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

192 JiDWJJf S. GOODBICH.
due to theoretical bias. It is true that I hold that the renal
organ of Amphioxus is a nephridium homologous with the
nephridia of Annelids and Platyhelminths, and not homologous
with the kidney tubules of the Oraniata (5, 7); but it
is now well known that the true nephridia of Annelids may
open into the coeloin. There is no a priori reason why
they should not do so in Amphioxus. However, no nephridium
has yet been found possessing both solenocytes and an
internal opening, though such intermediate stages must presumably
have existed.
The Relation of the Nephridium to the Bloodsupply.—The
general blood-supply has been well described
and figured by Boveri (1). But according to my observations
the vessels occur not so much as narrow capillaries, as
in the form of a large expanded vessel spreading over the
area occupied by the excretory organ. This is shown in
sections (figs. 7, 23), and also in the reconstructions given on
Plate 11. It will, moreover, be noticed that, although the
greater part of the bloodvessel lies on the inner or atrial
surface of the nepln-idium, yet several loops pass round to
the outer or coelomic surface. Thus a considerable part o£
the nephridial canal is entirely surrounded by the bloodvessels.
The soleuocytes radiate out from the canal, and
always lie on the wall of a bloodvessel, being attached to it
by a protoplasmic process (figs. 4, 15). The way in which
these cells are distributed is shown in figs. 14, 19, and
diagrams 2 and 3, and the text-figure. It will there be seen
that the longer tubes, which are of course those belonging to
cells furthest away from the canal, pass over the shorter
tubes to reach their destination. Never do the solenocytes
project freely into the ccelom; when they appear to do so in
sections this is, I believe, due to the cell having become
detached accidentally, either during the process of preservation
or of cutting. The tubes are therefore fixed at both
ends.
In the text-figure may also be seen the peculiar disposition
of the solenocytes at the top of the secondary gill-bar. Here