PHYSIOLOGY 111 



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 alwaj^s 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 trichophomm 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. 

 Recently Krijgsman (1925) studied Monas sp. (Fig. 46) which he 

 found in soil cultures, under the darkfield microscope and stated: 

 (1) when the organism moves forward with the maximum speed, the 

 flagellum starting from cl, 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 under- 

 goes 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 passes 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 

 permanently in the ciliates and vary in size and distribution among 



