"^ 



474 Journal of Comparative Neurology and Psychology. 



gravity, the difference in pressure on two sides or ends of the 

 animal being the determining factor. 



Does the unoriented animal react as Jensen supposed, by- 

 turning directly toward the side of least pressure ? This ques- 

 tion is not to be answered from a prion considerations ; only 

 actual observations of the movements of the animal in becom- 

 ing oriented can give us a reliable answer. With the Braus- 

 Druner stereoscopic binocular such observations can be made 

 without great difficulty. The best plan of experimentation 

 that I have found for giving many opportunities to observe the 

 animals at the time orientation takes place is as follows. The 

 animals are placed in a long U-tube (Fig. 12). The two open 



ends are covered with rubber 

 caps, and the tube is at first 

 placed with free ends upward. 

 The Paramecia collect at the free 

 ends. Now the tube is inverted; 

 the clouds of Paramecia at the 

 two ends move upward, toward 

 the cross piece of the u which 

 is now above (Fig. 12). Ar- 

 riving here, most of them do 

 not cease swimming, but move 

 across the cross piece of the 

 n and even start obliquely down- 

 ward. Here the reaction oc- 

 curs ; they turn around and swim upward again. At this point 

 (,r, Fig. 12) one has at any instant a large number of speci- 

 mens in the process of becoming oriented with anterior ends 

 upward. The binocular is now brought to bear upon this re- 

 gion, and the method of reaction is evident. The spiral course 

 becomes wide, the animals swerve strongly toward the aboral 

 side, so that the anterior end is moving about in a circle ; the 

 Paramecia appear to oscillate irregularly back and forth. In 

 other words, they are reacting in the usual "trial and error" 

 way — "trying" successively many different positions. This is 

 continued till they have gradually worked around into a posi- 



Fig. 12. Tube for study of the 

 reaction to graviy. jr, place where 

 the change of direction of movement 

 occurs. 



