38 



COLLEGE ZOOLOGY 



then move away, thus exhibiting a negative 

 reaction to contact or mechanical shock 

 (Fig. 16). If, however, a floating specimen 

 touches a solid object, it will react positively 

 and move toward the object. 



::'.:i^^^SaU 



Chemicals 



Choice of food by the amoeba is probably 

 largely the result of reactions to chemicals; 

 a positive reaction results in ingestion and a 

 negative reaction in movement away from 



Amoeba touches drop 

 of salt solufioi) 



Amoeba touches a rod 



}^-y(^y 



and quickly moves away 



forms pseudopodia around ifr 



and turns to 

 ovoid the obstacle 



Light 



Negative reaction of amoeba to strong 

 light. The direction of the beam of light 

 was changed at intervals. The amoeba in 

 each case changed its direction so as to 

 avoid the light. Time: 9:40 to 9:55 A.M. 



Figure 16. Amoeba. Reactions to various stimuli. Arrows show direction of movement. 



the food particle. The amoeba reacts nega- 

 tively to various chemicals such as table salt 

 (sodium chloride), acetic acid, cane sugar, 

 and methyl green (Fig. 16). 



Light 



The amoeba will orient itself in respect to 

 the direction of the rays of a strong light 

 and move away from it (Fig. 16), but it 

 may react positively to a very weak light. 



Temperature 



As noted above, the rate of locomotion of 

 the amoeba depends on the temperature of 



the medium. An increase in temperature re- 

 sults in movements away from the stimulus, 

 that is, in a negative response. If the tem- 

 perature is decreased sufficiently, movements 

 cease. 



Conclusions 



These examples of behavior of the amoeba 

 show that it is irritable and that stimuli are 

 conducted to all parts of its cell body. Its 

 reactions to stimuli are of undoubted value 

 to the individual and to the preservation of 

 the species since the negative reactions, pro- 

 duced in most cases by injurious agents such 



