Two shells lie on the beach, next to a clump of seaweed. Both shells are spirals. The shell on the left is the empty outer shell of an auger snail, which grew around and with its spiral-shaped body. Auger snails live in the surf zone of sandy beaches. They are predators, boring through the shells of other mollusks to feed on their soft bodies. The round bore hole in this empty shell tells us that its occupant was killed by another snail.

The shell on the right is a spirula shell, the internal shell of a small squid. When the squid was alive, the shell was coiled inside its body. Spirula squid are only about 3 inches long and live far out in the sea. Few people have seen them alive, but their shells wash up on beaches all over the world.

Why are there so many spirals in nature?

Spirals result from the uneven growth of two surfaces of a material. The outer surface grows the fastest, curling around the more slowly growing inner surface. As a snail's body grows, its shell grows, too. The outer surface of the shell grows faster than the inner surface and the newer, larger portions of the shell spiral around the older parts.

To see how this works, try making a "snake" from the paper tube wrapper of a plastic soda straw. Tear off the top of the wrapper. Then crumple the wrapper down to the base of the straw to form a wrinkled tube. Remove the straw from the tube and let the tube lie on the table while you slurp up a little of your drink with the straw. Touch the wet straw to the sides of the wrinkled tube and see the wet part grow, causing the tube to curve.

Related Virginia science Standards of Learning: 3.4; 3.6a; 4.5a; LS.10c; DM.10e.