210 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



normal to the cytoplasmic surface (Fig. 88, PL XXIV). Each 

 consists of two layers of tubular 15-m/x fibrils oriented perpendicu- 

 lar to each other. Minute clear vesicles typically are abundant in 

 the cytoplasm between these sheets. 



The esophagus, according to Noirot-Timothee, is a funnel, 

 open anteriorly, into which the peristomal canal, terminating in 

 the cytostome, depends eccentrically. The esophagus wall 

 consists of an outer, finely filamentous layer and an inner array of 

 15-m^ tubular fibrils, grouped as two-layered packets of 10 to 12. 

 Through the open anterior end of the esophageal funnel there is 

 continuity of ectoplasm and endoplasm, at least intermittently. 

 However, the fibrillar elements of the esophagus wall may 

 perhaps be continuous in some regions with either the fibrils of 

 the subperistomal sheets described above, or with retrociliary 

 fibers that descend in this direction. Along its dorsal surface the 

 esophagus wall is thrown into very straight, narrow, deep folds 

 protruding into its lumen (Fig. 88, PL XXIV); elsewhere it is 

 irregularly undulating. About midway down the body, the 

 esophagus spreads out over the dorsal surface of the endoplasmic 

 sac and merges with the latter. The wall of the endoplasmic sac, 

 which appears distinctly in the light microscope as a membrane 

 or fibrous sheet, is less discrete in most electron micrographs. In 

 some areas it seems to be a thinned-out version of the esophagus 

 wall ; elsewhere it consists of an ill-defined series of filamentous 

 cords. It apparently is not a continuous layer, and its only 

 membranous component is an array of granular endoplasmic 

 reticulum flattened against its inner surface. 



Endoplasm and ectoplasm are similar in structure, except that 

 Golgi bodies are limited to the endoplasm. The existence of 

 typical mitochondria is questionable; Noirot-Timothee believes 

 that membrane-limited spheroids with ill-defined internal contents 

 may represent quasi-mitochondria in these anaerobic cells. 



At the posterior end of the cell, the rectum is visible as a 

 distinct inpocketing of the pellicle. Where it meets the boundary 

 of the endoplasmic sac, the filamentous internal layer of the 

 pellicle seems to be continuous with the filamentous component 

 of the sac wall. The rectum itself is completely open to the 

 exterior; where it abuts against the endoplasm the latter is 

 bounded by a single unit membrane. 



