Coelenterates, also called Cnidaria, are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name cnidaria comes from cnidoblasts or cnidocytes, cells on the tentacles and body that contain stinging capsules called nematocysts. Cnidoblasts are used for anchorage, defence and capture of prey. Cnidarians show tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, the mouth on the hypostome. Digestion is both extracellular and intracellular. Some cnidarians, such as corals, have a skeleton composed of calcium carbonate. They exhibit two basic body forms. The polyp is sessile and cylindrical, as in Hydra and Adamsia. The medusa is umbrella-shaped and free-swimming, as in Aurelia or jellyfish. Cnidarians that exist in both forms show alternation of generations, called metagenesis, in which polyps produce medusae asexually and medusae form polyps sexually, as in Obelia. Examples include Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan) and Meandrina (brain coral). The key exam points are cnidoblasts, the dimorphic body forms and the single opening of the gastrovascular cavity, which makes the digestive system incomplete.