A SUBAQUEOUS CITY. 155 



which owes its own peculiar value to us by the 

 absence of anything hard. 



The beautiful unity and harmony which per- 

 vades all nature may be illustrated in endless 

 instances; but in none is it more manifest than in 

 the intimate relationship between the living forms 

 and the mineral atoms that surround them. Thus 

 silex, lime, iron, and other mineral substances are 

 held in solution in the water (the water may be in 

 the form of ponds, rivers, lakes, seas, or diffused in 

 the earth or in the air), and in this condition are 

 always available for the uses of animal and vegetable 

 life. The living protoplasm, which is the " physical 

 basis of all life," is ever taking up and assimilating 

 this fine mineral matter into the several structures 

 which constitute its bodily frame. Animals and 

 plants alike require it, in ever- varying proportions 

 according to their kind, in the formation of bone, 

 shell, horn, hair, and other matters , and thus our 

 Sponge largely avails itself of this ever present 

 material in the construction of its spicular invest- 

 ment or embryo skeleton. 



In SpongiUa the spicula are of two kinds, one 

 the amphidiscus, or double-disc cogged-wheel-like 



