82 THE MOST PRIMITIVE METAZOA 



(3) The Anthozoa, Scyphozoa and many of the higher animals 

 form their endoderm by invagination. This method is rarely 

 found in the Hydrozoa, where ingression is more usual, and this 

 latter situation has been regarded as being a specialised condition. 



If we accept these reasons for choosing the Anthozoa as the 

 primitive class of the coelenterates, what would the primitive 

 form look like? It might have been something between an 

 Antipatharian and a Protanthean. In the Antipatharia there are 

 only six, ten or twelve septa. The longitudinal musculature on the 

 septa is scanty or absent and the flagellated tracts are very simple. 

 The mesogloea is scanty and the siphonoglyph only weakly 

 developed. In Protanthea there are eight macrosepta and four 

 microsepta. There is a complete cylinder of longitudinal epidermal 

 muscles in the column and pharynx (these are much reduced in 

 other Anthozoa). The nerve net and ganglion cells are well 

 developed over the surface of the body — the ectodermal nerve net 

 being reduced in other anemones. The sphincter and basilar 

 muscles are absent. The retractor muscles are weakly developed 

 and there are neither septal filaments nor a siphonoglyph. 



Although forms such as Antipathes or Protanthea may be simple 

 Anthozoa, they are still very complex when compared to a 

 protozoan. We still know very little about the primitive anthozoans 

 but it requires a lot of imagination to bridge the gap between the 

 Antipatharia and the Protozoa. 



Are the Coelenterata the most primitive of the lower 

 Metazoa? We have to choose between the Mesozoa, Porifera, 

 Coelenterata, Ctenophora and the Turbellaria to find the most 

 primitive metazoan. It seems likely that the simplicity of the 

 Mesozoa can be discounted as due to their entirely parasitic 

 nature. Similarly the sponges can be discounted since their level 

 of organisation is quite different in nature from that present in the 

 other metazoan. It would be best to place the sponges on a side 

 line to the main line of origin of the Metazoa ; the time of origin 

 of the side line is not clear. 



There is no doubt that the simplest of the Hydrozoa are more 

 simple than either the Ctenophora or the Turbellaria. But as we 

 have already mentioned, we do not know that simple forms such as 

 Hydra are the most primitive of the Coelenterata. One of the 

 major clues that has been used to place the coelenterates has 



