110 



PROTOZOOLOGY 



case of carnivorous forms, the condition of food organisms may pro- 

 duce abnormalities in them, as was shown by Beers (1933) in Didi- 

 nium fed on starved paramecia (Fig. 40). 



Some thirty years ago, Robertson (1921-1927) reported that when 

 two ciliates, Enchelys and Colpoda, are placed in a small amount of 

 fresh culture medium, the rate of reproduction following a "lag pe- 



Fig. 39. Form and size variation in Tetrahymena vorax, due to differ- 

 ences in kind and amount of food material, as seen in life, X400 (Kidder, 

 Lilly and Claff). a, bacteria-feeder; b, c, saprozoic forms; d, individual 

 which has fed on killed Colpidium campylum; e, starved individual from 

 a killed-Colpidium culture; f-i, progressive form and size changes of 

 saprozoic form in the presence of living Colpidium; j, a young carnivore 

 which has been removed to a culture with living yeast. 



