ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



103 



red yeast cells being caught up by cilia which line the oral groove and 

 being forced through the gullet into the body. There the yeast cells 

 are rounded up into food vacuoles. As time passes the color of the 

 yeast within the vacuoles will change from a deep red to blue. This 

 indicates that enzymes have passed into the vacuoles and are digesting 

 the yeast. The color change means that the contents have changed 

 from alkaline to acid. A similar change takes place in the human 



Photo by Winchester 



Fig. 7.12. Small portion of living Paramecium greatly enlarged to show a contractile 



vacuole and its surrounding rays. 



stomach. The vacuoles become smaller as the food is digested and the 

 soluble part diffuses out into the protoplasm. When only an indi- 

 gestible residue remains it will be expelled through the anal pore 

 which lies slightly below the gullet. This pore is quite small and 

 cannot be seen except when the animal is in the act of egestion. 



There are two contractile vacuoles in Paramecium caudatum, and 

 these remain in a fixed position. There are radiating canals that bring 

 water to the vacuoles from other parts of the cell. When the vacuoles 

 become full they will contract rather quickly and force the water to 

 the outside. There is a small pore which connects with each vacuole 

 for this purpose. It requires only about fifteen seconds for a con- 

 tractile vacuole to fill and empty its contents. 



