 


goes an immediate change, its 
eolour turning from red to 
green. The digestion of a 
whole cropful of blood takes 
many months. The stomach 
is continued into a narrow in- 
testine (ant.): this passes into a 
somewhat dilated rectum (ret.) 
which turns shghtly upwards 
and opens by the anus (an.) 
in the last annulus, 


rk the metameres, the shorter lines the 
; v. l.j. ventro-lateral jaw. 


The excretory system 
consists of seventeen pairs of 
nephridia (nph. 1-17), situated x 
in segments 7-23. A typical e- 3 
nephridium (Fig. 409) has the x 2 


general form of a loop passing 


the longer vertical lines mz 


upwards from the ventral body- = e 
wall, produced into an_ off- x 
shoot which extends inwards x 
(mesially) to the correspond- = 


ing testis, and connected pos- 
terlorly with a small bladder = 
or vesicle (vs.). The principal 
loop is divisible into two chief 
parts, the main lobe (m. l.) and 
the apical lobe (a. 1.), connected 
with one another by a short 
recurrent lobe (r. 1): the off- 
shoot to the testis 1s known as 
the ¢estis-lobe (¢. 1.) ; it is absent 
in Hl. australis. 

All these parts are formed 
of a cloge-set mass of gland- 
cells, traversed by a complex 
system of minute intra-cellular = 
passages or ductules, which = 
finally unite mto 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 connection with 


ale gonopore ; yg. /. 


matic ; 


XV 


XIV 


Xill 


iil 


v.sem 
Xl 


/ 
Lettering as in Fig. 407, except d. j. dorsal jaw 


annuli. 


408.—Hirudo australis.—Dissection from the left side, semi-diagr: 


Fic. 


512 ZOOLOGY SECT. 


the testis. This lobed body is a modified ciliated funnel: it has a 
great nunber 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 
of 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 hemoglobin 


Fic. 409.—Nephridium of Hirudo medicinalis. «./. apical lobe; m. 1. middle lobe; 
vp. nephridiopore ; ast. ciliated funnel; 7.2. recurrent lobe; 4. /. testis-lobe; vs. vesicle ; 
rs. ¢. 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—dlood- 
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, @. 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. 


x PHYLUM ANNULATA 513 


The two principal sinuses are respectively dorsal (Figs. 405 and 
410, «. 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 gencral 
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 


Fic. 410.—Diagram of principal blood-channels of Leech. . s. dorsal sinus ; J. 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 cesophageal 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-csophageal 
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 LL 


514 ZOOLOGY SECT. 


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 1s 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 (pz.), sur- 
rounding a layer of large, clear, 
refractive cells (p.), which occupy 
the greater part of the organ. A 
nerve (7.) 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 
Fic, 411.—Section of eye of Leech. the segmental sense-organs is un- 


ce, cuticle ; d 3, gland-cells ; ep. epi- 

dermis; g, uerve-cells; 7. nerees known. 

p, refractive cells; pi. pigment. Reproductive Organs. — The 
(From Lang’s Comparative Anat- . : = 
ony.) Leech is moncecious. There are nine 


or ten pairs of testes (Figs. 407 and 
408, ¢s.), 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 
gaculatorius (d. ¢.). 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 1s 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 


x PHYLUM ANNULATA 515 


which are supplied with albumen-secreting gland-cells. The 
common oviduct opens into a curved muscular tube, the vagina (2), 
which opens in the middle line on the ventral surface of the 
fourth annulus of the twelfth segment, ic. 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 seginental : 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 Chaetopods, it will be remembered, the gonads 
he freely in the ccelome, their products—ova or sperms—are dis- 
charged from their external surfaces and carried off either by 
ceelomoducts 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 ccelome, and that their ducts are ccelomoducts. 

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 — Mi, 412.—The cocoon of Hirudo. 
takes place. Segmentation is unequal, oe 
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 
asegment. The anterior end of the body is suctorial, and several 
of the hindmost segments are fused to form a powerful sucking- 
dise, which is directed downwards and backwards. The mouth 

LL2 


516 ZOOLOGY SECT. 


lies in the anterior sucker, the anus is usually dorsal and imme- 
diately in front of the posterior sucker. The ccelome is always 
more or lexs 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 cesophageal 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 
au single pair. The testes have the form of sacs, and discharge 
their products internally : the ovaries cither 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, @.c. 
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 Clepsine (Glossiphonia), parasitic on Snails, 
Frogs, &c.; Piscicola, on fresh-water Fishes; Pontoddella and 
Branchellion, on marine Fishes (Fig. 413). 


ORDER 2.—ARHYNCHOBDELLIDA. 


Hirudinea in which there is no proboscis. 


Sub-order 1.—G@nathobdellida. 


Arhynchobdellida in which the mouth is provided with two, 
or more usually three, toothed jaws. 

This sub-vrder includes Hirudo, the common Leech, parasitic 
on Vertebrata; Aulostoma, the Horse-leech, free-living and 
carnivorous ; Hamadipsa, the Land-Leech. 


Sub-order 2.—Herpobidellida. 


_ Arhynchobdellida in which the mouth is not armed with true 
jaws. 

This group includes Herpobdella (Nephelis) Trocheta, Orobdella, 
etc.—all fresh-water or terrestrial forms. 


x PHYLUM ANNULATA 51 


=] 


Systematic Position of the Example. 


Hirwdo belongs to the family Hirudinide, of the sub-order 
Gnathobdellida. 

The presence of jaws places it in the sub-order Gnathobdellida : 
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 Hirudinide: the genus 
Hirudo is distinguished by the constant presence of 26 segments 
and of 102 annuli. 


2.Clepsine ae 
3.Branchellion 


1.Pontobdella 


Fic. 413.—Three Rhynchobdellida. b. gills ; pr. everted proboscis. (1, after Bourne ; 2and 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 Macrobdella valdivania is said 
to attain a length of 76 cm. (24 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 SECT. 


various species depending largely on colour and on the develop- 
ment of papillse, which in some cases are large and prominent. 
Sete are absent in all except one genus, Acanthobdella, 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 ¢nérovert, 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 gul.) 
leading from the 
| gud pharynx to the crop 
(cr.), and thrown 


alae into a coil when 
ce or the proboscis is 
hemi retracted. Among 


the Gnathobdellida 

Fic. 414.—Proboscis of Clepsine. A, retracted; B, everted ; 7 Seals H < 
er. crop; gui. gullet; wth, mouth; pr, introvert; s. ql. the median Jaw 48 
salivary glanas. (After Bourne.) absent in some 


land-leeches, and in 
other species all three jaws are rudimentary or absent, 

The varying development of the blood-vegsels 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 secn, there are lateral vessels with contractile muscular 
walls and dorsa] and ventral sinuses with non-contractile walls. 
In Pentobdella, one of the Rhynchobdellida, there are dorsal 
(Fig. 415 2, d.v.) and ventral (v.v.) as well as lateral vessels, and 
lateral (Z.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 (z.c.) and the ovaries (0v.), 
and oftshoots of it surround the testes (¢s.) and the nephrostomes 
(nst.). This arrangement clearly suggests the partial obliteration, 


PHYLUM ANNULATA 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 coelomic epithelium. Another 
interesting condition occurs in Nephelis (:?), in which the middle 
region of the body contains a series of paired, metamerically ar- 
ranged spaces (¢.), surrounded by botryoidal tissue and containing 
the nephrostomes. Development shows that these cavities are 
derived from true coelomic spaces in the embryo, formed, as in 
Cheetopoda, by a splitting of 

the mesoderm in each seg- 

ment. -leanthobdella, already 

referred to as exceptional in 

the possession of sete, is also Tar, 2 
the ae member of the class Es 
which has a well-developed 

and spacious ceelome, 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. 418, 3) a 
Rhynchobdellid parasitic on 
the Electric Rays (Torpedo and 
Hypnos) and on one of the Aus- 
tralasian Skates (Raja nasuta), 
differentiated respiratory or- 
gans or gills (br.) 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 Fie. 415.—Ciliated funnel of Herpobdella 


1: : : (Clepsine). cr. crown-cells of funnel; ex. 

those of Hirudo, but differ in terminal ecll of the nephridium ; lec. leuco- 

ey eytes ; st, duct leading to receptacle ; w. wall 

the structure of the cihated of receptacle. (From Meisenheimer, after L. 
funnels, which may be more von Graff.) 


or less modified, as in Hiruclo. 

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 coelomic 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 Hirndo and Herpobdella (Nephelis) the recep- 


Avo ZOOLOGY SECT. 


tacles appear to be the organs in which new blood-corpuscles are 
manufactured. The ciliated funnels of the Hirudinea correspond 
mure closely with the ecelomoducts or ciliated organs of the Poly- 
cheta 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 cach seg- 
ment a short branch terminating in 
a ciliated funnel, and a_ similar 
branch which opens externally (np.). 
A similar modification occurs in 
Branchellion. 
The nervous system always 
Cds a ee tees «= eLOSely Tésembles that of Hinido, 
of nerve-cord ; xp. nephridiopore ; ag also do the sense-organs. ‘The 
nph. nephridial network 3 nst. cili- 5 
ated funnel. (After Bourne.) 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 segmentally. In the Rhynchob- 
dellida the muscular penis is absent, its place being taken by an 
eversible sac or bursa copulatriz. 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 ccelomic spaces, 
probably making its way at last to the ovaries. This extraordinary 
process of hypodermic rmpregnation probably takes place in other 
genera, but has been most closely followed in Clepsine. 

It is in Clepsine that the early stages of development arc 
best known. Segmentation is unequal, the embryo consisting, in 
the cight-celled stage (Fig. 417, A), of four large ventrally placed 
megameres (mg.) and four dorsal micromeres (mz.). One of the 


x PHYLUM ANNULATA p21 


megameres, posterior in position, divides into two cells (B): 
the so-called newroncphroblast 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 subsequently 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. 


Fic, 417.—Six stages in the development of Clepsine. g. b. germinal bands ; mg. megameres 
mi. aicromeres ; 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 
pbarynx 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 Aead-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 
bemg 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, Himopsis vorux, 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, Briachellion oceurs 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 Heemopsis, the name Horse- 
leech is applied, is carnivorous, feeding on snails and other 
Molluscs; so also are Gloss/phonin (Clepsine), Herpobdella (Nephelis) 
and the gigantic Macrobdellu, The last-named genus and some 
others are of subterranean habits, living in moist earth. The 
Land-leeches (4Zwmadipsa) live in the forests of many parts of 


x PHYLUM ANNULATA 523 


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 (Hamadips) 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 metamerie 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 Rhabdoceles that multiply 
by budding, chains of zooids remaining connected together 
fora 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 zovids 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 ina 
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 SECT. 


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. 837) 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 


A B C 


ye | Of +E 
+ + ‘Or +€ 
+ + [Or +@)\ 
J+ + [@+ +@] 
et + St +@ 
+ + et +@ 
+ } (@+ +@/ 
+ 1 D+ +® 
3 
on (8 @ / 
pe Se 


Fic, 418.—Diagram to illustrate possible relations of the unsegmented to the metamerically 
segmented worm. A, unsegmentcd worm with differentiated head end; B, pseudo-meta- 
merism ; C, linear series of zovids in which the first zooid differs in character from the others, 
aud 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 
sete and the reduction of the ceelome, the Hirudinea diverge some- 
what widely from the Chetopoda; but a study of their earlier 
developmental stages shows unmistakably their close connection 
with the latter group, more particularly with the Oligocheta; and 
the existence of an undoubted Leech (Acanthobdella) with sete 
and with a well-developed coalome traversed by mesenteries helps 
still further to bridge over the gap between the two classes, 


x PHYLUM ANNULATA 525 


The following diagram will serve to illustrate this view of the 
relationships of the various groups referred to :— 


Polychaeta 


Oligochaeta 
Myzostomida 


Gephyrea 
Hirudinea 


Archi-Annelida 


Dinophilea Rotifera 


Gastrotricha 


Trochophore 


Fic, 419,—Diagram illustrating the relationships of the Annulata and the Truchelminthes. 


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 AI 
PHYLUM ARTHROPODA 


In this large and important group of animals we meet with a 
characteristic feature of the Cheetopoda, viz. metameric segmenta- 
tion, and also with more or less perfect bilateral symmetry; the 
mouth and anus are at opposite ends of the 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 Polycheta 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 ceelome, but to 
a hemocele,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, Myrtapropa, including the Centipedes and Millipedes. 

Class 4, INsEcrA, 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 


926 


SECT, X PHYLUM ARTHROPODA 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 species of either genus. 

External Characters.—The animal (Fig. 420) is from 20 to 
30 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. f.) 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 (£), 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 dorsal organ 
(d.o.). Passing trans- 
versely across the cara- 
pace, a short distance 
behind the dorsal organ, 
is a shallow furrow, the 


cervical Jold, immedi- Fic, 420.—Apus cancriformis, ee aspect. abd. 
P : abdomen ; a. f. caudal styles; ¢. 0. dorsal organ ; EZ. 
ately posterior to which paired eye ; ¢. medianeyv; sh. gl. shell-gland ; th f. 1, 
a pair of coiled tubes endites of first thoracic foot. (From Bronn’s Z'hier reich.) 


(sh. gl.) are seen, one on 
each side of the carapace: these are the shell-glands or excretory 
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 hea’. The 
segments have the form of chitinous rings, often produced into 


SECT. 


ZOOLOGY 


528 


Soqqepaeur spe SuanyTedey HS aka uvtpaut 


$ qooy [RULULOpAe YWpqe 


LD 


Ssap{qs Tepneo sp 


“qoay opovsoryy ‘TE LIT UP S eyyeusered 


‘9 ‘90 potted sg! fuvdi0 pestop “0 ) 


spaaomet sasvpua' 


dde oyy Jo ysow pue sovdvvo 


Ubd 
‘goedeana ‘do { ponod-poorg 


yo Jyey Uys “wora opis 


SS 


Saquyd peur-qsod «Eourd Seaypexeun (g “rue 67 rue 
dug S yori, 4g | oTUUEZUB Tun 


‘qryaTy snanpidet— ich UT 


XI PHYLUM ARTHROPODA 529 


small spines: each ring slightly overlaps its successor, and is 
connected with it by a narrow area, the wrtieular membrane, the 
chitinisation of which is less pronounced than that of the rings 
themselves. 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 picrced by the terminal 
anus (Fig. 424, an.). 

The ventral surface of the head is formed by a flattened sub- 
Jrontal plate (Fig. 422, s,fpl.), 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 (/br.), which over- 
hangs the mouth. From the sub- 
frontal plate also arise, on each 
side, two delicate processes, the 
innermost called the antennale 
(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 j 
from before backwards: theirnum- Fic. 422.—Apus glacialis, ventral 


EF aspect. abd. jf. abdominal feet; antl. 

ber varles from forty to nearly aieu Rule ; ané?. antenna ; lbr. labrum ; 

md. mandible ; mx. first maxilla; ov. 

seventy (th. f, abd. f.). aperture of oviduct ; s. f. pl. sub-frontal 

Appendages.— The antennule plate ; sh. gl. shell-gland ; th. thoracic 

: : feet ; th. /. 1, first thoracic foot. (After 
(Fig. 423, 7) consists of a bent Bernard.) 


rod bearing delicate chitinous - 
bristles or se¢e 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 wodomeres 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 (3) is also an unjointed appendage. It has the 
form of a deeply concavo-convex plate, strongly chitinised, and pro- 


VOL, I MM 


530 ZOOLOGY SECT 


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 maarlle, The 
first maxilla (4) consists of two curved chitinous plates, the second 
of a basal portion produced into two branches (4). Between the 
first maxilla and the mandibles are a pair of delicate unjointed 


3 EN ES a3 a eng €"2 
1.Antennule sll Pete a 5 aS an toga | 
Ay : ne ee: 
s (") en SOS 
2.Antenna x ee - - 
E 2, NO, 
ce yy, ' 
as ou 
3.Mandible \ 8S ; 
P : 9.11 Thoracic Foot 
lm: cae ms en.2 enw 
4.1% Maxilla i ene : 
are ‘y oy, % end en. 
CA {4 am tft Ma 
tL oe as 
ee ge a br 


: eireoe 
5.2" Maxilla - Bs 
‘a 
a fe 
fl a ; 10.1*'Abdominal Foot 
8.7 Thoracic Foot 


Fia. 423.—Typical appendages of Apus. 1—4, podomeres of axis; br. bract; ea. 1, en. 7, endites ; 
jl. flabellum ; ov, ova. (After Lankester.) 


processes, the paragnatha (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 (¢) 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 wavs formed of four podomeres (7-4), and bearing a number 
of offshoots: six of these, called endites (en. 1—en. 6), arise from 


XI PHYLUM ARTHROPODA DSL 


its inner or mesial border ; two, called cvites (br., f.), from its outer 
or lateral border. The proximal endite (ex. 7) 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 gnathobase. The 
distal endite is rudimentary (en. 6): the remaining four (en. 2-5) 
are long, jointed filaments. The proximal exite is nearly trian- 
gular, and is called the jlabellum (ji.); 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, 
zc, 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 flabellum (j2) 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 oostegopod 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 flabellum 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. cancriformis, 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 


39 


oe 


ZOOLOGY 


SECT. 


fact. that, while comparatively thick and strong in places where 


co 
Ji. 


oh gl 


At 


Beak $s 


digestive 
oviduct ; 


grammiatic) 


» Sagittal section (semi-dia 


Ss 


ovy. ovary ; p.. p. post-anal plate ; ped. s. p 


gland ; d. m. dorsal muscle 


Fic. 424.—Lepidurus kirkii 


no movemcnt 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 sete, 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 
sete of Chetopods, 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 
seginent. 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 lateral regions contract, 
the abdomen will be raised, 
lowered, or turned side- 
ways. In the lmb-bearing 
portion of the abdomen and 


in the thorax there is no longer a continuous muscular tube, but 


XI PHYLUM ARTHROPODA 533 


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 (c.ap.), 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 Pedi/ion (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 (br.), 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 maxille 
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 inéestine (int.) is continued 
back to the terminal anus (az). From each side of the 
stomach is given off a wide tube (d.gl.) which branches exten- 
sively, its ramifications finally ending in delicate ceca. The 
larger branches of these digestive glands contain food in process of 
digestion : their ultimate ceca 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 stomodzeum, the posterior end of the intestine 
from the proctodseum. 

The body-cavity is divided into several parts by membranous 
partitions (Fig. 425): there is a large median cavity in which the 
enteric canal (7) lies, called the intestinal sinus: on each side of this 
are luleral sinuses containing the muscles : and in the dorsal region 1s 
a median cavity, the pericardial sinus. All these spaces are 
devoid of an epithelial lining, and contain blood: as will become 
evident later (ef. p. 593), they “» not correspond with the cazlome 
of the higher worms. 

The central organ of the circulatory system is the /eart (Fig. 
424, ht, and Fig. 425, h), 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 hemoglobin, and contains amceboid corpuscles. 

As already mentioned, the function of respiration is discharged 
by the flabella and bracts of the feet, which are abundantly sup- 
pled with blood and the movements of which ensure a constant 


Fig. 425.—Transverse section of Apus. ci. muscles to fect ; uv, dorso-ventral muscles ; ¢. eggs ; 
dm. dorsal muscles ; g. ovary ; dv. dorso-ventral muscles ; h. heart ; é. intestine ; i. 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 (we.) 
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 (b.) 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 esophageal 
connectives pass backwards and downwards to join the ventral nerve- 
cord, which consists of « 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 


XI PHYLUM ARTHROPODA 535 


diminishes in size, and comes to an end at about the level of the 
last pair of abdominal fect (Fig. 42-4). 

The origin of the nerves given off from the central nervous 
system presents many points of interest. From the fourth ganglion 
of the ventral cord backwards each pair of appendages has its own 
par of ganglia, the metameric correspondence between the limbs 
and the nervous system being complete. The mandibles and the 
first maxille also receive nerves, each from their own pair of 
gangha, their serial homology with the more typical appendages 
being thus confirmed. But the second maxille receive their 
nerves (mn. 2) from the connectives between the third and fourth 
ganghia : the ganglion belonging to their segment may be assumed 


nes 


‘N 
Ni] 


Br: RS 
oe 
MO BEI ee NATE : Cf 
De ERE SSE ASRS ’ Boy hs 
ESSN ING pie 


Fic. 426,—Shell-gland of Apus, diagrammatic. ac. cephalic artery; b. bladder; hk. heart 
m. second maxilla ; ts. end-sac ; uc, urinary tube. (From Bernard.) 


to have atrophied. The antenna is supplied by a nerve (ant. 2) 
which springs from the cesophageal 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 antenne 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 pre-oral. The nerve of 
the antennule (ant. 1) also springs from the cesophageal connec- 
tive, but is traceable forwards to the brain, where it 1s 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 cesophageal connective 
and fused with the brain—a process which actually takes place 


536 ZOOLOGY SECT. 
with the ganglia of the antenns in the higher Crustacea. But it 
is also possible to consider the antennules as pre-oral appendages, 
belonging, like the prostomial tentacles of Chetopods, to the 
prostomial region, and therefore 
recelving 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 sete 
which occur on so many parts of 
the body, and especially as fringes 
to the limbs, are to be considered 
as organs of towch: the only other 
organs of special sense are the 
eyes. The patred 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 


Fia, 427.—Nervous 
cancriformis. 


of Apus 


system 
ant.’ nerve to an- 


tennule ; art.” to antenna; Ur. brain ; 
gn. 1-4, first four ganglia of ventral 


nerve-cord; md. imandibular nerve; 
max. 1, nerve of first maxilla; mx. 2, of 
second maxilla ; @s. con., oesophageal 
connective ; oj., optic nerve; th. f. 1, 
of first thoracic foot; v ga. visceral 
ganglion. (After Lankester and 
Pelsencer.) 


lens and vitreous humour, 


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 


sensory cells, constituting a retinula (re.), and enclosing a re- 
fractive rod, the rhabdome (rh.): the retinula is the actual per- 
cipient part of the ommatidium, its cells being comparable to our 
own rods and cones. The retinule 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 retinule. 

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 


XI PHYLUM ARTHROPODA 537 


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 orary (Fig. 424, ovy.) is a branched 
tube occupying a considerable portion of the body-cavity in 
sexually mature individuals. The walls of the tube are 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 


ce 


me 


Fic. 428.—Diagram of two ommiuatidia from the paired eyes of Apus. cr. vitreous cells ; er. vit- 
reous body ; el. counective-tissue fibre ; hy. epiderm cells ; p. pigment cells ; 7, inner parts 
of ommatidia ; re. retinuke; rh. rhabdome. (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 ccelome, 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 cntrolecithul, te, 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 SECT. 


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 (J-V) and a posterior bilobed 
anal region. The head-region bears a single median eye, and a 
pair of small unjointed appendages (Z), each with two large setze 
at its extremity : these become the antennules of the adult. The 
trunk region bears two pairs of appendages, the first of which (2) 


Fie. 429.—Three stages in the development of Apus. Js. frontal sensory organ ; L, digestive 
gland; s, carapace ; 1—4, cephalic appendages ; I—XILI, body-segments and appendages. 
(From Lang's Comparative Anatomy.) 


is very large and fringed with sete, 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 exopodite. 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 (2), so 
called because it becomes converted into the mandible of the 
adult: it 1s 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 mouth 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 nawpliws.! 

The nauplius swims freely, chiefly by vigorous strokes of 
the great antennary fvet, and after a time undergoes a series of 
moults or cedyses, 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 Chietopods, 
between those previously present and the anal region. The 
antennules have become shifted backwards, and rudiments of a 
fourth pair of appendages, the first maxille (4), have appeared. 
The carapace has grown out from the dorsal region of the head, 
and a peculiar paired sense-organ (/s.) 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 candal styles have 
attained a considerable size. Further moults occur, new seg- 
ments are added with their appendages, the antennules and 
antenne 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 antenne and mandibles are 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 cxopodite 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 fluviatilis). 


Astacus fluviatilis 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 metanauplius : the typical nauplius exhibits no segmentation 
of the trunk region. 


540 ZOOLOGY SECT. 


£ ae 
genera, but agreeing with Astacus in all essential features, are 
found in America,-Australia, and New Zealand. 
External Characters. Te body of the-Crayfish (Fig. 430, 4 
and £8) is divided into two regions—anzanterior, the cephalothorax 


Fic. 4304.—Astacus fluviatilus, side view of male. wu, antennule ; a2, antenna ; ab. abdomen j 
eth. cephalothorax ; Av, gill-cover; 7. rostrum; 3, third maxillipede ; 9, first leg; 10—13, 


remaining legs ; 1, uropod ; XIV, first abdominal segment ; XIX, sixth abdominal segment. 
(From Lang’s Compurative Anutomy.) 


Fic. 480 B.—Transverse section of abdomen of Crayfish. DA, dorsal abdominal artery; EM, 
dorsal muscles uf the abdomen; EP, space between the pleuron and the appendage; FM, 
ventral muscles of the abdomen ; M, muscles of the appendage ; N, endopodite ; NG, nerve- 
ganglion ; P, protupodite ; PL, pleuron; PR, hind-gut; 8, sternum; T, tergum; V, ventral 
abdominal artery ; X,exopodite, (From Parker's Preeticul Zoology, after Marshalland Hurst. 


(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 


XI PHYLUM ARTHROPODA 541 


one another in a vertical plane. The cephalothorax is again divided 
into two regions—an anterior, the ead ; and a posterior, the lhorax 
—by a transverse depression, the cervieal 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 
sides of the thorax, where it forms a flap or gill-cover (kd) 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 ¢erguwm, a narrow ventral region 
or sternum, and downwardly directed lateral processes, the plewra 
—the latter quite unrepresented in Apus. The ¢edson 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 
upon one another in a vertical plane—z.c, the whole abdomen can be 
extended or straightened, and fleved 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 


42 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 epistonea, 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 labrwim attached to the 
middle of its posterior border, The cephalic region of the cara- 
pace is produced im front into a large median spine, the rostrum 
(Fig. 430, 7): immediately below it is a plate from which spring 
two movably articulated cylindrical bodies, the eyc-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 (9-1/) 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 pleopods 
(Fig. 431, 10), the resemblance of which to the biramous limbs 
of the larval Apus is obvious. There is an axis or protupodite 
consisting of a very short proximal (pr. 1) and a long distal 
(pr. 2) podomere, and bearing at its free end two lps plates, 
fringed with setie, the endopodite (en) and eaopodite (cx). These 
appendages act as fins, moving backwards and forwards with a 
regular swing, and probably aiding in the animal’s forward 
movements. 

In the temale 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 (77) are alike in the two sexes: they are very 
large, both endopodite and exopodite having the form of broad flat 
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 wrapods or tail-feet. The telson itself 
bears no appendages. 

The thoracic appendages are very different. The four posterior 
segments bear long slender, jointed /egs (8), upon which the animal 


x1 PHYLUM ARTHROPODA 543 


walks: in front of these is a pair of very large legs terminating 
in huge claws or chel, and hence called chelipeds (Fig. 480, 9). 
The three anterior segments bear much smaller appendages 
more or less leg-like in form, but having their bases toothed to 
serve as jaws: they are distinguished as mvrillipeds or foot-jaws 
(Fig. 431, 5-7). 

The structure of these appendages is best understood by a con- 
sideration of the third mavilliped (7). The main portion of the 


\ F f ‘ 
E 7 
sgt Antennule - a Anronna 


BL 


PS br 
Sais. 
i.Uroped. 
Fig. 431.—Typical appendages of Astacus. ¢». 1—45, podomeres of endopodite ; ep. epipodite ; 


ec, exopodite ; jl. flagella ; g. gill; pr. 1, pr. 2, podomeres of protopodite ; 7—3, podomeres of 
axis of anterinule. (After Huxley.) 


limb is formed of seven podomeres arranged in a single series, 
strongly calcified, 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 (ez), and from the first springs a thin folded 
plate (ep) having a plume-like gill (g) attached to it. Obviously 
such an appendage is biramous, but with one of its branches 


44 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 (¢). The 
folded plate (e) 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 Jegs (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 
chelw : 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. 41), 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 
maxille in relation with the mouth, and in front of that aperture 
a pair of antennules and one of antenne. The hindmost appen- 
dage of the head is the second maazlla (5), a markedly foliaceous 
appendage : its protopodite (pr. Z, pr. 2) is cut up into lobes com- 
parable with the four proximal ‘endites in the thoracic feet of 
Apus: its endopodite (ex) corresponds with the fifth endite, while 
the sixth endite is represented. by the exopodite (ez), modified into 
a boomerang-shaped plate, which, as we shall see, is an important 
accessory organ of respiration. The first mala (4) is a very small 
organ, having neither exopodite nor epipodite. The mandible (2) 
is a large strongly caleitied 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-hke 
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 (/—°) ending 
in two many-jointed flagella (fl. 7. and %), which are sometimes 


XI PHYLUM ARTHROPODA 545 


considered as endopodite and exopodite. But in all the other limbs, 
as we have seen, the exopodite springs from the second segment 


of the axis, and the probabilities are 


that 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- 
tenne, 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, antennse, man- 
dibles, first maxille, and second 
maxillz: the next eight metameres 
(5th-12th) 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 


Fic. 432.—Portion of aleg of Astacus, 
with the exoskeleton partly re- 
moved, showing articulations and 
muscles. «art. m. articular mem- 
brane ; en. 2—5, podomeres of endo- 
podite ; ext. extensor muscles; J. 
flexors ; h. hinge. 


the same 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 articwlar membranes 


VOL, I 


NN 


546 ZOOLOGY SECT. 


(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 (h.), the two joints being at opposite ends of 
a diameter which forms the ais 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. Asa 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 
setze, which also exhibit a wide varia- 
tion in size and form. It is composed 
of a thick laminated chitinous mem- 
brane (Fig. 433, cw.), 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 (¢. ¢.) 
to which the muscles are attached. 

The muscular system, like the 
exoskeleton, shows a great advance in 
Pic. 493. —Verlical section of skin complexity over that of Apus. In the 


and exoskeleton of Lobster. 
e.t, connective-tissue ; cv.cuticle; abdomen (Fig. 434) the muscles are of 


etek Sa eS 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 (¢. m.), 
nearly circular in section, which sends off a slip (e2.) to be inserted 
into each segment above the hinge (h.): the contraction of this 


xI PHYLUM ARTHROPODA 547 


muscle must obviously tend to approximate the terga, and so aid 
the dorsal muscles in extending the abdomen. Around this central 
musele is wrapped, in each segment, a band of muscle (er. m.) in 
the form of a loop, the outer limb of which turns forwards and is 
inserted into a sternum, while the inner limb tums 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 


Fic. 434.—Fourrsegments of abdomen of Crayfish in sagittal section, with muscles (diagram- 
matic). A, extension; B, flexion; art. m., art. m’., articular membranes; ¢. m. central 
youscles; d. m. dorsal muscle; ex. extensor slip of ce1utral muscle; eav. a. enveloping 
muscle ; fl., fl.1, flexor slips ; 2. hinge; st. sternum ; ty. tergura. 


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. 
NN 2 


548 


Fia. 435.—Astacus fluviatilis, 


dissection from the right side. 
aa, antennary artery ; ab. abdo- 
men; «an. anus; b. d. aperture of 
duct of right digestive gland; 
bf. 4, cheliped ; in, ventral nerve- 
cord; cs. anterior division of 
gizzard; cth. — cephalothorax ; 
em, dorsal muscles; jm, ventral 
muscles ;) g, brain; kh. heart ; 
Ad, posterior purt of intestine ; 
tr. left digestive gland; in, 
mid-gut ; 0. ostium of heart ; oa. 
right lateral artery ; oa, dorsal 
abdominal artery; @, gullet ; 
gl. 1—5, pleopods ; pl. 6, uropod ; 
ps, posterior division of gizzard ; 
sa, sternal artery; ¢. testis and 
telson; waa, ventral abdominal 
artery; rd. vas deferens; «/o, 
male genital aperture. (From 
Lang, after Huxley.) 


ZOOLOGY SECT. 

It will be seen that the body- 
muscles of the Crayfish cannot be 
said to form a layer of the body-wall, 
asin Chietopods, the abdomen of Apus, 
&e., 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 (eat.), the larger on the flexor 
(jt.) 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 (oc) 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 museular fibres: within 
this is a single layer of columnar epi- 
thelial cells. In the gullet and gizzard, 


XI , 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 
stomodeum, 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 “ gastrie 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 setw, 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 bya pair of large glands (/7.), 
lying one on each side of the gizzard and anterior end of the 
intestine. They are formed of finger-like sacs or cwca, 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 ccelome. 

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. $38, «), externally by the | 
gill-cover or pleural region of the carapace (kd). Each gill con- 
sists of a stem giving off numerous branchial filaments, so that 


050 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 
rein. 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 


ath, arb, arf, arb, arb, arb, 
pee, p By 


2 A 3 e 
1 A) J Q 1 

arf, arb az azby azh, avbq 

Fic. 436.—Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed and 
the gills undisturbed ; in B the podobranchi are removed and the outer arthrobranchie 
turned down, @, antennule; «., antenna; aj, first; abe, second abdominal segment ; 
arb, 7—12, inner arthrobranchiz ; arb’. 7—12, outer arthobranchie ; ep. 5, scaphognathite ; 
plb, 11—13, pleurobranchize ; j/b. 7—13, podobranchs ; pl. 1, first pleupod ; 6—-13, thoracic 
appendages. (From Lang's Comparative Anatomy, after Huxley.) 


beneath this connective-tissue, hollowed out for the blood 
channels and containing gland-cclls, which will be referred to 
presently (p. 551). ’ 

According to their point of origin, the gills are divisible into 
three sets—first, podobraunchiw or foot-gills, springing from the 
epipodites of the thoracic appendages, from which they are only 


XI PHYLUM ARTHROPODA do 


partially separable ; secondly, arthrobranchie or joint-gills, spring- 
ing from the articular membranes connecting the thoracic 
appendages with the trunk; and thirdly, plewrobranchi«, 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 v for the vestige of a gill. 


TroraActc i’ e 
Sa MNTS I. Il. | III. | IV. Vi, VI. VII. | VIII. TorAaL. 
Podobranchie...;0+ep|1l+ep|l+ep|1l+ep|1+ep| l+ep | l+ep 0 6+7ep 
Arthrobranchie | 0 1 2 2 2 2 2 0 Td. 
Pleurobranchic 0 0 0 0 0 ‘i r 1 14+2r 
ToTAL ...,O+ep) 2-+ep| 3+ep| 34-ep| 3+ep|38+r+ep|3t+r+ep| 1 18+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 sacewle (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 wrinary 
bladder (bl.) which opens by a duct (d.) on the proximal segment of 
the antenna. The glands already referred to as occurring im 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 SECT. 


thorax, and ix a roughly polygonal muscular organ pierced by 
three pairs of apertures or ostia (0.), guarded by valves which open 
inwards. It is enclosed in a spacious pericardial sinus (Fig. 438, 
pe.), 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 


ol 


wp $ 
Fic, 437.—Diagram of kidney of Astacus fluviatilis. I, unravelled ; If, the parts in their 
natural relations. b/. bladder; ¢. p, cortical portion; ¢. duct; s. saccule; w. 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, antenne, green 
glands, &c., and sending off branches to the gizzard; and paired 
heputie urteries, 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 (uac.), which passes 
backwards beneath the nerve- 
cord, and supplies the ventral 
muscles, pleopods, Wc. 

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 dlood- 
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- Fic. 438.—Transverse section of thorax of Cray- 


Pet fish, diagrammatic. abm. ventral abdominal 
domen, and containing the muscles ; bf. leg; bm. ventral nerve cord; «. 
ventral nerve-cord and the intestine ; dom. dorsal muscles of abdomen ; 

ep. wall of thorax ;h. heart ; k. gills ; kd. gill- 
sternal and ventral abdom- cover; l. liver; ov. ovary; pe. pericardial 
: l : I h h sinus ;sa.sz., sternal artery ; vs. ventral sinus. 
inal arteries. n the thorax The arrows show the direction of the blood- 
the sternal sinus sends an current. (From Lang’s Comparative Anatomy.) 


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 (brc.v.), which pass 
dorsally in close contact with the, lateral wall of the thorax 
and open into the pericardial sinus (ped.s.). 
The whole of this system of cavities 1s full of blood, and the 
heart is rhythmically contractile. When it contracts, the blood 
VOL, I NN 2* 


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 


Fic. 439.—Diagram of the circulation in the Crayfish ; heart and arteries scarlet, veins and 
sinuses containing non-aérated blood, blue ; those containing aérated blood, pink. a. artery ; 
afle.e. afferent branchial vein ; br.c.v. branchio-cardiac vein; ef.br.v. efferent branchial 
vein; At. heart ;. ped.s. pericardial sinus; s. sinus ; sts. sternal sinus; vl, ostium with 
valves ; v2. 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 asteries, which carry the blood 
from the heart to the body generally; and (3) a system of return- 
ing channels, some of them, the simwses, 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 
Cheetopod, 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 


XI PHYLUM ARTHROPODA 555 


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. B 

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 
hemocyanin, which becomes blue when com- 
bined with oxygen; it is a respiratory 
pigment, and serves, like hemoglobin, as a 
carrier of oxygen from the external medium 
to the tissues. The hemocyanin 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 cesophageal 
connectives (sc). But the nght 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 not only the eyes and anten- 
nules, but the antenne 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 archa- 
cerebrum, the ganglion of the prostomium, 


and a secondary brain or syn-cerebruiy 
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 


Fic. 440.—Nervous system of 
Astacus fluviatilis. 
bg. sub-cesophageal gang- 
lion; cg. commissural 
ganglion ; g, brain ; s, vis- 
ceral nerve; sc, cesopha- 
geal connective ; y, post- 
cesophageal commissure ; 
IV—VIII, thoracic gang- 


nerve-cord, where the ganglia of the last 
three cephalic and first three thoracic seg- 
ments have united to form a large com- 
pound sub-esophageal 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 wsceral system of nerves (s) supplying the 
gizzard, originating in part from the brain and in part from the 
cesophageal connectives. 


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 1s constituted by a number of extremely delicate olfactory 
sete, borne on the external flagellum and supplied by branches of 
the antennulary nerve. The so-called audztory 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 sete, 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 sete on the body 
generally have a definite nerve-supply, and are probably tactdle 
organs. 

Reproduction.—The Crayfish is dicecious, 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, ¢, w) lies in the thorax, just beneath the 
floor of the pericardial sinus, and consists of paired anterior lobes 
(¢) 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 amceboid 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, v) 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 


XI PHYLUM ARTHROPODA 557 


sete on the 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 


Fic. 441. Reproductive organs of Astacus fluviatilis. A, female; B, male. od. vviduct ; 
oe, its external opening ; ov. ovary ; ¢. testis; u, unpaired posterior portion of gonad ; vd. vas 
deferens. (From Lang’s Cumpurative Anatomy, after Huxlcy.) 


depositing spermatophores on the ventral surface of the female’s 
body just before oviposition. 

Development.—The process of segmentation of the oosperm pre- 
sents certain striking peculiaritics. The nucleus (Fig. 442, A, 22) 
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 


Fic. 442.—Three stages in the formation of the blastoderm uf Astacus fluviatilis. nu. 
nuclei; yp. yolk-pyramids. (From Korschelt and Heider, after Morin and Reichenbach.) 


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 


558 ZOOLOGY SECT, 


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, A’), on which the eyes subsequently appear ; two some- 
what further back, the thoracico abdominal rudiments (TA); and 
one, posterior and unpaired, the endoderm-dise (#8), On the latter 


Fic. 443.—Early embryo of Astacus. BM, mesoderm; ES, endoderm dise ; 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 dlastopore. By this process the embryo passes 
into the gastrula-stage, which, however, differs from the corre- 
sponding stage in the types previously studied in the immense 
quantity of food-yolk filling up the space (blastoccele) between 
ectoderm and endoderm. Very soon the embryo become tri- 
ploblastic, or three-layered, by the budding of 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, 4): the thoracico-abdominal rudiments 
unite with one another, forming a well-marked oval elevation 


XI PHYLUM ARTHROPODA 559 


(Fig. 444, 71), and three pairs of elevations appear between it 
and the head-lobes. These are the rudiments of the first three 
pairs of appendages, the antennules (c,.), antenne («,.), 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 stomodwum (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 


Fic. 444.—Naunlius-stage of Astacus. 4, (above) eye; A, (below) anus; a). antennule ; 
dy, antenni; &, cerebral ganglion 5 ges. antennary ganglion ; gm, mandibular ganglion ; 
(. labrum ; m. mandible; 74, thoracico-abdominal rudiment. (From Lang’s Comparative 
Anatomy, after Reichenbach.) 


considerable time both stomodeum and proctodieum 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 backwards 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. Thusin Fig. 447 the ventral 
surface of the head and anterior thoracic region faces the observer, 


560 ZOOLOGY SECT. 


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 ecye-stalks appear 


Fics. 445 and -446.—Sections of embryos of Astacus. <A, Nauplius-stage (cj. Fig. 4445 B, after 
development of thoracic appendages (cf. Fig. 447). abd. abdomen; cx. anus; br. brain ; 
ect. ectoderm ; end. endoderm ; ent. enteron ; kt. heart ; mes. mesoderm; mes.’ splanchnic 
layer of mesoderm 3 wth. mouth 5 pedim. proctudeum ; sida stomodeum ; th. abd. thoracico- 
abdominal rudiment; v. nv. cd. ventral nerve-cord, (From Korschelt and Heider, after 
Reichenbach.) 

