TEXT-BOOK OF THE EMBRYOLOGY OF INVERTEBRATES. TEXTBOOK OF THE EMBRYOLOGY OF INVERTEBRATES Dr. E. KOESCHELT, PROFESSOR OF ZOOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF MARBURG. Dr. K. HEIDER, PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF BERLIN. TRANSLATED FROM THE GERMAN BY MATILDA BERNARD. REVISED AND EDITED WITH ADDITIONAL NOTES BY MARTIN F. WOODWARD, DEMONSTRATOR OF ZOOLOGY. ROYAL COLLEGE OF SCIENCE. Vol. II. PHORONIDEA, BRYOZOA ECTOPROCTA, BRACHIOPODA, ENTOPROCTA, CRUSTACEA, PALAEOSTRACA. LONDON: SWAN SONNENSCHEIN AND CO., Ltd. NEW YORK: THE MACMILLAN CO. 1899. 640079 PEEFACE. In presenting a second instalment of the translation of the Lehrbuch der vergleichenden Entivicklungsgeschichte der wir- bellosen Thiere to students of Zoology, I feel that an apology is due to them for the long interval of time that has elapsed since the appearance of the first part. Professors Mark and Woodworth, the translators of Vol. I., found themselves unable to spare time to continue the translation, and others who subsequently undertook the work were for similar reasons obliged to relinquish it. Consequently the book came into the hands of the present translator and editor only last year ; and although we have endeavoured to push the work forward as quickly as possible, the task of translating, printing, and editing has necessarily consumed a considerable time. The whole work, however, is now well in hand, and, in issuing Vol. II., we can confidently hope that no long period will elapse before the production of Vols. III. and IV., the former being already in the press. It will be noticed that the order of the subject-matter has been changed. In its original form the Text-book is divided into three parts of unequal size. That already translated (Part I.) is much smaller than either of the remaining parts. We have consequently found it necessary to divide the second and third parts of the original into three. In doing so we were faced by a difficulty. If the original sequence of the chapters was to be retained, each part could not be made complete in itself unless the relative size of the volumes was left out of consideration. It was therefore thought preferable to alter slightly the original sequence of the chapters. Those dealing with the Phoronidea, the Bryozoa Ectoprocta and VI PREFACE. Entoprocta, and the Brachiopoda have therefore been removed from the end of Part III. (Vol. IV. of the English edition), and placed at the commencement of Vol. II., which part contains, in addition, the Crustacea proper and the Palaeostraca. I do not think there can be any serious objection to this change, inasmuch as the four groups mentioned probably find a natural resting-place near to the Annelida, which were dealt with in Part I., while they have little, if any, relation to the Molluscan phylum treated in Part III. In all other respects the original text has been adhered to as closely as possible ; but it has occasionally been found necessary to rewrite certain paragraphs which, when translated, appeared somewhat in- volved and obscure. In such cases the motive of the original has been adhered to as far as possible, the sentences, however, being entirely recast. Additional notes have been added relating to the most important of the many fresh observations which have been made since the original work appeared. Such additional matter has been placed in footnotes, distinguished from the footnotes of the authors by being enclosed in square brackets. A considerable number of additional references have also been given; and these are placed with the Literature at the end of each chapter under the form of Appendices, and, as in Vol. I., numbered with Roman numerals. But even these additions could now be added to. For instance, since going to press a short note on the early development of Phoronis by E. Schultz* has appeared, which might with advantage have been incorporated in this work. Although desirous, as far as possible, to use the same terminology as the translators of Vol. I., I have been compelled to differ from them in the rendering of the ever-recurring word "Anlage." This important term in Vol. I. is rendered by the word " fundament." Exception, with which I concur, has already * "Ueber Mesoderm-bildung bei Phoronis," Otdyeln. Oppisk., Tryd. imp. St. Petersburg, Obshch. estestv., T. xxviii., Vuip. 1. The author deals mainly with the formation of the mesoderm, and controverts all Caldwell's observations relating to the origin of this layer, which he maintains is a mesenchyme, and • nis.s in the blastula and gas trul a stages, most conspicuously in the latter, as proliferation! from the entoderm. He could not find Caldwell's primitive groove, and thinks that this observer described as his posterior coelomic sacs the v.ntral invaginations which form the body-wall of the adult. PREFACE. Vll been taken to the use of this term,* on the ground that the word fundament implies the solid basis or foundation upon which a structure rests or is built, whereas an "Anlage" is essentially a changing, growing structure, which, though at one time the foundation, when only the foundation exists, eventually gives rise to, or rather itself becomes transformed into, the fully- formed organ. Having thus decided against the continued use of this term, I found myself face to face with the responsibility of selecting one of the numerous terms which have at one time and another been put forward as the English equivalent of " Anlage," at the same time knowing full well that, whichever word was adopted, I should find a large number of biologists against me, as nearly every teacher of note has proposed at least one word which he believes to be the only correct rendering of " Anlage." Eealising, then, the impossibility of satisfying everyone, I thought it advisable to pass over all the numerous terms which have been recently suggested, none of which are really satis- factory, and to revert to that much-abused word rudiment. Most biologists will agree that the term rudiment, if it had not been misused by some of our most eminent zoologists, would undoubtedly be the best word by which we could render the German term " Anlage." Unfortunately, following the lead of Darwin and others, we have acquired the habit of applying the terms rudiment and rudimentary to certain structures present in the adult which, in consequence of their small size and frequent loss of function, have retained a somewhat embryonic stamp, thus preserving the outward appearance of a rudiment but losing its essential character, viz., its inherent tendency to further growth. These, then, are not rudiments, but arrested, reduced, vanishing, or vestigial structures, and should be spoken of as vestiges. Why, because Darwin unfortunately * See Nature, 1896, p. 361. P. C. Mitchell, "Anlagen," Nat. Sci., vol. v., 1894. Dr. Willey (Nature, 1898, p. 390), who supports the use of the term primordium, objects to the word rudiment on the ground that the latter has been regarded as the first visible "Anlage" of an organ. But who is to decide when a growing structure is first visible to the eye ? Professor Wilder (Science, 1898, p. 793), in a reply to Willey, advocates the use of the term proton; but to define this term he has to make use of the word rudiment. Thus be states that proton was employed "to designate the primitive, undifferentiated mass or rudiment of a part." Vlll PREFACE. misapplied the word rudimentary, should we necessarily regard this misuse as hallowed, and ever after refuse to use the word in its common sense? To such an extent has this misuse of the word been carried, that even encyclopaedic dictionaries, after defining the word rudiment in such a manner as to prove that it is the very word we are seeking, as a rendering of the idea expressed by "Anlage," give us, under the technical use of the word, " In Zoology, a part or organ, the development of which has been arrested (see Vestige)." It would require but little trouble on the part of teachers of Biology to reinvest the word rudiment with its proper meaning. By carefully insisting on the use of the words vestigium and vestigial, or their equivalents, for all abortive or reduced structures met with in the adult animal, and restricting the terms rudiment and rudimentary to all growing and developing tissues and organs, they could insure this result in a few years. We have by no means always rendered "Anlage" as rudiment, for we find that the German use of the term is not at all precise, and it was often possible to express the meaning better by another English word. The extreme looseness with which some other terms, such as Vorderdarm, Mitteldarm, and Hinterdarm, are used in German was unfortunately not recognised until too late. These words are for the most part translated by the equally vague expressions fore-, mid-, and hind-gut. Throughout the Crustacea the fore-gut is co-extensive with the anterior ectodermal in- vagination, the stomoclaeum, and the hind-gut is similarly related to the corresponding posterior invagination, the procto- daeum ; the term mid-gut, however, is used in a more varied sense. While for the most part it is applied to the entire entodermal rudiment, it is at times used for the entodermal tube after the separation of the hepatic rudiment. As more precise terms, Professor E. Kay Lankester proposed the words enteron fur the entodermal rudiment before the separation of the various entodermal derivatives, and metenteron for what is left of the enteric sac as the central element of the alimentary canal after the separation of the outgrowths,* and * fik I" Bfeglisfc translation of Gtegenbaur's Elements of Comparative Anatomy, 1878. PREFACE. IX these terms might well be adopted in the Crustacea for the three divisions of the alimentary canal. In the Phoronidea, Bryozoa, and Brachiopoda, it is impossible to be so precise in our terminology, for the origin and homology of the various divisions of the alimentary canal in these forms are more or less obscure. This is notably the case in the adult Bryozoon, where it is quite impossible at present to interpret the parts of the canal, all of which appear to arise from the ectoderm. The use of the terms fore-, mid-, and hind-gut must not, consequently, in this group be regarded as implying any morphological homology with the similarly-named parts in the Crustacea, but only an analogy with those parts. All through this work the genital glands are spoken of as arising from, or in connection with, the mesoderm; and in places even the splanchnic or somatic layer of the mesoderm is specially mentioned as giving origin to the genital cells. These statements arose from the incompleteness of our know- ledge, at the time when this work was published, concerning the early history of the genital rudiment. A number of observations have, however, since been made on this point in various groups of Invertebrata, especially in different orders of worms and Arthropoda, which tend to show that the primitive genital cells are separated from the somatic portion of the embryo at an extremely early period in the cleavage of the egg. In fact, in some cases, directly after the first cleavage plane has appeared, it can definitely be pointed out which of the two spheres will give rise to the genital rudiment, although this cell will also form certain other rudiments, the actual isolation of the genital cell taking place somewhat later (as early as the 32-celled stage in Cyclops, according to Hacker). All these observations lend support to the belief that the invisible genital rudiment is from the first quite distinct from the somatic rudiment ; the former, at all events, attains a visible distinction at a much earlier period than is assigned to it in this work (see also Vol. i., p. 12, footnote). MARTIN" F. WOODWARD. Royal College of Science, London. December, 1898. CONTENTS OF VOL. II. Chapter XV. PHORONIDEA. By K. Heider I. Embryonic development . The Actinotrocha larva II. Metamorphosis Rise of the organs III. General considerations . Literature Chapter XVI. BRYOZOA ECTOPROCTA. By K. Heider Systematic ..... I. Formation of the egg, fertilisation, position of the embryo II. Embryonic development .... General account of the form of the larva . III. Metamorphosis .... 1. Type which develops an alimentary canal a. Larva of Alcyonidium . b. Larva of Tendra c. Larva of Membranipora (Cyphonautes) and Flustrella 2. Type of the intestineless Chilostomatous larva with sli developed corona 3. Type of the intestineless Chilostomatous larva with developed corona a. Structure of the larva . b. Metamorphosis 4. Type of the Vesicularian larva 5. Type of the Cyclostomatous larva 6. Type of the Phylactolaematous larva . IV. Development of the polypide V. Asexual reproduction of the Ectoprocta A. Budding B. Development of Statoblasts . C. Winter buds (hibernacula) . VI. Regeneration .... VII. General considerations Literature .... Chapter XVII. BRACHIOPODA. By K. Heider I. Testicardines . A. Embryonic development B. Metamorphosis II. Ecardines III. Changes in the shape of the shell htly highly PAGE 1 1 s 5 8 8 10 12 12 13 15 17 18 19 19 21 21 24 25 25 27 31 31 32 37 44 44 49 54 55 55 60 65 65 65 68 73 75 CONTENTS*. the Molluscoida K. Hkideil Chapter XVII. BRACHIOPODA— continued. IV. General considerations Literature Appendix — General considerations on Rhabdopleura and Cephalodiscus Literature Chapter XVIII. ENTOPROCTA. By Embryonic development and larva Metamorphosis Asexual reproduction General considerations Literature Chapter XIX. CRUSTACEA. By K. Heider Systematic I. Development of the embryo 1. Oviposition, care of the brood 2. Cleavage and formation of the blastoderm Vitelline membrane Types of cleavage Blastodermic cuticle, larval integument Paracopulation . 3. Formation of the germ-layers A. Copepoda . . , B. Phyllopoda . C. Cirripedia D. Decapod a . . . E. Schizopoda . F. Arthrostraca and Cumacea G. General considerations Types of development of the enteron Formation of the mesoderm . 4. Development of the external form of the body A. Entomostraca B. Arthrostraca and Cumacea Dorsal organ C. Leptostraca, Schizopoda, and Decapoda 5. Formation of the organs A. External integument . B. Endoskeleton C. Nervous system Development of the brain Primary segmentation of the cephalic i I). Sensory organs Development of the compound eye Development of the auditory organ S. Gills F. Intestinal canal Bntomoetraca Decapoda Arthrostraca Q. Heart CONTENTS. xiii Chapter XIX. CRUSTACEA- continued. PAGE H. Glands (antennal and shell-glands) 178 I. Genital organs . . ... 180 Literature referring to the embryonic development of the Crustacea 181 II. Metamorphosis of the Crustacea . . ... 190 1. The Nauplius stage . . ... 190 Primary segmentation of the Nauplius 191 Metanauplius . . . ... 192 Phylogenetic significance of the Nauplius 192 Further development of the Nauplius stage 193 2. Typical form of the Crustacean limbs . ... 193 Significance of the Phyllopodan limbs 195 3. Metamorphosis of the Phyllopoda . ... 196 A. Branohiopoda . • . . . 196 Apus . . . ... 196 Branchipus . . ... 199 Estheria . . . ... 201 General considerations regarding the Branchiopoda . 202 B. Cladocera . . . ... 203 4. Metamorphosis of the Ostracoda . ... 205 Cypridae . . . ... 205 Cytheridae, Cypridinidae . . ... 208 Halocypridae . . . ... 209 5. Metamorphosis of the Cirripedia . ... 209 A. Thoracica . . . ... 209 Nauplius . . ... 209 Metanauplius . . ... 213 Free-swimming Cypris stage . ... 214 Attachment and transformation of the Cypris larva . 216 Development of the adult Cirri pede shell 219 Metamorphosis of the Balanidae 219 B. Abdominalia . . ... 220 C. Rhizocephala . . ... 221 Nauplius of Sacoulina . ... 221 Cypris stage of Sacculina . ... 221 Kentrogon stage . . ... 223 Sacculina interna . . ... 226 Sacoulina externa . . ... 228 D. Ascothoracida . . ... 228 E. The morphological derivation of the complemental males 229 6. Metamorphosis of the Copepoda . ... 231 General considerations . . ... 231 A. Gnathostomata . . . 232 Character of the Nauplius stage 233 Metanauplius . . . . . 234 Cyclops stage . . ... 235 B. Parasita . . . ... 237 General considerations regarding parasitic Copepoda. 237 Chondracanthidae . . . . . 239 Philichthyidae . . . 239 Dichelestiidae . . ... 239 XIV CONTENTS. 9. 10. Chapter XIX. CRUSTACEA— continued. Caligidae (Chalimus stage) Lernaeidae Lernaeopodidae C. Branchiura .... 7. General considerations regarding the segmentation of the body and the metamorphoses of the Malacostraca Segmentation of the body Typical form of the limbs Metamorphoses of the Malacostraca . The Nauplius stage The Metanauplius stage The Protozoaea stage The Zoaea stage The Mysis and Metazoaea stages Final stages of metamorphosis (Mastigopus, Megalopa) Abbreviation of metamorphosis Metamorphosis of the Schizopoda and Stomatopoda 8. Metamorphosis of the Leptostraca Metamorphosis of the Schizopoda Nauplius and Metanauplius of the Euphausiidae Calyptopis stages Furcilia and Cyrtopia stages Development of the Mysidae and Lophogastridae Metamorphosis of the Decapoda A. Sergestidae . . ■ . Metanauplius of Lucifer Protozoaea and Zoaea of Lucifer Mysis and Mastigopus larvae of Lucifer Metamorphosis of Sergestes . Elaphocaris (Zoaea of Sergestes) Acanthosoma (Mysis larva of Sergestes) Metamorphosis of other Sergestidae B. Penaeidae .... Metamorphosis of Penaeus Ceratapsis Metamorphosis of Stenopus . C. Caridea Abbreviated development of some Caridea Amphion D. Astacidea .... Homarus .... Nephrops Astacus and Cambarus E. Loricata .... Embryos of Scyllarus Structure of the Phyllosoma . Metamorphosis of the Phyllosoma F. Thalassinidca Metamorphosis of Gebia Larva of Calliaxis (Trachelifer) CONTENTS. Chapter XIX. CRUSTACEA— continued. G. Anomura Zoaea and Metazoaea of the Anomura Larva of Porcellana (Lonchophorus) Larvae of the Hippidae H. Brachyura . Zoaea of the Brachyura Metazoaea Megalopa Abbreviated development of some Brachyura 11. Metamorphosis of the Stomatopoda Erich thoidina stage Erich thus stage . Alima stage Connection of the larval forms with definite genera 12. Metamorphosis of the Cumacea 13. Metamorphosis of the Anisopoda 14. Metamorphosis of the Isopoda Larvae of the Anceidae Larvae of the Bopyridae . Larvae of the Entoniscidae 15. Metamorphosis of the Amphipoda 16. General considerations regarding the development of the Crustacea Phylogenetic significance of the Zoaea Phylogenetic significance of the Nauplius Hypothetical derivation of the Crustacea from the Annelida Relations of the Protostraca to the Palaeostraca, Pantopoda, and Peripatus . Characteristics of the primitive Phyllopoda Relationships of the Entomostraca . Relationships of the Malacostraca Significance of Nebalia Literature of the metamorphoses of the Crustacea Chapter XX. PALAEOSTRACA. By K. Heider General considerations regarding the Palaeostraca I. Metamorphosis of the Trilobita II. Development of the Xiphosura 1. Oviposition, cleavage, and formation of the germ-layers in Limulus 2. Development of the external form of the body Trilobite stage of Limulus 3. Formation of the organs A. Nervous system and sensory organs B. Alimentary canal C. Formation of the mesoderm D. Respiratory organs . , 4. General considerations regarding the Palaeostraca Relations of the Palaeostraca to the Arachnida Literature of the Palaeostraca PAGE 285 286 289 290 290 291 293 294 296 297 297 301 301 303 303 304 304 305 306 307 309 309 310 313 313 315 316 317 318 318 319 332 332 334 340 340 343 347 348 348 352 353 355 356 356 360 CORRIGENDA. Page 10, line 2 from bottom— 1897 change to 1867. „ 57, in lettering of Fig. 28 — ma change to mg. „ 84, line 10— Denicker change to Deniker. „ 209, line 5 from bottom— Sluier change to Sluitor. „ 290, footnote *— enclose in square brackets. CHAPTER XV. PHORONIDEA. Although investigators early drew attention to the many structural features in which the genus Plioronis resembles the Bryozoa, this form has nevertheless hitherto usually been classed with the Sipunculids. Recently, however, greater stress has been laid on its relationship to the Bryozoa and the Brachiopoda (Ray Lankester, Caldwell (No. 1), Cori (No. 4a)). It is chiefly in the anatomy of the adult that the resemblance between these groups is found, but the larval forms may also without difficulty be compared with one another. I. Embryonic Development. Our knowledge of the first ontogenetic stages of Phoronis is due to the researches of Kowalevsky (No. 6), Metschnikoff (No. 9), Foettinger (No. 5), Roule (No. 9a), and Caldwell (No. 2). The latter author, whose description we shall follow in all important points, arrived at results which frequently differ from those of earlier investigators, so that many points seem to require re-examination. The eggs of Phoronis, according to Kowalevsky, are fertilised while still in the body-cavity of the parent.* They reach the exterior through the nephridial canals which open near the anus and function as genital ducts, and then, enveloped in a vitelline membrane, become attached to the tentacles of the parent, where the young develop up to the time of hatching. Cleavage is total and unequal ; the difference in size between the blastomeres of the animal and those of the vegetative pole is, how- ever, inconsiderable. As early as the four-celled stage, two smaller blastomeres can be distinguished from two larger; the eight-celled stage shows four smaller and four larger cleavage-spheres symmetri- cally arranged In the further course of this very regular cleavage, * Cori considers this statement improbable, and thinks that fertilisation takes place outside the parent, in the water. PHORONIDEA. D a small central cavity develops, and in this way a blastula forms which at first is spherical and then becomes elongated in the direction of the future longitudinal axis; in this blastula, a vegetative (ento- dermal) portion consisting of larger cells can be distinguished from a small-celled animal portion. The longitudinal axis coincides with the plane which divides the animal half from the vegetative half. A true invagination-gastrula then develops (Fig. 1), the blastopore being originally oval (Fig. 1 C), but its posterior portion becomes slit-like and soon closes. The anterior part that has remained open persists as the oral aperture of the larva (or, more cor- rectly, as the oeso- phageal aperture). The cells near the posterior closed part of the blastopore are found still later to be actively dividing, and, according to Caldwell, they take part in the formation of the mesoderm, this being especially the case near a depression (Fig. 1 D, g) at the most posterior part of the blastopore. Caldwell compares Phoronis in this re- spect to the Verte- brata, and calls the whole of this closed portion of the blasto- pore (Fig. 1 D) the primitive streaJi, and the depression appearing in the latter the primitive groove. Meanwhile, the embryo changes somewhat in shape. Its anterior part becomes swollen (Fig. 1 D) as the first indication of the future pre-oral lobe of the larva. The region of the primitive streak now Lengthens greatly, so that the depression mentioned above shifts quite to the posterior end of the embryo. The surface upon which OL^ sr Fig. 1. — Gastrula stage of Phoronis. A, younger, B, older stage in optical section (after Metschnikoff) ; x, proto- plasmic bodies in the blastocoele (mesenchyme cells ?). C and D, ventral superficial aspects Rafter Caldwell). C, stage with oval blastopore. D, stage with slit-like, partly closed blastopore ; m, anterior open part of the same, from which the mouth of the larva is derived ; g, posterior pit-like depression of the primitive groove ; a pre-oral lobe is distinctly marked off at this stage. EMBRYONIC DEVELOPMENT. the blastopore is situated may be defined as the ventral surface and the opposite as the dorsal surface. The mesoderm, according to Caldwell (No. 2) forms in a very- peculiar way. In the region corresponding to the most anterior part of the primitive streak, the entoderm-sac shows two lateral pocket-shaped outgrowths (Fig. 2 A, d), at the base of which meso- dermal elements {in') become detached through the proliferation of the entoderm -cells. When a considerable number of these meso- dermal elements have been formed, they arrange themselves into a Fig. 2.— Formation of the mesoderm in Phoronis (after Caldwell). A, transverse section through the anterior part of the embryo, showing the origin of the mesoderm. B, trans- verse section through the oral aperture of an older stage. C, horizontal section through an embryo, showing the formation of the posterior coelomic sacs (m")\ only parts of the anterior sacs (to') are cut through, ec, ectoderm ; en, entoderm ; d, paired archenteric diverticula; g, Binall pit at the posterior end of the primitive streak; m', anterior, m", posterior pair of coelomic sacs. pair of sacs, each enclosing a coelomic cavity (Fig. 2 B, in). This formation of paired coelomic sacs by the proliferation of cells from the lateral walls of the archenteron may, perhaps, be traced back to the type in which the enterocoeles arise through folding of the archenteron. Further back, single mesoderm-cells become separated from the primitive streak and pass into the space between the 4 PHORONIDEA. ectoderm and entoderm. At the most posterior part of the streak lies the small pit mentioned above (Fig. 1 B, g ; Fig. 2 C, g) ; this depression soon gives rise to two lateral diverticula that extend anteriorly between the ectoderm and entoderm (Fig. 2 C, m")3 and become the posterior coelomic sacs. The two pairs of coelomic sacs (mf and m") that thus arise are connected together by the isolated mesoderm -cells which have arisen from the anterior part of the primitive streak. After the formation of the posterior pair of coelomic sacs, which may, perhaps, be in some way connected with the formation of the nephridia, a shallow ectodermal depression forms posteriorly and fuses with the wall of the archenteron, and here the anal aperture arises. According to Metschnikoff (No. 9) and Foettinger (No. 5), the mesoderm forms much earlier than is stated by Caldwell, by a kind of mesenchyme- formation, single cells appearing in the cleavage-cavity of the blastula-stage (Fig. 1 A, B, x). These elements, according to the authors just named, are rather small, so that Caldwell's assumption that particles of protoplasm lying in the blastocoele have here been mistaken for mesoderm-cells appears somewhat probable. It is, however, possible that the formation of the coelomic sacs is preceded by the rise of a mesenchyme. According to Roule also (No. 9a), single mesenchyme-cells are found as. early as the gastrula-stage in the primary body-cavity. Later, after the anal aperture has formed, the cells of the primary entoderm are said to increase in number at the sides of the anus, and in this way to produce two solid mesoderm-bands, while single cells that become detached mingle with the rest of the mesenchyme. The anterior pair of coelomic sacs above described, which might be called cephalic cavities, now grow out anteriorly, and soon com- pletely fill the interior of the pre-oral lobe. These sacs seem to- yield only the lophophoral cavities (Fig. 5, lh) and the connected (1) cavity in the epistome (Fig. 5, eh) of the adult, which are separated by a transverse septum from the posterior part of the body-cavity.* This latter is yielded by the posterior pair of coelomic sacs, from which also is derived the median mesentery suspending the intestine (Fig. 5, iiis) which is retained throughout life. Secondary, lateral mesenteries, however, are also found. With the development of the pre-oral lobe, the principal sections of the alimentary canal and the coelomic sacs, the chief parts of the embryo are represented in rudiment, and now the whole surface becomes clothed with cilia. Thickening of the ectoderm now takes * [According to Masterman (No. II.), there are three perfectly distinct, coelomic cavities in the larva, viz. — a pre-oral or epistomal cavity, a collar- or lophophoral-cavity, and a trunk-cavity. Their origin is not described. — Ed.] EMBRYONIC DEVELOPMENT. 0 place at two definite points. The anterior thickening which occurs at the apical end of the embryo (Figs. 3, 4 B) may be regarded as the homologue of the neural plate of other larvae, and yields the ganglion known as the brain,* which here, throughout life, retains its original epidermal connection. A second thickening of the ectoderm appears behind the mouth as a semicircular, ciliated swelling. From this, which may be regarded as the equivalent of the post-oral ciliated ring, the row of larval tentacles develops (Figs. 3, 4 B), and also the nerve -strand that runs along their points of insertion. The tentacles begin to appear early as outgrowths of the body-wall; they rapidly increase in number, fresh pairs being added dorsally (Figs. 3, 4 B). If this row of tentacles is to be traced back to a transformation of a post-oral ciliated ring, we may regard a strongly ciliated swelling appearing at the edge of the pre-oral lobe as the pre-oral ciliated ring. The posterior part of the body which carries the dorsally displaced anal aperture now grows out into a large cone, the end of which is surrounded by a circum-anal ciliated ring (Figs. 3, 4 C and D). Three body-regions can be distinguished in the larva, which must now be called the Actinotrocha (Figs. 3, 4 B and C) — (1) the pre-oral lobe; (2) the post-oral section which carries the crown of tentacles and covers the posterior part of the body like an apron ; and (3) the posterior or anal section. The changes just described, which lead to the development of the Actinotrocha, take place after the commencement of free larval life. The youngest larva, just hatched from the egg (Fig. 4 A), is still without the crown of tentacles, two small projections lying near the anus being the only indications of the tentacle-rudiments, f II. Metamorphosis. The Phoronis larva (Figs. 3 and 4 B), the shape of which has been described above, was discovered by Joh. Muller and described as Actinotrocha branchiata, and was afterwards more carefully examined by Wagener, Gegenbaur, and others. Krohn (No. 7) and Schneider (No. 10) investigated the metamorphosis which led to the production of a Gephyrean-like form, Kowalevsky (No. 6) being * The pre-oral lobe, together with the brain of the larva, is, as we shall see below, thrown off during metamorphosis. Since, however, these parts are regenerated later, we may still theoretically trace them back to the corre- sponding parts of the larva. t [For the structure of the Actinotrocha, see Masterman's recent work (No. II.) on this larval form. These observations, if confirmed, would show that Phoronis is directly related to Balanoglossus, Cephalodiscus, and Rhabdo- pleura. — Ed.] PHOROXIDEA. the first to prove that the Actinotrocka was the young form of the Phoronis discovered by AVright. Since that time, the meta- morphosis of Phoronis has been more accurately investigated by Metschnikoff (No. 8), Wilson (Xo. 11), and Caldwell (Xo. 1). The first changes that take place in the Actinotrocha consist of a simple increase in size through growth and a continual increase in number of the tentacles. At the same time, a sensory organ develops in front of the neural plate, four eye-spots being added to it in one species. The pigment-spots characteristic of the different species now also develop on the pre-oral lobe and on the tentacles. Eudiments now appear of the de- finitive structures that are destined to replace the larval organs. Of these, the first to develop is an invagination of the body-wall on the ventral surface of the posterior region of the larval body (Fig. 4 C, iv), and in this the two layers of the body-wall (the ectoderm and the somatic mesoderm) can be distinguished. This invagination, which soon grows as a much-coiled tube into the larval body-cavity, represents, as we shall see, the rudiment of the greater part of the body-wall of the adult. Small truncated processes now develop at the base of the crown of tentacles, and from these are derived the adult tentacles (Fig. 4 D). When these structures have appeared as rudiments, the Actino- trocha sinks to the bottom, the critical moment of the commencement of metamorphosis having arrived, the whole process being accom- plished within a quarter of an hour. Metamorphosis is introduced by the evagination of the tube mentioned above (Fig. 4 D), this being protruded like the tentacle of a snail. Since the alimentary canal, together with its mesentery, is attached to the inner end of this tube, it soon has to follow the movement thus begun, and so comes to lie inside the completely evaginated tube (Fig. 4 K). Inuring these changes the rest of the larval body loses its tumes- cence. The oral and anal apertures therefore come to lie remarkably near one another. The pre-oral lobe of the larva is now thrown off, and the same fate overtakes the Larval tentacles and the circum-anal CC7V Fig. 3. — Larva of Phoronis (Actino- trocha), after Metschnikoff, from Balfour's Text-book, m, mouth ; an, anus. METAMORPHOSIS. 7 ciliated ring. The lost larval organs in the oral region consequently have to be replaced by extensive regeneration. It has already been mentioned that the rudiment of the permanent crown of tentacles which soon assumes the form of a horseshoe -shaped lophophore appears early. Fig. 4.— A series of stages in the metamorphosis of Phoronis from the Artinotroclia (after Metschnikoff, from Balfour's Text-book). A, young larva. B, larva after the develop- ment of the post-oral ring of tentacles. C, larva in which the invagination (iv) is com- mencing, from which is derived the greater part of the body of the Phoronis. D, invagination partially, and E, completely everted, an, anus ; m, mouth. By means of this remarkable metamorphosis, an animal is pro- duced the body of which is derived for the greater part from a prolongation of the ventral side of the larva. The dorsal side, on the contrary, has undergone considerable abbreviation, and can be recognised in the short tract lying between the mouth and the anus. 8 PHORONIDEA. There are a few points connected with the rise of the organs which must be described a little more in detail. The nephridia of Phoronis which have recently been carefully studied by Caldwell (No. 1) and Cori (Nos. 4 and 4a, and Ectoproc. Bryoz. Lit., No. 46) are, in the adult, paired, looped, ciliated canals opening externally near the anal aperture (Fig. 5, n). Each nephridium consists of a curved tube starting from the nephridiopore, passing outside the transverse septum, and opening through two ciliated funnels into the posterior part of the body-cavity. These organs thus belong essentially to the posterior section of the body. These nephridia, which in structure may be compared to the segmental organs of the Annelida, arise, according to Caldwell, through the metamorphosis of the larval nephridia discovered by him in the Actino- trocha. The latter, in their structure, recall rather the head-kidney of the Annelida. They are paired canals which open externally behind the transverse septum or diaphragm on either side of the invaginated ventral sac, the inner blind ends being connected with a number of excretory cells. These are star- shaped, a fine canal leading from each of them into the common duct. "With regard to the origin of this larval kidney, Caldwell thinks that we may regard the paired ducts as the remains of the communication established between the posterior coelomic sacs and the surrounding medium by means of the pit- like depression mentioned above (Fig. 2 C, g). The excretory cells, on the other hand, would have an independent origin in the somatic mesoderm-cells. Phoronis is specially distinguished by the possession of a closed blood-vascular system which ramifies on the intestine and in the tentacles, and in which a fluid containing red blood-corpuscles circulates. The details of the origin of this blood-vascular system are as yet not known, but it appears that the vessels originate by dehiscence in the splanchnic layer of the mesoderm. While, according to Cori, the vascular system of the adult is completely closed, there seems, in the larva, to be a communication between it and the cephalic part of the body-cavity. In the latter, the blood-corpuscles are said to arise in large agglomerations. III. General Considerations. The Actinotrocha may without difficulty be regarded as a some- what modified Trochophore. Indications are found in it of a pre-oral and of a post-oral ciliated ring, the latter being transformed into a row of tentacles, and of the characteristic neural plate. The development of the mesoderm is of special importance. This is arranged as two pairs of coelomic sacs which, however, do not appear to be fully equivalent to one another, since, according to Caldwell, they differ in their origin. The anterior pair of coelomic sacs yields the cephalic cavity (lophophoral cavity),* the posterior pair the whole body-cavity of the adult trunk. A transverse diaphragm dividing the two parts of the body-cavity is found in the Actinotrocha on a level with the crown of tentacles. The nephridia belong to the posterior coelomic section. We can thus distinguish, in the body of the Actinotrocha, as well * [In the light of Masterman's observations this point requires reinvesti- gation.— Ed.] GENERAL CONSIDERATIONS. as in that of the adult Phoronis, a cephalic section and a trunk section. Further, the body is unsegmented, and there are no indica- tions of the descent of Phoronis from a segmented ancestor. The metamorphosis of Phoronis affords important data for the orientation and interpreta- tion of the adult animal. We have seen that the principal parts of the body owe their origin to an excessive growth of the ventral side. The longi- tudinal axis of the adult therefore lies at right angles to that of the Actinotrocha. The dorsal surface is shortened, being restricted to the short tract lying between the mouth and the anus (Fig. 5, m-a). Al- though the larval organs are cast off, we may regard the organs that appear in their place as their full equivalents. We shall therefore have to derive the tentacle-crown of the adult from the post-oral ciliated ring of a Trocho- phor-e-like ancestor, and to regard the epistome as the transformed pre-oral lobe. The remarkable meta- morphosis of Phoronis certainly does not corre- spond to any phylogenetic condition. We shall have to assume that a very gradual shifting of the anal aperture along the dorsal middle line took Fig. 5.— Diagram representing a median longitudinal section of Phoronis (constructed after Cori). a, anus ; eh, epistomal cavity ; ep, epistome ; g, gan- glion ; i, intestine ; I, fenestrae in the mesentery ; Ih, lophophoral cavity ; m, mouth ; mg, second stomach [intestine of Benham] ; ms, dorso-ventral mesentery ; n, nephridium ; oe, oesophagus ; r, circular nerve ; t, tentacles ; vm, first stomach. place in the worm-like ancestor of Phoronis, which perhaps lived in 10 PHORONIDEA. sand or even in a tube, a shifting similar to that which takes place in the Siptmculidae (Vol. i., p. 363). The fact that, in the trunk region of the adult Phoronis the symmetrical arrangement of the body begins to be disturbed, is no doubt connected with the tubicolous manner of life. There is thus a longitudinal nerve-strand on the left side, and, according to Com (No. 4), a new plane of symmetry, different from the original one, is evident in transverse sections in the grouping of the longitudinal muscles in Phoronis psammopliila. Indeed, in the presence of secondary and tertiary mesenteries, which leave between them distinct intervals containing the groups of longitudinal muscles, there is a tendency to the development of the radial type. It should here be pointed out that, in Phoronis, the cephalic region is often lost and again regenerated. According to Com (No. 4a), not only the tentacle crown, but with it a part of the oesophagus, the epistome, the ganglion, the so-called lophophoral organs, the blood-vascular ring, and perhaps also the nephridia are. lost. The spontaneous throwing off of the cephalic section is specially liable to occur when the animals are placed in unfavourable conditions of existence. This process closely resembles the throwing off of the head in Pedicellina and in the Tubularia ; and the same process is found in the Ectoproctous Bryozoa, in the disintegration and subsequent regeneration of the polypide (p. 55). LITERATURE. 1. Caldwell, W. H. Preliminary note on the structure, develop- ment and affinities of Phoronis. Proc, R. Soc. London. Vol. xxxiv. 1882-1883. 2. Caldwell, W. H. Blastopore, Mesoderm, and Metameric Seg- mentation. Quart. Journ. Micro. Set. 1885. Vol. xxv. 3. Claparede, E. Beobachtungen iiber Anatomie und Entwick- lungsgeschichte der wirbellosen Thiere. Leipzig. 1863. 4. Cori, C. J. Beitrag zur Anatomie der Phoronis. Inaug.-Diss. Prog. 1889. 4a. Cori, C. J. Untersuchungen iiber die Anatomie und Histologie der Gattung Phoronis. Zeitschr. f. Wiss. Zool. Bd. li. 1891. 5. Foettinger, A. Note sur la formation du mesoderme dans la larve de Phoronis hippocrepia. Archiv. de Biol. Tom. iii. 1882. 6. Kowalevsky, A. On the anatomy and development of Phoronis (Inaug.-Diss. Russian). Petersburg, 1897. See Leuckart's Summary in Jahresb. Archiv. Naturg. Vol. xxxiii. 1867. LITERATURE. 1 1 7. Krohn, A. Ueber Pilidium und Actinotrocha. Archiv. f. Anat. u. Phys. 1858. 8. Metschnikoff, E. Ueber die Metamorphose einiger Seethiere. Zeitschr.f. Wiss. Zool. Bd. xxi. 1871. 9. Metschnikoff, E. Yergl. embryol. Studien. (3) Ueber die Gastrula einiger Metazoen. Zeitschr. f. Wiss. Zool. Bd. xxxvii. 1882. 9a. Roule, L. Sur le developpement des feuillets blastodermiques chez les Gephyriens tubicoles (Phoronis Sabatieri n sp.). Compt. Rend. Acad. Sci. Paris. Tom. ex. 1890. 10. Schneider, A. Ueber die Metamorphose der Actinotrocha branchiata. Archiv. f. Anat. u. Phys. 1862. 11. Wilson, E. B. The origin and significance of the metamor- phosis of Actinotrocha. Quart. Journ. Micro. Sci. Vol. xxi. 1881. See besides the older treatises of Joh. Muller, Wagener, Gegenbaur, Leuckart, Pagenstecher, etc. APPENDIX TO LITERATURE. I. Benham, W. B. The Anatomy of Phoronis Australis. Quart. Journ. Micro. Sci. Yol. xxx. 1890. II. Masterman, A. T. On the Diplochorda. Quart. Journ. Micro. Sci. Vol. xl. 1897. CHAPTEE XVI. BRYOZOA ECTOPROCTA. Colonial Bryozoa with the anus outside the lophophore, with a well-developed introvert and a spacious coelom. Systematic : — A. PHYLACTOLAEMATA. Fresh-water forms, with horse- shoe-shaped lophophore prolonged into two arms; with epistome overhanging the oral aperture (Oristatella, Plumatella, Pectinatella, Fredericella). B. GYMNOLAEMATA. Mostly marine; with circular lopho- phore; without epistome. I. Cyclostomata. Apertures of the zooecia terminal, circular and devoid of closing apparatus; without appendages (Crista, Diastopora, Horner a, Tubulipora, Frondipora). II. Ctenostomata. Apertures of the zooecia terminal, closable by means of tooth-like or seta-like projections of the tentacle-sheath. 1. Haley onellea (Alcyonidium, PJierusa, Flustrella). 2. Stolonifera (Vesicularia, Atnathia, Boiverbankia, Farella, Hypophorella ; the fresh-water forms Paludicella and Victorella also belong here). III. Chilostomata. Apertures of the zooecia not, as a rule, terminal, usually closable by a movable lid. 1. Stolonata (Aetea, JSucratea). 2. Radicellata. a. Cellularina (Cellularia, Scrupocellaria, Bugula). b. Flustrina (Flustra, Membranipora). c. Escharina (Retepora, Microporella, Eschar a, Lepra! i a, JSchizoporella). FORMATION OF THE EGG. 13 I. Formation of the Egg, Fertilisation, Position of the Embryo. The genital products of the Ectoproctous Bryozoa arise in cell- masses which originate as growths of the mesodermal parenchy- matous tissue (marine Ectoprocta), or of the peritoneal epithelium corresponding to the latter (Phylactolaemata), on the inner side of the body-wall (endocyst), or else in strands of the so-called funicular tissue. The ovaries are very generally found on the neural wall (dorsal side) in the anterior or middle part of the body; the testes lie at the base of the zooecium (i.e., in the proximal part). In the fresh-water Bryozoa, the genital rudiment frequently bears a relationship to that part of the mesenterial strand which is known as the funiculus. In Paludicella, for instance, the eggs lie on the body-wall near the point of insertion of the upper funiculus, while the spermatozoa arise on the basal portion of the lower funiculus. In the Phylactolaemata, on the contrary, the ovary lies on the oral body-wall, while the spermatozoa, as a rule, develop in aciniform masses at the upper part of the funiculus. In Cristatella, the spermatozoa arise on the mesodermal septa of the body-cavity. The genital products pass into the body-cavity, where, in some forms, fertilisation takes place. Since the Bryozoa are, as a rule, hermaphrodite, and as it is difficult to state in what way foreign spermatozoa can reach the body-cavity, self -fertilisation has been assumed to occur. The fertilised eggs either pass through the whole of their embryonic development, up to the time when the ciliated larva is formed, within the body-cavity of the parent, or else, through the dehiscence of the body-wall, reach the tentacle-sheath (Valkeria, Joliet, No. 17; Lepralia and Vesicularia, Ostroumoff, No. 26), in the cavity of which they pass through the embryonic stages till the larva hatches, or else the eggs (as in many Chilo- stomata) are received into special outgrowths of the zooecium which serve as brood-cavities. These are the ooecia and ovicells which have been described as individuals of the polymorphous Bryozoan stock peculiarly metamorphosed for the care of the brood. * In those cases in which the larvae pass through their earliest ontogenetic stages in the body-cavity of the mother, they escape either through the aperture of the zooecium, after the polypide to which they belong has undergone degeneration, or else there is a special aperture near the base of the tentacles for the passage of the * [In some Cyclostomata (Crista) the ovicells are undoubtedly modified zooecia in which the polypide is rudimentary or degenerate. — Ed.] 14 BRYOZOA ECTOPROCTA. embryos. Such an aperture was found in Farella by Van Beneden, and in Hypophorella by Ehlers. In a few Gymnolaemata (Alcyonidium gelatinosum, Membranipora pilosa) Farre, Schmidt, and Hincks found a flask-shaped ciliated canal {nephridium V) connecting the body-cavity with the surrounding medium and opening out between the tentacles. Prouho (No. 28a) recently observed in some species of Alcyonidium, that this inter- tentacular organ was connected with oviposition. In Alcyonidium albidum, the eggs probably undergo fertilisation within the body- cavity and become surrounded by a soft shell, being then ejected into the surrounding medium through the intertentacular organ while the parent polypide is extended, further development taking place in the water. More complicated conditions are found in Alcyonidium duplex. In this form, at the period of sexual maturity, the (male) polypide of a zooecium which is without an intertentacular organ develops spermatozoa. At the same time, on the aboral side of this zooecium, a second (female) polypide, provided with an ovary and an intertentacular organ, is developed; soon after fertilisation the male polypide degenerates. The fertilised eggs, which are provided with a shell, probably reach the tentacular sheath through the intertentacular organ ; here, each attached by a fine stalk, they pass through their further development. When the polypide is extended, the part of the tentacle-sheath carrying the eggs is evaginated. In this position the egg-shell bursts and the larva swims about freely. Within the ovaries, which originally consist of small, indifferent cells, a few (2-5) young egg-cells soon appear, the remaining cells becoming grouped round these to form a follicular epithelium (Vigelius). Of these young egg-cells, two at first develop more than the others ; but, as a rule, only one egg becomes fully mature. This egg remains at first connected with the ovary by means of a strand, while what remains of the ovary draws back to the body-wall so as to serve later as the place of origin of another egg. (On the conditions of the maturation of the egg and the ooecia of tin* Phylactolaemata, see below, p. 33.) Remarkable conditions of embryogenesis were found by Haumeii (No. 15) in Crista, a form in which the ripe ovicells contain a large number of embryos.* Beside these is found a protoplasmic nucleated network which sends out finger- * [This condition is brought about by a fission of the primary sexually- prodaoed embryos. As many as one hundred secondary embryos produced by budding are found in a single ovicell. Hakmkk, Quart. Jouni. Micro. Sci., Vols, xxxiv. and xxxix. — Ed.] EMBRYONIC DEVELOPMENT. 15 shaped processes, from the free ends of which the embryos are developed like Imds and eventually cut off. In quite young ovicells, on the contrary, a single ^egg-cell, surrounded by a follicle, is found ; this appears to give rise to the finger-shaped budding organ mentioned above as producing the embryos. In Crista, therefore, the number of embryos produced by the early division of the primary embryo is larger. The mature larvae swim out through the tubular aperture of the ovicell. According to Van Beneden and Pergens, the maturation of the egg is -connected in a certain regular way with the disintegration of the polypide* that produces it, and, in some forms (Flustra truncata, Microporella malusii, Bugula simplex and turbinata) with its later regeneration, so that, when the egg is fully mature, the polypide undergoes histolysis, and becomes changed into a brown body. While the ovary brings another egg to maturity, a new polypide forms. For details of these processes of regeneration see below, p. 55. Among these must also be reckoned the above observations of Protjho on Alcyonidium duplex. In the Phylactolaemata also, as a rule, during the development of the embryos and the statoblasts, the polypide to which they belong degenerates, but, in this case, there is no subsequent regeneration. II. Embryonic Development. The mature, spherical, or ellipsoidal eggs are surrounded by a hyaline membrane called by Pergens the chorion. In the ovum can be recognised a vesicular nucleus with spherical nuclear bodies, and a granular yolk often yellow or brown in colour. The two polar bodies, which are usually of unequal size, correspond in position to the animal pole of the egg. The first ontogenetic processes in the egg of the marine Ectoprocta are best known in Lepralia (Barrois, Nos. 6 and 7), in Tendra zostericola and Boicerbankia (Kepiachoff, Nos. 32 and 34), in Bugula calathus (Vigelius, No. 39), and in Micropoi*ella malusii (Pergens No. 27). In these eggs cleavage is total and almost equal (Fig. 6). The two-celled stage is attained by means of a meridional furrow, and the four-celled by means of another meridional furrow at right * The expressions "polypide" and "cystid'' correspond to an older view, according to which the cystid forming the wall of a chamber represents an individual which gives rise asexually through budding to the polypide. Each chamber of the Bryozoan stock, consisting of a polypide and a cystid, would then represent a double individual or a miniature colony. This view was founded on the great independence of the polypides as shown in the processes of degeneration and of regeneration above-mentioned. Although we do not share this view, we still retain in use these expressions which have become established. The cystid, then, means to us the lower part of the body- wall, while the polypide represents the retractile anterior section of the body with the intestinal canal attached to it. These two are merely parts of one individual. [Acting on the suggestion of Dr. Harmer, we have substituted the term "zooecium" for cystid in referring to the outer parts of the ordinary adult individual, as this term is of much more general use in English works on tin? Bryozoa. — Ed.] 16 BRYOZOA ECTOPROCTA. angles to the first ; this stage, in consequence of an equatorial furrow, passes over into the regular eight-celled stage (Fig. 6 ^1). A small blastocoele can frequently be observed as early as this stage, and the animal pole can be distinguished from. the vegetative pole by a differ- ence of size — often only very slight — in their respective blastomeres. In the further segmentation there is a peculiarity worthy of note. The sixteen-celled stage is reached through the appearance of two c y\ /$ '.® c®. ® © \®j U) i® ®J ^®J Fig. 6.— Cleavage and embryonic stages of Bugula calathus (after Vigelius). A, stage with eight blastomeres. B, stage with sixteen blastomeres. C, stage with thirty-two blasto- meres. All seen from above. The dotted lines in A and B indicate the direction of the next cleavage planes. D, blastula-stage in vertical section. E and F, gastrnla-stages. G, a later ontogenetic stage, c, corona-cells ; ec, ectoderm ; en, entoderm ; /, cleavage- cavity ; z, central tissue derived from the primary entoderm. furrows lying on either side of the first meridional plane and running parallel to it. An embryo thus results, each half of which consists of eight cells (in two rows of four, Fig. 6 B). A similar division leads to the thirty-two-celled stage (Fig. 6 G), two furrows forming parallel to and at the sides of the second meridional plane. Each half of tins embryo consists of four rows, each containing four cells. This stage is followed by less regular divisions and results in a EMBRYONIC DEVELOPMENT. 17 blastula bearing a general resemblance to a biconvex lens (Fig. 6 D). A more active division of the cells of the animal (later aboral) pole can now be remarked, while, from the vegetative pole, four entoderm- cells wander into the blastocoele (Fig. 6 E). As soon as the entoderm-cells have shifted inwards, the surrounding ectoderm-cells seem to join together. This type of entoderm-formation may be traced back to polar ingrowth. A shallow depression in the region of the vegetative pole is often noticeable during these processes. After the ectoderm has completely closed, the four entoderm-cells multiply, their derivatives becoming grouped so as to enclose a small slit-like cavity (archenteric cavity, Fig. 6 F). This cavity is usually obliterated later, the entoderm-cells multiply still more, and finally yield a mesenchyme or embryonic connective tissue (the inner mass, central tissue, " Fiillgewebe," Fig. 6 G, z), which completely fills the blastocoele. This tissue, which seems to be of little impor- tance in the later development, playing merely a passive role, soon assumes a reticular character "(Fig. 9 B, /). While these processes have gone on within, two rows of cells at the edges of the biconvex embryo have become distinguished by their larger size (Fig. 6 E and F) ; one of these rows at a later stage becomes very conspicuous, forming the rudiment of the ciliated ring or girdle of the larva, and is known as the corona or velum (Fig. 6 G, c). After the larva has attached itself, the central tissue gives rise to the meso- dermal portion of the first sedentary individual (primary zooecium), i.e., to the parietal mesoderm-layer on the inner side of the endocyst, and to the outer layer of the sac-like rudiment of the polypide (Fig. 12, b, p. 29). The rest of the central tissue unites with the cell-mass yielded by the degeneration of the larval organs to form a spherical mass (the so-called brown body)', which is composed of remains of cells, particles of food-yolk and detritus, and which, when the first polypide develops, is used up as food material. The central tissue is derived direct from the primary entoderm of. the k'astrula-stage. This germ-layer does not, as a rule, separate into mesoderm and definitive entoderm. Barrois (No. 7), indeed, maintains that such a separation does take place in Lepralia unicornis, the mesoderm there becoming arranged in paired mesoderm-bands. His observation, however, has received no further confirmation through researches made in connection with various other genera. In those few forms in which an alimentary canal is developed, there must of course be a separation of the mesoderm from the definitive entoderm. In quite early stages (corresponding to that in Fig. 6 F), a separation of the mesoderm from the entoderm was observed by Prouho (No. 28b) in the embryos of Alcyonidium and Membranipora pilosa (Cyphonautes) , but no further details as to the rise of the mesoderm could be observed. The embryo which originates in this manner (Fig. 6 G), in which two body-layers (ectoderm and central tissue = primary entoderm) 18 BRYOZOA ECTOPROCTA. can be distinguished, gives rise to the typical Ectoproctous larva by certain characteristic modifications of the ectoderm. The cells of the corona increase in size and become covered with cilia, giving rise to a transverse ciliated zone (Fig. 7, c), by means of which the body of the larva is divided into an upper aboral half (corresponding to the former animal pole) and a lower oral half. In the aboral half, a disc-like thickening beset at its edge with stiff setae develops (Fig. 7, r) ; this is the so-called retractile or ciliated disc (calotte of Barrois, cap), which has been regarded as the rudiment of the polypide that develops in the primary zooecium of the Bryozoan stock. At its periphery, this organ is encircled by a depression (Fig. 7, p, mantle-cavity Barrois). In the oral half, two organs develop first of all, the plane of symmetry of the larva being marked by their position. In the posterior portion of the oral side a deep ectodermal depression occurs; this originally has the form of an almost completely closed sac (Fig. 7, s), and from the fact that it serves as an attaching organ during the subsequent metamorphosis, it is called the sucker (sac interna, Barrois).* Further forward there is a median furrow beset with strong flagella, the so-called anterior ectodermal furrow (oral furrow of Nitschb, fente of Barrois, dorsal organ of Balfour) ; this is connected internally with a glandular tissue to which Barrois gave the name of pyriform organ (Fig. 7, o). In the space between the anterior ectodermal furrow and the sucker, the oral aperture (Fig. 7, m) is found in those larvae in which an alimentary canal develops. III. Metamorphosis. The metamorphosis of a Bryozoan larva comprises a more or less protracted free-swimming stage, during which no perceptible advance is made in the development of the animal, and further, the attach- ment of the larva, and the subsequent somewhat complicated changes which bring about its transformation into the first primary zooid of the young Bryozoan colony. Although there is no difficulty in tracing back the larvae of the Ectoprocta to a common type resembling that indicated above, we find certain distinguishable larval types, which, for the sake of clearness, must be treated separately. The description of each larva * It should here he pointed out that the name "sucker" is calculated to convey a false impression as to the function of this ectodermal invagination, which is in reality merely the basal plate of the future primary zooecium in pi. juration and in an invaginated condition. Fixation does not occur through suction, but only after the evagination of this basal plate, as will be seen later. TYPE WHICH DEVELOPS AN ALIMENTARY CANAL. 19 will be followed by the account of its transformation into the attached form, as far as this latter is accurately known. 1. Type which develops an Alimentary Canal. We shall commence our study of the Ectoproctous larvae with the consideration of a few isolated forms, which, in external struc- ture, do not greatly differ from the typical larva (especially from that of the Escharina), but which in the higher differentiation of their internal organs show a more primitive condition. In the possession of a well-developed (though also to some extent function- less) alimentary canal especially, these larval forms afford important points of comparison with the larvae of other groups, whereas, in most other Ectoproctous larvae, the primary entoderm gives rise merely to a parenchyma (central tissue), which in many forms contains a yolk-mass. a. Larva of Alcyonidium. The larva of Alcyonidium (Fig. 7) has already been described and figured by Farre and others, and later by Barrois (No. 6), its internal structure having been specially investigated by Harmer (No. 13). This larva differs little externally from the type which we shall find retained in the Escharina. When viewed from above, it appeal's perfectly spherical; when seen from the side (Fig. 7 A) the oral surface is distinguished by its greater convexity from the flat aborai side. Almost the whole of the latter is occupied by the retractile disc (Fig. 7, r), bordered with stiff cilia, this organ being separated by a circular depression (mantle-cavity, jp) from the ciliated corona (c) which is composed of large cells. On the oral surface, in the anterior region of the larva, we can recognise the anterior ectodermal furrow (o), at whose edge are cilia which, anteriorly, become grouped together to form a ciliated tuft (plumet). In the posterior part of the larva the sucker-invagination (s) is visible. Two pairs of flagella belonging to the posterior region of the oral surface should also be mentioned. In a longitudinal section (Fig. 7 B), we recognise first the large ■cells of the Giliated ring (c), then, in the anterior part of the oral •surface, the pyriform organ (o) lying at the base of the ectodermal groove, consisting of elongated ectodermal cells that possess a peculiar histological character. Between this organ and the aperture of the sucker (s), which lies further back, is the oral aperture ()//) leading through a narrow oesophagus, which may have arisen as 20 BRYOZOA ECTOPROCTA. an ectodermal invagination, into a wide gastric cavity that ends blindly. A comparison with the larvae of Tendra and of Cypho- nautes renders it probable that the part lying behind the aperture of the sucker corresponds to the anal region. This distinctly developed alimentary canal appears to be a vestigial organ, there being no indication that it actually functions. It degenerates in the further course of larval life. The thickness of the epithelium which lines the mantle-cavity (p) is remarkable, and contrasts with the somewhat thin ectodermal layer of the retractile disc. Closely con- nected with the latter layer, in the anterior part of the larva, there is a cell-mass (Fig. 7 B, g) which Harmer (No 13) regards as the brain. This author observed, arising within this cell-mass, fibrous strands, some of which are said to become connected with cells of the ectodermal furrow, while others can be followed into the neighbourhood of the large coronal cells. Should these statements be confirmed, we should appear justified in considering the pyriform organ as a sensory organ, whereas until now its histological constitution seemed to point to the conclusion that it functioned as a gland. The presence of a larval brain comparable in position to the neural plate would be of importance in tracing the l-Vtoproctous larvae back to the Trochophore. The statements of Hakmku hare recently been confirmed by the researches of Prouho- (Na. 28) in connection with Flustretta, but on account of the Sfer-C b5\P 771 Fig. 7. — A, larva of Alcyonidium mytili (after J. Barrois). B, longitudinal vertical section through the larva of Alcyonidium (diagram constructed after the figures of Harmer). c, coronal cells ; g, brain (?) ; m, oral aperture ; o, pyriform organ ; p, mantle-cavity ; r, retractile disc ; s, sucker. LARVA OF TENDRA. 21 difficulty of investigating this point, we are unable as yet to accept with certainty the interpretation that these fibrous strands are nerves. b. Larva of Tendra. If we leave out of account Cyphonautes, to which reference will shortly be made, Eepiachoff (No. 32) deserves the credit of having been the first to prove the existence of a well-developed alimentary canal in a typical Ectoproctous larva. This author found in the larva of Tendra (Memfo*anipora) zostericola, which in other respects follows the type of the ordinary Escharina larva, and also resembles the Alcyonidium larva, an intestinal canal, the oral aperture of which lies in front of the sucker, while the anal aperture lies behind that organ. The Tendra larva would thus approximate to the larva of Alcyonidium in the topography of its internal organs. c. Larva of Membranipora and Flustrella. The larva of Membranipora has frequently been investigated Ehrenberg placed it under the name of Cyphonautes among the Eotatoria; later, Joh. Muller compared it with the larva of Mitraria; and Semper, Leydig, and Claparede (No. 1) declared it to be a Lamellibranchiate larva, Schneider (No. 5) being the first to establish its true affinities as the young of Membranipora. Later observations were made by Allman, Metschnikoff (No. 19), Hatschek (No. 2), Barrois (No. 6), Eepiachoff (No. 4), and Prouho (No. 28) as to the structure of Cyphonautes, while its attachment and transformation were investigated by both Schneider and Ostroumoff (No. 3). It appears from the statements made by these authors that Cyphonautes agrees closely in all important points with the typical Ectoproctous larva (e.g., Alcyonidium larva), the apparent differences between them being of an unessential kind. Cyphonautes (Eig. 8.) resembles in shape a laterally compressed bell. The lower surface is bordered by cilia. If we compare this with the corona of the Alcyonidium larva, we find that we must regard the external surface of the bell as the aboral side, and the under surface of the bell as the oral side partly invaginated to form a vestibule or atrium. The whole body is covered externally by two triangular shell-valves connected together along their anterior edges, which run parallel to the fibrous strand (Fig. 8, /). The two valves that come into contact along this hinge-line cover the laterally compressed body on the two sides. Their posterior edges, which run parallel to the midgut and the hind-gut (the intestinal edges), are 22 BRYOZOA ECTOPROCTA. sharply bent round towards the middle line of the body. The shell- valves meet at this point like pincers. Near the lower aperture (the entrance to the atrium) the valves gape. At the aboral pole, a prominence (?*) covered with stiff cilia projects from the shell, but can be withdrawn into it. In this we recognise the homologue of the " retractile disc." Some uncertainty still prevails as to the ciliated ring that runs round the edge of the bell. According to Ehrenberg and Hatschek, there is a single com- plicated ring, forming many coils, but Claparede and Schneider, whose view has been adopt- ed by Prouho, think they can distinguish two distinct rings of cilia, an anterior ring in the neighbourhood of the pyriform organ (o) and a posterior, circum-anal ring, part of which sinks into the atrium. On the oral invagi- nated surface we notice first, in the anterior angle, a cell- mass provided with a tuft of cilia; this is the pyriform organ (o). According to Repiachoff, two parts may be distinguished in it; an invaginated, sucker-like epithelial thickening surrounded by a ring of cilia (ecto- dermal furrow), and a cell-accumulation lying above this (the actual pyriform organ). As in Alcyonidium, this organ is connected with the retractile disc (r) by fibrous strands (/), some of which have been claimed as muscle-fibres and some as nerve-fibres (Prouho). Cyphonautes has a fully-developed and functional alimentary canal. The oral aperture (in) is found in the base of the atrium, and leads * [The organs s and s' are not isolated structures as represented in the figure, but are situated underneath the body-wall, which latter should be indicated as a line extending forwards from the anus below s and *' and joining the ciliated ring near the mouth. — Ed.] Fig. 8.— Larva of Membranipora = Cyphonautes (combined from the figures of Claparede, Schneider, and Hatschek). a, atrium ; an, anal aperture ; /, fibres connecting the pyri- form organ (o) and the retractile disc (r) ; m, mouth ; s, glandular thickening (homologue of the sucker); s', similar, smaller organ (shell-adductor muscle).* LARVA OF MEMBRANIPORA AND FLUSTRELLA. 23 into a very expansible oesophagus, which ascends towards the aboral surface; here it bends sharply downwards and joins the dilated stomach which passes into the hind-gut. The anal aperture (an) opens into the atrium. In the space between the mouth and the anal aperture there is a paired glandular thickening (s). Near this kidney-shaped organ lies a similar smaller organ (s). Ostroumoff has shown that when the larva begins to change into the sedentary form, fixation takes place by means of this paired disc and the cementing substance secreted by it, so that we may regard it as the homologue of the sucker of other Ectoproctous larvae. The smaller organ seen near the sucker was assumed by Schneider and H. Prouho to be the transverse section of the adductor muscle of the shell. It is evident from the above description that Cyphonautes agrees in all important points with the typical Alcyonidium larva, from which it is distin- guished chiefly by the slighter development of the retractile disc and by the absence of a circular mantle-cavity surrounding that organ. In place of this oavity we find an extensive surface (mantle-surface) on the aboral half of the body, which secretes the two cuticular shell- valves. The oral surface does not here, as in Alcyonidium, bulge out downwards, but is withdrawn into the interior of the body, forming an atrium such as is also found in the Entoproctous genus Pedicellina. Intermediate between the Cyphonautes and the Alcyonidium type we may place the larva of Flustrella hispida, known through the researches of Metschnikoff (No. 20), Barrois (No. 6), and Prouho (No. 28). In this larva, the rudiment of the alimentary canal appears in the embryonic stages, but degenerates later. In other respects, the development of the Flustrella larva agrees closely with that of the Alcyonidium larva. An embryo develops in which the central part of the aboral surface becomes marked off from the corona by a circular mantle-cavity. Only secondarily does the corona bend round downwards in the form of two lateral lobes, apparently enclosing the oral surface in an atrium, while the circular mantle-cavity disappears. Paired shell-valves then appear and cover the whole aboral surface with the exception of the very small retractile disc. A similar though less complete trans- formation in the same direction is found in the larva of Eucratea chelata (Barrois). The researches of Prouho have led to interesting revelations as to the internal structure of the Flustrella larva. With regard to the presence of a nervous system, Prouho's researches agree essentially with those of Harmer. In the Flustrella larva, the retractile disc is connected, as in Cyphonautes, with the pyriform organ by a strand of muscle-fibres. Along this strand runs a bundle of nerve-fibres ; these start from the retractile disc and can be followed as far as to the cells of the ectodermal furrow and of the corona. In their course, single fibres of this bundle become connected with ganglion -cells. Prouho thinks that he can trace back the elements regarded by Harmer as ganglion-cells to simple mesoderm-cells, and considers certain elements which are provided with large nuclei as the true ganglion-cells of this region. When the complex organ connected with the ectodermal furrow is more carefully examined, an inner cell-complex (glandular organ, pyriform organ 24 BRYOZOA ECTOPROCTA. in a restricted sense) is found to be distinct from the ciliated cells of the ectodermal furrow. In this organ, which is evidently of glandular structure, three parts can be made out. The lateral parts show radially arranged cells opening into the ectodermal furrow, while the cells of the middle part run longitudinally and open into a small pit lying in front of the ciliated tuft of the ectodermal furrow (Prouho). Further, there is in this larva a highly developed musculature, consisting of lateral parietal and longitudinal muscles, and of a shell-adductor, already noticed by Metschnikoff. A mesodermal cell-layer lying below the ectoderm plays an important part in metamorphosis, the mesoderm -layer of the developing polypide and of the primary zooecium being formed in it. The transformation of the larva of this type into the sedentary form (primary zooid of the colony) takes place in the same way as in other Ectoprocta; the reader is therefore referred to the description of the metamorphosis of Bugula and Lepralia, given on p. 27. It need here only he mentioned that Cyphonautes, whose metamorphosis has been made known through the researches of Schneider and Ostroumoff, attaches itself by means of the paired, disc-like organ mentioned above on the oral side of the body, and on the degeneration (histolysis) of the internal organs, changes into a spherical mass covered by the shell-valves, this being the rudiment of the primary zooecium. While the ectocyst of the zooecium is secreted on the surface of this mass, the polypide of the first chamber is derived from the retractile disc through ontogenetic processes to be described later (Schneider and Ostroumoff). With regard to the last point, the recent researches of Prouho in connection with Flustrella have led to the remarkable discovery that the retractile disc, like all other larval organs, becomes invaginated and undergoes degeneration. During this process, a thickened disc forms out of the surrounding ectoderm, mesodermal elements joining it on its inner side. This bilaminar plate, which lies in the place of the degenerated retractile disc, now in its turn becomes invaginated, and thus gives rise to the sac-like rudiment of the polypide. Even though most other authors regard the polypide as derived directly from the retractile disc, it must be considered probable that the ontogenetic process here described in connection with Flustrella is the rule in the development of the polypides in all Ectoproctous larvae. Pherusa tubulosa, which, according to Prouho (No. 28a), is also distinguished by the possession of a bivalve shell, is allied to the above type, and the larva of Hypophorella, according to the same author (No. 28b), also approaches this type. 2. Type of the intestineless Chilostomatous Larva with slightly-developed corona. The larvae of this type, in outward appearance, almost exactly resemble the Alcyonidium larva. They chiefly occur in the Escharina tribe, an example being found in Lepralia Palladia ho, which TYPE OF THE INTESTINELESS CHILOSTOMATOUS LARVA. 25 has been described in detail by Barrois (No. 9). Here also we find the oral and aboral surfaces separated by a corona of somewhat large ciliated cells. The aboral surface shows the well developed retractile disc surrounded by a circular mantle-cavity ; on the oral surface, the ectodermal furrow with its ciliated tuft and the sucker- invagination are situated. Strong pigmentation is often found in the larvae of this and the following types, paired patches of pigment, like eye-spots, being found in individual cases. The most essential distinction between the larvae of this type and the Alcyonidium larva is the entire absence of the alimentary canal. We assume that the inside of this larva contains only the so-called central tissue. In the very imperfect state of our knowledge of the anatomy of these larvae, it must still be regarded possible that many of them may possess a well-developed alimentary canal. We are therefore unable at present to define exactly the distinctions between this type and the one last described. The metamorphosis of the larvae of this type will be described together with that of the following type. 3. Type of the intestineless Chilostomatous Larva with highly-developed corona. a. Structure of the Larva. We shall take as type of this group the larva of one of the Cellularhia, the Bugula larva (Fig. 9), which has become well known to us through the researches of Nitsche (No. 22), Claparede (No. 10), Salensky (No. 37), Barrois (No. 9), and Vigelius (Nos. 39 and 40). The intestinal canal here also is altogether wanting. The primary entoderm changes direct into the so-called central tissue (Fig. 9 B, f). The distribution of the ectodermal organs is the same as in the larvae of the Alcyonidium and the EscJiarina types; the whole form of the larva, however, is somewhat different, chiefly on account of the great growth of the coronal cells (c) in the direction of the principal axis. These cells lengthen greatly, the whole of their surface becoming covered with cilia, while in the larvae of the preceding types these cells usually bear only one row of cilia. This extension of the corona encroaches greatly on the aboral and oral surfaces. The retractile disc (r) is consequently smaller, and the ectodermal furrow (e) is so grown over by the increasing coronal cells that it at last appears enclosed by the coronal region. In this way a more or less spherical or rounded larva is 26 BRYOZOA ECTOPROCTA. produced, and in this the plane of symmetry is denoted by the position of the ectodermal furrow j the larva is thus peach-shaped. For details of the structure of the Bugula larva we are indebted chiefly to Yigelius (No. 39). Taking first the organs of, the oral surface, the sucker originates as an ectodermal invagination. In the later stages the aperture of the invagination appears to close, at least it was no longer visible. The upper part of the wall of the sucker becomes invaginated into the cavity of the organ itself, thus greatly compressing it. The whole rudiment in this way becomes cup-shaped. In the anterior part of the oral surface a second ectodermal invagina- tion appears ; this is found later between the cells of the corona, and is known as the ectodermal furrow (e). The ectodermal cells of this Fig. 9. — A, larva of Bugula plumosa (after J. Barrois). B, median section through a Bugula larva (diagram constructed from figures by Vigelius). c, cells of the corona (a single cell of this kind on each side) ; e, ectodermal furrow ; /, central tissue ; o, pyriform organ ; p, mantle-cavity ; r, retractile disc ; s, sucker. region are elongated, columnar, glandular cells. In direct connection with this modified glandular epithelium, there is a cell-mass which extends towards the interior of the larva and ends posteriorly in three processes. This is the pyriform organ (o). Yigelius is inclined to regard the ectodermal furrow and the pyriform organ as a common glandular complex derived from the ectoderm. The retractile disc (r) arises on the aboral surface as a simple ectodermal thickening, the cells of which lengthen greatly and become arranged in several layers. At the centre these elements seem to be wanting, while at the periphery of the organ they show a radial arrangement. This disc is protrusible beyond the surface of METAMORPHOSIS. 27 the larva. Its covering of stiff setae probably serves some sensory purpose, perhaps transmitting tactile impressions. b. Metamorphosis. The first indication that the larva is ready to attach itself is an alteration in its mode of progression. The larva now constantly moves in circles, with its retractile disc protruded to its utmost limit. By a sudden contraction of the body the sucker is shot out, and the fixation of the larva is accomplished by means of a sticky secretion from its lower sur- face. The evagi- J{ nated sucker (Figs. 10 and 11, s) in Bugula shows at first on its under surface a circular furrow, which sepa- rates an outer broader part from A narrower central portion that pro- jects downwards. At a later stage, the lower side of the sucker flattens into a broad cell- plate (plaque ad- hSsive, Barrois), from which is de- rived the basal sur- face of the endocyst of the primary zooecium. The whole of the rest of the body- wall of the primary zooecium, i.e., its upper part and its lateral walls, are yielded by the aboral portion which, in the larva, is comparatively small. This takes place chiefly by an extension of those parts of Fia. 10. — Metamorphosis of the larva of Lepralia unicornis (after J. Barrois). A, first stage of metamorphosis. The sucker (s) is evaginated, the corona (c) is beginning to bend round. B, next (so-called umbrella) stage. The corona (c) is completely bent round, c, coronal cells ; o, pyriform organ ; p, mantle-cavity ; r, retractile disc ; s, protruded sucker ; v, vestibulum ; x, paired organ of doubtful significance (according to Barrois, the mesodermal rudiment of the polyp'de). 28 BRYOZOA ECTOPROCTA. ^^^-s the ectoderm which previously formed the lining of the circular furrow (mantle-cavity, p\ and which in the larva are distinguished by the great depth of their cells (Fig. 10, A). The mantle-cavity (p) is in this way completely obliterated, the circular fold that bordered it externally, and was called the mantle, bending over downwards (Fig. 10, B) ; its inner thus becomes its outer surface, and gives rise to the body-wall of the primary zooecium. The corona at the same time bends round or unfolds (Fig. 10, B). The lower ends of the coronal cells (c) retain their position, while the upper ends first shift outward (Fig. 10, A) and then downward (Fig. 10, B), so that each coronal cell at the end of this process has rotated through an angle at first of 90° and finally of 180° (Fig. 1 1 c). We thus have a stage to which Barrois has given the name of the umbrella- shaped stage (Fig. 10, B). The upper surface is formed by the future body-wall, the lower by the downwardly rotated coronal cells. The edges of the umbrella become applied to the adhesive plate of the sucker (s) and fuse with it, the body- wall thus completing itself (Fig. 11). The displaced coronal cells at the same time fuse with the upper surface of the sucker, and in this way a circular cavity arises {vestibulum, Barrois, Fig. 11 v), the walls of which soon become completely detached from the body-wall and undergo degeneration. This is the fate not only of the corona, but of the ectodermal furrow, the pyriform organ, and (where such is present) the larval intestine. There is then found inside the sac-like larva a cell-mass derived from the degeneration of the most important larval organs; this unites with the mass that resulted from the degeneration of the central tissue, and forms the so-called broivn body. The rudiment of the body- wall of the primary zooecium is framed in the way just described. Its surface soon becomes covered with a chitinous secretion (often impregnated with lime salts) ; this is the ectocyst. During these ontogenetic processes, other changes can be seen to occur at the aboral pole which result in the development of the polypide of tin- primary individual. The retractile disc that lies Fio. 11. — Longitudinal section of attached larva of Bugula jlabdlata (after J. Barrois). c, coronal cells ; r, rudiment of the polypide (which has arisen by the invagination of the retractile organ) ; s, lower thickened surface of the evaginated sucker (adhesive plate) ; v, vestibulum. METAMORPHOSIS. 