106 PROTOZOOLOGY 



pushed out at the region where its elasticity is weakest and this 

 results in pseudopodial formation. When an amoeba is elongated 

 and undergoing movement, the elastic strength of the plasmagel 

 is the highest at its sides, lowest at the anterior end and inter- 

 mediate at the posterior end, which results in continuity of the 

 elongated form and in extension of the anterior end. If pressure is 

 brought against the anterior end, the direction of streaming of 

 plasmasol is immediately reversed, and a new hyaline cap is 

 formed at the posterior end which is thus changed into a new an- 

 terior end." 



Flagellar movement. The flagellar movement is only in a few 

 instances observable as in Peranema, but in most cases it is very 

 difficult to observe in hfe. Since there is difference in the number, 

 location, size, and probably structure (p. 45) of fliagella occurring 

 in Protozoa, it is supposed that there are varieties of flagellar 

 movements. The first explanation was advanced by Biitschli, who 

 observed that the flagellum undergoes a series of lateral move- 

 ments and, in so doing, a pressure is exerted on the water at right 

 angles to its surface. This pressure 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 move- 

 ment. There can be little doubt that this condition is satisfied in 

 flagellated organisms 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 direc- 

 tion of the transmission is always the same. A stationary wave, 

 as apparently contemplated by Biitschli, could not effect propul- 

 sion since the forces acting on the water are equal and opposite 

 during the two phases of the movement. 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 

 process has often not been observed. Verworn observed long ago 

 that in Peranema trichoyhoruni the undulation of the distal por- 



