THE MOST PRIMITIVE METAZOA 81 



a selection of authors and their choice of primitive form is shown 

 below. 



Haeckel Hydrida 



Moser Siphonophora 



Hyman Trachylina 



It is difficult to choose between the above groups; thus though 

 Hydra is more simple in its adult morphology, it is suggested 

 from a study of the range of form that the medusoid condition is 

 more primitive in the Hydrozoa and that development from this 

 led to the solitary polyp. 



On the other hand there is a growing body of opinion that the 

 Anthozoa are more primitive than the Hydrozoa. This view is 

 supported by Hadzi (1944), Ulrich (1950), Remane (1955), 

 Jagersten (1955) and Marcus (1958), who suggest the develop- 

 mental sequence went Anthozoa-Scyphozoa-Hydrozoa. No 

 closely reasoned account has yet been presented by the above 

 authors to show exactly how the morphology of the primitive 

 anthozoan would lead one to suppose that they are more primitive 

 than the hydrozoans, but the gist of the evidence is apparently as 

 follows. 



(1) If the Hydrozoa were the most primitive forms which later 

 gave rise to the Anthozoa this would not explain the marked 

 bilateral symmetry found in the Anthozoa. Bilateral symmetry is 

 usually associated with a mobile habit and one would not expect to 

 find it in a sessile form that had a long sessile history behind it. 

 This bilateral symmetry is found in the Ordovician Tetracorallia 

 and even in the arrangement of nagella in the zooxanthella larva. 

 From the symmetry as shown in the arrangement of the mesenter- 

 ies, the retractor muscles, septal filaments, siphonoglyphs and 

 sulcus, one would suppose that the Anthozoa arose from a free- 

 living mobile ancestor. 



(2) A second reason for choosing the Anthozoa as the most 

 primitive form lies in the range and structure of the nematocysts. 

 The Hydrozoa have over a dozen different types of nematocysts 

 whilst the Anthozoa have only about half a dozen different types. 

 Furthermore the cnidoblast that carries the nematocysts is more 

 simple in the Anthozoa; it lacks the cnidocil and instead has a 

 primitive ciliary cone (Pantin 1942). 



