how do shells grow?

I was walking on the beach when my son asked me this question, which stumped me. I could guess that the inside of the shell could be actively dissolved -redeposited like bone, but the marine creature would have difficulty accessing the outside of the shell to reform it. So how do they grow?

According to Wiki: The shell will grow over time as the animal inside adds its building material to the leading edge near the opening. This causes the shell to become longer and wider to better accommodate the growing animal inside.

This would apply to Mollusc shells, which might not be what you were looking for, but I’ll keep looking for answers.

Brendon

The mantle – the utmost outer edge of the creature in the shell – usually reaches over the end of the shell, or can be extended there, and that’s where the creature adds to the shell. Nautiluses and the extimct ammonites occupy the ends of their coiled shells, and only added onto the larger end of it (this is why those shells have the form of a logarithmic spiral – the living space increases regularlu as the shell extends). Clams seal the edge of their shell with a living rim, which is where the shell gets added to. Cowries have a mantle that extends up onto their shell and covers it (which is why the whole shell is “glossy” and smooth), and they add onto it everywhere. If you look at a snail shell or a nautilus or a clam or a mussel you can see the growth “rings”, and you can see exactly what the shell looked like at earlier periods in the life of the creature. With cowries, the new growth completely covers the older stuff, and you can’t.

Ah so it grows from one edge. I assumed that it grew all over. That explains a lot (and also how it makes all those pretty pattern in the shells)