
For some reason, this reminds me of a Far Side Cartoon. Something about "Look Mommy, the slugs are going south for the winter!"
Slugs are in the same family as snails, they seem to have evolved past snails, because some have internal or vestigial shells (around the part on their back called a mantle). They are made up mostly of water and need to secrete mucus in order to hold in their water and move over the terrain without injury. Sometimes they go over the windows on our sliding glass doors and leave little trails.
They have little feelers that look like antennae on their heads. These are called tentacles, and the upper pair are optical, while the lower pair is for smell/taste.
Slugs are hermaphrodites, meaning they are both male and female. They mate by curling in a ball around each other and lay eggs in the ground. Sometimes they crawl underground to get through the winter, but often they die in the winter. They also have problems in the heat of summer, if it's too dry and they have to find a dark, cool, moist place to seek refuge to get through the dry spell.
I was once with a friend when her cat walked in, working its jaw like a little franken-cat. His muzzle was covered with what we thought at first was foam (from drooling), but we soon discovered was a rubbery substance. The cat allowed me to peel the material out of his mouth, and somewhat off his face. However, it was like rubber cement, and I could not get it all out of the fur around his mouth. When the biggest plug came out of his mouth (I think it went into his throat and he was close to choking on it), the cat was back to normal and ran off to do more cat mischief.
I described the scene later to my brother, who is a veterinarian and he believes that the cat tried to eat a slug. The slug probably secreted some protective mucus when the cat started biting on it, and the mucus quickly congealed and became stuck on the cat's snout and in his mouth.
That's a hell of a defense mechanism.