CHAPTER III.- 



CCELENTERATA. 



Animals mora or less resembling in structure the 

 Freshwater Polyp (see p. 130). 



Classes.— PoKirERA, or Sponges. ^ 



AcTiNOzoA, or Sea-anemones and Corals. 

 Hydkozoa, or Jelly-fish and their'allies. 

 Ctenophoka, or Cydi.ppe and its allies. 



The Ccelbnterata are multicellular radiate animals 

 with no distinction between body-cavity and digestive 

 cavity (see pp. 40, 46). The body contains a simple 

 cavity, the enter on. 



Some of the lower forms of the Ccelenterata {Hydra) 

 possess practically only the two primary body layers, 

 ectoderm and endoderm, the mesoderm being present 

 only in a very rudimentary form : such an animal is 

 said to be dlploblastic, i.e. possessed of (only) two 

 body-layers, while the majority of animals are de- 

 scribed as triploblastic, i.e. consisting of three layers. 

 This is regarded as the most primitive and ancient 

 type of the Metazoa ; for it is comparable with the 

 two-layered gastrula, which has been described as a 

 stage of the embryonic development of mauy animals : 

 hence we may suppose that the rest of the iletazoa 

 are descended from an ancestor of this kind, and for 

 this hypothetical ancestor the name of gastrsea has 

 been proposed. So gi-eat indeed is the difference 



