The New International Encyclopædia/Fish

by Charles B. Davenport and Ernest Ingersoll

FISH (AS. fisc, Ger. Fisch; connected with Lat. piscis, Olr. iasc, fish). A backboned animal which lives in water, breathes by means of gills, and possesses paired fins. Such animals constitute the class Pisces, but popularly the term ‘fish’ includes, in addition to the above, certain other lower vertebrates, the lancelets (Leptocardii) and the round-mouths (Cyclostomata), not to mention the ignorant error of speaking of whales, etc., as ‘fish.’

TOPOGRAPHY OF A FISH.

1, Dorsal fin; 2, adipose fin; 3, caudal fin (tail); 4, anal fin; 5, pectoral fin (paired); 6, ventral fin (paired); 7, mandible (lower jaw); 8, maxillary (upper jaw); 9, operculum (gill-cover); 10, branchiostegals; 11, caudal peduncle; 12, lateral line; 13, series of crosswise scales at the point usually counted; 14, snout (nose); 15, eye; 16, head; 17, depth; 18, base of caudal fin; 19, distance from snout to nape or occiput.

Form. It is almost impossible to describe the form of a fish in terms that would include all the different varieties, notwithstanding the fact that the group as a whole presents a greater uniformity of form than other vertebrate groups, for instance, birds. The majority, however, have a more or less elongated body, tapering at both ends. The variations in form can usually be correlated with the habits of the fish. The great variety of habitats into which the fish have been crowded, and to which they have become adapted, has resulted in great diversity of form, though this diversity is mostly concerned with detail, leaving, as stated above, a characteristic fish form as a whole. The typical symmetry of a fish is embodied in such forms as the trout.

FORMS OF SCALES.

1, 2, Cycloid scales; 3, 4, 5, 6, ctenoid scales; 7, ganoid scales; 8, 9, dermal papillæ (from Monacanthus); 10, 11, cycloid scales from lateral line.

OPEN MOUTH OF A SALMON.

Shows arrangement of maxillary, palatine, and vomerine teeth in fishes.

Integument. Fishes are usually covered by scales or bony plates. These may become very minute, as in eels, or may be entirely wanting, as in the leather carp, in certain eels, and in many of the catfishes. Scales may be either bony or horny, and are generally imbricated like slates on a roof, the free end being backward. They arise from the deeper layer of the skin, the derma, grow outward and backward, and remain covered by a thin layer of epidermis. Bony plates are attached by the whole of one surface, and usually have a coat of enamel, which is derived from the epidermis, while the bony base arises from the derma. The differences of character in the scales have been made the basis of a classification of fishes by Louis Agassiz, according to whom all fishes are distributed into four orders—Cycloidei, Ctenoidei, Placoidei, and Ganoidei (qq.v.). This classification was very artificial and did not admit many intermediate cases, or the cases where more than one kind of scale was possessed by the same fish, and has long been disused, but it has been found very convenient in the study of fossil fishes. Here also it is giving way to a more natural classification. The dermal plates may become variously specialized, giving rise to spines, teeth, etc. The teeth vary greatly in size, shape, and arrangement. They may be flat, plate-like, as in the rays, or long and sharp, as in certain sharks. The conditions in the sharks, and in certain other groups, show in the clearest way by their structure and transitional forms that they are merely modified dermal plates or denticles. In the more recent fishes they are not restricted to the edge of the mouth, but may occur in the roof and floor, and on the tongue, gill-bars, and pharynx. The epidermis of fishes contains unicellular glands, which secrete the mucus covering their body, and pigment cells giving rise to the colors of the body.

VERTEBRA OF A BONY FISH.

Front and side views. c, Centrum; na, neurapophysis; pa, parapophysis; ha, hæmapophysis; ns, neural spine; hs, hæmal spine; za, zygapophysis.

