104 PEARLS [CH. VIII 



have a parasite I3 ing in a little sac (the pearl sac) of 

 epithelium which is decidedly the same as the shell- 

 secreting epithelium of the mantle, because it was 

 once continuous with it (fig. 9 d). This pearl sac 

 will have already commenced secreting layers of 

 shell-like substance round the parasite which has 

 been the cause of the sequence of events leading to 

 its formation. 



Thus pearls are formed in the common edible 

 mussel. This method, however, is not to be supposed 

 for a moment as applicable to other molluscs without 

 experimental evidence. As a matter of fact, it is not 

 the only method of pearl formation even in Mytilus. 



Now let us turn to the Ceylon pearl oyster which 

 produces such valuable pearls and see what processes 

 are involved there. 



The problem of the origin of the valuable oriental 

 pearls is obviously of greater economical importance 

 than that of the Mytilus pearls. The discussion of 

 the theories brought forward will be given in two 

 separate sections, for if the description of pearl forma- 

 tion in the fresh-water mussel is interpolated in the 

 middle of our account of the Ceylon pearl oyster 

 we shall be following the discoveries in their proper 

 sequence. 



In 1902 the British Government sent to Ceylon 

 a scientific commission (consisting of Prof. Herdman 

 and Mr J. Hornell) in order to report upon the 



