Page:Natural History, Fishes.djvu/22

8 membrane, more or less transparent, stretched over parallel or radiating rays. These rays are slender bones, sometimes consisting of a single piece, stiff and spinous; at other times composed of several pieces jointed together, and therefore flexible; the latter are frequently divided each into two or more branches at the tip. These characters of the fin-rays afford a ground for the division of one of the Sub-classes into Orders.

The fins of Fishes are of five kinds, and are designated according to their position, as pectoral, or breast-fins; ventral, or belly-fins; dorsal, or back-fins; anal, or vent-fins; and caudal, or tail-fins. The pectorals and ventrals are arranged in pairs, and correspond to the fore and hind limbs in other vertebrate animals; the pectorals, for instance, representing the wings of birds, the ventrals the feet. The other kinds have nothing corresponding to them in the other Classes, except it be the web-like expansion of skin that fringes the Newts, &c., or the cartilaginous dorsal in some of the Cetacea. The dorsal runs along the medial line of the back; it was formerly supposed to preserve the body in a perpendicular position in the water, but recent experiments have shown that it is not necessary for that object, though it may be accessory to it. Almost all fishes have this fin, and many have two; while a few, as the Haddock and Cod, have three dorsals. The anal fin corresponds to the dorsal, but is placed beneath the body, just behind the vent. The caudal is the most important fin of all, being the great organ of motion. It is the termination of the body, being expanded around the posterior extremity. In those fishes