STUDIES ON THE ECTOPARASITIC TREMATODES OF JAPAN. 51 



which have paired anterior suckers within the mouth ('• Mundsaug- 

 näpfe" of Braun), it freely communicates with the cavities of the 

 latter. In Microcotijle (PI. [V, figs. 1 and 2), Octocotijle, Dididophom^ 

 Onchocotijle, and Hexacotijle, the mouth-cavity expands more or less at 

 its posterior end and constitutes the prrphanjnx, into which the 

 anterior end of the pharynx protrudes. In Tristoiuum and Epibdella, 

 however, the mouth and the anterior end of the pharynx coincide so 

 that in these there is no mouth-cavity (PI. XXI, fig. 1 ; XXIII, 

 fig. 4 ; XXIV, fig. 12 ; XXV, fig. 7). 



The pharijnx is more or less ellipsoidal or almost spherical, or it 

 may have a constriction on each side, so that in horizontal optic 

 section it appears double, as in many species oî Trist omam (PI. XXIII, 

 fig. 1 ; VI XXIV, fig. 10 ; PI. XX^^, figs. 1 and 4). In Microco- 

 tijlc, Octocjtijic, Dididopluva, and Onchocotijle, it is traversed by a small, 

 tubular canal which puts the oesophagus in communication with 

 the cavity of the mouth, and is lined by a structureless membrane 

 of various thickness according to the species. In Monocotijlc, the 

 cavity of the pharynx is triangular in cross-section (PI. XVIIÏ, fig. 

 8) ; while in Tristomum and EpibdeUa, it is funnel-shaped and spacious. 

 Considerimx this internal cavitv as the axis for the sake of orientation, 

 we may distinguish the ventral and dorsal halves of the pharynx ; 

 and the ventral half thus distinguished is ahvays smaller than the other 

 half — resembling in this respect the oval anterior sucker of Oncliocotyle 

 and the distomes (PL IV, fig. 1 ; PI. X\', fig. 3 ; PI. XVIII, fig. 4; 

 PI. XXI, fig. 1 ; PI. XXIII, fig. 4 ; ri. XXIV, fig. 12; PI. XXV, 

 fig. 7). In nearly all species, the pharynx is an independent organ 

 entirely separated from the surrounding mesenchymatous tissue by a 

 distinct membrane of a cuticular appearance. In Tristomum, however, 

 this membrane is incomplete at some points at the posterior ends 

 of the ventral and dorsal halves, and thus aftbrds passage to the 



