Arion rufus (Linnaeus, 1758) is a large species of slug. It has an interesting defensive behiour: when irritated it can contract to hemispehrical shape.
Some authors consider Arion rufus as a subspecies of Arion ater (Linnaeus, 1758) or as the same species as Arion ater. Arion ater has the same defensive behaviour. Other Arion species can contact their body a little, but they can not contract their body so much. If you are unsure in determination, you can distinguish Arion rufus from Arion vulgaris (Moquin–Tandon, 1855) according to this defensive behaviour.
(in Czech) Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. 2010: Komentovaný seznam měkkýšů zjištěných ve volné přírodě České a Slovenské republiky. [Annotated list of mollusc species recorded outdoors in the Czech and Slovak Republics]. – Malacologica Bohemoslovaca, Suppl. 1: 1-37.
Taylor, J. W. (1907). Monograph of the land and freshwater Mollusca of the British Isles. Testacellidae. Limacidae. Arionidae. pt 8–14. Leeds: Taylor brothers. page 168, plate XVIII.