293] NORTH AMERICAN POLYSTOMIDAE—STUNKARD 13 



not homologous with Laurer's canal, concluded that it had arisen sui 

 generis. 



On the basis of these differences in the female genital ducts he 

 divided the Monogenea into two suborders: Monopisthocotylea and Poly- 

 opisthocotylea. The termer is characterized by the absence of the genito- 

 intestinal canal, the presence of a "true vagina" and a single pos- 

 terior organ of attachment ; the latter by the presence of the genito- 

 intestinal canal, "ductus vaginalis," many posterior adhesive organs, 

 and the absence of a "true vagina.*' In the Monopisthocotylea 

 he included the families Tristomidae, Monocotylidae, Udonellidae 

 and Gyrodactylidae ; and in the Polyopisthocot3'lea the families 

 Polystomidae, Microcotylidae and Octocotylidae. He pointed out that 

 by the removal of the genus Sphyranura, the Oligocotylea, the first of 

 Monticelli's tribes agrees entirely with his suborder Monopisthocotylea. 

 In the second of Monticelli's tribes, however, the Diclidophorinae, to- 

 gether with the genera Dactylocotyle and Hexacotyle, should be re- 

 moved from the Octocotylidae and placed with the Microcotylidae, since 

 they more nearly agree with the latter forms in internal structure. 



The next year Odhner (1913) reaffirmed his idea of the homology 

 of the vagina of the cestodes and Laurer's canal of the distomes, 

 but explained therewith that his denial of the homology of the 

 genito-intestinal canal and Laurer's canal had been based on an error of 

 Cerfontaine in describing an unpaired vagina as present in the genus Dac- 

 tylocotyle. On examination of this genus he had found that a "true 

 vagina" was absent, and concluded that the "true vagina" of the Mono- 

 pisthocotylea which he had homologized with Laurer's canal was never 

 present together with the genito-intestinal canal. From this he decided 

 that the "true vagina" was homologous with the genito-intestinal canal 

 and therefore with Laurer's canal. Now maintaining the homology of 

 the "true vagina" and the genito-intestinal canal he is in my opinion 

 obliged to dismiss the presence or absence of the genito-intestinal canal 

 as a basis of difference between his suborders, and explain why in one 

 group this canal opens to the exterior on the ventral side of the body 

 and in the other opens into the intestine. His homology of the "true 

 vagina" and the genito-intestinal canal is a most serious error since it 

 would invalidate the distinguishing feature which separates the two 

 suborders. 



I propose to show that the organ which functions as a vagina is 

 homologous in all the monogenetic trematodes, and that there can be no 

 division of the group on the basis of differences suggested by Odhner. 

 In fact, the work of Odhner is based on an incorrect assumption and 

 false homologies. Starting with the assumption that Laurer's canal is 

 homologous to the vagina of the cestodes, he has missed the truth in his 



