82 ELIZABETH LOCKWOOD THOMPSON 



perpendicular toward the right in 54.1% of the trials and toward 

 the left in 45.8% of the trials. This showed that the natural 

 path chosen by the three snails tested, in traveling over a flat 

 unrestricted substratum to the surface of the water for air, 

 in a comparatively small number of trials deviated somewhat to 

 the right. Table VII, p. 43. 



4. Experiments to test right and left preference : second series. 

 When the U apparatus (paragraph 1) is bordered by a " picket 

 fence " of wires, the snail is forced to travel on one side. When 

 it reaches the top of the stem it must turn toward either the 

 right or left end of the horizontal bar of the U to reach the 

 surface of the water. Fig. 14, Plate IV. 



5. In a series of 240 trials in which the two arms of the U 

 were identical and both reached the surface of the water, the 

 three snails tested followed the right arm in 51.6% of the trials 

 and the left arm in 48.3%, showing a slight right preference for 

 the snails tested. Table VIII, p. 45. 



6. Experiments to test preference for an ascending or descending 

 path. — In order to determine whether any preference existed 

 for an ascending or a descending path, the U-shaped labyrinth 

 was so tilted that the bar of the U deviated from a horizontal 

 at angles varying from 0-1 degree to 7-8 degrees. In 6()() trials 

 the three snails tested chose the descending arm in 43%) of the 

 trials and the ascending arm in 57%, showing a slight preference 

 for an ascending path. Table IX, p. 47. 



7. In subsequent experiments in which the U labyrinth was 

 used the horizontal bar was kept level. 



8. In the experiments to test learning, complete records are 

 presented for two individuals in each series of tests and sum- 

 maries of results are given for additional snails tested. 



9. Experiments on learning the U-shaped labyrinth. — In a 

 series of 888 tests on three individuals of Physa, using the 

 U-shaped labyrinth with the right arm shortened so that a 

 snail could not reach the surface of the water from its upper 

 end, thus making failure to obtain air serve as a punishment 

 incident to the choice of this arm, 58.8% of the turns were 

 toward the right, that is, incorrect, and 41.2% toward the left 

 or correct. There is then shown no evidence of ability in the 

 snail to distinguish right or left in connection with failure to 



