XIII ALCYONARIA SPICULES 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 sub- 

 divisions 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 Ghrysogorgiidae 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 calcare- 

 ous 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 Prim- 

 noidae. There are also many genera exhibiting spicules of 

 quite irregular form (Fig. 149, s). 



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 sub- 

 sequently be disintegrated into its constituent elements. In the 

 Precious corals (CoraUiidae) the spicules of the axial region fuse 

 too-ether to form a solid mass of lime almost as hard and com- 



