22 
The latter runs backwards near the dorsal surface of the body, 
and receives from below the unpaired duets formed by the union 
of the right and left uterine duets. Instead of terminating blindly 
or expanding into a reeeptaeulum seminis, as in most other Poly- 
elads, the vagina then bends downwards and opens on the ventral 
surfaee some little distanee behind the main female aperture. This 
posterior eontinuation of the oviduet has a thiek museular wall; 
its epithelium is raised up into a number of longitudinal ridges. 
Behind the point where the median uterine duet leaves it below a 
proeess of epithelium projeets into the lumen; this may, perhaps, 
aet as a valve for preventing the passage of the eggs baekwards to 
the posterior female aperture. “ Has well then speaks of the genera 
Trigonoporus, Bergendalia, Laidlawia, and Polyporus, and elassifies 
the Tripylocelis with the family Planoceridae. I have later presumed 
that it belongs to the family Leptoplanidae (Bock, 1913). 
Another Leptoplanid, Copidoplana paradoxa Bock, living on 
corals in the Gulf of Siam, has a very short ductus vaginalis, 
which runs almost straighl downwards from the entrance of the 
very small median uterine duet and has a separate aperture back 
of the joint opening of the vagina and the antrum masculinum. 
The outer part of the vagina is developed into a „vagina bulbosa“ 
(Bock, 1913, p. 216, text-fig. 46). 
From the list of Polyclads with a ductus vaginalis we can omit 
Polyporus coecus Plehn from Spitzbergen. As I will show elsewhere, 
this is a defeetive specimen, in which the duet to Lang’s glandular 
vesicle, as well as the intestinal coeca, seem to open outwards 
simply as a result of a mechanical injury to the margin of the body. 
If we summarize what has been said, we find in Polyclads that 
within the sub-order Acotylea there has developed, as a direct 
eontinuation of the inner end of the vagina, a duet, which either 
has a separate opening back of the normal female gonopore {Tri- 
gonoporus, Tripylocelis, and Copidoplana), or fuses with the distal 
end of the vagina, so that only a single orifice for the female 
apparatus oceurs (Bergendalia, Cryptophallus, and Ceratoplana). 
Thus, in the latter case the female genital apparatus has a cir- 
cular-like passage. Trigonoporus is classified with the family Lato- 
cestidae (Bock, 1913). Cryptophallus, as well as the genus Ber¬ 
gendalia, belongs to the family Stylochidae. The remaining Cera- 
