380 LKSLIH n. AllKV 



Ims liocii sii])(M'uni)()S(Ml ii])()n tlio more pi-iiuitive direct melaiio- 

 phore response, thereby making the behavior appear anomalous. 



It is evident, however, that the results enumerated in this 

 paper relative to the infhience of light and temperature upon 

 the retinal pigment of Planorbis, are conformable with the 

 previously quoted generalization of Parker. The existence 

 of a similar agreement in the behavior of the retinal pigment to 

 temperature was found by Congdon ('07) upon the prawn, 

 Palaemonetes, and by myself (Arey, 'lO"*) upon several fishes. 



The role played by body chromatophores in the economy of 

 an animal is supposedly adaptational with respect to its en- 

 vironmental coloration, and perhaps the regulation of its body 

 temperature as well. The significanace of movements of the 

 retinal pigment, on the contrary, are by no means patent, and 

 the agreement of its responses to light and temperature with 

 those of melanophores in general is of considerable speculative 

 interest. Is the reason for this unaminity of response a phylo- 

 genetic one, retinal pigment cells retaining a primitive behavior? 

 Or is it merely the similar but discontinuous expression of com- 

 mon physiological needs? Or is the agreement purely fortuitous, 

 not involving the fulfilment of any common need? Or is the 

 influence of light and temperature upon the pigment-containing 

 protoplasm necessarily similar wherever mobile pigment cells 

 are found, the pigment migration under any condition (when 

 not controlled by the nervous system), therefore, being always 

 predictable? By allowing one's fancy free rein, numerous 

 other possibilities may be conceived. 



Since w^e know many more instances of immobile than of 

 mobile pigment cells in the tissues of animals, it is reasonable 

 to suppose, and, moreover, evolutionary doctrines compel us to 

 assume, that the existence and perpetuation of the latter type 

 of cells is not without significance. Thus, ignoring all ques- 

 tions as to the reason for this or that behavior of pigment cells, 

 we at least can safely assume with Parker ('06, p. 411) : " . . . 

 it might be supposed that if a case arose in which a reversed 

 migration of pigment would be of service to the organism, such 

 form of migration would be evolved and a set of pigment cells 



