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Fish (Name applied to a class)

the name applied to a class of animals exclusively aquatic, and occupying the fourth and lowest station of the section Vertebrata. The head is large, and set on the body without the intervention of any distinct neck; the body is usually of a spindle-shape, tapering gradually toward the extremity; and the surface is usually smooth, without any irregularities which might impede the which m might impede the motion of t of the creature in its native element. In its general form the body is usually rounded, or slightly compressed at the sides; sometimes this flattening proceeds to a much greater extent, so that the animal presents the appearance of a broad band, or oval disk, of which the edges correspond with the dorsal and ventral surfaces; in other cases, the flattening takes place from above downward, producing a disk-like body, of which the upper and lower surfaces are dorsal and ventral. A fish may be shortly defined as an animal breathing through the medium of water by means of gills. This latter apparatus is the most important feature presented.

It is situated on each side of the neck, and consists of numerous laminæ fixed on arches. These laminæ are covered with numerous blood-vessels, and are so constructed as to present a considerable surface to the water, so that the blood may receive a sufficient portion of the oxygen FISH contained in that element. As the water in contact with the gills becomes denecessary that a constant current be caused to flow over them. In most fishes this is effected by their taking water in at the mouth and expelling it at the gill-covers.

The blood, which is constantly sent from the gills to the heart, is distributed by means of the arteries to every part of the body, whence it returns to the heart by means of the veins. Animals of this order are for the most part furnished with an air-bladder in the interior of the body, which, as it is often connected with the esophagus by a tube, must be regarded to a certain extent to the lungs of the air breathing Vertebrata. This sac or air bladder, however, has nothing to do with respiration; it receives blood from the arteries, and returns it into the veins and the air which it incloses is probably derived from this fluid.

By the dilatation or compression of this sac, the specific gravity of the fish is governed, and, acted on by a curious muscular apparatus, renders heavier than the surrounding element. The limbs of the fish are formed into fins; the forelegs what are termed the pectoral fins, and the posterior extremities, the ventral.

Besides these, ordinary fishes are furnished with one or two dorsal fins , an anal fin, and a caudal fin, or tail. per jaw The principal organ of motion is the caudal fin, or tail; by this it is propelled.

The dorsal and ventral fins serve to balance it, and the pectorals to arrest its progress when required . The bones of fishes are of a less dense and compact nature than in the higher order of animals, and always remain in an isolated state, similar to that of the embryo of the The head varies more in form than in any other class of verte- . The same bones as those found in other oviparous animals are almost always traceable. The upper consists of maxillary and intermaxillary bones. In the greater number of fishes, the intermaxillary bones constitute the chief portion of the upper jaw, the maxbones being placed behind and parallel to them. The lower jaw is composed generally of two bones on each side, portion in front, and the articular portion behind. The form of the body is for the most part such as mechanical principles teach to be best adapted for moving with least resistance through a liquid medium. The surface of the body is either smooth and lubricovered by closely imbricated scales, scales, rarely defended by bony plates or hard tubercles, still more Vol. IV-Cyc-Ј FISH rarely armed with spines. The power of touch can be but feebly developed in fishes. The organ of taste is a very inconspicuous one -the chief function of the framework supporting it, or the hyoidan apparatus , relating to the mechanthe organ of hearing there is no outward ism of swallowing and breathing. Of sign; but the essential part, the acoustic labyrinth, is present, and the semicircular canal, largely developed within the labyrinth, is without cochlea, and is rarely provided with a special chamber, but is lodged, in common with the brain, in the cranial cavity. usually large, but seldom defended by The eyes are eyelids, and ever destitute of a lachrymal organ. short and simple, with its divisions not The alimentary canal is commonly always clearly marked, the short and being hardly distinguishable red but cold, and is rarely elevated above from the stomach. The blood of fishes is the temperature of the surrounding element. The ment. The sexes of fishes, excepting the very decided external characters by which they may be distinguished.

The respiratory organs, however, occupy more space in the males than in the females , and, on the other hand, the abdomen is larger in the females than in the males . The differences of character in the scales have been are distributed into the following four of fishes by Agassiz, by whom all fishes made the foundation of a classification orders of cycloid, ctenoid, placoid, and are distributed into the following four ganoid fishes , having respectively cycloid, ctenoid, placoid, and ganoid scales; a classification which has been found particularly convenient with reference to fishes. diplomatist; born in New York City,