(Fig. 447, A), as well as the labrum (/.), and a fold on each side 

of the thorax, which is the rudiment of the carapace (¢s), and this 

gradually extends dorsally until it meets with its fellow of the 
upposite side and covers in the cephalothorax. The embyro now 


consists of a nearly globular cephalothorax with a small abdomen 


XI PHYLUM ARTHROPODA 561 


and a nearly complete set of appendages, all tucked in under the 
cephalothorax and closely packed together within the egg-mem- 
branes. In this condition the embryo is hatched, and for some 
time clings to the pleopods of the mothcr by means of the peculiarly 
hooked chelz 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 retinule of the eyes, and 


Fic, 447.—Embryo of Astacus after development of thoracic appendages. A, eyes; a. an- 
tennule ; dg. antenna; ab. abdomen; ¢, archicerebrum and ganglion of autennule ; yo, optic 
ganglion; /. labrum; a. mandible; my. mag. maxille; ¢, 1—S%, 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 antenne, 
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 maxilli:; and (5) the second maxille. 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 selevites, 
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 sete, 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 oftshoots of the thoracic wall or 
of the thoracic or abdominal limbs. The stomodzeum and proc- 
todeeum 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 
ccelome 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 maxill«, or of antennary (green) glands opening on the 
antenniv, , 

The nervous system consists of a brain united by cesophageal 
connectives with a ventral nerve-cord, formed of a double chain 
of ganglia joined together by commissures and connectives. The 
first three pairs of embryenic 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 amceboid with 


‘XI PHYLUM ARTHROPODA 563 


radiating pseudopodia, or 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 
nauphus-stage, which may be a frec-swimming larva or may 
be passed through before hatching, and is characterised by the 
presence of three pairs of appendages which become the 
antennules, antennz, 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 maxilla: 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 antenne 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 Branchipus and Artemia. 


ORDER 2.—NOTOSTRACA. 


Branchiopoda in which there is a large dorsal shield-shaped 
carapace. The eyes are sessile. The antenne are reduced. There 
are 40 to 63 pairs of trunk appendages. The caudal styles are 
many-jointed. 

Including Apus and Lepidurus. 


ORDER 3.—-CONCHOSTRACGA. 


Branchiopoda with a carapace divided into two lateral portions 
or valves like the shell of a bivalve mollusc, and enclosing the 
entire animal. The antenne are biramous and are used as 
swimming appendages. The eyes are sessile, coalescent. The 


od 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 Zstheria, Limnetis (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 antenne are biramous and used as 
swimming appendages. Only 4 to 6 trunk appendages ; caudal 
styles unjointed, claw-like. 

To this order belong Daphnia, Polyphemus, Leptodora (Fig. 
449), ete. 

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 antenne 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, Cythere, 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 antenne 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 
Cyvlops (Water-flea) (Fig. 451) and (0) parasitic forms or Fish- 
lice—e.g, Ergastlus, Chondracunthus, Lerncea (Pig. 452). 


XI PHYLUM ARTHROPODA 565 


ORDER 2.—BRANCHIURA. 


Parasitic Copepoda with compound eyes and a suctorial mouth. 
Some of the appendages of the body-segments arc usually provided 
with peculiar appendages—the flagella. The genital apertures are 
situated on the fifth body-segment. This order includes the 
Carp-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 cirriform 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 antenne 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 cypr'is 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 Balanus (Acorn-shell) (Fig. 455) and (0) 
parasites—e.g., Petrarca, Alcippe, Proteolepas. 


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 Peltogaster. 


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 OO 


566 ZOOLOGY SECT, 


marked off from one another. The abdomen is devoid of caudal 
styles. The exoskeleton of the head unites with that of inore or 
fewer of the thoracic segments to form a cephalothoracie 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 telseon—the last segment devoid of appendages, the telson 
bearing a pair of caudal styles. There isa 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 I] —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 <Anuspides, 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 clongated, 
tubular. 


XI PHYLUM ARTHROPODA 567 


ORDER 1.—MYSIDACEA. 


Peracarida in which, though the carapace extends over the 
greater part of the thorax, it does not coalesce dorsally with more 
than the first three segments. The eyes, when present, are 
supportcd on movable stalks ; the antennules are biramous, and 
the antenne have a scale-like exopodite or squame. The first 
pair of thoracic appendages are specialised as maxillipedes. The 
thoracic appendages (except sometimes the first and second pairs) 
are biramous. The uropods with the telson form a brvad fan-like 
tail-fin. 

This order includes Mysis (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 
vyes usually coalesce into one, which is not borne on a movable 
stalk. The antennules are sometimes biramous; the antennz 
have no exopodites. Some of the thoracic appendages are 
biramous. The telson may coalesce with the Jast segment of 
the abdomen. The uropods are styliform, and there is no fan- 
like tail-fin. 

Includes Cuma (Bodotria), Diastylis, ete. (Fig. 460). 


ORDER 3.—TANAIDACEA. 


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 antenne: 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, Leptochelia, ete. 


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 
antenne sometimes bear a minute exopodite. The thoracic limbs 
have no exopodites: the first pair are modified as maxillipedes ; 

00 2 


568 ZOOLOGY SECT. 


the rest are usually alike in character. The abdominal appendages 
are usually biramous; the rami function as branchiw. The body 
is nearly always dorso-ventrally compressed. This is a large order 
including many families, ey.—Asellus, Phreatoicus, 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 
inodified as prehensile organs (gnathopods) ; (8) there are vesicular 
or lamellar branchiz 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, Gammedrus (Fig. 461), Huperia, Caprella, 
Cyuimus (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 branchiz (podobranchs) 
attached to the bases of the thoracic limbs. The larva is a 
natuplius. 

This is a comparatively small order of pelagic Malacostraca, 
including Huphausia (Fig. 472), Thysanopoda, Nyctiphanes, and a 
few other genera. 


ORDER 2.—DECAPODA. 


Eucarida in which the first three pairs of thoracic appendages 
are modified as maxillipedes, with the branchie usually in several 
series—podobranchs, arthrobranchs, and pleurobranchs. 


Sub-order 1.— Mucrura. 


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 antenne 


XI PHYLUM ARTHROPODA 569 


are both large; the former are not sunk in pits, and the antenne 
usually have a seale-like v xopodite (squame). 

Included among the Macrura are (a) swimming forms—Penmus 
and Palemon (Prawns), Crangon (Shrimps), Luczfer, ete.; and ()) 
creeping forms—ZTomarus (Lobster), .tstacus, | stacopsis, Pura- 
nephrops, Cambarus (Fresh-water Crayfishes), Palinurus (Rock- 
lobsters), Sey//arus, ete. (Figs. 465, 466). 


Sub-order 2.—Anomura. 


Decapoda with the abdomen more or less reduced, usually held 
ina 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— 
Pagurus (Fig. 467), and other genera, the Cocoa-nut crab—Birgus 
—Galathea, Hippu, Porcellana, ete. . 


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 z-@ and 
megalopa stages. 

Including the true Crabs such as Cancer, Mein, Grapsus, ete. 
(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 branchi# 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 Sguil/a (Fig. 470), 
Gonodactylus, and other genera. 
Sytematic Position of the Examples, 


The genera Apus and Lepidurus belong to the family Apcdide 
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- 


A7v0 ZOOLOGY SECT. 


ments (considerably more than ten) and swimming-feet, together 
with the presence of the large shicld-like carapace, decides their 
position among the Notostraca. 

They are placed in the family Apodide in virtue of the elongated 
body with 40-60 pairs of swimming-fect, 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 
first thoracic feet are short. 

Astacus flwiatilis is one of several species of the genus Astacus, 
belonging to the family Potamobiide, tribe Astacoidea, sub-order 
Macrura, order Decapoda, and sub-class Malacostraca. 

The 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, together 
with that of movable eye-stalks, determines its position in the 
division Eucarida: the modification of the first three pairs of 
thoracic appendages as maxillipedes, and the arrangement of the 
branchi 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 Potamobiide is distinguished by 
having the podobranchie partly united to the epipodites, and by 
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. 

The 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. 
Ou 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. 

There is considerable diversity of form among the Branchiopoda. 
Apus has already been described. Branchipus (Fig. 448, 1) and 


XI PHYLUM ARTHROPODA o71 


Artemia (the Brine-shrimp) (4nostraca) are small shrimp-like 
forms, the former living in fresh-water lakes, the latter in brine- 
pools; they have no carapace, and the eyes are raised on unjointed 
stalks. In Limnetis (2), on the other hand, and in Estheria (3) 
the carapace takes the form of a shell, formed of two parts 
er ralees, united in Estheria by a hinge, and resembling the 
shell of a cockle or other bivalved mollusc. The limbs have the 
same gencral structure as those of Apus, but the antenne are 


3.Estheria 


Fic. 448.—Three Branchiopoda. In 3, « is the shell ; } the animal with one valve of the shell 
removed, «ntl, antennule; ant2, antenna; hf. heart; a. adductor muscle ; md. mandible ; 
ov. ovary; a. unpaired process from head; p. copulatory appendages ; sh.ql. shell-gland ; 
t. testis. (After Gerstaecker.) 


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 (br.p),in which the eggs are stored, The 


a72 ZOOLOGY SECT. 


paired cyes (£) have fused into a single organ, which cxhibits 
a contstant trembling movement. The antennules (ant. 7) 
are small, the antenne (ant. 2) very large, biramous, and con- 
stitute the chief organs of locomotion. The mandibles are large, 
the second maxille absent in the adult, and there are usually 
five pairs of leaf-lhke swimming-feet (/) on the thorax. The 
abdomen is devoid of appendages. Many of the Cladocera have 
an extraordinarily grotesque form (2, 3), owing to the peculiar 


a 


mus 


1.Duphnia 


ui. 449.—Three Cladocera, ant. 1, antennule ; ant. 2, antenna; lr. brain ; hr.p. brood-pouch ; 
E. eye; d.ql. digestive gland; 7, swimming-feet; ht. heart; md. mandible; sh.ql. shell- 
gland. (1 after Claus, 2 and 3 after Gerstaccker.) 


shape of the head, the immense antenne, and the great hump-like 
brood-pouch., 


The Ostracoda 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 Cypris, 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 


Xl PHYLUM ARTHROPODA HTS 


from one to 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 (¢), and in some forms 
compound eyes are present as well. There are only seven pairs 
of appendages. The antennules (ant.) and antenn® (ant.2) are 
large and uniramous. The mandible (md.) has a large leg-like palp 
and a flabellum-like offshoot. The first maxilla (m2.Z) also bears 
a large plate resembling a flabellum of Apus. The last cephalic 


Fie, 450.—A, external view of Cypris ; B, the same with the appendages exposed by the removal 
of the left valve of the shcll; C, transverse section; D, a single sperm, abd. abdomen ; 
ant.1, antennule; aat.2, antenna; <.gi. digestive gland; e. median eye; 7.1, 7%, thoracic 
feet ; int. intestine; m. adductor muscle; wd. mandible; maz.1, mr 2, maxilla; ov. ovary 5 
sh, shell; t. testis. (After Gerstaecker.) 


appendage (second maxilla, m2) is jaw-like in some forms 
(Cypris), leg-like in others (Cythere). The only thoracic appendages 
are two pairs of slender legs (71, 72). The abdomen («bd.) 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 Bucopepoda, and the Branchiura, 

The free-swimming Hucopepoda 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. 


ot 


TA ZOOLOGY SECT. 


Cyclops (Fig. 451, 1) has been compared in form toa split pear, 
the broad end being anterior, and the convex surface dorsal. 
The first thoracic segment is fused with the head, and the 


2.Calocalanus 


Fic, 451.—Frec-swimming Bucepepoda. Ja, female Cyelops, from the right side; b, dorsal 
view ; C, antenna of male; D, swimming-foot. abd.1, first abdominal segment; ant./, 
antennule; ant.2, antenna; e. th. cephalothorax ; e. median eye ; en. endopodite ; ¢.s. egg 
sac; ex. exopodite; or. ovary; pr.1, pr.2, protopodite ; 7. rostrum; «7 swimming-feet ; 
th.2, th.G, thoracic segments. (After Huxley, Gerstaccker, Hartog, and Giesbrecht.) 


cephalothorax (ec. 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 (¢). There are 
five free thoracic segments: the last (th, 6) bears the genital 


XI PHYLUM ARTHROPODA Td 


aperture, and is fused in the female with the first abdominal 
(abd. 1). There are four abdominal segments: the last bears the 
dorsal anus (a7), and a pair of candal styles produced into plumed 
setir. 

The antennules (wnt. /) 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 sette—as clasping organs, used 
for holding the female during copulation. The antennw (ant. :2) 
are comparatively short and uniramous. Mandibles and maxillie 
are present, and the first four thoracic segments bear biramous 
swimming-feet (Za sj.), 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 sete, especially those of the caudal 
styles. 

The parasitie Eucopepoda, or Fish-lice, present a very interesting 
series of modifications, illustrating the degeneration of structure 
which so often accompanies parasitism. rgasilus (Fig. 452, 1) is 
found on the gills of the Bass (Mforone labraa:); it is readily recog- 
nisable as a Copepod, but the appendages are greatly reduced, the 
antenne moditied into hooks for holding on to the host, and the eyes 
absent. Anthosoma (2), found in the mouth of the Porbeagle Shark 
(Lamna cornubica), has recognisable appendages, but the form of 
the body is much modified by the development of curious overlap- 
ping lobes. Nicothde (3), found on the gills of the Lobster, has 
antennee and mouth-parts modified for suction: the abdomen is 
normal, but the thorax is produced into huge lobes, which give 
it a curiously deformed appearance. In Chondracanthus (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 antenne (ant.2)—used for attachment—mandibles, maxillee, 
and two pairs of legs (7:1, 7.2) are present. The male (0) is of 
higher organisation than the female, but of minute size—about J; 
the length ofits mate—and is permanently attached to her body, 
close to the genital aperture (a, J/). — In Lerneea (7) and its allies 
the body is vermiform with a curiously lobed anterior end: the 
maxille are adapted for piercing the skin of the host and sucking 
its juices,and there are minute vestiges of feet. In Lesteira (5) 
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 SToT, 


(Genypterus blacodes), and the rest of the body hanging freely 
into the water. Lastly, in Tracheliastes (6) the second maxillie 


ezare™. 
fe fats: 
neetae, 


y 
1 
bs 


5. Lesteira 
Fic, 452,—Various forms of parasitic Bucopepoda. da, female; 4b, male. anf.1, antennule ; 


ant.2, antenna ; e. median eye ; es. egy-sac 3 £1, 7.2, thoracic feet ; A, male; wir.?, sccond 
maxillke. (After Gerstaecker, Claus, Cuvier, and G. M. Thempson.) 


(mz.2) are greatly enlarged, and form a characteristic organ of 
attachment. 


Argulus (Fig. 453) is the most familiar example of the Branchinra, or Carp- 
lice. It isan external parasite on fresh-water Fishes (Carp, Stickleback, &c.), not 


ne PHYLUM ARTHROPODA 577 


permanently attached like the degenerate forms just described, but crawling 
freely over the surface of the host. The body consists of an oval flattened 
cephalo-thorax, and a small bilobed abdomen (ab.). The mandibles and maxille 
are piercing organs enclosed in a sucking-tube or proboscis (r.), in front of which 
is a median tube ending in a spine (s/.). The second maxille are divided into 
two portions, the anterior of which (4/1) are modified into sucking-dises by 


Fui. 453.- Argulus foliaceus, young male. a), antennulc; dg, antenna; ab. abdomen ; 
b,—b4, thoracic fect 7. digestive glands connected with intestine ; &£/1, anteriur or suctorial 
feet ; &f2, posterior or leg-like portion of second maxille ; pa. paircd eye ; 7. rostrum : sd. shell- 
gland ; st, stylet ; ¢s. testis ; wa, median cye. (From Laug’s Comparative Anatomy.) 


which the parasite clings to the surface of its host, and there are four pairs of 
swimming-feet (b7—b4). 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 Luctrripedia are the Barnacles 
found on ships’ bottoms, piles, &¢., 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 anatifera) 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 scuéa (s); 
and two lateral and distal, the terga (¢). 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 biranious 
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 


Fic. 454.—Lepas anatifera. A, the cntire animal; B, anatomy. 4, antennule ; ¢. carina 3 
cd. coment-gland ; l. digestive gland ; m. adductor muscle ; od. oviduct 5 ov. ovary ; p. (im B) 
penis and (in A) peduncle ; s, scutum ; 4 tergum and testis ; ed. 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 maxille. There are no 
antenne: 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. 


XI PHYLUM ARTHROPODA 579 


The Sessile Barnacles or Acorn-shells (Balanus) have no stalk 
(Fig. 455), the head-region being short and broad. The scuta (s) 
and terga (¢) support a valvular carapace, through the opening of 
which the feet are protruded, and the whole animal is surrounded 
by a sort of parapet (sh) 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 Polliecpes. 

Many of the Eucirripedia are parasitic. Some of these (Petrarca, 
&e.), parasitic in Actinozoa, resemble the attached forms in 
essential respects; others (Alcippe), parasitic in the shells of 
Molluses and Cirripedes, have abdominal but no thoracic fect. 


Fic. 455.—Balanus. A. external view; B, anatumy. aj. antennules; ad. adductor muscle ; 
m. muscles of scuta and terga; 0, edge of parapet; ov. ovary ; ovi. oviduct; s. scutum ; 
sk, parapet; ¢ tergum; wo. female aperture. (From Lung’s Compurative Anatomy, after 
Darwin.) 


Proteolepas, also parasitic on other Cirripedes, has a maggot-like, 
segmented, limbless body, and a suctorial mouth. 

The Rhizocephula are represented by Sacculina (Fig. 456), parasitic 
on Crabs, and Peltogaster on Hermit-Crabs. Both genera have the 
appearance of an immense tumour (4s) on the abdomen of the host, 
showing no sign of segmentation, no appendages, no mouth or 
anus. From the attached end go off a number of delicate root- 
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 SECT. 


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. 


Fu. 456.-Saceulina carcini, on abdomen of crab. Ur. branchial region of crab; 1, 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 Conparutive Anatomy, after Delage.) 


The limbs are strikingly modified for the performance of various 
functions. 


The Phyllocurida 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 (7). 


XI PHYLUM ARTHROPODA 581 


The cyes (@) are large, compound, and raised on movably articu- 
lated stalks. The antennules («,) and antennie («,) are large, the 


mandibles (md.) have palps 
(mt), and the exopodite of 
the second maxilla (mzt) 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 (b7/), all of 
them leaf-like, and recalling 
those of Apus. The first 
four abdominal appendages 
(pl—p4) are large biramous 
swimming-fect, like those of 
Copepods; the fifth and sixth 
(po, pO) are small and uni- 
ramous. 

The Syncarida (<dneaspid- 
aren) (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 
and 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 with 
slender respiratory exopo- 
dites, and bear a double 
series of epipodites or bran- 
chiew; there are stalked eyes 
and a fan-like tail-fin formed 
of the telson and the ex- 
panded uropods. 

The Mysidacca (Fig. 459) 


Fic. 457.—Nebalia geoffroyi, male, a. eye; 
7, antennule >a», antenua ;¢, head ; bry. thoracic 
fect; d, intestine; 2. heart; Aim, gizzard; mel. 


mandible; m/, mandibular palp; mnt. exo- 
podite of second maxilla; pj—p4, pleopods ; 
vy. rostrum 3 8, carapace ; sm, adductor muscle ; 
t. testis; /—VI//, thoracic segments. (From 
Lang's Comparative Anatouy, after Claus.) 


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 


VoL. 1 


PP 


ds8v ZOOLOGY SECT. 


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 


Fic. 458.—Paranaspides lacustris, x4. al, antennules; a2, antenne; Ab.1, first 
abdominal segment: ep, epipodites or gills on the thoracic legs; md, mandible; Pl.1, first 
abdominal appendage ; 7’, telson; 7h.8, eighth free thoracic segment; UY, uropud. (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, 


Fic, 459.—IMysis oculata. cad. endopodite; er. exopodite ; of, otocyst. (After Gerstaccker.) 


except to a very slight degree in some forms. Moreover, the legs 
all possess exopodites (e7), 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 


is a uropod, z¢, assists the telson in the formation of the 


characteristic malacostracan — tail- 
fin: there is no tiace of the 
entomostracan caudal styles 

The Cumacea are also a very 
small group: Diastylis (Fig. 460) 
is a good cxample. They are 
little shrimp-like animals, differ- 
ing from all the Malacostraca pre- 
viously considered in having poorly 
developed sessile cyes, sometimes 
fused together, and in some genera 
altogether absent. The carapace 
(cth) is so small as to leave the 
five posterior segments (¢hJV— 
VITI) uncovered. The first two 
pairs of thoracic limbs are maxilli- 
pedes, the last six, legs: of these 
two or three pairs have cxopo- 
dites (ex). 

The Tunarducea, the Lsopoda and 
the Lmphipod« are often grouped 
together under the heading of 
Arthrostracu. 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 (Zalitrus, Orchestia) 
so common on the sea-shore. Of 
the Jsopoda very convenient ex- 
amples are Asellus (Fig. 462), 
common in fresh-water, and the 
well-known Wood-lice or Slaters 
(Oniscus, Fig. 464, 7), found 
under almost any picce of wood, 
stone, &e¢., which has lain undis- 
turbed on the ground for a few 
weeks. 

The body is usually compressed 
or flattened from side to side in 
Amphipods (Fig. 461), depressed 
or flattened from above down- 


ay ES 
\ x 


lic. 460.—Diastylis stygia. «1, an- 
tennule; a?, antenna; ub.1—ab.7, ab- 
dominal segments ; cth. cephalothorax ; 
en, endopodite; er, exopodite; p.1,p.6, 
pleopods ; IV-VIJ, th VIII, free thoracic 
segments. (From Lang’s Comparative 
Anatomy, after Sars.) 


PP2 


ast 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 (¢.t). 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, 
_lnisopoda—form a transition to the other Malacostraca, and 
especially the Cumacea. In the Amphipoda and Isopoda, the pos- 
terior seven thoracic segments (th.?—ihS) are free, and those of the 
short abdomen are usually free in Amphipods (Fig. 461, abd. 


Vic. 461.—Gammarus neglectus.  ¢//.f—«abd.ti, abdominal segments; wrt.1, antennule ; 
ant. 2, anteuna; cth. cephalothorax ; £. cye; j. f. 1, first jumping foot ; 7. 1—l. 7, legs; 
mep. maxillipade ; os. oostegite ; ov. ova; s.j.1, first swimming foot ; th.s—th.’, free thoracic 
segments. (After Gerstaccker.) 


1-4), often more or less fused in Isopods (Fig. 462, abd). In some 
Isopoda the thoracic segments are produced laterally into large 
and prominent pleura. 

The eyes (#) 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 antennze (azt.?) as well as the antennules (ant.Z) 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 (/./-1.7) which are usually arranged in 


XI PHYLUM ARTHROPODA 585 


two groups, four of them directed forwards and three backwards, or 
vice rersé. The legs end either in simple claws or in large sub- 
chele: vestigial exopodites are present in some Tanaidacea. 
In the female, certain of the legs bear flat plates, the ocstegites 
(Fig. 461, os), 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. 


Lp 

if 

Y 
LF G 


/, 


PA aG 


Fic. 462._Asellus aquaticus. A, dorsal; B, ventral view. «d/,abdomen ; ant.1, antennule ; 
ant2, antenna; bp. breod-pouch ; ¢.th, cephalothorax: £, eye; /.1—1.7, legs; pl.1—pl.?, 
pleopods ; th.2—th.S, free thoracic segments. (After Gerstaecker.) 


461, sf.), the last three peculiar stiff processes used for jumping 
(jf). In Isopods more or fewer of the pleopods have broad plate- 
like endo- and exopodites (Fig. 462, p/.3), the former thin and 
vascular and acting as gills: the sixth pair (p/.4) 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, 7) 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 


D86, ZOOLOGY SECT. 


brings up her young. Caprella (-’)is a singular creature in which 
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 Cyamus (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. 


Fic. 463.—Amphipoda. 3, u, male; b, female. (After Gerstaecker, and Bate and Westwood. 


Among the Isopoda, one of the most interesting forms is the 
common Wood-louse (Fig. 464, 2), which is almost unique among 
Crustacea for its perfect adaptation to terrestrial life. The allied 
“ Pill-bugs” (Armadillidium, ?) have the habit of rolling them- 
selves up intoa ball when disturbed. Cymothoa 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- 
rint, found in the gill-cavities of various Crustacea, parasitism 1s 
accompanied by great degeneration and asymmetry (2) as well as 
by a notable degree of sexual dimorphism, the males (<, 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) has 


XI 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 Kuphuusineew (Fig. 472) are 
pelagic forms in which none of the thoracic appendages are modi- 
fied so as to take the form of mavillipedes, and in which there 
ix only a single series of branchize (podobranchs). 

Amongst the Decapoda 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. 


Fic. 464.—Isopoda. 3,a, entire animal; b, posterior end with attached male (m); 4,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, antennz, and legs may attain extraordinary dimensions. 

The Lobsters and fresh-water Crayfishes agree with Astacus in 
all essential details, but the sea-Crayfishes (Palinwrus) present some 
striking modifications. There are no chelw, the legs all ending in 
simple claws: the antennz 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 Seyllarus 
(Fig. 466) and its allies the body is hroad and depressed, the bases 


588 ZOOLOGY SECT. 


of the legs widely separated from one another by the broad 
sterna, the antennw (cxé. :2) short and plate-like, and the eye-stalks 
(£2) enclosed in sucket-like grooves of the carapace. Most of 


ee, 


oe 
ne 
ee 


2.Palaemon. 
Fic. 465,—Shrimp (dorsal view) and Prawn (side view). (After Cuvier.) 


these characters show an approximation to what is found in 
the Crabs. 

Of the Anomiura, the Hermit-crabs (Pagurus, &c., Fig. 467) 
are very strangely modified in relation with their peculiar mode 


XI PHYLUM ARTHROPODA 589 


of life. They are always found inhabiting the empty shells of 
Gastropods (WwW helks, 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 (¢) on the dorsal side, and its appendages are more or less 
atrophied except the sixth pair (wp), 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 (25) are much reduced, and in 


Fic, 466.—Seyllarus arctus. Fic. 407.—Pagurus bernhardus. ch. chela of 
ant.1, antennule ; ant.2, antenna ; first right leg; 1.4, 24, fourth and fifth legs; 
EB, eye. (After Cuvier.) t, abdominal terga; vp. urupods. (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 (Birgus), 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 SECT. 


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 


Fic, 468.—Cancer pagurus. A, dorsal, B, ventral aspect. aat.1,antennule ; ant.2, antenna ; 
abd.1, abd.3, abd.7, abdominal segments ; BE, eye-stalk ; 1.1, 1.5, legs; wep.3, 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 (#) are contained in orbits or sockets of the carapace, 
which are so prolonged that the eyes appear to arise behind the 
antennules and antenne. Both pairs of feelers are small, and the 


(After Belliand de:Haan.) 


Fia. 469.— Typical Brachyura 
bases of the antennules are contained in sockets or /ossettes. 
third maxillipedes (map.) are broad, flat, and valve-hke, not leg- 


like as in the Macrura. 
of great size; the remaining legs generally end in simple claws, 


The 


The first legs (/.2) form chelipeds often 


592 ZOOLOGY SECT. 


but in the Swimming-erabs (Fig. 469, 7) the distal segment 
in the fifth pair is flattened and forms a fin. The range of 
variation in form, proportions, colour, markings, &e., among Crabs 
is very great (Fig. 469). 


Unlike the Decapoda, the Stomatopoda form a very small order, 
comprising a few genera varying from the size of a Shrimp to that 
of a Lobster. Sqiilla (Fig. 470) is the best known genus. 

The abdomen («f—a7) is very large in proportion to the 
cephalothorax, and the carapace (ch), which is thin and undalci- 
fied, leaves the last three thoracic segments (VJ—VJII) 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. 


Fic. 470.—Squilla. «i, antennule; a2, antenna; a1—a7, abdominal segments ; br, gills ; eth, 
cephalothorax ; p, copulatory organ ; pl— 5, pleopods ; p6, uropods ; VI—VIIJ/, free thoracic 
segments ; 1—8, thoracic appendages, (Fron Lang's Comparative Anatomy.) 


This arrangement appears to support the view that the anten- 
nulary region is a metamere distinct from the prostomium ; 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 (7) has three flagella ; the antenna (2) a single 
flagellum and a very large exopodite. The first five pairs of 
thoracic limbs (J—) are turned forwards towards the mouth, and 
act as maxillipedes; the second of these—corresponding with the 
second maxillipede of Astacus—is very large (7), 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 sib-chela is produced, both of the 
segments of which are produced into strong spines, The re- 


XI PHYLUM ARTHROPODA 593 


maining three thoracic appendages (G—8) 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 (pl, %) have gill- 
filaments (07) attached to their plate-like exopodites: the sixth 
(y@) 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 enticle of most 
Branchiopoda, Ostracoda, Copepoda, &., through the caleitied but 
still flexible cuticle of Astacus, to the thick, tuberculated, stony 
armour of many Crabs (Fig. 469, 3), or the shelly picecs of Cirri- 
pedes. The exoskeleton is secreted froma single-layered ectoderm, 
and undergoes periodical moults 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 Pulemonetes, 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 hamocele. : 

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 stomocdeum. A 
“gastric mill” is present in Malacostraca, and a rudiment of 
such an apparatus occurs in Ostracoda. The digestive glands 
are usually branched ceca formed as offshoots of the mesenteron : 
in the Isopoda and Amphipoda (Fig. 471, /) they are unbranched 
ceca extending into the abdomen: in Stomatopoda they consist 
of ten metamerically arranged 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 
Curipedia, respiration takes place bythe 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 


Fic. 471.-Orchestia cavimana, mile. «, eye; a, antennule; a, antenna; aoa, anterior 
aorta; op, posterior aorta ; bin, ventral nerve-cord ; br, gills; C-+7', cephalothorax ; de, vas 
deferens ; e/, rectuin; g, brain; h, heart ; hi, intestine ; kf, maxillipede ; /, digestive glands ; 
w, gullet ; p 1—p 7, abdominal segments 5 sim, “stomach” ; ud, intestinal c-cum ; vs. vesicula 
seminalis ; ¢, testis; (I—VII/, free thoracic segments. (Prom 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 podobranchix (Fig. 472) quite 


XI PHYLUM ARTHROPODA 595 


uncovered by the carapace. In the Decapoda the gills may be 
either plume-like, as in Astacus and its allies, or the delicate 
eylindrical 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 


av 


Fic, 472.—Antcrior portion of Buphausia pellucida. a), antennule; ané.2, antenna; ab.1, 
first abdominal segment ; au, eye; br. 1—8, podobranchia ; cth. cephalothorax ; en.1, en.2, 
endopodites of first two thoracic limbs; ex.1—ex.6, exopodites of first six thoracic limbs ; 
h. heart ; 1, digestive gland ; m, ‘‘stomach” ; ov. ovary ; ovd. oviduct ; J—VIII, protopodites of 
thoracic limbs. (From Lang's Comparative Anatomy.) 


of numerous forms leads to the conclusion that the typical or 
theoretical branchial formula for the group 1s as follows :— 


Tuoract: | ; ad ees . | | 
SRiahENTS: I. I. Tit. | IV. Vv. VI p ican aati 


i 


Podobranchiw |] + ep) l+ep.1l+ep, l+ep|) lt+ep' ltep 
| 


l+ep l+ep ! 8+ Sep 


Arthrobranchie| 2 2 2 ! 2 2 2 2 2 {16 


8 


Pleurobranchi| 1 1 1 | 1 1 | J | 1; 1 
a 
| 
| 


| Sod | 
Total ... 4+ep| 4+ep eerie Heap dep) Aten ttep ene 
i | 


( 


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 (Birgus) 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 SECT. 
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, dr) are outgrowths of 
the thoracic himbs: 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-flaments (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 
18 common among the lower groups, and is especially noticeable in 
Cyclops. 

The heart is absent in many Copepods (including Cyclops), in 
sume 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, 4); 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 antenne, and the maxil- 
lary or shell-glands opening on the bases of 
the second maxille. 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 1s 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 
ceca opening into the posterior end of the 
mesenteron. In sume of the Cirripedia the 
maxillary gland is described as opening into 


Fic. 478.—Nervous system of a 


Cra) (Maja squinado). 
ba, thoracic ganglion; cg. 
commissural ganglion; g, 
brain ; m, “stornach "5 se, 
cesophageal connective ; sg, 
visceral nerves ; 7, post-veso- 
phageal connective. (From 
Lang's Comparative Anat- 
omy, after Miluc-Edwards.) 


of gangha. 


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 


In the sessile Barnacles and in the Crabs (Fig. 478) 


this proccss reaches its limit, the whole ventral nerve-cord being 
represented by a single immense thoracic ganglion (bg). 


XI PHYLUM ARTHROPODA 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 setae 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, of) 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 (Cymothou). 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 swmmer 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 QQ 


598 ZOOLOGY SECT. 


which afterwards spreads over the whole yolk. But in most 
cases the egg is centrolecithal 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- 
acterasin 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, Leptodora (Fig. 449,37), while development of the summer 
eggs is indirect, the winter eggs give rise tu 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 Cirripedia, also, there is a free nauplius, the body of which 
is often produced into long spines. After several moults, the 


Fic. 474.—Cypris-stage of Lepas fascicularis. «). abdomen; pa. paired cye; rf, thoracic 
fcet ; wa, unpaired eye; 1,antennule. (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 (Z) also have become modified into organs 
of adhesion by the development of the penultimate segment into a 
disc, the antennee 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 


x1 PHYLUM ARTHROPODA 599 


of the adult. In Lepas 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 
protozowa-stage, 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 (Penceus and 
Lucifer) the embryo leaves the egg as a nauplius, and passes by 
successive moults through a protozoa stage, a zowa-stage, with 
segmented but limbless abdomen, and a mysis or schizopod-stage 
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 
zowa (Fig. 475, A), with an immense cephalothorax produced into 
spines, large stalked eyes, and a slender abdomen. This passes 
by successive moults into the megalopa-stage (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 (Homarus) both nauplius and zozea-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 (Palinwrus) and its allies, ihe newly hatched young 
is a strangely modified Mysis-form called a Glass-Crab or Phyllo- 


Q Q 2 


600 ZOOLOGY SECT. 


soma: it has broad, depressed cephalic and thoracic shields of 
glassy trausparency: 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 larve of Stomatopoda are grotesque little creatures with a 
very large spiny carapace. In Amphipoda there is no free larval 


> 


Rie 


Fiu. 47i.—Linvie of Crabs. A, Zuwa-stage of Maja; B, Megalopa-stage of Portunus. 
h, heart ; ¢y—u,, abdominal segments; J, antennule ; 2, antenna ; /—VJ//, thuracic append- 
ages. (From Liuug's Comparative Anatomy, after Claus.) 


form, but in Isopoda the young leave the egg in the form of a 
curlous 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, 
Pontellina mediterranca, may almost be considered as aérial: it 
is described as taking long flying leaps out of the water, after the 
manuer of a Flying-fish. Some, like Lobsters, Craytishes, &c., are 


XI PHYLUM ARTHROPODA 601 


solitary ; others, hke 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 oecur 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 Paleozoic 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 zozea-stage, many 
of the lower forms progressing no further. But in Malacostraca 
the zoza 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 larvee 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, 7.c., modifications in form, colour, &c., which serve 
to conceal them from their cnemies 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. Theimmensely enlarged and highly coloured chele 
of some male crabs (@elasimus, Fig. 469, 2) 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 (Palinurus 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 chele. 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. 


x 


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 ahead; 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 Phyllopeds 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 
Cheetopod. 

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 


XI PHYLUM ARTHROPODA 603 


separate lines, of an ancestral form common to them and the 
Branchiopoda. By a differentiation of the post-cephalic limbs, 
and 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-fin 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 
podobranchie 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 isin 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 
podobranchiz, 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, &., this process goes a step further, and the 
abdomen becomes permanently flexed under the cephalothorax, 
ae leading to the high degree of specialisation found in the 

rabs, 


604 ZOOLOGY SECT 
These relationships are expressed in the following diagam :— 


Brachyura 


Anomura 


Macrura 


Euphausiacea 


Arthrostraca 


Mysidacea 
Anaspidacea 
Stomatopoda 


a 


Phyllocarida 


Trilobita 
Branchiopoda 


ae Copepoda 


Ostracoda 


Annulata 


Fic. 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 
Paleozoic 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, ¢.sh), 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 


XI PHYLUM ARTHROPODA 605 


covered by a carapace or cephalic shield (c.sh), the elevated median region of which, 
known as the g/abella (g/), usually presents three or four transverse grooves, 
probably indicating the presence of four or five segments. The lateral regions 
of the carapace are divided hy an oblique line of separation, the frontal or facial 
suture (f.s), into an inner or mesial portion, the fiavd cheek (f.c), continues with 
the glabella, and an outer free portion, the morable cheek (mv); the latter 
bears the large paired compound eye (c). In some cases there is an indication 
of a dorsal 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.y), to the 
posterior edge of which is attached a large labrum or hypostome (/bv). In many 
Trilobites the hypostome lears a pair of small compound eyes. ‘The posterior 
angles of the carapace are often produced into spines. 


Fic. 477.—Dalmanites socialis, dorsal aspect; B, the same rolled up; C, under-side of 
head of Phacops fecundus. ¢.sh. cephalic shield ; e. eye; f.c. fixed cheek ; fs. frontal 
suture ; gl. glabella; lbv. labrum; m.r. movable cheek; p. pygidium; p/. pleura; s.f.p. 
sub-froutal 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 merlian region or axis, 
and of lateral plewra (pl) often produced backwards and downwards into spines. 
The abdomen is covered by a caudal shield or pygidium (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, hore a pair of appendages. 

The appendages are known only ina few cases. Quite recently a single pair of 
antennze (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 SECT. 


The larvw 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. 


— F 
Be25e) 


fe SST See 
Te At UES 
cl NS 
. ee =a 


NG 
SEN 


Matias er 


DFP 


uN y 
it 


Fic. 478.—Triarthrus becki, x2}. A, ventral surface with appendages; Ep, metastome ; 
Hy, hypostome. B, second thoracic appendage. ca. endopodite ; ex. exopodite x12. (From 
the Cambridge Natural History, after Beecher) 


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. 


XI PHYLUM ARTHROPODA 607 


CLASS II—-ONYCHOPHORA. 


The class Onychophora comprises only the aberrant arthropod 
genus Pertpatus, 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 


PN Se AANA 
Geo 


Fic. 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- 
pular-like animal of approximately cylindrical form, and not divided 
into segments: it has a fairly well-marked head, and a series (l4— 
42, according to the species) of pairs of short stumpy appendages. 


Fic. 480,—Ventral view of head of Peripatus capensis, with antenns, jaws, oral papille, 
and first pair of legs. (After Balfour.) 

The integument is thrown into a number of fine, transverse 

wrinkles. and is beset with numerous conical papillae, each 

capped with a little chitinous spine. The head (Fig. 480) bears 

a pair of antenne, a pair of eyes,a pair of jaws, and a pair of 

short processes known as the oral papille. The antenne are made 


608 ZOOLOGY SECT. 


up of anumber of short rings bearing minute spines. The eyes 
are constructed somewhat after the model of the chwtopod eye as 
described on p. 478. On the surface of the oral papille are 
situated the apertures of a pair of glands—the slime glands. 
Each jaw is composed of two 
curved, faleiform, pointed, 
chitinous plates, the inner 
toothed on their posterior con- 
cave edge ; they he at the sides 
of the mouth enclosed by a cir- 
cular lip. The jaws, as. well 
as the oral papille, are de- 
veloped as modified limbs. 

