12 ILLINOIS BIOLOGICAL MONOGRAPHS [292 



Discussing the classification of Monticelli, Odhner (1912) stated that 

 he considered the number of suckers of secondary importance and the 

 system based on them therefore lacking in fundamental systematic sig- 

 nificance. Accordingly he rejected the work of Monticelli, and using the 

 older classification of Monogenea, divided the forms within the group on 

 the basis of differences in the female reproductive ducts. He discussed 

 the relationship of the ducts of the female genital system in various trema- 

 tode and cestode genera, and stated that he was convinced as was claimed 

 by Stieda that Laurer's canal of the trematodes should be regarded as 

 homologous with the vagina of the cestodes. Intervening authors, Looss 

 (1893), Goto (1894) and several other writers, had considered Laurer's 

 canal of the Malacocotylea as homologous with the genito-intestinal canal 

 of certain Heterocotylea, and not with the vagina of the cestodes. Odh- 

 ner argued that Laurer's canal was the primitive vagina of the trema- 

 todes and that there had been a change of vaginal function from this 

 canal to the terminal part of the uterus, with the resulting degeneration 

 of the former duct. It now served in his opinion only to carry off excess 

 spermatozoa, together with yolk and shell substance not used in the for- 

 mation of the eggs. He adds that there is no evidence on which to base 

 an explanation of the transfer of the seat of vaginal function from 

 Laurer's canal to the terminal part of the uterus ; it must only be accepted 

 as a fact. 



According to Odhner, in the group of monogenetic trematodes, two 

 very different structures are included under the term vagina. One pres- 

 ent in the Tristomidae, Monocotylidae, and Gyrodactylidae opens to the 

 exterior on the left side of the ventral surface, and at the inner end is 

 enlarged to form the seminal receptacle. This tube he considered homo- 

 logous to the vagina of the cestodes and Laurer's canal of the digenetic 

 trematodes. The other structures which he did not consider homologous 

 to this true vagina were the ducts of the Octocotyliclae, Polystomidae, 

 and Microcotylidae, which function as vaginae and open into the vitelline 

 collecting ducts. These are paired and open to the surface either ven- 

 trally, laterally, or dorsally. Contending that they had arisen sui generis, 

 he proposed for them the name "ductus vaginalis." Considering 

 the question of whether the paired or unpaired condition of these 

 ducts was primitive, he argued that originally the duct was 

 unpaired and opened ventrally ; that the opening became divided 

 and the duct split, therefore the Y-shaped duct of Rajonchocotyle 

 must be considered as a stage in the development of the paired condition 

 of the ducts. A further separation would give the lateral openings of 

 Polystoma. In the Microcotylidae the openings have migrated dorsally 

 and fused producing a single dorsal tube. Odhner could find no homo- 

 logue for the genito-intestinal canal and since he maintained that it was 



