One of the peculiar stinging, cells found in Coelenterata (one of the major phyla of invertebrates, to which such forms as jellyfish belong); a nematocyst; a lasso cell.

A nematocyst is a stinging mechanism used for defense and to capture prey, by Hydra and other members of the Cnidaria (Coelenterata). It is located within a specialized cell, the nematocyte, and consists of a capsule containing a coiled tube.