it te cae coe ee Seels CORNELL UNIVERSITY LIBRARY BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 1891 BY HENRY WILLIAMS SAGE brary “iio 7 040 um DATE DUE GAYLORD TEXT-BOOK OF EMBRYOLOGY MACMILLAN AND CoO., Limirep LONDON + BOMBAY - CALCUTTA MELBOURNE THE MACMILLAN COMPANY NEW YORK + BOSTON - CHICAGO DALLAS - SAN FRANCISCO ‘HE MACMILLAN CO. OF CANADA, Lrp. TORONTO TEXT-BOOK OF EMBRYOLOGY VOL. I INVERTEBRATA BY e E. W. MacBRIDE, M.A., D.Sc. LL.D. F.R.S, PROFESSOR OF ZOOLOGY Al THE IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY SOUTH KENSINGLON EDITED BY WALTER HEAPE, M.A., F.RS. MACMILLAN AND CO, LIMITED ST. MARTIN’S STREET, LONDON 1914 Fas QOL G55 - Tea 5 a he Se A29n 08 TO THE MEMORY OF ADAM SEDGWICK BELOVED TEACHER AND FAITHFUL FRIEND THIS VOLUME IS DEDICATED BY THE AUTHOR f PREFATORY NOTE THE design of this Text-book of Embryology, of which this is the first volume, is to associate the structural development of embryos with broad generalizations of what is known of their physiology. Attention will be drawn, for instance, to the correlation between the function of certain organs of a larva and its habit of life, and, in a more general way, between function and habit and the course of development. Reference will be made to some of the more striking results obtained by Experimental Embryological research. Attention will be drawn to gaps in our knowledge which indicate promising fields for research. It is hoped that the interest of all students of Embryology will thus be stimulated, and the practical value of these volumes, especially for students of medicine, ensured. A second volume, by Professor Graham Kerr, in which the lower Vertebrata will be dealt with, will follow as soon as possible, and a third volume by Mr. Richard Assheton, on Mammals, will complete the work. The Authors are responsible for the facts and generalizations recorded, and to them is due all the credit which may be given to the work. THE EDITOR. vii PREFACE Tuis book has been written in order to achieve two objects, first to place before the reader in as succinct a form as possible the best ascer- tained results in the field of Invertebrate Embryology, and secondly to indicate some of the problems which as yet remain unsolved and the best means of attacking them. In order to attain the first object a number of typical life-histories, illustrating all the important groups of Invertebrata, have been described, and in selecting the types for special description two principles have guided us: first, the life-history of the type chosen must be thoroughly ascertained, and second, the type must be a common form easily accessible to students in temperate regions. Thus the spider has been chosen as a type of the Arachnida rather than the scorpion, and for the same reasons the life-histories of parasitic forms have been very slightly dealt with. The Trematoda and Cestoda have been entirely left out of consideration because it is difficult to obtain a complete series of the stages in the life-history of any one species—and though the external features of the life- history of members of these groups are known, their organogeny is still to be worked out. Moreover, the external features of the development of Trematoda and Cestoda are adequately described in ordinary text-books of zoology. In pursuit of the second object the methods used by the best investigators have been given in connection with the description of the life-history of each type examined by them, and we have ever striven to keep before the mind of the student the idea that the ultimate object of the Science of Embryology is not solely the ascertaining of facts but especially the determination of the laws of ix aes, weet en were wenn ey eee — as Invertebrate Embryology, very much must necessarily be omitted, but it seemed to us better to run the risk of criticism on this score and to bring our survey of the field to a conclusion within a reason- able period, rather than to attempt to give a complete account of all that is known of Invertebrate Embryology. Such an attempt would involve the task of writing not a single volume but a series of volumes; it would require for its accomplishment many years and would be beyond the powers of one man. Moreover, the first volume would be out of date long before the last volume was published. The literature lists have been purposely kept as brief as possible —as a rule only the most recent papers on the subject have been cited. Where earlier papers have been referred to it is chiefly because these papers, in laying the foundation of our knowledge, have not been superseded by later work. In conclusion, my best thanks are due to my colleagues, Professor Lefroy and Mr. Dobell, for valuable suggestions, and to my wife for much help in the tedious work of preparing the Index. IMPERIAL COLLEGE or ScIENCE, Sourm KENSINU'TON, July 29, 1914. CONTENTS CHAPTER I INTRODUCTION Scope of Embryology—Growth, maturation, and conjugation of the germ-cells—Sex- chromosomes and their bearing on theories of heredity—Recapitulation theory of development and the evidence in its favour—Embryonic and larval phases of development and their mutual relationship——Factors modifying the course of development. . : ‘ é ; : ‘ Page 1 CHAPTER II PRACTICAL HINTS Methods of preserving embryos— Methods of embedding embryo for section- cutting and of orientating them so that the sections shall be cut in a known direction , : s : 3 , ‘ 4 Page 32 * CHAPTER III PORIFERA The development of Sycandra raphanus as a type of the Calcarea: the Blastula, Amphiblastula, and Ascon stages in its development—The development of other sponges (Oscarella, Plakina, Esperia, Spongilla, Clathrina, Lewcosolenia) — The development of the gemmule of Ephydatia—The ancestral history of sponges . ; ; . ‘ : bg . Page 37 CHAPTER IV COELENTERATA The development of Z'ubularia as a type of the Hydrozoa—The development of the eggs of free-swimming medusae—The development of the gonophore of Tubularia and of the medusa of Podocoryne -- Other gonophores—The development of . Siphonophora, Narcomedusae, and Trachymedusac—The development of Aurelia asa type of the Scyphozoa—The development of Pelagia—The development of Urticina as a type of the Actinozoa—The development of Actinia bermudensis— The larvae of Zoanthidae and Cereanthidae—The development of Aleyonaria—The development of the skeleton of Caryophyllia—The development of Berde as a type x1 xi INVERTEBRATA of the Ctenophora—The formation of the tentacles of Callianira—The affinities of the Ctenophora with other Coclenterata—The experimental embryology of Coelenterata—The ancestral meaning of the Planula larva. . Page 58 CHAPTER V PLATYHELMINTHES The Polyclada the most primitive group of Platyhelminthes so far as their develop- ment is concerned—Development of Planocera inqutlina as a type of the Polyclada —The nature of spiral cleavage—Definition of cell-lineage and tlie nomenclature employed—Miiller’s larva in the development of Yungia and its metamorphosis— The interpretation of Miiller’s larva and the light which it throws on the ancestral history of Platyhelminthes . é , ‘ . Page 102 CHAPTER VI NEMERTINEA The development of Cerebratulus lacteus as a type of the Nemertinea—The Pilidium larva and its metamorphosis—Experimental embryology of the Nemertinea—The interpretation of the Pilidium larva and the light which it throws on the ancestral history of Nemertinea 4 : 3 ‘ . Page 118 CHAPTER VII ANNELIDA Methods of preserving and mounting the eggs of Annelida—The development of Polygordius appendiculatus as a type of the Annelida: its cell-lineage: its Trochophore larva and the metamorphosis thereof—The Trochophore larva of Polygordius lacteus and its metamorphosis—The development of various Polychaeta—The development of the nephridia of Criodrilus—The development of Nephelis as a type of the Hirudinea—The development of Clepsine—The inter- pretation of the Trochophore larva and the ancestry of the Annelida. Page 128 CHAPTER VIII ARTHROPODA The development of Peripatus capensis as a type of the Prototracheata—The develop- ment of Astacus as a type of the Crustacea—The formation of layers in Homarus, Lucifer, and Euphausia as types of the Eucarida—The formation of layers in Mysis as a type of the Peracarida—The formation of layers in Polyphemus and Branchipus as types of the Cladocera—The formation of layers in Lepas as a type of the Cirripedia—The larval history of Cyclops as a type of Crustacean larval history: the Nauplius and Metanauplius larval stages—The larval history of Phyllopoda: the Nauplius larva of Apus and Branchipus—The larval history of Cirripedia: the Nauplius and Pupa stages—The larval history of Ostracoda— The larval history of Euphausidacea and Penaeidea : the Nauplius, Protozoaea, and CONTENTS - xiii Zoaea larval stages—The ancestral meaning of the Nauplius larva—The develop- ment of the parasitic Copepod, Actheres ambloptitis, and the support which it gives to the recapitulation theory of development—The Zoaea larva in Euphausidacea, Penaeidea, Caridea, Anomura, and Brachyura—Ancestral meaning of the Zoaea larva—The Mysis larval stage: its modification in the Loricata into the Phyllo- soma larva: its modifications among the Crangonidae and Thalassinidae—The Megalopa larval stage of Brachyura—The brephic stages in Anomura—The larval history of Stomatopoda—The Metanauplius, Erichthoidina and Erichthus larval stages—The Alima larva—The larval history of Portwnion as a type of the parasitic Isopoda—The development of Agelena, a type of the Arachnida—The development of Limulus: its Trilobite larva and the significance thereof—The development of the Scorpion and of some other Arachnida—The development of Pantopoda—The development of Tardigrada—The early development of Donacia as a type of the embryonic development*of Insecta—The carly development of Doryphora—The embryonic development of Blatia and the supposed origin of its mid-gut epithelium—The development of the genital organs in Phyllodromia— The formation of the embryonic membranes in the primitive Aptera (Lepisma and Machélis)—The metamorphosis of Galerucella ulmi as type of the larval history of Insecta—The development of the wings in Doryphora—The development of the compound eye in Dytiscws—The metamorphosis in various families of Coleoptera, Diptera, and Lepidoptera—The complicated metamorphosis of the Muscidae—The meaning of metamorphosis as exemplified by observations on Bombyx —The development of Scolopendra as type of the Myriapoda—The development of Julus —The ancestral history of Arthropoda . ; ; : . Page 169 CHAPTER IX MOLLUSCA The development of Patella as a type of the Mollusca—The metamorphoses of Aemaea, Fissurella, and Haliotis—Experiments on the eggs of Patella—The development ‘of Paludina as type for the formation of the internal organs of Gastropoda—The development of Polyplacophora—Methods of preserving and staining the eggs of Gastropoda—Various modes of the development of the velum in Gastropoda— The larval kidneys of Gastropoda and their supposed homologies—The develop- ment of Solenogastres—The development of Dentaliwm as type of the Scaphopoda— Experiments on the eggs of Dentaliwn—The effect of cutting off the polar lobes of the egg—The development of Dreissensia asa type of the Pelecypoda—The larval development of the oyster—The development of Yoldia as a type of the Proto- branchiata—The mode of development of heart and pericardium in Cyclas—The development of Unionidae—The Glochidium larva—The segmentation of the egg of Sepia as a type of Cephalopod egg——The development of Loligo as type of the Cephalopoda—The development of the eye of Cephalopoda—General considerations on the ancestral history of the Mollusca ‘ ‘ 3 . Page 291 CHAPTER X PODAXONIA The development of Phascolosoma as type of the Sipunculoidea—The affinities of Podaxonia—The development of Sipunculus—The development of Phoronis : various views as to its affinities : F rae ‘ . Page 372 XVi INVERTEBRATA —The ancestral meaning of ‘the asymmetry of the larva—-The development of Cynthia partita as a type of the Urochorda—lIts cell-lineage and the segregation of coloured organ-forming substances—The process of gastrulation—The develop- ment of the nervous system—The free-swimming larva of simple Ascidians and its metamorphosis—The formation of heart, pericardium, and epicardium—The formation of the stigmata—The development of the genital organs in Molgula— . Experiments on the eggs of simple Ascidians—Development of Molgula ampul- loides—The Development of Ascidiae compositae—The development of Pyrosoma as a type of the Ascidiae luciae—The Cyathozooid—The development of Salpa ‘as a type of the Thaliacea—The formation of the placenta—The development of Doliolum—The development of buds in Urochorda—Stolonial budding —The organogeny of the Blastozooid—The Ascidiozooids of Pyrosoma—The buds of Salpa and of Doliolwm—Pallial budding in the Botryllidae—Budding in Diplosomidae —The reason for the different organogeny in the bud and in the larva—The affinities of the Urochorda with the Cephalochorda . : .. Page 568 CHAPTER XVIII SUMMARY Physiological explanation of recapitulation—Evidence in favour of the possibility of the inheritance of functional adaptations—Secondary modifications of the re- capitulatory life-history—The effect of yolk and of maternal secretions—The modus operandi of organ-forming substances—Sketch of the ancestral history of Metazoa . i ; ; - ; ; ‘ . Page 649 ERRATA Page 9, line 38, for ‘‘cell sap” read ‘‘cell and sap.” », 61, lines 5 and 6, for “‘medusa S. apicata” read ‘medusa of S. apicata.” », 125, line 22, for ‘‘first nucleus” read ‘nuclear matter of the sperm head.” ’ », 126, line 4, for * broken in two” read ‘‘ broken into.’ ¥ oe w Qo oe ana 15. 16. 17. . Two stages in the fixation of the larva of Sycandra ee ee 19. 20. al. 22. 23. 24, . Longitudinal sections of the Amphiblastula larva, the just-fixed larva, and 26. Qf 28. 29. ILLUSTRATIONS . Hight views of the maturation of the male cells of Lepidosiren paradoxa . Nine stages in the transformation of a spermatid into a spermatozoon - Four stages in the maturation and fertilization of the egg of Orepidulu plana . Two stages i in the first divition of lie: fertilized egg of Crepidule plona . Polar view of the first maturation division of the male germ cells of Alydus ptlosulus in order to show tetrads . . Second maturation division of the male germ cells ae Protenor beiane - Second maturation division of the male germ cells of Huschistus variolaris . . The larva and adult female of Portunion maenadis . The stalked larva and adult form of Antedon multispina 10. 11. 12, 13. 14, The larva and just-metamorphosed form of the Plaice (Pleuronectes stabassie Unripe egg of Limulus polyphemus. An example of a centrolecithal egg The ripe egg of Strongylocentrotus lividus. An example of an alecithal eg¢ Two stages in the segmentation of the egg of Sycandra raphanus View of the embryo of Grantia labyrinthica in the blastula stage igine ti in the embryonic chamber of the mother View of the embryo of Grantia labyrinthica in a fade stage ‘of development than that represented in Fig. 14 Section of a portion of Grantia labyrinthica . The Amphiblastula larva of Grantia labyrinthica An early stage in the metamorphosis of the Ascon stage of Seaniee raphanus into the adult : A late stage in the metamorphosis of the Aweon ties of Sueanaia ities into the adult condition Longitudinal sections through the free-swimming lay va of iseaetit atastiants in two stages of its development and its fixation Seven stages in the metamorphosis and fixation of the lerva and growth of the young sponge of Plakina monolopha ’ @ Two sections of the body-wall of the larva of Plakina monolopha to Siora the distinction between archaeocytes and mesenchyme Longitudinal section through the Amphiblastula larva of Esperia fevers the young sponge of Lewcosolenta variabilis Section through a gemmule-bearing individual of Ephy ydatia iiaenlan ate Three stages in the formation of the gemmules of Ephydatia blembingia Two methods of formation of the blastula in Tubularia mesembryanthenum Formation of endoderm in 7'ubularia mesembryanthemum xvii Ue? PAGE 10 11 12 14 14 23 24 25 27 27 38 389 39 40 40 41 41 42 44 45 46 46 48 49 50 55 xvili INVERTEBRATA FIG. 30. él. 32, 33. 34, 35. 36. 37. 38. 39. 40. 41. 42, 43. 44, 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. . The Arachnactis larva of Cereanthus membranaceus . Section of embryo of Lubularia mesembryanthemum showing the formation of the aboral tentacles Five views of external features of aiaeventt stapes in ‘iis derclonment of the embryo of Tubularia indivisa Two gonophores of Z'ubularia indivisa with dleweleping diedlaniad hele Stages in the development of the Actinula larva of T’ubularia indivisa Three longitudinal sections through the developing gonophore of Tubularia mesembryanthemum. . ‘ Four transverse sections through the Rea dion mango of Z abalone mesembryanthemum . Longitudinal section through very young ‘pouphese liad of Tubularia mesembryanthemum to show the origin of the genital cells Four stages in the development of the planula of Clytia Three stages in growth of the fixed planula of Clytia A young euleuy “ot Clytia reared from a planula in the aquarium Two longitudinal sections through the developing medusa of Podocoryne carned Four transverse sestions thr eet thie developing melita of Paden carnea in order to show formation of circular canal and endoderm lamella Two longitudinal sections through the developing gonophore of Clava squamata Three stages in the development ofa ‘sidlumnopibee (Cueintin ibatiagecsti tea) Embryo of a Geryonid (Carmarina fungiformis) in which endoderm cells are being budded off Early stages in the development of Aunt svonibe The fixation of the free-swimming larva of Aurelia aurita Two longitudinal sections through two Hydra-tubae of different ages Two transverse sections through a Hydra-tuba with four tentacles . ; A, a Hydra-tuba with eight tentacles. B, longitudinal section of a part of a similar specimen in order to show origin of septal funnels Oral view of Hydra-tuba with twenty tentacles. ve tentacles are repre- sented as cut off) < Two horizontal sections through the upper pat ie a Hydra: tuba, abot as old as that represented in Fig. 50, to show the formation of the ostium connecting the stomach pockets : Two stages in the strobilization of the Scyphistoma of dual aur site A strobilized Scyphistoma of Aurelia aurita. An Ephyra larva of Aurelia awrita a after liberation font the a dea Scyphistoma Longitudinal section through the s sense organ cal a ae Ephy ra Three stages in the development of the Ephyra larva into the adult Minelia An egg of Urticina crassicornis dividing into sixteen blastomeres . Four stages in the development of the ege of Urticina crassicornis as seen in longitudinal sections Two stages in the development of tives larva of tobi crassicornis : Two transverse sections through a larva of Urticina crassicornis in order to show the formation of mesenteries Transverse section through.a larva of Urticina crassicornis in which ithe tentacles have just been developed . Longitudinal section through the larva of Apisian wgaviiex in ‘aiilen ts show the ectodermal origin of the mesenterial filament Stages in the development of the larva of Actinia equina PAGE 56 56 57 57 58 59 59 60 60 61 62 63 64 65 67 68 68 69 70 71 71 72 73 74 74 75 75 v7 78 79 80 81 82 84 85 68. 69. 70. 71. 72. 73. 74, 75. 76. 77. 78. 79. 80. 81. 82. 83. 84, 85. 86. . A, dorsal, and B, ventral views of the free-swimming larva of Yungia 88. 89. 90. 91. 92. 93. 94, 95. 96. ILLUSTRATIONS » The Zoanthina larva of a Zoantharian ‘ . Transverse sections of two types of Actinozoan larvac : ‘ - Young living Caryophyllia cyathus, seen from above. ‘The calcareous skeleton shows through the transparent tissue Five stages in the development of the skeleton of Caryorhyliie eyes Side view of the segmenting egg of a Ctenophore (Cadlianira bialata) Two views of the developing egg of Berée ovata, seen from above Oral and aboral views of the embryo of Beréde ovata in a later stage of development Tilustrating the origin and ate of fl so- -called eanlemnr & ina Cignspliore embryo (Collianira bialata) Optical section of embryo of Berée forskalii seine the bestinins of the endodermal cavities ‘ : Optical section of embryo of Berée for: edie ina lates stage of dievalopalene: with a hollow endodermal sac . Two optical sections through the embryo of Bera Pusat : : Larva of Berée forskalit four days old, viewed from ‘‘stomach-plane ” Part of apical region of larva of Berée forskalii, viewed from stomach-plane. The free-swimming larva of Callianira bialata, viewed from the stomach-plane An embryo of Berée ovata with four ribs and two endodermal pouches, and a small extra third pouch ; obtained by isolating one of the first two blastomeres of Berée ovata . Developing egg of Planocera eaten: Eight-cell stage, viewed from - animal pole . : : ; : Developing egg of Planocera sagililings, Sixteen-cell stage, viewed from the animal pole . : a F : : 3 Developing egg of Planocera inguin Thirty-two-cell stage, viewed from vegetative pole Optical section of the dovdloying én of Bicniocere Tuo jean fier posterior pole . Diagrammatic frontal section vous the aoe of Penane inoutine ike a later stage of development than that represented in Fig. 83 Developing egg of Planocera inquilina ina late stage of segmentation, viewed from animal pole. Three longitudinal sections thr otigh developing embr, yos of Pilapovee japuitina aurantiaca . Lateral view of the Freee chaniigg lay va of ¥ uUngia ccnrplniisca A, dorsal, and B, ventral views of larva of vanes aurantiaca in vaibiels metamorphosis is beginning 5 A, dorsal, and B, ventral views of larva of Yungia aur tian in which metamorphosis is almost complete . : s Median sagittal longitudinal section through larva ae Yun, gia wieeintati , Median sagittal section through a young Polyclade worm (Yungia aur- antiuca) just after its metamorphosis Two stages in the segmentation of the egg of Crnebrib tin Vicie: vowel from the side The young gastrula of Ce ribratistios lactews : ; Two stages in the development of the Pilidium larva Lat Ce nah atulus lacteus showing the development of mesenchyme into muscles. A, earlier stage. B, later stage Two views of saivanead pilidsant laws of Cendtivatutus lectus in ovdse ts show the development of the muscles x1x PAGE 86 87 88 89 90 91 92 92 93 93 94 95 97 105 105 107 108 109 » 110 111 112 112 113 113 114 115 119 120 121 121 XX FIG. 97. 98. 99, 100. 101. 102, 103. 105. 106. 107. 108. 109. 110. 111. 112. 118. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124, 125. INVERTEBRATA A Pilidium larva shortly before its metamorphosis . Longitudinal section through a Pilidium larva of about ‘the age ef that represented i in Fig. 97 ‘ Two stages in the: development of the Wenertine rudiment within the Pilidium, viewed from above The Trochophore larva of Polygordius, viewed from ‘the Mie Stages in development of the blastula of Polygordius seen in spite! longitudinal section . Dorsal view of upper hemisphere of ese of Polygon die, in whieh weve: six cells have been formed . Three stages in the segmentation of ‘the lower or weelative sinitives of the egg of Polygordius . . 136, . Four views of the vegetative pole of the developing egg a Polygordius in order to show the processes of gastrulation and of the formation and closure of the blastopore Four diagrammatic transverse sections of lower jive of young Treehophote larva to show the mode of closure of blastopore Optical sagittal section of young Trochophore after gastr tion is coniplate Later stage in the development of Polygordius appendiculatus, in which the ‘‘ worm-body” is being formed by the growth of the trunk blastema. Longitudinal sections of stage i eiaay in preceding figure in order to show details Two sections of the anterior ue af’ a azote of Piteg odin dvoundieitintna in order to show the changes supervening on metamorphosis Polygordius appendiculatus immediately after metamorphosis Longitudinal sections of anterior portion of Polygordius ceppeasvaatittws immediately after metamorphosis in order to show details Late larva of Polygordius lacteus in optical frontal section . : Figure illustrating the origin of the mother cells of the adult mesoderm in Eupomatus , Diagrammatic ssigatbal Sactions of fully: epee Trachapiroue larva of ‘igi. matus in order to show the relative ee of the zal sca and of the coelomic rudiment Stage in the segmentation of the egg ‘of Nereis abihinti: iewed from aheue: showing a laeotropic spiral cleavage of the egg The free-swimming larva of Nereis limbata, three days old. A epled “Polytrochal ” larva Transverse section through the qoninal part of an ie of Cioetitos lacuwm Two longitudinal dentin thiough “the sentval poreien of an eibeyo ae Criodrilus lacuum . - Two longitudinal sections of eaibigos of HMuphetis hii ‘ A fairly advanced embryo of Clepsine (Glossiphonia), seen from behind Hinder view of a well-developed larva of Nephelis vulgaris Larva of Nephelis vulgaris, viewed from the side ‘ Stages in the segmentation and the gastrulation of the egg of Perinaine capensis Stages in the division: of the Blaatepore and i fenation of the ae of Peripatus capensis Diagrammatic transverse sections iu ‘he heeded of sucbinos of Prd: patus capensis of various ages in order to illustrate the mutual relation- ships of haemocoele (primary body-cavity) and coelom (secondary body- cavity) 5 : ‘i PAGK 122 123 124 130 132 134 141 142 143 145 146 149 150 151 152 154 155 155 156 158 159 160 161 162 162 170 172 FIG, 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142, 143. 144. 145. 146. 147. 148. 149. 150. 161. 152. 153.- 154. 155. 156. 157. 158. 159. 160. 161. 162. ILLUSTRATIONS The formation of the appendages in the embryo of Peripatus capensis Two sections through the developing egg of Astacus Sagittal section through the blastula of Astacus fluviatilis in order to show the primary yolk pyramids . Ventral view of an embryo of Astacus Auviatilis, the gastnala ates: in order to show the ventral plate 3 Sagittal section through a portion of the embryo of Aiowens fluviatilis in order to show the invagination of the endodermic rudiment Two sagittal sections through developing eggs of Astacus fluviatilis in order to show the development of the endoderm The “‘Nauplius” stage in the development of Astacus ‘Hudtatitis, ioned from the ventral side Three transverse sections through the developing nerve cord of “Aitacns fluviatilis Two views of developing eg: os of Hetaais ecwhaitibis i ne ental sieve Transverse section through the region of the heart in an embryo of Astacus fluviatilis in about the same stage as that represented in Fig. 134 B Longitudinal sation thrones cdvineell aula of Abrus ‘jeuciatiins parallel to the sagittal direction but to one side of the middle line Advanced einbryo of Astacus fluviatilis, viewed from the ventral side. The abdomen and hinder part of the thorax are cut off and spread out separately 3 Two ommatidia from the ee of a aay heteked Aitisore ‘iltokegtiis in longitudinal section é Two sagittal sections through advance sibpges ie tains fuuvlabiles Portions of two sagittal sections nee developing eges of Homarus americanus . Sections through the dewelepinig os of Hysts Saiaeteo Stages in the development of the egg of Polyphemus pediculus Four sagittal sections through the developing eggs of Lepas anatifera in different stages of development , The Nauplius larva of Cyclops canthocarnotdes from the eit eye Three stages in the further development of the larva of nae Two types of Eiauplins larva ‘ The ‘“‘Cypris” larva or Pupa of Lepas Sesstoitue’ is, seen from the gids The fixation of the Cypris larva of Lepas fascicularis The Nauplius larva of Cypris ovwm . The Protozoaea larva of Nyctiphanes inert Dorsal and ventral views of the ‘‘ Copepodid ” larva of Aether es senidlontilis Dorsal and lateral views of the just-fixed female of Actheres ambloplitis Lateral view of female Actheres ambloplitis after the adult characteristics have been attained . Enlarged view of gnathites aa line of fends Actheres cblopitis, seen from the side “Calyptopis ” Zoaea of Nystiphanes australis, later: al view Zoaea larva of Penaeus, ventral view Zoaea larva of Crangon vulgaris, lateral view ‘i Zoaea larva of Porcellana longicornia after the first moult . Zoava larva of the Crab Xantho “Mysis” larva of Homarus americanus, lateral view “‘Phyllosoma” larva of Palinurus vulgaris, ventral view “ Mysis” larva of Crangon allmannt, lateral view . XX1 PAGE 175 179 180 181 182 182 184. 185 186 187 187 188 188 190 191 192 193 195 197 198 199 200 201 202 203 205 206 206 207 208 209 210 211 212 218 214 215 xxii INVERTEBRATA Fa. PAGE 163. ‘ Megalopa.” larva of the Crab Pilumnus, dorsal view ; . 215 164. Two views of first post-larval stage of Zupagurus bernhardi, corresponding to the Megalopa stage of Brachyura : ‘ : . 216 165. Two stages in the development of a Stomatopod Tarra j é . 217 166. Two later larvae of Stomatopoda =. : : . ; . 218 167. Larva and adult female of Portwnton maenadis : » 219 168. Adult female of Portunion maenadis with appendages dissected at . 220 169. Three stages in the segmentation of the egg of Agelena . 222 170, Surface views of the developing egg of Agelena labyrinthica, shioeeta the primitive streak and the primitive cumulus 2 223 171. Section through the primitive streak of Agelena to illustrate the forma- tion of the germ layers : 224 172. Two views of embryo of Agelena at ‘the ported ae maximum extondlen of the ventral plate (4.¢. before reversion has begun). The coelomic cavities ave seen by transparence and are represented by lighter shading 225 173. Two sagittal sections through embryo of Agelena of different ages, but previous to reversion : : 226 174. A portion of a sagittal section of Ageians tan pliies z : . 227 175. Two views of embryos of Agelena undergoing reversion. 227 176. Transverse sections through the heart of Agelena iibpiuiaee in ise stages of development i : 4 : . 228 177. The ‘embryo of Agelena when reversion is enilees 228 178. Longitudinal section through part of the abdomen of 4g gelene labyr inthéea in order to show the origin of the genital organs . _ 229 179. Two sections through the developing stercoral pocket and Malpighian tubes of Agelena labyrinthica é 230° 180. Sagittal section through the hinder eae of ie spdatien of Apatina labyrinthica to show the hinder part of the mid-gut developing in connection with the stercoral pocket : 231 181. Longitudinal section through the abdominal appendages of an eanbiye ee Agelena labyrinthica in order to show the origin of the lung book -, 281 182. Surface views of the cut-off abdomen of three embryos of Agelena labyrinthica of different ages in order to show the modifications under- gone by the abdominal appendages 2 232 183. The condition of the brain in the eee of Bian aftes reversion hag taken place . : : ‘ . 288 184. Sections through the developing eyes of Agatenva ‘ 234 185. Two sagittal sections through embryos of the spider 7'her idiom seinen in two succeeding stages of development . ; . 234 186. Horizontal section through abdomen of an advanced arabrye of ‘Agelons labyrinthica in order to show the division of the yolk into lobes by septa 235 187. Ventral view of an embryo of Limulus longispina, twenty-one days old . 237 188. The Trilobite larva of Limulus Z ‘ 3 a 2 » 238 189. Two transverse sections through the ‘‘ germinal disc,” or developing area, of the egg of the Scorpion Luscorpius carpathicus in two stages ofdevelopment 239 190. Ventral view of embryo of the Scorpion Buscorpius carpathicus showing segments and appendages . ; : . 2 ‘ . 239 191. Larva of Ascorhynchus minutus : - 242 192. Dorsal view (optical frontal section) of the aang of Macrobiotus . 248 198. Portion of a sagittal section through the developing egg of Doryphora (Leptinotarsa) decemlineata before the formation of the blastoderm . 246 194. Surface view of the egg of Donacia crassipes at the conclusion of blastoderm formation ‘. 3 : ; ‘ ; ; : - 247 ¥IG. 195. 196. 197. 198, 199, 200. 201. 202. 203. 204, 205. 206. 207. 208. 209. 210. 211. 212. 218. 214, 215. 216. 217. 218. 219. ILLUSTRATIONS Section through the dorsal part of a developing egg of Donacta crassipes in order to show the primitive dorsal organ . « Diagrams showing the relations sustained to one another by amnion, serosa, and embryonic area in three succesive stages of the development of Donacia crassipes Diagrams to illustrate the fae mation of the pomenior wanton fold i in the egg of Donacia crassipes Two transverse sections through the gastral eroone of the egg of Donacia ~ erassipes after it is closed Three surface views of the embryo of Dea autnis a a Maye setter: the germinal streak begins to show division into segments Longitudinal section through the abdominal appendage of the embryo of Donacia crasstpes in order to show its glandular character Diagram of a longitudinal section parallel to the sagittal plane, but some- what lateral, through the embryo of Donacia crassipes in order to show the coelomic sacs Sagittal sections through the stomodaeum and proctodaenm of ‘Donacia crassipes 3 Diagrammatic repiesentation ‘of mid-gut of the emir yo of Bonet wheats showing its lateral pouches é Two transverse sections through embryos of Donasia re of differ ent ages, in order to show the modifications undergone by the coelomic sacs . Two diagrammatic sagittal sections through the egg of Periplaneta orientalis in two stages of development The ‘‘ grub” of Doryphora (Leptinotarsa) Ronenibmieake’ in ihe chika ‘estan, from the side Longitudinal section tieugt the gvbreante area ai an Satie yo of Phallodromta germanica Transverse sections through three sabes vot Phipitedroanses aprvnceniioa . three stages of development in order to show the nat of the coelomic sacs, the fat body and the generative organs The rudiment of the female genital organs of Phyllodromia ve manica reconstructed from sagittal sections thee Two views of the egg ne saccharina in dierent states of develop ment : Two diagrammatic Jatevail views through the egg of Moats Hiavaita in different stages of development ; Diagram of the anatomy of the larva of Galerucelle ulmi . é Portions of sections through the valve separating stomodaeum from aia gut in the larva of Galerucella ulmi Sections of the epithelium of the mid-gut in the pups of Calerucalti ‘aa showing the changes undergone i this tissue at the metamorphosis into the adult condition Portions of the Malpighian tubes of a larva aaa a pape of Gdiavnaeetto abies Portion of the abdominal muscles of the larva of Galerucella ulmi under- going histolysis Diagrammatic sections Jeon fie snadnal Fine of a wing in various stages of development showing how the wings ee in various groups of insects Two views of the head of the hieys of Dy isms marg innate in be staepe of development Longitudinal section through the simple eye caf larva ‘at Dy yitecus viasenlencdten parallel to its shortest diameter a XxX PAGE 247 248 249 250 251 252 254 255 255 256 259 260 263 264 264 265 266 267 269 270 270 XXIV INVERTEBRATA FIG. 220. 221. 222. 223, 224. 225. 234, 235, 236. 237. 238. 239. 240. 241, 242. 243, 244, 245. 246. 247. Lateral views of the head of the larva of Dytiscus marginalis in three stages of development in order to show the ee growth of the rudi- ment of the compound eye . Two stages in the development of ie éompound He of Dytisos manpinilie as seen in longitudinal section A small portion of the adult compound eye ae Dy wbiacll mar ee as seen in longitudinal section . Diagrams illustrating the mietetionp losis of " Musca woilidlorte Two diagrams illustrating the ee of the head of Hiaes vomitoria . Ventral view of the cmbapeuts area in a daysloning aoe of Sistonendrn cingulata - Development of Patella iiornitens Bight- ell tage shiowind the gpinales preparatory to the formation of the 16-cell stage . . Two stages in the development of the upper hemisphere al the anbagy a Patella coerulea, viewed from above . Stage in the development of the embryo of Patella sein: Heed from the vegetative pole of the egg, showing the first division of the third quartette and the beginning of bilateral symmetry as seen in the direction of the spindles in 3¢ and 3d . Two stages in the gastrulation of Patella coerulea in pineal lon gitedinal section . The ‘‘ Ctenophore” aiape ine the development of Patella aeniiles Seen from above . . View of the upper ‘areplions of an euibiy of Patella tirnilea diet bolore hatching . Lateral view of a ite eines of Patella eoniailew showing the nade a completion of the prototrochal girdle . Three diagrams of sagittal sections of the early ine of ‘Patella nies in order to illustrate the shift of the blastopore and the formation of the stomodaeum Lateral view of young Trochophore ie of Patella woes Ventral views of two stages in the development of the Trochophore larva of Patella coerulea . Side view ofa Veliger larva of Patella doriitea beloie torsion has taken place Side view of a Veliger larva of Patella coerulea after torsion has taken place ; Frontal section of a Veliger larva of Patella coer ‘tea in eden to show the mesodermic bands : Frontal section through the pre- Aroha ne ae an old Veliger larva of Patetla coerulea Two views of advanced aigeds of devclowmens of Actin eee. Three views of the just-metamorphosed Acmaca virginea in order to show the formation of the adult shell and the loss of the larval shell Two views of the young Haliotis tuberculata in order to show the forma- tion of the adult shell Illustrating the results obtained by ccna he blastorner es vat the developing egg of Patella coerulea . A Vertical section of the gastrula of Paludina vivipara Formation of the coelom in Paludina vivipara Optical frontal section of an embryo of Paludina mane a little older than those represented in Fig. 245. Two stages in the formation of the pericardium of Pedidos vivipara PAGH 274 275 276 277 278 282 293 294 295 298 299 300 301 3802 3808 3803 3804 305 306 308 310 311 311 312 FIG, 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258, 259. 260. 261. 262. 263. 264, 265. 266. 267. 268. 269. 270. 271. 272. 278. ILLUSTRATIONS Horizontal section through the visceral hump of an older embryo of Paludina vivipara than that represented in Fig. 247 in order to show the formation of the kidneys and the heart . Illustrating the development of the ureters of Paludina ieiceara end heir relation to the kidneys ‘ Two embryos of Paludina vivipara, viewed fr om the right ies’ in oteie is show the origin of the sense-organs and the beginning of torsion. : Just hatched Paludina vivipara, seen from the left side and viewed as a transparent object . Transverse sections through the viscer: sral humps of ibs 0 suibrpas of Paliisitin vivipara of different ages in order to illustrate the torsion of the organs, the development of the genital organs and their connexion with the right kidney, and the lengthening of the visceral hump associated with an increase in length of the liver . Apical region of an embryo of Crepidula shove ing the Molluscan + cross in a late stage of development ‘ Embryo of Limnaea stagnalis, viewed fice fake side as a transparent object, in order to show the larval kidney. a Vertical sections of the eggs of Dentaliwm before and oie fertilization i in order to show the flow of cytoplasmic substances . Stages in the cleavage of the egg of Dentalium Further stages in the cleavage of the egg of Dentaliwm : The Trochophore larva of Dentaliwm, twenty-six hours after fertilization ‘ Transverse section of the nee larva of Dentaliwm in the region of the prototroch Veliger larvae of Denandiiie 3 Larvae resulting from the davalupaient of oes of ‘Deenbeitaii eae whitch the polar lobe has been removed Vertical section of a larva of Dentaliwm developed from ope from which the first polar lobe had been removed in order to show the absence of mesoderm Longitudinal section a the 2. cell wane of Creienti poly re iia in order to show the blastocoele : Stages in the cleavage of the egg of Disisioaate piluni eens Sagittal sections of embryos of Dreissensia polymorpha showing the ite of gastrulation and of the formation of the shell gland Young Trochophore larva of Dreissensia polymorpha, seen from the ochital side . . Sagittal section throng a youe Trochophore jer of Devices ily morpha Transverse section af the ventral an ‘ion, of a Sotiny Veliger inne of Dreissensia polymorpha in order to show the origin of the mantle-groove and of the pedal ganglia Young Veliger larva of Dreissensia wel ere seen from he side Young Veliger larva of Dreissensia polymor me seen from the ventral surface Older Veliger larva of Tesi poliinnionntih seen from “the sil, This stage is the one which immediately precedes the metamorphosis . Transverse section through the dorsal region of an old Veliger larva of Dreissensia polymorpha in order to show the differentiation of the pericardium and the kidneys 5 é Sections through the cerebral pit of Veliger tars yae of Draenei poly- morphea XXV PAGE 313 314 316 317 318 322 824 326 327 328 829 329 330 331 331 333 334 338 339 340 341 342 343 XXVi INVERTEBRATA FIG, 274, 275. 276. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292, 293. 294, 295. 296. 298. Horizontal section through the anterior portion of an old Veliger larva of Dreissensia polymorpha in order to show the differentiation of the cerebral ganglia from the cerebral pit Side views of two young specimens of Basins pobiemmba afte ile metamorphosis has taken place : Horizontal sections through the anterior portions a ‘ee: just pvetation: phosed specimens of Dretssensia polymorpha in order to show the trans- formation of the cerebral pit into the labial palps . . Diagrammatic transverse sections of young specimens of Twaeweusie Holy: morpha in order to illustrate the development of the kidney . Transverse section through a young Dreissensia polymorpha in order to show the origin of the genital organs The late Veligor larva of Ostrea virginiana, view ad from hie cae. : The Veliger larva of Yoldia limatula, about three days old 3 Tonaituainal sagittal section through the Veliger larva of Yoldia timubate, three days old Diagrams illustrating the development of the penicudiint in hates and Dreissensia . . Glochidium larva of afokgatumne with widely opened valves, Hewed as a transparent object from the dorsal surface . Transverse section of a Glochidium larva of Unio eiioks 5 is sirens fed in the tissues of its host : Segmenting eg ae cell stage) of Sepia ee eiswed fone the | pole . A portion of ae margin of ile iaatedentn ot ihe: egg of Sens asfesinaalis at the conclusion of the process of segmentation in order to show the transformation of the blastocones into cells of the yolk-membrane Two sections through the edge of the blastoderm of Sepia officinalis in different stages of development in order to illustrate the development of the lower layer cells Two early embryos of Loligo inal Embryo of Loligo vulgaris, seen from the posterior side at the ebhieluaion of the period of development, termed by Faussek Stage I. ; Two sagittal sections of early embryos of Loligo vulgaris in order io illustrate the first formation of organs Embryo of Loligo vulgaris, seen from the side ‘sn behind, in aie to illustrate the development of the ganglia. The embryo is younger than that represented in Fig. 289 Embryo of Loligo wanna in the stage end by ‘Bans Stage 2, when the embryo begins to be grooved off from the yolk-sac ; posterior view Sagittal sections of two embryos of Loligo vulgaris to illustrate the forma- tion of internal organs Two diagrammatic transverse sections thr eiBh a satiny oat ‘yo of alige vulgaris in order to illustrate the origin of the coelom and the blood cavities Diagrams of three transverse scotivis thr ue an cupege of Loligo valet 4s much older than that represented in Fig. 294 Embryo of Loligo vulgaris at the a when the funnel is ‘formed, Howe from behind 7. Transverse sections of euibeyos of young auietle fish illustrating two stages in the development of the genital organs . Two early stages in the development of the eye of Kotige initgards, seen in transverse section : PAGE 344 3845 346 347 348 349 350 350 351 352 353. 355 356 357 357 358 359 360 360 361 362 363 364 365 366 FIG, 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 3809. 310. 311. 312. 313. 314, 315. 316. 317. 318. 319. 320. 321, 322. 323. 324, 325. 326. 327. 328. 329. 330. 331. 332, 333. ILLUSTRATIONS XXVii Sections through the developing eyes of young cuttle-fish to show the development of the lens Transverse section of the eye of a neatly ive embr #0 of Senin officinalis Early segmentation stages of the egg of Phascolosoma gouldti Later stage in the segmentation of the egg of Phascolosoma gouldit, viewed from the posterior aspect. Ne Two views of the apical region of ‘the sipnciting egg of Pigedloiie vulgare Nearly sagittal seston of an ‘euibige of Phosseatasoraa: iigahe at ithe stoge when gastrulation is beginning A Trochophore larva of Phascolosoma oulgare, a little more ‘than thirty- six hours old Nearly sagittal sections throng éibtationshusine Trechophors nevus of Phascolosoma Young specimens of Phaseotasiine ouutetet after the metanionphoss Lateral view of the Actinotrocha larva of Phoronis . : Diagrammatic frontal section of the Actinotrocha larva of Phoronts (sp) captured near Ceylon Longitudinal horizontal section of the embryo of an Aeration spocies of Phoronis Three stages in the metamorphosis of the Aettnstesdiia esta of Phorenis, seen from the side Early stages in the development of ie egg of oe enon edna The development of the larva of Membranipora pilosa Median sagittal section of the fully-grown Cyphonautes larva of Mentyané pora pilosa . ‘ Sections through fixed wid mebirnbe phosiig dared oe Menbrantpore pilosa Two degenerate beaek of lary: vae of Hetoprect Pilivion ‘ Stages in the development of the bud of Bugula avicularic. Early stages in the development of the egg of Pedicellina echinata Nearly sagittal section lying to the right of the median plane through the embryo of Pedivellina echinata Optical sagittal section of the free- aap macinting larva of Poacestttne echiGiats Median sagittal sections through two fixed and metamorphosing larvae of Pedicellina echinata : . Optical sections of early embryos of Persbeatalina wapienerviiciahe a Later embryos of Terebratulina scptentrionalis seen from the side (in optical section) Dorsal and ventral views of Jar va of ‘Y er" dirofuling soba cones Lateral view of a larva of Terebratulina septentrionalis Frontal and sagittal sections of the larva of Terebratulina sipiictanalls Two larvae of Zerebratulina septentrionalis just before and at the time of fixation Two young Tordbratulina sesikonirevedits fmntedindely ater the sttetae morphosis Young Terebratulina aeilendrtonadte. some little fine after rieiacioralvagts, viewed from the dorsal surface . Early stages in the segmentation of the oi of Cailidina paeet Further stages in the segmentation of the egg of Callidina russeola Sixteen-cell stage in the segmentation of the egg of Callidina russeola Two views of embryos of Callidina russeola showing the process of gastrulation PAGE 366 367 373 378 379 381 382 382 384 388 389 391 393 395 397 399 401 402 404 408 409 410 411 412 413 414 415 419 420 421 422 XXVlii INVERTEBRATA FIQ. 334, 335. 336. 337. 338. 339. 340. 341, 342, 343. 344. 345. 346. 347. 348. 349. 350. 351, 352. 353. 354. 355. 356. 357. 358. 359. Optical sagittal sections of two embryos of Callidina russeola after the process of gastrulation has been completed Ventral view of embryo of Callidina russcola somewhat iiet than those represented in Fig. 334 Optical sagittal sections through two late “annbeyes of Cultidina ieageslo in order to show the formation of organs . Ventral views of two embryos of Callidina russeola ata ‘stage not long before hatching : Optical sections of early sare of 85 gitta Ruwutale Cross section of the blastula of Sagitta bipunetata in the 16- eal stawo showing the determination of the mother cell of the genital organs Optical section of older embryo of Sagitta bipunctata showing the forma- tion of the ‘‘ head cavities ” and of the stomodaeum Ventral view of the larva of Sayitta enflata on the fourth day after hatghing. in order to show the origin of the nervous system and the separation af the trunk coelom from the tail coelom The 2-cell stage of the egg of Ascaris vabgaltenophaile leapuseuniatie) lien the spindles for the 4-cell stage are being formed, showing the diminution of the chromatin Early stages in the Sedmentatin of lie egg “at diet 1s inegatonapnote The 12-cell stage of the segmentation of the egg of Ascaris megalocephala, seen from the left and the right sides respectively, in order to show the rearrangement of the daughters of the cell AB Dorsal view of the segmenting egg of Ascaris megalocephala in the 20- cell stage . Ventral view of the demmcane ege of Nees nee in the 24-cell stage Dorsal view of the desmiontng egg af elms dagesbocaphaalo’ in fhe. 102- cell stage Optical median sagittal setiGh through the blastula of Haines Hegel: cephala at the time that the process of gastrulation is commencing Segmenting egg of Ascaris megalocephala in the 102-cell stage, seen from beneath : Sagittal sections through thr ee enbryas “6 Aseurts ineguitocophdiles in order to show the invagination of the genital cells and of the stomo- daeal rudiment Transverse section of eultigo of ae meg ecutadantinibe in . order to dow the invagination of the anterior mesoderm celis and of the mother cells of the epuital organs : A ‘*T-giant” of Ascaris Harataeieh dias seen a om. the left aide s Two diagrams of the 4-cell stage in the development of the egg of Ascaris megalocephala in order to show the cytoplasmic zones which Zur Strassen assumes to exist in the blastomeres : The stage corresponding to the 4-cell stage in he dayelapiiente of the normal egg which occurs in the development of the three types of ae fertilized eggs of Ascaris megalocephala Stage in the development of a ‘‘ball-egg” of Fenaves inegatouapheale cor- responding to the 8-cell stage in the development of a normal egg Stages in the early development of Asterias vulgaris Young larvae of Asterias vulgaris Fully developed Bipinnaria larva of Asterias Bilge about three wea old Larva of Asterias vulgaris, four days old, viewed from the dorsal surface, showing two madreporic pores PAGE 423 424 425 426 429 430 431 432 440 441 442 442 442 443 444 445 446 447 449 450 452 453 462 463 465 466 FIG. 360. 361. 362, 363. 364. 365. 366. 367. 368. 369. 370. 371, 372. 373. 374. 375. 376. 381. 382. 383. 384, 385. 386. 387. ILLUSTRATIONS Lateral views of an advanced Bipinnaria of Asterias se in which the brachiolarian arms are just appearing Ventral view of a Bipinnaria of Asterias vulgaris, of the same age as that shown in Fig. 360, in order to show the mutual relations of the coelomic cavities Lateral views of the bi aghiolarian ina ae the les of Astertels ‘ily wuts in various stages of fixation and metamorphosis Frontal longitudinal sections of two early embryos of Aster tn eiieadn in order to show the development of the coelom Views of a free-swimming larva of Asterina gibbosa, five or six days old Longitudinal frontal sections of larvae of Asterina gibbosa in order to show the segmentation of the coelom and the origin of the hydrocoele and madreporic vesicles Longitudinal frontal sections of havea of Sue wi S aes seven 49 dighi: days old, in order to show the beginning of the metamorphosis 5 Views from the side of a larva of Asterina gibbosa, seven days old, in the initial stages of metamorphosis Views from the side of a larva of Asterina gibboaa, sight days old, dhowiiz further progress in the process of metamorphosis . ‘i Longitudinal frontal section through a larva of Asterina gibbosa about the same age as those shown in previous figure Dorsal (¢.¢. aboral) views cf two young specimens of Asterone. g giBbose shortly after the metamorphosis Three figures illustrating the development 2 the ‘genital atolan, eonteal rachis, and gonad in Asterina gibbosa Longitudinal frontal section of the larva of Solaster bade, Early stages in the development of Ophiothriz fragilis Longitudinal frontal sections of early larvae of Ophiothria segues. Ventral views of young larvae of Ophiothria fragilis Ventral view of fully developed larva of Ophiothrix Frags, stivtennt days old . Three diagrams illastr ating fie internal changes which occur in the larvae of Ophiothriz fragilis up to the end of the first period of metamorphosis . . Longitudinal frontal section of a larva of Ophiothrix fragilis in the first stage of metamorphosis . Two larvae of Ophiothria fragilis in rte coucleding neon of stare phosis, viewed from the ventral side . Transverse sections through the discs of two young brittle- on in cndee be show the origin of the germ cells ‘ Two diagrams designed to elucidate the mutual selatinnshitys of ebinte- canal, dorsal organ, axial sinus, madreporic vesicle, genital rachis, etc., in an Asteroid and an Ophiuroid respectively Ventral view of the larva of Ophiura brevis during the mie bunibupheale Two stages in the segmentation of the egg of Strongylocentrotus lividus. (The oe of the egg of Echinus esculentus pursues an identical course) Early stages in the development of Echinus ‘patil Young larvae of Echinus esculentus, viewed from the dorsal surface Echinopluteus larva of Echinus esculentus, about eleven days old, viewed from the dorsal surface, in order to show the formation of the ciliated epaulettes Echinopluteus larva of Hehinus exeulontiss, twenty- -three days old, viewed from the dorsal surface : XX1X PAGE 468 469 470 473 474 480 482 485 486 488 489 493 494 496 497 500 502 507 508 509 510 512 XXX FIG. 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. 414. INVERTEBRATA Diagrammatic transverse sections through the ‘ Echinus-rudiment” of Echinoplutei larvae of Echinus eseulentus, ranging in age from twenty-one to fifty days . Diagrammatic side-views af the lar vae of an Aston oid, "Eohinold, ‘nil Balanoglossid in order to show the comparability of the apical plate of an Asteroid with the larval brain of the other two types of larvae Echinopluteus larva of Echinus esculentus, about fifty days old, just about to metamorphose, viewed from the left side Oral and aboral views of young sea-urchins immediately ‘afer the rite. morphosis A series of diapiinie siowne the changes which re su cnderzoes in Echinus esculentus after metamorphosis. The diagrams represent hori- zontal sections through the young imago . Types of Lithium larva The 32-cell stage in the nepnentatien of ‘the eee of Synapta digitata, viewed from the side ‘ ; The free-swimming gastrula of Sihenke dig tab Two young larvae of Synapta digitata, viewed from the sive Three views of young Auricularia larvae of Synapta digitata, viewed nem the dorsal] surface, showing the division of the coelom The fully- developed Auricularia larva of Synapta digitata, Aowed non the ventral surface and from the side Diagrams of theanterior aspect of metamor phosing inna of S ‘ynapla i, itate in ‘ordet to show the changes undergone by the longitudinal ciliated band Metamorphosing larva of Synapta digitata, viewed from the ventral aspect Pupa of Synapta digitata in two stages of development : Post-larval stage in the pee of Cucumaria saxicola, viewed from the side Early stages in the sib onic development of Anielon rosaceuw External views of the embryo and larva of Antedon rosacea Hae ihe mutual relations of stomodaeum and ciliated bands External views of larvae of Antedon rosacea at the time of Batehing dea after fixation in order to show the development of the calcareous assiolos Longitudinal sections through free-swimming larvae of Antedon rosacea . Fixed larva of Antedon rosacea, three and a half days after hatching, viewed from left side (decalcified) . Fixed larva of Antedon rosacea in which the veatibalea is Seek ‘ite in shies no trace of the arms have as yet appeared . ‘ Calyx of a larva of about the same age as that fereuoivied y in Fig. 408, decalcified and cleared in order to show the internal structures Fixed larva of Antedon rosacea, viewed from the ae in order to show the origin of the arms View of the calyx of a fixed ike va a Milan rosacea fond the upper ie in order to show the adhesion of the lobe of the hydr ocoele to the arm and the first dichotomy of the arm . Map showing the mutual relations of the ossicles in tebe fines of he ealyee of a fixed larva of Antedon rosacea when the adult condition has been nearly attained : The stalked larva and adult form of Aileen multispina Diagrammatic reconstruction of the common ancestor of the “siuaita Echinodermata, and diagrams showing the modifications which the descendants of this ancestor underwent in becoming the ancestors of the Eleutherozoon and Pelmatozoon stocks PAGE 513 516 517 519 521 527 531 531 532 534 535 - 537 538 540 543 546 548 549 550 551 553 555 556 557 559 563 FIG. 415. 416. 417. 418. 419. 420. 421. 422. 423, 424, 425. 426. 427. 428. 429. 430. 431. 432. 433. 434. 435. 436. 437. 438, 439. 440. 441, 442, 443, 444, ILLUSTRATIONS Early stages in the development of Balanoglossus clavigerus Later stages in the development of Balanoglossus clavigerus Still later stages in the development of Balanoglossus clavigerus Surface view of the young Tornaria larva of ae caste fein days old Illustrating the structure of he otra ‘slate and oe of a full- Lane New England ‘Tornaria larva Illustrating the origin of the cnfddle. and posterior uealunats veticleed in the New England Tornaria Full-grown New England Tornaria, seen fen the left ide. Tlustrating the metamorphosis of the Bahamas Tornaria . Illustrating the development of the dorsal nervous system in ithe shee. morphosing Bahamas Tornaria Sagittal section through the New England Pornatia jcavaediotely tie the metamorphosis into the Balanoglossid worm . Portions of transverse sections Ceuet! the trunk region of tine wong Balanoglossid worms of different ages in order to illustrate the develop- ment of the genital organs in the Bahamas species 2 The larva of Dolichoglossus pusillus in the act of escaping from ihe ege- membrane Longitudinal frontal sections through bie nate yos of Dolichoglvssus pusillus in order to illustrate the development of the body-cavities Stages in the segmentation of the egg of Amphioxus lanceolatus 4 Stages in the gastrulation of the egg of Amphiowus lanceolatus. All the figures represent sagittal sections . Diagrams showing the parts played by eetuiieca and endoderm in th process of gastrulation in Amphioxus lanceolatus, . - Transverse sections through an embryo of Amphioxus iwnisilaine jase after the completion of gastrulation in order to illustrate the develop- ment of nerve-cord, notochord, and coelom ‘ Transverse sections through an older embryo of Aianlitasns eioaclibus than that represented in the previous figure to show the further develop- ment of nerve-cord, notochord, and coelom é Illustrating the formation and development of the head. cavities ae Amphioxus lanceolatus Diagram of longitudinal section of Anepiions fumentatie parallel to he sagittal plane but lying to one side of the middle line in order to show the mutual relationships of head, collar, and trunk-cavities Illustrating the mutual relationships of collar-cavities and splaatinocodies in Amphiowvus lanceolatus . - Two young larvae of Amphioxus fowuntelns geen from ihe left sa Larva of Amphioxus lanceolatus with four primary gill- _ seen from the left side Diagrammatic transverse sactiboiis fue sie indies region of — young specimens of Amphiowus lanccolatus in order to show the origin of sclerotomes . Illustrating the develouuient of ite excr rotor y organs ‘of Avapiones lanceolatus . Illustrating the development of the genital or aS of Amphtonas ientestitin Illustrating the metamorphosis of the larva of Amphiowus lanceolatus The egg of Cynthia partita before and during fertilization . Stages in the segmentation of the egg of Cynthia partita Illustrating the gastrulation of the egg of Cynthia partita . XXX1 PAGE 571 572 573 574 575 576 577 578 579 580 581 583 584 588 590 592 593 594 595 596 597 598 599 . 600 601 602 603 610 612 615 XXxil INVERTEBRATA FIG. 445, 446. 447, 448. 449. 450. 451. 452, 453. 454. 455. 456. 460. 461. 462. 463. 464. 465. 466. 467. Sections through eggs of Cynthia partita in order to illustrate the changes in form undergone by ectodermal and endodermal cells during the pro- cess of gastrulation . - Illustrating the closure of the blactopore in the bast ula of ‘Oyneitite ponies Optical sopittal section of gastrula of Cynthia partita Dorsal, view of an embryo of Cynthia partita after the somite of dine blastopore in order to show the formation of the tail Transverse section through the middle of a gastrula of Cynthia ietie Longitudinal sagittal section of tadpole of Cynthia partita ce before it escapes from the egg-membrane Free-swimming tadpole of the simple ‘Autiiiton Clones esti te, seen febin the side Anterior portion of fie ee- Benne iadiwele of the sinple Aenidien Phalbisia mammilata, seen from above Illustrating the metamorphosis of the iadlisate of bind dacsaattvagtes Transverse sections through the brains of Ascidian tadpoles in order to illustrate the development of the hypophysial tube and of the adult ganglion Longitudinal sagittal epeions theoueti the baa wedales aia bypophysial tubes of just fixed tadpoles of Ciona intestinalis fs Transverse sections through the hinder part of the pharynx of a series of larvae and just_fixed young of the simple Ascidian Ciona intestinalis in order to illustrate the development of the heart pericardium and epicardium . . Illustrating the developrient of the satel araeee: in the ainaple Aecdian Molgula amputloides . Illustrating the result of the development of single (acionseies of the 2-cell stage of the egg of Cynthia partita . Section through the segmenting egg of ee soma Yt ntewm at the siden of thirty-six blastomeres Longitudinal sections through yo: embr oe of Py oe eiyaeica in order to show the formation of organs in the Cyathozooid Two embryos of Pyrosoma giganteum, viewed from the dorsal surface, in order to show the formation of organs in the Cyathozooid : The embryo of Salpa fusiformis attached to the maternal placenta, seen from the side ‘ Budding stolon of Perophora stort showing the development of Buds fee the septum of the stolon . Ze Sig Stages in the development of the bud of Disiaiew magnilarva Lateral view of old bud of Distaplia magnilarva Primordial colony of Pyrosoma giganteum consisting of fie Aeetiltonosids coiled round the degenerating Cyathozooid Bud of a Botryllid almost completely developed acta bearing two Sine buds, seen from the dorsal side ' 468. Four stages in the budding of a Dialosamid Ascidian PAGE 616 617 618 618 619 620 621 621 628 624 625 627 629 631 6383 633 634 636 638 639 640 641 642 648 CHAPTER I i INTRODUCTION THE science of Embryology has for its subject-matter the growth of animals from the time when they first appear as germs in the bodies of their parents until they reach the adult condition, and are able to produce similar germs themselves. It thus includes a study of the complete life-cycle, and is much more extensive in its scope than ordinary descriptive Comparative Anatomy which confines itself to a study of the adult forms. In practice, however, the study of the adult form precedes the study of all other stages of the life-history, because it is assumed that in the adult producing ripe germs, we have a stage which is the same whatever kind of animal we examine. A text-book of Embryology therefore assumes a knowledge of the adult forms of the animals whose life-histories it describes. Even with this limitation the scope of Embryology would be enormous were it not for a defect which is often overlooked, but which renders it possible to bring the most important results before the reader within moderate compass; this defect is the extreme difficulty of finding out with any com- pleteness the whole course of any given life-history. The life-history of an animal is only in the rarest cases directly observed ; it is deduced from a comparison with one another of in- dividuals of various ages, and only when we can examine’a large number of individuals belonging to stages separated from one another by very short intervals can we get any reliable results. Numberless mistakes have been made in the past and will continue to be made in the future, by the effort to re-construct a life-history from the observation of an insufficient number of stages. Thus, to give examples, the germ cells of Balanoglossus and of the Annelid Lopadorhynchus have been stated to arise from the ectoderm, whereas in reality in both cases they arise from the lining of the body-cavity or coelom. Sometimes the life-history has been actually read backwards: thus the later stages in the Tornaria larva have been regarded as the earlier. In the case of the vast majority of animals only bits and scraps of the life-history are known, and the number of cases in which the whole course of VOL. I B 2 INVERTEBRATA CHAP. this history is thoroughly known are very few. Hence a careful study of these few cases must suffice to include the certain and indisput- able results of the science of embryology. Outside this limited field are a number of suggestive facts from which no certain conclusions can be drawn, but which serve as clues to indicate those lines for future research which will probably give the most interesting results; some of these will be referred to in the following pages. The germs from which animals spring are of two kinds—(a) those that can develop directly, or asexual germs; and (0) those that under ordinary circumstances are incapable of development until they have united or “conjugated” with another germ. These latter are the sexual germs, and in all the animals which we shall have to consider they are unicellular, and are of two kinds, viz. comparatively large non- motile germs or ova (eggs), and small motile germs or spermatozoa. The organism which produces the ova is termed the female, that producing the spermatozoa, the male, and that which results from the union or conjugation of these two types of germ is called a zygote. If, as often happens, both kinds of germs are produced by the same adult, this is termed a hermaphrodite. In this case, however, the spermatozoa usually ripen before the ova and the animal is said to be protandrous. A case where the eggs ripen before the male germs is very rare amongst animals, but is commoner amongst flowering plants, such an organism being termed protogynous. Most asexual germs are multicellular, containing several nuclei and in some cases portions of more than one tissue of the parent. Such germs are called buds, and the laws of bud development are as yet most imperfectly known, but in many cases they seem to differ markedly from those governing the. development of the zygote. In this book our attention will be directed mainly to the development of the zygote, but the most important facts about bud development will also be given. Some asexual germs are called parthenogenetic ova, because in development and appearance they resemble true ova, from which they differ only in being able to undergo complete development without conjugating with spermatozoa. The develop- ment in this case is the same as that exhibited by true ova. The development of ova and of spermatozoa, from their first appearance until they become capable of union with one another, is known as gametogenesis, since gamete is a convenient word to designate both kinds of sexual germ. It is called oogenesis for the ova and spermatogenesis in the case of spermatozoa. Both forms of germ when first distinguishable are small, rapidly dividing cells which are often termed primitive germ cells. In these divisions the nuclear substance, which absorbs stain—the chromatin—hecomes arranged in the form of a definite number of rods or chromosomes—denoted by the symbol 2%. The actual number is characteristic of the species in question, and it is often assumed, and is taught in most text-books, that this number is characteristic of the nuclei of all the cells in the body when they I INTRODUCTION 3 proceed to division. But this is by no means universally the case ; in fact in several cases which have been subjected to detailed examina- tion it has been proved not to be the case. The germ cells in this state of rapid division are termed oogonia and spermatogonia respectively. Division is succeeded in both cases by growth and then by a state of rest, at the end of which the germs are known respectively as oocytes and spermatocytes of the first order. In most cases during this period the difference between the two kinds of germ becomes apparent. The spermatocytes increase only slightly in size as compared with the spermatogonia and undergo no diminution in number; but some only of the oogonia increase in size and become the large oocytes or unripe ova; the remaining oogonia are either absorbed as food by their successful sisters, or reduced in size so as to form “ follicle cells” which serve as a protective covering for the oocytes. The follicle cells in many if not all cases contribute nourishment to the oocyte, and a considerable portion of this food, termed deuteroplasm or food-yolk, is precipitated in the form of globules or platelets consisting chiefly of lecithin. A first stage in the storing of food material in the cytoplasm appears to be the emission into the cytoplasm of chromatin from the nucleus. This process has been studied in detail in Echinoderm eggs by Schaxel (1911). Yolk globules appear later, at first near the periphery of the egg, by the transformation of this cytoplasmic chromatin. Often indeed the modified cytoplasm infiltrated with chromatin is aggregated in a more or less spherical body which absorbs stain, termed the yolk- nucleus (Fig. 11, y.n); round this the yolk first appears, but after the completion of yolk formation it disappears. Occasionally, as Schaxel has shown to be the case in the egg of Strongylocentrotus, no yolk at all is formed, the deposits of cytoplasmic chromatin con- stituting the only reserve material present. Different eggs differ from one another in the amount and nature of the food-yolk. The follicle cells in many cases have as their final duty the secretion of an outer egg-shell or chorion (Fig. 11, ch). The inner egg-shell is secreted after fertilization by the cytoplasm of the egg itself and is termed the vitelline membrane. The nucleus of the unripe ovum is in nearly every case distinguished by the fact that it has the form of a vesicle con- taining a clear fluid termed the nuclear sap, within which is a dense mass of staining matter, different in chemical reactions from chromatin, which is termed the nucleolus. Stretched across the nuclear sap are a few fibres and on\these the true chromatin is situated as small inconspicuous grains. This arrangement of material gives the nucleus of the oocyte a peculiar look which is unmistak- able and which led to its being termed the germinal vesicle by the older writers, whilst the nucleolus was termed the germinal spot. Before male and female germ cells can unite both must mature, and this they do by undergoing two maturation divisions, The 4 INVERTEBRATA CHAP. I changes which the nucleus undergoes previous to, and during, these maturation divisions, have been studied with great minuteness by a large number of observers, and on many important points a general agreement has been arrived at. An excellent summary of the present stage of our knowledge has been given by Agar (1911), who has worked out the development of the male cells in the Dipnoan fish Lepidosiren. As maturation approaches, the chromatin granules in the nuclei of the spermatocytes of this species become aggregated into long ribbons, which are the chromosomes. This stage is termed leptonema, and the chromosomes in this stage are termed leptotene threads ; they appear in double the number that are found in the ripe ovum when ready to receive the spermatozoon. This double number is known as 2a. There is a large conspicuous nucleolus present as in the unripe egg (Fig. 1, A, »). The leptotene threads continue to shorten and become thicker and thus they pass into the stage of zygonema, in which the threads become opposed to one another in pairs and are termed zygotene threads (Fig. 1, B). Then the members of each pair fuse with one another. Thus the stage of pachynema is attained in which there are only « shorter thicker pachytene threads or chro- mosomes, arranged in the form of U’s in a “bouquet” at one side of the nucleus. : Here we arrive at a fundamental divergence of opinion between two groups of workers. The changes which we describe cannot, of course, be observed in the living nucleus, but must be inferred from the comparison with one another of fixed and stained nuclei. Some workers maintain that nuclei with # pachytene threads represent the first stage in maturation, and that the stage with 2 « zygotene threads represents an attempt at longitudinal division of these chromosomes, which is, however, abortive. Agar, however, points out that in the material which he studied the leptotene threads cross each other at all angles, and when the process of amalgamation in pairs or syndesis begins, at first only one end of each member of a pair of zygotene threads is parallel with its fellow, the other end passes into an irregular tangle. He, therefore, in common with a great number of workers, interprets the appearances seen in the beginning of the pachytene stage as the beginning of a side-by-side fusion of originally separate chromosomes—z.e. as parasyndesis. This stage is followed by one in which the two elements forming the pachytene chromosome separate in the middle, and the chromosome is trans- formed into an elongated ring which is twisted on itself (Fig. 1, C). This is the stage of strepsinema, the rings being termed strepsitene threads. The two sides of each ring separate from one another, first at one end, so that the ring is converted into an elongated V, and then at the other so that the two original constituents of the ring are again entirely separated from one another and the original number of chromosomes, 2 2, is restored. During the strepsinema stage a process called synizesis begins. Fie. 1.