CNIDARIA AS THE ONLY COELENTERATA 19' 



so-called gastrovascular system). I consider that I am justified 

 in deciding my interpretation as the more probable. 



Many other arguments and evidence could be given to sup- 

 port the thesis that the Ctenophora cannot be directly related 

 to Cnidaria. The ontogenies of the two groups in particular 

 are completely different from each other. It may be useful to 

 mention the so-called ciliary rosettes in Ctenophora for which 

 there is no homologue in Cnidaria. I think that these ciliary 

 rosettes developed out of the terminal organs of the proto- 

 nephridial system in Turbellaria. The examples which have 

 been given so far should suffice to prove that there are impor- 

 tant differences between Ctenophora and Cnidaria; yet at the 

 same time there are also some similarities which are such that 

 they make it probable that the two groups did not develop 

 one from the other, but instead independently from two an- 

 cestors that had been closely related to each other and that had 

 evolved from the same primitive form. This interpretation 

 could perhaps be also expressed with the dictum: si duo facuint 

 idem—non est idem. In both of them the evolution took a new 

 direction: it was the same in both cases and it tried to adopt 

 the characteristics of animals Uving a pure planktonic Hfe. In 

 the case of medusae, the evolution proceeded from a bilateral 

 symmetric and mobile stage, through a second stage which 

 was radial symmetric and sessile, to a third stage which was 

 radial symmetric and planktonic. In Ctenophora the develop- 

 ment went from a bilateral symmetric and mobile stage through 

 that of a planktonic larva influenced by the neoteny into the 

 final planktonic stage. In both cases the ancestors were bilateral 

 symmetric Turbellaria, but they belonged to two different sub- 

 groups (see Fig. 5). 



To the best of my knowledge no hypothesis that is more 

 probable, and therefore better, has been proposed to explain 

 the origin of Ctenophora which takes into consideration so 

 many of the numerous peculiarities of this small and now 

 isolated animal group, and which would make it possible to 

 understand them. Yet even if we do not bother about this 



