Stinging-cells, or CNIDOBLASTS or NEMATO-CYSTS, are characteristic of all Cœlenterata except Ctenophores. To them the jelly-fish, Portuguese-man-of-war, sea-anemones, and the like owe their power of stinging. They protect their possessors against some of their enemies and they serve to benumb or kill the small animals on which most of the Cœlenterates feed. Each stinging-cell contains a long coiled lasso or cnidocil bathed in poisonous fluid; at the base of the cavity in which the lasso lies there is a little living matter and a nucleus; projecting from the surface there is often a small trigger-like peak. When the cell is stimulated, in some cases at least by nervous impulse from adjacent nerve-cells, the lasso, which is many times the length of the cell, is rapidly everted. After this has taken place the cell dies. Often thecnidoblasts are grouped in little 'batteries' especially abundant on the tentacles or similar structures. They are usually situated on the external ectoderm, but are sometimes endodermic. Similar cells occur in some Turbellarian Worms. See CELENTERATA, HYDRA.
