Snails and Slugs
Nature Bulletin No. 271-A May 27, 1967
Forest Preserve District of Cook County
Richard B, Ogilvie, President
Roland F. Eisenbeis, Supt. of Conservation
SNAILS AND SLUGS
There is something irresistible, to a child or a collector, about the
marvelous coiled symmetry, the gay colors, and the endless variety of
snail shells -- some large and some small -- that wash up on beaches and
shores of both salt and fresh waters. But, once we come to know them,
the soft-bodied creatures who slowly built those spiral limy portable
houses, loop on loop, are equally interesting. In Florida, people search
for brilliantly colored tree snails in the subtropical forests. Elsewhere,
we find land snails, or their empty shells, in the leaf mold of damp
woodlands, under stones and rotting logs, and among rank vegetation.
In our cellars, gardens and truck farms we find slugs, which are land
snails without shells or with only a remnant of shell buried in their
flesh. Their soft but muscular bodies are straight, not coiled. They have
a keen sense of smell and some kinds do damage to certain flowering
plants, leaf crops, or mushrooms. The common garden slugs are black
or dark brown, and from one-half to two inches long when fully
extended. The great gray slug, a pest introduced from Europe, is light
gray marked with black and may be four inches or more in length.
The snails are related to the oyster, the clam, the mussel, the squid and
the octopus. All of these animals are called mollusks. More than 30,000
kinds of snails have been described, of which about two-thirds still exist
-- about half of them in salt water and the other half in fresh water or on
land. The remainder are known only as fossils and, in the limestone
quarries around Chicago, we find several kinds -- some as big as your
fist -- which have lain buried there since this region was on the floor of
the ocean, 150 million years ago.
Most freshwater snails have a distinctive head with two conspicuous
tentacles and a small black eye at the base of each, or eyes on the tips of
the tentacles. The land snails and slugs have four tentacles with the eyes
on the ends of the larger pair. When feeding or moving about, a snail
can come almost entirely out of the shell, which is permanently attached
to and carried on its back. It glides along on its "stomach foot" --
usually an elongated muscular disk on which it travels by wave-like
contractions that pass from front to rear. Some kinds of snail shells coil
to the right; some to the left; and others, such as the little limpets, have
a shell like a low tent.
On the underside of the head is the mouth which, when feeding on the
glass side of an aquarium, can be seen slowly opening and closing.
Inside is a horny file-like tongue with which it scrapes the algae and
other growths from the surfaces of rocks, submerged plants, and muddy
or sandy bottoms. Some freshwater snails have gills by which they
obtain oxygen from the water. Others, such as most of our common
pond snails, and the land snails and slugs, breathe with a lung and those
living in water must come to the surface frequently.
Land snails, slugs and air-breathing water snails do not have separate
sexes. Instead, each animal has both male and female organs. The land
snails and slugs lay their eggs, about 50 at a time, in summer or early
autumn, among grass roots or other damp places. The air-breathing
water snails lay tiny eggs, imbedded in clear blobs of jelly, at frequent
intervals throughout the year. Most of the gill-breathers have separate
sexes and bear their young alive.
Some snails have a solid plug to close the shell opening after the animal
retreats inside. The land snails, slugs, and most pond snails have no
plug but seal themselves in with a slimy covering when they hibernate
to pass the winter, or in order to survive a hot summer drought when
ponds go dry.
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Update: June 2012