Colonial (rarely solitary) Hexacorallia; colonies consist of numerous polyps arising from an encrusting coenenchymatous mass, polyps and coenenchyme usually encrusted with sand or other particulate matter. Tentacles arranged in two equal cycles, only one arising from each radius of the disc. Mesenteries arranged in couples and pairs; those subsequent to the first six couples arising serially in the ventro-lateral exocoels. Filaments with ciliated tracts present on all perfect mesenteries. Siphonoglyph single, ventral, and clearly differentiated. The cnidom consists of spirocysts, holotrichs, p- and b-mastigophores.
All the British species of this order are colonial. The form of the basal coenenchyme varies from a thin, creeping stolon to a thick, encrusting fleshy mass; occasionally it is reduced or absent. Some species live commensally on hermit crabs, forming a carcinoecium on the gastropod shell inhabited by the crab. In mature carcinoecia the mollusc shell is often completely dissolved away and replaced by the zoanthid colony.
Individual zoanthid polyps are remarkably uniform in their structure. The disc is a little wider than the column, concave, with the mouth on a rounded hypostome. A wide tentacle-free area surrounds the mouth, the tentacles being arranged in two distinct neat cycles at the edge of the disc. The column consists of a long cylindrical scapus terminating above in a definite rim or parapet which encloses a shallow groove, the fosse. The upper part of the scapus bears a number of longitudinal scapular ridges, each of which terminates on the parapet as a marginal tooth. In all British species except Isozoanthus sulcatus scapular ridges occur only on the endocoels, thus the marginal teeth correspond to the tentacles of the inner cycle.
The mesogloea of the scapus is often thick and usually contains isolated cells, or groups of cells, known as cell-islets. In some genera a system of canals, communicating with the ectoderm, forms a network within the mesogloea. This canal system may form a distinct ring-sinus in the upper part of the scapus, running just below the endodermal surface of the mesogloea, or parts of it may be separated as isolated lacunae. A sphincter muscle is always present in the parapet region; this may be mesogloeal or endodermal. Sections cut through an endodermal sphincter can be confusing: where the muscle passes through a mesentery it may give a false impression of being mesogloeal. Usually the ectoderm of the scapus and coenenchyme is encrusted with sand grains, sponge spicules, foraminiferan shells, etc., many of which may become deeply embedded in the mesogloea. A layer of periderm may also occur amongst this incrustation.
Mesenteries arrangement: Two pairs of directives are present, the ventrals being perfect and the dorsals imperfect. The lateral mesenteries form pairs, each pair (apart from those including the macrocnemic mesenteries) consisting of a perfect and an imperfect mesentery, each with a weak retractor muscle on the endocoelic face. New mesenteries arise as couples, eventually forming pairs, in the ventro-lateral exocoels. The mesenteries of the fifth couple may be perfect - the macrocnemic condition found in all British species, or imperfect (brachycnemic). Although the retractor muscles are weak the tentacles are always fully retractile.
The British species of Zoantharia are little known, largely because they are inconspicuous animals of mostly sublittoral occurrence and cryptic habits. The only systematic work on the British species is the monograph by Haddon and Shackleton (1891). Some of the systematic characters used by these authors, such as the height:diameter ratio in preserved specimens, are no longer considered to be of value.
Despite most generic and specific characters being internal, the British zoanthids are not too difficult to distinguish in life, although identification is not eased by their nervous disposition - they will retract at the slightest disturbance.