CHAPTER IV 



COELENTEEATA 



Classification adopted— 



Hydrozoa 



Scyphozoa 



Hydrida 



Hydromedusae.. 



Narcomedusae 



Trachymedusae 



Siphonophora 



Actinozoa 



Ctenophora 



/Alcyonaria 

 IZoantharia 



'^Cydippidea 

 Lobata 

 Cestidea 

 Platyctenea 

 Beroidea 



jG-ymnoblastea 

 \Oalyptoblastea 



(Edwardsiae 

 Hexactiniae 

 Cereanthidae 

 ZoantMdae 



The Coelenterata are considered by many zoologists to be closely 

 related to the parent group from which the other groups of Metazoa 

 have sprung. In simplicity of general organisation they rival the 

 Porifera, since the bodies of the adult Coelenterates, like those of 

 Porifera, are composed of two layers of cells with an intervening jelly. 

 Ever since in 1859, Huxley compared these two layers in a Ooelen- 

 terate to the two primary layers of the Vertebrate embryo, they 

 have been termed ectoderm and endoderm. 



The most interesting thing about the relationship between the 

 Porifera and the Coelenterata is that whilst the earliest stages of 

 development in the most primitive representatives of each group 

 are strikingly similar, and whilst in both cases a two-layered adult 

 condition is reached, yet the steps by which this goal is attained 

 differ so totally in the two cases that the two layers cannot be 

 regarded as corresponding to one another in the two groups. 



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