sea anemone, any of the relatively large, predominantly solitary polyps (see polyp and medusa) of the class Anthozoa, phylum Cnidaria. Unlike the closely related corals, these organisms do not have a skeleton. Sea anemones occur everywhere in the oceans, at all depths, but are particularly abundant in coastal waters. Many are beautifully colored (reds, pinks, yellows) and look like flowers when the oral, or feeding, end, equipped with many extensions called tentacles, is fully open. Some anemones are tiny, but most are from one to several inches (2.5-10 cm) long; the genus Stoichactis in the Great Barrier Reef off the coast of Australia may reach 3 ft (90 cm) in diameter at the expanded oral end. Most sea anemones attach temporarily to submerged objects; a few thrust themselves into the sand or live in furrows; a few are parasitic on other marine organisms. Some anemones feed on small particles, which are caught with the aid of a mucus secretion and moving currents that are set up by the tentacles. Most sea anemones are predaceous, immobilizing their prey with the aid of specialized stinging cells called nematocysts. Metridium is the genus most often studied in classrooms. The burrowing anemone, Cerianthuss, occurs on both Pacific and Atlantic coasts; some may reach nearly 2 ft (60 cm) in length. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia.

Any of more than 1,000 cnidarian species in the order Actiniaria, found from the tidal zone of all oceans to depths of more than 30,000 ft (10,000 m) and occasionally in brackish water. Species vary from less than 1 in. (3 cm) to about 5 ft (1.5 m) in diameter. The mouth, at the upper end of the cylindrical body, is surrounded by petal-like, usually colourful tentacles that bear stinging nematocysts for paralyzing prey such as fishes. Some species eat only microorganisms. Most species remain permanently attached to a hard surface such as a rock or the back of a crab.

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