The larvae of Stiliger ornatus

Dear Tzan-Chain,
I don't know of any study on the development of Stiliger ornatus but from the well-developed velum, with large cilia, seen in your lower photo, I would suspect that in this species the larva spends some time as a free-swimming planktonic veliger. Normally it would only lose its shell after it has settled down on to the substrate and begun to metamoprhose into a crawling slug.

I guess you are keeping egg masses in the laboratory to study? Unfortunately, I think in your case that the empty shells are a sign that the larvae have died and rotted away.
Best wishes,
Bill Rudman