42 E. J. LUND 



stance in question, and furthermore (b) the substance by virtue 

 of its chemical properties has, at least in some cases, a specific 

 action upon the mechanism of feeding and selection, as is shown 

 by a comparison of the results of the experiments with hema- 

 toxylin, saffranin and janus green. Additional evidence obtained 

 from observations upon the phenomena of cytolysis in Bursaria 

 also points to the correctness of this conclusion. A familiar in- 

 stance of a similar nature is the casting off of the peristome by 

 Stentor when stimulated or injured by chemicals. Another in- 

 stance is the fact found by Jennings that the anterior end in 

 Paramecium is more sensitive to mechanical stimulation than are 

 other parts of the body. 



That in feeding experiments with the Protozoa it is difficult 

 to discriminate closely between the effects of the medium and 

 those of the food substance itself is obvious, since (a) the amount 

 eaten depends upon so many factors other than the nature of 

 food and (b), since the organism selects on a chemical basis, 

 which involves a soluble substance or substances diffusing into 

 the medium from the food particle, hence necessarily involving 

 the external medium to a greater or less extent. It is of course 

 clear that differences in certain physical characters of food may 

 likewise determine whether or not it will be eaten. This is shown 

 most simply by objects which are too large, such as large yolk 

 grains and large individuals of Stentor, which cannot be swal- 

 lowed. 



From all the facts found from experiments upon food selection 

 by Bursaria, there is no evidence that active selection is based 

 upon either ''size, weight, form or surface texture" or any com- 

 bination of these, except in so far as simple mechanical condi- 

 tions would make them effective. All the facts show clearly 

 that the chemical nature of the food is the property upon which 

 the power of discrimination by Bursaria depends. Hence I find 

 no evidence from Bursaria to support Schaeffer's contention that 

 ''Stentor selects its food upon a tactual basis and apparently 

 not upon a chemical one" and that "Stentor reacts in selecting 

 food, to physical properties only or chiefly, and not to chemical 

 properties" (Schaeffer '10, page 131). On the other hand, the 



