The Class Pisces

Excerpt from The Class Pisces

Suspended in a fluid nearly as heavy as themselves, the former have no need of wings to sustain them in it. A great number of species have immediately under the spine an air vessel, which by compression or dilatation changes the specific gravity of the fish, and assists it in rising or descending in the water. Progression is executed by the motion of the tail, which strikes the water alternately right and left, and forces the fish forward; the gills discharging the water backward probably contribute also thereto. Hence, the limbs being but of little use are greatly reduced; the pieces analogous to the bones of the arms and of the legs are extremely short or entirely concealed; rays more or less numerous, supporting membranaceous fins in some degree represent fingers and toes. The fins corresponding to the anterior extremities are said to be pectoral, those which answer to the posterior are said to be ventral. Other rays attached to particular bones placed upon or between the extremities of the spinous processes support vertical fins on the back, under the tail, and at its extremity, which by being erected or depressed, extend or diminish at the will of the fish, the surface which strikes the water. The upper fins are said to be dorsal, the lower anal, and that of the end of the tail caudal.

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