44 The Under -Water World, 



method of reproduction known as 

 "alternation of generations/' a fixed 

 colony of polyps at certain seasons 

 develops special " buds " which detach 

 themselves from the parent stock and 

 swim away as little bell-shaped jelly- 

 fishes. These lead merry buccaneering 

 lives for a time, and then lay eggs that 

 likewise develop into larvae, but which, 

 instead of cruising at large, settle down 

 and give rise to a branching colony of 

 polyps. Some of these in their turn 

 produce free-swimming, egg-laying jelly- 

 fish, and so on, until presumably the end 

 of time. 



In certain hydroid colonies we find 

 a division of labour. Whilst the majority 

 of polyps are intent on feeding and thus 

 maintaining the general status of the 

 colony, a number of mouthless ones bud 

 off, giving rise to free-swimming medusae. 



The Hydra of our rivers and ditches 

 take the form of blobs of jelly with four 



