42 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



some manner related to the maintenance of fixed radial and cir- 

 cumferential distances in the complete bundle. Farther distally 

 the axial fibrils lose their regular spacing, subfibrils of the 

 peripheral nine end at random levels so that the double profiles 

 give way to single ones, and the total number visible in cross- 

 section diminishes to zero. The plasma membrane covers the tip 

 of the flagellum. 



A quite different kind of flagellar tip is present in the very tiny 

 green flagellate Micromonas pusilla studied by Manton (1959a). 

 Here the flagellum proper is reduced to a stub no more than 1 /x 

 in length, while a much longer distal thread consists of a pro- 

 longation of the two central fibrils only, still surrounded by the 

 flagellar membrane. 



It seems probable that Gibbons and Grimstone's description 

 will prove to be applicable to all flagella and cilia. Similar 

 components are evident in micrographs of the sperm tails of an 

 insect by Andre (1961), of sea urchin sperm tails and ctenophore 

 swimming-plate cilia by Afzelius (1959, 1961), and of rotifer 

 coronal cilia by Lansing and Lamy (1961). Published micrographs 

 of other species and unpublished micrographs from the author's 

 laboratory indicate that similar structures occur widely, but few 

 other materials are so favorable for detailed study as the hyper- 

 mastigote flagellates with their neatly aligned, close-packed 

 flagella, and few other authors have achieved such elegant 

 preservation and micrography as Gibbons and Grimstone. 



Cilia, whether of ciliated protozoa or of multicellular organisms, 

 tend to show minimum elaboration of the basic design just 

 described (although some unusual structures are present in long, 

 close-packed cilia of the swimming plates of ctenophores, as 

 shown by Afzelius, 1961; these probably are peculiar to these 

 extraordinary organs, since other cilia of the same organism are 

 more conventional). Flagella of flagellates frequently, but by no 

 means always, have expanded sheaths, often enclosing rods or 

 ribbons of peculiar structure, surrounding the conventional 

 fibrous axis. In addition, fine filamentous lateral appendages or 

 mastigonemes occur in characteristic patterns on the flagella of 

 many phytoflagellates. Since these features vary widely and tend 

 to be consistent within particular taxonomic groups, they cannot 

 be requisite to flagellar motility, and they will be discussed in 



