THE EVOLUTION OF ANIMAL COMMUNITIES 69 



numbers on the sixth to the eighteenth day; 

 their extinction is usually rapid and occurs from 

 the fifteenth to the thirtieth day of the life of the 

 culture. They are followed by certain of the 





Fig. 5. Hypotrich infusorians. 



infusorians that have strongly developed cilia on 

 their lower sides and are known as hypotrichs on 

 this account (Fig. 5). These are the third 

 set of organisms to develop jin considerable num- 



