230 



THE STUDY OF ANIMAL LIFE CHAP. 



or Antipatharians which are distantly related to sea- 

 anemones. The beautiful open-sea Ctenophores or Comb- 

 bearers, such as Beroe and Pleurobrachia, represent 

 the climax of activity among Ccelenterates. They move 

 by combs of fused cilia and are in many ways so divergent. 

 e.g. in having the stinging cells in almost all cases replaced 

 by adhesive cells, that many zoologists would place them 

 quite outside the series of Ccelentera. 



FIG. 65. THE ALTERNATION OF GENERATIONS IN THE COMMON JELLY- 

 FISH Aurelia. 



1, The free-swimming embryo ; 2, the embryo settled down ; 3, 4, 5, G, 

 the developing asexual stages, or hydra -tubae ; 7, 8, the formation of a 

 pile of individuals by transverse budding ; 9, the liberation of these indi- 

 viduals ; 10, 11, their progress towards the free-swimming sexual medusa 

 form. 



(From the Evolution of Sex ; after Haeckel.) 



Besides Hydra there are few other freshwater Coelentera, 

 such as the hydroid Cordylophora which occurs in brackish 

 water and in canals, a strange form Polypodium which is 

 parasitic in its youth on the eggs of the Russian sturgeon, 

 and a few remarkable freshwater jellyfishes, e.g. Limno- 



