172 S- <50T<^- 



menon. For, according to the view ofMonticelli and Pint ner, 

 the genito-intestinal canal is " un adattamento speciale " in the Mono- 

 genea, while according to that of Looss the vaginte of the same are 

 " Bildungen sui generis " — i. e., in logical language residual pheno- 

 mena. And as to the genito-intfestinal canal being a special adapta- 

 tion in certain members of the Monogenea, it is exceedingly difficult 

 to see how and from what necessity such a canal could have been 

 specially developed in a limited number of forms, while totally want- 

 ing in others. Whereas, if we regard the canal as in process of 

 abortion, we meet with no difficulty, and the various facts appear to 

 me capable of being brought into harmony with each other. Accord- 

 ing to my view, then, the Trematodes and the Cestodes are to be 

 derived phylogenetically from a form which — adopting the termino- 

 logy applicable at present only to the Monogenea — possessed the 

 paired vaginae, the uterus, and another duct, the homologue of the 

 canalis genito-intestinalis. In saying this, however, it should be observ- 

 ed that I by no means assert these organs to have had in the suppos- 

 ed ancestral form the same function as they have now in either of its 

 descendants. 



As an addendum to the above discussion I beg to make just one 

 short remark on the vagina of Diplozoon. In my paper on Dipl. 

 Nipponicum, I have described the vas deferens of one individual as 

 standing in direct connection with the yolk-duct of the other. In 

 doing so I stated merely what I could directly observe j for in my 

 sections I could not distinguish the vagina from the vas deferens. It 

 is true that the terminal portion of what I then considered the vas 

 deferens was exceedingly small, but this portion was not set off by 

 any demarcation from the proximal portion. I fully admit, how- 

 ever, the justness of Pintner's'^ criticism on this point. I have 



1). Pinfcuer — Noohmals fiber den Begattungsakt der parasitischen Plathelminthen. Cen- 

 tralbt. f. Bakter. u. Parasitenk., Bd. 9, 1891, No. 22. 



