12 
glandular cells. The various features mentioned show that in 
its type the prostate vesicle here more closely resembles that of 
Stylochoplana than ihat of Notoplana, but also that a tendency towards 
differentiation in direction of the conditions existing in Notoplana 
can be traced. 
I may at this point be allowed a deviation from the general 
description. A noteworthy feature is that no sperms are ever found 
in the prostate vesicle of Polyclads. In the numerous cases, when 
the prostate vesicle is separate (see Bock, 1913, p. 39) (for 
instance, in Stylochidae) and the ductus ejaculatorius, therefore, 
opens into the efferent duet of the prostate vesicle, this is hardly 
to be expected, although there might be a possibility of suction of 
sperms back into the vesicle when this expands after contraction. 
This absence of spermatozoa is, however, more marked when they 
must pass through the prostate vesicle during the ejaculation. This 
is generally the case in Leptoplanidae. In Stylochoplana the efferent 
duet of the vesicula seminalis opens direetly into the proximal end 
of the prostate vesicle, and thus the mass of spermatozoa passes 
along the entire length of the prostatic lumen. As an example I 
can point to Stylochoplana maculata (Quatrefages), the copulatory 
apparatus of which is shown in text-fig. 32 (p. 176) in my thesis 
of 1913. However, among the great numbers of Polyclads that I 
have examined, I have never run across a single case that con- 
tained any spermatozoa in the prostate vesicle; consequently, not 
even when the vesicula seminalis empties direetly into the prostate 
vesicle, and both lie close to each other. 
In the family Leptoplanidae the development of the prostate 
vesicle is such that the mass of spermatozoa passes through the 
vesicle without mixing with the prostatic secretion. In the genus 
Notoplana (see Bock, 1913) the efferent duet of the vesicula 
seminalis traverses the greater part of the lumen of the prostate 
vesicle and finally empties at its distal end. In more extreme 
cases (see, for instance, Notoplana stilifera, Bock 1923 c, text- 
fig. 7, p. 352), the epithelial wall in this part of the vesicle is not 
secretory. The glandular cells of the prostate are in these cases 
limited to the chambers which lie around the distal end of the effe¬ 
rent duet of the vesicula seminalis. Such an arrangement of cham¬ 
bers is advantageous, not only because the secretory epithelium 
