350 THORBORG MARIE BRUNDIN 



amounts of C0 2 to the water. It appears that increased amounts 

 cause increased stimulation which finally exhausts the organism. 

 A return to the optimum amount seems to set up again the 

 activity that was stopped temporarily. 



I did not find that carbon dioxide in itself, reversed the pho- 

 to taxis of Orchestia. I let the gas flow into the dish of speci- 

 mens that had been in the dark for twenty-four hours, until 

 they were unable to move. When I brought them to the light 

 they were negative. 



V. GENERAL CONCLUSIONS 



In studying the light reactions of the Amphipoda compara- 

 tively, we find that the aquatic Gammaridea* are permanently 

 negative to light under natural conditions, while the most 

 terrestrial forms are positive. The forms occupying a position 

 somewhere between the aquatic and the most terrestrial are 

 those which live on land in very moist conditions, such as the 

 0. agilis and 0. traskiana. These are positive, but have a pre- 

 liminary negative reaction. Talorchestia and 0. pitgettensis, 

 which live in the sand higher on the shore, are ordinarily posi- 

 tive at once to light. We find, therefore, that among the Amphi- 

 poda, the more terrestrial forms are the more positive. 



It was found by the experiments performed, that the condi 

 tions which bring about the positive phototaxis, are the con- 

 ditions which prevail in the environment of the more terrestrial 

 Amphipoda. Heat and dryness favor positive reactions, while 

 cold, moisture and quiet favor negative reactions. 0. pitget- 

 tensis lives in a dryer, warmer environment than 0. traskiana. 

 O. traskiana makes no hiding place for itself, and lives under 

 conditions of greater cold and moisture. 



The small individuals of 0. traskiana are very active. Their 

 periods of negative reactions are correspondingly short when 

 compared with the larger, more sluggish specimens. Yet even 

 these latter can be made positive to the light by enforced activ- 

 ity, and by dryness and heat. These factors produce metabolic 

 processes, through which the chemical reactions which may 

 perhaps be necessary to a certain response, take place. They 

 may also affect the nervous system in such a way that the shock 

 produced by a certain stimulus may be greatly increased or 



* Holmes: Phototaxis in the Amphipoda. 



