94 THE STORY OF LIFE IN THE SEAS. 



Jelly-fish and attach themselves to them as 

 parasites. 



In other species, however, the " alternation of 

 generations " is entirely lost, and the egg gives 

 rise directly to a free-swimming little Jelly-fish 

 which in time grows to be like its parent in 

 size and shape. In this case the fixed or sessile 

 form in the life-history is, as it were, omitted in 

 order that the animal may lead a life independent 

 of the coast and sea-bottom. 



The Jelly-fish, then, present us with an interest- 

 ing example of a manner in which the life- 

 history of an animal may be modified for or 

 adapted to this surface-swimming habit. 



There is also another point of interest 

 about these creatures in this connection. In 

 writing about the Copepods I pointed out the 

 mechanical contrivances they exhibit for keeping 

 themselves afloat, namely, the spines, hairs and 

 oil globules. Jelly-fish have neither spines nor 

 oil globules of the same nature, but still their 

 bodies are very light in the water and in the 

 absence of muscular movements sink but slowly 

 to the bottom. This lightness is due to the fact 

 that all the tissues and organs of which it is 

 composed are very largely distended with water. 

 When the body of a Jelly-fish is analysed it is 

 found that over 95 per cent, of it consists of 

 water. This power of absorbing large quantities 

 of fluid into the tissues, while it increases the size 

 of the body, proportionately diminishes its weight 

 in water. 



It has also another effect. It makes the 

 tissues of the body much more transparent and 



