338 THE EVOLUTION OF THE METAZOA 



of Cnidaria. The Cnidaria had evolved from the rhabdocoeloid 

 Turbellaria, and the Ctenophora by way of neoteny from the 

 planktonic larvae of Polycladida. 



It has been found that it has become necessary to submit 

 the phylogenetic relationships of other animal groups to a 

 critical revision in spite of the fact that all the findings made 

 by me and the conclusions reached on the basis of these fin- 

 dings have been limited above all to the lowest groups of 

 the Metazoa. This revision must be made from the same 

 standpoint and under the application of the same working 

 methods that have led me to a necessary reform of the 

 interpretation of the evolution of Metazoa from Protozoa, 

 and especially of the phylogenetic evolution of the Cnidaria. 

 Thus a revision has become necessary of the genealogic tree 

 and of the graphic representation of the phylogenetic con- 

 nections of the large groups, as well as of the systematic- 

 taxonomic classification. 



First I will make a brief explanation of those principles 

 which I used in my work on a reform of the animal system. 



The comparative morphology of recent animals and of those 

 animals that have been preserved as fossils was, is, and remains 

 the main method that can be used in our work when we 

 wish to construct a natural animal system and above all the 

 genealogical tree of the animal world. It can be expected that 

 in the future biochemistry will also make essential contribu- 

 tions to this field. All the facts should be taken into considera- 

 tion by the comparative morphology, both those that can be 

 observed by w^ay of a general and of a detailed analysis and 

 comparison of the adult forms, and of all the other stages 

 of development, beginning with the gametes; while at the same 

 time we must also take into account the physiological and 

 ecological moments and the changes of the environment. In 

 our morphological comparisons we pay special attention to 

 the adult stage, and only in the second line to the ontogenetic 

 stages; we have been led to this both by the experiences we 

 have had so far and by the theoretical conclusions. 



