Read about the sensory functions of slugs.
A slug has organs that it uses to smell, but they're tentacles -- not a traditional nose in the sense of an organ, situated on the face, with two nostrils that breathe and smell.
Slugs actually have two tentacles on the front of their sluggish bodies that they use to smell things.
A slug also has two larger tentacles that are topped with the tiny, sluggy eyeballs. The slug uses them to observe the slimy path of destruction as it ever-so-slowly goes through a garden.
The fact that a slug has four tentacles that perform sensory functions may have lead to the oft-repeated -- but totally erroneous -- factoid that "a slug has four noses." But first of all, a slug has NO noses, and secondly, it only uses two of the four tentacles to smell.