TREMATODE PAEASITES. 55 



mussel- eating ducks, such as the Scoter and the Eider ; but the 

 form he figured as the adult in 1902 from the Common Scoter, 

 which he referred to Gymiiophallus {Lecithodendrium) somaterice, 

 and which we are now describing below under the name G. oedemice, 

 is certainly not the adult of the larva which occurs in Mytilus, 

 but is a much smaller species *. 



In order to reconcile the small size of the larvse whicn he found 

 in the Scoter at Billiers in 1902 with the larger dimensions of 

 the larvae in the mussel, Jameson was obliged to have recourse to 

 an unlikely hypothesis. This was commented on by Odhner, 

 who suggested that Jameson had probably encountered a mixture 

 of two species, one probably being G. bursicola and the other a 

 new species of the same genus. As the present investigation will 

 show, the latter part of Odhner's supposition was correct ; while, 

 with regard to the former, until further specimens from Billiers can 

 be examined, it is impossible to say whether the larger examples 

 observed by Jameson at that station were G. bursicola or G. dapsilis. 



Since the publication of Jameson's paper, two species, namely 

 G7/mnoph(dlus bursicola Odhner and G. dapsilis Nicoll, have been 

 suggested as the possible adult form. Before the discovery of the 

 second of these species Odhner (1904) gave it as his opinion that 

 the probable adult was G. bursicola, a parasite of the Bursa 

 Fabricii of the Eider Duck [Somatei'ia moUissima). At the same 

 time he stated that a larva identical with that in MytiliLS occurs 

 in Saxicava rugosa in the Arctic regions. At that time there 

 could be little question as to the probable correctness of his view, 

 for no other known species was so likely to be the adult of the 

 parasite in Mytilus. The discovery by Nicoll (1907) in the Scoters 

 of a second bursicolous Gymno'phallus closely resembling, yet 

 distinct from, G. bursicola, showed that Odhner's conclusions 

 could not be accepted without further study, for G. dapsilis 

 appears to have even stronger claims than G. bursicola to bt 

 considered the adult of the larva in the Piel pearl-bearing Mytili f. 



The two chief facts in favour of such a view are that the sizes 

 of the suckers and the position of the ventral sucker, in the 

 Piel pearl-inducing Trematode, correspond more closely with 

 those in G. dapsilis, while the firm brittle consistency of the 

 body reminds one more of G. dapsilis than of the softer G. bur- 

 sicola. The position of the testis in relation to the ventral 

 sucker is also a feature of some moment. It is, of course, 

 quite possible that two closely allied Trematodes are concerned 

 with pearl -formation in Mytilus — the one derived from Tapes, 

 .the other from Cardium, — and it may be that the adults of these 

 two species are respectively G. bursicola and G. dapsilis. 



Jameson foresaw that it would be necessary to prove in some 



* [I distinctly remember the occurrence of a small number of larger Gymnoplialli 

 in the Billiers (Edemice, though it did not occur to me at the time that more than 

 one species might be present. 1 referred them all to the onlj' species then known to 

 me, G-. somateria. — H. L. J.] 



t We have not so far had an opportunitj' of re-examining the larva; found in the 

 Billiers mussels. 



