32 THE EVOLUTION OF THE METAZOA 



has been able to prove not only that in pterocorals the distribu- 

 tion of septa is bilaterally symmetric, but also that even 

 the first anlage of the septal apparatus was bilateral ("the six 

 protosepta show a distinct bilaterally symmetric develop- 

 ment" Schindewolf, 1950, p. 179). Later, a secondary radial 

 (hexamerous) symmetry has been developed from the 

 former. It must be emphasized that according to Schinde- 

 wolf (1950, p. 181-2) "the Palaeozoic pterocorals form the 

 ancestral forms of the now living stone-corals. Together with 

 the latter they must be included in the order of Madreporaria 

 which had been previously limited to the recent forms only." 



The fact is established by Schindewolf that this orthogenetic 

 course of evolution which goes from an internal bilateral 

 symmetry to a full cyclomeric radial symmetry has not been 

 completely carried through even in the present-day Madre- 

 poraria. It will be later seen that this has been first achieved 

 in Scyphozoa, and particularly perfectly in Hydrozoa. Here 

 we meet with a phenomenon which is especially important 

 to a phylogenist: it enables him to make well-founded con- 

 clusions about to the interrelationships on the basis of a 

 comparison of recent forms only. 



Again it can be stated that fortunately enough for phylo- 

 genists, the phyletic evolution of animals did not take such 

 a course that it evolved along a single progressively advan- 

 cing front so that the present day fauna would consist of the 

 most advanced vertebrata only, actually of only one 

 species: man. Even now we can find, so to say, in one and 

 the same pool, various forms living together: Flagellata, amoe- 

 bae, Turbellaria, snails, Annelida, insects, Bryozoa, fishes, 

 frogs, etc. Thus we have old types which themselves have 

 undergone further development and which in this way have 

 become more differentiated, together with new types which 

 are constantly emerging yet which are not necessarily always 

 the more progressive forms. The primitive characteristics have 

 not been preserved in the older types of structures only, but 

 also in structures of some younger forms. These latter 



