Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
formation of an external mantle and an external shell (Marschner et al.,
2012; 2013). During these investigations we noticed that in Pt 2+ -exposed
snails a horizontal rotation of the visceral sac does not take place. However,
our examinations of the nervous system of adult sluggish snails showed that,
despite the lack of this horizontal rotation, undoubtedly a streptoneurous
arrangement of the nervous system was established: on the left side of the
body the supraintestinal ganglion can be seen as a distinct ganglionic mass
from which emanates the supraintestinal nerve, which connects the supraintestinal
ganglion with the right pleuro-pedal ganglionic mass. This is what
has been described as the crossing of the pleurovisceral connectives (streptoneury,
chiastoneury), i. e., the crossing of the nerves that connect the left
pleural ganglion to the subintestinal ganglion on the right side and the right
pleural ganglion to the supraintestinal ganglion on the left side (e.g. Raven,
1966). In Marisa, this crossing of the two nerves is obscured because the
subintestinal ganglion is fused with the right pleuro-pedal ganglionic mass
and the second part of this crossing, the subintestinal nerve, “disappears”
(Demian and Yousif, 1975). What makes the interpretation of the 3D models
difficult, is the deformation of not only the nervous system but of all internal
organs caused by the undirected growth of the columellar muscle. This is
especially spectacular in P2, the oldest snail, in which the radula is almost
folded back on itself, nearly forming a loop.
However, despite this compression of the nervous system, the overall arrangement
is clearly identifiable in adults. Unfortunately, the 3D models
of the nervous systems of the embryos are not so easy to interpret. In the
control, the structure of the nervous system can be seen quite well, although
there are gaps in which there was no staining, maybe because serotonin was
not expressed there in these life stages. Those gaps were even more numerous
in the platinum-exposed embryos. In these specimens, only parts of
the nervous system could be recognized and the overall structure is almost
totally obscured. There are, however, structures that can be interpreted
as a complex consisting of the subintestinal ganglion fused with the right
pleuro-pedal ganglionic mass on the right side of the embryo. Despite all
the gaps, this observation is indicative for an arrangement of the nervous
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