Hexacorallia

Hexacorallia is a class 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, and zoanthids, arranged within five extant orders.^[2] The hexacorallia are distinguished from another class of Anthozoa, Octocorallia, in having six or fewer axes of symmetry in their body structure; the tentacles are simple and unbranched and normally number more than eight.^[3] 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 order of rugose corals.

Hexacorallia Temporal range: Fortunian–Present[1] PreꞒ Ꞓ O S D C P T J K Pg N
A stony coral, Acropora latistella
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Subphylum:	Anthozoa
Class:	Hexacorallia Haeckel, 1896
Orders
See text.

Aspidiscus cristatus from the Cenomanian (Upper Cretaceous) of southern Israel; oral view.

Aspidiscus cristatus from the Cenomanian (Upper Cretaceous) of southern Israel; aboral view.

Phylogeny

See also: List of prehistoric hexacoral genera

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, Antipatharia was considered to be in a separate class called Ceriantipatharia, though more recent genetic studies place it in Hexacorallia.^[2]

The class includes important coral reef builders such as the stony corals, sea anemones, and zoanthids. The recognized orders are shown below:^[4]

• Actiniaria – sea anemones
• Antipatharia – black corals
• Corallimorpharia – corallimorpharians aka "false corals"
• †Rugosa – rugose corals
• Scleractinia – stony corals
• †Tabulata – tabular corals
• Zoantharia – zoanthids