NEW INTERPRETATION OF CNIDARIA 269 



those of an actually existing species (the so-called Cydippe- 

 stage). Some Ctenophora, however, have become specialized 

 and they have descended to a life in the benthos. It is in 

 connection with these species and subgroups that the earlier 

 Cydippe stage has taken over the role of a true larva which 

 must even go through a metamorphosis during its subsequent 

 development. In this case, we can rightly speak of a recapitu- 

 lation; yet this is a recapitulation of a transitional ontogenetic 

 stage which is very similar to— even if not exactly the same 

 as, a species in its grown-up state. 



It should be mentioned, for comparison, that no larval form 

 had been developed by the Chaetognatha that had also evolved 

 by way of neoteny from the planktonic larvae of Brachiopoda 

 (Hadzi, 1959 a, 1959 d). No typical common transitional 

 stage of Chaetognatha has become known either. The pos- 

 sibility cannot be excluded, however, that in future a common 

 ontogenetic transitional stage of Chaetognatha will be found 

 after a detailed study of their ontogenies. 



The Origin of Turbellaria 



We will return now to our main problem. We have refused 

 to accept the attempt made by Jagersten who has tried to sup- 

 port the enterocoele theory by means of a supposed initial form 

 which he constructed ad hoc and which he called a bilatero- 

 gastraea. He equipped this form with all those elements which 

 could prove useful in his subsequent derivation of the Metazoa 

 from it. Our new task will be to show or, rather, to find the 

 most probable place where the origin of Turbellaria, and thus 

 also of all Eumetazoa, has to be sought. I must state at the 

 very beginning that no well-founded reason is known to me 

 which could justify considering Turbellaria as a product of a 

 retrogressive evolution. I share this opinion with the leading 

 experts in Turbellaria and I refer here among others to Otto 

 Steinbock (1958). The same standpoint regarding the Turbellaria 



