334 COELENTERATA ANTHOZOA chap. 



stomodaeum itself there is a groove called the " siphonoglyph " com- 

 posed of a specialised epithelium bearing long powerful cilia. But 

 the current produced by the siphonoglyph flows from the mouth 

 downwards into the coelenteric cavity and is thus in the opposite 

 direction to that produced by the dorsal mesenteric filaments. It 

 is very probable that these two currents on the opposite sides of the 

 zooids maintain the circulation of water in the deep-seated parts 

 of the colony which is necessary for the respiration of the tissues. 

 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, 



Fig. 148. Diagrammatic transverse sections of an Alcyonarian. A, through the stomo- 

 daeum ; B, below the level of the stomodaeum. 1)D, Dorsal directive ; dim/, dorso- 

 lateral mesenteric filament ; dm/, 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. 



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. 



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 con- 

 tinuous crystalline corallum of calcium carbonate. 



The spicules constitute one of the most characteristic features 

 of the Alcyonaria. They are not found in Comularia, 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 



