CNIDARIA AS THE ONLY COELENTERATA V 



phora, with a rich development of a jelly-like intermediate sub- 

 stance. Though cnidae are, on the whole, developed in Cni- 

 daria only, they have nevertheless been found also in a genuine 

 ctenophore, in Encblora rubra. Furthermore, a coelenterate has 

 been described which has been considered as an intermediate 

 form between the two subtypes of Coelenterata (Hydroctena 

 salenskii Dawydoff). Finally, not without significance is the dis- 

 covery of ctenophores which, though genuine, have begun to 

 resemble Turbellaria due to their transition to a benthonic way 

 of life; they have been explained as a transition form to Turbel- 

 laria, and in this way a connection of Colenterata with "higher 

 forms" has been won. 



All these facts have helped to support zoologists in their 

 belief that Ctenophora are real Coelenterata and as such the 

 closest relatives of Cnidaria. It is surprising to see how the 

 numerous and fundamental differences that actually exist bet- 

 ween the two groups of animals and touch not only their struc- 

 tures but even their ontogenies, make no deep impression on 

 the majority of zoologists. At the most, the absence of cnidae 

 in Ctenophora has been considered as an important mark of 

 difference and for this reason they have been called Acnidaria. 

 In our discussion we will limit ourselves to the essential points 

 only; we shall try to show that Cnidaria are basically different 

 from Ctenophora and that this difference is due to the fact that 

 both these types of animals have a different origin, even if 

 these origins are not widely separated from each other. For 

 this reason we will base our discussion on selected properties 

 that can be observed in Ctenophora. 



One of the principal characteristics of Ctenophora' is their 

 bisymmetry. This quaUty pervades the whole organization 

 of Ctenophora to such an extent that it can be observed 

 already in the fine structure of their ovum and also in their 

 ontogeny. Something identical— or even anything similar— 

 cannot be found in any medusa. The basically quadri-radial 

 symmetry which can be secondarily muhipHed through the 

 process of polymerization is typical for medusae. On the other 



