94 THE MOST PRIMITIVE METAZOA 



even certain at this stage that the ctenophores and coelenterates 

 had a common origin. It is possible that the nematocysts of the 

 ctenophores could have arisen independently of those in the 

 coelenterates ; after all there are some well-developed nematocysts 

 in the Protozoa. On the other hand there is almost nothing to 

 favour the view that the ctenophores are platyhelminthes. This is 

 particularly so because the turbellarians have a well- developed 

 reproductive system with accessory muscular sacs, whilst the most 

 that any ctenophore has is a small reproductive duct. There is 

 thus no clear indication that the ctenophores either gave rise to 

 or were derived from the Turbellaria. 



(5) The Platyhelminthes 



Though the platyhelminthes are usually considered as having 

 evolved after the Coelenterata, Hadzi (1944, 1953) has suggested 

 that this is not the case and that in fact the coelenterates evolved 

 after and from the platyhelminthes. The classification that Hadzi 

 gives of the lower Metazoa is as follows. 



This classification differs from the usual one in several respects. 

 First of all the platyhelminthes are considered to be the most 

 primitive of all the Ameria, more primitive than the Cnidaria. 

 Secondly the Ctenophora are placed in the platyhelminthes. 

 Thirdly the Anthozoa are considered to be the most primitive of 

 the Cnidaria. The evolutionary sequence devised by Hadzi is as 

 follows. 