—Eight views of the maturation of the male cells of Lepidosiren paradoxa. (After Agar.) A, leptonema stage. B, beginning of zygonema. C, strepsinema stage, beginning of synizesis. D, separation of two chromosomes which were united in syndesis completed ; nuclear membrane dis- appeared. H, second pairing of chromosomes begun, appearance of centrosomes for the first matura- tion spindle. F, later stage in second pairing of chromosomes, the centrosomes of the first spindle now situated at opposite poles of the nucleus. G, the anaphase of the first maturation division. H, the metaphase of the second maturation division. 61, a chromosome which has not yet paired with its fellow ; b2, two chromosomes united end to end ; 03, two chromosomes united in a ring; ¢, centrosome of spindle of first maturation division ; J (in D), the two longest chromosomes, (in F), the two longest chromosomes united end to end, (in G), the two longest chromosomes separating from each other in the first maturation division, (in H), one of the longest chromosomes dividing into two in the second maturation division ; n, nucleolus. 5 6 INVERTEBRATA CHAP. This is the aggregation of the chromosomes in a kind of bunch at one side of the nucleus. This persists for a time, but eventually the chromosomes separate out again from one another, a process known as diakinesis. The rings of the strepsitene stage are com- pletely broken up into their constituent parts, and the number 2 % is consequently restored by the time that diakinesis has reached its utmost limit. During diakinesis the chromosomes become driven apart from one another and become spread out under the nuclear membrane, and the nucleolus during the same time gradually loses its staining power and disappears from view. The nuclear membrane then disappears and the nuclear sap mingles with the cytoplasm (Fig. 1, D). At the same time, the centrosome, which forms the centre of the polar rays in every nucleus undergoing karyokinesis, and which lis at the side of the resting nucleus, is seen to have divided into two daughter centrosomes (Fig. 1, E ¢); they are already moving apart to take up positions at opposite poles of the nucleus, whilst between them the rays of the achromatic spindle are already developing. The chromosomes, which become much shorter and thicker, now join end to end in pairs; but their free ends then swing round and join one another, so that eventually parasyndesis again occurs (Fig. 1, F 6, 0?, and 63), and the number of chromosomes is again halved, and we thus obtain # short, thick, ring-like chromosomes which become arranged in a plane so as to form the equatorial plate of the first spindle. The division of the nucleus then takes place; each daughter nucleus receives one half of each ring, and these halves represent the separate chromosomes of the diakinetic stage which subsequently fused with another (Fig. 1, G). In the case of the testis the division of the nucleus is followed by the division of the germ cell into two equal cells, and in this way two spermatocytes of the second order are formed: the undivided germ cell with its nucleus and condensed chromosomes being termed the spermatocyte of the first order. The nuclei of the spermatocytes of the first. order do not undergo a resting stage, but in each of them a new spindle is formed at right angles to the first, and each chromosome becomes split longitudinally so that at the ensuing division of the nucleus one half of each chromosome goes into each daughter nucleus (Fig. 1, H). If this description has been followed it will be seen that, in the first maturation division whole chromosomes go to each daughter nucleus—this is termed the reducing division ; whilst in the second division one half of each chromosome goes to each daughter nucleus —this is termed the equating division. The division of the nucleus is again followed by the division of the cell, and in this way four cells are derived from each spermato- cyte of the first order. These cells are termed spermatids and each spermatid becomes converted into a spermatozoon. The manner in 1 INTRODUCTION 7 which this change is effected seems to be fundamentally the same in most animals which have been examined. One of the best and most recent accounts of it is that given by Duesberg (1909), who worked on the development of the spermatozoa of the rat. According to this author the spermatid is a small polygonal cell containing a large resting nucleus, at one side of which is the centrosome which functioned in the last maturation division. This centrosome has already divided into two minute centrioles lying one above the other (Fig. 2, A, c!, ¢). Besides the centrioles there is a peculiar body embedded in the cytoplasm termed the idiosome, apparently derived from the sphere or modified cytoplasm which sur- rounded the centrosome, although it is now widely separated from the centrioles. The distal or outer centriole begins to give rise to a thin filamentous flagellum which is the rudiment of the tail of the spermatozoon, whilst the idiosome travels to the opposite side of the nucleus from that on which the centrioles lie, and becomes applied to the nuclear membrane and forms a cap-like structure known as the acrosome. The nucleus sends out a prolongation which reaches the lower or proximal centriole, and this becomes applied to the nuclear membrane and forms a plate-like thickening on it. The nucleus carrying the acrosome then begins to emerge from the cytoplasm on the opposite side of the cell to that on which the centrioles are situated. Both centrioles are dragged after the nucleus. Round the upper or immersed half of the nucleus the cytoplasm is differentiated so as to form a funnel of clear substance called the “ruffle” (manchette) (Fig. 2, D, m). The nuclear sap is then apparently expelled and the nucleus converted into an almost uniform mass of staining matter. Its form changes—no longer spherical, it becomes sickle-shaped (Fig. 2, E). The distal centriole divides into two daughter centrioles, and to the lower and proximal of these, the tail filament, now greatly grown in length, is attached, whilst the upper or distal one forms a ring surrounding the filament. The nucleus now emerges completely from the cytoplasm. It forms the head of the spermatozoon; the ruffle disappears, but the acro- some is still distinguishable as a thickening on the convex side of the sickle. The cytoplasm now forms a thick mantle surrounding the lower part of the filament (Fig. 2,G). The ring-shaped centriole travels away from the nucleus along the filament till it reaches a definite position. The cytoplasm shrinks more and more and is eventually completely cast off. The ring-shaped centriole and the piece of the filament between it and the nucleus forms the middle- piece of the spermatozoon. When these changes are complete the spermatid is transformed into a spermatozoon, begins to exhibit active movement, and is capable of fertilizing a ripe egg. We must now turn our attention to the process of maturation of the egg. We left the egg in the stage when the nucleus was inordinately swollen with cell sap, and when there was a large nucleolus. All the changes exhibited by the ripening spermatocyte Fria. 2.—Nine stages in the transformation of a spermatid into a spermatozoon. (After Duesberg. ) A, spermatid with two centrioles, the proximal and the distal, at one side of nucleus and the idio- some at the other side. B, stage in which the distal centriole is giving rise toa filament, and in which the idiosome is applied to one side of the nucleus. C, stage in which the nucleus is beginning to emerge from the cytoplasm dragging after it the centrioles; the filament, now grown longer, is im- mersed in the cytoplasm, the ruffle has appeared, and the idiosome has become the acrosome. D, stage in which the nucleus has begun to become sickle-shaped. E, stage in which the distal centriole has divided into two and the outer one has become a ring. F, stage in which the ring is moving outwards along the filament. G, stage in which the ring has moved beyond the cytoplasm. UH, I, two stages in casting off the cytoplasm. ac, Acrosome ; c, proximal centriole ; cl, distal centriole ; id, idiosome ; m, ruffle (manchette) ; r, ring centriole. i & CHAP. I INTRODUCTION 9 are exhibited also by the oocyte of the first order, as we term the egg when it has reached its full size. Its nucleus passes through the stages of leptonema, zygonema, pachynema, strepsinema, etc. The nucleolus disappears, the nuclear membrane dissolves, and the nuclear sap mingles with the cytoplasm. The consequence of this is, that since the chromosomes form a small and inconspicuous mass, the egg, viewed under the low power of the microscope, appears to have lost its nucleus, and this is a ready way to distinguish unripe from ripe eggs, or in other words, oogonia from oocytes of the first order. The great difference between oocytes and spermatocytes of the first order is that in the case of the latter when the cell divides, it gives rise to two daughter cells of the same size which form spermatocytes of the second order, but when the oocyte divides, it forms two daughters of unequal size, the larger forming the oocyte of the second order whilst the smaller forms a rudimentary cell incapable of development, termed the first polar body. At the second maturation division the same phenomenon repeats itself, the oocyte of the second order divides into two unequal daughters—the larger is the ripe ovum, whilst the second is again a rudimentary cell which never develops and which is termed the second polar body. The first polar body often divides into two daughters of equal size which are, like the second polar body, to be regarded as sisters of the egg, or better, as rudimentary eggs (Figs. 3, B, and 4, B). The same sacrifice of quantity to quality therefore which is seen in the absorption of many oogonia by their more fortunate sisters, repeats itself in the maturation divisions. The ripe egg can now receive the spermatozoon. As soon as the head of the first spermatozoon has penetrated the egg an alteration of the surface of the latter usually takes place, which cuts off the tail of the spermatozoon and prevents any other spermatozoa from entering ; the middle piece of the successful spermatozoon, however, follows the head into the egg. This alteration in the surface of the egg has been diagnosed by Loeb as a kind of cytolysis: for it can be observed that a number of fine globules issue from the surface of the egg, and that their outer surfaces coalesce to form the inner egg-shell or vitelline membrane. i Meanwhile the spermatozoon head within the ovum swells up, it assumes a vesicular form, and nuclear membrane, cell sap and chromosomes can be again demonstrated in it. The middle piece~ takes on the character of a centrosome and around it the achromatic rays appear, forming what is termed an “aster.” The male pro- nucleus, as the spermatozoon head is now termed, moves towards the residual nucleus of the egg, termed the female pronucleus, which in its turn advances towards the male. Male and female pronuclei then meet and fuse and form a single nucleus, the zygote nucleus, and fertilization is complete. After a resting period of an hour or two the zygote nucleus begins to prepare for karyokinesis, and the spindle is so formed that 10 INVERTEBRATA CHAP. its equatorial plate lies at right angles to both female and male pro- nuclei, and so both are halved at the ensuing division and equal parts of both distributed to the first two cells into which the ovum Ch . oer Qo oe Fic. 38.—Four stages in the maturation and fertilization of the egg of Crepidwla plana. (After Conklin. ) A, formation of first polar body ; the spermatozoon has entered the egg and has begun to swell up into the male pronucleus. B, formation of second polar body ; the first polar body has divided into two. C, the male and female pronuclei have come together. D, formation of the first cleavage spindle ; the female pronucleus above and the male pronucleus below are still clearly distinguishable from oue another. fp, Female pronucleus ; mp, male pronucleus ; p! (in A), first polar body, (in B and C), two cells resulting from division of tirst polar body ; 2, second polar body. divides. In some few cases it is possible to distinguish in this first equatorial plate the chromosomes derived from male and female pronuclei respectively, for they are of different sizes and arranged in two different groups. This is especially clear in the egg of Crept- I INTRODUCTION 11 dula plana, a species in which the processes of maturation and fertilization have been worked out in great detail by Conklin (1902). The centrosomes of the first cleavage spindle are here stated to be derived from the division of the sperm centrosome. It is therefore reasonably certain that in all cases each daughter cell re- ceives a half of each male and female chromosome. It is by no means always true that the spermatozoon can only enter the egg after the formation of both polar bodies. In many cases it enters the egg whilst it is still an oocyte of the first order, and even before the nuclear mem- brane has been dissolved and the germinal spot has disappeared. This is true of the eggs of many Annelida and Mollusca. In other cases, such as in some Ascidians, the first maturation spindle is formed before the spermatozoon enters, but the first maturation division is not completed till the spermatozoon is inside the egg. Finally, in Dinophilus according to Shearer (1912), the spermatozoon enters the oogonium and remains passive during the growth and maturation of the A, the first cleavage spindle ; female chromosomes above germ cell. separated by an interval from male,chromosome below. B, the If the eggs are stale, division of the zygote nucleus = complete. f, cee a : 4 somes ; m, male chromosomes ; pl (in A), first polar body, (in B), he ban) We c its products of division of first polar body ; p?, second polar body. she 00 long trom e€ ovary before being fertilized, then more than one spermatozoon can enter them, and an extra centrosome is thus introduced, between which and one or both of the centrosomes resulting from the division of the centrosome of that sperm which has actually effected fertiliza- tion, extra achromatic fibres can be developed and irregular division Fic. 4.—Two stages in the first division of the fertilized egg of Crepidula plana. (After Conklin.) 12 INVERTEBRATA CHAP, of the egg results; in most cases this takes the form of simultaneous division into four equal parts. In the case of large eggs, like those of birds, it appears that normally a considerable number of spermatozoa enter the egg. One only unites with the female pronucleus and forms the zygote nucleus, the rest divide independently and form groups of small cells which are produced by the aggregation of the cytoplasm round the products of their division. Soon, however, the cells formed round the daughters of the zygote nucleus crush out and kill these other cells, and the former alone enter into the formation of the embryo. It appears therefore that the alteration of the surface of the egg so as to exclude supernumerary spermatozoa, which is so marked a feature in small eggs, must be due to some chemical influence radiating from the zygote nucleus, and that in large yolky eggs it does not reach sufficiently far to prevent the entry of extra spermatozoa at some distance from the first one. The sequence of events worked out by Agar for the maturation of the nuclei in the male cells of Lepidosiren agrees fairly closely with that described by other workers for other forms. But in many cases, before the first maturation division has taken place, when in the paired or bivalent chromosomes the components are beginning again to separate, a longitudinal split appears at right angles to the split separating the components; this is the anticipation of the final division of each chromosome into longitudinal halves which occurs in the second maturation division. We thus get quadripartite chromo- somes, which are termed tetrads. Further, many workers maintain that in the final pairing of chromosomes which takes place just before the first maturation division, there may in some species be an end-to-end junction, metasyndesis, instead of a side-to-side junction or parasyndesis. When a substance like chromatin appears in the ripening eggs and spermatozoa, in exactly the same forms, generation after generation, and when the masses of chromatin con- tinually undergo complicated changes Fie. 5.—Polar view of the first of shape in the same order, it is maturation division of the male natural to imagine that such a sub- germ cells of Alydus pilosulus, stance must be of great importance in in order to show tetrads. ars : . (After Wilson.) the main function of the germ cells, 2.¢. transmitting the hereditary qualities of the parents; as a matter of fact it was on the casting off of the two polar bodies as a preliminary to development, and on the nuclear changes which accompany this phenomena, that Weismann (1886) founded his famous theory of heredity. According to him the nucleus of the egg was supposed to have a portion of its material charged. I INTRODUCTION 13 with the function of producing the peculiarities of the cytoplasm of the unripe egg—the amount, colour, and composition of the food- yolk, ete. When this task was accomplished and the ovum was ripe, it was supposed that this portion of the nucleus, termed by Weis- mann histogenetic plasm, was extruded as the first polar body. eae oe of the second polar body he explained by his theory According to this theory, the material basis of the transmission of the parental qualities to the child is contained in the chromatin which is organized into a number of “ids.” Each “id” contains within itself the whole potentiality of the animal, i.e. one “id” alone is capable of causing the egg to develop into an adult animal. The “ids” are similar, but not exactly the same, and the animal which develops is a compromise between the potencies of the various “ids.” The “ids” are capable of assimilation and growth, and in each longitudinal division of the chromosome each “id” contained therein is divided into two precisely similar daughter “ids,” but in the transverse or reducing division (which in Weismann’s day was supposed to be the second, not the first division) different “ids” are separated from one another. This is what happens in the formation of the second polar body, and now the spermatozoon brings in an equal number of “ids” and thus the number originally present in the oogonia is restored. Since the “ids” of the spermatozoon are not exactly the same as those of the egg, and since Weismann assumed that the casting forth of half the maternal “ids” might take place at random, 2.e. might consist of any group of the “ids” amounting to half the number, the basis was given for inheritable variations, because different combinations of maternal and paternal “ids” might come about by a difference in the “ids” which were cast forth at the reducing division. It is one great merit of Weismann’s theories that, whatever may be thought of their truth or untruth, they have acted as powerful stimulants to research. Hertwig (1890) published a work on the spermatogenesis and oogenesis of the worm, Ascaris megalocephala, in which he showed, for the first time, the parallelism in the changes which occur during the ripening of both kinds of germ cell, and proved that the polar bodies were the degenerate sisters of the egg, thereby overthrowing Weismann’s theory of the histogenetic plasm. But Weismann’s theory of the meaning of the reducing division was not thus disproved, although of course it was shown that it was the first, not the second maturation division in which whole chromo- somes were separated from one another, and to which Weismann’s hypotheses must apply. This theory was shattered by the work which ensued on the discovery of what have been called sex- chromosomes, by McClung, Wilson, and other American workers. - Wilson has given an excellent summary of the whole subject (1911), and to this we must now address ourselves. In the spermatids of certain insects it was observed that in some 14 INVERTEBRATA CHAP. cases there was one more chromosome than in others. Let us denote these numbers by the formulae « and «+1. Further investigation revealed the fact that this extra chromosome, termed the accessory chromosome or hetero-chromosome, does not pair with any other chromosome before the first maturation division: in this division it divides longitudinally into two, whilst in the second maturation division it does not divide at all, but passes to one pole of the spindle, and is thus distributed to one of the two spermatids resulting from that division (Fig. 6). All the ripe eggs showed a number of chromosomes equal to the number in those Ss! px A spermatids which had the extra chromosome. i The zygotes which resulted from the con- APS jugation of these eggs with the two kinds of spermatozoa would show therefore 2 # +2 and | 2 «4+1 chromosomes in their nuclei. Examina- | tion of the tissues in the adult insect showed that the dividing nuclei of males possessed 22+1 chromosomes, whilst those of females \ W/Z had 2 a +2chromosomes. It seemed, therefore, Se robable that in the case of these insects, this Zs Deel chromosome contained some material Fra. 6.—Second matura. Which determined the production of the female tion division of the sex in the zygote to which it passed. a germ ee of In other cases it was found that the rovenor ee Trager. . = (After ‘Wilaon, ) spermatids all contained an abnormal chromo- h, Accessory chromosome, Some, but that this abnormal chromosome was in some spermatids large and in others small. It was shown that in the spermatocyte of the first order both abnormal chromosomes were present in the same nucleus, and that they both divide in the first maturation division by longitudinal splitting; but that before the second division they fuse together to form a bivalent chromosome, and that in the second maturation division they again - separate from one another, and that one proceeds to each daughter nucleus; so that in the case of these chromosomes alone the second maturation division is a “reducing division,’ whereas in the case of all the others the first maturation division is the reducing division. These abnormal chromo- pyc, 7,—Second maturation somes are termed idiochromosomes (Fig. 7). _ division of the male germ When two idiochromosomes are present, San ge eM : arts. (After Wilson.) the developing eggs always carry the larger _ - : one when maturation is complete, and the ea ae esicecieea nuclei in the tissues of the adult female have two large special chromosomes; whereas the nuclei in the tissues of the adult male carry one large and one small idiochromosome. Hence it is evident that the female grows from a zygote which has I INTRODUCTION 15 resulted from the fusion of an egg with a spermatozoon which contained the larger idiochromosome ; the formula of its nuclei therefore will be 2a + 2a, where a denotes the large idiochromosome. The male on the other hand has developed from a zygote which has resulted from the fusion of an egg with a spermatozoon containing the small idiochromosome, and the formula of its nuclei will be 2x+a+6, where 6 denotes the small idiochromosome. Hence the presence of two a chromosomes in the zygote determines the formation of a female, and we can reduce the case of idiochromosomes to that of the hetero-chromosome by supposing b diminishes until it dis- appears altogether. Other modifications have been described by Wilson; thus a may be represented by a group of chromosomes, but this group acts as a unit in the reducing division and passes to one of two daughter spermatocytes. The principle therefore is the same, and Wilson, in his final summary, suggests that what determines a zygote to be a female is an excess of peculiar trophic chromatin in its nuclei. Whilsta zygote which is characterized by a defect in this regard becomes a male. No such clear cases of differences between the sexes in the number of chromosomes have been found outside insects. Many statements as to the existence of such a dimorphism in other groups have been made but have not been proved to be true. It is quite obvious that such a dimorphism can only be demonstrated where the number of chromosomes is few and easily counted, and that where they are numerous the matter must remain in doubt. It is clear that the existence of special sex-determining chromo- somes is irreconcilable with Weismann’s conceptions of the chromo- somes as bundles of equipotential “ids” closely resembling one another, any one of which was able to direct the whole development. But the discovery of what Wilson calls sex-chromosomes has led to other results of a far-reaching character. The reducing division is a separation of whole chromosomes which immediately before this had paired with one another. Now the idiochromosomes, when present, always pair with one another, and the question arises whether the pairing of the other chromosomes is not just as definite a matter as that of the idiochromosomes. An examination of cases like the germ cells of insects, where the number of chromosomes is small, reveals the fact that in the nucleus, before “pairing” has taken place, the chromosomes are of different sizes, but that there are always—except in the case of idiochromosomes—two chromosomes of the same size, and that these two pair with one another. The pairing therefore, and the subsequent distribution of the members of the pair to different gametes at the reducing division, isa definite and not a haphazard phenomenon, and Montgomery (1904), to whom we owe this important | observation, has suggested that the two homologous groups of chromosomes, which he asserts can be seen in all the nuclei of the body, are derived from the male and female gametes respectively, to the union of which the adult, which produces the germ cells, owes its origin. 16 INVERTEBRATA CHAP. We thus reach the conception that male and female chromosomes remain side by side without fusing in the nuclei of the offspring during all its life, but that when this organism in turn produces germ cells these two kinds of chromosomes are segregated into different gametes. Now this conclusion appears at first sight to accord exactly with the theory to which the followers of Mendel have been led, and it entirely destroys the second half of Weismann’s theory of the pro- duction of variations at the reducing division, by the casting out of half the chromosomes, selected at random. This brilliant school of “Mendelians,” whose labours have been summarized by their most brilliant member, Bateson (1909), have been led to conclude that when two strains of animals are crossed, the hybrid produces two kinds of spermatozoa, or ova, as the case may be, one carrying the characters of the male and the other of the female with respect to each differentiating character. We should be wanting in our duty, however, if we allowed our readers to imagine that Montgomery’s theory had been fully estab- lished. Itis, on the contrary, only in the stage of a working hypothesis, and it labours under many difficulties. Thus, its superficial agreement with the Mendelian theory disappears under a deeper analysis. On the idea that male and female chromosomes are distributed to distinct gametes, each zygote should produce only two kinds of gametes (leaving out of sight the sex-chromosomes for the present), one with the maternal, one with the paternal characteristics. But the Mendelian hypothesis demands two kinds of gametes with regard to each differentiating character. Thus if a pea-plant have round and green seeds, and if it be crossed with another having yellow and angular seeds, we must expect the hybrid plants to produce seeds which give rise to plants bearing yellow and green seeds, and round and angular seeds; but all the yellow seeds will not be angular nor will all the green be round; on the contrary there will be four categories of seeds produced, viz.:—yellow round, yellow angular, green round and green angular. It is not easy to see how Mont- gomery’s hypothesis can be fitted in with this breaking up of the parental hereditary potencies into factors which are distributed among the germ cells independently of one another. Agar has, however, pointed out that in the stage of zygonema, when the first pairing takes place between chromosomes, there is opportunity for the exchange of material between them, and that when they again separate in the stage of strepsinema they may be different from what they were before the pairing took place.! Montgomery’s theory demands, further, the belief that the identity of each chromosome remains unimpaired during the resting period of the nucleus, when no trace of distinct chromosomes can be detected. 1 It has also been suggested that, previous to the reducing division, the bivalent chromosomes, each of which (ex hypothesi) consists of a paternal and of a maternal chro- mosome joined end to end, may not all be arranged so that their homologous ends point in the same direction. If this were so, one gamete might receive one chromosome from one parent and one from another. i I : INTRODUCTION 17 Occasionally, it is true, the sex-chromosome remains distinct during the resting period. We may imagine, by an act of faith, that the others too retain their identity although we cannot see them, but it seems to us that the only meaning which can be given to such an identity would be the persistence of a centre for the synthesis of some special substance. Lastly, it may be that in some cases sex is not irrevoc- ably determined in the germ, but can be determined by feeding. This at any rate has been asserted by Born (1881) in the case of tadpoles—though his results have not passed unquestioned. Wilson attempts to get over this difficulty by supposing that the sex- chromosome is only one of the factors which determine sex. Our final conclusion is that investigators have only touched the fringe of an intensely interesting and important subject, and that a great deal more research must be done before definite conclusions can be arrived at. The meaning of the process of fertilization has proved a fascinating subject for speculation. That the union of the two nuclei is not per se necessary for development, is proved by the experiments of Loeb and his pupils, who have caused the unfertilized eggs of Echinodermata, Annelida, and Mollusca to go through the early stages of their development by increasing the salinity or alkalinity of the water in which they lie, zc. by immersing the egg in what is called a hyper- tonic solution, or by causing the egg to form a vitelline membrane by rapid treatment with butyric and similar organic acids. By uniting these two methods, «.e. by first causing the egg to form a membrane through treating it with butyric acid and then treating it with a hypertonic solution, a close approach to normal development may be attained. The formation of the vitelline membrane is due, according to Loeb, to incipient cytolysis, 7.e. the peripheral protoplasm breaks down, forming minute globules which cohere together so as to form the membrane. Too long exposure to the acid causes the egg to die, by a continuation of the same process until the whole cyto- plasm is resolved into a mass of globules, but this process is arrested by the action of the hypertonic solution. Therefore Loeb concludes that the influence of the spermatozoon is primarily chemical ; in fact he supposes that it carries into the egg a ferment, “lysin,” which starts the process of cytolysis, and also another substance which arrests this process after it has resulted in the formation of a membrane. Loeb himself and his pupils Godlewski (1906) and Kupelwieser (1906) have shown that it is possible to fertilize the eggs of a Sea- urchin with the sperm of a Crinoid, a Star-fish, and even of a Mollusc. In these cases the resulting organism, as long as it lives, resembles exactly the normal larva which would have resulted if the egg of the Sea-urchin had been fertilized by its own sperm, and betrays not the smallest trace of the hereditary influence of the — foreign sperm which was used to evoke development. But of course, throughout the animal kingdom, offspring are as likely to resemble VOL. I c 18 INVERTEBRATA CHAP. the male parent as the female, and there must come a point when the hereditary influence of the male asserts itself. This, Loeb suggests, is when the foreign chromatin becomes dissolved and spreads its influence through the cytoplasm. Now the common experience of breeders, as collected by Darwin, bears witness to the beneficial effect on the vigour of the offspring which is gained by crossing two parents of slightly different strains. Their experience, moreover, is, that in what is termed inbreeding, that is, when the male is nearly related to the female in blood, the resulting offspring exhibit weakness of constitution. In the case of the higher plants, which have both kinds of germ in the same individual, self-fertilization produces in many cases similar results. This can be explained if we imagine that in the normal multiplica- tion of the cells in a developing organism, starting from the zygote and leading through many cell-divisions to the formation of germ cells, the wastage of the original chromatin is not quite made good. From experiments made on Protozoa we conclude that without chromatin no assimilation or building up of fresh living material can take place, and we can only imagine that the influence of chromatin is exerted through the substances which it is constantly giving off into the cytoplasm. Now if our assumption be just, the germ cells of each generation should become more and more imperfect in their chromatin equipment, and this imperfection should exhibit itself as a diminution in vitality. Hence we conclude that the prime object of conjugation is to maintain the vitality of the stock by adding together two chromatins of slightly different kinds, which will presumably not be deficient in the same places. On this view the purpose of reproduction by unicellular germs would be to render it possible for the two chromatins to be thoroughly mixed. Herbst (1900) has started an egg to develop by using hypertonic solutions and valerianic acid, and then, when the nucleus had divided into two, he has fertilized the bicellular organism with spermatozoa. As a result, one of the two nuclei has conjugated with the sperm nucleus, and an organism was produced, on one side of purely maternal character and on the other side showing the paternal influence. It has been asserted in contradiction to this that there are some plants, such as the Pea, which normally fertilize themselves and yet undergo no deterioration, and others like Hieraciwm, the hawkweed, in which sexual reproduction has been entirely replaced by asexual. But such isolated cases cannot be allowed to weigh in the balance against the great mass of evidence which tells in the contrary direction. The deterioration due to inbreeding may be very slow in showing itself, and a cross at long intervals may be sufficient to restore vitality; and he would be a rash man who would deny this possibility in the case of any of the species which apparently undergo continuous self-fertilization. Leaving now the question of the meaning of the sexual element I INTRODUCTION 19 in development, and turning to the developmental process itself, we find that every animal passes through two phases in its passage to the adult form. In the first of these phases the young organism is sheltered from external influences either by an egg-shell or by the body of the parent, or by both. Its food is supplied in the first instance by the deuteroplasm or food-yolk embedded in its own substance, supplemented in many cases by maternal secretions. In the second phase the young animal, after escaping from its shelter, is obliged to seek its own food, but it never is exactly like the parent, and some time elapses and considerable growth takes place before it attains the adult form. In the first phase it is known as an embryo, in the second as a larva. In different life-histories the embryonic and larval phases vary enormously in their relative lengths. Sometimes, as in Echinoder- mata, the young organism is thrown on its own resources at an extra- ordinary early period of development, a very long larval life ensues, and the young animal is at first utterly unlike the parent. In other cases, as in the case of Man, when the young organism leaves the parent it resembles the adult in all essential features. In such cases it is customary to say that the larval stage has been omitted. But the practice of confining the term “larva” to cases where the free-living young differ markedly from the parent is not logical. The baby is very different from a full grown man, and so is the young child; for example, the proportions of the limbs are markedly different, and a continuous series of stages can be found between differences of this kind and differences as great as those which divide the larva from the adult Echinoderm. We may assert with confidence that all animals pass through first an embryonic and then a larval phase of development, and nothing is gained by calling a larval stage which closely resembles the adult a “brephic” or “neanic” stage, as was originally suggested by Hyatt and has been adopted by some English zoologists. Of course, development goes on throughout both embryonic and larval phases, and the form of the organism is constantly changing ; but there is one great group of animals, the Arthropoda, in which the organism is confined within a rigid envelope derived from its own secretions, and in this case, for a definite period of time, the external form appears to be unchanged; only when the dead envelope is burst and cast off, do the internal changes which have been going on manifest themselves in a change of form. Hence we can appropri- ately speak of these periods of fixity of form as a series of larval stages, or, as Sharpe (1895) has suggested, we might call them instars. In other groups of the animal kingdom where this rigidity of form does not obtain, there are, nevertheless, crises in development when great changes take place very rapidly, accom- panied in many cases by the casting off of portions of the body of the larva. These crises are termed metamorphoses, and the stages of quiet growth preceding and succeeding them are looked 20 INVERTEBRATA CHAP. on as stationary in comparison, and spoken of as larval stages or instars. Now a great deal of the interest in the science of embryology has arisen from the fact that both the embryonic and larval phases of development show features which have been interpreted as being a reproduction of the characters of far-off ancestors of the species to which the adult belongs. This theory is the so-called fundamental law of biogenetics, and is summed up in the phrase, “ 7’he individual in its development recapitulates the development of the race.” If this “law” can be substantiated the interest in embryology becomes immense, it binds all the innumerable phenomena of development into one coherent scheme, and. opens the door to the hope that we may yet be able to sketch the main history of life on the earth. The direct evidence of this history, as contained in the fossil record, takes us back only a short distance. In the lower Cambrian rocks the great groups of Mollusca, Arthropoda, Echinodermata, and Brachiopoda are all as sharply marked off from one another as at the present day, and since only hard parts are preserved, the all- important “soft” parts which constitute the real living matter are irrecoverably lost, and no trace is left of an organism except it possessed some kind of skeleton. But an egg has many points of resemblance to the simplest animals, the Protozoa, and if development be really a recapitulation of ancestral history, then the whole of the ancestral history of an animal, from the Protozoan stage to the present, should be presented in outline in its life-history. But although most naturalists would agree that a life-history contains ancestral “elements,” all would be emphatic on the subject that it likewise exhibits many features which are purely secondary, and in no way reflect the characters of ancestors. With these general considerations the agreement stops. As the founder of the Naples © Biological Station has caustically remarked, it is a curious fact that every investigator is convinced that the type which he is studying has a monopoly of most of the primitive features, and that other types are secondarily modified. The endless wrangles about primitive and secondary features, which have made up so much of embryological writing for the past forty years, have so disgusted many leading workers in this field, that they have been inclined to go in the opposite extreme, and deny altogether the “biogenetic law.” Driesch may be mentioned as an example of this, and a very searching criticism of the whole hypothesis is given in the Darwin memorial ° volume by Sedgwick (1909), who in former years had done more than most workers to illuminate the hypothesis. Driesch’s (1907 and 1908) criticism leads him to the position that the development of an egg into an adult is not to be explained by physical and chemical laws, and he therefore attributes to each species of animal a peculiar “entelechy” or soul, which presides over the task of making its germs develop. Thus we are brought back to pre-Darwinian days, to a position indeed more primitive than that I INTRODUCTION 21 of the early nineteenth century, for it is surely easier to conceive of an All-embracing Intelligence, Whose myriad plans were realized in the different species, rather than of millions of uncaused and un- related intelligences. Why, if the entelechy of a Strongylocentrotus be entirely distinct from that of an Echinus, should their products so resemble one another? Has family resemblance in the animal kingdom no meaning? Our fathers attributed it to the Will of the Creator. Darwin taught us to believe that it was due to descent from a common stock. Driesch offers no explanation whatever, and it Seems to us that this final result is the reductio ad absurdum of his whole system. Driesch’s whole history has been that of the rebel against accepted opinions, and in so far his intervention is healthy, for nothing must be regarded as finally fixed, but pure reaction is equally unjustified. Sedgwick’s position (1909) is different. He formerly accepted the biogenetic law, but as its application seemed to yield the most dis- cordant results, he has been led to undertake a critical examination of the assumptions on which it is based. He points out that it is tacitly assumed that when a new feature appeared in the history of the race this showed itself only in the adult condition, whilst the previous adult condition was retained as a developmental stage; that, in a word, as evolution has proceeded the life-cycle of living matter has become more complex. Against such an assumption he points out that a careful examination of the embryos of related species force us to the conclusion that new features can appear at all stages of the life-history, and that as all living matter known to us undergoes cyclical changes, it is quite open to us to assume that this has always been the case since the first appearance of living matter on the globe, and that therefore the life-cycle has been moditied but not extended. Neither of these conclusions can be gainsaid @ priort, and it is therefore time to take stock of our data. It would lead us altogether too far to discuss the general proposi- tion that zoological affinity means blood-relationship: this, we take it, has been abundantly proved by the evidence, which Darwin has col- lected, of the relationship which breeds, varieties, and species sustain to one another; if this be so it will be conceded that this zoological affinity can be exhibited just as well by embryos as by adults, and that, therefore, for the elucidation of biological affinity the study of comparative embryology is necessary even if the biogenetic law be baseless. On looking into the question of the validity of this law, the first question which presents itself for solution is the mutual relationship of the embryonic and larval phases. On this subject Sedgwick himself (1894) threw light some years ago, when he pointed out that the embryonic phase is the remnant of a former larval phase, and that the . ancestral features which it exhibits are therefore features of a former larva; but these larval features, whether ancestral or not, consisted of organs adapted to the larval mode of life. If, then, these features 22 INVERTEBRATA CHAP. were really ancestral, what was being reproduced were primarily adaptations to an ancestral environment. . The proof that the embryonic stage is a concealed larval one exists widespread in the animal kingdom. When we find that the Nauplius stage of the shrimp Penaeus lives as a minute self-sustaining organism, using the tiny hooks at the bases of its second and third appendages as jaws to seize its prey ; and that the corresponding stage in the develop- ment of the crayfish is passed within the egg-shell, but that the embryo has the Nauplius limbs in the condition of useless stumps, although, just as in the case of the free-swimming larva, the passage into the next stage of embryonic life is initiated by a shedding of the cuticle, then we have no doubt which is the more primitive, the larva or the embryo. So too, when we find in the development of the Martinique toad Hylodes that the embryo within the egg has the tail of a tad- pole which is never used for swimming but is absorbed directly the animal hatches, we have no difficulty in concluding that the original condition of affairs was that in which the tadpole used its tail for the purpose for which its shape is adapted. Now the phase preceding the attainment of the adult form is always larval (it is often termed brephic or neanic), and this, according to the biogenetic law, should represent the last stage which the race has passed through before attaining its present condition, and will therefore be, generally speaking, the least modified stage in the life-history, since it is the most recently added to the series. Is there, then, evidence that this stage is of ancestral significance ? The answer to this question is that there is abundant evidence of it, and a few instances of this may now be given. The Oyster (Ostrea), contrary to the custom of the majority of bi- valves, lies on one side, and remains fixed thus through life; but the American species, at least in its so-called “brephic” stage, when it has terminated its free-swimming existence, creeps about for a short time, and possesses, like other bivalves, a “foot,” which is totally wanting to the adult oyster. Speaking broadly, when we examine the life-history of any aberrant member of a well-defined group in the animal kingdom, we find that in a late stage of its life-history it resembles the normal member of the group to which it belongs. Portunion, an Isopod para- sitic on Crustacea, iis distorted out of all resemblance to an Isopod, but when young it is an unmistakable Isopod, a trifle simplified in structure. The Crinoid Antedon when adult is devoid of a stalk, and swims by muscular movements of its arms; but when it is young it possesses a stalk like the vast majority of its congeners. The Plaice, Plewronectes, swims with one side down, and both eyes are twisted on to the upper side; yet the larva of this fish has both sides symmetrically formed with the eyes in the normal position, like the vast majority of Teleostei. This list could be extended indefinitely, and if the animals named are rightly classified in the groups in which I INTRODUCTION 23 they are placed, it is thereby implied that they once possessed the normal features exhibited by the typical members of these groups ; and therefore, beyond all question, their late larval stages must be of an ancestral character. It follows, then, that advances in evolution do, as a rule, manifest themselves when the animal is fully adult. But recent research in the laws of heredity has rendered it almost certain that inheritable variations are only those which affect the nature of the germ cell, and most zoologists refuse to believe that the adoption of a new mode Fig. 8.—The larva and adult female of Portunion maenadis. (After Giard.) A, larva’ just hatched. B, adult female. abd, ab- domen; atl, antennule ; a2, antenna; br, brood-sac composed of conjoined ovigerous plates of thorax; g, \ jaws or gnathites ; h, head ; pl, swimmerets or pleopods. of life by an adult could directly affect its germ cells. Lamarck’s idea that the change in the body induced by new habits could effect a corresponding change in the germ cells is rejected by them. How, then, is the adaptation effected ? If an animal assumes a new habit or mode of life with success, this can only be because a new and abundant food supply is thereby opened up to it. Now Darwin has suggested that a rich food supply is the proximate cause of the arrival of variations. Hence we may provisionally assume that the new food supply upsets the stability of heredity by altering the chemical constitution of the hereditary substance in the germ cells, and so variations in all directions are 24 INVERTEBRATA CHAP. produced, and those variations are preserved which adapt the organism at the right time to the new mode of life. Fia. 9.—The stalked larva and adult form of Antedon multispina. (After P. H. Carpenter.) A, larva. B,adult. 0b, Basal plate; cir, cirrus; pin, pinnule; r!, first radial plate ; : 72, second radial plate ; r3, third radial plate. Lf, then, the last stages in developmental history are, so to speak, I INTRODUCTION 25 the record of the last habits assumed by the species, the main frame- work of all developmental history must be the condensed record of ancestral experience ; for each stage in the development of an animal bears the same relationship to the one which immediately precedes it as the adult stage does to the last larval stage. We must now consider some of the influences which modify the ancestral character of developmental history. It is obvious that there is no a priori impossibility that the supply of rich food should produce variations which affect earlier stages of the life-history than SO laren Sc Sent NSS ey EE Ne RTI Fro. 10,—The larva and first metamorphosed form of the Plaice (Plewronectes platessa). (After Cole and Johnstone. ) A, larva. B, young Plaice just after metamorphosis. an, anus; pect, pectoral fin. the adult stage, especially if the alterations produced thereby have a “survival value.” Thus we may get secondary adaptations of the larvae to their environment, which are seen to be secondary for the reason that they differ widely from each other within the limits of a group in which the adult structure is constant. When, for instance, within the order of two-winged Flies we find some larvae adapted to living in water, others to living in earth, others in dung, and still others in dead bodies, and a few in living bodies, we cannot regard any of these methods of life as ancestral, and we find that in each case the larva is specially altered to suit it to the special conditions of its existence. 26 INVERTEBRATA CHAP. The most widespread alteration in the conditions of the larva which is met with is its transformation into an embryo through its retention within the egg-shell or the mother’s body. Since, as a race progresses from point to point in evolution, it should, according to the “biogenetic” law, leave behind it @ trail of larval stages, each corresponding to a condition of life which had formerly been the adult one, and in each of which the organism would have a distinct method of obtaining its food and a special set of enemies, a very long and complicated life-history should be produced. But the dangers of such a long larval life are very great, therefore a great advantage would be obtained by passing over some of these stages within shelter, and, as was pointed out above, in all life-histories we find an embryonic stage at the beginning. Now the food necessary for development during the embryonic phase is, in the vast majority of cases, furnished in the form of yolk platelets, embedded in the cytoplasm. This yolk sometimes distends the embryonic cells to enormous proportions ; it renders the process of cell-division difficult, and sometimes even impossible. In the ordinary process of cell-division each daughter nucleus becomes at, and immediately after the time of nuclear division, the centre of an attractive force which tends to collect the cytoplasm round it like a ball. In some cases, in consequence of this force, the first products of the division of the egg appear as spheres touching one another only at a point. But this period of activity is succeeded by a period of quiescence, and the centripetal force subsides, so that the cytoplasm of the two daughter cells tends to flow together again, unless a cell membrane has been formed between them in the meantime. When yolk is present it impedes the action of the centripetal force, apparently by rendering the cytoplasm less viscous; for cytoplasm devoid of yolk behaves like thick honey, whilst that which is loaded with yolk behaves more like a mixture of honey and water. Consequently, in yolky eges we find that in the first stage of their development they either divide into a few large clumsy cells, or else that cell division is represented by nuclear division only. Further, since the yolk is never uniformly distributed in the ovum, but is usually massed at one side, the first divisions result in the produc- tion of cells of unequal sizes or in the production of a cap of cells at one side of the egg, the rest remaining unsegmented. The pole of the surface of the egg which is relatively freer from food-yolk, and where the division into cells first occurs, is termed the animal pole of the egg; it is here too that the polar bodies are given off. The opposite pole, where most of the yolk is accumulated, is termed the vegetative pole. Such eggs are termed meroblastic, whilst eggs in which the yolk is sufficiently small in amount to allow of the division of the whole are said to be holoblastic. Further, whilst in most eggs the yolk is massed at one side (telolecithal) (vide Fig. 3 a), in a wide range of eggs it is massed at the centre, surrounded by a rind of comparatively yolk- I INTRODUCTION 27 free protoplasm (centrolecithal) (Fig. 11). The result of this latter dis- tribution is that a skin of cells is formed over an inert mass of yolk. But the clogging influence of yolk extends far beyond the first stages of develop- ment. The course of development can indeed be roughly divided into three stages:—(1) In the first the zygote becomes divided into a number of em- bryonic cells or blastomeres ; this stage is called segmen- tation; (2) in the second these cells are arranged so as to form the primary organs, the so-called germ layers, #.e. the skin, and the lining of the gut and of the body cavity; this stage is called the forma- tion of the layers; and (3) (After Munson. ) in the third stage these layers lecithal egg. Fic. 11.—Unripe egg of Limulus polyphemus. An example of a centro- are modified into the larval or ch, chorion; g.s, nucleolus (germinal spot); 9.v, permanent organs; this stage is called organogeny. Eggs with little or no last nucleus (germinal vesicle); per, peripheral cytoplasmic area free from yolk; y, central area of cytoplasm filled with yolk ; y.n, yolk nucleus, yolk are termed alecithal (Fig. 12). If yolk in the form of refringent globules should be totally absent, reserve stuffs in the shape Fic. 12.—The ripe egg of Strongylocentrotus lividus. (After Schaxel.) An example of an alecithal egg. chr, deposits of chromatin scat- tered through the cytoplasm and act- ing as reserve material ; , nucleus. of masses of chromatin are scattered about through the cytoplasm. In such eggs the building up of organs out of the first cells, or blastomeres which result from division, takes place by the simplest processes of unequally rapid growth of different parts, and of folding. Now in the folding of a layer of cells it is essential that the radius of curvature should bear such a relation to the size of the individual cell that the latter should not be deformed. When the layer consists of a few large yolky cells, folding becomes impossible and is replaced by proliferation of new cells at one point in the layer. Food, as we have seen, is usually supplied to the immature ovum by the sacrifice of the less fortunate oogonia or immature ova. In the case of the common polyp, Hydra, the im- mature egg comes at this stage to resemble an Amoeba. But in one 28 INVERTEBRATA CHAP. or two groups of animals the immature eggs destined to destruction are supplied to the zygote or fertilized ovum to be used as food. This leads to the strangest modifications of the early stages of development. The cells which result from the first divisions of the zygote may actually separate from one another and come together again in such a way as to surround the follicle cells, and this has led to the statement that, in certain cases, the egg dies and the embryo is developed out of follicle cells, but there is apparently no justification for this statement (cf. p. 635). But an embryonic stage may be, so to speak, intercalated between two larval stages. In the history of the race the change of habits which is recapitulated in the life-history, must have been con- tinuous, for no animal ever suddenly changed from one mode of life to another. Now the dangers incident to larval life and the opportunities of obtaining food, may vary very much, and will be much greater in some stages than others. If in one stage a large store of nourishment can be accumulated, it will be an advantage to the animal to pass quickly over the next stage, which is probably less favourable, and so we may get these intercalated embryonic, or, as they are usually termed, pupal stages. During these the animal is sometimes as quiescent as a true embryo, as in most insects ; in others, such as Cirripedia and Holothuroidea, it is active but takes no food. But there is one outstanding feature about most larvae which strikes the observer, and that is their extremely small size compared with that of the adult into which they eventually develop. This reduction in size is in all probability a secondary modification, but it has led to other modifications. An alteration of size produces an alteration in the physiological relations of the organism, and we find that where, from other evidence, we have reason to suspect that the ancestor had a long series of similar organs, the larva may only show one or two; for all these organs, if reduced to the same scale as that to which the whole body of the animal has been diminished, would become physiologically ineffective. Take, for instance, the gill slits in the larva of Amphiowus. These must have a certain minimum size if they are to work, on account of the viscosity of water, and therefore whilst they remain larger in proportion than the other organs of the body, their number becomes diminished, and so where the ancestor had almost certainly two rows of such slits, we find them represented in the larva by one row of slits which occupy the whole ventral surface. Lastly, it may be remarked that whereas it is true, generally speaking, that the more primitive features an adult exhibits, the more primitive features are found in the larva, yet the change from the larval to the embryonic method of development seems to take place quite independently of the status of the adult, and some animals preserving very primitive features have a development almost completely embryonic, whilst others higher in the scale retain a long larval history. 2 a I INTRODUCTION . 