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ICHTHYOLOGY (from Gr. LxObs, fish, and Xoyos, doctrine or treatise) , the branch of zoology which treats of the internal and external structure of fishes, their mode of life, and their distribution in space and time. According to the views now generally adopted, all those vertebrate animals are referred to the class of fishes which combine the following characteristics: they live in water, and by means of gills or branchiae breathe air dissolved in water; the heart consists of a single ventricle and single atrium; the limbs, if present, are modified into fins, supplemented by unpaired median fins; and the skin is either naked or covered with scales or with osseous plates or bucklers. With few exceptions fishes are oviparous. There are, however, not a few members of this class which show a modification of one or more of these characteristics, and which, nevertheless, cannot be separated from it.I. HISTORY AND LITERATURE DOWN TO 1880 The commencement of the history of ichthyology coincides with that of zoology generally. Aristotle (384322 B.C.) had a perfect knowledge of the general structure of fishes, which he clearly discriminates both from the aquatic animals with lungs and mammae, i.e. Cetaceans, and from the various groups of aquatic invertebrates. According to him: " the special characteristics of the true fishes consist in the branchiae and fins, the majority having four fins, but those of an elongate form, as the eels, having two only. Some, as the Muraena, lack the fins altogether. The rays swim with their whole body, which is spread out. The branchiae are sometimes furnished with an operculum, sometimes they are without one, as in the cartilaginous fishes. . . . No fish has hairs or feathers; most are covered with scales, but some have only a rough or a smooth skin. The tongue is hard, often toothed, and sometimes so much adherent that it seems to be wanting. The eyes have no lids, nor are any ears or nostrils visible, for what takes the place of nostrils is a blind cavity; nevertheless they have the senses of tasting, smelling and hearing. All have blood. All scaly fishes are oviparous, but the cartilaginous fishes (with the exception of the sea-devil, which Aristotle places along with them) are viviparous. All have a heart, liver and gall-bladder; but kidneys and urinary bladder are absent. They vary much in the structure of their intestines: for, whilst the mullet has ' a fleshy stomach like a bird, others have no stomachic dilatation. Pyloric caeca are close to the stomach, and vary in number; there are even some, like the majority of the cartilaginous fishes, which have none whatever. Two bodies are situated along the spine, which have the function of testicles; they open towards the vent, and are much enlarged in the spawning season. The scales become harder with age. Not being provided with lungs, fishes have no voice, but several can emit grunting sounds. They sleep like other animals. In most cases the females exceed the males in size; and in the rays and sharks the male is distinguished by an appendage on each side of the vent."Aristotle's information on the habits of fishes, their migrations, node and time of propagation, and economic uses is, so far as it has been tested, surprisingly correct. Unfortunately, we too often lack the means of recognizing the species of which he gives a description. His ideas of specific distinction were as vague as those of the fishermen whose nomenclature he adopted; it never occurred to him that vernacular names are subject to change, or maybe entirely lost in course of time, and the difficulty of identifying his species is further increased by the circumstance that sometimes several popular names are applied by him to the same fish, or different stages of growth are designated by distinct names. The number of fishes known to Aristotle seems to have been about one hundred and fifteen, all of which are inhabitants of the Aegean Sea. That one man should have laid so sure a basis for future progress in zoology is less surprising than that for about eighteen centuries a science which seemed to offer particular attractions to men gifted with power of observation was no further advanced. Yet such is the case. Aristotle's successors remained satisfied to be his copiers or commentators, and to collect fabulous stories or vague notions. With few exceptions (such as Ausonius, who wrote a small poem, in which he describes from his own observations the fishes of the Moselle) authors abstained from original research; and it was not until about the middle of the 16th century that ichthyology made a new step in advance by the appearance of Belon, Rondelet and Salviani, who almost simultaneously published their great works, by which the idea of species was established. P. Belon travelled in the countries bordering on the eastern part of the Mediterranean in the years 15471550; he collected Belon. rich stores of positive knowledge, which he embodied in several works. The one most important for the progress of ichthyology is that entitled De aquatilibus libri duo (Paris, 1JJ3). Belon knew about one hundred and ten fishes, of which he gives rude but generally recognizable figures. Although Belon rarely gives definitions of the terms used by him, it is not generally very difficult to ascertain the limits which he intended to assign to each division of aquatic animals. He very properly divides them into such as are provided with blood and those without ittwo divisions corresponding in modern language to vertebrate and invertebrate aquatic animals. The former are classified by him according to size, the further sub-divisions being based on the structure of the skeleton, mode of propagation, number of limbs, form of the body and physical character of the habitat. The work of the Roman ichthyologist H. Salviani (15141572), bears evidence of the high social position which the author Sealant. held as physician to three popes. Its title is Aquatilium animalium historia (Rome, 15541557, fol.). It treats exclusively of the fishes of Italy. Ninety-two species are figured on seventy-six plates, which, as regards artistic execution, are masterpieces of that period, although those specific characteristics which nowadays constitute the value of a zoological drawing were overlooked by the author or artist. No attempt is made at a natural classification, but the allied forms are generally placed in close proximity. The descriptions are equal to those given by Belon, entering much into the details of the economy and uses of the several species, and were evidently composed with the view of collecting in a readable form all that might prove of interest to the class of society in which the author moved. Salviani's work is of a high order. It could not fail to render ichthyology popular in the country to the fauna of which it was devoted, but it was not fitted to advance ichthyology as a science generally; in this respect Salviani is not to he compared with Rondelet or Belon. G. Rondelet (15071557) had the great advantage over Belon of having received a medical education at Paris, and especially Rondelet. of having gone through a complete.course of instruction in anatomy as a pupil cf Guentherus of Andernach. This is conspicuous throughout his worksLibri de piscibus ntarinis (Lyons, 1554); and Universac aqualilium historiae pars allera (Lyons, 1555). Nevertheless they cannot be regarded as more than considerably enlarged editions of Belon's work. For, although he worked independently of the latter, the system adopted by him is characterized by the same absence of the true principles of classification. His work is almost entirely limited to European and chiefly to Mediterranean forms, and comprises no fewer than one hundred and ninety-seven marine and forty-seven fresh-water fishes. His descriptions are more complete and his figures much more accurate than those of Belon; and the specific account is preceded by introductory chapters, in which he treats in a general manner of the distinctions, the external and internal parts, and the economy of fishes. Like Belon, he had no conception of the various categories of classificationconfounding throughout his work the terms " genus " and " species," but he had an intuitive notion of what his successors called a " species," and his principal object was to give as much information as possible regarding such species. For nearly a century the works of Belon and Rondelet continued to be the standard works on ichthyology; but the science did not remain stationary during that period. The attention of naturalists was now directed to the fauna of foreign countries, especially of the Spanish and Dutch possessions in the New World; and in Europe the establishment of anatomical schools and academies led to careful investigation of the internal anatomy of the most remarkable European forms. Limited as these efforts were as to their scope, they were sufficiently numerous to enlarge the views of naturalists, and to destroy that fatal dependence on preceding authorities which had kept in bonds even Rondelet and Belon. The most noteworthy of those engaged in these inquiries in tropical countries were W. Piso and G. Marcgrave, who accompanied as physicians the Dutch governor, Count Maurice of Nassau, to Brazil (16301644). Of the men who left records of their anatomical researches, we may mention Borelli (16081679), who wrote a work De motu animalium (Rome, 168o, 4t0), in which he explained the mechanism of swimming and the function of the air-bladder; M. Malpighi (16281694), who examined the optic nerve of the sword-fish; the celebrated J. Swammerdam (1637168o), who described the intestines of numerous fishes; and J. Duverney (16481730), who investigated in detail the organs of respiration. A new era in the history of ichthyology commences with Ray, Willughby and Artedi, who were the first to recognize the true principles by which the natural affinities of animals should be determined. Their labours stand in so intimate a connexion with each other that they represent but one great step in the progress of this science. J. Ray (16281705) was the friend and guide of F. Willughby (16351672). They found that a thorough reform in the method of treating the vegetable and animal kingdoms had become necessary; that the only way of bringing wi and order into the existing chaos was by arranging the lugbby. various forms according to their structure. They therefore substituted facts for speculation, and one of the first results of this change, perhaps the most important, was that, having recognized "species" as such, they defined the term and fixed it as the starting-point of all sound zoological knowledge. Although they had divided their work so that Ray attended to the plants principally, and Willughby to the animals, the Historia piscium (Oxf., 1686), which bears Willughby's name on the title-page and was edited by Ray, is their joint production. A great part of the observations contained in it were collected during the journeys they made together in Great Britain and in the various countries of Europe. By the definition of fishes as animals with blood, breathing by gills, provided with a single ventricle of the heart, and either covered with scales or naked, the Cetaceans are excluded. The fishes proper are arranged primarily according to the cartilaginous or the osseous nature of the skeleton, and then subdivided according to the general form of the body, the presence or the absence of ventral fins, the soft or the vinous structure of the dorsal rays, the number of dorsal fins, &c. No fewer than four hundred and twenty species arc thus arranged and described, of which about one hundred and eighty were known to the authors from personal examinationa comparatively small proportion, but descriptions and figures still formed in great measure the substitute for our modern collections and museums. With the increasing accumulation of forms, the want of a fixed nomenclature had become more and more felt. Peter Artedi ('705-1734) would have been a great ichthyologist if Ray or Willughby had not preceded him. But he was fully Arteat conscious of the fact that both had prepared the way for him, and therefore he did not fail to reap every possible advantage from their labours. His work, edited by Linnaeus, is divided as follows: (x) In the Bibliotheca ichthyologica Artedi gives a very complete list of all preceding authors. who had writtenon fishes, with a critical analysis of their works. (2) The Philosophia ichthyologica is devoted to a description of the external and internal parts of fishes; Artedi fixes a precise terminology for all the various modifications of the organs, distinguishing between those characters which determine a genus and such as indicate a species or merely a variety; in fact he establishes the method and principles which subsequently have guided every systematic ichthyologist. (3) The Genera pescsum contains well-defined diagnoses of forty-five genera, for which he has fixed an unchangeable nomenclature. (4) In the Species pisoium descriptions of seventy-two species, examined by himself, are_ givendescriptions which even now are models of exactitude and method. (5) Finally, in the Synonymia psscium references to all previous authors are arranged for every species, very much in the manner which is adopted'in the systematic works of the present day. Artedi has been justly called the father of ichthyology. So admirable was his treatment of 'the subject, that even Linnaeus Linnaeus. could only modify and add to it. Indeed, so far as ichthyology is concerned, Linnaeus has scarcely done anything beyond applying binominal terms to the species properly described and classified by Artedi. His classification of the genera appears in the 12th edition of the Systema thus: A, Amphibia nantia.Spiraculis compositis.-Petromyzon, Raia, Squalus, Chimaera. Spiraculis solitaries: Lophius, Acipenser, Cyclopterus, Baiistes, Ostracion, Tetrodon, Diodon, Centriscus, Syngnathus. Pegasus. B. Pisces aodes.-Muraena, Gymnotus, Trichiurus, Anarrhichas, Ammodytes, Ophidium, Stromateus, Xiphias. C. Pisces jugulares.Callionymus, Uranoscopus, Trachinus, Gadus, Blennius. D. Pisces taoracici.