An animals are classified on the following common features like –
cell arrangement
body symmetry,
nature of coelom,
digestive tract,
circulation system
reproductive system.

Without these common features it is very difficult to classify each organism individually, it is impossible to add new species which is detected every day without common features. For the study of diversity in animals, classification is based on common basic features which are –

Some difficulties are:

(i) Tracing of interdependence amongst various animals will become difficult

(ii) Difficulty in developing new species of animals