Solitary or colonial Anthozoa with an external calcareous skeleton (corallum) secreted by the base and walls of the polyp, hence their common name, stony corals. Structure of the polyps similar to those of Actiniaria and Corallimorpharia, more nearly resembling the latter in lacking basal musculature and ciliated tracts on the mesenteries. Mesenteries arise in pairs and are often arranged in cycles, as in the Actiniaria. Tentacles are retractile in all British species and are studded with tiny wart-like nematocyst batteries; in some species their tips are knobbed. Cnidom consists of spirocysts, holotrichs, b- and p-mastigophores.
The skeleton of an individual polyp (corallite in colonial forms) is constructed of a series of radial vertical plates, septa, arising from a thin basal plate. The septa fit into deep invaginations in the base of the polyp, between the mesenteries, forming cycles which are named in similar fashion to the mesenteries. A circular wall, theca, connects the outer edges of the septa and may bear longitudinal ridges, costae, which are external extensions of the septa. Septa which project above the level of the theca, usually being continuous with the costae, are exsert. The concavity of the corallum which contains the polyp is the calyx. Some species possess a central axial structure of variable form, the columella, originating from the lower inner edges of the septa. Genus Caryophyllia is the only British genus to possess pali, radially arranged plates forming an inner series separate from the septa, between the latter and the columella.
Colonies are formed by asexual budding which usually occurs in one of two ways: intratentacular budding which is basically equivalent to longitudinal fission and extratentacular budding, but in neither case do the new individuals become entirely separate, being united by a calcareous peritheca. Part of the polyp wall permanently overhangs the theca, this being the edge-zone where extratentacular budding occurs; in colonial forms the edge-zone usually connects adjacent polyps, then being termed coenosarc. Occasionally in some solitary forms two or more individuals become fused together to form a pseudocolony; this is not caused by budding but by the close proximity of developing young, or by planulae settling on existing coralla.
The higher classification of the Scleractinia, being largely based on minute details of skeleton formation, will not be considered here; for details see Wells (1956). Two families occur in the British Isles, one of which, the Caryophylliidae, has been divided into subfamilies, three of which are represented in the British fauna.