AN INTRODUCTION TO ZOOLOGY 



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. ment. Activity ceases 

 -* ^L_ altogether between 30 

 and 35 C., and the ani- 

 mal is killed by coagulation of the 

 protoplasm if the temperature is 

 raised to 40 C. 



Phototropism. If a strong light 

 strikes Ameba from the side, it will 

 orient itself in the direction of the 

 rays and move away from the source 

 illumination (Fig. 22). Ordinary white light 

 causes an Ameba to stop moving (35). 



Chromotropism. The effect of blue light upon 

 Ameba is similar to that of ordinary white light. 

 Red light apparently has no effect whatever upon 

 its normal activities, while other colors cause a vari- 

 able amount of decrease in its movement. 



Electrotropism. If an electric current is passed 

 through an Ameba, it contracts on the positive or 

 anode side of the body and moves toward the 

 cathode or negative pole. 



In Ameba there are no organs that can be com- 

 pared with what we call sense organs in higher organ- 

 isms, 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 



FIG. 22. Reaction of Ameba to light. The 

 arrows indicate the direction of the light 

 rays and the numbers the successive posi- 

 tions assumed by the animal. The Ameba 

 always moves away from the source of 

 light. (From Jennings after Davenport.) 



irritability or conductiv- 

 ity. Perhaps the most 

 primitive method in the 

 animal kingdom of a re- 

 action to stimuli is illns- 



