A class of phylum Cnidaria (formerly known as Coelenterata) whose members are marine, live exclusively as polyps, and occur as solitary individuals or in clonal or colonial groups. Most adult anthozoans are attached to a firm object or the sea bottom but some burrow into soft sediments, and rare ones are planktonic. As is typical of cnidarians, sexual reproduction commonly gives rise to a planktonic larva, the planula, which metamorphoses to a polyp, usually when it settles onto the substratum.