ANY different systems have 

 been adopted at various 

 times for the intelligent and 

 orderly arrangement of the 

 bivalve mollusks (Pelecy- 

 -^ ^ 2Joda). That followed in 



these pages is based upon the character of the 

 breathing organs, and, all things considered, this 

 appears to afford the most natural basis. In the 

 sketch of the Swan Mussel in the opening chapter 

 the position and structure of these organs were briefly 

 indicated, but in that species the gills have reached 

 a high stage of development. Starting from the 

 simplest form of such organs we find examples in 

 the little Nut-shells that burrow in the mud and 

 sand of our seashores. 



The most plentiful of these is the Common Nut- 

 shell {Nucula nucleus), whose empty shells may be 

 found along our beaches washed in from deeper 



54 



