450 
Marine Cercariae 
breadth 0‘22 mm., oral sucker small, round, 0'06 mm. across, prepharynx 
0'03 mm. long, pharynx 0‘04 mm., oesophagus 0'04 mm. Intestinal 
diverticula bend in towards the middle line in the vicinity of the 
genital sucker, then curve out and again approach each other posteriorly. 
Excretory vesicle clearly seen, consisting of two fairly straight lateral 
tubes, beginning near the pharynx and running into a simple, rather 
wide terminal sac at the posterior end of the body. Genital sucker 
about the middle of the body, not a true ventral sucker. Vestigial 
ventral sucker in front of genital aperture. Traces of two small testes 
in front of the ends of the intestinal diverticula. Cuticle covered with 
scale-like .spines. 
The authors regard this as the larval form of Cryptocotyle concava 
(Crepl.) which they have found in the adult state in the shag Phala- 
crocorax graculus which feeds, asGhey state, largely on small plaice, 
dabs and Bounders. 
T. Scott (1909, p. 78) records an encysted Distomid from the walls 
of the stomach of a witch Pleuronectes cynoglossus from the Moray 
Firth. He thinks it is very probably the cercaria of Distomiim 
cestoides Ed. van Beneden which he records from the oesophagus of the 
skate Raia batis, Aberdeen. He very kindly sent me specimens of 
the adults which are slightly different from those I have described from 
the stany ray R. radiata as Otodistomum veliforum (Creplin). 
Cercaria mytili sp. inq. (Lebour, 1906, p. 5). 
(PI. XXIX, fig. 10.) 
This occurs commonly encysted in the digestive gland of the mussel 
Mytilus edidis and cockle Gardium edule, chiefly in the mussel from 
Northumberland and the cockle on the West Coast of Scotland. The 
cyst is spherical and thick-walled but quite transparent, 0'20 mm. 
across. Very little could be made out of the structure of the cercaria 
as it was nearly always damaged when pressed out of the cyst. 
Cercaria 0'46 mm. long, covered with spines. Posterior end pointed, 
head end rounded. Oral and ventral suckers about the same size 
0‘06 mrn. across. Ventral sucker just behind the centre of the body. 
Alimentary system not seen. Excretory system very conspicuous in 
the cercaria curled up in the cyst. Two lateral canals full of clear 
granules. 
Further life-history unknown. 
