PROTOZOA 



85 



various parts of its body ; these temporary finger-like processes 

 are called pseudopodia. Movement takes place by the gradual 

 streaming of the entire substance of the amoeba into the pro- 

 jections from one side, and is thus practically a flowing process, 

 " the upper surface " (in A mceba verrucosa) ''continually pass- 

 ing forward and rolling under at the anterior end so as to form 

 the lower surface" (15). 



Its food consists of those entire minute animals or plants, or 

 those organic particles with which the pseudopods, or main 

 body portion have come into 

 contact. The amoeba pursues 

 such prey as long as it is in con- 

 tact with it, rolling it ahead in 

 its attempts to catch it, then 

 sending out pseudopodia on 

 each side to surround it. Fi- 

 nally if the prey has not escaped 

 the amoeba succeeds in sur- 

 rounding it with its side pseu- 

 dopodia and sends out from its 

 upper surface a thin film of 

 protoplasm over it, thus com- 

 pletely engulfing it. This food is pressed into the soft sub- 

 stance and passes gradually into the interior. 



Digestion. — With the food-victim a certain amount of water 

 is taken in by the amoeba ; this water with the victim forms a 

 '' gastric vacuole " or an improvised stomach, surrounded on 

 all sides by a wall of living protoplasm. From this protoplasm 

 there soon begins to form an acid secretion, probably hydro- 

 chloric acid, which thus gradually changes the chemical nature 

 of the water of the gastric vacuole and kills the prey which it 

 incloses ; with the first changes in chemical nature of the sur- 

 rounding water the prey begins to struggle and ceases its efforts 

 to escape only when killed by the secretion. It is probable that 

 the product of the digestive action of this acid is the formation 



Fig. 30. — The fresh-water protozoon, 

 Amceba proteus. It is now moving in 

 the direction of the arrow. «., nu- 

 cleus; C.V., contractile vacuole; p., 

 pseudopodia. Very much enlarged. 



