168 
Idioplanoides atlantica approaches Neostylochus with respect ta 
the prostatic vesicle, but shows a slight folding in the proximal 
end of it. In /. insignis Laidlaw rhere are three longitudinal tubes 
developed in the anterior half of the vesicle (Meixner’s revision 
1907), thus presenting a transition to a more complicated State. 
As related in the description above, a sirnplified prostatic vesicle 
is met with in Ilyplana. I regard this as a degenerated organ. 
In Stylochidæ the prostatic vesicle seems always to be provided 
with extra-capsular giands. It is at least valid for those that I have 
observed in a mature State. Direct information is in some cases 
lacking, but I think the statement is nevertheless true. Meixner 
has not been able to find them in Idioplanoides insignis but both 
the histological State and the staining were unfavourable for their 
detection. 
The removal of the granular giand cells from the epithelium 
to the parenchyma surrounding the muscular envelope of the 
vesicle has allowed both a far better supply of nutriment and 
sufficient Space for increase in size and numbers. The acquirement 
of this organization goes probably very far back in the history of the 
Polyclads. I have met the feature in the independent „muskulose 
Driisenorgane^ and it is widely spread in the Polyclad families as 
a characteristic of the vesicula granulorum. Even the most reduced 
vesicles occurring in Stylochidæ have retained the extracapsular 
giands. 
The treatment of the seminiferous system must be cut short. 
All the Stylochids possess the two large seminal canals and these 
lack always the commissure behind the female apparatus which is 
met with in Leptoplanidæ. The two canals unite to form an 
unpaired, median end duet, lying anterior to the penis. Provided 
with a thick muscularis it acts as an ejaculatory duet. Through 
local strengthening of the musculature at its proximal end a true 
seminal vesicle has been formed. As a compensation for this there 
appears in some other Stylochids {Meixneria, Cryptophallus etc.) 
through the same kind of process aso-called false seminal vesicle near 
the terminal end of each large seminal canal. A combination of 
the two features has given as result the three-lobed vesicula semi- 
nalis (Neostylochus, Stylochus maimoreus and others). In Enterogonia and 
Ilyplana vesiculæ seminales are absent as well as in a few other Acotyl- 
