BEHAVIOR OF THE LOWER ORGANISMS 



pletely cease, while the swerving toward the aboral side is farther 

 increased ; then the Paramecium swings its anterior end about a circle 



with its posterior 



i 



si 



Fig. 38. — More pronounced avoiding reaction. The anterior end 

 swings about a larger circle. 1-5, successive positions occupied. 



end near the cen- 

 tre (Fig. 39). In 

 this case the 

 animal may turn 

 directly away 

 from the stimu- 

 lating agent. 



Such varia- 

 tions are seen 

 when the infu- 

 soria are sub- 

 jected to stimuli 

 of different in- 

 tensities. If the 



animals come in contact with any strong chemical, or with water that is 



very hot, they respond first by swimming a long way backward, thus 



removing themselves as far as 



possible from the source of 



stimulation. Then they turn 



directly toward the aboral 



side, — the rotation on the 



long axis completely ceasing, 



as in Fig. 39. In this way the 



animal may turn directly away 



from the drop and retrace its 



course. But often the reac- 

 tion is so violent that the an- 

 terior end swings about in two 



or three complete circles before 



the animal starts forward 



again. Then the new path 



may lead it again toward the 



drop, when the reaction is 



repeated. 



In marked contrast with 



this violent reaction is the behavior when the stimulus is very weak. 



A weak stimulus is produced for example by -^ per cent to -^ per 



cent sodium chloride, or by water only three or four degrees above the 



normal temperature. The Paramecium whose oral cilia bring it a 



Fig. 30. — Avoiding reaction when revolution on 

 the long axis ceases completely. The anterior end 

 swings about a circle of which the body forms one of 

 the radii. 



