CHOICE OF FOOD IN AMEBA 241 



injurious particles and reject injurious substances. But when 

 the particles reach the pouch and funnel, selection is made on 

 a different basis, for here starch, glass and sulphur are rejected, 

 while food particles are eaten. When injurious substances affect 

 the disk feeding is for a longer or a shorter time more or less 

 completely inhibited. The decrease in the amount of food eaten 

 in such a case is not due to activity of the food selective 

 mechanism, for the stimulus resulting in partial or complete 

 cessation of feeding arose in another part of the organism where 

 selection between food and other materials has been shown not 

 to occur. It seems then that choice of food such as is observed 

 for example in stentor and in ameba results from the presence 

 in particles of qualities which induce feeding, and not from the 

 presence in other than food particles of qualities which prevent 

 the ingestion of these particles. 



In so far as selection is concerned, all of Lund's experiments, 

 including those where carmine, cinnibar, carbon and other 

 indigestible substances were eaten, can be explained by assuming 

 that choice is based upon the physical properties of the sub- 

 stances, and that all solid non-injurious particles of handleable 

 size are eaten. But if toxic substances are present, or if the 

 temperature is too low, or if an electric current is present, or if 

 the bursarias are violently agitated, the bursarias are disturbed 

 or injured and not only choice but feeding is suspended to a 

 degree corresponding to the strength of the stimulating agent. 

 This simple explanation covers all the recorded facts of feeding 

 in bursaria, and conforms with the explanation of the selective 

 processes in stentor and Paramecium, and to a considerable 

 extent with that of ameba. 



It has already been pointed out in several places that A. 

 dubia is readily stimulated by moving objects; and that almost 

 any object in vibratory motion is eaten, whether the particle 

 is composed of glass or carbon, or whether it is a living or a 

 dead organism. So ready is the response of these amebas to 

 objects in motion, that their seldom reaction to motionless 

 objects, or to lifeless objects, is readily overlooked. Thus Gibbs 

 and Dellinger ('08) whose plates II -and III indicate that they 

 probably worked with this species, concluded that "Amoeba 

 eats nothing dead." 



' The Amoeba shows distinct food preferences; with diatoms 



