222 UTILIZATION OF MINUTE LIFE. 



extensive traffic. In nature its stein, or the axis of 

 its polypidom, is calcareous, solid, and striated; it 

 is covered by a gelatinous porous envelope, in which 

 the coral polypes are seen implanted. 



Donati has thrown much light upon the orga- 

 nization of the coral stem, and the anatomy of the 

 gelatinous tunic which covers it, and places each of 

 its polypes, as it were, in connection one with the 

 other. It will be sufficient here to state that the 

 coral polypes produce the calcareous portion of 

 their polypidom, and also secrete this gelatinous 

 covering, which is of a very complicated nature. 

 The latter, when the coral is freshly taken from the 

 water, is easily peeled off; but if allowed to dry on 

 the stem, it becomes very difficult to detach it. 

 This cortex, or covering, presents numerous tuber- 

 cles or little eminences, each of which contains in 

 its cavity a white, soft, transparent polype, having 

 eight tentacles. As soon as the coral is withdrawn 

 from the water, each polype immediately contracts 

 itself, and withdraws into its cavity. 



The external portion of the solid coral stem is 

 generally much less compact than the interior. 

 When calcined, it loses its organic matter and its 

 colour, and is then seen to be composed of concentric 

 layers. Silliman, jun., has analyzed this substance ; 

 he finds that it is composed of carbonate of lime, 

 containing three to five per cent, of organic matter, 



