Though snails and slugs look very different from one another, they're practically the same thing -- except that snails carry coiled shells on their backs, while slugs don't.
If you're reading this where it's warm and rainy, you shouldn't have problems finding snails and slugs. They're found at the base of your house's foundation on the shady northern side, or maybe nibbling lettuce in the garden. If you're reading this during a drought, however, you may need to snoop around. Look for them early in the morning when dew wets everything. If it's really dry, you may just have to wait, for your backyard's snails and slugs may well be underground or beneath rocks or fallen trees, in suspended animation, waiting for rain. I found the one in the picture above resting on a slender bunch of grass flowers. In real life this snail's shell is less than half an inch long.
slug image by Bea Laporte
Some snail species live on land, while others live aquatic lives. If you've ever watched aquatic snails slowly moving across the glass walls of the inside of a water-filled aquarium, you know the snails' (as well as the slugs') basic features:
The picture at the right shows the head of a Banded Forest Snail, Monadenia fidelis, a common species in the US Pacific Northwest. Notice that this land snail's head bears four long appendages called tentacles, which are used to feel. The rounded tips are primarily odor-sensing chemoreceptors that enable the snail to smell its environment. The upper two tentacles also bear eye spots at their tips enabling the snail to detect movement and shades of gray.
At the left you see the breathing hole of the Banded Forest Snail shown above. At the hole's right you may notice a certain irregularity. That's the anus. Aquatic snails are somewhat different. For one thing, if you watch them in an aquarium you may see them send a tube up to the water's surface to get air. Also, aquatic snails bear only one pair of tentacles instead of two.
Here are some nice slug words to know: On the above slug, notice that where a snail's shell would be there's a long, leathery thickening almost as if the slug were wearing a cape. That's the mantle fold. Toward the back in the mantle fold you see a hole. Probably you can guess that that's the air hole. A more technical name for the air hole is pneumostome. The top pair of tentacles, the ones bearing the eye spots, are technically known as the cephalic tentacles. The lower power of tentacles are the oral tentacles. The lowest part of the slug, here showing many vertical stripes at its side, is the foot, and the foot's bottom surface, which glides over things, is -- you guessed it -- the sole.
The image at the right shows a cross section of a snail shell from outside my door. The top of the shell has been removed (not a side). It may take a while to understand what you see, but once you do you'll be able to visualize how the snail and its shell started out very small, and as the snail grew, so did the shell. Of course the shell is hard, so it had to grow in a special way, in a spiraling manner. The part of the shell that grew was the rim surrounding the protruding foot. The rim grew as if it were adding slender, ever larger rings, each below the last. In the image, the dark cavity on the shell's left is where the snail's foot emerged from the intact shell.
Actually, there's really nothing mysterious about how snails and slugs move. (Of course that's a slug above.) An individual propels itself by sending wave after wave of small contractions forward from the back of its foot toward the front. If you've lain on your back and inched yourself across the living-room floor by alternately curving and straightening your backbone, you understand the principle.
The mollusk phylum contains several classes and subclasses. One class holds octopuses and squids. Clams and oysters belong in another; and then there's the snail and slug class. The snail and slug class is itself huge and diverse, and all its members are known collectively as gastropods. The word gastropod is almost funny: It's derived from the classical Greek word gastros meaning stomach, and podos meaning foot. Watching aquarium snails grazing, you may think that "stomach-foot" is a good name for them!
The vast majority of gastropods are aquatic animals. In fact, snails and slugs are the only mollusk class found on land. The snails in our backyards should more precisely be called land snails.
One adaptation enabling land snails and slugs to survive on land is their ability to produce plenty of slimy mucous. If you've watched a land snail or a slug moving across a dry surface, you may have noticed that it left a silvery trail. This trail is mucous the creature lays down beneath it as it travels Land snails and slugs move upon a layer of mucous.
This mucous in wonderful stuff. For one thing, it prevents moisture in the animals' bodies from being soaked up by the dry terrain being traveled across. Also, it protects the animal's fleshy underparts from sharp objects. Snails and slugs can actually glide across the sharpest razor blade without cutting themselves.
The big danger in the lives of land snails and slugs is drying out. Gardeners know that one way to rid themselves of lettuce-eating snails and slugs is to sprinkle them with salt. The salt causes water to leave their bodies, and they shrivel up fast!
When dry weather comes, land snails and slugs bury themselves in the soil or find some other well protected spot. Snails plug up their shell holes with mucous, and slugs secrete a sort of mucousy cocoon for themselves. Then through the dry spell the animals remain in a state of suspended animation during which their body processes slow to a point almost like death. However, there's enough life in them for them to become active again once enough rain comes to dissolve the mucous and soak into their bodies.
Mucous also comes in handy when a predator such as a toad snatches up a seemingly defenseless slug. The slug secretes such quantities of the stuff that after the toad chews a few times it finds its mouth clogged with sticky, gooey slime.
On our Flower Pollination page we see that individual flowers often contain both male and female parts, but that usually such blossoms employ some kind of "trick" that keeps it from fertilizing itself. The same is true of land snails and slugs. When an organism, whether plant or animal, possesses sex organs of both genders, it's said to be hermaphroditic.
Most land snails and slugs possess both male and female parts. In some species, an individual may behave as male for a while, then as a female. When snails mate, as in the drawing at the right, two individuals pull up next to one another, arrange themselves so that the male part of one is opposite the female part of the other, and then each ejects male sperm into the female opening of the other. In a few snail and slug species, self-fertilization occurs -- an hermaphroditic individual mates with itself and produces offspring.
If you live in a rainy or drizzly area where gastropods are plentiful, you know that snails and slugs come in an amazing variety of sizes, shapes, and colors. Snail shells are a whole psychedelic world of their own.
On the Web, check out the Online Fieldguide to the Freshwater Snails of Florida and Kentucky Slugs.
You can review books about snails and slugs available at Amazon.com by clicking here.