The coral resembles the sea-anemone and is a polyp constructed on the radial plan (page 113). It lives in colonies, but, unlike the hydroid colonies, each polyp of the community is a complete organism, and in the reef-building corals all the individual polyps of a colony are alike. The home of each animal is called a /corallite/, and the aggregation of many corallites is a /corallum/. The corallite is composed of carbonate of lime secreted by the polyp, and, broadly speaking, may be called a skeleton. The secretion forms a basal plate and radiating partitions between the mesenteries in the cavity of the animal, and also surrounds the polyp like a cup. In some species this is a solid substance; in others it is like a network through which the animal substance (cœnosarc) of the zoöids of the colony is connected, as in /Madrepora/. The polyps reproduce by budding and by self-division (fission), in a manner similar to that of sea-anemones. The way in which the budding or the fission takes place determines the shape of the colony, or corallum, which has a great variety of forms. In some species the budding is confined to certain individuals of the colony. In this case the branched forms result. When growth takes place by fission, hemispherical masses are formed, which are often perfectly symmetrical, as in the so-called brain-corals. In /Astræa/ the polyps are inclosed separately, but in /Meandrina/ fission is confined to the upper half of the polyps, so that a complex polyp is formed, with several mouths opening into a common stomach, making long serpentine furrows on the corallum. Some of the genera of the second subdivision, the alcyonarians, differ materially from the ordinary idea of corals, as their framework is of a horny and more or less flexible material (chitin). Of these are the sea-fans, sea-whips, and sea-pens. The organ-pipe coral of the same subdivision is an example of an exceedingly fragile lime structure, while the red coral, /Corallina rubrum/, [pg147] the species so much used for ornamental purposes, has an exceedingly hard and stony character. The madrepore corals are called reef-builders, but not in the sense of constructors of reefs. They do not erect definite structures as bees do. It is the aggregation of the skeletons of the dead polyps, together with other agencies, which forms a reef. The coral polyps, though so minute individually, are almost infinite in numbers. It is estimated that the colonies rise one half of an inch in ten years.