On the numbering of peripheral doublets in cilia and flagella

The nine microtubular doublets of cilia and flagella have distinctive features that make it possible to assign an index number to each of them. Such an indexing has been used for a long time for animal cilia and flagella, whereas other indexing systems have been proposed recently for plant cilia. It is shown here that the similarity between cilia from animals and cilia from plants and protists is so great that the same indexing system can be used for all cilia regardless of their derivation.