29 here has changed into a plate formed of deep cells which becomes invaginated (Figs. 11, r and 12 A, a). The sac which thus arises soon becomes completely cut on from the body-wall (Fig. 12 B), and represents the rudiment of the ectodermal and also of the entodermal parts of the polypide. A second cell-layer (b) appears on the outer side of this sac, this being the mesodermal layer of the polypide. Various conjectures have been made as to the origin of this layer. According to Barrois (Xos. 7 and 9), there are, in Lepralia, two ectodermal thickenings at the sides of the anterior ectodermal furrow (Fig. 10, x) ; when the corona bends over, these reach the interior of the primary zooecium, and they alone are unaffected by the subsequent degeneration of the larval Fig. 12.— Two ontogenetic stages of the primary zooecium of Bugida calathus (after Vigelius). A, invagination of the retractile disc to form the rudiment of the polypide (a); b, cells of the external layer of the polypide ; e, epithelium of ectocyst. B, a somewhat older stage. The invagination forming the rudiment of the polypide has closed. In A the degenerating mass derived from the larval organs is omitted. organs, and remain to form the* mesodermal layer of the polypide. These statements have not been confirmed by later researches. Far more probable is the conjecture of Ostroumoff (Nos. 25 and 26) and Vigelius (No. 40), which has recently been confirmed by Prouho (No. 28), that this cell-layer is derived from the mesodermal larval tissue (central tissue). Of the remainder of this tissue, part seems to yield the somatic mesodermal layer (the so-called parenchymatous tissue), while another part joins with the granular masses which have arisen through degeneration in the formation of the so-called brown body. This body becomes connected with the stomach of the newly- formed polypide and is finally absorbed. These processes will be 30 BRYOZOA ECTOPROCTA. more fully discussed in a special section on the further development of the rudiment of the polypide (pp. 43, 44, and 55). In considering the metamorphosis of the Biujula larva, which is identical with that of the types hitherto described, we are at once struck by the fact that the larval intestine degenerates in all cases, and that the intestine of the primary individual is formed anew from an independent rudiment on the aboral surface. This fact, however, is explained by the capacity possessed by the Bryozoa in certain cases of degenerating and again regenerating the alimentary canal or even the whole polypide (see p. 55). We are therefore not obliged to regard the polypide of the primary individual as a second generation of the Bryozoan colony, derived by budding from the larva. The production of the primary individual from the larva rather comes under the category of metamorphosis, although this latter becomes very complicated in consequence of the far-reaching degeneration of the larval organs, and on account of the incon- spicuous form assumed by the rudiments of the future parts of the permanent body in the larva, these rudiments being somewhat comparable to the imaginal discs of the Insecta. With regard to these rudiments, that of the polypide is repre- sented in the larva by the retractile disc. The rudiment of the endocyst of the primary zooecium lies partly in the mantle-cavity and partly in the sucker-invagination. The body-wall of the primary individual is thus present in the larva in an invaginated condition, so as to afford the corona, as the locomotory organ of the larva, more room for development. While most of the parts of the primary zooecium are thus waiting in the larva in an invaginated and apparently functionless condition, it is a striking fact that the retractile disc, which has hitherto been regarded as the rudiment of the polypide, seems to be of functional importance to the larva (sensory organ ?). On this account, the observation of Phouho recorded above (j). 24) seems significant, according to which the retractile disc does not pass directly over into the rudiment of the polypide, but after its invagination undergoes a degeneration similar to that of the other larval organs, while, at the place formerly occupied by it, a new two-layered rudiment develops, viz., that of the polypide. The bending over of the corona and the formation of the vestibule, in the wall of which are included the coronal cells, the pyriform organ, and a part of the sucker, there to undergo degeneration, will appear less remarkable if we consider how often larval parts which have become useless, instead of being thrown out, sink into the interior of the individual undergoing metamorphosis, there, after degenerating, to be utilised as food-material. Under this aspect, these processes appear comparable to the retrogression of the embryonic mem- branes and the formation of the so-called dorsal organ in Insect embryos. TYrE OF THE VESICULARIAN LARVA. 31 4. Type of the Vesicularian Larva. In the larvae of this type an excessive lengthening of the coronal cells is found (Fig. 13). The whole larva in this way becomes greatly elongated; the aboral and oral areas are exceedingly circumscribed, and the ectodermal furrow (ec), as in the previous types, is covered by the cells of the corona. Another characteristic of these larvae which was observed by Barrois in Serialaria lendigera (No. 9) and by Ostroumoff in Vesi- cularia (No. 26) is the small size of the retractile disc (?*s) and the un- usual depth of the mantle- cavity (m) ; posteriorly, i.e., opposite to the ecto- dermal furrow, it is so deep as to extend beyond the middle of the body. Observers are not quite unani- mous as to the sucker. According to Barrois, only a functionless vestige of this organ (s) is retained ; Ostrou- moff, on the other hand, found a well-developed though not very large sucker. Fig. 13.— Larva of Serialaria lendigera (after Barrois). rs, retractile disc ; s, vestige of sucker ; ec, anterior ecto- dermal furrow ; m, mantle-cavity. In the same way, authorities differ as to the metamorphosis of the sucker, though in other respects metamorphosis here takes a similar course to that described for Bugula. Ostroumoff maintains that in this type also meta- morphosis commences with the evagination of the sucker, while Barrois holds that fixation takes place by means of two lobes that grow out from the lower end of the ectodermal furrow and which belong to the corona. For details of the remarkable manner in which the corona is reversed, its long cells being rotated and bent round, we refer the reader to Barrois (Nos. 8 and 9). The polypide is here said to arise not by invagination, but through the separation from the retractile disc of a cell-plate which becomes invaginated later (Ostroumoff). 5. Type of the Cyclostomatous Larva. The metamorphosis of the marine Cyclostomatous Bryozoa has been investigated chiefly by Metschnikoff (No. 21), Barrois (Nos. 6 and 9), and Ostroumoff (No 25). The larvae of this type (Fig. 14, .4) are distinguished by the presence of a large sucker- invagination (s), and by the vestigial condition of the retractile disc (r), which is found as an inconspicuous cell-mass at the base of the 32 I3RY0Z0A ECTOPROCTA. large mantle- cavity (p). A further peculiarity is exhibited by the corona, which here does not consist of long cells extending from the oral to the aboral area, but is composed of numerous small elements. The ectodermal furrow is hardly perceptible, but Barrois (No. 9) was able to identify it. This larva, in its metamorphosis, follows the usual course. Here also (Fig. 14, B) the first act of metamorphosis is the evagination of the sucker and its transformation into the basal plate (a) of the primary zooecium ; the bending over of the mantle then takes place, and the fusion of its periphery with the edges of the basal plate, by means of which the annular vestibular space (v) is closed in. While Fig. 14.— Longitudinal sections of two ontogenetic stages of a Cyclostomatous Bryozoon (after Ostroumoff). A, longitudinal section through the larva of Frondipora. B, metamorphosis of Ttibulipora serpens, a, adhesive plate ; /, central tissue ; p, mantle-cavity ; r, vestige of the retractile disc ; s, sucker ; v, vestibulum ; x, cell-p^te (rudiment of the polypide derived from the retractile disc). subsequently this whole structure degenerates, the polypide forms, not by invagination, but by the separation of a cell-plate (./•, Ostroumoff). This plate sinks in and becomes covered with mesoderm derived from the central tissue. Ostroumoff's observation of the appearance of a cavity lined with endothelium in the neigh- bourhood of the developing nutritive tube is worthy of mention. In this cavity, which degenerates later, we have perhaps a homologue of the body-cavity of the fresh-water Bryozoa. 6. Type of the Phylactolaematous Larva. The embryonic development and the metamorphosis of the Phylac- fcolaemata has been investigated by Metschnikoff (No. 20), Nitsche (No. 52), Reinhard (Nos. 54-56), Ostroumoff (No. 53), Korotneff TYPE OF THE PHYLACTOLAEMATOUS LARVA. 33 (No. 48), and Jullien (No. 47a), and more recently by Davenport (No. 46a), Kraepelin (No. 50), and Braem (No. 45a).* The ovary (Fig. 15 ov) is an aciniform growth of the inner meso- dermal layer of the body-wall, lying on the oral side of the polypide, immediately below the so-called duplicating bands (posterior parieto- vaginal muscles of Allman) and above the last daughter-bud. Single mesodermal cells increase in size, and become changed into young egg-cells, while other mesoderm-cells become grouped round them as the follicular epithelium. One embryo only develops in each individual (Keinhard, Kraepelin). Fertilisation and the commencement of cleavage take place within the egg follicle. Later, however, the young embryo passes into a peculiar brood-sac (ooecium. Fig. 15 x, 16 o), which functions as a uterus, and in which the further embryonic development takes place. The ooecium, when it first appears (Fig. 15 x), closely resembles a young polyp-bud, being a pocket-like invagination of the bilaminar body- wall. The outer or meso- dermal layer of this bilaminar invagination is destined to envelop the embryo, while the inner ectodermal layer undergoes degeneration in the course of further development. It was formerly somewhat doubtful whether the ooecium was altogether distinct from the primary egg-follicle. According to Reinhard, it was derived directly from the epithelium of the ovary. Whereas, according to Metschnikoff, the egg passed out of the original follicle into the body- cavity, where the ooecium grew round it secondarily, according to Kraepelin and Braem, the Fig. 15.— Section through a portion of the zooecial wall of Plumatellafungosa (after Braem). ec, ectoderm ; at, mesoderm ; ov, ovary ; x, rudiment of the ooecium. rudiment of the ooecium presses closely on the egg while still in the ovary, and receives it from the egg- follicle. According to the statements of most authors, cleavage is quite regular. Kraepelin, however, thought that he found certain differences in the blastomeres in the first stages. A cleavage-cavity early appears within the embryo, and a coeloblastula (Fig. 16, A, e) develops, which, by a solid ingrowth of cells at one pole (Fig. 16, B), passes into the gastrula stage. The point of ingrowth, according to * [The form in which the development of the larva and of the polypide has been best worked out is Plumatella {Alcyonclla) fungosa (=P. polymorpha, Kraepelin).— Ed.] 34 BRYOZOA ECTOPROCTA. the statements of Davenport, Braem, and Kraepelin, invariably corresponds to that pole of the embryo which is turned towards the wall of the zooecium. At the same point, at a later time, the first rudiment of the polypide is said to develop, so that we have here an important contrast to the Gymnolaemata, in which, so far as we know, the primary rudiment of the polypide always develops at the aboral pole of the larva. While the ingrowing cell-mass more and more fills the blastocoele, a new cavity develops with- in the former; analogy with other animals might incline us to consider this cavity as the primary enteric cavity, but we are led by the study of the further course of develop- ment to regard it as the body-cavity. The ingrow- ing cell-mass represents the mesoderm, and the cavity that develops within it the coelom. We thus find here a gastrulation in which the entoderm proper is apparently want- ing. As the coelomic cavity increases in size, the mesodermal epithelial layer becomes closely pressed against the ectoderm, and in this way a bilaminar, vesicular embryo develops (Fig. 16, C). '-£ Fig. 16. — Three stages in the embryonic development of Plumatella (after Kraepelin). A, blastula stage in the ooecium. B, stage of ingrowth of the meso- derm. C, bilaminar stage, e, embryo in the blastula stage ; ec, invaginated ectodermal part of the ooecium (cf. Pig. 15) ; o, ooecium. We shall only be able to interpret this stage correctly if we compare it with the stage that follows the gastrula in most marine Ectoprocta (Fig. 6, G, p). The primary entoderm there yields the so-called central tissue which represents the enteric rudiment of the larva, out of which, however, important mesodermal organs of the primary zooecium are also derived. In the embryo of the Phylactolaemata this tissue is represented by the inner epithelial layer. We must here assume that the larval intestine has undergone excessive reduction, so that not a vestige of it can be seen, and, taking into consideration the further development, we must regard this inner layer as mesoderm, and the central cavity enclosed by it as the coelom. TYPE OF THE PHYLACTOLAEMATOUS LARVA. 35 We shall have to call the external cell-layer of the embryo the ectoderm, but it must not be forgotten that this layer contains indifferent embryonic material from which will be produced the future rudiment of the polypide, including the ontodermal organs. The next ontogenetic phenomenon is the development of the rudiment of the first polypide. The larvae of some fresh -water Bryozoa (Plumatella) at the free - swimming stage contain two well - developed polypides (Fig. 19 A), one of which, however, always seems to develop as the older primary polypide, and the other as a precocious daughter bud. The larva of Plumatella Jruticosa, on the contrary, contains, according to Allman, only one primary polypide, and in Cristatella also, the second polypide does not seem to develop so early as in the above- named form. The polypides (Fig. 17, p) arise as simple invagi- nations of the bilaminar wall of the embryo. Their further development will be described more in detail below (p. 37). The first indi- cations of the polypide - invagi- nation are found in the form of a simple thicken- ing of the body- wall. Daven- port states that, in Plumatella, the rudiment of the second polypide appears from tin- first independent of the primary polypide. The conditions in Cristatella are, according to Davenport, somewhat diiierent. We shall see below (p. 47) that in this form every new bud is intimately connected at its commencement with an older bud, and that the Fig. 17.— Two later ontogenetic stages of the embryo ot Plumatella (after Kraepelin). ec, ectoderm of the embryo ; /, mantle-fold ; m, mesodermal layer ; o, ooecium ; p, polypide-rudiment ; pi, placenta. 36 BRYOZOA ECTOPROCTA. inner budding zone of the polypide is derived from a layer of embryonic tissue capable of growth, lying below the ectodermal epithelium. The ectodermal layer of the polypide -rudiment in the embryo also is overgrown by the surrounding ectoderm. The ectoderm at this part is therefore bilaminar. The inner layer represents the tissue capable of producing buds, from which the inner layer of the polypide-rudiment is derived. From this tissue is developed the inner layer of the first polypide-rudiment, and soon after, by its side, the invagination of the second polypide, which thus, in Cristatella, is directly connected with the primary polypide. While the polypide-rudiments are developing, we find, in Pluma- tella, a zone of ectodermal cells encircling the middle of the embryo and fusing with the wall of the ooecium (Fig. 17, pty This is the girdle-shaped placenta already recognised by Korotneff, by means of which the embryo is suspended in the uterus-sac and nourished. In Cristatella this is wanting ; nourishment here takes place through the ectodermal cell-plug which closes the neck of the ooecium, and which is in close juxtaposition to the anterior pole of the embryo. In the later stages a circular fold of the body -wall develops (Fig. 17 B,f), which surrounds the anterior half of the body, and may be regarded as the equivalent of the mantle-fold of other Bryozoan larvae. An ectodermal thickening, including the posterior pole (Fig. 18 A, x), was regarded by Ostroumoff as a vestigial sucker, while the anterior swollen part of the body in which the polypide-rudiments appear may be considered as the equivalent of the retractile disc. Although, in this way, the larvae of the Phylactolaemata may, without undue forcing, be compared with those of the marine forms, a difficulty arises from the above-mentioned circumstance that the primitive gastrula- ingrowth here occurs at the anterior pole, which has been regarded as the equivalent of the retractile disc. We are unable at present to overcome this obstacle in the way of comparing the larvae of the Phylactolaemata with those of the Gymnolaemata. The embryo, after developing in the manner described above> becomes covered externally with cilia, and, escaping from the parent, swims about freely. According to Kraepelin, it emerges through the aperture of a degenerated polypide (usually that of the parent). Braem, on the contrary, holds that the cavity of the ooecium opens directly upon the exterior in order to allow of the passage of the embryo. At this stage the larva is oval (Figs. 18 ^4 and 19 A), and the whole surface of its body is ciliated. At the anterior pole there is an aperture leading into the large mantle-cavity, from the base of which the two polypide-rudiments project. The attachment of the larva takes place at first by means of! DEVELOPMENT OF THE POLYPIDE. 37 modified glandular ectodermal cells at the posterior pole (Fig. 18-4, x) — sucker-rudiment of Ostroumoff. The projection which carries the polypides is evaginated from the mantle -cavity, the mantle-fold (/) at the same time bending back (Figs. 18 B and 19 B). The posterior pole of the body now becomes detached from the substratum, the edges of the mantle-fold (/) approach each other till they fuse (Fig. 18 C), and in this way a closed sac forms iu which is included the greater part of the external ciliated surface of the larva, which then undergoes degeneration. The essential distinction between the metamorphosis here described and that of the marine forms consists in the absence of the sucker. In consequence of this a basal adhesive plate does not develop. The whole wall of the young colony is derived exclusively from that part of the body-wall which, in the larva, lined the mantle-cavity. The condition of the mantle-fold consequently here resembles that in the marine forms. / Fig. 18.— Three consecutive stages in the fixation of the larva of Plumatella (after Bkaem). /, mantle-fold (in B and C reflected); k, buds; p, the two first developed polypides ; x, glandular part at the posterior end of the body, by means of which the first attachment takes place. IV. Development of the Polypide. We must now describe more in detail the development of the polypide, i.e., of the retractile cephalic section of the animal plus the intestinal canal appended to it. For our knowledge of the metamorphosis of the larva and the development of the polypide in tin: primary zooecium, we are indebted chiefly to the observations of Repiachoff (No. 29) on Tenclra, of Barrois (No. 9) on Lepralia, and of Prouho (No. 28) on Flustrella. The development of this primary polypide takes place in exactly the same manner as that of the polypides in the buds, which develop later in the colony, or in those zooecia in which the alimentary canal and polypide, as 38 BRYOZOA ECTOPROCTA. already mentioned (p. 15), have undergone degeneration, the polypide having therefore to be regenerated. In the forms mentioned above the regeneration of the polypide has repeatedly been investigated; notably by Nitsche (Nos. 23 and 52), Repiachoff (No. 30), Joliet (No. 17), Haddon (No. 12), Ostroumoff (No. 26), and more recently by Seeliger (No. 37a), Davenport (Nos. 11 and 46a), Braem (No. 45a), Oka (No. 52a), and Kraepelin (No. 50). * The way in which the polypide develops in the statoblasts has been specially described by Braem (No. 45a) and Oka (No. 52a). '7-^< Fig. 19.— Two ontogenetic stages of Plumatella fungosa (after Nitsche). A, an advanced embryo. B, larva with two polypides in the act of attaching itself; the mantle-fold (J) is already reflected, the apertures of evagination have moved far apart and the polypides are retracted, p, the two first formed polypides ; k, k', buds of future polypides ; /, mantle-fold. We have already seen the rudiment of the polypide arising in the primary zooecium of Bugula (Fig. 12, B, p. 29) in the form of a double-walled sac, the inner cell-layer of which is said to be derived through invagination from the retractile disc (see, however, the statements of Prouho, p. 24), while the outer layer has probably arisen from the cells of the central tissue. In the same way the first rudiment of the polypides in the buds (/.-, k', Fig. 19), in the regenerated individuals, and in the statoblasts are bilaminar sacs, which arise through invagination at one point of the bilaminar body-wall (endocyst). In the marine Bryozoa and in Plumatella, such an invagination lias, from the first, a lumen which passes direct * Cf. Harmeii, Qiiart. Journ. Micro. Sci., Vol. xxxiii., p. 123. DEVELOPMENT OF THE POLYPIDE. 39 over into the lumen of the polypide-sac. In other cases, e.g., in Paludicella (Davenport), we find at first a solid ingrowth within which a lumen forms secondarily. The bilaminar sac always remains connected by a longer or shorter strand with that part of the body- wall at which the invagination occurred. This strand (only a portion of which is represented in Fig. 20, A and B), is derived from the neck of the primitive polypide invagination, becomes hollow again at a later stage, and then represents the aperture of the tentacular sheath (Fig. 20, C, ts). s Fig. 20.— Diagram illustrating the development of the polypide (median sections, after figures by J. Barrois and Nitsche). A, sac-like rudiment of the polypide. The separation of the mid-gut rudiment (d) is marked by a constriction. B, development of the tentacle-buds (t) and the oesophageal invagination (oe). C, older ontogenetic stage in connection with the endocyst (e, e). a, external, I, internal layer of the polypide-rudiment ; d, mid -gut rudiment ; e, e, endocyst ; m, mouth ; n, in- vagination for the formation of the ganglion ; oe, oesophagus ; r, retractor muscle ; t, tentacle ; ts, ten- tacular sheath ; a (in C), anus. The outer layer of the polypide-sac (Fig. 20, A, a) is continuous ith the mesodermal layer of the body-wall (endocyst), and yields the mesodermal part of the polypide (the lining of the body-cavity, and the larger groups of muscles, &c). The inner layer (Fig. 20, A, i) is derived from the primitive ectoderm, and yields the ectodermal epithelium of the whole polypide and of the tentacle sheath, the nervous system, and the lining epithelium of the whole of the digestive tract. We should be justified in calling this layer the ecto- derm of the polypide, did not the mid-gut epithelium also originate from it.* * [Braem, in a recent work on the development of Plumatclla (Bibl. Zool., H. 23, 1897), states that entodermal tissue can be made out in the embryo. — Ed.] 40 BRYOZOA ECTOPROCTA. According to the more recent researches of Seeliger, Davenport, Kraepelin, Braem, and Oka on the development of the buds, there can be no doubt that the inner layer of the bilaminar sac is to be derived by invagination from the ectodermal layer of the parent, the outer layer, on the contrary, owing its origin to the mesoderm of the latter. Some of the earlier observers had derived the whole bud from the ectoderm of the parent (Nitsche, Ehlers, and Claparede), while Haddon held that all the three germ-layers of the parent take part in the formation of the bud. According to Joliet (No. 17), on the contrary, the first rudiment of the polypide in Eucratea is derived exclusively from the funicular tissue (endosarc), i.e., from a layer which we regard as belonging to the mesoderm. The first rudiment of the polypide is said here to consist of a mass of similar cells, which become arranged only secondarily into two layers, the inner cells acquiring an epithelial character and surrounding a central cavity, while the superficial cells yield the outer layer. The first change which can be remarked in the bilaminar polypide- sac consists in the formation of a diverticulum (Fig. 20 A, d ; 23 B, d), which represents the first rudiment of the enteric canal, especially that of the stomach and hind-gut. The origin of this diverticulum can be traced to a constriction or infolding of the wall of the primitive sac taking place from each side. In conse- quence of this, the lumen of the enteric rudiment at first remains connected along its whole length through a narrow slit with the lumen of the rest of the sac. As the intestine becomes more differentiated this slit becomes narrower, but never completely closed, for it persists as the anal aperture (Fig. 20 C, a). In the following stages a second diverticulum develops at the opposite (oral) side of the polypide-sac (Fig. 20 B, oe) ; this diverticulum, which represents the rudiment of the oesophagus, grows out towards the blind end of the first diverticulum and fuses with it (Fig. 20 C). After communication has been established at the point where the two diverticula come into contact, the intestinal canal is essentially complete, the so-called stomach-caecum developing at a later period. The description here given of the development of the enteric canal is founded upon the statements of Barrois (No. 9), Prouho (No. 28), Braem (No. 45a), Davenport (Nos. 11 and 46a), and Kraepelin (No. 50). A modification of tli is process was described by Nitsche (No. 23) in Plumatella and Flustra membranacea, and more recently by Ostroumoff (No. 26), the formation of the intestine taking place through two ingrowths which extend into the interior of th«- sue-like polypide-nuliment, from right and left, meeting and fusing in the median plane. The apertures left in front of and behind this fusion become the <»ral and anal apertures. "This process resembles what takes place if we bold .ui Indiarubber ball with a double wall in our two hands and press it on Idfl with a finger until the finger-tips are separated only by the four-fold wall of the ball." The oesophagus in this case would not have an independent origin, Seeliger observed a similar origin for the alimentary canal in the buds DEVELOPMENT OF THE POLYPIDE. 41 of Bugula. It should here be pointed out that the two different types of development of the intestine are not fundamentally opposed to one another ; in both cases a constriction pressing in from each side brings about the separation of the enteric canal from the atrium. Whether the rudiment of the oesophagus is completed at the same time is of comparatively little importance, and we may well imagine, even in nearly related forms, that in some cases the first and, in others, the second type of bud- development may be found. In Pectinella, on the contrary (Oka, No. 52a), in the statoblasts as well as in the buds, the diverticulum that forms first is said to represent the common rudiment of the oesophagus and the stomach, so that here the oral aperture seems to form first. The intestine is said here to grow out as a caecum from the stomach and to open towards the atrium (the upper cavity), the anal aperture seeming to form in this way. The ganglion can be traced back to an invagination which forms at the base of the upper cavity or atrium between the oral and the anal apertures (Fig. 20 C, n). The lumen of this invagination gives rise to the future brain-cavity. After the rudiment of the ganglion has been completely cut off from the surface layer, an ear-shaped diverti- culum forms on each side of it (Braem) ; these diverticula represent the rudiments of the lophophoral nerves. Each grows out as two nerves, one of which runs posteriorly into the corresponding arm of the lophophore, while the other runs forward and spreads out upon the oesophagus. The portion of the primary sac which remains after the intestine has been separated from it is known as the atrium (Fig. 23 B, at) or the cavity of the tentacular sheath. The greater part of its wall becomes modified into the tentacular sheath. The rudiments of the lophophore and of the tentacles (Fig. 23 B, at) develop early in the base of this cavity. The first rudiment of the lophophore takes the form of a swelling projecting into the atrium, and forming a semicircular border to the oral aperture. In the Gymnolaemata, this swelling closes in front of the anal aperture to form a ring, encircling the mouth, which carries the tentacles. Corresponding to the inwardly projecting swelling of the lophophore, there is, on the outer side of the polypide-sac, a groove which soon changes into a closed canal. This is the so-called circular canal, which must be regarded as part of the body-cavity. The tentacles arise as out- growths of the lophophore resembling the fingers of a glove. In the Gymnolaemata they are situated in a row on either side of the body (Davenport, Prouho). Only later do these two rows become connected through the development of the tentacle-buds in front of the mouth, while the lasfr tentacles to arise close the ring on the anal side. 42 BRYOZOA ECTOPROCTA. In the Phylactolaemata the posterior ends of the semicircular rudiment of the lophophore grow out as large finger-like structures (Fig. 23, C, I), projecting into the atrium. These are the rudiments of the two arms of the lophophore. The cavities within these processes (lophophore-cavities) are to be regarded as a part of the body-cavity. They communicate with one another through the semicircular canal that encircles the oesophagus. On the anal side, according to Braem, they are connected by the so-called forked canal, so that here also the system of lophophore-cavities forms a ring surrounding the oesophagus. Another significance has been attributed to this forked canal by Cori (p. 56). In the Phylacto- laemata the outer tentacles make their appearance before those on the inner side of the lophophore. The former appear first in the neighbourhood of the mouth and gradually fresh tentacles are added, each new one being nearer the free end of the lophophore, those at the apex forming last. The tentacles of the inner row develop in the reverse order, i.e., those nearest the apex arise first, whilst those near the epistome and above the forked canal are the last to appear. The epistome arises as a projecting fold on the anal side of the oral aperture, and into it is continued an outgrowth of the body- cavity — the epistomal cavity. ,It should be mentioned that several authors (Seeliger and others) maintain that a rudimentary epistome is to be found in the Gymnolaemata. The outer or mesodermal layer of the polypide-rudiment gives rise to the following parts : the peritoneal covering of the enteric canal, the muscle-layer of the intestine, and some of the body-muscles, especially the retractors. The development of the latter has been studied chiefly by Braem and Davenport. Groups of mesoderm- cells become detached at the neck of the bud, and become inserted at one end upon the polypide and at the other on the zooecial wall. These rudiments of the retractor muscles are originally inserted at a point on the zooecial wall quite near the neck of the bud, and only at a later stage, as the wall develops, do the points of insertion shift further from the aperture of the polypide- rudiment. In the Phylactolaemata, the outer mesoderm-layer of the polypide- biid ;ilso takes aiL important part in the development of the funiculus, which will Ik- f the four larval tufts of setae come to lie on the inner surface of the mantle (Fig. 32). The setae now soon fall off (Fig. 33), and, in those forms which, in the adult condition, possess setae on the margin of the mantle, arc replaced by the permanent setae. In METAMORPHOSIS. 69 Argiqpe, these latter are wanting. The two shell-valves now soon form on the outer surface of the mantle-lobes as cuticular secretions. The pedal region of the larva becomes the peduncle of the adult, and two large groups of muscles which can be recognised even in the larva (Fig. 32 B) become changed into the ventral peduncular muscles. In Liothyrina and Terehratulina there is also a pair of dorsal peduncular muscles in the larva. The two pairs of muscles which end at the bundles of setae become the shell-adductors. The pair of muscles which, in Fig. 32 B, lies on either side of the posterior end of the digestive tract, represents the rudiment of the divaricators. This pair divides later into a pair of dorsal and a pair of ventral divaricators. Fig. 31.— Stages in the attachment and metamorphosis of the larva of Terehratulina (after Morse), b, tufts of provisional setae ; c, cephalic region ; th, thoracic region ; p, peduncular region of the body. The metamorphosis of the cephalic region of the larva is the most obscure. In comparing this larva with that of Phoronis, we vshould expect that this section would give rise merely to the integu- mental fold above the mouth (epistome) and to the supra-oesophageal ganglion (see diagram, Fig. 34). According to Kowalevsky, however, it appears, on the contrary, that the rudiment of the oesophagus (Fig. 33 A, oe) develops as an ectodermal invagination in the region of the cephalic lobe, and the latter, with its eye-spots, is for some time longer recognisable within the attached larva (Fig. 33 B). If this is the case, we should have to attribute to the ciliated ring, which runs round the cephalic lobe of the larva, a postoral position. 70 BRACHIOPODA. It is, however, not impossible, indeed, taking into account the dorsal position of the lophophore-rudiment, it is probable that even before the oesophageal invagination appears in the cephalic lobe of the attached larva, certain displacements have taken place which alter the primitive conditions (cf. Fig. 34). t!UJff/ il . \H K —4- — . Yin, B Fio. 32.— Two stages in the metamorphosis of Argiope (after Kowalevsky). b, provisional setae ; d, rudiment of the mid-gut ; k, cephalic lobe ; m, mantle-fold ; st, peduncle. The rudiment of the lophophore (Fig. 33 A, t) arises in the form of an almost circular thickening on the inner surface of the dorsal mantle-lobe. As the mouth is later encircled by the lophophore, this circular thickening of the cephalic lobe must extend ventrally. The first tentacle-rudiments (t) are soon seen in the form of four swellings. At a later stage these develop into hollow tubular outgrowths METAMORPHOSIS. 71 (Fig. 33 B), their number being increased by the addition of new rudiments, the point at which the budding of tentacles first takes place being the most anterior or distal part of the lophophore, which later becomes its dorsal part. K B ~a St Fig. 33.— Two older ontogenetic stages of Argiope (after Kowalevsky). a, eye-spots ; gut ; oe, oesophagus ; st, peduncle ; t, tentacle-rudiments. raid- It may at first sight appear remarkable that the tentacles grow out on the inner side of the mantle-lobe. But if we consider the free-swimming larva, in which this inner side still functions as the outer side of the thoracic section, we shall see that the crown of tentacles here has a similar post-oral position as in the Adino- trocha (cf. diagram Fig. 34, t). K -.. Fig. 34.— Hypothetical scheme of the metamorphosis of the Brachiopoda for comparison with the Actinotrocha. A, free-swimming larva. The oral aperture (m) is depicted, although not actually present at this stage. The position in which it here appears, below the cephalic lobe (A;), is not in accordance with Kowalevsky's statements. The tentacle-buds also are not actually present at this stage. B, an imaginary transition stage. C, younger Brachiopud after the reversal of the two mantle-lobes. The epistome has arisen from the cephalic lobe (A:). The row of tentacle-buds (0 belongs for the most part to the dorsal lobe of the mantle. d, dorsal lobe of the mantle ; ep, epistome ; k, cephalic lobe ; m, mouth ; st, peduncle ; /, tentacle-buds ; v, ventral lobe of the mantle. 72 BRACHIOPODA. The later transformations in the lophophore have been investigated in Twebratulina by Morse. The lophophore here is originally circular, but, later, the anterior edge becomes indented. In this indentation, close to the median line, new tentacles form. The tentacular apparatus, which in this way has become horseshoe- shaped (Fig. 35), resembles the similar organ in Phoronis and the Phylactolaemata, this resemblance being heightened by the fact that here also a dorsal fold, the lip (e), can be seen over the mouth; this must be regarded as the epistome which, in later stages, is continued along the whole length of the row of tentacles. The mouth (m) lies in a ciliated furrow, the buccal groove, between the ventral row of tentacles and this fold, this furrow being con- tinued on to the arms as the brachial groove. The two processes Fig. 35.— Two ontogenetic stages of the lophophore of Terebratu- lina septentrionalis (after Morse), a, outer, b, inner arm ; d, alimentary canal ; e, epistome ; I, hepatic appendage ; m, mouth. of the horseshoe -shaped lophophore develop into the large lateral arms of the adult, while the little, spirally-coiled, inner arms (b) grow out only later in the dorsal indentation. The details of the development of the lophophore in later stages must vary on account of the varying shape of the adult organ in the different forms; but on this point we are still without accurate information. In Argiope, the lophophore retains the simple original horseshoe -shape throughout life. Both this form and Tltecidium show a primitive condition, inasmuch as the connection of the arms with the dorsal mantle-fold is per- manently retained. With regard to the external alteration of form in the attached larva, it should further be mentioned that, according to Morse, the ECARDINES. 73 youngest stage of Terebratulina septentrionalis strongly recalls the shape of the shell in Megerlia and Argiope. Later, a stage develops which, on account of the long, flattened shell- valves and the long peduncle, strikingly recalls Lingula, and this leads finally to the adult form. The only details known as to the development of the inner organs are such as are easily understood. The division of the intestine into sepa- rate sections and the development of the hepatic tubes as lateral diverticula of the anterior section of the intestine (Fig. 35 A, I) are among these, as also the rise of the nervous sys- tem from thick- enings of the ectoderm. The original connec- tion of the cen- tral nervous sys- tem with the ectoderm in va- rious forms of the Brachiopoda is, as in Phoronis, retained through- out life. Fig. 36. — Lingula larva (after Brooks), a, anus ; d, dilated anterior section of the intestine ; d1, narrowed terminal section of the same ; e, epistome ; m, mouth ; oe, oesophagus ; *-, rudiment of peduncle ; t, unpaired tentacle ; t', youngest tentacle-buds. II. Ecardines. The embryonic development of the Ecardines is up to the present unknown. Only the pelagic larvae and the youngest attached forms are known, the metamorphosis, briefly noticed by F. Muller and McCrady, having been chiefly investigated by Brooks in Lingula (No. 5). The most striking feature in the metamorphosis of this group is, that the pelagic larvae are at a very advanced stage of development which, in the Testicardines, is reached only after attachment. 74 BRACHIOPODA. The youngest larva of Lingula observed by Brooks was already enclosed in two flat, discoidal shell-valves, not articulating with one- another, but covering the animal dorsally and ventrally, their edges being free all round (Fig. 37, s and t). The anterior part of the mantle- cavity is occupied by the disc-like lophophore beset with tentacles (Figs. 36 and 37), in the centre of which can be recognised the oral aperture (Fig. 36, m) and an epistomal fold (e) overhanging it. In the lophophore, a striking feature is the presence of a dorsal, impaired tentacle (t), at the sides of which the youngest tentacle -rudiments grow out (Fig. 36, t'). The actual body of the animal is- small, and contains a body-cavity which is not spacious, an alimentary canal, and a few groups of muscles. Among these latter, we notice first a pair running near the oesophagus (oe) and an unpaired bundle extending from shell to shell. The alimentary canal at first appears divided into the bent oesophagus (oe), a dilated stomach (d), in connection with which can soon be seen the hepatic diverticula and a posterior intestinal outgrowth (d'), which bends round anteriorly and fuses with the body-wall on the right side, at which point the future anal aperture soon arises (Fig. 36, a). Other features to be noted in the youngest Lingula larva are the complete absence of the rudiment of the peduncle and the presence of a remarkable semicircular skeletal plate, which lies below the dorsal shell-valve and is con- nected with that valve. In the larva described by F. Muller (Fig. 38), which perhaps belongs to Crania, five pairs of strong provisional setae could be seen, the larva creeping by the help of these and the lateral movements of the shell -valves. In swimming, the shell -valves are opened and the lophophore is extended far beyond the shell, the swimming movements being brought about by the cilia covering the tentacles. The rudiment of the nervous system can be recognised in the form Fig. 37. — Diagrammatic median section through the Lingula larva (after Brooks, from Bal- four's Text -book), a, margins of the shell- valves ; b, thickened margin of the mantle ; c, mantle ; d, median dorsal tentacle; e, lophophore ; /, epis- tome; g, mouth; h, mantle - cavity; i, body- cavity; k, wall of the oesophagus; I, oesophagus ; m, hepatic chamber of the stom- ach; n, intestinal cham- ber of the stomach ; o, hind -gut; q, ventral ganglion; r, posterior muscle ; s, dorsal, t, ventral shell-valve. CHANGES IN THE SHAPE OF THE SHELL. 75 uf a ring surrounding the oesophagus; in this ring are found a ventral ganglionic thickening, two lateral ganglia and two dorsal otocysts. In Franz Muller's larva (Fig. 38), paired eye-spots (a) and auditory vesicles (o) were also observed. These sensory organs degenerate in the course of further development. The rudiment of the peduncle now arises at the posterior end of the body, and soon grows to a considerable length; by means of this the attachment of the larva takes place. In the further course of metamorphosis the lophophore assumes its final shape ; characteristic changes occur in the form of the shell-valves, and the mantle-sinuses arise as horn-shaped diverticula growing out from the body-cavity, while the rows of setae found in the adult appear on the margin of the mantle. III. Changes in the shape of the Shell. Certain transformations undergone by the shell of Terebratulina in the course of ontogenetic development were pointed out by Morse, who drew attention to the fact that the shell in this form, during its development, passes through stages in which it resembles the shells of Megerlia or Argiope (cf. p. 73). These developmental changes of the shell have recently been investigated by Beecher (No. 1), who proposes to name the first rudiment of the shell of a newly metamorphosed larva, which consists of a horny, chitinous, or cuticular secretion, the protegulum. The most primitive and the most widely distributed form of protegulum is a semi-circular or semi-elliptical plate (Fig. 39, p) with a straight hinge-line which, in length, corresponds to the greatest breadth of the plate. The shape of the protegulum may be modified in individual cases, the future form of the shell then exercising an influence. Only in rare cases does the shell, after, increasing in size by the formation of new layers, retain the original shape of the protegulum, the zones of growth here running parallel with the periphery. An example of this is afforded by Paterina (Obolus) labradorica. Other forms (e.g., Orbiculoidea) pass through a Paterina-\ike stage* in their ontogeny (Fig. 39 ^4). The phases through which the shell-valves pass in later stages are connected with the relative length of the peduncle, with the angle formed by the longitudinal axis of the animal and the substratum, and with the consequent greater or slighter mobility of the body. The manner of insertion of the peduncle is also of significance in connection with the transformations in the form 76 BRACHIOPODA. Fig. 38. — Pelagic Brachiopodan larva of Desterro, swimming by means of the extended lophophore (after F. Mdller). a, eye-spots; b, pro- visional setae ; e, epistome ; o, oto- cyst ; p, internal skeletal plate. of the shell. In Lingula, for instance, the peduncle projects as a prolongation of the longitudinal axis of the body between the posterior ends of the shell-valves, which consequently are of some- what the same shape. In most other forms, the peduncle is inserted more in the region of the ventral valve, the consequence being that, as it shortens, the two valves come into close but dissimilar relation to the substratum. Here we may perhaps find the reason for the valves having become more and more heteromor- phous. As a rule, forms in which the peduncle is long and the body com- paratively movable, have long and posteriorly pointed shells with a short hinge-line {Terebratulina). A shorten- ing of the peduncle brings the posterior end of the body into contact with the substratum, and this leads to the development of broad forms with lengthened hinge-line (e.g., Argiope, Terebratella). When, as in the Discinidae, the position of the body is strictly horizontal, the ventral valve resting on the substratum, and the peduncle emerging from an aperture near its centre, the shell resembles a circular disc (Fig. 39 B). Here, as in Anomia, among the Lamellibranchiata, there is a tendency to the development of a radial type of shell as a consequence of the attached manner of life. In the Oyster-like forms that fuse with the substratum (Thecidium, Crania), the diver- gence in the shape of the two valves reaches its highest limit. In one member of the Product id nc (Probo8cidella), the ventral shell-valve becomes lengthened by the growth of its frontal and lateral edges in such a way as to form a calcareous tube resembling that of Aspergillum. This form of shell may perhaps have arisen in adaptation to a boring or digging habit. A ,/> £ Fio. 39.^Two ontogenetic stages of the shell of Orbiculoidea minuta (after Beecher). A, Pater- ina-stage. B, older stage, p, protegulum. CHANGES IN THE SHAPE OF THE SHELL. 77 With regard to the- passage of the peduncle out of the shell, Beecher has distinguished four different types : I. Atremata. The peduncle passes out simply between the posterior edges of the two valves, retaining the direction of the axis of the body. Lingula, Obolus, Paterina. II. Neotremata. The peduncle arises from the ventral shell-valve and stands at right angles to the axis of the body. It lies either in an incision of the ventral valve (Schizomania), or becomes surrounded by the ventral valve as it grows, so that it then emerges from a subcentral orifice in the valve. Orbiculoidea, Discina, Acrothele. This group is perhaps nearly related to the Craniidae. III. Prototremata. The peduncle arises from a triangular del- tidial slit in front of the beak of the large shell, which may either be open (Orthis, Tropidoleptus) or closed by a pseudodeltidium (Orthisina, Leptaena, Strophomena, Chonetes, Strophaeodonta). The Tliecidiidae also belong here. IY. Telotremata. The peduncle arises from a foramen perfo- rating the beak of the ventral valve. The orifice from which it emerges is, as a rule, surrounded by a paired deltidium (Spiriferidae, Atrypidae, Khynclionellidae, Stringocephalidae, and Terebratulidae). In these groups a calcareous brachial skeleton develops. With regard to the development of the closing pieces of the beak-orifice known as pseudodeltidium and deltidium., little has been discovered. Beecher, following up Kowalevsky's observations on Thecidium, traces back the pseudodeltidium to a skeletal secretion of the peduncle. During the metamorphosis of the attached larva, the two folds of the mantle are reflexed anteriorly (Fig. 40 A) and yield the first rudiment of the shell (the protegulum, ds and rs, p. 78), the line at which the reflexion of the valves took place indicates the hinge-line (s) of the protegulum. In Thecidium, the dorsal side of the peduncle -rudiment now also becomes covered with a skeletal secretion (p), which in later stages fuses with the growing beak of the ventral valve (Fig. 40 B and G) and becomes the pseudodeltidium. The true deltidium of the Bhynchonellidae and the Terebratulidae, on the contrary, is probably to be derived from the reflected parts of the mantle of the ventral shell-valve itseli The two shell-valves, when they first appear, are completely separated from one another by the peduncle that emerges between them. They come into contact first at the outermost ends of the straight hinge-line (Fig. 36, p. 73), and at this point the first 78 BRACHIOPODA. rudiments of the hinge-teeth develop. These already occupy the position they retain throughout life, lying at either side of the original hinge-aperture, and thus, later, near the deltidium. When the hinge -line, at a later stage, lengthens out laterally over the teeth, as is specially the case in Orthis, Spirifer, and Strophomena, this is a consequence of secondary processes of growth. ds Fig. 40.— Development of the pseudodeltidium in Thecidmm mediterraneum (after Beecher). A, metamorphosed larva of Thecidium with the first rudiments of the shell (after Kowa- levsky). B, adult Thecidmm, seen from the dorsal side. C, the same, seen in profile. ds, dorsal shell-valve ; k, cephalic section ; p, pseudodeltidium ; s, hinge-line ; st, peduncle ; us, ventral shell-valve. IV. General Considerations. A serious obstacle to our comprehension of the pelagic Brachio- podan larva, as represented typically by the Argiope larva, is to be found in the circumstance that the oral and anal apertures are here wanting, their absence making the orientation of the body-surfaces and segments extremely difficult and uncertain. We are inclined to homologise the ciliated ring at the edge of the cephalic section with the pre -oral ciliated band of the Annelidan Trochophore, although Kowalevsky's observation as to the rise of the oesophagus (p. 69) seems to contradict this view. The position of the neural plate would be indicated by the eye-spots (and in Terebratulina, Fig. 31 B and 0, by a long ciliated tuft) on the cephalic pole of the larva. The anterior section of the body would thus possess characters in common with the Trochophore, and would be com- parable to the cephalic lobe of the Actinotrocha. Tn considering the significance of the posterior section of the body, we must institute comparisons with the larvae of the Bryozoa and of Phoronis. There can be no doubt that the point at which attachment takes place is identical in the Bryozoa and Brachiopoda, and corresponds to the posterior end of the body in the adult I'/">roni8. The fixation of the body is accomplished by means of the posterior pole of the body in the Brachiopodan larva; the pedal GENERAL CONSIDERATIONS. 79 section passes over into the peduncle of the adult. Consequently, the cavity which is surrounded by the posteriorly projecting mantle- fold of the Brachiopodan larva is to be homologised only with the •cavity within the so-called sucker of the Bryozoan larva. In the Bryozoa, also, that point of the sucker at which fixation first takes place forms a conical invagination in the main mass of the sucker (cf. the larva of Bugula, Fig. 9 B, p. 26). This conical portion would thus directly correspond to the pedal region of the Brachiopodan larva. This whole structure is evidently the equivalent of the invaginated ventral tube of the Actinotrocha. The aperture of this tube in the larva of Phoronis would be comparable to the aperture ■of the mantle-cavity in the Arglope larva. We can therefore without ■difficulty compare the Brachiopodan larva with an Actinotrocha in the later stages, in which the above tube-like invagination has already formed. The principal distinction between the two is the •complete absence of the anal region in the Brachiopodan larva. Here also metamorphosis commences (as in Phoronis and the Bryozoa) by the evagination of the formerly inturned part of the body through which fixation takes place. We thus find that a comparison of larval forms leads us to regard the Brachiopoda as nearly related to Phoronis and to the Bryozoa, and that a comparison of the anatomy of the adult Brachiopoda with that of adults belonging to the two other groups gives a general confirmation to this view. The agreement between these groups is specially clear if we compare the tentacle -bearing lophophore in Phoronis, the Phylactolaemata, and the Brachiopoda. In the latter also, the lophophore is originally horseshoe -shaped, there is an integumental fold known as the epistome (Fig. 41, ep) above the mouth (brachial fold), and a lophophore -cavity not in any way communicating (?) with the rest of the body-cavity. We shall have to regard the so-called small brachial sinus (Fig. 41, a) of the Brachiopoda as the equivalent of the lophophore-cavity proper of the Phylactolaemata (circular canal of the Gymnolaemata) while the large brachial sinus (b) perhaps corresponds to the epistomal cavity.* Certain differences between the cavity in the lophophore * In comparing the large brachial sinus with the epistomal cavity of the Phylactolaemata, we shall have to hear in mind that the large arm-sinus, as has recently been pointed out by Blochmann in Crania, is a paired structure, continuous in the median plane above the oesophagus. The median section (Fig. 41 B) represents the condition of the spiral arms laterally to the median plane. If we bear in mind the above-mentioned reservations, however, we shall De able to recognise in this diagram the agreement of the large brachial sinus with the epistomal cavity. 80 BRACHIOPODA. of the Brachiopoda and that of the Phylactolaematous Bryozoa have been pointed out by Blochmann (No. 4). The chief of these is that, in consequence of the extension of the epistomal cavity (larger brachial sinus) in the Brachiopoda, the lophophoral cavity is so circumscribed as to be reduced to a canal running below the row of tentacles (smaller brachial sinus) ; this canal then appears to be cut through twice in transverse sections (a, a!) through the lopho- Fig. 41.— Diagram illustrating the structure of a Brachiopod (after preparations of Terebratdla corianica; cf. Figs. 5, p. 9, and 28, p. 57). A, lateral view of the lophophore. Most of the tentacles are supposed to have been cut away. B, median section. The spiral arms are cut somewhat laterally to show the large brachial sinus, a, smaller brachial sinus ; a', the same in dorsal transverse section; b, larger brachial sinus (epistomal cavity); ds, dorsal shell- valve ; ed, hind-gut ; ep, epistome (brachial fold); ep', the same in dorsal transverse section ; h, heart; TO, mouth; mg, stomach; ms, mesentery; mt, mantle-fold; n, nephridium ; oe, oesophagus ; s, lateral arm ; sg, supra-oesophageal ganglion ; sp, spiral arm ; st, peduncle ; (, ventral tentacle ; t', dorsal tentacle ; vg, ventral ganglion ; vs, ventral shell-valve. phore-arms, as also does the epistomal fold (brachial fold ep, ep'). These differences are, indeed, very easily explained through the higher differentiation which has arisen from the simpler type retained in the Phylactolaemata. From the condition of the lophophore and the position of the epistome it follows that the shell-valve, known as the dorsal valve, corresponds in position to the anal side of the body in the Bryozoa, GENERAL CONSIDERATIONS. 81 although we should not be able to conclude this from the flexure of the alimentary canal (Fig. 41 B). We must here briefly return to the position of the anal aperture in the Ecardines. In Lingula and Discina the anus lies on the right side of the body, in Crania, as Joubin and Blochmann have proved, in the median plane at the posterior end of the body. Blochmann is inclined to regard this as a primitive condition. Since we, however, with Caldwell, regard the posterior end of the body in the Brachiopoda (in view of the conditions prevailing in Phoronis) as in reality belonging to the ventral side of the body, we must consider the position of the anus in Crania also as secondarily modified. As a rule, in the Brachiopoda, the intestine is bent in the direction opposite to that which it assumes in the Bryozoa (cf. Fig. 41 B, with Fig. 28, p. 57). The external apertures of the nephridia also appear to have undergone shifting in the ventral direction. Such changes in the position of individual organs may, however, easily be explained by the crowding of the body between the two shell- valves. Although these displace- ments are a certain obstacle in the way of comparing the Brachiopoda with the Phylactolaemata and Phoronis, the general agreement between these forms (which is specially marked in the body-cavity and the nephridial system) is so great, that we can hardly doubt their close affinity. On this subject we refer the reader to the statements of Blochmann. Since Steenstrup and others first pointed out that the Brachiopoda are in no way related to the bivalved Mollusca (Lamellibranchiata), repeated attempts have been made to ally them more nearly to the Annelida, by laying undue weight on the agreement that exists between the two groups with respect to the condition of the body- cavity, genital organs, and nephridia in the Brachiopoda, and to regard them, when possible, as Annelida adapted to a sedentary manner of life. Morse thus calls them "ancient cephalized Annelids," distinguishing them in this way from the sedentary Polychaetes (e.g., Serpula), which he names "modern cephalized Annelids." We must therefore refer briefly to those characters which have been claimed as evidences of segmentation. The division of the body of the larva into three or four regions is purely external, and, as far as is yet known, does not affect the coelomic sacs. Balfour pointed out that the order of appearance of the segments in the Brachiopodan larva differs from that prevailing in the Annelida. In the latter, the first segments to form are cut off from the posterior end of the body, while in the Brachiopoda the G 82 BRACHIOPODA. first segments become abstracted from the anterior part of the body. A consideration brought forward by Caldwell seems of still greater importance. It is evident that in Adinotrocha the principal axis of the larva differs from that of the adult. In the larva, this axis runs from the neural plate to the anal aperture, and corresponds to that of the Trochophore and of the Annelid that develops from the IVochophore. In Phoronis, after metamorphosis, on the contrary, a secondary principal axis is met with at right angles to the primary axis of the Actinotrocha. The principal axis of the Brachiopodan larva, however, corresponds to the secondary axis of Phoronis^ and runs from the neural plate to the point of attachment, which must be imagined as lying in the middle of the ventral side proper. When, therefore, segmentation takes place in a direction transverse to this principal axis, this cannot be compared to the segmentation of the Annelida, because the position of the latter is determined by the direction of another principal axis (the primary axis of an Actinotrocha). This example shows that the comparison with the Actinotrocha is of great utility in forming conclusions as to the Brachiopoda. If we believe in the comparison of the Brachiopoda with Phoronis and the Bryozoa, in which two groups the short line lying between the mouth and the anus is to be recognised as the dorsal middle line, we shall be in agreement with Caldwell, who considers that the two shell- valves in the Brachiopoda strictly speaking belong to the ventral side, and that here also attachment takes place at the middle of the ventral side. We have seen that in the Brachiopodan larva there is no true segmentation, and this is also the case with the adult. The presence of two pairs of segmental organs in Rhynchonella is the strongest evidence that can be brought forward in favour of segmentation. Yet we know that in the Annelida more than one pair of segmental organs may occur in a segment, so that too great stress should not be laid on this feature alone. In the same way two pairs of nephridia also appear in Phoronis australis (Benham).* There are many reasons for believing that the Ecardines must be regarded as the most primitive forms of the Brachiopoda. In this division a lateral anal aperture is retained in Lingula, and one lying posteriorly in the median plane in Crania. The peduncle in Lingula • [While pointing out that each of the two nephridia has a double opening into tin- body-cavity, BENHAM never commits himself to the assertion that there •ii. two pain <•!' nephridia. He considers Phoronis as more closely allied to the Gephyrea than to the Bryozoa.— Ed.] GENERAL CONSIDERATIONS. 83 is said often to be unattached and sunk into a sand-tube, so that in respect to this habit there is a marked similarity to Phoronis. It should here be mentioned that a few zoologists have assumed a near relationship to exist between the Chaetognatha and the Brachio- poda, chief among these is van Bemmelen (No. 2), and the view has also been advocated by Butschli,* 0. and R. Hertwig (coelom- theory). These assumptions are based upon the agreement in the first stages of development, the rise of the coelom by folding, the development of three body-segments, and on those anatomical features common in the two groups as a consequence of the similar origin and nature of the enterocoele. The adult forms differ altogether in manner of life, and show essential differences of structure, the cephalic section especially being developed in an altogether different way in the two groups. Further, the Brachiopoda, in consequence of the agreement of the larva with the Actinotrocha, seem to be allied to the Trochophore type, while no such affinity has as yet been propounded for the Chaetognatha. The establishment of relationships between these two groups seems therefore to be still very doubtful, and their structural resemblances appear to be mere analogies. In any case it would only be possible to admit some very remote connection between the two, LITERATURE. 1 Beecher, C. E. Development of the Brachiopoda. American Journal of Science. Part L, Vol. xli., 1891. Part II., Vol. xliv., 1892. 2. Van Bemmelen, J. F. Untersuchungen liber den anatomischen und histologischen Bau der Brachiopoda Testicardines. Jen. Zeitschr. f. Naturw. Bd. xvi. 1883. 3. Blochmann, F. Vorlaufige Mittheilung iiber Brachiopoden. Zool. Anz. Jg. viii. 1885. 4. Blochmann, F. Ueber die Anatomie und die verwandtschaft- lichen Beziehungen der Brachiopoden. Archiv. des Vereins der Freunde der Naturgesch. in Mecklenburg. Jg. xlvi. 1892. Cf. also Blochmann, F., Untersuchungen iiber den Bau der Brachiopoden. Jena. 1892. 5. Brooks, W. K. Development of Lingula. Chesapeake zoolog. Laboratory, Scientif. ResuHs of the Session 1878. Baltimore. tf. Hancock, A. On the Organisation of Brachiopoda. Trans. Roy. Soc, London. Vol. cxlviii. 1858. * Zeitschr. f. Wiss. Zool., Bd. xxvi. p. 393, footnote. 84 BRACHIOPODA. 7. Kowalevsky, A. On the Development of the Brachiopoda. Congr. of Russian Naturalists, Kasan, 1873. Sect. Anat. Phy. and Comp. Anat. (Russian. Ref. by Hoyer in Jahresb. f. Anat. und Phys. 1873.) 8. Kowalevsky, A. On the Development of the Brachiopoda. Izvyest. imp. ObsKch. Lyubit. Estestv. Antrop. i. Ethnog. Vol. xiv. Moscow, 1874. (Russian. Ref. in Journ. Amer. Sci. and Arts. 1874.) 9. Kowalevsky, A. Observations sur le developpement des Brachiopodes. Analysis by MM. Oehlert and Denicker. Archiv. Zool. Exper. (2) Tom. i. 1883. 10. Lacaze-Duthiers, H. de. Histoire cle la Thecidie. Ann. Sci. Nat. (4) Tom. xv. 1861. 11. Morse, E. S. On the Early Stages of Terebratulina septen- trionalis. Mem. Boston Soc. Nat. Hist. Vol. ii. 1871 ; also in Ann. Mag. Nat. Hist. (4) Vol. viii. 1871. 12. Morse, E. S. On the Embryology of Terebratulina. Mem. Boston Soc. Nat. Hist. Vol. iii. 1873. 13. Morse, E. S. On the Systematic Position of the Brachiopoda. Proc. Boston Soc. Nat. Hist. Vol. xv. 1873. 14. Muller, F. Beschreibung einer Brachiopodenlarve. Mutter's Archiv. f. Anat. u. Phys. 1860. 15. Muller, F. Die Brachiopodenlarve von Sta. Catharina. Zweiter Beitrag. Archiv. filr Naturgesch. 1861. 16. Shipley, A. S. On the structure and development of Argiope. Mittheil. Zool. Stat. Neapel. Bd. iv. 1883. 17. Zittell, K. A. Handbuch der Palaontologie. Bd. i. 1876-1880. APPENDIX TO LITERATURE OF BRACHIOPODA. I. Beecher, C. E. The Development of Terebratalia obsoleta. Amer. Geol. Vol. xi. 1893. General Considerations on the Molluscoida. As early as 1844, Henri Milne-Edwards (No. 11) pointed out the relationship existing between the Bryozoa and the Brachiopoda, and grouped them, with the Tunicata, as the "Molluscoida." In 1882, the true nature of the Tunicata having become known, Claus (No. 2) restricted the term Molluscoida to the Bryozoa and Brachio- poda. The near affinity between Phoronis and the Phylactolaemata was established chiefly by Caldwell (No. 1) and Ray Lankester GENERAL CONSIDERATIONS ON THE MOLLUSCOIDA. 85 (No. 8). These two authors are inclined to claim for the Sipun- culidae also affinity to these groups. A similar view has recently- been adopted by Ehlbrs (No. 3) and Roule (No. 12), the latter regarding the Brachiopoda, the Bryozoa, and the Phoronidae as a sub-group of the " Trochozoaires monomeriques," and suggesting distant relationship to the Sipunculidae, the Molluscs, and the Rotifera. Koule lays special stress on the Trochophoran characters of the Molluscoid larva. We ourselves have adopted the view of Hatschek (No. 6), who groups together the Phoronidae, the Bryozoa, and the Brachiopoda as Molluscoida, but excludes the Sipunculidae from the group. From all that is as yet known about the Ento- procta, we would, with Hatschek, separate them also from the Bryozoa and the Molluscoida generally. Further, according to the more recent researches made by McIntosh, Harmer (No. 10), Spengel (No. 13), and Ehlers (No. 3) in connection with Cephalo- discus, and by Fowler (No. 4) in connection with Rhabdopleura, these forms also must be removed from the group of the Molluscoida and probably grouped, as sedentary Enteropneusta, with Balano- glossus* The union of the Phoronidae, the Bryozoa, and the Brachiopoda to form a sub-group rests chiefly on the structural resemblance of the adult forms. We have already repeatedly pointed out anatomical features possessed in common by these forms, and refer the reader to the diagrams given in Figs. 5 (p. 9), 28 (p. 57), and 41 (p. 80), which, when compared, reveal so uniform a fundamental type that we can only refer the agreement in structure found here to homology. The Molluscoida are provided with a true coelom, a body-cavity lined with a flattened epithelium which is often ciliated, and in which the intestine is suspended by mesenteries (these, in the Phylactolaemata, being replaced by the funiculus). Only in the Gymnolaemata is the lining of the body-cavity no longer epithelial, while strands of con- nective tissue traverse the so-called funicular tissue of the body-cavity. The dorsal region of the body appears shortened, the oral and anal apertures of the looped alimentary canal are consequently approxi- mated, as is so often the case in tubicolous animals or those leading * [Masterman's interpretation of Adinotrocha, if correct, shows that Phoronis must for the future be associated with Cephalodiscus and Rhabdopl'ura. It is now fairly well established that these genera are closely related with Balano- glossus to the primitive chordate stock, and have no affinities whatever to the Molluscoida. Harmer, who has worked at the development both of the Ento- and Ectoproctous Bryozoa, still retains them in one group and considers that they have no affinities with Phoronis. See Harmer and Prouho, Appendix to Literature of Ectoprocta, Nos. III. and V. — Ed.] 86 MOLLUSCOIDA. a sedentary life. For the divergent position of the anal aperture and the looping of the alimentary canal in the Brachiopoda, we refer the reader to p. 81. Among the most typical features of the Molluscoida are the peculiar development of the cephalic, region, the presence of a horseshoe -shaped, tentacle -bearing lophophore having a distinct part of the body-cavity belonging to itself, the so-called lophophoral cavity (circular canal of the Gymnolaemata, smaller brachial sinus of the Brachiopoda), and the possession of a fold (epistome) above the mouth, into which another part of the body-cavity is continued (epistomal cavity, larger brachial sinus of the Brachiopoda). The relations of these parts of the body-cavity inter se and to the lower cavity which contains the intestine are in some respects still obscure. As centre of the nervous system, we have a ganglion lying above the oesophagus, which in the Brachiopoda, however, is less massive than the ventral ganglion. The excretory organs are represented by a pair of nephridia, which at the same time function as ducts for the genital products. (In Phoronis australis and in Rhynchonella there seem to be two pairs of nephridia; cf. on excretory organs of the Bryozoa, p. 56.) While we thus find far-reaching anatomical agreement between the adults of the various divisions grouped as Molluscoida, the similarity between the larval forms and their metamorphosis is less evident. We shall, however, have to lay stress upon the fact that, in the larvae of the three types to be distinguished {Actinotrocha, Bryozoan, and Brachiopodan larvae), the posterior section, by means of which the fixation of the adult is accomplished, first appears in an invaginated condition. This invagination in the Actinotrocha (Fig. 4 C, iv, p. 7) which yields the body- wall of the posterior section of the body, may be homologised with the sucker-invagination of the Ectoprocta and with the pedal section of the Brachiopodan larva, which is sunk into the mantle-fold. If we maintain this homology, we shall have to proceed to explain the metamorphosis of the Molluscoida from the Actinotrocha as the most primitive larval form. The Actinotrocha type is distantly allied to the Trochophore larva, being distinguished from the latter chiefly by the presence of a true coelom. We have pointed out above (pp. 57 and 78) that the derivation of the Bryozoan and Brachiopodan larva from an Actinotrochan form still presents some difficulties. We may well feel inclined to compare the posterior end of the body, which in the Molluscoidan larva appears in an invaginated condition, and by means of which fixation takes place later, with GENERAL CONSIDERATIONS ON THE MOLLUSCOIDA. 87 the foot of the Mollusca. Although, from the position of the organs, such a comparison is admissible, yet there do not appear to be sufficient grounds for this homology. The Sipunculidae show remarkable structural agreement with the Phoronidae, but this similarity may possibly be explained by a similar manner of life, and does not necessitate the assumption of a nearer relationship between the two groups. The larva of the Sipunculidae is closely allied to the Annelidan larva and shows the true Trocho- phore type, from which the Actinotrocha and the other Molluscoidan larvae are to some extent removed. It ought specially to be pointed out that the Sipunculidae agree with the Mollusca and the Annelida in the presence of two primitive mesoderm -cells which produce paired mesoderm-bands, while, in the Molluscoida, the origin of the mesoderm from the archenteron through folding is common. The rise of the coelomic sacs from the archenteron through folding is most clearly to be recognised in the Brachiopoda, but, in the Phylactolaemata also, the coelom must be understood to develop in this way, and the statements of Caldwell as to the formation of the mesoderm in Phoronis are favourable to such a view, although the observations of Koule do not appear to support it. Although we, as stated above in describing the development of the Chaetog- natha, are not inclined to lay great weight on these differences in the formation of the mesoderm, the facts must be pointed out. The shifting forward of the anus and the consequent shortening of the dorsal area lying between the mouth and the anus is brought about differently in the Sipunculidae (Vol. i., p. 363) and in Phoronis. In the Sipunculidae, the shifting of the anus is a consequence of very gradual alterations brought about by growth. The invagination which gives rise to the posterior end of the body in Phoronis is here altogether wanting. The principal point on which stress must be laid in comparing Phoronis and the Sipunculidae is the fact that the circle of tentacles surrounding the mouth has a different mode of origin in the two groups, so that the tentacles cannot be con- sidered as homologous. In Phoronis, this row of tentacles can be traced back to a formation of lobes in the region of the post-oral ciliated ring. In the Sipunculid larva, on the contrary, according to Hatschek (No. 5 ; cf. Vol. i., p. 363), the post-oral ciliated ring has nothing to do with the origin of the tentacles which surround the mouth. The post-oral ciliated ring of the Sipunculid larva lies still further back, and corresponds to the anterior edge of the sphincter, which, after the proboscis-like anterior part of the head 88 MOLLUSCOIDA. (the introvert) is withdrawn, closes the aperture of the invagination thus formed (Vol. i., Fig. 159, rm, p. 362). It appears from the above, that the Sipunculidae cannot be brought into any near relationship to the Molluscoida. Since, after careful consideration of the facts of ontogeny, we agree with Hatschek (No. 6) in doubting the near relationship of the Molluscoida and the Entoprocta, we are unable to adopt the view of those zoologists who use the structure and development of Pedicellina for explaining the conditions of the Molluscoida. This latter view has recently been adopted by Barrois, Seeliger, Davenport, and Ehlers. Among these, Ehlers approaches our standpoint, in so far as he also denies that the crown of tentacles in the Entoprocta is homo- logous with that in the Ectoprocta. For the reasons which compel us to exclude the Entoprocta from the Molluscoida, we must refer the reader to the following chapter (p. 101).* The remarkable genera Rhabdoplcura and Cephalodiscus have also repeatedly been brought into near relationship to the Ectoproctous Bryozoa and Phoronis. Meantime, the researches of Harmer, which have recently been confirmed by Spengel (No. 13), f and Ehlehs (No. 3) have revealed a striking similarity of organisation between Cephalodiscus and Balanoglossus. Young buds of Cephalodiscus show a distinct division of the body into three consecutive regions, comparable to the proboscis-, collar-, and trunk-regions of Balano- glossus. These regions correspond to a similar number of sections in the coelom, the proboscis - cavity being simple and unpaired, while the cavities of the collar and the trunk are each divided by a mesentery into paired halves. In the adult Cephalodiscus, the proboscis - cavity is restricted to the interior of the large oral disc, which lies like an epistome above the mouth. This cavity, according to Harmer, opens externally through two pores (proboscis- pores) which perforate the anterior part of the nervous system (Fig. 42, ex). These pores would correspond to the proboscis-pore or pair of pores {B. Kup- fferi, Bateson) of Balanoglossus. According to Ehlers, on the contrary, these pores represent the aperture of a special excretory organ, consisting of a canal (Fig. 42) which passes over into a wide terminal section lined with epithelium. The second section of the body- cavity also opens externally through two pores (collar-pores) beneath the apron-like fold, known as the operculum, which hangs down behind the mouth. To this region of the body * [In the foregoing account of the Bryozoa, considerable stress has been laid on the supposed relationship of the Ectoprocta to Fhoronis. Such a connection is by no means recognised by all students of the Bryozoa. HARMEB (Ectoprocta Lit., No. III.) does not consider that the Ectoprocta have any connection with Phoronis, and he would regard any structural resemblances as the result of coincidence rather than of close relationship. Both Harmer and Prouho (Ectoprocta Lit., No. V.) regard the Ectoprocta and the Entoprocta as nearly related. — Ed] + Cf. Spxnobl's monograph on the Enteropneusta, p. 753, etc. Prof. Spengel was kind enough to show us a number of drawings made from his preparation of Cephalodisc&fi. These confirmed in all essential points the statements of Ha i:\ikk. LITERATURE. 89 belongs the chief part of the central nervous system (n), which lies on the dorsal side continuous with the ectodermal epithelium, and at its anterior part is continued into the proboscis-region. Connected with the anterior part of the collar-region, on either side of the proboscis, are six biserial pinnate tentacles (t), each with a terminal knob, and into these the collar-cavity is continued. Further back are the pigmented apertures of the paired sac-like genital organs (g). The alimentary canal forms a dorsal loop, and the anus (a) is displaced far forward. The most anterior section of the alimentary canal shows a pair of lateral apertures, the so-called gill-slits (sp), and an f antero-dorsal diverticulum (x, noto- chord), which lies below the ner- vous system and extends into the proboscis. The body of Cephalo- discus is provided with a stalk, but the individuals are able to move about freely within the com- mon gelatinous coenoecium. The conditions of budding resemble those in Loxosoma. Fowler's more recent researches have re- vealed a considerable agreement between Rhabdopleura and Cephalo- discus Here also the intestinal diverticulum known as the noto- chord is found, as well as a pair of collar-pores, while the branchial clefts and proboscis - pores are not to be recognised. From all that is known, it appears that these genera are nearly related to Balano- glossus, while no connection with Phoronis and the Bryozoa can be proved to exist,* since, as shown by Lang (No. 7), the agreement in structure between these forms can easily be shown to result from similarity in their manner of life. Fig. 42.— Diagrammatic median section through Cephalodiscus (after Ehlers, combined from McIntosh and Harmer). a, anus ; ex, excre- tory organ (proboscis-pore) ; g, genital gland ; to, mouth ; n, nervous system ; sp, gill-slit ; t, tentacle ; x, intestinal diverticulum (noto- chord of authors). LITERATURE. 1. Caldwell, W. H. Preliminary note on the structure, develop- ment, and affinities of Phoronis. Proc. R. Soe. London. Vol. xxxiv. 1882-1883. 2. Claus, C. Grundziige der Zoologie (4). 1882. Bd. ii., p. 89. 4th Ed. 3. Ehlers, E. Zur Kenntniss der Pedicellineen. Abh. k. Gesellsch. Wixsensch. Gottingen. Bd. xxxvi. 1890. * [Masterman's researches on Adinotrocha, if confirmed, would show that Phoronis is undoubtedly nearly related to Cephalodiscus and its allies. — Ed.] 90 MOLLUSCOIDA. 4. Fowler, G. H. The Morphology of Bhabdopleura Normani Allm. Festschrift filr R. Leuckart. Leipzig, 1892. And Proc. Roy. Soc. London. Yol. clxxxviii. 5. Hatschek, B. Ueber Entvvicklung von Sipunculus nudus. Arb. Zool. Inst. Wien. Bd. v. 1884. 6. Hatschek, B. Lehrbuch der Zoologie. Jena, 1888. Part \.y p. 40. 7. Lang, A. Zum Verstandniss der Organisation von Cephalo- discus dodecalophus M'Int. Jen. Zeitschr. f. Nat. Bd. xxv. 8. Lankester, E. Ray. Article, Polyzoa in Encyclopaedia Britannica. ' 9th Ed. 1885. Part lxxiii., p. 429, etc. 9. Lankester, E. Ray. Rhabdopleura. Quart. Journ. Micro^ Sci. (2). Vol. xxiv. 1884. 10. McIntosh, W. Report on Cephalodiscus dodecalophus, etc. Challenger Reports. Vol. xx. 1887. With an Appendix by S. F. Harmer. 11. Milne-Edwards, H. Recherches anatomiques, physiologiques et zoologiques sur les Polypiers de France. 1841-1844. 12. Roule, L. Considerations sur l'embranchement des Trocho- zoaires. Ann. Sci. Nat. (7). Tom. xi. 1891. 13. Spengel, J. W. Monographic der Enteropneusten. Fauna und Flora des Golfes von Neapel. Bd. xvii. APPENDIX TO LITERATURE. I. Masterman, A. T. On the Diplochorda. Quart. Journ. Micro. Sci. Vol. xl. 1897. II. Delage, Y., and Herouard, E. Traite de Zoologie Concrete. Tom. v. 1897. III. Sedgwick, A. Text-book of Zoology. Vol. i. 1898. pp. 542- 585. Including a criticism of Masterman's Diplochorda. (No. I.). CHAPTER XVIII. ENTOPROCTA. The group of the Entoprocta, which, although small, is rendered of interest by the possession of many structural and developmental peculiarities, has hitherto usually been united with the Ectoproc- tous Bryozoa, but it is doubtful if the two should be connected. Of the forms belonging to this group, Pedicellina, and nearly allied genera such as Pedicellinopsis, Ascopodaria (Barcntsia), etc., and, further, UrnateUa, form colonies, but in Loxosoma, the buds do not remain connected with the parent, but become detached, so that in this genus only solitary individuals are met with in the adult condition. Our knowledge of the embryonic development of Pedicellina, apart from older statements, is principally due to the researches of Hatschek (No. 6), and Harmer (No. 5). The ontogeny of Loxosoma has more recently been described by Harmer (No. 4), whose results agree in essential points with those of Hatschek. The eggs of Pedicellina are fertilised while still in the ovary. The embryonic development takes place within the vestibule of the female,* which is transformed into a brood-cavity, the epithelium of which appears to become thickened and glandular for the nourishment of the embryos (Ehlers, No. 2). These are attached to the wall of the brood-cavity by the pointed end of the elongated, pear-shaped egg-envelope (a secretion of the epithelium lining the oviduct). The young larvae, when hatched, still remain in the brood-cavity, and are said to be attached to its wall. In the granular and somewhat opaque spherical egg of Pedicellina, enclosed in its vitelline membrane, a somewhat clearer animal pole can be distinguished, near which lies the germinal vesicle. Cleavage is, as a rule, total and unequal, but approaches the equal and regular * Pedicellina echinata is hermaphrodite. In other species the sexes appear to be separate. [According to Harmer. Pedicellina is sometimes dioecious and sometimes monoecious, and Loxosoma is perhaps always dioecious.— En.] 92 BNTOPROCTA. type. It is, however, not altogether regular, since the two-celled stage, at which the two blastomeres are not quite equal in size, is followed first by a three-celled stage, and only later by one showing four cells, this latter stage being reached by the separation from each of the two primary blastomeres of a somewhat smaller cleavage-sphere, near the animal pole. As the blastomeres of the animal pole now rapidly increase in number, and a central cavity appears, a blastula stage arises (Fig. 43 A) ; in this, the cells of the vegetative half are remarkable for their size and granular character. Further development leads to a formation of a true invagination gastrula (Fig. 43 B). The vegetative half of the embryo first flattens and then becomes invaginated towards the animal pole, the cleavage-cavity consequently being almost obliterated. The blastopore Fio. 43.