The lungs of lungfish are homologous to the lungs of tetrapods. Unlike most lungfish and tetrapods, their lungs are smooth sacs instead of alveolated tissue. Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Of extant lungfish, only the Australian lungfish can respire through its gills. In Uganda, lungfish and catfish were mainly fed to the young. Zuxu are carnivorous lungfish from the planet Ganlihk. The dentition of lungfish is different from that of any other vertebrate group. Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. Protopterus is the genus of four species of lungfish found in Africa. Obligate air breathers, such as the African lungfish, "must" breathe air periodically or they suffocate. It is one of only six extant lungfish species in the world. The body of the lungfish is covered with large, bony scales. The small settlement of Ceratodus, Queensland derives its name from that of the Australian lungfish. There are three major hypotheses as to how lungfish evolved their stubby fins (proto-limbs). This paleoenvironment also had abundant lungfish and clams.