The common fundamental features used for classifying animals include body symmetry, arrangement of cells, nature of coelom, level of organisation. Animal classification would be very confusing if fundamental features are not considered.
(i)Animals having different levels of organisation would have been placed in same group. E.g., Sponges and Cnidarians having cellular
and tissue level of organisation respectively.
(ii)Animals showing varied types of germinal layers would have been placed together, as diploblastic cnidarians and triploblastic platyhelminthes.
(iii)Animals having different body symmetry would have been placed together, as coelenterates with radial symmetry and platyhelminthes with bilateral symmetry.
(iv)There would have been no classification of animals based on with or without body cavity..
(v)Placing of oviparous and viviparous animals together.