ZOOFLAGELLATES 151 



Grasse, waves of contraction, necessarily involving the adjacent 

 cell surfaces as well, may be observed to pass down them. This 

 sort of activity must pose mechanical problems not met by free- 

 swinging flagella. Conceivably the dense strips on the outer half 

 of the flagellum might offer mechanical resistance, balancing the 

 drag of the body surface on the opposite side. They are rather 

 similar to some structures seen in animal sperm tails (see Fawcett, 

 1958). 



Within the cell, the kinetosomes apparently all originate in 

 contact with a homogeneous, rounded body of moderate density, 

 identified as the centrosome (Fig. 61, PL XVI). The latter is near 

 the extreme anterior tip of the organism, and all kinetosomes are 

 directed posteriorly. 



The axostyle of Pjrsonympha, unlike that of trichomonads, is 

 a contractile organelle; according to Grasse its movements are 

 clearly distinct from, and not synchronous with, those of the 

 flagella on the body surface. Its ultrastructure is also quite 

 different from what we have seen in Tritrichomonas and Foaina. It 

 is composed of a pile of longitudinally oriented lamellae (Fig. 60, 

 PI. XVI). In cross-section, anywhere from 14 to 74 more or less 

 concentrically curved sheets may be counted in the pile, each one 

 consisting of a single row of tubular fibrils (measured from 

 Grasse's published micrographs at about 23 m/x in diameter). 

 Lateral connectives are present between fibrils in a sheet, but none 

 between adjacent sheets. One sheet of fibrils, instead of parallel- 

 ing the others, is always seen perpendicular to them along one 

 side of the organelle ; in some specimens this may curve around, 

 partly encircling the pile. Anteriorly, the axostyle ends a micron 

 or so short of the centrosome, but its anterior tip is closely 

 applied to a finely striated fiber or axial ribbon that inserts on the 

 centrosome. The structure of this ribbon is somewhat enigmatic; 

 its striations appear longitudinal where it is in contact with the 

 axostyle, but closer to the centrosome it has a dark fiber along one 

 side and transverse striations along the other. One of Grasse's 

 micrographs, of a cell at a stage when the nucleus occupies an 

 anterior position (Fig. 61, PI. XVI) shows the same axial ribbon 

 also in close apposition with the pointed upper end of the nucleus. 



The paraxostyle is seen as a sinuous rod of very dense, perhaps 

 filamentous material, inserting on a lateral face of the centrosome 



