NEW INTERPRETATION OF CNIDARIA 271 



had already been developed. In any case Remane, too, sees in 

 Turbellaria a product of a retrogressive evolution (admitting 

 at the same time that some properties of Turbellarians also be 

 a result of a progressive development above all their complex 

 genital apparatus). Remane consequently does not attribute 

 any great phyletic significance to Turbellaria. The Turbellaria 

 must be content to represent a small side twig on the mighty 

 branch of Protostomia. 



Let us now pass over to the next problem. What is the situa- 

 tion regarding the line of evolution within the group of Tur- 

 bellaria itself? Do the Acoela represent the starting point of 

 the evolution ot Turbellaria, are they not only the simplest but 

 also the most primitive Turbellaria, or perhaps the most highly 

 developed Eucoela? We consider it as certain (the same opinion 

 is shared also by Steinbock, Hanson, Beklemischev, and the 

 majority of other zoologists, beginning with v. Graff) that the 

 Acoela do not only have the simplest structure or organi- 

 zation of all the Turbellaria that we know ; they are also prima- 

 rily the most primitive Turbellaria. If we disregard here all the 

 problems regarding the origin of Turbellaria, and if we ob- 

 serve Turbellaria as a whole as an independent animal group, 

 we cannot find a reason why we should not place the Acoela 

 at the beginning of a natural system of Turbellaria. The inter- 

 pretation which sees in a simple phenomenon a primitive ele- 

 ment also, is certainly justified here, even if it is not justified 

 in a number of other cases: a fact which I am well aware 

 of and which is repeated unnecessarily by competent 

 zoologists in their criticisms of my thesis. In spite of the fact 

 that the Acoela lead the same way of life as the remaining 

 typical Turbellaria nevertheless their organisation is far behind 

 the others. At the same time a clear trend towards a progressive 

 evolution can be observed in the Acoela. Beklemischev has 

 already shown this trend as particularly evident in their 

 hermaphroditic genital apparatus. We wiU presuppose all this 

 as a well-known fact and limit ourselves to the statement that 

 in reality there is no justification to consider the Acoela as 



