92 THE SMALLEST LIVING THINGS 



soluble waste matters in the protoplasm, such as carbon dioxide 

 (C0 2 ) and nitrogenous waste in the form of uric acid or urea, 

 will be dissolved in and disposed of with the water. Thus the 

 contractile vacuole can be regarded as a primitive excretory sys- 

 tem, a conclusion supported experimentally by recent workers 

 who showed that in Paramecium caudatum uric acid is actually 

 present and to an amount equal to approximately four to five 

 milligrams per liter of water. 



Nutrition 



The constant movement of ciliated protozoa, involving a 

 constant supply of energy derived through the burning of proto- 

 plasmic substances and leading to the constant formation of waste 

 matters which must be excreted, necessitates a constant renewal 

 of substances if the organism is to live. We have seen (page 89) 

 how a Paramecium disintegrates if this repair of body material 

 is prevented by starvation. The aggregate of processes con- 

 cerned in such repair are grouped together under the compre- 

 hensive term nutrition. 



Although the processes concerned with nutrition all operate 

 at the same time and are closely interwoven, we can examine 

 them individually under the headings of ( 1 ) food getting, (2) 

 digestion, (3) distribution and assimilation, and (4) defecation. 



Food Getting 



The methods employed by protozoa to obtain needed mate- 

 rials for repairing waste are all correlated with the phenomena 

 •of irritability. The special method used by any one type is the 

 result of many factors of the organization and adaptation to the 

 mode of life of the organism. It is probable that no two types 

 of protozoa employ an identical method. Nevertheless it is 

 possible, and certainly convenient, to group the manifold activi- 

 ties under a few main types, such as, (1) holozoic, (2) sapro- 

 zoic, (3) holophytic, and (4) mixotrophic* These four types 

 are usually given as the different methods of nutrition. The dif- 



* Holozoic, wholly or distinctively like an animal as to nutrition ; saprocoic, 

 living on proteins and carbohydrates in solution ; holophytic, wholly or distinctively 

 vegetable in nutrition — obtaining food after the manner of a green plant ; mixo- 

 trophic, deriving nourishment from a combination of any two or even of all three 

 of above methods. 



