164 S- <^o'ro- 



while in the Bothriocephalidce it is on the ventral or the dorsal side ac- 

 cording to the species. In contrast therewith the vaginal opening 

 of the Cestodes is very constant in its position and is situated close 

 to that of the vas deferens. The vagina of the Cestodes 

 and the uterns of the Trematodes I therefore regard 

 as homologous structures, having as they do their 

 external openings always near that of the vas deferens, 

 and both being the direct continuation of the oviduct. 



Which now of the two ducts, the vagina or the genito-intestinal 

 canal, of the ectoparasitic Trematodes -is the homologue of the Laurer's 

 canal of the Digenea? 



In the first place, the position of the genito-intestinal canal 

 corresponds very well with that of the Laurer's canal — and be it 

 remarked that I confine the latter term to the Digenea : both arise 

 from the oviduct between its origin and the ootyp, near or opposite 

 the opening of the yolk-duct. The striliing difference is that one 

 opens to the exterior while the other communicates with the intestine; 

 but this is in my opinion of quite a secondary importance, and 

 can not be regarded as a serious reason against the homology of 

 the two canals, any more than the absence of any external opening of 

 the uterus in Tcenia can be regarded as against its being homo- 

 logous with the uterus of Bothriocephaliis. In the second place the 

 Laurer's canal is admitted by most writers^' to present an abortive 

 character, and ifl many species of the Digenea it is totally wanting ; 

 and the same seems to me to hold true of the genito-intestinal canal of 

 the Monogenea. Thus it has no definite function; for the explanation 

 of Ijima seems to me, as it has seemed to others, to be rather forced, 

 and as Prof. Ij ima himself now admits it to be in conversation; then, 



1). Looss, Honticelli, Brandes, etc. 



