THE MOST PRIMITIVE METAZOA 



93 



often the most highly developed brain of all the turbellarians ; 

 they also have well-developed and numerous eyes, flame cells, 

 a complex reproductive system with a muscular penis, uterus, 

 seminal vesicle and prostate organ, and often show hypodermic 

 impregnation. The Ctenophora have nothing to compare with 

 this. 



It is interesting to mention here that Hadzi (1944) and de Beer 

 (1954) think that the Ctenophora arose from the Polycladida, a 

 viewed discussed in more detail on page 94. Hadzi places the 

 Ctenophora in the platyhelminthes though he agrees that 

 Coeloplana and Ctenoplana are aberrant forms. 



What conclusion can we come to regarding the position of the 

 Ctenophora? With regard to their level of organisation they are 

 in most respects at a similar level to that seen in the Anthozoa- 

 Scyphozoa line of the Coelenterata. It is not possible to place 

 them any closer than this until more research has been carried out 

 on the embryology and development of the ctenophores and until 

 the range of form of the coelenterates is better known. It is not 



' 1 j.lll L ! J J-l? - 1 .'.'. J J.U^ jJJJLL'-lLU-'-'-lJjj J-t JJji J JiJJJ JJJJ.J. 1 . 1 LLJAl i ! U-'.'.'IV. ciliated epithelium 



glandular cell 

 mesenchyme 



food vacuole 



— muscle fibre 



rr~ — "~ 



pharynx 



Fig. 34. Diagrammatic transverse section through the body of an 



Acoelan, Convoluta. There is a certain resemblance to the grade 



of organisation of the ctenophorans. (After von Graff.) 



