Coral
Coral, common name for members of a large class of marine invertebrates characterized by a protective calcium carbonate or horny skeleton. This protective skeleton is also called coral. Corals are divided into two subclasses, based on differences in their radial symmetry (symmetry around a central axis). One subclass consists of colonial, eight-tentacled animals, each with an internal skeleton. Among them are whip corals, gorgonians, and the red coral used in making jewelry. Members of the other subclass commonly have six tentacles, or multiples of six, but other patterns are also observed. They include the stony, or true, corals. Another class of the same phylum also contains forms known as coral that are not considered here. STRUCTURE True corals secrete calcium carbonate from the bottom half of the stalk of the individual animal, or polyp, forming skeletal cups to which the polyps are anchored and into which they withdraw for protection. In the flattened oral disc at the top of th...