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 centrales 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, 8vo). 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. : LEPTOGARDII.

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, and not 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.

AUTHORITIES.--The following selection from the extremely extensive ichthyological literature which has appeared during the period 1880-1906 will supplement the bibliographical notice appended to section I. I. The General Subject: A. Günther, _Introduction to the Study of Fishes_ (Edinburgh, 1880); B. Dean, _Fishes Living and Fossil_ (New York, 1895); T. W. Bridge and G. A. Boulenger, "Fishes," _Cambridge Natural History_, vii. (1904); D. S. Jordan, _Guide to the Study of Fishes_ (2 vols., New York, 1905). II. Palaeontological: A. Fritsch, _Fauna der Gaskohle und der Kalksteine der Permformation Böhmens_ (vols, i.-iii., Prague, 1879-1894); K. A. von Zittel, _Handbuch der Paläontologie_, vol. iii. (Munich, 1887); A. Smith Woodward, _Catalogue of Fossil Fishes in the British Museum_, vols. i.-iii. (London, 1889-1895); A. Smith Woodward, _Outlines of Vertebrate Palaeontology for Students of Zoology_ (Cambridge, 1898); J. S. Newberry, "The Palaeozoic Fishes of North America," _Mon. U.S. Geol. Surv._ vol. xvi. (1889); J. V. Rohon, "Die obersilurischen Fische von Ösel, Thyestidae und Tremataspidae," _Mém. Ac. Imp. Sc. St-Pétersb._ xxxviii. (1892); O. Jaekel, _Die Selachier von Bolca, ein Beitrag zur Morphogenie der Wirbeltiere_ (Berlin, 1894); B. Dean, "Contributions to the Morphology of Cladoselache," _Journ. Morphol._ ix. (1894); R. H. Traquair, "The Asterolepidae," _Mon. Palaeont. Soc._ (1894-1904, in progress); "Report on Fossil Fishes collected by the Geological Survey of Scotland in the Silurian Rocks of the South of Scotland," _Trans. Roy. Soc. Edin._ xxxix. (1899); L. Dollo, "Sur la phylogénie des Dipneustes," _Bull. Soc. Belge Géol._ vol. ix. (1895); E. W. Claypole, "The Ancestry of the Upper Devonian Placoderms of Ohio," _Amer. Geol._ xvii. (1896); B. Dean, "Palaeontlogical Notes," _Mem. N.Y. Ac._ ii. (1901); A. Stewart and S. W. Williston, "Cretaceous Fishes of Kansas," _Univ. Geol. Surv. Kansas_, vi. (Topeka, 1901); A. S. Woodward, "Fossil Fishes of the English Chalk," _Palaeontogr. Soc._ (1902-1903, etc.); R. H. Traquair, "The Lower Devonian Fishes of Gemünden.," _Roy. Soc. Edin. Trans._ 40 (1903); W. J. and I. B. J. Sollas, "Account of the Devonian Fish Palaeospondylus," _Phil. Trans._ 196 (1903); C. T. Regan, "Phylogeny of the Teleostomi," _Ann. & Mag. N.H._ (7) 13 (1904); C. R. Eastman, "Fishes of Monte Bolca," _Bull. Mus. C.Z._ 46 (1904); "Structure and Relations of Mylostoma," _Op. cit._ 2 (1906); O. Abel, "Fossile Flugfische," _Jahrb. Geol. Reichsanst._ 56 (Wien, 1906); L. Hussakof. "Studies on the Arthrodira," _Mem. Amer. Mus. N.H._ ix. (1906). III. Faunistic (recent fishes): (A) EUROPE: E. Bade, _Die mitteleuropäischen Süsswasserfische_ (2 vols., Berlin, 1901-1902). GREAT BRITAIN: F. Day, _The Fishes of Great Britain and Ireland_ (2 vols., London, 1880-1884); J. T. Cunningham, _The Natural History of the Marketable Marine Fishes of the British Islands_ (London, 1896); W. C. M'Intosh and A. T. Masterman, _The Life-Histories of the British Marine Food-Fishes_ (London, 1897); Sir H. Maxwell, _British Fresh-water Fish_ (London, 1904); F. G. Aflalo, _British Salt-water Fish_ (London, 1904). Numerous important researches into the development, life-conditions and distributions, carried out at the Biological Laboratories at Plymouth and St Andrews and during the survey of the fishing grounds of Ireland, have been published by