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 that are attached to each other by their outer skeletal walls. At some places they have formed coral reefs that are hundreds of miles long. The solitary corals are referred to as horn or cup corals. In colonial forms 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. Fossil corals are most common in limestone, where they sometimes make up a large part of the rock, but they also are found in shale and sandstone.