124 CrRIOUS DEFENCE. 



Xor is this all ; for when this first c^'^^^- 

 barrier is formed, it afterwards constructs 

 a second, of a membraneous nature, placed 

 more within, and at a little distance from the first. K. 

 too, at any other season, while the snail is in full 

 vigour, it be surrounded, for the sake of experiment, 

 with a freezing mixture, it will immediately set about 

 constructing a covering for its protection from cold ; 

 and with such diligence does it work, that in the 

 course of an hour or two it will have finished its 

 task, and a complete epiphragma will be formed. On 

 the genial warmth of spring penetrating its abode, 

 the snail prepares to emerge from its prison, by se- 

 creting a small quantity of a mucous fluid, which 

 loosens the adhesion, and the defence is thrown off by 

 the pressure of its foot. On every occasion when 

 another covering is required, this process of construc- 

 tion has to be renewed. By means of it snails and 

 other creatures may be preserved for months, and even 

 years, in a torpid but living state, ready to be restored 

 to their most active functions when sufficient warmth is 

 apphed. 



The argonaut, nautilus, and creatures of like habits, 

 require shells as hght as mav consist vdih the requisite 

 strength ; and, consequently, the relative specific gravity 



