Nematocysts are unique to the phylum; Cnidaria ( = Coelenterata)


Nematocysts are individual cells usually on the outer surface of the organism which have a variety of functions, most usually in defence or capture of prey species. These cells are known as stinging cells sometimes used to inject toxins which in some cases are toxic to man.


Nematocysts can be specialised to carry out a number of functions within the organism these include; Sticking to surfaces and wrapping around objects, Penetrating surfaces or secreting proteinaceous toxins. These functions are used in food collection,defense and to some extent, locomotion.


Nematocysts are usually most abundant on the feeding tentacles of all species, and within the digestive cavity of some species. The individual nematocyst rarely exceed 50 um (microns) in size, but is the great number that make them effective in providing protection or as a method to capture and stun prey species.
These cells are also important to the biologist in individual species recognition.

Nematocyst discharge is triggered by direct contact or other external stimulus. Once the cell is discharged a new nematocyst is formed as the mechanism in each cell can only be triggered once.