How do snails build their shells?

1. A baby snail hatches with a tiny shell already on its back. It never swaps that shell for a bigger one. Instead, the shell grows as the snail grows. But how?
2. Shell is made mostly from calcium. The snail gets it by scraping at chalky things with its rough tongue: bits of limestone, old shells, eggshell and the soil itself.
3. In the gut the calcium is taken into the snail's body. Its blood carries the calcium to a soft skin that lines the inside of the shell. That skin is called the mantle.
4. At the edge of the opening, the mantle first makes a thin, tough skin. Then it fills in hard calcium crystals behind it. Strip by strip, the edge of the shell moves forward.
5. New shell is only added at the opening, and each new part is a little wider than the part before. That is why the shell curls into a spiral that gets wider with every turn.
6. The finished shell is a strong home. The snail pulls itself inside to stay safe and to keep from drying out. If the shell cracks, the mantle patches it from the inside.

Tap each part to follow calcium from the mouth to the shell.

• Rasping tongue. The snail's tongue, called the radula, is covered in rows of tiny teeth. It scrapes food and chalky bits into the mouth.
• Gut and calcium store. Most of the snail's gut sits coiled up inside the shell. It takes calcium out of the food and passes it into the blood. Some is saved for later.
• Mantle. The mantle is a soft skin that lines the inside of the shell. It takes calcium from the blood and turns it into hard shell.
• Growing edge. The edge of the opening is where the shell grows. The newest strip is thin and pale. It hardens and thickens over the next days.
• Shell layers. The shell wall has layers. Outside is a thin, tough skin that carries the color. Under it are hard layers of calcium crystals, stacked in different directions.
• Whorls and growth lines. Each full turn of the shell is a whorl, and each is wider than the last. Fine growth lines mark where the edge used to be.
• First tiny shell. The very center of the spiral is the oldest part. It is the tiny shell the snail hatched with.

Feed the snail and watch the calcium travel to the edge of the shell. What kind of shell does it build with lots of calcium, and with very little?

Experiment: What will the new shell be like if the snail gets plenty of food but very little calcium?

1. Tap the eggshell and watch the new strip of shell appear.
2. Now tap the lettuce a few times and look at the new strips.
3. Compare the colors of the strips: dark and solid, or pale and thin?

What we found: With plenty of calcium, the snail builds thick, strong strips of shell. With very little, the new strips are pale, thin and narrow. A shell is built from what the snail eats, so no calcium means no proper shell.

Turn on JavaScript to try it yourself.

You have 9 months. Each month, eat lettuce to grow or eggshell to store calcium. Grow 5 new strips of shell, and keep them thick.

A snail's shell is part of its body, like your bones or your teeth. The snail cannot crawl out of it and find a new one. A baby snail hatches with a tiny shell, and that same shell has to grow with it for its whole life.

The shell is made mostly of calcium carbonate. That is the same material as chalk, limestone and eggshell. The snail gets its calcium from what it eats. It scrapes at plants, soil, stones and old shells with its rough tongue. Baby snails often eat the shell of their own egg first.

The builder is the mantle, a soft skin that lines the shell. Blood brings calcium to the mantle. At the edge of the shell's opening, the mantle lays down a thin skin and then fills in hard crystals behind it. Because new shell is only added at the opening, and each new part is a little wider, the shell curls into a spiral.

A snail that cannot find enough calcium builds a thin shell that breaks easily. That is why you find many snails on chalky soil and few on sour, acid soil. Gardeners who keep pet snails give them cuttlebone or crushed eggshell for the same reason.

Key words

Mantle

The soft skin under a snail's shell that makes the shell.

Calcium carbonate

The hard, chalky mineral that shells, eggshells, chalk and limestone are made of.

Radula

A snail's tongue, covered with rows of tiny teeth for scraping food.

Whorl

One full turn of a spiral shell.

Aperture

The opening of the shell, where the snail comes out and where new shell is added.

Logarithmic spiral

A spiral that gets wider by the same amount of times with every turn.

Check your thinking

1. Where on the shell is new material added?

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   At the edge of the opening. The mantle adds new strips at the edge of the opening. The center is the oldest part.

2. What is a snail's shell mostly made of?

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   Calcium carbonate, like chalk and eggshell. Shell is calcium carbonate, the same mineral as chalk, limestone and eggshell.

3. A snail lives where there is very little calcium. What will its shell be like?

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   Thin and easy to break. Without enough calcium the mantle cannot build a thick wall, so the shell stays thin.

4. Why does a snail's shell form a spiral that gets wider?

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   Each new part added at the opening is a little wider than the last. The shell only grows at the opening, and the opening gets wider as the snail gets bigger. That makes a widening spiral.

5. A snail's shell gets a small crack. What happens?

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   The mantle patches it with new shell from the inside. The shell is attached to the snail for life. The mantle can lay new shell under a crack to mend it.