-Cepola, Echeneis, Coryphaena, Gobi's, Cottus, Scorpaena,Zeus, Pleuronectes, Chaetodon, Spares, Labials, Sciaena, Perca, Gasterosteus, Scomber, Mullus, Trigla. E.. Pisces abdominales.Cobitis, Amia, Silures, Teuthis, Lori caria, Salmo, Fistularia, Esox, Elops, Argentina, Atherina, Mugil, Mormyrus, Exocoetus, Polynemus, Clupea, Cyprinus. Two, contemporaries of Linnaeus, L. T. Gronow and J. T. Klein, attempted a systematic arrangement of fishes. The works of Artedi and Linnaeus led town activity of research, especially in Scandinavia, Holland, Germany and England, such as has never been equalled in the history of biological science. Whilst some of the pupils and followers of Linnaeus devoted themselves to the examination and study of the fauna of their native countries,others proceeded on voyages of discovery to foreign and distant lands. Of these latter the following may be especially mentioned: O. Fabricius worked out the fauna of Greenland; Peter Kalm collected in North America,. F. Hasselquist in Egypt and Palestine, M. T. Brunnich in the Mediterranean, Osbeck in Java and China, K. P. Thunberg in Japan; Forskal examined and described the fishes of the Red Sea; G. W. Steller, P. S. Pallas, S. G. Gmelin, and A. J. Guldenstadt traversed nearly the whole of the Russian empire in Europe and Asia. Others attached themselves as naturalists to celebrated navigators, such as the two Forsters (father and son) and Solander, who accompanied Cook; P. Commerson, who travelled with Bougainville; and Pierre Sonnerat. Of those who studied the fishes of their native countries, the most celebrated were Pennant (Great Britain), O. F. Muller (Denmark), Duhamel du Monceau (France), C. von Meidinger (Austria), J. Cornide (Spain), and A. Parra (Cuba). The mass of materials brought together was so great that, not long after the death of Linnaeus, the necessity made itself felt for collecting them in a compendious form. Several compilers undertook this task; they embodied the recent discoveries in new editions of the classical works of Artedi and Linnaeus, but,they only succeeded in burying those noble monuments under a chaotic mass of rubbish. For ichthyology it was fortunate that two men at least, Bloch and Lacepede, made it a subject of prolonged original research. Mark Eliezer Bloch (1723-1799), a physician of Berlin, had reached the age of fifty-six when he began to write on ichthyological subjects. His work consists of two divisions:- Btoch (,1) Oconomische Naturgeschichte der Fische Deutsch- lends. (Berl., 1782-1784); (2) Naturgeschichte der ausltindischen Fische (Berl., 1785-1795). The first division, which is devoted to a description of the fishes of Germany, is entirely original. His descriptions as well as figures were made from nature, and are, with few exceptions, still serviceable; indeed many continue to be the best existing in literature. Bloch was less fortunate, and is much less trustworthy, in his natural history of foreign fishes. For many of the species he had to trust to more or less incorrect drawings and descriptions by travellers; frequently, also, he was deceivedas to the origin of specimens which he purchased. Hence his accounts contain numerous errors, which it would have been difficult to correct had not nearly the whole of the materials on which his work is based been preserved in the collections at Berlin. After the completion of his great work Bloch prepared a general system of fishes, in which he arranged not only, those previously described, but also those with which he had afterwards become acquainted. The work was ably edited and published after Bloch's death by a; philologist, J. G. Schneider, under the title M. E. Blochii Systema ichthyologiae . iconibus ex. illustratum (Berl., i8ox). The number of species enumerated amounts to 1519. The system is based upon the number of the fins, the various orders being termed Hendecapterygii, Decapterygii, &c. An artificial method like this led to the most unnatural combinations and distinctions. Bloch's Naturgeschichte remained for many years the standard work. But as regards originality of thought Bloch was far surpassed by his contemporary, B. G. E. de Lacepede, born at Agen,in France, in 1756, who became professor at the museum of natural history in Paris, where he died in 1825. Lacepede had to contend with great difficulties in the preparations of his Histoire des poissons (Paris, 1798-1803, 5 vols.), which was written during the most disturbed period Lacepede. of the French Revolution. A great part of it was composed whilst the author was separated from collections and books, and had to rely on his notes and manuscripts only. Even the works of Bloch and other contemporaneous authors remained unknown or inaccessible to him for a long time. His work, therefore, abounds in the kind of errors into which a compiler is liable to fall. Thus the influence of Lacepede on the progress of ichthyology was vastly less than that of his fellow-labourer; and the labour laid on his successors in correcting numerous errors probably outweighed the assistance which they derived from his work. The work of the principal students of ichthyology in the period between Ray and Lacepede was chiefly systematizing and describing; but the internal organization of fishes also received attention from more than one great anatomist. Albrecht von Haller, Peter Camper
E. Bancroft, John Walsh, and still more exactly by J. Hunter. The mystery of the propagation of the eel called forth a large number of essays, and even the artificial propagation of Sal monidae was known and practised by J. G. Gleditsch(1.764). Bloch and Lacepede's works were almost immediately succeeded by the labours of Cuvier, but his early publications were tentative, preliminary and fragmentary, so that some little time elapsed before the spirit infused into ichthyology by this great anatomist could exercise its influence on all the workers in this field. The Descriptions and Figures of Two Hundred Fishes collected at Vieagapatam on the Coast of Coromandel (Lend., 1803, 2 vols.) by Patrick Russel, and An Account of the Fishes found in the River Ganges and its Branches (Edin., 1822, 2 vols.) by F. Hamilton (formerly Buchanan), were works distinguished by greater accuracy of the drawings (especially the latter) than was ever attained before. A Natural History of British Fishes was published by E. Donovan (Lend., 1802-1808) ; and the Mediterranean fauna formed the study of the lifetime of A. Risso, Ichthyologie de Nice (Paris, 181o); and Histoire naturelle de l'Europe meridionale (Paris, 1827). A slight beginning in the description of the fishes of the United States was made by Samuel Latham Mitchell (1764-1831), who published, besides various papers, a Memoir on the Ichthyolagy.of New York, in 1815. G. Cuvier (1769-1832) devoted himself to the study of fishes with particular predilection. The investigation of their anatomy, Gtvter. and especially of their skeleton, was continued until he had succeeded in completing so perfect a frame-work of the system of the whole class that his immediate successors required only to fill up those details for which their master had had no leisure. He ascertained the natural affinities of the infinite variety of farms, and accurately defined the divisions, orders, families and genera of the class, as they appear in the various editions of the Regne Animal. His industry equalled his genius; he formed connections with almost every accessible part of the globe; and for many years the museum of the Jardin des Plantes was- the centre where all ichthyological treasures were deposited. Thus Cuvier brought together a collection which, as it contains all the materials on which his labours were based, must still be considered as the most important. Soon after the year r82o, Cuvier, assisted vatea_ by one of his pupils, A. Valenciennes, commenced clennes. his great work on fishes, Historic naturelle des Poissons, of which the first volume appeared in 1828. After Cuvier's death in 1832 the work was left entirely in the hands of Valenciennes, whose energy and interest gradually slackened, rising to their former pitch in some parts only, as, for instance, in the treatise, on the herring. He left the work unfinished with the twenty-second volume (1848), which treats of the Salmonoids. Yet, incomplete as it is, it is indispensable to the student. The system finally adopted by Cuvier is the following: A. POISSONS OSSEUX. I. A BRANCHIES EN PEIGNES OU EN LAMES. I. A Mdchoire Superieure Libre. a. Acanthopteryglens. Percoides. Sparoides. Branchies labyrinthiques. Polynemes. Chetodonoides. Lophioides. Mulles. Scomberoides. Gobioides. Joues cuirassees Muges. Labroides. Scienoides. b. Malacopterygiens. Abdominaux. Subbrachiens. A podes. Cyprinoides. Gadoides. Murenoides. Siluroides. Pleuronectes. Salmonoides. Discoboles. Clupeoides. Lucioides. 2. A Mdchoire, Superieure Fixee. Selerodermes. Gymnodontes. II. A BRANCHIES EN FORME DE HOUPPES. Lophobranches. B. CARTILAGINEUX OU CHONDROPTERYGIENS. Sturioniens. Plagiostomes. Cyclostomes. We have only to compare this system with that of Linnaeus if we wish to measure the gigantic stride made by ichthyology during the intervening period of seventy years. The various characters employed for classification have been examined throughout the whole class, and their relative importance has been duly weighed and understood. The important category of " family " appears now in Cuvier's system fully established as intermediate between genus and order. Important changes in Cuvier's system have been made and proposed by his successors, but in the main it is still that of the present day. Cuvier had extended his researches beyond the living forms, into the field of palaeontology; he was the first to observe the close resemblance of the scales of the fossil Palaeoniscus to thoseof the living Polypterus and Lepidosteus, the prolongation and identity of structure of the upper caudal lobe in Palaeoniscus and the sturgeons, the presence of peculiar " fulcra " on the anterior margin of the dorsal fin in Palaeoniscus and Lepidosteus, and inferred from these facts that the fossil genus was allied either to the sturgeons or to Lepidosteus. But it did not occur to him that there was a close relationship between those recent fishes. Lepidosteus and, with it, the fossil genus remained in. his system a member of the order of.Malacopterygii abdominales. It was left to L. Agassiz (1807-1873) to point out the importance of the structure of the scales as a characteristic, and to open a path towards the knowledge of a whole new subclass Agassiz. of fishes, the Ganoidei. Impressed with the fact that the peculiar scales of Polypterus and Lepidosteus are common to all fossil osseous fishes down to the Chalk, he takes the structure of the scales generally as the base for an ichthyological system, and distinguishes four orders: 1. Plaeoids.Without scales proper, but with scales of enamel, sometimes large, sometimes small, and reduced to mere points (Rays, Sharks and Cyclostomi,,with the fossil Hybodontes). 2. Ganoids. With angular bony scales, covered with a thick stratum of enamel: to this order belong the fossil Lepidoides, Sauroides, Pycnodontes and Coelacanthi; the recent Polypterus, Lepidosteus, Sclerodermi, Gymnodontes, Lophobranches and Siluraides; also the Sturgeons. 3. Ctenoids.With rough scales, which have their free margins denticulated: Chaetodontidae, Pleuronectidae, Percidae, Poly-acanthi, Sciaenidae, Sparidae, Scorpaenidae, Aulostomi. 4. Cycloids. With smooth scales, the hind margin of which lacks denticulation: Labridae, Mugilidae, Scombridae, Gadoidei, Gobiidae, Muraenidae, Lucioidei, Salmonidae, Clupeidae, Cyprinidae. If Agassiz had had an opportunity of acquiring a more extensive and intimate knowledge of existing fishes before his energies were absorbed in the study of fossil remains, he would doubtless have recognized the artificial character of his classification. The distinctions between cycloid and ctenoid scales, between placoid and ganoid fishes, are vague, and can hardly be maintained. So far as the living and post-Cretacean forms are concerned, he abandoned the vantage-ground gained by Cuvier; and therefore his system could never supersede that of his predecessor, and finally shared the fate of every classification based on the modifications of one organ only. But Agassiz opened an immense new field of research by his study of the infinite variety of fossil forms. In his principal work, Recherches sur les poissons fossiles, Neuchatel, 1833-1843, 4to, atlas in fol., he placed them before the world arranged in a methodical manner, with excellent descriptions and illustrations. His power of discernment and penetration in determining even the most fragmentary remains is astonishing; and, if his order of Ganoids is an assemblage of forms very different from what is now understood by that term, he was the first who recognized that such an order of fishes exists. The discoverer of the Ganoidei was succeeded by their explorer Johannes Muller (18or-i858). In his classical memoir Ober den Bau and die Grenzen der Ganoiden (Berl., 1846) he showed that the Ganoids differ from all the other osseous fishes, and agree with the Plagiostomes, in the structure of the heart. By this primary character, all heterogeneous elements, as Siluroids, Osteoglossidae, &c., were eliminated from the order as understood. by Agassiz. On the other hand, he did not recognize the affinity of Lepidosiren to the Ganoids, but established for it a distinct subclass, Dipnoi, which he placed at the opposite end of the system. By his researches into the anatomy of the lampreys and Amphioxus, their typical distinctness from other caftilaginous fishes was proved; they became the types of two other subclasses, Cyclostomi and Leptocardii. Muller proposed several other modifications of the Cuvierian system; and, although all cannot be maintained as the most natural arrangements, yet his researches have given us a much more complete knowledge of the organization of the 'teleostean fishes, and later inquiries have shown that, on the whole, the combinations proposed by him' require only some further modification and another definition to render them perfectly natural. The discovery (in the year 1871) of a living representative of a genus hitherto believed to be long extinct, Ceratodus, threw a new light on the affinities of fishes. The writer of the present article, who had the good fortune to examine this fish, was enabled to show that, on the one hand, it was a form most closely allied to Lepidosiren, and, on the other, that it could not be separated from the Ganoid fishes, and therefore that Lepidosiren also was a Ganoid,a relation already indicated by Huxley in a previous paper on " Devonian Fishes." Having followed the development of the ichthyological system down to this period, we now enumerate the most important contributions to ichthyology which appeared contemporaneously with or subsequently to, the publication of the great work of Cuvier and Valenciennes. For the sake of convenience we may arrange these works under two heads. I. VOYAGES, CONTAINING GENERAL ACCOUNTS OF ZOOLOGICAL COLLECTIONS A. French.I. Voyage autour du monde snr lei corvettes de S. M. 1' Uranie et la Physicienne, sous k cornmandement de M. Freycinet, ZoologiePoissons," par Quoy et Gaimard (Paris,, 1824). 