STUDIES ON THE ECTOPARASITIC TREMATODES OF JAPAN. 33 



In most species of this genus described in this paper, the invest- 

 ing membrane of the suckers is exactly similar to that of the other 

 parts of the body ; but in Trist. ovale the ventral (concave) surface is 

 raised into numerous conical papillae, at the top of which open' the 

 eiferent ducts of the numerous unicellular arlands afterwards to be de- 

 scribed. Along the inner border of the suckers also there are, in this 

 species, numerous larger papUlae crowded together (PI. XXIII, fig. 5), 



Posterior sucker — The posterior sucker of Tristomum is more or less 

 hemispherical in form, and is very similar in structure to that of 

 Calicotyle. In most species, however, a short stalk may be dis- 

 tinguished (PI. XX, fig. 5). The ventral (concave) surface is divided, 

 as in Calicotyle, by radiating elevations into a central and seven 

 peripheral polygons that surround the former ; the hindmost of the 

 peripheral polygons invariably occupying the median line. In some 

 species (Tr. sinuatum and Tr. rotundwn), the central area forms a regular 

 heptagon ; but in the majoi'ity of the species studied by me, its form is 

 that of a heptagon to one side of which an isosceles trapezoid has been 

 apposed by the shorter one of the two parallel sides and with the 

 boundary line between the two blotted out (Tr. ovale, Tr. foliacewn, Tr. 

 Nozawce, Tr. hiparasiticum). In Tr. sinuatum the two radial sides of 

 the hindmost peripheral area bifurcate near their outer ends and 

 there form with the margin of the sucker ^ich a small triangle (PI. 

 XX, fig. 1). 



A marginal membrane is always present, and is thrown into 

 wavy folds ; so that in a surface view, it seems as if it consisted of 

 numerous rectangular portions. Its investing membrane is very thin, 

 and its substance consists of a syncytium with its nuclei irregularly 

 scattered, and traversed by numerous fine, transverse, muscular fibres 

 (PI. XXIII, fig. 3). 



According to my observations on Tr. sinuatum, the species to 