W. L. Calderwood, J. T. Cunningham, E. W. L. Holt, W. C. M'Intosh, J. W. Fulton, W. Garstang and Prince in the _Journ. Mar. Biolog. Assoc._, _The Reports of the Fishery Board of Scotland_, _Scient. Trans. R. Dublin Soc._ and other periodicals. (B) DENMARK AND SCANDINAVIA: W. Lilljeborg, _Sveriges och Norges Fiskar_ (3 vols., Upsala, 1881-1891); F. A. Smith, _A History of Scandinavian Fishes by B. Fries, C. U. Ekström and C. Sundevall, with Plates by W. von Wright_ (second edition, revised and completed by F. A. S., Stockholm, 1892); A. Stuxberg, _Sveriges och Norges Fiskar_ (Göteborg, 1895); C. G. J. Petersen, _Report of the Danish Biological Station_ (Copenhagen, 1802-1900) (annual reports containing much information on fishes of and fishing in the Danish seas). (C) FINLAND: G. Sundman and A. J. Mela, _Finland's Fiskar_ (Helsingfors, 1883-1891). (D) GERMANY: K. Möbius and F. Heincke, "Die Fische der Ostsee," _Bericht Commiss. Untersuch. deutsch. Meere_ (Kiel, 1883); F. Heincke, E. Ehrenbaum and G. Duncker have published their investigations into the life-history and development of the fishes of Heligoland in _Wissenschaftl. Meeresuntersuchungen_ (Kiel and Leipzig, 1894-1899); (E) SWITZERLAND: V. Fatio, _Faune des vertébrés de la Suisse: Poissons_ (2 vols., Geneva and Basel, 1882-1890). (F) FRANCE: E. Moreau, _Histoire naturelle des poissons de la France_ (3 vols., Paris, 1881); _Supplément_ (Paris, 1891). (G) PYRENEAN PENINSULA: D. Carlos de Bragança, _Resultados das investigações scientificas feitas a bordo do yacht "Amelia." Pescas maritimas_, i. and ii. (Lisbon, 1899-1904). (H) ITALY AND MEDITERRANEAN: P. Döderlein, _Manuale ittiologico del Mediterraneo_ (Palermo, 1881-1891, not completed; interrupted by the death of the author); E. W. L. Holt, "Recherches sur la reproduction des poissons osseux, principalement dans le golfe de Marseille," _Ann. Mus. Mars._ v. (Marseilles, 1899); (I) WESTERN AND CENTRAL ASIA: L. Lortet, "Poissons et reptiles du lac de Tibériade," _Arch. Mus. d'Hist. Nat. Lyon_, iii. (1883); S. Herzenstein, _Wissenschaftliche Resultate der von N. M. Przewalski nach Central Asien unternommenen Reisen: Fische_ (St Petersburg, 1888-1891); L. Berg, _Fishes of Turkestan_ (Russian text, St Petersburg, 1905); G. Radde, S. Kamensky and F. F. Kawraisky have worked out the Cyprinids and Salmonids of the Caucasus (Tiflis, 1896-1899). (J) JAPAN: F. Steindachner and L. Döderlein, "Beiträge zur Kenntniss der Fische Japans," _Denkschr. Ak. Wien_, (vols. 67 and 68, 1883); K. Otaki, T. Fujita and T. Higurashi, _Fishes of Japan_ (in Japanese) (Tokyo, 1903, in progress). Numerous papers by D. S. Jordan, in collaboration with J. O. Snyder, E. C. Starks, H. W. Fowler and N. Sindo. (K) EAST INDIES: F. Day, _The Fauna of British India: Fishes_ (2 vols., London, 1889) (chiefly an abridgment of the author's _Fishes of India_); M. Weber, "Die Süsswasserfische des Indischen Archipels," _Zool. Ergebnisse e. Reise in Niederl. Ostind._ iii. (Leiden, 1894). Numerous contributions to the fauna of the Malay Peninsula and Archipelago by G. A. Boulenger, L. Vaillant, F. Steindachner, G. Duncker, W. Volz and C. L. Popta. (L) AFRICA: G. A. Boulenger, _Matériaux pour la faune du Congo: poissons nouveaux_ (Brussels, 1898-1902, in progress); and _Poissons du bassin du Congo_ (Brussels, 1901); G. Pfeffer, _Die Thierwelt Ostafrikas: Fische_ (Berlin, 1896); A. Günther, G. A. Boulenger, G. Pfeffer, F. Steindachner, D. Vinciguerra, J. Pellegrin and E. Lönnberg have published numerous contributions to the fish-fauna of tropical Africa in various periodicals. The marine fishes of South Africa have received special attention on the part of J. D. F. Gilchrist, _Marine Investigations in South Africa_, i. and ii. (1898-1904), and new species have been