90 
Distomum somaterie, a Trematode worm, the adult of 
which lives in the intestines of the Eider duck and the 
Seoter duck. He also stated that the larva inhabits Tapes 
or the cockle as a first host before getting into the mussel, 
and gave figures of the parasite in various conditions. 
Two very important matters are, however, left ina 
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 walls 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 pomt since 
Jameson's paper appeared, | 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 larve 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 
