Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that live on the seabed and build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc with a mouth and a fringe of tentacles. Although some species are solitary, most are colonial. The founding polyp settles on the seabed and starts to secrete calcium carbonate to protect its soft body. Solitary corals can be as much as 25 cm (10 in) across but in colonial species the polyps are usually only a few millimetres in diameter. These polyps reproduce by budding but remain attached to each other, forming a multi-polyp colony with a common skeleton, which may be up to several metres in diameter or height according to species.