Mtgration of the Retinal Pigment 547 



easily pointed out in the migration of the melanophore pigment of 

 reptiles. On the other hand, the migration of the retinal pigment 

 in crustaceans as caused by change of temperature seems to me not 

 to be adaptive, for it is always small in amount and it occurs at 

 temperatures higher and lower than those commonly experienced 

 by the animals. It seems reasonable that the migration is closely 

 associated with the accelerating effect of heat on the chemical 

 changes in the melanophore cell, and even if the migration 

 were more marked, it is difficult to see what advantage it would 

 give to its possessor. A probably related instance of lack of 

 adaptation has been described by Hess ('05, p. 423) in the 

 cephalopod eye, which often required one to two days in which to 

 complete che pigment migration. This length of time renders the 

 migration at best a very imperfect means of adaptation for sur- 

 roundings in which light and darkness follow each other at half-day 

 intervals. 



V SUMMARY 



In both Palaemonetes and Cambarus the proximal retinal pig- 

 ment migrates distally when the temperature is lowered and proxi- 

 mally when it is raised. 



In Palaemonetes the distal pigment migrates proximally when 

 the temperature is lowered, and distally when it is raised. 



In all cases increased temperatures cause a pigment movement 

 the reverse in direction to that produced by increased light. 



The effect of temperature is much weaker than that of light. 



In the eyes of crustaceans retinal pigment migration due to 

 temperature changes is probably not adaptive. 



BIBLIOGRAPHY 



ExNER, S., '91 — Die Physiologie der facettirten Augen von Krebsen unci Insecten. 



Deuticke, Leipzig und Wien, viii + 206 pp., 7 Taf. 

 Herzog, H., '05 — Experimentelle Untersuchungen zur Physiologie der Bewegungs- 



vorgange in der Netzhaut. Arch f. Anat. u. Physiol., Physiol. Abt., 



Jahrg. 1905, Heft 5-6, pp. 413-464, Taf. 5. 

 Hess, C, '05 — Beitrage zur Physiologie und Anatomic des Cephalopodenauges. 



Arch. f. ges. Physiol., Bd., 109, Heft 9-10, pp. 393-439, Taf. 5-8. 



