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 
