There are different groups of slugs not necessarily closer related to each other. But they can be discerned by their outer appearance.
NEW: Different kinds of slugs in pictures: The slugs described on this homepage.
Body length usually clearly more than 10 cm (4 in.):
Left: Lusitanian slug (Arion cf. vulgaris). Right: Black keel back slug (Limax cinereoniger).
Note the different form of the slugs' body and the respiratory hole situated in the front part of the Arion's mantle shield!
Left: A scared black slug (Limax cinereoniger). Note the respiratory hole situated in the rear part of Limax's mantle shield!
Middle: The keeled tail of a Limax slug. Limax cinereoniger is also known as black keel back slug, only this one is rather grey. But the characteristic white stripe along the tail keel is well visible.
Right: The foot sole of Limax cinereoniger. The foot sole of Limax maximus, which is easily mistaken for a cinereoniger, would be unicoloured white.
Body length between 5 und 10 cm (2 and 4 in., resp.):
Body length below 5 cm (2 in.):