576 



PHOTOPERIODISM IN INVERTEBRATES 



60 80 100 120 



TIME IN HOURS 



Fig. 6. Entrainment of the luminescence rhythm by periods of alternat- 

 ing light and dark which differ from 24 hr. Black bars show dark periods. 

 In subsequent experiments of this type a more uniform illumination sys- 

 tem has been used, and most of the apparent irregularities are no longer 

 observed. Note that when the cultures are placed in continuous dim light 

 (200 ft-c) subsequent to the entrainment, the rhythm reverts to its 

 natural period of approximately 24 hr. The importance of the light inten- 

 sity to the response observed in a particular entrainment schedule may be 



(Continued on p. 577) 



