CH. VI] THE FRESH-WATER MUSSEL 199 



most or nacreous layer of the shell. The determining 

 cause of the formation of a pearl is the presence of some 

 foreign body which serves as an irritant and forms, as it 

 were, a nucleus round which the nacreous deposit is laid 

 down. The pearly matter is produced at the expense of 

 the shell, and hence pearl-bearing specimens are usually 

 to some extent deformed or misshapen. Dr Lyster 

 Jameson 1 has recently shown that in the case of the 

 marine mussel Mytilus edulis pearls are formed in a closed 

 sac of the shell-secreting epithelium of the mantle, and 

 that those formed in the extreme margin of the mantle 

 are composed mainly of periostracum, while those which 

 occur elsewhere are composed of nacreous material. The 

 epithelial sac is first formed round a living trematode 

 larva, Distomum (Leucithodendrium) somaterice Lev., which 

 enters upon a resting stage in the tissues of Mytilus. 

 The trematode may become imprisoned within the pearl 

 and die or may effect its escape, but, the cyst having 

 been formed, the pearl is in either case completed. The 

 first host of this trematode is the cockle, Cardium edule, 

 or the " tapestry shell," Tapes decussatus j the final hosts 

 to which it gains access by the Mytilus are almost 

 certainly the eider-duck, Somateria mollissima, and the 

 black scoter, (Edemia nigra, both of which birds devour 

 quantities of Mytilus. It is highly probable that the 

 pearls found in the Unionidce, notably in the Scotch river- 

 mussel Unio margaritifer, will, if investigated, prove to be 

 of like origin and causation. 



Pearl fisheries are still existent in various Irish and 



1 Proc. Zool. Soc. vol. i. London, 1902. 



