1911 Encyclopædia Britannica/Ichthyology

ICHTHYOLOGY (from Gr. ἰχθύς, fish, and λόγος, 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 (384–322 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, mode 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 may be 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 1547-1550; he collected rich stores of positive knowledge, which he embodied in several works. The one most important for the Belon. progress of ichthyology is that entitled De aquatilibus libri duo (Paris, 1553). 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 it—two 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 (1514-1572), bears evidence of the high social position which the author held as physician to three popes. Its title is Aquatilium animalium historia (Rome, 1554-1557, fol.). It treats Salviani. 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 be compared with Rondelet or Belon.

G. Rondelet (1507-1557) had the great advantage over Belon of having received a medical education at Paris, and especially of having gone through a complete course of instruction in anatomy as a pupil of Guentherus of Andernach. Rondelet. This is conspicuous throughout his works—Libri de piscibus marinis (Lyons, 1554); and Universae aquatilium historiae pars altera (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 classification—confounding 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 (1630-1644).

Of the men who left records of their anatomical researches, we may mention Borelli (1608-1679), who wrote a work De motu animalium (Rome, 1680, 4to), in which he explained the mechanism of swimming and the function of the air-bladder; M. Malpighi (1628-1694), who examined the optic nerve of the sword-fish; the celebrated J. Swammerdam (1637-1680), who described the intestines of numerous fishes; and J. Duverney (1648-1730), 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 (1628-1705) was the friend and guide of F. Willughby (1635-1672). 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 Ray and Willughby. order into the existing chaos was by arranging the 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 spinous structure of the dorsal rays, the number of dorsal fins, &c. No fewer than four hundred and twenty species are thus arranged and described, of which about one hundred and eighty were known to the authors from personal examination—a 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 (1705-1734) would have been a great ichthyologist if Ray or Willughby had not preceded him. But he was fully Artedi. 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:—

(1) In the Bibliotheca ichthyologica Artedi gives a very complete list of all preceding authors who had written on 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 piscium contains well-defined diagnoses of forty-five genera, for which he has fixed an unchangeable nomenclature. (4) In the Species piscium descriptions of seventy-two species, examined by himself, are given—descriptions which even now are models of exactitude and method. (5) Finally, in the Synonymia piscium 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, Raía, Squalus, Chimaera. Spiraculis solitariis.—Lophius, Acipenser, Cyclopterus, Balistes, Ostracion, Tetrodon, Diodon, Centriscus, Syngnathus, Pegasus.

B. Pisces apodes.—Muraena, Gymnotus, Trichiurus, Anarrhichas, Ammodytes, Ophidium, Stromateus, Xiphias.

C. Pisces jugulares.—Callionymus, Uranoscopus, Trachinus, Gadus, Blennius.

D. Pisces thoracici.—Cepola, Echeneis, Coryphaena, Gobius, Cottus, Scorpaena, Zeus, Pleuronectes, Chaetodon, Sparus, Labrus, Sciaena, Perca, Gasterosteus, Scomber, Mullus, Trigla.

E. Pisces abdominales.—Cobitis, Amia, Silurus, Teuthis, Loricaria, 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 to an 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. Brünnich in the Mediterranean, Osbeck in Java and China, K. P. Thunberg in Japan; Forskål examined and described the fishes of the Red Sea; G. W. Steller, P. S. Pallas, S. G. Gmelin, and A. J. Güldenstädt 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. Müller (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 Lacepède, 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:— (1) Öconomische Naturgeschichte der Fische Deutschlands Bloch. (Berl., 1782-1784); (2) Naturgeschichte der ausländischen 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 deceived as 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 cx. illustratum (Berl., 1801). 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 Lacepède, born at Agen, in France, in 1756, who became professor at the museum of natural history in Paris, where he died in 1825.

Lacepède 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 of the French Revolution. A great part of it was Lacepède. 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 Lacepède 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 Lacepède 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 and John Hunter examined the nervous system and the organs of sense; and Alexander Monro, secundus, published a classical work, The Structure and Physiology of Fishes Explained and Compared with those of Man and other Animals (Edin., 1785). The electric organs of fishes (Torpedo and Gymnotus) were examined by Réaumur, J. N. S. Allamand, 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 Salmonidae was known and practised by J. G. Gleditsch (1764).

Bloch and Lacepède’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 Vizagapatam on the Coast of Coromandel (Lond., 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 (Lond., 1802-1808); and the Mediterranean fauna formed the study of the lifetime of A. Risso, Ichthyologie de Nice (Paris, 1810); and Histoire naturelle de l’Europe méridionale (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 Ichthyology of New York, in 1815.

G. Cuvier (1769-1832) devoted himself to the study of fishes with particular predilection. The investigation of their anatomy, and especially their skeleton, was continued until he had succeeded in completing so perfect a framework Cuvier. 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 forms, and accurately defined the divisions, orders, families and genera of the class, as they appear in the various editions of the Règne 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 1820, Cuvier, assisted Valenciennes. by one of his pupils, A. Valenciennes, commenced his great work on fishes, Historie 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.

 1. A Mâchoire Supérieure Libre. a. Acanthoptérygiens. Percoïdes. Sparoïdes. Branchies labyrinthiques. Polynèmes. .mw-parser-output .nowrap,.mw-parser-output .nowrap a:before,.mw-parser-output .nowrap .selflink:before{white-space:nowrap}Chétodonoïdes. Lophioïdes. Mulles. Scombéroïdes. Gobioïdes. Joues cuirassées. Muges. Labroïdes. Sciénoïdes. b. Malacoptérygiens. Abdominaux. Subbrachiens. Apodes. —— —— —— Cyprinoïdes Gadoïdes. Murénoïdes. Siluroïdes. Pleuronectes. Salmonoïdes. Discoboles. Clupéoïdes. Lucioïdes.

2. A Mâchoire Supérieure Fixée.
Selérodermes.Gymnodontes.

II. A Branchies en Forme de Houppes.
Lophobranches.

B. CARTILAGINEUX OU CHONDROPTÉRYGIENS.
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 those of 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 Agassiz. path towards the knowledge of a whole new subclass 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. Placoids.—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 Siluroides; also the Sturgeons. 3. Ctenoids.—With rough scales, which have their free margins denticulated: Chaetodontidae, Pleuronectidae, Percidae, Polyacanthi, 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, Neuchâtel, 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 Müller (1801-1858). In his classical memoir Über den Bau und 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 cartilaginous fishes was proved; they became the types of two other subclasses, Cyclostomi and Leptocardii.

Müller 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.—1. Voyage autour du monde sur les corvettes de S. M. l'Uranie et la Physicienne, sous le commandment de M. Freycinet, “Zoologie—Poissons,” par Quoy et Gaimard (Paris, 1824). 2. Voyage de la Coquille, “Zoologie,” par Lesson (Paris, 1826-1830). 3. Voyage de l'Astrolabe, sous le commandement de M. J. Dumont d'Urville, “Poissons,” par Quoy et Gaimard (Paris, 1834). 4. Voyage au Pôle Sud par M. J. Dumont d'Urville, “Poissons,” par Hombron et Jacquinot (Paris, 1853-1854).

B. English.—1. 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.—1. Reise der österreichischen Fregatte 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., 1838). 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.—1. H. Kröyer, Danmark's Fiske (Copenhagen, 1838-1853). 2. S. Nilsson, Skandinavisk Fauna, vol. iv. “Fiskarna” (Lund, 1855). 3. Fries och Ekström, Skandinaviens Fiskar (Stockh., 1836).

C. Russia.—1. Nordmann, “Ichthyologie pontique,” in Demidoff's Voyage dans la Russie méridionale, tome iii. (Paris, 1840).

D. Germany.—1. Heckel und Kner, Die Süsswasserfische der österreichischen Monarchie (Leipz., 1858). 2. C. T. E. Siebold, Die Süsswasserfische von Mitteleuropa (Leipz., 1863).

E. Italy and Mediterranean.—1. Bonaparte, Iconografia della fauna italica, tom iii., “Pesci” (Rome, 1832-1841). 2. Costa, Fauna del regno di Napoli, “Pesci” (Naples, about 1850).

F. France.—1. 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. Capello made contributions to our knowledge of the marine fishes on the coast of Portugal (Jorn. Scienc. Acad. Lisb.).

H. North America.—1. J. Richardson, Fauna Boreali-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., Washington, 1873-1879) and Reports and special publications of the U.S. Bureau of Fisheries contain valuable information. Numerous descriptions of North American fresh-water 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.—1. Fauna Japonica, “Poissons,” par H. Schlegel, (Leiden, 1850).

J. East Indies; Tropical parts of the Indian and Pacific Oceans.—1. E. Rüppell, Atlas zu der Reise im nördlichen Afrika (Frankf., 1828). 2. E. Rüppell, Neue Wirbelthiere, “Fische” (Frankf., 1837). 3. R. L. Playfair and A. Günther, The Fishes of Zanzibar (Lond., 1876). 4. C. B. Klunzinger, Synopsis der Fische des Rothen Meers (Vienna, 1870-1871). 5. F. Day, The Fishes of India (Lond., 1865, 4to) contains an account of the fresh-water and marine species. 6. A. Günther, Die Fische der Südsee (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 chiefly in the journals of the Batavian Society. Soon after his return to Europe (1860) Bleeker commenced to collect the final results of his labours in a grand work, illustrated by coloured plates, Atlas ichthyologique des Indes Orientales Néerlandaises (Amsterd., fol., 1862), the publication of which was interrupted by the author's death in 1878.

K. Africa.—1. A. Günther, “The Fishes of the Nile,” in Petherick's Travels in Central Africa (Lond., 1869). 2. W. Peters, Naturwissenschaftliche Reise nach Mossambique, iv., “Flussfische” (Berl., 1868, 4to).

L. West Indies and South America.—1. 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'expédition dans les parties central es de l'Amérique du Sud, “Poissons” (Paris, 1855). 3. L. Vaillant and F. Bocourt, Mission scientifique au Mexique et dans l'Amérique 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, issued by himself, under the title of Memorias sobre la historia natural de la isla de Cuba (from 1851), and Repertorio fisico-natural de la isla de Cuba (from 1865), partly in North American scientific journals. And, finally, F. Steindachner and A. Günther have published many contributions, accompanied by excellent figures, to our knowledge of the fishes of Central and South America.

M. New Zealand.—1. F. W. Hutton and J. Hector, Fishes of New Zealand (Wellington, 1872).

N. Arctic Regions.—1. C. Lütken, “A Revised Catalogue of the Fishes of Greenland,” in Manual of the Natural History, Geology and Physics of Greenland (Lond., 1875, Svo). 2. The fishes of Spitzbergen were examined by A. J. Malmgren (1865).

(A. C. G.)

II. History and Literature from 1880

In the systematic account which followed the above chapter in the 9th 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, 1880) was adopted by Albert Günther:—

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, Taeniiformes and Notacanthiformes.
Order II.: Acanthopterygii Pharyngognathi.
Order III.: Anacanthini.
With two divisions: Gadoidei and Pleuronectoidei.
Order IV.: Physostomi.
Order V.: Lophobranchii.
Order VI.: Plectognathi.
Subclass III.: Cyclostomata.
Subclass IV.: Leptocardii.

It was an artificial system, in which the most obvious relationships 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, andnot between the Ganoids and the Teleosteans”; 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 Günther), Ganoidei and Dipnoi of Müller 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.

Günther's classification would have been vastly improved 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 (1840-1897) 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, Glanencheli, Ichthyocephali, Holostomi, Enchelycephali, Colocephali.
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 relationships, 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-Müllerian 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 families of Vertebrata,” Amer. Natur., 1889, p. 849):—

 Class: Agnatha. I. Subclass: Ostracodermi. Orders: Arrhina, Diplorrhina. II. Subclass: Marsipobranchii. Orders: Hyperotreti, Hyperoarti. Class: Pisces. I. Subclass: Holocephali. II. Subclass: Dipnoi. III. Subclass: Elasmobranchii. Orders: Ichthyotomi, Selachii. 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, 1889-1901), and in his most useful Outlines of Vertebrate Paleontology (Cambridge, 1898), and was adopted by Günther in the 10th 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) Pleuropterygii (Cladoselache); (b) Ichthyotomi (Pleuracanthidae); (c) Acanthodii (Diplacanthidae, and Acanthodidae); (d) Selachii (divided from the structure of the vertebral centres into Asterospondyli and Tectospondyli). II. Subclass: Holocephali. With one order: Chimaeroidei. III. Subclass: Dipnoi. With two orders: (a) Sirenoidei (Lepidosiren, Ceratodus, Uronemidae, Ctenodontidae); (b) Arthrodira (Homosteus, Coccosteus, Dinichthys). IV. Subclass: Teleostomi. A. Order: Crossopterygii. With four suborders: (1) Haplistia (Tarassius); (2) Rhipidistia (Holoptychidae, Rhizodontidae, Osteolepidae); (3) Actinistia (Coelacanthidae); (4) Cladistia (Polypterus). B. Order: Actinopterygii. With about twenty suborders: (1) 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 (Cyprinidae, 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 Günther’s classification of 1880 still commended itself. Recent work on Palaeospondylus, 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 III. Teleostomi,

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 publication of Günther’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, but 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. Günther’s Catalogue of the Fishes in the British Museum therefore remains the only general descriptive treatise, but its last volume dates from 1870, 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 Günther’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 1880, 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 fresh-water 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. Günther (Challenger), A. Alcock (Investigator), R. Collett (Hirondelle), S. Garman (Albatross) and a general résumé up to 1895 was provided in G. B. Goode’s and T. H. Bean’s Oceanic Ichthyology. More than 600 true bathybial fishes are known from depths of 1000 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 Teleosteans 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 much discussed.

