1! i-n ru 01 CD a a : m a FISHES, LIVING AND FOSSIL Columbia SJniijersitg Biological Jetties. EDITED BY HENRY FAIRFIELD OSBORN. 1 FROM THE GREEKS TO DARWIN. By Henry Fairfieid Osborn. Sc.D Princeton. 2. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES. By Arthur Willey, B.Sc. Lond. Univ. 3. FISHES, LIVING AND FOSSIL. An Introductory Study, By Bashford Dean, Ph.D. Columbia. 4. THE CELL IN DEVELOPMENT AND INHERITANCE By Edmund B. Wilson, Ph.D. J. H. U. Frontispiece. — Head of DlNICHTHYS INTERMEDIUS, NEWBERRY, in front and side views. X fa. From photograph of specimen collected by Dr. William Clark, in the Waverly (Lower Carboniferous) of Ohio, now in the collection of Columbia College, New York. (V. p. 133.) p. COLUMBIA UNIVERSITY BIOLOGICAL SERIES. IIL FISHES, LIVING AND FOSSIL AN OUTLINE OF THEIR FORMS AND PROBABLE RELATIONSHIPS BY BASHFORD DEAN, PH.D. INSTRUCTOR IN BIOLOGY, COLUMBIA COLLEGE, NEW YORK CITY fforfe MACMILLAN AND CO. AND LONDON All rights reserved COPYRIGHT, 1895, BY MACMILLAN AND CO. 3 S 5 S Nortaoot) J. S. Gushing & Co. — Berwick & Smith Norwood Mass. U.S.A. MY FRIEND AND TEACHER JOHN STRONG NEWBERRY LATE PROFESSOR OF GEOLOGY IN COLUMBIA COLLEGE To)]/ 8' €l/l'8pWV £(iW TO TuJl/ l^OvwV y€I/OS IV OTTO TWV aAAwv dW ARISTOTLE, Z)^ Animalibus Historiae, Lib. II., cap. xiii. PREFACE A KNOWLEDGE of Fishes, living and fossil, is not to be included readily within the limits of an introductory study. In preparing the present volume it has nevertheless been my object to enable the reader to obtain a convenient review of the most important forms of fishes, and of their structural and developmental characters. I have also en- deavoured to keep constantly in view the problems of their evolution. At the end of the book a series of tables affords more definite contrasts of the anatomy and embryology of the different groups of fishes. And as an aid to further study has been added a summarized bibliography, including especially the works of the more recent investigators. My sincere thanks are due to my friend and colleague, Professor Henry Fairfield Osborn, for many suggestions during the early preparation of the book, and for the care with which he has later revised the proof. I must also express my indebtedness to Mr. Arthur Smith Woodward of the British Museum for his personal kindnesses in aiding my studies. My thanks are also due to my father, William Dean, for the preparation of the index. The figures, unless otherwise stated, are from my original pen drawings. B. D. BIOLOGICAL LABORATORY OF COLUMBIA COLLEGE, May, 1895- CONTENTS i PAGE Introductory. The form and movement of Fishes. Their classifica- tion; geological distribution; mode of evolution. The survival of generalized forms . i II The Evolution of Structures characteristic of Fishes: e.g. (i) gills, (2) skin defences, teeth, (3) fins, and (4) sense organs .... 14 III The Lampreys and their Allies. Their structures and probable relation- ships. The Ostracoderms and Palteospondylus 57 IV The Sharks. Their plan of structure; prominent forms, living and extinct; their interrelationships . . 72 V The Chimseroids. Their characteristic structures; their representatives and relationships -99 VI The Lung-fishes. Their structures. Extinct and recent forms. The evolution of the group .116 xi Xii CONTENTS VII The Teleostomes (i.e. Ganoids and Teleosts). Typical members; their structures and interrelationships; their probable descent .... 139 VIII The Groups of Fishes contrasted from the Standpoint of Embryology. Their eggs and breeding habits. Outlines of the development of Lamprey, Shark, Lung-fish, Ganoid, and Teleost. Their larval development 1 79 DERIVATION OF NAMES 227 BIBLIOGRAPHY 231 EXPLANATORY TABLES:. I. Classification of Fishes 8 II. Distribution of Fishes in Geological Time 9 III. Phylogeny of Sharks, Chimseroids, Dipnoans .... 98 IV. Phylogeny of Teleostomes 166 V. Characters of Vertebrae, Fins, Skull (Figs. 310-315) . . 252 VI. Relations of Jaws and Branchial Arches (Figs. 310-315) . 256 VII. Heart (Figs. 316-325) 260 VIII. Gills, Spiracle, Gill rakers (Figs. 9-12, 326-331) ... 260 IX. Digestive Tract (Figs. 326-331) 263 X. Swim-Bladder (Figs. 13-19) 264 XI. Genital System (Figs. 331-337) 266 XII. Plan of Circulation in Fishes (Fig. 338) 269 XIII. Excretory System (Figs. 331-337) 270 XIV. Abdominal Pores (Figs. 331-337) 271 XV. Central Nervous System (Figs. 339-344) 274 XVI. Sense Organs 276 XVII. Integument, Lateral line 278 XVIII. Developmental Characters 280 XIX. Comparison of Phylogenetic Tables of Authors .... 282 INDEX 285 LIST OF FIGURES FRONTISPIECE. Head of Dinichthys. FIG. PAGE i, 2. Moving fishes, shark and eel 2 3. Spanish mackerel .... 3 4. Front view of Spanish mackerel 4 5-8. Numerical lines of fishes • . 5 9-12. Gills of fishes . . . 17, 259 13-19. Air-bladder . . . .22, 265 20-31. Teeth and scales ... 24 32-38. Fin spines 29 39-43. Unpaired fins . . 32, 33 44-48. Caudal fin .... 37 49-54. Paired tins 42 55-60. Barbels and sense organs 47 61-68. Mucous canals . . . 50 69. General anatomy of Cyclo- stome 58 69 A. Skeleton of lamprey . . . 58 70-72 A-D. Bdellostoma, Myx- ine, Petromyzon . . 60, 61 73. Palceospondylus 65 74. Pteraspis .... 66 75. Palaeaspis ... ... 66 76. 77. Plates of Pteraspis ... 66 78, 79. Cephalaspis 66 80-82. Pterichthys 68 j 83. General anatomy of shark . 73 84. Skeleton of shark . . .75, 255 85. Vertebrae of shark .... 76 86. 86 A. Cladoselache . . .79 86 B. Teeth of Cladoselache . . So 87. Acanthodes Si 88. Acanthodes, shagreen . . . Si 88 A. Acanthodes, teeth . . 82 89. Climatius 82 FIG. PAGE 90. Pleuracanthus 83 90 A. Teeth of Pleuracanthus . 84 90 B. Head roof of Pleuracanthus 84 91. Cestracion 85 92. Chlamydoselache .... 87 93. Heptanchus 94. Squalus ... . . 89 95. Alopias . . 89 96. Lamna .90 96 A. Cetorhinus 90 96 B. Ltemargus 91 97. Squat ina 91 98. 98 A. Pristis 92 99. Pristiophorus .... 93 100. Rhinobatis 93 101. Raja . . 94 102. Torpedo 95 102 A. Dicerobatis (Cephalop- tera) 96 103. Shark phylogeny . . . 98 104. General anatomy of Chimasra 100 105. Skeleton of Chimsera . . 102 105 A. Ischyodus . . . 104 1 06. Myriacanthus 105 IO&A. Squaloraja ... . 105 io6B, C. Derm plates of Myria- canthus 105 107-112. Dental plates of Chimre- roids 106 113-116 A. Spines and claspers of Chimseroids . 107 117. Harriotta 108 1 1 8. Callorhynchus 109 119. Chimcera .... . 110 1 20. Chimsera, young . . . . 1 1 1 121. General anatomy of lung-fish 117 xin XIV LIST OF FIGURES FIG. PAGE 122. Skeleton of lung-fish . . . 119 1 22 A. Jaws and skull of Protop- terus 1 20 123. Dipterus 121 124. Derm bones of head of Dip- terus 121 125. 125 A. Jaws of Dipterus . . 121 126. Phaneropleuron . . . .122 127. Ceratodus 123 128. Skeleton of Ceratodus . . 123 1 28 A. Skull of Ceratodus . . .124 129. Lepidosiren 125 1 29 A. Protopterus 126 130. Coccosteus 131 131. Coccosteus, dorsal view . .132 132. Coccosteus, ventral . . .132 133. Dinichthys 133 134-137. Dinichthys, dorsal view 134 138-144. Mandibles of Arthrodi- rans 137 145. General anatomy of Teleost 140 146. Skeleton of Teleost . . . 142 147. Skeleton of Ganoid . . . 144 148. Polypterus 148 149. Polypterus, head of young . 148 150. Calamoichthys 150 151. Gyroptychius 151 152. Osteolepis 151 153. Holoptychius 151 154. Eusthenopteron . . . .152 155. Ccelacanthus 153 156. Diplurus 154 1 56 A. Undina 154 157. Lepidosteus 155 158. Elonichthys 156 159. Eurynotus ...... 156 1 60. Cheirodus 157 161. Semionotus 157 162. Aspidorhynchus . . . .158 163. Microdon 158 164. Palseoniscus 159 165. Acipenser 160 165 A. Chondrosteus .... 161 1 66. Scaphirhynchus . . . .162 FIG. PAGE 1 66 A. Psephurus 162 i66B. Polyodon 163 167. Amia 163 168. Amia, gular region . . . 164 169. Caturus 164 170. Leptolepis 165 171. Megalurus 165 171 A. Phylogeny of the Teleo- stomes 1 66 172-174. Deep-sea fishes . . .168 175. Fierasfer 169 176. Carassius 170 177. Amiurus 171 178. Callichthys 172 179. Mormyrus 172 I So. Anguilla 173 181. Perca 173 182. Gadus 174 183. Pseudopleuronectes . . -175 184. Chilomycterus 176 1 84 A. Lagocephalus . . . .176 185. Hippocampus 177 1 85 A. Syngnathus 178 186-199. Eggs of fishes . . . 181 200-215. Development of lam- prey 189 216-230. Development of shark . 194 231-248. Development of lung- fish 199, 201 249-268. Development of Ganoid 203 269-283. Development of Teleost 208 284-289. Larval sharks . . .216 290-295. Larval lung-fishes . .219 296-302. Larval sturgeons . . . 222 303-309. Larval Teleosts . . . 224 310-315. Skulls, jaws, and bran- chial arches .... 254 316-325. Heart and conus arte- riosus 258 326-331. Digestive tracts . . . 262 332-337. Urinogenital ducts and openings 267 338. Blood-vessels of shark . . 268 339-344- Brain 272 FISHES IN GENERAL INTRODUCTION FISHES, defined in a popular way, are back-boned ani- mals, gill-breathing, cold-blooded, and provided with fins. It is in their conditions of living that they have differed widely from the remaining groups of vertebrates. Aquatic life has stamped them in a common mould and has pre- scribed the laws which direct and limit their evolution ; it has compressed their head, trunk, and tail into a spindle- like form ; it has given them an easy and rapid motion, enabling them to cleave the water like a rounded wedge. It has made their mode of movement one of undulation, causing the sides of the fish to contract rhythmically, thrusting the animal forward. A clear idea of this mode of motion is to be obtained from a series of photographs of a swimming fish (Figs. 1-2) taken at successive instants : thus in the case of the shark (Fig. i) the undulation of the body may be traced from the head region backward, passing along the sides of the body, and may be seen to actually disappear at the tip of the tail. It is the press- ure of the fish's body against the water enclosed in these incurved places which causes the forward movement. The density of the living medium of fishes exerts upon them a mechanical influence ; they are, so to say, balanced in water, free to proceed in all planes of direction, poised B I 2 FISHES IN GENERAL with the utmost accuracy, enabled to rise to the surface or sink readily into deep water. A special organ, the 'air-,' or 'swim-bladder,' has even been acquired by the majority of living fishes, which, whatever may have been its origin or accessory functions (v. p. 21), has certainly to an extraor- RG. 1 Figs, i and 2. — Movement of fishes, — shark and eel. (After MAREY.) dinary degree the power of rendering the specific gravity of the fish the same as that of the surrounding water. In an example of a swift-swimming fish some of the most striking peculiarities of the aquatic form may be seen. The Spanish mackerel, Sconiberomorus (Fig. 3), shows admirably a stout spindle-like outline ; its entire sur- FORM AND FINS face is accurately rounded, and there appear no irregu- lar points which could re- tard the forward motion of the fish. Even in the wedge-shaped head the conical surface has been made more perfect by the tightly fitting rims of the jaws, by the smoothly closed gill shields, and by the eyes' accurate adjust- ment to the head's curva- ture. Viewed from in front (Fig. 4) the fish's outline appears as a perfect ellipse, and seems surprisingly small in size : the fins, which appear so prominent a feat- ure in profile, can now be hardly distinguished ; above and below they form keels, sharp and thin. In side view the vertical or unpaired fins are seen sur- rounding the hinder region of the body : they resolve themselves into dorsal (D), anal (A), and caudal (C) elements ; the former are low and stout, elastic in Fig. 3. — Type of swift swimming fish, their firm CUtwater margin, Spanish mackerel, Scomberomorus inacula- j tus (Mitch.), J. &G. X \. (After GOODR deeply notched and inter- in u. s. F. c.) 4 FISHES IN GENERAL rupted posteriorly, where useless elements have been dis- carded ; the caudal is broadly forked, stout in its support- ing rays, strong in power of propulsion. At its sides a remarkable ridge has been developed, functioning as a horizontal keel (R) and preventing the stroke of the cau- dal from varying from the vertical plane. The lateral, or paired fins, pectoral and -ven- tral (P and F), may rotate outward and arrange themselves in the line of the fish's motion, so that in a somewhat horizontal plane they may, like the unpaired fins, func- tion as keels. When thus erected, the paired fins present a firm anterior margin which serves as a cutwater. While thus somewhat similar in function to the vertical fins, the ventrals and especially the pecto- Fig. 4. — Front view of Spanish rals may acquire additional uses : they may serve as delicate balancers, or may aid in guiding or arresting the fish's motions. In further conformity to aquatic needs, the entire sur- face of the fish is notably slime covered, and although perfectly armoured by plates and scales, yet presents no point of resistance to forward motion. An internal balance, moreover, has been effected between the supporting, vis- ceral and muscular parts : the firm vertebral axis acquires its central position, and at its anterior end the head struct- ures form a compact, wedge-like mass : the body muscles which give the fish its form-contour thin away on the ven- tral side, permitting in the region between the head and the anal fin the space occupied by the closely compacted viscera : respiratory organs occupy a restricted space on either side of the gullet ; the heart and its arterial trunk are implanted closely in the throat in the median ventral NUMERICAL LINES 5 line ; the dorsal blood-vessel takes its position immediately below the vertebral axis, and the air-bladder in the most dorsal part of the abdominal cavity. FIG. 5 Pigs. 5-8. — Numerical lines of fishes and cetaceans. The " entering angle " begins at the snout-tip at the right, and extends as far as the vertical dotted line (36 %, about, of the entire length) ; the " run " then begins and is continued to the body terminal. 5. Striped porpoise, Phocaena lineata. 6. Spanish mackerel (Cuban), Scomberomorus caval/a. 7. Humpback whale, Megaptera. longimana. 8. Striped bass, Labrax lineatus. (All figures after PARSONS.) In acquiring this perfect outward symmetry it is inter- esting to note that the forms of fishes may be said to have actually evolved the practical solution of the most theoretical problems of curves and displacement in relation to sub- 6 FISHES IN GENERAL marine motion. A study of the " lines " of typical fishes by naval engineers * has led to some most interesting results as to the uniformity of their mathematical "normals." It is found, for example, that the "entering angles " of many and very different fishes are surprisingly similar (Figs. 6 and 8) : they thus terminate regularly (at the plane of the greatest cross-section of the body) at 36 per cent of the fish's total length; and the curves of the "run" (i.e. of the hinder part of the trunk, from the plane of the great- est cross-section to the body'terminal), similar for all, are smooth hollow curves, which in the forward motion of the fish permit the passage of the displaced water. It would be unreasonable to doubt that the fish form is adapted to the mechanical needs of its environment, even if there existed no further evidence than that of the meta- morphoses of aquatic mammals. Many of these have shown so complete an adaptation to water-living that it is scarcely remarkable that they were early included among fishes. And it is of further interest that there exist transitional forms between the land-living mammals on the one hand and the cetaceans on the other. In the Seal it is but the initial step in the transformation that has taken place ; the head and body have become bluntly tapering, the hind legs displaced backward, the foot and hand webbed, the hair adapted to submerged locomotion. A further stage in the acquisition of the fish-like form is shown in the Dugong and Manatee. And finally in the Dolphin and Whale (Figs. 5 and 7) have been actually attained the numerical lines of fishes (cf. Figs. 6 and 8). In these cases, the mechanical conditions of aquatic living have produced their result only at the greatest cost, — * '88. Parsons, Displacement and Area Curves of Fishes, Trans. Am. Soc. Mech. Engineers. CLASSIFICATION j enormous structural and physiological changes had of necessity to have been attained. The frame of the head and trunk has become moulded as in the fish's form, contours have been elaborately filled out and rounded, median dermal keels developed, vein valves lost, and the legs transformed into fin-like appendages. The form of the fish is accordingly to be looked upon as cast in a more or less common mould by its environment. Its internal structures, as in the cetacean, are also ob- served to be modified in accordance with its external form. This is a factor in the evolution of fishes which appears in every group and sub-group. And it has ever stood in the way of classifying them satisfactorily according to their kinships. " Fishes," used as a popular term, may include Lam- preys, Sharks, Chimaeroids, Lung-fishes, and "Modern Fishes" (Teleostomes), --the major groups to be dis- cussed in the present book. But the relative position of each of these divisions must at present remain more or less doubtful. The group of the Lampreys is certainly widely removed from the remaining ones, standing mid- way between the simplest chordate, Amphioxus, and the true fishes : it is usually given a rank co-ordinate with either of these, and, in fact, with all other groups of vertebrates, taken collectively. Sharks, Chimaeroids, Teleostomes, may be taken to represent true fishes ; and each might be assigned co-ordinate rank, although geneti- cally the Chimaeroids are certainly far more closely allied to the Sharks than are the Teleostomes. The Lung-fishes, as a widely divergent group, appear, as W. N. Parker has suggested, to be reasonably entitled to a rank equivalent to that of the three groups of true fishes taken together. 8 FISHES IN GENERAL The present writer has, however, retained in the main the classification of Smith Woodward, in which Fishes (Pisces) is looked upon as a class, and is made to include as sub-classes, (I.) Sharks, (II.) Chimaeroids, (III.) Lung- fishes, and (IV.) Teleostomes. A tabular grouping of the fishes is shown below. And on the opposite page their geological distribution is indicated. TABLE I A CLASSIFICATION OF FISHES Type: CHORDATA (VERTEBRATES). Class : Marsipobranchii, Lampreys, Palasospondylus, Hag, Lam- prey, Ostracodenns. Class: Pisces (True Fishes). I. Sub-class: ELASMOBRANCHII, Sharks and Rays. Order: Pleuropterygii (Dean), Cladoselachids (Dean). IcJiHiyotomi (Cope), Pleuracantliids. Selachii, Sharks and Rays. II. Sub-class: HOLOCEPHALI, Chimaeroids, Spook-fishes. Order : Chimaeroidei, Squaloraiids, Myriacantliids, Chimaerids. III. Sub-class: DIPNOI, Lung-fishes. Order : Sirenoidei, Dipterids, Phaneropleurids, Cte- nodonts, Lepidosirenids. ? Arthrodira, Coccosteids, Mylostoniids. IV. Sub-class : TELEOSTOMI, Ganoids and Bony Fishes (Teleosts). Order : Crossopterygii, Holoptychiids, Osteolepids, Onyc/iodonts, Ccelacanthids . Actinopterygii, Sub-order: Chondrostei (Ganoids), Palceoniscoids, Sturgeons, Garpikes, Amioids. Teleocephali, recent Bony Fishes (Tel- eosts). NOTE. — The groups italicized are represented only in fossil forms. The derivations of the scientific names are given on pp. 227-230. GEOLOGICAL DISTRIBUTION 9 TABLE II THE DISTRIBUTION OF FISHES IN GEOLOGICAL TIME The geological distribution of the prominent groups of fishes as here shown is in the main as given by Zittel (Palseontologie: Fische). The varying thickness of the lines denotes ap- proximately increase or diminution in the number of existing genera. .2 3 C/3 Devonian. <~ . '3 % Is « « o Permian. — , Triassic. 'J a 2 3 i—, ui , 3 sg a8 O U c 0> o £ Miocene. Pliocene. Recent. Marsipobranchii. Cyclostomes . . . ?Pteraspids .... ?Cephalaspids . . . Palaeospondylus . . ?Pterichthids . . . Elasmobranchii. Cladoselache . Acanthodians . . . Pleuracanthids Cestracionts Recent sharks (in- cluding Notidanids) Rhinobatus . . . Pristis Pristiophorus . . . Rays Chimaeroids .... Dipnoans. Dipterus .... Ceratodus .... PArthrodira .... Teleostomes. Crossopterygians . Acipenseroids . . . Lepidosteoids . Amioids 1 i • 1 1 mm mat fm ma m mm • sara mum = 1 aimm 1 DOB 1HIB KI •*- r^mmm • -• HB •• — Clupeoids .... Salmonids .... Perches and Bery- cids Siluroids .... Gadoids and other Teleosts .... •"i ^" — m I0 FISHES IN GENERAL Fishes hold an important place in the history of back- boned animals : their group is the largest and most widely distributed : its fossil members are by far the earliest of known chordates ; and among its living representa- tives are forms which are believed to closely resemble the ancestral vertebrate. The different groups of fishes appear especially favour- able for comparative study. Their recent forms are gen- erally well understood, both structurally and developmen- tally ; while a vast number of extinct fishes has been preserved to serve as a check, as well as an aid, to theoret- ical investigation. The remarkable permanence of the different types of fishes seems a striking proof of how unchanging must have ever been the conditions of aquatic living. From as early as the Devonian times there have been living mem- bers of the four sub-classes of existing fishes, — Sharks, Chi- maeroids, Dipnoans, and Teleostomes. Even their ancient sub-groups (orders and sub-orders) usually present surviving members ; while, on the other hand, there is but a single group of any structural importance that has been evolved during the lapse of ages, — the sub-order of Bony Fishes. There are many instances in which even the very types of living fishes are known to be of remarkable antiquity : thus the genus of the Port Jackson Shark, Ccstracion (Fig. 91), is known to have been represented early in the Mesozoic ; the Australian Lung-fish, Ccratodus (Fig. 127), dates back to Liassic times ; * the Frilled Shark, Clilainy- doselacJie (Fig. 92), though not of a paleozoic genus, as formerly supposed (Cope), must at least be regarded as closely akin to the Sharks of the Silurian. * Cf., however, Smith Woodward, The Fossil Fishes of the Hawkesbury Series at Gosford. Memoirs of the Geol. Surv. of N. S. W. Pal. No. 4, 1890. MODE OF EVOLUTION U The evolution of groups of fishes must, accordingly, have taken place during only the longest periods of time. Their aquatic life has evidently been unfavourable to deep- seated structural changes, or at least has not permitted these to be perpetuated. Recent fishes have diverged in but minor regards from their ancestors of the Coal Meas- ures. Within the same duration of time, on the other hand, terrestrial vertebrates have not only arisen, but have been widely differentiated. Among land-living forms the amphibians, reptiles, birds, and mammals have been evolved, and have given rise to more than sixty orders. The evolution of fishes has been confined to a note- worthy degree within rigid and unshifting bounds ; their living medium, with its mechanical effects upon fish-like forms and structures, has for ages been almost constant in its conditions ; its changes of temperature and density and currents have rarely been more than of local im- portance, and have influenced but little the survival of genera and species widely distributed ; its changes, more- over, in the normal supply of food organisms, cannot be looked upon as noteworthy. Aquatic life has built few of the direct barriers to survival, within which the ter- restrial forms appear to have been evolved by the keenest competition. It is not, accordingly, remarkable that in their descent fishes are known to have retained their tribal features, and to have varied from each other only in details of structure. Their evolution is to be traced in diverging characters that prove rarely more than of family value ; one form, as an example, may have become adapted for an active and predatory life, evolving stronger organs of progression, stouter armouring, and more trenchant teeth ; another, closely akin in general structures, may have acquired more 12 FISHES IN GENERAL sluggish habits, larger or greatly diminished size, and degen- erate characters in its dermal investiture, teeth, and organs of sense or progression. The flowering out of a series of fish families seems to have characterized every geological age, leaving its clearest imprint on the forms which were then most abundant. The variety that to-day maintains among the families of Bony Fishes is thus known to have been paralleled among the Carboniferous Sharks, the Mesozoic Chimseroids, and the Palaeozoic Lung-fishes and Teleostomes. Their environment has retained their gen- eral characters, while modelling them anew into forms armoured or scaleless, predatory or defenceless, great, small, heavy, stout, sluggish, light, slender, blunt, taper- ing, depressed. When members of any group of fishes became extinct, those appear to have been the first to perish which were the possessors of the greatest number of widely modified or specialized structures. Those, for example, whose teeth were adapted for a particular kind of food, 6r whose motions were hampered by ponderous size or weighty armouring, were the first to perish in the struggle for existence ; on the other hand, the forms that most nearly retained the ancestral or tribal characters --that is, those whose structures were in every way least extreme-- were naturally the best fitted to survive. Thus generalised fishes should be considered those of medium size, medium defences, medium powers of progression, omnivorous feed- ing habits, and wide distribution : and these might be re- garded as having provided the staples of survival in every branch of descent. Aquatic living has not demanded wide divergence from the ancestral stem, and the divergent forms which may culminate in a profusion of families, genera, and species, EVOLUTION do not appear to be again productive of more generalized groups. In all lines of descent specialized forms do not appear to regain by regression or degeneration the potential characters of their ancestral condition. A generalized form is like potter's clay, plastic in the hands of nature, readily to be converted into a needed kind of cup or vase ; but when thus specialized may never resume unaltered its ancestral condition : the clay survives ; the cup perishes. II THE EVOLUTION OF STRUCTURES CHAR- ACTERISTIC OF FISHES IT will be the object of the present chapter to review the gradations which occur in some of the characteristic structures of fishes and to follow in some degree the mode of their evolution. We may thus review the con- ditions of the (i) gills, (2) skin defences (including teeth), (3) fins, and (4) sense organs. The structures of the immediate ancestor of the fishes cannot be definitely inferred : the form, however, must have been elongate and transversely jointed, for this con- dition seems to have existed remotely before fishes --in the broadest sense --had become evolved. This segmen- tation, or metamerism, of the vertebrate body is best shown among water-living forms, sometimes indeed in so perfect a way as to suggest the jointed condition of an earth-worm. The segmented body of the eel-shaped Lamprey, shown in section in Fig. 69, illustrates an interesting condition of vertebrate metamerism. Its entire body, from the head region to the base of the tail, is composed of drum- like segments which closely correspond to one another in size and in component structures. Each segment thus resembles its neighbours in its equal portions of the vertebral column, digestive tract, nerve tube, muscle plates and blood canal, and in the arrangement of these parts with reference to bilateral symmetry. Motion in this form requires no more of each segment than that its EVOLUTION OF STRUCTURES je sides contract alternately to produce a rhythmical wave passing along the entire series of segments and giving the trunk an undulatory movement. Should this elongate body now acquire a more fish-like form, in attaining, for example, the power of more rapid movement, it is obvious that this simple type of meta- merism would undergo a series of changes. Every change of outward form would be reflected on the parts not only of each, but of all segments in their common relationships. To perform more perfectly the functions of their location, adjacent segments might become enlarged, folded, or blended, and cause the most puzzling complications of their component structures. One region of the body might thus appear to develop at the expense of another, as in the evolution of fin structures (cf. pp. 32-44), where a vertical fin fold, representing the sum of the dorsal and ventral out- growths of the hinder body segments, becomes reduced to the lappet-like dorsal and ventral fins ; the intervening substance of the fin web becoming drawn to the points where greater rigidity is required. The simple metameral character of the lamprey acquires an especial interest when the different groups of fishes are examined ; for it is found that all exhibit clearly body segmentg and segmental structures in the most varied stages of complexity. To trace metamerism seems, accord- ingly, a mode of determining to what degree the differ- ent groups have diverged from a common stem ; and to compare the sums of the archaic metameral characters in the different types of fishes may perhaps be looked upon as one of the safest aids in determining their genetic posi- tion. From the conditions of segmentation the lampreys must certainly be given a lowly rank ; even with due allow- ance for degeneration of structures they are clearly more l6 EVOLUTION OF STRUCTURES primitive than the most archaic sharks : while, on the other hand, to the metameral type of the sharks may the structures of the remaining groups of fishes be best referred. i. AQUATIC BREATHING Respiration in fishes is developed on the primitive chor- date plan of ejecting water through gill slits perforating the throat wall. The water taken in by the mouth is rich in absorbed air, and, as it passes out, is well calculated to oxygenate the blood suffusing the sides of the gill slits. Among the earliest chordates there seems evidence that the gill openings of the gullet were arranged with reference to some form of primitive segmentation. Per- haps they occurred as well in the region of the mid-diges- tive tract, before their location became restricted to the gullet. There has been as yet, however, little satisfactory evidence * as to the number or conditions of the gill slits in very primitive forms. In Amphioxus the gill arrange- ment seems clearly a most specialized one : its adult con- dition presents an atrium and an elaborate branchial basket,f which could hardly have occurred in the lowly ancestral chordate. Its early larva, however, is known to possess (but in a condition of assymmetry) but a few gill slits (seven to nine) from which the many openings of the adult branchial basket take their origin, — a developmental stage which most closely and most interestingly suggests the conditions of higher forms. * It has generally been inferred that the immediate ancestors of fishes had not many gill slits, probably notrfnore than eight or nine. A Liassic shark, a Cestraciont, Hybodus (p. 85), is known to have had but five; a Permian Pleu- racanthid, as in the recent Heptanchus, seven (p. 88) ; the Lower Carbonifer- ous Cladoselache probably seven. t C'f. Vol. II, of this series. Willey, Amphioxus and Other Ancestors of the Chordates. Figs. 9-12. —Arrangement of gills of Bdellostoma (9), Myxine (10), Shark (n), and Teleost (12). In each figure the surface of the head region is shown at the left. B. Barbels. BD. Outer duct from gill chamber, BS. BO. Common opening of outer ducts from gill chambers. BS. Branchial sac, or gill chamber. BS'. Branchial sac, sec- tioned so as to show the folds of its lining membrane. G. Lining membrane of gullet. GB. Gill bar, supporting vessels and filaments of gills. GC. Outer opening of gill cleft. GF. Gill filament. GK. Gill rakers. G V. Vessels of gill. J, J '. Upper and lower jaw. M. Mouth opening. N,N'. Anterior and posterior opening of nasal chamber. OP. Oper- culum. SP. Spiracle. ST. Tendinous septum between anterior and posterior gill filaments. * Denotes the inner branchial opening; — » , the direction of the water current. C 17 1 8 AQUATIC BREATHING In the singular group of lampreys and slime eels (Mar- sipobranchs, v. p. 57), the segmental arrangement of the gills seems of a primitive pattern. In the Californian Myxinoid (p. 59) the slits are as numerous as thirteen and fourteen on either side, each opening directly from the gullet to the neck surface (Fig. 9, G, *, BS'., BD). In the lamprey the conditions are similar, but the number of gill slits is reduced to seven. In Myxine (Fig. 10, G, BS', BD, BO} the outer portions of the canals becoming produced tail-ward have merged in a single pore (Fig. 71 *). In these forms each gill canal has become dilated at one point of its course, and in this sac-like portion the blood-suffused tissues have grouped themselves into leaf-like plates (gill filaments, or lamellae, BS') to increase their surface of contact with the out-passing water. The dilating power of this gill sac has then become specialized so that even should the animal's mouth be closed, water for respiration could be drawn in through the canal's outer opening: from this acquired function the elaboration of bran- chial muscles and a supporting framework of cartilage (branchial . basket, Fig. 69^, BB) may have taken its origin. Among fishes proper many stages in the evolution of gill organs are represented. They show altogether a marked advance over the conditions of Fig. 9. There has been a general tendency to press closely together the gill pouches and to elaborate into thinner and larger lamellae the blood-suffused tissue. In this process the gill chamber has become slit-like, bearing gill lamellae only on its front and rear margins ; its supporting tissue has con- solidated into stout vertical gill bars, the gill structures in general, becoming more highly perfected, tending to recede from the surface. These conditions may best be illustrated GILL CHARACTERS lg by contrasting the highly modified gill apparatus of a bony fish with the more archaic type of the shark. In the sharks (p. 73) the gill slits pierce separately the throat wall, as in the lamprey, and thus retain their primitive segmental arrangement (Fig. n). Their number is usually five on either side, but in an archaic form (Hep- tanchus, p. 88) may be increased to seven. Above and in front of the line of gill slits occurs a small opening leading into the gullet, the spiracle (SP). This, though at present possessing but few gill lamellae, and therefore of little respiratory value, was doubtless quite like its neighbours before its gill-supporting tissue became of value in suspending the lower jaw. It may now aid the mouth opening in admitting water to the gills. At the left of the figure (Fig. 11), the narrow slit-like openings of the gill clefts are seen at GC: at the right, where the upper portion of the head has been removed, the gill lamellae are shown at GF; the tissue intervening between the gill pouches is reduced to a thin tendinous septum, ST, at whose inner rim is the cartilaginous gill arch or bar, GB, supporting the branchial vessels, G V. In the gill region of a bony fish (Fig. 12) a number of modified characters are now evident : the spiracle has become obliterated; the number of gill bars reduced - in one form but two on either side remaining. These have become closely pressed together, and bent backward, receding from the surface of the head : their gill lamellae have become larger and more numerous, their intervening septum, ST, reduced in size. The gills no longer open separately at the surface, but into an outer branchial chamber formed and protected by a large overlapping scale, or opercle, OP. This shield-like organ is hinged at its anterior margin and opens or shuts rhythmically as 20 GILL CHARACTERS the throat muscles draw in or eject the water used in respiration. On the gullet wall, the gill bars, now seen to be closely drawn together, have acquired marginal outgrowths, or gill rakers, GR, which form an inter- locking screen across the gill openings and prevent the escape of food organisms. So perfect may this apparatus become that the opening and closing gill bars may retain even microscopic life.* Between the conditions of Figs, n and 12 there occur many transitional forms. To protect the gill region, specialized devices are known to have been evolved early in the history of fishes, — the more early if, as Garman has supposed, the gill fila- ments in primitive sharks protruded at the sides of the head. f There are thus the gill-encasing derm frills of the archaic sharks, Cladoselache, Clilamydoselaclie, and Acanthodes (pp. 78-83), or of Chimseroids (p. 100). These protective structures, the writer believes, may well have originated independently even within the limits of sub- groups. They have certainly no direct relation to the opercle of bony fishes. Modes of respiration by gill filaments have been found in endless variety among fishes, clearly dependent in the majority of cases upon environment. Thus fishes that require a temporary existence out of water will be found to have specialized spongy gill filaments and a closely fit- ting gill cover to keep moistened the respiratory organs (e.g. Callichthys, p. 172). * Thus in many bony fishes, e.g. mullet or Brevoortia (menhaden), the inner margins of the gill bars are fringed with what appears like the finest gauze, each gill raker giving off primary, secondary, and tertiary branches. A somewhat similar condition occurs in the shark, Selache (p. 90). f This condition appears to have been possessed by the Lower Carbo- niferous Cladoselache. SWIM-BLADDER 21 To live a longer time out of water has been rendered possible only by the appearance of a lung-like organ. Such a structure, however, would have been of too great impor- tance in the living economy of terrestrial vertebrates to have had a sudden origin : it may most reasonably have been derived from a similar structure occurring very gener- ally among fishes. The lungs certainly resemble the swim- bladder of fishes in so many important characters that it seems difficult to regard these organs as morphologically distinct. In itself the swim-bladder must be looked upon as an ancient and essentially a generalized structure, for within the groups of fishes it has already acquired a vari- ety of modified characters : appearing in a lowly condition in sharks, it acquires a balancing function in the majority of bony fishes ; in some forms (carp, siluroids) its function connects it with the auditory organ, often by a highly elaborated apparatus : while in other forms (Amia, Gar- pike, Dipnoans), it is unquestionably of respiratory value. The wide range in the characters of the air-bladder (cf. Figs. 13-19, and Table, p. 264), even among recent fishes, would naturally favour its homology with the lungs : it may thus be paired or unpaired, attached by its duct to either the dorsal, lateral, or ventral wall of the gullet : it may present the most varied characters in its lining membrane or in its vascular supply. When, moreover, it becomes of respiratory value (e.g. Dipnoans, Polypterus), the gills are known to become in part degenerate. The larval history of amphibians, presenting so perfect a transition between gill-breathing and terrestrial vertebrates, should alone seem to render more than probable the general homology of air- bladder and lung — an homology which a closer knowledge of the conditions of the lungs of the lower urodeles (e.g. Nccturns may well be expected to establish definitely. FIG. 13 STURGEON AND MANY TELEOSTS LEPIDOSTEUS AND AMIA ERYTHRINUS CERATODUS POLYPTERUS AND CALAMOICHTHYS LEPIDOSIREN AND PROTOPTERUS REPTILES BIRDS MAMMALS Figs. 13-19. — Air-bladder of fishes, shown from the front and sides. Cf. p. 264. A. Air- or swim-bladder. AD. Air duct. D. Digestive tube. (After WlLDKK.) 13. Sturgeon and many Teleosts. 14. Amia and Lepidosteus. 15. Erythrinus, a Cyprinoid Teleost. 16. Ceratodus. 17. Polypterus and Calamoichthys. 18. Lepi- dosiren and Protopterus. 19. Reptiles, birds, and mammals. The diagrams illus- trate the paired or unpaired character of the organ, its varied mode of attachment to the digestive tube, and the smooth or convoluted condition of its lining mem- brane. 22 SCALES AND TEETH 33 The mode of origin of the lungs as an unpaired divertic- ulum of the gullet is in every sense similar to that of the air-bladder. 2. THE DERMAL DEFENCES OF FISHES The dermal defences of fishes include scales, spines, fin rays, armour plates, and teeth, presenting in all a wide range of calcified structures. They have usually an outer, or surface layer of hard enamel-like texture and an inner substance heavy, stout, and bone-like. The former is de- rived from the outer layer of the skin (epidermis), the latter from the derma. The relation of these structural parts may be well seen in a section of shark skin which passes through one of its minute limy cusps, or dermal denticles (Fig. 20). The outer skin layer, E\ originally covered the denticle, which grew outward, papilla-like, beneath it ; its inner surface, in contact with the outgrow- ing papilla, secreted the enamel, £, and is known as the enamel organ, EO: at the cusp, however, the epidermis is early worn away. The bone-like substance of the tooth is clearly formed in the lower (dermal) layer of the skin, D' : it is formed by the calcification of the outer layers of the tip and base of the dermal papilla, leaving a vascular cavity, PC, within. This limy substance, "dentine," D, presents microscopically a columnar "cancellated" structure; in this and in its lack of bone cells it differs structurally from true (cartilage) bone. The dermal denticle of the shark is certainly the sim- plest form of a calcified skin defence : it appears to repre- sent the ancestral condition of the various scales, teeth, or bone plates which have been evolved in the groups of fishes. It is usually of minute size, and studs closely the entire surface of the skin, forming shagreen. In many 27 Figs. 20-31. — Mode of evolu- tion of (teeth and) dermal defences. 20. Shagreen denticle of shark, X 30, cross section. (After HOFER.) D. Dentine. /)'. Derma. E. Enamel. £'. Epidermis. EO. Enamel organ. PC. Pulp cavity, showing nutritive tubules passing into the dentine. 21. Shagreen denticle (" placoid scale ") of Greenland shark, Lcemar- gus, viewed from the side and (A) top, enlarged. 22. Shagreen denti- cles of shark, Scylliuin, showing mode of arrangement. X 30. 23. Shagreen of sting-ray, Urogymiuis, nat. size. (After SMITH WOOD- WARD.) 24. Ganoid dermal plates of Lepidosteus. A. Inner face of ganoid plates, showing tile-like device of interlocking. 25. Variation of ganoid plates in Aetheolepis. (After SMITH WOODWARD.) Plates from different regions vary in outline from cir- cular to 'lozenge shape. 26. Coalesced ganoid plates of the siluroid Callichihys. 27. Jaw of Port Jackson shark, Cestracion. 28. Dental plate of extinct cestraciont (?), Sandalodus. 29. Dental plates of jaw of sting-ray, Trygon (?). 30. Dental plates ofea.g\e-ray,Mytiodatis. 31. Scales of Teleost. A. A single scale enlarged. 24 EVOLUTION OF SCALES 2$ members of the shark group the denticles are scattered over the body without traces of metameral arrangement (Fig. 23) ; in others they acquire a segmental position (Fig. 22). Usually the denticles possess very definite shapes and regional characters ; their basal portion, where implanted in the skin, may thus become of enlarged size and regular outline (Fig. 21 A), their projecting cusps tapering, blunted (Fig. 23), or branched. Sometimes the fusion of contiguous denticles may occur (as in the en- larged blunted denticles of Fig. 23). The evolution of the more perfect body armouring of fishes from shagreen denticles has not been followed in minor details. It appears, however, that the calcifica- tion of the skin which occurs superficially in the dermal papillae of the shark may in other fishes be traced oc- curring in deeper and deeper layers of the derma : the papillae at the surface accordingly lose their functional importance, and tend to disappear, while the calcified tissue of the derma — representing morphologically the basal region of the denticles - - is coming to occupy more and more definite tracts. These processes have already taken their origin within the group of sharks. An interesting condition in the subsequent evolution of the dermal armouring is illustrated in Fig. 25, and has been described by Smith Woodward. The circular bone plate of the figure is a calcified dermal tract which still retains, scattered generally over its surface, traces of shagreen tubercles : from this shark-like condition a well-marked gradation in the form of the derm plates may be traced in different body regions of the same fish : according to metameral needs there are acquired rectangular or lozenge-shaped outlines. In Fig. 24 these bone or " ganoid " plates are seen to constitute a com- 26 EVOLUTION OF SCALES plete but flexible body armouring, made additionally strong by an interlocking articulation of its elements (24 A). In this form the enamel-like surface layer ("ganoine") of the ganoid plates is believed to be derived from the dentine substance, and not deposited by the epidermis : they bear numerous shagreen denticles during an early period of life. The most complete encasement of a fish's body by dermal plates is shown in Fig. 26, v. p. 172. The met- ameral conditions have here permitted extended fusions, a single dermal plate enclosing the upper, or lower division of the muscle-plate of either side. The thin horn-like scales of the majority of recent fishes, e.g. carp or perch (Fig. 31 A) are probably derived from a condition not widely different from that of Fig. 24. They take their origin, however, in a deeper layer of the derma, thence grow outward, arising as if from deep and flattened pockets. Their substance becomes horn-like, rather than limy, and they enlarge in outline, rather than in thickness. Their hinder margins, often crenulate, overlap widely the neighbouring scales ; their arrangement is in direct relation to the underlying metameres, and their surface is densely slime-coated. The dermal armouring they thus constitute is both light, tough, and flexible. Degeneration of scales is shown to occur in many, types. In some forms their size may become micro- scopic (eel), in others enormously enlarged (mirror carp). In cases they may entirely disappear (leather carp). The fusions of the dermal plates of the trunk-fish or of the sea-horse (p. 177) are probably degenerate. TEETH Teeth 27 Teeth have long been known to represent the dermal defences of the mouth rim. In this region they have become of especial value in the living economy of verte- brates— seizing, holding, cutting, or crushing the food- material. They have here accordingly been retained and specialized. In the sharks the dermal denticles of the mouth rim are often identical in shape and pattern with those of the entire body surface : they differ only in their larger size. Their arrangement in many rows still presents clearly their metameral character. The forms of teeth acquired among the different groups of fishes suggest closely the evolution of the more modi- fied dermal defences. In general, they are found to vary widely according to their function or location ; those near- est the dermal margin of the mouth usually retaining the cusp-like and more primitive features. Thus in the jaw of Port Jackson shark (Fig. 27, v. p. 85), the teeth of the symphysial region clearly represent shagreen denti- cles ; while those deeper in the mouth, large and blunt, serve as crushing or "pavement " teeth. These must evi- dently be looked upon as standing in the same relation to the anterior cusps, as do the bone plates of Fig. 25 to the derm denticles of Fig. 23 ; the fused crushing teeth have still retained their metameral arrangement. The dental plates (Fig. 30) of a ray, Myliobatis (p. 96) show more perfect conditions for crushing ; they are uniform in size, tightly set, and present a smooth, mosaic-like surface. A still more perfect fusion of the dental elements occurs in a ray, closely akin to Myliobatis ; all lateral elements have here been fused, but their metameral sequence has been re- tained (Fig. 29). In Fig. 28 is shown a dental plate of a 28 TEETH AND SPINES fossil shark (?), Sandalodus, which probably represents a condition of complete fusion ; it would accordingly cor- respond to the sum of the dental elements of half of the jaw of Fig. 27. In more highly modified fishes the tooth-producing region has become greatly extended ; teeth are present not only on the jaw rims, but deep in the mouth cavity, studding its floor and roof, and occurring even on the tongue, gill bars, and pharynx. Fin Spines Primitive dermal defences appear to have played a prominent part in the formation of fin spines. The clus- tering of dermal cusps on the exposed margin of a fin may have been an important initial step toward the for- mation of a rigid cutwater. The anterior margin of the fin of Fig. 49 is whitened with a fusion of dermal tuber- cles which must have formed a firm encrusting support ; the extension of the calcification of the bases of the tu- bercles would accordingly be the mode of origin of a fin spine. In Fig. 32 is shown a spine that appears largely of this origin. A similar spine (Fig. 33) shows its dermal tubercles not only at its sides, but in a most marked way at its hinder margins. In Fig. 34, representing the "sting" of the sting ray, a series of dermal spines, bear- ing rows of minute denticles are seen to arise in a meta- meral succession. A condition somewhat similar is known in the Carboniferous shark, Edestns (Fig. 35), whose spine, often of gigantic size, is of special interest, since it shows how important a part in spine-formation may be taken by the dermal defences of many successive metameres. The spine is clearly segmented, and as its separate elements (Fig. 37) are bilaterally symmetrical (Figs. 36 and 38), its FIN SPINES position was probably in the median line of the body. The well-marked, backward curve of the spine suggests 34 Figs. 32-38. — Fin spines. 