126 CCELENTERA. 



■which forms a gullet - tube. Between the ectoderm and 

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

 mesoglcea, of jelly-like consistency. In the simplest cases this 

 is quite devoid of cells, but secondarily, they may migrate 

 into it from the endoderm. Stinging cells of varying com- 

 plexity are almost invariably present, but in almost all the 

 Ctenophora their place is taken by adhesive cells. 



The Coslentera exhibit two divergent types of structure, 

 which recur constantly, in modified forms, throughout the 

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

 history, when they illustrate the phenomenon of alternation of 

 generations or metagenesis. Of the two, the more primitive 

 type is the sessile tubular hydroid, which may be compared to 

 a gastrula fixed by one end, and furnished with a crown of 

 tentacles placed round the central aperture of the other pole. 

 The other derived form, which has become specialised in 

 various directions, is the active medusoid or jellyfish type. 

 In several divisions the formation of a calcareous " skeleton" 

 by the hydroid type may result 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 cozlom, are often said to distinguish 

 Ccelentera and Sponges from the other Metazoa (Ccelomata), 

 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 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-ane- 

 mones 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 so characteristic of the series, 

 and illustrate, the former in the class Hydrozoa, the latter 

 in the Scyphozoa, the two physiological tendencies of the 

 Ccelentera. The delicate iridescent globes, which represent 

 the third class, the Ctenophora, illustrate the climax of 



