CXIDARIA AS THE ONLY COELEXTERATA 17 



and it is not able to form— even if this were useful— a nerve 

 centre, a brain ganglion, or even a sensory organ of the 

 type of statocyst. The facts show that this is really so; it 

 becomes particularly clear in those cases— which are by no 

 means rare— where a secondary, the so-called hypogenetic 

 development takes place, i.e. where the original generation 

 of polvps is left out so that the form of medusa develops as the 

 only type from the egg, and where, in spite of all this, the 



Fig. 4. Hydroctena salenskii Dawydoff. (After Dawydoff.) 



exumbrella remains empty and does not possess a vertical sen- 

 sory organ. This can be understood only if we take into con- 

 sideration the fact that in medusae their physiological anterior 

 pole is not their vertical pole, as is, on the other hand, the 

 case with Ctenophora. 



We will now pass to Hjdroctena salenskii which has been 

 discussed by C. Dawydoff. The name itself of this remarkable 

 animal shows that it was considered as an intermediate Hnk 

 between medusae and Ctenophora. Hydroctena has been observed 

 once and in three specimens only; these were neither sexually 

 ripe nor, as it seems, particularly well preserved. It has been 

 thought that it represents a mixture of elements characteristic 

 for Ctenophora and Hydromedusae, that is Hydromedusae 

 of the type of Narcomedusae (see Fig. 4). Various authors 



