Feature in Body Structure question?

Question

Group A:

Bird - sparrow, mynah

fish -goldfish, shark

reptile - snake, tortoise

amphibian - frog, toad

x

Group B

earthworm, housefly, crab

(a) All the animals in group A have a common feature in their body structure which group B does not have. What is the common feature?

(b) What does Group X represent?

Answer 1

(a). Vertebral column.

(b) Mammals.