— Three stages in the embryonic development of PediceMna echinata (after Hatschek, from Balfour's Text-book). A, blastula stage preparatory to gastrulation. Optical section seen from the side. B, gastrula-stage. Optical section from above. C, later stage, after the closing of the blastopore, seen from the side, a.e, archenteric cavity ; ep, ectoderm ; hy, entoderm ; me, mesoderm cells ; s.c, cleavage-cavity. closes in the form of a longitudinal slit corresponding to the median plane, and to the middle of the future ventral surface. A flattening of the ventral side can already be noticed (Fig. 43 C). At. that end of the slit-like blastopore which can be directly observed to give rise to the posterior or anal end of the adult, two symmetrically placed cells (me) now appear; these have retained the primitive character of cleavage-spheres, and are not yet externally covered by the ectoderm These are the primitive cells of the mesoderm. They are soon completely grown over by the ectoderm (Fig. 43 C), and then lie in a space between the ectoderm and the entoderm, which is derived from the cleavage-cavity, and is considered to be the primary body-cavity. This cavity develops further during the following stages, and gradually becomes filled with mesodermal elements. ENTOrROOTA. 93 During the next stages the development of the definitive form of the body takes place. The embryo lengthens at right angles to the flattened ventral side (Fig. 44). At the same time the ectoderm thickens over this ventral region, and thus assumes the form of a disc which, at its edge, is sharply marked off from the rest of the ectoderm, and later sinks in to form the vestibule. In the anterior half of this disc an invagination appears which soon becomes lined with ciliated cells (Fig. 44, oe) ; this is the rudiment of the oeso- phagus. At a somewhat later stage another similar invagination develops from the posterior part of the disc (Fig. 44 B, an.i) ; this is the rudiment of the hind-gut, which at first is a solid, inwardly projecting ectodermal thickening. The oesophageal invagination Fig. 44.— Two later stages in the development of Pedicellina (after Hatschek, from Balfour's Text-book), ae, archenteron ; an.i, anal invagination ; /, ectodermal fold ; f.g, ciliated disc ; oe, oesophagus ; x, dorsal organ. soon becomes connected with the rudiment of the mid-gut (Fig. 45 A). The establishment of communication between the mid-gut and the hind-gut, and the acquisition of an external aperture by the latter, occur only in the later stages (Fig. 45 B). The mesoderm- cells, meantime, have increased in number, the proliferation of the two pole-cells leading first to the formation of two short mesoderm-bands. Two larval organs specially character- istic of Pedicellina also appear. One of these, which we shall call the ciliated disc (Figs. 44 and 45, frj), is an ectodermal thickening which lies apically and consists of large glandular cells, its margin being beset with stiff cilia. The other larval organ, which is known as the dorsal organ (Figs. 44 and 45, x\ lies on the anterior side of 94 ENTOI'ROCTA. the cup-like aboral wall of the larva, and consists of a rather deep ectodermal depression, part of the anterior section of which can be evaginated. In the comparison which has often been made between the Pedicellina larva and the larvae of the Ectoprocta (especially with Cyphonautes), these two pro- visional organs have played a conspicuous part. The ciliated disc has usually been regarded as the homologue of the retractile disc of the Ectoprocta. Like the latter, it is retractile, and seems to function as a sensory organ. This, at least, seems to be proved by the observation that the larva of Pedicellina, whet swimming, always carries this organ directed forward. The structure of the Fig. 45.— Two later stages in the development of Pedicellina (after Hatschek, from Balkour's Text-book), a, anus ; an.i, anal invagination ; //77> f//f, etc., first, second, third, etc., thoracic limbs (in C, with the rudiments of Ihe branchiae); g, genital rudiment; gh1, primary brain; gh", secondary brain; vi, mouth; md, mandible ; vis, mesoderm ; vix1, first maxilla ; mx", second maxilla ; n, nuchal gland ; ob, upper lip ; oe, oesophagus ; s, shell ; sch, neural plate ; sd, shell-gland ; za, compound eye. the different regions of the body. The Nauplius limbs, for instance, often appear simultaneously or in quick succession, and the Nauplius stage is generally distinguished by a period of rest (and frequently by the development of a larval cuticle), while the posterior limbs, •characteristic of the later stages, usually develop in regular suc- cession. In the Phyllopoda, where the maxillae are but feebly developed, these limbs appear very late, when the embryo is fully -developed (Fig. 72 B and C, Zaddach). 148 CRUSTACEA. B. Arthrostraca and Cumacea. In the Arthrostraca, that region of the egg where later the embryonic rudiment develops is often indicated, even during cleav- age, by the small size of the blastomeres, or by the earlier formation of the blastoderm. A germ-disc forms which is at first round, but soon spreads out into a germ-band covering the whole ventral surface of the egg, and occasionally also extending dorsally over the anterior and posterior ex- ZJ Tf ^-«— -^ tremities (Fig. 74 A) The an- terior end of the germ-band is indi- cated by paired enlargements, the cephalic lobes (Fig. 69, o, p. 138), from which the rudiments of the eyes and of the brain arise, while the germ -band itself soon appears divided up by furrows into the separate body-seg- ments (Fig. 74 B). This segmentation and the first ap- pearance of the pairs of limbs takes place from before backwards; the segmentation, however, fre- quently occurs almost simultaneously in all regions of the body. As the lengthening germ-band has no room for free development, a characteristic curvature soon comes about. In the Isopoda, the curvature, which is originally dorsal (Fig. 74 B), only gives place to the reverse curvature at the end of embryonic life, whereas, in the Amphipoda, such a ventral curvature appears at an earlier Fio. 73.— Three stages in the development of parasitic Copepoda (after Van Beneden). A, Nauplius stage of Brachiella Thynni. B, older embryo of Anchorella, with the limbs of the first Cyclops stage. C, Cyclops stage of Hcssia colorata. a>, first antenna ; «", second antenna ; d, food-yolk ; en, entoderm (wall of the mid-gut) ; k, germ-band ; m, mandible ; mx, first maxilla ; mfl, first maxillipede ; mf", second maxillipede ; oe, oesophagus ; ol, upper lip ; p', pU, first and second pairs of thoracic (rowing) limbs. ARTHROSTRACA AND CUMACEA. 149 stage of development, the embryos consequently showing throughout embryonic life a ventral curvature of the abdomen (Fig. 75). Although, as a rule, the limbs appear to take their rise almost simultaneously, yet in Asellus, whose ontogeny exhibits several primitive features, we find a Nauplius stage* characterised by the presence of two pairs of antennae and the secretion of a larval cuticle (Fig. 74 A). The mouth-parts and six pair of thoracic limbs, and, finally, the abdominal appendages appear later (Fig. Fig. 74.— Two stages of development o« Asellus, seen from the side (diagrammatic). A, Nauplius stage (Van Beneden). B, older stage (Dohrn). a', first antenna; a", second antenna ; af, anus ; I, lobe-like appendages ; md, mandible ; mx1, first maxilla ; ma;", second maxilla ; mf, maxillipede ; I-VI, first six pairs of ambulatory limbs ; 1-5, first five pairs of pleopoda ; m, mouth ; x, vitelline membrane ; y, blastodermic cuticle. ,10 74 B). After the rudiments of the limbs have appeared, paired prominences form behind the mouth ; these become the rudiments of a bilobate lower lip {paragnatha). The course of development of the germ-band observed in Asellus is connected with that in Ligia by the presence of a distinctly recognisable Nauplius stage (Fig. 69, p. 138). The chief point worthy of notice in the later development of this form is that the rudiments of the thoracic limbs are originally biramose (Ntjsbaum, No. 85a), and that the adult limbs arise from these by the suppression of the exopodite. This is in har- mony with the presence of vestigial exopodites on the two anterior pairs of thoracic limbs of the Anisopoda, and supports the deduction that the Isopoda, and with them of all Arthro- straca, arose from ancestors resembling the Schizopoda. A feature apparently universal in the Isopoda is the temporary suppression in the embryo of the posterior of the seven pairs of ambulatory limbs (thoracic limbs), and its re-appearance in the young only after leaving * Former observers failed to find a rudiment of the mandible in the Nauplius stage of Asellus, but Boas (No. 4, Crustacean Metamorphosis) was able to prove the presence of such a rudiment in a few individuals. Fig. 75.— Embryo of an Amphi- pod (Cor&phium, F. Muller). K, dorsal organ. 150 CRUSTACEA. the egg. Larval integuments like that in the Nauplius of Asellus are of very common occurrence among the Arthrostraca. An apparent exception to the rule that the germ-hand in the Isopoda exhibits a dorsal curvature in its early stages is afforded by Cymothoa (Bullae, !N"o. 81). The germ-band, which here lies on the ventral side of the egg, whic.li is very large and richly supplied with food-yolk, shows for the greater part of its course the same dorsal curvature as that of other Isopoda. Its most posterior end (the rudiment of the telson) is, however, bent round ventrally. As the germ-band broadens in later stages, its lateral portions grow up over the dorsally superimposed food-yolk, and thus give rise to the lateral parts of the embryo. The further extension of this growth finally leads to the coalescence of the lateral edges of the germ-band in the dorsal middle line, whereby the enclosure of the mass of yolk within the embryo is completed. It should be noted that during this process that part of the ectoderm which formerly covered the dorsal accumu- lation of yolk is crowded into a smaller space, and finally degenerates. This process has apparently some connection with the de- velopment in the dorsal region of many Arthro- stracan embryos of an organ discovered by Mbissnbr in Gammarus and described as a mlcropyle apparatus; this was found later in many Amphipoda and a few Isopoda, and, as the spherical or dorsal organ, it has received many differ- ent explanations. In the Amphipoda, the dorsal organ appears during the differentiation of the germ-band as a dorsal disc-shaped thickening of the blastoderm (Fig. 75, K) projecting inwards towards the yolk ; when the larval integument develops, this organ adheres closely to it. A perforation is formed here (the micropyle), while the central parts of the disc form a small cavity by invagination (Fig. 76). In later ontogenetic stages, when the heart develops below the dorsal organ, the latter degenerates, mesoderm-cells wandering in between its cells and apparently taking an active part in its disintegration (Rossijskaya, No. 72). Fig. 76. — Diagrammatic transverse section through the embryo of Cymothoa (after Bullar, from Balfour's Text-book). Below is seen the germ- band in cross section, above, the dorsal organ ARTHROSTRACA AND CUMACEA. 151 A dorsal organ developed in this Avay has been proved by F. Muller to exist in many Araphipoda ; also in some Isopoda (Fig. 76) — Cymothoa, Claus and. Bullae, No. 81 ; Praniza, Dohrn. An interesting variation is found among the Amphipoda in the genus Orchestia, where the dorsal organ originally develops on the lateral edge of the germ-band, and only later shifts towards the median dorsal line (Uljanin, No. 75 ; Rossijskaya, No. 72). Another method of development of the dorsal organ is seen in Onisais (Dohrn, No. 83 ; Bobretzky, No. 80), where the cells of a large area thicken to form a saddle-shaped plate adhering closely to the larval integument (Fig. 71 A, do, p. 140). This plate, in the further course of development, becomes more and more separated, by an infolding of its margin, from the embryo, with which finally it is connected only by a thin, hollow column. The plate, when this connection is lost, undergoes disintegration. A similar dorsal organ is found in Ligia (Fr. Muller, No. 4 ; Rosalie Nusbaum, see J. Nusbaum, No. 39, p. 168). A dorsal organ similar to that in the Amphipoda has been observed in the Cumacea (Dohrn, No. 36). In Mysis it is originally paired (Fig. 77, d). The paired disc-like thickening found in Tanais (Fr. Muller, No. 4) connected with the larval integument, might also be thus interpreted. Among the Decapoda, the dorsal organ is found in a reduced condition in Crangon. (Kingsley, No. 53), and perhaps (?) also in Pandalus and Palinurus (Dohrn, No. 45), as well as in Homarus (Herrick, No. 50). Among the various explanations given of this organ, the one originated by Fr. Muller (No. 4), and put forward later in more detail by Grobben (No. 11), has met with the widest acceptance. According to these authors, the dorsal organ is the vestige of a once functional adhesive structure, equivalent to the nuchal gland {neck-gland, Fig. 72 C, n), which is still functional in the young stages of the Phyllopoda and is there sometimes retained throughout life. Such a nuchal gland was also found by Grobben and Urbanowicz in Copepoda (Cyclops, Ergasilus), and by Grobben in Euphausia. Although this homology appears very probable, we do not think it entirely established. It is possible, as already mentioned, that the dorsal organ is merely the expression of the com- plication of the blastodermic covering of the food-yolk caused by the lateral growth of the germ-band. This complication would then be expressed by an invagination in the Amphipodan type, and by a nipping off in the Oniscus type. These processes are perhaps analogous to the formation of the dorsal organ in the Insecta. Among those structures which have often been homologised with the dorsal organ, are the paired lobe-shaped appendages (Fig. 74, I) of the Asellus embryo. The true significance of these latter was first explained by Claus (No. 82) by a comparison with his observations on the young stages of Apseudes. In Apseudes, in the maxillary region, there are two wing-like integumental folds ; these are the rudiments of a shell which spreads over the small respiratory cavity, and under which the palp of the anterior maxilla and the vibratile epipodial lamella (scaphognathite) of the maxillipede come to lie. In Asellus the shell-fold is reduced to a triangular rudiment which (Rathke, No. 88) may function as an embryonic gill. The Anisopoda just mentioned (Tanais, Apseudes), in the conformation of their embryos, most resemble the Isopoda, having in common with them the dorsal curvature and the absence of the seventh pair of thoracic limbs. In other respects, they more nearly approach the Cumacea, all the abdominal limbs except 152 CRUSTACEA. the last (sixth) pair being still wanting when the young animal is hatched (Fr. Muller, No. 4 ; Claus, No. 78). It has already been shown (p. 151) that the paired disc-like thickenings observed by Fr. Muller in the Tanais embryo are probably to be referred to the dorsal organ. The embryos of the Cumacea also, through the absence of the seventh pair of thoracic limbs, and the presence of a dorsal organ (resembling that of Cymothoa) are allied to the Isopoda. As in the Anisopoda, the embryo when hatched possesses only one (the sixth) pair of abdominal limbs. C. Leptostraca, Schizopoda, Decapoda. There are two factors which, as a rule., influence the ontogenetic development of these groups. (1) The food-yolk which, in most cases, is present in very considerable quantities, and which determines the size of the egg and the entirely superficial extension of the embryo in the first stages; (2) the gradual development of the long germ-band out of an originally short rudiment consisting of a few segments (distinctly marking the Nauplius stage). Nothing further is known of the presumably very primitive condition shown in the development of the larval body of those genera which leave the egg in the Nauplius form (Eaphausia, Penaeus, Lucifer). We may take the ontogeny of Mysis as the starting-point of our description, following chiefly E. Van Beneden (No. 37) and Nusbaum (No. 39). The eggs of Mysis develop (as in the Cumacea and Arthrostraca) in a brood-sac covered by horizontally- placed lamellae (oostegites) of the thoracic limbs. The first development of the embryo appears at the point from which the formation of the blastoderm originated, and which corresponds to the future ventral surface. The formation of the blastoderm is commenced by the development of a rounded disc, the germ-integument, which ultimately spreads over the surface of the egg; a later stage shows a similar rounded blastodermic thickening (the germ-disc) which represents the first embryonic rudiment. This soon breaks up into three lobes, the middle one representing the rudiment of the caudal region, and the paired lateral lobes the rudiments of the germ -band. The latter soon grow forward as two diverging bands, from which the rudi- ments of the Nauplius limbs bud out as rounded prominences (Fig. 77 A). The broadened anterior ends (o) of these halves of the germ-band correspond to the cephalic lobes of the Entomostracan ami Arthrostracan germ-bands. As they give rise exclusively to the rudiments of the compound eyes and the optic ganglia, we shall give them the more precise name of optic lobes. On either side of the short germ-band, between but somewhat above the first and second pairs of antennae, lies the disc-shaped paired rudiment of the LEPTOSTRACA, SCHIZOPODA, DECAPODA. B 153 Fig. 77.— Five embryos of Mysis (after Nusbaum). A, youngest Nauplius stage. B, older Nauplius stage in profile. C, Nauplius after the disappearance of the vitelline membrane, in three-quarters profile. D, later Nauplius stage with the Nauplius integument (u). E, larva with thoracic limbs, a', first antenna; a", second antenna; c, vitelline membran d, dorsal organ ; md, mandible ; n, Nauplius integument ; o, optic lobe ; p, rudiment of the compound eye ; s, caudal region. 154 CRUSTACEA. dorsal organ (d). The plate-shaped rudiment of the caudal region («) is bounded anteriorly by the transverse depression (abdominal fold). The latter soon becomes covered over by the posterior end of the body (Fig. 77 B, s), which grows forwards to form the caudal fork. In this way the Nauplius stage, with the tail region bent over ventrally, is reached (Fig. 77 B). After this stage has been reached, the vitelline membrane is thrown off, a new cuticle (the Nauplius integument) being secreted simultaneously. The embryo now lies free in the brood-cavity, surrounded somewhat loosely by the Nauplius cuticle. The abdomen has meanwhile lengthened and straightened out (Fig. 77 6'), and becomes more and more filled with food-yolk evenly distributed within it, while the whole body finally assumes a distinct dorsal curvature. We can now (as in the Arthrostraca) distinguish a ventral germ -band from the dorsally-placed mass of food-yolk. The latter, especially in the cephalic region, swells considerably. The posterior parts of the embryo become distinctly segmented. The mouth - parts and the thoracic limbs appear simultaneously (Fig. 77 E), the rudiments of the abdominal limbs following in a later stage. During these changes the disc-shaped dorsal organs have coalesced in the dorsal middle line, and, as an invagination takes place there, they are here, both in position and form, comparable with the dorsal organ of the Amphipoda. The embryo of Nebalia, according to Metschnikoff's account (No. 33), strongly resembles in form that of Mysis. Here also we have the ventrally flexed Nauplius stage, as well as a lengthening and dorsal curvature taking place after the shedding of the vitelline membrane. The presence of a dorsal organ has, however, apparently not yet been observed in this form (p. 252). The embryos of the Decapoda differ from those of Mysis chiefly in the position of the parts of the body ; the ventral curvature which at first characterises the Nauplius of Mysis (i.e., the ventral flexure of the abdomen) is here retained to a much later stage until the larva is hatched, this being connected with the much later rupture of the egg-envelopes. In other respects the ontogenetic processes are very similar. In Astacus, of whose development a very detailed account can be compiled from the researches of Rathke (No. 63), Lereboullet (No. 58), Bobretzky (No. 41), and Reichenbach (Nos. 64 and 65), after the blastoderm has developed, the first embryonic rudiments are perceptible as five thickenings consisting of a simple layer of cells (Fig. 78). The anterior pair of the thickenings (K) corresponds to the optic rudiments of Mysis, and LEPTOSTRACA, SCHIZOPODA, DECAPODA. 155 may here also be described _as optic lobes. The posterior pair of formative centres (TA), which lie nearer one another, answer to the unpaired caudal rudiment of Mysis. But, since here not only the segments of the abdomen, but thoracic segments as well have begun to form, these discs may be called the thoraco-abdominal rudiments. The most posterior unpaired disc is the entoderm disc (ES). Anteriorly to this there is a point where active proliferation yields cells which spread out below the blastoderm-layer, this is the formative zone of the mesoderm (BM). While, by processes already described in detail (pp. 128, etc.), the invagination of the entoderm-disc and the gradual closing of the blastopore take place, the thoraco-abdominal rudiments come together to form an unpaired plate (Fig. 79, TA), in the middle of which the procto- daeal invagination (.4) is soon percep- tible. The anterior margin of this plate then becomes more sharply marked by a transverse depression (the caudal fold), which, in the fur- ther course of de- velopment, sinks deeper, sloping obliquely back- ward. The tho- raco-abdominal plate grows forward round and below this depression, forming the long posterior portion of the body ; this is curved ventrally and closely applied to the other embryonic rudiments (Figs. 80 and 63 B, p. 131). While these processes are going on in the posterior half of the body, active growth takes place on each side in a band connecting the optic lobes with the thoraco-abdominal rudiment (paired rudiment of the germ-band), which finally lead to the development of three pairs of limbs (Nauplius limbs, Fig. 79). These are the mandibles Fig. 78. — Part of the surface of an egg of Astacus ftuviatilis, with embryo beginning to form (after Reichenbach, from Lang's Text-book). BM, formative zone of the mesoderm; ES, ento- derm-disc ; K, cephalic lobes (optic lobes) ; TA, thoraco- abdominal rudiments. 156 CRUSTACEA. and the two pairs of antennae, the former, according to Bobretzky and Reichenbach, appearing somewhat earlier than the latter. The middle region between these rudiments long retains the character of an undifferentiated blastoderm ; in the anterior region, however, an unpaired swelling arises as the rudiment of the labrum (I), and behind it the stomodaeal invagination, while, mesial to the rudiments of the limbs, the pairs of ganglia belonging to them (ga2, gm) are recognisable as ectodermal thickenings. The Nauplius stage thus attained marks a natural period in the ontogenetic development of Astacus, this being also indicated by the appearance of a larval integument (Nauplius cuticle). It should be noted that, in the Nauplius stage, the different parts of the embryo lie nearer one another than when they first arise (cf. Figs. 78 and 79 drawn to the same scale). Such an ap- proximation of the rudiments in the em- bryo at these stages appears universal in the Decapoda. The Nauplius stage in other Decapoda develops in the same way as in Astacus. The embryonic rudi- ments, however, in most cases, only ap- pear after the closing of the mouth of the gastrula ; there is then found, in the immediate neigh- bourhood of the closed blastopore, an unpaired rounded prominence, in which the rudiment of the posterior end of the body can be discerned ; this is not, as in Astacus, paired. In the Loricata, according to Dohun (No. 45), as in Astacus, this may be described as the thoracoabdominal rudiment, whereas, in other cases, it seems to give origin exclusively to the abdominal segments. This rudiment soon shows wing-like outgrowths running forward, and establishing connection with the optic lobes which in the meantime have made their appear- ance. The rudiments of the Nauplius limbs then appear in the neighbourhood of these connecting strands. A transverse projection connecting the posterior parts of the optic lobes Fig. 79.— Embryo of Astacus fluvlatilis in the Nauplius stage (after Reichenbach, from Lang's Text-book). A (above), rudiment of eye ; alt first, and a2, second antenna ; G, cerebral ganglion ; ga2, ganglion of the antenna ; gm, ganglion of the mandible ; m, mandible ; I, upper lip ; TA, thoracoabdominal rudiment ; A (lying in TA), anus. LEPTOSTRACA, SCHIZOPODA, DECAPODA. 157 becomes the rudiment of the labrum, behind which the stomodaeal invagination soon appears. The latter, as a rule, lies in the space between the first and second pairs of antennae, but Kingsley has specially emphasised the post-oral position of the first pair of antennae in Crangon. Paired swellings arise near the posterior margin of the oral aperture ; these are the rudiments of the paragnatha or bilobed lower lip. The later stages (Figs. 80, 81, and 83) are characterised by the further growth of the thoraco-abdominal rudiment, which soon breaks up into segments, and also by the development of the posterior pairs of limbs which appear from before backward. At the same time the typical biramose character is found in the anterior limbs which were the first to develop, as well as a seg- mentation cor- respondingwith the developing joints (for the number and shape of the limbs of the hatching em- bryo, which vary greatly in the different groups of the Decapoda, see below, p. 257 : Metamorphosis of the Deca- poda). The optic lobes gain in independ- ence, bulging forwards and gradually rising from the surface below them, so that the club-like shape of the stalked eye can be recognised in the rudiment (Fig. 81). The posterior part of the body undergoes important alterations. There is here an early separation of the terminal or anal segment (Figs. 80, 81, T) from a budding zone composed of large cells which lies in front of this segment (in the embryo, however, owing to the forward curvature of the tail, further back). The anal aperture is TTIX, Fig. 80. — Embryo of Astacus fluviatilis, with the thoracic feet beginning to form (after Reichenbach, from Lang's Text-book). A, eyes; alf a,, first and second antennae; ab, abdomen; g, rudi- ment of the brain (procerebrum+antennal ganglion) ; go, optic ganglion; /-, upper lip; m, mandible; mxt, mx.2t first and second maxillae; T, telson ; t7, t8, thoracic limbs; tlt tn, maxillipedes ; t4, t8, ambulatory limbs ; ts, rudiment of the thoracic shield. 158 CRUSTACEA. originally formed on that surface of the terminal segment which afterwards becomes the dorsal surface, later it shifts into the space between the two lobes into which the terminal segment soon divides. Fig. 81.— Embryo of Astacus jluviatilis with (he rudiments of all the limbs (Fig. 83, p. 1(53) (after Reichenbach, from Lang's Text-book). The thoraco-abdomen, which in reality curves forward ventrally, is cut off and laid back, ad, antenna] gland ; ah, abdomen ; tt, first pair of ambulatory limbs (chelate feet); *s-£8, second to fifth pair of ambulatory limbs; T, telson. and through this space gradually reaches the later ventral side, and, in this way, its final position. Immediately behind the thoracoabdominal rudiment, at the point where this is connected with the rest of the body, in later stages, an FORMATION OF THE ORGANS. 159 accumulation of mesoderm-cells is discerned, forming the first rudi- ment of the heart (Figs. 63, 64, h, pp. 131, 132). In early stages the thoracoabdominal rudiment is surrounded by a -clear area bounded externally by closely contiguous blastoderm-cells. The latter soon become raised so as to form a fold which is specially distinct in its lateral parts ; this is the first rudiment of the thoracic shield (Fig. 80, ts). The clear area thus referred to is the rudiment of the branchial cavity. A remarkable structure lying like a dorsal disc on the embryo and called the "carapax" has been described in the early stages of Atyephyra by Ischikawa. The development of the Decapoda is principally characterised by the fact that the whole mass of food-yolk is confined to the anterior part of the dorsal side of the body, while the thoraco- abdominal rudiment is free from yolk. Even in late stages, in which the form of the hatching animal is already quite distinct, the cephalo-thoracic portion appears swollen up by food-yolk until almost spherical. Formation of the Organs. Our knowledge of the development of the different organs in the Crustacean embryo is still somewhat limited. The Decapoda are in this respect best known to us from the researches of Bobretzky (Xo. 41), Reichenbach (Nos. 64, 65), and Kingsley (No. 52-55). Besides these we have the observations of Nusbaum (Xo. 39) on Mt/sis, of Bobretzky (No. 80) and Xusbaum (Xo. 85) on Oniscus, of Grobben (Xos. 11 and 21) on Moina and Cetochilus, of Claus (Xos. 8 and 9) on Branchipus, Apus, and others. A. External Integument. As the superficial ectoderm of the embryo yields on its external surface the chitinous skeleton of the larva, it gradually acquires the character of the hypodermis or the matrical layer of this skeleton. It has been recently proved by T. Tullberg that, in the Lobster, the origin of this chitinous skeleton may be traced back to a direct transformation (chitinisation) of the body of the cell. It is an interesting fact that the hypodermal cells not only are able to change into chitin at their external ends, which are directed to the body surface, but that occasionally even their basal portions are similarly modified. Thus Reichenbach observed that, in Astacus, single hypodermal cells lengthen inwards and grow out into chitinised strands and pillars which function partly as supports for the carapace, and partly as points of attachment for the groups of muscles. In 1 60 CRUSTACEA. individual cases it is often impossible to distinguish between these ectodermal ingrowths and true connective tissue. This inner chitinous supporting tissue belonging to the ectoderm was found by Claus (No. 9) very richly developed in Branchipus. B. Endoskeleton. A further development of internal chitinous structures is brought about by infolding and invagination of the external integument. In this way are developed those tubular chitinised ingrowths which, as affording attachment for the more important muscles, have been called chitinous tendons, and some of which even, as was proved for the mandibular muscle of Astacus (Baur), are renewed at ecdysis. A striking development of chitinous tendons of this kind, formed from ectodermal invaginations (Reichenbach), is found, as is well known, in the penultimate joint of the pincer in Astacus fluviatilis. The inner sternal skeleton (endophragmal system) which bridges over the thoracic ganglia in Astacus develops in a similar manner, as a series of invaginations of the external integument (Bobretzky, No. 41), by a process of infolding of the inner wall of the branchial cavity and of the sternal surface of the thorax. Nusbaum (No. 85) was able to observe in Oniscus the origin from paired lateral invagi- nations of a similar chitinous diaphragm covering over the chain of ganglia in the thorax. A hemispherical, chitinous articular fold also arises as an ectodermal invagination in connection with the movable eye of the Cladocera and of most Branchiopoda (Grobben). C. Nervous System. Although probably belonging to a common rudiment,* the supra- oesophageal ganglion (brain) and the ventral chain of ganglia must be separately dealt with. The whole of the central nervous system arises as an ectodermal thickening. Even in early stages, paired ectodermal thickenings can be recognised on the inner side of the * Most authors agree that, in the Crustacea, the rudiment of the brain, from its first appearance, is connected with the primitive swellings of the ventral chain of ganglia by means of paired ectodermal thickenings (rudiments of the oesophageal commissures). This view, however, has been opposed. Urbanowicz, for example, found that in Cyclops (No. 23) the brain and a suboesophageal ganglion originate independently, and only become connected later by the development of the oesophageal commissures. This observation cannot, however, be considered as affording direct proof of Kleinenberg's views as to the original independence of the rudiments of these two parts of the central nervous system (Vol. i., p. 288), for it is easy to understand that the rudiments of the more massive parts of the central nervous system should become earlier perceptible as ectodermal thickenings, while those of the more delicate parts {e.g., of the oesophageal commissures) are only visible at a later stage. NERVOUS SYSTEM. 161 rudiments of the limbs ; these represent the rudiments of the paired ventral ganglia belonging to the individual segments. The consecutive pairs of ganglia are, however, connected by thickened ectoderm-bands, the rudiments of the longitudinal commissures, so that we may- regard two longitudinal ectodermal swellings (the " Primitivwulste," Hatschek, Fig. 82, pw) as the first rudiment of the ventral chain of ganglia ; these show segmental swellings (rudiments of ganglia), and are separated by the primitive groove (pr). In the later stages of development (Fig. 82 B), in the region of the primitive swellings, the ectoderm is seen to be composed of several layers, an outer one, which now is changed into the hypodermis (h) of this region, being separated from the inner layers. The latter now, as lateral sm £ Fig. 82.— Development of the ventral chain of ganglia in Astacus Jluviatilis (after Reichenbach). A, cross section through the mandibular segment of an embryo in which the maxillipedes have already appeared. B, cross section through the ganglionic rudiment in a maxillary segment of an embryo in which the maxillipedes have already developed, a", cross section of the second antennae ; hi, connective tissue covering over the inner side of the ganglionic rudiment ; ec, ectoderm ; en, entoderm ; /, bundle of fibres in cross section ; g, large ganglion-cells ; h, hypodermis ; m, invagiDated median strand of the ganglionic rudiment ; vis, mesoderm ; pr, primitive groove ; pw, primitive swelling of the ganglionic rudiments ; s, lateral strand ; sm, secondary mesoderm. strands (s) represent the rudiment of the ventral cord. Reichenbach, on whose description of the development of the nervous system in Astacus our account chiefly rests, was able to prove that, in the formation of each pair of ganglia of the ventral cord, there enters, besides the corresponding portion of the lateral strands, a median invagination (m) ; this is to be traced back to the primitive groove, and is known as the median strand. This agrees with the discoveries of Hatschek in connection with the origin of the ventral chain of ganglia in the Insecta. M 162 CRUSTACEA. The lateral strands are originally composed of simple embryonic cells. In later stages, however (Fig. 82 B) their structure is found to be more complicated, a cross section revealing three constituent parts. The first commencements of the formation of nerve-fibres (/) can soon be made out in the innermost (or basal) portion ; these run as two longitudinal bundles sbelow the lateral strands, and are connected with very fine processes of those cells of the strands which become changed into ganglion cells. Besides these paired bundles of fibres, there is, in the rudiment of every pair of ganglia, an unpaired mass of nerve-fibres which perhaps arises from the median strand and gives rise to the transverse commissures. The lateral strands at an early stage become invested with a layer of mesodermal tissue ; this covering, according to Reichenbach, repre- sents the neurilemma, and penetrates not only into the ganglia, but even into the central mass of fibres. The appearance of masses of fibres at the inner or basal side of the lateral strands has probably the significance of an onto- genetic recapitulation of a primitive condition, in which the whole nervous system was an epithelial structure, with the masses of fibres developed on its inner or basal side. Even in quite early stages, Reichenbach could distinguish, in the rudiments of the ganglia, larger and smaller cells of varying histological character. This distinction is also evident in the fully developed condition. The larger elements (Fig. 82 B, g) give rise to the so-called large ganglion -cells in the central nervous system of Astacus. Similar large cells were observed by Nusbaum even in early stages in Mysis. In the later stages, massive accumulations of pigment have occasionally been found in the ventral ganglia ; these, are probably deposited in mesoderm-cells. Instances of this are to be found in the ganglion which is connected with the sixth pair of appendages of Crangon (Kingsley), and in the thoracic ganglia of Mysis (Nusbaum). Reichenbach's view as to the participation of a median invagination in the formation of the chain of ganglia has since received only partial confirmation. Nusbaum, indeed, observed it in Mysis, and Grobben thought that it could be assumed for Moina. Claus, however, denied that a median invagination took part in the formation of the ventral cord in BrancMpus. On the other hand, Nusbaum recently recognised the presence of the median strand in Isopoda (Oniscus, No. 39), in which group Bobretzky (No. 80) and Bullar (No. 81) described the origin of the ventral cord from an unpaired thickening which only at a later stage divided into symmetrical halves. With regard to the development of the peripheral nerves, Reichenbach (No. 65) and Claus (No. 9) have shown that it is probable that these do not originate as outgrowths from the rudiments of the central nervous system, but that they appear as distinct ectodermal thickenings at a time when the whole nervous system is still connected with that layer. The transverse commissures which are double in each segment in BrancMpus arise in a similar way, according to Claus (No. 9). In tracing the development of the brain or supraoesophageal ganglion, we must first study Reichenbach's minute descriptions of its origin in Astacus (No. 65). According to this author, the whole central nervous system of the pre-oral part of the body arises in the form of three pairs of ganglia, equivalent to one another and belong- ing to three separate body-segments (Fig. 83). The most anterior NERVOUS SYSTEM. 163 of these ganglia, which develops in the proximal portion of the eye-stalk, yields the optic ganglion (or, o"), while the remaining two belong to the first and second pairs of antennae and enter into the formation of the brain proper or supraoesophageal ganglion. Of these last, that lying in the segment of the first antenna (antennule) very soon becomes divided by a transverse constriction into a pair of ganglia (a, b), the anterior of which (a) we will distinguish by Packard's name of procerebrum, while the posterior, since it gives off the nerve to the first antenna, has been called the antenmdar Fio. 83.— Embryo of Astucusfluviatilis with the rudiments of all the limbs (after Reichenbach). a, anterior, b, middle, c, posterior portion of the rudiment of the brain ; ab, abdomen ; an', first antenna; an", second antenna; d, mandibular ganglion; md, mandible; mx', first maxilla ; mx1', second maxilla ; mf'tmf", m/'", first, second, and third maxillipedes ; o, ruJi- lneut of the compound eye; o', the part of the optic ganglion which has arisen from the optic invagination ; o", inner part of the optic ganglion ; ol, upper lip ; r, rudiment of the rostrum ; t, telson ; th, fold of the thoracic shield ; I-V, first five pairs of ambulatory limbs. f/anglion (b). It should be mentioned that Reicbenbach believed that he observed, in late stages, a similar transverse division of the posterior pair of ganglia which give off the nerves to the second •antennae, and are therefore called the antennal garxjlia (c) ; this latter division, however, is not so striking, and also not of the some significance as the division which takes place in early stages between the procerebrum and the antennular ganglion. It is an important fact that the pairs of ganglia just described 164 CRUSTACEA. show, according to Reichenbach, in their development, great agree- ment both inter se and with the ganglia of the ventral cord. In each of these pairs of ganglia we can distinguish lateral strands and a median strand ; the lateral strands, in cross section, are seen to be broken up into three parts, as are the ganglia of the ventral cord. The median strand, however, varies in different regions. In the region of the optic ganglion, the two halves of the median strand shift far apart, and enter separately into connection with the corre- sponding ganglia. In the region of the procerebrum and the antennular ganglion, on the other hand, is found that median invagination of this strand which doubtless leads to the develop- ment of the commissural portions of the brain. In the region of the antennal ganglion, again, there is no median invagination. Reichenbach believes that this invagination has shifted forwards and k represented by that established between the antennular ganglia. If, however, we believe that the transverse commissure between the antennal ganglia was originally post-oral, and is perhaps still to be looked for in such a position (Claus, Ko. 78), we shall not be surprised at the absence of the median invagination between these ganglia. In later stages, the invaginations of the median strand are no longer distinct from one another in the region of the procerebrum and the antennular ganglion, a closer union between the different parts of the brain being then generally apparent. The procerebrum, according to Reichenbach, gives origin principally to the "anterior cerebral swelling," while the antennular ganglion is connected with the development of the "lateral swellings" (Krieger,. Dietl). These observations of Reichenbach are to a certain extent in agreement with those of Kingsley, who found in Crangon, apart from the optic ganglia, three consecutive pairs of ganglia taking part in the formation of the brain. The most anterior of these (the procerebrum) Kingsley, however, regarded from the first as an independent structure, it alone being originally pre-oral and homo- logous with the supraoesophageal ganglion of the Annelida. The two pairs of ganglia which follow (the antennular and antennal ganglia) are, when first developed, post-oral in position, and must thus be considered as ganglia of the ventral cord drawn into the cephalic region. The above observations lead us up to the question of the primary segmentation of* the pre-oral portion of the head in the Crustacea. REICHENBACH, with whom also Nusbaum (No. 39) is in essential agreement, has been led by his. NERVOUS SYSTEM. 165 ontogenetic researches to assume for this region of the body three component parts, homonomous with the other body-segments ; these are the optic, the antemmlar, and the antennal segments. In the optic segment, the optic ganglia would represent the segmental pair of ganglia, while the general position of the parts in the Astacus embryo incline Reichenbach to return to the old view of Milne-Edwards, according to which the eye-stalks represent the limbs of this segment. This last view, which subsequently found supporters in Huxley and Spence Bate,* has, however — and, as it appears to us, rightly — been disproved by Claus and Fr. Muller, by reference to the ontogeny of the stalked eye of the Phyllopodan larva (Branchipus) and of the Zoaea of Lucifer, which indicates that the eye-stalks are to be regarded as secondarily abstricted lateral portions of the head which have become indepen- dently movable, while the optic-ganglion, as a part of the brain shifted anteriorly, also attains only a secondary independ- ence. These conclusions remove all ground for assuming the presence of an independent optic segment. With regard to the segment of the body corresponding to the second pair of antennae, there can hardly be any doubt that we here have to do with an origin- ally post-oral body-segment, i.e., with a true trunk-segment, which only second- arily underwent displacement forwards, and thus attained a closer union with the pre-oral parts of the head. This view is supported by the changes brought about during embryonic development in the relative positions of the mouth and the second antennae (p. 157), and above all by the condition ot the nervous system, in which are to be found all transitions between the independent development of the pair of ganglia on this segment and their close fusion with the cephalic mass. It has been known, since the observations of Zaddaoh, that, in Apus, the origin of the pairs of antennal nerves is to be sought post-orally in the oesophageal com- missures, and later researches (Pelseneer, No. 14) showed the ganglia at this point to be connected by means of a post-oral transverse commissure, although this has to some extent been otherwise explained. Similar conditions are found in other Phyllopoda. Thus, in Claus' drawing of the brain of a Cladoceran (Fig. 84), three sections can be distinguished, only the two anterior being pre-oral in position. The most anterior section (c1, corresponding to Packard's * [See also C. Herbst, Uber d. Regeneration v. antennenahnlichen Organen an Stelle v. Augen, Archiv. f Entwickl. Mechanik, Bd. ii., 1895-96, p. 543 ; and B. Hofer, Ein Krebs mit einer Extremitat statt eines Stielauges, Verh. DeuU Zool. GcselL, 1894, p. 82.— Ed.] tut* Fig. 84. — Ventral aspect of the brain of Daphnia similis(after Claus). c1, anterior, &, middle, c3, posterior section of the brain ; go, optic ganglion ; n, nerve of the sensory organ of the neck ; na1, nerve of the first antenna; na", nerve of the second antenna ; n', second nerve of the second antenna ; sc, oesophageal com- missure. 166 CRUSTACEA. procerebium) gives off the nerves to the eyes and to the frontal and other sensory organs, the second section the nerves to the first pair of antennae (na'), while the posterior section (c3), which lies on the course of the oesophageal commissure (sc) behind the oesophagus, supplies nerves to the second antenna (na"). Among other Crustacea, the antennal ganglion undergoes more or less displacement forwards along the commissure and a subsequent fusion with the brain. The acceptance of this view of the alterations in position is attended with a certain amount of difficulty owing to the course of the transverse commissures (origin- ally running behind the oesophagus) connecting this pair of ganglia. The following alternative is offered us : either to suppose that a secondary pre-oral transverse connection is developed, or to assume that the transverse fibres, after the ganglia have completed their wanderings, retain their original course behind the oeso- phagus. Claus (No. 78) believes that this primary connective retains its original position in the adult, and he would identify as that structure the transverse post-oral connection between the oesophageal commissures, which is found in many Crustacea (Apseudes, Stomatopoda, Decapoda), lying in front of the mandibular ganglion. In other cases, this fibrous connective is perhaps fused with the transverse commissure of the mandibular ganglion. The question now arises whether we are justified in considering the part of the brain lying in front of the antennal ganglion as an originally single complex, or whether, in this also, a separation into (two) consecutive segments can be recognised. We must here mention Ray Lank ester's theory (No. 15), which distinguished in the Crustacean brain an anterior section connected with the optic ganglion under the name of archicerebrum, this only being enlarged into a syncerebrum when the ganglia of two following segments (the antennular and antennal segments) are drawn into it. This view has been accepted by Packard (No. 86), who distinguishes in the brain of Ascllus as separate sections : (1) the optic ganglia, (2) the procerebrum, (3) the antennular ganglia, (4) the antennal ganglia. In this case, we should have to find the homologue of the Annelidan brain, which develops out of the neural plate, in the pro- cerebrum alone, while the optic ganglia would be a secondary portion of the brain* arising during the later development of the paired lateral eyes, and the antennular and antennal ganglia would be ganglia of the ventral cord. This view stands opposed to that of Claits (No. 78), according to which the antennular ganglia, together with the procerebrum, formed an originally single complex, the primitive brain. This portion, which is to be deduced from the neural plate of the Annelidan larva, contains the ganglia of the former median sensory organs (Nauplius eye, frontal organ) and the anterior antennae, which are morphologically to be homologised with the Annelidan palps already con- nected with the neural plate. This latter view would be supported by the observations of Reichenbach, according to which the rudiment of the corre- sponding part of the brain is originally to be found as a single complex at the bases of the first antennae, and only later breaks up into two pairs of ganglia. A certain amount of support is also afforded by the peculiar structure of the first antennae, which, as carriers of important sensory organs, do not develop in accordance with the fundamental type of the Crustacean limb, a point specially emphasised by Claus and Boas. Such a heteromorphous structure of the antennule might, indeed, be secondarily acquired, and might be accounted for by its physiological significance already mentioned, as well as * A view first enunciated by Hatsohek (Beitriige zur Entwickelungsgeschichte <1< t Lepidopteren), and later accepted bj Obobben for the Crustacea. SENSORY ORGANS. 167 by its position at the anterior end of the body. The views of Ray Lankester and Packard are supported most strongly by Kingsley's statements con- cerning Crangon (No. 55) : not only was the procerebrum observed to originate independently of the antennular ganglion, but the antennules and their pair of ganglia were found in a distinctly post-oral position. In accepting this view we should have, indeed, to assume, with Ray Lankester, a backward wandering of the mouth. We must, however, await further researches as to the structure of the Crustacean brain, and above all, as to the development of the whole region now under consideration, before forming a decisive judgment. It is evident that, in a discussion as to the primary segmentation of the ante- rior region of the Crustacean body, the question of the morphological value of the first antenna comes to the front. Two alternatives are presented to us ; we must either regard it as a true limb, although somewhat modified in shape, or else, with Boas, deny that it has this significance, and consider it only as a stalked sensory organ (similar to the stalked eyes). Only in the latter case can we regard it as homologous with the primary cephalic tentacle of the Annelida. We, however, see many reasons for regarding the first antenna as a true body- appendage. We have only to recall the similarity between its position and development and those of the latter in the embryo, and its use for swimming purposes in the Nauplius stage and in many Entomostraca, in which the first antenna is sometimes diverted to other purposes {e.g., climbing and sucking). It is only in the higher Crustacea that this limb is distinctly set apart as a sensory organ. If these considerations incline, us to place the first antenna in the series of true trunk-limbs, we then have to ask whether the vestiges of the primary cephalic tentacle, so common among the Annelida, are not to be sought in some other structure. It is not difficult to assume this significance for the so-called frontal sensory organs (Fig. 123, fs, p. 269) found in the young stages of many Crustacea as paired peg-like or filamentous processes innervated from the procerebrum. This view gains in probability by a comparison with Peripatus, in whose embryos similar blunt processes have been observed, while the antennae of Peripatus, according to their development and their relation to the coelomic sacs, must be considered as modified trunk-limbs. If we held this view, to which, naturally, we can only ascribe a hypothetical value, we should be led to distinguish, with Ray Lankester, three sections in the anterior part of the Crustacean body which contains the brain. There would be one actual primary section, originally the only pre-oral section of the body, with the procerebrum, the eyes, and the frontal sensory organs, and two sections following posteriorly, trunk-segments drawn into the head (antennular and antennal segments), for which we must assume an originally post-oral position. We must, however, once more emphasise the fact, before accepting any one of these views, that in discussing these questions we are dealing entirely with hypothetical matters. D. Sensory Organs. Of the details of the development of the unpaired triple Nauplius or Entomostracan eye* nothing is as yet known, but mention should * According to Claus {Kaiserl. Akad. Wissensch. Anz. Wien, 1891), the Nauplius eye is composed of three inverted cup-shaped eyes, in which the nerves enter the retinal cells from the side turned away from, while the rods are directed towards, the pigment cups. A certain similarity with the median eye of the Arachnoida is thus brought about. [Bernard (The Apodidae) claims to have discovered paired rudiments for the Nauplius eye of Apus. — Ed.] 168 CRUSTACEA. be made of the observations of Leydig and Grobben, according to which this eye has a paired rudiment. According to Urbanowicz (No. 23), the eye in Cydops is formed "of three ectoderm-cells, each of which secretes pigment and. becomes a refractive sphere." The development of the paired compound eye has been best studied in the Decapoda (Bobretzky, No. 41 ; Reichenbach, No. 65 ; Kingsley, No. 52 ; Herrick, Nos. 48 and 49 ; and Parker, No. 62). It has also been observed in My sis (Nusbaum, No. 39), Para- podopsis (Boutchinsky, No. 37a), and Brancliipus (Claus, Nos. 8 and 9). The description of the development of the compound eye cannot be separated from that of the optic ganglion. The simplest example of the development of a compound eye is that of Brdnchipus. The rudiment of the compound eye, as well as of the optic ganglion, can here be traced back to a pad-like growth of hypodermis, the superficial parts of which become trans- formed into the eye, while the deeper parts contain the material for the optic ganglion, which is connected with the brain. The several layers of cells which represent the rudiment of the eye, and which must be regarded as a simple thickening of the hypodermis, soon show an arrangement of the elements into a superficial layer (which yields the corneal cuticle and the crystalline cones) and a deeper pigmented layer for the formation of the retinulae, the latter layer being connected with the rudiment of the optic ganglion by fibrous strands. In the lateral parts of the whole rudiment there soon occurs a histological differentiation of the optic ganglion and of the ommatidia which compose the eye, whereas, in the anterior and more median portions, a proliferating hypodermis of embryonic character (Fig. 85) is retained until a later stage ; this constantly yields new elements for the enlargement of the whole rudiment. Strictly speaking, two budding zones (k\ k"), which are distinct from, but in contact with one another, can be distinguished at this point ; one of these (k"), by the production of new ommatidia, adds to the size of the eye itself, while the other and more proximal (k') yields the elements corresponding to the optic ganglion. During these ontogenetic processes, the movable eye-stalks have arisen by simple outgrowth from the lateral parts of the head. The development of the eye in the Schizopoda and the Decapoda is very similar to that in Brancliipus. Here also the compound eye arises from a thickening of the hypodermis, which from the first is in close connection with the rudiment of the optic ganglion. SENSORY ORGANS. 169 Even in early stages, the outer and anterior part can be distinguished as the rudiment of the eye (Fig. 80 A, p. 157, and Fig. 83, o, p. 163), and the inner, posterior part as that of the optic ganglion (Figs. 80, go, and 83, o', o"). The latter is thus, from the very first, in close contact, not only with the rudiment of the eye, but, proximally, with that of the supraoesophageal ganglion. The rudiment of the eye is thus only a part of the ectoderm (Fig. 86 A) which becomes multilaminar and produces from its superficial layers the cells of the cornea and crystalline cones, while the lower layers give rise to the retinulae and the pigment-cells. A basal membrane secreted on the ,, ft' inner surface of this hypodermis (Fig. 86, mb) yields the membrana limi- tans which bounds the eye on the side of the optic gan- glion. Mesoderm- elements are de- posited on this membrane from within, and these, in Mysis, yield the pigment of the innermost of the three pigment-layers. In Mysis, according to Nusbaum (No. 39), differentiation occurs at the time when the stalked eye rises from its substratum, the first differ- entiation of the ommatidia appearing in that dorsally curved lamella which, by its curvature, causes the eye to separate from the food-yolk. In this lamella, a very regular arrangement of the cells both into horizontal layers and into vertical columns takes place early. The horizontal lamination separates the corneal cells from those of the crystalline cones, etc. The vertical arrangement gives rise to two alternating columns of cells, which we may distinguish as ommateal and intermediate columns. In each ommateal column, the most superficial transverse layer is formed by two corneal cells (Claus, No. 78) destined to secrete the corneal lens ; below this comes the layer of the crystalline cone, also consisting of two cells. In accord- ance with this number of cells, Grenacher was able to prove the origin of the crystalline cone from two separate segments, the Fig. 85.— Left eye of a young Branchipus, seen from the ventral side (after Claus). go, optic ganglion ; k', budding zone for the optic ganglion ; 7c", budding zone for the ommatidia ; m, muscle of the eye. 170 CRUSTACEA. boundaries of which can be recognised in the adult crystalline cone. The cells in the lower layers of the ommateal column no doubt give rise to the elements of the retinulae, but Nusbaum is inclined to- derive these from the intermediate columns. Claus derives from these columns the anterior and posterior pigment- cells which surround the crystalline cone in Mysis. The observation of these processes is rendered difficult by the early deposition of pigment which is to be found within the rudiment of the eye in two layers, and in a third mesodermal layer below it. tsc^reS5f^w5 Fig. 86.— Sections through the compound eye of the American Lobster (Homarus americamis) in three stages of development (after Parker). A, transverse section through the optic lobe in a young stage. B, older stage when the optic rudiment (r) and the optic ganglion (go) are beginning to be separated by the development of a basal membrane (mb). C, cross section of still older stage, c, rudiment of the brain ; go, optic ganglion ; mb, basal membrane ; r, rudiment of the eye (retinogen). The account here given of the ontogeny of the eye in Mysis resembles that given by Herrick (No. 48) in connection with Alpheus, and Parker (No. 62) for Homarus. The optic lobes here develop, by proliferation of the ectoderm, into a multilaminar rudiment (Fig. 86 A). (Herrick considers that indifferent elements out of the yolk also contribute to their formation). There then takes- place a separation into a superficial and a lower part (Fig. 86 B and C). The first layer (retinogen) becomes the rudiment of the eye (r), while that of the optic ganglion (go) arises from the cell-mass of the lower layer (gangliogen). In later stages the two layers are separated by a cuticular basal membrane, through which strands of nerve-fibres pass. In the rudiment of the eye itself, which we have called the retinogen, t\w separate ommatidia are said to develop (Herrick), the corneal cells forming into groups of two in the most superficial layer, and SENSORY ORGANS. 171 the crystalline cone-cells into groups of four in the subjacent layer, while, in the lowermost layer, the cells of the retinulae form bundles of seven each, these bundles reaching and enclosing the lowermost, pointed ends of the cells of the crystalline cone. The separate rudiments of the ommatidia are divided by numerous undifferentiated ectoderm-cells. In this last point the accounts of Parker and Herrick differ. In Homarus it is said (Parker) that the separate rudiments of the ommatidia lie close to one another, and are not separated by any kind of intermediate pillars. Three layers can be distinguished ; from the outermost arise the corneal hypodermis- cells and the anterior pigment-cells (distal retinulae), from the median layer the cells of the crystalline cone, and from the lowest the actual retinulae.* The paired lateral eye of the Isopoda also develops in a similar way. In Bullae's account of Cymothoa (No. 81) the rudiment of the eye stands in close connection with that of the optic ganglion. The two proceed from one and the same ectodermal thickening. While the inner layers of this thickening become detached for the formation of the optic ganglion which is connected with the brain, a superficial hypodermal thickening becomes marked off by a pigmented basal membrane. This hypodermal thickening represents the rudiment of the eye, in which the separate ommatidia seemed to be marked off from one another by a strongly pigmented mantle of cells. The details of the development of the ommatidia were not followed in this case. Iii the cases we have hitherto described, the development of the eye is comparatively simple, but in Astacus (Eeichenbach, No. 65) and Crangon (Kingsley, No. 52) complication occurs through an invagination which forms on the boundary between the rudiment of the eye and that of the ganglion. This invagination, which, according to Eeichenbach, is replaced at a certain stage of develop- ment by a more solid ingrowth, gives rise to the optic fold between the rudiment of the eye and the gangliogen ; in this fold an inner and an outer layer are recognisable. Although Reichenbach did not closely follow the future fate of the optic fold, it appeared probable to him that the outer layer enters into the formation of the eye and yields the retinular layer, while the inner fold enters into the formation of the optic ganglion. It was pointed out, especially by Carriere (No. 44), that in such a method of develop- ment of the retinular layer the position of its cells is reversed, their basal ends being directed towards the cells of the crystalline cone, and their upper ends towards the rudiments of the ganglion, and that we must thus assume a later re-arrangement in the retinular area, such as takes place among the Araneae, but has not yet been observed among the Crustacea. The suggestion made by Patten appears to us probable, viz. — that the optic fold has nothing to do with the formation of the eye, but merely yields the material * [Cf. Parke r, G. H., Retina and Optic Ganglia in Decapoda, Mittheil. Stat. Zool. Neapel., xii., 1895.] 172 CRUSTACEA. for the enlargement of the optic ganglion. If so, it would correspond to the proximal budding point (for the enlargement of the ganglion, Fig. 85, h') in the eye-stalk of Branehipus. This last view has recently been accepted by Kingsley also, who originally thought that the layer of the crystalline cones and the retinular layer arose from the outer wall of the optic invagination. The above view receives its chief support, as Carriere pointed out, from the position of a pigmented layer of mesoderm - cells, which, according to Reichenbach, develops between the outer wall of the optic fold and its crystalline cone-layer, and Avhich is, nevertheless, evidently identical with the layer of pigment-cells below the basal membrane of the eye, described above (p. 169) for My sis. With regard to the significance of the separate parts of the ommatidium, as to which Grenacher and Patten have recently taken opposite views, attention should be drawn to Parker's observations, which revealed a con- nection of the retinular cells with fine nerve -fibres, while the crystalline cone-cells, which doubtless reach to the basal membrane, end at that point. This is in agreement with the view of Grenacher, who saw in the retinular cells the percipient elements, while Patten considered that the crystalline cone-cells known as retinophorae were the elements connected with the nerves. In the development of the compound eye of the Cladocera, which was care- fully described by Grobben (No. 11), special interest is awakened by the formation of an integumental fold which grows over the eye, cutting off a hemispherical precorneal space. The movement of the sunken eye is thus assisted. Similar conditions are found in Apus, Esthcria, Limnaclia, and Limnetis. The compound eyes, in these forms and perhaps also in the Ostracoda, may be regarded as movable stalked eyes with degenerated stalks which have sunk below the surface. Where, as in the Cladocera, an unpaired compound eye is found, this must be considered to have arisen by the fusion of paired rudiments. An auditory organ was observed by Reichenbach (No. 65) in Astacus, as a dorsal invagination in the basal joint of the antennule. Even in early stages, the ectodermal sensory epithelium, which probably yields the auditory ridges, is distinguished by the multi- laminar and regular arrangement of its cells. Nusbaum (No. 39) similarly was able to observe the origin of the auditory sac in Mysis in the endopodite of the last pair of pleopoda out of an ectodermal invagination. E. Gills. The branchiae first appear as simple outgrowths of the superficial body- epithelium (ectoderm), within which lacunar blood-spaces traversed by connective tissue-strands develop (Reichenbach). We may with some probability regard all the branchial sacs or tubes which belong to the outer side of the basal joints of the limbs, INTESTINAL (,'ANAL. 173 and can thus be described as epipodial gills (p. 195), as homologous structures throughout the whole of the class Crustacea. We may also, perhaps, derive them from the branchial tubes of the Annelida. On the other hand, it must be pointed out that branchial outgrowths develop at other points also, e.g., on the exopodite of the pleopoda (Squilla), or on the endopodite of the same limbs (Siriella), as dorsal appendages in certain Ostracoda (Asterope), as mantle-folds in the Balanidae, etc. These naturally cannot be homologised with the epipodial gills. The epipodial branchial sacs present in a single row in the Phyllopoda are replaced in the Decapoda by three rows of branched tubes, which, according to their exact points of attach- ment, are distinguished by Huxley as podobranchiae, arthrobranchiae, and pleurobranchiae. Instead of these, we find in the Euphausidae and Lophogastridae only a single row of dendriform branched tubes, so that Claus raises the question whether the three rows of gills of the Decapoda may not be derived from the principal branches of the Schizopodan gill which have shifted apart. F. Intestinal Canal. The intestinal canal arises here, as in most groups of animals, from three separate rudiments, the fore-gut and hind-gut arising as ectodermal invaginations (the stomodaeum and the proctodaeum), while the mid-gut is formed from the cells of the entoderm. Whereas the two former approach their adult form by a series of comparatively simple changes, the development of the latter is brought about by far more radical ontogenetic processes, on account of disturbances due to the presence of the food-yolk, these processes also varying in the different orders of Crustacea. There is some variation among the sub-groups of the Crustacea with regard to the time of the appearance of the fore- and hind-guts. In the Entomostraca, the rudiment of the fore-gut, as a rule, appears first. This is also the case in Asellus, Gammarus, and Mysis, while, in Oniscus, the hind-gut develops first. In the Decapoda, the proctodaeal invagination usually appears first, a fact connected with the early development of the abdomen. The position of this invagination with relation to the closed blastopore is of importance. In Moina, according to Grobben, the point at which the blastopore closes corresponds to the oral aperture, while in the Decapoda, it always lies close to the future anal orifice. According to Reichenbach, in Astacus, it lies some- what behind the point at which the anal aperture is forming, as is also the case in Atyephyra (Ischikawa). The opposite, according to Lebedinsky, holds true for Eriphia. Here the proctodaeal invagination develops behind the blastopore. Kingsley, on the contrary, believes that, in Crangon, the procto- daeal invagination corresponds exactly to the point at which the blastopore 1 74 CRUSTACEA. closed, this being also maintained by Bobretzky for Astacus and agreeing with P. Mayer's account of JSupagurus. Hoek found that, in the free-living Copepoda, the point at which the blastopore had closed corresponded to the position of the future anal aperture, and Nassonow made the same observation in Balanus, so that, as a rule in the Crustacea, the position of the blastopore may be assumed to be in the neighbourhood of the anal aperture (p. 141). The time at which the three rudiments unite to form a single canal varies according to the stage of development at which the larva leaves the egg. In the free-living Copepoda (Cetochilus), the intestinal canal is completed, at an early stage, while in the Decapoda, communication is established between the fore- and hind-guts and the mid-gut usually at a later stage. The hind-gut here appears to show its adult structure sooner than the fore-gut. We have as yet very little accurate information as to the manner in which the mid-gut develops in the Entomostraca. In Moina, the entoderm-cells first form a solid strand, a cross section of which reveals a radial arrangement of the cells, but no lumen (Grobbex). In Cetochilus, on the contrary, the entoderm-sac which is formed by invagination seems to be transformed direct into the mid-gut. In many other Entomostraca, the mid-gut rudiment can be recognised as a central mass of cells filled with food-yolk (Balanus, Lang, Nassonow). In later stages, the nuclei of the entoderm-cells, with the protoplasm which surrounds them, migrate to the surface, and as the food-yolk is gradually assimilated, the cavity of the mid-gut appears within. This is also the case in Palaemon (p. 132). In the parasitic Copepoda also, the mid-gut, according to Van Beneden (Xo. 17), appears to develop in this way. The mid-gut, filled with food-yolk, is connected at its anterior end with the stomodaeal invagination, and at its posterior end with the proctodaeal invagi- nation. The development of the mid-gut is best known in the Decapoda. In Astacus, where the cells of the entoderm -vesicle absorb the whole of the food-yolk, without thereby disturbing the conformation of the vesicle, the epithelium of the mid-gut arises by the shifting of the nuclei to the surface of the yolk-bearing entoderm-pyramids, a separation of the cells from the food-yolk there taking place ; as the entoderm-cells increase in number they become arranged into an epithelium which now covers the surface of the disappearing yolk (p. 130). At the same time, the whole rudiment of the mid-gut, by constriction from without, assumes a lobate form. Paired anterior lobes form which become connected with the median rudiment of the mid-gut, at whose posterior dorsal portion can be recognised another swelling, the rudiment of the dorsal caecum of the mid-gut. The development of the mid-gut epithelium just described first takes INTESTINAL CANAL. 175 place at the point where the entoderm-vesicle and the rudiment of the hind-gut are in contact, and where soon an entodermal epithelial plate can be seen (Fig. 63 B, ep, p. 131). The same has been •observed at the point from which the formation of the hepatic tubes commences. Separate centres of epithelial formation are to be found at the anterior, lateral and posterior parts of the mid-gut correspond- ing to the anterior, lateral, and posterior hepatic lobes of the adult animal; at these points, the epithelium soon grows out to form the primary hepatic tubes. The rudiment of the posterior pair of tubes seems from the first to be connected with the entoderm-plate described above. As the epithelium continues to grow over the rest of the entoderm-vesicle, the central part of the alimentary canal arises, receiving the efferent ducts of the mid-gut gland (liver) ; this central part, in the adult, is of no great extent. Its musculature arises by the deposition of mesodermal elements. The stomodaeal invagination soon becomes divided into a narrower oesophageal portion and an inner and more swollen portion, the rudiment of the so-called stomach. In the latter can be recognised the rudiments of the tooth-plates as epithelial thickenings, and those of the gastrolith-sac as two diverticula diverging on the ventral side. The young Astacus hatches with two completely developed gastroliths (Reichenbach). The mid-gut becomes connected with the fore- and hind-guts only at a late stage. The mid -gut develops in a similar manner in those Decapoda in which the entoderm-sac does not retain its continuity, but breaks up into single cell- elements, which become distributed in the food-yolk (Palaemon, Eupaguriis, Eripliia, Atyephyra, Crangon, etc.). In these also, the entodermal elements finally rise to the surface, and yield the mid-gut in the way described above. Here also the first appearance of this epithelium was observed in contact with the blind inner end of the proctodaeal invagination (Fig. 64 C, ep). Three pairs of originally distinct hepatic rudiments, however, appear to be added to it {Crangon, Kingsley). The formation of the mid-gut in the Arthrostraca differs from the type above described as occurring in the Decapoda in that here the mid-gut epithelium does not proceed from elements scattered in the food-yolk, but from a paired lateral mass of cells which lies superficially on the yolk and gradually grows round it (p. 139, etc.), while within the yolk, vitellophags are found only in isolated cases (Oniscus, Nusbaum) ; in other cases -(Porcellio, Amphipoda), cell-elements are here altogether wanting. By the gradual circum- crescence of the yolk from the two sides by the paired entoderm- rudiment, the mid-gut vesicle closes, the very large primary hepatic 176 CRUSTACEA. sacs forming by constriction at its sides. These hepatic sacs give rise later, by longitudinal constriction, to four or six tubes. In some cases (Oniscus, Caprella, Sunamphithoe), the formation of the hepatic tubes precedes the development of the mid-gut vesicle. In most cases, the chief part of the entoderm is used up in the forma- tion of the hepatic rudiment, only a small part entering into the formation of the central portion of the intestinal canal. The latter is almost exclusively composed of the fore and hind-guts, only a short tract in the immediate neighbourhood of the openings of the hepatic tubes being entodermal in origin. As diverticula of the posterior section of the mid-gut, there arise in Gammarus (Pereyas- lawzewa) those paired tubes {urinary glands), the entodermal nature of which was recognised even in the anatomical researches of Nebeski. " The homologising of these urinary tubes with the Mal- pighian vessels of the Insecta is on this account inadmissible, the latter belonging to the hind-gut, and therefore having an ectodermal origin. The observations of Bullar (No. 81) on the development of the mid-gut in Cymothoa are in fairly close agreement with those of Nusbaum for Oniscus. There are, however, no so-called yolk-cells here inside the very considerable mass of food-yolk ; the formation of the mid-gut proceeds from the inner cell-layer of the germ-band. The first trace of a separate entodermal rudiment is found in a paired mass of cells lying somewhat behind the stomodaeal invagination ; this mass of cells gives rise to three pairs of hepatic tubes. At a later stage, a layer of cells connected with the epithelium of the hepatic tubes grows over the whole of the food-yolk. The rudiment of the mid-gut now consists of the yolk-vesicle thus formed and the hepatic tubes communi- cating with it. The anterior end of the yolk-vesicle is connected with the stomodaeal invagination. The proctodaeal invagination, on the contrary, is not in contact with the posterior end of the yolk-sac, but runs forward over its dorsal side so as to pass into it quite near the stomodaeal invagination. The yolk-vesicle appears now as a ventral diverticulum of the alimentary canal, but, as most of it undergoes absorption, it results that here also only quite a small part of the definitive intestinal canal near the points of entrance of the hepatic tubes belongs to the mid-gut (cf. p. 139 on Ligia). The above description of the formation of the mid-gut in the Arthrostraca, founded on the observations of Bullar, Nusbaum, Perf.yaslawzkwa, and Rossijskaya, applies, according to Nusbaum, to Mysis also (Figs. 65 and 66, p. 135). The entoderm here originally lies as a mass of cells in the most posterior section of the germ-band (p. 134). The entoderm-cells soon increase in number, and spread out over the whole ventral surface of the embryo. At a later stage they also reach the lateral and dorsal parts, the food-yolk in this way becoming surrounded by a layer of entoderm-cells While this circuni- crescence is going on in the posterior part of the embryo, the entoderm in the anterior part (behind the mandibular segment) forms two lateral grooves consisting of large granular cells (Fig. 66, I) ; these are the rudiments of the HEART. 177 hepatic tubes, which become connected later on the ventral side by entodermal epithelium. Two longitudinal folds arise and fuse with the inwardly curving edges of the grooves, and thus separate the hepatic tubes from the middle portion of the intestinal canal. At the same time, the hepatic tubes become divided through longitudinal folding into four secondary tubes in the same way as in the Arthrostraca. It appears that, in Mysis, when the mid-gut vesicle forms, the whole of the food-yolk is not taken up into it, but part of it comes to lie in the cephalic region outside of the intestine, and thus in the body-cavity. The food-yolk has a similar position in Moina. G. Heart. ' In describing the development of the heart, we must take as our starting point the observations made by Claus (Nos. 8 and 9) on Branchipus. The somatic layer of the mesoderm here forms a cell- stratum originating on the ventral surface, and now divided into separate segments; this mesoblast gradually grows upwards under the lateral parts of the integument. The dorsal edge of these .growing mesoderm-segments is formed by a single row of large cells (cardioblasts, Nusbaum, Figs. 87, c, and 88 A, c), which later assume a crescentic shape (Fig. 88 B, c), so that a channel is now formed on each side. These semilunar channels, meeting and fusing in the middle line, give rise to a dorsal tube (Fig. 88 C, c). The latter is from the very first divided up into separate segments (chambers) corresponding with the primitive mesoderm-segments ; the boundaries of these chambers persist as the lateral ostia. This origin of the heart seems distinctly to show that its lumen must be considered as a remains of the primary cleavage-cavity (Butschli, Schimkewitsch). The ostia develop at the boundaries of the mesoderm-segments. The cardio- blasts become transformed into the muscle-cells of the wall of the heart. During their development, the latter have their lower ends connected with the dorsal portion of the intestinal muscle-layer. From this point, a horizontal septum stretches out towards the body-wall ; this is the pericardial diaphragm (Fig. 88 C, s), which separates an upper portion of the coelom containing the heart from the rest of the body-cavity. This septum is found in all Crustacea. The heart of Oniscus develops in a similar way by the fusing of two grooves which arise from a single row of cells on each side (Nusbaum). The formation of the heart in the Amphipoda (Pereyaslawzewa, Kossijskaya) also passes through a similar stage. While, however, in Oniscus, the posterior parts of the l>o E. Ray. Appendages and Nervous System of Apus eaneriformia. Quart. Journ. Micro. Sci. Vol. xxi. 1881. LITERATURE. 183 16. Weismann, A., and Ischikawa, C. Ueber die Paracopulation im Daphnidenei, sowie liber Reifung und Befruchtung dessel- ben. Zool. Jdhrb. Bd. iv. Abth.f.Anat, 1891. APPENDIX TO LITERATURE ON PHYLLOPODA. I. Hacker, V. Ueber die Entwicklung der Wintereies von Moina paradoxa. Ber. Ges. Freiburg. Bd. vii. and viii. 1893. II. Kerherve\ L. B. De l'apparition provoquee des ephippies chez les Daphnies (Parthenogenesis of Daphnia). Mem. Soc. Zool. Tom. v. 1892. III. Samassa, P. Die Keimblaitterbildung bei den Cladoceren. Archiv. f. mikr. Anat. Bd. xli. 1893. Copepoda. 1 7. Beneden, E. van. Recherches sur l'embryogenie des Crustaces. IV. Developpement des genres Anchorella, Lernaeopoda, Brachiella et Hessia. Bull. Acad. Roy. Belgique (2). Tom. xxix. 1870. 18. Claus, C. Zur Anatomie und Entwicklungsgeschichte der Copepoden. Archiv. f. Natiirgesch. Bd. xxiv. 1858. 19. Claus, C. Die frei lebenden Copepoden etc. Leipzig, 1863. 20. Fritsch, J. A. Note preliminaire sur l'ontogenie de nos Cope- podes d'eau douce. Zool. Anz. Jahrg. v. 1882. 21. Grobben, C. Die Entwicklungsgeschichte von Cetochilus septentrionalis. Arb. Zool. Inst. Wien. Bd. iii. 1881. 22. Hoek, P. P. C. Zur Entwicklungsgeschichte der Entomostraken. II. Zur Embryol. der frei lebenden Copepoden. Niederl. Archiv. f. Zool. Bd. iv. 1877-1878. 23. Urbanowicz, Fel. Zur Entwicklungsgeschichte der Cyclopiden. Zool. Anz. Jahrg. vii. 1884. 24. Urbanowicz, FeL. Contributions a l'embryologie des Copepodes. Archiv. Slav. Biol. Tom. i. 1886. APPENDIX TO LITERATURE ON THE COPEPODA. I. Hacker, V. Die Keimbahn von Cyclops. Archiv. f. mikr. Anat. Bd. xlix. 1893. II. Schimkewitsch, W. Studien uber parasitische Copepoden. Zeitschr.f. Wiss. Zool. Bd. lxi. 1896. Cirripedia. 25. Beneden, E. van. Recherches sur l'embryogenie des Crustaces. III. Developpement de l'ceuf et de l'embryon des Sacculines (Sacculina carcini Thomps.). Bull. Acad. Roy. Belgique (2). Tom. xxix. 1870. 184 CRUSTACEA. 