I encountered this little fellow while digging in the sandy soil of northern Minnesota. He was unhappy about being exposed, and curled into this defensive - or threatening? - posture.
Most gardeners regard earthworms with something like reverence, but it's worth remembering that in other ecosystems the earthworm can be devastatingly destructive. Earthworms are not native to Canada and the Great Lakes region - basically to the area once covered by glacial ice. They are invasive species introduced during European settlement and during importation of materials from Asia. When they escape to a deciduous woods, they consume the leaf litter - the "duff" - that constitutes the future soil.
When earthworms invade a forested area, they consume the forest floor, and herbaceous plant diversity and tree seedling density decrease dramatically. Heavily impacted stands have been observed with only one species of native herb and virtually no tree seedlings remaining.
"Ninety-nine percent of the populations of native plant species normally found in hardwood forests, including large-flowered trilliums, yellow violets, and Solomon's seal, are destroyed in affected areas," said Frelich, a research associate in forest resources. "In many areas, the remaining bare soil is simply eroding away."
The best resource I know of online is the Great Lakes Worm Watch, maintained by the University of Minnesota.
The phrase "the worm turns..." seemed slightly familiar, but until I researched this post I didn't realize that it dates back to pre-Shakespearean times. The common literary citation is to Henry VI, part 3:
To whom do lions cast their gentle looks?But there are even earlier citations:
Not to the beast that would usurp their den.
The smallest worm will turn being trodden on,
And doves will peck in safeguard of their brood.'"
1546 J. Heywood Dialogue of Proverbs ii . iv. G4 V"Treade a worme on the tayle, and it must turne agayne."