30 STRUCTURE 



in composition to true cilia and flagella, for Tibbs (1957) found 

 that Polytoma flagellar protein contained the amino acids cystine 

 and tryptophan which are believed to be absent from bacterial flag- 

 ellar protein (see also p. 76). Neither of these amino acids was 

 found in Tetrahymena cilia by Watson et al.^ perhaps because 

 their hydrolytic technique would not permit the recognition of 

 these acids, but both were found in Chlamydomonas by Jones and 

 Lewin (1960); evidently the amino acid composition of cilia and 

 bacterial flagella requires further study. 



Root structures associated with cilia are doubtless also protein- 

 aceous in nature (Grimstone, 1961), although little work has been 

 done to confirm this. 



3. Variations in Shaft Structure 



Cilia 



Some cilia have been markedly specialized, especially where 

 they have a non-motile function. The shafts of normal motile 

 cilia are found to vary little from the pattern described above for 

 a typical cilium. Cilia are usually grouped to perform their 

 special functions and their co-ordinated action produces the 

 characteristic water movements. Usually many single separate 

 cilia work together with metachronal co-ordination, but, when 

 such single cilia cannot produce a large enough force, compound 

 structures may be built up of many cilia which can function as a 

 single unit; these compound cilia may still be metachronally 

 co-ordinated w4th other compound cilia, or each compound cilium 

 may beat independently. 



The details of structure of several types of compound cilia 

 that have been studied under the electron microscope show 

 features not found in typical cilia. In the ciliated protozoa, 

 compound cilia may be either cirri, which have the shape of a 

 truncated cone and occur in fairly small numbers usually without 

 metachronal co-ordination, or membranelles, shaped like triangular 

 plates, which occur in rows in which their beat is metachronally 

 co-ordinated. The shafts of the cilia in the ciliate type of undulat- 

 ing membrane (not to be confused with the completely different 

 undulating membrane of some flagellates and sperm tails, (see 

 pp. 38 and 42) occur close together in a long single row, but it 

 is not clear whether they are joined in any way (King, Beams, 

 Tahmisian and Devine, 1961). 



