(genus Cerianthus), any of a group of invertebrate marine animals of the class Anthozoa (phylum Cnidaria) characterized by an elongated polyp (i.e., a hollow stalklike structure with a mouth and tentacles at the upper end); the polyp lives in a tube of slime on the ocean bottom. The genus is widely distributed in tropical and subtropical waters. One species, Cerianthus americanus, found in shallow waters from New England to Florida, grows to about 60 cm (24 inches) in length.