324 THE EVOLUTION OF THE METAZOA 



It has later been found that there are no primary two- 

 layered animals, except some forms which show traces of a 

 strong retrogression due to their parasitic way of life. Haeckel 

 himself showed a considerable uncertainty on this point; he 

 wavered when he tried to find the actual division between the 

 two -layered and the three -layered animals (the Coelenterata 

 and the Coelomata, or the Diploblastica and the Triploblastica); 

 he even suggested once that the Platyhelminthes should be 

 classified among the Coelenterata. These animals had not 

 evolved a genuine middle layer, an actual mesoderm, but only 

 the so-called mesenchyme or the mesogloea. 



Finally, it was established that a majority of Coelenterata 

 posses a middle (the third) body layer and that they really repre- 

 sent a very motley group. Platyhelminthes were the first to have 

 definitely left this group; they were followed by Spongiae 

 (this, however, has not been accepted by all zoologists). 

 The Cnidaria and the Acnidaria (=Ctenophora) have thus 

 remained the last two groups which have still continued to be 

 classified as Coelenterata. Yet even in connection with the 

 Ctenophora it has become more and more clear that they do 

 not really belong here. The Cnidaria have thus been the last 

 group which have still been considered as Coelenterata, yet 

 even with these it has been found that they possess the third 

 (the middle) body layer even if it is only slightly developed. 

 They are therefore no real Diploblastica either. 



Evolution finally led, according to this interpretation, to the 

 formation of the third layer. The difficulties which emerge 

 together with it are even greater. The third body layer (actually 

 its solid part only because the sparse mesenchyme is usually not 

 considered as a part of the actual mesoderm) develops during 

 the ontogenies (which should serve as patterns for the phylo- 

 genetic conclusions) in widely different w^ays. Two such 

 developments are particularly conspicuous because they gene- 

 rally even if not completely, exclude each other. In one case 

 the mesoderm is developed from the ectoderm (the ecto- 

 mesoderm), or, if we are more precise, from the transitional 



