STUDIES ON THE ECTOPAfiASlTiG TREMATODES OF JAtAIf. 173 



indeed virtually admitted the presence of the vagina in Diplozoon in 

 my comparison '^ of it with Microcotyle, a genus which seems to me 

 to be closely allied in many respects to the former ; but I did not 

 then sufficiently so consider the theoretical aspect of the question as 

 to clearly express myself on the point. 



5. The Protrusion of the Penis — Although I have not had much 

 opportunity to observe it, there canscarcely be any doubt that the true 

 penis as well as the chitinous armature of the genital atrium (of 

 Microcotyle) can be protruded. In those forms which have the tubular 

 chitinous penis this is probably the sole part which is protruded and 

 introduced into the vagina of the other individual. On the other 

 hand, in Tristomum and Epibdella, or more generally in those forms 

 which have a well-developed conical or club-shaped connective-tissue 

 penis it is tTiis which is protruded ; but in this case it is not certain 

 whether it is actually introduced into the vagina of the other 

 individual. It has been a mistake on the part of some writers to draw 

 conclusions as to the inability of the penis to protrude from the small- 

 ness of the opening -of the genital atrium or the sliortness of the penis, 

 and, again, as to its incapability of being introduced into the vagina on 

 account of its great size. It should be remembered that the body in the 

 Trematodes is exceedingly soft and capable of both extension and con- 

 traction, as may be seen from Pintner's observation on the copula- 

 tion of the tapeworm, and mine on the protrusion of the penis in 

 Tristomum ovale. As already mentioned, in Tristomum some of the 

 diagonal fibres of the body are specially developed around the penis, 

 and no doubt assist in its protrusion ; but in most of the species studied 

 by me there are no protractors ; and in these cases the general 

 musculature of the body is, I think, entirely responsible for the protru- 



1). I. c, p. 187. 



