BRANCH CCELEKTERATA: THE POLYPS, ETC. 



99 



central stem are many medusoid structures, the swimming- 

 bells, by means of 

 whose opening and 

 closing the whole col- 

 ony is made to swim 

 through the water. 

 Each swimming - bell 

 is a modified medusa- 

 zooid, without tenta- 

 cles, without digestive 

 or reproductive or- 

 gans, but exercising 

 the power of swim- 

 ming by contracting 

 and forcing the water 

 out of the hollow bell 

 just as is done by 

 the free medusae. Be- 

 low the swimming- 

 bells, at the lower end 

 of the central stem, 

 are grouped many 

 structures presenting 

 at first sight a confu- 

 sion of variety and 

 complexity, but on 

 careful examination 

 revealing themselves 

 to be polyp- and me- 

 dusa-zooids modified 

 to form at least five 



kinds of particularly . 



FTG. 14. A colonial jelly fish (Siphonophora). 



functioning Struc- (After Haeckel.) 



tures. There are many flattened scale-like parts whose 

 function is simply that of affording a passive protection, 



