Immediately under the thin corium, and closely connected with it, we
find a thin muscle tube, as in the worms. On the other hand, the
Ascidia has a centralised heart, and in this respect it seems to be
more advanced than the Amphioxus. On the ventral side of the gut, some
distance behind the gill-crate, there is a spindle-shaped heart. It
retains permanently the simple tubular form that we find temporarily
as the first structure of the heart in the vertebrates. This simple
heart of the Ascidia has, however, a remarkable peculiarity. It
contracts in alternate directions. In all other animals the beat of
the heart is always in the same direction (generally from rear to
front); it changes in the Ascidia to the reverse direction. The heart
contracts first from the rear to the front, stands still for a minute,
and then begins to beat the opposite way, now driving the blood from
front to rear; the two large vessels that start from either end of the
heart act alternately as arteries and veins. This feature is found in
the Tunicates alone.

Of the other chief organs we have still to mention the sexual glands,
which lie right behind in the body-cavity. All the Ascidiae are
hermaphrodites. Each individual has a male and a female gland, and so
is able to fertilise itself. The ripe ova (Figure 2.221 o apostrophe)
fall directly from the ovary (o) into the mantle-cavity. The male
sperm is conducted into this cavity from the testicle (t) by a special
duct (vd). Fertilisation is accomplished here, and in many of the
Ascidiae developed embryos are found. These are then ejected with the
breathing-water through the cloaca (q), and so "born alive."

If we now glance at the entire structure of the simple Ascidia
(especially Phallusia, Cynthia, etc.) and compare it with that of the
Amphioxus, we shall find that the two have few points of contact. It