26. Bovallius, C. Embryologiska Studier I. Om Balanidernas Utveckling. Stockholm, 1875. _7. Hoek, P. P. C. Zur Entwicklungsgeschichte der Entomostraken. I. Embryologie von Balanus. Niederl. Archiv. f. ZooL Bd. iii. 1876-1877. 28. Lang, A. Die Dotterfurchung von Balanus. Jen. Zeitschr. f. Naturw. Bd. xii. 1878. 29. Nassonow, N. Zur embryonalen Entwicklung von Balanus. Zool. Anz. Jahrg. viii. 1885. 30. Nussbaum, M. Yorl Ber. iiber die Ergebnisse einer mit Unter- stiitzung der k. Acad, ausgef. Reise nach Californien. Sitz- ungsber. Akad. Wiss. Berlin. 1887. 31. Nussbaum, M. Anat. Studien an Calif ornischen Cirripedien. Bonn, 1890. APPENDIX TO LITERATURE ON CIRRIPEDIA. I. Groom, T. T. Early Development of ,the Cirripedia. Tr. Boy. Soc. London. Vol. clxxxv. 1894. Leptostraca. 32. Claus, C. Ueber den Organismus der Nebaliden und die system- atische Stellung der Leptostraken. Arb. Zool. Inst. Wien. Bd. viii. 1888. 33. Metschnikoff, E. On the Development of Nebalia (Russian). Zapiski imp. Acad. Nauk. St. Petersburg. Tom. xiii. 1868. 34. Metschnikoff, E. Sitzungsber. d. Verhandl. deutscher Natur- f or scher zu Hannover. 1865. p. 218. Cumacea. 35. Blanc, H. Developpement de l'ceuf et formation des feuillets primitifs chez la Cuma Rathkii Kroy. Becueil Zool. Suisse. Tom. ii. 1885. 36. Dohrn, A. Ueber Bau und Entwicklung der Cumaceen. Jen. Zeitschr. f. Naturw. Bd. v. 1870. APPENDIX TO LITERATURE ON CUMACEA. I. Uoutchinsky, P. Zur Embryologie der Cumaceen. Zool. Anz. Bd. xvi. 1893. Schizopoda. 37. Beneden, E. van. Recherches sur l'embryogenie des Crustaces. II. Developpement des Mysis. Bull. Acad. Roy. Belgique (2). Tom. xxviii. 1869. LITERATURE. 185 37a. Boutchinsky, P. Observations on the Development of Para- dopsis cornuta Czern. (Russian). Zapiski Novoross. Obshch. Estestv. Odessa, Tom. xiv., 2. 1890. 38. Nusbaum, J. Zur Embryologie der Schizopoden (Mysis cham- aeleo). Biol. Centralbl. Bd. vi. 1887. 39. Nusbaum, J. L'embryologie de Mysis chamaeleo (Thompson). Archiv. Zool. Exper. (2). : Tom. v. 1887. 40. Wagner, Jul. Sur le developpement des Schizopodes. Comm. prel. I. Sur la formation des feuillets embryonnaires de Neomysis vulgaris var. baltica Czern. Rev. Sc. Nat. Soc. Natural. St. Petersboury. Ann. i. 1890. II. La formation et la signification du sillon caudal de la Neomysis vulgaris var. baltica Czern/ Rev. Sc. Nat. Soc. Natural. St. Peters- bourg. Ann. i. 1890. APPENDIX TO LITERATURE ON SCHIZOPODA. I. Bergh, R. S. Beitrage zur Embryologie def Crustaceen. Zur Bildungsgeschichte des Keimstreifen von Mysis. Zool. Jahrb. (Anat.).' Bd. vi. 1893, II. Wagner, J. Zur Entwicklungsgeschichte der Schizopoden. Zool. Anz. Bd. xvii. 1894. Decapoda. 41. Bobretzky, N. On the Embryology of the Arthropoda (Development of Astacus and Palaemon). Zapiski Kievsk. Obshch. Estestv. Tom. iii., vuip. 2. See Hoyer, Jahresber. f. Anat. and Phys. von Hofmann and Schwalbe. Bd. ii. 1873. 42. Brooks, W. K. The Embryology and Metamorphosis of Sergestidae. Zool. Anz. Jahrg. iii. 1880. 43. Brooks, W. K. Lucifer, a study in Morphology. Tr. Roy. Soc. London. Vol. clxxiii. 1883. 44. Carriere, J. Bau und Entwicklung des Auges der zehnfiissigen Crustaceen und der Arachnoiden. Biol. Centralbl. Bd. ix. 1889. 45. Dohrn, A. Untersuchungen iiber Bau und Entwicklung der Arthropoden. 6. Zur Entwicklungsgeschichte der Panzerkrebse (Decapoda Loricata). Zeitschr. f. Wiss. Zool. Bd. xx. 1870. 46. Faxon, W. The development of Palaemonetes vulgaris. Bull. Mus. Comp. Zool. Vol. v. 1879. 47. Haeckel, E. Studien zur Gastraea-Theorie. II. Die Gastrula und die Eifurchung der Thiere. Jena, 1877. Jen. Zeitschr. f. Naturw. Bd. ix. 1875. 186 CRUSTACEA. 48. Herrick, F. H. The development of the compound eye of Alpheus. Zool. Anz. Jahrg. xii. 1889. 49. Herrick, F. H. Notes on the Embryology of Alpheus and other Crustacea, and on the development of the compound eye. John Hopkins Univ. Give. Vol. vi., No. 54, 1886 ; and Vol. vii., No. 63, 1887. 50. Herrick, F. H. The development of the American Lobster : Homarus americanus. John Hopkins Univ. Circ. Vol. ix. 1890. No. 80. 50a. Herrick, F. H. The development of the American Lobster. Zool. Anz. Jahrg. xiv. 1891. 51. Ischikawa, Ch. On the development of a freshwater macrurous crustacean Atyephyra compressa, De Haan. Quart. Journ. Micro. Sci. (2). Vol. xxv. 1885. 52. Kingsley, J. S. The development of the compound eye of Crangon. Journ. Morph. Vol. i. 1887. 53. Kingsley, J. S. The development of Crangon vulgaris. 2nd paper. Bull. Essex Inst. Vol. xviii. . 1887. 54. Kingsley, J. S. The development of the compound Eye of Crangon. Zool. Anz. Jahrg. ix. 1886. 55. Kingsley, J. S. The development of Crangon vulgaris. 3rd paper. Bull. Essex Inst. Vol. xxi. 1889. 56. Lebedinsky, J. On the development of Eriphia spinifrons (Russian). Zapiski Novoross. Obshch. Estestv. Odessa, 1889. 57. Lebedinsky, J. Einige Untersuchungen liber die Entwicklungs- geschichte der Seekrabben. Biol. Centralbl. Bd. x. 1890. 58. Lereboullet, A. Recherches d'embryologie comparee sur le deVeloppement du Brochet, de la Perche et de l'Ecrevisse. Mem. Savans. Etrang. Paris. Vol. xvii. 1862. 59. Mayer, P. Zur Entwicklungsgeschichte der Decapoden. Jen. Zeitschr. f. Naturiu. Bd. xi. 1877. 60. Mereschkowski, C. Eine neue Art von Blastodermbildung bei Decapoden. Zool. Anz. Jahrg. v. 1882. 61. Morin, J. On the Development of Astacus (Russian). Zapiski Novoross. Obshch. Estestv. Odessa. Tom. xi. 1886. 62. Parker, G. H. The history and development of the eye in the Lobster. Bull. Mus. Comp. Zool. Cambridge. May, 1S90. 63. Rathke, H. Ueber die Bildung und Entwicklung des Fluss- krebses. Leipzig, 1829. 64. Reichenbach, H. Die Embryonalanlage und erste Entwicklung des Flusskrcbses. Zeitschr. f. Wiss. Zool. Bd. xxix. 1877. LITERATURE. 187 €5. Reichenbach, H. Studien zur Entwicklungsgeschichte des Flusskrebses. Abhandl. Senkenberg. Nat. Gesellsch. Frank- furt. Bd. xiv. 1886. G6. Schimkewitsch, W. Einige Bemerkungen iiber die Entwick- lungsgeschichte des Flusskrebses. Zool. Anz. Jahrg. viii. 1885. 67. Schimkewitsch, W. Some Observations on the Development of Astacus leptodactylus (Russian). Isvyest. imp. Obshch. Lyubit. estestv. Antrop. i. Ethnog. Moscow. Tom. 1. 1886. 68. Weldon, W. F. R. Coelom and Nephridia of Palaemon serratus. Journ. Marine Biol. Assoc. Vol. i. 1889. APPENDIX TO LITERATURE ON DECAPODA. I. Allen, J. E. Nephridia and Body-cavity of the Larva of Palaemonetes varians. Proc. Roy. Soc. London. Vol. Hi. 1893. IT. Bouvier, E. L. Sur le Developpement embryonaire des Galatheides du genre Diptychus. Compt. Rend. Ac. Sci. Paris. Tom. cxiv. III. Bompus, H. C. The Embryology of Homarus americanus. Journ. Morph. Vol. v. 1891. IV. Boutchinsky, P. Zur Entwicklungsgeschichte von Gebia litoralis. Zool. Anz. Bd. xvii. 1894. V. Edwards, P. Milne-, and E. L. Bouvier. Sur le Developpe- ment embryonaire des Galatheides abyssaux du genre Diptychus. Compt. Rend. Ac. Sci. Paris. 1891-92. VI. Gorham, F. P. The Cleavage of the Egg of Virbius zosteri- cola. Journ. Morph. Vol. xi. 1895. VII. Parker, G. H. The Retina and Optic Ganglia in Decapoda. Mittheil. Stat. Zool. Neapel. Bd. xii. 1895. VIII. Roule, L. Developpement des Decapodes. 1, Du Paleinon serratus. Ann. Sci. Nat. Zool. (viii.). ii. 1896. IX. Weldon, W. F. R. The Formation of the Germ-layers in Crangon vulgaris. Quart. Journ. Micro. Sci. xxxiii. Amphipoda. 69. Bessels, E. Einige Worte iiber die Entwicklungsgeschichte und den morphologischen Werth des kugelformigen Organs der Amphipoden. Jen. Zeitschr. f. Naturw. Bd. v. 1870. 70. Pereyaslawzewa, Sophie. Le developpement de Gammarus poecilurus Rathke. Bull. Soc. Natur. Moscou (2). Tom. ii. 1888. 188 CRUSTACEA. 71. Pereyaslawzewa, Sophie. Le developpement de Caprella ferox Chrnw. Bull. Soc. Natur. Moscou (2). Tom. ii. 1888. 7l\ Rossijskaya, Marie. Le developpement d'Orchestia littorea. Bull Soc. Natur. Moscou (2). Tom. ii. 1888. 73. Rossijskaya - Koschewnikowa, M. Le developpement de la Sunamphithoe valida Czerniavski et de PAmphithoe picta Rathke. Bull. Soc. Natur. Moscou. 1890. 74. Uljanin, Y. Blastoderm- und Keimblatterbildung bei Orchestia Montagui und mediterranea. Zool. Anz. Jahrg. iii. 1880. 75. Uljanin, V. Zur Entwicklungsgeschichte der Amphipoden. Zeitschr. f. Wiss. Zool. Bd. xxxv. 1881. 76. Della Valle, A. Deposizione, fecondazione e segmentazione- delle uova del Gammarus pulex. Atti della Soc. dei Natu- ralisti di Modena (3). Vol. viii. 1889. 77. De la Valette St. -George, A. Studien liber die Entwicklung der Amphipoden. Abh, Naturf. Gesellsch. Halle. Bd. v. 1860. APPENDIX TO LITERATURE ON AMPHIPODA. I. Bergh, R. S. Die Drehung des Keimstreifens und die Anlage des Dorsal organes bei Gammarus pulex. Zool. Jahrb. (Anal). Bd. vii. 1893; ! Anisopoda. 78. Claus, C. Ueber Apseudes Latreillii Edw. und die Tanaiden. II. Arb. Zool. Inst. Wien. 1888. Bd. vii. Isopoda. 79. Beneden, E. van. Recherches sur Pembryogenie des Crustaces. I. Asellus aquaticus. Bidl. Acad. Roy. Belgique (2). Tom. xxviii. 1869. 80. Bobretzky, K Zur Embryologie des Oniscus murarius. Zeitschr. f. Wiss. Zool. Bd. xxiv. 1874. 81. Bullar, J. F. On the development of the parasitic Isopoda. TV. Roy. Soc. London. Vol. clxix. 1878. 82. Claus, C. Ueber die morphologische Bedeutung der lappen- formigen Anhiinge am Embryo der Wasserassel. KaiserL A bid. Wt8sensch. Anz. Wien. 1887. 83. Dohrn, A. Die embryonale Entwicklung des Asellus aquaticus. Zeitschr. f. Wiss. Zool. Bd. xvii. 1867. LITERATURE. 189 -84. Grosglik, S. Schizocol oder Enterocol ? Zool. Anz. Jahrg. x. 1887. •85. ISTusbaum, J. L'embryologie d'Oniscus murarius. Zool. Anz. Jahrg. ix. 1886. 85a. Nusbaum, J. Beitrage zur Embryologie der Isopoden (Ligia oceanica). Biol. Gentralbl. Bd. xi. 1891. 86. Packard, A. S. On the structure of the brain of the sessile- eyed Crustacea. Mem. National Acad. Sci. Washington, 1884. Bd. in. 87. Rathke, H. Untersuchungen iiber die Bildung und Entwick- lung der Wasserassel. Leipzig, 1832. 88. Rathke, H. Recherches sur la formation et le developpement de l'Aselle d'eau douce. Ami. Sci. Nat. Zool. (2). Tom. ii. 1834. 89. Rathke, H. Zur Morphologic Reisebemerkungen aus Taurien. Riga and Leipzig, 1837. 90. Reinhard, W. Sur le developpement de Porcellio scaber. Trav. Soc. Natur. Kharkoiv. Tom. xx. 1887. 91. Reinhard, W. Zur Ontogenie des Porcellio scaber. Zool. Anz. Jahrg. x. 1887. 92. Roule, L. Sur revolution initiale des feuillets blastodermiques chez les Crustaces isopodes (Asellus aquaticus L. et Porcellio scaber Latr.). Comp. Rend. Ac, Sci. Paris. Tom cix. 1889. APPENDIX TO LITERATURE ON ISOPODA. I. McIntosh, J. P. The Embryology of the Isopod Crustacea. Joum. Morph. Vol. xi. II. Nusbaum, J. Matergaly do embryogenii i histogenii rdwnonogou (Isopoda). Abh. Krakauer Akad. Bd. xxv. 1893. And Biol. Centralbl. Bd. xiii. 1893. III. Roule, L. Etudes sur le developpement des Crustaces. La segmentation ovulaires et le faconnement du corps chez l'Asellus aquaticus. Ann. Sci. Nat. Zool. (8). 1. 1896. IV. Roule, L. Etudes sur le developpement des Crustaces. Le Developpement du Porcellio scaber. Ann. Sci. Nat. Zool. Tom. xviii. 1894. 190 CRUSTACEA. II. Metamorphosis. 1. The Nauplius Stage. The most typical feature of the metamorphosis in the Crustacea, and therefore, perhaps, the most convenient one with which to commence our study, is the so-called Nauplius staqe. In those cases in which the young animal is hatched at a later stage of develop- ment (e.g., in the Cladocera, the Arthrostraca, and most Decapoda), the Nauplius stage is thrown back among the series of embryonic stages; nevertheless, we commonly find that this stage is indicated by a period of rest, such as an ecdysis of the Nauplius integument (p. 118). The Nauplius body (Fig. 89) is as a rule oval, the anterior end being the more rounded, and the posterior the narrower end of the body. In other respects, there is great variety in the shape of the body. Dorsally compressed, laterally compressed, long, and even broad Nauplii are to be found. The possession of three pairs of limbs — the future first antennae (a'), second antennae (a"), and mandibles (md) — points to the fact that, in the Nauplius, we have an already segmented larval form. This segmentation is, however, not externally recognisable in the body of the Nauplius, although the limits of the segments may be indicated in the ontogenetic stages which lead up to the Nauplius (free-living Copepoda, Cirri- pedia). Typically, the Nauplius has no shield-like reduplication of the dorsal integument, but such a fold (which usually only appears at a later stage) may be distinctly recognisable in individual cases (Cirripedia, Fig. 102 A), or at least may be indicated by a slight fold in the integument of the dorsal surface. The posterior end of the body is still without the paired furcal processes, but is armed with paired setae (furcal setae). The most anterior pair of limbs, the first antennae (a'), are uniramose, and consist of few joints or are still unjointed. They serve for locomotion, and are also of importance as bearing sensory organs. The two pairs of limbs which follow these are developed in the form of biramose swim- mi Dg limbs. The first of these, the second antennae (a"), lie at the sides of the mouth, and are distinguished by a strong, hook-like, masticatory process springing from the basal joint. The third pair of limbs, the mandibles (md), also function principally as locomotory organs. A masticatory blade is not, as a rule, developed on the basal joint, indeed, this structure, so characteristic of the appendage In the adult, is hardly indicated, although it may in some cases THE NAUPLIUS STAGE. 191 md^ occur here. The U-shaped antennal gland (at) opens externally on the basal joint of the second antenna. The oral aperture which lies between the two antennae is overhung by a large upper lip (lab rum), and leads into the alimentary canal, in which can be distinguished a short oesophagus, a widened mid-gut, and a hind- gut. The anal aperture may be wanting in the first Nauplius stages (Cttochilus, Cyclops). It has been observed in many cases that this aperture origin- ally opens on the dorsal surface (Cirripedia, Fig. 103, p. 212; Cetochilus, Fig. Ill B, p. 233; em- bryonic stage in the Cladocera, Fig. 72, p. 147), the aperture shift- ing later to the posterior end of the body between the furcal processes. The nervous system retains its original connection with the ectoderm, and consists of the supraoeso- phageal ganglion, circum- oesophageal commissures, and the first pair of gan- glia of the ventral cord. The second antennae are innervated from a pair of ganglia lying behind the mouth (Claus, Dohrn), an arrangement which is retained by the Phyllopoda in the adult. As a sensory organ, there is the unpaired Nauplius eye, which lies in the frontal region, and is composed of three parts. The most developed muscles are those of the limbs, which are attached to a point in the mid-dorsal region. The heart is not yet developed. Although there is no external segmentation in the Nauplius body, we can nevertheless recognise in it the following segments:— An anterior pre-oral or primary cephalic segment, a posterior terminal or anal segment at the posterior end of the body, and the true trunk-segments which He between these. As members of the last group, we recognise the section of the body belonging to the second antenna and the mandibular segment. We have, however, already pointed out (p. 165) that the region of the first antenna possibly also corresponds Fig. 89. — Nauplius of Cyclops (after Claus). a>, first antenna ; o", second antenna ; md, mandible ; o, Nauplius eye ; at, antennal gland ; ds, mid-gut with excretory cells. 192 CRUSTACEA. to a trunk-segment, which became early specialised and lost its independence. Tin- cephalic and terminal segments stand in a certain contrast to the trunk- .ents, in so far as only the latter have rudiments of limbs. The posterior portion of the Nauplios body contains anteriorly a budding zone from which ii.w trunk-segments are continually being produced, while its posterior ex- tremity eventually becomes the anal segment (telson) of the adult. The typical form of Nauplius above described undergoes many variations in detail, some of which will be described later. In most cases, at the Nauplius stage, not only the above-named trunk-segments are perceptible, but also the rudiments of other segments behind these. Such a stage, indicating as it does a higher grade of segmentation of the body, is more correctly described by Claus' name Of Metanauplius. This name denotes all those larval stages following upon the Nauplius stage, which, while retaining the general characters of the Nauplius, show an advance upon it in segmentation by the possession of trunk -segments bearing limbs behind the mandibular segment. Such a widening of the conception of the Nauplius appears the more admissible as O. F. Muller founded his hypothetical genus Nauplius on a Cyclops larva with four pairs of limbs, while to the younger stage with three pairs of limbs he applied the name Amymone. After it had been discovered that Muller's genus belonged to the series of ontogenetic stages of Cyclops {Jurine), the name " Nauplius " was adopted for all similarly-shaped Crustacean' larvae. The Nauplius, in accordance with the views of Fritz Muller (No. 16) was for a time regarded as a larval form of great significance (Haeckel, Dohrn, No. 9), as it was thought to represent the common racial form of all the Crustacea. In what way this racial form was to be connected with the lower groups of animals was not so clearly stated. It was, however, thought that the nearest relations of the hypothetical ancestral form were to be sought among the Rotifera or the Annelida. The first to oppose the prevalent view was IlvrscHEK (No. 15), who pointed out the resemblance in body -structure existing between the Crustacea and Annelida, and urged that the common ancestral form of the Crustacea must have had -a body already composed of many segments, and that such a racial form could be directly deduced from the Annelida. This view received great support when the structure of the two pairs of excretory glands (shell- and antennal glands) was better known ; the homology of these glands with the segmental organs of the Annelida had already been asserted by Leydig and Gegenbaur. Very gradually the following view has been adopted and is now generally accepted (Dohrn, No. 11), viz. — that the Nauplius does not represent a stage in the direct ancestral line of the Crustacea, but is a caenogenetic, adaptively modified larval form in which the following Crustacean characters are early developed : the form of tho limbs, the strong cuticularisation of the surface of the body, and in connection w ith this, the development of hair-like processes,. the absence of ciliated epithelia, the disappearance of the coelomic sacs, and the development of a lacunar body- 0 ivit y. The Nauplius thus shows typical Crustacean characters in its structure and in its histological constituents, while in its body-segmentation it stands at a level winch can best be compared with that of a polytrochan Annelid larva, The NaupUusi therefore, is a larval form which is secondarily modified by its early adoption pf the Crustacean manner of life, but which, phylogenetically, it ■ comparatively late stage. One of the principal reasons advanced for considering the Nauplius to be the form of the Crustacea was the common occurrence of this larval form TYPICAL FORM OF THE CRUSTACEAN LIMBS. 193 in the most varied groups of Crustacea. This general occurrence of the Nauplius in the ontogeny of all Crustacea shows that the hypothetical common ancestor had already passed through the stage at which the Nauplius was acquired, and that therefore the modification in the ontogeny of the Crustacea just •described took place in very early times. This appears the less surprising when we realise how greatly the ontogeny of a form is influenced by variation affecting the adult. Only in so far as the wide occurrence of the Nauplius larva justifies us in forming conclusions as to the ontogeny of the racial form of the Crustacea, can a certain phylogenetic significance be ascribed to it {Hatschek, Lehrbuch der Zool., p. 25). The further development of the Nauplius stage is accomplished in the Entomostraca, e.g., in the Phyllopoda and also in some Copepoda, by a series of very gradual changes of shape through many moults, the adult condition being reached by a continuous increase in the number of body-segments and of pairs of limbs, the growth of the dorsal shell-fold, the appearance of the lateral eye, and other transformations after the other, lowest forms, occurring one While, in these the course of metamorphosis is comparatively simple, it becomes more compli- cated among the Malacostraca, where independent larval forms, notably the Zoaea, are inter- calated. These do not belong to the direct series of transforma- tions between the Nauplius and the adult form, but are dis- tinguished by secondarily acquired peculiarities, resulting in a great increase in number of larval forms. (For the metamorphoses of the Malacostraca, see p. 246.) Fig. 90.— A, thoracic limb of a Copepod (after Claus). B, abdominal limb of Gam- mams locusta (after Boas). 1, first, 2, second joint of the protopodite ; en, endopodite : ex, exopodite. 2. Typical Form of the Crustacean Limbs. In the two posterior pairs of Nauplius appendages, we have a very primitive form of Crustacean limb. With the exception of the first antenna (antennule) which shows an altogether hetero- morphous structure, all the Crustacean limbs can be traced back to 2). The first of these joints (the proximal joint) (I) was called the basal joint by Claus, and the coxopodite by Huxley, while the second (the distal joint) (2) is known as the stem-joint (Claus) or basipodite (Huxley). It should be mentioned that in many cases (especially in the Malacostraca), the joints of the endopodite seem to form a direct continuation of Fio. &1.— Three Crustacean limbs (after Claus). A, maxilla of Calanella. B, thoracic limb of Nebalia. C, first maxillipede of an advanced Penaeus larva. 1, first, 2, second joint of the protopodite ; fc, masticatory processes of the same ; en, endopodite ; ex, exopodite ; q^ epipodial plate ; ep1, rudiment of a branchial tube. the protopodite (Fig. 91 J5), while the exopodite appears like a lateral appendage. This condition cannot, however, be regarded as primitive. The close proximity of the ventrally placed Crustacean limbs allows mutual inter-action of the two members of one and the same pair, and we therefore often find, on the inner side of the endopodite and protopodite, processes of very various kinds which act mechani- cally upon any foreign bodies (particles of food) that may come between the two approximated limbs. Such processes have received the general name of endites, but when found on the protopodite they may heir mow Bpecial names (Fig. 91, k), being known as masticatory TYPICAL FORM OF THE CRUSTACEAN LIMBS. 195 hooks, lamellae, or lobes, or else as masticatory blades. We have already observed the development of such a masticatory hook on the protopodite of the second antenna (Fig. 89, p. 191). Another component of the Crustacean limb which, however, is not always developed, is to be found on the external side of the protopodite, and is in most cases connected with the movement of the surrounding water and has a secondary respiratory function. Such appendages, several of which may occur together, we shall call epipodites (Fig. 91, ep), whether they are developed as lamellae (epipodial plate, ep) or as branchial sacs or tubes (ep1) provided with a rich circulation of blood (p. 172). The epipodial appendages, as a rule, belong to the coxopodite or basal joint. In the following pages we shall have to refer to the many modifiations and degenerations which these separate constituents of the Crustacean limb undergo. Of the two extreme types, the elongate form of limb appears to be the more specialised and to have developed very gradually, while the lamellate, broadened form, such as occurs in most Phyllopodan limbs and in the thoracic limbs of Nebalia, probably represents the more primitive type. It has not yet been satisfactorily demonstrated in what manner the separate parts of the leaf- like Phyllopodan limb can be homologised with those of the schematic Crustacean limb above described. Ray Lankester's explanation seems to have a certain degree of probability. Accord- ing to this author, among the six endites belong- ing to the inner side, the fifth represents the endopodite (Fig. 92, en5), and the sixth the exopodite (en6). This view is supported by the observations of Claus (Nos. 20 and 21), which show that, in the developing limb of Branchipus, these two lobes were the first to become distinct. The fan-like plate (ep), which arises much later, is to be regarded as an epipodial appendage as well as the branchial sac (ep1). The breaking up of the stem of the limb into separate joints shown in Fig. 92 is exceptional among the Phyllopoda. We feel tempted to deduce the typical biramose form of the Crustacean limb direct from the bilobed Annelidan parapodium. Such a deduction is supported by the fact just mentioned that the separation of the exopodite and endopodite takes place unusually early in the limb-rudiment in Branchipus (Claus). Whilst, however, the Annelidan parapodia are processes of the lateral sides of the body, the limbs of the Crustacea have shifted ventrally and have become approximated, a fact which indicates that we must imagine, as the intermediate form between the Annelida and the Crustacea, an animal which crept on the sea-bottom. The Fig. 92.— Second thoracic limb of Apus cancriformis (after Ray Lankester). 1, first, 2, second stem-joint; eni, en2, en^, en4, en5, en$, the six endites ; ep, epipodial lamella; ep', bran- chial sac. 196 CRUSTACEA. mutual action of the limbs and the development of endites connected with this is thus a new acquisition peculiar to the Crustacea. On the other hand, the epipodial appendages may with some probability be deduced from the dorsal cirri of the Annelida, although this view meets with a difficulty in the late appearance of these structures in the ontogeny of the Crustacea. We must not, however, forget that the respiratory requirements only, as a rule, increase when a certain size of body is attained, and that this appears to explain why small Crustacean larvae (as well as adults of small size) have no branchial appendages. 3. Phyllopoda. A. Branchiopoda. The larvae of the Branchiopoda, after hatching, bear a general resemblance to the Nauplius, but the body is divided into an anterior cephalic section carrying the Nauplius limbs and a posterior abdominal section. Since the rudiments of a number of body-segments are to be found in the latter, this larval stage of Branchipus must be described as a Metanauplius (Fig. 93). This stage is very little modified in the various Branchiopodan genera. The rudiment of the dorsal shell-fold is, as a rule, still wanting, only developing at a later stage. The further metamorphosis is exceedingly simple, new segments being successively yielded by the posterior end of the body, and the limbs budding out in the same order. An exception to this regular course of development is found in the limbs of the maxillary region which, in keeping with their slight development in the adult, appear late. While the segmentation of the body thus gradually approaches that of the adult, the paired compound eyes develop as well as the shell-fold, which latter must be counted as belonging to the maxillary region, the internal organs develop, and the three pairs of Nauplius limbs degenerate in size and become modified. As an example of the development of this group we may take that of Apus, which is so well known through the investigations of Zaddach (No. 31), Brauer (No. 18), and especially of Claus (Nos. 20 and 21). The Metanauplius of Apus (Fig. 93 A) is more or less oval in shape, narrowing posteriorly ; when it leaves the egg it still retains the original dorsal curvature of the body (Brauer) but soon straightens out. The only internal organs which can be recognised are the Nauplius eye and the intestine which widens anteriorly and opens out in a depression at the posterior end of the body. The three pairs of limbs are of the typical Nauplius form. The first antennae (i), which are inserted at the sides of the large, helmet- ghaped upper lip, are simple unjointed rods each carrying two large BRANCHIOPOD A. 197 setae at the tip. The second antennae (2) are large biramose swimming limbs, each carrying a movable masticatory hook on the basal joint. The endopodite of each of the second antennae is small and has setae at its end, while the five-jointed exopodite has five swimming setae on its inner side. The mandibles (3) are smaller than the second antennae, but agree with them in general structure. The masticatory hook is wanting in the mandibles; the future masticatory blade (often the sole representative of the mandible in the adult) is visible at this stage merely as a slight swelling on the inner side of the protopodite. The endopodite and exopodite are unjointed and carry setae at their tips. The anterior or cephalic region of the body, which carries the Nauplius limbs, is bounded posteriorly by a small dorsal prominence, in which we recognise the first rudiment of the dorsal shield. At the middle of this anterior dorsal region there is a more or less round, sharply circumscribed point, the rudiment of the nuchal gland so widely distributed among the Phyllopoda (p. 151). In the posterior, thoraco-abdominal region, five consecutive thoracic segments have begun to form as transverse swellings (I-V). After the first moult (Fig. 93 B), the form of the body is essen- tially changed by the broadening of the anterior region and the elongation of the narrow, cone-like, posterior section. The dorsal shield now covers the tergum of the first thoracic segment. The Nauplius limbs retain essentially the same character. The endo- podite of the second antenna is now two-jointed. On the basal joint of the mandible (3), a strong masticatory blade has developed with teeth on its inner edge, this being covered by the now relatively small upper lip. Behind the mandibles, the first pair of maxillae have begun to appear as simple plates (4). In the anterior thoracic region, two to four lobed rudiments of limbs can be made out. The number of thoracic segments is increased to eight by the addition of new rudiments. At the posterior end of the body, the large furcal processes have developed. Other points of interest in this stage are : the development of the frontal sensory organs as styloid filaments (fs) near the Nauplius eye, the rudiments of the hepatic outgrowths at the wide anterior part of the mid-gut, the greater distinctness of the antennal gland in the basal section of the second antenna, . and the rudiment of the shell-gland in the ventro-lateral part of the dorsal shield. The third larval stage (after the second moult), reveals lobed rudiments of limbs on the six anterior thoracic segments; on the 198 CRUSTACEA. seventh segment, a non-lobate rudiment is found, and behind this, the rudiments of two or three more segments. Behind the first pair of maxillae, the rudiment of a second pair can be distinguished as a transverse prominence. The shell-gland is distinctly visible in the lateral part of the dorsal shield, which has now slightly increased in size. At this stage the heart can already be made out, Fio. 93.— Three larval stages of Aptis cancriformis (after Claus, from Lang's Text-book). A, larva just hatched (Metanauplius). B, second larval stage with the anterior maxillae and seven (or eight) thoracic segments. C, fourth larval stage with about fifteen thoracic segments. 1, first, 2, second antenna; S, mandible; Uy first maxilla; I.-XIII., first thirteen trunk-segments ; fs, frontal sensory organ ; s, dorsal shield ; L, liver. reaching from the maxillary region posteriorly to the sixth thoracic segment. As the segments develop posteriorly, new chambers are added to the heart (Fig. 87, p. 178). In the fourth larval stage (Fig. 93 C), further rudiments of limbs are seen in the act of forming on the thoracic segments ; on the seven anterior segments these are distinctly lobate, on the next two, BRANCHIOPODA. 199 indistinctly lobate, and on the three or four following, they are unlobed. The thoracic shield (s) and the furcal processes have increased considerably in size. As important new rudiments, we have those of the paired eyes, which are situated dorsally above and behind the Nauplius eye, and in which the deposition of pigment has commenced These rudiments and those of the •developing optic ganglia become more and more distinct during the subsequent stages. At the fifth stage, there are nine distinctly lobate pairs of limbs, a tenth rudiment indistinctly lobate, and four following these not as yet lobate. Six more rudiments of segments are discernible behind these. At this stage, the formation of the heart and the branching of the liver by means of secondary diverticula progress gradually. In the later stages which succeed one another through numerous moults, the form of the adult is gradually approximated by the appearance of fresh segments and limbs, by the increase of the dorsal shield, which gradually covers almost the whole of the dorsal region, and by the lengthening of the furcal processes. At the same time a degeneration of the Nauplius limbs takes place, the first antenna remaining as a small two-jointed stump, and the second as a still smaller unjointed vestige; of the mandibles, only the basal joint is retained with its now strongly-developed masti- catory blade. The distal part of this latter limb is visible in the later larval stages as a gradually diminishing mandibular palp, which finally disappears. This degeneration of the limbs is accompanied by a change of the method of progression. In the first stages, the larva jerks itself forward by the powerful strokes of the second antennae, but now it moves more regularly by the gentler swimming action of the numerous rowing limbs. The above description applies to Apus carter if or mis. The develop- ment of Apus produdus, made known to us by Brauer (No. 18), is quite in agreement with the above account. The egg, however, is here considerably larger and the development more abbreviated. The number of moults and of separate ontogenetic stages is smaller. The second antennae, as early as the fourth stage, cease to act as rowing organs, and at the sixth stage are greatly degenerated. The Metanauplius which hatches out of the egg has not only a large number of segments, but already shows the rudiments of the paired eyes. The Metanauplius of Branchipus is somewhat longer, and the thoracoabdominal section of the body is more sharply separated from 200 CRUSTACEA. the cephalo-thoracic section. The maxillary segments and the first two thoracic segments, with the rudiments of the corresponding thoracic limbs, can be recognised, as well as the commencement of the metameric segmentation of the rest of the body (Claus, Xo. 20). The gradual development of these larvae, as well as that of Arte?nia, follows a course similar to that of Apus, if we bear in mind the differences caused by the absence of a dorsal shield and the consequent cramped position of the shell-gland, whose opening was found by Claus (No. 21) on the second maxilla. The course of development of the compound eyes can here be followed very clearly ; these, together with the optic ganglia, originate from a proliferation of hypodermis at the sides of the cephalic segment (p. 168). In this hypodermal growth, a separation into layers takes place, the inner layer being transformed into the Fig. 94.— So-called Nauplius stage of Estheria ticinenis (after G. Ficker). a', first, a", second antenna ; md, mandible ; 61, upper lip. ganglion, while the superficial layer yields the actual compound eye. A lateral outgrowth of this part of the head leads to the development of the eye-stalk, which later becomes a movable, jointed appendage (Claus). The method of development of the stalked eyes in this form affords the most weighty objections to the view that these organs are structures homologous with the limbs (p. 165). Until the stage at which the full number of segments and limb- rudiments is attained (first period of development), the antennae and the mandibular palps retain the larval character of the Metanaupliu8 stage. Only in the subsequent period of sexual • liflVrentiation, in which the genital segments develop in a manner characteristic of the two sexes, do the muscles of the swimming Mltennte and the mandibular palps degenerate. The mandibular pulps altogether disappear, while the second antennae shift to the BRANCHIOPODA. 201 frontal surface and become transformed into the so-called cephalic horns, varying in the different species. At the same time, a sensory organ (Claus, No. 21) develops between the forehead and median eye ; this must not be confounded with the paired frontal organ. It consists of an accumulation of club-shaped terminal nerve-cells which contain peculiar three -pronged, strongly refractive deposits. It corresponds to the nuchal organ described by Leydig in con- nection with the Cladocera. The development of the Estheridae also does not deviate in any essential point from that described for Apns and Branchipus. The so-called Nauplius stage (Fig. 94) is still entirely devoid of the shell- fold which, in the adult JEstheria, is bivalve. It is distinguished from the corresponding stage of Branchipus only by the remarkably large upper lip (ol) and the rudimentary condition of the first antenna (a'), which is a hemispherical swelling provided with a long seta (Ficker, No. 22). At a later stage, the two maxillary segments and six distinctly separate limb-segments, still, however, quite devoid of their limb-rudiments, can be recognised. At the posterior end of the body, the furcal processes have already developed. The rudi- ments of the paired eyes and of the ganglia belonging to them can be made out. At this stage the rudiment of the shell first appears as a paired outgrowth of the dorsal integument of the maxillary region (Fig. 96, s). Within this rudiment can be recognised the as yet slightly developed shell-gland. The later stages are chiefly characterised by the growth of the shell, the increase in number of segments and limbs, the changes which take place in the Nauplius limbs, and the gradual diminution of the upper lip. In these stages (Fig. 95), the larva strikingly resembles an adult Cladoceran. The head, which is not yet covered by the shell-valves, has a pair of small antennules beset with olfactory filaments (a'), and large swimming antennae (a") recalling in a striking manner by their structure the corresponding limbs in the Daphnidae. The mandibular palp (md) is much reduced. The shape of the shell-gland (sd), and the form of the posterior section of the body (post-abdomen), with its furcal hooks and two large sensory setae, so strikingly recall the Cladocera that we cannot doubt that the latter group has been derived from the Estheridae by a diminution of the number of segments and limbs, and a degeneration of the heart to a short sac (Claus, No. 8). In its larval development, Limnadia closely resembles Estheria (Lereboullet, No. 26). Here also the two shell-valves develop as originally distinct outgrowths of the dorsal integument of the 202 CRUSTACEA. maxillary region (Fig. 96, s), fusing in the median line only at a later stage. Although here, as in the Cladocera (Grobben), the two halves of the shell at first develop separately, we must, nevertheless, trace them back to an originally single dorsal shield, like that of Apus. In the youngest larval stages of Limnetis (Fig. 97) observed by Orube (No. 23), the shape of the body is very remarkable. In the number and structure of the limbs, this larva agrees with the so-called Nauplius of other Branch iopoda, but it is distinguished by a flat, circular, and somewhat transversely broadened dorsal shield. There Fig. 95.— Older (Cladocera-like) larval stage of Estheria (after Claus). a', first, o", second antenna ; d, intestine ; fr, frontal sensory organ ; h, heart ; 1c, gills ; md, mandible ; n, nervons system ; o, Nauplius eye ; sd, shell-gland. is a correspondingly large and transversely broadened upper lip on the ventral side. At the anterior cephalic region there are one unpaired and two paired lateral conical projections. The flat dorsal shield is replaced by a bivalve shell only at a somewhat late stage. Many very primitive features have been retained in the condition of the body among the numerous forms of the Branchiopoda. As such we may assume the presence of many body-segments, the comparatively homomorphic develop- ment of the limbs, the origin of the antennal nerves from a pair of •lia lying in the oesophageal commissure, the presence of separate ganglia in each maxillary segment, the ladder-like development of the CLADOCERA. 203 ventral chain of ganglia, the presence of a long chambered dorsal vessel, and perhaps also the lamellate character of the limbs. The fact that the great groups of the Malacostraca through their most primitive member {Nebalia) join on to the Branchiopoda, will still further incline us to see in the latter the now living representative of a series of forms which are least removed from the hypothetical ancestor of the Crustacea (Dohrn, No. 9). On the other hand, we must not forget that the existing Branchiopoda (Phyllopoda) in many respects seem undoubtedly to have undergone great secondary modifi- ■cation. In the first place we must here recall the reduced form of the oral limbs (mandibles and maxillae). With regard to these we shall have to turn to the larvae of the Copepoda and the Mala- costraca, so as rightly to complete the picture of the hypothetical racial form. Nevertheless, in forming a judgment of the phylogenetic relations of the Crus- tacea, we shall often have to return to the Phyllopoda as a very primitive group. Fig. 96.— Dorsal aspect of the larva of Limnadia (after Leke- boullet). a", base of the second antenna ; d, intestine ; md, mandible ; ol, upper lip ; s, rudiment of the right shell- valve. B. Cladocera. Whereas the Branchiopoda pass through a metamorphosis characterised by many moults, larval ecdyses are altogether wanting in their relations, the Cladocera, the young animal leaving the egg with a shape resembling that of the adult. The whole modelling of the body is here shifted back to the embryonic stages, among which a distinct Nauplius stage can be recognised ; this is in many cases even marked by a moult — development of the Nauplius cuticle — (Fig. 72, p. 147). Special peculiarities of shape, however, are met with in the newly- hatched young of the remarkable genus Leptodora. Whereas the young coming from the summer egg resemble the adult in shape {P. E. Muller), the larva hatched from the winter egg resemble an advanced Metanauplius (Fig. 98), and have therefore another stage to pass through (G. 0. Sars, No. 29). The body in this Meta- nauplius is long, without outwardly recognisable segmentation, and ends posteriorly in the two furcal processes. The first antennae (a) are short and club-shaped, the second (a") long biramose swimming 204 CRUSTACEA. appendages, which however are devoid of the special masticatory hook found in the Branchiopodan Nauplius. The mandibles {md} consist of the basal blade and a long unjointed oar-shaped palp. The oral aperture is overhung by a large upper lip {pi). In the thoracic section of the body the {PT-PVI) are visible in the form Fig. 97.— Nauplius stage of Limnetis brachyura (after Grube). a", second antenna ; k, masticatory process of the same ; md, mandible. rudiments of six' pairs of limbs of transverse swellings. In the cephalic region,, the Nauplius eye alone is recog- nisable, the com- pound eyes de- veloping at a later stage. Fur- ther metamor- phosis takes place- by the develop- ment of the limbs and of the body- segments, the growth of the dorsal shield, the diminution of the upper lip, the degeneration of the mandibular palp, and the acquisition of the characteristic curvature of the body. The Nauplius eye persists in the generation coming from the winter egg, whereas it is wanting in individuals developing from summer eggs. The genus Leptodora, in the metamorphosis of the winter eggs and in the Branchiopoda- like Metanau- plius stage, as well as in many other respects, has thus re- tained primi- tive characters among the Cladocera. Fio. 98. — Metanauplius stage of Leptodora hyalina developed from a winter egg (after G. O. Saks), a1, first, a", second antenna; md, mandible; pl-pVl, rudiments of the first six pairs of thoracic limbs; ol, upper lip. The summer and winter eggs differ not only with regard to the development of the embryo, but in other points as well. It has already been indicated (p. 114) that, in Bythotrcphes and Leptodora, the type of cleavage of the summer egg differs from that of the winter OSTRACODA. 205 -egg. Sars (No. 30) has pointed out in this connection that the embryos that develop (in ephippia) in the winter eggs remain during the whole duration of development enveloped in the egg-integument ("chorion"), while the summer eggs throw off the very thin vitelline membrane before reaching the last stage of development. The number of pairs of thoracic limbs which we found as rudiments in the Metanauplius was six, and this must be considered as the primitive number in the Cladocera. This number is also retained in the Sididae. In the embryo of Lynceus six pairs appear as rudiments, but the last of these degenerates later (Claus, No. 8). 4. Ostracoda. The Ostracoda, which are provided with a bivalve shell, often impregnated with calcareous salts, undergo more or less marked meta- morphosis. These animals were probably derived from Phyllopodan ancestors by a process of degeneration tending in a definite direction, the number of segments being reduced, and the powers of locomotion #* SM * Fig. 99. — Two larval stages of Cypris (after Claus, from Balfour's Text-book). A, Nauplius stage. B, second larval stage. A ', first, A", second antenna ; /", first pair of feet; Md, mandible ; OL, upper lip ; Mx', first maxilla ; SM, adductor muscle. diminished. In Cypridina, the Cytheridae, and the Halocypridae, whose development is as yet unknown to us, the metamorphosis appears to be to some extent abbreviated, since the larva when hatched has almost the form of the adult; the Cypridae, on the contrary, undergo a metamorphosis, passing through many moults from the Nauplius stage to the adult form. This metamorphosis has been observed by Claus (Nos. 32 and 34) in the cases of Cypris fasciata, C. ovum, and G. vidua. Reckoning by the number of consecutive moults, there are here nine different stages, some of which, however, differ only slightly. The Nauplius (Fig. 99 A) resembles the Phyllopodan larva at the same stage in that the three pairs of limbs are used as locomotory organs, and in the possession of a large helmet-shaped upper lip {OL), but in having a well-developed bivalve shell provided with an 206 CRUSTACEA. adductor muscle (SM ), it already shows a typical Ostracodan feature, the development of which has evidently been shifted back second- arily to this early stage. Further metamorphosis takes place, as in the Phyllopoda, by the gradual development in regular order of the posterior pairs of limbs, the growth of the posterior section of the body and of the corresponding internal organs. In accordance with these changes in the body, a change in the shell (observed by Zenker, Xo. 35) takes place. In the Nauplius, the anterior half of the shell is remarkably developed. Its greatest height, as well as its greatest r, mi L Fio. 100.— Two further stages of development of Cypris (after Claus, from Balfour's Text- book). A, fourth stage. B, fifth stage. Mx\ first maxilla ; Mx" and /', second maxilla ; /", first pair of feet ; L, liver. breadth, lies anteriorly to the middle of the body. The anterior section of the shell appears sharply inclined, while the posterior end of the shell runs out more gradually to a point. As the posterior part of the body develops, the posterior halves of the shell also increase in size (Figs. 99-101). An exception to the rule that the limits appear in order from before backward is found in the second maxillae which are somewhat retarded, not appearing until after the limbs which lie posteriorly to them have developed. There is here a recurrence of a feature found in the Phyllopoda and a few Copepoda ( Cyclojndae, Harpactidae). OSTRACODA. 207 The Nauplius stage {first free larval stage, Fig. 99 A) is, as already mentioned, distinguished by the presence of a bivalve shell provided with an adductor muscle (SM). At its highest point is found the Nauplius eye, which is here very large and provided with paired lenses. The alimentary canal, which consists of oesophagus, mid-gut, and hind-gut, is still without hepatic appendages. The oral aperture is overarched by a large upper lip (OL). The three pairs of limbs differ somewhat in shape from those of the Phyllopodan Nauplius, being here developed as uniramose crawling limbs. The first two pairs already approach the adult form. The first antenna (A') is bent inwards, and consists of four joints, the three terminal joints being beset on their upper surfaces with rowing setae. The second antenna (A") already shows the characteristic knee-like bend inwards. The mandible (Md) has still the form of a three-jointed crawling limb, its tip ending in a large hooked seta. The future masticatory blade is at this stage merely a small serrated process of the basal joint. In the second larval stage (Fig. 99 B) which follows, the mandible loses the primitive character just described. Besides this, behind the mandible, two more /'(Matf Fig. 101.— Sixth larval stage of Cypris (after Clacs, from Balfour's Text-book). Mx', first, Mx", and /', second maxilla ; /", /'", first aud second pairs of feet ; Fu, caudal fork ; L, liver ; SD, shell-gland. rudiments have arisen, those of the first maxilla (Mx) and of the first pair of feet (/"). The body has correspondingly lengthened. The basal joint of the third limb is now transformed into a large triangular masticatory blade (mandible) toothed on its inner edge, the rest of the limb persisting as a four- jointed mandibular palp. Attached to the basal joint of the latter can be recognised the first rudiment of a hand-shaped appendage beset with setae (exopodite). The next rudiment (that of the first maxilla) is a leaf-shaped curved plate (Mx'), while the first ambulatory limb (/") is an unjointed climbing organ provided with a terminal hook. In the third stage, the maxilla approaches its adult form, two strong, toothed, masticatory blades and the rudiment of a third being discernible. The rudi- ment of the movable gill-plate (exopodite) beset with hairs can also now 1 e recognised. In the following fourth larval stage (Fig. 100 A) there is no essential alteration in the antennae and the mandibular palp beyond a rich* r provision of setae ; on the other hand, on the first maxilla (Mx') four masticatory 208 CRUSTACEA. blades have developed, the upper one (endopodite) already showing the formation of a terminal joint. Behind these limbs a new rudiment {My!') can be made out occupying the space between the first maxilla and the first pair of feet ; this is the rudiment of the second maxilla which arises as a curved triangular plate after the first pair of feet has already appeared. The second pair of feet is still wanting, but the posterior end of the body is now marked by the development of two furcal setae. The 'fifth larval stage (Fig. 100 B) is distinguished by the further development of the second maxilla (Mx"), which now appears as a four-jointed limb directed backwards and carrying on its basal joint a projecting masticatory blade. It is an interesting fact that these limbs, during the ontogenetic stages of Cypris, not only resemble legs, but function as such, a condition which is retained throughout life in the Cytheridae. The two furcal appendages at the posterior end of the body are now more distinctly marked. Among the internal organs we must mention the development of the hepatic tubes (L), which extend into valves of the shell, and the first appearance of an organ lying below the eye and regarded by Claus as the shell-gland (Fig. 101, SD). The sixth stage (Fig. 101) is characterised by the appearance of the last (second) pair of feet (/'"). The second maxilla has become transformed by the increase in size of its masticatory blades and the diminution of its leg-like palp. The anterior pair of feet, until now somewhat undeveloped, has several joints. The posterior (last) pair of feet essentially resembles the rudiments of the limbs which precede it as found in earlier stages. The caudal fork already approaches its adult form and terminates in two strong hooked setae. In the seventh stage, which in the structure of the limbs shows the adult condition, the rudiments of the genital apparatus are recognisable. Among the alterations which have taken place in the limbs, the development of a small leaf-like appendage (exopodite) on the basal joint of the second maxilla, and the segmentation of the last pair of feet, deserve special mention. In the eighth stage, the development of the genital rudiment progresses, the way being prepared for the development of sexual differentiation, while the ninth and tenth seem to be stages for the development of sexual maturity. The Cytheridae, according to Zenker, hatch at a stage more or less comparable with the fourth stage of the Cypridae. Besides the two pairs of antennae, the mandibles and the first maxillae have appeared and are somewhat advanced in development. The rudiments of the two pairs of feet seem to be present behind the above. The mandibular palp still functions as a locomotory organ. In Cypridina, whose eggs, like those of the Cytheridae, pass through their embryonic development between the valves of the mother, the shell undergoes changes of shape resembling those pointed out in connection with the young form of Cypris. The compound paired eyes appear early. Embryos just before hatching possessed the furcal pro-appendages, but were without the three posterior pairs of appendages (the first pair of feet, the cleaning foot, and genital prominence). It is probable that the larva leaves the egg nearly resembling the adult in shape (Claus, No. 33). CIRRIPEDIA. 209 In the Halocypridae also metamorphosis has almost disappeared, the young when hatched being apparently already provided with all the limbs, and only being distinguished from the adult by the immature condition of the sexual organs, the small number of f ureal hooks, and the slight development of the sexual characters (Claus). 5. Cirripedia. The Cirripedia, which have been modified in a remarkable manner in connection with their attached mode of life, have often been placed in close relationship with the Copepoda, in accordance with the views of Claus (No. 8). We must, however, in consequence of the presence of the so-called Cypris stage (with a bivalve shell) which occurs in their metamorphosis and brings about the transition from the free to the attached life, assume for them a similarly-shaped ancestral form, which we must then seek among the Phyllopoda. Since the correspondence in structure and development between the Cirripedia and the Copepoda may also be explained as the result of adaptation in a similar direction (convergence), and as, on the other hand, the shell in the Copepoda has undergone great reduction, these two orders of Crustacea appear to us to be independent derivatives of the primitive Phyllopodan stock. The metamorphosis of the Cirripedia begins with a true Nauplius stage, characterised by the possession of the so-called frontal horns* and of a usually triangular dorsal shield. The series of Nauplius stages proceeding one from the other through several moults, ends in a Metanauplius in which can be recognised the rudiments of the paired eyes, a fourth pair of limbs, and the subsequent six pairs of thoracic limbs. Another moult and the larva appears in the free- swimming Cypris stage which, after attachment, prepares for the transition into the adult. This latter proceeds, after a further moult, from the attached Cypris stage {pupa). A. Thoracica. The series of Nauplii belonging to the Balanidae (Fig. 102 A) are, as a rule, simpler in shape than the much larger spined Nauplius of Lepas, so that we shall commence with a description of the former. The body shows no trace of external segmentation, and is covered dorsally by a flat, triangular or somewhat oval shell, which, at * We must here point out that the possession of frontal horns is not a universal distinctive feature of the Cirripedian Nauplius. Such horns are, for instance, wanting in the Nauplius of Laura Gerardiae (p. 229), and in a Rhizocephalan, found by Sluier, parasitic on an Ascidian {Sphaerothylacus polycarpac). They are also wanting in that remarkable pelagic larva, of whose systematic position we are still doubtful, which was obtained at Mindanao and at first thought by Willemoes-Suhm to be the larva of Limulus, but was later considered to be a Cirripede larva (No. 63). 210 the lateral angles of its frontal mar- gin, is produced out to form the " frontal horns " (h). The posterior end of this dorsal shell appears at first to be rounded, but at a later stage may have a pair of spines directed backwards and upwards. Each of the frontal horns contains a style-like process (Claus, No. 8), while in the sheath that covers this are found the openings of two large and several smaller unicellular glands (Fig. 102 B, di\ Claus, No. 8; WlLLEMOES- Suhm, No. 62). This is probably a protective appa- ratus and not a sensory organ. The posterior end of the body ( tho- racoabdominal section, Fig. 102 A,t) projects freely out beyond the l ' i2.— Larval stages of Balanus (after Claus). A, older Nauplius stage. B, MetanaupUus. " , llrst, a", second antenna; a/, anus; dr, glandular cells of the frontal horns ; da, dorsal • ufidiil spine ; f*-fvi, first six pairs of thoracic limbs ; fs, frontal sensory organ ; h, frontal boras; md, mandible; mx, rudiment of the maxilla; ol, upper lip: t, thoracoabdominal ■ppmd ™ * ' ' THORACICA. 211 dorsal shield and terminates in a caudal fork. The anal aperture {af) lies somewhat far forward on the dorsal side of this part of the body. Between it and the posterior edge of the dorsal shield there arises from the dorsal side of the thoraco-abdominal section a large spine running backwards and ending in a sharp point (the so-called caudal spine, ds). In later stages, on the ventral side of thoraco-abdominal region, in front of the caudal fork, are found a pair of thorn-like processes repeating the shape of the larger caudal fork, and in front of these another smaller pair. These thorn-like processes which, in some forms, are still more developed, may produce the appearance of segmentation on the thoraco-abdominal section, but cannot be regarded as indicative of a true segmentation of that region of the "body. In the anterior region of the body, the unpaired Nauplius eye which is closely applied to the brain is always distinctly perceptible. The oral aperture is overhung by a large helmet-shaped upper lip (ol), which, by its form, recalls the upper lip of the Phyllopodan larva. The limbs show the structure typical of the Nauplius stage, and, in the first stages, often appear only indistinctly jointed, while in the later stages the joints are more numerous and more distinct. The most anterior limb (a', first antenna) is invariably uniramose, and its distal portion is beset with swimming setae. The second and third limbs (a", md) are biramose. The longer exopodite consists of a large number of closely crowded joints beset with swimming setae ; the shorter endopodite has fewer joints. The protopodite in both these limbs carries several masticatory hooks directed towards the •oral aperture. The later stages are characterised by the gradual increase in size of the body, by the appearance of the above-mentioned spinous processes on the dorsal shield and in the thoraco-abdominal region, and above all by the presence of a paired frontal sensory organ (fs), which here •develops in the form of two filamentous processes inserted near the Nauplius eye. Among the internal organs, besides those already mentioned (eyes, brain, glandular tubes of the frontal horns), we should note the muscles for moving the limbs, the thoraco-abdominal appendages, and the alimentary canal. The latter consists of a short oesophagus, a wider mid-gut, and a hind-gut opening through the •dorsal anal aperture. As to the presence of the antennal gland (in the basal joint of the second pair of limbs) which occurs in the Entomostracan Nauplii, nothing is as yet known. 112 CRUSTACEA. The Nauplii of the Lepadidae, when hatched, seem closely to resemble the Nauplii of the Balanidae in shape, differing only in this one respect, that the long frontal horns are bent backwards. Soon after hatching they undergo a first moult, after which the spine-like thoraco-abdominal appendage (ventral spine) and the dorsal caudal spine first attain their full length. During later moults, the Nauplius grows to a great size (12 mm. long), and is then distinguished by a number of accessory structures. In Lepas fascicularis, seven moults take place before the Metanauplius stage is reached. These later stages (Fig. 103) show the thoraco-abdominal appendage (t), and the dorsal caudal spine (ds) drawn out as long, pointed processes beset with small hooks. A similar process (d) has arisen at the central point of the dorsal shell; this process, which runs obliquely backwards, is known as the dorsal spine. At the margin of the dorsal shield also, besides the frontal horns,, eight large spine-like processes can be recognised, the first pair lying anteriorly between the frontal horns, and the others being distributed laterally and posteriorly. Both the dorsal shield and the larger processes are further beset with a series of minute spines, which are wanting only on the frontal horns. Many of these minute spines, as well as- the marginal spines of the dorsal shield, are found to be perforated and connected with the ducts of unicellular integumental glands. On the ventral side of the thoraco - abdominal appendage, two more posteriorly-placed, immovable thorns can be distinguished, and, anteriorly, six pairs of larger movable spines (x), which do not appear simultaneously but successively, each new moult develop- 8 freeh pair, until, after the last moult, the full number of six is attained. The conjecture, made with some hesitation by Fig. 103.— Larva of Lepas australis (Archizoaea gigas) (after Dohrn). a, anal aperture; d, dorsal spine; ds, dorsal caudal spine; h, frontal horns; ol, upper lip; t, thoraco- abdominal appendage (Tentral spine); x, paired movable spines. THORACICA. 213 Dohrn (No. 42) and Willemoes-Suhm (No. 62), that these pairs of spines correspond to the rudiments of the six pairs of thoracic limbs in the Gypris-like larva appears to us somewhat plausible. This last stage would then have to be assumed to be the Meta- nauplius stage. For more recent observations on the larvae of the Lepadidae we are above all indebted to Willemoes-Suhm (No. 62). Dohrn (No. 42) has minutely- described a large Nauplius larva — probably to be referred to Lepas australis under the name of Archizoaea gigas. These stages have, however, absolutely nothing to do with the Zoaea larva of the Malacostraca. The development of long spine-like processes is a protective adaptation often occurring in pelagic animals (c/. the skeletons of the Radiolaria and the spines of the Pluteus larvae). In the whole series of Nauplius stages, except the last, there is no essential alteration apart from the increase in size of the body and the above-mentioned development of thorn- and spine-like processes. In the last stage which immediately precedes the Cypris stage (Metanauplius, Fig. 102 B), important new rudiments of organs appear. With respect to the general form of the body, it should be emphasised that the lateral parts of the dorsal shield now already begin to bend downwards, and, covering the body laterally, fore- shadow in position the valves of the shell in the Cypris stage. The three anterior pairs of limbs still bear a general resemblance to the typical Nauplius limbs, but already, within the first pair (first antennae), the rudiment of the sucking disc so important in the Cypris stage can be made out (Krohn, No. 50 ; Willemoes-Suhm, No. 62 ; Claus, No. 8). This fact affords a direct proof that the adhesive antennae of the later stages are actually derived from the most anterior pair of Nauplius limbs (Krohn, No. 50; Fr. Mullbr). Behind the mandible is to be seen the slightly developed rudiment of the fourth pair of limbs ; it is, however, probable, as we shall see further later, that we have here the rudiments of two con- secutive pairs of limbs (mx) (Metschnikoff, No. 53 ; Claus, No. 8). The thoraco-abdominal region of the larva has increased considerably in size; in it, beneath the Nauplius cuticle, can be recognised the rudiments of the six pairs of swimming limbs (f-fvl) of the larva (thoracic limbs), as well as the caudal processes of the short abdomen (furcal processes) (Krohn, No. 50; Claus, No. 8). Another im- portant point is that in this stage, near the Nauplius eye, the paired, movable compound eyes (Fig. 102 B) already appear as rudiments. In this Metanauplius stage, therefore, all the more important organs and limbs of the Cypris-like larva are formed, and the larva itself, 214 CRUSTACEA. by the casting of the Metanauplius integument, emerges from the preceding stage. The free-swimming Cypris stage (Fig. 104), which is only of short duration, derives its name from the bivalve shell that envelops the whole body. The actual segmentation of the body and the inner organisation have nothing in common with the Ostracoda. In respect to these the larva at the Cypris stage already very nearly approaches the adult. The two shell-valves which can be brought together by an adductor muscle, are directly continuous dorsally. The dorsal border appears arched, whereas the ventral edge is flattened. Anteriorly they are rounded, but run out to a point posteriorly. In the anterior section of the valves, near the ventral side, a small projection may be remarked on which can be recognised the frontal horn of the Nauplius. The ventral margins of the shell are closely approximated in the middle of their length. Posteriorly, a fissure opens between them for the passage of the swimming limbs of the larva (rf), and there is also an anterior aperture through which the first antennae, the adhesive organs (/), are protruded. The latter limbs, in the free Cypris stage, are used for the occasional attachment of the larva, which precedes and prepares the way for the final attachment. They consist at this stage of four joints, the basal joint joining the body by a broad base, and carrying various chitinous ingrowths (apodemes) for the attachment of muscles. The second joint, which is lengthened like an arm, is bent at an angle to the basal joint. The short third joint carries at its outer side the adhesive disc, at the centre of which the duct of the cement-gland opens, while the truncated fourth joint seems to be provided with one ordinary and one large olfactory seta (Claus, No. 39 j Willemoes-Suhm, No. 62). The cement-gland (Fig. 105, cd) shows various degrees of develop- ment in the Cypris larva. In the pupal stage which follows, it Fig. 104.-Ctypm-like stage of Lepas fascicularis (after Claus, from Lang's Text-book), ua, Nauplius eye ; pa, paired eye ; rf, thoracic (swimming) limbs ; ab, abdomen ; I, first antenna. THORACICA. 215 shifts to the base of the eye-stalk formed from part of the head of the larva, and here takes the form either of a closely-packed coiled gland (Lepas pectinata), or else of widely scattered cells connected only by the efferent duct. On account of its structure, the cement- gland must be regarded as an accumulation of unicellular glands on a much-branched common duct, the single glandular cells being sessile on this duct, like grapes. We are still very much in the dark as to the genesis of this gland and its morphological derivation. Attempts have been made to derive it from one of the two typical pairs of Crustacean glands (antennal and shell-glands), although such a homology is attended by considerable difficulty on account of the aperture of the gland occurring on another limb. Clatjs (No. 8) finds, within the shell- fold of the free-swimming Cypris stage of Lepas australis, a coiled gland-like cell strand, and is disposed to homologise this with the shell-gland of other Entomostraca, while he conjectures that the cement-gland which is recognisable in later stages is derived from this cell-strand. Willemoes-Suhm, on the contrary (No. 62), found, even in the Nauplius stage, a paired glandular mass lying at the sides of the upper lip, out of which, according to him, the cement- gland develops. While authorities thus hold conflicting views, we shall do well for the present to regard the cement-gland as a peculiar structure found in the Cirripedia, and to abstain from attempting to homologise it with the glands of other Crustacea. A great change occurs in the structure of the limbs which surround the oral aperture of the Nauplius. The second antennae seem altogether to disappear (if they are not to a certain extent, as Pagenstecher (No. 58) conjectured, retained in the palp-like appendages of the upper lip). The actual mouth-parts, together with the upper lip, are already shifted on to a slightly projecting oral cone, and appear in the form of three pairs of truncated rudi- ments, in which we recognise the future mandibles, the first maxillae, and the lower lip which results from the fusion of the second maxillae. In what way these mouth -parts are derived from the limbs of the Metahauplius stage is still far from clear. The most probable view appears to us to be that of Claus (No. 8), according to which the mandibles are derived from the basal joints of the third pair of Nauplius limbs, while the outer segment of the rudimentary limbs following these in the Metanauplius (Fig. 102 B, mx) yield the first maxillae. The under lip, on the contrary, is said to rise from a rudiment on the inner side of this limb. We should thus have to assume that, in this imperfectly developed limb, we have the rudi- ments of the first and second pairs of maxillae crowded together. Metschnikoff (No. 53) also derives two pairs of limbs from this rudiment, but identifies them with the mandible and maxilla of 216 CRUSTACEA. the adult Cirripede, the third Nauplius limb as well as the second being supposed to vanish completely. In the more posterior section of the body, we find six pairs of swimming limbs (rf), which, in their structure, strikingly recall the thoracic limbs of the Copepoda, and are also similarly used as swimming feet. Each of these consists of a two-jointed basal segment provided with an exopodite and endo- podite, each of these, again, being composed of two joints and clothed with long swimming setae. Posteriorly, the body terminates in a short abdomen (ab) composed of four segments, the last of which ends in two furcal appendages bearing long setae. With regard to the internal organs, it should be mentioned that, at this stage, sac-like outgrowths of that enlargement of the intestinal canal which is called the stomach begin to appear : these develop into the so-called liver ; further, that the rudiment of the ovaries is to be seen in the shape of paired tubes situated ventrally in the cephalic region. Fio. 105.— Pupa of Lepas pectinata (after Claus, from Lang's Text-book), iia, Nauplius eye ; pa, paired eye ; rf, thoracic limbs (with the tendril-like— cirriform— feet beginning to form inside) ; o, mouth ; d, intestine ; L, liver ; sm, adductor muscle ; sc, scutum ; t, tergum ; ca, carina ; cd, cement-gland ; I, first antenna with the sucking disc. The free-swimming Cypris-like larva, after becoming finally attached, gives rise to the fixed Cypris-Wie larva (Fig. 105) of the Cirripedia. As, at this stage, the larva ceases to feed and loses all power of locomotion, while important external and internal changes are going on in the body beneath the cuticle of the Cypris stage (as if beneath a pupal integument), this stage has been well called the pupal stage (Cy pis-like pupa). From this, by the casting of the cuticle of the Cypris-like stage, proceeds the young Cirripede. For details as to the processes through which these transformations are brought about, we are indebted chiefly to the THORACICA. 217 observations of Darwin (No. 40), Pagenstecher (No. 58), and Claus (Nos. 39 and 8). The larva attaches itself firmly with the sucking discs found on the first antennae and by means of a sticky fluid secreted by the cement-gland. At first the whole of the ventral surface is in contact with and parallel to the surface of attachment (Fig. 106 A and B). The changes which first attract attention are certain processes of growth by means of which different parts of the body approach the final shape. For example, the mouth-parts at the top of the buccal cone, which until now were very rudimentary, become distinctly M—4-m Fig. 106.— Diagrammatic illustration of the metamorphoses of Lepas. A, Cypris-Yike stage. B, attached pupa. C, young Lepas stage, still surrounded by the loosened Cypris shell (s). a1, first antenna ; ab, abdomen ; c, carina ; d, intestine ; to, mouth ; o, unpaired eye ; pa, paired eye ; rf, thoracic limbs ; *, Cypris shell ; sc, scutum ; t, tergum ; x, reflection of the dorsal integument ; y, ventral fold. developed ; in the Lepadidae the principal feature in this process is the great increase in size of the upper lip. The thoracic limbs also elongate and become (Fig. 106 B) the rudiments of the tendril-like feet (rf). All these parts develop under the somewhat loose Cypris cuticle. The long tendril-like feet naturally have not the room necessary for development in the short envelope of the thoracic limbs of the Cypris stage, and therefore become much curved, and even press back into the thorax (Fig. 105). The abdomen degenerates almost completely, while from its base (genital segment) the unpaired penis projects as an outgrowth. 218 CRUSTACEA. While these processes of growth are taking place, the whole thorax undergoes a significant change of position (Fig. 106 J. and B), Whereas, in the earlier stages, the thorax lay almost parallel to the ventral surface, it now rises into a more perpendicular position with regard to that surface, so that the anal aperture no longer lies behind the oral aperture, hut above it. Simultaneously with this change of position, a sharper distinction arises between the thorax and the cephalic region, the point (x) at which the wall of the thorax unites with the surface of the mantle shifting ventrally (cf. the position of x in Fig. 106 A, with its position in Fig. 106 B). Meanwhile many changes take place in the anterior cephalic region of the pupa, these being preparatory to its transformation into the stalk of the adult form. The broad basal joint of the adhering antenna first completely fuses with the head and is taken up into the latter, so that the adhesive antennae of the adult Cirripede each consist of only three joints. Further, somewhat behind this point, a deep infolding of the surface of the body occurs (Fig. 106 B, y), so that that portion of the head which forms the stalk is at this stage sharply bent on itself. This fold arises by the withdrawal of the stalk-integument from the cuticle of the Cypris larva. Some of the important organs of the larva, however, which are not to be taken over into the adult, remain attached to that cuticle. The chief of these are the paired eyes (the Nauplius eye, however, passes over into the adult Cirripede and is retained throughout life), and the chitinous processes called by Darwin the apodemes, which served for the attachment of the antennal muscles and are cast off in the moult which follows. These details are not represented in the figure. The most important change which now follows is in the position of the larva and the lengthening of the stalk which is connected with it. By an opening out of the fold above mentioned, the larva now rises from the surface of attachment, its ventral surface standing at right angles to the latter. At the same time the stalk passes out from between the shell-valves of the Cypris stage (Fig. 106 C), and lengthens to form the adult peduncle. Darwin (No. 40) pointed out the fact that the part by means of which the attachment of the larva at first takes place does not correspond to the frontal uity of the body, but to the most anterior portion of the ventral surface. Only after the upright position has been assumed does the frontal extremity come into contact with the surface of attachment, to which it is glued by a secretion. In Crytophialus, on the contrary, and also in Alcippe, Lithotrya, and Anelasma, this part is not firmly attached, but grows out further. This further growth can only take place when the substratum can give way correspondingly. THORACICA. 21 & In Anelasma, the substratum is affected by simple pressure, but in the other forms just mentioned by a boring action of the stalk. The passage of the stalk out of the Cypris shell is rendered possible by the fact that the latter at this stage surrounds the body very loosely. Whereas, in the Cypris stage, the whole of the cephalic region is contained within the bivalve shell, in the adult, the anterior part of that section (the stalk) is not covered by the shell, here, as in the Cladocera, only that part of the head which carries the mouth-parts being included within the shell. We may imagine this change to have arisen at the time when the stalk grew out by a secondary flattening out of that anterior part of the mantle-fold which, in the Cirripede pupa, covered the most anterior part of the head. The adult Cirripede shell now appears more distinctly beneath the Cypris shell, and in it can be made out the first rudiments (primary valves, Darwin) of the five calcareous plates (scuta, terga, and carina, Fig. 106 C, sc, t, c). These primary valves are distinguished by their sieve-like sculpturing, which is caused by the boundaries of the matrix-cells remaining evident in the calcareous secretion. The surfaces of the valves are covered by a thin cuticle. The primary valves do not directly increase in size, but new calcareous layers are continually being secreted below them, and these attain a size greater than that of the original valves. In a superficial view of the primary valves, they are now seen to be surrounded by concentric lines representing the subjacent calcareous layers. By this method of increase of the valves, the non-calcareous parts of the shell which spread out between the valves become more and more circumscribed, but in some cases these intermediate spaces are retained to a considerable extent (Conch oder ma). It should be mentioned that in those forms which have a large number of valves, only the five primary valves are at first formed. The metamorphosis of the Balanidae, in the first stages, resembles that of the Lepadidae. Here also the Cypris pupa gives rise to a young form attached by a short fleshy stalk. Only later does the broad base characteristic of the Balanidae develop and form that external secondary mantle-fold, within which the upper part of the mantle which carries the scuta and terga appears like an operculum. The first rudiments of the shell are here membranous, and the sculpturing mentioned above, which is characteristic of the Lepadidae, is wanting. While the external form of the adult is reached in this manner, the inner organs are also undergoing important changes, some of which are as yet little understood. Some organs are cast off at ecdysis (paired eyes, antennal apodemes), others simply degenerate (antennal muscles). Meantime the hepatic outgrowths appear in the intestinal canal. The cement-gland increases considerably in size, and the genital organs show special development. The ovaries undergo their characteristic displacement, shifting to a position within the stalk. The moult which follows, and in which the cuticle of the Cypris stage is cast, closes this period of development. In the moult, the outer cuticle of the two 220 CRUSTACEA. shell- valves is cast first, the thorax and the inner mantle- cavity losing their cuticle later. In most of the Thoracica, metamorphosis seems to run the course above described. Only in individual cases is it more abbreviated. Thus, according to Koren and Danielssex (No. 48), the larvae of Anelasma squalicola pass through the greater part of their metamorphosis within the mantle-cavity. Kossmann, however, describes the Nawplii of these forms whose larvae, according to the above authors, when hatched, are provided with six pairs of limbs. Pagenstecher has rightly connected this feature with the attachment of Anelasma to sharks. Still more abbreviated is the metamorphosis of Scalpellum Stromii, the Cypris stage of which, surrounded by the Nauplius cuticle, was found by Hoek (No. 45) even within the egg-envelope. B. Abdominalia. The metamorphosis of Alcippe, a form which bores into the columella of the shells of Fusus and Buccinum (especially when these are inhabited by Pagurids) seems to agree in essentials with that of the Thoracica. The Nauplii were first described by Hancock. The Cypris stage described by Darwin (No. 40) is distinguished by the possession of six pairs of thoracic limbs, a fact worth mentioning in contrast to the reduced number and shape of these limbs in the adult; only four pairs of these are retained, the first as a palp, the second biramose, and the third uniramose. In Kochlorine, which bores into the shell of Haliotis, Noll (No. 56) found two kinds of larval forms, a small form provided with adhering antennae, but without a mantle, and a larger form having a bivalve shell and resembling the Cypris stage. It is probable that the metamorphosis here is closely related to that of the nearly allied genus Cryptophialus. The metamorphosis of Cryptophialus, first described by Darwin (No. 40), is greatly abbreviated. The egg here passes almost direct into the Cypris-like larval form. There first arises from the oval egg a larva in which two processes can be recognised as the rudiments of the adhering antennae. A third process indicates the posterior end of the body. At a later stage, the adhering antennae are ap- proximated, while the body assumes generally a more pointed egg-like shape. From this stage the Cypris form emerges, in which can be distinguished the mantle-fold, the paired eyes, and the well-developed adhering antennae. The rudiments of the thoracic limbs are here wanting, but three pairs of setae are found on the abdomen. These larvae creep about by means of the adhering antennae in the mantle- cavity of the mother, and finally change into the adult form. For the development of the stalk in these forms, see p. 218. RHIZOCEPH AL A. 221 C. Khizocephala. The free-swimming stages of the Rhizocephala (Nauplius or Cypris stage) were early observed by Fr. Muller (Nos. 54 and 55), Kossmann (No. 49), and others. The later transformations, on the contrary, which bring about the transition from the Cypris stage to the parasitic form, were first observed by Delage (No. 41) in the case of Saceulina carcini. We shall, therefore, in our description, follow the statements of Delage. The Nauplius of Saceulina carcini leaves the egg with a more or less compact form, but by a moult which follows soon after hatching attains a greater length (Fig. 107 A). Otherwise it shows the normal Cirripede type. The two frontal horns with their glands (gl) are well developed, as also are the filamentous frontal organ (fs) and the Nauplius eye (wa), lying close to the brain. As a remains of the labrum a projection is found which is known as the rostrum; on the other hand, the oral aperture, the intestinal canal, and the anal aperture are wanting. The intestinal canal is here replaced by a large accumulation of food-yolk. The Nauplius limbs (J, 2} S) are developed in the typical manner, but the protopodites of the two posterior pairs are without the masticatory hooks usually found on them. A median mass of cells lying within the body between the brain and the rostrum is assumed by Delage to be the ovary (ov). Fritz Muller asserted the presence of a broad, oval, dorsal shield in the Nauplius of Saceulina. Kossmann, however, has pointed out that Muller's figures are of larvae about to undergo ecdysis, in which the Nauplius cuticle is no longer in contact with the body. The most important change found after a third moult is the further growth of the thoraco-abdominal region of the Nauplius> in which the six thoracic segments, with the rudiments of their limbs, soon appear distinct from the abdomen. At the same time the most anterior limbs begin to assume the character of the adhering antennae (Metanauplius stage). By the fourth moult the free Cypris stage (Fig. 107 B) is reached. In this moult the biramose limbs of the Nauplius stage are entirely cast off, and not only do the chitinous envelopes of these limbs adhere to the cast skin, but it appears that even some of the soft parts are thrown off in the moult. The Cypris stage in which, as in the former stages, there is no trace of an alimentary canal, closely resembles in form the similarly-named stage in the Thoracica. The bivalve shell has almost the same shape; the segmentation of the thorax, the form of the swimming limbs (I-VI) are in agreement. 