PROTOZOA. 45 



nuclei with most of the intersubstance have disappeared. 

 A difference in the size of the nuclei is apparent. This 

 difference is still more plainly seen in the later stage (fig. 

 8c) in which scarcely a trace of the intersubstance 

 remains. Finally the formation of anisospores takes 

 place. The clusters of fat fall apart into granules and 

 each spherical mass divides into as many wedge-shaped 

 pieces as there are nuclei (fig. 8d). These ultimately 

 fall apart when the large macrospores (fig. 2) and small 

 microspores (fig. 3) appear. Fig. 10 is a mature (usu- 

 ally called " old vegetative ") colony in which the flagel- 

 late young are ready to be set free by the disintegration 

 of the colony. It is distinguished from the young colony 

 (fig. 4) by its plain articulations. 



According to Haeckel the same Polycyttaria or colonial 

 Radiolaria which produce anisospores also produce, at 

 other times, the asexual isospores, so that it would seem 

 that these two forms of reproduction alternate with each 

 other, and if so, we have here in the simplest subkingdom 

 of animals the phenomenon of alternation of generations. 

 This variability in function as well as variability in struc- 

 ture is just what one would expect to find among organ- 

 isms which have not yet learned the ways of their more 

 specialized and therefore more stable descendants. 



MASTIGOPHORA. 



We now come to a group whose peculiar and more or 

 less constant characteristic is the possession of a whip or 

 flagellum, and which for this reason is known by some as 

 the Mastigophora and by others as the Flagellata. We 

 have found the flagellum among the Rhizopoda, Heliozoa, 

 and Radiolaria ; wherever, in fact, there has been need for 

 rapid motion it seems to have been developed as an adap- 

 tive character. In the typical Mastigophora it seems to 

 have become fixed in the organization and therefore an 



