30 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



be continuous with endoplasmic reticulum. The radial canal has 

 a round cross-section in diastole and apparently collapses like a 

 balloon at systole. At this time, a narrow layer of homogeneous 

 substance is present between the walls of the deflated radial canal 

 and the adjacent tubules, but during diastole this layer disappears 

 and some of the tubules appear to open into the canal. Around 

 the swollen medial ends, or ampullae, of the radial canals, the 

 surrounding sponge of nephridial tubules is less dense. A narrow 

 injector canal, around 0-5 /x long, leads from each ampulla 

 obliquely into the main contractile vacuole; nephridial tubules 

 are lacking around the injector canal and around the vacuole. 

 Fine fibrils, about 20 mit in diameter and appearing tubular in 

 cross-section, are seen singly on the outer surface of the membrane 

 of the ampullae. They pass along the walls of the injector canals, 

 joining to form increasingly wider ribbons, and continue, in tracts 

 of ten to 40 fibrils each, along the outer, or pellicle, side of the 

 contractile vacuole. The vacuole itself has a membrane indistin- 

 guishable from that of the radial and injector canals. In diastole, 

 the vacuole is round ; in systole the inner wall of the vacuole is 

 flattened but smooth while the outer wall is deeply folded in 

 regular waves, with the bands of fibrils occurring on one slope 

 of each wave. The fibril bands continue and describe a spiral 

 about the discharge canal, which is formed of invaginated cell 

 membrane. It is open externally and, during diastole of the 

 vacuole, closed at its inner end by a double septum consisting of 

 cell membrane on the outside and vacuole membrane on the inside. 



In the cytoplasm surrounding the zone of nephridial tubules 

 are mitochondria in moderate numbers and clusters of tube-like 

 elements (Fig. 12, PI. Ill) of unknown significance. Individual 

 tubes are about 50 m/x in diameter and are usually packed in 

 orderly rows within the cluster. 



This picture suggests several correlations with the observable 

 events of the contractile vacuole cycle. The cycle involves first 

 the filling of the radial canals while the main vacuole remains 

 collapsed; this would require an effective closure of the injector 

 canals. The ampullae expand considerably more than the elongate 

 radial canals; conceivably the spongy tubular zone surrounding 

 the latter may resist stretching. After maximal filling, the radial 

 canals and ampullae all empty rather rapidly through the reopened 



