CHAPTER XXVIII 



COELENTERATES IN GENERAL 



The hydra belongs to a third phylum, known as Coelenterata, made 

 up of animals which differ markedly from either Protozoa or Porifera 

 and which, though they are very simple, agree in the general plan of 

 structure with higher animals. This plan involves the existence of a 

 central digestive cavity or enteron with a mouth opening into it. The 

 wall of the body is made up of two cell layers, ectoderm and entoderm, 

 which occupy the same relative positions as the corresponding germ 



Meso^/ea 



Gastro- vascu/af 

 cctv/fy 



Fig. 58. — Diagrams illustrating the comparison of the structure of a polyp (A) with that of 



a medusa (5). 



layers do in the embryo. For the reason that only two germ layers 

 are considered to be represented, the coelenterates are termed diploblastic 

 and may be compared to the gastrula stage in the development of higher 

 animals. In some cases, as in certain sense organs, collections of similar 

 cells form simple tissues, but there is no development of true organs. 

 The coelenterates have a radial type of symmetry, the number of anti- 

 meres varying in different groups but tending to be of an even number. 

 Another characteristic of all coelenterates is the presence of nematocysts, 

 which have already been described. 



169. Polyps and Medusae. — Coelenterates exist in the form of two 

 general types. Those of the type known as polyps are attached to some 

 object. They have a mouth and almost without an exception possess a 

 ring of tentacles at the free end. Those of the medusa, or jellyfish, 



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