described by G. A. Boulenger and C. T. Regan. (M) NORTH AMERICA: D. S. Jordan and B. W. Evermann, _The Fishes of North and Middle America_ (4 vols., Washington, 1896-1900); D. S. Jordan and B. W. Evermann, _American Food and Game Fishes_ (New York, 1902); D. S. Jordan and C. H. Gilbert "The Fishes of Bering Sea," in _Fur-Seals and Fur-Seal Islands_ (Washington, 1899); The U.S. Bureau of Fisheries (since 1903) has published annually a _Report_ and a _Bulletin_, containing a vast amount of information on North American fishes and every subject having a bearing on the fisheries of the United States; S. E. Meek, "Fresh-water Fishes of Mexico," _Field Columb. Mus. Zool._ v. (1904). (N) SOUTH AMERICA: C. H. and R. S. Eigenmann, "A Catalogue of the Fresh-water Fishes of South America," _Proc. U.S. Nat. Mus._ 14 (Washington, 1891); the same authors, F. Steindachner, G. A. Boulenger, C. Berg and C. T. Regan have published contributions in periodicals on this fauna. (O) AUSTRALIA: J. E. Tenison-Woods, _Fish and Fisheries of New South Wales_ (Sydney, 1882); J. Douglas Ogilby, Edible Fishes and Crustaceans of New South Wales (Sydney, 1893); J. Douglas Ogilby and E. R. Waite are authors of numerous papers on Australian fishes in _Proc. Linn. Soc. N.S. Wales_ and _Rec. Austral. Mus._ (P) SOUTH PACIFIC: D S. Jordan and B. W. Evermann, "Shore Fishes of the Hawaiian Islands," _Bull. U.S. Fish. Comm._ 23 (1905). (Q) MADAGASCAR: H. E. Sauvage, _Histoire physique, naturelle et politique de Madagascar_, par A. Grandidier. xvi.; _Poissons_ (Paris, 1891). (R) OCEANIC FISHES: G. B. Goode and T. H. Bean, _Oceanic Ichthyology_ (Washington, 1895); A. Günther, _Deep-sea Fishes of the "Challenger" Expedition_ (London, 1887); C. H. Gilbert, "Deep-sea Fishes of the Hawaiian Islands," _Bull. U.S. Fish. Comm._ 23 (1905); R. Collett, _Norske Nordhavs Expedition: Fiske_ (Christiania, 1880); C. F. Lütken, _Dijmphna-Togtets Zoologisk-botaniske Udbytte: Kara-Havets Fiske_ (Copenhagen, 1886); L. Vaillant, _Expéditions scientifiques du "Travailleur" et du "Talisman": Poissons_ (Paris, 1888); A. Agassiz, _Three Cruises of the U.S. Coast and Geodetic Survey Steamer "Blake"_ (Boston and New York, 1888); A. Alcock, _Illustrations of the Zoology of H.M.S. "Investigator": Fishes_ (Calcutta, 1892-1899, in progress); A. Alcock, _Descriptive Catalogue of the Indian Deep-sea Fishes in the Indian Museum_ (Calcutta, 1899, contains references to all the previous papers of the author on the subject); R. Collett, _Résultats des campagnes scientifiques accomplies par Albert I^er prince de Monaco: poissons provenant des campagnes du yacht "l'Hirondelle,"_ (Monaco, 1896); R. Koehler, _Résultats scientifiques de la campagne du "Caudan,"_ (Paris, 1896); C. H. Gilbert and F. Cramer, "Report on the Fishes dredged in Deep Water near the Hawaiian Islands," _Proc. U.S. Nat. Mus._ xix. (Washington, 1896); C. Lütken, "Spolia Atlantica," _Vidensk. Selsk. Skr._ vii. and ix. (Copenhagen, 1892-1898); C. Lütken, _Danish Ingolf Expedition_, ii.: _Ichthyological Results_ (Copenhagen, 1898); S. Garman, "Reports on an Exploration off the West Coast of Mexico, Central and South America, and off the Galapagos Islands in charge of Alexander Agassiz, by the U.S. Fish Commission Steamer "Albatross," during 1891," _Mem. Mus. Comp. Zool._ vol. xxiv. (Cambridge, U.S.A., 1899). (S) ANTARCTIC FISHES: G. A. Boulenger, _Report on the Collections made during the voyage of the "Southern Cross": Fishes_ (London, 1902); L. Dollo, _Expédition Antarctique Belge_ (S.Y. "Belgica"). _Poissons_ (Antwerp, 1904); E. Lönnberg, _Swedish South Polar Expedition: Fishes_ (Stockholm, 1905); G. A. Boulenger, _Fishes of the "Discovery" Antarctic Expedition_ (London, 1906). (G. A. B.)

