face uppermost, a
difficulty at once arises with regard to the relative positions of the
"brain" and the mouth. In Vertebrates the brain, like the rest of the
nervous system, is dorsal to the mouth and the alimentary canal; in an
inverted Annelid, however, the brain is ventral to the mouth and is
connected with the dorsal nerve cord by commissures passing round the
oesophagus. It would seem, therefore, that the primitive Vertebrate must
have acquired either a new brain or a new mouth. Dohrn took the latter
view. He supposed that the original mouth of the primitive ancestor lay
between the _crura cerebelli_ in the _fossa rhomboidea_, and that in
Vertebrates this mouth has been replaced functionally by a new ventrally
placed mouth, formed by the medial coalescence of a pair of
gill-slits.[399] Probably the two mouths at one period co-existed, and the
older one was ousted by the growing functional importance of the newer
mouth.

The gill-slits were considered by Dohrn to be derived from the segmental
organs of Annelids, which were present originally in every segment of
the primitive ancestor. The gills were at first external, like the gills
of many Chaetopods at the present day. For their support cartilaginous
gill-arches naturally arose in the body-wall, and the superficial
musculature became attached to these bars. "There existed in all the
segments of the Annelid-ancestors of Vertebrates gills with
cartilaginous skeleton and gill-arches in the body wall. Each gill had
its veins and arteries, each had its branch of the ventral nerve-cord,
and between each successive pair of gills a segmental organ opened to
the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate
arose by the functional transformation of two pairs of these gills. The
anterior gills became the definitive internal gills of the Vertebrate,
for they gradually shifted into the mouths of the anterior segmental
organs, which had already acquired an opening into the pharynx and had
been transformed into true gill-slits. The posterior gills degenerated
and disappeared, but their arches remained as ribs. Gill-arches and ribs
were accordingly homologous structures and formed a _parietal_ skeleton.
The vertebrate anus, like the mouth, was probably secondary and formed
from a pair of gill-slits, the post-anal gut of vertebrate embryos
hinting that the original anus was terminal as in Annelids. The unpaired
fins of fish were originally paired and possi