Members of the phylum Cnidaria include hydras, jellyfish, sea corals, and sea anemones. Cnidarians live primarily in marine environments. They have tissue organization and a body plan displaying radial symmetry; that is, the organisms are circular with structures that radiate outward. The ends of the structures have tentacles with stinging devices called cnidocyte that help in defense and in trapping food. Digestion of food occurs within the central body cavity called the gastrovascular cavity. Cells lining this cavity produce enzymes to break down the food, and the products of digestion are taken into the cells. In addition, the animals have a loose network of nerve cells within their tissue. These nerve cells coordinate the animal’s activities.

There are two basic body plans in the cnidarians: a hollow, vaselike body called the polyp, and an inverted umbrella shape called the medusa (the plural is medusae). Hydras occur as polyps, while jellyfish appear as medusae. A three-layered body wall makes up the outer surface of the polyp or medusa.