386 THE EVOLUTION OF THE METAZOA 



yVn entirely different situation emerges if we view this 

 problem from the standpoint of the polykaryonic theory, i.e. 

 from the point of view which derives the Eumetazoa (the 

 primitive Turbellaria) from the primitive Ciliata. All the 

 "later" body cavities, the intestinal cavity included, had 

 been developed, more or less clearly, in their initial stages 

 as early as in the Ciliata, and that in the form of some 

 permanent or transitory vacuoles, slits, or similar spaces 

 that had appeared in their cytoplasm and that were filled with 

 the cell fluid. They have not appeared anew in the Eumetazoa, 

 they had only evolved, and differentiated in various ways. If we 

 understand as a coelom s.l. all the body cavities, we can consider 

 that all the Eumetazoa are actually Coelomata, at least primari- 

 ly, because many coeloms or even all of them can get lost 

 as a result of a secondary development. This is true both 

 if we accept the one or the other theory about the origin 

 of the Eumetazoa, because the coelenteron of the Coelenterata 

 could be considered as the first stage only of the enterocoelic 

 state of the "genuine" Coelomata. 



We can therefore see that those who adhere to the entero- 

 coelic version must necessarily meet with great difficulties as 

 soon as they try to propose a major division of the Eumetazoa. 

 They must propose such large taxons as the Acoela and the 

 Pseudocoela, because there are no Eumetazoa earlier than the 

 Annelida which had developed a coelom s. str.^ (a perigastro- 

 coele). At the same time they are uncertain w^hether the mor- 

 phological conditions that correspond to these categories, such 

 as the absence of an actual coelom, be a primary or a second- 

 ary phenomenon. This has been the reason why there are so 

 many zoologists who would like to explain all the Eumetazoa 

 of the Bilateria group, which show a simpler structure, as 

 a product of a retrogressive evolution, an interpretation which 

 has not been supported by any convincing arguments. 



When we see that there are such widely different body cavi- 

 ties and systems of cavities — which are different not only in 

 their forms but also in the ways that they are developed 



