CHAPTER V 

 THE COELENTERATES AND CTENOPHORES 



A. PHYLUM COELENTERATA 



Coelenterates are diploblastic, radially symmetrical animals, 

 usually bearing tentacles and provided with nettling cells. In 

 fundamental structure, the bodies are but slightly modified from 

 the plan of arrangement of the gastrula stage which occurs in 

 the embryology of all higher Metazoa. Because of their form, 

 Cuvier included them along with the echinoderms in his type 

 Radiata. Extreme differences in structure preclude the possi- 

 bility of uniting these two groups on the basis of superficial 

 agreement in disposition of their parts. The echinoderms 

 have a true body cavity or coelom in addition to the digestive 

 cavity. In the coelenterates, however, only one' cavity is 

 found. Phylogenetically, this single cavity represents the gas- 

 trula cavity from which in higher forms of life both digestive 

 cavity and body cavity arise during later development. Since, 

 in the coelenterates, these two cavities are not differentiated, 

 Leuckart suggested the name Coelenterata which they now bear, 

 carrying with it the idea of lack of specialization of coelom from 

 the enteron. 



Morphological Types. — Body form among the coelenterates is 

 usually referable to one of tw.o types, the hydroid or polyp form 

 and the medusoid or jellyfish form. While these two forms are 

 clearly differentiated one from the other, in fundamental struc- 

 ture they are essentially alike. In each, tentacles are usually 

 present and function in grasping food and bringing it into the 

 mouth. These tentacles, and in some instances the body also, are 

 supplied with nettling cells which are tubular threads coiled 

 within a small bladderlike structure. As a means of defence or of 

 offence, these threads are discharged from the nematocysts with a 

 force sufficient to carry them even through the chitinous body 

 covering of small crustaceans introducing into the wound an 

 irritating fluid. By their use, some coelenterates, the Portuguese 

 man-of-war for instance, are capable of inflicting painful injury 

 even to man. 



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