340 ALFRED GIBBS BOURNE. 
in many of the nephridial cells of Hirudo. It is not ciliated 
in any part of its course. It frequently loops back upon itself ; 
one such loop only is shown in fig.42. The lumen presents a 
somewhat irregular contour. 
The cells of the wide tube are similar to those of the middle 
tube, but the contour of the lumen is more regular. The wall 
of the vesicle is not lined inside by any layer of visible cells, 
but sections show scattered nuclei on the inner wall, and I 
think that the lumen must be intracellular, the cells being 
very much flattened out. There is a well-developed muscular 
layer which consists of an inner set of longitudinally placed 
fibres, and an outer set of more or less obliquely placed cir- 
cular fibres. Outside the muscle is a little connective tissue, 
and outside all a layer of pavement (ccelomic) epithelium cells ; 
these are very delicate, and, owing to the contraction of the 
muscle, somewhat difficult to demonstrate, but in a fortunate 
silver preparation they are very clear indeed. The vesicle is 
a tube closed at either end, and has a short prolongation on 
one side leading to the terminal duct. At the junction there 
is a well-developed sphincter, and the lining epithelium is 
ciliated all round the aperture. The terminal duct has very 
thin walls; these are like those of the vesicle, except that there 
is no muscle. The connective-tissue layer is very thin, but 
very strong, and appears structureless. 
I have already spoken of the very elaborate vascular supply 
of the nephridia. 
Nervous System. 
The cerebral ganglion lies as usual far forward on the dorsal 
wall of the pharynx. It is a single, somewhat square-shaped 
lobe (fig. 27). Arising from its anterior outer corners are the 
cesophageal commissures. These join together to form the 
subeesophageal ganglion, whence arise the ventral cords, 
which are united at intervals by ganglia. These cords are 
firmly bound up together, by a sheath composed of muscular 
and connective tissue, into a single cord; but as nerves they 
are really as separate and distinct from one another in the 
