2 INTRODUCTION. 



Testaceous animals inhalit their shells^ to which 

 they are only partially attached ; whereas the 

 Crustacea are indued with theirs, each limb being 

 invested with its own peculiar shield. 



Shells are either terrestrial, or found in rivers, 

 lakes, in shallows of the sea, or in the deep- 

 er beds of the ocean ; they are, however, all sub- 

 ject to the same arrangement, according to their 

 generic and specific characters, and not accord- 

 ing to their individual locality. 



On the physiology of testaceous animals, 

 which belong to a distinct Order, that of Mollusca 

 in the Class Vermes, it will not be expected that 

 much should be said in a mere elementary treatise. 

 Still only half the object would be attained, if it 

 were totally silent on a subject so fraught with 

 wonder, so indicative of the wisdom and the Pro- 

 vidence which first produced these little admirable 

 architects, and instructed them to form their 

 beauteous receptacles. Much as we delight in 

 viewing the deHcate and brilliant tones of colour, 

 the symmetrical structure, or the picturesque and 

 rugged surface which distinguish the different fa- 



