I checked on the flatworms a few days ago and found that:
1) I had a few less flatworms than I did before (specifically, the Dendrocoelum lacteum were missing)
2) I had 5 small brown balls attached to the upper sides of the tank, near but below the water line.
My first thought is that the Dendrocoelum had encysted somehow, as perhaps my tank was not to their liking, but after delving into the subject, I'm afraid it looks like Dendrocoelum has become the perfect meal for my Dugesia, supporting a round of egg-laying by that species. The eggs themselves are inside the cocoons, but i expect in a few weeks I'll have numerous baby Dugesia circling my tank.