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(organ-pipe coral), a genus of eight-tentacled, colonial coral polyps (Anthozoa) of the order Alcyonaria. The foundation of the colony is a skeleton of numerous parallel, dark-red limestone tubules that are up to 20 cm long and are joined by transverse platelike platforms. The corals resemble organ pipes. Inside the tubules are polyps with a bright-green crown of tentacles. Organ-pipe corals inhabit the tropical shallows of the Pacific and Indian oceans, usually among coral reefs.