1S1 BIOLOGY OF THE PROTOZOA 



irritability. The particular method employed by any one type of 

 organism is probably the result of many factors of organization and 

 adaptation combined with mode of life, all of which are traceable 

 to adaptations resulting from the effects of external stimuli and 

 response through irritability. It would indeed be remarkable, 

 considering the endless variety of endoplasmic and cortical differen- 

 tiations, were we to find a common method of food-getting amongst 

 the Protozoa. On the contrary, it is probable that no two types of 

 organism follow an identical method. Nevertheless it is possible, 

 and it is certainly convenient, to group these manifold activities 

 under a comparatively few main types which are designated: 

 (1) Holozoic nutrition; (2) saprozoic nutrition; (3) autotrophic or 

 holophytic nutrition; (4) heterotrophic nutrition. Many authori- 

 ties introduce a fifth type under the caption parasitic nutrition, 

 but as this does not differ in principle from saprozoic nutrition, it 

 is included with the latter type. 



While these terms apparently indicate different modes of nutri- 

 tion they are more applicable to methods of food-getting, and the 

 differences have to do in the main with the nature of the raw 

 materials taken in and the subsequent processes necessary for 

 their elaboration. Thus holozoic nutrition in Protozoa as in Metazoa 

 involves the ingestion of raw materials in the form of proteins, 

 carbohydrates and fats which are usually combined in the proto- 

 plasm of some other living organism eaten as food. It is an expen- 

 sive method of acquiring raw materials for it necessitates capture 

 and killing of living prey, preparation and secretion of digestive 

 fluids and ferments necessary to make the proteins and carbo- 

 hydrates soluble, and disposal of the undigestible residue. On the 

 other hand, it assures the supply of capital in the form of chemical 

 energy without the labor of storing it up. Saprozoic nutrition is, 

 so to speak, a more economical method, for the organism does 

 away with the elaborate processes of secretion and digestion and 

 relies upon the activities of other organisms for the preparation 

 of its raw materials and the "storage of energy." Dissolved pro- 

 teins and carbohydrates made soluble through the agency of bac- 

 teria agjjj other organisms in infusions, or prepared by the digestive 

 processes of the host in the case of parasites and some commensals, 

 are absorbed directly through the body wall or through special 

 receptive regions, by endosmosis. This type of food-getting may 

 be regarded as a degeneration or adaptation of the holozoic method, 

 the specialized absorptive areas being reminiscent of former mouths, 

 while the pathogenic effects of some types of parasites are inter- 

 preted as due to the secretion by the parasite of digestive fluids 

 which cause cytolysis of the host cells. Holophytic or autotrophic 

 nutrition, characteristic of plants, is quite different in principle 

 from the other two. Digestive processes typical of the majority 