32. Fin spine and pectoral fin of Acanthodian. 33. Hybodits (cestraciont shark). 34. Sting-ray, Trygon. 35. Edestus heinrichsii (Carboniferous shark, known only from its spine), side view of spine. < 1. 36, 37, 38. Dorsal view, separated element and transverse section of Edestus spine. that fin structures could not well have existed behind it. Each separate element has an elongated basal portion, 3Q EVOLUTION OF FINS which apparently was imbedded in the integument ; its gouge-like form (Figs. 37 and 38) permitted it to be firmly apposed to its anterior and posterior neighbours. Each median enamelled cusp represents apparently the sum of the shagreen papillae, occurring in the median-dorsal region of each metamere, its gouge-like underlying portion the metameral calcification of the bases of the denticles. What has been the mode of origin of the primitive derm cusps is a puzzling question. It is significant, per- haps, that they occur in primitive forms (sharks) in con- nection with the sense organs of the lateral line (p. 50), and that they are in this region retained in a number of archaic forms (Polypterus, p. 148, Callichthys, p. 172), which have in all other body parts evolved protective derm plates.* It is certain that for the sensory groove of the lateral line, no more simple, protective devices could have arisen than conical elevations of skin. Arising in this region, they may have extended their protective functions over the entire body surface. 3. THE EVOLUTION OF FINS Fins are the organs of progression adapted to the needs of aquatic living. A fish, balanced in its living medium, acquires, as has been seen, a boat-like form, enabling it to pierce the water in the least resisting manner. Its appendages, when they come to arise, must reasonably be looked to to fulfil the mechanical condi- tions of aquatic motion in order to propel to the best advantage the lightly balanced and boat-shaped mass. Fins might thus be expected to arise as keel-like struct- * In the sensory canals of the head of Chimasra, the presence of scattered bony plates, protective in function, v. p. 114, would suggest the concentration of the marginal cusp elements for more perfect protection. MEDIA. \ FLVS 3! ures, i.e. as ridges in the direction of the fish's axis or line of motion. Fish fins have long been distinguished as vertical (me- dian, or unpaired) or lateral (paired), the former function- ing both as keel and means of propulsion, the latter as accessory and specialized balancing organs. Median Fins Median fins are unquestionably the older. They exist in the simplest condition in those fishes whose axis is long and whose motion is undulating. Indeed, the sole swim- ming requisite is here the continuous dermal keel which passes down the back from the head to the body terminal, and extends thence forward on the ventral side. The undulatory motion of the body is well transmitted to the surrounding medium by the exaggerated undulation of this long, waving fin web. This condition was probably the ancestral one in the evolution of fishes. It represents the simplest metamerism ; it occurs as the adult condition in the lampreys (p. 57), and as the embryonic or larval stage in all fishes, appearing before any traces of paired fins are known ; it is even adverse to their specialization : should life habits require undulatory motion, paired fins must inevitably tend to disappear (eel, p. 173 ; Cala- moichthys, p. 150). From this condition the further evolution of the un- paired fins may thus be theoretically outlined. The primitive continuous dermal fin could have been of little value in active movement : its more rapid undu- lations could not have greatly increased the rate of motion, since its web, lacking in supports, would not have retained its rigidity. As the simplest means of strengthening the fin fold, " actinotrichia " (Ryder), appear to have been early EVOLUTION OF FINS evolved (Fig. 39, T) ; these are slender, unjointed fin sup- ports, passing from the body wall to the margin of the fin, appearing to arise without relation to the underlying body segments. The more rapid undulations of the contin- uous fin would next cause nodes to arise ; and at other points the greatest mechanical stress would occur. These portions of the fin web would accord- ingly become prominent, while the in- tervening or useless parts would dimin- ish in width and tend to disappear. The body terminal (tail, caudal fin) has now c become the seat of propulsion : dorsal and ventral fins arise as lobate elements of the fin fold, functioning as vertical keels in the region of the body where mechanical stress demands them (v. Fig. 40), increasing in size as the intervening portions of the web gradually disappear. Their rate of growth is doubtless af- fected by the appearance of the paired fins ; for even at an early period of de- velopment these are known to have an important function in balancing the fish. The lappet-shaped fins (Fig. 40) next acquire more rigid supports. Cartilagi- nous rod-like elements arise within the fin web, arranged in metameral sequence, representing, perhaps, fusions of actino- trichia. As shown in Fig. 40, these car- Fig- 39- — Hypothet- tilaginous "radials" R, appear to be ical ancestral shark. Let- ... ters as on p. 33. largest and stoutest in the widest por- MEDIAN FINS 33 tions of the fin lobe, and thence to taper in size toward the nodal points of the web. Each radial appears shortly to segment off a proximal joint, or "basal" cartilage, B, to secure a more perfect attachment with the wall of the body. The subsequent evolution of the fins appears to have been determined by two modifications of growth, --the clustering of the radial and basal elements, and the encroachment of newly formed marginal (distal) rays FIQ.40 B 43 Figs. 40-43. — Evolution of unpaired fins. 40. Plan of reduction of vertical fin web into its dorsal, anal, and caudal elements. 41. Arrangement of fin supports in primitive fin (Cladoselache). 42. Plan of archaic unpaired fin in (larval) shark. 43. Unpaired fin of fossil Crossopterygian, Holoptychius. (After SMITH WOOD- WARD.) A. Anal fin. B. Cartilaginous basal (fin support). C. Caudal fin. D. Dermal margin of fin. D ' . Anterior and D". Posterior dorsal fin. R. Cartilaginous radial (fin support). T. Actinotrichia. upon the functions of the older fin supports. Three stages in this metamorphosis will be seen in Figs. 41-43. The first illustrates the dorsal fin of an ancient shark (Cladoselache, p. 79), and will at once be seen to present most primitive conditions : it closely resembles the theo- 34 ANAL AND DORSAL FINS retical dorsal fin, D' or D" of Fig. 40. The form of the fin suggests the lobate constriction of the continuous fin web ; its radial supports, R, extend from the body wall to the margin of the fin, and between them traces of actino- trichia are to be seen. The anterior margin of the fin must now function as a strong cutwater, its supporting elements, both radial and basal, tightly clustering. A fin of this character could evidently have possessed a greater freedom of lateral movement in its hinder than in its an- terior part ; and thus the clustering of the fin supports becomes of especial significance. The region of move- ment, restricting itself to the hinder part of the fin, permits extensive fusions of the supporting cartilages anteriorly, and leads ultimately to exceedingly complex conditions. The dorsal fin of a Coal Measures fish (Ho- loptychius, p. 151) has thus (Fig. 43) specialized the power of lateral movement in the highest degree. The length of the fin has, in the first place, become greatly compressed, a process which seems to have resulted in implanting the anterior basals, B, deeply into the integument and in fusing them : the posterior basals then appear to have been everted from the surface of the body. Here they still retain their segmental arrangement, but are irregular in shape and reduce in size distally. An important part is taken by the dermal margin of the fin in modifying the size of the older fin supports. The simplest form of a dorsal fin of a recent shark (Fig. 42) has thus more than half of its functional area of a dermal origin, although in other regards it resembles closely the conditions of Fig. 41. The dermal margin of the fin has apparently increased to the detriment and consequent reduction of the cartilaginous elements ; it produces in its secondary structures light flexible horn- CAUDAL FIN 35 like rays, which prove stronger and more serviceable than the heavier radials ; it seems more capable of adapting the fin for special uses. Accordingly, in many forms of recent fishes, notably bony fishes, the .entire fin is found to become of dermal origin ; the radio-basals, greatly reduced in number and size, extend no further outward than the base of the fin ; they are usually small and irregular, and are often deeply sunken within the body wall. After this glimpse at the mode of origin of the vertical fins, i.e. dorsals and anals, the history of the final vertical fin, the tail, and of the paired fins may next be reviewed. The Caudal Fin The tail, or caudal fin, is the main organ of aquatic propulsion, and it is doubtless on this account that it presents so wide a range in its structure and outward form. From the earliest times there are found fishes of all groups whose tail shapes are tapering (diphycercal, Fig. 47), unsymmetrical (keterocercal, Figs. 45, 46), or squarely truncate (jwmocercal, Fig. 48), as the mechanical needs in swimming may have demanded. The following summary of the mode of evolution of the caudal fin seems to be warranted by study of fossil and embryonic forms. The vertical fin fold of the ances- tral fish was probably carried around the body terminal and strengthened by constant actinotrichia (Fig. 39 C), a condition similar to that (Fig. 44) of an early larval stage of living fishes (protocercy). This caudal structure, however, could have proven of value only in sluggish undulatory motion. The functional needs, which gave rise to radials anteriorly, have in the tail region produced firmer and stouter fin supports. These appear both on the 3 6 CAUDAL FIN dorsal and ventral sides, but, unlike the radials of the anal or dorsal fins, do not segment off basal elements. They first occur in the region of the base of the caudal, as in the embryonic stage (Fig. 44, R), since, perhaps, it is in this region that the greatest stress occurs in propulsion. It is not until a later stage that their metameral sequence is extended backward to the tip of the vertebral axis (Fig. 40, Q. With the origin of cartilaginous supports there seems to have arisen a mechanical need for enlarging the ventral lobe of the caudal ; it is here certainly that in the majority of early forms the radials appear longer and stouter, giv- ing rise to the condition of heterocercy of Figs. 45 and 46. The greater functional importance of the radials of the ventral region, R + H, is acquired contemporaneously with the upturning of the end of the vertebral axis. In the tail of a Lower Carboniferous shark (Fig. 46, v. p. 79), an extreme degree of heterocercy has been acquired before the radials of the lower lobe have extended themselves in the hindmost region of the vertebral axis ; the ventral web of the upper tail lobe, accordingly, is still strengthened by minute (dermal) rays, which the writer believes homol- ogous with actinotrichia ; on the fin's dorsal side the radials have been abruptly upturned with the notochord, and are fused into a compact cutwater. The plan of structure of the shark's caudal fin (Fig. 45) may in its most primitive form prove to be the ancestral one of fishes ; if this is the case it would give rise to the types of caudal fins of Figs. 47 and 48. That it has given rise to the latter form cannot be doubted, for even in the adult condition of the fin the notochord, N, may be seen passing to the upper lobe of the tail ; the essential out- ward form of this truncated, or homocercal, tail had already Pigs. 44-48. — Evolution of caudal fin. 44. Embryonic caudal of Amia. 45. Hetero- cercal caudal of shark, Cestracion. 46. Heterocercal caudal of Cladoselache, 47. Diphy- cercal caudal of Polypterus. (After L. AGASSIZ.) 48. Homocercal caudal of Teleost. (After RYDER.) D. Dermal fin supports. L. Lateral line. M. Spinal cord. MC. Membranous caudal. N. Notochord. N-\-, and R-\-N. Neural spines, including probably radial and basal elements. R. Radials. R-\-H. Haemal arch and spine ; includes as well, probably, radial and basal elements. 37 38 PRIMITIVE CAUDAL FIN been acquired in ancient sharks (Fig. 46). The fin of Fig. 47, however, has not generally been looked upon as derived from shark-like conditions ; it has, on the other hand, been thought to be most nearly of the ancestral form. The vertebral axis does not appear to be upturned, and the ventral and dorsal lobes of the fin remain nearly sym- metrical, or diphycercal. This form of the caudal fin, on the other hand, has been noted to present many degener- ate characters, and to the writer * it seems more reasona-_ .ble to regard the diphycercal condition as in many cases directly descended from the heterocercal. This might be effected by the terminal portion of the vertebral rod abort- ing (as in Fig. 47, N), and the upper and lower lobes of the tail becoming pressed backward until their hinder margins appose in the axial line.f The form of diphycercy which is seen in Fig. 119 is unquestionably of little morphological value ; it occurs commonly in deep-sea fishes of every group, and must be looked upon as a degenerate condition result- ing from impeded motion under the conditions of bathyb- ial, or deep-sea living. The cartilaginous supports of the caudal, like those of other unpaired fins, become greatly reduced in size by the encroachment of dermal rays. In the tail of the fossil shark (Fig. 46) the cartilaginous supports, R, extend to the very margin of the fin : in the modern shark (Fig. 45) a large part of the functional fin area has become of second- ary, or dermal origin, D. In the caudals of Figs. 47 and 48, distinct dermal rays, D, are seen, extending from the body wall to the fin margin, splitting and segmenting dis- tally in becoming more perfectly specialized in function. The cartilaginous supports, 7? + .Vand R + H, must now be * Journal oj Morphology, IX, I, 1894. f Gephyrocercy of Ryder. PAIRED FINS 39 looked upon as including the elements of both the radials and the haemal or neural processes and spines. The Paired Fins The paired fins of fishes claim an especial interest as the precursors of the limbs of the land-living vertebrates. In this light they have been widely studied, and many schemes have been devised for the comparison of the parts of the five-fingered extremity, or ckeiropterygium, of the amphibian with the fin structures of many fishes. The un- satisfactory character of these homologies, however, is felt at the present time more generally than ever, and many morphologists believe with Dr. Mollier * that the ancestral form of the terrestrial limb cannot be found in any of the known types of paired fins. Among fishes, on the other hand, there appears to be a well-marked unity of plan in the varied forms of the paired fins ; and there exists so perfect a gradation in structural characters in the different forms that it seems impossible to doubt their genetic kinship. Which fin, however, must be looked upon as the ancestral type is still disputed. Professor Gegenbaur has long maintained that the fin of Fig. 54 (or, better, the pectoral fin of Fig. 147!) is to be looked upon as the most primitive form, or Anhip- terygiuvi. It is a leaf-shaped fin, whose principal carti- laginous supports are arranged in a row from base to tip in the position of a mid-rib : and whose minor fin supports are grouped more or less symmetrically on either side of this axis (cf. Figs. 53, 54, 121, 123, 126). The archipteryg- ium is believed by Gegenbaur to have had a centrifugal origin : it arose behind the gill region, representing in its * SB. Gesell.f. Alorph. Milnchen, 1894, p. 17. t Gegenbaur, Das Flossenskelet der Crossopterygier. Cf. Morpk. JB, 1894. LATERAL FOLD FINS supporting substance the fusion of the cartilages of the hindmost gill bars ; in its outward growth the median axis of the fin was first produced, the minor supports then arrang- ing themselves on both anterior and posterior margins. The fin of Fig. 52 was believed to represent a specially evolved (or "monoserial") form of the archipterygium: the hindmost of its elements, B, was homologized with the primitive fin stem, along whose posterior (post-axial) mar- gin the elements, R, no longer occurred. The structures of Fig. 53 were adduced as a transitional stage in the dif- ferentiation of the biserial archipterygium (Fig. 54) into the monoserial form of Fig. 52. The theory of Gegenbaur as to the origin and evolution of the paired fins cannot be said to be in any way generally supported at the present time. The opposing view, that of their derivation from a continuous lateral dermal fin fold, based on the work of Thacher, Balfour, Mivart, Dohrn, Wiedersheim, and others, is widely accepted, and continues to gain supporting evidence on the sides both of embryology and palaeontology. In the following discussion of the paired fins the writer has mainly followed the recent studies of Wieders- heim.* The paired fins are believed to have arisen as balancing organs, accessory in function to the vertical fins. They probably occurred early in the line of descent as a response to a need for balancing the fish's body, at the time when the vertical fin was separated into caudal, dorsal, and anal elements. There can be little doubt that they first arose in the line of the fish's motion, and are known primitively (Figs. 49, 50), as a pair of keel-like lateral lappets arising somewhat ventrally, and directed outward and downward. * Das Gliedmassenskelet der Wirbelthiere, 1893. PAIRED FINS 4! The foremost pair appears anteriorly not far behind the gill region : from its position it has certainly the more im- portant mechanical function in balancing the fish's length -on this account becoming more widely modified in form and function as \\\Q pectoral fins. The hinder pair, or ven- tral fins, though in the plane of the pectorals, has a more ventral position, the hinder borders converging in the region of the anus. The ventral fins are certainly placed in the most motionless region of the fish: they are little affected by either the lateral or upward movements of the body ; and remain accordingly smaller in size and simpler in structure than the pectoral fins. That there may have existed in primitive fishes a third (post-ventral) pair of fins is by no means improbable (cf. T. J. Parker, Rcf. p. 244), although its presence has not as yet been satisfactorily demonstrated. The paired fins thus appear to have been derived from a continuous dermal fold, similar in every way to that giving rise to the vertical fins. They appear, moreover, to have undergone the same mode of evolution in their structures as have the dorsal or anal fins. The unpaired fin fold as it passed forward on the ventral side of the body may primi- tively have forked in the anal region, and given rise on either side to a lateral fold. In these might next appear an anterior and posterior pair of lappets, --pectoral and Central fins, — whose positions would be determined by mechanical needs, and whose size would increase as the intervening and useless portion of the dermal fold disap- peared. In the subsequent history of pectoral and ventral fins, supporting elements, actinotrichia, radials, and basals, would arise in the same way as in the unpaired fins, and a similar metamorphosis of the fin form would take place, owing to the concrescence of these elements and to the FIG. 49 Figs. 49-54. — Evolution of paired fins. 49-50. Pectoral and ventral fins of Cladose- lache. X 3. 51. Pectoral fin of Acanthodian, Parexus. (After SMITH WOODWARD.) 52. Pectoral fin of Heptanchus. (After GEGENBAUR.) 53. Pectoral fin of Xenacanthus (Plenmca>if/int.) (After A. FR1TSCH.) 54. " Archipterygial " pectoral fin of Ceratodus. (After HOWES.) B. Basal. D. Dermal. R. Radial. 42 PAIRED FINS 43 subsequent encroachment of the dermal fin margin. These conditions may be briefly illustrated. The paired fins of a primitive shark (Figs. 49, 50, v. p. 79) appear as the actual lappet-shaped remnants of a continuous dermal fold. The ventral fins (Fig. 50) have clearly retained even the out- ward shape of the fin fold ; the supporting elements are arranged in metameral order ; the radials, R, are unjointed, extending from body wall to fin margin ; the basals, agree- ing in number with the radials, are uniform in size, and as yet unfused. The pectorals, acquiring more special func- tions (Fig. 49), are enlarged in size, their basals, B, becom- ing compressed and obscure. In these fins the effect of concrescence is admirably marked ; the anterior fin margins, pressed tail-ward in their plane of growth, become firm and rigid, their elements stout and compact ; the basals, re- sponding to this outward need, cluster more firmly together, are compressed and fused, their anterior elements, largest and stoutest, become inturned, their posterior elements, slightest and most clearly metameral.* The next stage in the evolution of the paired fins is clearly comparable to that already noted as occurring in the dorsal fin of Holoptychius (Fig. 43), where the line of basals, fusing compactly into a plate-like mass, had in- turned its anterior, and protruded its posterior tip ; a change apparently slight, but great in functional impor- tance. Up to this stage the fin has been firmly implanted in the body wall ; its motion, probably slight upward or downward, served but to balance the fish, its fin rays, tending to concentrate anteriorly, functioned as an efficient cutwater. This process of concentration in the anterior fin margin may have resulted, the writer believes, in the * The effect of the enlarged and clustering dermal denticles in strengthen ing the cutwater margin of the fin has already been noted (p. 28). 44 PAIRED FINS formation of fin spine, as in Acanthodian * (Figs. 32, 51, and p. Si). But the protrusion of the line of the basals must have brought with it a new use in the economy of fish motion. The plane of the fin could now be directed upward or downward ; the fin would become a direct aid in propulsion ; it would acquire a paddle-like function ; it could also be extended sideways as a check to motion. Under these circumstances it is not unnatural that the region of the concrescence of the fin rays should now be transferred from the fin's anterior to the more useful pos- terior (now distal) margin, and that the fin rays, as well as the line of basals, should acquire a more jointed structure, suited to flexible motions. The course of the differentia- tion of fin structures may be traced from this point on- ward, as Wiedersheim has shown, by means of a series of gradational stages : from the conditions of Fig. 49 we may in the present figures pass to those of Fig. 52, thence to those of Figs. 53 and 54. In the pectoral fin of a modern shark (Fig. 52) the basal cartilages, B, may still be com- pared with those in the older form (Fig. 49 B) ; their distal element (B, at the right of the figure), however, protrudes from the body wall and is becoming surrounded by clus- tered radials, R ; the cartilaginous elements, it is here noted, have been placed in competition with the dermal elements, and have already yielded them over half of the fin area. In the next stage of the evolution, as in the pectoral fin of a Permian shark (Pleuracanthus, p. 83, Fig. 53), the line of the basals is seen to boldly protrude from the body wall and to have become distinctly jointed ; the radials have surrounded its distal end, and taken a position * This homology proposed by the writer has not been accepted by Smith Woodward; the spine is unquestionably encased outwardly by dermal den- ticles. PAIRED FINS 45 along the outer half of the hinder margin of the fin stem ; the dermal region of the fin, D, has notably increased. Indeed, the fin area in the modern bony fishes (Fig- 145, PF) may become entirely dermal, and the basal supports greatly reduced and metamorphosed. In a final type of fin (Fig. 54) the line of the basals has become widely spe- cialized, and the characters of the archipterygium have been attained : the fin stem is long, tapering, jointed ; the radials occur as clearly along the hinder as along the ante- rior margin; and, as in Figs. 52, 53, dermal rays contrib- ute largely to the fin area. This form of fin may be noted as most closely approximating in function the limb type of land-living vertebrates. It has recently been urged that the lateral fold origin of the paired fins as thus described is not confirmed by devel- opmental studies, --the especial ground for this belief being that in sharks these fins appear, even in very early stages, as paired lappet-like outgrowths, destitute of inter- vening fin membrane. The perfected fin fold is therefore claimed to represent nothing more than a specialization to bottom-living, since this condition is known to maintain in earlier stages and in more primitive metamerism in the development of skates : and as skates (p. 93) are well known to represent a comparatively recent offshoot from the stem of the sharks, it is accordingly inferred that the chief proof of the lateral fold doctrine is destroyed. Since these objections, however, were raised, the struct- ural conditions of the ancient shark of Figs. 49 and 50 have been described, and may be looked upon as the weightiest evidence of the origin of paired fins from lat- eral folds. Nor does it seem to the present writer that the early character of the fin-fold metamerism of skates is to be looked upon as an unexpected condition. Their 46 SEATSE ORGANS OF FISHES broad longitudinal fins, specialized to bottom-living, become fashioned in an ancestral mould; and it seems not unnatu- ral that they tend to reacquire their latent primitive form at an early period. On the other hand, the fin-fold condi- tion of the shark might be less perfectly shown on account of processes of accelerated development. 4. THE CHARACTERS OF THE SENSE ORGANS OF FISHES It has already been seen that the conditions of aquatic living have caused fishes to evolve adaptive structural char- acters, such as body form, specialized metamerism, organs of progression, and dermal investiture. It is not, accord- ingly, unnatural to expect that, from the same causes, the condition of the sense organs may have been strikingly modified. The sense of "feeling" -using the word in its general meaning --has been of especial value in fishes, and tactile organs appear to be independently developed in all fish groups whose living habits demand them. In the form of barbels they thus occur in members of the various divis- ions of bony fishes, as cod (cusk, Ophidinui) (Fig. 55), drum-fish, Pogonias (Fig. 56), or sculpin, Hemitripterus (Fig. 57). Their form may be lobate, thread-like, or villose ; they are often surprisingly similar in size, position, and innervation ; they usually appear on the inferior head surface, most often in the anterior throat region, in the position most exposed to tactile impressions. The thread- like barbels of the catfishes (Fig. 58, p. 171) are arranged in pairs about the margin of the mouth ; the longest lat- eral pair is connected with the marginal bone (maxillary) of the upper jaw and directed at will. In other mud-living forms, sturgeons (Fig. 160), the barbels have arisen on the under side of the shovel-like snout, directly in advance of Figs. 55-60. — Barbels and tactile sense organs. (After GOODE in U. S. F. C.) 55. Cusk, Ophidium. 56. Drum-fish, Pogonias. 57. Sea-raven, Hemitripterus. 58. Catfish, Amiurus. 59. Spoon-bill sturgeon, Polyodon (ventral view of snout). 60. Sea-robin (Gurnard) , Prionotus. 47 48 BARBELS AND LATERAL LINE the protractile sucking mouth. There can be little doubt that the most aberrant tactile organ in fishes is the long spatulate rostrum of the paddle-fish (Polyodon) of the Mis- sissippi (Fig. 59) : the sense organs are here known to be most highly specialized, although their intimate structure is as yet not understood. Tactile organs are often to be found upon fin structures, especially those of the anterior body region. In the sea-robin, Prionotus (Fig. 60), the sen- sory structures are borne by three anterior fin rays ; these are greatly enlarged, lose their connecting fin web, and can be moved at will in a variety of ways. In all cases the barbels appear to be true and highly specialized organs of touch, and the end organs are comparable ap- parently with the touch papillae of higher forms. Of their extreme sensitivity there can be no doubt, and as far as can be judged from their innervation, it would appear that their function is tactile rather than gustatory, as has been suggested. The limits of these processes, however, are no doubt poorly defined in aquatic living. TJie Lateral Line The sense organs, generally known as the lateral line, or mucous canal system, are looked upon as essentially peculiar to fishes. In the form of a ' lateral line,' they are arranged more or less segmentally along the median line of either side of the body and form a conspicuous feature in the outward appearance of the fish (Figs. 87, 104, LL, 121, LL, 145, LL). Often by striking colora- tion, the lateral line is rendered even more prominent, passing from the head to the tail as a pale or brightly coloured band, against the dusky side of the fish. In the region of the head, however, this sensory structure is, as a rule, no longer conspicuous : it dips below the skin sur- LATERAL LINE ORGANS face and becomes a series of interconnecting tubes, which pass along the most exposed ridges of forehead, cheek, orbit, and jaw rim. Here in different regions, these sen- sory mucous tubes may become dilated, constricted, or ramose, and may communicate with the surface by occa- sional or numerous pores. The mucous canal system has long been a subject of study and investigation. It is looked upon generally as a sensory organ, adapted to the conditions of aquatic living, but its function has not been definitely established. How it was acquired, or how its ancestral conditions have been modified in the present groups of fishes, must at present be looked upon as in many ways doubtful. The simplest conditions of the mucous canal system appear to exist in primitive sharks : and to these the writer believes that the modified sense canals in other fishes may best be referred. The ancestral condition of the lateral line of sharks appears to have been represented in an open continuous groove,* lined with ciliated sense cells, and protected only by an overcropping margin of shagreen denticles (Fig. 61). In this condition it at least exists in the ancient sharks of Figs. 86, 87, 92, and in the Chimaera (Fig. 104). That the canals of the head region were also primitively of this character appears exceedingly prob- able : they are thus retained in the adult Chimaera (Fig. 104, Jf.Q-t In the modern forms of sharks the condition of the * It is to be noted that this condition occurs in deep-sea fishes : it here is evidently an adaptation to their peculiar environment, which causes an early ontogenetic stage to be permanently retained. fin Callorhynchus this condition has been largely lost : the outer margins of the sensory groove have sealed over. 68 ' ' ''''' Figs. 6i--68. — Mucous canals (lateral-line organs). 61. Chlamydoselache, groove-like lateral line. (After GAKMAN.) 62. Plan of lateral line of sharks, longitudinal section. 63. Plan of sensory end buds (lateral line). 64. Sensory tracts of head of larval Amia. 65. Surface openings of tubules of sensory tracts of head of adult Amia. 66. Ramification of sensory tubules in dermal plate of Amia. 67. Cycloid scales of Amia, showing the openings of the tubules of the lateral line. 68. Cycloid scale of the lateral line of Amia, (Figs. 64-68 after ALLIS.) * Denotes an outer opening; — s- the direction showing the course of the sensory tubule. A7. Nerve supply. S. Sensory tissue. of an incoming stimulus. 5° LATERAL LINE ORGANS ij! sensory canals suggests the modifications to which the open sensory groove has been subjected. There are thus forms in which the canal becomes more and more deeply sunken in the integument, and acquires a tubular char- acter by the fusing together of its outer margins. The section of the lateral line of the Greenland shark, Lcs- margus (Fig. 62, v. p. 90), shows the tube-like sensory canal well sunken from the surface, but retaining met- ameral openings at the points. The sensory cells, S, are no longer, as in Fig. 61, scattered evenly along the floor of the canal ; they now occur in metameral masses supplied with a distinct nerve branch, IV, located in the region immediately below the external tubules. When sunken in the integument, the sensory canal is known to have acquired supporting structures to enable its tubular character to be maintained ; in the Cretaceous shark, Mesiteia, an elaborate series of surrounding calcined rings * were thus evolved. Further changes in the mucous canal are often accom- panied by the subdivision of the external apertures ; each of the openings of Fig. 62 might by this process give rise to a series of minute surface pores, as at 6" in Fig. 65, or enlarged, showing the collecting mucous canals in Fig. 66. This ramose mode of termination of the external tubules has been admirably described by Allis f in the ontogeny of a ganoid ; in a larval stage (Fig. 64, S, S, S), the condi- tion of the sensory canals is seen to differ little from those shown in section in Fig. 62 ; although imbedded in the integument, occasional pores are seen, S, S, to open to the surface ; these subsequently by repeated sub- division give rise to the great number of minute open- * A condition somewhat similar has been noted (Leydig) in Chimsera. t On the Lateral Line System of Amia calva. J. of Morph., 1889. EJ 2 LATERAL LINE ings already noted in Fig. 65. A process of this kind is carried to great lengths among the fishes which develop horn-like scales, as Amia, herring, or cod : in the scales of the lateral line region the distal tubules appear at the surface as a cluster of pores, as shown in Fig. 67, or in the detached scale of Fig. 66. The organs of the lateral line (of a bony fish) shown in section in Fig. 63 are regarded by the writer as of a highly modified character. They appear to have been derived from the conditions of Fig. 62 ; the end organ, S, corresponds with that, S, of the preceding figure ; its size, however, has greatly increased, and the intervening sensory tube has been lost ; its metameral opening at the surface corresponds with that of Fig. 62 ; the nerve supply, N, is now seen to have secured a more perfect relation to the end organs. The original significance of the lateral line system as yet remains undetermined. As far as can be judged from its development, it appears intimately, if not genetically related to the sense organs of the head and gill region of the ancestral fish : in response to special aquatic needs, it may thence have extended further and further backward along the median line of the trunk, and in its later differ- entiation acquired its metameral characters. A significant feature of its development is its peculiar innervation. Its lateral tract is innervated by a specially evolved root of the vago-glossopharyngeal group, but its head region is supplied by a similar root of the facial nerve (perhaps also by the trigeminus ; cf. Collinge, Rcf. p. 248). In view of this innervation, the precise function of this en- tire system of end organs becomes especially difficult to de- termine. Feeling, in its broadest sense, has safely been PLYEAL EYE 53 admitted as its possible use. Its close genetic relationship with the hearing organ suggests the kindred function of determining waves of vibration. These are transmitted in so favourable a way in the aquatic living medium, that from the side of theory a system of hyper-sensitive end organs may well have been specialized. The sensory tracts along the sides of the body are certainly well situated to deter- mine the direction of the approach of friend, enemy or prey. The Pineal Eye The presence or absence in fishes of \.}\Q pineal end organ, the "unpaired median eye of chordates," may finally be noted, since the condition of the epipJiysis and its associ- ated structures in fishes has an important bearing on general vertebrate morphology. It is -well known that in many forms of reptiles there exists, at the distal end of the epiphysis, a well-defined sensory capsule, whose structure shows unquestionably its optic function. It has seemed to many, therefore, that throughout the chordates the epiphysis has been primi- tively associated with a median eye, which has degenerated as the paired eyes became better evolved. That it has been retained in an almost perfect condition in reptiles has accordingly been looked upon as an outcome of a life habit which concealed the animal in sand or mud, and allowed the forehead surface alone to protrude : - the median eye thus preserving its ancestral value in enabling the animal to look directly upward and backward. If this view as to the presence of a parietal eye in the ancestral vertebrate is to be generally accepted, one would naturally suggest that the organ should be present, at all events to a recognizable degree, in some of the varied forms 54 PINEAL EYE of the lowest vertebrates extant, — fishes and amphibia. If there are no suggestions of its visual nature among these forms, one would be inclined to believe with O. Hertwig, that the epiphysis was originally of a different function and that its connection with a median eye may have been altogether of a secondary character. The evidence as to the presence, primitively, of a median eye in fishes is certainly far from satisfactory : * in all the forms of recent fishes, no structure has been found associ- ated with the epiphysis which, by the broadest interpreta- tion, could be looked upon as suggesting a visual function. It is possible that fishes and amphibia may, in their extant forms, have lost all definite traces of this ancestral organ on account of some peculiar condition of their aquatic living. On this supposition, evidence of its presence might be sought in the pineal structures of the earliest Palaeozoic fishes --whose terrestrial kindred, and probable descend- ants, may alone have retained the living conditions which fostered its functional survival. It is accordingly of interest to find that in a number of fossil fishes the pineal region retains an outward median opening, whose shape and position suggest that it may have enclosed an optic capsule. If the median eye existed in these forms, it may well have been passed along in the line of descent through the early amphibia (where substantial traces of a parietal foramen occur, e.g. as in Cricotus) to the ancestral reptiles. This view is greatly strengthened, as Beard has shown, by the presence in the lamprey of a pineal end organ (optic?). The evidence, however, that the median opening in the head shields of ancient fishes actually enclosed a pineal * Hertwig (Mark), Handbook of Embryology of Vertebrates, and Cattie, v. Ref. p. 250. PINEAL EYE 55 eye, is now felt by the present writer to be more than ques- tionable. The remarkable pineal funnel of the Devonian Dinichthys (Fig. 134) is evidently to be compared with the median foramen of Ctenodus and PaltzdapJins ( = Sire- noids, p. 122) ; but this can no longer be looked upon as having possessed an optic function, and thus practically renders worthless all the evidence of a median eye pre- sented by fossil fishes. It certainly appeared that in the characters of the pineal foramen of Dinichthys there ex- isted strong grounds for believing that a median visual organ was present : its opening was in the pineal plate, midway between the orbits (PN, Fig. 134). At the surface it was of minute size (X, Fig. 136), but below (Fig. 137) it flared out into a funnel-like form, shown in longitudinal section in Fig. 137 A. The peculiar character of this opening seemed to render it especially fitted for a visual function ; the minute external opening forms an image near the plane of the visceral opening of the funnel, with- out the specialization of a lens, — an image so perfect that it might readily be photographed. It is evident, accord- ingly, that if an optic capsule were enclosed by this fora- men, it would have enabled its possessor to have looked directly upward and backward ; and, without the need of developing lens-like and focussing structures, it could have readily received the images of all outer objects near or remote. But the function of this pineal foramen, unfortunately for speculation, could not have been optical. It occurs in a fish (Titanichthys) closely related to Dinichthys, and, as the writer * has recently found, is of a distinctly paired * He is obliged by accumulating evidence to abandon his former view that the pineal foramen of Dinichthys contained a specialized optic capsule (.V. }'. Rep. of Fisheries, 1891, pp. 310-314). ij 6 PINEAL EYE character, its visceral and outer openings bearing grooves and ridges which demonstrate that the pineal structures must not only have been paired, but must have entered the opening in a way which precludes the admission of the epiphysis. It is now, therefore, that the pineal fora- men which has been described in Siluroids * becomes of especial interest, since its contained structures are ap- parently connected with the lateral line system of paired nerves. It must for the present be concluded, accordingly, that the pineal structures of the true fishes do not tend to con- firm the theory that the epiphysis of the ancestral verte- brates was connected with a median unpaired eye ; it would appear, on the other hand, that both in their recent and fossil forms, the epiphysis was connected in its median opening with the innervation of the sensory canals of the head. This view, it is now interesting to note, seems essentially confirmed by ontogeny. The fact that three successive pairs of epiphysial outgrowths have been noted in the roof of the thalamencephalon, appears distinctly adverse to the theory of a median eye. * Dean, N. Y. Rep. of Fisheries, 1891, and Klinckowstrom, Anat. Anz., 1893, viii, p. 561. Ill THE LAMPREYS AND THEIR ALLIES THE relations of the more primitive chordates to the true fishes have not been considered in the present dis- cussion. A brief account, however, must be given of the Cyclostomes, or Marsipobranchii, which are represented in the recent lampreys and hags. The three prominent forms of Cyclostomes are figured on a following page (Figs. 70-72, A-D). They are eel- like in shape, but are lacking both in paired fins and in an under jaw. Their mouth is of a rounded form, and is suctorial ; when closing, its lateral margins draw to- gether. Their skeleton is of the simplest character, mem- branous rather than cartilaginous ; its elements are never more highly differentiated than those shown in the ac- companying figure (Fig. 69, A). Bdellostoma is shown in surface view in Figs. 70 and 72 A, and in sagittal section in Fig. 69. It is looked upon as the most archaic form of the living Cyclostomes. Barbel-like structures surround its mouth region ; its nasal canal (Fig. 69, N and C) has a forward opening at the snout, and a hinder one piercing the roof of the pharynx, -a very exceptional character in fishes; its tongue, stud- ded with rows of rasp-like teeth,* may be greatly everted, * The teeth of Myxinoids are cuticular structures, and may well have been evolved within the limits of the group. Beard has homologized them with the teeth of sharks, but his determination of the presence of true enamel has not been confirmed (Ayers). 57 a c E o bo -d ™> a .5 o '" '5 'S S .S > i t> rt ~ c/) OJ rt ^i O I-. —. c rt c . 5 Pi 2 1• " s!