Once a planulae polyp is settled on a hard surface, the young polyp creates a horizonatal layer of aragonite called the basal disk. As the polyp grows upward, the base's margin also turns upward, forming a cup called the epitheca, which contains daily growth bands. These and other sturctures called septa form the skeletal boundaries found at the bottom of the coral polyps, which are left behind as a result of the upward growth of the polyp.
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