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 differing in contents. In the female they
contain a quantity of simple ova (Figure 2.219 g); in the male a
number of much smaller cells that change into mobile ciliated cells
(sperm-cells). Both sacs lie on the inner wall of the atrium, and have
no special outlets. When the ova of the female and the sperm of the
male are ripe, they fall into the atrium, pass through the gill-clefts
into the fore-gut, and are ejected through the mouth.

(FIGURE 2.216. Transverse section of the lancelet, in the fore half.
(From Ralph.) The outer covering is the simple cell-layer of the
epidermis (E). Under this is the thin corium, the subcutaneous tissue