278 University of California Publications in Zoology [Vol. 19 



of six excretory systems in adult distomes which correspond to this 

 general type. Of the six adult distomes showing the "2X6X3" 

 formula for the excretory system, two, Haplometra cylindracea 

 (Zeder) (Looss, 1894, p. 68, pi. 8, fig. 163) and Opisthioglyphe ranae 

 (Frolich) (Looss, 1894, p. 87, pi. 8, fig. 157), are placed (Odhner, 

 1910, pp. 22 and 23) in the family Plagiorchiidae Liihe (=Lepoder- 

 matiidae Odhner). The other four, Pleurogenes medians (Olsson), 

 Pleurogenes claviger (Rudolphi ), Prosotocus co-nfusus (Looss) (Looss, 

 1894, pp. 95, 103, 107) and Anchitrema s<mguineum (Sonsino) (Looss, 

 1896, p. 110, pi. 8, fig. 77) are placed by Odhner (1910, pp. 76 and 

 177) in the family Lecithodendriidae Odhner. The first three of 

 these which are closely related form the basis of the subfamily Pleuro- 

 genetinae Looss. The excretory system of these six adult distomes 

 and that of Cercaria polyadena show fundamental homologies. They 

 correspond in the arrangement of the principal collecting tubes, in 

 the number and general position of the accessory collecting tubes, and, 

 finally, except in the case of Anchiotrema sang nine um and possibly in 

 Cercaria polyadena, in the number and grouping of the flame cells 

 in the capillary groups. Differences are to be found in the relative 

 lengths of the main stem and subdivisions of the bladder, which vary 

 from the condition of Haplometra cylindracea in which the divisions 

 are very short to the condition of Pleurogenes medians in which the 

 main stem has been almost completely reduced and the bladder has 

 assumed a V shape. These differences in the bladder produce also 

 differences in the relative positions of the capillary groups. I consider 

 the correspondence in number of flame cells and capillary groups and 

 in the number and arrangement of the collecting tubes in these six 

 adults and one cercaria to represent a fundamental homology, while 

 the differences in the shape of the bladder seem to me to be secondary 

 modifications. It is possible that the primitive type of bladder in 

 the "2 X 6 X 3" type of excretory system is that of Opisthioglyphe 

 ranae and Cercaria polyadena,, in which the divisions of the Y almost 

 equal the main stem and the common collecting tubes enter the horns 

 of the bladder back of the middle of the body. The other types of 

 bladder could easily be derived from this by a shortening or lengthen- 

 ing of the main stem or the horns. 



The excretory system of Cercaria polyadena agrees most closely 

 with that of Opisthioglyphe ranae. The ceroariae of this species and 

 Opisthioglyphe rastellm (Olsson) are stylet forms which Lithe (1909, 

 p. 192) places in his group of Cercariae armatae and which also belong 



