Air bladder

Air bladder
1. (Science: anatomy) 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 oesophagus.
2. A sac or bladder full of air in an animal or plant; also an air hole in a casting.