29 If it be asked why all animals have not exchanged the larval for the embryonic type of development, considering the advantage which the embryonic phase possesses, from the point of view of the safety of the young organism, it must be pointed out that the larval form of development offers compensating advantages from the point of view of wide dispersal of the species. The balance between these two alternatives seems to have been easily inclined one way or the other. It is therefore of the essence of Comparative Embryology to separate the fundamental ancestral traits of development from the superficial and secondary, and this is the task that has been patiently pursued for the last thirty years. As Sedgwick has pointed out, its results have been highly disappointing, and this has led many to doubt the validity of the ancestral explanation of development. But the reason for this disappointment is largely a human failing which will lead to equal disappointment in any branch of science. ‘This human failing is the ardent desire to settle fundamental questions in a few years. Obviously the most difficult pages of the embryonic record to decipher would be the earliest, for these have suffered most secondary modification, and yet it is precisely over such questions as the first differentiations of the embryo, such as the formation of the primary tissues or so-called “layers” of ectoderm, endoderm, and mesoderm, that most of the divergences of opinion have arisen. When we allow the mind to contemplate the vast profusion of living species at present in the world, each with its own peculiar life- history, and then reflect how few are at all known, we can see at once how small a clearing we have made in the forest of comparative embryology, and how premature it is to abandon the hope of finding a law underlying the likenesses and unlikenesses of the various modes of development. Where, as in the case of Vertebrata, the knowledge is more complete than in the case of other groups, the recapitulation of the structure of the lower members of the group in the young stages of the higher, is so plain as to be obvious to all. When the knowledge of other groups becomes equally complete the same thing will be obvious there also. Those who have abandoned Comparative Embryology for Experi- mental Embryology have set themselves the task of finding out the mechanism of the transformation of the apparently formless egg into the differentiated adult. But here again the impatience with delay, the determination to arrive at “basal” principles at once, will prepare disappointment for the workers in this branch also. Thus we find, as already pointed out, that whilst Driesch arrives at the conclusion that each kind of life-history owes its peculiarities to a non-material entelechy—-but leaves the resemblances between the life-histories utterly unexplained, Herbst arrives at the con- clusion that in each stage of development a substance is found which acts as a “stimulus” to cause the development to the next stage, while Loeb on the other hand maintains that until the conception of “stimulus” is utterly abandoned no real progress with the subject 30 INVERTEBRATA OMAP. will be made. Here we have divergences as great as those which exist between any upholders of rival phylogenetic theories. The real truth is that Experimental Embryology is an adjunct and uot an alternative to Comparative Embryology. It is a new and refined instrument of dissection: instead, for instance, of separating the blastomeres of a segmented egg by optical differences they are actually separated and their values tested by their powers of develop- ment. But the difference between the Echinoderm egg where any of the first eight blastomeres will develop into a whole larva, and the Annelid egg where the loss of a blastomere means the loss of a portion of the larva, still requires for its explanation the principle of affinity, that is to say that the ultimate explanation of the specific peculiarities of development is found in the chemical nature of the hereditary substance. So, before the future student of embryology stretches an almost limitless field of research. We must ultimately find out not only how the chemical quality of the germ-plasm determines the growth of the formless egg into the highly complex adult, but we must also find out howthis chemical quality can be altered, so that-variations can occur and evolution can take place, and this is the root-problem of biology. In order to make any attempt to solve this root problem we must, however, be able to control the whole life-cycle of the animal experimented on, and this is precisely where the work of Loeb, Herbst, and Driesch breaks down. All these workers have chosen as experimental objects the eggs of Echinoderms. These eggs are produced in enormous numbers and are easy to rear through the first stages of their development, but to reach even the adult form—to say nothing of the adult dimensions—they have to pass through a prolonged larval life during which there is an enormous mortality, and even when this metamorphoses into the adult shape is success- fully accomplished they have less than the millionth of the bulk of the fully ripe sexual form. Under the most favourable circumstances a year or two must elapse before they could produce germs and, therefore, before it could be possible to say whether the experiments had really altered the hereditary potentialities, or whether the distorted larva is merely the resultant of the new force applied and of the unaltered hereditary potentiality of the germ. The method of rearing these larvae until they attain sexual maturity has now been elucidated, but only a small proportion of the fertilized eggs can so far be reared; and - none of the workers mentioned above have attempted to rear the larvae beyond the earliest stages of their development. We pass now to consider the special embryology of the different groups of Invertebrata. In every case we shall, so far as possible, give examples of the fundamental laws of development laid down in this introductory chapter, and we shall indicate also what solid results have been gained from experiments performed on the developing eggs of animals belonging to each group. The group Protozoa are excluded, not because they do not I INTRODUCTION 31 in many cases exhibit a development showing larval and embryonic stages, but because in most cases it is not easy to determine what corresponds to the adult stage in Protozoa, and their life-histories are too imperfectly known for profitable comparison. LITERATURE . Most of the works contained in this list are of the nature of summaries or text-books, in which full reference to all the earlier works on the subject will be found. Agar, W. E. The Spermatogenesis of Lepidosiren paradoxa. Quart. Journ. Mier. Soc. vol. 57, 1911. Bateson, W. Mendel’s Principles of Heredity. Cambridge, 1909. Born. Experimentelle Untersuchungen iiber die Entstehung der Geschlechtsunter- schieden. Breslauer artzlicher Zeit., 1881. Conklin, E.G. Karyokinesis and Cytokinesis in the Maturation, Fertilization, and Cleavage of Crepidula and other Gastropoda. Journ. A.N.S. Phila. vol. 12, 1902. Darwin, C. The Variation of Animals and Plants under Domestication. London, 1868. Driesch, H. The Science and the Philosophy of the Organism. Gifford Lectures. Aberdeen, 1907 and 1908. Duesberg, J. La Spermiogénése chez le rat. Arch. Zellforsch. vol. 2, 1909. Godlewski, E. Untersuchungen iiber die Bastardierung der Echiniden- und Crinoidenfamilien. Arch. Ent. Mech. vol. 20, 1906. Herbst, C. Vererbungsstudien V. Ibid. Hertwig. Vergleich der Ei- und Samenbildung bei Ascaris megalocephala. Arch. Mikr. Anat., vol. 36, 1890. Kupelwieser. Entwicklungserregung bei Seeigeleier durch Molluskensperm. Arch. Ent. Mech. vol. 22, 1906. Loeb. Die chemische Entwicklungserregung des tierischen Eies. Berlin, 1909. Montgomery. Some Observations and Considerations upon the Maturation-phen- omena of Germ Cells. Biol. Bull. vol. 6, 1904. Schaxel, J. Das Zusammenwirkung der Zellbestandteile bei der Kireifung, Furchung und ersten Organbildung der Echinodermen. Arch. Mikr. Anat. vol. 76, 1911. Sedgwick. On the Law of Development commonly known as Von Baer’s Law, and on the Significance of Ancestral Rudiments in Embryonic Development. Quart. Journ. Micr. Sci. vol. 36, 1894. Sedgwick. The Influence of Darwin on the Study of Animal Embryology. Darwin and Modern Science. Cambridge, 1909. Sharpe. Insects. Camb. Nat. Hist. vol. 5, 1895. Shearer. The Problem of Sex Determination in Dinophilus gyroctiiatus. Quart. Journ. Mic. Sci. vol. 57, 1912. Weissman. Die Kontinuitit des Keimplasmas als Grundlage einer Theorie der Vererbung. Jena, 1886. Weissman. Das Keimplasma, eine Theorie der Vererbung. Jena, 1892. Wilson, E. B. The Sex-Chromosomes. Arch. Mikr. Anat vol. 77, 1911. CHAPTER II PRACTICAL HINTS THE object of this book is not merely to lay before the reader the best ascertained results of embryology, it is also designed to indicate the directions in which further research may be most ad- vantageously prosecuted, and to suggest reliable methods of pursuing such researches. Incidentally defects in the methods employed by some investigators, and the possible bearing of these defects on their results, will be pointed out. In the present chapter some general instruction will be given on methods of procedure which are applicable to all, or nearly all classes of embryo, while special methods will be described when each separate phylum is described. When one endeavours to work out the life-history of an animal the first step is to observe the larvae or embryos in the living state. In many cases a large number of points can only be made out in the living embryo, since the tissues are then in their natural state of tur- gescence, and living protoplasm is relatively transparent. The next step is to preserve or fiz the embryos, dehydrate and clear them and mount them whole. ; Fixing or preservation consists in adding some reagent to the specimen to be preserved which will form a stable and more or less solid compound with the protoplasm of the organism. This compound enables the form of the organism to be retained during the process of dehydration, and the macerating and deforming effects of the diffusion currents produced in this process to be resisted. Dehydration, is the removal of the water by successive immersion of the object in different grades of alcohol; clearing, is infiltration of the tissues by an oil like oil of cloves, cedar oil, etc., which renders them transparent. Now the reagent which forms the strongest compound with protoplasm and preserves in it the nearest resemblance to its living condition is the solution of osmium tetroxide in water, usually erroneously called osmic acid. For effective fixation a solution of at least 25 per cent must be used. “Osmic acid” has two disadvantages, it produces a very black stain which consists of the metal osmium, and it is apt to render the tissues brittle. Further, if applied to objects 32 CHAP. II \ PRACTICAL HINTS 33 of any size osmic acid forms a crust of hardened imperviable proto- plasm which prevents the penetration of the reagent into the interior. It is, therefore, a reagent eminently suited for the preservation of minute larvae and the permeable tissues of calcareous sponges. For the denser tissues of siliceous sponges other reagents would be more suitable; such, for instance, as a mixture of 3 parts concentrated solution of corrosive sublimate in water and 1 part of glacial acetic acid. This is one of the best and most universally employed preservatives: many investigators use, however, a smaller proportion of acetic acid (often as low as 5 per cent) than that just mentioned ; it is to be remembered that acid reagents are unsuitable for calcareous sponges and for other organisms which contain much calcareous matter, because the evolution of carbonic acid gas dissolves the calcareous matter, and so causes the formation of blebs in the tissues and of artificial rents and cavities which have no counterpart in the living animal. When it is desired to decalcify, this is best accomplished when the organism is in strong alcohol. If a drop or two of nitric acid be added to a small bottle (of two fluid ounces) full of strong alcohol and well shaken, a solution is produced which | will decalcify so slowly that the resulting gas is at once dissolved and never forms bubbles. A different method of decalcifying organisms which have been preserved in osmium tetroxide may be mentioned here. If, after being blackened by immersion in the solution and then rinsed in clean water, the specimens be immersed in Miller’s fluid, not only will the calcareous matter be slowly removed but also the excess of metallic: osmium, and the tissues will be rendered less brittle. Miiller’s fluid is a mixture of bichromate of potash, which contains unsaturated chromic acid and sulphate of sodium. Flemming’s fluid, which is a very favourite preserving medium, is really an attempt to combine the advantages of osmium tetroxide and chromic acid, for it is a mixture of these two fluids with acetic acid. It is an excellent preservative, but is intensely acid and open to the same objections as other acid reagents. The same remarks apply to Hermann’s fluid, which is a mixture like Flemming’s fluid, in which acid platinic chloride replaces the chromic acid. When it is desired to make whole mounts of minute forms it will generally be found that osmium tetroxide, corrosive sublimate, etc., render them too opaque. Strong formalin—that is a 40 per cent solution of the gaseous formic aldehyde in water—is a splendid reagent for this purpose. It kills small larvae instantaneously, with- out any shrinkage. It is apt, howeyer, to become acid by the oxidation of the aldehyde into formic acid; it is therefore advisable to carefully neutralize the solution before employing it. Further, the compound which it forms with protoplasm is soluble in water. Therefore, after a few minutes’ sojourn in the formalin solution, the specimens must be instantly transferred to absolute alcohol, and in this they must be stained. Eosin or methyl green dissolved in absolute alcohol are VOL. I D 34 INVERTEBRATA CHAP. very good stains. The transference to oil of cloves must be made by adding this substance drop by drop to the absolute alcohol at intervals of an hour or so for several days. After a sojourn in the pure oil the specimen is placed in the concavity of a hollowed slide and suddenly covered with thick solution of Canada balsam in xylol. The oil of cloves flies to the periphery of the balsam owing to surface tension and may be removed by blotting-paper. When all the information possible has been gleaned from whole mounts of embryos and larvae the next step is to cut them into series of sections arranged in order, but for this purpose they must be embedded in a block of paraffin so that the sections when cut by the microtome will be parallel to a known direction. To accomplish this placing, or orientation, as it is called, in the case of minute larvae is a matter of great difficulty, and unless the sections are cut in the right direction they are very difficult to interpret. The best way to overcome this difficulty is to embed the specimens in celloidin before embedding in paraffin. The solution of celloidin used for embedding vertebrate tissues, consisting of celloidin dissolved in a mixture of equal parts of absolute alcohol and ether, is not suitable for delicate larvae because too violent diffusion currents are produced in the process of changing from alcohol to the celloidin solution. If, however, the celloidin be dissolved in a mixture of four parts of absolute alcohol and one part of ether, then such currents are avoided. It is well to have three grades of this solution, one saturated, one made by diluting the saturated solution with an equal bulk of the solvent, and one by diluting it with two volumes of the solvent. The objects, if they are small, should remain in each grade for about one day. Then the thick solution with its contained embryos is poured into chloroform and the celloidin hardens to a cheesy consistence. After an hour’s sojourn in this fluid a piece of celloidin containing the embryo can be cut out and embedded in paraffin. The embedding may be done in one of two ways. (1) The piece of celloidin containing the object is placed in absolute alcohol, to remove any trace of moisture, and then immediately transferred to fresh chloroform to which fragments of clean paraffin are added. If the whole be heated to 60° for an hour all the chloroform will have evaporated and the object can now be poured, together with some of the paraffin, into a mould and allowed to cool. Before transferring to the mixture of chloroform and paraffin, the object can be studied under the lower power of the microscope and the celloidin shaped so as to direct the orientation of the block of paraffin. (2) The object and its surrounding celloidin may be transferred to cedar oil. If this be warmed for half an hour (by being placed on the top of the thermostat) all traces of moisture will be absorbed, and the cedar oil will render the celloidin absolutely transparent, so that the object can be examined as if it were mounted in oil of II PRACTICAL HINTS 35 cloves. The celloidin should then be cut as before so as to indicate the position of the object, and the latter, in its celloidin block, should be transferred to a mixture of cedar oil and hard paraffin and heated to 56° for fifteen minutes, and then for fifteen minutes to a bath of pure hard paraffin. This second method has the disadvantage of rendering the embryonic tissues rather brittle, but one great advantage of embedding in celloidin is that the tissues of the embryo become penetrated while cold by a substance which hardens and gives them support, before they are subjected to the ordeal of the hot paraffin bath, which has a tendency to cause shrinkage. When the sections are cut they are best mounted by first smearing the slide with Mayer's glycerine and albumen fixative, then laying the sections upon it and adding a layer of water heated to 60°; the hot water cools at once to about 45°, and this heat will flatten out the sections without melting the paraffin. The water is drained off and the slide dried on the top of the thermostat for forty minutes. Then the paraffin can be melted off and washed off in xylol. It should then be immersed in oil of cloves for one minute, which softens and dissolves the celloidin. Then it should be placed, not in absolute alcohol, but in 90 per cent alcohol, which washes off the oil of cloves and at the same time removes the glycerine from the glycerine and albumen fixative; it finally coagulates the latter and also hardens the semifluid celloidin, so that it forms an additional fixative for securing the adhesion of the sections to the slide. Another method is to transfer the slide from pure xylol to a mixture of equal parts of xylol and absolute alcohol. The absolute alcohol coagulates the albumen and removes the glycerine. The slide can then be transferred to 90 per cent alcohol and thereafter to alcohol of lower grades. Sections thus fixed will stand any further treatment without becoming loose. Mayer’s fixative is made by mixing white of egg strained through muslin with an equal volume of glycerine. A few drops of thymol are added to prevent the decomposition of the albumen. — In staining objects which have been preserved in osmium tetroxide it is often found that the black deposit of metallic osmium in the tissues prevents the stain from taking effect. The best general stain is Grenacher’s (sometimes called Delafield’s) haematoxylin. This is best used in a solution made by diluting the concentrated stain in three or four times its bulk of distilled water. This solution should be filtered before being employed. If the sections be previously immersed in a solution of borax-carmine in 70 per cent alcohol it will be found that they can remain in it for 24 hours without absorbing any stain, but if then they be transferred to the solution of haematoxylin described above, they stain rapidly and well. Excess of stain is removed by immersing the sections in-a solution of acid alcohol made by adding two drops of strong hydrochloric acid to 100 cc. of 70 per cent alcohol. If the 36 INVERTEBRATA CHAP. Il sections be examined from time to time under a microscope as the stain is being removed, a point will be detected at which the whole section takes on a reddish colour, and the nuclei stand out prominently. When this is observed the sections should be washed free from the acid alcohol by immersing them in 70 per cent alcohol. They should then be held inverted for a few moments over the mouth of a bottle of strong ammonia, the escaping fumes from which neutralize the last traces of acid, and the sections, now of a beautiful blue colour, may be dehydrated and mounted. The different tissues yield up their stain in different degrees, and a beautiful differentiation is effected by the different tints of blue. If a double stain be desired it can be effected by finally dehydrat- ing the sections in absolute alcohol to which eosin has been added, but if they remain too long in this solution all the haematoxylin will be washed out. The methods described in this chapter are general methods . applicable to all classes of embryos ; special methods will be described in the chapters dealing with special groups. CHAPTER III PORIFERA Classification adopted— Homocoela (= Asconidae) { Syconidae \ Leuconidae Triaxonia (= Hexactinellida) Demospongiae Catearea | Heterocoela THE group Porifera or Sponges stand apart from all the rest of the Metazoa, and their embryology is consequently of very great interest. We may suggest as a form for practical study the development of the Calcareous sponge Grantia compressa. This sponge, distinguished by its flattened shape, is a common denizen of the British coasts, and its embryology is being worked out by Professor A. Dendy. Allied species occur on the coast of North America; and the course of its development, so far as deter- mined, so closely resembles that of the Mediterranean species, Sycandra raphanus—the subject of Schulze’s classic research (18'75)—that the latter may be taken to represent that of Grantia and of Calcarea generally. SYCANDRA RAPHANUS The eggs are found embedded in the jelly which forms the sub- stance of the wall of the sponge, intervening as it does between the cells forming the dermal membrane and those lining the paragaster and the extensions of this latter cavity into the flagellate chambers. The spermatozoa occupy a corresponding position in the male. When ripe, they bore their way through into the flagellate chambers, and are discharged by the osculum. They swarm in the surrounding water, and, coming under the influence of the inhalent currents of the female, they penetrate through its pores and thence find their way to the eggs, which are thus fertilized in situ. The fertilized egg undergoes the first stages of its development in the maternal tissues. It is found to be contained in a cavity lined 37 38 INVERTEBRATA CHAP. by a definite layer of dermal cells. This cavity is wedged in between the layer of dermal spicules and a flagellated chamber. As it enlarges to suit the size of the growing embryo, it encroaches on the cavity of the flagellated chamber, since the layer of dermal spicules is unyielding. (Dendy, 1889.) The first stages of development must therefore be studied in transverse sections of the adult. When the larvae emerge they must be encouraged to settle on some convenient portable object. If it is desired simply to make whole mounts, the bottom of the vessel in which the parents are contained is strewn with coverslips, and these are removed when the young sponges have attached themselves to them, and immersed in 1 per cent solu- tion of osmic acid till fixation is effected, then stained in picro-carmine and mounted whole. If itis desired to cut sections of the larvae, the coverslips must be covered with a layer of paraffin wax or photoxylin, which can be scraped off and the larvae thus removed, when they can be dealt with by the methods described in the previous chapter. The egg divides into two, four, and eight blastomeres, which are arranged in one plane, and, from the 4-cell stage, they surround a central cavity open at both ends, which owes its existence to their mutual separation Fic. 13.—Two stages in the (Fig. 13). This stage is followed by a division segmentation of the egg Of all the cells into two tiers, so that sixteen of Sycandra raphanus. cells are formed in two rows, and then each pe tuer enue) of these rows is subdivided into two further A, 8-cell stage in which all the rows, and so we reach the 32-cell stage. blastomeres are in one tier with Bons : a central aperture. B, 16-celi Divisions now follow one another in the stage blastomeres arranged in individual cells somewhat irregularly, and two Riots of cight each round @ thus an oval vesicle is constituted, which central aperture. may be termed the blastula, one pole of which is rounded and one flattened, whilst inside it there is a cavity which is a development of the cavity formed by the separation of the first segments of the egg, and which is termed the blastocoele. One opening of this cavity to the exterior, that at the pointed end, is by this time closed, but that on the flattened “basal” surface persists for some time, though it too eventually closes. The cells immediately surrounding this latter pore are distinguished from the rest by becoming extremely granular. The granular cells increase by division to thirty-two, whilst the remaining cells become extremely long and columnar, and each develops a flagellum. The columnar half of the embryo is pressed against the wall of the yielding chamber, but the A III PORIFERA 39 granular cells encounter the resistance of the spicules, and therefore the embryo becomes hat-shaped. A few small cells are found in the segmentation cavity. Dendy (1889) thinks that these cells, which may be termed mesenchyme, have been budded from the flagellated cells, but this is not certain. The granular cells now proliferate rapidly, especially in the centre, and form a thick mass which becomes invaginated into the blastocoele. The embryo is now ready ‘ for birth. By the activity of its flagella it bores its way ak into the adjacent flagellated Ome chamber of the mother, and / then escapes through the osculum. During this pro- cess the blastocoele seems to absorb water, the invaginated “k-77eS cells are exserted, and thus ‘ the free-swimming larvae \\-gr acquire an oval form; but 8028 6020000008 i Y Q Se i @S @® gr /gooe 7 © SPic 6996 @ ee MCS Fic. 14.—View of the embryo of Grantia labyrinthica in the blastula stage lying in the embryonic Fic. 15.—View of the embryo of chamber of the mother. (After Dendy.) Grantia labyrinthica in a later oe ie tA stage of development than that col, collared cells lining a maternal flagellated chamber ; represented in Fig. 14. (After emb, embryonic chamber ; gr, granular cells of the embryo ; mes, cells, so-called mesenchyme budded into the blasto- coele ; spice, maternal spicules. Dendy.) Letters as in Fig. 14. the cells forming one half of the wall of this vesicle are granular and . rounded, whilst those forming the other half carry long flagella, and possess, in addition, a bright red pigment. The interior is half-filled up with granular cells. Sucha larva is termed an amphiblastula, and, as we shall see, this type recurs in all families of sponges (Fig. 17). After swimming for a day or two the amphiblastula comes to rest on the surface of a smooth stone, its ciliated half which preceded the other whilst the larva was moving, being directed downwards. Within a few minutes the larva has undergone an entire change of shape. Its anterior end flattens out; the ciliated cells which con- (After Dendy.) . Showing the escape of the larva from the tissues of the mother sponge into the flagellated chamber Section of a portion of Grantia labyrinthica. Fie, 16. escaping larva; 0, opening of flagellated chamber into central of the mother ; fl, flagellated chamber ; J, cavity (paragaster) of sponge. (After Dendy. ) Fic. 17.—The Amphiblastula larva of Grantia labyrinthica, , granular cells. > 97 0 4 f, flagellated cells ; CHAP, III PORIFERA 41 stitute it become invaginated into the posterior half, and the blasto- coele is thus reduced to a mere slit. The cells forming the edge TTT NY Fic. 18.—Two stages in the fixation of the larva of Sycandra raphanus. (After Schulze.) A, the flagellated cells are just retreating into the interior. B, the larva has assumed the form of a hemispherical cup and is attached by amoeboid processes of the outer granular layer. Seen in optical section. fl, flagellated cells ; gr, granular cells; », porocytes (?). of the cavity of invagination are granular, and when the ciliated cells become invaginated, these granular cells extend inwards Fic. 19.—An early stage in the metamorphosis of the Ascon stage of Sycandra raphanus into the adult. (After Maas.) A, external view of young sponge. B, diagrammatic longitudinal section of the same to show the gradual displacement of collar cells by granular cells. C, a small portion of such a section further enlarged. ch, radial flagellated chamber; fl, flagellated cells; gr, granular cells (in C the reference line points to a spot where the granular cells are migrating inwards) ; 0s, osculum ; p, inhalent pore ; ‘pg, paragaster. . along the substratum, and floor the cavity of the invagination (Fig. 18); they also extend outwards in irregular tongue-like pro- 42 INVERTEBRATA CHAP. cesses and adhere to the substratum; the process of attachment of the larva to the substratum is known as fixation. The larva is thus converted into a closed cylinder, the wall of Fic. 20.—A late stage in the metamorphosis of the Ascon stage of Sycandra raphanus into the adult condition. (After Maas.) Letters as in Fig. 19. which consists of an outer layer of flattened cells, and an inner layer composed of ciliated cells, on each of which the collar soon makes its appearance. This collar is characteristic of the cells lining the flagel- lated chambers in all Porifera. Between the two layers a layer of III PORIFERA 43 jelly makes its appearance, which is the real stiffening element in the sponge-wall. The formation of inhalent pores is now begun. Individual cells of the outer or dermal layer extend inwards through the jelly, and press asunder adjacent cells of the inner or gastral layer. These cells then become hollowed out, converted into drain-pipes, as one might term it, and the action of the flagella draws in water through them. Other cells migrate from the outer layer into the jelly, and form the characteristic calcareous needles or spicules. The first type of cells are called porocytes, the second scleroblasts. After the pores have been acting for some time, an exhalent opening is formed at the distal end of the cylinder. The formation of this osculum seems to be due in part to the hydrostatic pressure caused by the action of the pores. The tiny sponge is now quite comparable to the type of adult sponge exemplified by the genus Zeucosolenia. Its transformation into the adult Sycon is an affair of slow growth, and the process has not been observed in this Grantia, but there is no reason to doubt that it is essentially similar to what occurs in Sycandra raphanus, in which it has been described by Maas (1900). In Sycandra, pouches grow out horizontally from the cylinder which forms the body of the young sponge ; they are formed gradually, not all at once. As the pouches are formed the flagellated cells are taken up into them, and the dermal cells migrate inwards from the outside, pressing the flagellated cells asunder, and constitute the epithelium lining the central cavity of the sponge or “paragaster.” The interspaces between the openings of the horizontal pouches, that is to say, the niches left between the outer surfaces of these pouches, constitute the inhalent system of canals. In this sponge the re- productive cells seem also to be formed from the dermal layer; in their undifferentiated form they are full of yolk, and are known as archaeocytes. OTHER SPONGES To Maas (1898) we owe the demonstration that all sponge larvae are modifications of the type just described. Of the development of the Hexactinellida nothing is known; larvae, it is true, have been observed which seem to originate from unfertilized eggs, and which resemble the larvae of other siliceous sponges, but their history has not been followed. When we turn to the Demospongiae in which the spicules are arranged in cords, and which constitute the vast majority of sponges, we can trace a complete series from a development more primi- tive than that of Grantia to the most modified form. Beginning with Oscarella, which, although devoid of a skeleton, has its affinities with the Demospongiae, Maas shows that the embryo is hatched as an oval blastula, consisting of a uniform layer of flagellated cells. During the course of its free life as a larva, the cells of the posterior 44 INVERTEBRATA CHAP. half lose their flagella and become granular, so that the blastula is thus converted into an amphiblastula. The amphiblastula fixes itself and undergoes a metamorphosis like that of Grantia, but the resulting sponge, the “Rhagon,” is conical not cylindrical, and the Fig. 21.—Longitudinal sections through the free-swimming larva of Oscarella lobularis in two stages of its development and its fixation. (After Maas. ) A, early larva. B, larva in which posterior cells are becoming granular. C, Rhagon shortly after fixation. Letters as in two previous figures. flagellated chambers are pro- duced as hemispherical pouches of the inner layer (Fig. 21). In the development of the Tetractinellid Plakina, Maas (1909) describes the larva as beginning its free life as a blastula, since the cells constitut- ing its wall are at first al/ slender and ciliated, but the blastocoele contains a few rounded granular cells, termed archaeocytes, which seem to be the mother cells of the germ cells. The posterior half becomes granular by the alteration of the cells, which lose their cilia, but cells which are not to be confused with the archaeocytes are also budded from this half into the interior (Fig. 23). Fixation and metamorphosis occur as usual, but the resulting sponge has the form of a shorter cylinder than is the case with either Grantia or Oscarella. By downgrowths of dermal cells, the interior flagellated cells become divided into groups, which, although at first they retain a portion of the lumen of the sponge, eventually become solid; from these solid masses the spherical flagellated chambers are formed later (Fig. 22). Finally, in the _ siliceous sponge, Esperia (Maas, 1892), the larva is hatched as an amphi- blastula, but the flagellated cells cover four-fifths of the surface, and the granular cells form a solid plug projecting into the interior of the blastocoele and contain a sheaf of siliceous spicules, ready for distribution throughout the tissues of the young sponge as soon as fixation has occurred. Stretching across the blastocoele are I PORIFERA 45 branched ‘cells remindin gus of the mesenchyme cells of the larva of Grantia. Fic. 22.—Seven stages in the metamorphosis and fixation of the larva and growth of the young sponge of Plakina monolopha. (After Maas.) A, larva in amphiblastula stage ; granular cells budding off cells into interior. B, larva about to fix itself. C, larva just fixing itself, flagellated half flat. D, fixed larva, flagellated cells beginning to invaginate. E, ‘‘Rhagon” stage. F, G, two stages in subdivision of Rhagon cavity by downgrowth of septa; fl, flagellated cells ; gr, granular cells ; mes, ‘‘ mesenchyme”; s, septa dividing cavity of Rhagon. After fixation the flagellated cells collapse to form a solid mass, which is speedily separated into smaller masses by ingrowths of the 46 INVERTEBRATA CHAP. dermal cells, and these masses become hollowed out to form the spherical flagellated chambers. ‘ A B vi TH ' i f AY Uy. ; fi we 14, th cl Fic, 23.—T'wo sections of the body-wall of the larva of Plakina monolopha in order to show the distinction between archaeocytes and mesenchyme. (After Maas.) i A, a piece of wall of embyro not yet hatched. B, a piece of wall of free larva; arch, archaeocytes ; mes, mesenchyme. In this series, Grantia forms, not the beginning, but takes the second place, and, viewing the series. as a whole, we see a progressive shortening of the larval life joined to an anticipation of adult char- acters. We have, indeed, | before us, typical examples of the commonest form of the modification of develop- mental history from its primitive form. This consists in the reflecting back of structures characteristic of one period of the life-cycle to successive earlier periods in ontogeny. It is called heterochrony, and its pos- sible cause will be discussed in the summary. ‘The merit of having called attention to it, and of having emphasised its importance, belongs to Lankester. Fic. 24.—Longitudinal section through the Amphi- The development of the plastula larva of Esperia Eset (After Maas. ) most primitive sponges, the 4 une cle gy Sambi teen” Aseonidae, has been worked ae sp, pin-head Eau - out by Minchin (1896), and his results are of great interest but a little difficult to reconcile with the series determined by Maas. In the genus Clathrina, the embryo is hatched as an III PORIFERA 47 oval ciliated blastula, with two cells at its posterior pole which are interpreted as the mother cells of the archaeocytes or primitive ova. From these cells numerous granular cells are budded off and fill the interior of the vesicle, but other cells formed by the modification of individual flagellated cells here and there, which lose their flagella, also migrate inwards. These latter cells, at fixation, are stated to burst forth and surround the ciliated cells. In Leucosvienta, on the other hand, the posterior part of the interior of the blastula is filled with a mass of granular cells with small nuclei, and in front of these is a tube of flattened pigmented cells containing a lens-like body. This, according to Minchin, con- stitutes a rudimentary visual organ; it disappears at fixation. The cells of the posterior half of the blastula wall become granular in situ during the free life of the larva, and so an amphiblastula is produced. (Fig. 25.) Further details of these interesting life-histories are urgently called for. We wish to know what corresponds to the archaeocyte in Grantia. If in this form the archaeocytes are only differentiated from the dermal cells after fixation, this must surely be a more primitive arrangement than what obtains in the Asconidae or in the Tetractinellida, where these primitive ova are differentiated during the — segmentation of the egg. Minchin, indeed (1900), suggests that the granular cells, which are invaginated whilst the embryo of this sponge is in the tissues of. the mother, are archaeocytes and are quite distinct from the cells forming the one end of the amphiblastula which he regards as transformed flagellated cells; but this view is negatived by Dendy’s researches, the results of which have been described above. : : The development of the well-known freshwater sponge Spongilla, which has been worked out in great detail by Evans (1899), presents several features of great interest. This sponge belongs to the group Demospongiae and forms a larva somewhat like that of Hsperia, but the outer flagellate layer extends all round. One end of the larva is broader than the other and under this end is a cavity. The rest of the interior is filled with yolk-bearing “archaeocytes,” whilst just under the skin is a layer of flattened cells with dense nuclei, like those described in the interior of the larva of Grantia. It meta- morphoses in much the same way as Hsperia, ic. it fixes itself by the broad end; but Evans maintains that some of the “ flagellate chambers” are formed at the expense of groups of archaeocytes and do not owe their origin to the flagellated epithelium which invested the surface of the larva. Spongilla also reproduces itself by buds termed gemmules. The development of these in the allied genus Ephydatia has been worked out by Evans (1900). The gemmule first appears as a number of wandering cells in the jelly of the sponge, which are distinguished from their neighbours by possessing deposits of yolk in their cyto- plasm. These cells gradually collect at a fixed point in the tissues, 48 INVERTEBRATA CHAP, which is the centre for the formation of the gemmule. Here they become massed together so as to form a spherical lump. Other wandering cells with less yolk follow after them and form a layer Fic, 25.—Longitudinal sections of Amphiblastula larva, just fixed larva, and young sponge of Leucosolenia variabilis. (After Minchin.) A, free-swimming larva. B, just fixed stage. C, young sponge four days old; cent, central cells ; A, flagellated cells ; gr, granular cells ; int, intermediate cells; 1, lens; pig, pigmented cells. surrounding them, in which the individual cells, as a consequence of mutual pressure, assume a columnar form. The investment is for a considerable time not quite complete, and an opening therefore exists. In this opening are found some specially large cells called tropho- ur PORIFERA 49 cytes, devoid of yolk, but with plentiful dark granules in their cyto- plasm, which appear to act as carriers of nutriment to the enclosed yolk cells. The investing cells secrete a membrane on their inner surfaces. The columnar layer is then invaded by another class of wandering cells which come from the adjacent tissues of the sponge. These are clear cells carrying in their interior peculiar spicules called amphidiscs. The amphidisc resembles a pair of toothed wheels joined by an axle. Various stages in the development of amphidiscs can be seen in the maternal tissues. They first appear as little needles, similar in shape to the other spicules of the sponge. The ends of the needles thicken and eventually form wheel-like discs Fic. 26.—Section through a gemmule-bearing individual of Ephydatia blembingia. (After Evans. ) amph, Amphidise ; gemm, gemmule. (Fig. 26 amph). When the amphidises have taken up their position amongst the cells of the investing layer, the cells which carried them degenerate and disappear. Before the investment of columnar cells is quite complete, the trophocytes withdraw from the yolk cells and pass back into the mother sponge. The inner ends of the columnar cells, after having secreted the membrane, likewise degenerate and form a sort of network of fibres between adjacent amphidiscs. The outer portions of these cells, however, become segregated off from their inner de- generating portions and pass back into the mother sponge. Before doing so, however, they secrete on their inner ends another membrane, which may be called the outer membrane of the gemmule, since it unites together the outer ends of the amphidiscs. Where the investing VOL. I E 50 INVERTEBRATA CHAP. layer is finally completed no amphidiscs are found, and this forms a weak spot through which the inner mass, which is the real bud, eventually issues forth. The gemmules are set free on the decay of the parent in the autumn, fall to the bottom of the pool or stream and remain dormant till the spring. The inner mass then perforates the weak spot in the membranes, it streams forth in ameoboid Fic, 27,.—Three stages in the formation of the gemmules of Ephydatia blembingia. (After Evans. ) ' A, investment of columnar cells incomplete, trophocytes in contact with yolk cells, inner membrane being formed. B, investment of columnar cells complete ; immigration of scleroblast with amphidiscs. C, ripe gemmule. Yolk cells form a solid mass; amph, amphidisc; col, columnar cells; i.m, inner membrane ; 0, aperture for escape of embryo; 0.m, outer membrane; scl, scleroblast ; troph, tropho- cyte; y, yolk cells. fashion and forms a little mass of cells which develops into a sponge. Marshall (1884) has shown that in Spongilla lacustris the sponges to which the gemmules give rise reproduce themselves by sexual cells and then perish, whilst the larvae which arise from the fertilized eggs grow into sponges which produce gemmules ; thus there is in this sponge an alternation of generations similar to that with III , PORIFERA 51 which we shall become familiar when we study the next group, Coelenterata. Maas (1906) has reared the larvae of Calcareous Sponges in water artificially deprived of all carbonate of lime. The result was that no calcareous spicules were formed, and when the larva fixed the flagellated cells formed a solid mass and developed no lumen. Hence Maas concludes that the formation of spicules acts as a stimulus which determines the invagination of these cells to form a hollow cylinder. This may be true for Grantia and other Calcareous Sponges, but it is obviously untrue for Oscarella which has no spicules. : ANCESTRAL HISTORY In the introductory chapter it was pointed out that there is strong evidence that larval forms are, broadly speaking, reminiscent of ancestral conditions of the stock or phylum. When we find in the ontogeny of all sponges the blastula form cropping up, and further find that, in those with the longest larval history this stage is larval, becoming embryonic only in cases of a short free life, we feel justified in assuming that it represents in a rough sort of way the common ancestor of all Porifera. Such an ancestor—a hollow vesicle of flagellated cells—were it now living would be termed a colonial Protozodn. In Volvoz we have an organism which, if it did not possess chlorophyll and live like a plant, would correspond fairly closely to our idea of what this ancestral sponge must have looked like. Now the great interest attaching to the blastula is that it appears as a larva in the life- histories of at least two other primitive groups of Metazoa, and that as a more or less modified embryo it can be detected in the develop- ment of all the Metazoan groups. Hence the case for regarding it as representing the ancestral stock of Metazoa is greatly strengthened. — But such a stock when it existed must have been of world-wide distribution, swarming in all the seas. and waters of the globe. Such a world-wide stock would become adapted to different “stations ” and just as at the present day we have bottom-feeding as well as mid-water fish, so we may imagine that bottom-feeding blastulae were developed. These, instead of devouring the floating and swimming organisms like the rest, turned their attention to the microscopic forms lying on the bottom. Under these circumstances only the cells on the lower half of the blastula would be effective feeders, and the more flattened this part became the more effective would be their work. The other cells would become merely protective and would tend to lose their flagella, and so the spherical blastula would be modified into a cap-like form. Fixation would be the next step, and so far as we can tell from a consideration of the life-histories of fixed animals belonging to other phyla, fixation is an adaptation to withstand and at the same time take advantage of currents. The 52 INVERTEBRATA CHAP. III larva, instead of creeping about seeking fresh food, holds on with its protective cells and lets the current waft fresh food into its reach. So far our reasoning appears safe. But the porocytes baffle explanation ; we cannot picture to ourselves a process by which cells converted themselves into drain-pipes, when we remember that every step in the process must have been functional and must have had a survival value. We can only imagine that the hollowing out of a cell is perhaps a shortened reminiscence of the process by which gaps in the attached rim, which must have existed to allow the ingress of water, became surrounded. by protoplasm. The need for extending the surface of absorption, once fixation were accomplished, would account for the extension of the area of flagellated cells by their invagination, so that collectively they took on the form of a cylinder ; but the formation of the osculum is utterly obscure. In some few cases we can compare ancestral history as recorded by fossils, with ancestral history deduced from embryology ; we can then see, as compared with the record deduced from fossils, what an abbreviated sketch is constituted by the embryological record. In the present case it is true we have no fossils to guide us, but the abbreviation of ancestral history, as reflected in larval history, must be intense. The later stages of the history of the race, the gradual complication of the chamber system, is mirrored in the post-larval development: the first fixed Grantia is at first an Ascon, and only gradually takes on the Sycon characters as it grows in size. The history of all sponges just after fixation would be a most interesting field for research, and would throw much light on their mutual affinities. LITERATURE REFERRED TO Dendy, A. On the Pseudo-gastrula Stage in the development of Calcareous Sponges. Proc. Roy. Soc. Victoria (Australia), 1898. Evans, R. The Structure and Metamorphoses of the Larva of Spongilla lacustris. Quart. Journ. Mic. Sci., vol. 42, 1899. Evans, R. A description of Ephydatia blembingia, with an account of the formation and structure of the gemmule. Quart. Journ. Micr. Sci., vol. 44, 1900. Maas, O. Zur Metamorphose der Esperia lorenzi. Mitt. aus der Zool. Station zu Neapel, vol. 10, 1892. Maas, O. Die Keimblitter der Spongien und die Metamorphose von Oscarella (Halisarca). Zeit. fiir wiss. Zool., vol. 68, 1898. Maas, O. Die Weitcrentwicklung der Syconen nach der Metamorphose. Zeit. fiir wiss. Zool., vol. 67, 1900. Maas, O. Uber die Einwirkung carbonatfreier und kalkfreier Salzlésumgen auf erwachsene Kalkschwimme und auf Entwicklungsstadien derselben. Arch. f. Ent- wick., vol. 27, 1906. Maas, O. Zur Entwicklung der Tetractinelliden. Verh. der Deutschen Zool. - Gesell., 1909. . Marshall, W. The reproduction of Spongilla lacustris. Sitzungsber. Naturforsch. Gesell. Leipzig, 1884. Minchin, E. Note on the Larva and the Post-larval Development of Lewcosolenia variabilis, with Remarks on the Development of other Asconidae. Proc. Roy. Soe., vol. 60, 1896. Minchin, E. The Porifera. Lankester’s Treatise on Zoology, vol. iii., 1900. Schulze, F. E. Untersuchungen itiber den Bau und die Entwicklung der Spongien I. Mitt. Uber den Bau und die Entwicklung von Sycandre raphanus. Zeit. fiir wiss. Zool., vol. 25, Suppl. 1875. CHAPTER IV COELENTERATA Classification adopted— Hydrida Hydromedusae... ee ae rae Hydrozoa yy eA y ope a pes ROR = , i l i rn I i bh 5 if Fic. 96.—Two views of advanced Pilidium larva of Cerebratulus lacteus to show the development of the muscles. (After Wilson. ) A, viewed as a transparent object. .B, surface view. am, anterior amniotic invagination ; musc.per, so-called peritoneal muscles ; ret, retractor of the apical plate. On the posterior wall of the oesophagus a groove for conducting food appears, and numerous gland cells appear all over its wall. 122 INVERTEBRATA CHAP. Running round the edge of the larva under the prototroch a nerve- ring has been detected. The development of the Pilidium is now complete, and it swims about at the surface of the sea feeding on microscopic organisms which are whisked into its mouth by the action of its cilia. After Fic. 97.—A Pilidium larva shortly before its metamorphosis, (After Metschnikoff. ) Letters as before. In addition, a.im, anterior imaginal disc ; ¢.s, rudiment of cephalic slit ; pr, rudiment of proboscis ; p.im, posterior imaginal disc ; oes, oesophagus. about two weeks it begins its metamorphosis. This has been described by Metschnikoff (1869) and Salensky (1886). On the flattened under side four ciliated invaginations of the ectoderm are formed. These are termed the amniotic invaginations, and their deeper portions are the imaginal discs. Two of these invaginations are situated opposite one another on the right and left sides of the animal respectively, in front of the mouth, and two others are similarly situated behind the mouth. Each of them grows and deepens, extending upwards over the surface of the globular VI NEMERTINEA 123 stomach. Finally they meet one another, fuse and coalesce, the anterior and posterior on each side and the right and left on each side. The imaginal discs form the skin of the future worm whilst the outer walls of the coalesced invaginations form a temporary envelope known as the amnion. Before coalescence is quite complete the organs of the future worm are constructed, and as to the manner in which this is accomplished we have tantalizingly little information. It appears from Salensky’s account (1886) that the skin of the anterior part of the animal, as far back as the cephalic slits, originates from the anterior imaginal discs. The posterior imaginal discs form the skin of the hinder part of the body of the worm. The characteristic proboscis is formed as an ectodermal invagination. The proboscis sheath originates as a solid O0es Fic. 98.—Longitudinal section through a Pilidium larva of about the age of that represented in Fig. 97. (After Salensky.) br, rudiment of brain; sh, rudiment of sheath of proboscis. mass of mesoderm into which the proboscis invagination projects (Fig. 99). This mesoderm appears to be in close proximity to the ectodermal wall of the posterior imaginal disc on each side and possibly arises from it. Later the rudiment of the sheath becomes hollowed out and forms a sac lined by flattened cells and filled with fluid. The adult brain (dr, Fig. 99) arises as a thickening of the ectoderm of the anterior imaginal discs. The cephalic slits likewise arise as ectodermal ingrowths, not from the imaginal discs but from the larval ectoderm between anterior and posterior discs, and pouches grow out from the oesophagus to meet them (0e.p, Fig. 99 A). An anus must be formed, but as to how or when we have no information. In fact nearly all our information about this period of development is based on the examination, as whole objects, of larvae fished from the ‘sea, although Salensky has to some extent applied the method of sections. If once an appropriate food for the Pilidium larvae could be 124 © INVERTEBRATA CHAP. discovered, so that these larvae could be reared in large numbers through their metamorphosis, under experimental conditions, and if each stage in this change were thoroughly examined by sections, then a flood of much-needed light would be thrown on this period of Nemertine development. If the reader has followed the description so far given it will be evident that when all four amniotic invaginations completely coalesce they must cut the larva into an upper and a lower half. This is just what happens; and the lower and inner half, invested by the coalesced floors of the amniotic invaginations, and containing the alimentary canal, drops to the bottom of the sea and commences life as a young A Fic. 99,—Two stages in the development of the Nemertine rudiment within the Pilidium, viewed from above. (After Salensky.) e.s, cephalic slits ; 0, mouth ; oes.p, oesophageal pockets. Nemertine worm. The upper half consisting of the larval ectoderm, including the prototroch, lappets, and apical sense organ, and bounded inside by the coalesced roofs of the invaginations or amniotic invest- ment, continues to swim about for a little time before its energies are exhausted, and then it dies. EXPERIMENTAL WORK. E. B. Wilson and his pupils Yatsu and Zeleny have performed a most interesting series of experiments on the eggs and embryos of Cerebratulus, the general results of which may be shortly recounted here. The unfertilized egg was cut or shaken into fragments. If this be done before the membrane of the nucleus has disappeared, and if sperm be added to the fragments, only the fragment in which the VI NEMERTINEA 125 nucleus is situated develops intoa larva. But if the same experiment be performed after the nuclear membrane has faded, all the fragments will develop into larvae. It is, therefore, obvious, that when the nuclear membrane fades, some substance must pass into the cytoplasm which confers on any fragment of it the power to develop into a larva if a spermatozoon be added to it. If the same experiment be performed after normal fertilization has occurred, only the fragment containing the first invading spermatozoon will develop. All attempts to fertilize the other fragments by adding fresh spermatozoa failed. In the majority of cases the developing fragment is the one containing the zygote nucleus; but in some cases, when the frag- mentation of the egg had occurred before the spermatozoon had reached the nucleus, it is the fragment containing the spermatozoon and not that containing the nucleus which develops, while the fragment containing the latter can be seen to form the polar bodies, but it goes no farther in development. Therefore, just as some substance must exude from the egg nucleus’ which confers on all the cytoplasm the power to form a larva, so we are bound to conclude that some material is given off from the sperm head which inhibits development in the cytoplasm, except when under the influence of the first nucleus. When the egg was cut into fragments, however, and a piece was induced to develop, it gave rise to a perfect Pilidium larva of correspondingly reduced size. The segmentation occurred as in the normal larva, though the blastomeres were correspondingly smaller. But when the first two blastomeres of a normal egg were separated from one another, each divided as if it still formed part of the whole egg,—it formed two macromeres and two micromeres. The separation was effected by exposing the developing eggs to the influence of artificial sea-water, made up so as to entirely exclude lime; such water causes the blastomeres to lose their adhesion to one another and to fall apart, owing apparently to an alteration in the physical characteristics of the outermost layer of the cytoplasm. The separated blastomeres are then restored to normal sea-water and allowed to continue their development. When one of the first four blastomeres is separated it forms one macromere and one micromere by the first division, and continues to segment as if it formed one-fourth of the egg. Nevertheless in both these cases the half or quarter blastula closes its wound by narrowing and contraction of the edges, and develops into a Pilidium which is perfectly normal but of reduced size. The Pilidium which develops from one of the first four blastomeres, however, has its apical plate displaced forwards, a change which is probably due to the size of the cells, derived from the segmentation of the blastomeres, remaining the same as if they still formed part of a whole egg. Each cell has therefore to form a part of the larva proportionally four times as great as it would normally have done, and so it must be subjected to 126 INVERTEBRATA CHAP, much more severe curvature than usual, and these curvatures produce a series of strains which distort the resulting larva. A cell of the 8-cell stage is incapable of developing into a Pilidium. When the 8-cell stage is broken in two, its two constituent portions, viz. the macromeres and the first quartette of micromeres, each group of four cells can develop intoa larva. But the micromeric group form a larva with a very large apical sense-organ and no gut, whilst the macromeric group develops into a larva with an enormous gut and no apical organ. The same result is obtained by cutting the blastula along the equator, in this case the upper half forms a larva with enormous apical organ and vestigial gut, whilst the lower half forms a larva with large gut and no apical organ. Hence we conclude that whereas every one of the first four blastomeres contains all the substances necessary to form a perfect larva, after the occurrence of the third cleavage the substance necessary for the formation of the gut is restricted to the lower cells, whilst that destined to form the apical organ is confined to the upper four cells. Yatsu found that when the fertilized egg is cut into fragments abnormal Pilidia are produced, except where only a small fragment from the animal pole has been removed, and hence he concludes that the material destined to form the apical plate is situated not at the animal pole but in a ring a short distance beneath it. When we review the results of these experiments we are struck with the demonstration which they afford of the influence of the materials given off from the nuclei on the cytoplasm, and also with the proof that at the moment when sperm and egg nuclei approach one another a definite structure or arrangement of organogenetic materials is impressed on the cytoplasm. The outward and visible sign of this inward process may be the radiations which extend from the sperm nucleus outwards. This conclusion will be supported by evidence to which we shall call attention during our study of various . other invertebrate groups. The structure impressed on the cytoplasm reminds us of what was found to be the case with the Ctenophore egg, but it is not so definitely specialized as in the Ctenophore egg. In this respect the egg of the Nemertine occupies an intermediate position between the egg of the Hydromedusan and the egg of the - Ctenophore. AFFINITIES OF NEMERTINEA. We now approach the final question as'to what light the develop- ment of Cerebratulus throws on the ancestry of the Nemertinea as a whole. This question resolves itself into the problem: What is the ancestral significance of the Pilidium larva? We have to interpret a larva with a simple sac-like gut, opening by a mouth at its lower pole, whilst its upper pole is occupied by a cup-like sense-organ carrying long stiff cilia, and its locomotion is effected by a lobed band of cilia. Just as in the case of Miiller’s larva we are again reminded of a primitive Ctenophore. Miiller’s larva does not carry the apical tuft VI NEMERTINEA Lay of hairs, and in this respect is less like a Ctenophore than the Pilidium larva; but in having its ciliated band produced into eight processes instead of two, it is more like a Ctenophore than the Pilidium. We probably shall not go far astray in concluding that the Pilidium represents a free-swimming ancestor of the Nemertinea, belonging to the same great group as that containing the ancestor of the Ctenophores, but differing from the latter as a shark differs from a salmon, whilst both are fish. The metamorphosis into the Nemertine worm must be regarded as the immensely shortened recapitulation of the long development which occurred before this ancestor developed into a Nemertine; a development which must have been much longer than that which was necessary to convert the ancestor denoted by Miiller’s larva into the Polyclade worm. Proof will be given as our studies proceed, that such a cataclysmic metamorphosis as that of the Nemertine has been secondarily derived from a type of development that was originally slow and gradual. Nevertheless, as we have indicated above, if this meta- morphosis were thoroughly studied in detail we should know a great deal more about the steps by which that change was accomplished than we do at present. LITERATURE REFERRED TO Biirger. Die Nemertinen. Fauna u. Flora des Golfes von Neapel, vol. 22, 1895. _Metschnikoff. Studien iiber die Entwicklung der Echinodermon und Nemertinen. Mem. Acad. St. Petersb., series 7, vol. 14, 1869. Salensky. Bau und Metamorphose des Pilidium. Zeit. fiir wiss. Zool., vol. 43, 1886. Wilson, C.B. The Habit and Early Development of Cerebratulus lacteus. Quart. Journ. Micr. Sci., vol. 43, 1900. . ; Wilson. E. B. Experiments on Cleavage and Localization in the Nemertine Egg. Arch. Ent-Mech., vol. 16, 1903. ; : Yatsu. Experiments on the Development of Egg Fragments in Cerebratulus. Biol. * Bull., vol. 6, 1904. i : Zeleny. Experiments on the Localization of Developmental Factors in the Nemertine Egg. Journ. Exp. Zool., vol. 1, 1904. CHAPTER VII ANNELIDA Classification adopted Archiannelida Nereidiformia Spioniformia Terebelliformia Polychaeta, Capitelliformia | Scoleciformia Scabelliformia Hermelliformia Chaetopoda pene Acanthobdellidae Hirudinea + Rhyncobdellidae Gnathobdellidae THE group of segmented worms known as Annelida has furnished subjects for an immense amount of embryological study, but there are a great many points in their development still unsettled which offer a wide field for future research. Although widely diverse from each other in their adult structure the members of the group show a remarkable uniformity in their early development, so that the complete description of a single type will serve as a guide to what is known about the development of all. Annelida are divided into Archiannelida, including Polygordius and a few allied forms which never develop chaetae and are devoid of external circular muscles; Polychaeta, the central group, including worms with numerous chaetae, well-developed parapodia, and external circular muscles; Oligochaeta, freshwater and terrestrial worms, with few chaetae, complicated genital organs, no parapodia, but provided with external circular muscles; and Hirudinea, extremely modified forms with obscure segmentation, no chaetae or parapodia, but with external circular muscles, extremely complicated genital organs, and suckers used for progression. Of these forms the most primitive, and the one which shows the longest larval development, is the Archiannelidan Polygordius. The 128 CHAP. VII ANNELIDA 129 embryology of this form has been worked out in great detail recently by Woltereck (1902, 1903, 1905), and we select it as type for special description. As, however, although Polygordius occurs on both sides of the Atlantic and in both North Sea and Mediterranean, it is not very abundant or easy to obtain, some practical directions will be given as to the means of dealing with the eggs of Pomatoceros, a very common Polychaete belonging to the family Serpulidae. The develop- ment of Pomatoceros, in the early stages at least, is almost identical with that of Polygordius, and in one or two points even more primitive. The eggs of all Annelida undergo cleavage of the spiral type, which we have already studied in the case of the Platyhelminth Planocera. In Annelida, as in Planocera, the ectoderm is separated as three successive quartettes of micromeres. As in Planocera also, a blastula consisting of relatively few cells is formed, which, by invagination or epibole (see p. 92) is converted into a gastrula. METHODS Now for the study of such eggs the method of sections is of very little use. This method requires that the egg to be studied should consist of a large number of similar cells, so that a‘ sample such as a section presents would give a good idea of the whole; but where the egg consists of relatively few cells and these are in- dividualized at an early stage of development the method obviously fails. So there is nothing left but to make whole mounts and endeavour (as Surface did in the case of Planocera) to identify and trace the history of each individual blastomere. This procedure, as already mentioned (Chap. V. p. 104), is termed the study of Cell-lineage; it was introduced by the American zoologist Whitman (1878), who first employed it in the study of the eggs of Hirudinea, and it was taken up by a brilliant school, which ™ Whitman founded, one of the most prominent of which was Prof. E. B. Wilson (1892). Prof. Wilson applied the method to the study of the development of the Polychaete Nereis, a work which threw much light on Annelidan embryology. Other pieces of work of equal merit were those of Treadwell on Podarke (1901) and Child on Arenicola (1910). If, nevertheless, we select the work of a German for special description, when the credit of most of the investigations belongs to Americans, it is solely because the development of the type on which _he worked is so primitive and simple that, once it is known, all the others can easily be described in terms of it. In order that the cells may be identified in whole mounts of eggs, it is necessary that these should be rendered transparent, and that they should be examined from all sides. As the eggs of many species are opaque owing to the fact that they contain numerous yolk grains, this is not easy to do. Prof. E. B. Wilson employs a mixture of 3 parts of glacial acetic acid and 1 part glycerine. This mixture in many eggs dissolves the yolk granules and makes the whole of a VOL, I K 130 INVERTEBRATA CHAP. glassy transparency. The preparations made in this way, however, are not permanent, but they last long enough to enable good draw- ings to be made. Other authors make permanent preparations by preserving the eggs in “Hisig’s mixture,” @.e. 3 parts of saturated aqueous solution of corrosive sublimate and 1 part of glacial acetic acid, staining with haematoxylin and trusting, after dehydration by alcohol, to oil of cloves to clear them sufficiently to allow of complete examination. In order to examine the eggs from all sides Wilson rolls them about on the slide by moving the coverslip, which he supports on feet made of a mixture of beeswax and vaseline, the proper height of which can be ascertained by trial. Other workers attain the same end by introducing between slide and coverslip a piece of thin capillary glass rod ° or tube, drawn out to the requisite degree of tenuity. When the segmenta- tion is completed the embryo issues from the egg membrane and com- mences to lead a free life asa larva. The form of this larva resembles in broad outline the form of the Pilidium. Like it, it possesses an apical plate with a tuft of long cilia and a prototrochal girdle, and it is called a Trochophore. Fre. 100.—The Trochophore Larva of Polygordius, The Trocho p hore viewed from the side. (After Woltereck. ) differs from the Pilidium wit ata nmi pi pebaanen. suai in possessing an intestine t.tr, tetotroch. terminating In an anus, and in having a_post- trochal region of the body which projects behind the prototroch, instead of having only a concave surface in this position such as is found in the Pilidium. By the gradual growth and elongation of the post-trochal region, the body of the worm is formed. In the case of the Serpulid Pomatoceros the eggs and sperm are easily obtained by simply extracting the animals from their tubes and placing them in clean sea-water. If the genital cells are ripe they VII ANNELIDA 131 will immediately be shed, and in this way a natural fertilization of the eggs is accomplished. The Trochophore issues on the second day and rises to the top of the water. It can be reared through its entire development by supplying it with a pure culture of the diatom Mitschia. Such pure cultures can be obtained from Dr. Allen, Director of the Marine Biological Station at Plymouth, and they serve as pabulum for many different kinds of larvae. Pure diatom cultures were obtained originally by isolating under the microscope a single individual of the species of diatom desired, and then transferring it to a flask of sterilized and filtered sea-water. The sea-water is first shaken up with animal charcoal and decanted in order to remove all soluble toxins, and then passed through a Berkfeldt stone filter, which removes all organisms, even bacteria. To the sea-water is now added a certain amount of Miguel’s solution, about 2 drops per 100 c.c. of water, and the flask is stopped by a plug of sterilized cotton wool. In a month’s time a copious growth of the desired diatom is obtained. If a pipette-full of such a culture be added to an evaporating dish containing the larvae of Pomatoceros, these will develop normally and eventually metamorphose into the adult worms, which form tubes and attach themselves to the sides of the glass. In this way the whole life-cycle can be controlled, and such larvae can be examined living, or mounted whole, or examined by sections. The fixative found best is Hisig’s mixture (see ante). The methods of orientating, embedding, and cutting have been fully described in Chapter IT. POLYGORDIUS. CELL-LINEAGE Returning now to Polygordius we should remind the student that this is a minute worm which burrows in mud and sand. The eggs are excessively minute and very transparent, and the segmentation is remarkable for its extreme regularity. The eggs are dehisced into the sea by the breaking up of the parent’s body and are fertilized there. Up to the 64-cell stage all the cells divide at the same time, so that we have successive “cleavages ” which successively divide the egg into 2, 4, 8, 16, 32, and 64 cells, that, is six cleavages in all. More- over a 128-cell stage is very nearly realized, for all the cells of the 64-cell stage divide nearly synchronously, except those forming the prototroch and a few others of the upper hemisphere, which, having reached the summit of their development, divide no more. The macromeres are all precisely equal in size; it is therefore at first impossible to discriminate an A from a B, a C, or a D (see Chapter V.), but in the later cleavage stages this can be done, owing to the different way in which members of the second and third quartettes of micromeres, given off from the different macromeres, behave. Shortly after the 64-cell stage has been reached cilia appear on the cells destined to form the prototroch (Fig. 101, B). The 132 INVERTEBRATA CHAP. embryo then begins to rotate within the vitelline membrane, which it soon ruptures, and it then begins its free-swimming existence. Fic. 101.—Stages in development of the blastula of Polygordius seen in optical longitudinal section. A, 32-cell stage; B, 64-cell stage; C, 76-cell stage; D, 116-cell stage. Letters as before. In addition, mac, residual macromeres ; p, polar bodies ; v, vacuoles in cells forming the prototroch ; vit, vitelline membrane ; 1q11, the apical cells ; 1g112, the mother cells of the Annelidan cross. When about 140 cells have been formed the fully-segmented egg constitutes a thick-walled, extremely flattened blastula which is con- VII ANNELIDA 133 verted into a gastrula, and then into a Trochophore, by rearrange- ments of cells, without further cell-division. Further divisions of ‘oa only occur after the Trochophore has been feeding for some ime. Up to the 64-cell stage then, all the cells of the egg divide simultaneously, so that the first micromeres divide once as the second quartette of micromeres is given off. When the division to form the third quartette of micromeres is complete, the first quartette have divided twice; and when the fourth quartette, which gives rise to mesoderm and endoderm, is formed, they have divided thrice, and with this division the 64-cell stage is attained. The egg, which from the 16-cell stage had taken on the form of a hollow spherical blastula, now begins to flatten out. In the same period the second quartette of micromeres has divided twice and the third quartette once. At the next cleavage the cells of the prototroch fail to divide and so do certain other cells, descendants of the first group of micromeres, but all the other cells of the egg divide. A fifth quartette is given off, which is destined to form endoderm only, and the residual macromeres are now barely if at all larger than the micromeres to which they gave rise. The flattening of the embryo continues till it assumes the form of a flattened plate (Fig. 101, D). The next cleavage is participated in only by the macromeres, which divide not spirally but symmetrically with regard to the future median plane of the embryo. The fifth quartette of micromeres also divide, so do the fourth, and some cells of the second and third; and then invagination commences. In describing in detail the divisions of the cells it is most convenient to deal with the different quartettes of micromeres separately. Since, as has been already stated, it is impossible in the earlier divisions to distinguish one macromere from another, it is convenient to be able to refer to them collectively, and the letter q is used to denote a, b, c, and d. In all four quadrants of the egg the divisions of the first quartette of micromeres are exactly alike. We say that 1q divides into upper cells 1q!, mothers of the whole upper hemisphere of the Trochophore above the prototroch, and into lower cells 1, the mothers of the prototroch; 1q! divides again into upper cells 1q", mothers of the apical plate, or as it is sometimes termed the rosette, and of a St. Andrew’s cross of cells radiating from it, called the “ Annelidan cross” because it is conspicuous in the eggs of all Annelids ; and into lower cells 1q'?, mothers of a group of cells called by Wilson the intermediate girdle cells but termed by other authors the Molluscan cross, because in Molluscan eggs these cells take on the form of a conspicuous upright cross. 1q? divides into 1q”, and 1q22, one set of cells lying obliquely above the other. — ; At the next cleavage 1q!! divides into 1q", forming the apical cells or rosette which carry the tuft of long stiff cilia which con- 134 INVERTEBRATA CHAP. stitutes the apical sense-organ, and into lq", which form the rudi- ment of the Annelidan cross. 1q! bud off cells between them- selves and the apical cells, and in this way the arms of the cross are formed, that is to say, 1q!!? divide into 1q™”! and 1q™" Turning now to the other cells of the upper hemisphere we find that 1q!” behaves similarly. It divides into 1q™! and 1q™, and 1q!2! further divides into 1q!%4 and 1q¥% These three cells in each quadrant, 1q!”, 1q!”!2, and 1q!?4, are in four curved series, and this is also true of primitive Mollusca, but in higher Mollusca they are arranged in four straight lines and form the upright cross mentioned above. Fic. 102.—Dorsal view of upper hemisphere of egg of Polygordius, in which seventy-six cells have been formed. The rosette cells and the cells of the prototroch are left clear. The cells of the ‘‘ Molluscan” cross are cross-hatched. Those of the ‘‘ Annelidan” cross are marked with circles. The group of cells 1q?! and 1q” each divide into two sets of cells, so that we have four daughters of 1q? in each quadrant; and these sixteen cells acquire long powerful cilia and constitute the prototroch. They become large, clear, and vacuolated, and at first—and this is most interesting and important—they form four discrete groups; it is only later that these groups coalesce to form a complete ring. If this description has been followed it will be seen that the upper hemisphere of the egg, including the prototroch, when divisions temporarily cease, consists of forty cells. In the 64-cell stage it consists of course of thirty-two cells, since it forms exactly half the egg, but the cells constituting the “Annelidan” and “Molluscan ” crosses divide once again, and this brings the total number of cells up to forty. VII ANNELIDA 135 When we turn the flattened embryo over and view it from the vegetative pole, we are able, once the 64-cell [stage has been passed, to distinguish the various quadrants of the egg from one another, and to tell which is A and which is B, which C and which D, Now the embryo has a somewhat squarish outline and the rounded corners are formed by the groups of prototrochal cells belonging to the first quartette. These groups are directly opposite the respective macromeres from which they arose; for, if we take the group derived from A for example, all its members are daughters of la? and ultimately of la. But 1a itself was given off dexiotropically from A; that is to say, lay above and to the right of it. The next division is a laeotropic one, that is to say, as the name implies, Lal, the upper daughter, lies above and to the left of the lower daughter la’, Now this formation of the spindle with a left bend has the effect of causing 1a? itself to pass somewhat to the left, and thus undo to a certain extent its original right-hand twist, so that it eventually comes almost exactly opposite A. The line joining the prototrochal group of cells and the macromeres constitutes a radius of the figure, and cells or cell groups lying on this radius are said to be radial, and cells or cell groups alternating with them are said to be inter-radial. Now it is found that the third and fifth quartettes of micromeres are radial whereas the second and fourth are inter-radial. The following rule then is found to hold for the fate of cells forming these quartettes. In the radial quartettes the cells in quadrants A and B behave alike, but the cells in C and D while behaving like each other behave differently from those in A and B. In the inter-radial quartettes the cells in quadrants in A, B, and C, behave alike, but the cells in quadrant D behave differently from each of them. Turning our attention now to the second quartette, and taking a as example for a, b, ete. we find that 2a divides into 2a and 2a?, one directly above the other. Each then divides laeotropically into 2a" and 2a}, and 2a” and 2a, respectively, thus forming a lozenge- shaped group of four cells. Of these four the outermost divides radially into 2a} and 2a”, lying almost side by side; whilst 2a? and 2a! divide obliquely into 2a? and 2a}, 2a? and 2a?! respectively. These cells help to form a belt of flat clear cells lying just beneath the prototroch, and they divide no more till the Trochophore stage is reached. The fate of the innermost cell 2a” is different; it divides by a tangential cleavage into an outer cell 2a"! and an inner cell 2a”. Then each of these divides into a larger anterior cell 2a! and 2a? respectively and a small posterior cell 2a?2p and 2aP respectively. Exactly the same division takes place in quadrant C, and then the small posterior cells sink into the blastocoele and form that part of the mesectoderm or larval mesoderm, which will eventually give rise to the circular and radiating muscles of the larval oesophagus. In quadrant B, however, 2b”! divides into an outer and upper 136 INVERTEBRATA CHAP, cell 2b241, and an inner and lower cell 2b”?!2 whilst 2b?” divides into two sisters lying side by side, viz. 2b?" and 2b. Now these four cells in quadrant B, and the two cells in quadrant A, viz. 2a!* and 13°22 Qype pe A 2 a??? A Ae 3a! 3 im) a niles Bate 12 ENO z= I 3h? 3512 2ap2a 221 52 5b se 36 Op! 2b Qp2? | \” [ de Wes = Jo) Op. sae - 78) i a < 22! ‘ CBE 2 We OK~2 ay : PAX ES Oe a | septal" ce \ 2d" 72" 4a” 222 Ad! Fic. 103.