266 THE EVOLUTION OF THE METAZOA 



tion alone. In this way a further general evolution of the 

 larval stage has become completely unnecessary. Ephyrae 

 can occasionally be found mentioned as if they were larvae 

 of scyphozoan medusae; they are in reality their juvenile 

 stages only. 



No actinulae have been found among the Scyphozoa; we 

 find instead planulae which, however, have never been observ- 

 ed to reach the real plankton and to live there. Just as 

 we can speak here of a planula larva, we could also speak 

 of an ephyra larva of the metagenetic medusafe; the only 

 difference is that the ephyra as a larva of medusae does not 

 develop out of an egg but rather by way of asexual repro- 

 duction. In Narcomedusae, on the other hand, we find that 

 a so-called polypoid larva had been developed after the prima- 

 ry polyp generation had been lost; this larva even shows 

 an incUnation to a parasitic way of life. It is only superficially 

 similar to an actinula and it must be therefore treated as a 

 formation sm generis. 



Among the Siphonophora we find conditions that are some- 

 what more unusual; they stand in connection with the individu- 

 alization of the planktonic cormi and resemble slightly those 

 that can be found among the phylactolaematous Ectoprocta. 

 The planula of Siphonophora (e.g. the ratarula of Chondro- 

 phora) soon begins to form secondary individuals as well as 

 the impersonal parts of a polymorphous cormus (siphonula). 

 Here there are even provisional individuals (nectocalyces, etc.) 

 which are later abandoned, thus a development which is 

 similar to a metamorphosis. We must not forget that Sipho- 

 nophora represent the peak in the evolution of Cnidaria! 



A Comparison of Cnidaria Larvae with Other Larvae 



A comparison of larval conditions such as they occur in 

 Cnidaria with those that exist in the equally sessile Endoprocta 

 (Kamptozoa), among the Ameria is very illustrative in 



