PHOTOTACT1C REACTIONS IN HYALELLA 263 



remain negative ; even though the increase be gradual, the chemical 

 change in the eye or skin of the animal would take place as read- 

 ily when the necessary concentration was reached as when the 

 animal was dropped directly into the concentrated chemical. 

 Moreover, the duration of immerison in the chemical is greater 

 when the concentration is gradually increased; this would tend 

 to produce a more complete chemical change in the animal than 

 when it is dropped momentarily into the chemical. It is probable, 

 therefore, that these various changes of reaction are due, not to 

 chemical changes in the eyes or skin of the animal, but to a sud- 

 den stimulation or shock to the nervous system. 



I wish here to express my acknowledgments and thanks to Dr. 

 S. J. Holmes for his kindly criticisms and aid in preparing this 

 paper. 



Accepted by the Wistar Institute of Anatomy and Biology, March 21, 1910. Printed July S, 1910. 



