A TYPICAL ORGANELLE — SHAFT 17 



longitudinal sections that these central fibrils normally run straight 

 from one end of the cilium to the other without spiralling, a finding 

 that is corroborated by transverse sections in those cases where a 

 reference point is available in a series of sections, e.g. in compound 

 cilia of known shape. The presence of two central fibrils confers a 

 degree of bilateral symmetry on the ciliary axis (see p. 20 for 

 further discussion). Cross-striations with a periodicity of 130 A, 

 probably caused by spiral thickenings, were found in the central 

 fibrils of Pseiidotrichonyjnpha by Gibbons and Grimstone (1960) 

 and in Anodonta by Gibbons (1961b). 



In many transverse sections at high resolution some additional 

 central structure is visible. This takes various forms, but the most 

 commonly described is a membrane or sheath in the form of a 

 central tube surrounding or linking the two central fibrils (Fig. 4b). 

 Sometimes this sheath has been found to have a spiral structure 

 (Manton, 1956; Gibbons and Grimstone, 1960; Gibbons, 1961b). 

 It is believed that this sheath around the pair of central fibrils 

 could give rise to the appearance of a quartet of central fibrils that 

 is seen in a number of electron micrographs, e.g. Manton (1957) 

 and Grasse (1956). One or two further direct links were found 

 between the two central fibrils in Anodonta cilia by Gibbons 

 (1961b), as shown in Fig. 5a. An additional '* mid-filament " 

 (see Fig. 5) has been found by Afzelius (1961a) in Mnemiopsis cilia 

 and by Gibbons (1961b) in Anodonta cilia; in the latter it appears 

 in transverse sections as a dense dot some 50 A in diameter, but it is 

 not quite as clearly defined in Mnemiopsis cilia where a structure 

 about 100 by 180 A was found in transverse sections. Neither 

 structure has been seen in longitudinal sections, so that there is no 

 evidence that it is a longitudinal filament, although it consistently 

 appears in a particular position in transverse sections of the cilia. 

 Incidentally, the mid-filament was found to be nearest to periph- 

 eral fibril 1 in Anodonta and nearest to peripheral fibrils 5 and 

 6 in Mnemiopsis, and it is interesting that the normal direction of 

 the effective stroke of the ciliary beat is away from the side on which 

 the mid-filament occurs in both species, i.e. the effective stroke is 

 towards fibril 1 in Mnemiopsis and towards fibrils 5 and 6 in 

 Anodonta. 



Around the central fibrils the nine longitudinal peripheral 

 fibrils are arranged to form a cylinder of outside diameter about 



