i8o 



MANUAL OF ELEMENTARY ZOOLOGY 



out by the medusa and leads to the formation of polyps. 

 The asexual reproduction consists in the bud- 

 OonoratVoZ ' din S off of medusa; from the polyp stock. 

 Whereas, however, 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 succeeding one another. It will be remembered that 



a similar alternation of genera- 

 tions occurs in Monocystis and 

 that the malaria parasite has a 

 more complicated life history 

 of the same kind. The asexual 

 generation of Obelia is re- 

 latively inactive, gathering 

 much nourishment and spend- 

 ing little : the sexual genera- 

 tion is active, spending its 

 substance freely in locomotion, 

 which ensures the distribution 

 of the species, and thus open- 

 ing up fresh food supplies and 

 increasing the chances of 

 escape from enemies. Here, as in sexual reproduction, 

 we find a distribution of labour among individuals. 



The above explanation of the reproduction of hydroids 

 differs in one respect from that which is 

 Reproduction generally given. On the analogy of the budding 

 buiidin». ny " of Hydra, it is usual to regard the formation of 

 a hydroid colony by budding as a kind of 

 asexual reproduction in which there are formed numerous 

 "individuals" which do not separate. In that case the 

 alternation of generations contains an indefinite number of 

 acts of asexual reproduction between each two sexual acts. 

 We have preferred to treat the polyp stock as one individual, 

 containing a number of semi-independent parts— the 

 hydranths — each of which repeats the structure of the 

 whole body as it was at first, together with certain other 



A B 



Fig. 115. — A, Planula larva; 

 B, the young polyp into 

 which the planula grows 

 after settling. 



