The stinging cells present on the tentacles (and the bell of some species) of cnidarian. Each nematocyst consists of an coiled tube which may be bathed in venom. When the trigger on the outside of the nematocyst is touched, the tube rapidly inverts itself firing rather like a harpoon into the tissues of the prey. Venom on the outside of this tube is thus deposited in the tissues, and possibly into blood vessels during this trajectory. Finally, in some specialised nematocysts venom is then discharged through the open end of this thread tube and is deposited in the tissues o f the prey.
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