[Animal.] A minute creature, of low organization, which builds itself a framework of carbonate of lime, which is lined with the fleshy body of the living animal. Corals live in colonies in the warm seas, and their combined shells form great masses of coral rock, which in some regions become reefs or islands. They cannot exist at a depth greater than from 90 to 120 feet, and they also die by exposure to the air, so that from a depth of about 100 feet they work upwards until they reach low-water mark ; and when their progress is thus stopped in the upward direction, they begin to grow outwards, increasing on the outer edges of the reef, where they find food, carried by the ocean currents, most abundant. The action of the waves and the chemical action of the sea-water cement the materials into a firm steep slope. When a reef has been built on a shelving sea-bottom near a continent or round a volcanic island, the space of water inside is called the lagoon channel. When the reef has been built on a submarine ridge or peak, it forms a circular island, called an atoll, with a broad lake of sea-water inside, called a lagoon.