104 THE EVOLUTION OF THE METAZOA 



oldest Cnidaria. It has been quietly accepted that there is no 

 longer any medusoid form in Anthozoa and that such a form 

 had existed only in their ancestors. Yet in spite of this, this 

 "simple" fact should have been thought over. How is it that we 

 see Hydrozoa appear suddenly with both generations so that 

 they immediately develop the two "essential" forms; that the 

 Scyphozoa had been developed out of the Hydrozoa while at 

 the same time it is supposed that they possess only the medusoid 

 form so that they have been called Scyphomedusae, and their 

 scyphistoma considered to be a larval form; and, finally, in 

 the supposed subsequent evolution into the Anthozoa, the 

 highest developed Cnidaria, we suddenly see the poljrp form 

 now as the only form in existence ! One cannot help feeling 

 that here something must be wrong. How can we explain the 

 complete absence of the medusa form in Anthozoa, or, at 

 least, how can we make it probable? 



One would expect, if the Anthozoa had secondarily lost 

 their medusoid generation, that at least some traces of it could 

 still be found. Yet, as a matter of fact, not the slightest trace of 

 a medusoid generation can be identified. There have been nu- 

 merous enough opportunities for its preservation. The Antho- 

 zoa, too, have larvae (actinulae) that live in the plankton and 

 that partly show, like those of Echinodermata, an inclination 

 to bilateral symmetry. They do not show, however, any 

 trace whatever of a recapitulation of the medusoid formation. 

 Planktonic Actiniae are known (Minyidae) which even 

 possess a special apparatus that enables them to float more 

 easily in water and which, in spite of this, do not show any 

 common element with a medusa. Thus the name of "Antho- 

 medusae" has been used for a subtype of Hydromedusae with- 

 out fear lest it should be understood as a medusoid form of 

 a subtype of Anthozoa. If there were any gene or complex of 

 genes (any "trend") among the inherited properties of Antho- 

 zoa to form medusae this would have been apparent in their 

 abundant cormi (colonies) as traces of the polymorphism. In 

 these cormi both budding as well as fission can be observed. 



