Any polyp of the nearly 1000 anthozoan coelenterates belonging to the order Actiniaria. They occur intertidally and subtidally in marine and estuarine habitats, attached to solid substrates or burrowing into soft sediments. No fresh-water or truly planktonic species are known, though one genus, Minyas, secretes a float within its base and hangs suspended at the surface of the seas. Anemones may be very small, 0.04 in. (1 mm) long, to large individuals more than 3 ft (1 m) in length or diameter.