Solitary or colonial Anthozoa. Adult polyps possess more than eight tentacles and mesenteries, except for some Antipatharia which possess six. Form of the tentacles variable: in British species they are either simple and conical, or with terminal knobs; never branched or pinnate. One or two, exceptionally more, siphonoglyphs may be present. Retractor muscles of the directive mesenteries face outward, i.e. are located on the exocoelic face; lateral mesenteries variably arranged, usually including some imperfect ones, often paired. Skeletal structures occur in the orders Scleractinia and Antipatharia and in one genus of Zoantharia, the non-British Gerardia; such structures are either calcareous coralla or horny axial rods which are usually branched; sclerites never occur. No British orders are exclusively colonial and polymorphism does not occur. The cnidom consists of spirocysts (unique to this class), atrichs, holotrichs, p- and b-mastigophores.
Compared to the Octocorallia the polyps of the Hexacorallia are generally large and structurally complex. The range of morphological variation is wide and the typical structure of each order will be described separately in the introductions to these groups.
Five orders of Hexacorallia are known from British waters.