CILIATES 181 



units. Each cilium (in some species or body regions, each pair of 

 cilia) thus is surrounded by a pair of kidney-shaped alveoli 

 defining one polygon (Fig. 75, PI. XX). The closely apposed 

 membranes of the paired alveoli anterior and posterior to the 

 cilium form a double septum running between cilium and 

 trichocysts; these are the meridional silverlines. The polygonal 

 packing of the pairs of alveoli accounts for the polygonal silver- 

 lines, as silver is deposited along their contiguous margins or the 

 slight gaps between them. However, marked differences in 

 staining properties of the polygonal and meridional silverlines 

 constitute an unsolved puzzle (Parducz, 1958a). 



The ectoplasm underlying the pellicle is ridged in a pattern 

 reflecting the polygonal packing of the alveoli. Of the fibrous 

 structures seen in the tetrahymenids, only the kinetodesma have 

 clear counterparts in Paramecium. Metz, Pitelka, and Westfall in 

 1953 first demonstrated that these are striated (at about 40 m/x) 

 tapering fibrils proceeding from the kinetosome anteriad and to 

 the right and extending along the kinetodesmal bundle for about 

 five polygons (Fig. 76, PL XXI). The bundle itself may assume a 

 loose clockwise spiral. Where cilia are paired, the single kineto- 

 desmal fibril arises from the right anterior margin of the posterior 

 kinetosome of the pair. The parasomal sac, recognized and named 

 by Ehret and Powers, appears always to the right of the kineto- 

 some — again of the posterior one if they are paired — opening 

 to the exterior just to the right of the base of the cilium. 



No fibrils corresponding in position to the longitudinal, 

 transverse or postciliary fibrils of the tetrahymenids have been 

 seen. However, an intriguing network of fine filaments has been 

 demonstrated by Sedar and Porter, Schneider, and Roth. They 

 occur parallel to but below the pellicle, at about the level of the 

 lower ends of the kinetosomes, where they form a roughly 

 polygonal lattice. Tracts of these filaments also appear to make 

 contact with the innermost pellicle membrane. Their filamentous 

 nature is too distinct in many published micrographs to support 

 the suggestion of Ehret and Powers that they are merely tangential 

 sections of the bottoms of pellicle membranes. Roth (1958a) 

 reported that these, as well as a mat of similar filaments under the 

 membrane of the buccal cavity, were tubular fibrils with a 

 diameter of about 20 m/x; Schneider (1959) described them as 



