176 BIOLOGY OF THE PROTOZOA 



exhausted hy destructive metabolism. Groups of activities includ- 

 ing: (1) Food-getting; (2) digestion and secretion; (3) assimilation; 

 (4) defecation, find their place here. The specialized structures 

 adapted for these various activities ha\e been described for the most 

 part in the preceding chapters, and the following is suj)plementary 

 in nature dealing with the functions which these structures perform. 



]. Food-getting.— The varied methods by which Protozoa acquire 

 the needed materials for replenishing protoplasmic substances 

 reduced by oxidation are all correlated with the phenomena of 

 irritability. The particular method employed by any one type of 

 organism is probably the result of many factors of organization and 

 ada])tation 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, 

 ccmsiderihg the endless variety of endoplasmic and cortical differen- 

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

 the Protozoa. On the contrar^', 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 comparativel.y few main types which are designated: 

 (1) Holozoic nutrition; (2) saprozoic nutrition; (8) aut()tr()])hic or 

 h<)'()I)liytic nutrition; (4) heterotrophic nutrition. Many anthorities 

 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, 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 

 INIetazoa involves the ingestion of raw materials in the form of 

 proteins, car])ohydrates and fats which are usually combined 

 in the proto])lasm of some other living organism eaten as food. 

 It is an expensive method of acquiring raw materials for it neces- 

 sitates capture and killing of living prey, preparation and secre- 

 tion of digestive fluids and ferments necessary to make the proteins 

 and carbohydrates 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 prepara- 

 tion of its raw materials and the "storage of energy." Dissolved 

 proteins and carbohydrates made soluble through the agency of 

 bacteria and other organisms in infusions, or prepared by the diges- 

 tive processes of the host in the case of parasites and some com- 

 mensals, are absorbed directly^through the body^wall or through 