2. Voyage de la Coquille, " Zoologie," par Lesson (Paris, 1826-183o). 3. Voyage de l'Astrolabe, sous le commandment de M. J. Dumont d' Urville, " Poissons," par Quoy et Gaimard (Paris, 1834). 4. Voyage au Pole Sud par M. J. Dumont d' Urville, " Poissons,'' par Hombron et Jacquinot (Paris, 1853-1854). B. English.I. Voyage of H.M.S. Sulphur, " Fishes," by J. Richardson (Lond., 1844-1845). 2. Voyage of H.M.SS. Erebus and Terror, " Fishes," by J. Richardson (Lond., 1846). 3. Voyage of H.M.S. Beagle, " Fishes, by L. Jenyns (Lond., 1842). C. German.r. Reise der osterreichischen ' F'regatte Novara, " Fische," von R. Kner (Vienna, 1865). II. FAUNAE A. Great Britain.1. R. Parnell, The Natural History of the Fishes of the Firth of Forth (Edin.,, 18338). 2. W. Yarrell, 'A History of British Fishes (3rd ed., 'Lond., 1859). 3. J. Couch, History of the Fishes of the British Islands (Lond., 1862--1865). B. Denmark and Scandinavia.I. H. Kroyer, Danmark's Fiske (Copenhagen, 1838-1853). 2. S. Nilsson, Skandinavisk Fauna, vol. iv. " Fiskarna " (Lund, 1855). 3. Fries och Ekstroin, Skandinaviens Fiskar (Stockh., 1836).C. Russia.i. Nordmann, " Ichthyologie pontique," in Demidoff's Voyage dans la Russie meridionale, tome iii. (Paris, 1840). D. Germanyi. Heckel and Kner, Die Susswasserfische der Osterreichischen Monarchie (Leipz., t858). " 2. C.' T. E. Siebold, Die Siisswasserfische von Mitteleuropa (Leipz., 1863).' E. Italy and Mediterranean.r. Bonaparte, Iconografia della fauna italica, tom iii., " Pesci " (Rome, 1832-1841). 2. Costa, Fauna del regno di Napoli, " Pesci " (Naples, about 185o). F. France.I. E. Blanchard, Les Poissons des eaux douces de la France (Paris, 1866). G. Spanish Peninsula.The fresh-water fish fauna of Spain and Portugal was almost unknown, until F. Steindachner paid some visits to those countries for the purpose of exploring the principal rivers. His discoveries are described in several papers in the Sitzungsberichte der Akademie zu Wien. B. du Bocage and F. de B. Cappello made contributions to our knowledge of the marine fishes on the coast of Portugal (Jorn. Scienc. Acad. Lisb.). H. North America.I. J. Richardson, Fauna Bareali-Americana, part iii., " Fishes " (Lon 1836). 'The species described in this work are nearly all from the British possessions in the north. 2. Dekay, Zoology of New York, part iv., Fishes " (New York, 1842). 3. Reports of the U.S. Comm. of Fish and Fisheries (5 vols., Washing-ton, 1873-1879) and Reports and special publications of the U.S. Bureau of Fisheries contain valuable information. Numerous descriptions of North American freshwater fishes have been published in the reports of the various U.S. Government expeditions, and in North American scientific journals, by D. H. Storer, S. F. Baird, C. Girard, W. O. Ayres, E. D. Cope, D. S. Jordan, G. Brown Goode, &c. I. Japan.i. Fauna Japonica, " Poissons," par H Schlegel, (Leiden, 185o). J. East Indies; Tropical parts of the Indian and Pacific Oceans.i. E. Ruppell, Atlas zu der Reise im nordlichen Afrika (Frankf., 1828). 2. E. Ruppell, Neue Wirbelthiete, " Fische (Frankf., 1837). 3. R. L. Playfair and A. Gunther, The Fishes of Zanzibar (Lond., 1876). 4. C. B. Klunzinger, Synopsis der Fische des Rothen Meers (Vienna, 187o-1871). 5. F. Day, The Fishes of India (Lond., 1865, 4to) contains an account of the fresh-water and marine species. 6. A. Gunther, Die Fische der Sildsee (Hamburg, 4to), from 1873 (in progress). 7. Unsurpassed in activity, as regards the exploration of the fish fauna of the East Indian archipelago, is P. Bleeker (1819-1878), a surgeon in the service of the Dutch East Indian Government, who, from the year 1840, for nearly thirty years, amassed immense collections of the fishes of the various islands, and described them in extremely numerous papers, published chieflyin the journals of the Batavian Society. Soon after his return to Europe (186o) Bleeker commenced to collect the final results of his labours in a grand work, illustrated by coloured plates, Atlas ichthyologique des Indes Orientales Neerlandaises (Amsterd., fol., 1862), the publication of which was interrupted by the author's death in 1878. K. Africa.i. A. Gunther, " The Fishes of the Nile," in Petherick's Travels in Central Africa (Lond., 1869). 2. W. Peters, Naturwissenschaftliche Reise stitch Mossambique, iv., " Flussfische " (Berl., 1868, 4t0). L. West Indies and South America.--I. L. Agassiz, Selecta genera et species piscium, quae in itinere per Brasiliam collegit J. B. de Spix (Munich, 1829, fol.). 2. F. de Castelnau, Animaux nouveaux ou rares, recueillis pendant l'expcdition clans les parties centrales de l'Amerique du Sud, " Poissons " (Paris, 1855). 3. L. Vaillant and F. Bocourt, Mission scientifique an Mexique et dons l'Amerique centrale, " Poissons " (Paris, 1874). 4. F. Poey, the celebrated naturalist of Havana, devoted many years of study to the fishes of Cuba. His papers and memoirs are published partly in two periodicals
M. New Zealand.i. F. W. Hutton and J. Hector, Fishes of New Zealand (Wellington, 1872). N. Arctic Regions.--I. C. Laken, " A Revised Catalogue of the Fishes of Greenland," in Manual
H. HISTORY AND LITERATURE FROM 1880 In the systematic account which followed the above chapter in the qth edition of the Encyclopaedia Britannica, the following classification, which is the same as that given in the author's Introduction to the Study of Fishes (London, r88o) was adopted by Albert Gunther:- Subclass I. : PALAEICHTHYES. Order I. : Chondropterygii. With two suborders : Plagiostomata and Holocephala. Order II. : Ganoidei. With eight suborders : Placodermi, Acanthodini, Dipnoi, Chondrostei, Polypteroidei, Pycnodontoidei, Lepidosteoidei, Amioidei. Subclass II.: TELEOSTEI. Order I. : Acanthopterygii. With the divisions Perciformes, Beryciformes, Kurtiformes, Polynemiformes, Sciaeniformes, Xiphiiformes, Trichiuriformes, Cotto-Scombriformes, Gobiiformes, Blenniformes, Mugiliformes,Gastrosteiformes, Centrisciformes, Gobiesociformes, Channiformes, Labyrinthibranchii, Lophotiformes, Taeniifornies and Notacanthiformes. Order II. : Acanthopterygii Pharyngognathi. Order III. : Anacanthini. With two divisions Gadoidei and Pleuronectoidei. Order IV. : Physostomi. Order V. i Lophobranchii. Order VI. P ectognathi. Subclass Ill. : CYCLOSTOMATA. Subclass IV. : LEETOC.&itDII. It was an artificial system, in which the most obvious relation-ships of the higher groups were lost sight of, and the results of the already fairly advanced study of the fossil forms to a great extent discarded. This system gave rise to much adverse criticism; as T. H. Huxley forcibly put it in a paper published soon after (1883), opposing the division of the main groups into Palaeichthyes and Teleostei: " Assuredly, if there is any such distinction to be drawn on the basis of our present knowledge among the higher fishes, it is between the Ganoids and the Plagiostomes, and not between the Ganoids and the Teleos--teans "; at the same time expressing his conviction, " first, that there are no two large groups of animals for which the evidence of a direct genetic connexion is better than in the case of the Ganoids and the Teleosteans; and secondly, that the proposal to separate the Elasmobranchii (Chondropterygii of Gunther), Ganoidei and Dipnoi of Muller into a group apart from, and equivalent to, the Teleostei appears to be inconsistent with the plainest relations of these fishes." This verdict has been endorsed by all subsequent workers at the classification of fishes. Gunther's classification would have been vastly improved ICHTHYOLOGY [HISTORY [HISTORY FROM 1880 had he made use of a contribution published as early as 1871, but not referred to by him. As not even a passing allusion is made to it in the previous chapter, we must retrace our steps to make good this striking omission. Edward Drinker Cope (18401897) was a worker of great originality and relentless energy, who, in the sixties of the last century, inspired by the doctrine of evolution, was one of the first to apply its principles to the classification of vertebrates. Equally versed in recent and fossil zoology, and endowed with a marvellous gift, or " instinct " for perceiving the relationship of animals, he has done a great deal for the advance of our knowledge of mammals, reptiles and fishes. Although often careless in the working out of details and occasionally a little too bold in his deductions, Cope occupies a high rank among the zoologists of the 19th century, and much of his work has stood the test of time. The following. was Cope's classification, 1871 (Tr. Amer. Philos. Soc. xiv. 449). Subclass I. Holocephali. II. Selachii. III. Dipnoi. IV. Crossopterygia, with two orders: Haplistia and Cladistia. V. Actinopteri. The latter is subdivided in the following manner: Tribe I. : Chondrostei. Two orders : Selachostomi and Glaniostomi. Tribe II.: Physostomi. Twelve orders: Ginglymodi, Halecomorphi, Nematognathi, Scyphophori, Plectospondyli, Isospondyli, Haplomi, Glanen- cheli, Ichthyocephali, Holostomi, Enchelycephali, Colocephah. Tribe III. : Physoclysti. Ten orders : Opisthomi, Percesoces, Synentognathi, Hemibranchii, Lophobranchii, Pediculati, Heterosomata, Plectognathi, Percomorphi, Pharyngognathi. Alongside with so much that is good in this classification, there are many suggestions which cannot be regarded as improvements on the views of previous workers. Attaching too great an importance to the mode of suspension of the mandible, Cope separated the Holocephali from the Selachii and the Dipnoi from the Crossopterygii, thus obscuring the general agreement which binds these groups to each other, whilst there is an evident want of proportion in the five subclasses. The exclusion from the class Pisces of the Leptocardii, or lancelets, as first advocated by E. Haeckel, was a step in the right direction, whilst that of the Cyclostomes does not seem called for to such an authority as R. H. Traquair, with whom the writer of this review entirely concurs. The group of Crossopterygians, first separated as a family from the other Ganoids by Huxley, constituted a fortunate innovation, and so was its division into two minor groups, by which the existing forms (Polypteroidei) were separated as Cladistia. The divisions of the Actinopteri, which includes all Teleostomes other than the Dipneusti and Crossopterygii also showed, on the whole, a correct appreciation of their relation-ships, the Chondrostei being well separated from the other Ganoids with which they were generally associated. In the groupings of the minor divisions, which Cope termed orders, we had a decided improvement on the Cuvierian-Mullerian classification, the author having utilized many suggestions of his fellow countrymen Theodore Gill, who has done much towards a better understanding of their relationships. In the association of the Characinids with the Cyprinids (Plectospondyli) in the separation of the flat-fishes from the Ganoids, in the approximation of the Lophobranchs to the sticklebacks and of the Plectognaths to the Acanthopterygians, and in many other points, Cope was in advance of his time, and it is to be regretted that his contemporaries did not more readily take up many of his excellent suggestions for the improvement of their systems. In the subsequent period of his very active scientific life, Cope made many alterations to his system, the latest scheme published by him being the following ( Synopsis of the familiesof Vertebrata, Amer. Natur., 1889, p. 849):--49) Class : Agnatha. I. Subclass : OSTRACODERMI. Orders : Arrhina, Diplorrhina. II. Subclass : MARSIPOBRANCHII. Orders : Hyperotreti, Hyperoarti. Class : Pisces. I. Subclass : HOLOCEPHALI. II. Subclass : DIPNOI. IV. Subclass : TELEOSTOMI. (i.) Superorder: Rhipidopterygia. Orders : Rhipidistia, Actinistia. (ii.) Superorder : Crossopterygia. Orders : Placodermi, Haplistia, Taxistia, Cladistia. (iii.) Superorder : Podopterygia (Chondrostei). (iv.) Superorder : Actinopterygia. Orders : Physostomi, Physoclysti. This classification is that followed, with many emendations, by A. S. Woodward in his epoch-making Catalogue of Fossil Fishes (4 vols., London, 18891901), and in his most useful Outlines of Vertebrate Paleontology ( Cambridge , 1898), and was adopted by Gunther in the loth edition of the Encyclopaedia Britannica:Class : Agnatha. I. Subclass : CYCLOSTOMI. With three orders : (a) Hyperoartia (Lampreys) ; (b) Hyperotreti (Myxinoids) ; (c) Cycliae (Palaeospondylus). II. Subclass : OSTRACODERMI. With four orders : (a) Heterostraci (Coelolepidae, Psammosteidae, Drepanaspidae, Pteraspidae); (b) Osteostraci (Cephalaspidae, Ateleaspidae, &c.); (c) Antiarchi (Asterolepidae, Pterichthys, Bothrolepis, &c.) ; (d) Anaspida (Birkeniidae). Class : Pisces. I. Subclass : ELASMOBRANCHII. With four orders : (a) Pleuropierygii (Cladoselache) ; (b) Ichthyotomi (Pleuracanthidae) ; (c) Acanthodii (Diplacanthidae, and Acanthodidae) ; (d) Selachii (divided from the structure of the vertebral centres into AsterospondyV and Tectospondyli). II. Subclass : HOLOCEPHALI. With one order : Chimaeroidei. With two orders : (a) Sirenoidei (Lepidosiren, Ceratodus, Uronemidae, Ctenodontidae) ; (b) Arthrodira (Homosteus, Coccosteus, Dinichthys). IV. Subclass : TELEOSTOMI. A. Order : Crossopterygii. With four suborders: (I) Haplistia (Tarassius); (2) Rhipidistia (Holoptychidae, Rhizodontidae, Osteolepidae) ; (3) Actinistia (Coelacanthidae) ; (4) Cladistia (Polypterus). B. Order : Actinopterygii. With about twenty suborders: (I) Chondrostei (Palaeoniscidae, Platysomidae, Chondrosteidae, Sturgeons); (2) Protospondyli (Semionotidae, Macrosemiidae, Pycnodontidae, Eugnathidae, Amiidae, Pachycormidae) ; (3) Aetheospondyli (Aspidorhynchidae, Lepidosteidae) ; (4) Isospondyli (Pholidophoridae, Osteoglossidae, Clupeidae, Leptolepidae, &c.); (5) Plectospondyli (Cyprrnidae, Characinidae) ; (6) Nematognathi; (7) Apodes; and the other Teleosteans. There are, however, grave objections to this system, which cannot be said to reflect the present state of our knowledge. In his masterly paper on the evolution of the Dipneusti, L. Dollo