^ - <• . c" c "^ Hi O '^ •31 -> _o I ^ 8 -s I a e g O bo c O o O o c oja _ P O . o T3 rt - c v c CQ- O ^ I m. 6v 3 ^D C C I- OJ aj C tn bo C W 111 ft- S. 3 0 cn -d . P a g O rt 3 b SJ u. -S P -a a xi 0) w = -^ ^ - '5,E- <"' ••= SP . t bfl ^ 2 = C3 CJ THE LAMPREYS 59 as in Fig. 72, A, and then drawn in by stout tongue muscles, T (Fig. 69) ; its digestive tube is almost straight, terminating at the base of the tail region at A ; the region of the gullet, OE, is pierced by a number of branchial openings, varying from seven to fifteen, often assymmetrical. The body cavity is an extremely large one for the size of the contained viscera. An unpaired fin, supported by delicate, unbranched (dermal) rays is restricted to the hindmost part of the body. Passing down the side is a row of mucous pouches by which a remarkable supply of slime is secreted. The living animal is enabled, by the peculiar character of this slimy secre- tion, to render a pailful of water jelly-like in consistency. Bclellostoma occurs plentifully in the bays of the Pacific coast of America, notably at Monterey, California. It is active in its movements, is carnivorous, and is well known to take a baited hook. Its numbers make it an enemy of the fishermen, entangling and sliming their set lines, and destroying the captured fish. It is said to feed at night, although little is yet known of its general habits of living. None but adult specimens have thus far been observed. The Hagfish, Myxine glutinosa (Fig. 71, and 72, B}, is in many regards similar to Bclellostoma ; it differs mainly in the character of its unpaired fin and in its branchial struct- ures (Figs. 9, 10). As already noted, the outer ducts of the gills, instead of opening separately at the surface as in Fig. 70, are drawn together tail-ward, and terminate on either side in a common ventral opening (Fig. 71, at the point*). The unpaired fin is almost lacking in supports; its ventral origin is even as far forward as the branchial openings ; the anus, as a slit-like opening, pierces it in the tail region. As in Bdellostoma, the nasal canal begins at the snout, and at its hinder opening pierces the roof of '. *~ CJ ^ rO "~ •z <}> 0) ~ r1. ~* {/7 bo.-S 5 ft ^ in a! 3 « o t § 2 a t^ U 1) s-a . o S. "3 m\ O en &* U.^g U E ^ \ ^ E-ai o -S •> * LAMPREYS AND HAGS 61 the pharynx ; this, with other related conditions, has caused Myxine and Bdellostoma to be included in a sub-group of Cyclostomes, as Myxinoids, or Hyperotretcs* In each genus there is possibly no more than a single valid- species. Myxine is a well-known form : it occurs along the Atlan- tic coast at moderate depths. It is exclusively carnivorous, B Fig. 72. — A-D. Ventral aspects of heads of (A) Bdellostoma (after AVERS) ; (/>') Myxine (after GiJN- THER) ; (C) Ammoccetes (after GUNTHKR) ; (D) Pe~ trotnyzon (after GiJNTHER). often boring its way into the abdominal cavity of (diseased or injured) fishes, and with them is brought to market ; it is also taken not infrequently by line fisher- men. The smallest example that has thus far been described is 6 cm. in length ; it was recorded by Beard. (V. Ref. p. 239). The Lamprey, Petromyzon, is the most perfectly studied member of the Cyclostomes. Its species are common to the continents of the northern hemisphere ; and in South America and Australia there occur very closely allied genera, as Mordacia and Geotria. The largest lamprey, P. martinis (Fig. 72, and C, D), is known to attain a length of nearly four feet ; it occurs in the coast * v. Glossary, p. 228. 62 THE LAMPREYS rivers, ascending them in numbers in the springtime (April) on the way to the spawning grounds (v. p. 182). During its adult life it is supposed to be exclusively car- nivorous, to some degree, perhaps, parasitic, although many doubt that it is truly parasitic in the sense of entering the body cavities of healthy fishes. It certainly is often taken attached to other fishes, as shark, sturgeon, or salmon. Immature lampreys differ so strikingly from the adults that they were formerly regarded as species of a separate genus, Ammoccetes (v. p. 215). In feeding habits the am- mocoete is widely unlike the mature form ; it is toothless (Fig. 72, C), and in part mud-eating, i.e. vegetivorous. Petromyzon must be regarded as the most highly organ- ized of Cyclostomes. Its mouth has no longer the fring- ing barbels of Myxinoids, --which suggest, according to Pollard, the buccal cirrhi of Amphioxus, — it has acquired stout supporting cartilages and a funnel-shaped form, studded with a series of conical teeth, as shown in Fig. 72, C. The teeth of the hinder mouth region now appear almost as though they were supported by a mandibular cartilage ; the tongue, as in other Cyclostomes, bears the teeth which are probably of the greatest functional impor- tance. The nasal canal of Petromyzon has its outer opening on the dorsal surface of the head ; its inner end, however, does not perforate the roof of the mouth, although produced backward as a blind sac, closely apposed to the pharynx. Petromyzonts are, accordingly, arranged as the sub-group Hyperoartia, in contrast to the Myxinoids. Further structural characters, which the lamprey seems to have derived from simpler conditions, may be noted in its impaired fin, gill chamber, nervous system, and skele- ton. The unpaired fin has subdivided into dorsal and caudal elements, and is now supported by well-marked AFFINITIES OF LAMPREYS £3 rays, which (sometimes) bifurcate. The branchial region of the adult lamprey's gullet is restricted to a pouch-like diverticulum (v. p. 263 and Fig. 326). A 'sympathetic' nervous system, and a ' lateral line ' has appeared : the latter passes down the side in two branches, one above and one below the median lateral plane : its end organs are the pouches of nervous epithelium which in Myxi- noids are scattered generally over the body surface. The skeletal structures of the lamprey (Fig. 69, A) indicate well-marked advances : a stouter supporting tissue of car- tilage-like character has appeared ; the brain case is partly roofed over ; neural processes, NP, a branchial basket, BB, and a series of mouth cartilages are especially note- worthy. Affinities of the Cyclostomes The relations of the group, Cyclostomi, to the earlier chordates, and, on the other hand, to fishes, have been by no means definitely established. Dohrn and others have suggested that the Cyclostomes are greatly degenerate, and are even closely akin to the recent bony fishes, as perch or cod. Their views have been based upon several struct- ural characters, notably vestigial organs, such as the ap- pendages at the sides of the cloacal opening of Petromyzon which were believed to represent pelvic fins ; and there was further taken into consideration the belief that the entire group was one of degenerate life habits. The views of these writers, however, do not appear to be confirmed by later studies, and the belief is becoming more and more general that Cyclostomes represent a very ancient chordate stem whose ancestral form is most nearly exemplified by Bdel- lostoma. Parasitism has been acquired to a limited degree, but does not appear to have affected the general characters 64 KINSHIPS OF CYCLOSTOMES of the group. Among its primitive features are to be in- cluded : skeleton and muscles, continuous vertical fin, gill characters (p. 260), viscera (p. 263), urino-genital organs (pp. 266, 270), nervous and circulatory systems (pp. 260, 269, and 274). With these must be taken into account: absence of mandible* and of paired fins and girdles; and in addition the remarkable conditions of metamerism (p. 14). Little more that a vague kinship between lampreys and fishes has been established by the study of living forms. And, on the other hand, it would appear equally impracti- cable to obtain evidence bearing upon this problem from the side of palaeontology. All that is known of the recent Cyclostomes more than suggests that their soft body struct- ures would prove most unfavourable to fossilization. 'It would be only, therefore, in the event of some of their ancient members possessing calcified structures that palae- ontology would be able to offer a clue as to their ancient affinities. Upon the problem of their descent the evolution of fishes has, however, an undoubted bearing, in suggesting the lines and effects of aquatic evolution and the perma- nence of generalized types. It certainly tells of the ex- treme slowness of the evolution of aquatic forms and con- vinces us that the ancestral Cyclostome could only have occurred in a time stratum exceedingly remote. Palaeon- tology cannot perhaps hope to obtain more than sugges- tions of the ancestral forms, although these, from their generalized characters, may well have survived during geo- * The cartilages of the mouth region of Cyclostomes have been homologized with the structures of gnathostomes ; Pollard recently (Anat. Anz. ix, pp. 349-359) ascribes a cirrhostomial origin to the mouth parts of a Teleostome (catfish), which the writer cannot believe has been demonstrated; variations in the number, shape, and function of the cartilages of the mouth rim of Cyclostomes might well have occurred within the limits of this ancient group. A FOSSIL LAMPREY 65 logical ages. It can, however, show that Cyclostomes are not the degenerate descendants of shark-like forms ; and -if only by analogies in the evolution of fishes --it may still be able to demonstrate with fair probability their genetic kinships. It may, for example, prove that in the most ancient time there existed undoubted Cyclostomes, and that these in many and most specialized forms were even then branching-off twigs of a great descent tree. In such an event an inference would certainly be the more reasonable which derived the advancing line of fish descent from the genealogical tree of the more primitive Cyclostomes, than that vice versa. It is now accordingly of especial inter- est that the fossil remains of what seems undoubtedly a lamprey (Fig. 73) have been discovered in the Devonian ; and this, to- gether with a better knowledge of the ancient and curious chordate group, Os- tracoderms, may, it is hoped, lead to some ,•• •' solution of the Cyclostome puzzle. Fig. 73. — The De- vonian Cyclostome, The OstraCoderWlS Palceospondylus gimni, T. X 4. (After TKA- Ostracoderms, as they are called from QUAIR.) Achanarras quarry, N. Scotland. their shell-like, dorsal and ventral derm plates, are certainly the oldest known remains of verte- brates.* In their simpler forms they occur in the Upper Silurian ; they flower out in a variety of types in the De- vonian, and shortly become extinct. In the present con- * The earlier (Ordovician) vertebrate remains described by Walcott are as yet uninterpretable. F FIG. 74 75 Figs. 74-79. — Pteraspis (restored). X \. (After LANKESTER.) Lower Old Red Sandstone, Herefordshire. 75. Palcsaspis americana, Claypole. X 4. (Restoration after CLAYPOLE, somewhat modified by the writer.) 76. Pteraspis, dorsal shield, slightly restored. (After LANKESTER.) 77. Pteraspis, ventral shield (" Scaphaspis ") , showing mucous canals. (After SMITH WOODWARD.) 78. Cephalaspis lyelli, side view. (Re- stored by LANKESTER.) 79. Cephalaspis lyelli, dorsal aspect, x \. (After L. AGASSIZ.) Specimen from Old Red Sandstone, Forfarshire. A C. Rhomboidal scales from different body regions. B. Tessera from middle layer of head shield. 66 OSTRA CODE RMS 67 nection they may be described, if only to indicate that they are in no way closely connected with the ancient shark types (p. 78), and that they are accordingly of but indirect interest in the descent of jaw-bearing vertebrates. Ostracoderms may readily be reduced to three general types, Pteraspid, CcpJtalaspid, and PtericJitJiid. The first, oldest, and probably simplest occurs in the Lower Old Red Sandstone of Herefordshire. It was provided with arched back and breastplate (Figs. 74, 76, 77), from whose anterior lateral notches a pair of eyes protruded ; the sur- face of these plates (Fig. 77) appears to have been grooved for sensory canals. Pteraspis, as seen in the restoration, had a snout plate, a dorsal spine, and a body casing of rhomboidal scales ; its mouth was probably in the region immediately below the eyes, in front of the margin of the well-rounded ventral plate ; this was generally regarded as the dorsal plate of a kindred genus, " Scaphaspis" Closely related is the American Pteraspicl, Palceaspis (Claypole), from the Upper Silurian of Pennsylvania (Fig. 75) ; this form lacks the dorsal spine of the English species ; it has a well-marked lateral plate intervening between those of the back and ventral side, and, according to its discoverer, Professor Claypole, possessed pectoral fins similar to those seen in Fig. 123. Its hinder trunk region is unknown. ^? *-* ^j Cephalaspis, the second type of Ostracoderm, is from the Old Red Sandstone of Scotland (Figs. 78, 79). It was curiously suggestive of a trilobite, and with little doubt mimicked this ancient crustacean in its life habits. Its most prominent feature is a crescent-shaped head, with sharp rounded margin like a saddler's knife. This is protected dorsally by but a single plate, arching upward and backward ; at its summit was a pair of closely apposed eyes, and near its flattened rim were pouch-like sensory Fig. 80 SL Figs. 80-82. — Pterichthys testudinarhis, Ag. ; restored by R. H. TRAQUA1R, from the dorsal aspect (80), ventral aspect (8i),and lateral aspect (82). The double dotted lines indicate the grooves of the sensory canal system ; and in the trunk, the thick lines repre- sent the exposed borders of the plate, the thin line showing the extent of the overlap. ADL. Anterior dorso-lateral. AMD. Anterior median dorsal. A VL. Anterior ventro- lateral. EL. Extra-lateral (or operculum). L. Labial. MOCC. Median occipital. PM. Premedian. PDL. Posterior dorso-lateral. PMD. Posterior median dorsal. PVL. Pos- terior ventro-lateral. SL. Semilunar. (Figure from SMITH WOODWARD.) 68 PTERICHTHYS AND CEPHALASPIS 69 organs. The angles of the head plate are in some genera produced most acutely, and bear spines which served prob- ably in progression. The body walls were encased in metameral derm plates, which became arched in the median line to serve as a dorsal fin. A heterocercal tail and an anal fin were also present. Problematical opercu- lar flaps protruded at the sides of the head plate, and represented (as is now known) a continuation of the elastic middle layer of the head plate. Pterichthys must be looked upon as the culminating type of these anomalous forms (Figs. 80-82). As in some Cephalaspids, there are two body regions that are cui- rassed, --head and thorax. The tail portion is encased in dermal plates ; it bears a dorsal fin and a clumsy heterocercal tail. In the consolidation of its armoured parts the elements are usually clearly indicated. The curious arm-like jointed appendages at the lateral head angle were formerly regarded as homologous with the opercular flaps of Cephalaspid, but are now known to be nothing more than the lateral head angles produced and specialized (i.e. jointed for locomotion). The strengthen- ing spine of the dorsal fin is also but a primitive speciali- zation of the body integument ; it is formed by a pair of the bent scales of the dorsal ridge, and is not, therefore, homologous with the radial fin cartilages of fishes. In Cephalaspids and Pterichthids there occurs a pineal plate (or its equivalent) which may have been either movable or fixed. In this are to be found the paired eyes and the socket of a median unpaired eye (?). In all of these singular forms mouth parts* are wanting. In * Smith Woodward has since described a pair of inturned labial plates in the mouth of Pterichthys. Their position suggests that the sides of the mouth rim might become apposed, as in the Cyclostomes. 70 KINSHIPS OF OSTRACODERMS no instance has a trace of endoskeletal parts been ob- served. The more that is determined of the structural characters of Ostracoderms, the less is it possible to accept the views as to their affinities with forms other than "fishes," either (Cope) as to their permanent larval-ascidian char- acters, or (Patten) as to their relationships with arachnids. Their general kinship is certainly to the fishes. Accord- ing to Smith Woodward, the markings appearing on the visceral surface of head tests indicate the presence of gill pouches ; in some forms clearly marked furrows sug- gest the possession of vertical semicircular canals ; fish-like sense organs occur (Fig. 77) ; and their derm plates, in their cancellated and bone-like characters, cannot well be likened to the exoskeletal parts of invertebrates. The lamprey-like form, Pal&ospondylus gunni, Traquair (Fig. 73), in the Lower Devonian is by many looked upon as the actual solution of the Cyclostome, and even of the Ostracoderm puzzle. This interesting fossil was discov- ered by Dr. Marcus Gunn, in the Lower Old Red Sand- stone of Caithness, and was described in several papers by Traquair (Trans. Edin. Soc., 1892-1894). It is of very small size, commonly of about an inch in length, but is admirably preserved (Fig. 73). There can be no doubt that Palaeospondylus possessed a ring-like mouth sur- rounded by barbels like those of a Myxinoid, and that it lacked paired fins. But as a Cyclostome it must have highly specialized, having the same relation to the more primitive Cyclostomes of its day, as had the minute Acan- thodians (p. Si) to the existing sharks. It had thus a remarkably large caudal fin with elaborately bifurcating supports ; it had evolved stout, ring-like vertebrae, even in the caudal region, which had developed stout neural proc- 1 \ILALOSPOND YLUS 7 ! esses. Its skull was highly evolved : in its anterior part were represented, according to Traquair, the palatine car- tilages ; the brain case was complete, and the auditory capsules were of relatively enormous size. The lateral plates of the neck region are as yet uninterpretable. From the evidence of Palaeospondylus, accordingly, it may reasonably be inferred that lamprey-like forms existed in highly specialized conditions, even at the beginning of Devonian times. If they then existed, it is of course not impossible, and perhaps even not improbable, that their offshoots may have culminated in the Ostracoderms, as Smith Woodward has suggested. These can certainly belong to no gnathostome stem. Their organs, though often highly specialized, were yet of the most primitive order, — lack of paired appendages,* softness of axial parts, lowly sense organs ; even the dermal plates, elaborate in their subdivision or ornamentation, or in the special uses, as "opercula," "pectoral fins," or "fin rays,"f are yet but primitive specializations of the exoskeleton. * The presence of paired fins in Palaeaspis, as determined by Claypole, has not been confirmed. The present writer, to whom the type specimens were kindly shown by their describer, must regard these structures as elasmo- branchian (Chimgeroid ?) spines, in crushed condition, accidentally associated with the head region of the fossil. f It is obvious that these structures are but analogous to the opercular and fin structures of fishes, and would tend to separate, rather than closen, the ties of kinship of these groups. IV THE SHARKS ALL true fishes may conveniently be grouped into the four sub-classes that have been noted (p. 8) in the introduc- tory chapter. These are now in turn to be considered, and in this review the principal forms, fossil and recent, of each group must be exemplified. From the standpoint of their structural and developmental characters, a general idea of the mutual relationships of the fishes may finally be deduced. The sub-class Elasmobranchii, which includes the sharks and rays, is usually regarded as representing most nearly the persistent ancestral condition of fishes, and, indeed, of all other jaw-bearing vertebrates. As a group it should certainly be taken first in the present discussion, as a con- venient basis of comparison. Sharks and rays should be looked upon at the beginning as the representatives of the oldest, most widely diffused, and possibly largest group of fishes. In their living forms they suggest but faintly the number and variety of their fossil kindred. It is generally thought that the his- tory of this group, when more perfectly determined, is to furnish the most important evidence as to the general lines of descent of the fishes. 72 i- -C - r; - 3 OJ 3 £ 15 "? •> '5 "*^ c C • !•> ^ £3 Q.SJ -^ o C -4-T *^ ^ H C T3 > rt oj - rt U1-1 rH U io S S c/i O M S SE •§ g^ — « -a c .> o T o - I «° = c- c . -a rt M . -. ^ Ql >> ^> .= H! 15 -2 ^ 5 i 1 i 5 s o .s c o --. o 73 74 STRUCTURES OF SHARKS Structural Characters The definition of a shark emphasizes its cartilaginous skeleton, investiture of shagreen, uneven (heterocercal) tail, and its separate and slit-like gill openings. Its more defi- nite characters may well be summarized in the accompany- ing figure (Fig. 83). I. The SKELETON is cartilaginous (cf. Fig. 83, 84, and p. 252), sometimes calcified generally, but always (in recent forms) lacking in dermal bones. Behind the simple, trough- like brain case the vertebral rod, beginning at the occip- ital condyles, is clearly segmented ; the notochord is often retained, especially in the tail region, NC, but is encroached upon by the cartilaginous rings, centra, C, arising metamer- ally in its sheath (Fig 85). The vertical supports of each centrum include a well-marked ventral plate, the haemal arch and spine, HER, — which in the tail region probably represents as well the cartilaginous elements of the fin support, — and a pair of small dorsal plates, the neurals and interneurals, NP, 1C, each capped by a neural spine, NS. The fin supports compare closely in structure with the vertebral processes ; they form a large part of the functional fin, and preserve clearly, both in basal and radial parts, their metameral character. This segmental arrangement is also characteristic of the supporting ele- ments of the cavity of the mouth and throat. These con- stitute the "visceral arches" (cf. p. 256) which pass backward from the rim of the mouth to the region of the pectoral fin. The first visceral arch strengthens the rim of the mouth ; it is margined with teeth and functions as jaws,* * The writer believes that the upper element of the mandibular arch is to be regarded as the palatoquadrate cartilage, rather than the pre-spiracular ligament. 75 76 STRUCTURES OF SHARKS P and M. The second arch serves as the principal support of the jaw hinge, HM, while holding in position, ventrally, the hinder arches ; it also supports the tongue, and forms the hinder border of the spiracle (p. 19). The succeeding arches, usually five in number, are the bearers of the func tional gills, their jointed structure permitting the dilating and contracting movements of breathing. As a further skeletal element of the Elasmobranchs the sub-notochordal rod is to be mentioned. It is present in the larval stages of sharks, and appears to persist in the adult Cladoselache (p. 79). It is a prominent structure of the hinder body region, passing along, like a second notochord, immediately below the vertebral axis. Its significance is unknown. II. The INTEGUMENT of the sharks, as has been noted (p. 23), a a. is studded with shagreen denticles, Fig. 85. — Vertebras of often in metameral arrangement. shark (Squatitiii), longitudinal section. (After ZITTEL.) These have been shown to corre- ch. Notochord. rf. Calcified n(j dearly with the teeth> rim and anterior surface of J centrum. iv. Intervertebral The Soft Structures characteristic space, w. Centrum. i • 1 i of the Elasmobranchs include : — III. GILLS, arranged metamerally (p. 19) ; the most anterior one partly functional in the spiracle, SP. IV. SENSE ORGANS OF THE LATERAL LINE, in some forms in an open sensory groove, in others sunken and constricted in metameral pouches. V. BRAIN, simple in its segraental characters and cranial nerves (v. p. 274). VI. NASAL ORGAN, EYE AND EAR, as shown on p. 276. VII. RENAL AND REPRODUCTIVE SYSTEMS (p. 270), ab- dominal pores (p. 271). FOSSIL SHARKS nj VIII. DIGESTIVE TUBE with a single bend, S, /, the intestine provided with a spiral valve (p. 263), terminat- ing, together with the ducts of the renal and reproduc- tive organs, in a common cloaca, CL (p. 266). Liver, L, spleen, and pancreas large ; mesenteries simple but greatly fenestrated ; air bladder absent. IX. HEART with a contractile arterial cone, CA, con- taining several rows of valves (p. 260) ; circulatory system in general as described on p. 269. X. " CLASPERS " developed at the hinder margin of the ventral fins as the intromittent organ of the male. They are rudimentary in the female, CL' . Each clasper is the trough-like hinder rim of the fin, which becomes transformed into the compact, elongated, tube-like sperm canal. Its tip is often studded with elongated shagreen denticles whose recurved cusps retain it in copulo. Fossil SJiarks Of all fishes, sharks certainly suggest most closely in their general structures the metameral conditions of the Cyclostome: it should also be noted that they possess the greatest number of body segments, in some instances over three hundred, known among vertebrates. Little is known, however, of the primitive stem of the sharks, and even the lines of descent of the different members of the group can only be generally suggested. The development of the recent forms has yielded few results of undoubted value to the phylogenist : it would appear as if palaeon- tology alone could solve the puzzles of their descent. The history of fossil sharks has as yet been but imper- fectly outlined. The remains of the more ancient forms have usually proven so imperfectly preserved that little could be determined of their structural characters. Spines, 78 PALEOZOIC SHARKS teeth, shagreen denticles, have proven the antiquity of the shark stem and the wealth and variety of its fossil forms ; they have provided the evidence that even in Silurian times there lived sharks whose exoskeletal specializa- tions had progressed further than in their recent kindred : that in the Carbon there occurred the culminating-point in their differentiation, when specialized sharks existed whose varied structures are paralleled only by those of existing bony fishes, -- sharks fitted to the most special environment ; some minute and delicate ; others enormous, heavy, and sluggish, with stout head and fin spines, and elaborate types of dentition. But the detached fragments of the fossil sharks can give little satisfactory knowledge of their general structures. The simpler the form of the shark, indeed, the less liable is it to become fossilized. The more generalized of the ancient sharks must thus remain structurally unknown until more perfect fossils come to be found. To this event the discoveries of the past few years have certainly yielded most encouraging aid. Several forms of sharks of the Lower Carbon and Permian have been obtained in a con- dition of admirable preservation, and have already con- tributed materially to the morphology of Elasmobranchs. Other early forms may be forthcoming which will be found to have retained sufficient of the characters of their an- cestors to warrant more definite views as to the general relationships of fishes. Of the three primitive forms of fossil sharks lately described : the earliest, from the Ohio Waverly (Lower Carbon) is Cladoselaclic, Dean ; a later and puzzling form, from the Carboniferous, is CJiondrenchelys, Traquair ; the latest from the Permian and Coal Measures, is Pleuracan- tJins, Agassiz. The only early shark type that had previ- CLADOSELACHE 79 ously been structurally known was that of the aberrant and highly specialized Acanthodiau of the Coal Measures. Cladoselache is the most primitive, as well as the oldest, of these ancient sharks. It is relatively of small size, varying in the length of its species from two to six feet. Its outward form, as restored by the writer, is seen in Fig. 86, and in ventral view in Fig. 86 A. The shape is clearly Fig. 86. — Cladoselache fyleri, Newb. X \. Restoration by writer. After speci- men in the museum of Columbia College from Cleveland shales, Ohio. Fig. 86 A. — Cladoselache fyleri ; ventral aspect. that of a modern shark; the fins, too, in their size and position, have somewhat of a modern look ; and at the base of the tail occurs the small horizontal keel of many living forms. But in spite of these peculiarities, Cladose- lache must be looked upon as the most archaic, and, in many ways, the most generalized of known sharks ; its paired fins are but the remnants of the lateral fold (p. 43), serving alone as balancers ; the tail, curiously specialized, is widely heterocercal, its hinder web lacking supports in the upper lobe (p. 36) ; the vertebral axis is notochordal ; So CLAD O SELACHIAN SHARKS and the writer now finds that an exceedingly simple con- dition existed in the neural and haemal arches ; they prove to be of moderate size and thickness, each a tapering rod of cartilage, forked at its base ; each body segment con- tains a single neural and haemal spine, closely alike in size. Unlike modern sharks, Cladoselache was without claspers : its eggs must have been fertilized after their deposition, as in the majority of fishes other than Elasmosbranchs. The gill openings, at least seven (probably nine) in number, appear as in the restoration, to have been shielded anteriorly by a dilated dermal flap. A spiracle was probably present. The jaws were slender, and apparently hyostylic (p. 257) ; * the teeth are of the pattern of shagreen denticles, but occur Fig. 86 B. — Teeth of (" Cladodus") Cladoselache. X £. The above forms occur in different regions of the mouth. in clusters ("Cladodus" Fig. 86, B). The mouth was ter- minal in its position. The nasal capsule was apparently not connected with the mouth by a dermal flap. The eye was protected by several rings of rectangular plates, clearly shagreen-like in character. The integument was finely studded with minute lozenge-shaped denticles, and was everywhere lacking in membrane bones. The lateral line retained its groove-like character. The shark, Acanthodes (Fig. 87), of the Coal Measures is now to be regarded (Smith Woodward) as a member of a highly specialized Palaeozoic group. And its many spe- cialized structures — added to its greatly reduced size- * As Claypole's recent figure seems to demonstrate. Am. Geol.,Jan. ACANTHODES 81 may, perhaps, have been the cause of its extinction. The present writer believes that Cladoselache may well have represented the ancestral form of the Acanthodian. The generalized structures of the former have given place to a perfected dermal armouring, and a completed series Fig. 87. — Acanthodes wardi, Egert. X about -J. (Restoration slightly modified after SMITH WOODWARD.) Coal Measures, England. of balancing fins. In Acanthodes the shagreen denticles have thus become greatly enlarged and thickened, their flattened and enamelled surfaces wedging closely to- gether (Fig. 88) ; and on the roof of the head and mouth traces of membrane bones have appeared. Around Fig. 88. — Acanthodes gracilis, Beyr. Shagreen. X about rsc. (After ZlTTEL.) a. Outer face. b. Inner face. c. Isolated denticle. the eyes the many shagreen plates of Cladoselache have fused into a group of four. Supporting the dermal gill frills, there have also appeared rows of minute sculptured plates (corresponding, perhaps, to those, BR, of Fig. 145), homologous, apparently, with shagreen denticles. 82 ACANTHODIAN SHARKS Further resemblances to Cladoselache are to be traced in the position of the fins, gill slits, eyes, mouth, nasal cap- sule, and in the structures of the caudal fin (Kner), and of the lateral line. The teeth, however, are no longer of the derm-denticle pattern ; they have become few in number, large, and "degenerate" in their fibrous structure (Fig. 88, A). The fins are clearly more per- fect balancing organs than those of the older shark ; their anterior rim is Teeth of f°rmed by a stout spine, representing, the present writer believes, the con- ^^^^^^^ crescence of the radial fin supports ; it is heavily crusted over with the calcifications of shagreen denticles. The functional fin area has thus become dermal, and is lacking in supports, excepting in the pectoral fin. This, as the most highly specialized of all the body fins (p. 41), appears in some cases to have evolved strengthening (dermal) rays in its proximal portion (as in Figs. 87 and 32). Fig. 88 A. Acantkodopsis wardi. X I. From sketch after speci- men in British Museum. Fig. 89. — Climatius scutiger, Egert. X I. (From ZlTTEL, after POWRIE.) Old Red Sandstone, Forfarshire. In connection with these fin structures the remarkable Acanthodian, Climatius (Fig. 89), should finally be men- tioned. In this form the paired fins are represented by a series of fin spines whose size grades backward from the pectoral region ; a series of paired fins appear, therefore, PLE URA CANTHUS O ,03 o 5 Ct, D C^ a •o •a o £ o •a T) (/) Cj Ji <" "rt 0. s z -a — ~~' a ^ o Od P 5 £ . o 3 c ro j— i— i CJ g — u 5 3 '-5 -H C > rt "u P OH 84 PLEURACANTHID SHARKS Fig. 90 A. — Teeth of Pleuracant/ius. (After DAVIS.) X essential to sharks. That it was actually a shark cannot be doubted ; its gills, six or seven in number, opened separately to the surface ; its teeth (Fig. 90 A) were typically shark-like, arranged in many rows on Meckelian and palatoquadrate cartilages ; a tuberculated dorsal spine was present ; claspers occurred in the male ; the vertebral column, although notochordal, N, presented intercalary plates, 1C, and the jaw was essentially hyostylic, HM. On the other hand, many of its struct- ures are clearly tran- sitional to the Dip- noan : the pelvic fins are shark-like, with the radial supports, R, arising from but one side of the line of basals, B ; but the pectoral fin is typi- cally archipterygial, and the caudal diphy- cercal, as in the lung- Fig. 90 B. — Dermal bones of the head roof of fishes 111 this re- Pleuracanthus. X 5. (After DAVIS.) gard the continuous dorsal fin, with its separate basals and radials, B and R, is again noteworthy. But most singular of all the features of this lung-fish-like shark were its integumentary charac- ters ; shagreen tubercles had disappeared on the body sur- face, and derm bones had appeared roofing the head : their arrangement (Fig. 90 B} is strikingly similar to that of the lung-fish of Fig. 124. ( 'HONDRENCHEL YS 85 The final form of Palaeozoic shark whose structural char- acters have in any way been described is Chondrenchelys. It appears to have somewhat resembled the Pleuracanthid in its elongate form and tapering tail ; but as yet the details of its structure have not been discovered. In its vertebral characters it had certainly made a marked ad- vance ; the notochord had become greatly constricted ; and well-marked centra and arches were present. These appear to have been highly calcified, and show a peculiar Fig. 91. — Port Jackson shark, Cestracionphilippi (?). X TV (After GARMAN.) Australia. A. Ventral. B. Anterior, and C. Dorsal aspect of head. beaded or fretted structure which in this form is appar- ently unique. Other ancient sharks, as far as can be inferred from fragmental structures, appear to have closely resembled forms that are still extant. Such unquestionably were the Cestracionts, a group of sharks especially abundant in the early Palaeozoic seas, judging from the numbers of their fin spines and 86 PORT JACKSON SHARKS pavement teeth that have been preserved. Their bygone role was certainly a long and important one. In some of their forms they could have differed but little from their single survivor, the Port Jackson shark, Ccstracion (Heterodontus) (Fig. 91, A, B, C). In others, the denti- tion and dermal defences suggest a wide range in evo- lution. Their general character appears to have been primitive, but in structural details they were certainly specialized ; thus their dentition had become adapted to a shellfish diet, and they had evolved defensive spines at the fin margins, sometimes even at the sides of the head. In some cases the teeth remain as primitive shagreen cusps on the rim of the mouth, but become heavy and blunted behind ; in other forms the fusion of tooth clus- ters may present the widest range in their adaptations for crushing ; and the curves and twistings of the tritoral sur- faces may have resulted in the most specialized forms of dentition (e.g. Janassas, Petalodonts, CocJiliodonts, and Psam- modonts of the Coal Measures) which are known to occur not merely in sharks but among all vertebrates. Equally interesting may prove the evolutional details of other cestraciont structures when they come to be known. Ray-like proportions may well have been evolved even among the earliest Palaeozoic forms. The surviving member of this group, Cestracion, sug- gests in itself the adaptations of a bottom-living form in its greatly enlarged pectorals. Its genus, however, has not been traced earlier than the Mesozoic, although its comparatively generalized dentition (Fig. 27) suggests a far more remote descent. It is of interest to note that Cladoselache approaches in its dentition the characters of the primitive Cestracionts (e.g. Synechodus). 88 PRIMITIVE LIVING SHARKS Recent Sharks The forms of Sharks and Rays common at the present time are generally looked upon as closely related genetically, although their lineage cannot be defi- nitely traced. As far as palae- ontological evidence goes, they may well have been derived from a single Palaeozoic an- cestor. Perhaps of all recent forms, Chlamydoselache (Fig. 92), and Notidanus (Heptanchus, or Hep- tabranchias) (Fig. 93), which are universally regarded as " primi- tive," have inherited most di- rectly the features of this gen- eralized Palaeozoic form. But which of these two sharks must be regarded as resembling its re- mote ancestor the more closely seems to the writer a very doubt- ful matter. Chlamydoselache derives its great interest from its late discovery (1884, Gar- man), rareness, and Pleuracan- thid type of teeth (Fig. 92, A) ; but now that it has been taken in numbers — comparatively - in deep water, one is inclined to believe that many of its RECENT SHARKS 89 "primitive" features, like its eel-like shape, may partly be due to its environment : its resemblance, moreover, to the Pleuracanth has since been found to be of a superficial character. Notidanus, on the other hand, adds to its primitive characters the presence of no less than seven Fig. 94. — The horned dog-fish, Squalm acanthias, L. in U. S. F. C.) Atlantic. X J. (After GOODE gill slits, — a feature which morphologists generally are inclined to regard as of great significance. The many forms of recent sharks have certainly not diverged widely from the stem of descent which Notidanus may well represent : they retain the sub-cylindrical body form, specializing more or less to environment ; in deep- sea genera the body length appears proportionally in- Fig- 95- — The thrasher shark, Alopias vulpes (Gmel.), Bonap. 2. >< T'O. Atlantic. creased : predatory forms, such as Squalns, Alopias, Lanuui (Figs. 94, 95, 96), acquire great size and strength, travel great distances, and are enabled to become cosmopolitan. Among the minor details to which their evolution has been carried, may be noted : the pattern, size, and arrange- RECENT SHARKS ment of teeth and shagreen denticles ; the calcification of the vertebras (great differences sometimes occurring in the same genus, e.g. Scyllium), the size, disposition, and num- Fig. 96. — The mackerel shark, Lamna cornubica (Gmel.) , Fleming, x sV. North Atlantic. her of the fins, the more or less pouch-like character of the sensory canals. In the basking shark, CctorJiinns (Selache) (Fig. 96 A), widely specialized conditions occur in the gill rakers, which enable the throat to retain the smallest food organ- Fig. 96 A. — The basking shark, Cetorhinus maximus, (L.) Blainville. cf. X E'O. (After GOODF. in U. S. F. C.) isms. In another shark, Lcemargus (Fig. 96 B}, the eggs are probably fertilized after being deposited, --a condition unique among recent Elasmobranchs. SQUATINA AND PRISTIS gj The different families of the existing sharks appear to to have been already differentiated during the early Meso- zoic times. The ancient shark-like form had then given place to the flattened and rostrated types, adapted to the Fig. 96 B. — The Greenland shark, Lamargus borealis, L. X 35. (After GiJNTHER.) conditions of bottom living and to the special character of their shell-fish or crustacean diet. One of the earliest offshoots from the main selachian stem appears to have been Squatina (Fig. 97), popularly known as the monk-fish, or angel-fish. As early as the Mesozoic times it was existing, differing but little from the recent species. Its general shape is shark-like, although its Fig. 97. — The monk-, or angel-fish, Rhina squatina. 9. X 12. Atlantic, Mediterranean, Pacific. head and trunk are clearly depressed. This, together with its enlarged pectoral fins, enables it to take a position closer to the bottom. The recent saw-fish, Pristis (Figs. 98, 98 A\ is next to 92 ->•-< '•' be mentioned as a form somewhat transitional from shark to ray. Its body, as may be seen in the figure, has been strikingly flattened, the gill openings changing their posi- tion from the lateral to the ventral side, but the fins re- taining in general the selachian characters. Its singular rostrum with lateral spike-like teeth is unquestionably a Fig. 98. — The saw-fish. Pristis pectinattts, Latham. 5. x seas. (After GOODE in U. S. F. C.) Tropical highly specialized organ. Pristis is thus far known not earlier than the Eocene, but its close connection genetically with the ancient and more generalized Pristiophorus is usually conceded. Pristiophorus (Fig. 99) is certainly more closely allied to the sharks : its gill slits have not as yet acquired their ventral position, and its rostrum suggests the ancestral Fig. 98 A. — Saw-fish, ventral view. conditions of that of Pristis. Its barbel-like structures, however, distinguish this form clearly from all other Elasmobranchs. It is known to have occurred as early as the Jura. The Skates or Rays are well known to represent the most highly modified survivors of the ancient stem of the SA'.-l TES 93 sharks ; they appear comparatively late in time, and may well be regarded as the culminating forms of the specializ- ing bottom-living sharks of the Mesozoic. Whether they are directly descended from forms like Squatina or Pristio- Fig. 99. — Pristiophorus (cirratus) . 9. (After JAEK EL.) Australia. phorus must be looked upon as exceedingly doubtful, as the depressed body form may possibly have arisen independently in these different families. The most nearly ancestral form of the skates appears to have sur- survived in RJiinobatus (Fig. 100). The shark-like body form is here most nearly retained, and its fin structures GO Fig. 100. — Rhinobatits planiceps. % . x £. (After CARMAN.) (The lower portion of the figure showing ventral side.) 5. Spiracle. GO. Gill slits. are the least specialized ; these transitional characters of Rhinobatus become more prominent in view of its ancient occurrence : its genus was clearly defined as early as the Oolite. 94 FLATTENED SHARKS The body form of the Skate (Fig. 101) has become admirably adapted to bottom living ; it is exceedingly flattened anteriorly, its head and trunk and paired fins fusing so perfectly that from the surface view one could not define their limits ; the tail region, on the other hand, has dwindled away to rod-like or whip-like proportions. Fig. 101. — The barn-door skate, Raja latvis, Mitch. in U. S. F. C.) x £. (After GOODE In the process of flattening, the gill openings take their appearance early in the ventral side of the body, and the pectoral fins, enlarging rapidly, press closely forward at the side of the flattened head, fusing with its tissues. Motion is now accomplished by the gentle undulation of the long horizontal fin margin : and the enlarged AFFINITIES OF THE ELASMOBRANCHS 95 of the glides anterior element of the fin stem, by being raised or de- pressed, comes to direct the upward or downward motion of the fish. In this mode of movement seems to have been paralleled the undulation of the ancestral fin fold. On the fish's dorsal side colour adaptations have become marked, the ventral aspect becoming de- ficient or wanting in pigment. In its hab- its the skate mimics the colour bottom and along inconspicuous- ly, apparently with- out movement ; when alarmed, it will press its enlarged and flat- tened fins so closely to the bottom that it appears to adhere, and is to be dislodged only with the great- est efforts. Two of the aber- rant forms of rays are shown in Figs. 102 and 1 02 A. The for- mer, the Torpedo, is remarkable on account of its electric organs ; the latter, Dicerobatis, on account of the great breadth of its pectorals, and its enormous size. Fig. 102. — The torpedo, Torpedo occidcnta- lis, Storer. tf. X |. (After GOODE in U. S. F. C.) 96 KINSHIPS OF SHARKS Affinities In concluding the present chapter, the probable affini- ties and interrelationships of the Elasmobranchs may be summarized as follows (v. Fig. 103) :- 1. Of all known stems that of the shark is most nearly ancestral in the line of jaw-bearing vertebrates. 2. A generalized form not unlike Cladoselache might well represent the ancestor of Pleuracanthid, as well as of the primitive Cestraciont, of Acanthodes, and of the modern sharks and rays. Fig. 102 A. — The mantis, or devil ray, Cephaloptera (Dicerobatis] draco. X sV (After GUNTHER.) Tropical seas. 3. On the evidence of the Permian Pleuracanthids, lung-fishes (Dipnoans) and the earliest bony fishes (Crossopterygians) are to be derived from an advancing shark type. 4. From the ancestral stem of the recent sharks Cestracionts were the most early differentiated : it is one of their more generalized forms, Cestracion, that has alone A' INS HI PS OF SIIAKKS 97 survived among the widely evolved genera and families of Palaeozoic times. 5. The more primitive types of modern sharks, Chlamy- doselache, Notidanus, represent in an almost differentiated condition the Palaeozoic phylum. 6. The modern rays are derived in early Mesozoic times from the main shark stem, not (in the opinion of the writer) descended from Cestracionts, Pristids, Pristiopho- rids, or even (?) Rhinids. 7. Chimaeroids, next to be discussed, represent the most ancient of known offshoots from the (Pre-Silurian ?) sharks : they are not degenerate in their essential structures, nor are they connected with the ancestral phylum of the lung- fishes, save through a common descent from early shark- like ancestors. These results the writer has expressed in the diagram on the following page. The diverging phyla are indicated as they are represented historically ; their primitive con- currence with the main line of descent is suggested by dotted lines. H Ancestral Elasmobranch. (TABLE III) PALXEO- ZOIC. MESO- ZOIC. CXENO- zoic. a -C CO _c JH cc. Q. Q. cc C£ 2 8 E Ic O Q- b Fig. 103. — Scheme suggesting the interrelationships of Sharks, Chimseroids, and Lung-fishes. 