RETINAL PIGMENT OF PLANORBIS 363 



in mid-winter and dredged Planorbis from the deeper water 

 to which they resort; under these conditions he has always found 

 them to be in an exceedingly active condition. 



This interesting migratory behavior to what is presumably 

 the stimulus of low temperature has a definite 'protective' 

 value to the snail, since an animal thereby removes itself to 

 a depth at which ice is not formed. It is probable that the 

 situation is not one in which an annual rhythm, independent 

 of temperature, has been impressed upon the species, for when 

 Planorbis, during its retreat to deeper water, is removed to 

 laboratory aquaria and kept for some time at room temperature 

 it neither manifests a tendency to seek the deepest water, nor 

 does it exhibit negative geotropism. On the other hand, I 

 do not recall that anijnals introduced into jars of ice water showed 

 any marked tendency to seek the bottom of the dish. It is 

 possible that there is, in fact, a temperature response which, 

 however, comes on but slowly, as evinced by the gradual with- 

 drawal under natural conditions. 



Except for minor changes, the following is quoted directly from 

 an enlightening statement made by Mr. Clapp during a corre- 

 spondence in which I sought his aid in solving the migratory 

 behavior of this animal : 



Wliat you write in regard to the migration of Plaiiorl)is is my idea 

 also. There is little doubt that Planorbis lias a rest period comparable 

 to that of nearly all other gasteropods — the effeet of inactive pha-ses 

 in the life cycle is often clearly marked on the shell. In many species 

 of land shells there are two annual rest periods, the shorter occurring 

 in the summer, the longer in the winter. I have never observed any 

 general inactivity or burrowing during the summer on the part of 

 our New England species of Planorbis, and therefore have thought it 

 probable that their rest period occurs during the winter months. 



The winter rest period of the land shells in this section of the coun- 

 try is governed entirely by the temperature. When spring arrives 

 earlj' the shells appear early — the hardier species first, the more deli- 

 cate species later — but all appear earher than in other years when 

 the warm weather comes on tardily. It seems probable that Plan- 

 orbis, Physa, L>nnnaea and similar shells should follow the same prin- 

 ciple, in other words, become inactive and bury themselves when the 

 temperature of the water falls below a definite point. To a certain 

 extent this may be true, but if so, the rest period must be much shorter 

 than in land shells, and, I think, not compulsory as with the latter 



