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

STRUCTURE 0.1;' THE EXCRETORY ORGANS Ob' AMl'EIOXUS. 197
The nephridium of Hatschek reaches its maximum development
in the adult, where it is indeed the largest nephridinm
in the body, some 2 mm. in length. Lying on the left side,
below and parallel to the notochord, it opens just behind the
velum into the pharynx, 1 and runs forward a long distance
to a point just in front oE the ciliated groove (Raderorgan).
Here it ends blindly, and along its course are given off short
blind diverticula (figs. 27, 42, 43, 44). Solenocytes are set
on the dorsal and lateral sui'faces of the organ along almost
its whole length, being especially numerous on the diverticula
(fig. 28). Altogether au enormous number of solenocytes
are present on this nephridinm in the adult Amphioxus.
The canal runs along the floor of a narrow cavity beside
the aoi-ta (figs. 42—44). It is to the wall of this cavity that
the solenocytes are attached, and it appears to be of coclomic
nature; at all events it is in open communication with the
myocoele of the first myotome in larval stages (figs. 25, 26,
33). In the adult, however, it is closed off, and the lining
epithelium seems to be very irregularly developed, forming
no distinct layer of cells (fig. 28).
In the larva of about 13 gill-slits, of the left series only
(fig. 33), the nephridium can be well seen by transparency
as a short tube opening behind into the pharynx (fig. 24).
Its dorsal surface is entirely beset with solenocytes in several
closely packed rows (fig. 29). An optical section of the
organ at this stage is represented in fig. 38, showing clearly
the way in which the tubes of the solenocytes pierce the thin
dorsal wall.
We may summarise as follows the observations recorded
above :—The nephridium of Hatschek is a true nephridium,
similar in structure to the posterior paired nephridia. In
the adult, where it reaches its maximum development, it
extends along the left aorta from in front of the ciliated
1 On one occasion only I have found an opening from the canal into
tlie hinder region of the buccal cavity itself, as well as the posterior
opening into the pharynx.