Anthozoa which are colonial with the exception of a single family. The colony is sometimes free. The tentacles are eight in number, similar and pinnate, the mesenteries also eight and complete. The retractor muscles are well developed and placed on the ventral aspect of each mesentery. The siphonoglyphe, when present, is single and ventral. The zooids are sometimes dimorphic and then are knozun as autozooids ( = polypes) and siphonozooids ( = zooids), the latter being of simplified structure. Calcareous spicides in the mesoderm are very rarely absent.
The colonial Alcyonaria are distinguished from one another by the form of the colony and the character of the skeleton. As to the first, two principal groups are distinguishable. (1) The zooids originate from a system of basal tubular stolons (Clavidaria), from a narrow band-like stolon (Sarcodictyon), or a disc-like expansion (Sympodium and Tubi-poridae), as well as from tubes connecting the zooids at different heights as in Clavularia viridis, or from platforms s. external tabulae as in Tubi-poridae. The consequence is that the individual zooids remain independent and separate. In certain fossil forms (Favosites) which appear to belong here the zooids are closely apposed. (2) The zooids are imbedded in a well-developed coenosarc, and the fore-part or oral extremity of the body, which is sometimes invaginable, e. g. Corallium, sometimes not so, e. g. Primnoa, is the only region which projects freely. The colony however constituted is either attached or free. When attached the base is extended, if the colony is massive as in the Alcyonidae1 and Helioporidae, or relatively small as in the Pseudaxonia and Axifera, the colony of which is a more or less branched and spreading structure, the branches of which are either free or only accidentally fused where in contact, e. g. in Coral-lium, or united to form a lattice-work. In the latter case they are all disposed in one and the same plane.
The colony is free in the Pennatu-lidae. It has an elongated slender axis, the basal portion of which is more or less pointed and sunk in sand or mud. The exposed portion bears the zooids, (i) at its apex (Umbellula); (2) in a single row (Protocaulidae, Protoptilidae), or in numerous and irregular rows along one, the dorsal aspect and the sides, e. g. Funiculina, sometimes however leaving a narrow dorsal streak free; (3) aggregated on latero-dorsal leaflets as in Pteroeides and Pennatula; or (4) confined to one aspect of a terminal kidney-shaped expansion as in Renilla.
1They occur also in the Hydrocorallina among Hydrozoa, and are consequently of no systematic importance.
2Scytophorus striatus among Hexactiniae, certain Zoanthidae, the Cereantheae, as well as some specimens of Corallium among Alcyonaria, are undeniably hermaphrodite. The question of sex is complicated by the fact that it is possible for the male and female organs to be developed at different times or on different mesenteries. Such at least is the case with certain Hexactiniae, according to de Lacaze Duthiers: see A. Z. Expt. i. 1872, pp. 309, 371. The instance of Coralliiun shows that one individual or one colony may be uni-sexual, another hermaphrodite.
The skeletal structures are not less distinctive than the character of the colony. The discontinuous or spicular skeleton is only absent in the non-colonial Monoxenia and the colonial Helioporidae. It is the only skeleton present in the Tubiporidae and Alcyonidae, the spicules in the first-named being united by minute serratures into a continuous tube for each zooid, except near its oral extremity where they are free. It is present in other Alcyonaria in conjunction with other forms of skeleton mentioned below. The spicules themselves are lens-like, cylindrical, acicular, flattened or stellate, sometimes smooth, or roughened with pointed or warty processes. They are composed of Calcium carbonate with traces of Magnesium carbonate and are tinged with Iron, hence often imparting colour to the colony. The calcite has the typical form of minute rhombohedra; and in the spicule, layers of rhombohedra alternate with fine layers of an organic substance which is most developed superficially. It is rare for the spicules to project freely beyond the surface of the body as they do in the leaflets of Pteroeides.
The organic horny skeleton may form either an external sheath or an internal and central branched axis. The first form is a cuticular secretion of the ectoderm, and occurs only in Clavidaria (Cornularidae), and Sarco-dictyon, where it is thin near the oral extremities of the zooids but thickens towards their bases. Inasmuch as the spicules of Clavularia near the base may also be coated with horny layers which become connected to one another and with the external sheath as well, the zooids acquire a firm support in this region. A horny axial skeleton is found as a simple rod in Pennatididae, or as a branching rod in Pseudaxonia and Axifera where, however, it is attached by a basal expansion to some foreign object. It is lamellate and then may consist of horny and calcareous layers alternating; or be divided into successive joints of alternately horny and calcareous nature, e. g. in Isis; or it may contain cavities filled with a spongy or calcareous mass; or finally be impregnated throughout with calcareous matter. Its centre is sometimes hollow, filled with a spongy material or secondary calcareous deposits. In some instances, e. g.
Sclerogorgia, the horny joints of Melithea and Mopsea, the axial skeleton appears to be composed of spicules with horny sheaths which fuse together, whilst the hard calcareous joints of the two last-named genera consist almost exclusively of coherent spicules. Cor allium has an entirely calcareous axial skeleton which is formed by the fusion of calcareous spicules at first separate. The horny or partially horny, partially calcareous axis of the Axifera differs from that of the Pseudaxonia in being covered by a superficial epithelium which is invaginated basal ectoderm. An epithelium also covers the surface of the axis in Pennatididae but its origin is uncertain1. The horny sheaths of the spicules, like the spicules themselves, are derived from mesoglaeal cells which have in the first instance an ectodermic origin; and the calcareous cementing material which unites the spicules of Coralliutn is probably similarly derived2. The calcareous skeleton of Heliopora is peculiar, and is composed of tubes large and small the adjacent walls of which appear to fuse. The large tubes form calycles for the autozooids whilst the small tubes lodge the siphonozooids. The calcite is laid down in lamellae and it is coloured blue by an organic pigment.