84 THE MOST PRIMITIVE METAZOA 



now see, we do not know the relationship between the Coelenterata 

 and the Ctenophora. 



(4) The Ctenophora 



There are three questions that should be discussed concerning 

 the Ctenophora. (1) What is their ancestry? (2) Are they 

 coelenterates? (3) Are they ancestral Turbellaria? These are all 

 difficult questions to answer and involve a careful consideration of 

 the structure of the ctenophores. 



The ancestry of the Ctenophora 



Nothing definite is known about the ancestry of the ctenophores. 

 It is generally suggested that they arose from a basic stock that 

 gave rise to the coelenterates; thus there are certain resemblances 

 and certain differences between the ctenophores and the coelenter- 

 ates, as can be seen from the lists below. 



Coe 



(1 



enterate characteristics of the Ctenophora 



There are two primary layers ; the ectoderm and endoderm 

 are well developed and there is no definite mesoderm — just a 

 mesenchyme. 



The main body cavity is the gastro-vascular cavity. 

 There is only one opening to the gut — the mouth. There is 

 no true anus. 



They have a stomodeum at the entrance to the gut as in the 

 anemones and some medusae. 



The gut is divided like that of the Scyphozoa but the eight 

 divisions are more like the symmetry of the Alcyonaria. 

 They are radially symmetrical. 



They have mesenchyme muscles like some of the coelenter- 

 ates, e.g. Trachylina, Scyphozoa. 

 The gonads are derived from interstitial cells. 

 The outer surface has cilia ; as comb plates in Pleurobrachia, 

 as a ciliated surface in Coeloplana. 

 The tentacles are like those of some Scyphozoa. 

 The lasso cells may take the place of nematocysts, but 

 Euchlora has true nematocysts. 

 There are no nephridia. 

 They have a subepidermal nerve net. 