(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.

III. Teleostomi, with the skull well developed and with membrane bones, with paired nasal apertures, primarily with median and paired fins, including all other fishes.

(G. A. B.)

IV. Anatomy[1]

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 side 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 sub-divisions of the body—head, trunk and tail—as 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.

The tail, or postanal region, is probably a secondary development—a 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 categories—the median or unpaired fins and the paired fins.

 Fig. 1.—Heterocercal Tail of Acipenser. a, Modified median scales (“fulcra”); b, bony plates.

 From Cambridge Natural History, vol. vii., “Fishes, &c.,” by permission of Messrs. Macmillan & Co., Ltd. Fig. 2.—Cladoselache. (After Dean.)

The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement was by waves of lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The system of median fins consists in the first instance of a continuous fin-fold extending round the posterior end of the body—as persists even in the adult in the existing Dipneusti. A continuous median fin-fold occurs also in various Teleosts (many deep-sea Teleosts, eels, &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 there is a greatly increased size of the fin-fold—both 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 tip—the 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 ventral side, and correlated with this the skeletal axis is turned upwards as in the heterocercal 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).

 From “Challenger” Reports Zool., published by H.M. Stationery Office. Fig. 3.—Chlamydoselachus. (After Günther.)

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 protocercal 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 portions—the 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, Eckeneis) the anterior dorsal fin is metamorphosed into a sucker by which the creature attaches itself to larger fishes, turtles, &c.

From Cambridge Natural History, vol. vii., “Fishes, &c.,” by permission of Messrs. Macmillan & Co., Ltd.

Fig. 4.Tilapia dolloi, a teleostean fish, to illustrate external features. (After Boulenger.)

 A, Side view. g.r, Gill rakers. B, First branchial arch. l.l, Lateral line organs. a.f, Anal fin. n, Nasal opening. c.f, Caudal fin. p.f, Pelvic fin. d.f, Dorsal fin. p.op, Preoperculum. g.f, Gill lamellae. pt.f, Pectoral fin.

The paired fins—though more recent developments than the median—are yet of very great morphological interest, 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 fin—the 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) or jugular (Gadus—cod, haddock, &c., fig. 5).

Fig. 5.—Burbot (Lota vulgaris), with jugular ventral fins.

The median fin, especially in its caudal section, is the main propelling 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: (1) 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.[2] Both views rest upon the assumed occurrence of stages for the existence of which there is no direct evidence, viz. in the case of (1) 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 gills—are 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 Malopterurus 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.[3] 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 Protopterus[4] the crescentic cement organ lying ventrally behind the 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[5] as an outgrowth from the wall of the alimentary canal, closely resembling a gill pouch. In Amia[6] the same appears to be the case. In a few Teleosts similar organs occur, e.g. Sarcodaces, Hyperopisus,[7] 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 undergone 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.

 From Trans. Zool. Soc. of London. Fig. 6.—Larva of Polypterus. (After Budgett.)

 From Phil. Transactions, Royal Society of London. Fig. 7.—Thirty Days’ Larval Lepidosiren. (After Graham Kerr.)

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 on each side—on visceral arches III., IV., V. and VI. (It may be recalled that in Urodeles they occur on arches III., IV. and V., with vestiges[8] 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 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 the 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 cloacal opening. Corresponding with structural and functional differences it is for descriptive purposes divided into the following regions—(1) 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.

From Bridge, Cambridge Natural History, vol. vii., “Fishes, &c.” (by permisson of Macmillan & Co., Ltd.). After Boas, Lehrbuch der Zoologie (by permission of Gustav Fischer).

Fig. 8.—Diagrams to illustrate the relations of branchial clefts and pharynx in an Elasmobranch (A) and a Teleost (B); 1, 2, &c., Branchial septa.

 b.c, Opercular cavity. b.l, Respiratory lamellae. c, Coelom. e.b.a, Opercular opening. hy.a, Hyoid arch. hy.c, Hyobranchial cleft. l.s, Valvular outer edge of gill septum. n, Nasal aperture. oes, Oesophagus. op, Operculum. p.q, Palato quadrate cartilage. Ph, Pharynx. sp, Spiracle.

In certain cases (e.g. Lepidosiren)[9] the buccal cavity arises by secondary excavation without any actual pushing in of ectoderm.

It is highly characteristic of the vertebrata that the pharynx—the 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.

The visceral clefts arise as hollow pouches (or at first solid projections) of the endoderm. Each pouch fuses with the ectoderm at its outer end and then becomes perforated so as to form a free communication between pharynx and exterior.

The mesenchymatous packing tissue between consecutive clefts forms the visceral arches, and local condensation within each gives rise to important skeletal elements—to which the name visceral arches is often restricted. From the particular skeletal structures which develop in the visceral arches bounding it the anterior cleft is known as the hyomandibular cleft, the next one as hyobranchial. 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 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 Teleostomes—including the ordinary Teleosts—the gill lamellae are attached only at their extreme inner end.

In the young of Selachians and certain Teleosts (e.g. Gymnarchus and Heterotis)[10] 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.[11]

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,[12] and apparently also in Amia[13] 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.[14]

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 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, Fig. 9.—Lung of Neoceratodus, opened in its lower half to show its cellular pouches. a, Right half; b, Left half; c, Cellular pouches; e, Pulmonary vein; f, Arterial blood-vessel; oe, Oesophagus, opened to show glottis (gl.). an outgrowth of the pharyngeal wall intimately associated with gaseous interchange. In the Crossopterygians and Dipnoans this pharyngeal outgrowth agrees exactly in its mid-ventral 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 hydrostatic 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: (1) it arises in the young fish as a dorsal instead of as a ventral diverticulum, and (2) it derives its blood-supply not from the sixth aortic arch but from branches of the dorsal aorta.

These differences are held by many to be sufficient to invalidate the homologizing of the swimbladder with the lung. The following facts, however, appear to do away with the force of such a contention. (1) In the Dipneusti (e.g. Neoceratodus) the lung apparatus has acquired a dorsal position, but its connexion with the mid-ventral glottis is asymmetrical, passing round the right side of the gut. Were the predominant function 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 should 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 case 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 swimbladder 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 hydrostatic 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 90% 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 Teleostomes, shorter and straighter in the Selachians, Dipnoans and lower Teleostomes. 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 as in 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 pyloric valve. In the lower gnathostomatous fishes (Selachians, 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.[15] 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—being still found, however, in a reduced condition in Amia and Lepidosteus, and possibly as a faint vestige in one or two Teleosts (certain Clupeidae[16] and Salmonidae[17]). In the majority of the 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 canal—the cloaca—is 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 independently 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, originally—as we may assume—mainly 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 lining—there 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 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.[18]—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 previously predominant function of contraction and an increase of their previously subsidiary function of producing electrical disturbance; so that the latter function is now predominant.

From Gegenbaur, Untersuchungen zur vergleich. Anat. der Wirbeltiere, by permission of Wilhelm Engelmann.

Fig. 10.—View of Torpedo from the dorsal side: the electric organs are exposed.

 I, Fore-brain. II, Mesencephalon. III, Cerebellum. IV, Electric lobe. br, Common muscular sheath covering branchial clefts (on the left side this has been removed so as to expose the series of branchial sacs). f, Spiracle. o.e, Electric organ, on the left side the nerve-supply is shown. o, Eye. t, Sensory tubes of lateral line system.

In the skates (Raia) the electrical organ is a fusiform structure derived from the lateral musculature of the tail; in Gymnotus—the electric eel—and 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 viewed from above lying on each side of the head and derived from the musculature of the anterior visceral arches. In Torpedo the nerve-supply is derived from cranial nerves VII. IX. and the anterior branchial branches of X.

The electric organ is composed of prismatic columns each built up of a row of compartments. Each compartment contains 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.[19] 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 enteron—the whole forming what is known as the archinephros (Lankester) or holonephros (Price). Like the other segmented 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 products 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 skein—a 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 coelom—to form a Malpighian body. The separation may be incomplete—the 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 continuations—the 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[20] (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 Polypterus[21] 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[22] the rudiment of the duct forms a backward continuation of the 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. (1) Female.—In the Selachians and Dipnoans the oviduct is of the type (Müllerian 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 Müllerian 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 Müllerian duct as elsewhere.

From Arch. zool. expérimentale, by permission of Schleicher Frères.

Fig. 11.—Urino-Genital Organs of the right side in a male Scyllium. (After Borcea.)

 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.

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 Müllerian 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 Müllerian ducts, though it has been argued that the converse process has taken place. The genital pores mentioned must not be confused with the abdominal pores, which in many adult fishes, particularly in those without open peritoneal funnels, lead from coelom directly to the exterior in the region of the cloacal opening. These appear to be recent developments, and to have nothing to do morphologically with the genitourinary system.[23]

(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 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[24] 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.

Graham Kerr, Proc. Zool. Soc. London.

Fig. 12.—Diagram illustrating Connexion between Kidney and Testis in Various Groups of Fishes.

 A, Distributed condition of vasa efferentia (Acipenser, Lepidosteus). B, Vasa efferentia reduced to a few at the hind end (Lepidosiren). C, Reduction of vasa efferentia to a single one posteriorly (Protopterus). D, Direct communication between testis and kidney duct (Polypterus, Teleosts). c.f, Nephrostome leading from Malpighian coelom into kidney tubule. T1, Functional region of testis. T2, Vesicular region of testis. WD, Mesonephric duct.

Teleostomi.—In the actinopterygian Ganoids a well-developed testicular network is present; e.g. in Lepidosteus[25] numerous vasa efferentia arise from the testis along nearly its whole length and pass to a longitudinal canal lying on the surface of the kidney, from which in turn transverse canals lead to the Malpighian 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 explained[26] 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 character—it may be infiltrated with salts of lime—giving rise to the supporting tissues of the skeletal system. The other is characterized by the intercellular 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—(1) 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 notochord—a cylindrical rod composed of highly vacuolated cells lying ventral to the central nervous system and dorsal to the gut. Except in Amphioxus—where the condition may probably be secondary, due to degenerative shortening of the central nervous system—the 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.[27] 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 arches—the 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.

(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 it is shaped like the cavity of a dice-box. When free from notochord and surrounding tissues such a cartilaginous centrum presents a deep conical cavity at each end (amphicoelous).

From Wiedersheim, Grundriss der vergleichenden Anatomie, by permission of Gustav Fischer.

Fig. 13.—Diagrammatic transverse sections to illustrate the morphology of the vertebral column.

 A, Primitive conditions as seen in any young embryo. B, Condition as it occurs in Cyclostomata, sturgeons, embryos of bony Actinopterygians. C, Condition found in Selachians and Dipnoans. D and E, Illustrating the developmental process in bony Actinopterygians and higher vertebrates. c, Centrum. d.a, Dorsal arcualia. n.a, Neural arch. nc, Notochord. nc.ep, Chordal epithelium. n.sp, Neural spine. sh.1, Primary sheath. sh.2, Secondary sheath. sk.l, Connective tissue. tr.p, Transverse process. v.a, Ventral arcualia.

A secondary modification of the centrum consists in the calcification of certain zones of the cartilaginous matrix. The precise arrangement of these calcified zones varies in different families and affords characters which are of taxonomic importance in palaeontology where only skeletal structures are available (see Selachians).

(b) Arcicentrous type. Already in the Selachians the vertebral column is to a certain extent strengthened by the broadening of the basis of the arcualia so as partially to surround the centra. In the Teleostomes, with the exceptions of those ganoids mentioned, the expanded bases of the arcualia undergo complete fusion to form cartilaginous centra which, unlike the chordacentrous centra, lie outside the primary sheath (figs. 13, D and E). In these forms no invasion of the secondary sheath by cartilage cells takes place. The composition of the groups of arcualia which give rise to the individual centrum is different in different groups. The end result is an amphicoelous or biconcave centrum in general appearance much like that of the Selachian.

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, i.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 ventral 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 complete fusion of the basal portions of a number of radialia (dorsal fins of Holoptychius and various Selachians, and anal fin of Pleuracanthus). In view of the identity in function it is not surprising that a remarkable resemblance exists between the mechanical arrangements (of skeleton, muscles, &c.), of the paired and unpaired fins. The resemblance to paired fins becomes very striking in some of the cases where the basal fusion mentioned above takes place (Pleuracanthus).

Trans. Roy. Soc. Edinburgh.

Fig. 14.—Chondrocranium of a young Lepidosiren, showing the suspension of the lower jaw by the upper portion of the mandibular arch. (After Agar.)

 H, Hyoid arch. M, Mandibular arch. o.a, Occipital arch. ot, Auditory capsule. q, Quadrate = upper end of mandibular arch. tr, Trabecula.

