152 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



beginnings of maturation, as shown in the formation of 

 polar nuclei in some Heliozoa, Sporozoa, Flagellata, and 

 Lobosa ; {c) the beginnings of sex, in the difference of size 

 and of constitution sometimes observed between two con- 

 jugating units (e.g. in Coccidium) ; {d) the beginnings of 

 many-celled animals, in the associated groups or colonies 

 which occur in several of the Protozoan classes. These 

 colonies show a gradation in complexity. Raphidiophrys 

 and other Heliozoa form loose colonies, which arise by the 

 close coherence of the products of fission. Among the 

 Radiolarians there are 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 repro- 

 ductive 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. 



