108 HYDRA AND OBELIA. PHYLUM CCELENTERATA 



ALTERATION OF GENERATIONS 

 It will be seen that Obelia, like Hydra, reproduces itself both 

 sexually and asexually. Sexual reproduction is carried out by 

 the medusa and leads to the formation of polyps. The asexual 

 reproduction consists in the budding off of medusae from the polyp 

 stock. But whereas in Hydra, the two processes go on side by 

 side sometimes in the same individual, and succeed one another 

 quite irregularly, in Obelia there are two different types of 

 individual— the polyp stock and the medusa— which follow 

 one another regularly and are each confined to one method of 

 reproduction. Thus we have a definite alternation of generations, 



a sexual and an asexual form suc- 

 ceeding one another. It will be 

 remembered that such generations 

 also alternate in Monocystis and 

 that the malarial parasite has a 

 more complicated life-history of the 

 same kind. The asexual generation 

 of Ohelia is relatively inactive, 

 gathering much nourishment and 

 spending little : the sexual genera- 

 tion is active, spending its sub- 

 stance freely in locomotion, which 

 ensures the distribution of the 

 species and thus opens up fresh 

 food supplies and increases the 

 chances of escape from local dangers. The gist of the story is the 

 distribution of labour among individuals of different kinds. 



su 



Fig. 74. — A diagram to show the 

 development of medusae as 

 buds on a blastostyle. 



bl., blastostyle ; s.u.c. subumbrellar cavity ; 

 1-6, successive stages in the develop- 

 ment of a medusa. 



METAGENESIS 



The designation ' alternation of generations ' has been applied 

 to a number of different types of life-history which have in com- 

 mon only the fact that reproduction is accomplished differently 

 in successive phases of the reproducing organism. It is a useful 

 ' omnibus ' term but should not be taken to imply more than 

 superficial resemblance between the processes it covers. The type 

 met with in hydroids is known as metagenesis. We shall observe 

 a quite different process, which does not involve asexual repro- 

 duction, in the Uver fluke and again in a nematode worm. Botanists 

 restrict the term to life-histories, such as those of mosses and 



