362 LESLIE B. AREY 



corresponding conditions of light and darkness. His results 

 concerning a photomechanical influence were entirely negative, 

 although he states his behef (p. 255) in the existence of an appre- 

 ciable pigment migration caused by certain unknown factors. 

 Assuming that the retinal pigment of these animals does 

 undergo migratory movements, it is reasonable to expect that 

 the behavior of pigment cells, having no demonstrable nervous 

 connections (Smith, '06), is dependent upon definite environ- 

 mental stimuli, hence I decided to reinvestigate the influences 

 that simple stimulating- agents such as light and temperature 

 might have upon the retinal pigment of these snails. 



MATERIAL AND METHODS 



Planorbis was obtained in abundance at certain localities 

 on the bank of the Charles River, Cambridge, Mass. During 

 warm weather it is easily procurable within arm's reach of the 

 shore, but as the water grows colder a gradual withdrawal of 

 animals occurs toward the deeper bed of the river. My experi- 

 ence in collecting during the fall of 1914 will illustrate this 

 behavior. In the second week of October a great wealth of 

 material was discovered at a location where the previous June 

 no specimens had been seen; at the time of this collection the 

 snails were distributed as far up as the water's edge. The 

 next collecting trip was made on November 21, and, in the 

 same spot, where several weeks previously hundreds of animals 

 were obtainable, there now remained a few stragglers only, 

 and these were withdrawn from the bank almost beyond reach. 

 Ten days later scarcely a specimen was left within sight. 



I was interested to discover whether this withdrawal was due 

 to an active migration of individuals, or to the effect of the 

 low temperature which might cause the snails to become inac- 

 tive, whence they would be washed by the current down the 

 sloping bank into deeper water. Mr. W. F. Clapp of the Mu- 

 seum of Comparative Zoology informs me that the former of 

 the alternatives is undoubtedly correct. Moreover, low tem- 

 perature does not necessarily cause these animals to become 

 inactive, since he reports having often broken through the ice 



