Mastigonemes are tiny. You are unlikely to come across one unless looking very hard. Here is a summary of definitions for this obscure piece of cellular micro-
anatomy important enough to help rest a kingdom upon
As a piece of anatomy, Mastigonemes create a brush of fine fibers on some eukaryotic flagella. The fibers can be:
Stiff, which can change the hydrodynamics of flagellar propulsion.
Flexible, altering the effective diameter of the flagellum (flimmer filaments).
Taxonomically, they are tripartite hairs, sometimes used as a useful character for identification in Phycology.
They are also one feature, of a new Superphylum/Kingdom Stramenopiles. The ninth of what was once five kingdoms. A group identified by close friends as
"tubulocristate protists with tripartite flagellar hairs or those derived from such organisms".
Stramenopiles includes Chromophyte (algal) and Oomycete (pseudofungal) taxa.
Two other evocative descriptions would have to be:
"Stiff, glycoproteinaceous, usually three parted, tubular, flagellar hairs"
and
"tripartite tubular hairs consisting of basal segment, long tubular shaft, and 2-3 terminal filaments"
With this knowledge feel more confident to explore a new uncharted Kingdom.