has conclusively shown that the importance of the autostyly on which the definition of the Holocephali from the Elasmobranchii or Selachii and of the Dipneusti from the Teleostomi rested, had been exaggerated, and that therefore the position assigned to these two groups in Gunther's classification of 188o still commended itself. Recent work on Palaeospondylui,.on the Ostracoderms, and on the Arthrodira, throws great doubt on the propriety of the positions given to them in the above classification, and the rank assigned to the main divisions of the Teleostomi do not commend themselves to the writer of the present article, who would divide the fishes into three subclasses:- I. Cyclostomi II. Selachii the characters and contents of which will be found in separate articles; in the present state of uncertainty as to their position, Palaeospondylus and the Ostracodermi are best placed hors cadre and will be dealt with under these names. The three subclasses here adopted correspond exactly with those proposed in Theo. Gill's classification of the recent fishes (" Families and Subfamilies of Fishes," Mem. Nat. Ac. Sci. vi. 1893), except that they are regarded by that authority as classes. The period dealt with in this chapter, ushered in by the publica- much discussed. tion of Gifnther's Introduction to the Study of Fishes, has been one of extraordinary activity in every branch of ichthyology, recent and fossil. A glance at the Zoological Record, published by the Zoological Society of London, will show the ever-increasing number of monographs, morphological papers and systematic contributions, which appear year after year. The number of new genera and species which are being proposed is amazing, hut it is difficult to tell how many of them will simply go to swell the already overburdened synonymy. Perhaps a reasonable estimate of the living species known at the present day would assess their number at about 13,000. It is much to be regretted that there is not a single general modern systematic work on fishes. The most important treatises, the 7th volume of the Cambridge Natural History, by T. W. Bridge and G. A. Boulenger, and D. S. Jordan's Guide to the Study of Fishes, only profess to give definitions of the families with enumerations of the principal genera. Gunther's Catalogue of the Fishes in the British Museum therefore remains the only general descriptive treatise, but its last volume dates from 187o, and the work is practically obsolete. A second. edition of it was begun in 1894, but only one volume, by Boulenger, has appeared, and the subject is so vast that it seems doubtful now whether any one will ever have the time and energy to repeat Giinther's achievement. The fish fauna of the different parts of the world will have to be dealt with separately, and it is in this direction that descriptive ichthyology is most likely to progress.North America, the fishes of which were imperfectly known in 188o, now possesses a Descriptive Catalogue in 4 stout volumes, by D. S. Jordan and B. W. Evermann, replacing the synopsis brought out in 1882 by D. S. Jordan and C. H. Gilbert. A similar treatise should embrace all the freshwater species of Africa, the fishes of the two principal river systems, the Nile and the Congo, having recently been worked out by G. A. Boulenger. Japanese ichthyology has been taken in hand by D. S. Jordan and his pupils. The fishes of the deep sea have been the subject of extensive monographs by L. Vaillant (Travailleur and Talisman), A. Giinther (Challenger), A. Alcock (Investigator), R. Collett (Hirondelle), S. Garman (Albatross) and a general resume up to 1895 was provided in G. B. Goode's and T. H. Bean's Oceanic Ichthyology. More than 60o true bathybial fishes are known from depths of 'Goo fathoms and more, and a great deal of evidence has been accumulated to show the general transition of the surface fauna into the bathybial. A recent departure has been the exploration of the Antarctic fauna. Three general reports, on the results of the Southern Cross, the Belgica and the Swedish South Polar expeditions, had already been published in 1907, and others on the Scotia and Discovery were in preparation. No very striking new types of fishes have been discovered, but the results obtained are sufficient to entirely disprove the theory of bipolarity which some naturalists had advocated. Much has been done towards ascertaining the life-histories of the fishes of economic importance, both in Europe and in North America, and our knowledge of the larval and post-larval forms has made great progress. Wonderful activity has been displayed in the field of palaeontology, and the careful working out of the morphology of the archaic types has led to a better understanding of the general lines of evolution; but it is to be regretted that very little light on the relationships of the living groups of Telcostcans has been thrown by the discoveries of palaeontologists. Among the most remarkable additions made in recent years, the work of R. H. Traquair on the problematic fishes Palaeospondylus, Thelodus, Drepanaspis, Lanarkia, Ateleaspis, Birkenia and Lanasius, ranks foremost; next to it must be placed the researches of A. S. Woodward and Bashford Dean on the primitive shark Cladoselache, and of the same authors, J. S. Newberry, C. R. Eastman, E. W. Claypole and L. Hussakof, on the Arthrodira, a group the affinities of which have been AuTaoRITIEs.The following selection from the extremely extensive ichthyological literature which has appeared during the period 188o1906 will supplement the bibliographical notice appended to section I. I. The General Subject: A. Gunther, Introduction to the Study of Fishes (Edinburgh, 1880) ; B. Dean, Fishes Living and Fossil (New York, 1895) ; T. W. Bridge and G. A. Boulenger, " Fishes," Cambridge Natural History, vii. (1904); D. S. Jordan, Guide to the Study of Fishes (2 vols., New York, 1905). II. Palaeontological: A. Fritsch, Fauna der Gaskohle and der Kalksteine der Perm-formation Bohmens (vols. i.-iii., Prague, 18791894) ; K. A. von Zittel, Handbuch der Paliiontologie, vol. iii. (Munich, 1887) ; A. Smith Woodward, Catalogue of Fossil Fishes in the British Museum, vols. i.iii. (London, 18891895) ; A. Smith Woodward, Outlines of Vertebrate Palaeontology for Students of Zoology (Cambridge, 198) ; J. S. Newberry, " The Palaeozoic Fishes of North America," Mon. U.S. Geol. Surv. vol. xvi. (1889) ; J. V. Rohon, "Die obersilurischen Fische von Osel, Thyestidae and Tremataspidae," Mem. Ac. Imp. Sc. St-Petersb. xxxviu. (1892); O. Jaekel, Die Selachier von Bolca, ern Beitrag zur Morphogenie der Wirbeltiere (Berlin, 1894) ; B_ Dean, " Contributions to the Morphology of Cladoselache," Journ. Morphol. ix. (1894) ; R. H. Traquair, " The Asterolepidac," Mon. Palaeont. Soc. (1894-1904, in progress); " Report on Fossil Fishes collected by the Geological Survey of Scotland in the Silurian Rocks of the South of Scotland," Trans. Roy Soc. Edin. xxxix. (1899); L. Dollo, " Sur la phylogenie des Dipneustes," Bull. Soc. Belge Geol. vol. ix. (1895); E. W. Claypole, ;' The Ancestry of the Upper Devonian Placoderms of Ohio," Amer. Geol. xvii. (1896) ; B. Dean, " Palaeontological Notes," Mem. N.Y. Ac. ii. (1901) ; A. Stewart and S. W. Williston, " Cretaceous Fishes of Kansas," Univ. Geol. Surv. Kansas, vi. (Topeka, 1901); A. S. Woodward, " Fossil Fishes of the English Chalk," Palaeontogr. Soc. (19021903, etc.); R. H. Traquair, " The Lower Devonian Fishes of Gemunden," Roy. Soc. Edin. Trans. 40 (1903) ; W. J. and I. B. J. Sollas, " Account of the Devonian Fish Palaeospondylus," Phil. Trans. 196 (1903); C. T. Regan, " Phylogeny of the Teleostomi," Ann. &' Mag. N.H. (7) 13 (1904); C. R. Eastman, "Fishes of Monte Bolca," Bull. Mus. C.Z. 46 (1904) ; " Structure and Relations of Mylostoma," Op. cit. 2 (1906); 0. Abel, " Fossile Flugfische," Jahrb. Geol. Reichsanst. 56 (Wien, 1906); L. Hussakof. " Studies on the Arthrodira," Mem. Amer. Mus. N.H. ix. (1906). III. Faunistic (recent fishes) : (A) EUROPE: E. Bade, Die mitteleuropaischen Silsswasser-Jlsche (2 vols., Berlin, 1901-1902). GREAT BRITAIN: F. Day, The Fishes of Great Britain and Ireland (2 vols., London, 1880-1884) ; J. T. Cunningham, The Natural History of the Marketable Marine Fishes of the British Islands (London, 1896); W. C. M'Intosh and A. T. Masterman, The Life-Histories of the British Marine Food-Fishes (London, 1897) ; Sir H. Maxwell, British Fresh-water Fish (London, 1904) ; F. G. Aflalo, British Salt-water Fish (London, 1904). Numerous important researches into the development, life-conditions and distributions, carried out at the Biological Laboratories at Plymouth and St Andrews and during the survey of the fishing grounds of Ireland, have been published by W. L. Calderwood, J. T. Cunningham, E. W. L. Holt, W. C. M'Intosh, J. W. Fulton, W. Garstang and Prince in the Journ. Mar. Biolog. Assoc., The Reports of the Fishery Board of Scotland, Scient. Trans. R. Dublin Soc. and other periodicals
Untersuch. deutsch. Meere (Kiel, 1883); F. Heincke, E. Ehrenbaum and G. Duncker have published their investigations into the life-history and development of the fishes of Heligoland in Wissenschaftl. Meeresuntersuchungen (Kiel and Leipzig, 1894--1899) ; (E) SWITZERLAND: V. Fatio, Faun.e des vertebras de la Suisse: Poissons (2 vols., Geneva and Basel, 1882189o). (F) FRANCE: E. Moreau, Histoire naturelle des poissons de la France (3 vols., Paris, 1881) ; Supplement (Paris, 1891). (G) PYRENEAN PENINSULA: D. Carlos de Braganca, Resultados das investigacoes scientificas feitas a bordo do yacht "Amelia." Pescas maritimas, i. and ii. (Lisbon, 18991904). (H) ITALY AND MEDITERRANEAN: P. DOderlein, Manuale ittiologico del .11editerraneo (Palermo, 18811891. not completed; interrupted by the death of the author); E. W. L. Holt, " Recherches sur la reproduction des poissons osseux, principalement dans le golfe de Marseille," Ann. Mus. Mars. v. (Marseilles, 1899) ; (I) WESTERN AND CENTRAL ASIA: L. Lortet, " Poissons et reptiles du lac de Tiberiade," Arch. Mus. d'Hist. Nat. Lyon, iii. (1883); S. Herzenstein, Wissenschaftliche Resultate der von N. M. Przewalski nach Central Asien unternommenen Reisen: Fische (St Petersburg, 18881891); L. Berg, Fishes of Turkestan (Russian text, St Peters-burg, 1905) ; G. Radde, S. Kamensky and F. F. Kawraisky have worked out the Cyprinids and Salmonids of the Caucasus (Tiflis, 1896-1899). (J) JAPAN: F. Steindachner and L. Doderlein, Beitrage zur Kenntniss der Fische japans," Denkschr. Ak. Wien, (vols. 67 and 68, 1883) ; K. Otaki, T. Fujita and T. Higurashi, Fishes of Japan (in Japanese) (Tokyo, 1903, in progress). Numerous papers by D. S. Jordan, in collaboration with J. O. Snyder, E. C. Starks, H. W Fowler and N. Sindo. (K) EAST INDIES: F. Day, The Fauna of British India: Fishes (2 vols., London, 1889) (chiefly an abridgment of the author's Fishes of India) ; M. Weber, " Die Siisswasserfische des Indischen Archipels," Zool. Ergebnisse e. Reise in Niederl. Ostind. iii. (Leiden, 1894). Numerous contributions to the fauna of the Malay Peninsula and Archipelago by G. A. Boulenger, L. Vaillant, F. Steindachner, G. Duncker, W. Volz and C. L. Popta. (L) AFRICA: G. A. Boulenger, Materiaux pour la faune du Congo: poissons nouveaux (Brussels, 1898-1902, in progress); and Poissons du bassin du Congo (Brussels, 1901); G. Pfeffer, Die Thierwelt Ostafrikas: Fische (Berlin, 1896); A. Gunther, G. A. Boulenger, G. Pfeffer, F. Steindachner, D. Vinciguerra, J. Pellegrin and E. Lonnberg have published numerous contributions to the fish-fauna of tropical Africa in various periodicals. The marine fishes of South Africa have received special attention on the part of J. D. F. Gilchrist, Marine Investigations in South Africa, i. and ii. (18981904), and new species have been described by G. A. Boulenger and C. T. Regan. (M) NORTH AMERICA: D. S. Jordan and B. W. Evermann, The Fishes of North and Middle America (4 vols., Washington, 189619oo) ; D. S. Jordan and B. W. Evermann, American Food and Game Fishes (New York, 1902); D. S. Jordan and C. H. Gilbert " The Fishes of Bering Sea," in Fur-Seals and Fur-Seal Islands (Washington, 1899) ; The U.S. Bureau of Fisheries (since 1903) has published annually a Report and a Bulletin, containing a vast amount of information on North American fishes and every subject having a bearing on the fisheries of the United States; S. E. Meek, " Fresh-water Fishes of Mexico," Field Columb. Mus. Zool. V. (1904). (N) SOUTH AMERICA: C. H. and R. S. Eigenmann, " A Catalogue of the Fresh-water Fishes of South America," Proc. U.S. Nat. Miss. 14 (Washington, 1891); the same authors, F. Steindachner, G. A. Boulenger, C. Berg and C. T. Regan have published contributions in periodicals on this fauna. (0) Aus-TRALIA: J. E. Tenison-Woods, Fish and Fisheries of New South Wales (Sydney, 1882); J. Douglas Ogilby, Edible Fishes and Crustaceans of New South Wales (Sydney, 1893) ; J. Douglas Ogilby and E. R. Waite are authors of numerous papers on Australian fishes in Proc. Linn. Soc. N.S. Wales and Rec. Austral. Mus. (P) SOUTH PACIFIC: D S. Jordan and B. W. Evermann, " Shore Fishes of the Hawaiian Islands," Bull. U.S. Fish. Comm. 23 (1905). (Q) MADAGASCAR: H. E. Sauvage, Histoire physique, naturelle et politique de Madagascar, par A. Grandidier. xvi.; Poissons (Paris, 1891). (R) OCEANIC FISHES: G. B. Goode a.Id T. H. Bean, Oceanic Ichthyology (Washington, 1895); A. Gunther, Deep-sea Fishes of the " Challenger " Expedition (London, 1887) ; C. H. Gilbert, " Deep-sea Fishes of the Hawaiian Islands," Bull. U.S. Fish. Comm. 23 (1905); R. Collett, Norske Nordhays Expedition: Fiske (Christiania, 188o) ; C. F. Laken, Dijmphna-Togtets Zoologisk-botaniske Udbytte: Kara-Havets Fiske (Copenhagen, 1886) ; L. Vaillant, Expeditions scienti-fiques du "Travailleur'' et du "Talisman": Poissons (Paris, 1888) ; A. Agassiz, Three Cruises of the U.S. Coast and Geodetic Survey Steamer " Blake " (Boston and New York, 1888) ; A. Alcock, Illustrations of the Zoology of H.M.S. " Investigator ": Fishes (Calcutta, 18921899, in progress); A. Alcock, Descriptive Catalogue of the Indian Deep-sea Fishes in the Indian Museum (Calcutta, 1899, contains references to all the previous papers of the author on the subject); R. Collett, Rcsultats des campagnes scientifiques accomplies par Albert I^* prince de Monaco: poissons provenant des campagnes du yacht l'Hirondelle," (Monaco, 1896); R. Koehler, Resu tats scientifiques de la campagne'du " Caudan," (Paris, 1896); C. H. Gilbert and F. Cramer, " Report on the Fishes dredged in Deep Water near