of which is thickened; it sends connective-tissue partitions between
the muscles (M1) and to the chorda-sheath. N medullary tube, Ch
chorda, Lh body-cavity, A atrium, L upper wall of same, E1 inner wall,
E2 outer wall, Lh1 ventral remnant of same, Kst gill-reds, M ventral
muscles, R seam of the joining of the ventral folds (gill-covers), G
sexual glands.)

Above the sexual glands, at the dorsal angle of the atrium, we find
the kidneys. These important excretory organs could not be found in
the Amphioxus for a long time, on account of their remote position and
their smallness; they were discovered in 1890 by Theodor Boveri
(Figure 2.217 x). They are short segmented canals; corresponding to
the primitive kidneys of the other vertebrates (Figure 2.218 B). Their
internal aperture (Figure 2.217 B) opens into the body-cavity; their
outer aperture into the atrium (C). The prorenal canals lie in the
middle of the line of the head, outwards from the uppermost section of
the gill-arches, and have important relations to the branchial vessels
(H). For this reason, and in their whole arrangement, the primitive
kidneys of the Amphioxus show clearly that they are equivalent to the
prorenal canals of the Craniotes (Figure 2.218 B). The prorenal duct
of the latter (Figure 2.218 C) corresponds to the branchial cavity or
atrium of the former (Figure 2.217 C).

(FIGURE 2.217. Transverse section through the middle of the Amphioxus.
(From Boveri.) On the left a gill-rod has been struck, and on the
right a gill-cleft; consequently on the left we see the whole of a
prorenal canal (x), on the right only the section of its fore-leg. A
genital chamber (ventral section of the gonocoel), x pronephridium, B
its coelom-aperture, C atrium, D body-cavity, E visceral cavity, F
subintestinal vein, G aorta (the left branch connected by a branchial