The legs are not jointed, but 
rows of papille 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 
Fie. 481,—Dorsal view of the internal organs of layer of circularly arranged 

Peripatus, aa. anus; ant. antemmm; Jn. muscular fibres, and a layer of 
; cox. gid. coxal gland of the seven : : eke ie 
teenth leg; g gen. male genital aperture ; longitudinal fibres divided into 
ne. co, nerve-cord ; neph. nephridia; or. pap. a series of bundles. A layer of 


Std Ow ae More, Wok eed eet elena ll 
dint ee: -eplthelman 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 heemoceele, and is filled with blood. 
The enteric canal (Fig. 481) begins with a small buccal cavity, 


ay 


ane 


: io 
- ayn 


xI PHYLUM ARTHROPODA 609 


enclosed by the circular lip raised up into a number of papille 
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 (phar.) leading to a narrow «asophagus. 
The part which follows, the mesenteron 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— 
Ze, 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 EEE 
rarely branched tracheal 6) 
tubes, lined with a thin = Fic. 4s2.—Section through a tracheal pit and diverg- 


ors xk ss ing bundles of tracheal tubes of Peripatus. ty. 
chitinous layer exhibiting trachew; tv, c. cells in walls of trachee; tr.v. 
fine transverse striations. tracheal stigma; tr. p. tracheal pit. (From Cum. 


: fi Nat. Hist., after Balfour.) 
Groups of these open 1n 


little depressions of the integument, the tracheal pits (¢7.p.), the 
external openings of which are known as the stigmatu (¢7.0.). 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, 
cou. gid.), 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. edwardsit—they 
are only developed in the male. A pair of larger glands—the 
slime glands (si. gid.)—opening at the extremities of the oral 
papilla, 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 (bin.) situated in the 
head, and of two longitudinal nerve cords (ne. co.) which run parallel 


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, 
(2.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 antenne., 
The nerves to the jaws are just where the brain passes into the 
longitudinal nerve cords. 

The cxcretory 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.¢.), 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. 


Fic. 483.—Peripatus capensis, nephridium frum 


the ninth pair of legs. 0.s, external opening ; Reproductive organs. 
p-f, internal opening into the lateral compartment > 
uf the body-cavity ; s, vesicle of nephridium; —Peripatus has the sexes 


sc. 1, s.c. 2, s.e. 8, sc. 4, successive regions ot ee 
coiled portion ; s.0.t., third portion of nephridium distinct. In the female 


is not shown,” (kro the Con. Net waivanee there are two tubular 
Balfour.) : ovaries and two wtert, 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 receptaculum seminis 
and receptaculum ovornm. 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 vesicula seninalis ; 
this is followed by a long, narrow, coiled vas deferens. The two 
vasa deferentia unite together to form a median tube—the ductus 
gjaculatorius—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 


x1 PHYLUM ARTHROPODA 611 


complicated cases which enclose masses of sperms to form 
spermatophores. 

Development.—The differences between the species of Peripatus 
as 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 egg encloses a considerable amount of food-yolk, in others the 
quantity of food-yolk is smalJ, and nutriment is obtained from the 
parent. 

In P. nove-zealandiw there is a superficial segmentation. The 
first segmentation-nucleus is itself superficial, and segmentation 


Fic, 484.—T'wo early stages in the development of Peripatus nove-zealandiz. A, transverse 
section of an ovum in which the yolk is nearly covered by the blastoderm (bl); B, transverse 
section of an ovum in which the blastopore (blp.) 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. ‘lhe peripheral nuclei multiply rapidly and grow round 
the yolk so as completely to enclose 16 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—/. 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 surtace 
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 appled 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 wterine 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 bands, 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 cwlome. 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 


XI PHYLUM ARTHROPODA 613 


Arthropods, and presents some striking points of resemblance to 
the Chitopoda. 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 


Fic. 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 hlastopore (bl.) 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 (tn.); the whole embryo has now become strongly curved towards the dorsal 
side, (After Balfour.) 


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 RR 


614 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 Cheetopoda. 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.’ 


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 antenns, a pair of eyes, and two or three pairs of 
jaws; the body is not distinguishable into regions, but consists 
of anumber of similar segments, each bearing either one pair of 
legs or two pairs. A system of air-tubes or trachez, 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 antenne and jaws, anda 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. 


OrnvDER 1.—PAuROPODA. 


Progoneata with ten trunk-segments and nine pairs of legs, one 
pair to each segment except the first. Antenna with several 
flagella. Trachez not known. The order includes only the single 
genus Pauropus (Fig. 489). 


1 Ag 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, 


XI PHYLUM ARTHROPODA 615 


ORDER 2.—DIPLopop, (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 seginents, 
and in which there is only a single 
pair of branching trachex, 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- Fi. 486, Seolopenarella 
: immaculata. (From Leuck- 
tremity of the body. art, after Latzel.) : 


ORDER 1.—CHILOPODA (SYNGNATHA). 


Opisthogoneata with numerous (15—173) trunk-segments, 
each bearing a single pair of legs. Numerous trachez 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 off asin an Insect; it appears to be composed of about 
four fused segments. The antenna consist sometimes of many, 
sometimes of comparatively few segments; in Puuwropus they are 
branched. <A pair of eyes, situated on the dorsal surface of the 
head, consist of aggregations of ocelli except in Scwéigera, in which 
there are compound eyes, differing, however, in their structure 
from those of Insects. There is a movable labrum, a pair of 
mandibles, and two pairs of maxille. The mandibles have no 

RR 2 


616 ZOOLOGY SECT. 


palps; one or both pairs of inaxille usually possess palps; the 
second pair of maxilla 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 poisonsjaws (maxillipedes), by 
means of which the Centipede inflicts its poisonous bite. 


Path. wre Lay, 
sor (aes Cena brn 


stone 


unt 


z : a SS 

A AS 
awe 
ESS SN 
ee, fay 
fo Wd] aly 

y LH 
fi 


ve side. ant. antenne ; brn. brain: cor. ap. 

coxe of appendages ; ft. 15, fifteenth pair of legs ; 

int. intestine; mal. Malpighiin tubes; mrp. 

Fic 487.—Scolopendra. (From maxillipedes ; ne. co. nerve cord; os. cesophagus ; 
Cuvier's Animal Kingdom.) stom, stomach. (From Leuckart.) 


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 


XI PHYLUM ARTHROPODA 617 


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 (maillipedes), 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 fitth 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. 636). 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 tubcs, 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 trachew, There is one pair of stigmata in 
each segment in the Diplopoda, and the branch- 
ing trachee do not anastomose. In the Chilo- 
poda the number of stigmata is in most cases 
less than the number of segments, and the 
trachee anastomose, often forming longitudinal 
trunks which may extend throughout the body. 
In Scutigera, the stigmata are unpaired and 
dorsal, and lead, not into trachez, but into air- 
sacs or lungs. 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 cesophageal connectives, 
and a ventral nerve-cord consisting of a series 4... sso panropus 
of double nerve-ganglia, one in each segment, — huxleyi. nttm 
with double connectives between them. The 74 e"b “ter bse 
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. 


618 ZOOLOGY SECT. 


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-nuclens 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 


Fic. 490.—Two stages in the development of Strongylostoma, one of the Diplupoda. 
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. Stomodeum and proctodeum are developed as invagina- 
tions of the surface layer. The thickening of the blastoderm gives 
rise to a germinal band in which 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 


XI PHYLUM ARTHROPODA 619 


capable of being grouped in the same orders as the living repre- 
sentatives of the class, and are 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 Insecta—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-tlies, 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, antennxe, mandibles, and two pairs 
of maxille, 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 trachez 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 
orventalis or P. americant). 


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 SECT. 


lurking-places in the day-time, and is almost omnivorous in its 
diet. It isa 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 1s the head, elongated verti- 
cally, bearing the very long slender feelers or antenna and the 


Fic, 491.—Periplaneta orientalis, male. A, dorsal view. B, ventral view x2}. abl. ab? 
a)9, abl, first, second, ninth, and tenth segments of abdomen ; ant. antenne; ¢. cerci; el. 
clypeus ; cx. coxa of third log; Z&. eye; el. elytra; ep. epicranium; j: fenestra; fe. femur of 
third leg; kid. head; fgl. ly. lg. legs; Up. labial palp; i. labrum; mn. mandible; mp. 
maxillary palp ; p.p. style on ninth abdominal segment, internally to which a podical plate 
is seen; th]. (th. in B) th2. th®. segments:of thorax; ti. tibia; tv. 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 elypeus, 
or portion extending vertically downwards ; and two lateral parts, 


XI PHYLUM ARTHROPODA 621 


the gene, in front. The eyes are a pair of reniform black patches 


lor 


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- 
tennee, 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 


: MX, 
known, but which may: Fic. 492.—Mouth parts of the Cockroach. /br. labrum ; 
be an abortive repre- m mentum ; md. mandible ; ml. anterior pair of maxille } 
S : me, and ai, outer and inuer divisions of the first and 
sentative of the simple second pair of maxilla; mg. second maxille; pl. labial 
Ser eye , 4 palp ; pm. maxillary palp; st. stipes; sim. submentum: 
eyes or o¢ ella found im (From Lang's Comparative Anatomy.) 


most Insects. 


Movably articulated with the lower or ventral end of the 
clypeus is a broad plate, the dabrwm or upper lip (Fig. 492, br.) 


Fic. 493.—Periplaneta americana. 
Lateral view of the head and its ap- 
pendages. cerv. one of the cervical 
sclerites; cy. eye; gen. gena; man. 
mandible ; maxl., first pair of max- 
illee ; max?, second pair of maxille 
(labium), 


overhanging the aperture of the 
mouth. Below the gene 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 
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 maxille 
(ma), mie.t), Each maxilla exhibits 
a structure comparable to the funda- 
mental type of the appendages of 
the Crayfish :—a basal part or proto- 
podite, consisting of two segments 
(podomeres), supporting an internal 


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 (mzt.), and an outer, softer, more elongated—the galea (me.). 
The exopodite forms a palp, the maxillary palp (nm.), consisting 
of five podomeres. Behind these are the second maxille, 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 (m.) and submentwn (sm.), so that the two appendages 
form a sort of lower lip called the labiwm. The endopodites taken 
together constitute what is termed the /zgula ; each is divided into 
two parts like the endopodite of the first maxillz. The exopo- 
dites form three-jointed palps, the labial palps (pl.). 

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 sclerdtes (cerv.). 

Each of the three segments of the thorax—known respectively 
as prothorax, mesothorax, and metathorax—is covered over dorsally 
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 ait 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 cova ; 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 sete 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 sternwm, both of which are thinnish and flexible— 
the terga and sterna of succesive segments overlapping one another 
trom 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 


XI 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 cerct. 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 antenne probsbly correspond to the antennules of the Crayfish 
—the antenne 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. Antenne. 
Antenne. Absent. 
Mandibles. Mandibles. 
First: maxilla, First maxilla. 
Second maxille. Second maxillze (labium). 
First maxillipedes. First legs. 
Second maxillipedes. : Second legs. 


Third mavyillipedes. 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. ‘T'wo 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. 


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 (obl, stern.), 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 
oe strongly developed. 

7 Between the body-wall and 
the alimentary canal is a cavity 
taking the place of the ceelome, 
but in reality forming a speci- 
ally developed part of the 
blood-vascular system (hamo- 
cle). This is bounded extern- 
ally by an irregular wall, formed 
of a mass of polygonal cells 
constituting the fut-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 cach gland is 
Fic, 494.—Ventral portion of the muscular an elongated thin-walled sac— 

system of the Cockroach. add. cor. ad- the salivary receptacle (sal. 7ree.). 


ductor of coxa; abd. cor, abductor of coxa ; 


ext. fem. extensor of femur ; Ist tera. stern. The duct given off from the 
first tergo-sternal ; long. stern. longitudinal 7 as sis 
sternal ; obl. stern. oblique sternal. (After salivary receptacle joins that 
sasciaiieb seams of the opposite side, and the 
median duct thus formed is 
joined by a single duct (sad. 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 
esophagus (ws.), which leads to an elongated saccular dilatable sac 
—the crop (cr.). On this there follows the proventriculus 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 


addcor 


aba.cox 4 


exh sem 
Mtergstern 


g 


long.stern 


Ne 
i.) 
| ns 


ALE 
a 


obl stern 


lerg. stern 


XI PHYLUM ARTHROPODA 625 


given off eight tubular hepatic cea (hep. cw.)—blind tubes 
somewhat narrower than the stomach. The point of junction 
of the stomach with the intestine is marked by the presence of 


orn 
“Ry ad = TENET 
ope } [ sal du thor.gang! 
ant~ We uy gang 
A Ped 
f N)-ee~ 
J = 
oho plp 


Fic. 495.—Semi-diagrammatie view of the internal organs of female Cockroach, dissected 
from the left side. The heart is not represented. abd.!, abd 5, first and fifth abdominal segments; 
abd. gang.6 sixth abdominal ganglion; an. anus; ant. antennary nerve ; brn. brain; cer. 
cercus ; cee, ceca; coll. colleterial glands; cr. crop; gizz. gizzard; gon. gonapophyses ; 
inf. yang. sub-cesophageal ganglion; int. intestine; lv. plp. labial palp; l.ov. left ovary ; 
malp. Malpighian tubes 3 wx. plp. maxillary palp. ; od, points to the external opening of the 
median oviduct (vagina); os. cesophagus ; opt. optic nerve; rec, rectum; 7.0v. right ovary ; 
sal. gld, salivary glands ; sal. vec. salivary receptacle (left) ; sal. du. salivary ducts, indicating 
the point at which the median duct of the salivary glands unites with the median duct of 
the salivary receptacles; spi. stigmata; st. 7, sternum of the seventh segment; te. 10, 
tergum of the tenth segment ; th1, th?, 113, first, second, and third segments of the thorax ; 
thor. gang}, 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 caca, 
is of the nature of a mesen- 
teron, the region in front = © 428 ss 
being a stomodrum, and “\idetreoéuroach. {After Minlhand Denny) 
that behind a proctodaum. 

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 


626 ZOOLOGY SECT. 


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 museles (Fig. 522, 2.). 
Respiration takes place through the instrumentality of a 
system of air-tubes or trachew (Figs. 497 and 498), opening on the 
surface at the stigmata, to which reference has already been made. 
These trachee form a richly ramifying system extending to all 
parts of the body. They possess a chitinous internal lining, 
supported by means of a spirally- 
ae wound, fibre-like thickening. By 
== means of this system of air-tubes 
( yy air is conveyed throughout the 
i body to all parts, and there is 


N =| thus ensured the rapid and com- 
2 eS. plete oxygenation which the 
‘A= 9 3/ functional activity of the Insect 
Aes requires, 

[= The nervous system consists 
= of a brain (Fig. 495, lrn., and 


KS 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 
S&S head, and divisible into two parts, 

F “ie. eo Seana anterior and posterior, From the 
layer; 6, nuclei. “(From Gegenbaur.) anterior part 1s 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 antenne. It is supported by 
a chitinous framework—the tentorium. From the brain there run 
backwards a pair of wsophageal connectives (conn.), passing, one on 
each side of the cesophagus, downwards and backwards to the sub- 
esophageal ganglia. The latter, which are situated between the 
submentum and cesophagus, give off a pair of connectives, passing 
backwards to the first thoracic ganglia. From the sub-cesophageal 
. ganglia are given off the nerves to the labrum, the mandibles 
and both pairs of maxille. 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 


XI 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 cesophageal connectives by two nerves, which 
join above the cesophagus to form a median frontal ganglion. 

The organs of special sense are the eyes, the antenne, 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 antennz and palpi, 


br 
oh opt 
Foon 


| f ree a” 
Yar a 
fr 0700 A : 7 Pl 
Be s:: 
l f \ 
: 4 { 
4 i 


Fic. 498.—Cockroach. View of Fic. 499.-Coekroach. General view of the 
the arrangement of the principal nervous system. abd, sixth abdominal gang- 
trunks of the tracheal system. lion ; ant. antennary nerve ; br. brain; conn. 
(After Miall and Denny.) cesophageal connective ; inf. sub-cesophageal 

ganglion ; opt, optic nerve ; thor,! thor,2 thor,3 
first, second, and third thoracic ganglia. 
(After Miall and Denny.) 


together with the anal cerci, act as organs of touch. ‘In addition, 
certain sete: on the antenne 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 vesiewle 
seminales, two tufts of whitish ceca, which together constitute 
what is known as the “mushroom-shaped gland”; these open 
into the anterior end of the gaculatory duct (duct. ¢7.), an un- 


628 ZOOLOGY SECT. 


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 gonapophyscs, which subserve copula- 
tion, 

In the female there are two groups of ovarian tubes or ovartoles, 
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 (ed.). Each ovarian 
tube has a beaded appearance, owing to its containing a row of 
ova, which inerease 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 


Fic. 500.—Cockroach. Male 
reproductive organs, lateral 
view. duet. ej. ductus ejacula- 
torius with mushroom-shaped 
gland; stern. 7, sternum of 


seventh segment of abdomen ; Fic. 501—Cockroach. Female re- 
terg. 7, tergum of the same productive organs. coll. gld. colleterial 
segment; fest. testis. (After glands; od. oviducts; ov. ovaries. 
Miall and Denny.) (After Miall and Denny.) 


constitute the spermatheca or receptaculum seminis. A pair of 
ramifying glandular tubules, the colleterial glands (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 cclls, 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 amesboid cells, which are 


xI PHYLUM ARTHROPODA 629 


distributed through a considerable portion of the yolk. These 
(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  j,,, 


502.—A—D, successive stages in the seg- 


into relation with the begin- mentation of the ovum of an Insect ; Ulast. 
3 blastoderm ; peri. peripheral protoplasm ; seg. 
nings of the stomodeum, segmentation cells ; yk. yolk; yk. c. yolk-cells. 


: . . : F Ki helt and Heid fter Block i 
which arises as an invagina- (From Korschelt and Heider, after Blochmann. ) 


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. he ventral plate early becomes divided by a 


VOL. I S338 


ZOOLOGY SECT. 


630 
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 antenne appear is at first post-oral in position, 


Fic. 503 —Ventral plate of embryo Cockroach 
(Blatta germanica),-isolated from the 
yolk. as, amnion and serosa; at, antennary 
lobe; cgl. brain; cpl. caudal plate; lb. 
labrum 3 md. mandible ; mel, m%, first and 


Fic. 504.—Embryo Cockroach just after the 
rupture of the amnion and serosa, lateral 
view of entire egg. Letters as in preceding 
figure. In addition, uf, fatty body; ast. 
caudal styles; b. cephalic end of yolk ; oc. 


second rmaxille; pl, p?, p3, legs. (After 
Wheeler.) 


eye. (After Wheeler.) 
but subsequently becomes fused with a pre-oral segment (pros- 
tomium), so that the antenne 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 
maxillee, 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, 


XI PHYLUM ARTHROPODA 631 


forming a complete two-layered covering for it The outer Jayer 
is termed serosa, the inner amnion re 

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 forma somatoplewre, 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 (Awmocele) is not derived from the 
ccelome 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 antenne situated on 
the prostomium, mandibles, and two pairs of maxille; 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. 


ss 2 


632 ZOOLOGY SECT. 


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 trachee, 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 
Fis. 505.—Lepisma. (After Guerin and ave absent, and the surface 1s 
i 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 je, s0e.-Poaura. (After 
the mouth are masticatory. The prothorax Guerin and Percheron.) 
is not united with the other segments of 
the thorax. Development is direct, or there is a gradual and 
incomplete metamorphosis. 


XI PHYLUM ARTHROPODA 633 


; This order includes Earwigs, Cockroaches, Stick- and Leaf- 
insects, Grasshoppers, and Locusts (Fig. 507). 


Fig 507.—Locusta. (From Cuvier’s Fic.508.—Ephemera (May-fly) and 
Animal Kingdom.) larva, (After Guérin and Percheron.) 


ORDER 3.—NEUROPTERA. 


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-fiies (Fig. 
508), Dragon-flies, Ant-lions, and Caddis-flies. 

ORDER 4,—HEMIPTERA. 


Insects in which wings are usually present, sometimes similar, 
sometimes dissimilar, and in which there is a jointed suctorial 


Fia. 509.—Aphis rogz and larva. (From Cuvier’s Animal Kingdom.) 


634 - ZOOLOGY SECT. 


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. 


Fig, 510,—Cieada. (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. 511.—Culex (mosquito) and larva, Fic, 512.—Bot-fly of thejhorse (Gastro- 
(After Guérin and Percheron). philus equi). «a, -mature insect; J, 

egg attached toa hair; c, d, and e, stages 

in the larval development. (After Brehm.) 


adapted for piercing and sucking. ‘The prothorax is fused with the 
other segments of the thorax. There is a complete metamorphosis, 


XI PHYLUM ARTHROPODA 635 


This order includes Fleas, Gnats, and Mosquitoes (Fig. 511), 
House-tlies and Blow-flies, Bot-flies (Fig. 512), Crane-tlies, and 


“ Daddy-long- legs.” 


Fic. 513,—Butterfly (Pieris), with caterpillar and chrysalis stages, (After Guérin and Percheron.) 


ORDER 6.—LEPIDOPTERA. 


Insects with both pairs of wings well developed and covered 
with scales (modified hairs). The maxille 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 pro- 
thorax is fused with the mesothorax. 
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 folded-up membranous pos- 
terior wings. The prothorax is movable 
on the other segments. The jaws are fully 


oy Wi )) 


PLL 


Fic. 514.— Beetle (Crioceris) 
with larva. (After Guérin 
aud Percheron.) 


developed, and adapted for biting and chewing. The metamor- 


phosis is complete. 


This order includes the true Beetles (Fig. 514), 


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 genwina, which comprises all the members of 
the order with the exception of the aberrant group of the Earwigs 
(sub-order Dermaptera). Of the Orthoptera genuina there are 
three divisions, the Cursoria, to which the Cockroaches belong ; 
the Gressoria, comprising the Mantide and Phasmide, or Stick- and 
Leaf-insects and their allies; and the Saltatoria, including the 
Grasshoppers, Locusts, and Crickets. The division Cursoria com- 
prises the single family of the Cockroaches (B/attidw), characterised 
by the deflexed head, the flat oval body, the large prothoracic 
tergum, the long antenns, the three pairs of legs similar, with 
large coxee entirely covering the sternal surface of the thorax, the 
five-jointed tarsi, and the presence of anal cerci. Periplaneta 
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 (ewt.), 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- 

onal markings; sometimes it is perfor- 

] kings ; t t f 

ated by numerous pores; sometimes it 

is covered with thin scales; in many 

cases it 1s developed into tactile haiis or 

sete, which may be scattered over the 

Fic, 615.—Section throngh the body, or may be located only on certain 

integument of an Insect. of the appendages—the antenne, the 

base. basement membrane ; e + 7 . 

cut, layers of the cuticle; maxillary and labial palpi, and the tarsi 

epi, epidermis; set. seta, f tl ] I land t 

(After Miall and Denny.) 0 the egs. n some, ganas are presen 

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 larve. 


XI PHYLUM ARTHROPODA 637 


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 epicraniwm is the most extensive; the clypeus, 
situated in front of it, supports the labrum ; the gen are situated 
laterally, and a median piece, the gu/a, 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 aeck, 
covered with soft skin. 
supported only by iso- 
lated cervical sclerites, on 
the ventral aspect. 

The three segments of 
the thorax—pro-, meso-, 
and = meta - thoraa — 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 
dorsal or tergal and ven- 
tral or sternal elements, 
sometimes separate from — Frc, 516.—A, month parts of the Honey-bee (Apis 


mellifica) ; B, the two pairs of maxille. au. eye; 

one another laterally, a. anteriiia: 6: eardo 3 ep. epiphary ns er ae S 
j i a li. ligula; m. mentum 5 wm, ml, first pair of maxilke ; 
sometimes united to md, mandible ; pl. labial palpi; pm. palp of the first 
gether in such a way as pair of maxille; prg. paraglossa ; sm. submentuin ; 


to form complete rings stm. stipes of the first maxille. (From Lang.) 


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 


638 ZOOLOGY SECT. 


4 
an additional pair—the so-called maaillule—occur between the 
mandibles and first maxille. 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 antenne 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 sete, 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 B 


Fic. 517.—Mouth parts of the Diptera. A, of Tabanus; B, of Culex. Lettering as in pre- 
ceding figure : pp. 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 the Hymenoptera (Fig. 
516) the mouth-parts are adapted both for biting and tor licking 
and sucking ; the mandibles (md.) and maxill (mz.) 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 paraglosse (prg.). In the Hemiptera there 
is a proboscis formed from the labium and enclosing the stylet-like 


XI PHYLUM ARTHROPODA 639 


mandibles and maxillz. 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 (mi?) 
enclosing a spine or seta (ip.)—which is a process from the hypo- 
pharynx, a chitinous process on the floor of the mouth—and 
sometimes stylet-lke maxilla (ma), In the Lepidoptera (Fig. 
518) the mandibles are aborted in the adult, and the maxille are 
developed into elongated half-tubes, which when applied together 
form a complete tube (s7.) 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 ae : 
slender; in some ‘hey ond male bettering as n preceding gues 
are expanded to enable pl. labial palp; pm. palp of the anterior maxille ; 
them to act as swimming hy RUPE INE Wibhiee, (CENORN Eaeey 
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 traches, 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 


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 e/ytra—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. 

Hemocele.—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 coslome of other groups; but is 
found, when we study its mode of development, to be a hemocale 

—an extended part of the blood-vascular system. The ccelome 
is apparently represented only by the lumen of the reproductive 
organs. 

i fat-body is always present, either in the larval condition or 
throughout life. It consists of a mass of polygonal cells bounding 
the hemoccele 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 tat-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 (Zphemertdw). 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) 1s present in some Insects ; 
in others it is replaced by a projec- 
tion from the floor, the hypopharyna 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 
esophagus (Figs. 519 and 520 «.)— 
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- 
ventriculus or gizzard (pv). Appended sc. 519—Digestive apparatus of a 
to the stomach at its anterior end are, Bectle (Carabus: auratus). 


e 2 ad, anal glands: ab, their muscu- 

in many Insects, a varying number of lar appendages; ci, stomach ; 

. 2 : ed, hind gut; in, crop; k, head 

tubular blind pouches, the hepatic cca. with mouth ‘parts; @.’cesopha- 
: 1 

2 « . : * = F tri 35 vm. 

At its junction with the small intes- Tualplen tan yelees, (Reana Lane. 


tine, or further back, there open a after Dufour.) 

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 wrinary bladder. The intestine is usually 


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 atr-sacs or air- 
Fic. 520.—Nervous, tracheal, and digestive systems of the PCSCTUOUTS (¢6). In the 

Honey-bee. «. «antenna; av, compound eye; by, be, b3, aquatic larvee of some 


the three pairs of legs; em, stomach; ¢/, hind-gut; him, : 
honey stomach (crop); rd, rectal glands ; st, stigmata; Insects there 1s a 


dy roils of tracheal exptem om, Malpighian week series of soft external, 

simple or divided, 

processes—the tracheal gills (Fig. 521)—attached to the abdominal 

segments and richly supplied with trachee, 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 tracheze. The blood is colourless or faintly yellowish 
or greenish, and contains colourless corpuscles. A contractile 


xI PHYLUM ARTHROPODA 643 


dorsal vessel or heart (Fig. 522) extends through the abdomen—and 
sometimes thorax—immediately below the terga. 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-esophageal 


7 


\F 


tk, 


We 


yy 
te 

Fic. 521.—Thorax and anterior abduininal segments Fic. 522,—Heart of Cockchafer 

of a larval Ephemerid with tracheal gills. (Melolontha). «a aorta; 

HF, hind wings; trl, tr?, tr3, tracheal gills; ¢, m, m, alary muscles. (From 


longitudinal tracheal trunks; VF, fore wings. Gegenbaur.) 
(From Lang's Comparative Anatomy.) 


ganglion or brain, a sub-esophageal ganglion, also double, and a series 
of thoracie 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 antenne, 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 wsophageal connectives pass backwards on either 
side of the mouth from the brain to the sub-cesophageal ganglia. 
These connectives are very short, and, as a consequence, the brain 
and sub-cesophageal ganglia are closely approximated. From the 
latter there originate nerves to the appendages of the mouth—the 
mandibles and the two pairs of maxille. 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. 528), 
and in some of the Diptera this reaches such an extreme that all 
the ventral ganglia, with the exception of the sub-cesophageal, are 
united into one continuous elongated mass. The Insects, like the 
higher Crustacea, possess a visceral or sympathetic nervous system, 
connected with the cesophageal connectives, and passing backwards 
on the cesophagus 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 namber of 
minute hexagonal facets, each of which represents one of the 


Fic. 523 —Nervous systems of four species of Diptera to illustrate various degrees of concentratiun. 
A, non-concentrated nervous systems of Chironomus plumosus with three thoracic, and 
six abdominal ganglia; B, nervous system of Empis stercorea with twu thoracic and five 
abdominal ganglia ; C, nervous system of Tabanus bovinus, with onc thoracic ganglion 
and with the abdominal ganglia closely approximated ; D, nervous system of Sarcophaga 
carnaria, with all the ganglia of the ventral chain united together with the exception 
of the sub-cesophageal. (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 tound 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 retinule. 


XI PHYLUM ARTHROPODA 645 


The ocellt, or simple eyes (Fig. 524), consist of a bi-convex 
transparent thickening of the cuticle—the /ens—and beneath it 
of a group of specially modified epidermal cells. Some of these, 
situated beneath the 
lens, form a transparent 
mass, the vitreous body, 
another set of elongated 


cells being arranged to WZ — Ni 
form the retina. ONO WA Lope ad SS i 
- , SOO WWE qT) 
The antennx and palpi ¢y§ == di Pi NN wt 
are the organs of touch, Cee) I) WA 
and these appendages A004, ip d 
seem to be also the seat \\ WV 
of the olfactory sense. A i 
number of minute pro- Fig. 524.—Section through the occllus of a young 
: Dytiscus larva. ct. cuticle: gk, cells of the vitreous 
cesses sometimes sunk body; hy, epidermis; /, cuticular lens; no, optic 
nerve; re, retinal cells; st, rods. (Froin Lang, after 


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 maxillze and the 
epipharynx are perhaps connected with the sense of taste. The 
results of experiments on the action of the antenns 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. 

Peeuliar nerve-endings, 
supposed to be audrtory, 
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 (se.). Groups 
of these are associated to- 


Fic, 523,—Chordotonal (auditory) organ in the tibia gether to form the auditory 
of Isopteryx apicalis. 6k, bluod-corpuscles ; 


Grenacher.) 


ec. integument; es. terminal fibrous strands at- organ. F 

tached to the integument ; gz, nerve-cells : sc. ter- In certain Insects—the 
minal rods; tr. trachea, (From Lang, after v. 7 

Graber.) 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 TT 


646 ZOOLOGY SECT. 


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 trachee 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 


Fic, 526,—A, female and B, male sexual apparatus of the Honey-bee ; ad, accessory glands ; 
de, cmmon ejaculatory duct ; gd, poison-glands ; gb, poison-vesicle ; ks, bulb of the stinging 
apparatus ; id, rectum, twisted back aud cut off ; nva, accessory sac of the vagina (bursa 
copulatrix); od, oviduct ; ov, ovary ; p, penis ; rs, receptaculum seminis ; sd, colleterial gland ; 
t, testes ; va. vagina ; vd, 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 ovarioles ; 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 extreinity. 
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 


xt PHYLUM ARTHROPODA 647 


this median oviduct, or opening close to it, are receptacula seminis 
(rs.) and colleterial or cement- glands (sd.). Sometimes there is a 
copulatory sac, or bursa copwlatrix (nva.). In the male the paired 
testes (B, t.) vary greatly in form: sometimes each isa 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 ejaculatory 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 ; %.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 females) occasionally produce 
ova which, without fertilisation, develop into drones (males). In 
one or two groups, including the Scale-Insects (Coccidw) and 
Gall-Insects (Cynipide), males are never developed, so that repro- 
duction is exclusively parthenogenetic. Padogenesis accompanies 
parthenogenesis in certain Diptera ; ¢¢., the /arvw 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. 


4 
4 
lo 


648 ZOOLOGY SECT, 


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 
Fis. 427,—A—D, successive stages in the seg- formation of the amnion and 


mentation of the ovum of anInsect U/ast. 
blastoderm; peri. peripheral protoplasm ; the development of the meso- 


sahis “(vont Korschelt and Heider, citer derm and endoderm. In some 
teen 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 segmentally arranged ectodermal involutions 
(Fig. 530, st). 


XI PHYLUM ARTHROPODA 


649 


Metamorphosis.—In some instances the young Insect, when 


it escapes from the egg-mem- 
branes, has exactly the form of 
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 emago, 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 


blast 


Fic. 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; aun. f. fold of the amnion ; 
anuin. cav. cavity of the amnion ; blast. blasto- 
derm covering the yolk ; ect. ectoderm ; end. 
endoderm; gast, invagination of ventral 
plate; ser. serosa; vent. pl. ventral plate ; 
yk. yolk. (After Korschelt and Heider.) 


of jointed legs, and the abdominal segments short unjointed pro- 
leys (Fig. 513). In most instances the larve differ widely from the 


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. 


E 


Tic. 529.—A—E, ventral view of five stages in the development of Hydrophilus. « 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; *, procephalic lobes; 7, groove-like medio-ventral in- 
vagination ; s, germinal bands covered by the amnion. (From Lang, after Heider.) 


Fic. 530.—A 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; mx, second 
maxilla; 1, pe, py, thoracic legs; p4, 75, 7, P9, rudiments of the appendages of the first, 
second, fourth, and sixth abdominal appendages; st, stigmata; vk, 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 imaginal 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 larvee (“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 larve 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 larvee of the Ichneumons, which develop in the interior of the 
bodies of other insect-larve ; also of the larve 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 IT, 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°77 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 28. Insects are also dis- 
tinguished among the Invertebrata by the keenness of their 
senses. The sense of sight is, as we should expect from the 
elaborate character of the optic organs, the most highly developed, 
many Insects having been shown by experiment to have a keen 
sense of colour; but a sense of smell, the seat of which is in the 
antennie and palpi, can be shown to exist in a high degree, and 


a b c 


Fic. 531.—Honey-bee (Apis mellifica). «a, queen (perfect female); b, worker (imperfect 
female), and c, drone (male). (After Brehm.) 


the parts about the mouth bear nerve-endings concerned in a well- 
developed sense of taste. A sense of hearing does not appear to 
be universally present, but is well marked in such forms as produce 
sounds, At the same time Insects are remarkable for the instincts, 
often leading to results of an elaborate character, which guide 
them in the pursuit of food and the protection and rearing of 
their young. Among the insects which are the most highly 
endowed in this respect are some—the Ants, Bees, Wasps, and 
Termites—which live together in organised associations or com- 
munities, the various individuals composing which are distinguish- 
able into seawal individuals, neuter workers, and soldiers (Figs. 531 


Tic, 582.—Red Ant (Formica 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. 

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; but they only 
become abundant in the Carboniferous. All the Paleozoic Insects 
belong to a group which has been regarded as a distinct order, 
and has been named the Paleodictyoptera. The members of this 
group are characterised rather by the absence of the special 
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 Phasmide among the Orthoptera ; others repre- 
senting the modern Day-flies among the Neuroptera; 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, 
isa 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 1s typically composed of a number of distinct 
segments ; in some cases cephalothorax and abdomen are amalga- 
mated. There are no antenne in the adult Arachnid, though 
rudiments of them have been found in the larvee of some species. 
The first pair of appendages of the cephalothorax (probably repre- 
senting the antenne of the Crayfish) are the chelicere ; 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 trachew, 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 (Huscorpiv 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 


oh) 
CP emer (eee Cem (ERG ‘ 3 H Nt c- be. 


= 


Fic. 533.—Euscorpio. (From Fic. 534.—Scorpion. Ventral surface of the 
Cuvier’s Animal Kingdom.) cephalothorax and pre-abdomen. chel. cheli- 
cere; op. operculum ; pect. pectines; ped. 
pedipalpi; stig. 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 


x 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 porson-glands. The wnal 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 /abrwm. 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 pedipalpr (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- 
cere, 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 (pect.)\—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). Pra, pee reer 
All the appendages of the Scorpion are post- i Scorpion. (Alter 


oral in position, and the most anterior—the 
cheliceree—are probably best regarded as cor- 
responding to the antennae of the Crayfish, the equivalent of 
the Crayfish’s antennules and of the antennz of the Cockroach 


656 ZOOLOGY SECT 


not being present. ‘The pedipalpi would then be the homologues 
of the mandibles of the Insect and the Crustacean. 


CRAYFISH. CocKROACH. SCORPION. 
Antennules. Antenne. Absent. 
Antenne. Absent. Chelicere. 
Mandibles. Mandibles. Pedipalpi. 
First maxillee. First maxillee. First legs. 
Second maxillie. Second maxillee. Second legs. 
First maxillipedes. First legs. Third legs. 
Second maxillipedes. Second legs. Fourth legs. 
Third maxillipedes. Third legs. 


Digestive system.—The narrow mouth leads into a large 
chamber with elastic walls, the phavyna:; 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 cesophagus 
leads, receives the ducts of a pair of salivary glands (Fig. 537, 
sal. gid.). 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. dw.) 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 (mal.)\—-which act as the organs of renal 
excretion. 

Circulatory organs.—An elongated tubular heart (Fig. 536, 
hrt.) hes 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 aorte (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. 


657 
penings 
ry sac ls 


in the Scorpions are in the form of 
Each pulmona 


(pul.), the stigmata or external o 


PHYLUM ARTHROPODA 
The organs of respiration 
pulmonary sacs or book-lungs 
eady been referred to. 


of which have alr 


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Into the 


The lining 
paces between the Jaminw the air penetrates, 


membrane is raised up into numerous delicate lamin lying 
rallel with one another like the leaves of a book. 


a compressed chamber lined with a thin cuticle. 


numerous Narrow 8s 


pa 


658 ZOOLOGY SECT. 


and oxygenates the blood which enters the interior of the lamin 
from the ventral sinus. 

A pair of coaal 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 isa bilobed cerebral ganglion or brain (Fig. 536, 
ira.) from which nerves are 
given off to the eyes; a nerve- 
collar formed of a pair of 
asophageal connectives unites 
ventrally in a sub-wsophageal 
ganglion, forming the anterior 
part of a ventral nerve-cord 
(ne. co.). The connectives and 
sub-cesophageal 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. 


Fic. 537.—Dorsal view of the internal organs The organs of special sense 
of Scorpion. che/. chelicerze ; hep. liver; é 
hep. du. hepatic ducts; mal. Malpighian are the eyes and pectines. The 


tubes ; mesent. mesentcron ; proct. intestine ; 


sal, gid. salivary glands ; stomo. stomodzeum. lateral eyes (Fig. 556) are 
(From Leuckart, after Blanchard.) 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 ¢estes 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 1s made up of 
three longitudinal tubules with transverse connecting branches ; 
the oviducts open on the operculum. 


XI 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 discoidalis applied. On one side are formed a number of cells 
in the form ofa 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 


Fic. 538.—Three surface views of the ventral 


plate of a developing Scorpion. A, pius italicus), later stage. ap. 
before the appearance of segments ; B, after Il.—VI., abdominal appendages; ch. 
five segments have become formed ; C, after chelicere ; p. 1—4, legs; m. mouth ; 
the appendages have begun to be formed. ped. pedipalpi; pab. post-abdomen, 
(From Balfour, after Metschnikoff.) (From Korschelt and Heider, after 


Metschnikoff.) 


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 cf 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 (ap, J.—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 cheliceree and pedipalpi—and 
four pairs of legs. There are no antenne. The organs of respira- 
tion, when present, are usually either traches 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 1s 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 cheliceree and large chelate 
pedipalp1. 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 chelicerz are very small, the pedipalpi 
similar to those of the Scorpions. The organs of respiration are 
a system of trachee. 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 chelicerwz are simple, the pedipalpi simple or chelate, and the 


XI PHYLUM ARTHROPODA 661 


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 chelicerw are chelate ; the 


pedipalpi elongated and leg-like. The organs of respiration are 
trachee, 


This order includes Galeodes (Fig. 542). 


ORDER 5,—PHALANGIDA. 


Arachnida with an unsegmented cephalothorax, and an abdomen 
of six segments. The chelicerz are chelate, the pedipalpi leg-like. 
The organs of respiration are trachee. 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 chelicersze are sub-chelate, with poison-glands; the pedipalpi 
simple. The organs of respiration are book-lungs alone, or book- 
lungs combined with trachev. 

