The protoconch or embryonic shell has a different sculptural pattern to the rest of the shell. This is the part of the shell the snail has when it is born.
The protoconch is visible even on adult shells.
As snails grow, they enlarge their shell and more whorls can be seen. In many species, when they fully mature, some species develop a ‘lip’ on the edge of the shell opening.
Slugs and semi-slugs follow a similar development.