414 THE EVOLUTION OF THE METAZOA 



of the Polymeria in spite of their progressive evolution com- 

 bined with a strong specialization. 



The suggestion to use the Spiralia as a systematic notion 

 and the group of the Protostomia as a part of the same must be 

 resolutely rejected. Such a suggestion was made, for example 

 by Kaestner (1954) and by others. Such a group would 

 include besides some clear Articulata (Annelida, Echiuroidea) 

 also many groups of our Ameria, above all the MoUusca, 

 Nemertinea, and the Platyhelminthes. The spiral type of 

 cleavage appears in widely different animal groups and it is in 

 no way connected with the problem of the closer relationship. 

 It belongs exclusively into the sphere of rationalizations of the 

 mechanism of the early ontogenies. Or in other words, the 

 spiral type of cleavage can appear in all those cases where 

 similar conditions of the egg structure appear together with 

 the mechanism of cleavage which is connected with the 

 former. Attempts have also been made to justify the existence 

 of the Spiralia as a natural group by referring to the similarities 

 that can be observed in their larvae (Kaestner, 1954:13). 

 Such an argument has already been sufficiently discounted in 

 our present study. 



It would be in no way difficult to show the unacceptability 

 of the division of the groups which are covered by the notion 

 and the taxon Polymeria into several (six) phyla as we find 

 this in the Zoological Names (edited by A. S. Pearse). The 

 classification which comes closest to the system which we 

 consider as the best one is the system which was proposed by 

 Beklemischew. It has already been mentioned that his cha- 

 racterization of the w^hole high phylum Articulata (the seventh 

 high phylum, according to Beklemischew) corresponds exactly 

 in its sphere with our Polymeria. The main difference exists in 

 the fact that Beklemischew divides his high phylum into five 

 subphyla (AnneUdes, Tardigrada, Pentastomida, Onychophora, 

 Arthropoda) while we beHeve that a completely sufficient 

 subdivision can be made by means of two subphyla (Annelida, 

 Arthropoda). We consider that the three remaining groups 