The palatopterygoid bar (p.pt) is represented by a faint vestige which disappears before the stage figured.

(B) Chondrocranium[28].—In front of the vertebral column lies the cartilaginous trough, the chondrocranium, which protects the brain. This consists of a praechordal portion—developed out of a pair of lateral cartilaginous rods—the trabeculae cranii—and a parachordal portion lying on either side of the anterior end of the notochord. This arises in development from a cartilaginous rod (parachordal cartilage) lying on each side of the notochord and possibly representing a fused row of dorsal arcualia. The originally separate parachordals and trabeculae become connected to form a trough-like, primitive cranium, complete or nearly so laterally and ventrally but open dorsally. With the primitive cranium there are also connected cartilaginous capsules developed round the olfactory and auditory organs. There also become fused with the hinder end of the cranium a varying number of originally distinct neural arches.

After W. K. Parker, Trans. Zool. Soc. London.

After Gegenbaur, Untersuchungen zur verg. Anat. der Wirbeltiere, by permission of Wilhelm Engelmann.

After Hubrecht, Brown’s Tierreich, by permission of Gustav Fischer.

Fig. 15.—Chondrocranium, &c. of Scyllium (A), Notidanus cinereus (B) and Chimaera (C).

 Br.A, Branchial arches. olf, Olfactory capsule. c.h, Ceratohyal. ot, Auditory capsule. e.p.l, Ethmopalatine ligament. p.pt, Palato-pterygoid bar. Hm, Hyomandibular. p.s.l, Prespiracular ligament. M, Meckel’s cartilage. r, Rostrum. o, Orbit.

(C) Visceral Arches.—The skeleton of the visceral arches consists essentially of a series of half-hoops of cartilage, each divided in the adult into a number of segments and connected with its fellow by a median ventral cartilage. The skeleton of arches I. and II. (mandibular and hyoidean) undergoes modifications of special interest (figs. 14 and 15). The lower portion of the mandibular arch becomes greatly thickened to support the lower or hinder edge of the mouth. It forms the primitive lower jaw or “Meckel’s cartilage.” Dorsal to this an outgrowth arises from the anterior face of the arch which supports the upper or anterior margin of the mouth: it is the primitive upper jaw or palato-pterygoquadrate cartilage. The portion of the arch dorsal to the palato-pterygo-quadrate outgrowth may form the suspensorial apparatus of the lower jaw, being fused with the cranium at its upper end. This relatively primitive con-arrangement (protostylic, as it may be termed) occurs in Dipneusti among fishes (cf. fig. 14). More usually this dorsal part of the Fig. 16.—Fore-limb of Ceratodus. mandibular arch becomes reduced, its place being occupied by a ligament (pre-spiracular) uniting the jaw apparatus to the chondrocranium, the upper jaw being also attached to the chondrocranium by the ethmopalatine ligament situated more anteriorly. The main attachment, however, of the jaws to the chondrocranium in such a case, as holds for the majority of fishes, is through the enlarged dorsal segment of the hyoid arch (hyomandibular) which articulates at its dorsal end with the chondrocranium, while its ventral end is attached to the hinge region of the jaw by stout ligamentous bands. A skull in which the jaws are suspended in this manner is termed a hyostylic skull (e.g. Scyllium in fig. 15).

In Notidanus (fig. 15, B) there is a large direct articulation of the upper jaw to the chondrocranium in addition to the indirect one through the hyomandibular: such a skull is amphistylic. In Heterodontus the upper jaw is firmly bound to the cranium throughout its length, while in Holocephali (fig. 15, C) complete fusion has taken place, so that the lower jaw appears to articulate directly with the cranium (“auto stylic” condition). In Dipneusti[29] (Lepidosiren and Protopterus) the cartilaginous upper jaw never develops (except in its hinder quadrate portion) beyond the condition of a faint rudiment, owing doubtless to its being replaced functionally by precociously developed bone.

(D) Appendicular Skeleton.—The skeleton of the free part of the limb is attached to the limb girdle which lies embedded in the musculature of the body. Each limb girdle is probably to be looked upon as consisting, like the skeleton of the visceral arches, of a pair of lateral half-hoops of cartilage. While in Pleuracanthus the lateral halves are distinct (and segmented like the branchial arches), in living Selachians generally the two halves are completely fused ventrally with one another. The part of the girdle lying dorsal to the articulation of the limb is termed scapular in the case of the pectoral limb, iliac in the Fig. 17.a, Skeleton of pectoral limb of Pleuracanthus. (From Gegenbauer, after Frisch.) b, Skeleton of pectoral limb of Acanthias. (After Gegenbauer.) case of the pelvic, while the ventral portions are known respectively as coracoid and ischio-pubic.

In most Teleostomes the primitive pelvic girdle does not develop; in the Dipneusti it is represented by a median unpaired cartilage.

The skeleton of the free limb is probably seen in its most archaic form amongst existing fishes in the biserial archipterygium of Ceratodus (fig. 16). This is indicated by the relative predominance of this type of fin amongst the geologically more ancient fishes. The biserial archipterygium consists of a segmented axial rod, bearing a praeaxial and a postaxial series of jointed rays.

In Protopterus and Lepidosiren the limbs are reduced and the lateral rays have less (Protopterus) or more (Lepidosiren) completely disappeared.

In such an archaic Selachian as Pleuracanthus the fin is clearly of the biserial archipterygial type, but the lateral rays are reduced (pectoral) or absent (pelvic) (fig. 17, a) on one side of the axis. In a typical adult Selachian the pectoral fin skeleton has little apparent resemblance to the biserial archipterygium—the numerous outwardly directed rays springing from a series of large basal cartilages (pro-, meso- and metapterygium). The condition in the young (e.g. fig. 17, b, Acanthias) hints strongly, however, at the possibility of the fin skeleton being really a modified biserial archipterygium, and that the basal cartilages represent the greatly enlarged axis which has become fixed back along the side of the body. In Crossopterygians (Polypterus) the highly peculiar fin skeleton (fig. 18) while still in the embryonic cartilaginous stage is clearly referable to a similar condition. In the Actinopterygians—with the increased development of dermal fin rays—there comes about reduction of the primitive limb skeleton. The axis becomes particularly reduced, and the fin comes to be attached directly to the pectoral girdle by a number of basal pieces (Teleosts) probably representing vestigial rays (cf. fig. 19).

 From Budgett, Trans. Zool. Soc. London, xvi, part vii. From Wiedersheim’s Verg. Anat. der Wirbeltiere, by permission of Gustav Fischer. Fig. 18.—Skeleton of Pectoral Limb of Polypterus. a, 30 mm. larva. b, Adult.
 From Wiedersheim’s Verg. Anat. der Wirbeltiere, by permission of Gustav Fischer. Fig. 19.—Skeleton of Pectoral Fin of Amia.

Views on the general morphology of the fin skeleton are strongly affected by the view held as to the mode of evolution of the fins. By upholders of the lateral fold hypothesis the type of fin skeleton described for Cladoselache[30] is regarded as particularly primitive. It is, however, by no means clear that the obscure basal structures figured (Fig. 20) in this fin do not really represent the pressed back axis as in Pleuracanthus.

 From Bashford Dean, Mem. N.Y. Acad. of Science. Fig. 20.—Skeleton of Pectoral Fin of Cladoselache.
 Fig. 21.—Placoid elements of a male Thorn-back, Raia clavata.

The pelvic fin skeleton, while built obviously on the same plan as the pectoral, is liable to much modification and frequently degeneration.

Osseous or Bony Skeleton.—The most ancient type of bony skeleton appears to be represented in the placoid elements such as are seen in the skin of the Selachian (fig. 21). Each placoid element consists of a spine with a broadly expanded base embedded in the dermis. The base is composed of bone: the spine of the somewhat modified bone known as dentine. Ensheathing the tip of the spine is a layer of extremely hard enamel formed by the inner surface of the ectoderm which originally covered it. Such typical placoid scales are well seen on any ordinary skate. In the groups of fishes above the Selachians, the coating of placoid elements shows various modifications. The spines disappear, though they may be present for a time in early development. The bony basal plates tend to undergo fusion—in certain cases they form a continuous bony cuirass (various Siluroids, trunk-fishes) formed of large plates jointed together at their edges. More usually the plates are small and regular in size. In Crossopterygians and Lepidosteus and in many extinct forms the scales are of the ganoid type, being rhomboidal and having their outer layer composed of hard glistening ganoine. In other Teleostomes the scales are as a rule thin, rounded and overlapping—the so-called cycloid type (fig. 22, A); where the posterior edge shows toothlike projections the scale is termed ctenoid (fig. 22, B). In various Teleosts the scales are vestigial (eel); in others (as in most electric fishes) they have completely disappeared.

 Fig. 22.—A, Cycloid Scale of Scopelus resplendens (magn.). B, Ctenoid Scale of Lethrinus (magn.).

Teeth.—Certain of the placoid elements belonging to that part of the skin which gives rise to the lining of the stomodaeum have their spines enlarged or otherwise modified to form teeth. In the majority of fishes these remain simple, conical structures: in some of the larger sharks (Carcharodon) they become flattened into trenchant blades with serrated edges: in certain rays (Myliobatis) they form a pavement of flattened plates suited for crushing molluscan shells. In the young Neoceratodus[31] there are numerous small conical teeth, the bases of which become connected by a kind of spongework of bony trabeculae. As development goes on a large basal mass is formed which becomes the functional tooth plate of the adult, the original separate denticles disappearing completely. In the other two surviving Dipnoans, similar large teeth exist, though here there is no longer trace in ontogeny of their formation by the basal fusion of originally separate denticles. In the Selachians the bony skeleton is restricted to the placoid elements. In the Teleostomes and the Dipnoans the original cartilaginous skeleton becomes to a great extent unsheathed or replaced by bony tissue. It seems highly probable that the more deeply seated osseous elements occurring in these as in the higher groups arose in the course of evolution by the spreading inwards of bony trabeculae from the bases of the placoid elements. Such a method has been demonstrated as occurring in individual development in the case of certain of the more superficially placed bones.[32]

The placoid element with its cap of enamel secreted by the ectoderm is probably originally derived from a local thickening of the basement membrane which with the external cuticle may be looked on as the most ancient skeletal structure in the Metazoa. The basal plate appears to have been a later development than the spine; in the palaeozoic Coelolepidae[33] the basal plate is apparently not yet developed.

Only a brief summary can be given here of the leading features in the osteology of fishes. Care must be taken not to assume that bony elements bearing the same name in fishes and in other groups, or even in the various sub-divisions of the fishes, are necessarily strictly homologous. In all probability bony elements occupying similar positions and described by the same anatomical name have been evolved independently from the ancestral covering of placoid elements.

Teleostei.—It will be convenient to take as the basis of our description the bony skeleton of such a Teleostean fish as the salmon. In the vertebral column all the cartilaginous elements are replaced by bone. The haemal spines of the turned-up tip of the tail are flattened (hypural bones) and serve to support the caudal fin rays.

In Argyropelecus and in one or two deep-sea forms the vertebral column remains cartilaginous.

 From Parker & Haswell’s Text-book of Zoology, by permission of Messrs. Macmillan & Co., Ltd. Fig. 23.—One of the radialia of the salmon, consisting of three segments, ptg¹, ptg², ptg³, and supporting a dermal fin ray, D.F.R.

Apart from the ossification of the radialia which takes place in the adults of bony fishes there exist special supporting structures in the fins (paired as well as median) of all the gnathostomatous fishes and apparently in nature independent of the cartilaginous skeleton. These are known as dermal fin-rays.[34] Morphologically they are probably to be looked on (like placoid elements) as local exaggerations of the basement membrane.

In their detailed characters two main types of dermal fin-ray may be recognized. The first of these are horny unjointed rays and occur in the fins of Selachians and at the edge of the fins of Teleostomes (well seen in the small posterior dorsal or “adipose” fin, particularly in Siluroids). The second type of dermal fin-ray is originally arranged in pairs and forms the main supports of the fin in the adult Teleost (fig. 23). The members of each pair are in close contact except proximally where they separate and embrace the tip of one of the radialia. The fin-rays of this second type are frequently branched and jointed: in other cases they form unbranched rigid spines.

In the angler or fishing-frog (Lophius) the anterior rays of the dorsal fin become greatly elongated to form small fishing-rods, from which depend bait-like lures for the attraction of its prey.

From Wiedersheim, Verg. Anat. der Wirbeltiere, by permission of Gustav Fischer.
Fig. 24.—Chondrocranium of Salmon, seen from the right side.
 alsph, Alisphenoid. orbsph, Orbitosphenoid. basocc, Basioccipital. proot, Prootic. ekteth, Lateral ethmoid. psph, Parasphenoid. epiot, Epiotic. ptero, Pterotic. exocc, Exoccipital. socc, Supra occipital. fr, Frontal. sphot, Sphenotic. opisth, Opisthotic. vo, Vomer.

