230 MARY O. McGINNIS 



animal, swimming rapidly toward the light, directs the head for- 

 ward, and places the body parallel with the light rays. 



A superficial examination of the eyes with a dissecting microscope 

 does not reveal any unequal distribution of the pigment to account 

 for this peculiar orientation of the body with respect to the source 

 of light. 



Effect of previous exposure to darkness 



Two sets of experiments were undertaken to determine the 

 effect of more or less prolonged exposure to darkness on the char- 

 acter of the phototropic response. One series was intended to 

 test animals that had been subjected to short exposures of from 

 twelve to fifty-six hours. The other series was concerned with 

 animals reared in the dark from early embryonic stages. 



After 12 hours in the dark the two specimens experimented 

 with reacted to the light in a normal way. After an exposure 

 of twenty-eight hours, four other specimens responded in the usual 

 manner. Of four males exposed to darkness for 56 hours, three 

 were positive to the light, while one appeared negative. 



In the case of four specimens reared in the dark, it was found 

 that they responded positively to light just as do those reared 

 under ordinary conditions of illumination. 



It seems clear from these observations that previous exposure 

 to darkness does not change the positive phototropism. The 

 single exception mentioned above is probably referable to another 

 cause. When the animals were moulting they were very inactive, 

 responding very slowly, not at all, or negatively. 



Kinetic effect of light 



Branchipus seemed to move faster in bright than in less intense 

 light, and experiments were made to test more carefully this 

 apparent photokinetic response. 



One of the most noticeable of the movements of Branchipus is 

 the constant vibratory motion of the swimming feet. Whether 

 the animal is swimming or lying quiet, its feet are continuously 

 engaged in this motion, which is more rapid when the animal is 



