Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
tropods (Crofts, 1937, summarized by Page, 2006a). During this process,
the visceropallium, which includes most of the visceral organs, rotates 180 ◦
anticlockwise relative to the cephalopodium (head and foot), leading to a reorganization
of tissues and organs. The post-torsional state is characterized
by an anterior mantle cavity with anus and ctenidium and the crossing of
the pleurovisceral nerve connectives, which is known as streptoneury. This
horizontal, anticlockwise rotation by 180 ◦ can be observed in developing embryos
of patellogastropods and some vetigastropods (“ontogenetic torsion”).
Such a rotation can also be observed in our test animal, M. cornuarietis
(Demian and Yousif, 1973a), although it undergoes a direct development inside
a capsule instead of the indirect development with a veliger larva like
vetigastropods and patellogastropods. In contrast to extant gastropods, the
molluscan archetype is hypothesized to have a posterior mantle cavity with
posterior ctenidium and anus as well as uncrossed pleurovisceral nerve cords
(Garstang, 1929, summarized by Page, 2006a) and, thus, could be described
as untorted.
Two different mechanisms have been discussed as possible causes for torsion
in gastropods. Crofts (1937, 1955) suggested a process involving larval
retractor muscles attached to the protoconch, whereas Haszprunar (1988)
and Ponder and Lindberg (1997) suggested differential growth to be the
cause for torsion. In the case of M. cornuarietis, Demian and Yousif (1973a)
reported that the rotation of the visceral sac must be the result of differential
growth of the two sides of the embryo, as it has not yet developed any
muscles on the onset of this rotation.
Subsequent to the description of conditions causing the mentioned body
plan shift (Osterauer et al., 2010b), this article gives details of the platinuminduced
alteration of developmental processes in M. cornuarietis embryos. It
presents scanning-electron micrographs and histological sections of both normal
and “shell-less” embryos and compares the morphology and anatomy of
the developmental stages of platinum-treated embryos with untreated ones.
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