Exclusively colonial Anthozoa, colonies fixed or free. Typical polyps possess eight tentacles, each of which bears two lateral rows of tiny offshoots, pinnae, eight mesenteries, all perfect, and a single, ventral siphonoglyph. Retractor muscles of the directive mesenteries face inward (cf. Hexacorallia), retractors of other mesenteries face ventrally. Polymorphism occurs in some groups. Most species possess calcareous sclerites, embedded in the flesh. Some forms also have an internal rod-like axis of calcareous or horny substance. The cnidom consists entirely of small nematocysts resembling b-mastigophores (Schmidt, 1974); spirocysts and other types being absent.
A typical polyp, called an autozooid where polymorphism occurs, is usually differentiated into two regions: a distal retractile part, the anthocodium, and a more rigid proximal part, the anthostele into which the anthocodium retracts. Often the distal rim of the anthostele forms a calyx, usually reinforced with sclerites, which closes over the retracted anthocodium. The anthosteles are generally embedded within the common coenenchyme and interconnected by a network of endodermal tubules, solenia, arising from the column walls.
The only British group in which polymorphism occurs is the Pennatulacea: in addition to the normal autozooids, short, tentacle-less polyps with enlarged siphonoglyphs occur, these are siphonozooids, which are usually visible as low protruberances on the colony wall.
The form of the sclerites is an important feature in classification but most British species are easily identified without examining these.
Octocoral colonies develop by asexual budding from sexually produced ancestral polyps. Colony structure is very variable and often distinctive; in some tropical forms a very large size is attained. Members of this class, which include such forms as the sea-fans, sea-pens, and "dead men's fingers" have a worldwide distribution and are found at all depths, from the lower shore down to at least 9000 m. There are few British species but some of these may be abundant in favourable localities.