minute, elongated, or spherical capsule produced exclusively by members of the phylum Cnidaria (e.g., jellyfish, corals, sea anemones). Several such capsules occur on the body surface. Each 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 by chemical or mechanical cues, a lidlike structure on the top of the capsule pops aside, and the thread everts explosively 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 some jellyfish (qq.v.) species can be extremely painful to humans and may cause paralysis, shock, and even death.

Nematocysts are a type of cnidae, and it is the presence of cnidae that separates jellyfish and other cnidarians from other animals. Cnidae are among the most complex intracellular secretion products known.