This order comprises all the true Spiders (Fig. 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 trachee. 

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 UU 


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 (Paleozoic) forms 
of large size (Fig. 550). 


38. GENERAL ORGANISATION. 


The external form in the Scorpionida has already been sufti- 
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 chelicerae are small ; 
the pedipalpi are large, and resemble 
those of the Scorpions in their chelate 
form. Spinning glands are present. 

The LPedipalpi, or Scorpion-spiders 
(Fig. 541), are intermediate in some of 
their external features between the 
Fic, 540-—Chelifer bravaisii, ©C0“PIons and the Spiders. The abdomen 

16, second to sixth pairs of is broad and marked out into a series 

eemaatee Anatomy) "®* 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 cheliceree 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 antenne 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 


X1 PHYLUM ARTHROPODA 663 


% 
& 
a7 . re 


7 Oe 


Fig, 541.—Phrynus. (From Cuvier’s Animal Kingdom.) 


4 
je 
4 


Fic. 542.—-Galeodes dastuguei ?, natural size. 1—6, the six pairs»uf appendages ; 
1, cheliceree ; 2, pedipalpi ; ¢, head ; th. thorax; ab. abdomen. (From Lang, after Dufour.) 


wu 2 


664 


ZOOLOGY SECT. 


appearance of being intermediate between the Insecta and the 


other groups of Arachnida. 


lic, 543,_Npider (Epeira diadema), 


thorax by a constriction. 


The cephalothoracic region is divided 
by a constriction into two parts, 
head and thorax, the latter made up 
of three segments. The chelicere 
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 chelicere. 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- 

The chelicere (Fig. 544, 4) are sub- 


chelate, and the duct of a large poison-gland opens at the extremity. 


TT wy 
wk 


Fie, 544.—4, Chelicere, and B, pcdipalpi of female of Bpeira diadema. (After Leuckart.) 


The pedipalpi (Fig. 544, B) are elongated, and end in simple 
extremities ; in the male (Fig. 545) the terminal joint 1s modified 


to serve for the reception 
ference of the sperms. 


and trans- 
At the ex- 


tremity of the abdomen is the spinning 
apparatus or arachnidium (Fig. 55], 
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 


Fic. 


545,—Pedipalpi of male of 
Epeira diadema. /!/. III. 
IV. V. podomeres: bb, sac; sph. 
spiral tube. (After Leuckart.) 


is composed. The fine threads of viscid secretion issuing from 
the ducts harden on exposure to the air, and are worked up 


xt 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 Phalangidu, 
or “ Harvest-men,” the cephalo- 
thorax is not constricted off 
from the abdomen. The cheli- 
cerze are chelate, the pedipalpi 
short and leg-hke, 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. 2 a 
Sometimes the basal portions of Fia. seas CA pie ie scabizi). 
the pedipalpi form a suckin 
pobnes anche the stplet ike chelicerwe, modified to form 
piercing organs; sometimes these appendages are claw-like or 


Fic, 547.—Water mite (Prombidium fuliginosum), female chel. chelicera ; pe. pedipalpi. 
“ (After Leuckatt.) 


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 claws, between which 
there may be discs or stalked suckers. 

In the -Yiphosura 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 chelicere (1), 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 
(opere.), overlapping all the 
posterior appendages ; on its 

Fra. 548.—Limulus. Dorsal aspect. posterior face are the two 

CE) 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 
chilaria. 

In the Lurypterida (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 


XI PHYLUM ARTHROPODA 667 


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 branchis, 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,insome Acarida, 
and in the Xiphosura. ! 
In the Solpugida and 
Phalangida they oc- 


cur on the bases of Fic. 549.—Ventral view of Limulus. 1—6, appendages of 


7 . cephalothorax ; abd. abdomen ; ceph. cephalothorax ; opere. 
the last pair of legs . operculum, behind which are seen the series of abdominal 
in the Araneida and appendages : tls, caudal spine or telson, (After Leuckart.) 


Xiphosura, as in the 
Scorpion, they are found on the bases of the fifth pai of 
appendages. 

Alimentary system.—The cesophagus (Fig. 551, es.) of the 
Spiders is expanded behind into a special sucking stomach (suck. st.). 
The» mesenteron (mesent.) gives off in the cephalothorax a pair 
of large diverticula from each of which arise five narrow 
diverticula (cwe.) which enter the bases of the pedipalps and legs ; 
in the abdomen it is surrounded by a mass of cells commonly 


668 ZOOLOGY SECT. 


termed “ liver” (hep.), the ducts of which open into it. The rectum 
or proctodewm (rect.) is dilated; the dilated portion (rect. cae.) 
gives off two pairs of Malpighian tubes (mai.). 

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 
Ze ee tubes. 
EZ LSE ile In the Xiphosura, 
5 eee NG the mouth (Fig. 552, 
mo.), Which is situ- 
ill eas ated some distance 
EN rea behind the anterior 
6\ pied extremity of the 
7 Bes body, leads into a 
re 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 
Se) een “liver” (2. div.). 
ane Be ea AR ea aoe A i 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. 

In the various orders the organs of respiration differ a good 
deal in their character. In the Pseudoscorpionida they take the 
form of branching ¢rachew similar to those of Insects. In the 
Pedipalpi there are two pulmonary sacs or book-lungs similar to 
those of the Scorpions. In the Solpugida there is a system of 


PHYLUM ARTHROPODA 669 


XI 


ry sacs 
ig. 554), 


are either four pulmona 
ary sacs and a system of trachew (Fi 


trachee. In the Spiders there 


(Fig. 553), or two pulmon. 


[vp1O¥ ‘OM fSaAIOU [RUTTMOpgR ‘av “pqn 
4 I 


‘Ieptids snowowmnoudip v jo sues 


Seetaorpatya “ayo : wR. { UIMIpIUypRae 


1O [WUAIJUL aq} JO MATA [AOI] OYLUMUTRASEIGE— "Tee “HT 


Tracheve are present in the Phalangida and also in the majority 


In the Xiphosura the organs of respiration are 


of the Acarida. 


670 ZOOLOGY ‘ SECT, 


external appendages or gills (book-gills), in the shape of delicate 
lamin 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 


ver.stit 


Fia. 552.—Diagrammatic view of a median longitudinal section of Limulus. ald.app. abdominal 
appendages ; an. anus; brn. brain: chil. chilaria; hep. du. opening of one of the hepatic 
ducts ; ht. heart ; int. intestine ; l. liv. “liver” ; mo. mouth ; ne. co. nerve-cord ; ws. esophagus ; 
operc. operculum ; tels. telson ; ven. sinus, venous sinus ; 1—4, 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 


Fia. 553.—Book-lung of a Spider Fic. 554.—Main branches of the tracheal 
(Zilla callophylla). a, system of a Spider. st. stigma, 
axis; b, laminz; st, stigma. (From Hertwig, after Bertkau.) 


(From Hertwig.) 


cephalothoracic ganglia to form a single mass perforated by the 
cesophagus, 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 


XI PHYLUM ARTHROPODA 671 


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.) Fi, Sion of the ook gis of Emus with 

In Limulus the com- Lankester.) 
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 papille, 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 
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- 
Fic. 556.—Section of the lateral eye off Euscorpius face of which follicles 

italicus: iat. intermediate cells; lens, cuticular ei : 

lens; nere. ce terminal nerve-cells ; nerv. f. nerve- project prominently 3 

Ws Bere rhabd. rhabdomes. (After sometimes they unite 

into a single circular 
ovary. There are two short oviducts even when the ovary is 
single; these unite in a median vagina, which opens on the 


WH CO tnd 


672 ZOOLOGY SECT. 


exterior by a median genital aperture at the base of the abdomen. 
One, tio, or three receptacula seminis (ree. sem.) are present, and 
either open into the vagina or independently on the surface. In 
the male there are two elongated tubular ¢es¢es 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 


Fie. 597. Section of the central eye of Euscorpius. other Arachnids. To 
Let’ i i re. pigi. ini 
etters as in preceding figure. pigm. cells containing capture the Insects 


pigment ; «tr. vitreous body (a specialised part of the 
ectoderm). (After Lankester and Bourne.) 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 materia! 
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. 


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 Paleozoic, ranging from the Lower Silurian rocks to the 
Devonian. 


APPENDIX TO THE ARACHNIDA. 


THE PYCNOGONIDA, 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 PycnoGonipa. 


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 ua 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 superticial resemblance to a nauplius ; but the 


674 ZOOLOGY SECT. 


appendages are simple, and in other respects the larva has no essential likeness 
to the nauplius form. Additional segments with their appendages are formed 


Fic. 558._Nymphon hispidum. 1—7, appeudages ; ab, abdomen ; s. proboscis. 
(From Lang, after Hoek.) 


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. 


Fic. 559.—Pentastomum tezenioides, 
young female. an. anus ; gang. ganglion ; 
hk. hooks; mo. mouth; os. cesophagus ; 
ov. ovary; ovd. oviduct; vec. sem. re- 
ceptaculum seminis; ser. ap. sexual 
aperture; stom. stomach; ut. uterus. 
(After Leuckart.) 


The larve of the species of one genus are 


internal parasites in certain hydroid 
Zoophytes. 


THE LINGUATULIDA OR PENTASTOMIDA. 


The Linguatulida (Fig. 559) are para- 
sitic animals, which, when superficially 
examined, present little appearance of 
athnity 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 (Ak.) 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 cesophagus, 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 
tenioides) inhabits certain cavities—the 
frontal sinuses and maxillary antra con- 
nected with the nasal chambers—in the 


Iv PHYLUM ARTHROPODA 675 


Dog and 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 « 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 
pairs of short legs. 


Tur 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 papille; the buccal cavity 
contains a pair of horny, sometimes partly 
calcified, teeth (styl.). The ducts of a pair 
of salivary (?) glands (sali) open into the 
cavity of the mouth ; there is a muscular 
pharynx (ph.), a narrow cesophagus, 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 Fic. 


560.—IMlacrobiotus hufelandi. 


3 1—IV, appendages ; buee. buccal cavity; 
both sexes are saccular, and open into gid. accessory gland ; mal, Malpighian 
the terminal part of the intestine. Seg- tube; ov. ovary; rert. rectum ; sult. 

. : let 1 recular. The salivary glands ; stom. stomach ; styl. 
mentation is complete and reg : teeth. (From Hertwig’s Lehrbuch, 
young animal at one stage has only two after Greef and Plate.) 


pairs of rudimentary legs, but develops the ate 
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. 


676 ZOOLOGY SECT. 


RELATIONSHIPS OF THE AIR-BREATHING ARTHROPODA.? 


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 trachez, 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 trachee 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 bcok-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 trachez were developed without genetic relationship 
with 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 
ae., 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 antenne 
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 
Chetopoda. The elongated, homonomously segmented body, the 
well-defined head with its antenne, 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 xX 
i 


ee f 


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 
Arachnids 
Insecta 
; Pycnogonida Xiphosura 
Myriopoda Crustacea 


Eurypte rida 


Onychophora 


Tardigrada 


Pentastomida 


Primitive Arthropods 


Annelida 


Fic. 561 


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. 


XI PHYLUM ARTHROPODA 679 


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. 


xX 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. 

Class 1—PELEcyPoDA, including the bivalved shell-fish, such 
as Mussels, Cockles, Oysters, &c. 

Class 2.—AMPHINEURA, including the Chitons and their allies. 

Class 3.—GaAsTROPODA, including the univalved Shell-fish, such 
as Periwinkles, Whelks, Snails, Slugs, &e. 

Class 4.—ScaPHopPoDa, including the Tooth-shells. 

Class 5—CEPHALOPoDA, including the Cuttle-fishes, Squids, 
Octopods, and Nautil1. 


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 margaritifer, 1s 
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 

6380 


SECT. NII PHYLUM MOLULUSCA 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 (ea. sph.) smooth-walled, the other (in. sph.) 
beset with delicate processes or fimbriw. 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 (/t.) 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 


2 A 


ex. sph 
tr.sph 


Fic. 562.—Anmodonta cygnea. The entire animal. A, from the left side; ‘+B, from the 
posterior end; d. p. «. dorsal pallial aperture ; ex. sph. exhalant siphon ; jt. foot; in. sph, 
inhalant siphon ; /g. ligament; m. mantle; un. 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 SECT. 


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 


115 Ay ~ ¢ 


yall ae qa =} Wy 


Fic. 563.—Anodonta cygnea. A, interior of right valve; B, the animal removed from th ' 
shell. «. ad. anterior adductor or its impression ; a. 7 anterior retractor or its impression ; 
d. gl, digestive gland, secn through mantle ; ex. spk. exhalant siphon ; /t. foot; gl. gills, seen 
through mantle ; k. (. hinge-line ; in. sph. inhalant siphon ; kd. kidney, seen through mantle ; 
k. o. Keber'’s organ, seen through mantle; m. mantle; p. «d. posterior adductor or its 
impression ; pe. pericardimn, seen through mantle; pl. l. pallial line ; pl. m, pallial muscles ; 
», r. posterior retractor or its impression ; prc. protractor or its impression. 


hinge-line (Fig. 563, A, h.1.), by a tough, elastic substance, the 
hinge-ligament( Figs. 562 and 568, lg.) 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 MOLLUSCA 683 


ridges, the hinge-teeth, separated by grooves or sockets, and so 
po 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 wmbo (wm.), 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 @ch 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 fa 2S E 
of the carbonic acid in the EZR SE 
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 (pl. 1.) caused 
by the insertion into the 
shell of muscular fibres 
from the edge of the 


en emaniiiniiel 
— pre 


mantle : the streak a Fic. 564.—Vertical section of shell and mantle of 
hence called the pallial Anodonta. c. ¢. connective-tissue layer of 
line Beneath the anterior mantle ; ep. 1, its outer epithelium; ep. 2, its 

Rt inner epitheliam ; nM nacreous layer of shell; 
end of the hinge the Claus: ice prs. prismatig layer. (After 


pallial 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. 7.) 
and of the protractor (pre.) of the foot: one connected with the 
posterior adductor impression is that of the posterior retractor (p.7.) 
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 shel] consists of three layers. Outside is a brown horn-like 
layer, the periostracum (Fig. 564, pre.), composed of conchiolin, a 
substance allied in composition to chitin. Beneath this is a 

VOL. I x ox 2 


684 ZOOLOGY SECT. 


prismatic layer (ys.) 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- 
hgament 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 (/¢.), 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, a.c. 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.”)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 coelome 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.7.) and posterior (p. 7.) 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. 568, pl. m.), which, by their 
insertion into the shell, give rise to the pallial line already seen. 

The ceelome is reduced toa single ovoidal chamber, the pert- 
cardiwm (Fig. 565, pe.), 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 


coelomic 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 inéernal and eaternal labial palps 


(J. int, plp., 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, 
gul.) into a large stomach (st.), which receives the ducts (d.d.) of a 
pair of irregular, dark-brown digestive glands (d.gl.). The intestine 


l.exé. pl, a 
laden “gt 


Fic. 565.—Anodonta cygnea. The animal with most of the left mantle-lobe removed. 
a. 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 ; kd, kidney ; 
l. ext. gl. left external gill-lamina ; 1. ext. plp. left external labial palp; /. int. gl. left internal 
gill-lamina; /. iat. pip. left internal labial palp;/. m. cut edge of left mantle-lobe; ith. 
mouth ; p. ad. posterior adductor ; pe. pericardium ; p. 7. posterior retractor ; pre. protractor ; 
ret, rectum ; +. 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 tirst 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 ndges 
begin in the stomach and are continued into the first portion of 
the intestine. The stomach contains, under certain conditions, 
a gelatinous rod, the erystalline style. 

On each side is a single gill or ctenidium composed of two 
plates or lamine,an inner andan outer, We have thus right outer 


686 ZOOLOGY SECT, 


and inner gill-laminz, and left outer and inner gill-lamine (Fig. 
565, Lext. 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 ranning 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 lamelle, 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 (4. 1.7.), which extend between the 


SS 


TT 
«=: 
y 


gon Mm 


kd WE 


Fic. 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 ; 
ce. pl. gn. cerebro-pleural ganglion ; d.d. duct 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 ; gu/. gullet; i. J. j. inter-lamellar junction ; in. sph. inhalant siphon ; int. intes- 
tine ; kd. kidney ; m. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ; 
pe. pericardium; pd. gn. pedal ganglion ; 7. ap. renal aperture; 7. au. right auricle ; 
ret. rectum; 7. p. a. reno-pericardial aperture; st. stomach; ty. typhlosole; +. ventricle ; 
ce. gn. Visceral ganglion ; w. t. water tubes. 


two lamelle, 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 lamine is: 
due to the fact that each lamellais made up of a number of close- 
set gill-filaments (f.): the longitudinal striation to the circumstance 
that these filaments are connected by-horizontal bars, the inter- 
filamentar junctions 1. f.7.). Atthe thin free or ventral edge of the 
lamina the filaments of the two lamelle 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 PHYLUM MOLLUSCA 687 


minute apertures, or ostia (0s.), 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 (7.), 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 


Fic. 567,-Anodonta cygnea. A, transverse section of outer, and B, of inner gill-lamina ; 
C, diagram of gill-structurc; D, transverse section of gill filament. 4. ¢. blood-corpuscle ; 
6. v, blood-vessels ; ch. chitin; f branchial filaments; ep. epithelium; 7. f. j. inter- 
filamentar junction ; é. /. inner lamella; i. J. j. inter-lamellar junction ; 0. /. outer lamella; 
os. external ostium ; 0s’ internal ostium ; 7, chitinous rods ; w. ¢. 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 (0. 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 


688 ZOOLOGY SECT. 


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 _ 


Fic, 568.—Anodonta cygnea. Three transverse sections, A. B.C. au. auricle; bl. urinary 
bladder ; eat. gl. external gill-lamina ; ft. foot ; i. 1. j. inter-lamellar junction } int. intestine ; 
int. gl. internal gill-lamina ; kd. kidney ; k. 0. Keber’s organ ; lg. ligament; m. mantle; p.ad. 
posterior udductor ; pe. pericardium ; rct. rectum ; 8. br ec. supra-branchial chamber ; sh. shell ; 
ty. typhlosole ; v. ventricle ; ve. vena cava; v. yn. visceral ganglion. (After Howes, slightly 
altered. - 


mantle: in this region the inner lamelle of the right and left 
inner laminz are united with one another, and the dorsal edges 
of all four laminz constitute a horizontal partition between the 
pallial cavity below and the exhalant chamber or cloaca above., 
Owing to this arrangement it will be seen that the water-tubes all 


xu 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 1s 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 aération 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 
feces 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 (0l.), 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 (7. 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 
Bayanus,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 Keber’s 
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 (auw.). 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 
aorte the blood passes into arteries (Fig. 569, art.) avrt.?) 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 (af. 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: ils blood (a7¢.t) is 
returned directly to the auricles without passing through either 


a 


~— v. 
; , | a Od 
NENG SY’ I 
a SE Le 


i\ 
ae | 
| | | af. brv 


wv “= 
| 
i 


aH 


Fic. 569,—Diagram of the circulatory system of Anodonta. Vessels containing aérated blood 
red, non-aérated blue. aj. br. v. afferent branchial veins ; ao. aorta; art. 1, artery to mantle ; 
art, 2, artery to body generally ; aw. 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 isa 
small cerebro-pleural ganglion (Fig. 566, ¢. pl. 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 toa 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- 
visceral connective which passes directly backwards, through the 
kidney, and ends in a visceral ganglion (v. gn.) placed on the ventral 


XI 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-cegophageal nerve mass 
corresponding in part with the brain of Annelids and Arthropods, 
and the pedal ganglia as an infra-cesophageal 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 
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 


i Fic. 570._Statocyst of Anodonta. a,b,c,c’, 
probably derived from the cere- cellular layers surrounding the statocyst ;'ot 

: tolith. (EF Cambridge Natural 
bral ganglion. Sensory cells Wistory.) ec a 


—probably tactile—also occur 
round the edge of the mantle, and especially on the fimbriz 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-lamine, which they 
distend enormously. Thus the outer gill-lamine act as brood- 
pouches, and in them the embryo develops into the peculiar larval 
form presently to be described. 


692 ZOOLOGY SECT, 


Development.—Segmentation of the oosperm is complete, but 
unequal. <A gastrula is formed by the invagination of the mega- 
meres into the micro- 
meres, but the archen- 
teron (Fig. 571, ent.) 
thus formed is quite 
small and insignifi- 
cant, and has no 
physiological import- 
ance untila late period 
of larval life. Certain 
of the cells of the 
gastrula are budded 
off into the blasto- 
cele, where they ac- 
cumulate and form the 
mesoderm (mes.). At 


m- about the same time a 

Fic. 571.—Early embryo of Anodonta. ¢h, vitelline deep invagination (sd.) 
membrane ; ent. archenteron ; m. micropyle ; mes. meso- : re S 

derm ; 7k, polar cells; sd, shell-gland ; sk, lateral cells ; 18 formed, which might 

w, cilla. (From Korschelt and Heider’s Embryology.) easily be mistaken for 


the archenteron, 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.) 1s 


(aes Se 
Pen rate 


Ss 


TO. 


Fic, 572,—Two later stages in the development of Anodonta. ent. archenteron ; ies. meso- 
derm,; s. shell; sd, shell-gland ; so. sense-organs ; 2, 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-lke 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 (f). The mesoderm 
cells give rise to a single immense 4dductor 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 


D.-°7 


Fic, 673.—A, advanced embryo of Anodonta. B, free glochidium ; f, provisional byssus 
s. shell ; sh, hooks ; sm, adductor muscle ; so. sensc-organs ; w, cilia, (From Korschelt and 
Heider’s Embryology.) 


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 inme- 
diately posterior to the former an invagination, the stomodwum (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 proctodzeum. The foot (/w) arises 
as a median ventral elevation behind the mouth, and on each side of 


694 ZOOLOGY SECT. 


it two papille (/:) 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. 


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- 
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 
byssus-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 céenidea, 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 
Fia. 574.—Three stages in the metamorphosis of Ano- system 18 well-de- 

donta. d, enteric canal; f. provisional byssus ; fu, veloped, the largest 


foot ; g, lateral pits ; k, rudiments of gills; m. mouth; 1 ‘en eh 
sh. shell; sm, adductor muscle; so. sense-organs ; v, muscles being either 


cilia. (From Korschelt and Heider’s Embryology.) 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 


XII PHYLUM MOLLUSCA 695 


serve 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 aorte, 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 accessory 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.—PROTOBRANCHIA. 


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-flaments. 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 (Mylilus, 
Fig. 585), Anomia, Trigonia, &e. 


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 (Meleugrina), Lima, Pinna, &e. 


ORDER 4.—EULAMELLIBRANCHIAS. 


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 a 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 (Cardiwm), 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 Poromya, Cuspidaria, &e. 


Systematic Position of the Faamples. 


Anodonta and Unio are two genera belonging to the family 
Unionide, sub-order Integripalliuta, order Hulamellibranchia. 

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 Jntegripalliata. 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 Unionide. 
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 
vf the siphons are correlated important variations in the shell 


XII PHYLUM MOLLUSCA 697 


which are 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 
Arca, 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 (Mytilus 
edulis), the anterior adductor becomes greatly reduced and the 
posterior correspondingly enlarged ; and in another species of the 
same genus (Jf. Jatus) the anterior adductor has completely 
atrophied, the function of closing the shell being performed by the 


Fic. 575.—Anatomy of Peeten. I, palpi; IJ, foot; III, aperture of gonad into kidney ; IV, 
external renal aperture; V, male, and VI, female portion of gonad; VII, pallial cye; 
VII, 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 isa 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 yY Y 


698 ZOOLOGY SRT. 


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 Lsomyaria (A), 


i . 1 dotted line passes 
Fic. 576.—Left valves of A, Mya; B, Modiola; C, Vulsella. The upper 
7 itniagty Uaestainige ViTteR, 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) Heteromyaria (B), and those with large centrally 
placed posterior and no anterior adductor (Pseudolamellibranchs 
and Anomia among Filibranchs) Monomyania (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 


a. O77. ium edule. A, exhalant siphon; B, inhalant siphon; F, foot. (From the 
Bigs a Saree ‘Cambridge Natural fHistory.) 


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 


Al PHYLUM MOLLUSCA 699 
these circumstances the posterior pallial muscles become en- 
larged to form retractors of the siphons, and the portion of the 
palhal line from which they arise becomes, as it were, pushed 
forwards so as to form a bay or pallial sinus (Fig. 578, p.s). Thus 
the shells of species with well- 
developed siphons are sinupal- 
liate, or have an_ indented 
pallial line, while those with 
small or no siphons are %n- 
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 


Fic. 578.—Venus gnidia, inner surface of 
left valve. al, anterivr lateral tooth; am, 
anterior adductor impression; c, cardinal 
teeth ; 1. ligament; tu. dunule; », pallial 
line ; p. 1, posterior lateral tooth ; p. 1, pos- 
terior adductor impression; p. s. pallial 
sinus; uv. umbo. (From the Cambridge 
Natural History.) 


able to remain buried in the 
mud or sand, only the ends of 
the siphons being exposed for the supply of aérated 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 


Fic, 579, Serobicularia piperata, inits natural position, partly buriedin sand, A, exhalant 
siphon ; B, inhalant siphon. (From the Camridge 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 niuscular 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. Hach valve is inequilateral, being divided into 


YY 2 


Vic. 580.—Solecurtus 
strigillatus. = s. af, 
inhalant siphon, s. ef, 
exhalant siphon, the 
two united at Ss. 
(From the Cambridge 
Natural History.) 


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 (Pecten), and 
the inequivalve 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 Reguwienca (A) the left valve is 
spirally coiled, so that it resembles a snail- 
shell, and its aperture is closed by the flat 
lid-lke nght 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 


Fru. 581,—Diagram illustrating the various degrees of union of the mantle-lobes. b.0, byssal 
aperture ; f. fuot; s. a, exhalant siphon; s. 4, inhalant siphon ; J, first point of union between 
siphons ; 2, second, between inhalant siphon and foot ; 3, 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 


XII 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 
Aspergillum (Fig. 584), which lives 
buried in sand, there is a similar, but 
wider 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 


Fic. 582.—Requienia ammonea; B, Hip- 
purites cornu-vaccinum. 4, right 
valve; f, point of fixation. (From the Cam- 
bridge Natural History.) 


sometimes, though not always, dis- 
tinguishable at the apex of each 
valve in the Pelecypoda in general. 

In Nucula, Arca, &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 jr. 5s3.weredo navalis, in 


j -li a piece of timber. P. pallets 
more common 18 the ploughshare like (small calcareous plates support- 
form we are already familiar with in ing the siphons); ss. siphons ; 

‘ T. tube; V. valve of shell. 
Anodonta and Unio, adapted for slowly (From the Cambridge Natural 
History.) 


making its way through sand or mud. 
In a few forms, eg. 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, 


702 ZOOLOGY SECT. 


the foot is sometimes provided with a levator muscle, particularly 
well developed in Nucula and its ailies. 

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 (AZytt/us) 
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 
ee a eee of the byssus. It must be borne in mind 

“(Aftersowerby,.) sd 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 
(¢.g. Nucula) the filaments are short, compressed, and free from one 
another (Figs. 586, 4, 
and 587, A). In Amu- 
stwm (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 "%,"P"piece of woot F- fost: 8, exhalant. siphon, 
U, and therefore con- (From the Cambridge Natural History.) 
sisting of a proximal 
or fixed limb and a distal or free hmb. The flexure takes place 
in such a way that the free limb is external in the outer row 
of filaments, internal im the inner row. Adjacent filaments are 
loosely united by groups of large interlocking cilia (see Fig. 588), 


XII PHYLUM MOLLUSCA 703 


placed at regular intervals, and in this way the outer and 
inner limbs of the filaments are respectively joimed 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 lamin, each 
formed of an outer and an inner lamella: the inner lamella of the 
outer and the outer lamella of the inner lamine are united along 
their dorsal edges, the line of junction representing the axis of the 


pap 


Fic. 586.—Adult specimen of Nucula delphinodonta, represented as seen from the right 
side. Reconstructed to show internal organs. Fully grown specimens are 4 mm. long, 
aa. anterior adductor muscle; bg. byssal gland; cg. cerebral ganglion; /. foot; g. gill: 
h. heart ; int. intestine; (p. labial palp ; qs. cesophagus ; ot. statocyst ; pa. posterior adductor 
muscle; pap. falp-appendage ; pg. pedal ganglion; sto. stomach; vy. visceral ganglion. 
(After Drew.) 

ctenidiuin: the outer lamella of the outer and the inner lamella 

of the inner Jamin: are free dorsally. 

In Mylilus (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 interfilamentar 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 


704 ZOOLOGY SECT. 


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 Jamelle of the night and 
left inner laminz 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 


Via. 587.—Half transverse sections of various Pelecypoda to show the chief kinds of gill. 
A, Nucula; B, Amusium; C, Arca; D, Mytilus; E, Anodonta; F, Poromya. 
a. aperture in branchial septum ; 6. «. lood-vessel ; ft. foot; i. f. inner row of filaments ; 
i. g. inner lamina; 7. 1. inner lamella; i. (. j. interlamelar junctions ; m. mantle; 0. f. outer 
row of filaments; 0. q. outer lamina; o. 2. 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 


XII PHYLUM MOLLUSCA 705 


anterior adductor. It is usually bounded by two pairs of labial 
palps which sometimes attain an immense size (Fig. 586); there is 
never any trace of jaws or other masticatory apparatus. The 
convolutions of the mtestine are some- 
times very complex. The crystalline style 
either les freely in the stomach and 
anterior part of the intestine, or is con- 
tained in a cecal 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. Gi | 
It is possible also that the dissolved tion ; J. filaments. (Prom the 
substance of the style may play the part pence Saleen 
of a digestive secretion, since 1t 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 


Fic, 589.—Dissection of Poromya. I, anterior palp; II, foot ; III, lamella on branchial septum ; 
IV, valve of branchial aperture ; 1V’, anal siphon; V, posterior adductor ; VI, posterior re- 
tractor of foot ; VII, heart ; VIII, ovary ; 1X, branchial septum ; X, anterior adductor. (From 
Pelseneer.) 


tubes or nephridia, opening at one end into the pericardinm 
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 SHOT. 


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 Keber’s organs, glandular 
developments of the wall of the pericardium. 

Circulatory Organs.—The heart is usually perforated by 
the rectum, but les altogether. above it in Nucula (Fig. 586, /) 
and some other genera; the ordinary arrangement seems to have 


Fic. 590.—Sagittal section of part of enteric canal of Donax. I, lower lip; II, intestine; II], 
pyloric ceeun; 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 Arca the ventricle is divided into 
two by a constrictim. 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 (¢. g., Arca) owing to hemoglobin in the 
corpuscles; in some cases it is of a bluish tint owing to the 
presence of hamocyanin. 

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, 


XIT PHYLUM MOLLUSCA 


The most characteristic sense-organs arc 
The  statocyst 


(‘ otocysts”) and the osphradia. 


707 


the statocysts 
“auditory” or 


directive organ—is always placed in the foot, in close relation to 


the pedal ganglion, sometimes embedded in the latter. 


statocysts are developed as involutions of 
the ectoderm and retain their connection 
with the exterior in Nucula (Fig. 586 of) 
and some others. In most cases they 
become closed sacs. The cavity is usually 
cihated, 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 atdominal sense- 
organs, occur one on each side of the 
anus in Arca 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 (Peeten), which has 
a single row (Fig. 575, VIT) all round the 
mantle-border. 


The 


vi A= (" 


Fia. 591.—Nervous system and 
“auditory” organs of Nu- 
cula. I, pleural ganglion ; 
UI, pleuropedal-connective ; 
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 counec- 
tive; XIV, anterior pallial 
nerve; XV, nerve to palps; 
XVI, cerebral ganglion. (From 
Pelseneer.) 


Each has a cornea (Fig. 592 7), a cellular (not 


cuticular) lens (2), 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 


708 ZOOLOGY SECT, 


like that of Vertebrates. The pallial 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 


Wie Mi 
ees 
Za SS eee 


Ws 
oe sh) 


TAR Wee 


Fia. 592.—Vertical section of eye of Pecten. 1, cornea; 2, lens; 3, external epithelium ; 
4, blood-sinus ; 4, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and Heider.) 


filaments of the inner lamina of the gill in Mytilus and some other 
genera. 

Reproduction and Development.—Most Pelecypoda are 
dicecious, 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 


xt PHYLUM MOLLUSCA 709 


frequently takes place in the water after the eggs are laid. Seg- 
mentation is total but unequal, and the gastrula is formed either 


Fic. 593.—Five stages in the development of Ostrea. « anus; bl. blastopore ; m. mouth 3 
ma, stomach ; mes. mesoderm : rk, polar bodies ; 3. shell; sd, shell-gland ; sm. anterior adductor ; 
w, pre-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 stomodseum (7) 


as an invagination of the 
ventral surface, and the 
larva of most forms, un- 
hike that of Anodonta or 
Unio, passes into a stage 
in which it closely re- 
sembles the trochophore 
of Chetopods (Fig. 598), 
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 


Fic, 594,—Veliger larva of Ostrea. «a. anus; dm. 
dorsal longitudinal muscle ; 2. “liver” ; m. mouth ; 
ma, stomach; s. shell; sm. adductor muscle; ss. 
hinge of shell; Vel. velum; xm. ventral longitu- 
dinal muscle, (From Korschelt and Heider.) 


rise to the pedal ganglion and corresponds with the rudi- 
ment of the ventral nerve-cord in Polycheta. The pelecypod 


710 ZOOLOGY SECT. 


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 vel.): 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 (/) arise as a row of delicate processes, at first simple 


Fic. 595.—Two embryos of Cyelas. «. anus; by. byssus-gland; 7. foot; yg. gonad; k, gill; 
m. mouth; m+. stomach and “liver”; mr. edge uf mantle; n. kidney ; p. pericardium ; s’. 
unpaired shell; 8”. rudiment of paired shell; sd. shell-gland ; ed. gullet ; vel. velar areca. 
(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 Nwewla, about 4 mm. long, to the Giant Clam (7ridacna 
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 7rigonia. As far as is known, the colours are all what 
are called “ non-significant,” ¢.e. are of no physiological or ethological 
importance. In this connection the formation of pearls by some 


xi PHYLUM, MOLLUSCA 71i 


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” (Meleagrina margaritifera) 
and by the Pearl-mussel (Unio margaritiferu). 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 (Cardium), Trigonia, &e., 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 Arca 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) inwhich 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 
Kulamellibranchia, 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 


PSEUDO-LAMELLIBRANCHIA 


INTEGRIPALLIATE 
EULAMELLIBRANCHIA 


HETEROMYARIAN 
FILIBRANCHIA 


ISOMYARIAN 
FILIBRANCHIA 


PROTOBRANCHIA 


Fic. 506.—Diagram illustrating the mutual relationships of the Pelecypoda. 


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 : 


XII 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 Neomenic, Proneomenia, Chatoderma, 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 Chetoderma (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 


ri Fic, 597.—Chetoderma 
merely represented by a narrow strip from yh Cater as 


which the cuticle and spicules are absent. m. mouth. (From the 
Cambridye Natural His- 
The ventral groove, when present, usually tory) 


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 Chetoderma it is 
entirely absent. With the ventral groove is connected in front 
an anterior ciliated groove, while behind it isin 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 
VOL. I “A 


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-ventrally 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- 
Fic. 598.-Neomenia Vided with eyes or tentacles. The most re- 

ages poe markable external feature of Chiton is the 

m mouth. (from the presence on the dorsal surface of a calcareous 

History.) ow 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 


CLIL 
Fic, 509.—Chiton spinosus, dorsal view. Fic. 600.—Chiton, ventral view. an. anus ; 
(From the Cambrulge Natural History.) clen. ctenidia ; Jt. foot ; mant. mantle edge ; 


mo. mouth ; plp. palp. (After Pelsencer.) 


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, cfen.) to the number of 


xu 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 (Chetodcrina) a transverse, slit, situated 
ventrally near the anterior extremity. There is a buccal cavity, 
with a radula! insome 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 cecum or 
a double row of cxca. 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. 608, /iv.). 

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 (pert), from the 
rest. In the Placophora the cclome 
(Fig. 603) is an extensive cavity, lined 
with a ccelomic epithelium, and divided 
into three completely separated parts— 
the pericardium, the genital cavity, and 
the general body-cavity. 

Vascular System.—The vascular sys- t 
tem of the Aplacophora is very rudi- — Fis, 601.—Chiton, valves of 

: : shell. (From the Cumbridge 
mentary. There is a heart enclosed in a Natural History.) 
pericardium, and composed, when_ best 
developed, of an auricle and a ventricle (Figs. 602 and 605, per). 

In Chiton there is a well-developed heart (Fig. 608, A/.) 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 
ee 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 two pallial. 
These are connected together by an cesophageal ring, thickened 
dorsally into a single or double cerebral ganglion; and in front of 
this is ‘a second, more slender stomatogastiic nerve-ring with sinall 

1 For a description of the structure of this characteristic organ, sve the 
account of ‘l'riton (p. 726). 


ZZ2 


716 ZOOLOGY SBOT. 


ganglia. The pedal cords (vv) may present in front a pair of 
ganglionic thickenings connected by a commissure, and further 
back there may be a series of enlargements united by commissures. 
The pallial cords (/, 2) are connected behind, above the rectum, 
by a commissure (~, ¢) 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. 


Fic. 602.—Cheetoderma nitidulum, longitudinal section. an. anus; bin. brain; eee. gland- 
ular ceca of mesenteron ; cten. ctenidium ; dia. diaphragm separating off the posterior portion 
of the body; mo. mouth; peri. pericardium and heart; rad. radula; rect. rectum. (After 
Simroth.) 


In the Placophora (Fig. 604, D) there is an cesophageal 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 ctenidia, 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, 


XII PHYLUM MOLLUSCA 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 


7eph ye ent ltu 


Fic. 603.—Diagrammatic longitudinal section of Chiton, specially intended to show the 
relations of the parts of the ccelome, which, except the genital parts, are bordered with a 
thick line. «wa. anus; ent. enteric cavity; ft. foot; gor, gonad; hd. head-lobes; h¢. heart ; 


Halen) mo, mouth; neph. nephridium ; peri. pericardial cavity. (From Simroth, after 
aller. 


Aplacophora. A pair of processes situated in front, at the sides of 
the mouth, have the character of labial palps. In the buceal 
cavity there are cup-shaped gustatory vrgans supplied with 
nerves from the cerebral commissures, and in front of the 
odontophore is a thickening of the epithelium—the subradulur 
organ—containing nerve-endings. Remarkable sensory organs, 
the méercesthetes and the megalesthetes, lie in the canals already 


c c 

1 U U L 
Pe pe pe 
B c D 


Fic. 604,—Nervous system of AMphineura. 4A, Proncomenia; B, Neomenia,; C, Chetoderina ; 
D, Chiton. c, cerebral ganglia; é, 4, pallial cords ; pe. posterior commissure ; s, stomatogastric 
commissure or ring, with ganglia ; v, v, pedal cords. (From the Cumbridge Natural History, 
after Hubrecht.) 


mentioned as occurring in the superficial layer of the shell-valves. 
The megalesthetes may take the form of eyes, with cornea, 
lens, pigment-layer, iris, and retina; in some cases the lens is 
absent. 


ZOOLOGY SECT. 


718 

Reproductive and Renal Organs.—In the Placophora 
the sexes are distinct: in the Aplacophora, with the exception 
of Cheetoderma, they are united. In the Aplacophora (Fig. 605), 
with the exception of Chetoderma, the gonads are paired. 
The sexual products pass into the pericardial cavity and thence 
are carried to the exterior by a pair of ducts (ccelomoducts) 
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. pert. 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 


PHO ANCCONANNCONN ren. 


Fie. 


605,—INeomenia carinata, reproduc- 
tive organs. cop. copulatory organs; gon. 
gonads enclosed in extensions of the peri- 
cardial cavity ; gonod, gonoducts ; peri. peri- 
cardium ; ref. receptaculum seminis, (From 
Simroth, after Wiren.) 


give rise to a gastrula by in- 
vagination. This develops into 
a form of trochophore with a 
ciliated ring, the prototrach. 
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 cight 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 


XII PHYLUM MOLLUSCA 719 


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 


606,—Chiton, nephridial and genital’systems. an. anus; cten. ctenidiag gen. ap genital 
aperture ; gon. gonad ; gonod. gonoduct ; mo. mouth ; neph.ap. nephridial aperture ; ». per ap. 
aperture from nephridia to pericardium. (From Simroth, after Haller and Lang. 


into the segmentation-cavity and speedily assume a_ bilateral 
arrangement. 

