l'l.AKL PRODUCTION. 



29 



Fig. 6. Aggregation of encysted 

 Cestode larvae in the mantle of 

 Planum phii-i ill, i : magnified. 



The encysted Cestode larvae found in the pearl-oyster are, however, also present in 



Placuna sometimes in great numbers, so as to be densely crowded together in the 



superficial layer of the mantle, as shown in fig. 6. Compound cysts, where one larva 



occurs within the vesicle of another, are sometimes seen. Similar larvae, both alive 



and as nuclei of pearls, are also found in specimens 



of Placuna placenta from the Gulf of Kutch.* In 



fact, a fuller experience is causing us to incline to 



the view that various parasites may act as pearl 



nuclei even in the same mollusc. Some pearls are 



certainly formed round intrusive Nematodes. We 



have a complete cyst pearl, free and unattached, of 



which the nucleus is a coiled Cheiracanthus unci- 



natus, on which the pearl deposit is not sufficiently 



thick and opacpie to obscure the coils so as to render 



identification difficult. 



But although Kelaart's statement, half a century old, that various kinds of worms 

 are concerned in pearl -formation may be correct, still we hold that our investigation 

 has shown that in Margaritifera vulgaris, at Ceylon, the production of the orient 

 pearl is dependent upon Cestode infection and that the species mainly concerned is 

 Tetrarhynchus unionifactor. 



The next question that naturally arises is Can we profitably follow up Kelaart's 

 suggestion that it might be possible to increase the number of pearls by infecting the 

 molluscs with the appropriate parasites ? This " margarose artificielle " has been 

 tried, as we have shown above, by Dubois in a case where the parasite was supposed 

 to migrate from one mollusc (a Mytilus) to another of a different genus (JUargariti- 

 />')'(). Giard and others have pointed out the difficulties in the way of accepting 

 this case, and the doubts that naturally arise ; and we are probably correct in 

 concluding that the method has not as yet resulted in a marked success on the 

 southern coast of France ; although it is quite possible that similar methods with 

 other shell-fish elsewhere may give good results. 



On the Ceylon pearl banks, however, it is probably quite unnecessary to take 

 any steps to ensure infection with the appropriate parasite. Oysters, wherever they 

 appear, when they are old enough contain pearls, and encysted parasites are even more 

 abundant. Even when new beds are formed artificially by transplanting to unoccupied 

 ground, as we do not doubt will be the case in the future, that operation may be 

 carried out with perfect confidence that when the four-year-old oyster is fished it will 

 contain the normalf supply of pearls. The parasites are probably so widely spread 

 that every pearl oyster in the Gulf of Manaar, or, for that matter, around the coast 

 of Ceylon, runs a fair chance of becoming infected. Cyst pearls are found in the 



* See HoitNELi.'s Reports from the Ceylon Marine Biological Laboratory, Part II., 1906. 

 t Of course some beds are richer in pearls than others and some years are better than others. 



