170 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Distomum somaterice, a Trematode worm, the adult of 

 which lives in the intestines of the Eider duck and the 

 Scoter duck. He also stated that the larva inhabits Tapes 

 or the cockle as a first host before getting into the musseh 

 and gave figures of the parasite in various conditions. 



Two very important matters are, however, left in a 

 somewhat unsatisfactory condition by Jameson's paper. 

 The first of these is the mode of origin of the epithelial 

 sac which encloses the larval parasite, and which secretes 

 from its cellular wall s layer after layer of nacreous material 

 so as to form a pearl. The presence of this sac was known 

 before (Von Hessling, 1858, and Diguet, 1899), but no one 

 has yet satisfactorily traced its origin. Jameson several 

 times compares it with the epithelium on the outer surface 

 of the mantle, using such terms as "similar to" and 

 "indistinguishable from" but he evidently considers that 

 it has nothing to do with that epithelium, although it 

 produces an identical pearly secretion. He describes the 

 sac round the parasite as formed by the proliferation of a 

 few cells which " are basally continuous with fibres of 

 connective tissue." He also says of it, " This epithelium 

 appears to arise quite independently of the outer 

 epidermis." Now such a mode of origin as this is very 

 unlikely, and although I have not had the opportunity of 

 re-examining pearl- bearing mussels on this point since 

 Jameson's paper appeared, I think there can be little or no 

 doubt that the cells of the pearl sac are directly and 

 genetically connected with the exactly similar cells on the 

 outside of the mantle. It is almost certain that the 

 parasite in burrowing into the mantle carries in with it one 

 or more epidermal cells which proliferate to form the sac. 

 As the Distomid larvse are found moving on the inner 

 surface of the shell before coming to rest in the mantle 

 they must traverse the epidermis, and it is natural to 