In the skull of the adult salmon it is seen that certain parts of the chondrocranium (fig. 24) have been replaced by bone (“cartilage bones”) while other more superficially placed bones (“membrane bones”) cover its surface (fig. 25). Of cartilage bones four are developed round the foramen magnum—the basioccipital, supraoccipital and two exoccipitals. In front of the basioccipital is the basisphenoid with an alisphenoid on each side. The region (presphenoidal) immediately in front of the basisphenoid is unossified, but on each side of it an orbitosphenoid is developed, the two orbitosphenoids being closely approximated in the mesial plane and to a certain extent fused, forming the upper part of the interorbital septum. In the anterior or ethmoidal portion of the cranium the only cartilage bones are a pair of lateral ethmoids lying at the anterior boundary of the orbit. A series of five distinct elements are ossified in the wall of the auditory or otic capsule, the prootic and opisthotic more ventrally, and the sphenotic, pterotic and epiotic more dorsally. The roof of the cranium is covered in by the following dermal bones—parietals (on each side of the supraoccipital), frontals, dermal ethmoid and small nasals, one over each olfactory organ. The floor of the cranium on its oral aspect is ensheathed by the large parasphenoid and the smaller vomer in front of and overlapping it. The cartilaginous lower jaw is ossified posteriorly to form the articular (fig. 25) with a small membrane bone, the angular, ventral to it, but the main part of the jaw is replaced functionally by a large membrane bone which ensheaths it—the dentary—evolved in all probability by the spreading outwards of bony tissue from the bases of the placoid elements (teeth) which it bears. The original upper jaw (palatopterygoid bar) is replaced by a chain of bones—palatine in front, then pterygoid and mesopterygoid, and posteriorly metapterygoid and quadrate, the latter giving articulation to the articular bone of the lower jaw. These representatives of the palatopterygoid bar no longer form the functional upper jaw. This function is performed by membrane bones which have appeared external to the palatopterygoid bar—the premaxilla and maxilla—which carry teeth—and the small scale-like jugal behind them. The quadrate is suspended from the skull as in the Selachians (hyostylic skull) by the upper portion of the hyoid arch—here represented by two bones—the hyomandibular and symplectic. The ventral portion of the hyoid arch is also represented by a chain of bones (stylohyal, epihyal, ceratohyal, hypohyal and the ventral unpaired basihyal), as is also each of the five branchial arches behind it. In addition to the bony elements belonging to the hyoid arch proper a series of membrane bones support the opercular flap. Ventrally there project backwards from the ceratohyal a series of ten overlapping branchiostegal rays, while more dorsally are the broader interopercular, subopercular and opercular.

From Wiedersheim, Verg. Anat. der Wirbeltiere, by permission of Gustav Fischer.
Fig. 25.—Complete Skull of Salmon from left side.
 art, Articular. op, Opercular. branchiost, Branchiostegal. pal, Palatine. dent, Dentary. par, Parietal. epiot, Epiotic. pmx, Premaxilla. eth, Dermal ethmoid. preop, Preopercular. fr, Frontal. pt, Pterygoid. hyom, Hyomandibular. pter, Pterotic. intop, Interopercular. Quad, Quadrate. Jug, Jugal. socc, Supraoccipital. mpt, Mesopterygoid. sphot, Sphenotic. mtpt, Metapterygoid. subop, Subopercular. mx, Maxilla. sympl, Symplectic. nas, Nasal. Zunge, Tongue.

In addition to the bones already enumerated there is present a ring of circumorbital bones, a preopercular, behind and external to the hyomandibular and quadrate, and squamosal, external to the hinder end of the auditory capsule.

In the salmon, pike, and various other Teleosts, extensive regions of the chondrocranium persist in the adult, while in others (e.g. the cod) the replacement by bone is practically complete. Bony elements may be developed in addition to those noticed in the salmon.

In the sturgeon the chondrocranium is ensheathed by numerous membrane bones, but cartilage bones are absent. In the Crossopterygians[35] the chondrocranium persists to a great extent in the adult, but portions of it are replaced by cartilage bones—the most interesting being a large sphenethmoid like that of the frog. Numerous membrane bones cover the chondrocranium externally. In the Dipneusti[36] the chondrocranium is strengthened in the adult by numerous bones. One of the most characteristic is the great palatopterygoid bone which develops very early by the spreading of ossification backwards from the tooth bases, and whose early development probably accounts for the non-development of the palatopterygoid cartilage.

Appendicular Skeleton.—The primitive pectoral girdle, which in the Dipneusti is strengthened by a sheath of bone, becomes in the Teleostomes reduced in size (small scapula and coracoid bones) and replaced functionally by a secondary shoulder girdle formed of superficially placed membrane bones (supraclavicular and cleithrum or “clavicle,” with, in addition in certain cases, an infraclavicular and one or two postclavicular elements), and connected at its dorsal end with the skull by a post-temporal bone.

The pelvic girdle is in Teleostomes completely absent as a rule.

The skeleton of the free limb undergoes ossification to a less or greater extent in the Teleostomes.

In Polypterus the pectoral fin (fig. 18, B) shows three ossifications in the basal part of the fin—pro-, meso- and metapterygium. Of these the metapterygium probably represents the ossified skeletal axis: while the propterygium and also the numerous diverging radials probably represent the lateral rays of one side of the archipterygium.

In the Teleostomes the place of the pelvic girdle is taken functionally by an element apparently formed by the fusion of the basal portions of several radials.

Vascular System.—The main components of the blood vascular system in the lower vertebrates are the following: (1) a single or double dorsal aorta lying between the enteron and notochord; (2) a ventral vessel lying beneath the enteron; and (3) a series of paired hoop-like aortic arches connecting dorsal and ventral vessels round the sides of the pharynx. The blood-stream passes forwards towards the head in the ventral vessel, dorsalwards through the aortic arches, and tailwards in the dorsal aorta.

The dorsal aorta is single throughout the greater part of its extent, but for a greater or less extent at its anterior end (circulus cephalicus) it consists of two paired aortic roots. It is impossible to say whether the paired or the unpaired condition is the more primitive, general morphological conditions being in favour of the latter, while embryological evidence rather supports the former. The dorsal aorta, which receives its highly oxygenated blood from the aortic arches, is the main artery for the distribution of this oxygenated blood. Anteriorly the aortic roots are continued forwards as the dorsal carotid arteries to supply the head region. A series of paired, segmentally-arranged arteries pass from the dorsal aorta to supply the muscular body wall, and the branches which supply the pectoral and pelvic fins (subclavian or brachial artery, and iliac artery) are probably specially enlarged members of this series of segmental vessels. Besides these paired vessels a varying number of unpaired branches pass from dorsal aorta to the wall of the alimentary canal with its glandular diverticula (coeliac, mesenteric, rectal).

The ventral vessel undergoes complicated changes and is represented in the adults of existing fishes by a series of important structures. Its post-anal portion comes with the atrophy of the post-anal gut to lie close under the caudal portion of the dorsal aorta and is known as the caudal vein. This assumes a secondary connexion with, and drains its blood into, the posterior cardinal veins (see below). In the region between cloaca and liver the ventral vessel becomes much branched or even reticular and—serving serving to convey the food-laden blood from the wall of the enteron to the capillary network of the liver—is known as the hepatic portal vein. The short section in front of the liver is known as the hepatic vein and this conveys the blood, which has been treated by the liver, into a section of the ventral vessel, which has become highly muscular and is rhythmically contractile. This enlarged muscular portion, in which the contractility—probably once common to the main vessels throughout their extent—has become concentrated, serves as a pump and is known as the heart. Finally the precardiac section of the ventral vessel—the ventral aorta—conveys the blood from heart to aortic arches.

In addition to the vessels mentioned a large paired vein is developed in close relation to the renal organ which it serves to drain. This is the posterior cardinal. An anterior prolongation (anterior cardinal) serves to drain the blood from the head region. From the point of junction of anterior and posterior cardinal a large transverse vessel leads to the heart (ductus Cuvieri).

From Boas, Lehrbuch der Zoologie, by permission of Gustav Fischer.

Fig. 26.—Diagram to illustrate the condition of the Conus in an Elasmobranch (A), Amia (B) and a typical Teleost (C).

 a, Atrium. b.a, Bulbus aortae. c.a, Conus arteriosus. s.v, Sinus venosus. v,v′, Valves. v.a, Ventral aorta. vt, Ventricle.

Heart.—Originally a simple tube curved into a somewhat S-shape, the heart, by enlargements, constrictions and fusions of its parts, becomes converted into the complex, compact heart of the adult. In this we recognize the following portions—(1) Sinus venosus, (2) Atrium, (3) Ventricle. A fourth chamber, the conus arteriosus, the enlarged and contractile hinder end of the ventral aorta, is also physiologically a part of the heart. The sinus venosus receives the blood from the great veins (ductus Cuvieri and hepatic veins). It—like the atrium which it enters by an opening guarded by two lateral valves—has thin though contractile walls. The atrium is as a rule single, but in the Dipnoans, in correlation with the importance of their pulmonary breathing, it is incompletely divided into a right and a left auricle. In Neoceratodus the incomplete division is effected by the presence of a longitudinal shelf projecting into the atrial cavity from its posterior wall. The opening of the sinus venosus is to the right of this shell, that of the pulmonary vein to the left. In Prototerus and Lepidosiren a nearly complete septum is formed by the fusion of trabeculae, there being only a minute opening in it posteriorly. The atrium opens by a wide opening guarded by two or more flap valves provided with chordae tendineae into the ventricle.

The ventricle, in correspondence with it being the main pumping apparatus, has its walls much thickened by the development of muscular trabeculae which, in the lower forms separated by wide spaces in which most of the blood is contained, become in the Teleostomes so enlarged as to give the wall a compact character, the spaces being reduced to small scattered openings on its inner surface. In the Dipnoans the ventricle, like the atrium, is incompletely divided into a right and left ventricle. In Ceratodus this is effected by an extension of the interauricular shelf into the ventricle. In Lepidosiren the separation of the two ventricles is complete but for a small perforation anteriorly, the heart in this respect showing a closer approximation to the condition in the higher vertebrates than is found in any Amphibians or in any reptiles except the Crocodilia. The conus arteriosus is of interest from the valvular arrangements in its interior to prevent regurgitation of blood from ventral aorta into ventricle. In their simplest condition, as seen e.g. in an embryonic Selachian, these arrangements consist of three, four or more prominent longitudinal ridges projecting into the lumen of the conus, and serving to obliterate the lumen when jammed together by the systole of the conus. As development goes on each of these ridges becomes segmented into a row of pocket valves with their openings directed anteriorly so that regurgitation causes them to open out and occlude the lumen by their free edges meeting. Amongst the Teleostomes the lower ganoids show a similar development of longitudinal rows of valves in the conus. In Amia (fig. 26, B), however, the conus is shortened and the number of valves in each longitudinal row is much reduced. This leads to the condition found in the Teleosts (fig. 26, OC), where practically all trace of the conus has disappeared, a single circle of valves representing a last survivor of each row (save in a few exceptional cases, e.g. Albula, Tarpen, Osteoglossum, where two valves of each row are present).

After Newton Parker, from Trans. of the Royal Irish Academy, vol. xxx.

Fig. 27.—Venous System of Protopterus, as seen from ventral side.

 a, Atrium. ac, Anterior cardinal. an.v, Anastomotic vein. c, Intestine. c.v, Caudal vein. f.v, Femoral vein. g.b, Gall-bladder. h.v, Hepatic vein. i.j.v, Inferior jugular vein. i.v.c, Posterior vena cava. k, Kidney. l, Liver. ov.v, Ovarian veins. p, Pericardium. p.c.v, Left posterior cardinal. p.v′, Parietal veins. r.p.v, Renal portal. s, Stomach. s.b.v, Subclavian.

In Front of the conus vestige of the Teleost there is present a thick walled bulbus aortae differing from the conus in not being rhythmically contractile, its walls being on the contrary richly provided with elastic tissue.

The Dipnoans[37] show an important advance on the conus as in atrium and ventricle. The conus has a characteristic spiral twist. Within it in Neoceratodus are a number of longtitudinal rows of pocket valves. One of these rows is marked out by the very large size of its valves and by the fact that they are not distinct from one another but even in the adult form a continuous, spirally-running, longitudinal fold. This ridge projecting into the lumen of the conus divides it incompletely into two channels, the one beginning (i.e. at its hinder end) on the left side and ending in front ventrally, the other beginning on the right and ending dorsally. In Protopterus a similar condition occurs, only in the front end of the conus a second spiral fold is present opposite the first and, meeting this, completes the division of the conus cavity into two separate parts. The rows of pocket valves which do not enter into the formation of the spiral folds are here greatly reduced.

These arrangements in the conus of the Dipnoans are of the highest morphological interest, pointing in an unmistakable way towards the condition found in the higher lung-breathing vertebrates. Of the two cavities into which the conus is partially divided in the Dipneusti the one which begins posteriorly on the right receives the (venous) blood from the right side of the heart, and ending up anteriorly dorsal to the other cavity communicates only with aortic arches V. and VI. In the higher vertebrates this cavity has become completely split off to form the root of the pulmonary arteries, and a result of aortic arch V. receiving its blood along with the functionally much more important VI. (the pulmonary arch) from this special part of the conus has been the almost complete disappearance of this arch (V.) in all the higher vertebrates.

