Question -
Answer -
If fundamental characters are not taken into account then the classification of animals is difficult due to difference in characters in different animals which are as following:
(i) Grade of organisation
(ii) Symmetry
(iii) Coelom
(iv) Segmentation
(v) Notochord
Common fundamental features help in grouping animals in certain categories or sub-categories. For examples, all birds have fore-limbs modified to assist in flying. This gives us one clue to categorise a particular animals among aves.