ZOOFLAGELLATES 167 



dictyosomes arranged, according to Noirot-Timothee, in two 

 concentric strata but without any detectable connection with the 

 kinetosomes. In the endoplasm are found the numerous nuclei, 

 with dense peripheral nucleoli. Bodies with the characteristic 

 structure of protozoan mitochondria but with much finer micro- 

 tubules are common (Fig. 68, PI. XVIII); Noirot-Timothee 

 suggests that these mitochondria may be the granules described 

 as Zeller bodies by light microscopists. In addition, clouds of 

 tiny membranous vesicles are encountered, as well as occasional 

 clusters or cisternae of granular membranes. 



None of the electron-microscope studies has revealed any 

 intracellular fibers of a type frequently described by light micro- 

 scopists (see Wessenberg, 1961, and Grasse, 1952). Nor has the 

 falx been identified with any certainty. In view of the large 

 number of sections examined by the various investigators, it 

 seems probable that falcular kinetosomes are not associated with 

 fibers or other structures peculiar to them. 



The pellicular structure of Opalina is reminiscent of that of the 

 gregarines, which likewise are mouthless animal symbiotes. If 

 absorption of nutrients can occur all over the cell membrane, the 

 convolutions of the surface clearly would increase its effective 

 area. However, the fibrous construction of the ribs does not 

 make this possibility particularly inviting. The cell is elastic and 

 flexible under mechanical distortion, but normally maintains a 

 constant characteristic shape. Conceivably the fibrous ribs, lying 

 perpendicular to the cell surface, might provide a degree of 

 mechanical support while not interfering with intake of nutrients 

 through the cell surface between them. 



Electron microscopy does not yet add any fuel to arguments 

 concerning opalinid affinities. This is not for lack of precision in 

 information on the ultrastructure of adult Opalina; thanks to 

 Noirot-Timothee's work this is relatively satisfying. Data at 

 the electron-microscope level on nuclear phenomena, gametes, 

 and young stages will be helpful, but only when we have similar 

 data on many series of ciliates and flagellates to compare them 

 with. For the time being Wessenberg's conclusion that their 

 position is unique appears to be the most satisfying one. 



