THE NEW GENEALOGICAL TREE 421 



of modified trochophores; this, however, does not have 

 any sense if viewed from the phylogenetic point of view. 

 Greater differences can be observed during the embryonic 

 development. 



The most important conclusion we can reach on the basis 

 of our present discussion is that the polymerous state of the 

 perigastrocoele must be considered as a primary, primitive 

 phenomenon, and that the oligomerous state had only evolved 

 secondarily due to a reduction of the segmentation. This 

 reduction had been caused by a changed way of life. This de- 

 velopment had finally led to a complete uniformity of the 

 perigastrocoele. 



The Phylum Oligomeria and Its Classes 



We come now to a point of the construction of the natural 

 animal system and the formation of the phylogenetic tree. 

 This is considered quite generally as disputable but it 

 can easily be solved it viewed from our standpoint. All zoolo- 

 gists agree that the Arthropoda represent a climax in a gene- 

 rally progressive evolution of the animal w^orld and that for 

 this reason no further evolution had been possible along this 

 line. We must therefore presuppose that the evolution which 

 led to the emergence of the Chordoma and of the Ver- 

 tebrata, had started from a lower level than that reached by the 

 Arthropoda. It has already been indicated how various zoolo- 

 gists believed they found the point of separation of 

 the two lines of evolution in various parts of the genealogical 

 tree. In the scheme of the genealogical tree we can find this 

 evolution represented by two strong branches, the protostomic 

 and the deuterostomic branches. As a typical example 

 we can mention here the genealogical tree as proposed 

 by Cuenot (see Fig. 50). Without any intention to deal 

 here in detail with all the attempts that have been made so far 

 to represent the phyletic relationship between the Protostomia 

 28 



