Class Mammalia

Mammalia (Latin mamma, breast) is found in essentially every habitat, and is defined by two features unique among animals: the body is covered by hair, and every member possesses mammary glands, which nourish the young with milk — the single most distinctive feature of the class. Some mammals are secondarily adapted to fly (bats) or live fully in water (whales, dolphins). Mammals have two pairs of limbs, variously adapted for walking, running, climbing, burrowing, swimming or flying.

The skin is glandular, bearing sweat glands, scent glands and sebaceous glands. The exoskeleton, where present, includes horny epidermal horns, spines, scales, claws, nails, hooves and bony dermal plates. Teeth in mammals are thecodont (set in individual sockets), heterodont (differentiated into distinct types — incisors, canines, premolars, molars) and diphyodont (replaced once, from milk teeth to permanent teeth) — three dental features found together in no other vertebrate class. External ears (pinnae) are present. The heart is four-chambered with a left systemic aortic arch, and mature red blood cells are circular, biconcave and non-nucleated (unlike the nucleated RBCs of every other vertebrate class covered in this chapter). …