42 



GUIDE TO THE CORAL GALLERY. 



corresponding alteration of form (dimorphism) among the units of 

 the colony. 



The fertilised egg of the Medusa develops into the fixed Hydroid 

 colony, the latter forming buds in which the eggs are produced. 

 This is an instance of alternation of generations, a phenomenon very 

 common among the Hydrozoa, but especially marked in cases where 

 the asexually formed generative polyp or individual becomes detached 

 and swims away. 



Fig. 3.1 



Portion of a colony of BougainviUea fruticosa, magnified. (After 



AUman.) 



In cases where the generative polyps are free-swimming, advan- 

 tage would result from the eggs being scattered over a wide area, and 

 not crowded in the neighbourhood of the parent stock ; and, further, 

 the egg-carrying polyp (Medusa) can swim to the surface where food 

 is plentiful. The free generative polyp, or Medusa of ClavateUa 

 prolifera (Fig. 6), shows a transition between a feeding polyp and 

 Medusa. The umbrella can scarcely be said to exist in this case, the 



^ From " Encyclopaedia Britannica." 



