ALTERNATION OF GENERATIONS. 



203 



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 preHminary 

 phase of the Cladonema, reproducing itself asexually, 



