284 LIGHT AND THE BEHAVIOR OF ORGANISMS 



pods. Minklewicz (1907, p. 50) says that Lineus ruber, 

 which is ordinarily negative in the regions of the spectrum 

 toward the violet end and positive in those toward the red 

 end, becomes positive in the former and negative in the 

 latter if subjected to a solution consisting of 25 to 80 c.c. 

 of distilled water and 100 c.c, of sea water, but that the 

 reactions to white light remain negative. There may then 

 be, according to Minkiewicz, a reversal in the sense of reac- 

 tion to light of given wave lengths without a reversal in the 

 sense of reaction to white light. 



In working on the light reactions of Temora longicor- 

 nis, a copepod, Loeb (1905, p. 282) noticed that the animals, 

 ordinarily negative, were frequently positive immediately 

 after being caught. This change in the sense of reaction 

 was due probably to mechanical agitation. Miss Towle 

 (1906, p. 345) obtained similar results. She found that 

 the light reaction of Cypridopsis could be temporarily 

 changed from negative to positive by taking the animals 

 up in a pipette or by making them pass through a maze 

 constructed with needles, but that they could not be 

 changed in the opposite direction. In certain organisms 

 however precisely the opposite change takes place. Holmes 

 (1905, p. 319) observed that Ranatra becomes negative if 

 it is handled under water or taken from the water and 

 dropped in again. He also (1901) thinks that the fact 

 that Orchestia gracilis is positive in air and negative in 

 water may be due to the contact stimulus of the water. 

 It is of interest to note that while these animals are per- 

 manently negative in sea water they become positive in 

 fresh water shortly before they die. The copepod Labi- 

 docera, which is ordinarily positive to light, can, according 

 to Parker (1902, p. 117), be made temporarily negative by 

 vigorously ejecting it from a pipette into sea water several 

 times. 



e. Effect of internal changes. — There are many organ- 

 isms which respond to light in one way during part of their 

 existence and in a different way, or perhaps not at all, 



