STUDIES ON THE ECTOPARASITIC TREMATODES OF JAPAN. ^49 



exposed to strong mutual pressure (PI. Ill, fig. 8; PI. XY, fig, 8; 

 PI. IX, fig. 8). In this process the portions of the investing 

 membrfine of the inner surface of the suckers, which have in some 

 species been clritinously changed, would materially assist the elasticity 

 of the wall. 



The bulbous penis of Oclocotijle, Dlclidopliora, and Calicotijle are 

 evidently composed of fibres the same in nature as those of the suckers' 

 of the first-named two genera, and therefore in my opinion not 

 muscular. 



If, again, my view as to the origin of the genital atrium be 

 true, that it is formed by the invagination of the surface of the 

 body, it unifies in an unexpected manner the relation of the suckers 

 of -the genera so often mentioned above to the hemispherical or 

 cylindrical organs about the genital atrium' in some species of Mi- 

 crocotijle^ in being composed of exactly the same substance, for since, 

 according to it both kinds of organs fall under the same category 

 of local modifications for special purposes of the meseuchyma near 

 the external surface of the body, they therefore consist of the same 

 substance. 



2. Penis— After what I have said about the penis in diiferent 

 species and genera I believe that the relation of its different foi-ms 

 can be made out more satisfactorily than has hitherto been done. 



The penis seems to have its most complicated structure in 

 Tristoinum and Epibdella. It is here, as already described, a hol- 

 low club-shaped organ projecting by its distal portion into the 

 genital atrium with which its internal cavity is directly continuous, 

 and is provided with muscular fibres of its own arranged in two 

 ways, riz., circularly and longitudinally. In my opinion, it is 

 to be regarded as formed by an elevation of the wall of the 

 genital atrium around the opening of the vas deferens and a simul- 



