What's with the snails?

In a blog article for Smithsonian, Colin Schultz wonders why in English manuscripts from the 13th and 14th centuries, knights are always fighting snails. (photos)

Schultz writes:

It’s a great unsolved mystery of medieval manuscripts. As Got Medieval writes, “You get these all the time in the margins of gothic manuscripts.” And I do mean all the time. They’re everywhere! Sometimes the knight is mounted, sometimes not. Sometimes the snail is monstrous, sometimes tiny. Sometimes the snail is all the way across the page, sometimes right under the knight’s foot. Usually, the knight is drawn so that he looks worried, stunned, or shocked by his tiny foe.