Corals, jellyfishes, sea anemones, and hydras are members of the phylum Cnidaria. The name Cnidaria (from the Greek term knide, meaning “nettle,” and Latin term aria, meaning “like” or “connected with”) refers to the stinging structures that are characteristic of some of these animals. These organisms have a digestive cavity with only one opening to the outside; this opening is surrounded by a ring of tentacles used to capture food and defend against predators. Cells in the tentacles and outer body surface contain stinging, harpoonlike structures called nematocysts. Cnidarians are the first group in the animal hierarchy to have their cells organized into tissues.