Arionidae, common name

Common name

A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 the "roundback slugs" or "round back slugs" are a taxonomic family of air-breathing land slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s, terrestrial

Terrestrial animal

Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

 pulmonate gastropod mollusks in the superfamily Arionoidea

Arionoidea

Arionoidea is a taxonomic group, a superfamily of air-breathing land slugs, shell-less terrestrial pulmonate gastropod mollusks.- Families :Families within the superfamily Arionoidea include:* Arionidae* Anadenidae* Ariolimacidae* Binneyidae...

.
The distribution of this family of slugs includes Nearctic

Nearctic

The Nearctic is one of the eight terrestrial ecozones dividing the Earth's land surface.The Nearctic ecozone covers most of North America, including Greenland and the highlands of Mexico...

, Palearctic

Palearctic

The Palearctic or Palaearctic is one of the eight ecozones dividing the Earth's surface.Physically, the Palearctic is the largest ecozone...

 and Oriental regions.
Unlike some slugs, European Arionidae have no internal shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

. They also have no keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

 on the back. The caudal mucous pit

Caudal mucous pit

The caudal mucous pit, or caudal mucous horn, is an anatomical structure on the tail end of the foot of various land snails and slugs, terrestrial pulmonate gastropod mollusks....

 is above the tip of the tail.