Charles B. Davenport and Ernest Ingersoll

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”