CORAL AND CORAL ISLANDS, the solid support or skeleton of the coral polyps. (See POLYPS). It was formerly supposed that the coral was a calcified portion of the soft parts of the animal, but this has been disproved. Recently Bourne has proved by examination that the skeleton is forrned as a secretion by certain cells and that there is no deposition of crystalline carbonate of lime in the actual cells. The calcareous septa or partitions are deposited by the soft septa of the animal in the radial chambers. The coral particles begin to be deposited in the embryo polyp before it becomes fixed to the bottom. In the very young polyp of the Mediterranean Astroides, as soon as it becornes stationary 12 calcareous parti tions are deposited, and these enlarge and finally become jointed to the external walls (theca) of the coral, forming a groundwork or pedestal, on which the young polyp rests, as if on a limestone foundation. If isolated needles or rods of lime are distributed beneath the outer layers of the polyp-stock, the latter be comes horny or leathery and more or less flexible, as in the alcyonarian IdoIyps. The en tire slceletal mass is called the corallium or coral-stock. In the common red coral (Coral lium rubrum) of the Mediterranean, the solid unjointed coral-stock has a thin cortical layer of spicules in which the polyps are retractile.
Coral-like masses, encrusting reefs, are also formed by the animals of the hydroid millepore, which may be distinguished by the multitude of minute cells or openings, much smaller than those of any genuine coral polyp. See MILLE PORE.
The simpler, most primitive corals, are cup shaped, forming a single calicle or corallite, con taining but one polyp, as in the early Paleozoic cup-corals, and the existing species of Flabel !um, Caryophyllia and Dettocyathus, which live buned in the mud in deep cold water in all seas, from Greenland to the tropics.
Microscopic Structure of There are two kinds. In the aporose or poreless corals such as Flabellum, Astrma and the like, the coral-stock is throughout its mass solid and stony. In the perforate kinds, such as Madre pora, all parts of the corallium, including the connecting ccenenchym, or common calcareous stock or stem, has a structure like a meshworlc, consisting of delicate rods of carbonate of lime so united as to leave interstices, which in the living coral are traversed by a network of inter lacing tubes, representing the ccenosarc.
Rate of Growth Little is Icnown regarding the rapidity of growth in corals. A specimen of Mceandrina labyrinthica, measur ing a foot in diameter and four inches thicic in the most convex part, was taken from a block of concrete at Fort Jefferson, Tortugas, which had been in the water only 20 years. Hunt calculated that the average growth of a Mceandrina observed by him at Key West was half an inch a year. Verrill states that a Madrepora found growing on the wrecic of the ship Severn grew to a height of 16 feet in 64 years, or at the rate of thfee inches a year. See
POLYP.
Coral The reef-building corals are the Madrepora and Mceanclrina, the latter dome shaped and massive. They are confined to waters in which through the coldest winter months the temperature does not fall below 68' F., though usually the water is warmer than this, the mean annual temperature of the north Pacific being about 73.5° F., and of the south Pacific 70° F. Coral reefs are abundant in the West Indies and occur on the coast of Brazil as far sopth as Cape Frio, but still more so on the central Pacific. None occur on the western coast of Europe and Africa and the two Amer icas. In depth reef-building species do not oc cur below 15 to 20 fathoms. Coral reefs are divided into fringing, barrier and circular reefs, or atolls. Fringing reefs encrust the shores on which they grow. Barrier reefs have a lagoon between the reef and the main land. A fringing reef may ultimately become a bar rier reef either by a sinking of the land and the successive upbuilding of the seaward face of the reef, or by gradual dying of the coral polyps next to the land where they do not get enough fresh water and food, the stony skele ton being redissolved while the outer part of the reef continues to grow. Atolls are roughly circular, with a central lagoon, with or without central islands. Contrary to the views of Darwin and Dana, it has been found by A. Agassiz that modern coral reefs form as a rule but a thin crust, those of Florida not more than from 50 to 75 feet in thickness; yet Tertiary reefs and those geologically older may be much thicker. According to Darwin and Dana atolls were supposed to have been formed on sub marine banks over a subsiding sea-bottom, first starting as a fringing reef about an island or slightly submerged peak. After they had grown to the surface, the inner face of the reef could not get food, and ceased to grow. As the island or peak gradually sank, the outer face of the reef grew up in a ring. Owing to the observation of Semper, Agassiz and others, it is now evident that atolls may be deposited under any conditions, whether the sea-bottom be stationary, rising or subsiding. We will, however, have to hold on to the Dar win subsidence theory to account for coral reefs over about 100 feet in thickness, since corals do not grow at greater depths than this. From his prolonged explorations in the West Indies and the Pacific Ocean, A. Agassiz concludes that the barrier reefs of Fiji, the Hawaiian Is lands and the West Indies usually flank vol canic islands and are underlaid by volcanic rocks. Those of New Caledonia, Australia, Florida, Honduras and the Bahamas are under laid by outliers of the adjoining land-masses, which crop out as islands and islets in the outer edge of the barrier reefs. Some of the barrier reefs of the Society, Fiji and of the Carolines, show that the wide and deep lagoons, separating them from the main islands, have been formed by erosion, from a broad, fringing reef-flat, as explained above.