The poisons are dissolved in a highly concentrated (2 molar) saline solution contained inside a vesicle within the nematocyst. The vesicle is held shut under the tension created by a rigid molecular collagen structure with a strength equivalent to that of steel, Holstein explains.Link (via Digg)
When the tentacle touches its prey the lid of the nematocyst flips open, triggering the folded collagen spring to release outwards. This ejects a sharp spine, known as a stylet. The stylet pierces the external barrier of the prey with a pressure of more than 7 billion Pascals – in the range of that generated by a bullet fired from a gun.