368 THE EVOLUTION OF THE METAZOA 



the Acoela had in all probability passed quickly through the 

 intermediate stages of evolution while there had been in all 

 probability no third type of evolution. It is therefore complete- 

 ly improbable that any corresponding transitional forms will 

 be found in the future. Even if such forms had existed in the 

 most remote past they have certainly not left any identifiable 

 traces of their existence. 



Otto Steinbock (in his report at the symposium held in 

 California, 1960) has recently made an interesting suggestion 

 of considering the Ciliata as actual Mesozoa. This suggestion 

 is in fact nothing more than an ingenious observation; the 

 actual Ciliata are in reality genuine even if complex Protozoa, 

 similarly as are, for example, the Nummulites, the Hyper- 

 mastigina, and the Opalinidae. The evolution of the Metazoa 

 had gone parallel to the evolution of these forms and there 

 are the hypothetical Proacoela only which could be considered 

 earnestly as "Mesozoa." 



The Classification of the Metazoa 



The Parat(pa as an Independent Suhregnum 



In spite of the fact that the evolution of the complex "poly- 

 cellular" forms had been so widely different in the plant and 

 animal worlds we find that this difference does not have an 

 absolute character. All the known facts if viewed objectively 

 disprove that the evolution of these complex forms— which 

 had made possible their differentiation and a larger size of their 

 bodies — had been monophyletic within the sphere of animals 

 and that it had been basically different from the evolution of 

 plants. There are the Spongiae which have been given such 

 wddely different scientific names and which have been found 

 to be problematic in several respects ; in a certain sense they 

 are more similar to plants than are the remaining polycellular 

 animals. Originally they were considered as Zoophyta; the 



