240 THE EVOLUTION OF THE METAZOA 



in both these cases from their respective polyps. In Scyphozoa 

 again this development had pursued two completely different 

 paths ; in the first type of development the medusoid distal 

 half of the polyp had remained attached and this was followed 

 by a reduction and by a change of the medusa form. In the 

 second type the planula continued to exist in the plankton, 

 the polypoid generation has been omitted and it is in this 

 way that the scyphomedusa can develop directly from a pla- 

 nula. This second variant has been repeated in Hydrozoa— we 

 speak here of the hypogenetic species or even groups, using 

 the expression as it was proposed by Haeckel (Trachylina). 

 The Hydromedusae remain attached to their cormi because 

 here medusae are developed from the whole hydranth (and 

 not out of half only), and that by way of budding; these 

 medusae are afterwards changed into a more and more reduced 

 medusoid form which can finally completely disappear, the 

 polyp generation which was earlier sterile is now^ changed 

 into a sexual and the only existing generation. 



The classification of Hydrozoa, and especially that of Hydro- 

 idea, w^hich tries to express the evolutionary path these 

 animals had taken, must be based on conditions that can be 

 observed in their polyp generation; yet at the same time it 

 must also take into consideration the younger medusa gene- 

 ration as regards the details of morphology. It is important 

 that we adhere, in our endeavours to work out the details 

 of this system, to the fact that Hydroidea do not include any 

 primarily solitary species, and that in these animals solitariza- 

 tion has been evolved several times, along several lines of 

 evolution (Fig. 45). This is not a theoretical supposition but it 

 can be also easily observed in the recent material (in the 

 so-called Limnomedusae). 



It seems that three orders are sufficient for the division of 

 Hydrozoa, i.e. Hydroidea, Trachylinea, Siphonophora. Since 

 we. do not know which, taken geologically, of Siphonophora 

 and Trachylinea is the older (it is even possible that both of 

 them had evolved diphyletically), so it is the same if the first 



