Hydras are small and very common, most living in shallow, freshwater ponds. Hydras are usually less than 1 cm tall, preying on smaller invertebrates among the filaments and leaves of freshwater plants and algae. Hydras are sessile, attaching to some sort of substrate. Anthozoa are the sea anemones and corals. 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.