Arionidae

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Arionidae are a family of air-breathing land slugs, often called roundback slugs. They are terrestrial mollusks in the superfamily Arionoidea.

Distribution
Arionidae live in the Nearctic, Palearctic, and Oriental regions, meaning parts of North America, Europe/Asia, and nearby areas.

Anatomy in brief
- They have no keel on their backs (unlike some other slugs).
- The caudal mucous pit sits above the tail tip, and the breathing pore (pneumostome) is in front of the middle of the mantle.
- They can reach up to about 25 cm in length.
- The mantle covers only part of the body and sits toward the front.
- The jaw is ribbed (odontognathic). The radula teeth include a central tricuspid tooth with lateral and marginal teeth that have broad bases.
- The digestive system forms two loops.
- The heart is tilted to the left; the kidney is circular and surrounds the aorta.
- The shell is greatly reduced or buried in the mantle.
- Reproduction: the penis is present in some species, but many rely on an epiphallus to help produce spermatophores. Male reproductive organs are often reduced.
- Haploid chromosome numbers range from 21 to 30.

Genera
The family Arionidae has no subfamilies. The type genus is Arion. Genera include:
- Arion
- Anadenulus
- Ariunculus
- Carinacauda
- Geomalacus
- Gliabates
- Hesperarion
- Letourneuxia
- Magnipelta
- Securicauda
- Udosarx
- Zacoleus

Ecology and parasites
Parasites of Arionidae slugs include Sciomyzidae flies.

Taxonomy and notes
- Arionidae was named by J. E. Gray in 1840.
- Synonyms include Tetraspididae Hagenmüller, 1885.
- The fossil record for this family extends from the Miocene to the present.