M. V. Lebouk 
423 
rediae and ceicariae are formed most abundantly in the spring, summer 
and early autumn,—scarcely at all in the winter. The encysted stages, 
however, are to be found through the winter, and it seems that these 
are real resting stages in which the larva has ceased to feed and is 
gradually forming its reproductive organs and getting ready for its 
adult life. 
Effect of Parasite on Host. 
The presence of sporocysts and rediae certainly injures the Molluscan 
host. The outer appearance of the digestive gland is usually enough 
to show if it is inhabited by these parasites. It looks unhealthy and is 
a grey, sickly yellow or a peculiar unnatural orange. It is generally 
completely riddled with the sporocysts or rediae which feed upon its 
substance. If the gonad is infected it is quite destroyed. 
On the other hand encysted cercariae seem to do little or no harm 
to their hosts even when present in great numbers; they may even be 
of economic importance, as in the formation of pearls. 
Many larval Trematodes infest our edible shell-fish. For instance 
Buce'phalus haimeanus entirely spoils the cockles which it occupies. 
Many sporocysts and rediae occupy the digestive gland of periwinkles 
and Biiccinum rendering it not a digestive gland at all but simply an 
organ full of parasites. The edible crab Cancer pagurus is also very 
often crowded with the cysts of Spelotrema excellens although this does 
not appear to affect the tissues in any way. 
The following is a list of Trematode lai vae occupying some of our 
most important edible shell-fish ; 
(1) In Littorina littorea (the periwinkle) 
Cercaria linearis (kidney and mantle cavity) 
Echinostomuni secundum (digestive gland) 
Cercaria lop>hocerca (digestive gland) 
(2) In Buccinum undatum (the “ buckie ”) 
Cercaria huccini (digestive gland) 
C. neptuneae (digestive gland) 
Acanthopsolas lageniformis (digestive gland) 
(3) In Cardium edule (the common cockle) 
Bucephalus haimeanus (throughout body) 
Gymnop>hallus margaritae (under umbo) 
Lepodora rachiaea (?) (visceral mass) 
Echinostomuni secundum (foot) 
