12 ILLINOIS BIOLOGICAL MONOGRAPHS [12 



found only one was infected with Cercaria cribrata and one with C. metentera, 

 or, in each case, only a 0.06 per cent infection. The heaviest infection found 

 by Ssinitzin among these twenty-one species of parasites was that of C. zernowi 

 in Cardium exiguutn, 7.0 per cent. The average for the twenty-one species is 

 only 1.34 per cent. In the cases of the worms found in Cerithiolum exile and 

 Rissoa venusta, the percentage of infection is so low that no parasites would 

 have been found had not a large number of snails been collected and examined. 

 The record of Iturbe and Gonzalez (1917) for the cercariae of Schistosoma 

 mansoni in Venezuela shows a heavy infection. 



The infection in moUusks of the Bitter Root valley is decidedly heavy, 

 altho it varies within wide limits under different factors of place and season. 

 The lowest percentage of infection found was that with Cercaria gracillima in 

 Lymnaea proxima at Buckhouse Bridge, 3.1. From a different slough in the 

 same locaUty one month later a 46.5 per cent infection with this species was 

 found in Physa. Thus the percentage of infection is found to fluctuate with- 

 in a very circumscribed area. The least infection of Physa with C. gracillima 

 was from the Maclay Sloughs farther down the River, 5.5 per cent. On the 

 other hand, C. trisolenata was found as a hundred per cent infection in both 

 Physa and Planorbis collected from these same sloughs. Taken as a whole 

 the infection average during the fall of 1916 for the Bitter Root moUusks is 

 29.02 per cent. The average by host species is somewhat different, 24.8 per 

 cent for Lymnaea, 25.16 per cent for Physa, and 75.0 per cent for Planorbis. 

 Leaving Planorbis out of consideration because of the few specimens collected 

 there is an average infection of over 24 per cent in Lymnaea and in Physa. 

 The per cent infection for May 1917 (not included in table II) gives an average 

 of 11.5 for Lymnaea, 16.6 for Physa, and 50.0 for Planorbis. 



