During these important processes, that take place in just the same way
in the Amphioxus, a tail-like projection grows out of the posterior
end of the larva-body, and the larva folds itself up within the round
ovolemma in such a way that the dorsal side is curved and the tail is
forced on to the ventral side. In this tail is developed--starting
from the primitive gut--a cylindrical string of cells, the fore end of
which pushes into the body of the larva, between the alimentary canal
and the neural canal, and is no other than the chorda dorsalis. This
important organ had hitherto been found only in the Vertebrates, not a
single trace of it being discoverable in the Invertebrates. At first
the chorda only consists of a single row of large entodermic cells. It
is afterwards composed of several rows of cells. In the Ascidia-larva,
also, the chorda develops from the dorsal middle part of the primitive
gut, while the two coelom-pouches detach themselves from it on both
sides. The simple body-cavity is formed by the coalescence of the two.

When the Ascidia-larva has attained this stage of development it
begins to move about in the ovolemma. This causes the membrane to
burst. The larva emerges from it, and swims about in the sea by means
of its oar-like tail. These free-swimming larvae of the Ascidia have
been known for a long time. They were first observed by Darwin during
his voyage round the world in 1833. They resemble tadpoles in outward
appearance, and use their tails as oars, as the tadpoles do. However,
this lively and highly-developed condition does not last long. At
first there is a progressive development; the foremost part of the
medullary tube enlarges into a brain, and inside this two single
sense-organs are developed, a dorsal auditory vesicle and a ventral
eye. Then a heart is formed on the ventral side of the animal, or the
lower wall of the gut, in the same simple form and at the same spot at