Vlll INTRODUCTION 



On the Formation of the Shell, in Testaceous Animals. 



If we were to be asked why one slug goes naked, 

 while another is provided with a habitation ; why 

 two animals of the same manners, the same appe- 

 tites, the same internal, and nearly the same exter- 

 nal conformation, should be so differently dealt with, 

 it would be difficult to find a satisfactory answer. 

 The powers of nature are generally sufficient for 

 the purposes she intends to fulfil. Now and then 

 there appears to be something wanting in her ope- 

 rations ; but when we tax her with deficiencies, it is 

 still possible that she may be right, and we be 

 wrong. In the present instance, there seems to be 

 either profusion in one case, or parsimony in the 

 other, since the common slug has merely the rudi- 

 ments of a shell ; it can lay only the first stone, while 

 the garden snail can build a house. 



It is not to our purpose to consider who first dis- 

 covered the thin testaceous plate on the back of the 

 slug ; it is enough to be assured that it exists, and to 

 describe it accordingly. There is an oval, warty 

 substance, extending from the neck, partly down 

 the back of the slug, called the lesser mantle, or 



