STUDIES OX THE ECTOPARASITIO TEEMATODES OE JAPAN'. 121 



surface views. The num,eroas ducts tlint proceed from each half 

 of the vitellarium at Jast unite with one another and form a single 

 large duct, the paired yolk-duct. These are in most species sym- 

 metrically disposed with respect to the median line of the bod}'', 

 but in Hexacotijle that of the right side is situsited much more 

 in front of the left (Pi. XIV, figs. 1 & 7). In Dididophom a 

 similar asymmetry is also observable, but is not so striking (PL X, 

 figs, 1, 5, & 9); and in this genus the relative positions of the ducts of 

 the two sides of the body are the i-everse of what is found in 

 Hexacotijle, the duct of the left side being situated anterior to 

 its fellow of the opposite side. During the summer season the 

 paired yolk-ducts are almost always filled Avith yolk-cells, and 

 are consequently very conspicuous, being sometimes perceptible 

 with, the naked eye thi'ough the transparent tissue of the worm. 

 In Axine heterocerca, Microcotyle, Hexacotijle, and Onchocotijle they 

 run for the greater part of their lengths parallel to the long axis of 

 the body, just on the inner side of the intestinal trunks; but in Cali- 

 coti/le, Monocotijle, Tristoimim, Epibdella, and DicUdophora the dncts of 

 both sides are situated on the same sti\aight line, which is directed at 

 a right angle to the long axis of the body. In Axiiie aherrans and in 

 Octocotiile the paired ducts unite Avith each other in the mediiin line of 

 the body immediately after leaving the vitellarium, and form the 

 single yolk-duct (PL VII, fig. 5). In Tristomum, Calicotyle, and Mono- 

 cotijle the paired yolk-duct of each side is again formed by the union 

 of two smaller ducts which come respectively from the anterior and 

 posterior part of the vitellarium; sometimes, however, it is formed 

 by the simultaneous union of numerous ducts, as in Trist. ovale 

 (PL XXIII, fig. 1). In Microcotyle truncata the paired yolk-ducts appear 

 to unite with each other at the very beginning and then to separate again 

 into two ducts similar to those of the other species (PL II. fig. 1); 



