n larva, having no cilia
and being necessarily motile, requires for the insertion of its
muscles or contractile organs ... a central flexible axis, a true
chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This
point of view is strengthened by the fact that in _Molgula_, studied
by Lacaze-Duthiers, the embryo is practically stationary, and forms no
notochord, nor ever develops sense-organs in the cerebral vesicle.

Giard's general conclusion is that "the true homology with Vertebrates
ceases after the formation of the cavity of Rusconi and the medullary
groove: the homologies established by Kowalevsky for the notochord and
the relations of the digestive tube and nervous systems are not
atavistic, but adaptive, homologies" (p. 282). There is accordingly no
close genetic relationship between Ascidians and Vertebrates.

Giard's criticisms did not avail to check the vogue of the new theory,
which soon became an accepted article of faith in most morphological
circles.[397] The fall of the Ascidians from their larval high estate
provided the text for many a Darwinian sermon.

Some years after the genetic relationship of Ascidians and Vertebrates
had been established, a rival theory of the origin of Vertebrates made
its appearance--a theory which was practically a rehabilitation in a
somewhat altered form of the old Geoffroyan conception that Vertebrates
are Arthropods walking on their backs. This was the so-called Annelid
theory of Dohrn and Semper. Both Dohrn and Semper started out from the
fact that Annelids and Vertebrates are alike segmented animals, and it
was an essential part of their theory that this resemblance was due to
descent from a common segmented ancestor. Both laid great stress on the
fact that the main organs in Vertebrates are arranged in the same way as
in an Annelid lying on its back, the nervous system being uppermost, the
alimentary system coming next, and below this the vascular.

Dohrn's earlier views are contained in the fascinating little book
published in 1875, which bears the title _Der Ursprung der Wirbelthiere
und das Princip des Functionswechsel_ (Leipzig). He followed this up by
a long series of studies on vertebrate anatomy and embryology,[398] in
which he modified his views in certain details. We shall confine our
attention to the first sketch of his theory.

If the Vertebrate is conceived to have evolved from a primitive Annelid
which took to creeping or swimming ventral sur