Fishes are cold blooded vertebrates. This means that they remain at approximately the same temperature as the water surrounding them, in contrast to the whale or the water rat, which like us maintain a much higher temperature. Aquarium fishes share with ourselves and other mammals, however, the possession of a backbone, or vertebral column, and are built on the same fundamental plan, having the same basic system of bones and organs as we do. Fishes breathe oxygen, but it is usually absorbed only from solution in water by the gills, which are leaf like organs, normally four on each side of the neck in a pouch covered by the operculum, or bony gill cover. The gills are richly supplied with blood vessels, and water is swallowed from the mouth and forced over the gills, leaving by a slit between the operculum and the body. The rate of fishes` respiratory movements is partly determined by the need for oxygen and its concentration in the surrounding water.

Fins There are two paired and (in all but fancy goldfish and a few other fishes) three unpaired fins. The paired pectoral and pelvic (ventral) fins correspond, respectively, to the arms and legs of human beings and connect with bony girdles in the body which correspond to our own pectoral and pelvic girdles. The unpaired fins are the dorsal, the anal, and the tail or caudal fins, as shown in the accompanying figure. These fins are supported by rays, sometimes bony and sometimes made of cartilage. In some families the dorsal fin is split entirely into two parts, the forepart with spiny rays and the hind part with soft rays. In the characins and some others, there is a small adipose fin, composed of fatty material with no fin rays.

Body The fish body is composed mainly of a large lateral muscle on each side of the backbone, divided by sheets of connective tissue into segments corresponding to the vertebrae, which give rise to the typical flaking of the cooked fish. This is the main organ for swimming. The internal organs often occupy a very small volume, toward the front, so that much of the apparent trunk of the fish is really its tail (as distinct from the tail fin). This is indicated by the forward position of the beginning of the anal fin, which marks the end of the digestive tract. Fishes possess the usual organs familiar to students of human anatomy, with the exception of lungs and chest cavity; they have a stomach, intestines, a liver, a spleen, kidneys, and so forth.

Skin and Scales. The skin may be naked, or it may be covered by scales or by bony plates which in turn have an outer layer over them. The scales may be opaque or transparent; if they are transparent, the appearance and color of the fish may be due to skin pigments, not to scale color or formation, as in the calico goldfish. Bony plates may be seen in the Corridors, or South American armored catfishes.

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