Coelenterata

Coelenterata is a term encompassing the animal phyla Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). 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 (ectoderm and endoderm), along with a middle undifferentiated layer called mesoglea, and radial symmetry. Some examples are corals, which are typically colonial; hydrae, jellyfish, sea anemones, and Aurelia, which are solitary; Pennatula; Portuguese man o' war; Gorgonia; and Physalia. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.