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 {335}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.