Sea anemone is the popular name for some 1,000 species of polyp or coelenterate of the order Actiniaria in the class Anthozoa. These brightly colored, flowerlike animals are found in all seas, but they are generally more numerous and larger in temperate and warm waters. Most of them live singly, but some form colonies.
Unlike the closely related stone corals, sea anemones never form an internal limy skeleton but may form a superficial one in the ectoderm. Generally they live attached to rock pilings or the like, but a few burrow in sand or mud. Some fix themselves as commensals to shells inhabited by hermit crabs. A few are truly parasitic in jellyfishes.
Despite their popular name, only a few of these animals resemble anemone blossoms. The majority look like dahlias or chrysanthemums. The typical sea anemone has a stout, more or less cylindrical, body that is topped by a broad, flat disk. The disk contains a slitlike mouth surrounded by whorls of simple hollow tentacles, which resemble flower petals. From the mouth a short, tubular gullet, or esophagus, reaches into the cavity of the body. It is connected to the body walls by radiating septa, or mesenteries, that divide the body cavity into a corresponding number of sacs.
The esophagus communicates with the sacs through a central space into which the septa do not reach. A pair of ciliated grooves, or siphonoglyphs, extend along opposite sides of the esophagus and into the corresponding corners of the mouth. These always remain open and are the seat of inflowing and outflowing currents of water. The grooves provide both a means of respiration and an outlet for the removal of waste matter from the body.
The body walls, as well as the tentacles, which are outgrowths from them, are very contractile and largely composed of muscles arranged in a circular and a longitudinal layer. The circular layer is for extension; the longitudinal layer, for retraction. These muscles have special relations to the mesenteries. The mesenteries are vertical radiating septa reaching from the mouth disk or peristome to the base, and from the body wall to the esophagus, but ending freely below the esophagus. They are not strictly radial in arrangement but are grouped in pairs, almost always, like the tentacles, in some multiple of six. The mesenteries corresponding to the siphonoglyphs differ in structure from all of the others and are termed directive; the others form different classes according to the order and degree of development. The intermesenteric sacs may further communicate by one or two pores in each mesentery.
Along the edges of the septa the testes and ovaries are developed from the cells lining the gastric cavity. Digestive cells are also found in the same region, as well as an area filled with stinging thread cells, or nematocysts.
Sea anemones may be distinguished by the arrangement of their septa and tentacles and, less importantly, by their color and form. Most species, particularly the tropical varieties, are beautifully colored. The average size of a sea anemone is 21/2 to 3 inches (64–76 mm) in diameter and 4 inches (100 mm) in height.
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