924 OYSTERS, AND ALL ABOUT THEM. 



or layers, in the same way that the solar rays are distributed 

 in divers colours in the rainbow, by the surface of drops of 

 clear water. 



The Chinese and other eastern nations, knowing the 

 details of the Pearl-producing process, are in the habit of 

 introducing small particles under the mantle of the mol- 

 luscs, and of thus procuring in due time artificial Pearls. 

 Small metallic figures or images may sometimes be seen, 

 which have been coated with a pearly or nacrous deposit 

 through their introduction within the shells of the Pearl 

 Oyster and others. Pearls being a calculus, or morbid 

 concretion, are also formed in consequence of some exter- 

 nal injury which the shell receives, particularly from the 

 operations of certain minute worms, which occasionally 

 bore even quite through to the animal. The pearls are 

 formed in the inside on these places. Hence it is easy to 

 ascertain, by the inspection of the outside only, whether a 

 shell is likely to contain Pearls. If it be quite smooth, 

 without cavity, perforation, or callosity, it may with cer- 

 tainty be pronounced to contain none. If, on the contrary, 

 the shell be pierced or indented by worms, there will 

 always be found either Pearls or the embryos of Pearls. 

 It is possible, by artificial perforations of the shells, to 

 cause the formation of these substances. The process 

 which has been chiefly recommended is to drill a small 

 hole through the shell, and to fill this hole with a piece of 

 brass wire, rivetting it on the outside like the head of a 

 nail ; and the part of the wire which pierces the interior 

 shining coat of the shell will, it is said, become covered 

 with a Pearl. In connection with the Pearl-bearing shells 

 above mentioned, there is also the Unios, or fresh-water 

 mussels of our own rivers, to which I will presently allude. 

 As the reader has already been given to understand, in the 



