12 TERRKSTRIAL AIR-BREATHING MOLLUSKS. 



visible in the daytime that thousands may he near without being known. 

 The injury which they commit in kitchen-gardens, for this reason, is 

 often vaguely ascribed to worms or to birds ; and no measures are taken 

 against the real culprits. Their habits, in general, coincide witli those 

 "which have been described as distinguishing the order ; and we shall 

 therefore mention here only those which are peculiar to them. They 

 differ from the other families in not possessing the faculty of hiberna- 

 tion, or suspension of their organic functions during the cold season. 

 In temperate latitudes the snails hibernate, 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 iiaunts until 

 severe frosts set in, when they retire into the earth and other sheltered 

 retreats. 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 situations are in motion throughout 

 the year ; and individuals of all the genera and species which we have 

 kept in confinement have continued active, fed freely, 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 accu- 

 mulating a quantity of tenacious mucus at the posterior extremity of 

 the foot, which they attach to the object from which they are to com- 

 mence their descent ; then, loosing their own hold, they hang suspended 

 by this point. Continuing 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 eye- 

 peduncles and its whole body in various directions, as if seeking some 

 object on which to make a lodgment. The faculty of suspending them- 

 eelves in this manner indicates that they pass some part of their lives 



