72 



PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



which take place in the plant's protoplasm. Animals solve the problem 

 of nutrition by living on plants, or by eating- other animals which, 

 either directly or indirectly, live on plants. Still other types live as 

 parasites upon other animals, some, like the intestinal worms, using 

 freely the foods that are prepared by, and for the use of, the host, 

 while others, like some insects, suck the blood, or, like trichina, 

 invade the cells and tissues, and live at the expense of the living- 

 protoplasm. 



In the group of protozoa all of these methods of food getting are 

 found. Many forms possess chlorophyl, and like the green plants, 



FIG. 22 



Synura uvella, a colony of phytoflagellates, often a source of disagreeable odors and 

 tastes in drinking waters. (After Calkins.) 



manufacture their food directly from simple elements. These protozoa 

 are of considerable theoretical interest, for they stand upon the border- 

 line between the animal and the plant kingdoms, and are sometimes 

 classed as one, sometimes as the other. They are thus involved in 

 what has been one of the most contested of biological problems, the 

 limits of the animal and plant kingdoms, and the problem is the more 

 difficult because some types of this intermediate group may on occa- 

 sions make their food, while at other times they eat like undoubted 

 animals and take in solid food (Chromulinaflavicans,a,i\d some forms 

 of dinoflagellata). The problem has but little significance in the present 



