Most people live in homes that are firmly fixed in one spot. They may be huge apartment
blocks or small private residences, mud-walled huts or houses on stilts, but they are alike in that
they are not easily moved. For some people; however, who like to wander, a permanent dwelling
is a handicap. They prefer a mobile home, one that they can pick up and take along with them on
their travels. Tents are conveniently portable, and for many centuries they have been a favourite
form of shelter for nomadic tribesmen. Tents are also frequently used in military operations, and
on ordinary camping trips. Other types of mobile homes, such as caravans or houseboats, are
roomier and more comfortable to live in than canvas tents.
Certain animals also enjoy the advantages of living in a mobile home. The snail, which
carries its house about on its back, manufactures for itself a durable shelter of exactly the right
size. As the snail grows, it adds to its shell, so that it never needs to search about for larger
quarters. A snail does not shed or exchange its shell; it simply goes on enlarging it. The shell is
composed of a lime-like substance secreted by a fold of skin called the mantle. The mantle
completely lines the shell and wraps around the snail’s body. Secretions flow from the interior
margin of the mantle throughout the life of the snail, and then evenness of the flow determines the
smoothness of the shell. An even flow produces a smooth, thin shell while an uneven flow results
in a ridged and fairly thick shell. The snail begins constructing its shell at the small, pointed end,
and as it grows, it continuously lengthens and widens the shell, by adding further coils or whorls.
Most snail shells are right-handed spirals, although a few twist naturally to the left. If you hold a
snail shell with the opening downward, and look at it from above, you will find that nearly all of
your specimens spiral clockwise from the tip to the opening. This is what is meant by a right-
handed spiral.