(i)Two characteristic features of vertebrates are as follows:
(a) Presence of an internal skeleton.
(b) Triploblastic, coelomic and segmented body.
(ii) (a) Echidna and Platypus are considered mammals because they have mammary glands to nourish their young ones.
(b)Birds have modified forelimbs to reduce body weight for flight.
Crocodiles are reptiles because they are cold¬blooded, lay eggs and have scales on their body.