Page:Natural History, Fishes.djvu/17

Rh gills, through orifices for the purpose, called the gill-openings. The breathing apparatus is protected by large bony plates.

In most of the Bony Fishes there is found a membranous bladder, commonly of a lengthened form, placed along the body between the spine and the bowels. It is filled with air, and is well-known as the air-bladder, or swimming-bladder. It varies in appearance; sometimes, as in the Hedgehog-fishes (Diodon), and their allies, it is two-lobed, more rarely it is double; in some genera, as in the Electric Eels, and the Carp family, it is divided by a transverse partition, which, in the latter, allows of intercommunication through a narrow orifice. In one of the Catfish family (Pangasius) it is divided into four compartments. In many species there are closed or blind tubular processes sent off from various parts of the surface; and in others it is subdivided into many irregular cells. From this structure it appears evident that the air-bladder is the lingering remnant of the lungs of air-breathing animals.

In some instances this bladder is found to be connected with the organs of hearing; but its chief function is the regulation of the specific gravity of the animal, aiding it in rising or sinking in the water, or maintaining any particular depth that its exigencies may require. In general, those species that swim at the surface, or that rove freely through the water, are furnished with this organ, while deep-water fishes are destitute of it; but there are many unaccountable exceptions. The air contained is found to vary in its character; but in marine fishes oxygen