Corals of the sea

   For nearly half a billion years tiny animals called corals have lived in the earth's warm seas. We know about the corals of long ago because they left behind them millions upon millions of the rock "houses" they built for themselves.
   Corals are simple animals, with no bones, no heads, and no legs. They wave food into their mouths with a circle of tiny feelers. Corals look much like little flowers, And as a rule they grow much as plants do. New animals branch off from old ones. Soon a whole colony is built up. Each animal takes in lime from the water and walls itself up with it. The older animals die, but their limestone houses remain.
   There have been so many billions of billions of corals that in many places in the Pacific Ocean coral reefs rise in circles above the surface of the sea. These circles are called atolls. Many islands of the Pacific are bordered by coral reefs.
   There are many different kinds of coral. The tiny animals themselves are all much alike. But the rock houses they build are quite different in shape. Brain coral, organ pipe coral, and staghorn coral are common kinds. Precious coral is red. Beads are often made of it.