PROTOZOA AS CELLS 27 



exterior in the two peritrichs ; these have thick (65 m/x), apparently 

 homogeneous walls and appear during diastole to be closed at 

 both proximal and distal ends by membranes. In Tokophrya (Fig. 

 10, PI. Ill), a permanently open external channel leads to the body 

 surface from a small papilla projecting into the contractile vacuole ; 

 in the papilla the lumen of the canal narrows to a very fine tubule 

 during diastole but is widely expanded during systole. Fine 

 fibrils, about 18 m/x in diameter, radiate from the channel to the 

 adjacent vacuole membrane and might, if they are contractile, be 

 responsible for changes in the diameter of the excurrent tubule. 



In the astome Metaradiophrya gigas the discharge canal consists 

 of a distal invagination of the cell surface, with annular fibers in 

 one side of its wall and a proximal cone-shaped part ending in a 

 papilla in the main vacuole wall. This proximal end is closed off 

 by a septum from the vacuole cavity. Radially arranged fibrils 

 that look tubular in section pass from the canal wall to the 

 vacuole's cortex (Fig. 11, PI. III). Some other astomes have a 

 contractile vacuole apparatus that is a permanent long tube. This 

 undergoes rhythmic waves of contraction, and discharges by a 

 series of well-marked pores. The latter have longitudinal fibers 

 in their walls and, again, radial fibers leading from their proximal 

 parts toward the main vacuole wall. 



Contractile vacuoles of an unusual sort have been observed by 

 Noirot-Timothee (1960) in several entodiniomorph ciliates. These 

 intestinal symbiotes of ungulates have vacuoles that pulsate at 

 rather infrequent intervals, with a prolonged resting stage inter- 

 rupting diastole. Light microscopy shows a basophilic and 

 osmiophilic cortex about the vacuoles. In electron micrographs 

 this zone during the resting stage appears as a sparse halo of tiny, 

 clear, spherical vesicles. In some micrographs, scraps of ergasto- 

 plasm are present in the adjacent cytoplasm. 



The most complex contractile vacuole apparatus yet examined 

 is that of Paramecium. In most species of this genus the contractile 

 vacuole is a reservoir fed by pulsating radial canals. Schneider 

 (1960a, 1960b) in a thorough electron-microscope study found in 

 P. aurelia and P. caudatum the apparatus diagrammed in Text-fig. 2. 

 A network of fine canaliculi, 1 5-20 m/x in diameter, forms a 

 cylindrical field around each radial canal (Fig. 12, PL III). At the 

 periphery of the field these nephridial tubules appear in places to 



