THE BEHAVIOR OF AMCEBA 



greatly increases the chances of coming in contact with a solid body, and 

 it is equally evident that contact with a solid is under the circumstances 

 exactly what will be most advantageous to the animal. As soon as the 

 tip of one of the pseudopodia does come in contact with something 

 solid, the behavior changes (Fig. 14). The tip of the pseudopodium 



a a c 



FIG. 14. Method by which a floating Amreba passes to a solid. 



spreads out on the surface of the solid and clings to it. Currents of 

 protoplasm begin to flow in the direction of the attached tip. The 

 other pseudopodia are slowly withdrawn into the body, while the body 

 itself passes to the surface of the solid. After a short time the Amoeba, 

 which had been composed merely of a number of long arms radiating 

 in all directions from a centre, has formed a collected flat mass, creep- 

 ing along a surface in the usual way. This entire reaction seems a re- 

 markable one in its adaptiveness to the peculiar circumstances under 

 which the organism has been placed. 



Positive reactions toward solid bodies are particularly common in 

 the process of obtaining food. In our account of the food reactions we 

 shall give examples of striking and long-continued reactions of this sort. 



B. Reactions to Chemicals, Heat, Light, and Electricity 



Reactions to Chemicals. If a strong chemical in solution diffuses 

 against one side or end of the body, the Amceba contracts the part af- 

 fected, releasing it from the substratum, while the protoplasmic cur- 

 rents start in some other direction. The animal has thus changed its 

 course. The reaction to chemicals can best be shown in the following 

 way. The tip of a capillary glass rod is moistened, then dipped in some 

 powdered chemical, preferably a colored one, such as methyline blue. 

 This tip is then, under the microscope, brought close to one side of an 

 Amoeba in an uncovered drop of water. As soon as the diffusing 

 chemical comes in contact with one side of the body, the reaction occurs. 

 Chemicals that are fluid may be drawn into an excessively fine capillary 

 tube and the tip of this held near the Amceba. Some of the variations 

 in the reactions to chemicals are shown in Fig. 15. 



