VIII] THE ORIGIN OF PEARLS 99 



His paper followed in 1902, and confirmed the 

 discovery of Dubois. Both these workers definitely 

 established the fact that in the edible mussel from 

 Billiers, pearl formation was caused by larval worms 

 which were parasites in the shellfish. Moreover, the 

 larvae were recognised as the young of the flatworms, 

 known as trematodes (related to the familiar species 

 which occurs in the liver of sheep and causes liver 

 rot). Dubois simply followed up the methods of 

 previous investigators. Jameson gave a much more 

 detailed account of pearl formation, and pointed out 

 the great importance of discovering the method by 

 which the larval parasite caused the formation of the 

 pearl sac. In addition to this he attempted to trace 

 the young parasite to the adult worm. Like many 

 other parasites, this worm requires several hosts in 

 order that it may live its normal life. Just as the 

 trematode parasite (the liver fluke) of the sheep 

 requires two hosts, a pond snail and the sheep, and as 

 the well-known malaria-causing parasite requires both 

 mosquitos and man, so does the worm parasite in the 

 mussel require at least one other host. Jameson 

 attempted to piece together by expei'iments this life- 

 history, and came to the conclusion that the adult 

 form lived in a bird, the common Scoter, which feeds 

 on the mussels and destroys large numbers of them 

 at Billiers. His investigations gave the following 

 interesting life-cycle for the parasite. 



7—2 