Skeleton. The skeleton of fishes consists of the skull with its visceral skeleton; the vertebral axis with its processes; the pelvic and pectoral girdles; and the supporting elements of the various fins. This in the lower cartilaginous fishes consists only of cartilage, no true bones being present. The skull, which in the higher fishes is a complicated structure, in the elasmobranchs consists of a rather simple cartilaginous hollow case, the chondrocranium, inclosing the brain, and not composed of distinct pieces. As one ascends the scale, bones are added to this chondrocranium from the outside, arising as dermal ossifications; these are probably merely highly modified dermal plates. In the ganoids the chondrocranium generally persists with centres of ossification present, and the whole head is incased in dermal bones. In the higher bony fishes the chondrocranium is usually replaced by cartilage bones with many dermal bones added. To the lower part of the skull in all fishes a series of arches are attached. These form the lower jaw and the hyoid and gill arches. The backbone generally consists of a series of vertebræ which, with the exception of Lepidosteus, are biconcave. Dorsally they bear neural arches inclosing the spinal cord, and these are prolonged dorsally as a neural spine, varying in length. Ventrally, the vertebræ bear ribs in the anterior portion, and in the caudal region there are hæmal arches inclosing the caudal artery and vein. These arches are prolonged ventrally as a hæmal spine. In some elasmobranchs, in the chimæras, in the lung-fishes, and in some ganoids, there are no such definite vertebræ developed, but the notochord, which in the teleosts persists only as remains in the cavities between the adjoining centra, is a continuous rod. The neural and hæmal arches and the ribs are variously developed. In most of the elasmobranchs there are present definite biconcave vertebræ with neural and hæmal arches, transverse processes, and rudimentary ribs, but they remain cartilaginous or become only slightly ossified. The centra are pierced by a canal through which remains of the notochord are continuous. The posterior end of the spinal column forms the basis of the caudal fin. See Skeleton.

VERTEBRÆ OF A CARTILAGINOUS FISH.

1. Side view. 2. Longitudinal section. 3. Transverse section of caudal vertebra of a shark.

a, Centrum; b, neurapophysis: c, intercrural cartilage; d, hæmapophysis; e, spinal canal; f, intervertebral cavity; g, central canal of persistent portion of notochord; h, hæmal canals for blood-vessels.

The fins are supported by cartilaginous or bony rods. In the dorsal and anal these rods do not join those of the internal skeleton directly, but, imbedded in the flesh, are interposed between the spinous processes of the vertebræ. The paired fins, not always all present, represent the typical fore and hind limbs of quadrupeds. They consist of a basal set of bones, varying in number and arrangement in the different groups, bearing the radiating fin-rays, and articulating proximally with the pelvic or shoulder girdle. The girdles are cartilaginous in elasmobranchs, lung-fishes, and sturgeons; but in teleosts this, like the chondrocranium, has additions in the way of dermal bones, which have become associated with it. The pelvic bones may be imbedded in the muscles of the abdomen, or may occur farther anterior and become fastened to the pectoral girdle. In some fishes the pelvic fins, which answer to the hind feet of quadrupeds, are actually farther forward than the pectoral fins, and are then called jugular. In some fishes, as in the common eel, the ventral fins are wanting, while in others both pairs may be absent. In lung-fishes “the skeleton of the pectoral fin consists of a stout basal cartilage, an elongated, tapering central axis made up of a number of short cartilaginous segments, and two rows of jointed cartilaginous rays extending out on either side of the axis.” See Fin.

Internal Structure. The respiratory organs of fishes consist of gills, and in the case of Dipnoi, of gills and lungs. In the region of the pharynx the alimentary canal communicates with the exterior on each side by a series of slits called gill-clefts. The water passing through the mouth into the pharynx escapes to the exterior through these gill-clefts. The bars bounding these clefts have attached to them the gills, which are merely the mucous membrane of the bars raised up into a number of ridges, called branchial filaments. These are highly vascular, the blood entering them being venous in character, and they constitute the true respiratory organs. The water passing through the clefts bathes the filaments and effects the necessary interchange of gases. In the lung-fishes the air-bladder has assumed the function of a lung. This organ is not a smooth-walled bag, as in other fishes, but a highly vascular, much-chambered organ. The air enters it through a connection with the pharynx. See Gill; Respiratory System.