the Hawaiian Islandss," Proc. U.S. Nat. Mus. xix. (Washington, 1896); C. Lutken, " Spolia Atlantica," Vidensk. Selsk. Skr. vii. and ix. (Copenhagen, 18921898) ; C. Lutken, Danish Ingolf Expedition, ii.: Ichthyological Results (Copenhagen, 1898); S. Garman, " Reports on an Exploration off the West Coast of Mexico, Central and South America, and off the Galapagos Islands in charge of Alexander Agassiz, by the U.S. Fish Commission Steamer "Albatross," during 1891," Mena. Mus. Comp. Zool. vol. xxiv. (Cambridge, U,S.A., 1899). (S) ANTARCTIC FIsuEs: G. A. Boulenger, Report on the Collections made during the voyage of the " Southern Cross ": Fishes (London, 1902) ; L. Dollo, Expedition Antarctique Beige (S.Y. " Belgica "). Poissons (Antwerp, 1904) ; E. Lonnberg, Swedish South Polar Expedition: Fishes (Stockholm, 1905) ; G. A. Boulenger, Fishes of the " Discovery " Antarctic Expedition (London, 1906). (G. A. B.)III. DEFINITION OF THE CLASS Pisces. ITS PRINCIPAL DIVISIONS Fishes, constituting the class Pisces, may be defined as Craniate Vertebrata, or Chordata, in which the anterior portion of the central nervous system is expanded into a brain surrounded by an unsegmented portion of the axial skeleton; which are provided with a heart, breathing through gills; and in which the limbs, if present, are in the form of fins, as opposed to the pentadactyle, structure common to the other Vertebrata. With the exception of a few forms in which lungs are present in addition to the gills, thus enabling the animal to breathe atmospheric air for more or less considerable periods (Dipneusti), all fishes are aquatic throughout their existence. In addition to the paired limbs, median fins are usually present, consisting of dermal rays borne by endoskeletal supports, which in the more primitive forms are strikingly similar in structure to the paired fins that are assumed to have arisen from the breaking up of a lateral fold similar to the vertical folds out of which the dorsal, anal and caudal fins have been evolved. The body is naked, or scaly, or covered with bony shields or hard spines. Leaving aside the Ostracophori, which are dealt with in a separate article, the fishes may be divided into three subclasses I. Cyclostomi or Marsipobranchii, with the skull imperfectly developed, without jaws, with a single nasal aperture, without paired fins, and with an unpaired fin without dermal rays. Lampreys and hag-fishes. II. Selachii or Chondropterygii, with the skull well developed but without membrane bones, with paired nasal apertures, with median and paired fins, the ventrals bearing prehensile organs (claspers) in the males. Sharks, skates and chimaeras. IV. ANATOMY1 The special importance of a study of the anatomy of fishes lies in the fact that fishes are on the whole undoubtedly the most archaic of existing craniates, and it is therefore to them especially that we must look for evidence as to the evolutionary history of morphological features occurring in the higher groups of vertebrates. In making a general survey of the morphology of fishes it is essential to take into consideration the structure of the young developing individual (embryology) as well as that of the adult (comparative anatomy in the narrow sense). Palaeontology is practically dumb excepting as regards external form and skeletal features, and even of these our knowledge must for long be in a hopelessly imperfect state. While it is of the utmost importance to pay due attention to embryological data it is equally important to consider them' critically and in conjunction with broad morphological considerations. Taken by themselves they are apt to be extremely misleading. External Features.The external features of a typical fish are intimately associated with its mode of life. Its shape is more or less that of a spindle; its surface is covered with a highly glandular epidermis, which is constantly producing lubricating mucus through the agency of which skin-friction is reduced to an extraordinary degree; and finally it possesses a set of remarkable propelling organs or fins. The exact shape varies greatly from the typical spindle shape with variations in the mode of life; e.g. bottom-living fishes may be much flattened from above downwards as in the rays, or from sidq to side in the Pleuronectids such as flounder, plaice or sole, or the shape may be much elongated as in the eels. Head, Trunk and Tail.In the body of the fish we may recognize the three main subdivisions of the bodyhead, trunk and tailas in the higher vertebrates, but there is no definite narrowing of the anterior region to form a neck such as occurs in the higher groups, though a suspicion of such a narrowing occurs in the young Lepidosiren. i For general anatomy of fishes, see T. W. Bridge, Cambridge Natural History, and R. Wiedersheim, Vergl. Anat. der Wirbelliere. The latter contains an excellent bibliography. The tail, or postanal region, is probably a secondary developmenta prolongation of the hinder end of the body for motor purposes. This is indicated by the fact that it frequently develops late in ontogeny. The vertebrate, in correlation perhaps with its extreme cephalization, develops from before backwards (except the alimentary canal, which develops more en bloc), there remaining at the hind end for a prolonged period a mass of undifferentiated embryonic tissue from the anterior side of which the definitive tissues are constantly being developed. After development has reached the level of the anus it still continues backwards and the tail region is formed, showing a continuation of the same tissues as in front, notochord, nerve cord, gut, myotomes. Of these the (postanal) gut soon undergoes atrophy. Fins.The fins are extensions of the body surface which serve for propulsion. To give the necessary rigidity they are provided with special skeletal elements, while to give mobility they are provided with special muscles. These muscles, like the other voluntary muscles of the body, are derived from the primitive myotomes and are therefore segmental in origin. The fins are divisible into two main categoriesthe median or unpaired fins and the paired fins. The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median scales (" fulcra ") ; b, bony plates. skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement
there is a greatly increased size of the fin-foldboth dorsally and ventrally. There is further developed a highly characteristic asymmetry. In the original symmetrical or protocercal (=diphycercal) type of tail (as seen in a cyclostome, a Dipnoan and in most fish embryos) the skeletal axis of the body runs straight out to its tipthe tail fold being equally developed above and below the axis. In the highly developed caudal fin of the majority of fishes, however, the fin-fold is developed to a much greater extent on the From "Challenger " Reports Zool., published by ventral side, and cone- x. \?. Stationery of ce. skeletal axis is turned upwards as in the hclerocercal tail of sharks and sturgeons. The highest stage in this evolution of the caudal fin is seen in the Teleostean fishes, where the ventral tail-fold becomes developed to such an extent as to produce a secondarily symmetrical appearance (homocercal tail, fig. 4). The sharks have been referred to as possessing heterocercal tails, but, though this is true of the majority, within the limits of the group all three types of tail-fin occur, from the protoeercal tail of the fossil Pleuracanthids and the living Chlamydoselachus to the highly developed, practically homocercal tail of the ancient Cladoselache (fig. 2). The praecaudal portion of the fin-fold on the dorsal side of the body becomes broken into numerous finlets in living Crossopterygians, while in other fishes it disappears throughout part of its length, leaving only one, two or three enlarged portionsthe dorsal fins (fig. 4, d.f.). Similarly the praecaudal part of the fin-fold ventrally becomes reduced to a single anal fin (a.f.), occasionally continued backwards by a series of finlets (Scombridae). In, the sucker-fishes (Remora, Echeneis) the anterior dorsal fin is metamorphosed into a sucker by which the creature attaches itself to larger fishes, turtles, &c. The paired finsthough more recent developments than the medianare yet of very great morphological interest, c.f..-- Fmm Cambridge Natural History, vol. vii., " Fishes, &c.," by permission of Messrs. Macmillan & Co., Ltd. &c.), though the highly specialized features in other respects make it probable that we have here to do with a secondary return to a condition like the primitive one. In the process of segmentation of the originally continuous fin-fold we notice first of all a separation of and an increase in size of that portion of the fin which from its position at the tip of the tail region is in the most advantageous position for producing movements of the body. There is thus formed the caudal fin. In this region ~/llllllll11l.9D!"" f From Cambridge Natural History, vol. vii., " Fishes, &c.," by permission of Messrs. Macmillan & Co., Ltd. g.r, Gill rakers. 1.1, Lateral line organs. n, Nasal opening. p.f, Pelvic fin. p.op, Preoperculum. pt. f, Pectoral fin. A, Side view. B, First bronchial arch. a.f, Anal fin. c. f, Caudal fin. d.f, Dorsal fin. g.f, Gill lamellae. as in them we are compelled to recognize the homologues of the paired limbs of the higher vertebrates. We accordingly distinguish the two pairs of fins as pectoral or anterior and pelvic (_" ventral ") or posterior. There are two main types of paired finthe archipterygial type, a paddle-like structure supported by a jointed axis which bears lateral rays and exists in an unmodified form in Neoceratodus alone amongst living fishes, and the actinopterygial type, supported by fine raylike structures as seen in the fins of any ordinary fish. The relatively Less efficiency of the archipterygium and its predominance amongst the more ancient forms of fishes point to its being the more archaic of these two types. In the less highly specialized groups of fishes the pectoral fins are close behind the head, the pelvic fins in the region of the cloacal opening. In the more specialized forms the pelvic fins frequently show a more or less extensive shifting towards the head, so that their position is described as thoracic (fig. 4) of jugular (Gad uscod, haddock, &c., fig. 5). Flo. 5.Burbot (Lola vulgaris), with jugular ventral fins. The median fin, especially in its caudal section, is the main propel-ling organ: the paired fins in the majority of fishes serve for balancing. In the Dipneusti the paired fins are used for clambering about amidst vegetation, much in the same fashion as the limbs of Urodeles. In Ceratodus they also function as paddles. In various Teleosts the pectoral fins have acquired secondarily a leg-like function, being used for creeping or skipping over the mud (Periophthalmus; cf. also Trigloids, Scorpaenids and Pediculati). In the " flying " fishes the pectoral fins are greatly enlarged and are used as aeroplanes, their quivering movements frequently giving a (probably erroneous) impression of voluntary flapping movements. In the gobies and lumpsuckers (Cyclopteridae) the pelvic fins are fused to form an adhesive sucker; in the Gobiesocidae they take part in the formation of a somewhat similar sucker. The evolutionary history of the paired limbs forms a fascinating chapter in vertebrate morphology. As regards their origin two hypotheses have attracted special attention: (I) that enunciated by Gegenbaur, according to which the limb is a modified gill septum, and (2) that supported by James K. Thacher, F. M. Balfour, St George Mivart and others, that the paired fins are persisting and modified portions of a once continuous fin-fold on each side of the body. The majority of morphologists are now inclined to accept the second of these views. Each has been supported by plausible arguments, for which reference must be made to the literature of the subject.' Both views rest upon the assumed occurrence of stages for the existence of which there is no direct evidence, viz. in the case of (t) transitional stages between gill septum and limb, and in the case of (2) a continuous lateral fin-fold. (There is no evidence that the lateral row of spines in the acanthodian Climatius has any other than a defensive significance.) In the opinion of the writer of this article, such assumptions are without justification, now that our knowledge of Dipnoan and Crossopterygian and Urodele embryology points towards the former possession by the primitive vertebrate of a series of projecting, voluntarily movable, and hence potentially motor structure on each side of the body. It must be emphasized that these the true external gillsarc the only organs known actually to exist in vertebrates which might readily be transformed into limbs. When insuperable objections are adduced to this having actually taken place in the course of evolution, it will be time enough to fall back upon purely hypothetical ancestral structures on which to base the evolutionary history of the limbs. The ectoderm covering the general surface is highly glandular. In the case of the Dipneusti, flask-shaped multicellular glands like those of Amphibians occur in addition to the scattered gland cells. A characteristic feature of glandular activity is the production of a slight electrical disturbance. In the case of Malopterurur this elsewhere subsidiary function of the skin has become so exaggerated as to lead to the conversion of the skin of each side of the body into a powerful electrical organ.' Each of these consists of some two million small chambers, each containing an electric disk and all deriving their nerve supply from the branches of a single enormous axis cylinder. This takes its origin from a gigantic ganglion cell situated latero-dorsally in the spinal cord between the levels of the first and second spinal nerves. Cement Organs.The larvae of certain Teleostomes and Dipnoans possess special glandular organs in the head region for the secretion of a sticky cement by which the young fish is able to attach itself to water-plants or other objects. As a rule these are ectodermal in origin; e.g. in Lepidosiren and Protopterus3 the crescentic cement organ lying ventrally behind the '- Cf. J. Graham Kerr, Proc. Camb. Phil. So'. x. 227. 2 For electric organs see W. Biedermann, Electro-Physiology. 