114 PEARLS [CH. 



separated and lies free in the tissues (%. 1 2 c). So 

 the pearl formation in this fresh-Avater mussel com- 

 mences actually in the outer epithelium and is caused 

 b}^ a product of the mollusc itself. Thus ^ve arrive 

 at a very important point, which is that there is no 

 obvious diflPerence between "muscle pearls" and other 

 true pearls, for both are formed round nuclei which 

 are products of the shellfish. Any difference in 

 structure may be simply due to their difterence in 

 position. 



The pearl sac appears to possess the power of 

 secreting all the various kinds of shell substance, one 

 after the other. Parts of it may even be secreting 

 one kind of substance whilst other parts secrete 

 another. What determines the character of the sub- 

 stance to be secreted is quite unknown. 



Thus we have dealt with a second method and 

 cause of pearl formation, which has now been demon- 

 strated by Rubbel to be of frequent occurrence. 



In conclusion let us return once more to the 

 oriental pearls from the Ceylon pearl oyster. We 

 have seen that Herdman recognised various causes, 

 and that the muscle pearls were supposed by him to 

 be formed round a substance which was a product of 

 the shellfish itself The chief cause, however, in the 

 case of cyst pearls was supposed to be a cestode 

 parasite which was found infecting the oysters in 

 abundance. 



