VIII 

 FRESH-WATER JELLY-FISHES 



MOST people nowadays know a jelly-fish when they 

 see one and recognise that it is eminently a 

 product of the sea one sees them washed up on the sea- 

 shore, soft discs of transparent jelly of the size of cheese- 

 plates (Fig. 2). They have a mouth in the centre of the 

 disc, often at the end of a depending trunk, like the 

 clapper of a bell. Some have tentacles, sometimes yards 

 long, which sting like nettles. They also have eye-spots, 

 an internal system of canals and muscles which enable 

 them to swim by causing the edge of the disc or bell to 

 contract and expand in alternate strokes. There are 

 hundreds of kinds of marine jelly-fish varying in size from 

 a sixpence to that of a dinner table, and until twenty-five 

 years ago none were known to live in ponds, lakes, or rivers. 

 Although they often are carried up estuaries, and may 

 stay for a time in brackish water, or even in fresh water, 

 none were known which really lived and bred in fresh 

 water. They were regarded, as are star-fishes and sea- 

 urchins, as distinctively marine, and debarred by the 

 delicacy of their watery jelly-like substance from tolerat- 

 ing the change from sea water to fresh water as a per- 

 manent thing. All fresh-water animals fishes, shell-fish, 

 cray-fish, worms, and polyps are derived from closely 

 similar marine animals, are in fact sea-things which have 

 suffered a change, and been able to stand it 



