A nematocyst consists of a pocket inside of which is a tiny spine connected to a minute supply of toxin (not necessarily mild, I have been stung by jellyfish). The pocket lies just beneath the skin of the nematode. There is a pore through the skin, and into the nematocyst. The action of the nematocyst is reflexive. When something brushes against the skin around the cyst, the pocket contracts and the spine is forced out through the pore and into the unsuspecting victim. The toxin flows through the spine. Nemtocysts are tiny. They can be found arrayed in vast numbers in the skin of nematodes.
Jellyfish are the most common users of nematocysts. This is how they defend themselves and prey on their meals. If you have ever brushed against a jellyfish, then the pain you feel is the result of hundreds or thousands of nematocysts going off and spitting a good bit of poison into your hide.
It's pronounced: knee-MAH-toe-sist, where the 'a' in MAH is the same as in "bat".
Huh?! ooh! ow! owowowowowow! crap! Does anyone have the meat tenderizer?