Nematocysts are specialized capsules that serve as homes for special cells called cnidoblasts, which typically contain poison or venom and help species within the phylum Cnidaria drive away enemies or catch prey. Nematocysts are relatively small cells and are found only in cnidarians, such as jellyfish, sea anemones and corals. Although small, nematocysts serve an important role in helping cnidarians feed and survive.
Inside each nematocyst is a special cell called a cnidoblast, which contains a small thread, typically filled with venom or poison, that launches into action when stimulated by external cues, such as chemical or mechanical triggers. These small threads protrude through the closed coverings of nematocysts, which look and act much like container lids or garbage can tops. When stimulated, the threads within the cells generate enough force to open the protective lid formed by the nematocyst, then eject from the surrounding capsule with a forceful twisting motion.
After leaving the nematocyst, these tiny threads generally travel at high speeds into the nearby object, which may be either a predator or prey. Many threads contain potent toxins that set to work paralyzing tissues within the targeted organisms. These toxins are quite powerful, and produce severe pain upon injection, and may even cause shock or death in humans.