STUDIES ON THE ECTOPARASITIC TREMATODES OF JAPAN. ^49 



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

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

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

 species been chitinmisly changed, would materially assist the elasticity 

 of the wall. 



The bulbous penis of Oclocotijle, Diclidophora, and CidicotijJe are 

 evidently composed of fibres the same in nature as tlKx^e 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 invaginati(jn 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 abtnit tlie genital atrium in some species of Mi- 

 crocotijlc, in being composed of exactlv the same substance, for since, 

 accordinof to it both kinds of orofans fall under the same cateo;orv 

 of local modifications for special purposes of the meseuchyma near 

 the external surf ice of the body, they therefore consist of the same 

 substance. 



2. Fenis — After what I haA'e said about the penis in different 

 species and genera I believe that the relation of its different forms 

 can be made out more satisfactorily than has hitherto beendone. 



The penis seems to have its most complicated structure in 

 Tristoimun and EpibdcUa. 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, viz., circularly and longitudinally. In my opinion, it is 

 to be regarded as formed l)y an elevation of the wall of the 

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



