Hydras, jellyfishes, sea anemones, and corals belong to the phylum Coelenterata, which derives from two Greek words meaning "hollow gut". Thus, coelenterates live up to their name, for they have a digestive system that consists of a hollow sac, with one opening that serves as both a mouth and an anus. The digestion is mainly delegated to cells lining the gut. Although it is a primitive form of an arrangement, it serves these animals well.
Other features that characterize the coelenterates are the presence of three layers of cells in their bodies and tentacles, and the characteristic stinging cells, which have microscopic harpoons called nematocysts. Zoologists once believed that the body wall of coelenterates was made of only two cell layers; an outer epidermis (ectoderm) and an inner gastrodermis (endoderm). Zoologists now consider the jelly-like middle layer between the epidermis and the gastrodermis to be true mesoderm.
Most coelenterates have the same basic body plan as hydra and are radially symmetrical. This has advantages for an animal that is attached to the bottom and does not move much, because it enables it to receive input from its environment in all directions. Some sponges are also radially symmetrical.
The coelenterates make up a diverse and widespread phylum, with 9,000 living species, including relatively familiar creatures, such as jellyfish, sea anemones and corals among the marine forms, and the freshwater hydras.