Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
Herewith, we were able to show that torsion is not a prerequisite for the
development of a streptoneurous arrangement of the nervous system in M.
cornuarietis.
Introduction
Torsion in gastropods is a difficult topic although the theory behind it is simple
enough. It is defined as an anti-clockwise rotation of the visceral sac and
mantle by 180 ◦ relative to head and foot, and takes place during the ontogeny
of all snails and slugs (e.g. Bieler, 1992) as the so-called “ontogenetic torsion”.
It is the common morphological feature of gastropods (e.g. Garstang, 1928)
and supposed to be responsible for several anatomical conditions that can be
found in snails, some of which are the anterior position of anus and mantle
cavity as well as the crossing of the pleurovisceral nerve connectives known
as streptoneury or chiastoneury (Bieler, 1992; Haszprunar, 1988; Page, 2006;
Raven, 1966). According to the theory, extant gastropods are descendants
of an untorted so-called “hypothetical primitive mollusc” (Yonge, 1947) in
which the mantle cavity with gills and anus was positioned posteriorly (reviewed
in Page, 2006). Supposedly, this untorted “hypothetical primitive
mollusc” exhibited a rotation of the visceropallium by about 180 ◦ , leading
to a twisting of the internal organs, including the nervous system and the
intestine and thus became a torted gastropod (Raven, 1966). We recently
reported that exposure to bivalent platinum ions during embryogenesis stops
the growth of mantle edge and shell gland in Marisa cornuarietis embryos
(Marschner et al., 2012; 2013). As a result, these embryos do not only exhibit
no exterior shell but do also not show a horizontal rotation of the visceral sac
by 180 ◦ , as can be seen in “normally” developing Marisa embryos (Marschner
et al., 2012; 2013). However, the ensuing snails possess an anterior anus
which is typical for the torted condition of gastropods while, at the same
time, the gills can be found posteriorly on the visceral sac. These individuals
both display characteristics typical for torted gastropods (anterior anus)
and characteristics that are supposed to apply to the untorted hypothetical
ancient mollusc. Another anatomical condition that is, by theory, supposedly
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