nematocyst

1. (biology) A capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing stingExamples: "Often one sees, on surface view, a distinctly rounded vacuole, occupying a position in the cell occupied in others by a large nematocyst."; "The young nematocyst decreases in size and becomes coarsely granular, the granules arranging themselves in a spiral line close under the external membrane."; "Nematocyst discharge is another example of the Rubicon principle – it either fires or it does not."Synonyms: cnida, cnidocyst, cnidocyte