Nematocysts are found in the phylum Cnidaria, which includes jellyfishes and sea anemone. Also called cnidocyte or cnidoblast, these are specialized cells found in the tentacles of jellyfish capable of projecting a thread-like structure as a form of self-defence or to capture prey. Most members carry venom and if stung, can cause excruciating pain or paralysis. These cells superficially resemble barbed wires capable of penetrating flesh in a few milliseconds.
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Further Reading: Coelenterata