The air or swimming bladder of fishes is a sac, usually unpaired, filled with gas and lying dorsal to the intestine. Embryologically it corresponds with the lungs, as it arises as a diverticulum of the intestine, and in this connection may persist as the pneumatic duct, or in other cases may be wholly lost. The function of the air-bladder is not always clear. When it is supplied with venous blood, as in Dipnoi and Amia, and its gases are periodically exchanged for outside air, it doubtless functions as a lung. When it is supplied with arterial blood, or when it is a closed sac, its function is supposed to be hydrostatic. It may, in addition, serve as a storehouse for oxygen taken in by the gills. The contraction and expansion both of the bladder and of the body musculature serve to condense and expand the air in the bladder, and thus may aid the fish in rising or sinking in the water. Unequal anterior, posterior, or lateral pressure on the bladder may likewise aid the fish in directing its course. In some fishes the forked anterior end of the air-bladder fits closely against the posterior wall of the auditory capsule. In carps and siluroids the bladder and auditory organs are connected by a chain of bones. Such connection) doubtless enable the fish to become more keenly sensitive to any change in hydrostatic pressure in the bladder.

Except in teleosts, where a conus arteriosus is wanting, the heart of fishes consists of (1) a sinus venosus, (2) one auricle, (3) one ventricle, and (4) a conus arteriosus. In the teleosts the conus is represented by the bulbus arteriosus, which, however, is a part of the aorta and does not undergo rhythmical contraction like the conus. The sinus venosus is a thin-walled expansion of the afferent veins, and a sort of antechamber to the thin-walled auricle. From the latter the blood passes into the thick-walled, muscular ventricle; thence either into the ventral aorta (teleosts) or into the conus. From the conus the ventral aorta extends forward a short distance, and then divides on each side into a number of branches (afferent branchial arteries), which pass through the gill-arches, breaking up there into capillaries in the gill-filaments which re-collect into the efferent branchial arteries. These unite above the pharynx as a single large artery, the dorsal aorta, which passes backward through the entire length of the body, supplying the different organs. The most important branches given off are the carotids to the head, the subclavian to the pectoral fins, the mesenteric and cœliac to the digestive organs, the renal to the kidneys, the spermatic or ovarian to the reproductive glands, and the iliac to the pelvic fins. Posteriorly the aorta is continued as the caudal artery. From the anterior part the blood is returned by the jugular vein; from the pectoral fin by the subclavian; from the digestive system by the hepatic-portal to the liver; thence by the hepatic; and from the other portions of the body by the cardinal. All these enter the sinus venosus. Thus in all except the lung-fishes the heart contains only venous blood. All the blood on its course to the system passes through the gills first and is there purified. In the lung-fishes, where the air-bladder functions as a lung, some arterial blood reaches the heart from the air-bladder by the pulmonary vein. This empties into the left side of the sinus venosus. The sinus venosus, the auricle, and the conus are imperfectly divided in the lung-fishes, suggesting the condition in amphibia. The blood-corpuscles of fishes are nucleated.

The central nervous system in fishes consists, as in other vertebrates, of a brain and spinal cord and the sympathetic system. The brain presents the usual divisions of the higher forms. It lies in the same plane with the spinal cord, and exhibits no flexures. The brain does not completely fill the cranial cavity, and the intervening space is filled with the gelatinous arachnoid tissue. In the teleosts the optic lobes and the cerebellum constitute the largest divisions, the cerebrum remaining very poorly developed. In the elasmobranchs the olfactory lobes may be enormously developed. Ten cranial and many spinal nerves leave the brain and spinal cord. The sympathetic system presents the usual character and relations found in vertebrates. The emotions of fishes (manifestations of anger, fear, etc.), indicative of the mental status, are extensively considered in the Proceedings of the Zoological Society of London for 1878.

