THE WEB OF LIFE 313 



But these are not ' fine pearls '. The experiment has 

 often been made of boring a hole through the shell and 

 inserting a minute fragment of mother-of-pearl between the 

 shell and the mantle ; this makes a centre for pearl- form- 

 ation, and a more valuable semi- artificial pearl results. 

 Gradually it began to be suspected that the really fine 

 clear pearls, with translucent centres, were formed around 

 minute intruding parasites, which the skin of the mollusc 

 imprisoned, somewhat in the same way as the oak imprisons 

 the larva of a gall- wasp within an ' oak-apple '. 



In 1902 H. Lyster Jameson showed that the agent in 

 forming the pearls in the common Edible Mussel (Mytilus 

 edulis) is the larva of a parasitic Trematode, which, instead 

 of secreting a cyst of its own, as is usual with such larvae, 

 stimulates the mussel to form around it a sac of epidermal 

 cells. These cells possess the same physiological properties 

 as the outer shell- secreting epidermis, and eventually, on 

 the death of the Trematode larva, secrete conchiolin and 

 calcareous salts, which, deposited in concentric layers around 

 the remains of the worm, become the pearl. But the life- 

 history remains obscure. It is possible that the early stages 

 of the Mytilus parasites live in the cockle (Cardium 

 edule), where closely related forms certainly occur. It is 

 possible that the adult form of the Mytilus parasite is to be 

 found in the Scoter Duck, but the experiments made to 

 test this have not yielded any conclusive result. 



It has been suggested that the fine pearls of the Ceylon 

 pearl-oyster are due to the larvae of a tapeworm, Tetra- 

 rhynchus unionifactor, but the searching work of Lyster 

 Jameson does not confirm this conclusion. There is no 

 doubt that the young stages of this tapeworm occur in 

 the pearl-oyster, along with pearls, but it does not follow 



