SEASONAL INFECTIONS 209 



and Cable ( 1932) . Portions of the life cycles of certain other species have 

 been described by Martin and Gregory (1951) and Maxon and Peque- 

 gnat (1949). 



Most of the trematode larvae included in this study develop in the 

 digestive gland of Cerithidea californica but certain species, such as the 

 large strigeid, Cercaria buchanani, Catatropis sp., and the Y-bladder 

 cercaria develop in the mantle w^all or in organs of the snail anterior to 

 the digestive gland. The fact that these last named species do not compete 

 for a place in the digestive gland of the snail may explain, in part, their 

 success in establishing infections of the multiple type. 



Multiple infections involving echinostomes were relatively rare. In 

 this connection it should be noted that Cort, McMullen, and Brackett 

 (1937) found no cases of double infections involving echinostomes in 

 their study of 7,259 Stagnicola emarginata angulata. Various reasons for 

 the nonconformity to expectancy, based on chance, have been offered by 

 Cort, et al. (1937), and others. Inhibition of one infection upon the de- 

 velopment of another and possible lethal effects of certain combinations 

 have been suggested but actually we know essentially nothing about this 

 intriguing problem. Plans to make an analysis of experimentally induced 

 multiple infections are being formulated in our laboratory. 



SUMMARY 



A study has been made of trematode infections in the marine or 

 brackish-water snail, Cerithidea californica Haldeman, over a twelve- 

 month period. A total of 12,995 snails were studied, of which at least 

 1000 were examined each month. 



The percentages of infection ranged from 54 to 74 with "peaks" in 

 December, January, and May and "lows" in February, June, July, and 

 October. 



Heterophyid and xiphidiocercariae occurred more frequently than 

 other types. 



Six hundred sixty-seven double and twenty-three triple infections were 

 found. The combination of species in these multiple infections did not 

 always fit the frequency pattern which should have resulted if only chance 

 were operative. 



LITERATURE CITED 

 Cort, W. W., McMullen, D. B. and Brackett, S, 



1937. Ecological studies on the cercariae in Stagnicola emarginata angulata 

 (Sowerby) in the Douglas Lake Region, Michigan. Jour. Parasit. 

 23:504-532. 



1939. A study of larval trematode infections in Helisoma campanulatum 



