Coral reefs are one of the oldest types of living systems on earth, and certainly one of the most spectacular. They are massive underwater structures formed by the limestone skeletons of tiny invertebrate animals. Reefs house a greater diversity of body forms, chemistry, and animal phyla (thirty-two compared to the eight that inhabit the most biodiverse ecosystems on land). Phyla comprise the second largest category of living things, after kingdoms. Coral animals begin life as free-floating larvae, but settle on the sea floor in sedentary colonies. The term "coral" applies both to these animals and to their ske