The growth of F. is very rapid when supplies of food are abundant, but becomes slow in less favorable circumstances, or is arrested for a long time, in a manner to which there seems to be nothing similar Oolong other vertebrate animals, The skin of F. is generally covered with scales (q.v.), which, however, are sometimes minute and imbedded in the skin, and sometimes altogether wanting. The scales are either horuy or bony, and are generally imbricated, like the slates of a roof, their free ends backwards; but sometimes form bony plates, fixed by the whole of their lower surface. They usually exhibit beautiful symmetrical markings •and inequalities of surface of various kinds, and in some are covered with a thick coat of enamel. The differences of character in the scales have been made the foundation of a classification of F. by Agassiz, by whom all F. are distributed into the four orders of Cycloid, etehoid, Plaeoid, and Ganoid FiAes (see these heads), having respectively cystoid, ctenoid, placoid, and ganoid scales; a classification which has been found particularly convenient with reference to fossil F., although other systems maintain their ground against it as preferable for recent species. It is not, however, wholly artificial, for a relation can be very generally traced between the character of the scales and the general structure and economy of a fish.
The scales of a row extending from the head to or towards the tail on each side of the body of osseous F. in a somewhat waved line, called the lateral line, are pierced for the transmission of a slimy matter, with which the whole body is lubricated, The colors of F. depend upon a substance consisting of small polished laminm, secreted by the skin.
As F. need no covering, like fur or feathers, to prevent the dissipation of their animal .heat in the surrounding medium, their scales must be regarded chiefly as defensive armor. Sonic of them are also defended by large bony plates, which are either on the head alone or also on the body, and some by spines connected with the fins, gill-covers, etc. Few have any other offensive weapons than their teeth, but the spine attached to the tail of some rays is a remarkable exception, as is also the elongated snout or beak of the sword-fish, saw-fish, and a few others. But a much more remarkable kind of armor —probably both offensive and defensive—is possessed by a few F., in an electrical apparatus, by which they can give severe shocks. It is also an interesting fact, that the electrical apparatus is quite different in different F..possessing it, the gymnotus, or electric eel, the torpedo, and the electric si]urus or malapterurus. See ELECTRICITY, ANIMAL.
Many F. arc gregarious, swimming in shoals, which in some species consist of immense multitudes. Some also make periodical migrations; salmon, for example, as3ending our rivers, and herrings and pilchards visiting our coasts, but the long migration formerly ascribed to these F. is now doubted or disbelieved. The occasional
overland migrations of eels, and the more frequent overland migrations of some tropical F., cannot but be regarded with peculiar interest; and the instinct is very wonderful by which, when fleeing from a pool that is about to be dried up, they direct their course towards a place where water is more abundant. This faculty is, however, rare, although possessed by tropical F. both of the eastern and western hemispheres; but more gener ally the F. destined to inhabit tropical ponds which are liable to be dried up, are capable of living dormant, imbedded in the mud, till they are liberated again by the rains, when they reappear in their former multitudes.
Of the uses of F. to man, by far the most important is that of supplying him with food. F. form an article of food in almost all countries, and in sonic a principal part of the food of the inhabitants. Many F. are highly esteemed for the table, which are not procured in sufficient abundance to be a principal part of food in any country. Some F., on the contrary, are unpalatable; and some, mostly tropical, are poisonous, whilst others are poisonous only at particular seasons.—The skin of some cartilaginous F. yields shagreen (q.v.), and the air-bladder of some F. yields isinglass (q.v.). The minute laminre 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 is obtained from a number of F., and the medicinal value of cod liver oil is now well known.
The classification of F. most generally adopted is that of envier, who divides them into Ossots FIRMS (having true bones), and cartilaginous fishes(q.v.), and divides osseous fishes into acanthopterous F. (acantkopterygii, q.v.) and malacopterous F. (malacopterygli, q.v.). The system of Agassiz has already been noticed. That of Muller and Owen differs from both. • Fossil MItes.—The medium in which F. live, and the hard and almost indestruc tible nature of sonic portions of their skeletOns—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 sting ray, and the scales in the bony pike, would survive the total destruction of the cartilaginous skeleton as well as the soft portions of these fish, and would alone remain to testify to their existence.