PHYLUM PROTOZOA 37 



tact with the substratum, the animal moves in the direction 

 of that pseudopodium until the normal creeping position has 

 been attained. Contact with food also results in positive re- 

 actions. Ameba, therefore, reacts negatively to a strong me- 

 chanical stimulus and positively to a weak one. 



Chemotropic reactions prove that fimeba is sensitive to changes 

 in the chemical composition of the water surrounding it. " It 

 has been shown to react negatively when the following sub- 

 stances come in contact with one side of 



its body; methylene blue, methyl green .,""" 



(Fig. 14), sodium chloride, sodium car- 

 bonate, potassium nitrate, potassium 

 hydroxide, acetic acid, hydrochloric acid, 

 cane sugar, distilled water, tap water, FlG - J 4- Chemotro- 



,. ,. pism of Ameba. The ani- 



and water from other cultures than that ma i mov es away when a 

 in which the Amoeba under experimenta- little methyl green diffuses 



against the advancing end. 

 tion lives. (From Jennings.) 



Negatively thermotropic reactions result 



if Ameba is locally affected by heat, since the animal will move 

 away from heat stimuli. Cold and excessive heat retard its 

 activities, which cease altogether between 30 and 35 C. 



Ameba is negatively phototropic, since it will orient itself in 

 the direction of the rays of a strong light and move away from 

 it (Fig. 15). 



In Ameba there are no organs that can be compared with what 

 we call sense organs in higher animals, and we must attribute its 

 reactions to stimuli to that fundamental property of protoplasm 

 called irritability. The superficial layer of cytoplasm receives 

 the stimulus and transfers the effects to some other part of the 

 body; thus may be shown the phenomenon of internal irritabil- 

 ity or conductivity. The stimulus causing a reaction seems to 

 be in most cases a change in the environment. The behavior of 

 Ameba in the absence of external stimuli, for example when it 

 is suspended freely in the water (p. 36), shows that some of its 

 activities are initiated by internal causes. 



