272 GENERAL ZOOLOGY 



Coelen'tera are Hydrozoa, Scyphozoa, Actinozoa, and Cte- 

 nophora. The classes constitute a fairly distinct phylum by 

 including animals with radially symmetrical bodies (at least 

 in external appearance) in which there is but one cavity. 

 This cavity, joining the mouth-opening, is the digestive 

 cavity, and is not separated from a body-cavity, as in higher 

 animals (whence the derivation of the name of the phylum). 

 The body develops from an embryo that has two germ-layers, 

 - the ectoderm and the endoderm. In this particular it is 

 different from every phylum of animals heretofore described. 

 The characteristic organs of offense and defense are the net- 

 tling-capsules. By some authorities the Ctenophora are 

 separated into a phylum by themselves, partly because they 

 do not possess nettling-capsules. 



