68 The Animal Mind 



ency. Jennings (378) reports no such rejection of unsuit- 

 able particles in the case of Paramecium, but Metalnikow 

 (485, 486) says that when Paramecia have been kept for 

 some time in water containing carmine grains they cease 

 to swallow them ; the evidence being that fewer and fewer 

 grains are found in the animals. Schaeffer (656) thinks 

 this result is due to the mechanical change in carmine 

 grains that have been long in the water, which become 

 stuck together in the mucus excreted by the Paramecia. 

 Metalnikow (487) however finds that when fresh carmine 

 is used the Paramecia avoid it apparently as a result of 

 their previous surfeit, and that when particles of aluminum 

 are used instead of carmine they acquire a discrimination 

 against these even more quickly. He therefore feels con- 

 vinced that the discrimination is a chemical one. 



Stentor is a ciliate protozoon which spends a part of its 

 existence anchored by a long extension of its body, like 

 the stem of a flower : at times it pulls this up and swims 

 off. Food is taken in by the whirl of cilia around the 

 mouth, and may be rejected by a reversal of the direction 

 of this whirl. Schaeffer (656) says that Stentor discrim- 

 inates not only between organisms and inedible particles, 

 but between different kinds of organisms; he thinks, 

 however, that the basis of discrimination is not chemical, 

 because food soaked in a variety of chemicals is readily 

 taken, while jelly made of food organisms is rejected. He 

 believes the discrimination rests probably on several me- 

 chanical factors in combination, for example, size, weight, 

 form, and surface texture, no one of which is alone suf- 

 ficient to determine the choice. On the other hand Lund 

 (446), observing another ciliate named Bursaria, finds that 

 this organism will reject yolk of egg particles if they have 

 been treated with certain dyes, and concludes that the basis 



