PREVIOUS INTERPRETATIONS OF CNIDARIA 89 



who have firmly accepted the interpretation (or who have been 

 more or less inclined to accept it) which considers the form 

 of the medusa to be the primary form. The adherents to this 

 theory refer above all to its complete radial symmetry and 

 to its life in plankton which are both considered to be a sign 

 of primitiveness. Moreover, there are many species of Cnidaria 

 which occur in the form of medusa only, where this form is 

 therefore directly developed out of a planktonic planula. All the 

 elements which occur in Siphonophora can be traced back 

 to a modiiEed form of medusa (according to the medusa 

 theory). The Anlagen of medusoids appear very early in their 

 ontogenies. Yet all these and many other facts which are 

 referred to by those who support the theory of the primacy 

 of the medusa form can be explained even more easily and 

 probably, on the basis of the first interpretation. Even if 

 we disregard the fact that a polyp has never been developed 

 from a medusa (either by way of proliferation or by way of 

 fission) and that the development has always been exactly 

 in the opposite direction so that medusae have been developed 

 out of polyps, we are hardly able to imagine how a highly 

 developed creature with a complicated structure as can be 

 observed in medusae could have evolved out of a planuloid 

 form. One of the most recent advocates of the primacy of 

 the medusa form is L. H. Hyman (I: 634 ff); her arguments 

 can be used here as an illustration of this thesis. Initially, she 

 admits that "nothing definitive can be said" about the origin 

 of Cnidaria. Yet in spite of this she later expresses the follow- 

 ing opinion, "It seems unavoidable to assume that they come 

 from a gastrea type ..." Then she tries to help herself with 

 Naef's '^Metagastraea'^ as the next higher stage ; this had been 

 further evolved into an "^^//W^" which corresponds, according 

 to her, to the present-day actinula larva. What, however, is 

 an actinula? It is clearly a larval stage of a polyp, and it is 

 already highly specialized so that it can be found in Hydroidea 

 only, e.g. only in the highly specialized Tubulariidae. The 

 Actinula descends, so to speak, to the bottom and develops 



