above; the ascidia is attached at the tail end. The branchial gut
(br), which is pierced by a number of clefts, continues below in the
visceral gut. The rectum opens through the anus (a) into the atrium
(cl), from which the excrements are ejected with the respiratory water
through the mantle-hole or cloaca (a); m mantle. (From Gegenbaur.)

FIGURE 2.221. Organisation of an Ascidia (as in Figure 2.220, seen
from the left). sb branchial sac, v stomach, i small intestine, c
heart, t testicle, vd sperm-duct, o ovary, o apostrophe ripe ova in
the branchial cavity. The two small arrows indicate the entrance and
exit of the water through the openings of the mantle. (From
Milne-Edwards.))

The hind end, which corresponds to the tail of the Amphioxus, is
usually attached, often by means of regular roots. The dorsal and
ventral sides differ a good deal internally, but frequently cannot be
distinguished externally. If we open the thick tunic or mantle in
order to examine the internal organisation, we first find a spacious
cavity filled with water--the mantle-cavity or respiratory cavity
(Figure 2.220 cl). It is also called the branchial cavity and the
cloaca, because it receives the excrements and sexual products as well
as the respiratory water. The greater part of the respiratory cavity
is occupied by the large grated branchial sac (br). This is so like
the gill-crate of the Amphioxus in its whole arrangement that the
resemblance was pointed out by the English naturalist Goodsir, years
ago, before anything was known of the relationship of the two animals.
As a fact, even in the Ascidia the mouth (o) opens first into this
wide branchial sac. The respiratory water passes through the
lattice-work of the branchial sac into the branchial cavity, and is
ejected from this by the respiratory pore (a apostrophe). Along the