|A stony coral, Acropora latistella|
Hexacorallia is a subclass of Anthozoa comprising approximately 4,300 species of water-based organisms formed of colonial polyps generally with 6-fold symmetry. This includes all of the stony corals, which are vital for coral reef formation, as well as all sea anemones, tube anemones, and zoanthids within six extant orders. They are distinguished from the other subclass of Anthozoa, Octocorallia, in having six or fewer lines of symmetry in their body structure and only single rows of tentacles. These organisms are formed of individual soft polyps which in some live in colonies and can secrete a calcite skeleton. As with all Cnidarians these organisms have a complex life cycle including a motile phase when they are considered plankton and later characteristic sessile phase. It also includes the significant extinct orders of the rugose corals and tabulate corals
Hexacorallia is considered to be monophyletic, that is all contained species are descended from a common ancestor, however it has been suggested that many of the current orders are not. Historically Ceriantharia and Antipatharia were considered to be a separate subclass called Ceriantipatharia, though more recent genetic studies place them in Hexacorallia with Ceriantharia as the oldest, or basal, group.
The subclass includes important coral reef builders the stony corals (Scleractinia), sea anemones (Actiniaria) and related tube-dwelling anemones (Ceriantharia), and zoanthids (Zoantharia). Antipatharia contains the black corals and Corallimorpharia are similar to anemones. The extant orders are shown below:
A number of extinct orders of corals have been classified as their calcium skeleton forms a prolific fossil record. These are generally thought to be close to the ancestors of modern Scleractinians and existed during the Paleozoic Era 541-242 million years ago:
- Numidiaphyllida †
- Kilbuchophyllida †
- Heterocorallia †
- Rugosa †
- Heliolitida †
- Tabulata †
- Cothoniida †
- Tabuloconida †
Hexacorallia or Zoantharia
The terms Hexacorallia and Zoantharia have been used synonymously to avoid confusion with some of the extinct species not showing sixfold symmetry. For example, Rugosa † is also called Tetracorallia, for its fourfold symmetry. Therefore, Hexacorallia might appear misleading, and Zoantharia is sometime preferred in paleontological contexts. However, with Zoantharia being the name used since 1899 until recent days  for the hexacorallian order Zoantharia, the use of the term Hexacorallia is strongly recommended[by whom?] in order to avoid confusions.
- Daly, M.; Brugler, M.P., Cartwright, P., Collins, A.G., Dawson, M.N., Fautin, D.G., France, S.C., McFadden, C.S., Opresko, D.M., Rogriguez, E., Romano, S.L. & Stake, J.L. (2007-07-21). "The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus". Zootaxa 1668: 1–766. ISSN 1175-5326.
- Oliver, W. A., Jr. (1996). "Origins and relationships of Paleozoic coral groups and the origin of the Scleractinia". In G. D. J. Stanley (ed.). Paleobiology and Biology of Corals. Columbus, Ohio: The Paleontological Society. pp. 107–134.
- Ben Kotrc (2005). "Anthozoa: Subgroups". Fossil Groups. University of Bristol. Retrieved 2009-03-23.
- Carlgren O. 1899 Actiniaria und Zoantharia. Symbolae Physicae supp. pp.13-16.
- Sinniger F., Häussermann V. 2009 Zoanthids (Cnidaria: Hexacorallia: Zoantharia) from shallow waters of the southern Chilean fjord region with the description of a new genus and two new species. Org. Div. Evol. 9:23–36
- Reimer J.D., Todd P.A. 2009 Preliminary molecular examination of zooxanthellate zoanthid (Hexacorallia, Zoantharia) and associated zooxanthellae (Symbiodinium spp.) diversity in Singapore. Raffles Bulletin of Zoology. (in press).