PHYLUM PROTOZOA 29 



It probably plays an important role in the metabolic activity of 

 the cell. 



A clear space filled with a fluid less dense than the surrounding 

 protoplasm may be seen in favorable specimens. It is called the 

 contractile vacuole (Fig. 9, 2), since its walls contract at more or 

 less regular intervals and force the fluid contents out of the body. 

 It serves to get rid of the water taken in through the surface of 

 the body, thus regulating the tension between the protoplasm 

 and the surrounding medium. It is also considered a primitive 

 excretory organ. 



The solid particles of food engulfed by Ameba cause the for- 

 mation of food vacuoles (Fig. 9, 4), which are temporary structures 

 for the digestion of organic material. Besides the nucleus, con- 

 tractile vacuole, and usually one or more food vacuoles, there are 

 often undigested particles, and foreign substances, like grains 

 of sand (Fig. 9, 5), embedded in the endoplasm. 



Metabolism. — Metabolism is the term applied to the series 

 of processes concerned with the manufacture and breaking down 

 of protoplasm. The term anabolism is used for the constructive 

 processes such as the ingestion, digestion, absorption, and as- 

 similation of food. The term katabolism means the breaking 

 down of protoplasm into simpler products, and includes the 

 processes of secretion, excretion, and respiration. 



Food. — The food of Ameba consists of very small aquatic 

 plants, such as Oscillaria and diatoms, Protozoa, Bacteria, 

 and other animal and vegetable matter. A certain amount of 

 choice of food is exercised, or the A meba's body would become 

 overloaded with particles of sand and other indigestible mate- 

 rial among which it lives. 



Ingestion (Fig. 10). — The ingestion or taking in of food oc- 

 curs without the aid of a mouth. Food may be engulfed at any 

 point on the surface of the body, but it is usually taken in at 

 what may be called the temporary anterior end, that is, the part 

 of the body toward the direction of locomotion. A small amount 

 of water is taken in with the food, so that there is formed a 



