Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
first tended to invaginate and its central portion tentatively evaginated, thus
forming the mantle anlage. However, this anlage stopped growing before
leading to a rotation of the visceral sac and to mantle formation. In older
embryos, it could be observed that the whole complex associated with shell
secretion and mantle formation (consisting of mantle edge, shell gland, and
mantle anlage) did not remain on the lateral side of the embryo but moved
to the ventral side of the visceral sac. This movement may be due to a
redirection of the counter-clockwise rotation program of the visceral sac.
Apparently, the body axis around which differential growth lets the tissues
rotate had changed (Fig. 10) and caused the whole visceral sac to rotate vertically
by 90 ◦ . The described movement alteration is also in accordance with
an observation we made during our investigations of the internal anatomy of
adult shell-less snails (not yet published): the osphradium, which originates
on the right dorso-lateral side of the visceral sac, was finally located on the
left ventro-lateral side of the visceral sac in Pt 2+ - exposed M. cornuarietis
(unpublished data). This is not a surprise if its anlage rotated vertically by
90 ◦ relative to its origin. The orientation of this altered rotation also explains
why the ctenidium that differentiates next to the osphradium on the
right dorso-lateral side shifted sinistrad relative to its origin.
As Osterauer et al. (2010b) described, the shell gland secreted calcium
carbonate despite its abnormal position. This calcium carbonate precipitation,
called “internal shell”, was found to cup the digestive gland inside the
snail’s body. How these internal shells are secreted exactly is unknown. We
also do not yet know about the future of the mantle anlage during postembryonic
development. With time, the shell grows larger inside the snail, but
shell gland and mantle edge do not expand much. Normally, it would not be
possible for the shell to grow into the body because of the underlying tissue
(mantle anlage). So for this to happen, the invaginated mantle anlage either
followed the growth of the internal shell or it was torn by it as a result of
shell growth, two possibilities one may speculate upon. Demian and Yousif
(1973c) reported that the mantle first appeared in Stage VII, when the rotation
of the visceral sac had begun, as a result of the excessive growth of the
mantle anlage. Lacking this outgrowth, Pt 2+ -exposed embryos neither de-
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