KEVERSIOX IN ORIENTATION TO LIGHT 505 



('14), on the other hand, holds that a sudden decrease makes 

 Daphnia positive and a sudden increase negative. 



3. Increase in temperature generally makes organisms positive 

 to light and decrease usually makes them negative. But Loeb 

 ('05, p. 276) maintains the opposite holds for Polygordius larvae 

 and Holmes ('05) comes to the same conclusion in reference to 

 Rarlaltra; moreover, in some organisms temperature appears to 

 have no effect on reversion. 



4. Nearly all of the observations on the effect of chemicals on 

 reversion in orientation have been made on Crustacea (larvae and 

 adults) and on Arenicola and Balanus larvae. None of the 

 unicellular forms have been tested in respect to this and only one 

 colonial form, Volvox, has been studied, and in this one only 

 the effect of acids. These observations lead to the following 

 conclusions : 



Acids tend to make all of the forms studied positive with the 

 exception of Arenicola, but in many instances some of the salts, 

 alkalis, and narcotics have the same effect as the acids. It is 

 thus evident that chemicals which are fundamentally different 

 in properties may have the same effect on reversion. Conse- 

 quenth^ reversion in the Crustacea cannot be specifically related 

 to the chemical constitution of the environment. It is therefore 

 probably related to the physiological state of the organisms as a 

 whole, but as to the nature of the physiological states involved 

 in reversion we are as yet in total darkness. The interesting 

 experiments of Allee ('18, p. 95) show that in the May-fiy nymphs 

 it is not specifically associated with stimulation or depression. 

 Allee found that hydrochloric acid and ethyl alcohol cause re- 

 version from negative to positive, but that in some cases the 

 nymphs were stimulated and in others depressed as indicated by 

 the rate of production of carbon dioxid. Thus it is evident that 

 in these relatively complex forms the problem is greatly involved. 

 In the simpler forms it is not unreasonable to expect a more direct 

 relation between the environmental factors and reversion. 



In a few of the investigations on the effect of chemicals on re- 

 version solutions of known concentration were used, but in most 

 of them the chemicals to be tested were merelv added in indefinite 



