COELENTEEA (JELLY-FISHES, CORALS, ETC.). 43 



COELENTERA. 



The rest of the Gallery is occupied by the fossil G-alleryX. 

 COELENTERA (Hollow-guts). This great division of the Table-cases 

 animal kingdom, which comprises sea-anemones, corals, wall-cases 

 jelly-fishes, and their allies, owes its name to the fact that 1-6. 

 the body-wall of each individual encloses a single hollow 

 chamber (not many as in sponges) which forms the digestive 

 cavity of the animal, and is not traversed by any separate 

 gut such as is found in echinoderms and all the other groups 

 that follow. The body-wall consists of an inner and an 

 outer layer of cells, the " endoderm " and " ectoderm " as 

 they are respectively called, and between them a jelly-like 

 layer, the " mesogloea," which is very thick and noticeable in 

 an ordinary jelly-fish. In a jelly-fish, too, one may readily 

 observe that the organs of the body are regularly arranged on 

 lines radiating from the centre (see Fig. 18). A radiate 

 symmetry of this kind is found in most Coelentera. In 

 many Coelentera the individuals or zooids unite to form 

 colonies. 



Setting aside the Ctenophora, which some exclude from 

 the Coelentera, and of which no fossils are known, the 

 Coelentera may be divided into three classes : Hydrozoa, 

 Scyphozoa, and Anthozoa. 



The Hydrozoa include the fresh water polyp Hydra, the 

 Hydroid zoophytes such as Scrhdaria the sea-fir, the corals 

 Millepora and Stylaster, and various forms without fossil 

 allies. All of these have a simple tubular body-wall, 

 enclosing a cylindrical cavity which is not divided by 

 partitions ; the mouth is not pushed inwards ; the repro- 

 ductive cells are derived from the ectoderm and are set free 

 directly to the exterior. 



The Anthozoa (Flower-animals, also called Actinozoa 

 or Eayed animals) include the sea-anemones, nearly all the 

 stony corals, precious coral, sea-fans, sea-pens, and dead 

 men's fingers. In these the mouth is pushed inwards, so as 

 to form a tube; the body-cavity is divided by partitions, 

 called mesenteries, which consist of mesogloea covered with 

 endoderm; the reproductive cells are derived from this 

 endoderm and are set free into the body-cavity whence they 

 pass to the exterior through the mouth. 



The Scyphozoa (Cup-animals) comprise the larger jelly- 

 fishes. Some place them with the Hydrozoa, which they 



