Hi Karen,
The seeds look different depending on the variety.
The base of the flowers swells and gets dry then brown. Some varieties get little holes that open near the base of the flower where it meets the stem when ripe. C. latifolia and C. rotundifolia do this. Probably other ones do also.
The link below shows a good variation of pods and campanula seeds, better than I can describe: )
Remy

Here is a link that might be useful: Campanula Seed Pod and Seeds page from The Seed Site