I am trying to learn more about the internal, vestigial shells of some slugs. Surprisingly, very little research has been done with them and there seem to be only a handful of relevant papers. In situations like this, it is often best to start from the beginning. By coincidence, or perhaps not by coincidence, the oldest original scientific paper I own, George B. Simpson's "Anatomy and physiology of Polygyra albolabris and Limax maximus" published in 1901 in the Bulletin of the New York State Museum also happens to mention the formation of the internal shell of Limax maximus during the slug's embryonic development.
Simpson describes the development of Limax maximus starting from an egg:
The body of the animal as first observed consists of a slight swelling of the upper side of the cell mass...The swelling...very soon shows a tendency to divide into two parts (pl. 24, fig. 17), the anterior part of which is the foot proper, the posterior part the mantle, shell sac, etc. Even at this early stage the embryonic shell can be observed, consisting of a few dark colored crystalline plates, not yet united.Here is pl. 24, fig. 17:
We skip ahead a bit:
The body now rapidly develops, as shown in figures 15 and 16, plate 25. The shell has also increased in size, consisting of numerous crystalline plates, not yet unified.Here is pl. 25, fig. 16 (the arrow I added is pointing at the shell within the future mantle):
As development proceeds a movement of the cells takes place from the ectodermal sac into the constantly enlarging body (pl. 26, fig. 1).Here is pl. 26, fig. 1:
It still doesn't quite look like a slug. The yellow arrow is pointing at the mantle and the red one to the shell, which, although Simpson doesn't mention it, still consists of separate pieces.
Finally, after a little bit more development, the creature starts to resemble a slug.
The right tentacle is recognizable (blue arrow) and so is the pneumostome, the breathing hole (green arrow). The shell (red arrow) still has a granular appearance; the individual plates have not fully fused yet. Simpson doesn't mention when the pieces of the shell finally unify into one piece, but, presumably, this happens at around the time of hatching.
There are 2 important pieces of information we can gather from this account:
1. The shell starts to form very early during the development of the embryo within the mantle and there it remains in adults. It is truly a vestigial structure left over from the snail ancestors of Limax.
2. The shell does not start out as a tiny bit of crystalline substance that grows bigger and bigger; instead, it is formed from the union of many bits of crystalline plates. In adult Limax maximus, the shell is a flat, elongated plate (this post has a picture of the shell). Interestingly, in some other slug genera, for example, Arion, the shell remains as a mass of granules of calcium carbonate, which makes sense as the next stage in an evolutionary sequence.