FEEDING REACTIONS IN DILEPTUS GIGAS. 115 



prey comes to them, and take what they can get, others are pre- 

 datory and go in search of food. These are the most interesting 

 of all protozoa, for they are occasionally too fastidious apparently 

 to take the' ordinary run of microscopic wilds, but seem to select 

 their food with all the care of a gourmand." As an example of 

 this type he describes the reactions of Actinobolus radians. 



Miss Moody ('12) in her study of Actinobolus and SpatJiidium 

 asserts that they " subsist exclusively on a special type of ciliate. 

 Actinobolus awaits the coming of Haltcria grandinella before mak- 

 ing use of its weapons of offense," while Spat hid him swims about 

 " with seeming indifference to all food material except the little 

 ciliate, Colpidium colpoda." She concludes that " the protoplasm 

 of these organisms has become modified chemically and physiologi- 

 cally to such an extent that a reaction to one kind of protoplasm 

 only is possible ; in other words, forms like Actinobolus and 

 Spathidium have become " educated through ' error ' to the selection 

 of one species of food each, namely, Halteria grandinella and 

 Colpidium colpoda." 



Metalnikow ('12) contends that if paramecia are fed for some 

 time on a non-digestible substance, they take in gradually less and 

 less, until finally they refuse it entirely under all conditions, but 

 that they nevertheless take in other substances just as before. He 

 shows that in the case of feeding on carmine this power of selection 

 is lost at the time of division. He also shows that there is a de- 

 cided power of discrimination between substances already within 

 the body; for some substances are quickly excreted, while others 

 remain within the body for a considerable length of time. 



In his studies of one of the Suctoria, Podophrya collini, Root 

 ('14) maintains that there are several definite factors which deter- 

 mine the selection of food in this organism. He shows that the 

 character of the outer surface of certain organisms as to physical 

 and chemical constitution, mucus secretion, etc., prevents the 

 attachment of the seizing apparatus. He shows, moreover, that 

 the size, the activity, and the characteristic behavior of certain 

 organisms in relation to the sessile habits of Podophrya collmi are 

 also determining factors. 



From this brief review of some of the more general literature in 

 this field it is evident that selection of food has thus far been 