98 V THE CHIMyEROIDS CHIM^ROIDS are shark-like in their general characters, but cannot be looked upon as in any strict sense closely associated with the Elasmobranchs. They constitute the second of the more important groups of fishes. Their typical representative is the Chimaera, spook-fish, or sea- cat (Fig. 119). Structural Characters The typical structures of Chimaera are shown in the dis- section given in Fig. 104. Its thick, round, and blunted head tapers away gradually to the tip of a diphycercal tail, C. The body surface is generally smooth. The paired fins are somewhat shark-like, but their dermal margins have be- come greatly enlarged, tapering distally to an acute point ; the foremost dorsal fin provided with an anterior spine folds like a fan and may be depressed into a sheath, SH, in the body wall ; this fin and the hinder ones are largely dermal, D', basal and radial supports existing only at /?', R'. The gill arches, BA, may be seen to be closely drawn together; their outer openings are now reduced to the slit-like aper- ture beneath the dermal flap, OP. Teeth exist in the form of dental plates, closely fused with the jaws ; as shown in the figure, D, three of these occur in each side, a single one on the mandible, an anterior and posterior on 99 100 STRUCTURES OF CHIMMROIDS the upper jaw (" premaxillary " and "palatine") ; they are studded with hardened points, or "tritors" (Figs. 109-112). The sense organs are similar to those of sharks ; the nasal capsule, NAS, has both an anterior and a posterior open- ing, O, O', the latter within the cavity of the mouth. The visceral parts are decidedly shark-like ; the diges- tive tube is straight (p. 263) ; the intestine, /, with a spiral valve of three turns ; the liver, L, is prominent ; the kidney, K, reproductive organs, T, and their ducts, VD, SS, VS, and abdominal pores are as in sharks ; the intes- tine, however, opens directly to the surface, A, separating an anal from a urogenital aperture, UG. The mesenteries are string-like. The male fish is provided with a highly specialized intro- mittent organ, CL ; it has a supplemental clasping organ, VC, at the front margin of each ventral fin, F(cf. also Fig. 116 and Fig. 116^), and a retractile spine in the region of the forehead, MSP (cf. Figs. 113 and 115). The skeleton of a Chimaeroid is shown in the following figure (Fig. 105). Its structure is cartilaginous. The ver- tebral axis is notochordal ; its sheath, lacking in definite centra, is strengthened anteriorly by a series of calcified rings. In the anterior region of the trunk, neural proc- esses, interneurals, and neural spines, NP, IN, NS, to- gether with haemal processes, occur as in sharks ; toward the tail region they fade away, and before joining with the head at the occipital condyles, OC, they fuse into a compact mass, joining with the basal supports of the dorsal fin. The cranium is of a highly compacted structure ; its vertical height has been greatly produced ; the orbits, OR, are of great size and are separated from each other by a membranous septum. The snout region is greatly meta- "« - o> • >- C- '— .s • « g- g g O " — ^ o — • " w n ^OCrSo^O0" ^ •" tf ^ - a " £CQQ =Z ^ j2 ^"*< _aj ft 2 o 0-3 •[- rt 5^ § — . o — - Cd 2 .Q ^ C 'H bfl S * -S S « C C -ZJ « a, ID in -v- •t: 3 a ^ « ^=5 ~ >^ tc . OJ >-* 6/3 S- ^ C (LJ ^ S m "B O - ^ £_ rt -*~> 0 rt S2 -g - I a Q | s rt o . « - . M in (S 3 ni o o -a -r 2 "° iJ > . 5 H r i < M ^" ?. *> ^' a £ o 3 o b rt bi T; i2 Si 52 o s S tg: 8 a a Jo ^ a 1 20 L UNG-FISHES (Fig. 122), this plan of structure is somewhat obscured by the rudimentary character of the radial and basal elements, R + D, although the fin stem presents a well-marked jointed character, B. The pelvic girdle, a solid plate of cartilage, is produced anteriorly into a narrow median out- growth, PG, and laterally into a pair of dorsal spurs, PG' . The shoulder girdle is composed on either side of a large ventral element, SG, which meets its fellow in the median ventral line, and of a short dorsal element, SG', which connects it with the skull. Fig. 122 A. — Jaws and skull of Protopterus annecfans, figured in front and side aspects, showing paired dental plates. X I. (After NEVVBERRY.) M. Dental plates of (dentary) mandible; P. of palatopterygoid; V. of vomer. In the head region (v. pp. 252, 254), the brain case is cartilaginous, with, however, a few true bone centres (e.g. epiotic) appearing ; the roofs of the skull and mouth, together with the mandible, are well sheathed by dermal bones, as FP, N, PP, DN, AG. Paired dental plates fringe the rim of the mandible (Fig. 122 A, M), the vomerine region (V}, and the anterior end of the palato- pterygoids (P). Fossil Lung-fishes The structures of the recent Dipnoans can as yet be but imperfectly compared with those of fossil forms. Their ancestral conditions can only be determined when more 121 122 LUNG-FISHES perfect evidence is discovered as to their kinship and the lines of their descent. In the history of fishes, Dipnoans are known to have been early a dominant group. In some regards, one of their ancient forms bore many resemblances to the Pleura- canthid shark, which, although known at present only in a later period, may well have been its contemporary. But the range in the forms of Dipnoans occurring in the early Palaeozoic indicates the remote antiquity of their origin. They had even then evolved exoskeletal characters which are scarcely less specialized than those of existing forms. Fig. 126. — A restoration of the Devonian lung-fish, Phaneropleuron. X I. Dipterns, of the Old Red Sandstone (Fig. 123), had a complete body armouring of cycloidal scales, a head roofing of dermal plates (Fig. 124), and well-calcified jaw rims (Figs. 124, 125, \2$A). Its fin rays were dermal in structure, its paired fins were archipterygial, its tail and its dorsal fins separate and lobate. Its mucous canals had become elaborately adapted to the body scales (lateral line, Fig. 123) and head plates, piercing the latter with minute pores, as in Figs. 65, 66. Anterior and posterior nares are indicated under the rim of the upper jaw (Fig. 125, 1-2). Marginal teeth have disappeared ; a pair of elaborate dental plates on the mouth roof (palatine) are apposed by a simi- lar pair in the hinder part of the mandible (splenial). The Carboniferous Cteuodns was a nearly allied form. Another Devonian lung-fish, Phaneropleuron (Fig. 126), '23 124 LUNG-FISHES was similar to Dipterus in its skeletal characters. Its elon- gate diphycercal tail was continuous with the dorsal and anal (?) elements ; in this, and in the retention of marginal cusp-like teeth, it resembled the Pleuracanthid sharks. Living Forms The three forms of living lung-fishes may reasonably be looked upon as the survivors of the more generalized Palaeo- zoic forms. Ceratodus, the Australian genus, appears to have retained most perfectly the ancestral conditions ; it has probably remained almost unmodified from the early Mesozoic times,* and presents close affinities to the Coal Measure family, Ctcnodontidce, and even to the Devonian Dipterids. Its outward ap- pearance is shown in Fig. 127, and its skeleton in Fig. 128. The latter is seen to resemble closely the charac- Fig. 128 A. — Skull of Ceratodus. . . Seen from the ventral side. (After terS Of Tig. 122; its paired ZITTELO ^ns are archipterygial ; the c. Occipital rib. d. Dental plates. na. Anterior and posterior nares. P. mOUtll is lacking in marginal Palatine. PSph. Paraspenoid. Pt. . . , . T . Pterygoid. Qu. Quadrate. I'o. Vomer. Cutting plates (ct. //, Tig. 122 A). The dental plates of the palatine and splenial regions (Fig. 128 A) are seen to correspond clearly with those of Figs. 125, 125 A. Ceratodus had long been known to the colonists of * v. p. 10. The recent genus, according to Dr. Gill, is to be distinguished as Neoceratodus. THE RECENT LUNG-FISHES i2$ Queensland as a plentiful food-fish, a "salmon" in size and taste, although, curiously enough, it remained undescribed until 1870 (Krefft, and Giinther). After this its develop- mental history was eagerly awaited, in the hope that it would reveal the affinities of the Dipnoans to the sharks, amphibians, and in general to the early chordates. About ten years ago Caldvvell was sent to Queensland by the Fig. 129. — The South American lung-fish, Lepidosiren paradoxa. Natter. X *. (From NICHOLSON, after NATTERER.) A front view of the mouth is shown at /?. Royal Society, and succeeded in securing a set of the embryonic stages, but his results still remain unpublished. A second set of embryos was collected in 1891 by Semon, from whose recent paper a summary is later given (p. 198). The development of Ceratodus, however, as far as it is at present known, has proven in many ways unsatisfactory to the phylogenist ; its abbreviated growth stages cannot be 126 LUNG-FISHES looked upon as furnishing clearly the ancestral history of Dipnoans. The two remaining forms of recent lung-fishes, Lepi- dosiren and Protopterus, resemble each other so closely that Ayers has contended that they should be regarded as distinct only specifically. Lepidosiren,the South American form (Fig. 129), was discovered by its describer, Natterer, in 1837 in the upper Amazon. It then, for many years, succeeded in eluding the collectors, and was known as one of the rarest specimens of foreign museums. In 1887 it was, however, re- discovered in Paraguay, where it appears to have long been known as a food-fish. Its structures are now regarded as en- titling it unquestionably to the rank of a distinct genus. Protopterus, common in the White Nile and Congo (Fig. i2gA), has long been the " Lepido- RELATIONSHIPS OF LUNG-FISHES I27 siren" of dealers, often of museums. It is the best known of Dipnoans, on account, partly, of the ease with which it may be transported alive. In the hardened mud cocoons with which it encases itself during the dry season, it is readily dug out of the stream bed and packed for exporta- tion. When placed in tepid water, the cocoon dissolves and the fish shortly revives. Relationships A review of our knowledge of Dipnoans gives but little satisfactory suggestion as to their relations as a group. They must certainly be looked upon as an advancing phylum from which the amphibia may early have diverged. Their many amphibian characters have been lately em- phasized by W. N. Parker. On the other hand, the evidences of the kinship of Dipnoans to the other types of fishes can only be interpreted as the common con- vergence of the ancient phyla toward the structures of the ancestral form of fish. Thus we find that the types of Devonian lung-fishes can only be distinguished from those of the contemporary Teleostomes by the pattern of arrangement of the plates of the head roof,* a condition which has led Smith Woodward to believe that these groups had already diverged before the appearance of dermal bones. Lung-fishes have unquestionably many structures which may have been derived from the more generalized condi- tions of the sharks ; and as a group they may not unrea- sonably be looked upon as descended from the primitive elasmobranchian stem. Their ties of kinship to the sharks * The present writer regards this distinction as somewhat provisional; median head plates are nominally characteristic of Dipnoans (Fig. 124), but, as in the sturgeons and siluroids, they are also well known among Teleostomes. Protopterus has, moreover, a symmetrical arrangement of the head plates. 128 RELATIONSHIPS OF LUNG-FISHES have now been closened by the proof that their paddle- shaped fins may be directly deduced from a " monoserial archipterygium," and that their diphycercal caudal, formerly regarded as most primitive in plan, may have been acquired secondarily after a condition of heterocercy (W. N. Parker, Traquair, Dean). The resemblances of Dipnoans to Elasmobranchs might be summarized in the following structures : - I. VERTEBRAL AXIS. Its notochordal condition and simple metameral, neural, and haemal elements suggest the conditions of Cladoselache (p. 80) ; in that ancient form, however, the vertebral processes had not come into rela- tion with the unpaired fins. II. SKULL. The chondrocranium is as yet largely re- tained ; as yet no dentigerous membrane bones of the mouth rim (maxillary and premaxillary) have appeared. III. TEETH. These are clearly of an elasmobranchian order; the tubercles of the dental plates (Fig. 125) suggest closely a shagreen pattern ; in Phaneropleuron, marginal cusps have even been retained. The palatine and splenial plates parallel strikingly some of the forms of Cestraciont dentition. IV. BRAIN. Its structures are of an advancing elasmo- branchian order, annectent with reptilian (Ceratodus) and amphibian types (Protopterus).* V. VISCERAL CHARACTERS. Heart, gills, digestive tract, vessels, mesenteries. The closely corresponding characters of Phaneropleuron and Pleuracanthus might be looked upon as independently acquired ; bnt in view of the many nearnesses of their phyla, these characters may reasonably be regarded as proof of genetic kinship. * Burckhardt. A R THR ODIRAN L UNG-FISHES 129 The advancing structures of the Dipnoan include, in addition : - I. EXOSKELETAL SPECIALIZATIONS. Head-roofing der- mal bones (cf., however, Pleuracanthid) and cycloidal scales. In early forms (Dipterus) these appeared at the surface and were apparently enamelled. In recent forms they are deeply sunken in the integument (Prototerus). They suggest closely the structures of Crossopterygian (p. 149). II. ARTICULATION OF THE MANDIBLE. This is auto- stylic, somewhat as in Chimaeroid (v. p. 256). Its homol- ogy is obscure. III. AIR-BLADDER, (v. p. 264). IV. ABSENCE OF VENTRAL "CLASPERS" (cf., however, Cladoselache). V. TRUE POSTERIOR NARES (amphibian). VI. THE GREAT SIZE OF THE CELLULAR ELEMENTS OF ALL TISSUES (amphibian) ; THE GLANDULAR STRUCTURES OF THE EPIDERMIS (amphibian). VII. CIRCULATORY CHARACTERS : the three-chambered heart ; aortic arches. VIII. LIMB STRUCTURE. This, however, is not to be interpreted as in any way directly transitional to cheirop- terygium. The Arthrodiran Lung-fishes The ARTHRODIRA, as Smith Woodward has shown, may provisionally be regarded as an order of extinct and highly specialized lung-fishes. They occur geologically among the earliest fishes, and include a number of (Devonian) forms whose peculiar characters and gigantic size must have made them among the most striking members of ancient fauna. The group might be regarded as standing in the same rela- tion to the ancient Dipnoans as Acanthodians to the Cla- K 130 A R THR ODfRAN L UNG-FISHES doselachian sharks. As recently as 1887 its members were associated by Traquair with Pterichthys, but the discovery of jaws, specialized dentition, fin spines, and highly evolved pelvic fins at once separate this group from the lowly Ostracoderms. American Arthrodirans, described mainly by Newberry and by Claypole, have proven of especial interest. They occur from the Silurian to the Coal Measures. The giant predatory member of this group, Dinichthys (Frontispiece, and Figs. 133-137), attained a length of ten feet. Titan- ichtJiys, less formidable in armour and dentition, may well have been twenty-five feet in length. These forms occur almost exclusively in the Waverly of Ohio. Their discovery has here been due to the efforts of Dr. William Clark of Berea, Rev. William Kepler of New London, and Mr. Jay Terrell of Linton ; and most of the type specimens have been preserved in the museum of Columbia College, New York. The European member of this group is a small, fresh- water (?) form, Coccosteus, especially abundant in the Old Red Sandstone of Scotland. It has thus far yielded the most complete material for study, and its structural char- acters might accordingly be described, since they are probably common to all members of the group. The lateral view of Coccosteus is shown in Fig. 130, the dorsal aspect of the anterior region in Fig. 131, and the ventral view of the visceral region in Fig. 132. It will accordingly be seen that the general shape of the body of this Arthrodiran was somewhat depressed ; that the head, shoulder, and stomach regions were protected by bony plates ; and that the. trunk region was lacking in armouring, and short in relative length. In well-preserved fossils the space occupied by the notochord, N, is seen to COCCOSTEUS l^l pass from the region of hinder plates of the body armour to that of the tip of the tail. This is seen to be bordered by neural and haemal processes, IV, //, which in size and character are somewhat comparable with those of Protop- terus or Pleuracanthus. The dorsal fin presents a meta- meral series of supporting cartilages (radial and basal, DR, DB). The basal supports of each pelvic fin have become compressed into a flattened plate, VB. Pelvic fins were present, but there have as yet been found no traces of pectoral appendages. In Dinichthys Newberry believed that a pectoral fin spine was present, and that this fin was N nH DR MC Fig. 130. — The Devonian Arthrodiran, Coccosteus decipiens, Ag. X 1. Old Red Sandstone, Scotland. (Side view, restored ; slightly modified, after SMITH WOOD- WARD.) A. Articulation of head with trunk. DB. Cartilaginous basals of dorsal fin. DR. Cartilaginous radials of dorsal fin. H. Haemal arch and spine. MC. Mu- cous canals. N. Neural arch and spine, f/. Median unpaired plate of hinder ventral region. VB. Basals of ventral fin. VR. Radials of ventral fin. probably Siluroid-like (p. 171), but this view has not been confirmed. The head of Coccosteus was clearly flattened, with orbits and nasal openings near its anterior margin ; it was roofed by a stout buckler of closely fitted dermal plates (Fig. 131), whose outer surface was tuberculate, enamelled, and furrowed by sensory grooves, MC. The arrangement of the dermal plates of Coccosteus was early (1861) compared by Huxley with that of recent Siluroids, 132 ARTHRODIRAN LUNG-FISHES an analogy afterward supported by Newberry, Dean, and recently, on account of the similar characters of the sen- sory canals, by Pollard. In their conclusions, however, fundamental characters of structure seem to have been overlooked in the unlikeness of Arthrodiran to Tele- ostome. The inner structure of the cranium of Arthro- PM MC PO SO FIG. 131 Figs. 131, 132. — Coccosteus decipiens. Dorsal view of dermal armouring. X 5. (After TRAQUAIR.) 132. Ventral plates. (After TRAQUAIR.) ADL. Antero-dorso-laterai. AL. Anterolateral. A VM. Antero-ventro-lateral. C. Central. E. Ethmoid. EO. Epiotic. IL. Inferior lateral. At. Marginal. MC. Mucous canals. MD. Median dorsal. MO. Median occipital. MV. Me- dian ventral. O. Opercular. PDL. Postero-dorso-lateral. PL. Posterior lateral. PM. Premaxillary. PN. Pineal. PO. Preorbital. PTO. Postorbital. PVL. Postero-ventro-lateral. SO. Suborbital. dira was evidently entirely cartilaginous ; in a Russian Coccosteid, according to Smith Woodward, the base of the brain case (parachordal cartilages) has been preserved and shows a " tubular canal originally occupied by the anterior STRUCTURES OF ARTHRODIRAN 133 extremity of the notochord." Gill arches and opercula are not definitely known. The mandible was attached directly to the skull (autostylic). The jaws were shear-like, their margins usually with pointed teeth, whose bases fuse with the tissue of the jaw and constitute dental plates. In all forms, as in Dinichthys (Frontispiece), there appear to have been three pairs of these "plates," those forming the rim of the mandible below, and those of the vomerine and palatine regions (" premaxillary " and "maxillary") above.* This arrangement of the dental plates somewhat resembles the Dipnoan's. Those of the Arthrodiran, how- ever, appear to have been movable, and suggest a dental condition elsewhere unknown among vertebrates. Fig. 133. — Restoration of Dinichthys hitermedius, Newb. X -}*• Cleveland Shales, Ohio. The body armouring of dermal plates is characteristic of the group. A carapace, cape-like in shape, begins at the head angle and broadens out backward and dorsally towards the median line. It consists of a single median spade-shaped element, which forms the strong ridge of the back, and a flanking of lateral plates, all compactly joined. The rigid shield that is thus formed is movably connected with the head ; an elaborate joint, formed on either side between the anterolateral dorsal plate, Fig. 131, ADL, and the "epiotic," EO, — whence the name Arthrodira, — must * According to Dr. Clark, an additional symphysial pair of dental plates was present in both upper and lower jaw (Dinichthys). 134 ARTHRODIRAN LUNG-FISHES have permitted the head to be thrown backward to a degree which suggests the thoracic joint of an Elater. On the ventral side of the trunk there occurs a flattened plastron (Fig. 132) : its dermal elements are connected by overlapping margins ; they are lighter, and in some forms (Fig. 135) lack the tuberculate surface of the dorsal plates. Dorsal and ventral shields are connected by stout lateral elements (Fig. 132, IL), which, passing ventrally, FIG. 135 137 A Figs. 134-137. — Dermal plates of Dinichthys. 134. Associated plates of head and shoulders. 135. Plates of ventral armouring. (After A. A. WRIGHT). 136. Pineal plate of Dinichthys intermedia;, surface view. 137. Pineal plate of Dinich- thys terrelli, visceral aspect. 137 A. Pineal plate, in sagittal section. ADL. Antero-dorso-lateral. A VL. Antero-ventro-lateral. A VM. Antero- ventro-median. E. Ethmoid. EO. Epiotic. MO. Median occipital. PN. Pineal. PO. Preorbital. PTO. Postorbital. PVL. Postero-ventro-lateral. SO. Suborbital. X. External aperture, and -> , the axis of the pineal funnel. meet in the median line, and become the anterior support- ing rim of the plastron. By some writers these have been homologized as "clavicles." In further detail little is known of the anatomy of 4 STRUCTURES OF ARTHRODIRAN 135 Arthrodirans. Sensory canals have been described chan- nelling the surface of the dermal plates of the dorsal side. In the body region of Coccosteus evidence of a lateral line occurs (Smith Woodward) in a white calcified band fossilized in the region of the space of the notochord. In this form, too, an endoskeletal plate is known, (Fig. 130, U) occurring in the median line in the region of the vent, which must be regarded as " suggesting an internal element of support occurring in the vertical septum between the right and left halves of some paired organ (S. IV.)." The character of the dermal investiture of the trunk has apparently not been described ; it may therefore be of interest to note that the museum of Columbia College has recently acquired two of the hinder dorsal plates of Dinichthys which clearly indicate the presence of integument. The plates are covered by a crinkled epidermis, whose irregular surface traceries resemble the roughened finish of Turkey morocco. This leather-like surface is seen to have been continued over the margin of the plates along the side of the trunk ; traces of scales or tubercles are altogether lacking, and its appearance suggests that it may have been degenerate in structure. Among Arthrodirans there occurs a series of most inter- estingly evolved forms ; and it is found more and more evident that they, with other lung-fishes, may have repre- sented the dominant group in the Devonian period, as were the sharks in the Carboniferous, or as are the Teleosts in modern times. There were forms which, like Coccosteus, had eyes at the notches of the head buckler ; others, as Macropetaliclitliys, in which orbits were well centralized ; some, like Dinichthys and Titan- ichthys, with the pineal foramen present ; some with pectoral spines (?) ; some with elaborately sculptured derm 136 AR THR GDI RAN L UNG-FISHES plates. Among their forms appear to have been those whose shape was apparently sub-cylindrical, adapted for swift swimming ; others (Mylostoma} whose trunk was depressed to almost ray-like proportions. In size they varied between that of a perch and that of a basking shark. In dentition (Figs. 138-144) they presented the widest range in variation, from the formidable shear-like jaws of Dinichthys to the lip-like mandibles of Titan- ichthys, the tearing teeth of TracJiostcns, the wonderfully forked, tooth-bearing jaw tips of DiplognatJins, to the Cestraciont type, Mylostoma. The latter form has hitherto been known only from its dentition, but now proves to be, as Newberry and Smith Woodward suggested, a typical Arthrodiran. The puzzling characters of the Arthrodirans * do not seem to be lessened with a more definite knowledge of their different forms. The tendency, as already noted, seems to be at present to regard the group provisionally as a widely modified offshoot of the primitive Dipnoans, bas- ing this view upon their general structural characters, dermal plates, dentition, autostylism. But only in the latter regard could they have differed more widely from the primitive Elasmobranch or Teleostome, if it be ad- mitted that in the matter of dermal structures they may clearly be separated from the Chimseroid. It certainly is difficult to believe that the articulation of the head of Arthrodirans could have been evolved after dermal bones had come to be formed, or that a Dipnoan could become so metamorphosed as to lose not only its body armouring * The writer believes that the Arthrodirans may as well be referred to the sharks as to the lung-fishes; as far as existing evidence goes, they certainly differed more widely from the lung-fishes than did the lung-fishes from the ancient sharks. They may, perhaps, be ultimately regarded as worthy of rank as a class. FIG. 138 144 Figs. 138-144. — Mandibles of Arthrodirans : Cleveland Shale, Ohio. 138. Mylostoma variabilts, Newb., visceral aspect. 139. Titanic lit hys clarki, Ne\vb., visceral aspect. X \. 140. Trachosteus, Newb., outer aspect. >< J. 141. Diplo- gnathus, Newb., outer aspect. < \. 142. Diplognathus, seen from dorsal side. 143. Diplflgiiathiis, visceral aspect. 144. Dniichthys intermedius, outer aspect. X \. 137 138 ARTHRODIRAN LUNG-FISHES but its pectoral appendages as well. The size of the pectoral girdle is, of course, little proof that an anterior pair of fins must have existed, since this may well have been evolved in relation to the muscular supports of plastron, carapace, trunk, and head. The inter-movement of the dental plates, seen especially in Dinichthys, is a further difficulty in accepting their direct descent from the Dipnoans. VII THE TELEOSTOMES ALL fishes not to be grouped among Sharks, Chimae- roids or Lung-fishes, have been included in the fourth sub-class, Teleostomi. In this are to be merged the two time-honoured groups, Ganoids* and Teleosts, since it is now found that there are absolutely no structures of the one group that are not possessed by members of the other. The terms, therefore, "Ganoid" and " Teleost," must be used in a popular and convenient, rather than in an accurate sense; the former to denote the "old-fashioned" Teleostome, with its rhombic bony body plates, and carti- laginous endoskeleton ; the latter, the modern "bony .fish," with rounded, horn-like scales and its calcified endo- skeleton. Teleostomes present so wide a range of variation that it becomes exceedingly difficult to include in a single definition their minor structural characters. As a basis for the comparison of the Teleostomes, the characteristic structures of a single type, e.g. the Perch, might conveniently be taken. From these conditions, typical of a modern and highly specialized form, the simple structures of the ancient, more primitive, and ancestral Ganoids may afterward be readily understood. * The term Ganoid, as here used (as far as p. 147), includes the Crossopte- rygians as well. 139 140 STRUCTURES OF TELEOST General Anatomy In the Teleost (Fig. 145) the shortened and muscular body appears admirably adapted to the conditions of aquatic motion. Anteriorly it is broad and deep, its trunk muscles firmly attached to the bony prongs of the enlarged base of the skull, DCR, and to the solid, compact, calcified vertebrae, V, and their stout processes. The fish's tapering sides are encased in horn-like cycloidal scales, CS, a light, flexible armour, whose elements over- lap, defending every point, and whose smooth and slime- coated surface provides the least possible resistance to motion. The fins, D, C, A, PF, VF, are light and strong, erectile and depressible; their rays are thin, narrow, spine- like, strong ; they are entirely dermal, their cartilaginous supports sinking within the body wall, RB. The caudal is large and fan-shaped (homocercal), its crowded rays providing admirably its needed strength ; its stout basal supports, compacted beneath the tip of the notochord, NC, show that its form is modified heterocercy. The pectoral fin, PF, has now taken its position high in the side of the body ; its basi-radial supporting elements are reduced to a proximal row of a few small irregular plates. The skeleton is completely calcified. The vertebral axis has undergone entire segmentation, the notochord persist- ing only between the cup-shaped faces of the centra ; the vertebral arches and processes have merged with the centra, and those of the hinder region, YY, H, with prob- ably the basal fin supports as well. Ribs, R, usually with intersupporting processes, strengthen the walls of the visceral cavity, and represent calcifications of the myocom- mata, rather than transverse processes of the vertebrae. The skull is formed of compact bony elements ; its carti- •-1 w §; 5, L_, ri rt •a E 3 « 3 f~v ^ -— ^ t/> o < o c cd -H : . i> -c c^- ^8.= ts cu _ -= £ 3 h o _• " • rt o >, o I HI o c « ^ K J -£ £ § ^ o -^ O r "S S > "5 • - •fe § Qi § I S C *»«« rn f1 *^ rr Q • R <, P HT a; « °=i d g. £ .8 O !° '-3 ^ D- . ^5 ^ sieSl^l- - b/j W GJ C -C1 o **~ rt o _ u "—(So . o ° Q.55C1 fc^c^'S ^"^^.•E Sc/3 g. o .2 . n^ o "o "" ^•^^O^o «^ £ £ S ^ ^- W £ „ H _, o •, o « — ^ ,; h >, cs _ij u rt i-Ji 3 rS b/i ^ ^Ss^s QJ ^>n £ l-J ^c P| ^N (Figs. 151, 152), are certainly most nearly in .a° the ancestral line of the recent forms. Like fe many sharks and fossil Dipnoans, they present a heterocercal tail, a single anal fin, and a pair of dorsals. The pectoral fin of Osteolepis is becoming a typical archipterygium. HoloptycJiins, another Devonian form (Fig. 153), approaches even more closely the dipnoan types : the scales are cycloidal ; its paired fins are distinctly archipterygial ; and the caudal region, reduced in length, is becoming meta- morphosed into the typical diphycercal form by the ten- dency of the second dorsal and anal fin to coalesce with FOSSIL CROSSOPTERYGIANS i$i the caudal. In these forms a number of paired gular plates may occur. In a closely related genus, Eustkenoptcron, also of Fig. 151 152 Fig. 151. — Gyroptychius. X \. Old Red Sandstone, Scotland. (After SMITH WOODWARD.) Fig. 152. — Osteolepis. X 4. Old Red Sandstone, Scotland. (Restoration from SMITH WOODWARD, after PANDER.) Devonian age (Fig. 154, A, B), the structure of the basal parts of the unpaired fins is exceedingly interesting ; the radial supports are unfused, while the basals, merged in a Fig. 153. — Holoptychius andersoni. Old Red Sandstone, Scotland. single plate, have come into especial relation with the axial skeleton ; the subsequent stage of their differentia- 152 FOSSfL CROSSOPTERYGIANS '53 tion has been noticed in Fig. 43. The condition of the caudal fin of Eusthenopteron is also worthy of note ; the tip of the notochord is retained although the functional portion of the fin is derived from the more anterior body region. The vertebral arches are here clearly sug- gestive of the conditions of the Dipnoan. Ccelacantkus, common in the Coal Measures (Fig. 155), is the most specialized of the Crossopterygians ; it has retained all of the archaic structures of its kindred, yet has concealed them under the outward appearance of a recent bony fish ; the general contours of its head, trunk, scales and fins resemble strikingly those of a dace or T' - - Fig. 155. — Ccelacanthus elegans, Newb. x J. Coal Measures, Ohio. A. Position of calcified swim-bladder. chub ; but on closer view the paired fins are found to be archipterygial, the scales enamelled and sculptured, the true caudal fin the degenerate stump of the notochord ; the functional caudal has been formed of the enlarged fin rays of the dorsal and anal region. Traces of a calcified air-bladder, A, are often preserved. Diplurus and a closely related genus, Undina (Figs 156, 156 A), may finally be noted among the highly evolved Crossopterygians. They appear in the Mesozoic when the majority of their kindred have disappeared ; they have as- sumed peculiar characters and have apparently reached the point of differentiation when they shortly become extinct. 154 TELEOSTOMES Diplurus has become excessively shortened in its body length ; the head is of relatively enormous size ; its derm bones are squamous, and appear to have been deeply implanted in the integument ; teeth have disappeared ; Bfl Fig. 156. — Diplurus longicaudatus, Nevvb. X \. Triassic, Boonton, N.J. A. Position of calcified swim-bladder. A". Second anal fin (now the ventral portion of the functional caudal). BR. Radial and basal fin supports. C. Caudal fin (degenerate). D. Hindmost dorsal fin (now the dorsal portion of the func- tional caudal). J. Jugular. scales have become exceedingly thin and are rarely pre- served. Fin structures are apparently of a degenerate character ; their cartilaginous bases, when showing, appear Fig. 156 A. — I hidina gulo, Egert. (Restoration after SMITH WOODWARD.) X j. Lower Lias of Lyme Regis. to have become reduced to single plates, as BR ; the caudal is the elongate tip of the vertebral axis ; the functional caudal, now elongate and diphycercal, is formed GANOIDEAN FOKMS ^5 by dorsal and anal elements, D, A", as in Coelacanthus. The boundary line of the calcified air-bladder, A, is often preserved. II. ACTINOPTERYGIANS A. Chondrosteans (Ganoids}. Ganoids agree with the Crossopterygians in their exoskeletal characters, although usually lacking in gular plates. The most important differences between these groups have been reduced to those of fin structures ; the Ganoids have no longer the lobate form of the paired fins ; their basal fin supports have become greatly reduced and are usually represented by a single row of a few metamorphosed elements in the Fig. 157. — The short-nosed gar-pike, Lepidosteus platystomus, Raf. < |. Mis- sissippi basin. (After GOODE in U. S. F. C.) most proximal region of the fin. The transitional stages -if they exist --between the lobate and the monoserial fins have not as yet been demonstrated. Fossil Forms From the middle of the Palaeozoic period to the end of the Mesozoic there seems to have been a culminating time of forms like the still existing Gar-pike (Fig. 157) ; their fossils are generally the most numerous, and, on account partly of their strong body armouring of interlocking rhombic plates, the most perfectly preserved of fossil fishes. They usually exhibit the structural characters 56 TELEOSTOMES which Lepidosteus has retained, while diverging widely on all sides in matters of shape, size, special dentition, and Fig. 158. — Elonichthys (Rhabdolepis) macropterus (Giebel), Bronn. x J. (After L. AGASSIZ.) Lower Permian, Rhenish Prussia. features of the body armouring, — characters, apparently, of minor morphological importance. But a few of the char- acteristic types of the early Ganoids can be noted in the present connection. Some of the more important have been figured in Figs. 158-164. Fig. 159. — Eitrynotus crenatus, Agassiz. x |. (After TRAQUAIR.) Calcif- erous Limestone, Scotland. Thus ElonichtJiys (Fig. 158) was a form which had evolved a small size and narrow sculptured body plates ; FOSSfL GANOIDS Eurynotus (Fig. 159) had attained a great depth of body and prominent dorsal fin ; Cheirodus (Fig. 160) was dis- tinctly flattened; Scmionotns (Fig. 161) was small, with Fig. 160. — Cheirodus granulosus, Young. X 5. Coal Measures, Scotland. (After TRAQUAIR.) elaborate fin conditions ; Aspidorhynckus (Fig. 162) had a remarkable pointed snout and a reduced number of body Fig. 161. — Scmionotus kapffi, Fraas. X f. (From ZlTTEL, after FRAAS.) Keuper, Stuttgart. plates ; Microdon (Fig. 163), flattened like Cheirodus, had evolved an admirable series of crushing teeth (-Pycnodont). And, finally, is to be mentioned Palaoniscus (Fig. 164), a form whose abundance, numerous species, and long sur- 158 TELEOSTOMES viva! (Palaeozoic-Mesozoic) have made it the most widely known of fossil fishes. Of all extinct Ganoids there mrt ^ pmi Solenhofen. . — Aspi-is,Aga.ss\z. X §. (After ZlTTEL.) Jura, appears to attach to Palasoniscus the greatest morphological interest ; on the one hand, it seems closely akin to the ^Fig. 163. — Microdon wagneri, Thiolli£re. X |. (From ZlTTEL, after THIOL- LIERE.) Jura, Cerin. LIVING GANOIDS recent gars, and, on the other, even as evidently to the sturgeons ; of all fossil kindred of these living forms, it seems most nearly in the ancestral line. Fig. 164. — Palaoniscus macropomus, Agassiz. X |. (After restoration of TRAQUAIR.) Upper Permian. Ganoids certainly outrank the Crossopterygians in the number and variety of their ancient forms. Their few living representatives give but little idea of the importance of the group, and can suggest but faintly the lines of its evolution. Living Types The recent Ganoids include the Gar-pike, the Sturgeons, and Amia. The first is of especial interest in connecting the group most closely with the Crossopterygians, the last as best illustrating the intermediate stage between the Ganoids and Teleosts. The Gar-pike, Lepidosteus (Fig. 157), resembles Polyp- terus in many characters of skeleton and dermal defences. It is a form not uncommon in the fresh waters of North America, and is especially abundant in the Mississippi, Great Lakes, and rivers of the Southern States. In South Carolina the writer has known the gar-pikes to occur in such numbers that they would fill the shad nets, and for many days render this fishery impracticable. They some- times attain a length of six feet, and are said to become 1 60 TELE OS TOMES as aggressive as sharks. They are remarkably tenacious of life, and their complete armouring of dermal plates renders them practically invulnerable. In development Lepidosteus has apparently more prim- itive features than Acipenser (v., p. 207 ; also Jour, of Morph. XI, No. i). Of all recent Ganoids, Lepidosteus must certainly be looked upon as retaining most perfectly the structural characters of the most abundant and probably the most generalized Palaeozoic and Mesozoic forms. Its genus, it is true, is not known to occur earlier than the Eocene, but its structures — scales, fins, labyrinthine teeth and partially calcified skeleton --are known to have been possessed, ^*t& Fig. 165. — The sturgeon, Acipenser sturio, L. X *V- Streams entering North Atlantic. (After GOODE in U. S. F. C.) even in their details, by a number of the older genera and families. The Sturgeons, Acipcnscr, ScapJiirJtynchns, PscpJiurus, Polyodon, must in many ways be looked upon as of a highly adaptive or even retrogressive character. There is strong evidence that in their descent a large proportion, and, in cases, all of their dermal armouring has been lost, and that their cusp-like ancestral teeth have either disappeared or are retained in a rudimentary condition. The interrelationships of the four surviving forms of sturgeons have not as yet been definitely suggested ; transi- tional fossil forms have thus far been lacking, and the relative importance of the different structures in the recent THE STURGEOXS T6, genera cannot, therefore, be determined for purpose of comparison. The genus of the common sturgeon, Acipenser, is the most completely studied of the recent forms. It includes twenty or more "species," varying in length from one (A. brevirostris, of the Eastern United States) to ten yards (A. Jinso, of Russia), and is altogether one of the most valu- able food-fishes of the rivers, lakes, and coasts of the north- ern hemisphere. It is a sluggish, bottom-feeding fish, common in muddy streams. Its broad and pointed snout, sensory barbels, and greatly protractile jaws are the most striking differences from the Palasoniscoid ; its dermal Fig. 165 A. — Chondrosteits acifrseroides. X J- From Lias of Lyme Regis. (Restoration of skeleton after SMITH WOODWARD.) armouring has become reduced to the five longitudinal bands of body plates,* but is more perfect in the tail region ; its skeleton retains an entirely cartilaginous con- dition. In its larval stage conical teeth are known to be present, and the entire series of dermal plates are much larger in relative size. A figure of Ckondrosteus, a Liassic sturgeon, may here * It is interesting to note that in Palaeoniscoids there is sometimes a notice- able tendency for the five rows of plates, dorsal, and the paired lateral and ventral, to increase in size, suggesting the first steps in the origin of the derm plates of Acipenser. M 1 62 TELEOSTOMES parenthetically (Fig. 165 A) be inserted ; it is of especial interest as suggesting an approximation of the type of the modern sturgeon to that of the Palaeoniscoid ; its snout is shorter than in Acipenser ; its jaws larger, and apparently less protrusible ; its dermal plates of the head region, including the branchiostegals, are clearly of the ancient pattern, and the fins, fin supports, and vertebral characters, Fig. 166. — The shovel-nose sturgeon, Scaphirhynchus platyrhynchus (Raf. ), Gill. X J. Mississippi basin. (After GOODE in U. S. F. C.) together with the general small size of the fish, suggest intermediate conditions. Of the remaining sturgeons, the shovel-nose, Scaphi- rhynchus (Fig. 166), of the Mississippi and of Central Asia, seems to possess the closest relations to Acipenser ; although it is apparently a more modified form, on account of its elongate body shape and flattened snout, it still retains many interesting and archaic features. Among Pig. i66A. — Psephurus gladiHS, Gun. v 1. Rivers of China. (After GtiNTHER.) these it includes the most complete dermal armouring of recent forms, its hinder body region being entirely encased. Psephnrus (Fig. i66A), of the Chinese rivers, and Poly- odon, or Spatularia (Fig. 166 B), of the Mississippi, are the other forms of living sturgeons. Their greatly elon- gate snouts, giving them the popular names of Spoonbills, Paddle-fish, Spear-fish, are among the most remarkable STURGEONS AND AM [A 163 sensory appendages of fishes. They have been but little studied, and their relations to Acipenser have never been satisfactorily determined. They have certainly many feat- Fig. 166 B. — The spoon-bill sturgeon or paddle-fish, Polyodon spatula (Walb.), J. and G. X ?. Ventral and side view. Mississippi basin. (After GOODE.) ures in skeletal parts, fin structures, lateral line organs, jaws, teeth, which can only be looked upon as of primitive character ; on the other hand, their highly specialized ros- trum, degenerate opercula, and want of dermal amouring would suggest an early divergence from the main stem of the sturgeons. To the writer, Psephurus seems the more generalized of these peculiar forms. Fig. 167. —The bowfin, Amia calva, L. X \. (After GOODE in U. S. F. C.) Central and Eastern United States. Amia calva (Fig. 167) is the last of the recent Ganoids to be noted. Its distribution corresponds closely with that of the gar-pike ; it is a common form, worthless as 164 TELEOSTOMES a food-fish, but deemed worthy of a host of local names, as : Bowfin, Grindle, Dog-fish, Mud-fish, Sawyer, Joseph Grindle, Lawyer-fish. Its interest, as already sug- gested, is in its close kin- ship to the Teleosts on the one hand, and to the sturgeons and gars on the other. Its cycloidal scales, its fin structure, and cal- cified skeleton seemed of so modern a character, that it was long included among the members of the herring group; only after a closer examination did its primitive struct- ures become apparent. It is one of the few Gan- oids which possess a gular plate (Fig. \6$,jitg). Like that of Lepidosteus, its air-bladder is cellular, and of respiratory value (Wilder). Fig . 168. — Amia. Ventral view of jaw region. X i. (After ZlTTEL). brs. Branchiostegal rays. h. Cerato- hyal. jug. Jugular plate, md. Mandible. Fig. 169.— Caturus furcatus. x |. (From SMITH WOODWARD, after AGAS- SIZ.) Lithographic stone (Upper White Jura), Solenhofen. The relations of Amia become of especial interest, in view of the number and range of its fossil kindred. Its TELEOST-LIKE GANOIDS I65 group is known to have attained its prominence at a later geological time than the other Ganoids ; it is doubtless derived, more or less directly, from the main ganoidean stem. Three of the more typical Mesozoic forms are shown in Figs. 169, 170, 171, in Cat urns, Leptolepis, and Fig. 170. — Leptolepis sprattiformis. X f. (From SMITH WOODWARD.) Lith- ographic stone, Solenhofen. Megalurns. To these amioid forms the ancestry of the (majority of the) Teleosts is reasonably to be traced. A general scheme of the phylogeny of the Teleostomes is suggested on the adjoining page (Fig. 171 A). B. Teleocephali (Teleosts.) This group, popularly known as that of the bony fishes, or Teleosts, includes as great a proportion perhaps as 95 per cent of the kinds of fishes Fig. 171. — Megahirus elegantissimus, Wagner. Solenhofen. X |. (After ZITTEL.) Jura, living at the present time. The immense number of their genera and species is doubtless suggestive of the form changes which occurred during the flowering periods of the sharks, chimasroids, or lung-fishes. Teleosts have diverged most widely of all fishes from 1 66 TELEOSTOMES what seem to have been their primitive structural condi- tions. Their skeleton has become highly calcified, its ele- ments multiplying, fusing, and specializing. The notochord has practically disappeared, owing to the complete formation of bony vertebrae. The derm bones of the head, which in ANCESTRAL TELEOSTOME (TABLE IV) PAUAE020IC PALAEONISCOID PALAEOZOIC CROSSOPTERVGIAN , MESOZOIC CATURID STURGEON LEPIDOSTEUS SILUROID POLYPTERUS AMIA Fig. 171 A. UOPHOBRW4CH ACANTHOPTERYGIAN The Phylogeny of the Teleostonies. SYNSNATM the ancestral Ganoid were at the surface, enamel-coated,* are now deep-seated in the head, resembling true cartilage bones ; their surfaces are usually deeply furrowed or ridged, * The enamel of Ganoid plates (ganoine) appears to be derived from the underlying bony tissue, not deposited by the overlying epidermis (enamel organ). EVOLUTION OF TELE OS TS ify and their character is often squamous. Scales are widely specialized, thin, horn-like, ornate, overlapping their outer margins, their inner rims set deeply but loosely in dermal pockets (Fig. 31). Fins are dermal structures, their ancient basal supports hardly to be distinguished ; the primitive tail structure is so masked by clustered and fused skeletal elements that its heterocercy is scarcely apparent. In short, the most widely modified conditions can be shown to exist in Teleosts in almost every structural character, as in gills, teeth, opercula, circulatory and urinogenital organs, sensory structures, and nervous system. They have evidently been competing keenly in the struggle for survival, for in every detail of form or structure the most varied conditions exist. In addition to these structural adaptations of Teleosts, changes in coloration have been rendered possible by the transparency of their scales ; and in their different families these changes have taken place often with striking results : adaptive coloration, brilliant, dull, mottled, inconspicuous, occurs with a range of varia- tion which is not surpassed even by the colours of birds. It is not remarkable, therefore, that members of the different groups of Teleosts should often parallel each other in structural likenesses, when placed under the same environmental conditions. Each organ, in fact, may be- come a centre of variation, and confuse the line of the descent of the minor groups ; for the keenest judgment cannot select of all these varying structures those which can definitely be made the standards of general comparison. Environment, like a mould, has impressed itself upon forms genetically remote, and in the end has placed them side by side, apparently closely akin, similar in form and structure. A striking instance of changes due to environment is i68 TELEOSTOMES well known in the case of Deep-sea Fishes, in their acquir- ing a characteristic shape under the conditions of abyssal life. The head region of these forms becomes greatly exaggerated in size, and the trunk tapers suddenly away toward the tip of the pointed tail. The tissues become extremely modified, soft, porous, delicate, often trans- parent ; skeletal parts are deficient in lime, and loosely articulated. Many organs are retained in curiously unde- veloped or aborted conditions ; the vertebral axis is noto- Figs. 172-174. — Deep-sea fishes. (After GiJNTHER.) 