COMMONER FORMS OF SHELLS. 17 



Mr. Gwyn Jeffreys in the first volume of his 

 " British Conchology." One of them is the habit 

 which the animal has of emitting its purple- 

 coloured blood on being irritated, apparently as a 

 means of defence. This is especially noticeable 

 from its large size in P. corneus; and although 

 experiments have been made with the view of fix- 

 ing and utilizing the purple dye thus yielded, they 

 have hitherto proved unsuccessful. Another pecu- 

 liarity is, that several of the vital organs are placed 

 on the left side of the body, instead of on the 

 right (as in most other univalves), while the shell 

 is coiled the other way, that is, from left to right. 



Some of the smaller species, found in marshy 

 ground which becomes dried up in summer, close 

 the mouth of their shell with an epiphragm or filmy 

 covering, and live in retirement until a return of 

 moisture tempts them forth again. They are all 

 herbivorous in their nature. 



It would not be possible within the limits of the 

 present chapter to attempt a description of all the 

 genera of land and freshwater mollusca. In the 

 above remarks we have directed attention only to 

 the more prominent forms. We have not described 



c 



