THE FUNCTIONS OF THE PARTS OF CILIA 



145 



beating of such a cilium is shown in PI. XVIIb. This pattern 

 of beat was described and figured by Gray (1922a) from obser- 

 vations on cilia from the same source. A very similar form of 

 beat is shown by ciUa on the branchial lobes of the tubicolous 

 polycheate Sabellaria (Fig. 40a), and the beat of the vestibular 

 cilia oi Paramecium appears basically the same (Fig. 40b), although 

 the amplitude of beat may be a little larger. The slight difference 

 in the appearance of the beat may be related to the metachronism 

 of the cilia, for that of the cilia of Sabellaria is diaplectic, while 

 that of these cilia of Paramecium is antiplectic (see p. 174); the 

 latter cilia may interfere with each other in the recovery stroke. 



Fig. 40. Diagrams showing the sequence of movements of cilia 

 of (a) Sabellaria and (b) Paramecium. 



Notice particularly that the recovery stroke starts before the 

 effective stroke is complete. 



Cilia do not all show the same form of beat, and the beating 

 of many more types of cilia must be accurately described before 

 we can attempt any classification of beating patterns. No 

 " pendular " movements of the type figured by Gray (1928) have 

 been described from cinematograph studies, and the evidence 

 points to the fact that all ciliary beating cycles come under Gray's 

 heading of '* flexural " movements. 



3. The Functions of the Parts of Cilia 



The structure of cilia is well-known and consistent, but there 

 is little evidence as to the functions of the various parts. A 

 motile cilium must contain both contraction and compression 

 elements, and certain structures seem well placed for the 



