The bony fishes. They breathe by drawing water over four or five pairs of gills located in chambers covered by a protective flap called the operculum . Species of bass, trout, perch, tuna, and herring are examples. The remaining classes are collectively called tetrapods because most animals in these classes have two pairs of limbs that support them on land. Most amphibians maintain close ties with water and are most abundant in damp habitats such as swamps and rain forests.
Most amphibians rely heavily on their moist skin to carry out gas exchange with the environment.
Some terrestrial species lack lungs and breathe exclusively through their skin or oral cavity. Amphibian eggs lack a shell and dehydrate quickly in dry air. Amphibians generally lay their eggs in ponds or swamps or at least in moist environments. Some urodeles are entirely aquatic, but others live on land as adults or throughout life. Anurans are more specialized than urodeles for moving on land.