152 THE WORLDS LUMBER ROOM. 



hours, each individual must in that time imbibe at least 

 19-2 grains (about nineteen drops), or 33,333 times its own 

 weight, of water, in order to obtain the silica required for 

 its case or sheath.* Nineteen drops ! The quantity sounds 

 insignificant ; but if human beings drank at the 

 same rate in proportion to their size, each would 

 swallow five million pounds in twenty-four hours, 

 or enough to drive a mill-wheel. 



This microscopic population is so vast that we 

 cannot form the faintest idea of its numbers, but 

 we do know that one species is capable of 

 multiplying four-fold in from twenty-four to thirty 

 hours, so that in ten days a million individuals 

 might spring from one parent. 



But one little animal, the Rotifer, multiplies 

 faster still, and in thirty days might have a trillion 

 descendants, the weight of whose silicious sheaths 

 Fig. 32. would be 65,ooolbs. ; and supposing the mass to 

 ROTIFER N be of the densit y f " mountain-meal," it might 

 form a bed of silica twenty-five square miles in 

 extent, and about one foot and three-quarters thick. 



The Rotifers, or wheel-animalculse (Fig. 32), so called 

 from the wheel-like arrangement of cilia with which they 

 are furnished, are much higher up in the scale of life than 

 the Infusoria, though so minute that one species, having 

 a single ruby-like eye, measures only yj-gth of an inch in 



* As already stated, 1,000 grains of sea-water contain but a minute 

 quantity of dissolved silica, and as it is unlikely that the diatom or in- 

 fusorian extracts the whole amount, they probably swallow much more 

 than nineteen drops. 



