FIN, in Natural History, a well known part of fishes, consisting of a membrane supported by rays, or little bony or cartilaginous ossicles.
The office of the fins has commonly been supposed to be analogous to that of feathers in fowls; and to assist the fish in its progressive motion, or swimming; but the later naturalists find this a mistake.
The tail is the great instrument of swimming: the fins only serve to keep the fish upright, and prevent vacillation or wavering. See ICHTHYOLOGY Index.