Hexacorallia is a subclass of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are colonial and reef-forming, as well as all sea anemones, tube anemones, and zoanthids, ranged within six extant orders. The hexacorallia are distinguished from the other subclass of Anthozoa, Octocorallia, in having six or fewer axes of symmetry in their body structure and only single rows of tentacles. These organisms are formed of individual soft polyps which in some species live in colonies and can secrete a calcite skeleton. As with all Cnidarians, these organisms have a complex life cycle including a motile planktonic phase and a later characteristic sessile phase. Hexacorallia also include the significant extinct orders of the rugose corals and tabulate corals