Coelenterata is a rejected phylum encompassing the animal phyla Cnidaria and Ctenophora. The name comes from Ancient Greek κοῖλος (koîlos) 'hollow' and ἔντερον (énteron) 'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, along with a middle undifferentiated layer called the mesoglea, and radial symmetry. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.