develops, as we shall see, in the shape of a pouch-like blind sac,
that forms out of the alimentary canal behind the stomach.

The formation of the circulatory system in this animal is not less
interesting. All the other vertebrates have a compressed, thick,
pouch-shaped heart, which develops from the wall of the gut at the
throat, and from which the blood-vessels proceed; in the Amphioxus
there is no special centralised heart, driving the blood by its
pulsations. This movement is effected, as in the annelids, by the thin
blood-vessels themselves, which discharge the function of the heart,
contracting and pulsating in their whole length, and thus driving the
colourless blood through the entire body. On the under-side of the
gill-crate, in the middle line, there is the trunk of a large vessel
that corresponds to the heart of the other vertebrates and the trunk
of the branchial artery that proceeds from it; this drives the blood
into the gills (Figure 2.210 l). A number of small vascular arches
arise on each side from this branchial artery, and form little
heart-shaped swellings or bulbilla (m) at their points of departure;
they advance along the branchial arches, between the gill-clefts and
the fore-gut, and unite, as branchial veins, above the gill-crate in a
large trunk blood-vessel that runs under the chorda dorsalis. This is
the principal artery or primitive aorta (Figure 2.214 D). The branches
which it gives off to all parts of the body unite again in a larger
venous vessel at the underside of the gut, called the subintestinal
vein (Figures 1.210 o and 2.212 E). This single main vessel of the
Amphioxus goes like a closed circular water-conduit along the
alimentary canal through the whole body, and pulsates in its whole
length above and below. When the upper tube contracts the lower one is
filled with blood, and vice versa. In the upper tube the blood flows
from front to rear, then back from rear to front in the lower vessel.