202 



THE EVOLUTION OF SEX. 



ova an embryo develops, which eventually settles down to start 

 a new sessile colony. And thus through the seasons we have 

 hydroid asexually producing sexual medusoids, and these again 

 producing hydroids. The life-history for two complete rhythms 

 may be written in the formula, in which M, F, and A stand for 

 male, female, and asexual forms respectively, — 



J- -^ y ^ 1? 



A, asexual hj-droid ; S, sexual medusoid ; fertilised ova at base. 



Or take, in slight contrast, the life-story of the common jelly- 

 fish Aurelia. Large free-swimming sexual animals produce ova 

 which are fertiHsed by sperms ; the embryo develops, not how- 



The alternation of generations in the common jelly-fish Aurelia ; i, the free-swimming embryo, 

 or planula ; 2, the embryo settled down ; 3, 4, 5, 6, the developing asexual stage, or hydra- 

 tuba ; 7, 8, the formation of a pile of individuals; 9, the liberation of these ; 10, 11, the 

 acquisition of the free-living sexual medusa form. — From H^ckel. 



