8 



INTRODUCTION 



common function of cilia, for in animals of most groups mucous 

 strands are transported by ciliated epithelia. The moving strands 

 of mucus may carry entangled debris where the cilia are perform- 

 ing a cleansing function, or may also carry food particles in the 

 case of cilia used in feeding and in some digestive systems. Under 

 this heading also come the locomotory cilia of some forms, such as 

 the unspecialized somatic cilia of some protozoa and some larval 

 forms, and the locomotory cilia of some nemertines, molluscs, 

 turbellarians and gastrotrichs. In ciliated tubes where the 

 diameter of the tube is large compared with the cilium length, the 

 cilia may function in a similar way, with either a cleansing function 

 or a special transport function, e.g. in the respiratory and female 

 reproductive tracts of mammals. Where the tube diameter is 

 narrow the whole of the fluid contents of the tube may be moved. 

 Cilia which create currents at a greater distance from the 

 surface generally have a function in locomotion, feeding or 

 respiration. The compound cilia that make up the comb -plates of 

 ctenophores are perhaps the extreme example of locomotory cilia; 



Fig. 3. The arrangement of cilia on the gill filament of 

 Mytilus and some of the water movements that they create 



(from Gray, 1928). 



their size is such that they move through a large body of water, 

 and a succession of plates beat the water like a paddle-wheel. 

 Many of the more specialized larval forms like the molluscan 



