160 H. C. STEVENS 



continue into the open space, exclusively lighted by one of the 

 windows. It is to be noted that up to the time that the middle 

 partition is met with, locomotion is in a side-wise direction, with 

 reference to the long axis of the crab's body. On orienting to 

 the light, after coming in contact with the partition, rotation 

 occurs about the posterior mid-point of the carapace, in such a 

 way that the anterior end of the animal is directed towards the 

 light. As the crab moves towards the light, with the long axis 

 of its body obliquely disposed with reference to the length of 

 the box, as a result of the rotation just described, further rota- 

 tion of the long axis of its body with reference to the long axis 

 of the box takes place, with the result that the anterior end of 

 the body is turned directly towards the source of light and both 

 sides of the body are equally illuminated. This type of orien- 

 tation is shown in Fig.' 2. This rotation of the body so that 

 the light fell equally upon both sides, was very striking with 

 the w 7 hite and green glasses. With red it was least. Blue prob- 

 ably stands next to red in this respect, with yellow next to 

 green. In a considerable proportion of the reactions, the crabs, 

 upon emerging from the pen, rotated towards the light (from 

 the transverse position with reference to the mid-line of the 

 reaction box, wnich they always occupied on leaving the pen, 

 to the right or left according as the head was pointed to the 

 left or right), immediately, without advancing to the partition 

 in the manner which has just been described, and struck out 

 in an oblique direction towards the right or left source of light. 

 This type or orientation is shown in Fig. 1. As the animal 

 advanced towards the light, the anterior end turned more and 

 more as the window was approached, until, when it arrived, 

 both sides were equally exposed to the light. An analysis of the 

 reactions has been made with view to finding out what propor- 

 tion of the reactions ended in the manner just described and what 

 sort of glass caused such reactions. Out of a total of 105 reac- 

 tions in which the head was directed towards the source of 

 light with each side of the body equally exposed to it, 47 were 

 reactions to white, 24 to green, 17 to yellow, 14 to blue and 

 3 to red, light. Without entering into a discussion of theories 

 at this point, it may be pointed out that these results seem to 

 show that unequal intensity of stimulation of the two sides 

 of the crab's body is the chief factor in orientation. 



