Sea Anemone, common name for marine, flowerlike polyp having a cylindrical, or vaselike, body. Many species are colored; large specimens may attain a diameter of 1 m (3 ft). The body is closed and attached to rocks or coral at one end and, at the other end, has a central mouth surrounded by tentacles armed with nematocysts (stinging cells and thread cells that paralyze and entangle the small fish and marine animals that constitute its prey). The slitlike mouth opens into a short esophagus opening into the body cavity. At each end of the mouth, a permanent pore opens into a ciliated groove, called a siphonoglyph, in the side of the gullet, through which a continuous current of water flows, carrying oxygen to the tissues and removing waste matter. The body cavity is divided into a number of sacs by septa extending from the body wall. These septa increase the surface available for the secretion of digestive juices and the absorption of nourishment, and they contain the gonads that produce the sperm and eggs.

Most sea anemones reproduce sexually; budding and fission are comparatively rare. The eggs are usually fertilized in the gastric cavity, and the young are discharged from the mouth as free-swimming larvae, which soon attach themselves to surfaces. Hermit crabs sometimes attach sea anemones to their shells (see Symbiosis). Some anemones become completely parasitic on certain species of jellyfish.

Scientific classification: Sea anemones make up the order Actiniaria of the class Anthozoa.