78 THE MOST PRIMITIVE METAZOA 



Scyphozoa and the Actinozoa, and in effect revolves around which 

 of these can be considered as being the most primitive. As we 

 have seen, Huxley considered that the Hydrozoa were the more 

 primitive, but many zoologists now think that the Actinozoa are 

 the more primitive. 



Let us now consider the simple and the complex characters of 

 the coelenterates. Some of these, as shown in the following lists, 

 are contradictory, but this is due to the wide range of structure 

 occurring within the coelenterates. 



Simple coelenterate characteristics 



(1) There are only two well-developed epithelial layers, 

 ectoderm and endoderm. 



(2) They have a mesogloea. 



(3) They have musculo-epithelial cells. 



(4) The ectoderm may be ciliated. 



(5) The gut has only one opening. 



(6) There is a hydraulic skeleton. 



(7) They are free living forms and not parasitic. 



(8) Digestion is both intra- and extra-cellular. 



(9) There is no respiratory or excretory system. 



(10) They are polymorphic. 



(11) They have a high regenerative capacity. 



(12) They have a planula larva. 



(13) The nerve net shows little concentration. 



(14) They show radial symmetry. 



Complex coelenterate characteristics 



(1) They may develop cells in the mesogloea to form mesen- 

 chyme and mesoderm. 



(2) The body layers may become quite complex, e.g. three types 

 of cells in the ectoderm: (a) sensory and mucus cells; (b) 

 interstitial cells; (c) muscle cells. 



(3) They may have separate muscle cells (Trachylina and 

 Scyphozoa) which may be striated. The musculature can 

 be complex, e.g. circular, longitudinal and oblique muscle 

 bands. 



(4) They develop a skeletal system. This may be an exoskeleton 

 in Obelia or Heliopora or an endoskeleton as in Cor allium. 