3 J. Graham Kerr, Quart. Journ. Mier. Sci. vol. xlvi.mouth consists of a glandular thickening of the deep layer of the ectoderm. In young ganoid fishes preoral cement organs occur. In Crossopterygians there is one cup-shaped structure on each side immediately in front of the mouth. Here the glandular epithelium is endodermal, developed' as an outgrowth from the wall of the alimentary canal, closely resembling a gill pouch. In Arnia s the same appears to be the case. In a few Teleosts similar organs occur, e.g. Sarcodaces, Hyperopisus,s where so far as is known they are ectodermal. Photogenic Organs.The slimy secretion produced by the epidermal glands of fishes contains in some cases substances which apparently readily undergo a slow process of oxidation, giving out light of low wave-length in the process and so giving rise to a phosphorescent appearance. In many deep-sea fishes this property of producing light-emitting secretion has under-gone great development, leading to the existence of definite photogenic organs. These vary much in character, and much remains to be done in working out their minute structure. Good examples are seen in the Teleostean family Scopelidae, where they form brightly shining eye-like spots scattered about the surface of the body, especially towards the ventral side. External Gills.In young Crossopterygians and in the young Protopterus and Lepidosiren true external gills occur of the same morphological nature as those of Urodele amphibians. In Crossopterygians a single one is present on each side on the hyoid arch; in the two Dipnoans mentioned four are present From Trans. Zool. Soc. of London. on each sideon visceral arches III., IV., V. and VI. (It may be recalled that in Urodeles they occur on arches III., IV. and V., with vestiges' on arches I. and II.). Each external gill develops as a projection of ectoderm with mesodermal core near the upper end of its visceral arch; the main aortic arch is prolonged into it as a loop. When fully developed it is pinnate, and is provided with voluntary muscles by which it can be moved freely to renew the water in contact with its respiratory surface. In the case of Polypterus a short rod of cartilage projects from the From Phil. Transactions, Royal Society of London. hyoid arch into the base of the external gill. Their occurrence with identical main features in the three groups mentioned indicates that the external gills are important and archaic organs of the vertebrata. Their non-occurrence in at least some_ of the groups where they are absent is to be explained by die presence of a large vascular yolk sac, which necessarily fulfils in a very efficient way the respiratory function. Alimentary Canal.The alimentary canal forms a tube traversing the body from mouth to cloacaI opening. Corresponding with structural and functional differences it is for descriptive J. Graham Kerr, The Budgett Memorial Volume. J. Phelps, Science, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, Amer. Journ. Anat. v. (1906) p. 154. J. S. Budgett, Trans. Zool. Soc. Lond. xvi., 1901, p. 130. ' L. Definer, Zool. Jahrbiicher Anat. Band xix. (1904), S. 434. purposes divided into the following regions(r) Buccal cavity or mouth cavity, (2) Pharynx, (3) Oesophagus or gullet, (4) Stomach, (5) Intestine, and (6) Cloaca. The buccal cavity or mouth cavity is morphologically a stomodaeum, i.e. it represents an inpushing of the external surface. Its opening to the exterior is wide and gaping in the embryo in certain groups (Selachians and Crossopterygians), and even in the adult among the Cyclostomata, but in the adult Gnathostome it can be voluntarily opened and shut in correlation with the presence of a hinged jaw apparatus. The mouth opening is less or more ventral in position in Cyclostomes and Selachians, while in Dipnoans and Teleostomes it is usually terminal. In certain cases (e.g. Lepidosiren)t the buccal cavity arises by secondary excavation with-out any actual pushing in of ectoderm. It is highly characteristic 9f the vertebrata that the pharynxthe portion of the alimentary canal immediately behind the buccal cavity--communicates with the exterior by a series of paired clefts associated with the function of respiration and known as the visceral clefts. It is especially characteristic of fishes that a number of these clefts remain open as functional breathing organs in the adult. e.b.a The visceral clefts arise as hollow pouches (or at first solid From Bridge, Cambridge Natural 21islory, of the endoderm. vol. vii., "Fishes, &c." (by permission of projections) Macmillan & Co., Ltd.). After Boas, Lehr- Each pouch fuses with the buck der Zoologie (by permission of Gustav Fischer). ectoderm at its outer end and pharynx in an Elasmobranch (A) between pharynx and exterior. and a Teleost (B) ; 1, 2, &c., Bran- chial septa. The mesenchymatous pack- b.c, Opercular cavity. ing tissue between consecu- b.l, Respiratory lamellae. Live clefts forms the visceral c, Coelom. arches, and local condensation e.b.a, Opercular opening. hy.a, Hyoid arch. within each gives rise to im- hy.c, Hyobranchial cleft. portant skeletal elementsto 1.s, Valvular outer edge of gill which the name visceral arches septum. is often restricted. From the n, Nasal aperture. oes, Oesophagus. Particular skeletal structures op, Operculum. which develop in the visceral p.q, Palato quadrate cartilage. arches bounding it the anterior Ph, Pharynx. cleft is known as the hyoman- sp, Spiracle. dibular cleft, the next one as hvobranchial. In common usage the hyomandibular cleft is called the spiracle, and the series of clefts behind it the branchial clefts. The typical functional gill cleft forms a vertical slit, having on each side a gill septum which separates it from its neighbours in the series. The lining of the gill cleft possesses over a less or greater extent of its area a richly developed network of capillary blood-vessels, through the thin covering of which the respiratory exchange takes place between the blood and the water which washes through the gill cleft. The area of respiratory surface tends to become increased by the development of outgrowths. Frequently these take the form of regular plate-like structures known as gill lamellae. In the Selachians these lamellae are strap-like structures (Elasmobranch) attached along nearly their ' J. Graham Kerr, Quart. Journ. Mier. Sci. xlvi. 423. whole length to the gill septum as shown in fig. 8, A. In the Holocephali and in the sturgeon the outer portions of the gill septa have disappeared and this leads to the condition seen in the higher Teleostomes (fig. 8, B), where the whole of the septum has disappeared except its thick inner edge containing the skeletal arch. It follows that in these higher Teleostomesincluding the ordinary Teleoststhe gill lamellae are attached only at their extreme inner end. In the young of Selachians and certain Teleosts (e.g. Gymnarchus and Heterotis)2 the gill lamellae are prolonged as filaments which project freely to the exterior. These must not be confused with true external gills. The partial atrophy of the gill septa in the Teleostomes produces an important change in their appearance. Whereas in the Selachian a series of separate gill clefts is seen in external view each covered by a soft valvular backgrowth of its anterior lip, in the Teleostean fish, on the other hand, a single large opening is seen on each side (opercular opening) covered over by the enormously enlarged valvular flap belonging to the anterior lip of the hyobranchial cleft. This flap, an outgrowth of the hyoid arch, is known as the operculum. In the Teleostomi there are usually five functional clefts, but these are the survivors of a formerly greater number. Evidence of reduction is seen at both ends of the series. In front of the first functional cleft (the hyobranchial) there is laid down in the embryo the rudiment of a spiracular cleft. .In the less highly organized fishes this survives in many cases as an open cleft. In many sharks and in sturgeons the spiracle forms a conspicuous opening just behind the eye. In rays and skates, which are modified in correlation with their ground feeding habit, the spiracle is a large opening which during the great widening out of the body during development comes to be situated on the dorsal side, while the branchial clefts come to be ventral in position. In existing Crossopterygians the spiracle is a slit-like opening on the dorsal side of the head which can be opened or closed at will. In Dipneusti,as in the higher Teleostomes, the spiracle is found as an embryonic rudiment, but in this case it gives rise in the adult to a remarkable sense organ of problematical function.' Traces of what appear to be pre-spiracular clefts exist in the embryos of various forms. Perhaps the most remarkable of these is to be found in the larval Crossopterygian,4 and apparently also in Amiee at least, amongst the other ganoids, where a pair of entodermal pouches become cut off from the main entoderm and, establishing an opening to the exterior, give rise to the lining of the cement organs of the larva. Posteriorily there is evidence that the extension backwards of the series of gill clefts was much greater in the primitive fishes. In the surviving sharks (Chlamydoselachus and Notidanus cinereus), there still exist in the adult respectively six and seven branchial clefts, while in embryonic Selachians there are frequently to be seen pouch-like outgrowths of entoderm apparently representing rudimentary gill pouches but which never develop. Further evidence of the progressive reduction in the series of clefts is seen in the reduction of their functional activity at the two ends of the series. The spiracle, even where persisting in the adult, has lost its gill lamellae either entirely or excepting a few vestigial lamellae forming a " pseudobranch " on its anterior wall (Selachians, sturgeons). A similar reduction affects the lamellae on the anterior wall of the hyobranchial cleft (except in Selachians) and on the posterior wall of the last branchial cleft. A pseudobranch is frequently present in Teleostomes on the anterior wall of the hyobranchial cleft, i.e. on the inner or posterior-face of the operculum. It is believed by some morphologists to belong really to the cleft in front.' Phylogeny.The phylogeny of the gill clefts or pouches is uncertain. The only organs of vertebrates comparable with them morphologically are the enterocoelic pouches of the entoderm which 2 J. S. Budgett, op. cit. 3 W. E. Agar, Anat. Ana., 1905, S. 298. J. Graham Kerr, The Budgett Memorial Volume. J. Phelps, Science, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, Amer. Journ. Anat., v. 1906, p. 154. ' F. Maurer, Morphol. Jahrb. ix., 1884, S. 229, and xiv., 1888, S. 1 j5. give rise to the mesoderm. It is possible that the respiratory significance of the wall of the gill cleft has been secondarily acquired. This is indicated by the fact that they appear in some cases to be lined by an ingrowth of ectoderm. This suggests that there may have been a spreading inwards of respiratory surface from the external gills. It is conceivable that before their walls became directly respiratory the gill clefts served for the pumping of fresh water over the external gills at the bases of which they lie. Lung.As in the higher vertebrates, there develops in all the main groups of gnathostomatous fishes, except the Selachians, an outgrowth of the pharyngeal wall intimately associated with gaseous interchange. In the Crossopterygians and Dipnoans this pharyngeal outgrowth agrees exactly in its midventral origin and in its blood-supply with the lungs of the higher vertebrates, and there can be no question about its being morphologically the same structure as it is also in function. In the Crossopterygian the ventrally placed slit-like glottis leads into a common chamber produced anteriorly into two horns and continued backwards into two " lungs." These are smooth, thin-walled, saccular structures, the right one small, the left very large and extending to the hind end of the splanchnocoele. In the Dipnoans the lung has taken a dorsal position close under the vertebral column and above the splanchnocoele. Its walls are sacculated, almost spongy in Lepidosiren and Protopterus, so as to give increase to the respiratory surface. In Nexeratodus (fig. 9) an indication of division into two halves is seen in the presence of two prominent longitudinal ridges, one dorsal and one ventral. In Lepidosiren and Protopterus the organ is completely divided except at its anterior end into a right and a left lung. The anterior portion of the lung or lungs is connected with the median ventral glottis by a short wide vestibule which lies on the right side of the oesophagus.; In the Teleostei the representative of the lung, here termed the swimbladder, has for its predominant function a hydro-static one; it acts as a float. It arises as a diverticulum of the gut-wall which may retain a tubular connexion with the gut (physostomatous condition) or may in the adult completely lose such connexion (physoclistic). It shows two conspicuous differences from the lung of other forms: (r) it arises in the young fish as a dorsal instead of as a ventral diverticulum, and (2) it Flo. 9.