Arionidae Gray, 1840
The center of diversity for the family is the western Palearctic, especially the Iberian peninsula (Wiktor 1996).
About 35 species are in the genera Arion, Geomalacus, and Letourneuxia (Barker & Efford 2004).
External: Plain or striped body; mantle lies on the anterior part of the body; pneumostome anterior to midpoint on right side of mantle; granular skin on mantle; round tail with caudal pit at tip, no keel; wide foot fringe; sole one part.
Internal: Internal shell small and plate-like or just vestigial calcareous granules; Jaw odontognathic, ribbed; radular teeth: central tricuspid, lateral bi- or tricuspid, marginal bicuspid, all having broad bases, teeth often accreted; heart, in relation to body axis, is titled to the left; Kidney circular (surrounding aorta); cephalic retractors tend to divide into separate branches attached independently to the posterior part of pallial complex; 2-loop digestive system; male copulatory organs generally reduced, their role being taken over by a well-developed atrium and epiphallus that produces spermatophores; thus, no penis, just long epiphallus; no vagina; ligula in atrium or end of oviduct (Wiktor 2001).
Up to 250 mm long, but usually much shorter.
Ecology and Distribution
Native throughout Europe and north to Scandinavia, N Africa down to the Sahara, with some species in Asia east to the Urals (and Siberia); major speciation center in Iberian Peninsula (De Winter 1986; Wiktor et al. 2000); not in the Caucasus or Black Sea region (Wiktor 1983).
For its defense, an arionid slug will retract its head, become hemispherical, and stick down to the substrate. Many Arion species have extra colored pigment as a deterent in the slime that they produce when perturbed (Rollo & Wellington 1979).
Consumes plant, fungus and sometimes feces, carrion (Herbert 1997).