Figure 1. Series of light micrographs showing progress of vacuoles through the digestive cycle in the ciliate Tetrahymena vorax. The vacuoles contain particles of India ink, which are not degraded by digestive enzymes. (a) Nascent vacuole attached to cytostome (arrow) and newly formed vacuole (V) near oral region. (b) Two vacuoles undergoing acidification. The decrease in the diameter of the vacuoles condenses the contents into a compact mass. (c) A vacuole at a time when the pH of the vacuole is the most acidic. (d) A vacuole after fusion with lysosomes. The addition of the lysosomal membrane results in a clear region between the compact India ink mass and the vacuole membrane (arrow). (e) A vacuole at the posterior end of the cell late in the digestive cycle. (f) A mass of India ink particles recently released from the cytoproct (arrow) at the posterior end of the cell. Bars=10 m. Reproduced with permission from Mislan and Smith-Somerville (1986).

Figure 2. Electron micrographs of vacuoles with ingested food. (a) Partially digested bacteria in vacuole (V) in the ciliate Tetrahymena vorax after fusion of lysosomes with the phagosome. (b) Wood in vacuole in Pyrsonympha vertens, an intestinal flagellate of the termite Reticulitermes flavipes. Arrows indicate sites of endocytosis at the cell surface. Bars=1 m.

Figure 3. Macrostomal cells of the polymorphic ciliate Tetrahymena vorax. The microstomal form (shown in Figure 1) differentiates into this cell type in response to a signal released by potential prey. (a) A prey protozoan (P) in a vacuole before separation from the cytostome. (b) Two vacuoles (V) with protozoa in different stages of digestion. Bar=10 m.