CNIDARIA AS THE ONLY COELENTERATA 13 



be brought forward as a proof of the direct relationship bet- 

 ween Ctenophora and Cnidaria. The tentacles of medusae have 

 always and without exception, an intestinal diverticulum in 

 their axes, i.e. a continuation of the intestine with an ento- 

 dermal coat and with an intestinal lumen. It is not rare for the 

 intestinal lumen to be secondarily obliterated. In this case the 

 entoderm forms a solid entodermal axis which is not unUke a 

 rouleau of money or a primitive chorda dor sails. In this way the 

 entodermal axis develops into an internal skeleton. The structure 

 of the tentacles of Cnidaria is usually described as if they were 

 composed of an ectoderm and of an entoderm only. Later we 

 shall try to show in more detail and in connection with the 

 whole body of Cnidaria that the intermediate layer of the 

 body participates also in the structure of the tentacles. This 

 layer usually bears the unsuitable name of a mesoderm or 

 mesogloea which again creates the impression that we have 

 here something different from the intermedial layer of the body 

 of other Eumetazoa. I have therefore proposed for it the name 

 mesohyl. It is better to speak about the mesoderm only in con- 

 nection with early ontogenies. 



A typical feature of all Ctenophora is the fact that their 

 tentacles never have an intestinal diverticulum or any entoderm 

 whatever. As important as this difference is, it is, however, by 

 no means the only one. Even the way that these tentacles grow 

 is completely different in the two cases. In Ctenophora the 

 tentacles actually grow constantly during their whole Ufe time; 

 their growth continually takes place at the considerably 

 thickened bases deeply sunk into a cutaneous follicle, while at 

 the same time the free ends slowly break away. In connection 

 with this we find in Ctenophora the base is organized in a 

 completely different way from that of Cnidaria. Without finding 

 it necessary to enter into further details w^e have the full right 

 to maintain and conclude on the basis of what has already been 

 mentioned, that the tentacles of Ctenophora and Cnidaria do 

 not have the same origin and that we have here a case of 

 functionally similar, and therefore analogous, organs. 



