STEUCTUKE OE THE EXCEETOEY OEGANS OF AMriIIOXDS. 189 
nuclei do not gradually decrease in number, but suddenly 
stop in the immediate neighbourhood of the solenocyte tubes 
(figs. 6, 20). Here, where these tubes spring out of the 
canal, there are no nuclei; but the wall itself is continued as 
a sheet of more or less granular cytoplasm completely closing 
off the lumen of the canal (figs. 6, 9, 20, 21). This canal wall 
may be thick or thin, the variation in thickness depending, 
I believe, chiefly on the state of tension of the fluid inside 
the canal. In good thin sections the wall is always visible. 
Indeed, the better the section, and the more perfect the 
stain, the clearer becomes the limiting wall, whatever may 
bo the direction in which it is cut. 
Figs. 19 and 20 represent two sections taken parallel to 
the surface of the nephridium, sagittal sections of the animal. 
The first just shaves through the outer wall of the canal, and 
shows many solenocytes lying on the blood-vessel. The 
second, which ouly corresponds to the left hand portion of 
the first figure, cuts deeper into the canal through the 
extremity of one of the branches, where may be seen the 
solenocyte tubes piercing the closing wall. In the next section 
the nuclei of the opposite side begin to appear, the whole 
thickness of the small solenocyte-beariug offshoot having 
been nearly cut through, 'fhe following section would show 
only a slice of the wall. There is no opening. Fig. 21 gives 
a similar view of another nephridium in the same animal. 
Two consecutive sections through the lowermost tip of the 
anterior limb of the nephridium are drawn in figs. 13, 14. 
Here again are seen the tubes piercing the wall, in which 
there is no trace of an opening. 
Figs. 5 and G represent sections from a series nearly 
transverse to the animal and parallel to the bar. That in 
fig. 5 passes through the external- pore, and shaves off the 
wall of a diverticulum. The next section (fig. 6) cuts through 
the extremity of this diverticulum. It is seen that the lumen 
is closed off from the coelom by a distinct cytoplasmic wall, 
through which pass solenocyte tubes. In fig. 7 is drawn a 
portion of the same section when the microscope has been 
VOL. 54, I'AKT 2. — NEW SEKIES. 14 
