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.
Source:
GOC:rph,
PMID:15175283,
GOC:jid,
PMID:1624463