Fig. 28.—Venous System of Polypterus 30 mm. larva (dorsal view).

 a.c.v, Anterior cardinal vein. d.C, Ductus Cuvieri. h.v, Hepatic vein. i.j.v, Inferior jugular vein. ir.v, Inter-renal vein. l.v, Lateral cutaneous vein. p.c.v, Posterior cardinal vein. p.n, Pronephros. p.v, Pulmonary vein. s, Subclavian vein. s.v, Sinus venosus. th, Thyroid. v, Vein from pharyngeal wall. * Anterior portion of left posterior cardinal vein.

Arterial System.—There are normally six aortic arches laid down corresponding with the visceral arches, the first (mandibular) and second (hyoidean) undergoing atrophy to a less or greater extent in post-embryonic life. Where an external gill is present the aortic arch loops out into this, a kind of short-circuiting of the blood-stream taking place as the external gill atrophies. As the walls of the clefts assume their respiratory function the aortic arch becomes broken into a network of capillaries in its respiratory portion, and there is now distinguished a ventral afferent and a dorsal efferent portion of each arch. Complicated developmental changes, into which it is unnecessary to enter,[38] may lead to each efferent vessel draining the two sides of a single cleft instead of the adjacent walls of two clefts as it does primitively. In the Crossopterygians and Dipnoans as in the higher vertebrates the sixth aortic arch gives off the pulmonary artery to the lung. Among the Actinopterygians this, probably primitive, blood-supply to the lung (swimbladder) persists only in Amia.

Venous System.—The most interesting variations from the general plan outlined have to do with the arrangements of the posterior cardinals. In the Selachians these are in their anterior portion wide and sinuslike, while in the region of the kidney they become broken into a sinusoidal network supplied by the postrenal portion now known as the renal portal vein. In the Teleostomes the chief noteworthy feature is the tendency to asymmetry, the right posterior cardinal being frequently considerably larger than the left and connected with it by transverse anastomotic vessels, the result being that most of the blood from the two kidneys passes forwards by the right posterior cardinal. The Dipnoans (fig. 27) show a similar asymmetry, but here the anterior end of the right posterior cardinal disappears, being replaced functionally by a new vessel which conveys the blood from the right posterior cardinal direct to the sinus venosus instead of to the outer end of the ductus Cuvieri. This new vessel is the posterior vena cava which thus in the series of vertebrates appears for the first time in the Dipneusti.

Pulmonary Veins.—In Polypterus (fig. 28) the blood is drained from the lungs by a pulmonary vein on each side which unites in front with its fellow and opens into the great hepatic vein behind the heart. In the Dipnoans the conjoined pulmonary veins open directly into the left section of the atrium as in higher forms. In the Actinopterygians with their specialized air-bladder the blood passes to the heart via posterior cardinals, or hepatic portal, or—a probably more primitive condition—directly into the left ductus Cuvieri (Amia).

Lymphatics.—More or less irregular lymphatic spaces occur in the fishes as elsewhere and, as in the Amphibia, localized muscular developments are present forming lymph hearts.

Central Nervous System.—The neural tube shows in very early stages an anterior dilated portion which forms the rudiment of the brain in contradistinction to the hinder, narrower part which forms the spinal cord. This enlargement of the brain is correlated with the increasing predominance of the nerve centres at the anterior end of the body which tend to assume more and more complete control over those lying behind.

Spinal Cord.—A remarkable peculiarity occurs in the sun fishes (Molidae), where the body is greatly shortened and where the spinal cord undergoes a corresponding abbreviation so as to be actually shorter than the brain.

Brain.—It is customary to divide the brain into three main regions, fore-, mid-, and hind-brain, as in the most familiar vertebrates there is frequently seen in the embryo a division of the primitive brain dilatation into three vesicles lying one behind the other. A consideration of the development of the brain in the various main groups of vertebrates shows that these divisions are not of equal importance. In those archaic groups where the egg is not encumbered by the presence of a large mass of yolk it is usual for the brain to show in its early stages a division into two main regions which we may term the primitive fore-brain or cerebrum and the primitive hind-brain or rhombencephalon. Only later does the hinder part of the primitive fore-brain become marked off as mid-brain. In the fully developed brain it is customary to recognize the series of regions indicated below, though the boundaries between these regions are not mathematical lines or surfaces any more than are any other biological boundaries:—

 Rhombencephalon (Hind-brain) ${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}}$ Myelencephalon (Medulla oblongata). Metencephalon (Cerebellum). Cerebrum (Primitive Fore-brain) ${\displaystyle \scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}}$ Mesencephalon (Mid-brain). Thalamencephalon (Diencephalon). [Hemispheres (Telencephalon).]

The myelencephalon or medulla oblongata calls for no special remark, except that in the case of Torpedo there is a special upward bulging of its floor on each side of the middle line forming the electric lobe and containing the nucleus of origin of the nerves to the electric organ.

A and B from Wiedersheim, by permission of Gustav Fischer.

Fig. 29.—Brain of Scyllium (A), Salmo (B) and Lepidosiren (C).
The three figures are not drawn to the same scale.

 cer, Cerebellum. c.h, Cerebral hemisphere. th, Thalamencephalon. f.b, Primitive fore-brain (in B the line points to the thickened wall of the fore-brain, the so-called “basal ganglia”). G.p, Pineal body. m.b, Roof of mid-brain, optic lobes, tectum opticum. o.l, Olfactory lobe. IV.v, Fourth ventricle.

The cerebellum occurs in its simplest form in lampreys and Dipnoans (fig. 29, C), where it forms a simple band-like thickening of the anterior end of the roof of the hind-brain. In Selachians it is very large and bulges upwards, forming a conspicuous organ in a dorsal view of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is also large. It projects back as a great tongue-like structure over the roof of the fourth ventricle, while in front it dips downwards and projects under the roof of the mid-brain forming a highly characteristic valvula cerebelli. A valvula cerebelli occurs also in ganoids, while in the Crossopterygians a similar extension of the cerebellum projects backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).

The mesencephalon is a conspicuous structure in the fishes from its greatly developed roof (tectum opticum) which receives the end pencils of the optic nerve. Normally it projects upwards as a pair of large optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is not sufficiently great to cause obvious lateral swellings in external view.

Fig. 30.—Median Longitudinal Section through the brain of Lepidosiren and Polypterus. In the upper figure (Lepidosiren) the habenular ganglion and hemisphere are shown in outline though not actually present in a median section.

 a.c, Anterior commissure. par, Paraphysis. cer, Cerebellum. pin, Pineal body. d.s, Dorsal sac. p.c, Posterior commissure. g.h, Habenular ganglion. s.v, Saccus vasculosus. h.c, Habenular commissure. t.o, Tectum opticum. i.g, Infundibular gland. v.III, Third ventricle. l.p, Lateral plexus. v.IV, Fourth ventricle. o.c, Optic chiasma. vel, Velum transversum. pall, Pallium.

The thalamencephalon is one of the most interesting parts of the brain from its remarkable uniformity throughout the Vertebrata. Even in Amphioxus the appearance of a sagittal section strongly suggests vestiges of a once present thalamencephalon.[39] The roof—like that of the myelencephalon—remains to a great extent membranous, forming with the closely applied pia mater a vascular roof to the III. ventricle. Frequently a transverse fold of the roof dips down into the III. ventricle forming the velum transversum (fig. 30).

The side walls of the thalamencephalon are greatly thickened forming the thalamus (epithalamus and hypothalamus), while a ganglionic thickening of the roof posteriorly on each side forms the ganglia habenulae which receive olfactory fibres from the base of the hemisphere. The habenular ganglia are unusually large in the lampreys and are here strongly asymmetrical, the right being the larger.

The floor of the thalamencephalon projects downwards and backwards as the infundibulum. The side walls of this are thickened to form characteristic lobi inferiores, while the blind end develops glandular outgrowths (infundibular gland, fig. 30) overlaid by a rich development of blood sinuses and forming with them the saccus vasculosus. The optic chiasma, where present, is involved in the floor of the thalamencephalon and forms a large, upwardly-projecting ridge. Farther forwards on the floor or anterior wall is the anterior commissure (see below).

Passing forwards from the mid-brain (cf. fig. 30) a series of interesting structures are found connected with the roof of the primitive fore-brain, viz.—posterior commissure (intercalary region), pineal organ, habenular commissure with anterior parietal organ, dorsal sac ( = pineal cushion), velum transversum, paraphysis. The posterior commissure is situated in the boundary between thalamencephalon and mid-brain. It is formed of fibres connecting up the right and left sides of the tectum opticum (?). The habenular or superior commissure situated farther forwards connects the two ganglia habenulae. In the immediate neighbourhood of these ganglia there project upwards two diverticula of the brain-roof known as the pineal organ and the parapineal (or anterior parietal) organ. The special interest of these organs[40] lies in the fact that in certain vertebrates one (parapineal in Sphenodon and in lizards) or both (Petromyzon) exhibit histological features which show that they must be looked on as visual organs or eyes. In gnathostomatous fishes they do not show any definite eye-like structure, but in certain cases (Polyodon, Callichthys, &c.) the bony plates of the skull-roof are discontinuous over the pineal organ forming a definite parietal foramen such as exists in lizards where the eye-like structure is distinct. It is also usual to find in the epithelial wall of the pineal organ columnar cells which show club-shaped ends projecting into the lumen (exactly as in the young visual cells of the retina[41]) and are prolonged into a root-like process at the other end. Definite nerve fibres pass down from these parietal organs to the brain. It is stated that the fibres from the pineal organ pass into the posterior commissure, those of the parapineal organ into the habenular commissure.

The facts mentioned render it difficult to avoid the conclusion that these organs either have been sensory or are sensory. Possibly they represent the degenerate and altered vestiges of eye-like organs present in archaic vertebrates, or it may be that they represent the remains of organs not eye-like in function but which for some other reason lay close under the surface of the body. It would seem natural that a diverticulum of brain-tissue exposed to the influence of light-rays should exhibit the same reaction as is shown frequently elsewhere in the animal kingdom and tend to assume secondarily the characters of a visual organ. The presence of the rod-like features in the epithelial cells is perhaps in favour of the latter view. In evolution we should expect these to appear before the camera-like structure of a highly developed eye, while in the process of degeneration we should expect these fine histological characters to go first.

Selachians.—No parapineal organ is present. The pineal body (except in Torpedo where it is absent) is in the form of a long slender tube ending in front in a dilated bulb lying near the front end of the brain in close contact with, or enclosed in, a definite foramen in the cranial roof.

Holocephali and Crossopterygii.—Here also the pineal body is long and tubular: at its origin it passes dorsalwards or slightly backwards behind the large dorsal sac.

Actinopterygian Ganoids resemble Selachians on the whole. In Amia a parapineal organ is present, and it is said to lie towards the left side and to be connected by a thick nerve with the left habenular ganglion (cf. Petromyzon, article Cyclostomata). This is adduced to support the view that the pineal and parapineal bodies represent originally paired structures.

Teleostei.—A parapineal rudiment appears in the embryo of some forms, but in the adult only the pineal organ is known to exist. This is usually short and club-shaped, its terminal part with much folded wall and glandular in character. In a few cases a parietal foramen occurs (Callichthys, Loricaria, &c.).

Dipneusti.—The pineal organ is short and simple. No parapineal organ is developed.

The dorsal sac is formed by that part of the roof of the thalamencephalon lying between the habenular commissure and the region of the velum. In some cases a longitudinal groove is present in which the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal sac is particularly large and was formerly mistaken for the pineal organ.

The velum transversum is a transverse, inwardly-projecting fold of the roof of the primitive fore-brain in front of the dorsal sac. To those morphologists who regard the hemisphere region or telencephalon as a primitively unpaired structure the velum is an important landmark indicating the posterior limit of the telencephalon. Those who hold the view taken in this article that the hemispheres are to be regarded as paired outpushings of the side wall of the primitive fore-brain attribute less morphological importance to the velum. Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III. ventricle.

In Petromyzon and Chimaera the velum is not developed. In Dipnoans there are present in its place paired transverse folds which are probably merely extensions backwards of the lateral plexuses.

The Paraphysis is a projection from the roof of the primitive fore-brain near its anterior end. It is well seen in Dipnoans[42] (Lepidosiren and Protopterus) where in the larva (exactly as in the urodele larva) it forms a blindly ending tube sloping upwards and forwards between the two hemispheres. In the adult it becomes mixed with the two lateral plexuses and is liable to be confused with them. In the other groups—except the Teleosts where it is small (Anguilla) or absent (most Teleosts)—the paraphysis is by no means such a definite structure, but generally there is present a more or less branched and divided diverticulum of the brain wall, frequently glandular, which is homologized with the paraphysis. The morphological significance of the paraphysis is uncertain. It may represent the remains of an ancient sense organ, or it may simply represent the last connexion between the brain and the external ectoderm from which it was derived.

