In the reef-building corals the living viscous substance that covers the surface and connects the polypes into a mass, is in process of time so completely consolidated by abstracting the small quantity of carbonate of lime that the sea-water contains, that little if any animal matter remains; and as this process is continually repeated, one generation of polypes perishes after another, the inert matter increases indefinitely, and the surface at which the consolidation is actually going on is the only part that is alive. The surfaces of the dense convex masses of many of these Astræan corals are entirely covered with deep hexagonal stars, whose rays extend upwards all round, and end in narrow, sharp, and elevated lines formed by the junction of the rays of the adjacent stars; in other species the rays are often crowded together, and the columella only shows a few points in the deep hollows. Through these deep cups the polypes protrude their circular disks and tentacles in quest of food, the nutritious products of which maintain the polypes as well as the general living fabric which unites them, and the refuse is ejected from their mouths; for each polype has an independent life of its own besides the incidental life that it possesses as part of a compound being. In many of the corals the polypes show great sensibility, shrinking into their cells on the slightest touch, yet no nervous system has been discovered. The variety of compact and branching corals far exceeds description: 120 species are inhabitants of the Red Sea alone, and an enormous area of the tropical Pacific is everywhere crowded with the stupendous works of these minute agents, destined to change the present geological features of the globe, as their predecessors have done in the remote ages of its existence. Four distinctly different formations are due to the coral-building polypes in the Pacific and Indian Oceans, namely, lagoon islands or atolls, encircling reefs, barrier reefs, and coral fringes, all nearly confined to the torrid zone.