BRANCHIÆ, or GILLS, in the anatomy of fishes, the parts corresponding to the lungs of land-animals, by which fishes take in and throw out again a certain quantity of water, impregnated with air. All fishes, except the cetaceous ones and the petromyzum, are furnished with these organs of respiration; which are always eight in number, four on each side the throat. That next the heart is always the least, the rest increasing in order as they stand near the head of the fish.

Each of these gills is composed of a bony lamina, in form of a semicircle, for the most part; and on its convex side stand the leaves or lamellæ, like so many sickles. The whole convex part of the lamellæ is befit with hairs, which are longest near the base, and decrease gradually as they approach towards the point. There are also hairs on the concave side of the lamellæ, but shorter than the others, and continued only to its middle. The convex side of one lamina is fitted into the concave side of the next superior one; and all of them are connected together by means of a membrane, which reaches from their base half-way their height, where it grows thicker, and in some measure resembles a rope. The rest of the lamina is free, and terminates in a very fine and flexible point.

As to the use of these gills, they seem to be designed to receive the blood protruded from the heart into the aorta, and convey it into the extremities of the lamellae; from whence being returned by veins, it is distributed over the body of the fish.

BRANCHIARUM oramina, apertures of the gills. In most fishes there is only one aperture; in the cartilaginous ones, these apertures are ten in number, five on each side; and in the petromyzon or lamprey, there are no less than fourteen of these apertures, seven on each side.

As to the cetaceous fishes, they have no aperture of this kind; and the reason seems to be, because they are furnished with lungs.