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You might have wondered why I’m paying every time attention to the coiling direction of the shell. Is it left (sinistral) or right (dextral)?

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Stephen Jay Gould has been one of the scientists that wondered why snails are one of the few major groups that do not present a basically bilateral symmetry in their body, or more precisely, in their shell [1]. While most species are dextral, there are exceptions. In some groups (e.g. the family Clausiliidae) sinistral shells are the rule, and dextral species the minority. And there are also groups where in some genera a number of species do exist where both sinistral and dextral shells do occur (this is called enantiomorphy).
Gould realised that with the introduction of early printing techniques (woodcuts, etchings) the original had to be made as mirror-image to have the final printed image correctly shown. If one forgot this principle of reversion, a dextral shell could look sinistral and vice versa; of course this only applied to non-bilateral objects. In some cases clearly errors have been made, e.g. the Conus shell image by Rembrandt [2].

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When Gould examined early printed books, he came sometimes across sinistral illustrations of dextral shells in works published before 1700. “In fact, almost all snail illustrations from this period are reversed”, and finally became to the conclusion that “if standard sources and noted artists all drew snails in mirror image from their natural occurrence, they must have been following a well-accepted convention of the time, not making an error” [3]. In this context, it is interesting that the final example that led him to this conclusion was the work by Augustino Scilla, an Italian painter who also engraved his own plates for his book De Corporibus Marinis Lapidescentibus. This finally led him to postulate “the conceptual world of pre-eighteenth-century zoology must have accorded little importance to the orientation of a shell”. I will refer to this as the ‘Gould hypothesis’.

Allmon attempted to test this hypothesis by examining illustrations of both snails and crabs (another group of asymmetrical animals) in major illustrated works printed before 1758 [4]. He concluded that Gould was correct. However, his research was limited to a number of natural history works. Thus remain questions like: Is this indifference also noticeable in contemporary general works? How are snails illustrated with a different, direct technique like painting or drawing? Is there a difference between different artists? And how does this compare to much earlier illustrations in medieval manuscripts?

These are questions which hopefully may attempted to be answered in the future with the data collected in this blog.

Notes:
[1] Gould, S.J. (1995) Left snails and right minds; were early conchologists choosing sides when they held the mirror up to nature? Natural History (New York) 104 (4): 10–18.
[2] http://hdl.handle.net/10934/RM0001.COLLECT.36288.
[3] Gould, o.c.: 16.
[4] Allmon, W.D. (2007). The evolution of accuracy in natural history illustration: reversal of printed illustrations of snails and crabs in pre-Linnean works suggest indifference to morphological detail. Archives of natural history 34: 174–191.

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