228 THE EVOLUTION OF THE METAZOA 



In this way only Cnidaria remain, as the last of the Mohicans, 

 in the large group of Coelenterata. As a matter of fact, we 

 can even now find books on zoology (cf. D'Ancona, 'Let^ioni 

 diBiologia eZoologia Generale, 1939) where Cnidaria only appear 

 under the heading of Coelenterata. Nevertheless, they have left 

 an impoverished Coelenterata in the old place of the natural 

 system. 



Important problems arise as soon as we place Hydrozoa 

 with their secondarily solitary and simplified species at the end 

 of the system of Cnidaria and when we put the primarily soli- 

 tary Actiniaria in their stead; such a problem is, for example, 

 the question of how to connect Actiniaria "downwards," or 

 where to derive them from? Can we still continue to use the 

 gastraea theory which is irrevocably connected with the hy- 

 pothesis that Coelenterata consist of two body layers? Or 

 should we rather take into consideration the next "lower" 

 Eumetazoa, the Turbellaria, which scholars have tried again 

 and again somehow to connect with Coelenterata, usually in 

 the same way as they have tried to derive Turbellaria from 

 Coelenterata. If, however, we accept the opposite thesis, w^hich 

 seems a priori to be the more probable one, that Cnidaria had 

 developed from Turbellaria, then suddenly a new difficulty 

 arises; though repeatedly mentioned in passing, nobody (with 

 one exception, O. Steinbock) has ever earnestly attempted to 

 solve the problem of the origin of Turbellaria. This again 

 opens the important question of the origin of the eumetazoan 

 Turbellaria from their protozoan ancestors. Were these Pro- 

 tozoa which stood closest to the present day Euciliata, or colo- 

 nies of Flagellata? 



The zoologist who proposes a new interpretation of the 

 evolution of Cnidaria which he wishes to be generally accepted, 

 is also obliged to give rational answers to all the questions 

 raised above; these answers must be at least more probable 

 than the answers that have been given and defended till now. 



In a series of sactions I will endeavour to discuss the most 

 important consequences of the new interpretation of Cnidaria, 



