EXISTING ORGANISMS— CLASSIFICATION 



33 



[uplotes 



Spirostomum 



lionotus 



Fig. 14. — Some common fresh water protozoa, greatly magnified. (From 



Woodruff, after Curtis.) 



the cells of the different layers carry on different functions, and 

 this structure is simpler than one in which a third layer is differ- 

 entiated. 



On this basis we are able to group the fourteen phyla. Among 



