02 TORPIDITY AMONG MOLLUSKS. 



no air can pass out from or into the shell, and its occupant settles down 

 to undisturbed repose. The circulation of his blood seems quite to cease, 

 and it cannot be detected that he breathes — at least a chemical test will 

 show that the air within the shell is as pure as that outside. It is wonder- 

 ful what extremes of temperature these little creatures are able to bear. 

 The fiercest heat of summer does not harm them, nor excessive cold. 

 Professor Verrill, of Yale College, told me the other day that he once 



THICK-SHELLED PALUDIN/E. 



placed eighteen snails in a box containing sand two or three inches in 

 depth, and left it exposed in his garden during the winter. The mercury 

 went to twenty degrees below zero, yet in the spring half of them were 

 alive, though the)' must have been frozen solid many times. 



The same is true of the bubble of air with which the water-spider 

 surrounds himself in his torpid rest under the ice of the pond. If yon 

 should break in the drum-head of the snail he would freeze to death ; and 

 if you draw the air out of the bubble around the spider, he will drown 

 before he awakes. It is necessary, then, to the safety of both, though 

 neither seems to breathe it; perhaps it serves as a kind of blanket, within 

 which snails and other little creatures are able to pass without exhaustion 

 the long, cold months, during which, if active, they would be unable to 

 get any proper food, and therefore would starve to death. 



Some snails, however, seem to hibernate very little. Thus the small, 

 glassy mollnsk, Vitrina, has often been seen crawling about when there 

 was snow on the ground. It is perhaps in consequence of this hardihood 

 that it extends its wanderings into polar regions, and can be collected 

 upon very high mountain-tops. I have seen it lively in the high sierras 

 of Colorado, when the grass was stiff and white with frost, all the brooks 



