4 E. J. LUND 



for as long as five hours before they are killed. It is, howe\'er, 

 plainly evident when one follows the development of wild cul- 

 tures from day to day that some forms are eaten in greater num- 

 bers than others and if the smaller forms, such as Colpidium, 

 Vorticella and Arcella, are present in abundance along with such 

 forms as Paramecium, Stentor and Stylonychia, the former kinds 

 serve exclusively as food for Bursaria while the latter are rarely 

 eaten. When the cycle of development of the culture comes to 

 the stage where, for example, Paramecium is in superabundance, 

 then the body of Bursaria may be more or less filled with Para- 

 mecia. In contrast to the above mentioned forms, Spirostomum 

 ambiguum was always rejected. It was often seen to be taken 

 into the oral pouch but invariablj^ was thrown out again, while 

 Paramecia present in the same culture were readily eaten by the 

 same Bursaria individuals at the time of the observations. This 

 is the only case where Bursaria was seen definitely to discrimi- 

 nate between two different forms of Protozoa. 



By simple methods of observation like the above, it would 

 be impossible to determine just what the principle and the fac- 

 tors are that determine whether Bursaria will feed on only one 

 or several or all of these forms if they be present in all the cul- 

 tures simultaneously, which of course they often are. It is with- 

 the object of elucidating these and certain related questions that 

 the following comparatively simple experiments hsLve been per- 

 formed, by limiting and determining to a high degree the condi- 

 tions under which this organism will react to food. 



ACTION OF THE STRUCTURAL MECHANISM FOR FEEDING AND 

 THE SELECTION OF FOODS 



An account of the food relations of Bursaria requires us to 

 examine in some detail the objective processes involved in feed- 

 ing; these are very striking. The highly developed oral appa- 

 ratus with its large cilia, when in operation during the feeding 

 process, may easily be observed. When the organisms are fed 

 on such substances as yolk or starch they usually sooner or 

 later become quiet for a time, and settle to the bottom of the 

 dish or stick to the surface fihn of the water, then they may be 