An important derivative of the primitive fore-brain is seen in the pair of cerebral hemispheres which in the higher vertebrates become of such relatively gigantic dimensions. The hemispheres appear to be primitively associated with the special sense of smell, and they are prolonged anteriorly into a pair of olfactory lobes which come into close relation with the olfactory organ. From a consideration of their adult relations and of their development—particularly in those groups where there is no disturbing factor in the shape of a large yolk sac—it seems probable that the hemispheres are primitively paired outpushings of the lateral wall of the primitive fore-brain[43]—in order to give increased space for the increased mass of nervous matter associated with the olfactory sense. They are most highly developed in the Dipneusti amongst fishes. They are there (cf. fig. 29, C) of relatively enormous size with thick nervous floor (corpus striatum) and side walls and roof (pallium) surrounding a central cavity (lateral ventricle) which opens into the third ventricle. At the posterior end of the hemisphere a small area of its wall remains thin and membranous, and this becomes pushed into the lateral ventricle by an ingrowth of blood-vessel to form the huge lateral plexus ( = plexus hemisphaerium). In this great size of the hemispheres[44] and also in the presence of a rudimentary cortex in the Dipnoi we see, as in many other features in these fishes, a distinct foreshadowing of conditions occurring in the higher groups of vertebrates. The Cyclostomes possess a distinct though small pair of hemispheres. In the Selachians the relatively archaic Notidanidae[45] possess a pair of thick-walled hemispheres, but in the majority of the members of the group the paired condition is obscured (fig. 29, A).

In the Teleostomes the mass of nervous matter which in other groups forms the hemispheres does not undergo any pushing outwards except as regards the small olfactory lobes. On the contrary, it remains as a great thickening of the lateral wall of the thalamencephalon (the so-called basal ganglia), additional space for which, however, may be obtained by a considerable increase in length of the fore-brain region (cf. fig. 30, A) or by actual involution into the third ventricle (Polypterus).[46] The great nervous thickenings of the thalamencephalic wall bulge into its cavity and are covered over by the thin epithelial roof of the thalamencephalon which is as a consequence liable to be confused with the pallium or roof of the hemispheres with which it has nothing to do: the homologue of the pallium as of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.[47]

Associated with the parts of the fore-brain devoted to the sense of smell (especially the corpora striata) is the important system of bridging fibres forming the anterior commissure which lies near the anterior end of the floor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in the Dipnoans (Lepidosiren and Protopterus) of a corpus callosum (cf. fig. 30 B) lying dorsal to the anterior commissure and composed of fibres connected with the pallial region of the two hemispheres.

Sense Organs.—The olfactory organs are of special interest in the Selachians, where each remains through life as a widely-open, saccular involution of the ectoderm which may be prolonged backwards to the margin of the buccal cavity by an open oronasal groove, thus retaining a condition familiar in the embryo of the higher vertebrates. In Dipnoans the olfactory organ communicates with the roof of the buccal cavity by definite posterior nares as in the higher forms—the communicating passage being doubtless the morphological equivalent of the oronasal groove, although there is no direct embryological evidence for this. In the Teleostomes the olfactory organ varies from a condition of great complexity in the Crossopterygians down to a condition of almost complete atrophy in certain Teleosts (Plectognathi).[48]

The eyes are usually of large size. The lens is large and spherical and in the case of most Teleostomes accommodation for distant vision is effected by the lens being pulled bodily nearer the retina. This movement is brought about by the contraction of smooth muscle fibres contained in the processus falciformis, a projection from the choroid which terminates in contact with the lens in a swelling, the campanula Halleri. In Amia and in Teleosts a network of capillaries forming the so-called choroid gland surrounds the optic nerve just outside the retina. As a rule the eyes of fishes have a silvery, shining appearance due to the deposition of shining flakes of guanin in the outer layer of the choroid (Argentea) or, in the case of Selachians, in the inner layers (tapetum). Fishes which inhabit dark recesses, e.g. of caves or of the deep sea, show an enlargement, or, more frequently, a reduction, of the eyes. Certain deep-sea Teleosts possess remarkable telescopic eyes with a curious asymmetrical development of the retina.[49]

The otocyst or auditory organ agrees in its main features with that of other vertebrates. In Selachians the otocyst remains in the adult open to the exterior by the ductus endolymphaticus. In Squatina[50] this is unusually wide and correlated; with this the calcareous otoconia are replaced by sand-grains from the exterior. In Dipnoans (Lepidosiren and Protopterus) curious outgrowths arise from the ductus endolymphaticus and come to overlie the roof of the fourth ventricle, recalling the somewhat similar condition met with in certain Amphibians.

In various Teleosts the swimbladder enters into intimate relations with the otocyst. In the simplest condition these relations consist in the prolongation forwards of the swimbladder as a blindly ending tube on either side, the blind end coming into direct contact either with the wall of the otocyst itself or with the fluid surrounding it (perilymph) through a gap in the rigid periotic capsule. A wave of compression causing a slight inward movement of the swimbladder wall will bring about a greatly magnified movement of that part of the wall which is not in relation with the external medium, viz. the part in relation with the interior of the auditory capsule. In this way the perception of delicate sound waves may be rendered much more perfect. In the Ostariophysi (Sagemehl), including the Cyprinidae, the Siluridae, the Characinidae and the Gymnotidae, a physiologically similar connexion between swimbladder and otocyst is brought about by the intervention of a chain of auditory ossicles (Weberian ossicles) formed by modification of the anterior vertebrae.[51]

Lateral Line Organs.[52]—Epidermal sense buds are scattered about in the ectoderm of fishes. A special arrangement of these in lines along the sides of the body and on the head region form the highly characteristic sense organs of the lateral line system. In Lepidosiren these organs retain their superficial position; in other fishes they become sunk beneath the surface into a groove, which may remain open (some Selachians), but as a rule becomes closed into a tubular channel with openings at intervals. It has been suggested that the function of this system of sense organs is connected with the perception of vibratory disturbances of comparatively large wave length in the surrounding medium.

Peripheral Nerves.—In the Cyclostomes the dorsal afferent and ventral efferent nerves are still, as in Amphioxus, independent, but in the gnathostomatous fishes they are, as in the higher vertebrates, combined together into typical spinal nerves.

As regards the cranial nerves the chief peculiarities of fishes relate to (1) the persistence of the branchial clefts and (2) the presence of an elaborate system of cutaneous sense organs supplied by a group of nerves (lateralis) connected with a centre in the brain which develops in continuity with that which receives the auditory nerve. These points may be exemplified by the arrangements in Selachians (see fig. 31). I., II., III., IV. and VI. call for no special remark.

From Bridge, Cambridge Natural History, vol. vii. “Fishes” (by permission of Macmillan & Co., Ltd.). After Wiedersheim, Grundriss der vergleichenden Anatomie (by permission of Gustav Fischer).

Fig. 31.—Diagram of Cranial nerves of a Fish. Cranial nerves and branchial clefts are numbered with Roman figures. Trigeminus black; Facialis dotted; Lateralis oblique shading; Glossopharyngeal cross-hatched; Vagus white.

 bucc, Buccal. c, Commissure between pre- and postauditory parts of lateralis system. d.r, Dorsal roots of spinal nerves. g.g, Gasserian ganglion. gn.g, (Geniculate) ganglion of VII. hy, Hyomandibular. l.n.X, Lateralis vagi. m, Motor branches of hy. md, Mandibular. md.ex, External mandibular. mk.c, Meckel’s cartilage. mx, Maxillary. oc, Occipitospinal. ol.o, Olfactory organ. op.p, Ophthalmicus profundus. op.s, Ophthalmicus superficialis. pn, Palatine. pq., Palatopterygo-quadrate cartilage. s, Spiracle. st, Supra-temporal branch of lateralis system. t.a, Lateralis centre in brain. v.n, Visceral nerve. v.r, Ventral roots.

Trigeminus (V.).—The ophthalmicus profundus branch (op.p.)—which probably is morphologically a distinct cranial nerve—passes forwards along the roof of the orbit to the skin of the snout. As it passes through the orbit it gives off the long ciliary nerves to the eyeball, and is connected with the small ciliary ganglion (also connected with III.) which in turn gives off the short ciliary nerves to the eyeball. The ophthalmicus superficialis (cut short in the figure) branch passes from the root ganglion of V. (Gasserian ganglion), and passes also over the orbit to the skin of the snout. It lies close to, or completely fused with, the corresponding branch of the lateralis system.

The main trunk of V. branches over the edge of the mouth into the maxillary (mx.) and mandibular (md.) divisions, the former, like the two branches already mentioned, purely sensory, the latter mixed—supplying the muscles of mastication as well as the teeth of the lower jaw and the lining of the buccal floor.

The main trunk of the Facialis (VII.) bifurcates over the spiracle into a pre-spiracular portion—the main portion of which passes to the mucous membrane of the palate as the palatine (pnVII.)—and a postspiracular portion, the hyomandibular (hy.) trunk which supplies the muscles of the hyoid arch and also sends a few sensory fibres to the lining of the spiracle, the floor of mouth and pharynx and the skin of the lower jaw. Combined with the main trunk of the facial are branches belonging to the lateralis system.

Lateralis Group of Nerves.—The lateralis group of nerves are charged with the innervation of the system of cutaneous sense organs and are all connected with the same central region in the medulla. A special sensory area of the ectoderm becomes involuted below the surface to form the otocyst, and the nerve fibres belonging to this form the auditory nerve (VIII.). Other portions of the lateralis group become mixed up with various other cranial nerves as follows:

(a) Facial portion.

(1) Ophthalmicus superficialis (op.s.VII.): passes to lining of nose or to the lateral line organs of the dorsal part of snout.

(2) Buccal (bucc.VII): lies close to maxillary division of V. and passes to the sensory canals of the lower side of the snout.

(3) External mandibular (md.ex.): lies in close association with the mandibular division of V., supplies the sensory canals of the lower jaw and hyoid region.

Lateralis vagi (l.n.X.) becomes closely associated with the vagus. It supplies the lateral line organs of the trunk.

In the lamprey and in Dipnoans the lateralis vagi loses its superficial position in the adult and comes into close relation with the notochord.

In Actinopterygians and at least some Selachians a lateralis set of fibres is associated with IX., and in the former fishes a conspicuous trunk of lateralis fibres passes to some or all (Gadus) of the fins. This has been called the lateralis accessorius and is apparently connected with V., VII., IX., X. and certain spinal nerves.[53]

Vagus Group (IX., X., XI.).—The glossopharyngeus (IX.) forks over the first branchial cleft (pretrematic and post-trematic branches) and also gives off a palatine branch (pn.IX.). In some cases (various Selachians, Ganoids and Teleosts) it would seem that IX. includes a few fibres of the lateralis group.

Vagus (X.) is shown by its multiple roots arising from the medulla and also by the character of its peripheral distribution to be a compound structure formed by the fusion of a number of originally distinct nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.), one of which forks over each gill cleft behind the hyobranchial and which may (Selachians) arise by separate roots from the medulla; (2) an intestinal branch (v.n.X.) arising behind the last branchial and innervating the wall of the oesophagus and stomach and it may be even the intestine throughout the greater part of its length (Myxine).

The accessorius (XI.) is not in fishes separated as a distinct nerve from the vagus.

With increased development of the brain its hinder portion, giving rise to the vagus system, has apparently come to encroach on the anterior portion of the spinal cord, with the result that a number of spinal nerves have become reduced to a less or more vestigial condition. The dorsal roots of these nerves disappear entirely in the adult, but the ventral roots persist and are to be seen arising ventrally to the vagus roots. They supply certain muscles of the pectoral fins and of the visceral arches and are known as spino-occipital nerves.[54]

These nerves are divisible into an anterior more ancient set—the occipital nerves—and a posterior set of more recent origin—(occipito-spinal nerves). In Selachians 1-5 pairs of occipital nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and 2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5 pairs occipito-spinal; in Teleosts finally the occipital nerves have entirely disappeared while there are 2 pairs of occipito-spinal. In Cyclostomes no special spino-occipital nerves have been described.

The fibres corresponding with those of the Hypoglossus (XII.) of higher vertebrates spring from the anterior spinal nerves, which are here, as indeed in Amphibia, still free from the cranium.

Sympathetic.—The sympathetic portion of the nervous system does not in fishes attain the same degree of differentiation as in the higher groups. In Cyclostomes it is apparently represented by a fine plexus with small ganglia found in the neighbourhood of the dorsal aorta and on the surface of the heart and receiving branches from the spinal nerves. In Selachians also a plexus occurs in the neighbourhood of the cardinal veins and extends over the viscera: it receives visceral branches from the anterior spinal nerves. In Teleosts the plexus has become condensed to form a definite sympathetic trunk on each side, extending forwards into the head and communicating with the ganglia of certain of the cranial nerves.

(J. G. K.)

V. Distribution in Time and Space

The origin of Vertebrates, and how far back in time they extend, is unknown. The earliest fishes were in all probability devoid of hard parts and traces of their existence can scarcely be expected to be found. The hypothesis that they may be derived from the early Crustaceans, or Arachnids, is chiefly based on the somewhat striking resemblance which the mailed fishes of the Silurian period (Ostracodermi) bear to the Arthropods of that remote time, a resemblance, however, very superficial and regarded by most morphologists as an interesting example of mimetic resemblance—whatever this term may be taken to mean. The minute denticles known as conodonts, which first appear in the Ordovician, were once looked upon as teeth of Cyclostomes, but their histological structure does not afford any support to the identification and they are now generally dismissed altogether from the Vertebrates. As a compensation the Lower Silurian of Russia has yielded small teeth or spines which seem to have really belonged to fishes, although their exact affinities are not known (Palaeodus and Archodus of J. V. Rohon).

