A large, solitary polyp (order Actiniaria) with many tentacles surrounding an oral disc, at the centre of which is the mouth which is also the anus. The animal lives attached by its adhesive foot to a solid surface, although a few swim freely. The tentacles have specialized cells that can evert suddenly on contact from prey or an aggressor, releasing stinging threads (nematocysts). Most are harmless to humans, but a few species deliver a potentially lethal sting. Sea anemones feed on small fish and crustaceans. There are more than 800 species found worldwide but most abundant on coral reefs.