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 
larvee 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 alge become positive at a higher 
temperature and negative at a lower. The flagellate Chro- 
mulina, according to Massart, is positive at 20°C. 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. 
Larve of Palemonetes 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 Paramecium bur- 
saria, 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 
by Loeb to cause positive phototaxis in Gammarus and Cy- 
4 
