This is the largest class of the Cnidaria phylum, with over 6000 species. Anthozoans are often colonial having the polyp body plan with no alternation with the medusa form. Anemones are sessile, attaching themselves, like the hydras, to a substrate. This attachment is not permanent as sea anemones can slowly slide on a film of mucus. Corals have polyp structures similar to anemones but corals are usually colonial and much smaller.
Most corals secrete a hard skeleton of calcium carbonate.
Corals have animal symbionts called zooanthellae. The zooanthellae are dinoflagellates that receive phosphorus and other minerals as wastes of the coral, and in return the zooanthellae provide sugars to the coral through photosynthesis. If the coral becomes stressed it can spit the animal symbionts out and die, known as coral bleaching.