Case 3. 
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 Bougainvillea fruticosa, magnified. (After 
Allman.) 
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 Glavatella 
prolifera (Fig. 6), shows a transition between a feeding polyp and 
Medusa. The umbrella can scarcely be said to exist in this case, the 
1 From “ Encyclopaedia Britannica.” 
