00 THE EVOLUTION OF THE METAZOA 



two forms exist in Cnidaria, and at the first sight they appear 

 to be rather different; above all they differ considerably in 

 their ways of life. These two forms are that of the polyp and 

 that of the medusa. In the literature three possible explanations 

 can be found supported by various scholars: (1) The polyp 

 is the initial form which has been further developed into 

 the medusa as a secondary form; this type of evolution 

 can frequently be observed in recent species. (2) The form 

 of medusa is the primary form; out of this form the polyp 

 has been developed as a larval or post-larval ontogenetic 

 stage; it is possible that this polyp finally takes over the 

 reproductive function. (3) The forms of polyps and of medusae 

 have been evolved in parallell out of an indifferent form; 

 in this way we can imagine "Polyp und Meduse (seien) essen- 

 tiell . . . fiir den Typus des Nesseltieres," as this has been 

 expressed by E. Reisinger (1957, p. 695). 



The last (neutral) of the above m^entioned three possibilities 

 seems to be the one least probable. It cannot be proved and 

 it is not supported by any facts. The existence of numerous 

 species of Cnidaria with a complete metagenesis which can 

 be observed particularly among Hydrozoa, somewhat less 

 among Scyphozoa, and not at all among Anthozoa, can be 

 explained by way of any of the above three possibilities. 

 As regards their origin, the conditions in Lucernariidae among 

 Scyphozoa, and in Narcomedusae among Hydrozoa seem to 

 support the third interpretation. Yet these two cases can also 

 be explained on the basis of the first variant, as will be 

 later explained in more detail. Neither Lucernariidae nor 

 Narcomedusae can with certainty be considered to be primitive 

 forms. On the contrary, they are both highly specialized and 

 their position is at the periphery of the groups they belong 

 to. In this way the two opposite possibilities finally remain ; 

 polyp or medusa as the primitive form. Let us first take into 

 consideration the second possibility. 



There are some zoologists— even if they do not represent 

 a majority of them— who have investigated the Cnidaria and 



