Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
side of the visceral sac forward and into the mantle cavity, this right-hand
side epithelium in Pt 2+ -exposed snails became the outer epithelium of the
snail’s visceropallium instead of just lining the mantle cavity. The ctenidium
developed on the right-hand side of the visceral sac and should conventionally
also be pushed into the mantle cavity by the growing mantle epithelium.
Instead, in Pt 2+ -exposed M. cornuarietis, it remained at the posterior end
of the snail and even shifted a bit to the left.
All this suggests that in a shell-less and clearly “nonrotated” M. cornuarietis
embryo characteristic traits of both “torted” and “untorted” (in the
sense of the ancient mollusc type) gastropods could be found. Interpretations
are difficult, because the concept of ontogenetic torsion was described
in species with planktonic veliger larvae in which a horizontal rotation is
clearly visible. Nevertheless, based on the facts that the M. cornuarietis visceropallium
also shows a clearly visible horizontal rotation and that torsion
was defined to be a common feature of all gastropods, we presume that the
rotation of the visceral sac in M. cornuarietis corresponds at least to some
extend to the conventional torsion in vetigastropods and patellogastropods.
Our observations lead us to the conclusion hat this torsion-like rotation in
M. cornuarietis is not a unitary process in one fell swoop and only partly a
result of differential growth, as intestine and anus reached their normal positions
without a horizontal anticlockwise rotation of the visceral sac (and,
furthermore, in spite of a vertical rotation), and that this differential growth
affects particularly the outer organs like the ctenidium.
There have been reports on several gastropod species, in which not all components
of the visceropallium rotated synchronously (summarized by Page,
2003); and Page (1997) herself reported a “partial torsion” in Haliotis kamtschatkana
due to a dislocation of pallial tissue which resulted in the mantle
cavity and the anus being located on the right side of the embryo despite
a full 1808 rotation of the shell. She also found this phenomenon in the
gastropods Trichotropis cancellata, Lacuna vincta, Haminoea vesicula, Pleurobranchea
californica, and Diodora aspera (Page, 2003). In T. cancellata,
she also observed full streptoneury of the pleurovisceral connectives despite
a rotation by mere 90 ◦ of the visceral sac. She proposed that torsion should
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