438 



EVOLUTION OF ANIMALS 



Part V 



a strangely active spatter of peppered egg white. It gives no sign that it is 

 carrying on the same basic essentials of living as are one's own cells. Amebas 

 are generally colorless, or gray to black {Pelomyxa palustris) from the bac- 

 teria that live in the cytoplasm. Two regions are distinguishable in the body, 

 a clear outer layer of ectoplasm and the central endoplasm which contains 

 the vital organelles and the nucleus, separated from the endoplasm by the 

 nuclear membrane (Fig. 21.11). There may be clusters of green particles in 

 the endoplasm, bacteria and diatoms in the food vacuoles which are temporary 

 stomachs in which digestion prepares the food for absorption and assimila- 

 tion. The contractile vacuole widens and vanishes only to appear again in 

 nearly the same place. Such vacuoles eliminate metabolic waste products and 

 are important guardians of the water content of the body. They are active 

 when the animal contains too much water, and disappear when it contains too 

 little. Neither marine nor parasitic amebas have contractile vacuoles. Since 

 their bodies and the sea water or the protoplasm of their hosts contain about 

 the same proportions of salt, the sea water does not flood into their bodies as 

 it does into the amebas of fresh water. 



Locomotion. Amebas commonly move about by means of pseudopodia 

 although these are usually greatly reduced in parasitic species. 



Emptying ond reforming 

 of contractile vocuole 



Amebo cut through 

 contractile vocuole 



(O^^ 



4 3 2 



Food vacuole 



Contractile vocuole 



Temporory front end 



, Pseudopodium forming 



Ectoplasm 



Endoplasm 

 nner 

 toplosm 



Clear outer protoplasm 



Fig. 21.11. An ameba, showing its principal structures. Inset, section of ameba 

 with the contractile vacuole in successive stages of emptying and refilling. 



