Trichocysts

Trichocysts (from ancient Greek τρίχα - hair + σύστημα - thread; “thread-like fibers”) are nematodes more than 2 mm long; some species live in the intestines of invertebrate animals. Touch causes contraction even in intact stinging threads (a person who accidentally touches it experiences a short-term painful spasm).

These organs function identically both in representatives of the class of roundworms and in representatives of flatworms.