BOOK V. 

 MOLLUSCOUS ANIMALS. 



I. Bivalves, or those having two shells. 



THE PEARL OYSTER. (Avicula Margaritifera.) 



WHO that sees the beauty and delicacy of pearls would 

 imagine that they were the production of disease ? 

 Such, however, is the case, as they are either formed in 

 the body of the oyster which inhabits the shell ; or they 

 rise from cracks in the shell itself, the delicate, silvery, 

 half-transparent lining of which forms the substance 

 generally called Mother-of -Pearl, or Nacre. Their for- 

 mation is generally caused by the introduction of some 

 foreign body between the mantle or skin of the animal 

 and its shell ; the irritation thus produced causes succes- 

 sive coats of pearly matter to be deposited on the in- 

 truding object, and thus the pearl is formed. The best 

 pearls are those which are fairly imbedded in the sub- 

 stance of the mantle. These shells are found in the 



