506 journal of Comparative Neurology and Psychology. 



tube was then placed on a movable carriage, so the animal could 

 always be kept in the field of the microscope no matter what part 

 of the tube it might be swimming through. 



Once in the tube the Paramoecium swims to the forward end and 

 upon reaching the meniscus jerks backward for several times its 

 own length, then approaches again in a wider spiral than before. 

 This backing and approaching takes place at least a dozen times 

 and later the Paramoecium settles down to a pecking movement, 

 revolving anti-screwAvise about the meniscus and attacking about 

 five places in its circumference. 



In the original approaching and retreating both movements may 

 be either screwwise or anti-sere wwise. In approaching, both the 

 screwwise and anti-screww^ise movements give about the same 

 width of spiralj namely, a very slight one. If the retreat is made 

 anti-screwwise a relatively straight course is followed, the spiral 

 being hardly noticeable. If the retreat is screwwise a very wide 

 spiral results. 



I 



Fig. I. Paramoecium turning in capillary tube. 



In most cases the animal after a varying time bends its anterior 

 end around toward the aboral side (fig. i), forming a "U" with 

 its body, and after a number of jerks succeeds in reversing the posi- 

 tion of its body in the tube. In all cases it turns toward the aboral 

 side, thus using the long creeping cilia near the buccal groove to 

 obtain a hold on the side of the tube. 



Due to these movements being of no fixed type but varying 

 greatly from time to time under the same conditions, a satisfactory 

 explanation cannot be made in terms of tropism. The "trial and 

 error" explanation, although the principle is no doubt involved, as 

 it is in the gross movements of all animals, does not seem to satisfy, 

 because the movement of reversing in the tube requires a great 

 deal of effort and perseverance on the part of the Paramoecium 

 and a relatively long time to accomplish. The law of trial and 

 error describes the organism as avoiding any great diflftculty and 

 turning to a more easily accomplished movement. The summa- 

 tion of stimuH of many failures probably becomes adequate to 

 cause this unusual reaction. 



