544 Comparative Animal Physiology 



concavity of the mildly curved cilium in advance. The recovery stroke in- 

 volves a bending back again of the base of the cilium and then a progressive 

 passage of this flexion to the tip. The efTective stroke, therefore, appears to 

 involve movement of a rigid cilium, whereas recovery appears to be con- 

 cerned with a progessive stiffening from base to tip of an initially Hmp 

 structure. 



Combinations of pendular and undulatory movements are found in the 

 vibratile organs of the mammalian epididymis*^- and in a number of flag- 

 ellates as, for example, in Trypanosoma.''^' In such organisms as the latter 

 the undulatory and pendular movements need not occur in a single plane, 

 nor need the two occur simultaneously in the same plane. Thus the tip of 

 the flagellum in the course of its beat may trace out an elliptical orbit, a 

 figure 8, or a more complicated figure. 



The single vibratile element of the flagellate, Monas, has been described 

 to possess the capacity to carry out numerous types and combinations of 

 activities."*- Forward movement is accomplished by a rapid pendular move- 

 ment of the flagellum from a position directed forward to one at about right 

 angles to the direction of progression. The recovery stroke is a typical flex- 

 ural recovery (Fig. 244, A). The effective stroke may, instead, be initiated 





b 



Fig. 243. Movement of a frontal cilium of Mytilus: a, effective and h, recovery stroke. 



From Gray."' 



near the base and rapidly pass toward the tip. In slower forward movement, 

 the total sweep of the flagellum may be reduced to about half the normal 

 amplitude, but with the typical form of beat seen in rapid progression (Fig. 

 244, B). Backward progression is brought about by undulatory activity with 

 the waves passing from base to tip of the organ (Fig. 244, C). Lateral move- 

 ments are the result of undulatory movements of a flagellum flexed'^^ at about 

 90 degrees (Fig. 244, D). Directing the flagellum backward results in for- 

 ward movement. The undulatory activity may involve only the tip of the 

 flagellum, or practically its whole course. 



The activity of the flagellum has been investigated with the aid of high 

 speed cinema photomicrography.'^^' •^" Undulatory activity of the flagellum 

 exerts only a pushing action on the medium. Forward locomotion in such 

 common species as Peranevia and Eiiglena, in which the flagellum was for- 

 merly believed to pull the organism and hence function as a tractellum, ap- 

 pears to result from a bending of the whole flagellum, or at least of its active 

 tip, backward. "^^^ '^^ The undulatory activity typically passes from base to tip 



