Coelenterata is a term encompassing the animal phyla Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies) and Ctenophora (comb jellies). The name comes from Ancient Greek: κοῖλος, romanized: koilos, lit. 'hollow' and ἔντερον, enteron, 'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells (external and internal), and radial symmetry. Some examples are corals, which are typically colonial, and hydra, jelly fish and sea anemones which are solitary. Coelenterata lack a specialized circulatory system relying instead on diffusion across the tissue layers.