CHAPTER 



10 



RADIAL ANIMALS: 



The Phyla Coelenterata 

 and Ctenophora 



The phyla Coelenterata and Ctenophora contain the relatively simple types 

 of animals characterized by radial to biradial symmetry and by a generally 

 primitive plan of organization. This plan centers about a single internal 

 digestive cavity, typically with a single opening to the exterior. The bodies 

 of these animals are frequently composed of only two cellular layers — one 

 covering the external surface, the other lining the digestive cavity. Cellular 

 differentiation has given rise to tissues, but the organs that are present are 

 very primitive. Although the coelenterates and ctenophores are sometimes 

 considered as subphyla within a. single phylum Coelenterata, their funda- 

 mental diflferences justify placing them in separate phyla. 



The Phylum Coelenterata 



The Coelenterata are the simplest many-celled animals possessing the diges- 

 tive cavity characteristic of all Metazoa except mesozoans and sponges. The 

 digestive cavity with its single opening, the mouth-anus, is responsible for 

 the name Coelenterata, literally "hollow intestine." Coelenterates may be 

 defined as radially symmetrical animals consisting in the simplest cases of two 

 layers of cells, epidermis and gastrodermis, separated by a non-living, non- 

 cellular, secreted supporting lamella. This is true only of the most primitive 

 forms; all others have developed a middle layer which contains cells and is 

 therefore comparable with the mesenchyme, of mesodermal origin, in higher 

 animals. Although this third layer is rudimentary at best, its presence casts 

 doubt on the common general statement that coelenterates as a group are 



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