Coral
Coral, small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral. Although most corals form colonies by budding, there are some solitary corals; in both types the individual animals, called polyps, resemble the sea anemone in form. Corals grow in warm and temperate climates and in the cold water found at greater depths, but they are most abundant in warm, shallow water; over 200 coral species are found in the Great Barrier Reef of Australia. In many shallow-water species the polyps contain unicellular plants, which may provide the high oxygen concentration required by such corals. Stony Corals In the large group known as stony corals, or true corals (Madreporaria), each polyp secretes a cup-shaped skeleton, the theca, around itself. Some solitary corals of that group may reach a diameter of 10 in. (25 cm); in the colonial forms the individual polyps are usually under 1