Most fish breathe by extracting oxygen from water, by means of their gills. Gills are delicate filaments of tissue through which blood passes. As water flows over the gills, oxygen diffuses into the blood.
However, some fish have an additional method of obtaining oxygen; they use a lung, just as we do. Fish which breathe in this way are, not surprisingly, called lungfish. The lung is formed from a modified swim bladder, the organ usually used to help a fish maintain buoyancy in the water.
Versatility founds in Australia, South America and Africa, lungfish live in places which may become very dry during drought. They use their gills to breathe when water is plentiful, but as drier conditions prevail they take in air from the surface into their lungs, or they sometimes bury themselves in the mud and breathe through a small tube until water returns. Some lungfish live in stagnant water, and use their lungs as well as their gills all the time.