Structures linking the tips of ciliary and flagellar microtubules to the membrane.

The distal tips of both central pair and outer doublet microtubules are linked to flagellar membranes of Chlamydomonas and to ciliary membranes of both Tetrahymena and Aequipecten by the central microtubule cap and by distal filaments, respectively. The distal filaments are linked to ciliary membranes at their distal ends and, at their proximal ends, form a carrot-shaped plug which inserts into the lumen of the A-tubule of each outer doublet. Both types of tip structures are present in regenerating cilia and flagella. At least 2 different types of distal filaments are present in Tetrahymena. Certain cilia in Tetrahymena oral apparatuses and in certain molluscan gills contain distal filaments which join with the central microtubule cap and with the ciliary membrane to form a cap-filament-membrane complex.