274 THE EVOLUTION OF THE METAZOA 



jectures. We must be content with the degrees of probability, 

 and we have no right to declare any attempt to represent or 

 reconstruct another path of evolution as impossible. 



We could make our task very easy if we accepted the inter- 

 pretation that was proposed by v. GraflF, and, in a somewhat 

 modernized forrn, by Beklemischev. According to this inter- 

 pretation a planuloid being which resembled a gastrula (Grob- 

 ben) has been the starting point for the, evolution of Turbel- 

 laria (and at the same time of Coelenterata and of Eumetazoa 

 generally). Beklemischev (1958 : 109) modified this widely 

 accepted interpretation stating that the Turbellaria had deve- 

 loped "von einem Zweig der Urcoelenteraten, die durch Neo- 

 tenie im Planulastadium geschlechtsreif vvurden." I consider 

 such a deduction as completely improbable and, moreover, as 

 a purely auxiliary solution. Those who accept this inter- 

 pretation like to refer to larvae as recapitulations of their adult 

 ancestors. The planula appears in reality among the Cnidaria 

 as a larval form without being at the same time a recapitulation 

 of some ancestral form, as it has been already stated. The 

 latter, however, could not at all be found among the Turbella- 

 ria, and especially not among the Acoela where one would 

 expect above all such a recapitulating planula. Yet even if it 

 had been found, this could not be considered as a proof for a 

 planuloid origin ot the Acoela, and thus of Turbellaria. Matters 

 being such as they are, we have no real basis of deriving the 

 Acoela from some planuloid ancestors. 



G. S. Garter (1954), who agrees with several points of 

 my interpretation, considers that I underrate the facts of 

 ontogeny and that in this I make a mistake. It is true that in my 

 opinion we must place more emphasis in our phylogenetic 

 conclusions and constructions on the conditions of com- 

 parative morphology of the adult forms, than on the data that 

 can be obtained by means of a comparative ontogenetics 

 (embryology). I especially insist that we must be more than 

 careful in our evaluations of the freely living larval forms. I 

 support my view not only with the well-founded abrogation 



