10 INTRODUCTION 



which food particles can be filtered. Many endoparasitic forms 

 show a loss of at least the external cilia, and often many internal 

 cilia also ; it is understandable that many gut parasites particularly 

 should produce a cuticle for protection. It is also important that 

 cilia cannot function in air, while muscles can be used to move air 

 and can function to move structures in air. Although it appears 

 that there are no biological functions which can only be performed 

 by cilia, except perhaps for the cleansing of the surfaces of living 

 cells, many animals which rely mainly on muscles have retained 

 cilia for some purposes. 



