Yup, you are correct. I believe Pericelididae family. How were you able to trap and keep it intact? When I had to catch one I panicked and it broke into 2 pieces... nasty buggers.
Turkey baster would not work. This thing was large and held onto rocks very well.
I had to remove the rock it was on from thw display, and hold it just above the water, with anothet small rock just below it. The flatworm moved to the smaller rock just below the rock it was on (so it could get back into water). Then i took that small rock out of the water and put it in a tupper ware container which the FW eventually stayed in.