undulating membrane

[ŭnjə-lā′tĭng, -də-]
n.
  1. An organelle of locomotion in certain flagellate parasites consisting of a finlike extension of the cytoplasmic membrane with the flagellar sheath.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.