210 THE EVOLUTION OF THE METAZOA 



the polyp generation has been abandoned as completely 

 as in the hypogenetic Scyphozoa. The larval stage has 

 been prolonged (it is only externally polypoid) by means of 

 a transition to a temporarily parasitic way of life, it has been 

 more strongly developed, and it has even become able to 

 reproduce asexually. 



It should be mentioned in passing that it has not been 

 proved that the planula either of Scyphozoa or of Hydrozoa 

 has evolved into the adult form of a medusa by way of a 

 prolonged life in plankton. We always have a case of a secon- 

 dary direct change due to the abolition of the benthonic 

 polyp generation wherever a planula has actually been changed 

 directly into a medusa, a fact which can always be proved if 

 we make comparisons with the closely related species. All 

 hypotheses which suggest that the medusa form, or that the 

 planktonic planula or actinula represent the primary form in 

 the phylogeny of Cnidaria must therefore be rejected. 



We can state, w^hen w^e now^ compare the ontogenetic 

 development of Cnidaria with those of Turbellaria, that little 

 actual material is available especially about the Acoela. 

 Furthermore, we can see (as was also the case with the genital 

 apparatus) that the ontogenetic morphogeneses show a strong 

 inclination to a specialization which has reached its climax in 

 groups whose eggs represent extreme cases of exolecithality. 

 On the other hand, a trend of evolution can be observed to- 

 wards a strictly determined and differentiated type of develop- 

 ment that can be found as early as among the Acoela (e.g. 

 in the species of the genus Convoluta). This trend, combined 

 with the formation of a duet (in Acoela) and of a quartet 

 (in higher Turbellaria), as well as the formation of an Anlage 

 of a middle body layer which is closely connected with the 

 Anlage of the intestine from a strictly fixed blastomere (4 d) , 

 had been inherited by Ctenophora by way of Polycladida. 

 Here already the mesohyl is formed during the ontogeny out 

 of two sources, evidently a contrivance of the evolutional 

 technique. 



