Dictionary of Ichthyology  

air bladder = gas bladder, the preferable term since the composition of gases may not be identical to that of air (a thin membranous, sometimes alveolated sac in the dorsal portion of the abdominal cavity. Contains a varying mixture of gases and 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, a less appropriate term).

vn and vn1 = air bladder of Cyprinus carpio from H. G. Seeley, "Freshwater Fishes of Europe: a History of their Genera, Species, Structure, Habits, and Distribution", 1886. Photograph by Brian W. Coad.

Canadian Museum of Nature
www.briancoad.com