AIR-BLADDER, the name given to a structure present in all bony fishes (Teleosts) and homologous with the lungs of land vertebrates. The functions of this organ, which contains gas having a high percentage of oxygen, are obscure ; hydrostatic uses have been attributed to it with doubtful validity. In some fishes (especially Siluroidea), it is connected with the internal ear by a chain of bones, the Weberian ossicles, and in these forms it is possibly concerned with hearing. It usually retains a connection with the alimentary canal (except in most Acanthopterygii, q.v.), but is normally dorsal to it. (See FisH.)