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

198 EDWIN S. UOOPRIOH.
groove backwards to the pharynx into which it opens.
Very numerous solenocytes are set chiefly on short blind
diverticula. It has no internal opening, and lies in a cavity,
which is in communication with the myocoele of the first
myotome in the larva.
That this nephridium is in every way similar to and homologous
with the paired posterior nephridia there can be no
doubt. Van Wijhe's suggestion, mentioned above, must
therefore be abandoned. Two peculiarities, however, still
remain to be explained; its unpaired character and its opening
into the alimentary canal. No one, so far as I am aware,
has yet worked out the exact relation of the gill-slits to the
somites in the larva of Amphioxus, and my own observations
on this point are very incomplete. Bat judging from the
course of the dorsal spinal nerves (fig. 30), the first gill-slit
of the left (on the right side) series, which is the first to
appear in the larva, corresponds to the third myotome.
Probably its true morphological position is between the
second and third myotome. Presumably Hatschek's nephridium
would correspond to the next gill-slit in front, between
the second and first myotomes, did such a slit exist. As for
its unpaired character, I can for the present offer no better
explanation than this, that it is the left of an original
anterior pair of nephridia, the one-sided development of
which is no doubt correlated with the general asymmetry of
the anterior region so conspicuous in the larva. But this
question can only be profitably discussed after an exhaustive
study of the development, and must therefore be put aside
for the present. In the same way a detailed knowledge of
tho development of this organ, and of the posterior nephridia,
is necessary before one can discuss the significance of the
anomalous position of the opening.