158 H. C. STEVENS 



down by a constant but unknown amount. When the head 

 of the observer was introduced into the opening in the lid of 

 the box and all light excluded by means of the hood, the only 

 sources of light were the two windows which appeared brightlv 

 illuminated. Even with the least bright colors, red and blue, 

 with dark adaptation of the eyes of the observer, the move- 

 ments of the crab could be seen. . 



In the study of the reactions to light, the mode of orientation 

 of the animal and its final position with reference to the source 

 of light were qbserved and two times were measured. The 

 two times were those which elapsed from the moment when 

 the crab was placed in the V-shaped pen until the moment it 

 emerged from trie pen ; the other time was that which elapsed 

 from the moment of emergence from the pen until the source 

 of light was reached. See Fig. 2. Inasmuch as the crab is an 

 exceedingly thigmotropic individual, the length of time which 

 passed in the pen before emergence was very variable. On the 

 other hand, the time between emergence from the pen and the 

 reaching the source of light, which I shall speak of as the reac- 

 tion time to light, was much more constant, although subject 

 to considerable variation both with respect to the lights and 

 to the animals used. The times of emergence from the pen 

 and the attainment of the light were signalized by the observer 

 whose head was in the reaction box by snapping the fingers. 

 An assistant outside who held a watch recorded the time. The 

 movements of the crabs are very deliberate so that the second 

 is a sufficiently small unit in which to measure the reaction 

 time. In the study of the orientation of the animal each change 

 of position, from the position in which it was placed in the 

 pen, until it reached the source of light, was recorded by the 

 observer upon a diagram of the interior of the reaction box. 

 A record was thus kept of each crab's route as it passed from 

 the pen to the windows. 



Under the conditions of the experiments two variable factors 

 entered into the behavior of the crabs in such a way as to modify 

 their reaction times and their modes of orientation. These fac- 

 tors were thigmotropism or its more special form of "goniotro- 

 pism," as Minkiewicz calls it, and habituation. At the beginning 

 of a reaction experiment, the crab was placed in the middle of 

 the V-shaped pen with its cephalothorax away from the source 



