Unlike other phyla, cnidarians possess nematocysts, specialized cells that turn these seemingly passive animals into weapon-wielding underwater predators.
The word cnidaria comes from the Greek word cnida, which means nettle. Often mistaken for organelles, nematocysts are actually long, hollow threads coiled inside the body-sac or tentacles. Specialized cells secrete these harpoon-like stingers in response to chemical or physical stimuli. Like the feathers of a bird or the horns of a deer, nematocysts are not living material. They may be inanimate, but they are not ineffective. A cnidarian needs only to fire a tubule to release the nematocyst and pierce or ensnare its prey. This aquatic weaponry can also be used to obtain food, defend against predation, or in the case of polyps, adhere to solid substrates during locomotion.
There are three types of nematocysts, and while all species of cnidarians possess at least one form, an organism may possess several at one time.