554 ASA A. SCHAEFFER 



behaves in this way it is commonly said that the appetite be- 

 comes more intense upon repeated stimulation, and finally be- 

 comes strong enough to induce eating. As far as this may be 

 translated into physiological terms, the statement also applies to 

 ameba; that is, that repeated stimulation is frequently necessary 

 to induce an ameba to eat. This is not merely an interpretation 

 of what takes place but a plain statement of observation. The 

 ameba has been changed in some way so that at the second stim- 

 ulation the food object, refused at first, is eaten. The only 

 demonstration of this invisible change that is at present possible 

 is the result following a second stimulation. 



The reactions toward motionless organisms and toward parts 

 of organisms, may be readily observed only among granular 

 amebas. Those of the raptorial type have not with certainty 

 been seen to eat an entirely motionless organism; but judging 

 from their behavior toward carmine, it seems likely that they 

 may occasionally do so. One may observe a raptorial ameba 

 move over motionless diatoms again and again without any 

 reaction toward them, but if the ameba comes near an actively 

 moving diatom, it is usually ingested. A number of similar 

 observations have been made. Inactive living diatoms and also 

 a Phacus triqueter in the same condition, were very slightly 

 agitated with a very fine glass needle, with the result that a food 

 cup was formed over the agitated organism but was arrested in its 

 further development when agitation ceased. In the case of the 

 phacus, agitation was continued until the food cup was completed 

 almost down to the surface of the glass. Further agitation was 

 of course impossible and the food cup, in consequence, was de- 

 veloped no further. The sides of the food cup did not become 

 attached to the glass, nor was a layer of protoplasm thrown out 

 under the phacus. The ameba finally moved on over the phacus 

 without further reaction. 



It is evident from these experiments and observations that a 

 food particle, if not in motion, is not a sufficient stimulus in many 

 cases, among the raptorial amebas at least, to bring about the 

 feeding reaction; and that if a moving particle of food, partly 

 enclosed in a food cup, then stops moving, the feeding process is 



