146 THE HISTORY OF CREATION. 



originated. Scarcely distinguishable from the Hydra are the 

 adherent Hydroid polyps (Campanularia, Tubularia), which 

 produce freely swimming medusae by buddiug, and out 

 of the eggs of these there again arise adherent polyps. 

 These freely swimming Hood-jellies are mostly of the form 

 of a mushroom, or of an umbrella, from the rim of which 

 many long and delicate tentacles hang. They are among the 

 most beautiful and most interesting inhabitants of the sea. 

 The remarkable history of their lives, and especially the 

 complicated alternation of generation of polyps and me- 

 dusae, are among the strongest proofs of the truth of the 

 theory of descent. For just as Medusae still daily arise out 

 of the Hydroids, did the freely swimming medusa-form 

 originally proceed, phylogenetically, out of the adherent 

 polyp-form. Equally important for the theory of descent is 

 the remarkable division of labour of the individuals, which 

 among some of them is developed to an astonishingly high 

 degree, more especially in the splendid Sipliono'phoraP 

 (Plate VII. Fig. 13). 



The third class of Sea-nettles — the peculiar division of 

 Comb-jellies (Ctenophora), probably developed out of a 

 branch of the Hood-jeUies. The Ctenophora, which are also 

 called Ribbed-jellies, possess a body of the form of a cu- 

 cumber, which, like the body of most Hood-jeUies, is as clear 

 and transparent as crystal or cut glass. Comb or Ribbed- 

 jellies are characterized by their peculiar organs of motion, 

 namely, by eight rows of paddling, cilikted leaflets, which run 

 in the form of eight ribs from one end of the longitudinal axis 

 (from the mouth) to the opposite end. Those with narrow 

 mouths (Stenostoma) probably developed later out of those 

 with wide mouths (Eurystoma). (Compare Plate VII. Fig. 16.) 



