14 ILLINOIS BIOLOGICAL MONOGRAPHS [294 



entire discussion and when at a loss to explain a structure lias derived it 

 sui generis. His later paper (1913) admitting the homology of Laurer's 

 and the genito-intestinal canals corrected one mistaken contention, but 

 his separation of the female copulatory ducts into a true vagina and 

 "canalis vaginalis" seems entirely without foundation. There is no evi- 

 dence to support the idea that the single vagina is not homologous to the 

 paired vaginae. In fact, Odhner described the paired vaginae as arising 

 by the division of a single unpaired tube, probably ventral in position. 

 He derived this tube sui generis, and cited no reason why it is not homo- 

 logons with the ventral unpaired vagina of the Monopisthocotylea. Fur- 

 ther he gives no means of distinguishing between the two. 



Looss (1893) presented a strong argument to prove that Laurer's 

 canal is not a vagina, nor homologous to the vagina of the cestodes. 



Goto (1894) reveiewed the literature up to that date and gave a 

 careful and detailed study of the canalis genita-intestinalis. Making a 

 very clear and comprehensive analysis of the question and summarizing 

 evidence from a wide study of ectoparasitic forms, he concluded that the 

 genito-intestinal canal and Laurer's canal are homologous and that 

 neither are homologous with the vagina of the Monogenea. He showed 

 that in the group there is a perfect series of vaginae from a truly paired 

 to a truly unpaired condition. He discussed the idea of Braun who re- 

 garded the presence of a single vagina as the result of a simple atrophy 

 of one of th.' originally paired vagina, with the conclusion that the rela- 

 tions of the duets ''point strongly to the view that the unpaired vagina 

 has been formed by the union and subsequent displacement of the ori- 

 ginally paired vaginae, and not as Braun supposes by the atrophy of one 

 of them." 



In the present study, the histological character and the relative posi- 

 tion and relationships of the ducts of the female system support the con- 

 tention of Looss and Goto that Laurer's canal is homologous with the 

 genito-intestinal canal, and affords no evidence that these ducts have any 

 further homologue. A review of the literature and the study of the duets 

 in the three families discussed in this paper has convinced me that 

 Laurer's canal is homologous to the genito-intestinal canal ;and the vagina 

 of the Monopisthocotylea is homologous with the originally single, sub- 

 sequently paired, and secondarily fused vaginae of the Polyopisthocotylea. 

 It makes no difference whether the single or paired condition is regarded 

 as primitive. Given a single unpaired vagina as described by Odhner 

 for the Monopisthocotylea ; by a division of the external part and subse- 

 quent lateral migration of the openings, the paired vaginae of the Poly- 

 opisthocotylea are explained. These ducts entering the body from the 

 sides, lying parallel with the vitelline ducts and discharging into the 