Sense Organs. Unequally scattered over the body of fishes there are the so-called ‘end buds’—modifications of the epidermis. In structure these sense organs largely resemble taste buds, which in the vertebrates above fishes are restricted to the mouth-cavity. In fishes these ‘end buds’ are probably also taste organs, since it has been shown that a fish can taste with its skin. Besides these there are other aggregations of sense cells, probably tactile in function. Situated within longitudinal grooves or pits are sense cells, probably largely tactile in function, known as the lateral-line organs. These grooves open by definite pores to the surface. There is usually one series of such along each side, known as the ‘lateral line,’ but there may also be developed a more or less complicated system of grooves on the head. Many fishes have filamentous appendages, more or less definitely arranged around the mouth and nose, known as barbels. Cave-dwelling fishes, which have lost their power of sight, have strongly developed tactile papillæ on the head. The organs of smell are a pair of pits in the skin at the anterior dorsum of the head, lined with sense cells. There are no internal nares except in the Dipnoi, but the pits open to the surface by the external nares, each more or less completely divided into two, to permit the water to enter one, bathe the sense surface, and escape by the other. Fishes have no external and middle ear, but merely the inner, consisting of the semicircular canals, with their ampullæ, a sacculus, and utriculus. The otoliths are large. Various experiments point to the conclusion that the ear in fishes is merely an organ of equilibrium. The eyes have the usual structure of the vertebrate eye. The accessory organs, like the lids and lachrymal glands, are poorly developed. Eyes may be absent in cave and deep-sea forms. See Nervous System, Evolution of.

The digestive system consists, as in other vertebrates, of the alimentary canal, with its more or less definitely marked divisions (mouth, pharynx, gullet, stomach, and intestine), and its glands, liver, gall-bladder, and the pancreas. The mouth and its teeth present, the greatest variety in form and arrangement. The pharynx opens to the surface by the gill-clefts as above indicated. The gullet and stomach vary with the food habits of the fish. Predatory fishes swallow and stow away large objects. An extreme instance is the deep-sea fish Chiasmodon niger, which has been taken with a fish in its distensible stomach larger than itself. At the junction of the stomach and intestine, in ganoids and in nearly all teleosts, are given off a number of blind sacs, the pyloric-cæca. These may be very numerous. The intestine in all fishes except teleosts has a spiral valve in the form of a ridge running spirally along the wall and projecting into the interior of the intestine. The alimentary canal opens either with the urino-genital ducts into a common chamber, the cloaca, or, as in teleosts, ganoids, and Dipnoi, separately to the exterior. There are no salivary glands. See Alimentary System, Evolution of.

The excretory organs in all fishes are a pair of glands situated just under the backbone and protruding into the body-cavity. The excretion is carried away by a ureter, which empties variously in the different groups of fishes. In the elasmobranchs and Dipnoi the kidneys extend for about two-thirds the length of the body-cavity, and the ureters, having united, open into the cloaca as a common duct. In the teleosts and ganoids the glands may occupy the entire length of the body-cavity. The ureters open into a urinary bladder, and this into the urino-genital sinus, the latter opening separately to the exterior. See Excretory System, Comparative Anatomy of.

Reproduction. The sexes are separate. The testes and ovaries are paired organs varying in shape and position in the abdominal cavity with the different groups. The products of the male and those of the female may or may not be led to the exterior by a duct. In the male this may be a more or less convoluted vas deferens or a simple continuation of a bag. In teleosts the testes or ovaries are simply continued posteriorly as a duct which empties into the urino-genital sinus. In case there is no oviduct the eggs break free into the body-cavity and pass into the urino-genital sinus by a pair of slits in the anterior wall. In ganoids there is always an oviduct. In the elasmobranchs and Holocephali there is an oviduct, usually quite highly developed, opening into the cloaca. In many teleosts and in nearly all elasmobranchs and Holocephali the eggs are fertilized in the body of the mother. In many instances the egg develops there to quite an advanced stage. In all other fishes the milt is poured over the eggs as they are extruded, or into the water in the immediate vicinity. In elasmobranchs and Holocephali the ‘claspers’ act as intromittent organs, by which the milt is introduced into the body of the female. In the ovoviviparous teleosts the anterior portion of the anal fin is modified into an intromittent organ.

