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Most corals are made up of hundreds of thousands individual polyps like this one. Many stony coral polyps range in size from one to three millimeters in diameter. Most coral polyps have very simple bodies. Much of the polyp's body is taken up by a stomach filled with filaments that digest the food. Open at only one end, the polyp takes in food and expels waste through its mouth. A ring of tentacles surrounding the mouth aids in capturing food, expelling waste, and clearing away debris. Most food is captured with the help of special stinging cells called nematocysts, which are inside the polyp's outer tissues (called the epidermis). Calcium carbonate is secreted by reef-building polyps and forms a protective cup called a calyx within which the polyps sits. The base of the calyx on which the polyp sits is called the basal plate. The walls surrounding the calyx are called the theca. The coenosarc is a thin band of living tissue that connects individual polyps to one another and helps make it a colonial organism. Figure credit: NOAA
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