Corals are animals that belong to the phylum Cnidaria, which also includes creatures such as sea anemones, jellyfish, and Hydra. Because they create skeletons out of calcium carbonate, corals have a very good fossil record, but their soft-bodied relatives like jellyfish do not. Modern corals are known as Scleractinians; some are solitary, while others form vast colonies. Fossil corals species can be very difficult to differentiate. One reason is that their growth form may be closely tied to the water energy in their surrounding environment, so otherwise identical corals that grow up under different conditions may have very different appearances This is called ecophenotypic variation. Because of this, special attention needs to be paid to the fine structure of individual corallites: the places where individual coral polyps live in the colony.