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The black or thorny corals (Antipatharia) make up an order of about 230 recognized species, many found in deep water. The colonies often grow in whiplike or branching tree-like formations up to 6 meters tall, supported by a skeletons are made of a hard protein called antipatharin. The skeletons of many species contain dark pigments, and most produce spine-like structures on the surface of the skeleton. Usually the polyps have six nonretractable tentacles, but they can have multiples of six up to 24, arranged in an irregular branching pattern; this is key to distinguishing these corals from the gorgonian corals (octocorals, having eight tentacles), which they otherwise resemble (Kozloff 1990).