284 ZOOLOGY 



268,000,000 might be developed in one month by the process 

 of division. Apparently the division does not ordinarily go on 

 indefinitely, but from time to time the Paramoefia unite tem- 

 porarily in pairs and undergo an exchange of some of their 

 nuclear matter. This is doubtless the beginning of what is 

 known in the higher animals as sexual reproduction. 



The Protozoa' are divided into four principal classes ar- 

 ranged according to the character of the locomotion. In the 

 simplest forms, called Rhizop'oda, the entire body changes 



Fig. 270. — Amceba, the protcus iininialfiilo. Greatly magnified, n, the 

 nucleus; wa\ water \'iicuoles ; c.o, contractile vacuoles; /.r, food vacu- 

 oles. E. B. Wilson, "The Cell." 



shape and the appendages can be retracted so that the whole 

 body assmnes a spherical mass. The simplest of the Rhi- 

 zopods is the Amoeba.^ This ty]:ie varies greatly in size, from 

 0.02 mm. to about 0.3 mm. It appears as a clear, glistening 

 body of changing outline. The l)ody, when viewed with a high 

 power, shows a complicated structure. It contains various 



1 Greek, prufco, earliest ; zuuii, animal. ~ amoihc, alteration. 



