Inspired and encouraged by Hekebolos' spider diaries, I'll try to make this a weekly feature on marine animals (probably posted on Fridays) if you all are interested. This diary will be fairly simple. I'm not going into a whole lot of details about snail anatomy, but just the basic differences between left and right-handed shells.
Snails, or univalves, comprise the largest group of mollusks. Nearly all are characterized by a hard shell, made primarily out of calcium carbonate, which rotates around a central columella.
The chirality, whether the shell is right-handed or left-handed, varies from species to species, although some types can twist either way. (This however causes problems with reproduction since the reproductive organs are opposite as well.)
A basic shell structure is shown below:
First of all, let's discuss what it actually means for a snail to be "left-handed" or "right-handed". If you hold the shell in your hand, with the opening (the aperture) facing toward you, if the opening is on the right side it is "right-handed" (or dextral) and if it is on the left side it is "left-handed" (or sinistral). So a dextral shell grows in spirals in a clockwise fashion and a sinistral shell grows counter-clockwise. In the photo below, the snail on right is dextral and the one on the left is sinistral.
As you can see, the diagram at top is of a dextral snail. The central columella is shown in the cutaway and forms the structural base for the shell. The opening is surrounded by a lip, where the growth of the shell occurs, and each segment of the shell is called a "whorl". The majority of species are dextral, by the way.
One question I'm often asked is how the shell actually grows. The layers of the shell are put down by an organ called the mantle, which secretes the limey substance that the shell is made out of. The minerals are added to the lip of the aperture, essentially making the shell longer as it spirals, giving more room for the growing animal inside. But here's the tricky part; the layers of shell are not secreted uniformly along the lip of the opening. One side of the lip gets more attention than the other. That's what makes the shell coil, otherwise you would end up with a snail shell that more resembled a walrus' tusk.
The most common marine snail in southern New England is the common periwinkle (Littorina Littorea). It is dextral, but there are several reports of sinistral individuals found in Europe. I've looked for twenty-five years and I've yet to find one. One of these days. Also, these snails can't turn themselves over if flipped upside down so I can't for the life of me figure out why they are so damn common.
So it's rare that a dextral species will have an individual that is sinistral, and visa versa. However, shells can vary greatly in other ways within a species depending on the environment it lives in, as you can see here. Of course, there is always the possibility of giant mutants that appear to be neither dextral or sinistral. But these are usually just sinister creations by cartoonists who don't study molluscan biology.