THE MOST PRIMITIVE METAZOA 99 



There is also little assurance that the Acoela are the most 

 primitive of the Turbellaria. They are a comparatively unstudied 

 group of animals; we know next to nothing about their physiology, 

 little experimental embryology has been performed on them and 

 we do not know their range of morphological forms. Though von 

 Graff (1904) was of the opinion that their structure was primitive, 

 a great deal more research will have to be carried out before such a 

 position can be justified and even more research will be necessary 

 before they can seriously be derived from the ciliates. For 

 further details concerning the phylogeny of the platyhelminthes 

 and their relationship to the Ctenophora one should consult 

 Bresslau (1933). 



There is a school of thought represented by Marcus (1958) which 

 suggests that the platyhelminthes are an advanced group of 

 animals that were once coelomate and more complex morphologic- 

 ally than, say, the Nermertini, Phoronidea or Brachiopoda. The 

 platyhelminthes according to this view are secondarily simplified. 

 They have lost their coelom, anus and circulatory system. They 

 have reduced their nervous system, and altered their reproductive 

 system to make up for the loss of the coelom. It is suggested that 

 one well-known example of a coelom being lost when an animal 

 takes up parasitic habit is that of the Hirudinea, and that the 

 platyhelminthes have gone even farther along this course. 



We thus have two conflicting views concerning the status of the 

 platyhelminthes. Hadzi suggests that they are the most primitive 

 of all the Metazoa, being more primitive than the coelenterates 

 and derived from the ciliates. Marcus suggests that the platyhel- 

 minthes are an advanced group of animals whose simplicity of 

 structure is due to their parasitic habit and that they arose some 

 time after the Nemertea. 



What can one conclude about the most primitive of the Metazoa? 

 There are, as we have seen, five contestants, Porifera, Mesozoa, 

 Coelenterata, Ctenophora and the Platyhelminthia, for this title. 

 These groups are almost completely isolated from each other 

 though a few tenuous connexions can be made. It is quite clear 

 that the available evidence is insufficient to allow us to come to 

 any satisfactory conclusion regarding their interrelationships. At 

 the same time it is also clear that a great deal of work remains to 

 be done on all of these groups. We are still very ignorant about the 



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