THE EVOLUTION OF ANIMAL COMMUNITIES 71 



bers ; sometimes they reach their maximum abun- 

 dance before the Colpoda but they usually remain 

 much longer. They are normally present in 

 greatest numbers after the monads have passed 

 their maximum. During this period the medium 

 is usually becoming more alkaline but this fact 

 may have nothing to do with the changes in 

 animal life which we are following. 



The hypotrichs are succeeded by another infu- 

 sorian, the common Paramecium (Fig. 6) . These 

 usually attain their greatest numbers at the 

 surface after the hypotrichs have passed their 

 maximum. They may appear early, as early 

 as the third day, but usually come in much 

 later; their usual maximum comes about the 

 seventeenth to the twentieth day. If the culture 

 becomes infested by their small but voracious 

 enemy, Didinium, they may go into rapid decline; 

 in cultures without this Paramecuim-esiiing ani- 

 mal, they live on for sixty -five days or more. 



Paramecia are present in cultures along with a 

 stalked form called Vorticella (Fig. 7). This 

 animal appears irregularly in the cultures from 

 about the second to the eighteenth day and 

 reaches its maximum in from sixteen to fifty -nine 

 days. The Vorticella have been known to become 

 extinct in sixty-seven days but in other cultures 



