236 THE EVOLUTION OF THE METAZOA 



(Fig. 43). The ending "-medusae" should rather be avoided 

 since in Scyphozoa there is— as far as we know on the 

 basis of our present research— not a single large systematic 

 group (e.g. an order) \\ hich includes the hypogenetic species 

 (i.e., the medusa forms) only. I have discussed this problem 

 at a greater length in another study (Hadzi, 1955 a). 



No radical changes are necessary because of the new inter- 

 pretation of Cnidaria in connection with orders that belong 

 to Hydrozoa. The order Hydroidea has been, and still remains, 

 the basis order of Hydrozoa and there can be no doubt that 

 other orders had evolved from it. No primarily solitary hydro - 

 polyps exist any longer. All those species that live now as 

 solitary animals, as well as those that do not have any ten- 

 tacles, are a result of a secondary development. For the present 

 two additional orders, besides that of Hydroidea, are sufficient, 

 i.e., Trachylina, as a group of all the hypogenetic species 

 which have been wrongly interpreted to be highly primitive 

 animals yet which are, in fact, highly specialized forms (just as 

 in the case of the benthonic Hjdra which occurs in the polypoid 

 form only); and Siphonophora. As for Trachylina, it seems 

 very probable that they evolved diphyletically, once as Narco- 

 medusae, and the second time as Trachomedusae. It can 

 therefore be expected that sooner or later it will become 

 necessary to make a sharper distinction between these two 

 groups of Hydrozoa if we continue to make taxonomic dis- 

 tinctions between two "sister groups" that had evolved 

 separately even if they look similar, because their evolutions 

 pursued two parallel ways and because their roots were closely 

 related to each other. 



With regard to Siphonophora it can now be considered as 

 certain that they had evolved from the benthonic cormi of the 

 polypo-medusoid athecate Hydroidea (Had:zi, 1918, 1954). 

 They therefore represent the climax in the progressive evolu- 

 tion within the order Hydrozoa and thus also of Cnidaria as a 

 whole. In these the polymorphism is finally extended to the 

 medusoid form also. The individualization of cormi, the way 



