These pits detect water currents and predators or prey that may be causing water movements near the fish. Click on the picture below to be taken to an internal view. Amphibians were the first land vertebrates, arising from fish with stout, fleshy fins. Most amphibian adults are terrestrial, but they lay eggs in water. The eggs are fertilized externally and each hatches into an aquatic larval stage called a tadpole.
Tadpoles undergo a dramatic metamorphosis of body shape as they become adults.
Development of legs and lungs in amphibians was a major evolutionary event. However, primitive lungs had already developed in some fish. In addition to lungs, the soft moist skin of some amphibians is highly vascularized and accounts for as much oxygen diffusion as the lungs. Class Mammalia Mammals are covered with insulating body fat and hair, and maintain a constant body temperature as birds do. The circulatory system distributes oxygen, nutrients, and heat.