Breeding Habits. Fishes that lay eggs show no parental care, as a rule, either for their eggs or young. The eggs are fastened to rocks or weeds or other objects, and the eggs and young are left to shift for themselves. In many marine species the eggs are extruded into the water, and during their first period of development float at the surface and are carried about by the currents. In all such cases the loss both of eggs and young must be very great, and to meet this loss such species usually produce enormous numbers of eggs. Thus a single, large cod may produce in a single year 10,000,000 eggs. On the other hand, in some species there is considerable care bestowed upon the eggs and young by the parents—this duty usually falling to the male. The sticklebacks (q.v.) are well-known instances. The male builds a nest of sticks, grass, etc., cementing them together with a sticky excretion, and guards the nest and eggs during incubation. In some of the Siluridæ, after the young are ready to leave the nest, the male may be seen leading the brood about, guarding it until the individuals are better able to shift for themselves. This instinct is found in other families. The cave blindfish (Amblyopsis) retains the eggs during incubation in its capacious gill-cavity. The seahorse develops a brood-pouch along the ventral side of the body, in which the eggs and young are harbored. Many marine fishes, like the shad and the salmon, ascend the rivers each season to spawn. These migrations may be for great distances, and against the greatest difficulties, such as rapids and falls. Such migratory species are known as anadromous fishes. The reverse process takes place in the case of the eel, which goes to the ocean to spawn.

The spawning season of most species is during the spring months. In the tropics, where the rainy and dry seasons alternate, these are determining factors in the time of spawning of certain species. Many species spawn during the colder months, for example the Salmonidæ. Many fishes show on the approach of the breeding season a noticeable sexual difference, the male being marked by more brilliant colors, or by the temporary growth of tubercles on the head and other portions of the body. Many species, however, do not exhibit this sexual dimorphism.

The eggs of fishes vary greatly in size and shape. The typical fish-egg is globular, more or less transparent, having a tough protective membrane, within which the yolk-laden egg proper lies. The yolk is present in sufficient amount to maintain the embryo until it can swim about and feed for itself. The outer protective membrane is very commonly sticky, to enable it to cling to stones, weeds, etc. Sometimes eggs stick together in clusters. In other cases the outer coat has tufts of fine filaments with which the egg fastens itself to weeds. The shark's eggs are inclosed in large, horny, purse-shaped cases within which the embryo is developed. The period of incubation is very various. In certain pelagic eggs the embryo emerges from the eggs in 24 to 48 hours after deposit, while in other cases, as the trout, the period extends over three to six months. See Egg; Embryology.

Food. The food of fishes includes all sorts of vegetable and animal matter and forms. Some are omnivorous. Others are exceedingly choice about their food, living almost exclusively upon certain species of Crustacea, for instance. Some, like the carp, are vegetarians, and the smaller fishes are the principal food of the larger, predaceous forms, like the trout and bluefish. Many species subsist entirely upon the minute organisms they can strain out of the water.

Distribution in Space and Time. About 10,000 species of living fishes are known. In their distribution fishes are almost coextensive with water. The greatest variety is found in the tropics. Many families are exclusively marine, others as exclusively of the fresh water, while many have representatives in both, or spend a portion of their time in each. Certain groups, like the Cyprinodontinæ, are distributed only along the shallow shore waters; others, like the sharks and bluefish, are pelagic, living on the high seas, and such usually have a wide distribution. The ocean depths have their peculiar fish fauna—species modified for these peculiar conditions, and unable to subsist at the surface. These species, living in darkness, often have no eyes, and many are phosphorescent. The coldest latitudes harbor their fishes. Some families, like the cod, are prevailingly distributed in colder waters, and certain species have been taken in lakes above the line of perpetual snow. Temperature is one of the important factors in determining the distribution of fishes. Deep-sea forms, where the temperature is uniform, have a wide distribution. The geological history of any region, with the changes in the river systems, etc., it has brought about, is another important factor. See Deep-Sea Exploration; Distribution of Animals; Cave Animals; Evolution.