—(Cuntinued on opposite page.) 2a? and in quadrant C, viz. 2c?" and 2c” (Fig. 103, C), are destined to form the stomodaeum, but for the complete history of this structure we must wait until we have considered the history of the third quartette of micromeres. VII _ ANNELIDA 137 In the quadrant D the divisions at this stage are similar, but 2d’ and 2d”! do not divide until the Trochophore stage is reached. It follows that the so-called larval mesoderm is formed from the second quartette in three of the four quadrants of the egg. We now pass to the consideration of the third quartette, and we would remind our readers that this quartette is radially situated, whereas the one we have just been considering was inter-radial; and C Qprer Qpe2ei2 3a'2 > emerges a4 sal | f DAT ot ose e: 3c2pp Bq?" Fic. 103 (continued).—Three stages in the segmentation of the lower or vegetative surface of the egg of Polygordius. A, stage of about 76 cells; B, stage of about 112 cells; C, later stage in which a mass of rapidly dividing cells at the lower pole is sharply distinguished from an outer zone of clear cells. The heavy black iine surrounds the cells which later will take part in the process of invagination and the formation of the lips of the blastopore. The cells belonging to the second quartette are dotted, those belonging to the third quartette are marked by vertical lines. The cells belonging to the fourth quartette are marked by little circles, those belonging to the fifth quartette by horizontal lines. The residual macromeres, and those belonging to the first quartette, are left white. The names of the cells which form the larval mesoderm are surrounded by circles. Cf. (2a2Ip). further, as in eggs with spiral cleavage in general, the second and third quartettes of micromeres come to lie about the same parallel of latitude, so to speak, on the globe represented by the whole egg, since the quadrants of one quartette occupy the gaps between the quadrants of the other. Taking then the quadrant A first (and what applies to A is true also of B) we find that 3a divides into 3a! and 3a”; and each of these now divides into an anterior and posterior cell, i.e. 3a!4, 3a1P, 3a74 and 3a?, respectively. Of these the first two 3a! and 3a/?, remain 138 INVERTEBRATA CHAP, undivided and help to complete the belt of broad flat cells, the other parts of which are formed by the cells of the second quartette, to which allusion has already been made. The other two cells each divide into an anterior outer large cell, 3a"! and 3a7P1, respectively, and a posterior inner smaller cell, 3a”? and 3a7P?, respectively. The last two eventually sink into the blastocoele and help, like the similar cells of the second quartette, to form larval mesoderm, whilst their two larger sisters enter into the formation of the stomodaeum. Weare now able to take a more general survey of the cells which enter into the formation of this structure. The front wall of the stomodaeum is formed by the four cells 2b??44, 2b?!?2, 2b?" and 2b?#l, Its right side is constituted by the cells 2c?#* and 2c”, and its left side by the corresponding cells 2a?244 and 2a”r, In its right anterior corner we find the cells 3b”! and 3b??!, in its left anterior corner the corresponding cells 3a741 and 3a?Pl. In the quadrants C and D the micromeres of the third quartette divide, at first, similarly to those belonging to quadrant A, B. Thus, taking 3d for example (and remembering that all said about it is equally true of 3c), we find that it divides into 3d! and 3d?, and each divides into anterior and posterior cells: of these 3d! and 3d!P become broad and flat and remain undivided, and thus complete the band of this kind of cell right round the egg; though at a later period, as we shall see, they form part of the posterior lip of the mouth. 3d divides into 3d! and 3d, and these cells will help to complete the hinder wall of the stomodaeum in a manner to be described later. 3d’, however, divides into an anterior and a posterior cell, 3d?P* and 3d°PP, and both these cells undergo another similar division, so that we get an antero-posterior directed line of four cells, 3dP44, 3d?Pap, 3d2pP, and 3d?pPP (Fig. 103, C). The last two of these cells constitute the rudiment of one of the larval kidneys or archinephridia, the other being formed by the corresponding cells in quadrant C. The more anterior cell of each pair at a later stage sinks into the blastocoele and is transformed into a flame cell or solenocyte, with a cavity and a tuft of cilia waving within it; whereas 3d?eep forms the excretory tube and remains in connection with the ectoderm (Fig. 106). Passing now to the fourth quartette we find that all four cells divide radially, each giving rise to two daughters lying side by side; so that we have 4a? and 4a, 4b" and 4b!, 4c and 4cP, and 4d" and 4d', as we pass round the egg. Of these all but 4d" and 4d! enter into the formation of the gut wall: the last named will eventually give rise to those longitudinal streaks of cells known as the mesodermic or germinal bands. These bands will eventually become hollowed out to form the coelom or true body-cavity, the walls of which constitute the adult mesoderm. The fifth quartette divides also at first evenly, 5a into 5a! and 5a, 5b into 5b! and 5b?, 5c into 5c! and 5c?, and 5d into 5d! and 5d2. In quadrants A and B the division stops, but it goes on in quadrants VII ANNELIDA 139 Cand D; in these 5c! and 5c? divide into 5c! and 5e!2, and into 5c and 5c”, respectively ; and the same thing happens to 5d! and 5d?. This greater growth in the hinder quadrants of the egg, which occurs both in the 3rd and 5th quartettes, has the effect of pressing the pairs of cells 5a1 and 5a?, and 5b! and 5b’, out of their original arrangement of two lines converging to the lower vegetative pole of the egg, into a position of two lines parallel to one another, and they will eventually form the sides of the mid-gut (Fig. 106). ; The residual macromeres 5A, 5B, 5C, and 5D, lastly divide, each into two equal daughters; 5C and 5A into anterior and posterior cells, 5B into right and left cells by radial divisions; but 5D divides into an inner and an outer cell 5D! and 5D?, and the outer 5D! divides again into 5D™ and 5D”, so that here, as in the fifth quartette, we have increased multiplication of cells in the hinder part of the egg. These divisions complete all the divisions of cells which take place in the flattened plate-like blastula. We have, as we have already seen, 40 cells of the first quartette. Of the second we have 8 stomodeal, 4 larval mesoderm, and 26 cells forming the belt of flattened cells; 7e¢. 38 in all. Of the third quartette 8 enter into the formation of the stomodaeum, and 4 form larval mesoderm, 4 form larval kidneys, and 4 form ventral ectoderm (3c7P* and 3cP, + 3d2Pa# and 3d?p4P, respectively), making 28 in all. The fourth quartette contains only 8 cells, the fifth 12, and there are 9 residual macromeres. So that the grand total of all the cells at this stage is, 40 + 38+ 284+8+124+9; ae 135 in all. At this point of development invagination of the cells of the lower surface begins, and the blastula is converted into a gastrula which, in virtue of its apical plate and its four groups of prototrochal cells, may be already termed a Trochophore. We shall now study how invagination is brought about. The nine residual macromeres form a plate at the vegetative pole. The two cells forming the centre of this plate, namely 5D? and 5D”, rise upwards into the blastocoele, in consequence no doubt of altered chemical conditions here, that is, of altered cytotaxis. As the centre of the plate thus sinks in, two lateral ridges of cells become prominent and outline the edges of the indentation so formed; in a word they outline the blastopore. These lateral ridges are, on the right side, 5b!, 5b?, 4c*, 5c, and 5c"; on the left side 5a, 5a?, 4a? 5d¥ and 5d). . The blastopore takes on the form of an oval opening, elongated in an antero-posterior direction. The front of the blastopore is formed by the cells 4b" and 4b!, and the hinder end at this stage by the cells 4d and 4d. The centre cell in each row (4a? and 4c*) approaches its opposite partner and so the ridge of the oval is converted into a figure of eight. These two latter cells finally meet one another and the oval opening is thus cut into two openings, the primitive mouth and the primitive anus respectively. The primitive mouth persists, but the primitive anus is temporarily 140 INVERTEBRATA CHAP. VII closed by the union of the four cells 5c!, 5d¥, 5c™, and 5d respectively. At a later period of development, however, the permanent anus re-opens at the same spot, so that the temporary closure is an event of no importance. . We have thus the problem solved before our eyes how, out of a single primitive opening used both for injestion and egestion or defaeca- tion, such as we find in Coelenterata and Platyhelminthes, separate openings for injestion and egestion were formed. When the primitive anus is closed the blind end of the gut remains in close contact with the cells 4d‘ and 4d. Two outer columns of cells parallel to the first two are then formed. These consist on the right side of 2c?4, 2c?8p, 30742, and 3c?P, and on the left side of 222%, 2a222p, 3d282, and 3d%94, The hinder cells of these - two outer ridges also meet; 7.e. first 3c”? and 3d”, then 2a?” and 3c%”P, and lastly 3c?p24 and 3d?°**; but their front cells, 2c?” and 2a”, do not meet; they, as we have already seen, help to form the sides of the stomodaeum (Fig. 104, B). As these outer columns of ectodermal cells meet, the endodermic . pouch shrinks away from them and leaves a blastocoelic space between it and them; so that the process of closing the ventral wall of the gut is completed before the ventral ectoderm is complete, and thus, for a brief moment, the blastocoele is actually in open com- munication with the external world (Fig. 105, C). The final closing of the outer part of the blastopore is effected by the rotation inwards and backwards of the cells 3c?P* and 3d?P4, These cells rotate through an angle of 180°, and so come to lie actually within and behind the cells 3c?P@P and 3d?pp (Fig. 104, D). It is at this stage of development that the cells 3c?PP? and 3d*Pp* wander like amoebocytes into the blastocoele and form the solenocytes of the two archinephridia. The pre-anal tuft of cilia, the telotroch, is formed by the cell 3d%P2*, The lower'lip of the large mouth is formed by the cells 3c'4, 3d!, 3e%1, 3d?1, 302, 3d%?, which swing through a right angle to occupy that position. The cells 3c!?, 3d'P, become elongated in an antero-posterior direction, acquire short cilia, and form the metatroch, 7.¢. the circular band of feebler cilia, which runs parallel to the prototroch behind the mouth (Fig. 104, D). Turning our attention now to the second quartette in quadrant D, we find that the cell 2d222 wanders like an amoeba over the ventral surface of 4d" and 4d!. Each of these cells has by this time budded off a small anterior cell, 4d"? and 4d", which is the beginning of the adult mesoderm on each side. We find now, when we look at the under side of the Trochophore behind the mouth, two large thin plate-like cells 3c?p¢P and 3d?”P in front. These constitute what Woltereck calls the hyposphere, or under surface of the almost spherical larva. To the sides of these, lie 3c?PPP and 3d?PPP, the tubal cells of the archinephridia. Behind them are a group of three compact cells covering the adult mesoderm ; and these cells which, for reasons to be explained later, we call the *euleysetq yunIy “gL $qooryope} ‘u7'2 $snue saryrurad ‘od fygnou ‘w f yooryeyom ‘up-w fsnuy yearel “v7 { erodoqseq ‘d7q Sumpiuydeurore ‘ap f ysnoryy syvolq snue queued ayy er1atTA uoltsod ‘9 ‘gq pue Vy sjyuvipenb ul eyjoqrenb pay} ey} 07 pue ‘gq yuvipenb ur 04j9j1enb ptodes el[} 0} Sulsuofeq s]jso poweqqyey UA PIG St o}YM ay eovds oy, ‘qd pue ‘Oo ‘¥ syuvipenb oy Ur eqqeqzenb puocoes ay} 09 Sursuojeq s]j9o0 peueyyep squesetder indy ey} punor sjop esieds Jo a[pals sy. ‘ainSy Surpsoeid ul sv peysmsuly -sIp ale soqqjeyienub quareyip eb “paqeurseaul ewulodeqd aABYy sI[9d [eopowio}s YOIYA UL pure ‘paulIoj ere YooryepoUL pUe YOOIOTe4 YOY UL aSeqs ‘q ‘pesoyo sI snue said YOY ULVeYS ‘OC ‘eedgpg PUL vedgdg ST[E9 OY} JO WOTZV}OL JO UOTZIOITp otf} AOYS SMOIIB oY, ‘snue satyuTId puy yynour earywd o7Ul peplarp euooeq «sey «arodoqseiq = eZOTAL eseys ‘gq “qyste jo einSg v exI] padvys e1odoysetq WIM o8e4s ‘W ( poytpout ‘yooreyTo MA Jeqzy) “eaodo4 -SB[q 9} JO oNsOpO pue TOT} -eMof JO pue woTyepnayses jo sassoooid oy} MOYs 04 snipsobhjog jo 38a surdojea -ep eq} Jo sjod satyeyoSea ay} Jo SMOIA IMOY—'FOL ‘OLA 142 INVERTEBRATA Fic. 105.—Four diagrammatic transverse sections of lower part of young Trochophore larva to show mode of closure of blastopore. (After Woltereck. ) A, gastrulation beginning ; B, constriction of blastopore, the cells of fourth quartette approach one another; C, inner lips of blastopore closed; D, outer lips of blastopore closed. The figures refer to the quartettes to which the cells thus designated belong. M, residual macromeres. CHAP. trunk blastema, are 3e?paa, 3d?Pa4, and 20222, Outside and behind these are thin plate-like cells, belonging to the quadrant D of the second quartette, but immediately behind them lie the cells 2d?!2 and 2071, which mark the point at which, about this time, the anus 1s re-formed. Proceeding now to examine the cells which enter into the gut-wall, we find that the ven- tral surface of the gut is formed (1) by the union of the pair 4a* and 4c? which, as we have seen, by join- ing with one another, cut the original blasto- pore into two openings ; and (2) by the union of the following pairs of cells belonging to the fifth quartette, viz. 5d” and 5c!2, 5d!! and 5c, and at a slightly later period, the pair 5d# and 5c24, which also unite with one another. As the dorsal wall of the gut is invaginated the Trochophore swells out and becomes arched dorsally, recovering in this way from its flattened shape. The cells which form the dorsal wall, from being cubical become flattened and converted into thin arched shells of cyto- plasm. The front wall of the mid-gut where it joins the stomodaeum is formed by the VII ANNELIDA 143 cells 4b! and 4b', which, it will be remembered, were originally situated at the front rim of the long oval blastopore. Cells 5a! and 5c} also form part of the front wall of the mid-gut, being situated below 4b! and 4b". The lateral walls of the mid-gut are formed on the left by 5B! above, by 5a2, in the middle, and 4a? below; and on the right by the corresponding cells 5Br, 5c?, and 4c*. The hinder wall of the mid-gut, where it joins the intestine, is formed on the left by 5A, on the right by 5C*, and in the mid-dorsal line by 5D*, The valve which projects into the cavity of the gut oes. ret Fic. 106.—Optical sagittal section of young Trochophore after gastrulation is complete. (After Woltereck. ) Letters as in preceding figures. In addition, V, intestinal valve (this is represented in outline) ; oes.ret, larval retractor muscle of oesophagus. and tends to separate the cavity of the mid-gut from that of the intestine, is formed by the following pairs of cells, one member of each pair being situated on the right side and the other on the left,—4cP and 4a?, 5Be and 5C?, 5c” and 5d”; and in addition by 5D! in the mid-dorsal line. The intestinal wall is formed by three pairs of cells, 5c 5c! and 5c! on the right, and 5d¥, 5d, and 5d?! on the left side, and by the single cell 5D" in the mid-dorsal line. Its hinder wall is formed by 4d!2, 4d, which at this period form part of the gut wall but which later are the mother cells of the adult mesoderm. Between and slightly behind them, a little later, the permanent anus arises, and since they formed the hinder rim of the original 144 INVERTEBRATA CHAP, | blastopore and later of the primitive anus, it will be seen that the permanent anus corresponds to the hindermost piece of the primi- tive one. The Trochophore is now complete and can begin to feed. It is the only larva in which the ancestry of every cell has been completely worked out, and hence its cell-lineage has been given with a completeness which it will not be necessary to repeat in the case of other forms. As now constituted it is very similar to the Pilidium, for the ventral surface is still almost flat and no trace of the projection which is to form the body of the future worm, has yet appeared. It is true that the Pilidium possesses no anus, but this, as we have seen, is also true of the young Trochophore. The distinctive features of the Trochophore at this stage are the metatroch, the archinephridia, and the telotroch, all of which are wanting in the Pilidium. FURTHER DEVELOPMENT OF POLYGORDIUS There are two species or varieties of the European Polygordius, one found in the North Sea, P. dactews, and one in the Mediterranean, P. appendiculatus. The segmentation of the egg and the early development up to the attainment of the Trochophore stage is strikingly similar in both varieties, but the later development, till the attainment of the adult form, is very different in the two cases. The adult worms according to Woltereck are practically indistinguish- - able from each other, so that we have here a curious instance of specific peculiarities being developed during the later larval history. We shall describe the later development of the Neapolitan species first, as it is the simplest. The external features of this stage of development were exhaustively described by Fraipont (1887), but its true significance has only been made clear by Woltereck (1902 and 1905). In this species, after feeding has gone on for some time, a rapid period of cell division and growth sets in in the three cells forming the trunk blastema (these it will be remembered are 2d?” and 3cP3* and 3d?P*); and in two cells, 4d'P and 4d'?, the mother cells of the adult mesoderm which lie internal to the three cells just men- tioned. These two latter cells had already, in the Trochophore, each budded off a small cell 4d" and 4d") in front of them: this process is now repeated many times, and in this way two long strings of cells, the mesodermic bands, are formed. The original mother cells of the bands are termed teloblasts. The multiplication of the cells of the trunk blastema furnishes new ectoderm to cover these bands, and in this way a post-trochal outgrowth of the body is formed. The intestine elongates at the same time by the growth and division of the cells forming its walls, for the anus is situated at the termination of this post-trochal projection. VII ANNELIDA 145 By longitudinal divisions the mesodermic bands become several rows of cells thick, and a faint indication of the division of the post- trochal “body” into segments is now discernible. These segments are indicated in the ectoderm by faint transverse grooves parallel to one another; in the mesodermic bands by the appearance in each band of a set of cavities, which we may term somites,. situated one behind the other, corresponding to the grooves in such a way that, in each segment, one pair of cavities is formed. The right and left somites in each segment rapidly expand and displace the blasto- coele, and finally meet one another both above and below the intestine. Then, where their walls impinge on one another, absorption takes place, and so instead of two coelomic sacs one coelomic ring-shaped space is found in each somite. Whilst these changes have been taking place the archine- phridia have undergone further development, by the adhesion to them of further cells budded from the third quartette. The original pore cell divides and gives rise to a string of cells. The original flame cell, or soleno- cyte, persists, but the newly- added cells form additional solenocytes. The transformed fc. 107. Tate sbies (athe Gann st eae iculatus, in a are now known asthe =?’ Cay a being formed by the rst_ pair of protonephridia growth of the trunk blastema. (After (P.N}, Fig. 108, A). Each consists Woltereck. ) of a tube which forks internally Letters as before. In addition, a, permanent into two branches, and each anus; 4.b, hind te Heat a branch terminates in two peculiar Winaon; 12, trout limit tunk besten. solenocytes. Each of these ; ; ; solenocytes consists of a head studded all over with blind cuticular tubes, each of which contains a flagellum (sol, Fig. 108, A). Bebind this pair of protonephridia a second pair arises. — Each of these consists of a tube opening by a pore on one side in the region of the first somite formed from the trunk blastema. | The tube is terminated internally by solenocytes which are situated in the swollen body of the Trochophore, and which | are derived from wandering cells, descendants probably of the third quartette. The tube itself owes its origin to a pore cell situated in the ectoderm of VOL. I L oye -aqyetd [voids at[3 JO SeyOSUU 104 -OVIJOI UIVUL Ja. $ 4nd jo reyouryds ‘yds named by rapid division A Ce d give rise to the ectoderm, the latter to the endo- derm. When segmentation is completed, the endo- derm consists of a num- ber of larger cells loosely connected one with another by strings of cytoplasm which occupy most of the space within the egg membrane, the ectoderm on the other hand forms a cap of small closely aggregated cells which are also connected together by filaments of cytoplasm (Fig. 123 A). Sedgwick justly at- Fic. 123.—Stages in the segmentation and the gastrula- tion of the egg of Peripatus capensis. (After Sedgwick.) A, conclusion of segmentation. The ectodermic cells form a small cap resting on the endoderm cells which are loosely dis- persed within the egg membrane. B, the endodermic cells are contracted so as to form a compact mass, and the ectodermic cells have begun to grow over them (epibole). OC, the covering-in of the endoderm cells is almost complete—a few endoderm cells protrude through the blastopore. ect, ectodermal blastomeres ; end, endodermal blastomeres. tached considerable im- portance to these con- necting filaments, and held that they upset the popular conception of a cell as an isolated unit, and of a Metazoon animal as a mere collocation of such units, or as a colonial Protozoon. He was inclined to regard a multinucleate Protozoon, such as Actinosphaerium, -as giving a better idea of the common ancestor of Metazoa. Most vin ARTHROPODA 171 colonial Protozoa are found, however, when closely examined, to exhibit similar strings of cytoplasm connecting together the various individuals which make up the colony, and so the opposition between the two views tends to disappear. _The mass of scattered endoderm cells undergoes contraction, its units being drawn closely together, so that it forms a compact group of cells, and then the ectoderm grows over its sides and completely Invests it, leaving only a small area in the centre of the vegetative pole uncovered. At this spot the large rounded endoderm spheres protrude for a time (Fig. 123 C). Soon a cavity is formed in the centre of the endodermic mass, by the formation of vacuoles which coalesce with one another. This cavity, which is the archenteron, opens to the exterior by an aperture, the blastopore, in the centre of the uncovered area of endoderm, and so the process of gastrulation is completed. The gastrula, which had become nearly spherical, now again elongates, and the blastopore becomes elongated also. Behind it there appears a darker area which seems to be an area of rapid proliferation in the endoderm, this is named by Sedgwick the primitive streak. From this area there is produced a crescentic mass of cells lying beneath the endoderm, the two horns of which grow forwards at the sides of the blastopore and constitute the two mesodermic bands. In the meantime the elongated blastopore becomes divided by a constriction into two apertures, the anterior of which persists as the mouth whilst the posterior remains as the anus. The mesodermic bands then became divided into blocks termed somites, in each of which a cavity, the coelomic cavity, appears (Fig. 124). For some time the blastopore is considerably less in length than the embryo, so that there is a prae-oral as well as a post-anal gut, or to put it in another way, there is a short ventral surface and a very long arched dorsal one. The prae-oral and post-anal gut finally disappear owing to the greater relative growth of the ventral surface. The reader will not fail to observe that up to this stage there is a remarkable general resemblance between the development of _Peripatus and that of an annelid. The formation of a cap of small ectoderm cells resting on larger endoderm cells and gradually investing the latter by the process termed epibole; the division of the blastopore into mouth and anus; the formation of mesodermic bands from endoderm cells in the posterior lip of the blastopore, and their division into metamerically arranged somites, in each of which a cavity appears ;—all these are features which have become familiar to us in our study of the development of Annelida. But from this stage onwards distinctively arthropodan features make their appearance. The rudiments of appendages appear as pairs of protrusions of the ventral ectoderm arranged metamerically behind one another in correspondence with the somites; the first to appear are the antennae which are at first situated at the sides of the mouth, but which later, along with the corresponding somites, shift forwards 172 INVERTEBRATA CHAP. VIII to a prae-oral position ; the other appendages develop in order from before backwards. Into each rudimentary appendage an outgrowth of the corresponding somite with its coelomic cavity extends. Then the endodermic tube shrinks away from the ectoderm and leaves A B SONU Fic. 124.—Stages in the division of the blastopore and the formation of the mesoderm of Peripatus capensis. (After Sedgwick.) A, the blastopore elongated but unconstricted ; the primitive streak is seen behind the blastopore. B, the blastopore has just divided into mouth in front and anus behind. The mesodermic’ bands have been formed and have already budded off somites in front. C, the embryo has become concave ventrally. The appendages are beginning to grow out from the somites. a, anus; ap, appendage ; at, rudiment of antenna ; blp, blastopore ; m, mouth; p.s, primitive streak ; som, somites. spaces which eventually form the body-cavity of the adult, a cavity which is totally distinct from the coelom and is termed the haemo- coele since it becomes filled with blood (Gr. haema, blood). It corre- sponds exactly to the blastocoele or primary body-cavity of the Annelid larva. Of these spaces three primary ones may be dis- tinguished, namely, one median ventral and two dorso-lateral (Fig. “sUBSIO TRIQUeA ‘o'a £ efaodOMNERY] ayy JO snuls [eIneu-qns ‘w'gns { e4yru0S 9} JO WOISIATP [BIVW9A ‘ZruWos § ayLWMOS 844 JO UOISIAIp [Bsiop ‘pwos { MoOTe00 *a"2 ‘aqTUMOS ayy} JO AQIAvD ‘wos fsqyueUTIpNr peaquoA pue [esIop JO sdUaUSETvOD 9Y4 Aq PoUTIOJ Aqtavo ywraostAtsed jeroued ‘acd £ Aquaeo [ersostalied [elaues Jo yUSWIPNI [eaqzUeA ‘ward $AtAwO eIeostalsod yeseuas jo quaulpni jesiop ‘pra'd ‘!wmnqdes jerpreo -wod ‘sod $ wmtpavotsed ‘od { paoo-earou ‘ow Sejeoooulsry ey} JO SNUIS [BIE9R] ‘s-7 {(@Ja00oTMEBY ey} JO SUOISIAIP [eIEq¥T -OSIOP OXY OYJ JO BOUGdSa]vOD Sq PetHIO}) qzeoy ‘WH f9n3 “9 Sumipmydeu perro -OS—UBSIO ALOJeIOXe ‘xa { efeoooWRY JO UWOISIAIP [Bs9JV[-osiop “yp ‘moqsds SHOAIOU OY) JO eIsues Jo ated v Suyoeu -W09 BINSSIUIMMOD SIEASURIy “UWUWOD f BTI00 -OUlsvq JO UOISIATp aefnorpuedde ‘s-dv ‘srgpdweg ynpe ue jo uorser peyued 949 Ysnoryy uoyves ‘Mm ‘ayTUIOS ey} JO UOISIAIP [VS1Op 9y4 JO Jno Surusyqey ony jo q[usel @ se ‘pewIoy st umtpavoled ey} YOIyA Ul pue ‘paulsoy st ues10 £10} -o10Xe OY} YOIYA ul oAIquia ue ySno1yy UWOTJOeS ‘(fT “peudOJ ST AVI OY} YoryM UL pue ‘suoryiod [e1yUaA puUv [BSIOP OFUT peprlarp ATa7oTAwWOd st o41WOs oT COTTA UT ofiquie we YSnory4 Uoydes ‘QO «‘suos0d je1qUeA pue [esIOop OFU Surprarp ysnf st OPUWOS oY} YOTYA UI OAIQUIO Ue YSnoiYy uoyoes ‘gq ‘parvedde sey s[s0oomery arojeq osIQte UB YSno1y4 uoToes ‘Y (3PIASpeg r0yy¥,) *(Aytavo-kpoq Areptoses) wWops0d pue (Aq1avo-Apoq Arewtid) ojsoo -owey Jo sdrysuonepar penqnur oy AYVIYSN[[I OF JAapIO UI sase snowea Jo swsuadvo sngodiiag Jo sokiqwue JO soIpoq oy} Ysno1q, suoloes OSIOASUVAL OTFEUMIUIVISEIG—"GZI “OL 178 174 INVERTEBRATA CHAP. 125 A). By the further shrinkage of the endoderm the two dorso- lateral spaces fuse into one median dorsal space which subsequently forms the cavity of the heart. Inasmuch as this space is wedged in between the dorsal apices of a pair of somites, it corresponds both in origin and position to the dorsal blood-vessel of Annelida. "Meanwhile each somite has become divided into a dorsal marion lying at the side of the median blood-space, and a ventral portion lying in the base of the corresponding appendage. The walls of both portions, but especially of the latter, give rise by proliferation to a great mass of cells which fills up the appendage and clings to the side of the gut. In this mass other blood-spaces make their appearance. First appears a space in each appendage which embraces the tip of the ventral division of the coelom and forms the cavity of the leg in the adult. This we may term the appendicular sinus. Then comes a space nearer the mid ventral line, above the spot where the nerve-cord is formed. The nervous sytem arises in the typical annelidan manner as two ventral band-like thickenings of the ectoderm which remain widely separated in the middle line, but which meet one another in front of the mouth and behind the anus. The brain arises as two thickenings of the prae-oral lobe where these bands meet in front. The blood-space above each half of the nervous system forms the lateral sinus of the body cavity of the adult, and it remains separated from the appendicular sinus and also from the median ventral blood- space by a strand of cells. Above it, at the sides of the gut and external to the dorsal divisions of the coelom, two other spaces appear at each side. The dorsal division of the coelom, in most of the segments of the body, collapses and forms a flat plate of cells from which the side of the heart and one half of the pericardial septum are formed. One pair of the spaces which lay externally to these parts of the coelom, meet above the plates of cells which result from the collapse of the coelomic cavities and form the pericardium; the other pair meet above the gut and form the dorsal division of the general body-cavity. This dorsal division of the haemocoele coalesces with the median ventral space and forms the general perivisceral cavity of the adult. The ventral division of the cavity of the somite (som?, Figs. 125 C and D)—~z.. the true coelom—persists as a thin-walled vesicle from which a coiled tube, the excretory organ, the so-called “nephridium,” grows out and, fusing with the ectoderm on the inner side of the leg, forms there a pore which is, the external opening of the nephridium. Finally the nervous system separates from the ecto- derm, forming two parallel nerve-cords, and between them and the ectoderm, of which they originally formed a part, a sub-neural sinus is formed. The ectodermic thickenings from which the nerve-cords have broken away, remain for a long time visible, and are termed by Sedgwick ventral organs. They gradually approach each other in VIII | ARTHROPODA 175 the mid-ventral line, and it has been surmized that they are a last reminiscence of the ventral ciliated groove which extends between mouth and anus in many Annelid larvae. In certain of the hinder segments of the body the dorsal divisions of the coelom, after giving rise to the lateral walls of the heart and to the pericardial septum, do not utterly collapse, but retain narrow cavities. The somites belonging to several successive metaineres fuse with one another so as to form two longitudinal tubes which constitute the genital organs (Fig. 125 D). In the penultimate segment the division of the somite into dorsal and ventral portions does not take place; the excretory organ which belongs to this segment forms the lower portion of the genital duct, whilst the upper portion of the same duct is formed by the undivided coelom belonging to that segment. Fic. 126.—The formation of the appendages in the embryo of Peripatus capensis. : (After Sedgwick. ) : A, side view of an embryo in which the appendages are in process of formation, to show the ventral concavity and the dorsal hump. B, ventral view of the head of a much older embryo, to show the cerebral groove and the lip which surrounds the buccal cavity. at, antenna ; C9, cerebral groove ; gn, gnathite (jaw); gn.b, swelling at base of jaw ; J, lip enclosing buccal cavity ; or.p, oral papilla. It remains to be added that a stomodaeum and proctodaeum are formed by ingrowths of ectoderm round the lips of the original mouth and anus, which displace these openings inwards; that on the under side of the ectodermal thickenings, which give rise to the brain, two deep pits are formed termed the cerebral grooves (c.g, Fig. 126 B) which later become closed off from the exterior, and form for a time hollow appendages of the brain; that the outer buccal cavity which envelops the jaws is formed by the growth of a semi- circular fold (/, Fig. 126 B); that the salivary glands are the excre- tory organs belonging to the jaw segment, from the tubes of which glandular pouches grow out, which project backwards; and finally, 176 INVERTEBRATA CHAP. that the slime glands, which open on the oral papillae and secrete the silk with which Peripatus spins its web, are of ectodermal origin, as are also the crural glands which open on the inner side of the legs external to the openings of the excretory tubes. The tracheae develop from simple ectodermal ingrowths which arise very late in development. The eyes are simple and formed like the eyes of Mollusca, as vesicles which become closed off from the exterior. A cuticular lens is secreted by the cells of the anterior wall, and the cells forming the posterior wall become the visual cells. The whole organ resembles the “ocellus” of an Insect larva and the cuticular lens may be compared to the “glass body” of the latter (see Fig. 219), From this necessarily brief and condensed sketch of Sedgwick’s results, it will be seen that the change from the annelid to the arthropod type of structure must have been accompanied by a suppression of the coelom and an enlargement of the blood-spaces, the latter forming the functional perivisceral cavity of the adult, while remnants of the former persist in the end-sacs of the excretory organs and the cavities of the generative organs. This change was also accompanied by an intensification of the secretion of cuticle, and it is just conceivable that this intensifica- tion of the secretory powers of the ectoderm entailed the other changes which supervened. If chitin be allied to uric acid, as has been asserted, and if the production and casting off of chitin can be likened to nitrogenous excretion, then we may understand how the coelomic wall, which had previously undertaken a considerable portion of this function, might become relatively unimportant and might tend to dwindle and disappear. In Peripatus the chitinous cuticle is thinner and more flexible than in any other known Arthropod, and in no other Arthropod is a continuous series of “ nephridia” retained. In all others the cuticle is thicker and the “nephridia” are reduced to one or a very few pairs; in some cases they seem to be absent altogether. Cuticle and “nephridia ” seem therefore to vary in development inversely to one another, and since increase in cuticle seems to entail decrease in “nephridia,” it may well be that the same factor has led to the decrease and disappearance of the coelom. VIII ARTHROPODA 177 CRUSTACEA Classification adopted— (The new terms invented by Calman (1909) have not been universally adopted. Lhey are given in brackets.) Branchiopoda I. Phyllopoda{ (vetcene II. Cirripedia III. Ostracoda IV. Copepoda V. Malacostraca Leptostraca : s ; 3 i . (Phyllocarida) Anaspida : : : ; : : . (Syncarida) Stomatopoda . : : : ; 3 . (Hoplocarida) ( Isopoda Arthrostraca, Anisopoda Amphipoda : = : . (Peracarida) Cumacea ‘ J Mysidacea, selizopoda, \ Euphausidacea Penaeidea Macrura rbeaed Nephropsidea . (Bucarida) Decapoda Loricata Anomura Brachyura When we now turn to survey what is known of the development of other Arthropoda we find that Insects and Arachnida exhibit, clearly and obviously, a comparatively slight modification of the type of development exemplified by Peripatus. But Crustacea have a development which is not so obviously referable to this type. One or two Crustacea are said to have total segmentation of the egg. The best known case of this is the Penaeid shrimp Lucifer as described by Brooks (1882). With these exceptions the eggs of all Crustacea, Myriapoda, Insecta, and Arachnida have incomplete segmenta- tion, and all, including those which have total segmentation, have the peculiar disposition of yolk known as centrolecithal. The eggs of Peripatus capensis and of allied species are the only Arthropodan eggs which could properly be described as telolecithal. In a centrolecithal egg the yolk is densest in the interior of the egg, and it is surrounded by a skin or rind of cytoplasm. Often the nucleus of the ripe egg is situated near the centre in a sort of island of cytoplasm, but when it divides the daughter nuclei wander outwards and take up their places on the exterior; consequently a segmentation of the egg results which is apparently total but is in reality superficial, for the cleavage planes dividing the blastomeres from one another extend only a limited distance inwards, so that VOL, I N 178 INVERTEBRATA CHAP internally all the blastomeres merge in an unsegmented mass of yolk. If this superficial segmentation occurs only on one side of the egg— ae. if the daughter nuclei migrate only to one part of the surface— the segmentation becomes meroblastic, although, as we shall see, this meroblastic segmentation differs most markedly from that found in Cephalopoda, which is derived from the telolecithal method of development. ASTACUS FLUVIATILIS We select as type of Crustacean development that of the common river crayfish Astacus fluviatilis, A full description of the develop- ment of this form is given by Reichenbach (1888), and no such thorough account of the development of any other form has been given before or since. As every one knows, the eggs are carried throughout their entire development by the mother, attached to her swimmerets by a glutinous secretion. Reichenbach found that when the eggs were removed from the parent they quickly degenerated and died. In the case of the allied genus the Lobster (Homarus), it is perfectly feasible to rear the eggs after they have been removed from the swimmerets of the mother, but in order to do this an elaborate arrangement must be provided so as to secure a constant supply of fresh aerated water to bathe the eggs and to ensure that they shall be constantly agitated. Such an apparatus is provided in the various lobster hatcheries built and maintained by the Canadian and United States govern- ments. In default of such apparatus the plan adopted by Reichenbach seems to be efficient and simple, viz. to keep a large number of females carrying eggs in an aquarium, and from time to time to remove a portion of their brood for examination. By means of very simpJe arrangements the females can be kept for a long time in a state of perfect health. If the bottom of the tank be covered with only a few inches of water, and provided with an overflow; if a slender stream be kept constantly falling into the tank from a tap; if the tank be provided with.a covering of wire- netting in order to prevent the crayfish escaping; and if the whole tank be kept shielded from direct sunlight—then all the conditions will be fulfilled necessary to maintain the crayfish in a healthy condition. They are easily fed on earth-worms, scraps of fish, etc. All the eggs belonging to any one female are in the same stage of development at one time, but the period required for complete development is a very long one, extending over several months. Thus, by keeping together a large number of females with eggs in very different stages of development, a complete series of stages can be picked out in a very much shorter time than would be required if the eggs of the same female were taken for all stages. The eggs of the crayfish, like those of most Arthropoda, are very difficult to deal with, as they are composed chiefly of semifluid yolk enclosed in a very tough resistent membrane, and if an attempt be VIII ARTHROPODA 179 made to remove this membrane, the fluid mass flows out and the egg is destroyed; so Reichenbach recommends the following pro- cedure. The eggs are carefully removed from the parent and are placed in water which is slowly heated to 70° C.; then they are further hardened by being immersed in a 2 per cent solution of bichromate of potash for twenty-four hours; then they are soaked for twenty- four hours in distilled water, which is often changed ; and finally, they are transferred to 70 per cent to alcohol. By careful manipula- tion with needles the thick “chorion” can now be removed and the hardened egg escapes without injury. Reichenbach was accustomed to remove the embryonic rudiment from the egg by means of a sharp knife, then to stain it in picrocarmine, thoroughly dehydrate it and mount it in Canada-balsam; and he also studied the eggs by means of sections cut parallel to and also transverse to the long axis of the embryonic rudiment. Fic, 127.—Two sections through the developing egg of Astacus. (After Morin, from Korschelt and Heider.) A, stage with few nuclei situated near the centre of the egg. B, stage where tho nuclei have reached the surface, and the formation of the primary yolk pyramids has begun. m, nucleus; y.p, primary yolk pyramid. As will transpire immediately, there are many points of the greatest interest in the development of the crayfish on which Reichenbach’s account throws insufficient light. If, as we hope, this life-history should become the object of renewed investigation, the method of imbedding in celloidin and paraffin, described in Chapter II., would be of the greatest assistance in dealing with eggs like those of the crayfish, which, owing to the number of yolk grains they contain, are exceedingly brittle when hardened. The earliest stages in the development of Astacus were not seen by Reichenbach, whose work begins with the stage when the nuclei which result from the division of the zygote nucleus have reached the surface of the egg, where they form a uniform layer all over its surface. A Russian naturalist, Morin (1886), has, however, figured earlier stages, and from him we learn that the zygote nucleus, as in many other Arthropodan eggs, occupies at first a central position and divides there; and that the daughter nuclei are at first internal but gradually migrate outwards till they reach the surface (Fig. 127). 180 INVERTEBRATA CHAP. Reichenbach found that in the first stage observed by him the egg was imperfectly divided by radiating planes into a series of radially arranged pillars, in each of which was contained one of the daughter nuclet. These pillars were referred to by previous authors as “primary yolk pyramids.” Reichenbach regards them correctly as an imperfect division of the egg into columnar blastomeres; the cleavage planes which separate adjacent pillars correspond to the planes which divide adjacent blastomeres in other eggs. He shows, indeed, that each pillar of yolk is capped on its external surface by cytoplasm containing a nucleus, and is clothed also on its sides with cytoplasm. In Reichenbach’s first stage, then, we have a blastula in which the blastocoele is filled with unsegmented yolk. The yolky part of the blastomeres, the yolk pyramids, persist as such for a very short time; the dividing planes disappear, and we are left with a skin of flattened cells surrounding an immense mass of yolk. Such a skin is termed a “blastoderm.” The formation of the gastrula is initiated by an increase in num- ber of the blastoderm cells on one side of the egg. They press on each other laterally and become columnar in character, and so the “ ventral- plate” is formed. This ventral plate indicates the future neural side of the embryo. Strictly speak- ing, all cells within the confines of Fic. 128.—Sagittal section through the the plate have not the columnar blastula of Astacus fluviatilis to show character; this is confined to five the primary yolk pyramids. (After ‘ : i Reichenbach.) circular areas, in each of which inet esieiaetiastune Beene: the cells are arranged in elegant a concentric curves and in lines radiating from a central point. Of these five areas the two anterior and widest apart are termed the “cephalic lobes.” They are the rudi- ments of the paired eyes and of the cerebral ganglia, and in the centre of each is to be found a pair of cells larger and clearer than the rest. Behind the cephalic lobes, and situated go close together as almost to touch one another, are two similar areas, which Reichenbach terms the thoracico-abdominal rudiments; and behind these again, in the middle line, is a single circular area, the endodermic rudiment. At the front border of the endodermic rudiment the cells are engaged in active proliferation, and here they are not in a single layer but in several layers of small rounded cells. This is the point of origin of the mesoderm. In the next stage the areas of the ventral plate which intervene between the five circular areas shrink go as to bring these latter closer together. This shrinkage is almost certainly due to a change in form VIII ARTHROPODA 181 of the blastodermic cells from a flattened to a more columnar shape. The cephalic lobes, which have increased in size, are brought nearer to each other so that they are only separated by a groove, and they are also approximated to the thoracico-abdominal rudiments. The endo- dermic disc is indented in its anterior portion by a deep, semicircular groove; this groove is the beginning of the process of gastrulation (Figs. 129 and 130), and may be termed the endodermic groove. The mesoderm which lies in front of this consists of a limited number of large cells termed primary mesoderm, mingled with a larger number of small cells. The former will give rise to masses corresponding to the somites of Peripatus, from which the muscles and probably the genital organs arise; the latter constitute Reichen- bach’s so-called secondary mesoderm, they wander widely and occur every- where between ectoderm and endoderm, and appear to give rise to blood and connective tissue cells. Reichenbach emphasizes the fact that these cells originate both from ecto- derm and from endoderm, but it seems probable that the primary mesoderm has an endodermic origin, while the secondary springs from the ectoderm. Soon the endodermic groove becomes a complete circle and the periphery of the endodermic disc is invaginated. Just as we Fic. 129.—Ventral view of an embryo of Astacus have found to be the aban Aeriokti heetrls dagen peor teh in other eggs, the process BG a datney eaueh a : < 3 a c.l, cephalic lobe ; inv, invaginated area of blastoderm ; of invagination can be th.abd, thoracico-abdominal thickening. analysed into (@) an in- crease in the number of cells and (b) an inwardly directed cytotaxis. The result of this kind of process is that the centre of the endo- dermic disc projects for a time as a kind of endodermic button, but as the process continues this button is also carried inwards, and a circular blastopore is left where once there was a superficial disc of endoderm. The anterior part of the periphery undergoes the most rapid invagination, and so the endodermic sac projects forward beneath the thoracico-abdominal rudiments. These rudiments are now connected with one another by a bridge of high columnar cells, and each is also connected with the cephalic lobe of its side by a streak consisting of parallel lines of columnar 182 INVERTEBRATA CHAP. cells; but between the cephalic lobes there is still a groove of flattened indifferent cells. As a result of these changes we have now a heart- shaped, coherent, ventral plate of columnar cells. In the next stage the blastopore changes from a circular to Fic. 180. — Sagittal section through a portion of the em- bryo of Astacus fluviatilis to show the invagination of the endodermic rudiment. (After Reichenbach.) end, endoderm, the two references to end mark the anterior and posterior limits of the endodermic plate ; mes, ‘‘ primary” mesoderm ; mes1, second- ary mesoderm. an elliptical shape, with its long axis coincident with the long axis of the embryo. The thoracico-abdominal rudiments become thoroughly united with one another in the middle line, and become arched upwards so as to project over the open blastopore and partially conceal it from view. Simultaneously the blastopore begins to close Fic. 131.—T wo sagittal sections through developing eggs of Astacus fluviatilis in order to show the development of the endoderm. (After Reichenbach. ) A, stage before the closure of the blastopore. B, stage in which the hind-gut has appeared. dip, blastopore ; end, endodermic sac ; mes, mesoderm ; proct, proctodaeum opening by anus; th.abd, rudi- ments of thorax and abdomen ; yp, endodermic cells swelling up to form secondary yolk pyramids, by the lateral union of its sides, the process beginning in front, and its hinder border begins to grow forwards and thus assists in the process of closing. Reichenbach’s account of this matter and his figures illustrating it are most unsatisfactory. He denies that the backward growth of the VIII ARTHROPODA 183 abdominal rudiment has anything to do with the closing of the blastopore, and his figures show that this is closed by the union of two flat sheets of endoderm cells, uncovered by ectoderm. Now the lip of the blastopore is a spot where ectoderm passes into endoderm ; it is difficult to imagine that in the process of closing there is a dissolution of this continuity, and the suspicion is aroused that if these stages were worked over by the celloidin-paraffin method ditterent results would be obtained. In all probability the dissolution of continuity is due to the method of section cutting. At this same time the cells which formed the endodermic button and which now form the floor of the endodermic sac become more columnar in shape. This increase in size is due to the fact that they begin actively to ingest the yolk granules; and they continue to do so In successive stages till all the yolk granules, which made up the unsegmented mass in the centre of the egg, are contained in the yolk cells. The endodermic cells increase enormously in length during this process and were termed by the earlier authors the secondary yolk pyramids ; their growth is, however, little advanced in the stage which we are now discussing. As the thoracico-abdominal rudiment advances over the blastopore it becomes obviously bilobed, and in the notch between the lobes is seen the last rudiment of the blastopore. In front of this, according to Reichenbach, 7.e. in the bridge which connects the two halves of the rudiment, a new invagination makes its appearance; it is the ‘rudiment of the adult intestine or proctodaeum, which opens by the anus. It is by no means improbable that further investigation would show that the proctodaeum arises just where the last vestige of the blastopore disappeared. At the same time the two cephalic lobes have become connected in their hinder region by a curved bridge of columnar cells. This is the rudiment of the labrum or upper lip; behind it, in a slightly later stage, an invagination appears which will mark the position of the mouth and of the oesophagus (stomodaeum), but of these there is, at this period, no trace. In the streaks of cells connecting the cephalic lobes and thoracico-abdominal rudiments, three outwardly directed, semicircular thickenings are observable, of which the hinder- most pair are the furthest advanced. These are the rudiments of the first three pairs of appendages, viz. the antennules, antenne, and mandibles of the adult. The mesoderm when last considered consisted of a small number of large and of a large number of small cells. In this stage the large cells form a mass beneath the thoracico-abdominal rudiment, whilst the smaller have extended and spread all over the surface of the ventral plate and form special aggregations in the cephalic lobes and in the lip rudiment. As the rudiments of the appendages become more marked the ventral plate continues to shrink in size and takes on an oval outline. On the median side of each appendage is to be seen a mass of cells 184 INVERTEBRATA CHAP. with large clear nuclei ; these are the rudiments of the ganglia of the nervous system. The first of these pairs of ganglia is connected with a similar mass of cells which forms a kind of focal line, surrounded by the concentric parabolic curves of cells which make up the cephalic lobes. This focal mass of cells is the rudiment of the primary cerebral ganglion or protocerebrum, to which later the anten- nulary ganglion or deuterocerebrum adds itself. The two cerebral ganglia are connected by a bridge in front of the labrum. To the compound mass on each side there is added, at a later period, the an- tennary ganglion or tritocerebrum (rc, pre. Fig. 137). The outer lab part of the cephalic o lobe gives rise to the eye-stalk, the ecto- derm covering which gives rise to the visual thabd cells of the compound eye; at its base there is a deep groove, the cells lining which, in later stages, bud off the cells which form the optic ganglion. This groove may be termed the cerebral car Fig. 132.—The “Nauplius” stage in the development of ae hh Astacus fluviatilis viewed from the ventral side. (After e€ mout 2.8 DOW Reichenbach.) made its appearance an, anus ; atl, rudiment of first antenna ; a2, rudiment of second AS a groove behind antenna; car, ridge marking the first trace of the carapace; cl, the labrum and leads cephalic lobe ; lab, labrum ; m, mouth ; mn, rudiment of mandible; - pr.c, protocerebrum ; th.abd, rudiment of thorax and abdomen. into a narrow stomo- daeum, which descends vertically towards the endodermic sac but does not yet reach it. Behind the mouth there is found a median groove of ectoderm extending backwards between the ganglia of opposite sides. The cells forming this groove proliferate and form between each pair of appendages a thickening, two or three cells deep, which later enters into the formation of the transversé commissures between the ganglia of the double ventral nerve cord (Fig. 133). The primary mesoderm forms a compact mass, in which, however, some indications of a division into segmental masses corresponding to the appendages are to be seen. This is one of the points on which a renewed investigation is very desirable, because Reichenbach’s statements on this point have been overlooked by subsequent workers, and it has been generally assumed that Crustacea are distinguished VIII ARTHROPODA 185 from other Arthropoda by the non-segmentation of the mesoderm (Balfour, 1880). The anus is still situated on the dorsal aspect of the thoracico-abdominal rudiment. It is, however, shifted somewhat forwards as compared with its former position, and will eventually pass into the terminal notch and so on to the ventral surface of the’ abdominal rudiment; but this does not happen until a later stage has been reached. Finally, on the surface of the egg, outside the thoracico-abdominal rudiment, there is to be seen a semicircular ridge—very faintly marked. This is the first trace of the head-shield or carapace. When this stage of development is reached the ectoderm secretes a thin cuticle ‘which is detached from the surface of the egg before further growth occurs, and we may interpret this as the first moult or ecdysis, and as marking the completion of a stage of development. Now when we survey what is known of the life-histories of other Crustacea we find that, in the majority of Copepoda, Cirripedia, and Ostracoda, and in the more primitive Phyllopoda as well as in a few Schizopoda and Decapoda, the embryo, when it has attained this stage of development, bursts the egg-shell and escapes as a free-swimming larva, to which the name Nauplius has been given, and Fic. 133.—Three transverse sections which is distinguished by possessing a tzesghthe dering pens on large upper lip and only three pairs of bach.) appendages. We can scarcely doubt comm, mass of cells derived from ventral ~ that the formation and exuviation groove destined to form the transverse of this cuticle in the embryo of femme: 1, ikerng fo fom oe of Astacus is a reminiscence of a con- Fon les EG dition when the embryo, at this stage of development, became a free-swimming Nauplius larva—and we regard this as one of the many proofs that the embryonic phase of development is secondarily derived from the larval, and not vice versa. We therefore term this stage of development the Nauplius stage. Following the Nauplius stage a different form of development begins; the ventral shield, which had been undergoing contraction, begins to grow vigorously, and in the angle where this shield passes into the ventral surface of the thoracico-abdominal rudiment there is found a zone of rapidly growing cells, and as a result of their activity the point of origin of this rudiment is carried farther back. Thus it becomes bent under the ventral shield, just as a crab carries its abdomen permanently bent underneath it. Then if we turn our attention to the appendages, we find that the 186 Fic. 134.—Two views of developing eggs of Astacus Jluviatilis seen from the ventral surface. Reichenbach. ) A, stage in which the rudiments of maxillae have appeared, and in which the caudal fork is visible. B, stage in which the rudiments of thoracic appendages are appearing, and in which theabdomen is segmented. ab, abdomen ; atl, tirst antenna; atl.g, antennulary ganglion (deuterocerebrum) ; at2, second antenna ; car, fold which becomes edge of the carapace; caud.f, caudal fork ; ¢.g, cerebral groove which gives rise to the optic ganglion ; lab, labrum; mn, rudiment of mandible; ma}, first maxilla ; mz, second maxilla ; mapl, mxp2, mxp3, first, second, and third maxillipedes ; oc, eye-stalk; op.g, optic-ganglion; pr.c, proto- cerebrum ; ret, retinulae of the compound eye; th, rudiments of thoracic appendages. (After INVERTEBRATA CHAP. original three have be- come longer; and that the second, which is the rudiment of the antenna, has become bifurcated at the end, which is an in- dication of the branching of the limb into exopo- dite and endopodite. Behind the mandible is the region of the ventral shield, which owes its origin to this budding zone; on it are to be found the rudiments of five new pairs of appen- dages, viz. those corre- sponding to the first and second maxilla and to the three pairs of maxil- lipedes. All except the first of these are very faintly marked indeed. The thoracico- abdominal rudiment has grown in length and has become marked out into segments by grooves. On the dorsal side, just at its point of origin, there is seen an ectodermic thickening. Below this there is a plate-like mass of mesoderm, the ends of which in the next stage become bent upwards and attached to the ecto- derm, so as to enclose a space which is the cavity of the heart (H, Fig. 135). In the sides of this mesodermal mass, just as in the case of Peripatus, irregular cavities appear (pe, Fig. 135). These are the rudiments of the peri- cardial cavity and they VII ARTHROPODA 187 eventually meet above the heart and separate it from the ectoderm. The primary mesoderm becomes divided into a double series of spheri- cal masses, one pair corresponding to each of the segments into which the thoracico-abdominal rudiment is divided. In these somites cavities Fic. 185.—Transverse section through the region of the heart in an embryo of Astacus fluviatilis in about the same stage as that represented in Fig. 134 B. (After Reichenbach.) end.p, endodermal plate ; H, heart; pce, spaces destined to form the pericardium, appear which represent the coelomic cavities of Peripatus. At the termination of the thoracico-abdominal rudiment there is a deep in- dentation and the anus is no longer visible; in other words, the tail is divided into two lobes which are termed the tail lobes or caudal fork. The anus has in fact been pushed forwards till it lies in the Fic. 136.—Longitudinal section through advanced embryo of Astacus fluviatilis parallel to the sagittal direction but to one side of the middle line. (After Reichenbach.) ab.g, abdominal ganglion ; cer, cerebral ganglion ; coe, coelomic cavities ; end.p, endodermal plate ; ext.m, extensor muscles ; proct, hind-gut; stom, fore-gut ; th.g, thoracic ganglia ; v.s, ventral sinus. notch between these lobes, and it then passes on to the ventral surface by the partial fusion of the two tail lobes above it. Just in front of these lobes there is a crescentic area of rapidly growing cells. This is a second budding zone, and it is to its activity that the increased length of the thoracico-abdominal rudiment is due. 188 INVERTEBRATA CHAP. In the next stage the yolk has been completely ingested by the endoderm cells, which have become in consequence very tall and columnar in shape, and the fold which gives origin to the carapace is strongly marked. Beneath it there is a deep groove on each side which gives rise to the branchial cavity of the adult. Five additional pairs of appendages, the rudiments of the so-called ambu- latory legs, the pos- session of which causes Astacus to be reckoned as a Decapod, are de- veloped, and the first and second maxilla and the three maxillipedes have become bifurcated. Fic. 188.—Two ommatidia from Fic, 187.—Advanced embryo of Astacus fluviatilis the eye of a newly-hatched viewed from the ventral side. The abdomen and crayfish in longitudinal sec- hinder part of the thorax are cut off and spread out tion. (After Reichenbach.) separately. (After Reichenbach.) corn, cornea ; corn.c, corneal cells ; ab, abdomen; br.c, rudiment of branchial cavity; dt.c, crys, crystalline cone; erys.c, crys- deuterocerebrum ; ex, opening of excretory organ ; lab, labrum; talline cone cells; rh, rhabdome pr.c, protocerebrum; oc, eye-stalk; th, thoracic legs; th.g, shimmering through the retinula; thoracic ganglia ; tr.c, tritocerebrum. pig, pigment cells. In the basal part of the rudiment of the antenna a sac appears, which is the rudiment of the excretory organ (ez, Fig. 137). This sac appears to be similar to one of the coelomic sacs of the abdomen, to it is added a large ectodermic pocket which forms the thin-walled ureter. Soon afterwards the rudiments of the abdominal feet, or pleopods, make their appearance; and in the bifurcated antennule there is an ecto- dermic pit to be seen, which is the beginning of the auditory organ, VIII ARTHROPODA 189 The cephalic lobes now project freely from the surface of the egg as the eye-stalks, and the ectoderm cells covering them have become several layers deep. These ectoderm cells then become arranged in radially directed strings, each of which forms an ommatidium or eye element; the outermost cells giving rise by their secretion to the corneal lens, those beneath them to the crystalline cone, whilst from the innermost cells the retinula is derived (see Figs. 220-222). The coelomic cavities of the mesoderm disappear as the primary mesoderm cells form themselves into flexor and extensor muscles (ext.m, Fig. 136). The arched dorsal region of the egg begins to flatten in consequence of the gradual digestion and diminution of the masses of yolk stored up in the endoderm cells. The outer ends of these cells, in which the nuclei are situated, gradually separate from the yolky portions. These latter break up into rounded masses and are gradually digested. The first place where the cytoplasm separates from the yolk is in the dorsal surface of the mid-gut, just where the proctodaeum impinges on it. Here a flat stretch of epithelium, the “endodermal plate,” becomes separated from the yolk (end.p, Fig. 135). Soon the rostral spine begins to be differentiated between the anten- nules in the head region. The gills appear under the branchiostegite as outgrowths from the basal joints of the limbs. The two halves of the caudal fork fuse to form a simple rounded telson. The ectoderm everywhere sends inwards solid pegs; they form the supports for the tendons and ligaments of connective tissue which are formed by the wandering cells. The embryo is now ready to break open the egg-shell and enter upon its free life. For some time the store of yolk in the endoderm suffices ; but gradually the extreme convexity of the dorsal hump dis- appears, as the remaining store of yolk is used up, and the endoderm cells shrink in size. The flaccid endodermic sac becomes indented by folds, and is gradually fashioned into the complex structure of tubes known as the adult liver. Its median portion persists as the adult mid-gut. Just before the embryo hatches Reichenbach was able to detect the rudiments of the genital organs. These appear in the 14th, 15th, and 16th segments in the dorsal region, and appear to consist of rounded masses of cells, in each of which a lumen appears. The masses seem to be arranged metamerically in accordance with the segments, and at the hinder end of the rudiment of each side there is to be seen a tube, which is presumably the rudiment of the genital duct. Reichenbach’s imperfect observations, so far as they go, fit in admirably with Sedgwick’s results on Peripatus. OTHER CRUSTACEA We shall now take a brief survey of what is known of the development of other Crustacea, and shall direct our attention to 190 INVERTEBRATA CHAP. two points: (1) the mode of formation of layers, 7.e. the differentiation of ectoderm, endoderm, and mesoderm, and (2) the larval history. Fic, 189.—Two sagittal sections through advanced embryos of Astacus fluviatilis. (After Reichenbach.) A, stage in which secondary yolk pyramids are complete. B, stage in which the endodermic sac is ~ divided into lobes—the rudiments of the liver tubes. ab.a, abdominal artery ; ab.g, abdominal ganglia; ag.m, anterior gastric muscle; an, anus (the reference lint goes to a point some little distance inside the proctodaeum) ; cer, cerebral ganglion ; end, endodermic sac (mid-gut) ; H, heart ; hep, liver saccule ; lab, labrum ; i, mouth; p.g.m, posterior gastric muscle; proct, proctodaeum (hind-gut); 7, rostrum; st.a, sternal artery ; stom, stomodaeum (fore-gut); tel, telson ; th.abd, thoracico-abdominal rudiment ; th.g, thoracic ganglia ; op.a, opthalmic artery ; yp, secondary yolk-pyraimid. FORMATION OF LAYERS With regard to the first point, all the fragmentary knowledge which we possess of the early history of other Crustacean eggs seems to show that they agree in all essentials with the egg of Astacus as to the mode in which the layers are differentiated. If we VII ARTHROPODA 191 look at the nearer allies of Astacus, we tind that in Homarus (the lobster) the egg is much larger than that of Astacus, owing tu the presence of a larger amount of yolk, and that the endodermic area is relatively small. The invaginated cells form at first a nearly solid mass projecting into the yolk; they multiply and spread through the yolk, ingesting it as they proceed, and a cavity appears in the interior of the mass filled with disorganized cells. Eventually they reach its Fic. 140.—Portions of two sagittal sections through developing eggs of Homarus americanus. (After Herrick.) A, stage in which the endoderm cells form a solid inass. B, stage in which the endoderm cells are spreading through the yolk. deg, degenerate remains of more central cells; end, endoderm; inv, cavity of invagination. surface, and here form an investing layer; thus secondary yolk pyramids are not formed. Much the same process occurs in the prawn Palaemon; but here the endoderm cells become detached from one another and wander through the yolk and eventually arrange themselves in an epithelial layer outside it; when they have reached this position they become columnar but never attain the length of the endoderm cells in Astacus. This kind of development seems to be general throughout the Decapoda. In Lucifer, however, as we have seen above, Brooks asserts that the segmentation of the egg is total, that a hollow blastula con- sisting of relatively few cells is formed, and that an invagination takes 192 INVERTEBRATA CHAP. place by which an archenteron is formed which is large and occupies most of what was the interior of the blastula. The development of the Schizopod Huphausia, as far as the gastrula stage, has recently been worked out by Taube (1909). Here, as in Lucifer, the egg undergoes total segmentation and the blastomeres are all of nearly the same size. In this way a hollow blastula is formed. After the 32-cell stage, however, the cells do not all divide ; two remain undivided, and form the rudiment of the endo- derm, and these, at the 112-cell stage, pass into the interior of the Fic. 141.—Sections through the developing egg of Afysis chamaeles. (After Wasbaum.) A, ventral part of egg showing the solid ingrowth of cells which replaces invagination. B, trans- verse section of embryo showing the formation of the epithelium of the mid-gut. end, endoderm ; hep, liver saccule ; v.nc, ventral nerve cord. blastula. The blastopore is surrounded by a ring of special cells, and of these two are said to give rise to the mesoderm. In Mysis and its allies, and in Amphipoda and Isopoda, in a word in all the Peracarida, however, the invagination is replaced by a solid ingrowth of endoderm cells, and when these detach themselves and wander through the yolk, they form an endodermal epithelium, at first only on the ventral side of the yolk (Fig. 141). Only very gradually does this epithelium extend so as to enclose the yolk on the upper side also. In these cases too we have meroblastic seg- mentation, 2.¢. the zygote nucleus, whether it is in the interior of the egg, as in the Isopoda, or on the surface as in Mysis, gives rise to VIII ARTHROPODA 193 daughter nuclei, which form a blastoderm, at first only on the ventral surface of the egg ; only at a later period do cells come to the surface of the yolk on the dorsal side also. When we descend to the lower groups of Crustacea we find that amongst Phyllopoda the development of the Cladoceran genus Polyphemus has recently been worked out by Kiihn (1912). In fundamental characters it agrees with that of Luphausia; the egg undergoes total segmentation. A 2-cell stage is followed by a 4-cell stage and this by an 8-cell stage in which there are two tiers of four cells, and in which a segmentation cavity or blastocoele makes its appearance. The four cells nearer the animal pole of the egg are larger and clearer than those nearer the vegetative pole, but the latter contain most of the yolk, aud in one of them are embedded the remains of the sister cells of the egg, i.e. oocytes, which do not ripen, but serve as nourishment. In the 16-cell stage we get two Fig. 142.