AbstractThe members of the phylum Cnidaria (corals, sea anemones, medusae) are all equipped with stinging cells (cnidocytes, nematocytes), which serve mainly in prey capture and defense. The secretory product of these cells is a most complicated extrusome consisting of a cyst containing a tubule and a liquid matrix. Mechanical stimulation of the cell's cnidocil apparatus by a prey or an offender leads via bioelectrical signal transduction to the explosive discharge of the cnidocyst. In stenoteles of Hydra this process, during which the tubule is everted out of the cyst, takes less than 3 msec. The forces involved are partly due to spring‐like tensions stored in the collagenous structural compartment, and partly to an osmotically generated intracapsular pressure, which can amount to 150 bar (1.5 × 107 Pa). The osmotic machinery depends on the presence in the cyst's matrix of inorganic cations (either K+, Mg2+ or Ca2+) and rare polyanions (poly‐γ‐L‐glutamates), which, so far, have not been reported from recently evolved eukaryotes. The discharging cyst acts like a self‐reloading syringe, injecting poison and other components into the target. Since the cnidocytes are incapable of regenerating their exocytosed cysts, they have to be replaced by new cells derived by differentiation from pluripotent stem cells (interstitial cells).