38 THE EVOLUTION OF THE METAZOA 



been discovered in this way have had an enduring value 

 in spite of the fact that they have been arrived at by way 

 of some wrong initial presumptions. 



It was soon found that the larval stages, as well as complete 

 ontogenies, proved useful for the construction of the natural 

 system even if one did not employ the "fundamental bioge- 

 netic law;" it is only necessary to change the basis of such 

 comparisons and to make these comparisons exclusively 

 betW'een various stages of development, even if the results 

 reached in this way seem to be less important and certain. 

 We can now state— and this with considerable certainty— that 

 in a comparison of ontogenetic stages the chances of suc- 

 cess are the greater, when the older stages are compared. 

 It must be initially expected that in comparison to the 

 primitive forms, the greatest changes will be observed in 

 the youngest or initial stages of the ontogeny, since these 

 are not only phyletically the oldest but also in their organi- 

 zation, the simplest stages. Here Caenogeneses may often take 

 place. This is particularly true in the case of those animals 

 whose development occurs from the very beginning in free 

 water. It is therefore better to omit the early phases of 

 ontogenesis from phyletic constructions, or, as is fre- 

 quently heard, from our speculations. 



Let us bypass the intermediate ontogenetic phases and 

 discuss the phyletically oldest larvae. The planula of Cnidaria 

 has generally been considered to be such a larva partly because 

 the Cnidaria are believed to be the oldest Eumetazoa and 

 partly because the planula has the simplest structure, i.e. it 

 corresponds to HaeckePs gastraea. It consists of two layers 

 only and it has neither an intestine nor an oral opening. The 

 next form which follows the planula is the famous trocho- 

 phore. This development frequently takes place by w^ay of 

 an intermediate form of protrochophora. 



Because of its simplicity the planula has given rise to 

 conjectures regarding the primitive form of all Metazoa. The 

 planula has generally been considered as a very primitive 



