(a) Echidna and platypus lay eggs, but are considered as mammals because
(i) They are warm blooded animals
(ii) They all have four chambered heart
(iii) Mammary glands for the production of milk for their off springs
(iv) Their skin has hairs as well as sweat and oil glands.
(b) Crocodile has four, chambered heart, but still is a reptile because they have dry, non- glandular skin, epidermal scales are also present. They are also cold blooded and oviparous.
(c) Birds have light weighted bones with internal spaces filled with air. These bones are called pneumatic bones. This feature reduces the corporal density of the animal facilitating the flight.