It is not until we reach the Upper Silurian that satisfactory remains of unquestionable fishes are found, and here they suddenly appear in a considerable variety of forms, very unlike modern fishes in every respect, but so highly developed as to convince us that we have to search in much earlier formations for their ancestors. These Upper Silurian fishes are the Coelolepidae, the Ateleaspidae, the Birkeniidae, the Pteraspidae, the Tremataspidae and the Cephalaspidae, all referred to the Ostracophori. The three last types persist in the Devonian, in the middle of which period the Osteolepid Crossopterygii, the Dipneusti and the Arthrodira suddenly appear. The most primitive Selachian (Cladoselache), the Acanthodian Selachians (Diplacanthidae), the Chimaerids (Ptyctodus), and the Palaeoniscid ganoids (Chirolepis) appear in the Upper Devonian, along with the problematic Palaeospondylus.

In the Carboniferous period, the Ostracophori and Arthrodira have disappeared, the Crossopterygii and Dipneusti are still abundant, and the Selachians (Pleuracanthus, Acanthodians, truesharks) and Chondrostean ganoids (Palaeoniscidae and Platysomidae) are predominant. In the Upper Permian the Holostean ganoids (Acanthophorus) make their appearance, and the group becomes dominant in the Jurassic and the Lower Cretaceous. In the Trias, the Crossopterygii and Dipneusti dwindle in variety and the Ceratodontidae appear; the Chondrostean and Holostean ganoids are about equally represented, and are supplemented in the Jurassic by the first, annectant representatives of the Teleostei (Pholidophoridae, Leptolepidae). In the latter period, the Holostean ganoids are predominant, and with them we find numerous Cestraciont sharks, some primitive skates (Squatinidae and Rhinobatidae), Chimaerids and numerous Coelacanthid crossopterygians.

The fish-fauna of the Lower Cretaceous is similar to that of the Jurassic, whilst that of the Chalk and other Upper Cretaceous formations is quite modern in aspect, with only a slight admixture of Coelacanthid crossopterygians and Holostean ganoids, the Teleosteans being abundantly represented by Elopidae, Albulidae, Halosauridae, Scopelidae and Berycidae, many being close allies of the present inhabitants of the deep sea. At this period the spiny-rayed Teleosteans, dominant in the seas of the present day, made their first appearance.

With the Eocene, the fish-fauna has assumed the essential character which it now bears. A few Pycnodonts survive as the last representatives of typically Mesozoic ganoids, whilst in the marine deposits of Monte Bolca (Upper Eocene) the principal families of living marine fishes are represented by genera identical with or more or less closely allied to those still existing; it is highly remarkable that forms so highly specialized as the sucking-fish or remoras, the flat-fish (Pleuronectidae), the Pediculati, the Plectognaths, &c., were in existence, whilst in the freshwater deposits of North America Osteoglossidae and Cichlidae were already represented. Very little is known of the freshwater fishes of the early Tertiaries. What has been preserved of them from the Oligocene and Miocene shows that they differed very slightly from their modern representatives. We may conclude that from early Tertiary times fishes were practically as they are at present. The great hiatus in our knowledge lies in the period between the Cretaceous and the Eocene.

At the present day the Teleosteans are in immense preponderance, Selachians are still well represented, the Chondrostean ganoids are confined to the rivers and lakes of the temperate zone of the northern hemisphere (Acipenseridae, Polyodontidae), the Holostean ganoids are reduced to a few species (Lepidosteus, Amia) dwelling in the fresh waters of North America, Mexico and Cuba, the Crossopterygians are represented by the isolated group Polypteridae, widely different from any of the known fossil forms, with about ten species inhabiting the rivers and lakes of Africa, whilst the Dipneusti linger in Australia (Neoceratodus), in South America (Lepidosiren), and in tropical Africa (Protopterus). The imperfections of the geological record preclude any attempt to deal with the distribution in space as regards extinct forms, but several types, at present very restricted in their habitat, once had a very wide distribution. The Ceratodontidae, for instance, of which only one species is now living, confined to the rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic and Cretaceous rocks of Europe, North America, Patagonia, North and South Africa, India and Australia; the Amiidae and Lepidosteidae were abundant in Europe in Eocene and Miocene times; the Osteoglossidae, now living in Africa, S.E. Asia and South America, occurred in North America and Europe in the Eocene.

In treating of the geographical distribution of modern fishes, it is necessary to distinguish between fresh-water and marine forms. It is, however, not easy to draw a line between these categories, as a large number of forms are able to accommodate themselves to either fresh or salt water, whilst some periodically migrate from the one into the other. On the whole, fishes may be roughly divided into the following categories:—

I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea fishes.

II. Brackish-water fishes.

III. Fresh-water fishes.

IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn); B. catadromous (descending to the sea to spawn).

About two-thirds of the known recent fishes are marine. Such are nearly all the Selachians, and, among the Teleosteans, all the Heteromi, Pediculati and the great majority of Apodes, Thoracostei, Percesoces, Anacanthini, Acanthopterygii and Plectognathi. All the Crossopterygii, Dipneusti, Opisthomi, Symbranchii, and nearly all the Ganoidei and Ostariophysi are confined to fresh-water.

The three categories of marine fishes have thus been defined by Günther:—

“1. Shore Fishes—that is, fishes which chiefly inhabit parts of the sea in the immediate neighbourhood of land either actually raised above, or at least but little submerged below, the surface of the water. They do not descend to any great depth,—very few to 300 fathoms, and the majority live close to the surface. The distribution of these fishes is determined, not only by the temperature of the surface water, but also by the nature of the adjacent land and its animal and vegetable products,—some being confined to flat coasts with soft or sandy bottoms, others to rocky and fissured coasts, others to living coral formations. If it were not for the frequent mechanical and involuntary removals to which these fishes are exposed, their distribution within certain limits, as it no doubt originally existed, would resemble still more that of freshwater fishes than we find it actually does at the present period.

2. Pelagic Fishes—that is, fishes which inhabit the surface and uppermost strata of the open ocean, and approach the shores only accidentally or occasionally (in search of prey), or periodically (for the purpose of spawning). The majority spawn in the open sea, their ova and young being always found at a great distance from the shore. With regard to their distribution, they are still subject to the influences of light and the temperature of the surface water; but they are independent of the variable local conditions which tie the shore fish to its original home, and therefore roam freely over a space which would take a freshwater or shore fish thousands of years to cover in its gradual dispersal. Such as are devoid of rapidity of motion are dispersed over similarly large areas by the oceanic currents, more slowly than the strong swimmers, but not less surely. An accurate definition, therefore, of their distribution within certain areas equivalent to the terrestrial regions is much less feasible than in the case of shore fishes.

3. Deep-Sea Fishes—that is, fishes which inhabit such depths of the ocean that they are but little or not at all influenced by light or the surface temperature, and which, by their organization, are prevented from reaching the surface stratum in a healthy condition. Living almost under identical tellurian conditions, the same type, the same species, may inhabit an abyssal depth under the equator as well as one near the arctic or antarctic circle; and all that we know of these fishes points to the conclusion that no separate horizontal regions can be distinguished in the abyssal fauna, and that no division into bathymetrical strata can be attempted on the base of generic much less of family characters.”

A division of the world into regions according to the distribution of the shore-fishes is a much more difficult task than that of tracing continental areas. It is possible perhaps to distinguish four great divisions: the Arctic region, the Atlantic region, the Indo-Pacific region and the Antarctic region. The second and third may be again subdivided into three zones: Northern, Tropical and Southern. This appears to be a more satisfactory arrangement than that which has been proposed into three zones primarily, each again subdivided according to the different oceans. Perhaps a better division is that adopted by D. S. Jordan, who arranges the littoral fishes according to coast lines; we then have an East Atlantic area, a West Atlantic, an East Pacific and a West Pacific, the latter including the coasts of the Indian Ocean. The tropical zone, whatever be the ocean, is that in which fishes flourish in greatest abundance and where, especially about coral-reefs, they show the greatest variety of bizarre forms and the most gorgeous coloration. The fish-fauna of the Indo-Pacific is much richer than that of the Atlantic, both as regards genera and species.

As regards the Arctic and Antarctic regions, the continuity or circumpolar distribution of the shore fishes is well established. The former is chiefly characterized by its Cottids, Cyclopterids, Zoarcids and Gadids, the latter by its Nototheniids. The theory of bipolarity receives no support from the study of the fishes.

Pelagic fishes, among which we find the largest Selachians and Teleosteans, are far less limited in their distribution, which, for many species, is nearly world-wide. Some are dependent upon currents, but the great majority being rapid swimmers able to continue their course for weeks, apparently without the necessity of rest (many sharks, scombrids, sword-fishes), pass from one ocean into the other. Most numerous between the tropics, many of these fishes occasionally wander far north and south of their habitual range, and there are few genera that are at all limited in their distribution.

Deep-sea fishes, of which between seven hundred and eight hundred species are known, belong to the most diverse groups and quite a number of families are exclusively bathybial (Chlamydoselachidae, Stomiatidae, Alepocephalidae, Nemichthyidae, Synaphobranchidae, Saccopharyngidae, Cetomimidae, Halosauridae, Lipogenyidae, Notacanthidae, Chiasmodontidae, Icosteidae, Muraenolepididae, Macruridae, Anomalopidae, Podatelidae, Trachypteridae, Lophotidae, Ceratiidae, Gigantactinidae). But they are all comparatively slight modifications of the forms living on the surface of the sea or in the shallow parts, from which they may be regarded as derived. In no instance do these types show a structure which may be termed archaic when compared with their surface allies. That these fishes are localized in their vertical distribution, between the 100-fathoms line, often taken as the arbitrary limit of the bathybial fauna, and the depth of 2750 fathoms, the lowest point whence fishes have been procured, there is little doubt. But our knowledge is still too fragmentary to allow of any general conclusions, and the same applies to the horizontal distribution. Yet the same species may occur at most distant points; as these fishes dwell beyond the influence of the sun’s rays, they are not affected by temperature, and living in the Arctic zone or under the equator makes little difference to them. A great deal of evidence has been accumulated to show the gradual transition of the surface into the bathybial forms; a large number of surface fishes have been met with in deep water (from 100 to 500 fathoms), and these animals afford no support to Alexander Agassiz’s supposition of the existence of an azoic zone between the 200-fathoms line and the bottom.

Brackish-water fishes occur also in salt and fresh water, in some localities at least, and belong to various groups of Teleosteans. Sticklebacks, gobies, grey mullets, blennies are among the best-known examples. The facility with which they accommodate themselves to changes in the medium in which they live has enabled them to spread readily over very large areas. The three-spined stickleback, for instance, occurs over nearly the whole of the cold and temperate parts of the northern hemisphere, whilst a grey mullet (Mugil capito) ranges without any appreciable difference in form from Scandinavia and the United States along all the Atlantic coasts to the Cape of Good Hope and Brazil. It would be hardly possible to base zoo-geographical divisions on the distribution of such forms.

The fresh-water fishes, however, invite to such attempts. How greatly their distribution differs from that of terrestrial animals has long ago been emphasized. The key to their mode of dispersal is, with few exceptions, to be found in the hydrography of the continents, latitude and climate, excepting of course very great altitudes, being inconsiderable factors, the fish-fauna of a country deriving its character from the headwaters of the river-system which flows through it. The lower Nile, for instance, is inhabited by fishes bearing a close resemblance to, or even specifically identical with, those of tropical Africa, thus strikingly contrasting with the land-fauna of its banks. The knowledge of the river-systems is, however, not sufficient for tracing areas of distribution, for we must bear in mind the movements which have taken place on the surface of the earth, owing to which present conditions may not have existed within comparatively recent times, geologically speaking; and this is where the systematic study of the aquatic animals affords scope for conclusions having a direct bearing on the physical geography of the near past. It is not possible here to enter into the discussion of the many problems which the distribution of fresh-water fishes involves; we limit ourselves to an indication of the principal regions into which the world may be divided from this point of view. The main divisions proposed by Günther in the 9th edition of the Encyclopædia Britannica still appear the most satisfactory. They are as follows:—

I. The Northern Zone or Holarctic Region.—Characterized by Acipenseridae. Few Siluridae. Numerous Cyprinidae, Salmonidae, Esocidae, Percidae.

1. Europaeo-Asiatic or Palaearctic Region. Characterized by absence of osseous Ganoidei; Cobitinae and Barbus numerous.

2. North American or Nearctic Region. Characterized by osseous Ganoidei and abundance of Catostominae; but no Cobitinae or Barbus.

II. The Equatorial Zone.—Characterized by the development of Siluridae.

A. Cyprinoid Division. Characterized by presence of Cyprinidae, Mastacembelidae. Anabantidae, Ophiocephalidae.

1. Indian Region. Characterized by absence of Dipneusti, Polypteridae, Mormyridae and Characinidae. Cobitinae numerous.

2. African Region. Characterized by presence of Dipneusti, Polypterid and Mormyrid; Cichlid and Characinid numerous.

B. Acyprinoid Division. Characterized by absence of Cyprinidae and the other families mentioned above.

1. Tropical American or Neotropical Region. Characterized by presence of Dipneusti; Cichlidae and Characinidae numerous; Gymnotidae and Loricariidae.

