On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

STRUCTURE OP THE EXCRETORY ORGANS OE A3IPHIOXOS. 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
be the direction in which it is cut.
Figs. 19 and 20 represent two sections takeu parallel to
the surface of the nephridiutn, 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 nest section
the nuclei of the opposite side begin to appear, the whole
thickness of the small solenocyte-bearing offshoot having
been nearly cut through. The 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 nephridinm 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 6 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 cceloni 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
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