REGULATION OF REACTIONS 279 



to light In low temperature takes them from the surface 

 and prevents their freezing, while in Polygordius larvae 

 and those entomostraca which become negative when the 

 temperature increases the reactions take them out of the 

 warm surface water. I am however not positive that 

 the surface water becomes warm enough to injure these 

 creatures. If it does not, then those reactions are appa- 

 rently not adaptive. 



c. Effect of chemicals. — That the reactions to light in « 

 some organisms are closely associated with the chemical 

 constituents of the environment was clearly shown by the 

 experiments of Englemann referred to elsewhere. In these 

 experiments, Englemann found that the green ciliates, 

 Paramecium bursaria and Stentor viridis, respond to light 

 only when the oxygen pressure is below normal, but he did 

 not note any actual reversal in reaction due to changes in 

 the chemical condition of the medium. Loeb (1904, p. 2) 

 however states that Gammarus pulex, which is " naturally 

 negatively heliotropic " can be made positive by adding 

 small quantities of any of the following substances to the 

 water: carbon dioxid, hydrochloric, oxalic or acetic acid, 

 various narcotics, " such as ether, chloroform, paralde- 

 hyde, alcohol or esters" and ''all the ammonium salts, 

 ammonium hydrate included." The alkalis, excepting 

 NH4OH, urea, oxygen and hydrogen, on the other hand, 

 only excite these creatures; they do not cause reversal in 

 the reaction. Similar results were obtained in experiments 

 on Cyclops and Daphnia. The former however can also 

 be made negative, if it is in the positive state, by the addi- 

 tion of NaOH. "Attempts to make sea-water Gammarus 

 positively heliotropic by CO2 have failed." Holmes (1901) 

 observed that the amphipod Jassa becomes positive when 

 placed into foul sea water. 



The fact that chemicals so very different in their general 

 properties as acids, alkalis and narcotics may have the same 

 effect on the sense of the reactions of these organisms seems 

 to show that the effect of the different chemicals is not 



