The cubozoan nematocysts have the most dangerous venom. Among the enemies of the cnidarians, for which their defenses have been built, are the nudibranch snails, who then seem to use the nematocysts for their own defenses. The cnidarians are very versatile and can modify their tentacles or polyps, depending upon the space in which they live and the presence of nearby competitors for it. These competitors, which may consist of sea anemonies, might induce the cnidarian to modify marginal tentacles or acrorhagi, so that they are able to drive off, injure or kill the intruder. The modification of these structures are under investigation (Elliot 57-72).

The cnida, or nematocyst, within the phylum, is contained within and then secreted when necessary by the Golgi of a cell called the cnidoblast. A cnida is the most complex secretory product known in biology. Once it receives an appropriate chemical and/or physical stimulus, the cnida fires, throwing out a tube many times longer than the capsule. The tube may carry a poison, may stick to its prey, or may become entangled with the object, depending on which type of cnida it carries. The cnida can only fire once. The three types of cnidae are nematocysts, ptychocysts and spirocysts. Nematocysts exist in all the Cnidarian classes, but some types of nematocysts are found only in certain members of the various classes. In Anthozoa are found the Spirocysts; since they are adhesive. Ptychocysts occur only in the Ceriantharia anthozoan order and are the restricted in their distribution taxonomically; their function being to gather bits of mud in their robust tubes to form on the exterior shell of these burrowing cnidae.

The most infamous...

The extent to which cnidarians use chemicals to defend themselves is in proportion to which the nematocysts are used by the animals. Some jellyfish are extremely dangerous, while most of the others may have painful stings, yet are not dangerous, unless the victim is allergic and suffers anaphylaxis, similar to the reaction to bee stings. Not only the largest and the smallest of jellyfish are threatening to humans, but the ordinary box jelly appears to present the most threat to human life.
It is possible that box jellies actually hunt down their prey. They have light sensitive organs (ocelli) or "eyes" and move very quickly (3 to 3.5 knots), compared to the true jellyfish, which drift aimlessly on the ocean. The box jelly's 24 eyes are located in the center of each side of its head, or bell in clusters of 16 pits of light-sensitive pigment. Only one pair in each cluster has a lens, cornea, retina, and iris which contracts in one of the eyes in each set. Only four of this creature's eyes can make out light levels, which help it to discern up from down by where the sunlight is on the surface of the water and it possibly cannot see anything else but blurred images.

The box jellies (Chironex fleckeri) are the most poisonous of cnidae; their powerful venom has caused at least 5,567 deaths, since such deaths started being recorded in 1884. Fatal envenomations are caused by the biggest box jellies, whose high concentration of nematocysts makes it most deadly (Williamson 279). But the tiny Irukandji jellyfish, the size of a penny, have been the cause of at least two deaths