THE MOST PRIMITIVE METAZOA 79 



(5) They have a gut. (The Mesozoa and Acoela have no gut.) 



(6) The gut may have subdivisions (pharynx, mesenteries). 



(7) The gut develops a circulation in Aurelia and Alcyonium. 



(8) They have nematocysts. 



(9) They have specialised sense organs such as eyes and 

 statocysts. 



(10) Some coelenterates are bilaterally symmetrical. 



(11) Some coelenterates such as Velella and Porpita show 

 division of labour. 



As is well known, there is considerable diversity of structure 

 within the coelenterates, and even though various coelenterates 

 can be derived from a common plan there is still difficulty in 

 deciding which are the most primitive coelenterates as opposed to 

 the most simple. Thus though one can arrange a series going 

 from, say, Hydra to Physalia, or from a diploblastic radially 

 symmetrical form to one that is triploblastic and bilaterally 

 symmetrical, there is no historical justification for either such 

 series. We must find some collateral evidence to help determine 

 which are the most primitive of the coelenterates. 



The most primitive coelenterates 



There is hardly a group of the coelenterates that has not at one 

 time or another been claimed to have been the most primitive. 

 Perhaps the two most prevalent claims are (1) that the polyp is 

 the most primitive form, and (2) that the medusa is the most 

 primitive form. 



The view that the polyp is the most primitive form in the 

 coelenterates has been supported by Haeckel, de Beer and Hadzi 

 as well as various other writers. Whilst Haeckel suggests that 

 Hydra is primitive, Hadzi thinks that the anemones are more 

 primitive and that evolution within the coelenterates has gone 

 from the Anthozoa to the Scyphozoa and Hydrozoa. This view 

 is supported by de Beer (1954), who writes, " It follows and is 

 generally recognised, that the polypoid person which is the only 

 one represented in the Anthozoa, is more primitive than the 

 medusoid person found in the Scyphozoa and Hydromedusae, 

 which is clearly an adaptation to dispersal on the part of the sessile 

 form." 



