CHOICE OF FOOD IN AMEBA 247 



very slightly muddy water, paramecia would starve to death 

 if careful selection should be practiced, for the time consumed 

 in rejecting silt would permit but very little food to be eaten. 

 It is therefore of advantage to Paramecium not to examine 

 finely divided particles individually, since the maximum amount 

 of food can be obtained under normal conditions without selec- 

 tion, and the few particles not of food value that may be 

 ingested are harmless. 



It may be supposed then that Paramecium has lost the power 

 to a greater or less degree, of selecting its food as it is brought 

 to its mouth. Everything that is finely divided is eaten, whether 

 poisonous or not. Metalnikow's experiments ('12) where powd- 

 ered glass, sulphur, aluminium, arsenic salts, etc., were eaten, 

 prove this point very strikingly. The physical quality of solidity 

 of a substance is sufficient to set off the ingesting mechanism, 

 provided the particle is of such size that Paramecium can take 

 it down the gullet. 



The positive responses of Paramecium to many chemicals, 

 as worked out by Jennings and others, may be interpreted as 

 a reaction to a stimulus which has become associated with the 

 presence of food, most of these chemicals probably stimulating 

 Paramecium in a manner similar to carbon dioxide, which is 

 always found where bacteria are found, and with which Para- 

 mecium continually comes into contact. It is not necessary 

 to suppose that each of the positively stimulating chemicals 

 has a characteristic stimulus. The reactions do not indicate 

 this, and it is quite an adequate explanation to assume that 

 the sensation is the same in quality for most or all of the chemi- 

 cals in solution producing positive behavior. The intensity of 

 the sensation may vary, however, for different substances, as 

 it seems to vary with the degree of concentration of any one 

 of certain chemicals. The fact that Paramecium reacts to 

 chemicals does not at all indicate that the composition of the 

 chemical substance is sensed by the Paramecium, or even that 

 a qualitatively characteristic response is produced in the sense 

 organs by each substance. It is likely that some physical 

 property, common to carbon dioxide and to other substances 

 causing positive behavior, stimulates the Paramecium. There 

 are a number of instances in which it has been definitely shown 

 that a physical quality of an object stimulates amebas, stentors 



