198 NUTRITION AND METABOLISM 



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 

 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 vacu- 

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

 concerned with respiration. Contractile vacuoles are usually absent 

 in protozoa which are parasitic within other animals. 



*" The process of respiration in the protozoa is in general similar to 

 that of higher animals. Most of them require oxygen and eliminate 

 carbon dioxide. The contractile vacuole which is found in certain 

 forms is believed to have a respiratory function. Respiration may 

 consist of the liberation of energy through oxidation or through the 

 breaking down of complex molecules. In organisms of an anaerobic 

 habit the respiration is probably through internal molecular changes 

 affecting material stored in the cytoplasm. 



*"ln addition to the expulsion of solid undigested material from 

 the cytoplasm there is evidence that waste products other than CO2 

 are excreted by contractile vacuoles. Many organisms also secrete 

 material either of the nature of chitinous membranes on their surface 

 or metabolic products in the form of granules, etc., within their bodies. 



*" Derangement of function may be produced associated with it 

 are visible degenerative changes. It has also been found that certain 

 protozoa have the ability to recover from injury and to regenerate lost 

 parts." 



* Prepared by J . L. Todd. 



