546 Edgar Davidson Congdon 



was found clustered around the rhabdoms or between them. Some- 

 times a little was scattered in the region between the rhabdoms 

 and the basement membrane. Eyes of animals that had been 

 kept at 34° C. (Fig. 7) frequently showed no pigment at all above 

 the basement membrane. In some cases a small amount of 

 pigment was scattered near the rhabdoms. Although there were 

 frequent individual differences, the remaining experiments yielded 

 a natural series of results between the two extremes mentioned. 

 The proximal pigment of Cambarus moves therefore, like that in 

 Palaemonetes, distally with decreasing temperature and proximally 

 with increasing temperature. Light and heat have opposite effects. 

 As in Palaemonetes, the movement in response to temperature is 

 much less than in response to light. 



IV DISCUSSION 



Parker ('06, p. 410) in a recent paper summarized the effects of 

 light on the melanophores of the reptile skin and on the proximal 

 retinular pigment in the arthropods in the statement that all 

 migration due to increased light is distal and so toward the source 

 of light, and all migration due to decreased light is proximal. 

 He also gives experimental evidence that increased temperature 

 produces a proximal migration and decreased temperature a distal 

 one in the melanophore pigment of reptiles. The proximal pig- 

 ment of the decapod crustaceans Palaemonetes and Cambarus, as 

 shown in this paper, falls under the same rule. Herzog's obser- 

 vations on the influence of temperature on the migration of the 

 pigment in the frog's retina agrees with this statement for tempera- 

 tures below 18° C, but not for those above this point, where the 

 reverse is said to be true. Aside from this observation of Herzog's, 

 which needs confirmation, all evidence points toward a general 

 law for temperature the reverse of that for light; namely, increase of 

 temperature causes proximal migration, decrease of temperature, 

 distal migration. 



In most instances of the migration of retinal pigment, the proc- 

 ess has an adaptive value in controlling the amount of light that 

 reaches the receptive organs. Possible adaptations may also be 



