8 CEYLON PEARL OYSTER REPORT. 



common mussel (Mytilus edulis*) is the larva of Distomum somaterice, a Trematode 

 worm, the adult of which t lives in the intestine 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 mussel, 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 

 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 had 

 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 from our own observations upon pearl-bearing 

 mussels obtained from the same locality as Jameson's, we think there can be little or 

 no doubt that the cells of the pearl sac are directly or indirectly, but at least 

 genetically, connected with the exactly similiar cells on the outside of the mantle. 

 It is very probable 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 

 larvae 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 suppose that in their 

 migration they may push some epidermal cells in before them. Even in the absence 

 of direct evidence of this, it will be admitted that it does not involve such a violent 

 assumption as that the connective tissue in the centre of the mantle can produce an 

 epithelial sac, the cells of which are indistinguishable both in structure and in function 

 from the epidermis outside. 



In giving a preliminary account of jjearl-formation in the Ceylon pearl oyster to 

 Section D of the British Association in September, 1903, we took up the position 

 that the sacs enclosing the pearls were in all cases of ectodermal (epidermal) origin ; 



* JAMESON also states that he had found a Trematode in a sac in an example of the Ceylon pearl 

 oyster ('Nature,' January 22, 1903, p. 281). 



t Odhner, however, has shown that Jameson's larval stages and his sexually mature form cannot 

 belong to the same species, and that both belong to the genus Gymnophallus. The adult, according to 

 ODHNEE (' Fauna Arctica,' iv., 2, p. 291, 11)05) is Gymnophallus somateria (Levinsen), and the larval form 

 which catises the pearl-formation in Mytilm belongs to Gymnophallus bursicola, Odhnek. In a recent paper, 

 "TJber die Entstehung der Perlen," Dr. M. LiiiiE also refers Jameson's stages to different species of 

 Gymnophallus, and considers it probable that the parasite that causes pearl-formation in the mussel is a 

 distinct species which must then lie called Gymnophallus mwrgaritarwm (Duuois). 



