PREVIOUS INTERPRETATIONS OF CNIDARIA 185 



from some sessile ancestors; this pentamerous symmetry, how- 

 ever, is not a property which occurs 100 per cent in all Echino- 

 dermata; neither is the tetramerous radial symmetry such a 

 property in Cnidaria. 



Among the recent Cnidaria the tetramerous radial symmetry 

 can be found especially strongly developed in Scyphozoa and 

 in Hydrozoa, both in their polyp as well as in their medusa 

 forms. It is least developed in Anthozoa. This could lead us 

 to a conclusion, if we accept the view that the evolution took 

 the course Anthozoa -> Scyphozoa — > Hydrozoa, that this in- 

 clination to the tetramerous radial symmetry had not been 

 developed earlier than during the phylogeny of Cnidaria. On 

 the other hand, if we consider the opposite interpretation to 

 be true we are obliged to accept that in Hydrozoa the tetra- 

 radial symmetry had been developed for an unknown reason, 

 and that it had been inherited from them by Scyphozoa. We 

 will not find this problem so simple if we accept the 

 interpretation which sees in Anthozoa the most primitive 

 Cnidaria, yet the problem is still such that it can be rationally 

 solved. 



No recent subgroup of Anthozoa with a tetramerous radial 

 symmetry has been known. Neither can such a group be found 

 among the fossil species, even if one large subgroup which 

 is now extinct bears the name Tetracorallia or Rugosa (palaeo- 

 zoologists prefer now to call this subgroup Pterocorallia). 

 The number four does not refer here to the individual 

 scelerosepta, but rather to their groups (see Fig. 10). All this 

 is due to the fact that those forms only have been preserved 

 as fossils whose bodies had already reached a considerable 

 degree of specialization and a well-developed set of 

 sclerosepta. This state (which was reached during the Ordo- 

 vician) was preceded by a long evolution, when anthopolyps 

 did not yet possess a calcareous skeleton and when in all 

 probability they still continued to live as mainly solitary ani- 

 mals. Stages can be observed in the ontogenies of both extinct 

 as well as of the still living Anthozoa which are equipped 



