POLYPES. 233 



of lime, oxides of iron and alumina, O99 of sulphate 

 of lime, and traces of fluoride of calcium. 



All these salts are extracted, by the polypidom- 

 making polypes, from the water of the sea. If we 



Fio. 30. Caryophyllea fastigiata. 



analyse the water of the ocean near ' ( coral-reefs/' 

 we find a considerable deficiency of lime. Thus, 

 Dr. Forchhammer, in an interesting paper, has lately 

 shown that where madrepore polypes abound, the 

 salts furnished by the sea only contain 2 per cent, 

 of lime. But, on the other hand, these polypes can 

 never extract the whole of the lime from the sea- 

 water, as this author and others appear to think, 

 for Nature has established here one of her beautiful 

 rotations : as the little polypes extract lime from 

 the water to form the new portions of their poly- 

 pidom, the water, by means of the carbonic acid it 



