Most people recognize land snails and slugs when they see them: They have moist, soft, elongated bodies with a definite “foot” and a head with paired tentacles. Slugs lack visible shells, while snails have coiled shells composed of hard calcium carbonate. In a generalized way, the basic body form of snails and slugs is like that of other molluscs (limpets, squids, octopuses, clams, mussels, and so on), but snails have the distinctive spiral shell.
Slugs are essentially the same thing as land snails; they are simply land snail species that do not develop visible shells. Slug species occur in several snail families, so slugs and snails do not represent different taxonomic groups. Different slug species are usually identified by the different positions and configurations of the mantle and body openings.
Mollusc shells are often used for identification because they are durable and have easily-seen characteristics, such as bumps, ridges, and spiraling traits. The aperture (opening) often has distinctive folds, teeth, and other projections. Most are drab white, tan, gray, or brown, but some species have red, yellow, or striped shells. Long after a snail has died, and even after the shiny, colored outer layer (periostracum) of a shell has been eroded, the different species can be identified by the bleached, white mineral portion of the shell.
Look closely at the structure of a snail shell: One key identifier for any snail is how the shell is coiled. Most snail species are “right-handed,” but some are “left-handed.” Here’s how to tell: If you hold the shell with the tiniest whorls (apex) facing you, and the spiral develops and widens in a clockwise direction, the shell is right-handed (or dextral). If it goes counterclockwise, then it’s left-handed (or sinistral). Each turn of the shell is called a “whorl.”
After the shell, the most obvious part of a snail is its muscular foot, with which the snail moves by waves of muscle contractions, aided by a layer of mucus. At the front of the body is the head, which has one or two pairs of retractable tentacles: The longer, upper pair bears the snail’s eyes; the shorter, lower pair are used for chemical sensing — taste and smell. The mouth faces downward; in it, the radula, a hard, microscopically toothed structure, scrapes at food like a little rasp.
Another organ, the mantle, is the material connecting the snail’s body to its shell; it protects the animal from drying out, and it secretes the calcium, proteins, and other shell-making molecules that, over time, enlarge the shell. You may see a hole in the mantle — that’s the pneumostome (NOO-mo-stome), which functions like a nostril; the animal breathes through it. Just behind the pneumostome is another hole — the anus.
Keep in mind that many snails are amphibious and can move in and out of water. Most land snails, and several types of aquatic snails, are pulmonates that breathe air via a lunglike pulmonary cavity protected by the mantle. Another group of snails, the prosobranchs, or gilled pond snails, are more strictly aquatic, though 3 species of land snails in Missouri are prosobranchs. They have tubelike siphons that draw water (not air) in and out of the mantle cavity, where gills extract oxygen. Another identifier for gilled snails is that most of them have a hardened operculum (“trapdoor”) that closes the opening when they retract into the shell; this helps them to prevent drying out.