--Lung ofNeoceratodus, derives its blood-supply not from opened in its lower half to show the sixth aortic arch but from its cellular pouches. as Right branches of the dorsal aorta. half; h, Left half ; c, Cellularfunction of the lung in such a form to become hydrostatic we might expect the course of evolution to lead to a shifting of the glottis dorsalwards so as to bring it nearer to the definitive situation of the lung. (2) In Erythrinus and other Characinids the glottis is not mid-ventral but decidedly lateral in position, suggesting either a retention of, or a return to, ancestral stages in the dorsalward migration of the glottis. (3) The blood-supply of the Teleostean swimbladder is from branches of the dorsal aorta, which may be distributed over a long anteroposterior extent of that vessel. Embryology, however, shows that the swimbladder arises as a localized diverticulum. It follows that the blood-supply from a long stretch of the aorta can hardly be primitive. We shogid rather expect the primitive blood-supply to be from the main arteries of the pharyngeal wall, i.e. from the hinder aortic arch as is the ease with the lungs of other forms. Now in Amia at least we actually find such a blood-supply, there being here a pulmonary artery corresponding with that in lung-possessing forms. Taking these points into consideration there seems no valid reason for doubting that in lung and swim-bladder we are dealing with the same morphological structure. Function.In the Crossopterygians and Dipnoans the lung is used for respiration, while at the same time fulfilling a hydro-static function. Amongst the Actinopterygians a few forms still use it for respiration, but its main function is that of a float. In connexion with this function there exists an interesting compensatory mechanism whereby the amount of gas in the swimbladder may be diminished (by absorption), or, on the other hand, increased, so as to counteract alterations in specific gravity produced, e.g. by change of pressure with" change of depth. This mechanism is specially developed in physoclistic forms,where there occur certain. glandular patches (" red glands ") in the lining epithelium of the swimbladder richly stuffed with capillary blood-vessels and serving apparently to secrete gas into the swimbladder. That the gas in the swimbladder is produced by some vital process, such as secretion, is already indicated by its composition, as it may contain nearly go % of oxygen in deep-sea forms or a similar proportion of nitrogen in fishes from deep lakes, i.e. its composition is quite different from what it would be were it accumulated within the swimbladder by mere ordinary diffusion processes. Further, the formation of gas is shown by experiment to be controlled by branches of the vagus and sympathetic nerves in an exactly similar fashion to the secretion of saliva in a salivary gland. (See below for relations of swimbladder to ear). Of the important non-respiratory derivatives of the pharyngeal wall (thyroid, thymus, postbranchial bodies, &c.), only the thyroid calls for special mention, as important clues to its evolutionary history are afforded by the lampreys. In the larval lamprey the thyroid develops as a longitudinal groove on the pharyngeal floor. From the anterior end of this groove there pass a pair of peripharyngeal ciliated tracts to the dorsal side of the pharynx where they pass backwards to the hind end of the pharynx. Morphologically the whole apparatus corresponds closely with the endostyle and peripharyngeal and dorsal ciliated tracts of the pharynx of Amphioxus. The correspondence extends to function, as the open thyroid'groove secretes a sticky mucus which passes into the pharyngeal cavity for the entanglement of food particles exactly as in Amphioxus. Later on the thyroid groove becomes shut off from the pharynx; its secretion now accumulates in the lumina of its interior and it functions as a ductless gland as in the Gnathostomata. The only conceivable explanation of this developmental history of the thyroid in the lamprey is that it is a repetition of phylogenetic history. Behind the pharynx comes the main portion of the alimentary canal concerned with the digestion and absorption of the food,, This forms a tube varying greatly in length, more elongated and coiled in the higher Teleostornes, shorter and straighter in the Selachians, Dipnoans and lower Teleostornes. The oesophagus or gullet, usually forming a short, wide tube, leads into the glandular, more or less dilated stomach. This is frequently in the form of a letter J, the longer limb being continuous with the gullet, the shorter with the intestine. The curve of the J may be asin Polypterus and the perch produced backwards into a large pocket. The intestine is usually marked off from the stomach by a ring-like sphincter muscle forming the pouches; e, Pulmonary vein; These differences are held by f, Arterial blood-vessel; oe, many to he sufficient to invalidate l)esunhagus, opened to show the homologizing of the swim- glottis (gl.) bladder with the lung. The follow- ing facts, however, appear to do away with the force of such a contention. (t) In the Dipneusti (e.g. Neoceraladies) the lung apparatus has acquired a dorsal position. but its ctuu,cxion with the mid-ventral glottis is asym,netri' el, passing round the right side of the gut. Were the predominant pyloric valve. In the lower gnathostomatous fishes (Selachians, I Crossopterygians, Dipnoans, sturgeons) the intestine possesses the highly characteristic spiral valve, a shelf-like projection into its lumen which pursues a spiral course, and along the turns of which the food passes during the course of digestion. From its universal occurrence in the groups mentioned we conclude that it is a structure of a very archaic type, once characteristic of ancestral Gnathostomata; a hint as to its morphological significance is given by its method of development.' In an early stage of development the intestinal rudiment is coiled into a spiral and it is by the fusion together of the turns that the spiral valve arises. The only feasible explanation of this peculiar method of development seems to lie in the assumption that the ancestral gnathostome possessed an elongated coiled intestine which subsequently became shortened with a fusion of its coils. In the higher fishes the spiral valve has disappeared--b'ng still found, however, in a reduced condition in Amid and :Le idosteus, and possibly as a faint vestige in one or two Teleosts (certain Clupeidue 2 and Salmonidae3). In the majority of the l Teleosts the absence of spiral valves is coupled with a secondary elongation of the intestinal region, which in extreme cases (Loricariidae) may be accompanied by a secondary spiral coiling. The terminal part of the alimentary canalthe cloacais characterized by the fact that into it open the two kidney ducts. In Teleostomes the cloaca is commonly flattened out, so that the kidney ducts and the alimentary canal come to open in-dependently on the outer surface. The lining of the alimentary canal is throughout the greater part of its extent richly glandular. And at certain points local enlargements of the secretory surface take place so as to form glandular diverticula. The most ancient of these as indicated by its occurrence even in Amphioxus appears to be the liver, which, originallyas we may assumemainly a digestive gland, has in the existing Craniates developed important excretory and glycogen-storing functions. Arising in the embryo as a simple caecum, the liver becomes in the adult a compact gland of very large size, usually bi-lobed in shape and lying in the front portion of the splanchnocoele. The stalk of the liver rudiment becomes drawn out into a tubular bile duct, which may become subdivided into branches, and as a rule develops on its course a pocket-like expansion, the gall-bladder. This may hang freely in the splanchnocoele or may be, as in many Selachians, imbedded in the liver substance. The pancreas also arises by localized bulging outwards of the intestinal liningthere being commonly three distinct rudiments in the embryo. In the Selachians the whitish compact pancreas of the adult opens into the intestine some little distance behind the opening of the bile duct, but in the Teleostomes it becomes involved in the liver outgrowth and mixed with its tissue, being frequently recognizable only by the study of microscopic sections. In the Dipnoans the pancreatic rudiment remains imbedded in the wall of the intestine: its duct is united with that of the liver. Pyloric Caeca.In the Teleostomi one or more glandular diverticula commonly occur at the commencement of the intestine and are known as the pyloric caeca. There may be a single caecum (crossopterygians, Ammodytes amongst Teleosts) or there may be nearly two hundred (mackerel). In the sturgeons the numerous caeca form a compact gland. In several families of Teleosts, on the other hand, there is no trace of these pyloric caeca. In Selachians a small glandular diverticulum known as the rectal gland opens into the terminal part of the intestine on its dorsal side. Coelomic Organs.The development of the mesoderm in the restricted sense (mesothelium) as seen in the fishes (lamprey, Lepidosiren, Protopterus, Polypterus) appears to indicate beyond i J. Ruckert, Arch. Entwickelungsmech. Band iv., 1897, S. 298; J. Graham Kerr, Phil. Trans. B. 192, 1900, p. 325, and The Budgett Memorial Volume. 2 Cuvier et Valenciennes, 'Hist. nat. des poiss. xix., 1846, p. 151. 7 J. Rathke, Ub. it. Darmkanal u.s.w. it. Fische, Halle, 1824, S. 62. doubt that the mesoderm segments of vertebrates are really enterocoelic pouches in which the development of the lumen is delayed. Either the inner, or both inner and outer (e.g. Lepidosiren) walls of the mesoderm segment pass through a myoepithelial condition and give rise eventually to the great muscle segments (myomeres, or myotomes) which lie in series on each side of the trunk. In the fishes these remain distinct throughout life. The fins, both median and paired, obtain their musculature by the ingrowth into them of muscle buds from the adjoining myotomes. Electrical Organs.'It is characteristic of muscle that at the moment of contraction it produces a slight electrical disturbance. In certain fishes definite tracts of the musculature show a reduction of their previ- ously predominant function of contraction and an increase of their previously subsidiary function of producing electrical disturbance; so that the latter function is now predominant. In the skates (Raia) the electrical organ is a fusiform structure derived from the lateral musculature of the tail; in Gymnotusthe electric eeland in Mormyrus it forms an enormous structure occupying the place of the ventral halves of the myotomes along nearly the whole length of the body; in Torpedo it forms a large, somewhat kidney- shaped structure as From Gegenbaur, Untersuchungen zur vergleich, viewed from above Anat. der Wirbeltiere, by permission of Wilhelm lying on each side of the Engelmann. head and derived from Fin. to.View of Torpedo from the dorsal the musculature of the side: the electric organs are exposed. anterior visceral arches. In Torpedo the nerve- I, Forebrain. supply is derived from II, Mesencephalon. cranial nerves VII. IX. III, Cerebellum. and the anterior bran- IV, Electric lobe. chiai branches of X. br, Common muscular sheath covering The electric organ branchial clefts (on the left side this has been removed so as to expose the is composed of pris- series of branchial sacs). matic columns each f, Spiracle. built up of a row of o.e, Electric organ, on the left side the compartments. Each nerve-supply is shown. o, Eye. compartment contains t, Sensory tubes of lateral line system. a lamellated electric disc representing the shortened-up and otherwise metamorphosed muscle fibre. On one face (ventral in Torpedo, anterior in Raia) of the electric disc is a gigantic end-plate supplied by a beautiful, dichotomously branched, terminal nervous arborization. The development of the mesoderm of the head region is too obscure for treatment here. The ventral portion of the trunk mesoderm gives rise to the splanchnocoel or general coelom. Except in the Myxinoids the anterior part of the, splanchnocoel becomes separated off as a pericardiac cavity, though in adult Selachians the separation becomes incomplete, the two cavities being in communication by a pericardio-peritoneal canal. Nephridial System.The kidney system in fishes consists of segmentally arranged tubes leading from the coelom into a longitudinal duct which opens within the hinder end of the enteronthe whole forming what is known as the archinephros (Lankester) or holonephros (Price). Like the other segmented Cf. W. Biedermann, Electro-Physiology. e Literature in N. K. Koltzoff, Bull. Soc. Nat. Moscou, 1904, p. 259 t organs of the vertebrate the archinephros develops from before backwards. The sequence is, however, not regular. A small number of tubules at the head end of the series become specially enlarged and are able to meet the excretory needs during larval existence (Pronephros): the immediately succeeding tubules remain undeveloped, and then come the tubules of the rest of the series which form the functional kidney of the adult (Mesonephros). The kidney tubules subserve the excretory function in two different ways. The wall of the tubule, bathed in blood from the posterior cardinal vein, serves to extract nitrogenous pro-ducts of excretion from the blood and pass them into the lumen of the tubule. The open ciliated funnel or nephrostome at the coelomic end of the tubule serves for the passage outwards of coelomic fluid to flush the cavity of the tubule. The secretory activity of the coelomic lining is specially concentrated in certain limited areas in the neighbourhood of the nephrostomes, each such area ensheathing a rounded mass depending into the coelom and formed of a blood-vessel coiled into a kind of skeina glomerulus. In the case of the pronephros the glomeruli are as a rule fused together into a single glomus. In the mesonephros they remain separate and in this case the portion of coelom surrounding the glomerulus tends to be nipped off from the general coelomto form a Malpighian body. The separation may be incompletethe Malpighian coelom remaining in connexion with the general coelom by a narrow peritoneal canal. The splanchnocoelic end of this is usually ciliated and is termed a peritoneal funnel: it is frequently confused with the nephrostome. Mesonephros.The kidney of the adult fish is usually a compact gland extending over a considerable distance in an anteroposterior direction and lying immediately dorsal to the coelomic cavity. Peritoneal funnels are present in the adult of certain Selachians (e.g. Acanthias, Squatina), though apparently in at least some of these forms they no longer communicate with the Malpighian bodies or tubules. The kidneys of the two sides become fused together posteriorly in Protopterus and in some Teleosts. The mesonephric ducts undergo fusion posteriorly in many cases to form a median urinary or urinogenital sinus. In the Selachians this median sinus is prolonged forwards into a pair of horn-like continuationsthe sperm sacs. In Dipnoans the sinus becomes greatly dilated and forms a large, rounded, dorsally placed cloacal caecum. In Actinopterygians a urinary bladder of similar morphological import is commonly present. Gonads.The portion of coelomic lining which gives rise to the reproductive cells retains its primitive relations most nearly in the female, where, as a rule, the genital cells are still shed into the splanchnocoele. Only in Teleostomes (Lepidosteus and most Teleosts) the modification occurs that the ovary is shut off from the splanchnocoele as a closed cavity continuous with its duct. In a few Teleosts (Salmonidae, Muraenidae, Cobitis) the ovary is not a closed sac, its eggs being shed into the coelom as in other groups. The appearance of the ovary naturally varies greatly with the character of the eggs. The