264 THE EVOLUTION OF THE METAZOA 



observed in the planula is a primary phenomenon, in compa- 

 rison with numerous Ectoprocta where the same phenomenon 

 is clearly a result of a secondary development. No meta- 

 morphosis is necessary for the further development of the 

 planula. It is well known that in a few aberrant cases the 

 planula does not develop directly into a polyp, but rather 

 into an actinula or into an Anlage of a cormus. 



The question finally arises whether this planula could 

 represent a recapitulation of an ancestral state? Our answer 

 must already be negative for reasons of principle alone. More- 

 over, such a creature is clearly unable to live as an independ- 

 ent organism because it has neither a mouth nor an intestine. 

 We would be obliged, if we tried by all means to see in a 

 typical planula a recapitulated ancestral form, to make further 

 suppositions, and explain above all the absence of a mouth 

 and of an intestine as secondary phenomena. This would 

 finally lead us to the question what had been the reason that 

 such a development had taken place. It should be mentioned 

 in passing that this planula in no w^ay corresponds to a gastrula 

 (nor does it correspond to a gastraea) ; its contents is not a 

 "pure" entoderm such as is known in a gastrula, but rather 

 a mixture of entoderm and of the mesohyl (mesoderm), and 

 thus an entomesohyl. The planula is therefore not a coelenter- 

 ate larva. 



In the Anthozoa, which we consider to be the most primitive 

 Cnidaria, a trend can be observed which tries to prolong the 

 free life in the plankton of the younger stage of their develop- 

 ment. Two main lines of evolution can be identified in this 

 connection, if we disregard a possible third line of evolution 

 where the free stage no longer leaves the benthos, a case 

 that can be observed in some Zoantharia (Rakovec, 1960). 

 In Ceriantharia a real beginning can be observed of the 

 subsequent progressive evolution of the plankton stage which 

 has even induced scholars to give special names to this stage 

 (cerinula, cerianthula, etc.). Unfortunately, little is known of 

 the subsequent development of these already specialized larvae. 



