4.08 WILLIAM NICOLL. 
epithelium. It is thus histologically part of the cesophagus. 
This condition is of some structural importance. In other 
species which I have been able to examine, and particularly 
in the ALLocrEADIIN», the layer of epithelial cells is continued 
right up to the junction with the cesophagus. The musculature 
of the diverticula is the same as that of the cesophagus, but 
the fibres are more irregularly spaced. 
The excretory vesicle consists of. a single undivided 
terminal sac, lying just under the dorsal surface of the body 
and extending as far forwards as the anterior border of the 
anterior testis, where it divides into two much narrower 
tubules running forwards to the level of the oral sucker. 
Over the testes the vesicle is compressed transversely, but 
elsewhere it is nearly isodiametric. The excretory pore is 
not quite terminal but is displaced very slightly dorsally. 
In section the wall presents an irregular outline. The lining 
of the vesicle consists of a distinct but thin membrane, which 
stains like cuticle, and under this there is a regular layer of 
oval nuclei with their long axes parallel to the wall of the 
vesicle. No cell outline could be discriminated. The nuclei 
possess a distinctive character in the apparent absence of 
nucleoli. They contain numerous chromatophil threads and 
granules staining rather’ dark blue, while the rest of the 
nucleus has a lighter tint. These nuclei can be traced 
throughout the greater part of the excretory system and 
appear to be peculiar to it. This lining membrane evidently 
does not conform to the usual type, in which the epithelial 
cells are prominent in the wall of the vesicle and are sharply 
marked off from the surrounding tissues. Here the cells are 
not prominent, and they are inclose relation to the adjoining 
parenchyma. 
Genital system: The testes are two rounded or ovoid 
bodies, situated directly behind each other in the middle line 
of the body. The posterior testis is about one quarter 
of the body length from the posterior end, while the anterior 
testis is separated from the ventral sucker by a shorter space. 
They, may be closely apposed to each other, or more usually 
