112 PROTOZOA THE SIMPLEST ANIMALS. 



several colonial forms ; in these the individuals are united by 

 their extra-capsular protoplasm, but are all equivalent. In 

 Proterospongia the cells show considerable morphological 

 distinctiveness ; some are flagellate, some amceboid, some 

 encysted and spore - forming. Again, in Volvox, as we 

 noticed above, the cells of the colonies show a distinction 

 into nutritive and reproductive units. 



Lastly, in their antithesis of passivity and activity, con- 

 structive and destructive preponderance, anabolism and 

 katabolism, the Protozoa illustrate the phases of the cell- 

 cycle, and so furnish a key to the variation of higher 

 animals. 



