158 ^"^ ANIMAL KINGDOM 



Fiqure 8 9 An ameba capturing a large flagellate The flagellate hits the side of the 

 ameba and slips into the crevasse at the base of a pseudopod. The ectoplasm of this 

 region erupts and rapidly engulfs the prey. Stages shown are at about one second 

 intervals. 



much of the inner endoplasm is a sol. Where pseudopods are forming 

 the outer endoplasm is also a sol, becoming a gel along the sides of the 



advancing lobes. 



Amebas eat a wide variety of materials. They crawl slowly about, 

 engulfing inactive food such as plant cells and debris by flowing slowly 

 around them and enclosing them in food vacuoles. They may also 

 capture active prey such as flagellates (Fig. 8.9) in a somewhat different 

 manner. When a swimming flagellate bumps into an ameba, it not only 

 tends to slide into a crevasse between pseudopods but also stimulates the 

 ameba to flow rapidly in its direction. If the anterior end of the flagellate 

 becomes wedged, the ameba's protoplasm engulfs the entire prey in a 

 second or two. If the flagellate is not wedged it is simply pushed away 

 by the advancing protoplasm. Sometimes the flagellate appears to be- 

 come attracted to the ameba and returns again and again, so that the 

 ameba may have several opportunities to be successful. Certain species 

 of ameba are quite particular about their food, and eat only flagellates 

 or only plant cells. 



The fate of food vacuoles has been studied closely. At first they 

 become acid, owing to the secretion by the protoplasm of inorganic 

 acids (such as hydrochloric acid, secreted in our own stomachs). This 

 kills the prey and initiates digestion. Later the vacuole becomes alka- 

 line, enzymes are secreted into it, and digestion continues. Enzymes for 

 the hydrolysis of proteins, fats and carbohydrates have been found in 

 food vacuoles. The food particles swell, become indistinct, and the 

 vacuole enlarges. As digestion is completed, both nutrients and water 

 are absorbed by the protoplasm and the vacuole shrinks to a very small 

 size. As the ameba continues its slow locomotion, the indigestible rem- 

 nants are expelled and left behind. This digestive process in the food 

 vacuole has been observed in all of the major groups of protozoa. 



A few amebas, such as the genus Diffiugia (Fig. 8.10), cement sand 

 particles together to make a protective case. Other species secrete a 

 membranous covering. Pseudopods project through a lower opening, 

 and by means of these the animals move about. 



He/iozoa. Related to the amebas is the order Heliozoa, a group of 

 fresh-water sarcodinids with numerous delicate pseudopods projecting 

 from a bubbly center (Fig. 8.10). They float in the water and capture 



