INTRODUCTION 



above. Flagella attached to the outside of small bodies like some 

 protozoa and plant spores, or the sperm of animals and some 

 plants, are used to move water from flagellar base to tip, and by 

 their action they move the body forwards either directly (PI. XV,) 

 or indirectly by causing a gyration and rotation of the body (Fig. 

 34, p. 135). Flagella inside the body generally seem to function 

 in moving fluids along channels as in the sponge or flame cells 

 (Fig. 2a, b). Here the flagella lie in the lumen of the channel and 



b. 



Fig. 2. a, The arrangement of flagella and the water move- 

 ments that they cause in a sponge (from Bidder, 1923). b, 

 Diagram of a flame cell system showing the position of the 



flagella. 



move water from base to tip. Flagella have been described in 

 other places in the bodies of animals, but their motion has not 

 been thoroughly studied. 



Cilia may have mechanical functions in moving fluids over open 

 surfaces and in tubes. In both situations there may be cilia whose 

 function is to move a shallow film of fluid over the cell surface, or, 

 alternatively, the cilia may be responsible for more distant 

 currents in the fluid. Generally, those cilia which produce these 

 more distant currents are longer and larger (frequently compound) 

 and are able to sweep through a larger volume of water than the 

 cilia which move the surface film. 



The movement of a thin film of fluid is perhaps the more 



