8 LAND AND FRESH-WATER SHELLS OF N. A. [PART I 



uate, under all circumstances, on the approach of cold weather ; 

 the slugs, on the contrary, having the power of resisting extreme 

 cold, continue in their usual haunts until severe frosts set in, 

 when they retire into the earth and other sheltered reti'eats. 

 Here they remain in a state of inaction and partial torpidity ; 

 the functions of the body, however, still going on, though slowly 

 and with diminished force. A slight increase of heat arouses 

 them and stimulates their organs to renewed action, and they 

 accordingly often come abroad in mild weather, even during the 

 winter. Those which inhabit cellars and other protected situa- 

 tions, are in motion throughout the year ; and individuals of all 

 the genera and species which we have kept in confinement have 

 continued active, fed frecl}^, and increased in size as much in the 

 coldest months as in the summer. 



All the species which have yet come under our notice possess 

 the power of suspending themselves in the air by a gelatinous 

 thread. This they effect by accumulating a quantity of tenacious 

 mucus at the posterior extremity of the foot, which they attach to 

 the object from which they are to commence their descent ; then, 

 loosing their own hold, they hang suspended by this point. Con- 

 tinuing the secretion, their own weight attenuates the mucous 

 attachment, and draws it out into a thread. As this dries and 

 hardens, a fresh supply is afforded, the thread is lengthened, and 

 the animal lets itself down any desirable distance. At this time, 

 also, the margin of the foot pours out mucus freely, and during 

 the whole operation the locomotive disk is in active undulatory 

 motion, in the same manner as when in ordinary progression. It 

 appears in this way to guide and force towards the extremity the 

 mucus which is secreted on its surface, and which, collected at 

 its extreme point, forms the thread. The slug often pauses in its 

 descent, and extends its tentacles and its whole body in various 

 directions, as if seeking some object on which to make a lodg- 

 ment. The faculty of suspending themselves in this manner 

 indicates that they pass some part of their lives on trees, from 

 which they can thus make a convenient descent to the earth ; 

 there are some species, indeed, which are stated to inhabit trees 

 almost exclusively. It may serve also as a means by which they 

 can suddenly escape from the attacks of their enemies, and par- 

 ticularly of birds. It is mostly, however, when they are young, 

 or at least not grown to their full size, that they eyjoy this power. 



