38 The Animal Mind 



Amoeba presents itself as a good subject. Structurally, it con- 

 sists of a single cell, as dp all the Protozoa, the lowest group 

 of animals ; it is so small that it can be studied only through 

 the microscope; its form, at least that of Amceba proteus, the 

 most typical species, is irregular and constantly changing in 

 locomotion or in response to stimulation. While the internal 

 substance of its body shows a certain amount of differentiation, 

 there is no trace whatever of special modifications that might 

 be supposed to serve for the conduction of stimuli to different 

 parts of the body, and thus represent the prototype of a ner- 

 vous system. Nor have any structures been found that could 

 conceivably be used for the special reception of stimuli ; that 

 is, there are no sense organs. So far as the anatomy of the 

 animal is concerned, then, it differs so widely from our own 

 that we could only conclude from it the absence of all those 

 features which our conscious experience involves. 



Turning from structure to behavior, we find the external 

 activities of Amceba, that is, those not confined to the inner 

 processes of its cell body, to be superficially, at least, divisible 

 into two classes : movements of locomotion and responses to 

 stimulation. Amceba, though a water-dwelling animal, is 

 not a free-swimming one, but moves by crawling on a solid 

 body. This method of locomotion involves in A mceba proteus 

 changes of form on the animal's part, projections, called pseu- 

 dopodia, being sent out in advance of the movement of the 

 whole body. The protoplasm of the body shows in this pro- 

 cess certain flowing movements which are differently described 

 by different observers, and doubtless vary in different species : 

 thus Rhumbler finds that the protoplasmic currents move 

 backward along the sides of the animal and forward through 

 the middle in a way quite comparable to the behavior of cur- 

 rents in a drop of any fluid where the tension of the surface 

 is diminished in front, i.e., at the point toward which the drop, 



