LANDEU: ANATOMY OF HEMIURUS CREXATUS. 13 



Immediately outside the basement membrane of the coeca I find a 

 layer of circular muscle fibres (Plate 3, Figs. 35, 36), and outside these 

 a layer of longitudinal muscle fibres (Plate 2, Fig. 25). The fibres are 

 .thread-like and lie some distance apart in both layers. 



Excretory System. 



The excretory system consists of a posterior sack-like reservoir, or vesicle 

 (vsl. exc, Plate 1, Fig. 1 ; Plate 2, Fig. 14,), into which opens a main 

 ventral trunk that bifurcates above the ventral sucker into a pair of 

 vessels which reunite dorsal to the pharynx, thus forming a loop that 

 encircles the anterior portion of the digestive system. Aside from the 

 small vessels the excretory system has in general the form of the frame 

 of a tennis racket. 



The excretory vesicle is from one-fifth to one-fourth as long as the 

 appendix and has a diameter about twice as great as that of the excretory 

 trunk which leads into it. The wall of the vesicle (Plate 2, Fig. 14, 

 vsl. exc.) consists of three layers : internally a structureless membrane, 

 immediately surrounding this a layer of delicate longitudinal muscle 

 fibres, and outside the latter a layer of very large band-like circular 

 muscle fibres. At its posterior end the vesicle opens to the exterior by 

 a terminal pore, which, so far as I can make out, is not provided with 

 a sphincter muscle, nor does there appear to be any vestibule present. 

 At its anterior end the vesicle communicates with the excretory trunk 

 by a narrow opening that is provided with a thick spool-shaped wall, 

 which may possibly act as a valve (Plate 2, Fig. 14, vlv. exc). This 

 valve-like structure consists of a homogeneous substance resembling 

 cuticula, and the short lumen through it, connecting the excretory vesicle 

 with the excretory trunk, is very narrow. The wall of the valve is so 

 thick as to render any great enlargement of the lumen highly improbable ; 

 moreover it appears to be distinct from the thin structureless membrane 

 that lines vesicle and trunk, and passes continuously from one to the 

 other. As there are no circular muscles surrounding the valve, I do not 

 see how the lumen through it can be closed ; but it seems probable that 

 by a forcible contraction of the excretory vesicle, brought about by 

 means of its strong circular muscle fibres, the greater part of the con- 

 tents would be forced to the exterior through the much larger terminal 

 pore, by which very little resistance would be offered. 



The median excretory trunk extends from the excretory vesicle to a 

 point dorsal to the ventral sucker, where it bifurcates ; the branches turn 



