Chapter 18 



UNICELLULAR, MICROSCOPIC ANIMALS 

 (PHYLUM PROTOZOA) 



Amoeba; Paramecium; Euglena; Volvox; Plasmodium 



AMOEBA 



Amoeba (a-me'ba) (Gr. amoihe, change) is a common fresh-water 

 protozoan (pro to -zo' an) (Gr. protos, first; zoa^ animals) about 1/100 

 inch (0.2 mm.) in length. Under the microscope it appears as an irregu- 

 lar, colorless, jellylike, granular mass which is changing its shape by the 

 formation of small fingerlike processes called pseudopodia (su do -po' di a) 

 (Gr. pseudes, false; pous, foot). A disk-shaped nucleus, containing 

 chromatin granules, is not easily observed in living specimens. With 

 high power, it will be observed that the living protoplasm has a flowing 

 (streaming) movement. Many structures are more easily observed if the 

 protozoan is killed and stained with a dye. Some of the more important 

 characteristics of the common species {Amoeba proteus) are given briefly 

 (Figs. 157 to 162). 



Integument (Covering). — An outer, thin, clear ectoplasm layer (ek' to- 

 plazm) (Gr. ektos, outside; plasma, mould) is just external to the inner, 

 granular endo plasm (en'doplazm) (Gr. endon, within). Within the 

 endoplasm are the nucleus, granules, vacuoles, etc., described below. 



Ingestion and Digestion. — Food may be ingested at any point on the 

 body surface but usually at the anterior end (part toward the direction of 

 locomotion) . Minute animals and plants are selected and surrounded by 

 the pseudopodia. Then thin sheets of cytoplasm cover the food, even- 

 tually forming a jood vacuole (vak' u ol) (L. vacuus, empty) . This tem- 

 porary structure contains water and digestive enzymes. Digestion within 

 the food vacuole takes place in an acid environment (as in the stomach 

 of a higher animal) and later in an alkaline environment (as in the in- 

 testine of a higher animal). The digested foods are absorbed into the 



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