CNIDARIA AS THE ONLY COELENTERATA 15 



is their combined aboml sensory organ (see Fig. 3). Anything 

 identical or even similar is completely absent in medusae. 

 Hjdroctena has been mentioned again and again as if it was an 

 exception and at the same time an intermediate link between 

 those Cnidaria which resemble Narcomedusae and Ctenophora. 



Fig. 3. Diagram of the Ctenophore organization. (After Hertwig 

 and Kaestner.) 



Hjdroctena, however, as this \\-ill be shown in the ensuing pages 

 of the present study, cannot be considered to have such a role. 

 The absence of a static organ in the exumbrella of medusae is 

 not a mere chance. Its absence in medusae and its presence in 

 Ctenophora can be easily and convincingly explained. 



We shall later see how the part of the body which is turned 

 upwards in a ctenophore that is freely poised in water corres- 

 ponds to its anterior part. In Turbellaria, a static organ situated 

 close under the skin and in the centre of their central nervous 



