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

200 EDWJN S. GOODRICH.
of sphincter (figs. 33 and 36). A shallow branchial chamber
lined with epidermis is thus formed, leading from the external
to the internal opening. It is in this chamber that the
nephridium opens, at a place corresponding apparently to the
point of junction of the ectoderm with the endoderm (fig. 41).
The position of the nephridiopore can be seen in figs. 36 and
39. When the atrium becomes formed by the closing off of
the space between the metapleural folds, with which the
branchial cavities become merged, the pores open into the
atrium. A ventral view of a stage where the atrium has just
begun to be formed posteriorly shows one or two nephridia
behind the last open gill-slit (fig. 35). Probably these
nephridia belong to the posterior gill-slits, which have closed
up (Willey, 15); they open now directly on the surface
(fig. 30).
The young nephridium is a flattened sac, without internal
opening (figs. 36 and 39). From its inner end spring a large
number oE solenocytes; their tubes pierce its wall, and their
flagella pass into the lumen of the sac. The majority of the
solenocytes spread over the blood-vessel which runs along
the future dorsal edge of the slits. The solenocytes of the
first few slits scarcely extend beyond this limit; but, passing
backwards to more posterior nephridia, we find that the
solenocytes spread farther and farther up towards the dorsal
aorta, the tubes lengthening out as the cells lie farther from
the nephridial sac. At about the fifth or sixth nephridium
some of the solenocytes actually reach the aorta (fig. 40).
The tubes in this case may attain a really astonishing length,
stretching right across the field of a T^-th oil-immersion
objective with oc. 8.
Fig. 34 represents the posterior gill region of a living
larva, in which the remarkable development of the solenocytes
is well shown. Here a group of the longest solenocytes,
some twelve to eighteen in number, spread out over the aorta
in a most beautifully regular fan-like arrangement in each
segment. A section of this region is shown in fig. 31; the
fan-like disposition is found in each segment to the hindmost