Cnidarian, or, Coelenterate, an animal of the phylum Cnidaria, or Coelenterata. The phylum includes the corals, hydras, jellyfish, Portuguese man-of-war, and sea anemones. Cnidarians are water-dwelling animals. The body is typically cylindrical or bell-shaped and has a single opening used for taking in food and excreting wastes. Arranged around the opening are tentacles armed with cnidocytes, cells used to sting prey and enemies. Some cnidarians live singly, others in large colonies.The cnidarian has tentacles armed with cnidocytes, cells used to sting prey and enemies.