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Probably an oligochaete. The "Y" shaped mouth is typical of leeches (though I think this one is in the wrong orientation), but only one order of leeches (Acanthobdellida) bears setae on the body, and they don't have an anterior sucker. So I would say an oligochaete or one of the few freshwater polychaetes (family Aeolosomatidae) - although this latter group do not appear to have setae on the prostomium.