CCELENTERATA, (Neo-Lat. nom. pl., from calenteron, front (.l•:. noiXos, hollow + hrepov, enteral. intestine). One of the phyla of the animal kingdom, usually ranked as next to the lowest of the types of Aletazoa: the polyps and jelly-fishes. It is characterized by the absence of a body-cavity and a separate cir culatory system, both of these being functionally replaced by the system of chanffiers or tubes into which the mouth opens. I /wing to the fact that this system is not only digestive, hut that it takes the place of the body-cavity, it is some times called the calenteron. Shire commonly the aelenteron is called the gastro-vascular cavity. since it not only serves as a stomach for diges tion, but, by means of its branches, as a vascu la• or circulatory system in conveying food (and perhaps oxygen) to all parts of the body. All of the Crele»terata are more or less perfectly radially symmetrical, and generally on the number 4 or ri; that is, they can be divided in either four or six vertical planes, and the result ing halve: will be approximately similar. The Oelenterata may therefore be defined as radial ly symmetrical, unsegmented Aletazoa, having a gastro-vascular cavity. In most Cffilenterata there is no anus, waste matter from the digestive system hieing ejected through the mouth. Be tween the endodermal lining of the gastro-vascu lar canals and the ectodermal covering, of the body is a structureless layer, known as the supporting layer or `inesoglrea.' This may he very thin and firm, or it may be very thick and gelatinous. It contains scattered nerve and muscle cells, skeletal cells, and pigment-cells.
The month is usually surrounded with a circle of tentacles, which function both as organs of touch and as capturing organs. The number of these oral tentacles varies from two to sev eral hundred, and their size and shape vary quite as much. On these tentacles, and often on other parts of the body, there are batteries of remarkable stinging cells. (See NEMATOCYST.) These are wanting in only one class, the Cteno phores (q.v.).. The sense-organs of the Ccelen terata are very simple, as a rule, and many forms have no other sense than that of touch. Pigment eyes and positional organs of several kinds occur in the free-swimming forms.
The sexes are separate, and the eggs are set free and fertilized in the water. In their life history the Ceelenterata show some of the most interesting phenomena in the whole animal kingdom. To understand them we must remem ber that these animals not only reproduce by means of eggs. but even more frequently, like plants, by budding. and that oftentimes, as in plants, the buds do not become detached, but re main connected with the parent stock as long as they live. Moreover. in the type of Ccelenterata we have two very different sorts of individuals— those which are bell-shaped and free-swimming, known as (q.v.). and those which are more or less cylindrical, and are attached to ,oine object. known as polyps (q.v.). Now, it so happens that the polyp form of a species may give rise to a medusa by budding, and that me dusa break, away and becomes free-swimming; it gives rise to eggs which in turn develop into attached polyps. This process is simple alterna
tion. of generations (q.v.)—that is, each genera tion is like its grandparents, and not like its parents. The process may become simplified, until the medusa generation is entirely lost, by the medusa bud never getting free from the polyp and ultimately losing even the appearance of a medusa. Or the process may become very much complicated by additional generations pro duced by budding coming in between the others. The forms which produce the eggs are of course the sexual generations, while those that produce the buds are the uses/eat.
The interesting and important question as to whether the first CMenterata were medusoid or potypoid forms has been vigorously debated, but is by no means fully determined yet. Owing to the fact that the buds so often remain inti mately connected with the parent, we find more or less complicated colonies or stocks very com mon. These may be free-swimming, as in the siphonophores having then arisen from medusa•: or they may be fixed as in all corals (q.v.), and in such cases have arisen from polyps. In most such colonies, owing to the division of labor that has taken place, we have differently appearing individuals in one colony, some being adapted to locomotion. seine to cap turing prey. some to digesting food, some for de fensive purposes, and some simply for reproduc tion.
The Co?lenterata are a large group. of many hundred species, widely distributed in the oceans of all parts of the world. Only three or four species are known as occurring in fresh water. As individuals most of the species are small, though a few sea-anemones and some me duste reach a diameter of over a foot ; in colo nies, however, some of the reef-building corals cover an extraordinary area. As for color, the Cadenterata are among the most gorgeous of animal,: no shades are too exquisite or delicate. Nearly all are carnivorous, though perhaps some forms may use vegetable matter in part. The only species of any direct use to man are the precious and the lime-produeing corals.
In classification the Ccelenterata present dif ficulties. The ctenophores are so different in some important ways from other cadenterates, that some prefer to regard them as a separate type, especially since two genera are known that connect them directly with the flat wo•ms (Platoda) ; but as they are radially sym metric-al and have a gastro-vascular system, they really come within the definition of cadenterates. The type then may he divided into three great classes: (1) Hydro:oa (q.v.). which have no gastral filaments and have an eudodermal (esoph agus. if any; (2) Reyphozoa (q.v.). which have endodermal gastral filaments and give rise to medusa?: (3) Aetinona, or i.athozou (q.v.), which are polyp-form. possess mesenteries. and never give rise to tneduste: and (4) Ctraonhora (q.v.). distinguished by having eight meridional rows of swimming-plates.