Nematocysts (Gk. nemato, thread + cystos, cell) are individual cells (unique to the phylum Cnidaria) usually located on the outer surface of the organism. These cells are also known as stinging cells sometimes used to inject toxins (greenish area of the animation) which, in some cases are toxic to man (e.g. Marine stinger = Chironex fleckeri). Nematocysts are most abundant on the tips of feeding tentacles. The individual nematocyst rarely exceed 50μ in size, but their great number make them quite effective.

 

Purpose: The various functions include: sticking to surfaces and wrapping around objects, penetrating surfaces or secreting proteinaceous toxins. These functions are used in food collection, defense and to some extent in locomotion (e.g. anemones). Mechanism: To discharge the nematocyst of the cnidocyte, a cilium protruding from the surface (the cnidocil) is used as the triggering mechanism. Once stimulated, the lid (operculum) springs open in an explosive manner and the coiled tube turns inside out and pierced into the dermal layer of the prey. The blades and barbs act as inverted "tweezers" thereby enabling easier penetration of the shaft into the victim.