10 ILLINOIS BIOLOGICAL MONOGRAPHS 



TABLE I 

 DISTRIBUTION OF CERCARIAE IN THE BITTER ROOT VALLEY 



[10 



a, Physa gj;rina b, Lymnaea proxima 



Compare with map on page 8. 



c, Planorbis trivolvis d, Ptychocheilus oregonensis 



not be specific. In the case of Cercaria trisolenata, where the infection ranged 

 from 22 to 100 per cent in Physa and from 50 to 100 per cent in Planorbis, 

 the host must be considered facultative. 



In several instances the same species of snail from the same collection 

 harbored two or more cercariae (cf. collections nos. 3, 6, 7, 13, 16, 17, 18). 

 For example (Table I), at the Maclay Sloughs both Cercaria trisolenata and 

 C. gracillima were found in the same host species, Physa, in fact in the same 

 individual. This case is paralleled by the record of Cort (1915:55), where 

 the sporocysts of Cercaria polyadena and C. reflexae were found within the same 

 liver tissue of Lymnaea reflexa. However, Ssinitzin (1911) in an examination 

 of several thousand snails of six species, in which he discovered twenty-one 

 species of cercariae, makes no record of two species in the same host individual. 



In the collection of Lymnaea proxima from Buckhouse Bridge three species 

 of trematodes were found as parasites, Cercaria dendritica, C. racemosa, and 