222 CRUSTACEA. The abdomen (ab) is very rudimentary, consisting of a single joint bearing two f ureal appendages. The first antenna (1) is without sucking disc and cement-gland. It consists of three joints : a basal joint which is broadened and connected with the chitinous apodemes, a long middle joint, and a short, terminal joint provided with three Fio. 107.— Consecutive larval stages of Sacculina carcini (after Delage, from Lang's Text-book). A, Nauplius after the first moult. B, free-swimming Cypt^is stage. C, Cypris stage after the larva has become attached to a seta (bb) of the host. D, formation of the Kentrogon larva. K, the Kentrogon larva after the Cypris shell has been thrown off and the arrow has formed. F, the arrow has bored through the chitinous cuticle of the host. 1, 2, 3, the three pairs of Nauplius limbs ; I-VI, thoracic limbs ; ab, abdomen ; bb, seta of the host ; /, fat globules ; /«, frontal sensory organ ; gl, glands of the frontal horns ; ov, rudiment of the ovary ; pf, arrow ; ua, Nauplius eye. hooked setae. The Nauplius eye (ua) persists, but the paired eyes are absent. Among the internal organs present at this stage, we note the very distinct ovarian cell-mass (ov), the strongly-developed musculature, the persistence of the glands of the frontal horns (yl)t and the accumulations of pigment and of food-yolk (/). RHIZOCEPHALA. 223 The next stage which we may compare with the pupa of the Thoracica is called by Delagb the Kentrogon stage. In this stage the attachment of the larva to the body of the host (Carcinus maenas), and its passage into the body-cavity of the latter are accomplished. After the free Cyjpris-like larva has swum about for three or four days, it seeks out a host, a young shore-crab (from 3 to 12 mm. broad), and attaches itself to one of the integumental setae (bb), one of the adhering antennae (1) of the Cypris larva surrounding such a seta near its point of insertion (Fig. 107 G). The point at which the larva attaches itself is not, as we should expect, a priori, on the ventral surface of the abdomen (of the host), but seems to be in- discriminately chosen. Fixation often takes place on the back of the host, or on one of the legs. The next change to take place may be described as a moult in which many important parts of the body are cast off (amputated). First the soft contents of the adhering antennae are drawn in, the apodemes (chitinous tendons of the antennal muscles) being at the same time expelled from within the body. These latter remain attached for a long time to the envelopes of the adhering antennae, which are also retained for some time (Fig. 107 Z>), as they are of importance in bringing about the attachment of the larva to the host. While the soft parts are everywhere withdrawn from the chitinous envelope, the thorax is protruded far beyond the shell -valves and amputated in toto (Fig. 107 C). This can only be accomplished by a somewhat extensive rupture of the body-wall, and, through this, remnants of internal organs are thrust out into the larval shell and lost. Thus a great part of the pigment found in the larva, as well as remains of food-yolk, are thrown out, the frontal glands and the whole body musculature undergoes degeneration, and the masses of detritus thus produced, together with the Nauplius eye, are now eliminated. The remainder of the body left after the separation of all these organs draws together to form a solid oval sac (Fig. 107 D), which soon becomes surrounded with a chitinous envelope. This latter is closely contiguous to the sac; only at its most anterior end, that turned towards the adhering antennae, it can be noticed that the soft body seems to lie naked against the inner sides of those organs. The newly-formed envelope is in this region probably exceedingly delicate, and very closely apposed to the inner surface of the antennae. The layers into which the contents of the sac break up are at this stage not at all clear. A superficial ectodermal cell-layer can, however, be distinguished from the inner mass which, in all probability, is meso- 224 CRUSTACEA. dermal, and the chief constituent of which is the cell-mass of the ovary. Besides remains of pigment and food-yolk, other mesodermal elements no doubt enter into the formation of this second layer, from which are to be derived the rudiments of the testes as well as the musculature and other organs of the adult Sacculina. It is important to bear in mind that the encysted sac thus produced has been derived, after the expulsion of the whole thorax, exclusively from the cephalic section of the Cypris larva. The soft body of the sac-like larva now begins to develop a small fine point at its anterior end (Fig. 107 D), which protrudes into the inner cavity of the R. ;m- „„ antenna used for at- tachment, and this is followed by the secret- ing of a new chitinous envelope at the surface of the soft body (second moult of the Kentrogon stage, Fig. 107 E). As the new cuticular layer thickens considerably over this anterior point, it forms the arrow-like tube which character- ises the Kentrogon stage. This tube, in- creasing in length and becoming somewhat bent, invaginates the anterior surface of the sac (Fig. 107 E). At this stage the cast off Cypris shell is either only very loosely at- tached to the sac, or is even completely severed from it. The protrusion of the arrow now takes place (Fig. 107 F, pf), the invagination just described being re-evaginated. The arrow first entert the inner cavity of the adhering antenna, and thence, con- ducted by the latter, passes into the soft articular membrane of the myy flnx Fio. 108.— Two sections through the nucleus of a Sacculina Interna (after Delaoe). A, younger stage. B, older stage, am, outer mantle-layer ; im, inner mantle-layer; m, mesoderm-cells ; o, aperture of the perivisceral cavity; ov, rudiment of the ovary ; p, perivisceral cavity. RHIZOCEPHALA. 225 seta to which the Cypris-like larva attached itself. Through this membrane the arrow bores, thus establishing a communication between the inner cavity of the sac and the body-cavity of the host. During these processes the soft inner body becomes surrounded with another very delicate cuticle (third moult of the Kentrogon stage). There is at this point a gap in our knowledge of the development of Sacculina. There can, however, be no doubt that the soft body Fig. 109.— Longitudinal sections through two stages of development ot Sacculina carcini (after Delage). A, Sacculina interna. B, Sacculina externa, a, atrium (widening of the oviduct) ; am, outer mantle-layer ; b, brood-cavity (mantle-cavity) ; B, basal membrane ; C, central tumour ; cl, cloacal opening ; D, intestinal wall of the host ; dr, glands of the ovarian sac ; /, aperture of the perivisceral cavity ; g, ganglion ; im, inner mantle-layer ; L, body- wall of the host ; ov, ovary ; p, perivisceral cavity ; pe, perivisceral ectodermal layer ; 11, root- processes (some in cross section) ; t, rudiment of testes. of the larva passes through the canal of the arrow, so as, in this way, to reach the body-cavity of the host. The Sacculina thus becomes an endoparasite (Sacculina interna). Q 226 CRUSTACEA. Sacculina interna. The endoparasitic larva now wanders from the point at which the attachment of the Cypris larva occurred, further into the host, until it reaches the ventral side of the intestinal canal where its final fixation takes place. At the same time it sends out an exceedingly wide-spread network of rootlets which penetrate throughout the whole body of the host, covering superficially all the organs and only leaving the heart and the gills unaffected. At the point where the actual Sacculina lies, all the rootlets gather together to form a plate (basal membrane, Fig. 109, B), in the middle of which can be seen a swelling (central tumour, Fig. 109, C). The body rudiment of the Sacculina lies sunk in this central tumour as its so-called nucleus. The rootlets, the basal membrane, and the central tumour show essentially the same histological structure. They consist of a superficial epithelium (ectoderm) and an inner cavernous tissue composed of star-like anastomosing cells of connective tissue. The nucleus (Fig. 108) is completely sunk in the central tumour, and therefore lies in a cavity which is called by Delage the perivisceral cavity (p), and which opens externally through one small aperture. Even this aperture closes (Fig. 108, B), to re-appear later in the form of a transverse fissure (Fig. 109, /, fente de sortie). The point at which the nucleus is in contact with the wall of the perivisceral cavity is now known as the stalk (peduncle) of the nucleus. In the nucleus itself, a superficial ectodermal layer can be dis- tinguished (Fig. 108 A) which, near the stalk, passes into the wall of the perivisceral cavity. The inner mass of the nucleus at this stage consists almost exclusively of the rudiment of the ovary (ov) ; but in the stalk there are some mesoderm-cells which are of importance in connection with the development of the testes, the musculature, the connective tissue, &c. Delamination next takes place in the ectodermal layer of the nucleus, which divides into two layers that shift apart (Fig. 108 B). Into the space between the two layers, some of the mesoderm-cells just mentioned wander to yield the musculature of the mantle. The two layers of ectoderm which have thus arisen are known as the outer (am) and inner (im) mantle-layers, on account of their relation to the future mantle of the Sacculina. A second similar process of delamination now takes place in the inner mantle-layer, an inner ectodermal layer surrounding the central part of the nucleus being thus split off. The latter, as surrounding the future visceral sac, is distinguished as the perivisceral ectodermal layer (pe). Between it and tin- inner mantle-layer there now appears a cavity lined with chitin (Fig. 109, b); this is the so-called brood-cavity (cavite incubatrice). RHIZOCEPHALA. 227 We thus see that, by changes in the nucleus, the most important parts of the body of the adult Sacculina begin to be formed ; viz., the inner visceral sac, the brood-cavity, and the mantle-fold which •surrounds it. The visceral sac is not completely encircled by the brood-cavity, for, at the point of attachment of the sac, the inner mantle-layer passes over into the perivisceral ectodermal layer ; this transition point is the so-called mesentery, which Delage assumes to have a ventral position in the body. Having now obtained a general idea of the development of the form of the "body in Sacculina, we must add a few words as to the rise of its most important •organs. In the mantle region the changes are not great. In the late stages, the mantle-cavity breaks through into the perivisceral cavity, and thus arises the cloacal aperture (Fig. 109, cl), which lies almost opposite the stalk of the Sacculina, but somewhat to the left side of the body. While the ectodermal •cells of the mantle lengthen to form transverse connective fibres (Fig. 109 B), the mesoderm-cells change into longitudinal muscle-strands and the sphincter of the cloaca. More important alterations occur in the region of the visceral sac. The ■ganglion (g) here forms by an immigration of ectoderm-cells, in which not only the perivisceral ectodermal layer, but also the inner mantle-layer (by means of the mesenterial margin ?) are said to take part. Whereas, in earlier stages, the whole inner space of the visceral sa*c was almost exclusively occupied by the ovarian rudiment, numerous mesenchyme- cells now wander from the stalk into the perivisceral sac, surround the ovary, forming a peritoneal envelope around it, and fill the space between the body-wall, the ganglion, and the ovary. The ovarian rudiment simultaneously breaks up into two lateral lobes connected by a commissure. The manner in which the short oviduct arises is not quite •clear, but Delage believes that he can trace it back to a paired lateral ecto- dermal invagination. This latter, widening inwardly, gives rise to the so-called atria {a), on whose walls the glands of the ovarian sac (cement glands, dr) appear as lateral outgrowths. The vasa de/erentia arise in the same way through ectodermal invaginations near the stalk of the visceral sac, while the actual testes (t) are derived from mesoderm-cells which become attached to the ■ends of these ducts. After the Sacculina, completely enclosed within the central tumour {in the perivisceral cavity), has in almost every respect attained the grade of development of the adult, it rises to the surface of the central tumour, passing out through the widened opening of the perivisceral cavity (Fig. 109 A, /). The fold by which this cavity was formed now draws back to the base of the stalk and soon entirely disappears. An increase in size takes place in the Sacculina after leaving the central tumour, and this causes constant pressure on the ventral wall of the abdomen of the host (Fig. 109 B, L) leading to gangrene of the part thus affected, and to the consequent formation of an opening through which the body of the Sacculina passes out freely, its stalk 228 CRUSTACEA. still connecting it with the basal plate and the network of rootlets within the host. The Sacculina is, by these processes, changed into a Sacculina- externa (Fig. 109 B). The parts that lie outside of the host now become strongly chitinised, there is further increase in size, and the stage of sexual maturity is reached. The metamorphosis of Sacculina above described is without doubt one of the most remarkable processes of transformation in the animal kingdom. The intercalation of a temporary endoparasitic condition must no doubt be referred to the protected position thus obtained,, and, indeed, the whole process of development of this form has undergone marked coenogenetic changes. Although, considering the unusual simplification in structure of the Kentrogon larva, we may not be ontogenetically justified in tracing back the various parts of the adult body to those of the Cypris-like larva, the consideration of other forms (e.g., Anelasma) leaves us not a moment in doubt as- to how the body of the adult Sacculina is to be interpreted. Such consideration would lead us to compare the peduncle of the Sacculina, running out at its base into root-like processes, to the stalk of the Lepadidae, the mantle of the Sacculina to the shell of the latter,, and the brood-cavity of Sacculina to the mantle-cavity of the Lepadidae. The cloaca! aperture would then correspond to the shell-cleft in the latter family. This interpretation is supported most of all by the similar position of the ovarian sacs in these cavities. It becomes probable, on comparison with Anelasma, that Delage's definition of the mesenterial margin as the ventral side of the Sacculina is actually correct. Frequent attempts have recently been made to oppose the Khizo- cephala to all other Cirripedia as an independent group (sub-order). On the other hand, it must be pointed out that they, in the Nauplius and the Gypris stage, show such complete agreement with other Cirripedes that too great stress must not be laid on changes of structure which have evidently arisen secondarily as the result of parasitic life. D. Ascothoracida. This group comprises a few forms (Laura Gerardiae, Synagoga mira, Petrarca bathyactidis) which live parasitically in Anthozoa, and Dendrogaster astericola, an endoparasite in the Asteroidea (Solaster, Kr/t master). In the most important features of their organisation > these forms are true Cirripedes, although they claim a special position within that group. Laura Gerardiae, thanks to the re- THE MORPHOLOGICAL DERIVATION OF THE COMPLEMENTAL MALE. 229 searches of Lacaze-Duthiers (No. 51), is the member of this group which we know best. The animal is surrounded by a large mantle which here shows direct relation to the shell-valves of the Cypris stage ; each half contains between its two lamellae not only the hepatic outgrowths of the intestinal canal, but also the ovaries. The body proper appears very much reduced, but is still distinctly segmented, the mouth-parts are adapted for sucking, the six (or five) pairs of thoracic limbs have degenerated, and the abdomen is short. It should be mentioned as a characteristic of the group that the first antennae are here never used, as in other Cirripedes, for the attachment of the body. For the general morphological elucidation of these forms indeed, we must compare them not with the adult Lepas, but rather with the free-swimming Cypris larva. Very little has been made known, up to the present time, of the ontogeny of these forms. The cleavage of the egg in Laura seems to resemble that in Balanus. The Nauplii of Laura show very little similarity with V the typical Cirripede Nauplius, on account of the absence of the very characteristic frontal horns. A small form observed by Lacaze-Duthiers, and no doubt representing a stage in the course of development of Laura, may possibly be regarded as a complemental male. The Cypris stages of Dendrogaster (Fig. 110), however, are known {Knipowitsch, No. 47), the metamorphosis in this form appearing to be abbreviated, no free Nauplius stage being passed through. The larva, in which an anal aperture, as in the adult, is wanting, bears a general re- semblance to the Cypris stage of the Cirripedia. Both the single and the compound eyes are, however, wanting. A very large olfactory filament {a') is developed on the first antenna. There are five biramose pairs of thoracic limbs ; the first abdominal segment carries the rudiment of a penis (p). The abdomen, (ab) which is distinguished by its length, consists of five joints and the furcal appendages. Fig. 110.— Free-swimming Cypris stage of Dendogaster astericola (after Knipowitsch). a', first antenna ; ab, abdomen ; d, intestine ; m, buccal cone ; n, nervous system ; p, rudiment of penis. E. The Morphological Derivation of the Complemental Male. The sexual differentiation of the Cirripedia is very complicated and difficult to explain. As a rule the Cirripedia are hermaphrodite. We shall not err if, considering the almost universal separation of 230 CRUSTACEA. the sexes in all other Crustacea, we regard this hermaphroditism as secondarily acquired in consequence of the attached manner of life. We must assume that the free-swimming ancestors of the Cirripedia were of separate sexes, and that hermaphroditism was only gradually acquired after the fixed sedentary mode of life had become fully established as a characteristic of this group. While, in the Balanidae and Rhizocephala, hermaphroditism has become the exclusively prevailing condition, there is a tendency in many groups of the Lepadidae to retrogression in the direction of the separation of the sexes. Male forms here appear either side by side with herma- phrodite individuals, being then called complemented males, or. else, in cases of the complete separation of the sexes, in company with true females. These complemental males are always smaller than the hermaphrodite individuals or the females ; they are found attached parasitically to the bodies of the hermaphrodites or females. In isolated cases, the shape of the body in the male deviates only slightly from that in the hermaphrodite (Scalpellum villosum and S. Peronii), but in other cases there is striking sexual dimorphism in this respect, the male undergoing a process of degeneration in which the calcareous portions of the skeleton, the limbs, the mouth, and the alimentary canal have been lost, thus sinking to a very low grade of organisation, and becoming actually a dwarf or complemental male. The following degrees of degeneration of the male are found in the genus Scalpellum (Hoek, No. 46). I. True hermaphrodite forms (Scalpellum balanoides, Hoek). II. Large hermaphrodite forms with small complemental males. (a) The males resemble the hermaphrodites in structure. The division into capitulum and peduncle is recognisable, mouth and alimentary canal are present (Scalpellum villosum, S. Peronii). (b) The males have degenerated ; they are without mouth and intestine; without a shell, or have only a vestigial shell; without a peduncle (Scalpellum vulgare, S. rostratum). III. Separate sexes. The female is large and resembles the hermaphrodite individuals of the other species. The male is very small (Scalpellum ornatum, S. regium, Hoek, etc.). The Abdominalia (Alcippe, Cryptophialus) show a differentiation akin to that of this last group. Here also we find separation of the sexes with highly developed sexual dimorphism. The complemental males appear very much reduced. They have no tendril-like feet, no mouth, and no alimentary canal. In other respects, if we take into account the reduction that has commenced, their structure can be traced back to that of the female. A condition resembling that above described for Scalpellum is found in the COPEPODA. 231 genus Ilia. Injbla quadrivalvis, side by side with the hermaphrodite form, there is a small complemental male with a large peduncle, but a very reduced capitulum and a diminished number of thoracic limbs, while in Ibla Cumingii a similar male is found side by side with a true female, complete separation of the sexes being here obtained. It follows from the above that, in accordance with Hoek (No. 46), in tracing the sexual condition of the Cirripedes, we start from the hermaphrodite form, proceeding to pothers with dwarf males in company with hermaphrodite individuals, and reach finally a complete separation of the sexes in which marked sexual dimorphism has secondarily developed. The series of com- plemental males, as well as the dwarf males, would thus be derived from the hermaphrodite form by the degeneration of the female genital rudiment. It is perhaps difficult to imagine how the want of complemental males can in the first instance have made itself felt in hermaphroditic forms. But we must bear in mind that, according to F. Muller, cross -fertilisation appears to be the rule even among the true hermaphroditic Lepaclidae. This cross-fertilisation is of the greatest importance in securing the vitality of the race ; but owing to the fixed mode of life of the Cirripedia there is some danger that it may not take place. All such danger is precluded, by the development of these dwarfed forms living semi-parasitically on the larger hermaphrodites. The dwarfs, by the atrophy of their ovaries, have become males, and by the attachment to the hermaphrodites ensure cross -fertilisation. We must regard this condition as a partial retrogression in the direction of separation of the sexes, which has again been reached in individual cases in the further course of this development. The view sketched above is in opposition to that of Claus (No. 8), according to which the sexually distinct forms {e.g., Alcippe and Cryptophialus) have re- tained the primitive condition. From this originally dioecious condition, by a transformation of the females into the large hermaphrodite form, the condition found in most Lepadidae was developed, while the males were only retained in isolated species as complemental males. Consequently the males would be a vestigial remainder from those times when hermaphroditism had not yet become the rule among the Cirripedia. The occurrence of dwarf males side by side with hermaphrodite individuals in a number of forms would be easily explicable by this ingenious theory (not, however, in Scalpellum villosum and S. Peronii). Hoek, however, has pointed out in opposition to it, that, in this case, the dwarf males ought to show a greater resemblance in structure to the Cypris form than they actually do show. The dwarf males in reality appear to be connected by gradual transitions with the complemental males of Scalpellum villosum and S. Peronii, which latter forms are evidently to be derived from the hermaphrodite form. The view held by Claus would apply to the Rhizocephala if the statements of Fr. Muller and Delage were to prove true that complemental males occur in these forms which, throughout life, do not develop further than the Cypris pupa. But this last view, which rests upon the discovery in the cloacal aperture of a young Sacculina externa of an attached dead Cypris envelope, must still be considered doubtful, and has actually been denied by Giard. ; st ^^CJ7£i, ^ *~o /*-»- o-v*-**- 6. Copepoda. t*wv^Uv,^.ft«^p'.w^ The Copepoda are very numerous and rich in varieties of form, ^ - ^^cxS^ in spite of the simplicity of their body segmentation; they never- dmi* theless show morphological characters which, in relation to those 232 CRUSTACEA. of the hypothetical ancestor of the Entomostraca, must be regarded as decidedly degenerate. Among these we must reckon the small size of the body and the comparatively small number of its segments, the reduced form or entire absence of the heart, the want of separate respiratory organs (branchiae), the loss of the paired lateral eyes, which are retained only in the families Cory- caeidae and Po?itellidae, and perhaps also the slight development of the dorsal shield. On the other hand, there are certain indications that the Copepoda ought to be counted among the most primitive of existing Crustacea. Among these characters we should specially mention the use of the two pairs of antennae as locomotory and clasping organs, the very primitive structure of the mouth-parts in the free-living forms (occurrence of a biramose mandibular palp, the segmentation of the first maxilla, Fig. 91 A, p. 194), and the meta- morphosis which, in the free-living forms, shows very primitive features. With regard to the segmentation of the body, we must distinguish as the most anterior region of the body a simple cephalic region carrying the antennae and the mouth-parts. The latter consist of three pairs of appendages (mandibles, first and second maxillae), the last pair separating into two appendages (the exopodite which shifts forward is called the first maxillipede, the endopodite which shifts backwards yields the second maxillipede). The thoracic region con- sists of five segments provided with biramose swimming limbs (Fig. 90 A, p. 193); the last of these segments may be vestigial, -whilst the anterior segment often fuses with the cephalic segments, this union giving rise to an anterior region known as the cephalo-thorax. The abdomen consists of five segments, the most anterior of which alone carries the rudiment of a limb (genital prominence). A fusion of the two anterior abdominal segments usually gives rise, in the female, to a double genital segment which bears the genital aperture. In a few Pontellidae, the cephalo-thoracic region, by the appearance of demarcations between the segments, becomes subdivided into regions (each consisting of two segments). This peculiarity stands almost alone in the whole series of Crustacea. Such a segmentation can only be regarded as a secondary re-appearance of the long-lost independence of the cephalic segment. It has, nevertheless, a certain interest. A. Gnathostomata. The metamorphosis of the free-living Copepoda is accomplished as a very gradual transition from the Nauplius to thei adult form through many moults. There is, however, at a certain period, a more sudden GNATHOSTOMATA. 233 -change of shape, and this enables us to separate the course of metamorphosis into two periods, the first comprising the series of Nauplius and Metanauplius forms, while the second is distinguished by a name taken from the metamorphosis of the Cyclopidae as the series of Cyclops-like larval forms. In the first series, the Nauplius limbs show a general resemblance to the primitive form, the abdomen is not yet distinctly marked off, and the furcal processes have not yet Fio. 111.— Larval stages of Cetochilus septentnonalis (after Grobben, from Lang's Text-book). A, Nauplius. B, Metanauplius. C, older Metanauplius. I and 2, first and second antennae ; 3, mandible; h, maxilla; 5, 5, exopodite and endopodite of the second maxilla ( = first and second maxillipedes) ; I, II, first and second pairs of thoracic limbs ; an, anus ; g, brain ; gz, genital cells ; m, mouth ; me, primitive mesoderm-cells ; 61, upper lip. developed. In the second series, these last advances in development are made, while the antennae and mouth-parts approach the adult form. The development of the free-living Copepoda has been specially investigated by Claus (Nos. 64 and 67). We take, as the foundation of our description, the metamorphosis of Cetochilus, which has been minutely described by Grobben (No. 73). The free-living Copepod leaves the egg as a strikingly unspecialised Nauplius (Fig. Ill A). 234 CRUSTACEA. The body, which is usually oval (only in individual cases long, trans- versely broadened or barrel-shaped), shows no traces of external segmentation, and carries on its ventral side the large upper lip (ol), as well as the three typical pairs of Nauplius limbs (1, £, 3). The anterior limb (first antenna) is uniramose, the next (second antenna) biramose and provided with a masticatory hook on the protopodite. The mandible in Cetochilus is without such a masticatory process,, and is a simple biramose swimming limb. The limbs are beset at their ends with long setae. In the alimentary canal we can distinguish a stomodaeum, a long enteron, and a proctodaeum. The latter, in Cetochilus, in the first Nauplius stage, is still found as a solid ingrowth of the ectoderm, the anal aperture not yet having broken through. The nervous system is still connected for its whole course with the ectoderm. As a sensory organ, we find the Nauplius eye. The coiled antennal gland, which is probably developed at this stage, functions as excretory organ, besides which the cells of the enteron seem to have undertaken an excretory function ; urinary concretions have, at least, been proved to exist in certain projecting cells in the enteron of the Nauplius of Cyclops (Leydig, Fig. 89, ds, p. 191). The terminal region of the Nauplius is sharply bent ventrally,. and provided with two setae. We here find, internally on each side, a large mesoderm-cell (me) ; these are assumed by Grobben to be primitive mesoderm-cells. Later Nauplius stages are distinguished by the greater length of the body and by the outgrowth of its posterior region. During this latter process, the more strongly chitinised integument of the dorsal parts becomes marked off as the cephalo-thoracic shield by the de- velopment of a fold at its margin. The proctodaeum has now become perforated, a distinct dorsally-placed anal aperture being apparent. The brain is connected behind the Nauplius eye with a paired ectodermal growth, in which can be recognised the rudiments of paired lateral eyes and their optic ganglia (secondary brain) ; these, at a later period, become vestigial. The rudiment of the genital organs is to be recognised in large mesoderm-cells lying one on each side of the alimentary canal (Fig. Ill B). There now appears, behind the mandible, a small biramose limb,, the rudiment of the first maxilla (4), and the larva passes into the first Metanauplius stage (Fig. Ill B). A later Metanauplius stage (Fig. Ill C) reveals three more rudi- GNATHOSTOMATA. 235 ments of limbs, viz., the second maxillae (5), from the two branches of which the so-called maxillipedes of the Copepoda are derived, and the first two pairs of thoracic limbs (I, II). This stage still has a distinct ISTauplius appearance. The body has grown in length, but still, when seen from the side, shows the characteristic ventral curvature. The posterior end of the body is still without the furcal processes. The two pairs of antennae have not essentially changed from their condition in the former stage, except in the increased number of their setae. The masticatory hooks are still present on the basal joints of the second antennae. In the mandible (3) a large masticatory blade can be seen projecting from the basal joint. The first maxilla (4) is like a small lobed plate, while, in the second maxilla (5), indications of the separation of the exopodite (so-called anterior or outer maxillipede) from the endopodite (so-called posterior or inner maxillipede) are to be seen. The rudiments of the two anterior pairs of thoracic limbs are found as bilobed swellings (I, II). At this period, several moults take place, no essential change occur- ring in the shape of the larva, beyond the appearance behind the second thoracic segment of the rudiment of a third. The whole series ends by a moult through which the larva passes on into the series of Cyclops-like stages. The first of these stages, which, in accordance with the accepted terminology of the Copepodan metamorphoses, we shall call the first Cyclops stage (more correctly named Cetochilus stage by Grobben), reveals essential changes of form. The body is no longer flexed ventrally, but is straight. Its most posterior region is sharply marked off from the anterior portion of the body by a constriction ; the furcal appendages and the rudiment of a fourth thoracic segment have developed. The limbs approximate in shape to those of the adult, although they have not so many joints. The first antenna has passed from the short leaf- shape into the long, cylindrical, oar-like form ; it stands out laterally from the body, and consists of many joints. The second antenna has remained biramose, but has lost its masticatory process; on the mandible, the masticatory blade has greatly increased in size. The maxilla is larger and more richly jointed, the maxillipedes have become transformed into large prehensile organs. The two anterior pairs of thoracic limbs have developed as swimming limbs; the basal segment of each already consists of two joints, but the two rami are still unjointed; the third pair of thoracic limbs, on the contrary, can only be recognised as a pair of bilobed rudiments. 236 CRUSTACEA. Among the transformations which take place in the internal organs during the Cyclops stages, we must mention the degeneration of the paired optic rudiment and of the secondary brain. Now for the first time the nerves which run to the paired frontal organs are distinctly recognisable. The antennal gland degenerates, while the shell-gland, which opens externally at the base of the anterior maxillipede, becomes functional in its stead. The anal aperture no longer lies dorsally, but shifts to the posterior end of the body between the two furcal appendages. The genital organs show an advance by an increase in number of genital cells and by the development of the efferent ducts. The paired genital rudiments now meet above the intestine and fuse to form a single gland. The heart develops between the first and second thoracic segments out of a paired rudiment of mesoderm-cells. Fig. 112.— Two stages of development of Canthocamptus staphylinus (after Claus). A, Meta- nauplnis stage. B, Cyclops stage with three pairs of swimming limbs, a', first antenna ; a", second antenna ; md, mandible ; mx, maxilla ; to/, second maxilla (rudiment of the two so-called maxillipedes) ; to//, first so-called maxillipede ; to/", second so-called maxillipede ; P't P"t P,nt first, second, and third pairs of thoracic limbs. In the second Cyclops stage (Fig. 112 B), there first appear the rudiments of the limbs of the fourth thoracic segment and the delimitation of the fifth thoracic segment. Thus we find, at this stage, behind the cephalo- thorax, four free thoracic segments, and, following these, the as yet unsegmented abdomen. Of the thoracic limbs, the three anterior pairs are well developed. In the third Cyclops stage, the fourth pair of thoracic limbs has attained full development and the first abdominal segment has formed ; in the following Cyclops stages, the gradual segmentation of the abdomen takes place, as well as the complete transition of the limbs to their PARASITA. 237 final shape. Besides the formation of a more richly segmented body, in the families of the Cyclopidae and Corycaeidae, the (inner) accessory branch of the second antenna is lost as early as the first Cyclops stage, and the mandibular palp degenerates. The metamorphosis of the Calanidae (Cetochilus) , which has here been considered as the type, is distinguished by the regular development of the limbs from before backwards, but in the Harpactidae (Fig. 112) and Cyclopidae an exception to this regular order is found in the second maxilla (mf), which in the later Nauplius stages has, indeed, begun to form, but is still in an exceedingly rudimentary condition, so that the pair of limbs which follow it precede it in development. We have here a parallel to the condition of the maxilla in the Phyllopoda. B. Parasita. The free-living Copepoda are connected with the more specialised parasitic genera by many transitional forms which mark the various degrees of parasitism, and we consequently also find various stages in the transformation and degeneration of the segmentation of the body. It may be laid down as a general rule that the female, on account of the part played by her in reproduction, shows a greater tendency to adopt the parasitic mode of life, and consequently to undergo degeneration of the locomotory organs, obliteration of the boundaries between the segments and deformation of the body. Thus, even in the little modified Sapphirina, we find that the females become parasitic in the respiratory cavity of the Tunicata, or in the umbrellar cavity of Diphyes, while the males are always found swimming freely about. An extreme example is found in Lemaea (Fig. 114 A and B), the metamorphosis of which ends in a Cyclops-like free-swimming stage, during which copulation takes place ; the female afterwards becomes attached to a fish (often one of the Gadidae), and undergoes great deformation of the body (Fig. 114 0 and D). Heteromorphism of the sexes develops in this group, in so far as the male is only slightly removed in the segmentation of its body from the later Cyclops stages, while the female is greatly transformed in accordance with her parasitic habit. The same is the case in the Philichthyidae and the Chondracanthidae. In some forms, on the contrary, the male also departs from the Cyclops-like shape of the later larval stages by a secondary transformation. Whereas, in the Caligidae and Dichelestiidae, the two sexes are not strikingly different in shape of body or size, in the Lernaeopoda (Fig. 115 D and K), the heteromorphous development of the two sexes takes place in a way different from that shown in Lemaea. The males here 238 CRUSTACEA. participate in the degeneration of the body -segments as a result of their parasitic life, and they further undergo arrest of growth, so that an enormously large female is contrasted with a dwarf male. This kind of heteromorphous development of the two sexes must be regarded as an excessive adaptation to the different sexual functions, which is rendered possible by parasitic life. The parasitic forms can be deduced from the free-living forms by imagining that the latter have, in consequence of parasitism, under- gone certain changes. Thus the parasitic Copepoda first passed through Metanauplius and Cyclops stages to a stage approaching the shape of the free-living form, and then, through a series of further stages, attained parasitic deformation. The metamorphosis of the parasitic Copepoda has thus been lengthened by the addition of final parasitic stages. The first two series of stages, however, seem to be correspondingly shortened. The larvae of the parasitic Copepoda frequently do not hatch in the Nauplius form, but in an advanced Metanauplius stage, or even in the Cyclops stage (Fig. 73, p. 148; Chondracanthus, Tracheliastes, Achtheres, Anchorellay Brachiella, etc.). On the other hand, the metamorphosis may be shortened by the suppression of the later Cyclops stages, since, in cases of the most specialised parasitic forms, the very first Cyclops stage passes at once into the parasitic form (Chondracanthus, Lernaeopodidae). A further distinction between the metamorphosis of the parasitic Copepoda and that of the free-living forms arises from the circum- stance that, even in the larval condition, a sedentary manner of life (on the gills of a host) is adopted, and that, in keeping with this, there is a development of a peculiar attaching organ (the frontal band of the larvae in the Caligidae, Lernaea, and Lernaeopoda) and of resting stages with reduced limbs (so-called pupal stages). It would take us beyond the limits we have assigned to ourselves to give a complete enumeration of the very scattered notices of individual larval forms among the parasitic Copepoda, especially as there are still many gaps in the observations made on the development of these forms. We must content ourselves with selecting a few of the more important forms, of whose meta- morphosis a more accurate knowledge has been obtained. We must here, in the first place, separate those larvae which apparently are not provided with tbe larval adhering apparatus (frontal band) from those in which such an organ has been observed. In bhOM families in which the adult retains more or less of the body seg- mentation of the free-living Cupupoda {e.g., Corycacidae and the Notodelphyidae, which latter is placed among the Gnathostomata), the metamorphosis appears ""' to differ essentially from that above described for free-living forms. In the PARASITA. 239 €hondracanthidae, on the contrary, we find the above-mentioned abbreviations •of metamorphosis. The young larvae which hatch from the egg of Chondra- ■canthus gibbosus already show behind the Nauplius limbs the rudiments of two other pairs of limbs, and must therefore be described as Metanauplii (Claus, No. 71). The youngest parasitic females remain essentially at the level of development of the first Cyclops stage. Of the four distinct thoracic segments seen at this stage, only the two anterior ones carry bilobate rudiments of limbs without setae, while the posterior region of the body (abdomen) is small and divided into two parts. No further Cyclops forms follow these first •stages, but while the small male remains during life at this grade of develop- ment, the female undergoes a secondary transformation, the region of the third and fourth thoracic segments increases in size and forms the main portion of the body. The large thoracic region now becomes transformed in an extraordinary manner, dorsal and ventral swellings and lateral projections appearing as secondary outgrowths on each of the thoracic segments (except the first). Somewhat similar phenomena are found in the family of the Philicthyidae. Here the larval form which hatches from the egg in the genus Lernaeascus is a Nauplius with a large provision of food-yolk, whose second antennae are without masticatory hooks, the adhering apparatus (frontal band) being absent, as in the Metanauplius of Chondr acanthus. The parasitic form arises from a Cyclops stage in which the thorax and abdomen are distinctly segmented, but only the two anterior thoracic segments show a well-developed rudiment of a swimming limb, while the third thoracic segment has only the vestigial remains of a limb. The male retains the shape of one of these developmental stages, while the female is parasitically transformed, the thoracic region lengthening, while a peculiar development of asymmetrical rows of chitinous scales makes its appearance (Claus, No. 69). In the family just described, the ascending series of larval forms does not appreciably rise above the level of the first Cyclops stage, but in the Dichelestiidae, where the body of the adult undergoes less modification, the later Cyclops stages are also passed through. The young which hatch from the egg are true Nauplii. The frontal band appears to be wanting (?) in the larvae of this group. An adhering apparatus of this kind is found in the larvae of the Caligidae, which, in their earlier stages, strikingly resemble the Cyclops stage (pupae) of Lcrnaca (Clatjs, No. 70, see below). The later larvae, which more nearly Fig. 113.— Two larval stages of Lernaea branchialis (after Claus). A, first Cyclops stage. B, so- called pupal stage. a\ first antenna ; a", second antenna ; //, first, ///, second pair of thoracic limbs ; k, adhesive mass ; oc, eye. 240 CRUSTACEA. resemble the adult in form, but are still distinguished by the possession of the frontal band, were described by Bijkmeister as distinct forms under the name of Chalimus. Later, however, F. Muller proved that these forms belong to the ontogeny of Caligus, as had already been conjectured by Kroyer. The Lemaeidae are very interesting with respect not only to the parasitism of the larval forms, but to the deformation that occurs in the body of the female after copulation. The metamorphosis ' of Lernaea branchialis has been made known by Metzger and Claus (No. 70). It is probable that the larva which hatches from the egg is a Nauplius resembling that of Achtheres, in which the segmentation of the body of the first Cyclops stage appears to be commencing beneath the cuticle. In these stages there is a free-swimming period, during which a search is made for the first host {Platessa flesus). The youngest forms found attached to the gills still show in all respects the segmentation of the first Cyclops stage (Fig. 113 A). They correspond to the first stage of development of Achtheres. The cephalo-thoracic region is followed by three free thoracic segments and a posterior unsegmented region carrying the furcal processes. Two pairs of well -developed swimming limbs (/-*", f11) can be recognised, one attached to the cephalo-thorax and the other to the first free thoracic segment, as well as a third pair of truncated limbs (on the second free segment). The mouth-parts are already of the true Siphonostomatous type. The upper and lower lips (labrum and paragnatha ?) have fused to form a sucking tube which contains the stylet- shaped mandibles, while the pointed palp-like maxillae are attached to its sides. The first antennae {a') are beset with setae, the second antennae (a"), as well as the anterior maxillipedes, are changed into hooks for attachment. The posterior maxillipedes have completely degenerated, a point in which this larva differs from that of Achtheres. The later Cyclops stages which succeed each other (Fig. 113 J?) show decided adaptation to parasitic life. A hardened mass of secretion (k), projecting from the head and comparable to the frontal band of the larva of Caligus, brings about the attachment of the larva to the gills of the host ; this permanent attachment precedes the degeneration of the locomotory organs. Almost all the limbs, and especially the swimming limbs (fl, fn), are now unjointed and truncated ; they have no setae and are immovable. These stages in which independent movement is lost have also received the name of pupal stages. In this pupal condition the segments of the body and the pairs of limbs which were still wanting are developed. A stage can be distinguished with three pairs of swimming limbs and four free thoracic segments ; at this period, in the male, the posterior maxillipede, until now suppressed, becomes distinct; then a further stage is developed with four pairs of swimming limbs ; this last stage leads through another moult to the free-swimming stage in which copulation takes place (Fig. 114 A and B). Apart from the slight segmentation of the abdomen, the body shows the full development attained by free-living Copepoda. The first antennae (a') are now jointed and beset with setae and sensory filaments, the four pairs of swimming limbs (f1,/11^), with their clothing of setae, are adapted for active swimming, while in the structure of the second antennae {a") and of the mouth-parts, the Siphonostomatous type is marked. The female (Fig. 114 B) is distinguished by a great lengthening of the genital segments, which gives the whole abdomen the appearance of a long, vermiform appendage. The female genital organs are not yet sufficiently developed for the production of eggs capable of fertilisation ; the receptaculum seminis, on the other hand, with the two pores (g) for the reception of the seminal masses from the PARAS ITA. 241 spermatophores, has attained full development. This free-swimming copulatory Cyclops stage is the last stage of the male, the fertilised female, however, seeks a new host (one of the Gadidae), in which she undergoes marked trans- formation of the body (Fig, 114 C and D). The genital segment, which has enlarged for the development of the eggs, is now a large doubly -curved portion of the body, the small abdomen, with its truncated furcal appendages, forming its termina- tion. The cephalo- thorax is changed by the addition of three horns which function as barbed hooks, carrying at their points fork-like out- growths. During these transforma- tions, the limbs are all retained, but are to a certain extent trans- formed by strong chitinisation. The remarkable form Spliacronella, Leuckartii, which is parasitic in the brood- cavity of ArupTrithoe, is connected with Lemaea by its metamorphosis. Salfnsky (No. 80) found in Sphaeronella an extremely degene- rate pupal stage fol- lowing the first free- swimming Cyclops stage. Neither seg- mentation nor limbs were recognisable in the sac-like body which was attached by a larval adhering apparatus to the epimeral plates of the host, led, through gradual transitionary stages, to the adult form. Fig. 114. — Sexually mature stage of Lemaea branchialis (after Claus). A, male. B, female at the copulatory stage. C and D, later condition of the female transformed by parasitism, slightly magnified, a', first, a", second antenna ; fi-fiv, first four pairs of thoracic limbs; g, opening of the receptaculum seininis ; mxf, maxillipede ; oc, eye ; sp, spermatophoral sac ; t, left testis. This stage The metamorphosis of the Lernaeopodidae is best known through the works 242 CRUSTACEA. of Kollak (No. 77), v. Nordmann (No. 79), Claus (No. 66), VEJDOWSK (No. 81), and others. Its course in the various forms seems to show gre agreement, so that AchtJieres, described by Claus, may be selected as a typ The young animal which hatches from the egg (Fig. 115 A) exactly resembles Nauplius, swimming about with difficulty by means of its two anterior pairs limbs (first and second antennae). Closer examination, however, reveals the fa that the body which is hidden within the Nauplius cuticle already shows th degree of organisation characteristic of the first Cijdops stage. Not only tl mouth-parts, but two pairs of thoracic swimming feet (p1, p'2), lie hidden with the Nauplius integument. The mandibles (md) and first maxillae {mx) lie small stumps at the sides of the upper lip, which enters into the formation Fio. 115.— Metamorphosis of Achtheres percarum (after Claus, from Balfours Text-book). A, so-called Nauplius stage. B, first Cyclops stage. C, older stage of the male larva. J>, sexually mature female. E, sexually mature male. at\ afi, first and second antennae ; md, mandible ; mx, maxilla ; pm1, pvifi, first and second maxillipedes ; p1, p2, first and second swimming limbs ; z, frontal organ ; i, intestinal canal ; o, Nauplius eye ; b, glandular body ; t, organ of touch ; ov, ovary ; /, spine derived from the fused maxillipedes ; g, cement gland ; rs, receptaculum seminis ; n, nervous system ; tc, testis ; v, vas deferens. the adult rostrum. The position of the two maxillipedes (pml, pm2) is of Interest, in so far as it proves distinctly that they develop as the exopodite and cndopodite of one and the same limb (second maxilla). Besides the above organs, we recognise the future adhering organ in the form of a spirally-coiled filament ending in a spherical swelling which grows out from a highly-refractive frontal process (2). Claus considers that this apparently homogeneous filament is a tube filled with a fluid secretion, and is the duct of a glandular mass which secretes a cementing medium. This first stage which seems to cover the whole series of Naicplius stages, undergoes ecdysis after a few hours, the larva which follows possessing the organisation of the first Cyclops stage (Fig. 115 B). It PARASITA. 243 has a long cephalo-thorax, followed by three free thoracic segments and an unsegmented abdomen. In the thoracic region, two pairs of well-developed swimming limbs (p1, p2) and a third rudimentary pair (p3) can be recognised. The first antennae (at1) are cylindrical three-jointed limbs beset with setae. The second antennae are still biramose (at2), but are already changed into the adhering organs of the larva, the longer branch ending in a hook bent like a claw, while the shorter branch is beset with papillae. The upper lip has united with a channelled lower lip (derived from the paragnatha ?) to form a conical sucking proboscis, from the outer sides of which project the short, conical mandibles, representing the transition from the masticatory jaws of the Cyclo- pidae to the piercing stylets of the Parasita ; here also the palp-like first maxillae are found. These are followed by the two pairs of maxillipedes transformed into adhering hooks (pm1, pm2), the outer one already assuming a more forward position, and the inner one lying more posteriorly. Among the internal organs, the intestine, the Nauplius eye shifted far back, and two bean-shaped bodies (b, glands ?) at the sides of the latter can be recognised. It is probable that the larva, after a short life of free activity in the water, becomes attached as early as this stage to the mucous membrane of the jaw of the perch (v. Nordmann). The peculiar adhering organ, however, appears to become free, and to be used for its special purpose only after a further moult. In this and two other attached stages, which probably follow through further moults, the mandibles shift to a position within the sucking proboscis, while a reduction in the setae on the swimming feet probably takes place. These stages, however, did not come under observation, but in a somewhat later stage which was observed (Fig. 115 C), there was already considerable approxi- mation in the structure of the body to the adult Achtheres. The body has now become almost vermiform, the first thoracic segment having separated from the head and having united with the four following segments to form a sac-like region, at the end of which the small, pointed, furcal appendage can be recognised. The antennae and the mouth-parts already essentially resemble the corresponding parts of the adult. The frontal adhering organ, with the exception of a vestige of its basal section (z), has vanished ; on the other hand, a new provisional adhering organ in the shape of a stiff filament (/) has arisen on the outer (anterior) maxillipede (pyn1) ; this filament starts from the tips of the fused outer maxillipedes. It is an interesting fact that at this stage sexual differentiation begins to be noticeable. Smaller individuals (young males) have remarkably strong outer maxillipedes (pm1), which are only united at the point of insertion of the adhering filament, and carry a strong terminal hook. When this filament is thrown off in the next moult, these structures give rise to the anterior maxilli- pedes of the male, which remain distinct and function as hooks for attachment (Fig. 115 E). The posterior maxillipedes (pm2) are also very large and each carries a small anchoring hook. In the female, on the contrary, the anterior maxillipedes (pm1) are rather long and retain the fused condition, ending in a sucking disc (Fig. 115 D) ; the posterior maxillipedes can in the same way be distinguished from those of the male by a large, hook-like, terminal joint With regard to the inner anatomical peculiarities of this larval form, we must first mention the degeneration of the Nauplius eye, which, indeed, is not universal among the parasitic Crustacea. The Nauplius eye is retained in the dwarf males of Chondracanthus and of Lcrnacopoda, as well as in many females (e.g., Chondracanthus comutus). In the posterior cephalic region of the larvae under consideration, at the sides of the intestine, there are two pairs 244 CRUSTACEA. of glands derived from the bean-shaped bodies above mentioned ; these glands, whose efferent ducts open out at the base of the maxillipedes, yield a still' secretion. Between these glandular bodies a dorsally-placed, pulsating organ can be noticed ; this is probably a short, sac-like heart. A similar organ has been observed in Tracheliastes by Vejdowsky, and in the larva of Lernaea by Hesse. The rudiments of the genital organs are already clearly recognisable. At the next moult the animal becomes sexually mature. The male does not increase in size, but, in the female, the posterior region of the body becomes very much enlarged. C. Branchiura. The Branchiura are. usually considered to be nearly related to the Copepoda. This supposed relationship is based principally on the similarity existing between the swimming limbs of Argulus and those of the free-living Copepoda, and on the structure of the mouth-parts-, which, according to the investigations of Claus, are very like those of the parasitic Copepoda (Siphonostomata). Hook- shaped mandibles and stylet -like maxillae can be distinguished. These appear to be enclosed in a proboscis formed by the fused upper and lower lips, together with lateral parts which must be regarded as derived from the mandibles. These are followed by twro pairs of maxillipedes which serve as adhering organs. The view that these latter are the twro rami of the second maxilla which have become independent receives special support from the position of the ducts of the shell-gland, discovered by Claus on the basal portion of the second maxillipede. Such a derivation of the maxilli- pedes would constitute a fresh link between these forms and the Copepoda. Argulus, indeed, shows in the structure of the genital organs, as well as in other points of its organisation, some remark- able peculiarities, and by the possession of paired, movable, lateral eyes and of branched, hepatic tubes approaches the Phyllopoda, so that we must probably consider the Branchiura as an offshoot from the common ancestor of the Copepoda. The eggs, which are rich in food-yolk, and in which the germ-band attains a ventral curvature, are attached by the female in rowrs to stones, etc. The young, when hatched (Claus), already greatly resemble the adult in shape (Fig. 116), having the same body- segmentation as well as the same number of limbs (apart from the maxillae). The shield-like anterior portion of the body (cephalo-thorax) consists of the cephalic segments fused with the most anterior limb- bearing segment which carries a pair of swimming limbs. Three five thoracic segments follow, each having a pair of swimming BRANCHIURA. 245 limbs, and these again are followed by an unsegmented abdomen with terminal fnrcal appendages (in the young form). In the adult, these latter have shifted dorsally. The young are further distinguished from the adult by the small extent of the cephalo-thoracic shield (Fig. 116), which does not yet cover the thoracic segments dorsally. The further metamorphosis principally concerns the transformation 116.— Newly-hatched larva of Argulus foliaccus (after Claus). a', first antenna; o", second antenna ; md, mandible ; mdt, mandibular palp ; mf, first, mf", second maxilli- pede ; p1, p*, first four swimming limbs. the different limbs. In the newly-hatched larva, the first antenna [a') is short, three-jointed, with a large hook-like appendage on its basal segment. The second antenna (a) is distinctly larger and biramose, an endopodite ending in a hook, and an exopodite provided with setae being distinguishable in it. In the mandible (md) are found a basal segment (masticatory blade), which becomes included 246 CRUSTACEA. in the proboscis, a middle portion forming the lateral wall of the proboscis, and a long, palp-like appendage provided with setae and projecting freely. This latter, the outer branch of the second antenna and the anterior pair of swimming appendages, are the most important locomotory organs of the larva. No trace could be found in the larva of the stylet-like maxillae which are contained within the adult proboscis. The two pairs of maxillipedes terminating in hooks (mf, mf) serve for adhering organs. Of the four pairs of swimming limbs (pl-pi)f only the anterior pair is free and movable, and somewhat resembles in shape a biramose Copepodan limb. The biramose rudiments of the three posterior swimming limbs are still unjointed and immovable. During the course of metamorphosis, after several moults, the basal hook-like processes of the two pairs of antennae grow stronger • the exopodite of the second antenna and the palp of the mandible disappear, while the endopodite of the second antenna, losing its terminal hook, is transformed into a simple palp. The large sucking disc develops on the first maxillipede. The swimming limbs become biramose and are provided with setae, thus resembling the limbs of the Copepoda. In the two anterior pairs of these limbs the rudi- ments of the inner branches, known as the flagella, appear, while sexual differentiation is evident in the two posterior pairs, a char- acteristic transformation of the protopodite taking place in the male. 7. General Consideration regarding the Segmentation of the Body and the Metamorphoses of the Malacostraca. In the Malacostraca the body consists of three primary regions, each of which, in all the divisions of this sub-class, contains a definite number of segments. In the anterior cephalic region, to which belong five pairs of limbs (the two pairs of antennae and the three pairs of jaws), there is, with few exceptions, no trace of a separation into distinct segments. In the thoracic region, which follows posteriorly and consists of eight limb-bearing segments, while the boundaries of the different segments are still more or less marked, only a slight power of movement between the individual segments is retained. But in the abdominal region, which consists of six limb-bearing segments and a terminal portion (telson), the full mobility of the separate segments is, as a rule, preserved, a fact connected with the development of the posterior end of the body into a swimming tail of great importance for the locomotion and the steering of the body. GENERAL CONSIDERATION. 247 The terga of the cephalic segments are thickened to form the carapace (dorsal shield); this, at its lateral and posterior margins, passes into an integumental fold which runs back over the thoracic region, and thus covers some or all of the thoracic segments dorsally. Only rarely do the terga of the thoracic segments covered by the dorsal shield retain any degree of independence (Stomatopoda, Fig. 141, p. 298, a few Schizopoda and Nebalia) ; in most cases the terga of the thoracic segments fuse closely with the integumental fold of the dorsal shield lying above them. A fusion of the cephalic and thoracic segments thus takes place, forming a common region of the body (ceplialo-tlxorax). In one series of Malacostracan forms, however, the marginal fold of the dorsal shield has undergone degeneration (Arthrostraca) ; here, as a rule, only the anterior thoracic segment fuses with the cephalic region forming the small cephalo-thorax of this group, which is followed by seven free and movable thoracic segments. The obliteration of the boundaries and consequent loss of move- ment between the segments of the cephalic and thoracic regions just mentioned has its elfect on the condition of the limbs. Only in rare cases (Nebalia, Euphausiidae) do all the eight pairs of thoracic limbs agree more or less in structure. As a rule, one or more pairs belonging to the anterior part of the thorax enter into close relation with the mouth and become modified for the purpose of mastication. These are then distinguished as maxillipedes, while the succeeding thoracic limbs which serve for locomotion receive, in many groups, the name of ambulatory limbs. In the Arthrostraca, only the anterior pair of thoracic limbs is changed into a pair of maxillipedes, but in the Decapoda there are three pairs of maxillipedes, and in the Stomatopoda as many as five anterior pairs of thoracic limbs are thus transformed. We must assume as the fundamental form of Malacostracan limb a biramose swimming limb with basal epipodial appendage, such as is retained as a thoracic limb in the Schizopoda. The shape of the thoracic limb in Nebalia (Fig. 91 B, p. 194) suggests to us that this form may perhaps have been developed from a more lamellate type of limb resembling that of the Phyllopoda. A two-jointed protopodite passes into a five-jointed endopodite, while the exopodite (flagellate branch), which often undergoes degeneration, frequently exhibits a large number of closely-crowded joints beset with setae. If we compare the metamorphosis of most Entomostraca (especially that of the Phyllopoda) with that of the Malacostraca (p. 193), 248 CRUSTACEA. we find, in the first group, a more gradual transition through many moults from the Nauplius to the adult form, while in the Mulacostraca the metamorphosis has attained a higher degree of specialisation, inasmuch as the separate stages appear more distinct from one another, and larval stages are intercalated which are not on the direct path of transition from the young to the adult form. These latter, by the development of secondary peculiarities, attain a certain independence and lead on to the adult condition only through further important changes in form. The metamorphosis of the lower Crustacea thus bears the same relationship to that of the Malacostraca as does incomplete to complete metamorphosis among the Insecta. As examples of these newly-introduced stages in the ontogenetic process, we must mention especially the Zoaea of the Decapoda and the Zoaea-like stage of the Schizopoda (Calyptopis) and the Stomatopoda, which are distinguished by the fact that although they possess the full number of body-segments, those of the middle region of the body are in a rudimentary condition. In this larva the posterior (five to seven) thoracic segments seem to be unusually backward in developing, and to have either no limbs or only very rudimentary ones, while the segments of the abdomen are already highly developed. The Zoaea is evidently a larval form secondarily changed by marked adaptation to a pelagic manner of life. From this standpoint it seems appropriate enough that the body retains a compressed form, that the most important locomotory organs (the maxillipedes and in some cases the antennae) are developed in the anterior region of the body, and that a posterior movable region (abdomen) develops early for swimming and steering purposes. The rudimentary condition of the middle region of the body thus ap- pears to some extent explicable. In regarding the Zoaea as a larval form secondarily intercalated in the course of development, which has attained a certain independent value and significance, special interest attaches to the development of the heart. Bearing in mind the condition of the heart in Ncbalia and the Schizopoda, we should expect to find, in the larval forms of the Decapoda, a long tubular heart. We should also presuppose that the three pairs of ostia occurring in the Decapodan heart would already be found in the heart of the Zoaea. This, however, is not the case. Tin- heart of the Zoaea is a short sac, recalling to some extent that of the Copepoda. It has only two pairs of lateral ostia (in individual cases, such as PmUMUti Kiiplnitislti, only one). The missing pairs of ostia only appear later. This proves clearly that the heart has undergone secondary modification corre- •ponding to the requirements of the Zoaea. The phylogenetic stages in the development of the heart have been modified to suit the altered conditions of organisation of the Zoaea it GENERAL CONSIDERATION. 249 In the complete series of developmental stages of the Decapoda, which however is only retained in full in a very few cases, the following stages can usually be distinguished passing one into the other by a series of moults. 1. The Nauplius stage (Fig. 122 A, p. 267). This stage shows great agreement with the Nauplius of the Entoanostraca in form and in the possession of the three typical pairs of limbs, the anterior limb (first antenna) being uniramose, but the two posterior limbs (second antenna and mandible) biramose. A free Nauplius stage is found in Penaeus and in Euphausia, among the Schizopoda. 2. The Metanauplius stage (Fig 117, p. 254, and Fig. 118, p. 258), in the form of the body, closely resembles the last stage, but shows, behind the Nauplius limbs which have shifted somewhat forward, the rudiments of four more pairs (in Euphausia only three). An integumental fold, which arises laterally and posteriorly, is the first rudiment of the dorsal shield. The posterior end of the body is marked by two short prominences bearing setae (furcal processes). The Metanauplius stage is the starting-point in the metamorphosis of Lucifer. 3. The Protozoaea stage (Fig. 119 A, B, p. 260, and Fig. 121 ^4, p. 264). The pairs of limbs which appeared as rudiments in the Metanauplius (first and second maxillae and second maxillipede) have now developed fully. The anterior region of the body is covered by the dorsal shield ; posteriorly, the body passes into a narrower region combining the rudiments of the thorax and the abdomen, and already showing segmentation anteriorly, while the posterior (abdominal) region is not yet fully segmented. The antennae still possess the Nauplius character and function as oars, as also do the biramose maxillipedes. The mandibles have greatly altered ; the basal joint is retained as a masticatory blade, while the distal part (palp) has disappeared. The Protozoaea stage occurs in the metamorphoses of the Penaeidae and Sergestidae. It is distinguished by the presence of distinct furcal processes. In individual cases (Sergestes) the third pair of maxillipedes may also be developed at this stage. 4. The Zoaea stage (Fig. 119 E, p. 260; Fig. 123 G, p. 269; Fig. 124, p. 272, and Fig, 136, p. 291). This stage agrees in all important characters with the preceding stage, from which it is distinguished by the distinct segmentation of the posterior or abdominal region. The sixth abdominal segment, it is true, often remains united with the telson for some time. The limbs of the Zoaea stage are the same as those of the preceding stage. In the more primitive Decapoda, 250 CRUSTACEA. the antennae also serve as oars, while, in the Zoaea of the Brachyura, these limbs are kept in the background, and locomotion is carried on exclusively by the two pairs of biramose maxillipedes in con- junction with the movable abdomen. In many cases, the third pair of maxillipedes has already begun to function. The limbs which follow these may be present as sac-like, unjointed rudiments in close contact with the body, but they never function in the Zoaea. The pleopoda are still altogether wanting, except the sixth pair (uropoda), which may, in individual cases, develop as early as this stage. The spine-like process, which rises from the cephalo-thorax and occurs typically in the Brachyuran Zoaea, was formerly considered as a characteristic of this stage, and too great stress was laid upon this point. A really important characteristic of this stage, however, is the fact that the six posterior thoracic segments (commencing with that carrying the third pair of maxillipedes) are generally rudimentary and often difficult to recognise, while the abdominal segments are very apparent on account of their large size and distinct boundaries. The Zoaea stage indicates, in many Decapoda, the beginning of metamorphosis, many leaving the egg in this condition. The Protozoaea and Zoa,ea, in contrast to the Metanauplius, are dis- tinguished by the gradual development of the paired stalked eye which, as in Branchipus, first arises as a lateral outgrowth of the cephalic region (rf. the Zoaea of Lucifer, Fig. 119 E, p. 260), the eye-stalk developing very gradually. 5. The Mysis stage (Fig. 120 A, p. 262, and Fig. 123 D, p. 269) and Metazoaea stage (Fig. 133 B, p. 287). By the development of the posterior thoracic limbs, the Zoaea passes into the Mysis- or Schizojwda-Uke stage. These limbs, which now begin to function, resemble the maxillipedes in being biramose swimming appendages provided with setae; they assist the maxillipedes in propelling the body, and recall the limbs of the Schizopoda. At this stage the pleopoda develop. In the Brachyura and Anomura, the process of development seems to be simplified so far as the rudiments of the ambulatory limbs are concerned; these limbs, which are present in the late Zoaea stage, never resemble the Schizopodan limbs, but from the first are uniramose and pass direct into the adult form. An exopodite never develops on the rudiments of these limbs. Consequently, the Zuam stage in these animals is followed by a stage in which tie- general form of the body resembles that of the Zoaea, but, in addition, the rudiments of the five pairs of ambulatory limbs are GENERAL CONSIDERATION. 251 more or less developed, though still closely pressed against the body. This stage, which replaces the Mysis stage in the Anomura and Brachyura, has been called by Claus (No. 7) the Metazoaea. 6. Last stages of metamorphosis. These stages are now only distinguished from the adult form to which they lead by unim- portant characteristics. In the Sergestidae, the loss of the exopodite on the thoracic limbs and the enlargement of the abdomen leads from the Mysis stage to the Mastigopus stage (Fig. 120 C, p. 262). In the Penaeidae and Caridea the corresponding stage is called the first shrimp stage. The last stages of metamorphosis in the Anomura and Brachyura are known as the Megalopa (Fig. 133 ^4 and B, p. 287), the transition from this stage to the adult form involving considerably greater changes in the Brachyura than in the Anomura, because the latter remain throughout life at a stage of development nearer that of the Megalopa. If we review the series of larval stages here described, we see that the order of development of the segments and limbs from before backward is as a rule retained. Only in details are there certain characteristic deviations from this order. For instance, the develop- ment of the thoracic segments in the Zoaea stage is usually retarded as compared with the development of the abdominal segments, and, among the limbs, an exception occurs in the early appearance of the sixth pair of pleopoda. These exceptions to the rule can be shown to result from the adaptation of the larva to a pelagic existence. The whole series of ontogenetic stages described above is only passed through in very few cases in the Decapoda. Penaeus and Lucifer may serve as examples of such complete metamorphosis. As a rule, metamorphosis is more or less abbreviated, the early stages being obscured or hurried through within the egg. The Sergestidae, for instance, hatch at the Protozoaea stage, most of the Caridea at the Zoaea stage, the marine Astacidea at the Mysis stage. The most extreme case of abbreviated metamorphosis is found in many forms living in fresh water and on land (Astacus, Telphusa, and Gecarcinus). The abbreviation of metamorphosis is also attained in another way, viz., by the tendency towards the obliteration of the charac- teristics of the special ontogenetic stages. Thus we shall see that, in the Caridea, the Zoaea stage appears altered by anticipating certain of the peculiarities of the Mysis stage. The complete dis- appearance of the Mysis stage in the metamorphosis of the Brachyura and Anomura can be explained in a similar way. 252 CRUSTACEA. The series of ontogenetic stages described above refers to the Dccapoda, but the Schizopoda (Euphausiidae) and the Stomatopoda are in this respect closely related to this order. The Calyptopis stage of the Euphausiidae might be assumed to be a Protozoaea or a Zoaea stage if it did not differ from the latter in the absence of the second pair of maxillipedes. In the metamorphosis of the Stomatopoda, on the other hand, we see that a suppression of the thoracic segments and the limbs belonging to them, similar to that in the Decapodan Zoaea, leads to larval forms which have been called the Pseudozoaeae of the Stomatopoda. A second series of forms among the Malacostraca, however, including the Cumacea and the Arthrostraca, shows, in connection with the care of the brood which there prevails, an entire dis- appearance of metamorphosis. In these groups we have conditions related to those found in the Mysidae and Leptostraca. We do, indeed, find in the belated appearance of the last pairs of thoracic limbs in the Isopoda a last remains of that ontogenetic tendency which, in the Decapoda, led to the development of the Zoaea stage. 8. Leptostraca. The Leptostraca (Nebalia), like the Mysidae, have no free- swimming larval stage. When the young animals leave the shell- cavity of the mother, which is used as a brood-chamber, they have in all essential points (Metschnikoff, No. 82) attained the final shape. Metamorphosis is therefore here, as in the Mysidae, confined to those stages which were passed through in the brood-cavity after the shedding of the egg-integument. With regard to the appearance of the different limbs, the order from before backward is retained. In this respect, and in the absence of a distinctly marked Zoaea stage, the Leptostraca nearly approach the Phyllopoda. The three pain of Nauplius limbs appear first. There then follows a stage in which these three pairs of limbs have advanced in development, while behind them can be recognised four other pairs (two pairs of maxillae and the two anterior pairs of thoracic limbs). This stage thus shows, in the number of limb-rudiments, a certain agree- ment with the Zoaea form. At a later stage, the rudiment of a third pair of thoracic limbs can be made out. The embryo lies in the egg; curved in such a way that the ventral surface of the caudal ii is in contact with the ventral surface of the anterior part "1 I lie body. The egg-integument then splits, and the larva, which ms freed, hut is still enveloped in the larval cuticle, and which SCHIZOPODA. 253 shows the rudiments of all the thoracic limbs, not merely straightens out, but becomes even somewhat dorsally curved. We thus have, in Nehalia, a repetition of the changes with regard to the relative position of the body regions which we saw (p. 154) commencing in Mysis, but with the difference that, in the latter, the rupture of the egg-integuraent and the straightening of the body take place as early as the Nauplius stage, whereas here they occur at a later stage. The pleopoda now gradually appear in regular order from before backward, the body approaches the adult form, and the young leave the brood-cavity (Metschnikoff, No. 82). 9. Schizopoda. A very primitive form of metamorphosis through many moults is retained among the Schizopoda by the Euphausiidae. The Mysidae, on the other hand, emerge from the Nauplius cuticle 'in a condition closely resembling the adult in form, and at this stage they leave the brood-pouch of the mother and swim about freely (p. 1 53). The different larval stages of the Euphausiidae, none of which can be exactly identified with the Protozoaea and Zoaea of the Decapoda, were described by Dana as separate genera under the names of Calypiopis, Furcilia, and Cyrtopia, Claus (No. 97) being the first to prove that these forms belonged to the ontogeny of the Euphau- siidae. The youngest stages were made known by Metschnikoff (Nos. 93 and 94), who established the important fact that the larva of Euphausia leaves the egg as a true Nauplius. The most important points in the later ontogenetic stages were made known to us chiefly by Claus (Nos. 91 and 8) for Euphausia, and more recently the process of development in various forms has been traced in greater detail by G. 0. Sars (No. 95), Brook and Hoyle (No. 90). The Nauplius of Euphausia, on leaving the egg, has an oval unsegmented body still without a shell-fold, and carrying on its anterior half the three pairs of typical Nauplius limbs. The anterior pair of these is uniramose, the two others biramose ; their distal ends are beset with setae. A segmentation of the limbs is not yet distinctly evident. Only a very small oral aperture can be seen. Later stages (Fig. 117 A) are distinguished by the development of three more limb-rudiments (two pairs of maxillae (4 and 5) and the first maxillipedes), and must therefore be regarded as Metanaup/iu.< stages. The three anterior pairs of limbs (1, 2, 3) have still the Nauplius character. We can recognise the rudiments of the Nauplius eye, the upper lip (o), the paired paragnatha (u), and a shield-like 25 1 CRUSTACEA. fold covering the lateral parts of the posterior limb-rudiments. The posterior end of the body, below the anal aperture which is now becoming more distinct, is lengthened into two rounded f ureal processes fringed with setae. Later ' Metanauplius stages show a change in the third limb, which has entirely lost the form of ! Pld. 117. -Three stages of development of Eitphausia (from Lang's Text-bool). A, Meta- pltM (after Mktschnikoff). B, Calyptopis (after Claus). C, older Calyptopis (utter Claus). 1 and "i, first and second antennae; 3, mandible ; It, 5, first and second maxillae ; /, first pair of niaxillipedes ; ab, abdomen; (a^a^) first five abdominal segments; o6, sixth pair of pleopoda ; an, anus ; fs, frontal sensory organ ; o, upper lip ; u, paragnatha ; th, thoracic Mgraent*. a iwimming appendage and has developed into a masticatory blade (mandible) with a greatly reduced palp. The shield-like fold has now developed in the anterior region of the body, and thus :<>unds the cephalic portion of the larva (rf. the later stages, Fig. 117 B and C). In the posterior limbs, the first indication of SCHIZOPODA. 