42 
In this way the loop of the vagina lying rostrally to the prostate 
vesicle can thus be developed as shell giand duet in complete 
agreement with the condition existing in the three Limnostylochus- 
species and the two Idioplanoides-spQc'iQS. If Woodworth had men- 
tioned anything about the openings of the shell giands, doubts 
would have been out of the question, and accordingly Meixner 
would hardly have drawn the median uterine canal to the lower 
limb of the vagina, as is the case in his diagram. 
We shall now return to the histological structure of the female 
apparatus in Discosiylochus. 
Pars externa vaginae is at a short distance from its outer 
opening lined with a low, glandless, slightly ciliated epithelium, 
which is only about one third the height of the ventral epidermis. 
The muscularis is here about as thick as the muscular wall of the 
body, which it also resembles; in faet, the median sections suggest 
that the musculature is a direct continuation of this wall. The 
Shell giand duet has an epithelium almost twice as high, and the 
ciliated covering is noticeably thicker. This is necessary for its 
special duty, that of distributing the shell secretion elements. 
Contrary to those of the foregoing part of the vagina, the muscular 
fibres, which are here of a coarser calibre, lie more sparsely, and 
the secretion duets seem to hide, at leåst on the stained specimen, 
the existence of the muscular wall. Dense aceumulations of shell 
giands terminate especially in the upper part of the shell duet, 
and here is a small dilation, the epithelial wall of which has a 
characteristic wave-like outline. The large number of secretory 
duets present suggests that the actual work of forming the shells 
of the eggs takes place here. The furrows in the underlying 
epithelium are suited for storing the shell giand secretion. It is 
not improbable that the epithelial walls may be of some importance 
for the retardation of the sperma during the act of copulation, 
particularly as the vagina immediately after makes its sharp turn 
in a posterior direction. I might call to mind the faet that 
Enterogonia possesses shell giand duets equipped with „a peculiar 
transverse cut of the distal part of one of the uteri. Near the vagina 
they run medianwards. The unpaired median uterine duet, probably 
existing also here, would then run backwards and upwards, as in the 
two known species of Idioplanoides. 
