Reef-building polyps are confined to the tropical seas, where the winter
temperature is not below sixty-eight degrees. Coral formations are most
extensive in the Pacific Ocean, especially south of the Equator, and in
the two great archipelagoes of the East and West Indies; but a large
number of coral islands also occur in the Indian Ocean. The Coral Sea,
east of northern Australia, is particularly remarkable for the great
extent of its coral reefs.