INTRODUCTORY CHAPTER. 



3 



mode by which it forms its shell, its place of 

 residence, and habits, before entering upon a 

 minute and classified division. The present 

 chapter will therefore have these objects in 

 view. 



The animals which inhabit and form shells 

 are called Mollusca *, and, as their name signi- 

 fies, have a soft body, most of them being en- 

 tirely destitute of bones, though some are 

 provided with internal shells, more or less per- 

 fectly formed ; some species of slug, for in- 

 stance, have internally a rudely shaped hard 

 body, and others a mass of small grains. To 

 compensate for the apparent deficiency in this 

 respect, and to enable them to provide them- 

 selves with a defence against any injury to 

 which they might be liable from so delicate a 

 structure, they have been furnished with an ex- 

 ternal skin or envelope, covering them more or 



* From mollis, Lat., soft, and esca, Lat., flesh. La- 

 marck gives the name of Mollusca only to those which 

 inhabit univalve shells, but naturalists now generally com- 

 prise under that appellation the inhabitants of both uni- 

 valves and bivalves. 



B 2 



