Air Bladder, an organ in some kinds of fishes, commonly called by fishermen the "swim.'1 Fishes endowed with great powers of locomotion, and accustomed to pass rapidly from the surface to the bottom of the ocean, and vice versa, are provided with an air bladder or a swim, by which they can modify at will the specific gravity of their bodies in the water, as birds do in the atmosphere by admitting air when they wish to rise, and by expelling it as they descend. Not that fish draw air into their swims and expel it, as birds do in their quills, etc, but they have the power of generating gas to fill the swim like a balloon within the body when they wish to ascend in the water, and expelling it when they descend. Fishermen are well acquainted with the functions of the bladder in the cod and other species, which require to be brought fresh to market at a great distance from the place where they are caught; they perforate the air bladder with a fine needle, allowing the air to escape, and thus rendering the fish unable to rise from the bottom of the well-boats where they live for a considerable time, while brought to market. Cod sounds are the salted air bladders of these fishes.

The Iceland fishermen, and those of Newfoundland, prepare isinglass from cod sounds; and the Russians make a superior kind of isinglass from the sounds or swims of the sturgeon. The swim is composed of a lengthened sac, sometimes simple, as in the common perch, or divided into several compartments by transverse ligature, as in the trout and salmon; sometimes furnished with appendices, more or less numerous in different species. It is composed of a thick internal coat of fibrous texture, and a thin external coat, the whole being enveloped in the covering of the intestines. The swim has in many species no external opening, and the air or gas with which it is distended is supposed to be secreted in such cases by a glan-dulous organ with which it is always provided. In fresh-water fishes the air bladder communicates sometimes with the oesophagus and sometimes with the stomach, by means of a small duct or tube; and in these instances no secreting gland is found. A very few species, among which is the common eel, have air bladders opening by an external duct, and also provided with secreting glands. Fishes deprived of their air bladders sink helpless to the bottom of the water, and there remain.

All the different species of flat fish, such as skates, soles, tur-bots, brills, etc, which live only on the coasts and on sand banks at the bottom of the ocean, where they find their food, have no air bladders; their bodies are heavier than water, and their mode of life does not require them to ascend. Mackerel and other species, which find their food entirely on the surface, and remain there, have no air bladders; their bodies are comparatively light, and they need not sink low down in search of food. Some zoologists have supposed that the air bladder of fishes may be connected with the respiration, and it is now generally admitted to be a rudimentary lung. Much remains to be yet observed with regard to the relation of this organ to the general conformation of fishes; for it is sometimes found in one species, and entirely absent in another which belongs to the same genus.