[Cnidaria] Stinging organelle characteristic of the Cnidaria, it consists of a double-walled capsule, secreted by a particular cell called cnidocyte, containing a refringent fluid, a distal opeculum, and a coiled and folded tubule (shaft, thread or internal tube) which everts and straightens on discharge. Following the structure of the internal tube different types of cnidocysts are recognised, they are of great use in taxonomy. Cnidocyst are used for prey capture, defence, and attachment.

Alternative forms for cnidocyst : nematocyst, stinging cell.