SEASONAL INFESTATION WITH LARVAL NEMATODES. 495 



infestation. This operates, according to him, to raise the per- 

 centage of infestation by removing old snails which had formerly 

 been infested and had recovered and become immune. He 

 believes that the majority of fresh water mollusks may die from 

 natural causes in June and July. This is borne out by the 

 observations of Manson-Bahr and Fairley, Annandale, and others. 

 This factor is probably an important one among the fresh water 

 snails of India which Sewell studied, for the percentage of infes- 

 tation was frequently very high and the number of old, immune 

 individuals was probably relatively quite large. But among 

 marine gastropods, where usually only a small percentage harbors 

 larval trematodes, it would not be expected that this factor 



TABLE I. 



INFESTATION RECORD OF 8,875 SPECIMENS OF Nassa obsoleta. 



1 Not isolated for emergence. 



