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nematocyst

A structure found within a specialised cell, the nematocyte (also known as cnidocytes, or cnidoblasts), and 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.

involved in the stinging mechanism for defense and prey-capture that is unique and present in all animals of the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.).