THE MOST PRIMITIVE METAZOA 97 



(4) Haeckel suggested that the Coelenterata represented the 

 hollow gastrula stage found in the embryology of the Echinoderms, 

 Sagitta and Amphioxus. This would indicate that the coelenterates 

 are a primitive group. But we now know that the blastula is more 

 often a solid form and that the endoderm is not always formed by 

 the invagination and development of a hollow gut. In fact the 

 hollow gut is an advanced condition when compared with that 

 present in the Acoela. 



Objections can be raised to all of Hadzi's points. 



(1) There is no evidence that the radial symmetry is secondarily 

 acquired by the coelenterates. The detection of an external 

 symmetry depends upon having some external organs; the 

 coelenterates do not have any such organs along the length of the 

 polyp and thus they cannot display this external bilateral sym- 

 metry. It cannot therefore be established that they had an 

 external bilateral symmetry at some stage which was later lost. 

 Furthermore there is no evidence that bilateral symmetry is 

 acquired first of all by the external organs and later by the internal 

 organs. 



(2) It is not generally accepted that the polyp is the most 

 primitive form in the coelenterates. In fact there is quite a 

 body of opinion that holds the medusa to be the primitive form 

 (see p. 80). 



(3) Though it is correct that not all the coelenterates are 

 diploblastic, the mesogloea of many of the Hydrozoa shows little 

 or no development. They at least can be considered as having an 

 effective diploblastic condition. As for homologising the cell 

 layers, it is not strictly correct to assert that the gonads arise 

 from the ectoderm in the Hydrozoa. It is more correct to state 

 that they arise from interstitial cells. In this way, therefore, one 

 can homologise the germ layers within the coelenterates. 



(4) Finally the fact that the blastula may often be solid in no 

 way indicates that the adult must have had a solid gut in 

 the most primitive metazoans. The larval form merely indicates 

 what the primitive larval condition was like, not the adult 

 condition. 



Having asserted the non-primitive nature of the coelenterates 

 Hadzi presents the following evidence that the Acoela are more 

 primitive than the coelenterates. 



