96 THE MOST PRIMITIVE METAZOA 



colonial animal might have had. The syncytial level of organisa- 

 tion would correspond to the solid blastula, the stereoblastula, 

 that is sometimes found in the embryology of the Metazoa. The 

 simplest Metazoa did not have a hollow gut and corresponded in 

 structure to the present-day Acoela (Fig. 35). 



The Acoela gave rise to the other platyhelminthes, amongst 

 which were the Rhabdocoelida with their straight gut, and the 

 Polycladida with their branched gut. The Rhabdocoelida gave 

 rise to the Anthozoa which in their turn gave rise to the Hydrozoa 

 and the Scyphozoa. The Polycladida have a swimming larval 

 form, the Miiller's larva, and this became neotenous and gave 

 rise to the Ctenophora. Neotenous Miiller's larvae have been 

 described; thus Heath (1928) described Grafizoon lobata which 

 resembled a sexually mature Miiller's larva. 



The arguments that Hadzi puts forward in favour of his views 

 are as follows and he is supported by de Beer (1954, 1958). 



The Coelenterata are not primitive 



(1) The radial symmetry shown by the coelenterates is second- 

 arily acquired by them. The development of bilateral symmetry is 

 shown first of all in the external parts of the higher animals and is 

 then impressed on the internal organs. In the coelenterates such 

 as the Anthozoa, the bilateral symmetry is only found internally 

 as in the mesenteries. Therefore it must have lost its external 

 bilateral symmetry and be in the process of acquiring a radial 

 symmetry concomitant with a sessile habit. 



(2) The polyp is more primitive than the medusoid form. 

 Since the polyp is found in the Anthozoa whilst the Hydrozoa have 

 both polyp and medusa it follows that the Anthozoa are the most 

 primitive of the Coelenterata. 



(3) The Coelenterata are not diploblastic. They have a well- 

 developed middle layer, the mesogloea, which often contains 

 cells. Furthermore the cell lavers in the Anthozoa and the 



j 



Hydrozoa are not strictly comparable since in the Hydrozoa the 

 germ cells are formed from the ectoderm whilst in the Anthozoa 

 and the Scyphozoa they are formed from the endoderm. The 

 germ layers in the coelenterates are not comparable to the germ 

 layers of the higher animal and are not even homologous within 

 the coelenterates. 



