Gas bladder is a term with more universal acceptance than swim bladder or air bladder. The latter terms are misnomers in the sense that gas bladder is not really used for swimming nor the gases it contains are of the same proportion as the gases in air. The bladder is a hollow, retroperitoneal organ lying against the peritoneum ventrally and the kidneys dorsally. The bladder is an evagination either from the middorsal or lateral walls of the foregut. In some fishes the connection with the foregut still persists. In general, a pneumatic duct, the duct connecting the gas bladder to foregut, persists in soft-rayed fishes; in spiny-rayed fishes this duct is degenerated and the bladder loses its connection with the alimentary canal thus completely isolating it from the outside. Fishes with pneu-matic ducts are called physostomous fishes, or physostomes; those without are the physoclistous fishes, or physoclists. Both terms, physostomous and physoclistous, can also be used to describe the gas bladders.