p 



FOSSIL REMAINS CF SPONGES. 565 



are scarcely at all changed. It is evident, then, that some 

 peculiar attraction for silicious matter exists in the Sponges ; 

 and this is readily explained upon well-known chemical prin- 

 ciples. If in a saturated solution of two salts, a crystal of one 

 of those salts be placed, it will be increased by a crystalline 

 deposit of its own composition ; whilst the other salt will not 

 form any deposit around it. If two organic structures, there- 

 fore, one containing silicious spicula, and the other calcareous 

 crystals, be exposed to a fluid holding both these substances in 

 solution, they will each attract from it their own ingredient. 



