214 C. F. PHIPPS. 



tions" entered the field of intense light they were plainly stimu- 

 lated and usually darted back quickly to the dark area. 



The typical reaction for the species, negative to intense light, 

 was that given by a majority of the untreated amphipods in the 

 series of readings taken, and the group was considered to give 

 reversed reactions only when a majority, in a series of readings, 

 was found in the region of greatest light intensity. Such a 

 reversal was found with one group of Gammarus, fresh stock, 

 where in 60 readings the majority, 54 per cent., were found to 

 be positive to intense light, while but 20 per cent, were negative, 

 at the time of the readings. Another group of Gammarus fresh 

 stock, however, was just as strongly negative to intensity. In 

 this case of reversed reaction the animals remained in the inten- 

 sity field much more constantly than did those of other groups 

 which showed strong negative reactions. These few reversals 

 with Gammarus may be due to the intense light, or to some factor 

 not recognized. Mast ('ii) has reported cases where long con- 

 tinued or increased light intensity has reversed the phototactic 

 reactions of certain animals. The terrestrial form of amphipods, 

 Orchestia agilis, is negatively phototactic when first exposed to 

 light, but becomes positively phototactic with bright light, the 

 stronger the light the quicker the reaction. 



When the tank was placed in an oblique position so that it was 

 at an angle of 45° to the direction of light rays with the dark 

 end nearer the lamp (position 2), the majority of reactions in all 

 the experiments performed were normal. As stated above, this 

 normal reaction is based upon both light intensity and direction 

 of rays, and as the direction of rays exerts a stronger influence 

 than does intensity, the animals were forced to the light end of 

 the tank, when the tank was in this position, and so appear 

 positive to intense light. The percentages of normal reactions 

 (positive readings), with the tank in position 2, are not so high 

 as in position i (negative readings). This undoubtedly is due 

 to the fact that the influences of intensity and direction of rays 

 were working against each other when the tank was in position 2. 



The position of the tank was again changed so that it was at 

 an angle of 45° to the direction of rays with the light end nearer 

 the lamp (position 3). In all except one series of readings. 



