INTRODUCTION 



water in through the ostia and pass it out through the oscula. 

 Each flagellum causes a current of water to pass in through the 

 pahsade of microvilH that makes up the collar of the choanocyte ; 

 these microvilli filter food particles from the water current 

 (Fjerdingstad, 1961). Also, in the flame cells found in some 

 invertebrates the flagella run along the tubes in such a direction 

 that their movement attempts to propel fluid towards the external 

 opening. Movement of small bodies and the movement of fluid 

 along tubes can be carried out with at least equal efficiency by 

 cilia, but the ciliary organelles would be arranged with their long 

 axes more or less at right angles to the direction of motion of the 

 fluid. 



The widely-held belief that flagella are longer than cilia is 

 certainly not correct. Cilia may be compounded together into 

 large structures that may be much longer than any flagellum. 

 However, there is a mechanical reason whv a non-compound 

 organelle with a true ciliary beat is normally short (see p. 172), 

 while a flagellar structure is not limited in length except by factors 

 such as the transport of metabolites to provide energy for contrac- 

 tion. The fact that a structure beats like a cilium does not mean 

 that it must be short, although it is frequently said that flagella are 

 longer than cilia because they show several waves of bending 

 within their length. 



Where many of these organelles occur together, they almost 

 alw^ays show the ciliary beating pattern; it seems more efficient 

 than the flagellar pattern, and is certainly more adaptable in those 

 cases where a reversal of beat may be required. In such large 

 groups of cilia the continuous even flow of the fluid over the surface 

 is ensured by the rhythmic beating of the cilia which are co- 

 ordinated metachronally in such a way that at any instant there 

 will be some cilia at all phases of their beat ; some cilia are in the 

 active phase of beat and others nearby are recovering from their 

 active beat and preparing to perform another. Flagella which 

 occur in groups may beat together, but in their motion it appears 

 that the bending waves travelling down adjacent flagella may be 

 synchronized by mechanical interaction through the water 

 (Pitelka and Schooley, 1958). 



The mode of functioning of flagella has been briefly referred to 



