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how do snails grow since they have shells?

Snails add new growth on the outside of the shell coil. See this site for more:

J. Elliott

Snails and other mollusks begin their life with a very small shell which they expand as they grow. As the snail grows it adds new layers to the outer edge of the shell, making the shell just a little bit larger. If you look carefully at the shell of a snail, or a shell fish, you will see ridges which are the result of growth at different rates - If the snail grows slowly, it makes a thicker shell - in fast growth periods the shell is thinner.

Nigel Skelton
Tenant Creek High School

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