What is the function of the nematocysts in the cnidarians?

The stinging cell of a cnidarian contains a sac that houses a coiled up tube, immersed in venomous toxins. When triggered to fire by touch, chemical stimuli, and/or nerve excitation, a hinged lid opens, the tube rapidly shoots out of the sac by eversion (turning inside out), penetrates the skin of its victim, and introduces its venom along the entire length of the tubule. When large numbers of these stinging cells deliver a sufficient amount of venom, the prey (or predator) is paralyzed and can no longer cause damage to the fragile jellyfish.

The entire evagination process only takes a millisecond or two. These stinging cells are some of the most sophisticated in the entire animal kingdom. They fire only once and then are re-adsorbed, to be eventually replaced by a brand new stinging cell. New nematocyte stinging cells (containing the nematocyst sac organelle) are created near the base of a tentacle from interstital stem cells. They migrate down the tentacle while developing the complex internal nematocyst structure, and eventually distribute themselves into a species specific pattern along the tentacle.