208 THE EVOLUTION OF THE METAZOA 



individual but instead it immediately begins to form cormi, a 

 phenomenon which can be observed above all in Siphonophora. 



The conditions that can be observed in the ontogenies of 

 Cnidaria as a whole, show that the phylogenetic development 

 must have followed the direction Anthozoa -> Scyphozoa -> 

 Hydrozoa, and not the opposite direction, as has been believed 

 till now. The modes of evolution show considerable variety 

 and they are therefore not uniform. The younger pelagic 

 larva, the planula, seems to be most characteristic in the 

 ontogeny of Cnidaria; it had been evolved parallel to the 

 sessility of the adult form of polyp. It would be misleading 

 if we tried to see in this planula a recapitulation of a common 

 adult ancestral form. This planula does not feed independently, 

 and it can therefore not live long when swimming freely. 

 A slight prolongation of the planula stage can be achieved by 

 means of a reserve food given to the egg before it starts on 

 its way; yet in this way the mode of evolution is changed. 



In Cnidaria a significant prolongation of the pelagic stage— 

 without the usage of a medusoid generation (primary or 

 secondary)— has been reached, as it seems polyphyletically, so 

 that the subsequent development (yet not a real metamorpho- 

 sis) takes place in the pelagic zone; the larva as such becomes 

 able in this way to feed independently. Such a larva has been 

 called an "actinula;" attempts have been made to suggest that 

 the actinula represents the primitive form of Cnidaria, an equally 

 misleading interpretation. The more progressively developed 

 planktonic larvae have no longer a freely moving medusoid 

 generation. 



Actinulae of a very specified form can be found as early 

 as among the primarily solitary Anthozoa, i.e. in Ceriantharia 

 (Fig. 46). They are known as Semper's larvae, while at the 

 same time their relationship to certain adult forms could 

 not be determined. They develop some properties of the 

 adult form preserving simultaneously, as it seems, certain 

 archaic characteristics, e.g. a phase with four tentacles only, 

 with six sarcosepta, and with a well-developed bilateral sym- 



