PREVIOUS INTERPRETATIONS OF CNIDARIA 165 



Thirdly, the digestive system of Ctenophora seems at the 

 first glance to resemble that of medusae; in reality, however, 

 it is quite different from the latter: and is, in our opinion, not 

 a new formation but rather a transformation of conditions 

 that had existed in their turbellarian ancestors, in Polycladida. 

 It has been repeatedly emphasized that Ctenophora are not 

 Coelenterata. The similarities that exist in their digestive 

 systems are due to the fact that both Cnidaria as well as Cteno- 

 phora come from Turbellaria whose intestine shows a charac- 

 teristic inclination to form diverticula or excrescences. The 

 roots of the two groups, however, are separate. It was quite 

 wrong when Haeckel tried to deduce the "gastrovascular con- 

 ditions" of Ctenophora from those that occur in the hydro- 

 medusa (anthomedusa) Ctenaria ctenophora. On the basis of this 

 wrong hypothesis we should assume that Turbellaria, as 

 descendants from Ctenophora, had inherited from the latter the 

 tendency of their intestine to form diverticula. In reality the 

 development went in just the opposite direction. 



When we pass to the discussion of the digestive organs 

 of individual potyps of the Cnidaria (Fig. 32) we meet the gene- 

 rally accepted opinion that they followed a progressive Une 

 of development. If this were true, then it would be neces- 

 sary to consider both the ectodermal gullet (pharynx) as well 

 as sarcosepta as newly acquired complications. We must here 

 raise the question whether we are allowed to suppose that 

 the intestinal apparatus has been progressively developed in 

 individuals in spite of their consistently sedentary way of life 

 and the constant diminution of the size of the corm-forming 

 zooids? If we take into consideration our experiences with 

 other sessile animal groups we must straight away deny the 

 probability of such a supposition; we must do this especially 

 because this supposition is contradicted by our analyses of 

 other systems of organs. 



The situation becomes much more clear if we reverse the in- 

 terpretation and if we derive the conditions that can be obser- 

 ved in the digestive organ of Anthozoa from those of 

 12 



