Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
system in which the right intestinal ganglion is fused with the right pleuropedal
ganglionic mass as it is normal for Ampulariidae (Demian and Yousif,
1975). Although the 3D models of the Pt 2+ -exposed embryos are difficult to
interpret, they definitely do not contradict the results of the investigations
of the adult snails. Furthermore, it is likely that the position of ganglia has
been influenced by the growth of the columellar muscle even in very young
Pt 2+ -exposed individuals. Despite the uncertainties remaining for early neurogenesis,
in Marisa it has become clear that in adult non-torted snails both
intestinal ganglia are in their “normal” positions, which means, that either
this exchange of positions of the two intestinal ganglia can occur without
the rotation of the visceral sac or the two ganglia do not exchange their positions
but simply “assimilate” each other’s identity, which is only defined
on the basis of the ganglia’s positions anyway. Whichever possibility is true,
our results show that, at least in Marisa, ontogenetic torsion, the horizontal
rotation of the visceral sac, is not necessary for the nervous system to
establish a physical arrangement that is supposed to be caused by torsion
(Haszprunar, 1988).
However, although the theory says that the crossing of the pleurovisceral
connectives is caused by torsion, there are several accounts on embryonic
development in snails showing that the situation is not so simple. Raven
(1966) summarized the details of neurogenesis of different gastropods and
stated that in Acmaea, Haliotis, Trochus, and Littorina the parietal and
visceral ganglia were only formed after ontogenetic torsion and that the connections
between the intestinal ganglia and the opposite pleural ganglia were
emanated from their definite positions. Page (2003) investigated ontogenetic
torsion in different species and did not only find that the visceropallium does
not always rotate as a single unit, but that a full streptoneurous condition
can be found in embryos of Trichotropis cancellata, in which the visceropallium
has only rotated by 90 ◦ . The available data on ontogenetic torsion are
rather scanty and show that there is a quite high degree in variability between
different gastropod groups (Jenner, 2006) which caused Page (2006) to
suggest an alternative hypothesis called “asymmetry hypothesis”. This hypothesis
explains the development of what is considered as the post-torsional
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