CNIDARIA AS THE ONLY COELENTERATA 27 



perigastrocoel. Later I will show that the assertion according 

 to which Cnidaria are built of two body layers does not agree 

 with actual fact. The true coelom, which I call perigastro- 

 coel, and which alone would allow us to call the bearer of 

 such a coelom, a coelomate, does not occur earlier than in 

 AnneHda, the lowest subgroup of Polymeria. 



Even if the Cnidaria remain, on the basis of their actual situ- 

 ation and not on that of a hypothesis, isolated from the two 

 other supposed members of Coelenterata (Spongiae and 

 Ctenophora), they are not yet distinguished from other lower 

 invertebrates, i.e. from my Ameria or from the primarily 

 unsegmented animals. We must search for their connection 

 downwards, i.e. for the point where they have branched off; 

 that is, we must search for it among the freely moving bila- 

 terally symmetric ancestors, that lived on the bottom of the sea 

 and whose organization shows a somewhat higher develop- 

 ment. This must be proved, or at least made probable, and 

 it will be my duty to try to do this in the present study. 



A detailed comparison of Cnidaria with Ctenophora shows 

 that these two groups of animals, which for a long time have 

 been grouped together with Spongiae into the phylum 

 Coelenterata, do not even stand in a direct mutual relationship. 

 In my opinion they have evolved independently, each from 

 its own ancestor. In this way Cnidaria are the only animals 

 that remain in the large group of Coelenterata. 



Yet before we begin to tackle our main problem, to try 

 to identify the origin and the true nature of Cnidaria, it may 

 be useful to discuss first those methods w^hich have been 

 used in connection with the solution of our problem. 



