Coral polyps are short-lived microscopic organisms, which live in colonies. They flourish in shallow, mud free and warm waters. They secrete calcium carbonate. The coral secretion and their skeletons form coral deposits in the form of reefs. They are mainly of three kinds - Barrier reef, Fringing reef and Atolls. The Great Bather Reef of Australia is a good example of the first kind of coral reefs. Atolls are circular or horse shoe-shaped coral reefs.