PROTOZOA AS CELLS 43 



connection with the species concerned. Interesting peripheral 

 structures, often paralleling the nine outer fibers of the axis, are 

 found in many animal spermatozoa (see Fawcett, 1958, 1961). 



The basal regions of flagella and their intracellular parts, the 

 basal bodies or kinetosomes, differ somewhat in the various 

 groups of organisms studied. Additional fibrous structures of 

 diverse pattern and construction are attached to kinetosomes in 

 different species, and will be discussed later. But a definite and 

 highly significant pattern is common to the morphology of 

 kinetosomes in general. 



The kinetosome consists of a cylinder of nine fibrils that 

 continue distally as the nine peripheral fibrils of the flagellum 

 (Text-fig. 3). Evidence from some pioneer papers {e.g., Fawcett 

 and Porter, 1954; Bradfield, 1955) indicates that a short interrup- 

 tion may occur in these nine fibrils where they emerge from the 

 cell surface in certain metazoan ciliated epithelia, but this work 

 needs to be repeated using more recent techniques. For the most 

 part, the continuity is unquestionable. The kinetosome almost 

 always is short, about 0-5 /x, but in hypermastigote flagellates such 

 as Trichonympha it may extend up to 4 ^ in length. At least in the 

 proximal part of the kinetosome the nine fibrils typically are not 

 double, but triple, and the row of three is skewed from the 

 circumferential direction somewhat more than is the doublet in 

 the flagellum (Fig. 15, PI. IV). This probably accounts for an 

 apparent thickening of the proximal parts of the fibrils as seen in 

 longitudinal section. These features have been noted several 

 times {e.g., Rhodin and Dalhamn, 1956; Noirot-Timothee, 1959; 

 Afzelius, 1959; Gibbons and Grimstone, 1960) and are visible by 

 hindsight in many other published micrographs. 



In metazoan ciliated epithelia, kinetosomal fibrils generally 

 converge basally and mingle in a dense matrix material; the 

 kinetosome thus is closed off at its tapering inner end. Protozoan 

 kinetosomes with few exceptions are open proximally and main- 

 tain a nearly constant diameter throughout. Within the low-density 

 core of the kinetosome, rods, granules, radial fibrils, or tubular 

 structures may appear, or the entire interior may look empty. A 

 cartwheel-like configuration in the center of cross-sectioned 

 kinetosomes is not uncommon. Regularly arranged fine filaments 

 connecting specific subfibrils of the nine fibrils with each other or 



