CNIDARIA AS THE ONLY COELENTERATA 



51 



is really the only form which survives. Thus we can return by 

 way of an indirect course to the initial situation. Finally, the 

 generation of medusae as the only form that had survived, can 

 develop into an animal that resembles polyps and lives as such 

 on the sea bottom (Armohydra see Fig. 7, Hala?nmohydra see Fig. 

 8, Otohydra see Fig. 1 8 .) 



Fig. 7. A reduced trachyline medusa Armohydra janomci^i Swed- 

 mark & Tesnier; A, longitudinal section; B, habit; g, gonad; 

 e, exumbrella; m, mouth; sc, subumbrellar cavity; v, velum; 

 (after Swedmark and Tesnier). 



In connection with all these developments the question 

 necessarily arises how can we in a given case differentiate 

 and correctly interpret the path evolution had taken? A broad- 

 er and deeper comparison can help us here while we take 

 into consideration on one hand the ecologic conditions and 

 on the other Dollo's rule of the irreversibility of evolution. 

 Many mistakes have been made in this connection. Many 

 zoologists, among them even specialists in Cnidaria, are inc- 

 lined to see in the planktonic form of medusae the original, 



