The Evolution of Man, vol 2 | Page 7

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. 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 otherwise
in the head, the foremost third (Figure 2.216). Here we find a number
of complicated structures that cannot be understood until we have
studied them on the embryological side in the next chapter (cf. Figure
1.81). The branchial gut lies free in a spacious cavity filled with water,

which was wrongly thought formerly to be the body-cavity (Figure
2.216 A). As a matter of fact, this atrium (commonly called the
peribranchial cavity) is a secondary structure formed by the
development of a couple of lateral mantle-folds or gill-covers (M1, U).
The real body-cavity (Lh) is very narrow and entirely closed, lined with
epithelium. The peribranchial cavity (A) is full of water, and its walls
are lined with the skin-sense layer; it opens outwards in the rear
through the respiratory pore (Figure 2.210 c).
On the inner surface of these mantle-folds (M1), in the ventral half of
the wide mantle cavity (atrium), we find the sex-organs of the
Amphioxus. At each side of the branchial gut there are between twenty
and thirty roundish four-cornered sacs, which can clearly be seen from
without with the naked eye, as they shine through the thin transparent
body-wall. These sacs are the sexual glands they are the same size and
shape in both sexes, only