428 
Marine Cercariae 
The final host is almost certainly one of the ducks. Jameson 
believes it to be the scoter Oedemia nigra L. which eats mussels 
voraciously. Nicoll’s G. dapsilis was also from the scoter 0. nigra and 
the velvet scoter 0. fusca. 
Gercaria scrivenensis sp. inq. 
(PI. XXV, fig. 9.) 
This was found on one occasion in Tapes p)uUastra from Loch 
Scriven, the Clyde, in August. Two examples occurred in one individual 
between the mantle and shell. It appears to be different from the 
other Gymnophallus cercariae so far known. Length 04 mm., breadth 
0'24 mm. Body covered with spines. Oral sucker 0'09 mm. across, 
ventral sucker 0'03 mm. across. Intestinal caeca reaching a good way 
beyond the ventral sucker. Pharynx 0'03 mm. long. Excretory system 
conspicuous, forks reaching to pharynx. 
Further life-history unknown. 
Gercaria glandosa mihi (1908 A, p. 4). 
(PI. XXV, fig. 10.) 
This occurs in Paludestrina stagnalis from Fenham Flats and is 
the only Gymnophallus cercaria -known that inhabits a Gastropod. It 
is always free between mantle and shell. Only one to six specimens 
occur in the same individual. It is not certain whether this is the first 
or the intermediate host as no sporocysts were found. From the 
scarcity of specimens it seems more likely that it is the intermediate 
host. 
The cercaria measures 0'20 mm. in length, very contractile, head 
region contains numerous gland cells filling up the space between body 
wall and oral sucker and opening by ducts round the dorsal margin of 
the oral sucker. It is possible that these are cystogenous glands and 
that in this form there is an encysted stage. Oral sucker O'Oo mm. 
across, ventral sucker 0'033 mm. with inner margin crenulated. Small 
pharynx leading to short oesophagus, intestinal caeca extending to 
beyond the centre of the ventral sucker, usually black with contained 
food. Excretory vesicle large, forks not reaching as far as pharynx. 
Testes developed on each side of excretory vesicle. 
Further life-history unknown. Possibly this and not C. margaritae 
is the cercaria of G. somateriae which inhabits the intestine of the Eider 
Duck Somateria mollissima. 
