| |
|____________________________|
|
Proto-Amniota. Amphibia.
| |
|_____________________|
|
Proto-Pentadactyloidei.
|
Teleostei. |
| |
Ganoidei. |____________Dipnoi
| |
|__________________|
|
Proto-Ganoidei.
|
|____________Holocephali.
|
|____________Elasmobranchii.
|
Proto-Gnathostomata.
|
____________________|
| |
_Cyclostomata_. |
|
|
Proto-Vertebrata.
|
|
|
|
____________________|______________________
| |
_Cephalochorda_. Protochordata. _Urochorda_.

The hypothetical ancestral forms (Protochordata) possessed a notochord,
a ventral suctorial mouth and numerous gill-slits, and were presumably
descended from the common ancestor of Annelids and Vertebrates.
Amphioxus and the Ascidians found their place in this schema as
degenerate offshoots of the ancestral Protochordates, while the
Cyclostomes were in the same way the degenerate modern representatives
of the ancestral Protovertebrates.

Balfour's suggestion, that the nervous system in Annelids and
Vertebrates might have arisen by the dorsal or ventral coalescence of
the lateral nerve cords found in their common ancestor, bore fruit in
the speculations of Hubrecht,[409] on the relation of Nemertines to
Vertebrates.

The Annelid theory was firmly supported by Eisig, who in his elaborate
monograph on the _Capitellidae_[410] maintained against Fuerbringer the
genetic identity of the Annelidan nephridia with the kidney tubules of
Vertebrates. The independent discovery by E. Meyer[411] and J. T.
Cunningham,[412] of an internal segmental duct in _Lanice_, into which
several nephridia opened, s