72 THE LIFE OF THE MOLLUSCA 



The limits of molluscan endurance, however, are 

 said to be a maximum of 52° C. (= 125*6° F.) and a 

 minimum of -3 C. ( = 26*6° F.). 



Seasonal changes do not, of course, cause such slow- 

 moving creatures as molluscs to migrate, as birds 

 and some mammals do ; but their appearances and 

 disappearances in a given locality may generally 

 be accounted for, as in the case of plants, by the 

 prevalence of favourable or unfavourable conditions 

 leading to their rapid multiplication or the converse 

 in the area under consideration. Cephalopods, 

 perhaps, being more capable of locomotion, may 

 migrate along a coast, and the observed departure 

 and reappearance of the Octopus may in part be due 

 to this as well as to causes connected with their en- 

 vironment affecting their multiplication. 



The extremes of seasonal changes are, on the other 

 hand, for the most part passively resisted. Some of 

 the more active marine forms may in very cold 

 weather retire into deeper water, but the ordinary 

 freshwater kinds simply burrow into the mud and 

 pass the winter in a torpid state. On the land the 

 Slugs burrow into the earth and form a small 

 chamber which they line with mucus; while the 

 Snails bury themselves in the ground, or hide under 

 dead leaves and vegetable refuse, sometimes singly, 

 and sometimes, as in the case of the common Garden 

 Snails, in colonies. The most remarkable hiber- 

 nacula, or winter abodes, are, however, those formed 