Two rings of cells surrounding the embryo develop cilia (czd.), 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 stomodeum, A 


720 ZOOLOGY SECT. 


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 


Fic. 607.—Chiton, development. 4, general view of larva; B, section of early, and C, of later 
trochophore. calc. calcifications (rudiments of shell) ; rev. g. cerebral ganglion ; cil. ciliary ring ; 
cil. t. ciliary tuft at apical pole ; eve, eye ; ft.gl. foot-gland ; mes. mesoderin ; imesent. mesenteron ; 
mo. roouth ; rd. radular sac; sp. spines; visc. 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, cale.), 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 varions colonial 
Ccelenterates. The Placophora are all vegetable feeders, their food 
consisting of minute alge 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 of 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 (Buccinwm wn- 
datum) will be found to conform to the description. 

The shell (Fig. 608) isa 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—z. 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 colwmella — 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 sutwre, 
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 
strize which run transversely 
Fic. 08,—Shell of Triton nodiferus. to the stronger lines. At 
ioe 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 


XII 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 siphonal 
process is for the lodgment 
of a spout-like process of 
the edge of the mantle— 
the siphon. 

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 fits 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 — Fie. 609.—Longitudinal median section of the shell 
with a thin layer of uncal- calcd 
cified chitinoid material, the 
periostracum, beneath which is a thick prismatic layer, and, ining 
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 colwmellar 


724 ZOOLOGY SECT. 


masele (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 
a aan eee found to present a distinct head, which 
G. 610,—Operculum o : 
Triton nodiferus. bears dorsally a pair of appendages—the 
tentacles (tent)—ot a sub-cylindrica] 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), 1s extended forwards, bearing 
the mouth at its anterior end; at other times the introvert is 
completely imvoluted 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 


mantecav coll 


stph 


" tent 


metapa mesopa propa 


Fic. 611,—Lateral view of the body of a female specimen of Triton nodiferus, removed from 
the shell, moderately extended. col, m. columellar muscle ; co/l, collar of mantle; eye, eye ; 
lor, liver; mont. eae, mantle-cavity ; meso. mesopodium ; neph. nephridium ; op. operculum ; 
ov. ovary ; prop.!yropodium ; stom. 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. 


XI 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 vperenlum 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, rt 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 
marked in the part which is capable 
of being protruded from the shell, but 


Fic. 612.—Diagram of the introvert of 
Triton, in longitudinal section, 


is still recognisable ; and an examina- 
tion of the internal organs shows a 
marked excess of development on the 
left-hand side, zc. the side which cor- 
responds with the longer outer side 
of the spiral of the shell. The sur- 


as it appears when almost com- 
pletely extended. The black lines 
represent the wall of the ali- 
mentary canal; the cross-hatched 
part the wall of the introvert ; 
the dotted linc marks the position 
of the opening through which the 
introvert passes. intro. introvert ; 
mo. mouth; ws. lumen of cwso- 


. : shagus: 
face of the part of the animal which atc 


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 pallial cavity. In it are to be 
found the ctenidium, the osphradium, and the 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, cen.) 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 
lamine 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, 7.c. nearer the middle of the body. It presents a central 
axis, connected at right angles with which are two rows 
of close-set delicate lamelle. Like the ctenidium, it is closely 
applied to the wall of the mantle-cavity throughout its length. 
The laminz 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 (bue.). At the sides of the entrance to this cavity 
the investing cuticle 1s thickened to form two distinct horny plates 
—the jaws (Figs. 613, 614, and 615, yaw). 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, ed., 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 esophagus (Figs. 613 and 615, ws.). 
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 


—- post 0€9 


BW, 


& 
yy ovid 


NP ant.aort 


“post.aore 


Fu: 618.—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 
flaps laid out, that’on the left bearing the ctenidium and osphradium, and that on the right the 
rectum and terminal part of the oviduct. The muscular dorsal wall of the body 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 opened so as 
to show the odontophore, and the cesophagus 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 scen, being hidden by the nephridium, and 
the ovary is not represented. an. anus; ant. aort, anterior aorta; wur. auricle ; buc, buccal 
cavity ; cer. buc. con. cerebro-buceal connective ; cer. g. cercbral ganglia ; crop, crop ; clen. 
ctenidium ; int. intestine; jaw, jaw; l. buc. g. left buccal ganglia; l. sul. gl. ieft salivary 
gland; neph. nephridium ; xeph. ap. nephridial aperture ; od. odontophore 3 ws, wesophagus ; 
ws’, anterior end of same, cut and turned aside; osph. osphradium ; ovid. oviduct; ovid’. 
terminal thick-walled portion of oviduct; pleur. g. pleural ganglion ; post. wort. postcrivr 
aorta ; post. ws, postericr wsophagus ; rad. s. radulasac ; ret, rectum ; 7, sal. g/. right salivary 
gland; sal. du. salivary duct; siph. siphon; supru, g. supra-cesophageal visceral ganglion ; 
tent. tentacle; tent. n. tentacular nerve; vent. ventricle, 


728 


SECT. 


direction ; and, when the cavity of the organ is opened, it is found 
that these correspond to numerous delicate folds which extend 


rad 


Fic. l4.—Triton nodiferus. 
buceal muss, fron above, magnified. 
jaw : odont. lateral surface of odontophore ; rad. 
radula; 7. jaw, right jaw. 


far inwards, and almost 
completely block up the 
lumen. On either side of 
the crop is a large gland 
—the salivary gland (Fig. 
613, 1 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 


cesophagus, where the two come into close apposition, becoming 
embedded in the wall of the cesophagus, 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 wsophagus. 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 duets 
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 


mag bod.cav 


Fic. 


care 7ad.sac 


615.-Triton nodiferus. Diagrammatic longitudinal ver- 
tical section of buccal cavity. bod. cur. body-cavity ; cart. car- 
tilage of odontophore; jaw, right jaw; ws. ceesophagus; rad. 
radula; rad. sac. radula sac. 


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 reetwm (reet.), embedded in the glandular wall of the 


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, awr.) 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 if 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 
cesophagus, crop, and anterior cesophagus, 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, and 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 (pl. g.)—lie close 
together over the posterior part of the cesophagus, 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. buc. con.) 
to a buccal ganglion (7. 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 3A 


30 ZOOLOGY SECT. 


nbucg 


cerrbuc.con 


pedg 


fen£Lir 


i -UYTAG 


nimant.rn 


Poteau? 
one INF 
visen Tabd.g 


Fic. 616.—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-cesophageal visceral ganglion ; 1. abd. g. left abdominal ganglion ; 1. br. n. left branchial 
nerve ; l, br. n’. nerves to branchia and osphradium ; J. mant. n. left mantle-nerve ; opt. 2. 
optic nerve; ped, can. cerebro-pedal and pleurv-pedal connectives ; ped. g. pedal ganglia ; pl. y. 
pleural ganglion ; r. abd. gq. right abdominal ganglion ; +. br. 7. right branchial nerve; 7. buc. 
gang. right buceal ganglion ; 7. mant. ». right mantle-nerve ; supra. 4. supra-intestinal visceral 
ganglion ; tent. n. nerve to tentacle ; ¢ésc, n. visecral nerve-branches 


XII PHYLUM MOLLUSCA 731 


downwards and forwards a stout cerebro-pedal connective, and from 
each pleural ganglion a plewro-pedal 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 cesopbagus, and, some distance to the left of the ali- 
mentary canal, expands into a triangular supra-intestinal visceral 
ganglion (supra. g.), situated below the superficial layer of muscular 
fibres. The left pleural ganglion gives off an infra-intestinal 
viscerul 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 


cr 


= cerbuc.con 
GELS, 
Fic. 617.—Triton nodiferus. Lateral view of nerve-ganglia and related parts. Letters as in 


Fig. 616; in addition—ant. aort. anterior aorta; cr. crop; ws. cesophagus; sal. du. salivary 
duct 37. sal. gl. right salivary gland. 


runs towards the osphradium and ctenidium, which it supplies 
with branch nerves, and unites with a stout manéle-nerve (J. 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 (1. abd. g.; 7. abd. g.), and these 
are joined by a transverse commissure, from which a number of 
visceral nerves (vise, 7.) 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 osphradium, which have been already referred to, 
‘are the eyes and the statocysts. The eye (Fig. 618) is a rounded 
invagination of the epidermis with an inner wall or redina (ret.) 

3A 2 


732 ZOOLOGY SECT. 


composed of pigmented and sensory cells. The latter (ve¢inaphores) 
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 (reéinule@) have their free 
extremities much .enlarged, and 
surround the slender ends of the 
retinophores. <A layer of short 
rods (rds.) lies within the retina 
roper. The outer wall is thin, 
and, with the overlying epidermis, 
forms a transparent cornea. In 
the interior of the eye is a clear 
rounded Jens (d.) of dense cuti- 
cular matter secreted by the cells 
ras of the retina ; this is surrounded 
by a less dense vitreous body. 
The sexes are distinct. There 
is a single gonad—ovary or testis 


Si 


GSH 


opt. ¢ 
ta as the case may be—lodged in 
Fic. 618.—Triton. Section of the eye- i is ie3 
co. cornea ; ep. epidermis ; l. lens; opt. the visceral spiral. The F sper m- 
optic nerve; rds. layer of rods (the line duct is a white tube, thickish and 
is not continued far enough inwards) ; ? é. 
ret. retina. 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. 


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 


XIL 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 
veliger 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 (Patellide). 


Sub-Order 2.—Rhipidoglossa. 


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 (Haliotide), 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 lamellw, attached throughout its length to the wall of the 
mantle-cavity. 

Sub-Order 1—Platypoda. 


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 Aplysiide, 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 Pleropoda. 


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 branchize usually arranged in a circlet 
around the anus, or in rows on the dorsal surface, or laterally under 
the edge of the mantle. 


This sub-order includes Doris, Holis, Tethys, 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 7'riton, 
which is the only member of the family Z’ritonide, belonging to 
the sub-order Platypoda, The family Tritouide 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, 2.¢., towards the head-end of the animal. The mantle- 
cavity and associated parts (pallial 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. 


B 


A 


Fic. 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 
symnmetrical 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. auus: aort. aorta; cer. g. cerebral ganglion; l. cten. left 
ctenidium ; 1. vise. com. left visceral connective ; mant. mantle ; imo. mouth; neph. ap. nephri- 
dial apertures ; ped. y. pedal ganglion ; pl. y. pleural ganglion; r. cten. right ctenidiuin; r. 
aaa right visceral connective; visc. com. visceral connectives. (After Korschelt and 

eider. 


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 


XII 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 (vise. 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-intestinal. 

Universally accompanying the process of forward displacement 
of the pallial complex, except in Huliotis 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 Fre. 620.—shell of Solarium per- 


] 1 spectivum, from the under 
becoming restricted to the basal side. (From the Cambridge Natural 
portion; or several such partitions History.) 


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 form of the shell 


738 ZOOLOGY SECT. 


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 
(Heteropoda and Pteropoda); in others, though 

Fic, t21.—Shell of present and external, it is too small to enclose 

PERREEEOOEEONTT. ihrer uiniemtall (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 flat ventral 
surface on which the animal creeps by wave-like contractions 
of the muscular tissue. An exceptional case is that of Czecum, 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 


XII PHYLUM MOLLUSCA 739 


or mesopodium, which is the most important, with a smaller 
anterior  propodium 
and posterior meta- 
podium. In many bur- 
rowing forms (Fig. 
626) the propodium 
is well developed and W777 
sharply marked off to 
ach as a burrowing Fia. 622,—Cypraa moneta (Cowrie). Showing the mantle, 
organ. Ina few cases tg provided with marginal tentacles, partly enveloping the 


: shell. Br. siphon ; AZ. M. mantle ; F. foot ; £7. tentacles at the 
a par of tentacles— edge of the mantle, (From Cooke, after Quoy and Gaimard.) 


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 
that remain fixed. The metapodium 
very usually in the Streptoneura bears 


Br 


Fia. 623.—Doris (Archidoris) Fic. 624.—Carinaria mediterranea. «. anus; 
tuberculata. a anus; br. br. branchia; f. foot; i. intestine; m. mouth; p. 
branchie# ; m, penis; rh, 7h, penis ; s. sucker; sh. shell; ¢. tentacles. (From the 
tentacles. (From the Cambsulye Cambridge Natural History.) 


Nutural 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- 


" or pods (Fig. 628) which are 
Fic. 625.—A Slug (Limax). pulmonary : i 
aperture. (eroua the Cambridge Natural History.) specially modified for a 


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 


a ¥ sap 


Fic, 626.—Sigaretus levigatus, exemplifying great development of propodium (pr.) and 
metapodium (met.), ina burrowing Gastropod. The shell has been removed, ik mesopodium F 
1. “liver”; s. ap. aperture of siphon ; ¢. ¢, 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 papilla 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 itrovert, 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 
Fie. 627.—Aplysia, dorsal usually cylindrical or club-shaped. In most 

Kelerstem) A" 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 lap— 


XII PHYLUM MOLLUSCA TAL 


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 


Fic, 628.—Shell-bearing Pteropoda. /. 7. fins; J. liver; 0. 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 


Fic. 629.—Atlanta peronii. «a, cerebral ganglia; b, pedal ganglia; e. eye; g, ctenidia ; 
h, heart; k, nephridium ; l. “liver” ; m. mouth ; 0. ovary ; p, operculum ; t. testis 


stphon—which lies 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 SECT. 


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- 


Fic, 630.—Pterotrachea scutata. «ali. alimentary canal; cten. gills; eve, eye; fl. float; 
mo. mouth ; prob. proboscis ; repr. gonad ; sh. shield covering a portion of the dorsal surface ; 
su. 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 beats 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. 

In the Nudibranchs true ctenidia are absent, but their place 
as ae organs is taken by a number of secondary branchie, 


i 
i 


am 


pulm 


Fic. 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 Molis; 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. 


XII 2 PHYLUM MOLLUSCA 743 


In the limpets (Patella and its allies, Fig. 633) the true 
ctenidia are represented only by a pair of vestiges, and respiration 


ee 


Fie, 632,—Pleurophyliidia‘lineata, Fic.§633.—Patella vulgata ,iseen}from the ven 


from the ventral surface. «. anus; tral side. jf. foot; y. d. circlet of gill-lamelle ; 
by. secondary branchiz: m. mouth; nn. e, edge of the mantle; mu. attachment-muscle; 
s. 0. sexual opening. (From the Cum- si, slits in the attachment-muscle; sh. shell; 
bridge Natural History.) v. efferent branchial vessel ; v’. aorta ; ve. smaller 


vessels. (From the Cambridge Natural History.) 


is carried on by a number of secondary branchie (g./.) 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, 
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- 


r Fic. 634.—Pulmonary cavity and related parts in a slug 
pe ently ae much (Limax). aort. aorta; aur. auricle; neph. nephbridium ; 


pulu 


hydrostatic as re- peric, pericardium, laid open ip. oo Pulmonary Peis : 
= - pul. v. pulmonary vein with its ramifications ; rect. rectum ; 
spiratory. In one ur. ureter ; vent. ventricle. (After Pelseneer.) 


family of Pulmonata, 
the pulmonary chamber gives off a number of branching air-tubes 
or trachece. In some of the Pulmonata there is a return to 


744 ZOOLOGY SECT. 


a completely aquatic mode of respiration accompanied by the 
development of secondary gills—vascular processes of the wall 
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 cecum 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, ¢.g. Haliotis, it traverses the ventricle, in others 
the pericardium ; in others it passes through the nephridium. In 
Holis (Nudibranchia) the stomach gives off a number of glandular 
ceca which penetrate into the interior of the secondary branchie 
or cerata on the dorsal surface; these cieca take the place of 
the “liver” of other Gastropoda. In some of the Pectinibranchia 
there is a peculiar adrectal gland, situated at the side of the rectum 
and secreting a colourless fluid, which in Mfurex 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 pericardiwm—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 


XII PHYLUM MOLLUSCA 745 


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 aorte. 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 varices 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 cerebrac 
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 
isa pair of visceral ganglia. 
A pair of pedal ganglia 
united together by a trans- 
verse commissure and joined 
to the cerebral ganglia by 


Fic. 635.—Nervous system of Patella. cer. g. 


connectives, give off behind cerebral ganglia; mant. 7, mantle-nerves 3 

: d ] osph. g. osphradial ganglia; ped. gy. pedal 
one or two pairs of peda ganglia and pedal nerve-cords; pl.g. pleural 
nerves, as already mentioned. ganglion. (etvor pbeugels) 


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 Fisswrella 
there is a similar pair of pedal cords which are connected together 
by transverse commissures, and, in the latter genus, Jom one 
another posteriorly. ; 

In the Euthyneura (Fig. 636) except in Acteon and Chilina, 
the visceral cords are not caught up in the twist of the visceral 
mass, and do not cross one another. 


740 


SECT. 


ZOOLOGY 


In the Snails and other Pulmonata (Fig. 637) the ganglia of the 


nervous system are more closely aggregated together. 


Fic, 636.—Nervous system 
of Aplysia (Opistho- 
branchia) abd. abdo- 
minal ganglion; cer. y. 
cerebral ganglion ; osphr. 
y. osphradial ganglion ; 
ped. g. pedal ganglion ; 
pl. g. pleural ganglion. 
(After Spengel.) 


instead of towards it, as in other Mollusca. 


A pair 
of cerebral ganglia overlie the cesophagus, 
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 Oncidiwm, 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, 
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 peng 

close relation to the pedal , ceng 
ganglia, but are always in- 

nervated from the cerebral. Figs Pte 


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. 


Osplnng 

Fic, 637.—Nervous system of Limnzeus (Pul- 
monata). abd. g. abdominal ganglion 3 cer. y. 
cerebral ganglion ; osphr. g. osphradial gang- 
lion; ped. gy. pedal ganglion; pl. g. pleural 
ganglion, (After Spengel.) . i 


XII PHYLUM MOLLUSCA 747 


primitively right osphradium, which is alone developed, assumes 


the form of a pectinate body Rs a ridge, on either side 


Gy 


Fic. 638,—Eyes of Gastropoda, A, Patella; B, Trochus; C, Turbo; D, Murex. 
ep. epidermis; 1. lens; op. n. optic nerve; 7. retina; v. k. vitreous humour, (From the 
Cambridge Natural History, after Helger.) 


of which is a row of close-set lateral lamine, and is commonly 
termed the parabranchia 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 


tam 


Fic. 639.—Transverse scction of osphradium of 


exterior, either directly or area br. n. branch nerve passing to lamina ; 
. lamine ; osphr. n. mai radi rve. 
through a duct—the ureter. ieee 7. m. Main osphradial nerve. 


Both right and left neph- 

ridia may be present, though unequal in size, the one situated 
to the mght of the anus being larger than that situated to 
the left; or the former may alone be developed (Euthyneura). 


748 ZOOLOGY SECT. 


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 Euthynetra. 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 1s observ- 
able. There is an 
ovotestis or “ herma- 
phrodite gland” (herm. 
gl., Fig. 641, 4)—some 
of the follicles of which 
produce ova, while 
others produce sperms, 
a convoluted “ herma- 


> a ” he 
Fic. 640.—Reproductive organs of Helix. all. gl. albumen- pha odite duct (her m. 


gland; d. s. dart-sac; flag. flagellum of the penis ; d.), an albumen -gland 
herm. gl. hermaphrodite gland or uvotestis ; her. . duct of 2 a ? 
vvotestis ; muc. gl. mucous gland ; mue. gl. ap. apertures of in which the albumen 


mucous glands into vestibule ; orid. oviducal part of the s 
common duct; ovid. ap. aperture of oviduct into vesti- of the relatively large 


bule ; pen. penis; vec. sem. receptaculum seminis ; vec. eggs is formed. and 
sem. ap. aperture of receptaculum seminis ; sp. d. sperm : 2 
duct; sp, d', spermiducal part of common duct. (After sometimes a separate 
Pelsenecer.) 7 
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 (me. gl.) ; 
frequently a gland termed prostate is connected with the sperm duct, 
and there is an eversible sac—the see 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 


XII PITYLUM MOLLUSCA 749 


may be simple or may be incompletely divided longitudinally into 
two parts. In the Pulmonata the first part (“hermaphrodite duet ” 


proper) is simple, and 
serves for the pas- 
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 


Ftc, 641.—Follicles of the ovotestis of the Gastropoda. 4 
of Helix hortensis (Pulmonata): B, of the Bolide. 
a, @, ova; 4, masses of sperms; ce. common efferent duct. 
(From Gegenbaur.) 


off a long, slender, tapering diverticulum, the flagellum (j/lag.), 


in which the sperms are made up 


Fic. 642.—Forms of egg-cases in Gastropoda. 
D, Pyrula or Busycon; 8, Conus; (, 
Voluta musica; £, Ampullaria. 
the Cambridge Natural History.) 


into elongated masses or 
spermatophores. 
Development.—The 
limpets (Patella) are ex- 
ceptional in laying the 
eggs one by one and un- 
fertilised—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 2 


750 ZOOLOGY SECT. 


float about frecly. 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 micremeres 
become constricted off, the larger cells being the megameres ; 
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-mescderm cell) are formed two 
primitive mesoderm cells, from which the cells of the mesoderm 
are developed. In some cases (Paludina) the mesoderm is formed 


NII 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 


Mes 
Fic. 643 —Diagram of the segmentation and formation of the germinal layers of the Gastropoda. 
Aand B, lateral view; C—F, viewed from the animal (upper) pole ; H, from the vegetal (lower) 
pole ; G, in optical section ; ect. ectoderm ; ¢nd. enduderm ; mic. micromeres ; meg. megameres ; 
mes. mesoderm ; pol. 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 (4) which has on one side the large megamcres 
The latter becume enclosed by the micromeres, and the foundation 

3B 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 velwin—is formed. Subsequently 


Fic. 644.—Earlier stages in the development of Patella. 4, blastula; B, beginning of endo- 
dermal invagination ; (, cowpletion of gastrula; D, frontal section of somewhat later stage. 
ap. pl. apical plate ; b/. blastopore ; endim. endo-mesoderm cell; end. endoderm ; mes. meso- 
derm ; mesent. mesenteron ; 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 


XII PHYLUM MOLLUSCA 753, 


ectoderm which pushes the blastopore inwards and forms the 
rudiment of the stomodieum. 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. 


Fic, 645.—A and B, Trochophores of Patella at different stages. In 4 are to be seen the 
circular blastopore and the two fvot-elevations ; in B the blastopore is drawn out, at the sides 
of it are the two mesoderm bands. (From Korschelt and Heider, 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. pl) 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 (un. c). 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 (rad) at the same time forms the rudiment of the radi” 
sac. The statolith-sacs appear as depressions of the ectoderm 


——— 


754 ZOOLOGY SECT. 


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 Palidina 
it becomes converted into the anus; in some the mouth is 
developed from its anterior portion ; in others the stomodival 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, vel.) 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 
Pie, 64 Tate truchophore of lava of Ratella i» lugs: of locomotion for the 
plate ; f. foot ; ies. mesoderm cells ; imesent, mesenteron ; larva. With the full 
mo. mouth ; v«/ rudiment of radula-sac ; sh. shell. (From 
Korschelt and Heider, after Patten.) 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 dorsally. 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 (7) may attain a considerable development 
during the veliger stage, and on its posterior and dorsal part appears 


XM PHYLUM MOLLUSCA visys) 


the operculum. Two little processes on the velar area develop into 
the tentacles (¢ent.), and the cyes (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 


<j: 
Yon Mi 


Fic. 647.—Veliger stage of Vermetus. cer. g. cerebral ganglia; ev. eye; f. foot; mo, mouth ; 
ot, statocyst ; sh. 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, Wc., 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 


/ | 


iS 5) ZOOLOGY SECT. 


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 
arly 3,000 fathoms. Many forms, however, are 
inhabitants of fresh water, while many Pulmonata are terrestrial, 
and oceur 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 Paleozoic age. 

The mutual relationships of the various groups of Gastropoda 
are shown in the following diagram (Fig. 648) :— 


Platypoda Heteropoda 


Rhipidoglossa 


ee 


x 


Pulmonata 
Tectibranchia 


Nudibranchia 


Scaphopoda 


Fig, s48,—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, Siphonodentalinm, and Pulsellum. The 
body is elongated so as to be 
almost worm-like, with complete a 
bilateral symmetry. The mantle- 


folds are almost completely united “aX > 
to form a cylindrical tube en- aR ee 
closed by the shell (Fig. 649), SSS 
which is in the form of a delicate, Fic. 649.—Dentalium, longitudinal section of 
curved tube, open at both ends shell. (After Keferstein.) 


and wider at the anterior or 

oral end than at the other. The foot (Fig. 650, f) 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 


NIL PHYLUM MOLLUSCA 757 
situated on a short oral proboscis, and is sometimes surrounded by lobed pro- 
cesses or 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 
with the mesenteron is a large bilobed digestive gland (/.). 
The anus is situated ventrally behind the base 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 anumber 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 the pre- 
oral circlet of other molluscan larve ; at the same time 


Fic. 650.—Dentalium, 


a bunch of cilia previously developed at the apical anatomy. a. anterior 
pole becomes more conspicuous and a considerable part. aperture of mantle; 7. 

z : lester foot; ¢. gonad ; i, neph- 
of the general surface covered with more delicate cilia. ridium; JU. digestive 
The blastopore, at first terminal, is shifted forwards on gland. (From the Cram 
the ventral surface until it comes to be immediately Union Natuhal History, 


after Lacaze-Duthiers.) 


behind the ciliated circlet. At its anterior end an in- 
yagination gives rise to the mouth and stomodzeum. 


‘i es 

Fia, 651.—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; v, v. 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 


Tos 


ZOOLOGY SECT. 


far the lonvest 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 


Fie. 652.—Rhodope ver- 
anii. General view. The 
scattered curved bodies 


are the spicules. @ «ap. 
male aperture; Q@ «ap 
female aperture; — bue. 


buecal cavity; bra. cen- 
tral nervous system ; cac. 
cecum; int, intestine; 
mth, mouth; or. ovary ; 
pam. layer of pigment ; 
sal. gid. salivary gland ; te. 
testes. (After von Graff.) 


margins. The pre-oral circlet or velum changes its 
form—at first it is conical, later it becomes plate-like, 
and is then gradually reduced, the larva sinking 
to the bottom; and though still occasionally swim- 
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. 


B. RHODOPE. 


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, 
cesophagus, mid-gut with a ecum, 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 buecal— 
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. 


XII PHYLUM MOLLUSCA 


-T 
or 
~ 


Class V.-CEPHALOPODA. 


The Cephalopoda, including the Cuttle-fishes, Squids, Octopods, 
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 capable of containing 
the body of the animal. 


1. EXAMPLES OF THE CLASS. 
1. THE CurrLe-Fisu (Sepia), 


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 posterior 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. cultrata 
—but 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 SECT. 


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 
surtace of each (2.c. 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 (s¢.); 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 
Fic. 653.-—Sepia cultrata. Entire animal fifth arm on the left side presents 

viewed from the antero-dorsal aspect. a slight modification, some of 

the suckers being absent. This 
is an indication of a change termed hectocotylisation, 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. 


XII 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 
cephalopodium. 

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, 
imf.)—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 fecal 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 


ZOOLOGY SECT. 


concave aborally, and bounded laterally by thin prominent wing- 
like ridges which converge to meet at the aboral extremity. The 


Fig. 654.—Shell of 
Sepia cultrata, 
posterior view. Re- 
duced. 


main mass of the shell consists of numerous, 
closely-arranged, thin lamin 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. (55) 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, 
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 zidocysts. 
When the mantle-cavity 
is laid open (Fig. 658) there 
is seen on each side of it one 
of the two  plume-shaped 
clenadia (cten.). In the middle 
line of the posterior surface, 
close to the internal opening 
of the funnel, is the anal 
aperture (un.) situated at the 
oral extremity of a longi- 
tudinal tube—the rectwm. 


Fre. 655.—Chromatophore of Sepia, magnified. 
vuc. nuclei in wall of sac; piyi. pigment; 
rad, wus. 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 


XII PHYLUM MOLLUSCA 763 


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, eye 
and gives support to the eyes. ae 
Other cartilages support the bases 
of the arms. A thin shield- 
shaped plate—the nuchal 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 Vises 
each on a thin plate of cartilage, — Fiu.s%0.—Sepia eultrata, cranial car- 


: : tila een fi th terior a ot, 
and other thin cartilages support with the oanities of the wong ae 
the bases of the fins erie eye, position of eye indicated by 

aS otted line; of. statocyst; pall. n. 
Alimentary System. == he pallial nerve; cise. n. visceral nerves. 


mouth is surrounded by a thin 
peristomial membrane, within which isa circular lip beset with 
numerous minute elevations. Lodged within the circular lip is a 
pair of powerful horny jaws (Fig. 659, Fig. 660, jaw}, jaw?; Fig. 
661, 7.; 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 esophagus (Figs. 660 and 661, @ ; 
Fig. 663, @s), 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 
ied ih = etic stomach (st.), and, close to the pyloric aperture 
cultrata,nuchal leading from the latter into the intestine, opens 
ss a wide caewm (e.). The alimentary canal at this 
point bends sharply round upon itself, and the 
intestine runs nearly parallel with the cesophagus to open into 
the mantle-cavity as already described. 

A pair of glands (Fig. 661, sg.; Fig. 663, sql.), which are 
commonly termed sa/ivary, though their functional correspondence 
with salivary glands has not been proved, are situated in the 
head behind the cranial cartilage. The ducts of these two glands 
run inwards and unite tu form a median duct, which opens into 


764 ZOOLOGY SECT. 


the buccal cavity. The name of “ liver” (Fig. 660, LU. ; 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 


0 ty 

Wo, | fA 

\. 20/0) oF 
Soy Be 

Rt) [2a 
\ So) [2G 
t ANA 
SIN 
ie) 


C 


mart.cart rt \\ 


liv 
I 
inkd 


acmid 


Zz 
DW) 


Fic. 658.—Sepia cultrata, female seen from the posterio-ventral aspect, the wall of the mantle 
cavity divided along the middle line and the twu flaps thus formed spread out go as tu expuse 
the contents. ac. nid. accessory nidamental glands ; av. anal aperture with its lateral append- 
ages; f. membranous fold attaching the ctenidium to the wall of the mantle-cavity; inf. 
external opening of funnel; inf. cart. infundibular cartilage ; ink. 7. ink-duct ; tak. s. ink-sac ; 
liq. ligamentous band which extends from the anterior wall of the mantle-cavity to the ovary, 
cut across ; dir. “liver”; l. cfen. left ctenidium ; /. neph. left nephridial aperture ; l. nid. left 
nidamental gland ; (. si. y. loft stellate ganglion ; manf. cart. mantle-cartilage ; mo. mouth ; 
mus. neck-muscles ; or, ovary 3 ovid. oviduct ; rect. rectum. 


and left portions, each of which has a duct opening into the 
cavity of the alimentary canal opposite the point where stomach, 
cecum, and intestine meet. Surrounding the ducts and opening 
into them are masses of minute vesicles (Fig. 661, 6, d.); -the 
secrction of these has the property of converting starchy matters 


XII PHYLUM MOLLUSCA 765 


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-sae (Fig, 
658, ink. s.; Fig. 661, 2). 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 


s : y Fiu, 65.—Sepia officinalis, jaws. A 
arteries which lead off from the fy situ: a removed and slightly en. 
i ] Jarged. (From the Cambridge Nuturat 

aorte communicate by their ivory.) 


ultimate branches with a system 
of capillaries, and these with a 
system of veins. A large median vein, the vena cava (¢. ev.), runs 
from the head to the neighbourhood of the rectum, in front of 
which it bifureates to form the left and right afferent branchial 
veins (J. aff. br. v., r. aff. br. v.), each running through the cavity of 
the corresponding renal organ to the base of the gill, where it 1s 
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.¢ 


766 ZOOLOGY SECT. 


Peleeypoda. 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 (lL. aur., 7. aur.). 

The celome (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 


Fic. 660.—Sepia, median section through Fic, 661,—Sepia officinalis, entcric 


the buccal mass. yg. luc. buccal ganglia ; canal. a. anus; b. d. duct of one of 
uv. stom, stomatogastric ganglia; gust. sup- the portions of the digestive gland ; 
posed gustatory organ; jawl, posterior b. m. buccal mass ; ec. cecum ; 7. ink- 
jaw ; jaw, anterior jaw; w. esophagus ; sac; i. d. ink-duct; j. jaws; l0. 
p-rist. peristomial membrane ; rad. radula. digestive gland; w. cesophagus; p. 
(After Keferstein.) pancreatic appendages ; 7. rectum 3 


s. g. salivary glands ; sf. stomach. 
(From the Cambridge Natural History.) 


the cavities of the nephridia or renal sacs. The aboral part of 
the coelome forms the capsule (gonocele) which encloses the ovary 
or testis. 

The paired, plume-shaped ctenidium les 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 lamella, the surface 
of which is increased by the presence of a complex system of fold- 
ings. Internally the lamelle are not completely in contact, an 


XII PHYLUM 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 lamell, and is gathered up again into vessels which open into 
the main efferent vessel leading to the auricle. 


Sad ea 


ar 
reas 


Fic. 662.—Sepia cultrata, male specimen seen from the postcro-ventral aspect, the mantle- 
cavity opened as in Fig. 658, the posterior body-wall partly dissected off, so as to expose the 
organs in the visceral sac, the ink-sac and duct removed. aort. main aorta; aort.’ aboral 
aorta ; app. appendage of left branchial heart ; ewe. cecum; inf. cart. funnel cartilages ; fir. 
digestive gland ; l. ald. v. left abdominal vein ; 7. «ff. bv. left afferent branchial vessel ; /. wur. 
left auricle; /. br. ht. left branchial heart; l. cfen. left ctenidium; /. st. g. left stellate 
ganglion ; mant. cart. mantle-cartilage ; mo. mouth; pen. penis; prost. prostate; 1. abd. v. 


right abdominal vein ; +. e/en. right ctenidium ; vert. rectum; +. ven. app. appendages of 
right afferent branchial vessel ; te. testis; te. «. vein to testis; va. valve of funnel; vent, 
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 plewro-visceral ganglia (Fig. 666), all of 
relatively large size, are closely aggregated together around the 

3c 2 


768 ZOOLOGY SECT. 


esophagus, supported and protected by the cranial cartilage. The 
cerebral ganglia (ver. g.) ave 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 


Fic. 663.—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 funncl and the anterior and posterior walls of the mantle-cavity are 
likewise bisected longitudinally. The left ctenidium with the left nephridial sae 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 3 buc. buccal mass ; br. 
cart, section of cartilage supporting the arms; cer. . cerebral ganglia; giz. gizzard ; ink. 8. 
ink-sac 3 taf, funnel; jaw, jaw; l. aur. left auricle; l. br. kt. left branchial heart ; l. cten. 
left ctenidium ; liv. position of digestive gland ; l. neph. left nephridial sac ; 2. cart. nuchal 
cartilage ; «s. esophagus; of. cavity of statocyst laid open; ped. g. section of pedal gang- 
lion ; perist. peristomial membranc ; post. v. abdominal vein ; 7. aur. right auricle; r. cten. 
right ctenidium ; rect. rectum; sal. salivary gland; sh. shell; st. stomach; te. testis; va. 
valve of funnel; ¢. car. vena cava; rent, ventricle. 


a pair of short thick processes—the optic nerves or optie 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 cesophagus to unite with the 


XII PHYLUM MOLLUSCA 769 


pedal and pleuro-visceral ganglia, which lie behind. The pedal 
ganglia (Fig. 667) are, like the cerebral, united into a single 
mass; orally this is prolonged and expanded into a broad mass 
from which the ten brarhial nerves (br. n.) are given off to the 
arms. The plewro-visrervl ganglia, also united into one, are in 
immediate contact with the pedal behind 
the cesophagus. 

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, bwc.), 
situated close to the buccal mass on the 
anterior aspect of the cesophagus. 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. stom.), also closely 
united, situated on the posterior aspect 
of the csophagus. 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 


Fia. 664.—Sepia officinalis, 


ganglia of considerable size—-the visceral longitudinal section of ink: 
: : SAC. tL, 3 & ink- 5 
and the gastric—occur in the course of ngs inkigland {7 auvity 
‘ aT ASU < 17 of ink-sac; 0. orifice of ink- 
this system. The pleuro-visceral ganglia a a a 
also give off two very stout pallial nerves ter muscles. (From the Cum- 
. si tiidge Natural History, after 

(pall. n.) which run through the neck to Girod.) 


the inner surface of the mantle-cavity, 
where each expands into a large, flat, pallial or stellate ganglion 
(Fig. 658, J. 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 


wal) ZOOLOGY SECT. 


supported by curved plates of cartillage forming a sort of orbit, 
connected with the cranial cartilage. The significance of the 


) 


nabdv. A ver. app 


’ 
SOUYV 


Fic, 665,—Sepia cultrata, heart and main blood-vessels from the pusterior aspect. «ant. wo, 
aort, aorta; aort’. aboral aorta; app. appendage of right branchial heart; ef. br. v. right 
efferent branchial vessel; ink. a. artery to ink-sac ; ink. v, vein from ink-sac ; Ll. aff. bv, v. left 
afferent branchial vessel; l. aur. left auricle; or. 7. deep ovarian vein ; ov. v'. superficial 
ovarian vein; pall. v. pallial vein; r. ud. v. right abdominal vein; +. af. br. v. right 
afferent branchial vein ; 7. cten. right ctenidium; +. br. ht, right branchial heart; v. cae. 
vena cava: ven. app. venous appendages ; vent. ventricle. 


bri 


VESC.TL vese.n 


Fia. 666.—Sepia cultrata, cephalic gang- 


Fic, 667.—Sepia cultrata, anterior vicw 
lia from the anterior aspect. uo. aorta; 


buc, buceal ganglion ; cer. bue. con, cere- 
bro-buceal connective; cer. g. cercbral 
ganglion ; opt. g. optic ganglion (removed 
on the left side); opt. st. optic stalk ; 
pall. n. pallial nerve ; pl. a. pleural gung- 
lion ; vise. n, visceral nerves, 


of pedal and pleuro-visceral ganglia after 
removal of the cerebral and optic. hr. 2. 
brachial nerves; conn. connectives be- 
tween the cerebral and the pedal and 
pleuro-visceral ganglia (cut across); inf. n. 
nerve to funnel; pall. 2. pallial nerves; 
vise. 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 


XII 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 (sel. cart). Externally, 
«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 


etl proc f 


scl.cart 


hey 
SN STATINS y 
CAI SZ 


orb.cart 


Fia. 668.—Sepia, section of eye. cil. proc. ciliary processes ; conn. false cornea ; ir. iris; lens, 
lens ; opt. y optic ganglion ; orb. cart. orbital cartilage ; rds. rods 3 ref. retina; sel. cart, sclerotic 
cartilage. (From Vogt and Jung, after Hensen.) 


process—the ciliary process (ctl. prec.) —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 


Ti2 ZOOLOGY SECT. 


immediately bounds the internal cavity of the eye is a layer of 
short narrow prismatic bodies—the lwyer 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 imnediate 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 
organs, 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 erisia 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 
comphcated form. The function of the stato- 
cysts as organs of hearihg 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 

Pe ta piacul- surface behind each eye; among the ciliated 

highly magnified.’ cells lining the pit are numerous narrow sensory 

cells connected at their bases with the fibres of 

anerve derived from a small ganglion situated close to the optic 

ganglion. A small elevation (Fig. 660, gust), covered with papule, 

on the floor of the buccal cavity just in front of the odontophore, 
is perhaps an organ of taste. 

