24 STRUCTURE 



(PL I, II and Illb), or a little above it, where there is usually a 

 transverse membrane or plate (Figs. 4 and 7). This appears to 

 extend the v^hole width of the cilium in at least some cases, and is 

 often associated with a neck-like narrowing of the ciliary shaft for 

 a short distance (Fig. 7H and PI. Ila). Above this membrane 

 there is usually a spherical or discoidal granule (the axosome) 

 about 800 A in diameter at which the central fibrils end (PI. IVb), 

 while one of the central fibrils of Pseudotrichonympha ends at a 

 crescentic body which is fused to some of the peripheral fibrils at 

 the level of the transverse membrane. 



The transition from peripheral doublets in the shaft into 

 triplets in the base w^as first thoroughly worked out in Pseudotricho - 

 nympha by Gibbons and Grimstone (1960). In a transverse section 

 of the shaft, the line joining the centres of the two subfibrils of any 

 peripheral doublet is tilted at about 5° to the tangent to the circle 

 of peripherals, with subfibril A slightly closer to the centre of the 

 shaft. No arms are present on fibrils in the transition region or 

 immediately above it, but a third subfibril appears at the transition 

 beside each of the nine doublets which are continued into the base 

 from the shaft. Afzelius (1959) suggested that the third subfibril 

 replaces the arms, but Gibbons and Grimstone found that the 

 third subfibril appears on the opposite side of the armless subfibril 

 (i.e. in position C), and so cannot be a continuation of the arms. 

 The three subfibrils of each triplet appear tubular and about the 

 same size as the subfibrils of the doublets in the shaft. Over a 

 distance of about 1 jit around the transition region each peripheral 

 fibril twists, so that in the basal body the line joining the centres of 

 the three subfibrils of any triplet makes an angle of about 45° with 

 the tangent to the circle of triplets (PL I). According to Gibbons 

 (1961a), each triplet is inclined inwards in a clockwise direction as 

 seen by an observer looking outwards from the inner end of the 

 basal body. 



The details of a rather more complex transition zone, which has 

 been found in cilia of Anodonta by Gibbons (1961b), are illustrated 

 in Fig. 7. Gibbons considers that the basal body proper is that 

 region in which the peripheral fibrils are triplets, and that the 

 region of the cilium between the distal end of subfibril C and the 

 proximal end of the typical shaft structure is the transition zone. 

 This transition zone is about -8 /x long in Anodonta. The changes 



