Give reasons for the following:
$(a)$ Echidna and Platypus lay eggs but are considered as mammals.
$(b)$ Birds have pneumatic bones.

(N/A) Echidna and Platypus are classified as mammals because they possess mammary glands to feed their young with milk,have hair on their bodies,and are warm-blooded (homeothermic),despite the fact that they lay eggs (oviparous).
$(b)$ Birds have pneumatic (hollow and air-filled) bones to reduce their body weight,which makes them light and facilitates flight.