CNIDARIAN

also called coelenterate any member of the phylum Cnidaria (Coelenterata). Mostly marine animals, the cnidarians include the corals, hydras, jellyfish, Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans. The phylum Cnidaria is made up of four classes: Hydrozoa (hydrozoans); Scyphozoa (scyphozoans); Anthozoa (anthozoans); and Cubozoa (cubozoans). The nearly 9,000 species share several attributes, supporting the theory that they had a single origin. Variety and symmetry of body forms, varied coloration, and the sometimes complex life histories of cnidarians fascinate layperson and scientist alike. Inhabiting all marine and some freshwater environments, these animals are most abundant and diverse in tropical waters. Their calcareous skeletons form the frameworks of the reefs and atolls in most tropical seas, including the Great Barrier Reef that extends more than 2,000 kilometres along the northeastern coast of Australia. Only cnidarians manufacture microscopic intracellular stinging capsules, known as nematocysts or cnidae, which give the phylum its name. The alternative name, coelenterate, refers to their simple organization around a central body cavity (the coelenteron). As first defined, coelenterates included not only the animals now designated cnidarians but also sponges (phylum Porifera) and comb jellies (phylum Ctenophora). In contemporary usage, coelenterate generally refers only to cnidarians, but the latter term is used in order to avoid ambiguity. also called Coelenterate (phylum Cnidaria, or Coelenterata), any of about 9,000 species of aquatic, mostly marine invertebrates including the corals, hydras, jellyfish, Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans. The Cnidaria include some of the most beautifully coloured marine species, many having elegant and symmetrical body forms. Cnidaria are found both along the coastline (coral and sea anemones) and farther out in the ocean (jellyfish and Portuguese men-of-war). The cnidarians are the most primitive types of animals whose cells are organized into different layersthe ectoderm, or outer skin, and the endoderm, or inner skin. Between these two layers is a noncellular film called the mesoglea. Cnidarians are roughly cup-shaped, and the tissues composed of these two layers surround a cavity known as a coelenteron, or hollow gut. For this reason, the phylum is sometimes called the Coelenterata. The cnidarians are the only phylum whose members have only two layers of cells; besides the protozoans, sponges, and mesozoans, whose members have only one layer, the members of all other phyla also have a third layer of skin, called the mesoderm. The gastrovascular cavity, or coelenteron, is the basic internal organ of the cnidarians. There is no well-defined respiratory, circulatory, or excretionary organ apart from this, nor is there a well-defined head or nervous system. At one end of the body there is the mouth, the only opening into the coelenteron. Tentacles, which surround the mouth, radiate outward. The radial symmetry of cnidarian bodies also characterizes the ctenophores; for this reason, the two phyla are sometimes grouped together as the Radiata. Cnidarian bodies vary greatly in size from nearly microscopic, as in the case of some hydromedusae, to several feet in diameter, as with certain sea anemones, or more than a ton in weight, as in the case of certain jellyfish. The body may be of various colours. The tentacles surrounding the mouth are employed in the capture and ingestion of food. Cnidarians are carnivorous, and most feed on zooplankton. Others feed on small crustaceans, fish eggs, worms, and smaller cnidarians, while still others consume even larger animals, up to the size of small fish. The tentacles possess numerous toxin-containing capsules called nematocysts, which are used to paralyze the prey. When these nematocysts come into contact with another animal, a fine-pointed thread or hollow spear is ejected from each capsule, piercing the tissue of the prey and allowing the poison to penetrate. Many capsules are triggered at once, and the toxin is quickly replaced. A number of cnidarians have toxins powerful enough to cause severe rashes on the skin of swimmers who encounter them, and the toxins of a small number of cnidarians are lethal to humans. Once immobilized, the prey is conveyed to the coelenteron, where it is broken down and absorbed into the body. There are two basic body forms of cnidarians, the polyp form and the medusa form. The polyp is sessileit fixes itself to another body or surface. A polyp body is cylindrical, with the mouth and tentacles directed upward. Many polyps are colonial, occurring in bunches. Coral is the most notable example of a cnidarian colonial polyp. A coral secretes a calcareous exoskeleton to which other corals may also attach themselves. Atolls and reefs in certain parts of the world, including the 1,000-mile- (1,600-kilometre-) long Great Barrier Reef along the northern coast of Australia, are made of these skeletons. The other body form is the medusa, which is usually free-swimming but may also be colonial. The medusa body resembles a bell or saucer, with the tentacles hanging down from the base or margin. A medusa swims by a form of jet propulsion powered by successive contractions of muscles around the mouth. In certain cnidarian species, the members may be of either body form. This may happen within the same colony or sometimes even in alternating generations. The four classes of Cnidaria are Hydrozoa, Scyphozoa, Anthozoa, and Cubozoa. Hydrozoans are thought to be the most primitive of the cnidarians. They may be polyps, medusae, or both and may occur singly or in colonies. There are about 2,700 known species of Hydrozoans. Individual members are relatively small. This class includes the Portuguese man-of-war. Scyphozoans are more familiarly known as jellyfish. They are mostly medusae and generally range in size from 1 to about 16 inches (2 to 40 cm). There are about 200 known species, and they inhabit all seas, from coastal to deeper waters. Anthozoans are polyps, either solitary or colonial. These are found along coastal waters and include sea anemones, coral, sea fans, and sea pansies. This is the largest class of Cnidaria, with about 6,000 known species. This class is unique in that some of its members, such as various types of coral, secrete calcareous exoskeletons. Cubozoa contains only one order, Cubomedusae, which was, until recently, placed under the class Scyphozoa. Cubozoans belong evolutionarily between the Scyphozoa and the Hydrozoa. They take their name from the boxlike shape of the medusa, which is the dominant form of the class. Additional reading The standard technical treatment of the Cnidaria is Libbie Henrietta Hyman, The Invertebrates, vol. 1, Protozoa Through Ctenophora (1940). The history of the phylum as deduced from the fossil record is covered in Raymond C. Moore (ed.), Treatise on Invertebrate Paleontology, part F, Coelenterata (1956, reprinted 1967). C.E. Mills and R.L. Miller, Ingestion of a Medusa (Aegina citrea) by the Nematocyst-Containing Ctenophore Haeckelia rubra (formerly Euchlora rubra): Phylogenetic Implications, Marine Biology, 78(2):215221 (1984), offers information on the relationship between Ctenophora and Cnidaria. The following proceedings of international symposia on cnidarians provide useful information: W.J. Rees (ed.), The Cnidaria and Their Evolution (1966); Takasi Tokioka and Saburo Nishimura (eds.), Recent Trends in Research in Coelenterate Biology (1973); G.O. Mackie (ed.), Coelenterate Ecology and Behavior (1976); and P. Tardent and R. Tardent (eds.), Developmental and Cellular Biology of Coelenterates (1980). See also the following proceedings of international coral reef symposia: Proceedings of the Third International Coral Reef Symposium, 2 vol. (1977); The Reef and Man: Proceedings of the Fourth International Coral Reef Symposium, 2 vol. (1982); and French Polynesian Coral Reefs: Proceedings of the Fifth International Coral Reef Congress, 6 vol. (1985). The major justification of recognizing class Cubozoa is B. Werner, Bau und Lebensgeschichte des Polypen von Tripedalia cystophora (Cubozoa, class. nov., Carybdeidae) und seine Bedeutung fr die Evolution der Cnidaria, Helgolnder wissenschaftliche Meeresuntersuchungen, 27:461504 (1975). Cadet H. Hand, Jr. Daphne Gail Fautin