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.
HOW SNAILS & SLUGS MOVE:
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!
LAND SNAILS & MUCOUS
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
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.
photos by Tom Searcy
of Couer d'Alene, Idaho
Tom Searcy was having a coffee on his
patio when he saw the two above slugs hanging from a 6-8-inch string of slime. He came
back 10-15 minutes later and found the bluish "bloom" below.
The bluish "bloom" consists of
the snails' joined sex organs, which have been extruded from the snails' bodies and
wrapped around one another.
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
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
You can review books about snails and slugs available at Amazon.com by clicking here.