—Stages in the development of the egg of Polyphemus pediculus. (After Kiihn.) A, passage from 16-cell stage to 30-cell stage, from the side. B, 118-cell stage, from below. OC, _ sagittal section through a stage of between 236 and 452 cells. end, endodermal cells ; gen, vells of genital rudiment ; mes, mesoderm cells. tiers of eight cells each, since every cell except one divides by a meridional cleavage. This exceptional cell is the one of the four situated in the vegetative half of the egg, which has received the remains of the nutritive cells. It divides, not meridionally, but into an upper and a lower cell; the lower contains the remains of the nutritive cells, it is the rudiment of the genital organs, and is termed the generative cell; the upper is the endoderm cell, and gives rise to the lining of the mid-gut. At the next period of cleavage these two cells do not divide, but all the other cells divide each into an upper and a lower daughter cell (Fig. 142, A). In this way we get in the animal half of the egg two tiers of eight cells, and in the lower half of the egg an upper tier of six cells and a lower tier of six cells. This lower tier lies at the vegetative pole and forms a horse- shoe-shaped group surrounding the endoderm cell and the generative cell. There are thus thirty cells in the egg. Shortly after the endoderm cell divides into right and left halves, thus raising the number of cells to 31. VOL. I 0 194 INVERTEBRATA CHAP. At the next period of cleavage the endoderm cells divide into upper and lower daughters, and the generative cell divides into right and left halves, and in this way a 62-cell stage is attained. The two generative and four endodermic cells do not, however, divide in the following period of cleavage, so that instead of 124 cells we have only 108 in the next stage. In this stage the group of endodermic and generative cells is surrounded by a horseshoe - shaped group of six cells, descendants of the similar group in the 30-cell stage. This group constitutes the mesoderm (Fig. 142, B). When the endodermal group becomes invaginated, as a result of inwardly directed cytotaxis, the mesoderm cells are also invaginated (Fig. 142, C). Kiihn derives the mesoderm from the ectoderm because in the 30-cell and 62-cell stages the mother cells of the mesoderm are cells which give rise also to daughters which eventually form part of the ectoderm. But this comparison is misleading and unjust. The wall of the blastula is differentiated into regions, an endodermic below, a mesodermic above this, and above this again an ectodermic. The mere fact, that when the blastular wall consists of few cells, mesodermic and ectodermic regions happen to find themselves contiguous to one another so as to be within the territory of one of the few nuclei, is of no importance. The mesoderm of Polyphemus corresponds in position and origin to that of Astacus. The early differentiation of the genital rudiment is a common feature in animals of small size and short life-cycle. About the development of the other division of Phyllopoda (we. Branchiopoda) very little is known. The egg of Branchipus is stated to undergo total segmentation, but the inner ends of the blastomeres. are said to coalesce into a yolky mass, on the surface of which is a blastoderm. Doubt has recently been cast on this statement, and there is no doubt that it requires reinvestigation. The development of Copepoda and Cirripedia seems to be funda- mentally of a similar type. It may be regarded as a modification of the type described for Astacus, a modification which is produced by the diminution in absolute size of the egg, due to the smaller adult size of the species, coupled with the fact that the nucleus and its daughter nuclei are not diminished in the same proportion as is the whole egg. Therefore the amount of nuclear matter relative to the size of the egg is greater in these forms than in Decapoda, and the nuclei are also far fewer in numbers. The result of this is to produce a form of segmentation which might be variously described either as holoblastic or meroblastic, according as one regarded the nucleus which remains nearer the centre of the egg, as either—(1) the nucleus of a huge blastomere whose cell territory includes all the egg which is not marked out into blastoderm, or (2) as a nucleus in unsegmented yolk which has not as yet had its cell protoplasm delimited. The development of Zepas and its allies has been studied by Groom (1894), and the development of Lepas in its earlier stages has been studied in great detail by Bigelow (1902). In this case the mother VILL ARTHROPODA 195 nucleus divides three times successively, and at each division gives off a daughter which migrates to the surface and segregates round itself a blastoderm cell, the pre-existing blastoderm cells also dividing each time. In this way an investment of the yolk by blastoderm cells is effected. At the fourth cleavage the mother nucleus gives rise to a primary mesoderm cell in front and then comes to the surface itself as the first endoderm cell. Subsequently both mesoderm and endoderm cells divide into right and left halves, and the endoderm cells withdraw from the surface. This inwardly directed cytotaxis is the process of gastrulation. The cells bordering the blastopore at the anterior end bud off cells which sink inwards; these may be termed mesectoderm, and they are perhaps equivalent to the secondary mesoderm of Astacus. A mesodermal band is formed which extends forwards and upwards, and along its course three outwardly directed transverse grooves delimit the three pairs of appendages of the end mes mcs.ect Fic. 143.—Four sagittal sections through the developing eggs of Lepas anatifera in different stages of development. (After Bigelow. ) A, 15-cell stage, formation of mesoderm. B, 16-cell stage, mesoderm formed. C, 62-cell stage, formation of mesectoderm. D, 250-cell stage, formation of mesodermic band, dlp, blastopore ; end, endo- dermal nucleus ; mes, mesoderm ; mes.ect, mesectoderm ; mes.b, mesodermic band ; p2, second polar body. Nauplius. The dorsal extension of these appendages is in reality the same phenomenon as their outward direction in Astacus, and the apparent difference in direction is due to the smaller quantity of yolk in the egg of Lepas. The development of Copepoda, to judge from the somewhat con- flicting accounts which we possess, seems to be essentially similar to that of Lepas. A renewed study by modern methods of the development of a primitive form like Branchipus would throw a flood of light on the development of Crustacea generally, and perhaps enable us to under- stand the contlicting accounts given of the development of Copepoda. LARVAL HISTORY—THE NAUPLIUS With regard to the larval history, we may take as type the development of the common fresh-water Copepod Cyclops, of which various species are found in fresh water all over the world. If females of Cyclops carrying egg-sacs are isolated and kept in small shallow 196 INVERTEBRATA CHAP. glass vessels in pond-water with a little pond-weed, they will live tor a considerable time, and the eggs will hatch out into larvae, and these larvae can be reared through their complete development till they attain the adult condition. When the larvae escape from the egg membrane they have an oval outline, and are provided with a large, swollen, almost square upper lip and three pairs of appendages. Of these, the first pair are inserted in front of the lip, and each consists of a single branch divided into three joints, of which the centre one is the largest. All three carry long hairs at their ends. The second pair of appendages on each side consists of a broad basal piece (protopodite); it carries a long, inwardly directed hook, which nearly meets its fellow beneath the lip. This pair of appendages is postoral. The distal portion of the linb consists of two branches, an exopodite or outer branch, composed of a basal piece and four joints, and an endopodite or inner branch, com- posed of basal piece and two joints. The basal pieces of exopodite and endopodite are more or less adherent. The third pair of append- ages are quite similar to the second, but smaller in size; the proto- podite is longer in proportion than in the second pair of appendages, and it has on its inner side a triangular outgrowth, which carries one or two long, inwardly-directed bristles. This, like the corre- sponding process on the second appendage, is masticatory in function. The exopodite and endopodite consist as before of four and two joints respectively, but the distal joint of the endopodite projects inwards. These three pairs of appendages are moved by long, back- wardly-directed muscles, which converge towards and are inserted in a small area in the dorsal integument. The mouth leads into a vertical oesophagus which is provided with constrictor and dilator muscles. This, from its cuticular lining, is obviously an ectodermal stomodaeum. It opens into the true endodermal gut, which runs backwards nearly to the posterior end of the animal; here it opens by an orifice, guarded by a sphincter muscle, into ashort proctodaeum lined by cuticle and derived from the ectoderm. In front of this opening the midgut gives rise to two ventro-lateral pouches, which have an excretory function and are filled with granules of uric acid. In the base of the second pair of appendages is the opening of a sac which projects backwards at the side of the mouth. This sac, the antennary sac, is also excretory in nature, and is homologous with the similarly situated sac in Astacus. The nervous system consists of a praeoral brain, on which rest two simple eyes, and a sub-oesophageal ganglion connected with it by a pair of cords. The nerves for the second and third pair of appendages are connected with the sub-oesophageal ganglion. The little larva was baptized Nauplius by Claus (1858). The name had been previously employed by the Danish naturalist, O. F. Miiller, for a later stage in the development, when four pairs of appendages were formed; he imagined this to be an independent VIII ARTHROPODA 197 organism. When the Nauplius is just hatched it remains still for a few seconds, until its cuticle hardens and becomes strong enough to resist the pull of the muscles. Then it starts on its active career, swimming by a series of darts through the water, each dart being caused by a synchronous back- ward blow of all the appendages. At each blow of the legs the masticatory hooks seize any food particles that they may encounter and drive them into the mouth ; feeding and swimming are thus performed by the same move- ments. If we compare this larva with the stage in the development of Astacus when a cuticle is first formed, we can see that there is a fundamental resemblance be- tween the embryo in one case and the larva in the other; the difference between the two being, first, that the appendages of the embryo, since they are not func- tional, are represented by mere stumps; and secondly, that the embryo possesses cephalic lobes, and the rudiments of compound Fic. 144.—The Nauplius larva of Cyclops canthocarnoides from the ventral surface. (After Claus, ) : an, anus ; atl, first antenna; at2, second antenna; enp, endopodite ; exp, exopodite ; gnl, gnathobase of second antenna; gn?, gnathobase of mandible; lab, labrum ; mn, mandible. eyes, which are absent in the larva. But the adult Copepod also has no rudiments of compound eyes, and the origin and significance of the compound eye is still an unsettled question. The little creature, as it begins to feed, grows, and, like all Arthro- poda, can only grow by casting its cuticle. Just like the embryo of Astacus, there are two growing regions, one at the posterior end of the animal, which gives rise to a pointed prolongation of the body equivalent to the thoracico-abdominal rudiment of Astacus, and one immediately behind the third appendage. At the end of the first moult the larva passesinto what has been called the Metanauplius condition, when the small rudiments of the two pairs of maxillae and the so-called maxillipede, or third maxilla, appear behind the large third appendage. They are concealed by it, and hence Claus, in his original communication (1858), came to the astounding conclusion that not only the adult mandibles but the two first pairs of maxillae were derived from the division of the third appendage of the Nauplius. The posterior end of the larva becomes bilobed, the anus is in the bay thus formed, and just beneath the spot where the excretory sacs of the gut are situated there are developed a pair of stumps, which eventually form the first thoracic feet. 198 INVERTEBRATA CHAP. By three successive moults the length of the posterior part of the body is increased, and at each moult a new pair of stump-like Fie. 145.—Three stages in the further develop- ment of the larva of Cyclops. (After Claus.) A, stage in which gnathites have appeared. B, stage in which rudiments of thoracic Jegs have ap- peared. OC, stage after critical monlt. Letters as in previous figure. In addition, caud.f, caudal fork; er, excretory matter in the walls of the alimentary canal ; mai, first maxilla; ma2, second maxilla; map, maxilli- pede; thl-4, first, second, third, and fourth thoracic appendages respectively ; oc, eye. rudiments of thoracic feet appears. Then comes a critical moult, at which the larva changes its whole appearance and practically attains the adult form (Fig. 145, C). The breadth is diminished and the length increased; the first appendage has become many jointed and approxi- mates in shape to the first antenna of the adult. The second appendage loses its masticatory hook and its endopodite, and has now become shorter than the first: it becomes the second antenna of the adult. The third appendage loses everything but the basal joint, and so is VIII ARTHROPODA 199 converted into the mandible. The first two pairs of thoracic legs are each now distinctly divided into two branches, but both exopodite and endopodite are as yet un- divided. The bifurcation of the caudal end has now deepened, so that the anus is guarded on each side by a rod-like appendage—one half of the caudal fork. In the base of the second maxilla may be seen the sac-like rudiment of the shell-gland—the adult excretory organ, whilst the antennary sac has disappeared. The thoracico- abdominal rudiment is now dis- tinetly divided into segments. At successive moultsadditional joints are added to the first antenna, the branches of the thoracic legs become jointed, and the posterior thoracic legs B Fic. 146.—Two types of Nauplius larva. i i i i figs the the Nauplius larva of Branchipus stagnalis (after Claus) combined from two ligures. B, 1 , augue lane Oe Lepas (after Groom). Letters as in preceding figure. In addition, c.sp, caudal spine ; d.l.p, dorso-lateral spine ; jr.f, frontal tilament; g/, glandular mass in the base of the dorso-lateral spine ; gni, gnathobase of antenna; gn2, gnathobase of mandible ; seg, segments in the thoracico- abdominal rudiment of the Nauplius of Branchipus ; v.sp, the pre-anal spine. 200 INVERTEBRATA CHAP. become first forked, then jointed, and so the adult form is attained. Claus, in the paper cited, has given an interesting sketch of the differences between Nauplii of different species, and one of the most interesting facts which he brings out is that in the larvae of Cyclopsinidae, in which more yolk is contained in the egg, the masticatory hooks on the second and third appendages are absent, and the animal at first takes no food. When we examine the life history of the Phyllopoda, we find that in the Branchiopoda and in one or two cases amongst the Cladocera, the young animal enters on its free-swimming existence as a Nauplius larva. In most Cladocera, however, the whole development is completed within the egg-shell, and the animal Fic. 147.—The “ Cypris” larva or Pupa of Lepas fascicularis seen from the side. (After Willemoes-Suhm. ) add, adductor muscle of carapace; atl, first antenna; caud.f, caudal fork; ex, excretory organ (shell-gland) ; fiz, dise for fixation; jix.gl, fixing gland; gn, gnathites (t.e. mandible, first maxilla, second maxilla); int, intestine; m, mouth; lab, labrum; ocl, simple eye; oc2, compound eye; th, thoracic legs. ; hatches out with all the adult features already developed. The Nauplius larva, when it appears, shows the same general features as the larva of Cyclops, but the upper lip is very long and projects backwards, covering the ventral surface. If we turn to the Branchiopoda we find that the Nauplius larva is characterized by the great development of the post-oral portion of the body, and by the fact that the third appendages are not forked (Fig. 146, A). After the first moult, before any more appendages appear, the post-oral region becomes marked by a series of four or five transverse grooves, an indication of as many segments, and in the larva of Apus these are evident when the larva first escapes from the egg-membrane. Just as in the case of Cyclops so in Apus and other genera of Branchiopoda there comes a critical moult, at which antennae and mandibles are reduced to their adult proportions. The Cladocera are remarkable for retaining throughout life the VIII ARTHROPODA 201 forked nature and swimming function of the second antenna, so that in their case the “critical” character of the moult is reduced to the loss of the distal joints of the mandible. _ The Cirripedia also begin their free life as Nauplius larvae, which in all essentials, and even in such minute points as the many-jointed exopodite and feebly-jointed endopodite, agree with the larvae of Cyclops. They differ in the development of the dorsal integument into a great triangular shield, with two antero-dorsal horns, the dorso-lateral spines, and one postero-dorsal horn, the caudal spine, Fic, 148.—The fixation of the “Cypris” larva of Lepas fascicularis, (After Willemoes-Suhin. ) A, the larva just in the act of ecdysis after fixation. B, the young Barnacle fixed to a piece of dead shell. Letters as before. In addition, cn, carina; l.car, valves of larval carapace ; Ith, larval thoracic appendages being cast off; sc, scutum ; ter, tergum ; th, adult thoracic app. forming its three angles, and in the possession of a ventrally directed pre-anal spine. Further, we find in front of the first pair of appen- dages a pair of flexible antennae, the frontal filaments. As the larva grows the thoracico-abdominal portion of the body becomes divided into segments, on which six pairs of bilobed appendages are successively developed, whilst in the angle between this rudiment and the head the two pairs of maxillae appear as buds. Beneath the edges of the dorsal shield the rudiments of the paired eyes appear as dark areas. Then comes the critical moult, when the second pair of appendages completely disappears, and the third is reduced to 202 INVERTEBRATA CHAP. its basal blade, whilst on the first antenna there is developed a disc for fixation. A bivalve carapace now appears which replaces the old three-cornered larval shield, the six pairs of thoracic legs acquire swimming-hairs and take over the swimming function, the compound eyes become functional, and the larva has now passed into what is termed the Pupa stage. The pupa swims actively about for some time, but it takes no food. It finally settles on a suitable spot, and attaches itself by the disc on the first antenna in which a gland with a glutinous secretion opens; by the copious effusion of this secretion the larva is attached. An ecdysis now takes place and, by the preponderant growth of the skin of the ventral surface, the animal is rotated into a position in which it may be described as standing on its head. The praeoral part of the body growsvery much in size and becomes the stalk, and shelly plates, the scutum, the tergum, and the carina, are secreted by the folds of skin which constitute the carapace ; these calcareous plates replace the chitinous shields of the pupa. In this way the adult Barnacle condition is attained (Fig. 148). Fre. 149.—The Nauplius larva of Cypris ovwin. In the development of the (After Claus. ) parasitic forms like Sacculina, aid, adductor muscle of carapace; atl, first which draw nourishment from antenna ; af, second antenna ; cer, cerebral ganglion; the blood of their host through ieee mandible; oc, eye; st, stomo- root-like extensions of the stalk, the Nauplius possesses no mouth and is fed by the yolk in its endoderm, and the pupa on fixation amputates the hinder part of the body. The Ostracoda also enter on their free-life as creatures with the three appendages of the Nauplius, but the two flaps of skin con- stituting the bivalve carapace of the adult are already developed, and the second and third appendages consist of one branch only, the outer branch or exopodite being lost. The passage into the adult condition here is so gradual that one cannot speak of a critical moult. In the development of the carapace and the unforked character of the second and third appendages we have an anticipation of adult characters (Fig. 149). When we turn our attention to the higher Crustacea we find that the Mysidacea among the Schizopoda and all the Cumacea carry the eggs in a brood pouch beneath the body of the mother, and from these eggs young Crustacea hatch out with all the essential features of the adult. But in the Euphausiadacea among the Schizopoda, and in some genera at least of the tribe Penaeidea among Decapoda, to which Lucifer belongs, the young leave the egg membrane as VIII ARTHROPODA 203 N auplii, showing, however, the ridge-like rudiments of two or three pairs of postoral appendages. In the case of both Zuphausia and Penaeus a succession of moults leads, as in other Nauplii, to the development of the thoracico- abdominal rudiment, and to the appearance in it of ring-like segments which first appear in the anterior or thoracic portion. The appendages belonging to these segments, which are the maxilli- pedes, are strongly developed, especially the first, which has a long exopodite used in swimming. A cephalic shield or carapace makes its appearance as a frill or fold round the head region, under- neath which the future paired eyes appear as dark areas. In Penaeus the anterior part of this shield is produced into a sharp rostral spine. The larva is now in the stage known as the Protozoaea (Fig. 150). Its further development into what is known as a Zoaea larva involves the growth and Fic. 150.—The Protozoaea larva of segmentation of the abdominal Nyctiphanes australis, (After Spence- portion of the body, the thoracic Bate.) segments remaining extremely Letters as in preceding figure. cl, simple narrow, especially the posterior aed oc2, compound eye; th.s, thoracic ones, and the paired eyes become stalked. In Euphausiadacea the stalks are so short that the paired eyes do not project beyond the edge of the carapace, and the larva is consequently known as a Calyptopis. The majority of Decapoda pass through the Nauplius stage during their embryonic life and only enter on their larval life as Zoaeae. ANCESTRAL CRUSTACEAN ‘We may now pause to consider how far the Nauplius larva may be regarded as representing an ancestral Crustacean form. Since it occurs in all the lower groups of Crustacea with no greater modifications than are found, for instance, in the different types of larva amongst Echinoidea; and since it also occurs in isolated cases amongst the higher Crustacea; and since, furthermore, an embryonic stage corresponding to the Nauplius is clearly marked in the development of every Crustacean egg which has been so far studied ; 204 INVERTEBRATA CHAP. it is quite clear that the common ancestral stock of Crustacea passed through a larval stage corresponding to the Nauplius. Now, on the principles laid down in the first chapter of this book, we are driven to conclude that the Nauplius represents a common ancestor of all Crustacea in however modified a form. Fritz Miiller, in his work, Fiir Darwin (1864), concluded that all Crustacea were actually the descendants of a small oval, unsegmented species of animal with three pairs of legs. Hatschek (1877 and 1878), on the contrary, pointed out that such a conclusion implied that the Crustacea had no affinity with Annelida, nor with Pertpatus, Arachnida, and Insecta, in all of which the early embryo was comparatively long and distinctly segmented, with a double series of ccelomic cavities. This conclusion Hatschek rightly thought to be incredible, and he therefore adopted the opposite opinion, namely, that the Nauplius had no ancestral significance at all, but that since in all Arthropoda—and Annelida, for that matter—the segments were developed from before backwards, so that the first was the oldest, there must in all exist a stage in which there were only three segments in the embryo, and, according to him, it was due entirely to a secondary modification that the eggs of some Crustacea were hatched when they reached this stage. Korschelt and Heider, agreeing in the main with Hatschek, suggest that the Nauplius is an “ Arthropodized Trochophore ”—that it represents the Trochophore of Annelida with certain Arthropodan features precociously added. Balfour (1880), finally, whilst believing that the Nauplius in its present form was much modified, yet believed that it exhibited ancestral features, and that the hinder part of the body had formerly exhibited a segmentation which it had secondarily lost. ' Amongst all the views we have recounted, that of Balfour seems to come nearest the truth. It was reserved for his pupil and successor, Sedgwick, to enunciate clearly what Balfour instinctively felt, viz. that the embryonic phase is secondarily developed out of the larval stage, and not vice versa. Indeed, Hatschek’s view is thoroughly inconsistent with the fact that, when the larva does not hatch out as a Nauplius, a cuticle is produced and shed by the embryo whilst still within the egg-shell when it reaches the Nauplius stage, thereby showing that formerly this stage must have been passed through in the open, in the ancestors of the forms in which it is now purely embryonic. ‘We saw in Chapter I. that a larva, as compared with the actual ancestor which it represented, is usually greatly diminished | in size, and that this diminution in size is not accompanied by a representation of all the organs which the ancestor possessed, also diminished to scale, because such diminution would render them ineffective; on the contrary, we saw that those organs which were functionally dominant in the ancestral condition are reproduced by the larva, while the others are suppressed. We are, therefore, probably nearest the truth when we suppose VIII ARTHROPODA 205 that the Crustacean ancestor was like a Polychaete Annelid, with fairly numerous segments bearing somewhat feeble and membranous appendages like parapodia, but possessing greatly enlarged append- ages attached to the first three segments, which fulfilled the major part of the work of locomotion, the first pair of these appendages alone having passed in front of the mouth. Corresponding to the diminution in size of the whole body in the larva as compared with its original size in the ancestor, the comparatively functionless posterior appendages have been suppressed. But from such a form, with only one pair of appendages in front Fig. 151.—Dorsal and ventral views of the “ Copepodid” larva of Actheres ambloplitis. (After Wilson.) A, dorsal view. B, ventral view. afl, first antenna; at?, second antenna; caud.f, caudal fork ; Jr.gl, frontal gland ; lab, labrum ; mn, mandible; mal, first maxilla ; mx2, second maxilla ; mrp, maxilli- pede ; th, thoracic legs ; w.7, under lip. of the mouth, with the two next pairs in the form of powerful locomotor organs not specialised for either mastication or sensation, the Arachnida can be derived; and the fact that Onychophora and Insecta likewise have only one pair of antennae shows that they too could be traced back to such an ancestor. Finally, the extinct Trilobita, whose jaws bear long forked palps, and which possess only one pair of antennae, seem clearly to belong to the same cycle of affinity. That this reasoning is justifiable and not far-fetched we may illustrate by taking a case where we may almost say that the ancestor is known, and where we are therefore in a position to compare the ancestor and its representation in the larva. This is the life-cycle of the parasitic Copepod, Actheres ambloplitis, which 206 INVERTEBRATA CHAP. has been worked out by H. Wilson (1911). The adult lives on the gills of the rock-bass, Ambloplites rupestris : it is a sac-like organisin fixed by two econjoined arms to the host; it shows no trace of . Copepod structure except the long egg-tubes, which the female hears protruding from the end of her body. If we were to classify by adult structure alone, no one would dream of regarding Ae- theres as a Copepod; but yet every zoologist is fully convinced that Actheres is a modified Copepod—that is to say, that it is descended from an ancestor which was like Cyclops or Calanus or some other typical Copepod genus. Now the Nauplius and Meta- nauplius stages are completed inside the egg membrane, and Fic, 152.—Dorsal and lateral views of just- the young animal hatches out as fixed female of Actheres ambloplitis. what is termed a Copepodid— ee ee namely, in a form which every A, Hotsal View. B, Jeteral view. Letters as in one would recognize at a glance preceding figure ; fr.f, frontal filament. : 5 : as showing the typical structure of a Copepod, that is, of the ancestor. When, however, we look closely at this Copepodid larva we find that it differs from an ordinary Copepod in the following points:—(1) There are but two free segments in the thorax each carrying a pair of forked swimming appen- dages, whereas five such segments on the normal Copepod carry four pairs of forked swimming appendages and one rudimentary pair; (2) the exo- podites and endopodites of these legs are not divided into joints, while the corresponding members in an ordinary Copepod are many-jointed; (3) the first antennae are short, stumpy, and few-jointed, as contrasted with those in an ordinary Copepod, where they are normally long and composed of Fic. 153.— Lateral view of female many joints ; (4)the second antennae = {Heres | ambloplitis | after | adult in the Copepodid are likewise exceed- (After Wilson.) es ingly short, and although forked each branch is unjointed and the inner one terminates in a hook, whereas in the normal Copepod this hook-like termination is not found; (5) in the jaws, ze. mandible, maxillae, and maxillipede, there is nothing which could be described cauaf Letters as in preceding figures, VII ARTHROPODA 207 as atypical, but a long convoluted glandular tube lies in the mid- dorsal region of the head and opens at the front end of the carapace, and this tube secretes a long gelatinous filament. The larval stage of the Copepodid only lasts thirty-six hours at most. The larva is swallowed by the rock-bass, and has the instinct to burrow into the mucous membrane of the pharynx of its host. When the pointed front end of the carapace of the parasite comes into contact with the bone of the gill arch of the host, the distal end of the filament already referred to is extruded from the frontal gland, and adheres thereto. The larva backs off, and the filament draws out; but before it is completely extruded the larva grasps the end of it by the incurved hooks of the second maxillae, and holds on. A moult follows, in which the maxillipedes are shifted forward so that their bases are now situated between those of the second maxillae. The second maxillae are greatly enlarged, and have lost their segmentation though their two incurved ends still tightly grasp the filament. A sucking tube is formed by the union of a projecting labrum with an “ ee peo ese mnt orn nee er the mandibles, though they can side. (After Wilson. still be forced out through eee ; a saber lateral slits between these lips. ee oe pe eee The minute first maxillae are attached to the sides of the sucking tube, and the two pairs of antennae are much shortened; they are reduced in fact to mere mps (Fig. 154). a The aa as now stab the vascular gill of its host with its needle-like mandibles and suck its blood, and it grows rapidly in size, moulting frequently. At the first of these moults practically the adult form is attained, all trace of the thoracic appendages is lost, and the maxillipedes are transformed into blunt shapeless lobes. The filament of the female shortens till the claws of the second maxilla are actually in contact with the skin of the host; that of the male, however, remains long, and he appears to crawl around like a tethered goat until he finds a female; then he relaxes his hold on the filament and seizes the female with his claws, and so is in a ition to effect sexual union. pe of the differences between the Copepodid larva and the typical Copepod shows examples of all the changes we postulated in explaining the Nauplius ;—we have the reduction in segmentation, and the disappearance of appendages, or rather the replacement of a homologous series of appendages by a smaller number of similar ones; in fact there is a functional rather than 208 INVERTEBRATA CHAP. a proper morphological representation of these in the Copepodid larva. If, then, it be a sound principle of science to reason from the known to the unknown, we are justified in regarding the differences between the known ancestor of Actheres and the larva by which it is represented, as a means to deduce the unknown ancestor of all Crustacea from the Nauplius larva, by which we believe that ancestor to be represented. A similar problem confronts us when we consider the significance of the typical larva of the higher Crustacea, the Zoaea. We have already seen that this larval form develops out of the Nauplius larva in the Euphausiadacea amongst Schizopoda, and the Penaeidea amongst Decapoda. But all the Anomura and Brachyura, so far as is known, begin their free-swimming life with the Zoaea stage, and amongst the Macrura this is true of the Caridea. All Zoaea larvae agree in possessing (1) paired stalked compound eyes, (2)a carapace which overlaps and conceals. part of the thorax, and (3) a segmented abdomen. All agree further in possessing (4) three pairs of limbs (7.e. mandibles, and first and second maxillae) which are thoroughly modified to form jaws, (5) in having the first few segments of the thorax well developed, and / =a se ay carrying forked limbs termed vnslrals, lateral view. (After Spenee-Bate) ™maxillipedes on which a large part of the locomotor function falls; and finally they agree (6) in having the hinder segments of the thorax either entirely suppressed or very thin, and without appendages or with mere rudiments of appendages. But whilst the general features of the Zoaea may be regarded as constant, its specific features vary from group to group. The Zoaea of the Euphausiadacea and of the Penaeidea is characterized by retaining a large forked second antenna like that of the Nauplius, which assists in swimming. The Zoaea of Euphausiadacea has one pair only of maxillipedes well developed, and it has several other well-marked peculiarities: thus the last pair of abdominal appendages is developed, and the border of the carapace projects considerably and conceals the short eye-stalks from view when the larva is seen from above, hence, as already mentioned, it is termed Calyptopis (Fig. 155). The Zoaea of the tribe Penaeidea, amongst Decapoda, differ from car, carapace ; hep, liver lobules ; oc, compound eye. VIII ARTHROPODA 209 the Calyptopis in having two pairs of maxillipedes developed, and a rudiment of the third, and in having small rudiments of appendages even on the thin posterior thoracic segments; also in having a median rostral and two postero-lateral spines on the carapace, and in having long-stalked compound eyes. \\ Vy Ve A \ an \ caud, Uf Fic. 156.—Zoaea of Penaeus, ventral view. (After Claus. ) mxp)-3, first, second, and third maxillipedes ; pl, rudiments of pleopods (i.e. abdominal appendages) ; r, rostrum ; th, rudiments of hinder appendages of thorax ; wr, uropods (last abdominal appendages). The Zoaea of the shrimps and prawns (t.e. Caridea) agrees with the Zoaea larva of the Penaeidea in possessing these spines on the carapace, but it differs from them and agrees with the Zoaea of the higher Decapoda in two respects; (1) in having the exopodite of the second antenna converted into a flat unjointed scale (squame), as in the adult Decapod, (2) in having the endopodite of this appendage much shortened so that this limb is no longer locomotor in function. VOL. I P 210 INVERTEBRATA CHAP. Three pairs of maxillipedes. are, however, developed, and on them consequently devolves the whole function of locomotion. The hinder segments of the thorax are completely suppressed, and no trace of appendages is found on the abdomen (Fig. 157). The Zoaea of the Anomura is very similar in general appearance to the Zoaea of Caridea, but it only possesses two pairs of maxillipedes, and, generally speaking, no trace of the hinder thoracic appendages is present at birth although they appear after the first moult, and the rostral spine is always very long and sometimes enormously elongated. Finally, the Zoaea of the Brachyura, while agreeing in most points with the Zoaea of the Anomura, differs from it and all other Zoaeas in possessing a long mid-dorsal spine sloping backwards. The very same controversy which developed concerning the meaning of the Nauplius, raged over the significance of the Zoaea. Some, like Dohm (1870), held it to represent an ancestor of the Fic. 157.—Zoaea larva of Crangon vulgaris, lateral view. (After Sars. ) Letters as in previous figure. Schizopoda and Decapoda ; others, like Claus (1878), pointed out that such a conclusion would imply that in the ancestral Decapod the hinder part of the thoracic region was unsegmented, and that these segments, when they did appear, must have been secondarily inter- calated. If such reasoning were justified it would sever the higher Crustacea from all connection with the lower Crustacea, for in these latter the segments follow one another in development in un- interrupted series from before backwards; and so the conclusion was drawn that the Zoaea had no significance whatever. Balfour then pointed out that in the more primitive types of Zoaea the posterior thoracic segments, although very thin, are distinctly present, and therefore surmised that the Zoaea might represent in a modified form the ancestor of higher Crustacea. On the principles laid down in this book, we must agree with him. The Zoaea was clearly a larval form in the life-history of an ancestor common to the Schizopoda and Decapoda, in a word, we might say, to the primitive Malacostracan, and therefore represented an ancestor of this Mala- costracan. But in that case, what stage in the evolution of the VIII ARTHROPODA 211 polychaete worm into the shrimp does the Zoaea represent ? Obviously one in which (1) the first antenna had become purely sensory, (2) the second antenna had moved in front of the mouth and had lost its masticatory function, (3) the mandible had become purely masticatory, (4) the next two pairs of appendages (maxillae) had been modified into jaws, (5) the main swimming function had been thrown on the first two or three appendages of the thorax, and, (6) compound eyes and a carapace had been developed. If we read over this list we might conclude that the Phyllopod genus Apus, if it had possessed better developed antennae, would have given us a good idea of what the ancestor represented by the Zoaea looked like. The first two pairs of thoracic appendages of Apus are developed into long antenna-like organs. All the others, of which Fic. 158.—Zoaea larva of Porcellane longicornia, after the first moult. (After Sars, ) d.l.sp, dorso-lateral spines of the carapace ; 7, enormously elongated rostral spine ; th, radiments of hinder thoracic appendages. there is a great number, are thin and parapodia-like. These, which must have existed in the ancestor, are not represented in the Zoaea, owing doubtless to their physiological unimportance and the diminu- tion in size of the larva. We cannot, however, well imagine that the abdomen in the ancestor was devoid of appendages, although it is so ' in most Zoaeae. In the Zoaeae of the Penaeidea, indeed, the append- ages of the last segment are developed, and there are vestiges of appendages on the other segments of the abdomen. The abdomen, as a region with peculiar appendages, is character- istic of the Majacostraca, and the physiological necessity which led to its evolution can be inferred by watching the way in which it is used by Zoaeae. Many of these swim on their backs, using the long spines which project from the carapace as a keel. The maxillipedes are used as oars, and the abdomen functions as a rudder. We can now form to ourselves a picture of the course which evolution followed in transforming the ancestor represented by a 212 INVERTEBRATA CHAP. Nauplius into a primitive Crustacean. The second antenna became gradually shifted forward and lost its masticatory function, while the third appendage became exclusively masticatory and its distal joints shrank to an insignificant palp. At this stage of evolution: the animal was assisted in mastication by the modification of the append- ages of the next two or three segments, which formed maxillae, but which were never greatly changed from their original parapodia-like condition. When these changes had been effected the ancestor was definitely a Crustacean, and from this level the Ostracoda may well have branched off. In the Ostracod the number of pairs of inaxillae varies from one to three in different genera, and what in one genus is a maxilla in a neighbouring genus may be a small thoracic limb. Swimming is mainly performed by the antennae. Finally, in this group alone among Crustacea, there are retained throughout life two pairs of excretory organs, viz.,a pair of antennary glands as in the higher Crustacea, and a pair of maxillary “shell ”-glands as in the lower Crustacea. The Cladocera also must have branched off about the same period of evolution from the common stock, and this is true also of some of the Phyllopoda. Those genera, however, like Apus and Branchipus, in which the antennae have lost their swim- ming function, represent the higher stage of development. Following the stage of evolution which we have been discussing, a new stage supervenes in which the swimming function began to be handed on to the first thoracic appendages, while the hinder part of the body became specialized to form a rudder Fia, 159.—Zoaea larva of the by the diminution in size of its appendages. Crab Xantho. (After Cano.) The Copepoda and Cirripedia seemed d.sp, median dorsal spine of the to have diverged at this point. In them fefewed ‘and acting a6 2 iene, 28 in the Zoaea larva the appendages spine, of the hinder segments are suppressed altogether—a phenomenon doubtless due to the diminution in size, which affects these Crustacea in the same way as it affects the larvae. A condition just previous to this stage is also represented by the Zoaea of Penaeidea and Euphausiadacea, in which a large portion of the swimming function is still carried out by the second antenna. But the process of “handing on” the swimming function to the thoracic appendages, once initiated became progressive, and soon the second antenna became relieved entirely of its swimming functions, which were then exclusively performed by the thoracic appendages, whilst the second antenna was set free for sensory functions. VIII ARTHROPODA 213 This stage is represented by the Zoaea larvae of Caridea, Anomura, and Brachyura. The Zoaea is transformed by several moults; first into a Metazoaea, in which the rudiments of the appendages of the abdomen and of the hinder segments of the thorax appear; secondly into a so-called “Mysis” stage, in which, typically, the hinder segments of the thorax bear forked limbs designed to assist in swimming, and in which the first appendages of the thorax tend to become somewhat diminished in size and degraded from the rank of locomotor organs of prime importance to that of maxillipedes. _ This Mysis larva, as its name implies, is of such obvious ancestral significance that no one has ever attempted to deny that it represents a Schizopod ancestor. We can, however, even here trace the work Fic. 160.—“ Mysis” larva of Homarus americanus, lateral view. (After Herrick.) exl-ex7, the seven exopodites borne by seven of the thoracic legs. of the same modifying tendencies which have obscured the ancestral significance of the Nauplius and the Zoaea. In such of the Nephrops- idea as do not complete their development within the egg-shell (cf. Homarus, the lobster, and Nephrops, the Norway lobster) the larva emerges in the Mysis condition, with this difference, that the abdominal appendages are at first quite suppressed. In the Loricata, of which the rock-crayfish, Palinurus, and the square-nosed lobster Scyllarus are the best known representatives, the larva emerges from the egg-shell in a singularly moditied Mysis stage (Fig. 161). In this larva the thorax is broad and flat and of a glassy transparency ; the abdomen, though distinctly divided into segments, is very small and has no rudiments of appendages. The thorax has only six of the eight pairs of appendages which it should possess if normally developed, and of these, those representing the first two pairs of maxillipedes are small but those representing the third maxillipede and the first three pairs of walking thoracic 214 INVERTEBRATA CHAP. legs, have enormously long endopodites and short outer forks (exopodites). Here we see, as has been repeatedly emphasized in this book, that Nature treats as a single organ an apparatus consisting of several pairs of metamerically arranged organs, which co-operate in the performance of a single function; and when the whole organ is diminished in size—as when it is reproduced in a larva—the number of component similar organs is reduced also. This modified Mysis is known as a Phyllosoma or glass-crab. As Fic. 161.—‘‘ Phyllosoma”’ larva of Palinurus vulgaris, ventral view. (After Cunningham. ) ab, abdomen ; hep, liver saccules seen through ex, exopodite of thoracic appendages. it grows and moults the first maxillipede grows larger, and the last two pairs of thoracic legs also make their appearance; so also do the appendages of the abdomen, and thus the adult condition is approached. The explanation of the singular appearance of this larva is, that in the case of the Loricata the Mysis has ceased to be an actively-swim- ming organism and has become a surface-drifter ; the long legs being widely spread out and acting as supports. Another series of moditications of the Mysis stage have been described by Sars (1891) in the case of the Crangonidae amongst Caridea. The Zoaea of these forms it will be remembered has, when hatched, three pairs of maxillipedes developed as forked swim- VIII ARTHROPODA 215 ming appendages. In subsequent moults a varying number, but in no case do all the remaining segments of the thorax develop swimming appendages. Thus in Crangon only one extra segment develops forked appendages (Fig. 162), in Cheraphilus two segments develop Fic. 162.—“ Mysis”’ larva of Crangon Allmanni, lateral view. (After Sars.) tel, telson ; expl4, the four exopodites borne by the three maxillipedes and great chela respectively ; pl, pleopods (abdominal appendages) ; ur, uropods (last abdominal appendage); th+8, the appendages _ which will form the five pairs of walking legs. appendages with exopodites, in Pontophilus two, and in Sabinea one only. Those segments that do not develop swimming appendages give rise to simple, unforked appendages, which at the last moult develop directly into the hinder walking legs of the adult, as in the case of the Loricata, and we may add, as in the case of the Mysis larva which develops out of the Zoaea larva of Thalassinidae, another family of the Caridea. Finally, in the Brachyura al/ the thoracic segments behind the first two which bear the swimming appendages of the Zoaea, develop only rudimentary bud-like appendages whilst the larval swimming stage persists, but when this is over and the larva begins life at the bottom, then these appendages develop directly into the walking legs without ever passing through a forked stage. Thus, in the life-history of the crabs Pea a ee the Mysis stage has been completely 716-9 Vesmopa | lama or we siniineeed bur nevertheless the Zoaea oe oo ents does not change into the adult stage Titeareaeah aessediny Denies, but into a form called a Megalopa, in which the carapace is longer than broad, and in which all the seg- ments of the abdomen possess well-developed swimming appendages. This larva obviously represents a Macruran stage in the ancestry of 216 INVERTEBRATA CHAP. crabs, just as clearly as the Mysis larva represents a Schizopod stage in the ancestry of Macrura. The Megalopa is transformed into the adult by one or two moults. The life-history of the Anomura closely resembles that of the Brachyura; in their case also the Mysis stage is omitted, but the third appendage of the thorax, the third maxillipede, becomes func- tional before the critical moult which ends the free- swimming life. The post- larval stage of the Paguridae or hermit crabs, which corre- sponds to the Megalopa stage ot Brachyura,is distinguished by the possession of a sym- metrically-developed ab- domen—an indication that these asymmetrical forms are descended from ancestors which were bilaterally sym- metrical. It has been found that the abdomen of the young Pagurid becomes asymmetrical before the animal seizes on an empty shell in which to shelter its abdomen. This fact is of extraordinary interest on account of its bearing on the nature of heredity. The curious groups of Stomatopoda, which agree with Schizopoda in having only three pairs of appendages moditied as jaws, and in Fic. 164.—Two views of first post-larval stage having exapolines eaten of Lupagurus bernhardi corresponding to the of the thoracic appendages ; Megalopa stage of Brachyura. (After Sars.) but which differ from them, A, dorsal view. B, lateral view. br, branchiae attached and indeed from all other Groat prisot once linia, whnestinsie, hebacostaoa; In havang the first five pairs of thoracic appendages modified into grasping claws, and in having gills developed on the abdominal appendages, present a life-history which affords further confirmation of the laws of larval modification, laws which have made themselves clear in the course of our study. There is some reason to believe that the life-history of some Stomatopoda begins with a Nauplius larva. At any rate Lister (1898) has captured in the open sea a Metanauplius in which the mandible is reduced to its blade, and in which the rudiments of the maxillae VIII ARTHROPODA 217 are developed. This larva is shown to belong to a Stomatopod by the fact that it possesses already two stalked compound eyes, which distinguish it from the Metanauplii of all other groups of Crustacea. It has a triangular carapace resembling that of the Zoaea larva; and the hinder part of the body is formed of an unsegmented abdomen terminating in a jointed caudal fork. The next stage in the life-history which is known is the Erich- thoidina stage, which Balfour compared to a Zoaea larva. In this stage _the larva has a precisely similar carapace and stalked eyes, but the first antenna has developed a second flagellum and so become forked, whilst the second antenna is now unforked. There are the usual three pairs of jaws, and these are followed by no less than five pairs . Fic. 165.—Two stages in the development of a Stomatopod larva. A. After Lister. B. After Hansen (from Lister). A, Early ‘‘ Metanauplius” stage. B, So-called ‘‘Zoaea” or Brichthoidina stage. d.sp, dorsal spine ; d.lsp, dorso-lateral spines ; mxp1%, the five maxillipedes ; pl, the rudiment of the appendage of the first segment of the abdomen. of forked swimming appendages. The abdomen is, however, almost unsegmented, only the jist segment with rudiments of its appendages being present, and it ends like the abdomen of most Zoaeae in a broad tail-fan. Thus the larva is more like a Mysis larva than a Zoaea, but differs from both these types of larvae in the character of the abdomen. In subsequent moults the abdomen becomes segmented and develops its appendages, whilst the endopodite of the second appendage of the thorax develops into a great hooked claw, and the first appendage develops into a long slender unforked limb, the hinder three pairs of appendages dwindle into insignificant rudiments or vanish altogether. In this way the Erichthus stage is reached, which is sometimes termed a Pseudozoaea because it has only the first two pairs of thoracic appendages well developed, the next three being represented by mere stumps, and the appendages of the hindermost thoracic appendages being totally absent. This is transformed into 218 INVERTEBRATA CHAP. the adult by a series of moults in which the diminished or vanished appendages reappear in the form of grasping claws, and in which the hindermost segments of the thorax develop their appendages as long legs with the rudiments of exopodites. In the case of other Stomatopoda the embryo develops within the ege until it has reached the Pseudozoaea stage; it then emerges as an Alima larva, which differs only in unimportant details from the Pseudozoaea. The subsequent develop- ment of the Pseudo- zoaea is the same as that of the larva. Now these life- histories justify us in regarding the Stom- atopoda as derived from Schizopod ances- tors, in which the anterior thoracic appendages were gradually converted into grasping claws whilst the swimming function was thrown on the abdominal appendages; just as we believe that Deca- poda are derived from Schizopoda, in which Fic. 166.—Two later larvae of Stomatopoda. (After Claus.) the posterior thoracic A, Pseudozoaea stage of Erichthus Edwardsi, ventral view. segments were de- B, Alima larva of unknown stomatopod. (This larva has moulted veloped into ambula- since birth.) In A, map4 and mxp5, rudiments of fourth and fifth tory legs whilst the maxillipedes. In B, th6-8, rudiments of last thoracic legs; 7, : ¢ = rostrum. swimming function is equally relegated to the abdomen. The five pairs of grasping claws constitute, from the point of view of heredity, a single organ which is reproduced in the Pseudozoaea and Alima larvae as two large functional pairs only. The Sessile-eyed Decapoda, including Cumacea, Anisopoda, Amphipoda, and Isopoda, and the division Mysidacea of Schizopoda, which are all grouped by Calman (1909) under the name Peracarida, enter on their free life similar to their parents in all essential features ; but the just-born young of all these groups agree in having the last segment of the thorax and its appendages suppressed; another, if less well-marked, example of the same rule as that exemplified by the larvae of Stomatopoda. Amongst the Isopoda, however, there are a considerable number of genera which have developed suctorial mouths by a union of upper VIII ARTHROPODA 219 and under lips, and which become parasitic on other Crustacea. The shapes of some of these parasitic forms have become distorted out of all recognition, especially in the female sex. In Portunion (Fig. 168), for instance, which belongs to the family of the Entoniscidae, the “oostegites” of the thoracic legs become enlarged into long leaf- like folds which are packed together like the leaves in a bud, the legs themselves having completely disappeared. The head, with its sucking apparatus, forms a small rounded knob, whilst the abdomen Fic. 167.—Larva and adult female of Portunion maenadis. (After Giard.) A, larva, dorsal view. 3B, adult female, lateral view. abd, abdomen; atl, antennule; a#2, antenna; br, brood-sac composed of conjoined ovigerous plates of thorax; g, jaws or gnathites; h, head; pl, swimmerets or pleopods. is bent back over (not under) the thorax, and its appendages take the form of crimped laminae. The larvae of these extraordinary forms have the depressed body and segmentation of a normal Isopod (Fig. 167): a short pair of first and a long pair of second antennae, and six pairs of unforked thoracic legs followed by six pairs of forked abdominal ones. The jaws are already lancet-like and the lips united. In all but these last two points they resemble the young of normal Isopoda when they leave the brood-pouch, and not even the most determined 220 INVERTEBRATA CHAP, opponent of the recapitulation theory could deny their ancestral significance. It is a tacit assumption of the recapitulation theory that 2 z PAE, Castes Gi cost, Fic. 168.—Adult female of Portunion maenadis, with appendages dissected out. (After Giard. ) ab, abdomen; wntl, vestige of first antenna; ant?, vestige of second antenna; ceph, swollen head; map, rudiment of maxillipede; enp2, the endopodite of the second abdominal appendage ; enp?, the endopodite of the third abdominal appendage; oostl, the three lobes of the first oostegite of the right side; oost?, the second oostegite on the right side; oost3, the third oostegite on the right side; oost4, the fourth oostegite on the right side ; oost5, the fifth oostegite on the right side. when the environment is changed ? advance in evolution is corre- lated with a change in the environment, and with the consequent acquisition of new kinds of food by the animal in the adolescent stage of its life-history. The evidence that this theory is well founded comes out more and more strongly the more the embryology of the various members of the animal kingdom is studied ; but it is exceedingly difficult to reconcile it with modern work on the subject of heredity, which appears to point to the conclusion that changes in morphology are due to changes occurring in the nuclear matter of the germ cells before fertilization. When such chemical changes have been effected—why, it may be asked, should their influence appear just at that moment of development Light might be thrown on this question by a careful and systematic study of the life-histories of parasitic and abnormal forms, belonging to large families or orders in the animal kingdom, which show well-established and stereotyped normal features. Work on these questions will form one of the most fascinating features of future embryological study. VIII ARTHROPODA 221 ARACHNIDA Classification adopted— Delobranchiata Xiphosura ; Scorpionidea Pedipalpi Araneina Acarina Pentastomida (incertae sedes) Embolobranchiata The most primitive Arachnid living is undoubtedly the horse- shoe crab Limulus, the development of which has been studied by many authors, but most recently by Kingsley (1892, 1893) and Kishinouye (1893). The distribution of this genus renders it unsuit- able as a type of the Arachnida, for it is practically inaccessible to European students. A similar objection applies to the Scorpion which, on the whole, must be regarded as the most primitive of exist- ing land Arachnids; its absence from the temperate regions of both hemispheres is a serious drawback, and hence we select our type for special description from amongst the ubiquitous spiders, and take as our chief authority Kishinouye (1891-94). This observer has not only published the most thorough work on the subject but has examined the course of development in different genera belonging to different families, and found it identical in all important points. AGELENA Amongst the types described by Kishinouye there is one Agelena, a cellar spider, representative species of which are found all over the northern hemisphere, and one of which, A. labyrinthica, formed the subject of an embryological research by the late Prof. Balfour. Quite recently another author, Kautsch (1909, 1910), has also studied the development of Agelena labyrinthica. His conclusions in the main confirm those of Kishinouye, but in some points he has penetrated further in the analysis of the development of this species than Kishinouye; in other points, again, it seems likely that his variations from Kishinouye’s account will turn out to be incorrect. We shall therefore select Agelena as a type. The species of this genus can be kept and will breed in captivity. The eggs of is found in Polygordius, not by a massive inflow of large cells as in Patella. The blastopore, according to him, persists as the anus, and the stomodaeum is formed in front of it, so that the mouth is a new perforation. The prototroch appears, as in Patella, as a double circle of cilia carried by two rows of cells. On the ventral side of the intestine a median bilobed pouch is formed, which is the Fic. 244.—Vertical section of OT UM Of the mesoderm (Fig. 245). This pouch the gastrula of Paludina becomes cut off from the gut, loses its cavity, vivipara, (After Evlan- and gives rise to two irregular mesodermal ger.) bands which extend forwards at the sides of ee ae the gut. Each of these bands gives off a small compact mass at its anterior end (/.n, Fig. 246), which becomes converted into a larval kidney (Erlanger, 1894); while the rest of the streak breaks up into an irregular mass of stellate cells 1X MOLLUSCA 311 which span the blastocoele, extending from gut to ectoderm. In Fic. 245.—Formation of the coeclom in Paludina vivipara. (After Erlanger.) A optical frontal section of embryo in the stage when the coelom is being formed. B, sagittal section of embryo in this stage. C, transverse section in this stage. coe, coelomic pouch; g, gut; p.tr, prototrochal cells. the hinder end of each streak there is, however, a compact mass which becomes hollowed out to form a coelomic vesicle, the rudiment of one of the pericardial sacs. The two meso- dermic bands then fuse together in the middle line behind, and the pericardial sacs become pressed against one another so that their conjoined walls form a septum (sept, Fig. 247, B). Tonniges denies point-blank the existence of this ventral sac. According to him the adult mesoderm arises as an ectodermal proliferation, on each side of the middle line, which gives rise to the irregular mass of cells seen by Erlanger. Fic. 246.—Optical frontal section of an embryo of Paludina vivipara a little older than those represented in figure 245, (After Erlanger.) Ln, larval kidney ; m.b, mesoblastic band ; p.tr, prototrochal cells. Now, we may quite 312 INVERTEBRATA CHAP. confidently say that, whatever may be the true state of affairs, Tonniges is most certainly wrong. For what he figures as the earliest stages of the formation of the mesoderm in Paiudina are precisely similar to later stages in the development of the mesoderm in Physa, and other forms, whose cell-lineage has been worked out in the greatest detail. In all these cases the origin of the adult Fic. 247.—Two stages in the formation of the pericardium of Paludina vivipara. (After Erlanger.) A, horizontal section through visceral hump, of stage in which the two mesodermic band sare still separate—a rudiment of the pericardium has appeared in each. B, horizontal section through visceral hump of later stage, in which the two mesodermic bands have fused in the middle line to form the septum separatiug the right and left pericardial sacs. g, gut; l.per, left pericardial sac ; r.per, right pericardial sacs ; sept, septum formed by the opposed walls of the pericardial sacs ; tr, trabecula of cells crossing right pericardial sacs, mesoderm has been traced to the cells of the fourth quartette, which, as in Annelida, are part of the endoderm. The later products of the division of the mother cells of the mesoderm, it is true, often come into such close contact with the ectoderm that, if one had not a complete series of the earlier stages to examine, one would believe that there was demonstrative proof that the mesoderm was derived from the ectoderm; and indeed this very mistake has been made by other German workers in the case of other Mollusca (Meisenheimer, 1898, 1901, and Harms, 1909). The 1X MOLLUSCA 313 difficulty in the case of Paludina is that the complete series of earlier stages is not at all easy to obtain, since the earlier stages of development are passed through rapidly while the later stages of growth take much longer to accomplish. The chances are, therefore, that in any one womb nearly all the embryos will belong to the post-trochophoral or veliger stages; and Erlanger himself once told us that to find material for the adequate study of these earlier stages would require at least two months’ search. The opportunities for coming to a decision in the matter are far fewer than in the case of an ordinary Gastropod, whose eggs are laid by thousands, and where every desired stage can be had in abundance. ee : ae int Fic. 248.— Horizontal section through the visceral hump of an older embryo of Paludina vivipara than that represented in Fig. 247, to show the formation of the kidneys and the heart. (After Drummond.) H, rudiment of heart; hep, liver; int, intestine ; 1%, left kidney; l.per, left pericardial sac; r.ir, right kidney; r.per, right pericardial sac ; sept, evanescent septum between the pericardial sacs ; st, stomach. Erlanger’s account of the matter agrees in principle with what is known of the development of Annelida with very small eggs, like Eupomatus (Hydroides), where the mesodermal cells are budded out from the intestine. But of course the formation of a definite pouch is a far more primitive method of development than that so far described for any Annelid or Mollusc, and it is a somewhat strange thing that this mode of development should be found in Paludina, which cannot be described as a very primitive member of the class to which it belongs. There the matter must rest, since Erlanger has been cut off by an untimely death, till some other embryologist has the patience to thoroughly investigate this difficult subject. When we reach the stage of the development of the pericardium 314 INVERTEBRATA CHAP, of Paludina, agreement happily reigns among observers. The foot appears as a mid-ventral protrusion, the shell gland as a mid-dorsal shallow invagination. Just as in the case of Patella, the shell gland is everted and converted into a shell-forming area covering a visceral hump. The mantle fold and mantle groove appear in the same way as in Patella, and the torsion process takes place apparently slowly, as all stages in its com- pletion are often found. Before this happens, however, the rudiments of many organs appear. To begin with, the com- pact mass at the hinder end of each mesodermic band becomes hollowed out, as we have already seen, to form a small pericardial vesicle (Fig. 247), and from each of these vesiclesan evagina- tion, the rudiment of a kidney, is found. The two coelomic vesicles are at first separated by a septum, but this is soon absorbed and a single vesicle,the pericardium, results (Fig. 249). This lies ventral to the in- testine near the posterior end of the embryo. The Fic. 249.—Tlustrating the development of the ureters of rudiment of the heart Paludina vivipara and their relation to the kidneys. appears as a dip in the After Erlanger. (etter Helene) dorsal wall of thissac,and A, cut-off visceral hump of an embryo, rather older than ; . that represented in Fig. 248, viewed from below. B, horizontal in this Way a bag full section through the visceral hump of an embryo of the same of blastocoelic fluid is age as that represented in A. a.p, anal papilla; hep, liver; formed which hangsdown lk, left kidney ; l.wr, left ureter; m.f, mantle fold; per, peri- = t h c cigs cardial sac (the right and left pericardial sacs of the earlier into ti é pericardium and stage have fused) ; r.k, right kidney ; r.wr, right ureter. constitutes the heart Sate ed with its contained blood. The kidney on the right side becomes marked off from the pericardium by a constriction, and this narrow communication forms the reno-pericardial canal of the adult. On the left side the kidney rudiment remains small and thick-walled, and is also marked off from the pericardium by a constriction. The ureters or external sections of the kidneys arise as IX MOLLUSCA 315 ectodermal invaginations. They are in reality, however, only deeper portions of the mantle groove which intervenes between the mantle fold and the body wall (Fig.' 249). The embryonic stomach, whose cells are gorged with albuminous matter, has been shifted into the visceral hump, and in this way the alimentary canal takes ona U shape. The intestine is lined by small cubical cells, and, by an extension of cells of this description along the mid-dorsal and mid-ventral lines to meet the stomodeal cells, the embryonic stomach becomes divided into two lobes which, in later life, become converted into the two lobes of the liver. The median portion lined by small cells forms the adult stomach, and the radula sac arises as a ventral pocket of the:stomodaeum. Meanwhile the rudiments of the sense organs appear. The otocysts (ot, Fig. 250) arise as pocket-shaped invaginations of the ectoderm at the side of the foot. The eyes arise as similar invagina- tions of the pretrochal area (oc, Fig. 250); they are formed at the bases of two conical projections which are rudiments of the tentacles (ten, Fig. 250). The principal ganglia of the central nervous system arise as separate ectodermic thickenings, the commissures connecting them being formed only afterwards. The cerebral ganglia arise later as two thickenings of the velar region close to the eyes. The pedal ganglia arise similarly from the post-velar region close to the otocysts, at the sides of the foot. The pleural ganglia arise on the sides of the body higher up and further back; and lastly the visceral ganglia arise from the ectoderm of the mantle cavity, that is, the deepest and most posterior part of the mantle groove. All these ganglia and their commissures are established before torsion begins. Torsion now takes place, and the mantle cavity, with the opening of the anus and the two visceral ganglia, is rotated upwards and to the right, so that it passes along the right side in an oblique line until it reaches its permanent position on the back of the neck. This torsion involves the lengthening of the intestine into a recurrent loop, and the passage of the original right visceral ganglion upwards and to the left, where it forms the supra-intestinal ganglion, whilst the original left one is displaced to the right side and forms the sub-intestinal ganglion. The right ureter passes -upwards and to the left, and the left one eventually takes up a position on the right below it (Fig. 250, B). The pericardium and the persistent right kidney are displaced from their original position, underneath the gut, to a lateral position in which the kidney is above the pericardium. When the torsion is complete the velar cells disappear and the tentacles become long, while the foot develops its crawling surface, and on the upper aspect of its posterior portion the operculum appears. The gill appears as a series of outgrowths from the roof of the mantle cavity, and the embryo then takes on the general appearance of the adult (Fig. 251). 316 INVERTEBRATA CHAP. After the embryo has escaped from the womb of the mother, the genital organs develop. According to Miss Drummond (1902), the genital cells are budded from the pericardial wall, close to where the original left kidney joins it, and this kidney forms the first part of the genital duct. Coincidently with this development the peri- Fic, 250.—Two embryos of Paludina vivipara viewed from the right side in order to show the origin of the sense-organs and the beginning of torsion. (After Erlanger.) A, stage when the foot and visceral hump are both short, and the embryo is almost bilaterally symmetrical. B, stage when foot and visceral hump have become elongated, and in which torsion has begun. The arrow shows the direction in which torsion takes place. a, anus; f, foot; H, heart; hep, liver ; int, intestine ; J.n, larval kidney ; l.per, left pericardial sac ; l.wr, left ureter ; m.c, mantle cavity ; m.f, mautle fold; ac, eye; oes, oesophagus ; of, otocyst; r.per, right pericardial sac; 7.s, radula sac ; r.ur, right ureter; sh, shell; st, stomach; ten, tentacles; V, velum. cardium is shut off from the genital rudiment. The outer and longer portion of the genital duct is formed by the left ureter (Fig. 252). The torsion of the visceral hump, which results in the transference of the mantle cavity from the posterior aspect of the hump to its anterior face, is to be carefully distinguished from the spiral twisting of the hump, which is shown by the spirally twisted shell; since the Ix MOLLUSCA 317 shell may never be spirally twisted at all, as in Acmaea, and presumably in Patella, and yet the torsion may reach its extreme limit ; moreover, torsion is always complete before the spiral twisting of the shell begins, as is well seen in the development of Paludina. Both changes are due to the unequal growth of the two sides of the animal; but whereas torsion affects the whole area of the side of the body above the insertion of the foot, the inequality of growth resulting a pe Fic, 251.