4 Laidlaw, Classification of the Polydaa Turbellarza. 



joins the former usually at the base of the penis. This 

 type of apparatus is found in the majority of the Cotylea, 

 and in many Acotylea. 



In the second type' the prostate lies about the 

 vesicular duct, between the vesicula seminalis and the 

 penis, and passes its secretion into that duct. This 

 arrangement is found in many genera of the Acotylea, 

 e.g., Stylochoplana, Cestoplana ; but in the Cotylea it 

 occurs only in Pericelis (:02 p. 292) and there in a modified 

 form. 



As already stated the variations presented by these 

 two types are much greater than those found in connection 

 with the vagina. Thus the shape of the penis differs 

 greatly in different genera. It reaches its greatest speciali- 

 zation in Planocera and its immediate allies, and in 

 Cryptocelis. In Discocelis and related genera it is large 

 and blunt, in Enantia it is a rounded vesicle-like body. 

 A good idea of the differences in form exhibited by this 

 organ may be obtained by a study of Plate 30 of Lang's 

 monograph ('84). 



The vesicula seminalis is found usually immediately 

 beyond the union of the two vasa differentia. It is, in 

 fact, a widened part of the ductus ejaculatorius, provided 

 with muscular walls. It is occasionally absent altogether 

 as in Discocelis tigrina, PJiylloplana lactea, etc. In some 

 such cases its place is taken by the terminal parts of the 

 vasa differentia, which become swollen and provided with 

 muscular walls at their distal ends. This condition is 

 found in PJiylloplana lactea, and also in the Latocestidse, 

 where we must suppose it to have arisen independently. 

 In this latter case it is carried so far that there is a well 

 defined pair of secondary vesicles. Such are found, too, 

 and again quite independently, in the genus Paraplanocera. 

 ^See Fig. 4, p. 8. 



