Fishes are vertebrates fitted to live in water. Their typically fusiform shape is that best adapted to progress through the rather dense medium they inhabit; and their limbs are swimming organs, or "fins." These are of two kinds, "paired" and "median." The former are the pectorals, one on each side of the forward part of the body, and the pelvic, or ventral fins on the belly and near together; these four serve, like the bilge keels of a ship, to maintain stability—prevent rolling over—rather than for progression. The median fin is vertical, and extends around the tail from the middle of the back to the end, when it is complete; but in most cases it is represented by an upright fin, the "dorsal" on the back, by the "caudal" fin fringing the tail, and by the "anal" fin at the vent. The powerful caudal fin is the principal agent in swimming, aided by undulatory movements of the dorsal and ventral fins; and it has a twisting action that drives the animal forward as does the rolling of the oar in "sculling" a boat. The median fins are developed from the skin, and are supported by a skeleton system of their own, not connected with the spine. In most fishes the upper and lower halves of the caudal fin are alike, and the tail is symmetrical, but in sharks and some others the end of the spine curves upward and the lower wing of the tail is much larger than the upper; in the former case the caudal fin is said to be "homocercal," and in the latter "heterocercal."
The fins of fishes are in many species modified and adapted to purposes remote from swimming or balancing. Thus it is the first dorsal fin of the remora that has become the sucker on its crown; in the angler the first rays of the back fin are lengthened and lobed to form its "lure," and elongation of various fin rays as feelers, or light bearers, etc., may be found elsewhere. The pectorals are enormously enlarged to make wings for the flying fish and the gurnard, and to give a substitute for legs to the Oriental gobies that like to go ashore, while the ventrals are transformed in certain fishes of swift streams into organs by which they can fasten themselves to the bottom or climb against a cataract.
The skin of fishes is rather thick and tough, and abounds in glands that secrete mucus, and in cells that secrete the hardening, or protective, denticles and scales that form the coat of most species, and which differ widely.
Louis Agassiz distinguished four kinds of scales—placoid, ganoid, cycloid and ctenoid. The first named occur only in the selachians (sharks and rays) and are variously shaped particles of lime that prick through the skin, which makes excellent polishing material when prepared as "shagreen." These "denticles" in the skin become teeth in the mouth without change of structure, and the great spurs with which the "saw" of the sawfish is armed are only extreme instances of this special adaptation.
Ganoid scales are such as formed the armor of the great extinct tribe of ganoid fishes, a remnant of which survives in our gar pikes, or billfish. In some of the fossils they are roundish, and overlap, but in modern ganoids they are rhombic in shape and plate the body edge to edge, connected by toothlike processes that articulate with the adjacent scales, and permit flexibility in the body. The outer face of the scales is enameled, like teeth, beneath which is a layer of bone substance and the teeth in the mouth are only modified scales.
Cycloid and ctenoid scales are those of ordinary fishes, and are precisely alike, except that the hinder, or attached, end of the latter is split into a comblike fringe. They have a rounded or often polygonal form, are composed of lime, and are translucent, thin, elastic, and overlap like shingles on a roof. The scales of fishes increase in size with the animal's growth by additions to the exposed rim, and as these accessions may be observed, by counting them the age of the fish may be computed, when checked by certain other considerations.
The colors of fishes are produced by pigment cells, both in the skin and on the outside of the scales; and by a peculiar tissue composed of secretion products called "iridocytes." These, by their various ways of reflecting light, and by the color elements contained in them, give rise to the different hues of fishes.
Fish show their inferiority as a class by retaining the method of respiration by means of gills characteristic of the aquatic invertebrates. The gills are composed of bright red tassels set on hoops that encircle the throat, and are usually covered by a movable flap—the "gill cover." Under this flap, the neck of the fish is perforated by crescentic slits. The fish normally breathes by taking gulps of water into the mouth and throat, and squeezing it out through the gill slits; during its rhythmical passage over the thin gills the oxygen of the dissolved air is absorbed by the hæmoglobin of the red blood, and is carried away to incessantly revivify the body; and at the same time carbon dioxide is set free and got rid of in the outgoing stream.
An organ peculiar to fishes is the air bladder—a sac lying under the backbone and communicating by a duct with the stomach. It is not only of service in respect to buoyancy, but is accessory to respiration. In spite of its name, however, it does not contain air, but a gas rich in oxygen and nitrogen which is secreted by certain arteries and is carried away when needed by other blood vessels, as fat and starchy substances are stored elsewhere and may be drawn upon when food falls short. Nevertheless, the chief function of the "swim bladder," which is exceedingly varied in shape, is to render the fish of the same weight as the water in which it lives. In this condition of equilibrium the fish swims with a minimum of muscular effort. A consequence of the organization, however, is to restrict the vertical range of each fish and kind of fish, because any considerable movement up or down means a change of pressure. This will bring about the expansion or contraction of the volume of gas in the air bladder and thus alter the specific gravity of the animal. Such automatic adjustment is limited, however, and practically prevents a fish rising or falling far above or below the depth to which it was born; and the fatal effects of violent change are seen in those fishes brought up in explorers' dredges from great depths, the air bladders of which are invariably so distended as to kill the animal. Nevertheless, some species seem able to migrate from and to great depths; and temperature is perhaps a greater factor in vertical distribution than the air bladder, the adjustments of which must be slow. The great body of fish life in the sea resides within about 300 fathoms of the surface.