Ill] 



THE PEARL OYSTER 



21 



occurs at places where the closing (or adductor) 

 muscles are attached. It will be necessary to describe 

 these layers in some detail, for the same structures 

 are present in pearls. In fact, pearl structure is very 

 similar to shell structure. 



The outermost layer of the shell is known as the 

 Periostracum (see fig. 2). It is a more or less horny 

 layer which is normally secreted by the cells of the 



Fig, 2. Diagram of section of a typical molluscan shell. P, Peri- 

 ostracum, Pris., Prismatic layer. Nac. , Nacreous layer. 

 H, Hypostracum. 



edge of the mantle only. Consequently it does not 

 increase in thickness after once it has been formed. 

 In young shellfish the periostracum is relatively much 

 thicker than in old specimens, for the other layers 

 increase in thickness. Very often, too, the peri- 

 ostracum is really thinner in the adults, because it 

 gets rubbed ofi*. 



The periostracum is sometimes continued in the 

 form of numerous lamellae, into the next deeper 



