The brain of F. differs very considerably from that of other vertebrate animals. See BRAIN. In general, they possess the nerves and organs of all the senses, although time senses of touch and taste are commonly supposed to be more dull than in many other animals; and a few F., living chiefly in mud, or in the waters of caverns, are destitute of eyes, and consequently of sight, although even they possess optic nerves, and seem sensitive to light. But in most of them, the eyes are large, and vision is evi dently very acute; and some have cirri or barbules near the mouth, filaments proceed ing from some of the fin-rays, etc., which are regarded as delicate organs of touch, adapted to the wants and habits of the particular species. The eyes are covered by the skin, modified in its character, and have no eyelids nor nictitating membrane. They are very variously placed in different kinds. There is no external ear.
The mouth is the only organ of prehension. It is very different in different kinds— sometimes very small, sometimes extremely large, sometimes forming a sucker by which the fish'ean both fix itself and pump up the fluids of the animal on which it preys. The snout is also abbreviated, prolonged, or otherwise modified in very various, ways. The teeth are far more various in form, number, position, than iu any other class of animals. They never have any roots, but are fixed to the bones which support them: they fall off, however, and are replaced. Some F. have no teeth; some have very small teeth; some have teeth in great number, but •so fine as to resem ble the hairs of a brush; some have short thick teeth; some have long sharp teeth, either straight or crooked; some have teeth so flat and closely set that they resemble :A regular and beautiful pavement; and the teeth of F. are sometimes situated not only on the jaw-hones, but on the roarer or bone extending along the middle of the roof of the mouth, and indeed, also, on other parts of the palate to the very throat, and very com monly on the tongue. The food of F. is various: a few subsist on vegetable food of different kinds, but most of them on animal food, of which there is no kind that does not seem to be particularly to some of them, from the mere animal cule or the most minute crustacean to the flesh of the mammalia. In general, they are excessively voracious, and seem to spend most of their lives in seeking food. Many of Ahem prey on other F., and many seem equally willing to devour other species or the younger and weaker of their own. Some of them swallow their food almost or absolutely alive; others subject it to processes of comminution, trituration, and mastication in the mouth. Salivary glands are not found in F., although they
exist in some of time invertebrate animals. The digestive process seems to formed very rapidly. The stomach and intestines vary very much in different kinds. The kidneys are in general extremely large, extending through the whole length of the abdomen.
The air-bladder is found in many F., but not in all; and is present or absent in differ ent F. even of the same genus or family. See AIR-BLADDER. Its uses, and its connection with the habits of particular species, have as yet been but partially ascertained.
F. are oviparous (egg-producing); a few are ovoviviparous (eggs hatched within the body, and young produced alive). The chief reproductive organs are generally two elongated lobes of a fatty substance, milt, in the males, and of rudimentary eggs, roe, in the females. Impregnation usually takes place after the roe or spawn is deposited, time male accompanying the female to the place of spawning. In some cartilaginous F., it takes place before the deposition of the eggs; and male sharks and rays are furnished with organs called elaspers, time, use of which is well indicated by time name. The fecundity of F. is generally very great, and their eggs very small in proportion to the size which they ultimately attain, although this is not so much the case, in the carti laginous F. already mentioned. Some• of thc.Ie. most valuable to man, as the salmon, herring, and cod, are remarkable for their fecundity. Nine millions of eggs have, according to Leuwenhoek, been ascertained to exist in the roe of a single cod; and provision is thus made both for the preservation of the species amidst all the dangers to which the spawn and the young are exposed, and for the wants of man. The spawn of F. is deposi`ed in very different situations, according to the different kinds—as by some on aquatic plants, by some on beds of sand and gravel; but many species leave the depths of the ocean in order to deposit it in shallower waters, and some, usually marine, ascend rivers for this purpose. Very few F. take any care of their eggs or young; but there are remarkable exceptions to this rule, and some of the gobics and sticklebacks are known to tend their young with great. care. Sticklebacks also construct nests. See STICKLEBACK. It is not since this curious fact was discovered, although these little F. have been so long familiarly known; and it is therefore not improbable that many other F. may have the same habit.