56 



ELEMENTS OF BIOLOGY 



makes its appearance periodically, distends with liquid, then expels 

 its contents into the water, disappearing only to reappear again 

 slowly. This is the contractile vacuole (Fig. 12). It is not a 

 feature common to all animal cells, nor to all species of Amceba, 



PSEUDOPODIUM ■ 



\r.v\- 





NUCLEUS 



^\^ ■/,■■■■ >-;'-i^:i:^':-i .V'? ;■'■ ■: ■ -iA 



if. r ■>.; ,af V ■: •; ~V-.\ \ ■■■.',■ ■ 



/•00£) VACUOLE 



ENDOSARC 



ECTOSARC 



CONTRACTILE VACUOLE 



Fig. 12. — An amoeba as commonly observed in the laboratory'. As it was being 

 sketched the animal was withdrawing the pseudopodia shown at the bottom of 

 the figure and advancing those at the top. 



but is found in many varieties of Protozoa. Its chief function ap 

 pears to be the maintenance of the proper water content of the cell, 

 for it is lacking in marine Protozoa, which are surrounded by water 

 with a relatively high osmotic pressure. The nucleus of the amoeba 

 may be seen in the living animal as a somewhat dense central body, 

 approximately spherical in shape. 



