ABSTRACT
Fishes show an interesting diversity of approach to the problem of acquiring oxygen and transporting it to the tissues. Even though most fishes obtain oxygen from the water, they do so using gills (and sometimes other surfaces) of varied designs, causing water to flow over them in a variety of ways. There are also air-breathing species in no less than 70 freshwater genera; some of these have rather curious methods of aerial gas exchange, involving unexpected structures such as the body scales and the hindgut. As well as lungfish, many phyletically ancient freshwater fishes breathe air, such as Amia, Megalops, Polypterus, and Lepisosteus, testifying to its adaptive advantage in some freshwater environments which contain little oxygen or are at risk of drying up.
