rding to Darwin is the barrier reef. It is different thanthe fringing reef in one main way: the area between the living coral of a fringing reef andthe shore is the reef flat, but the area between the barrier reef and the shore is a lagoon(Stafford-Deitsch 21). The lagoon is a relatively shallow area of water, only a few metersin depth. However, its width can be enormous as in Australias Great Barrier Reef, someforty miles wide.The final type of reef distinguished by Darwin was the Atoll. It is a more-or-lesscircular crest of coral with a lagoon located at its center, only about a few kilometers indiameter. They are usually far from human influence and very deep, making them ideal fordiving. There might be areas on an atoll reef where sand is exposed to the air. The reefacts like a trap for sand and with the occasional storm, and persistent currents, banks ofexposed sand and small sand islands can be formed.Coral is formed in an astonishing way. The animals that form coral belong to thesame animal group as the hydras, jellyfish, and sea anemones. Most individual coralanimals, polyps, are less than one inch in diameter, but a small percentage of them measureone foot. A coral polyp has a cylinder-shaped body with a mouth surrounded by tentaclesat one end, and the other end is used to attach itself to the hard surfaces at the sea bottom. Most coral polyps live in colonies. The stony corals attach themselves to each other witha flat sheet of tissue that connects to the middle of each body. Half of the coral polypextends above the sheet and half below. The polyps build their limestone skeletons bytaking calcium out of the seawater. They than deposit calcium carbonate (limestone)around the lower half of the body. As new polyps grow, the limestone formation becomeslarger and larger.The polyps feed primarily on tiny swimming organisms like the larvae of manykinds of shellfish. Reef corals cannot live without algae. They use so...
