"are secreted by the of all ns and only cnidarians. Of the three categories of cnidae (also called cnidocysts), s occur in all cnidarians, and are the means by which cnidarians defend themselves and obtain prey; s and s are restricted to a minority of major . A cnida discharges by eversion of its tubule; venom may be associated with the tubule of a nematocyst. About 30 major morphological types of nematocysts are recognized, but no single nomenclature for them is accepted."