Page:The American Cyclopædia (1879) Volume V.djvu/345

 CORAL 341 would be a significant name. Each leaf has a surface covered with polyp flowers, and was formed by the growth and secretion of these polyps. Clustered leaves of the acanthus and oak are at once called to mind by other species ; a sprouting asparagus bed by others. The mushroom rs here imitated in very many of its fantastic shapes, and other fungi, with mosses and lichens, add to the variety. Vases of madrepores are common about the reefs of the Pacific ; they stand on a cylin- drical base, which is enveloped in flowers when alive, and consist of a network of branch- es and branchlets, spreading gracefully from the centre, covered above with crowded sprigs of tinted polyps. The actinias may well be called the asters, carnations, and anemones of the submarine garden ; the tubipores and alcyonia form literally its pink beds ; the gor- goniae and melitaeas are its flowering twigs; the madrepores its plants and shrubbery ; and astrseas often form domes amid the grove a dozen feet or more in diameter, embellished with green or purple blossoms which stud the surface like gems, while other hemispheres of meandrina appear as if enveloped in a network of flowering vines." Over the surface of all these corals each depression is the site of a polyp ; and the radiated form of this cell cor- responds in its plates to the similar structure of the animal. As young polyps are produced, they communicate for a time or permanently with the parent stock, through the internal cavity, in some species each having in the early period of growth nothing externally to mark its separate existence but a new mouth and inci- pient tentacles. In a living polyp, the tenta- cles are expanded and made rigid by injection with sea water. When disturbed, the water is ejected, the tentacles contract and disappear beneath the margin of the disk which is rolled inward over them, and conceals also the mouth. In many even of the larger corals the living portion is but a thin outer part of the mass, the rest having become dead by the drying up of the tissues as growth went on. The various forms of coral are produced by as many species of polyps. A large number of kinds are de- scribed and figured by Prof. Dana in his great work " On Zoophytes," and a general review of the subject, illustrated by many figures, is ?'ven in his " Coral and Coral Islands " (New ork, 1872). Among the tribes of corals, some species or other are found in all oceans from the equator to the polar regions, and to the lowest depths explored by man. But the range of individual species and families is limited by the physical conditions of light, heat, pressure, <fec., appropriate to their organization. Those tribes which produce the great coral reefs, as the astrseas, madrepores, meandrinas, &c., are developed with peculiar luxuriance in the warmest parts of the Pacific, where the tem- perature varies from 75 to 85 ; but they are also found in waters the temperature of which during the coldest winter months does not fall below 68, and in other oceans and seas. Two isothermal lines of 68, one N., the other S. of the equator, near the parallel of 28, but vary- ing therefrom according to the marine currents and the vicinity of continents, will include all the growing coral reefs of the world. The higher the temperature, the greater is the pro- fusion and variety of the coral reefs. The range in depth of the reef-forming corals appears to be limited to 120 ft, and comparatively few are found below half that depth. This state- ment may at first sight seem inconsistent with the fact that coral is often found extending from a few feet to hundreds and even thousands of feet below the surface of the sea. Various theories have been advanced to account for this, but they have all been rejected, and the explanation first offered by Mr. Darwin is now very generally adopted by the scientific world. Mention has already been made in the article ATOLL of the coral reefs of the Pacific, where they are traced in barriers for hundreds of miles; and also of the annular form of many of them, the reef surrounding shallow basins, while the waters outside are often of unfathom- able depth. According to Mr. Darwin's theory, which is supported and more fully developed by Prof. Dana in his recent work above men- tioned, the bottom of the ocean where these atolls are found has been for ages slowly sub- siding, while the coral reef has pari passu been growing upward. Hence, while the living coral has never existed more than 50 or 100 ft. below the surface of the water, the coral rock, the product of former ages, exists at im- measurable depths. The dead corals and shells of the coral seas become ground up by the waves as they sweep over the reef, and thus the beds of coral debris are made which be- come by consolidation the coral reef rock. This reef rock differs in no important particular from the great limestone strata which are spread over large portions of the western states, and which testify by the corals they contain, and the other fossils associated with them, to a similar mode of production. The fine white mud which is made by trituration much resembles when dried the ordinary white chalk of Europe. It is often spread over the bottoms of the lagoons or channels, and is also drifted over the shallow seas outside of the reefs by currents. Throughout the long series of the geological formations produced by aque- ous deposition, back to those of the Silurian era, the agency of the marine polyps is traced in the production of limestones. At one pe- riod, in the lower Devonians, there were true coral reefs of great extent over the Missis- sippi basin, and along the northern borders of New England ; and the corals of those old reefs may be now seen making the old reef rock at the falls on the Ohio near Louisville, and near Lake Memphremagog, in northern Vermont. Unmistakable coral rocks of recent production are met with in the islands of the Pacific and Indian oceans, sometimes hundreds