Some of the more familiar and better-known gastropods are terrestrial gastropods (the land snails and slugs) and some live in freshwater, but more than two thirds of all named species live in a marine environment.
In habitats where there is not enough calcium carbonate to build a really solid shell, such as on some acidic soils on land, there are still various species of slugs, and also some snails with a thin translucent shell, mostly or entirely composed of the protein conchiolin.