1 68 PHYLUM CCELENTERA 



primary opening of this cavity becomes the mouth of the adult ^ 

 but in the more specialised types there is an (ectodermic) oral 

 invagination, which forms a gullet-tube or stomodteum. 

 Between the ectoderm and endoderm of the body wall there 

 is a supporting layer, or mesogloea, often of jelly-like con- 

 sistency. In Ctenophora, however, a more definite mesoderm 

 is established at an early stage in development. In the 

 simplest cases the mesogloea is a secretion quite devoid of cells, 

 but secondary cells may migrate into it from the endoderm. 

 Stinging cells of varying complexity are almost always present, 

 but in most of the Ctenophora their place is taken by adhesive 

 cells. 



The Coelentera exhibit two types of structure — polypoid 

 and medusoid — which recur in modified forms throughout the 

 group, and may be both present in the course of one life- 

 history, when they illustrate the phenomenon of alternation of 

 generations , or metagenesis . The more primitive type is the 

 sessile tubular polyp, which, at its simplest, may be com- 

 pared to a gastrula fixed by one end, and furnished with a 

 crown of tentacles round the central aperture of the other pole. 

 The other derived form, which has become specialised in 

 various directions, is the active medusoid or jelly-fish type. 

 In several divisions the formation of a calcareous " skeleton " 

 by the polypoid type results in the production of " corals.'* 

 Multiplication by budding is common, and often results in the 

 formation of colonies, some of which show considerable divi- 

 sion of labour. 



The preservation of the primary axis, the absence of true 

 mesoderm and of a ccelom, are often said to distinguish 

 Coelentera and Sponges from the other Metazoa {Coelomata), 

 but the results of recent researches on the nature of the meso- 

 derm seem to rob this distinction of part of its precision. 



General Survey 



The CcElentera or " Stinging animals " include a large 

 number of familiar and beautiful forms. The graceful 

 zoophytes which fringe shells and stones, and the tiny 

 transparent bells which float in the pools ; the sea-anemones 

 which cluster in the nooks of the rocks, and the active jelly- 

 fish which swim on the waves, are but different expressions 



