302 THE EVOLUTION OF THE METAZOA 



It certainly is welcome and useful for my concept when 

 Hanson called my attention to these forms of Euciliata which 

 show some combined primitive characteristics, provided that 

 the plurality of the two types of karyons be a primary pheno- 

 menon. This makes my thesis of the origin of the Acoela, 

 and thus of the Eumetazoa generally, from some infusoria- 

 like ancestors even more probable and more easily acceptable. 

 Yet at the same time I do not think that it is of much 

 importance. The two Holotricha are genuine Infusoria, and 

 not some transitional forms. They suit the scheme of the 

 Mosaic theory by w^ay of which the characteristics that can 

 be observed in one descendant can be attributed to different 

 ascendants so that these characteristics become united and 

 combined in a certain descendant which develops thus into 

 a new type. It cannot be understood how our concept which 

 derives the Acoela from the Infusoria could depend on this 

 addition which was made by Hanson. 



The second difficulty of the original concept which makes 

 its acceptance hard or even impossible was formulated by 

 Hanson in the following question, "Why is the macronucleus, 

 one of the most important of all ciliate characters, absent 

 in the Acoela?" (Hanson, 1958 : 33). This has in my opinion 

 not been the most fortunate formulation of this question; its 

 words should rather be, "what corresponds, what in the 

 Acoela is homologous to the macronucleus of the Eu- 

 ciliata?" I was not able to discuss this problem in detail in 

 my very short article w^hich appeared in the Systematic Zoolo^^ 

 yet even from this article, and even more so from my other 

 publications it appears clear that I consider the macronucleus 

 to be homologous to the somatic, or vegetative ("indifferent") 

 karyons. The macronucleus is therefore a result of a second- 

 ary oligomerization which succeeded during the evolution 

 of Infusoria, the polymerization that had taken place in the 

 Flagellata, the ascendants of Infusoria. The polymerization, 

 however, was continued in the evolution of the Acoela, it 

 was even further developed not only in the quantitative 



