FUNCTIONAL ACTIVITIES 93 



ferences between them, however, have mainly to do with the 

 nature of the raw materials taken in by the organism and with 

 the subsequent processes necessary for their assimilation. 



Holozoic nutrition in protozoa, as in higher animals, involves 

 the ingestion of food in the form of proteins, carbohydrates, 

 and fats, which are usually obtained from the protoplasm of the 

 other living organisms that are eaten. It is an expensive mode 

 of feeding since it requires labor in the capture and killing of liv- 

 ing prey, preparation and secretion of digestive fluids and fer- 

 ments necessary to dissolve the proteins and carbohydrates, and 

 defecation of the indigestible remains. 



Saprozoic nutrition is a more economical method, for in this 

 process the organism does away with the elaborate processes of 

 digestion and relies upon the activities of other organisms to 

 prepare the dissolved proteins and carbohydrates that are in- 

 gested. Such types live upon dissolved foods, prepared mainly 

 by the bacteria found in infusions or by the hosts of such forms 

 as are parasitic. The food is taken in through the body wall 

 or through special receptive regions by endosmosis. 



Holophytic nutrition, characteristic of the chlorophyll-bear- 

 ing types, is quite different in principle from the other two. The 

 ingestion of solid or dissolved proteins and carbohydrates is ab- 

 sent. Instead, the very unstable substance, chlorophyll, is manu- 

 factured in the presence of light by specialized plastids (chloro- 

 plastids) of the cell. Chlorophyll is very sensitive to light and 

 in some way not yet fully understood is instrumental in utilizing 

 the radiant energy of sunlight to bring about the union of carbon, 

 oxygen, and hydrogen in proper proportions to form the group 

 of chemical compounds known as carbohydrates. 



Finally, mixotrophic nutrition is characteristic of those forms 

 which combine any of the above methods of acquiring raw ma- 

 terials. Some forms combine holozoic and saprozoic methods, 

 others holozoic and holophytic, still others holophytic and sapro- 

 zoic, and some combine all three methods. 



Continuous Feeders 



The great majority of protozoa are holozoic in their food 

 getting; many of them apparently eat all the time, and according 

 to the French biologist, Emile Maupas (1844-1916) are glut- 



