CHAPTEE IV. 



COELENTERATA.* 



Radially symmetrical animals with only one cavity in the body 

 the gastrovascular space which serves alike for digestion and circula- 

 tion. The generative cells are always either ectodermal or endodermal. 



The Coelenterata, which include polyps, corals, sea-anemones, jelly- 

 fishes, etc., are multinucleate animals, in which the greater number 

 of the nuclei are arranged in regular layers at the body surfaces, and 

 constitute, with the protoplasmic layer which contains them, the 

 external and internal epithelia, commonly called the ectoderm and 

 endoderm respectively. Between these two layers the protoplasm is 

 reduced to a sparse reticulum, without or with only a few nuclei, the 

 spaces between the strands of the reticulum being filled by a gelatinous 

 matter, the jelly, f When this interposed gelatinous layer is thin and 

 inconspicuous, as in most polyps, it is called the supporting lamella, 

 or structureless lamella ; when it is thick and bulky, as in some parts 

 of the jelly-fishes, it is called simply the jetty. Further, these layers 

 are always differentiated to a greater or less extent into functional 

 tissues contractile tissues and nervous tissues ; and nematocysts are 

 always present as differentiations of the ectoderm or endoderm, or of 

 both layers ; the generative cells are always products of one of these 

 layers ; and finally, the ectoderm or the protoplasm of the jelly very 

 commonly secretes a skeletal tissue, which may be either cuticular, 

 horny, or calcareous. On the other hand, the internal surface of the 

 body is not differentiated into organs of circulation, of digestion, or 

 of coelom distinct from each other. The vegetative processes are 

 performed by the internal surface of the enteric cavity, or gastro- 

 vascular space as we shall call it, of which the central part func- 

 tions as stomach and intestine, the peripheral as vascular system. 



* R. Leuckart, Zoologischc Untersuclmngen, I., Giessen, 1853. C. Chun, 

 Coelenterata in Bronn's Klassen u. Ordnungen, Bd. 2, Abth. 2. 



t Sometimes called the mesoglaea an unsuitable term, because it suggests an 

 ectoglaea and entoglaea, which do not exist. 



