34 A. M. REESE 



It is apparent, therefore, that low temperatures, not far above 

 the freezing point of water, cause these animals to become so 

 sluggish as to be more or less indifferent to differences of light 

 and darkness. As the temperature rises they become active and 

 seek the dark region of the aquarium. When the temperature 

 reaches about 36° C. they become abnormally active and again 

 become indifferent to light and shade differences. At somewhat 

 less than 40° C, about the temperature of human blood, (though 

 they could doubtless be acclimated to higher temperatures') they 

 are seriously affected or possibly killed. 



Experiment III. — Another variation of experiment I was to 

 determine whether the animals would seek the dark half of the 

 aquarium when the illumination was from below. 



The same aquarium and eleven animals were used as in the 

 preceding experiments, but the light was thrown from below 

 by the same tungsten lamp, placed six inches below the bottom 

 of the aquarium. In all, 60 observations, at three-minute in- 

 tervals, were made, with a rest of three and one-half hours be- 

 tween the 30th and 31st observations. The temperature of the 

 water was about 27.5° C. and the animals were active throughout 

 the experiment, those in the light being the more active, on the 

 average. The total number of animals counted in the light was 

 266; those in the dark, 360. 



It is evident then, that Diemyctylus tends to come to rest 

 in the dark region of the aquarium when the light comes from 

 below, but that the tendency is not so strong as when the source 

 of light is above the water. 



Experiment IV. — This experiment was to determine the re- 

 action of Diemyctylus in relation to the direction of white light. 



In this and similar experiments both the region of the aqua- 

 rium where found and the position of the animal in relation 

 to the direction of the light were noted. It was noticed that 

 when the aquarium, described on page 29, was placed with one 

 end about eight feet from a window, but not in the direct sun- 

 light, on a fairly bright day, a large proportion of the animals 

 stayed in the end of the aquarium towards the light and swam 

 against the glass as though trying to get nearer the window. 

 No actual counts were made in this observation. 



