Cnidarians (also known as Coelenterata)

The coelenterates were the first multicellular animals with true tissues. The body is sac-shaped and radially symmetrical, with an opening at the top forming the mouth. Surrounding the mouth are a circlet of tentacles. The cavity of the sac-shaped coelenteron is where digestion takes place. Food enters the sac, and wastes leave via the mouth. The tentacles often possess stinging cells, nematocysts, to stun or kill prey. The inside body wall is folded into mesenteries which increase the surface area to aid in food digestion and absorption. No heart, circulatory, excretory, or respiratory systems are present, and the nervous system is rudimentary.

Two basic feeding orientations exist. Medusa (e.g. jellyfish) are free floating with the mouth and tentacles pointing downward. Polyps (e.g. corals, sea anemones, & hydra) are attached to the bottom with the mouth and tentacles pointing upward. In many coelenterates, the sexual medusa and asexual polyp stages alternate with each other in the life cycle. In other cases, only the medusa or polyp stage is present. Polyps may grow in large colonies.

Most coelenterates are marine, and many prefer warm tropical shallow water. Many coelenterates (jellyfish, many hydrozoans, and sea anemones) do not secrete a skeleton and have a negligible fossil record. Others, especially many anthozoan corals, secrete an extensive skeleton and are well-preserved.

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Last updated on February 24, 1997-jlc.