Ancestry. The medium in which fishes live and the hard and almost indestructible nature of some portions of their skeleton, as their teeth, spines, and scales, would lead us to anticipate their frequent occurrence in the sedimentary rocks; but inasmuch as the soft parts of the animal are liable to speedy decomposition, the remains of fish must often exist in a fragmentary and scattered condition. Thus the teeth in the shark, the spine defense in the stingray, and the scales in the bony pike would survive the total destruction of the cartilaginous skeleton as well as the soft portions. Many quite complete casts of skeletons, however, have been obtained, so that not a little is known of the past history of the group. The earliest fishes occurred in the Upper Silurian. Remains of all of the main groups, excepting the higher teleosts, have been found from this period. Among the elasmobranchs the earlier forms were quite distinct from any now living, with the possible exception of the Port Jackson shark (q.v.). These forms flourished to the Triassic period, and in the case of the cestracionts to the Eocene. The recent elasmobranchs appeared in the late Triassic or early Jurassic, and were more abundant in the past than at present. The Dipnoi flourished in the Triassic. The ganoids were a dominant group up to Miocene times, but at present exist in mere remnants. The dominant fishes of to-day, namely, the higher teleosts, first appeared in numbers during the Jurassic and Cretaceous periods. These at the present day exhibit the greatest diversity in type. In times past the other groups presented this great variety of form, and it is mainly those species that retained the more generalized characters that survived and are present with us now. See Extinction of Species.

Economic Value. By far the most important use of fishes to man is in supplying him with food, and in some regions they form the principal means of subsistence. Some fishes, nevertheless, are unpalatable, and even poisonous to a greater or smaller extent. The skin of some cartilaginous fishes yields shagreen, and the air-bladder of some species yields isinglass. The minute laminæ which give brilliancy of color to some, and the similar substance found in the air-bladder of others, afford the materials of which artificial pearls are made. Oil useful for lamps, etc., is obtained from several species, and the medicinal value of cod-liver oil is now well known. See Fisheries.

Classification. History of Ichthyology.—Among the ancient students of ichthyology, that branch of natural history which treats of fishes, the first to be mentioned, as usual, is Aristotle. In modern times ichthyology began to be cultivated about the middle of the sixteenth century by Belon, Rondelet, and Salviani. Their work was of value locally only. The first work of real value, and which marks the beginning of a system based on scientific principles, was that of Willughby and Ray, which appeared in 1686 under the title Historia Piscium. Here a distinct effort at classification was made. They divided all fishes into two classes, Cartilaginei and Ossei. Each of these classes was divided into two groups, on the basis of the form of the body—the Cartilaginei into Longi, including the sharks, and Lati, including the skates; and the Ossei into Plani, including the flatfishes, and Non-plani, including all others. It is at once evident how artificial this classification is. Artedi, whose writings, on account of his death, were published by Linnæus, worked out a system of classification considerably influenced by Willughby and Ray. He included the cetaceans among the fishes. His system was adopted by Linnæus in his earlier editions of Systema Natura, but later (1758) Linnæus devised an original classification, which, among other changes, eliminated the Cetacea from the fishes and placed them with the mammals. The classification worked out by Bloch and Schneider was superficial in the extreme. The number of fins present was the basis of their division into Monopterygia, Dipterygia, etc. This work was published in 1801. Bloch, in 1782-95, published a large and important work on fishes, comprising nine volumes with fine illustrated plates, in which he described about four hundred species. Several other authors wrote extensively on fishes during the last half of the eighteenth century and the beginning of the nineteenth. Among these is to be mentioned Lacépède, Histoire naturelle des poissons (5 vols., Paris, 1803), in which 1400 species were described. During the first quarter of the nineteenth century Cuvier did much on the classification of fishes, his system appearing in his Regne Animal (Bonn, 1830). The anatomist Johannes Müller published in 1846 a natural classification which influenced the systems to a very high degree. He divided fishes into Leptocardii, Marsipobranchii, Elasmobranchii, Ganoidea, Teleostei, and Dipnoi. Louis Agassiz (q.v.) advanced our knowledge both of living and fossil fishes. Influenced by the latter, he divided the class into four groups on the character of their scales: placoid, ganoid, cycloid, and ctenoid. This classification, though convenient in many ways for the study of fossil remains, was adopted by scarcely any of the authorities. Albert Günther, in his Catalogue of Fishes in . . . the British Museum (London, 1859-70), has largely modeled the modern system of classification. Among the recent more influential American ichthyologists are Theodore Gill, the late E. D. Cope, and David Starr Jordan, whose historical review of ichthyology in the Proceedings of the American Association for the Advancement of Science for 1902 is very complete.

