a mealtime surprise

Tonight at dinner, I glanced over at my terrarium on our dining table only to see a small slug sliming its way up the side of the glass. Gross!!! What in the world? These were tiny plants I purchased at a nursery greenhouse, and the potting soil was bought there too, sealed in a bag. WHY was there a SLUG in there?! Yuck. I'm not sure what I'm going to do now.