How do nematocysts fire?

Nematocysts, which are specialized capsules produced by cnidarians, fire in response to specific mechanical or chemical cues.12

When a cnidarian contacts potential prey or a predator, a lidlike structure on the capsule pops open. This triggers the explosive eversion of a coiled, hollow tubule—often armed with barbs—from within the capsule. As the thread turns outward with a twisting motion, the barbs act like a drill to penetrate the target.12

Once the tubule has successfully attached to or penetrated the object, toxins contained within the capsule are injected through the hollow thread to paralyze the victim. Alternatively, some tubules function by wrapping around and ensnaring prey. Each individual nematocyst can be fired only once.12