26 THE EVOLUTION OF THE META20A 



considered— as it has been already mentioned— to have evolv- 

 ed from the Cnidaria, and not directly from the blastea. The 

 Tubellaria, finally, have been thought to have developed from 

 the Ctenophora. The main idea has been to derive the lowest 

 Metazoa from a ciliated gastrula-like primitive form, which 

 itself has been supposed to go back to an organism resembling 

 a blastula, and this finally to a colony of Flagellata. Even 

 nowadays this same theory is proposed w4th an admirable 

 ardour by A. A. Sachwatkin (1956) on the basis of Metschni- 

 koff's parenchymella theory. 



Since in a classification which should be natural, i.e. phyloge- 

 netically founded, a similar or even identical level of organiza- 

 tion cannot be considered sufficient to bring together several 

 groups into a higher unit ; it is also necessary to prove that 

 thev have descended from the same ancestral form. In the case 

 of the Coelenterata it has been attempted to prove again and 

 again— or at least to make it seem probable— that they have a 

 common origin. These attempts, however, cannot be conside- 

 red to have been successful. The completely artificial construc- 

 tions such as those proposed by Remane (1950) and by those 

 who followed him, e.g. by Alvarado and Jagersten— to 

 mention three recent attempts only— have been completely 

 unconvincing. 



The isolation of Cnidaria alone, after the withdrawal of 

 Spongiae and Ctenophora, would not suffice to cause the 

 abandonment of the higher category of Coelenterata. It 

 would now be necessary to prove that there is a basic difference 

 between Cnidaria and the animal groups that stand closest 

 to them, and, then, all other animal groups (the so-called 

 Coelomata, or, according to Ray Lankester, Coelocoela), as 

 has been the case with Spongiae. This difference was be- 

 lieved to have been found in the fact that Cnidaria are 

 real "Gastraeadae," i.e. animals with their body built of two 

 body layers. For this reason they have been called Diblastica, 

 Didermique, etc. Furthermore, and in connection with this, 

 the Cnidaria have no coelom cavities and in particular no 



