togastric commissure
bearing two small ganglia arises from the cerebral ganglia and
surrounds the oesophagus.

The development is at present entirely unknown.

_General Remarks on the Amphineura._

The most important theoretical question concerning the Amphineura is how
far do they represent the original condition of the ancestral mollusc?
That is to say, we have to inquire which of their structural features is
primitive and which modified. Their bilateral symmetry is obviously to
be regarded as primitive, and the nervous system shows an original
condition from which that of the asymmetrical twisted Gastropods can be
derived. But in many other features both external and internal the three
principal divisions differ so much from one another that we have to
consider in the case of each organ-system which condition is the more
primitive. According to Paul Pelseneer the Polyplacophora are the most
archaic, the Aplacophora being specialized in (1) the great reduction of
the foot, (2) the disappearance of the shell (_Cryploplax_ among the
Polyplacophora showing both reductions in progress), (3) the
disappearance of the radula. But it is a widely recognized principle of
morphology that a much modified animal is by no means modified to the
same degree in all its organs. A form which is primitive on the whole
may show a more advanced stage of evolution in some particular system of
organs than another animal which is on the whole more highly developed
and specialized. Thus the independent metamerism of certain organs in
the Chitons is not primitive but acquired within the group: e.g. the
shell valves and the ctenidia. And although embryology seems to prove
that the Neomeniomorphs are derived from forms with a series of
shell-valves, nevertheless it seems probable that the calcareous
spicules which alone are present in adult Aplacophora preceded the solid
shell in evolution.

It is held by some morphologists that the mollusc body is unsegmented,
and therefore is to be compared to a single segment of a Chaetopod or
Arthropod. In this case there should be only one pair of coelomoducts in
the adult, the pair of true nephridia which should also occur being
represented by the larval nephridia. There should also be only a single
coelom, or a pair of lateral coelomic cavities. On this view then the
Aplacophora are more primitive than the Polyplacophora in the relations
of coelom, gonad and coelomoducts; and the genital ducts