134 PHYLUM C(ELENTERA. 



primary opening of this cavity becomes the month of the adult, 

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

 invagination, which forms a gullet-tube or stomod&um. 

 Between the ectoderm and endoderm of the body wall there 

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

 sistency. In the. simplest cases this 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 Cozlentera 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 polype, 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 wliich 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 

 Ccvlentera and Sponges from the other Metazoa (Cculomata), 

 but the results of recent researches on the nature of the 

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



GENERAL SURVEY. 



The Coelentera 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 of the antithesis between sedentary polypoid 

 and active medusoid types which is characteristic of the 



