Basis of Classification

Despite all the differences in shape, structure and outward form among animals, a number of deep, fundamental features recur across many groups — and it is these shared features, not superficial appearances, that animal classification is actually built on. The features used as the basis of animal classification include:

• The level of organisation of cells (how the body's cells are arranged and how far the division of labour has gone).
• The body symmetry (whether, and how, the body can be divided into matching halves).
• The number of germ layers formed in the embryo (diploblastic versus triploblastic organisation).
• The nature of the coelom, or body cavity — whether it is present, absent, or only partially lined.
• The presence of segmentation of the body.
• The presence or absence of a notochord. …