Hexacorallia

Hexacorallia (synonym: Zoantharia; older synonyms: Hexacoralla, Hexaradiata, Hexactinia, Actinanthida; in English: hexacorals, hexacorallians, zoantharians) is a class of Anthozoa.

Both names, i. e. Hexacorallia and Zoantharia, and their English equivalents, also have narrower meanings - see below in the chapter Taxonomy. The name Hexacorallia also has a rare meaning unrelated to the content of this article - it is used as a synonym of the name Ceriantipatharia.

Hexacorals are aquatic organisms formed of polyps, generally with 6-fold symmetry. The number of hexacoral species is approximately 4,300 (as estimated in 1987). Extant hexacorals are composed of 6 orders: the tube anemones (Ceriantharia), zoanthids (Zoanthiniaria), sea anemones (Actiniaria), corallimorpharians (Corallimorpharia), the black corals (Antipatharia) and the stony corals (Scleractinia). The first four orders are skeleton-less, while the last two orders have a skeleton. Some of the Scleractinia are coral-reef builders.

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. 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 includes the significant extinct order of rugose corals.

The taxon Hexacorallia is considered to be monophyletic, that is, all contained species are descended from a common ancestor.

Certain or all skeleton-less hexacorals are called sea anemones in the literature. More specifically, sea anemones can mean one of the following:

The current system is shown below:

class (or subclass) Hexacorallia [aka: Zoantharia]: