94 Pearls. 



especially layers of weed are thus found embedded 

 in shells, forming unnatural excrescences on the 

 surface. 



These ''blisters" may be found apparently empty 

 and discoloured within, or full of water and half 

 decayed substances ; in both cases an offensive 

 odour points out that the enclosed substance was 

 of animal or vegetable character. Pearls are also 

 frequently found in ''blisters;" they escape from 

 the body of the oyster, and are then treated in 

 exactly the same manner as a stone or other 

 intruder, being covered over by each successive 

 layer of nacre. These unnatural protuberances on 

 the inner surface of the shell are gradually removed 

 by the oyster secreting thinner layers of nacre on 

 the top of them than at the base, until the surface 

 becomes again level. Slowly but steadily the ex- 

 terior surface of the shell decays and disappears, until 

 the blister and its contents come within reach of 

 the advancing dissolution, and then these in turn 

 disappear. Any inorganic or insoluble matter in 

 the blister falls to the bottom ; and thus the oyster 

 literally passes a stone or other intruder through its 

 shell. 



This process is illustrated by Figs. 2, 3 and 4, i^See 

 opposite page). In Fig. 2, a small Pearl, or other foreign 

 body fallen into the shell, has become cemented to the 



