Freshwater Annelid?

Freshwater Annelid?

Observation - Freshwater Annelid? - UK and Ireland. Description: No idea where to start with this one. Seen swimming in freshwater (pic 2)in S wave motion. Approx 8cm long.

No idea where to start with this one. Seen swimming in freshwater (pic 2)in "S wave" motion. Approx 8cm long.