PHOTOTAXIS 49 



become apparently oblivious to any other stimulus except 

 the light which they slavishly follow. 



The effect of temperature on phototaxis is by no means 

 uniform. Loeb found that certain marine copepods and the 

 larvaB of Polygordius which are positively phototactic may 

 be rendered negative when subjected to a higher tempera- 

 ture. Strasburger found, on the other hand, that the 

 swarm spores of many algae become positive at a higher 

 temperature and negative at a lower. The flagellate Chro- 

 mulina, according to Massart, is positive at20C. but negative 

 at 5 C. Orchestia agilis has been found by the writer to 

 become strongly negative if dipped into water, but if the 

 water is heated nearly to the point of producing death the 

 reaction becomes positive. 



The concentration and chemical nature of the medium 

 also influence phototaxis. It was found by Loeb that 

 negative specimens of Polygordius and certain copepods were 

 rendered positive by increasing the salt content of the sea 

 water, while the addition of fresh water rendered specimens 

 negative which previously showed a positive response. 

 Larvae of Palaemonetes which are normally positive become 

 negative if the sea water is diluted with half its volume of 

 distilled water (Lyon). 



Certain infusoria, Stentor viridis and Paramoecium bur- 

 sana, which contain chlorophyll go toward the light only 

 when the supply of oxygen is insufficient (Engelmann), 

 but whether the response is phototactic or photopathic is 

 uncertain; at all events it seems to be an adaptation to 

 lack of oxygen, for in the light oxygen becomes produced as 

 in plants by the chlorophyll in the organism. The amphipod 

 Jassa which is usually negative becomes markedly positive 

 in foul sea water. Carbon dioxide and other acids were found 

 bv Loeb to cause positive phototaxis in Gammarus and Cy- 



