SEASONAL INFESTATION WITH LARVAL NEMATODES. 493 



December only immature rediae were present, while in the 

 succeeding months there were always some apparently mature, 

 but non-emerging cercariae. In July and August, 1925, many 

 fully developed cercariae together with a few early germ balls 

 were present in all of the rediae. A fourth species, C. setiferoides 

 sp. nov. was found in every collection but that of May. Although 

 usually less than one per cent, of Nassa was infested, in early 

 July (1925) 5.9 out of a total of 8.4 per cent, infestation was due 

 to this species. Some mature, or at least apparently fully formed, 

 cercarise were found in every collection, which would seem to 

 indicate that Nassa obsoleta is continually being infested by the 

 miracidia of this trematode. There was no emergence of C. 

 setiferoides from isolated snails until June and July, 1925; this 

 seems to be the season of maturity of this larva, although some 

 infestations with immature rediae were found both in late July 

 and in August, 1925. 



Much the greatest part of the total infestation was due to the 

 presence of a tailless larva, Cercarixum lintoni sp. nov., which 

 Linton (19156) briefly described but did not name. It was 

 present in every collection, and except in that of July 6, 1925, 

 the percentage of infestation with this species was greater than 

 the total for all other trematodes present (Text-fig, i ) . Although 

 fully developed larvae were found throughout the year, there was 

 a striking seasonal variation in the percentage of snails infested 

 with mature l larvae. In Text-figure 2, the total percentage of 

 infestation of Nassa obsoleta with Cercarixum lintoni and the 

 percentage of snails from which mature larvae emerged are shown 

 graphically. The two maxima of the total infestation graph 

 might be ascribed to the semi-annual visitation of Woods Hole 

 by the definitive vertebrate hosts, whether migratory birds or 

 fishes. It is presumed that the majority of the definitive hosts 

 of marine cercariae are fishes, concerning the migrations of which 



1 The emergence of fully formed larvae from the snail when isolated in sea water 

 for forty-eight hours is taken as evidence of the full maturity of the trematode. 

 In some cases the larvae in crushed snails were apparently fully formed, but did 

 not emerge, for reasons unknown. Whether or not emergence is a fair test of 

 maturity, the same procedure has been followed with all collections (except the 

 May shipment, when they were not isolated due to lack of time). It is possible 

 that in the case of some snails only a few larvae emerged and that these escaped 

 observation. 



