CHAPTER 6 



THE MOST PRIMITIVE METAZOA 



We have seen so far that the Metazoa can be derived either from 

 syncytial protozoans, from multicellular protozoan colonies or 

 from the Protophyta. To some extent the theory that one chooses 

 as the most probable will depend upon which group is considered 

 to be the most primitive of the Metazoa. Thus if one considers the 

 Sponges as the most primitive of the Metazoa then one could 

 suggest a link between the Protozoa and the Metazoa via the 

 Choanoflagellata. If on the other hand one thinks that the 

 Acoelous Platyhelminthes are the most primitive metazoans, then 

 one could consider that the link with the Protozoa was via the 

 complex ciliates. It is therefore important to decide which are the 

 most primitive of the Metazoa, but before this can be done one 

 has to consider four questions. 



(1) Which of the metazoan groups can be considered the 

 earliest to have evolved? 



(2) Which are morphologically the most simple of the Metazoa? 

 (This will not necessarily be the first group to have evolved.) 



(3) What is the relationship between the major groups of the 

 lower Metazoa? 



(4) Can the Metazoa be considered as a polyphyletic group 

 with more than one origin from the simpler living forms? 



The metazoans that will be considered here are the five groups : 



(1) Porifera. 



(2) Mesozoa. 



(3) Coelenterata. 



(4) Ctenophora. 



(5) Platyhelminthes. 



Before discussing these questions it will be as well to indicate 



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