130 CCELENTERA. 



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

 is quite devoid of 'cells ', but secondarily, these may migrate into 

 it from the endoderm. Stinging cells of varying complexity 

 are almost invariably present, but in most members of the 

 class Ctenophora they are modified into adhesive cells. 



The Ccelentera exhibit tivo 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 jelly fish 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 shoiv considerable divi- 

 sion of labour. 



The preservation of the primary axis, the absence of true 

 mesoderm and of a ccelome, are often said to distinguish 

 Ccelentera and Sponges from other Metazoa, or 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 " Nettle animals " of the Germans, 

 include a large number of familiar and beautiful forms of 

 life. 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 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 irridescent globes, which 

 represent the third class, the Ctenophora, illustrate the 

 climax of activity, for among them there is no sessile hydroid 

 type. 



In our survey of the series, however, we must pass over 



