Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
The nervous system of the adult
Fig. 3A,B shows two views of a submodel of C1 that focuses on the head and
foot region. The columellar muscle follows the shape of the columella. The
odontophor takes up a great part of the head and the radula curves slightly
along the dorsal side of the head from the mouth to the transition to the
visceral sac. This most distal part is the radular sac. The ganglia can easily
be distinguished: the cerebral ganglion and pedal ganglion fuse to a large
ganglionic mass on either side of the head. From the left cerebral ganglion a
ganglionic streak emanates, but does not connect to the supraintestinal ganglion.
The supraintestinal ganglion is connected to the pedal ganglion and
the osphradial ganglion in the mantle cavity. In Fig. 3C,D a submodel of P1
is shown. All organs inside the head are compressed in a craniad direction.
The odontophor is only about 1/3 of its usual size and the the curvature
of the radula is increased. The supraintestinal ganglion is shoved craniad
and now positioned laterally of the left cerebral ganglion. This compression
seems to be caused by the columellar muscle which curves upward too much
and thus takes up space that is usually occupied by nerves, radula, and odontophor.
Unlike in C1, the osphradial ganglion in P1 is not positioned above
the head. The osphradium and its ganglion originate on the right lateral side
of the visceral sac. As was shown by Marschner et al. (2012; 2013), differential
growth of shell gland and mantle edge stops and the visceral sac rotates
vertically by about 90 ◦ . It is this rotation that makes the osphradium and its
ganglion end up on the left lateral side of the visceral sac slightly above the
mantle edge and shell gland. This kind of visceral sac rotation does not correspond
to ontogenetic torsion - as mentioned, these snails remain untorted.
P2, shown in Fig. 3E,F, is the older of the two sluggish snails. Here, the
odontophor is also compressed and the radula is bent even stronger, crudely
resembling the shape of a bass clef. The radula enwinds the odontophor in
a way that the usually most distal part, the radular sac, is now positioned
on the oral pole of the head and not on the aboral one. The odontophor in
P2 is not surrounded by the ganglia like in C1 (Fig. 3A,B), but is pushed
in front of them along with the radula. Like in P1, the columellar muscle in
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