Can someone tell me why all shells of the mollusks I know in Europe
screw on from left to right (like a clock)?

Szphly ?

You?ve received some excellent information in the comments from both
Dops-GA and Hfshaw-GA but I have to bring up an essay on the topic by
the late Stephen Jay Gould, a professor of geology and zoology at
Harvard until his death in 2002.

Gould was best known as an essayist for ?Natural History? magazine
(and being a die-hard New York Yankees fan).  His essays appear in
collections like ?Bully for Brontosaurus? and ?Wonderful Life.?

The essay that covers your topic is ?Left Snails and Right Minds,?
which as it turns out was one of the 3 articles that Gould said drew
the most comments from readers.  It was published in the book
?Dinosaur in a Haystack ? Reflections in Natural History,? 1995.  By
the way, it?s worth mentioning that Gould?s particular zoological
expertise was in gastropods or snails.

Gould points out that higher-order animals (such as humans) are symmetric
about their central axis but that snails are not.   

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SOME TERMINOLOGY

Amazing Facts on Molluscs
?The Snails? Shell,? (undated)
http://www.weichtiere.at/Mollusks/Schnecken/morphologie/schale.html

Aperture: the shell opening, positioned at the bottom in scientific illustrations
Apex: tip of the shell (top, in illustrations)

Right-handed: ?dextral? shells ? and the diagram here is ?dextral? ?
Amazing Facts on Molluscs
?The Snails? Shell,? (undated)
http://www.weichtiere.at/Mollusks/Schnecken/morphologie/schale.html

Left-handed: ?sinistral? shells, from Latin for ?left,? sinister

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WHAT NATURE FINDS

For some unknown reason, most species are clockwise (right-handed) or
dextral.  Gould, an ardent Darwinist, refused to speculate as to the
competitive advantage for mollusks ? or why there?s an imbalance
between dextral and sinistral mollusks.

But he does observe the following:
?	in his own technical research on West Indian land snails there are
only 6 specimens ever found among millions studied
?	a few species are sinistral ? but related species will likely be dextral
?	in India, the conch shell Turbinella pyrum is a symbol of the god
Vishnu.  Left-handed shells are very rare ? but being rare are prized
highly.
?	the sinistral species are so uncommon that often get named for the
contrariness, such as Busycon contrarium or Busycon perversum.

---


SNAILS IN BOOKS

Despite the fact that almost all snails are
dextral/right-handed/clockwise, if you examine diagrams in books
printed from the 17th Century to the 20th Century, you?ll find the
snails are sinistral ? or counter-clockwise.

He cites D?Arcy Wentworth Thompson?s book, ?Growth and Form,?
published in 1917 and still in print.  And he cites both Ceruti?s
?Musaeum Calceolarinum? in 1622 and Michele Mercati?s ?Metallotheca,?
published in 1719.  Only on pages that have other animals pictured
does the engraver have the mollusks oriented properly (then because
the engraver was reversing the image, so that the mammal was oriented
properly).

Engravers routinely reversed pages (both print and images) but the
orientation for snails wasn?t considered important enough, so
engravings aren't reversed.  Which results in a print world of
counter-clockwise snails.  And which inspired Gould to write the essay
? and for hundreds of scientists and printers to respond.

Hfshaw?s cellular development link may come closer to explaining this
phenomenon than I did here but I hope that you find this helpful.

Google search strategy:
Apex aperture ?right-handed? snail

Best regards,

Omnivorous-GA

You may only be coming across the clock-wise shells (the other
direction being rare), but my recollection from introductory biology
is that there is variation in the direction in coiling and that it is
genetically determined. This site supports that idea:

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio.section.1734  

"The orientation of the cleavage plane to the left or to the right is
controlled by cytoplasmic factors within the oocyte. This was
discovered by analyzing mutations of snail coiling. Some snails have
their coils opening to the right of their shells (dextral coiling),
whereas other snails have their coils opening to the left (sinistral
coiling). Usually, the direction of coiling is the same for all
members of a given species. Occasionally, though, mutants are found.
For instance, in species in which the coils open on the right, some
individuals will be found with coils that open on the left. Crampton
(1894) analyzed the embryos of such aberrant snails and found that
their early cleavage differed from the norm. The orientation of the
cells after the second cleavage was different in the sinistrally
coiling snails owing to a different orientation of the mitotic
apparatus (Figure 8.28). All subsequent divisions in left-coiling
embryos are mirror images of those in dextrally coiling embryos. In
Figure 8.28, one can see that the position of the 4d blastomere (which
is extremely important, as its progeny will form the mesodermal
organs) is different in the two types of spiraling embryos.
Eventually, two snails are formed, with their bodies on different
sides of the coil opening.

The direction of snail shell coiling is controlled by a single pair of
genes (Sturtevant 1923; Boycott et al. 1930). In the snail Limnaea
peregra, most individuals are dextrally coiled. Rare mutants
exhibiting sinistral coiling were found and mated with wild-type
snails. These matings showed that there is a right-coiling allele D,
which is dominant to the left-coiling allele d. However, the direction
of cleavage is determined not by the genotype of the developing snail,
but by the genotype of the snail's mother. A dd female snail can
produce only sinistrally coiling offspring, even if the offspring's
genotype is Dd. A Dd individual will coil either left or right,
depending on the genotype of its mother."

In addition to the excerpted material there's a lot of other very
interesting material on mollusc development.

Hope this helps

See http://pharyngula.org/index/weblog/spiral_cleavage/

Great site, hfshaw-ga!

Thank you all of you! That's very interesting!
Regards from Switzerland