Which gastropods do not have a shell?

In slugs and opalsobranch (or nudibranch) molluscs, the shell is absent. This is due to the fact that they no longer need a sink. For example, for some (slugs) it prevented them from moving on a certain terrain, while others lost the need for protection due to their toxicity. Nevertheless, some slugs have preserved a rudimentary shell in the form of a small plate, in which calcium is stored.
Nevertheless, in these gastropods, the shell is reduced secondarily, i.e. initially it is laid, and then disappears as it grows up.