25 
In the Family Leptoplanidae, besidcs the case of Ceratoplana, 
the ductus vaginalis is, as mentioned, present in Copidoplana para- 
doxa Bock and in Tripylocelis typica Haswell. In both cases the 
duet has a special outward opening. Copidoplana resembles Noto- 
plana most closely in its organization, if one leaves out of account 
that there is an atrium genitale commune into which the antrum 
masculinum and the vagina open. Tripylocelis has its male and 
female orifices well separated, and this Polyclad seems to resemble 
Discoplana. With its single female gonopore Ceratoplana is no 
doubt most related to Stylochoplana. Therefore, this similarity in 
the organization, i. e. the presence of a ductus vaginalis, is not 
sufficient reason to warrant bringing the divergent genera Copidoplana^ 
Ceratoplana^ and Tripylocelis more closely together. An examination 
of the numerous species that belong to Notoplana, Discoplana, and 
Stylochoplana reveals that in each of the three genera all the various 
stages in the reduction of Lang’s glandular vesicle, from a very 
good development to almost complete disappearance, are found. In 
almost every case, however, the duet is retained. It is sufficient 
to mention only a few cases: In Discoplana pacificola (Plehn, 1896) 
the glandular vesicle is quite large; in D. subviridis (Plehn, 1896) 
horse-shoe shaped; in D. concolor (Meixner, 1907) small; in D. ma- 
layana (Laidlaw, 1903) almost completely rudimentary, with only 
the very muscular duet retained. A glance at pages 187—214 
in my treatise of 1913 furnishes many additional examples in regard 
to the genus Notoplana, and numerous new species of the Family 
Leptoplanidae in my later collections from the Pacific confirm this 
idea. In the sub-order Acotylea most species possess Lang’s glan¬ 
dular vesicle with its duet as a continuation inwards of the vagina 
interna; this duet has, consequently, during the reduction of the 
glandular vesicle either received an outward opening of its own 
on the ventral side back of the female orifice, or joined the distal 
end of the vagina. In Trigonoporus the ductus vaginalis has the 
characteristic beaded appearance that so often is noticed in the 
duet of Lang’s glandular vesicle. Moreover, in the sub-order Co- 
tylea, which almost without exception lacks a Lang’s glandular vesicle 
(compare Bock, 1923 b), there is, in accordance with this 
faet, no continuation of the vagina in the shape of a ductus vaginalis. 
This circumstance is in perfeet agreement with the theory. 
