PROTOZOA 129 



cell depend upon the direction and the amount of the 

 flow. If the protoplasm touches food it flows around 

 the particle, engulfs it, and after the nourishing part 

 has been assimilated, flows away, leaving the residue 

 behind. The animal thus moves from place to place and 

 eats by means of projections of flowing protoplasm that 

 appear and disappear at any point of the constantly 

 changing outline. The ameba does not differ from 

 other members of the group in its method oi assimila- 

 tion and reproduction. 



More Highly Specialized Forms. — Other one-celled 

 forms are more highly developed. A delicate wall sur- 

 rounds the protoplasm, making the outline of the cell 

 definite. Different regions are specialized for the per- 

 formance of special work. The surface is more or less 

 covered with fine, hair-like appendages called cilia 

 which by their movement propel the animal from place 

 to place as oars propel a boat (Fig. 72). At a definite 

 point is an opening comparable to a mouth, surrounded 

 by a circlet of cilia. This opening leads through a short 

 passageway, comparable to a gullet, into the protoplasm. 



The food consists of the bodies of smaller organisms. 

 They are caught in the whirlpool formed by the move- 

 ment of the cilia about the mouth and are drawn 

 through the gullet into the body, where they may be 

 seen with the aid of a compound microscope (Fig. 73). 



Food Substances. — The protoplasm of these organ- 

 isms contains the food substances which all animals 

 require — water, salts, carbohydrates, fats and proteids. 

 Surrounded by a drop of fluid these are carried about 

 in the stream of circulating protoplasm until they are 



