MOLLUSCOUS ANIMALS. 507 



mistry of the ocean in its connexion with animal 

 life which requires our attention. A large 

 number of marine creatures derive from the 

 waters the solid matter forming their hard outer 

 case. The shells of innumerable molluscous 

 animals, the hard shields of countless mil- 

 lions of animalcules, and the solid substance 

 secreted by the coral animal, are all derived from 

 the water by the processes of vital chemistry. 

 This hard matter consists chiefly of carbonate 

 of lime. It was found in the researches in the 

 ^Egean Sea, that a most important influence 

 was exercised by the composition of the coast 

 and sea-bottom. Great tracts of a cretaceous 

 limestone border the sea, and by their constant 

 degradation fill its Avaters with a white sediment 

 of the carbonate of lime. In such water, there- 

 fore, those creatures which require this sub- 

 stance for the purposes of their economy, may 

 be expected to abound, and accordingly it was 

 found that large numbers of molluscous ani- 

 mals existed in these regions. On the contrary, 

 where the islands and coast consisted of ser- 

 pentine, the waters bathing them were almost 

 devoid of molluscous or testaceous animals, 

 owing doubtless to the comparative absence of 

 the necessary mineral constituents of the sea- 

 water. 



Some idea may be formed of the vast extent 



