noun, plural: cnidocysts
A capsule containing thread-like tube (commonly barbed and toxic) found inside the cnidocyte of Cnidarians
Cnidocysts are a distinguishing feature of Cnidarians (hence the name). These capsules are found inside the cnidocytes, which are specialized cells found in their tentacles. These capsules will evert thread-like tube upon stimulation. The tube is barbed and often contains toxin, and thus delivers a paralyzing sting to capture a prey or to ward off enemies.