The Mind of the Simplest Animals 39 



the Amoeba presents itself as a good subject. Structurally, 

 it consists of a single cell, as do 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 nervous 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-dwell- 

 ing animal, is not a free-swimming one, but moves by crawl- 

 ing on a solid body. This method of locomotion involves 

 in Amceba proteus changes of form on the animal's part, 

 projections, called pseudopodia, being sent out in advance of 

 the movement of the whole body. The protoplasm of the 

 body shows in this process certain flowing movements which 

 are differently described by different observers, and doubt- 

 less 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 currents in a drop of any 



