g6 Pearls, 



internal surface of the valve by a single layer of nacre. 

 In Fig. 3, the obstruction has become buried in the 

 substance of the shell, and the inner surface is again 

 level, because the successive layers of nacre are 

 thinner on the top of the foreign body. At the 

 same time, the decay of the outer surface has been 

 advancing, until in Fig. 3, it has nearly reached the 

 Pearl or other intruder ; and finally in Fig. 4, this 

 foreign body has entirely disappeared, together with 

 its encircling layers of nacre, having been set free 

 by the destruction of the external part of the 

 shell. 



Besides the possession of this faculty, it is evi- 

 dent that the oyster is sensible to vibrations of 

 either sound or motion. When it is lying agape 

 in the boat, any slight concussion will cause it im- 

 mediately to close tightly, like a vice. Many a 

 shell is discovered by bubbles of gas arising from 

 it, in the act of closing, on account of the near ap- 

 proach of a diver, and a consequent tremor of the 

 water. The most remarkable fact, however, is that 

 it is so far conscious of the point of attack of a 

 "borer" (Fig. i, p. 85), eating its way through the 

 shell, that, in order to guard against the danger of 

 being pierced, the oyster secretes extra-thick layers of 

 nacre at that point, and thus, in course of time, 

 seriously disturbs the natural shape of its pearlx- 



