undulating membrane

noun

un·​du·​lat·​ing membrane ˈən-jə-ˌlāt-iŋ-, ˈən-d(y)ə- 

: a vibratile cytoplasmic membrane:

a

: a lateral expansion of the plasma membrane in some flagellates that is usually associated with a flagellum

b

: a row of laterally fused long cilia associated in many ciliates with the oral structures