It is assumed that most of the phyla in the Eumetazoa diversified from a flatworm-like descendant in the Precambrian. The Parazoa are evolutionarily the earliest forms of animals, but there is no evidence from the fossil record that any other forms evolved from them.

The Eumetazoa is divided according to the symmetry of the animals. Two Phyla are radially symmetrical, the Cnidaria and the Ctenophora. The other Phyla are bilaterally symmetrical. However some animals in the Phylum Echinodermata are secondarily radially symmetrical, e.g. sea urchins and starfish; it is believed that they evolved from bilaterally symmetrical animals.

Certain structural features are associated with bilateral animals, they include:
· Elaboration of antagonistic musculature with circular and longitudinal muscles.
· Acquisition of a body cavity and peristalsis to move food along the gut.
· Cephalisation, with the mouth and nervous tissues concentrated at the front.
· Segmentation, this is found in all animals from the Platyhelminthes onwards.
· Elaboration of the mesoderm with connective tissues, a blood system and muscles.

The Bilateral Phyla can be further subdivided into three groups. The Acoelomates have no body cavity surrounding the gut. Pseudocoelomates have a body cavity surrounding the gut but the gut itself is not surrounded by mesoderm. Coelomates have a true gut where the whole cavity is lined by mesoderm. The table below shows how the bilaterally symmetrical phyla are distributed according to the presence or absence of a coelom.