366 THE EVOLUTION OF THE METAZOA 



There Are No Well- Warranted Mesozoa 



Before we begin to deal with the taxon Metazoa it may 

 prove useful if we discuss briefly the problem whether the 

 taxon jMesozoa, which is frequently used by zoologists, be 

 justified or not. 



Without any intention to go here into details we wish to 

 state that w^e understand as "Mesozoa" only the Orthonecto- 

 idea (Orthonectida + Dicyemida). In all the other cases which 

 were also considered as Mesozoa it has been possible to 

 establish with considerable certainty that they were not real 

 Mesozoa s. l. We wdll therefore not discuss them in this 

 connection. So far the Mesozoa have been given various 

 taxonomic evaluations; as a maximum they have been consi- 

 dered as a sub -kingdom (cf. Zoological Names ^ etc., published 

 by AAAS, edited by A. S. Pearse, 1949); as a minimum they 

 have been classified as an appendage only of the Platyhel- 

 minthes (e.g. by Cuenot). Moreover they have also been evalu- 

 ated as a phylum (e.g. by L. H. Hyman, The Invertebrates^ 

 L, 1940) which is a middle-way found between the two 

 extremes. So far the whole problem has remained unsolved. 

 The most extreme solution can be found in Beklemischew 

 (1958) who simply does not mention the Mesozoa at all in his 

 systematic survey of the animal groups because of the un- 

 certainty which exists regarding their systematic position. 

 In this way he is not obliged to accept the auxiliary solution 

 and to represent the Mesozoa as a kind ot appendage. Bekle- 

 mischew's solution is quite naturally purely formalistic. We 

 must decide which interpretation appears at the present 

 moment (on the basis of our present knowledge) as the most 

 probable. 



The main problem is whether the simplicity of the body 

 structure which we find in these animals can be considered 

 as a primary phenomenon, or whether it is a result of a 

 secondary development which had taken place as a conse- 

 quence of their parasitism. It appears as much more probable 



