CHAPTER 32 



PHYLOGENETIC CONSIDERATIONS 



Any explanation of the origin of groups of animals must of necessity- 

 be speculative. At best the geological history of any one group is frag- 

 mentary, and the living representatives are themselves highly modified 

 individuals who have solved the problem of existence in their own in- 

 dividualistic way. The only possible approach to this problem is the 

 examination of living representatives of the various phyla and then a 

 •careful consideration of their relationships. 



Origin of the Metazoa. — It is not at all illogical to assume that the 

 metazoans must have originated from some single-celled organism. From 

 this it is only reasonable to look at the protozoans for some evidence of 

 possible metazoan ancestry. 



Among the protozoans, the most varied and diverse class is that of 

 the flagellates. Within this class are holophytic, holozoic, and parasitic 

 forms, and also some which group together to form colonies. No other 

 class of the protozoans has members that show such plasticity. From this 

 fact it has generally been conceded that it was either this group or some 

 similar one from which not only the metazoans, but even the plant king- 

 dom took its origin. 



Many types of colonies are formed by the various species of flagel- 

 lates ; Goniiini colonies are flattened squares, Pandor'ma forrhs a sphere 

 of 16 closely packed cells which closely resemble certain cleavage stages, 

 Eudorina has 32 cells near the surface of a gelatinous sphere, Platydorina 

 is a fiat oval, jellylike mass with 32 cells and a scalloped posterior end, 

 and Volvox with its many cells is a hollow sphere not unlike the blastula 

 of metazoan embryology. With such evidence at hand, it is not impos- 

 sible to imagine the metazoans as arising from some such aggregation of 

 flagellated protozoans. 



The possible origin is this : a flagellated, spherical colony gradually 

 developed within which there was differentiation into somatic and repro- 



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