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Coral polyp mouth. Close-up of the mouth of a hard coral polyp. The central mouth of the polyp is surrounded by a ring of tentacles which are normally retracted during the day. Each individual polyp is then embedded in a skeleton of calcium carbonate and is linked to the rest of the colony with tissue covering the outside as well as through channels set into the stone-like skeleton. Photographed in the Lembeh Strait, Sulawesi, Indonesia. (KEYSTONE/SCIENCE PHOTO LIBRARY/MATTHEW OLDFIELD)