The whole of the long tube that runs along the ventral side of the
alimentary canal and contains venous blood may be called the
"principal vein," and may be compared to the ventral vessel in the
worms. On the other hand, the long straight vessel that runs along the
dorsal line of the gut above, between it and the chorda, and contains
arterial blood, is clearly identical with the aorta or principal
artery of the other vertebrates; and on the other side it may be
compared to the dorsal vessel in the worms.

(FIGURE 2.214. Transverse section of a young Amphioxus, immediately
after metamorphosis, through the hindermost third (between the
atrium-cavity and the anus).

FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B
medullary tube, C chorda, D aorta, E visceral epithelium, F
subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4
outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre
plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.)

The coeloma or body-cavity has some very important and distinctive
features in the Amphioxus. The embryology of it is most instructive in
connection with the stem-history of the body-cavity in man and the
other vertebrates. As we have already seen (Chapter 1.10), in these
the two coelom-pouches are divided at an early stage by transverse
constrictions into a double row of primitive segments (Figure 1.124),
and each of these subdivides, by a frontal or lateral constriction,
into an upper (dorsal) and lower (ventral) pouch.

These important structures are seen very clearly in the trunk of the
amphioxus (the latter third, Figures 2.212 to 2.215), but it is