portion of coelomic lining which gives rise to the male genital cells (testis) is in nearly, if not quite, all cases, shut off from the splanchnocoele. The testes are commonly elongated in form. In Dipneusti' (Lepidosiren and Protopterus) the hinder portion of the elongated testis has lost its sperm-producing function, though the spermatozoa produced in the anterior portion have to traverse it in order to reach the kidney. In Yul yplerus 2 the testis is continued backwards as a " testis ridge," which appears to correspond with the posterior vesicular region of the testis in Lepidosiren and Protopterus. Here also the spermatozoa pass back through the cavities of the testis ridge to reach the kidney duct. In the young Teleost' the rud ment of the duct forms a backward continuation of the 1 J. Graham Kerr, Proc. Zool. Soc. Lond. (1901), p. 484. z J. S. Budgett, Trans. Zool. Soc. Lond. xv. (1901), vol. p. 24. H. F. Jungersen, Arb. zool. toot. Inst. IVurzburg, Band ix., 1889.testis containing a network of cavities and opening as a rule posteriorly into the kidney duct. It is difficult to avoid the conclusion that the testis duct of the Teleost is for the most part the equivalent morphologically of the posterior vesicular region of the testis of Polypterus and the Dipneusti. Relations of Renal and Reproductive Organs. (r) Female. In the Selachians and Dipnoans the oviduct is of the type (Mullerian duct) present in the higher vertebrates and apparently representing a split-off portion of the archinephric duct. At its anterior end is a wide funnel-like coelomic opening. Its walls are glandular and secrete accessory coverings for the eggs. In the great majority of Teleosts and in Lepidosteus the oviduct possesses no coelomic funnel, its walls being in structural continuity with the wall of the ovary. In most of the more primitive Teleostomes (Crossopterygians, sturgeons, Amia) the oviduct has at its front end an open coelomic funnel, and it is difficult to find adequate reason for refusing to regard such oviducts as true Mullerian ducts. On this interpretation the condition characteristic of Teleosts would be due to the lips of the oviduct becoming fused with the ovarian wall, and the duct itself would be a Mullerian duct as elsewhere. A departure from the normal arrangement is found in those Teleosts which shed their eggs into the splanchnocoele, e.g. amongst Salmonidae, the smelt (Osmerus) and capelin (Mallotus) possess a pair of oviducts resembling Mullerian ducts while the salmon possesses merely a pair of genital pores opening together behind the anus. It seems most probable that the latter condition has been derived from the former by reduction of the Mullerian ducts, though it has been argued that the con-verse process has taken place. The genital pores mentioned must not be confused with the abdominal pores, which in many adult fishes, particularlyinthosewithout open peritoneal funnels, lead from coelom directly to the exterior in the It. region of the cloacal opening. These appear to be recent developments, and to have nothing to do morphologically with the genitourinary system.' (2) Male.It seems that primitively the male reproductive elements like the female were shed into the coelom and passed thence through the nephridial tubules. In correlation probably with the greatly reduced size of these elements they are commonly no longer shed into the splanchnocoele, but are conveyed from the testis through covered-in canals to the Malpighian bodies or kidney tubules. The system of covered-in canals forms the testicular network, the individual canals being termed vasa efferentia. In all probability the series of vasa efferentia was originally spread over the whole length of the elongated testis (cf. Lepidosteus), but in existing fishes the series is as a rule ' E. J. Bles, Proc. Roy. Soc. 62, 1897, p. 232. From Arch. zool. experintenlale, by permission of Schleicher Frees. m.n. 1, Anterior (genital) portion of mesonephros with its coiled duct. m.n. 2, Posterior (renal) portion of mesonephros. s.s, Sperm sac. T, Testis. u," Ureter " formed by fusion of collecting tubes of renal portion of mesonephros. u.g.s, Urino-genital sinus; v.s, Vesicula seminalis. restricted to a comparatively short anteroposterior extent. In Selachians the vasa efferentia are restricted to the anterior end of testis and kidney, and are connected by a longitudinal canal ending blindly in front and behind. The number of vasa efferentia varies and in the rays (Raia, Torpedo) may be reduced to a single one opening directly into the front end of the mesonephric duct. The anterior portion of the mesonephros is much reduced in size in correlation with the fact that it has lost its renal function. The hinder part, which is the functional kidney, is considerably enlarged. The primary tubules of this region of the kidney have undergone, a modification of high morphological interest. Their distal portions have become much elongated, they are more or less fused, and their openings into the mesonephric duct have undergone backward migration until they open together either into the mesonephric duct at its posterior end or into the urinogenital sinus independently of the mesonephric duct. The mesonephric duct is now connected only with the anterior part of the kidney, and serves merely as a vas deferens or sperm duct. In correlation with this it is somewhat enlarged, especially in its posterior portion, to form a vesicula seminalis. The morphological interest of these features lies in the fact that they represent a stage in evolution which carried a little farther would lead to a complete separation of the definitive kidney (metanephros) from the purely genital anterior section of the mesonephros (epididymis), as occurs so characteristically in the Amniota. Dipneusti.In Lepidosiren 1 a small number (about half a dozen) of vasa efferentia occur towards the hind end of the vesicular part of the testis and open into Malpighian bodies. In Protopterus the vasa efferentia are reduced to a single one on each side at the extreme hind end of the testis. Teleostomi.In the actinopterygian Ganoids a well-developed testicular network is present; e.g. in Lepidosteus 2 numerous vasa efferentia arise from the testis along nearly its whole length and pass to a longitudinal canal lying on the surface of the A.' B C 'D Graham Kerr, Proc. Zool. Sac. London. A, Distributed , ondition of vasa D, Direct communication be- efferentia (Acipenser, Lepi- tween testis and kidney dosteus). duct (Polypterus,Teleosts). B, Vasa efferentia reduced to a c.f, Nephrostome leading from few at the hind end (Lepi- Malpighian coelom into dosiren). kidney tubule. C, Reduction of vasa efferentia T,, Functional region of testis. to a single one posteriorly T2, Vesicular region of testis. (Protopterus). WD, Mesonephric duct. kidney, from which in turn transverse canals lead to the Mal- pighian bodies. (In the case of Amia they open into the tubules or even directly into the mesonephric duct.) In the Teleosts and in Polypterus there is no obvious connexion between testis and kidney, the wall of the testis being continuous with that of its duct, much as is the case with the ovary and its duct in the female. In all probability this peculiar condition is to be 1 J. Graham Kerr, Proc. Zool. Soc. Lond. (1901) p. 484. ' F. M. Balfour and W. N. Parker, Phil. Trans. (1882). explained 3 by the reduction of the testicular network to a single vas efferens (much as in Protopterus or as in Raia and various anurous Amphibians at the front end of the series) which has come to open directly into the mesonephric duct (cf. fig. 12). Organs of the Mesenchyme.In vertebrates as in all other Metazoa, except the very lowest, there are numerous cell elements which no longer form part of the regularly arranged epithelial layers, but which take part in the formation of the packing tissue of the body. Much of this forms the various kinds of connective tissue which fill up many of the spaces between the various epithelial layers; other and very important parts of the general mesenchyme become specialized in two definite directions and give rise to two special systems of organs. One of these is characterized by the fact that the intercellular substance or matrix assumes a more or less rigid characterit may be infiltrated with salts of limegiving rise to the supporting tissues of the skeletal system. The other is characterized by the inter-cellular matrix becoming fluid, and by the cell elements losing their connexion with one another and forming the characteristic fluid tissue, the blood, which with its well-marked containing walls forms the blood vascular system. Skeletal System.The skeletal system may be considered under three headings(r) the chordal skeleton, (2) the cartilaginous skeleton and (3) the osseous skeleton. 1. Chordal Skeleton.The most ancient element of the skeleton appears to be the notochorda cylindrical rod composed of highly vacuolated cells lying ventral to the central nervous system and dorsal to the gut. Except in Amphioxuswhere the condition may probably be secondary, due to degenerative shortening of the central nervous systemthe notochord extends from a point just behind the infundibulum of the brain (see below) to nearly the tip of the tail. In ontogeny the notochord is a derivative of the dorsal wall of the archenteron. The outer layer of cells, which are commonly less vacuolated and form a " chordal epithelium," soon secretes a thin cuticle which ensheaths the notochord and is known as the primary sheath. Within this there is formed later a secondary sheath, like the primary, cuticular in nature. This secondary sheath attains a considerable thickness and plays an important part in strengthening the notochord. The notochord with its sheaths is in existing fishes essentially the skeleton of early life (embryonic or larval). In the adult it may, in the more primitive forms (Cyclostomata, Dipneusti), persist as an important part of the skeleton, but as a rule it merely forms the foundation on which the cartilaginous or bony vertebral column is laid down. 2. Cartilaginous or Chondral Skeleton.(A) Vertebral column.' In the embryonic connective tissue or mesenchyme lying just outside the primary sheath of the notochord there are developed a dorsal and a ventral series of paired nodules of cartilage known as arcualia (fig. 13, d.a, v.a). The dorsal arcualia are commonly prolonged upwards by supradorsal cartilages which complete the neural arches and serve to protect the spinal cord. The ventral arcualia become, in the tail region only, also incorporated in complete archesthe haemal arches. In correlation with the flattening of the body of the fish from side to side the arches are commonly prolonged into elongated neural or haemal spines. The relations of the arcualia to the segmentation of the body, as shown by myotomes and spinal nerves, is somewhat obscure. The mesenchyme in which they arise is segmental in origin (sclerotom, which suggests that they too may have been primitively segmental, but in existing fishes there are commonly two sets of arcualia to each body segment. In gnathostomatous fishes the arcualia play a most important part in that cartilaginous tissue derived from them comes into special relationships with the notochord and gives rise to the vertebral column which functionally replaces this notochord in most of the fishes. This replacement occurs according to two different methods, giving rise to the different types of vertebral column known as chordacentrous and arcicentrous. ' J. Graham Kerr, Proc. Zool. Soc. Lond. (1901), p. 495. ' H. Gadow and E. C. Abbott, Phil. Trans. 186 (1895), p. 163 cave centrum in general appearance much like that of the Selachians In Lepidosteus the spaces between adjacent centra become filled by, a secondary development of intervertebral cartilage which then splits in such a way that the definitive vertebrae are opisthocoelous, (a) Chordacentrous type. An incipient stage in the evolution of a chordacentrous vertebral column occurs in the Dipneusti, where cartilage cells from the arcualia become amoeboid and migrate into the substance of the secondary sheath, boring their way through the primary sheath (fig. 13, C). They wander throughout the whole extent of the secondary sheath, colonizing it as it were, and settle down as typical stationary cartilage cells. The secondary sheath is thus converted into a cylinder of cartilage. In Selachians exactly the same thing takes place, but in recent forms development goes a step further, as the cartilage cylinder becomes.. broken into a series of segments, known as vertebral centra. The wall of each segment becomes much thickened in the middle so that the notochord becomes constricted within each centrum and the space occupied by iti.e. concave behind, convex in front. Ribs.In the Crossopterygians a double set of " ribs " is present on each side of the vertebral column, a vential set lying immediately outside the splanchnocoelic lining and apparently serially homologous with the haemal arches of the caudal region, and a second set passing outwards in the thickness of the body wall at a more dorsal level. In the Teleostomes and Dipnoans only the first type is present; in the Selachians only the second. It would appear that it is the latter which is homologous with the ribs of vertebrates above fishes. Median Fin Skeleton.The foundation of the skeleton of the median fins consists of a series of rod-like elements, the radialia, each of which frequently is segmented into three portions. In a few cases the radialia correspond segmentally with the neural and haemal arches {living Dipnoans, Pleuracanthus tail region) and this suggests that they represent morphologically prolongations of the neural and haemal spines. That this is so is rendered probable by the fact that we must regard the evolution of the system of median fins as commencing with a simple flattening of the posterior part of the body. It is only natural to suppose that the edges of the flattened region would be at first supported merely by prolongations of the already existing spinous processes. In the Cyclostomes (where they are branched) and in the Selachians, the radialia form the main supports of the fin, though already in the latter they are reinforced by a new set of fin rays apparently related morphologically to the osseous or placoid skeleton (see below). The series of radialia tends to undergo the same process of local concentration which characterizes the fin-fold as a whole. In its extreme form this leads to |
