Biology and Genetics

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 1/8 in. (3 mm) long, but the colonies may be enormous. The body of each polyp is saclike, consisting of a wall of jellylike material surrounding a digestive cavity, with a single opening, the mouth, at the unattached end. The mouth is surrounded by tentacles used to capture small prey and is invaginated to form a pharynx leading into the body cavity. Thin sheets of tissue (mesentaries) extend radially from the wall to the pharynx, dividing the cavity. A second set of radial divisions is created by...