A specialized cell found only in the ectoderm of the Cnidaria. It contains a nematocyst, a fluid-filled sac within which lies a long hollow coiled thread. When a small sensory projection (cnidocil) on the surface of the thread cell is touched, e.g. by prey, the thread is shot out and adheres to the prey, coils round it, or injects poison into it. Numerous thread cells on the tentacles of jellyfish produce their sting.