nematocyst /nem″ətōsist′/ [Gk, nema, thread, eidos, form, kystis, bag] , a capsule containing a barbed, threadlike process found in certain cells on the external surface of cnidarians, such as the Portuguese man-of-war and jellyfish. The nematocysts of some cnidarians can penetrate the skin and inject a poison, causing painful and potentially fatal injury.
