[Hydromedusae] Stinging organelle characteristic of the Cnidaria, consisting of a capsule secreted by a cell called a cnidocyte, containing a refringent fluid and a coiled and folded tubule (thread) which everts and straightens on discharge. According the structure of the internal tube different types of cnidocysts are recognised, this being an important taxonomic trait (Desmoneme cnidocyst, Microbasic eurytele cnidocysts, Macrobasic eurytele, Cnidocyst types). Cnidocysts are used for prey capture, defence, and attachment.

Alternative forms for Cnidocyst : Nematocyst, Stinging cell.