Their hard, external scales are covered with mucus, and the operculum or gill cover acts as bellows. A swim bladder provides buoyancy and the lateral line can detect water currents and predators or prey that may be causing water movements near the fish. Have a two-chambered heart.
Most adults are terrestrial, but lay eggs in water.
They breath with lungs but the larval stages and some adults use gills. The soft moist skin of some amphibians allows as much oxygen to diffuse into the body as the lungs. The three-chambered heart is specialized for high delivery of aerated blood.