220 C. F. PHIPPS. 



V. Conclusions and Discussion. 

 (A) Summary. — i. In an experimental tank set at right angles 

 to the direction of light rays and graded from intense light to 

 darkness, pond, stream and river amphipods, as a group, seek the 

 dark area, therefore are negative to light intensity. 



2. When the same tank is set obliquely, at an angle of 45°, to 

 the direction of rays with the dark end nearer the lamp, the 

 amphipods are forced to the light area, even though they are 

 negative to intense light. The stimulus of the direction of rays, 

 to which the amphipods react negatively, has a stronger effect 

 than the stimulus of light intensity. 



3. If the experimental tank is set at an angle of 45° to the 

 direction of rays so that the light end is nearer the lamp, normal 

 amphipods, as a group, seek the dark end. In this case the 

 direction of rays exerts the same stimulating effect as light 

 intensity in forcing the animals to the dark area. This again 

 shows a negative reaction to intensity and to direction of rays. 

 A larger percentage show negative reaction, with the tank in this 

 position, than when the tank is at right angles to the light rays. 



4. When treated with certain depressing agents many of these 

 amphiphods become reversed in their reactions to light intensity 

 and to direction of rays. 



5. Freshly obtained amphipods give a larger percentage of 

 negative reactions, both to intensity and direction of rays, than 

 do amphipods which have been kept in the laboratory for some 

 time. 



6. In these experiments there is no evidence of orientation of 

 amphipods either to light intensity or to direction of rays. 



7. Changes in the metabolic processes (physiological states) 

 of the amphipods were undoubtedly the cause of reversed 

 reactions in this series of experiments. 



Some of the above results have been obtained also by other 

 investigators. Holmes ('01) says that all aquatic amphipods 

 studied by him were negatively phototactic, although three 

 species of land amphipods studied were positively phototactic. 

 Loeb ('04), writing about experiments on Gammarus pulex and 

 other animals, says that "whatever increases the activity tends 

 to increase the positive reaction to light, while anything which 



