| Hexacorallia Temporal range: | |
|---|---|
 | |
| A stony coral, Acropora latistella | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Anthozoa |
| Class: | Hexacorallia Haeckel, 1896 |
| Orders | |
Hexacorallia (synonym: Zoantharia [2] [3] [4] [5] [6] [7] [8] ; older synonyms: Hexacoralla, [9] [10] Hexaradiata, [11] Hexactinia, [12] Actinanthida [13] ; in English: hexacorals, [14] hexacorallians, [14] zoantharians [15] ) 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. [16] Some of the Scleractinia are coral-reef builders. [17]
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. [18] 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. [16]
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]: [22] [23] [6] [24] [25] [2] [7] [26] [16] [27] [28] [29] [30] [31]
The order †Heterocorallia (heterocorals), until recently considered related to or included in Rugosa, is now included in octocorals. [35]
Ceriantharia and Antipatharia were formerly jointly classified in a taxon called Ceriantipatharia, which was considered a separate (sub)class outside Hexacorallia. [16] [27] [36] [24] [20]
Based on the type of their skeleton, the hexacorals are divided into sea anemones, antipatharians and (Hexacorallian) corals. More specifically: [6] [30] [36] [7]
In the 19th century, this skeleton-based division into three parts was also the standard formal way of subdividing the taxon hexacorals, i. e. the Hexacorallia [aka Zoantharia] consisted of the taxa (orders): [37] [38]
