How does a snail grow its shell?
Shell deposition is quite a complicted thing, but basically the shell has 4 layers the outer layer being the periostracum which is tanned and hard. The other layers are the calcareous layers and the shell is enlarged by addition of mineral from the outer edge of the mantle to the lips of the aperture. The mantle epithelium (outer layer) has shell secreting glands in it and there are also mantle edge glands that secrete periostracum around a periostracal groove. For a picture see Barnes and Ruppert – Invertebrate Zoology page 383 (6th ed).[Claire Reader] There’s also a bit about the shell on the anatomy page.
