They are terrestrial pulmonate gastropod mollusks in the superfamily Arionoidea (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005).

Members of this family most obviously differ from related slugs in that their mantles are broadly rounded, and very large, covering the entire body.

Pilsbry (1948) stated that "the enormously developed mantle, the large empty shell sac, and the insertions of the free retractor muscles along the margins of the foot cavity, instead of dorsally as in the Arionidae are special to the Philomycidae".

[2] A further anatomical oddity of the group, shared with certain helicid and zonitid snails, is their creation and use of calcareous love darts during mating.

[4] Genera within the family Philomycidae include: A cladogram based on sequences of cytochrome-c oxidase I (COI) genes showing phylogenic relations of genera in the family Philomycidae by Tsai & Wu (2008)[5] (simplified): Meghimatium Philomycus Pallifera Megapallifera Arion Deroceras Arion and Deroceras were used as outgroup.