Semi-slug

Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but not quite vestigial.

[1] The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered over with the mantle.

This is a type of gastropod that is intermediate between a slug (without an external shell) and a land snail (with a large enough shell to retract completely into).

There exist a number of gastropod families that have semi-slug species.

[1] Semi-slugs have a worldwide distribution and have evolved in several families; genera include:

Side and top view of a Cryptella canariensis from the Canary Islands
Shell of Cryptella canariensis
An unidentified semi-slug from Uganda