ORIGIN OF ORIENTAL PEARLS 13 



oyster is quite young and hardly fixed on the sub- 

 merged reefs, whilst the over-fishing takes place when 

 the animal is fully matured and perhaps growing old. 

 The fact that Professor Herdman and Mr. Hornell 

 conveyed the young oysters from Manaar in the north 

 of the island by boat to Colombo and then on by train 

 to Galle in the south, and there succeeded in rearing 

 them, shows that there would be little difficulty in 

 artificially rearing oysters in convenient localities and 

 then transplanting them to such fishing-grounds as 

 show danger of depletion. With regard to over- 

 fishing, if the grounds are under the charge of a 

 trained zoologist there is no reason why this should 

 go on. 



When Professor Herdman was called in to advise 

 the Government, he saw at once that it was the oyster 

 that had failed in the last ten years, not the pearls 

 within the oysters. Microscopic examination of thin 

 sections made through decalcified pearls showed that 

 they are almost in all cases deposited around a minute 

 larval cestode or tapeworm. These larvae make their 

 way into the oyster, and the irritation they set up 

 induces the formation of the pearl, just as was the 

 case with the cercaria-formed pearls of the mussel. 

 Where do these larvae come from ? We cannot say 

 with absolute certainty. Older specimens of tape- 

 worms belonging to the new species, Tetrarhynchus 

 unionifactor, also live in the oyster ; and it may be that, 

 were a larva to escape entombment in a pearl, it would 

 grow up into one of these. But even these never 

 become mature in the oyster; to attain sexual maturity 

 they must be swallowed by a second host. What is 

 the second host of the pearl-forming cestode ? This 

 question we are only recently able to answer, and 

 here, again, without absolute certainty. I have 

 recently described the adult form of T. unionifactor 

 from a large ray, Rhinoptera javanica. In this fish, 



