Coelenterates comprise more than 9000 species, including coral, hydra, jellyfish, Portuguese man-of-war, and sea anemone distributed in all oceans. Only a few species are known to inhabit fresh water. Two forms of coelenterates exist, the polyp and the medusa. The phylum is divided into three classes: one in which the polyp form predominates; a second consisting solely of polyp forms; and a third composed primarily of medusa forms. The polyp form, such as the reef-building coral, occurs in colonies. It has a plant like appearance and attaches itself to rocks or debris on the sea bottom. By contrast, with rare exceptions, the medusae swim freely. Coelenterates grow only if well fed and shrink if starved.