swim bladder
Thin-walled, air-filled sac found between the gut and the spine in bony fishes. Air enters the bladder from the gut or from surrounding capillaries (see capillary), and changes of air pressure within the bladder maintain buoyancy whatever the water depth.
In evolutionary terms, the swim bladder of higher fishes is a derivative of the lungs present in all primitive fishes (not just lungfishes).
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