2 INTRODUCTION 



forward the idea that the ciUa actually ** contain muscular sub- 

 stance throughout a greater or less part of their length, by which 

 thev can be bent or extended". He had indeed noticed that cilia 

 may bend in the main part of their length as well as at the base, 

 although the significance of this observation was not appreciated 

 by some of his contemporaries. The same author observed 

 metachronal waves (although the term metachronal was not used 

 until later), and compared the appearance of these waves to those 

 produced by wind in a corn-field; he explained the waves correctly 

 as being the result of the undulations of the individual cilia 

 composing the waves. Sharpey knew that in some cases cilia were 

 compounded together, and he records that cilia varied considerably 

 in length from -^\-q in. on the branchiae of Buccinum to perhaps less 

 than 0.000075 in. In his review he mentions results of experiments 

 o ilia with electricity, varied temperature, acids, alkalis and 

 v&x-jus other substances carried out by himself and others, and, 

 although the experiments were not recorded in detail, they 

 demonstrate that the physiology of cilia was already exciting 

 attention. 



In spite of this flowering of interest in cilia, little serious work on 

 ciliary activity was carried out until the end of the nineteenth 

 century, when Verworn, Schafer, Parker and Heidenhain were 

 notable contributors to our knowledge of ciliary functioning, and 

 there was renewed interest in ciliary structure. Much of this 

 earlier work has been summarized by Gray (1928) in his well- 

 known monograph ; only the more important of these older works 

 will be mentioned here, while more recent work will be treated in 

 greater detail. 



Since the time of Sharpey many workers have added further to 

 our knowledge of the distribution and functions of cilia and 

 flagella. Some idea of the wide occurrence of cilia and flagella may 

 be gained from Table 1 . The information given there can only be 

 of a provisional nature in that some records require checking, and 

 in some cases the fine structure of the organelle requires to be 

 examined in the electron microscope to show that it conforms to 

 the standard pattern. This applies particularly to the sensory 

 cilia, which are only recorded as present when there is evidence 

 both that the structures are cilia and that they have a function in 

 the receptor organ, while a question mark indicates that one or 



