92 Herbert Eugene Walter 



The existence of such a decided geotactic tendency should not 

 be forgotten when trying to determine the part Hght plays in 

 planarian behavior. 



Again, it was found that there was less accuracy of orientation 

 to directive light while the planarians were on the sides of the 

 aquarium in a position parallel to the light rays than while they 

 were on the bottom. 



Their behavior in the former case was the resultant of at least 

 two known stimuli, gravity and light, whereas gravity was prac- 

 tically eliminated when they glided on the floor of the aquarium. 

 In the experiment cited under the preceding paragraph 92 per 

 cent of the distance traversed by the worm on the bottom of the 

 aquarium was in a direction in general away from the light, as 

 contrasted with only 79 per cent when it was traveling on the sides 

 of the aquarium. This difference of 13 per cent may represent 

 roughly the necessary correction for geotaxis, in order to ascertain 

 the influence of light alone. 



Thigmotaxis. Contact with the substratum is an almost con- 

 stant condition of planarian activity. Occasionally worms may 

 be seen dangling free at the end of a mucus-thread, as commonly 

 occurs among many fresh-water snails; sometimes they may fall 

 helplessly from the surface-film to the bottom, but definite con- 

 tact with something firm is the rule during their ordinary loco- 

 motion. 



A change in the degree of this contact, and consequently a pro- 

 duction of thigmotactic stimulation, may come about in two ways: 

 the surface on which the animal glides may present irregularities, 

 such as increased roughness or a different degree of solidity, or 

 the worm itself may vary in the extent of body-surface which it 

 brings into contact with the substratum. This latter method of 

 causing thigmotactic stimulation applies especially to Bdelloura, 

 which has the habit of frequently alternating a leech-like looping 

 movement with ordinary gliding, thus changing its contact rela- 

 tions and probably producing a thigmotactic stimulus in conse- 

 quence. 



As already mentioned, Bdelloura, when subjected to sudden 

 dark, usually detaches itself from its support and wriggles vio- 



