CHAPTER III 

 INTRODUCTION TO THE METAZOA 



In the foregoing chapter, attention has been called to the tend- 

 encies which some Protozoa display toward the specialization 

 of their cells. Most of this specialization is expressed in increased 

 complexity of intracellular organization. In some instances, 

 however, groups of cells, termed colonies, act as the unit or indi- 

 vidual. Within colonies some of the cells frequently become 

 specialized as gametes for reproductive purposes but all of the 

 somatic cells remain alike. In the Protozoa, any specialization 

 of somatic cells affects all equally, so there is no differentiation. 



It seems probable that the Metazoa must, at some time, have 

 originated from protozoan ancestors, but all evidences of the 

 means of this transition have been lost and both Protozoa and 

 Metazoa have gone far along independent lines of evolution 

 since the Metazoa came into existence. The lack of fossil forms 

 bridging this gap is not surprising, for both the Mastigophora 

 and the Ciliata, through the ancestors of which this genesis 

 might have taken place, have no hard parts which could be 

 preserved as fossils. 



There are a few present-day animals which have been termed 

 the Mesozoa because they have been considered as intermediate 

 between Protozoa and Metazoa, but their simplicity is probably 

 due to degeneracy, as is shown in the section of this chapter 

 devoted to them. 



Embryology frequently offers a clue to and outlines the steps 

 in phylogeny (recapitulation theory). All Metazoa, in their 

 development, typically pass through a single-celled stage — the 

 fertilized egg. This fact readily furnishes the basis for an analogy 

 of metazoan origin from the single-celled organisms. In the 

 development of the fertilized egg into the adult organism, there 

 is a time when large numbers of fundamentally similar cells (the 

 blastomeres) are produced through the partition of the single 

 cell. It is only in later development that these cells through 

 histological differentiation assume different forms and come to 



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