Venom is ejected via a nematocyst

Venom is ejected via a nematocyst, a whip-like hollow tubule, which lies coiled under high osmotic pressure. When mechanical or chemical stimuli activate an external trigger, the lid of the cell pops open and sea water rushes in. This forces a microscopic barbed harpoon to shoot out, penetrate and inject venom into its victim. Nematocyst discharge can occur in less than a millionth of a second, making it one of nature's fastest biomechanical processes.