Scleractinia

Scleractinia, also called stony corals or hard corals, an order in the Cnidaria. It contains the polyps which build most coral reefs. 
Most of these corals are colonial. They secrete calcium carbonate, the base of every reef. Very often, the corals form a symbiosis with unicellular dinoflagellates. These zooxanthellae are usually of the genus "Symbiodinium". 
The closest relatives to stony corals are the sea anemones. Stony corals are only distantly related to fire corals and soft corals. There are about 1500 species, in 35 families.