108 PEARLS [CH. Yiii 



wliich we shall return later. Suffice it to say that 

 Herdmaii and Hornell believed them to be formed 

 owing to internal causes and not to any parasite or 

 other body of external origin. 



With regard to the other really fine oriental 

 pearls, Herdman and Hornell came to the conclusion 

 that the chief agent concerned in their formation was 

 the tapeworm larva, which occurred so frequently in 

 the pearl oysters. The oriental pearls are of course 

 like all others formed by the activity of a pearl sac, 

 and Herdman recognised the great resemblance of 

 this to the outer shell-secreting epithelium. The 

 origin of the pearl sac has, however, never been 

 observed in the Ceylon pearl oyster, but Herdman 

 thought that the parasite boring into the mollusc 

 might carry in certain epithelial cells from the 

 surface. 



What is the life-story of this jmrasite which occurs 

 in the pearl oyster? It must be confessed that so 

 much doubt exists concerning the relation of the 

 encysted larvae to both the pearls and the adult 

 tapeworms that the technicalities are beyond the 

 scope of this little book. The conclusions of Herd- 

 man, Hornell and Shipley (who described the para- 

 sites found in the Ceylon pearl oysters and fishes) 

 can be expressed in the following form : 



