STUDIES OX THE ECTOPABASITIC TEEMATODES OE JAPAX. 123 



it occupies a position doi'sal to the vagina. 



The wall of the larger yolk-ducts, like that of the primary ones, 

 consists of a thin structureless membrane wholly destitute of nuclei. 



Vagina — The vagina is very generally present in the mono- 

 ojenetic Trematodes ; so far as I have observed it is wanting: onlv in 

 Octocoti/le and Diclklophom. Dieckhoff'^ indeed describes it in 

 Odohofliriwii lanceolatum ; but from his general description of this 

 species and especially from the structure of the posterior suckers and 

 of the genitjil organs as described by the same writer, I doubt whether 

 this species is to be included in either of the two above mentioned 

 genera as I shall define them in the systematic part of the present 

 paper. The vagina is paired either throughout its whole extent, or 

 in only its -proximal part, or again it may be truly unpaired; in 

 most species it opens by its proximal end into the yolk-duct. In 

 MoiLOCotijle, however, it opens directly into the oviduct at the same 

 level, and side by side, with the paired yolk-ducts, so that in this 

 genus three separate ducts come to open at the same point into the 

 oviduct. In Calicotyh, again, the paired vaginal canals '^ unite with 

 each other in the median line of the body, and form a short unpaired 

 duct which then opens into the veceptaculum semiids (which is in this 

 species nothing 'else than an enlarged portion of the oviduct) at a very 

 short distance from the opening of the unpaired yolk-duct. 



Having thus given a general orientation I shall now proceed to 

 describe the vagina in different species. 



As I have already stated elsewhere,^' a truly paired vagina is 

 present in Calicotijle and Oncliocotyle. In the latter genus the vaginal 

 openings lie on the ventral side of the body near the median line only 



1). Dieckhoff — Beitriige zur Kenntuiss der eoboparasifcischen Treinatoden. AroMv f. 

 Katurgesohiolite, 57. Jahrg., I. Bd., 1891. p. 264. 



2). By " vaginal canal " I mean tte canal into which the vaginal opening leads. 

 3). Centralblatt fur Bakteriol. u. Parasitenkunde, Bd. X.IV, 1893. p. 798. 



