gg S. G0T6. 



of its length, and opening to the exterior on the dorsal surface of the 

 body (PL XVII, fig. 2 & PI. XVIII, fig. 5). In Tristomum, finally, 

 the larger of the two main vessels of the excretory system com- 

 municates, in the anterior part of the body a little behind the anterior 

 sucker, with a large but short vessel, which proceeds thence anteriorly 

 and towards the dorsal surface of the body, on which it opens a little 

 behind the sucker, almost midway between the lateral border of the 

 body and the internal ventral nerve (P\. XX, fig. 1 ). This terminal 

 excretory vessel is sometimes much swollen so as to deserve the name 

 of a terminal vesicle. 



In Microcotyle and Onclwcotijle, the two vessels run roughly parallel 

 to the lateral borders of the body, and remain quite distinct from each 

 other throughout their entire lengths, the smaller vessel proceeding past 

 the excretory opening towards the anterior part of the body, and there 

 dividing into numerous smaller vessels. This is also the case in Hexaco- 

 tyle ; but in this genus, the vessels, on reaching the posterior suckers, 

 turn towards the median line of the body, keeping close to them, and 

 passing between the innermost small sucker and the next one, turn 

 again towards the latei'al border of the body, and become continuous 

 with each other directly behind the outermost sucker (PI. XIII, fig. 

 4). In Diclidophom, the main vessels of each side become con- 

 tinuous with each other at the posterior end by a cross commissure at 

 about the level of the second pair of suckers (PI. X, fig. 5). In 

 Axine again, the main vessels of one side of the body are very much 

 longer, in accordance with the asymmetrical form of the body ; and 

 the vessels of the two sides approach very near each other at the pos- 

 terior extremity of the body, and are there united by a very short 

 transverse vessel (PL VII, fig. 1). The connection of the vessels of 

 the two sides are also efiected in a similar way in Monocotyle near the 

 posterior extremity of the intestine. Close to the point of connection, 



