^4 THE EVOLUTION OF THE META20A 



is not without some fantasy (cf. the "Hypothetical Diagram" 

 in vol. I., p. 38 of Hyman's work). The reader should also 

 then compare how I explain and understand the origin of 

 bisymmetry in Ctenophora and how this is done by 

 Hyman; she simply writes, "The biradial symmetry of cteno- 

 phores is obviously derivative of tetramerous radial symmetry, 

 as seen in Hydrozoan medusae." I should only like to ask 

 why the bisymmetry should be developed from a radial 

 symmetry during the life of the animal in the plankton? 

 We must be aware of the fact that in Ctenophora the paired 

 system can be observed not only in their tentacles. 



Here it should be emphasized that no facts are available — 

 as far as I can see them— which disprove the validity of my 

 hypothesis. For some time it seemed that there was an essential 

 difference between Ctenophora and Turbellaria; this difference 

 was thought to be found in the ontogenetic development of 

 the mesoderm (mesohyl). Ctenophora have, according to 

 Metschnikoff, a "genuine mesoderm," such as it also occurs 

 in Turbellaria, particularly in Polycladida among the latter. 

 Hatschek, however, who later investigated the development 

 •of Ctenophora maintained that Metschnikoff had made a 

 mistake and that those groups of cells which had been consi- 

 dered by Metschnikoff to be an Anlage of a genuine mesoderm 

 belonged in reality to the entoderm. This study by Hatschek 

 which had been completed and illustrated was unfortunately 

 never published. The two authors of the standard work on 

 the ontogeny of invertebrates (Korschelt and Heider, 1936. 

 p. 265) were able to see Hatschek's pictures and they were 

 convinced that his explanations were right. In this they have 

 been followed by Hyman. If no further progress had later 

 been made in this field then the close connection between 

 Ctenophora and Cnidaria would have seemed to be a firmly 

 established fact. I selected this theme for a doctoral thesis 

 to be investigated by one of my students, Peter Us; he made 

 a new investigation of this point in the ontogeny of Cteno- 

 phora. This investigation was constantly made under my 



