Most have the typical lung arrangement described above, but in the Athoracophoridae, the mantle cavity is replaced by a series of blind tubules, while the Veronicellidae respire through their skin, and have lost the lung altogether.
In these aspects they are anatomically distinct from most other types of terrestrial slugs, which typically belong to the order Stylommatophora, and which have a forward located anus, and retractile tentacles.