This is a picture of a hydra tentacle. The cool thing is, if you look closely you can see a discharged nematocyst. This special cell is made by all members of the cnidarians (jellies, anemones, and hydras). It has a coiled up thread inside, that when triggered releases and stings the prey. When the trigger is ‘touched’ the bulb of the nematocyst is filled with calcium. This causes an osmotic reaction. In other words, the water from outside the bulb forces its way in to try and dilute the calcium. This causes the barb and ‘whip’ to evert. The firing of the nematocyst takes only a few microseconds.
After the nematocyst has fired, that cell is useless and falls off. New nematocysts are constantly being produced. This process is taken advantage of by some nudibranchs, who eat cnidarian tentacles. The mature nematocysts fire, but the immature ones are incorporated onto the back of the nudibranch where they finish maturing. These pilfered nematocysts can then be used to sting fishes trying to eat the nudibranch.
There are several types of nematocysts, depending on their function. There is a harpoon-like one used to inject venom, a sticky one for capture, a lasso-like one for capture and injection, and one that is only found in tube anemones which makes up the parchment structure of their tubes.