58 



COLLEGE ZOOLOGY 



Anterior 



Yellow-green 



Red (food vacuole forming) 



Blue-green (acid) 



-Yellow (alkaline) 



■Orange 



■Red-orange 



-Waste material 

 Red-orange 



Posterior 



Figure 27. Paramecium caudatum. Diagram illustrating cyclosis and the process of digestion. 

 To indicate the path of the food vacuole within the paramecium, some yeast cells stained with 

 Congo red (a red dye) should be placed near the animal. The yeast cells are taken into the 

 body, where a food vacuole is formed. The arrows in the figure of the paramecium indicate the 

 path of the food vacuole within the body (cyclosis). The change in color of the Congo red 

 to blue-green indicates that the vacuole became acid soon after it was formed. As the vacuole 

 with the yeast cells circulates through the body, it changes back to a red-orange color, indicating 

 that the vacuole and its contents are becoming less acid. Chemical tests have proved that the 

 vacuole with its contents actually becomes definitely alkaline even though the red-orange color 

 would not prove this, for Congo red is an indicator of acidity only, and cannot be used to test 

 for alkalinity. We can assume from experiments that in the digestion of food the vacuole is 

 first acid and then alkaline. 



the surrounding cytoplasm; and the forma- 

 tion of another vacuole is begun. 



A food vacuole is a droplet of water with 

 food particles suspended within it. As soon 

 as one is separated from the cytopharynx, it 

 is swept away by the rotary streaming move- 

 ment of the endoplasm, known as cyclosis 

 (Fig. 27). This carries the food vacuole 

 around a definite course, which begins just 

 behind the cytopharynx, passes posteriorly, 

 then forward and aborally, and finally pos- 

 teriorly to near the oral groove. In the course 

 of this journey, digestion takes place. During 

 the cyclosis the food is digested by enzymes 



from the endoplasm, and the food vacuoles 

 become smaller. The digested food is either 

 stored, used for a vital activity, or is built 

 up into protoplasm. 



Regulation of water content 



Two contractile vacuoles are present in 

 Paramecium caudatum, occupying definite 

 positions, one near each end of the body. 

 They lie in the inner layer of the ecto- 

 plasm and communicate with a large por- 

 tion of the body by means of a system of 

 radiating canals, 6 to 11 in number (Fig. 

 25). These canals fill with liquid, then dis- 



