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That group of the Radiata whose members typically bear tentacles and possess intrinsic nematocysts. The name Cnidaria is also used for this phylum and is preferred by some because the name Coelenterata, as first used, included the sponges (Porifera) and the comb jellies (Ctenophora), as well as the animals called coelenterates. See Ctenophora, Porifera
The coelenterates are mainly marine organisms and are best known as jellyfish or medusae, sea anemones, corals, the Portuguese man-of-war, small polypoid forms called hydroids, and the fresh-water hydras. Taken together, the phylum is divisible into three classes as follows: (1) Hydrozoa, the hydroids, hydras, and hydrozoan or craspedote jellyfish (hydromedusae); (2) Scyphozoa, the acraspedote jellyfish; and (3) Anthozoa, the sea anemones, corals, sea fans, sea pens, and sea pansies. See Anthozoa, Hydrozoa
It is convenient to recognize two basic body forms in this phylum, the polyp and the medusa, into which all coelenterates can be classified. The polyp and the medusa, however, have many features in common (see illustration).
The polyp is a radially, biradially, or radiobilaterally symmetrical individual having a longitudinal oral-aboral axis and is usually sessile. The mouth is at the free end and is surrounded by one to many whorls or sets of tentacles which may be hollow or solid. The aboral end is commonly developed as an adhesive device for attachment and is conveniently referred to as a base. The central body cavity is the gastrovascular cavity, also called the enteron or coelenteron.
The medusa is a tetramerously or polymerously radial individual and is free-swimming. The body is usually bell- or bowl-shaped with the mouth suspended in the center of the underside of the bell on a stalk. Instead of directly surrounding the mouth as in the polyp, the tentacles are located at the margin of the bell. The outer or aboral part of the bell is recognized as the exumbrella and the under or oral part as the subumbrella. The mouth leads to the central stomach which in turn gives rise to four or more radial canals. These radial canals run through the umbrella, on the subumbrellar side, and commonly lead to a ring canal at the margin which is continuous around the margin.
The unique and most distinctive feature of coelenterates is the possession of intracellular, independent effector organelles called nematocysts, but also known as stinging cells or nettle cells. A coiled thread tube in each cell may be rapidly everted under proper stimulation and used for food gathering and for defense against predators, intruders, or enemies. Nematocysts are produced within cells called cnidoblasts. The morphologically simplest coelenterates, the Hydrozoa, have nematocysts limited to their outer epidermis whereas the more complex Scyphozoa and Anthozoa bear nematocysts in both the outer epidermis and inner gastrodermis.
The phylum is characterized by its carnivorous diet, made possible first by the possession of nematocysts which make the predaceous habit successful. After food has been trapped, movements of the tentacles carry it to the mouth, where with the help of ciliary and muscular devices the food is moved to the coelenteron. Here extracellular proteases prepare the way for final intracellular digestion. No herbivorous coelenterates are known.
The reproductive system of coelenterates consists of specialized areas of epithelia, the gonads, which periodically appear and produce gametes. There are no ducts for the sex products or any accessory sexual structures. Fertilization usually occurs in the water surrounding the animal, although a few coelenterates have their eggs fertilized in place and may then brood their young.
The ability to regenerate lost parts is characteristic of coelenterates. Pieces cut from almost any part of polyps will in time grow into new polyps. The regenerative powers of medusae are much less well developed, and not only will the excised piece not develop but it may not even be replaced by the medusa. Gradients of regenerative ability in polyps exist with the ability for a piece to reconstitute a new whole organism decreasing from the mouth to the base. See Regeneration (biology)
a phylum of primitively organized multicellular animals. Coelenterates are aquatic, mostly marine, and either solitary or colonial; only a few maintain a parasitic mode of life (for example, Cunina, suborder Narcomedusae). Individuals are either polyp-shaped and sessile or medusoid and free-swimming; however, this division is not systematic, since polypoid and medusoid generations alternate (metagenesis) in the life cycle of many coelenterate species.
