Cnidarians Consist of Several Groups

Cnidarians consist of several groups that sometimes are divided into four or six categories. Most agree, however, that Anthozoa (corals), Scyphozoa (jellyfish), Cubozoa (box jellies), Hydrozoa (medusae, siphonophores, hydroids, fire corals) and Scyphozoa (true jellyfish) comprise the four main groups. Staurozoa (stalked jellyfish) and Polypodiozoa (a single specie: Polypodium hydriforme Ussow, 1885 - a parasite) may be added to the previous four. Their name comes from the Greek word "cnidos," meaning stinging nettle. Cubuozoa jellyfish have also been called "sea wasps."

All of these species are armed with stinging cells called nematocysts. They may have all inherited this characteristic from a single ancestor, yet as a group, cnidarians are extremely diverse and would not seem to be related at all. Some are attached to rocks, others float freely in the sea with stinging tentacles. Some have no tentacles, yet if touched emit stinging chemicals that are poisonous and may even kill a human who meets up with too many of them at one time. Another characteristic is common among them. They are all round, with parts of their body extending out from the center; they are "radially symmetrical." A third characteristic they all share is that they all have hydrostatic skeletons, whether or not they have mineralic or organic endoskeletons or exoskeletons (Shick 270).

The phyla Cnidaria live exclusively in water and are polyps, as the sea anemones, corals and medusae. Cnidaria, being polypoid or medusoid are biradially or radially symmetrical, and is uncephalized, with a single opening in its body, the mouth. Around the mouth are tentacles with microscopic capsules of stinging toxic nematocysts, which act offensively or defensively. This is what makes the phylum distinctive, this cnidae (or threads) (Fautin 5).

Sting of Cnidarians

The original reason for the sting is to capture and paralyze prey. The stinging cell is called a cnidocyte, which is located within a structure called a nematocyst. The nematocyst is the "stinger" and is shaped like a thread coiled and ready to strike and deliver the stinging toxin into the body of the prey. Some react to touch, others spring forward on their own, on an impulse from the animal which tells it to fire. Most deliver a smarting, harmless sting, but there are jellyfish which can deliver an extremely harmful dose of stings, which, with the wrong person, could become fatal. This form of fatality is common on the northern coast of Australia, where humans sometimes tangle with the jellyfish which live there in abundance. These jellyfish include the giant Lion's Main (Cenae Capillata), whose bell can reach 96 inches with tentacles as long as 98 feet in length. (Oceanside 3).

The capsule which contains the thread has a "hair trigger" which, when touched, makes the capsule explode as it shoots the poisonous thread out. The poison penetrates the prey or threatening animal and once it is poisoned, the tentacles of the cnidarian wrap around the prey and the prey is eaten with a mouth located in the very center of the tentacles. Only the clownfish is immune to the poisonous effect of the cnidocyte. The clownfish actually lives among the tentacles of the anemone and hydra cnidarians.

The stinging cnidarians (or cnidae) may be divided into three groups, based on the type of defense that they use. The stinging nematocysts, which are well-known and familiar, the spirocysts, which adhere themselves to rock, such as corals, and sea anemones, and the ptychocysts, which form tubes for themselves out of sand. Among these are the different variations of strength of their toxins. 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 jellyfish, the Portuguese Man o' War (Physilia), is really a colony of hydrozoans and is not a jellyfish. 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.