[AS., from root of L. penna, a feather.] The parts of a fish by which it balances itself and moves forward in the water. The paired fins on the opposite sides of the body are the true limbs of the fish. Those near the gills behind the head are the pectoral fins, and represent the fore limbs of quadrupeds ; those below are the ventral fins, and represent the hind limbs ; those on the back are dorsal or first and second dorsal; those underneath near the tail are the anal fins, and the tail itself is the caudal fin. These are used as its means of motion and the tail also serves as a rudder.