NEW INTERPRETATION OF CNIDARIA 265 



Frequently an "actinula" is developed also by other species of 

 Anthozoa; it represents a more progressive stage in the 

 development towards an anthopolyp whose most important 

 requisites can already be found here during the life in free 

 water (the mouth together with gullet [pharynx], septa, ten- 

 tacles). Hardly any development can be observed in special 

 larval organs (e.g. in the tuft of sensory cilia that occur at 

 the aboral end of larvae). We have here a prolongation of the 

 plankton phase only, or an early development in the direction 

 of the adult state. Neither can we speak here of a metamor- 

 phosis (the primary tentacles are not, as it seems, abandoned). 



The occurence of a true actinula is limited to Anthozoa. 

 The form which has been developed by Hydrozoa and which 

 has also been called an actinula is not identical to a genuine 

 actinula and it should be given a different name (perhaps a 

 hydrula). The latter form has neither a gullet nor septa; it 

 has only an oral opening, and intestinal cavity, and few ten- 

 tacles whose ends are thickened. It seems that larvae similar 

 to an actinula had repeatedly been evolved by Cnidaria, a 

 development which had perhaps even been repeated by 

 Hydrozoa several times (e.g. the actinula of Tubularidae, and 

 the actinula of Narcomedusae). 



If we adhere to our interpretation of an evolution of 

 Cnidaria in the direction Anthozoa -> Hydrozoa we could 

 rightly expect that a progressive evolution of larvae could 

 be observed in Scyphozoa and in Hydrozoa. This, however, 

 is not the case. How can this fact be explained? I believe I 

 have found an acceptable interpretation of this situation in 

 the fact that during the subsequent evolution a completely 

 new generation of individuals that live in plankton had been 

 invented both by Scyphozoa and by Hydrozoa, i.e. medusae 

 in their two "editions." The medusa generation had taken 

 over not only the sexual function but also the "task" of 

 distributing its own species, a development which is especially 

 characteristic in those cases where as a result of a secondary 

 development the species is represented by the medusa genera- 



