Inspite of differences in structure and form of different animals, there are fundamental features common to various individuals in relation to the arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems.

These features are used as the basis of animal classification and some of them are discussed here. Animals are classified on the basis of certain fundamental features; they are:

Levels of organization of cells, body symmetry, diploblastic and triploblastic organization, nature of coelom, protostome and deuterostome development, segmentation of the body and presence or absence of notochord.

One of the distinguishing feature of animals is presence of notochord. Notochord is a mesodermally derived rod –like structure formed on the dorsal side during embryonic development in some animals. Animals with notochord are called chordates and those animals which do not form this structure are called non–chordates.

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