146 THE HISTORY OF CREATION. 
originated. Scarcely distinguishable from the Hydra are the 
adherent Hydroid polyps (Campanularia, Tubularia), which 
produce freely swimming medusze by budding, 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- 
dusze, are among the strongest proofs of the truth of the 
theory of descent. For just as Medusz 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 Siphonophora.™ 
(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-jellies. The Ctenophora, which are also 
called Ribbed-jellies, possess a body of the form of a cu- 
eumber, which, like the body of most Hood-jellies, 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, ciliated 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.) 
