190 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



to the greater sensitiveness of the tentacles as special tactile organs. In 

 these experiments every one of the minus and zero results was due to 

 stimulation of the right tentacle. 



TABLE II. 

 Response to Thigmotactic Stimulation of the Sides of the Body. 



These facts clearly prove that, under ordinary circumstances, the slug 

 is negatively thigmotactic. In our consideration of the animal's responses 

 to other stimuli, we shall have to take this into account, as causing 

 occasional vagaries, and therefore endeavor to eliminate it as much as 

 possible from the experiments. 



II. Geotaxis. 



What determines whether the head end of the slug shall be directed 

 up or down ? 



Methods. — The same apparatus was used as in the preceding experi- 

 ment. A circular glass plate was employed so that the animal could be 

 rotated into any desired position without the necessity of its being 

 handled. The plate was set in a box at an augle of about 45° with the 

 horizon. In each test the animal was so placed on the plate that the 

 long axis was horizontal, different sides being directed downward in 

 different trials. At first the experimentation consisted mostly of watching 

 the animals in order to obtain some clue for further work. Later, rough 

 sketches of the pigment patterns of the individual animals were made, so 

 that it was possible to identify individuals with certainty ; the same 

 animal could then be subjected to experiments at different times and the 

 difference in results noted. The methods used in working out particular 



