Corals are small, brightly colored marine animals that look much like flowers. The animal grows an external stony skeleton, connected on the inside with radial partitions that divide the body into chambers. The animal itself is called a polyp, and the skeleton is called coral.
Some corals live together in colonies made up of hundreds of individuals attached to one another by their outer skeletal walls. At some places, they form coral reefs hundreds of miles long.
The skeletons of solitary polyps may be shaped like cushions, horns, or tubes, each with a depression in the top in which the animal lived. The solitary corals are referred to as horn or cup corals.
In colonial polyps, the skeletons may be either branching or closely packed and massive. Corals live mainly where the seas are warm and shallow. They are numerous in today's tropical seas. The animals have been common throughout geologic time, so it is easy to collect fine specimens in almost any part of Illinois.