THE MOST PRIMITIVE METAZOA 



95 



Hydrozoa 



t 



Anthozoa 



t \ 



Ciliata — > Acoela -> Rhabdocoelida Scyphozoa 



\ . 



Polycladida — > Ctenophora 



Hadzi thinks that the Metazoa arose by the formation of cell 

 walls in a syncytial ciliate. This would then lead to a multicellular 

 animal with a complete organisation, i.e. an antero-posterior axis 

 and without the difficulties of reorganisation that a multicellular 



brain 



glandular 

 secretion 



.statocyst 



0" 



p&, 





:'Lfood vacuole 



mesenchyme 





Ff^:':7. v.-.SM: : - ift- '.•':'•• ' : , --mouth 

 • ■ ■■ •'• & ••':..•. • ••• :•",.. • ■A.I 



food-5 

 vacuole 



oenis 



ovary 



bursa 

 seminalis 



___ Reproductive 

 opening 



cilia 



Fig. 35. Diagrammatic longitudinal section through an Acoelan 

 to show the order of complexity of its structure. Note the 

 development of a complex reproductive system. (From Bronn.) 



