Phylum Cnidaria

Abstract

The possession of an internal cavity that is lined by a special layer of cells, embryologically the endoderm, and in which the digestive and absorbtive functions are centred, distinguishes all the remaining groups of cellular animals from the Porifera or sponges. The simplest of these groups has an internal cavity in which there is no separation between the enteric or digestive cavity and the general body cavity. Both cavities are represented by one continuous space, the coelenteron, which opens to the exterior by the aperture of the mouth. These animals constitute the phyla Cnidaria and Ctenophora and are all at a low level of organisation with a conspicuous radial symmetry which, in some instances, becomes markedly biradial.