172. Paralipans bathy- bius. 640 fathoms. 173. Bathyonus compressus. 1400 fathoms. 174. Notacant/ius sexspinis. 1800 fathoms. chordal ; gill arches, as many as six (?) in number, may open freely to the surface, never enclosed by opercula ; sensory canals remain as open grooves as in the most generalized fishes ; paired fins are retained either in an undeveloped condition or are not produced at all. Absence of light has been not without its effects ; body colours are usually dark and meaningless ; while, on the other hand, when eyes still DEEP-SEA TELEOSTS AND FIERASFER 169 occur, a widely modified series of integumentary phos- phorescent organs are often evolved as lures by predatory forms. It is evident, in the case of deep-sea fishes, that the simple condition of their structures does not separate them widely in point of descent from more specially evolved Teleosts. Intermediate forms, occurring in shal- lower water, often connect them clearly with different, and widely distinct, groups of bony fishes. In this way the Fig. 175. — Fierasfer acus, Kaup. X §. (After EMERY.) Commensal of sea- cucumber in southern waters. forms which are shown in Figs. 172, 173, 174 are severally connected with the cottid, the cod and the salmon, al- though the striking similarity of their outward structures would naturally lead one to regard them as far more intimately related. Another interesting instance of the modification of a fish's form by its living conditions has often been noted in the case of Fierasfer (Fig. 175). This small Teleost lives as a commensal in the branchial chamber of the sea-cucum- 170 TELEOSTOMES her, and from its peculiar life habit retains permanently a number of its embryonic characters ; it has thus its elongated larval form, a functional pronephros, a noto- chordal skeleton and immature fin conditions (Emery, Ref. p. 249). To what degree the structures of fishes may be varied by artificial selection is an interesting question, but one that has as yet received little attention even from those who have made artificialization an especial study. In the instance of the GoldfisJi it is well known how wide a Fig. 176. — Goldfish, Carassius aura/us ("Telescope" variety). X i. GiJNTHER.) Japan. (After variation has been produced in colour, size, and proportions. Fin structures are elaborately developed, long, drooping, lace-like, often to a degree which must render progression both slow and difficult. Even the eyes have been made to become large and protruding (Telescope-fish, Fig. 176). In carp the variation in scale character, due to artificializa- tion, is also to be mentioned. It is natural, perhaps, that artificial selection has been most successfully practised CA T FISHES 171 among these forms which compete most actively for survival. To conclude the present chapter, several forms of Tele- osts may be briefly discussed as especially characteristic of the group, namely the catfish, Mormyrus, eel, perch, cod, flounder, porcupine-fish, sea-horse. The catfish, representing the Silnroids, has, as already noted, many structural affinities to the sturgeon, and is, perhaps, a direct descendant of some early type of Mesozoic Palasoniscoid. It is a representative of a large and wide- spread family, usually of river fishes. Its habits are slug- Fig. 177. — The bull-head (catfish), Amiiinis me las (Raf.), Jord. and Cope- land. < i. (After GOODE in U. S. F. C.) Eastern North America. gish and mud-loving. Its trunk is heavy, rounded, and without Teleostean scales; its broad mouth margin is pro- vided with barbels ; the fin rays of its dorsal and pectoral fins fuse into a stout, serrate, erectile spine. In North American forms armouring derm plates are developed only on the head roof (Fig. 177). Closely akin to these are the Asiatic genera, and the single European species, Silnrus glanis, the gigantic Wels of the Danube. The Nile is of interest if only for its forms of catfish to parallel the shapes and structures of the recent Teleosts. 172 TELEOSTOMES In South America the catfish is a regnant type, and is remarkable for the variety as well as for the number and size of its forms. Many, completely armoured (Fig. 178), are strongly suggestive of Ganoids. Their armouring is Fig. 178. — South American Siluroid, Callicht/iys arm a tits. X I. (After GiJNTHER.) Upper Amazon. metameral and archaic, their sensory canals primitive in structure and arrangement. Mormyrns, like the catfish, appears to have long been divergent from the main stem of the Teleosts. Its species Fig. 179. — Mormyrus oxyrhynchus. X J. (After GUNTHER.) Nile. are restricted to the Nile, one — the long-nosed M. oxyrliyn- chns (Fig. 179) -- figuring prominently in Egyptian myth. In many of its structures it is archaic, as in axial skeleton, fins, dermal characters, sensory canals ; in others, e.g. hear- EEL-LIKE FORMS 173 ing organ, it is most highly specialized. Its group is an interesting one, and has been but little studied. The Eel (Fig. 180) might well be taken as one of the Fig. 180. — The eel, Anguilla vulgaris, Turton. X \. (After GOODE in U. S. F. C.) Europe, South Asia, North Africa, North America. fish forms evolved by special environment. Living in soft river bottoms, a serpent-like movement in progression has gradually been acquired ; its form has, therefore, become elongated and rounded, and the internal structures corre- O spondingly modified. Fin structures have accordingly been Fig. 181. — The perch, Per en americana ( = JluviaUlis ?) , Schrank. X 1. (Alter GOODE in U. S. F. C.) metamorphosed, ventral fins lost, tail degenerated, and a continuous dorsal and ventral secondarily evolved; scales have become reduced in size, supplanted by mucous layers. 174 TELEOSTOMES Similarity in eel-like form, e.g. as of Mur&na, is not in itself indicative of direct kinship. (Afodcs.} The Perch (Fig. 181) has long been taken as a repre- sentative Teleost. Perfect in its "lines," its compact, wedge-like shape cleaves the water by vigorous thrusts of a strong broad caudal ; its fins are stout, supported by spinous rays ; its dermal armouring light, smooth, and flex- ible ; its colour is brilliant under its transparent scales. So adapted is it to its environment that its organ of static equilibrium, the air-bladder, has lost its valvular connec- tion with the gullet. Of existing fishes about one-half are essentially percoid. (Acanthopterygii.) Fig. 182. — The codfish, Gad us morrhua, L. X J. (After GoODE in U. S. F. C.) North Atlantic. The Cod (Fig. 182) is scarcely less important as a repre- sentative Teleost. Its structural differences may perhaps represent the result of a competition less active than that of the perch in the struggle for survival. Heavy in body, its sluggish form has become blunted and rounded ; its fins are depressed, their rays soft and yielding; its scales are reduced in size, colours less .vivid ; its swim-bladder loses its connection with the gullet. As many, perhaps, as one quarter of the existing genera of fishes may be assigned to this type. (Anacantkini.) The Flounder -(Fig. 183) should be mentioned as a singu- FLOUNDERS AND PORCUPINE-FISHES 175 lar instance of environmental evolution, its flattened body adapting itself both in shape and colour to its bottom living. Its entire side, --not the ventral region, as in the rays, — is flattened to the bottom. The unpaired fins now become of especial value ; they increase in size, and their undulatory movements enable the fish to swim rapidly yet retain its one-sided position ; ventral fins become useless, and degenerate. The further adaptations of the flat fish include its pigmentation only on the upper or light-exposed side, and the rotation of the eye from the blind to the upper Fig. 183. — The winter flounder, Pseudopleuronectes americanus (Walb.), Gill. X \. (After GOODE in U. S. F. C.) North Atlantic. side,-- in this giving one of the most remarkable cases of adaptation known among vertebrates. (Heterosomata?) The Porctipine-fish (Fig. 184) may be referred to as another singular result of environmental evolution. Its globular and inflatable form bespeaks slowness of motion and helplessness if exposed to changes of temperature or current. Its fins are reduced and feeble, suited, how- ever, to its tranquil habitat ; its fused jaws, parrot-like, show in how special a way its food is best secured. It has evolved a protective casing of enormous needle-like scales, whose shape parallels that of the derm denticles 176 TELEOSTOMES of the shark. As a somewhat transition form to the more usual conditions of the Teleost, the Rabbit-fisJi has been figured (Fig. 184 A). Fig. 184. — The porcupine-fish, Ckilomyctertts geometricus (Schn.), Kaup. x §- (After GOODK in U. S. F. C. report.) Warmer Atlantic. Fig. 184 A. — The rabbit-fish, Lagocephalus Icrvigatus (L.), Gill. X 5. (After GOODE in U. S. F. C.) Northeast Atlantic. A final, perhaps the most bizarre, instance of adapta- tion among Teleosts is that of the Sea-Jiorsc (Fig. 185). In spite of its many structural oddities, its genetic kin- ship with the Sticklebacks (Hemibranchiates) cannot be doubted. Yet to have attained its present form its evolu- tion must have been carried along a widely divergent path. It may, in the first place, have fused the lines of its meta- meral scales, dividing off the surface of its elongate body SEA-HORSE AND PIPE-FISH 177 in sharp-edged rectangles, whose corners became produced as spines. At this stage of evolution its appearance might well be represented by (Fig. 185 A) the kindred Pipe-fish. To secure more perfect anchorage in its algous feeding- ground, its body terminal must now have discarded its fin membranes and become prehensile, - - probably the most remarkable adaptation in the entire class of fishes, since it causes metameral organs to change the plane in which they function from a horizontal to a vertical one. As a probable de- velopment of prehensilism, three changes may next have been wrought : the flexure of the neck region, the thickening of the trunk, and the metamorphosis of the fins. The first change may have been brought about by the normal position of the fish's axis becoming, as is well known, vertical ; the head then assumes its normal horizontal plane and thus parallels mildly the cranial flexure of higher ani- mals. The enlargement of the . Fig. 185. — The sea-horse, Hip- trunk region is evidently of static pocampus heptagonus, Raf. x \. 1 -ru i«- <-• f 4-u (After GOODE in U. S. F. C.) East value. The alteration of the po- Joast of North America, sition, size, and degree of move- ment of the pectoral fins, the loss of the ventrals and the changed function, now one of propulsion, of the dorsal, appear clearly the result of the altered plane of the fish's motion. Further structural changes might with interest TELEOSTOMES be followed, as in characters of viscera, gills, and endo- skeleton. In its life habits mimicry is strongly evinced; Fig. 185 A. — The pipe-fish, Syngnathus acus J, L., showing abdominal pouch. X i. (After GiJNTHEK.) Coasts of Europe and Africa. the well-known genus Phyllopteryx, whose entire body surface develops pigmented appendages, is with difficulty to be distinguished from a rough-shaped seaweed. (LopJio- brauckii.} VIII THE DEVELOPMENT OF FISHES THE groups of fishes have hitherto been contrasted in the structures of their living and fossil forms. They should next be reviewed in the light of their mode of development ; for the developmental stages of the Shark, Lung-fish, or Teleostome might be expected, according to time-honoured belief, to furnish important evidence as to their descent and interrelationships. The younger stages of the various forms of fishes should thus suggest their ancestral characters : the developing Teleost should approach the Ganoid ; the Lung-fish and the Ganoid should resemble their supposed elasmobranchian ancestor. But the embryology of fishes is in this regard very inconclusive, if at present in any important way sugges- tive. The majority of the forms, including some of the most important, are developmentally unknown ; yet suffi- cient is known of the representative members of the groups to show the most perplexing characters. On the one hand, the developmental .processes of forms which are regarded by the morphologist as closely akin seem often widely distinct ; and, on the other hand, the fishes which should, a priori, exhibit an archaic mode of development actually present complex processes of early growth which can only be interpreted as highly specialized. In fact, there are far greater differences in the developmental plans 179 jgO DEVELOPMENT OF FISHES of the closely related Ganoid and Teleost, than in those of a Reptile and a Bird ; and even among the members of the single group, Teleosts, there are more striking embryolog- ical differences than those between Reptiles and Mammals. Adaptive characters have entered so largely into the plan of the development of fishes that they obscure many of the features which might otherwise be made of value for comparison. And until the controversies regarding some of the most fundamental principles in embryology — e.g. the importance of the loss or gain of food yolk — shall be decided, it seems impracticable to use the plan of develop- ment as in any strict sense a guide in phylogeny. It is, accordingly, rather with the view of contrast- ing the groups of fishes, whose external features have hitherto been compared, that the present chapter seems of especial importance. They may briefly be reviewed in their (A) spawning habits, (B) the mode of fertilization of their eggs, (C) their embryonic, and (D) larval de- velopment. A. EGGS AND BREEDING HABITS The eggs of typical fishes in Figs. 186-199, illustrate how wide a range occurs in their shapes and sizes. All are of about actual size, except Figs. 189-191, which have been reduced about two-thirds. From the figures the character of the egg membranes may also be contrasted. Among Cyclostomes, which are usually looked upon as of close genetic kinship, there appears a striking dif- ference in the characters of the eggs. Those of Bdello- stoma and Myxine (Figs. 186, 187) are large and bluntly spindle-shaped, encased in a horn-like capsule ; those, on the other hand, of Petromyzon are minute, spherical, and enclosed in delicate and jelly-like membranes (Fig. 188). FIG. 189 Figs. 186-199. — ^-nas ar>d egg cases of fishes. Ail of about actual si/.e except 189-91; these have been reduced about two-thirds. 186. Bdellostoma: germ disc (?) at upper pole and in 186 A terminal hook processes and micropyle. (After AYERS.) 187. Myxine. (After STEENSTRUP.) 187 A. Terminal process. 188. Petromyzon mar inns. 189. Shark, Scy Ilium. (After GUNTHER.) 189 A. Skate, Kaja. 190. Port Jackson shark, Cestnicion. (After GtJNTHER.) 191. Chimaeroid, Callorhyncluts. (After GiJNTHER.) 192. Lung-fish, ( 'riatfldus. (After SEMON.) 193. Ganoid, Lepidosleus. 194. Ganoid, Acipetiser. 195. Siluroid, Arius, showing larva. (After GO NTH ER.) 196. Teleosts : sea-bass, .Stv/./v/cr, and 197. shad, Alosa. 198. Blennv, ftiemiiits, showing attached egg capsules. 199. Enlarged Blennius (after GUITEL), showing mode of attachment of capsule. 181 1 82 DEVELOPMENT OF FISHES The eggs of Myxinoids are probably deposited at a single time ; at first extruded by pressure of the body wall ; then drawn out string-like, one egg following another, attached by hooked and thread-like processes (Figs. i86A, 187^). Little is known, however, of the actual breeding habits of Myxinoids, either as to locality, mode, or season ; individuals of Myxine and Bdellostoma with ripe spawn have never been taken even in the most favourable regions. It is supposed that their spawn- ing does not occur in the immediate neighbourhood of the shore, since detached eggs have been dredged in the deeper water. Their breeding time is probably in the early spring, although possibly intermittent spawning takes place. In Myxine, according to Putnam,* the bulk of the eggs may be deposited as late as the beginning of winter. The spawning habits of Petromyzon, on the other hand, have been especially favourable for observation. The eggs are deposited in shallow and clear water and the move- ments of the fish may readily be followed. In the small stream at Princeton,! f°r example, the lampreys make their appearance about the middle of May and remain on the spawning grounds two or three weeks. Their "nests" are seen scattered thickly on the gravelly shoals, often but a few feet apart. Each will be occupied by several males and a single female, the latter conspicuous on account of greater size. When spawning, the lampreys press together and cause a flurry in the water at the moment when the eggs and milt are emitted. This portion of eggs will now * As observed at Grand Menan. Pro. Bust. Soc. Nat. Hist. Feb. '74. t Professor McClure and Dr. O. S. Strong have here repeatedly observed the spawning lampreys; it is to their account that the writer is here indebted. Compare, also, the excellent account given recently by Professor Gage. Kef. p. 234. EGGS OF ELASMOBRANCHS 183 be covered with a thin layer of sand or gravel, --the spavvners always returning to the same nest, --and a sec- ond, third, and more tiers of eggs will be added. When the eggs have finally been deposited, the nest is fortified by a dome-like mass of pebbles and stones, which the lam- preys carefully drag to the spot. The nest is thus marked out as well as protected, and is said to be made of partial use during the following season. The hatching of the eggs takes place within about a fortnight. The eggs which Sharks and Rays deposit are usually enclosed in a stout, horn-like capsule ; this is in general of oblong or rectangular outline, its surface smooth or ridged ; the case of the egg of Scy Ilium (Fig. 189), shows thread- like terminal processes, while these in the ray (Fig. 189^) are stout and spine-like. A great variation may exist in the size of the egg and in the character of its envelopes among the different groups of Elasmobranchs. The egg of the Port Jackson shark, Cestmcion (Fig. 190), is of enor- mous size and possesses an extremely thick, spiral-rimmed, pear-shaped capsule ; that of the Greenland shark, Lcemar- gus, is said to be spherical and relatively small, and to be deposited unprotected by capsule. The breeding habits of Elasmobranchs are but imper- fectly known. With the exception, perhaps, of Laemargus, the sexes copulate.* The clasping appendages of the male are inserted either singly or together into the cloaca and oviduct of the female, and the eggs appear to be fertilized in the uppermost portion of the oviduct. The egg then becomes surrounded by a glairy albuminous envelope, and thereafter by the secretion of the oviducal gland, which in the lower oviduct hardens into the horny capsule. The * The copulation of sharks has been but rarely observed (e.g. by Bolau in Hamburg ; cf. Kef. on p. 241). !84 DEVELOPMENT OF FISHES majority of sharks and rays are viviparous ; the eggs are retained in the lowermost portion of the oviduct (uterus) and the embryo establishes a "placental" circulation, the vascular yolk sac becoming adherent to the walls of the uterus. Other sharks deposit their eggs, and their mode of oviposition has been observed. The egg (Fig. 189), when slightly protruded from the cloaca, is rubbed against brush-like objects, and when its terminal processes become finally entangled, the egg is withdrawn. The processes of the egg case which leave the body last, the longer ones, are often greatly straightened out when the egg is depos- ited ; subsequently their elastic character causes them to curl tightly, and often to secure a firm attachment to neighbouring objects. The eggs of oviparous skates (Fig. 189 A) are said to be deposited on sand flats near the mark of low water. Mr. Vinal N. Edwards of Wood's Holl, Massachusetts, believes that they are implanted ver- tically in the sand, and, from the occurrence of "beds" of skate eggs, that the fishes are singularly local in their places of spawning. Eggs of Elasmobranchs* are often many months in hatching ; the young fish finally escapes through a slit at the end of the egg case. Nothing is known definitely of the breeding habits of Chimaeroids. The mode of copulation of the sexes is doubtless similar to that of sharks. Their clasping organs are highly specialized sperm ducts, and the hook-bearing organs at the anterior margin of the ventral fin, and on the forehead of the male, function in all probability in retaining the female. The forehead spine could certainly prove of such service if the position of the fishes during mating was at all similar to that figured for Scyllium by * In the case of Scyllium the eggs are deposited about six days after they have been fertilized ; they then hatch in from 200 to 275 days. EGGS OF FISHES I85 Bolau.* The egg case of Callorhynchus (Fig. 191) is essentially shark-like ; it is of spindle-shaped outline, and its broad, fringing margin gives it an almost seaweed-like appearance. The egg is believed to be deposited in deep water. The spawning of but one of the three existing Lung- fishes has been recorded. Ceratodus, according to Setnon, has a spawning season extending over several months ; it deposits its eggs in shallow water, scattering them broad- cast. The female fish is attended by several males, and the emission of eggs and milt appears to be simultaneous. The egg (Fig. 192) lacks a horny capsule, but is amply protected by a thick, jelly-like hull. It hatches during the second week. Eggs of Ganoids are shown in Figs. 193, 194. They are encased in a jelly-like envelope, especially viscid in the case of sturgeon. When deposited, they speedily adhere to whatever they touch, and often remain attached until the time of hatching. The spawning grounds are in shallow water ; the fish occur in numbers during a few days of May and June, each female attended by several males : ova and milt are emitted simultaneously, at short intervals. The eggs develop rapidly, hatching in about a week. The eggs of Teleosts present the utmost variety in number, form, membranes, and mode of deposition. In some forms (Embiotocids, Blenniids, Cyprinodonts) they may even develop within the ovarian tissue, establishing there a "placental" circulation. They have been fertilized within the fish, the anal fin spine of the male having in some cases been metamorphosed into a copulatory organ. The eggs of Siluroids (Fig. 195) are generally of large size, * v. Ref. p. 241. 1 86 DEVELOPMENT OF FISHES and somewhat adhesive; they are deposited in "nests," i.e. bowl-like depressions, and are attended by the male fish.* Other adhesive eggs are those of carp, Christiceps, Batra- chus. Eggs of Saltnonids are deposited loosely in " nests " on a clean, gravelly bottom ; their membranes are thick and parchment-like. On the other hand, the majority of pelagic fishes produce eggs which float (Figs. 196, 197) ; of these the membranes are extremely hygroscopic and transparent, and an oil globule, located in the yolk region of the egg, serves to diminish, its specific gravity. The egg membranes of a number of Teleosts, e.g. Blennies (Fig. 199), appear essentially shark-like ; a horn-like cap- sule is evolved, whose terminal processes afford it a firm attachment. Aberrant modes of oviposition are not lack- ing ; the South American Siluroid, Aspredo, as is well known, carries its eggs attached to its ventral surface ; the pipe-fishes and sea-horses, Siphostoma, Solenostoma, Hip- pocampus, have specialized a pouch-like fold of the abdo- men and of the ventral fins, which serves to retain the eggs and larvae. It is curious to note that this remark- 'able condition occurs only in the male. The breeding habits of Teleosts are in general like those of Ganoids ; their spawning season is usually during the spring and summer, but is seldom of very brief duration. The hatching of the eggs depends largely upon water temperature, and may vary from a few days to several months (Salmo). B. THE FERTILIZATION PHENOMENA The processes of the maturation and fertilization of the egg have as yet shown but minor differences in the * In several genera they are carried about in the gill chamber of the male, thus ensuring aeration. EARLY DEVELOPMENT ^j groups of fishes. In the forms which have thus far been studied * there have been few noteworthy variations from what appear the normal conditions of vertebrates. The sperm usually gains admission to the egg through a micro- pyle in the egg membranes which becomes formed imme- diately after the extrusion of the polar bodies. A sperm cell, invariably a single one, participates in the actual fertilization. This may occur directly by the formation of a single male pronucleus, as e.g. in Petromyzon, Teleosts ; while in the sharks, on the other hand, Ruckert describes a multiple fertilization (polyspermy), where many male pronucleif are formed, the one nearest in position fusing subsequently with the female pronucleus. An inter- mediate condition seems to be retained in the sturgeon, where several (six to nine) micropyles have been noted, although but a single one occurs in the kindred Ganoid, o o Lepidosteus (Mark, Ref. p. 249). C. THE KMMKYONir DEVELOPMENT When the egg of a fish is deposited, it contains but the elements of a single cell. Its size and its enveloping membranes may vary widely, but its constituents are con- stant,— cytoplasm and nucleus. The size of the egg in different fishes varies with the amount of food material, or yolk, stored away in its cytoplasm ; the enormous egg of the shark differs from the minute egg of the lamprey strikingly in this regard. But even in the minute lamprey egg there is a certain amount of yolk material present. In every egg there can usually be distinguished at sight * Lamprey by Kupffer and Bohm, and Calberla ; Sharks by Ruckert ; Te- leostomes by Hoffman, Agassiz and Whitman, Kupffer, Bohm, and others. t These appear later to undergo karyokinesis, and are thereafter to be regarded as supplemental merocytes (p. 195). 1 88 DEVELOPMENT OF PISHES an upper and a lower zone : the latter rich orange in colour, caused by the settling of the heavier yolk material ; the former lighter in colour, containing the nucleus of the egg, and originating the growth processes. The less the amount of yolk in the lower, or vegetative, region, the smaller is naturally the egg, and the more obscure becomes the limit of the upper zone, or germ, or animal pole, as it is indifferently called. In the yolk- filled egg of the shark, on the other hand, the upper zone becomes reduced to a mere "germ disc " on the surface of the egg (Fig. 216, GD}. If but little yolk is present, the early growth processes, i.e. the splitting of the germ cell, or egg, into many cells, or blastomeres, to give rise to the embryo, affect the entire egg. If, however, much yolk is present, the cells at first multiply only at the animal pole, and the yolk-filled region, remaining unsegmented, fur- nishes the nutriment for the cell growth above. In the present outline of the development of fishes, the following types are reviewed : - I. Petromyzon ; II. Shark; III. Lung-fish; IV. Ganoid; V. Teleost. I. The Development of Petromyzon The egg of Petromyzon is of small size (Fig. 188), and is poorly provided with yolk material ; in surface view one can only distinguish the germinal from the yolk region by its slightly lighter colour. In the side view of the egg of Fig. 200, the beginning of the first cleavage plane is seen ; a vertical plane, passing through the egg, completes the stage of the two blastomeres of Fig. 201. The nuclei were at first close to the upper, or animal, pole, but they shortly take their position somewhat above the plane of the egg's equator. A second cleavage plane is again vertical, ap- FIG. 200 201 204 EN Figs. 200-215. — Development of lamprey, Petromyzon planeri. Figs. 200-204, 208-212 X 18, others X about 30. 200, 201. First cleavage, beginning and concluded. 202. Third cleavage. 203. Fourth cleavage, in section, showing beginning of segmentation cavity. 204, 205. Early and late blastulas, in section. 206, 207. Early and late gastrulas, in section. 208, 210, 212. Early embryos showing growth of head end. 209, 211. Sagittal sections of early embryos showing differentiation of organs. 213, 214. Transverse sections of early embryos. 215. Sagittal section of newly hatched larva, Ammocastes. (Figs. 211, 215, after GOETTE, others after v. KUPFFER.) BP. Blastopore. C. Coelenteron. CH. Notochord. DL. Dorsal lip of blastopore. EC. Ectoderm. EN. Entoderm. EP. Epiphysis. G. Gut. H. Heart. M. Central nervous system. MES. Mesoblast. N. Nasal pit. NC. Neurenteric region. J>". Mouth pit, stomo- daeum. SC. Segmentation cavity. T. Thyroid gland. Y. Yolk and yolk cells. 189 190 DEVELOPMENT OF FISHES proximately at right angles to the first ; the third, which shortly appears, is horizontal (Fig. 202), giving rise to the stage of eight blastomeres ; this plane, passing slightly above the equator; causes the upper blastomeres to be slightly smaller in size than those of the lower hemisphere. The amount of yolk in the egg, it is accordingly inferred, although not sufficient to prevent the passage of cleavage planes, is enough, nevertheless, to retard the nuclear cleav- ages in the region of the lower, or vegetative, pole. In Fig. 203, showing a vertical section of the following stage, another horizontal cleavage has been established in the upper part of the egg ; the segmentation cavity is seen in the centre of the figure arising as the central space between the blastomeres. This is seen to have become greatly enlarged in Fig. 204, a slightly later stage where in vertical section is seen a greatly increased number of blastomeres. Repeated cleavage of all blastomeres now continues regularly, and results in the production of a b/astnla, a smooth-surfaced cell mass containing the seg- mentation cavity, SC (in section, Fig. 205) ; this is seen to be located in the region of the animal pole. In the next developmental stage, gastmla, seen in section in Fig. 206, the primitive digestive tract, ccelentcron, C, is appearing ; it arises as an indentation of the side of the blastula. The coelenteron, soon greatly increasing in depth, reduces in size and finally obliterates the segmentation cav- ity, taking the position, C, shown in section in Fig. 207. Here the segmentation cavity has practically disappeared ; the surface opening of the coelenteron is the blastopore, BP; the cell layer of the gastrula's surface is the ecto- derm, EC\ the cell layer lining the coelenteron is the en- toderm, EN: the coelenteron, it will be seen, is closely apposed to the ectoderm at the left of the figure, — the DEVELOPMENT OF LAMPREY IQI future dorsal region of the embryo ; on this side the margin of the blastopore is known as the dorsal lip, DL, while to the right the ventral lip is seen greatly enlarged by the yolk-bearing cells, K A somewhat later stage (Fig. 208) shows the blastopore as a narrowly constricted opening, BP, whose dorsal lip is slightly raised at its left- hand margin. The head of the embryo is to arise near the opposite pole (as in Fig. 210), and is thence to elon- gate into neck and trunk (Fig. 212). A sagittal section of a stage, slightly older than Fig. 208, shows admirably the structures of the embryo that have thus far been differ- entiated (Fig. 209). Contrasting with Fig. 207, it will thus be seen that the coelenteron, arising at BP, has become greatly elongated ; at its blind end its lining mem- brane, entoderm, EN, is in contact with an indented por- tion of the ectoderm, at S, where later the opening of the mouth will be established ; and that ventrally the coelen- teron has given off a pouch which passes into the yolk, and will later be differentiated as the liver. That the entire dorsal wall of the coelenteron has become thickened, con- stitutes the main difference between the sections of Figs. 207 and 209 ; there have, in other words, arisen between the entoderm and ectoderm of Fig. 207 the central ner- vous system, or medullary cord, M, and the notochord, CH. The origin of these structures may best be traced in the cross-section of a slightly earlier stage (Fig. 213) ; the coelenteron, or gut, is at G, the ectoderm at EC, the yolk cells intervening at Y; and the notochord and medullary cord, CH, and M, in the sagittal region immediately be- tween the gut and the ectoderm. In the medullary region the ectoderm cells are seen pressed together, growing down- ward and sidewise, forming altogether a compact cell cord * * As in Teleosts, but unlike other vertebrates. ICj2 DEVELOPMENT OF FISHES passing clown the back of the embryo ; the notochord is aris- ing from the differentiating cells of the roof of the gut. In the cross-section shown in Fig. 214, the subsequent con- ditions of these structures may be seen ; the medullary nerve cord, M, is now in section elliptical, separated dor- sally from the ectoderm, and its cellular elements are of more uniform size, arranged with bilateral symmetry, its central lumen having not as yet appeared ; the notochord, now constricted off from the wall of the gut, takes upon it its characteristic form and structure. It is, however, in the differentiation of the walls of the gut that this section is of especial interest ; the gut is seen to have greatly enlarged, and at the expense of the yolk material ; its lining membrane, entoderm, EN, is now directly ap- posed to the outer germ layer, ectoderm, EC. The middle germ layer, mesoderm, MES, — out of which cartilage, muscular and connective tissue, are formed, --is now seen taking its origin as paired evaginations of the dorsal wall of the gut. The mesoderm shortly loses its connection with the entoderm, and by the rapid increase of its cellular elements rapidly invests the remaining embryonic struct- ures ; its segmental character may be seen in the surface view shown in Fig. 210, its dorsal portions appearing as the primitive segments. Later developmental stages are shown in the sagittal sections, Figs. 211, 212. These may best be compared with Fig. 209. In Fig. 211 the head end of the body has greatly elongated, and with it the gut cavity has dilated ; entoderm is now composed of very minute cells, whose nuclei are suggested by dots ; the yolk has become more definitely restricted to the region of the hinder gut ; the blastopore is still seen ; at its lips the germ layers are alone fused. DEVELOPMENT OF FI SPIES 193 II. The Development of the Shark On the side of embryology a shark presents many points of striking contrast to the lamprey ; yet it may in many regards be looked upon as archaic in its developmental characters. Its contrasting structures (together with those of lung-fish, Ganoid, and Teleost) may best be reviewed in the table, p. 280. The egg of the shark is of large size, richly provided with yolk material. When removed from its membranes, it is seen to be of a bright orange colour ; its form is elon- gated, and the weight of its pasty substance causes it to assume a flattened ovoid (Fig. 216). At the upper pole of the egg is a small, light-coloured spot, the germ disc, GD, which figures prominently in the early stages of develop- ment. It would represent the lamprey's entire egg, if one could imagine a point of the lower pole of the latter hugely dilated with yolk. It is in the region of this germ disc alone that every process of development as far as gastrula- tion occurs. The segmentation of the germ disc is shown in Figs. 217-220. In the first of these (Fig. 217) the germ is seen to be sharply marked off from the surrounding yolk by a circular band ; two cleavages have traversed it in the form of narrow grooves separating the blastomeres. In Fig. 218 the fifth cleavage has been completed; the furrows dividing irregularly the surface of the germ disc fade away at its periphery. Fig. 219 represents a vertical section of the germ disc at this stage ; the upper, finely dotted layer, thinning away at either side, is the germ disc ; the coarsely granular material below is the yolk ; the depth of the cleavage furrows is seen, and it will be noted that up to this stage of development there have been no horizontal FIG. 216 217 218 219 CH Figs. 216-230. — Development of shark, ScyUinni (mainly). (All but 216 after BAL- FOUR.) 216. Egg freed from case showing germ disc GD. 217. Germ disc at second cleavage. 218. Germ disc at fifth (?) cleavage. 219. Vertical section of similar stage. 220. Vertical section of slightly older germ disc. 221. Blastula. 222. Early gastrula. 223. Blastoderm showing early growth of embryo. 224-226. Slightly later stages of growth of embryo. 227. Stage showing early embryo and mode in which the blastoderm sur- rounds yolk. 228. Early embryo viewed as a transparent object. 229, 230. Transverse sections of early embryo. A. Anal invagination. AU. Auditory vesicle. BP. Dorsal lip of blastopore. C. Coelenteron. CF. Tail folds. CH. Notochord. CP, Cephalic plate. EC. Ectoderm. EN. Entoderm. G. Gut. GD. Germ disc. GS. Gill slits. H. Heart. HE. Head eminence. M. Central nervous system. M' . Yolk nuclei, merocytes. AfRS. Mesoblast. NC. Neurenteric canal. OP. Optic vesicle. PS. Primitive segments. »S'. Mouth pit, stomodseum. SC. Segmentation cavity. 194 DEVELOPMENT OF SHARK 195 cleavages. A stage in which early horizontal cleavages are represented is shown in Fig. 220. This may well be compared with the last figure ; the germ disc, while not increasing in diameter, is now seen to have multiplied its blastomeres by horizontal cleavages ; it is converted into a plug-shaped mass of cells, sunken into the yolk material. At M' are cell nuclei, which have found their way into the adjacent yolk, and which there acquire a developmental importance. They become the so-called merocytes, or yolk nuclei. The section of the germ shown in Fig. 221 represents a subsequent stage of development ; the blastomeres, by continued subdivision, have become greatly reduced in size, and are clearly to be distinguished from the smooth-sur- faced, yolk-like material lying beneath. Merocytes, M', are apparent in the superficial layer of the yolk ; they are supposed to serve a twofold function, — on the one hand, to elaborate the yolk material and fit it for the embryo's use ; on the other, to supply the cells which are being con- tinually added to the germ's margin. In the figure a large cavity is shown to exist between the yolk and the mass of blastomeres. This cavity has been identified as the seg- mentation cavity, SC, and the developmental stage as the blastula ; it is as though the lower hemisphere of the lamprey's blastula (Fig. 205) had become enormously enlarged, and all traces of the cells in the floor of its segmentation cavity lost, except in the layer of the metamorphosed cells, the merocytes. In the next growth process the extent of the germ area becomes greatly increased ; the thick blastula is now thinned out into a surface layer of regular cells, an en- larging disc-like blastoderm, which will eventually grow around and enclose the entire egg. The blastoderm of 196 DEVELOPMENT OF FISHES Fig. 223 is a pale-coloured circular membrane of about a half inch in diameter lying on the surface of the egg. Sectioned at an earlier stage (Fig. 222) the blastoderm is seen to present the following contrast to the blastula of Fig. 221 : the floor of the segmentation cavity has flattened, and a sharp rim forms the outline of the blastoderm ; at one side this rim is seen to protrude over the yolk mass, leaving a narrow, fissure-like cavity between. This stage is identified as the gastrula ; the fissure-like cavity, the ccelenteron ; its marginal blastoderm, the dorsal lip of the blastopore ; its ventral lip, the entire yolk mass. The growth of the embryo's form takes its origin at the blastopore's dorsal lip. In Fig. 223 the rim of the blasto- derm is seen indented near the point CF, and its thicken- ing at this region becomes more and more marked in subsequent stages ; on the other hand, the anterior por- tion of the blastoderm, growing continually on all sides, becomes excessively thin, flattening itself tightly to the yolk, and reducing the segmentation cavity to the small area indicated at SC. The growth of the embryo in the mid-region of the blastopore's dorsal lip may next be followed in the stages, Figs. 224, 225, 226. The inden- tation of the rim may thus be seen to assume a creese- like thickening, thrusting forward its blunt end, the head eminence, HE, over the blastoderm ; at the points CF, the tail eminences, the rim of the blastoderm is thick, protruding, appearing to be pressing together in the median line, and causing the body of the embryo to be actually pushed into form and thrust above the level of the blastoderm. In Fig. 225 the sides of the embryo are separated dorsally by a deep groove, the medullary furrow, the future canal of the central nervous system. In Fig. 226 this is seen at a more advanced stage ; its hinder DEVELOPMENT OF SHARK portion has been roofed over by the coalesced sides, and the process of enclosing the groove is being continued anteriorly, although the head end of the embryo is now flattened out as the prominent cephalic plate. In the stage figured in 227, the form of the embryo has been acquired : the head in the manner already outlined, the tail by the coalescence and subsequent outgrowth of the tail folds, CF. The entire embryo now rises above the blastoderm, as this continues to enclose the yolk. In the figure the yolk has thus been more than half enclosed ; its final appearance is seen in the oval space outlined by a dotted line behind the embryo. The origin of the germ layers is not as readily traced as in the Cyclostome. Ectoderm is the most clearly marked; even in the blastula (Fig. 221) it has appeared as an outer single-celled stratum clearly differentiated from the underlying cells. Entoderm is only to be seen on the dorsal wall of the ccelenteron : the ventral entoderm (cf. Fig. 222) is merged with the yolk. Meso- derm takes its origin from the inner layer on either side of the median line, but it arises as a solid cell mass instead of as the pouch-like diverticula in Petromyzon. Cross-sections of an embryo represented by Fig. 224 have been figured in Figs. 228 and 229 ; the former is of the hinder region and illustrates the mode of growth of the mesoderm, MES ; the latter across the head region, shows that in this region the mesoderm is separated from the inner layer. Both sections show the simple character of the medullary groove, and the latter section the mode of origin of the notochord, CH, i.e. as an axial thickening of the entoderm. An embryo of about the stage of Fig. 227 is extremely delicate and may readily be viewed as a transparent object. 198 DEVELOPMENT OF LUNG-FISH By this time (Fig. 230) it will be seen that its prominent organs have already been differentiated. There are thus : medullary canal, M, with optic, OP, and auditory, AU, vesicles ; gut with gill slits, G S, neurenteric canal, NC, and suggestion of mouth, S, and anus, A ; notochord, CH ; segmented mesoderm (primitive segments), PS, and heart, H. The medullary groove was converted into a canal, as has been already suggested, by the overroofing and fusion of the summits of the medullary ridges ; its anterior dilatation is the brain ; the gut, G, communicates freely below with the yolk mass ; it is a cavity, a portion of the coelenteron that has been constricted off with the embryo ; its openings, the mouth, anus, and gill slits, are secondary, acquired after there have been established in these regions fusions of entoderm and ectoderm ; the neurenteric canal, NC, a communication between medul- lary tube and gut, is a structure acquired in the stage of Fig. 226, where the hinder medullary groove was roofed over, allowing, in the region of the tail folds, a communi- cation to exist between medullary canal and coelenteron. The notochord has by this stage been completely sepa- rated from the entoderm ; it already assumes a supporting function. III. TJie Development of Ceratodus The development of a Lung-fish has thus far been de- scribed (Semon) only from the outward appearance of the embryo. The egg of Ceratodus (Fig. 192) is seen without its covering membranes, enlarged, in Fig. 231. Its upper pole is distinguished by its fine covering of pigment. The first fine planes of cleavage are shown in Figs. 232-236; and from these it will be seen that the yolk material of the lower pole is not sufficient to prevent the egg's total seg- FIG. 231 232 233 234 235 236 237 238 240 242 239 244 245 246 247 Figs. 231-247. — Development of lung-fish, Ceratodus. (After SEMON.) X 4-7. 231. Egg immediately before cleavage. 232, 233. First cleavage, seen from above and from the side. 234. Second cleavage, seen from above. 235, 236. Third cleavage, seen from above and from the side. 237. Blastula. 238, 239. Gastrulae showing closure of bListopore. 240. Early embryo, seen from the side. 241. Early embryo showing medullary folds (head). 242. Tail region of same embryo. 243. Tail region of slightly later stage. 244. Head region of same embryo. 245-247. Later embryos. AV. Auditory vesicles. BP. Blastopore. GS. Gill slits. J\f. Mouth pit. A/F. Medullary folds. O. Olfactory lobes. OP. Optic vesicles. PN. Primitive kidney, proncphros. PS. Primitive segments. Y. Yolk mass. 199 2QO DEVELOPMENT OF FISHES mentation. The first plane of cleavage is a vertical one, passing down the side of the egg (Fig. 233) as a shallow surface furrow, not appearing to entirely separate the sub- stance of the blastomeres, although traversing completely the lower hemisphere (Fig. 232). A second vertical furrow at right angles to the first is seen from the upper pole in Fig. 234 ; it is essentially similar to that of Fig. 233. The third cleavage of Fig. 235 is again a vertical one (as in all other fishes, but unlike Petromyzon), approximately meridi- onal ; its furrows appear less clearly marked than of earlier cleavages, and seem somewhat irregular in occurrence. The fourth cleavage is horizontal above the plane of the equator. Judging from Semon's figure (Fig. 236), at this stage the furrows of the lower pole seem to have become fainter, if not entirely lost. A blastula showing complete segmenta- tion is seen in Fig. 237 ; the blastomeres of the upper hemisphere are the more finely subdivided ; the conditions of the segmentation cavity may be expected to prove similar to those of Fig. 205. Two stages of the gastrula are shown in Figs. 238 and 239, showing a full view of the blastopore. In the earlier one (Fig. 238) the dorsal lip of the blastopore is crescent-like ; in the later (239) the blastopore acquires its oblong outline, through which the yolk material is apparent ; its conditions may later be compared to those of a Ganoid (Figs. 254, 255). The growth of the embryo is illustrated in the remaining figures (Figs. 240-248). A side view of an early embryo is shown in Fig. 240 ; at the top of the egg to the right is the head region, to the left the blastopore and tail. The surface view of the head region (Fig. 241), the medullary folds, MF, may be compared with those of Fig. 225, although they are low and widely separated ; the axial seam is referred to by Semon as a demonstration of the DEVELOPMENT OF LUNG-FISH 2OI theory of the embryo's concrescence. In the hinder region of the same embryo (Fig. 242) the blastopore is still apparent, BP, reduced to a narrow, fissure-like aperture ; around it is the tail mass, corresponding generally to CF of Fig. 226 ; and encircling all is the hinder continuation of the medullary folds. The next change of the embryo is strikingly amphibian- like ; the medullary folds rise above the egg's surface, and, arching over, fuse their edges in the median dorsal line. In Fig. 243, the tail region of a slightly older embryo, this process is clearly shown ; the medullary folds, MF, are seen closely apposed in the median line ; hindward, how- ever, they are still separate, and through this opening the blastopore, BP, may yet be seen. At this stage primitive segments are shown at PS; in the brain region in Fig. 244 the medullary folds are still slightly separated (cf. CP, Fig. 226). Two views of an older embryo are fig- ured (Figs. 245 and 246), where the fish- like form may be rec- ognized. The medul- Fig- 248.