70 THE BEHAVIOR OF PROTOZOA 



to form a stream which issues in another direction. Stimu- 

 lation of the posterior end causes the animal to quicken its 

 pace. Stimulating a pseudopod causes it to be withdrawn; 

 and by repeated stimulation the animal may be driven 

 about at will. 



To the weaker stimuli which are received by coming into 

 gentle contact with solid objects, Amoeba often reacts 

 positively. If, when floating about in the water, one pseu- 

 dopod comes in contact with a solid it adheres to it; there 

 is a flow of granules toward the point of contact, from the 

 rest of the body; this pseudopod enlarges, the others contract 

 until most of the body has flowed into the attached pseudo- 

 pod, when the Amceba crawls along the surface of the object. 

 The utility of the negative response to strong mechanical 

 stimuli is obvious since it enables the organism to avoid 

 injurious agencies. The positive reaction tends to keep 

 the Amceba in contact with solid objects where most of its 

 food is secured and where it receives protection. Probably 

 also it plays a part in determining the behavior toward food. 



To injurious chemicals Amoeba reacts much as to strong 

 mechanical stimuli; it creeps away from regions of higher 

 temperature, it reacts negatively to light. All these responses 

 are purposive in that they are adapted to the preservation 

 of the organism. Simple as Amceba apparently is it manages 

 to cope very effectively with the conditions of its existence. 

 One might conceivably construct a machine which would 

 run of itself, gather the food needed to supply the energy 

 used in its workings, avoid automatically contact with 

 obstacles which would impair its running, move away from 

 regions too hot or too cold for its efficient operation, protect 

 itself by producing coverings in unfavorable situations, and 

 guide itself into the most favorable regions for its mainte- 

 nance; but what a wonderfully complicated mechanism it 



