6 BEHAVIOR OF THE LOWER ORGANISMS 



(Fig. 8). Many other special peculiarities of movement are described 

 in the great work of Penard (1902) on these organisms. 



2. Reactions of Amceba to Stimuli 



The conditions under which Amceba lives are not always the same, 

 and as the conditions change, the behavior of Amceba changes also. 

 Such changes in behavior are usually called reactions, while the external 

 agents that induce them are called stimuli. 



A. Reaction to Contact with Solids 



One of the commonest stimuli is that due to contact with a solid 

 object. If a solid body strikes strongly against one side or one end of a 



moving Amceba, the part affected contracts 

 and releases its hold on the substratum, 

 and the internal currents start away from 

 it. The Amceba changes its course and 

 moves in another direction. We may call 

 this a negative reaction, since it takes the 

 animal away from the source of stimulation. 

 This reaction can be produced experi- 

 mentally by touching the animal, under the 

 microscope, with the tip of a glass rod 

 drawn to a minute point (Fig. 9). The 

 animal does not, as a rule, move directly 

 away from the side touched, but merely in 

 some other direction than toward this side. 

 If we touch it at the anterior edge, the part 

 touched stops and contracts, while the cur- 

 rent turns to one side at this point, so that 

 the animal moves at an angle with its for- 

 mer course (Fig. 9). Often the course is 

 altered only a little in this way. But if all of one side or one end is 

 strongly stimulated, then a pseudopodium may be sent out on the side 

 opposite, so that the animal moves almost directly away from the stimu- 

 lated region (Fig. 10). 



By repeatedly stimulating Amceba it is possible to drive it in any 

 desired direction. The advancing edge is touched with the rod ; it 

 thereupon withdraws. A new pseudopodium is sent out elsewhere. 

 If this does not lead in the direction desired, it is touched, causing retrac- 

 tion, whereupon the Amceba tries a new direction. This continues 



Fig. q. — Negative reaction to 

 mechanical stimulation in Amceba. 

 An Amceba advancing in the direc- 

 tion shown by the arrows is stimu- 

 lated with the tip of a glass rod at 

 its anterior edge (a). Thereupon 

 this part is contracted, the currents 

 are changed, and a new pseudo- 

 podium sent out (&). 



