On each of the eight mesenteries there is a longitudinal ridge due to the presence of a band of retractor muscles. The position of these muscles on the ventral surfaces of the mesenteries only is one of the characteristic features of the sub-class (Fig. 148, and p. [329]). They vary considerably in thickness and extent according to the power of retractility possessed by the zooids, but they never vary in their position on the mesenteries.
Fig. 148.—Diagrammatic transverse sections of an Alcyonarian. A, through the stomodaeum; B, below the level of the stomodaeum. DD, Dorsal directive; dlmf, dorso-lateral mesenteric filament; dmf, dorsal mesenteric filament; gon, gonad; Si, siphonoglyph; V.D, ventral mesentery; V.L, ventro-lateral mesentery. The upper half of the section in B is taken at a higher level than the lower half.
The skeleton of Alcyonaria may consist of spicules of calcium carbonate, of a horny substance frequently impregnated with calcium carbonate and associated with spicules of the same substance, or in Heliopora alone, among recent forms, of a continuous crystalline corallum of calcium carbonate.
The spicules constitute one of the most characteristic features of the Alcyonaria. They are not found in Cornularia, Stereosoma, in a recently discovered genus of Gorgoniidae (Malacogorgia), in certain Pennatulacea and in Heliopora; and it is probable that they may be absent in some local varieties of certain species of Clavularia.
The spicules of Alcyonaria consist of an organic matrix supporting a quantity of crystalline calcium carbonate. In some cases (Xenia) the amount of inorganic salt is so small that the spicule retains its shape after prolonged immersion in an acid; but generally speaking the relative amount of calcium carbonate is so great that it is only by the careful decalcification of the spicules in weak acetic acid that the delicate fibrous organic matrix can be demonstrated.
The spicules vary in size from minute granules to long spindles 9 mm. in length (Spongodes, sp.). They exhibit so many varieties of shape that an attempt must be made to place them in groups. The most prevalent type perhaps is that called the spindle. This is a rod-shaped spicule with more or less pointed extremities. They are usually ornamented with short simple or compound wart-like tubercles (Fig. 149, 5). Spicules belonging to this type are found in all the principal subdivisions of the group except the Pennatulacea.
In the Pennatulacea a very characteristic form of spicule is a long rod or needle marked with two or three slightly twisted ridges, frequently a little knobbed or swollen at the extremities. In the same group, in Xenia and Heteroxenia among the Alcyonacea, and in the family Chrysogorgiidae the spicules are in the form of minute discs or spheres, and in some genera the discs may be united in couples (twins) or in threes (triplets) by short connecting bars (Fig. 149, 10). More irregular calcareous corpuscles of minute size are found in some genera of Pennatulacea.
Other characteristic spicules are the warted clubs of Juncella, the torch-like spicules of Eunicella (Fig. 149, 3), the clubs with irregular leaf-like expansions at one extremity ("Blattkeulen") of Eunicea, and the flat but very irregular scales of the Primnoidae. There are also many genera exhibiting spicules of quite irregular form (Fig. 149, 8).
In the greater number of cases the spicules lie loosely in the mesogloea and readily separate when the soft tissues of the colony decay or are dissolved in a solution of potash. In a few noteworthy examples the spicules become in their growth tightly wedged together to form a compact skeleton, which cannot subsequently be disintegrated into its constituent elements. In the Precious corals (Coralliidae) the spicules of the axial region fuse together to form a solid mass of lime almost as hard and compact as the substance of a pearl.