372 University of California Publications in Zoology [Vol. 19 



How long an encysted agamodistome may live in a living snail has 

 also never been determined. Since cereariae are not produced during 

 the winter months, and since cysts are found throughout the year, it 

 is obvious that they live for at least four months. Doubtless they exist 

 for a year and perhaps several years in the tissues of the snail before 

 deterioration and death. By raising eggs of Physa occidentalis in 

 aquaria and by placing with these young snails the cereariae of 

 Echinostoma revolutum, the duration of the cyst could be determined 

 by examination of the snails from month to month. 



The following .table gives an analysis of the various stages of 

 Echinostoma revolutum found in the four hundred snails of various 

 sizes of Physa occidentalis taken from Stow Lake, Golden Gate Park, 

 San Francisco : 



Mother-rediae 13 



Daughter-rediae and cereariae 234 



Encysted agamodistomes or cysts 282 



Cysts only _ - 95 



Cysts within rediae _ 13 



Uninfected snails 53 



These snails were examined during every month of the year except 

 July. The cyst and daughter-redia stages were found during every 

 month, but the mother-redia stage was found only in the months of 

 December, January, and February. Since the mother-redia stage was 

 not known or even suspected until December, 1918, it is obvious that 

 more than thirteen snails were so infected. Also, since as stated, no 

 effort was made to see if all the snails had some rediae that contained 

 cysts, the number so parasitized was also probably much greater than 

 recorded. The outstanding feature of the above table is that such a 

 large percentage of Physa occidentalis contained some stage of the 

 parasite. Perhaps the next most important feature is that more snails 

 were found to be infected with cysts than with rediae. This, as pre- 

 viously stated, is due to this snail being both the intermediate and 

 secondarv intermediate host. 



