Nematocysts are microscopic spherical chambers which hold a small barb that are exclusive to Cnidarians (jellyfish, coral, anemones). Nematocysts are found on the bodies or tentacles on Cnidarians. Each is produced by a special cell called a cnidoblast containing the barb which quickly turns outward 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 touch, a lid-like structure on the top of the capsule pops aside, and the thread everts explosively. As eversion and twisting proceed, the barbs act like a drill, penetrating into the object. If a toxin is present, it goes through the thread, penetrating the victim’s tissues. After eversion, the thread separates from the nematocyst. The threads of some nematocysts capture small prey by wrapping about them. The stinging effect of nematocysts in the Portuguese man o´war and some jellyfish species can be extremely painful to humans and may cause paralysis, shock, and even death.