294 University of California Publications in Zoology [Vol. 19 



In Pleurogenes medians, Pleurogenes claviger, and in Prosotocus 

 confusus, Looss describes the same "2-6-3" arrangement in the flame 

 cells capillaries and accessory collecting tubes (Looss, 1894, pp. 95, 

 103, 107). Since he does not figure the excretory systems in these 

 three forms it is more difficult to make detailed comparisons. The 

 chief variation from the species already described seems to be in the 

 shortening of the main stem of the bladder, which in Pleurogenes 

 median and Prosotocus confusus is so much reduced that the bladder 

 becomes V-shaped instead of Y-shaped (Looss, 1894, pi. 2, figs. 33, 36). 



I consider the correspondence in number of flame cells and 

 capillary groups and in the number and arrangement of the collecting 

 tubes of the seven species having the "2-6-3" type of excretory 

 system to represent a fundamental homology. The differences in the 

 shape of the bladder I consider to be secondary modifications. It 

 seems probable that the primitive condition of the excretory bladder 

 in the "2-6-3" type of excretory system is that found in Opisthio- 

 glyphe ranae or in Cercaria polyadena (Cort, 1919), where the 

 divisions of the Y are about equal to the main stem. From this type 

 of bladder by lengthening the main stem and reducing the sub- 

 divisions would be derived that of Haplometra cylmdracea and finally 

 by the complete elimination of the divisions would be produced 

 the club-shaped bladder of Margeama calif orniensis. An intermediate 

 condition in bladder structure between Haplometra cylindracea and 

 Margcana calif orniensis is described by Looss (1902, p. 851) for Brachy- 

 coelium salamandrae (Frolich) (= Distomum crassicolle Rudolphi), 

 in which the divided lobes are reduced to little pockets at the anterior 

 tip of the bladder. Brachycoelium salamandrae, as I will show later, 

 is very closely related to Margeama calif orniensis, and although the 

 details of its excretory system have never been worked out, I would 

 expect on account of this close relationship to find that its excretory 

 system is of the "2-6-3" type. On the other side the progressive 

 reduction of the main stem of the bladder would finally produce the 

 V-shaped condition found in Pleurogenes medians. The above con- 

 siderations indicate that it is dangerous to lay as much stress as is 

 at present done on differences in the shape of the excretory bladder, 

 in the classification of trematode families and subfamilies when the 

 whole pattern of the excretory systems is known in so few forms. 



I consider the fundamental homology of the pattern of the excre- 

 tory systems of those forms having the ' ' 2-6-3 ' ' type to indicate some 

 degree of relationship. The excretory system is so conservative that 



