PREVIOUS INTERPRETATIONS OF CNIDARIA 131 



actively moving animal I have been able to show that 

 the nervous system has again become locally dense, especially 

 in the aboral foot plate, with a clear direction (radial or circular) 

 of nerve fibres (Hadzi, 1908). 



A distinctly progressive tendency can be observed in the 

 stem (siphonosome) of Siphonophora, among Hydrozoa. They 

 have developed, as polypersonal cormi, the ability to move 

 freely. This development of nervous system is parallel to the 

 development of muscles in this strongly contractile stem of 

 cormus (according to K. C. Schneider, 1896-1898). 



Now let us pass over to the medusoid form. Since we derive 

 the Cnidaria fiomthe Turbellaria we must consequently consi- 

 der the medusoid form as secondary and as evolved from the 

 polypoid form. The same, however, has also been considered 

 as the correct interpretation by numerous experts in Cnidaria 

 who have believed at the same time that the Hydrozoa were 

 the most primitive Cnidaria because there were such numerous 

 and convincing proofs which seem to support such an inter- 

 pretation. The development of the medusoid form, which has 

 twice taken place (Scypho- and Hydro -medusae), is above all 

 a secondary liberation, or a readoption of mobility with a 

 simultaneous transition of the sexual function from one form 

 to the other. This is the quintessence of the metagenesis, of the 

 alternation of generations. It is senseless to deny the existence 

 of an alternation of generations in Cnidaria, as has been 

 recently attempted by Hyman: this is a consequence of an 

 inco:rect interpretation of the polyp form and of the polyp 

 generation as if these were a larval stage which had not been 

 developed into an adult form earlier than in Anthozoa. And 

 what are Hydra and other Hydrozoa that have become second- 

 arily solitary animals under a simultaneous loss of their medusa 

 generation, in spite of the fact that they become sexually ripe ? 

 Is this a case of neoteny ? This cannot possibly be so, in view of 

 the fact that in the Hydroidea we can find all possible degrees of 

 reduction of the medusoid generation; we must therefore have 

 here a secondary omission of the medusoid generation. 