—Just hatched Paludina vivipara, Seen from the left side and viewed as a transparent object. (After Erlanger.) a.p, anal papilla; b.g, buccal ganglion; br, rudiments of the gill; eg, cerebral ganglion; gon, rudiment of genital organ; H, heart; hep, liver; int, intestine; l.wr, left ureter; m.f, mantle fold; oc, eye; op, operculum ; osph, osphradium ; ot, otocyst ; ped, pedal nervous cords; per, pericardium ; 7.k, right kidney ; 7.s, radula sac; r.ur, right ureter ; sal, salivary gland ; st, stomach ; v.1, visceral loop of the nervous system. in spiral twisting of the shell affects only a more dorsal region, leaving the floor of the mantle cavity unaffected. It is accompanied by a lengthening of the visceral hump—and this is associated by Miss Drummond (1902) with the outgrowth of the embryonic stomach so as to form the adult liver. The most plausible explanation of the inequality of growth, in both torsion and twisting, is that in the ancestral gastropod the lengthened visceral hump fell over to one side as the animal took to crawling over uneven ground. As a 318 INVERTEBRATA | CHAP. result the skin on one side would be stretched and stimulated to grow, while the skin on the other side would be crushed and its growth inhibited. Fia. 252.—Transverse sections through the visceral humps of two embryos of Paludina vivipara of different ages to illustrate the torsion of the organs, the development of the genital organs and their connection with the right kidney, and the lengthening of the visceral hump associated with an increase in length of the liver. (After Drummond.) A, younger stage in which the gonad is beginning as a thickening of the wall of the pericardium, and in which the left kidney is still distinct. B, older stage in which the gonad is connected with the rudiment of the left kidney. The right kidney has passed completely to the left. Letters as in previous figures. Such is, in outline, the organogeny of a Mollusc; and, as_ mentioned above, Erlanger’s work, except in so far as concerns the origin of the mesoderm, has been confirmed by the most recent workers on Mollusca. IX MOLLUSCA 319 METHOD OF RECONSTRUCTING ORGANS FROM SERIES OF SECTIONS A word on certain methods is here advisable. The method of handling and making sections of minute embryos has been fully described in Chapter II.; but in the examination of sections, the observer, in looking through a series, mentally synthesises the pictures presented by successive sections, and thus forms a conception of the organ of which they each form part. This, with a little practice, is a comparatively easy matter when one is dealing with a bilaterally symmetrical animal or any animal whose body can be compared to a cylinder; when, however, one is dealing with an animal which is twisted in a spiral fashion, like Paludina, the mental synthesis is a very difficult matter, and, to assist it, the following method of reconstruction is followed. The usual way of applying this method is to draw outline sketches of successive sections at a constant magnification, for example 200 diameters. If the sections are 5, thick, z.e. 35,5 of a millimetre, which is a usual thickness in dealing with minute objects, then, such a section, being magnified in all its dimensions to the same extent as the picture that is drawn of it, would be 1 millimetre thick. The drawings are therefore transferred to wax plates 1 millimetre thick, and the outline of the particular organ whose course it is desired to study is boldly drawn in a conspicuous colour. Round this outline the wax plate is cut away; and the pieces of successive plates are piled on one another in the proper order, and in this way the solid form of the organ is reconstructed. In theory the pieces from successive plates should fit exactly, but in practice it is found necessary to adapt them to each other by melting the edges with a hot scalpel. Prof. Graham Kerr has, however, elaborated a method by means of which practically as good results are obtained with infinitely less labour. Instead of wax plates he used square plates of fine ground glass of appropriate thickness. On these the outlines of successive sections of the organs which it is desired to reconstruct are drawn with a pencil of coloured chalk. The plates are now piled upon each other in proper order in a square glass vessel which is filled with cedar oil of the same refractive index as that of the glass used— both oil and plates being, of course, specially made for the purpose. The result of this arrangement is that the glass becomes absolutely invisible when immersed in the cedar oil, the coloured outlines of successive sections stand out boldly, and the solid form of the organ is conspicuous at the first glance. One great advantage of this method is that the materials, glass plates and cedar oil, can be used over and over again, since the pencil outlines are easily washed off. OTHER GASTROPODA We shall now examine how far the developmental history which we have described in Patella and Paludina is exemplified in the case 320 INVERTEBRATA CHAP. of the other Gastropoda which have been studied. We find that those primitive forms which preserve the original bilateral symmetry (the Polyplacophora, Chiton and its allies) possess a typical Trochophore larva, similar in all respects, so far as our knowledge goes, to that of Patella, The cell-lineage of Ischnochiton, as worked out by Heath (1899), seems to be exactly similar to that of Patella, and the formation of organs in the European Chiton polit, as described by Kowalevsky (1883), wears an even more primitive aspect than in Paludina; for example, the two pericardial sacs in Chiton poldi are large, and occupy most of the post-trochophoral region. A re-examination of the later larval history of Polyplacophora by the aid of refined modern methods would be of rare interest. A Trochophore larva is also found in the case of such primitive forms as Yrochus, which has been worked out in great detail by Robert (1902), and in Acmaea, Fissurella, and Haliotis, all of which retain two auricles in the heart and two kidneys. In all other Gastropoda, so far as is known, the embryo becomes a larva only when the Trochophore stage has been passed through and the coiled shell has been formed. Since, in almost every case, the eggs only develop after they have been laid, this involves their being laid in capsules secreted by the oviduct of the mother. Sometimes many eggs are contained in a capsule (Prosobranchiata generally), sometimes only one (Opisthobranchiata and Pulmonata), and in this latter case the capsules are very small and generally embedded in a jelly which is difficult to get rid of. The capsules of the Proso- branchiata are usually large, they are unprotected by jelly and attached singly to submarine objects such as stones. It is easy enough to detach them and slit them open, and in this way a supply of embryos can readily be obtained. It is interesting to see in some such cases (cf. Purpura) the beginnings of the same process which has led to such distortions of development as are seen in the case of Platyhelminthes. Some of the embryos in a capsule develop imperfectly and are swallowed by their successful sisters, to whom they serve as pabulum. In dealing with the egg-capsules of Pulmonata and Opisthobranchi- ata, which are immersed in jelly, various methods are employed. Sometimes, as in the case of the Opisthobranch Fiona, it is found possible to use reagents (for example, picro-acetic and picro-sulphuric acids) which will preserve the whole mass in bulk, and the jelly with the contained egg-capsules can then even be embedded in paraffin; but usually it is necessary to remove both jelly and capsules. We may give Wierzejski’s method of dealing with the eggs of Physa (1905) as a good example of how this can be accomplished. The egg-capsules are dissected out of the jelly by needles, then they are immersed for two or three minutes in a mixture of the solution of corrosive sublimate in water and glacial acetic acid, and the adhering jelly is then removed by the action of Ix MOLLUSCA 321 distilled water, in which it is soluble. The egg-capsule is then opened by a prick with a needle, and the embryo, as it floats out, is taken up in a pipette and put into 30 per cent spirit, which is after a time exchanged for 50 per cent spirit, and so by degrees the embryo can be immersed in absolute alcohol. This method is employed when it is desired to cut sections of the embryos. For the study of cell-lineage, however, another method is pre- ferred. The whole mass of jelly with its contained egg-capsules is thrown into a mixture of equal proportions of Perenyi’s fluid and water. The jelly at first turns milky but gradually becomes clear. Then the capsules are opened by needles and the embryos float out. They are put into 15 per cent alcohol, and then into 30 per cent for twenty-four hours, and then gradually brought through higher grades of alcohol into absolute alcohol. The right moment to open the capsules must be carefully observed; if that time be allowed to pass the jelly turns milky again. A method of staining with silver nitrate was also employed by Wierzejski, and when successful it caused the outlines of the cells to be indicated by brown lines. A-75 per cent solution is used; in this the capsules are allowed to stay until they are brown, then they are opened and the embryos washed with alcohol. Wierzejski mounts his embryos in a mixture of balsam and clove oil, which remains sufficiently fluid to allow the embryos to be rolled about by the motion of the coverslip. The coverslip is supported by thin pieces of paper, or by thin pieces of glass tube drawn out to the proper degree of tenuity. By slight modifications of this method the eggs of all Opisthobranchiata and Pulmonata can be dealt with. The development of a Gastropod in which the larval stage begins at the time when shell and foot have been formed, bears the same relation to that of Patella as the development of Nereis sustains to that of Polygordius. The most important differences are the ex- tremely early indications of the future asymmetry, and the reduction of the prototroch. In Crepidula, which has been studied by Conklin (1897), and in Fiona, which has been worked out by Casteel (1904), for example, only the anterior primary trochoblasts (7.e. the de- scendants of la? and 1b?) develop cilia, the greater part of the ciliated band being formed from “ secondary trochoblasts,” which include the tip cell 2b" and descendants of 2b! and 2b”, and even (in Fiona) of certain cells of the third quartette. If this description is followed it will be seen that the velum consists principally of two anterior lobes; the circle is completed in Orepidula by a band of ciliated cells which runs across the anterior hemisphere of the larva in front of the apical cells, in Fiona by an ill-defined ciliated area extending over the posterior part of the front hemisphere, and in Physa not at all. When, as in many Prosobranchs, the state of affairs is as in Crepidula, the eyes and ’ tentacles are excluded from the velar aréa, and hence the homology of the velum with the Annelidan prototroch has been seriously VOL. I Y¥ 322 INVERTEBRATA CHAP. questioned; but when the structure of the embryos of Patella, Trochus, and Chiton was elucidated, the real homology of velum and prototroch became apparent. Another difference is seen in the development of the “cross.” This becomes far more conspicuous in the case of most Gastropods than it does in Patella. The terminations of its four arms are formed by the “tip cells” 2a1, 2b", 2c4, and 2d4. The basal cells are of course 1a!2!, 1b!21, 1cl2!, and 1d, while tucked away between the four basals and the apical cells 1a, ete., are the so-called “peripheral rosettes” 1a!!2, ete, which represent the Annelidan cross, but which do not divide more than once in Mollusca and hence do not attain any great development. The basal cells of the Mollus- can cross, on the contrary, divide several times transversely, and then the daughter cells in the a, b, and ¢ quadrants become longi- tudinally divided into two and even into four rows of cells (Fig. 252). In the d quadrant they remain undivided longitudinally for a considerable time, but also Fic, 253.—Apical region of an embryo of Crepidula showing the Molluscan cross in a late stage of development. (After Conklin. ) The apical cells are unshaded, as are also the primary trochoblast cells. The ‘peripheral rosette ” cells (Annelidan cross) are marked with small circles. The cells of the Molluscan cross are ruled with horizontal lines, except the derivatives of the tip cells, which, since they belong to the second quartette, are dotted. The most interesting thing eventually divide, filling up the gaps between the apical and the prototrochal cells; the latter usually divide only once, forming eight “turret cells,” of which only the anterior, as we have seen, develop cilia. These points can be well seen in the segmenting ege of Crepidula (Fig. 253). that has been elucidated in the development of these more modified Gastropods is the relation of the organs of the veliger to certain groups of cells in the cell-lineage. Thus, in Planorbis it is found that the cerebral ganglia arise by internal proliferation from the lateral arms of the cross, except from their tip cells and the cells immediately adjoining these; from the anterior arms, except from their tip and basal cells; and from the two hinder arms of the Annelidan cross. In Crepidula, Physa, and Fiona it is found that the mother cell of the mesoderm, 4d, divides as usual into right and left cells, 4d" and 4d. These then bud off two small anterior cells whose fate is to become endodermic, while the mesodermic mother cells divide into equal parts; so that we have four large mesodermic mother cells, two on each side. From each of the inner pair of mother cells a second small cell is given off; and these, with the first two cells, form a group of four small cells which lie close to the macromeres behind. IX MOLLUSCA 323 When the macromeres, by further division, have formed the larval stomach, these four cells seem to give rise to the hinder part of the intestine. They may be termed mesendoderm. There is no doubt at all that a similar state of affairs will be found in Patella when the cell-lineage has been fully worked out. It is another proof that the coelomic cells are essentially endodermic in origin. The so-called mesectoderm or larval mesoderm, consisting of ectodermal cells which wander inwards and are converted into the muscles of the larval oesophagus, is derived in Mollusca generally from the anterior quadrants (a and b) of the third quartette. The stomodaeum in Miona, Crepidula, and Physa arises from 2b222 and 2b? in front, and at the sides from cells of the third quartette, to a certain extent, as in Polygordius. In these Mollusca, however, these third-quartette cells succeed in excluding 2a”? and 20222 from the sides of the stomodaeum, whilst behind it is closed entirely by third-quartette cells. In this case 2a”? and 2c”? give rise to lateral ridges of cells, between which and the opening of the mouth there are grooves filled up by third-quartette cells. In these grooves two pits appear (Physa) which form the rudiment of the radula sac. The ridges formed by 2a”? and 2c”? meet behind the mouth so as to enclose the pits and the mouth in a common atrium or outer stomodaeum ; so that eventually 2a?” and 2c”? do form the outer stomodaeum. The foot in primitive forms, such as Trochus and Patella, is at first double; it arises from descendants of 2d, in the region of the ciliated groove. The most puzzling things about Gastropod Mollusca are the larval kidneys. These appear to be absent in Marine Proso- branchiata, but are found both in Pulmonata such as Limnaea and in fresh-water Prosobranchiata such as Paludina. In these forms they consist of V-shaped tubes with the apex of the V directed forwards, and they are formed of one huge giant cell. The internal end, we. the upper limb of the V, is a solenocyte; the lower limb opens to the exterior not far behind the head and a long way in advance of the opening of the permanent kidneys. At first sight one would be strongly inclined to regard these kidneys as equivalent to the archinephridia of Annelida. But the painstaking analysis of Wier- zejski (1905) has conclusively proved that, in the case of Physa at any rate, the larval kidney arises from three cells at the anterior end of a row which is budded from the outer mesodermic teloblast on each side; and Erlanger (1894) found that in Paludina the larval kidney was segmented off from the anterior tip of the coelomic vesicle on each side. Therefore this type of kidney really belongs to the type which Goodrich terms coelomiduct, and its appearance would seem to indicate that Gastropoda originally possessed two pairs of coelomic kidneys—a conclusion which on other grounds may be regarded as extremely probable. In Opisthobranchiata, on the other hand, there is found either 324 INVERTEBRATA CHAP. one or a pair of dark excretory vesicles situated extremely far back in the neighbourhood of the anus. This vesicle, in Aplysia, has been supposed by Mazzarelli (1898) to be the rudiment of the permanent kidney, but Holmes (1900) declares that in Hiona the main part of it is formed from a large cell which he identifies as 3c. In Umbrella Heymons (1893) finds a pair of these organs which arise from 3c} and 3d4. They thus roughly correspond to the archinephridia of Polygordius in position. The whole uncertainty in the matter arises from the fact that _ in no single species of Opisthobranchiata have we the complete developmental history of the organ from its earliest origin in the embryo until the larva has metamorphosed into the adult; and Fic. 254.—Embryo of Limnaea stagnalis viewed from right side as a transparent object in order to show the larval kidney. (After Erlanger.) a, anus; ¢.o, external opening of the larval kidney ; f, foot; hep, lobes of liver; Um, larval kidney ; n.c, nerve collar; 0, mouth; r.s, radula sac; sh, shell; sol, solenocyte of the larval kidney; sé, stomach. Comparative Embryology, so long as it is based on bits and scraps of development, is bound to be full of obscurities and apparent contradictions. In Marine Prosobranchiata there are frequently present two external protuberances of ectoderm cells, situated on each side and behind the velum. These become filled with excreta, and are eventually cast off. When it is remembered that the archinephridia of Polygordius owe their origin to ectodermal cells, it will be seen that it is quite possible to regard these external nephridia as homologous with them. SOLENOGASTRES We now pass on to consider the developmental history in other classes of Mollusca. The Solenogastres are an extremely primitive IX MOLLUSCA 325 and at the same time a degenerate group, in some of which the ventral ciliated groove is retained throughout life; but they have been shown in one case to possess a typical Trochophore larva. This case is that of Dondersia, and the Trochophore is gradually converted into the adult form by the elongation of the post-trochal region. SCAPHOPODA (Dentalium) - The Scaphopoda, with tubular shell and mantle, are represented by Dentalium and a few closely allied genera. The development of Dentalium bas been worked out by Wilson (1904). It is practically of world-wide distribution, being found in muddy bottoms. Wilson found the eggs of the Mediterranean species ripe in June, and he gives the following description of them. ‘They are yolky and deeply coloured by pigment which varies in tint from olive-green to brownish-red. When dehisced from the ovary the egg is almost as flattened as a biscuit, though one side is more flattened than the other, and this side is proved subsequently to be the vegetative pole. In the centre of each flattened surface is a white non-pigmented area. After remaining in sea-water for from twenty to thirty minutes the egg becomes spherical and bursts its ovarian membrane or chorion. A jelly-like layer which surrounds the egg then swells up, and the egg now looks like a sphere with white poles and a broad ring of pigment. But when the egg is fixed in picro-acetic and cut into sections, the two poles are seen to be widely different. At the vegetative pole there is a dense mass of cytoplasm, devoid of yolk, which is continuous with a thin layer of clear cytoplasm surround- ing the egg. This mass of cytoplasm also extends upwards through the egg to the germinal vesicle or nucleus, which is situated near the animal pole and surrounds it. At the animal pole there is a minute dise of cytoplasm free from yolk, which is far too small to account for the large white area seen in the living egg in this region. This latter must owe its appearance therefore to the presence of yolk granules of white colour. As the egg lies in sea-water the cytoplasm of the animal pole slowly increases in amount, seemingly by a radial inflow from sur- rounding regions. The wall of the nucleus now breaks down and the first polar spindle is formed. Things are now at a standstill until the egg is fertilized, when the two polar bodies are formed one after the other. The spermatozoon enters at the vegetative pole. From this pole a pillar of granular cytoplasm extends upwards and becomes temporarily confluent with the cytoplasmic area at the upper pole. This pillar is in large measure produced by the material which was contained in the nucleus of the unripe egg, and which was extruded when the nuclear wall broke down. _ The first cleavage occurs half an hour after fertilization, and is vertical. At the same time the lower white pole of the egg is cut off 326 INVERTEBRATA CHAP. from the rest by a horizontal constriction. Sections show two things —first, that this lower sphere contains, besides the vegetative cyto- plasm, a certain amount of yolk; and secondly, that it remains 1n connection with one of the two spheres produced by the vertical cleav- age, by means of a thin pedicle never completely severed. As the cleavage of the two blastomeres from one another becomes complete, the lower sphere coalesces with one of the two upper spheres, and the blastomere so formed is shown afterwards to be CD. The lower sphere has been somewhat inappropriately named: the yolk lobe, for Fic, 255.—Vertical sections of the eggs of Dentaliwm before and after fertilization in order to show the flow of cytoplasmic substances. (After Wilson.) A, before fertilization—after extrusion from the oviduct. B, after fertilization—formation of the first polar spindle. C, division into two blastomeres—extrusion of the first polarlobe. g.s, germinal spot, i.c. nucleolus; g.v, germinal vesicle ; 1.w.s, lower white substance; n, the first two daughter nuclei separating from each other; 1, first polar lobe; u.w.s, upper white substance.: which Wilson substitutes the name polar lobe, and its fusion with one of the two blastomeres is known as the retraction of the polar lobe. When the next cleavage occurs, AB of course divides into A and B, and the polar lobe is again constricted from CD, but at the con- clusion of the cleavage it fuses with D, which is thus rendered by far the biggest of the first four blastomeres. A and C, moreover, as usual, meet in an upper cross furrow. Each of the four cells contains a portion of the white area which was situated at the animal pole of the egg, but only D has the white material of the vegetative pole. IX MOLLUSCA 327 At the next cleavage the first quartette of micromeres are given off. These consist entirely of the white material, though some of this still remains in the macromeres. The polar lobe is again constricted off from D, but it is much smaller than before and the constricting furrow does not extend so deeply. When the cleavage is complete the polar lobe again fuses with 1D. Before the next cleavage occurs the white material derived from the animal pole, part of which was left in each macromere, increases Fic. 256.—Stages in the cleavage of the egg of Dentalium. (After Wilson.) A, completion of the first cleavage. B, beginning of the second cleavage, seen from the side. C, tlic second cleavage in its most intense period, seen obliquely from above and the side. pl, the first polar lobe ; 2, the second polar lobe ; p.b, polar bodies. in amount, moves over to the right side of each cell, and extends somewhat down the side. In 1D this also occurs, but in this cell the white material from the animal pole is joined by the white material from the vegetative pole, which moves over and fuses with it. Of the second quartette of micromeres 2d is formed first, and it is composed of the white material derived from both poles, whereas 2a, 2b, and 2c, which are formed soon afterwards and likewise consist of white material, have received only white material which was originally at the animal pole of the egg. At the same time the first-quartette cells divide into the trochoblasts (1q?) and the upper cells, the latter 328 INVERTEBRATA CHAP. being slightly larger. The third quartette is formed as usual at the next cleavage; 3d is larger than its sisters, and entirely composed of white material. After this cleavage gastrulation begins by the macromeres passing bodily into the blastocoele, just as in Patella. Of the fourth quartette 4d alone was clearly observed; it is smaller than 3d and very much smaller than 2d, and is pure whte. Penis As in Patella, cilia are developed about ten hours after fertilization, Fic. 257.—Further stages in the cleavage of the egg of Dentalium. (After Wilson.) A, beginning of third cleavage (8-blastomere stage), seen from the lower pole. 3B, the formation of the second quartette of micromeres, seen from the lower pole. The greater part of the substance which formed the polar lobes passes into 2d. C, the formation of the third quartette of micromeres, seen from the lower pole. D, the division of the macromere which gives rise to the mother cell of the meso- deri. p*, third polar lobe. and in twenty-four hours well-developed Trochophore larve are set free. These are remarkable for their very broad prototroch, which consists of three complete circles of large cells with cilia. The pre-trochal region is short and conical; it is covered all over with short cilia, and it bears at its apex an apical plate with a long tuft of motionless but flexible cilia. The post-trochal region is also short and conical, and at its posterior end there is a telotroch consisting of a tuft of short rigid hairs. The stomodaeum has not yet opened into the gut. This latter consists of a sac-like stomach and a short blind intestine; the anus IX MOLLUSCA 329 is not yet formed. At the sides of the intestine are seen two short mesodermal bands. are seen lying to the right and left. Theseare proliferated from the ectoderm, and are almost certainly the beginnings of the cerebral ganglia. Very soon after the beginning of larval life the rudiment of the shell gland can be made out, and the everted edge of this already foreshadows the future mantle fold, which is at first double, like that of a Pelecypod. During the course of the next day the larva sinks to the bottom ; the pre-trochal or velar region becomes relatively smaller whilst the post-trochal region grows very much in length, and then the velar region becomes finally com- pletely invaginated, and in this way the larva attains the stage of a veliger., By the end of the second Fic. 259.—Transverse section of the Trocho- phore larva of Dentaliwm in the region of the prototroch. (After Wilson.) . Letters as in previous figure. In the pre-trochal region two masses of cells Fic. 258.—The Trochophore larva of Den- talium—twenty-six hours after fertilization. (After Wilson.) : a.p, apical plate ; mes, mesoderm ; p.tr, prototroch ; st, stomach ; stom, stomodaeum ; t.tr, telotroch. day not only is the shell gland everted but a delicate hyaline shell has been formed, and into this the diminished prototroch or velum can be withdrawn. The foot has now made its appearance as ® median ridge. At the end of the third day the foot has become large, protrusible, and bilobed at its free end; and the mantle lobes have partially united beneath the animal. By the fifth day the prototroch has disappeared and the otocysts and pedal ganglia can clearly be seen; the metamorphosis may now be said to be complete. It is worthy of note that the northern species of Dentaliwm, which was studied by Lacaze-Duthiers, took twenty-five days to reach the same stage. - 330 INVERTEBRATA CHAP. EXPERIMENTAL EMBRYOLOGY OF DENTALIUM This peculiar development offers abundant opportunity for experiment, as Wilson was not slow to perceive. Some of these experiments were quite similar to those which he performed on Patella, and led to similar results; but the most interesting results were those obtained by removing the polar lobe, which can be readily done by means of a fine scalpel. When this is done at the time of the first cleavage the embryo continues to develop, but all the cells at the second cleavage are equal in size and possess no lower white area, and no polar lobe is subsequently formed. At the subsequent cleavages the micromeres given off in the D quadrant are precisely similar in size to their sisters, and the Fie. 260.—Veliger larvae of Dentaliwm. (After Wilson.) A, Veliger larva, thirty-two hours old. B, Veliger larva, three days old. Letters as in previous figure. In addition, f, foot ; mf, mantle fold ; sh, shell; V, velum. embryo becomes a larva with a normal prototroch and a conical pre-trochal region ; but there is no projecting post-trochal region, the posterior surface of the larva being almost flat. The pre-trochal region is covered, as normally, with fine cilia, but the apical tuft is absent, and so is the thickened apical plate which is present in normal larvae. On the other hand, the lateral ingrowths of ectoderm, which we suppose to represent the cerebral ganglia, are present. Such larvae live four days and then disintegrate. Occasionally a post-trochal protuberance appears to be formed, but when this is examined by sections it is seen to be a plug of solid endoderm pro- jecting through the open blastopore. No mesodermal bands are ever _ pre-trochal region, has too large a IX _ MOLLUSCA 331 seen. The conclusion is therefore inevitable that the first polar lobe contains the material necessary not only for the formation of the whole post-trochal region, but also for the formation of the apical plate. It must also contain the material necessary for the formation of the mesodermic bands. This conclusion is confirmed by separation of the first two blasto- meres ; both of these, when isolated, continue to segment as if they Fic. 261.—Larvae resulting from the development of eggs of Dentaliwm from which the polar lobe has been removed. (After Wilson.) A, larva, twenty-four hours old, developed from egg from which the first polar lobe has been re- moved. B, Larva, twenty-four hours old, developed from egg from which the second polar lobe had been removed. Letters as in Fig. 258. formed part of a whole egg, but both subsequently give rise to larvae which swim about, though they possess a confused irregular proto- troch. The larva derived from AB, however, in its general structure, resembles the larva developed from a whole egg from which the first polar lobe has been cut off, because it has neither apical plate nor post- trochal region. The larva derived from CD, on the other hand, which carries the polar lobe, though it is asymmetrical and has too small a post-trochal one and possesses a well-defined apical plate. If the egg be allowed to reach the 4-cell stage, and if the polar lobe that is then protruded, that is Fic, 262.—Vertical section of a larva of the second one, be removed, a larva Dentalium developed from egg from is produced in most respects similar which dr gee eaaie e e tape 1. . 4 removed, to show e absence oO to the one which arises from an egg iuesodentay (ater Wilin) from which the first polar lobe is end, protruding plug of endoderm. removed; it possesses neither meso- dermic bands nor post-trochal region, but it possesses an apical plate and the characteristic apical tuft of cilia. Therefore the second polar 332 INVERTEBRATA CHAT lobe does not contain the specific organ-forming material for the apica plate; in the interval between the formation of the first and secon polar lobes it has been distributed to a different region of the egg. Where that region is it is not difficult to determine. If th micromeres of the first quartette be separated from each other by allowing the embryo to develop in artificial sea-water devoid o calcium, then each micromere will develop into a closed ectodermi vesicle; but only the micromere 1d develops an apical plate, and th apical “stuff” is therefore transferred to this micromere. Now, in Patella the apical plate is formed in larvae developed from each o the four micromeres of the first quartette; we have therefore in th development of Dentalium a case of specialization, similar to tha which we often meet with in eggs with spiral cleavage, in whicl one member of a quartette does the work normally undertaken by al the sisters in other species. A case of this kind was met with ir the first case of spiral cleavage which was studied, namely, in the development of Planocera as compared with that of other Polyclad: Platyhelminthes. These remarkable experiments of Wilson establish in the most incontrovertible manner the existence of specialized organ-forming substances in the egg of Dentaliwm. It is but fair to add that th first experiments of this kind were made by Crampton (1896) on th egg of the Gastropod lyanassa, where a similar polar lobe is found. PELECYPODA— Dreissensia We must now consider the development of that great group o Mollusca familiarly known as bivalves and scientifically as Pelecy: poda or Lamellibranchiata.” The most complete and satisfactory study of the development of any form belonging to this group is that by Meisenheimer (1901) on the life-history of Dreissensia polymorpha This type we may therefore select for more special study. Dreissensia is a genus found in brackish and fresh water both ir England and on the continents of Europe and America. In form i closely resembles the marine genus Mytilus, the common mussel, t¢ which it is regarded by many authorities as nearly allied, and: fron which it differs in having the two mantle lobes firmly united for part of their length in the mid-ventral line, and in having thé posterior opening prolonged into two separate tubular siphons. Ii is interesting from the fact that, though a fresh-water species, it retain: a long larval development of very primitive facies, whereas most fresh water species have a shortened, modified,and mainly embryonicdevelop ment. Dreissensia is clearly a recent immigrant into fresh water. Meisenheimer obtained his material from one of the small fresh water lakes of Germany (the Ploner See). The eggs of Dreissensi polymorpha are laid in June, and are cast forth from the mother it masses, bound together with a slight amount of slime which is easil washed away. The eggs have no chorion of any kind, and hence ar 1X MOLLUSCA 333- quite easily preserved. For the earlier stages corrosive sublimate and picro-sulphuric acid were the reagents used, but for the later stages and for the free-swimming larvae Hermann’s mixture of osmium tetroxide, platinum chloride, and acetic acids gave the best results. It was necessary to paralyse the larvae by cautiously adding cocaine to the water in which they lived, before attempting to preserve them, otherwise they contracted themselves into shapeless lumps in which the natural relationship of the various organs could not be made out. The larval stages swarmed in the lake and were captured by using a fine-meshed Plankton net, so that the difficulties connected with artificial rearing were entirely avoided. A striking feature of the early development of Dreissensia is the intermittent appearance of the blastocoele. This cavity is large and well developed in the 2-cell stage (Fig. 263); it subsequently disappears, but reappears in later stages, such as the 8-cell and 16-cell stages. ee iw coele serves as a reservoir of excreta ps which are periodically voided. ane c The egg divides into the usual four macromeres A, B, C, and D, but of \ oe a these D is so much ‘larger than the rest Nea” ; like micromeres budded from one large fic. 263.—Longitudinal section of macromere. This state of affairs is the 2-cell stage of Dreissensia worth bearing in mind in view of the oa ae ae extraordinary statements which have is lathecs ten seine been made about the development of 1d appears first and is the largest, though the disparity in size between it and its sisters, la, 1b, and 1c, which appear subsequently, is not great. But at the next cleavage, when these micromeres divide, each into two daughters of equal size, and when the second quartette of micromeres is formed, one of these latter, 2d, is relatively enormous first and second quartettes but its own sister macromere, 2D. This huge “micromere” corresponds to the one which Wilson, in the development of Nereis, has termed the first somatoblast, from which most if not all the ectoderm covering the body of the adult worm is derived. X, is derived the shell-gland, and we have strong reason to suggest, although this is not quite proved, the foot; it has, however, been proved by Lillie (1895) in the case of Unio. The first somatoblast now gives off a cell below and to the right. This cell is of course 2d?: it is denominated by Meisenheimer x,, since he calls the parent Meisenheimer supposes that the blasto- that the remaining three appear much ble. other Pelecypoda. When the first quartette of micromeres is formed, in size; it overshadows not only all the micromeres belonging to the From the first somatoblast of Dreissensva, termed by Meisenheimer somatoblast X. At the next cleavage all the daughters of the first 334 INVERTEBRATA CHAP. quartette divide again, so that we get four concentric circles of cells, 1g", 1q¥, 1q2, and 1q”. The third quartette of micromeres now begins to be formed, 3d being formed before its sisters. X gives rise to a small cell on the left, the proper title of which is 2d!?, but which is called by Meisenheimer x, The somatoblast has thus acquired at its lower border a wreath of three cells, x,,3d, and x,. Of the second quartette of micromeres, which should have divided when the third quartette was being formed, only Fic, 264. —Stages in the cleavage of the egg of Dreissensia polymorpha. (After Meisenheimer.) A, upper hemisphere of egg in the 16-cell stage. B, upper hemisphere of egg just passing into the 54-cell stage. The formation of the apical cells is seen. C, egg seen from the vegetative pole in the 8-cell stage at the moment when 2d is being formed. D, Posterior view of egg in a somewhat later stage than that shown in B, to show the primary mesoderm cells and some of the products of the division of X. M, primary mesoderm cell ; p.b, polar body. one (2d=x) has as yet divided. Another member of this quartette now divides, 2.e. 2¢; whilst 22 divides into 2d2! and 2d” —or, according to another notation, x, gives x,, and x,,. It is quite clear therefore that in Dreissensia, unlike Patella, the radial symmetry of the spiral type of cleavage is very early interfered with, and that the prospective importance of the organs derived from 2d is reflected back into a very early stage of ontogeny; this is testified to by the precocious divisions and development of the cells derived from this blastomere. The remaining members of the third quartette of ectoderm cells, IX MOLLUSCA 335 3a, 3b, and 3c, are now budded off from their respective macromeres. Only after this has happened do the anterior cells of the second quartette, viz. 2a and 2b, divide into 2a! and 2a?, 2b! and 2b?, respec- tively; whilst the somatoblast X buds off from its upper border a small cell x,. All the cells of the first quartette now undergo renewed cleavage, so that we have eight circles of cells, viz. 1qg™, 1q™, 1q!!, 1q!22, 1q24, 1q?, 1q”!, and 1q. In these divisions the members of each circle belonging to the D quadrant divide before their sisters. We have thus in Dreissensia the same typical divisions of the cells of the first quartette which are found in Patella and Polygordius; but Meisenheimer does not refer to or figure any conspicuous cross-like arrangement of any of these cells; on the contrary, he seems to imply that they continue to have a concentric arrangement. At the lower pole of the egg a single representative of the fourth quartette is now given off. This is 4d, which Meisenheimer calls the “second somatoblast”; but it is uf course homologous with the mother cell of the mesoderm in both Polygordius and Patella. At first the mother cell of the mesoderm, which we may designate as M, touches the second somatoblast; but the latter gives off a cell towards the vegetative pole which Meisenheimer calls x,, and this, along with x, and x,, completely separates X and M (Fig. 264, D). After a few more divisions in the cells of the first quartette, the first unmistakable traces of bilateral symmetry make their appear- ance by the division of both X and M into right and left halves. Then from each half of X a small cell, x,, is budded off posteriorly, and the arrangement of the derivatives of the first somatoblast is as shown below, viz. : X-4#,- X a BH, — %,— Hy — He — wy. Each half of M also buds off a small cell, and then, by repeated transverse divisions, a longitudinal plate of large cells which is the rudiment of the shell gland is developed out of the two halves of X. Following the stage which we have just described, the process of gastrulation begins. The residual macromeres 4A, 4B, 4C, and 4D sink inwards towards the blastocoele. The small cells given off from the mother cells of the mesoderm sink in with them and go to build up the wall of the mid-gut. The mother cells should therefore be termed mesendoderm, not true mesoderm ; they themselves lie posterior to the lip of the blastopore, and are partly invaginated with the endoderm in the process of gastrulation. By repeated division they give rise later (just as in Paludina) to a loose mesenchymatous mesoderm, out of which the connective tissue and muscles of the adult bivalve are formed. The invagination of the mid-gut cells proceeds at first very slowly, because their progress is impeded by the much more rapid and conspicuous invagination of the cells forming the shell gland. This latter deep invagination lasts only a short time. Soon the cells forming the shell gland are again everted and form, as in Patella, a 336 INVERTEBRATA CHAP. saddle-shaped plate with thickened edge, on the dorsal surface of the larva. On this plate a thin horny secretion, the first rudiment of the shell, appears. As the process of eversion takes place the in- vagination of the endoderm goes on rapidly, and soon a sac is formed whose wall is composed of large columnar cells, and which opens to the exterior by a constricted opening, the blastopore (Fig. 265). Fic. 265.—Sagittal sections of embryos of Dretssensia polymorpha, showing the process of gastrulation and the formation of the shell gland. (After Meisenheimer. ) A, stage in which the endoderm and the shell gland are both beginning to be invaginated. B, stage in which the invagination of the shell gland has reached its maximum. , stage in which the blasto- pore is closed and the shell gland is beginning to be evaginated. D, stage in which the shell gland is completely evaginated and the stomodaeum is beginning to be formed. a.p, apical plate ; bl, blastopore ; end, endoderm ; hep, cells which will eventually form the liver ; M, primary mesoderm cell ; p.tr, proto- troch ; s.g, shell gland ; stom, stomodaeum. The blastopore becomes shifted forwards and finally closed in the position where the mouth afterwards opens. This forward shift seems to be largely due to the growth of the band of small cells, x, - x,, derived from X, which separated originally X and M on the posterior surface of the embryo. ‘This band thus comes to occupy the region immedi- ately behind the mouth; and as the fuot is later developed in this region, it probably owes its origin to these cells. When the blastopore has been completely closed, the stomodaeum originates as IX MOLLUSCA 337 an ectodermic invagination just where the last trace of the blastopore was situated. The wall of the mid-gut, after the blastopore has become closed, undergoes a characteristic differentiation. The cells forming its anterior wall acquire large clear nuclei with conspicuous nucleoli, whilst those forming the lateral and posterior walls retain small deeply staining nuclei. Soon the peculiar cells of the anterior wall become confined to two slight outpouchings of the wall of the stomach, to the right and the left of the mid-ventral line. These pouches of the larval stomach will eventually give rise to the adult liver. From the posterior wall of the stomach is developed the intestine, and this grows 3 backwards and becomes attached to the ectoderm behind the mouth. Here a very shallow invagination is formed, the proctodaeum ; and at a slightly later stage, by the union of the procto- daeumand intestine, the anus becomes opened. In front of the anus is formed the teloroch, consisting of a 3 couple of cells carrying stiff \ 2) —~stom hairs. . “OBIS : So far we have not men- SI tioned the prototroch and the apical plate. Both these structures appear about the time when the shell gland is everted ; the proto- Fic. 266.---Young Trochophore larva of Dreis- troch is in the form of a sensia polymorpha, seen from the ventral side. girdle of cells carrying (After Meisenheimer. ) powerful cilia, and the apical Letters as in previous figure. In addition, t.tr, telotroch, plate in the form of a group aes of cells at the animal pole bearing a wisp of long stiff cilia. The cell- lineage of the cells forming these organs Meisenheimer was not able to determine, but there is no reason to doubt that it is, in the main, the same as in Patella. The prototrochal cells develop vacuoles in their interior, as is the case with the prototrochal cells of Polygordius. Lastly, situated just behind the spot where the anus will develop, there is a group of small cells which Meisenheimer believes to be of ectodermal origin, which will give rise, at a later period, to the coelomic sacs and to their derivatives, the kidneys and genital organs. This cell-group occupies precisely the same place as does the first rudiment of the pericardium in Paludina, and as do the mother cells of the mesoderm in an earlier stage of development in Dreissensia. When this stage of development has been attained, the VOL. I Z 338 INVERTEBRATA CHAP. embryo bursts the egg-membrane and enters on its free-swimming life as a Trochophore larva (Fig 266). In Physa, as we have already seen, Wierzejski has traced the pericardium back to its origin in the derivatives of the mother mesoderm cells, through an unbroken series of stages. For these reasons we reject Meisenheimer’s view of the origin of these cells, and believe that they are derived from the mother mesoderm cells after the latter have given off the mesenchymatous tissue alluded to above. This view would bring the develop- ment of Pelecypoda into harmony with that of other Mollusca, and should be definitely tested. The Trochophore larva soon Fic. 267.—Sagittal section through a young passes into the condition of a Trochophore larva of Dreissensia polymorpha. Veliger larva. This change (After Meisenheimer. ) takes place by the enlargement Letters as in Figs. 265 and 266. In addition, coe, of the prototroch into the group of ala rom whic the clon Gercardivm) velum and by the growth of adult hinge). the bivalve shell. Behind the prototroch several rows of Fic. 268.—Transverse section of the ventral portion of a young Veliger larva of Dreissensia polymorpha to show the origin of the mantle-groove and of the pedal ganglia. (After Meisenheimer. ) m.c, mantle-groove ; p.g, thickenings of ectoderm which will give rise to the pedal ganglia ; sh, shell. large cells are differentiated; they are covered with numerous fine cilia and reinforce the action of the prototrochal girdle; this enlarged x MOLLUSCA 339 structure is now known as the velum. These additional cells remind us of some of the “secondary trochoblasts” of Patella. The shell of the Trochophore is merely a thin horny cuticle secreted by the cells of the everted shell gland. This cuticle adheres closely to the ectoderm in the mid-dorsal line, and the ectoderm cells here become columnar ; this region constitutes the hinge of the adult shell. The cuticle adheres loosely towards the sides, but at the edges of the shell gland a renewed deposition of cuticle takes place in two small circular areas which rapidly extend in dimensions, and in this way the valves of the bivalve shell are laid down. Not merely horny, but also calcareous material is secreted by these shell-forming areas. LY Fic. 269.—Young Veliger larva of Dreissensia polymorpha, seen from the side. (After Meisenheimer.) a, anus ; add.a, adductor muscle ; a.p, apical plate (afterwards becomes the anterior adductor muscle); coe, rudiment of coelom ; ¢.p, cerebral pit ; cr.s, crystalline sac ; hep, lobes of liver ; int, intestine; I.n, larval kidney ; m.tr, metatroch ; 0, mouth; p.g, pedal ganglion; r.d, dorsal retractor muscle; r.m, middle retractor muscle ; 7.v, ventral retractor muscle; sh, shell; ¢.tr, telotroch ; v.g, visceral ganglion; V, velum. The larva now changes its shape and instead of being cylindrical becomes more or less laterally compressed. As the newly formed shell valves extend towards the mid-ventral line, the mantle-cavity appears as two longitudinal invaginations on the ventral surface (Fig. 268). By the appearance of these grooves the edges of the area formed from the everted shell gland are changed into right and left mantle-lobes. The valves of the shell have a characteristic shape which appears to be practically universal amongst the veliger larvae of- Pelecypoda. The hinge-line is straight and horizontal and the lower margin of the valve is curved, so that the shape of the whole may be described as semicircular (Figs. 269, 271). Behind the mouth, which is situated on a projecting oral cone, is 340 INVERTEBRATA CHAP. a post-oral tuft of cilia, the sole representative of the metatroch of Annelida. The oesophagus is ciliated, and small organisms are whisked into the stomach. The modification of the anterior wall of the stomach into the liver-pouches has already been mentioned. From its posterior ventral wall a short pouch grows out on the left side, whose cells secrete rod-like excrescences. This is the rudiment of the crystal- line sac which secretes the crystalline style (Figs. 269, 270). A larval kidney, consisting of a straight tube opening at the Pues / , Fic. 270.—Young Veliger larva of Dreissensia polymorpha, seen from the ventral surface. (After Meisenheimer. ) Letters as in previous figure. In addition, m.c, mantle-cavity. side of what is afterwards the foot, and terminating internally in a solenocyte situated near the liver-pouch, makes its appearance at the same time as the shell and disappears as the foot grows out. Meisenheimer derives it from an ingrowth of the ectoderm, but he has no convincing evidence to prove this. We think it more probable that it arises, as Wierzejski (1905) has proved that it arises in Physa, from cells budded off from the mother mesoderm cells. Three sets of powerfully developed muscles are formed, consist- ing of spindle-shaped cells which arise from the proliferation of the mesoderm cells. All three are inserted into the cuticle of the hinge 1x MOLLUSCA 341 in the posterior dorsal region, and all pass forwards and slant down- wards. The uppermost of these is the dorsal retractor, the fibres of which pass forwards and diverge to the right and left and are inserted into the upper parts of the velum; below it lies the median retractor, which sends fibres to the lateral and ventral parts of the velum ; whilst below this again lies the ventral retractor, which is inserted into the anterior portions of the right and left mantle-lobes. All these muscles are of a transitory character and disappear when Fig. 271.—Older Veliger larva of Dreissensia polymorpha, seen from the side. This stage is the one which immediately precedes the metamorphosis. (After Meisenheimer. ) Letters as in two previous figures. In addition, add.a, anterior adductor muscle’; add.p, posterior adductor muscle ; br, rudiments of gill-papillae ; byss, byssus gland ; k, rudiment of kidney ; ot, otocyst ; per, rudiment of pericardium. the free-swimming life is given up; but the anterior adductor muscle, passing from one valve of the shell to the other, is already formed at this stage, and it persists into the adult. The three pairs of ganglia characteristic of Mollusca make their appearance at this stage. Of these the cerebral ganglia owe their origin to a bilobed pit, termed the cerebral pit, situated within the velar area in front of and below the ciliated apical plate. This pit evidently corresponds to the two lateral thickenings of the velar area in Patella. From the bottom of this pit a bilobed mass of cells is 342 INVERTEBRATA CHAP. separated off, which differentiates itself into two lateral masses of nerve cells with fibres between them (c.p, Figs. 269, 270, and 271). The pedal and visceral ganglia arise as two pairs of thickenings ‘of the ectoderm of the ventral surface, one pair being situated close behind the other (Fig. 269). In addition a pair of pleural ganglia make their appearance as a small pair of thickenings of the lateral ectoderm of the body, half-way between the rudiments of the cerebral and pedal ganglia; in later life they fuse with the cerebral ganglia (Fig. 274). The otocysts arise as small spherical invaginations of ectoderm at the apex of the mantle-groove. They are situated in the region which is afterwards converted into the foot. Towards the end of larval life the foot makes its appearance. It Fic. 272.—Transverse section through the dorsal region of old Veliger of Dreissensia poly- morpha in order to show the differentiation of the pericardium and the kidneys. (After Meisenheimer. ) coe, first rudiment of pericardial cavity ; H, heart; h.a, hinge area of shell; int, intestine ; 1k, left kidney ; per, ring of cells which gives rise to pericardium ; r.k, right kidney ; sh, shell. is defined by two transverse furrows, an anterior and a posterior, the latter cutting in deeply between the pedal and the visceral ganglia and separating them from one another. On the posterior aspect of the foot a deep invagination occurs which is lined by columnar cells. This is the rudiment of the byssus gland, which secretes the cords of horny material by means of which the adult Dreissensia anchors itself. The forepart of the foot grows into a finger-like process covered with minute cilia, and the primitive kidney disappears. The intestine becomes bent into a slight loop; it runs upwards from the stomach and bends downwards and forwards to reach the anus. The posterior adductor muscle is formed by a modification of some of the spindle-shaped cells of the mesenchyme, and so also is the retractor of the foot. This retractor muscle is a mass of fibres which project downwards from the posterior part of the mid-dorsal Ix MOLLUSCA 343 region surrounding the end of the intestine and extend into the hinder region of the foot. The first rudiments of the gills appear as a row of short, knob-like, ciliated protrusions from the roof of the mantle-groove, on each side and parallel with the pos- terior surface of the foot (br, Fig. 271). The coelomic rudiment becomes divided into a rounded mass of cells on each side of the intestine, which are the rudiments of one kidney, and into an arch of cells above the gut connecting these two rudi- ments. In the very last stage before metamorphosis this arch becomes a ring of cells surrounding the intestine (Fig. 272). The rudiment of the cerebral ganglia becomes detached from the cerebral pit, and the cells forming the apical plate degenerate, cast off their cilia, and dis- appear (Fig. 273). The loop of the intestine becomes very long, so as to extend upwards parallel to the left side of the stomach. The metamorphosis of the Veliger into the mussel takes place with startling rapidity ; it is as sudden as the change which converts the late free - swimming Fic. 273.--Sections through the cerebral pit of Veliger larvae of Dreissensia polymorpha. (After larva of Polygordius into the adult worm. As in that case, so here, the velar cells die and are cast off, the larval muscles break up and disappear, the whole anterior region in which Meisenheimer. ) A, longitudinal section through the pit of a larva, in which the apical plate is fully developed. B, longitudinal section through the pit in a larva in which apical plate is degenerating. CO, transverse section through the anterior part of the pit in a Veliger to show its bilobed character. a.p, apical plate; c.g, rudiment of cerebral ganglion; c.p, cerebral pit. the mouth was situated shrinks, and the cells forming the mouth cone degenerate and disappear. The result of this change is to bring the mouth and the anterior adductor closer to one another, and thus to swing the foot round so that its apex points forwards instead of downwards. The rows of gill papillae are swung round from a vertical 344 INVERTEBRATA CHAP, to a horizontal position, and the loop of the intestine is straightened out. After metamorphosis the labial palps, so characteristic of the adult mussel, are formed in an amazing manner. The cerebral pit, from which the rudiments of the cerebral ganglia have been separated, flattens out and forms two lateral bands of ciliated epithelium above and at the sides of the mouth. These develop into the upper and outer labial palps, while downgrowths from their inner ends give rise to the inner and lower labial palps (Fig. 276). The shell begins to alter its larval shape by a preponderant growth of its posterior and lower angle, and by a certain amount of growth Fsc. 274, Horizontal section through the anterior. portion of an old Veliger of Dreissensia polymorpha in order to show the differentiation of the cerebral ganglia from the cerebral pit. (After Meisenheimer.) c.g, cerebral ganglion ; ¢.p, cerebral pit ; oes, oesophagus ; pl.g, rudiment of pleural ganglia ; sh, shell; V, velum. of its anterior angle also. The gill papillae grow into long filaments, which become locked together by the longer cilia on their lateral faces. The development of an additional outer row of gill filaments, and the bending up of the ends of the filaments to form the reflected portions of the gill lamellae, take place later in life (Fig. 275). The rudiments of the kidneys acquire cavities. The ring of cells surrounding the intestine becomes double and the two layers separate from one another, the cavity between them being the pericardium, ae. the coelom. The cavity of the heart is the space between the inner of these layers and the intestine. The kidney becomes U-shaped, and the inner limb of the U on either side (which will form the ureter) coalesces with the corresponding part of the other kidney so as to form a transverse space beneath the intestine. Into this space opens, on either side, a diverticulum of the mantle-cavity, and in this IX MOLLUSCA 345 way the external opening is formed. The greater part of the ureter Fic. 275.—Side views of two young specimens of Dreissensia polymorpha after the metamorphosis has taken place. (After Meisenheimer.) A, young specimen in which the shell retains the shape it possessed in the veliger. B, older specimen in which the shell is beginning to assume its adult proportions. a, anus; add.a, anterior adductor muscle ; add.p, posterior adductor muscle ; br, rudiments of gills; ¢.g, cerebral ganglion ; c.p, eerebral pit; f, foot; H, heart; hep, liver; k, kidney; lp, labial palp; ot, otocyst; per, pericardium ; p-g, pedal ganglion ; 7.f, retractor of foot ; st, stomach ; v.g, visceral ganglion. is therefore not ectodermal but coelomic in origin. The limb of the 346 INVERTEBRATA CHAP. kidney which was originally the outer limb, bends inwards and fuses with the lower part of the pericardium, and here the reno-pericardial canal is formed (Fig. 277). 172..C Fic. 276.—Horizontal sections through the anterior portions of three just metamorphosed specimens of Dreissensia polymorpha in order to show the transformation of the cerebral pit into the labial palps. (After Meisenheimer. ) A, specimen in which the velum is just being thrown off. B, old specimen in which the velum is already discarded, in which the cerebral pit is beginning to open out. C, specimen in which the cerebral pit has given rise to two ciliated lobes. c.g, cerebral ganglion ; ¢.p, cerebral pit; lp, labial palps; mc, mantle-cavity ; oes, oesophagus ; V, discarded velar cells. The genital organ arises from a median ventral strip of the pericardial wall, in front, just between the openings of the reno- pericardial canals. It consists of peculiar large cells with pale nuclei. These cells multiply, become detached from the pericardial wall, and Ix MOLLUSCA 347 Fic. 277.—Diagrammatic transverse sections of young specimens of Dreissensia polymorpha in order to illustrate the development of the kidney. (After Meisenheimer.) A, the kidney a round sac. B, the kidney assumes a U-shape—the outer limb develops into the glandular portion, the inner limb into the ureter. C, the outer limb becomes bent inwards toward the middle line. D, the two inner limbs—the rudiments of the ureters fuse in the middle line. E, the internal and external openings are formed. ¢.o, external opening of the kidney ; int, intestine ; i.0, in- ternal opening of the kidney ; J.k, left kidney ; m.c, mantle-cavity ; 7.4, right kidney ; r.p, reno-peri- cardial canal; wr, ureter. - 348 INVERTEBRATA CHAP. divide into masses which, in the latest stages examined by Meisen- heimer, are found beneath the pericardium and lying not far from the lateral ectoderm on each side. The formation of the genital ducts was not observed by him (Fig. 278). Fic. 278.—Transverse section through a young Dreissensia polymorpha in order to show the origin of the genital organs. (After Meisenheimer.) G, median mass of genital cells forming a thickening in the floor of the pericardium ; /, heart ; int, intestine; k, kidney ; per.gl, pericardial gland ; per, pericardium ; r.p, reno-pericardial canal. Our account of the development of Dreissensia is now complete. We must pause, however, and glance at what is known of the develop- ment of other Pelecypoda before considering the development of the highest Mollusca. OTHER PELECYPODA The development of no other Pelecypod has been worked out with anything like the same completeness as Dretssensia. What we know of other life-histories are mainly bits and scraps. From the accounts, however, given by Horst (1882) of the development of Ostrea, by Drew (1906) of that of Pecten, by Hatschek (1885) of that of Z'eredo, by Sigerfoos (1895) of that of Pholas, and by Loven (1848) of that of Cardium, we can only conclude that the development of all these forms is practically identical with that of Dreissensia. The figures given of the veliger larvae are so similar that one would almost be driven to con- clude that there is a veliger larva of definite type common to all marine Pelecypoda, and that the differentiation of the various genera from one another takes place during post-larval life. Indeed, the researches of Stafford (1910) on the veliger larvae found in the lagoons of Prince Edward Island, and off the New Brunswick coast, have gone far to bear out this conclusion. Amongst other things he has shown that the late veliger larva of Ostrea virginiana possesses a well-marked foot which is used for locomotion’ in the early post-larval stages, before the definitely fixed life of the adult is assumed (Fig. 279). 1X MOLLUSCA 349 The only point which requires some comment is the description given by the earlier workers of the segmentation stages. Thus Horst (1882) and Hatschek (1883) both describe the endoderm as represented by one huge macromere, which buds off the micromeres which give rise to the ectoderm; instead of there being, as in all other Mollusca, four macromeres. There is strong ground for believing that this is a misinterpretation, and that in all cases four macromeres are really formed, but that, as is the case with Dreis- sensia, one is much larger than the rest. To the statement that the development of Pelecypoda, up to the veliger stage, pursues a uniform course in all genera, two marked exceptions must be made. The first of these concerns the group of the Protobranchiata, including the genera Nucula, Leda, Yoldia, etc., whose develop- ment has been studied by Drew (1899, 1901). In this group the velum acquires enormous dimensions, and consists of circles of large vacuolated cells placed one above the other, forming a barrel-shaped structure. The first and last circles bear numerous small cilia all over their surface, and the central three circles have each a narrow band of long cilia (Fig. 280). Fic, 279.—The ete aoe Cee A sagittal section through this Stafford.) viewed from the side, (After extraordinary sirugsure reveals add.a, anterior adductor muscle ; add. osterior inside it a saddle-shaped shell adductor muscle; br, rudiments of nee toot ; Wes gland, a long narrow stomo- lobes of liver; of, otocyst; V, retracted velum. daeum leading up toa stomach, — and a cerebral ganglion arising in Yoldia as a pit in front of the apical plate (Fig. 281). The foot appears later, and when the meta- morphosis occurs and the velar cells are cast away, the cilia covering the foot are sufficiently powerful to enable the animal to glide over the mud in which it lives before any burrowing movements are carried out. The general plan of the development is therefore the same as in Dreissensia. At the other pole of variation are freshwater forms like Cyclas, Pisidium, etc., and the family of the Unionidae, where the early stages of development are passed between the lamellae of the gill of the mother, and where is therefore no free-swimming stage and neither prototroch nor velum is developed. aos In Ziegler’s account of the development of Cyclas (1885) it is stated that there is only one large macromere from which all the micromeres are budded off. As we pointed out above, this is probably 350 INVERTEBRATA CHAP. a misinterpretation of the early stages of development. Cyclds is further remarkable for the fact that the coelom makes its appearance as two vesicles situated at the sides of the intestine. These vesicles become constricted into dorsal and ventral halves, but they meet one another both above and below the intestine, and hence, in another Fic. 281.—Longitudinal sagittal section of the Veliger larva of Yoldia limatula, Fic. 280.—The Veliger larva of Yoldia three days old. (After Drew.) oa about three days old. (After a.p, apical plate; ¢c.g, rudiment of cerebral reWe ganglia (representative of cerebral pit); s.g, ‘ a.p, apical plate ; bl, position of blastopore ; shell gland; st, stomach; stom, stomodaeum ; V, velum. V, velar cells. way, the same end results as that attained in Dreissensia, The space between dorsal and ventral halves forms the auricle of the heart, that between the vesicles and the intestine forms the ventricle (Fig. 282). The Unionidae give rise to an extraordinary larva, known as a Glochidium ; it is devoid of mouth, velum, and foot, but provided with a bivalve shell, and the lower borders of the valves. are each IX MOLLUSCA 351 provided with a sharp, inturned tooth. This larva is capable of only a few spasmodic flappings of the valves of the shell, which propel it through the water for a short distance. It is ejected from the gills by the parent when a freshwater fish happens to pass in the vicinity, and a successful larva contrives to fix itself on the gills or fins of the passing fish by grasping them by means of the valves of the shell. The bite of the valves stimulates growth of the soft vascular gill, so that the Glochidium is soon enclosed in a cyst in which it completes its development, and from which it emerges only when it has attained the adult condition (Fig. 283). The best account of the early development of Unionidae has been given by Lillie (1895), and the most recent worker at the post-larval OO YR eee per H So 2 Le Wy a We int Fic. 282. —Diagrams illustrating the development of the pericardium in Cyclas and Dreissensia. (After Meisenheimer. ) A, development of pericardium in Cyclas, B, development of pericardium in Dreissensia. aur, space which forms the auricle of the heart; H, heart; int, intestine ; per, pericardium. development is Harms (1909). Lillie’s account is interesting in — making it quite clear that, in spite of its aberrant appearance, ie development of the embryo of Unio conforms to the scheme given for Dreissensia. There is, it 1s true, no prototroch, and the first quartette of micromeres divide only once or twice and form the “head vesicle.” On the other hand, as in Dreissensia, the tirst somatoblast, 2d (X), is enormous, and it divides in exactly the same way as in Dreissensia. The group of small cells along its lower edge (x,-x;) give rise to what Lillie calls the ventral plate, a thickened region of the ectoderm from which the foot is formed in post-larval life. There is a primary mesoderm cell, 4d, which divides into right and left halves, from each of which a packet of cells is formed, parts of which break up into mesenchyme. The shell gland is enormous and the endodermic rudiment very small. As the shell gland becomes everted the 352 INVERTEBRATA CHAP. endodermic rudiment is invaginated and the blastopore closes, but the stomodaeum remains as a thickened plate of ectoderm during the Glochidium stage. Rudiments of three pairs of ganglia are sometimes present (Anodon), and sometimes not (Unio). On each side the mantle-lobe bears three sense cells with long sense hairs. There is a powerful adductor muscle connecting the valves of the shell. In the post-larval life the most marked feature is the modifica- tion of the cells forming the larval mantle. These cells develop into huge vacuolated columnar structures which actually absorb and digest fragments of the blood cells and other tissues of the host Fia. 283.—Glochidium—larva of Margaritana with widely opened valves, viewed as a transparent object from the dorsal surface. (After Harms.) add, adductor muscle; by.f, byssus thread ; cil, ciliated patch on the ventral surface ; sh, larval ~ shell; st, stomach ; t, teeth of shell. which enter the mantle-cavity of the parasite. The cells destined to form the adult mantle arise from the apices of the two mantle-grooves and spread downwards, displacing the larval mantle cells, which are eventually shed (Fig. 284). We can only compare these rudiments of the adult mantle to the imaginal discs of insects. The single larval adductor muscle disappears and is replaced by the adult adductor, of which sometimes the anterior and sometimes the posterior is the first to be formed. The foot, gills, and otocysts arise exactly as in Dreissensia. The coelom arises as two little packets of cells, one on each side, closely attached to the ectoderm, from which Harms supposes them to have been derived; but this view, for reasons given above, we cannot accept. The later development of these rudiments is exactly the same as in Dreissensia; the main mass on each side IX MOLLUSCA 353 hollows out to form the kidney, whilst a band of cells grows out from each and forms a ring round the gut, splits into two layers, and forms the pericardium. The accounts of Harms and Lillie leave no doubt in the mind that in the embryo of the Unionidae we are merely dealing with an ordinary Pelecypod veliger modified for a parasitic existence. Fic. 284.