PHYSIOLOGY 127 



lum undergoes a series of lateral movements and, in so doing, a pres- 

 sure is exerted on the water at right angles to its surface. This pres- 

 sure can be resolved into two forces: one directed parallel, and the 

 other at right angles, to the main body axis. The former will drive 

 the organism forward, while the latter will tend to rotate the animal 

 on its own axis. 



Gray (1928), who gave an excellent account of the movement of 

 flagella, points out that "in order to produce propulsion there must 

 be a force which is always applied to the water in the same direction 

 and which is independent of the phase of lateral movement. There 

 can be little doubt that this condition is satisfied in flagellated organ- 

 isms not because each particle of the flagellum is moving laterally to 

 and fro, but by the transmission of the waves from one end of the 

 flagellum to the other, and because the direction of the transmission 

 is always the same. A stationary wave, as apparently contemplated 

 by Biitschli, could not effect propulsion since the forces acting on 

 the water are equal and opposite during the two phases of the move- 

 ment. If however the waves are being transmitted in one direction 

 only, definite propulsive forces are present which always act in a 

 direction opposite to that of the waves." 



Because of the nature of the flagellar movement, the actual proc- 

 ess has often not been observed. Verworn observed long ago that in 

 Peranema trichophorum the undulation of the distal portion of flagel- 

 lum is accompanied by a slow forward movement, while undulation 

 along the entire length is followed by a rapid forward movement. 

 Krijgsman (1925) studied the movements of the long flagellum of 

 Monas sp. (Fig. 48) which he found in soil cultures, under the dark- 

 field microscope and stated: (1) when the organism moves forward 

 with the maximum speed, the flagellum starting from c 1, with the 

 wave beginning at the base, stretches back (c 1-6), and then waves 

 back (d, e), which brings about the forward movement. Another type 

 is one in which the flagellum bends back beginning at its base (/) 

 until it coincides with the body axis, and in its effective stroke waves 

 back as a more or less rigid structure (g) ; (2) when the organism 

 moves forward with moderate speed, the tip of the flagellum passes 

 through 45° or less (h-j) ; (3) when the animal moves backward, the 

 flagellum undergoes undulation which begins at its base (k-o) ; (4) 

 when the animal moves to one side, the flagellum becomes bent at 

 right angles to the body and undulation passse along it from its base 

 to tip (p); and (5) when the organism undergoes a slight lateral 

 movement, only the distal end of the flagellum undulates (q). 



Ciliary movement. The cilia are the locomotor organella present 



