ALCYONARIAN CORALS 



109 



swims within a polyp's length of the coral can possibly 

 escape the batteries of nematocysts with which the tentacles 

 are armed. 



Between the autozooids a number of small yellowish- 

 white spots can be seen, each of which is provided with a 

 little mouth when the coral is alive and expanded. Until 

 recently these spots were thought to be young polyps which 

 develop into autozooids, but it was shown by Moseley that 

 they are a different kind of polyp, and perform a different 

 function from the polyps which expand (Figs. 46 and 



47, S.). 



They are called the siphonozooids. They have no 

 tentacles and the mesenteries are very much reduced, but 



Fig. 48. — Diagram of a transverse 

 section through an Alcyonarian polyp. 

 St., stomodaeum ; Dm., dorsal mesen- 

 teries ; I'm., ventral mesenteries. 



Fig. 49. — The same, taken at a 

 lower level than the stomodaeum. 

 g., the gonads situated on the lateral 

 ventral mesenteries. 



the stomodaeum is provided with a broad groove armed with 

 very powerful cilia, by means of which the currents of water 

 in the canal system are maintained. 



The bark or coenenchym of Corallium is of a dark- red 

 colour, due to the presence of a large number of red spicules 

 of calcium carbonate about -07 mm. in length (Fig. 50). 

 The spicules are formed by certain specialised cells in the 

 ectoderm covering the bark. These cells become detached 

 from the rest of the ectoderm and sink dow^n into the sub- 

 stance of the bark, where the spicules continue to grow, 

 until they become jammed together to form a solid mass of 

 coral. In this way the axis is formed and grows. The 

 increase in diameter of the axis of the stem and branches 

 does not seem to take place by the addition of newly formed 

 layers of jammed spicules, but continuously, so that in a 



