nematocyst nem·a·to·cyst (něm'ə-tə-sĭst', nə-māt'ə-)
A capsule within specialized cells of certain coelenterates containing a barbed, threadlike tube that delivers a paralyzing sting.
One of the minute capsules in the tentacles of cnidarians, such as jellyfish, hydras, or sea anemones, used for stinging. The capsule is produced by a special cell (called a cnidoblast) and contains a tightly coiled barbed thread that quickly shoots forth if the capsule's lid is disturbed. The thread often contains poison.