III. DEFINITION OF THE CLASS _Pisces_. ITS PRINCIPAL DIVISIONS

Fishes, constituting the class _Pisces_, may be defined as Craniate Vertebrata, or Chordata, in which the anterior portion of the central nervous system is expanded into a brain surrounded by an unsegmented portion of the axial skeleton; which are provided with a heart, breathing through gills; and in which the limbs, if present, are in the form of fins, as opposed to the pentadactyle, structure common to the other Vertebrata. With the exception of a few forms in which lungs are present in addition to the gills, thus enabling the animal to breathe atmospheric air for more or less considerable periods (Dipneusti), all fishes are aquatic throughout their existence.

In addition to the paired limbs, median fins are usually present, consisting of dermal rays borne by endoskeletal supports, which in the more primitive forms are strikingly similar in structure to the paired fins that are assumed to have arisen from the breaking up of a lateral fold similar to the vertical folds out of which the dorsal, anal and caudal fins have been evolved. The body is naked, or scaly, or covered with bony shields or hard spines.

Leaving aside the Ostracophori, which are dealt with in a separate article, the fishes may be divided into three subclasses--

I. Cyclostomi or Marsipobranchii, with the skull imperfectly developed, without jaws, with a single nasal aperture, without paired fins, and with an unpaired fin without dermal rays. Lampreys and hag-fishes.

II. Selachii or Chondropterygii, with the skull well developed but without membrane bones, with paired nasal apertures, with median and paired fins, the ventrals bearing prehensile organs (claspers) in the males. Sharks, skates and chimaeras.

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.

[Illustration: FIG. 1.--Heterocercal Tail of _Acipenser_. a, Modified median scales ("fulcra"); b, bony plates.]

[Illustration: 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).

[Illustration: 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.

[Illustration: 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).

[Illustration: 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.

[Illustration: From _Trans. Zool. Soc. of London_.

FIG. 6.--Larva of Polypterus. (After Budgett.)]

[Illustration: 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.

[Illustration: 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, 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.

[Illustration: 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.)]

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.

[Illustration: 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.]

_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.

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.

[Illustration: 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.

[Illustration: 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_).

[Illustration: 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_).

[Illustration: _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.

[Illustration: _A._ After W. K. Parker, _Trans. Zool. Soc. London_.

_B._ After Gegenbaur, _Untersuchungen zur verg. Anat. der Wirbeltiere_, by permission of Wilhelm Engelmann.

_C._ 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 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).

[Illustration: FIG. 16.--Fore-limb of _Ceratodus_.]

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 case of the pelvic, while the ventral portions are known respectively as coracoid and ischio-pubic.

[Illustration: FIG. 17.--a, Skeleton of pectoral limb of _Pleuracanthus_. (From Gegenbauer, after Frisch.) b, Skeleton of pectoral limb of _Acanthias_. (After Gegenbauer.)]

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.

[Illustration: From Budgett, _Trans. Zool. Soc. London_, xvi,