ALTERNATION OF GENERATIONS. 203 
Stauridium. The Medusa is the sexually mature form of 
the cycle of the species ; its ova develope into polypes, 
which constitute the intermediate form in their develop- 
ment; that is to say, it is not transformed into the 
FIG. 14. 
animal from which it is derived, but produces gemmules. 
Only in this generation does the species revert to the 
sexual form. 
We shall understand this alternation of generations 
if we begin with the simplest Medusa polypes. Such 
a one is the annexed Hydractinea carnea, of which 
the female individual is portrayed. Compared with 
the intermediate form, Stauridium, the preliminary 
phase of the Cladonema, reproducing itself asexually, 
