A TYPICAL ORGANELLE — SHAFT 13 



2. The Structure of a Typical Ciliary Organelle 



Shape and Size 



Most cilia have a constant diameter from the base to within a 

 short distance of the tip, where there is a gradual taper to a blunt 

 point. Some cilia show a greater or lesser length of thinner whip- 

 lash region at the end of the cilium, e.g. the somatic cilia of Stentor 

 (PL VII h) (Randall and Jackson, 1958), or the posterior flagellum 

 of spermatozoids of Hhnanthalia (Manton, Clarke and Greenwood, 

 1953). The special shape of some sperm tails is caused by sheaths 

 of mitochondria and other thickenings around the main axis. 



The diameter of ciliary organelles, which is more constant than 

 the length, usually lies between 0-15 and 0-3 ^u,, although some 

 sperm tails and flagella with additional structural components may 

 have a diameter larger than 1 ju. Single cilia commonly have a 

 length of 5 to 20 /u,, but where they are combined into compound 

 structures they may be very much longer, e.g. up to 2000 ju, in the 

 ctenophore Mnetniopsis (Afzelius, 1961a). Flagella have been 

 found to have lengths of the order of 5 to 150 /x, while sperm tails 

 tend to be longer, up to 200 /x or more. It seems that the dimen- 

 sions of ciliary organelles must be closely bound up with their 

 functioning (see p. 172). The lengths and diameters of some cilia, 

 flagella and sperm tails are quoted in Table 2. 



The Shaft 



The membrane which completely encloses the cilium is con- 

 tinuous with the plasma membrane of the cell. It often appears 

 wrinkled in fixed material, but in life it is probably smooth except 

 in special cases like compound cilia and *' flimmer-flagella," where 

 it bears surface appendages. From high-resolution electron 

 micrographs it appears that this membrane is three-layered, with 

 a dense layer 20 to 30 A thick on either side of a less dense 30 A 

 layer, giving a total thickness of about 70 to 90 A. 



The two central fibrils, which run parallel for the length of 

 the shaft of the cilium, occupy a central position with about 300 

 to 350 A betw^een their centres, as shown in Fig. 4b. In transverse 

 section they appear tubular with a total diameter of 150 to 250 A 

 (see Table 3), and a wall thickness of 40 to 50 A. It appears from 



