SEASONAL INFESTATION WITH LARVAL NEMATODES. 499 



lightly stained with intra-vitam neutral red, but not with 

 toluidine blue. Large excretory vesicle, the most conspicuous of 

 internal structures, slightly to right of median line normally, and 

 extending nearly to region of bifurcation of gut; very wide and 

 bent more or less in shape of Z; filled with spherical refractile 

 concretions, ranging from very small ones up to 3 fj. in diameter. 

 Main lateral collecting tube of either side entering excretory 

 vesicle at a point about two thirds of distance from anterior end; 

 entrance of anterior and posterior branches posterior to level of 

 ventral sucker. Succession of single excretory flagella in wall of 

 main lateral collecting tube; many flame cells observed, but 

 exact pattern of excretory system not solved; apparently more 

 highly developed posteriorly. Several cell masses present in 

 anlage of reproductive system; no interpretation ventured as 

 to parts of adult system represented. Development of cercariae 

 in rediae in visceral mass of Nassa obsoleta. Immature redias 

 slender, without locomotor appendages; average length 440/1, 

 and diameter at widest part 74^1; long coiled gut extending 

 beyond middle of redia. Germinal epithelium localized in 

 posterior end. More mature rediae with both mature cercariae 

 and germ balls. 



C. setiferoides is similar to C. setifera Joh. M tiller 1850 (de- 

 scribed by Monticelli, 1914, and redescribed by Odhner, 1914, 

 from one of Monticelli's slides), but differs in a number of im- 

 portant structures. It is obviously different from C. lutea 

 Giard 1897, C. pectinata Huet 1891, and C. setifera Pelseneer, 

 1906; C. fascicularis Villot 1875 is not completely described. 

 The cercaria with caudal setae which Fewkes (1882) found free 

 near Newport, R. I., may be identical with C. setiferoides; a 

 detailed description of it was not published. The present species 

 may also be identical with the larval form of Pharyngora bacil- 

 laris, recorded by Nicoll (1910) and Lebour (1917) as free in the 

 plankton from Plymouth. The latter investigator (1916, 1917) 

 found the metacercarial stage in various medusae and in Sagitta 

 bipunctala; her brief description and single figure of the tricho- 

 cercous cercaria (1917) are not sufficiently detailed to determine 

 whether it and the present species are the same. 



