CCELENTERATA. 114 CCELENTERATA. contains scatterod nerve and muscle cells, skeletal cells, and pigment-cells. The mouth 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 Coelen- 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. FEATURES OF C(ELENTERATA. 1. General anatomy : ;i, mouth; s, gullet; 6, cavity of the body (enteron) ; e, integument, composed of ectoderm and eudoderm, separated by a space; c c, convoluted cords (craspeda) containing thread-cells (see 2), and form- ing the free edges of the mesentery (m) ; 1 1, tentacles: r. reproductive organ contained within the mesentery. 2. Thread-cell or stinging-cell (nematocyst) of a hydra, un- discharged : a, formative matrix (cnidoblast): h, coiled thread within the cell ; c, trigger hair (cnidocil). 3. The same everted and discharged" by rupture of the cell follow- ing a touch upon the cnidocil. The sexes are separate, and the eggs are set free and fertilized in the water. In their life history the Coelenterata show some of the most interesting plienomena 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 Coelenterata we have two very different sorts of individuals — those which are bell-shaped and free-swimming, known as medusre (q.v.). and those which are more or less cvlindrical, and are attached to some object, known as polyps (q.v.). Now, it so happens that the polyp form of a species may give rise to a medusa l)v budding, and that me- dusa breaks away and becomes free-swinnning; it gives rise to eggs which in turn develop into attached polyps. This process is simple altenia- lioii 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 ase-riial. The interesting and important question as to wliellicr the first Coelenterata were medusoid or polypoid 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 (q.v. J. having then arisen from medusa»; or they may be fixed as in all corals (q.v.), and in such cases have arisen from ])olyps. 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, some to cap- turing prey, some to digesting food, some for de- fensive purposes, and some simply for reproduc- tion. The Coelenterata 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 fresli water. As individuals most of the species are small, though a few sea-anemones and some me- dusa; 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 C(elenterata are among the most gorgeous of animals ; no shades are too exquisite or delicate. Nearly all are carnivorous, though perhaps some fonns' may use vegetable matter in part. The only species of any direct use to man are the precious and the lime-producing corals. In classification the Coelenterata present dif- ficulties. The ctenophores are so dift'erent in some important ways from other coelenterates, that some zoologists prefer to regard them as a separate type, especially since two genera are known th.Vt connect them directly with the flat- worms (Platoda) ; but as they are radially sym- metrical and have a gastro-vascular system, they reallv come within the definition of coelenterates. The "type then may be divided into three great classes: (1) Uiidrozoa (q.v.), which have no gastral filaments and have an endodermal oesoph- agus, if any; (21 Scypho::oa (q.v.). which have endodermal gastral filaments and give rise to medusa;: (.3) Acthwzoa, or Antho-oa (q.v.), which are polyp-form, possess mesenteries, and never sive rise to niedusiP; and (4) Vienophora (q.v.). 'distinguished by having eight meridional rows of swimming-plates. FossTT, Forms;. The Coelenterata comprise some of the oldest fossils found in the Lower Cambrian rocks (Archaeocyathus and Etlimophyl-