— Embryo of Ceratodus, near the time " of hatching. lary folds have com- cs. GUI slits. M. Mouth pit. OP. Optic vesi- pletely fused in the deS' PN' Primitive kidney- Pronephros. T. Tail J eminence. median line, and the embryo is coming to acquire a ridge-like prominence ; optic vesicles and primitive segments are apparent, and at BP the blastopore appears to persist as the anus. The continued growth of the embryo above the yolk mass, Y, is apparent in Fig. 247; the head end has, however, grown the more rapidly, showing gill slits, GS, auditory, optic, and nasal vesicles, AU, OP, and O, at a time when 202 DEVELOPMENT OF GANOID the tail mass has hardly emerged from the surface. Pro- nephros has here appeared at PN (cf. with Fig. 247, Fig. 210). It is not until the stage of the late embryo of Fig. 248 that the hinder trunk region and tail come to be prominent. The embryo's axis elongates and becomes straighter ; the yolk mass is now much reduced, acquiring a more and more oblong form, lying in front of the tail, T, in the region of the posterior gut (cf. Figs. 211 and 212). The head, and even the region of the pronephros, PN, are clearly separate from the yolk sac ; the mouth, M, is coming to be formed. IV. The Development of Ganoids The development of Ganoids is next to be outlined. The eggs of the sturgeon and gar-pike are poorly provided with yolk. They have still, however, a greater amount than those of the lamprey or lung-fish, and in many regards of development suggest nearnesses to the Elasmo- branchs. The egg of the sturgeon shown in Fig. 249 shows clearly two distinct zones ; the upper, blotched with pig- ment at the animal pole, is pale in colour; the lower, rich in yolk, is orange-coloured, well speckled with pigment. The early cleavages appear at first only in the upper pale- coloured area which corresponds apparently with tfye germ disc of the shark's egg. In Fig. 250 there have been two cleavages, vertical and at right angles to each other ; these have sharply traversed the germ area, the earlier one being now produced slightly into the yolk region of the egg — only, however, as a slight surface furrow. The third cleavage (Fig. 251) presents a stage closely corre- sponding with that of Ceratodus of Fig. 235, its plane tend- ing to pass parallel to the first cleavage : the germ disc FIG. 249 250 251 252 253 VL 257 258 EC,MITSENNC 259 260 M H Figs. 249-268. — Development of Ganoids, Acipenser and (last four figures) Lepi- dosteus. x about 12. 249. Egg immediately before cleavage. 250. Second cleavage. 251. Third cleavage. 252. Blastula. 253. Vertical section of blastula. 254. Early gastrula. 255. Late gastrula. 256. Vertical section of late gastrula. 257. Early embryo. 258. Sagittal section of same stage. 259, 260. Head and tail regions of slightly later embryo. 261. Transverse body section of hinder body region of same stage. 262, 263. Head and tail regions of late embryo. 264. Embryo immediately before hatching. 265. Lepidosteus' blastula. 266. Vertical section of early gastrula. 267. Late gastrula. 268. Embryo, showing mode of separation from yolk. BP. Dorsal lip of blastopore. C. Ccelenteron. EC. Ectoderm. EN. Entoderm. F. Pectoral fin. GS. Gill slits. H. Heart. HE. Head eminence. KV. Kupffer's vesicle. LC. Marginal limit of ccelenteron. M. Mouth pit. i\fC. Medullary canal. MES. Mesoblast. NC. Neurenteric canal. OL. Olfactory pits. OP. Optic vesicles. PN. Primitive kidney, pronephros. PS. Primitive segments. SC. Segmentation cavity. 71 Tail eminence. VL. Ventral lip of blastopore. Y. Yolk, yolk mass. YP. 'Yolk plug. 203 2O4 DEVELOPMENT OF FISHES is deeply cut by the furrows ; the yolk area, however, only superficially ; the shallow furrow of the first cleavage on the yolk hemisphere now passes through the lower pole ; the second cleavage, passing downward, has made a shal- low groove extending half-way between the rim of the germ area and the lower pole of the egg. It is the great amount of yolk in the lower hemisphere that retards the cleavage of the blastomeres. In Fig. 252 the entire germ area has become subdivided into a mass of small cells, while the large, irregular blastomeres of the yolk hemisphere are separated only by superficial furrows. This stage, the blastula, is seen in section in Fig. 253: the yolk, unsegmented, occupies the lower hemisphere ; the germ area contains a segmentation cavity, SC, with a roofing of small cells, and a floor of irregular cells half engulfed in a deep, underlying zone transitional between germ and yolk. An early gastrula is seen in Fig. 254 : the more rapid multiplication of the cells of the germ region has given rise to a down-reaching cap of cells, whose boundary is here sharply marked off from the large and imperfect yolk cells of the lower hemisphere. At BP, the rim of the cell cap, or blastoderm, is sharply distinct from the yolk ; it is the dorsal lip of the blastopore ; the remaining portion of the rim is, generally speaking, the remainder of the rim of the blastopore ; more accurately it is the circumcres- cence margin of Hertwig. The late gastrula of Fig. 255 shows the greatly increased extent of the blastoderm : its margin is continually reducing the size of the blastopore, BP '; on its dorsal lip at HE, the outline of the embryo is appearing. A sagittal section of this stage (Fig. 256) shows at BP the dorsal, and at VL the ventral, lip of the blastopore ; at YP the yolk material appears at the egg's DEVELOPMENT OF GANOID 205 surface as a plug-like mass ; at SC is the segmentation cavity. The dorsal lip of the blastopore is seen to be far longer than the ventral lip ; its rim is the more inflected, at KV occurring a recessus which the writer compares to the Kupffer's vesicle of Teleost development; the cavity, C, coelenteron, between the wall of the blastopore and the yolk mass is in this region the largest. The germ layers in this stage, EC, MES, EN, are seen to be confluent at the blastopore's rim ; at the termina- tion of the coelenteron, entoderm and mesoderm are merged ; the ectoderm forms the roof of the segmenta- tion cavity. The form of the embryo next becomes more definitely established. In Fig. 257 the blastopore, much reduced in size, is seen at BP ; its thickened rim is whitish in colour; the darkened area, whose boundary is LC, is the ccelenteron, seen faintly through the translucent margin of the blastopore ; the embryo is the opaque area of the blastopore's dorsal lip, terminating anteriorly in the dilated tract, H, the head region. In a sagittal section of a slightly later stage (Fig. 258), the relations of germ layers, EC, MES, EN, coelenteron, C, and yolk mass, Y, may be compared with those of the section (Fig. 256), wherein the region YP corresponds to that of NC. A thin ectoderm will now be seen to have enclosed the entire egg ; the segmentation cavity has disappeared ; the rim of the blastopore, becoming continually constricted, causes the yolk material to recede from the surface, and leaves the blastopore disappearing, as the blunt diver- ticulum of NC. The neurenteric canal, NC, is the last communication between the surface of the egg and the coelenteron ; this has become established before the blas- topore closes in the stage of Fig. 257 at its dorsal lip; 206 DEVELOPMENT OF FISHES the medullary furrow of the embryo has here been the deepest, and has been bridged over by a coalescence of its margins. At the anterior end of the embryo the inner, EN, and middle, MES, germ layers become greatly thinned, in the region where the heart is shortly to arise. The next stage of development is represented in Figs. 259, 260, showing front and hinder regions of the same embryo. The curiously flattened mode of growth char- acteristic of the sturgeon is here very apparent ; the embryo has surrounded over three-fourths of the egg's circumference, yet has not risen above its surface curva- ture ; the head region is especially flattened ; mouth, M, heart, //, gill slits, GS, brain, and optic vesicles are broadly spread out : the fourth ventricle at MC, the pronephros at PN, the primitive segments at PS. In the tail region the medullary folds appear at M, the pronephric duct at PN, the neurenteric canal at NC. A favourable section through the hinder body region of an early embryo is shown in Fig. 261 ; it illustrates the mode of origin of the following structures : the notochord as an axial thickening of entoderm, EN, immediately under MC '; the medullary canal, as an infolding of (an under, or formative layer of) the ectoderm, its sides, folding over dorsally, coming to fuse in the median line; the mesoderm, MES, as in sharks, arising (partly) from the entoderm on either side of the notochord. The later stage, shown in Figs. 262 and 263, may be con- trasted with Figs. 259 and 260 ; the head region, though still greatly flattened out, is now rising above the surface ; the trunk region is becoming prominent ; the tail is bud- ding out, and separating from the egg surface ; sense organs are well outlined, and pectoral fins, F, elasmobran- DEVELOPMENT OF TELEOST 2O/ chian in character, are appearing. An embryo shortly before hatching is next figured (Fig. 264) ; the head has now entirely lost its flattened character; the mouth in- vagination occurs at M '; the tail, much elongated, is compressed laterally, and already presents the dermal embryonic fin ; the yolk sac is attached along the an- terior body region, in a position more nearly that of the shark than of the lung-fish. Of the two Ganoids, sturgeon and gar-pike, the latter, as the writer has pointed out, * has the more shark-like developmental features. Its segmentation is incomplete, since the yolk pole of the egg is at no time traversed even by superficial furrows. The blastoderm, or cell cap, is early apparent, and is clearly marked off by a furrow from the irregular marginal blastomeres (Fig. 265). It resem- bles closely the segmented germ disc of an Elasmobranch, and the irregular marginal blastomeres may be compared to merocytes. The section of a late blastula of Fig. 266 does not differ widely from that of the shark of Fig. 221 ; a segmentation cavity is present, whose floor is smooth, and contains a well-marked zone of merocytes, M\ the smaller quantity and firmer consistency, perhaps, of the yolk do not, on the other hand, permit the blastula to occupy the sunken position of that of the shark. In the gastrula of the gar, further, a well-marked notch appears at the dorsal lip (as in this stage, Fig. 223, of the shark), representing the primitive blastopore. And, finally, the form of the embryo rises boldly from the surface, and early presents the well-marked head and tail eminences, HE and T, of Fig. 268, comparable with Figs. 225 and 227. * Am. y. Morph., Vol. XI, No. I. FIG. 269 270 272 SC Figs. 269-283. — Development of Teleost, Serramis atrarius. (After H. V. WILSON.) Fig. 276 X 25. 269. Egg immediately prior to segmentation, showing position of germ disc and of oil globule. 270. Germ disc after first cleavage. 271. Germ disc after third cleavage. 272. Vertical section of blastula. 273. Vertical section of blastula, showing origin of periblast. 274. View of marginal cells of blastula of similar stage. 275. Growth of blastoderm around yolk mass. 276. A slightly later stage, showing growth of embryo. 277. Continued growth of embryo and reduction in size of the blastopore. 278. Sagittal section of tail region of embryo of last figure. 279, 280, 281. Cross-sections of embryos, ^Imwing successive stages in the development of notochord, gut, neuron, mesoblast. 282. Cross-section of young embryo, showing the mode of formation of gill slit. 283. Embryo shortly before hatching. A. Anus. AU. Auditory vesicle. BP. Dorsal lip of blastopore. CH. Notochord. }-:C. Ectoderm. EN. Entoderm. G. Gut. GD. Germ disc. G R. Germ ring. GS. Gill slit. H. Heart. HP. Head process. AT. Kupffer's vesicle. M. Spinal nervous system. MES. Mesoblast. MP. Marginal periblast cells. OG. Oil globule. OL. Ol- factory pit. OP. Optic capsule. P. Periblast. PS. Primitive segments. SC. Segmen- tation'cavity. SCH. Subnotochordal rod. TM. Tail mass. Y. Yolk. 208 DEVELOPMENT OF TELEOST V. The Development of Teleost 209 The mode of development of bony fishes differs in many and apparently important regards from that of their nearest kindred, the Ganoids. In their eggs a large amount of yolk is present, and its relations to the embryo have become widely specialized. As a rule, the egg of a Teleost is small, perfectly spheri- cal, and enclosed in delicate but greatly distended mem- branes (Fig. 269). The germ disc, GD, is especially small, appearing on the surface as an almost transparent fleck ; it may occupy the same position as in the other fishes, or, as in the figure, it may occur at the lowermost pole. Among the fishes whose eggs float at the surface during development, as of many pelagic Teleosts, e.g. the Sea-bass, Serranus atrarius,--to which all the accom- panying figures refer, --the yolk is lighter in specific gravity than the germ ; it is of fluid-like consistency, almost transparent. In the yolk at the upper pole of the egg an oil globule, OG, usually occurs ; this serves to lighten the gravity of the entire egg, and from its position must aid materially in keeping this pole of the egg uppermost. The early segmentation of the germ is seen in Figs. 270, 271. In the former, the first cleavage plane is estab- lished, and the nuclear divisions have taken place for the second ; in the latter, the third cleavage has been com- pleted. As in other fishes these cleavages are vertical, the third parallel to the first. A segmentation cavity, SC, occurs as a central space between the blastomeres, as it does in the sturgeon and gar-pike. Stages of late segmentation are seen in section in Figs. 272, 273. In both the segmentation cavity, SC, is greatly 2io DEVELOPMENT OF FISHES flattened, but extends to the marginal cells of the germ disc ; in Fig. 272 its roof consists of two tiers of blasto- meres, its floor a thin film of the unsegmented substance of the germ ; the marginal blastomeres are continuous • with both roof and floor of the cavity, and are produced into a thin film which passes downward, around the sides of the yolk. In Fig. 273 the segmentation cavity is still further flattened ; its roof is now a dome-shaped mass of blastomeres ; the marginal cells have multiplied, and their nuclei are seen in the layer of the germ, P, below the plane of the segmentation cavity. These are seen at MP in the surface view of the marginal cells of this stage (Fig. 274) ; they are separated by cell walls only at the sides ; below they are continuous in the superficial down- reaching layer of the germ. The marginal cells, MP, shortly lose all traces of having been separate ; their nuclei, by continued division, spread into the layer of germ flooring the segmentation cavity, and into the delicate film of germ which now surrounds the entire yolk. Thus is formed the periblast of teleostean development, which from this point onward is to separate the embryo from the yolk; it is clearly the specialized inner part of the germ, which, becoming fluid-like, loses its cell walls, although retaining and multiplying ,its nuclei. It would accordingly corre- spond to that portion of the germ of the sturgeon in Fig. 253 which lies below the plane of the segmentation cavity, and which extends downward at the sides of the yolk ; in this case, however, the surface outlines of the cells have not been lost. It will be seen from later figures (Figs. 278-282) that the periblast, P, comes into intimate rela- tions with the growing embryo ; it lies directly against it, and appears to receive cell increments from it at various regions ; on the other hand, the nuclei of the periblast, DEVELOPMENT OF TELE OS T 2II from their intimate relations with the yolk, are supposed to subserve some function in its assimilation. Aside from the question of periblast, the growth of the blastoderm appears not unlike that of the sturgeon. From the blastula stage of Fig. 273 to that of the early gastrula (Fig. 275), the changes have been but slight ; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent at SC. The rim of the blastoderm has become thickened, as the 'germ ring;' and immediately in front of BP, the dorsal lip of the blastopore, its thickening, as in Fig. 255, marks the appearance of the embryo. In Fig. 276 the germ ring, GR, continues to grow downward, and shows more prominently the outline of the embryo ; this now terminates at HP, the head region ; while on either side of this point spreads out tail-ward on either side the indefi- nite layer of outgrowing mesoderm, MES. In the stage of Fig. 277 the closure of the blastopore, BP, is rapidly becoming completed ; in front of it stretches the widened and elongated form of the embryo. A sagittal section through a late stage of the blastopore appears in Fig. 278 ; with it may be compared the corresponding region of the sturgeon of Fig. 256; the yolk plug, YP, of the latter is now replaced by periblast, P, the dorsal lip at BP, by TM, the tail mass, or more accurately the dorsal section of the germ rim ; the coelenteron under the dorsal lip has here disappeared, on account of the close approxima- tion of the embryo to the periblast ; its last remnant, the Kupffer's vesicle, KV, is shortly to disappear. At TM, the germ layers become confluent as at BP in Fig. 256, but, unlike the sturgeon, the flattening of the dorsal germ ring, TM, does not permit the formation of a neu- renteric canal. 212 DEVELOPMENT OF FISHES The process of the development of the germ layers in Teleosts appears an abbreviated one, although in many of its details it is but imperfectly known. In the develop- ment of the medullary groove, as an example, the follow- ing peculiarities exist : the medullary region at HP (Fig. 276) is but an insunken mass of cells without a trace of the groove-like surface indentation of Fig. 261 or 229. Its condition is figured at M in Fig. 282. It is only later, when becoming separate from the ectoderm, EC, that it acquires its rounded character (Fig. 279), M; its cellular elements then group themselves symmetrically with refer- ence to a sagittal plane, where later by their disassocia- tion (?) the canal of the spinal cord is formed (Fig. 280), M. The growth of the entoderm is another instance of special- ized development. In the section of the embryo of Fig. 279, the entoderm exists in the axial region, its thickness tapering away abruptly on either side ; its lower surface is closely apposed to the periblast ; its dorsal thickening will shortly become separate as the notochord. In a fol- lowing stage of development (Fig. 280), the entoderm is seen to arch upward in the median line as a preliminary stage in the formation of the cavity of the gut. Later, by the approximation of the entoderm cells in the median ventral line, the condition of Fig. 281 is reached, where the completed gut cavity exists at G. The formation of the mesoderm in Teleosts is not defi- nitely understood. It is usually said to arise as a process of ' delamination,' i.e. detaching itself in a mass from the entoderm. Its origin is, however, looked upon generally as of a specialized and secondary character. The mode of formation of the gill slit of a Teleost does not differ from that in other groups ; an evagination of the entoderm, GS (Fig. 282), coming in contact with an LARVAL FISHES 213 invaginated tract of ectoderm, EC, fuses, and at this point an opening is later established. In Fig. 283 has been figured a late embryo. This may be compared with that of the sturgeon of Fig. 264. The Teleost, though of rounded form, is the more deeply im- planted in the yolk sac ; it is transparent, allowing noto- chord, primitive segments, heart, and sense organs to be readily distinguished ; at about this stage both anus, A, and mouth, M, are making their appearance. D. THE LARVAL DEVELOPMENT OF FISHES When the young fish has freed itself from its egg mem- branes, it gives but little suggestion of its adult form. It enters upon a larval existence, which continues until matu- rity. The period of metamorphosis varies widely in the different groups of fishes — from a few weeks' to longer than a year's duration ; and the extent of the changes that the larva undergoes are often surprisingly broad, invest- ing every organ and tissue of the body, — the immature fish passing through a series of form stages which differ one from the other in a way strongly contrasting with the mode of growth of amniotes ; since the chick, reptile, or mammal emerges from its embryonic membranes in nearly its adult form. The fish may, in general, be said to begin its existence as a larva as soon as it emerges from its egg membranes. In some instances, however, it is difficult to decide at what point the larval stage is actually initiated : thus in sharks, the excessive amount of yolk material which has been pro- vided for the growth of the larva renders unnecessary the emerging from the egg at an early stage ; and the larval period is accordingly to be traced back to stages that are still enclosed in the egg membranes. In all cases the 214 DEVELOPMENT OF FISHES larval life may be said to begin when the following con- ditions have been fulfilled : the outward form of the larva must be well defined, separating it from the mass of yolk, its motions must be active, it must possess a continuous vertical fin fold pass-ing dorsally from the head region to the body terminal, and thence ventrally as far as the yolk region ; and the following structures, characteristic in outward appearance, must also be established, the sense organs, — eye, ear and nose, — mouth and anus, and one or more gill clefts. Among the different groups of fishes the larval changes are brought about in widely different ways. These larval peculiarities appear at first of far-reaching significance, but may ultimately be attributed, the writer believes, to changed environmental conditions, wherein one proc- ess may be lengthened, another shortened. So too the changes from one stage to another may occur with sur- prising abruptness. As a rule, it may be said the larval stage is of longest duration in (I) the Cyclostomes, and thence diminished in length in (II) Sharks, (III) Lung- fishes, (IV) Ganoids, and (V) Teleosts ; in the last-named group, a very much curtailed (i.e. precocious) larval life many often occur. I. Larval Cyclostomes The Cyclostome larva is represented in a stage as early as that of Fig. 212 : its form is here retort-shaped ; the yolk material is concentrated in the ventral region immediately in front of the blastopore (the anus ?), but is distributed in addition in the cells of other body regions. In the section of a slightly older larva (Fig. 215), in which the mouth is all but established, the form outline has become regular, the bulk of the yolk, Y, restricted to the LARVAL SHARKS 215 cavity of the intestine, the only instance of this condition known among fishes (Ceratodus ?), and, with but a single exception (Ichthyophis),* among all other vertebrates. The larval lamprey is by this time a quarter of an inch long, yellowish white in colour ; its movements are slug- gish, rarely more than to cause it to wriggle worm-like from the bottom. A few weeks later it has acquired its brownish grey colour, its fin fold is well marked, and its habit is active ; it now feeds on muddy ooze rich in organic matter. It by this time possesses the essential characters of the well-grown larva, long looked upon as a distinct genus, Ammoccetes. In its larval stage the lamprey appears to live a number of years ; in Petromyzon planeri the adult stage is said to be sometimes deferred until the autumn of the fourth or fifth year. The trans- formation is then a surprisingly sudden one ; the head attains its enlarged size, the mouth its ring-like and suc- torial character, losing its more anterior position, and its lip-like flaps (cf. Fig. 72, C, D} ; teeth are developed in place of the numerous mouth papillae ; gills, formerly simpler in character, opening directly from neck surface to gullet, now enter the branchial chamber, a ventral diverticulum of the gullet ; eyes become prominent, complete their development, and attain the head surface ; unpaired fin, formerly of great extent, is now reduced to its adult position and proportions. II. Larval Sharks The larval history of Sharks has been summarized in Figs. 284-289 : the younger of these stages (Figs. 284, 285, 286) have not as yet escaped from their egg mem- branes. The hatching, in fact, of the young shark is * The writer has not confirmed Salensky's observation upon the sturgeon. FIG. 284. 287 Figs. 284-289. — Larval sharks. (Figs. 284-287 after BALFOUR.) 284. Pristiurus (embryo, X 5) with yolk sac (X 2). 285, 286. Larvas of Scyllium. X 4. 287. Ventral view of head of larval Scyllium, slightly younger than that of last figure. X 8. 288. Larva of Acanthias. x 4. 289. Late larva of Acanthias. X §. G. Gills. GS. Gill slits. PP. Pectoral fin. SP. Spiracle. Y. Yolk sac. VS. Stalk of yolk sac. 216 LARVAL SHARKS 217 an exceedingly slow one ; Pristiurus emerges from the egg in about nine months, Scyllium in about seven. And in consequence of the large amount of yolk stored in the yolk sac, the young shark, as in Fig. 289, has fully acquired its adult outward characters by the time the yolk is exhausted and its sac absorbed. In Fig. 284 is figured a stage in the development of Pristiurus which may be regarded as either embryonic or larval ; the form of the larva is well established ; gill clefts, muscle-plates, mouth, and sense organs are present ; but, on the other hand, unpaired fin and anus are lacking. There is shown the abrupt constriction, characteristic of Elasmobranchs, which separates the animal from the yolk sac, — a construction which in later stages becomes narrow and tubular. The relatively larger size of the yolk sac in later stages is, of course, the result of the bulkier elabo- ration of the yolk material. The youngest stage (Fig. 284) shows prominently the great enlargement of the anterior end of the embryo, a marked cephalic flexure, large optic capsule, and irregular gill slits of graded sizes ; a tubular tail end, bulbous at the terminal, where the neurenteric canal occurs ; as yet the nasal pits are in close proximity to the mouth. In the next stage (Fig. 285), the elongated trunk has its unpaired fin, the neurenteric canal disappearing ; the beginnings of the pectoral fins are noticeable ; gill clefts are of more uniform size ; and the anal region is indicated. In the stage of Fig. 286, further advances are seen in the con- stricting off of the unpaired fins, the appearance of the ventral and the continued growth of the pectoral fins ; in the reduced foremost gill slit (spiracle) ; in the jaw region, and, in fact, in the entire shaping of the head ; in the appearance of the lateral line. In the ventral head 2iS LARVAL DEVELOPMENT region (Fig. 287), is to be noted the prominence of the mouth cavity, and the enlarged gill arches, showing by this time the outbudding branchial filaments. In the stage of Fig. 288, the larva begins to appear shark-like ; the fins are longer and more noticeable, the anus has appeared, and the branchial filaments by continued growth protrude at all gill openings. The external gills thus acquired are seen in a later stage (Fig. 289) to have disappeared ; they have aided, however, as Beard, Turner, and others have shown, in absorbing nutriment, and must be looked upon as an especial organ of the larval life of the animal. Fig. 289 illustrates a final larval stage : in it there appear all of the structures of the adult outward form, e.g. shagreen, fin spines, nictitating membrane, anterior and posterior nasal openings. This larva has been esti- mated to be about a year older than that of Fig. 284. III. Larval Lung-fisJi The larval history of the lung-fish, Ceratodus, as recently described by Semon, seems to offer characters of excep- tional interest, uniting features of Ganoids with those of Cyclostomes and Amphibians. The newly hatched Ceratodus (Fig. 290) does not strikingly resemble the early larva of shark (Fig. 284). No yolk sac occurs, and the distribution of the yolk material in the ventral and especially the hinder ventral region is suggestive rather of lamprey or amphibian ; it is, in fact, as though the quantum of yolk material had been so reduced that the body form had not been con- stricted off from it. The caudal tip in this stage appears, however, to resemble that of the shark, and as far as can be inferred from surface views a neurenteric canal persists. Like the shark there then exists no unpaired fin ; the FIG. 290 PS M 291 292 MC H PF ; iyB 293 Figs. 290-295. — Larval lung-fishes, Ceratodiis. (After SEMON.) X 6. 290. Embryo at about the time of hatching. 291. Young larva. 292. Larva of two weeks. 293. Larva of four weeks, ventral side. 294. Larva of six weeks. 295. Larva of ten weeks. A. Anus. A U. Auditory vesicle. EG. External gills. GS. Gill slits. //.Heart. M. Central nervous system. MC. Mucous canals. O. Opercular flap. OL. Olfac- tory organ. 'PF. Pectoral fin. PN. Pronephros. PS. Primitive segments. 5. Mouth pit, stomodaeum. 219 22O DEVELOPMENT OF FISHES gill slits, five (?), GS, are well separated, and there is an abrupt cephalic flexure. In this stage pronephros and primitive segments, PS, are well marked, and are out- wardly similar to those structures in Ganoid ; the mouth, S, is on the point of forming its connection with the digestive cavity ; the anus is the persistent blastopore ; the heart, well established, takes a position, as in Cyclo- stomes, immediately in front of the yolk material. In a later stage the unpaired fin has become perfectly established, the tail increasing in length ; the gill slits have now been almost entirely concealed by a surrounding dermal outgrowth, the embryonic operculum ; a trace of the pectoral fin, PF, appears ; the lateral line is seen pro- ceeding down the side of the body ; near the anal region the intestine* becomes narrower and the beginnings of the spiral valve appear. In a larva of two weeks (Fig. 292), a number of developmental advances are noticed : the fish has become opaque, the primitive segments are no longer seen; the size of the yolk mass is reduced; the anal fin fold appears ; sensory canals are prominent in the head region ; lateral line is completely established ; the rectum becomes narrowed ; and the cycloidal body scales are already outlined. Gill filaments may still be seen beyond the rim of the outgrowing operculum. In the ventral view of a some- what later larva (Fig. 293), the following structures are to be noted : the pectoral fins which have now suddenly budded out,f reminding one in their late appearance of the mode of * The yolk appears to be contained in the digestive cavity as in Ichthy- ophis and lamprey. t The abbreviated mode of development of the fins is most interesting ; from the earliest stage they assume outwardly the archiptervgial form ; the re- tarded development of the limbs seems curiously amphibian-like ; the pec- torals do not properly appear until about the third week, the ventrals not until after the tenth. LARVAL GANOIDS 221 origin of the anterior extremity of urodele ; the greatly en- larged size of the opercular flap ; external gills, still promi- nent ; the internal nares, OL, becoming constricted off into the mouth cavity by the dermal fold of the anterior lip (as in some sharks) ; and finally (as in Protopterus and some batrachian larvae) the one-sided position of the anus. The larva of six weeks (Fig. 294) suggests the outline of the mature fish ; head and sides show the various open- ings of the tubules of the insunken sensory canals ; and the ' archipterygium ' of the pectoral fin is well defined. The oldest larva figured (Fig. 295) is ten weeks old ; its operculum and pectoral fin show an increased size ; the tubular mucous openings, becoming finely subdivided, are no longer noticeable ; and although the basal supports of the remaining fins are coming to be established, there is as yet little more than a trace of the ventrals. IV. Larva! Ganoids The larval forms of a Ganoid, Acipenser (Figs. 296- 302), resemble far more closely those of the shark than of the lung-fish. When newly hatched, the young sturgeon (Figs. 296, 297) is attached to the well-rounded yolk sac situated in the throat region, in exactly the position one would expect the yolk stalk to be situated if the yolk mass were larger ; it resembles the shark larva of Fig. 295 in its unpaired fin, in gill slits, in olfactory, OL, optic, OP, and auditory, A U, organs, and in the fact that it possesses even at this stage a trace of the neurenteric canal ; on the other hand, it suggests the Ceratodus larva of Fig. 291 in its stout trunk region, prominent muscle segments, pro- nephros, PN, and anus, A ; at the foremost corner of the yolk sac are mouth pit (stomodaeum, S} and heart. A larva of the second day resembles in many features the 298 30O B SP H 301 B SP Figs. 296-302. — Larval sturgeons. (All but Fig. 302 after KUPFFER.) Fig. 299, X 18 ; 296-300, x 10 ; 301, X8; 302, X 3. (Enlargement approximate.) 296, 297. Larvae shortly after hatching. 298. Larva two days old. 299. Mouth region of larva of third day. 300. Larva of fourth day. 301. Larva of twenty-eight days. 302. Sturgeon of twelve months. A. Anus. AU. Auditory vesicle. B. Barbel. GS. Gill slit. H. Heart. OL. Ol- factory pit. OP. Optic vesicle. PP. Pectoral fin. PN. Pronephros. 5. Mouth pit. SP. Spiracle. 222 LARVAL TELE OS TS 223 shark larva of Fig. 286: dorsal, caudal, and anal regions are outlined in the unpaired fin ; a pectoral fin of a fin-fold character, PF, has appeared ; the spiracle, SP, is becom- ing established. The mouth region is more clearly indi- cated in this stage, S, but may better be seen in ventral view in a slightly later larva ; here (Fig. 299) the posterior lip is constricted off from the yolk region, and the anterior lip is budding off near the median line a pair of the tactile barbels ; the dermal fold (operculum) enclosing the gills is in a condition very similar to that of Ceratodus in Fig. 293. A larva of the fourth day (Fig. 300) shows well-marked advances : the snout is elongated ; the opercle is enclosing the gills, which are now seen to protrude as external branchial ; the pectoral fin elongates and is tend- ing to protrude its fin axis ; body segments and heart are encroaching into the region of the now elongate yolk sac ; the lateral line has been formed. In a larva of four weeks (Fig. 301), the essential outlines of the sturgeon may be recognized, although the head appears of strikingly larger proportions : barbels, nares, mouth, operculum, and spiracle are as in the adult ; fins, of the mature outlines, are want- ing in all save basal supports ; yolk material has long since been exhausted. A very late larva (Fig. 302), supposed to be twelve months old, differs outwardly from the sexually mature form in but its colouring and dermal plates : those of the regular rows are of great size, conspicuous in their abrupt spines and well-roughened borders ; and those of the remaining trunk integument are remarkably prominent ; the tail of the larva shows clearly its palaeoniscoid character. V. Larval Te leasts The metamorphoses of the newly hatched Teleost must finally be reviewed ; they are certainly the most FIG. 303 Figs. 303-309. — Larvag of Teleost, Ctenolabrus. (After A. AGASSIZ.) Fig. 309 X about 7, other figures X about 14. 303. Larva shortly after hatching. 304, 305. Larvae of first few days. 306, 307. Larva of one week. 308. Larva of two weeks (?). 309. Final larval stage, four (?) weeks. A. Anus. A U. Auditory vesicle. CH. Notochord. GR. Gill protecting der- mal rays. H. Heart. M. Central nervous system. OL. Olfactory capsule. OP. Optic vesicle. PF. Pectoral fin. .S. Stomodaeum. 224 LARVAL TELE OS TS 22$ varied and striking of all larval fishes, and, singularly enough, appear to be crowded into the briefest space of time ; the young fish, hatched often as early as on the fourth day, is then of the most immature character ; it is transparent, delicate, inactive, easily injured; within a month, however, it may have assumed almost every detail of its mature form. A form hatching three mille- metres in length may acquire the adult form before it becomes much longer than a centimetre. The larval life of the common Sea-bream, or Gunner, Ctenolabnts ccerulcns, has been admirably figured by A. Agassiz. The newly hatched fish (Fig. 303) has the yolk sac appended at the throat, as a large, transparent, if slightly tinted, globule ; save for its great delicacy and transparency, it may generally be compared to the corre- sponding larva of Acipenser (Fig. 296). By the third day (Fig. 304), the yolk sac has become greatly reduced, the trunk elongated, the fin fold less conspicuous ; primitive segments have appeared ; the pectoral fin has arisen, but is not of the elasmobranch form of the similar stage (Fig. 298) of sturgeon ; it is long, thin, transparent, and its rapid growth indicates its metamorphosed character. The mouth, S, is in this stage on the point of formation. In a slightly older larva (Fig. 305), the yolk has almost dis- appeared ; its gill slits, GS, and mouth have now been formed, and with the latter the nasal apertures. In a fol- lowing stage (Figs. 306, 307), a well-marked opercular fold makes its appearance ; pectoral fins acquire their com- pleted outline and the fin fold undergoes changes : ante- riorly it acquires supporting actinotrichia, posteriorly the dermal supports of the caudal fin appear and at their bases the coalesced radio-basals ; a ganoidean heterocercy is here apparent, its distal tip the membranous opisthure, O. Q 226 LARVAL TELEOSTS The later larva (Fig. 308) is characterized by the appear- ance of abundant pigment masses (not shown in the figure) in all regions of the trunk ; branchiostegal rays, GR, and traces of pelvic fins are noted ; the caudal fin has become separated from the dorsal and anal elements. And finally, in the stage of Fig. 309, the fish, although still of very small size, has acquired almost perfectly its mature features ; the outward differences are only those of pigmentation and fin proportions. LIST OF DERIVATIONS OF PROPER NAMES Acanthodes, aKuv&uSr/s, provided with spines. Acanthopterygii, aKavOa, spine, vrepv^, fin(ned). Acipenser, d/«7r>7aAr/, head. Lamna, Aa/xva, classic name for a shark. Lepidosiren, A£TTI'S, scale (d), siren, salamander. Lepidosteus, ACTUS, scale, ovriov, bone. Leptolepis, ACTTTOS, smooth or delicate, ACTTIS, scale(d). Lophobranchii, Ados, tuft, ^Spay^iov, gill(ed). Marsipobranchii, /xapowiov, pouch, ^pay^tu, gills. Megalurus, p-eyas, large, oupa, tail (eel). Microdon, p.ikpos, small, dSous, tooth(ed). Mormyrus, classic name of a (sea) fish ( — from ju,op/u,vpa>, I murmur). Myliobatis, /ruAt'us, pavement (toothed), part's, skate. Mylostoma, p.uAos, mill(like), oro/xa, mouth. Myriacanthus, /nvpias, ten thousand, aKavda, spine. Myxine, ^-u^tvos, slimy-fish. Onychodus, ovv£, claw; oSou?, tooth(ed). Ophidium, ot'8tov, a snake. Osteolepis, ocrreov, bone, AETTC'S, scale (d). Ostracoderm, ocrrpa/aov, shell, Sepp-a, skin. Palaeaspis, TraAouos, ancient, acrTrts, shield. Palasoniscus, TraAatos, ancient, OVI'O-KOS, a sea-fish. Palasospondylus, TraAatds, ancient, crTrdvSvAo?, vertebras. Parexus, ? Trape^w, have as one's own (referring to the peculiar nature of the fish?). Perca, classic name of fish. Petromyzon, TreVpos, stone, /u.v£aco, to suck. Phaneropleuron, <£avepds, well marked, TrAevpd, side (fins) or ribs(?). Pisces, fishes. Plagiostomi, TrAayios, transverse, 0-To/x.a, mouth. Plectognathi, irAeKTos, twisted, yva^o?, jaw. Pleuracanthus, TrAevpa, side, aKavOa, spine. Pleuropterygii, TrAeupa, side, Trrepv^ fin(ned). Pogonias, Trwyoji/ta?, bearded. Polyodon, TroAv's, many, oSwi/, tooth(ed). Polypterus, TroAv's, many, TrrepoV, fin(ned). Prionotus, vrptW, saw, VWTOS, back. Pristiophorus, Trptcrris, a saw, e^ope'w, to carry. Pristis, Trpurns, a saw-fish. Protopterus, 7rpu»Tos, ancient, TrrcpoV, fin(ned). 230 FISHES, LIVING AND FOSSIL Psammodus, ^ajUfios, sand, oSous, tooth (ed). Psephurus, i//^os, a little stone, oupa, tail. Pseudopleuronectes, i/'euSos, false, TrAeupoV, side, VT/KT^S, swimmer. Pterichthys, 7rre'pu£, fin or wing, LX@V<;, fish. Raja, classic name of skate. Rhabdolepis, pa/3Sos, nail, ACTTIS, scale(d). Rhina, pivrj, a rasp. Rhinobatus, pu/a, Rhina, part's, skate. Rhynchodus, puyx°s> snout, oSou's, tooth (ed). Scaphirhynchus, Nat. Nos. 99. 100, 102. '76 HUXLEY, Ceratodus, P. Zool. S. 24-58. '64 KLEIN, Protop. Wiirt. n't'rwiss. Jahresber. 134-144. '78 Mi ALL, L., Cerat. and Protop. Palaeont. S. xxxii, 1-32. '88 PARKER, W. N., Ber. d. Naturforsch. Gesell. Friburg, VB. iv, H. 3, Nat. xxxix, 9-21, and Tr. Cardiff Nat. S. xx. '91 Protop. P. Roy. S. xlix, 549-554. '92 Protop. (Large memoir), Tr. R. Irish Acad. xxx, 115-227. '66 PETERS, Monatsber. Ak. Wiss. Bed. 12-13. SKELETON. — '93 KLAATSCH, H., Wirbel, VH. Anat. Gesell, 130- 132. '91 TELLER, F., Skull of Ceratodus, AH. Geol. Reichanst. xv, H. 3. MUSCLES. — '72 HUMPHREY, G. M., Ceratodus and Protop. J. Anat. and Phys. vi. FINS AND GIRDLES. — '86 ALBRECHT, P., Protop. Fin forked, SB. Ak. Bed. 545-546. '91 BOULENGER, Protop. Renewed pectoral. '83 DAVIDOFF, M., Cerat. Pelvic fin. '84 GILL, T., Shoulder girdle, Ann. N. H. xi, 173-178. '83 HA SWELL, W. A., Cerat. Paired fins, P. Linn. S. N. S. Wales, vii, 2-11. '91 HOPLEY, C., Protop. Renewed pectoral, Am. Nat. xxv, 487. '87 HOWES, G. B., Cerat. Paired fins compared with sharks', P. Zool. S. 3. '94 LANKESTER, E. Ray, Lepid. Villous processes of hind limbs, Nat. Apr. 12. '86 SCHNEIDER, A., Zool. Anz. ix, 521-524, and ('87) Zool. Beitr. ii, 97-105. '71 TRAQUAIR, R. H., Protop. Tail restored, Br. Ass. R. '90 VANHOFFEN, Cerat. VH. Gesell. D. Naturf. ii, 134. 246 FISHES, LIVING AND FOSSIL INTEGUMENT AND TEETH. — '87 BOCKLEN, H., Cerat. Denti- tion, JH. Ver. WUrt. xliii, 76-81. '60-'61 KOLLIKER, A., Protop. Histol. of Skin, Z. Naturwiss. Wiirzb. i. '65 PAULSON, M., Protop. Histol. of epidermis, B. Acad. Sci. St. Petersb. viii, 141-145. '92 ROSE, C., Zahnbau u. Zahnwechsel, Anat. Anz. vii, 821-839. '89 WALTHER, G., Prot. Skin, Z. f. Phys. Chem. xiii, H. 5. '80 WiEDERSHEiivr, R., Scales, A. mikr. Anat. xviii. VISCERA, VESSELS, GLANDS. — '80 BOAS, E. V. (Heart and arteries), Morph. JB. vii, 321-354. '78 FURBRINGER (Excretory), Morph. JB. iv, 60. '76 HUXLEY, Anterior nares, P. Zool. S. 180. '45 HYRTL, J., Lepid. AH. d. bohm Gesell. Prag. '78 LANKESTER, E. R., Heart, P. Zool. S. 634, and ('79) Tr. Zool. S. x, 493-506. '89 PARKER, W. N., Veins (L. cardinal), P. Zool. S. 145-151. '94 SPENCER, W. B., Cerat. Vessels (complete memoir), Macleay Mem. Vol. Linn. S. N. S. Wales, 2-32. NERVOUS SYSTEM, END ORGANS. — '82 BEAUREGARD, H. (Cranial), J. de TAnat. Phys. xvii, 230-242. '91 BURCKHARDT, R., Zirbel. Anat. Anz. vi, 348-349. '92 Cent. nerv. sys. Berlin, 64 pp. Also Zool. Gesell. ii, 92-95, and SB. Nat. Fr. Berl. 23-25. '94 (Zwischenhirndach). Anat. Anz. 152. '86 FUL- LIQUET, G. (Brain), A. Sci. Naturelles, xv, 94-96, and Rec. Zool. Suisse, iii, 1-130. '94 PINKUS, F. (Undescribed nerve), Anat. Anz. ix, 562-566, and (Cranial nerves of Protop.) Morph. Arb. (Schwalbe), 275-346. '89 SANDERS, A., Cent. nerv. sys. Cerat. Ann. N. H. iii, 157-188. '80 WIEDERSHEIM, Skel. and cent. nerv. sys. Jen. Z. xiv, and Morph. Stud. Heft i, Jena. '82 WIJHE, J. VV. van, Visceralskel. u. d. Nerven. Cerat. Nied. A. Zool. v, 207-320. '87 WILDER, B., Brain, Am. Nat. xxi, 544-548. EMBRYOLOGY.— '86' BEDDARD, F. E., Ovarian ovum, P. Zool. S. 272-292, and Zool. Anz. ix, 635-637. '84 CALDWELL, W. H., (Preliminary), J. and P. Roy. S. N. S. W. xviii, and ('87) in Phil. Trans, clxxviii. '93 HASSE, C., Wirbelsaule, Z. wiss. Zool. Iv, 533-542. '93 SEMON, R. (Habits and development — surface views of eggs and larvae), DS. d. Med. Nat. Gesell. Z. Jena, pp. 50. THE GANOIDS GENERAL (NATURAL HISTORY). — '70 DUMERIL, Aug. Tome ii, pp. 625, Roret, Paris. '35 HECKEL, J., Scaphirhynchus, SB. Akad. Wien. '71 LUTKEN (Classification), Transl. in Ann. N. H. 329-339. '85 ORR, H. (Phylogeny) Inaug. Dissert. Jena, 37 pp. '65-'66 SMITH, J. A., Calamoichthys, P. Roy. S. Edinb. v, 654- BIBLIOGRAPHY: GANOIDS 24? 659, and ('66) 457-479. '69 STEINDACHNER, F.. Polypterus, SB. Wien. Akad. 1.x. GENERAL ANATOMY. — '87 TWAXZOW, N., Scaphirhynchus, B. S. Mosc. 1-41. '54 LEYDIG (Histology of Polypterus), Z. wiss. Zool. v. '50 LITTANY, M., Acipenser, B. S. Mosc. xxiii, 389-445. '44 MULLER, J., Bau u Grenzen, A. f. Anat. and ('46) AH. d. Berl. Akad. d. Wiss. '92 POLLARD, H. B., Polypterus, Anat. and phylogeny, Morph. JB. V, 387-428, and preliminary in ('91) Anat. Anz. vi, 338-343. '48 WAGNER, A., de Spatulariarum Anat. Inaug. Diss. Berol. 175 WILDER, B., Notes on Am. Gan, I. Respir. of Lepid. and Amia. II. Tail formation of Lepid. III. Pect. fin formation of Lepid. IV. Brains of Amia, Lepid., Acip., and Polyod. P. Am. Ass. Adv. Sci. xxiv, 151-193. '76 Brains, Philadel. Acad. P. xxxviii, 51-53. '78 Amia and Lepid. rudimentary spiracle, P. Am. Ass. (unpub'd), and Am. Nat. xix, 192. And in the respiration of Amia, P. Am. Ass. 306- 313. '85 WRIGHT, R. R., Notes on anat. of fishes: A. Cutan- sense organs. B. Spiracular cleft of Amia and Lepid. C. And. organ of Hypophthalmus, Am. Nat. xix, 187-190 and 513. D. Hyomand. clefts and pseudobranchs of Amia and Lepid. and Amia, J. Anat. Phys. xix, 477-497. E. Amia's serrated append- ages, Sci. iv, 511. '87 ZOGRAFF, N., Monograph (Russians) on Sturgeon, Tr. S. Nat. Mosc. Hi, pp. 72. '87 Affinities of ganoids, Nat. xxxvii, 70. SKELETON.— '77 BRIDGE, T., Cranium Amia J. Anat. Phys. xi, 605-622. '78 Polyodon, Phil. Trans, clxix, 683-734. '89 Cranial anat. Polypterus, P. Birmingham, Phil. S. vi, 118-130. '83 CAFAUEK, F. (Prag.). '47 FRANQUE, H., Amia, Folio, Berolini. '78 GOETTE, A., Wirbelsaule, A. mikr. Anat. x, 442- 641. '93 HASSE, C., Wirbelsaule, Z. Wiss. Zool. 76-90. '60 KOLLIKER, Ende d. Wirbelsaule, Leip. '20 KUHL u. HASSELT Ost. of Sturgeon, Kuhl's Beitr. Zool. in Vergl. Anat. 2 Abth. 188-202. '51 MOLIN, R., Scheletro dell. Acipenser, SB. Acad. Wien, vii, 357-378. '82 PARKER, W. K., Skull (and develop.) of Acip. P. Roy. S. 142-145, of Lepidos. 443-491. '83 SAGEMEHL, M. (Skull of Amia), Morph. JB. ix, 177-227. '92 SCHMIDT, L. (Vertebrae of Amia), Z. wiss. Zool. liv, 748-764. '85 SHUFELDT, R. W., Amia, R. U. S. F. C. ('83) 747-834. '70 TRAQUAIR, R. H., Calamoichthys, J. Dub. Geol. Soc. June 8. '70 Skull of Polypterus, J. Anat. Phys. v, 166-183. 82 WIJHE, J. W. VAN, Visceralskelet (u. Nerven) — includes Ceratodus, — Nied. A. Zool. v, 207-320. 