Why there was a need of change in classification system? 

Explanation

There are different animals with different structures and forms.
As over a million species of animals have been described till now the need for classification becomes all the more important.
The classification also helps in assigning a systematic position to newly described species.
Basis of Classification :
Inspite of differences in structure and form of different animals these are fundamental features common to various individuals in relation to the arrangement of cells.
Body symmetry
Nature of coelom
Patterns of digestive system
Circulatory system
Reproductive system
These features are used as the basis of animal classification.