158 THE EVOLUTION OF THE METAZOA 



Parallel to this we find in the Scyphozoa a characteristic 

 retrogression of the polyp generation. As a consequence of 

 this retrogression the whole class has been given the name of 

 Scyphomedusae, because in this class the medusoid generation 

 has become the prevalent one. No primarily solitary polyps 

 have been known so far. The soUtarization of the polyp 

 generation took place by way of a secondary reduction of 

 cormi. The periderm which w^as originally preserved by 

 scyphopolyps was the same as had been inherited by the 

 corm-forming species from their anthozoan ancestors of the 

 type of the recent Stolonifera, among Octocorallia (e.g. 

 Cornularia cormicopiae, [Fig. 42]). The cuticular external skeleton 

 had been reduced during the soUtarization and the elimination 

 of the polyp generation so that finally there are only resting 

 states, as for example podocysts (Hadzi, 1912) that have 

 remained covered with this cuticle. The sexual function, the 

 formation of gonads, has been entirely transfered to the 

 freely living medusoid generation. The completely weakened 

 primary generation has finally been completely relinquished, so 

 that it is the very progressively developed medusoid generation 

 only w^hich has been preserved. It has already been mentioned 

 that a side branch has also been evolved in the Scyphozoa 

 which has become completely benthonic so that cilia do 

 not even occur in its planula (Fig. 35). These are the Lucer- 

 nariidae whose medusoid generation remains hanging on the 

 maternal polyp and is united with it into a new whole w^hich 

 represents a special case of hypogenesis (polypomedusae). 



As among Scyphozoa, there are no primarily solitary species 

 of polyps among Hydrozoa which should naturally also be 

 sexually mature, and we dare maintain that in the Hydrozoa 

 such species of polyps have really never existed. In all proba- 

 bility they wxre corm-forming from the very beginning. The 

 sexual function has been taken over by the medusoid generation 

 which has become secondarily free. We have already mentioned 

 that in the more primitive species the primitive sex cells 

 develop first in the cormus, after which they migrate actively 



