28 PROCEEDINGS OF THE 



to the life-history, since the rays also feed upon pearl-oysters, 

 still there is nothing in the observed facts to forbid the existence 

 of such a stage, and it is not unusual in Tetrarhynchids to have 

 two fish-hosts, an intermediate Teleostean which is devoured by a 

 final Elasmobranch. 



The huge fish, then, which devastates our Pearl Banks is all- 

 important in passing on to future generations of the oysters which 

 it devours the parasite upon which pearl-production depends. No 

 one knowing the facts could advocate the destruction or even the 

 exclusion of this particular enemy of the pearl-oyster. If the rays 

 increase much in number the beds of oysters w"ill be decimated if 

 not exterminated, and the pearl-fisheries ruined; and, on the othei- 

 hand, if the rays are greatly reduced in number the parasites will 

 be likewise reduced, and although the oysters may flourish they 

 will have few pearls and the fisheries again will be ruined — so 

 intricate and nicely balanced are sometimes the processes of 

 Nature. 



The further question then arises — Can we profitably follow up 

 Ivelaart'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 Mytilns) to another of a 

 different genus (Margaritifera). 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 Fi'ance ; 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 un- 

 necessary under present conditions to take any steps to ensure 

 infection with the appropriate parasite. Oysters wherever they 

 appeal', 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, this may be done with 

 perfect confidence that when the four-year-old oyster is fished 

 it will contain the normal * 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 

 oysters at Trincomalee ; the fishes that are, in all probability, the 

 hosts of the parasite in its more advanced stages also abound at 

 various points. It is the molluscan host, and not the parasite, 

 that stands in need of artificial aid in Ceylon. If we can increase 



* Of course, some beds are richer in pearls than others and some years are 

 better than others. 



