Class Mammalia

Class Mammalia (Latin mammae = breasts/nipples) includes animals such as bat, Rattus (rat), Macaca (monkey), Camelus (camel), whale, human beings, Cannis (dog), Felis (cat), Elephas (elephant), Equus (horse) and Pteropus (flying fox), along with the egg-laying Ornithorhynchus (platypus) and the pouched Macropus (kangaroo).

Mammals are defined by the presence of mammary glands, which produce milk to nourish their young. They are found in almost every habitat: most are terrestrial, some are aquatic (like whales), and a few are aerial (bats) or arboreal (tree-dwelling, like monkeys). Limbs are adapted for a wide range of locomotion — walking, flying, climbing, burrowing or swimming. The body is differentiated into head, neck, trunk and tail, and mammals are homeothermic.

The exoskeleton takes the form of hair, fur, nails, hooves or horns. The skin is glandular, bearing both sweat glands and sebaceous (oil) glands; mammary glands themselves are a specialised type of modified sweat gland.

Mammals possess an external ear (the pinna), unlike the other vertebrate classes discussed here, and show heterodont dentition — different types of teeth (such as incisors, canines, premolars and molars) specialised for different functions. Red blood cells are biconcave and lack a nucleus (enucleated), though blood remains red in colour. The heart is ventrally placed and perfectly four-chambered.

Respiration occurs through the lungs. The brain is highly developed; the cerebrum bears a band of nerve fibres called the corpus callosum, connecting its two hemispheres — a feature not seen in the other classes. …