‘he 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 MOLLUSCA 773 


their posterior junction is given off a median diverticulum 
(Fig. 671, med. s), into which the pancreatic follicles (panc.) project. 
Through each excretory sac runs the corresponding «ferent 
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 


med.s 


Fic. 670.—Sepia officinalis, excretory organs. abd. v. abdominal vein ; «pl, funnel-like opening 
from the pericardium; ap2, aperture of communication between the left and the median 
nephridial sac ; ink. s. v. ink-sac vein ; mel. s, median sac ; pall. v. pallial vein ; wr. ureter 5 
+, cav, Vena cava; tea, 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, ¢e.) 
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 vesicula seminalis (ves.), to which is appended a glandular 
body, the prostate (pr.). In the interior of the vesicula seminalis 


v4 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 spermatophore (Fig. 673, B); 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— 


WP) sap 


Ziv 


0) 


ov Be 


Fic. (71.—Sepia officinalis, diagram of a median vertical section of a female specimen, to 
show tbe relations of the cavities. «ap. aperture between the secondary body-cavity (peri- 
cardium) and the lateral nephridial sac ; br. if. branchial heart ; nf. funnel; ivi s. ink-sac ; 
iat, intestine; lat. s. lateral nephridial sac; lir. liver; med. s. median nephridial sac ; or. 
ovary ; ov. ap. aperture leading from oviduct to secondary body-cavity ; pane, pancreatic 
topemiag ee ; sh, shell; st, stomach ; ws. ureter ; rent. ventricle. (From Vogt and Jung, after 
xrobben.) 


the spermatophoral sac or Needham’s 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, 


XII PHYLUM MOLLUSCA 779 


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 


Fic. 672.—Sepia, reproductive organs of male. pn. penis; Fic. 673.—Sepia. A, sperms, 
pr. prostute; sp. x. sperm-sac; te, testis; v. def. vas highly magnified ; B, sperma- 
deferens ; ves. vesicula seminalis. (After Keferstein.) tophore. sp. mass of sperms ; 


spr. spring apparatus by which 
the wall of the spermatophore 


mdamental glands (nid), situated to the — Frupiured. (Prom Vogt and 
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 lamelle ; 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 SECT. 


i, THE Pearty Nautitus (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 


Fic. 674.—Section of the shell of Nautilus pompilius, showing the septa (s, s), the septal 
necks (s. 7., 8. 2.), the siphuncle, si. (represented by dotted 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 siphwnele (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 
cicatriz, which may indicate the original presence of the larval 
shell, or protoconch, which has fallen off in the course of develop- 
ment. 


XII 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 (endogastric), in Nautilus it is towards the dorsal side 
(exogastric). 

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 SECT. 


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 


Fic. 675,—Nautilus pompilius, diagrammatic lateral view of a female specimen enclosed 
in its shell. cart. cartilage ; cfen. ctenidia ; hd. hood ; ins. funnel ; jaws, jaws ; mant. mantle ; 
mant’, dorsal mantle fold overlapping the coil of the shell ; us. position of lateral mass of 


muscle: nid. nidamental glands ; sept. first septum ; siph. siphuncle. (After Keferstein.) 


hood bears two tentacles, and has the appearance of being com- 
posed of the immensely developed sheaths 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 


xIT 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 ancther on the aboral side of each 
eye. The latter (ophthalmic) differ from the rest in being highly 


Fic. 676.—Nautilus macromphalus, adhering to the substratum in a vertical position by 
means of its tentacles. ¢. eye; h. hood; n. w. nuchal membrane detached from coil of shell ; 
o. t. ophthalmic tentacles ; sh. shell; w. f. wing of funnel. (After Willey.) 


sensitive, ciliated, and with the ridges on the inner side produced 
into lamelle. The tentacles of the inner series difter 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 SECT. 


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), 


Fic. 677.—Inner posterior lobe of foot of female of Nautilus pompilius, with neighbouring 
parts of cephalopodium. ow. organ of Owen; ¢. 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 (2) 
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. 


XI PHYLUM MOLLUSC'A 781 


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, vel.). 

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 bemg 
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. 


3~ 


Fic, 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; 38, the flattened tentacle 
with the rows of minute cavities ; 
xz, patch of modified integument. 
Two-thirds of the natural size. 
(After Haswell.) 


Mantle and Mantle-Cavity.—The mantle is produced around 
the head into a free flap, longer and looser than the mantle-flap of 


Sepia. 


Fic. 679.—Nautiius pom- 
pilius, cartilaginous in- 
ternal skeleton. (After 
Keferstein.) 


Dorsally this splits into two layers, 
reflected over the convexity of the shell, 
which fits into a hollow behind the hood. 
Ventrally and posteriorly the mantle en- 
closes a large mantle-cavity (Fig. 680), cor- 
responding to that of Sepia. 
lodged two pairs of ctenidia (cten.), having 
the same general structure as the single 
pair present in Sepia. 
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. 0s.). 


In this are 


Between the bases 


In the iniddle 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 «hora! for those situated towards the opposite extremity, the 
same terms being also used to indicate re/ufive 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 


3D 


782 ZOOLOGY SECT. 


aperture with minutely lobed margin, situated on a slight eleva- 
tion, but by no means so prominent asin Sepia. On each side 
are two apertures, the oral and aboral nephridial apertures (Fig. 
680, a. 2. neph., pt. neph.), corresponding to the single pair of 
Sepia, but not elevated on papilla. Close to each posterior 


ry, 


Rar re A, 


Fic, 680.—Nautilus pompilius, interior of mantle-cavity of a male specimen with the 
postero-ventral wall reflected. «. /. neph. oral left nephridial aperture; an, anus; cten. 
ctenidia ; ey. eye; 7. funnel; /. g ap. left reproductive aperture indicated by a bristle passed 
through it; /. vise. ap left viscero-pericardial aperture ; mn. s. Needham’s sac; pen. penis 5 
pl. neph. aboral left nephridial aperture ; ». os. aboral osphradia 31. «nt. os. right oral osphra- 
dium ; v. ». visceral nerves. (After Willey.) 


nephridial aperture is an opening—the viscero-pericardiul (/. vise.ap., 
r, vise, ap.)—leading into the pericardial section of the cceelome; 
these are not represented in Sepia. In both sexes there are two 
reproductive ducts, right and Icft; but in both the right alone 
appears tu be functional, and the left is much the smaller. The 


XIL 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 papille. There is a pair of 
jaws (Fig. 681, jaw) 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 
(odont.), 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 
cesophagus, are one large median and two lateral tongue-like pro- 
minences beset with papille; on the inner surface of the latter 
are the apertures of a pair of salivary glands. 

The cesophagus («s.) 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 (znt.), shortly after it leaves the stomach, develops 
a rounded cecum (ewe.) 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,” 0. du.), opening 
as above mentioned into the cecum, have a series of small 
diverticula which may represent the pancreatic appendages of 
Sepia. 

The celome consists of the pericardium and the gonocwle—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 hemoceele. The latter consists of three chief parts—the peri- 
stomial, peri-cesophageal and peri-hepatic heemocceles, the first sur- 
rounding the buccal mass, the second the cesophagus, 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 


3D 2 


Fic. 


wf 
tng 


pedg 
Pag, 


VUSCIL 


palln 


rliw 
bait 
coec 


tnl.7 


6sl.—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. acc. gl. vesicula seminalis; 
an, anus ; dort, oral aorta ; wort’, posterior pallial artery; b. du. bile-ducts ; bue. ». buccal nerves ; bue. 
pap. papille of peristomial membrane ; cer. y. cerebral ganglion ; ewe. cecum ; er. crop; hd. hood; ins. 
funnel ; inf. 2. infundibular nerve ; it.) part of intestine between stomach and cecum ; in/.2 part of 
intestine following cvcum ; jaw, larger (pusterior) jaw ; 1. off. br. v. left efferent branchial vessels ; l. tev. 
int. left internal tentacular lube ; need. s. Necdham’s sac ; odont, odontophore ; «’. style passed from buccal 
cavity into the opening of the wsophagus ; «s, wsophagus ; olf, x. olfactory nerve ; opl. n. optic nerve ; 
olo, statocyst 5 pall. ». pallial nerves ; ped. g. pedal ganglion; pl. y. pleural ganglion ; 7. ef. br. v. right 
efferent branchial vessel; rer. retractor muscle of the buccil mass; 7. liv. right lobe of “liver” 5 stoi. 
stomach ; (est, testis ; tong. tongue-shaped elevation of the flour of the mouth; va. valve of funnel ; 
ven. c. Vena vaya; vent. ventricle. 


SECT, NIT 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 aorti (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 
lesser aorta, which immediately bifurcates. One of its branches 
the posterior pallial artery (Fig. 682, post, pall. a.)—passes to the 
area of the mantle applied to the septum, bifurcates to supply this 
area, and gives off a branch to the siphuncle. The other—anterior 


rect 


‘SI 


Fic, 682.—Nautilus pompilius (male), origin of pallial and genital arterics. aat. pal. a. 
anterior pallial artery; ef. bv. 7. efferent branchial veins ; gen. «, 1, artery to vesicula 
seminalis (7. sem.) ; gen. a. 2, testicular artery and its branches ; gen. a. 3, artery to pyriform 
sac; 7. 8 spermatophore-sac ; post, pall. a. posterior pallial artery ; pyr. pyriform sac ; rect. 
rectum ; ‘est, testis. (After Willey.) 


pallial (ant. pall. a.\—after giving off arteries to the intestine 
and rectum, and to the branchize 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. ¢.) 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 


786 ZOOLOGY SECT. 


hemoceele. At its aboral end it presents a dilatation from which 
four afferent branchial veins (Fig. 683 «laf, p.Llaf, p.r.aff, 
rant. 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 (/. neph. s., 7. neph. s., l. post. neph. s., 7. post. neph. 8.) 
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 


pane ieee ee 


2. u.ap 
\ ‘Lpost.neph.ap 
Shp 
—— 


eee a 
rpost.neph.ap Ee . ee 
PLY : E eerenny ee | 
rpost.neph.s | 


rpost.aur | vent renapp 
TE.app _ Lpost.neph.s 
veSe,pers 


Fic. 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. l, aff. left oral afferent vessel; cten. right ctenidia ; l. neph. s. left nephridial sac ; 
l. neph. ap. left oral nephridial aperture; l. ost. neph. ap. left aboral nephridial 
aperture ; 7. post. neph. s. left aboral nephridial sac ; l.v.ap, left viscero-pericardial aperture 5 
p. lL. aff. left aboral afferent vessel; ». 7. aff. right aboral afferent vessel; 7. ant. af. right 
oral afferent vessel; 7. ant. aur. right oral auricle; ren. app. renal appendages ; 7. neph. ap. 
right nephridial aperture; +. post. aur. right aboral auricle; 7. post. neph. s. right aboral 
nophridial sac; +. v. ap. right viscero-pericardial aperture ; ven. c. vena cava; vent. ventricle ; 
vise, per. 8, 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 celome. They have been compared 
with the appendages of the branchial heart of Sepia, but differ in 
their relations to the renal appendages. 


XII PHYLUM MOLLUSCA 787 


Nervous System.—Nautilus differs strikingly from Sepia, and 
somewhat resembles Chiton (p. 716, 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 cesophagus 
just behind the buccal mass. The anterior part of the collar 
(Fig. 681, cer.y.) represents the cerebral ganglia, the oral 
portion of the posterior part the pedal (ped. g.), the aboral 
portion the pleuro-visceral (pl. y.); 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 statocones. 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 aslightly 
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 
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, ¢est., 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 (acc.) 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 


78S ZOOLOGY SECT. 
a receptacle, the spermatophoral sac or Needham’s sae (sp. 8.), 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 


Trgelr.op 


Fic, 684.—-Nautilus pompilius, male reproductive organs. «ec. vesicula seminalis ; ef”. vess. 
efferent branchial vessels; /. yen. op. left genital opening; post. ae, posterior aorta; pyr. 
pyriform appendage; 7. gen. op. right genital opening; sj. 8s. spermatophore-sac ; fest, 
testis; vent, ventricle. 


Fic. 685.—Nautilus pompilius, female reproductive organs. alb. albumen-gland ; 1. gen. op. 
left genital opening ; ov. ovary ; pyr. pyriform appendage ; 7. 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 


XII PHYLUM MOLLUSCA 789 


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 * te oe aueiea 
of the foot forms a funnel for the in its capsule. (After Willey.) 
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 
cesophagus. 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, 
andsurrounding 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. 


van) ZOOLOGY SECT. 


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 cight 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 forma 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 Sepiide 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- 


XII PHYLUM MOLLUSCA TOL 


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 
funnel 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 


Fic. 687.—Octopus vulgaris. 4, at rest; B, in motion; f. funnel, the arrow showing the 
direction of the propelling current through the water. (From Cvoke, 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 hectocotylised) 
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 


792 ZOOLOGY SECT. 


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- 


Fic, 688,—Loligo 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 uot united. 


XI 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. 


Fic. 689.-Argonauta argo, female, showing the relations of the animal to the shellin the living 
state, the arrow showing the direction of movement. 7. funnel; . mouth, with jaws project- 
ing; sk. shell, with arms as seen through it; wa, webbed arm clasping the shell. (From 
Cooke, after Lacaze-Duthiers.) 


Chromatophores, 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 


Fic. 6u0.—Octopus lentus, wale 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 
siphwncle—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, Spirula 
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) 
(iis Wixi TERS ER SBARASHES instead of being, like the initial 

an Octoped with the arms united by a Chamber in Nautilus, similar to the 

enantio (hem Cocke ate Homey 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 


Fic. 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 

“ ead the convexity of the inner side of the septa; EB, portion of a septal neck. (After 
‘ooke. 


the soft parts to the shell is the reverse of what obtains in 
Nautilus, the shell of Spirula curving backwards (endogastric 


XII PHYLUM MOLLUSCA 795 


curvature), that of Nautilus forwards (¢vogastric curvature). More- 
over the shell of Spirula 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 swtwres, 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- Fu. 693—Spirula peronii, 


7 . lateral view. «, terminal 
monite was also characterised by the sucker; /. famel; s,s], #. 
possession of a paired or unpaired struc- ee oo oe ie 
ture, sometimes horny, sometimes cal- which is indicated hy dotted 


lines, (From Cooke.) 
careous, called the aptychus,not 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-ostracui, a 
calcareous guard, and a part termed 
the phragmacone. The last, which 

os alone represents the shell of Spirula, 

Fic, 604,—An Ammonite(Ceratites has the form of a cone divided intern- 
nee ally by a series of septa perforated by 

a siphuncle. These parts are most 

completely developed in the extinct genus Belemnites, 1n which 
the shell’ (Fig. 695) consists of a straight, conical, chambered 
phragmacone (phr.), with a siphuncle, enclosed in a calcareous 


796 ZOOLOGY SECT. 


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 lamelle. In the Squids 
(¢.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 
Fig, 693.—Shell ofa Belem. clivided into chambers. It is not secreted 

mites an te by the mantle like the shells of other 


pro-ostracum ; phr, phrag- 4 
mocone. (From Nicholson Mollusca, but by the surfaces of a pair of 
an ydekker’s Pulwonto- E ‘ 2 ig 
louy-) 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 


SERCH, | Ze 
\ix cs 
lta Sy a> oe 


Fic, 606.—Shell of Argonauto argo. 


Chivtopoda (p. 472), Crustacea, and Arachnida (p. 667)—but 
attains a much more elaborate character in the present group than 
in any othcr Invertebrates. 


XU PHYLUM MOLLUSCA 797 


The plume-shaped gills, lodged in the mantle-cavity, are two 
in number 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 ceelome 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, 
cesophagus, 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 cesophagus is dilated to form a sort 
of crop, and the stomach is gizzard-hke. In that genus also the 
ink-gland, general in the Dibranchiata, is absent, and there is a 
cecal 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 cesophagus, 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 ofa 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 


TOS ZOOLOGY SECT. 


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. Statocysts 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 (Hledone 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 1s 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- 
face of the yolk on the side of 
the egg at which the micropyle 
is situated. Continuous with 
this germinal dise 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 
nee eee es Lace nearly circular blastoderm, the 

(From orschelvand Heider, after Watast)” 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,C) appears below the margin of the blastoderm, and 


vent 


XIL PHYLUM MOLLUSCA 799 


extends inwards until it comes to underlie the whole of the 
embryonic part of the blastoderm : separating this from the yolk 


Fic, 698.—Sepia, blastoderm at a late stage of segmentation. b/. blastoderm; yk. yolk. (From 
Korschelt and Heider, after Vialleton.) 


isa thin layer of uncertain derivation—the yolh-cpithelium (Fig. 
699, yk. ep.). There is some doubt as to the nature of the second 


an 


CR OO CE 
LEE SNe RTS 


Yyh-—P 
—Secti through the edge of the blastoderm of Sepia at three successive stages ; 
asa wile yolk ita 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 
3B 2 


800 ZOOLOGY SECT. 


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 stomodeum eventually opens into this; a proctodeum is 
merely represented by the ectodermal pit forming the anus. 


siege 


Fic. 700.—Early stages in the development of Loligo. 4, stage at which the rudiments of the eyes 
and of the shell-gland are first distinguishable ; B, later embryo from the oral side; Cand D, 
from the anal side. ant. f. 7. anterior funnel-fold; ar. rudiments of arms; cfen. ctenidia ; 
eye, eye; mo. mouth; mant, rudiment of mantle ; of. statocyst ; post. f. f. posterior funnel-fold ; 
sh. yl. 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.gi.). 
Below the mantle—ze. nearer the vegetal pole—appear two eleva- 
tions each with a pit-like depression, which are the rudiments of 


XII PHYLUM MOLLUSCA 801 
the eyes; and still nearer the vegetal pole a serics of paired 


elevations, the rudiments of the arms. 


necart 


Are 


Yes yk.s 


Fic. 701.—Two later stages in the development of Loligo. A, fromthe funnel side. B,obliquely 
from above. Letters as in preceding figures; ne. cart. nuchal cartilage. (After Korschelt and 
Heider.) 


Yes Yes 
Fic. 702.—Two stages in the development of Loligo, later than those represented in Fig. 701. 


From the anal or funnel side. Letters as in preceding figure ; in addition, jin, fins. (After 
Korschelt and Heider.) 


After the complete enclosure of the yolk by the blastoderm, the 
mouth (70.) is developed as an oval depression between the rudi- 


802 ZOOLOGY SECT, 


ments of the eyes. Immediately in front of the edge of the mantle 
appear two short ridges, the beginnings of the gills (e¢en.), and a pair 
of folds—the postervtor funnel-folds (post. f. 7.)—which are formed 
between these and the eyes, are the first rudiments of the funnel, 
the greater part of which, however, 1s formed from a second pair 
of folds—the anterior funnel-folds (ant. f. 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 


yrs Ys 
Fic. 703.—Two late stages in the development of Loligo, seen from the funnel side, Letters as 
in preceding figures. (After Korsche]t and Heider.) 


from a part only of these elevations; each is a pit which sub- 
sequently becomes closed to forma vesicle—the optic vesiele : 
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 dise was originally situated—a constriction, which soon 
hecomes 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, forins a rounded appendage of the embryo—the 


XII PHYLUM MOLLUSCA 803 


yolk-sac (yk. s.). The yolk-sac undergoes contractions, which are 
due to the action of contractile cells in the thin mesoderm 
hning 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 ona 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 
lime; 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 (Lolago, 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 Paleozoic epoch, during which there 
lived a great variety of forms of this group, the shell 
being straight (Orthoceras), 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. Unhke 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 


Nautlloids Octopoda 


Ammonites 


Fic. 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 Cheetopoda. 

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 


XII 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 larve (Miiller’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 Miiller’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 proctodeeum ; and (5) the develop- 
ment of a celome. 

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 ainong 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, 1s 
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 1s 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. Paleontology 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. 


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 « definition of the 


term or of the group. 


Aas 376*, 418 

Abdomen, of Apus, 531: Astecus, 541: 
Periplaneta, 620, 622 

Aboral, 376*, 418 

Absorption, 34 

Abyssal species, 8* 

lcantharia, 60 

Acanthin, 61* 

Acanthocephala, 297* - External charac- 
ters, 312, 313 : Body-wall, 312: Body- 
cavity, 313: Proboscis, 313: Vessels, 
313: Nervous-system, 313: Excretory 
organs, 313, 815: Reproductive 
organs, 814, 315: Development, 315 

Acanthobdella, 518, 519 

Acarida, 661*, 665, 667, 668, 669, 672, 
673 

Achromatin, 17* 

Aciculum, 441* 

Acineta, 99, 100, 101 

Acela, 252, 267, 272 

Acontia, 188*, 201 

Acorn-shells, 565 

Actaon, 745 

Actinal, 376*, 418 

Actinia, 198, 228 

Actiniaria, 194*, 196, 197 

Actinobolus, 93, 94 

Actinodactylella, 259, 260, 267, 273 

alctinometra, 425 

Actinomma asteracanthion, 61, 62 

Actinophrys sol, 56, 57, 69 

Actinospherium, 57% 

Actinostome, 376* 

Actinotrocha, 358, 359, 503 

Actinozoa, 128: Example, 185: Distine- 
tive characters and classification, 193 : 
Systematic position of example, 196: 


General organisation, 196: Budding, 
197 : Structure of poly pes, 198 : 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 

Adjustors, 364* 

Adradius, 139 

Adrectal gland, 744 

Ajginopsis, 155, 156 

Aiginura, 155 

Equorea, 143 

Affinities—See Relationships 

Agalma, 163 

Agamobium, 140*, 176 

Aggressive characters, in Crustacea, 601 

Air-sacs, of Insects, 642 

Albertia, 335 

Alciopide, 488 

Alcippe, 565, 579 

Alcyonacea, 195, 197, 198, 199, 203 

Alcyonaria, 195*, 196, 197, 199, 200, 201, 
202, 203, 227 

Alcyonide, 208 

Alcyonidium, 348 

Alcyonium, 195, 208 

Alecithal, 219* 

Alimentary canal—See Digestive system 

Alimentary system—NSee Digestivesystem 

Allolobophora antipe, 481 

Alpheus, 602 


810 


Alpine forms, s* 

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 Asteriax, 383: 
Eichinus, 396, 399: Sea-cucumber, 402, 
403: Anfedon, 408: Echinodermata, 
415 

Ainitotic division, 19* 

Ammonites, 790, 795, 803, 804 

Amnion, of Peripatus, 612: Periplaneta, 
631: Scorpion, 659 

Ama@sBa, 10, 11, 12, 18, 14: Pseudopods, 
47: Endosarc, 46: Ectosarc, 46: Con- 
tractile vacuole, 46: Encystation, 
46: Fission, 46: Systematic position, 
48 

Amebidw, 48 

Ameebocytes, 382, 399 

Amebophyra, 233 

Amcebula, of Didymium, 66, 67° of 
Gregarina, 82, 83 

Amphiblastula, 124*, 125 

Amphidises, 121*, 122 

Amphilina, 262, 287 

Amphineura, 680, 712* : Distinctive char- 
acters and classification, 712: (eneral 
organisation, 713: External features, 
718,714: 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: thology, distribution, &c., 
720 


Amphinomider, 467 

Amphipoda, 568*, 583, 584, 585, 586, 593, 
596, 603 

Amphiptyches, 202, 268 

Amphistomum, 257 

Amphitretus pelayicus, 794 

Amphiura, 232 

Amphinra squamata, 429 

Ampullx, 158*, 378*, 399 

Ampullaria, 749 

Amusium, 702, 704 

Anal filament, 663 

Anal glands, of Periputus, 610 

Anal respiration, 596 

Anal spot, of Paramecium, 90 

Anaspidacea, 566*, 581, 582, 596, 603 

Anaspides, 566, 

Anatomy, 3* 

Anchors, 177 

Angiillula, 305 

Anisopoda, 584” 

Ankylostoma duodenale, 306 


INDEX 


Annulata, 439%: General remarks on, 
523 : Relationships, 525 

Annuli of Leech, 506 

Axoponra, 680: Shell, 682: Body, 684: 
Muscles, 684 : Ccelome, 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, 698, 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 
Cwlome, 407: Enteric canal, 407: 
Ambulacral system, 408: Nervou: 


system, 408: Perihemal and hemas 
system, 409: Sacculi, 409: Reproducl 
tive organs, 409 : Metamorphosis, 409- 
432, 483: Systematic position, 414: 
Development, 432 

Antenna, of Asfacus, 544—See also Ap- 
pendages 

Antennary gland, of Astacus, 551, 552 

Antennary glands, 55], 596 

Antennule, of Astacus, 544—See also 
Appendages 

ANTHENEA FLAVESCENS, 884, 386, 887, 
388 

Anthomeduse, 141%, 143, 144, 149 

Anthophysa, 576 

Anthosoma, 575, 576 

Anthura, 568 

Antimeres, 42* 415 

Antipatharia, 195", 201, 202, 203, 204, 
210 

alntipathes, 201 

Antispadix, 780 

Ant-lions, 633 

Ants, 636, 652 

Aorta—See Vascular system 

Aphides, 647 

alphis rose, 683 

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 

Aplysiidiv, 734 

Apoda (Holothuroidea), 413*, 426, 428 

Apodiidw, 569, 570* 

Apopyle, 108, 109, 110* 

Appendages, of Rotifera, 330: Apus, 
529, 580: Asfucus, 542, 548, 544: Crus- 
tacea, 570: Per/patus, 607, 608: Peri- 


INDEX 


plancida, 621 . Insecta, 637: Scorpion, 
655 

Apseudes, 567 

Aptera, 632*, 640, 677 

aAptychus, 795* 


Avus, 526: External characters, 527, 
528: Appendages, 529, 530: Body- 
wall, 531: Muscular system, 532: 


Digestive organs, 532, 533: Body- 
cavity, 6383: Circulatory system, 533: 
Respiration, 334: Renal organ, 534, 
535: Nervous system, 584, 536: 
Organs of sense, 536, 587: Reproduc- 
tive organs, 537: Development, 537, 
538: Systematic position, 569 

Apus, 584, 535, 537, 538 

aApus cancriformis, 528, 536 

clpus glacialis, 529 

Aquatic pupa, 651 

Arachnida, 526, 653°, 677: Example, 
653: Distinctive characters and classi- 
fication, 660: General organisation, 
662: External form, 662-667: Endo- 
sternite, 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, 673 . 
Appendix, 673 

Arachnidium, 664 

Araneida, 661*, 664, 667, 669, 670 

Arbacin punctulata, pedicellaria, 422 

Arca, 695, 697, 701, 702, 704, 706, 711 

Arcella, 49, 50 

Archeocytes, 122 

aArchenteron 23* 

Archi-Annelida, 439, 503*, 504, 505, 524 

Archi-cerebrum, 555 

Archi-cerebrum, of Periplaneta, 631 

Archi-Chetopoda, 465°, 477 

Archigetes, 262, 263, 287 

Architeuthis, 803 

aAryiope, 366 

Aryonautu argo, 798, 796 

Argonauts, 790, 791, 796 

Argulus, 565, 576, 577 

Arhynchobdellida, 516* 

Aricia, 479 

Aristotle’s lantern, 397 

Ark-shell, 702 

Armadillidium, 586, 587 

Armadillo, 524, 546 

Armata, 496*, 497, 498, 499, 502 

Arrow worms, 297 

Artemia, 563, 571 

Arthrobranchie, 551 

Arthropoda, 526%: 
breathing, 677 

Arthrostraca, 583* 

Articulata, 366*, 367, 368, 369: Shell, 
398 

Ascarida, 304° 


Affinities of air- 


811 


ASCARIS LUMBRICOIDES, 297: External 
characters, 297: Body-wall, 298: 
Digestive organs, 299, 300: Cclome, 
301: Excretory system, 301: Nervous 
system, 301, 802: Reproductive 
organs, 302, 803: Development, 303: 
Systematic position, 304 

Ascaris meyalocephala, 297 

Ascaris niqrovenosa, 308, 309, 310 

Ascnris suilla, 297 

Ascetia, 116 

Ascon, 117, 118* 

Ascopodaria, 354 

Asellus, 568, 583, 585 

Asexual reproduction, 40: in Ameba, 
46: Heliozoa, 59: Radiolaria, 63: 
Euglena, 69: Flagellata, 74: Choano- 
flagellata, 78: Dinoflagellata, 79: 
Cystoflagellata, 79: Sporozoa, 80: 
Coccidiidea, 84: Hzemosporidea, 86: 
Myxosporidea, 87: Paramecium, 90: 
Ciliata, 98: Tentaculifera, 101: 
Sponges, 121: Actinozoa, 197: Platy- 


helminthes, 283: Bugula, 346: 
Cheetopoda, 486 

Aspergillum, 696, 701, 702 

Aspidobranchia, 733* 

Aspidochirote, 415 

Aspidocotylea, 253", 260, 273 

Aspidogaster, 284 

Asplanchna, 328, 330, 331 

Astacopsis, 569 

Astucus, 569 

AsTacus FLUVIATILIS, 539, External 
characters, 540: Abdomen, 541, 
Thoracic region, 541: Head, 542: 


Appendages, 542, 548: Articulations, 
545: Body-wall, 546: Muscular 
system, 546, 547: Digestive organs, 
548: Respiratory organs, 549, 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, 570 

Astasiopsix, 71 

ASTERIAS RUBENS, 375: General external 
features, 375, 876, 377: Transverse 
section of an arm, 378: Vascular and 
nervous systems, 379 : Structure of the 
disc, 380: Body-wall and celome, 381 : 
Digestive system, 382: Ambulacral 
system, 383: Reproductive system, 
386 : Systematic position, 414 

Asteriide, 414* 

Asterina, development, 388, 389, 390, 
391, 392, 393 

Asterina yibbosa, 429, 432 

Asteroidea, Example, 375: Development, 
388 : Distinctive characters and classi- 
fication, 410: Apical system, 417: 
Modifications of form, 418, 419: 


812 


Celome, 425: Ambulacral system, 


425: Blood-vascular system, 426: 
Hemal system, 427: Axial organ, 
428: Enteric canal, 429: Nervous 


system, 429: Reproductive organs, 
429 : Development, 430: Ethology, 434 

Asthenosoma, 421 

Astracoidea, 570* 

Astra, 197, 198, 207 

Astropecten, 419, 420 

Astrophyton, 421 

Astrosphere, 17, 18” 

Atlanta peroni, 741 

Atrium, 249, 346 

Atrochal, 486* 

Attraction-sphere, 18* 

Auditory organs, 39, 645 

Anlactinium 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: Nense-organs, 172: Development 
and life-history, 1738, 174, 175: 
Systematic position, 177 

Auricle, of heart, 36* 

Auricles, of Sea-urchin, 397 : 
phora, 223* 

Anriciaria, 404, 413, 4381, 432 

Australian region, 9* 

-Autolytus 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", 80 


of Cteno- 


B 


Deke 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 

Buelloura, 284 

Bear-animalcules, 673 

Bee-parasites, 65] 

Bees, 526, 619, 636, 647, 652 

Beetles, 525, 619, 635, 640 

Belemnites, 790, 795, 796, 805 

Benthos, 8* 

Berenice, 150 

Beroé’’, 224, 228 

Beroida, 221*, 224 

Bicellariide, 348* 

Bicellular glands, 493* 


INDEX 


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, 401 

Black coral, 195*, 202, 210 

Blastoccele, 23* 

Blastoidea, 414*, 435, 437 

Blastomeres, 22* 

Blastopore, 22, 23* 

Blastosphere, 23* 

Blastostyle, of Obelia, 129*, 180: Lepto- 
linee, 151 : Porpita, 165, 166 

Blastula, 23* 

Blattu—See Periplaneta 

Blattide, 636, 653 

Blepharoblast, 70*, 72 

Blood, 29*, 34* 

Blood-corpuscles, 30 

Blood-vascular system—See 
system 

Blood-vessels, 34 

Blow-flies, 635 

Blue coral, 195 

Bodotria, 567 

Body-cavity—See Ceelome 

Body-wall, of Sea-anemone, 185: Hormi- 
phora, 215: Liver-fluke, 241: Platy- 
helminthes, 262: Nemertinea, 290: 
Ascaris, 298: Nematoda, 305: Cheeto- 
gnatha, 316: Brachionus rubens, 325 : 
Bugula, 343 : Ectoprocta, 351 : Mayel- 
lania, 362: Asterias, 381: Sea-cucum- 
ber, 402: Nereis, 443: Earthworm, 
456 : Cheetopoda, 473 : Sipunculus, 493 : 
Gephyrea, 497: Hirudo, 508: Apus, 
531: Astacus, 546: Crustacea, 594: 
Peripatus, 608: Myriapoda, 617: In- 
secta, 636 

Bojanus, organs of, 689 

Bolina hydatina, 224 

Bone, 25, 27*, 28 

Bone-corpuscles, 27* 

Bonellia, 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 

Bothriocephalus latus, 285 

Botryoidal tissue, of Leech, 509 

Bougainvillea, 143, 144, 152 


Vascular 


INDEX 


Brachial disc, of Discomeduse, 183 

Brachial ossicles, 407 

Brachiolaria, 411, 432 

Brachionide, 329* 

Brachionus, 324, 325, 326, 328, 329 

BRACHIONUS RUBENS: External charac- 
ters, 323, 324: Body-wall, 325: Diges- 
tive organs, 325: Ccelome, 325: Ex- 
cretory system, 326: Nervous system, 
and sense organs, 326: Reproduction 
and development, 326, 327 : Systematic 
position, 329 

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 

Branchellion, 516, 517, 519, 522 

Branchie, 35*- of Asterias, 376. Sea- 
urchin, 395: Polychwta, 471 : Oligo- 
cheta, 473: Branchellion, 519: 
Astacus, 549; Crustacea, 593: Ano- 
donta, 686, 687, 688 : Pelecypoda, 702 : 
Triton, 725: Gastropoda, 742 

Branchial formula, of Astacus, &e., 551 

Branchiopoda, 563*, 569, 570, 571, 593, 
594, 596, 597, 598, 602, 603, 677 

Branchipus, 563, 570, 571 

Branchiura, 565*, 573, 577 

Brine-shrimp, 571 

Brisingide, 418 

Brood-pouch, 370, 531 

Brood-cavity, 101 

Brown body, 346 

Buccal cavity, 32 

Buccinum undatum, 722 

Budding, 40*, 41—See Asexual repro- 
duction 

Budding, in Turbellaria, 257 

Buffon, 5 

Bugs, 634, 640, 651 

Bugula, 347 

Bueuba AVICULARIA, 341, 342: Body- 
wall, 343: Ceelome, 343: Alimentary 
canal, 343: Nervous system, 343: 
Excretory organs, 343: Reproductive 
organs, 344: Development, 344, 345, 
346 : Systematic position, 348 

Bugula plumosa, 346 

Bursa copulatrix, 270, 271, 272, 273, 647 

Busycon, 149 

Buruvs, 653: External features, 654, 
655: Digestive system, 656, 657, 658 : 
Circulatory organs, 656, 657 : Organs of 
respiration, 657 : Nervous system, 657, 


VOL. I 


813 


658 : Organs of special sense, 658 : Re 
productive organs, 658 : Developinent, 


5: 
Butterflies, 526, 619, 635 
Byssus, 694, 702 
Byssus-gland, 694, 702 
Byssus, provisional, 693 


C 


Cieade 633 

Cake-urchins, 412*, 417, 423 

Calcarea, 112*, 120, 122, 123 ; 

Calcareous spicules, of Sponges, 107, 108, 
120, 122 

Calciferous glands, 458 

Callianira, 218, 219, 222 

Callitiara, 150 

Calocalanus, 574 

Calotte 230*, 344 

Calymma, of Radiolaria, 60 

Calyptoblastea 143* 

Cambaru-, 569 

Cambrian, 7 

Campanulariide, 142* 

Canalicule (bone), 27* 

Canals, Haversian, 27*, 28: incurrent, 
radial or flagellate, excurrent of 
Sponge, 107, 108, 109*, 117: of Medusa, 
135, 136 

Canal system of Sponges, 117, 118 

Cancer, 569, 590 

Cannostome, 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: 
of Scorpion, 654 

Carboniferous, 7 

Cardiac sac, of Polychxta, 476 

Cardinal process, 360* 

Cardium, 696, 698, 701, 711 

Carina, of Cirripedia, 578 

Curinaria mediterranea, 739 

Carp-lice, 565 

Carpoidea, 414*, 435 

Cartilage, 25, 26*: Hyaline, 26, 27: 
Fibrous, 26, 27: Yellow elastic, 26: 
Calcified, 27 

Caryophyllaus, 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: Amceboid, 
45: Encysted, 45 


Astacus, 540: 


3 °F 


14 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 Medusr, 151 

Centro-dorsal ossicle, of Antedon, 406 

Centrolecithal, 587, 219* 

Centrosome, 17* 

Cephalic apodeme, of Apus, 533: Asta- 

. cus, 542 

Cephalopoda, 680, 759*. Examples, 759, 
776: Distinctive characters wud <lassi- 
fication, 789: Systematic positicn 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 Astacus, 541 

Cerata, 744 

Ceratella, 146 

Ceratella fusca, 145 

Ceratites nodosus, 795 

Ceratium, 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, 268, 264, 266, 
268, 269, 270, 273, 279, 280, 281, 282, 
285, 286, 287, 288: Example, 245 

Cestus veneris, 228 

Cetonia aurata, 316 

Cheta, 440—See Seta 

Chetoderma, 713, 715, 716, 717, 718 

Chetogasta, 486 

Chetognatha, 297*, 316: External char- 
acters, 316: Body-wall, 316: Enteric 
eanal, 317: Caelome, 317: Nervous 
system, 317: Sensory organs, 318: 
Reproduction, 318: Development, 
318 

Cheetonotus, 386 

Cheetopterus, 469, 481 

Chetosoma, 319 


Chetosomide, 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 sete, 468, 469, 470: Bran- 
chie, 471: Body-wall, 473: Cclome, 
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, 
&e., 488: Appendix, 489 

Chetosomidiw, 319 

Chalk, 56 

Charybdwa marsupialis, 181 

Cheilostomata, 347*, 348, 349, 351, 352, 
354 

Chel, 543* 

Chelicerz, 653*, 662, 664, 665, 666 

Chelifer bravaisti, 662 

Chelipeds, 543* 

Chilaria, 666 

Chilina, 745 

Chilognatha, 615*, 617, 618 

Chilopoda, 615*, 616, 617, 618 

Chironomus, 644 

Chitin, 31, 46* 

Chiton, 680, 713, 714, 715, 717, 718, 719, 
720 

Chitonellus, 714 

Chlamydomyxa, 64, 65 

Chloreemide, 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 

Chetopterus, 469 

Chondracanthus, 564, 575, 576 

Chordotonal organ, 645 

Chorion, of Cephalopoda, 798 : of Insects, 
628 

Choristida, 126 

Chromatin, 16*, 17 

Chromatophores, A ctinosphirrium, 57, 58: 
Chlamydomyxa, 65: Flagellata, 71, 72: 
Dinoflagellata, 78, 79: Sepia, 762: 
Cephalopoda, 793 

Chromosome, 17, 18* 

Chrysalis, 651 

Cicada, 634, 646 

Cicatrix, 776 

Cidaris, 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: Un- 
dulating membranes, 92, 94: Mega- 
nucleus, 93, 94: Micronuclei, 93, 94: 
Contractile vacuole, 93, 94, 96, 97- 
Non-contractile vacuoles, 93, 94: 
Trichocysts, 93, 94: Digestive ap- 
paratus, 94, 95: Skeleton, lorica, 94, 
95, 96: Operculum, 95, 96: Colonies, 
94, 95, 97: Reproduction, 96, 97, 98: 
Conjugation, 99 

Ciliated chambers, 118 

Cinclides, 188, 201 © 

Circulation—See Vascular system 

Circulatory system —See Vascular system 

Cirri, 405*, 439 

Cirripedia, 565*, 593, 594, 596, 597, 598, 
601, 602 

Cirrus, 240 

Cirrus sac, 249 

Cistella, 366, 368, 369, 370, 371 

Cladocera, 564*, 572, 573, 596, 598 

Cladophiure, 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, 
251: Nemertinea, 295: Nematoda, 
303: Rotifera, 327: Polyzoa, 347: 
Brachiopoda, 366: Jchinodermata, 
410: Chetopoda, 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 

Clathrina blanca, 124, 

Clathrozoon, 146 

Clathrulina, 58 

Clavatella, 145 

Clavule, 417* 

Cleaning foot, 581 

Clepsine, 516, 517, 518, 519, 521, 522 

Cliona, 122, 126 

Clitellum, 455, 468, 509 

Cloaca, 249 

Clypeaster subdepressns, 423 

Clypeastridea, 412*, 417, 423 

Clypeus, 620*, 637 

Cnidoblast, 133* 

Cnidocil, 188, 134* 

Coccide, 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 

Celenterata, Classes, 128: Examples, 
128, 168, 185, 211: Relationships, 226- 
229: Appendix, 230: Relationships to 
Sponges, 228 

Coeliac canal, 407 

Cwlome and body-cavity, of Ascaris, 
301: Nematoda, 307 : Acanthocephala, 
313: Chetognatha, 317: Bugula, 343 : 
Endoprocta, 354: Phoronis, 356: 
Magellania, 365: Asterias, 381: Sea 
urchin, 399: Sea-cucumber, 403: 
Antedon, 407: Echinodermata, 425: 
Nereis, 442: Chetopoda, 474: sipun- 
culus, 493: Gephyrea, 498: <Apus, 
533 : Crustacea, 593: Peripatus, 608 : 
Insecta, 640: Anodonta, 684: Am- 
phineura, 715: Sepia, 766: Cephalo- 
poda, 797 

