46 THE NAUTILUS. 



Litorina rudis and L. pulliata are two other inhabitants of the 

 brackish waters which must migrate annual]}^ to escape the 

 ground ice of winter. These forms inhabit the Fucus fronds 

 and rocks along the shore between tides. They do not leave 

 these situations until driven out by the cold. Hence, they do 

 not leave their stations until the middle of November and then 

 move very slowly down over the Fucus and rocks to some point 

 below the level of ice formation. In February they are most 

 frequently discovered beneath the lowest living fronds of Fucus 

 which hang down over the lowest line of exposed bowlders 

 forming a hover for the snails. They may also be found packed 

 into the crevices between the rocks and under the over-hanging 

 ledges. The larger snails apparently lead an active life during 

 the severest winter weather, for I have found them still feeding 

 at 12° C. I have also discovered them in active copulation in 

 every month of the winter. The younger individuals are less 

 fortunate in their migration. Apparently they become ex- 

 hausted and crawl beneath the stones of the upper tide limits 

 where they hibernate during the winter months. They are the 

 first of the snails to appear, however, coming forth this year 

 during the week of March 1st. Active migration of the larger 

 snail did not begin until the fourteenth of the month and the 

 upper limits were reached on the twenty-first of March. The 

 migration continued for some time later though less perceptibly 

 and less actively. 



I have not observed any one cause for the migration of these 

 snails except temperature. The Ilyanassa offers a perplexing 

 problem indeed. If it isn't absolutely necessary, why do they 

 go into very deep water ? They leave the flats and return when 

 the water at low tide is at or near 13.5° C. L. litorea apparently 

 wanders about until by pure accident he stumbles onto a place 

 favorable for hibernation. L. rudis and palUata migrated most 

 actively when the temperature of the water at high tide was 

 11.5° C. Density changes in the water were so slight that they 

 cannot be held responsible for the migrations. A shortage of 

 food material may be important ; but the causes, whatever they 

 are, seem complex enough for a special study of the subject. 



