Feeding Habits of Ameba. 63 



Not only do amebas react to beams of light at a distance, but 

 also to what we may call for convenience, dark beams. The 

 technic is more difficult, however, in working with dark beams 

 and the results are less definite. The amebas react positively in 

 most cases to beams of light, but to dark beams the reactions are 

 more frequently negative. It remains unknown why there should 

 be this difference. 



Particles of glass, carbon, and various food substances were 

 placed in light and in dark beams to see what the effect would be 

 on ameba. In general it may be said that the amebas are not 

 disturbed when a food object lies in a beam of bright light, but 

 when lying in a dark beam the ameba is clearly disturbed and 

 feeding is often forgone. 



These observations extend considerably our information con- 

 cerning reactions to objects at a distance, but just what the quali- 

 ties are in objects which are thus sensed is still entirely unknown. 



One of the most interesting observations that I have made in 

 connection with the feeding experiments is, that ameba sometimes 

 moves neither directly toward a stimulating object, nor directly 

 away from it, but moves around it, encircling it, while it is still 

 some distance from the object. All classes of substances are 

 thus encircled : glass, carbon, food substances, beams of light, 

 etc. The reason for this peculiar behavior can not then be laid 

 to the nature of the particle but must be due chiefly to the internal 

 condition of the ameba, guided to some extent by the stimuli 

 received from the particle. 



Encircling seems to take place whenever the stimuli coming 

 from a particle are not sufficiently strong to produce movement 

 directly toward the source of the stimulus, nor weak enough to 

 be ignored by the ameba. But these conditions would not be 

 sufficient to produce encircling such as we see in ameba. Such 

 conditions would doubtless produce uncertain behavior, but it 

 would not necessarily express itself in encircling the test object. 

 There must therefore be another factor present in ameba which 

 is concerned with encircling. This factor is a tendency to con- 

 tinue moving forward after movement is once started. The 

 ameba seems to acquire some sort of a momentum of reaction, 

 which tends to keep amebas moving in more or less straight paths. 

 Such a tendency balanced against stimuli producing mild posi- 



