CILIATES 199 



of water occurs when Stentor becomes fully extended ; the vacuoles 

 suggest a mechanism for this intake. 



Arising at apparently regular intervals from the inner surface 

 of the buccal cavity membrane and passing posteriad in the scant 

 cytoplasm between vacuoles is a system of fine tubular fibrils 

 aligned in sheets (Fig. 82, PI. XXIII). The fibrils are about 25 m/x 

 in diameter and in section resemble fibrils of the membranelle 

 roots (see below). Their terminus is unknown. 



Cilia and kinetosomes are of conventional structure. Often the 

 kinetosome appears to be closed off at the bottom where attached 

 fibers arise. In addition, many kinetosomes — most frequently 

 in conjugant or dividing cells — contain in their lumina very 

 dense granules, about 25 m/x in diameter, aligned in neat rows 

 parallel to the kinetosome axis. 



Somatic kineties have microscopically visible kinetodesma. In 

 the electron microscope (Faure-Fremiet, Rouiller, and Gauchery, 

 1956b; Faure-Fremiet and Rouiller, 1958a; Randall and Jackson, 

 1958; Inaba, 1959) the fibers are seen to consist of up to 24 

 parallel longitudinal ribbons, each ribbon composed of 24 to 30 

 fine tubular fibrils about 20 m/x in diameter, joined together 

 laterally by thin bridges. The electron-microscope image is 

 strikingly like that of the contractile axostyle of the zooflagellate 

 Pjrsonjmpha (p. 151). Careful study shows unequivocally that 

 each fibril arises from one of the kinetosomes along the kinety 

 (Fig. 83, PL XXIII). At its base the fibril is broadly joined to the 

 kinetosome and often appears striated, like the kinetodesmal 

 fibrils of Tetrahymena and Paramecium. It tapers very abruptly, 

 however, loses all trace of striation, and becomes a tubular filament 

 of uniform diameter. All published pictures indicate that each 

 sheet of the kinetodesmos consists thus of overlapping fibrils that 

 enter the bundle from kinetosomes on one side and end at intervals 

 on the other side. They clearly are much longer than the kineto- 

 desmal fibrils of the hymenostomes. Randall and Jackson 

 calculated that on the average there were within the bundle at any 

 one level about 500 fibrils, while they estimated 1000 kinetosomes 

 per kinety. This would suggest that a single fibril may extend 

 for as much as half the length of the animal. However, there 

 were some indications that each kinetosome might contribute 

 more than one fibril to different sheets within the bundle. 



