The stony substance of corals is chiefly carbonate of lime, which the polypes have the power of abstracting from the sea-water, combined with a small quantity of animal matter, and a still smaller quantity of phosphate of lime, with a trace of silver and magnesia. This stony substance takes the crystalline form of needles. By the successive deposition of these needles, a network is formed round the body of the animal, which by a series of these deposits is condensed into a hard impervious coat or wall. During this formative process many characteristic forms may be produced by division and building, depending upon the genus and species of the polype; but they do not lay eggs till they come to maturity. Some corals increase both by budding and division, but by far the greater number grow in size by budding, as the Astræa, which constitutes a portion of the reef-building corals of the tropical seas. They form groups, in which the whole of the polypes, except their starry summits, are soldered or pasted together by a living viscous substance, consolidated by carbonate of lime, abstracted from the sea-water, so that the resulting coral frequently becomes a rounded mass, the surface of which is more or less covered with stars, which may be circular or angular, large or small, deeply set or prominent, according to the genera or species, both of which are exceedingly numerous. In fact the forms produced vary according as the buds spring from the base of the polypes, from the sides of the cylindrical body, from the summit or disk, from the limits of these three parts, or from the whole animal. In all these varieties the buds are the result of a superabundance of vital activity in the part. When the buds proceed from the sides of the polypes the corals are rounded masses; but when they spring from the disk or cups of the star, the consequence is the death of the parent polypes, and the development of a new layer of living individuals above the dead ones. No part of the new polypes is seen except their stars, their bodies being enclosed in the common tissue. As this process may be continued indefinitely, the coral may increase to any size; but the size becomes still greater when successive buds are formed over every part of the polypes, and when all the successive generations are soldered together by the common tissue. In every case the polypes are alive only on the surface where they have free access to light, heat, and air, which is furnished by the sea-water in which they live.[31]