It closely resembles Homalozoon vermiculare (a creature I see often and know fairly well), except for the apparent lack of trichites (nematodesmata) behind the oral aperture, the absence of a "parapharyngeal mass," and the overall appearance of the area around the oral aperture (the Spathidiid-like truncation, that curious raised "lip").
I would consider it a mutant or damaged Homalozoon, except for the vigor and competence with which it forages, grazing at organic detritus in a manner very much like that of a worm.
Any thoughts, speculations, or opinions would be welcome.

http://www.youtube.com/watch?v=YQiekya_Kjo