Cnidarians include the corals, jellyfish, sea anemones, and hydras. The name Cnidaria refers to the stinging structures that are characteristic of some of these animals. These organisms have a vase-shaped body plan and 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. The tentacles—and sometimes the outer body surface—contain longer, harpoonlike structures called stingers or nematocysts; the stinging cells are called cnidocytes.