11] 



LIFE HISTORY OF TREMATODES— FAUST 



11 



C gracillima. In such a case as this, one species, C. dendritica, was present 

 in each host in large numbers, while C. racemosa was less frequent, and the 

 third species of worm, C. gracillima, constituted a very light infection in only- 

 one of the thirty-two snails examined. Hausmann (1897:16) in referring to 

 the dominance of one parasite species in the individual host, regards this 

 phenomenon as a biological antagonism. 



Turning to the per cent of infection in the snails collected in the fall of 

 1916 (Table II), a heavy parasitism is found to exist. The data are especially 

 significant when compared with the records of other investigators. Cort 

 (1915) gives detailed data for eleven species of moUusks collected from nine 

 localities. His collections were made in the fall of 1913. The least per cent 

 of individuals infected was 1.4, that for Pleurocerca elevatum, secured from 

 the Sangamon River at Mahomet, Illinois. This moUusk contained Cercaria 

 megalura. The heaviest infection recorded by Cort was that with C. isocotylea, 

 where an 18 per cent infection was found in Planorbis trivolvis from Urbana, 

 Illinois. The average infection from Cort's eleven species records is 8.5 per 

 cent. Ssinitzin (1911) has recorded data from twenty-one species of cercariae 

 described by him for the vicinity of the Black Sea at Sebastopol. In many 

 cases his records show a uniquely low parasitism, practically insignificant from 

 a pathological point of view. Out of 1159 individuals of Rissoa venusta he 



TABLE II 

 INFECTION RECORD FOR CERCARIAE OF THE BITTER ROOT VALLEY 



a, Physa gyrina 



b, Lymnaea proxima c, Planorbis trivoivii 



d, Ptychocheilus oregooensis 



