Fig. 1. — Microscopic views of a shelled amoeba. 



A medium-sized amoeba is over a million million times smaller than the 

 smallest mammal. 



Shells of two diverse races of Difflugia corona, showing characteristics 

 that become hereditarily altered as generations pass. The individual shown 

 at a and i? (a side view and a view looking at the mouth) bears about sevefl 

 long spines and has sixteen teeth around the mouth; the individual shown at 

 c and d has four small spines and fourteen teeth. 



