Stinging mechanism used for defence and prey capture by Hydra and other members of the Cnidaria ( Coelenterata). It is located within a specialised cell, the nematocyte consists of a capsule containing a coiled tube. When the nematocyte is triggered, the wall of the capsule changes its water permeability and the inrush of water causes the tube to evert explosively ejecting the nematocyst from the cell. The tube is commonly armed with barbs and may also contain toxin.

The cells at the tip of an anemones' (corals) tentacles which "sting" when touched.

fluid-filled capsules (some contain poisons) used to capture organisms or repel predators; present in large numbers in stinging cells; often injected into prey