226 GARY N. CALKINS 



cc. of clear pond water. Twelve individuals of Paramecium 

 caudatum were picked out and placed with each of them, this 

 number being purely arbitrary. After a few days' trial it was 

 found that a smaller number of Paramecium gave better results 

 and a standard daily diet of 9 Paramecium caudatum was es- 

 tablished and maintained throughout the experiments, which 

 are still under way. Five lines of X and five of Y were estab- 

 lished on the second day and one individual from each of the ten 

 lines was picked out and placed with 9 Paramecium caudatum 

 in 0.25 cc. fresh spring water daily. The usual history of Didin- 

 ium in such an environment at the end of twenty-four hours 

 is 8 Didinium and no Paramecium. At times we find only 2 or 4 

 Didinium and no Paramecium, showing that the appetite was 

 good but the dividing power reduced. Again we find occasion- 

 ally 2 or 4 Didinium and from 2 to 5 Paramecium, or some- 

 times, only 1 Didinium and from 8 to 10 Paramecium, indicat- 

 ing what I shall speak of as loss of appetite. In still other 

 cases the single individual does not divide at all but, notwith- 

 standing daily changes of water and food, dies, usually by the 

 fourth day. Finally encysted individuals which have not di- 

 vided are occasionally found at the end of 'twenty-four hours, 

 together with from 9 to 13 Paramecium. The rate of division 

 of Paramecium is of course very low owing to the scarcity of 

 bacterial food. 



GENERAL DESCRIPTIONS 



Feeding habits of Didinium nasutum 



Next to the capture of Halteria grandinella by Actinobolus 

 radians I know of nothing more spectacular or amazing in the 

 whole realm of microscopy than the seizure and ingestion of Para- 

 mecium by Didinium. Described by Balbiani (73), by Thon 

 ('05), by Jennings ('06) and by Mast ('09) there is Uttle in the 

 process for me to dwell on. The actively rotating carnivore 

 swims vigorously through the water, occasionally limiting its 

 activity to side or bottom of the culture dish, making vicious 

 jabs downwards or sideways until it hits something soft enough 

 for its proboscis to penetrate. As earlier observers have pointed 



