Yours, Etc.: Origins and Uses of 8 Sign-Offs
minute, elongated or spherical capsule that is found chiefly in members of the phylum Cnidaria (e.g., jellyfish, corals, sea anemones). The capsule, which occurs on the body surface, is produced by a special cell called a cnidoblast and contains a coiled, hollow, usually barbed thread, which quickly turns outward (i.e., is everted) from the capsule upon proper stimulation. The purpose of the thread, which often contains poison, is to ward off enemies or to capture prey.