swim bladder

A thin membranous, sometimes alveolated gas-filled sac lying in the upper part of the body cavity of many bony fishes just beneath the vertebral column. Contains a varying mixture of gases, not identical to the composition of air. May be one, two or three chambered. May be connected to the gut by a tube, the ductus pneumaticus (then called physostomous) or unconnected (then called physoclistous). May function as one or more of:- hydrostatic organ, sound producing organ, sound receptor, respiratory organ. Found in Actinopterygii. Often lacking in bottom fishes. Sometimes called swim bladder or air bladder, less appropriate terms.