NEMATOCYST

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. When stimulated, a lidlike structure on the top of the capsule pops aside, and the thread is everted with a twisting motion. As eversion and twisting proceed, the barbs act like a drill, penetrating into (and pulling the thread into) the foreign object. If a toxin is present, it passes through the hollow thread, penetrating and paralyzing the victim's tissues. After eversion, the thread separates from the nematocyst. The threads of some nematocysts ensnare small prey by wrapping about them. The stinging effect of nematocysts in the Portuguese man-of-war and other jellyfish (qq.v.) can be extremely painful to humans and may cause paralysis, shock, and even death.