104 DIATOMS 



unlike pill-boxes, each with a lid, bottom, and 

 rim. Some are oblong, others sigmoid, ellip- 

 tical, triangular, &c. 



Diatoms were formerly placed in the animal 

 kingdom, chiefly because they are able to 

 move. A more exact study of their structure 

 and mode of nutrition shows that they must 

 be classed with the lower Algae. 



Diatoms increase in number by successive 

 division into two, in such a manner that suc- 

 ceeding generations are diminished in size. 

 This diminution is owing to the fact that the 

 wall of the older portion overlaps that of the 

 younger, and the diminution therefore corre- 

 sponds in extent with the thickness of the 

 wall. This process goes on through a series 

 of generations, the minimum size being about 

 half the maximum. When the minimum is 

 reached, the wall is cast off, the contents 

 become rounded in form and increase in 

 size, and a new individual of the maximum 

 size is produced. The late Professor W. 

 Smith estimates that no less than a thou- 

 sand million individuals may be produced 



