CHAPTER IX. 



CCELENTERA. 



Class i. Hydrozoa 'Hydromedusae. 



(biphonopnora. 

 Class 2. Scyphozoa -f Acraspeda. 



(.Actinozoa. 



Class 3. Ctenophora. 



< Hydromedusse. 

 or Class 1. Hydrozoa •< Siphonophora. 

 ( Scyphomedusx. 

 Class 2. Actinozoa. 



Classy. Ctenophora. 



The Ccelentera — including zoophytes, jelly-fish, sea-ane- 

 mones, corals, and the like — form a very large series of 

 Accelomate Metazoa, i.e. multicellular animals without a 

 body cavity. Their simplest forms are not much above the 

 level of the simplest sponges, but the series has been more 

 progressive. Thus many illustrate the beginnings of definite 

 organs. In their variety they seem almost to exhaust the 

 possibilities of radial symmetry, and some types {e.g. 

 Ctenophora) may be regarded as pioneers of the yet more 

 progressive bilateral "worms." Many are very vegetative, 

 deserving the old name of zoophytes (which should rather 

 be read backwards — Phytozoa), and in their budded colonies 

 afford interesting illustrations of organic co-operation and 

 division of labour. 



General Characters. 



The Ccelentera are simple, almost wholly marine, forms in 

 which the primary long axis of the gastrula becomes the long 

 axis of the adult, which is almost always radially symmetrical 

 about this axis. There is no body cavity or cozlom, distinct 

 from the primitive digestive cavity (enteron) and its outgrowths. 

 In the lower members of the series, the primary opening of 

 this cavity becomes the mouth of the adult, but in the more 

 specialised types there is an (ectodermic) oral invagination, 



