158 REPRESENTATIVE SINGLE-CELLED ANIMALS 



Within the past fifty years a number of different interpretations 

 have been placed upon the processes to be observed in the move- 

 ments of various species of amoeba. Thus, amoebas have been 

 supposed to send out pseudopodia, hke jets from a fountain with a 

 current flowing outward in the center and backward on all sides 

 (Berthold, 1886, and others) ; again, it has been thought that the 

 amoeba flows by a roUing motion (Jennings, 1904); or "walks" on 

 stiff pseudopodia (Bellinger, 1906) (Fig. 87). According to the most 



Fig. 87. — Outlines of successive stages in the movements of a single amoeba 

 as seen laterally. Such "walking" upon the pseudopodia, which can flow 

 out and be retracted, is apparently a method of locomotion in some species. 



A marks a fixed point on the surface. (Outlined from the photographs by Dellinger.) 



recent theory (Mast, 1923), an amoeba moves by contractions of 

 the body, which cause a flow of the central fluid portion out into 

 the pseudopodia. This movement is accompanied by character- 

 istic changes of the protoplasm from a sol to a gel state and from 

 gel to sol. Further explanation appears in the legend of Fig. 86. 

 While Mast's theory supersedes many of the earlier interpretations, 

 one hesitates to accept it as final in the absence of confirmations in 

 various species. It is an interpretation of observations upon 

 Arnceha proteus alone and may not be applicable to all amoebas. 

 It seems, however, to be an important step forward in the under- 

 standing of what actually happens in amoeboid movement. 



The term behavior is used to designate the activities of an organ- 

 ism as a whole in relation to external and internal conditions. 

 Fundamentally, we are deaUng with irritability, or sensitivity, 

 which is one of the important properties of protoplasm; but the 

 unicellular amoeba is an individual and hence may be studied as 

 any other animal that " behaves " in certain ways under given 

 conditions. The feeding reactions are of particular interest in 

 this connection. The amoeba captures living prey in the form of 

 smaller unicellular animals and plants, which are abundant 

 wherever there are many amoebas. The various species differ in 



