INTRODUCTION. 3 



same way, these protozoa flow away from waste particles which are to be 

 excreted. Other protozoa have definite mouth areas for the ingestion of 

 food, and definite anal areas for its excretion. Those protozoa which 

 ingest solid food, digest it within gastric vacuoles; the food is digested in 

 these vacuoles, as it is in many-celled animals, by the aid of enzymes and 

 of acids. The most important of the disease-producing protozoa live 

 within nutrient fluids, for example the blood, and they obtain their nour- 



A 



FIG. 47. A, Amoeba proteus; Na, a food particle; Cv, contractile vacuole; N, nucleus. 



(After Doflein.) 



ishment from the fluid in which they live, by osmosis; consequently, 

 they have no definite mouth area, nor gastric vacuoles, Fig. 47. 



"Some of the protozoa, for example, the ciliata, possess contractile 

 vacuoles. A contractile vacuole is a clear area which appears, grows 

 slowly, empties itself, by a rapid contraction, of the water which has 

 drained to it, and forms again. The water which it ejects contains the 

 soluble waste products resulting from the metabolism of the protozoon. 

 One function of the contractile vacuoles is, therefore, excretion; in some 

 protozoa, they are probably also concerned with respiration. Contractile 



