PREVIOUS INTERPRETATIONS OF CNIDARIA 163 



to disappear. The polypoid form (generation) must serve as 

 basis for the construction of the definitive system of the 

 Hydroidea. We cannot begin to discuss here the details of 

 such a system and we must limit ourselves to a few general 

 observations only. 



We have here a progressive trend in evolution which has 

 reached its climax in Hydroidea. All these phenomena can be 

 completely understood only if we accept the standpoint which 

 considers Anthozoa to be "the most primitive," and Hydrozoa 

 as the most specialized, i.e. phylogenetically the youngest 

 Cnidaria. 



The Digestive System 



While the external form of all three classes of Cnidaria 

 remains essentially identical with their basic form, i.e. with 

 the form of polyps (a cylindrical or a cup -shaped form) w^e 

 find such differences in the form of the digestive system (of 

 the internal layer or of the entoderm) that by means of these 

 differences we are able to make clear-cut distinctions between 

 the three classes. It was under the influence of the thesis of a 

 generally progressive evolution of all living beings that the 

 polyps of Cnidaria with a simple and smooth intestinal tube 

 generally considered as the most primitive ones, those which 

 show the formation of longitudinal septa in their "initial" 

 stages as the next more highly developed ones, and finally 

 those with strongly developed sarcosepta as the most highly 

 developed polyps, an interpretation which agrees with the 

 generally supposed line of evolution H^^drozoa -> Scyphozoa 

 -> Anthozoa. 



In addition to this there is one more complication in their 

 digestive systems. On one hand we can observe in cormi 

 that the digestive organs of zooids develop mutual communi- 

 cations; on the other hand, in medusae the tubes grew out 

 of the central part of their digestive system, thus out of the 

 "stomach," and this has finally led to the formation of a 



