C42 Edgar Davidson Congdon 



observed, for a reflecting apparatus. These pigment migrations 

 plainly tend to protect the eye from over-stimulation by strong 

 light and to increase its chances of stimulation in weak light. 



It is evident from the foregoing that light conditions must be 

 taken into account in testing the effects of temperature. To do 

 this, a series of experiments in the dark and another in the Hght 

 w^ere planned, each of which included three temperatures: 10°, 

 20° and 30° C. One extreme of temperature in each series would 

 increase and the other decrease the effect of light, if indeed tem- 

 perature is a factor in the migration of pigment. As the photo- 

 mechanical changes in Palaemonetes are ordinarily completed in 

 about two hours, I extended my experiments to only two and a half 

 hours. The three experiments of each series were performed at 

 the same time. Care was taken in the light series to have the 

 three aquaria for the three experiments placed close together so as 

 to receive equal illumination. Six to twelve individuals were 

 taken for each experiment. All of the animals came from a com- 

 mon supply and were similar in size and sex. One set of experi- 

 ments was conducted twice, once with each sex. No difference 

 in responsiveness was found between males and females. 



The animals were put into quart glass jars filled with water and 

 these jars were placed each in a two-gallon cylindrical aquarium. 

 The latter was filled with water at the desired temperature; thus 

 the water in the inner jar containing the animals was brought in 

 half an hour to the required temperature. Preliminary trials 

 showed that 10° C. and 33° to 35° C. were not harmful to the 

 prawns when thus gradually produced, though the higher tem- 

 perature would cause at times the death of the animals if suddenly 

 applied. In the dark series there was no easy means at hand for 

 maintaining the desired temperature in the dark-proof box with- 

 out admitting light. Consequently the box was kept closed dur- 

 ing the whole of the experiment, the water thus being allowed to 

 change gradually toward room temperature. This resulted in a 

 variation of about 3° C. during the experiment, an amount not 

 sufficient to predjudice the result. At the end of treatment the 

 animals were plunged into water at 80° C. for a fraction of a min- 

 ute, until fixation was accomplished. The eyes were then prepared 



