Updated by: Arabinda Naik
The class-Mammalia (mamma - breast) includes about 5,499 species. The examples of Mammalia are bats, tigers, lions, elephants, whale, rat, rabbits, horses, sheep, monkeys, etc. Mammals are found in a variety of habitats, such as forests, grasslands, mountains, deserts, polar ice caps, caves, etc. Some of them have adapted to fly or live in water. The body is divided into head, neck, trunk, and tail. The presence of mammary glands is the most characteristic feature of mammals. Their number varies from two to many. Mammary glands are well-developed in females. Skin is covered with sweat glands and sebaceous glands. Exoskeleton contains feathers, hairs, fur, nails, claws, horns, hooves, etc. External car called pinnae are present. They are the skin derivatives. Endoskeletons possess skull and vertebrae. Vertebrae are connected by intervertebral discs. Two pairs of pentadactyl limbs are present for walking. running: climbing burrowing lying, and swimming. Forelimbs are modified as flippers in whales. The digits end with horny nails, claws, or hoofs. Digestive system comprises of complete digestive system. Different types of teeth are present in the jaw. Lungs are spongy and clastic. A sound producing organ called larynx is also present in mammals. Heart is four-chambered with two auricles and two ventricles. RBCs are oval, biconcave, and enucleated. Excretory organs are kidneys. They excrete urea. Brain is well developed with large, convoluted cerebrum and cerebellum. 12 pairs of cranial nerves are present. Sense organs are well developed. Ear comprises of three regions i.e.., internal, middle, and external. External opening is protected by a large fleshy pinna. Eyes are protected by lids of which upper one is movable. Mammals are unisexual and show prominent sexual dimorphism. Gonads are paired. The testes present in males descend into scrotal sacs. Most of them are viviparous. Fertilisation is internal. Embryo develops in the uterus. The foetus is connected to uterine wall through placenta and gets nutrition and respiration. Placenta formation is not found in prototherians. Development is direct. Larval stage is not present. They are amniotes, i.e., the embryo develops four embryonic layers during development. Parental care is highly developed. They feed their young ones on their milk and protect them from enemies.