coelenterata

Phylum Coelenterata (Gr., koilos = hollow ; enteron = intestine), also known as Phylum Cnidaria, is a group of aquatic, mostly marine animals characterized by a simple body structure with a single opening serving as both mouth and anus. Members of this phylum have a diploblastic body (two tissue layers: ectoderm and endoderm) and radial symmetry. They possess specialized stinging cells called cnidocytes containing nematocysts, used for defense and capturing prey. Coelenterates exhibit two main body forms: the sessile polyp (e.g., Hydra) and the free-floating medusa (e.g., jellyfish). This phylum includes organisms like corals, sea anemones, and hydroids, playing important roles in marine ecosystems, such as reef-building.