(i) They are found in a variety of habitats like Polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them are also adapted to fly or live in water.
(ii) They have milk producing mammary glands to nourish their young ones.
(iii) They possess two pairs of limbs which are adapted for walking, running, climbing, burrowing, swimming or flying.
(iv) They possess hair on the skin.
(v) External ears or pinnae are present.
(vi) They possess different types of teeth in the jaw.
(vii) The heart is four-chambered. They are homoiothermous
(viii) They respire through lungs.
(ix) Sexes are separate and fertilisation is internal.
(x) They are viviparous with few exception and development is direct.s
