Class Anthozoa

Anthozoans are cnidarians that exist exclusively in the polyp form, completely lacking a medusa stage; they include sea anemones, corals, sea pens, and sea fans. They exhibit radial symmetry, having either hexacorallia (6-fold symmetry) or octocorallia (8-fold symmetry). Their cylindrical bodies have a central mouth surrounded by tentacles that are armed with nematocysts. Their gastrovascular cavity is divided by septa, which increase their surface area and provide structural support. It also consists of a stomodeum, a tubular pharynx that extends from the mouth into the digestive cavity. Most anthozoans are marine and either solitary or colonial, including many corals. Corals produce and release calcium carbonate skeletons that accumulate and form coral reefs, some of the most diverse ecosystems on Earth. Many of these corals have mutualistic relationships with zooxanthellae, providing them with nutrients through photosynthesis.