The term slug refers to snails that have shown a drastic reduction in the size of the shell. This has happened on several independent occasions, so slugs are not a natural 'monophyletic' group with a single evolutionary source. However, it is convenient to deal with them as a single unit for identification purposes. Thirty-six slug species representing seven families were known from the British Isles in 2012, though some are introductions that do not readily exist outside of greenhouses and much taxonomic confusion surrounds a number of groups.
Of the slugs you are likely to encounter, four families have no sign of an external shell (Arionidae, Limacidae, Agriolimacidae and Milacidae). A fifth family, the Testacellidae has a tiny external shell at the hind end of the body. This is how you identify those five families:
Arionidae (round-back slugs). Easily identified by the breathing aperture (pneumostone) being in front of the mid point of the mantle (located in the posterior third of the mantle in limacids, agriolimacids and milacids). The top of the body never bears a keel (in contrast to milacids, limacids and agriolimacids) and the mantle is rough and shagreened without the fingerprint-like pattern of limacids and agriolimacids. The tail is blunt when viewed from above and bears mucus glands. Internally, the remains of the shell are represented by a few calcareous crystals. Most species contract into a hump when disturbed, though the ability varies between species. Two British genera: Arion and Geomalacus.
Limacidae. Pneumostone located in hind third of the mantle. Mantle with a pattern of ridges resembling fingerprints centred upon the midline. A keel is present but does not extend forward to the mantle and can be very short in Limacus. The internal shell is assymetrical in contrast to the symmetrical ones of milacids. These slugs do not contract into a tight hump when disturbed. Three British genera: Lehmannia, Limacus and Limax.
Agriolimacidae. Very similar to limacids and only recently split. The keel is truncated and turns down sharply at the tip of the tail. The pattern of concentric rings on the mantle are centred to the right of the mid-line, just above the pneumostone. Just a single British genus: Derocerus, though this is the largest family of slugs viewed on a world basis.
Milacidae (keeled slugs). These have a distinct keel running down the midline of the back between the mantle and the tip of the tail. The mantle bears a distinct horse-shoe shaped groove (except for Boetgerilla). The internal shell resembles a clam valve and is symmetrical. British genera: Boetgerilla, Milax and Tandonia.
Testacellidae (shelled slugs). These lack a mantle and have a small external shell towards the tip if the tail. Only a single British genera: Testacella.
Many of our most familiar slugs are naturalised introductions, especially those associated with cultivated land or buildings. The British fauna is quite dynamic with a number of species currently expanding their range.
Some useful on-line resources are available including:
A PDF of the '1983 AIDGAP (FSC) key 'A Field Key to the Slugs of the British Isles':
Ireland's molluscs (with good coverage of slugs):
A new FSC AiDGAP key is in preparation at the time of writing, and will place greater emphasis on the need for dissection, which is essential for the accurate determination of many species.