Figure 2: All corals have skeletons made of calcium carbonate, the same material that composes sea shells. This diagram shows the various forms of coral skeletons. Tropical reef corals secrete their skeletal material in the form of cups that hold each polyp. As the colony grows, more calcium carbonate is secreted below the polyp. In soft corals and gorgonians, the calcium carbonate occurs in the form of small needles, flowers, dumbbells, or plates, all with the general name sclerite. The sclerites are usually embedded in the outer layer of tissue around the polyp but can also be found throughout the colony. They may be arranged in a variety of ways, a typical one being in the shape of a V-shaped bar. In some groups, the sclerites occur as plates on the outside of the colony, forming protective cups around the polyps. No matter what the shape or arrangment, most are truly beautiful and intricate structures.