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.
Mammals nourish their young with milk produced by the mother's mammary glands.