144 



NUTRITION AND METABOLISM 



rABC 



food, digest it within gastric vacuoles by the aid of enzymes and of 

 acids, just as is the case in many-celled animals. The most important' 

 of the disease-producing protozoa live within nutrient fluids, for ex- 

 ample the blood, and they obtain their nourishment from the fluid in 

 which they live, by osmosis;. consequently, they have no definite mouth 

 area, nor gastric vacuoles. 



"Some of the protozoa, for example, some amoebae and ciliata, pos- 

 sess contractile vacuoles. A contractile vacuole is a clear cavity which 



Fig. 99. — A,Amasba proteus; Na, a food particle; Cv, contractile vacuole; N, nucleus. 



{After Doflein.) 



appears in the cytoplasm, grows slowly, empties itself by a rapid con- 

 traction of the fluid which has drained into it and forms again. The 

 fluid which it ejects contains the soluble waste products resulting from 

 the metabolism of the protozoon. One function of the contractile vac- 

 uoles is, therefore, excretion; in some protozoa, they are probably also 

 concerned with resjJiration. Contractile vacuoles are frequently absent 

 in protozoa which are parasitic within other animals. 



"Organisms which feed upon solid food, for example the bodies of 

 other organisms, are said to be holozoic in their mode of life. Others 



