Selection of Food in Stentor Cceruleus {Ehr.) 129 



But whatever the basis is, it is constantly varying. The degree 

 of hunger, as we have seen, has a marked influence upon selection. 

 Hunger perhaps does not form part of the real basis of selection 

 inasmuch as it only influences the degree of accuracy in discrimi- 

 nation. The same statement may be made as regards mere con- 

 tact with food. As was shown in experiments described on page 

 119, the mere contact of paramecia with Stentor seems, like hun- 

 ger, to have increased the degree of accuracy in discrimination. 



The experiments then in this part show that Stentor probably 

 selects its food upon a tactual and not upon a chemical basis. 

 Further the experiments seem to indicate that more than one of 

 the factors, weight, form, or surface texture, serve as a basis for 

 discrimination. 



THE SELECTIVE MECHANISM 



We have seen that there are two mechansims by means ot 

 which discrimination is directly brought about. The ingesting 

 mechanism is set into action by certain qualities in particles, when 

 the Stentor is hungry. The absence of these qualities, or the 

 presence of qualities which are objectionable to Stentor, call into 

 action the rejecting mechanism. These two mechanisms, taken 

 each by itself, are constantly varying with regard to the strength 

 of stimulus required to set either into action. As the Stentor 

 passes from a hungry to a satiated stage the ingesting apparatus is 

 continually less, and less easily set off". But with the rejecting 

 apparatus the reverse is true. As the Stentor grows more and 

 more satiated, constantly weaker stimuli set off" the rejecting 

 mechanism until finally all particles stimulate it into action. 



These changes are due to the constantly varying physiological 

 state, brought about by the constantly accumulating food in the 

 body of Stentor. That is, the amount of food ingested regulates 

 solely the ease with which the ingesting or the rejecting mechan- 

 isms are set off^. But in some cases there is evidently a direct 

 eff"ect produced in the stimulus-receiving organ by the stimulus. 

 Such cases are those experiments in which carmine and food par- 

 ticles were fed, sometimes mixed, and sometimes each substance 



