INTRODUCTION. IX 



shield ; in the thickness of this mantle there is a 

 cavity which contains the shell. The under part of 

 the shell is separated from the heart of the animal, 

 by a thin membrane ; above, it is covered by the 

 skin of the mantle. It lies in its cavity perfectly 

 detached, without any organic union, and is a sin- 

 gle deposition of carbonate of lime, connected by 

 gluten, exactly like any other shell. It is to be 

 found in the small slugs, as well as the large ; but in 

 the great brown slug, it is half an inch long, and a 

 quarter broad. There is no doubt that the calca- 

 reous matter is secreted from the lower part of the 

 cavity, and that, the operation being continued at 

 different times, the shell accumulates as the animal 

 advances in growth. The different accessions of 

 shell may be distinguished by the striae seen in its 

 substance. 



We have here a very simple process, confined to 

 the mere secretion of calcareous matter, without any 

 mechanical contrivance whatever. It is not so with 

 the garden snail; her powers are by no means so 

 limited. She can construct a habitation which 

 unites two distinguishing properties, strength and 

 lightness. It is a well turned arch, that can defend 

 her body from all common accidents, and be trans- 

 ferred, without inconvenience, wherever she pleases. 

 The snail is oviparous, and the young leave the egg 

 completely covered with a very thin transparent 



vol. i. h 



