208 MARTIN 



numbers of double infections, 62, 58, and 12. Although the infection 

 numbers for the latter three species are considerably lower than that of 

 the large pigmented echinostome, their frequencies of double infections 

 are higher. Obviously, something other than chance is involved. 



The seasonal distribution of triple infections was: in November, 

 1 in December, 4 in January, 1 in February, 8 in March, 2 in April, 

 4 in May, in June, in July, 1 in August, in September, and 2 in 

 October. The following table lists the triple infections in order of their 

 frequency. 



TABLE III 

 Triple Infections 



Species Frequency 



small strigeid -\- Y-bladder cercaria + E. californiensis 9 



small strigeid -{-Y-bladder cercaria -{- large xiphidiocercaria 3 



schistosome -\- Y-bladder cercaria -\- E. californiensis 3 



small strigeid -\- Y-bladder cercaria -\- small xiphidiocercaria 2 



Catatropis sp. + schistosome -{- small xiphidiocercaria 2 



Catatropis so. + schistosome -f- E. californiensis _. - 1 



fin-tailed echinostome -{- schistosome -f- E. californiensis.... 1 



small xiphidiocercaria -|- schistosome -|- Y-bladder cercaria 1 



small strigeid -j- schistosome -f- Y-bladder cercaria 1 



23 



In the triple, as in the double infections the Y-bladder cercaria is 

 most frequently involved. 



DISCUSSION 



As shown in Figure I, the percentages of total infections varied from 

 54 to 74 during the year. December — January and May were the peak 

 months. These peaks probably reflect the increased exposure of the snails 

 to trematode infections which must occur during those parts of the year 

 when the local bird population is augmented with migratory species. The 

 graph of total infections does not indicate the seasonal variation of in- 

 fections for particular species of trematodes. Certain species, i.e. Cata- 

 tropis, which probably use only migratory birds as hosts, show marked 

 peaks during the period or periods of bird migration and marked depres- 

 sions between these periods. Other species may maintain a fairly uniform 

 infection rate throughout the year, which probably indicates that definitive 

 hosts are available each month. Another factor which possibly may effect 

 the infection rate per month and which has not been investigated, is the 

 duration of each infection. The evaluation of this factor would involve 

 the elucidation of all the life cycles, the infection of parasite-free snails 

 and, of necessity, the conduction of a long-term project. Some of the life 

 cycles of the trematodes considered here have been worked out experi- 

 mentally by Martin (1950 a, b, c), Robinson (1952), and Stunkard 