Ceelomoducts, 439, 480 

Caloplana, 225, 286 

Ccenenchyma, 207* 

Ccenosarc, 131* 

Coleoptera, 635*, 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, 68 

Corocuirus, 401: General external fea- 
tures, 401: Structure of body-wall, 
402: Ambulacral system, 402: Nerve- 
ring, 402: Perihemal and hemal 
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: 
Leptolinw, 143: Actinozoa, 197: Poly- 
zoa, 340: Bugula, 341: Ectoprocta, 
348: Endoprocta, 355 

Colpoda, 98 

Columella, of Coral, 205*: of Triton, 
722 

Column, 185* 

Comatule, 434, 435 

Comatulidw, 415 

Comb-jellies, 128 

Combs, of Hormiphora, 211*, 212 

Comb-ribs, 213* 

Commensalism, in Sponges, 126: Hy- 
dractinia, 144: Actinozoa, 208: 
Platyhelminthes, 284: Chaetopoda, 488 : 
Crustacea, 602 

Complemental males, 597 

Conchiolin, 683* 

Conchostraca, 563*, 569 

Condylostoma, 94 


3 °F 2 


816 


Cone, of Coronata, 179 

Cones, 733 

Conjugation of Amba, 47: Foraminifera, 
56: Heliozoa, 60: Cystoflagellata, 79: 
Ciliata, 99: Flagellata, 75: Para- 
mecium, 90 

Connective tissue, 25*: Gelatinous, 25, 
26: Fibrous, 25: Retiform, 25, 26 

Connective tissue cells of Sponges, 11] 

Contractile vacuole, 11*, 18, 47, 66, 68, 
77, 88, 89 

Contractility of muscles, 28, 37* 

Conus, ‘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* 

Cordylophora, 167 

Cornea, 771: false, 771 

Corona, of Polyzoa, 344: Sea-urehin, 
396 

Coronary groove, 176, 179* 

Coronata, 176*, 179, 180 

Corpuscles, 30: amceboid, 30: Miescher’s 
or Rainey’s, 88 

Cortex, of Actinospherium, 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: 
pion, 658: Arachnida, 667 

Crabs, 526, 569, 587, 590, 593, 595, 596, 
599, 603 

Crane-flies, 635 

Crangon, 569, 588 

Crania, 366, 368 

Crayfish, 5389—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: Celome, 425: Ambulacral 
system, 426: Blood-vascular system, 
427 : Hemal system, 427: Axial organ, 


Scor- 


428: Enteric canal, 428: Nervous 
system, 429: Reproductive organs, 
429: Development, 430: Ethology, 


434 


INDEX 


Crioceris, 685 

Crisia, 347 

Crista acustica, 772* 

Cristatella, 348, 350 

Crop, 32 

Crown, of Coronata, 179 

Crustacea, 526, 678: Example a, 526: 
Example b, 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: 
FEixcretory organs, 596: Nervous 
system, 596: Sense-organs, 597: Re- 
production, 597: Development, 597 : 
Ethology, 600: Affinities and mutual 
relationships, 602: Appendix, 604 

Qryptocephala, 465*, 468, 475, 476, 484, 
488 


Cryptomonas, 71 

Cryptoniscus, 586, 587 

Cryptozonia, 411*, 414 

Crystalline style, 685: of Gastropoda, 
744 

Ctenaria, 150, 227 

Ctenidium, 685, 687, 688, 702: of 7'riton, 
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, 228, 224: Appendix, 225: Re- 
lationships, 226 


“Ctenoplana, 225, 286 


Ctenopteryx, 791 

Ctenostomata, 348*, 350, 351, 353 
Cubomeduse, 176%, 180, 181 
Cucumaria planci, 401 
Cucumaria—See Colochirisx 
Culex, 586, 634, 638 

Cuma, 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 Vo 
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 


Cydippe, 210 

Cydippida, 221*, 222 

Cymothoa, 586, 597 

Cyntpide, 647 

Cyprea, 739 

Cypris, 564, 572, 573, 596 

Cypris stage, of Cirripedes, 565, 598 

Cyst, Amba, 46*, 47: Didyminm, 67 ; 
Euglena, 68, 69: Monocystis, 80 

Cysticercoid, 280* 

Cysticercus, 251, 280, 281* 

Cysticercus cellulose, 285, 286 

Cystoflagellata, 69*: Characteristic 
features, 79 

Cystoidea, 413*, 435, 437 

Cythere, 564, 572 

Cytoplasm, 16* 


D 


Dee of Afillepora, 156* : 
Stylaster, 159 

Dactylozooids, 149%, 163: of Mil/epora, 
158: Stylaster, 158: Halistemma, 160, 
161 

Daddy long-legs, 635 

Dahlia wartlet, 185 

Dallingeria, 71 

Dalmanites socialis, 605 

Daphnia, 564, 572 

Darwin, 6 

Dasychone, 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, 587, 594, 596, 597, 
599, 602, 603 

Decapoda (Cephalopoda), 790*, 791, 804 

Degeneration, in Copepoda, 575: in Cir- 
ripedia, 579: in Isopoda, 586 

Deiopea, 223 

Deltidium, 360* 

Deltoid plates, 414 

Demospongia, 112* 

Dendrochirote, 415* 

Dendrocelum—See Planaria 

Dendrocometes, 100 

Dendrophyllia, 207 

Dendrosoma, 100, 101 

Dentalium, 756, 757 

Denticles, 442 

Depastridce, 178 

Derm (dermis), 31* 

Dermal branchiz of Echinoidea, 395 

Dermal branchiz, 376, 395, 425 


817 


Dermal cortex, 108, 111*, 119 

Dermal pores, 376 

Dermaptera, 636 

Dermis, 31* 

Desmoscolecidw, 320, 321 

Desmoscolex, 320 

Destructive metabolism, 13* 

Deutomerite of Gregarina, 82*, 83 

Development, of Sycon, 124, 125: Sponges, 
122: Obelia, 139: Leptolinew, 152: 
Trachylinw, 155: Aurelia, 173, 174, 
175: Sea-anemone, 192: Hormiphora, 
217: Dicyemida, 231: Rhopalura, 
233: Planaria, 239: Liver-Fluke, 243: 
Tenia, 250: Platyhelminthes, 273-282 : 
Nemertinea, 294, 295: Ascaris, 303: 
Nematoda, 308, 809: Chetognatha, 
318: Brachionus rubens, 326 : Rotifera, 
334: Bugula, 344: Ectoprocta, 352: 
Endoprocta, 355: Phoronis, 358, 359 : 
Brachiopoda, 370: Asterina, 388, 389: 
Sea-urchin, 400: Sea-cucumber, 404 : 
Antedon, 482: Echinodermata, 430: 
Nereis, 450, 451, 453: Lumbricus, 463, 
464: Cheetopoda, 483: Gephyrea, 500 : 
Hirudo, 515: Hirudinea, 520, 521: 
Apus, 588: Astacus, 557: Crustacea, 
597: Peripatus, 611, 612, 613: Myria- 
poda, 618: Periplaneta, 628: Scor- 
pion, 659: Anodonta, 692: Pelecypoda, 
708: Amphineura, 718: Gastropoda, 
749: Scaphopoda, 757: Cephalopoda, 
798 

Devonian, 7 

Diastylis, 567, 583 

Diatomin, 78* 

Dibothriocephalus, 273 

Dibranchiata, 789*, 792, 793, 794, 795, 
797, 798, 804 

Diceras, 700 

Dichyocysta, 95 

Dicyclica, 413* 

Dicyema, 230, 231 

Dicyemide, 230, 231 

Didinium, 93, 94 

D1IDYMIUM DIFFORME, 66, 67 

Differentiation, 23* 

Difflugia, 49, 50 

Digenetica, 252*, 257, 259, 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, 236, 237: 
Liver-Fluke, 241: Platyhelminthes, 
265, 266, 267: Nemertinea, 290: As- 
caris, 299, 300: Nematoda, 305, 306: 
Chetognatha, 317: Brachionus rubens, 
325: Rotifera, 333: Bugula, 343: 
Ectoprocta, 351; Endoprocta, 354; 


818 


Phoronis, 357: Magellania, 362: As- 
terias, 382 : Sea-urchin, 399, 400: Sea- 
cucumber, 408, 404: Antedon, 407 : 
Echinodermata, 428: Nereis, 442: 
Lumbricus, 457: Chetopoda, 475: 
Sipunculus, 493, 494: Gephyrea, 498 : 
Hirudo, 409, 410: Hirudinea, 518: 
Apus, 582, 533: Astacus, 548: Crus- 
tacea, 593: Peripatus, 608: Myria- 
poda, 617: Periplaneta, 624, 625; In- 
secta, 640, 641: Scorpion, 656, 657, 
658: Arachnida, 667: Anodonta, 684, 
685: Pelecypoda, 704: Amphineura, 
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 

Diecious, 40*, 139* 

Diophrys, $2, 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 

Discomedusez, 176*, 182, 183, 189 

Discorbina, 53 

Discosoma, 208, 210 

Discrelium lanceolatum, 285 

Dissepiments, 205* 

Distinctive characters—See Classification 

Distomida, 253*, 254 

Distomum, 265 

DIsTOMUM HEPATICUM, 
Fasciola hepatica 

Distomum rathousii, 285 

Distribution, 8: of Sponges, 126: Hydro- 
corollinz, 159: Scyphozoa, 184: Ac- 
tinozoa, 209: Ctenophora, 224: Platy- 
helminthes, 283: Ectoprocta, 353: 
Brachiopoda, 371: Gephyrea, 502; 
Hirudinea, 524: Onychophora, 612: 
Pelecypoda, 710: Amphineura, 720 : 
Gastropoda, 755: Cephalopoda, 803 

Distribution, geographical, 8 


240 — Nee 


INDEX 


Distribution, geological, 8* : of Forami- 
nifera, 56: Sponges, 126 : Corals, 210 : 
Brachiopoda, 371: Chatopoda, 488: 
Insecta, 652: Arachnida, 673 : Cephalo- 
poda, 803 

Distribution, vertical, 8* 

Divaricators, 364 

Dochmius duodenalis, 306 

Docoglossa, 733%, 756 

Doctrine of descent, 6 

Donaz, 106 

Doris, 734, 742 

Doris (Archidoris) tuberculata, 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 


Hie, 39* 


Ear-shells, 733 

Earthworm—See Lumbricus 

Earthworms, 439 

Karwigs, 636 

Ecdyses, of Apus, 539 

Echiniide, 414* 

Echinoderes, 319, 320 

Eehinoderidiwe, 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: Heemal system, 
427; Axial organ, 428: Enteric canal, 
428 : Nervous system, 429; Sexes, 429: 
Development and metamorphosis, 430 : 
Echinopedium, 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 branchiz, 425: 
Stewart’s organs, 425: Ambulacral 
system, 426: Blood-vascular system, 
427: Hemal system, 427: Axial organ, 
428: Enteric canal, 428: Nervous 
system, 429: Reproductive organs, 429: 
Development, 430: Ethology, 434 

Echinopedium, 432* 

Kchinorhynchus, 312, 318, 314, 315 


INDEX 819 


Ecutnxvs, 393; General external features, 
393, 394, 395: Corona, 396: Aristotle's 
lantern, 397: Nervous system, 398: 
Ambulacral system, 399: Enteric canal, 
399, 400 : Ccelome, 399 : Blood-vascular 
system, 400: Reproductive organs, 
400: Development, 400: Systematic 
position, 414 

Echiuroidea, 496*, 497, 498, 499, 502 

Echiurus, 496, 497, 500, 501 

Ectobranchiata, 415* 

Ectocyst, 341* 

Ectoderm, 23*-——-See Body-wall 

Ectoprocta, 347*, 348 : Structure of body- 
wall, 351. Alimentary canal, 351 : 
Nervous system, 351 : Nephridia, 352 : 
Avicularia, 352: Vibracula, 352: Re- 
production and development, 352: 
Ethology and distribution, 353 

Ectosarc, 46 

Edriasteroidea, 414*, 435 

Hdwardsia, 200, 202, 227, 229 

Eggs—See Development 

Himeria, 84 

Ejaculatory duct, 243 

Elasipoda, 412*, 428, 434 

Eledone moschata, 798 

Elephant’s tusk shells, 7.56 

Eleutherozoa, 410* 

Elk-horn coral, 156, 157 

Elytra, of Cockroach, 622: Polychwta, 
469 : Coleoptera, 635, 640 

Embryology, 3*—See Development 

Empis, 644 

Emulsions, 34 

Encystation, 45*, 46, 59 

Endites, 530 

Endocyst, 341* 

Endoderm, 23* 

Endoderm-dise, of Astucis, 558 

Endoderm lamella, 136* 

Endogastric, 777* 

Endophragmal system, of As/ucus, 542 

Endopodite, 538* 

Endoprocta, 340, 348*, 354: Vestibule, 
354: Nephridia, 354: Cloaca, 354: 
Ganglion, 354: Testes and ovaries, 
354: Foot-gland , 354: Development, 
355 

Endosare, 46 

Endoskeleton, 31* 

Endosternite, 655, 667 

End-sac, 534 

Enteric canal—See Digestive system 

Enteroccele, 389 

Enterozoa, 105 

Entobranchiata, 415* 

Entovalva, 711 

Environment, 9* 

Eocene, 7 

Folis, 734, 742, 744 

Epeira diadema, 664 

Ephelota, 100, 101 


Ephemera, 633 

Ephemeride, 641, 653 

Ephippium, 597 

Ephyropsis, 179, 180 

Ephyrula, of Aurelia, 175* 

Epiblast, 23* 

Epibolic gastrulation, 275 

Epiboly, 218* 

Epicranium, 620*, 637 

Hpidermis, 24, 31—See Body-wall 

Epimerite of Gregarinida, 82, 83* 

Epineural canals, 398* 

Epipharynx, 641 

Epiphragm, 738 

Epiphysis, of Sea-urchin, 398 

Epipodite, 544* 

Epipodium, 740 

Epistoma, of Astucus, 542 

Epistome, 340, 351, 356 

Epistylis plicatilis, 92, 94 

Epistylis umbellaria, 93 

Epitheca, 205* 

Epithelia, 24*: 
Stratified, 24* 

Epithelium, enteric, 33* 

Equatorial plate, 18* 

Equivalve, 699* 

Ergasilus, 564, 575, 576 

Eistheria, 564, 571 

Ethiopian, 9* 

Ethology, 9*: of Platyhelminthes, 283; 
Rotifera, 335: Ectoprocta, 353: Echi- 
nodermata, 434: Chetopoda, 488: 
Hirudinea, 522: Crustacea, 600: In- 
secta, 651: Arachnida, 672: Pelecy- 
poda, 710: Amphineura, 720: Gastro- 
poda, 755 

Kuchlanis, 328 

Huchlora, 222 

Eucirripedia, 565*, 577, 579 

Hucope, development, 152 

Eucopepoda, 564*, 573, 574, 575, 602 

Endendrium, 140 

EvUGLENA VIRIDIS, 67, 68: Systematic 
position, 70 

Eulamellibranchiata, 696*, 697, 698, 711, 
712 

Eucarida, 568%, 587 

Eucopella, 184 

Huglenide, 70 

Euglenoidea, 70 

Eumalacostraca, 566*, 581 

Huphausia, 568, 595, 599 

Euphausiacea, 568*, 587, 594, 596, 599, 
603 

Buphrosyne, 468 

Euphyllopoda, 596 

Euplectella, 120 

Hupomatus, 485 

Euryalida, 417, 420, 421 

Eurypterida, 662*, 666, 668, 673, 678 

Eurypterus fischeri, 668 

Evscorrio—See Buruus 


Non-stratified, 24*: 


820 


Euscorpius italicus, 659, 671, 672 

Kuspongia, 115, 121 

Euthyneura, 734%, 
748 

Evolution, 6 

Examples, of Rhizopoda, 46 : Mycetozoa, 
66: Mastigophora, 67: Sporozoa, 80: 
Infusoria, 88: Porifera, 105: Hydro- 

+ 20a, 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: 
Cheetopoda, 440, 454: Gephyrea, 492: 
Crustacea, 526, 539: Insecta, 619: 
Arachnida, 653: Pelecypoda, 680: 
(Gastropoda, 721: Cephalopoda, 759- 
776: Holothuroidea, 401 

Excretion, 14*, 37 

Excretory pores, 213, 29, 298 

Excretory system, of Leptolini, 151: 
Planaria, 238: Liver-Fluke, 241. 
Teenia, 248: Platyhelminthes, 269 : 
Nemertinea, 292: Ascaris, 301: Behino- 
-rhynchus, 313: Brachionus rubens, 326: 
Rotifera, 333 : Eetoprocta, 352: Endo- 


737, 742, 745, 747, 


procta, 354: Phoronis, 356: Mage/- 
lania, 365: Brachiopoda, 369: Neress, 
448: Lumbricus, 460, 461 : Chietopoda, 


479, 480: Sipunculus, 495: Gephyrea, 
499: Hirudo, 510, 512: Hirudinea, 
519, 520: Apus, 534, 585: Astacus, 
551, 552: Crustacea, 596: Peripatis, 
610: Periplaneta, 625: Insecta, 641 : 
Scorpion, 656: Arachnida, 667: Ano- 
donta, 689: Pelecypoda, 705: Amphi- 
neura, 718, 719: 7'riton, 729: Gastro- 
poda, 747: Scaphopoda, 757: Sepia, 
772, 773: Nautilus, 786: Cephalopoda, 
798 

Excurrent canals, of Sponges, 108, 109* 

Exhalant siphon, 681 

Exites, 531 

Exogastvric, 777* 

Exopodite, 538* 

Exoskeleton, 31*--See Body-wall 

External features, of Sycon, 105, 106: of 
Porifera, 114, 115, 116: Obelia, 180: 
Aurelia, 168, 169: Sea-anemone, 185, 


186: Actinozoa, 196: Hormiphora, 
211, 212: Planaria and Dendrocelum, 
236: Liver-Fluke, 240, 242: Tiwnia 


solium, 245: Platyhelminthes, 254: 
Nemertinea, 288: Ascaris lambricoides, 
297: Nematoda, 305: Hchinorhynchus, 
312: Chetognatha, 316: Brachionus 
rubens, 323, 324: Rotifera, 330: Bugula 
aricularia, 341, 8342 : Ectoprocta, 349, 
350: Endoprocta, 354: Phoronis, 356 : 
Magellania, 362: Brachiopoda, 366 : 
A sterias rubens, 375, 376,377: Anthenea 
flavescens, 386: Sea-urchin, 393, 394, 


INDEX 


395: Sea-cucumber, 401: Anfedon 
rosacea, 405, 406: Echinodermata, 415 : 
Asteroidea, 418: Ophiuroidea, 415: 
Echinoidea, 421 . Holothuroidea, 416 : 
Crinoidea, 424: Nereix, 440, 441: 
Lionbrieus, 454: Chetopoda 467, 468 : 
Sipunc alas nudus, 492, 493 : Gephyrea, 
496: Archi-annelida, 503: Hirudo, 
506, 507, Hirudinea, B17 : Apus, 527 : 
Astarus, 540: Crustacea, 570: Peri- 
patus, 607, 608: Myriapoda, 615: 
Periplancta, 6 619, 620: Insecta, 636: 
Seorpion, 654, 655: Arachnida, 662: 
Anodonta, 684 : Pelecypoda, 696: 
Amphineura, 718, 714: T'riton, 723: 
Gastropoda, 735: Scaphopoda, 756 : 
Sepia, 759, 760: Nautilus, 777- 
Cephalopoda, 790 

Ex-umbrella, 134* 

Eyes, 39*: of Euglena, 68: Medusz, 
155: Planaria, 236: Platyhelminthes, 
268: Nemertinea, 294: Nematoda, 
307: Chetognatha, 318: Brachionus, 
326: Rotifera, 334: Dinophilus, 337: 
Brachiopoda, 369: Asterias, 377: 
Nereis, 447 : Chietopoda, 478: Hirudo, 
514: Apus, 536, 537: Astacus, 556: 
Crustacea, 597: Periplaneta, 627 - 
Insecta, 644, 645: Arachnida, 670): 
Pelecypoda, 707: Chiton, 717: Triton, 
731: Gastropoda, 746: Sepia, 771: 
Nautilus, 787 


K 


7 

i ABRICIA, 478 

Facets, 556* 

Facial suture, 605* 

Feces, 33* 

Falciform young — See Sporozoites 

Family, 4* 

Fasciota HEeATICA, 240: General fea- 
tures, 240, 242: Body wall, 241: 
Digestive system, 241: Water-vessels, 
241: Nervous system, 241; Repvro- 
ductive organs, 242, 243: Development, 
243, 244: Systematic position, 254 

Fascioline, 254 

Fat, 26*, 27 

Fat body, of Periplaneta, 624 

Fauna, 8* 

Feather-star—See A nfedon rosucea 

Feeding, method of, Amba, 12, 46: 
Actinophrys, 57: Chlamydomyxa, 65: 
Huglena, 68, 69: Flagellata, 72: 
Choanoflagellata, 78: Monocystis, 80: 
Tentaculifera, 101 

Femur, of Cockroach, 622 

Fenestre, of Periplaneta, 621* 

Ferment, 12* 

Fertilisation—See Impregnation 


INDEX 


Fever, parasite of, quartan, 86: Tertian, 
86 


Fibres, nerve, 29*, 30 

Fibro-cartilage. 26*, 27 

Filaria bancrofti, 312 

Filaria medinensis, 305, 312 

Filariasis, 312 

Filibranchia. 695*, 697, 698, 711, 712 

Fimbriz, of Fresh-water Mussels, 681 

Fire-flies, 645 

Fission, 14, 40", 46, 60, 59, 65, 69, 73, 74, 
78, 79, 81, 90, 101 

Fissurella, 737, 745 

Five-chambered organ, 409* 

Fixed cheek of Trilobites, 605* 

Flabellum, 205, 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, 51*, 56, 58, 60, 63, 65, 66, 
69, 78, 79 

Flagellum, 24*, 68, 69 

Flagellum, of Astacus, 544 

Flame-cells, 238, 269 

Flat-worms—See Platyhelminthes 

Fleas, 635, 640, 651 

Flies, 526, 619 

Float, 159*, 160 

Floscularia, 328, 330, 331 

Flustra, 347 

Folliculina, 94 

Food-yolk, 30 


Food vacuole, of Actinophrys, 57: Para- 
mvcium, 90* 
Foot, of Anodonta, 681. Pelecypoda, 


701: Amphineura, 713: Triton, 724: 
Gastropods, 738, 739: Sepa, 760: 
Nautilus, 777 + Cephalopoda, 791 

Foot-gland, 354 

Foraminifera, 48* : General structure, 50, 
51. Skeleton, 51, 52, 53: Protoplasm, 
54: Dimorphism, 55, 56*: Repro- 
duction, 56: Distribution, 36 

Formica rufa, 652 

Fossettes, 591* 

Fossils, 7* 

Fossula, 210 

Fredericella, 348 

Fresh-water Crayfish, 539 

Fresh-water Mussel—See 4 nodontu 

Fresh-water Sponges, 126 

Fresh-water Worms, 439 

Fritillaria, 233 

Frondicularia, 58 

Frontal suture, 605* 

Fulcrum (of Rotifera), 325 

Pungia, 205 


821 


Funic, 343" 

Funiculus, 343, 351 

Funnel, 792 

Funnel folds, 792 eee 


Gia: 622 


Galathea, 569 

Galeodes, 663, 661 

Galls, 647 

Gall-flies, 636, 647 

Gall-insects, 647 

Gametes, of Flagellata, 74, 75* : Sporozoa, 
80, 81 

Gametocytes, 80, 86 

Gammarus, 584, 568, 583 

Gamobium, 140", 176 

samogenesis, 21* 

Ganglia, 38, 242 

Gastral cortex, 108, 111*, 119 

Gastric filaments, of Aurelia, 172* 

Gastric mill, 593: of Astacus, 549 

Gastric ostium, 110* 

(xastric ridges, of Aurelia, 173* : pouches, 
170* 

Gastrolith, 549* 

Gastrophilus equi, 684 

Gastropoda, 680, 721": Example, 721. 
Distinctive characters and classitica- 
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, 
755: Relationships, 756: Appendix, 
756 

Gastropores, of Jfillepora, 156* 

Gastrotricha, 323, 335*, 3386 

Gastrozooids, 163, 202 

Gastrula, 23* 

Gastrulation, 22, 23* 

Gelasimus, 591, 602 

Gemmules, 112, 121* 

Gene, 621* 

Generic, 1* 

Genital cloaca, 248*: rachis, 385, 430: 
stolon, 385, 430: plates, 396, 397: 
burs, 430: operculum, 655* 

Genital system—See Reproductivesystem 

Genus, 1* het 

Geological distribution—See Distribu- 
tion, geological 

Geoplanide, 253 : 

Gephyrea, 439, 491* : Example, 492: Dis- 
tinctive characters and classification, 


822 


495: Systematic position of the ex- 
amples, 496: General organisation, 
496: Body-wall, 496: Colome, 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 

(zerminal disc, 798 

yerminal layers, 23* 

Germinal spot, 19*, 30 

Germinal vesicle, 19*, 30 

Germ-vitellarium, 270 

Giant Clam, 710 

Giant fibres, 477 

Giant nerve-cells, 478 

Gigantorhynchus, 312, 318, 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 

Globigerina, 58, 56 

Glochidium, 693 

Glomeris, 282 

Glossiphonia, 516 

Glossocodon, 154 

Glow-worms, 645 

Glycera, 472, 474 

Glyceride, 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* 

Gonoceele, 783* 

Gonodactylus, 569 

Gonodendra, 163* 

Gonopodaria, 354 

Gonopore, 298 

Gonotheca, of Obelia, 129*, 180 

Gonozooids, 202* 

Gordius, 304, 306, 308 

Gorgonacea, 195*, 203, 204 

Gorgonia, 197, 204 

Granule glands, 270, 271 

Grapsus, 569 

Graptolithida, 142*, 166, 167 

Grasshoppers, 619, 633, 636, 645 

Green gland, of Astacus, 551 

Gregarina, 82, 83 

Gregarina blattarum, 82 


INDEX 


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* 

Gymnolemata, 347*, 348, 351, 352 

Gyractis, 200, 201 

Gyrocotyle, 262, 268, 287 

Gyrodactylide, 259 

Gyrodactylus, 277, 258 


H 


= Ethology 

Hemadipsa, 516, 522 

Hemal system, 380 

Hematochrome, 68, 72 

Hematococcus, 72, 78 

Hemocele, of Crustacea, 526, 593*: 
Peripatus, 624: Insecta, 640 

Hemocyanin, 555 

Hemoflagellata, 72 

Hemoglobin, 30, 36 

Hemopsis vorax, 522 

Hemosporidea, 81* : 
tures, 85, 86 

Halicystus, 177 

Haliotide, 733 

Haliotis, 737, 744, 745 

Halistemma, 159, 161, 162, 168 

Halistemma tergestinum, 160, 168 

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 Astacus, 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 


Characteristic fea- 


INDEX 


Felix, 748, 749 

Helix nemorals, ‘742 

Hemipneustes radiatus, 423 

Hemiptera, 633*, 638, 653 

Hemisomes, 43* 

Hepatic cca, 407 

Hepato-pancreas, 34, 549 

Hermaphrodite, 39* 

Hermit-crabs, 569, 588, 589 

Hermit-crabs and Hydractinia, 144 

Herpobdella, 516, 519, 520, 522 

Herpobdellida, 516* 

Hesionida, 475 

Heterocela, 112* 

Heterocotylea—See Monogenetica, 252 

Heterocyemide, 230, 232 

Heterogamy, 41* 

Heterogeny, 277*, 287, 310* 

Heteromita, 71, 72, 75, 76 

Heteromyaria, 698* 

Heteronemertini, 296* 

Heteronereis, 440, 450 

Heteropoda, 733*, 738. 740, 756 

Heterotrichous, 92*, 94 

Hexacanth embryo, of 
Cestodes, 280 

Hexactinellida, 112*, 120, 123, 126 

Hezxactinice, 196, 200 

Hexarthra, 328, 330, 332, 335 

Hinge-ligament, 682 

Hinge-line, 361, 682 

Hinge-teeth, 361, 683 

Hippa, 569, 589, 603 

Hippurites, 700, 701 

Hirudinea, 439, 506: Example, 506: 
Distinctive characters and classifica- 
tion, 515: General organisation, 517: 
Form and size, 517: Set, 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 

Hirudinide, 517* 

Hirudo, 520, 522 

Hirudo australis, 506, 510, 511 

Hirgvupo MEDICINALIS, 506: 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 

HIRUDO QUINQUESTRIATA—See H. MEDI- 
CINALIS 

Hirudo sanguisuga, 522 

Histology, 3* 

Histriobdellea, 336, 338, 339, 524 

Holarctic, 9* 

Holoblastic, 22* 


Tenia, 250: 


Holophytic, 65*, 69, 72, 78 

Holothuria— See Sea-cucumber 

Holothurian—See Colochirus 

Holothuroidea: Example, 401: Distinc- 
tive characters and classification, 412: 
General form, 416: Modifications of 
form, 423: Coelome, 425: Ambulacral 
system of vessels, 426: Blood-vascular 
system, 426: Hemal 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 

Homarus, 569, 599 

Homocela, 112* 

Hood, 779* 

Hook-headed worms, 297 

Hoplocarida, 569* 

HoRMIPHORA pPLUMOSA, 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 

House-flies, 619, 635 

Hyaline cartilage, 26*, 27 

Hyalonema, 120, 127 

Hyalosphenia, 49 

Hybrids, 2* 

Hydatids, 282* 

Hydatina, 328, 331 

Hydra, 133, 147, 148, 153, 167, 168 

Hydractinia, 144, 145, 149, 167 

Hydranths, of Obelia, 129", 148, 187 

Hydrocele, 390 

hae ame 142*, 156, 157, 158, 159, 
16 

Hydrocoralling, distribution, 159 

Hydroctena, 227 

Hydroids, naked-budded, 143* : 
budded, 143* 

Hydrophilus, 650 

Hydrophyliia, of Halistemma, 160*, 160 

Hydrorhiza, 128* 

Hydrospires, 414* 

Hydrotheca, of Obelia, 129*, 130, 167 

Hydrozoa, 128: Example, 128: General 
structure and classification, 140: Alter- 
nation of generations, 141: System- 
atic position of example, 142: General 
remarks on, 167 

Hydrula, 139*, 140, 153, 229 

Hymenoptera, 636*, 638, 642, 646, 649, 
653 

Hyperia, 568 

Hypoblast, 23* 

Hypodermic impregnation, 334, 52Q 


covered- 


824 


Hypopharynx, 641 
Hypostome, of Obelia, 129* 
Hypotrichous, 92*, 94 


I 


eeennes 636, 651 

Ichthyophthirius, 102 

Idmonea, 347 

Idotea, 568 

Tlloricata, 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 

Individual differences, in Nereis dumerilit, 
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 embryos, of Dicyemide, 
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: Hemoceele, 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 


INDEX 


Inter-mesenteric chambers, 187* 

Internodes, of Siphonophora, 161 

Inter-radius, 188, 139* 

Interstitial cells, of Obelia, 132 

Intertentacular tube, 352 

Intestinal ceca, 383, 388 

Introvert, of Polyzoa, 341: Sipunculus, 
493: Gephyrea, 496: Triton, 724: 
Gastropoda, 744 

Invagination, 23* 

Tridocytes, 762* 

Iris, 39, 771* 

Isomyaria, 698* 

Isopoda, 567*, 583, 584, 585, 586, 587, 
593, 596, 597, 603, 677 

Isopteryx, 645* 

Isopteryx apicalis, 645 


4 ena 32 


Jelly-fish, Common (Aurelia aurita), 168 
Jelly-fishes, 128 
Jurassic, 7 


K 


aus 59 


Karyokinesis, 16, 17 

Keber’s organ, 689, 706 
Kidneys—See Excretory system 
King-Crabs, 653, 662, 666 
Kingdom, 5* 

Koonunga, 566 

Kraussina, 368 

Krohnia, 316 


L 


Lae PALPI, of Pelecypoda, 685 

Labial palps, 622 

Labium, 622 

Labrum, 620 

Labrum, of Apus, 529: of Astacus, 542: 
of Scorpion, 655: of Periplaneta, 620 

Labyrinthula, 65, 66 

Lacinia, 622 

Lacrymaria, 94 

Lacteals, 34 

Lacune (bone), 27*° 

Lacunar system, 380 

Lagena, 58 

Lamellez (of bone), 27, 28 

Lampetia, 217, 222 

Lamp-shells, 360 

Lancet plate, 414 

Land-snails, 734 

Lantern ccelome, 400 


INDEX 


Laomedea, 140 

Lappets, of Sea-anemone, 187 

Larva, of Desor, 295 

Larval membranes, of Periplaneta, 630* 

Larval organ, 391 

Latreillia, 591 

Laurentian, 7 

Laurer’s canal, 243 

Laverania, 86 

Leaf-insects, 633, 636 

Leda, 695 

Leech—See Hirudo 

Leeches—See Hirudinea 

Lemnisci, 313* 

Lemnobdella—See Hirudo 

Lens, 39—See Eye 

Lepas, 565, 577, 578 

Lepas anatifera, 578 

Lepas fascicularis, 598 

es 635*, 639, 647, 649, 651, 
65: 


Lepidurus—See A pus 

Lepidurus kirkii, 528, 582 

Lepisma, 682 

Leptochelia, 567 

Leptochone, 478 

Leptodiscus, 79 

Leptodora, 564, 572, 598 

Leptoline, 141*: General structure, 142, 
143, 144, 145: Perisarc, 143: Meduse, 
143, 144, 145: Cenosare, 143: Repro- 
ductive zooids, 149: Development, 152 

Leptomeduse, 141*, 143, 144, 149 

Leptostraca, 566%, 580, 581, 595, 596, 601, 
603 


Lernea, 564, 575, 576 

Lesteira, 516 

Leucilla convexa, 117 
Leucocytes, 30 

Leucodore, 209 

Lice, 634, 640, 651 

Ligula, 262, 288 

Ligula, 622* 

Lima, 696, 702 

Limax, 139, 743 

Limneus, 746 

Limnetis, 564, 571 
Limnobdella—see Hirudo 
Limnocnida, 167 

Limnocodium, 167 

Limpets, 721, 733, 743—See Patella 
Limulus, 666, 667, 670, 671 
Lineus, 232 

Lingua, 641 

Lingual ribbon, 726 
Linguatulida, 673, 674*, 675, 678 
Lingula, 366, 367, 368, 369, 371 
Linneus, 1, 3 

Liquid tissues, 29 

Lithite, 186* 

Lithites, of Aurelia, 173 
Lithobius forficatus, 615 
Lithocircus annularis, 60 


825 


Lithocysts, 135* 

Littoral forms, 8* 

Liver, 34* 

Liver-Fluke—See Fasciola hepatica 

Liver-pancreas, 34 

Lobata, 221*, 228, 224 

Lobosa, 48*, 49: (seneral structure, 48, 
49: Skeleton, 49 

Lobsters, 569, 587, 599 

Locomotion, of Amba, 11, 46: Heliozoa, 
57: Huglena, 68: Flagellata, 72: 
Choanoflagellata, 78: Sporozoa, 80: 
Paramecium, 90: Tentaculifera, 101 

Locomotor rods, 319 

Locusta, 633 

Locusts, 526, 633, 636, 646 

Lohmanella, 233 

Loligo, 796, 798, 800, 801, 802, 803 

Loliyo vulgaris, 192 

Lophogaster, 567 

Lophomonas, 93, 94 

Lophophore, 340, 341, 362, 368 

Lophopus, 351 

Lorica, of Ciliata, 96: Flagellata, 71, 
73: Choanoflagellata, 77, 78: Tenta- 
culifera, 101. Rotifera, 324 

Loricata, 328*, 329 

Loxosoma, 348, 354 

Lucernarida—-See Stauromeduse, 176, 

Lucernaria, 176, 177, 178 

Lucifer, 569, 599 

Lnucina, 703 

Lumbricide, 467* 

Loumepricus, 454: General external fee- 
tures, 454: Body-wall, 456: Seta, 
4154, 457 ; Setigerous sacs, 457: Enteric 
canal, 457: Vascular system, 458: 
Nervous system, 459: Organs of ex- 
cretion, 460, 461: Reproductive organs, 
461, 462: Development, 463, 464; 
Systematic position, 466 

Lumbricus rubellus, 483 

Lumbricus trapezoides, 483 

Luminous organs, 645 

Lung, 35*: of Scorpion, 657: of Pulmo- 
nata, 743 

Lymph, 29* 

Lysophiure, 411* 


M 


|) Genes VALDIVIANA, 517 

Macrobiotus hufelandi, 675 

Macula acustica, 772 

Macrura, 568*, 601, 602, 603, 670 

Madrepora, 207 

Madreporaria, 194*, 197, 198, 200, 205, 
207, 210, 227 

Madrepores, 207, 210 

Madreporic canal, 383, 399, 402 

Madreporite, of Starfish, 376, 419: of 
Sea-urchin, 397 


826 


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: 
Ceelome, 365: Blood system, 365: Ex- 
cretory organs, 365: Nervous system, 
365 : Reproductive organs, 366: Posi- 
tion of example, 367 

Magellania flarescens, 360 

Maggot, 649* 

Mai, 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* 

Mantide, 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 Avrelia, 168%, 
169 

Marine Annelids, 439 

Mastax of Rotifera, 325 

Mastigameba, 70, 71 

Mastigophora, 45* : Example, 67: Clas- 
sification, 69: General organisation, 
69: Systematic position of the example, 
70 

Matrix of connective tissue, 25* 

Maturation, 19*, 20 

Maxilla—See Appendages 

Maxillary palp—See Appendages 

Maxillipeds—See A ppendages 

Maxillute, 638* 

_May-flies, 633 

Measly pork, 310 

-Medulla, of Actinospherium, 57, 58: 
Monocystis, 80: Paramecium, 88, 89 

Medullary sheath, of Nerve-fibre, 29*, 30 

-Medusa-buds, of Obelia, 129 

-Meduse, of Obelia, 134, 135 

‘Megadrili, 466*, 467 

-Megagametes, of Flagellata, 76*: Spo- 
rozoa, 84, 85, 86: Ciliata, 99 

-Megagametocyte, 84, 85 

-Megalesthetes, 717 

-Megalopa stage, 599 

-Megameres, of Ctenophora, 217, 218: 
of Polyclad, 274 


INDEX 


Meganucleus, of Ciliata, 93: Para- 
mecium, 88, 89: Dinoflagellata, 78 

Megaspheric forms, 55, 56 

Megaspores, of Radiolaria, 63 

Megazooid, of Vorticella, 96, 98 

Meleagrina, 696, 710 

Meleayrina margaritifera, 711 

Melicerta, 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 Flabellum, 2C6: 
of Sea-anemone, 187* 

Mesenteries, of Sea-anemone, 187, 182* 

Mesenteries, development of, in Sea- 
anemones, 192, 1938: Arrangement in 
Actinozoa, 200 

Mesenteron, 187 

Mesoblast, 23* 

Mesoceele, 356* 

Mesoderm, 23*, 219* 

Mesoderm bands, of Periplaneta, 631 

Mesogleea, 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* 

Metaceele, 356* 

Metacrinus interruptus, 424 

Metagenesis, 40*: of Foraminifera, 56: 


Obelia, 139: Leptoline, 152: Tra- 
chyline, 156: Aurelia, 173: Liver- 
Fluke, 243: Tenia, 250: Platyhel- 


minthes, 273: Nematoda, 310 

Metameres, 43*, 523 

Metamerism, 523 

Metamorphosis, of Trachyline, 155: Sea- 
anemone, 193: Platyhelminthes, 273: 
Nemertinea, 295: Phoronis, 359: As- 
terina, 388: Antedon, 432: Echino- 
dermata, 430: Chetopoda, 487: Apus, 
537: Crustacea, 597: Insecta, 649 

Metamorphosis, retrogressive—See De- 
generation 

Metanauplius, 539 

Metanemertini, 296* 

Metapodium, 725, 739 

Metathorax, 622 

Metazva, 19", 105 

Metentera, 187* 

Micresthetes, 717 


INDEX 


Microdrili, 466* 

Microgametes, of Flagellata, 76* : Spo- 
rozoa, 84, 85, 86: Ciliata, 99 

Microgametocyte, 84, 85 

AMMicrogromia, 50, 56 

Microhydra, 148, 167 

Micromeres, of Ctenophora, 217, 218 

Micromeres, of Polyclad, 274 

Micronucleus, of Ciliata, 93: of Dino- 
flagellata, 78 

Micronucleus, of Paramecium, 88, 89 

Micropyle, 20 

Micropyle, of Cephalopoda, 798 

Microspheric forms, 55, 56 

Microspores, of Radiolaria, 63 

Microstomum, 282, 283 

Microzooid of Vorticella, 96, 98 

Miescher’s corpuscles, 88* 

Migration, 285* 

Miliola, 52 

Millepora, 156 

Millepora alcicornis, 157, 158 

Millipedes, 526, 614, 615 

Minyas, 202, 226 

Miocene, 7 

Miracidium, 244* 

Mites, 526, 653, 661, 665, 668, 673. 

