Sea anemone body plan.
Longitudinal section and cross sections. After Barnes, 1980
The construction of anemones is accomplished with three tissue layers: an outer ectoderm, inner endoderm, and a mesogloea sandwiched in between. The three tissue layers are essentially laminar but highly folded to form the large surface of mesenteries that partition the coelenteron. The body plan is radially symetrical, though the radial symmetry of anthozoa is properly termed "bira-dial symmetry" because their mesenteries occur as couples on opposite sides of a plane bisecting the mouth, actinopharynx, and siphonoglyph(s). This bilateral symmetry is related to the hydrostatic design (Pantin, I960) and the need to ventilate the coelenteron, especially in large anemones (Hyman, 1940).
The mesenteries not only provide a large surface area for gas exchange, they are also an important site of digestion of food (see the topic feeding), and development of gametes (see topic reproduction). Descriptions of mesenteries often refer to them being "complete1' or "incomplete." If one imagines an anemone cut in cross section, the mesenteries form what looks like spokes, with the body wall that forms the column in the wheel position and the pharynx located at the hub. Complete mesenteries are attached to the column wall on one side and the wall of the pharynx on the other. Their presence provides structural support that facilitates the hydrostatic skeleton achieved by muscular contraction against the water filled cavities. Incomplete mesenteries are attached to the column wa 11 only, extending part of the way into the gastrovascular cavity, the lower part of the coelenteron below the pharynx, where prey items are digested. Mesenteries occur in "cycles" that are usually in multiples of twelve. The primary cycle
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