The locomotory organs of aquatic vertebrates. In fish there are typically one or more dorsal and ventral fins (sometimes continuous), whose function is balance; a caudal fin around the tail, which is the main propulsive organ; and two paired fins: the pectoral fins attached to the pectoral (shoulder) girdle and the pelvic fins attached to the pelvic (hip) girdle, which are used in steering. These paired fins are homologous with the limbs of tetrapods. Fins are strengthened by a number of flexible fin rays, which may be cartilaginous, bony and jointed, horny, or fibrous and jointed.