SEASONAL INFESTATION WITH LARVAL NEMATODES. 49! 



contributed most extensively. There are only a few scattered 

 references on North American cercariae. .Fewkes (1882) briefly 

 described a cercaria with caudal setae found free near Newport, 

 Rhode Island. Tennent (1906, 1909), worked out the life history 

 of Bucephalus haimeanus and described its gasterostome cercaria. 

 Linton (19150, 19156) found three species of cercariae in the 

 Woods Hole region : two f urcocercous forms, one from Ilydroides 

 dianthus and one from Pecten irradians, and one tailless larva 

 from Nassa obsoleta which has been designated Cercarixum 

 lintoni in this paper. More recently one of us (Miller, 19250, 

 19256) has made surveys of the larval trematodes infesting 

 marine gastropods from Puget Sound and from the Dry Tortugas. 



MATERIAL AND METHODS. 



Snails received at Saint Louis were maintained until used in 

 synthetic sea water made up according to the formula of Ditmar. 

 All individuals of a collection, except that of May, were isolated 

 for forty-eight hours to determine those from which mature 

 larvae were emerging. These infested snails were placed in 

 separate aquaria as a constant source of living cercariae for study. 

 At convenient times the remaining snails were crushed indi- 

 vidually, and their tissues examined under a binocular dissecting 

 microscope in order to find non-emerging cercariae and their 

 parthenitae; these were usually present in the digestive gland. 

 After the living material had been studied, the infested viscera, 

 freed cercariae, and parthenitae were fixed in Bouin's, or hot 

 corrosive sublimate fluid. Cercariae and parthenitae were stained 

 with Ehrlich's acid hematoxylin and mounted in Canada balsam. 

 Unless otherwise noted, all measurements recorded were taken 

 from these preserved specimens. Sections of snail viscera, two 

 and one half and five micra in thickness, were variously stained. 



SEASONAL INFESTATION. 



The graph for total infestation (Text-fig, i) with all five species 

 of larvae has two maxima, practically equal, occurring in De- 

 cember and in July. Between these maxima are low areas, in 

 each of which the percentage of infestation averages not more 

 than one half that of the maximum. This plainly shows a 



seasonal fluctuation in the presence of larval trematodes in this 

 22 



