air bladder

air· blad·der

air bladder
  1. An air sac, sometimes double or variously lobed, in the visceral cavity of many fishes. It originates in the same way as the lungs of air-breathing vertebrates, and in the adult may retain a tubular connection with the pharynx or esophagus. (Anat.)
  2. A sac or bladder full of air in an animal or plant; also an air hole in a casting.