Lungfish lives in fresh water. Its body is spindle-shaped, its hard bones are underdeveloped, and its movements are flexible. It has internal nostrils connected to the mouth, unlike other fish. When moving in the water, it uses its gills to breathe, but in a dry environment, it can use its swim bladder to breathe air directly, hence the name lungfish.

The swim bladder of lungfish is very similar to a lung in structure. It has a honeycomb structure on the inside and a network of blood vessels with many branches. The swim bladder wall has stretchable muscles, so the swim bladder can suck in air and water like a lung. Release carbon dioxide.

The existing lungfish include the Australian barramundi with a single swim bladder and the African and American lungfish with double swim bladders. When the water is completely dry, the African lungfish hides itself in the mud and forms a solid "hut" around it. It also forms a capsule to wrap itself up, opening a small hole opposite the mouth or nostrils to let in air. The swim bladder can be breathed through the nostrils. They can spend several months in this house. When the rainy season arrives and the mud walls are washed away by water, the lungfish returns to the water and lives an active life.