Diatoms 



109 



Diatoms — Diatoms are among the most abundant of 

 living things in all the waters of the earth. They occur 

 singly and free, or attached by gelatinous stalks, or 



Fig. 34. Miscellaneous diatoms, mostly species of Navicula; the filaments 

 are blue-green algae, mostly Uscillaloria. 



aggregated together in gelatinous tubes, or compactly 

 grouped in more or less coherent filaments. AH are 

 of microscopic size. They are most easily recognized by 

 their possession of a box-like shell, composed of two 

 valves, with overlapping edges. These valves are 

 stiffened by siHca which is deposited in their outer walls, 

 often in beautiful patterns. The opposed edges of the 



