The stinging cell organelle that injects venom in jellyfish and other cnidarians is the nematocyst.
A nematocyst is a specialized capsule inside a cnidocyte, the characteristic stinging cell of cnidarians. Jellyfish, sea anemones, corals, hydroids, and related animals use these microscopic structures to capture prey and defend themselves. When triggered, the capsule rapidly everts a hollow, often barbed tubule into the target.
The discharge is extraordinarily fast and forceful. Pressure generated inside the capsule helps propel the tubule, which can pierce tissue and deliver venom. Some nematocysts also anchor the target, allowing the cnidarian’s tentacles to bring prey toward its mouth.
Terminology causes confusion because “cnidocyst,” “cnida,” and “nematocyst” are sometimes used broadly or interchangeably. More precisely, cnidocyst is the general capsule term, while nematocysts are the penetrant, venom-injecting type. Other cnidae include spirocysts, which entangle prey, and ptychocysts, which help build protective tubes. Cnidocytes are single-use cells and must be continually replaced.