248 FISHES, LIVING AND FOSSIL MUSCLES. — '85 McMuRRiCH, J. P., Head of Amia, Stud. Biol. Lab. J. Hop. Univ. iii, 121-153. '82 SCHNEIDER, H., Augenmuskeln, Jen. Z. xv, 215-242. INTEGUMENT, TEETH. — '78 BARKAS»W., Teeth of Lepicl. Tr. Roy. S. N. S. Wales, xi, 203-207. '77 MACKINTOSH, H. W., Scale of Amia. (?). '80 PAWLOW, H., Teeth of Sturgeon, Arb. St. Petersb. Nat. Gesell. No. 9, 494-508. '59 REISSNER, Schup- pen v. Polyp, and Lepid. A. f. Anat. '87 ZOGRAFF, N., Za'hne d, Knorp. gan. Biol. Centralb. vii, 178-183 and 224. VISCERA. — '86 CATTANEO, G., Glandula gastriche nell' Acip. Rend. Inst. Lomb. xix, 676-682. '78 FURBRINGER, Excretory sys. Morph. JB. iv, 56-60. '72 HERTWIG, R., Lymph. Driisen d. Storherzens, A. mikr. Anat. ix, 62-79. HOEVEN, J. v. D. (Air-bladder of Lepid.) 4to. (?). '91 HOPKINS, G. S., Structure of stomach of Amia, P. Am. Micr. S. xxii, 165-169. '92 Diges. tracts of N. A. Gan. P. Am. Ass. xli, 197. '69 HYRTL, J., Blutgefasse d. aus. Kiemendeckel-Kieme v. Polyp. SB. Ak. Wien, Ix, 109-113. '86 MACALLUM, A. B., Diges. tract and pancreas of Acip., Amia, Lepid. J. Anat. Phys. xx, 604-636. '91 SEMON, R., Zusammen- hang d. Harn- und Geschlechtsorgane, Morph. JB. xvii, 623-635. '77 STOHR (Valves in conus — compares sharks'), Morph. JB. ii, 197-228. '90 VIRCHOW, H., Spritzlochkieme v. Acip. A. Anat. Phys. (Phys. Abt.) 586-588. '86 WILDER, Serrated appendages of Amia, P. Am. Ass. xxxiv, 313-315. FINS. — '94 GEGENBAUR, Flossenskelet d. Crossopterygier, Morph. JB. xxi, 119-160. '82 RAUTENFELD, E. V., Skel. hint. Glied- massen, Inaug. Diss. Dorpat, 47 pp. '77 THACHER, J., Ventral fins, Tr. Connec. Acad. iv, 233-242. '80 DAVIDOFF, M. V., Skel. d. hint. Gliedmassen, Morph. JB. vi, 126-128 and 433-468. '66 HUXLEY, Illus. of struc. of Crossopt. 4to, Lon. NERVOUS SYSTEM, END ORGANS. — '89 ALLIS, Lateral line of Amia, J. of Morph. '83 CATTIE, J. T., Epiphysis, Z. wiss. Zool. xxxix, 720-722, and A. cle Biol. iii, 101-196. '94 COL- LINGE, W. E., Sensory canals, of Polypterus, P. Birmingh. S. viii, 255-262; of Lepid. op. cit. 263-272 ; of Polyodon Q. J. M. S. xxxvi, 499-437. '83 DOGIEL, A., Retina, A. mikr. Anat. xxii, 419- 472, and '84 Naturf. Ges. Kasan, xi, 124 pp. '86 Geruchsorgan, Tr. Kasan. Univ. xvi, 82 pp. '88 Retina, Anat. Anz. iii, 133-143. '79 Gisow, A., Gehororgan, Bonn. '81 Gisow, A., Gehororgan, A. mikr. Anat. xviii, 484-519. '88 GoRONOWiTSCH, N., Gehirn u. Cranialnerven v. Acip. Morph. JB. xiii, 427-514. '70 BIBLIOGRAPHY: TELEOSTS 249 MiKLUCHO-MACLAY. N. v., Mittelliirn, Leip. 410, pp. 74. '81 RETZIUS, Gehb'rorgan v. Polyp. Stockh. '81 SCHNEIDER, H., Augenmuskelnerven, Jen. Z. viii, 215-242. '87 WALDSCHMIDT, J., Centr. nerv. u. Geruchsorg. v. Polyp. Anat. Anz. ii, 308-322. DEVELOPMENT.— '78 AGASSIZ, A. (Larvae of Lepicl.), P. Am. Acad. A. and Sc. xiii, 65-76. '89 ALLIS, E. P., Lateral line, Amia, J. of Morph. '81 BALFOUR and PARKER, Str. and devel. of Lepid. P. Roy. S. xxiii, 112-119, and '82 in Phil. Trans, (large memoir). '89 BEARD, J., Early devel. of Lepid. P. R. S. xlvi, 108-118. '95 DEAN, B., Early devel. gar and sturgeon, J. Morph. xi, No. I, 1-62. '82 DUNBAR, G., Breeding of Lepid. Am. Nat. May. '94 FULLEBORN, F. (Breed, habits Amia and Leipd.), SB. Akad. Wiss. Berl. xl, 1-14. '67 GEGENBAUR, Wir- belsaule d. Lepid. Jen. Z. iii, 359-414. '93 JUNGERSEN, H. F. E., Embryonalniere d. Stors, Zool. Anz. 464, and ('94), Amia, op. cit. No. 451. '70 KOWALEWSKY, OWSJANNIKOW, U. WAGNER, Stor, B. Acad. St. Petersb. xiv, 287-325, and Mel. Biol. du B. Acad. St. Petersb. vii, 171-183. '91 KUPFFER, K. v., Kopf. v. Acip. SB. Gesell. Morph. Mlinchen, 107-123, and ('93) memoir, Leh- man, Miinchen. '90 MARK, E. L., B. Mus. Comp. Zool. xix, 1-127. '82 PARKER, W. K., Skull of Lepid. and Acip. P. Roy. S. '87 PELTSAM, E. D., Segmentation (Russian), MT. Gesell. Mosc. Univ. I, Heft i, and Protocolle d. SB. Zool. Sect. Mosc. ('86) I, Heft i, 206. '89 RYDER, J. A., Sturgeon, Am. Nat. xxii, 659-660, and ('90) B. U. S. F. C. viii, 231-281. '78 SALENSKY, W., Sturgeon, SB. Gesell. Nat. Kasan ('77), 34 (Russian). Also Post-Emb. Entwickel op. cit. ('78) 21 (Rus- sian). (Segmentation) Zool. Anz. ('78) 243-245, and (Skeleton) 266-269, 288-291. (General) Mem. S. Nat. Univ. Kasan, vii 1-226 (Russian). '80 Pt. II, Post-Emb. and Organogeny, op. cit. x, 227-545. Abstract in HOFMAN and SCHWALBE'S JB. vii. 213, 217-225. '81 (French), A. de Biol. ii, 233-278. THE TELEOSTS (Literature greatly summarized.) GENERAL ANATOMY. — '93 PARKER, T. J., Zobtomy (Cod). '88 ROLLESTON, Forms of animal life, 83-102, and '95 VOGT and JUNG, Anatomic comparee, Vol. II. '80 EMERY, C., Fierasfer, Fauna u. Flora d. Golfes v. Neapel, ii. SKELETON. — '83 BROOKS, H. S., Haddock, P. Roy. S. Dub. iv, 166-196. '90 GILL, T., Skeleton, notes, P. U. S. Nat. Mus. xiii, 250 FISHES, LIVING AND FOSSIL 157-170, 231-242, 377-38°- 179 GOETTE, A., Wirbelsaule u. Anhange, A. mikr. Anat. xvi, 117-142. '84 GOLDI, E. A. (Derm bones of Catfish, Balistes, Acipenser), Jen. Z. xvii, 401-447. '82 KOSTLER, M., Knochenverdickungen, Z. wiss. Zool. xxxvii, 429- 456. '73 VROLIK, A., Verknockerung, Nied. Arch. Zool. 219-314. TEETH, INTEGUMENT.— '78 BOAS, J. E. V. (Scarid dentition), Z. wiss. Zool. xxxii, 189-215. '78 CARLET, M., Ecailles, Ann. Sci. Naturelle, viii, Art. 8. '86 SCHAFF, E.. Lophobranchier, Inaug. Diss. Kiel. VISCERA, GLANDS, CIRCULATORY. — '80 BOAS, J. E. V., Conns, Morph. JB. vi, 527-533. '87 BROCK, J., Urogenital, Z. wiss. Zool. xlv, 532-541. '91 CALDERWOOD, W. L., Head kidney, J. Mar. Biol. Ass. ii, 43-46. '82 EMERY, C. (Kidney), A. Ital. Biol. ii. 135-144, Atti. Ace. Rom. xiii, 43-49, ('85) Zool. Anz. viii, 742-744. '77 FURBRINGER (Excretory), Morph. JB. iv, 43- 49. '83 MAURER, F., Pseudobranchien, op. cit. ix, 229-251. '86 Thymus, op. cit. xi, 129-172. '86 WEBER, M., Abdominalporen (Geschlechtsorgane), op. cit. xii, 366-406. SWIM-BLADDER. — '90 BRIDGE, T. W., P. Birm. Phil. S. vii, 144- 187. '89 BRIDGE and H ADDON, A. C., Siluroids, P. Roy. S. xlvi, 309-328, Phil. Trans. ('93) clxxxiv, 65-333. '88 CORNING, H. K., Wundernetz, Morph. JB. xiv, 1-53. NERVOUS SYSTEM, END ORGANS.— '82 CATTIE, J. T., Epiph- ysis, A. Biol. iii, 101-196. '88 CHEVREL, R., Sympathetic, C. R. cvii, 530-531. '91 GUITEL, F-., Ligne laterale, A. Zool. exper. ix, 125-190, 671-697. '92 HERRICK, C. L., Fore-brain, Am. Nat. xxvi, 112-120, and Anat. Anz. vii, 422-431. '87 LEN- DENFELD, R. v., Phosphorescent organs, Challenger, xxii, 277- 329. '81 MAYSER, P., Gehirn, Z. wiss. Zool. xxxvi, 259-364. '84 SEDE DE LIEOUX, P. DE, Ligne laterale, Paris, 115 pp. EMBRYOLOGY, GENERAL. -- '91 WILSON, H. V., Sea-bass, U. S. F. C. B. ix, 209-277 (with references). '81-'91 RYDER, J. A., U. S. F. C. R. and B. Larval Teleosts: '77 AGASSIZ, A., P. Am. Acad. v, 117-126, ('78) xiv, pp. 25, ('82) 271-303, and Mem. Mus. Comp. Zool. xiv, 1-56. '87 CUNNINGHAM, J. T., Tr. Roy. S. Edinb. xxxiii, 97-136, ('91) J. Mar. Biol. Ass. ii. '83 HILGEN- DORF, SB. Nat. Fr. 43-45. '90 HOLT, E. W. L., Sci. Tr. R. Dub. S. 432-474. '80 LUTKEN, C., Dan. Selsk. xii, 413-613. '91 MclNTOSH, W. C., R. Fish. Scot, ix, 317-342. '90 MclNTOSH and PRINCE, Edinburgh, 4to. '87 RAFFAELE, F., MT. z. Stat. Neap, viii, 1-84, ('90) ix, 305-329. BIBLIOGRAPHY: TELEOSTS 251 HERMAPHRODITISM. — '91 HOWES, G. B., J. Linn. S. xxiii, 539- 558. '67 JACKEL, H., AH. Nat. Gesell. Num. Hi, 245. '76 MALM, A. W., CE. v. Ak. Fbrh. Stockholm. '67 SMITH, J. A., P. Roy. S. Edinb. '6* '65, 300-302, ('70) J. Anat. Phys. iv, 256-258. '91 SMITH, W. R., R. Fish. Scot, ix, 352. '84 WEBER, M., Ned. Tijdschr. Amst. 21-43, ('87) 128-134. VIVIPAROUS DEVELOPMENT. — '85 RYDER, P. U. S. Nat. Mus. viii, 128-156 (with references). 252 FISHES, LIVING AND FOSSIL • —• C/l *•£ i t/i o ^_ ^3 Co O c3 ."H i-"t*H 2 bjo -Q "^ "IrJ ^ ^ -T-l «^H -^ **-* C *7^ (/] C3 Q ?t !=•£ o 'S <3 b S-o . ?i ^-S fe °^1||.|S 4J 1 pi E? T-I " tn D c3 ol ff % OJ 0) JD "2 ^•gi c^-^2 1 'S-*5 > jj> 2 5 ^o jj-'g j« "1 ^ g ^ '5/J^ ^ T3 CO W o In 't! 53 rt o •£ tn "•S-S - Ji «"« •~ 4> fe 4J £ u1^ T3 M jj *- ^ 3 >SJ ra - j- o oc tn *Q *O rt "*-* 1^ "tn W 'o HH o- £ i2 'G c u HH r^ - 0 c >-. o ~ .2 - o "^ 'oJ ^ QJ *"* ^- "^ ' — ' CO rt ^3 in o •" -2 ~ ^! ^ •" ^H w<£ >-5 rt ^ c'^j 1— 1 fe rC 03 o.'5b jj1 if £ '§ 6 I'll S-^| 3 ^lllllll tL, •£ b/j^u r_j* c3 3 r1 « £ o "5 ^ ^J.scg^-au^ O ? c/3 "£ "^ G 3 : *0 "S ^ §•"§ |1 o'S,H|l_J'B E C) jg^-2 § « o rt E «--9 2 « .- 03 3 cs A §-I-|| g'l'il s V. SKELETO 0* 1— 1 CO en E u. in .— . m" 3 Cranium a trench of fibro-cartilage, at the n gnathostomes, trabeculos and parachordals. By ular" ; also of two branchial cartilages in Myxine surrounding the cyclostome mouth forming its fr homologues of the cirrhial cartilages of Amphiox group. In Petromyzon a large " basi-hyobranchi is present, which has been compared to the extr; above noted, has been described in Myxine. O O . j f~)-Q,J O-r^ ^ Vertebral axis a stout notochord ; i a neural sheath of fibrous cartilage, sn: Unpaired fins supported by dermal (?) r; face by a longitudinal ligament. Fin ray Vertebral Column. Notochordal in several forms. Well- developed biconcave vertebras usually present. Stout, cartilaginous haemal and neural arches, interneurals. Centra represent the fusion of the surround- ing neural and haemal arches with the sheath ; cartilage cells migrate from arches into the sheath at definite points ; sheath becomes greatly thickened ; ossi- fications of centra take place concentri- cally and radially. Ribs rudimentary, the homologues of transverse processes. in CO 3 O 1 •i 'w> ctf s£ 2 .Cn Q -^ •o ^ *0) & iS o f } D SKELETON OF FISHES 253 «, rt "£ £ >. t/> ,Orirt •— J^'H" '" c s-^o— ^Ti'hn P 2 ^ H w c 3 W 1-1*^* >rt t"J OJJ t* ' G C ^ U ' • r~^ U QJ ^ rt 3J '""' G C ^ O f~ • ~ ^•a ofg x£-32-S^ S°aJ5 o c 53 3 c d u c G/0 3 -3 g — ' ) _^ g g^"? b rtjxi-s^ 0 -0 (U s^o, §^^'s§ "* *^i :D ^ ^ ^" . ^* '^^ Ul *^ tr~? "^ — • - - - "1 O •« r-* i "> O rt £ .E o E, •o = o 5 2 3 ._ e8 - j o • "O U 2 '-3= ^ a rt o •S K g B, 2 ^ I •a 3 cj aj 2 - ..J por- ents s in in ; con al e m e fin on) asals §«*- g-3 gS'S'SI 'IgtA^ oj - * cu -^ e ^ ~ ~~" rt g ^ ^ "d jj 1* •g MOJ-X-^'S rt rt-S'-T P M •S-5.0^ H'55 «^|I-S •B fe = "" ^-a sup- ypes, ainly ossified, ortion of s greatly il homo- fins y t m red man ting pes al po asals Tail of unpaire rays of m arts representi f older fin typ ions. External rmal; radio-ba uent, ossified. tio mal o 5 o o 'K 5 tn &-a ~«^-o e UQ S.2«-o S x s ..2 c -E3bS S S '=- ^ S n D t: s - JS o -' G-u o " c . z 2 u 8~ g g^ ,M C 0 . o to T3 3 O «c5 3 O fl S — i t-3 £ c o a u 4-i O CO XI o 2^ H 3 FIG. 310 312 Figs. 310-315. — Skulls of fishes, to illustrate the mode of articulation of jaws and branchial arches. 310. Skull of Scyllium. (After MARSHALL and HURST.) 311. Hep- tanchus (Notidanus). (After HUXLEY.) 312. ChiiiMsra. 9 313. Ceratodus. (Slightly modi- fied after HUXLEY.) 314. Pofypterus. 315. Salmon. (After PARKER.) A. Articular. AG. Angular. BR. Branchiostegal rays. CHY. Ceratohyal. D. Dentary. EHY. Epihyal. EPH, LG. Epihyal ligament. EPO. Epiotic. F. Frontal. GHY. Glossohyal. HHY. Hypohyal. HM. Hyomandibular. IO. Interoperculunru y. Jugal. LC. Labial cartilages. MCK. Meckel's cartilage. MPT. Metapterygoid. MSPT. Mesopterygoid. MX. Maxillary. N. Nasal. NC. Nasal capsule. O. Opercu- hun. OC. Opercular cartilage. OR. Suborbital ring. P. Parietal. PAL. Palatine. l'.\rX. Premaxillary. PO. Preoperculum. PTO. Pterotic. PTQ. Palatoquadrate. PTY. Palatopterygoid. Q. Quadrate. SQC. Supraoccipital. SE. Supra-ethmoid. SM. Symplectic. SO. Supraorbital. SP, Splenial. UMC. Upper median cartilage (not frontal spine of male). Figs. 310, 314, 315 are regarded by HUXLEY as "hyostylic " (i.e. the hyoid element, //.I/, attached by ligaments to the jaw hinge, taking an important part in the suspension of the jaw ; 311, a modified hyostylic condition ; the hinder upper margin of PTQ becom- ing greatly enlarged, and attached by ligaments to the skull, is spoken of as "amphistylic " ; 312-313, were "autostylic," i.e. the upper jaw element fused with the skull. 254 255 256 FISHES, LINING AND FOSSIL | &^J= *-" e P _i >* o s re — aj o rt -^ 'En £P ij g u S . rt e— ' of "^ M^ 3 5 M (H ^ (UaJ^^*^^15 B CO c'"'s ""5 ^ W 3j ^ •»" -^ "in E •1 ^B c ^ £, c 3 "H. L vS -s««ra^o -, B 3 ^J -4J5 0) O O *r-i ^J OJ « £ ..J2 OJ Jj 2. ° " " g .£., g 3 B jg m £> s — ' P v a'S tT ^ " g 1 J3 "B ~ ., rt S l_ ^ 5 3 O >^ ~" P 3 '3 «3 g iT-o g^to "5 •£ ."2 ^D u 2 "? -^ *- !£ J ~ «'j2 J="o .2 o 1) C . ^."H ° tu o Si. 0^5 a.h'*PS P a v c * S'"1 k- „ f. ^ a.5 c •1 S" u ? 8 g O •- tn J3 2 S •<> '-g 5 b H o «j ^ | .£, g_ u rt « <*J ^ — c"*3 fe c&^s"^ oJS S* £ gS|s.S o,^' boJS .S j- B " S D ^ Jj >,> n. | Si-0 °-&- "lei10! rt ri S ^ ^ w o o *TJ tifl [/] QJ C/3 Q_J bJO O **"* O l*H OT '^ — I/I O -^ O . OJ r-. 0 co * O ,_ O '5 ^) 0 O ^"' O a|*^.g ^ -5 o 2'H |-:Ili! Ctf B— rnt) D- &g rt 11 o 0.5-3 £ d ^ ££° B°^.2 2 >-T t3 >» B ^ « >^"2 S| ^3 JJ-Q 5 > .2 ^ ol i_ 'hr— « 2 i -° c j? >~rt-.2co lit ii g'§ J ^ oltij i— i B C * — • C u H ••3 8 T3 S £ .^ 0 C "S « S ^rt 'T 3 S « C^ •£ S J3 o o Q « -g 8 """^ ^- B O — — cd §< a S BS 15 u £ ^ c3 U hJ "? b Si ,c t^ ,_ LO » ^ .5 ra o .3 O PJn O ~— *— ' ~~^ £ ^ — s Ir C- ^ -Sr S.I'S'BB - 3 -o ^H CX"*^ '^ I— J gllll ^.""(S ^ Jffi"-^ 6 5 ^> CO ^ co -5, 2» ^ c -c S i l« " „, ! U U "*" m CJ j_ 2 • § 3 t: •sill 0. E ^ s '? M E 9 •a £g,g.l 3 •*- £ •-( O C C tn tr S T3 "*" 8.1 O -^ u Q e u :s is <£ $ .- O _>, •a,.s38o T3 1> 3 "2 XI -~- o Q^ C « in " B £ 1-1 C.«- yn ill 3 s\ii.i "H >^ vi ^ tu oi t-U"g r- T3 S E^ ^ cu p-g-g i/5 ex 02 ^1— . — C £ « P. •— J— i C CX oi a! s S "3 «X 3 .i J3 £ M i i « ,J .|2 T3 •s. .ts o c o ~ e j5 . C "> '> rt ?C?"2 ci O f l- O 3 t-i -o A « •« *? s ^^ S -s "" M u •£ ™~ rt ^ S3" <« i-d-:§^|8r •— U !- OJ ° •= £ C 3 " rC "-> ™ « I. ^ 4J_4 u '5 C8 J3 O J3 IlP-a " -3 S a 4 „ x O"B S « e •§l^|«-i^lii 3~98-g,1'8-gJHl|. ^ -^ qj is-Slfl'3! C rt o T3 M ^ rf tg^^g*-2-! s u-£ p« js-jT; *-• en . cd ra TO ±5 *«o£gc>-g U rt ~ i?r< V .- fi 1_ •— H, --; t|°°^ ^-^ > 1/1 " ti *- -rt »-• HjKo.*! o '™( t*H S bjo"^ ^ j-j -^ cti O -*^ ""^ s-« QJ G '•5 'g =r.-s I -3 " ^ s'J .^ -| u ^ gS|-3 ScSg tn *Q Cy D "1 rrt t/3 *•* •foi,-§:£3"rt£3M ||- « ^ 1 1 "3 S" S E ii 2 u "S is l^xll^-o .0 .C i! 01 u ~ 0^ C .5 __ - ^ (•*! ^ c^-< 2 •2 o _4J « ctf rtrt-2 t*H *J ,n *S O* - ? i: b_ -3 ->• 1 1 'S3? 3 c li'iiWiii ^ cj *^ if, ^ . C j/i >, >^S S "c ^ p<8 a "S"^ O O £" '^ o -"I >>._ ^ I- 60 (0 >> >?-^ _^ ^ " " £ •a >-S u-c S 3 ffi "3 — ciH-^ o ** aj-2 (3 K "3 ° 5 o, S3 c ffi'Saju'i'^'^KajP siS'S'S 5"° £ c.'u « 3 E 2 U S £ oj '-T2 — ir4 r- O OS O C O C ,Q § H.'S 2 o o E — "^ — . r- . C *5 >> r. a c - . v . used by Huxley (Pro. Zool. Soc., '76, pp. 40-44), ma : " the Mandibttlar arch is attached directly to the sV the hinge of the jaw is mainly supported " by the dors :re the " palatoquadrate cartilage is quite distinct fro ular being small and contributing little to its support, ifactory condition, especially in view of the results rec be seen to incline to the older view, but cannot regard Palatoquadrate cartilage functions as upper jaw ; its mesial border fused with chondrocranium, and bearing anteriorly the dental plate; quadrate element imperfectly ossified. Meckelian cartilage flattened, en- sheathed on ectal surface with dentary and angular; membrane bones (removed in the figure) ; ensheathed on ental surface with splenial. Palatoquadrate fused at mesial border with chondrocranium. In it the following mem- brane bones appear: quadrate, pterygoid, mesopterygoid, metapterygoid, and anteriorly a small palatine element. The dentigerous portion of the upper jaw is now composed of membrane bones arising near the mouth rim, in front of the palatoquadrate, premaxilla, maxiila, vomer, and jugal. Meckelian carti- lage, reduced in size, is encased in membrane bones, dentary, articular, and splenial. Palatoquadrate not widely different from Polypterus ; membrane bones more sharply differentiated. Meckelian cartilage reduced to a rod-like rudiment, encased in stout membrane bones; an additional ossification is present — the angular — and in kindred fishes several additional ones may occur. w £ aj_n •—••-•« rt v >- ~> -o cs ' > --C Ji •" C in •* 1/1 i X ," rt C l-i $rN o t/i ^r^ S 5T | . * ^ E 3 a a~-a ^ i 2^ 1 C^J fl» tl° " ?*. O >»'^ '^ "^,'^t ,*^» ^ CJ} ^ g,rt ^ o g;f 0— to (U ™ ^ slt^slg °US £^ S^g *^^|I| FIG. 316 AU 318 AU 322 320 325 VEN VEN Figs. 316-325. — Heart and arterial cone of fishes. 316. Heart of shark; 317, Heart of catfish, Siliirus glanis ; 318, Heart of shark, shown opened at the side. 319. Conus arteriosus (inner view) of Chimizra. 320. Conus of Ceratodus. 321. Conus of Protopte- rus. 322. Conus of Lepidosteus. 323. Conus of Atnia. 324. Conus and bulbus of the Teleost, Butriinis. 325. Conus and bulbus of the Teleost, Clupea. (Figs. 316-318 after WlEDERSHElM, 320-325 after BOAS.) A. Aorta. A U. Auricle. B. Bulbus. C. Conus. V. Valves. VEN. Ventricle. 258 Figs. 9-12. —Arrangement of gills of Bdellostoma (9), Myxine (10), Shark (n), and Teleost (12). In each figure the surface of the head region is shown at the left. B. Barbels. BD. Outer duct from gill chamber, BS. BO. Common opening of outer ducts from gill chambers. BS. Branchial sac, or gill chamber. BS '. Branchial sac, sec- tioned so as to show the folds of its lining membrane. G. Lining membrane of gullet. GB. Gill bar, supporting vessels and filaments of gills. GC. Outer opening of gill cleft. GF. Gill filament. GR. Gill rakers. GV. Vessels of gill. J,J'. Upper and lower jaw. M. Mouth opening. N, N'. Anterior and posterior opening of nasal chamber. OP. Oper- culum. SP. Spiracle. ST. Tendinous septum between anterior and posterior gill filaments. * Denotes the inner branchial opening; -*, the direction of the water current. 259 260 FISHES, LIVING AND FOSSIL co osus or bulbus. peritoneum. (Chlamydoselache). Pericardium connected dium communicating with ccelome by single le rudimentary. Sinus venosus and atrium d from ccelome. th ccelome. Pericardium separated from ccelome. rows of valves. Pericardium separated from ;n ventricle and arterial trunk. Pericardium W E co i— i ]_• aj t-i cS i/i C (D <*4 O C 0 in > « > v« o Pericar c o 8 u o 2T5 . N d r s u boS* •" a c > cj &-S-3 M " S > 5 u t; J3 in 0) h fe O H P^ < W i^ M m ^O r«1 I/) _bX) UH O o IH U .C 'S '5 jy~ o LI c D Ticardium a continual (8) transverse rows c in o _> "c3 > '"o in p CO ure of pericardium f valves. Pericardiurr Pericardium commun 0^ in 3 11 ^e M£? C '"^ s| ^^ S3 _> 15 > Vt-l O fH 1 0) M '35 rt E i^ V CX tx in" t o ^ 2 • - U E U o 3 .3 PH c •£ S £ 4» O E? W s — ' 3 h 'c? 3 " 0 in 2-^ £,« ^3 E H rt 3" uj O u5 o > "rt C ti"rt ^§ rt 0 "rt 1 in (U 2 ° in ^3 3 fH > "rt "o ° QJ ro M "3 ,t l ^^ t^ ^j r- S 3 — 2§ U > S t^ . > ^.t5 O u c i .s '^ **" 3 c O " •^ *-T ^ 'C o rt.° in ? J=5 o 1-1 c 2 ^ *- 0) 03 > 8 "o o" •r -° u * •5 oo '5 ^ rt g >," It consists on-contract lontractile, v ccelome b> o £ "rt tf 2 S wisted, with ded longitu rotopterus ; — "O S rt o §& O C U •i-"" O II 'tn.2 ome. is practical arated from c •5 1/1 D "*~i '> OL, ^^ 'in in 8 ex in ^ 5 3 & in ^ -5 c 3 C 3 3 o 3 S! — ' r~ _, ^ .^^ C "- C C c O U o U 0 U 0 U U) O in u< O 0 uu o u in in' •5* 3 r« . ^ 'o ilostomes 03 fc* 13 o< 4> 0 O inoans. Protopter Ceratodui ro .£ " •*•* O ^-C*- vi w u PH CO CO" O o o co bXJ E >. -g o. rt O U I GILLS AND GILL DEFENCES 26l o-='£ o3 "^ CL . •Sue u OJ ^ T) B t- C *-* rt j~ « ^ T- td *^ CX 1/3 CIS 2 5 -S TJ CJ ^N O c O ^ 3 O Ji tn w a« •o t_ o ••-» T> ji I S J ^ T3 C 3 3 o c -r; o S "."a rt e rt l/l r- — ' in u T3 i u l-l (U [^ -£ ca w" j_ O m 2- 9 — r* ri J a rt o. a : : r s c -^ rt Q T3 5 w -43 S " 0 _ *- e ^ 'Q in b cj cs c E" E E E rt' *° rt =: •*•* **"' Q ' CJ ^ C1J3 3 3 p 3 3 ¥ *-"£}•_-: G "3 '•" •— >~> ti 'bi^S 3 • "3 3 .- "3 " *o ~~ ' ^ 2 ^ r-— — — ^ , '|~! cj _ra o o ™ O QJ c==-| J J3 rt .."S tn rt g CU 2 ^ 3 *~* "^ CJ ^ II * M'~ '— Q c E *— ^n 'bj} V- Pi •" d bJC s C3 U 1 •— -2 ID >»? Sr rt c ex 0 n! bn J3 C '£ 9 late-like vascular filaments ; its filaments thus protrude sterior projections of the en openings and preventing th Gills. filaments on both sides o decrease little in size hindx rms (Selache) a screening rakers is developed. last bar with filaments only le ; an imperfect screening it. te gills ; closely interlocking 42 "So II rt •*-! rH "^ CJ S^1*-1 ^ O ' — -— ^ O !~~j (L> 1 -j *~ "^ *C ^ p2 ••-* "S M O D- 3 A rt *o rt 5 *S Q OJ o^ ™ i-2 v_ ^ ^ ' W _ ~^ ~. CJ ° S" b£ 'tn 'S ^s in — " o .£ >- i M in i-< 3 r3 (D O 3 3 3 iO QJ n ^-H c >- — — 0 3, a'So 1/1 3 C in P. as a rt QJ QJ CJ o >? /"^** = OJ o 0 H l^ll 1 1 O U) ° I-H" _C CJ • — • ,^ QJ 'T; eex « - g2g _c "" e c •*-" •- n CJ O o ^ •Js s i" " ^ li.s^ u, i- c TS a> Present branch. S^ g-S .§^1 ~ C in C T3 | O 13 oj CJ rt 3 T3 3 ^ "-1 "-1 fV ., 3 (^ ^3 Ci J3 », .0 ^ 0 rt y 0.&S O O. v- ,Q "O O bJD cd o 13 O O o in -a TT cs a. CO CO en 3 , CO l~ 3 «> t^ . 2 =) •S o +j a> S m CO CO CO 0 1 o (X o •4-> b a ta fe O< T3 CO p rt j3 H 4-1 '£< 5* " "3 — i U OT £ en 3 o IH 0) o £ "3 cm 4) 'o "o 6 "S ^ <3 Ou >- -1 ° — £ " 3 o o o J_, r- Q_J •-a, 5 c ° •^ .3 5 1/7 »-H * £ c aj If! ^ e re in K '~ rt W -g >> a. e C C/2 rt 1 ^ o _o o co "o 0 "rt o T3 a> ^ re 3 0 5 Q _3 |_, IU > •3 o rt o "o E 3 o Ui "re in 1-1 O •a 'S re 0 _o ^0) i_ < g 3 o 're o D u intestine ; short, large intestine, with , Teeth labyrinthodont. , small intestine ; large intestine, with :le present. o" C e o E? _rt 3 0 1_ O > "°' e long and narrow (with spiral valve ns ; no cloaca. Many conditions of TD '£ C « £ e rt 3 1C to ~ >, "rt 1-1 "^ m in 1-1 a. 0 'in o U4 2 0 •a rt a; £ c o .2 f.w 3 rt ° t O in — 0) '-3 1/1 IA • — S •- e '-3 IX. DIGESTIVE TRACT in •o re bo b re 75 'O «i ££ r •* ^ I ir. .5f> =5 fe <~ o •— — c S .S ^ M '"a, | '-3 g. r£ 14-. I/) > ° _ 1 "« 'Sf D< -ti " -° ~- •S c e •— re o < i/i bo S ^3 _C c "3 o >S '5n ^ — <^ o in ^ _rt •g g s § , no marked stomachic dilation ; intestine with sj 5 0 in a> o re To _CJ "o rt u 'a, in E ii 3 "u in O in p, 3 p, O •" 'bo D _re ^T ^" bn ^ c ° '£ t- in rt 1/1 C b 3 ON _. *o 'So >s a -a > 'o — >. 2 f. CX w tO QJ •S "§ c '"^ ~ in (/i o • ~ e ^ .S 0 re "5 'S, re "> 11 1/3 c3 ^ iach small ; intestine with spiral valve of 9 turns, si which the hinder 3 present external branchiae. dc caecum, spiral valve of 8 turns, no cloaca, single (o - in •a u £ E in ^ "> rt S in *- i- - O i- -a ll re 2 ° £ K 5 0 "S •Si* _0 "?> rt D. o •sl in ° Iz rt t/3 •« 1 c S ui g D ^ ho H •3 3 E o — ^ o o m g [5j rt 't c 2 i •3 -a 2 °. ">i a 3 | - 3 1 'i j~)T • gj & C S3 •a ij Js "i I '5 u J ^ -a .c in 0 11 •a xi 5 —" M s •C w .Sf rt 'rt — JL if- ll = ^~ S g « g r i p, ^ 0 Cfl 'O a. o o *- E u — — C 'K 'M -o E 1/1 u 3 o '2 U '" r- 5 g •— '?^ t/1 re >, * 2 ^ -i-" 0 w 1 "« ^ C. -> ^ M £-* "*^ £ r! u 4J~ CJ *T3 O (L)" tuO -^ rt O S - rt 2 5J — ^^ o 0. •a 1^ g g _E "u '2 'S C _ "§."« — 75 *" .S c QJ (U S M To -3 2 CD ^ O £ ?. ui C >»!/! •- -^ p g — •§ t3 in MO D. g - O -2 O &. ^3 •fclw ^ S; -4->>,-;;Q. 60 O) ™ ^ O W • — ' *-" •« O*-^ o OUt-XO'L)^-J t3J3 c|Oo-iiuCLii-)<; u" 5 we H 264 FISHES, LIVING AND FOSSIL w Q Q PQ W) to 3 0 lining membrane thrown into irregular ridges ; its vascular supply of union of efferent branchial artery, thence returns by vein to left a. D e 0, o _rt 1 7i >n O (A t/1 g (/I ^ <" 0 rt o in nj > 13 0> O 03 ^ c o i- -i .5 £ rt •^ » — O i— U d. "o 1*-, "rt mi r-1 ,_ S — .§ £ >> | aj "3 M U <4 [_r 01 C1 d 3 0) T3 .— 0 a 1 1 §•* l/l C ,_J p l oi c3 • — ' t-. , . - *^ _rl U T3 >? ~ bo <"' '> Cd S H in .1 Kn S c inting. (Caecal pouches o bb c > C o in rt Ul C T3 x-. i_ rt >, O "3 0 sinus venosus. in 3 ^-. 1) 2 o" p £ C 0 'S cT in terus; pulmonary artery, 3 1 QJ C O Ul C £. 0 •a O) FH systematic veins (portal ? rt 13 en 0 T3 O in 1) °-g t« -r; O • — ti/) >- ygos ; opens to dorsal wal lining membrane irregula O t/J o .a > t/i c 0 a 0 VI O bO returns by systematic veir in sturgeon. Often subdi the side of the gullet ( Acanthopterygians, Anac; Siluroids, and in the latte and in fossil Crossopteryf c3 n *5^ '"^ t/1 N ' ^ N* N is;' ui £ ^ ^ OH < "^ (X ^ ^ ^ '/i c in 3 V Ul 3 en , 1-J H CO 3 n • clostome CO •a rt locephali [moans. OJ 1 0 £ 'ul O T3 5T Ceratodi leostome >- 11 E >, 0 CM in O T3 'a. U rt 'g 0 & 3 en C/l O 0) S >, 0 .^* D o GO a ft H STURGEON AND MANY TELEOSTS LEPIDOSTEl/S AND AMIA ERYTHRINUS CERATODUS POLYPTERUS AND CALAMOICHTHYS LEPIDOSIREN AND PROTOPTERUS REPTILES BIRDS MAMMALS Figs. 13-19. — Air-bladder of fishes, shown from the front and sides. Cf. p. 264. A. Air- or swim-bladder. AD. Air duct. D. Digestive tube. (After WILDER.) 13. Sturgeon and many Teleosts. 14. Amia and Lepidosteus. 15. Erythrinus, a Cyprinoid Teleost. 16. Ceratodus. 17. Polypterus and Calamoichthys. 18. Lepi- dosiren and Protopterus. 19. Reptiles, birds, and mammals. The diagrams illus- trate the paired or impaired character of the organ, its varied mode of attachment to the digestive tube, and the smooth or convoluted condition of its lining mem- brane. 265 266 FISHES, LIVING AND FOSSIL life?); the younger adults are thus (Nansen, lital products fall from gland to body cavity, horny processes, and appear to be emitted in a gh abdominal pores. Fertilization takes place much convoluted vas deferens and vesicula igh urinogenital papilla and grooves of claspers raries to coelome, thence to common opening of except in Lsemargus ; in Trygonorhina 2 eggs us-like portion of oviduct by unpaired aperture, vitelline sac fuse with oviduct, establishing ind dilated terminal portion of segmental duct enter paired openings of oviducts (Miillerian) water. DUgh coelome and genital tubes, vasa deferentia be secondary structures. (Oviduct continuous ifications.) A vas deferens (Wolffian ?), which f ureter. An intromittent organ (modified anal , A single oviduct is often present, opening in oviduct — this dilates in the case of viviparous ;sion of young. Ovaries or testes (occasionally rring may exceptionally be dioecious, sometimes :o be regularly hermaphrodite. "o £•;; .^g •g,0 O^v ° OJ I- >.T3 O +1 •- V ^aJoJ-C^^^t: XI. GENITAL SYSTEM (Cf. Figs. 332-337) irmaphrodite; ova and sperm, however, ripened at different periods 1 Cunningham) males, the older, females ( = Protandric condition). ( thence out by abdominal pores. Eggs fasten together at ends by th string. Fertilization unknown ; perhaps occurring in abdominal cavil xes separate. Eggs and sperm fall into body cavity, thence emitted thr in water. xes separate. Testes paired, rounded ; sperm passes outward throu seminales (= Wolffian duct) to sperm sac and urinogenital sinus; tin into cloacal opening and oviduct of female. Eggs pass from paired paired oviducts (Miillerian), are at once fertilized and receive shel to i shell) in upper oviduct, thence pass outward through dilated u opening behind ureter, to cloaca. In viviparous sharks the walls " placental " circulation. nilar to oviparous shark. xes separate. Sperm passes outward through sperm duct (Wolffian i through cloaca and vent. Eggs drop from ovaries to coelome, them and escape through a common opening into cloaca : are fertilized in milar to Dipnoan, genital opening separate behind anus. ii u ii u u it u II II II II II II II xes separate in nearly every instance. Sperm and ova pass outward 1 and oviducts, to unpaired genital pore. Oviduct and vas deferens m with ovarian capsule, although there appear many and noteworthy m may fuse more or less with its fellow, sometimes opens into or in fron fin spine or elongated urinogenital papilla) may sometimes be prese front of ureter. Ovary sometimes enclosed in sac continued into tl fishes (Einbiotocidse), and is widely specialized for attachment and e single) paired, may be confluent. Cod, flat-fish (Pleuronectid), and testis on one side, ovary on the other. Some forms (Serranus) appe; u . H o ._* U h* X M Cfl c/) C/3 tn T) 2 c" i/) P C u in |-=^ o •d 8 in T) • 2 Se- as . 0 Mrt _OJ t/5 t/J || 03 >" |j ^ « o & ||E S"u |SCQ 0, 1 6 £ gu Pi ^H CJ CO o H H 333 MD 334 U AP UGS 335 AP M 336 UGS OVD 337 UGP Figs. 332-337. — Urinogenital ducts and their external openings. 332. Cyclostome, Petromyzon, (After W. K. PARKER.) 333. Shark, J. 334. Chimseroid, juv. 9 . 335. Ceratodiis. 336. Ganoid, 9- 337- Teleost (Salmonoid), 9. (After BOAS.) A. Anus. AP. Abdominal pore. CL. Cloaca. G. Genital opening. MD, MD' . Left and right Miillerian ducts. OVD, OVD'. Left and right oviducts (not Miillerian ducts). K. Rectum. U, U' . Left and right ureters. UG. Urinogenital opening. UG' , UG". Left and right Urinogenital ducts. UGP. Urinogenital papilla, showing distal opening. UGS. Urinogenital sinus. UP. Urinary papilla. 267 268 CIRCULATION IN FISHES 26$ ^ C 2 y ~ S -— • t- —- r- — * O ^ k C » OS d •^ "?5 "*~* tJDo^Cr^c: ""jurt n ^2 ^ "H-Tj g*"rt § g ^-a "y1^ u s E ti,-^ c ^ "M I « "^ '**--••- O 0>- g rt a; --^ •a ^ • - > t; _ ^; rt-S-rt o b -^ .? - c . e •" - ~ c ., J?*3 ^ij^^c o^J"M Ifi-gsg^S °^^=ls ss"- fc.-Jitn.i^^g J^jjOrtJi.S o~uo -c^^"^"1^ a;P^_n="3 E •- "" OJ S — ; -^ J3 E J3 P fl M LTO-: c ' eS hn ™ ^ 2r - - t; 5 o o £ = £ 0 tO •— " - " - "^ --* . i^ ^T -^ 2 fe- _- • - D ^i ^ r^'> O- 3 ^ 'aJ r^ ^ rt ^ "~^^ *" ^3 -— -^ c CX > tn ^ p ^ "^5 I "Q "5 O ^7S "^ •« |3J ^ S *^- y D--™ ^^ r- ^ S C/3 is a vascular bulb connecl lial vessels appearing to ar No cephalic circle. Effer ; from each collecting tru > — m «i 3 •£ '« "3 M _z in '.- ~ W3 .S T3 3 o 3-13 *C C -1 C V D.J jr > CU D O C£i £• V S3 o rt 2-SS -s-g rt w C . c i> . 0) 2 rt *^ ^ —, ftj S 5 5S 0 " •s c d 1) •t^ *O Q. o o u « t_ rt c^ W EC H 0 2 »v *Vi M "> •~ O •d" C U -t-a rt — gupS S3* ^-S s ~-s S - •- CU 2 £ - c v •.£" 3 O *- 5^o «£ c .-'5i O g-g.^ „ a. - un O ^ > rt u -o W) <" i3 rt — bO -^0 — C/2 J cc '5< rt 0 JS in r" '^j (/; '^^ ,,. M o .. Mc-c m .5 'S D 3 M •5 c° O g rt 0 ? "o >-M g « •5 ^~ lowev O15 0) C 5 rt UJ C/3 C/3 W > > -(3 rt u c O aorta mud y O. (U in £ ^0 U ^j s o ^"5 c B rt o J3 C •" rt S_^ o o o < I/} "5 "J ££ u s^d .— C Is apparent arch arisin E .^ -1- ^ O en 5 u ^ ^ U ^ 'S ^ arteries, r « 0 r^T3 5 S s^ •j u CULATOR m obranch. se ; afferent ^-- — i »L. ""• S S 2 c .- 3 £ o .S - -j > ^o^l St.E^ 0 '« u — • g b =« '35 3 .H .Jj§l 1°! 0) - > phatic vesse t branchial £p 0 °"5 V-. ^ o £ •a > rt « rt 5 o /, > i. 2 ' Sag S « 73 rt '_" rn Oi U w ffi H b o z C' c« >— OJ in rt W <4-l O u l/l o _G 0 u _rt | 'in sympathetic system, rtened by its twisted cour: in Amphibia) ; branches ted on each side into a •y. Veins scarcely large he right posterior cardina :o represent as well the 1 e right posterior cardinal ing thus into the grooved ,vith venous blood. Lym uced, bulb prominent, las :nted by single tubes inste r tops a horizontal efferen dorsal aorta. The princ efferent branchial vessels e ; Cuvierian sinus-ductu: .s venosus. < cu X ~r« " 2 iJ ° rt o ^Z \n 1 •*-• i/j rt -1 >w .— « 1) u _a u n 1) 'in z: = s 0 U ils essenti; ""^ LO S3 i§ '5. Is ¥1 'eU 0) j; S g ,S.H ^rt^sf Z« .S*l§8-32 ~.23 ~ bfliJ-? -T— — t/J l/l J- CJJ-n 0 « V >r tj ^ J> *3^ — 15 rt y i ^ - gj in ^ 13 z: '_ 7: -3 T3 3 *^ o - C in rt 1/1 rt U '-< rt Q s S S P 2 - •n o "c "rt o> "o § -2 3 ? S 0) a p] rt -i: rt X! 73 rt -S o sMT"3rfSll1'|Slsa|^S'i,3 I i« ^ ^g^-j.Q' >^i>%-'3Qj-'o ^ja— - f(j ™Q. cn_-j rt-^'?s£ 't' S^aj-ESui* ^S^^SS .S oH CO O H W c* u X w *> V v' ~j- U) ryj **^ g +J . . ^•; » °| Remarks. " 3 c " " ft "5s o. •£•- c'c "re 3 u ^ •- — S S & 2S "'S u i ^|£8| SJ* : |gS|g. | rt « M,. l~l G CJ " ^ 4J »•" Q."1^ ^ ^ c«">cM>Sj S "13 -«""«'§ I .sM-§s £«s § = - N § 8.1 fil ^S's u u « u re •£ -g-|3 |^3 «s>« rt — o T!1 t-^ i T3 /^ C G O T3 /-^ 3 C'w S S "^C- 0 (-'«o.°« '-1 !-, tjj D • L. oa-t au u +j »u 3 jz: .S^ OJ 3^1 g* 5 rt ^ 2lc G ti | at *j »*j 6 g s- s :« '-a ."2 £-s^ *^, 3 > fO "^ PS 0 M ro •*•! S Ci •3 c 3 |i || ° ° i 3 gr. 1 _C .1^ • .1^ S f1"" C 'a w c^T w c rt F^H g s . - ^*j w — ^ ^ rt * ^ ^ c g So 5-a •-^ C tfl r± C ui CX re *-• P< TO -^ 4J 'O i-> U ^H v PH ", we o we " $£, to '> v ^ V *c '> "^? *^x *. 5 S u c *^ •o ^ 2 "S >> o W ^ -^ £> n c <; '•'•.'S r t/i d( ui t) C ui £>-Z Sn " ^ "S1 5 — C , (A 0 lii H 11 liJi * - i—i C fe'ls hi| ^ fc|Ss 1 '*" V 1*1 ld . *?* w (U ui I) c u u "OP. SX £ U3 •- 5x « > g : 14; v5 ..S : o cT - IH flj " ;£?S3H 3 s « O O- >-« -5 8.3*8,51 &.is.s 0=35 52 •- c G U o E U ab •o o 0) — s.e-3 4) ^ T3 U) ^ | C-r u S 53 g fccS? rt i/i C >> 10 2'C * Probably not the metanephros 0 Q PQ HH X S S c 0) — 0) T3 ide of (somewhat behind) anus, hitiids ; may be absent in malt 2^'/lor^^ 4J4JO - jlj Q.— 1) IH > bfi *^ **" aS 2 ^ "3 J S 2u = Scg S • - ^-^o: 3^ .MO c « n § :i rt fel S -a rt a >• "rt 3 7i rt '? *c3 '3 '3 c "_^ tn OH PU CL, OH D tomes. . CO w d -M w CO 2 S 0 P< 'H 09 o "o M a -u rt tH 0 'o c O CD ^j j^ j M Cj "a! U CO 0 0 0. O H FIG. 339 340 341 ML Figs. 339-344. — The brain of fishes. The dorsal view of each brain is shown in the upper figure, the ventral view immediately below. 339. Bdellostoma. (After JOH. MULLER.) 340. Petromyzon (Ammoc&tes stage). (After ZlEGLER'S model.) 341. Shark (angel-fish, Squatina). (After DUMERIL.) 342. Chim&ra. (After WILDER.) 343. Lung-fish, Protopterus. (After BURCKHARDT.) 344. Perch, Perca. (After T. J. PARKER.) AQS. Aqueduct of Sylvius. DSE. Diverticula of saccus endolymphaticus. £. 272 342 IV Epiphysis. EP. Epencephalon. IN. Infundibulum. LH. Lobus hippocampi. LI. Lobi inferiores. L T. Lamina terminalis. M. Mesencephalon (optic lobes). ML. Myelen- cephalon (spinal cord). MT. Metencephalon (medulla). MT. Anterolateral lobes of metencephalon. P. Prosencephalon (cerebral hemispheres). PT. Pituitary body. R. Olfactory lobes. SV. Saccus vasculosus. 71 Thalamencephalon. V 4. Fourth ven- tricle. Numbers I-X. Cranial nerves, i. First spinal nerve. T 273 274 FISHES, LIVING AND FOSSIL CO W E CO I— I fe fe O W H CO > CO CO t3 O > P^ W ^ hJ < p^ H ^ W L) W E H > X partly separated ; the region of ptic lobes (mesencephalon) and nd sharply tapering, separated dorso-ventrally, its nerves with al branches derived from each ded as represented by the long 0 c 1 i long epiphysial stalk with end well-marked infundibulum and 6/ia/on small ; behind its rim a B ; its nerves alternate anterior ,3 •o 2 V d ID 13 o O o c 13 u ID W) ^ — . . . .-. , O g co 42 5: * 1 § ~ | O 43 c« co rj li il I co 2 bn « 1 1 1 1 1 - * § J! ! 0 Ln 43 «« ^ ° °* •* "I u .§ 43 ^ .M i* in .-£ > CO O Z u. OJ OJ 2 > S § T3 ? s-f - a P3 O -4; •£ ||^| - O ^ 4> 'O rt ^. ^ p C fl|f^ 1 -f.f-8 & o 'a & ^ -*--*-• ~ S ^ '3 a; t/i o ^3 *- * w a s ^ •£ _rt .£- « D OS C 43 £j; o/) ft v en q oi 3 g | ~ - r^ 1^~ rt i D O T3 t3 "g B B LO- g5 rt. jn e o •§ •- ^ i 'S 'o oi 'oj ^t3 C en 43 > .S 2 2 '3 p M cS u 43' Oj C3 CJ 03 0 01 O ri •*-> 09 _O CO _0 73 B 2 1 09 O 1 "o >> ffl ti, CO NERVOUS SYSTEM OF FISHES 27$ 5 '5 73 ^ jr e d C- C _>> 73 M c JC1 ^ *- cs -—" .•"• jj -j S o u 3 -» yj 3 o **< S •-"dSSoo0- st < M < ~ -S S •§ § TS •§ •« r « iltl^j a£ 5i "S ' o §—<_,. s;m:co^^rt.y •§, S '2 -S „ .g gj| c o rt £ O a -G Cu . — 1| l| fore-brain, not outward nditions of lamprey ai 3 C Si O S > •a o u "r; c s. 01 •d u d CJ '-3 C .s a •« d — — ^3s 0 ° ss .S c d — 51 11 d j^ ^ <- c * o S c 1 1 .2 f-« rr^ 3° » 8 ~ ^ c en- rt 73 r; 'o -S C bJO 03 C :s dorsal region distinc1 epiphysis is rudimentai On u Sr u. 3. 'd c Cu ** 1.1 M .2 ™73 .— iT S .- 'in c/f ^ t! rt -; c> c c If) « ^C '^5 (/7 ~ 2 o | D« ~~ ^^o-S^o1" li-s § if §>•§ :d olfactor I choroid t- C u >< ^, C s Ul - £J °l (/i • — p; 'O 8 E V 1) D O ^ u c ^ 73 O 1> o — 3 reduced, il .1 view ; its O i^ g M c. '"o 3 wl P C '"* 1) (/I > _«_ — D i- > •2 i 1 73 >^ 1 "* d 1^ o > > .a d -a ra >% in _g 75 rC d 'S u *- u S O d ^3 * ' en ^0 h u *'— i ^J — tn C ^ t/1 d i> 3J ~* ^ J5 ti u 73 <2 £ gS w il e c d 'o >, 5 73 c H. i*- a 0 1- c erior lateral outgrowths immediately belo ly open. Spinal cord and spinal nerves ess the vagus. An optic chiasma occurs. T cattered throughout the entire mantle, with markable form ; the thalamencephalon grea ng brain parts, concealing the mesencepl, ed, its marginal rims have greatly widened, hiasma of optic nerves. Spinal cord, ner into halves, which terminate anteriorly in and long, with prominent epiphysis and linent pituitary body ; mesencephalon far s 'i (in Ceratodus ?) not differentiated, su en O £X 4^ 2 I7i 'o u ul O ^2 0 J 1 '(/) {/] D fl _0 ~. !_ « CJ o ^' 0 3 d J3 O, B S ^ "3 o 'Z c If £ £ en tu >*. _-: be regarded as an extreme modification obranchian conditions. The brain greatl tb.S S S •^ 5 _O en £•0 2 2 o <£; -^ O i-i d t/i ^ „- 3 d -^ r* CJ t/1 73 §'5 e I 1^ *sJ <\, ^J •%* 3 5 i- s: — O § d jy U5 _, o P3 _JL ^J -S -01 c/1 a "•^ — *» 11 S ^ > ^ S ^ *^ ^ C 0) 0 •£ 73 ^~ OJ ^ 2 1 O ^ X b ~ 2 ng and concealing posteriorly the region i system connects anteriorly with the third c ome Ganoids. The substance of the brai races of differentiation of the cortical cells *; u „ uj 4> -3 73 o c 73 ~ „ "- E 3 rt '£ •- S d d 0 < x '" •— E > -*-73'r1t/)Oflji:- •gg^^T^^SS S:s-S"5ji>,o 5 £• c c - •£ ^ 'C ^^g-S^^S^ S5 8 "M 3 > Sj.S ^j1 'tj^ C fc> ^ ,_ gbflS^rt^^O re-brain separated nencephalon narrow inferiores, and pron lalves ; epencephalo, 5 T O J_ c « ^ 4 t- ^ "3 c 11 in 3 O O rt t/i V <41 O like. A systematic the corpus striatum ~. o t^ O 0 o ^ 1> O •= m of Teleosts may referable to elasm ely modified ; the } ippocami ?) ; thala> """__ '5. CU oj E c •« £ % 2 g a. ^ d £" 73 C rominent, overlappi . The sympathetic as in sharks and s m-nervous, or with t •v* C w - - -£- v>^(u^^(/i-C5:(U S -2 S. ^ •= •£? -5 M tu *^ 5» _c ^ C3 •-•" « .3 S | j « 1 2 s S 1 « t I S'SSSMrt§* •" >T3 s* £ tn^Cfa«S4^-M II spicuous lobi i divided into 1 Urodela; mete. with diverticuls general shark- differentiated ; substance and nervous syste structures are characters wid lobed (lobus h and the choroi lobi inferiores ; epencephalon p does not occur > c X Auditory Organ. 'O Z .. • £- )"^""'ctf H ""* t/1 3 t/) 'p Ci w Cj "H 3 • - "^ i- H c o nr ~ "* « "<3 o> y1 -ri ! 12 .g ,3 'x — ~ 3 c o " ^ ^ « ) ' w 03 r^ 5 ITU ^i OJ f* Q in O ^_ rt C ) Cu O «^ w OJ3 ^— •— ' ^^ 4 Contains the usual elements in the iult Petromyzon ; in Myxinoids lacks ^e muscles, sclerotic, iris, lens ; these re also lacking in Ammoccete, the verlying skin functioning as cornea, lo tapetum is present. O | u o U) — "ft g I in ffi u •s "* 1 g - III a-3 o o T) 'S S ^ (— ' rf ^ lit j— • g_) ^ c 'J " M 2 •^ c o rt V ci O i t-> if> • I-H ^*^ .11. •^1 ,nout region yxinoids) a £ 0 m ft rt 0 o ft o V M 0) M 13 o II ^ o "a. -a p, O £ •£ H 'S . 13 »- 0) §| •S. c uter opening i i O rt c- £ O '3 •C c i! '« OT 3 0 l~ it I 1 «s B.S, 3" O "o "5 ft u in ' — 0) "S 8 ^ si % 1-1 O t3 vS "o x: o oi w shaped, with stout radiating sensory epithelium ; its ne medullated, as in all forms) enormous olfactory bulb, clo: nasal organ may or may not ^ "5 £ j_ D is subdivided into anterior tions by an overlapping (in d. "3 c 't/j CO 0> g p to w s o 73 <3 o CO THE SENSE ORGANS 277 Vestibule and its duct as in shark ; the sacculus, recessus, and amnullce appear more perfectly evolved. Orean separated from brain by membranous wall only. i Vestibule and its duct as in sharks : recessus little developed, sacculus re c u in C a! SJ 'o (8 r • 3 15 • With many specialized structures ; u o I* IH in "rt o of M ^ I/) the cornea, lacking, r. S u E? rt O Z in _i; E3 in 3 £ Eye highly speciali U ft ra 0 0) £? re >> C O .a process (pigmented vous, muscular) aris :ni_°jl - c ^ g 15 o a. T3 S JJ o 3 C in s 2- •£ re 22 u •- rt 1 - c 1 S .0 8 ° •- •« £ 3 llili!l ra X> n-1 > *^ ^^g-21 S3 o ~ ^^ u ft t, c z; j>> _>, X u o ^c 3 J~ "in in re 3 JS f T3 1 (U £ j_i u (U in D, •£ >> in o — OJ~ 1) 3 u in to c £ penings become e specialized lal or naris issomev O C_ r C 0 0 o 'C re c 0 i_i 0) in O ft ft 'o a i-. t! 5 u .c in o re "ft , somewhat tubi transverse (slig V O •d S j>> ^5 13 M w rt in aj i_i ni C 1« o 'n ^ ,C "3 ~ O c l/l ^ u E S c c tn 15 £ S •a 0 c S re °rt £ -a" 0) c V rt 5~ > L in ~. in re •a OJ U) O 2 o V u 13 in 3 so -a .£ o g ft _C in rt re o £ "5 2 o 15 in re C u _C plicated by th es. Theanteri O bo C-, o c in c3 c '& re £ c_ merine " dentc w re M _o u o "3 in — "i in ^ '« u u > F-i ID C « rt u-, 0 D, — c « ^0 ^ "3 "r a; *0 1 0) c , ni £ in" _o tn c _2 T3 rt 0) t_> -S I/J cS ous secretion. In Petrom fa c _o "S "o to [rt to I* 3 •d in are connected with the cori ith fine pores. 2 H— 1 ClH O 01 ti W Sk relatively thick ; cle of large and defii ito monocellular g nsists of many layei 3 a ^c M 3 JS in 3 •d" O ^ d >^ 3 _a> "o in cuticular border is ated condition. It rt T3 1) u 2 is contains superfic D s, whose processes border is pierced w H — "d a! a) .~ o o "°_ 0 V ^ 0 •s B! ^ £ ;=: "3 b [ARAC ir! 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I i O (J >, S u O m • — ' O J3 ° in : 03 rt 13 i- C 4J 03 T) <-.- 4) = X 4) ** LI 1H rt .C C OT Shagreen and d' Epidermis of rC ^ 1/3 Q; i'f t— i i - 0 3 "° S >- C rt _4> 3 CX • -Q 0) *C -a 3 4; ^ 0) 4) •£ E I. -SP rt O< Ganoidean or h< 03 TO 0> 03 B 2 | 3 § « t) 1 .& 1 U (0 Q 0 H 280 FISHES, LIVING AND FOSSIL CO W X CO *-H ii( W S OH O J w > w Q W w n: H > X *j ~ i -^ 3 2 i u^ 3 3 t) & • •° l"°-§ 3 Ul 3 3 2 rt 3 a i- . « rt rt oc u 0 ft 3 h 0.6 G O E c ^> ii-- rt rt " « S 0 £„, >, . 9 *J C tn .9 a a «T8 S X |4 1 tn SJ ^ L> '*j "3 ""j ^ 0 rt S ^3 '— . o -^ X ^i? S c^ o ^ 3 i^ og > w *_. ^ O< ~a rt o '3 E >^jj i. ^.... M •-.i rt 6 >.2 - j; „; •a 1 t S rt ,!£ g jj "> ^2 en -G rt 3 X X ^ ." 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H all ^ ssi 1<3g < ^j"H 2--'- u u.S rt o1 O a s IH ^«- w > 6 13 O >, O *^ *- M W)j3 S to t« S3 a I u ^ u c 1C 282 FISHES, LIVING AArD FOSSIL XIX. THE SUPPOSED DESCENT Interrelationships and lines of descent as suggested by a number of noted on each scheme. * Denotes that the diagram is the present writer's Howes ('91)* (Oil T'rino Genital System) Smith Woodward ('!)«)* (On Palaeontology) Nephrorchidic (Ela.-mobranchs Batrachians Amniotes) Euthurchidic (Ganoids Teleosts Marsipobranchs Dipnoans) Ostracoderms Haeckel C98)* (On Genera Anatomy) Proselachian Cyclostome Teleosts Selachian . ^v. Chimaeroid \ Palaedipneusten \ \ Proganoid \ \ Dipnoan Ganoid and \ Ganoids Crossopterygians 'Sharks K I a,i I -< h ('98) (On Aiial Skeleton) Cartilaginous /\ PerichorHnl Cartilaginous Chordal Verte- f ( Vertebrae. brae Teleost Amphibian Shark W.N.Parker C5*2)* (On General Anatomy) Ancestral Stem of Amphibians and Fishes Acipeueer' \ Teleost Lepidosteus Cope ('85)* (On General Anatomy and Palaeoutology) Amphibian Dipnoan Ganoid Ichthyotome Selachian Ganoid - Burckhardt ('92) (On Central Nervous System) Selachian / v Petromyzon . Dipnoan ~^J Chimaeroid Dipnoan \ Amphibian Elasmobraneh Crossopterygian Teleost' XChondrostei (Nervous Syste Cerato'duB Protopterus Teleost Reptile Progauoid Acipeuser, Amphibia ^^ Bony Gauoids Teleosts ami End Organs) Petroruyzun Proselachian Shark SUPPOSED DESCENT OF GROUPS 283 OF THE GROUPS OF FISHES observers ; their views have been based on the different lines of investigation interpretation of the text of the author cited. Bailout- 080) (On Embrvology and Anatomy) ISouR ( 9O) Ancestral Elasmobrancn ,/-» r1;.,,,, . (un ^jrcu Protoganoid Dipuoan GaDoid Bony Ganoids \ 080)* (On Anatomy) Includes Lancelet and Cyclostomes as 2 Sub classes of Fishes Palaeichthys t Amphibia > Protopterus Ceratodus Teleost Gill 096) (On Structural Characters) \ Teleos Ganoids (With Dipnoans) Chondrosteans (Sharks and Chjmaeroids) •-Pleuraeanthid Myxinr / I \ \ \Teleostome Petromyzon / \ \Dipnoan Chimaeroid Elasmobranch Bridge 0?8) (On Osteology of Ganoids) Davldoff('80) (On Extremities and Girdles) Primitive Gnathostome I I Scaphyrhynchus Acipenser Polyodon Selachiau Shark Rays(?) I Chimaeroid Apueumato- — cue la Elasniobrancli HepUnchus _Pneumato- coela Polypterus Aiuia Lepidosteus Amphibia Physostome Selachoidei Teleos teoidei Kabl 089) (On Embryology) A Cyclostomes Ampniox Beard 090) (On Embryology and Brain) Selachians Ganoids Selachodichthyidae Pollard I (On Anatomy of Head) \Shark Crossopterygian Ctenodipterini (Devon) Selachians Teleosts \Ampliibia and Prutamnia Amphibia Ischyodus (Jura) Holocephali Ceratodus (Trias) / Ceratodus Protopterus Dipnoans and Amphibia INDEX Abdominal pores, 271. Acantkias, larva of, 216 (Figs. 288, 289). Acanthodes, gill shields, 20; a fossil shark of the Coal Measures, 79; structure of, So, Si; A. wardii, Si (Fig. 87) ; shagreen and denticle of A. gracilis, Si (Fig. 88) ; affinities of, 95; diagram of affinities, 98 (Fig. 103); gill arches, 114. Acanthodians, antiquity of, 9; fin spine and pectoral fin, 28, 29 (Fig. 32) ; pectoral fin of Parexus, 42 (Fig. 51), 44. Acanthodopsis ivardii, teeth of, 82 (Fig. 88 A}. Acanthopterygian, 166 (Fig. 171 A). ACANTHOPTERYGII, in classification, 9 ; as a subdivision of Teleocephali, 174. Acipenser, in classification, 8 ; antiquity of, 9, 166 (Fig. 171 A}; swim-blad- der of, 22 (Fig. 13); description of, 159-161; A. stitrio, 160 (Fig. 165); eggs and breeding habits, 181 (Fig. 194), 185; fertilization, 187; devel- opment of eggs, 203 (Figs. 249- 264), 207; larval development of, 221-223 (Figs. 296-302); heart, conus and bulbus arteriosus, tables, 260; gills, spiracle, gill rakers and opercula, tables, 261 ; digestive tract, tables, 262 (Figs. 326-331) ; swim- bladder, tables, 264, 265 (Fig. 13); genital system, tables, 266; urino- genital ducts and external openings, tables, 267 (Figs. 332-337); excre- tory system and urinogenital ducts, ACTINOPTERYGII, in classification, 8, 147; description of, 155-178 (Figs. 157-185 A); Chondrosteans (Gan- oids), 155; fossil forms, 155-159 (Figs. 158-164); living types, 159- 178 (Figs. 165-185^). Actinotrichia, 31, 33 (Fig. 39). sEtheolepis, ganoid plates of, 24 (Fig. 25). 