255 the formation of lobes is evident, while, near the Nauplius eye, a paired frontal organ (fs) has developed, similar to that already described in the Phyllopod larva. The Metanaupllus gives rise through further moults to the series of Galyptopis stages (Fig. 117 J? and C), which are characterised by the development of the six most anterior pairs of limbs, and of the long posterior (thoraco-abdominal) region of the body. The two pairs of antennae still retain essentially the Nauplius character, although they are now jointed. The first antenna now exhibits a three-jointed shaft, at the end of which two short processes (the rudiments of the future flagella) are inserted. In the second antenna, at the end of the exopodite 'which is covered with setae, a distinct segmentation into closely-crowded rings is evident. The two maxillae (4, o) and the first maxillipede (/) appear as richly-lobed appendages, showing in their form considerable agreement with the Phyllopodan limbs. The first maxilla (4), besides its two masticatory blades and its endopodite, shows a short truncated portion bearing setae (exopodite), the rudiment of the future fan-like plate. In the second maxilla (5) the exopodite is in quite a rudimentary condition, while, on the inner side of the protopodite, four masticatory processes have developed. The first maxillipede (1) has the character of a biramose swimming limb (especially in Nyctiphanes). At the beginning of the thoraco- abdomen, which lies behind these appendages, the closely-crowded rudiments of the other thoracic segments can already be distinctly seen (Fig. 117 B, th), while the abdomen (ab) still appears un- segmented. The posterior end of the abdomen has already been transformed into the middle plate of the caudal fin, and is beset with strong spines along its posterior edge. In front of the anal aperture the first rudiments of the lateral parts of the caudal fin (sixth pair of pleopoda, a6) can be recognised. The cephalo-thoracic shield, which covers the anterior part of the body, has undergone great development. In Euphausia it is distinguished by the presence of an unpaired dorsal spine directed backwards, by the delicate dentation of its edges, and by a notch in its ventral margin which recalls the excavation in the edges of the shell in Cypridlna and Halocypris. In other genera (Nyctiphanes) the shell has no dorsal spine and the margins are not toothed; there are also only very indistinct lateral notches in the margin of the shell. The internal organs, which should be noticed at this stage, are the gradually developing paired rudiments of the eye, the hepatic outgrowths of the alimentary canal, and the short sac-like heart which is continuous 256 CRUSTACEA. with a well-developed arterial system, and is provided with one pair of venous ostia. Later Calyptopis stages (Fig. 117 C) are distinguished from those just described by the more distinct development of the rudiments of the eyes which are still hidden under the dorsal carapace, and by the full segmentation of the body. Not only is the thorax (th) divided into seven (short) segments, but the abdomen also (a-^-a^) appears fully segmented In the last of these stages, the sixth pair of pleopoda (a6) are developed as freely projecting lateral wings of the caudal fin. If we compare the series of Calyptopis stages with the other stages of Malacostracan larvae, we must class the younger Calyptopis stage with the Protozoaea, the older with the Zoaea. The Calyptopis stages, however, differ from these in the absence of the second pair of maxillipedes which have not yet developed. The later stages, known as the Furcilia stages, are characterised above all by the complete development of the stalked eye which is now movable, and which from this time onwards is not covered by the forward extension of the dorsal shield, but projects freely through an indentation at its edge. A corresponding change is found in the part of the cephalic shield lying between the eyes, which is gradually transformed into a frontal plate running out into a point to form a rostrum. While the six anterior pairs of limbs still retain for the time the shape seen in the Calyptopis stage, the missing posterior limbs are developed, the first Furcilia stage showing the rudiments of the second pair of maxillipedes and of the first pair of abdominal limbs. The other abdominal limbs develop very soon, while the third maxillipede and the ambulatory thoracic limbs, as well as the branchial rudiments belonging to them, appear more gradually in order from before backward. At the same time, in Euphausia, the rudiments of the eye-like luminous organs at the bases of the limbs develop. The most characteristic feature of the Cyrtopia stage is the change in the shape of the antennae, which from this time are no longer used as oars, but approach the adult form. The two flagellate branches of the first antenna have lengthened considerably and have become Biented into many rings. In the second antenna, the transformation of the endopodite into the flagellum, and of the exopodite into a scale, are noticeable. Through the completion of the number of limb-rudiments and the development of the last thoracic limbs, the Cyrtopia larva passes gradually into the adult form. DECAPODA. 257 The Mysidae, ike Nebalia, only undergo metamorphosis within the brood- cavity of the mother, the young which leave that cavity already showing the form of the adult. We have mentioned above (p. 154) that the egg-integument in My sis is shed at the Nauplius stage. The larva, which is then enveloped in the Nauplius cuticle alone, is essentially embryonic in character. It is maggot- shaped and can move but little, the limbs are soft and have no fringe of setae. The larval cuticle, beneath which the remaining limbs develop, is distinguished, in Mysis vulgaris and M. jiexuosa, by the fact that, at its posterior end, it runs out into two setose furcal processes. The next ten pairs of limbs (two pairs of maxillae and eight pairs of thoracic limbs) appear simultaneously. The first of the pleopoda to appear is the sixth pair which enters into the formation of the caudal fin (Fig. 77 E, p. 153) ; the five anterior pairs grow out only after the shedding of the Nauplius cuticle (P. J. and E. Van Benedest, Nusbaum). The course of development in the Lophogastridae seems to be in complete agreement with that in the Mysidae. Sars, at least, has figured an ontogenetic stage of Lop/wgaster, which completely resembles a late larval stage of Mysis, the only distinction being that, in the former, all the pairs of pleopoda are already present ; these limbs perhaps appearing somewhat earlier in Loplw- gaster than in Mysis. The development of the Mysidae and the Lophogastridae may in a general way be described as essentially abbreviated in comparison with that of the Euphau- siidae. In this respect, as well as in the innjer ontogenetic processes, the Mysidae and Lophogastridae approach the Cumacea and the Arthrostraca. 10. Decapoda. A. Sergestidae.* Among the Decapoda, the Sergestidae and the Penaeidae are distinguished by the primitive conditions recalled by their meta- morphoses, which begin with a very simple stage [Nauplius or Metanauplius), and also by the regular order of appearance (from before backward) retained by the body-segments. In the family of the Sergestidae, the metamorphosis of the genus Lucifer has been specially accurately observed. The Protozoaea of this ontogenetic series was called by Dana Ericthina demissa ; Claus (No. 8) afterwards found the Zoaea stage belonging to it, but Willemoes-Suhm (No. 157) first established its connection with the development of Lucifer, while Brooks (No. 109) observed the complete course of metamorphosis, from the hatching of the egg to the attainment of the adult form. His observations have been found to agree with those made on the Challenger material by Spence Bate (No. 100) and Willemoes-Suhm. The actual Nauplius stage is passed through in the egg; the young Lucifer larva hatches at a stage which we must describe as a Meta- nauplius (Fig. 118 ^4). On the short oval body we can recognise the * [The Sergestidae and Penaeidae form the tribe Penaeidea comparable to the Caridea, etc. — Ed.] 258 CRUSTACEA. Nauplius eye, the projecting upper lip (ol), and a few furcal setae J indicating its posterior end. There is as yet no trace of a shield-likt fold. In the anterior region of the body, the three pairs of Nauplius limbs (a', a", md) are inserted. Of these, the first (a!) is uniramose : it consists of five joints and carries swimming setae at its end. The second antenna (a") has a two-jointed protopodite and two swimming rami ; when this appendage is compared with that of other forms, we| are inclined, in opposition to Brooks, to regard the ramus, which lias; several joints and is provided with setae, as the exopodite and the simpler, unjointed branch as the endopodite. The third pair of i limbs (mandibles) resembles the second in structure, but is smaller and less jointed. It has an unsegmented protopodite, a single-jointed Fig. 118.— Two Metanauplius stages of Lucifer (after Brooks). A, larva just hatched. B, somewhat older stage, a', first antenna ; a", second antenna ; rf, shell-fold ; md, mandible ; mx, first maxilla ; mx", second maxilla ; m/', first maxillipede : ol, upper lip ; p, paragnatha. endopodite, and an exopodite consisting of three joints, both rami carry a few swimming setae. Following these posteriorly are four pairs of swellings, which represent the rudiments of the two pairs of maxillae (mx', mx") and the first and second pairs of maxillipedes. Later Metanauplius stages (Fig. 118 B), in which the Nauplius limbs show a slight degeneration, have at the sides of the body the rudiment of a shield-like fold (d), and besides this, on the basal joint of the mandible, a stiff masticatory process. The Protozoaea stages which now follow show the seven anterior pairs of limbs characteristic of the Zoaea stage completely developed and functional. The anterior part of the body is covered by a dorsal shield which is continued anteriorly above the Nauplius eye into a SERGESTIDAE. 259 long rostrum (r) ; this carries on its posterior margin one unpaired dorsal and two somewhat longer lateral spines. The posterior region of the body has developed into a large thoraco-abdomen, the most posterior end of which is transformed into the middle plate (telson) of the caudal fin, and is provided with strong spines. Behind the second maxillipede, four more segments have become distinct — that carrying the third pair of maxillipedes (Fig. 119 A, mp") and those carrying the three anterior pairs of ambulatory limbs (tl-tz). The heart (h) and the hepatic outgrowths of the intestinal canal (/) have developed. The first antenna (a') now consists of two joints only, a long basal and a short terminal joint beset with setae ; the second antenna (a") has essentially the same shape as in the preceding stage ; it has an exopodite of several joints beset with many setae and a simpler endopodite ; it still functions as the chief locomotory organ of the larva. The mandible (Fig. 119 D, md) now consists exclusively of a masticatory blade toothed along its inner edge. All trace of the palp found on the mandible at earlier stages is now lost. The two pairs of maxillae already foreshadow their adult form. The first maxilla (Fig. 119 5) shows, on the inner side of the protopodite, two projecting masticatory blades, a short, two-jointed endopodite, and a truncated exopodite beset with feathered setae. The second maxilla (Fig. 119 C) is principally distinguished by the large number of masticatory processes borne on its inner border. The two pairs of maxillipedes (Fig. 119 D, mf, mf") are shaped like biramose swimming limbs with long endopodites composed of several joints and short, unjointed exopodites. In the maxillary region a shell- gland is seen developing. Later Protozoaea stages (Fig. 1 1 9 D), which correspond to Dana's Erichthina demissa, are distinguished by the commencing develop- ment of the paired eyes (o), as well as by the further advance of segmentation in the thoraco-abdomen. At the antero-lateral margin of the dorsal shield a swelling becomes apparent, within which an accumulation of pigment denotes the rudiment of the paired eye. Among the new segments that have appeared in the posterior region of the body are those carrying the fourth ambulatory limbs (t4) and the four anterior abdominal segments (1-4). The last thoracic segment (that carrying the fifth pair of ambulatory limbs) never attains distinct development in Lucifer. The abdomen is not yet segmented. The next stage (Fig. 119 E) is marked by the complete segmenta- tion of the abdomen, and must therefore be called a Zoaea stage. 260 CRUSTACEA. The completely developed limbs are the same as those of the Protozoaea stage, but behind these we find that the third maxilli- pede {mf") and the four anterior ambulatory limbs (tl^-th*) havej appeared as small biramose rudiments. Besides these, rudiments of I E Fio. 119.— Protozoaeac and Zoaea of Lucifer (after Brooks). A, first Protozoaea stage. B, first maxilla of the same. C, second maxilla of the same. D, later Protozoaea stage ( Erichthina). /:, Zoaea stage, a', first antenna; a", second antenna; abc, sixth pair of pleopoda (lateral tidal I'm); en, endopodite; ex, exopodite ; h, heart; /, hepatic outgrowths ; , mandible; mf, first maxillipede ; mf", second maxillipede ; m/'", third maxillipede; vip, protopodite of the third pair of maxillipedes ; mx', first maxilla ; mx", second maxilla ; o, paired C< e ; oc, Nauplius eye ; ol, upper lip ; r, rostrum ; tM-th*, rudiments of tm- first four pain of ambulatory limbs; fi-t*, the four segments carrying ambulatory limbs; 1-C, tb^ lis abdominal segments. SERGESTIDAE. 261 the lateral portions of the caudal fin (sixth pair of pleopoda) have now appeared. The next moult of the larva produces a very decided change in the external shape of the body. The larva now passes into the so-called Mysis or ScMzojwd-like stage (Fig. 120 .4), which was described by Dana under the name of Sceletina armata. The antennae have lost their locomotory functions and tend to assume their adult form ; the paired eyes (o) have become movable and stalked, and the Nauplius eye (oc) is still retained. The seven pairs of biramose swimming limbs (viz., the three pairs of maxillipedes and the four anterior pairs of ambulatory limbs) .function as loco- motory organs. The larva, from this time onward, ceases to progress in a jerking manner, as in the preceding stage, and swims evenly and swiftly. The dorsal shield has still the same general shape, but, in its anterior part, the stalked eyes have become cut off from it. The indentation of the shield which contains these latter is marked by a pair of antero-lateral spines, while the spines at the posterior margin of the shield have disappeared. The cephalo-thoracic shield remains short in comparison with the abdomen which, in the next stages, increases still more in length. In the abdomen, we can recognise six perfectly distinct segments and the caudal fin, consist- ing of the telson and the greatly enlarged sixth pair of pleopoda (ab6). The first antenna (a') now consists of a two- (later three-) jointed protopodite and a short terminal joint which is to be considered as the representative of the nagellum and is richly provided with feathered setae. The second antenna (a") is a reduced biramose limb almost without setae. In the later Mysis stages this appendage makes a fresh growth, its endopodite being transformed into the flagellum and its exopodite into the scale -like appendage. The mandibles (md) are simple masticatory blades without palps. The maxillae (m$c\ mx") have essentially the same form as in the preceding stages. This is also the case with the first pair of maxillipedes (mf), the segmentation of which is somewhat less distinct than in the pre- ceding stages. The six pairs of swimming limbs which follow (second and third maxillipedes and first four pairs of ambulatory limbs) are biramose and very similar in form. Each limb consists of a two-jointed protopodite, a long, four-jointed endopodite, and a shorter exopodite Avhich i?, however, indistinctly divided into numerous joints. An ample provision of setae enables these limbs to act as powerful swimming organs. Later Mysis stages (Fig. 120 B\ which are principally distin- 262 CRUSTACEA. I i . Il'O.— Three later larval stages of Lucifer (after Brooks). A, younger Mysis- or Schizopod- like stage. /;, older Mysis- or Sckizopod-like stage less highly magnified. C, Maatigopua stage, a', first antenna j a", second antenna; ab,-n&fi, the six pleopoda; c, cephalo-thoracic shield; dr, antenna] gland; en, flagellnm, and ex, scale of the second antenna; md, mandible ; vif, first maxillipede ; m/", second maxillipede ; mx', first maxilla ; mx", second maxilla; o, compound eye; oc, Nauplius eye; ol, upper lip; r, rostrum; 1-6, six abdominal MgaMBjts, SERGESTIDAE. 263 guished by the segmentation of the second antenna and the great development of the abdomen, show, on the five anterior abdominal segments, the bud-like rudiments of the pleopoda. The transition from the Mysis stages to the adult form takes place through the Mastigopus stage (Fig. 120 C), which in length of body already approaches the adult Lucifer, but is distinguished from the latter by the absence of the neck-like prolongation of the cephalo-thorax. This stage is marked by the shortness of the flagellum of the first antenna (a'), while the flagellate appendage of the second antenna {en) has considerably lengthened. The mouth- parts and the thoracic limbs have attained their final condition. The mandible has no palp, the first maxilla has lost the exopodite, the latter portion of the second maxilla is transformed into a large, fan-like plate. The first maxillipede is changed into a short, two- jointed appendage ; the second maxillipede {mf") has, like all the other thoracic limbs, lost its exopodite and has lengthened out and become geniculate. The third maxillipede and the three anterior pairs of ambulatory limbs form a row of short, simple appendages covered with setae. The fourth pair of ambulatory limbs has entirely disappeared. The first pleopod (ab-^ is uniramose, while the four following pleopoda {2-5) have the usual biramose form. The Nauplius eye and the shell-gland have now disappeared ; on the other hand, the antennal gland {dr), which opens on the base of the second antenna, can be recognised as a coiled canal. The adult form, as contrasted with the Mastigopus stage, is char- acterised by the lengthening of the flagellum of the first antenna, in the basal joint of which the auditory organ develops, as well as by the neck-like prolongation of the head. The flagellum on the second antenna has also undergone considerable elongation. The sexual differentiation now develops, the male being distinguished by accessory structures on the first and second pairs of pleopoda, by spines on the ventral side of the fifth abdominal segment, as well as by certain differences in the caudal fin, while the female has in these respects retained the characters of the larval form. The development of Lucifer just described is, in all essential points, repeated in the metamorphosis of Sergestes, although the two larvae are somewhat unlike in outward appearance. The Zoaea of Sergestes, which is remarkable for the dorso- ventral flattening of its body and its extraordinarily large, branched, spinous processes, was described by Dana under the name of Elaphocaris, the later stages already being known as Acanthosoma (Claus, No. 91) and Mastigopus (Leuckaut, Claus, No. 91). Claus and Willemoes-Suhm simultaneously proved that these forms belonged to the ontogeny of Sergestes, and discovered 264 CRUSTACEA. the Protozoaea stage of this genus. More recently, many larval forms belonging to this series have been described from the Challenger material (Spence Bate No. 100). The youngest Protozoaea of Sergcstes is only known from a drawing b] Willemoes-Suhm (No. 100, p. 354). It possesses the seven anterior pairs o limbs, but the third pair of maxillipedes has not yet developed. Only the firs PlO. 121.— Two larval stages of Sergestes (after Claus). A, Protozoaea stage. B, Acanthotom stage, seen from the dorsal side, a', first antenna ; a", second antenna ; mf, tirstmaxillipede mj", second maxillipede; ?»/'", third niaxillipede ; mx\ first maxilla; mx"y second maxilla ol, upper lip ; sd, shell-gland. rudiments of the paired eyes can be made out. Within the alimentary canal a large amount of yolk remains, und this no doubt allows the larva to leave the egg at this stage. In all other respects the larva drawn by Willemoes-Siiim agrees with the youngest stage described by Claus (Fig. 121 A). This latter (Fig. 121 A) can be compared in all its essentials with the later SERGESTIDAE. 265 Protozoaea stage of Lucifer, from which, however, it is distinguished by the compact form of its body, the early development of the stalked eyes, and the appearance of the third pair of maxillipedes (mf"). The seven anterior pairs of limbs agree very closely in structure with those of the Protozoaea stage of both Lucifer and Penaeus. The first antenna (a') especially shows a segmentation of the base into live joints which recurs in Penaeus. The mandibles have no palps, the first maxilla (mx') has its exopodite developed as a fan-like plate, such an exopodite again being found on the long leg-like second maxilla (mx"). At the base of the latter, Claus found the coiled shell- gland (sd). The three pairs of maxillipedes {mf'-mf") are biramose limbs, the third pair, however, being somewhat small. Behind these are found the five narrow segments which later carry the ambulatory limbs, while the abdomen still appears altogether unsegmented. The latter terminates in a plate which runs out into prongs, and is here much more marked than in Lucifer. This terminal part seems to find its homologue in the anal segment of the Phyllopoda with its two furcal processes. A marked feature of this stage is the great develop- ment of the movable stalked eyes and the development of protective processes on the broadened dorsal shield. There are here one anterior rostral spine starting from a broad base, one dorsal and two lateral spines, each of which appears branched through the formation of secondary processes. These spines no doubt correspond to the much shorter ones of the Lucifer larva, with which they agree in position. The Zoaea which emerges from the stage just described (Elaphocaris) is still more abundantly armed with spines. The frontal, dorsal, and lateral spines have, greatly increased in size. The frontal spine has become comparatively slender, and a pair of branched antero-lateral spines have developed on its base. The separate segments of the abdomen also, which are now distinct (except that the sixth segment is still indistinguishable from the forked telson), are also well armed with lateral spines. While the anterior limbs, including the three pairs of maxillipedes, have the same character as in the former stage, only the rudiments of the biramose limbs are to be recognised on the five following thoracic segments. The sixth pair of pleopoda has also appeared. In the transition to the Mysis stage (Acanthosoma, Fig. 121 B) the complicated armature of the dorsal shield is lost, only the basal portions of the branched processes found in the Elaphocaris being retained as short s] tines, but the spines on the abdomen are still retained. The furcal processes of the telson are also reduced to two short spines projecting backwards. Significant changes are found in the development of the limbs. The first antenna in younger Acantho- foma has a long, still unjointed protopodite (provided with a basal toothed process) and two short terminal processes which appear to be the rudiments of the principal and accessory flagella of later stages. In the second antenna (a") the formerly many-jointed exopodite is transformed into a rod-like appendage, while the endopodite has developed into a long flagellum. The two pahs of maxillae still show the character described for the earlier stage. The anterior niaxillipede is remarkable for the reduction of its exopodite ; but the seven following pairs of limbs (the two remaining maxillipedes and the limbs which have developed from the five rudiments on the thoracic segments) are changed into conspicuous biramose limbs in which the exopodite, richly clothed with setae, is specially apparent. The limbs of the abdomen are short rudiments, except those of the sixth segment which, on the contrary, are well developed as the long lateral divisions of the caudal fin, and are provided with setae. Later 266 CRUSTACEA. Acanthosoma stages are noteworthy on account of the development of the auditory vesicle at the base of the first antenna and the further growth of the rudiments of the pleopoda. The Mastigopus form which develops from the Acanthosoma is very like that of Lucifer. Here also we find great developmont of the abdomen as opposed to relatively small size of the cephalo-thorax. The armature of spines, with the exception of the persistent rostrum, is reduced to small vestiges. The exopodites are lost on the thoracic limbs (maxillipedes and ambulatory limbs), and the last pair of these limbs have altogether disappeared. The pleopoda are now greatly developed, but still have no exopodites ; these, however, are evident as buds on the two posterior pairs, but only attain functional development at a later stage. The changes by which the Mastigopus passes into the adult form chiefly affect the limbs (above all the mouth-parts), which now approach the adult form ; the mandibular palp, for instance, grows out, the two last pairs of thoracic limbs reappear, and the gills develop. The developing mandibular palp remains two-jointed in Sergestes; the anterior maxilla in the later Mastigopus stages still shows a short rudiment of the palp. In the posterior maxilla, on the contrary, the exopodite is transformed into a large, fan-like plate. In the anterior maxillipede, the exopodite and endopodite are short appendages, while a large, plate-like, masticatory blade has developed on the protopodite. The second maxillipede is short and geniculate, while the third has retained the form of a long limb. Even in early stages, the rudiments of the pincers are recog- nisable on the second and third ambulatory limbs. The loss of the Nauplius eye and the shell-gland, and the development, on the other hand, of the antennal gland are noticeable features in the internal anatomy. In the absence of gills and of the two posterior pairs of thoracic limbs, Lucifer shows features which are present in Sergestes at the Mastigopus stage. Lucifer has thus retained certain larval characteristics. The many larval stages of Sergestes found among the Challenger material agree in the most essential points with the stages above described, but show great variability in the armature of the cephalo-thorax and abdomen. The Sergestid Zoaea called Platysaccus crenatus is interesting ; its dorsal shield, which is rounded and provided with marginal spinous lobes, leaves the four posterior thoracic segments (already provided with limbs) completely uncovered, a feature in which this form agrees with the Penaeus and Lucifer larvae. Some larvae of the Mi/sis and Mastigopus stage called Sciocaris telsonis are remarkable for the shape of the telson, which has segmented furcal processes. The contrasts in outward appearance presented by the short, broad, and spiniferous larva of Sergestes and the slender larval form of Lucifer are brought about by a series of ontogenetic stages, which Brooks (No. 109) has traced back to a meeting point in the genus Acctcs. These larvae are more compact than the Lucifer larva, and are also somewhat better supplied with spines. Brooks is inclined to refer to this series a Protozoaea larva described by Dohrn (No. 121, "Larva of an unknown Crustacean," Plates 29 and 30, Figs. 62-67) and Ci .vis (No. 8, " Phyllopoda-like Protozoaea of unknown ancestry," Plate 4, Figs. 2-7). The latter larva is principally characterised by the great development of the still unstalked lateral eyes, which, as in Lucifer, cause a buckle-shaped projection of t lie dorsal carapace. With respect to the condition of the stalked eye, which is already wry highly developed in the Protozoaea of Sergestes, this larva holds a position intermediate between the latter and Lucifer. PENAEIDAE. 267 B. Penaeidae. Our knowledge of the metamorphosis of Penaeus is due mainly to the researches of F. Muller (Nos. 141 and 142). It is of great significance in forming an estimate of the Decapodan metamorphosis, on account of the presence of the Nauplius stage, as well as the primitive order retained in the development of the seg- ments and limbs. Claus (No. 8) has since added further de- tails, especially with regard to the Protozoaea stage. More recent investigations have been made by Brooks (No. 110), who was able to observe the origin of all the stages follow- ing the first Protozoaea in specimens cultivated by him. A series of larval forms chiefly belonging to later stages are described by Spence Bate, who utilised the observations of WlLLEMOES-SUHM (No. 100). Penaeus, like the Euphau- aidae, leaves the egg as a true Nauplius (Fig. 122 A). The long, pear-shaped body is still without a shell-fold, and ter- minates posteriorly in two long, furcal setae. At the anterior end of the body, the Nauplius eye is visible. The three typical pairs of Nauplius limbs are still unjointed, and are provided with swimming setae. The succeeding Metanauplius stage, in outward appearance, still greatly resembles the Nau- plius, but is distinguished from the latter by the appearance of a transverse dorsal integumental fold (rudiment of the dorsal shield) Fig. 122.— Two stages of development of Penaeus (after F. Muller, from Lang's Text-book). A, Nauplivs. B, Protozoaea. 1, first, 2, second antenna ; 5, mandible ; 5, second maxilla ; /, first, II, second maxillipede ; (III-VIII), rudiments of the third to eighth thoracic segment; ab, abdomen. 268 CRUSTACEA. and two truncated processes provided with setae at the posterior end of the body (furcal processes). This stage greatly resembles the Metanauplius of Lucifer (Fig. 118 B, p. 258). Like the latter, it possesses a massive, helmet-shaped upper lip. The three pairs of Nauplius limbs appear shifted somewhat forward, and four pairs of bud-like limb-rudiments have arisen. The third Nauplius limb shows an important alteration, a thickening of its basal portion being evident as the rudiment of the masticatory blade of the future mandible, while, from the two swimming rami, the living contents have been withdrawn, an indication that these parts will be cast off in the next moult. Near the eye, the paired frontal organ (Fig. 122 B), which also occurs in the Zoaean series, can be recognised as a small, conical projection. The next stage observed is the Protozoana (Fig. 122 B), with well- developed, more or less rounded, cephalo-thoracic shield, seven pairs of limbs and an unsegmented abdomen (ab) terminating in distinct furcal processes. The antennae are still locomotory organs. The first antenna (1) is divided up into four joints. On the posterior antenna (2) a three-jointed endopodite and a four-jointed exopodite can be recognised. The upper lip is helmet-shaped and marked by a spine which points forward, this is also found in the larvae of the Sergestidae. The mandibles are now palpless, toothed, masticatory blades. The maxillae agree to a great extent in shape with those figured for Lucifer (Fig. 119 B and C, p. 260). In the first maxilla, two inwardly projecting cutting blades can be recognised on the protopodite, there is a jointed endopodite and a leaf-like exopodite provided with setae. The second maxilla resembles the first in shape, but has four blades on the protopodite and a somewhat longer endopodite. The two anterior pairs of maxillipedes (Z, II) are developed as biramose swimming limbs. At the base of the succeeding (thoraco-abdominal) region of the body, transverse rings mark a division into six segments (those bearing the third maxillipede and the five ambulatory limbs, III-VIII). The position of the anal aperture is worthy of mention ; it is, at this stage, exactly terminal, lying between the two furcal processes, and only later shifts to the ventral side of the telson. The presence of three pairs of hepatic outgrowths from the intestine is already evident. The heart, which is situated at the anterior extremity of the thuraco-abdomen, has only one pair of ostia (1). At the anterior margin of the cephalo-thorax, the rudiments of the paired eyes are visible as swellings in close contact with the frontal organ. PENAEIDAE. 269 Later Protozoaea forms (Fig. 123 A) which Claus (No. 8) observed are chiefly characterised by the development of five abdominal segments (ax-ab) still hidden under the larval cuticle, they also show a greater development of the paired eyes and the division of the protopodite of the first antenna (1) into five joints (a feature which recurs in the Protozoaea of Sergestes). The sixth Fig. 123.— Later larval stages of Penaeus (after Claus, from Lang's Text-hook). A, older Protozoaea, dorsal aspect. B, thoracoabdominal region of a somewhat older larva with rudiments of limbs, ventral aspect. C, Zoaea, ventral aspect. D, Mysis stage, lateral aspect. 1, 2, first and second antennae; 3, mandible; U, 5, first and second maxillae; J, II, III, first three maxillipedes ; IV-VIII, first five ambulatory limbs; (IV-VIII), five posterior thoracic segments; (a^a,.), the six abdominal segments ; at-a6, the six pleopoda ; ab, abdomen ; en, endopodite ; ex, exopodite ; fr, frontal sensory organ ; /,, liver ; t, telson. abdominal segment is still fused with the telson. Besides the rudiments of limbs already mentioned, the truncated rudiment of the third maxillipede (III) can now be seen. The Zoaea which proceeds from the above (Fig. 123 B and C) is marked by the development of the movable stalked eyes, and 270 CRUSTACEA. of a frontal spine found between these organs, as well as by that of the thoracic limbs. The abdominal segments now increase greatly in size, so that they are soon longer than those of the thorax. The third pair of maxillipedes can now be recognised as a short biramose limb (Fig. 123 B, III). Behind this, the rudiments of the ambulatory limbs (IV- VIII) are just visible, and still slighter prominences indicate the limb-rudiments on the five anterior abdominal segments. It is thus evident that, in Penaeus, the limbs have retained the order of development from before backwards. An exception to this order, however, is afforded by the sixth pair of pleopoda, which can at this stage already be recognised as bilobed rudiments (a6), thus preceding the other abdominal appendages in development. This pair of pleopoda develops as the lateral portions of the caudal fin (Fig. 123 C, a), while the small limb-rudiments on the five anterior segments of the abdomen appear to be temporarily suppressed during the later Zoaea stage (Fig. 123 (7, aj-a5). In such a later Zoaea stage, the five joints in the protopodite of the first antenna seem also to have disappeared ; the rudiments of the five pairs of ambulatory limbs can now be recognised as small biramose appendages (IV-VIII). In the Mysis stage which follows (Fig. 123 D), the antennae have ceased to function as locomotory organs, the locomotory function being now performed by the biramose thoracic limbs. The cephalo- thorax is now small as compared with the abdomen, which develops greatly, and in which the pleopoda have now reappeared. Dorsal spines appear on the abdominal segments from the second to the fifth. The anterior antenna (1) now exhibits a three-jointed protopodite and two still unjointed flagella; the latter become jointed at a future period ; olfactory hairs develop, and an auditory organ forms at the base of this limb. In the second antenna (#), the exopodite has changed into a scale (squame) fringed with setae, while the endopodite becomes transformed into the adult flagellum. The upper lip has lost its spinous process. On the mandible, the two- jointed palp of the adult develops. In the anterior maxilla, the endopodite degenerates into a short truncated limb, while the exopodite is completely lost. The exopodite of the second maxilla changes into the large respiratory plate. The first maxillipede resembles that of Sergestes, the protopodite growing out into a large cutting plate, while the endopodite and exopodite are retained as small appendages, and a branchial pouch sprouts from the basal joint (Fig 91 a, i>. 194). The second and third maxillipedes (//, III), CARIDEA. 271 as well as the five following thoracic limbs (IV-VIII) are still large biramose limbs with well-developed exopodites. In the transition from the My sis stage to the shrimp form, the exopodites of the thoracic limbs are reduced, while the flagella of the antennae become jointed, and the pleopoda which appear in the Mysis stage attain their full development. The second and third maxillipedes retain the character of biramose limbs. The second maxillipede has a geniculate endopodite, a smaller exopodite (flagellate appendage), and gills ; the third maxillipede has its endopodite developed as a long ambulatory limb, while, in it also, the exopodite persists as a flagellate appendage. The three following limbs (first, second, and third ambulatory limbs) are provided with rudimentary pincers as early as the Mysis stage. The various ontogenetic stages of Penaeus, which have become known through the examination of the Challenger material, can easily he brought into agreement with the process of development just described. They, however, show many variations which may be regarded as generic and specific differences ; these chiefly affect the armature of spines (especially in the abdomen) and the relative development of the limbs (the thoracic limbs and the pleopoda). The remarkable Peteinura gubernata, which SPENCB Bate (No. 100), on account of certain points of agreement with Aristeus, regarded as possibly belonging to the Penaeidae, must no doubt also be considered as a larval form. This form agrees closely with that described by Dohrn (No. 121) as Ceratapsis longiremis, in the armature of the cephalo-thorax, the shape of the limbs, and the excessive development of the exopodite of the sixth pleopod. Ceratapsis is regarded by Boas and Clatjs as a larva of some form which must be related to the Penaeidae on account of the structure of its gills, made known by the latter author. The metamorphosis of Stenopus, which is allied to Penaeus and shows some relation to that of the Sergestidae, has been described by Brooks and Herrick (No. 111). The larva which hatches from the egg is a Protozoaea with sessile eyes, antennae serving as swimming limbs, a deeply-cleft telson, and a long ros- trum, and with all the pairs of limbs as far back as the first thoracic appendage (inclusive) already developed. The posterior region of the body is only indis- tinctly segmented, and has no appendages. From this form a true Zoaea develops, showing the characters of a Caridid Zoaea (p. 272). A later stage is characterised by the enormous lengthening of the fifth pair of ambulatory limbs, which function as the principal locomotory organ of the larva and equal the whole body in length. These limbs are reduced in the following Mastigopus stage to small vestiges, and the preceding pair (fourth ambulatory limbs) also degenerate for the time only. In this respect, Stenopus recalls the Sergestidae (p. 266). C. Caridea. The metamorphosis of the Caridea (Palaemonidae, Alpheidae, Crangonidae, etc.) is, as compared with that of Penaeus, essentially abbreviated. The Nauplius and Protozoaea are never free stages 272 CRUSTACEA. in the Caridea, but are merged in the general embryonic develop- ment which takes place in the egg. The embryo leaves the egg, as a rule, as a peculiarly-shaped Zoaea (Fig. 124), which in certain characters anticipates the Mysis stage, and thus in many respects is more advanced than the typical Zoaea. We may take as an example a larva described by Claus (No. 113), and referred to the ontogeny of Hippolyte. In this larva we can distinguish an anterior cephalo-thoracic and a posterior abdominal region. The latter is long and already shows its full number of segments (in forms related to this Zoaea, however, the separation of the sixth segment from the telson is often at first indistinct). The telson is, as a rule, no longer cleft (furcal structure), but has the form of a broad plate with a spinous posterior margin ; in individual cases, however {e.g., Pontophilu*. G. 0. Sars, No. 151), a furcate condition with two lateral wings is found. The dorsal shield carries a simple pointed rostrum and short supra-orbital and antennal spines, but there is no further development of spinous processes. In many larvae of this group, however, there is a very prominent dorsal spine on the second abdominal segment, as well as smaller ones on the three following segments. In other re- spects, the spinous armature of this region of the body undergoes great variation in the forms belonging to this group. Near the Nauplius eye, the paired eyes are distinct and already stalked. In the anterior region of the body, besides the seven pairs of limbs occurring in the Zoaea, the eighth pair (third maxillipedes, Kg, 124, Mf") appear in the form of well developed swimming limbs. The region following this, which comprises the five segments carrying ambulatory limbs, is almost unrepresented, its segments and limbs only becoming apparent at a later period. The pleopoda are Fio. 124.— Zoaea of Hippolyte (after Claus, from Balfour's Text-book). Mx', first, Mx'i, second maxilla ; Mf, Mf", Mf"1, first, second, and third maxillipedes. CARIDEA. 273 still altogether wanting. In the second antenna, the transformation of the exopodite into the scale and of the endopodite into the flagellum of the adult can be recognised, the mandibular palp is wanting as well as the leaf-like appendage (exopodite) of the first maxilla, which is found in Penaeus. The heart has two pairs of ostia, a normal condition in the Zoaea, the third pair developing during the My sis stage. While an apparent irregularity is produced in the order of development of the segments by the suppression of the fifth thoracic segment, the rudiments of the limbs appear in the regular order Fin. 125.— Older larva of Hippolyte, after the development of the thoracic appendages (after Claus, from Balfour's Text-book). from before backwards. The only exception is afforded by the sixth pair of pleopoda, which frequently develop . precociously. The ambulatory limbs develop gradually from before backwards, in contrast with those of the Sergestidae and of Penaeus, which appear simultaneously. The development of the thoracic limbs frequently shows variations in detail which are not all as yet accurately known. In Hippolyte, the first two pairs of ambulatory limbs and the sixth pair of abdominal limbs (uropoda) appear simultaneously. In an older stage (Fig. 125) these become biramose, while the buds of the three posterior pairs of ambulatory limbs also appear. In Palaemon, the larva which hatches from the egg has the rudiments of the three following thoracic limbs developed behind the third pair of maxillipedes (Bobretzky). The larva of Crangon, when hatched, shows the bud-like rudiments of the two anterior pairs of ambulatory limbs (Claus, No. 113 ; Ehrenbaum, No. 123*), but the rudiments of the three following * G. 0. Sahs (No. 151), on the contrary, states that in Cravgon vulgaris and in Chcraphilus and Pontophilus only the buds of the first pair of ambulatory limbs are present, while the four remaining pairs appear simultaneously in the next stage. 274 CRUSTACEA. pairs develop very soon after. The earliest free larva of Palaemonetes vulgaris shows the rudiments of the two anterior pairs of thoracic limbs, while the posterior pairs arise independently in succeeding stages ("W. Faxon). Only after the thoracic limbs have developed do the pleopoda appear as buds. By the development of the thoracic limbs into biramose swimming appendages, the larva passes into the Mysit stage, which varies in its appendages in the different forms, inasmuch as the exopodite may be suppressed in individual thoracic limbs. In Hippolyte, Caridina, and Palaemonetes vulgaris, for instance, the last pair has no exopodite (ihis is perhaps the case in most Caridid larvae), while, in Cherapliilus and Pontophilus (G. 0. Sars, No. 151), and in the fresh-water form Palaemonetes varians, the rudiment of the exopodite seems to be suppressed on the last three thoracic limbs, in Grangon vulgaris (Ehrenbaum and G. 0. Saks) and in Sabinea (Sars) on the last four. In the latter forms there is thus an evident tendency to the elimination of the Mysis stage from the metamorphosis. The tendency to simplification of the ontogenetic processes, which finds expression in the above conditions, has led, in individual cases, to a mucli more marked abbreviation of metamorphosis. The embryo of the arctic form Hippolyte polaris, as observed by Kroyer (No. 136), showed the five pairs of ambulatory limbs as simple (uniramose), jointed appendages, the rudiments of pincers being already manifest on the anterior pair. The live anterior pairs of abdominal limbs could also be recognised as biramose rudiments, while the sixth pair was still wanting. The metamorphosis of Sabinea (G. 0. Saks, No. 151) seems similarly abbreviated, the youngest larva {Myto Gaimardii of Khoyer) hatched from the egg already showing the rudiments of all the five ambulatory limbs and of the five anterior (and already biramose) pairs of pleopoda. Only the first ambulatory limbs have exopodites, the second being uniramose and very small. In Sclerocrangon boreas, the eggs of which are distinguished by their large size, there appears to be entire suppression of metamorphosis (Spence Bate, No. 100, G. 0. Sars, No. 151), and the same is the case with the deep sea forms Cryptocheles and Bythocaris (G. 0. SARS, No. 151). Many of the fresh -water Caridea (e.g., Caridina Desmarestii) have, in inntrast to their marine relations, no abbreviation of metamorphosis, but such abbreviation is distinctly evident in Palaemon Potiuna (F. Mui.lkk, No. 143) and Palaemonetes varians (P. Mayer, No. 138). Palaemon Potiuna h-uves the egg at a stage of development very similar to that of HippolyU polaris, being only distinguished from the latter by the backward condition of the mouth-parts (for, owing to the larger amount of food-yolk present, the larva does not commence to feed until some time after hatching), and by the presence of gill-rudiments. The variety of Palaemonetes varians, which occurs in fresh water in the South of Europe, hatches, according to P. Mayer, at an advanced Zoaea stag! representing the transition to the Mysis stage, and possesses all the limbs Bpt the last pair of pleopoda. The two anterior pairs of ambulatory limbs ASTACIDEA. 275 already show the rudiments of pincers, and are provided with exopodites, which are wanting on the three following ambulatory limbs. The gills (pleurobranchiae) are well developed on all the segments bearing the ambulatory limbs. The abdominal limbs are found as biramose buds. The telson, in shape and armature, resembles that of the Caridid Zoaea, and is not as yet sharply marked off from the sixth abdominal segment. In contrast to the above, it is an interesting fact that the variety of Palaemonetes varians, which occurs on the shores of Northern Europe (but also in brackish or fresh water), shows far less abbreviation of metamorphosis. According to Boas (No. 105), the Zoaeae which hatch from the eggs of this form have only truncated, unjointed ambulatory limbs, while the abdominal limbs are altogether wanting. In all forms with abbreviated development, the eggs are considerably larger and more richly provided with food-yolk, and the number produced by the female is consequently smaller. The young, when hatched, still retain a considerable amount of food-yolk, which is only gradually absorbed, and consequently they do not take food from outside until very late. In keeping with this, the mouth -parts of the otherwise highly-developed larva are in a very rudimentary condition. According to Herrick (No. 133) and Packard (No. 144) a similar abbrevia- tion of metamorphosis is found in two species of Alplieus. In other species of this genus the metamorphosis does not deviate from that of other Caridea, but in Alpheus hcterochelis it is abbreviated, and in Alplieus praecox, which lives in Sponges, it is almost altogether lost. In connection with the Caridea we should mention the genus Amphion, a form whose systematic position is still very doubtful. It agrees in the form of its Zoaea larva and in the possession of branchial rudiments which show the character of phyllobranchiae (Spence Bate, No. 100) with the Caridea. The oldest known stages of Amphion appear (in the deficient segmentation of the antennae) not to have attained full development, and must probably be >nsiclered as larvae, although Dohrn (No. 120) and Willemoes-Suhm, on account of the statement that genital rudiments appear in them, were inclined to regard them as adult forms. The long body moves by means of six pairs of biramose limbs, which correspond to the second and third maxillipedes and the four following thoracic limbs, while the last pair of thoracic limbs is present in an undeveloped condition. In the shape of its biramose limbs, as well as in the presence of branched hepatic tubes in the anterior part of the cephalo-thorax, AmjtMon shows striking agreement with the Phyllosoma, and Boas (No. 103) actually conjectured that it might be the larva of the genus Polycheles, which is related to the Loricata. The youngest known stages of Amphion are Zoaeae resembling in appearance the Caridid Zoaeae. The second and third pairs of maxillipedes, developed as biramose swimming limbs, are the principal loco- motory organs, while the first pair of maxillipedes already appear drawn towards the mouth. The abdomen ends in an oval telson ; the pleopoda are wanting, but the sixth pair very soon develops. Other forms nearly related to Amphion have been described by Willemoes-Suhm, their similarity in shape of body to Scrgcstes being pointed out. These forms are known as Amphiones. D. Astacidea. In the tribe of the Astacidea there is still greater abbreviation of metamorphosis than in most Caridea. There is no longer any free Zoaea stage. In Homarus, whose metamorphosis has become known 276 CRUSTACEA. Fjg. 12G.— Larva 01 the American lobster, just hatched (after Smith). «', first, a", second antenna; in/"', third maxillipede ; pf-p^, first five ambulatory limbs. by the works of Kroyer (No. 136), G. 0. Sars (No. 148), S. J. Smith (No. 153), and Kyder (No. 147), the stage at which the young leaves the egg (Fig. 126) is the Mysis stage, which moves about by means of the exopodites of the third pair of maxillipedes and of the five pairs of ambulatory limbs (mf'"-py) developed as strong swimming organs. The dorsal shield passes anteriorly into a simple rostrum. The ab- domen is distinguished by the presence on its segments of dorsal and lateral spines, but is still devoid of limb- rudiments. The telson is a triangular toothed plate, in- dented posteriorly. The first antenna (a) is still un jointed, the second (a") is biramose, with an exopodite changed into a scale and a slender endopodite (rudiment of the flagellum). The mandible carries a three-jointed palp, there is no exopodite on the first maxilla. The second maxilla and the maxillipedes resemble in essentials those of the adult, but are still in many respects rudimentary. The endopodites of the three anterior pairs of ambulatory limbs already terminate in rudimentary pincers. In the next stage, the limb-buds appear on the second, third, fourth, and fifth abdominal segments. On the first antenna, the segmentation of the principal flagellum and the rudiment of the accessory flagellum can already be recognised. The three anterior pairs of ambulatory limbs have increased in size, and the pineer- rudiments are more clearly developed. In the third larval stage, the Schizopodan features gradually dis- appear, while the adult characters begin to develop. The exopodite of the third maxillipede, as well as that of the fifth ambulatory limb, degenerate, while the limbs of the sixth abdominal segment attain development. The abdominal spines begin to degenerate. In the next stage, the Schizopodan characters have been entirely lust. The exopodites of the ambulatory limbs are mere vesti The larva, in all essentials, resembles the adult, but still retains its pelagic manner of life, and swims by means of its abdominal limbs. ASTACIDEA. 277 Only in a later young form, which has sunk to the bottom, does the asymmetry of the pincers become apparent ; the first abdominal limbs which, to begin with, are similar in form in the two sexes, appear at this same time. At a still later stage sexual differentiation is attained. The metamorphosis of Nephrops norvegicus, a single stage of which was made known long ago by Claus (No. 113), and which has more recently been completely described by G. 0. Saks (No. 149), agrees in all its details with that of Homarus. Here, as in the latter, the four stages described above can be distinguished, being similarly characterised in both forms. The larva of Nephrops, however (Fig. 127), has a peculiar appearance, being distinguished by the spines on its abdomen. On the third abdominal segment there is a email dorsal spine, on the fourth and fifth a much larger one, while the sixth abdominal segment has a pair of long backwardly directed spines. The telson which, in the Fig. 127.— Later Mysis stage of Nephrops norvegicus (after Sars). a:, first antenna ; a", second antenna; mf", third maxillipede ; p^-p5, first five ambulatory limbs; p!2, pleopod of the second abdominal segment ; pi5, pleopod of the fifth abdominal segment ; r, rostrum. first and second stages, Avas not distinctly marked off from the sixth abdominal segment, runs out into two very large movable spinous processes which are retained even in the third stage, together with the lateral plates of the caudal fin (sixth pair of pleopoda), and only disappear in the fourth stage, in which the telson is transformed into the middle plate of that fin. The young of Astaeus, when hatched, are distinguished only in unessential points from the adult. The cephalo-thorax is swollen by the mass of nutritive yolk beneath it, and the rostrum is curved downwards between the eyes. The first pair of abdominal limbs is still undeveloped. The pleopoda of the sixth segment have also not attained free development. The telson has a peculiar oval shape. In other respects the young of Astaeus, which still remain for some time hanging to the abdominal limbs of the mother and are protected by her, entirely resemble the adult. Metamorphosis here, as in so many fresh-wafer forms, has entirely disappeared. The young of Cambarus (W. Faxon, No. 127) closely resemble in shape the above forms, and, like those of Astacus, recall in certain respects the Paraslaci(/<>e. The first and sixth pairs of pleopoda are still wanting, and the transverse segmentation in the region of the telson-plate has not yet developed. The 278 CRUSTACEA. latter, unlike that of Astacus, has no fringe of setae. Development seems still more abbreviated than in Astacus, inasmuch as the caudal fin very soon attains completion. E. Loricata. The larvae of the Loricata hatch in a form long known as the Phyllosoma, which was formerly regarded as an independent genus and placed either among the Stomatopoda or among the Decapoda. That this form belonged to the ontogeny of the Loricata was first rendered probable by the experiments in cultivating specimens made by Couch (No. 116), who was able to hatch, from the eggs of Palinurus, larvae which Gerstacker had already assumed to be Phyllosoma. Coste and Gerbe arrived at the same results almost simultaneously. The embryonic development of Scyllarus and Palinurus, as well as the transition into the young Phyllosoma, was made known by Dohrn (No. 119), while the metamorphosis of the Phyllosoma was established chiefly by Claus (Nos. 91 and 8) and Kichters (No. 146). Various Phyllosoma have recently been described by S pence Bate (No. 100). The Phyllosoma must be regarded as a peculiarly-shaped My sis stage. The Loricata thus leave the egg at the stage at which many of the Astacidea hatch. The leaf-like, flatly compressed shape of the body and the slight development of the abdomen must be regarded as adaptations to a pelagic existence, while, on the other hand, apparently primitive characters are retained in the independence of the thorax and the presence of furcal processes on the telson. Other features, however, can only be explained as degeneration- phenoniena, the correctness of this view being confirmed by com- parison with the embryonic stages. The embryos of Scyllarus, whose ontogeny was worked out by Dohrn, pass through a Naupllus stage which bears a general re- semblance to that described above (p. 156) for Astacus, and is marked by the secretion of a larval integument (Nauplius cuticle). The later stage also resembles the corresponding stage of Astacus, in that the posterior region of the body which has now developed (thoraco-abdomen) appears flexed ventrally. The anterior region of the body, in which the food-yolk is deposited, bears the two pairs ntennae and the mouth-parts, including the first maxillipedes. second and the third maxillipedes are found on the recurved thoraco-abdominal section, together with the rudiments of the three iior ambulatory limbs. Behind the last pair of these rudi- ments there follows an unsegmented terminal region in the form LORICATA. 279 of a square plate. Most of the limbs are still of a somewhat embryonic stamp, but it should be mentioned that the three anterior pairs of limbs of the thoraco-abdomen (second and third maxilli- pedes and first ambulatory limbs) consist of uniramose rudiments, while the two following pairs (second and third ambulatory limbs) are still simple bud-like outgrowths. At a somewhat later stage, the second antennae, which are very little shorter than the first, show the rudiments of the antennal glands at their The mandible, in which the palp is wanting, is slightly bilobate at the tip; the first maxilla is bilobate, and the second has incisions on its inner edge which break it up into three lobes. The first maxillipede also, which was simply truncated at an earlier stage, is now a short bilobed rudiment. On the other hand, the exopodites of the second and third maxillipedes have become reduced, while the second and third ambula- tory limbs appear distinctly biramose. Further back are found the small trun- cated rudiments of the fourth and fifth ambulatory limbs. A similar condition of the limbs is found in the Palinuras embryos at later es (Fig. 128). The first antenna («') is simple and unjointed, while the somewhat longer second antenna (a") shows the rudiment of a lateral branch. The first maxilla (mx') is trilobate, the second bilobate. In the two anterior maxillipedes (mf, mf") as well as in the third ambulatory limb (//"), the exopodite is vestigial ; the third maxillipede {mf"), on the contrary, and the first and second ambulatory limbs (f, f) are provided with large exopodites. The posterior region of the body contains the segments carrying the fourth and fifth pairs of ambulatory limbs (t4, t5), as well as the abdominal segments (1-6), and ends in a cleft telson. Fig. 128.— An embryo of Palinurus qundricornis with thoraco-abdomen turned back (after Claus). a, anus; a', firat antenna; a", second antenna; md, mandible ; mf, mf", mf", first, second, ami third maxillipedes ; mx', mx", first and second maxillae ; oc, Nauplius eye ; ol, upper lip ; p, parag- natha ; V, t", t"', first three ambulatory limbs ; 0, t5, segments carrying the fourth and fifth ambu- latory limbs ; x-x, boundary of the cephalo-thorax ; 1-6, first six abdominal segments. 280 CRUSTACEA. The last stages of embryonic life already distinctly show the seg- mentation of the body characteristic of the Phyllosoma larva. The body falls into three sections, the anterior broadened section being still swollen by the food - yolk deposited in it, and carrying antennae, man- dibles, and maxillae. This is followed by a moderately broad region, consisting of the segments which carry the maxillipedes and ambulatory limbs, and be- hind this comes a short narrow abdomen. Be- fore the Phyllo- soma leaves its embryonic en- velope, certain processes of de- generation (Fig. 1*29) take place in the body. The second an- tennae («") are r educ e d t o simple, mi- branched pro- cesses, and the second maxillae (mx") to short ■■•■jointed stumps. The first maxillipede in Scyllams is altogether lost, while in Palinurus it is retained in the form of a much reduced vestige. The exopodites of the second and third maxillipedes Fio. 129.— Young Phyllosoma of Scyllarus (after Claus). Only the basal joints of some of the limbs are drawn, a', first antenna ; a", second antenna; d, antennal gland; a, brain; /, liver; md, mandible; mx', lirst maxilla; mx", second maxilla; mf", second maxillipede; mf", third maxillipede; », ventral chain of ganglia; w, Nauplius eye; ol, upper lip; p', /', [>'", lirst three ambulatory limbs. LORICATA. 281 (/>?/", mf") were similarly reduced at a still earlier stage. The segments carrying the fourth and fifth ambulatory limbs and those of the abdomen become less distinct. The youngest hatched Phyllosoma (Figs. 129 and 130) very closely resemble the oldest embryonic stages. The clear, transparent body is flattened out like a leaf, and divided into the three sections just described. The anterior (cephalic) section, which is usually ellipsoidal or oval in outline, carries at its anterior end the two pairs of antennae and the stalked eyes, while the Nauplius eye (oc) lies just above the brain. The mouth lies almost in the middle of the ventral surface, surrounded by the mandibles and first maxillae ; the reduced second maxillae (mx") have shifted further back. There is, in the Phyllo- soma of Scyllarus (Fig. 129), no trace of the first maxillipedes, whereas, in Palinurus, these are present as vestigial prominences. The thoracic region (Figs. 129 and 130) somewhat resembles a broadened disc, and carries the maxillipedes and ambulatory limbs. The two anterior pairs of these limbs (the second and third maxilli- pedes) in the youngest Phyllosoma of Scyllarus are uniramose and five-jointed, while the three anterior ambulatory limbs consist of six joints and carry exopodites. The rudiments of the fourth and fifth ambulatory limbs, together with the segments to which they belonged, have almost entirely disappeared. In the youngest Phyllo- soma of Palinurus (Fig. 130), on the contrary, the third maxillipede has an exopodite, and the fourth and fifth pairs of ambulatory limbs are found as minute buds. The thoracic region of the body is originally simply joined on posteriorly to the anterior cephalic section. Later, however, the dorsal surface of the cephalic region grows back over the thorax, thus forming the dorsal shield, while the thorax itself gives rise principally to the " sternal plastron." The abdomen is a short, indistinctly- segmented, rudimentary appendage ending in two furcal processes. Between these, lying at the posterior end of the body, is the anal aperture, which only secondarily takes up a ventral position through the fusing of the basal parts of the furcal processes. This position of the anus recalls that described in Penaeus (p. 267) and Astacus (p. 157), and shows that the Phyllosoma, in the development of the posterior end of the body, has retained a peculiarity of an earlier larval stage (Protozoaea of Penaeus). The metamorphosis of the Phyllosoma consists in the appearance (or rather reappearance) of the missing body-segments and pairs of CRUSTACEA. limbs, and in a transformation of those already present. The first antenna becomes jointed and develops the rudiment of the accessory flageUum (Fig. 131), as well as the olfactory filaments of the prin- cipal flagellum and the auditory organ in the basal joint. The second antenna also (Fig. 131) becomes jointed, and, in the Phyllo- goma of Scyllarus, already shows indications of the lamellate shape typical of the adult. In the mandible, a palp, at first simple and later three-jointed, develops. The maxillae approach the adult form. The first maxillipede and the fourth and fifth ambulatory limbs now develop, while the penultimate ambulatory limb, as well as the second maxilli- pede, develop still further. From the basal joints of the maxillipedes and ambulatory limbs, the branchial rudiments now grow out. The abdomen becomes distinctly seg- mented, and six pairs of rudiments of pleopoda appear on it, the last pair participating in the formation of the caudal fin. Internally, the formation of diverticula loads to the development of the hepatic tubes (/). This, in the PhyUowma of ScyUanu (Fig. L29), is commenced by the development of three pairs of caeca, the middle pair Boon showing secondary ramifications. The PhyUosoma of Valinurun (Fig. 129), on the other hand, shows, from the very beginning, richer ramification of the so-called hepatic tubes. The organ! of the circulatory system are already, as GbOBNBAUR has d, of the typical Decapodan character. There is a heart pro- vided with three pairs of ostia, from which the arterial vessels sd in a form characteristic of the Decapods (Claus, No. 6). The change of tin* PhyUowma larva into the young Btages of the adult form, whieh are considerably smaller than the oldest PhyUosoma^ has nol yet been directly observed. Fio, 180. — Young Phyttotoma of Palinurus, shortly before hatching (after Oxa.ua, from Lino's Tat-book). ah, abdomen; L, liver; //-///, aeoond and third maxillipedes; iv-vi, first three ambu- latory limbs, LOR10ATA. The Phyllosoma of PalinurHa is to be distinguished from that of Sctfttmrmhf lin features, some of which have already been pointed out In the youngest Phyllosoma of Palinnrns, the second antenna is shorter than the first (Fig, 130), hut this condition is soon reversed (Fig. 181), and henceforth tl antenna remains longer. In Scyllarus, on the other hand (Fig. TJi>). the seeond antenna is always smaller than the first, and in later stages is transformed into the lamellate organ. Again, the presence of a rudiment of the first pair ol maxillipedes is characteristic of the Pln/llosoma of Palinurus, and so is the advanced development of the maxillipedes and ambulatory limba in the i-i . l l. -Older Pkyllotoma of Pcdinurui (aft i ' MxHUpide; »/)/"', third maxillipede ; pi, | '. ■■ Bv« Ambulatory Itmba, youngest free stages. While it is in this way not difficult to idrntifx tht Phyllosoma of Palinurus, there yet remain many other Phyllommm larraa, tome of which arc very remarkable forms, which can only conjeotnrally and with uncertainty be referred (<» the different genera of the iMntltioorn Lori llarus, TJienus, Ibacus, Paribacus, etc., Ri< HTl as, No, 146 . among I HA8WELL (No. 131) h.i (h crihod a Phyllosoma probabl) be] is. This author considt i it to 1"' a furtlu i erryi Guerin, which belongs to Mi LNl Edv | of tht * Phylk) 284 CRUSTACEA. laticaudes," in which the abdomen arises with a broad base as the direct pro- longation of the thorax. Richters, on the other hand, has conjectnrally referred the Brevicaudes to Ibacus and Paribacus. Fossil Phyllosoma have been recognised in the Solenhofen slates. F. Thalassinidea. The larvae of the Thalassinidea (Gebia, Calocaris, Callianassa, Calliaxis), known to us from the treatises of G. 0. Sars (No. 149) and Claus (Nos. 6, 7, and 8), are allied to the Caridid larvae by the shape of the body, the possession of a long rostral spine, and the peculiar armature of the; abdomen (wanting in Gebia) ; this latter consists of a long dorsal spine on the second and shorter spines on the three following abdominal segments. Their metamor- phosis is, however, of special interest on account of the complete transition which they exhibit between the manner of development of the Caridids and that of the Anomura and Brachyura. The youngest larval stage of Gebia (Fig. 132 A) is a Zoaea wdiich is distinguished from the Zoaeae of other Macrura chiefly by the fact that only the two anterior maxillipedes (mf, mf") func- tion as biramose swimming limbs, while the third maxil- lipedes, as well as the four following ambulatory limbs, are found as rudiments devoid of setae, the four anterior pairs being biramose, while the most posterior pair is still simple. The fifth pair of ambulatory limbs, as well as the pleopoda, are still altogether wanting. The telson, which is not yet marked off from the sixth abdominal segment, is a spade-shaped plate somewhat indented ai itfl posterior edge and beset with setae. The paired eyes, which till immovable, recall those of the Anomuran larva, in the I'm;. 132.— Two larval stages of Gebia littoralis (after G. O. Sars). A, Zoaea stage (dorsal view). /-', Mijs'is stage (side view), a', first antenna ; «", second antenna; cfl-cfi, rudiments of the live posterior abdominal appendages; mf, mf", mf", the three maxillipedes; pl-p^, the five ambula- tory limhs ; or, Nanplins eye. ANOMURA. 285 great elongation of the posterior crystalline cones, causing the eye to project somewhat backwards. The My sis stage (Fig. 132 B), which results from the above through a transitionary stage, shows the fifth ambulatory limb as well as the rudiments of the pleopoda (except those of the first abdominal segment). The exopodites on the third maxillipede and on the three anterior ambulatory limbs (mf"'-pIn) are provided with setae and function as swimming limbs. The endopodites of these limbs, on the contrary, are still quite embryonic in appearance; they are unjointed and have no setae. The telson, which has now changed into a long rectangular plate, has, at its posterior margin, a small unpaired spine and seven longer spines on each side. The lateral limbs of the caudal fin (a6) are already developed. In later stages, the endopodites of the ambulatory limbs become more developed and the rudiments of pincers become evident on the first ambulatory limbs, while the exopodites gradually degenerate. The agreement between the metamorphosis of Gebia and that of the Anomnra consists in the form of the paired eyes and telson, in the hiramose maxillipedes, and above all in the condition of the third maxillipede, which is first used as a swimming limb in the Mysis stage, while its endopodite is still rudi- mentary. While, in this respect, the larva of CaMiaxis agrees with the Zoaea of Gebia, the Zoaeae of Callianassa and Calocaris agree with those of the Caridea, inasmuch as all the three maxillipedes here function as biramose swimming limbs. As a contrast to the features which connect the metamorphosis of the Thalassinidea with that of the Anomura and Brachyura, the distinct develop- ment of the Mysis stage in the former group should be pointed out. This stage, which is characteristic of the Macruran metamorphosis, is, as we shall see, suppressed in that of the Anomura and Brachyura. The Zoaea of the remarkable deep sea form Calocaris Macandreae is provided, like the other forms, with well developed paired eyes. The degeneration of these organs, which characterises the adult, takes place only after the Mysis stage (G. 0. Sars, No. 149). The Calliaxis larva (Claus, Nos. 6 and 7) is distinguished by a curious neck-like elongation of the cephalic region, which causes a certain resemblance to Lucifer. Another feature characteristic of this form, which was described by Brook (No. 107) as Trachclifer, is the lengthening of the telson into two wing-like lobes bearing spines posteriorly. G. Anomura. The Anomura in their metamorphosis show a somewhat primitive condition, in which, in many respects, they agree with the Brachyura. In most cases the young leaves the egg as a Zoaea (Fig. 133 A), in which, as in the Brachyura and a few Thalassinidea, the two anterior maxillipedes function as the chief locomotory organs. 286 CRUSTACEA. The third pair of maxillipedes is only present in a rudimentary condition (Fig. 133 (J). The next ontogenetic stage which develops from the Zoaea (Fig. 133 B), and which must be regarded as equivalent to the Mysis stage of the Macrura, already shows all the ambulatory limbs and the rudiments of most of the pleopoda. The ambulatory limbs have no exopodites, and already approach their final shape. We shall, in accordance with Claus (No. 7) and in agreement with the similarly-shaped stage of the Brachyura, define this stage as the Metazoaea. The characters of the Myais stage here seem suppressed. The Metazoaea must be regarded as a transitionary form between the Zoaea and the youngest adult form (Megalopa of the Brachyura). In one important point the Metazoaea of the Anomura is distinguished from that of the Brachyura, viz., the condition of the third maxillipede which here, though possessing a rudimentary endopodite, has a well-developed exopodite functioning as a swimming limb, while, in the Metazoaea of the Brachyura, this limb still remains altogether rudimentary. This relation of the third maxillipede to the locomotory function is a link connecting the Anomura with the Caridea, in which the third maxillipede functions as a locomotory organ as early as the Zoaea stage (p. 272). We must consider this as a character of the Mysis stage precociously developed, in which the Caridid Zoaea departs from the typical Zoaea, while in the Metazoaea of the Anomura it must be regarded as a last vestige of a lost Mysis stage. The development of the Anomura has recently been described in detail by G. 0. Sars (No. 150), while fragmentary notices concerning it were published earlier by Claus (Nos. 6, 7, 8, and 113), Spexce Bate (No. 98), Dohrn (No. 120), Faxon (No. 126), F. Muller (No. 140), Smith (No. 152), and others. The development of Eupagurus Jkrnhardus may serve as a type. The Zoaea of Eupagurus (Fig. 133 ^4) has a somewhat compressed body, and is chiefly remarkable for the shape of its dorsal shield, which runs out anteriorly into a long rostral spine, but is deeply indented posteriorly, the indentation separating two points which are directed backwards. A similar form of dorsal shield is found in all Anomuran larvae. The short, stalked, and as yet immovable paired eyes are remarkable for the posterior projections already described in connection with Gebia (p. 284). Between them the Nauplius eye can be recognised. The two anterior abdominal seg- ments are covered by the dorsal shield, the sixth abdominal segment it yet marked off from the telson. The postero-dorsal margins ANOMURA 287 of the intermediate segments of the abdomen are serrated. The telson is slightly cleft (a condition which reappears in the Brachyura) and is armed on each side with six setae. The first antennae (a') are simple and unjointed processes with setae at their tips. In the second antenna (a), the unjointed endopodite is still continuous with the protopodite, while the small exopodite with setae on its inner side is already separated. The mandible has no palp. Behind the two pairs of maxillae there are two pairs of biramose swimming maxillipedes (mf, mf"), while the third pair (Fig. 133 C) is to be Fig. 133. — Larval stages of Eupagxmis Bemliardus (after G. O. Sars). A, Zoaea (dorsal view). B, Metazoaea (side view). C, ventral view of the segment carrying the third maxillipedes in the Zoaea. D, caudal fin of the Metazoaea. a', first antenna; a", second antenna ; a^, rudi- ment of the fifth pleopod; a®, the sixth pleopod (uropod); mf, mf", mf", the three maxillipedes ; pi-piv, the first four ambulatory limbs ; k, gill-rudiments ; ol, upper lip ; t, mandibular palp ; r, rostrum. found only in the form of a very small two-jointed appendage pressed against the ventral surface. The five pairs of ambulatory limbs and the rudiments of the pleopoda are still altogether wanting. In a later stage, known as the Metazoaea (Fig. 133 B), which still resembles the Zoaea in appearance, the rudiment of the third maxillipede (mf") has developed further. It has a two-jointed exopodite beset with setae, which, like the exopodites of the pre- ceding maxillipedes, functions for swimming, while the indistinctly 288 CRUSTACEA. segmented endopodite, still devoid of setae, appears rudimentary and functionless. The same character is shown by the rudiments of the four pairs of ambulatory limbs which follow (jp'-pIV), the largest of which are those of the first pair, and show distinct rudiments of pincers. A similar very small rudiment is also carried by the fifth thoracic segment (not visible in the figure). In the abdomen, th.] I Wki.hon has pointed out the significance of these spines in checking move- ment in certain directions. Joitm. Mar. Biol. Assoc, N. S., Vol. i. BRACHYURA. 291 the dorsal shield and sloping upwards and backwards, and a pair of lateral spines at the posterior lower angles of the shield which slope outwards. At the anterior part of the body, the large, short- stalked lateral eyes rise from broad bases; between them lies the Nauplius eye. The movable abdomen which is used as a rudder for steering, is ventrally flexed, and consists of five free segments, while the sixth still appears to be united in one piece with the .telson. The posterior thoracic region, which later bears the ambulatory limbs, appears in the Zoaea very little developed. It is quite short and rudimentary, is hidden under the dorsal shield and shows either no indications at all of limbs (peraeopoda), or only short truncated rudiments. In the last case it would perhaps be more correct to call the stage at which the larva leaves the egg the Metazoaea. The seven anterior pairs of limbs are present in the Zoaea, well developed and functional. The antennae are distinguished by great simplicity of shape. The first antennae are short, unjointed processes, at the ends of which only a few setae develop (in the youngest stage of Carcinus maenas only two (Spence Bate)). The second antenna consists of a protopo elite which often grows out into a very long spinous process which is seen less strongly developed in the Anomuran larvae {e.g., Eupagurus). Another spinous process clothed at its end with setae must be regarded as the exopodite (squame, st). The endopodite (rudiment of the future flagellum) is at first altogether wanting, but soon appears in the form of a small prominence which grows out between these two spinous processes. The mandible still consists exclusively of the masticatory blade ; the mandibular palp is altogether wanting. The maxillae already show the structure typical of the Decapoda (Fig. 137 D and E). The protopodite of the first maxilla has two masticatory blades turned inwards and provided with setae, and a two-jointed palp (endopodite). On the second maxilla there are four lobate masticatory processes on the protopodite, two occurring on each joint, an endopodite also divided up into lobes, and an Fig. 138.— Zoaea of Thia polita (after Clacs). mf, first maxillipede ; mf", second maxillipede. 292 CRUSTACEA. exopodite developed as a respiratory plate fringed with setae. The two anterior maxillipedes (Fig. 136, mf\ mf") have developed as biramose swimming limbs. The endopodite of the first maxillipede has four joints, while that of the second maxillipede is more rudi- mentary and usually consists of three short joints. The exopoditea (flagellate branches) have long swimming setae at their ends. The rudiments of the succeeding limbs (third maxillipedes and first five ambulatory limbs) seem entirely wanting in the youngest Zoaea of many Brachyura (e.g., in Pinnixa, W. Faxon), in other cases some or all of them appear as short, truncated outgrowths (Maja, Inachus). The pleopoda are still altogether wanting. The abdomen is generally distinguished by a definite development of spines, the second segment A '* Fig. 137.— Limbs of an older Zoaea of Portumis (after Glaus). A, first antenna. B, secoi antenna. C, mandible. D, first maxilla. E, second maxilla, en, endopodite ; at, exopodit ex, spinous process ; t, mandibular palp. having a pair directed anteriorly, and the three succeeding segments each having a pair directed posteriorly. The telson, as a rule, has a characteristic forked form, and is produced out laterally into a long spine. On the inner side of these two processes on the telson there are, in most cases, three strong setae. The form of the Brachyuran Zoaea undergoes, in isolated cases, variations from the type here described, which are chiefly connected with the develop- ment of Hit; spinous processes and the form of the telson. In Gelasimus, for instance, the spinous processes of the dorsal shield are unusually short. In Achaeus, there is no frontal spine, but a short dorsal spine is retained. In Tnachua also the frontal spine is wanting (Claus, No. 8; Gourmet, No. 130) In Maja (Couch) and in Eurynome (Kinahan) all the spinous process.- BRACHYURA. 