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The term ‘coral’ is commonly used for both ‘soft’ and ‘hard’ corals and sometimes includes other colonial Cnidaria (also commonly called Coelenterata). The term ‘Coral’, used without a qualifying term most commonly refers to hard or skeletonised corals and it is used this way here. The taxonomic relationship between these taxa is indicated in the box below. This classification does not include many extinct orders of skeletonised Cnidaria with uncertain affinities.

            Phylum Cnidaria
              Class Hydrozoa
                      Order Hydroidea (hydroids)
                      Order Milleporina* (including Genus Millepora)
                      Order Stylasterina* (including Genera Distichopora and Stylaster)
              Class Scyphozoa (jellyfishes)
              Class Cubozoa (sea wasps)
              Class Anthozoa
                  Subclass Octocorallia
                      Order Helioporacea* (Heliopora coerulea)
                      Order Alcyonacea*  (soft corals, Tubipora, sea fans and relatives)
                      Order Pennatulacea (sea pens)
                  Subclass Hexacorallia
                      Order Actiniaria (simple sea anemones)
                      Order Zoanthidia (colonial anemones)
                      Order Corallimorpharia (corallimorpharians)
                      Order Scleractinia*  (true stony corals)
                      Order Rugosa*  (Paleozoic corals)
                      Order Tabulata* (Paleozoic corals)
                  Subclass Ceriantipatharia
                      Order Antipatharia (black corals)
                      Order Ceriantharia (tube anemones)

              *Groups having some or all species with stony skeletons are indicated in bold.

Of the orders listed above, the three hexacoral orders (shown in bold) are or were, dominant faunas of coral reefs as well as non-reef habitats. Scleractinia occur in the Mesozoic and Cenozoic; Rugosa and Tabulata occur in the Paleozoic. The four octocoral and hydrozoan orders (shown in bold) are all primarily or entirely Cenozoic.

Of the four extant orders, all except the Stylasterina are partly or wholly zooxanthellate (corals which have symbiotic blue-green algae called zooxanthellate in their tissues, see below) and are therefore restricted to sun-lit depths and warm water. Whether or not Rugosa and Tabulata were also zooxanthellate has long been debated without definite outcome.

J.E.N. Veron