Fishes, the lowest class of vertebrated animals, red-blooded, breathing through the medium of water by means of branchioe or gills. Like other vertebrates, they have an internal skeleton, the brain and spinal cord protected by a bony cavity and canal, muscles external to the bones, never more than four extremities, and the organs of special sense in the cavities of the head. Living in a medium heavier than air, and very nearly of the same density as their bodies, locomotion is comparatively easy, and their form, tins, and smooth surface are admirably calculated for rapid progression; breathing by means of air contained in the water, their blood is cold, and consequently their vital energy is less than that of mammals and birds. The brain is very small, and the organs of sense calculated to receive only the simplest impressions of sight, smell, hearing, taste, and touch; generally unable to make any sounds, with an inflexible body, simply articulated limbs, fixed and staring eyes, living in comparative darkness and silence, there is no change in their countenance, no expression of feeling or emotion, no apparent motives in their monotonous existence beyond the necessity of supplying themselves with food, escaping from their enemies, and providing for the continuance of their species.

Their chief pleasure is that of eating, and their only danger is from the superior strength and quickness of other inhabitants of the waters or from the artifices of man; to eat, and to avoid being eaten, are the great occupations of their lives, and the varieties of their forms, their instincts, and their favorite haunts are intimately connected with these objects; the movable filaments of the lophius or angler, the prolonged snout of the pipe fish and chaetodon, the winglike expansions of the flying fish, and the electric armature of the torpedo and gym-notus, are all instruments either for offence, defence, or escape. Cold-blooded, they are little sensitive to changes of temperature, and their migrations and seasons of propagation are less influenced by thermometric conditions than are those of the higher vertebrates; many fishes spawn in winter, and it is in the cold northern waters that the innumerable individuals of the cod and herring species are pursued by man. Even the loves of fishes are marked by the same sang froid; very few species have sexual union; in most, the males pursue the eggs rather than the females, and coldly fecundate the spawn of unknown adults, from which arise young which they will never recognize and probably never see.

A few females, as the stickleback, deposit eggs in nests made by the males; some carry their eggs and even their young with them for a short period, and feed and protect their little ones like true mothers; but, as a general rule, the joys of maternity are unknown among fishes, and the sexes care nothing for each other even in the breeding season. With all this apparent lack of enjoyment, and low position in the vertebrated series, the class of fishes displays as much and perhaps more variety and elegance of form and beauty of coloration than the more psychically favored birds and mammals; there is not a color of the rainbow, nor a metallic reflection, nor the hue of a precious stone, which may not be seen in the bands, spots, and scales of fishes. Many tribes of men, both savage and .civilized, obtain their principal nourishment from the sea; the countless numbers of cod, mackerel, herring, and other migrating fishes, give employment to thousands of men, and prove important items of national wealth.

The habits of fishes, even of the most common species, are comparatively little known from the difficulty of observing them in their native haunts; we know that some are solitary, and others gregarious; some great wanderers, others restricted within narrow limits; some surface swimmers, others remaining at the bottom, or at great depths; some living on sandy bottoms, others in rocky, others in muddy localities; some found only in salt water, others only in fresh, others in both or in brackish waters; some seen only near the shore, others in very deep water far from land; some sluggish like the skates, others active like the sharks and scomberoids; some perish quickly out of the water, as those with widely open gills like the herring, others live a long time after being caught, like the eel, or can travel over land, or climb trees, like the climbing perch {anabas scandens).-The external form of fishes is very various, but the head is not separated from the body by a distinct neck, and the trunk generally is continued gradually into the tail; in the skates the tail is long and distinct from the body.

The body may be rounded as in the diodon, cylindrical as in the eel, compressed horizontally as in the rays, or flattened vertically as in most fishes; the head may be larger than the body as in the angler, compressed, angular, and obtuse as in the bullhead, prolonged into a beak as in the pipe fish, or the upper jaw may project over the mouth as in the sword fish and sharks; the mouth may open on the under or upper surface, or, as is usual, at the end of the snout, with a greater or less extent of gape. The nostrils may be single as in the sharks and rays, or double as in most fishes. The eyes vary greatly in size and in direction; generally on the sides of the head, in the uranoseopns they look upward, and in the flounder family both are on one side. In the cartilaginous fishes the external borders of the gills are attached to the skin, and the gill openings correspond in number to the intervals between the branchiae; but in the osseous fishes there is a single large gill opening on each side, just behind the head, serving for the exit of the water, after it has been swallowed and made to pass over the gills, the flapping of the gill covers assisting the respiratory process.

Some of the apodal or muraenoid fishes have hardly the rudiments of fins; in others, the fins are either vertical and on the median line, or lateral and in pairs. The lateral fins are the pectorals and the ventrals, corresponding to the anterior and posterior limbs of higher animals; the pectorals are attached behind the opening of the gills; the ventrals are generally on the lower surface of the body, and may be variously placed from under the throat, even in advance of the pectorals, to the origin of the tail. The vertical tins serve the purposes of keel and rudder, and are the dorsal on the back, the anal under the tail, and the caudal at the end of the body. All these fins vary in size and in the number of rays which sustain them, being sometimes spiny, sometimes soft, branched, and composed of many small joints. In the old system of nomenclature, the mala-copterygians are bony fishes with soft articulated fin rays; the acanthopterygians, bony fishes in which some of the rays are spiny; and the chondropterygians, the so-called cartilaginous fishes.