1 86 



THE BEHAVIOR OF LOWER ORGANISMS. 



Among the sweeping statements that one finds current in regard to 

 the behavior of these low organisms is one to the effect that the Protist 

 does not avoid an obstacle in its path. This statement is made for 

 example by Ziehen, in his excellent Leitfaden der physiologischen 

 Psychologies* It is worth while, therefore, to describe in connection 

 with the reactions to mechanical stimuli just how Amoeba avoids an 

 obstacle. Let us take a concrete case. An Amoeba creeping with a 

 broad, flat anterior end came in contact at the middle of its anterior 

 edge with the end of a long filament of some sort (Fig. 69). The 

 particular spot touched (c) ceased to move forward, becoming entirely 

 quiet (reaction to a weak mechanical stimulus). On each side of it 

 motion continued as before, so that after a time the filament projected 

 into a notch in the middle of the anterior edge. Then gradually the 



FIG. 68. f 



forward movement ceased on the side x and increased at _y, the pseudo- 

 podium x contracted, and its endosarc passed into y. The animal then 

 continued its course in the direction indicated by y. It had thus 

 changed its path so as to avoid the obstacle presented by the filament. 

 Such cases are often seen. 



* " Hindernissen weichen dieselben nicht aus " (Y. c., p. nj. 



tFiG. 68. An Amoeba moving in the direction shown by the arrows in the 

 unbroken outline is stimulated by drawing the tip of a glass rod along one side 

 from a to b. Thereupon a pseudopodium bursts out of the opposite side, as 

 shown by the broken outline, and the Amoeba continues locomotion in the direc- 

 tion so indicated. 



\ FIG. 69. Method by which Amoeba avoids an obstacle. The Amoeba a-b-c-d 

 comes in contact at c with the end of a filament. Thereupon motion at c ceases, 

 while elsewhere it continues, so that after a time the Amoeba has the position 

 shown by the broken outline. Then the currents become changed in #; its sub- 

 stance passes into the pseudopodium y, and the Amoeba continues to move in the 

 direction indicated by the arrows in the lower figure. 



