S6 Pearls. 



two valves, the upper one rounded and the lower one 

 flat. They are composed of consecutive layers of 

 ''nacre" secreted by the oyster, each layer trending 

 away from the hinge and over the horny lip more 

 than its fellow below ; these layers are thicker at 

 the hinge than at the lip, and they are continually 

 in process of formation. This continued growth is 

 necessary, in the first place to enlarge the young 

 shell so as to accommodate the increasing dimen- 

 sions of the owner, and secondly in order to renew 

 and keep up the necessary thickness of the shell, 

 which through external influences decays away ; the 

 nacre is also secreted in order to cover any foreign 

 substance, such as mud or weed, that may drift 

 into the shell, and being unable to escape may 

 cause inconvenience to the delicately constructed 

 mollusc ; it likewise serves to arrest the progress of 

 the "borers," which attack the shell and seek to effect 

 an entrance. Conchologists assert, as stated in the 

 preceeding chapter, that the nacre is secreted by 

 the mantle, but it is probably that the adductor 

 muscle itself contributes to the formation of that 

 portion of the shell which bears the adductor 

 impression. 



The resplendent play of colour which the inner 

 surface of the Pearl shell exhibits — a display that 

 defies any attempt at artificial imitation — is not 



