Flagellar filaments located in the periplasmic space; characterized in spirochetes, in which they are essential for shape and motility. Composed of a core surrounded by two sheath layers, the flagella rotate to allow migration of the cell through viscous media, which would not be possible using external flagella. [ http://www.ncbi.nlm.nih.gov/pubmed/15175283 http://www.ncbi.nlm.nih.gov/pubmed/1624463 ]