204 THE EVOLUTION OF THE METAZOA 



gressive development can also be observed. It is probably a 

 unique case in the whole animal world that animals that use 

 Cnidaria (this is true, as it seems, for Hydrozoa only) as their 

 food, e.g. Turbellaria, Ctenophora, Gastropoda, are able to 

 use cnidae which have been taken from their prey, as the 

 so-called kleptocnidae (Martin, 1914). It seems that the cnidae 

 of only the Hydrozoa can be used as kleptoknidae because 

 they are of the highest quality. 



Ontogeny 



Initially, I gave a general explanation of my interpretation of 

 the relationship betw^een ontogeny and phylogeny. I agree 

 with Garstang, de Beer, Steinbock, Sacarao, and numerous 

 other zoologists who have resolutely rejected as wTong and 

 misleading the so called "fundamental biogenetic law" as it 

 was formulated by Ernst Haeckel. According to this law, 

 ontogeny is supposed to be a shortened and somew^hat 

 changed ("falsified!") phylogeny; the phylogeny did not repre- 

 sent some old ontogenies (ontogenetic stages), but rather a 

 chain of adult stages or forms. Even if basically wrong, this 

 interpretation as proposed by Haeckel was originally useful 

 for propagating the idea of Evolution. This quasi-law has now 

 been banished for all times from the demesne of biology; this, 

 however, should not mean that ontogenies cannot serve as a 

 useful material when we are attemptirg to come to some 

 phylogenetic conclusions by means of a comparative method. 

 It is only necessary that we proceed cautiously in order to 

 avoid the pitfalls ot the so called caenogenesis. 



We are aw^are of the fact that w^e will have to overcome 

 considerable difficulties in the struggle for a new interpreta- 

 tion of the phylogeny of Cnidaria; this is due especially to a 

 widely believed, yet false, combination of tw^o concepts that 

 were proposed by Haeckel; this is, on the one hand, the con- 

 cept of a general repetition of the phylogeny in individual 

 ontogenies; and, on the other, the concept of a gastraea as the 



