416 THE EVOLUTION OF THE METAZOA 



intermediate categories in our system of the Arthropoda such 

 as the superclasses. All the recent Arthropoda can therefore 

 be subdivided directly into the corresponding classes. The 

 number of these classes varies, if we take into consideration 

 the recent animals only, between thirteen (e.g. according to 

 Comstock), eleven (e.g. according to Beklemischew), and 

 five. I believe that five classes are sufficient because there are 

 many suggested classes here which can be easily considered 

 as subclasses. These five classes are, the Onychophora (with 

 the Protracheata and the Tardigrada), Crustacea, Chelicerata 

 (among these are also the Xiphosura and the Pentastomida), 

 Myriapoda (here are also included the Symphyla, Pauropoda, 

 Diplopoda, and the Chilopoda with the old root of the Hexa- 

 poda), and the Insecta (with the two subclasses, Apterygota 

 and Pterygota). 



We can easily see how well-founded is a reduction of the 

 number of classes if we study the classification as it was pro- 

 posed by Comstock who suggested that there should be thirteen 

 classes of the Arthropoda (yet without the Trilobita; his 

 classification goes back to the year 1925!). He considers the 

 Onychophora and the Tardigrada as two separate classes; 

 separate classes are also the four groups of the Myriapoda 

 (this can be found quite frequently even in modern zoologists 

 because of the differences which exist in the relationship 

 connections between the four groups). The Palaeostraca 

 CLimulus), Pycnogonida, and the Pentastomida are also con- 

 sidered as independent classes which in reality must be 

 grouped together with the Arachnida into the class Cheli- 

 cerata. Comstock separates, quite unnecessarily, a part of the 

 Apterygota as Myrientomata from the remaining insects as 

 Hexapoda, a classification which is no longer in use. In this 

 way we find that five classes sufficiently represent all the ma- 

 jor subdivisions of the Arthropoda. I will not discuss 

 here the still open problem of how the Arthropoda had 

 actually evolved from the Chaetopoda, and how the present 

 classes evolved from the primitive Arthropod, because I 



