\OTKS OX 'I II I HI 1 1 \\lok (>l I II I HDDLKR CRAM. \K^ 



phototaxis does not play any marked role in its life. According 

 to Schwartz and Safir on cloudy days the tiddlers remain inactive. 

 On the contrary, in my laboratory hundreds of fiddlers displayed 

 the most activity in the dark which may depend on being un- 

 disturbed when nobody remained in the room. The inactivity 

 on cloudy days is surely connected with heat, not with the light. 

 What we call phototaxis in Uca may be observed only under 

 very unnatural conditions when the crab is struggling blindly 

 for escape. Its activities are then uncontrolled by the inhibitory 

 centers and automatic correlations may drive the animal towards 

 the light. Such cases are very interesting for a study of auto- 

 matic nervous connections but they are scarcely valuable for 

 psychology. 



As to the thigmotaxis some observations point to its existence. 

 In the dark Uca also dig their holes near the wall but of course 

 no particular spot of it is preferred to the others. And again a 

 control experiment shows that the thing is not so simple. I put 

 several crabs into a large crystalliser (n inches in diameter) 

 filled with sand. I thrust in the sand several small glass plates 

 at different spots, covered the crystalliser and put the whole in 

 the dark. If remaining close to the wall be only a matter of 

 thigmotaxis we might expect that the crabs will dig just as often 

 near the glass plates scattered through the whole surface of the 

 sand, as they dig near the outer wall. But in fact from 26 crabs 

 tested several times only one made the burrow near a peripheral 

 glass plate, all others dug invariably close to the outer wall. 

 And yet the sum of contact surfaces of all plates with the sand 

 was even superior to the surface of the crystallizer's wall. Once 

 more the thigmotaxis proves to be not an adequate explanation. 

 The tendency of the crab is not to remain by the wall but to 

 remain at the very periphery of the vessel. Under similat 

 conditions practically all animals show the same behavior, 

 Paramcecium as well as man. As I showed (6) Paramceciiun 

 swims along the periphery of the vessel in spite of its thigmo- 

 taxis being negative while swimming. In a closed place 

 which we wish to leave we will be oftener found at the periphery 

 than in the middle of it. The reaction has nothing to do with 

 thigmotaxis. 



