Growth, and Habits of Coral Zoophytes. 47 



over the surface, and sometimes these stars form the centres 

 of small prominences, called calicles (little cups). Besides 

 the polyp cells, which mark the position each of a separate 

 polyp, there are pores or cellules penetrating the texture of 

 the coral mass ; yet in some zoophytes, the coral secretions 

 continue increasing in the animal till the pores are almost 

 or quite obliterated, and the texture is nearly compact, 

 the polyp cells alone remaining. In many species, wherever 

 there are concavities of much depth in the surface of a zoo- 

 phyte, the coral of these concavities is loose or more spongy 

 than elsewhere, for the reason, apparently, that the polyps 

 in such parts have a poorer chance for securing food and 

 fresh portions of water. 



In the Gorgoniae, and other species forming a distinct axis 

 to the branches, this axis is solid, without a trace of a cell, 

 and usually with faint evidences of a concentric structure. 

 It is thus that the red coral of commerce used in jewellery, 

 differs from the Madrepore or common white coral ; it is the 

 axis of a species of Corallium ; and the polyps constituted a 

 layer about it, in the same manner as the polyps of Gorgoniae 

 cover the horny axis of these species. 



In hardness^ the common calcareous corals are a little above 

 ordinary limestone or marble, the degree being represented 

 in the mineralogical scale of Mohs, by 35 to 4, while in lime- 

 stone it is about 3. The ringing sound given when coral is 

 struck with a hammer, indicates this superior hardness. It 

 is a common error of old date to suppose that coral when first 

 removed from the water is soft, and afterwards hardens on 

 exposure. But, in fact, there is scarcely an appreciable dif- 

 ference ; the live coral has a slimy feel in the fingers, but if 

 washed clean of the animal matter, it is found to be quite 

 firm. The waters with which it is penetrated, may contain 

 a trace of lime in solution, which evaporates on drying, and 

 adds slightly to the strength of the coral, but the change i* 

 hardly appreciable. A branched madrepore rings on being 

 struck when first collected ; and a blow in any part puts in 

 hazard every branch throughout it, on account of its elasticity 

 and brittleness. Its specific gravity varies from 2*5 to 2^^ : 



