Fin is an appendage of fish, which is an organ for swimming and maintaining body balance. * * * is divided into two kinds: one kind of fin is paired, called lateral fin, which is divided into pectoral fin and ventral fin, equivalent to forelimbs and hind limbs of terrestrial vertebrates. The basic function of lateral fin is to maintain the balance of the body and change the direction of movement. Another kind of fin is unpaired, located on the longitudinal axis of the body, called odd fin, which is divided into dorsal fin, gluteal fin and caudal fin. The basic functions of dorsal fin and gluteal fin are not only to maintain the balance of the body, to prevent tilting and swaying, but also to help swimming. The caudal fin has the functions of balance, propulsion and steering.