UPDATE: It seems slugs are not as easy as I assumed. This article could be totally wrong in identification. Unfortunately this fellow escaped his jar due to a too big slit in the aluminum foil cap I had on the jar and so I can't go back and, now a little more knowledgeable, make a definitive identification. So take anything written below with a grain of salt.
Slugs seem to be a little easier to get to species. There are 17 known species in Pennsylvania, most of them non-native. Also it helps that there is a great and current slug key of the known PA slugs on the Carnegie Museum site (http://www.carnegiemnh.org/mollusks/palandsnails/key.html).
This slug is Philomycus flexuolaris, the Winding Mantleslug. It is a native of Pennsylvania and common in high elevation wooded areas. It is identified to family first by looking at the mantle. This one has the mantle covering the entirety of the back and over the head versus slugs that have a sectioned appearance. After that you can identify by the mottled dorsal line and mottled lateral lines on both sides.
I found this little guy (not that little, this one measures 66mm) under a log at Moon Lake Park while on a walk with my older daughter, Emma, and one of my dogs, Sasha.
|Range map from Land Snails of Limestone Communities and Update of Land Snail Distributions in Pennsylvania (Pearce) with my own county added to range (in orange). Formerly unknown to Luzerne County.|
Mantle: Full back and over head
Other: Mottled Dorsal Line and mottled Lateral lines on both sides
Moon Lake Park
Macrohabitat: Mixed Deciduous (mostly oak)
Microhabitat: Under a decaying log used as the side of a path