STtJDIElS OS THE ECTOPAHASITIC TKEMATODES OF jAPAJf. 51 



which have paired anterior suckers within the mouth (" Mundsaug- 

 -napfe" of Braun), it freely communicates with the cavities of the 

 latter. In Microcotyle (PL IV, figs. 1 and 2), Octocoiijle, DicUdophom, 

 Oncliocotijle, and Hexacotyle, the mouth-cavity expands more or less at 

 its posterior end and constitutes the prepharynx, into which the 

 anterior end of the pharynx protrudes. In Tristomwn and JSpibdella, 

 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 pharynx 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 of Tiistomum (PI. XXIII, 

 fig. 1 ; PL XXIV, fig. 10, ; PL XXV, figs. 1 and 4). In Microco- 

 tyle, Octocotyle, Diclidopliora, and Oncliocotyle, 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 Monocotyle, the 

 cavity of the pharynx is triangular in cross-section (PL XVIII, fig. 

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

 Considering this internal cavity 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 always smaller than the other 

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

 and the distomes (PL IV, fig. 1 ; PL XV, fig. 3 ; PL XVIII, fig. 4; 

 PL XXI, fig. 1 ; PL XXIII, fig. 4 ; PL XXIV, fig. 12 ; PL XXV, 

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

 entirely separated from the surrounding niesenchymatous tissue by a 

 distinct membi'ane 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 affords passage to the 



