Coral are a marine invertebrate in the class Anthozoa of phylum Cnidaria. It typically live in compact colonies of many identical/individual polyps. The group include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.
A coral "group" is a colony of one myriad genetically identical polyp. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps.