68 



INVERTEBRATE MORPEOLOGT. 



CHAPTEE V. 



TYPE CGELENTERA. 



The Coelentera include the diploblastic Metazoa, only 

 two germ-layers, the ectoderm and endoderm, being repre- 

 sented in their organization (Fig. 33). Between these two 

 layers, however, a third (Fig. 33, mg) 

 is invariably present, which in its 

 primitive condition is not cellular, but 

 consists of a gelatinous or fibrous sub- 

 stance secreted by one of the two 

 cellular layers. Usually, however, 

 cells from the endoderm or ectoderm 

 wander into it, and sometimes are so 

 numerous as to give it the appearance 

 of a cellular layer. Even in such 

 cases, however, the gelatinous matrix 

 is the fundamental substance of the 

 Pig. 33.— Diagram op Hy- layer, which it seems preferable to 

 dra TO SHOW the Gen- ^gj,j^ ^j^g mesoglcea, rather than to imply 

 an homology which does not exist by 

 designating it the mesoderm. 



In consequence of the absence 

 of the mesoderm the Coelenterates 

 present in the interior only a single cavity. Consequently 

 it may be said that the coelom is not represented in the 

 Coelentera, though their central cavity is usually regarded 

 as equivalent to both coelom and enteron of the higher 

 forms. The so-called endoderm, however, seems to be homol- 

 ogous with their mesoderm plus endoderm, and may be more 

 accurately termed the mes-endoderm, and it seems preferable 

 to regard the coelom as not yet differentiated. 



Another feature which obtains throughout the group is 

 the radiate ground-form. In many but one axis can be de- 



EKAL StKUCTUKE OF 

 CCBLBHTERATE. 



ec = ectoderm. 

 en = eadoderm. 

 mg = mesoglcEii. 



