Swimming Bladder
the swim bladder or air bladder of fishes; a hollow sac, formed of several tunics, containing gas, situated in the abdominal cavity, but outside the peritoneal sac, entirely closed or communicating by a duct with intestinal tract. The special function of the swimming bladder is to alter the specific gravity of the fish, or to change the center of gravity. It is absent in the Leptocardii, Cyclostomata, Chondropterygii, and Holocephala, but occurs in all the Ganoidei, in one suborder of which (Dipnoi) it possesses anatomical characters, and assumes, to some extent, the functions of a lung; in the genus Ceratodus, the swimming bladder, though a single cavity, has symmetrically arranged internal pouches, while in the other genera of the sub-order (Lepidosiren and Protopterus) it is laterally halved, is supplied with venous blood by a true pulmonary artery, and by its cellular structure closely approaches the lungs of a reptile. are