293 wanting. Claus, however, has described the presence of a long frontal spine on the Zoaea of Maja. In a larva described by Dohen (No. 121) as Fissocaris, which has a long frontal spine and two pairs of large lateral spines, the dorsal spine is wanting. In other cases the dorsal and lateral spines may be exceed- ingly long, and may end in balloon-shaped swellings. Larvae thus provided with lateral spines projecting backwards were described as Pluteocarialae by Claus (No. 8). A Zoaea form distinguished as Pterocaris is remarkable for the wing -like elevations of the lateral parts of the carapace, and for the transversely-broadened form resulting from this. Many of the Zoaeae of the Brachyura are distinguished by the great develop- ment of the spinous process on the second antenna ; e.g., those of Xantho rividosus (Goukret, No. 130) and of Panopeus Sayi (W. Faxon, No. 125), in which this process equals in length the long frontal spine. When the youngest Zoaea leaves the egg, it is not altogether free, but appears surrounded by a somewhat loose and detached larval integument (p. 119) ; this has been claimed by Conn as the cuticle of the Protozoaea stage passed through during embryonic life. Only after a moult, which usually takes place early, does the Zoaea become free. A similar condition is found in many other Decapoda (e.g., in all Anoumra and many Macrura). F. Mullei: (No. 16) has pointed out the morphological interest of a study of this larval integument, the form of the tail in this youngest integument in Achaeus, and perhaps also in Maja, recalling that of the shrimp larvae. This larval cuticle Avas utilised later by Paul Mayer (No. 137) in an attempt to derive the different forms of telson and for phylogenetic purposes. More recently, further investi- gations with regard to it have been made by W. Faxon (No. 125) and Conn (Nos. 114 and 115). The spinous processes of the dorsal shield are always wanting in the larval integument, but are often present as rudiments beneath the cuticle, withdrawn in a telescopic manner. While, in the group of the Grapsoidea (Sesarma), the larval integument, apart from the spinous processes, is a fairly true cast of the Zoaea which proceeds from it, in most other Brachyura it shows no inconsiderable deviations from that form. The antennae, hilly, appear in the larval integument in a higher degree of development. The first antenna consists of a protopodite and two terminal blanches provided with setae, one of which is of considerable length. The second antenna is specially remarkable for the presence of laige processes provided with setae on the exdpodite. In the posterior region of the body, the caudal fork is usually characterised by possessing seven feathered setae on each side. The number :i seems to be typical in the setae of the telson throughout the Decapoda, and renders the study of the hatched Zoaea before the casting of the larval integument a valuable aid in tracing back the shape of the telson, which often varies in a later stage, to the fundamental form already described (Tauj. Mayer). The series of ontogenetic stages which follow after the Zoaea have usually received the same name, but have been more suitably < -a! l<'il Metazoaeae by Claus (No. 7). They do not, in general shape (';/*. the somewhat younger stage, Fig. 138), entirely resemble the Zoaea, but are distinguished by the greater development of the limb-rudiments. In the first antenna (Fig. 137 A) there is now an unjointed protopodite showing the rudiment of the auditory organ as a vesicular swelling, and two rudiments of flacella, the inner still 294 CRUSTACEA. being short and unjointed, while the outer flagellum, beset at its end with olfactory filaments, has broken up into short rings. The second antenna (Fig. 137 B) has the rudiment of the flagellum (endopodite) greatly developed ; but the softer portions have with- drawn from the spinous process and the exopodite as a sign that these appendages will be lost in the next moult. On the mandible ((7), a finger-shaped and still unjointed mandibular palp has developed. The two maxillae (D and E) have undergone changes which are comparatively slight, while in the two anterior maxillipedes the exopodites are divided up at their ends into short joints, and seem beset with numerous swimming setae. The rudiments of the third pair of maxillipedes and of the five ambulatory limbs (Fig. 138, IV- VIII) are already conspicuous. They, however, have no clothing of setae and are still functionless, being carried closely pressed against the sternal surface of the body. These rudiments develop gradually, but always in the direction of the form of the adult appendages. The third max- illipede thus soon shows all the parts of the future limb, a two- jointed exopodite and the branchial appendages, which also develop on the three following pairs of limbs. The most anterior pair of ambu- latory limbs with their pincers develop greatly. On the abdomen, the pleopoda have now developed as indistinctly two-jointed append- ages, while the sixth pair are still simple stumps. An exopodite does not develop on the'ambulatory limbs. The My sis stage is thus suppressed in the metamorphosis of the Brachyura, and is repla l>!l the Metazoaea stage. This is an interesting case of simplification of the course of development. The Metazoaea passes into the young form of the Brachyuran known as the Megalopa (Fig. 139 A and B), which brings about the Fio. ISS.—Zoaea of Maja after ecdysis (after Claus, from Lang's Text-book). 1, 2, first and second antennae ; I, II, III, first three maxillipedes; IV-VIII, first five ambulatory limbs; a„-aG, last five pleopoda ; h, heart. BRACHYURA. 295 transition from the swimming to the creeping manner of life, and in the most important points of its segmentation already agrees with the adult. In the condition of the abdomen, the Megalopa is some- Avhat on a level with an adult Anomuran Decapod. The anterior part of the body with the limbs already shows the typical Brachyuran character. The youngest Megalopa stages, however, still in most cases carry on the dorsal shield traces of the former Zoaean spines (Fig. 139 A). The limbs have now attained their final shape; the maxillipedes have lost the locomotory function and are com- A Fig. 139. — Three ontogenetic stages of Carcinvs maenas. A, younger, and B, older Megalopa stage. C young crab. (A, after Spence Bate ; B and C, after Brook), d, dorsal spine ; r, rostrum. paratively smaller. The ambulatory limbs, on the contrary, have developed greatly. The abdomen is still stretched out backwards, and shows the pleopoda as swimmerets provided with long setae ; these consist of a basal joint and an oval terminal plate provided with setae (exopodite), while the quite short endopodites furnished with small hooks, as retinacula, serve to connect the right and left swimmeret, and consequently bring about a simultaneous movement of the two limbs. The forked telson of the Zoaea has changed into a rounded caudal plate. 296 CRUSTACEA. The different Brachyuran Megalopae also show variations ir details, hading to the formation of distinct genera by Dana (Mare&iia, MoiiolepUt Oyllcne, Triloba). The youngest Megalopa stages of different forms vary with regard to the remains still to be found of the Zoaean spines. While tin- still to a considerable extent retained in Carcinus maenas (Spence Bai i . No. 97), they are, in other cases, more degenerate, and may even (Portun altogether wanting in the youngest Megalopa which develops from the Metaz The Megalopa passes gradually through several moults into the final form (Fig. 139 (T). The changes during this period, which • described for Carcinus maenas by Brook (No. 106), are chiefly in the shape of the dorsal shield and the degeneration of the abdomen typical of the Brachyura, this part of the body being from this time curved ventrally and applied to the thoracic sterna. While, in "by far the greater number of Brachyura, metamorphosis takes the course described above, it undergoes considerable abbreviation in individual cases, by the suppression of certain stages. An interesting example of this is found in Pinnixa, where the Mctazoaea stage, at f\ m^-v /J the last moult, givr- direct to a young crab, the Megalopa stage being entirely wanting in this form (W. Faxon, No. 126). The metamorphosis of some land and fresh- water crabs appears ab- breviated in another manner. West w ooi (No. 156) states that the young of a specie! of Gecarcinus leaves tin- egg in a condition which, apart from the want of pleopoda, altogether resembles that of the adult. Thompson, on the other hand, found that, in other s] of Gecareinus, the young hatch as Zoacae. This is also the case with other land crabs (Ocypoda, Gelasimus) ; it therefore appears that, in most crabs that li- land, metamorphosis is not abbreviated, and that the young Zoacae are hatched in the sea, a fact connected with the regular migration of the land crabs to the sea-shore (F. MULLBR, No. 16). Tin- abbreviation of metamorphosis in fresh-water crabs [Trichodactylus, F. MOlLBR, No. 143; Dilocarciturs, Goldi, No. 129; Telphusa, Mercakti, No. 139) is, on the other band, in agreement with what is known of other fresh-water Decapods (e.g., Palaemonetes, Astaciis, &c). The young (Fig. HO) here leave the egg in a form already greatly resembling the adult. The •till appear comparatively large, and the cephalo-thorax, on account of the ice of food-yolk, is much swollen. The abdomen has no pleopoda. In Wcinus, the segments of the abdomen are still quite distinct, and do not "\v the fusion characteristic of the adult. Fio. 140.— Youngest stage hatching from the egj; of Telj>Misa fluviatilis (after Mercanti). A, dorsal aspect, ]:, lateral aspect. STOMATOPODA. 297 11. Stomatopoda. The Stomatopoda are a branch of the higher Crustacea which very early separated from the common Malacostracan stock, and which retain in their organisation very primitive features, side by side with peculiar developmental forms. Among the former we may reckon the long dorsal heart provided with many pairs of ostia, and the condition of the dorsal shield which covers the segments carrying the maxillipedes, but does not fuse with them. Another primitive character is perhaps found in the presence of ten pairs of segmentally-arranged hepatic tubes, some of which belong to the abdomen. The position of the more important organs (hepatic tubes, genital organs, heart) in the large abdomen is a distinctive characteristic of the Stomatopoda, as opposed to other Malacostraca, in which this region of the body serves almost exclusively as a muscular locomotory organ. On the other hand, the metamorphosis of the Stomatopoda shows peculiar characters, although it cannot be denied that it has a certain ontogenetic tendency in the same direction as that of other Malacostraca. In spite of the important treatises of Claus (No. 86) and Brooks (Nos. 83 and 84), our knowledge of the metamorphosis of the Stomatopoda is still somewhat incomplete, especially with regard to the first stages which hatch from the egg, and the con- nection between the larval forms, that often vary greatly, and the sexual animals to which they belong, these last differing less from one another than do the larvae. A general distinction may be made between two types of larvae which, however, are connected together by intermediate types; these two were formerly classed as separate genera under the names of Ericldhus and Alima. Of these two forms the Ericldhus shows the more primitive metamorphosis, so that we shall describe this larva first. The youngest known stage of the Ericldhus series, probably the one at which the young animal leaves the egg, is called the Ericldhoidina. The youngest larva known (Fr. Mtjller and Claus, No. 87) is 2 mm. long, and is seen to be divided into three regions : (1) an anterior, unsegmented cephalic region which carries the eyes, antennae, and mouth-parts, and gives origin to the fold of the dorsal shield which projects backwards; (2) a middle thoracic region hidden under the dorsal shield and consisting of eight segments, the five anterior carrying Copepod-like biramose limbs, while the three posterior segments are limbless; (3) a posterior region in which 298 CRUSTACEA. the still unsegmented abdomen is developed in the form of a flat caudal plate. The spines of the dorsal shield recall the Protozoaea stage of Lucifer. There is a long, forwardly-directed rostrum, a short, unpaired median spine projecting backwards from the posterior edge of the dorsal shield, while, springing from the sides, are two Fig. 141. —Several consecutive Erichthoidina stages (after Claus). a', first antenna ; <<". second antenna; ai-a.6, the six pleopoda; I-V, the five niaxillipedes ; 6, 7, S, the threa posterior thoracic segments which, in these stages, have no limbs. long postero-lateral spines (Fig. 141 A). Besides the Nauplius eye, the paired stalked eyes are found on either side of the rostrum below the dorsal shield. The two pairs of antennae are still short and uiiiramose. The mandible has no palp, and the two pairs of maxillae are present in a very rudimentary condition as small lobes. The five STOMATOPODA. 299 pairs of swimming limbs which follow (Fig. 141, I-V) are biramose, and provided at their ends with setae ; these appendages correspond to the five pairs of maxillipedes found later, while the next three limbless segments (6, 7, 8) carry at a later stage the biramose ambulatory limbs. In the stage now under consideration, all the thoracic segments are quite distinct, the five anterior ones being provided with limbs, while the abdomen is still unsegmented. In the following stages, the abdominal segments appear in regular order, as also do the pleopoda belonging to them, while still no trace of limb-rudiments can be seen on the last three thoracic segments. It results from this, that, in the Stomatopodan meta- morphosis, the primitive order of development of the segments (from before backward) is retained, whereas this order is broken through in the formation of the limbs by the belated appearance of limb-rudiments on the three posterior thoracic segments. In the next stage (3 mm. long, Fig. 141 .4), the first abdominal segment is marked off, and the rudiment of the first pair of pleopoda, still devoid of setae, are already to be seen on it (a1). In the first antenna, the rudiment of an accessory flagellum can be recognised as a short conical process. The five pairs of swimming limbs are also modified. In the second swimming limb particularly, the endopodite has enlarged as the rudiment of the future raptorial claw. In the next stages (Fig. 141 B and (7), the different abdominal segments and their limbs continue to appear in regular order. The anterior pair of pleopoda are already developed as biramose lamellate appendages furnished with setae, while those of the posterior segments are more rudimentary in shape (al-cfi). Even the sixth pair of pleopoda (a6), which later attains great development as lateral limbs of the caudal fin, is no exception in this respect, but develops last and in a shape exactly resembling the other pleopoda. .Meantime, the limbs of the anterior portion of the body, especially those of the maxillary region, undergo important alteration. In the anterior antenna (Fig. 141 D, a), we can distinguish a three-jointed protopodite, a short exopodite beset with olfactory setae, and a longer inner branch (the rudiment of the accessory flagellum appearing at a later stage). The second antenna, besides a fan-like plate which has developed on its extremity, shows the bud-like rudiment of a flagellum. Whereas the mandible is still for a long time without a palp, short rudiments of palps have already appeared on the two maxillae. The two anterior maxillipedes (Fig. 141 B, C, D> I, II) change in the direction of their final shape; the exopodite which 300 CRUSTACEA. serves as a swimming organ degenerates and finally altogether disappears. The endopodite of the first maxillipede remains com- paratively small, and gradually develops the rudiment of a small prehensile claw at its end. The endopodite of the second maxillipede, on the contrary, early develops into a powerful, clawed raptorial limb. On the basal joints of both these limbs the rounded epipodial plates, which are still without setae, appear simultaneously. The three folloiving pai?s of hiramose limbs meantime undergo an unusually interesting jirocess of degeneration, by which their transformation into the adult form is introduced. Here also the exopodite gradually vanishes, but, in addition, the endopodite becomes an unjointed rudiment without setae, out of which, only in later stages, is produced the adult limb ending in a short pre- hensile claw. Indeed, the degeneration of these three limbs may go so far that they disappear altogether, only to re- appear in later stages simultaneously with the rudiments of limbs of the three following thoracic segments (sixth, seventh, and eighth thoracic segments). In the latter case we have a larva which, in the possession of the seven anterior pairs of limbs and the absence of the six following thoracic limbs, shows a certain agreement with the Zoaea of other Malacostraca, and which has there- fore been called the Pseudozoaea of the Stomatopoda (Fig. 143). This larval type, first described by Fr. Muller, chiefly occurs (Claus) in the larvae belonging to the genera Pseudosquilla and Gonodactylu8 which were described by Brooks as Pseuderichthus and Gonerichthus ; it is, however, found (BROOKS) in the ontogenetic series (Lysioerichthus) of -mis T/yeioaquilla. Pro. 11.'. Older Erichthtu (after •iais). „', first antenna; a", Mcond antenna; i-V, maxilli- pedes; VI-Vill, rudiments of the three ambulatory limbs ; "'-",;, the six pleopoda; br, branchial rudiments. STOMATOPODA. 301 Later stages, which are distinguished by the transformation of the sixth pair of pleopoda into the lateral appendages of the caudal fin, bring about the transition to the actual Erichthus stage (Fig. 142), the three posterior maxillipedes gradually reappearing and the hitherto missing pairs of limbs of the last three thoracic segments growing out as buds. When the rudiment of the rounded prehensile claw can be recognised on the last three maxillipedes, and the three posterior thoracic appendages are seen as rods which soon become biramose, the Erichthus stage appears to be reached. This stage thus possesses all the limbs of the adult form. The transition into the sexually mature form is accomplished very gradually, the abdomen continually increasing in size, and the branchial filaments (br) growing out on the exopodites of the abdominal limbs. The larval forms which bring about this transition to the sexually mature animal, when exhibiting broad and compact bodies and retaining the appearance of the Erich- thus, are known as Squillerichthus, while certain ontogenetic forms which, even in the Erichthoidina and Erichthus stages, are remarkable for their slender bodies, pass from the latter into a Squilloid stage (Claus) which more nearly resembles the adult form. A second Stomato- podan ontogenetic series consists of the Alima forms. These larvae (Fig. 144) are distin- guished by their great size, the length of the body, and the broad, Hat cephalo- thoracic shield, which usually does not cover the posterior thoracic segments; further, by the position of the mouth, which has shifted far back, and by the slight forward extension Fig. 143.— Squilloid larva (so-called Pseudozoaea), after Claus. a', first antenna ; a", second antenna ; I, II, first and second maxillipedes ; ep, epipodial appendage ; ai-a6, the six pleopoda. 302 CRUSTACEA. of the shield, owing to which the bases of the eye-stalks appear to be covered by the rostrum only. None of these features, taken alone, is to be relied upon in distinguishing between the Alima and the Urkhthus, since forms are known which, by the anterior position of the mouth and the covering of all the thoracic segments by the dorsal shield, show affinities with the Erickthus, while, by the uncovered condition of the eye -stalks and the flattening of the cephalo - thorax, th e y show Alima character- istics. Such transition- ary forms have been called Alimerichthus. The youngest known Alima stages (Fig. 144) probably correspond to the Pseudozoaea de- scribed above in connec- tion with the ErtchthuB series. The anterior an- tenna already shows the rudiment of the acces- sory flagellum, while in the second antenna, which ends in an oval plate, the rudiment of the flagellum is still wanting. The first pair of maxillipedes (I) are long and palp-like, while the second pair (II) have already assumed their final form as prehensile organs. The three following pairs of maxillipedes (III-V) as well as the three pairs of biramose ambulatory limbs are entirely wanting. The tegmenta to which these latter belong may still be indistinctly ) narked off. The four anterior pairs of pleopoda are well developed, while the fifth and sixth abdominal segments are still but slightly developed as limbless rudiments. It appears that the Alima leaves the egg in the form just described (c/ P. Mayer, No. 138, l'u:. 144.— Youn- Alima larva (after Brooks), a', first antenna ; a", second antenna ; /, II, first and second maxillipedes; 0, 7, S, three posterior thoracic segments ; "' ":j, first live pleopoda. CUMACEA. 303 p. 219); the metamorphosis of the Alima series would thus appear abbreviated by the suppression of the Erichthoidina stages. The further course of development in the Alima form agrees with that in the Erichthus forms. The connection of the various ontogenetic forms with definite genera and species of Stomatopoda can hardly be accomplished on account of the difficulty of obtaining continuous series of stages. It must, however, be considered probable (Brooks, No. 84) that the Alima and Alimerichthus forms represent the larvae of the genus Squilla. W. Faxon, at least, succeeded in producing from an advanced Alima the young stage of Squilla empusa. Brooks believes that he can recognise in Alimerichthus the larvae of species closely related to Squilla microphthalmia. Still more difficult to decide is the question in what way the Erichthus forms which merge one into the other are to be distributed among the other Stomatopoda. Claus, however, has related certain Erichthus forms with high, laterally compressed frontal spine and short carapace (Pseude- richthus, Brooks) to the genus Pseudosquilla, while, on account of the slightly arched dorsal shield, the long rostrum and the closely approximated, posterior lateral spines, and, above all, the absence of teeth on the terminal joint of the large prehensile maxillipede, he has related other forms (Gonerichthus, Brooks) to Gonodaciylus. Another series of larval forms {Erichthus Duvauccllei and multispinosus) characterised by a strongly arched dorsal shield, a flat abdomen, widely separated posterior lateral spines, as well as ventrally incurved lateral edges of the dorsal shield, may be referred, with Brooks, on account of the presence of numerous teeth on the terminal joint of the second maxillipede, to Lysiosquilla, and may therefore be called Lysioerichthus. Brooks was able to observe, in the case of a larva nearly related to Lysioerichthus multispinosus, the direct transition into Lysiosquilla excavatrix. With regard to the difficulty of classifiying the other larval forms, it must be borne in mind that our knowledge of the adult forms is by no means complete, as has been proved by occasional discoveries (such as that of the remarkable Pterygosquilla found by Hilgendorf). Fossil Stomatopoda have also recently become known. 12. Cumacea. The Cumacea, which occupy an intermediate position between the Schizopoda and the Arthrostraca (especially the Anisopoda), show an abbreviated and fairly direct form of development. As in the Mysidae (p. 257), metamorphosis is confined almost entirely to stages passed through within the brood-cavity of the mother. The embryos, by their dorsal curvature, as well as by the presence of the dorsal organ, recall the Isopoda. The compound eye, which in the adult is usually unpaired, arises by the fusion of paired rudiments. The young that emerge from the brood-cavity are still without the last pair of thoracic limbs, and, in this respect, resemble the Isopoda. Only the sixth pair of pleopoda is well developed. The five anterior pairs are wanting in the young (as is also the case in the Anisopoda), and are, as a rule, only partly developed in the adult males (Dohrn, No. 96). M4 CRUSTACEA. 13. Anisopoda. Among the Arthrostraca, the Anisopoda (Apsendes, Tanais) show the most primitive condition, somewhat suggestive of that found in the Schizopoda. Embryonic development and the greater part of the metamorphosis here takes place within the brood-cavity of the mother (as in Mysis and the Cumacea). The young on emerging from the brood-cavity (Fig. 145), like those of the Isopoda, are dis- tinguished from the adult by the absence of the last pair of thoracic limbs. As in the Cumacea, all the pleopoda, ex- cept the sixth pair, which are filamentous caudal appendages, are wanting. A point of great interest is the presence of a wing-like shell-fold (ps) stand- ing out laterally from the cephalo-thorax. This renders possible the derivation of the Arthrostraca from a racial form provided with a dorsal shield, and also seems to establish the significance of the lobe -like appendages of the Asellus em- bryo (cf. above, p. 151). l'ui. 145. — Young of Apsmdes Latreillii from the brood-cavity (after Claus). 77- VII, second to seventh thoracic segment ; 1-6, the six abdominal segments ; a', first antenna ; "", swond antenna ; ab®, sixth pleopod ; ep, epipodial appendage of the maxillipede ; ex, (ixopodites of the first and second thoracic limbs; mxf, maxillipede; p1, 2'2, first and second thoracic limbs ; ps, carapace 14. Isopoda. We have already had occasion to mention some features of the development of the Isopoda (p. 150), especially the dorsal curvature and the development of the dorsal organ in these animals. In the Isopoda also, after the full number of segments have appeared and the limb-rudiments have developed, the egg-envelope splits, and the larva, still incapable of motion, maggot-like, and surrounded by the Nauplius cuticle, under- its further development in the brood-cavity of the mother (Asellus). When it leaves that cavity, it already resembles the adult isopoda. 305 in the general segmentation of the body, being still distinguishable from the latter by the relatively large size of the head and the eyes, the incomplete segmentation and the non-existence of setae on the limbs, and above all by the absence of the last pair of thoracic limbs. This young form passes gradually through several moults into the adult stage. Schiodte and Meinert (No. 175) distinguished in the Aegidae (and Cymothoidae generally) three consecutive larval stages, the youngest of which, found still in the brood-cavity of the mother, is marked by the absence of setae on the limbs and the telson. The second free-swimming stage has already developed setae, and during the third stage the limb-rudiments develop on the last thoracic segment. In the Cymothoidae we meet with indications of degeneration owing to the parasitic life adopted by these forms ; these even appear during metamorphosis, finding expression in the shortening of the antennae and the changing of the thoracic limbs into adhesive organs. In individual cases, these changes lead, through the reductions caused by parasitism, to a much more distinctly marked metamorphosis, the sexual heteromorphism also being more accentuated than in the free-living Isopoda. This is the case in the families of the Anceidae, Bopyridae, and Entoniscidae. In the genus Gnathia, the female form (Praniza) is very different from the sexually mature male (Anccus). In the female, the head is small and triangular, and three of the posterior thoracic segments fuse to form a swollen region (brood-chamber) ; in the male, the head is broad and square, and the seizing pincers are branched like the horns of the stag-beetle. The young forms of this family show the elongate Praniza type, but from the very earliest stages there are indications of sexual dimorphism, the larvae which will become females already showing signs of the fusion of three thoracic segments, which, in the young males, are quite distinct from one another. These Praniza-\ikc larvae lead a parasitic life (on fishes). Consequently, mouth-parts which are adapted for piercing and for suction project forward under a large upper lip. The mandibles and maxillae are adapted for piercing ; they are palpless, and pointed like stylets, and the mandible and second maxilla are toothed at their ends. A pair of maxilli pedes follows, the segment carrying them being fused with the head. This anterior maxillipede, which is somewhat long, forms a kind of lower lip, while the second gnathopod ends in an anchoring hook. The five following thoracic segments (the third to the seventh) carry live pairs of limbs changed into anchoring organs, the fifth to seventh being fused in the female.* The eighth thoracic segment is retained in a very rudimentary condition, and is followed by a well-segmented abdomen furnished with biramose pleopoda used for swimming. During the transformation into the adult form, the upper lip and the maxillae are lost, while the maxillipedes undergo important alteration. They become lamellate organs for promoting circulation of the surrounding water. In the * [According to Stebbing (A History of Crustacea, London, 1893) it is the fourth, fifth, and often the sixth thoracic segments which fuse and form the brood-chamber. — En.] 306 CRUSTACEA. female larva the head decreases in size, the eyes undergo degeneration, while in the male larva the head grows out into a large square region of the body, which is also provided with degenerate eyes, and from which project, anteriorly at the sides of the rudimentary upper lip, two strong seizing pincers. We should feel inclined to derive these latter from the mandibles of the young form, had not Dohrn observed that they have an independent origin (c/. the treatises of Spence Bate, No. 161 ; Hesse, No. 168, and Domix, No. 164). In those much specialised parasites, the Bopyridae, which have sucking and very much reduced mouth-pai'ts, striking sexual dimorphism develops in a manner similar to that found in many parasitic Copepoda (Lernaeo- podidae), the less degenerate but smaller dwarf males (Fig. 147 A) appearing attached to the large and much deformed females. The males retain in general the Isopod appearance, the body remains symmetrical and distinctly seg- mented, the eyes are retained, although in a reduced condition. In the female, on the contrary, the eyes are almost entirely lost ; the disc-like broadened body is asym- metrical, and its segments often indistinctly marked off one from the other. The segmentation of the abdomen may in both sexes be reduced. The larvae of the Bopyridae, when they leave the brood-cavity (Fig. 146), have well-developed jointed antennae, the second pair being principally used for locomotion. The mouth-parts already show the structure characteristic of the adult. The six pairs of thoracic limbs are developed as anchoring organs. The last thoracic segment is still, as in all Isopod larvae, without limb-rudiments. Most of the abdominal segments are distinct from one another, only the last two are fused with the telson. There are five biramose (in many forms uniramose) pairs of pleopoda used as swimming limbs, while the limbs of the sixth segment (uropoda) have developed as caudal hooks. There are no distinctions of sex at this stage between the larvae, which swim about freely and seek for their future host. After attachment has taken place in the branchial cavity of the host (commonly a prawn), the larva develops further through the appearance of the last pair of thoracic limbs, and the gnat reduction of the antennae and the pleopoda, till, finally, the reduced adult form above described is reached. The abdomen of the adult, Fig. 146.— Larva of Bnpyrus with six pairs of thoracic limbs (after Walz). a', first antenna ; a", second antenna; md, mandible ; ul, lower lip; abs, first abdominal segment. ISOPODA. 307 in many forms, carries unjointed tubes or lamellae corresponding in position to the pleopoda ; these are, perhaps, of importance as respiratory organs, and were formerly repeatedly claimed as trans- formed pleopoda. Kossmann, however, has pointed out that they appear as new structures only after the entire disappearance of the IV;. 147.— A, male of an Entoniscid (Cancrion miser). B, young larva of an Entoniscid (Portunion muenadis), after Giard and Bonnier, from Lang's Text-book, a', first antenna ; a", second antenna ; ab, abdomen ; au, eye ; h, testis ; he, heart ; I, hepatic tubes ; pl^pl^, the six pleopoda ; ?•, rostrum ; t.,-t., the second to the seventh thoracic limb ; th, thorax. latter. This, however, does not disprove the pleopodan character of these appendages, since individual appendages often completely degenerate in the metamorphosis of the Crustacea, and reappear again later. The most marked parasitic transformations of the female are found in t lie Entoniscidac, which, as has been confirmed by the recent researches of Giaud and Bonnier (No. 167), are very closely related to the Bopyrvlae. These 308 CRUSTACEA. parasites are found within the body-cavity of the host (crabs, Paguridac), but must be described as ectoparasites, as they are enclosed in a chitinous sac derived from an invagination of the outer surface of the body (wall of the branchial cavity) of the host. The body of the female (Fig. 148 B) is very peculiar in shape and dorsally curved ; it has a rounded cephalic region (eg) with piercing mouth-parts and vestigies of antennae (ae, ai), an unsegmented thorax (th) which carries the ventral brood-cavity formed by the lamellae of the limbs (Fig. 148 A), and a segmented abdomen (ab) with sabre-shaped or lamellate pleopoda (en3). The small males (Fig. 147 A) become attached to the bit £i /^%rs,, TV!3 W v\ gMpMMNi p - Fio. 148.— Adult female of an Entoniscid (Portunion mnenadis), after Giard and Bonnier, from Lang's Text-boolc). A, with the brood-cavity partly opened in the ventral median line and the brood-lamellae separated. The abdomen (ab) is so placed that the ventral side is seen. B, brood cavity not opened, showing the dorsal surface of the abdomen (ab) Ir, the anterior, middle, and posterior lobes of the first brood-lamella on the right side ; same of the first brood-lamella on the left ; Ilr, III, second brood lamellae (right and left) ; ////•, ////, third brood-lamellae (right and left); IV, fourth brood-lamella; Vr, VI, fifth brood-lamellae (right and left) ; ab, abdomen ; ae, outer, ai, inner antennae ; ex.,, exopodite of the second pleopod ; en3, endopodite of the third pleopod ; co, cephalic region (so-called cephalogaster) ; h, cardial prominence; mf, maxillipede ; pi, pleural lamella of the flrsl abdominal segment; ov, ovary; th, thorax. females and resemble in appearance the Bopyrid males, being, however, distin- guished from these by the absence of the last pair of thoracic limbs (which have degenerated) and of the second antennae. The young larvae (Fig. 147 /•') very closely resemble those of the Bojiyridae, and always possess paired (au), and sometimes also a Nauplius eye (Grapsion). They are distinguished from the Bopyrid larvae chiefly by the shape of the penultimate thoracic Limbs (/7), which varies in different genera,, and also differs from that of the other AMPHIPODA. 309 thoracic limbs. The last pair is, as in all Isopoda, wanting. In a later stage (Cryptoniscus stage) this missing pair of limbs develops. The larvae of this stage become sexually mature as complemental males, protandrous hermaphro- ditism occurring here as in all other Isopoda (Bullar, P. Mayer). At a later stage they change into females or into the final degenerated male forms (Giard and Bonnier, No. 167). 15. Amphipoda. The embryos of the Amphipoda, which, as has been previously mentioned (p. 148), are distinguished from those of the Isopoda by the ventral curvature of the body within the egg, already have the full number of segments and limbs of the adult. Even the fusions which take place between special segments in some forms are already present in the embryo (F. Muller, No. 16). Metamor- phosis is consequently reduced to small alterations of shape, increase in number of the joints and olfactory filaments of the antennae, and additions of setae and teeth. A somewhat more pronounced metamorphosis is found in the Hyperidea. In the newly hatched young of Hyperia, F. Muller found no rudiments of pleopoda, whereas Claus, in a Hyperia parasitic on a Discomedusa, found, at a similar stage, the pleopoda and uropoda already developed. As a rule, the young of the Hyperidea, as compared with the adults, in which the eyes are often excessively developed, are remarkable for the smallness of the eyes, and consequently of the head. They are often further distinguished by the shape of the limbs. The young of Phronima, for example, have no powerful pincers on the antepenultimate pair of limbs (Pagenstecher). Spence Bate (No. 2) and more recently Claus (No. 177) have published accounts of remarkable differences between the young and the adults, some of which can be referred to the general manner of life. The larvae of Rhabdosoma thus appear remark- ably compact ; those of Eutyphis, on the contrary, are long. The Rhabdosoma larva recalls in its structure the genus Vibilia. The abdominal limbs first appear in the form of small rudiments. The Eutyphis larvae resemble in appearance the Gammaridae, so that the derivation of the Hyperidea from the Crevettina seems supported by the young stages (Claus, No. 177). 16. General Considerations Eegarding the Development of the Crustacea. The study of the metamorphosis of a group so varied and so rich in forms as the Crustacea is one of the most attractive and interesting pursuits for the morphologist. Great importance has repeatedly been attached to individual larval stages from a phylogenetic point of view. Although, in recent times, the Natqrtius and Zoaea have been robbed of their glory as racial forms of the Crustacea, the study of Crustacean metamorphosis does not lose all phylogenetic significance, inasmuch as very distinct indications as to the relationships of the different groups are afforded by the manner of their development. 310 CRUSTACEA. Great interest also attaches to the consideration of the causes which have brought about secondary modifications in the metamorphosis of the Crustacea. The view that the Nauplius stage corresponds to the racial form of the Crustacea is associated above all with the name of Fr. Mulleb (No. 16), and received considerable support from his discovery that even among the Malacostraca there is a form (Penaeus) whose meta- morphosis begins with a free-swimming Nauplius. After Haeckel, in his Generelle Morphologie, had adopted this view, it received the support of the most prominent investigators of the Crustacea (Dohrn, Claus). For a long time it remained the prevailing view. As to the way in which the Nauplius was to be deduced from lower forms of Invertebrata, only careful conjectures were hazarded. Worms devoid of segmentation, or with only a few segments, had to be investigated, and from this point of view the Eotatoria or simply-shaped Annelid larvae naturally came first under consideration. In the same way as the Nauplius was said to be the racial form of all Crustacea, the Zoaea was regarded as the racial form of the higher Crustacea or Malacostraca. This view was due chiefly to the state of knowledge at that time concerning the structure of the Brachyuran Zoaea. Starting from the view that the segments of the central part of the body (the five posterior thoracic segments) are only rudimentary in the Zoaea, or, as was repeatedly asserted, are not present, F. Muller (No. 16) put forth the view that the Malacostraca were separated from the Entomostraca by the entirely different order of formation of the segments. He distinguished in the body of the Malacostraca four regions, each said to consist of five segments : the primitive body, the anterior body, the middle and the posterior body. The primitive body is a direct derivative of the Nauplius body, and yields the three anterior segments (those of the first and second antennae and mandibles) and the two posterior segments (those of the uropoda and of the telson). Later on the more newly-acquired regions of the body appear intercalated between the anterior and posterior portions of the primitive body, the segments of the anterior body (maxillae and maxillipedes) being formed first, next those of the posterior body (five anterior abdominal segments), and finally those of the middle body (segments of the five ambulatory limbs). This view was opposed as early as 1871 by Claus from the study of the ontogeny of the Stomatopoda, in which, as in the Phyllopoda, the different segments appear successively in regular order from before backward. GENERAL CONSIDERATIONS. 311 The idea that the Zoaea had significance as a hypothetical racial form was widened and modified by Dohrn (No. 9). Relying on certain features found among the Entomostraca, which were inter- preted as Zoaean characteristics, and supported above all by the consideration of the Nauplius of Lejpas, which was taken for the Archizoaea because of its spinous structures, Dohrn thought himself justified in claiming the Zoaea as the racial form of all the Crustacea, which, proceeding from the Nauplius, had brought about the transition to a Phyllopod-like ancestral form of the Crustacea. Dohrn was the principal founder of the view that the most primitive Crustacean forms are retained in the central groups of the Phyllopoda, and that all other Crustacean groups can be derived from the Phyllopoda, a view which still prevails and which we ourselves accept, although, with Claus, we do not regard the hypothetical racial form as possessing exclusively the characters of the living Phyllopoda, but would construct a hypothetical racial group of primitive Phyllopods, in many respects, especially in the structure of the mouth-parts, more primitively constituted than the existing forms. Dohrn's assertion of the importance of the Phyllopoda as the central group from which all Crustacea can be derived was, in any case, a distinct advance, in so far as it removed the contrast made by F. Muller between the Malacostraca and the Entomostraca by providing a possible common derivation for all classes of Crustacea. Indeed, Dohrn's view smoothed the way for further advance, since it led easily, by logical sequence, to the direct deduction of the Phyllopoda from ancestral forms resembling the Annelida. Such a derivation of the Crustacea was, however, only very gradually accepted. The Nauplius at first remained unshaken in its position as racial form of all Crustacea, but the supposed phylo- genetic significance of the Zoaea fell into the background. To Claus (No. 8) is due the credit of having, in consequence of his compre- hensive investigations, recognised and established the nature of the Zoaea as a secondarily modified larval form. The ontogeny of the Stomatopoda and, above all, the metamorphosis of Penaeus, most distinctly show that there is no essential difference between the larval development of the Malacostraca and that of the Ento- mostraca, with regard to the order in which the new segments appear, that order, in both cases, being from before backwards. One of the most important peculiarities of the Zoaeae of the higher Macrura and Brachyura, viz., the retarded development of the 312 CRUSTACEA. segments in the middle region of the body, was thus recognised as being only a secondarily acquired character. But even for these last forms it has been distinctly shown by the more recent researches of Claus (No. 6), that these segments are not, as was. formerly assumed, altogether wanting, but that they are present, although in a form difficult to recognise externally, and in a very compressed condition. Thus Claus has observed that, in the Zoaea stage, all the pairs of ganglia of the apparently undeveloped segments in the middle of the body were already present as the closely crowded thoracic ganglionic mass, already perforated by the sternal artery, a relationship which foreshadows the adult condition, as does the whole of the vascular system of the Zoaea. Balfour, however,* believed that certain phenomena of Malacostracan onto- geny, especially the disappearance and reappearance of some of the appendages (the mandibular palp, the last two pairs of thoracic limbs in the Mastigqpus stage of the Sergestidae, the three posterior pairs of maxillipedes in the Stomatopoda), could only be explained with the help of a racial form which, in many respects (above all, in the rudimentary condition of the middle region of the body), showed Zoaean characters. According to this view, there would have developed from the primitive Phyllopods, first Ncbalia-\ikf independent movement. It is certain that ;i more perfect form of movement with less expenditure of force was thus attained. The transformation of the Annelidan parapodia into independently movable oars indicates the reason for the modifications in shape undergone by these appendages, which finally led to the development of the biramose Crustacean limb. In view of the fact that many of the parapodia of pelagic Annelida actually assume a lamellate form, we may luppose that the hypothetical ancestor of the Crustacea had similarly shaped limbs. We shall therefore be inclined to accept the view that the lamellate and GENERAL CONSIDERATIONS. 315 as yet unlengthened form of Phyllopodan limb, which recurs in Ncbalia and in the maxillae of the Copepoda and Malacostraca, approaches the primitive type, and that the elongate forms of limb have been secondarily developed from this. We shall then be able to derive the biramose character of the Crustacean limb direct from the correspondingly shaped Annelidan parapodium, and should perhaps be justified in deriving the epipodial appendages of the Crustacean limb from the dorsal gills of the Annelidan parapodia. On the other hand, the greater mobility of the segmental appendages led to the development of a new function, viz., the interaction of the two limbs of one and the same pair, this again leading to the development of corresponding lobate structures (endites, masticatory processes) on the mesial sides of the limbs. Such processes occur on all the trunk-limbs of the Branchiopoda, and are there used for the purpose of passing on particles of food. In most Crustacea, on the contrary, the accessory structures are limited to the limbs surrounding the mouth. It is a fact worthy of special attention that the second antenna in the Nauplius stage also participates in the work of mastication by means of the masticatory process on its basal joint, and attains its pre-oral position only at a later stage, when it loses its masticatory function. If we imagine a Protostracan serving as a racial form of the Crustacea produced out of an Annelidan type by the modification above described in the method of locomotion, and the changes in the limbs (with which is connected a modification in the condition of the body-cavity), it becomes evident that we cannot attribute to it all those characters by which the group of the Crustacea is distin- guished. The union of the five anterior limb-bearing segments to form a common region of the body (head), the transformation of the two anterior pairs of limbs into typical Crustacean antennae, the development through the Nauplius — these are characters which occur in all Crustacea, and which we shall also attribute to the primitive Phyllopod, but which need not necessarily be claimed for the Protostracan form. We may, on the contrary, claim a great latitude of variation for this latter. We shall have to assume that the hypothetical Protostracan group comprised forms of life far removed from the typical Crustacean structure. As such stocks produced independently from the Protostracan group, we may point to the Palaeostraca (Trilobita, Gigantostraca, Xiphosura), as also to the Pantopoda, of which we shall treat later. It should here be pointed out that Peripatus also, in a few respects, shows wonderful agreement with the Crustacea. The researches of Sedgwick have revealed a great similarity in the structure of the nephridia in the two groups. The relation of the maxillary ganglion to the brain in Peripatus recalls the corresponding relation of the antennal ganglion in the Crustacea. It has further become probable that a structure in Peripatus, until recently little observed, 316 CRUSTACEA. is the homologue of the Crustacean frontal organ. In consequence of these points of agreement, we may conjecture that the ancestral forms of the Arthropodan series leading to the Myriopoda and the Insecta also sprung from the Protostracan grouj}. If we try to give a sketch of those ancestral forms which led on from the more generally circumscribed Protostraca to the actual Crustacea, and which we are accustomed to call the primitive Phyllopoda, we shall have to presuppose in them a more homono- raously segmented body, and a slighter differentiation of the various sections of the body, than are found in the Crustacea of to-day. Each of the similar trunk-segments which together constituted the greater part of the body had a pair of ventral ganglia, a pair of biramose, lamellate, Phyllopod-like limbs, and perhaps also (like Peripatus) a pair of nephridia. Then, as we have to regard the antennal glands and the shell -glands as well as the genital ducts as transformed nephridia, the very varying position of these latter ducts in the different groups of Crustacea seems to indicate that we must attribute to the ancestral form of the Crustacea a large number of pairs of nephridia. We may perhaps assume for the limbless posterior region of the body (terminal or anal segment) paired furcal processes as inherited from the common ancestral form of the Crustacea. The most typical Crustacean characters had, how- ever, evidently already found expression in the primitive Phyllopoda in the structure of the anterior region of the body. We find here the fusion of the five anterior, limb-bearing segments (to which, as a sixth, should apparently be added an anterior primary cephalic segment with the eyes and the frontal organ) to form a common ■region of the body, the dorsal integument of which, enlarged by folding, formed the protective dorsal shield. Among the five pairs of limbs belonging to this region, the antennules, which were in all cases originally uniramose, assumed an exceptional position as bearers of the more important sensory organs. The second antennae, which followed them, were biramose and functioned chiefly as oars, perhaps also participating in the work of mastication. For this latter purpose, the mandibles lying behind the upper lip were specially suited through the modification of the basal joint, while the other part of the limb was retained in the Copepoda as a two-jointed palp. Next came two pairs of maxillae approaching in structure tin- subsequent trunk-limbs, and perhaps still retaining their original characters in the existing Crustacea. The presence behind the mandibles of a paired lower lip (paragnatha) in various groups of GENERAL CONSIDERATIONS. 317 Crustacea enables us to ascribe such a structure to the common racial form. The anterior cephalic region comprised the frontal organ (primary cephalic tentacle of the Annelida 1), the unpaired (so-called Nauplius) eye, and the paired compound eyes, which we must evidently assume to be inherited from the common racial form. This development of the cephalic region and its limbs gave those characters by which the Crustacea proper (primitive Phyllopoda) were distinguished from the Palaeostraca and the other Arthropodan stocks. In the primitive Phyllopoda the sexes were probably separate ; they possessed a long dorsal blood-vessel with segmental pairs of ostia, and perhaps also a pair of hepatic out- growths in each segment. The presence of this last character is supported by the organisation of the Stomatopoda (cf. on .the common racial form of the Crustacea, Lang's Text-book of Com- parative Anatomy, Yol. i., p. 406). In conclusion Ave must briefly refer to the interrelationships of the different groups of Crustacea. Among the Entomostraca the Branchiopoda, among the Alalacostraca Xcbalia, and in many points of inner organisation the Stomatopoda, stand nearest to the primitive Phyllopoda. Among the Entomostraca, the Copepoda were probably the first to branch off independently ; these while undergoing, in adaptation to pelagic life, a certain degeneration (that of the dorsal shield, the heart, and the respiratory organs, and the loss of the paired eyes), in other respects, especially in the structure of the mouth-parts, retained a very primitive condition. The other Entomostraca (Phyllopoda, Ostracoda, and Cirripedia) seem to stand somewhat nearer one another. Among the Phyllopoda, the small Cladocera, consisting of few segments are evidently a degenerate form of the Estheridae. For the Ostracoda we shall have to take as a starting point a very primitive Phyllopod completely enclosed in a bivalve shell, a form which thus must have resembled in appearance the Estheridae. In the most primitive form among the Ostracoda, Cypridina, the structure of the limbs clearly indicates a relationship with the Phyllopoda. Since we were obliged to presuppose for the primitive Phyllopoda a body of many segments, we shall have to assume for the Ostracoda a secondary diminution in number of the segments. The Cirripedia also must be deduced from a racial form similar to that of the Ostracoda, starting from the free-swimming Cypris- like larva. According to Claus, a nearer relationship to the Copepoda must often be assumed. This latter assumption rests upon the similarity of the thoracic limbs, and the similar number of segments in the thorax of the two groups. These features may, however, have been acquired independently in the two groups, since they actually recur in other Crustacea (the Cladocera, for example, having six thoracic segments), we cannot therefore consider the question as decided. For instance, we do not forget that the typical I lopepodan characters (degeneration of the lateral eyes and of the dorsal shield, breaking up of the second maxillae into two pairs of maxillipedes) are not found in the Cypris larva of the Cirripedes. In judging of the systematic position of these latter, we shall have to lay special stress on the presence of a large bivalve shell from which the mantle of the adult develops. It therefore appears to 318 CRUSTACEA. us that there is only a distant relationship between this group and the Cope- poda, and, taking into account the Cypris larva, we are justified in assuming a derivation from a primitive Phyllopod with a bivalve shell. In these points we agree with the view of Balfour {Text-book of Comparative Embryology, Vol. I., p. 509) and Fowler (No. 43). Among the Malacostraca, the Leptostraca which, besides the now living genus Nebalia and its relations, probably comprises a series of fossil forms such as Ceratocaris, Dictyocaris, and Hymenocaris, occupy the most primitive position and show morphological characters which connect this group directly with the Phyllopoda. Nebalia, for the accurate knowledge of whose structure and the definition of whose systematic position we are indebted to Claus, shows decidedly the Malacostracan type in the division of the body into regions, in the structure and number of the limbs, and in many points of internal anatomy (presence of a masticatory stomach as part of the stomodaeum, number and distribution of the hepatic tubes), so that there can be no doubt about its connection with the Malacostraca. Even the eight similarly shaped pairs of thoracic appendages, resembling the Phyllopodan limbs, are related to those of the Malacostraca by the number of joints in the endopodites. Contrasted with this, the presence at the posterior end of the abdomen of a supernumerary limb- less segment is of smaller importance. Among the features which connect the Ncbalidae with the Phyllopoda and thus constitute them direct descendants of the hypothetical group of primitive Phyllopoda must be enumerated : the presence of a long heart with a large number of venous ostia (four large and three small pairs), the very primitive structure of the ventral chain of ganglia, the maxillary ganglion having, as in Branchipus, remained separate, the flat, lamellate form of the eight pairs of thoracic limbs which recalls the Phyllopoda, and in which the separation of maxillipedes from the ambulatory or from the swimming limbs has not yet taken place, the presence of a large bivalve shell closing by means of a special adductor muscle, and finally the possession of two long, independently movable furcal processes strongly resembling those of Branchipus. Among the other Malacostraca, the Stomatopoda occupy an unusually isolated position. We evidently have here to do with a stock which separated very early from the primitive Malacostraca. While the form of the heart, and perhaps also the arrangement of the hepatic tubes, indicate a primitive condition, there is a development in many other directions of new and evidently independently acquired characters. The principal stock of the Malacostraca, on the other hand, is to be derived from the Leptostraca through the Schizopoda, among which, again, the Buphmuiidae must be regarded as the most primitive group. Their primitive character can chiefly be recognised in the shape of the thoracic limbs, which are all developed as biramose swimming limbs, and more or less resemble one another. The view that the Decapoda are descended from Schizopoda is supported eh idly by the presence of a Schizopoda-like stage in the metamor- phosis of many Decapods. Among these the Penaeidea, to which also the cidea are nearly related, approach the Schizopoda most closely. The other Decapoda seem to lie derived forms of the ontogenetic series proceeding from the Bchixopoda. The Brachyura, which are connected with the Macrura through many tranaitionary forms known as Anomura, must be regarded as the most highly developed but most specialised group of this series of forms. A second scries of forms proceeding from the Schizopoda leads through the LITERATURE. 319 Mysidae and the Cumacea to the Arth rostra ca, which we must regard as derived from the Schizopoda by the degeneration of the dorsal shield, the stalked eyes, and the exopodites of the thoracic limbs. This derivation has recently been strongly advocated by Boas (No. 4). It is supported by the structure of the Anisopoda, in which rudimentary exopodites are retained on. the two anterior thoracic limbs, and by the presence of a remnant of the dorsal shield. It receives further countenance from Nusbaum's observation in the embryo of Ligia (Lit., p. 189, No. 85a) of a biramose rudiment in the case of all the thoracic limbs. The Anisopoda are nearly related to the Isopoda, among which Jsellus especially has retained a primitive condition, while the Amphipoda must be regarded as a more specialised group.* LITERATURE OF THE METAMORPHOSIS OF THE CRUSTACEA. Crustacea in General. 1. Bate, C. Spence. Report on the present state of our know- ledge of the Crustacea. Report of the British Association Adv. Sci. 1878. 2. Bate, C. Spence, and Westwood, J. 0. A History of the British Sessile-eyed Crustacea. 2 Vols. London, 1861-1868. 3. Bell, Th. A History of the British Stalk- eyed Crustacea. London, 1853. 4. Boas, J. E. V. Studien liber die Verwandtschaftsbeziehungen der Malacostraken. Morph. Jahrb. Bd. viii. 1883. 5. Claus, C. Die morphologiscben Beziehungen der Copepoden, Phyllopoden, Cirrhipedien etc. Zeitschr. Naturwiss. Wiirz- burg. Bd. iii. 1862. * [Anaspides, a most remarkable fresh -water Crustacean from Tasmania, combines in many respects the characters of the Schizopoda and Arthrostraca. In general form it resembles the Amphipoda, but it possesses stalk eyes, auditory organs in the two-jointed protopodite of the antennule, exopodites and epipodial plates on the thoracic limbs, and typical biramose swimmerets on the abdomen. It was placed by Thomson (Trans. Lin. Soc. Zool. (2), vi. 3) among the Schizopoda. Great interest lies in the comparison made by Calm an [Trans. Roy. Soc. Ldinb., xxxviii., p. 787) between Anaspides and the Palaeozoic forms Palaeocaris, Gampso?iyx, and Acanthotelson. — Ed.] [Since the above was published in 1891, a new attempt has been made to deduce the Crustacea from the Phyllopoda, Apus being the form which, it is claimed, supplies all the requirements of the common ancestor. BERNARD'S arguments are too numerous to be detailed here, and should be studied in the originals (App. to Lit. on Crust, in Gen., I. and II.). We will confine ourselves to remarking that this derivation would account for the very general pretence of the Nauplius larval form, the Nauplius being merely the young Amu stage in the development of its derived forms. Bernard's method of deducing Apus from an Annelid with its prostomium and mouth bent ventrally so as to use its parapodia as jaws seems, as he claims, to find support from the varying month- parts of the Trilobita and the Gigantostraca, and, in general, it must be admitted that the recent discoveries of the appendages of Trilobites have largely helped to confirm his argument. On the other hand, it has led him to conclusions about the origin of the shell- and antennal-glands, which most investigators of the ontogeny of these organs will not regard as justifiable. — Ed.] 320 CRUSTACEA. 6. Claus, C. Zur Kenntniss.der Kreislaufsorgane der Schizopoden und Decapoden. Arb. Zool. Inst. Wien. Bd. v. 1883. 7. Claus, C. jSeue Beitriige zur Morphologie der Crustaceen. Arb. Zool. Inst. Wien. Bd. vi. 1885. 8. Claus, C. Untersuchungen zur Erforschung der genealogischen Grundlage des Crustaceen-Systems. Wien. 1876. 9. Dohrn, A. Geschichte des Krebsstammes. Jen. Zeitschr. f. Naturw. Bd. vi. 1871. 10. Dohrn, A. Untersuchungen etc. 8. Die Uberreste des Zoea- Stadiuras in der ontogenetischen Entwicklung der verschie- denen Crustaceen-Famiiien. Jen. Zeitschr. f. Naturw. Bd. v. 1870. 11. Dohrn, A. Die Pantopoden des Golfes von Xeapel und der angrenzenden Meeresabschnitte. Leipzig, 1881. (On the significance of the Nauplius and the Phylogeny of the Crustacea.) 12. Faxon, W. Selections from Embryological Monographs. Com- piled by Alex. Agassiz, W. Faxon, and E. L. Mark. 1. Crustacea. Mem. Mils. Comp. Zool. Harvard College. Cambridge. Vol. ix. 1882. 13. Faxon, W. Bibliography to accompany Selections from Embryo- logical Monographs compiled by Alex. Agassiz, W. Faxon, and E. L. Mark. I. Crustacea. Bull. Mus. Comp. Zool. Harvard College. Cambridge. Vol. ix. 1882. 14. Gerstaecker, A. Crustacea in Bronn's Classen und Ordnungen des Thierreichs. Bd. v. Abth. 1. 1. Halfte, 1866-1879. 2. Halfte, 1881-1898, unfinished. 15. Hatschek, B. Beitriige zur Entwicklung der Lepidopteren. Jen. Zeitschr. /. Naturw. Bd. xi. 1877. Studien zur Entwicklungsgeschichte der Anneliden. Arb. Zool. List Wien. Bd. i. 1878. (On the significance of the Naupliua and the Phylogeny of the Crustacea.) 16. Muller, F. Fur Darwin. Leipzig, 1864. 17. Packard, A. S. On the Homologies of the Crustacean Limbs. Amer. Natural. Vol. xvi. 1882. APPENDIX TO LITERATURE ON CRUSTACEA IN GENERAL. I. Bernard, H. M. The Apodidae. Nature Series. London, 1892. U. Bernard, H. M. The Systematic Position of the Trilobiics. Quart. Journ. Geol. Soc. Vols. 1. and li. LITERATURE. 321 Phyllopoda. 18. Brauer, F. Vorl. Mittheilung iiber die Entwicklung und Lebensweise des Lepidurus (Apus) productus. Sitzungsber. Acad. Wiss. Wien. Bd. lxix. 1874. 19. Chambers, Y. T. The Larva of Estheria mexicana. Amer. Nat. Vol. xix. 1885. 20. Claus, C. Zur Kenntniss des Baues und der Entwicklung von Branchipus stagnalis und Apus cancriformis. Abh. k. Acad. Wiss. Gottingen. Bd. xviii. 1873. 21. Claus, C. Untersuchungen iiber die Organisation und Ent- wicklung von Branchipus und Artemia. Arb. Zool. Inst. Wien. Bd. vi. 1886. 22. Ficker, G. Zur Kenntniss der Entwicklung von Estheria tici- nensis Bals. Criv. Sitzungsber. Acad. Wiss. Wien. Bd. lxxiv. 1876. 23. Grube, E. Bemerkungen iiber die Phyllopoden nebst einer Uebersicht etc. Archiv. f. Naturg. Bd. xix. 1853. 24. Joly, ]Sr. Histoire d'un petit Crustace (Artemia salina Leach.) etc. Ann. Sci. Nat. (2). Tom. xiii. 1840. 25. Joly, N. Eecherches zool. anat. et physiologiques sur l'lsaura cycladoides ( = Estheria) nouveau genre etc. Ann. Sci. Nat (2). Tom. xvii. 1842. 26. Lereboullet, A. Observations sur la generation et le developpe- ment de la Limnadia de Hermann. Ann. Sci. Nat. (5). Tom. v. 1866. 27. Leydig, F. Ueber Artemia salina und Branchipus stagnalis. Zeitschr. f. Wiss. Zool. Bd. iii. 1851. 28. Packard, A. S. A Monograph of North American Phyllopod Crustacea. 12th Ann. Rep. U. S. Geol. Geogr. Survey. Washington, 1883. 29. Sars, G. 0. Om em dimorph Udvikling samt Generationsvexel hos Leptodora. Vidensk. Selskab. Forliandl. 1873. 30. Sars, G. 0. On some Australian Cladocera raised from dried mud. Vidensh. Selskabs. Forliandl. Chrisiiania, 1885. 31. Zaddach, G. De apodis cancriformis Schaeff. anatome et historia evolutionis. Diss, inaug. zootomica. Bonnae, 1841. Ostracoda. 32. Claus, C. Zur naheren Kenntniss der Jugendformen von Cypris ovum. Zeitschr. f. Wiss. Zool. Bd. xv. 1865. Y 322 CRUSTACEA. 33. Claus, C. Ueber die Organisation der Cypridinen. Zeitschr. f. Wiss. Zool. Bd. xv. 1865. 34. Claus, C. Beitrage zur Kenntniss der Ostracoden. Entwick- lungsgeschichte von Cypris ovum. Schrift. d. Ges. z. Befoul. d. gesammt. Naturiu. Marburg. Bd. ix. 1868. 35. Zenker, W. Monographie der Ostracoden. Archiv. f Natur- gesch. Bd. xx. 1854. Cirripedia. 36. Bate, C. Spence. On the development of the Cirripedia. Ann. Mag. Nat. Hist. (2). Vol. viii. 1851. 37. Burmeister, H. Beitrage zur Naturgeschichte der Rankenfiisser. Berlin, 1834. 38. Claparede, E. Beobachtungen iiber Anatomie und Entwick- lungsgeschichte vvirbelloser Thiere. Leipzig, 1863. pp. 98-101. 39. Claus, C. Die cypris-ahnliche Larve der Cirripedien. Marburg, 1869. Schrift. d. Ges. z. Befbrd. d. gesammt. Naturw. Bd. ix. 5th Suppl. 40. Darwin, Ch. A Monograph of the Sub-class Cirripedia. Ray. Society. 1851-1854. 41. Delage, Yves. Evolution de la Sacculine. Archiv. Zool. Exper, (2). Tom. ii. 1884. 42. Dchrn, A. Untersuchungen iiber Bau und Entwicklung der Arthropoden. IX. Eine neue Nauplhisform (Archizoea gigas). Zeitschr. f. Wiss. Zool. Bd. xx. 1870. 43. Fowler, G. H. A Remarkable Crustacean Parasite and its Bearing on the Phylogeny of the Entomostraca. Quart. Journ. Micro. Sci. (2). Vol. xxx. 1890. 44. Hesse, M. Memoire sur les metamorphoses, que subissent pendant la periode embryonnaire les Anatifs appeles Scalpels obliques. Ann. Sci. Nat. (4). Tom. xi. 1859. 45. Hoek, P. P. C. Report on the Cirripedia, etc. Challenge* Reports. Vol. viii. 1883. 46. Hoek, P. P. C. Report on the Cirripedia, etc. (Anatomical Part). Challenger Reports. Vol. x. 1884. 47. Knipowitsch, N. Dendrogaster astericola n. g. et n. sp., eine neue Form aus der Gruppe Ascothoracida. Biol. Centralbl. Bd. x. 1891. 48. Koren, J., and Danielssen, D. Zoologiske Bidrag. Magazin for Naturvidenshaberne. Bd. iii. 1847. (Also in Isis. 1848.) LITERATURE. 323 49. Kossmann, R. Suctoria und Lepadidae. Arb. Zool. Instit. Wurzburg. Bd. i. 1873-74. 50. Krohn, A. Beobachtungen iiber die Entwicklung der Cirri- pedien. Archiv. f. N aturgeschichte. Bd. xxvi. 1860. 51. Lacaze-Duthiers, H. de. Histoire de la Laura Gerardiae. Institut de France. Mem. de VAcademie Sciences. Tom. xlii. 1882. See also Archiv. Zool. Exper. Tom. viii. 1879- 1880. 52. Lang, A. Ueber die Metamorphose der Naupliuslarven von Balanus mit RUeksicht auf die Gestaltung der Gliedmaassen und die Verwandlung in die Cypris-ahnliche Larve. Mitth. Aargauische nat. Gesellsch. Bd. i. 1878. 53. Metschnikoff, E. Sitzungsber. Versamml. deutsch. Naturforscli. zu Hannover 1865. p. 218. (Balanus balanoides.) 54. Muller, Fr. Die Rhizocephalen, eine neue Gruppe schmarot zender Kruster. Arcliiv. f. Naturgesch. Bd. xxviii. 1862. 55. Muller, Fr. Die zweite Entwicklungsstufe der Wurzelkrebse (Rhizocephalen). Arcliiv. f. Naturgesch. Bd. xxix. 1863. 56. Noll, C. Kochlorine hamata, ein bohrendes Cirriped. Zeitschr. f. Wiss. Zool. Bd. xxv. 1875. 57. Norman, A. M. Synagoga mira. Report of the British Assoc. 1887 (1888). 58. Pagenstecher, A Beitrage zur Anatomie und Entwicklungs- geschi elite von Lepas pectinata. Zeitschr. f. Wiss. Zool. Bd. xiii. 1863. 59. Schultze, Max. Zoolog. Skizzen. Zeitschr. f. Wiss. Zool. Bd. iv. 1853. p. 189. 60. Thompson, J. V. Zoological Researches and Illustrations. Vol. i. Part i. Memoir 4 : On the Cirripedes or Barnacles, etc. Cork, 1830. 61. Thompson, J. V. Discovery of the Metamorphosis in the second type of the Cirripedes, viz., the Lepades, etc. Phil. Trans. London, 1835. Part. ii. 62. Willemoes-Suhm, It. von. On the development of Lepas fascicularis. Phil. Trans. London. Vol. clxvi. 1876. Part. i. 63. Willemoes-Suhm, R. von. On a Crustacean Larva at one time supposed to be the Larva of Limulus. Quart. Journ. Micro. Sci. Vol. xxiii. 1883. 324 CRUSTACEA. Copepoda. 64. Claus, C Zur Anatomie und Entwicklungsgeschichte dor Copepoden. Archiv. f. Naturg. Bd. xxiv. 1858. 65. Claus, C. Untersuchungen liber die Organisation und Ver- wandtschaft der Copepoden. Wurzburger Naturw. Zeitschr. Bd. iii. 1362. 66. Claus, C. Ueber den Bau und die Entwicklung von Achtheres percarura. Zeitschr. f. Wiss. Zool. Bd. xi. 1861. 67. Claus, C. Die freilebenden Copepoden mit besonderer Beriick- sichtigung der Fauna Deutschlands, der Nordsee und des Mittelmeeres. Leipzig, 1863. 68. Claus, C. Ueber die Entwicklung, Organisation und system- atische Stellung der Arguliden. Zeitschr. f. Wiss. Zool. Bd. xxv. 1875. 69. Claus, C. Ueber Lernaeascus nematoxys Cls. und die Familie der Philichthyden. Arb. Zool. Inst. Wien. Bd. vii. 1887. 70. Claus, C. Beobachtungen liber Lernaeocera, Peniculus und Lernaea. Ein Beitrag zur ISfaturgeschichte der Lernaeen. Schrift. Ges. zur Beford. d. Gesammt. Naturw. Marburg. 1868. Bd. ix. 2nd Suppl. 71. Claus, C. Ueber den Bau u. d. Entwicklungsgesch. parasitiscber Crustaceen. Cassel, 1858. 72. Claus, C. Zur Morphologie der Copepoden. 3. Ueber die Leibesgliederung und die Mundwerkzeuge der Schmarotzer- krebse. Wurzburger Naturw. Zeitschr. Bd. i. 1860. 73. Grobben, C. Die Entwicklungsgeschichte von Cetochilus sep- tentrionalis Goodsir. Arb. Zool. Inst. Wien. Bd. iii. 1881. 74. Hartog, M. M. The morphology of Cyclops and the relations of the Copepoda. Trans. Linn. Soc. (2). Vol. v., Part i. 1888. 75. Hartog, M. M. The morphology of Cyclops and the relations of the Copepoda. Zool. Anz. Jahrg. viii. 1885. 76. Kellikott, D. S. A larval Argulus. North Amer. Entomol Vol. i. 77. Kollar, V. Beitrage zur Kenntniss der lernaeenartigen Crus- taceen. Ann. d. Wien Mas. Bd. i. 1835. 78. Leydig, Fr. Ueber Argulus foliaceus. Archiv. f. Mikr. Anat. Bd. xxxiii. 1889. 79. Nordmann, Alex. v. Mikrographische Beitrage zur Xatur- geachichte der wirbellosen Thiere. Heft. ii. 1832. LITERATURE. 325 80- Salensky, W. Sphaeronella Leuckartii. Archiv.f. Naturgesch. Bd. xxxiv. 1868. 81. Vejdowsky, F. Untersuchungen iiber die Ana'tomie und Meta- morphose von Tracheliastes polycolpus. Zeitschr. /. Wiss. Zool. Bd. xxix. 1877. Leptostraca. 82. Metschnikoff, E. On the Development of Nebalia (Russian). Zapiski imp. Akad. St. Petersburg. Tom. xiii. 1868. Stomatopoda. ' V 83. Brooks, W. K. The larval stages of Squilla em'pusa Say. Chesapeake Zool. Lab. Scientific Results of the Session of 1878. Baltimore, 1879. 84. Brooks, W. K. Report on the Stomatopoda collected by H. M.S. Challenger. Challenger Reports. Vol. xvi. 1886. 85. Brooks, W. K. Notes on the Stomatopoda. Ann. Mag. Nat. Hist. (5). Vol. xvii. 1886. 86. Claus, C. Die Kreislaufsorgane und Blutbewegung der Stomato- poden. Arb. Zool. Inst. Wien. Bd. v. 1883. 87. Claus, C. Die Metamorphose der Squilliden. Abh. d. konigl. Ges. Wiss. Gottingen. Bd. xvi. 1871. 88. Muller, F. Bruchstiick zur Entwicklungsgeschichte der Maul- fiisser. Archiv. f. Naturg. Bd. xxviii. 1862. 89. Muller, F. Ein zvveites Bruchstiick aus der Entwicklungs- geschichte der Maulfiisser. Archiv. f. Naturg. Bd. xxix. 1863. Schizopoda. 90. Brook, G., and Hoyle, W. E. The Metamorphosis of Britisli Euphausiidae. Proc. Roy. Soc. Edinburgh. Vol. xv. 1887- 1888. 91. Claus, C. Ueber einige Schizopoden und niedere Malaco- straken. Zeitschr. f. Wiss. Zool. Bd. xiii. 1863. 92. Dohrn, A. Untersuchungen iiber den Ban und die Entwick- lung der Arthropoden. Zeitschr. f. Wiss. Zool. Bd. xxi. 1871. p. 375. Penaeus zoaea = Larva of Euphausia. 93. Metschnikoff, E. Ueber ein Larvenstadium von Euphausia. Zeitschr./. Wiss. Zool. Bd. xix. 1869. 94. Metschnikoff, E. Ueber den Naupliuszustand von Euphausia. Zeitschr./. Wiss. Zool. Bd. xxi. 1871. 95. Sars, G. 0. Report on the Schizopoda. Challenger Reports. Vol. xiii. 1885. 326 CRUSTACEA. Cumacea. 96. Dohrn, A. Ueber Bau und Entwicklung der Cumaceen. Jen. Zeitschr. f. Naturw. Ed. v. 1870. Decapoda. 97. Bate, C. Spence. On the development of Decapod Crustacea. Phil. Trans. London. Vol. cxlviii. 1858. 98. Bate, C. Spence. Carcinological Gleanings. No. iv. ^4?/?^. Mag. Nat. Hist. (4). Vol. ii. 1868. (Larvae of Porcellana, Pagurus, Palinurus, etc.) 99. Bate, C. Spence and Power, W. H. On the development of the . Crustacean Embryo and the Variations of forms exhibited in the Larva of thirty-eight genera of Podoph- thalmia. Ann. Mag. Nat. Hist. (4). Vol. xviii. 1876. 100. Bate, C. Spence. Report on the Crustacea Macrura collected by H.M.S. Challenger. Challenge?' Reports. Vol. xxiv. 1888. 101. Birge, E. A. The first Zoaea Stage of Pinnotheres ostreum. Amer. Natur. Vol. xvi. 1882. 102. Birge, E. A. Notes on the development of Panopeus Sayi. Stud. Biol. Lab. Johns Hopkins Univ. Vol. ii. 1883. 103. Boas, J. E. V. Amphion und Polycheles (Willemoesia). Zool. Anz. Jahrg. ii. 1879. 104. Boas, J. E. V. Studier over Dekapodernes Slagtskabsforhold. Kong. Danske Vidensk. Selskabs Skrifter. Naturvid. og. mathem. Afd. (6). Bd. ii. Copenhagen, 1880. 105. Boas, J. E. V. Kleinere carcinologische Mittheilungen. (Spengels. Zool. Jahrb. Abth. f. System. Bd. iv. Heft iv. 1889. 2). Ueber den ungleichen Entwicklungsgang der Salzwasser- und der Susswasserform von Palaemonetes varians. 106. Brook, G. On the rate of development of the common shore-crab (Carcinus maenas). Ann. Mag. Nat. Hist. (5). Vol. xiv. 107. Brook, G. Lucifer-like Decapod Larva. Proc Bog. Soc. Edinburgh. Vol. xv. (Calliaxis larva). 1887-1888. 108. Brooks, W. K. The Embryology and Metamorphosis of Sergestidae. Zool. Anz. Jahrg. iii. 1880. 109. Brooks, W. K. Lucifer, a study in Morphology. Phil. Trans. London. Vol. clxxiii. 1882. LITERATURE. 327 110. Brooks, W. K. The Metamorphosis of Penaeus. Johns Hopkins Univ. Circ. Vol. ii. 1882. See also Ann. Mag. Nat. Hist. (5). Vol. xi. 1883. 111. Brooks, W. K. A preliminary abstract of researches by W. K. Brooks and F. H. Herrick on the life history of Stenopus. Johns Hopkins Univ. Circ. Vol. viii. Abstr. in Journ. R. Micro. Soc. 1889. 112. Brooks, W. K. and Wilson, E. B. The first Zoaea of Por- cellana. Stud. Biol. Lab. Johns Hopkins Univ. Baltimore. 1881. 113. Claus, C. Zur Kenntniss der Malacostrakenlarven. Wilrz- burger Naturw. Zeitschr. Bd. ii. 1861. 114. Conn, H. The significance of the larval skin of decapods. Stud. Biol. Lab. Johns Hopkins Univ. Vol. iii. 115. Conn, H. Evidence of a Protozoaea stage in crab development. Johns Hopkins Univ. Circ. Vol. iii. See also Arm, Mag. Nat. Hist. (5). Vol. xiii. 1884. 116. Couch, R. Q. On the Metamorphoses of the Decapod Crus- taceans. 11th Ann. Report of the Roy. Cornwall Polytechnic Society. Falmouth, 1848. 117. Czerniawski, W. Megalopa-like Crustacean larvae (Russian). Trud. Russk. Entomolog. Obshch. Estestv. Tom xi. 1880. 118. Czerniawski, W. Crustacea decapoda Pontica littoralia (Russian). Zapiski Charkoffsk. Obshch. Estestv. Suppl. II. 1884. 119. Dohrn, A. Untersuchungen etc. 6. Zur Entwicklungsgesch- ichte der Panzerkrebse (Decapoda Loricata). Zeitschr. f. Wiss. Zool. Bd. xx. 1870. 120. Dohrn, A. Untersuchungen etc. 10. Beitrage zur Kenntniss der Malacostraken und ihrer Larven. (Amphion, Lopho- gaster, Portunus, Pandalus, Galathea, Elaphocaris.) Zeitschr. f. Wiss. Zool. Bd. xx. 1870. 121. Dohrn, A. Untersuchungen etc. 11. Zweiter Beitrag zur Kenntniss der Malacostraken und ihrer Larvenformen (Zoaeae). Zeitschr. f. Wiss. Zool. Bd. xxi. 1871. 122. Du Cane, C. On the Metamorphoses of the Crustacea. Ann. Mag. Nat. Hist. Vol. iii. 1839. 123. Ehrenbaum, E. Zur Naturgeschichte von Crangon vulgaris. Fabr. Sonderbeilage zu den Mittheil. der Sektion f. Kilsten- u. Hochseefischerei des Deutschen Fischer ei-Vei'eins. Berlin^ 1890. 328 CRUSTACEA. 124. Faxon, W. On the development of Palaemonetes vulgaris. Bull. Mus. Comp. Zool. Harvard College. Cambridge. Vol. v. 1879. 125. Faxon, W. On some points in the structure of the embry- onic Zoaea. Bull. Mus. Comp. Zool. Harvard College. Cambridge. Vol. vi. 1880. 126. Faxon, W. On some young stages in the development of Hippa, Porcellana, and Pinnixa. Bull. Mus. Comp. Zool. Harvard College. Cambridge. Vol. v. 1879. 127. Faxon, W. A revision of the Astacidae. Mem. Mus. Comp. Zoology Harvard College. Cambridge, 1885. Vol. x. (Young stages of Cambarus.) 128. Gegenbaur, C. Organisation von Phyllosoma etc. Midler's Archiv. f. Anat. und Phys. 1858. 129. Goldi, Em. A. Studien iiber neue oder weniger bekannte Podophthalmen Brasiliens. Archiv. f. Naturgesch. Jahrg. lii. 1886. (Young of Dilocarcinus septemdentatus.) 130. Gourret, P. Considerations sur la faune pelagique du Golfe de Marseille etc. Ann. Mus. H. Nat. Marseille. Tom. ii. (Pontonia, Inachus, Xantho. Porcellana, Dromia, Pisa, Lambrus, Pilumnus, Pinnotheres.) 131. Haswell, W. A. Note on the Phyllosoma Stage of Ibacus Peronii, Leach. Proc. Linn. Soc. Neio South Wales. Vol. iv. 1879. 132. Herrick, F. H. The development of Alpheus. Johns Hopkins Univ. Circ. Vol. vii. 1888. 133. Herrick, F. H. The abbreviated metamorphosis of Alpheus, and its relation to the conditions of life. Jolms Hopkins Univ. Circ. Vol. vii. 1888. 134. Huxley, T. H. The Crayfish. An introduction to the study of Zoology. Internal. Scientific Series. Vol. xxviii. London, 1880. 135. Joly, N. Etudes sur les Moeurs, le Developpement et les Metamorphoses d'une petite Salicoque d'Eau douce (Caridina Desmarestii) etc. Ann. Sci. Nat. (2). Tom. xix. 1843. 136. Kroyer, H. Monographisk Fremstilling af Slaegten Hippo- lytes Nordiske Arter etc. Copenhagen, 1842. K. DansH Vidensk. Selskab. Naturvvh og mathem. Af. Bd. ix. (Larvae of Hippolyte, Homaras, and Cymopolia.) 137. Mayer, P. Zur Entwicklungsgescbichte der Decapoden. Jen. Zeitschr. f. Naturw. Bd. xi. 1877. LITERATURE. 329 138. Mayer, P. Carcinologische Mitth. 9. Die Metamorphosen von Palaemonetes varians Leach. Mitth. Zool. Stat. Neapel. Bd. ii. 1881. (With a note on the newly-hatched Squilla larvae.) 139. Mercanti, F. Sullo sviluppo postembrionale della Telphusa fluviatilis. Laf. Bull. Soc. Entom. Ital. Anno xvii. 1885. See also Archiv. Ital. de Biol. Tom. viii. 140. Muller, F. Die Yerwandlung der Porcellanen. Archiv. f. Naturgesch. Bd. xxviii. 1862. 141. Muller, F. Die Verwandlung der Garneelen. Archiv. f. Naturgesch. Bd. xxix. 1863. 142. Muller, F. Ueber die Naupliusbrot der Garneelen. Zeitschr. f. Wiss. Zool. Bd. xxx. 1878. 143. Muller, F. Palaemon potiuna. Ein Beispiel abgekiirzter Entwicklung. Zool. Am. Jahrg. iii. 1880. 144. Packard, A. S. Notes on the early larval stages of the fiddler Crab (Gelasimus pugnax) and of Alpheus. Amer. Natur. 1881. See also Ann. Mag. Nat. Hist. (5). Vol. viii. 145. Kathke, H. Zur Entwicklungsgeschichte der Decapoden. Archiv. f. Naturgesch. Bd. vi. 1840. (Homarus, Pagurus, Galathea, Hyas.) 146. Richters, F. Die Phyllosomen. Ein Beitrag znr Entwick- Ilungsgeschichte der Loricaten. Zeitschr. f. Wiss. Zool. Bd. xxiii. 1873. 47. Ryder, J. A. Metamorphoses of Homarus americanus. Amer. Nat. Vol. xx. 48. Sars, G. 0. Om Hummerens postembryonale Udvikling. Forhandl. Vidensk. Selshahs. Christiania, 1874. 149. Sars, G. 0. Bidrag til Kundskaben om Decapodernes For- vandlinger. I. Nephrops, Calocaris, Gebia. Archiv. f. MatUem. og Naturvid. Bd. ix. 1884. 150. Sars. G. 0. Bidrag til Kundskaben om Dekapodernes For- vandlinger. II. Lithodes, Eupagurus, Spiropagurus, Gala- thodes, Galathea, Munida, Porcellana (Nephrops). Arrht'r. f. Mathem. og Naturvid. Bd. xiii. 1889. 151. Sars, G. 0. Bidrag til Kundskaben om Decapodernes For- vandlinger. III. Fam. Crangonidae. Archiv. f. Mathem. og Naturvid. Bd. xiv. 1890. (Crangon, Cheraphilus, Pontophilus, Sabinaea und Sclerocrangon boreas). 152. Smith, Sidney J. The early stages of Hippa talpoidea. Tranf. Connecticut Acad. Arts and Sci. Vol. iii. 1877. 330 CRUSTACEA. 153. Smith, Sidney J. The early stages of the American Lobster (Homarus americanus Edwards). Trans. Connecticut Acad. Arts and Set. Vol. ii. 1873. 154. Smith, Sidney J. The metamorphoses of the Lobster and other Crustacea. Rep. U.S. Fish. Comm., 1871-1872. Washington, 1873. 155. Stuxberg, A. Karcinologiska Iakttagelser. Ofvers. k, Svenska Vetensk. AJcad. Fbrhandl. Bd. xxx. 1874. (Stenorhyn- chus, Carcinus, Portunus, Galathea, Hippolyte, Palaemon, Pachybdella.) 156. Westwood, J. 0. On the supposed Existence of Metamor- phoses in the Crustacea. Phil. Trans. London, 1835 (Gecarcinus). 157. Willemoes-Suhm, E. v. Preliminary Remarks on the Develop- ment of some Pelagic Decapods. Proc. Roy. Soc. London. Vol. xxiv. 1876. APPENDIX TO LITERATURE ON DECAPODA. I. Brooks, W. K., and Herrick, F. H. Embryology and Meta- morphosis of the Macrura (Stenopus, Alpheus). Johns Hopkins Univ. Circ. Vol. xi. 1892. II. Cano, G. Sviluppo postembryionale dei Cancridi. Bull. Ent. Ital. Tom. xxiii. III. Cunningham, J. T. The Development of Palinurus vulgaris. Journ. Mar. Biol. Assoc. Vol. xi. 1891. IV. Hensen, H. J. On the Development and the Species of the genus Sergestes. Proc. Zool. Soc. London. 1896. V. Rosenstadt, B. Untersuchungen iiber die Organisation und postembryonal Entwicklung von Lucifer Reynaudii. Zool. Jahrb. (A?iat.). Bd. ix. Anisopoda. 158. Claus, C. Ueber Apseudes Latreillii Edw. und die Tanaiden. Arb. Zool. Inst. Wien. Bd. v. 1884. 159. Claus, C. Ueber Apseudes Latreillii Edw. und die Tanaiden. II. Arb. Zool. Inst. Wien. Bd. vii. 1887. 160. Dohrn, A. Untersuchungen etc. 7. Zur Kenntniss vom Bau und der Entwicklung von Tanais. Jen. Zeitschr. f. Natuno. Bd. v. 1870. Isopoda. 161. Bate, C. Spence. On Praniza and Anceus and their Affinity to each other. Ann. Mag. Nat. Hist. (3). Vol. ii. 1858. LITERATURE. 331 162. Beneden, E. van. Recherches sur Fembryogenie des Crust,! I. Asellus aquaticus. Bull. Acad. Roy. Delg. (2). Tom. xxviii. 1869. 163. Dohrn, A. Die embryonale Entwicklung des Asellus aquations. Zeitschr. f. Wiss. Zool. Bd. xvii. 1867. 164. Dohrn, A. Untersuch. etc. 4. Entwicklung und Organisation von Praniza (Anceus) maxillaris. Zeitschr. f. Wiss. Zool Bd. xx. 1870. 165. Fraisse, P. Die Gattung Cryptoniscus Fr. Miiller. Arb. Zool. Inst. Wilrzburg. Bd. iv. 1878. 166. Fraisse, P. Entoniscus Cavolinii n. sp. nebst Bemerkungen iiber die Umwandlung und Systematik der Bopyriden. Arb. Zool. Inst. Wilrzburg. Bd. iv. 1878. 167. Giard, A. et Bonnier, J. Contributions a l'utude des Bopyriens. Trav. Inst. Zool. Lille. Tom. v. 1887. 168. Hesse, E. Memoire sur les Pranizes et les Ancees. Ann. Sci. Nat. (4). Tom. ix. 1858. 169. Kossmann, R. Die Entonisciden. Mitth. Zool. Stat. Neapel. Bd. iii. 1882. 170. Kossmann, R. Studien iiber Bopyriden. II. Zeitschr. f. Wiss. Zool. Bd. xxxv. 1881. 171. Kossmann, R. Studien iiber Bopyriden. III. Mitth. Zool. Stat. Neapel. Bd. iii. 1882. 172. Muller, F. Entoniscus Porcellanae, eine neue Schmarotze rassel. Archiv. f. Naturgesch. Bd. xxviii. 1862. 173. Muller, F. Brucbstiicke zur Naturgeschichte der Bopyriden. Jen. Zeitschr. f. Naturw. Bd. vi. 1871. 174. Rathke, H. Untersuchungen iiber die Bildung und Entwick- lung der Wasserassel. Leipzig, 1832. 175. Schiodte, J. C. and Meinert, Fr. Symbolae ad monographiam Cymothoarum crustaceorum isopodorum. Nat. Tidskrift. (3). Bd. xii. and xiii. 1879-1883. 176. Walz, R. Ueber die Familie der Bopyriden etc. Arb. Zool. Inst. Wien. Bd. iv. 1882. Amphipoda. 177. Claus, C. Die Platysceliden. Wien, 1887. CHAPTEli XX. PALAEOSTRACA. Under the name Palaeostraca, Steinmann and Doderlein have united the groups of the Trilobita, the Gigantostraca, and the Xiphosura. There can hardly be any doubt, as Dohrn (No. 11) and others have shown, that these three groups are closely connected. The Xiphosura (among which the genus Limulus, as sole living representative, is of special interest) show, in the shape of the cephalo-thorax, especially in Belinurus, a striking resemblance to the Trilobites, which are also recalled in the ontogenetic stages of Limulus. On the other hand, the Gigantostraca (Eurypterus, Pterygotus), in the division of the body into regions and in the structure of the limbs, are closely allied to Limulus. Like the latter, they possess an anterior cephalo- thoracic region, with six pairs of limbs, some, being chelate, taking part, by means of their broadened coxal portions, in the work of mastication. The masti- catory region is bounded posteriorly by a lower lip known as the metastoma (in Limulus by paired chilaria, Fig. 158, cJi, p. 345, and Fig. 159 B, p. 346). The region which follows the cephalo-thorax, the pre-abdomen, consists of six segments, the leaf-like limbs of which served for respiration, Avhile there follows posteriorly a post-abdomen, consisting of six limbless segments and the telson. This latter region, in Limulus, is in a reduced condition. If wo attempt to classify the Palaeostraca under the Crustacea, as has repeatedly been done, we shall be compelled to widen our conception of the latter group. The Crustacea, as a group, appear to be charac- terised by the possession of two pre-oral pairs of antennae, which, in the adult, take no part in the work of mastication, and besides their locomotory function serve chiefly as sensory organs.