STUDIES ON THE ECTOPAEASITIC TREMATODES OF JAPAN. 99 



tion with the genital atrium. In Hexacotyle the homologue of the 

 connective-tissue penis of such genera as Tristomum, Epibdelki; and 

 Monocotyle is present in the form of a conical papilla at the top of 

 which opens the vas deferens, as will be explained under ' General 

 Considerations.' 



Glandula prostatica — So far as I know, the prostate gland has 

 hitherto been described only in Tristomum among the ectoparasitic 

 Trematodes ; but according to my observations it is present in all the 

 genera described in this paper. Only in Octocotyle I have not been 

 able to demonstrate it owing to scantiness of the material and its bad 

 preservation. Even early in the course of my present studies my 

 attention was drawn to cells of a peculiar appearance around the vas 

 deferens, which were very unlike those of the mesenchyma ; but their 

 nature long remained to me a problem. In Axine (PL VIII, fig. 1) 

 and Microcotyle (PI. Ill, fig. 10 ; PI. V, fig. 6) they always occupy 

 the median portion of the body, and in some species this portion is al- 

 most wholly occupied by these cells to the exclusion of all others (PL 

 III, fig. 10). They are of a rounded or more frequently of a polygo- 

 nal outline, with a finely granular, deeply staining protoplasm destitute 

 of any distinct external membrane. In Microcotyle the nucleus is 

 usually very large and contains one or more nucleoli ; but in Axine it 

 is smaller (PL YIII, fig. 1 ). In Monocotyle also, peculiar cells can 

 always be observed in the mesenchyma around the vas deferens 

 (PL XVIII, fig. 2, fros. gl.) which always form a group very con- 

 spicuous in sections. The individual cells are in this case provided 

 with a membrane of its own, and are more or less vacuolated, the 

 granular protoplasm forming simply a thin layer just inside the cell- 

 membrane, or radiating in threads from the nucleus towards the peri- 

 phery, or sometimes distributed uniformly but thinly through the cell- 

 body. The form of the cells is either polygonal or globular, and the 



