What is "semi-slug"

Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but the shell is not quite vestigial. 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-slugs species. There exist about 1000 species of semi-slugs in comparison to about only 500 species of slugs.