Lungfish are members of the subclass Dipnoi. They are a sister group to the Coelacanths and to the early lobe-finned fish which gave rise to the tetrapods. All these fish have paired appendages, fins or limbs, attached to a pelvis or shoulder by means of a single bone, the femur or humerus.
The living members of the group are special freshwater fish. They are the only living fish that have both lungs, as well as gills. They need this because they live in small lakes and rivers that sometimes dry up. When this happens, these fish don't die, but they can survive in the mud, for up to 4 years. They use their lungs during this time. This is called aestivation, which is a type of dormancy.
Currently there 8 species of lungfish, 6 of them in Africa, one in South America and one in Australia.