eli5: How seashells are made?

Question

I know they are made of CaCO3, but I don’t understand how they know the shape in which they are supposed to grow.

In: Biology

Answer 1

The mantle of the mollusk secrets the material. Where it secretes material a shell is laid down. That is really all there is to determining the shape.

Four a snail with a spiral shell, as a lava, the snail twists itself into a spiral shade and then secretes the beginning of the shell. As it gets bigger, it pushes out and has to secrete more and larger shell on top of what it has already produced, which eventually makes a spiral.