Mitosis, 16*, 17 

Mitotic division—See Mitosis. 

Modiola, 698, 702 

Mollusca, 680*: 
804 

Molluscoida, 340* 

Molluscoida, mutual relationships of the 
classes, 372 

Mongrels, 2* 

Moniezia, 288 

Monocyclica, 413* 

Monocystis aciiis, 80, 81: Systematic 
position, 82 

Moneecious, 39* 

Monogenetica, 252*, 258, 259, 265, 268, 
269, 272, 273, 277, 284, 286, 287 

Monomyaria, 698* 

Monosiga, 77 

Monotus, 255 

Monozoa, 253* 

Morphology, 3* 

Morula, 22* 

Moruloidea, 230* 

Mosquitoes, 635, 651 

Mother-cyst, 282 

Mother-of-pearl, 684 

Moths, 635 

Mouth papille, 376* 

Movable cheek, 605* 

Mulberry body, 22° 

Multicellular, 19* é 

Multicellular gland, 25 

Miiller’s as stg 

i i lia, ? 

Multicle fission, of Luglena, 69 

Murex, 744, 747 


General remarks, 


827 


Musele, striated, 28*, 29: non-striated, 
28*, 29 

Muscle processes, Hydra, 148 

Muscles, 37* 

Muscular fibres of Sponges, 111 

Muscular system, of Awrelia, 172: Sea- 
anemone, 186, 188: Magellania, 364: 
Brachiopoda, 368: Hirudo, 508, 509: 
Apus, 5382: Astacus, 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 ae 

Myriapoda, 526, 614", 677, 678: Distinc- 
tive characters and classification, 614 < 
General organisation, 614: External 
features, 615 : Integument amd 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, 590, 
603 

Mysis, 567, 582 

Mytilus, 695, 697, 702, 703, 708 

Mytilus edulis, 697, 702 

Mytilus latus, 697 

Myxidium lieberkiihnii, 87 

Myzxobolus miilleri, 87 

Myxospongie, 120, 126 

Myxosporidea, ST 
features, 87 

Myzostoma, 489, 490, 491 

Myzostomida, 489*, 490, 491 


N 
Nie 684* 


Naiidomorpha, 473 

Nuis, 486 

Narcomeduse, 142*, 154, 155 

Natural History, 1* 

Nauplius, 538, 539 

Nausithée, 180 

Nautiloids, 680, 790 

Nautilus macromphalus, 779, 789 

Navutizus pompiuius, 776: Shell, 776: 
External characters of soft parts, 777 : 
Mantle and mantle-cavity, 781: En- 
teric canal, 783 : Ccelome, 783: Heart 
and circulation, 783: Renal organs, 
786: Nervous system, 787: Sense- 


Characteristic 


828 


organs, 787: Reproductive organs, 
787, 788: Systematic position, 790 

Neartic region, 9* 

Nebalia, 566, 579, 581 

Neck, of cockroach, 620 

Nectocalyx, 159* 

Necturus, 304 

Needham’s sac, 774*, 788 

Nekton, 8* 

Nematomorpha, 304*, 305, 307 

Nemathelminthes, 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: Coelome, 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, 298: 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 

Nereide, 466*, 488 

Nereidiformia, 466” 

Nereis, External features, 440, 441: 
Enteric canal, 442, Ceelome, 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: Leptoline, 151: Aurelia, 172: 
Tealia, 192: Hormiphora,216; Planaria, 
238 : Fasciola, 241: Tenia, 248: Platy- 
helminthes, 266, 267: Nemertinea, 
291, 292, 293: Ascaris, 301, 302: 
Nematoda, 307: Acanthocephala, 313 : 
Chetognatha, 317: Brachionus, 326: 
Rotifera, 334: Dinophilea, 337: Gastro- 


INDEX 


tricha, 336: Polyzoa, 351 : Endoprocta, 
354: Phoronis, 357: Magellania, 365: 
Brachiopoda, 369; Starfish, 379: Sea- 
urchin, 398: Holothurian, 402: Ante- 
don, 408: Echinodermata, 429: 
Nereis, 446: Lumbricus, 459: Cheto- 
poda, 476: Myzostomida, 490: Sipun- 
evlus, 494: Gephyrea, 498: Archi- 
annelida, 504: Hirudo, 513; Hiru- 
dinea, 520: Apus, 534, 586: Astacis, 
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* 

Nicothée, 576 

Nidamental gland, 775* 

Noah’s Ark shell, 695 

Norctiluca, 79 

Nodes, of Siphonophora, 161 

Nodosaria, 58 

Nomenclature, binomial, 1* 

Non-contractile vacuoles, 11*, 60 

Voteus, 331 

Notholca, 331 

Notommata werneckii, 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 

Nucua, 695, 698, 701, 702, 708, 706, 707, 
710 

Nudibranchia, 734*, 738, 742, 744, 756 

Nummulites, 53 

Nyctiphanes, 568 

WNyctotherus, 92, 94, 95 

Nymphon hispidium, 674 


O 
een General structure, 128, 130: 
Microscopic structure, 131, 182: 


Medusz, 134, 185, 186: Comparison 
of polype with medusa, 136, 187, 138: 


INDEX 


Reproduction, 139: Development, 139 : 
Systematic position, 142 

Occlusor muscle, 350 

Ocelli, 148, 173 

Octopoda, 680, 790*, 791, 793, 797, 804 

Octopus, 791, 793, 796 

Octorchandra, 150 

Ocular plates, 396, 397 

Odentophora, 676*, 678 

Odontophore, 721, 726, 727 

(Esophagus—See Digestive system 

Oikomonas, 71, 72 

Olfactory organs, 39*: pits, 173* 

Oligocheta, 465*, 468, 469, 471, 473, 474, 
475, 478, 479, 481, 482, 483, 486, 488, 
489 

Olynthus, 116 

Ommatidium, 536*, 587 

Onchidium, 746 

Oniscus, 568, 583, 587 

Onychophora, 526, 607*, 677, 678—See 
Peripatus 

Ocecium, 341 

Ookinete, 87* 

Oosperm, 21* 

Oostegites, 585* 

Oostegopod, 531* 

Ootype, 243, 270, 271 

Opalina, 93, 98, 99 

Opalinopsis, 94 

Operculum Radiolaria, 61, 62: Ciliata, 
95, 96: Gastropoda, 723*, 724, 739 

Ophioglypha, 420 

Ophiuroidea, Distinctive characters and 
classification, 411* : General form and 
symmetry, 415: System of plates, 
418: Modifications of form, 420, 421 : 
Celome, 425: Ambulacral system, 
426; Blood-vascular system, 426: 
Hemal system, 427: Axial organ, 
428: Enteric canal, 428: Nervous 
system, 429: Reproductive organs, 
429: Development, 430: thology, 
&e., 434 

Ophrydinm, 95, 97 

Ophryodendron, 100 

Ophryoglena, 94 

Opisthobranchia, 734*, 738, 744, 745° 
755 

Opisthogoneata, 615*, 619, 677 

Opisthorchis sinensis, 285 

Opossum-Shrimp, 523 

Optic gland, 772 

Optic vesicle, 802 

Oral, 376* : Arms, 170* 

Orchestia, 583, 594 

Order, 4* 

Organ of Bojanus, 689": cf. Owen, 780* 

Organic evolution—See Evolution 

Organism, 1* 

Organ-pipe Coral, 195 

Organs, 31* 

Oriental region, 9* 


VOL. I. 


829 


Orobdella, 516 

Orthoceras, 803 

Orthonectid, 230, 232, 238, 234 

Orthoptera, 632*, 638, 640, 653 

Othoptera genuina, 636 

Oscarella, 117 

Oscaria, 115 

Oscular sphincter, 111* 

Osculum, 106* 

Osphradium, 691, 707, 731, 797 

Ossicles, 375 

Ostia, 106*, 109, 533* 

Ostium, 187* 

Ostracoda, 564*, 572, 578, 593, 594, 597, 
598, 601, 602 

Ostrea, 696, 698, 701, 706, 708, 709 

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, 36 

Oxeole spicules, 108, 110* 

Oxyuris, 807 

Oysters, 680, 696 


P 


P ACHYCHALINA, 121 

Predogenesis, 41*, 224*, 647 

Pagurus, 569, 589 

Pelaumon, 569, 588 

Palemonetes, 593 

Palearctic region, 9* 

Paleodictyoptera, 652 

Paleontology, 6* 

Palinurus, 569, 587, 595, 599, 602 

Pallial line, 683* 

Pallial groove, 344 

Pallial sinus, 365 

Pallial muscles, 684 

Pallial complex, 735 

Pallium—See Mantle 

Palpus, 470 

Paludicella, 348, 353 

Paludina, 754 

Palus, 205* 

Palythoa, 127, 197 

Pancreas, 34* 

Pancreatic juice, 34* 

Pandorina, 72, 73, 74, 75 

Papula, 424* 

Parapodium, 439, 440, 441, 468, 739 

Paragastric cavity, 106*, 108 

Paragnatha, 530*, 548 

Paramwcide, 91 

PaRAmMacium, 88, 89, 90: Systematic 
position, 91 


38a 


830 


Paranithras, GOL 

Paramitome, 15* 

Paramylum, 68* 

Paranaspides, 566, 582 

Paranephrops, 569 

Parasitism, Protozoa, 72, 81, 82, 85, 87, 
5: Hydrozoa, 167: Mesozoa, 230: 
Rotifera, 335 

Parenchyma, 236*, 264, 351 

Parenchyma muscle, 265 

Parenchymula, 124* 

Parthenogenesis, 21*, 40*, 287, 647 

Parthenogonidium, 75* 

Parthenope, 591 

Patella, 737, 148, 745, 746, 747, 749, 751, 
752, 753, 754 

Puraseison axplanchnus, 829 

Parazoa, 105 

Patellidw, 733 

Pauropoda, 614", 618 

Pauropus, 614, 615, 617 

Paxilla, 414 

Peachia, 202 

Pea-crab, 602 

Pearl-mussel, 710 

Pearl-oyster, 696 

Pearls, 71] 

Pébrine, 87* 

Pecten, 696, 697, 700, 708, 710 

Pectines, 655* 

Pectinibranchia, 733,* 744 

Pedal gland, 725* : Lobes, 179 

Pedalion, 328, 330, 382, 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 

Pelomywa, 49 

Pelagic, 8* 

Prlayohydva, 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*, 484 

Peltogaster, 565, 579 

Pen, 795* 

Penwus, 569, 599 

Penial setie, 298, 304 

Pennatula, 196, 197, 198, 199, 204, 208 


INDEX 


Pennatulacea, 195", 197, 199, Lut 
Pentacrinoid larva, 410, 433 
Pentacrinus, 410 

Pentastomida—See Linguatulida 

Pentastomum tenioides, 674 

Peptones, 34 

Peptonephridia, 481” 

Peracarida, 566*, 6U1 

Perforate corals, 207* 

Pericardial sinus, 533* 

Perichondrium, 27* 

Pericolpa, 179, 180 

Perihemal system, 380* 

Periosteum, 28* 

Periostracum, 683* 

Perrpatts, 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 

Peripatus nove-zealandiv, 611 

Periphylla, 179 

PERIPLANETA AMERICANA, 619, 620: 
Head, 620, 621: Neck, 622: Thorax, 
622: Abdomen, 622: Respiratory 
movements, 623 : Muscles, 624: Hemo- 
cele, 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 orientalix, 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 

Peromedusz, 180 

Per-radius, 139* 

Petaloid ambulacra, 422 

Petasus, 154 

Petrarca, 565, 579 

Phacelle, 172* 

Phacops fecundus, 605 

Pheodium, 61, 62 

Phalangida, 661%, 665, 667, 669, 670 

Phanerocephala, 465*, 466, 476, 484, 
488 

Phanerozonia, 411* 

Pharynx—See Digestive system 

Phasmide, 636, 653 


INDEX 


Pheronema carpenter’, 120, 128 

Philodina, 328, 381, 334 

Pholas, 696, 700, 711 

Phoronida, 340, 355* 

Phoronis, 355, 356, 357, 358, 359 

Phragmoceras, 803 

Phragmocone, 795*, 796 

Phreatoicus, 568 

Phronima, 585, 586 

Phrynus, 663 

Phylactolemata, 348*, 349, 351, 352, 353 

Phyllocarida, 566%, 580, 581, 594, 596, 
601, 603 

Phyllodoce paretti, 479 

Phyllosoma, 600 

Phylogeny, 8* 

Phylum, 5*, 43* 

Physalia, 163 

Physiology, 9* 

Pieris, 685 - 

Pigment, 68, 119 

Pilema, 188 

Pilidium, 295 

Pill-bug, 586 

Pinna, 696, 702 

Pinnotheres, 602 

Pinnules, 407* 

Piscicola, 516, 522 

Placophora, 713*, 715, 716, 717, 71s, 
720, 721 

PLANARIA, 236: General features, 236, 
237, 240: Digestive system, 236, 287: 
Water vessels, 287, 238: Nervous 
system, 287, 238: Reproductive sys- 
tem, 238, 239: Systematic position, 
253 

Planaridiv, 253 

Plankton, 8* 

Planorbulina, 53 

Plant lice, 634, 647 

Planula, 139*, 140, 153, 173, 192 

Plasma, of blood, 30 

Plasmodium, 30*, 66, 67 

Platowm sterco eum, 51 

Platyctenea, 226* 

Platyhelminthes, 235, 251*: Examples, 
236, 240, 245: Systematic position, 
253: General external features, 254: 
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, 283 : 
Appendix, 288 

Platypoda, 733*, 734 

Pleopod, 542* 

Pleurobrachia, 211 

Pleurobrachiide, 221 

Pleurobranchia, 550, 551* 


831 


Pleuron, 541* 

Pleurophyllidia, 742, 148 
Ploima, 328*, 329, 330 
Plumatella, 348, 349 
Plumuluria, 149 

Pluteus, 400, 411, 412, 431, 432 


‘Pneumatophore, 159* 


Podical plates, 623* 

Podobranchia, 550* 

Podomere, 526* 

Podophrya, 99, 100 

Podura, 632, 633 

Polar body, 19*, 20 

Polar plates, 216* 

Polian vesicle, 383*, 399, 402, 426 

Pollicipes, 579 

Polyarthra, 328, 330, 331 

Polycelis, 255 

Polycheta, 465*, 470, 471, 474, 478, 479, 
480, 481, 483, 484, 485, 486, 488, 489 

Polycladida, 252*, 255, 256, 257, 263, 
264, 266, 267, 268, 271, 273, 274, 287 

Polycolpa, 154 

Polydora, 488 

Polygordiidw, 503 

Polygordius lacteus, 504 

Polygordius neapolitanns, 508, 504, 505 

Polykrikos, 78, 79 

Polymorphism, 141* 

Polynesian region, 9* 

Polynde, 469, 484 

Polynée extenuata, 471 

Polynée setosissima, 467 

Polynoidw, 467, 481 

Polyeca, 77, 78 

Polyophthalmus, 479 

Polype, 129*, 148 

Polyphemius, 564, 572 

Polyphyletic, 286* 

Polypodium, 167 

Polyspermy, 21* 

Polystomatous, 183* 

Polystomew, 257, 258 

Polystomella, 55 

Polystomum, 258, 277, 284 

Polytrochal, 486* 

Polyzoa, 340*: Example, 341: Distinc- 
tive characters and classification, 347 : 
See Hetoprocta and Endoprocta 

Polyzoa (Cestoda), 253* 

Pontellina mediterranea, 600 

Pontobdelia, 516, 517, 518, 520 

Porcellana, 569 

Pore-membrane, 108, 109* : 

Porifera, Example, 105: Distinctive 
characters and 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, 
&e., 126, 228 ‘ 

Porocyte, 109*, 124 


3G 2 


832 


Poromya, 696, 785 

Porpita, 165, 166 

Port-hole—See Cinclis 

Portuguese Man-of-War, 163 

Portunus, 591 

Post-abdomen of Scorpion, 654 

Potamobiide, 570* 

Poterion, 115 

Pra-abdomen of Scorpion, 65+ 

Prawns, 587, 588 

Priapulide, 496 

Priapulus, 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 

Prorocentrum, 78, 79 

Prorodon, 94, 95 

Proscolex, 250* 

Prosiphon, 741* 

Prosobranchia—See Streptoneura 

Prosoceele, 356* 

Prosoma, 654 

Prosopyle, 108, 109% 

Prostate, 270, 271, 483 

Prostomium, 440%, 455, 469 

Protameha, 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 

Protomyaxa, 67 

Protonemertini, 295* 

Protonephridial system, 236", 269 

Protoplasm, 11*, 14, 15*, 16 

Protopodite, 538*, 542 

Prototroch, 322, 451 

Protozoa, 45 

Protozowa, 599 

Protractor muscle, 683 

Proventriculus, 624 

Psammoclema, 115 

Pseudo-gastrula, 124* 

Pseudo-lamellibranchia, 695*, U97, 698, 
G00, Fly 712 

Pseudo-manubrium, 155* 


INDEX 


Pseudo-metamerism, 255" 

Pseudopod, 10*, 45% : of A meba, 46 _ 

Pacuie nearpionida, 660*, 662, 668, 670 

Psolus, 423 

Psorosperms, 87 = 

Pteropoda, 734°, 741, 756 

Pterotrachea, OF a 

Pulmonary.sac, e 

Pinon 734", 738, 740, 743, 744, 746, 
748, 749,°750, 795, 756 

Pulsellian, 756 ; 

Pupa (of Curipedia), 598 

Pupa (of Insects), 649 

Purpura, 744 

Pyenogonida, 673%, 674, 678 

Pygidium, 605* 

Pyloric czca, 382 

Pyrenoids, 73 

Pyriform organ, 344 

Pyriform sac, 787 

Pyrula, 749 

Pyxicola, 95 


c=) 


Q UADRULA, 49 


R 


est: 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 

Radular, sac, 726", 782 

Rainey’s corpuscles, 88* 

Raphidiophrys, 58 

Razor-fish, 711 

Receptacula 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, G76: Pelecypoda, 
712: Cephalopoda, 804: Mollusca, 804 

Relationships, diagrams of : of Protozoa, 
103: Crelenterata, 229:  Platyhel- 
minthes, 287: Echinodermata, 437 - 
Annulata and Trochelminthes, 525 . 


INDEX 


Crustacea, 604: 
Pelecypoda, 712: 
Cephalopoda, Sv4 

Renal organs—Nce Excretory system 

Reproduction, Reproductive System, 14, 
30, 39: Anweba, 14, 46: Foraminifera, 
56: Heliozoa, 59: Radiclaria, 63: 
Mycetozva, 67+ Luglena, 69: Flagel- 
lata, 74: Choanotlagellata, 78: Dino- 
flagellata, 79: Cystoflagellata, 79: 
Monocystis, 80: Gregar nida, 83: 
Coccidiidea, 84: Heemosp.ridea, 80: 
Paramecium, WO: Ciliata, 97: Ten- 
taculifera, 101: Porifera, 120: Obelix, 
139: Leptolinz, 149: Trachylinz, 155: 
Hydrocorallina, 158: Siphonophora, 
162, 164, 166: Anrelia, 171: Tealiu, 
192: Actinozoa, 197: Hormiphora, 
216: Mesozoa, 231, 234: Planaria, 
238, 239: Fuscrola, 242, 248: Teniu, 
248, 249, 250: Platyhelminthes, 270: 
Nemertinea, 295: Ascaris, 302, 303: 
Nematoda, 307: Acanthocephala, 314, 
315: Chetognatha, 318: Brachionus, 
326: Rotifera, 334: Dinophilea, 337 : 
Bugnila, 344: Ectoprocta, 352: Endo- 
procta, 3855: Phoronis, 357: Magel- 
lania, 366: Brachiopoda, 369: Star- 
fish, 886: Sea-urchin, 400: Holothurian, 
404: Antedon, 409: Echinodermata, 
$29: Nereis, 449: Lumbricus, 461, 
462, 481: Chietopoda, 481: Myzosto- 
mida, 490, 495: Sipunculus, 495 : 
Gephyrea, 499: Archi-annelida, 504 : 
Hirudo, 514: Hirudinea, 520: Apus, 
5387: <Astacus, 556: Crustacea, Peri- 
patus, 610: Myriapoda, 618: Peri- 
planeta, 627, 628: Insects, 646: Scor- 
pion, 658: Arachnida, 671: Mussel, 
691: Pelecypoda, 70S: Amphineura, 
718, 719: Vriton, 732: Gastropoda, 
748 : Scaphopoda, 757; Rhodope, 758 : 
Sepia, 773, 775: Nautilus, 787, 788: 
Cephalopoda, 798 

Requicnia, 700, 701 

Reservoir (Euglena), 68 

Respiration, 13%, 35 

Respiratory organs, 35° : Starfish, 376 : 
Sea-urchin, 399: Holothurian, 404: 
Chetopoda, 471: MHirudinea, 502: 
Apus, 534: Astacus, 549, 550: Crus- 


Arthropoda, 678: 
(sastropoda, 756 : 


tacea, 594: Peripatus, 609: Myria- 
poda, 617: Peripluucta, 626, 627: 
Insects, 641, 642: Scorpion, 657: 


Arachnida, 668 : Mussel, 685: Pelecy- 
poda, 702, 704: Triton, 725: Gas- 
tropoda, 742: Sepia, 766: Nautilus, 
781: Cephalopoda, 797 

Respiratory trees, 404, 428 

Retina, 39 

Retinophore, 732* 

Retinula, 536*, 537, 556 

Retractor muscles, 683 


833 


Rhabdites, 263 

Rhabditis-form, 310 

Rhabdocelida, 252*, 255, 257, 263, 265, 
256, 267, 268, 269, 271, 272, 273, 276, 
283, 284 ' 

Rhabdogaster, 319 

Rhabdome, 563*, 587 

Rhabdonema nigrovenosum, 308, 310 

Rhagon-type of Sponge, 118* 

Rhinophore, 787* 

Rhipidodendron, 71, 73, 74 

Rhipidoglossa, 733*, 756 

Rhizocephala, 565*, 579, 580 

Rhizopoda, 45*: Example, 46: Clas- 
sification and general organisation, 
47 

Rhizostome, 177*, 182, 183, 227 

Rhizota, 328*, 330, 333 

Rhodope, 758 

Rhombogene, 232 

Rhopalura, 282 

Rhynchobdellida, 516*, 517, 513, 
520 

Rhynchodemus, 255* 

Rhynchonella, 366, 369 

Rhynchota, 100 

Rhyncocele, 289* 

Ring-vessel, 270*, 283 

Rock-lobster, 569 

Rock systems, 7 

Rocks, igneous aud aqueous, 7* 

Rosette, 407* 

Rosette (Earthworm) 461 

Rosette plate, 418 

Rostellum, 246* 

Rostrum, 542* 

Rotalia, 52 

Rotation, sense of, 39 

Rotifer, 328 

Rotifera, 323": 


o19, 


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 

Rotule, 398* 

Round-worms, 
thes 

Rugosa, 210* 


297—See Nemathelmin- 


Sse 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, 283, 234 


834 


Saliva, 34* 

Salivary glands, 34* 

Salivary receptacle, 624 

Salmacina, 486 

Salpingwea, 77 

Saltatoria, 636 

Sand-hopper—See Orchestia, 568 

Saprophytic, 69*, 72 

Sarcocystidea, 81*, SS 

Surcocystis, 88 

Sarcolemma, 28* 

Sarcophaga, 644 

Sarcoptes scabiei, 665 

Sarsia, 145 

Scale-insects, 634, 647 

Scallop, 696 

Scaphoda, 756*, 757 

Scaphopoda, 680, 756%, 757 

Schistosomum hematobinm, 285 

Schistoxomum japonienm, 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 

Scolopendrella, 615, 617 

Scorpron—See Butuus. 

Scorpions, 526, 653 

Scorpionida, 660*, 673, 676, 677 

Scorpion-spiders, 632 

Scrobicularia piperata, 699 

Scuta, 578 

Scutariella, 259 

Scutiyera, 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 

Seyphula, 174*, 175, 229 

Sea-anemones, 128, 185, 194, 196, 200— 
See Tealia 

Sea-blubbers, 182 

Sxa-cucumBrer, External features, 401 . 
Structure of body-wall, 402: Ambu- 
lacral system, 402: Nervous-vascular 
systems, 402: Coelome, 403: Enteric 
canal, 408, 404: Reproductive organs, 
404: Development, 404: Systematic 
position, 414 

Sea-cucumbers, +12—See Holothuroidea 

Sea-fans, 195 

Sea-firs (Sertularians), 143 

Sea-hares, 721 


INDEX 


Sea-mats, 340—See Polyzoa 
Sea-mice, 479 _ 
Sea-mussel, 695 


Sea-pens, 196 : 
ioe External features, 393, 394, 
395 : Corona, 396 : Aristotle’s lantern, 
397: Nervous system, 398 : Ambu- 
lacral system, 399 : Enteric canal, 399, 


: . lar 
400: Coelome, 399: Blood vascu 

system, 400: Reproductive organs, 
£00: Development, 400: Systematic 


osition, 414 re A 

Gs sonchine, 412*See Echinoidea 

Secondary axis, 41*, 42 

Secretion, 25* 

Segment, 43* : 

Segmental organ—See Nepredium 

Segmentation of oosperm, 22* 

Segmentation-cavity—See Blastoccle 

Segmentation-nucleus, 20° 

Setson, 335 

Seisonida, 329 

Selenaria, 347 

Selenariidiv, 349, 352 

Self-mutilation, 435 

Semostome, 177*, 182, 227 

Sense-organs, 39* : Obelia, 185: Trachy- 
line, 154: Aurelia, 172: Cubomeduse, 
182: Hormiphora, 216: Ctenoplana, 
225: Platyhelminthes, 268: Nemer- 
tinea, 294: Chetognatha, 318: Bra- 
chionus, 326: Rotifera, 334: Phoronis, 
357: Starfish, 377: Sea-urchin, 395: 
Nereis, 447: Chetopoda, 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 papille, 302, 493 

Sepia cultrata, 759 

Serra, 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, 778: Repro- 
ductive organs, 773, 775: Systematic 
position, 790 

Sepiide, 790* 

Septal funnel, 173% 

Septal neck, 776* 

Septibranchia, 696*, 697, 698 


INDEX 


Septum, 205* 

Serialaria, 348 

Serosa, 631, 659 

Serpulu, 472, 476, 477, 488 

Serprulidie, 468, 472, 485, 486 

Nertularians, 143 

Seta, 489, 440, 441 

Seta, provisional, 452 

Netigerous sac, 441*, 452, 453, 457 

Sexual dimorphism, 40* 

Sexual generation, 140 

Shell-gland, 248, 529, 692, 800 

Shell-gland (Apus), 534, 535% - 
tacea), 596 

Shell, Magellania, 360. 361 : Brachiopoda, 
367, 368: Musse?, 682: Pelecypora, 
697: Chiton, 714: Triton, 721, 722, 
723: Gastropoda, 737: Scaphopoda, 
756: Sepia, 761, 762: Neeutilus, 776, 
Cephalopoda, 793 

Shelly loop, 360 

Ship-worm, 696 

Shrimp, 526, 569, 587 

Sicula, 167* 

Sigaretus, 740 

Silicispongie, 126* 

Silverfish, 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*, 159, 160, 162, 163, 
164, 165, 166, 226 

Siphonozooid, 202* 

Siphons, inhalant and exhalant, 681*, 698 

Sipbuncle, 776* 

Sipunculidee, 496, 498, 502, 503 

Sipunculoidea, 496*, 497, 498, 499, 502 

SipuncuLus Nupus, General external 
features, 492, 493: Body-wall, 493: 
Ceelome, 493: Blood-vascular system, 
493: Alimentary canal, 493, 494: 
Nervous system, 494: Nephridia and 
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: Sepia, 763 : 
Cephalopoda, 796—See also Shell and 
Body-wall 

Skin, 31*—See Body-wall 

Slime glands, 608 

Slugs, 680, 734 

Snails, 680, 734 

Solariiun, 7 7 

Solecurtus strigillatus, 700 

Solen, 711 

Solenocytes, 338, 479 


(Crus- 


835 


Solenogastres, 713*, 715, 717, 718, 720, 
721, 805 

Solenomya, 695 

Solpugida, 661*, 662, 663, 667, 668, 670 

Somatoblast, 450 

Somatopleure, 631* 

Spadella, 316 

Spadix, 780*, 781 

Spatangoidea, 412*, 417, 422 

Species, 1*, 113* 

Specific name, 2* 

Sperm, 20, 30* 

Spermary, 39* 

Spermatidia, 344 

Spermatozoon—See Sperm 

Spermiduct, 40% 

Spermotheca, 308* 

Spheeridia, 395*, 417 

Spheroma, 568 

Spherophrya, 100 

Spicules, 32°, 203 

Spiders, 526, 653 

Spinnerets, 664 

Spinning-glands, 664 

Spinules, 240* 

Spireme, 18* 

Spirifera, 664, 

Spiroloculina, 58 

Spirorhis, 489 

Spirorbis levis, 484 

Spirula, 790, 794, 795 

Splanchnopleure, 631* 

Spongelia, 121 

Spongilla, 118 

Spongillide, 121, 126 

Spongin, 112, 120%, 121 

Spongin-blasts, 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, 51, 83, 84, 85, 86, 88 

Springtails, 632, 640 

Squame, 544* 

Squammulina, 51, 52 

Squids, 790 

Squilla, 592 

Steecharthrum, 232 

Statoblasts, 353 

Statocones, 707%, 787 

Statocysts, 268, 295, 691, 707, 79S 

Statolith, 268, 707* 

STARFISH, External characters, 375, 376, 
377: Transverse action of arm, 878: 
Vascular and nervous systems, 379: 
Structure of disc, 380: Body-wall and 
ccelome, 381: Digestive system, 382: 


836 


Ambulacral system, 383: Reproduc- 
tive system 386: Development, 388, 
389, 390, 391, 392, 393: Systematic 
position, 414 

Starfishes, 374—See Asteroidea 

Stauromeduse, 176*, 177, 178, 227 

Stenocyphus, 178 

Stentor, 92, 94 

Stephanoceros, 328, 330, 331 

Sternaspis, 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, 628, 612, 655 

Stinging capsule—See Nematocyst 

Stolon, 196% 

Stomach—Nee Digestive system 

Stomatogastric nerves, 447 

Stomatopoda, 9697, 592, 593, 596 

Stomidium, 201* 

Stomodeal canal, 213 

Stomodeum, 173* 

Stone-canal, 383*, 399 

Stony-corals, 128, 195 202 

Stratiodrilus tasmanicus, 388 

Strepsiptera, 640, 651, 

Streptophiure, 411* 

Streptoneura, 733", 735, 7 
744, 745, 746, 748, 750, 7 

Strichotricha, 95 

Strobila, 246", 247, 262 

Strongylocentrotus, 393, 895, 897, 414— 
See Sea-urchin 

Strongylostoma, 615 

Stronygylis, BOS 

Srylarioides, 488 

Stylaster, 157, 158, 159 

Style, 158* 

Stylonychia, 99 

Subcortical cavity of sponges, 11{* 

Subdermal cavity of sponges, 118, 119 

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 (Rhizostome), 183 : 
Discomedusiv, 183 

Summer eggs, 597 

Supplemental skeleton, 53, 54* 

Supporting lamella—See Mesogliwa 

Swimming-hell—See Nectocalyx 

Swimming ovaries, 315 


36, 739, 742, 
a) 


INDEX 


Swimming-plate, 213* 

Sycantha, 118 

Sycetta, a es 

Syeettide, le a2 

a ase 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” 

Syllida, 475, 487, 488 

Syllis ramosa, 487 

Symbiosis, 63° 

Symmetry, 41", 42°: Polype and Medusa, 
138: Tealia, 189, 192 

Symphyla, 615*, 617, 619, 677 

Synapte, 424, 434, 438 

Synaptidir, 429 

Synapticula, 205" 

Syncarida, 566*, 581 

Syn-cerebrum, 555*, 631 

Syncoryne, 145 

Synerypta, TW 

Syneytium, 298* 

Syngnatha, 615°, 616, 617, 618 


T 


T 

ABANUS, 638, 644 

Tabula, 157°, 205, 208 

Tactile cones, 260%, 268 

Tactile organs, 39 

Tenia cvnurus, 252 

Tonia crassicollis, 285 

Tenia creumerina, 270 

Treniada, 254 

Tenia echinococcus, 261, 262, 282, 286 

Tenia mediocanellata, 285 

Tenia saginate, 285 

Tenia serialis, 282 

Tenia serrata, 285 

TANIA soOLIUM, General features, 245, 
246, 247- Nervous system, 248: 
Excretory organs, 248: Reproductive 
organs, 248, 249, 250: Development, 
250, 251: Systematic position, 254 

Teeniole, 173* 

Tatitrus, 583 

Tanaidacea, 567*, 583, 584, 585, 603 

Tondais, 5607, 583 

Tape-worm—See Tenia and Cestoda 

Tardigrada, 673, 675", 678 

TreaLia: 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 


Tealidia, 1u* 

Tectibranchia, 734°, 7565 

Teloblast, 464* 

Telolecithal, 219* 

Telotrochal, 486° 

Telson, 541* 

Temnocephala, 259, 200, 269, 277, 278, 
279 


9 Ty 


Temnocephalea, 2 
278, 279, 284, 2 

Tendon, 37* 

Tentacle sheath, 210% : 

Tentacles, 32°, 9] 

Tentacular canal, 213 

Tentaculifera, 91* : Body and tentacles 
99, 100: Nucleus, contractile vacuoles 
shell, colonies, reproduction, 100, 101 

Tentaculocyst, 155*, 172 

Tentorium, 626* 

Terebella, 474 

Terebellidw, 472, 474 

Terebra, 738 

Tercbratula, 366, 368 

Terehratulidie, 367* 

Terebratulina, 372 

Teredo, 696, 701 

Terga, 578* 

Tergum, 541* 

Termites, 633 

Tessera, 177, 178 

Testis—Nee Spermary 

Tethys, 734 

Tetrabranchiata, 790*, 792, 795, 797, 798, 
803, 804 

Tetramita, 71 

Tetrarhynchus, 261, 202 

Tetrastemma, 291 

Thalassoplancta, 61 

Thallasema, 499 

Theea, 205% 

Thomson, J. Vaughan, 3 

Thorax, of Apus, 531: Astacns, 541 
Periplancta, 622: Solpugida, G4 

Thread-worms, 207—See Nemathel- 
ininthes 

Thysanopoda, 568 

TLhysanozoon, 255 

Thysdnura, 640) 

Thuricola, 95 

Ticks, 653, 661—See Acarida 

Tiedemann’s vesicle, 383*, 426 

Tintinnidinm, 94 

Tissues, 23* 

Tomopteris, 488 

Tooth-shells, 680—See Scaphopoda 

Trachew, 35", 609, 743 

Tracheal gills, 642, 643 

Pracheliastes, 516 

Trach lins, 9& 

Trachclomonds, 71 ; 

Trachyline, 142°: General 
153: Sense organs, 154: 


*, 259, 260, 269, 273, 


> 
y 
4 


Xoots, 155% 


’ 
> 


structure, 
Tentacles, 


155: Reproductive organs, 154, 155: 
Development, 155, 156 

Trachymeduse, 142”, 154, 155, 156 

Translation, 285* 

Trap-loor Spider, (72 

Trapeia, 209 

Trematoda, 252", 257, 258, 259, 260, 264, 
265, 266, 207, 268, 269, 270, 272, 273, 
277, 278, 279, 284, 285, 286, 287, 288 : 
Example, 240 

Treploplaa, 233 

Triarthrus beckii, 606 

Trichina, 310, 311 

Trichinella, 306, 311 

Trichiniasis, 312 

Trichocyst, 89, 93 

Trichoplax, 233 

Trichostomata, 91 

Tricladida, 252*, 253, 255, 257, 263, 264, 
267, 268, 269, 271, 275, 276, 283, 284 

Tridacne yiguts, 710 

Trigger-hair—See Cnidocil 

Trigonia, 695, 701, 710, 711 

Trilobita, 604, 605, 606, (77 

Trimorphism, 129* 

Trimyaria, 290 

Tristivothata, 319 

TRITON NODIFERUS, 721. Shell, 721, 722, 
723: External features of soft parts, 
723: Foot, 724; Visceral spiral, 725: 
Mantle, 725; Ctenidium, 725; Os- 
phradium, 726: Digestive system, 726, 
727, 728: Vascular system, 720: Ex- 
cretory system, 729: Nervous system, 
729, 730, 731: Sensory organs, 731 : 
Reproductive organs, 732: Systematic 
position, 734 

Tritonidie, 733, 734", 745 

Trivium, 377*, 416 

Trochal disc, 324 

Trochelminthes, 322": Appendix, 336 

Trocheta, 516 

Trochophore—Sce Trochosphere 

Trochosphura, 328. 832, 335 

Trochospherida, 328* 

Trochosphere, 322, 440 

Trochus, 733, 747 

Trombidinm fuliyinosum, 665 

Trophozoite, 80 

Trypanosomes, 71, 72 

Tube-feet, 377%, 376, 395, 401 

Tubifea, 478, 483 

Tibificidic, 488 

Tubipora, 195, 198, 199, 203, 208, 210 

Tuhulasia, 145, 158 

Trhidaviv, 143 

Turbellaria, 252*, 255, 256, 257, 202, 
263, 264, 2065, 266, 267, 268, 269, 271, 
272, 273, 274, 275, 276, 284, 286: Ex- 
imple, 236 

Lurbo, 733, 747 

Typhlosole, 458°, 475, 685 


838 INDEX 


U 


O sine, ii 

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 
Unionide, 696* 

Unisexual, 40* 

Urea, 36 

Uric acid, 13, 36 

Urinary organs, 37* 
Urnatella, 354 

Urns, 493* 

Uropod, 542* 

Uterine bell, 313 

Uterus, 40* 


vy 


\ 


Leen contractile, 11*, 18, 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: Nemertinca, 291: 
Acanthocephala, 313: Phoronis, 357: 
Magelania, 365: Brachiopoda, 369: 
Starfish, 379: Holothurian, 426: 
Antedon, 409: Echinodermata, 426: 
Nereis, 444; Lumbricus, 458: Sipun- 
culus, 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: Arachnida, 668: Mussel, 689: 
Pelecypoda, 706: Amphineura, 715: 
Triton, 729; Gastropoda, 744: Scapho- 
poda, 757: Sepia, 765: Nautilus, 783 ; 
Cephalopoda, 797 

Vas deferens—NSee Spermiduct 

Vegetal pole, 751 

Velarium, 170*, 182 

Velella, 166 

-Veliger, 709, 710%, 751 

Velum, 135*, 186, 170, 710, 752 

Ventral, 42* 

Ventricle, 36* 

Venus gnidia, 699 

Venus’s Flower-basket, 120 

Venus’s Girdle, 223 

Vermes, 235 


Vermetes, 733 

Vermetus, 755, 756 

Vermiform embryos, 231] 
Vermilia cespitosa, 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 
Vulselia, 698 


Ww 


We ene 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 Onixeis 


x Fy 


ee 661*, 666, 667, 668, 669, 
673, 677, 678 


Y 


y nes Cretis—See Zoochlorella. 
Yellow elastic cartilage, 26* 

Yoldia, 695 

Yolk, 19* 

Yolk-glands, 236* 

Yolk, epithelium, 799: reservoir, 243 
Yolk-sac, 803 


INDEX 839 


Zoochlorella, 60, 63, 208, 341 


Z Zoo-geographical Regions, 8 
Z Zooid, 40* 50, 75, 283 
ILLA CALLOPHYLLA, 670 Zoology, 1* 
Zoxa, 599* Zoophyte, 128 
Zoantharia, 194* 227 Zoothamnium, 97 
Zoanthus, 196, 197, 200, 201 Zygophiure, 411* 
Zoceciun, 341 Zygote, 74, 75* 80, SI 


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