—Transverse section of a Glochidium larva of Unio which is already fixed in the tissues of its post. (After Harm.) a.m, cells which will form the adult mantle ; int, intestine; I.m, vacuolated cells of larval mantle, which absorb food material from the host ; m.c, mantle-cavity ; p.g, rudiments of pedal ganglir; ¢, teeth of larval shell. CEPHALOPODA—Loligo, Sepia We must now turn to the study of the embryology of the highest Mollusca, the Cephalopoda (lit. head-footed), so called because the fore part of the foot has grown into a frill surrounding the head. Two genera are represented by common species, both on the English coasts and in the Mediterranean; these are Sepia officinalis and Loligo vulgaris. So far as is known, the development of both pursues a practically identical course. We shall select. Loligo as‘a type for special study, because its development has been more completely worked out, and because species of this genus are common on the American coast; but we shall not hesitate to fill up lacunae in our . knowledge of the development of Zoligo by the description of corre- sponding stages from the development of Sepia, when these are better known. The eggs of both genera, like all Cephalopod eggs so far described, contain an abundance of yolk, the cytoplasm being mainly restricted to a small disc at the animal pole of the egg, in which the nucleus is VOL. I 2A 354 INVERTEBRATA ‘CHAP. situated. The Cephalopod egg is, in fact, the beaw ideal of a telo- lecithal egg. The egg of Sepia is nearly spherical, about the size of a pea; it is enclosed in a tough black chorion of the consistence of india-rubber, difficult to remove. The egg of Loligo, on the contrary, is about the size of an apple pip, and is of an elongated oval shape. Many eggs are laid together immersed in a somewhat tough jelly, which can be partly dissolved by exposure to the action of Eau de Javelle for fifteen minutes. For the study of segmentation stages surface views are essential, and for this purpose the most superficial layer, including all the cytoplasm, is removed from the animal pole of the egg by means of a sharp knife, and the skin thus obtained is spread out flat. Even when sections are desired it is inadvisable to endeavour to cut through the whole mass of yolk; only a small part of the upper half of the egg should be removed and cut into sections. The segmentation of Sepia has been carefully described by Vialleton (1888). Minchin began a renewed study of the subject and made a series of exquisite preparations from eggs preserved in Hermann’s fluid, which, however, he did not describe; but these have been described by Koeppern (1909), and his results confirm in every detail those of Vialleton, whose account we follow here. No such ex- haustive account of the early stages of development of Zoligo is available, but, from what is known of it, it agrees with that of Sepia in every particular. According to Vialleton, then, the cleavage of the egg of Sepia is meroblastic, that is to say, it is only the protoplasmic end of the egg which is divided by the cleavage furrows, the yolk being quite un- affected. When the nucleus has divided into four, and two cleavage furrows at right angles have been formed, we have obviously a stage which corresponds to the stage of division of other Molluscan eggs into four macromeres. The next cleavage furrow is a circumferential one and cuts off four inner cells, termed blastomeres by Vialleton, from four outer large cells whose lower ends fade into the yolk, which he terms blasto- cones. ‘This stage corresponds roughly to the stage of the formation of the first quartette of micromeres in other Molluscan eggs, but no one has attempted to work out the cell-lineage in a Cephalopod egg, and it will become obvious that many more cleavages are necessary to separaté one from another the specific materials of the germinal layers than is the case with the vastly smaller eggs of other Mollusca. For example, in the 32-cell stage of Sepia (Fig. 285), if the blasto- cones be regarded as corresponding to the macromeres, it is obvious that there must be at least 20 macromeres and only 12 micromeres, whereas in a normal Molluscan egg there would be 4 macromeres and 28 micromeres in the 32-cell stage. By further radial cleavage furrows the number of blastocones is greatly increased, and by new circumferential furrows the inner portions of these are continually cut off as new blastomeres, whilst IX MOLLUSCA 355 the already existing blastomeres undergo division, and in this way a one-layered sheet of cells, or “blastoderm,” becomes spread over the upper surface of the egg. When the blastoderm has extended so as to form a sheet of cells covering about one-eighth of the surface of the egg, the formation of new blastomeres from the blastocones ceases: these latter now appear as a series of narrow, spoke-like pillars radiating from the blastoderm asahub. The basal. portions of these spokes become narrower and narrower, as their nuclei wander farther and farther away from the blm Fic. 285.—Segmenting egg (32-cell stage) of Sepia officinalis viewed from the animal pole. (After Koeppern, from Minchin’s preparations. ) ble, blastocone ; blm, blastomere. blastoderm, till they become mere threads. Finally the portion with the nucleus separates altogether from the blastoderm and begins to divide, and in this way a membrane is formed consisting of a single layer of extremely flattened cells, which rapidly extends all over the yolk; it extends also underneath the blastoderm so as to cover the yolk there also. This sheet of cells is termed the yolk-membrane ; it has been rightly compared to a portion of the endoderm. — Its cells exercise some digestive influence on the yolk and gradually change it into a form that can be used for the nourishment of the embryo. But no part of this yolk-membrane is incorporated in the permanent epithelium of the mid-gut. The cells from which the mid-gut epithelium will be formed, arise later by divisions of the most 356 INVERTEBRATA CHAP. peripheral blastomeres of the blastoderm, at right angles to the surface. These divisions take place along what afterwards are seen to be the posterior and lateral edges of the blastoderm. In this way an incomplete ring of what are called “lower layer cells” is formed, which former observers have termed mesoderm but which yh So 0) i, ss 9 a ey ey are eres we may term mesendoderm. miereaTe ys Lankester (1875) maintains that rs a 7 (e\ kee} Poe} there is a “primitive streak ” in the segmenting eggs of Cephalo- poda, by which he means a re- I stricted area of the surface over which the proliferation of cells which gives rise to the mesendo- derm takes place. A priori this is very likely, although so far this observation has not been confirmed. This mesendoderm A Bor IG D) is Fic. 286.—A portion of the margin of the blastoderm of the egg of Sepia officinalis at the conclusion of the process of seg- mentation ; to show the transformation of the blastocones into cells of the yolk- membrane. (After Koeppern, from Minchin’s preparations. ) grows inwards under the blasto- derm to a considerable extent, but does not invade, except in the form of a few loose scattered cells, its front edge or its centre. The formation of definite organs in the embryo of Loligo has been worked out in com- paratively recent times by Korschelt (1892) and by Faussek (1900). These workers found that Hermann’s fluid, and similar mixtures containing osmium tetroxide, render the tissues too brittle for section- cutting although giving excellent histological detail. They therefore employed picro-sulphuric acid and Perenyi’s fluid, which gave excellent preservation of form, although the histological detail is not so perfect as that obtained by the use of Hermann’s fluid. The first organ to make its appearance is the shell gland. This appears as a heart-shaped area of columnar ectoderm in the mid- dorsal line. This thickening in fact occupies the apex of the egg; in front of it is what we may call the anterior slope, behind it the posterior slope. Soon an invagination appears in the centre of this thickening; but, as Lankester long ago pointed out (1875), this does not correspond to the invagination of the shell gland which we encounter in other Molluscan embryos. That invagination precedes the condition of the gland when it is a saddle-shaped area, and lasts only a short time, but the invagination in the shell gland area of a Cephalopod lasts throughout life; it constitutes, in fact, the shell-sac which encloses the shell (a horny pen in ZLoligo, but a complex calcareous structure in Sepia). From a comparison with ble, blastocones ; y.m.c, yolk-membrane cells. IX MOLLUSCA 357 the primitive genus Nautilus, we know that this covering in of the shell by mantle flaps is a secondary phenomenon which was not present in the earlier Cephalopoda, A é (of ole PROawEsy YiMe. Fic. 287.—Two sections through the edge of the blastoderm of Sepia officinalis in different stages of development ; to illustrate the development of the lower layer cells. (After Koeppern, from Minchin’s preparations.) A, Younger stage. B, Older stage. A is more highly magnified than B. me, mesoderm, i.e. lower layer cells; y.m.c, yolk-membrane cell. The eyes now appear as shallow cups on the sides of lateral protrusions of the body, which may be termed eye-stalks. The edges of the Cephalopod shell gland constitute, as in other Mollusca, the rudiment of the mantle, and underneath them appears a groove, Fic. 288.—Two early embryos of Loligo vulgaris. (After Korschelt. ) A, An embryo seen from the posterior side. B, A slightly older embryo seen from the anterior side. ar, rudiments of arms; m.f, mantle-fold; 0.c, optic cup; o.st, optic stalk ; s.s, shell-sac ; stom, stomodaeum. : deepest behind, which is the rudiment of the mantle-cavity. Just within this groove two buds appear to the right and left of the middle line; these are the rudiments of the gills: whilst below them appear two pairs of ridges converging towards the ventral surface and 358 © INVERTEBRATA CHAP. constituting the rudiment of the hind foot or funnel. These are termed the anterior and posterior funnel folds respectively. The rudiments of the fore foot, the arms, appear as a series of thickenings on the lower edge of the blastoderm (Tigs. 288, 289). Whilst these changes have been going on, the first trace of the mid-gut makes its appearance as a thickening of the “ mesendoderm” on the posterior slope of the egg; whilst the stomodaeum appears as an invagination in the middle line of the anterior slope of the embryo. In the endodermal thickening a cavity is formed by the separation of its constituent cells from each other. This cavity is at first closed internally merely by the yolk-membrane; but it soon independently acquires its own internal wall, whilst externally it raises the ectoderm into a slight papilla, the anal papilla, at the apex of which the anus appears a little later. There is no procto- daeum. Before the anus is per- forated the incipient mid-gut gives off a diverticulum which is the rudi- ment of the ink gland (Fig. 290, B). The stomodaeum is at first a shallow cup, but it soon extends up towards the mid-dorsal line and past it, where, in a much later period.of development, it fuses with and@pens into the mid-gut. The radula sac appears as a ventral outgrowth of Fie, 289.—Embryo of Loligo vulgaris the stomodaeum, and a still ore seen from the posterior side at the ventral outgrowth is the rudiment conclusion of the period of develop- of the salivary gland. The oto- ment, termed by Faussek Stage 1. . ‘ a (After Faussek.) cysts now arise as open pits near a,anus; af.f, anterior funnel fold; ar, the Junction of the anterior and mont ds ns, pri fone ods os boo Cais of foldg, as already noteh pi te arama 3284 two pairs of folds, as already noted, eye-stalk ; ot, otocyst; s.s, shell-sac. anne Ve mudiments of the el In this stage the rudiments of the three principal pairs of ganglia arise. The cerebral ganglia appear as long streak-like ectodermic thickenings running below the eyes; the pedal ganglia as much shorter streaks parallel to the hindermost portions of these and above them, extending to the otocysts; and finally the pleural ganglia are represented by short vertical streaks extending up towards the mantle. The shape of the rudiments reminds one of the band-like condition of the ganglia found in Nautilus (Fig. 291), and we may add that the appearance of the funnel as two un- connected ridges also recalls its condition in Nautilus. The first trace of the genital cells appears in this stage as a packet of large pale cells with pale nuclei, situated on the posterior aspect of the embryo between the rudiment of the gills (Fig. 297, A). Ix MOLLUSCA 359 Fic. 290.—Two sagittal sections of early embryos of Loligo vulgaris in order to illustrate the first formation of organs. (After Faussek.) A, Stage in which the endoderm is first visible as a differentiation of the mesoderm. B, Stage in which the ink sac is formed. end, endoderm; i.s, ink sac; mes, mesoderm s.s, shell-sac; stom, stomodaeum ; y.m, yolk- membrane. 360 INVERTEBRATA CHAP. As development proceeds the upper portion of the egg, covered by the blastoderm, begins to be separated by a constriction from the lower part, which consists merely of yolk covered by the yolk- membrane, and so we are enabled to distinguish between “embryo” and yolk-sac. It must be remem- bered, however, that a considerable portion of the yolk is contained within the confines of the embryo; this is known as the internal yolk- sac, and is disconnected from the external yolk-sac, which appears as an appendage. The two are, of course, joined by a neck of com- munication. Fic. 291.—Embryo of Loligo vulgaris seen from the side and behind in order to illustrate the development of the ganglia. The embryo is younger than that repre- sented in Fig. 289. (After Faussek.) Fic, 292.—Embryo of Loligo vulgaris in the stage termed by Faussek Stage 2, when the embryo begins to be grooved off from the yolk-sac ; posterior view. A, Embryo seen from behind. B, Embryo seen from the side. «ff, anterior funnel folds ; ar, rudiments of arms; br, rudiment of gill; ¢. Me rudiment of cerebral ganglion ; oe, eyecus: aa (After Korschelt.) eye-stalk; ot, otocyst; pf, posterior funnel Letters as in previous figure. In addition, folds ; p.g, rudiment of pedal ganglion ; s.s, shell- u.p, anal papilla; 7.f, retractur muscle of sac; v.g, visceral ganglion. funnel; t.ar, tentacular arms, When the grooving off of the external yolk-sac has become distinct, many other changes take place. The eye-stalks grow in length, while the eye-pits become closed, and the inner segment of the lens is developed as a secretion from the anterior wall of each pit. The otocysts also become closed and sink into the ridges which IX MOLLUSCA 361 are the rudiments of the funnel. The retractor muscles of the funnel appear as two ridges which stretch from the middle of the funnel, where anterior and posterior funnel folds have united, upwards to the edge of the mantle. The mantle edge extends over the rudiments of the gills, and the mantle cavity deepens; whilst the shell-sac closes, and the central region of the mantle becomes arched up so as to constitute a visceral hump. The rudiments of the arms lengthen; those of the tentacular arms exceed the others Fic. 293.—Sagittal sections of two embryos of Loligo vulgaris to illustrate the formation of internal organs. (After Korschelt.) A, Section of younger embryo with incomplete alimentary canal. B, Section of older embryo in which stomodaeum and mid-gut have joined. a, anus; ¢.g, cerebral ganglion ; e.y.s, external yolk-sac ; f, funnel; fn, fin rudiment; H, heart; int, intestine; i.s, ink sac; i.y.s, internal yolk-sac ; m.c, mantle- cavity ; m.f, mantle-fold ; 0, mouth ; ot, otocyst; p.g, pedal ganglion ; per, pericardium ; 7.s, radula sac ; sal, salivary gland ; s.s, shell-sac ; stom, stomodaeum ; v.g, visceral ganglion ; y.m, yolk-membrane. in length. The rudiments of the ganglia become constricted off from the thickenings of the ectoderm; that part of the thickenings giving rise to the cerebral ganglia which lie beneath the eyes, becomes infolded and gives rise to the so-called white bodies (Fig. 292). The internal changes which occur are best made out by combining the views obtained by sagittal sections with those obtained by horizontal sections through the embryo. The most marked features of the stage which we are discussing are the appearance of the coelomic cavities and of the blood spaces, both of which arise as - 362 INVERTEBRATA CHAP, splits in the mesoderm. The coelomic cavities contain a clear fluid, and have a definite epithelial arrangement of the cells forming their walls; whilst the blood spaces contain an albuminous serum which stains, and their walls are often irregular and in some places formed only by the yolk-membrane. The coelom arises as two vesicles lying beneath the mid-gut. Each of these vesicles becomes divided by a con- striction into a dorsal part, which is the rudiment of the pericardium, and a ventral part, the rudiment of the kidney ; the connec- tion between the two, though narrow, persists, and forms the reno-peri- cardial canal. Beneath the shell-sac, on the posterior slope of the embryo, a wide blood space arises whose cavity is traversed by cords of mesen- chyme. This isthe posterior sinus (Fig. 294). It ex- tends anteriorly round the sides of the gut and there constitutes the two forks of the vena cava. From this a branch extends into the rudiment of the gill, which is the beginning of the branchial heart. In Fic. 294.—Two diagrammatic transverse sections front, these forks unite through a young embryo of Loligo vulgaris to beneath the gut to form illustrate the origin of the coelom and the blood the unpaired vena Cava. cavities. (After Faussek.) The heart arises independ- A, The more posterior; B, the more anterior section. ently of this sinus as two br, gill; brik, rudiment of branchial heart ; coe, tudiments hollow tubes lyin inter- of coelom ; H, rudiments of systemic heart ; int, intestine ; y g p.s, posterior blood sinus; s.s, shell-sac; v.c, vena cava. nally between the coelomic rudiments. Behind, these tubes unite to form the ventricle, but in front, where kidney and coelom join, they diverge so as to form the auricles (Fig. 295). In the next period of development, the end of which is represented in Fig. 296, the embryo becomes as large as the external yolk- sac, and the funnel is definitely constituted by the union of the free edges of the folds in the mid-ventral line. The arms have now acquired suckers and have extended round the head to the mid- dorsal line, so that the encircling of the head by the fore foot is 1X MOLLUSCA 363 completed. The anterior chamber of the eye is formed as an ecto- dermic fold, which arises from the base of the eye-stalk and encloses it. In this way the outer surface of the adult head is formed and the eye-stalks are no longer prominent. This en- veloping fold we may term the corneal fold. From the spot where the primary eye vesicle closed the rudiment of the outer segment of the lens is formed as an external secretion. The eyelids are constituted by a pair of folds similar to those which wall in the anterior chamber, but lying out- side the latter. Lying inside the anterior chamber is still another pair of folds which con- stitute the iris. The alimentary canal, which had become a completed tube in the previous period, now shows further develop- ment. The radula sac of three transverse sections through an embryo of Loligo vulgaris, much older Fie. 295,.—Diagramis (After Faussek, ) A, The most posterior; C, the most anterior of the three sections. Letters as in previous figure. In addition, aur, auricle; k, kidney; m.c, mantle-cavity ; m.v, mantle-vein ; per, pericardium. than that represented in Fig. 294. becomes sharply marked off from the stomodaeum. In front of it two 364 INVERTEBRATA CHAP. ventral diverticula grow out, which are the rudiments of the salivary glands. The mid-gut becomes differentiated into a stomach and an intestine, and the diverticulum which forms the ink sac opens into the latter. The liver arises as two lateral outgrowths of the stomach. The surfaces of these outgrowths become folded, and this is the first indication of the formation of the liver tubules. The coelom, which is already constricted into kidney and pericar- dium, now increases greatly in volume. The two pericardial rudiments fuse behind and enclose the two rudiments of the heart; these latter likewise fuse together. In front the pericardial rudi- ments remain separate and are applied to the paired rudiments of the heart which here con- stitute the auricles. The paired portions of the pericardium communicate with the kidneys and give rise to the reno- pericardial canals. Behind, the single pericardium grows backward and extends into the growing genital organ, which becomes divided up into the genital folds. This portion of the pericardium becomes, later, divided off from that sur- rounding the heart and forms the genital coelom (Fig. 297). he kidney sacs develop a high columnar epithelium on : ; their inner walls, where they *iSyeriod when the tunel is formed, views @£@ in contact with the forks from behind. (After Korschelt.) of the vena cava; the epithelium ar, arms; en.f, cornetl fold which envelops the lining their outer walls becomes eye-stalk, and forms the outer chamber of the eye ; very thin. The posterior sinus J, completed funnel; fn, fins; m.f, mantle edge; oc, ; See ab VolneAd, is reduced to very small dimensions by the expansion of the shell-sac, and is cut off from the vena cava. The portions of the vena cava which extend into the gills and constitute the branchial hearts, develop great thickenings of their walls on one side. These thickenings, as experiment has proved, are excretory in nature and consist of vacuolated cells; they are covered externally by thin peritoneal epithelium where they touch the coelom. These are of course the appendages of the branchial heart. The cartilage so characteristic of Cephalopoda is formed by the modification of mesodermic connective tissue, and is first visible in the neighbourhood of the foot. The chromatophores likewise, IX MOLLUSCA 365 according to Faussek, are of mesodermal origin. During this period the whole of the ectoderm, except that covering the inner surfaces of the arms, undergoes mucous degeneration—ie. the cytoplasm of its cells degenerates into slime and is cast off. How the adult ectoderm is regenerated Faussek was unable to determine. no INC Fic. 297.—Transverse sectious of embryos of young cuttle-fish, illustrating two stages in the development of the genital organs. (After Faussek.) A, Section through young embryo of Loligo vulgaris, showing the first appearance of the genital cells in the mesoderm. B, Section through older embryo of Sepia officinalis, showing the migration of these cells into a fold projecting into the coeloin, br, rudiments of gills; gen, genital cells ; gf, genital fold ; I, heart ; m.c, mantle-cavity ; p.a, posterior aorta ; per, pericardium ; 7~.s, posterior sinus ; s.s, shell-sac. At the time that the external yolk-sac is absorbed, a large diverticulum, the spiral caecum, grows out from the Stomach ; and this circumstance, together with the enlargement of the internal yolk-sac, into which some of the yolk from the external sac passes, is responsible for the almost entire suppression of the cavities of the kidneys and genital organs which takes place at this period. Later, when the yolk is finally absorbed, the kidneys and pericardium 366 INVERTEBRATA CHAP. reacquire their cavities, and then the kidneys become fused together in the mid-ventral line; this fusion is characteristic of Loligo, and NI \ fi HN D Fic. 298.—Two early stages in the development of the eye of Loligo vulgaris seen in transverse section. (After Lankester, from Balfour.) oc, eye-cup; r, rudiment of retina. c does not occur in Sepia. By far the most complicated organ in the Cephalopod is the eye, the general features of the development of which have already been described. Some details may now be added. As soon as the primary eye pit closes the inner segment of the lens begins to be formed. It first appears as a thin cuticle spreading Fic. 299.—Sections through the developing eyes of young cuttle-fish to show the development of the lens. (After Faussek.) A, Section through the eye of Loligo vulgaris. B, Section through the eye of Sepia officinalis. c.ep, large cells of the corpus epitheliale ; c.epl, small cells of the periphery of the corpus epitheliale which grow over the larger cells and secrete the fibres of the lens; i.l, inner segment of the lens ; ir, iris; 1.f, lens fibres. over a considerable portion of the inner surface of the closed eye- sac, but it becomes thickened at one point in the centre, and IX MOLLUSCA 367 projects inwards as a rod-shaped structure. The cells forming this part of the wall of the eye-sac are enlarged and cubical, whilst those forming the more peripheral portions of the eye-sac wall are small. Zhe large cells, which secreted the primary part o the lens, disappear in the centre, probably as a result of lens secretion ; towards the sides they persist as the characteristic cells of the corpus epitheliale of the ciliary body. The further growth of the lens is Fic. 300.—Transverse section of the eye of a nearly ripe embryo of Sepia officinalis. (After Faussek. ) c.ep, epithelial body; c.ep1, small cells of c.ep forming lens capsule; ¢.f, corneal fold constituting the outer wall of the outer chamber of the eye; e¢.l, thickening, the rudiment of the lower eye-lid ; g-c, ganglion cells of the retina; i.ch, inner chamber of the eye; 7.1, inner segment of the lens; ir, iris ; o.ch, onter chamber of the eye; 0.1, outer segment of the lens; o.st, eye-stalk ; v.c, visual cells of the retina; w.b, mesodermal rudiment of the white body. effected by means of the small cells forming the more distant portions of the wall of the eye-sac. These cells grow forward on each side towards the lens, asa kind of fold overspreading the large cells of the corpus epitheliale ; and by them the lens is added to, both in thick- ness and depth. The iris folds and the outer segment of the lens are formed before the fold which walls in the anterior chamber of the eye, and constitutes the corneal fold, is formed. The outer segment of the lens is formed like the inner segment; at first it is a cuticle which thickens in the 368 INVERTEBRATA CHAP. centre, then the cells beneath this thickening disappear and it is added to by the more lateral cells. If this description be followed it is clear that the primary eye vesicle, with its contained inner segment of the lens, corresponds to the eye as we find it in Gastropoda; and that the outer segment of the lens and the outer chamber of the eye are subsequent additions. The ciliary body consists of the adpressed posterior wall of the outer chamber and anterior wall of the inner chamber of the eye. The hinder wall of the primary eye vesicle forms the retina. This consists at first of a single layer of columnar ectodermal cells with the nuclei at different levels, bounded externally by a basement membrane. Laterally it is continuous with the layer of small cells which forms the lens. Soon the single layer constituting the retina changes into many layers of small rounded cells; of these the outer layers begin to pass outwards through the basement membrane, and they constitute the nervous layer of the retina. From the innermost layer of cells visual rods grow out (vc, Fig. 300), pointing into the cavity of the eye-sac; but these cells do not all undergo this transforma- tion; alternating with the visual cells are cells which secrete pigment. The inner portions of the visual cells, that is, the portions turned towards the cavity of the eye-sac, and these pigment cells, alone retain their primary position with regard to the basement membrane. The nervous portion of the retina is thus seen to consist of two layers of nuclei with a clear space between them (almost certainly occupied by dendrites of the nerve cells), and the whole presents a striking analogy to the layers of cells in the human retina, except that the layers occur in the reverse direction so far as the incidence of light is concerned. GENERAL CONSIDERATIONS ON THE ANCESTRAL HISTORY OF MOLLUSCA When we review the account of the development of Mollusca given in this chapter, certain facts stand out clearly. First, in the early larvae of Patella, Dentalium, and Dreissensia we are evidently dealing with a single type, and this type must be classed as a Trocho- phore larva, similar in all essentials to the Trochophore larva of Annelida. Therefore the common ancestral group from which Gastropoda, Scaphopoda, Pelecypoda (and we may add Solenogastres) spring, must have had a Trochophore larva. In a word, all Mollusca are thus shown to be descended from an ancestor represented by the Trochophore, #.e. the same ancestor as gave rise to the Annelida. What, it may be asked, was the factor which caused two families of the descendants of this ancestor to diverge so widely from one another in structure? We must surely look for this factor solely in a divergence of modes of life. Now, the fundamental type of habits common to all Annelida is a burrowing mode of existence; and from that, coupled with a wriggling method of locomotion during their occasional excursions into the upper water, we were able to deduce the main peculiarities of their adult structure. But the habits of IX MOLLUSCA 369 Mollusca do not lend themselves to such easy generalization. The Pelecypoda and Scaphopoda burrow, the Gastropoda crawl, and the Cephalopoda propel themselves by projecting squirts of water through the funnel. But if we take into account Drew’s statement (1899) that Yoldia —surely one of the most primitive of Pelecypoda,—when just meta- morphosed, glides over the mud by means of its cilia, we might be inclined to conclude that the primitive habits of the original Mollusca consisted in crawling or gliding over the surface, in contradistinction to the burrowing mode of life adopted by primitive Annelida. It is highly probable that the most primitive living Cephalopod, Nautilus, in which the constituent folds which*make up the funnel are not united, can flatten out this organ and crawl. But if we are entitled to conclude that the Trochophore larva represents the common ancestor of Annelida and Mollusca, we must regard the various Veliger larvae as representing an anticipation of adult conditions ; a telescoping of development, in all respects similar to that shown by the post-trochophoral stages of development in Annelids. The Veliger of Gastropoda, with its spirally twisted shell, can hardly represent an ancestral stage; because, as we have seen, the unequal growth of the mantle edge which causes the twisting is most plausibly explained by the overbalancing of a tall visceral hump, such as would surely occur in an animal crawling over uneven ground, not in one which was free-swimming. The Veligers of Pelecypoda and Scaphopoda exhibit in.the free-swimming stage the distinguish- ing adult characters of their respective groups. As has been mentioned several times already, this reflection of adult characters into success- ively early stages of life-history is a phenomenon which meets us everywhere in embryology, and it is one of the most suggestive features in the whole process of development. Turning now to the development of Cephalopoda, it is at first sight difficult to find any points of contact whatever between their development and that of other Mollusca. Thus, the gills are amongst the earliest organs to be formed in Cephalopoda, whilst they are the latest in Gastropoda and Pelecypoda. All trace of the Trochophore stage has been eliminated from Cephalopod ontogeny, and there is nothing corresponding to the veliger stage. Even the early history of the shell must be greatly hastened through, for the shell-sac and shell gland are two different things. Lankester (1890) has pointed out that, in the later stages of the development of the snail (Helix), a large ventral protrusion of the foot filled with yolk is produced. This he rightly compares to the external yolk-sac of the Cephalopod embryo, for this certainly represents a median protrusion of the foot, since the rest of the foot is formed all round it. We have here, how- ever, a case of analogous development, not of real homology, for the heavily yolked egg of the asymmetrical snail has not been derived ~ from the heavily yolked egg of the Cephalopod, or vice versa. ‘VOL. I 2B 370 INVERTEBRATA CHAP. If, and when, the development of Nautilus is worked out, we shall probably gain points for comparison of the development of Cephalopoda with that of other Mollusca; in the meantime we can only conclude that the accession of large stores of nourishment has almost obliterated the traces of ancestral history in their development, leaving only the most general resemblance in the formation of the layers and the development of the sense-organs, as links between them and other Mollusca. In fact, the reflection of the development of organs which become important in adult life, into successively earlier periods of development, termed by Lankester heterochrony, has, in Cephalopoda, reached its maximum. LITERATURE CONSULTED Boutan. La Cause principale de l’Asymétrie des Mollusques Gastéropodes. Arch. Zool. Exp., 3rd series, vol. 7, 1899. Casteel. The Cell-lineaye and Early Larval Development of Fiona marina, a Nudibranchiate Mollusc. Proc. Acad. Nat. Sci., Philadelphia, 1904. Conklin. The Embryology of Crepidula. Journ. Morph., vol., 18, 1897. Crampton, H. E. Experimental Studies on Gasteropod Development. Arch. Ent. Mech., vol. 8, 1896. Drew. Yoldia limatula. Mem. Biol. Lab., Johns Hopkins Univ., vol. 4, 1899. Drew. The Life-History of MNucula delphinodonta. Quart. Journ, Micr. S8c., vol. 44, 1901. Drew. ‘The Habits, Anatomy, and Embryology of the Giant Scallop Pecten tenwt- costatus. Univ. Maine Studies, Nov. 1906. Drummond. Notes on the Development of Paludina vivipara, with special refer- ence to the urino-genital organs and theories of Gasteropod Torsion. Quart. Journ. Micr. Se., vol. 46, 1902. Erlanger. Zur Entwicklung von Paludina vivipara. Morph. Jahrb., vol. 17, 1891-92. Erlanger. Zur Bildung des Mesoderms bei der Paludina vivipara. Jbid. vol. 22, 1894. , Erlanger. Etudes sur le développement des Gastéropodes pulmonés. (1) Etude sur le rein Jarvaire des Basommatophores. Arch. Biol., vol. 4, 1895. Faussek, V. Untersuchungen iiber die Entwicklung der Cephalopoden. Mitt. Zool. Stat. Neapel, vol. 14, 1900. Harms. Postembryonale Entwicklungsgeschichte der Unionidae. Zool. Jahrb., (Abt. fiir Ont.), vol. 28, 1909. Hatschek. Uber die Entwicklungsgeschichte von TYeredo. Arb. Zool. Inst. Wien, vol. 6, 1885. Heath. The Development of Jschnochiton. Zool. Jahrb. (Abt. fiir Out.), vol. 12, 1899, Herbst. Uber das Auseinandergehen von Furchung und Gewebeszellen in kalk- freien Medium. Arch. Ent. Mech., vol. 9, 1900. Heymons, R. Zur Entwicklungsgeschichte von Umbrella mediterranea. Zeit. f. wiss. Zool., vol, 56, 1893. Horst. De Ontwikkelingsgeschiedenes van de Oester. Tijdschr. Ned. Dierk., Dec. 1, 1882. Holmes. The Karly Development of Planorbis. Journ. Morph., vol. 16, 1900. Koeppern. Notes on Prof. E. A. Minchin’s preparations of the early stages in the developwent of Sepia. Proc. Roy. Soc. Edin., vol. 18, 1909. Korschelt. Beitrige zur Entwicklungsgeschichte der Cephalopoden. YTestsch. zum 70ten Geburtstag von Leuckart, 1892. Kowalevsky. Embryogénie du Chiton polii, avec quelques remarques sur le développement des autres Chitons. Ann. Mus. Nat. Hist. Marseilles, vol. 1, 1883. Lankester. Observations on the Development of Cephalopoda. Journ. Micr. Sci., vol. 15, 1875. Lankester. Zoological Articles, Mollusca (rept. from Encyclop. Brit. 1890). IX MOLLUSCA 371 Lillie. The Embryology of the Unionidae. Journ. Morph., vol. 10, 1895. Loven. Beitrige zur Kenntniss der Mollusca Acephala Lamellibranchiata. Transla- tion, 1879, from Abk. Kang. Schwed. Akad. Wiss., 1848. Mazzarelli. Bemerkungen iiber die Analniere der freilebenden Larven der Opisthobranvhiern. Biol. Centrbl., vol. 18, 1898. Meisenheimer, J. Entwicklungsgeschichte von Limax maximus, Pt. I. Zeit. f. wiss. Zool., vol. 63, 1898. Meisenheimer, J. lintwicklungsgeschichte von Dreissensia polymorpha. Zeit. f. wiss. Zool., vol. 69, 1901. Patten. The Embryology of Patella. Arb. aus d. Zool. Instit. der Univ. Wien, vol. 6, 1885. Robert. Recherches sur le développement des Troques. Arch. Zool. Exp., series 3, vol. 2, 1902. Stafford. On the Recognition of Bivalve Larvae in Plankton Collections. Cont. to Biology, Ottawa, 1910. i Sigerfoos. The Pholadidae : I. Note on the Early Stages of Development. Johns Hopkins Univ. Cire., vol. 14, 1895. Tonniges. Die Bildung des Mesoderms bei Paludina vivipara. Zeit. f. wiss. Zool., vol. 61, 1896. Vialleton. Recherches sur les premiéres phases du développement de la Seiche (Sepia officinalis). Aun. Sci. Nat. Zool., vol. 6, 1888. Wierzejski. Die Entwicklungsgeschichte von Physa fontinalis. Zeit. f. wiss. Zool., vol. 83, 1905. Wilson. Experimental Studies in Germinal Localization. (1) The Germ Regions in the Egg of Dentaliwm. (2) Experiments on the Cleavage-Mosaic of Patella and Dentalium. Journ. Exp. Zool., vol. 1, 1904. Ziegler. Die Entwicklung von Cyclas cornea. Zeit. f. wiss. Zool., vol. 41, 1885. CHAPTER X PODAXONIA Classification adopted— { Sipunculoidea Podaxonia (Gephyrea nuda) \ Ehorsaides In the old group of the Gephyrea, which used to be regarded as a subdivision of the Annelida, there were included several families or sub-orders of such diverse structure that it has been recently customary to separate them entirely from each other and to regard them as belonging to quite distinct phyla. Of these families one, the Echiuroidea, is undoubtedly to be regarded as a modified group of Polychaeta; and about another, the Priapuloidea, nothing can be said until the development has been worked out, and we know more about the adult anatomy of its members. A third group, the Sipunculoidea, constituting the Gephyrea nuda, agrees with Annelida and Mollusca in possessing a Trochophore larva, and hence must be regarded as descendants of the same Ctenophore-like ancestor, from which, as we have seen reason to believe, these two phyla originated. They differ, however, from both Annelida and Mollusca in the fact that the principal extension of the body takes place in a direction almost at right angles to the line joining mouth and anus, and further- more in a ventral direction. Hence the name Podaxonia, coined for them by Ray Lankester (1890) with his customary insight. It is probable that the group of Ectoproct Polyzoa is allied to the Podaxonia, but the full proof of that is a matter to be settled by future investigation. The group of the Phoronidea, however, con- stituting the old division of Gephyrea tubicola, is almost certainly closely allied to the Podaxonia, of which it will be considered a sub- division. Evidence in favour of this view will be offered in this chapter. PHASCOLOSOMA The genus Phascolosoma has representative species on both sides of the Atlantic. The cell-lineage and larval development of the 372 CHAP. X PODAXONIA 373 American species P. gouldit has been worked out by Gerould (1907), who also confirmed his results by work done on the European species P. vulgare. The eggs of the American species, when they mature, are dehisced ‘into the coelom and pass into the nephridia. They are finally laid in the sea and there fertilized. They are provided with a strong “chorion” or “ yolk-membrane,” which persists until the close of embryonic and the beginning of larval development. The spermatozoon penetrates this membrane through a micropyle. After the eggs have been fixed in picro-sulphuric acid, it is possible to dissolve the chorion by exposing them to Laburraque’s solution for two hours, and according to Gerould no harm is done to the egg itself by this treatment. The cleavage reminds us in many ways of that of Dentaliwm. The egg divides into the usual four macromeres, but D is, from the Fig. 301.—Early segmentation stages of the egg of Phascolosoma gouldit. (After Gerould.) A, 4-cell stage viewed from the side. B, 8-cell stage viewed from the side. C, 8-cell stage viewed from above. ch, chorion; p.b, polar bodies. first, very much bigger than its sisters A, B, and C. It has, in fact, five times the volume of any one of its three sisters. In the 8-cell stage a first quartette of micromeres is formed, and these are relatively large cells, as big as the smaller macromeres. In the 16-cell stage la, 1b, 1c, and 1d divide as usual into 1a’, 1b}, 1c}, and 1d1, and 1a?, 1b?, 1c”, and 1d? respectively. These two sets of cells are about equal in size, and they are larger than the residual macromeres 2A, 2B, and 2C. Of the second quartette of micromeres which, with these macromeres, form the lower half of the egg, 2a, 2b, and 2c are small, but 2d and its sister cell, the residual macromere 2D, are both enormous and of about equal size. In attaining the 32-cell stage, the upper eight cells of the egg divide equally, so that the quartettes of cells 1q™, 1q”, 1q”, and 1q” are all of about equal size. So far as the second quartette of micromeres are concerned, each divides into a small upper and lower larger cell. The residual macromeres 2A, etc., give off the third 374 INVERTEBRATA CHAP. quartette of micromeres; 3a, 3b, and 3c being formed first and 3d later. They are all comparatively small cells (Fig. 302). The upper half of ob the egg continues to divide more rapidly than the lower half. 1g" divides into 1q™, the apical cells, and 1q', which are the so-called “ peripheral rosettes” or the An- nelidan cross, whilst the so-called “ inter- mediate girdle cells,” 1q!, divide into 1q!, the basal, and 1q!?, the intermediate cells of the arms of the “Molluscan cross.” 1g” and 1q” also each Fia. 302.—Later stage in the segmentation of the egg of Phascolosoma gouldti, viewed from the posterior aspect. (After Gerould.) divide, so that in each Cells belonging to the second quartette are dotted ; those belonging quadrant of the egs to the third quartette are ruled with vertical lines. there are four daugh- ters of 1q?, and these cells are, of course, as in Mollusca and Annelida, the primary trochoblast cells. Fic. 303.—Two views of the apical region of the segmenting egg of Phascolosoma vulgare. (After Gerould.) The apical and the prototrochal cells are left white. The ‘ peripheral rosettes” or ‘‘ Annelidan cross” cells are covered with circles, whilst the ‘‘intermediate girdle cells” or ‘‘ Molluscan cross” are ruled with horizontal lines. A, early stage. 3, 48-cell stage. p.l, polar bodies. These cells in /hascolosoma are very large and extend backwards so as to overlap and cover the cells of the second and third quartettes. They become thickly covered with somewhat small cilia. The three x PODAXONIA 3°75 intermediate cells of the Molluscan cross 1a!22, 1b!22, 1c”, also acquire cilia and are incorporated in the prototrochal girdle. They are the secondary trochoblasts (Fig. 303). Of the fourth quartette of micromeres 4d is formed long before the others, and, as usual, immediately divides into right and left sisters 4d" and 4d!, which are the mother cells of the mesoderm. The residual macromeres eventually form a plate of endoderm which is situated beneath the “b” arm of the Molluscan cross. As in Patella and Dentaliwm the process of gastrulation begins by the sinking-in of this plate, and this in-sinking is caused by a Fie. 304.—Nearly sagittal section of an embryo of Phascolosoma vulgare at the stage when gastrulation is beginning. (After Gerould.) a.p, apical plate ; d.c, dorsal cord ; end, endodermal cells derived from residual macromeres ; end 1, accessory endodermal cells, derived from the mesodermal band ; h.b, head-blastema ; mes, mesodermal band; p.tr, prototrochal cells; stom, cells which give rise to the stomodaeumn ; ¢.b, cells forming the trunk-blastema. change of shape in its component cells. These elongate and become flask-shaped, the bulb of each flask passes into the segmentation- cavity or blastocoele whilst the neck remains for a time connected with the surface (Fig. 304). Further divisions at the upper pole of the egg result in the pro- duction of a diamond-shaped apical plate of cells surrounding four central cells, the apical cells, which acquire long stiff hairs and form the apical sense-organ. The cells surrounding this apical plate then begin to sink inwards so as to form a ring-shaped invagination which is, as a matter of fact, the head-blastema. The trunk-blastema lies on the ventral surface, behind where the blastopore is situated and 376 INVERTEBRATA CHAP. where the stomodaeum is eventually formed. It is a triangular plate of cells with the apex directed forwards and is formed of descendants of 2d. Trunk-blastema and head-blastema are con- nected in the mid-dorsal line by a narrow cord of cells—the dorsal cord (d.c. Fig. 304). The mother cells of the mesoderm, by this time, have each given rise to an anterior band of four cells and to a minute posterior cell, which lies against the endoderm, and which, as in Mollusca, probably gives rise to the intestine. The closure of the blastopore is effected partly by the forward growth of the trunk-blastema and partly by the in-sinking of the descendants of 2a, 2b, and 2¢ which give rise to the stomodaeum. The embryo becomes a larva by beginning to swim. This happens about twenty-four hours after fertilization of the egg. The larva does not burst the egg-membrane, but carries it about with it. The prototroch, composed of the primary and secondary trocho- blasts, is a broad belt of cells covered with minute cilia. There is a well-marked metatroch consisting of a girdle of long cilia. Between prototroch and metatroch is situated the opening of the stomodaeum, which is surrounded by a special girdle of small cilia. In front of the apical plate, to the right and left of the middle line, eye-spots are found (oc, Fig. 305); these are situated just above the region where the cells are being invaginated to form the head-blastema. Until thirty-six hours after fertilization have elapsed, the larva remains spherical. After that time the posterior portion of the body elongates more rapidly than the anterior portion, and at the beginning of the third day, in the case, at any rate, of Phascolosoma vulgare, it sinks to the bottom. At this period the egg-membrane is at last shed, and underneath it a fine cuticle is now to be seen, which has been mistaken for the persisting egg-membrane but is in reality quite distinct from it. Before, however, this happens, very considerable changes occur in the Trochophore larva. The appearance of the apical plate changes, since the sense-organ appears to move to its anterior edge; this is due to the anterior part of the plate becoming invaginated to form the cerebral ganglion. Round the edge of the apical plate is found the prae-oral band of cilia; this has nothing to do with the prototroch, the cells composing which carry quite minute cilia. Behind the proto- troch there is a narrow band of ectoderm from which the mesecto- derm is formed; that is to say, from this band cells are budded inwards into the blastocoele. Some of these cells are transformed into longitudinal accessory retractors (ret.acc, Fig. 306), others become changed into circular muscles. The principal retractors are formed from the cells of the apical plate which bear the prae-oral circle of cilia. These retractors retain throughout life their insertion into the ectoderm at the point where they originated from the apical plate (ret.d, ret.v, Fig. 306). ‘ Before the Trochophore larva sinks to the bottom the rudiment x PODAXONIA 377 of the ventral nerve cord makes its appearance as an unpaired ecto- dermic thickening. This thickening becomes detached from the ectoderm and sinks inwards, and in Phascolosoma gouldii (but not in Phascolosoma vulgare) it becomes divided into two to four segments. These may be regarded as ganglia of the nerve cord. In Phascolosoma gouldit the mesoderm also becomes divided into segments (mes!, mes?, mes*, Fig. 306). The anus is formed, about the forty-fifth hour, by a narrow cone of endoderm cells growing out dorsally and becoming Fic. 305.—A Trochophore larva of Phascolosoma vulgare a little more than thirty-six hours old. (After two figures by Gerould combined. ) ap, apical plate with its tuft of cilia ; ch, chorion still investing the embryo ; h.b, head-blastema ; mtr, metatroch ; p.tr, prototroch ; oc, eye-spots ; stom, stomodaeum; t.b, trunk-blastema. attached to a cluster of ectoderm cells which become slightly invagin- ated. Somewhat later the coelomic cavity appears; in Phascolosoma gouldti spaces appear in each of the mesodermal segments, which fuse together and form one undivided cavity. In P. vulgare, however, in which the mesoderm is unsegmented, the coelom appears from the beginning as an undivided space. When the Trochophore sinks to the bottom the prototroch is got rid of by a most peculiar process. The inner ends of the large cells of which it is constituted break down into yolky granules; these 378 INVERTEBRATA CHAP. (y.g, Fig. 306) are shed into the coelom and are there taken up by amoebocytes and absorbed. In this way, gradually, the whole of the prototroch is disposed of. During this process the larva takes on a cylindrical form with a diminishing ring-shaped swelling in front; this swelling is the disappearing prototroch. A nerve strand runs from the cerebral ganglion to the apical sense-organ, and from this ganglion originate a pair of muscle cells Fic. 306.—Nearly sagittal sections through metamorphosing trochophores of Phascolosoma, A, sagittal section to one side of the middle line of the larva of Phascolosoma gouldti, about fifty-seven hours old, to show the segments of the mesodermal band. (After two figures by Gerould combined.) B, sagittal section nearly median of the larva of Phas- colosoma vulgare, about forty-eight hours old. (After Gerould.) a, anus; ap, apical plate; coe, coelomic cavity ; end, endodermic tube; mesl, mes?, aves, segments of the mesodermic band ; mes.ect, point of origin of the mesectoderm ; mtr, metatroch ; n.coll, nerve collar ; pr.c, prae-oral circle of cilia; ptr, degenerate prototrochal cells ; ret.ace, accessory retractors 5 ret.d, dorsal retractor muscles formed from ectoderm; ret.v, ventral retractor muscles formed from ectoderm ; 8.0.9, supra-oesophageal ganglion ; stom, stomodaeum ; v.2.c, ventral nerve cord; y.g, yolk- granules, remains of prototrochal cells. which run backwards and are inserted into the dorso-lateral region of the skin behind the prototroch. At the sides of the ventral nerve plate there are situated two series of clusters of muscle cells. The more ventral of these run backwards towards the posterior insertion of the retractors; the more dorsal extend forwards to the region between prototroch and post-oral circlet. The invagination of the anterior region of the body, so as to form x PODAXONIA 379 an introvert, is begun as soon as the coelomic cavity appears in the mesodermic bands. Then the retractor muscles, whose formation was described above, begin to act and to pull in the whole apical region. Up till the end of the sixth day there is a freely projecting flattened prostomium, ciliated on its under surface. In the second week this prostomium grows out into a dorso-lateral extension on each side, on which ciliated tentacles are developed. Beneath the mouth a ciliated under-lip is formed. Fic. 307.— Young specimens of Phascolosoma youldii after the metamorphosis. (After Gerould. ) A, young worm six and a half days old. B, young worm thirty days old. Letters as in preceding figure. In ad- dition: neph, nephridium ; ten, ridge-like outgrowth of prostomium, on which ciliated tentacles of the adult appear. Both species of Phascolosoma then develop a circle of hooks round the base of the introvert; these can still be made out as minute hooks in the adult P. vulgare, but in P. gouldii they entirely dis- appear. The papillae, so characteristic of the adult skin, appear as oval clusters of ectoderm cells which project slightly inwards towards the coelom. Large yellow cells (chlorayogen cells) are disposed in lines along the coelom. The two nephridia (neph, Fig. 307) appear to originate as solid ectodermal ingrowths, in each of which a cavity appears later. They come to open into the coelom by the intervention of cells of 380 INVERTEBRATA CHAP. coelomic origin, from which the internal funnel or nephrostome is formed. As the body grows longer and longer the anus appears to move forwards, but this appearance is simply due to the fact that the part of the body intervening between the apical plate and the anus does not grow nearly so fast as the portion situated behind the mouth, on the ventral surface; this disparity of growth is the essential characteristic of all Podaxonia. If we review the development which has just been described, we shall find ourselves driven to the conclusion that, not only are Phascolosoma and its allies descended from the common Ctenophore- like ancestor of Annelida and Mollusca, but that they have diverged from the Annelid stem after the beginnings of segmentation had been acquired, and that they represent one mode in which the descendants of the primitive Annelida were adapted to a burrowing life. SIPUNCULUS The development of the well-known Mediterranean genus Sipunculus has also been worked out by Hatschek (1883) though not at all in the same detail as Gerould has worked out Phascolosoma, It agrees in all essentials of its embryonic and larval history with Phascolosoma, the chief differences being the form which the proto- troch assumes and the mode of disposing of it. In Sipunculus the prototroch, instead of being represented by sixteen primary prototrochal cells, is represented by a broad mantle of comparatively small cells carrying short cilia. This mantle is, however, incomplete in the mid-dorsal line, where a narrow line of sunken cells connects the head- and trunk-blastema. This line of cells corresponds to the dorsal cord of Phascolosoma. When the larva metamorphoses the whole of the mantle is cast off, as in Vucula amongst Mollusca, and is not absorbed into the coelom as in Phascolosoma. PHORONIDEA A form of great interest, which was placed in the old group of Gephyrea under the division Gephyrea tubicola, is Phoronis. Phoronis is now made the type of a special family, the Phoronidea, which Lankester considers to belong to the Polyzoa, but which we regard as more nearly related to the Sipunculoidea whose develop- ment we have just discussed. Phoronis agrees with the typical Podaxonia in the ventral development of the body, but instead of living in sand and mud it inhabits a leathery tube which it secretes for itself. It possesses, like most Podaxonia, a curved lateral extension of the lips of the mouth bearing ciliated tentacles, but these, instead of being prae-oral as in Phascolosoma, are post-oral. x PODAXONIA 381 The full embryonic history of Phoronis has not been satisfactorily made out, although a preliminary account of the subject has been given by Caldwell (1883 and 1885), and further work on the subject has been done by Masterman (1898), Ikeda (1901), de Selys Long- champs (1902), and Shearer (1906). The free-swimming larva of Phoronis is termed Actinotrocha, and was regarded as an independent organism before its life-history was known. Its remarkable metamorphosis into the adult form was described by Metschnikoff (1871), while a minute description of the structure of the adult larva was given by Goodrich (1905). a Masterman’s paper awakened widespread interest and created a lively controversy. He en- deavoured to show that Actino- trocha, like the larva of Balano- glossus (p. 575), possessed five coelomic sacs, viz. a prae-oral and two pairs of lateral sacs ; and that these sacs were developed as outgrowths from the gut, and that consequently Phoronis was allied to the Protochordata, and in particular to Cepha- lodiseus, which has ciliated tentacles like those of the : NAY AAS Actinotrocha larva. He even 4//7 ANI, | \ lr endeavoured to find the homo- y logue of the notochord in two glandular pouches which project forwards from the stomach of Frc. 308.—Lateral view of the Actinotrocha Actinotrocha (gl, Fig. 311). larva of Phoronis. (After Metschnikoff. ) The Actinotrocha larva ap, ee bv, Blood-veseel 5 m, mouth ; m.tr, metatroch ; t.tr, telotroch. possesses a hood-shaped prae-oral lobe covered with minute cilia and carrying a thickened apical plate. Somewhat below the centre of its upper surface the prae-oral lobe contains a cavity, called by Masterman the prae-oral coelom. Behind the mouth there is an oblique ciliated band, in other words a metatroch, which is drawn out into a series of hollow tentacles (Fig. 309). The tentacles contain cavities which open into right and left loop-shaped vessels, situated at the sides of the oesophagus, which were compared by Masterman to the “collar coelomic cavities” of Cephalodiscus. There are a pair of nephridial tubes which, according to Masterman, open internally into the collar-cavities and are compared by him to the collar pores of Cephalodiscus. Behind these a pair of coelomic sacs flank the alimentary canal, which correspond to the trunk coelomic cavities of Cephalodiscus. A ciliated girdle or telotroch encircles the hinder end of the larva. 382 INVERTEBRATA CHAP. Untortunately Masterman’s fascinating hypothesis has not been sustained by subsequent workers. Thus, Goodrich shows clearly that the so-called prae-oral coelom is merely a portion of the blastocoele, or primary body-cavity, corresponding to the cavity surrounding the gut in a Trochophore larva; and that the nephridia cannot be homo- logous with collar pores, because they end blindly internally and are beset with solenocytes projecting into the blastocoele; they are in fact archinephridia like those of Annelida. Goodrich admits the existence of collar and trunk coelomic cavities, but Ikeda, de Selys Longchamps, and Shearer deny that these arise as endo- dermic diverticula. In justice to Masterman it ought to be noted that Fie. 310.—Longitudinal — hori- zontal section of the embryo of an Australian species of Fia. 309.—Diagrammatic frontal section of the Phoronis. (After Caldwell.) Actinotrocha larva of Phoronis (sp") captured near Ceylon. (After Goodrich. ) arch, archenteron ; mech, mesen- chyme cells; mes, diverticula of the ap, apical plate ; col.c, collar coelom ; int, intestine ; archenteron giving rise to the meso- mtr, tentacles of the metatroch; neph, nephridium ; derm of the trunk-cavities according oes, oesophagus ; pr.bs, prae-oral blood space; s.n.p, to Caldwell. According to Shearer, subneural pit; sol, solenocytes of the nephridia; st, an ectodermic pit giving rise to the stomach ; tr.c, trunk coelom ; t.tr, telotroch. tubes of the two nephridia. Caldwell described the coelom as arising from two posterior outgrowths from the gut (Fig. 310); and though Shearer asserts that the bilobed ingrowth seen by Caldwell is an ectodermic pocket which gives rise to the tubes of the nephridia, yet the fact that the first trace of the coelom seen by him was an unpaired bilobed sac, lying close to the dorsal surface of the hinder end of the gut, renders it possible that after all Caldwell and Masterman are right and that the trunk coelom does arise as a pair of posterior diverticula of the gut, at least in some species. Further work is needed to obtain complete certainty on this point. x PODAXONTA 383 In any case, however, we fear that Masterman’s hypothesis cannot be upheld. Even if the “trunk” coelom does arise in the way which he degcribes, this does not necessarily prove a close affinity between Phoronis and the Protochordata, because we have already seen that the origin of the mesoderm from 4d in Annelidan eggs must be regarded as a modification of such a mode of development, and Erlanger has actually described the mesoderm as arising as a pouch in Paludina. The fatal flaw in Masterman’s theory is the absence of a prae-oral coelom in the Actinotrocha larva, and though it is conceivable that the notochord of Cephalodisews should be represented by a paired structure in Actinotrocha, yet the glandular pouches of Actinotrocha have no resemblance to a notochord. The notochord in all Proto- chordata, and in Vertebrata, is a modification of the endoderm into a supporting tissue, by an increase in thickness of the cell-walls of its component cells and the degeneracy of their contained protoplasm. Such a change can be seen in the endoderm of the solid tentacles of the hydroids of Zubularia, for instance, as compared with the endoderm of the hollow tentacles of Hydra. But the mere fact that the cells composing these glandular pockets in Actinotrocha contain large vacuoles, does not create any special resemblance to a notochord. On the whole, the early development, so far as it is known, creates the impression of being a modified form of the development described for Phascolosoma. In both forms the nephridia arise as ectodermal pockets which, subsequently, after the metamorphosis has been accomplished, acquire openings into thecoelom. The ciliated prae-oral lobe of Phoronis may be compared to the prototroch of Phascolosoma. In both forms the metatroch is prominent. After the Actinotrocha has led a free-swimming existence for some time, and has increased in size, an invagination of the ectoderm appears on the ventral surface, mid-way between mouth and anus. This pouch increases in depth until it reaches the intestine of the larva, to which it becomes adherent. The intestine increases greatly in length and is thrown into several loops (Fig. 311); the pouch is also thrown into folds as it grows longer. At length a critical point of growth is reached, at which meta- morphosis suddenly supervenes. The ectodermic sac is everted and forms a huge evagination which constitutes the main part of the body of the “ worm.” As the intestine was attached to the apex of this sac, when this is evaginated the intestine is drawn out in a U shape. The ciliated tentacles of the post-oral band fall off, but from their bases grow out stumps from which the adult tentacles are later developed. The prae-oral lobe disappears, according to Caldwell (1883) it is bodily amputated and falls into the gaping mouth and is there digested. The whole metamorphosis occupies only a quarter of an hour. We ourselves can testify that on one occasion we left an advanced Actinotrocha in a watch-glass, left the room for a short time, and on coming back found a young Phoronis. 384 INVERTEBRATA CHAP. If Caldwell has given the details of the metamorphosis correctly it is exceedingly diflicult to interpret, for his account would seem to imply that the apical plate and subjacent ganglion are sacrificed, in which case the cerebral ganglion of the adult must be a new A Fic, 311,—Three stages in the meta- morphosis of the Actinotrocha Jarva of Phoronis, seen from the side. (After Metschnikoff. ) A, stage in which the ventral ecto- dermie invagination has just made its appearance. B, stage in which the ventral invagination is partly everted. C, stage in which the metamorphosis is almost complete. a.ten, rudiments of adult tentacles; gl, glandular pocket of the stomach regarded by Masterman as a homologue of the notochord ; int, intes- tine ; inv, ectodermic invagination which becomes everted to form the body of the worm; l.ten, larval tentacles of the metatroch; pr.l, prae-oral lobe; st, stomach ; t.tr, telotroch. formation. Now in the metamorphosis of every Trochophore so far studied, the apical plate and the associated ganglion form the head- blastema, and persist through larval life into the adult condition. It is possible that Caldwell has made a mistake in this matter, and that it is the hood in front of the ganglion, which we have already compared to a broad prototroch bearing minute cilia, which is x PODAXONTA 385 amputated ; and that this proceeding is equivalent to the casting off or absorption of the prototroch in other forms. If this supposition be justified, then the rest of the metamorphosis can be viewed as a modification of the process of gradual growth of the ventral part of the body, already observed in Phascolosoma and Sipunculus. It is a very instructive modification, showing the kind of secondary change which may be expected to occur in ontogeny. It is another example of the complete omission of the intermediate stage of development between the Trochophore stage and the adult condition. This intermediate stage must have existed in the history of the race and doubtless occurred at one time in the history of the individual. The general conclusion then, to which we are led by a review of the development of Phoronis, is that it really does belong to the group of Stpunculus and Phascolosoma, and that the classification of the older authors is so far justified. This conclusion, however, raises another series of most interesting questions. The structure of Phoronis is so similar to that of the Phylactolaematous Polyzoa that Lankester (1890) regarded Phoronis as a Polyzoan ; and it seems difficult to evade this conclusion. But in that case Phoronis would be the only solitary Polyzoan known, and all the true Polyzoa (leaving out of account the anomalous Entoprocta) must be regarded as modified Podaxonia. LITERATURE CONSULTED Caldwell, W. H. Preliminary Note on the Structure, Development, and Affinities of Phoronis. Proc, Roy. Soc. (Lond.), vol. 34, 1883. Caldwell, W. H. Blastopore, Mesoderm, and Metameric Segmentation. Quart. Journ. Mie. Sci., vol. 25, 1885. ; . Gerould, J. H. Studies on the Embryology of the Sipunculoidea, I. The Embryonic Envelope and its Homologue. Mark Anniversary Volume, 1903. ; Gerould, J. H. Studies on the Embryology of the Sipunculoidea, II. The Development of Phoscolosoma. Zool. Jahrb. (Anat. u, Ont.), vol. 23, 1907. Goodrich, E. On the Body-cavities and Nephridia of the Actinotrocha Larva. Quart. Journ. Mic. Sci., vol. 46, 1903. : Hatschek. Uber die Entwicklung von Sipunculus nudus. Arb. Zool. Inst. Wien, vol. 5, 1883. . Ikeda. Observations on the Development, Structure, and Metamorphosis of Actinotrocha. Journ. Coll. Sci. Imp. Univ., Tokyo, vol. 13, 1901. Lankester, E. RB. Zoological Articles ‘‘Polyzoa,” yepub. from Encyel. Britt., 1890. aR: Masterman. On the Diplochorda, Parts I. and II. Quart. Journ. Mic. Sci., vol. 40, 1897; Part III., zbid. vol. 48, 1900. aft ; ; Metschnikoff, El. Uber die Metamorphose einiger Seetiere, III. Actinotrocha. Zeit. Wiss. Zool., vol. 21, 1871. ; ; de Selys-Longecbamps. Recherches sur le développement de Phoronis. Arch. de biol., vol. 18, 1902. ae . Shearer-Cresswell. Studics on the Development of Larval Nephridia (I. Phoronts). Mitt. Zool. St. Neapel, vol. 17, 1906. VOL. 1 206 CHAPTER XI POLYZOA Classification adopted— Phylactolaemata Cyclostomata poly2oa: Hevoprocty Gymnolaemata eee, Cheilostomata Polyzoa Entoprocta THE group of the small colonial animals known as the Polyzoa includes two divisions, known respectively as the Ectoprocta and the Entoprocta, about whose affinity with one another there is very considerable doubt. Both groups agree in being colonial, in possessing a ring of ciliated tentacles surrounding the mouth, by the action of which they obtain their food, and in having the principal nerve ganglion situated between mouth and anus on the surface which is normally turned upwards. In the Ectoprocta the coelom is spacious and well developed, and from its walls the genital cells are developed; whilst the body is divided into a posterior part (the zooecium) and an anterior introvert (the polypide). The ring of ciliated tentacles surrounds the mouth alone, and is therefore morphologically a metatroch. In all these features the Polyzoa Ectoprocta resemble the Podaxonia. In the Entoprocta, on the other hand, the coelom is entirely suppressed, except in so far as it is represented by the minute cavities of the genital organs. There are distinct nephridia, ending internally in blind ciliated ends; the body is divided into an upper cup-like part called the calyx, and a lower solid stalk. The ring of ciliated tentacles surrounds both mouth and anus, and is, morpho- logically, a prae-oral band or prototroch. Prouho (1892), Harmer (1896), Seeliger (1906), and Czwiklitzer (1909) regard the two groups as closely allied, but Korschelt and Heider (1892) regard them as totally distinct phyla. We shall deal with them separately in this chapter, and, after having studied both, indicate our opinion as to which side in this controversy has the greater weight of evidence in its favour. We begin with the Polyzoa Ectoprocta. 386 CHAP. XI POLYZOA 387 POLYZOA ECTOPROCTA In the vast majority of Polyzoa Ectoprocta the egg is fertilized whilst it is still in the maternal tissues, and undergoes the first stages of its development there. It finally emerges as a free- swimming larva, with more or less degenerate gut, and, after a short free existence, fixes itself and grows into the first person of the future colony. In a few cases, however, the eggs are shed into the sea and are fertilized there, and a comparatively long larval development ensues; this type of larva has a well-developed gut, and can feed itself. In these latter cases we have obviously the primitive type of Ecto- proctan development, and it is they which deserve our closest attention. They have been most carefully studied by Prouho (1892), and we select for special description one of the forms described by him which has a long larval development. MEMBRANIPORA PILOSA Membranipora pilosa is a species occurring abundantly around the coasts of Europe as a delicate lace-like incrustation on the fronds of Laminaria. Closely allied species are found in similar situations all over the world. Membranipora is easily kept living in vessels of clean sea-water; the eggs are freely discharged, and develop into the young free-swimming larva, which is termed Cyphonautes, a name bestowed on it when it was supposed to be an independent organism. Its true nature was shown by Schneider (1869), who captured it in the sea and watched it metamorphose into Membrani- pora. To rear Cyphonautes in captivity up to this stage would require arrangements for feeding it with diatoms such as have been employed with success in the case of many other larvae. Tf the vessels in which the larvae are kept have been previously coated with a layer of transparent photoxylin, then, when the larvae fix themselves, they can be removed from the sides of the vessel together with the photoxylin to which they are adherent, and cut into sections. Kupelwieser (1905), to whom we owe this method, has given us the best account of the metamorphosis of the larva. He paralysed the free-swimming larvae by adding drops of hydro- chlorate of cocaine to the sea-water in which they were swimming ; the larvae were then preserved in Flemming’s fluid or a mixture of the solution of corrosive sublimate and glacial acetic acid. In the development of the egg of Membranipora the division into blastomeres takes place in an absolutely even and regular manner, and recalls in a good many ways the segmentation of the egg of Polygordius. At the 16-cell stage all the blastomeres are equal to one another in size; the embryo, however, does not form a sphere, but a biconvex lens (Fig. 312), the axis joining the animal and vegetable poles being very much shortened. The blastocoele is excessively narrow. At the 32-cell stage the flattening is still more 388 INVERTEBRATA CHAP. marked, and four blastomeres situated in the centre of one face are distinguishable by their granular contents. These blastomeres are the rudiment of the endoderm, and the blastula stage may be said to be now completely attained. At the next stage the endodermal cells sink inwards, filling up the blastocoele, whilst the other cells meet beneath them, and so the gastrula is formed. One is thus reminded forcibly of the extreme flattening which the blastula of Polygordiws undergoes just prior to gastrulation. The blastopore, or aperture left by the in-sinking of the endoderm cells, is almost immediately closed. The face on which it was situated will be called the oral face. Two cells, placed symmetrically to the right and to the left of the middle line, are found at the next stage in the development of Alcyonidium albidwm, which also gives rise to a Cyphonautes-like larva; but they have not as yet been observed in the case of B end C Fic, 312.