THE MOST PRIMITIVE METAZOA 83 



been that of embryological development. Haeckel suggested that 

 the adult coelenterates such as Hydra were at a stage comparable 

 to the gastrula seen in Ampkioxus. Even in Haeckel' s time it was 

 pointed out that the embryology of the coelenterates did not 

 follow that of the higher animal. The blastula of the Hydrozoa 

 is most often a solid larva, the interior of which is filled with 

 cells. Hadzi and de Beer take this solid larva to indicate that the 

 primitive larva had a solid gut and they think that the coelenterates 

 cannot be primitive since they have a hollow gut. It may be 

 correct that the most common type of planula has a solid 

 interior. But this in no way indicates that the adult also had a 

 solid gut. 



The Acoela are not the only animals to have a solid gut in the 

 adult condition. Within recent years the Pogonophora, a group 

 related to the Pterobranchiate Protochordates, have been 

 described by Ivanov (1954-7). They have a solid gut filled with 

 endoderm cells. The Pogonophora are coelomate animals and it 

 is not clear whether the solid gut is here a primitive condition or 

 one that is due to the small size of the animal. At any rate it leads 

 one to wonder about the precise conditions that lead to the 

 retention of the solid gut if it is a primitive condition, or the 

 development of a solid gut if it is an advanced condition. Jagersten 

 (1955) thinks it highly unlikely that the primitive metazoans had a 

 solid gut since this would mean that the hollow gut arose at least 

 twice, once in the Coelenterata and again in the Turbellaria. It is 

 perhaps worth noting that certain coelenterates such as Clytia, 

 when they feed, fill the gastro-vascular cavity with endodermal 

 processes so that the gut takes on a solid mesh-like appearance. 

 Thus Hadzi's view that the Coelenterata cannot be the most 

 primitive of the Metazoa since they have not a solid gut is open to 

 two objections: firstly we do not know that the solid gut is a 

 primitive condition and secondly some coelenterates can at times 

 show a condition resembling a solid gut. 



In conclusion, then, it is apparent that we do not know whether 

 the coelenterates are more or less primitive than other lower 

 metazoans such as the Turbellaria. We do not know if the hollow 

 gut is a primitive condition. We do not know if the Hydrozoa are 

 more primitive than the Anthozoa. W T e do not know which is the 

 more primitive form, the medusa or the polyp, and as we shall 



7— IOE 



