On the Ancestral Form of the Chordata.

354 A. A. W. HUBEECHT
Nemertines (as was already advocated in my paper " zur Anatomie
und Physiologie des Nervensystems der Nemertinen,"
Amsterdam, 1880), then the double proposition above enunciated
necessarily leads to the conclusion that the spot just
mentioned corresponds to that part of the vertebrate
brain where the hypophysis (proboscis) bends
upwards towards the central nervous apparatus and
where the notochord (proboscidian sheath) terminates,
i.e. the region of the primitive fore-brain.
This proposition at the same time implies the homology between
the vertebrate fore-brain and part of the nervous lobes
of Platyelminth ancestors.
It remains to be further inquired into—and the facts as they
lie before us are very suggestive in this direction—whether
perhaps the distinction between the two pair of lobes as they
are present in most Nemertines may not have been perpetuated
in the vertebrates,, these superior lobes (after dorsal coalescence
of the tvvo halves of the nervous system) becoming the forebrain,
the inferior ones the equivalents of mid- and hind-brain.
The following two points are in favour of such an interpretation:
(1) the nerves for the higher sense organs, eyes, 1 and
olfactory (?) pits start from the superior brain lobes;
(2) the strong nerve which on both sides supplies the anterior
(respiratory, M'Intosh!) region of the oesophagus,
and for which in a former paper I have proposed the
name of N. vagus, takes its origin in the inferior lobes
(figs. 3 and 5).
Upon the dorso-median coalescence of these inferior lobes
and of the lateral stems, above the intestine and the proboscidian
sheath, the latter must have become severed anteriorly
from its connection with both nerve-system and proboscis.
Might not the fact of the anterior end of the notochord being
bent upwards in several of the lower Elasmobranchs (cf.
1
It is of course understood that the ectodermal eyes of Nemertines are
not directly comparable to the myelonic vertebrate eye. However, it is important
that Graff has already succeeded in demonstrating true cerebral eyes
in other Platyelrflinths ('Monogr. der Turbellarien'}!