25- Agassiz, L., 37, 66, 107, in. Agassiz, A., 224. Air-bladder, v. Swim-bladder. Allis, E. P., 50, 51. Alopias,%<); A.vulpes (thrasher shark), 89 (Fig. 95). Alosa, eggs and breeding habits, 181 (Fig. 197), 1 86. American Arthrodirans, 130. American Geologist, 80. Amia, in classification, 8; antiquity of, 9, 1 66 (Fig. 171 A); swim-blad- der of, 21, 22 (Fig. 14); sensory tracts in head dermal plates, and scales of, 50-52 (Figs. 64-68) ; A. calva, 51 note; a Ganoid with her- ring-like scales, 145; description of, 163-165 (Figs. 167, 168); Mesozoic forms, 164, 165 (Figs. 169-171); heart, conus and bulbus arteriosus, tables, 260; gills, spiracle, gill rakers, and opercula, tables, 261 ; digestive tract, tables, 263; swim-bladder, tables, 264, 265 (Fig. 14) ; genital system, tables, 266; excretory sys- tem and urinogenital ducts, tables, 271. Amiitrus, barbels of, 46, 47 (Fig. 58). tables, 271. Ammocates, head of, 61 (Fig. 72 Q, 285 286 INDEX 62; development of egg, 189 (Fig. 215). Amphibian affinities of the shark, 98 (Fig. 103). AMPHIOXUS, in classification, 7; gills of, 1 6. ANACANTHINI, 174. Anal fins, v. Fins. Anatomy, v. Shark, Cladoselacke,Acan- thodes, Climatius, Pletiracanthus, Chondrenchelys, Chimara, Dipnoan, etc. Angel-fish, v. Rhina. Angnilla, v. Eel. APODES, 173. Aquatic breathing, 16-23; modes of, 2O. Archipterygium, 39. Arius, eggs and breeding habits, 181 (Fig. 195), 185, 186. Armour plates, 23; evolution of, 25. ARTHRODIRA, in classification, 8; de- scribed, 129-138 (Figs. 130-144); geological position of, 9, 129; asso- ciated with Pterichthys by Traquair, 130; American, described by New- berry and by Claypole, 130; Din- ichthys, 130 (Frontispiece and Figs. tjS-'S?); vary ing size of, 136; den- tition, jaws, and mandibles, 136, 137 (Figs. 138-144); affinities, 136-138; differing from lung-fishes and from sharks, 136 note. Aspidorhynchus, 157; A. acutirostris, 158 (Fig. 162). Aspredo, eggs and breeding habits, 1 86. Authors, comparison of phylogenetic tables of, 282, 283; v. Bibliogra- phy. Ayers, H., 57, 60, 181. Balfour, F. M., 40, 193, 216; phylo- genetic table of, compared, 283. Barbels, 46-48 (Figs. 55-60). Basking shark, v. Cetorhinus. Bass, striped, numerical lines of, 5 (Fig. 8). Bathyonus compressus, 168 (Fig. 172). Batrachus, eggs of, 186. Bdellostoma, gills of, 17 (Fig. 9); anatomy and general description of B.dombeyi, 57, 58 (Fig. 69 A), 59, 60 (Fig. 70), 61 (Fig. 72 A); eggs of, 180, 181 (Fig. 186); genital sys- tem, tables, 266; excretory system and urinogenital ducts, tables, 270; brain of, tables, 272 (Fig. 339) ; cen- tral nervous system, tables, 274. Bean, T. H., 103, 108, no. Beard, ]., 57, 6l, 146, 217; phylo- genetic table of, compared, 283. Berycids, antiquity of, 9. Bibliography, 231-251. Blenniids, eggs of, 185 (Figs. 198- 199), 1 86. Blenny, v. Blenniids. Blood-vessels, v. Fishes, circulation in, Heart, Chimceioids, etc. Boas, ]. E. V., phylogenetic table of, compared, 283. Bohm, A. A., 187. Bolau, H., 185. Bony fishes, v. Teleosts. Bow-fin, v. Amia calva. Brain, of Chimceroids and sharks, 114; resemblances between lung-fishes and Elasmobranchs, 128; compari- son tables of, 272 (Figs. 339-341), 273 (Figs- 342-344), 274-275. Branchial arches, table of relations of, 254 (Figs. 310-315), 256-257. Breathing, aquatic, 16-23. Breeding habits, 180-186; table of the early development of fishes, 280-281 . Brevoortia (menhaden), gills of, 2O. Bridge, T., phylogenetic tables of, compared, 283. Bulbus arteriosus, comparative tables of, 258 (Figs. 316-325), 260. Bull-head, v. Catfish. Burkhardt, R., 128; phylogenetic table of, compared, 282. Butrinus, heart, conus and bulbus ar- teriosus, 258 (Fig. 323) ; compari- son tables of, 260. Calamoichthys, swim-bladder of, 22 INDEX 287 (Fig. 17); median fins of, 31; an- tiquity of, 148; described, 150; C. calabaricus, 147, 150 (Fig. 150). Calberla, E., 187. Caldwell, W. H., 125. Callichthys, respiration of, 2O; ganoid plates of, 24 (Fig. 26), 26; origin of dermal cusps, 30; C. armatus, 172 (Fig. 178); eggs and breeding habits, 1 86. Callorhynchus, lateral line lost, 49; description of, 104, 109; mandibu- lar, 106 (Fig. no); bottle-nosed Chimcera, 109 (Fig. ilS); eggs and breeding habits of, 181 (Fig. 191), 185. Canals, v. Lateral line. Carassius auratus, 170 (Fig. 176). Carp, scales of, 26 (Fig. 31 A}; eggs of, 187. Catfish, barbels of, 46, 47 (Fig. 58) ; description of, 171, 172; Amiurus melas, 171 (Fig. 177). Cattie, J. T., 54. Caturus, 164-165; C. furcatus, 164 (Fig. 169); Mesozoic caturid, 166 (Fig. 171^). Caudal fins, 35; evolution of, 35-39 (Figs. 44-48). Central nervous system, v. Nervous system. Cephalaspis, antiquity of, 9; described, 67; C.lydli, 66 (Figs. 78, 79). Cepkaloptera, v. Dicerobatis. Ceratodus, antiquity of, 9, 10; swim- bladder of, 22 (Fig. 1 6); archip- terygial pectoral fin of, 39, 40, 42 (Fig. 54), 44, 45; description of, 123 (Fig. 127), 124; skeleton of, 123 (Fig. 128); skull of, 124 (Fig. 128.^); embryonic stages, 125; eggs and breeding habits, 181 (Fig. 192), 185; development of egg, 198-202 (Figs. 231-248) ; larva of, 218-221 (Figs. 290-295) ; skeleton of, tables, 253; jaws and branchial arches, tables, 254 (Fig. 313), 257; heart, conus and bulbus arteriosus, tables, 258 (Fig. 320) ; comparison tables of heart, etc., 260; gills, spiracle, gill rakers, and opercula, tables, 261; digestive tract, tables, 263; swim-bladder, tables, 264, 265 (Fig. 16); genital system, tables, 266; urinogenital ducts and external openings, tables, 267 (Fig. 335); excretory system and urinogenital ducts, tables, 270; abdominal pores, tables, 271. Cestracion, antiquity of, 10; jaw of, 24 (Fig. 27); caudal fin, 36, 37 (Fig. 45), 38; anatomy of, 85 (Fig. 91), 86; Port Jackson shark, 181 (Fig. 190), 183. Cestraciont, antiquity of, 9, 10; gills of, 1 6 note; anatomy of, 85, 86; dentition of, 86; affinities of, 95, 96; dental evolution, 112. Cetacean, fish-like form of, 5 (Fig. 7), 6. Cetorhinus, go (Fig. 96^). Challenger report, quoted, 87, 103. Characteristic structure of fishes, 14. Cheirodus, 157; C. granulosus, 157 (Fig. 1 60). Cheiropterygium, 39. Chilomycterus geometriciis, 175, 176 (Fig. 184). ' Chimara, sensory canals of the head, 30; lateral line of, 49, 51 note; affinities to shark, 98 (Fig. 103); anatomy of, 99-101 (Fig. 104); skeleton of, 101-103; skeleton of C. rnonstrosa, 102 (Fig. 105) ; genus, 104; mandibular, 106 (Fig. 109); palatine plate, 106 (Fig. 109 A~); clasping spine of forehead, 107 (Fig. 113); ventral fin and clasping organ, 107 (Figs. 1 1 6, 117); bottle-nosed Chimaera, 109 (Fig. 118); general description, no (Fig. 119), in (Fig. 120); dermal plates, 113 (Fig. 104) ; comparison tables of skeleton of, 253; jaws and branchial arches, tables, 254 (Fig. 312), 256; urino- genital ducts and external openings, 288 INDEX tables, 267 (Figs. 332-337); ab- dominal pores, tables, 271; brain of, 273 (Fig. 342). CHIM/EROIDS, in classification, 7, 8; antiquity of, 9, 10; gill shields, 20; affinities to shark, 96; general de- s/ri.ption of, 99-115 (Figs. 104- 120); anatomy of, 99-101 (Fig. 104); skeleton of, 101-103 (Fig. 105); embryology and larval his- tory of, 103; fossil Chimasroids, 103, 104 (Fig. 105,4); living Chi- mceroids, description of, 104-111 (Figs. 117-120); spines and clasp- ing organs, 107 (Figs. 113-116); affinities, 111-115; dental plates, in (Fig. ill); history of fossil forms, 112; dental evolution, 112; structural affinities to shark, 112- 115; divergences from elasmo- branchian structure, 113; skull and mandible of, 1 1 3 ; fins and fin spines, 113; skin defences and teeth, 113; gill arches, 114; brain of, 114; lat- eral line, 114; clasping spine, 114; descent of, 115; diphycercal tail compared with that of sharks, 115; separated from Arthrodirans, 136; eggs and breeding habits, 181 (Fig. 191), 184, 185; list of authors and works on the Chimeeroids, 244; gills, spiracle, gill rakers, and oper- cula, tables, 271; genital system, tables, 266; circulation in, tables, 269; central nervous system, tables, 275; sense organs of, tables, 277; integument and in- tegumentary sense organs, 279; early development of, tables, 280- 281. Chlainydoselache, antiquity of, 10; gill shields, 2O; lateral line, 49, 50 (Fig. 61); C. anguineus, 87 (Fig. 92); affinities to shark, etc., 96; gill arches, 114. Chondrenchelys, 78; anatomy of, 85. CHONDROSTEI, in classification, 8, 161, 162. Chondrosteus, 161, 162; C acipense- roides, 161 (Fig. 165.4). Chordates, ancestors of, 1 6 note; de- scription of, 63-65. Christiceps, eggs of, 186. Circulatory characters in Dipnoans, 129. Cladodus, teeth of, So (Fig. 86 B). Cladoselache, in classification, 8; an- tiquity of, 9; gill slits, 16; gill shields, 20; dorsal fins of, 33 (Fig. 41); caudal fin of, 36, 37 (Fig. 46), 38; pectoral and ventral fins of, 42 (Figs. 49, 50), 43-46; a primitive form of, 78; description of, 79; anatomy of, 79 (Figs. 86 and 86,4 and 86 j?) ; dentition of, 86; affini- ties of, 95, 98 (Fig. 103); gill arches, 1 14. Clark, W., 130, 133 note, Frontispiece. Clasping spine of Chimceroids, 114; absence of, in Dipnoans, 129. Claypole, E. W., 66, 67, 71, 80, 130. Climatiiis, anatomy of, 82 (Fig. 89). Clupeoid, antiquity of, 9; heart, conus and bulbus arteriosus, 258 (Fig. 320) ; heart, etc., comparison tables of, 260. Coccosteus, in classification, 8; locali- ties, 130; anatomy of C. decipiens, I3I~I33 (Figs- 130-132); dermal and ventral plates of, 132 (Figs. I3l> J32); lateral line in, 135; eyes of, 135- Cochliodonts, 86; dental evolution of, 112. Cod, barbels of, 46, 47 (Fig. 55), 171; description of Gadus morrhna, 174 (Fig. 182); circulation in, tables of, 269. Ccelacanthus, in classification, 8; dor- sal fin of, 33, 34 (Fig. 43), 43; de- scription of, 87 (Fig. 92), 153; as a Crossopterygian, 147; C. elegans, 153 (Fig. 155). Columbia College Museum, 130, 135, Frontispiece. Conus arteriosus, comparison tables of, INDEX 289 258 (Figs. 316-325), 260; v. Sharks, etc. Cope, E. D., 8, 10; phylogenetic table of, compared, 282. Cricotus, 54; parietal foramen of, 54. CROSSOPTERYGII, in classification, 8; antiquity of, 9; unpaired fins of, 33 (Fig. 43) ; affinities to shark, 96; included in the term Ganoid, 139; ancestry of, 147; a group of Teleo- stomes, 147; description of, 148- 155 (Figs. 148-156^); habits of living and breeding, 150; fossil forms, 150-155 (Figs. 151-156^); pakeozic, 166 (Fig. 171 A}. Ctenodus, in classification, 8; median foramen of, 55; affinity to Cerato- dus, 122, 124; ancestry of, 147. Ctenolabrus caruleus, larval develop- ment of, 224 (Figs. 303-309), 225. Curves of fishes, 5, 6. Cusk, barbels of, 46, 47 (Fig. 55). Cusps, v. Derm cusps. CYCLOSTOMES, in classification, 7, 8; antiquity of, 9; metamerism in, 14- 16; gills of, 18; lampreys, 57-63; their affinities, 63-65; pakeichthyic affinities, 70; eggs and breeding habits of, 180, 181 (Figs. 186, 187); fertilization of eggs, 187 note; larval development, 214, 215 (Figs. 212, 215, p. 189, and 72, p. 60) ; names of authors and works, list of, 234-238; skeleton of, tables, 252; heart, conus, and bulbus arteriosus, tables, 260; gills, spiracles, gill rakers, and oper- cula, tables, 260; digestive tract, tables, 262 (Fig. 326), 263; swim- bladder, tables, 264; genital system, tables, 266; urinogenital ducts and external openings, tables, 266, 267 (Fig. 332) ; abdominal pores, tables, 271, 272 (Fig. 340); central ner- vous system, tables, 274; sense or- gans, tables, 276; integument and integumentary sense organs, tables, 278. Cyprinodonts, eggs of, 185. u Davidoff, M., phylogenetic table of, compared, 283. Davis, J. W., 84. Dean, B., 8, 78, 128, 132. Deep-sea fishes, lateral line in, 49. Defences, v. Dermal and Teeth. Dental plate, of Sandalodus, 24 (Fig. 28), 28; of sting-ray, 24 (Fig. 29); of eagle-ray, 24 (Fig. 30), 27; of Arthrodirans, 136, 137 (Figs. 138- 144); of Dinichthys, 136-138. Denticle, v. Dermal defences. Dentine, v. Shark, skin of. Derm cusps, origin of, 30. Dermal defences of fishes, 23-30; of shark, 23, 24 (Figs. 30, 31); evolu- tion of, 24 (Figs. 24-26), 25; of Chimseroids, 113; of Coccosteus de- cipiens, 132 (Fig. 131); v. Fin spines. Dermal sense organs, v. Sensory or- gans, integumentary. Development, v. Fishes, Eggs, larval, etc. ; comparison table of early, 280, 281. Devil ray or mantis, v. Dicerobatis. Dicerobatis, 95, 96 (Fig. 102 A). Digestive tract, comparison tables of, 263 (Figs. 326-331). Dinichthys, Frontispiece; pineal fun- nel' 555 general description, 130- 138; type specimens in Columbia College Museum, 130 (Frontispiece and Figs. 133-137); fin and fin spine, 131; D. intermedius, resto- ration of by Newberry, 133 (Fig. 133 and Frontispiece); elater-joint of, 134; dermal, ventral, and pineal plates of, 133 note; dorsal plates in Columbia College Museum, 135; jaws of, 136, 137 (Figs. 138-144); in- ter movement of dental plates of, 1 38. Diphycercal-shaped fin, 35, 37 (Fig. 47)- Diplognalhus, jaw of, 136, 137 (Figs. 141-143). Diplnrus, 147, 153, 154; D.longicau- datus, 154 (Fig. 156). 290 INDEX DIPNOANS, in classification, 7, 8; an- tiquity of, 9, 10, 147; swim-bladder of, 21; affinities to shark, 96, 98 (Fig. 103); general description of, 116-129 (Figs. 121-129); structural characters and general anatomy of, 116-120 (Fig. I2i); skeleton of, 118 (Fig. 122), 119; fossil forms, 120-124 (Fig5- 123~I26); living forms, 123-127 (Figs. 127-129^); relationships, 127-129; amphibian characters of, 127, 129; kinship to sharks, 127; the advancing struc- tures of, 129; the Arthrodiran lung- fishes, 129-138 (Figs. 130-144); arthrodiran affinities, 136; eggs and breeding habits, 181 (Fig. 192), 185; larval development of, 218- 221 (Figs. 290-295); names of authors and works on, list of, 244- 246; comparison tables of skeleton, 253; skeleton of Protopterus annec- tans, 119 (Fig. 122); skull and branchial arches, table of relations of, 257; heart, conus and bulbus arteriosus, tables of, 258 (Figs. 320, 321 ); comparison tables of heart, etc., 260; digestive tract, 262 (Fig. 329) ; comparison tables of digestive tract, 263; genital system, tables, 266; urinogenital ducts and external openings, tables, 267 (Figs. 332- 337); circulation in, tables, 269; brain, 272 (Fig. 343); central ner- vous system, tables, 275; sense or- gans, tables, 277; integument and integumentary sense organs, tables, 279; early development of, compari- son tables, 280-281. Dipterzts, in classification, 8 ; antiquity of, 9; description of, 121 (Figs. 123-125), 122. Dohrn, A., 40, 63. Dolphin, fish-like form of, 6. Dorsal fin, v. Fins. Drum-fish, barbels of, 46, 47 (Fig. 56). Dugong, fish-like form of, 6. Eagle-ray (Myliobatis), dental plates of, 24 (Fig. 30), 27. Early development, v. Development. Edestus hcinrichsii, fin spine of, 28-30 (Figs. 35-38). Edinburgh Society, Transactions of, quoted, 70. Edwards, V. N., 184. Eel, movement of, 2 (Fig. 2) ; gills of, 18; median fins of, 31; description of Anguilla vulgaris, 171, 173 (Fig. 1 80). Eggs of fishes, 180-186 (Figs. 186- 199), v. Comparison tables of the early development of fishes, 280. ELASMOBRANCHII, in classification, 8, 9; antiquity of, 9; description of, 72-97 (Figs. 83-102); affinities of, 95; resemblances to lung-fishes, 128, 129; to Athrodirans, 136, v. Shark; eggs and breeding habits of, 183, 184 (Figs. 189,189^); circulation in, 268 (Fig. 338), 269; central ner- vous system, tables of, 274, 275. Elonichthys, 156; E. {Rhabdolepis) macropterus, 156 (Fig. 158). Embiotocids, eggs of, 185. Emery, C., 169, 170. Enamel of shark skin, 23, 24 (Fig. 20) ; enamel organ of shark, 23, 24 (Fig. 20). Entering angle of fishes, 5, 6. Environment, changes due to, 167- 169 (Figs. 172-174). Erythrinus, swim-bladder of, 22 (Fig. 15)- Eurynotus, 157; E. o-enatus, 156 (Fig. 159). Eusthenopteron, 151-153; E. foordi, IS2 (F'g- J54)- Evolution, of fishes, slowness of, 1 1 ; of fins, 30-46; of unpaired fins, 31- 39 (Figs. 39-43) J of paired fins, 39- 46 (Figs. 49-54). Excretory system, tables of, 270, 271 (Figs. 332-337. P- 267). Exoskeletal specializations of Dip- noans, 129. INDEX 291 Eye, v. Pineal eye. Feeling, sense of, 46-48. Fertilization phenomena, 186, 187, v. comparison tables of the early devel- opment of fishes, 280. J-ifrasfer, 169, 170; F. acits, 169 (Fig. 175)- Fins, location of, 3, 4; evolution of, 30-46 (Figs. 39-54) ; unpaired, 31- 39 (Figs. 39-43); dorsal and anal, 31-35 (Fi§s- 39-43); caudal, 35- 39 (Figs. 44-48); paired, 39-46 (Figs. 49-54); pectoral, 41-43 (Figs. 49. 5I~54); ventral, 41-43 (Fig. 50); of Chimseroids, 113; primitive dermal, 31 ; of Cladoselache, 33 (Fig. 41) ; of Calacanthns, 34 (Fig. 43) ; of Crossopterygian (Holopty chins), 33 (Fig. 43)- Fin spines, 23; description of, 28-30 (Figs. 32-38) ; of Acanthodian, 29 (Fig. 32) ; of Hybodits, 29 (Fig. 33); of sting-ray, 28, 29 (Fig. 34); of Edestus heinrichsii, 28, 29 (Figs. 35-38); of Chimseroids, 113. Fishes, defined, I; movement of, 1,2 (Figs. I, 2); type of swift swim- ming fish, 3, 4 (Fig. 3) ; balanced in water, 1,4; symmetry of, 4; nu- merical lines of, 5, 6 (Figs. 5-8) ; effect of environment of, 7; classifi- cation of, 7, 8; geological distribu- tion of, 9; importance of group, 10; permanence of, 10; evolution of, II ; generalized, 12; characteristic struc- ture of, 14-56 (Figs. 9-60); meta- merism, 14-16; aquatic breathing, gills, etc., 16-23 (Fig5- 9-!9)'» der- mal defences of, 23-30 (Figs. 20- 38) ; teeth in highly modified fishes, 28; development of, 179-225 (Figs. 186-309); embryology of, 1 79 ; eggs and breeding habits of, 180-186 (Figs. 186-199); fertilization of eggs of, 1 86, 187; development of eggs of, 187-214 (Figs. 200-283); larval development of, 213-225 (Figs. 284-309); names of authors and works, on the general subject, 231-234; skeletons, table of, 252, 253 (Figs. 69, 84, 105, 122, 146, 147, and 310-315); skull, jaw, and branchial arches, tables, 254 (Figs. 310-315); heart of, 258 (Figs. 316-325), 260; comparison tables of heart of, 260; gills, spiracles, gill rakers, and opercula, tables, 259 (Figs. 9-12), 260, 261; di- gestive tract, tables, 262 (Figs. 326- 331), 263; swim-bladder, tables, 264, 265 (Figs. 13-19); genital system, tables, 266, 267 (Figs. 332- 337) ; circulation in, tables, 268 (Fig. 338), 269; excretory system and urinogenital ducts, 270, 271 (Figs. 332-337, p. 267); abdominal pores, 271 ; brain of, 272 (Figs. 339- 341), 273 (Figs. 342-344); central nervous system, tables, 274, 275; sense organs, tables of, 276, 277; characters of integument and in- ' tegumentary sense organs, 278, 279; early development, compari- son tables of, 280, 281. Flounder, 171; description of, 174, 175; Pseudopleitronectes a inert ca- ntts, 172 (Fig. 183). Fossil forms, v. Sharks, Chimceroids, etc. Fraas, 157. Fric, 102, 119. Frilled shark, v. Chlamydoselache, etc. Fritsch, A., 42, 83. Gadoid, 9. G 'adits, v. Cod. Gage, S., 182. Ganoid plates, in sEtheolepis, 24 (Fig. 25); in Lepidosteus, 24 (Fig. 24); in Callichthys, 24 (Fig. 26). GANOIDS, in classification, 8, 148; an- tiquity of, 9; dermal plates, 24 (Fig. 25), 25; Ganoid includes the Cros- sopterygians, 139 note; the term " Ganoid " used in the popular sense to denote the Teleostomes, 139; con- 292 INDEX trasted with Teleost, 144 (Fig. 147) ; air-bladder like that of a Dipnoan, | 145; J. Miiller as to structural differ- ences between Ganoids and Tele- osts, 145; recent Ganoids, 159; Mesozoic, 166 (Fig. 171 A) ; eggs and breeding habits, 181 (Figs. 193, 194); fertilization of eggs of, 187; development of eggs of, 202-207 (Figs. 249-268) ; larval development, 211-223 (Figs. 296-302); names of authors and works on, 246-249; skeleton, tables of, 253 ; skeleton of Polypterns bichir, 144 (Fig. 147); digestive tract, tables, 262 (Figs. 326-331); urinogenital ducts and external openings, tables, 266, 267 (Figs. 332-337) ; abdominal pores, tables, 271; tables of early devel- opment, 280, 281. Ganoine, 166 note. Carman, 87, 93, 109, no. Gar-pike, v. Lepidosteus. Gegenbaur, C., 39, 40, 42, 146. Generalized fishes, defined, 12. Genital system, comparison tables of, 266 (Figs. 332-337)' 270, 271. Geological distribution of fishes, 9. Geologist, American, quoted, 80. Gill, T., no; phylogenetic table of, compared, 283. Gill rakers, 20; comparison tables of, 260. Gill shields, 20; v. Sharks, Chiiriceroids, etc. Gills, 16-23; evolution of, 18; of Amphioxus, 16; of Bdellostoma, 17 (Fig. 9); of Myxi ne, 17 (Fig. 10) ; of shark, 17 (Fig. n); of Teleost, 17 (Fig. 12); of Cyclostomes, 18; of Heptanchus, 16, 19; of mullet, 20; of Brevoortia (menhaden), 20; of Selache, 20; number of gill slits, 1 6, note; table of comparison of, 260, 261 (Figs. 9-12, p. 259). Goette, A., 189. Goldfish, 170; Carassius auratus, 170 (Fig. 176). Goode, G. B., 3, 47, 89, 90, 92, 94, 95, 103, 108, 155, 160, 162, 163, 171, Graf, A., 75, 102, 119. Greenland shark, v. Lixmargus. Guitel, F., 181. Gunn, M., 70. Gunther, A., 60, 90, 96, 103, 123, 125, 146, 162, 168, 170, 172, 178, 181; phylogenetic table of, compared, 283- Gurnard, v. Prionotus. Gyroptychius, 150, 151 (Fig. 151). Haeckel, 146; phylogenetic table, compared, 282. Hagfish, in classification, 8; v. Myxine. Harriotta, 103, 104, 108 (Fig. 117); clasping spine of, 115. Heart, v. Sharks, etc. HEMIBRANCHIATES, 176. Hemiiripterus, barbels of, 46, 47 (Fig. 57)- Heptabranchias, v. Notidamts. Heptanchus, v. Notidanus. Hertwig, O., 54, 204. Heterocercal caudal fin, 35, 37 (Figs. 45,46). HETEROSOMATA, 175. Hippocampiis, 176; H. heptagonus, 177 (Fig. 185) ; eggs and breeding habits, 1 86. Hofer, B., 24. Hoffman, 187 note. HOLOCEPHALI, v. Chimaeroids; heart, conus and bulbus arteriosus, tables, 260; digestive tract, tables, 263; swim-bladder, tables, 264. Holoplychius, in classification, 8; un- paired fins of, 33 (Fig. 33) ; ances- try of, 147; description of, 150; H. andersoni, 151 (Fig. 153). Homocercal caudal fin, 35, 37 (Fig. 48). Howes, G. B., 42; phylogenetic table of, compared, 282. Huxley, 131, 257. Hybodus, number of gill slits, 16 note; fin spines of, 28, 29 (Fig. 33). INDEX 293 Hydrolagits colliei, general anatomy of, 100 (Fig. 104), no. HYPERORAKTIA, 62. ICHTHYOMI, in classification, 8. Innes, W., 149. Integument, v. Shark, sense organs, etc. Intestine, v. Digestive tract. hchyodus, 103 (Fig. 106) ; mandibular of, 106 (Figs, ill, 112), 112. Jaekel, O., 92, 113. Janassa, 86. Jaws of fishes, 24, 27; of Port Jackson shark, 24 (Fig. 27), 27; table of relations of, 254 (Figs. 310-315), 256, 257. Journal of Morphology, quoted, 51 note, 160. Kepler, W., 130. Klaatsch, phylogenetic table of, com- pared, 282. Kner, 82. Kreft, 125. Kupffer, K. v., 187 note, 189, 222. Labrax lineatus, v. Bass. Lamargus, shagreen denticle of, 24 (Fig. 21 ), 25; described, 90 (Fig. 96 ^5) ; breeding habits of, 183 and note. Lagocephalus, description of L. lavi- gatus, 176 (184 A}. Lamna, 89, 90 (Fig. 96). Lamprey, classified, 8; metamerism in, 14-16; gills of, 17; v. Petromy- zon, Cyclostomes, etc. Lampreys, v. Cyclostomes, etc.; com- parison table of the early develop- ment of, 280, 281. Lankester, E. R., 66. Larva, v. Fishes, larval development of. Lateral line, 48-53 (Figs. 61-68) ; of Chimreroids and shark, 114; in Coc- cosieus, 135. Lepidosiren, in classification, 8; swim- bladder of, 22 (Fig. 18); account of, 125 (Fig. 129), 126; swim- bladder, tables of, 264, 265 (Fig. 18). Lepidosteus, in classification, 8; an- tiquity of, 9, 166 (Fig. 171 A); swim-bladder of, 21, 22 (Fig. 14); ganoid dermal plates of, 24, 25 (Fig. 24) ; especial interest of gar-pike in connecting the Ganoids with the Crossopterygians, 159; gar-pike, L. platystomns, described, 1 59- 1 60 (Fig. 157); eggs and breeding habits of, 181 (Fig. 193), 185; fertilization of, 187; development of egg of, 203 (Figs. 265-268), 207; heart, conus and bulbus arteriosus, 258 (Fig. 322) ; comparison tables of heart, etc., 260; gills, spiracle, gill rakers, and opercula, tables, 261 ; digestive tract, tables, 263; swim-bladder, tables, 264, 265 (Fig. 14); genital system, tables, 266; excretory sys- tem and urinogenital ducts, 271. Leptolepis, 165; L. sprattiformis, 165 (Fig. 170). Leydig, F., 51 note. Limb structure in Dipnoans, 129. List of names of authors and of their works, 231-251. List of the derivations of proper names, 227-230. LOPHOBRANCHII, 1 66 (Fig. 171 A), 178. Lung-fishes, v. Dipnoans. Lungs, v. Swim-bladder. Mackerel shark, v. Lamna. Mackerel, Spanish, movement and fins of, 2, 3 (Fig. 3); front view of, 4 (Fig. 4); lines of, 5 (Fig. 6). Macropetalichthys, eyes of, 135. Manatee, fish-like form of, 6. Mandibles of Chimaeroids, 113; articu- lation of in Dipnoans, 129. Mantis, or devil-ray, v. Dicerobatis. Marey, 2. 294 INDEX MARSIPOBRANCHS, v. Cyclostomes ; tables of the early development of, 280, 281. McClure, 182. Mechanical adaptation of the fish's form, 5, 6. Median fins, v. Fins. Megalurus, 165; M. elegalitissimus , 165 (Fig. 171). Megaptera, v. Whale; M. longii/iana, numerical lines of, 5 (Fig. 7), 61. JMenaspis, skin defences of, 113. Menhaden, v. Brevootia. Metamerism, vertebrate, of fishes, 14- 16; of lampreys, 15; of sharks, 16. Miall, L., 126. Microdon, 157; M. -wagneri, 158 (Fig. 163). Mivart, St. G., 40 Modern fishes, v. Teleostomes. Mollier, S., 39. Monk-fish, v. Rhino.. Mormyrus, 171, 172; M. oxyrhynchus, 172 (Fig. 179). Morphology, Journal of, quoted, 51 note. Mouth of fishes, v. Jaws, Teeth, etc. ; of catfish (a Teleostome), 64 note. Movement in water, I, 2 (Figs. I and 2). Mucous canal system, v. Lateral line. Miiller, Johannes, 145. Mullet, gills of, 20. Jlfni-icna, 173. Myliobatis, v. Eagle-ray. Mylostomids,\\\ classification, 8; trunk of, 136; jaws of Mylostoma varia- bilis, 136, 137 (Fig. 138). Myriacanthits, in classification, 8 ; restoration of, 104; head region of, 105 (Fig. 106); dermal plates of head and snout, 105 (Figs. 106, A and B), 113; mandibular, 106 (Fig. 107); dorsal spine, 107 (Fig. 114); dental evolution, 112; sha- green tubercles and dermal bones and plates, 105 (Fig. 106), 107 (Fig. 114), 113. Myxine, classification, 8; gills of, 17 (Fig. 10), 18; general description, of M. glutinosa, 59, 60 (Fig. 71), 61 (Fig. 72 B~); eggs of, 180-182 (Fig. 187); genital system, tables of, 266; excretory system and urino- genital ducts, 270. Myxinoid, Californian, gills of, 18; teeth of, 57; eggs of, 182 (Figs. 186 A and 187 A) ; comparison tables of the early development, 280, 281. Names, list of authors and their works, 231-251. Names, list of derivations of, 227-230. Nares, in Dipnoans, 129. Natterer, J., 125. Necturus, swim-bladder of, 21. Nervous system, central, 272 (Figs. 339-340' 273 (Figs- 342-344), 274, 275. Newberry, J. W., 78, 106, 120, 130, 131, 132, 136. Newton, 106. Nicholson, H. A., 125. Notacanthus sexspinis, 168 (Fig. 174). Notidamis, antiquity of, 9; gill slits, 1 6, 19; pectoral fin, 40-42 (Fig. 52), 44, 45; described, 87-89 (Fig. 93); affinities, 96; skull, jaws, and branchial arches of, 254 (Fig. 3")- Numerical lines of fishes, 5, 6 (Figs. 5-S). Onyckodus, in classification, 8. Operculum of Teleosts, 19; comparison tables of, 260. Ophidium, barbels of, 46, 47 (Fig. 55)- Opisthure, ill. Osteolepis, in classification, 8; descrip- tion of, 150, 151 (Fig. 152). OSTRACODERMS, classified, 8; antiquity of, 9; description of, 65-71; types of, 67; affinities of, 66 (Fig. 77), 70; list of authors and works on Ostracoderms, 238. INDEX 295 Paddle-Hsh, v. Polyodon. Paliraspis americana, 67 (Fig. 75); paired fins or spines, 71 note. Pal&daphus, median foramen, 55. Fahconiscus, in classification, 157, 158 (Fig. 164); Pakeozic, 166 (Fig. 171 A). Paheospondylus, in classification, 8; antiquity of, 9, 71 ; P. gunni, 65 (Fig- 73), 70; patosichthyic affini- ties, 70; list of authors and their works on Palaospondyhis, 238. Pander, 121, 151. Paraliparis bathybius, 168 (Fig. 172). Parexus, pectoral fin of, 42 (Fig. 51), 44- Parker, W. N., 7, 117, 127, 128. - T. J., 41, 58. Parsons, 5, 6. Perca, v. Perch. Perch, antiquity of, 9; scales of, 25 (Fig. 31 A), 26, 171; described, 174; Perca americana {=flnvia- talis?), 173 (Fig. iSi); digestive tract, tables of, 262 (Figs. 326-331). Petalodonts, 86. Petromyzon, 61 ; P. marinus, 60 (Fig. 72), 61 (Fig. D), 62; skeleton of, 58 (Fig. 69); eggs of, 180-183; eggs of P. marinus, 181 (Fig. 188); fertilization of eggs, 187; development of, 188-192; develop- ment of P. planer i, 189 (Figs. 2OO- 214); digestive tract, tables of, 262 (Fig. 326) ; genital system, tables of, 266; urinogenital ducts and ex- ternal openings, 267 (Fig. 332) ; excretory system and urinogenital ducts, 270; brain of, 272 (Fig. 340) ; central nervous system, 274. Pkaneroplettron, in classification, 8; description of, 122 (Fig. 126). Pkoctena litieata, v. Porpoise. Phylogeny, tables of, 98 (Fig. 103), 166 (Fig. 171 .4} ; comparison of the phylogenetic tables of the differ- ent authors, 282, 283. rhyllopteryx, 178. PHYSOSTU.ME, 166 (171 A). Pineal eye, 53-56. Pipe-fish, v. Syngnathus. PISCES, v. Fishes. PLECTOGNATHI, 176. Pleuracantktis, in classification, 8; gill slits, 16; a fossil shark, 78; anatomy and skeleton of, 83 (Fig. 90) ; dermal bones of head roof, 84 (Fig. 90 A}; teeth of, 84 (Fig. 90 £}; affinities of, 95, 98 (Fig. 103); anterior spine of dorsal fin, 114; tail of, 115; Coccosteus com- pared with, 131. PLEUROFrERYGii, in classification, 8. Pogonias, v. Drum-fish. Pollard, H. B., 64, 113, 132. Polyodon, barbels of, 46, 47 (Fig. 59), 48; described, 160-163; P- spatula, 162 (Fig. 1 66 .Z?) ; gills, spiracle, gill rakers, and opercula, tables of, 261. Polypterus, swim-bladder of, 21, 22 (Fig. 17) ; origin of derm cusps, 30; caudal fin of, 36, 37 (Fig. 47) ; tail of, 115; skeleton of P. bichir, 144, 147 (Fig. 147); contrasted with Teleosts, 144; P. bichir described, 148 (Fig. 148), 149 note; P. lap- radei, 149 (Fig. 149) ; fh table of phylogeny, 166 (Fig. 171^); skull and branchial arches, 254 (Fig. 314) ; table of relations of skull and branchial arches, 257; comparison tables of gills, spiracle, gill rakers, and opercula, 261; digestive tract, tables, 263; swim-bladder, tables, 264, 265 (Fig. 17) ; excretory system and urinogenital ducts, tables, 270. Porcupine-fish, v. Chilomycterus. Porpoise, striped, lines of, 5 (Fig. 5). Port Jackson shark, v. Cestracion. Powrie, 82. Prionotus, barbels of, 46, 47 (Fig. 60), 48. Pristiophorus, antiquity of, 9; descrip- tion of, 92 (Fig. 99) ; affinities of, 96-98 (Fig. 103). 296 INDEX Pristis, antiquity of, 9; description of, 91 (Figs. 98 and 98^); affinities of, 96-98 (Fig. 103). Pristiurus, larval development of, 215, 216 (Fig. 284). Protocercy, 35. Protopterus, swim-bladder of, 22 (Fig. 18); anatomy of, 116 (Fig. 121); paired fin structure, 118 (Fig. 122), 119; jaws and skull, 119 (Fig. 122 A)\ account of, 126 (Fig. 129.4); Coccosteus compared with, 131; heart, conus and bulbus arte- riosus, 285 (Fig. 325); comparison tables of heart, etc., 260 ; gills, spiracle, gill rakers, and opercula, tables, 261 ; digestive tract, tables, 262 (Fig. 329), 263; swim-bladder, tables, 264, 265 (Fig. 18); circula- tion in, tables, 269; excretory sys- tem and urinogenital ducts, 270; abdominal pores, 271; brain of, 273 (Fig. 343) ; central nervous system tables, 275. Psammodus, dentition, 86. Psephurus, 160-163; P- gladius, 162 (Fig. 166.4). Pseudopleuronectes, v. Flounder. Pteraspis, antiquity .of, 9; described, 67 (Figs. 74, 76, 77). Pterichthys, antiquity of, 9; described, 69 (Figs. 80-82) ; Arthrodira associ- ated with by Traquair, 1 30. Putnam, 182. Pycnodont, 157, 158. Rabbit-fish, v. Lagoccphalus. Rabl, C., 146; phylogenetic table of, compared, 283. Raja, v. Ray. Rat-fish, v. Chimtzra. RAY, in classification, 8; antiquity of, 9; shagreen of, 24 (Fig. 23); de- scription of, 93-95 (Figs. 100-102) ; barn-door skate (R. Icevis) , 94 (Fig. 101); affinities, 95, 96, 98 (Fig. I03); eggs and breeding habits, 181 (Fig. 189.4), 183, 184. Recent sharks, v. Sharks. Relationships, v. Affinities, under the family and species. Respiration, v. Aquatic breathing. Retzius, G., phylogenetic table of, com- pared, 282. Rhina, 91 (Fig. 97); affinities to shark, 96, 98 (Fig. 103) ; brain of, tables of, 272 (Fig. 341). Rhinobatus, antiquity of, 9; descrip- tion of, 93 (Fig. 100) ; affinities to shark, 98 (Fig. 103). Rhyncodus, mandibular of, 106 (Fig. in), in. Rtickert, J., 187. Ryder, J. A., 31, 37, 115. Salensky, W., 214 note. Salmonid, antiquity of, 9; eggs and breeding habits, 186; skull and branchial arches, table of, 254 (Fig. 3i5)i 257. Sandalodus, dental plates of, 24 (Fig. 28), 28. Scales, 23; of Teleost, 24 (Fig. 31); degeneration of, 26. Scaphaspis, 66 (Fig. 77), 67. Scaphirhynchus, 160; S. platyrhyncus, 162 (Fig. 166). Scomberomorus maculatus, 2, 3 (Fig. 3) ; front view of, 4 (Fig. 4) ; lines of, 5 (Fig. 6). Sculpin, barbels of, 46, 47 (Fig. 57). Scy Ilium, shagreen of, 24 (Fig. 22), 25, 9°; eggs of, 181 (Fig. 189), 183, 184 and note; development of egg of, 193 (Figs. 216-230); larvae of, 215, 216 (Figs. 285-287); skull, jaw, and branchial arches of, 254 (Fig. 310), 256. Sea-bass, v. Serramts. Sea-cat, v. Chim&ra and Callorhyn- chus. Sea-horse, v. Hippocampus. Sea-raven, v. Hemitripterus. Sea-robin, v. Prionotus. Seal, fish-like form of, 6. Selache, gills of, 2O. INDEX 297 SELACHII, in classification, 8. Semionotus, 157; S. kapjfi, 157 (Fig. 161). Semon, R., 125, 181, 199, 200, 219. Sense organs, characters of, 46-56; tables of, 276-277; integument and integumentary sense organs, tables of, 278, 279. Sense of feeling, 46-48. Sensory canals in head of Chimaera, 30- Sensory tubules, v. Lateral line. Serranus, eggs of, 181 (Fig. 196), 186; development of egg of S. atrarius, 208 (Figs. 269-283). Shad, v. Alosa. Shagreen denticle of shark, 23-25 (Figs. 20-22) ; of sting-ray, 24 (Fig. 23)> 25. SHARKS, movement of, 2; in classifi- cation, 7, 8; antiquity of, 9, 10, 72; gills of, 17 (Fig. n), 19; spiracle of, 19; gill shields of, 2O; skin, enamel, and dermal denticle of, 23- 26 (Figs. 20-22) ; shagreen denticle of the Greenland shark (Lcemargus), 24 (Fig. 21 ); jaw of Port Jackson shark (Cestracion), 24 (Fig. 27), 27; evolution of the dermal armour- ing, 25, 26 (Figs. 25, 26) ; unpaired fins of, 33, 34 (Figs. 39-43) ; caudal fin of, 36-39 (Figs. 45-47) : lateral line of, 49, 50 (Figs. 61, 62), 51, 76; description of, 72-98 (Figs. 83-103) ; position of, 72; general anatomy of, 73 (Fig. 83); skeleton of, 74-76 (Fig. 84) ; sub-notochordal rod in skeleton, 76 (Fig. 85); integument of, 76; brain of, 76; nasal organ, eye, and ear, 76; renal and repro- ductive system of, 76; digestive tube, viscera, 77; heart, 77; clasp- ers, 77; fossil sharks described, 77- 86 (Figs. 86-91) ; teeth of fossil, 86; recentsharks, 87-95 (Figs. 92-101); affinities of, 95-98 (Fig. 103) ; eggs and breeding habits, 181 (Figs. 189- 190), 183, 184; fertilization of eggs, 187 note; development of egg of, 194-198 (Figs. 216-230); larval de- velopment of, 215-218 (Figs. 284- 289); list of authors and their works on sharks, 238-244; comparison tables of the skeleton of, 252; skel- eton of Cestracioii galeatus, 75 (Fig. 84), 255; skull, jaws, and branchial arches, tables, 256; heart, tables, 258 (Fig. 317), 260; gills, spiracle, gill rakers, and opercula, tables, 262 (Fig. II, p. 259); swim-bladder, tables, 264; genital system, tables, 266; urinogenital ducts and exter- nal openings, 267 (Fig. 333), and tables, 270; plan of circulation in, tables, 268 (Fig. 338), 269; ab- dominal pores, tables, 271 ; brain of, 272 (Fig. 341); sense organs of, tables, 276; integument and integ- umentary sense organs, tables, 279; comparison tables of the early devel- opment of, 280, 281. Siluroicl, antiquity of, 9; affinity and phylogeny of, 147, 166 (171 A}, 171 ; South American Siluroid (Cal- lichthys armatus~], 172 (Fig. 178); eggs and breeding habits of, 181 (Fig. 195), 185, 186 and note; heart, conus and bulbus arteriosus, tables of, 258 (Fig. 318). Siphostoma, eggs and breeding habits of, 1 86. SIRENOIDEI, in classification, 8. Skates, description of, 93-95 (Figs. 100-102) v. Ray. Skeleton, v. Shark, Pleuracanthus, Chi- meeroid, Dipnoan, Ceratodus, etc. Skin defences, v. Dermal and Teeth. Skull of fishes, dermal bones of head root of Pleuracanthus, 84 (Fig. 90 A) ; of Chimaeroids, 113; resem- blances of skull of lung-fishes to Elasmobranchs, 128; of Dinichthys intermetiius, 133 (Fig. 133 and Fron- tispiece) ; table of relations of skull, jaws, and branchial arches of, 254 (Figs. 310-315), 256. 298 INDEX Smithsonian Institution, Heptanchus, 88 (Fig. 93). Solenostoma, eggs and breeding habits, 1 86. South American lung-fish, v. Lepido- s ire n. South American Siluroid, v. Callichthys. Spatularia, v. Polyodon. Specialized fishes, defined, 12. Spines, 23; v. Fin spines, Clasping spines. Spiracle of shark, 18; comparison tables of, 260. Spook-fish, v. Chimsera and Chimse- roids. Spoon-bill sturgeon, v. Polyodon. Squaloraja, in classification, 8; affini- ties of, 98 (Fig. 103) ; restoration of, 104, 105 (Fig. io6^4); mandibular of, 106 (Fig. 108) ; frontal spine of, 107 (Fig. 115); dental evolution of, 112; skin defences of, 1 1 3. Sgualus, 89 (Fig. 94). Squatina, v. Rhina. Steindachner, F., 149, 150. Sticklebacks, v. Hemibranchiates. Sting-ray, shagreen of, 24 (Fig. 23) ; dental plates of jaw, 24 (Fig. 29), 25; fin spine of, 28, 29 (Fig. 34). Stomach, v. Digestive tract. Strong, O. S., 112. Structure, characteristic, of fishes, 14. Sturgeon, v. Ac ipe nser ; spoon-bill sturgeon, v. Polyodon and Psephu- rus ; shovel-nose sturgeon, v. Sca- phirhyncus ; a Liassic sturgeon, v. Chondrosteus. Swim-bladder, hydrostatic, I, 21, 22 (Figs. 13-19); of Amia, 21, 22 (Fig. 14); of gar-pike, 21, 22 (Fig. 14) ; of Dipnoans, 21 ; of Polypterus and Calamoichthys, 21,22. (Fig. 1 7) ; of Necturus, 21; of sturgeon, 22 (Fig. 13) ; of Teleosts, 22 (Fig. 13) ; of Erythrinus, 22 (Fig. 15); of Ceratodns, 22 (Fig. 1 6); of Lepido- siren. 22 (Fig. 18); of Protopterus, 22 (Fig. 18) ; of Dipnoans, 129; compared with reptiles, birds, and mammals, 20 (Fig. 19); comparison tables, 264, 265 (Figs. 13-19). Swimming: eel, shark, mackerel, 2. Symmetry of fishes, 4. Synechodus, dentition of, 86. Syngnathus, 166 (Fig. 171 A~) ; de- scription of, 177, 178; S. acus, 178 (Fig. 185 ,-/); eggs and breeding habits of, 186. Tail, v. Caudal fins. Teeth, general, 23, 24 (Figs. 27-30) ; description and evolution of, 27, 28; of Port Jackson shark, 24 (Fig. 27), 27, 86; of highly modified fishes, 28; of Myxinoicls, 57; of Cladodus, 80 (Fig. 86 £); of Acanthodopsis, 82 (Fig. 88 A}; of Pleuracanthus, 84 (Fig. 90 B) ; of fossil sharks, 86; of Chimseroids, 113; resemblances of lung-fishes to Elasmobranchs as to teeth, 128. TELEOCEPHALI, included in Actinop- terygians, 8, 148; description and phylogeny of, 165, 166 (Fig. 171 .-/). TELEOST, antiquity of, 9, 147; gills of, 17 (Fig. 12), 19; operculum of, 19; gill rakers of, 20; swim-bladder of, 22 (Fig. 13); swim-bladder of Ery- tkrinus, 22 (Fig. 15); scales of, 24 (Fig. 31); caudal fin of, 36, 37 (Fig. 48) ; the term " Teleost " used in the popular sense to denote the modern " bony fish," 139; the perch a convenient type, 139; general anatomy of, 141-145 (Figs. 145, 146); skeleton of Perca flnviatilis, 142 (Fig. 146); relationship and descent, 145-147; description and phylogeny of, 165, 166 (Fig. 171 A); modified conditions of, 167-171; eggs and breeding habits, 181 (Figs. 196-199), 185, 186; fertilization of, 187 and note; development of egg, 207-212 (Figs. 269-283) ; larval development, 223-225 (Figs. 303- 309) ; list of authors and their INDEX 299 works, 249-251; comparison tables of the skeleton of, 253; heart, conus and bulbus arteriosus, tables, 258 (Figs. 324, 325), 260; digestive tract, tables, 262 (Fig. 331), 263; urino- genital ducts and external openings, 267 (Fig. 337), and tables, 271; circulation in, tables, 269; abdomi- nal pores, tables, 271; brain of, 273 (Fig. 344) ; central nervous system, tables, 275; comparison table of the early development of, 280,281. TELEOSTOMES, in classification, 7, 8; antiquity of, 9, 10; mouth of, 64 note; opercular apparatus of, 114; tail of, 1 15 ; affinities to Arthrodirans, 136; general description of, 139- 178 (Figs. 145-185 A); skeleton of, 141-143 (Fig. 146); visceral parts of, 143; contrasted with Ganoids, 144 ( Fig. 147) ; Teleosts and Ganoids merged into one group by Prof. Owen, 146; descent of, 146; affinities with the Dipnoans generally admitted, 146; Rabl de- rives them from a selachian stem, 146; Beard and Woodward as to their descent, 146; two principal subdivisions of, 147; phylogeny, scheme of, 165, 166 (Fig. 171 A}\ comparison tables of skeleton of, 253; table of relation of skull, jaws, and branchial arches, 257; heart, conus and bulbus arteriosus, tables, 260; gills, spiracle, gill rakers, and opercula, tables, 261 ; digestive tract, tables, 263; swim-bladder, tables, 264, 265 (Fig. 13); genital system, tables, 266; sense organs, tables, 277; integument and integu- mentary sense organs, tables, 279. Telescope-fish, v. Carassius. Terrell, J., 130. Thacher, J., 40. Thiolliere, 58. Thrasher shark, v. Alopias. Tissues, cellular elements of, in Dip- noans, 129. Titanichthys, pineal foramen of, 55, 56, 135; size and localities of, 130; lip-like mandibles of, 136; mandi- bles of T. clarki, 136, 137 (Fig. 139)- Torpedo, 95 (Fig. 102). Trachosteus, jaws of, 136, 137 (Fig. 140). Transactions of Edinburgh Society, quoted, 70. Traquair, R. H., 65, 68, 70, 71, 78, 128, 130, 132, 156, 157, 159. Trygon, dental plates of jaw of, 24 (Fig. 29); fin spine of, 28, 29 (Fig. 34)- Turner, W., 217. Undina, 147, 153; U. gulo, 154 (Fig. 156^). United States Fish Commission Re- ports, quoted, 3, 89, 90, 92, 94, 95, 155, 160, 162, 163, 171, 173-177- United States National Museum, Pro- ceedings of, quoted, 103. Urinogenital system, comparison tables of, 266, 267 (Figs. 332-337), 270, 271. Urogymnus, shagreen of, 24 (Fig. 23). Ventral plates of Coccosteus decipiens, 132 (Fig. 132). Vertebral axis of lung-fishes, resem- blance to Elasmobranchs, 128. Vienna collection, 149 note. Visceral characters, resemblance be- tween lung-fishes and Elasmo- branchs, 128; of Teleost, 143; of Ganoids, 145. Walcott, 65. Ward, H. A., 75. Whale, fish-like form of, 6. Whale, humpback, numerical lines of, 5 (Fi§- 7)- Whiteaves, 152. Whitman, C. O., 187 note. Wiedersheim, R., 40, 113. Willey, A., 16. 300 INDEX Wilson, H. V., 208. Woodward, A. S., 8, 10, 24, 25, 33, 42, 66, 68-71, 80, 81, 106, 107, 112, T2I, 127, 129, 131, 132, 135, 136, 146, 151, 154, 161, 164, 165; phylogenetic table, compared, 282. Works on the general subject, fishes, 231-234; on the Cyclostomes, 234- 238 ; on the Ostracoderms and Palceospondylus, 238; on the sharks, 238-244; on the Chimseroids, 244; on the lung-fishes, 244-246; on the Ganoids, 246-249; on the Teleosts, 249-251. Xenacanthus, pectoral fin of, 39, 40, 42 (Fig. 53), 45; v. Pleuracan- tkus. Zittel, K. v., table of geological dis- tribution of fishes, 9; quoted, Si, 82, 104, 124, 157, 158, 164, 165. Zoological Society, Proceedings of, 257 note. Columbia University Biological Series. EDITED BY HENRY FAIRFIELD OSBORN, Da Costa Professor of Biology in Columbia College, This series is founded upon a course of popular University lectures given during the winter of 1892-3, in connection with the opening of the new department of Biology in Columbia College. The lectures are in a measure consecutive in charac- ter, illustrating phases in the discovery and application of the theory of Evolution. Thus the first course outlined the de- velopment of the Descent theory; the second, the application of this theory to the problem of the ancestry of the Vertebrates, largely based upon embryological data; the third, the applica- tion of the Descent theory to the interpretation of the structure and phylogeny of the Fishes or lowest Vertebrates, chiefly based upon comparative anatomy ; the fourth, upon the problems of individual development and Inheritance, chiefly based upon the structure and functions of the cell. Since their original delivery the lectures have been carefully rewritten and illustrated so as to adapt them to the use of Col- lege and University students and of general readers. The vol- umes as at present arranged for include: I. From the Greeks to Darwin. By HENRY FAIRFIELD OSBORN. II. Amphioxus and the Ancestry of the Vertebrates. By ARTHUR WILLEY. III. Fishes, Living and Fossil. By BASHFORD DEAN. IV. The Cell in Development and Inheritance. By EDMUND B. WILSON. Two other volumes are in preparation. MACMILLAN & CO., 66 FIFTH AVENUE, NEW YORK. 1. FROM THE GREEKS TO DARWIN. THE DEVELOPMENT OF THE EVOLUTION IDEA. BY HENRY FAIRFIELD OSBORN, Sc.D, PRINCETON, Da Costa Professor of Biology in Columbia College. Ready in September. This opening volume, " From the Greeks to Darwin," is an outline of the development from the earliest times of the idea of the origin of life by evolution. It brings together in a continu- ous treatment the progress of this idea from the Greek philoso- pher Thales (640 B.C.) to Darwin and Wallace. It is based partly upon critical studies of the original authorities, partly upon the studies of Zeller, Perrier, Quatrefages, Martin, and other writers less known to English readers. This history differs from the outlines which have been pre- viously published, in attempting to establish a complete conti- nuity of thought in the growth of the various elements in the Evolution idea, and especially in the more critical and exact study of the pie-Darwinian writers, such as Buffon, Goethe, Erasmus Darwin, Treviranus, Lamarck, and St. Hilaire, about whose actual share in the establishment of the Evolution theory vague ideas are still current. TABLE OF CONTENTS I. THE ANTICIPATION AND INTERPRETATION OP NATURE. II. AMONG THE GREEKS. III. THE THEOLOGIANS AND NATURAL PHILOSOPHERS. IV. THE EVOLUTIONISTS OF THE EIGHTEENTH CENTURY. V. FROM LAMARCK TO ST. HILAIRE. VI. THE FIRST HALF-CENTURY AND DARWIN. In the opening chapter the elements and environment of the Evolution idea are discussed, and in the second chapter the re- markable parallelism between the growth of this idea in Greece and in modern times is pointed out. In the succeeding chap- ters the various periods of European thought on the subject are covered, concluding with the first half of the present century, especially with the development of the Evolution idea in the mind of Darwin. II. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES. BY ARTHUR WILLEY, B.Sc. LOND., Tutor in Biology, Columbia College ; Balfour Student of the University of Cambridge. Ready in September. The purpose of this volume is to consider the problem of the ancestry of the Vertebrates from the standpoint of the anat- omy and development of Amphioxus and other members of the group Protochordata. The work opens with an Introduction, in which is given a brief historical sketch of the speculations of the celebrated anatomists and embryologists, from Etienne Geoft'roy St. Hilaire down to our own day, upon this problem. The remainder of the first and the whole of the second chapter is devoted to a detailed account of the anatomy of Amphioxus as compared with that of higher Vertebrates. The third chapter deals with the embryonic and larval development of Amphioxus, while the fourth deals more briefly with the anatomy, embryology, and relationships of the Ascidiaus; then the other allied forms, Balanoglossus, Cephalodiscus, are described. The work concludes with a series of discussions touch- ing the problem proposed in the Introduction,. in which it is attempted to define certain general principles of Evolution by which the descent of the Vertebrates from Invertebrate ancestors may be supposed to have taken place. The work contains an extensive bibliography, full notes, and 135 illustrations. TABLE OF CONTENTS. INTRODUCTION. CHAPTER I. ANATOMY OF AMPHIOXUS. II. Ditto. III. DEVELOPMENT OF AMPHIOXUS. IV. THE ASCIDIANS. V. THE PROTOCHORDATA IN THEIR KELATION TO THE PROBLEM OF VERTEBRATE DESCENT. III. FISHES, LIVING AND FOSSIL. AN INTRODUCTORY STUDY. BY BASHFORD DEAN, PH.D. COLUMBIA, Instructor in Biology, Columbia College. This work has been prepared to meet the needs of the gen- eral student for a concise knowledge of the Fishes. It contains a review of the four larger groups of the strictly fishlike forms, Sharks, Chimaeroids, Teleostomes, and the Dipnoaus, and adds to this a chapter on the Lampreys. It presents in figures the prominent members, living and fossil, of each group; illustrates characteristic structures; adds notes upon the important phases of development, and formulates the views of investigators as to relationships and descent. The recent contributions to the knowledge of extinct Fishes are taken into special account in the treatment of the entire subject, and restorations have been attempted, as of Diuichthys, Ctenodus, and Cladoselache. The writer has also indicated diagram matically, as far as generally accepted, the genetic relationships of fossil and living forms. The aim of the book has been mainly to furnish the student with a well-marked ground-plan of Ichthyology, to enable him to better understand special works, such as those of Smith Wood- ward and Grimther. The work is fully illustrated, mainly from the writer's original pen-drawings. TABLE OF CONTENTS. CHAPTER I. FISHES. Their Essential Characters. Sharks, Chimaeroids, Teleo- stomes, and Lung-fishes. Their Appearance in Time and their Distribution. II. THE LAMPREYS. Their Position with Reference to Fishes. Bdel- lostoma, Myxine, Petromyzon, Palaeospondylus. III. THE SHAHK GROUP. Anatomical Characters. Its Extinct Members, Acauthodian, Cladoselachid, Xenacanthid, Cestracionts. IV. CHIMAEROIDS. Structures of Callorhyuchus and Chimaera. Squalo- raja and Myriacanthus. Life-habits and Probable Relationships. V. TELEOSTOMES. The Forms of Recent " Ganoids." Habits and Dis- tribution. The Relations of Prominent Extinct Forms. Crosso- pterygiaus. Typical " Bony Fishes. " VI. THE EVOLUTION OF THE GROUPS OF FISHES. Aquatic Metamerism. Numerical Lines. Evolution of Gill-cleft Characters, Paired and Unpaired Fins, Aquatic Sense-organs. VII. THE DEVELOPMENT OF FISHES. Prominent Features in Embryonic and Larval Development of Members of each Group. Summaries.