Quick aside, fun with word roots: The nema root means thread (Greek origin), while cnida is Latin for nettle. Science often has several terms for the same thing, one Greek in origin and one Latin. Consider the hormone that increases heart rate, epinephrine or adrenaline, they are the same thing. So named because it is released from the adrenal glands, which sit on the kidney (ad = near; or next to, renal = kidney. This is the Latin word root version. Where as epinephrine mean basically the same things (epi = on, nephros = kidney. From the Greek word roots). Back to cnidocytes (cyte BTW means cell).
The cell is called a cnidocyte, and most often the structures inside are called cnidae (plural for cnida); some specific types of cnida are called nematocysts. Cnida that penetrate are called penetrant nematocysts, while others are spirocysts or ptychocysts. The nematocysts variety are the most familiar and the ones I’ll be considering here. They are the pesky barbed tubes that are the reason people don’t like jellys.
A cnidocyte looks something like this:
Be wary of jellys that have washed up on the shore because the cnidocyte is rigged and ready to go. Even if the jelly is dead the cnidocytes can be active.
Cnidarians, the phylum that contain the jellies, sea anemones, and corals are a primitive group. Yet these relatively simple animals produce toxins that form holes in cells, or act as neurotoxins, vasodilators, or enzymes. A huge array of different types of chemicals. This is quite amazing considering the status of cnidarians as “simple” among the animals. Maybe they aren’t so simple after all.
The pore forming toxins, do just that. The molecules make their way into and through the cell membrane leaving holes. Not only will the integrity of the cell be compromised but other molecules inside the cells, the cell juice, will rush out. You do not want to lose your cell juice.
Neurotoxins usually mess with sodium or potassium channels of neurons, the functional cells of your nervous system. An exchange of sodium and potassium are vital to the function of a neuron. If the channel is blocked by a chemical (the toxin) then function is impaired. If too many channels are blocked then functions stop and ....well, in order for you to function your nerves need to function.
Vasodilators cause the opening of blood vessels. Vasodilation causes blood pressure to drop. If too much of this happens, you can guess this next part.
Enzyme toxins actually digest the tissue they contact. Some of them digest blood cells; I think that might be a bad thing.
The complexity of cnidarian venoms (and venoms of other organisms) may have important medical uses. Most often they are being investigated for analgesic (pain relief) properties but other possibilities, such as treatment of neurodegenerative diseases such as dementia or insulin resistance, are also being targeted.
Finally let me include something from the animal-using-other-animal's-parts department:
Some species of nudibranchs, sea slugs, can eat and use these cells for their own protection. Managing, somehow, to get the cnidocytes out of the digestive system safety (that is without firing) and up into the soft projections on its back (called cerata). The stinging cells are used for protection, firing into potential predators of the sea slug.
The sea slug manages to get the nematocyst into the tops of their cerata, white tips in the drawing below, called cnidosacs.
Next time you’re at the beach do not eat the jellys and try to sequester the nematocyst for yourself. I said do not!
Further Reading:
Greenwood, P.G. 2009. Acquisition and use of nematocysts by cnidarian predators. Toxicon 54: 1065-1070.
Jouiaei, M., A.A. Yanagihara, B. Madio, T.J. Nevalainen, P.F. Alewood, and B.G. Fry. 2015. Ancient venom systems: A review of cnidaria toxins. Toxins 7: 2251-2271.
Schlesinger, A., E. Winter., and Y. Loya. 2009. Active nematocyst isolation via nudibranchs. Marine Biotechnology 11: 441-444.