2. Tropical Pacific Region. Includes the Australian as well as the Polynesian Region. Characterized by presence of Dipneusti. Cichlidae and Characinidae absent.

III. The Southern Zone.—Characterized by absence of Cyprinidae and scarcity of Siluridae. Haplochitonidae and Galaxiidae represent the Salmonids and Esoces of the northern zone. One region only.

1. Antarctic Region. Characterized by the small number of species; the fishes of

(a) The Tasmanian subregion;

(b) The New Zealand subregion; and

(c) The Patagonian or Fuegian subregion being almost identical.

Although, as expressed in the above synopsis, the resemblance between the Indian and African regions is far greater than exists between them and the other regions of the equatorial zone, attention must be drawn to the marked affinity which some of the fishes of tropical Africa show to those of South America (Lepidosirenidae, Characinidae, Cichlidae, Nandidae), an affinity which favours the supposition of a connexion between these two parts of the world in early Tertiary times.

The boundaries of Günther’s regions may thus be traced, beginning with the equatorial zone, this being the richest.

Equatorial Zone.—Roughly speaking, the borders of this zoological zone coincide with the geographical limits of the tropics of Cancer and Capricorn; its characteristic forms, however, extend in undulating lines several degrees both northwards and southwards. Commencing from the west coast of Africa, the desert of the Sahara forms a boundary between the equatorial and northern zones; as the boundary approaches the Nile, it makes a sudden sweep towards the north as far as northern Syria, crosses through Persia and Afghanistan to the southern ranges of the Himalayas, and follows the course of the Yang-tse-Kiang, which receives its contingent of equatorial fishes through its southern tributaries. Its continuation through the North Pacific may be indicated by the tropic, which strikes the coast of Mexico at the southern end of the Gulf of California. Equatorial types of South America are known to extend so far northwards; and, by following the same line, the West India Islands are naturally included in this zone.

Towards the south the equatorial zone embraces the whole of Africa and Madagascar, and seems to extend still farther south in Australia, its boundary probably following the southern coast of that continent; the detailed distribution of the freshwater fishes of south-western Australia has been little studied, but the tropical fishes of that region follow the principal watercourse, the Murray river, far towards the south and probably to its mouth. The boundary-line then stretches to the north of Tasmania and New Zealand, coinciding with the tropic until it strikes the western slope of the Andes, on the South American continent, where it again bends southward to embrace the system of the Rio de la Plata.

The four regions into which the equatorial zone is divided arrange themselves into two well-marked divisions, one of which is characterized by the presence of Cyprinid fishes, combined with the development of Labyrinthic Percesoces (Anabantidae and Ophiocephalidae) and Mastacembelids, whilst in the other these types are absent. The boundary between the Cyprinoid and Acyprinoid division seems to follow the now exploded Wallace’s line—a line drawn from the south of the Philippines between Borneo and Celebes, and farther south between Bali and Lombok. Borneo abounds in Cyprinids; from the Philippine Islands a few only are known, and in Bali two species have been found; but none are known from Celebes or Lombok, or from islands situated farther east.

The Indian region comprises Asia south of the Himalayas and the Yang-tse-Kiang, and includes the islands to the west of Celebes and Lombok. Towards the north-east the island of Formosa, which also by other parts of its fauna shows the characters of the equatorial zone, has received some characteristic Japanese freshwater fishes. Within the geographical boundaries of China the freshwater fishes of the tropics pass gradually into those of the northern zone, both being separated by a broad, debateable ground. The affluents of the great river traversing this district are more numerous from the south than from the north, and carry the southern fishes far into the temperate zone. Scarcely better defined is the boundary of this region towards the north-west, in which fishes were very poorly represented by types common to India and Africa.

The African region comprises the whole of Africa south of the Sahara. It might have been conjectured that the more temperate climate of its southern extremity would have been accompanied by a conspicuous difference in the fish fauna. But this is not the case; the difference between the tropical and southern parts of Africa consists simply in the gradual disappearance of specifically tropical forms, whilst Silurids, Cyprinids and even Anabas penetrate to its southern coast; no new form, except a Galaxias at the Cape of Good Hope, has entered to impart to South Africa a character distinct from the central portion of the continent. In the north-east the African fauna passes the isthmus of Suez and penetrates into Syria; the system of the Jordan presents so many African types that it has to be included in a description of the African region as well as of the Europaeo-Asiatic.

The boundaries of the Neotropical or Tropical American region have been sufficiently indicated in the definition of the equatorial zone. A broad and most irregular band of country, in which the South and North American forms are mixed, exists in the north.

The Tropical Pacific region includes all the islands east of Wallace’s line, New Guinea, Australia (with the exception of its south-eastern portion), and all the islands of the tropical Pacific to the Sandwich group.

Northern Zone.—The boundaries of the northern zone coincide in the main with the northern limit of the equatorial zone; but they overlap the latter at different points. This happens in Syria, as well as east of it, where the mixed faunae of the Jordan and the rivers of Mesopotamia demand the inclusion of this territory in the northern zone as well as in the equatorial; in the island of Formosa, where a Salmonid and several Japanese Cyprinids flourish; and in Central America, where a Lepidosteus, a Cyprinid (Sclerognathus meridionalis), and an Amiurus (A. meridionalis) represent the North American fauna in the midst of a host of tropical forms.

There is no separate arctic zone for freshwater fishes; ichthyic life becomes extinct towards the pole wherever the fresh water remains frozen throughout the year, or thaws for a few weeks only; and the few fishes which extend into high latitudes belong to types in no wise differing from those of the more temperate south. The highest latitude at which fishes have been obtained is 82° N. lat., whence specimens of char (Salmo arcturus and Salmo naresii) have been brought back.

The Palaearctic or Europaeo-Asiatic Region.—The western and southern boundaries of this region coincide with those of the northern zone. Bering Strait and the Kamchatka Sea have been conventionally taken as the boundary in the north, but the fishes of both coasts, so far as they are known, are not sufficiently distinct to be referred to two different regions. The Japanese islands exhibit a decided Palaearctic fish fauna with a slight influx of tropical forms in the south. In the east, as well as in the west, the distinction between the Europaeo-Asiatic and the North American regions disappears almost entirely as we advance farther towards the north. Finally, the Europaeo-Asiatic fauna mingles with African and Indian forms in Syria, Persia and Afghanistan.

The boundaries of the North American or Nearctic region have been sufficiently indicated. The main features and the distribution of this fauna are identical with those of the preceding region.

Southern Zone.—The boundaries of this zone have been indicated in the description of the equatorial zone; they overlap the southern boundaries of the latter in South Australia and South America, but we have not the means of defining the limits to which southern types extend northwards. This zone includes Tasmania, with at least a portion of south-eastern Australia (Tasmanian sub-region), New Zealand and the Auckland Islands (New Zealand sub-region), and Chile, Patagonia, Tierra del Fuego and the Falkland Islands (Fuegian sub-region). No freshwater fishes are known from Kerguelen’s Land, or from islands beyond 55° S. lat.

The Tropical American region is the richest (about 1300 species); next follow the African region (about 1000), the Indian region (about 800), the Europaeo-Asiatic region (about 500), the North American region (about 400), the Tropical Pacific region (about 60); whilst the Antarctic region is quite insignificant.

Of the migratory fishes, or fishes travelling regularly from the sea to fresh waters, most, if not all, were derived from marine forms. The anadromous forms, annually or periodically ascending rivers for the purpose of spawning, such as several species of Acipenser, Salmo, Coregonus, Clupea (shads), and Petromyzon, are only known from the northern hemisphere, whilst the catadromous forms, spending most of their life in fresh water but resorting to the sea to breed, such as Anguilla, some species of Mugil, Galaxias and Pleuronectes, have representatives in both hemispheres.

(G. A. B.)

1. For general anatomy of fishes, see T. W. Bridge, Cambridge Natural History, and R. Wiedersheim, Vergl. Anat. der Wirbeltiere. The latter contains an excellent bibliography.
2. Cf. J. Graham Kerr, Proc. Camb. Phil. Soc. x. 227.
3. For electric organs see W. Biedermann, Electro-Physiology.
4. J. Graham Kerr, Quart. Journ. Micr. Sci. vol. xlvi.
5. J. Graham Kerr, The Budgett Memorial Volume.
6. J. Phelps, Science, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, Amer. Journ. Anat. v. (1906) p. 154.
7. J. S. Budgett, Trans. Zool. Soc. Lond. xvi., 1901, p. 130.
8. L. Drüner, Zool. Jahrbücher Anat. Band xix. (1904), S. 434.
9. J. Graham Kerr, Quart. Journ. Micr. Sci. xlvi. 423.
10. J. S. Budgett, op. cit.
11. W. E. Agar, Anat. Anz., 1905, S. 298.
12. J. Graham Kerr, The Budgett Memorial Volume.
13. J. Phelps, Science, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, Amer. Journ. Anat., v. 1906, p. 154.
14. F. Maurer, Morphol. Jahrb. ix., 1884, S. 229, and xiv., 1888, S. 175.
15. J. Rückert, Arch. Entwickelungsmech. Band iv., 1897, S. 298; J. Graham Kerr, Phil. Trans. B. 192, 1900, p. 325, and The Budgett Memorial Volume.
16. Cuvier et Valenciennes, Hist. nat. des poiss. xix., 1846, p. 151.
17. J. Rathke, Üb. d. Darmkanal u.s.w. d. Fische, Halle, 1824, S. 62.
18. Cf. W. Biedermann, Electro-Physiology.
19. Literature in N. K. Koltzoff, Bull. Soc. Nat. Moscou, 1901, P. 259.
20. J. Graham Kerr, Proc. Zool. Soc. Lond. (1901), p. 484.
21. J. S. Budgett, Trans. Zool. Soc. Lond. xv. (1901), vol. p. 324.
22. H. F. Jungersen, Arb. zool. zoot. Inst. Würzburg, Band ix., 1889.
23. E. J. Bles, Proc. Roy. Soc. 62, 1897, p. 232.
24. J. Graham Kerr, Proc. Zool. Soc. Lond. (1901) p. 484.
25. F. M. Balfour and W. N. Parker, Phil. Trans. (1882).
26. J. Graham Kerr, Proc. Zool. Soc. Lond. (1901), p. 495.
27. H. Gadow and E. C. Abbott, Phil. Trans. 186 (1895), p. 163.
28. For development cf. Gaupp in Hertwig’s Handbuch der Entwickelungslehre.
29. Cf. W. E. Agar, Trans. Roy. Soc. Edin. xlv. (1906), 49.
30. Bashford Dean, Journ. Morph. ix. (1894) 87, and Trans. New York Acad. Sci. xiii. (1894) 115.
31. R. Semon, Zool. Forschungsreisen, Band i. § 115.
32. O. Hertwig, Arch. mikr. Anat. xi. (1874).
33. R. H. Traquair, Trans. Roy. Soc. Edin. xxxix. (1899).
34. Cf. E. S. Goodrich, Quart. Journ. Micr. Sci. xlvii. (1904), 465.
35. R. H. Traquair, Journ. Anat. Phys. v. (1871) 166; J. S. Budgett, Trans. Zool. Soc. Lond. xvi. 315.
36. T. W. Bridge, Trans. Zool. Soc. Lond. xiv. (1898) 350; W. E. Agar, op. cit.
37. J. V. Boas, Morphol. Jahrb. vi. (1880).
38. Cf. F. Hochstetter in O. Hertwig Handbuch der Entwickelungslehre.
39. C. v. Kupffer, Studien z. vergl. Entwickelungsgeschichte der Cranioten.
40. Cf. F. K. Studnička’s excellent account of the parietal organs in A. Oppel’s Lehrbuch vergl, mikr. Anatomie, T. v. (1905).
41. F. K. Studnička, S.B. böhm. Gesell. (1901); J. Graham Kerr, Quart. Journ. Micr. Sci. vol. xlvi., and The Budgett Memorial Volume.
42. J. Graham Kerr, Quart. Journ. Micr. Sci. vol. xlvi.
43. F. K. Studnička, S.B. böhm. Gesell. (1901); J. Graham Kerr, Quart. Journ. Micr. Sci. vol. xlvi., and The Budgett Memorial Volume.
44. G. Elliot Smith, Anat. Anz. (1907).
45. F. K. Studnička, S.B. böhm. Gesell. (1896).
46. J. Graham Kerr, The Budgett Memorial Volume.
47. F. K. Studnička, S.B. böhm. Gesell. (1901); J. Graham Kerr, Quart. Journ. Micr. Sci. xlvi., and The Budgett Memorial Volume.
48. R. Wiedersheim, Kölliker’s Festschrift: cf. also Anat. Anz. (1887).
49. A. Brauer, Verhandl. deutsch. zool. Gesell. (1902).
50. C. Stewart, Journ. Linn. Soc. Zool. (1906), 439.
51. T. W. Bridge and A. C. Haddon, Phil. Trans. 184 (1893).
52. For literature of lateral line organs see Cole, Trans. Linn. Soc. vii. (1898).
53. For literature of lateral line organs see Cole, Trans. Linn. Soc., vii. (1898).
54. M. Fürbringer in Gegenbaur’s Festschrift (1896).