6 PEARLS AND PARASITES 



larvae of these flukes form the last stage in a complex 

 series of larval forms which occur in the life-history of 

 a trematode or fluke, and they differ from the adult in 

 two points their generative organs are not fully de- 

 veloped, and they usually have a tail ; but this organ 

 is wanting in our pearl -forming cercaria, called a 

 cercariaeum by Mr. Jameson. Such a larva has only 

 to be swallowed by a scoter to grow up quickly into 

 the adult trematode capable of laying eggs. Now 

 this bird, called by the French fishermen the ' cane 

 mouliere,' is the greatest enemy to the mussel-beds ; 

 it is not only common around the French mussel-beds 

 of Billiers (Morbihan), but occurs in numbers at the 

 mouth of the Barrow channel, close to our English 

 pearl-bearing mussel-beds. With its diving habits it 

 destroys and eats large quantities of the mollusc. 

 Those cercarise which are already entombed in a 

 pearl cannot, of course, grow up into adults, even if 

 they gain entrance to the alimentary canal of the 

 scoter ; but those that are not ensheathed may do so. 

 Further, the fluke may possibly live in other hosts 

 where no pearl is formed. At any rate, there seems 

 no lack of larvae successful in their struggle to attain 

 maturity, for it has been calculated that the alimentary 

 canal of an apparently healthy scoter may harbour as 

 many as six thousand adult flukes. 



Thus there are two courses open to the cercaria 

 when it has once found its way into the mussel ; it 

 either forms the nucleus of a pearl and perishes, or it 

 is swallowed by a scoter, becomes adult, and prepares 

 to carry on the race. But how do the cercariae make 

 their way into the mussel, and whence do they come ? 

 At present their birth, like that of Mr. Yellowplush, is 

 ' wrapped up in a mistry.' We may presume that the 

 eggs make their way out of the scoter into the sea- 

 water, and that there they hatch out a free-swimming 

 larva, which, after the manner of trematodes, swims 



