CHAPTER XIII 



THE ORIGIN AND CHARACTERISTICS OF THE 

 METAZOA 



ORIGIN OF MANY-CELLED ANIMALS 



There is every reason to believe that Metazoa arose from 

 Protozoa; i.e., that single-celled animals gave rise to many- 

 celled animals. There is no fossil record of such a trans- 

 formation because the earliest geological strata on the earth 

 have all been modified so that their records are destroyed. 

 The first records show single-celled animals living with 

 many-celled types, as they do today. Yet the develop- 

 ment of every metazoan starts with a single cell and the 

 law of biogenesis (Chapter V), therefore, indicates that the 

 origin of Metazoa was from single cells. Furthermore, 

 all the processes that take place in the development of a 

 metazoan, up to the formation of tissues, are duplicated 

 in protozoans. There is a general direction which all cell 

 associations take, and metazoans have simply gone farther 

 than protozoans. 



With these things in mind, it is possible to give a synop- 

 sis of the probable steps in the origin of many-celled animals. 

 First, groups of cells arose as a result of the ordinary multi- 

 plicative processes in protozoans binary fission. The cells 

 after dividing did not separate but remained united. After 

 a time such behavior became the regular procedure in cer- 

 tain strains of protozoans and colonies of cells always re- 

 sulted from cell-division. This gave opportunity for the 

 next step division of labor between the cells. Some were 

 set aside to carry on cell multiplication and reproduction, 

 others looked after the nutritive and protective functions. 

 This specialization was a convenience for the whole colony, 

 for each individual cell had fewer things to attend to. This 



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