Present Arrangement. The subphylum Vertebrata includes as the lowest in rank of its groups several series of fish-like vertebral divisible first into Acraniata (the lancelets [Leptocardii] only; see Amphioxus), and Craniata, which includes all the remainder. The fish-like Craniata fall into two classes:

I. Cyclostomata. Characterized chiefly by having ‘a suctorial mouth devoid of functional jaws,’ and by the absence of paired fins; these are the lampreys and hagfishes (orders Petromyzontes and Myxinoidei).

II. Pisces. Characterized by having the organs of respiration (gills) and the organs of locomotion (paired fins) adapted for an aquatic life. The class is divided into subclasses, as follows:

(1) Elasmobranchii.—Pisces with a skeleton composed essentially of cartilage—the sharks, rays, etc., divided into three orders, Cladoselachea, Pleuracanthea, Acanthodea, and Selachii. The first three are represented by Paleozoic fossil forms. The last includes many extinct and all the existing forms.

(2) Holocephali.—Shark-like Pisces, with a large compressed head and a single external branchial aperture. It includes only the family Chimæridæ (chimæras).

(3) Teleostomi.—Pisces ‘distinguished from the Elasmobranchii and Holocephali by having the primary skull and shoulder-girdle complicated by the addition of membrane bones, and by possessing bony instead of horn-like fin rays.’ This includes all of the common ‘bony fishes,’ as well as the so-called ganoid fishes. Its orders are: Crossopterygii (bichir, etc.); Chondrostei (sturgeons); Holostei (gar-pikes, etc.); Teleostei (bony fishes generally). The first three orders are frequently grouped together as ‘Ganoidei.’

(4) Dipnoi.—Pisces with lung-like respiratory organs as well as gills, and the fins constructed on the type of the archipterygium. It includes the lung-fishes, and by some authors is made a separate class altogether. Its orders are Monopneumona and Dipneumona.

(5) Ostracodermi (q.v.).—A group of uncertain limits and affinities, known only from Paleozoic fossils, “characterized by the extraordinary development of the exoskeleton (bony plates) of the head, and the absence, in all the fossil remains hitherto found, of endoskeleton, including jaws.”

Bibliography. Bloch, Allgemeine Naturgeschichte der Fische (Berlin, 1782-95); Cuvier and Valenciennes, Histoire naturelle des poissons (22 vols., Paris, 1828-49); Günther, Catalogue of Fishes in British Museum (London, 1859-70); Günther, Introduction to the Study of Fishes (Edinburgh, 1880); Dean, Fishes, Living and Fossil (New York, 1895). Consult also various works on the comparative anatomy of vertebrates, such as Huxley, Gegenbaur, Owen, Parker and Haswell, Wiedersheim, etc. Of faunal works the principal are: Jordan and Evermann, Fishes of North and Middle America (4 vols., Washington, 1896-1900); Goode and Bean, Oceanic Ichthyology (Washington, 1895); Goode, American Fishes (New York, 1888); annual Reports and Bulletins of the United States Commission of Fish and Fisheries and of the National Museum (Washington, 1870 onward); governmental documents issued by Canada and Newfoundland; Eigenmann, South American Fishes (San Francisco, 1893); Yarrell, History of British Fishes (3d ed., London, 1859); Couch, History of British Fishes (London, 1865); Houghton, Freshwater Fishes of Great Britain (London, 1879); Siebold, Die Süsswasserfische von Mitteleuropa (Leipzig, 1863); Blanchard, Les poissons des eaux douces de la France (Paris, 1866); Day, Fauna of British India: Fishes (London, 1889); Hutton and Hector, Fishes of New Zealand (Wellington, 1872). For fossil fishes, consult Woodward, Outlines of Vertebrate Paleontology (Cambridge, 1898), in which is a full bibliography to the fossil forms; Agassiz, Recherches sur les poissons fossiles, vols, i.-iii. and supplement (Neufchâtel, 1833-44); Woodward, Catalogue of Fossil Fishes of the British Museum, vols, i.-iii. (London, 1889-95); and for American forms, Newberry, “Paleozoic Fishes of North America,” in Monographs of the United States Geological Survey, vol. xvi. (Washington, 1890).