The coelenterates have only a single gastral cavity, or coelom, either simple or divided into chambers or canals and used to digest food. The intestinal cavity communicates with the external environment through a mouthlike opening. The mouth is usually surrounded by tentacles, which are used to seize prey. Undigested food is eliminated through the same opening.
The body of a coelenterate consists of two layers of cells: ectoderm and endoderm. Between these layers there is a more or less developed interlayer of jellylike acellular mesoglea, produced as a result of the vital activities of the two principal layers. The ectoderm consists chiefly of epithelial muscle cells, performing protective and motor functions; characteristic stinging cells, or nematocysts, used for attack and defense; and undifferentiated interstitial cells, from which other cellular elements develop. The endoderrn is made up of epithelial-muscle and digestive-glandular cells. The digestion of food starts in the coelom and ends in the cells of the endoderm (intracellular digestion), which seize the particles of food with outpouchings of cytoplasm that resemble the pseudopods of protozoans.
The nervous system consists of a subepithelial neural plexus, somewhat denser around the mouth and in the tentacles. In medusae this plexus forms two loose rings of nerves along the edge of the body. Sensory organs—photosensitive eyespots and statocysts—are found only in the medusae. Statocysts are used to regulate the rate of motor contractions and as an organ of equilibrium.
Coelenterates are generally dioecious. In most, the gametes are released to the external environment, where they are fertilized and the characteristic larvae, or planulae, develop. A polyp (less commonly, a medusa) results from the metamorphosis of a planula. Fertilization is internal in a few species (for example, in the hydra). In some actinians, all developmental stages take place in the maternal organism; the young animals emerge through the mouth.
Vegetative reproduction—budding or strobilation, followed by transverse division—is also characteristic of many coelenterates. In the hydra and certain other forms the young animals, on reaching a certain stage of growth, separate from the parent. More often, however, budding results in the formation of colonies. Coelenterates are structurally varied, a result both of the differences between polyps and medusae and of the great diversity in shape of colonies. Many coelenterates are brightly colored.
There are about 9,000 extant species of coelenterates, about 500 of them in the USSR. Coelenterates are found in all seas, from the surface to the deepest levels, living both unattached in the water and on the bottom. They play a major role in marine biocenoses. All are predators, and many compete for food with commercial fish. Some coelenterates serve as food for fish.
The skeletons of reef-forming madreporic corals are used to obtain lime and as building stone. The red and black corals are highly prized in jewelry. Some species of jellyfish are salted and used as food in Southeast Asia. Some coelenterates (for example, Gonionemus) can badly injure swimmers by poisoning them with their stinging cells. Some tropical jellyfish can cause severe poisoning and even death.
Phylum Coelenterata includes three classes: Hydrozoa, Scyphozoa, and Anthozoa. Order Ctenophora is also sometimes classified with the Coelenterata.
The coelenterates probably appeared at the end of the Proterozoic; representatives of all classes have been found in Cambrian deposits. Stromatoporoids (Hydrozoa) and Tabulata and tetracorals (Anthozoa) flourished during the Ordovician and Carboniferous periods. Several groups of coelenterates became extinct at the end of the Paleozoic, but many new groups, similar to modern forms, appeared during the Mesozoic. All told, there are about 20,000 known extinct species. Most had a massive skeleton, and they contributed to the formation of thick layers of limestones. Many species and genera of Coelenterata are important in detailed stratigraphy.
REFERENCESNaumov, D. V. Gidroidy i gidromeduzy morskikh, solonovatovodnykh i presnovodnykh basseinov SSSR. Moscow-Leningrad, 1960.
Naumov, D. V. Stsifoidnye meduzy morei SSSR. Moscow-Leningrad, 1961.
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Osnovy paleontologii: Gubki, arkheotsiaty, kishechnopolostnye, chervi. Moscow, 1962.
Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.
Hyman, L. The Invertebrates, vol. 1. New York-London, 1940.
D. V. NAUMOV