The most notable feature of the garden snail is its shell, a beautiful globe of yellowish brown with the speckled markings that give it the designation "aspersum". This carapace is the snail's home, its armour - and a miracle of natural engineering.
All the snail's organs are packed inside whilst, at the same time, the shell protects them from predators. (We will say nothing of clever birds that use stones as anvils to crack it open and devour the soft interior). The shell has 3 layers: a thin periostracum made largely of protein which gives it a leathery finish known as shagreen - a new word to me! Then comes a thick stratum made of calcium carbonate crystals packed together for strength. That mineral composes over 95% of the shell and is the reason why snails need to eat calcium, which, in the wild, takes the form of soil, chalk, limestone or even (avert your eyes if you are sensitive) the shells of other snails. Finally, protecting the body is an inmost smooth lining.
During the life of the snail, its mantle (the tissue that covers its visceral organs) secretes new liquid calcium and proteins into the aperture, the opening at the edge of the shell. This hardens quickly and forms growth lines, not unlike those of a tree. The result is the typical dextral spiral with patterns that act as camouflage against a background of leaf litter - or your garden borders. It has 4 to 5 whorls with an apex which was the juvenile creature's starting point, its protoconch. which often survives as a fossil. I love the idea that this snail does not lose its baby form entirely but carries it around for life. Perhaps we all do! A thickened edge at the lip is a sign of a mature Cornu aspersum.
The fact that the snail lives inside its house means that it can avoid both loss of moisture and threats. It can withdraw fully within and, if necessary, secrete a layer of mucus called the epiphragm. This seals off the aperture and conserves moisture. It uses this technique during hibernation and the summer equivalent, aestivation, when the snail is inactive. If the shell is damaged, but not too much, and the inner organs are still sound, the mantle can produce fresh calcium and conchiolin to repair the wound with a patch. Beats reaching for an Elastoplast!
I feel that the humble garden snail has one up on homo sapiens here: in extreme weather I would love to create my own protection and retire from the world.