They're probably harmless unless you are raising night crawlers for bait - they are a serious pest in earthworm rearing beds in the deep south. In the garden, I doubt they are going to have a significant impact on either worms or slugs - mine haven't.

Ryan

Something has had an impact on slugs/snails--I find much less damage to strawberries than in the past. In fact, that's where I found the flatworms, in strawberries.

Are you talking about the ones that look like an earthworm, but are flat and have a "Hammerhead"? I was told to kill these since they kill regular earthworms.

The worms didn't have the shovel-shaped head; the anterior end was actually really pointed and the worms could redistribute their bodies to be either short/chubby or stretched out/thin--kind of looked like a leech. They were completely black and very shiny.

They are smooth, not segmented like an earthworm. And the anterior end is tapered to a point, not shovel or shield or triangular shaped. Next time I see one I'll take a picture.