452 THE EVOLUTION OF THE METAZOA 



The planktonic larvae had in all probability again been 

 developed after the animal had returned to a life in the sea. 

 Yet this planktonic phase of the ontogeny now reached 

 a much higher level of organization, and we are therefore 

 fully justified to speak here of a secondary larval form. Such 

 a larva shows the structure of a chordate, and not of an 

 amerous animal as is the case with the trochophores and 

 with the dipleurulae. It is wrong to see in the primary larvae 

 (the various trochophores and dipleurulae) a recapitulation 

 of an adult ancestral form; the same is also true for those 

 larvae of the Chordata that can be justly called tadpoles. 

 Viewed from this standpoint we must reject any attempt (e.g. 

 the attempt made by Berrill) to derive the Chordoma from 

 some kind of planktonic larvae. The evolution of the Larvacea 

 from the "tadpole" of the Ascidia belongs as a clear case 

 of neoteny in a completely different chapter. 



A Comparison of the present Attempt to Derive the 



Chordoma from an Ancestral Form Resembling the 



Present Day Enteropneusta with the Similar Attempt 



Made by Hans Steiner 



Hans Steiner has also tried to derive the Chordoma from 

 the Enteropneusta in a very interesting study, Geschichte der 

 Initialgestaltung der Chordaten (1956) where we can find numerous 

 correct observations. In this study he takes into consideration 

 the comparative morphology and the way of life of these 

 animals. As regards the derivation itself there is a considerable 

 agreement between his and my interpretation; and in this 

 point Steiner therefore does not show in principle anything 

 new. There is, on the other hand, a great difference in the 

 way we interpret the evolution which led to the emergence 

 of the Echinodermata and of the Enteropneusta. Steiner tries 

 to suggest something which I must consider as completely 

 impossible, he correctly rejects the opinions proposed by 



