Cilia 645 



around the flagellum, as well as along it. Such movement imparts rotational 

 and gyrational components alopg with a usual backward thrust. The re- 

 sulting rotation and gyration of the body of the flagellate around an axis 

 which constitutes the direction of locomotion is considered to provide an im- 

 portant, if not the chief, force propelling the organism forward. The body 

 proper acts under these circumstances as a screw-propeller.'^**- ''•' 



From the preceding paragraphs we see clearly that ciliary activity varies 

 greatly in its complexity and variability in a single cilium. Some cilia appear 

 to show a simple and uniform activity, others show considerable variability, 

 more or less under the control of the general response mechanism of the 

 body. 



Cilia are able to propel an organism through the water, or propel the sur- 

 rounding medium past a stationary ciliated cell as a result of a directed 

 thrust upon the medium. More work must be done upon the medium dur- 

 ing the effective phase of a stroke than during recovery. In those cases 

 where the eff^ective stroke is of the pendular type and recovery is of the 

 flexural type, the mechanism is obvious. Simple pendular ciliary activity 

 would be expected to be quite an inefficient type for directed movement. In 

 undulatory activity the thrust upon the medium is dependent on the progres- 

 sion of a wave along the vibratile organ. A standing wave would obviously 

 exhibit an equalization of all pulls and thrusts. A wave passing from base to 



Fig. 244. Some activities of the flagellum of Monas during locomotion. Arrows indicate 

 direction of movement of the organism. (Redrawn from Krijgsman).^" 



tip would exert a thrust away from the cell upon the medium, proportional 

 to its rate of transmission along the flagellum. 



Although cilia appear, on observation, to move through the medium at a 

 very high rate, they are actually moving very slowly, as a simple calculation 

 will show. The angular velocity of the cilia is high, as is seen from the 

 fact that a cilium may show ten or twelve cycles per second, but the tip of a 

 cilium is actually moving through the medium at a maximum rate of only 

 a few feet per hour, a rate quite slow in terms of propulsive instruments of 

 the type of which we ordinarily think. The rate of the effective stroke is 

 usually 2 to 5 times that of the recovery. 



The reversal of the direction of effective beat of cilia has often been re- 

 ported. The most evident instances of this phenomenon lie among •^he ciH- 

 ated protozoans and small turbellarians, such as Stenostovmin, \> liose re- 

 versal of direction of locomotion may reflect this reversal of active fv. Some 

 early reported cases of ciliary reversal among metazoans appear to i.ad their 



