Figure 3. Air bladder and lungs of various fishes and tetrapods (schematic). A. Typical dorsal air bladder in actinopterygian (ray-finned) fishes. Primitively, the air bladder is connected to the pharynx by a pneumatic duct, a condition which allows the animal to rise to the surface and gulp or discharge air, thereby adjusting its density. In advanced forms, the bladder is closed and contains specialized areas for gas production and resorption. B. Air bladder of extant freshwater holostean fishes, Lepidosteus (gar pike) and Amia (bowfin) has a folded inner surface and is capable of some gas exchange. C. Erythrinus, an unusual teleost type possibly related to the bowfin, with a lateral opening suggestive of the transition from lung to air bladder. D. Australian lungfish, a living sarcopterygian (lobe-finned fishes) with the bilobed lung rotated dorsally. The connection to the pharynx remains ventral. E. Polypterus, the so-called "bichir" of central Africa, and probably representative of the primitive state which was antecedent to all other lung and bladder types. Living chondrosteans trace to the Paleozoic Paleoniscoids which were close to the basal stock from which all other groups of ray-finned fishes evolved. F. Tetrapod lung with complex internal structure. To view some the various lung / bladder types superimposed on the evolutionary tree in Figure 2, click here. From Romer, A. S. 1964. The Vertebrate Body. W. B. Saunders. Philadelphia.