CHAPTER IX. 



CCELENTERA. 



Class ^. HvDRozoA I ">'^''°'"\^"-^=>=' Class I. HydrozoaI"^"^™'"'^''"^^"^- 



i biphonophora;, \ Scyphomedusie. 



Class 2. SCVPHOZOA / Acraspeda, or Class 11. Actenozoa. 



\ Actinozoa. 

 Classy. Ctenophora. Class III. Ctenophora. 



The Coelentera — including zoopliytes, jelly fish, sea 

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

 Acoelomate Metazoa, i.e., of 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 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 most in- 

 teresting illustrations of organic co-operation and division 

 of labour. 



General Characters. 



The Cxlentera are simple, usually 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 ccelome, distinct from 

 the primitive digestive cavity (enteron) and its outgroivths. 

 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 the7-e is an (ectodermic) oral invagination, 

 which forms a gullet tube. Betweeti the ectoderm and 

 endoderm of the body wall, there is a supporting layer, or 



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