Mother-oJ'Pearl Shell. 97 



bed. This is seen in Figs. 5, 6 and 7. {^See 7iext 

 page). The borer is represented in Fig. 5, attacking 

 the exterior of the shell, and the oyster has secreted 

 an unusually thick layer of nacre opposite the point 

 of attack. In Fig. 6, the borer has penetrated into 

 the substance of the shell, but successive deposits of 

 nacre have been formed to resist the intrusion. In 

 Fig. 7, the decay of the external part of the shell 

 has removed the borer, while the internal surface 

 has resumed its natural form. The three figures are 

 placed at different levels in order to shew that as 

 external decay proceeds, the oyster adds to the 

 internal surface. All these figures are taken from 

 photographs of the Pearl shell. 



The oyster's sole means of protection against 

 crabs and other enemies, is that of closing its shell ; 

 if, however, this shell is pierced completely through 

 by a " borer," (Fig. i, p. 85), a breach of its walls is 

 effected, and the defenceless citadel is open to attack. 

 Thus dead shells generally exhibit the cause of their 

 late owner's disappearance. A large species of 

 Murex, a Univalve, is also an inveterate enemy of 

 the Mother-of-Pearl oyster, attaching itself to the 

 shell, and boring through it. When, therefore, a 

 shell is found with any unnatural protuberance on 

 its interior surface, the pearler scrapes away a little 

 of the decaying shell at the back of this spot, and if 