* The * The Crustacea are further distinguished from the Palaeostraca by tbe structure <>f the mouth-parts, which are developed typically in the form oi' mandibles and maxillae, and in the position of the paired lower lip (parag- oatha), which appears in various groups behind tin; mandibles and in front • •f the maxillae. In the adult Crustacean, Hie maxillae can hardly he regarded as locomotory organs. PALAEOSTRACA. 333 Xauplius stage, further, is typical of their development. The Palaeo- straca, on the contrary, seem to lack both these characteristics. We shall therefore, perhaps, be more accurate in our determination of the systematic position of the Palaeostraca if we consider them, not as true Crustacea, but as a distinct group, nearly related to the Crustacea, which branched off independently from the ancestral form (Protostraca) before the first typical characters (the two pairs of antennae and the Nauplius stage) had developed (p. 315). A feature which is very common among the Palaeostraca, but does not occur to the same extent among true Crustacea, is the frequent fusing of the posterior segments of the body to form a distinct region (pygidium). This is evidently an adaptation to the habit of rolling up that part of the body. Among the Palaeostraca, the lowest grade of development is found in the Trilobites, as is evident from the more homonomous segmentation of the post-cephalic region, and, according to Walcott, the uniform character of the numerous limbs. The same author has shown (No. 5) that the limbs of the Trilobites exhibit in their structure remarkable agreement with the typical Crustacean limb. The former are biramose (Fig. 149), with a five- or many- jointed endopodite (en) ending in a claw, and a two- to three-jointed exopodite (ex). On the outer side of the coxal (basal) joint, spiral epipodial appendages (ep), thought to be gills, are attached.* It may be pointed out that, in Limulus also, the biramose character of the Crustacean limb finds expression in the presence of an * [The recent researches, more especially those of BexCHXB (No. II.), on the limbs of Trilobites show that these appendages, with the exception of the highly specialised antennae, were but slightly modified in the different body-regions. Typically each appendage was biramons, consisting of a two-jointed protopodite, whose coxal joint in the more anterior limbs formed a masticatory blade, i five jointed endopodite, and a strongly setiparous exopodite with an expanded 1ms.i1 joint, and a multiarticulate palp. On the pygidium, these appendages were more lamellate, and closely resemble those of the larval Apns, being typically phyllo- podiform, and it is these limbs that Beechkii regards as indicative of the primitive type of limb-structure. Anteriorly, the endopodites assume a more cylindrical form. — Ed.] Fig. 140. — Diagrammatic cross-section through a trunk -limb of a Trilobite (after Walcott, from Lang's Text-book), en, endopodite; ep, epipodial appendages; ex, exopodite; d, intestinal canal; r, tergum ; p, pleura. 334 PALAEOSTRACA. appendage which may he regarded as an exopodite, on the sixth pair of limhs (Fig. 158, x, p. 345), as well as in the form of the abdominal limbs (av a2). While the Palaeostraca in one direction join on to the Crustacea and to their hypothetical ancestors, the Protostraca, they are further of great interest as probably the original group from which the air-breathing Arachnida developed. The view that the Arachnida, and above all the Scorpiones, are closely related to Limulus — first put forth by Strauss-Durkheim, and more recently established on a firmer basis by Kay Lankester (No. 16) — appears supported by so many points of agreement in the structure and the development of the two groups, that we cannot refuse to accept it; it will be more fully discussed below. Only a few of the ontogenetic stages of the fossil Palaeostraca have been preserved through the favourable character of the stone enclosing them. The ontogenetic stages of many Trilobites are, however, known, and in some cases it has been possible to put together complete series, so that Ave are able to solve many important problems in the metamorphosis of the Trilobites. The ontogenetic stages of the Gigantostraca, on the contrary, have not as yet been observed. I. Trilobita. Notwithstanding the more recent researches of Ford (Nos. 2 and 3), Walcott (No. 6), Matthew (No. 4), the earlier investigations of Barrande (No. 1), which are here followed, still provide the founda- tion for our knowledge of the metamorphosis of the Trilobites.* Baiuiaxde distinguished four ontogenetic types which, however, he considered as merely provisional. Three of these methods of development may be regarded as modifications of one type, while the Agnostus type seems quite distinct from the others. 1. Type in which the adult pygidium develops late. A very complete metamorphosis, consisting of many consecutive stages, was established by Barrande for Sao hirsuta (Fig. 150), a form belonging to his first 'ontogenetic type. The youngest known stages (^4) still differ greatly in appearance from the adult. They are exceedingly small (two-thirds of a millimetre in diameter), rounded, and disc-.shaped, and as yet show no distinct segmentation of the body. The latter consists chiefly of the rudiment of the future cephalic region, in which the glabella is already marked oil* from the cheeks by dorsal furrows. The posterior border of the * LSee BbBCBSR, Append. Lit. Trilobita, No. I. — Ed.] TYPE IN WHICH THE ADULT PYGIDIUM DEVELOPS LATE. 335 glabella is not yet distinct. At its sides, close to the anterior margin of the body, two arched indentations can be recognised; these, it may be conjectured, are connected with the rudiments of the eyes (a). The rudiment of the cephalic shield forms the chief part of the body at this stage. The posterior section is small and shows the rudiments of a few indistinct segments. A few tooth-like projections of the posterior margin are to be regarded as the pleura of these segments. This region contains the rudiment of the whole of the future thorax and pygidium. JL B k Fig. 150. — Five ontogenetic stages ot Sao Mrsuta (after Barrandk). A, youngest stage, B, somewhat older stage with more distinct demarcation of the cephalic region. C, stage with two free thoracic segments. D, stage with seven free thoracic segments. E, stage with twelve free thoracic segments, a, rudiment of eye ; g, facial suture ; p, transitory pygidium. In a later and somewhat larger stage (B) the cephalic region has become marked off by a distinct boundary from the posterior region. In this latter, the rudiments of the segments have become more distinct, and they have increased in number. In the succeeding stages (C, D, E) the thorax first makes its appearance as a distinct body-region, being formed from the anterior segment-rudiments of 336 PALAEOSTRACA. the posterior growing mass, the most anterior of these rudiments becoming segmented off as the free, movable thoracic somites. Fresh thoracic segments are successively set free from the anterior end of the posterior mass, which itself continues to grow and form new segment-rudiments. This posterior mass of incompletely sepa- rated segment-rudiments is nothing more than the budding zone of the still undeveloped thoracic segments, and must not be confounded with the pygidium of the adult. It was called by Barrande the "pygidium transitoire," and is distinguished externally from the adult pygidium by the fact that the latter, in Sao and Dahnanites, has an unbroken margin, while the posterior margin of the transitory pygidium shows freely-projecting teeth which are connected with the Fio. 151.— Five ontogenetic stages of Olenellus asaphoides (after Fobd). A and B, younger stages. C and D, older stages. E, adult form, a, cheek-spine ; b, inner spine ; c, optic rudiment ; d, swelling within that rudiment ; p, transitory pygidium. development of the pleura of the free thoracic segments. Only after the full number of free thoracic segments (in Sao 17) is attained does the adult pygidium develop; this, in Sao, is very short. While the full segmentation of the adult is in this way gradually attained, the cephalic region, apparently through modifications brought about simply by growth, undergoes transformation which causes it to approach nearer to the shape of the adult head-shield. The limbs, the occipital furrow, and the cheek (genal) spines appear. The glabella becomes more distinct, and the transverse furrows appear »n it, giving a suggestion of segmentation. Finally, the facial suture (i?, A in pyx, Ogygia, and Triarthrus. A still more abbreviated meta- morphosis seems to characterise PlO. l.r>2.— Three stages in the development of Trinucleus crnwtus (after Barrande). A, youngest stage, consisting merely of the cephalic shield and the pygidium. /-', stage with one free thoracic seg- ment. C, stage with four free thoracic segments. TYPE WITH EARLY DEVELOPMENT OP THE ADULT PYGIDIUM. 339 Barrande's fourth type. The cephalic region and the thorax are here already completely developed; the pygidium, on the contrary, is somewhat undeveloped, the number of segments composing it being still incomplete. We must class under this type Paradoxides, the species of Dalmanites which belong to the Hausmann group, a few species of Phacops, Proetus, Asaphus, etc. 2. Type with early development of the adult pygidium. This division corresponds to the second of Barrande's ontogenetic types, to which belong Agnostus and Trinudeus. The youngest known stages consist merely of the rudiments of the cephalic shield and the pygidium. The latter, although still incomplete, has essen- tially the characters of the adult. Metamorphosis is thus limited to the development of the thoracic region, which takes place in such a way that, as in forms already described, successive free segments become cut off from the anterior part of the pygidium. The other modifications consist in the increase in the number of the rudiments of the segments in the pygidium, and in the more perfect shaping of the cephalic region. In Trinudeus, for example, the characteristic rows of pores of the "limb" of the head-shield are developed. This form of development, as contrasted with the earlier type, must be regarded as more specialised. In view of the evident importance of the presence of the pygidium, jE. B. we cannot greatly wonder that the modification of the posterior segments of the body to form this structure was shifted to quite an early stage. In the small number of segment - rudiments in the first stages, the abstriction of the tho- racic segments and the development of new rudi- ments of segments at its anterior end, the pygidium, in the younger stages of this type, closely resembles the transitory pygidium of the types described above. It is, however, distinguished from these *by the fact that, in form, it more nearly approaches that of the adult. It is evident that no sharp distinction can be made between these two types of development The ontogenetic stages of Agnostus and of Trinudeus strikingly recall certain Fig. 153 —Four stages in the development of Agnoti (after Barrande). A, youngest stage, consisting of the cephalic shield and the pygidium. B, stage with the rudi- ments of two thoracic segments. 0, stage with two fully formed thoracic segments. D, adult form. 340 PALAEOSTRACA. early stages in the development of the germ-band in the Scorpiones, as described by Metschnikoff and others (Vol. iii.). In the latter also are found at first an anterior and a posterior region of the body, a few small free segments, arising successively, being cut off from the posterior region and intercalated between the two regions. Similar stages are found in the Araneae. We must, however, bear in mind that the free segments of this germ-band are later drawn into the head, and are evidently represented in the Trilobites by the segments of the glabella. These cannot, therefore, be homologised with the free thoracic segments of the Trilobites. A certain similarity is, however, brought about by the presence of a large posterior section from which the trunk-segments arise later. Barrande pointed out that most species of Trilobites in which metamorphosis occurs belong to the older strata of the Bohemian Silurian, while, in the more recent strata, young stages of Trilobites are much less frequently found, although, in these same strata, the number of species is richer, and the conditions for the preservation of these delicate forms are also to some extent favourable. Barrande therefore seems justified in conjecturing that metamorphosis was perhaps replaced by direct development in the later and perhaps more specialised Trilobites. II. Xiphosura. Limulus polypkemus is the only Xiphosurid whose ontogeny has as yet been investigated.* The metamorphosis of L. mollucamis is still unknown, hut it should he mentioned that Willemoes-Suhm (No. 31) believed that a pelagic Crustacean larva obtained off the Philippines by the Challenger, which resembled the Cirripede larvae (p. 209), could be referred to L. mollucamis. He, however, changed his opinion later, and classed the larva among the Cirripedes. 1. Oviposition, Cleavage, and Formation of Germ-layers. The eggs of L. polypJiemm are laid on the seashore in holes dug in the sand half-way between high and low water mark (Kikgsley, No. 14), or even near the former. L. rotandicauda and L. mollu- canus, on the contrary, do not deposit their eggs, but carry them about attached to the swimmerets. The eggs, when laid, are enclosed in a very thick leathery membrane composed of several layers, which has been described as the chorion, and is perhaps not formed in the ovary itself (Dohrn, No. 11), but in a special portion of the genital ducts. As the embryo increases in size, this membrane bursts, bo that, in the later stages, the embryo is covered solely by the later formed blastodermic cuticle. For information as to the first ontogenetic processes in the egg of Limulus wc arc entirely dependent on the short accounts of OsBORN • L. longiqwnus has also recently been investigated by KlSHlNOUYE [Zool. . 1 1, No. 869), but we are not here able to give his statements in detail [see App. Lit. on Xiphosura, No. II. See note, p. 343.] OVIPOSITION, CLEAVAGE, AND FORMATION OF GERM-LAYERS. 341 (No. 22), Brooks and Bruce (No. 10), and Kingsley (Xo. 15). Judging from these, considerable agreement with the ontogeny of the Arachnida, especially the Scorpiones, appears to prevail. In tracing the first stages of development we shall chiefly follow the more recent observations of Kingsley. The first cleavage-nucleus lies, surrounded by formative yolk, near the centre of the egg.* Repeated division gives rise to a large number of cleavage-nuclei which become distributed within the egg before cleavage of the egg-contents (formation of definite cell-areas) takes place. This distribution is not regular, for the cell-nuclei wander to the surface most abundantly at the point where, in later stages, the first rudiment of the embryo will appear. Since a demarcation of the cleavage-cells takes place first at this point, the cleavage is apparently meroblastic (discoidal) in character, and thus recalls the processes observed in the Scorpiones (Vol. iii., Figs. 1 and 2). Finally, however, the whole egg breaks up into segmentation -spheres which are chiefly composed of food-yolk elements ; each, however, contains a cleavage-nucleus surrounded by protoplasm. The wandering to the surface of the cleavage-nuclei mentioned above leads, by the demarcation of cells in that region, to the formation of the blastoderm. The latter is thus first completely formed at the special point referred to, and in these stages presents the appearance of an accumulation of closely b packed cells at this pole, while the rest of the surface of the egg is — -s^*^ •vp^j^^ - w Still covered with large Fjo 154-_Cr0S3 section through the germ -disc of Cells filled with food- Limulus showing the formation of the germ -layers ,, T ,, , (after Kingsley). b, blastopore (primitive groove); yolk. It appears that, ^ yoik_cens . eCj ectoderm < m, mesoderm. in later stages also, a blastodermic thickening is retained at this spot after the blastoderm- formation has advanced over the rest of the surface ; this thickening may be compared with the primitive cumulus (Claparede) of the spider's egg. While the formation of the blastoderm is still going on, a cuticular secretion takes place at the surface of the blastoderm-cells, this leading, as in many Crustacea (p. 118), to the development of the * [Kingsley surmises this, but was unable to find the first cleavage-nucleus. — Ed.1 342 PALAEOSTRACA. blastodermic cuticle. This cuticle, which attains a considerable thickness and, at later stages, when the chorion has been shed, is the only covering of the embryo, is marked out into polygonal areas which correspond to the limits of the individual blastoderm-cells concerned in its secretion. Not all the cleavage-cells enter into the formation of the blasto- derm. Many remain, filled with food-yolk, near the centre of tbe egg. The sum total of these so-called yolk-cells represents the entoderm. The blastoderm, on the other hand, contains the elements of the future ectoderm and mesoderm.* The next processes, which must be described as gastrulation, and which lead to the formation of the germ-band, now start from the primitive cumulus. At the middle of this latter there first appears a round depression, the blastopore, which soon becomes triangular and then lengthens. A primitive groove thus forms (Fig. 155 A). Simultaneously, behind the primitive cumulus, a second blastodermic thickening arises, connected with the former; this, in superficial figures, appears as a white patch. The primitive groove, which lengthens posteriorly, soon stretches into this second prominence. During this process the proliferation of mesoderm-cells takes place from the primitive groove, these cells spreading out below the ectoderm (Fig. 154). The groove here yields only mesoderm and no entoderm. The extension of the mesoderm-elements below the ectoderm, which proceeds from the primitive groove, appears in superficial figures as a clear area surrounding the latter (Fig. 155 A). This area soon extends, and the whole of the clear region under which the mesoderm lies must be regarded as the commencing embryonic rudiment, and may now be called the germ-disc. At its centre, the primitive streak can be recognised, although this is much less distinct in the stages which now follow than when it first appears. * [According to the latest researches of Kingsley (No. I.), each surface-cell divides tangentially into a smaller external and a larger internal cell ; the latter cells are rich in yolk and yield the entoderm, while the smaller cells yield the mesoderm. In surface view the primitive cumulus appears like a pit, but this is merely an optical effect produced by the greatly thickened centre. From this point and from the primitive streak which extends backwards from it, we find a proliferation of mesoderm-cells. KlSHINOUYB (No. II.) regards the ectoderm as having a double origin, that of the ventral surface arising from the blastoderm, while that of the dorsal surface arises from the yolk. The mesoderm, he states, arises in three ways \ that of the cephalo-thorax from the deeper cells of the blastoderm, and thai of the abdomen from the primitive streak; a third portion arises from the yolk-cells and possibly gives rise to the blood-corpuscles.— Ed.] DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 343 2. Development of the external form of the body. The germ-disc, whose form is now that of an elongated oval, and the bilateral symmetry of which is marked by the remains of the indistinct primitive groove, next becomes divided by a transverse furrow into an anterior cephalic area and a posterior post -oral thoracoabdominal region. A second transverse furrow very soon occurs behind the first, cutting off from the thoraco-abdominal region the most anterior thoracic segment. This stage, in which the body consists of a rounded cephalic portion, a still unsegmented posterior region, and a trunk-segment intercalated between these two, strikingly recalls a similar stage observed in Scorpio. The median furrow extends anteriorly into the cephalic segment, while posteriorly it is lost in the unsegmented region of the body. New thoracic segments now successively separate from the posterior region, until the number of free thoracic segments amounts to six. A similar stage has been observed in the Araneae (Yol. iii., Fig. 25 A). Fig. 155. — Two embryonic stages of Limulus (after Kingsley). A, stage with primitive groove. B, stage with rudiments of limbs. 1-6, the six thoracic limbs; a, anus; b, primitive groove (blastopore) ; do, dorsal organ ; me, germ-disc with subjacent mesoderm- layer ; m, mouth ; n, neural groove ; alf rudiment of the operculum (first abdominal limb). We now distinguish (Figs. 155 B and 162) a semicircular, anterior cephalic region, six thoracic segments, and a posterior abdominal region, also semicircular in outline. The rudiments of the limbs (the chelicerae, 1, and the five following pairs, 2-6) very soon appear on the thoracic segments, at first as button-like prominences. The smallest of these are the rudiments of the chelicerae, and each subsequent pair is larger than the one in front of it. The oral and anal apertures (m and a) are marked by ectodermal depressions. The first of these lies in the cephalic region, and consequently in front of the first pair of limbs (chelicerae), which belong to the first thoracic segment (Packard, Kingsley). In later stages, the 344 PALAEOSTKACA. mouth changes its position, shifting further back, so that it comes to lie behind the chelicerae, between the second pair of limbs (Fig. 156). Between the oral and anal apertures, which, according to Kingsley (No. 14) are peculiarly elongate, runs the neural groove (Fig. 155, ?i), which occupies the place of the primitive groove, that has now disappeared. The whole of the long germ-disc is bordered laterally by a thickened wall (Fig. 156, r), in which we recognise the first rudiment of the cephalo-thoracic shield. The first rudi- ments of the most anterior pair of limbs of the abdomen (a,, operculum) very soon appear. Even at this stage, on either side and beyond the limits of the germ-disc, on the level of the fourth thoracic segment, two rounded thickenings (Fig. 155 B, do), can be recognised; these probably correspond to the so-called dorsal organ (Watase). The following stage (Fig. 156) shows the thoracic limbs more strongly developed and each bent on itself inwards and downwards. There are now the rudiments of two pairs of leaf-like abdominal limbs (av a2) which are distin- guished from the thoracic limbs by their form, by their position near the median line, and by the manner of their appearance. They become separated from the germ-disc by an infolding of the body-sur- face taking place below and behind them (Kingsley). At this stage, Packard thought that he could recognise an indication of the boundaries of the seg- ments on the lateral parts of the yolk near the germ-disc ; this, however, Kingsley was unable to confirm. The embryonic rudiment lies, up to this time, as a gradually extending flat disc on the sphere of yolk. The latter now begins to change into the dorsal half of the embryo by the absorption of the nutritive materia] (Fig. 157). The abdominal region (ab) is PlO. r,<;.— Embryo of Limulus (after Dohrn). 1-6, the six pairs of thoracic limbs ; a,, fust abdominal limb (operculum); a.,, second abdominal limb ; 06, Abdomen flexed rentrally; brj> ventral chain of ganglia ; c, blastodermic cuticle ; d, chilaria ; w, mouth ; r, margin of the future cepbalo-thoracic shi.ld ; x, exopodite of the sixth thoracic limb. DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 345 still very short, its posterior end being flexed downwards (Dohrx, cf. Fig. 156, ab). The embryo now becomes covered with a delicate cuticular integument, which is cast off by ecdysis in the next stage and then comes to lie within the space which is enclosed by the blastodermic cuticle. The next stage (Fig. 158) is characterised by the transverse splitting of the chorion, the two halves of which remain for a long time attached to the egg as a hemispherical shell. The embryo moves within the space enclosed by the blastodermic cuticle, which is now increased by the sea-water taken into it. The cuticle is thus stretched, and the cell-like mosaic on its surface disappears. The limbs now gradually develop more in the direction of their final C Fig. 157. — Embryo of Liviulus (after Watase). 1-6, the six thoracic limbs ; o„ first abdominal limb (operculum) ; az, second abdominal limb; ab, abdo- men ; do, dorsal organ ; an, position of the rudiment of the lateral eye (Watase) ; c, blastodermic cuticle ; x, exopodite of the sixth thoracic limb. Fio. 158.— Limulus embryo, lateral view (after Kingsley). 1-6, the six thoracic limbs; «,, first abdominal limb (opt- rculum) ; a2, second abdominal limb ; ab, abdomen ; th, chilaria ; do, dorsal organ ; x, exopodite of the sixth thoracic limb. shape, their segmentation and the rudiments of the pincers becoming apparent. The outer appendage of the coxa (x) of the sixth pair of limbs, usually regarded as the exopodite, appears. Behind these limbs the rudiments of the paired lower lip, known as the chilaria (ch, metastoma), become apparent close to the middle line of the body ; since this appendage has no ventral ganglion of its own, and does not correspond to any mesoderm-segment (Kingsley, No. 14), it should not be regarded as a limb.* An indication of segmentation now appears in the abdominal region (Figs. 157, 158, ab). * [According to Kishinouye, this pair of appendages has a ventral gangHmi ,,| its own, and is therefore to be considered as a pair of limbs. Kingsley (No. III.) believes that Kishinouye mistook the operculum for the metastoma. — Ed.] 346 PALAEOSTRACA. With the gradual diminution of the food-yolk and the develop- ment of the dorsal region, the adult form becomes evident in the embryo at the next stage. The anterior region of the body, which consists of the cephalic segment and the six thoracic segments added to it, has now the form of a shield, although its dorsal side is still swollen and hemispherical. The dorsal side of this region of the body now exhibits a segmentation corresponding to the six thoracic segments; this is brought about by the re-distribution of the food-yolk (rudiment of the enteron) and by the formation of the mesodermal septa which grow inwards. On each side of the body, six outgrowths of the yolk-laden entoderm (hepatic rudiments), separated by mesodermal septa, can be recognised, the distal end of the lobes being again secondarily branched (Fig. 158). The \\> ^ '--*. Fig. 159. — Two stages in the development of Limulus (after Kingrley, from Lang's Text-book). A, young embryo, just hatched in the Trilobite stage, seen from the dorsal side. B, embryo a short time before hatching, seen from the ventral side. anterior lobes now no longer lie transversely, but radially. The rudiments of the eyes have also become distinct. Those of the middle eyes are originally situated ventrally (Packard, No. 23), but soon shift over the anterior margin of the cephalo-thorax on to its dorsal surface. The rudiments of the lateral eyes correspond in position (Watase) to the fourth hepatic lobe, and lie on the inner side of the dorsal organ described by this author (Fig. 157). The abdominal region now shows distinct segmentation, which, however, according to Kingsley, only affects the internal organs, while the ectoderm appears untouched by it (?). Nine abdominal segments in all can be distinguished (Fig. 160), the last representing the tudiment of the caudal spine, which is surrounded laterally by DEVELOPMENT OP THE EXTERNAL FORM OF THE BODY. 347 ma the incurved segments in front of it (Fig. 159 A). At the base of this segment lies the anal aperture. The limbs now grow more and more like those of the adult, but the teeth are still wanting on the coxal joints, which are modified into masticatory ridges. The two anterior pairs of abdominal appendages, behind which a third pair has become recognisable, have attained the typical biramose form by the development of a small inner lobe (endopodite — Fio\ 158, a a Fig. 159 B). The stage at which the embryo emerges from the blastodermic cuticle is known as the Trilobite stage (Figs. 158 A, 159). The cephalo-thoracic shield is now natter and wider, and has lost all traces of segmentation. It is distinctly divided by two longitudinal furrows into a central region with a well-marked keel and two lateral regions. The large eyes are situated in the furrows separa- ting the median from the lateral lobes, and a semicircular ridge connects the median with the lateral eyes. The abdominal region still appears divided up into segments. Six movable thorns can be recognised on its lateral margins (Fig. 160) from the second segment to the seventh. The rudiment of a fourth pair of abdominal limbs now appears. The first )air is changed into the oper- culum and a fusion of the inner margins of the two exo- )odites takes place, accom- mied by a degeneration of the endopodite. In the second and following abdominal limbs the rudiments of the branchial lamellae now appear, only four )eing at first apparent on each limb. Their number is increased later yy the budding of new lamellae at the bases of the limbs. The mdopodites of these limbs become divided up into three segments. The young which hatch at the Trilobite stage are endowed with L'eat activity, and already burrow in the sand like the adults. By leans of their abdominal limbs they can swim about freely, and an msequently occasionally taken in tow-nets, A. Agassiz having found Fig. 160.— Larva of Limulus, at the Trilobite stage (after Watase). do, dorsal organ ; I, hepatic diverticula; la, lateral eye ; ma, median eyes ; s, rudiment of the caudal spine. 348 PALAEOSTRACA. a specimen three miles from the coast. After the first moult, they pass into a stage which is distinguished from the preceding by the richer ramification of the hepatic tubes, by the closer fusion of the abdominal segments, and by the lengthening of the caudal spine. This stage (Fig. 161) is worthy of notice on account of its resemblance to the Hemiaspid genus Prestwichia. The later stages already show the characters of the adult in all respects except sexual differentiation. The latter seems to develop very late (according to Lockwood, in the third or fourth year). Until this occurs, the males resemble the females, they then develop strong terminal claws on the second pair of limbs instead of pincers. 3. Formation of the Organs. A. Nervous System and Sensory Organs. The ventral chain of ganglia develops in the form of two _._ ectodermal thicken- ings (Fig. 15G, bg) on either side of the median line, which enclose be- tween them a thin- ner portion of the ectoderm. In a superficial view, the latter appears as a neural groove, al- though it is not actually sunk below the surface. The lateral halves of the ventral ganglionic chain become the future longitudinal commissures, which thicken segmentary to form the ganglia, and become de- tached from the ectoderm from be- fore backward. The transverse commissures seem to arise by the Invagination of the enclosed median ectodermal area (neural groove), FlO, Kil.— Older larval stage of Lirmdus (after Watask), hepatic, diverticula; la, lateral eye; ma, median eyes. NERVOUS SYSTEM AND SENSORY ORGANS. 349 Kingsley. The ventral chain of ganglia thus develops essentially in the manner described above as typical for the Crustacea (p. 160), and apparently for all Arthropoda. There are, in all, eight distinct pairs of ventral ganglia in the embryo, the six anterior pairs occurring on the six thoracic segments. As the oral aperture shifts back,°the anterior pair of post-oral ganglia, those belonging to the chelicerae, moves gradually forward, so that finally these ganglia become incor- porated with the oesophageal commissures proper, the nerves which run to them originating close to the edge of the brain. The thoracic portion of the ventral chain of ganglia is at first elongated. Only in later stages does it become concentrated anteriorly, the circular form characteristic of the adult arising at the same time by the shifting apart of its two halves. Xothinc nn of the vascular is as yet sheaths of ..-• so, JO, to,. — so. accurately known as to the this part of the nervous system in Limulua, but Kingsley (No. 15) states that they pass through a stage in their de- velopment which, in the Scorpiones, persists through- out life, where they are found lying upon the oesophageal com- missures without completely envel- oping them. As to the development of the supra- oesophageal ganglion proper, or brain, which gives off the optic and some integumental nerves (the frontal nerves \ we have only a few short statements by Patten (Nos. 28 and 29) and Kingsley (No. 15), which throw but little light on the actual facts concerning the com- plicated processes which here appear. According to Pattf.n, with whom Kingsley agrees, the rudiment of the brain consists of three consecutive pairs of ganglia, which represent a pre-oral continuation of the ventral chain of ganglia lying in the cephalic region of the body. (This is in agreement with the Bohemc drawn up by Patten for the Scorpiones and Acilius, and described in the section on the nervous system of the Insecta, Vol. iii. ) Each of these three pairs corresponds to a pair of ectodermal invaginations which originally lie OB t he outer sides of the ventral chain, and give rise to the optic ganglia. In LtmuUll, the two anterior pairs of invaginations are said to unite to form the median JO, Fig. 162. — Diagram of the germ-disc of Limulvs with the lateral, segmental sensory organs (after Patten), a, anus ; m, mouth ; r, margin of the cephalo-thoracic shield ; s, sensory organ of the third cerebral segment, so^sog first six lateral, segmental sensory organs; soit dorsal organ, 1-6, the six pairs of thoracic limbs. 350 PALAEOSTRACA. eyes and the nerves connected with them, while the third pair, in Scorpio, yields the optic ganglia of the lateral eyes, but, in Limulus, is related to a small sensory organ (Fig. 162, s). The lateral eyes belong to the third (Patten), fifth (Kingsley), or, according to other authors, the fourth thoracic segment, and thus represent post-cephalic structures. Each optic ganglion is. continued posteriorly into a nerve strand, a kind of lateral nerve running along the outer side of the limb-rudiments, and connected with a sensory organ in each segment (Fig. 162, sox-soe). According to Patten, these sensory organs (with the exception of the rudiments of the lateral eyes) have usually only a temporary importance, and soon disappear. Kingsley, on the contrary, holds that the first pair (sox) yields the median eyes, the second a peculiar and as yet undescribed sensory organ, while the third disappears ; from the fourth is developed the dorsal organ (so4) of Watase, which persists for a long time ; the fifth passes into the compound lateral eyes, and the sixth, finalty, degenerates. Until these statements have been confirmed by more detailed observations, they must be regarded Avith some scepticism.* We must refer the reader to Patten's treatise cited above for details as to the development of the brain in Limulus, his account of which, in the absence of satisfactory figures, is hardly com- prehensible. We are also unable, on account of the fragmentary character of the statements made on the subject, to decide how far Packard's more recent researches (No. 27) as to the structure of the brain in Limulus can be brought into agreement with Patten's views. Packard emphasises the fact that the cheliceral ganglion in Limulus does not fuse with the brain, but remains distinct. The brain proper sends off only the nerves to the median and lateral eyes, and two pairs of integumental nerves (frontal and inferior frontal nerves). It consists of three pairs of lobes : those of the lateral eyes, the median eyes, and the cerebral lobes proper. In the absence of figures, it is impossible to obtain a clear idea of the true relationships of these cerebral lobes. Patten considers that the development of the median eyes in Limulus closely resembles the processes observed in Scorpio. Here, as in the latter, there are two (or perhaps, according to Patten, four) invaginations which, shifting backward, unite in the median line to form a sac with a common posterior aperture. This sac gives off anteriorly two tubular processes, the blind ends of which, becoming apposed to the hypodermis of the cephalo-thorax, are transformed into the median eyes, while the parts of the processes that remain change into the optic nerves. The development of the lateral eyes, now made more fully known by Watase (No. 30), is much simpler. We can trace the lateral eyes back to a highly -differentiated part of the ectoderm (hypo- dermis). The actual rudiment of the compound lateral eye (Fig. 163, la) is a thickened point of the ectoderm (hypodermis) near the so-called dorsal organ (do), which apparently belongs to * [Kingsley, in his latest work, withdraws his account of the segmental sense organs, and would now regard those structures as glandular. KlSHlNOUYB (No. 111.) funis four parts in the developing brain, and regards the lateral i phalio, not thoracic. — Ed.] NERVOUS SYSTEM AND SENSORY ORGANS. 351 a posterior thoracic segment (according to Packard and Patten the third, according to Kingsley the fifth; cf also Fig. 157, au and do). The optic area proper (Fig. 163, la) is bounded at its dorsal (median) and ventral (lateral) margins by folds (df and vf), which, converging posteriorly till they meet, form the letter V. At the point at which the two folds meet, a short tubular invagination of the ectoderm is formed, extending below the surface, so that the V-shaped rudiment becomes Y-shaped. These folds, which are composed of very large cells (Fig. 164, df and vf), yield new cell- material for the formation of younger ommatidia at the margin of the optic area. Each ommatidium (om) arises in the form of a Fig. 163.— Transverse section through the Trilobite stage of Livmlvs (after Watase). 1> flrst ancl second gill-bearing limbs ; o, operculum ; K, branchial lamellae ; m, muscle ; d, food-yolk. 356 PALAEOSTRACA. 4. General Considerations. We have already (p. 352) pointed out that the Xiphosura are evidently somewhat closely related to the Trilobites. Certain characters which tend to connect the two are specially apparent in the young larvae of Limulus (Trilobite stage), their presence in the adult also being undeniable. Here, as in the Trilobites, the anterior region of the body is divided by two longitudinal furrows into a middle and two lateral portions. The position of the lateral eyes agrees in the two groups, but the occurrence of ocelli (median eyes) has not yet been fully established in the Trilobites. The two groups further agree in the general configuration of the cephalo-thorax, in the bending under of the anterior part of the cephalic shield, and in other points. Our present knowledge of the cephalic limbs of the Trilobites seems to indicate that they resembled in structure those of the Gigantostraca.* Four pairs of jaw-feet have been found in the latter, the last pair having been specially developed, and having grown out into broad, oar -like limbs. Various fossil forms of Xiphosura connect Limulus with the Trilobites ; among these, Belinurus, by the shape of its cephalo-thoracic (cephalic) shield, which is continued into long cheek -spines, strikingly recalls the Trilobite family, Trinudeidae. Although the Palaeostraca thus appear to form a single group based upon natural relationship, a certain distant relationship to the Crustacea is, as we have already pointed out (p. 315), undeniable. This is supported chiefly by the structure of the Trilobite limbs, described more in detail by Walcott (No. 5),* and by the biramoee character of the abdominal limbs of Limulus. We have already stated why we regard the Palaeostraca and the Crustacea as groups of equivalent value, but have abstained from uniting them. We, however, think ourselves justified in assuming that they both had their origin in a common racial group, the Protostraca, which may perhaps also be regarded as the racial group of the Onychophora — Myriopoda series. The relationship which appears to exist between the Palaeostraca and the air-breathing Arachnida deserves closer consideration. As early as 1829, Strauss-Durkheim emphasised the near relationship between Limulus and the Arachnida. He based his view chiefly on the radial arrangement of the limbs, on the common sternal plate, and on the presence of an inner endoskeleton (the endosternum), ■ * See Editorial note, pp. 333 and 360. GENERAL CONSIDERATIONS. 357 lying between the ventral cord and the intestine, which affords attachment to numerous groups of muscles. The views of Strauss- Durkheim were supported on embryological grounds by Ed. van Beneden (No. 8) in 1871, and J. Barrois in 1878. Claus also, in 1876,* expressed his belief that "the air-breathing Arachnida may have been derived from the polygnathan Merostomata (Trilobites, Eurypterida, and Xiphosura)." Huxley had also expressed himself in a similar way as to the genealogical connection between the Arachnida and the Merostomata. The near relationship of the two groups has been recently demonstrated in more detail by Ray Lankester (No. 16), by means of a careful comparison of the structure of Limulus with that of Scorpio. Although Ray Lankester, as we think, goes decidedly too far in insisting that Limulus should be regarded as an Arachnid, he still deserves credit for having established on a broader base the view that the two forms belong to the same phylogenetic series. It appears to us that the structure of the limbs used for respiration and adapted to aquatic life, the absence of the Malpighian vessels, and, moreover, their connection with the Trilobites, which are further removed from the Arachnida, afford sufficient cause for giving the Xiphosura a more independent position. We cannot here undertake to enter further upon the palaeonto- logical evidence in favour of the genealogical connection between the Arachnida and the Palaeostraca. We can only mention that imong the Gigantostraca, which are nearly related to the Xiphosura, forms are found which, in appearance and in the segmentation of the posterior region of the body, stand still nearer to the Scorpiones than does Limulus itself. We must confine ourselves to a short consideration of the points of comparison between Limulus and the Scorpiones. In both forms we recognise an anterior region of the body (cephalo-thorax) carrying six pairs of limbs, and covered by a dorsal shield, having on its upper side two median eyes, and, nearer the edge, paired lateral eyes. The median eyes of Limulus and of Scorpio agree so closely in structure, that we cannot doubt that bhey are homologous. We might take the same view of the lateral >yes, even though the numerous unicorneal lateral eyes of Scorpio essentially differ from the remarkable lateral eyes of Limulus, the composition of which is very primitive. In this case we should lave to regard the lateral eyes of Scorpio as of a modified type. * Untera. zur Geneal. Grundl. dcr Crustac. Systems. 358 PALAEOSTRACA. Of the six pairs of limbs belonging to the cephalo-thorax, the most anterior (the chelicerae) shift during development in front of the oral aperture, while the pair of ganglia belonging to them enter into closer connection with the brain. The five pairs of limbs behind these serve for locomotion and mastication. While, in Limulus, the coxae of all the limbs appear enlarged and have toothed masticatory ridges, in Scorpio, only the pedipalps and the first two pairs of ambulatory limbs have basal blade-like structures. An upper lip (rostrum, camerostome), lying in front of the mouth between the chelicerae, is common to the two forms ; as also is an original paired projection behind the sixth pair of limbs, which, in Limulus, is represented by the chilaria, but, in Scorpio, fuses to form a small pentagonal plate found in front of the operculum. Behind the cephalo-thorax, in Scorpiones, comes a pre-abdomen consisting of seven segments, which is followed by a post-abdomen of five segments, with a terminal poisonous spine. If we regard the long caudal spine of Limulus as the representative of the poisonous spine, we shall consider the region usually described as the abdomen to be the equivalent of the pre-abdomen and the post-abdomen of Scorpio. This region, in Limulus, consists of eight fused segments. Taking into account certain fossil forms (Belinurus), however, we may conjecture that the last of these segments, strictly speaking, corresponds to several segments which have not separated. The resemblance between Limulus and Scorpio finds expression in the development of the abdominal limbs. In both forms, rudiments of limbs appear in the embryo on the six anterior abdominal segments. Of these, the most anterior pair changes, in Limulus, into the large plate-like structure known as the operculum, which is also slightly developed in Scorpio, and on the inner side of which lie the genital apertures. The five posterior pairs of limbs, in Limulus, are leaf- like, and carry gills, and thus serve for respiration. In Scorpio, the most anterior pair develops into pectines, while the four other pairs seem to disappear at the time when the lung-sacs develop. In the assumption of a near relationship between Limulus and Scorpio, an important part is played by the supposed transformation of the gills of the former into the lungs of the latter. In structure the two organs show remark- able agreement. While, however, careful consideration shows how a transition may well have taken place from the book-like gills of Limulus to the lung-book of Scorpio, there are certain difficulties which Ray Lankester (Nos. 16 and 20), Kincslf.y (No. 14), and MacLeod (No. 21) have sought to set aside by means of various hypotheses. Ray Lankestee himself gave up his original, very artificial theory, and has since derived the lung of Scorpio from the gill of GENERAL CONSIDERATIONS. 359 Limulus by complete invagination. The limb was not only, according to this view, invaginated as a whole, but each single branchial lamella was separately invaginated, the limb, as it were, growing into the body instead of growing out, so that the interstices between the lamellae then became the lamellae of the book-lung. This view somewhat resembles that of Kingsley. It appears to us that MacLeod's view is the simplest explanation, and the one best agreeing with the facts. MacLeod (No. 21) starts from the assumption that the lamellae of the book-like gills are homologous with those of the book-like lungs. The gill- bearing limbs of Limulus are usually closely pressed against the ventral side of the abdomen. The branchial lamellae develop only on those upper surfaces which are pressed against the body. The ventral side of the limbs, in Limulus, already shows a depression corresponding to the branchial lamellae. If we imagine the respiratory limbs shifted further apart than they are in Limulus, and the edges of the depression just alluded to fused with the edges of the leaf- like limb, a closed space, the lung-sac, will thus be formed. The free posterior edge of the limb would then become the anterior margin of the stigma belonging to this sac. By this assumption, MacLeod is able to explain certain features in the structure of the Arachnid lung : e.g., the facts that some of the lung-lamellae are free not only at their posterior edges, but at their lateral edges also, and that the corresponding lung-sacs of the two sides are connected, etc.* The agreement in the internal anatomy of the two forms is no less remarkable than that in the outer segmentation of the body and in the structure and functions of the limbs. The presence of an endosternum has already been mentioned. We shall here merely recall to mind the large, branched liver, opening through several efferent ducts into the intestine, the retiform rudiment of the genital glands, the presence of a circum-oesophageal, arterial vascular ring accompanying the oesophageal commissure (and in Limulus developing into an actual vascular sheath), and, finally, the presence of a gland (brick-red gland of Limulus, coxal gland) on the coxae of the fifth pair of limbs (third ambulatory limbs). The agreement which we have thus pointed out in the structure and development of Limulus and of the Arachnida is so remarkable that we can hardly avoid the conclusion that the two forms are genetically related. We therefore accept the view that the Arachnida have developed from the Palaeostraca through adaptation to terrestrial life. It may be further mentioned here that adaptation to life in fresh water, and possibly on land (?) had, perhaps, taken place even in the Gigantostraca them- selves. According to Zittel (No. 7), they are found in the coal formations associated with land plants, scorpions, insects, fish, and fresh water amphibia. * [Bernard (App. to Lit. on Trilobita IV.), on the other hand, has endea- voured to show that the lung-books of Scorpio developed in situ, as adaptations to the circulatory system. He further claims that the Arachnids originally had more stigmata than there were respiratory limbs in Limulus.— Ed.] 360 PALAEOSTRACA. A striking feature in some of the forms (Eurypterida) in this group is the scale- like marking of the body-plates. A homology is naturally suggested, and has been carried out by Ray Lakkkstek, between the coxal glands of the Arachnida and Xiphosura and a pair of Crustacean nephridia. This could only apply to the shell-gland, which belongs to the segment of the second maxillae, i.e. to the fifth limb-bearing segment. We should then have to homologise the chelicerae of the Arachnida with the first pair of antennae of the Crustacea, an assumption which seems to us somewhat daring, and not sufficiently supported by the structure and development of the brain in the two groups. There is the less need for a homology between the shell-gland of the Crustacea and the coxal gland of Limuhis and Scorpio, as we have to imagine each body-segment originally provided with a pair of glands of this kind, an assumption which appears to be justified by a comparison with Peripatus.* LITERATURE. I. Trilobita. 1. Barrande, J. Systeme Silurien du Centre de la Bohem<\ Prague et Paris. Premiere Partie. Tom. i. 1852. Ami Supplement Tom. i. 1872. 2. Ford, S. W. On some Embryonic Forms of Trilobites from the Primordial Rocks at Troy, N. Y. Amer. Journ. Sci. (3). Vol. xiii. 1877. 3. Ford, S. W. On additional Embryonic Forms of Trilobites from the Primordial Rocks of Troy, N. Y., etc. Amer Journ. Sci. (3). Vol. xxii. 1881. 4. Matthew, G. F. Sur le developpement des premiers Trilobites. Ann. Soc. Roy. Malacologique de Belgique. Tom. xxiii. 1888. 5. Walcott, C. D. The Trilobite: New and Old Evidence relating to its Organisation. Bull. Mus. Comp. Zool. Harvard Coll. Cambridge. Vol. viii. 1880-1881. 6. Walcott, C. J). Fossils of the Utica Slate. Trans. Albany Inst. Vol. X. 1879 (?). Contains statements as to the metamorphosis of Triarthrus Becki. 7. ZlTTEL, K. A. Handbuch der Paliiontologie. Bd. i. Abtheil. ii. Milnchen and Leipzig, 1885. [* The new discoveries of the antennae and limbs of a Trilobite ( Triarthrus) have advanced our knowledge of these fos&ila and of their affinities so greatly, that the student must not rest content with the foregoing. The recent observa- tions of Bl i < HER and BERNARD (App. to Lit. on Trilobita, Nos. I., II., III., IV.) have taken the point off many of the author's comments on the valuable infor- mation given above. The text is translated as it stood on account of the matter it contains and the reader is merely referred to the more recent works en the subject. This .seems to be the only alternative to re-writing the whole chapter, Ed ] LITERATURE. 361 APPENDIX TO LITERATURE ON TRILOBITA. I. Beecher, C. E. Larval forms of Trilobites from the lower Helderberg group. Amer. Journ. Sci. (3). Vol. xlvi., pp. 142-147. A larval form of Triarthrus. T. C, pp. 378 and 379. The Larval Stages of Trilobites. Amer. Geol. Vol. xvi., pp. 166-197. II. Beecher, C. E. Thoracic Legs of Triarthrus. Amer. Journ. Sci. (3). Vol. xlvi., pp. 467-470. Appendages of the pygidium of Triarthrus. Op. tit. xlvii., pp. 298-300. Structure and Appendages of Trinucleus. Op. tit. xlix., pp. 307-311. The Morphology of Triarthrus. Op. cit. (4). Vol. i., pp. 251- 256. IIT. Bernard, H. M. The Systematic position of the Trilobites. Quart. Journ. Geol. Soc. Vols. 1. and li. IV. Bernard, H. M. The Comparative Morphology of the Galeo- didae. Trans. Linn. Soc. 1896. V. Matthew, W. J). Antennae and other Appendages of Triar- thrus Beckii. Amer. Journ. Sti. (3). Vol. xlvi. VI. Walcott, C. D. Note on some of the Appendages of the Trilobites. Proc. Biol. Soc. Washington. Vol. ix. II. Xiphosura. 8. Beneden, E. van. De la Place que les Limules doivent occuper dans la Classification des Arthropodes d'apres leur developpe ment embryonnaire. Ann. Soc. Entom. Belgique. Tom. xv. 1871. Ann. Mag. Nat. Hist. (4). Vol. ix. 1872. 9. Beneden, E. van. Beobachtungen iiber die ersten Stadien der Embryonalentwicklung von Limulus. Tageblatt der Jf.6. Versammlung deutsch. Naturf. u. Aerzte in Wiesbaden. 1873. p. 58. 10. Brooks, W. K., and Bruce, A. T. Abstract of Kesearches on the Embryology of Limulus polyphemus. Johns Hopkins Univ. Circ. Vol. v. 1885. 11. Dohrn, A. Zur Embryologie und Morphologie des Limulus polyphemus. Jen. Zeitschr. f. Nat. Bd. vi. 1871. 12. Gerstacker, A. Crustacea. Bronrt's Classen und Ordnungen des Thierreichs. Bd. v. Abth. i. 1866-1879. 362 PALAEOSTRACA. 13. Gulland, G. L. Evidence in favour of the View that the Coxal Gland of Liniulus and of other Arachnida is a Modified Nephridium. Quart. Journ. Micro. Sci. (2). Vol. xxv. 1885. 14. Kingsley, J. S. Notes on the Embryology of Limulus. Quart. Journ. Micro. Sci. (2). Vol. xxv. 1885. 15. Kingsley, J. S. The Ontogeny of Limulus. Zool. Anz. Jg. xiii. 1890. Also Amer. Natur. Vol. xxiv. 1890. 16. Lankester, E. Ray. Limulus an Arachnid. Quart. Journ. Micro. Sci. (2). Vol. xxi. 1881. 17. Lankester, E. Ray. On the Coxal Glands of Scorpio hitherto undescribed, and corresponding to the brick-red Glands of Limulus. Proc. Roy. Soc. London. Vol. xxxiv. 1882-1883. 18. Lankester, E. Ray, and Bourne, A. G. The minute Structure of the Lateral and the Central Eyes of Scorpio and of Limulus. Quart. Journ. Micro. Sci. (2). Vol. xxiii. 1883. 19. Lankester, E. Ray. On the Skele to-trophic Tissues and Coxal Glands of Limulus, Scorpio, and Mygale. Quart. Journ. Micro. Sci. (2). Vol. xxiv. 1884. 20. Lankester, E. Ray. New Hypothesis as to the Relationship of the Lung-book of Scorpio to the Gill-book of Limulus. Quart. Journ. Micro. Sci. (2). Vol. xxv. 1885. 21. MacLeod, J. Recherches sur la structure et la signification de l'appareil respiratoire des Arachnides. Arch. Biol. Tom. v. 1884. 22. Osborn, H. L. The Metamorphosis of Limulus polyphemus. Johns Hopldns Univ. Circ. Vol. v. 1885. 23. Packard, A. S., jr. The development of Limulus polyphemus. Mem. Bost. Soc. Nat. Hist. Vol. ii. 1872. 24. Packard, A. S. Further Observations on the Embryology of Limulus, etc. Amer. Nat. Vol. vii. 1873. 25. Packard, A. S. The Anatomy, Histology, and Embryology of Limulus polyphemus. Anniversary Mem. Bost. Soc. Nat. Hist. 1880. 26. Packard, A. S. On the Embryology of Limulus polyphemus. III. Amer. Nat. Vol. xix. 1885. 27. Packard, A. S. Further studies on the brain of Limulus polyphemus. Zool. Anz. Jg. xiv. 1891. 28. Patten, W. Segmental Sense Organs of Arthropods. Journ. Morph. Vol. ii. 1889. 29. Patten, W. On the Origin of Vertebrates from Arachnids. Quart. Journ. Micro. Sci. (2). Vol. xxxi. 1890. LITERATURE. 363 30. Watase, S. On the Morphology of the Compound Eyes of Arthropods. Stud. Biol. Lab. Johns Hopkins Univ. Vol. iv. 1890. Extr. in Quart. Journ. Micro. Sci. (2). Vol. xxxi. 1890. 31. Willemoes-Suhm, E. v. On a Crustacean Larva at one time supposed to be the larva of Limulus. Quart. Journ. Micro. Sci. (2). Vol. xxiii. 1883. APPENDIX TO LITERATURE ON THE XIPHOSURA. I. Kingsley, J. S. The Embryology of Limulus. Journ. Morph. Vols. vii. and viii. II. Kishinouye, K. On the Development of Limulus kmgispina. Journ. Sci. Coll. Imp. Univ. Japan. Vol. v. 1 892. III. Packard, A. S. Further Studies on the Brain of Limulus polyphemus, with Notes on its Embryology. Mem. Acad. Washington. Vol. vi. IV. Patten, W. On the Morphology and Physiology of the Brain and Sense Organs of Limulus. Quart. Journ. Micro. Sci. Vol. xxxv. V. Patten, W. Variations in the Development of Limulus polyphemus. Journ. Morph. Vol. xii. VI. Tower, E. W. The external opening of the brick-red Gland in Limulus polyphemus. Zool: Anz. xviii. ADDITIONAL LITERATURE ON PALAEOSTRACA IN GENERAL, I. Laurie, M. Eecent additions to our knowledge of the Euryp- terida. Nat. Sci. Vol. iii. II. Laurie, M. Anatomy and Eelation of the Eurypterids. Tr. Roy. Soc. Edinb. Vol. xxxvii. SUBJECTS INDEX. A. Abdominalia, 220. Acanthosoma, 263, 266. Acanthotelson, 319. Acetes, 266. Achtheres, 238. — percarum, 242. Acidaspis, 338. Acrothele, 77. Actinotrocha, 5. — branclriata, 5. Aegidae, 305. Aeglina, 338. Aetea, 12. Agnostus, 339. — nudus, 339. Albunea, 290. Alcippe, 218, 220, 230. Alcyonella = Plumatella. — f'ungosa, 33. Alcyonidium, 12. — albidum, 14. — duplex, 14. — gelatinosum, 14. — larva, 19. — mytili, 20. Alima, 297, 301-303. Alimerichthus, 302. Alpheidae, 271. Alpheus, 133, 170. — heterochelis, 275. — praecox, 275. Amathia, 12. Amphion, 275. Amphiones, 275. Amphipoda, 104, 118, 309. Amphitboe, 140, 241. Ampyx, 338. Amymone, 192. Anaspides, 319. Anceidae, 305. Anceus, 305. Anchorella, 117, 119,148, 238. Anelasma, 218, 228. — squalicola, 220. Anisopoda, 104, 304. Anomia, 76. Anomura, 104, 285. Apoderae, 218. Apodidae, 167. Apseudes, 104, 151, 166, 304. — Latreillii, 304. Apterura, 290. Apus, 104,105,159,165, 172, 196, 319. — cancriformis, 195,199. — productus, 146, 199. Archicerebrnm, 166. Archizoaea, 311. — gigas, 213. Aretlmsina, 338. Argiope, 65, 66. Argulus, 104, 105, 244. — foliaceus, 245. Arionellus, 338. Aristeus, 271. Artemia, 125, 200. Arthobranchiae, 173. Arthrostraca, 104, 136, 148, 247. Asaplius, 339. Ascopodaria, 91. Ascothoracida, 228. Asellus, 114, 118, 149, 173, 319. Aspergillum, 76. Astacidea, 251, 275. Astacus, 109, 113, 118, 128,154,172-175,178. — fluviatilis, 129-131, 155-163. Asterope, 173. Atremata, 77. Atrypidae, 77. Atyephyra,104,110,133, 159, 173. Avicularia, 49. B. Balanidae, 173, 209. Balanoglossus, 5. Balanoglossus Kupff'eri, 88. Balanus, 111, 126, 174, 210. — improvisus, 126. Barentsia, 91. Basipodite, 194. Belinurus, 332, 356. Birgus latro, 289. Blastodermic cuticle, 118. Bopyridae, 305. Bopyrus, 306. Bowerbaukia, 12. Brachiella, 117, 238. — Thynni, 148. Brachiopoda, 65-84. Brachyura, 104, 290. Branchiopoda, 104, 196. Branchipus, 104, 110. 125,146,159,162,16.^ 168,178,181,195,199, 318. Branchiura, 104, 244. Brick-red gland, 355. Brown body, 17, 28, 43, 44, 55, 58. Bryozoa Ectoprocta, 12- 64. Bryozoa Entoprocta, 91- 103. Buccinum, 220. Bugula, 12. — calathus, 15. — flabellata, 28. — larva, 25. — plumosa, 26. — simplex, 15. — turbinata, 15. Bythocaris, 274. Bythotrephes, 111, 114, 120, 204. C. Calanella, 194. Calanidae, 237. Caligidae, 237. Caligus, 117, 240. 366 SUBJECTS INDEX. Callianassa, 1 1 5, 284,285. — subterranea, 112, 114. Calliaxis, 284. Calocaris, 284, 285. Calyptopis, 253. Calyptopisstage,248,255. Cambarus, 277. Cancrion miser, 307. Canthocaniptus staphy- linus, 236. Caprella, 118, 140, 176. Carcirms maenas, 223, 291, 295, 296. Cardioblasts, 177. Caridea, 251, 271. Caridina Desmarestii, 274. Carina, 219. Cellularia, 12. Cellularina, 12. Centropages, 105. Cephalodiscus, 5, 88. Ceratapsis longiremis, 271. Ceratocaris, 318. Cetochilus,105,lll,120, 141,159,174,180,190. — septentrionalis, 233. — stage, 235. Chalimus, 240. Cheraphilus, 273. Chilaria, 332. Chilostomata, 12. Cliondracanthidae, 237, 239. Chondracanthus, 106, 111, 238. — cornutus, 243. Chonetes, 77. Chorion (Bryo. Ect.), 15. — (Crust.), 106, 205. Cirri pedia, 104, 126, 209. Cladocera, 104. Clavella, 117. Cleavage, types of (Crust), 108, 115. Complemental male, 230. Conchoderma, 219. Conocephalites, 338. Copepoda, 104, 119, 231. Copulation-cell, 119. Corona (Bryo. Ect.), 19. Corycaeidae, 232, 238. Coxopodite, 194. Crangon, 104. 118, 133, 162,167,173,175,273. — vulgaris, 274. Orangonidae, 271. Crania, 76. Craniidae, 77. Crisia, 12. Cristatella, 12 — mucedo, 53. Cromus intercostatus, 337. Crustacea, 104-332. Cryptocheles, 274. Cryptoniscus stage, 309, Cryptophialus, 218, 220. 230. Ctenostomata, 12. Cuma, 116. Cumacea, 104, 118, 119, 136, 148, 303. Cyclatella annelidicola, 100. Cyclopidae, 206. Cyclops, 120, 151, 160, 190. — stage, 119. Cyclostomata, 12. — larva, 31. Cyllene, 296. Cymothoa, 137,150,171, 176. Cymothoidae, 305. Cyphaspis, 338. Cyphonautes, 17. Cypridae, 105. Cypridina, 105, 205, 208, 255, 317. Cypris, 105, 206. — fasciata, 205. like larva, 216, 317. — ovum, 205. — reptans, 111. — stage, 209, 318. — vidua, 205. Cyrtopia, 253. Cystid, 15. Cvstigen half, 51. Cytheridae, 205, 208. D. Dalmanites, 336. — socialis, 337. Daphnia, 114. — longispina, 119, 146. — similis, 125, 165, 180. Daphnidae, 201. Daphnella, 114. Decapoda, 104, 118, 127, 152, 168. Deltidium, 77. Deudrogaster astericola, 228. Dias, 105. Diastopora, 12. Dichilestiidae, 237, 239. Dictyocaris, 318. Dilocarcinus, 296. Diphyes, 237. Disc, ciliated (Bryo. Ent), 93, 94. or retractile (Bryo. Ect.), 18. Discina, 77. Discinidae, 76. Dorsal organ ( Bryo. Ent. ) , 93, (Crust.), 141, 150, (Pal.), 344, 350. Dromia, 290. Drominea, 290. E. Ecardines, 73. Echinaster, 228. Ectocyst, 28. Edriopthalmata, 104. Elaphocaris, 263, 265. Endites, 195. Endocyst, 13. Endopodite, 194. Entomostraca, 104, 146. Entoniscidae, 305. Ephippium, 105. Epipodites, 195. Epistome, 65. Ergasilus, 151. Erichthina demissa, 257. Erich thoidina stage, 297. Erichthus stage, 297, 300, 303, Eriphia, 114, 173, 175, 178. — spinifrons, 133, 180. Eschara, 12. Escharina, 18. Estheria, 104, 105, 172, 201. Estheridae, 201, 318. Eucratea, 12, 40. — chelata, 23. Eucopepoda, 104. Eupagurus,118,132,l74, 286. — Bernhardus, 287. — Prideauxii, 110. Euphausia, 104,151,248, 254. Euphausiidae, 173, 247, 252, 253, 257, 318. Euryj items, 332. Eutyphis, 309. Exopodite, 194. Farella, 12. Klusl ra, 12. niniibranacea, 40. — truncata, 15. Flustrella, 12. SUBJECTS INDEX. 367 Flustrella hispida, 23. — larva, 21. Forked canal (Bryo. Ect.), 42. Formative mass, 51. Fredericella, 12. Frondipora, 12. Funiculus, 13, 50. Furcilia, 253. Fusus, 220. G. Gadidae, 237, 241. Galathea, 288. Galatliodes, 289. Gammaridae, 309. Gammarus, 104, 173. — fluviatilis, 111. — locusta, 111-118. — poecilurus, 140. — pulex, 111. Gangliogen, 170. Gebia Kttoralis, 284. Gecarcinus, 251, 296. Gelasimus, 296. Gigantostraca, 332. Glaucothoe, 288. Gnathia, 305. Gnathostomata, 104, 232. Gonerichthus, 300. Gonodactylus, 300. Grampsonyx, 319. Grapsion, 308. Gymnolaemata, 12. H. Halcyonellea, 12. Halocypridae, 205, 209. Halocypris, 255. Harpacticus, 111. Harpactidae, 206. Hessia colorata, 148. Hibernacula, 54. Hippa, 110. — talpoidea, 289. Hippolyte, 272, 273. — polaris, 274. Homarus, 114, 132, 151, 275. — americanus, 170, 276. Homola, 290. Horn era, 12. Hydrocephalus, 338. Hynienocaris, 318. Hyperia, 309. Hyperidae, 309. Hypopliorella, 12. Il)acus, 283. Ibla, 230. Ibla Cumingii, 230. — quadrivalvis, 230. Illaenus, 338. Intertentacular organ, 14, 56. Isopoda, 104, 118, 304. Jurine, 192. K. Kentrogen stage, 223. Kochlorine, 220. Labrum (Crust.), 156. Larval integuments, 118. LauraGerardiae,209,228. Lepadidae, 105. Lepas, 209, 217, 311. — australis, 212. — fascicularis, 212, 214. — pectinata, 215, 216. Lepralia, 12. — metamorphosis, 27. — Pallasiana, 24. — unicornis, 17. Leptaena, 77. Leptodora, 114, 146,203. — hyalina, 204. Leptostraca,104,152,252. Lernaea, 117, 237. — branchialis, 240, 241. Lernaeascus, 239. Lernaeopoda, 117, 119, 238. Lernaeopodidae, 241,306. Ligia, 104, 116, 149, 319. — oceanica, 117, 136. Limnadia, 172, 201, 203. Limnetis, 172, 202. — brachyura, 204. Limulus, 118, 209, 332, 340-360. — mollucanus, 340. — polyphemus, 340. — rotundicauda, 340. Lingula, 73-76. Liothyrina, 69. Lithodes, 289. Lithotrya, 218. Lonchophorus, 289. Lophogastridae,l73, 257. Lophophore (Brach. ), 70. — (Bryo. Ect), 41. Loricata, 278. Loxosoma, 91. — annelidicola, 100. — cochlear, 100. — Kefersteinii, 100. Loxosoma phascolosoma- tum, 100. — Raja, 100. — singulare, 100. Lucifer, 107, 127, 152, 165, 249, 257, 312. Lynceus, 205. Lysioerichthus, 300, 303. Lysiosquilla, 300, 303. M. Macrura, 104, 290. Maja, 294. Malacostraca, 104. Marestia, 296. Mastigopus stage, 263. Megalopa stage, 251. Megerlia, 73. Membranalimitans, 169. Membranipora, 12. — larva, 21. — pilosa, 14. — zostericola, 21. Merocytes, 112. Mctanauplius, 196, 209, 249. Microporella, 12. — malusii, 15. Micropyle apparatus, 150. Mitraria, 21. Moina, 106, 114, 123, 146, 159, 162, 173, 177, 180. — rectirostris, 147. Molluscoida, 84-90. Monolepis, 296. Munida, 289. Mysidae, 105, 257, 319. Mysis, 104, 117, 151 153, 159, 162, 168- 173, 177. — chamaeleo, 134. — fiexuosa, 257. — stage, 250. — vulgaris, 257. My to Gaimardii, 274. N. Nauplius stage, 119, 190, 249, 310-313. Nebalia, 104, 105, 118, 154, 194, 203, 247, 252, 312, 317. Neck gland (Crust.), 151. Neotremata, 77. Nephrops norvegicus, 277. Notodelphyidae, 105, 238. 368 SUBJECTS INDEX. Nuchal gland (Crust.), 151. Nyctiphanes, 253, 255. Obohis, 77. — labradorica, 75. Oeypoda, 296. Og'ygia, 338. Olenellus asaphoides, 336-338. Oniscns, 104, 116, 118, 140,151,159,160,177. Oniscus, 173, 175. — murarius, 140. Ooecia, 13, 49. Oostegites, 105. Orbiculoidea, 75, 77. Orchestia, 118, 140, 151, 181. Orthis, 77. Orthisina, 77. Ostracoda, 104. Ovicells, 13, 49. P. Palaemon, 104, 110, 115, 118, 131, 174, 273. — Potuini, 274. — serratus, 179. Palaemonetes, 110. — varians, 274. — vulgaris, 274. Palaemon idae, 271. Palaeocaris, 319. Palaeostraca, 315, 332- 363. Palimmis, 151, 278. — quadricornis, 279. Paludicella, 12. — Ehrenbergii, 48. Pandalus, 151. Pantopoda, 315. Paracopulation, 119. Paradoxides, 338. Paragnatha, 148. I'.napodopsis, 168. — eornuta, 117. Parasite, 104, 237. Parastacidae, 277. Paribacus, 283. Paterina labradorica, 75. Peotinatella, 12. Pectinella, 41. Pedicellina, 91. — echinata, 92, 98. iVdin-llinopsis, 91. Penaeidoa, 249, 257,267. Peaaena, 114, 152, 194, 248, 267, 273, 310. Peripatus, 167, 179, 314, 355, 360. Peteinura gubernata, 271. Petrarca bathyactidis, 228. Phacops, 339. Pherusa, 12. — tubulosa, 24. Philicthyidae, 237. Phoronidea, 1. Phoronis, 1-11, 84-89. — psammophila, 10. — australis, 82. Phronima, 309. Phylactolaemata, 12. — larva, 32. Phyllopoda, 104, 123, 196. Phyllosoma, 278-284. — Duperryi, 283. Pinnixa, 296. Platessa flesus, 240. Platysaccus crenatus, 266. Pleopoda, 106. Pleurobranchiae, 173. Plumatella, 12. — fruticosa, 35. — fungosa, 33, 38. — polymorpha, 33. Podobranchiae, 173. Podophthalmata, 104. Polyphemus, 111, 114. Polypide, 15. — (Bryo. Ect ), develop- ment of, 37. Pontellidae, 232. Pontophilus, 271, 273. Porcellana longicornis, 289. Porcellio, 114, 139, 175. Portunion maenadis,307, 308. Portunus, 118, 296. Praniza, 151, 305. Prestwichia, 348. Proboscidella, 76. Productidae, 76. Proetus, 338. Protegulum,- 75. Protopodite, 193. Protostraca, 314, 333. Prototremata, 77. Protozoaea stage, 119, 249. I'scuderichthus, 300. Pseudodeltidium, 77. Pseudosquilla, 300. Pterygotus, 882. Pterygura, 290. Ptychoparia Linnarssoni, • 337. Pygidium, 333. Pyriform organ (Bryo. Ect.), 18. R. Radicellata, 12. Ranininea, 290. Rathke's pyramids, 109, 128. Regeneration (Bryozoa Ect.), 55. Retepora, 12. Retinogen, 170. Rhabdopleura, 5, 88. Rhabdosoma, 309. Rhizocephala, 105, 221. Rhynchonella, 82. Rhynchonellidae, 77. S. Sabinea, 274. Sacculina, 112. — carcini, 221-228. Sagitta, QQ. Sao hirsute, 334-337. Sapphirina, 237. Scalpellum (balanoides, ornatum, Peronii, re- gium, rostratum, vill- osuni, vulgare), 230. — Stromii, 220. Scaphognathite, 151. Sceletina armata, 257. Schizomania, 77. Schizopoda, 104, 134, 152, 247, 253. Schizopoda - like stage, 250. Schizoporella, 12. Sciocaris telsonis, 266. Sclerocrangon boreas, 274. Scorpio (compared with Limulus), 357-360. Scrupocellaria, 12. Scuta, 219. Scyllarus, 278-284. Sergestes, 249, 263. Sergestidae, 257, 273. Serialaria lendigera, 31. Sida, 114. Sididae, 205. Siriella, 173. Solaster, 228. Spliaeronella Leuckhartii, 241. Sphaerothylacus poly- oarpae, 209. SUBJECTS INDEX. 369 Spherical organ (Crust.), 150. Spiriferidae, 77. Spiropagurus, 288. Sqnilla, 173, 303. Squillerichthus, 301. Squilloid stage, 301. Statoblasts, 49. Stenopus, 271. Stolonata, 12. Stolonifera, 12. Stomatopoda, 104, 247, 297. Stringocephalidae, 77. Stringocephalus, 66. Strophaeodonta, 77. Strophomena, 77. Sunamphithoe, 140, 176. Synagoga mira, 228. Syncerebrum, 166. T. Tanais, 104, 118, 119, 151, 304. Telotremata, 77. Telphusa, 251. — fiuviatilis, 296. Tendra zostericola, 15. — larva, 21. Terebratella corianica, 80. Terebratula, 65. Terebratulidae, 77. Terebratulina 65. — septentrionalis, 73. Terga, 219. Testicardines, 65. Thalassiiiidea, 284. Thecidiidae, 77. Thecidium, 65. — mediterranean!, 78. Thenus, 283. Thia polita, 291. Thoracica, 209. Thoracostraca, 104. Tracheliastes, 238, 244. Trachelifer, 285. Triarthrus, 338. Trichodactylus, 296. Triloba, 296. Trilobita, 332-334. Trin u cleidae, 356. Trinucleus, 339. — ornatus, 338. Trochophore larva, 86. Tropidoleptus, 77. Tubulipora, 12. — serpens, 32. U. Urnatella, 91. V. Valkeria, 13. Vesicularia, 12. — larva, 31. Vibilia, 309. Vibracularia, 49. Victorella, 12. Vitelline membrane, 106. Vitellophags, 134. W. Winter buds, 54. X. Xipliostira, 332, 340. Zoaea stage, 119, 193, 249, 310-313. Zooecium, 13. Zoontocaris, 288. AUTHORS INDEX, A. Agassiz, A. Linmlus, 347. Allman, G. J. Cyphonautes, 21, Pluraatella, 35. B, Baer, K. E. von. Crustacea, 313. Balbiani, E. G. Myriopoda, 119. Balfour, F. M. Brachiopoda, 81. Bryozoa Ect., 18. Crustacea, 318. Malacostraca, 312. Barrande, J. Trilobita, 334-340. Barrois, J. Bryozoa Ect., 15-40. Bryozoa Ent., 101. Liniulus, 357. Molluscoida, 88. Bate, C. Spence. Amphipoda, 309. Crustacea, 165. Decapoda, 257, 264, 271, 274, 275, 278, 286, 288, 291, 296. Isopoda, 306. Bate, C. Sfence, and Power, W. H. Crustacea, 326. Baub, G. Astacus, 160. Beechbr, C. E. Brachiopoda, 75. Trilobita, 333. Bell, Th. Crustacea, 319. BlMMBLEN, .1. K. VAN. Brachiopoda, 83. Beneden, E. van. Bryozoa Ect., 14, 15, 54. Br3vozoa Ent., 96. Copepoda, 147, 174. Crustacea, 106, 118, 122. Gammarus, 107, 115. Isopoda, 114-118,149. Limulus, 357. Mysis, 116, 152. Sacculina, 112. Beneden, E. van, and Bessels, E. Copepoda, 111, 116. Gammarus, 111, 115, 118. Beneden, P. J. and E. Mysidae, 257. Benham, W. B. Brachiopoda, 82. Phoronis, 11. Bergh, R. S. Mysis, 185. Oligochaeta, 138. Bernard, H. M. Apus, 167, 319. Limulus, 359. Bessels, E. Gammarus, 107, 115. Birge, E. A. Crustacea, 326. Blanc, H. . Crustacea, 106, 118. Blochmann, F. Bryozoa Ect., 42, 56. Crania, 79, 81. Insecta, 119. Boas, J. E. V. Asellus, 149. ( Sruttacea, 166. Decapoda, 271, 275, 290. Bobretzky, N. Astacus, 109, 127-134, 154, 159, 160, 174. Decapoda, 111, 168, 181. Oniscus, 116-118, 138, 140, 151, 159, 162. Palaemon, 110, 118. 131, 273. Bov/tchinsky, P. Crustacea, 117. Nebalia, 116. Parapodopsis, 168. Bovallius, C, 184. Brash, F. Bryozoa Ect., 33, 38, 39, 44, 49. Brauer, F. Apus, 146, 196, 199. Brancbipus, 108. Brook, G. Bracbyura, 296. Bkook, G., and Hoyle, W. E. Scbizopoda, 253. Brooks, W. K. Decapoda, 257, 266, 284. Lingula, 7->. Lucifer, 108, 127, 257- 263, 312. Stomatopoda, 302. Bkooks, W. K., and Bruce, A. T. Limulus, 354. Brooks, W. K., and Herrick, V. H. Decapoda, 271. Brooks, W. K., and Wilson, E. B. Crustacea, 827. AUTHORS INDEX. 371 Bullae, F. J. Cymothoa, 140, 150, 171, 176. Isopoda, 309. Oniscus, 162. BuRMEISTER, H. Chalimus, 240. Butschli, 0. Brachiopoda, 83. Crustacea, 177. Caldwell, W. H. Brachiopoda, 81. Molluscoida, 84. Phoronis, 1-8. Calman, W. T. Anaspides, 319. Cano, G. Crustacea, 330. Carriere, J. Crustacea, 171. Chambers, V. T. Estheria, 321. Claparede, E, Bugula, 25. Cyphonautes, 21. Limulus, 341. Loxosoma, 100. Phoronis, 10. Claus, C. Amphipoda, 309. Apseudes, 151, 304. Apus, 159, 196. Branchipus, 125, 159, 162/168, 177, 178, 181. Cirripedia, 209-220, 231. Cladocera, 107, 165, 205. Copepoda, 111, 233- 246. Crustacea, 106, 164, 173, 191, 194, 200, 314. Cymothoa, 151. Cypris, 205-207. Decapoda, 257, 263, 269, 271-296. Estheria, 201, 202. Halocypridae, 209. Lynceus, 205. Metazoaea, 293. Molluscoida, 84. Nauplius, 310. Schizopoda, 253, 254. Stomatopoda, 297-303. Zoaea, 312. Conn, H. W. Decapoda, 293. Cori, C. J. Bryozoa Ect., 55. Phoronis, 1, 8, 10. Phylactolaemata, 42. Coste, F., and Gerbe, Z. Decapoda, 278. Couch, R. Q. Decapoda, 278, 292. Cunningham, J. T. Crustacea, 330. Cuvier, G. Crustacea, 313. CZERNIAWSKI, W. Crustacea, 327. D. Dana, J. Decapoda, 257, 261, 263, 296. Schizopoda, 253. Danielssen, D. Cirripedia, 220. Darwin, Ch. Cirripedia, 217-219. Davenport, C. B. Bryozoa Ect., 38, 44. Bryozoa Ent., 103. Chilostomata, 55. Gymnolaemata, 38-41 , 48. Molluscoida, 88. Phylactolaemata, 33, 51. Delage, Yves. Cirripedia, 221-231. Della Valle, A. Gammarus, 105, 107, 111. Demade, P. Bryozoa Ect., 63. Deniker, J. Brachiopoda, 67. Dietl, E. Crustacea, 164. Doderlein, L. Palaeostraca, 332. Dohrn, A. Cirripedia, 212. Crustacea, 191, 192, 203. Cumacea, 151, 303. Dohrn, k.—contd. Daphnia, 146. Decapoda, 151, 266, 271, 275, 278, 286, 289, 293. Isopoda, 306. Limulus, 340, 344. Nauplius, 310, 313. Oniscus, 151. Palaeostraca, 332. Praniza, 151. Tanais, 118. Zoaea, 311. Du Cane, C. Crustacea, 327. E. Edwards.H.de Milne-. Crustacea, 165. Decapoda, 283, 288. Molluscoida, 84. Ehlers, E. Bryozoa Ect., 14, 21, 40. Cephalodiscus, 88. Molluscoida, 85, 88. Pedicellina, 91, 101. Ehrenbaum, E. Crangon, 273, 274. Ehrenberg, C. G. Cyphonautes, 22. Eschscholz, J. F. Decapoda, 289. F. Farke, A. Bryozoa Ect., 14, 56. Faxon, W. Decapoda, 277, 286, 289, 290, 292. Palaemonetes, 1 1 1 ,274. Stomatopoda, 303. Ficker, G. Estheria, 201. Foettinger, A. Bryozoa Ent., 96. Phoronis, 1, 4. Ford, S. W. Trilobita, 334. Fowler, G. H. Crustacea, 318. Molluscoida, 85. Rhabdopleura, 89. Fraisse, P. Crustacea, 331. Fritsch, J. A. Copepoda, 122. 372 AUTHORS INDEX. G. Gegenbaur, C. Crustacea, 179, 282. Phoronis, 5. Gerbe, Z. Decapoda, 278. Gerstaecker, A. Crustacea, 182, 278. Palaeostraca, 361. Giard, A., and Bonnier, J. Isopoda, 307-309. Goldi, Em. A. Crustacea, 328. GOITRRET, P. Decapoda, 290, 292. Grenacher, A. - Crustacea, 169, 172. Grobben, C. Cetochilus, 111, 120, 141, 159. Cirri pedia, 127. Cladocera, 105, 179, 202. Copepoda, 119, 233. Crustacea, 151, 166, 168. . Moina, 123, 146, 159, 173, 174. Groom, T. T. Cirripedia, 112, 126, 127. Grosglik, 9. Crustacea, 189. Grube, E. Limnetis, 202, 204. Gulland, G. L Limulus, 355. H. Hacker, V. Cyclops, 180. Hadpon, A. C. Bryozoa Ect., 38. Gymnolaemata, 55. Haeckel, E. Nauplius, 192, 310. IVnaeus, 114,127, 133. Hancock, A. Brachiopoda, 83. Cirripedia, 220. Babmsr, s. v. Bryozoa Ect., 14, 15, 19, 23, 44. 55. Harmer, S. F.— contd. Bryozoa Ent., 91, 97, 102. Cephalodiscus, 88. Loxosoma, 91. Molluscoida, 85, 88. Hartog, M. M. Cyclops, 324. Haswell, W. A. Decapoda, 283. Hatschek, B. Cyphonautes, 21. Crustacea, 166, 192, 313. Molluscoida, 85,87,88. Pedicellina, 91-96, 99, 101. Phylactolaemata, 47. Hensen, H. J. Crustacea, 330. Herbst, C. Crustacea, 165. H^rouard, E. Molluscoida, 90. Herrick, F. H. Alpheus, 110, 170,275. Decapoda, 133, 168. Hippa, 110. Homarus,114,132,151. Palaemonetes, 110. Hertwig, O. and R. Brachiopoda, 83. Hesse, M. Copepoda, 244. Isopoda, 306. HlLGENDORF, F. M. Stomatopoda, 303. Hincks, T. Bryozoa Ect., 14, 56 HOEK, P. P. C. Balanus, 111. Cirripedia, 220, 230. Copepoda, 111, 119, 174. HOFER, B. • Crustacea, 165. Hoyer, H. F. Brachiopoda, 67. Huxley, T. H. Crustacea, 165, 173, 1<»3. Herostomata, 357. II v \it, A. Bryozoa Ect., 63. I. Ischikawa, C. Atyephyra, 105, 134, 173, 178. See Weismann, A., and Ischikawa, C. Joliet, L. Bryozoa Ect , 13, 38, 40. Bryozoa Ent , 102. Joly, N. Phyllopoda, 321. JULLIEN, J. Phylactolaemata, 33. Jurine, L. Cladocera, 105. K. Kellikott, D. S. Argulus, 324. Kerhkrve, L. B. Crustacea, 183. Kin aha n, G. H. Decapoda, 292. KlNGSLEY, J. S. Crangon, 118,133, 151, 157-179. Decapoda, 133, 159, 168. Limulus, 340, 342, 350, 355. KlSHINOUYE, K. Limulus, 340, 342, 345, 350, 352. Kreinenberg, N. Annelida, 160. Knipowitscjh, X. Deiulogaster astericola, 229. KOLLAR, V. Copepoda, 242. KORRN, J., and Danielssen, D. Cirriitedia, 220. KOROTNEFF, A. Phylactolaemata, 32. Kossmann, R. Cirripedia, 220. Isopoda, -"507. Sacculina, 112. Kowa i. i:\sky, A. Brachiopoda, 65, 68. Crustacea, 179, 180. Phoronis, 1, 5. AUTHORS INDEX. 373 Kraepeltn, K. Bryozoa Ect., 33-36, 38, 40, 43, 49, 51, 54, 56. Krieger, K. R. Crustacea, 164. Erohn, A. Cirripedia, 213. Phoronis, 5. Kroyer, H. Copepoda, '240. Decapoda, 274, 276. Lacaze-Duthiers, H. DE. Laura Gerardiae, 229. Thecidium, 65, 67. Lang, A. Balanus, 111, 174. Cephalodiscus, 89. Lankester, E. Ray. Crustacea, 166, 167, 195. Limulus, 334, 355-360. Molluscoida, 84. Phoronis, 1. Lankester, E Ray, and Bourne, A. G. Limulus, 362. Laurie, M. Eurypterus, 363. Lebedinsky, J. Cladocera, 179. Daplmia, 125, 180. Eriphia, 133, 173, 178, 180. Lereboullet, A. Astacus, 109,118,154. Limnadia, 201, 203. Leuckart, R. Decapoda, 263. Phoronis, 11. Leydig, F. Araneae, 119. Copepoda, 234. Crustacea, 168, 179. Cyphonautes, 21. Gammarus, 111. Ludwig, H. Crustacea, 106. M. McCrady, J. Bracliiopoda, 73. McIntosh, J. P. Crustacea, 189. McIntosh, W. Molluscoida, 85. MacLeod, J. Limulus, 358. Marchal, P. Crustacea, 179. Masterman, A. T. Cephalodiscus, 89. Molluscoida, 85. Phoronis, 4, 5, 8. Matthew, G. F. Trilobita, 334. Matthew, W. D. Trilobita, 361. Mayer, P. Decapoda, 274, 293. Eupagurus, 110, 118, 132, 174. Isopoda, 309. Stomatopoda, 302. Meissner, G. Gammarus, 150. Merc ant i, F. Decapoda, 296. Mereschowski, C. Callianassa, 114. Metschnikoff, E. Bryozoa Ect, 21, 23, 31, 32. Cirripedia, 213. Crustacea, 180. Leptostraca, 252. Nebalia, 154. Phoronis. 1-6. Schizopoda, 253. Metzger, A. Copepoda, 240. Milne-Edwards, H. de. Decapoda, 283, 288. Molluscoida, 84. Morin, J. Astacus, 113, 114. Moi:se, E. S. Bracliiopoda, 81. Terebratulina, 65, 68. Muller, F. Amphipoda, 151, 309. Bracliiopoda, 73. Copepoda, 240. Corophium, 149. Crustacea, 151, 165, 192. Decapoda, 267, 286, 289, 290, 296. Lepadidae, 231. Muller, Y.—contd. Ligia, 151. Nauplius, 310. Stomatopoda, 297. Tanais, 151, 152. Zoaea, 310. Muller, Joh. Cyphonautes, 21. Phoronis, 4. Muller, O. F. Cirripedia, 213, 221. Nauplius, 192. Muller, P. E. Leptodora, 146, 203. N. Nassonow, N. B. Balanus, 111,126,174. Crustacea,lll,126,l74. Nebeski, O. Crustacea, 176. Kitsch e, H. Bryozoa Ect., 18, 25, 32,38-40,44,46,48, 49, 51. Loxosoma, 100. Noll, C. Cirripedia. 220. NORDMANN, A. V. Copepoda, 242. Norman, A. M. Synagoga mira, 323. Nusbaum. J. Cirripedia, 127. Crustacea, 108, 164, 168-172, 177. Ligia, 116, 136-140, 149, 319. Mysis, 134-136, 152, 159, 162, 176, 181, 257. Oniscus, 159,174,176. Nusbaum, Rosalie. Ligia, 151. Nussbaum, M. Cirripedia, 111, 127. O. Oehlert, D. P. Bracliiopoda, 67. Oka, A. Bryozoa Ect., 38, 43. Osborn, H. L. Limulus, 340. Ostroumoff, A. Bryozoa Ect., 13-56. 374 AUTHORS INDEX. Pack A it d, A. S. A sell us, 166. Caridea, 275. Crustacea, 163. Limuhis, 343-355. Pagenstecher, A. Amphipoda, 309. Cirripedia, 215-220. Phorouis, 11. Parker, G. H. Decapoda, 168, 171. Honiarus, 170. Patten, W. Crustacea, 172. Cymothoa, 137. Limuhis, 349-352. Pelseneer, P. Crustacea, 165. Pereyaslawzewa, Sophie. Amphipoda, 111. 140, 177, 181. Arthrostraca, 176. Gammarus, 176. Pergens, E. Bryozoa Ect., 15, 49. Prouho, H. Bryozoa Ect , 14, 17, 20-24,29,37,38,40, 41, 44, 56-58. Bryozoa Ent ,96, 102. Molluscoida, 85, 88. R. Ratiike, H. Asellus, 151. Astacus, 154. Copepoda, 147. Reichenbach, H. Astacus, 109,114, 118, 128-134, 154-164, 171, 175, 178. Crustacea, 166, 178. Decapoda, 168, 181. Reinhard, W. Phylactolaemata, 32, 33, 49, 51. Porcellio, 114, 140. Jvl.l'IACHOFF, W. Bryozoa Ect, 15, 21, 22, 37, 38, 43. I Ik ii i Kits, F. Decapoda, 278, 283. R()SKNSTA1>T, 1'). Crustacea, 330. Rossuskaya, Marie. Amphipoda, 111, 140, 150, 151, 177, 181. Arthrostraca, 176. Roule, L. Asellus, 114. Molluscoida, 85. Phorouis, 1. Porcellio, 114. 139. Ryder, J. A. Decapoda, 276. S. St. George, A. de la Y ALETTE. Gammarus, 107, 111. Salensky, W. Bugula, 25. Copepoda, 241. Samassa, P. Crustacea, 183. Sars, G. 0. Decapoda, 272-289. Leptodora, 203, 204. Schizopoda, 253. SCHIMKEWITSCH, W. Astacus, 130. Crustacea, 177. Schiodte, J. C, and Meinert, F. Isopoda, 305. Schmidt, O. Gymuolaemata, 14. Loxosoma, 103. Schneider, A. Bryozoa Ect, 21-24, 49. Phorouis, 5. Sedgwick, A. Bryozoa, 102. " Molluscoida, 90. Peripatus, 179. Seeliger, O. Bryozoa Ect., 38. Bryozoa Ent., 95, 99, 100. Molluscoida, 88. Pedicellina, 98. Semper, C. Cyphonautes, 21. Schultze, Max. Crustacea, 323. Shipley, A. S. Argiope, 67. Braohiopoda, 65. Sluiter, C. P. Rhizocephala, 209. Smith, Sidney J. Decapoda. 276, 286, 289, 290. Smitt, J. A. Bryozoa Ect., 56. Spence Bate, C. See Bate. Spengel, J. W. Cephalodiscus, 88. Molluscoida, 85. Stebbing, T. R. R. Crustacea, 805. Decapoda, 290. Steenstrup, J. J. S. Brachiopoda, 81. Steinmann, C. G. G. Palaeostraca, 332. Strauss- Durkheim, H. Limulus, 334, 356. Stuhlmann, F. Insecta, 119. Stuxberg, A. Crustacea, 330. Suess, E. Brachiopoda, 66. Thompson, J. V. Cirripedia, 323. Thomson, G. M. Anaspides, 319. Tower, R. W. Palaeostraca, 363. Tullberg, T. F. H. Homarus, 159. U. Uljanin, V. Amphipoda, 111, 118, 151. Pedicelliua, 103. Urbanowicz, F. Copepoda, 119, 151. Cyclops, 120, 160, 168. V. Valette, St. George, A. de la. Gammarus, 107, 111. Valle, A. della. S.r Della Valle. Yk.idovsky, F. Copepoda, 242. AUTHORS INDEX. 375 Verworn, M. Phylactolaemata, 42, 51, 52. VlGELIUS, W. J. Bryozoa Ect., 14-16, 25, 26, 29. VOGT, -C. Loxosoma, 103. W. Wagever, G. Phoronis, 5. Wagner, Jul. Mysis, 135. Walcott, C. D. Trilobita, 333, 334, 356. Walz, R. Crustacea, 331. Wasiljefe, E. Oniscus, 140. Watase, S. Limulus, 344-352. Weismann, A, Crustacea, 107. Weismann, A., and ISCHIKAWA, C. Bytliotryphes, 120. Cladocera, 107, " 111, 119. Crustacea, 114. Ostracoda, 111. Weldon, W. F. R. Decapoda, 290. Palaemon, 179. Westwood, J. O. Decapoda, 296. WlLLEMOES-SUHM, R. V. Cirripedia, 209-215. Decapoda. 257, 263, 264, 267, 275. Limulus, 340. Wilson, E. B. Oligochaeta, 137. Phoronis, 6. Wright, E. Strethill-. Phoronis, 6. Z. Zaddach, G. Apus, 165, 196. Phyllopoda, 147. Zenker, W. Cytheridae, 208. ZlTTELL, K. A. Brachiopoda, 66. Liniulus, 359. PLYMOUTH : W. BRENDON AND SON, PRINTERS. **rt PLEASE DO NOT REMOVE CARDS OR SLIPS FROM THIS POCKET UNIVERSITY OF TORONTO LIBRARY