The external anatomy of a slug includes the following: •
Tentacles: Like other pulmonate land gastropods, the majority of land slugs have two pairs of 'feelers' or
tentacles on their head. The upper pair is light-sensing and has eyespots at the ends, while the lower pair provides the
sense of smell. Both pairs are retractable in
stylommatophoran slugs, but contractile in
veronicellid slugs. •
Mantle: On top of the slug, behind the head, is the saddle-shaped
mantle. In stylommatophoran slugs, on the right-hand side of the mantle is a
respiratory opening, the
pneumostome, which is easier to see when open; also on the right side at the front are the genital opening and anus.
Veronicellid slugs have a mantle covering the whole dorsal part of the body, they have no respiratory opening, and the anus opens posteriorly. •
Tail: The part of a slug behind the mantle is called the 'tail'. •
Keel: Some species of slugs, for example
Tandonia budapestensis, have a prominent ridge running over their back along the middle of the tail (sometimes along the whole tail, sometimes only the posterior part). •
Foot: The bottom side of a slug, which is flat, is called the 'foot'. Like almost all gastropods, a slug moves by rhythmic waves of
muscular contraction on the underside of its foot. It simultaneously secretes a layer of
mucus that it travels on, which helps prevent damage to the foot tissues. Around the edge of the foot in some slugs is a structure called the 'foot fringe'. •
Vestigial shell: Most slugs retain a remnant of their shell, which is usually internalized. This organ generally serves as storage for calcium salts, often in conjunction with the digestive glands. An internal shell is present in the
Limacidae and Parmacellidae. Adult
Philomycidae, and
Veronicellidae lack shells. ==Physiology==