92 



THE MOST PRIMITIVE METAZOA 



due to convergence and that Coeloplana and Ctenoplana are in fact 

 specialised aberrant and advanced forms. In particular there are 

 considerable differences between the embryology of the cteno- 

 phores and the polyclads which make it improbable that the two 

 groups are related. These differences are not due to the presence 

 or absence of yolk but instead reflect a more fundamental differ- 

 ence. The ctenophore egg cleaves into four and at the next 

 cleavage it divides to form a small group of cells, the micromeres, 

 and a large central group, the macromeres. These form a flat 

 plate of cells. Cleavage continues till there are eight macromeres 

 and many micromeres. In the polyclads, on the other hand, after 

 the four-cell stage the embryo shows a definite spiral cleavage 

 pattern. The cells can all be classified in terms of the spiral cleavage 

 pattern found in the Rhabdocoelida, Tricladida, Annelida and 

 Mollusca. This spiral cleavage is not found, nor is there any 

 indication of spiral cleavage, in the development of the 

 Ctenophora. 



There are other differences between the polyclads and the 

 ctenophorans. Thus the polyclads have a well-developed brain, 



mesenchyme 



lumen of 

 food canal 



muscle fibre — 



eosinophil_ 

 body 



° O o 



1 Q.° J. o o ,o 



• o o ° • -P • » ° °„ 



° O • n r, °"o ° n° °»° 



gland cell-- -£- 



ciliated 

 epidermis 



Fig. 33. Diagrammatic transverse section through the body of a 

 ctenophore, Coeloplana. (After Komai.) 