—Early stages in the development of the egg of Membranipora pilosa, (After Prouho.) A, stage of sixteen blastomeres seen from the side. B, stage of thirty-two blastomeres seen from the underside. C, gastrula seen from the side. blp, blastopore ; end, cells which form the endoderm. Membranipora, though it is quite probable that they exist there also. In the case of Alcyonidiwm they help to form larval muscles, which traverse the blastocoele, and these muscles also exist in the larva of Membranipora. These two cells, whose exact origin Prouho could not determine, he calls the mother cells of the mesoderm. They are, however, situated in front of the mass of endoderm, and have, in all probability, nothing whatever to do with the true pole cells of the mesoderm in Polygordius which give rise to the coelomic wall, but are rather to be compared to the mesectoderm of Polygordius, i.e. cells derived from the second quartette of micromeres, 4c. from the ectoderm which gives rise to the blastocoelic muscles in the larva. Coming now to the next changes observed in the embryo of Membranipora, we find that the whole embryo takes on a conical shape, the oral face forming the base of the cone, whilst at the upper end, where the point of the cone should be, a thickening of the ecto- derm becomes visible, which is termed the apical organ and which is homologous with the apical plate of Annelidan and Molluscan larvae. On the oral face now appears a wide depression. This is XI POLYZOA 389 the beginning of the enormous stomodaeum which pushes the endodermic mass towards the posterior end of the embryo. The embryo now adheres by its oral face, and also by the apical organ, to the vitelline membrane, and at the same time it alters its shape, so that it becomes compressed from side to side. The vitelline membrane seems to be absorbed where it is in contact with the embryo; stiff cilia or sense-hairs appear on the apical organ, and A Fic. 313.—The development of the larva of Membranipora pilosa. (After Prouho.) _A, young larva just free from the egg-membrane ; end, solid mass of cells which will be hollowed out to form the stomach. The stomodaeum is formed, but does not join the stomach as yet. B, young larva in which the stomodaeum has joined the stomach. C, larva a little older than that repre- sented in B, in which the proctodaeal invagination has been formed. ap, apical plate; co, corona ; proct, proctodaeal invagination ; mes, ectomesoderm. powerful locomotor cilia on certain cells of the thickened ridge of ectoderm, termed the mantle, which forms the border of the oral face. The space surrounded by the mantle has become concave, and it is termed the atrium; into it the stomodaeum opens. The ring of ciliated cells is termed the corona. The embryo now becomes a larva and swims about ; the remnants of the vitelline membrane which still envelop it in the middle are brushed off. After the free life has begun the mass of cells forming the endoderm becomes hollowed out and forms the larval stomach. 390 INVERTEBRATA CHAP. Mesoderm cells in front of this multiply and form a string leading from the aboral thickening to the ventral surface; this string is the rudiment of the main dorsal muscle of the larva. Shortly afterwards an ectodermic invagination is formed in the posterior part of the oral face. This is the proctodaeum, the rudiment of the anus and of the larval intestine. It grows in length and joins the stomach, and the latter opens into the stomodaeum, in which cilia become developed, and so the definite alimentary canal is completed and feeding begins. A delicate bivalve shell is secreted by the larva; each valve is tri- angular, and the apical organ protrudes between the apices of the valves, whilst their bases flank the corona. Soon afterwards the ring of ciliated ectoderm which we have termed the corona, and which we may compare to the prototroch of the Trochophore larva, begins to exhibit modifications. In front of the mouth a pair of transverse ridges grow inwards from it at right angles to its course, and constitute a transverse band of cilia across the ventral face of the larva. A pair of similar ridges also grow in- wards from the oral band in front of the anus, and constitute a second transverse band of ciliated ectoderm there. When the Cyphonautes is fully grown it possesses two other organs: a so-called “piriform organ” in front of the mouth, con- sisting of columnar ectoderm cells, and an “internal sac,” which is an invagination of the ectoderm between mouth and anus. The “ piriform ” organ arises as an ectodermal invagination, which becomes almost shut off from the exterior, but remains connected therewith by a narrow longitudinal slit, the cells lining which are covered by powerful cilia. This slit is termed the vibratile cleft (v.cl, Fig. 314). Though these ciliated cells afterwards meet those of the corona they originate quite independently of it, and only subsequently come into contact with it. The cells of the piriform organ itself take on a glandular appearance, and emit a secretion into its cavity. The main muscle, alluded to above, leads from the apex and sides of the piriform body to the apical organ, and then passes beneath this to run down the posterior aspect of the Jarva to the most posterior cells of the corona. In front of the ciliated groove which forms the opening into the piriform organ there is a rounded ciliated pit which is delimited from the groove by a blunt prominence. On the hinder aspect of this pit there is a small group of cells which carry exceptionally long cilia— cilia which, moreover, are bent in a peculiar hook-like manner, and which swing backwards and forwards in the middle line. These are termed the vibratile plume. A special branch of the main dorsal muscle pierces the glandular sac of the piriform organ and continues its course to end at the base of the cells carrying this special “ vibratile plume.” A strand of nerve fibres accompanies this muscular strand. But the main mass of the muscle and nerve proceed downwards, and whilst the nerve fibres become more numerous the muscle fibres decrease in number, Many of the nerve fibres go to the ciliated cells XI POLYZOA . 391 of the vibratile cleft, but after these have been given off the main mass of the nerve fibres proceeds to the ciliated cells of the prototroch or corona. Muscle fibres also go to the cells of the vibratile cleft, apgl apyis y _,MUSC.CITC.0€S uscd =—“__esmuscsk. *\o. ese an CK Add _jnuse lata an nf he : I mt 4 Hi Ly i i i WN | dl - Fig. 314.—Median sagittal section of the fully grown Cyphonautes larva of Membranipora pilosa on the top of which have been traced certain structures (muscle fibres and corona, etc.) lying to the right of the median plane. (After Kupelwieser, slightly altered.) Addi, main adductor muscle of the valves ; Add?, accessory adductor muscle of the valves; ap.g, cells of the apical plate of the ganglionic nature ; ap.gl, cells of the apical plate of a glandular nature ; ap.vis, cells of the apical plate of a visual nature; cil, cells forming anterior and posterior borders of ‘the larva carrying long cilia ; col, anterior section of the corona; co, posterior section of the corona ; gr.e, granular cells; i.s, internal sac; it, intestine; m.e, mucous cells of mantle edge; musc.circ. oes, circular muscles surrounding the oesophagus ; muse.d, dorsal muscle ; musc.d1, branch of dorsal muscle which is inserted in the apical organ ; muse.d2, branch of dorsal muscle which curves back under the apical organ and is inserted in the posterior section of the corona; musc.lat.a, anterior part of lateral muscle ; muse.lat.p, posterior part of lateral muscle; musc.sk, sucker muscle ; n.f, nerve fibres leading from apical organ to vibratile plume and to corona; oes, oesophagus; /p.o, piriform organ; sec, secretion produced by the cells of the internal sac ; v.cl, vibratile cleft; v.pl, vibratile plume. musclatp and to the cells of the corona, but in addition muscle fibres are given off which are inserted in the upper part of the piriform organ, and others which encircle it after the manner of circular fibres. Kupelwieser regards the function of the piriform organ as skeletal, 392 INVERTEBRATA CHAP. that is to say, he thinks that it affords a convenient insertion for muscle fibres; those inserted in its upper part he regards as retractors, the encircling fibres as protractors, and the vibratile plume he con- siders the real sense-organ. He believes that this sense-organ comes into play just before the fixed life is taken up, and that its function is to select a suitable spot for the fixation of the larva. The internal sac, or sucker, arises just in front of the anus, and, according to Kupelwieser, it begins as a solid thickening of the ectoderm, which soon splits into two layers separated by a cavity. Prouho, however, says that it arises as an invagination just in front of the anus. This little sac develops, as larval life proceeds, into a wide, spacious sac, which is drawn out into two horns. The upper wall of the sac remains thin, but its lower wall, where it abuts on the atrial cavity, becomes glandular, and produces great masses of a slimy secretion. Eventually this wall breaks down and allows the sucker to open into the atrial cavity, into which the secretion is then discharged. Two muscles arise, one on each side, from the ectoderm of the central parts of the flat sides of the larva, and are inserted into the upper wall of the sac. These muscles, termed the sucker muscles, only come into play at the metamorphosis. The ectoderm of the sides of the larva, as we have already noted, secretes two thin valves of shelly material, which thus form a bivalve shell protecting the larva. Round the edges of each triangular shell-bearing area there runs a cushion of large swollen cells, filled with a mucoid secretion, thus taking on the outline of a triangle. From the base of this triangle a ridge of the same material projects upwards a short distance. The ectoderm covering the narrow sides of the larva, between the valves, is covered with short cubical ciliated cells. From this description it follows that the apical organ is bounded laterally by the cell-cushions, and front and back by cubical epithelium. The apical organ itself is a two-layered, slightly concave plate, or shallow cup of cells. The rim of this plate is composed of converging ~ columnar cells, each bearing a single stiff cilium or sense-hair. Inside this outer ring comes a second ring of cells bearing pigment, to which Kupelwieser assigns a visual function, while in the centre there is a mass of clear rounded cells. From the fact that a bundle of nerve fibres proceeds from these inner cells, Kupelwieser draws the conclusion that they are of a ganglionic nature. The fibres of the dorsal muscle penetrate between the cells of the apical organ in order to attain their insertion ; but not all the fibres of the dorsal muscle, in fact only the minority, have this insertion. The majority of the fibres of the dorsal muscle pass back under the apical organ and diverge into bundles, right and left, and are inserted into the ciliated cells of the posterior part of the corona. Most of the fibres belonging to the dorsal muscle are striated, but some muscles have smooth fibres. There is an adductor muscle connecting the two valves of the shell POLYZOA 393 XI beneath the stomach, and above it there is a similar smaller muscle, *[[9YS ay] JO Sartva ‘cys “ys f puq apidAjod jo waraposaut ‘saw-pod [esiop ‘posnw $SIsATOYSI UT ayOsSNUI Jo QueUISeay ‘osnw S yoRulogs pas{jo Wf WOd s[[99 WaAapopus Jo yuatseay ‘pus ‘ vUOIOD ‘oo : AQIAvO [elaye “47 Jeorde ‘dp *paouvape YOU ST SISA[OJSIY] YOTYAL UL BAAR] B YSNOIYY UoIgoas [eyo “q “UWOIYRXY Isqje AjoyRIpsWUll BAIL] B YS £pnq eprdcjod ‘ ‘uv8i0 waojlatd ‘o'd { susvydosao ‘so { ayosntt CaSpe apjueur Jo s[[eo snoonut ‘aw {azZd ealseype UF WIJ 0} YNO poueyqey OVS [BUEZUL ‘sy apiddjod 91 wW4of 0} Japio Ul UOI4QeUISBAUT JO ssaooid ur aed O14} WOTQDes [BVWWSeS “WZ (aasormpedny sayy) “vsopd nuodiuniquayy Jo evarry Susoqdxomezout pur pexy Ysnosyy swooeS— “GT g “Oly LX RAE hee LiZt — At oe 2 era, i oe e , \e Two lateral muscles, an anterior and the accessory adductor. f the shell which posterior, arise on each side from the same area o 394 INVERTEBRATA CHAP. gives rise to the sucker muscles, and are inserted in the cells of the corona. During its active life the larva swims with its apical organ directed forwards; but when the free life draws to an end it glides over the bottom, with its oral surface directed downwards, and during this period the vibratile plume can be seen to carry out tactile move- ments. Finally, the sucker is everted and forms a thin flat plate of cells which adheres to the substratum (Fig. 315). All the muscles except the sucker muscles contract strongly, and the piriform and apical organs are in consequence strongly retracted. The outer edges of the adhesive sucker turn upwards and unite with the edges of the mantle, and the remnant of the atrial cavity is converted into a ring-shaped space, towards the inner side of which the cilia are directed. Then the muscles connecting the sucker with the valves of the shell contract, and with great strength, so that the valves of the shell are, so to speak, flattened out over the compressed larva. Histolysis of the larval tissues now begins. First the cushion cells disintegrate and their mucoid contents are cast into the ring- shaped atrium. In this way the ciliated cells of the corona become cut loose from the mantle edge, the cells of which join the edge of the adhesive plate formed from the sucker; and the remnants of the coronal cells are found floating in the ring-shaped atrial cavity. The apical organ is very deeply invaginated and broken loose from the flanking cushion cells; the adjacent ordinary ectoderm cells meet above it, and from it, afterwards, the polypide of the first bud, i.e. the alimentary canal and ciliated tentacles, are developed. The coronal cells and larval muscles are attacked by wandering amoebo- cytes. The stomach and intestines excrete brown granules into their respective cavities, and finally lose their cavities and become solid clumps of degenerating cells. The whole animal is thus reduced to a thin-walled sac containing, invaginated into it at one point, a thick-walled sac, which is the former apical disc and is the rudiment of the future polypide of the mother bud of the colony. TYPES OF POLYZOAN LARVAE Before studying the further development of the polypide it will be well to cast a brief glance at the other types of larvae which have been described in Polyzoa. All are modifications, one might add modifications in the direction of degeneracy, of the Cyphonautes type. Prouho has indeed shown that the species Alcyontdiuwm albidum, which belongs to quite a different division of Polyzoa (Ctenostomata) from that to which Membranipora belongs (Cheilostomata), has a larva which can be distinguished only by minute specific differences from the larva of Membranipora; and the same is true of the larva of Hypophorella expansa, which also belongs to the Ctenostomata but to a different division from that to which Aleyonidiwm belongs. The larva of Flustrella, a genus allied to Alcyonidiwm, is very « «ee XI POLYZOA 395 similar to the Cyphonautes. It has a bivalve shell, a well-developed pyriform organ, and a complete set of muscles; but the alimentary canal is somewhat degenerate, the intestine being wanting. The corona forms a complete ring without cross ridges. In some species of Aleyonidium, such as Alcyonidium polyoum, described by Harmer(1887) (Fig. 316, A), further de- generacy can be seen; a stomodaeum and stomach alone are present, as in the larva of Flustrella, but the bivalve shell is gone, and the apical organ is a wide, flat disc. The corona con- sists of a single ring of large ciliated cells. When we pass to Cheilo- stomata, like Lepralia, Bugula, etc., we find that the gut has entirely disappeared, and is repre- sented by a mass of mesen- chyme cells. The corona of Lepralia resembles that of Alcyonidiwm, but that of Bugula consists of enormously tall cells, each extending through the whole height of the larva (Fig. 316, B). In the Cyclostomata there is also a gutless larva, but now the apical organ is represented by @ Fic. 316.—Two degenerate types of larvae of Ecto- deep invagination devoid proct Polyzoa, (Combined from figures given by of sense cells; the corona —_ Srschelt and Heider.) A, optical section of the larva of Alcyonidium polyowm. 18 represented by a broad B, optical section of the larva of Bugula plumosa. The dark belt of ciliated cells of streaks represent coronal cells lying between the reader comparatively small size, 2nd the median plane of the larva. «ap, apical disc ; ap.g, ; ganglion cells beneath the apical disc; co, corona; i.s, and the pyriform organ is internal sac ; m, mouth ; p.o, pyriform organ ; st, stomach. absent. Finally in the freshwater Phylactolaemata, where, as in many other freshwater animals, there is an extremely shortened develop- ment, we find an oval larva, most of whose surface is covered with fine cilia, but which has an invagination at the anterior pole whilst the posterior pole is glandular. The broad ciliated band represents the corona, the apical invagination the apical organ, which is, however, entirely devoid of sense-hairs, and from which one or two 396 INVERTEBRATA / CHAP. polypides are already being developed. Fixation takes place by the posterior glandular pole, and then the walls of the anterior invagina- tion are suddenly turned back so that the polypide area is exposed. The retroverted folds adhere to the substratum and force the larva away from its primary attachment. In this way a huge sucker-like organ is formed at the posterior pole; this sucker becomes a completely closed sac, and then its contents are devoured by amoebocy tes. It will be seen that in the series of larvae which we have just described we have to deal with a progressive disappearance of larval structures, and a progressive hurrying on of adult structures. Thus in the Phylactolaemata, which constitute the culminating point of the series, the larval body has become merely a skin enclosing the first two or three buds. It is obvious, therefore, that in seeking for light on the past history of the Polyzoan stock we must confine our attention to the primitive type of larva represented by Cyphonautes. BUDDING The metamorphosed Cyphonautes consists of a simple ectodermic. sac with a closed- vesicle of columnar cells projecting into it. This ectodermic vesicle is termed the polypide, and from it the ectodermic parts of the tentacles, and the whole alimentary canal of the first person of the colony, are derived. The mesodermal portions of the tentacles, including the walls of the coelomic canals which they contain, are derived from a layer of mesoderm cells (pol.mes, Fig. 315) which clothes the external surface of the polypide. The exact origin of these mesoderm cells from pre-existing larval mesoderm has not been determined. The ectodermic sac-like body of the metamorphosed larva constitutes the zooecium of the first polypide. The valves of the larval shell are soon shed and are replaced by the continuous cuticle which constitutes the ectocyst of the zooecium. The first person of the colony originates therefore as a bud on the body of the metamorphosed larva; and, so far as is known, the development of this bud is quite similar to that of the later buds, by which the colony increases in size. It follows that in Polyzoa Ectoprocta, we have not the continuous life-history of an individual proceeding from the larval to the adult condition, but an alternation of generations by which a sexually produced form, the larva, gives rise to an asexually produced form, the first person of the colony. The manner in which the buds of Polyzoa Ectoprocta develop has been investigated by many authors. Seeliger (1890), who investigated the buds of Bugula, has given the clearest account of the matter, and as his results have been confirmed in almost every point by the latest observer, Romer (1906), we shall follow Seeliger in our account. When a new bud is about to be formed the new zooecium arises as an out-pouching of the old one. The cavity of this pouch is iets XI POLYZOA 397 eventually cut off from that of the parent zooecium by a mesodermic septum, but before this happens the first rudiment of the new polypide appears in the new zooecium as an ectodermic thickening, which later becomes an ectodermic pouch open to the exterior, except in so far as it is roofed over by the common cuticle or OCECnO CS: <0. OQ 200.099900 900 Fic. 317.—Stages in the development of the bud of Bugula aviewlaria, (After Seeliger.) A, the early rudiment of the polypide in the form of an ectodermic invagination. B, a later stage: the rudiment of the polypide is almost shut off from the exterior, and has increased in depth, and its mesodermic covering has become continuous. C, D, two sections through an older polypide in which the constriction of the rudiment into atrium and gut has begun. C, is through the opening which remains as the anus. &, section through a still older polypide in which the tentacles have appeared as ridges in the atrial wall. The section goes through the opening which remains as mouth. at, _ rudiment of atrium ; pol.mes, mesoderm ; pol, polypide ; oes, oesophagus ; ten, tentacles. ectocyst. This pouch deepens and its mouth closes, and the resulting sac becomes divided by a constriction into an upper region, which is the rudiment of the future atrium or tentacle sheath, with its contained tentacles, and a lower region, which is the rudiment of the entire gut of the new person. The sac becomes clothed externally by mesoderm cells; these 398 INVERTEBRATA CHAP. cells eventually form a coherent layer, but appear to arise as wandering cells which adhere individually to the external surface of the polypide. From this mesodermic layer the coelomic canals of the tentacles and the ring canal which unites them, are derived. The constriction between gut rudiment and atrial rudiment becomes so deep at one place as to completely sever the two from each other, but before and behind this place two openings are left by which the two rudiments still communicate, and these openings form the mouth and anus of the new person. The gut rudiment becomes divided by constrictions into oesophagus, stomach, and intestine. The atrial rudiment develops the lophophoral tentacles as ridges projecting into its cavity, and at the completion of development it reacqtires an opening to the exterior. The retractor muscles, funiculus, etc., are derived from scattered mesoderm cells, which originate from the mesoderm cells of the mother. Romer, who investigated the buds of Alcyonidiwm, differs only from Seeliger in finding that in Zz Lik coe Fia. 356.—Stages in the early development of Asterias vulgaris. (After Field.) A, blastula in optical longitudinal section. B, gastrula in optical longitudinal section showing the formation of mesenchyme. C, older gastrula in optical longitudinal section. D, transverse section of a larva slightly older than that represented in C, showing the formation of the coelom. ap, apical disc; coe, coelomic sac originating as pouches from the archenteron ; mes, mesenchyme; ves, vesicle at the apex of the archenteron. : (Fig. 357, B), and thus completes the larval oesophagus. Along the sides of this oesophagus a V-shaped band of strongly ciliated epithelium is differentiated, which is termed the adoral ciliated band (Fig. 357, C). It seems to be formed from both the ectodermal and the endodermal region of the oesophagus. The angle of the V is situated behind in the mid-ventral line. The limbs of the V pass up the sides of the oesophagus, and their terminations are XVI ECHINODERMATA 463 connected by a much less strongly ciliated band, which passes round the dorsal side of the oesophagus just behind the mouth. It is commonly taken for granted that the function of the adoral band is to direct a stream of water carrying minute organisms into the mouth, and that it is in this way that the larva secures its nourishment. Some years ago we made some observations on the function of the homologous band in the larva of Zchinus, and it seems to us that the main function of the adoral ciliated band, like the function of the cilia in the transverse grooves running across the labial palps of Pelecypod Mollusca, is to remove excess of food from the neighbourhood of the mouth. The minute organisms, which constitute the bulk: of the food, may be seen to be carried in by a current which passes into the stomodaeum at its dorsal border. This current seems to be caused by the cilia of the Fic. 857.—Young larvae of Asterias vulgaris. (After Field.) A, about three days old, from the side. B, about four days old, from the ventral surface. ©, about tive days old, from the ventral side. a, anus; cil.ad, adoral band of cilia ; cil.long, longitudinal ciliated band ; int, intestine ; oes, oesophagus ; st, stomach ; stom, stomodaeum. principal longitudinal band (v. infra), aided no doubt by the cilia of the dorsal side of the stomodaeum. At the ventral end of the stomodaeum particles may be seen to be flung outwards violently— hence it is apparent that the current produced by the adoral band is directed outwards. The food accumulated in the outer end of the stomodaeum is transferred to the stomach, not by the action of cilia but by peristaltic muscular contractions... Whilst these changes have been taking place other events have been occurring. The cilia, which covered the whole surface of the blastula and gastrula, become specially abundant and long over the course of a sinuous band of thickened epithelium which is termed the longitudinal ciliated band (cil.long, Fig. 357, A), and which is the principal locomotor organ of the larva. Over the rest of the surface they do not disappear, but become very sparse. This is due to the passive stretching of the epithelial cells in these regions, due to the increase in the pressure of the blastocoelic fluid. The longitudinal ciliated band is also found in the larvae of 464 INVERTEBRATA CHAP. Ophiuroidea, Echinoidea, and Holothuroidea. It consists of two sides, and of an anterior and of a posterior cross-bar. The anterior cross-bar is situated in front of the mouth, and the posterior cross- bar in front of the anus. Neither cross-bar is straight, both are bent into the form of loops. The loop formed by the anterior cross- bar is bent back along the ventral surface of the prae-oral region or forehead of the larva, and it is termed the prae-oral loop, and the area which it surrounds is termed the frontal field. The loop formed by the posterior cross-bar bends forward along the ventral surface in front of the anus, and the area which it surrounds is called the anal field. When the alimentary canal has been completed by the union of the stomodaeum and oesophagus the larva is able to feed, and, if suitable diatoms be provided, it will live and grow, even in a com- paratively small aquarium, without any change of water. Dr. Gemmill has reared the larvae of Asterias rubens for over two months in his laboratory at Glasgow, until they had completed their meta- morphoses ; and this feat has also been accomplished with the larvae of Asterias glacialis by Professor Yves Delage in his laboratory at Roscoff. As the larva increases in size the tissue of the longitudinal ciliated band grows more quickly than adjacent regions of the ectoderm, and the band becomes thrown into folds. These folds form lobe-like out- growths termed larval arms, which correspond in number, position, and size on the two sides of the body, and confer on the larva a “ bipinnate ” appearance, whence the name Bipinnaria. Mortensen (1898) has invented a nomenclature for the larval arms of Asteroidea and the homologous structures in other Echinoderm larvae, and we shall follow his nomenclature in this book. Before the larval arms have attained any size the prae-oral loop becomes separated from the rest of the longitudinal ciliated band, and this primary band becomes, in this way, divided into two secondary bands, which we shall term the prae-oral and the post-oral bands respectively (pr.o.6, p.0.b, Fig. 358). In the Bipinnaria larva of Asterias vulgaris the prae-oral band carries only two larval arms, which are termed the prae-oral arms (pr.o.a, Figs. 358, 359), and the frontal field is somewhat quadrangular in shape. But in the larvae of the European species, Asterias rubens and A. glacialis, the prae-oral band develops in addition a median anterior arm, directed forwards, termed the median ventral arm, and the frontal field is consequently triangular in shape. From the post-oral band, in all three species of Asterias, there is developed an anteriorly directed arm, which is termed the median dorsal arm (m.d.a, Fig. 358). From the sides of the post-oral band, about one- third the length of the larva from its anterior end, there are given off two arms, termed the antero-dorsal arms (a.d.a, Fig. 358). Still farther back, at rather more than two-thirds the length of the larva from its anterior end, two similar arms arise termed the postero- XVI ECHINODERMATA 465 dorsal arms (p.d.a, Fig. 358). Where the sides of the post-oral band pass into the anal loop two long arms are developed, termed the postero-lateral arms (p.l.a, Fig. 358). Finally, from the sides of the anal loop two short arms are developed, termed the post-oral arms (p.0.a, Fig. 358). The two coelomic sacs are, for a considerable time, two somewhat rounded pockets lying at the sides of the oesophagus. The one on A Fig. 358.—Fully developed Bipinnaria larva of Asterias vulgaris, about three weeks old. (After Field.) A, from the ventral surface. B, from the dorsal surface. a, anus; a.d.a, anterior dorsal arm ; cil.ad, adoral ciliated band ; fus, anterior fusion of right and left coelomic sac; int, intestine ; l.coe, left coelomic sac; m.d.a, median dorsal arm; m.p, madreporic pore ; oes, oesophagus ; p.d.a, postero-dorsal arm; p.1.a, postero-lateral arm ; p.0.a, post-oral arm ; p.o.b, post-oral band of cilia; pr.o.a, prae-oral arm; pr.o.b, prae-oral band of cilia; 7.coe, right coelomic sac ; st, stomach ; stom, stomodaeum. the left side sends up a short vertical outgrowth which fuses with a slight inward dip of the ectoderm, and in this way forms a canal leading to the exterior. The opening of this canal is termed the primary madreporic pore (m.p, Fig. 359), and the canal itself is termed the pore-canal. Its wall is covered with cilia which beat inwards and striye to distend the coelomic sac with sea-water. Twenty years ago Field (1894) stated that, in the case of Asterias vulgaris, the right coelomic sac formed a similar pore-canal, terminat- ing in a right madreporic pore (m'p', Fig. 359), which soon, how- VOL. I 2H 466 INVERTEBRATA CHAP. ever, became obliterated. No subsequent observer recorded the exist- ence of this right madreporic pore, although the larvae of Astertas were raised by thousands for experimental purposes by Driesch, Herbst, and others; but quite recently Dr. Gemmill has been able to confirm Field’s statement. He finds that a right madreporic pore is formed in about one in every ten larvae of Asterias rubens, and in one out of every two larvae of Asterias glacialis. In all cases it very soon closes. The appearance of a right and left madreporic pore is the first indication of what rs really the key cillong to the understanding of Echinoderm development, viz. the fact that the two sides of the larva originally gave rise to precisely similar organs, but that some of these organs grew and de- veloped on the left side while they atrophied on the right, and that thus an asymmetry was produced. The coelomic sacs now begin to grow in length until they form long, narrow, cylindrical cavities, reaching from the prae-oral region mp1 of the larva to the posterior end ; and by their form and relations they merit the name “ water-tube,” bestowed on them by Agassiz (1864). The right and left water- tubes meet one another in the prae- oral lobe and fuse into one (fus, Fic. 359.—Larva of Asterias vulgaris Fig. 358, B), but elsewhere they four days old, viewed from the dorsal remain separated fromoneanother by surface, showing two madreporic pores. (After Field.) es alimentary canal, and above and Names as in the preceding figure. In be pa this by a vertical mesentery. addition, l.coe, left coelomic sac; m.p (left), Now a constriction appears persistent madreporic pore; mip! (right), tran- just behind the madreporic pore, nad madreporic pore; 7.coe, right coelomic which almost, but not quite, divides the left water-tube into anterior and posterior portions. On the right side no such constriction is formed until considerably later. The front division of the left water- tube may be termed the left anterior coelom, whilst the hinder division is named the left posterior coelom (J.p.c, Fig. 360, A). The posterior portion of the left anterior coelom swells out slightly, and begins to form five lobe-like outgrowths arranged in an open curve. This swelling and its lobes are the rudiment of the water vascular system, and are termed the hydrocoele (hy, Fig. 360). The most dorsally situated lobe is numbered (1), the next (2), and so on. Occasionally a similar five-lobed outgrowth, which we may term the right hydrocoele, is formed as an outgrowth from the right water- XVI ECHINODERMATA 467 tube. In this case this water-tube becomes completely divided into anterior and posterior portions, which we may term right anterior and right posterior coeloms respectively, just as the left is normally. This formation of a second hydrocoele was first described by us in Asterina (1896). Somewhat later, near the mid-dorsal line but to the right of it, a small sac is formed. According to Field (1894) it just appears as a solid bud of cells in the blastocoele, but in Asterina gibbosa (Fig. 365) it is certainly budded off from the posterior wall of the right side of the anterior coelom, with which it remains connected for some time by a solid string of cells. The bud is, from the first, nearly but not quite solid; it would be correctly described as a very thick-walled evagination of the right anterior coelom. It is soon cut off from the anterior coelom, and then its cavity rapidly enlarges and it becomes thin-walled. In Asterias Goto (1897) has seen this cavity connected by a string of cells with the Jeft anterior coelom, but it is practically certain that a re-examination of this point will show that Asterias and Asterina agree in essentials. Gote’s observations are exceedingly in- complete, and it isiby no means clear that the “scattered string of cells,” which he saw connecting the sac with the left anterior coelom, represents the original connection of the sac with the coelom. This sac may be termed the madreporic vesicle. According to Gemmill it executes slow pulsations. It may be compared to the pericardial vesicle of Balanoglossus (see p. 575). We formerly (1896) regarded the madreporic vesicle as the vestigial right hydro- coele, but the observations of Gemmill, who has seen this vesicle and a well-developed right hydrocoele present in the same larva, render this view untenable. METAMORPHOSIS OF ASTERIAS With the formation of the madreporic vesicle, and of the rudiment of the water vascular system, the Bipinnaria has reached the summit of its development as a free-swimming organism. It now begins to prepare to take up a fixed life, and with this change in habits the metamorphosis may be said to begin. From the anterior end of the larva, between the prae-oral and post-oral bands, there grow out three clubbed arms which are not ciliated in the larva of A. vulgaris, but in the larva of A. rubens and of A. glacialis the median ventral “ arm” of the prae-oral ciliated band is continued on to them. These arms contain diverticula of the anterior coelom. One is median and dorsal (br.med, Fig. 360), and the other two are situated symmetrically to the right and left of it (brat, Fig. 362). These processes are termed the brachiolar arms, and the larva is now termed a Brachiolaria. It still swims, but it occasionally attaches itself to the side of the vessel in which it is contained by the brachiolar arms, which appar- ently act as suckers. Goto states that the cells forming the walls of 468 INVERTEBRATA CHAP. the coelomic vesicles develop muscular fibrils at their bases, which are for the most part disposed circularly, but some of which pursue a longitudinal course. Now the brachiolarian arms differ from the other larval arms in possessing hollow outgrowths from the coelom within them, and it appears certain that the longitudinal muscular fibres accompanying these outgrowths can cause a retraction of the central portions of the tips of the brachiolarian arms, and thus enable them to act as suckers. As we have noted above, the right and left coelomic sacs fuse with one another in the prae-oral lobe, and the left becomes almost divided A B bra. mda Fic. 360.—Lateral views of an advanced Bipinnaria of Asterias vulgaris, in which the brachiolarian arms are just appearing. (After Goto.) A, outline view to show the segmentation of the coelom. B, more detailed sketch. Letters as in Fig. 358. In addition, a.c, anterior coelom ; br.a, rudiment of anterior median brachiolarian arm ; hy, hydrocoele ; 1-5, the five lobes of the hydrocoele ; 1.p.c, left posterior coelom. into two by a constriction which appears just behind the hydrocoele. Somewhat later a similar constriction appears in the right coelomic sac, and by these two constrictions two posterior regions, the left posterior coelom and the right posterior coelom, become marked off from the left and right coelomic sacs. The left posterior coelom begins to extend beneath the gut over to the right side ; this extension is known as the right ventral horn of the left posterior coelom, and its formation causes the whole sac to take on the form of a U (Fig. 361). It fuses with the right coelomic sac in front of the constric- tion, separating it into anterior and posterior portions—in a word, it fuses with the right anterior coelom. Soon, left and right anterior coelomic sacs, already fused in front, become completely merged in one another so as to form a single anterior coelom. The right XVI ECHINODERMATA 469 posterior coelom becomes entirely cut off from the anterior coelom : ib is termed the epigastric coelom by Goto, who thought that it was formed by the growth of an independent longitudinal septum, but this error has been corrected by Gemmill. By the time that these changes have been accomplished the brachiolar arms have been formed; and in the centre of the circle formed by them a circular disc of thickened glandular ectoderm appears. This is the organ for permanent fixation (fix, Fig. 362). Holding on by its brachiolar arms the larva brings this disc into close contact with the substratum and thus permanently fixes itself. The larva may now be said to be differentiated into a posterior region f~ containing the stomach, the intestine, the hydrocoele, and the right and left posterior coeloms; and into an anterior region, consisting of the prae-oral lobe, containing the mouth, oesophagus, and the anterior coelom. The anterior region may be termed the stalk, the more posterior the disc. Once the larva has become firmly attached the stalk is pro- gressively shortened (Fig. 362, B). The stone-canal makes its appear- ance aS an open groove of ciliated epithelium, situated on the anterior aspect of the septum dividing anterior from left posterior coeloms. It begins just beneath the inner end of the pore- canal, and it runs down to the spot where the anterior coelom is beginning to be pinched from the hydrocoele. On the posterior aspect of the septum there appears a groove-like outgrowth of Epc Fic. 3861.— Ventral view of a Bipinnaria of Asterias vulgaris of the same age as that shown in Fig. 360, in order to show the mutual relations of the coelomic cavities. (After Goto.) Letters as in preceding figure. In addition, @plel, right ventral horn of left posterior coelom; r.p.c, right posterior coelom. the left posterior coelom. This groove is the rudiment of the peri-oral coelom. It grows into a tube which extends in the form of a slight crescent beneath a faint bulge of the stomach, which is the rudiment of the adult stomach. The stomodaeum becomes disconnected from the endodermal portion of the oesophagus: it persists for a brief time as a shallow pit of the ectoderm, but eventually disappears entirely, and the oesophagus becomes, as metamorphosis proceeds, a less and less conspicuous appendage of the stomach. The anus of the larva is also obliterated, and the intestine becomes shortened till it forms a very short tag attached to the stomach. According to Gemmill this tag persists throughout metamorphosis, and from it the rectum of the adult is developed. Five thickened lobes now appear on the ectoderm covering the right posterior coelom. These are the first traces of the arms of the 470 INVERTEBRATA CHAP. future star-fish and are termed collectively the aboral disc. As the ventral horn of the left posterior coelom extends further and Fic. 362.—Lateral views of the brachio- larian phase of the larva of Asterias vulgaris in various stages of fixation and metamorphosis. (After Goto.) A, earliest stage: temporary fixation by means of the brachiolarian arms: prae-oral lobe undiminished. B, later stage : permanent fixation by means of the fixing disc: the prae-oral lobe is much shrunk. C, still later stage: the prae-oral lobe has almost dis- appeared: metamorphosis almost complete. Lettering as in Fig. 360. In addition, fix, the disc by which the larva fixes itself; roman numerals I-V, the rudiments of the five arms. further round to the right, and grows in size, the right posterior coelom and the disc covering it become displaced backwards, and XVI ECHINODERMATA 471 eventually come to occupy a position at the posterior pole of the larva. At the same time other changes are occurring. The hydro- coele becomes more and more grooved off from the anterior coelom. The stone-canal is changed from a groove into a tube by the meeting of its edges. As the hydrocoele becomes grooved off it exhibits a dorsal horn and a ventral horn, and a central piece where it remains for a time in connection with the anterior coelom. The ventral horn extends round underneath the larva to the right side, in fact it grows parallel with the left posterior coelom. Thus the metamorphosis of the larva may be roughly summed up as consisting in the preponderant growth of the organs of the left side as compared with their antimeres on the right side (right hydro- coele and right posterior coelom), together with the gradual atrophy of the prae-oral portion of the larva which forms the stalk. The lobes of the hydrocoele are the rudiments of the radial water-vascular canals of the adult star-fish, and the completed arm consists of the process of the aboral disc, into which an outgrowth from the left posterior coelom extends, and to which the process of the hydrocoele becomes applied. There is little doubt that the process of the aboral disc, with its contained coelom, is to be regarded as an outgrowth of the body, secondarily developed, in order to give support to that long hydrocoele lobe, or radial canal, which was originally a free tentacle. Each primary lobe of the hydrocoele develops lateral lobes in pairs, as branches; of these two pairs are formed before metamorphosis is complete. These are the rudiments of the paired tube feet. The adult stomach appears, we have seen, as an outgrowth from the larval stomach on the left side, in the region where the peri-oral coelom has made its appearance as an outgrowth from the left posterior coelom. As the new stomach grows out the peri-oral coelom extends round it; whilst outside it the left posterior coelom, whose dorsal and ventral horns meet and fuse with one another, forms an outerring. At the same time short pouches grow out from the larval stomach into the developing arms. These pouches are the rudiments of the pyloric caeca of the adult, and the larval stomach becomes the pyloric sac, whilst the adult stomach is really the eversible sac which the star-fish wraps round its prey. The adult mouth is formed by the fusion of the adult stomach with the ectoderm. Only when the metamorphosis is nearly complete does the rectum make its appearance. It grows out in the mesentery separating the right posterior coelom from the left posterior coelom, in the same mesentery, that is to say, in which the larval intestine was situated, and, as mentioned above, Gemmill has shown that it is formed from the stump of the larval intestine. The adult anus appears later still; it is eventually perforated in the right dorsal inter-radius, using the word “right” in reference to the sagittal plane of the larva. ; The shrinkage of the prae-oral lobe is largely due to the change in form of the ectoderm cells covering it—they change from a flat to a 472 INVERTEBRATA CHAP. columnar form. The ectoderm covering the brachiolar arms is involuted into pockets, and these involuted portions are attacked and devoured by phagocytes. In Asterina all the ectoderm of the prae-oral lobe is disposed of in this way; but in Asterias, as the prae-oral lobe shrinks in size, a good deal of the ectoderm which originally covered it is drawn into the covering of the oral disc of the star-fish. As the shrinkage of the prae-oral lobe goes on, the sinuosities of the ciliated band, the “arms” of the larva, become straightened out and thus obliterated. Finally, all that is left of the prae-oral lobe is a small button projecting from the oral surface of the star-fish (Fig. 362, C), but when the star-fish begins its free life, as it wrenches itself free, the neck of this button is pulled out into a long filamentous stalk.which eventually breaks through and sets the star-fish free. DEVELOPMENT OF OTHER ASTEROIDEA We may now glance at the peculiarities of the larvae of other Asteroids whose life-history has been studied. Quite a number of “species” of Bipinnaria are known, but few of them can be assigned to any definite species of Asteroid. The complete life-cycle is only known in the case of Asterias rubens, A. glacialis, and A. vulgaris. It has been doubted whether all Bipinnaria larvae develop brachiolar arms, 7.e. pass through a fixed stage. Gemmill has recently shown that the larva of Porania pulvillus has such a stage ; but extraordinary statements are made about the large Bipinnariae be- longing to Asteroids of the family Astropectinidae, in which brachiolar arms have never been observed. M. and C. Delap (1905) state that in these larvae the star-fish rudiment is amputated from the posterior half of the larva, the front half of which goes on living for a long time after. Here is a matter which urgently requires reinvestigation. We may now turn to the consideration of the development of Asterina gibbosa, which has been worked out by us in considerable detail (MacBride, 1896). This development has already been alluded to more than once. Its main peculiarities concern (a) the formation of the larval gut and of the coeloim, and (0) the external appearance of the larva. With regard to (a) the first point, we find that Asterina gibbosa has a prolonged embryonic life and only escapes from the egg-mem- brane on the fourth day, when not only coelom, but also stomodaeum and madreporic pore have been formed. The archenteron is spacious and nearly fills the blastocoele. The coelom arises as an enormous unpaired vesicle, constituting more than half the archenteron, and from this vesicle prolongations, “tongues,” extend backwards at the sides of the gut (Fig. 363, B). Then transverse septa appear, which divide off right and left posterior coeloms from an anterior unpaired coelom. These septa are found in the “ tongues” of the coelom, so that there is an anterior portion of each tongue which belongs to the anterior XVI ECHINODERMATA 473 coelom. The larval intestine is straight, and both it and the larval anus disappear shortly after the animal enters on its larval existence ; that is on the fifth day. The coelom is only separated from the gut after the stomodaeum has broken through. As for the second point (6), the larva has the form of a boot. The sole of the boot is the prae-oral lobe, which is enormous, and the “upper” of the boot is the body. The back of the boot corresponds to the ventral surface of the larva, and here the larval mouth is situated; while the front of the boot is the dorsal surface. The prae-oral lobe is surrounded by a thickened ridge which bears specially long cilia, by the aid of which, and of the cilia of lesser length which cover the ectoderm everywhere, the larva glides about on the bottom. It uses the prae-oral lobe as a sucker, attaching the thickened rim to the substratum on which it is moving, and then retracting the centre. In the centre there appears, on the seventh day, Fic. 363.—Frontal longitudinal sections of two early embryos of Asterina gibbosa, in order to show the development of the coelom. (Original.) A, stage in which the coelom is a spherical vesicle. B, stage in which the coelom is growing back in tongues at the sides of the gut. al, rudiment of gut: blp, blastopore ; coe, rudiment of the coelom. a circular area of thickened glandular epithelium (Fig. 364). This is the fixing disc, and by means of the secretion produced by it the larva effects a permanent fixation to the bottom. Once this has been accomplished the rim is destroyed by the same process as that by which the brachiolar arms are removed in the Bipinnaria larva. The whole prae-oral lobe shrinks, until final atrophy takes place and the | larva wrenches itself free and walks away as a little star-fish. The internal changes which take place during the larval life, and the metamorphoses, are known in detail. Let us go back to the time when the transverse septa are found in the coelom. The septum is formed on the left side before it is formed on the right, and in both cases it begins at the dorsal side and grows down to the ventral surface. On the left side, the septum, after formation, becomes perforated by two holes, a dorsal and a ventral one. In this way a free passage of fluid between anterior and posterior coeloms is allowed ; and as the cells of the coelomic wall, as in the Bipinnaria larva, secrete muscular fibrils, and the larva can change its shape very much, 474 INVERTEBRATA CHAP. this is very necessary. Similar holes are formed in the transverse septa in the Bipinnaria larva, as we have already seen. ; The hydrocoele arises as a bulge on the left side of the posterior part of the anterior coelom, whilst the madreporic vesicle is formed as a bud from the posterior end of the anterior coelom, a little to the right of the median line. It becomes hollowed out, and is for a time attached to the wall of the anterior coelom by a string of cells, but this is soon broken and the vesicle detached. A right hydrocoele with five well-developed lobes, or sometimes with only three or only one lobe, is sometimes developed, and often in this case the madreporic vesicle is suppressed, which is the reason why we formerly regarded the two structures as homologous. Even before metamorphosis begins the left posterior coelom is wider than the right, and begins to send out a ventral horn which underlies the right coelom, and the peri-oral coelom originates as a pocket of the left posterior coelom. Fic. 364.—Views of a free-swimming larva of Asterina gibbosa tive or six days old. (After Ludwig.) A, from in front. B, from the side and above. fix, fixing disc. pr.l, prae-oral lobe. In Asterina gibbosa, as soon as metamorphosis commences the stone-canal makes its appearance as an open groove on the anterior face of the transverse septum, as in Asterias. This groove becomes closed in the middle, but opens at one end into the hydrocoele, and at the other end into the anterior coelom just below the opening of the pore-canal. Then five outgrowths of the coelom shaped like inverted wedges are formed. Of these outgrowths, one arises from the anterior coelom and four from the left posterior coelom (p.h, Fig. 366). They project against the ectoderm and alternate with the five lobes of the hydrocoele. These outgrowths are soon cut off from the coelom, and lie between ectoderm and coelomic wall as flattened vesicles. They are the rudiments of the perihaemal system of spaces. The arms grow out as blunt outgrowths from the region of the body occupied by the left posterior coelom, and into each of them an XVI ECHIN ODERMATA 475 outgrowth from this coelom extends. It is noteworthy that, counting the arms from before backward, No. 1 arm is really situated over No. 2 lobe of the hydrocoele, and eventually fuses with it; and later in the metamorphosis, when the ring-shaped growth of the left hydrocoele and the left posterior coelom is complete, No. 1 hydro- coele lobe comes to lie under No. 5 arm. The neighbouring angles of adjacent perihaemal spaces grow into the arms beneath the hydrocoele lobe, and in this way the two radial perihaemal canals, which are found in each adult arm, are formed. The external peri- haemal ring-canal is formed by the fusion of the main portions of these spaces. The internal perihaemal canal is formed by a circular extension of the hinder part of the anterior coelom which is included within the Fic. 365.—Longitudinal frontal sections of larvae of Asterina gibbosa, to show the segmenta- tion of the coelom and the origin of the hydrocoele and madreporic vesicles. (Original.) A, section of larva about five days old. B, section of larva about six days old. OC, section of Jarva about six and a half to seven days old. ae, anterior coelom; a.st, rudiment of adult stomach; hy, rudi- ment of the hydrocoele ; 1, 2, etc., its lobes; l.p.c, left posterior coelom ; m.v, madreporic vesicle ; p.0.c, rudiment of peri-oral coelom. 7.p.c, right posterior coelom ; st, larval stomach. body of the star-fish when the stalk finally disappears. This portion of the anterior coelom, into which pore-canal and stone-canal open, is known as the axial sinus (Fig. 366, a'c!). The septum, which divides it from the general body-cavity surrounding the stomach in the adult star-fish, is nothing but the old transverse septum which separated the anterior coelom from the left posterior coelom in the larva; and it follows, therefore, that the general body-cavity of the adult is only the ring-shaped left posterior coelom, which, with Goto, we may term the hypogastric coelom. The right posterior coelom of the larva becomes the epigastric coelom of the adult. In Asterias, Goto maintains that the perihaemal spaces appear as solid masses of mesenchyme, lying on the ventral surfaces of the arms when metamorphosis is complete, and that these spaces subsequently 476 INVERTEBRATA CHAP. become hollowed out; but Gemmill has shown that this is an error and that these spaces originate in Asterias in the same manner as they arise in Asterina. ; As metamorphosis proceeds the prae-oral lobe shrinks more and more, and the neck of the lobe becomes constricted. The effect of A B Fic. 366.—Longitudinal frontal sections of larvae of Asterina gibbosa seven to eight days old, to show the beginning of the metamorphosis. (Original.) A, section through a larva in the dorsal region of the body. B, section through a larva near the ~ median region of the body. C, section through a larva in the ventral region of the body. Letters as in previous figure. In addition, p.h, rudiments of perihaemal spaces; p.h.1.2, rudiment of perihaemal space intervening between lobes 1 and 2 of the hydrocoele; p.h.2.8, rudiment of perihaemal space between lobes 2 and 3 of the hydrocoele, and so on; st.c, stone-canal (still an open groove). this is to bring arm No. 5 closer and closer to hydrocoele lobe No. 1 (Fig. 368). At the same time each hydrocoele lobe gives rise to two pairs of lateral branches springing from its base. These are the rudiments of the paired tube feet (Fig. 368, ¢./), while the tip of the primary lobe forms the azygous tube foot in which the radial canal terminates. XVI ECHINODERMATA 477 The larval oesophagus or stomodaeum, as in Asterias, becomes disconnected from the larval stomach, shallows out and disappears. The larval stomach, which, as we have seen, forms the adult pyloric sac, begins to give off blunt outgrowths into the cavities of the nascent arms: these are the rudiments of the pyloric caeca (Fig. 369). The adult stomach, begun as we have noted as an out- growth on the left side of the larval stomach, increases in size and comes in contact with the ectoderm at a spot between the dorsal and ventral horns of the left posterior coelom. These horns meet above it, and so the left posterior coelom is converted into a ring. Within this ring lies the ring formed by the hydrocoele, and beneath this the ring formed by the peri-oral coelom. The holes in the septum dividing the anterior coelom from the left posterior coelom become healed up, and with the progressive constriction of the neck of the Fic. 367.—Views from the side of a larva of Asterina gibbosa seven days old in the initial stages of metamorphosis. (After Ludwig.) A, from the right side. B, from the left side. 1-5, lobes of hydrocoele. I-V, rudiments of arms ; pr.l, prae-oral lobe. prae-oral lobe the anterior coelom becomes divided into a transitory portion, situated in the stalk, and a permanent portion, the axial sinus, which is included within the disc of the star-fish. The hydro- coele communicates with the anterior coelom not only through the stone-canal, which has become a closed tube, but through an opening in the neighbourhood of its third lobe which does not become closed until metamorphoses is nearly complete. The adult nervous system of Asterina arises as a plexus of ganglion cells and fibres, beneath the ectoderm which overlies the perihaemal spaces and the lobes of the hydrocoele. Goto states that, in Asterias, part of this ectoderm is derived from the longi- tudinal ciliated band of the larva. If this statement could be confirmed it would be a matter of great interest. Of the development of the adult calcareous skeleton of Asterina we have a full account from Ludwig (1882), and we have also some information about the origin of the calcareous skeleton in Asterias rubens from Bury (1895). ‘The first traces of calcareous plates in 478 INVERTEBRATA CHAP. Asterina, and in all other Echinoderms which have been studied, are little triradiate spicules embedded in and produced by the mesenchyme cells intervening between coelomic wall and ectoderm ; each arm of the spicule, as it grows, bifurcates, and the forks of Fic. 868.—Views from the side of a larva of Asterina gibbosa eight days old, to show the progress of metamorphosis. (After Ludwig.) A, from the right side. B, from the left side. Letters as in previous figure. In addition, tf, rudiments of paired tube feet. adjacent arms join one another, and in this way a mesh is formed. From the junction of the two forks another arm is given off, and, by a repetition of the processes of forking and of union of forks, a network of calcareous meshes is slowly built up. The first spicules to appear in Astervas and Asterina are the rudiments of the terminal plates (T, Fig. 370), which overarch and protect the azygous tentacles in which the primary lobes of the hydrocoele, or radial water-vascular canals, terminate. Alternating with these terminal plates arise five basal plates (B, Fig. 370), one of which surrounds the madreporic pore and is the rudiment of the madreporite of the adult. In the centre of the circle of basals there arises the so-called dorso-central plate (D.C, Fig. 370). This plate does not he over the right posterior coelom but rather to one side of it, and the adult anus, which appears at one side of the dorso-central plate, is conse- quently situated over the mesentery separating the left posterior and right posterior Fig. 369.—Longitudinal frontal section through coeloms. ; a larva of Asterina gibbosa about the same The rectum, which is age as those shown in previous figure. formed as an outgrowth from the larval stomach, lies in this mesentery. On the ventral caecum. side of the disc there appear (Original. ) Letters as in Figs. 365 and 366. In addition, alel, rudiment of axial sinus; py.c, rudiment of pyloric pairs of spicules alternating with the rudiments of tube feet. These spicules are the rudiments of the ambulacral plates. The muscles XVI ECHINODERMATA 479 connecting these plates with one another, by means of which the arm can be bent and the ambulacral groove closed, are derived from the cells forming the walls of the perihaemal canals. As metamorphosis approaches completion, the septum dividing the peri-oral coelom from the encircling left posterior coelom is largely absorbed, and the two cavities coalesce; as remnants of this septum there remain ten bands, two in each arm, which constitute the retractor muscles of the adult stomach. The adult mouth is formed by the fusion of the wall of this stomach with the ectoderm; there is no adult stomodaeum. When the stalk has been almost absorbed the little star-fish wrenches itself loose from the substratum by pulling with its tube feet, and it walks away. Fic. 370.—Dorsal (z.e. aboral) views of two young specimens of Asterina gibbosa shortly after the metamorphosis, (After Ludwig.) A, young star-fish ten days old. B, young star-fish sixteen days old. a, adult anus; az.t, azygous tentacle of water-vascular system ; B, basal plate; D.C, dorso-central plate; R, radial plate; 7’, terminal plate. In the specimen shown in figure B, only four basals are developed. The post-larval development has been followed in Asterina. The main points which have been determined concern the further development of the skeleton and the development of the genital system. We shall deal with the development of the skeleton first. As the arms grow in length new tube feet are added in pairs, the first formed tube feet remaining at the base of the arm, and, alternating with the new tube feet, new ambulacral ossicles are added. At the same time the terminal plates are carried out to the tips of the arms, and new plates are intercalated between them and the central plate. The most important of these, and the first to appear, are the radials situated at the bases of the arms. The names basals, dorso-central, and radials, it may be remarked, have been bestowed on these plates from a suggested homology with the 480 INVERTEBRATA CHAP. plates which make up the skeleton of a Crinoid, the value of which will be examined when we consider the development of a Crinoid. The origin of the genital organs is very peculiar. About the time that the metamorphosis is completed a peculiar fold appears in the wall of the axial sinus which abuts against the left posterior, yy f Dy cot j Sie aS or ie pa 9G. Bom 2a inv, Fic. 371.—Three figures to illustrate the development of the genital stolon, genital rachis, and gonad in Asterina gibbosa, (Original. ) A, vertical section of the disc of a specimen that has just metamorphosed. Dise about ‘75 mm. in diameter. B, horizontal section through one of the inter-radial septa of a specimen about 1:2 mm. across the dise. C, Section through the incipient gonad of a specimen about 6 mm. across the disc. ab.s, aboral sinus ; alcl, axial sinus; d.o, dorsal organ; gen.r, genital rachis; gen.st, genital stolon; gon, gonad; gon.s, gonadial sinus; m.v, madreporic vesicle; ph, perihaemal space; pr.g.inv, primitive germinal involution ; st.c, stone-canal. or, aS We may now term it, the hypogastric coelom. This fold is the rudiment of the strand called axial organ, dorsal organ, or genital stolon, and formerly regarded as a heart (d.o, Fig. 371, A). At the dorsal edge of the hypogastric coelom, where the remains of the mesentery which separated it from the epigastric coelom are still to be seen, an involution of the coelomic epithelium takes place which XVI ECHINODERMATA 481 projects into this fold. The involution, at first hollow, soon becomes solid and is termed the primary germinal involution (p.y.inv, Fig. 371, A). The solid bud thus produced proliferates downwards into the axial organ and forms the peculiar cells characteristic of this structure, but it also proliferates laterally, to the right and to the left, and forms a cord of primitive germ cells known as the genital rachis (g-7, Fig. 371, B), which grows as a freely projecting rod right round the disc of the star-fish until the two ends meet and a complete circle is formed. The rod is, of course, covered with a thin layer of peritoneum which is reflected over it; it is supported in a sling of peritoneum which grows out parallel with the rod and underneath it, as a flap, which fuses with the body wall on both sides and encloses a space called the aboral sinus (ad.s, Fig. 371, B). At each side of the base of each arm a branch is given off from the rachis enclosed in a branch of the aboral sinus. This branch extends downwards and then along the arm, where it enlarges and forms a bunch of diverging branches which constitute the genital organ. Round each branch, of course, is a branch of the aboral sinus, but this branch becomes cut off from the main part of the sinus by the ingrowth of a septum (gon, Fig. 371, C). The genital duct is formed as a solid outgrowth from the base of the genital organ, where it joins the rachis. This outgrowth fuses with the ectoderm and then becomes hollowed out. The process by which a genital gland discharges itself bears a considerable resemblance to the way in which an abscess finds its way to the surface. Both Solaster and Cribrella agree, as far as the general form of their larvae is concerned, with Asterina, though differing in details of shape and size of the prae-oral lobe, and they have a quite similar metamorphosis. But they differ in one most important respect, they never develop a larval mouth, and the larval gut remains in a most rudimentary condition, being merely a collar of more columnar cells round the middle portion of the archenteron. The hinder portion of the archenteron gives rise to the left posterior coelom, whilst the front portion gives rise, not only to the single anterior coelom, but also to the right posterior coelom. From the anterior coelom are also given off the hydrocoele and the madreporic vesicle. The credit of elucidating this extraordinary form of development belongs to Masterman (1902), whose observations on Cribrella have been confirmed in almost every point by those of Gemmill on Solaster (1912). Masterman (1902), viewing this development as primitive, regards the right posterior coelom as the antimere of the left hydrocoele, and the left posterior coelom as a median structure containing right and left elements. He terms the madreporic vesicle the central coelom. According to Gemmill, in Solaster the madre- poric vesicle arises to the right of the middle line, as in Asterina. There are obvious objections to regarding the shortened develop- ment of an Asteroid, with a yolky egg, as capable of throwing light VOL. I 21 482 INVERTEBRATA CHAP. on the primitive condition of the development of Asteroidea. Gemmill’s researches (1912) on Soluster allow of another interpreta- tion of the type of development. In Solaster the posterior coelom is not symmetrical, but inclined to the left, and the area which eventually forms the gut does not extend evenly all round the archenteron. We might therefore derive the condition of affairs in the larva of Solaster from the condition of things in the larva of Asterina, by imagining that the preponderant growth of the organs of the left side has been pushed — so far back into the embryonic period, that the gut-rudiment is swung out of the longitudinal into a transverse position, and so the open end, from which the coelom is cut off, is directed to the left instead of anteriorly. Solaster is further remarkable for the fact that, although it is a star-fish with many rays, the left hydrocoele has at first only five lobes which all develop simul- taneously, and additional lobes are developed much later; so that in one and the same specimen, when the more dorsal lobes of the hydrocoele have already developed lateral tube feet, and the peri- haemal spaces in connection therewith have been completely Fic. 872.—Longitudinal frontal section of separated from the coelom, the the larva of Solaster endeca, (After more ventral lobes will still be coe quite undivided, and the ad- oo eeca. he ee jacent perihaemal spaces will rp.c, Tight posterior coelom. have the form of shallow evagina- tions of the coelom. This circum- stance seems to indicate that the number five, so characteristic of the rays in all Echinodermata, was characteristic also of the ancestral Echinoderm, and that where many rays are found this is not a survival of a primitive state of affairs, but is a secondary modification. EXPERIMENTAL EMBRYOLOGY OF ASTEROIDEA A great many experiments have been performed on the eggs of Asteroidea, but in the case of many of these quite the same results have been obtained with the eggs of Echinoidea, which have been classic subjects of experimentation since the study of Experimental Zoology started. So far as space will allow they will be mentioned XVI ECHINODERMATA : 483 when dealing with the Echinoidea, but there are some which are, so to speak, peculiar to the Asteroidea. It has been already mentioned that the fully developed gastrula of Asterias is a long, sausage-shaped structure, and that the archenteron only reaches half-way through it (Fig. 356). Driesch (1895) made a thick culture of these gastrulae, accumulating hundreds in a very small quantity of water, and snipped at random in this water with a fine pair of scissors about 200 times. In this way he succeeded in cutting a number of the gastrulae in pieces. Sometimes he found that they were cut longitudinally and sometimes transversely. In both cases, if the fragment included both endoderm and ectoderm, it healed up by the approximation of its edges and formed a miniature gastrula, which then developed further into a small but perfect Bipinnaria. If, however, the gastrula was transversely bisected after the thin-walled vesicle at the apex of the archenteron had made its appearance, so that this vesicle was removed, then the truncated larva healed up and went on developing, but it never formed a new coelomic vesicle, although it takes on externally the form of a Bipinnaria. The appearance of this vesicle, therefore, according to Driesch, “negatively determines,” z.e. limits the potency, that is, the power of development possessed by the archenteric wall. Before that vesicle appears a small fragment of this wall will grow and rearrange its cells so as to form the three segments of the asteroid gut, viz. oesophagus, stomach, and intestine, and in addition the terminal vesicle; but, when this has been once formed, then, even a larger fragment of the archenteron is incapable of moulding itself into more than oesophagus, stomach, and intestine. Driesch regards this limitation of power as due to a progressive “ stiffening” of the protoplasm, which renders it less and less amen- able to the regulating influence of the “entelechy,” or indwelling power, which, according to the vitalistic principle held by Driesch, knows and wills what tt wants to do with the material at its disposal. A more humble explanation, and one more in accord with what we know of other eggs, is that there exists a definite coelom-forming substance which is at first diffused through all the cells of the gut wall, but which becomes, as development proceeds, definitely localized in one spot; and if that portion of the gut be removed, the remainder has no material which will allow of the development of the coelom. Herbst (1896) showed that if a solution of 3°7 per cent of sulpho- cyanide of potassium be made, and then three parts of this solution be added to 100 parts of sea-water, the eggs of Asterias will develop in this medium and will live on for four weeks, but that they do not get beyond the blastula stage. The beginning of an archenteron may be formed, but it degenerates into a granular mass of cells and is absorbed. These persistent blastulae, though possessing the clear trans- parence which indicates health, differ from the normal blastulae in possessing abundance of mesenchyme in the interior. The normal 484 INVERTEBRATA CHAP, blastula of