A MICROSCOPIC POPULATION. 151 



of Portugal and elsewhere, so there is no longer any doubt 

 of their genuineness. 



It is, however, the smaller and chiefly invisible or- 



Fig. 31. SPONGES: 



A, VENUS'S FLOWER-BASKET; B, GLASS-ROPE 

 SPONGE. 



ganisms which use up most of the silica carried into the 

 ocean, and which have left the most extensive remains. 



Professor Bischof compares them with an extensive 

 pumping apparatus, so enormous are the quantities of water 

 perpetually passing through them. A hundred millions 

 weigh less than one grain, but being full-grown in twenty-four 



