Many land snails seal their shell mouth with a dried mucus film in dry weather. Helix pomatia goes considerably further before overwintering. The animal burrows into soil or leaf litter, retracts fully, and secretes a plug of calcium carbonate mixed with mucus across the aperture, which hardens into a rigid white lid a millimetre or so thick.
Behind that seal the snail's metabolism slows sharply, and it survives on stored reserves for months. The plug reduces water loss and, being solid rather than filmy, gives some protection against predators and cold. A small pore allows limited gas exchange. In spring the snail pushes the epiphragm off and often eats it, recovering the calcium.
This is a genuine winter dormancy tied to temperature, distinct from the lighter summer inactivity, aestivation, that the same species enters in drought behind a thinner seal. The animal is also long-lived by land snail standards, with individuals recorded past ten years, and it is legally protected in several European countries where collection for food has reduced populations.