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i. Corals are marine invertebrates in the class Anthozoa of phylum Cnidaria.

ii. Coral comprises reef which secretes calcium carbonate to form hard skeletons.

iii. They flourish in shallow, mud free and warm waters. The coral secretion and their skeletons from coral deposits in the form of reefs: they are mainly of three kinds: barrier reef, fringing reef and atolls.

iv. The Great Barrier Reef of Australia is a good example of the first kind of coral reefs.